ACTA FACULTATIS XYLOLOGIAE ZVOLEN 62 1/2020

TECHNICKÁ UNIVERZITA VO ZVOLENE DREVÁRSKA FAKULTA

ACTA FACULTATIS XYLOLOGIAE ZVOLEN

VEDECKÝ ČASOPIS SCIENTIFIC JOURNAL

62 1/2020

Scientific journal Acta Facultatis Xylologiae Zvolen publishes peer-reviewed scientific papers covering the fields of wood: structure and properties, wood processing, machining and drying, wood modification and preservation, thermal stability, burning and fire protection of lignocelluloses materials, furniture design and construction, wooden constructions, economics and management in wood processing industry. The journal is a platform for presenting reports and reviews of books of domestic and foreign authors.

Vedecký časopis Acta Facultatis Xylologiae Zvolen uverejňuje pôvodné recenzované vedecké práce z oblastí: štruktúra a vlastnosti dreva, procesy spracovania, obrábania, sušenia, modifikácie a ochrany dreva, termickej stability, horenia a protipožiarnej ochrany lignocelulózových materiálov, konštrukcie a dizajnu nábytku, drevených stavebných konštrukcií, ekonomiky a manažmentu drevospracujúceho priemyslu. Poskytuje priestor aj na prezentáciu názorov formou správ a recenzií kníh domácich a zahraničných autorov.

VEDECKÝ ČASOPIS DREVÁRSKEJ FAKULTY, TECHNICKEJ UNIVERZITY VO ZVOLENE 62 1/2020 SCIENTIFIC JOURNAL OF THE FACULTY OF WOOD SCIENCES AND TECHNOLOGY, TECHNICAL UNIVERSITY IN ZVOLEN 62 1/2020 Redakcia (Publisher and Editor’s Office): Drevárska fakulta (Faculty of Wood Sciences and Technology) T. G. Masaryka 24, SK-960 01 Zvolen, Slovakia Redakčná rada (Editorial Board): Predseda (Chairman): Vedecký redaktor (Editor-in-Chief): Členovia (Editors):

Jazykový editor (Proofreader): Technický redaktor (Copy Editor):

prof. Ing. Ján Sedliačik, PhD prof. Ing. Ladislav Dzurenda, PhD. prof. RNDr. František Kačík, PhD. prof. RNDr. Danica Kačíková, PhD. prof. Ing. Jozef Kúdela, CSc. prof. Ing. Ladislav Reinprecht, CSc. prof. Ing. Jozef Štefko. CSc. doc. Ing. Pavol Joščák, CSc. doc. Ing. Hubert Paluš, PhD. Mgr. Žaneta Balážová, PhD. Antónia Malenká

Medzinárodný poradný zbor (International Advisory Editorial Board): Pavlo Bekhta (UA), Nencho Deliiski (BG), Vlado Goglia (HR), Denis Jelačić (HR), Bohumil Kasal (USA), Wojciech Lis (PL), Remy Marchal (FR), Miloslav Milichovský (CZ), Róbert Németh (HU), Peter Niemz (CH), Kazimierz A. Orlowski (PL), Franc Pohleven (SI), František Potůček (CZ), Włodzimierz Prądzyński (PL), Alfréd Teischinger (AT), Jerzy Smardzewski (PL), Mikuláš Šupín (SK), Richard P. Vlosky (USA), Rupert Wimmer (AT) Vydala (Published by): Technická univerzita vo Zvolene, T. G. Masaryka 2117/24, 960 01 Zvolen, IČO 00397440, 2020 Náklad (Edition) 150 výtlačkov, Rozsah (Pages) 188 strán, 16,36 AH, 16,50 VH Tlač (Printed by): Vydavateľstvo Technickej univerzity vo Zvolene Vydanie I. – apríl 2020 Periodikum s periodicitou dvakrát ročne Evidenčné číslo: 3860/09 Časopis Acta Facultatis Xylologiae Zvolen je registrovaný v databáze (Indexed in): Web of Science, SCOPUS, ProQuest, AGRICOLA, Russian Science Citation Index Všetky práva vyhradené. Nijaká časť textu ani ilustrácie nemôžu byť použité na ďalšie šírenie akoukoľvek formou bez predchádzajúceho súhlasu autorov alebo vydavateľa.

© Copyright by Technical University in Zvolen, Slovak Republic.

ISSN 1336–3824

CONTENTS

01 JOZEF KÚDELA – RASTISLAV LAGAŇA – TOMÁŠ ANDOR – CSILLA CSIHA: VARIATIONS IN BEECH WOOD SURFACE PERFORMANCE ASSOCIATED WITH PROLONGED HEAT TREATMENT ..................................................................................................... 5 02 LADISLAV DZURENDA  MICHAL DUDIAK: THE EFFECT OF THE TEMPERATURE OF SATURATED WATER STEAM ON THE COLOUR CHANGE OF WOOD ACER PSEUDOPLATANUS L. ........................................ 19 03 VLADIMIR SHAMAEV: COMPUTER SIMULATION OF THE PROCESS OF END-GRAIN WOOD TREATMENT UNDER PRESSURE ........................... 29 04 NENCHO DELIISKI – LADISLAV DZURENDA – NATALIA TUMBARKOVA – DIMITAR ANGELSKI: MATHEMATICAL DESCRIPTION OF THE LATENT HEAT OF BOUND WATER IN WOOD DURING FREEZING AND DEFROSTING .............................................................. 41 05 JOZEF FEKIAČ  JOZEF GÁBORÍK  MÁRIA ŠMIDRIAKOVÁ: 3DFORMABILITY OF PERFORATED MATERIALS BASED ON VENEER ... 55 06 MIROSLAV REPÁK – LADISLAV REINPRECHT: PHYSICOMECHANICAL PROPERTIES OF THERMALLY MODIFIED BEECH WOOD AFFECTED BY ITS PRE-TREATMENT WITH POLYETHYLENE GLYCOL ............................................................................................................ 67 07 MIKULÁŠ SIKLIENKA - ANDREJ JANKECH: THE EFFECT OF SELECTED FACTORS ON OUT-OF ROUNDNESS DURING THE GREEN WOOD DRILLING ............................................................................................. 79 08 PIOTR TAUBE – KAZIMIERZ A. ORŁOWSKI – DANIEL CHUCHAŁA – JAKUB SANDAK: THE EFFECT OF LOG SORTING STRATEGY ON THE FORECASTED LUMBER VALUE AFTER SAWING OF PINE WOOD ................................................................................................ 89 09 ALENA OČKAJOVÁ  MARTIN KUČERKA  RICHARD KMINIAK  TOMASZ ROGOZIŃSKI: GRANULOMETRIC COMPOSITION OF CHIPS AND DUST PRODUCED FROM THE PROCESS OF WORKING THERMALLY MODIFIED WOOD ................... 103 10 ALENA ROHANOVÁ: NON-DESTRUCTIVE PENETRATION METHOD FOR DETERMINING THE QUALITY OF STRUCTURAL SPRUCE WOOD (PICEA ABIES KARST. L.) IN SITU .................................................. 113 11 NELLY STANEVA  YANCHO GENCHEV  DESISLAVA HRISTODOROVA: DEFORMATION COMPARISON OF UPHOLSTERED FURNITURE FRAMES WITH SIDE PLATES FROM PB, OSB AND PLY BY FEM ................................................................................... 125

12 JOZEF MITTERPACH  RASTISLAV IGAZ  JOZEF ŠTEFKO: ENVIRONMENTAL EVALUATION OF ALTERNATIVE WOOD-BASED EXTERNAL WALL ASSEMBLYS ................................................................... 133 13 MILOŠ HITKA – MARTINA LIPOLDOVÁ – JARMILA SCHMIDTOVÁ: EMPLOYEES’ MOTIVATION PREFERENCES IN FOREST AND WOODPROCESSING ENTERPRISES ......................................................................... 151 14 MARIANA SEDLIAČIKOVÁ – ZUZANA STROKOVÁ – JARMILA KLEMENTOVÁ – ANNA ŠATANOVÁ – MÁRIA MORESOVÁ: IMPACTS OF BEHAVIORAL ASPECTS ON FINANCIAL DECISIONMAKING OF OWNERS OF WOODWORKING AND FURNITURE MANUFACTURING AND TRADING ENTERPRISES .................................. 165 15 ERIKA LOUČANOVÁ – MIRIAM OLŠIAKOVÁ: IDENTIFICATION OF CUSTOMERS DRIVERS FOR THE WOOD BUILDING AS AN ECOLOGICAL INNOVATION IN BUILDING CONSTRUCTION IN SLOVAKIA ........................................................................................................ 177

ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 5–17, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.01

VARIATIONS IN BEECH WOOD SURFACE PERFORMANCE ASSOCIATED WITH PROLONGED HEAT TREATMENT AT 200 °C Jozef Kúdela – Rastislav Lagaňa – Tomáš Andor – Csilla Csiha ABSTRACT The research was aimed at analysing the discolouration, morphology and wetting of beech wood thermally modified under influence of a temperature of 200 °C acting during three heating periods (1, 3, and 5 hours). The results show that with progressing heating time, the lightness L* decreased significantly, while the coordinates a* and b* increased as long as for three hours. Subsequently, a moderate decrease followed. Just after one-hourlasting thermal modification, the total colour difference E value was much higher than 12. It means that a totally new colour resulted from the thermal treatment as short as onehour. Further treatment prolongation induced further E increase, nevertheless, more moderate. Enhanced roughness was indicated by an inspection of the changes in the beech wood surface morphology. But this fact was not confirmed unambiguously in all three heating periods. The results show that the beech wood thermal treatment resulted in significantly improved resistance of beech wood surface to wetting proven by the wetting contact angle values  ˃ 90°. Moreover, the time necessary for the complete drop soaking into the substrate increased by one or two orders. The wetting varied mainly due to the treatment temperature. The importance of treatment duration was not confirmed unambiguously. The reduced wood surface wetting significantly affected lowering the wood surface free energy, which was mainly due to a decrease in the polar component of this energy. This fact may negatively affect the surface treatment quality if the coating materials are applied on the wood modified this way.

Keywords: beech wood, thermal modification, colour, roughness, wetting, contact angle, surface free energy.

INTRODUCTION During the last two decades, considerable interest has been devoted to effects of heat energy on wood performance in the context of wood thermal, hydro-thermal and hydrothermal-mechanic treatment. The aim of these wood treatment technologies is a purposedriven modification of specified physical and mechanical properties of the treated wood.

The original purpose of wood thermal treatment was to enhance the wood hydrophobicity and, consequently, its dimensional stability. At the same time, caution was devoted to prevent substantial changes in the mechanic performance of wood treated in this way. The lowered sorption capacity of thermally modified wood is responded by lower values of equilibrium moisture content, FSP included (HILL 2006, BOONSTRA et al. 2007, SRINIVAS and PANDEY 2012, VOBOLIS and ALBREKTAS 2014, AYTIN et al. 2015, HORVÁTH 5

et al. 2016). The lowered wood hydrophilicity also impacts the dimensional stability of the thermally modified wood. The results presented in the papers MYES and OKSANEN 2003, AYTIN et al. 2015, ESTEVES and PEREIRA 2009, KAČÍKOVÁ and KAČÍK 2011) suggest that the thermally modified wood shows improved dimensional stability, with reduced swelling and shrinkage.

Lower water uptake capacity, swelling and shrinkage in thermally modified wood are supposed resulting largely from the chemical changes in the main wood components. The temperatures associated with wood thermal treatment (150–260 °C) cause first of all degradation of highly hygrophilous hemicelluloses (REINPRECHT and VIDHOLDOVÁ 2011, BOONSTRA and TJEERDSMA 2006, KAČÍK et al. 2016). This drop in hemicelluloses content leads to an increase in fragility (NAVI and PIZZI 2015).

The hydrophobicity of thermally modified wood is enhanced also as a result of degradation of amorphous cellulose and the allied increase in the relative proportion of crystalline cellulose. These two facts reduce the accessibility of free hydroxyl groups.

The reduction of the number and accessibility of free sorption sites has a decisive influence on dropping the equilibrium moisture content value and improving the associated dimensional stability (BHUIYAN and HIRAI, 2005, ESTEVES et al. 2008).

Cellulose crystallinity increases as far as 200 °C and exceeding this threshold is followed by a decrease in the crystalline cellulose proportion (ESTEVES and PEREIRA 2009).

Thermal treatment also induces a decrease in the cellulose polymerising degree, which causes weakening of bending and tensile strength of the thermally modified material (ESTEVES and PEREIRA 2009, HILL 2006, ZAWADSKI et al. 2016).

The lignin structure consists of covalent bonds that stabilize thermal oxidation and reactions during thermal treatment. Lignin degrades in lower intensity than polysaccharides. Carboxyl and free phenolic hydroxyl groups are created during the thermal treatment. These groups allow networking of lignin, which also contributes to hydrophobicity after the treatment (ESTEVES and PEREIRA 2009, HILL, 2006, KAČÍKOVÁ and KAČÍK 2011). Moreover, lignin incrusts fibrils in the secondary cell walls and its hydrophobicity hampers access of water to free hydroxyl groups (BOONSTRA and TJEERDSMA 2006).

Dealing with wood thermal modification, it is also needed to identify the changes in wood surface properties. Characteristic wood surface property is its colour. The colour is important for differentiating between the species and it also serves as a wood quality indicator (BABIAK et al. 2004, HRČKA 2010). Wood colour can be considerably modified under heat during thermal, hydro-thermal and thermo-hydro-mechanic treatment (ČERMÁK and DEJMAL 2013, DZURENDA 2014, BEKHTA et al. 2014, KÚDELA et al. 2018, KÚDELA and ANDOR 2018, KUČEROVÁ et al 2019 and others).

Up to present, the variation in the wood colour space has been dealt on empiric background only, without possibility to attain a pre-defined required discolouration. This is especially true in the case of thermal treatment (thermo-wood), hydro-thermal treatment (steaming, boiling) and thermos-hydro-mechanic treatment (pressing). Unveiling the mechanism backing up the discolouration of wood subjected to these processes should facilitate the development of methods for attaining pre-defined wood colour modifications and, at the same time, ensuring better colour stabilisation. The colour obtained through thermal treatment is not stable and varies significantly under the influence of environmental factors in outdoor conditions (ANDOR 2018). KUČEROVÁ et al. (2019) even propose that the discolouration, as a relatively easy to measure parameter, could serve as a predictor for the surface quality of thermally treated wood.

The results obtained by TODARO et al. (2015), BEKHTA et al. 2018, KÚDELA et al.

(2018) also point at the fact that the thermally treated wood exhibits restricted wetting with 6

water and other liquids. The enhanced resistance against liquid water is the result of chemical changes and the surface morphology that is modified by the thermal treatment too. On one hand, the enhanced resistance of thermally treated wood against wetting with liquids seems like a profit; on the other hand, there may be negative impacts on the glued joint strength and on the adhesion of coating films to the wood substrate.

It is indisputable that this subject area has been exploited rather poorly and that there are numerous questions to answer and problems to solve. This is obvious not only based on the very variable quality of thermally treated wood, but also based on the up-to-present used methods and approaches for calculation of thermodynamic characteristics, differing in the mathematical tools applied and also in the number of liquids necessary to carry out the experiments (GINDL and TSCHEGG 2002, BLANCHARD et al. 2009, PIAO et al 2010, HUBBE et al. 2015, PETRIĆ et al. 2015). As liquid standards, there are used liquids with limited wood surface wetting: non-polar and non-polar-polar ones, with an additive characteristic of the surface free energy. The differences in the chemical composition of liquids initiate the formation and the nature of the interface between liquids and wood, which is reflected in different values of surface free energy and its components (KÚDELA 2014).

The referred works allow us to conclude that all the discussed changes in the performance of thermally modified wood are affected by the technique of the thermal treatment, temperature, heating period, wood species and others.

The aim of this work was to evaluate the influence of duration of thermal treatment on changes in selected surface properties in beech wood. Under specific setup conditions, there were studied changes in beech wood surface colour and morphology. There were also performed experimental studies focused on wetting with standard polar and non-polar liquids, from which surface free energy values were determined.

MATERIAL AND METHODS The study was performed on beech (Fagus sylvatica L) wood material. For experiments, there were prepared four series of test specimens, each with the following dimensions: radial face 50  100 mm2 and thickness – 15 mm (Fig. 1). Before the experiment, the specimen surface was sanded with a sandpaper grit P180. The first series consisted of control specimens. The other three series were firstly dried down to the zero moisture content, and then there were treated thermally in a laboratory hot-air drying kiln without moisturizing climate, with an open system, at a temperature of 200 °C. These series differed in their treatment periods: one, three and five hours. The measurement of roughness, colour and contact angles at the wetting process followed the methods proposed

by KÚDELA et al. (2019).

Fig. 1 Specimen for a thermal treatment, dimensions and shape.

Colour measurement The colour coordination values in the colorimetric space CIE L*a*b* before and after thermal treatment and the differences in the colour coordinates L*, a*, b* related to the control specimens were measured with a spectre-photometer Spectro-guide 45/0 gloss

7

(BYK- GARDNER GmbH, Germany). The discolouration extent was determined through the total colour difference E calculated according to the equation: ∆𝐸 = √∆𝐿2 + ∆𝑎2 + ∆𝑏 2 ,

(1)

∆𝐿∗ = 𝐿𝑇 − 𝐿𝑅

(2)

∆𝑎∗ = 𝑎 𝑇 − 𝑎𝑅

(3)

∆𝑏 ∗ = 𝑏𝑇 − 𝑏𝑅

(4)

Note: the index „T“ is used for the colour value after wood surface treatment, „R“ means so-called referential (control) value obtained for the surface of original, untreated wood.

Roughness measurement The morphological changes in the thermally treated wood surface were assessed through roughness parameters. The roughness was measured with a profilometer Surfcom 130A (Carl Zeiss, Germany). The measurements were carried out on thermally treated radial faces of beech specimens, parallel and perpendicular to the grain. There were measured the following roughness parameters: Ra – arithmetic mean deviation, Rq – root-mean-square deviation, Rz – maximum height of the assessed profile within a sampling length, Rt – maximum height of the assessed profile within the total length, RSm – mean distance between the valleys. The sampling length, representing 2.5 mm, was determined based on the preliminary measured values of roughness parameters Ra and Rz. The total measured length consisted of five sampling lengths.

Wood wetting with liquids and assessment of its surface free energy During the wetting process, contact angles were measured up to the complete soaking of the testing-liquid drop into the substrate. The used measuring equipment was a goniometer Krüss DSA30 Standard (Krüss, Germany). There were two applied testing liquids, differing in their polarity – redistilled water and diiodomethane. The reasons for using namely these liquids can be found in KÚDELA (2014). Diiodomethane is a non-polar liquid with its non-polar component of surface free energy higher than the disperse component of this energy in the wood. Redistilled water represents polar-non-polar liquid with its polar component of surface free energy higher than the polar component of wood. The parameters of these two liquids are in Table 1.

Tab. 1 Surface free energy and its components for the testing liquids.

Testing liquid

Liquid character

water diiodomethane

polar non-polar

𝛾

𝛾𝑑

72.8 50.8

21.8 50.8

𝛾𝑝 [mJ·m2] 51.0 0.0

𝛾+

𝛾−

25.5 0.0

25.5 0.0

From the moment of the contact of the testing drop (volume 0.0018 ml) with the wood surface, the wood wettability and drop spreading along the fibre direction were inspected. The history of the drop shape, from the first contact up to the complete soaking, was recorded with a camera. The scanning frequency (number of scans per second) was adjusted according to the wetting duration.

The drop shape was analysed, and the contact angle was determined based on two methods: drop perimeter (circle) method and drop height and diameter (height-width) method.

8

The contact angle value θ0 was measured at the beginning of the wetting process, immediately after the drop applying onto the wood surface. The moment of reversing the advancing angle to receding one was determined based on the parameter d (drop width) values. The contact angle measured at this moment was considered as „equilibrium“ contact angle – e. The contact angle values 0 and e were then used for calculating the contact angle w for an ideally smooth surface, following the method proposed by LIPTÁKOVÁ and KÚDELA (1994). This angle was subsequently applied for calculation of surface free energy and its components. The contact angle measurements were carried out on each specimen, at six points.

As the wood was wetted with two different liquids, the wood surface free energy was determined separately for wetting with water and with diiodomethane, according to the adjusted equation originally proposed by NEUMANN et al. (1974). with the disperse and polar components Sd and Sp calculated in the following way, as suggested by KLOUBEK (1974).

The resulting surface free energy of thermally treated beech specimens was determined as the sum of the polar component of this energy quantified with water and the disperse component obtained with diiodomethane.

RESULTS AND DISCUSSION Colour change The first change detectable visually on beech wood surface subject to thermal treatment was discolouration. With prolonged treatment time, the colour coordinates L*, a* and b* exhibited significant changes, which is also in accord with the results of the two-way variance analysis performed. The basic statistic characteristics for these coordinates are in Table 2. The colour spaces of thermo-wood varieties obtained with particular thermal treatment modes can be considered clearly separated.

Tab. 2 Basic statistic characteristics of colour coordinates L*, a*, b* related to different heating periods at 200 °C. Duration of thermal treatment [hours] 0 1

Colour coordinates

Basic statistic characteristics

*

L 81.13 0.76

x̄ SD

a* 4.91 0.21

b* 16.09 0.38

x̄

55.16 6.39 16.58 1.84 0.56 0.44 3 44.71 8.21 16.69 x̄ SD 1.30 0.30 0.69 40.34 7.79 14.61 x̄ 5 SD 0.97 0.31 0.81 x̄ – average, SD – standard deviation, the number of measurements performed for each series was 60 SD

The colour coordinates of the original, non-treated beech specimens were from the interval reported for beech wood by BABIAK et al. (2004), HRČKA (2010), KÚDELA et al.

(2017). The variability was relatively low, which means that the beech wood specimen surface was homogeneous in colouration. With prolonged thermal treatment, the lightness values L* decreased distinctly.

The steepest descent in this coordinate, in comparison with referential specimens, was observed after one-hour heating at 200 °C. Later, at the same temperature, the significant decrease in lightness was continuing, nevertheless, the rate was getting lower 9

with the progressing time. The values of coordinates a* and b* were moderately increasing with progressing treatment time, up to three hours. This change was not as conspicuous as in the case of lightness, however, the statistical differences were significant.

The drop in lightness and shift of coordinates a* and b* towards red and yellow caused that the wood was getting more and more dark-brown saturated. After five treatment hours, the values of coordinates a* and b* began to decrease moderately. The impact on the beech specimen discolouration is documented in Fig. 2.

Control sample 0 hours

Heating time at 200 °C 3 hours

1 hour

5 hours

Fig. 2 Tint of the beech specimen surface after thermal treatment at 200 °C, treatment duration 1, 3 and 5 hours.

The differences in the colour coordinates L*, a*, b* and the total colour difference E* are represented in Fig. 3. The total colour difference E was much over the value of 12 as early as after one-hour-treatment. According to the assessment scale reported by ALLEGRETTI et al. (2009), the value of 12 means, a completely new colour compared to the original. Qualitatively similar colour variation in beech wood was also observed by other authors (GONZALES et al. 2009, DZURENDA 2014, KÚDELA and ANDOR 2018 and others). The observed quantitative differences were the result of different methods used for thermal treatment.

Fig. 3 Colour coordinates L*, a*, b* and the total colour difference E* after thermal treatment at 200 °C, treatment duration 1, 3 and 5 hours.

GONZALES et al. (2009) report that the beech wood discolouration during thermal treatment is participated by all wood components, with the highest correlation between discolouration and modification of hemicelluloses. There have also been identified links between colour modification and modification in the lignin structure. The colour coordinate b* after 5 hours of treatment dropped back under the reference sample colour.

According to KUČEROVÁ et al. (2019), this alteration of colour coordination b* is related to the rapid decrease of saccharide content and relative increase of lignin content.

10

Surface morphology Beech wood thermal treatment had also effects on this wood surface morphology assessed through specified roughness parameters. The impacts of the treatment duration and the anatomic direction was evaluated with the aid of two-way variance analysis. The results of this analysis confirmed that the two tested parameters had significant effects on the roughness parameters values. The values of roughness parameters Ra, Rq, Rz, and RSm obtained parallel to grain and perpendicular to the grain, for all the treatment modes are presented in Fig.4.

The results suggest that in all cases, the Ra, Rq and Rz values were lower parallel to the grain than perpendicular to the grain. In the case of RSm, the opposite was true. The roughness parameters values logically result from the interaction between the internal wood structure and mechanic treatment of wood surface (in our case, sandpaper with a grit size P180). Due to the high heterogeneity of beech wood structure, the roughness of this wood displayed a rather high variability. During thermal treatment, the two interacting factors were supplemented with a third one – heat.

The heating temperature of 200 °C and treatment duration of one hour caused enhanced roughness in both anatomic directions. With prolonged treatment time under the same temperature, there followed a moderate smoothing, and after five treatments hours, the roughness of the treated specimens was again almost the same as the roughness of the untreated control. This roughness variation is in good correspondence with the changes to the chemical structure, explicitly the relative increase in lignin content (KUČEROVÁ et al. 2019).

It could be assumed that incrustation of lignin into fibrils throughout the cell wall smoothed out the surface, namely in the case of longer heating.

Wilks lambda=.83569, F(6, 470)=7.3556, p=.00000

Wilks lambda=.86247, F(6, 470)=6.0148, p=.00000

10

10 perpendicular to grain parallel to grain (weigted mean and 0.95 confidence intervals)

9

7

8 Rq [m]

Ra [m]

8

9

6 5

7 6 5

4

4

3

3

2

0

1

3

2

5

0

Time [hours]

9

600

8

550

7

500

6 5

450 400

4

350

3

300

1

3

5

Wilks lambda=.98696, F(6, 470)=.51583, p=.79646

650

RSm [m]

Rz [m]

Wilks lambda=.86247, F(6, 470)=6.0148, p=.00000

0

3

Time [hours]

10

2

1

250

5

Time [hours]

0

1

3

5

Time [hours]

Fig. 4 Changes in beech wood roughness parameters after thermal treatment performed at 200 °C for one, three and five hours.

11

Wood surface wetting with water and diiodomethane The thermal-treatment-induced chemical changes signalised through beech wood surface discolouration as well as the morphological changes on the beech wood surface affected significantly the wood surface´s wetting with standard liquids, primarily water. In all cases, water drop applied on the wood surface was spreading continually over the surface, and, at the same time, soaking into the substrate. The differences in wood wetting performance before and after thermal treatment were mostly detected in the values of contact angles 0, e, w, as well as in the duration of the wetting process up to the complete soaking of the drop into the wood substrate.

The best water-wetting performance was observed in beech wood surface before thermal treatment. Sanding prior the treatment enhances the proportion of polar hydroxyl groups, and, consequently, the wood surface becomes more hydrophilous, in comparison, for example, with a milled surface (LIPTÁKOVÁ et al. 1995, KÚDELA et al. 2016). This fact was reflected in notable drops in contact angle values 0, e and w and in faster spreading over and soaking of the drop into the wood. The average contact angle 0 value at the moment of the application was 21°, the value of the equilibrium contact angle e was somewhat lower (13°). The angle for an ideally smooth surface w had an average value of 15° (Fig. 5). The values of these angles confirm the appropriate wetting of the wood surface before the thermal treatment. The time necessary for the drop to soak completely into the substrate was several seconds. a)

b)

Fig. 5 Duration of beech wood thermal treatment performed at 200 °C, and the resulting contact angle values.

For all three treatment periods, thermal treatment at 200 °C yielded significantly better beech wood resistance against surface wetting with water. This was also evident from the evidently higher values of all three contact angles 0, u and w (Fig. 5). The average values of contact angle 0 ranged between 110°116°, the average values of contact angles e and w were in all cases above 90°. There was also observed more time necessary to reach the equilibrium state at the interface wood – water (te), and the time of duration of the entire wetting process until the complete soaking of the drop into the substrate was multiplied by two orders (Fig. 6). These facts give evidence for the hydrophobic nature of the beech wood surface after thermal treatment. The different wetting capacities were mainly to the heating.

In this case, the heating duration was not confirmed as an important factor unequivocally.

The thermal treatment also eliminated differences in wetting caused by different mechanic pre-treatments of the wood surface before thermal treatment (milling, grinding).

The beech wood wetting with diiodomethane was different from water. In the first case, the contact angle values of the liquid with thermally modified wood were 12

significantly lower, and there were also significantly shorter times necessary for the complete drop soaking into the substrate (Fig. 5b and 6). On the other hand, the treatmentmode-dependent differences in the contact angle values were small.

Fig. 6 Heating time and complete soaking of the drop into the wood substrate

Degraded wetting of thermally modified beech wood has also been reported by other authors (TODARO et al. (2015), MIKLEČIĆ et al. 2016, BEKHTA et al. 2018). These authors, however, observed smaller changes in the contact angle values in the thermally modified wood compared to our results. The reasons may be several, such as a method used for wood wetting with liquid, a method for contact angle value evaluation, and quality of the experimental material used. MIKLEČIĆ et al. (2016) point at the importance of the fact whether or not the thermally treated wood was subsequently mechanically treated. The work referred as well as our observations suggest that the additional wood surface sanding after thermal treatment may, to some extent, restore the surface hydrophilicity. We can see that there are manifold factors possibly masking the influence of the thermal treatment as such on the wetting process.

According to LIPTÁKOVÁ and Kúdela (1994), the values of contact angles o and u are the results of both wood surface morphology and wood chemical composition, while the w values depend only on the chemical structure of the wood surface. Consequently, the different w values confirm the chemical modifications induced by the thermal treatment.

80 70 60 50 40 30 20 10 0

Surface free energy [mJ.m-2]

Surface free energy [mJ.m-2]

Surface free energy in thermally modified beech wood For the particular thermal modification modes, the values of beech wood surface free energy S with its disperse and polar components ds and sp were firstly determined, according to the methods discussed above, separately based on the wetting with water and the wetting with diiodomethane. For the surface free energy calculation, the contact angle w was used. The obtained results are in Fig. 7.

Water Surf ace f ree energy Disperse component Polar component

0

1

3

5

80 70 60 50 40 30 20 10 0

Diiodomethane

0

Time [hours]

1

3

5

Time [hours]

Fig. 7 Duration of thermal treatment at 200 °C and the related values of the surface free energy and its components determined based on wetting with water and diiodomethane.

13

The beech wood surface free energy determined using the parameter values obtained with wetting with water was the highest before the thermal treatment. Thermal treatment lasting one hour at a temperature of 200 °C considerably reduced these energy values, primarily due to the reduction of its polar component. With advancing treatment time, no significant changes occurred any more. For all the treatment modes, the dispersive component was dominant (Fig. 7). The surface free energy values determined before the thermal treatment with the aid of diiodomethane were lower than the corresponding values obtained using water; in case of diiodomethane, the free energy values were practically equal to the values of a disperse component of this energy. The thermal treatment did not cause any significant changes.

The obtained results show that the liquid standards used for assessment of wood surface properties at the interface between wood and liquid differed, as for the values of surface free energy and its disperse and polar components. KÚDELA (2014) demonstrates that the polar component of surface free energy in wood is best fitted with water the polar component of which is higher than the polar component of wood. For obtaining the disperse component values, diiodomethane was identified as the most suitable liquid, as the disperse component of surface free energy of diiodomethane is higher than the disperse component of wood. Then the surface free energy is the sum of the polar and disperse components determined as described just above. The surface free energy results obtained in this way are in Tab. 3. Such determined surface free energy values for beech wood are somewhat higher, with dominant disperse component. Similar results for surface free energy have been reported in MIKLEČIĆ et al. (2016). However, for all the thermal treatment modes, the surface free energy values obtained in this way were relatively low, which may have negative impacts on the adhesion of the film-forming materials applied on thermally modified wood.

Tab. 3 Surface free energy (γs) of thermally modified beech wood determines as the sum of the polar component (γ sp) based on wetting with water and the disperse component (γsd) based on wetting with diiodomethane.

Surface free energy and its components

Heating period at 200 °C 3 hours 5 hours 47.34 48,65

γs [mJ.m-2]

Control 0h 84.99

1 hour 47.66

γsd [mJ.m-2]

44.24

44.39

44.51

45.55

γ sp [mJ.m-2]

40.75

3.275

2.826

3.095

CONCLUSIONS The results and their analysis indicate that the beech wood thermal treatment running at a temperature of 200 °C during three different treatment periods (1, 3, and 5 hours) was noticeably responded through changes in the studied surface properties.

With prolonged treatment time, the lightness L* was decreasing significantly. After five-hour-lasting treatment, the lightness was reduced to one half of the initial value. The coordinates a* and b* were increasing during the first three hours, then, there was a moderate decrease. The drop in lightness and shift of colour coordinates a* and b* towards red and yellow caused that the wood was gradually coloured saturated deep brown. As early as after one-hour thermal treatment, the value of the total colour difference E markedly exceeded the value of 12, which means a completely new wood tint compared to the referential specimens.

14

The thermal treatment also induced changes to the beech wood surface morphology.

These changes were observable as enhanced roughness. In all cases, the roughness parameters Ra, Rq and Rz values were lower parallel to the grain than perpendicular to the grain. However, in the case of RSm, the reverse was true.

The results disclosed that the thermal treatment of beech wood improved significantly this wood surface resistance against wetting with water. This was evident from the contact angle values  ˃ 90°. The time necessary for the complete soaking of the drop into the substrate was one order of magnitude longer than in untreated wood. In the case of thermowood wetting with non-polar diiodomethane, the influence of thermal treatment was not found important. The resistance to liquid water can be considered to be a profit. However, it is necessary to address the expected durability of the resistance.

Reduced wood surface wetting had a significant influence on lowering the wood surface free energy, primarily due to the substantial decrease in the polar component of this energy. This may imply negative consequences for the quality of a surface coating process of thermally modified wood.

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KÚDELA, J., ANDOR, T. 2018: Beech wood discoloration induced with specific modes of thermal treatment. Ann. WULS-SGGW, For. and Wood Technol. No 103: 64–69.

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MIKLEČIC, J., JIROUŠ-RAJKOVIČ, V. 2016: Influence of Thermal Modification on Surface Properties and Chemical Composition of Beech Wood (Fagus sylvatica L.). In Drvna Industrija 67(1): 65–71, 2016.

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ACKNOWLEDGMENTS This research was funded by the Slovak Research and Development Agency under the contract No. APVV-16-0177 (50 %) and by the Scientific Grant Agency of the Ministry of Education SR and the Slovak Academy of Sciences Grant No. 1/0822/17 (50 %).

ADDRESSES OF AUTHORS Prof. Jozef Kúdela Dr. Rastislav Lagaňa Dr. Tomáš Andor Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Wood Science T. G. Masaryka 24 960 01 Zvolen Slovak Republic kudela@tuzvo.sk Dr. Csila Csiha University of Sopron The Simonyi Karoly Faculty of Engineering Wood Sciences and Applied Arts 9400 Sopron, Bajcsy-Zs. u. 4.

Hungary

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 1928, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.02

THE EFFECT OF THE TEMPERATURE OF SATURATED WATER STEAM ON THE COLOUR CHANGE OF WOOD ACER PSEUDOPLATANUS L. Ladislav Dzurenda  Michal Dudiak ABSTRACT The colour change of the wood of tree species Acer pseudoplatanus L. resulting from the process of thermal treatment with saturated water steam at the temperatures of tI = 105 ± 2.5 °C, tII = 115 ± 2.5 °C, tIII = 125 ± 2.5 °C, tIV = 135 ± 2.5 °C for τ = 7 hours are presented in the paper. The white to pale white-yellow color of native maple wood varies from pale brown to pinkish-yellow to brownish-red, depending on the temperature of the saturated water steam. In the CIE L*a*b* color space, the changes shown are at the L* - coordinate within the range of L* = 86.0 - 65.3 and the chromatic coordinates: red a* = 5.9  10.8 and yellow b* = 16.4 - 19.4. The effect of the temperature of saturated water steam on the variation of the lightness L *, red a * and yellow colour b * of maple wood in the colour space CIE L* a* b* is given in the equation: L* = –0.489∙t + 132.86; a* = 0.081∙t + 0.105; b* = 0.059∙t + 11.445. Significant colour changes of maple wood are observed as a result of technological process at the temperatures above 122 °C. Irreversible changes in colour of the maple wood resulting from one of the modes of colour modification of wood with saturated water steam extend the possibilities of its use in the field of construction-joinery, construction-art and design.

Keywords: maple wood, colour, thermal treatment, saturated water steam.

INTRODUCTION The colour of wood is one of the macroscopic features to identify wood of individual tree species visually. Chromophores are molecules responsible for the colour of wood, i.e. functional groups of: >C=O, -CH=CH-CH=CH-, -CH=CH-, aromatic compounds absorbing light in the UV/VIS spectra present in the chemical components of wood (lignin and extractive substances such as pigments, tannin, resin, etc.).

Wood placed in the environment of hot water, saturated water steam or saturated humid air is heated and its physical, mechanical as well as chemical properties change.

Thermal treatment of wood, besides physical and mechanical changes applied in the process of manufacturing veneers, plywood, bentwood furniture or pressed wood are accompanied with the changes in chemical properties and colour of wood (KOLLMANN and GOTE 1968, NIKOLOV et al. 1980, SERGOVSKIJ and RASEV 1987, TREBULA 1986, TOLVAJ et al. 2010, DZURENDA and ORLOWSKI 2011, DZURENDA 2013, BARANSKI et al. 2017, SIKORA et al 2018). In the past, colour changes when wood becoming darker during the steaming process were used to remove the undesirable colour differences between light coloured sapwood and 19

dark coloured heartwood or to eliminate wood stain colours as a result of mould. In recent times, research into thermally modified wood has been focused on the issue of the colour change of specific wood species into more or less bright hues or wood imitation of domestic or exotic tree species (MOLNAR 2002, TOLVAJ et al. 2009, DZURENDA 2014, 2018a, b, c, BARCIK et al. 2015, BARANSKI et al. 2017).

Sycamore is an example of diffuse porous tree species. Maple wood is hard, medium heavy, elastic with good mechanical properties. It is easy to work with, easy to cut, plane, chisel, sand and polish. The colour of dry wood of sycamore is white to yellow-white.

Natural aging of maple wood due to UV radiation results in changing the colour, wood is getting darker. Maple wood is used for making furniture, musical instrument, toys, home utilty products and sports equipment, as well as flooring. Joiners and cabinetmakers can imitate wood of other tree species by staining the maple wood.

The aim of the paper is to determine the effect of the temperature of saturated water steam on the colour change of the wood of tree species Acer pseudoplatanus L. resulting from the thermal treatment – colour modification with saturated water steam at the temperatures of: tI = 105 ± 2.5 °C, tII = 115 ± 2.5 °C, tIII = 125 ± 2.5 °C, tIV = 135 ± 2.5 °C for τ = 7 hours.

MATERIAL AND METHODOLOGY Materials The wood of Acer pseudoplatanus L. in a form of sawn timber with the thickness of h = 40 mm and the moisture content above the fibre saturation point was thermally modified with saturated water steam in the pressure autoclave APDZ 240 (Himmasch AD, Haskovo, Bulgaria) in the company Sundermann s.r.o. Banská Štiavnica.

Methods Mode of thermal treatment in order to modify the colour of maple wood with saturated water steam is illustrated in Fig. 1. The conditions of thermal treatment of individual modes of colour modification are described in Tab.1.

Fig. 1 Mode of colour modification of maple wood with saturated water steam.

20

Tab. 1 Modes of colour modification of maple wood with saturated water steam.

Temperature of saturated water steam [°C] tmin tmax t4 102.5 107.5 100 112.5 117.5 100 122.5 127.5 100 132.5 137.5 100

Modes Mode I Mode II Mode III Mode IV

Time of operation [h] τ1 -phase I τ2-phase II Total time 6.0

1.0

7.0

The colour of wood was measured using 35 pieces of sawn timber thermally untreated (green wood) and 35 pieces of sawn timber of maple wood thermally treated with individual modes after being dried to the moisture content of Wp = 12 ± 0.5 % in a conventional wood drying kiln KAD 1 × 6 (KATRES Ltd.). Subsequently, woodturning blanks with the dimensions of 40 × 80 × 800 mm were prepared from sawn timber. Flat surfaces and edges were processed using Swivel spindle milling machine FS 200.

Color Reader CR-10 (Konica Minolta, Japan) was used to assess the colour of maple woodturning blanks in the CIE L*a*b*colour space. The light source D65 with lit area of 8mm was used.

Lightness coordinate L* and coordinates a* and b* of CIE L*a*b* colour space, as well as chroma C* and total colour difference ∆E* were measured using a randomly selected samples n = 50 of maple woodturning blanks of thermally untreated wood and using the sample set n = 50 of woodturning blanks of thermally treated sawn timber with individual modes. Measurement of the coordinates L* , a* and b* , chroma C* and total difference ∆E* using samples of maple wood was carried out in the centre of the blank width and in the centre of the blank edge, 400 mm far from the face. The values of colour coordinates are mentioned in a form of formula x  x  s x i.e. the mean value and standard deviation.

Total colour difference ΔE* is determined according to the following formula, as a result of the difference in the colour coordinates ΔL*, Δa*, Δb* set following the surface measurements of thermally untreated as well as treated maple woodturning blanks: E  

where:

L

* 1

 L*   a1*  a *   b1*  b *  2

2

2

(1)

L*, a*, b* coordinate values in the wood colour space prior to the process of thermal modification of wood.

L*1, a*1, b*1 coordinate values in the wood colour space of thermally modified maple wood.

The change in wood colour, besides changes in the chromatic coordinates in the CIE L*a*b*colour space, was assessed also following the changes in the lightness ∆L*, chroma ∆C* and hue angle h° in the CIE L*C*h° colour space using cylindrical coordinates.

Chroma C* is an integration of the values of the coordinates of red colour a* and yellow colour b* projected onto the chromatic plane of cylindrical colour space: 2

𝐶 ∗ = √𝑎∗2 + 𝑏 ∗2

(2)

where: b* the value of the chromatic coordinate of yellow colour, a* the value of the chromatic coordinate of red colour.

Hue angle h° is expressed in positive degrees starting at the positive a* axis and progressing in a counterclockwise direction and is described with the formula: 𝑏∗

° ℎ𝑎𝑏 = 𝑎𝑟𝑐𝑡𝑎𝑛 (𝑎∗)

(3)

where: b* the value of the chromatic coordinate of yellow colour, a* the value of the chromatic coordinate of red colour.

21

RESULTS AND DISCUSSION White and white-yellow colour of maple wood changed during the process of thermal modification with saturated water steam. The wood of tree species of Acer pseudoplatanus L. was getting darker and browner. The changes in thermally modified maple wood are illustated in Fig. 2.

Fig. 2 The colour of maple wood prior to and after thermal treatment by individual modes.

The values of the coordinates in the CIE L*a*b*colour space, chroma C* and total colour difference ∆E* describing the colour of maple wood prior to and after the thermal treatment by individual modes are mentioned in Tab. 2 Tab. 2 The values of the coordinates in the CIE-L*a*b* colour space describing the maple wood prior to and after thermal treatment with saturated water steam by individual modes.

Temperature of saturated water steam Not thermally treated wood tI = 105 ± 2.5 °C tII = 115 ± 2.5 °C tIII = 125 ± 2.5 °C tIV = 135 ± 2.5 °C

Coordinates of the CIE L*a*b*colour space L* 86.0 ± 2.6 80.3 ± 1.8 77.5 ± 1.8 73.6 ± 1.6 65.3 ± 1.4

a* 5.9 ± 1.3 8.4 ± 1.5 9.6 ± 1.7 10.5 ± 1.5 10.8 ± 1.3

b* 16.4 ± 1.7 17.7 ± 1.6 18.1 ± 1.5 18.9 ± 1.5 19.4 ± 1.3

Chroma C* 17.4 19.6 20.5 21.6 22.2

Total colour difference ∆E* --6.5 9.2 13.4 25.1

The thermal treatment of maple wood with saturated water steam of a temperature of tI = 105±2.5°C for τ = 7 resulted in the light white-brown-pink hue, in the CIE L*a*b*colour space defined by the coordinates: LI* = 80.3 ± 1.8; aI* = 8.4 ± 1.5; bI* = 17.7 ± 1.6.

According to the colorimetric classification of the change in the colour of wood during thermal treatment (CIVIDINI et al. 2007), the total colour difference ∆EI* = 6.5 is not considered the high colour difference but only the colour difference visible with light quality screen. The mode of thermal treatment of maple wood at a temperature of tII = 115±2.5°C resulted in light brown-pink hue with the coordinates: LII* = 77.5 ± 1.8; aII* = 9.6 ± 1.7; bII* = 18.1 ± 1.5. More significant changes in wood colour, i.e. getting more darker, more 22

browner, resulted from the thermal treatment by the modes II and III with a temperature of saturated water steam of tIII = 125±2.5 °C, or a temperature of tIV = 135±2.5 °C. Rate of change in the wood colour and hues during the processes of thermal treatment of maple wood by the mode III is defined by the values of coordinates: LIII* = 73.6 ± 1.6; aIII* = 10.5 ± 1.5; bIII* = 18.9 ± 1.5. Unique brown-red colour of maple wood with the values of coordinates: LIV* = 65.3 ± 1.4; aIV* = 10.8 ± 1.3; bIV* = 19.4 ± 1.3 resulted from the thermal treatment with the saturated water steam at a temperature of tIV = 135±2.5°C. According to the colorimetric classification of the change in the colour of wood during thermal treatment (CIVIDINI et al. 2007), the total colour difference ∆EIV* = 25.1 is considered high colour difference.

Following the visual control of the wood colour on edges of woodturning blanks as well as the measurement of the colour on mentioned faces, the fact that the colour of maple wood cross section is uniform could be seen. The same changes in the colour through the volume of wood is due to fast heating of wood to the required temperature with saturated water steam in the cross section of woodturning blanks (DZURENDA 2018d). This way, good conditions for the processes of hydrolysis and extraction of water soluble substances modifying the chromophoric system of wood were created.

Mentioned findings is beneficial for practice allowing the thermally treated woodturning blanks to be used for making lamellae for flooring or for other 3D processing of solid timber without risk of the change in the wood colour between the surface and the centre.

The effect of the temperature of saturated water steam in the range between t = 105 – 135 °C on the change in the values of individual coordinates in the CIE L*a*b* colour space are illustrated in Fig. 3.

Fig. 3 Correlation between a decrease in lightness, an increase in the values of red and yellow colours of thermally treated maple wood in the CIE L*a*b* colour space and the temperature of saturated water steam.

Following the assessment of the change in the colour of thermally treated maple wood according to the parameters of the CIE L*C*h° colour space, the fact that an increase in the temperature of wood in the technological process caused a significant decrease in changes in lightness ∆L* and slight increase in chroma ∆C* can be stated. Mentioned changes are shown in Fig. 4.

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Fig. 4 Correlation between the changes in lightness ∆L*, chroma ∆C* and the temperature of saurated water steam in the technological process.

Changes in chroma C* in the chropmatic plane a*, b* and the hue angle h° of wood of Acer pseudoplatanus L. due an increase in a temperature of saturated water steam ranging between t = 105 – 135 °C is given in Fig. 5.

Fig. 5 Changes in chroma C* and the hue angle h° in the chromatic plane a*,b*.

Correlation between the change in the total colour difference ∆E* of maple wood and the temperature of saturated water steam in the technological process is given in Fig. 6.

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Fig. 6 Correlation between the change in total colour difference ∆E* of maple wood and the temperature of saturated water steam in the technological process.

A decrease in lightness of thermally modified wet wood is in compliance with the knowledge about changing the colour of wood, its darkening, during technological processes like steaming the wood mentioned in works: (TOLVAJ et al. 2009, 2010, DZURENDA 2018 b,c), drying in the environment of hot humid air, or overheated water steam (KLEMENT and MARKO 2009, DZURENDA and DELIISKI 2012a, b, BARANSKI et al. 2017), or during thermowood production processes (BARCIK et al. 2015, PINCHEVSKAJA et al. 2019).

The hydrolysis of wood, and following the works (FENGEL and WENEGER 1989, SOLÁR 2004, LAUROVA et al. 2004, BUČKO 2005) subsequent polysaccharide degradation, as well as oxidation of saccharides and pectin, dehydration of pentoses to 2-furaldehyde and condensation of polysaccharide cleavage products, chemical changes in lignin (free radicals, increase in the phenolic hydroxyl groups) and also forming the phenolic extracts are the main causes of mentioned changes resulting from the higher temperature of wet wood. Mentioned reaction result in forming new chromophoric groups causing the changes in wood colour. It is confirmed (Fig. 4.) by a decrease in the lightness ∆L* from 5.7 to 20.7 and an increase in chroma ∆C* from 2.2 to 4.1, as well as by the visual changes in the colour of maple wood in Fig. 2.

The fact that chroma C* moves away on the colour plane a*, b* (Fig. 5) from the centre of the coordinates of red colour a* and yellow colour b* proclaims the saturation of the colour of thermally treated wood. This findings is important for the wood workers and for the users of products from thermally treated wood as well, as intense colours in interior are more acceptable by a human than range of colours from greys. Moreover, the mode of thermal treatment of maple wood at a temperature of saturated water steam tIV =135 ± 2.5 °C resulted in highlighting the texture of maple wood with the darker hue of brown colour in annual ring of earlywood in comparison to the latewood in the tangential and radial section and the structure of maple wood is highlighted this way.

The values of total colour difference of the colour of maple wood ∆E* caused by the processes of thermal treatment with saturated water steam at the temperature ranging from 105 °C to 135 °C were ∆E* = 6.5 ÷ 21.5. The correlation describing the change in the colour of maple wood in Fig. 6 is caused, to a large extent, by changes in the coordinates of lightness L* and to a lesser extent, by the changes in coordinates of red colour red a* and yellow colour b*. Following the colorimetric classification of the change in the colour of wood during thermal treatment mentioned by CIVIDINI et al. (2007), according to a mentioned

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correlation, significant changes in the hues – brown-red of maple wood occurred at the temperature of saturated water steam above t ≥ 122 °C. According to the description of physical and mechanical properties resulting from the thermal treatment of wood mentioned by the authors: KOLLMANN – GOTE 1968, Trebula 1986, the changes in the colour of maple wood resulting from the thermal treatment by presented modes are in the group of irreversible changes in wood. Irreversibility of the changes in the colour of maple wood is confirmed by the differences in the analyses of ATRFTIR spectroscopy in lignin-carbohydrate complex of thermally treated wood as well as untreated wood (VYBOHOVÁ et al. 2018), and by the presence of monosaccharides, organic acids and basic structural elements of guaiacyl-syringyl lignin in the condensate mentioned in the works: (BUČKO 1995, DZURENDA and DELIISKI 2000, KAČÍK 2001, LAUROVA et al.

2004, KAČÍKOVÁ and KAČÍK 2011, SAMEŠOVA et al. 2018).

Irreversible changes in colour of the maple wood resulting from one of the modes of colour modification of wood with saturated water steam extend the possibilities of its use in the field of construction-joinery, construction-art and design.

CONCLUSION 1. The results of experiments focused on observing the changes in the colour of tree species Acer pseudoplatanus L. during the process of thermal treatment with saturated water steam at the temperatures ranging between: t = 105135 °C for τ = 7 hours are presented in the paper.

2. The colour of maple wood thermally treated with saturated water steam changes, gets darker and becomes from white-brown-pink to brown-red.

3. The effect of the temperature of saturated water steam on the change of lightness L*, red colour a* and yellow colour b* of maple wood in the CIE L*a*b* colour space are given in following formulae: L* = –0.489∙t + 132.86, a* = 0.081∙t + 0.105, b* = 0.059∙t + 11.445.

4. The dependence of the total colour difference ∆E*on the temperature of wood in the technological process of colour modification of maple wood with saturated water steam is described with the formula: ∆E* = 0.0132∙t2 – 2.6746∙t + 141.87.

5. Significant changes in the colour of maple wood occurs at a temperature above 122 °C. REFERENCES BARAŃSKI, J., KLEMENT, I., VILKOVSKÁ, T., KONOPKA, A. 2017. High Temperature Drying Process of Beech Wood (Fagus sylvatica L.) with Different Zones of Sapwood and Red False Heartwood. In BioResources 12(1),18611870. DOI:10.15376/biores.12.1.1761-1870.

BARCIK, Š., GAŠPARÍK, M., RAZUMOV, E.Y. 2015. Effect of thermal modification on the colour changes of oak wood. In Wood Research 60 (3), 385396.

BUČKO, J. 1995. Hydrolýzne procesy. (Hydrolysis processes). Zvolen: Techniská univerzita vo Zvolene. 116 p.

CIVIDINI, R., TRAVAN, L., ALLEGRETTI, O. 2007. White beech: A tricky problem in drying process.

In International Scientific Conference on Hardwood Processing, Quebec City, Canada.

DZURENDA, L., DELIISKI, N. 2000. Analysis of moisture content changes in beech wood in the steaming process with saturated water steam. In Wood Research 45(4), 18.

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DZURENDA, L.,ORLOWSKI, K. 2011. The effect of thermal modification of ash wood on granularity and homogeneity of sawdust in the sawing process on a sash gang saw PRW 15-M in view of its technological usefulness. In Drewno 54(186): 2737.

DZURENDA L., DELIISKI N. 2012a. Convective drying of beech lumber without color changes of wood. In Drvna industrija, 63, 2: 95–103. doi:10.5552/drind.2012.1135 DZURENDA L., DELIISKI N. 2012b. Drying of beech timber in chamberdrying kilns by regimes

preserving the original colour of wood. Acta Facultatis xylologiae Zvolen [54] 1: 33 – 42 DZURENDA, L. 2013. Modification of wood colour of Fagus sylvatica L. to a brown-pink shade caused by thermal treatment. In Wood Research, 58(3), 475482.

DZURENDA, L. 2014. Colouring of beech wood during thermal treatment using saturated water steam.

In Acta Facultatis Xylologiae Zvolen, 56(1),1322.

DZURENDA, L. 2018a. The Shades of Color of Quercus robur L. Wood Obtained through the Processes of Thermal Treatment with Saturated Water Vapor. In BioResources. 13(1): 15251533.

DOI: 10.15376/biores.13.1.1525-153.

DZURENDA, L. 2018b. Hues of Acer platanoides L. resulting from processes of thermal treatment with saturated steam. In Drewno, 61(202): 165176. DOI: 10.12841/wood.1644-3985.241.11 DZURENDA, L. 2018c. Colour modification of Robinia pseudoacacia L. during the processes of heat treatment with saturated water steam. In Acta Facultatis Xylologiae Zvolen, 60(1): 6170. DOI: 10.17423/afx.2018.60.1.07 DZURENDA, L. 2018d. The Effect of Moisture Content of Black Locust Wood on the Heating in the Saturated Water Steam during the Process of Colour Modification. In MATEC Web of Conferences 168, 06004. doi.org/10.1051/matecconf/201816806004.

FENGEL, D., WEGENER, G. 1989. Wood: Chemistry, Ultrastructure, Reactions; Walter de Gruyter: Berlin.

KAČÍK, F. 2001. Tvorba a chemické zloženie hydrolyzátov v systéme drevo-voda-teplo. (Formation and chemical composition of hydrolysates in wood-water-heat system). Zvolen: Technická univerzita vo Zvolene, 75 p.

KAČÍKOVÁ, D., KAČÍK, F. 2011. Chemické a mechanické zmeny dreva pri termickej úprave.

(Chemical and mechanical changes of wood during thermal treatment). Zvolen: Technická univerzita vo Zvolene, 71 p.

KLEMENT, I., MARKO, P. 2009. Colour changes of beech wood (Fagus sylvatica L.) during high temperature drying process. In Wood research 54 (3): 4554.

KOLLMANN, F., GOTE, W. A. 1968. Principles of Wood Sciences and Technology. Vol. 1. Solid Wood, Springer Verlag, Berlin – Heidelberg - New York,592 p.

LAUROVA, M., MAMONOVA, M., KUČEROVA, V. 2004. Proces parciálnej hydrolýzy bukového dreva (Fagus sylvatica L.) parením a varením, (Process of partial hydrolysis of beech wood (Fagus sylvatica L.) by steaming and boiling). Zvolen: Technická univerzita vo Zvolene, 58 p.

MOLNAR, S., TOLVAJ, L. 2002. Colour homogenisation of different wood species by steaming. In Interaction of wood with various Forms of Energy. Zvolen: Techniská univerzita vo Zvolene. 119– 122 p.

NIKOLOV, S., RAJČEV, A., DELIISKI, N. 1980. Proparvane na drvesinata, (Steaming wood). Sofia: Zemizdat,174 p.

PINCHEVSKA, O., SEDLIAČIK, J., HORBACHOVA, O., SPIROCHKIN, A., ROHOVSKYI, I. 2019.

Properties of hornbeam (Carpinus betulus) wood thermally treated under different conditions. In Acta Facultatis Xylologiae Zvolen 61(2): 2539. DOI: 10.17423/afx.2019.61.2.03.

SAMEŠOVÁ, D., DZURENDA, L. JURKOVIČ, P. 2018. Kontaminácia kondenzátu produktmi hydrolýzy a extrakcie z tepelného spracovania bukového a javorového dreva pri modifikácii farby dreva, (Contamination of condensate by products of hydrolysis and extraction from the heat treatment of beech and maple wood with modification of wood color). In Chip and Chipless Woodworking Processes 2018. 11(1): 277–282.

SIKORA, A., KAČÍK, F., GAFF, M., VONDROVA, V., BUBENÍKOVA, T., KUBOVSKÝ, I. 2018, Impact of thermal modification on color and chemical changes of spruce and oak wood. In Journal of Wood Science (2018) 64:406–416, doi.org/10.1007/s10086-018-1721-0.

SERGOVSKIJ, P. S., RASEV, A. I. 1987. Gidrozermicheskaya obrobotka i konservironaniye drevesiny,

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(Hydrothermal processing and conservation of wood). Moskva: Lesnaja promyslennost. 360 p.

SOLÁR, R. 2004. Chémia dreva. Zvolen: Technická universita vo Zvolene. 102 p.

TOLVAJ, L., NEMETH, R., VARGA, D. MOLNAR, S. 2009. Colour homogenisation of beech wood by steam treatment. In Drewno. 52 (181): 517.

TOLVAJ, L., MOLNAR, S., NEMETH, R., VARGA, D. 2010. Color modification of black locust depending on the steaming parameters. In Wood Research 55(2), 8188.

TREBULA P. 1986. Sušenie a hydrotermická úprava dreva, (Wood drying and hydrothermal treatment). Zvolen: Techniská univerzita vo Zvolene. 255 p.

VÝBOHOVA, E., GEFFERET, A. GEFFERTOVA, J. 2018. Impact of Steaming on the Chemical Composition of Maple Wood. In BioResouces 13(3): 58625874.

ACKNOWLEDGMENT This experimental research was prepared within the grant project: APVV-17-0456 “Termická modifikácia dreva sýtou vodnou parou za účelom cielenej a stabilnej zmeny farby drevnej hmoty” as the result of work of author and the considerable assistance of the APVV agency.

AUTHORS’ ADDRESSES Ladislav Dzurenda Technical University in Zvolen T. G. Masaryka 24 960 01 Zvolen Slovakia dzurenda@tuzvo.sk Michal Dudiak Technical University in Zvolen T. G. Masaryka 24 960 01 Zvolen Slovakia xdudiak@is.tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 29−39, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.03

COMPUTER SIMULATION OF THE PROCESS OF END-GRAIN WOOD TREATMENT UNDER PRESSURE Vladimir Shamaev ABSTRACT The process of introducing modifiers into wood from the end-grain under pressure was studied for the development of treatment technology by computer modeling. A physical model of meso-level wood treatment was developed taking into account temperature and pressure variables differing by cavities, annual layers and core rays. Analytical expressions are obtained for describing the wood impregnation process on the basis of a mathematical model. It was found that with increasing vessel diameter dV–fluid speed νl increases according to the following law: 𝜐𝑓 = 𝜐0 + 𝛽√𝑑𝑉 ,where ν0–fluid speed in the smallest vessel; β–coefficient of proportionality.

Keywords: wood, vessels, treatment, computer simulation, fluid, modifier, treatment equations.

INTRODUCTION Currently, various methods of poplar, aspen, eucalyptus, and other species wood modifying are used in order to replace hardwoods and use low-value softwood rationally (WESTINet al. 2010, GUBANOVA et al. 2013, LEKOUNOUGOU et al. 2011,WEBSITE OF ACCOYA et al. 2011). These species are used to replace valuable tree species (oak, boxwood, the mahogany, and palisander). All these methods include technological operations of impregnation, drying and pressing the wood (SHAMAEV et al. 2018, SHAMAEV et al. 2014, ROWELL et al. 2012, SHAMAEV et al. 2018) from which the operation of treatment is the least studied. All common methods of impregnation, as a rule, are based on the autoclave treatment method, when wood dried with a moisture content of not more than 30%, is treated under pressure with a modifying solution and then subjected to secondary drying to a moisture content of 612% (SANDBERG et al. 2013, SHAMAEV et al. 2018a, SHAMAEV et al.

2018b). A common disadvantage of this method is double drying of wood, which takes a lot of time (14 weeks). Diffusion impregnation of raw wood is not used, since the process lasts 26 weeks. New opportunities have been opened after the development of the end-grain wood treatment under pressure (WEBSITE OF KEBONY et al. 2018). According to this method, a raw rounded log is loaded into the installation, where an impregnating solution is fed from one end of the log under pressure of 0.83.5 MPa, and water (tree sap) flows from the other end until the entire log is completely filled (through impregnation) with the modifying solution. A pressure pulse is used to accelerate the process, and the solution is activated by ultrasound. Oily antiseptics, stabilizers of wood shapes and sizes, hardeners-nanocrystal line

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cellulose, cashew nut shell liquid and antifriction lubricants (Biol, ceresin), various dyes are used as modifiers.

However, the process of end-grain wood treatment under pressure is practically not studied; all the modes are empirical ones.

Earlier attempts to model the process did not find practical application (TSHABALALA et al. 2012). Therefore, the mechanism of end-grain wood treatment under pressure taking into account technical models of hardwoods and softwoods by computer simulation is studied in this paper.

EXPERIMENTAL PART Mathematical modeling in fluid mechanics is based on the finite element approach (ZANKEVICH et al. 1974).

With regard to modeling the process of wood impregnation, we proposed to split the entire volume of the moving fluid into a large number of balls (in the three-dimensional case) or circles (in the two-dimensional case), which are elements of the liquid that interact with each other. The diameter of the elements could be 110 microns (depending on the problem being solved), while the fluid flow was simulated quite well and, at the same time, high speed of computer calculations is provided. The spherical (or circular) shape of the elements was adopted in order to achieve isotropy of the properties of the model fluid. The basic properties of fluid (density, modulus of elasticity, thermal conductivity, etc.) were recalculated for one fluid element. The introduction of various types of balls (with appropriate properties) enabled to simultaneously consider different media in the model: various types of fluids, gases, modifiers, wood elements.

Wood and liquid are made up of circles of the same diameter to increase the versatility of the model. However, the circle elements of wood kept motionless in the process of modeling. Distributing the elements of wood in space, it is possible to achieve reproduction of both the structure of coniferous species (pines, figure 1a) and the structure of deciduous species (birch, figure 1b). The model reproduces the main structural elements of wood: tracheids, vessels, partitions, fringed and non-enlarged pores, scalariform perforation, etc.

b Fig. 1 Representation in the models of softwoods (a) and hardwoods (b).

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It suffices to take into account the minimum properties of the fluid, in particular, its mechanical properties, for estimated calculations. In this case, it is necessary to describe the Newtonian motion of a large number of bodies (fluid elements) interacting with each other by viscoelastic forces.

It was decided to use a two-dimensional model, since it significantly accelerates the calculations, which is important during the first stage.

The state of each element-circle i is determined by four variables: the Cartesian coordinates of its center (xi, yi) and Cartesian components of speed (νxi, νyi;). The interaction of the elements with each other will be considered viscoelastic, which enables to adequately take into account the occurrence of elasticity during the compression of the fluid and the loss of energy during the fluid flow.

The developed model (as a whole) is a system of a large number of differential and algebraic equations, as well as the conditions for inclusion of certain forces. The system of differential equations was solved numerically. The modified Euler-Cauchy method was used, which is especially effective in solving second-order differential equations. The coordinates and velocities were calculated using the formulas: 𝑥𝑖+1 =

𝑥𝑖 +𝜈𝑖 ·𝛥𝑡+𝑎𝑖 ·(𝛥𝑡)2 2

νi+1=vi+aiΔt,

(1) (2)

where х,ν, а– coordinate, speed and acceleration of an element; i–integration step number (i- current step, i+ 1 — next step); Δt– integration step.

Numerical integration step Δt of differential equations system was determined by repeated experiments with successively decreasing step (in 2 times). We stopped at that step, after which the simulation results remained almost unchanged (the change is no more than 12%). The determined step was Δt = 106 s and it was used in all calculations in this work.

A computer program “Program for simulating the process of fluid wood treatment” was compiled in Object Pascal in integrated programming environment Borland Delphi 7.0 for computer experiments with the model and the convenience of theoretical analysis.

A computer experiment consisted of calculating the fluid penetration into a wood structure for a certain period of time (1000 integration steps). Distribution of liquid concentration inside the wood C(х) and C(y), as well as pressure distributions Р(х)in the direction of impregnation were defined during the computer experiment. Biol lubrication with nanocrystalline cellulose: kinematic viscosity – 138 mm2/s. The temperature during the impregnation was taken as 20 ± 2°C.

RESULTS AND DISCUSSION Since the initial stage of end-gain treatment under pressure (then the process becomes stationary and laminar) is the most difficult one, the main attention was paid to the first minutes of impregnation.

To test the performance of physical and mathematical models of impregnation, we conducted the first computer experiments with the model. In the course of a computer experiment, the fluid moved along the tracheids (pine) and to the wood vessels (birch), flowing from the vessel (tracheid) into the vessel (tracheid) along the pores. Thus, the processes occurring in the model were in good agreement with the ideas about the mechanism of wood treatment.

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Since a piece of wood in the physical model had small length along the direction of impregnation (about 100 microns), the model enabled to investigate the initial stages of impregnation in the most accurate way. Figure 2 shows the sequence of wood treatment. The drawings were made at the following time from the beginning of impregnation: 0, 4, and 10s.

Fig. 2 Liquid filling of a fragment of the physical structure of pine over time.

By analyzing the sequence of patterns, it could be verified that the fluid moved faster with increasing diameter of the tracheid dc, and the dependence was approximately as

32

follows νf = ν0+ β√dc, where νf – fluid speed; ν0 - fluid speed in the smallest tracheid; β coefficient of proportionality.

Analysis of the concentration profiles of the liquid along the direction of impregnation (Figure 3) showed that, even though the piece of wood was gradually filled with fluid, the mass concentration of the liquid decreased in the direction of impregnation according to an approximately linear law.

When fluid moved, there were several characteristic pressures near the front of the fluid (Figure 4). The first characteristic pressure P1 (the smallest one) caused the fluid to move freely along the tracheid. The second characteristic pressure P2 caused fluid to flow through the pores.

Fig. 3 Changes in the concentration of the impregnating fluid along the model fragment of pine over time.

Fig. 4 The change in pressure of the impregnating fluid along the model fragment of pine over time.

Hardwood species have different microscopic structure, so wood impregnation occurs differently. Figure 5 shows the sequence of wood impregnation. The drawings were made at the following time from the beginning of impregnation: 0, 2and 8 s.

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Fig. 5 Liquid filling of a fragment of birch structure over time.

The change in the percentage of the total impregnation fluid over time is presented in Figure 6. The change in pressure over time is represented in Figure 7.

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Fig. 6 The change in percentage of the total impregnation liquid along the model fragment of birch over time.

Fig. 7 The change in pressure of the impregnating liquid along the model fragment of birch over time.

Comparing the results of computer experiment with the results of actual impregnation of pressed birch wood along the fibers on specimens of 50×50×100 mm (the last dimension is along the fibers) at initial impregnation stage (the duration is 8 s) showed their close convergence (Figure 8). The maximum discrepancy during the initial and final impregnation stages does not exceed 7%.

Fig. 8 The change in pressure over time is represented after 8 s from beginning of impregnation at a pressure of 0,8 MPa according to the results of computer (1) and active (2) experiment.

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It is necessary to choose analytical formulas that describe the dependencies obtained as a result of experimental studies and computer experiments for an analytical description of the impregnation process.

The considered concentration profiles ω (x) and pressure P (x) are “step-like” (“sigmoidal”) curves: transitions of the function from a higher level to a lower one took place. Such a transition is well described by the Boltzmann sigmoidal function, which is often used in the description of chemical processes (Figure 9): F(x)=F2+

(F1– F2) 𝑥−𝑥0

(3)

1 +𝑒 𝑑∙𝑥

where F1andF2 – initial and final function values; d – multiplication factor; х о – inflection point of sigmoidal Boltzmann function.

Modifying the Boltzmann function as applied to the wood impregnation problem, we obtained the following impregnation equation: 𝜔(𝑥, 𝜏) = 𝜔𝐾 −

𝜔𝐼 −𝜔𝐶 𝑥−𝜏∙𝜈01 ∙𝑒−𝑘1𝜏 ) 𝑥∙𝑎𝜏𝑏

(4)

1+exp(

where х–coordinate along the direction of impregnation; τ–impregnation time; ωI and ωC–the initial concentration of fluid in the wood and the concentration corresponding to the full impregnation; ν01–initial speed of the front of impregnation; k1–coefficient of impregnation front speed reduction; а and b–coefficients determining the expansion of the front of impregnation over time.

Fig. 9 Comparison of theoretical curve with the results of computer experiment.

The method of least squares was used to determine the coefficients of the proposed dependencies, according to the results of a theoretical study. The one-criterion optimization problem was solved in the six-dimensional parametric space R6 without any restrictions on variables. The objective function in this formulation of the optimization problem was unimodal in the whole definition domain: it had a single minimum that is global one.

Minimum search was performed by iterative numerical method using Microcal Origin 5.0 program. There were constants that were included in the approximating expression within the framework of the method, according to the condition of the minimum of the sum of

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squares of graph deviations from the points obtained in computer experiments. That is, the next optimization problem was solved: 2 𝑆(𝑎1 , 𝑎2 ,… ) = ∑𝑁 𝑖=1(𝑓(𝑎1 , 𝑎2 , … , 𝑥𝑖 ) − 𝑦𝑖 (𝑥𝑖 )) → min ⟹ 𝑎1 , 𝑎2 , …

(5)

wherea 1 , a 2 ,…– constants (parameters) included in the approximating expression; N–number of computer experiment points; x i , y i –set of computer experiment points; i–computer experiment number; f (a1,a2,….,x.) –approximating function.

Function limitations.

The one-criterion optimization problem was solved in the six-dimensional parametric space R6 without restrictions on variables (the unconditional optimization problem). The objective function in this formulation of the optimization problem is unimodal in the whole definition domain: it has a single minimum that is global one. Minimum search was performed by iterative numerical method using Microcal Origin 5.0 program.

The solution of the optimization problem in the case of complex approximating expressions was made by an iterative method. In the framework of this work, the search for approximating expressions was performed using Microcal Origin 5.0 program, which is widely used in processing the results of the experiment.

As a result of approximation, the following equation of impregnation was obtained: 𝜔(𝑥, 𝜏) = 0 +

100 𝑥−𝜏∙40∙𝑒−0,2𝜏 1+exp( ) 2+0,5𝜏

(6)

wherew is measured in percent of total impregnation; х–in millimeters; τ–in hours.

Figure 10 shows a series of concentration profiles calculated by the last equation.

During the first hours of impregnation (1-2 hours), the front of the fluid moves quickly, however, starting from τ = 3 hours, the front movement slowed down, and the front of the impregnating liquid practically stopped in the wood approximately by τ = 5 hours, death of penetration was approximately 70 mm.

Fig. 10 Concentration profiles w (x) at different time values τ and experimental curve (--●--●--), obtained in 1 hour of natural birch wood treatment.

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CONCLUSIONS 1. A physical model of the interaction of wood – liquid system in the form of balls (volumetric) and circles (flat) of the corresponding diameter, differing in the static position of the circles - wood elements and the dynamic state of the circles - liquid elements was developed.

2. It was developed by means of a computer experiment program and a system of differential equations was created and solved enabling to create a mathematical model of softwood (pine) and hardwood (birch) species treatment at micro level.

3. A physical model of meso-level wood treatment was developed taking into account temperature and pressure variables, differing in consideration of cavities, annual layers and core beams. On the basis of a mathematical model, analytical expressions were obtained for description of wood impregnation process.

4. With an increase in vessel diameter dV, the fluid speed νf increased approximately according to the following law: 𝜐𝑓 = 𝜐0 + 𝛽√𝑑𝑉 , where ν0 - fluid speed in the smallest vessel; β - coefficient of proportionality.

5. Comparison of computer modeling results with the results of an active experiment showed their high resemblance.

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SHAMAEV, V, PARINOV, D. 2018. Patent 2646612 Russian Federation IPC B27K 3/02 (2006.01) Wood impregnation method; Applicant and patent holder Federal State Budgetary Educational Institution of Higher Education Voronezh State University of Forestry and Technologies named after G. F. Morozov. 2018, 6 P. SHAMAEV, V., MANAEV, V., KONDRATYUK, V., VOSKOBOYNIKOV, I., SCHELOKOV, V., KONSTANTINOVA, S., VARNAKOV, A. 2014. Patent №2511302 Russian Federation IPC A device for impregnating wood from an end face under pressure. 2014, 6 P. SHAMAEV, V., MEDVEDEV, I., PARINOV, D, SHAKIROVA, O., ANISIMOV, M. 2018. Investigation of physical and mechanical properties and microstructure of modified wood produced by self–pressing metod. In Acta Fakultatis Xylologiae Zvolen, 2018, 60(2): 2532.

SHAMAEV, V., PARINOV, D., MEDVEDEV, I. 2018. Wood Modification by Pressing. In Engineering Studies. 2018, vol. 10, issue 3 (2), p. 708717.

TSHABALALA, M., MCSWEENY, J., ROWELL R. 2012. Рeat treatment of wet wood fiber: a study of the effect of reaction conditions on the formation of furfurals. In Wood Material Science and Engineering. 2012, vol. 7, p. 202208.

WEBSITE OF ACCOYA [electronic resource]. 2014. Access mode: https://www.accoya.com/whyaccoya/benefits/ - title from the screen.

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WEBSITE OF KEBONY COMPANY [electronic resource]. 2018. Mode of access: http://kebony.com/en/products/ – title from screen.

WESTIN, M. 2010. Durability of Modified Wood – Laboratory vs Field Performance. Technical Research Institute of Sweden, Boras, Sweden, 2010, 142 P. ZANKEVICH, O. 1974. The finite element method in the theory of structures and in continuum mechanics kevich, 1974, 283 P.

ADDRESSES OF AUTHORS Vladimir Shamaev, doctor of engineering, professor of the Department of Wood Science Voronezh State University of Forestry and Technologies named after G.F. Morozov Voronezh Voronezh region Russian Federation drevstal@mail.ru Phone: +79802454092, +7(473)253-67-22.

Home address: 96, Kholzunova str, apt. 105, Voronezh Voronezh region Russian Federation

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 41−53, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.04

MATHEMATICAL DESCRIPTION OF THE LATENT HEAT OF BOUND WATER IN WOOD DURING FREEZING AND DEFROSTING Nencho Deliiski – Ladislav Dzurenda – Natalia Tumbarkova – Dimitar Angelski ABSTRACT Some basic characteristics and terms of the specific latent heat of crystallization of the water in wood and of melting the ice formed by it are presented in the paper. The change in the temperature of ice and water during their heating and cooling occurred in the phase transitions were described and analyzed. Using data from the specialized literature a mathematical description of the specific latent heat of crystallization and melting the bound water in wood were suggested. Based on that description, an update of an available equation for the specific heat capacity of the frozen bound water in wood above the hygroscopic range was carried out. The information about the specific latent heat and specific heat capacity of the water in wood materials is needed for computing the non-stationary temperature distribution and energy consumption during their thermal treatment, and also for the model based automatic control of that treatment.

Key words: latent heat, mathematical description, specific heat capacity, wood, bound water, freezing, defrosting.

INTRODUCTION It is known that latent heat is the thermal energy absorbed or released by a body or a thermodynamic system during a constant-temperature process of phase transition of a given substance (https://www.britannica.com/science/latent-heat, https://www.engineeringtool box.com/latent-heat-melting-solids-d_96.html). During such a process, the temperature of the system stays constant as heat is added: the system is in a mixed-phase regime in which some parts of the system have completed the transition and others have not. A familiar example is the melting of ice, which does not convert suddenly into a liquid but for a definite period of time there is a mixture of crystals and liquid water.

The term “latent” was introduced around 1762 by British chemist Joseph Black. It is derived from the Latin latere, which means to lie hidden (https://en. wikipedia.org/ wiki/Water/data_page). Consequently, the latent heat represents a thermal energy in hidden form, which is supplied or extracted to change the aggregate state of a given substance without changing its temperature.

During the calculation of the duration and energy consumption of the defrosting processes of frozen wood materials, the heat needed to carry out the phase transition, i.e. for the melting of the ice, formed both from the free water, as well as from the frozen part of the 41

bound water in the wood must be taken into account. When relating these heat consumptions to 1 K they can be expressed by their corresponding specific heat capacities, i.e. through the specific heat capacities of the frozen free water, cice-fw, and of the frozen bound water, cicebw, in the wood.

In the expressions to determine cice-fw and cice-bw in the specialized scientific literature the latent heat of the free and of the bound water in wood participate respectively (CHUDINOV 1966, STEINHAGEN 1986, 1991, STEINHAGEN – LEE 1988, GOSS – MILLER 1989, SHUBIN 1990, KHATTABI – STEINHAGEN 1992, 1993, 1995, POŽGAJ et al. 1997, SIMPSON – TENWOLDE 1999, TREBULA – KLEMENT 2002, VIDELOV 2003, DELIISKI 2004, 2009, 2011, 2013, PERVAN 2009, DELIISKI – DZURENDA 2010, DZURENDA – DELIISKI 2011, DELIISKI et al. 2013, 2015, 2019, HADJISKI – DELIISKI 2015, 2016, DELIISKI – TUMBARKOVA 2017a, 2017b, 2018, 2019).

According to these expressions, there is widely accepted that the latent heat of both the free and bound water in the wood is the same and it is equal to 3.34·105 J·kg1.

The research on freezing water and melting the ice formed from it in different capillary-porous materials shows that the latent heats of the free and bound water in these materials differ from one another quantitatively (EFIMOV 1985, LANGE et al. 1994, CHO et al. 1996, OLIEN – LIVINGSTON 2006, SZEDLAK et al. 2009, KIANI – SUN 2011, KRANIOTIS – KRISTINE 2017). That gives us reason to investigate the difference between the latent heat of free and of bound water in wood, which are typical representatives of capillary-porous materials.

The aim of the present paper is to suggest a mathematical description of the latent heat of the bound water in wood and to incorporate it in an updated version of the equation to determine the specific heat capacity of the frozen bound water in wood.

MATERIAL AND METHODS Basic characteristics and terms of the latent heat When a substance changes phase, i.e. it goes from either a solid to a liquid or a liquid to a gas state, it requires energy to do so. The potential energy stored in the interatomic forces and between molecules needs to be overcome by the kinetic energy of the motion of the particles before the substance can change phase.

In Figure 1, example schemes of the change in the temperature T of the ice during its heating and also of water during its freezing are given.

Starting at point A (see Fig. 1-left), the ice was in its solid phase, heating it by the energy Q brings the temperature up to its melting point but the ice was still in a solid state at point B. As it was heated further, the energy from the heat source Q went into breaking the bonds holding the atoms in place. This took place from B to C where the water absorbed the energy QLat without a change of its temperature. At point C the whole amount of the ice was transformed into water. The further heat addition from C to D went into the kinetic energy of the water, which caused an increase in its temperature.

During cooling the water (see Fig. 1-right) its temperature decreased from A to B. When the melting point was reached in B, a crystallization of the water started. From B to C, the freezing of the whole amount of the water into ice occurred without change in its temperature. During that process a release of the absorbed energy QLat into the surrounding was carried out. The further cooling of the ice from C to D causes a decrease in its temperature.

42

In the International System of measuring units (SI) the latent heat is represented by the parameter of specific heat of phase transition, which is also called by the term specific latent heat L. It is expressed by the amount of energy in the form of heat Q (in J), which is required to completely effect a phase change of mass m (1 mol or 1 kg) of a given substance, i.e.

Fig. 1 Change in T of ice during its heating (left) and change in T of water during its freezing (right).

L

Q m

(1)

Using eq. (1), the thermal energy denoted as QLat can be determined, as follows:

QLat  m  L

(2)

where QLat is the amount of the energy absorbed or released during the change of phase of the substance, J; m – mass of the substance, mol or kg; L – specific latent heat for a particular substance, J·mol-1 or J·kg-1.

In the specialized literature the following indexed options of L are mostly used: • Lf – specific latent heat of fusion or melting. It is used in models of processes with phase transitions from solid to liquid state of the substances; • Lcr – specific latent heat of crystallization. It is used in models of processes with phase transitions from liquid to solid state of the substances.

• Lv – specific latent heat of vaporization. It is used in models of processes with phase transitions from liquid to gaseous state of the substances.

• Lc – specific latent heat of condensation or liquefaction. It is used in models of processes with phase transitions from gaseous to liquid state of the substances.

Dependence of the latent heat of water on the temperature In the specialized literature it has been proven that the latent heat L of a given substance is equal to the difference ΔH between the enthalpy of the two phases of the substance (EFIMOV 1985, SZEDLAK et al. 2009, PAHI 2010, https://en.wikipedia.org/wiki/ Latent_heat). In our case of freezing of the water in wood and melting of the ice in wood this means that Lf  Lcr  H (3) where H  H water  Hice (4) is the difference between the enthalpies of the water, Hwater, and the ice, Hice, J·kg1.

43

Fig. 2 Change in ΔH between water and ice, depending on T (acc. OLIEN and LIVINGSTON 2006).

The difference Lf = ΔH can be determined using the graphs given in Fig. 2.

The temperature dependence of the difference in enthalpy between water and ice at a constant pressure p can be expressed by the following equation of Kirchhoff (SZEDLAK et al.

2009, PAHI 2010, https://www.physics.info/heat-latent/):

 H     cp  T p

(5)

where

cp  cwater  cice

(6)

is the difference between the specific heat capacities of the liquid water, cwater, and the ice, cice, at atmospheric pressure, J.kg-1·K-1.

Taking into account eqs. (4) and (6), for the case of the ice melting, the solution of eq.

(5) is equal to (SZEDLAK et al. 2009) Lf (T )  Lf (Tf ) 

Tf

 cwater  cice dT

(7)

T

where Lf is the specific latent heat of melting of the ice, J·kg1; Тf – temperature at the point of the ice melting, K; T – temperature lower then Tf, K; cwater and cice – specific heat capacities of the liquid water and ice respectively, J·kg1·K1.

In Fig. 3, experimental data given in SZEDLAK et al. (2009) for Lf of overcooled water in the range from 236 to 263 K are presented. They were obtained by three scientific teams using different measuring methods and equipment. In that Figure, the change in Lf depending on T calculated by eq. (7) is also given.

44

Fig. 3 Change in Lf of water, depending on Т (acc. SZEDLAK et al. 2009).

Latent heat of water in wood during its freezing and defrosting The information about the change in the specific latent heat of crystallization and melting of both the free and bound water in wood materials is of significant interest during the modeling and model-based automatic control of the freezing and defrosting processes of these materials.

As it was noted in the Introduction, in the specialized scientific literature it is widely accepted that the specific latent heat of crystallization of the free water and of melting of the ice formed by it in capillary porous materials, including wood, do not depend on the temperature and are equal to, as follows: Lcr -fw  Lf -fw  3.34  10 5 J  kg 1 .

(8)

The reason for the constant value of Lcr-fw and Lf-fw is the circumstance that the free water freezes at constant temperature, equal to 273.17 K, i.e. 0 oC or in very narrow temperature range below 273.15 K. The temperature conditions for the freezing of the bound water differ significantly from these for the freezing of the free water. It is known that the bound water freezes gradually in a very wide range below 273.15 K and even at extremely low climatic temperatures on the earth part of it remains in a liquid state in the wood.

During studying of the impact of T on the latent heat of melting of the ice at constant pressure and temperatures below 273.15 K, EFIMOV (1985) derived the following equation, which represents a modification of the classical equation of Clausius-Klapeyron:

L L  cp  T T

(9)

If the condition Δcp = const is fulfilled, the solution of eq. (9) is equal to Lf  bw  Lf  fw 

T Tf  fw

 cp  T  ln

T Tf  fw

(10)

where Тf-fw = 273.15 K is the melting temperature of the ice formed from the free water at atmospheric pressure, K; T – temperature of overcooling of the water subjected to freezing: T < Тf-fw, K; L – latent heat of melting of the ice formed from bound water at temperature T, 45

J·kg1; Lf-fw = 3.34·105 J·kg1 – latent heat of melting of the ice formed from the free water; Δcp – difference between the specific heat capacities of the water and ice at atmospheric pressure, J·kg1·K1.

According to EFIMOV (1985), equation (10) can be used for the calculation of the latent heat of melting of the bound water, Lf-bw, in capillary-porous materials.

The results obtained by the equation (10) show that a decrease in T causes a decrease in Lf-bw, which could be explained with the circumstance that the absorption of the bound water in capillary-porous materials including wood is accompanied with heat release and with an increase in the alignment of the water molecules in the materials.

RESULTS AND DISCUSSION After substitution in eq. (6) of cwater = 4218 J·kg1·K1 and cice = 2116 J·kg1·K1 (https://en. wikipedia.org/wiki/Water/data_page), Δcp = 2102 J·kg1·K1 at atmospheric pressure and temperature T =273.15 K (i.e. at 0 oC) was obtained.

Replacing in eq. (10) the values of Тf-fw = 273.15 K, Lf-fw = 3.34·105 J·kg1, and Δcp = 2102 J·kg1·K1, the following equation was obtained for the calculation of the specific latent heat of melting the ice formed from the bound water in wood materials:

Lf bw  1.223  103Т  2.102  103T  ln

T 273 .15

(11)

Update of the mathematical description of the specific heat capacity of frozen bound water in wood above the hygroscopic range According to DELIISKI (2004, 2009, 2011, 2013) the suggested mathematical descriptions of the effective specific heat capacities of wood during its freezing and defrosting, the following equations for the calculations of the specific heat capacities of the frozen free and bound water in wood above the hygroscopic range, cice-fw and cice-bwm respectively, were given in DELIISKI et al. (2019): cice -fw  3.34  10



272.15 5 u  ufsp

1 u

 exp0.05671Tu 272.15

272 .15 cice-bwm  1.8938 10 4 ufsp  0.12 

272.15 293 .15 ufsp  ufsp  0.021

(12) (13) (14)

where cice-fw is the specific heat capacity of the frozen free water in wood, J·kg-1·K-1; cicebwm – specific heat capacity of the frozen bound water in wood when the maximum possible amount of bound water in the wood species is present, J·kg1·K1 (that condition is always 293.15 fulfilled above the hygroscopic range); u – wood moisture content, kg·kg-1; ufsp – 272.15 standardized value of the fiber saturation point at T = 293.15 K (i.e. at t = 20 oC), and ufsp

is the fiber saturation point at T = 272.15 K (i.e. at t = –1 oC), kg·kg1. At that temperature, melting the frozen bound water in wood is fully completed and melting the free water in wood starts during heating the frozen wood (DELIISKI – TUMBARKOVA 2017a, 2017b, 2018, 2019).

46

The multiplier 3.34·105 in eq. (12) is the value of the specific latent heat of the free water in wood (refer to eq. (8)), which is needed for the phase transition of 1 kg liquid free water into ice.

The number 1.8938·104 in the right part of eq. (13) represents the result from multiplying of the number 0.0567 by 3.34·105, which was obtained in DELIISKI (2011, 2013) during the mathematical description of cice-bwm. That description was based on the widely accepted assumption that the specific latent heats of melting both the bound and free water in wood are the same and equal to 3.34·105 J.kg-1.

Using eq. (11), the following update and more precise version of eq. (13) was suggested in the present paper:





T  272 .15 exp0.0567 T  272 .15   (15) cice - bwm   69 .344 Т  119 .183Т  ln  0.12    ufsp 273 .15  1 u 

The expression given in the first brackets in eq. (15) was obtained after multiplying of eq. (11) by the number of 0.0567.

In order to solve the eqs. (11), (12), (14), and (15) with different values of the variables in them, a software program was prepared, which was an input in the calculation environment of Visual FORTRAN Professional. With the help of that program, calculations were carried out for the determination of Lf, Lf-bw, cice-fw, and cice-bwm.

In Fig. 4 the change in the specific latent heat Lf in the range from 233.15 to 273.15 K (i.e. from –40 to 0 оС) was given, which was calculated according to 3 different approaches.

The calculated by SZEDLAK et al. (2009) according to eq. (7) change in Lf is shown in Fig. 4 through the top line “Szedlak et al.”.

Through the middle line „Acc. eq. (11): Lf-bw” the calculated by us according to eq.

(11) change in Lf-bw is given.

The bottom line “Olien & Livingston” shows the change in Lf, which was obtained according to eqs.(3) and (4) using the graphs of Hwater = f(T) and Нice = f(T) given in Fig. 2.

The comparison of the graph of Lf-bw in Fig. 4 with the two graphs of Lf shows well their compliance with each other. The slope of Lf-bw occupied an intermediate place between the slopes of Lf determined by SZEDLAK et al. (2009) and OLIEN – LIVINGSTON (2006). The differences between the analogous values of Lf-bw and Lf in the studied temperature range were as follows: ± 9 at Т = 243.14 K; ± 6% at Т = 253.15 K, ±3 % at Т = 263.15 K and 0% at Т = 273.15 K. It can be noted, that the graph of Lf-bw in Fig. 4 is more relevant to the experimental data in Fig. 2 in comparison to both graphs of Lf in Fig. 4. This fact gave reason to recommend eq. (11) for implementation as a mathematical description of Lf-bw in the models of wood thermal treatment processes, which include freezing and defrosting of materials from different wood species.

In Figure 5 the calculated according to eq. (12) change in the specific heat capacity of the frozen free water, cice-fw, in beech and poplar wood, depending on the wood moisture 272.15 to u = 1.0 kg·kg1 is presented. For the determination content u in the range from u = ufsp 272.15 according to eq. (14) the standardized values of the fiber saturation point of ufsp

293 .15 293 .15 ufsp  0.31 kg·kg-1 for the beech wood and ufsp  0.35 kg·kg1 for the poplar wood

was used (VIDELOV 2003, DELIISKI – DZURENDA 2010).

47

Specific latent heat L f , J.kg

-1

340000 320000 300000 280000 260000 240000

Szedlak et al.

220000

Acc.eq.(11): Lf-bw

200000

Olien&Livingston

180000 230

240

250 260 270 Тemperature Т , K

280

120000 Beech

-1

Specific heat capacity c fw, J.kg .K

-1

Fig. 4 Change in Lf at Р = const, depending on Т

100000 Poplar

80000 60000 40000 20000 0 0,3

0,4 0,5 0,6 0,7 0,8 0,9 Wood moisture content u , kg.kg -1

1

Fig. 5 Change in cice-fw of beech and poplar wood, depending on u.

In Figures 6 and 7, the calculated according to eqs. (13) and (15) change in the specific heat capacity of the frozen bound water, cice-bw, in beech and poplar wood respectively, depending on the temperature in the range from 213.15 to 272.15 K (i.e. from –60 oC to –1 o C) is shown. The derived by eqs. (13) and (15) lines of cice-bw have labels “old” and “updated” respectively in the legend of these figures.

The analysis of the obtained simulation results, part of which are presented in Fig. 5 to Fig. 7 lead to the following statements: 1. The specific heat capacity of the frozen free water, cice-fw, increases curvilinearly from 0 to 111890 J·kg1·K1 for the beech wood and from 0 to 105210 J·kg1·K1 for the 272.15 to u = 1.0 kg·kg1. poplar wood when u increases from u = ufsp

48

Beech

u = 0,4-old u = 0,4-updated u = 0,6-old u = 0,6-updated u = 0,8-old u = 0,8-updated u = 1,0-old u = 1,0-updated

-1

Specific heat capacity c ice-bwm, J.kg .K

-

3000 2500 2000

1

1500 1000 500 0

-60

-50

-40

-30

-20

-10

0

o

Temperature t , C

Fig. 6 Change in cice-bw of beech wood, depending on t.

Poplar

u = 0,4-old u = 0,4-updated u = 0,6-old u = 0,6-updated u = 0,8-old u = 0,8-updated u = 1,0-old u = 1,0-updated

3000 2500 2000 1

-1

Specific heat capacity c ice-bwm, J.kg .K

-

3500

1500 1000 500 0

-60

-50

-40

-30

-20

-10

0

o

Temperature t , C

Fig. 7 Change in cice-bw of poplar wood, depending on t.

2. An increase in the standardized value of the fibre saturation point of the wood 293.15 causes a decrease in cice-fw because of the circumstance that with an increase in ufsp 293.15 272.15 decreases (see eqs. the amount of the free water in the wood equal to u – ufsp ufsp (12) and (14)).

293.15 Each increase in ufsp by 0.01 kg·kg1 causes an exponential increase in the specific

heat capacity of the frozen bound water cice-bwm. When t increases from –60 oC to –1 oC, cicebwm increases several tens of times at the given value of the wood moisture content u. At t = –1 oC the capacity cice-bwm reaches its maximum values, which according to eq. (15) are equal to the following: 49

• at u = 0.4 kg·kg1: cice-bw = 2826 J·kg1·K1 for beech and cice-bw = 3362 J·kg1·K1 for poplar wood; • at u = 0.6 kg·kg1: cice-bw = 2473 J·kg1·K1 for beech and cice-bw = 2942 J·kg1·K1 for poplar wood; • at u = 0.8 kg·kg1: cice-bw = 2198 J·kg1·K1 for beech and cice-bw = 2615 J·kg1·K1 for poplar wood; • at u = 1.0 kg·kg1: cice-bw = 1978 J·kg1·K1 for beech and cice-bw = 2354 J·kg1·K1 for poplar wood.

4. The increase of u causes a decrease in cice-bwm. When u at a given t increases from 0.4 to 1.0 kg·kg1, cice-bwm decreases by 30.0% for both the beech and poplar wood. This means that each increase of u by 0.01 kg·kg-1 reduces cice-bwm by approximately 0.5%.

293.15 5. An increase in ufsp for separate tree species causes a linear increase in cice-bw.

293.15 Each increase in ufsp by 0.01 kg·kg1 causes an increase in cice-bwm by approximately 4%.

6. The calculated by eq. (15) updated values of cice-bwm are found to be smaller in the range of up to 20% compared to the analogous old values of cice-bwm, which are calculated by eq. (13) (see Fig. 6 and Fig. 7). This means that the application of the more precise eq.

(15) instead of eq. (13) will lead to smaller calculated values of the energy, which is needed for the melting of the frozen bound water in wood materials during their thermal treatment.

CONCLUSIONS Some basic characteristics and terms of the specific latent heat of crystallization of the water in wood and of melting of the ice are being considered in this work. Based on the analysis of data in the specialized literature and in the Internet sources the following were determined: • the specific latent heat of fusion and crystallization of free water in wood does not depend on the temperature and is equal to Lf-fw = Lcr-fw = 3.34·105 J·kg1; • the specific latent heat of fusion and crystallization of the bound water in wood, Lfbw = Lcr-bw, decreases with a decrease in the temperature T. This can be explained with the circumstance that the absorption of the bound water in capillary-porous materials, including wood, is accompanied with heat release and with an increase in the alignment of the water molecules in the materials.

• an equation was derived, which can be used as a mathematical description of the specific latent heat Lf-bw = Lcr-bw in wood materials during the phase transition of the bound water in them. That equation is based on the classical equation of Clausius-Klapeyron, which has been modified by EFIMOV (1985).

Based on that description, an update of the available equation for determination of the specific heat capacity of the frozen bound water in wood above the hygroscopic range, cicebwm, were carried out. The calculated by that equation updated values of cice-bwm are found to be smaller in the range of up to 20% compared to the analogous values of cice-bwm, which are calculated by a previous version of that equation. The application of the more precise updated equation for cice-bwm will lead to smaller calculated values of the energy, which is needed for the melting of the frozen bound water in wood materials during their defrosting.

The obtained results can be used for scientifically based determination of the specific latent heats Lf-fw = Lcr-fw, Lf-bw = Lcr-bw, and also of the specific heat capacity of the frozen bound water cice-bwm in mathematical models of freezing and defrosting processes of wood 50

materials. They could support the development of improved software of systems for modelbased automatic control of such processes (HADJISKI – DELIISKI 2016).

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STEINHAGEN, H. P., LEE, H. W. 1988. Enthalpy Method to Compute Radial Heating and Thawing of Logs. In Wood Fiber Science, 20(4): 415421.

SZEDLAK, A., JOHNSON, A., KOSTINSKI, A., CANTRELL, W. 2009. The Temperature Dependence of Water's Latent Heat of Freezing. Dept. of Physics and Atmospheric Sciences Program. Michigan Technological University, Houghton MI 49931.

TREBULA, P., KLEMENT, I. 2002. Drying and Hydrothermal Treatment of Wood. Zvolen: TU vo Zvolene, Slovakia, 449 pp.

VIDELOV, H. 2003. Drying and Thermal Treatment of Wood. Sofia: University of Forestry, 335 pp.

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ACKNOWLEDGEMENTS This document was supported by the grant No BG05M2OP001-2.009-0034-C01 "Support for the Development of Scientific Capacity in the University of Forestry", financed by the Science and Education for Smart Growth Operational Program (20142020) and co-financed by the European Union through the European structural and investment funds.

This document was also supported by the APPV Grant Agency as part of the project: APVV-170456 as a result of work of authors and the considerable assistance of the APVV agency.

AUTHORS’ ADDRESSES Prof. Dr. Nencho Deliiski, DrSc. University of Forestry Faculty of Forest Industry Kliment Ohridski Blvd. 10 1797 Sofia Bulgaria deliiski@netbg.com Prof. Ing. Ladislav Dzurenda, PhD. Technical University in Zvolen Faculty of Wood Science and Technology T. G. Masaryka 24 960 01 Zvolen Slovakia dzurenda@tuzvo.sk Eng. Mag. Natalia Tumbarkova, PhD. Assoc. Prof. Dimitar Angelski, PhD. University of Forestry Faculty of Forest Industry Kliment Ohridski Blvd. 10 1797 Sofia, Bulgaria ntumbarkova@abv.bg, d.angelski@gmail.com

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 55−65, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.05

3D-FORMABILITY OF PERFORATED MATERIALS BASED ON VENEER Jozef Fekiač  Jozef Gáborík  Mária Šmidriaková ABSTRACT The paper is focused on the impact of perforations on 3D-formability of two-layer birch veneer materials. Two-layer material consisted of one perforated and one solid veneer.

The veneers were glued together with low density polyethylene film. The grain of veneers in the board ran at a 90 degree angle to each other. Veneer perforation was done using CO2 laser. Several perforation varieties were designed taking into account the shape of perforation, the direction of perforations with respect to the wood fibres and spacing between the perforations. 3D formability was determined by pressing a ball punch into the hole in the matrix and evaluated according to the detected deepening. The results indicate a negative impact of the perforations on 3D formability when compared to non-perforated material; only the variety with “I” shape perforations oriented parallel to wood fibres approached the non-perforated material. Lightening of the material was the secondary benefit of the perforations.

Keywords: 3D formability, gluing, lightening, modification, perforation, veneer.

INTRODUCTION Veneer-based layered materials were created to change the unwanted properties of native wood. Layering and bonding of veneers create composite materials with various properties and a wide range of applications. Considerable attention is paid to modification both laminated veneer materials and modification of the veneer itself. Some progress has been made in increasing of 3D formability of veneers by modification of them: WAGENFÜHR et al. (2006), ROSENTHAL (2009), BUCHELT et al. (2010), HEROLD and PFRIEM (2013), SLABEJOVÁ and ŠMIDRIAKOVÁ (2013, 2014), FEKIAČ et al. (2015), FEKIAČ et al. (2016), SLABEJOVÁ et al. (2017), GAFF et al. (2017). One of the most effective ways to modify veneer for 3D forming is based on targeted disruption of its integrity by cutting and subsequent joining by hot melt fibre (NAVI and SANDBERG 2012, KRENZ 2013).

Cutting the material to increase its formability is practically used in production of furniture and decorative items. Two principles to disrupt material integrity are known: mechanically using a cutting tool or using a concentrated energy (laser). Mechanical cutting of flat materials into a certain depth to achieve a desired bend radius is commonly used technology. The cutting is not observable in the final product at all. At present, materials modified by disruption of their integrity into their full thickness (perforated materials) are commercially available. The perforations are aesthetically pleasing and also provide excellent sound absorption at all frequencies (DUKTA, s. a.).

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Laser technology is largely used for decorative finishing of wood and wood based materials. The dominant application for decorative purposes is engraving the surfaces. The incising (perforating) is also a significant application.

Laser incision is applied successfully in production of so-called “living hinge materials”. These are perforated flat materials with a specific pattern of perforations that allow the material to be bend without breaking. A typical example is a book cover made from plywood modified by perforations (Fig. 1). To improve design of the “living hinge materials”, FENNER (2011) proposed a mathematical model that allowed to reduce the number of trials and errors when designing. It made it possible to create “living hinge materials” fatigue-resistant so they can be used not only for static bends, but also for moving parts of objects.

Fig. 1 Living hinge in book cover made from plywood (LASERCUTCRAFT, s. a.).

The method of incision materials to change their formability can also be applied for spatial forming. The principle of incision is based on creation of two-side perforated grid which allows the material to be bend three-dimensionally. When laser incision, different shapes of perforations are designed (Fig. 2).

Fig. 2 Perforations used to increase material formability; they also fulfill an aesthetic function (OBRARY, s. a.).

An indisputable advantage of the perforated materials is the reduced weight. Recent interest in reduction the weight of materials is confirmed by the research on properties of lightweight wood materials DUDAS and VILHANOVÁ (2013), VILHANOVÁ and GÁBORÍK (2018), BARBU et al. (2010), EBNER and PETUTSCHNIGG (2005), MEDRI et al. (2015) and other authors.

An important factor affecting the properties of layered materials is the adhesive applied. The most commonly used adhesives for production of layered veneer materials are urea-formaldehyde and phenol-formaldehyde adhesives. Nowadays, when there are increased requirements for the ecological and hygienic safety of products and materials, it is necessary to reduce emissions resulting from applied formaldehyde containing adhesives.

One of the possibilities to reduce the emissions from materials and finished products is 56

modification of the formaldehyde based adhesives. This issue was dealt by LYUTYY et al.

(2017), MATYAŠOVSKÝ et al. (2014), RUŽIAK et al. (2017), ŠMIDRIAKOVÁ et al. (2012).

Another possibility to reduce formaldehyde emissions is replacing the formaldehyde based adhesives with the adhesives formaldehyde free. At present, also PVAC adhesives are used for production of materials or for finishing materials e.g. veneering. Successful replacement of the formaldehyde based adhesive was reported by SONG et al. (2017). They used a polypropylene foil (PP) as the adhesive for gluing plywood. KAJAKS et al. (2012) used a polyethylene foil (PE). The properties of plywood glued with PE foils was also reported by BEKHTA and SEDLIAČIK (2019), CHANG et al. (2017), IBRAGIMOV et al. (2017), KARRI et al.

(2018), FANG, et al. (2013).

The aim of the paper is to evaluate 3D formability of partially perforated materials.

The tested materials were veneer based, two layered, with cross layering, glued with polyethylene foils.

MATERIAL AND METHODS Based on results of veneer formability published by FEKIAČ et al. (2015), sliced veneer of birch (Betula Pendula Roth) was chosen to prepare two-layer material. The thickness of the veneer was 0.58 ± 0.04 mm and the average humidity of 8.5 ± 0.5%. Formats of 90 × 195 mm were cut using CO2 laser; a half of the formats having wood fibres in direction identical to the longer format size and the other half to the shorter format size. A half of the veneer formats was modified by targeted perforations using CO2 laser. The violation of the integrity of the veneer by perforations could increase the deformability of wood.

For the experiments, the perforations in the shape of letter “I“ (line length of 10 mm) and the letter “S” (dimensions of 10 × 5 mm) were designed. The cutting width was 0.5 mm.

Three variants of the arrangement of perforations, with respect to wood fibres in the veneer, were designed – parallel to the fibres (marking “P”), perpendicular to the fibres (“K”) and mixed arrangement of both directions (“M”). For each arrangement, two spacing intervals between perforations were designed: 2 mm and 5 mm. The designed perforation variants are shown in Fig. 3.

Fig. 3 Variants of perforation on test specimens with respect to wood fibres in veneers; where: I – perforations in shape of letter “I“, S – perforations in shape of letter “S“, P – direction of perforations parallel to wood fibres, K – direction of perforations perpendicular to wood fibres, M – mixed arrangement with respect to wood fibres, 2 – spacing between perforations of 2 mm, 5 – spacing between perforations of 5 mm.

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Marking perforation variants as well as marking the test specimens was a combination of letters and a number: the shape of perforation, arrangement of the perforations, spacing between perforations.

The two-layer material for the experimental testing was made from two veneer sheets by cross-laminating. In the glued file, one veneer was modified by perforations and the other was solid (non-perforated). To join the two veneers, low density polyethylene (LDPE) foil with the thickness of 0.12 mm was applied; the dosage was 100 g.m-2. The created assembly was pressed in a heated hydraulic press (FONTIJNE TP 400) under pressing conditions: temperature 150 °C, pressure 1.8 MPa, time 4 minutes. Under the same conditions, the reference material (reference sample) made from solid (non-perforated) veneers was pressed.

According to the above mentioned technology, two-layer materials with an average thickness of 1.02 mm and the surface dimensions of 90 × 195 mm were made. After 7 days of conditioning, the weight of each perforated and solid (non-perforated) material was measured using laboratory balance of accuracy of 0.001g. Subsequently, from the prepared materials, test specimens of circular shape with a diameter of 60 mm were cut using a punch (Fig. 3). Nine test specimens were prepared for each variant of perforated material and also for the reference material.

The 3D formability testing was carried out on the principle of pushing the test specimen using a ball punch into a hole in a matrix. The ball with a diameter of 40 mm was moving at speed of 3 mm.min-1 while the circumference of the test specimen was hold.

Holding the test specimen prevented curling on perimeter of the test specimen; but at the same time, the specimen could move freely between the matrix and the holder. The testing was proposed and presented in detail by ZEMIAR and FEKIAČ (2014). It is schematically shown in Fig. 4.

Fig. 4 The testing of 3D formability of veneers using a ball punch – with the specimen hold; where: 1 – test specimen Ø 60 mm, 2 – ball punch, 3 – matrix, 4 – holder, 5 – spacer, F – forming force (loading).

The testing of 3D formability was performed using the test machine LabTest 4.050.

The specimens were loaded in two ways with respect to the perforated surface: the perforated surface of the test specimen was loaded by the ball punch (Fig. 5a), or the solid surface was loaded (Fig. 5b).

During the 3D formability testing, the monitored parameters were: the deepening (hmax) and the forming force (F); they were directly recorded by the test machine through integrated sensors. Based on the mentioned parameters, the 3D formability was evaluated.

The main indicator to assess 3D formability was maximal deepening measured (hmax), i.e. deepening at the moment of failure of the test specimen.

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Fig. 5 Test specimens loaded during the 3D formability testing: a) on perforated surface, b) on solid surface (non-perforated); where: hmax – maximum deepening at the moment of failure of the specimen, 1 – perforated veneer, 2 – solid veneer, 3 – matrix, 4 – holder, F – forming force (loading).

The additional indicator of 3D formability was the force required to change the shape of the test specimen by per unit of deepening (Fh). The force per unit of deepening (Fh) was calculated as a ratio of the force measured at the moment of failure of the test specimen (Fmax) and the associated deepening (hmax) according to the relation (1): 𝐹

𝐹ℎ = ℎ𝑚𝑎𝑥

(1)

𝑚𝑎𝑥

Where:

Fh – force per unit of deepening [N.mm-1], Fmax – the force measured at the moment of failure of the test specimen [N], hmax – maximum deepening [mm].

The lightening of the material (weight loss) was also monitored; lightening is a result of perforating and is related indirectly to 3D formability. To compare the impact of the variants of perforation on the lightening of the material, percentage of weight reduction was calculated according to the relation (2): Δ𝑚 = 100 − Where:

𝑚𝑝𝑒𝑟𝑓 𝑚𝑟𝑒𝑓

∙ 100

(2)

∆m – lightening of the material [%], mperf – weight of the perforated material [g], mref – weight of the solid (non-perforated) material [g].

RESULTS AND DISCUSSION The maximum deepening and forces per unit of deepening were evaluated using the program STATISTICA 12 by multi-factor analysis of variance and the Duncan test. Based on significance level (p), the impact of variants of perforation on 3D formability of the tested two-layer materials was assessed. The effect of perforations on 3D formability was evaluated in view of the shape of perforation, arrangement of perforations with respect to direction of wood fibres in the veneer, spacing between perforations, and direction of loading.

Influence of load direction The analyses showed that the change in deepening was not significant under loading both the solid and perforated surface for any variant of perforation. This is demonstrated by the Duncan test. The significance level “p” exceeded the standard significance level of 0.05 for each perforation variant.

When designing the perforations, we assumed that the perforation in veneer would allow the material to deform more, without observable breaching. If the perforated veneer in material was on the side loaded by pressure of the punch (Fig. 5a), the perforations would allow the material to be compressed more, without breaching. Conversely, if the perforated veneer was on the opposite side (Fig. 5b), even a slight widening of the perforations would 59

allow the material to become more deformable. The results indicate that 3D formability was not affected by the positioning of the perforated veneer significantly.

The comparison of measured values of maximum deepening (hmax) depending on the variants of perforation and direction of loading is shown in Fig. 6. The analysis did not confirm better 3D formability of one-side perforated veneer material.

Fig. 6 Deepening (hmax) at loading on solid surface and perforated surfaces; note: I and S – material with perforations in the shape of the letters “I“ or “S“; 2 and 5 – spacing between perforations (mm).

Influence of perforation shape When evaluating the impact of the shape of perforations on 3D formability, no significant difference between perforations “I” and “S” (except for the perforations parallel to wood fibres) was shown. What was confirmed by Duncan's test.

From the view point of the shape of perforations, it can be stated that the perforations do not affect the 3D formability significantly. However, also other factors must be considered.

Influence of direction of perforations with respect to wood fibres Comparing the perforation variants in terms direction of perforations, the most noticeable difference in 3D formability was shown in the variants with perforations of “I”.

When comparing the variants with parallel direction of perforations with the variants with perpendicular direction (IP2 with IK2; IP5 with IK5), under load on the perforated surface, the 3D formability of the material with perpendicular perforations was lower by 40 % to 52 %. Under load on the solid surface, the 3D formability of the material with perpendicular perforations was lower by 29 % to 37 %.

The variants with mixed arrangement of perforations (IM2 and IM5), when compared with the parallel arrangement, showed a reduced 3D formability by an average of 35 % under load on the solid surface and by 49 % under load on the perforated surface.

If taking into account the perforation direction, no significant difference was shown between the perforations “S”, except for case (SK2 a SM2 under load on the solid surface).

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The difference between the perforation variants “S” varied from 3 to 22 % depending on their direction.

In general, the highest 3D formability was measured for the variants with perforations parallel to wood fibres, lower 3D formability for perforations perpendicular to fibres, and the lowest one for mixed arrangement.

Better 3D formability of the tested materials with perforations parallel to wood fibres can be explained by the fact that these perforations disturb the integrity of the fibres less (less amount of cross-cut fibres) and so weaken the veneer less. This was also confirmed by the measured forces required for 3D forming (Fig. 7). The lowest forces were measured for the variants where the perforations had cut the wood fibres (the arrangement perpendicular to the fibres and the mixed arrangement). The cross-cutting of wood fibres can be a reason why there was no significant difference in 3D formability in “S” perforations (depending on direction of the perforations).

Influence of spacing The 3D formability tended to increase with increasing spacing between perforations in cases of perforations along the fibers or the mixed arrangement with respect to wood fibres. The 3D formability was in the interval from 1% under load on the perforated surface (variant IM5) to 29.7 % under load on the solid surface (variant SM5).

An opposite tendency was recorded if direction of the perforations was perpendicular to wood fibres; the material with “I” perforations and the spacing of 5 mm (variant IK5) showed smaller deepening (3D formability) by 5.2 % under load on the solid surface than the variant IK2. Under load on the perforated surface, the deepening was lower by 21.1 %.

A possible cause for reduction in 3D formability can be in the perforations themselves as they affect the size of the cross-sectional area of the material and impair the integrity of the material.

Comparison with reference (non-perforated) material A comparison of deepening measured for perforated two-layer materials with the deepening measured for the reference (non-perforated) material is shown in Fig. 6. The reference specimens reached the deepening of 5.79 mm on average. The range of confidence intervals for the reference specimens is shown by the distance between a pair of pink dashed lines. As can be seen, the perforated test specimens of all the designed perforation variants showed a smaller deepening than the reference specimens. The deepening the most similar to the reference was measured for the specimens with “I” perforations parallel to wood fibres, under load on the perforated surface. The reduction in deepening was by 6.3 % on average (deepening of 5.43 mm) for spacing of 2 mm and by 8.1 % for 5 mm spacing (deepening of 5.32 mm). Under the load on the solid surface, the reduction in 3D formability for the same perforation variant was by 16.9 % for spacing of 2 mm (deepening of 4.81 mm) and by 10.8 % for 5 mm spacing (deepening of 5.17 mm). It can be stated that the expected increasing in 3D formability due to the perforations has not been confirmed.

Force per unit of deepening The additional indicator of 3D formability was the force per unit of deepening. When evaluating the impact of loading on the solid or the perforated surfaces, the analysis did not confirm significant differences in force per unit of deepening. Evaluation of the measured force per unit of deepening for the individual perforation variants is shown in Fig. 7.

In terms of force per unit of deepening required for 3D forming, the designed perforation variants were positive. When compared to the reference (solid, non-perforated) material, for all the perforated materials, a lower force per unit of deepening was required (ranging from 18.5 % for IP5 to 78.3 % for IK2). The confidence interval of the reference

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specimens is defined by the distance between the pink dashed lines (Fig. 7). The average force per unit of deepening for the reference specimens was 71.92 N·mm1.

If compared to the reference (non-perforated) material, the most significant decrease in force per unit of deepening was recorded at the perforated variant IK2. If the perforated surface was loaded, the force of 15.7 N·mm1 was needed; if the solid surface was loaded, the force was 15.6 N·mm1. On the other hand, the smallest decrease in the force per unit of deepening was recorded at the variants with “I” perforations parallel to wood fibres. If the perforated surface was loaded, the decrease was by 34 % (force 47.4 N·mm1) for 2 mm spacing and by 21.2 % (force 56.7 N·mm1) for 5 mm spacing. If the solid surface was loaded, the decrease was by 34.3 % (force 47.3 N·mm1) for 2 mm spacing and by 18.5 % (force 58.6 N·mm1) for 5 mm spacing. The significance of difference between directions of the perforations in the designed variants, what was confirmed by Duncan's test.

The significant difference in the force per unit of deepening was shown between the perforation shapes only if the perforations were parallel and perpendicular to wood fibres Fig. 7 shows that the force per unit of deepening required for 3D forming of perforation variants with spacing 2 mm was lower by 6.8 % to 34.4 % if compared to 5 mm spacing.

Fig. 7 Force per unit of deepening at loading on solid and perforated surfaces; note: I and S – materials with perforations in the shape of the letters “I“ or “S“; 2 and 5 – spacing between perforations (mm).

Lightening of material Another change in the material properties due to the perforation was lightening of the material (weight reduction). Depending on designed perforation variant, different percentage of lightening was achieved (Table 1). No significant weight loss was achieved. The greatest lightening was found for “I” perforations with 2 mm spacing (5.8 %). This was expected, since the mentioned variants have the greatest number of perforations per unit area. If the orientation was perpendicular to wood fibres (variant IK2), the cutting of summer (denser) wood was more frequent which also affected the weight reduction. The slightest lightening was recorded for the materials with “I” perforations with 5 mm spacing (lightening of 1.5 %).

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This was also expected, since there were the least number of perforations per unit area (least of all designed variants).

The best formable perforated material “IP2” reached the second largest lightening of 5.1 %, which makes the two-layer material with IP2 perforations very interesting.

Tab. 1 Percentage of lightening of the materials depending on perforation variant.

Lightening [%]

IP2 5.1

IK2 5.8

IM2 3.3

IP5 2.0

IK5 1.7

Perforation variant IM5 SP2 SK2 1.5 3.4 3.8

SM2 3.3

SP5 2.3

SK5 2.2

SM5 2.1

CONCLUSIONS The paper is focused on changes in 3D formability of two-layer cross-layered material made from veneers if one of the veneers was perforated. Lightening of the material was also interesting as a secondary benefit of the perforating. Based on the experimental results, the following conclusions can be stated: 1. The 3D formability of the materials with perforations in the shape of the letter “I” oriented in direction parallel to wood fibres (perforation variants IP2 and IP5), was the most similar to the material with no perforation.

2. The best 3D formability was recorded for the materials with perforations oriented parallel to wood fibres, followed by the materials with perforations perpendicular to fibres and then materials with mixed arrangements of the perforations.

3. Almost all the designed perforation variants showed better 3D formability at 5 mm spacing if compared to 2 mm spacing (by 1 % to 29.7%).

4. No effect of the shape of perforations on 3D formability was shown (excluding the perforations parallel to wood fibres.

5. If compared with the reference (solid, non-perforated) material, none of the designed perforated materials showed increased 3D formability.

6. The force recalculated per unit of deepening was lower for all the designed perforation variants (from 18.5 % to 78.3 %) if compared to the force per unit of deepening for the reference (solid non-perforated) material.

7. In terms of lightening of material, the perforation variant with “I” perforations perpendicular to wood fibres and spacing of 2 mm was the most significant; the material lightening was 5.8 %.

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BEKHTA, P., SEDLIAČIK, J. 2019. Enviromentally-friendly high-density polyethylene-bonded plywood panels. In Polymers, 2019, Vol. 11, No. 7, pp. 1166.

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ACKNOWLEDGEMENT This work was supported by Slovak Research and Development Agency under the contract No. APVV-18-0378, APVV-14-0506, and APVV-16-0177, and by the Scientific Grant Agency of the Ministry of Education SR Grant No. VEGA 1/0556/19. This publication was supported by the Operational Programme ‘Research and Innovation’, the project: LIGNOPRO -Progresívny výskum úžitkových vlastností materiálov a výrobkov na báze dreva (Progressive Research into Utility Properties of Materials and Products Based on Wood), ITMS project code: 313011T720, co-funded by the European Regional Development Fund (ERDF).

AUTHORS' ADDRESS Ing. Jozef Fekiač, PhD. doc. Ing. Jozef Gáborík, CSc. Ing. Mária Šmidriaková, PhD. Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Furniture and Wood Products T. G. Masaryka 24 960 01 Zvolen, Slovakia jozef.fekiac@tuzvo.sk gaborik@tuzvo.sk smidriakova@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 67−78, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.06

PHYSICO-MECHANICAL PROPERTIES OF THERMALLY MODIFIED BEECH WOOD AFFECTED BY ITS PRE-TREATMENT WITH POLYETHYLENE GLYCOL Miroslav Repák – Ladislav Reinprecht ABSTRACT The European beech (Fagus sylvatica L.) wood – natural, as well as pre-treated with 20 wt.% water solution of polyethylene glycol (PEG 6000) – was thermally modified at the temperatures of 170, 190 or 210 °C for 1, 2, 3 or 4 hours. The selected properties of thermally modified beech wood, i.e. soaking, volume swelling, impact bending strength and Brinell hardness changed more intensively at using more severe thermal regimes from 170 °C / 1 h to 210 °C / 4 h. The presence of polar, white-waxy consistency of polyethylene glycol macromolecules in the thermally modified beech wood specimens had different effects on their properties. PEG 6000 participated in supressing the volume swelling and soaking in water of the thermally modified beech wood specimens, and also inhibited a decrease of their impact bending strength. On the contrary, polar PEG 6000 macromolecules together with thermal loads participated in decreasing the Brinell hardness.

Key words: beech wood, polyethylene glycol, thermal modification, swelling, soaking impact bending strength, hardness.

INTRODUCTION Thermal modification of hardwoods and softwoods performed at high temperatures usually from 160 °C to 260 °C, is a physico-chemical process connected with changes in their structural characteristics and selected properties (REINPRECHT and VIDHOLDOVÁ 2011, SANDBERG et al. 2017, VIDHOLDOVÁ et al. 2019). Thermowood commercial products, such as ThermoWood, PlatoWood, RectifiedWood, OHT-Wood, and others, have a better dimensional stability and durability – mainly higher resistance against wood destroying fungi and insects (LUNGULEASA et al. 2018, TAŞDELEN et al. 2019). Modulus of elasticity in bending of thermally modified woods is obviously unchanged, however, their bending strength and other strength properties are decreased usually from 5% to 20% (KAČÍKOVÁ et al. 2013, ANDOR and LAGAŇA 2018, WANG et al. 2018).

Due to thermal modification processes firstly their molecular structure associated primarily with the hemicelluloses degradation, creation of hemicelluloses-lignin linkages, and extinction of some hydroxyl groups in various wood species is changed (TJEERDSMA et al. 1998, SRINIVAS and PANDEY 2012, CAI et al. 2018). A range of changes in the molecular structure and partly also in the anatomical structure (e.g. composition and thickness of the individual cell wall layers) and geometry structure (e.g. volume and cracks) of thermally modified woods, connected with following changes in their physical, mechanical and 67

biological properties, is related: (a) to the type of used heating medium, e.g. air, nitrogen, steam, plant oil, wax, or other; (b) to the thermal modification technology, e.g., regulation of the temperature, pressure, and time; (c) to the tree species used and its initial moisture content (TJEERDSMNA et al. 1998, HILL 2006, YILDIZ et al. 2006, ESTEVES and PEREIRA 2009, KOCAEFE et al. 2015, REINPRECHT 2016).

European beech (Fagus sylvatica L.) is one of the most used commercial hardwood species in Central Europe – as its logging is great and its workability and impregnability is well (KÚDELA and ČUNDERLÍK 2012). However, beech wood has also some disadvantages – as a great volume swelling or shrinkage (KLEMENT et al. 2019), and a lower resistance to fungi and insects (REINPRECHT 2016).

Polyethylene glycols are polar macromolecules. At a room temperature they are liquids (e.g., from PEG 300 to PEG 1000) or solid substances with a waxy consistency (e.g., from PEG 1500 to PEG 10000). In practice, various PEG types, containing smaller and also higher polyethylene glycol macromolecules, are successfully used for a dimensional stabilization of archaeological waterlogged wooden artefacts (HOCKER et al. 2012, MAJKA et al. 2018).

However, polar PEGs usually decrease strength properties of wood, because the wood treated with them has permanently swollen cell walls and the strengthening effect of hydrogen bonds inside the cell walls consisting from polysaccharide and lignin molecules is suppressed (ALMKVIST et al. 2016).

The aim of this work was based on an assumption, that combining a primary technology of wood pre-treatment with polar polyethylene glycol macromolecules with a followed technology of wood exposition to high temperatures could effectively improve selected properties of the thermally modified wood.

MATERIALS AND METHODS Wood High quality specimens of European beech (Fagus sylvatica L.) heart-wood, i.e. without rot, insect gallery, growth defects, tension wood and red-false wood, were prepared from the sawn timber naturally seasoned to a moisture content of 13.5% ± 2%. Three dimensional types of specimens (longitudinal × tangential × radial) were used in the experiment – type (a) 25 mm × 25 mm × 5 mm in testing the Brinell hardness; type (b) 5 mm × 50 mm × 25 mm in testing the soaking and swelling; and type (c) 120 mm × 10 mm × 10 mm in testing the impact bending strength. The top and bottom surfaces of specimens of the type (a) and type (b) were milled.

The beech wood specimens were dried at 103 ± 1 °C to the oven-dry state in the kiln Memmert UNB 100 (Memmert, Schwabach, Germany), subsequently cooled in desiccators to a temperature of 20 ± 2 °C and weighed with an accuracy of 0.001 g.

Pre-treatment of wood with polyethylene glycol Pre-treatment of the beech wood specimens was performed in stainless steel containers by immersion technology with 20 wt.% water solution of polyethylene glycol PEG 6000 (HiMedia, Ltd., Mumbia, India). Water solution of PEG 6000 had a dynamic viscosity of 21.8 × 103 Pa·s at 20 °C. Pre-treatment process at atmospheric pressure and at a temperature of 100 °C lasted 3 hours.

The weight percent gain (WPG) of PEG 6000 by beech wood specimens was determined after their 4-week conditioning in a moist-air ( = 95%; t =20 °C; xylene = antimold component) and following drying to the oven-dry state, using Eq. 1:

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𝑊𝑃𝐺𝑃𝐸𝐺 =

𝑚0−𝑃𝑟𝑒𝑡𝑟𝑒𝑎𝑡𝑒𝑑 − 𝑚0 × 100 [%] 𝑚0

(1)

where: m0-Pretreated – mass of the oven-dried specimen containing PEG 6000 [g], m0 – mass of the oven-dried original specimen [g].

Thermal modification of wood The natural as well as with PEG 6000 pre-treated beech wood specimens were thermally modified in the kiln Memmert UNB 100 (Memmert, Schwabach, Germany) at the temperatures of 170, 190 or 210 °C for 1, 2, 3 or 4 hours.

Thermally modified beech wood specimens were cooled in desiccators to a temperature of 20 ± 2 °C, then their weights with an accuracy of 0.001 g and dimensions with an accuracy of 0.01 mm were determined, and repeatedly were transferred to desiccators.

Specimens used for testing the impact bending strength and hardness were airconditioned for 14 days at the temperature of 20 ± 2 °C and a relative air humidity of 60 ± 5%. Oven-dry specimens with 0% moisture content were used in soaking and swelling tests.

Soaking and swelling The soaking test according to the Standard STN 49 0104 was performed, using Eq. 2, in order to determine the ability of the reference and modified beech wood specimens (5 mm × 50 mm × 25 mm) to absorb distilled water (Si),: 𝑚𝑖 − 𝑚0 (2) 𝑆𝑖 = × 100 [%] 𝑚0 where: mi – weight of the moist specimen at the defined time of soaking [g], m0 – weight of the oven-dried specimen [g].

At the same time, volume swelling of wood (Vi) was evaluated, using Eq. 3: 𝛽𝑉𝑖 =

𝑉𝑖 − 𝑉0 × 100 [%] 𝑉0

(3)

where: Vi – volume of the moist specimen at the defined time of soaking [mm3], V0 – volume of the oven-dried specimen [mm3].

Impact bending strength Impact bending strength of specimens in tangential direction (I) was determined according to the Standard ISO 3348, using Eq. 4: (4) 𝑊 −2 [𝐽. 𝑐𝑚 ] 𝐼= 𝑏 × 𝐻 where: W – work done for cutting the specimen [J], b, h – specimen cross section dimensions [cm].

Brinell hardness Brinell hardness of specimens in radial direction (HB) was evaluated according to the Standard EN 1534, using a steel ball with a diameter of 11.284 mm impressed into the wood surface with a force of 500 N. It was calculated by Eq. 5: (5) 𝐹 2×𝐹 [𝑀𝑃𝑎] 𝐻𝐵 = = 𝑆 𝜋 × 𝐷 × (𝐷 − √𝐷2 − 𝑑 2 ) where: F – force on the ball [N], D – ball diameter [mm], d – diameter of the impressed area [mm].

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RESULTS AND DISCUSSION Weight percent gain (WPG) The WPG values of polyethylene glycol PEG 6000 by beech wood specimens ranged from 5.49% to 8.26% depending on wood dimensions (Fig. 1).

Specimens having a relatively high portion of axial surfaces, i.e. of type (a) 25 mm × 25 mm × 5 mm (L × T × R) and type (b) 5 mm × 50 mm × 25 mm (L × T × R), had greater WPG values in comparison with specimens of type (c) 120 mm × 10 mm × 10 mm (L × T × R) having the smallest portion of axial surfaces.

Fig. 1 WPG of PEG 6000 by beech wood specimens of types (a), (b) and (c).

Swelling and soaking At the more severe thermal loads of beech wood specimens, i.e. from 170 °C / 1 h to 210 °C / 4 h, more apparent decrease in the kinetics of their volume swelling and soaking determined from 1 to 24 hours (Figs. 2 and 3), and also after 24 and 336 hours (Tabs. 1 and 2) occured.

For example, after modification of beech wood specimens at a temperature of 210 °C for 4 h, the volume swelling determined after 24 hours decreased approximately about 50% (absolute decrease from 20.95% to 10.39%) and the soaking approximately about 25% (absolute decrease from 60.97% to 45.73%).

As already mentioned, in wood heated at high temperatures the hemicelluloses are depolymerized preferentially and the number of hydroxyl groups responsible for wood hygroscopicity is prominently decreased. LUNGULEASA et al. (2018), for beech plywood thermally treated at 200 °C for 3 h determined a reduction in the water absorption and thickness swelling about 70% and 60%, respectively. REINPRECHT and REPÁK (2019) found that as a result of beech wood paraffin-thermal modifications performed at 190 or 210 °C for 1, 2, 3 or 4 hours, there markedly decreased its soaking in water (in all cases by more than 30%) and volume swelling (from 26.8% to 62.9%). Similarly, REINPRECHT and VIDHOLDOVÁ (2008) found out that the soaking and swelling of beech and spruce woods were reduced due to thermal modification processes performed by the oil heat treatment (OHT) technology with rapeseed oil – maximally reduced in the case of the highest temperature of 220 °C acting for the longest time of 6 h.

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25

Reference TM 170/1

20

TM 170/2 TM 170/3

TM 190/1 TM 190/2

βVi (%)

TM 170/4

15

10

TM 190/3 TM 190/4 5

TM 210/1 TM 210/2

0

TM 210/3

0

1

3

5

8

24

5

8

24

Time (h)

TM 210/4

a) 25

Reference PEG + TM 170/1 PEG + TM 170/2

20

PEG + TM 170/3 PEG + TM 170/4

PEG + TM 190/2

15

βVi (%)

PEG + TM 190/1

10

PEG + TM 190/3 PEG + TM 190/4 PEG + TM 210/1

5

PEG + TM 210/2 PEG + TM 210/3

0 0

1

3

PEG + TM 210/4

Time (h)

(b) Fig. 2 Volume swelling kinetics of the reference and thermally modified beech wood specimens – Not pre-treated = TM (a); Pre-treated with PEG 6000 = PEG + TM (b).

The polar polyethylene glycols (PEGs) are hydrophilic substances. PEG macromolecules can be in the pre-treated woods located not only in the cell lumens, but also in the cell walls depending: (a) on the dimensions of PEG macromolecules and also of new substances created from them at high temperatures; (b) on the additional conditionings of pre-treated woods in air-humid environment. Generally, PEGs with smaller macromolecules can better penetrate to the cell walls (REINPRECHT 1995). The polymerization degree and 3D-size of PEG macromolecules can be decreased for example due to their thermal oxidative degradations already at temperatures below 100 °C, e.g., in connection with a production of low molecular weight esters (HAN et al. 1996). With a pro-longed impregnation/pretreatment time and also with a pro-longed conditioning time, there better conditions for diffusion processes of PEG macromolecules (and also substances created from them at high temperatures) into micro-pores of the water-swollen cell walls of wood can occur. This fact shall apply for dimensional stabilization of archaeological wooden artefacts, when in the cell walls and cell lumens of wood are small vaporizable water molecules replaced with greater non-evaporable PEG macromolecules, and thus conserved wood is a permanently swollen.

HOCKER et al. (2012), MAJKA et al. (2018) and several other researchers studied moisture properties of wood containing polyethylene glycols in dependence on the different 71

polymerization degree of PEG macromolecules, and on their different concentration in water, ethanol or other organic solvent. BRODA (2018) treated archaeological waterlogged oak wood with PEGs 400 and 4000 water solutions. The results of liquid water absorbability experiment clearly showed approximately about 50% decreased uptake of water by specimens treated with PEGs in comparison with untreated ones.

Reference TM 170/1 TM 170/2 TM 170/3 TM 190/1 TM 190/2 TM 190/3 TM 190/4 TM 210/1 TM 210/2 TM 210/3

Si (%)

TM 170/4

65 60 55 50 45 40 35 30 25 20 15 10 5 0 0

TM 210/4

1

3

Time (h)

5

8

24

5

8

24

(a) PEG + TM 170/1 PEG + TM 170/2 PEG + TM 170/3 PEG + TM 170/4 PEG + TM 190/1 PEG + TM 190/2 PEG + TM 190/3 PEG + TM 190/4 PEG + TM 210/1 PEG + TM 210/2

Si (%)

Reference

65 60 55 50 45 40 35 30 25 20 15 10 5 0 0

1

3

Time (h)

(b) Fig. 3 Soaking kinetics of the reference and thermally modified beech wood specimens – Not pre-treated = TM (a); Pre-treated with PEG 6000 = PEG + TM (b).

The PEG 6000 (HO-/CH2-CH2-O/n-H) with molecular weight of 5000–7000 consists from relatively long linear macromolecules, which have polymerisation degree “n” from 130 to 140.

Transport of the polar PEG 6000 from the lumens into the cell walls of wood is limited by dimension of its macromolecules and by dimension of the micro-pores in water swollen cell walls. In this experiment, the transport of PEG 6000 into the cells walls of beech wood specimens was, by our opinion, influenced and limited: (a) during the pre-treatment/impregnation process performed at 100 °C, i.e., by a lower penetration possibility of greater PEG macromolecules and also from them at a temperature of 100 °C potentially created esters and other polar substances, in comparison to a good penetration possibility of small water molecules, so in a situation when the wood structure behaves like chromatography column (REINPRECHT and HUDEC 1995, REINPRECHT 1995); (b) during the air-humid conditioning of pre-treated wood specimens, i.e., the diffusion possibility of PEG macromolecules (and eventually also substances created from PEG 6000 at thermal loads) from the cell lumens into the cell walls of wood primary saturated and swollen with small water molecules.

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In accordance with the above mentioned facts, indeed the starting dimensions of the thermally modified beech wood specimens containing PEG 6000 were greater before the soaking and swelling tests comparing to the not pre-treated specimens. Subsequently, the volume swelling of the thermally modified beech wood should be and also was smaller for specimens pre-treated with PEG 6000 (Fig. 2, Tab. 1). Due to the presence of PEG 6000 in the thermally modified beech wood specimens, also more or less important decrease in their soaking in water occurred. This knowledge can be explained by a lower portion of empty pores and micro-pores in wood specimens containing PEG 6000 (Fig. 3, Tab. 2).

Tab. 1 Volume swelling of the reference and thermally modified beech wood specimens – Not pre-treated and Pre-treated with PEG 6000.

Volume Swelling - βVi (%) Thermal Modification

Not pre-treated

Reference 170 °C / 1 h 170 °C / 2 h 170 °C / 3 h 170 °C / 4 h 190 °C / 1 h 190 °C / 2 h 190 °C / 3 h 190 °C / 4 h 210 °C / 1 h 210 °C / 2 h 210 °C / 3 h 210 °C / 4 h

20.95 (1.04) 19.74 (0.56) c 18.91 (0.48) a 18.27 (0.20) a 17.36 (0.76) a 17.21 (0.21) a 16.32 (0.52) a 15.96 (0.38) a 14.87 (0.39) a 14.60 (0.64) a 12.74 (0.13) a 11.38 (0.56) a 10.39 (0.13) a

24 h Pre-treated with PEG 6000 10.27 (0.28) a 9.65 (0.37) a 9.13 (0.42) a 8.93 (0.32) a 9.64 (0.23) a 9.22 (0.44) a 8.96 (0.34) a 8.68 (0.35) a 9.06 (0.39) a 8.97 (0.16) a 8.43 (0.28) a 8.20 (0.80) a

Not pre-treated 21.25 (1.28) 19.99 (0.67) d 19.10 (0.48) b 18.88 (0.20) b 17.82 (1.00) a 17.35 (0.66) a 17.24 (0.81) a 16.68 (0.30) a 14.93 (0.54) a 14.65 (0.32) a 13.34 (0.37) a 12.11 (0.57) a 11.11 (0.65) a

336 h Pre-treated with PEG 6000 10.37 (0.83) a 9.74 (0.55) a 9.42 (0.56) a 9.40 (0.99) a 9.72 (0.89) a 9.52 (0.51) a 9.17 (0.43) a 8.75 (0.68) a 9.10 (0.27) a 9.01 (0.57) a 8.51 (0.54) a 8.32 (0.33) a

Note: Mean values are from four replicates. Standard deviations are in parantheses. The Duncan test, with significance levels a = 99.9%, b = 99%, c = 95% and d < 95%, was performed in relation to reference specimens.

Tab. 2 Soaking of the reference and thermally modified beech wood specimens – Not pre-treated and Pre-treated with PEG 6000.

Soaking - Si (%) Thermal Modification Reference 170 °C / 1 h 170 °C / 2 h 170 °C / 3 h 170 °C / 4 h 190 °C / 1 h 190 °C / 2 h 190 °C / 3 h 190 °C / 4 h 210 °C / 1 h 210 °C / 2 h 210 °C / 3 h 210 °C / 4 h

24 h Not pre-treated Pre-treated with PEG 6000 60.97 (0.49) 57.07 (2.18) c 46.84 (3.08) a 56.57 (1.28) c 45.17 (1.41) a 55.09 (1.07) b 43.45 (2.25) a 54.94 (1.80) a 42.15 (1.07) a 56.04 (1.04) b 47.31 (3.12) a 54.77 (2.84) a 46.02 (1.08) a 52.95 (1.53) a 43.79 (2.24) a 49.60 (1.69) a 40.42 (2.48) a 48.96 (1.27) a 45.25 (0.73) a 47.56 (1.63) a 42.21 (2.19) a 47.26 (2.50) a 41.25 (3.39) a 45.73 (1.26) a 39.54 (0.96) a

Not pre-treated 90.65 (4.09) 81.93 (4.10) b 81.71 (5.36) b 80.66 (2.37) b 78.45 (4.06) a 81.27 (1.80) b 81.15 (3.11) b 77.74 (5.25) a 67.27 (1.90) a 66.32 (1.54) a 65.93 (3.93) a 65.79 (1.16) a 60.33 (0.53) a

336 h Pre-treated with PEG 6000 64.52 (3.36) a 63.84 (2.16) a 61.84 (3.96) a 57.30 (0.95) a 62.09 (1.87) a 61.41 (4.06) a 60.92 (1.90) a 56.78 (3.10) a 61.80 (2.08) a 61.24 ( 2.84) a 59.78 (2.30) a 55.13 (2.53) a

Note: Mean values are from four replicates. Standard deviations are in parantheses. The Duncan test, with significance levels a = 99.9%, b = 99%, c = 95% and d < 95%, was performed in relation to reference specimens.

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LUO et al. (2012) found out that water absorption and thickness swelling of flour-wood polypropylene composites decreased with increased concentration of PEG 1000 in the wood substance. Treatment of wood with 30% water solution of polyethylene glycol reduced the 24 h water uptake and thickness swelling of this type of composite by 34.4% and 64.6 %, respectively. A following heat treatments of this type of composite with content of PEG provided an extra reduction in the thickness swelling (additional absolute decrease about 12.128.6%). Obtained results showed, similarly with our experiment, that the PEG pretreatment and the following thermal modification can better improve the dimensional stability of wood composites as the alone thermal modification.

Impact bending strength and Brinell hardness The impact bending strength of the thermally modified beech wood decreased in the range from 14.36% to 71.11% (absolute decrease from 5.92 J·cm2 to 5.07 J·cm2 until 1.71 J·cm2) – proportionally to an increase in the temperature and time of thermal modification (Tab. 3). It is especially caused by a significant thermal degradation of hemicelluloses situated between cellulose microfibrils in cell walls of wood and also due to organic acids resulting from the hemicellulose decomposition catalyzing the cleavage of lignin– polysaccharide matrix of wood (ZAMAN et al. 2000).

Similar results obtained REINPRECHT and REPÁK (2019) for beech wood thermally modified in a paraffin melt at 190 or 210 °C from 1 to 4 hours when its impact bending strength decreased proportionally to increase in the temperature and time in the range from 17.84% to 48.33%. By REINPRECHT (1992), the impact bending strength of poplar wood treated at a temperature of 210 °C for 3 h decreased as well as evidently about 61%. A strong decrease of the impact bending strength at several degradation processes, mainly of hardwoods containing 2–2.5 times more of pentosans in comparison to coniferous woods (FENGEL and WEGENER 2003), is owing to decomposition of pentosans and loss of their elastic-mechanical function in cell walls.

Tab. 3 Impact bending strength and Brinell hardness of the reference and thermally modified beech wood specimens – Not pre-treated and Pre-treated with PEG 6000.

Thermal Modification Reference 170 °C / 1 h 170 °C / 2 h 170 °C / 3 h 170 °C / 4 h 190 °C / 1 h 190 °C / 2 h 190 °C / 3 h 190 °C / 4 h 210 °C / 1 h 210 °C / 2 h 210 °C / 3 h 210 °C / 4 h

Impact bending strength I (J.cm-2) Pre-treated with Not pre-treated PEG 6000 5.92 (0.97) 5.07 (0.85) c 5.37 (0.58) d 4.73 (0.55) a 5.26 (0.21) c 4.63 (0.23) a 5.23 (0.48) c 4.55 (0.18) a 5.17 (0.32) c 4.05 (0.30) a 5.18 (0.32) c 3.76 (0.35) a 5.01 (0.42) c 3.51 (0.33) a 4.72 (0.56) a 3.38 (0.45) a 4.57 (0.26) a 2.83 (0.23) a 3.50 (0.22) a 2.33 (0.99) a 3.37 (0.17) a 2.12 (0.18) a 3.23 (0.30) a 1.71 (0.31) a 2.89 (0.49) a

Brinell hardness HB (MPa) Pre-treated with Not pre-treated PEG 6000 38.71 (3.40) 37.75 (3.86) d 28.47 (1.96) a 35.07 (2.68) d 26.22 (3.67) a 35.02 (4.24) d 25.96 (4.88) a 34.63 (3.51) d 24.85 (1.96) a 36.11 (4.97) d 25.39 (2.31) a 35.02 (4.29) d 22.60 (2.33) a 33.61 (4.87) d 23.16 (3.81) a 30.72 (3.99) b 22.06 (3.84) a 27.85 (4.33) a 22.81 (2.73) a 27.00 (1.97) a 22.14 (3.43) a 24.75 (2.43) a 21.25 (3.51) a 22.68 (2.08) a 20.03 (2.55) a

Note: Mean values are from six replicates. Standard deviations are in parantheses. The Duncan test, with significance levels a = 99.9%, b = 99%, c = 95% and d < 95%, was performed in relation to reference specimens.

On the contrary, the presence of PEG 6000 in the thermally modified beech wood partly slowed down decrease of the impact bending strength, when its decrease ranged only from 9.29% to 51.18% (absolute decrease from 5.92 J·cm2 to 5.37 J·cm2 until 2.89 J·cm2) 74

(Tab. 3). This finding can be explained by a protection effect of the polyethylene glycol macromolecules against the oxygen transport inside the cell walls of thermally exposed wood, in a connection with supressing the oxidation depolymerisation reactions in the ligninpolysaccharide matrix of wood (HILL 2006, REINPRECHT and VIDHOLDOVÁ 2011).

The Brinell hardness of the thermally modified beech wood decreased in the range from 2.48% to 41.41% (absolute decrease from 38.71 MPa to 37.75 MPa until 22.68 MPa), proportionally with the pro-longed time and increased temperature, especially at the highest temperature of 210 °C (Tab. 3). However, it should be stressed, that hardness of all thermally modified beech wood specimens containing PEG 6000 decreased more apparently, it means in presence of polar polyethylene glycol waxy-consistency macromolecules the hardness of thermally modified beech wood decreased in a greater margin from 26.45% to 48.26% (absolute decrease from 38.71 MPa to 28.47 MPa until 20.03 MPa).

Results of our experiment can be compared to results obtained by LUNGULEASA et al.

(2018) who searched reduced Brinell hardness of beech plywood thermally modified at the temperatures of 160, 180 and 200 °C for 1, 2 and 3 hours. The Brinell hardness decreased about 16% at 180 °C for 3 h, or maximally about 40.6% at 200 °C for 3 h. On the contrary, the Brinell hardness slightly increased for specimens thermally modified at 160 °C. REINPRECHT and VIDHOLDOVÁ (2008) also determined a decrease in the Brinell hardness of beech wood by 1.5% to 33% when this wood species was modified in rapeseed oil at the temperatures of 180 or 220 °C for 3 or 6 hours. BORŮVKA et al. (2018) determined different changes in the Brinell hardness of thermally modified beech and birch woods at a temperature of 210 °C for 3 hours – hardness for beech wood decreased by 37%, bur for birch wood it increased by 9%.

CONCLUSIONS 



Beech wood had changed selected physical and mechanical properties, i.e., soaking, volume swelling, impact bending strength and Brinell hardness due to thermal modifications performed at the temperatures of 170, 190 or 210 °C for 1, 2, 3 or 4 hours.

These properties changed more intensively using more severe thermal regimes from 170 °C / 1 h to 210 °C / 4 h.

The presence of polar and waxy-consistency polyethylene glycol macromolecules “PEG 6000” in beech wood before its thermal loads differently affected its final physical and mechanical properties: - PEG 6000 significantly reduced the volume swelling in water of the thermally modified beech wood specimens, and their soaking kinetics apparently decreased as well.

- PEG 6000 slowed down decrease in the impact bending strength of beech wood caused due to high temperatures at its thermal modifications.

- On the contrary, PEG 6000 participated in a more evident decrease in the Brinell hardness of the thermally modified beech wood.

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ISSN 0735-6161.

ACKNOWLEDGMENTS This work was supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic Grant No. VEGA 1/0729/18, and by the Slovak Research and Development Agency under the contracts No. APPV-17-0583 and APVV-16-0177.

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ADRESSES OF AUTHORS Ing. Miroslav Repák Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Wood Technology T. G. Masaryka 24 960 01 Zvolen Slovak Republic xrepak@tuzvo.sk Prof. Ing. Ladislav Reinprecht, CSc. Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Wood Technology T. G. Masaryka 24 960 01 Zvolen Slovak Republic reinprecht@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 79−88, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.07

THE EFFECT OF SELECTED FACTORS ON OUT-OF ROUNDNESS DURING THE GREEN WOOD DRILLING Mikuláš Siklienka - Andrej Jankech ABSTRACT The issue of the out-of roundness during the process of green wood drilling is presented in the paper. The focus is placed on analysing the moisture content, direction and position during drilling and revolutions in respect to selected commercial tree species. The aim of the research was to determine the correlation between the accuracy of roundness and the changes in the mentioned factors and parameters associated with drilling. The results can be used to optimise the process of the CNC drilling. The samples of the following tree species – European beech, English oak and Norway spruce were used to measure the roundness. The data were gathered when drilling the samples in longitudinal, tangential and radial directions at the revolutions of 710, 1000 and 1400 min1, feed rate of = 0.225 mm and the moisture content of 12 %, (in the case of European beech also 18%).

Key words: drilling, tree species, out-of roundness, drill, operational direction and position, revolution, moisture content.

INTRODUCTION Innovations and developments in machinery and technology in order to increase the productivity at work, to improve quality and at the same time, to reduce the production costs must be implemented in the manufacturing industry to stay competitive on the domestic and international markets (CHOUHAN et al. 2016, KALITA and NATH 2016, BAKO and BOŽEK 2016, PRADEEP KUMAR et al. 2017, KAMBLE et al. 2019). Therefore, CNC technology has been widely applied so far. Optimizing the machining operation of CNC machines requires the optimal settings for the cutting parameters of the processes under given conditions of machining. Wood is orthotropic and anisotropic material, i.e. cutting parameters changes depending upon the tree species, its physical and mechanical properties (KOLEDA et al. 2016, KÚDELA et al. 2018, SIKLIENKA et al. 2017). Quality indicators of machining such as accuracy and surface roughness, indicators relating to the forces and energy (cutting force and cutting performance) are considered the machining process parameters (KRILEK et al.

2014, KMINIAK et al. 2015, KORČOK et al. 2015 and KOLEDA et al. 2019, SZWAJKA and TRZEPIECIŃSKI 2017).

Drilling the commercial tree species typical in temperate zone is mentioned in scientific literature very rarely. In foreign literature, attention is paid to exploring tropical tree species, tree species of North America and Japan. Drilling refers to the process of creating round holes using a drill. During drilling, the cutting tool is fed in a direction parallel to its axis of rotation. The function of the face of cutting edge is the most important. Drill 79

construction depends on many factors, such as drilling conditions in regard to wood grain direction, drill diameter and geometry, drilling depth, hole positional accuracy, productivity at work. In the case of drilling wood across the grain, taper drill bit and centering drill bit can bend and the grains are cut in the direction perpendicular to their longitudinal axes. On the contrary, when drilling wood along the grain direction, grains are cut in the direction parallel to their longitudinal axes. Drilling parallel to the grains is defined by parallel axial movement with the grains. Cutting edges are inclined to drill axis, and because of the same conditions in any positions during one revolution, the taper drill bit is not necessary. The research of most authors are focused on observing the torque and axis power depending upon selected drilling parameters. MCMILLIN and WOODSON (1972) dealt with the quality of drilled holes in the Borealis pine wood using the twist drill with centering bit and taper drill bit and flat drill. They found out that the chip thickness and revolution did not affect the hole quality. Flat drill was excellent in the hole quality during drilling dry wood in longitudinal direction and the holes drilled in longitudinal direction in the wet wood were of better quality using the twist drill. When drilling in the direction across the grain, twist drill was excellent in drilling wet as well as dry wood. KOMATSU (1976) investigated the quality of holes drilled using the Japanese drill with taper drill bits and centering bit and the twist drill with flat bit at the revolutions of n= 250, 1000, 1500, 2500 and 5000 min1. When drilling using the Japanese drill, out-of roundness and roughness were almost constant at the revolutions ranging from 1500 to 5000 min1. Little out-of roundness and the roughness of drilled holes drilled using the twist drill with V-bit was in the case of the revolutions of 1500 or 2500 min1. KOMATSU (1978c) investigated drilled hole inclination angle ranging from 80o to 180o. In the case of a top angle of 100o, the value of out-of roundness of the drilled hole was the smallest. The hole size changes with variation in wood moisture content. An increase in the moisture content can result in an increase in the hole size. An increase across the grains is more marked than an increase parallel to the grains.

The aim of the paper is to analyse the effect of selected factors on the out-of roundness during drilling the selected commercial tree species. Timber is drilled in three basic anatomical directions – longitudinal, radial, tangential.

MATERIAL AND METHODS Description and preparation of samples Samples from tree species European beech (Fagus sylvatica L.), English oak (Quercus robur L.) and Norway spruce (Picea abies L.) were used to observe the phenomena generated during drilling. The samples with the dimensions of 50 × 50 × 50 mm were used in the experiment (Fig. 1). The dimensions of the samples were according to the requirements and construction of the test equipment. 81 samples were from beech wood and 54 samples were from oak and spruce wood respectively.

Fig. 1 Samples and their dimensions.

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All samples were air-conditioned to the moisture content of 12 ± 1%. The density of samples was as follows: qEB = 0.711 g.cm-3, qEO = 0.720 g.cm-3, qNS = 0.421 g.cm-3. Some of the samples from beech wood were subsequently air-conditioned to the moisture content of 18 ± 1%.

Description of drills and drill conditions Twist drills with taper bits and centering bits made of alloy steel were used for drilling the samples in the experiment. Drill used marked as Art. 517.200.31 were produced by the company Pesaro with diameter 20 mm.

Drill direction: longitudinal (L), radial (R), tangential (T) Revolution: n = 710, 1000, 1400 min-1 Feed rate: f = 0.225 mm Wood drilling Wood drilling was carried out using the vertical milling machine FA 4 AV in three basic directions (radial R, tangential T and longitudinal L) as it is illustrated in Fig. 1. Four holes with the depth of 2D were drilled into the samples with the dimensions of 50 × 50 × 50 mm.

The samples were placed in the milling machine (SIKLIENKA and ŠAJBANOVÁ 2002, 2003).

In the case of samples air-conditioned to the moisture content of 12 + 1 %, two samples of each tree species for one drill diameter, one value of revolution and one drill direction were used, thus 8 holes were drilled.

Measuring the out-of roundness of the drilled hole The out-of roundness of the drilled hole was observed on the holes drilled in the samples of all tree species using the drill with the diameter of 20 mm. The holes were drilled in longitudinal, radial and tangential directions. Selected holes were divided by 15°, i.e.12 diameters D1–1´ – D12–12´ were measured. These diameters were measured using the calliper. In the case of the holes drilled in the longitudinal direction, the out-of roundness of the drilled hole was measured in the transverse plane (Fig. 2). Fig. 2 shows that the diameter D1–1´ was measured in the radial direction and the diameter D7–7´ in the tangential direction.

Fig. 2 Measuring the out-of roundness of the drilled hole A in the longitudinal direction to the transverse plane.

The holes drilled in the tangential direction were drilled in the longitudinal-radial plane. The diameter D1–1´ was measured in the longitudinal-radial plane in the direction parallel to the grain direction and the diameter D7–7´ was measured in the longitudinalradial plane in the direction across the grain (Fig. 3).

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Fig. 3 Measuring the out-of roundness of the drilled hole A in the tangential direction to the longitudinalradial plane.

In the radial direction, the holes were drilled to the longitudinal-tangential plane. The diameter D1-1´ was measured in longitudinal-tangential plane in the direction parallel to the grains and the diameter D7–7´ was measured to the longitudinal-tangential plane in the direction across the grains (Fig. 4.)

Fig. 4 Measuring the out-of roundness of the drilled hole A in the radial direction to the longitudinaltangential plane.

Measured results were evaluated using the program STATISTICA. In the case of beech wood samples, the effect of the drill direction, revolution, operation location and moisture content on the out-of roundness of drilled hole was observed. In the case of other tree species, the effect of revolutions, operation direction and location was observed.

Moreover, the effect of tree species on the out-of roundness of the drilled hole was evaluated.

RESULTS AND DISCUSSION Evaluation of the out-of roundness of the drilled hole in the case of beech wood samples The effect of the moisture content, operation direction, revolutions and operation location on the out-of roundness of the drilled hole are evaluated in this part. Tab. 1 shows that the effect of the moisture content, opeartion direction, revolutions and operation location on the out-of roundness of the drilled hole was statistically significant. Fig. 5 illustrates the effect of the moisture content, operation direction, revolutions and operation location on the outof roundness of the drilled hole in all operation directions. In the case of the holes drilled in samples of other tree species, the fact that the effect of individual parameters associated with drilling on the out-of roundness of the drilled hole was almost the same can be stated.

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Tab. 1 Four factor analysis of variance for the out-of roundness of the drilled hole A. Factor 1 2 3 4 12 13 23 14 24 34 123 124 134 234 1234

SS 0,042 0,042 0,022 0,133 5,14E-05 0,001 6,08E-05 0,006 0,004 0,001 3,29E-04 0,001 4,74E-04 9,11E-04 6,05E-04

df

MS 1 2 2 11 2 2 4 11 22 22 4 22 22 44 44

0,042 0,021 0,011 0,012 2,57E-05 6,79E-04 1,52E-05 5,80E-04 1,62E-04 4,55E-05 8,23E-05 5,93E-05 2,15E-05 2,07E-05 1,37E-05

F 2494,56 1246,46 668,86 726,37 1,55 40,80 0,91 34,81 9,71 2,73 4,94 3,56 1,29 1,24 0,83

p - level 0,000 0,000 0,000 0,000 0,214 0,000 0,456 0,000 0,000 0,000 0,001 0,000 0,166 0,139 0,782

1 – the effect of the moisture content on the out-of roundness of drilled hole, 2 – the effect of the operation direction on the out-of roundness of drilled hole, 3 - the effect of the revolutions on the out-of roundness of drilled hole, 4 - the effect of the operation location on the out-of roundness of drilled hole, 12, 13, 23, 14, 24, 34, 123, 124, 134, 234, 1234 – mutual interaction of the factors.

The effect of the moisture content on the out-of roundness of the drilled hole: The fact that the moisture content affected the out-of roundness of the drilled hole in a significant way can be stated. Moreover, higher values of the out-of roundness of the drilled hole were measured in the case of higher moisture content. KOMATSU (1976) also mentions that there is a correlation between the moisture content and the hole size. Bigger increase was observed when drilling across the grains than along the grains.

The effect of the revolutions on the out-of roundness of the drilled hole: In our experiment, there was a decrease in the out-of roundness of the drilled hole when the revolutions increased. KOMATSU (1976) found out that when drilling using the twist drill with flat bit, the out-of roundness of the drilled hole were almost constant with the revolutions ranging from 1500 to 5000 min1.

The effect of the drill direction and location on the out of roundness of the drilled hole: The effect of both factors are evaluated together because they are interconnected. The fact that the highest values of the out-of roundness of the drilled hole were measured in the case of the holes drilled in the radial direction, lower values were measured in the tangential direction and the lowest in the longitudinal direction can be stated. In general, the results were gained in the case of all revolutions and moisture contents. In terms of quality, the size of the drilled hole should be accurate in a shape of cylinder. However, the accuracy of machining is affected by lots of parameters and factors. In our experiment, the fact that the holes in the point entering the wood were in a shape of ellipse was found out. This conclusion is confirmed by the experiment of KOMATSU (1978c, 1979b, d) who changed the top angle of the drill and the feed speed.

83

1,98

1,97

1,96

A [cm]

1,95

1,94

1,93

1,92

a

w=12 ± 1%

D11-11'

D9-9'

D10-10'

D7-7'

D8-8'

D5-5'

D6-6'

D3-3'

D4-4'

D1-1'

D2-2'

D12-12'

D10-10'

D11-11'

D8-8'

D9-9'

D6-6'

D7-7'

D4-4'

D5-5'

D1-1'

D2-2'

D3-3'

1,91

D12-12'

n=710 n=1000 n=1400

w=18 ± 1%

2,01 2,00 1,99

A [cm]

1,98 1,97 1,96 1,95 1,94 1,93

b

w=12 ± 1%

D12-12'

D9-9'

D10-10'

D8-8'

D7-7'

D6-6'

D5-5'

D4-4'

D3-3'

D2-2'

D1-1'

D12-12'

D11-11'

D10-10'

D9-9'

D8-8'

D7-7'

D6-6'

D5-5'

D4-4'

D3-3'

D2-2'

D1-1'

1,92

D11-11'

n=710 n=1000 n=1400

w=18 ± 1%

2,00 1,99 1,98

A [cm]

1,97 1,96 1,95 1,94 1,93 1,92

c

w=12 ± 1%

D12-12'

D10-10'

D9-9'

D8-8'

D7-7'

D6-6'

D5-5'

D4-4'

D3-3'

D2-2'

D1-1'

D12-12'

D11-11'

D10-10'

D9-9'

D8-8'

D7-7'

D6-6'

D5-5'

D4-4'

D3-3'

D2-2'

D1-1'

1,91

D11-11'

n=710 n=1000 n=1400

w=18 ± 1%

Fig. 5 The effect of the moisture content w, revolutions n and the location of measurement on the out-of roundness of drilled hole A in the case of beech wood samples (a – longitudinal direction, b – radial direction, c – tangential direction).

The smallest diameters in the holes drilled in the longitudinal direction to the transverse plane were measured in the tangential direction (the diameters D7–7´) and the largest diameters were measured in the radial direction (the diameters D1–1´).

In the case of the holes drilled in the tangential direction to the longitudinal-radial plane, the diameters D1–1´ were the largest ones and the diameters D7–7´ were the smallest ones. The diameters D1–1´ were measured in the longitudinal-radial plane in the direction parallel to 84

the grains and the diameters D7–7´ were measured in the longitudinal-radial plane in the direction across the grains.

The fact that in the case of the holes drilled in the radial direction to the longitudinalradial plane, the diameters D1–1´ were the largest ones and the diameters D7–7´ were the smallest ones can be mentioned. The diameters D1–1´ were measured in the longitudinaltangential plane in the direction parallel to the grain direction and the diameters D7–7´ were measured in the longitudinal-tangential plane in the direction across the grains. In the scientific literature, there are data associated with the dimensional accuracy of the hole measured using the beech wood samples. KOMATSU (1976) measured the largest diameters of the holes along the grains and the smallest ones across the grains.

Evaluation of the out-of roundness of the drilled hole in the case of other tree species Graphs illustrating 95 % confidence intervals for the out-of roundness of the drilled hole for the individual tree species are in Fig. 6. n=710

2,02

A [cm]

2,00

1,98

1,96

L R T

1,94

1,92 1

2

3

1

D1-1'

2

3

1

D2-2'

2

3

1

D3-3'

2

3

1

D4-4'

2

3

1

D5-5'

2

3

1

2

D6-6'

3

1

2

D7-7'

3

1

D8-8'

2

3

1

D9-9'

2

3

1

D10-10'

2

3

1

D11-11'

2

3

D12-12'

n=1000

2,00

A [cm]

1,98

1,96

1,94

L R T

1,92

1

2

3

1

D1-1'

2

3

1

D2-2'

2

3

1

D3-3'

2

3

1

D4-4'

2

3

D5-5'

1

2

3

D6-6'

1

2

3

D7-7'

1

2

3

D8-8'

1

2

3

1

D9-9'

2

3

1

D10-10'

2

3

1

D11-11'

2

3

D12-12'

2,00

n=1400

A [cm]

1,98

1,96

1,94

L R T

1,92

1 D1-1'

2

3

1

2 D2-2'

3

1

2 D3-3'

3

1

2 D4-4'

3

1

2 D5-5'

3

1

2

3

D6-6'

1

2 D7-7'

3

1

2 D8-8'

3

1

2 D9-9'

3

1

2

3

D10-10'

1

2

3

D11-11'

1

2

3

D12-12'

Fig. 6 The effect of the location of measurement, tree species, and revolutions on the out-of roundness of the drilled hole A in the case of the holes drilled in the L - longitudinal direction, R – radial direction, and T – tangential direction, 1-European beech, 2-English oak, 3-Norway spruce.

85

The quality of drilled holes is affected significantly by wood hardness and elasticity.

In our experiments, softwoods were drilled: Norway spruce, as well as hardwood: European beech and English oak. Fig.6 shows that the highest values of the out-of roundness of the drilled hole were measured in the case of the holes drilled in spruce wood with the density of 0.421 g·cm3. Thus the fact that when drilling softwood, the quality of the hole is lower comparing to the holes drilled in hardwood with higher density can be stated. The holes of the highest quality resulted from drilling beech wood samples. Fig.6 also shows that the largest diameters D1-1´ were observed in the case of all values of revolution in other tree species and the diameters D7–7´ were the smallest ones. The fact that in the case of all other tree species, there was a correlation between the revolutions and diameters can be seen.

When the revolutions increased the out-of roundness of the drilled hole decreased.

CONCLUSIONS Knowledge associated with observing the phenomena generated during drilling were presented in the paper. Especially the effect of technological factors on the out-of roundness of the drilled hole was analysed in the paper. In the case of the holes drilled in the beech wood samples, the effect of the moisture content, revolutions, operation direction and measurement location on the values of the out-of roundness of the drilled hole. In the case of the holes drilled in the samples of other tree species, the effect of revolutions, operation direction and measurement location were investigated. The effect of the tree species on the out-of roundness of the drilled hole was evaluated as well. Measured results can be summarised as follows: - When measuring the hole accuracy, the fact that the holes in the point drill entering the wood were in a shape of ellipse can be stated.

- When the evolutions increased, the values of the out-of roundness of the drilled hole decreased slightly.

- When the moisture content increased, the values of the out-of roundness of the drilled hole increased as well.

- The highest value of the out-of roundness of the drilled hole were measured in the case of the holes drilled in the radial direction, lower in the tangential direction and the lowest ones in the longitudinal direction.

- The smallest diameters of the holes were measured in the case of the holes drilled in the longitudinal direction to the transverse plane and the largest diameters were in the case of the radial direction.

- In the case of the holes drilled in the tangential direction to the longitudinal-radial plane, the largest diameters were measured in the longitudinal-radial plane in the direction parallel to the grains. On the contrary, the smallest diameters were measured in the longitudinal-radial plane across the grains.

- In the case of the holes drilled in the radial direction to the longitudinal-tangential plane, the largest diameters were measured in the longitudinal-tangential plane in the direction parallel to the grains. On the contrary, the smallest diameters were in the longitudinal-tangential plane across the grains.

- Higher values of the out-of roundness of the drilled hole were measured in the case of the holes drilled in samples generated from the softwoods, In the case of hardwood, the values of out-of roundness were lower. More accurate holes were drilled in the hardwood samples.

86

REFERENCES BAKO, B., BOŽEK, P. 2016. Trends in simulation and planning of manufacturing companies. In International Conference on Manufacturing Engineering and Materials, ICMEM 2016, Procedia Engineering, 6 June 2016 Vol. 149, p. 571575.

CHOUHAN, Y. S., SALODA, M. A., JINDAL, S., AGARWAL, C. 2016. Optimalization of drilling process parameters for thrust force. In International Journal of Fracture and Damage Mechanics 1, p. 17.

KALITA, B., NATH, T. 2016. An experimental investigation and optimization of cutting parameter in drilling AISI B1113 using M2 HSS drill bit. In International conference on explorations and innovation in engg. tech. (ICEIET) KAMBLE, N., JATTI, V. 2019. Optimalization of drilling process parameters during of drilling of AISI 317L stainless steel. In Engineering Research Express vol. 1, N. 2., p. 9. http://doi.org/10.1088/26348695/ab2dcl.

KMINIAK, R., GAŠPARÍK, M., KVIETKOVÁ, M. 2015. The Dependence of Surface Quality on Tool Wear of Circular Saw Beades during Transversal Sawing of Beech Wood. In BioResources 10(4). p. 71237135.

KOMATSU, M. 1976. Machine boring properties of wood. II. : The effects of boring conditions on the cutting forces and the accuracy of finishing. In Journal of the Japan Wood Research Society, vol.

22, 1976, no. 9, p. 491497.

KOMATSU, M. 1978. Machine boring properties of wood. V. : The effects of the point angle of twist drill on the boring forces. In Journal of the Japan Wood Research Society, vol. 24, 1978, no. 8, p.

526532.

KOMATSU, M. 1979 a. Machine boring properties of wood. VIII. : The effects of grain angle of wood on the boring properties of Japanese boring bit. In Journal of the Japan Wood Research Society, vol.

25, 1979, no. 2, p. 117124.

KOMATSU, M. 1979. b. Machine boring properties of wood. IX. : The effects of grain angle of wood on the cutting force of twist drill. In Journal of the Japan Wood Researcch Society, vol. 25, 1979, no.

9, p. 573581.

KOMATSU, M. 1979. c. Machine boring properties of wood. X. : The effects of grain angle of wood on the cutting accuracy of twist drill. In Journal of the Japan Woos Research Society, vol. 25, 1979, no. 9, p. 582587.

KOLEDA, P., BARCÍK, Š., SVOREŇ, J., NAŠČÁK, Ľ., DOBRÍK, A. 2019. Influence of Cutting Wedge Treatment on Cutting Power, Machined Surface Quality, and Cutting Edge Wear When Plane Milling Ofak Wood. In BioResources 14(4), p. 92719286.

KORČOK, M., KOLEDA, P., BARCÍK, Š., VANČO, M. 2018. Effects of technical and technological parameters on the surface quality when milling thermally modified European oak wood. In BioResources 13(4), p. 85698577.

KRILEK, J., KOVAČ, J., KUČERA, M. 2014. Wood crosscutting process analysis for circular saws. In BioResources 9(1). p.

14171429.

ISSN: 1930-2126. http://dx.doi.org/10.15376/ biores.9.1.14171429 KÚDELA, J., MRENICA, L., JAVOREK, Ľ. 2018. The influence of milling and sanding on wood surface morphology. In Acta Facultatis Xylologiae Zvolen, Vol. 60, no. 1, p. 7183, ISSN 1336-3824.

MC MILLIN, CH. W., WOODSON, G. E. 1972. Moisture content of southern pine as related to thrust, torque, and chip formation in boring. In Forest Product Journal, vol. 22, 1972, no. 11, p. 5559.

PRADEEP KUMAR, B., INDRA KIRAN, N. V. N., PHANI KUMAR, S. 2017. Effect of cutting parameters in drilling of EN8 (080M40) Carbon steel to obtain max. MRR and min. temp. By using RSM (under dry condition). In International Journal of Engineering and Management Research 7, p. 533539.

SIKLIENKA, M., ŠAJBANOVÁ, D. 2002. The influence of chosen factors on torque and thrust during boring of some kinds of woods. In Trieskové a beztrieskové obrábanie dreva ´02, 2002, Zvolen: TU Zvolen, 2002, s. 231–234. ISBN 80-228-1190-4.

SIKLIENKA, M., ŠAJBANOVÁ, D. 2003. The influence of chosen factors on torque and thrust during boring of pine wood. In Forest and wood processing technology and the enviroment, 2003, Brno: MZLU; Praha : Ministerstvo zemědelství ČR, 2003, s. 385392. ISBN 80-7157665-4.

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SIKLIENKA, M., KMINIAK, R., ŠUSTEK, J., JANKECH, A. 2017. Delenie a obrábanie dreva. Zvolen: TU vo Zvolene, 357 s., 2017. ISBN 978-80-228-2845-1.

SZWAJKA, K., TRZEPIECIŃSKI, T. 2017. An examination of the tool life and surface quality during drilling melamine faced chipboard. In Wood Research, 62(2), p. 307318. ŠAJBANOVA, D., SIKLIENKA, M., VACEK, V. 2002. Vplyv vybraných faktorov na osovú silu a krútiaci moment pri vŕtaní smrekového dreva. In Valivé ložiská a strojárska technológia 2002, Žilina: TU Žilina, 2002, s. 136–140. ISBN 80-7135-999-7.

ACKNOWLEDGEMENTS This article was created with the support of VEGA 1/0485/18 „Machining strategy for special cutting models of agglomerated materials in nesting machining on a CNC machining center“.

AUTHORS ADDRESS Prof. Ing. Mikuláš Siklienka, PhD. Technical University in Zvolen Faculty of Wood Science and Technology Department of Woodworking T. G. Masaryka 24 960 01 Zvolen Slovakia siklienka@tuzvo.sk RNDr. Andrej Jankech, PhD. Technical University in Zvolen Faculty of Wood Science and Technology Department of Mathematics and Descriptive Geometry T. G. Masaryka 24 960 01 Zvolen Slovakia jankech@tuzvo.sk

88

ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 89102, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.08

THE EFFECT OF LOG SORTING STRATEGY ON THE FORECASTED LUMBER VALUE AFTER SAWING PINE WOOD Piotr Taube – Kazimierz A. Orłowski – Daniel Chuchała – Jakub Sandak ABSTRACT The optimal transformation path for the resource is determined by the quality of a log combined with its dimension. The commercial value of derived products is also closely connected with the size and extent of containing wood deficiencies. The results of studies with three diverse strategies for log sorting are presented in the paper. Resource assessment by a worker without extensive experience in sorting logs, the certified grading expert, and the automatic in-line system including optical scanner with dedicated software are compared. It was shown that the lack of experience of the person performing the sorting operation results in reducing the potential economic profits of a sawmill. On the contrary, the overall efficiency of the log conversion process is considerably improved by the automated sorting systems with scanners. Early identification of logs optimal for specific lumber production is assured by reducing the human errors and subjective evaluation. Both, the yield of produced timber and profits of the sawmill are directly affected this way. It was demonstrated that the log sorting rate performed by the scanner is four times higher in comparison to grading by the certified expert, as well as three times higher compared to employee with no experience. The finding that the volume of high-quality lumber of elevated value is the lowest in the case the log is sorted by a human was proved.

Key words: pine, log sorting method, log quality, shape scanner, sawmill.

INTRODUCTION A rise in demand for sawn timber in Poland has been noticed recently as a boost use of construction timber and expansion in the furniture manufacturing. Unfortunately, an increase in demand is not combined with the sufficient increase in log supply. High quality timber is used not only for high-value furniture and single-family houses but also for multistory buildings, including both rafter framing (traditional and prefabricated) and crosslaminated timber structures (GOTYCH et al. 2009, KRZOSEK 2011). The shortage of supply and refined technical requirements for the construction material determine need for rationalization of the log use and consequent optimization of supply chains.

The quality sorting is relatively well developed routine for the output stream of the sawmill production, including grading diverse assortments of sawn timber. The visual assessment is still most widely implemented sorting method, especially in a small and medium size sawmills (WIERUSZEWSKI et al. 2019). However, fully automatic quality sorting systems become standard in high throughput mills due to limitations of the humanbased assessment (SANDAK 2009). The quality aspects evaluated in such automatic systems 89

include diverse material properties, such as dimensions, grain/fibre direction, mechanical properties, presence of wood defects, colour pattern density among the others. Specific technical solutions differs between scanner and include mechanical testing (KRZOSEK 2011), stress wave velocity/attenuation, as well as radiometric absorbance/reflectance /transmittance in different ranges of electromagnetic spectrum among the others (KROHN and PALM 1981, KOLB and GRUBER 1981, GÖRGÜN and DÜNDAR 2018, SANDAK and SANDAK 2017).

Even if sorting of timber is essential for assuring expected product quality and maximizing profits, the overall efficiency of the sawmill production can be substantially improved by appropriate sorting of logs and following optimization of the sawing pattern.

The basic approach for quantifying log quality is a manual method described in standards, such as PN-92/D-95017 or EN1927-2:2008. In this case, an expert person performs set of manual measurements of the log geometry and identify presence of selected wood deficiencies noticeable on the log surface. It is clear that the correctness and repeatability of the grading decision, as well as time necessary for scrutinizing assessment will depend on the level of the operator training and his overall experience. In any case, the full objectivity and repeatability may not be guaranteed. As a consequence, the costs associated to the quality sorting at the log yard are considered as high (HAN-SUP et al. 2011). Alternative solutions for more efficient sorting were proposed to measure simultaneously lot of logs arranged in stacks (GEJDOS et al. 2019, GUTZEIT and VASKAMP 2012). Free vibrations, stress wave velocity or acoustic analysis (TSEHAYE et al. 2000) in combination with machine vision (GEJDOS et al. 2019, GUTZEIT and VASKAMP 2012) were identified as most suitable.

The highest economic gain and optimal use of raw resources along the supply chain requires decision regarding the resource quality and usage suitability as early as possible.

For that reason, some prototype solutions for quality grading aligned with the tree felling by the processor directly at the forest stand were proposed (SANDAK et al. 2019). However, till now the automated log sorting methods implemented in the sawmill yard are considered as more practical solution. In majority of state-of-the-art systems dedicated scanners allow measurement of the log dimensions, shape variation and consequently to determine the quality quantifier. This kind of scanning is carried out during the separation of incoming logs (sorting according to dimensional/quality class) or before sawing operation to optimize the sawing pattern. Set of laser triangulation sensors scanning the log from different directions are usually used for raw data acquisition. A great advantage of this approach is minimal requirement regarding the yard area for storing logs as well as capacity for sorting high number of logs with feed speeds up to 200 m·min1 (SIEKIAŃSKI et al. 2019). The trend of further upgrade of the log scanners by integrating additional to triangulation measurements is noticeable these days. Machine vision systems working in visible or infrared ranges, light scatter detectors, ultrasonic or microwave scanning modules are combined allowing multi-sensor evaluation and consequently more effective detection of wood defects and/or quality sorting. The latest technological developments provide a possibility to visualize defects in the log interior (such as knots or cracks) that are not visible on the log surface. In that case affordable X-ray radiography module is integrated with industrial scanners (FREDRIKSSON et al. 2014). The number of modules may vary from one to a few, resulting in better representation of the raw resource. The X-ray scanning of logs may be extended to the fully functional Computed Tomography (CT) scanners that are capable to real-time measure, grade or optimize logs at full industrial speeds (RAIS et al.

2017). The combination of diverse sensing techniques leads to improvement of the produced timber quality. RIDOUTT et al. (1999) reported that proper sorting of logs have an impact on the strength of the lumber produced.

90

The goal of this study was to quantify an effect of the log sorting strategy on the estimated yields and economic gain for the lumber obtained after sawing process in a real case of the middle-sized sawmill. The critical comparison of three alternative sorting schemes was performed with a special emphasis on the variety of constrains common in the industrial environments.

MATERIALS AND METHODS Materials Scotch pine (Pinus sylvestris L.) logs harvested in the Błędno Forestry, Lubichowo District in the Pomeranian Region in Poland were used as experimental samples. The sample lot corresponded to a typical single transport delivered by logistic operator to the premises of the Sylva sawmill. The wood was ordered from the forest as a large-size logs with dimensions in accordance to PN-92/D-95017 standard, assuming log length Lw = 4 m and a minimum available diameter of the top end without bark dtop = 14 cm. The total number of logs in the delivery lot was Nt = 143, however, subset Na = 60 of logs was randomly selected for the sorting simulation. Each chosen log was marked with a unique number to assure proper tracking and further comparison of results (Figure 1). Microsoft Office Excel 2007 was used for randomized selection of logs. Table 1 presents a list of logs used for experimentation.

Fig. 1 Marked Scotch pine logs ready for sorting operation.

Tab. 1 Identifiers of randomly selected logs used for sorting simulation. log number (dtop - measured with an optical scanner) 1 3 13 14 16 17 19 20 (27) (26) (23) (26) (22) (25) (25) (33) 38 39 41 43 46 47 51 53 (29) (32) (23) (29) (29) (22) (26) (28) 69 72 77 78 79 81 84 85 (25) (22) (24) (23) (24) (23) (22) (23) 99 101 103 105 111 112 114 115 (23) (26) (22) (27) (31) (24) (24) (23)

21 (26) 54 (26) 87 (27) 117 (30)

22 (23) 55 (26) 89 (29) 119 (28)

25 (31) 59 (26) 90 (27) 124 (23)

28 (26) 63 (26) 92 (27) 135 (22)

29 (25) 64 (27) 93 (22) 137 (24)

30 (23) 65 (22) 95 (29) 139 (27)

35 (25) 68 (23) 98 (27) 140 (25)

Strategies for log sorting The lot of experimental logs was subjected to dimensional and qualitative assessment implementing four sorting strategies.

91

Forester in the forest The first quality assessment was performed by the certified forest worker who sorted logs to quality classes in accordance with the PN–92/D–95017 and PN–D–95000:2002 standards.

The assessment was performed before transporting logs to the sawmill and was a basis for the estimation of the market values of logs. The set of information recorded in the transport documentation included: wood species (Scotch pine), log length (large-size logs with Lw = 4 m) and minimum quality class of log within the lot (class C). The summary of such characteristics as extracted from the formal documentation is presented in Table 2.

Unfortunately, due to the lack of tracing procedure for identification of a single log, it was impossible to link this information with a specific logs. Consequently, the formal trade documentation had only limited value for the sawmill sorting operations as the whole procedure must be repeated on-site. In addition, a common practice of the sawmill is to perform debarking operation before sorting and storing logs. The result of debarking is a clean log that allows more precise dimension measurements and better sight to wood defects present on its surface.

Tab. 2 The set of information available to extract from the forest logistics documentation when lot of logs reaches sawmill destination.

Symbol of logs SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W SO - W Sum total

Quality class A A A B B B B B B B B B C C C C C C C C C C C

Length. Lw (m) 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

Diameter. dtop (cm) 31 32 34 23 24 25 26 27 28 29 31 33 20 21 22 23 24 25 26 27 28 29 31

Quantity (pieces) 2 1 1 8 5 7 5 6 3 1 1 1 2 9 21 24 12 10 6 6 2 3 1 141

Volume. Vwood (m3) 0.66 0.35 0.39 1.52 1.00 1.54 1.20 1.50 0.81 0.29 0.33 0.37 0.28 1.44 3.57 4.56 2.40 2.20 240 1.50 0.54 0.87 0.33 30.05

Unexperienced person The set of experimental logs after debarking process was spread out on the storage yard (Figure 2) to allow easy access to each log for taking required measurements. The following assessment was performed by the team of two sawmill workers who were not specifically 92

trained for the quality sorting of logs. Both operators were instructed to perform simple measurements of log dimensions by using standard tools (measuring tape and ruler), including small-end diameter of log dtop and its length Lw. The unexperienced employees did not take into account a taper of the logs. In addition, the bulk volume of each log Vwood was calculated following equation 1 adopted for determining the volume of a simple cylinder. 𝑉𝑤𝑜𝑜𝑑 =

𝜋∙𝑑𝑡𝑜𝑝 2 40000

∙ 𝐿𝑤

(1)

The unexperienced workers did not sort logs into specific quality classes and therefore the whole lot was assumed to be of same but unknown quality. As a rule, all these logs were destinated for the follow-up sawing operation as a diameter-based sorts where the optimal sawing pattern is defined to maximize the volume of generated products without wood quality consideration.

Fig. 2 Experimental logs ready for manual grading after debarking and spacing on the storage yard.

Certified grading expert The second mode of sorting was conducted by the certificated grading expert. This experienced employee measured and classified quality of selected logs following rules defined in PN-92/D-95017 and PN-D-95000:2002 standards. The volume of log was estimated with the use of formal tables included in GM-900-7/2013 (Ordinance of General Director of State Forests in Poland no 74) and calculated according to equation 2, considering the log as a bevelled cone with a taper z. 𝜋

𝑉𝑤𝑜𝑜𝑑 = 40000 ∙ (𝑑𝑡𝑜𝑝 + 𝑧 ∙ (2)

𝐿𝑤 2 ) 2

∙ 𝐿𝑤

In-line automatic scanning system The scanner JORO-3D-800 produced by Jörg Elektronik GmbH (Oberstaufen, Germany) was used for automatic sorting of experimental logs following the manual quality assessment procedures. It is equipped with three sets of triangulation sensors combined with video cameras. Those are arranged all around the measured log passing through the scanner on the conveyer. The system allows continuous and complete characterization of each log, providing reliable and repeatable sorting decisions derived on the basis of objectively measured log properties. The continuous and high resolution recording of the log surface features allows extraction of several relevant characteristics, such as: diameter, length, ovality, taper, curvature, as well as cross-section’s flatness of log ends (Figure 3).

The software expert system of the scanner determines the quality class on the basis of all available information collected from each log separately (Web Source 1). However, the grading decision derived by scanner is considered as only a suggestion that has to be

93

confirmed or adjusted by the scanner operator. He was a highly skilled worker supervising the whole sorting line. The operation is able to observe the log appearance before decision.

All the data collected from scanning experimental logs were recorded and stored on the computer hard disk for further analysis. a)

b)

Fig. 3 The in-line automatic scanning system for logs: laser line illuminating the log during scanning (a) and software for controlling, visualization and sorting (b) (Web Source 1).

Optimization of the log sawing pattern The optimal pattern for sawing logs is critical to assure the best value of produced assortments as well as the highest production volume. Optiscie 2.0 (Etablissements Mauchamp SAS, Quetigny, France) software package was used to determine best sawing strategy for each sorted log. Two product lines are in the portfolio of the sawmill, where timber elements of highest (A) quality are sawn to the thickness of 27 mm, while moderate quality (class B and C) to the thickness of 52 mm. The kerf width of circular saw St used in analysis was 4 mm. Various sawing scenarios were generated assuming dimensional class graduation of the small-end diameter of log dtop every 2 cm and the maximum dtop = 28 cm.

An example of two alternative sawing patterns for logs of diameter class 20.0–21.5 cm for different quality assortments are presented in Figure 4. The width of each board matched the allowed dimension matrices defined by production engineers of the Sylva sawmill. a)

b)

190 157 78

27

52

27

27

52

27

27

52

27

190 110

low log quality

waste

high log quality

Fig. 4 Examples of alternative sawing patterns used for processing logs of varying quality but corresponding top diameter (range from 20 to 21.5 cm).

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RESULTS AND DISCUSSION Comparison of the cumulative volume Vlogs of sorted pine logs estimated after implementing three diverse sorting scenarios is shown in Figure 5. It is evident that the volume of logs measured by the grading expert corresponds to that estimated by the optical scanner.

The result of the cumulative volume obtained from the measurements performed by an unexperienced employee was clearly differencing from other approaches. This is related to the taper of logs neglected when calculating volume (equation 1). The summary of all log characteristics as measured by both workers, not skilled and expert, is reported in Table 3.

The sorting time is an important issue increasing overall operational costs of the sawmill.

The result of this research indicates that it varied considerably in each studied scenario. The optical scanner realizes sorting process with conveyor feed speed of 100 m·min1. The unit sorting time tu for logs of length Lw = 4 m, and with a standard distance between logs on the conveyor 2 m, corresponds to tu = 3.6 seconds per single log. The sawmill worker’s time necessary for the loading of the sorting line with logs from the entrance pile as well as unloading of sorted logs from the collection boxes should be taken into consideration when estimate the total cost of the sorting operation. This is defined as preparatory and finishing time tpf that in the case of present study was estimated at 15 minutes (0.25 hour) for the lot of 60 logs (FELD 2003). The time tpf increases even more to 30 minutes (0.50 hour) for manual sorting operations due to the necessity of spreading logs over the wide area before sorting and following collection along with separations of graded logs. It was noticed that the average time for quality sorting of a single log by the grading expert varied between 30 to 60 seconds, depending on the extent of wood defects and overall log quality. Therefore, an average value (tu = 45.0 seconds) was adopted for the following calculations. The worker without experience required approximately 15.0 seconds for each log to properly measure its dimensions. It is possible to express the sorting efficiency as a log sorting rate (LSR) indicator, computed as a ratio of the sorted logs volume and cumulative time necessary for this operation (equation 3).

The resulting LRS for three tested sorting scenarios are shown in Figure 6. 𝐿𝑆𝑅 = 𝑡

𝑉𝑙𝑜𝑔𝑠

(3)

𝑝𝑓 +(𝑁𝑎 ∙𝑡𝑢 )

volume of sorted logs Vlogs, m3

13,5

13,4

13,4

grading expert

in-line scanner

13,0 12,7 12,5

12,0 unexperienced worker

Fig. 5 The estimated total volume of sorted pine logs assessed with three sorting scenarios.

Despite the fact that the certified grading expert measures logs dimensions and determines their volume with similar accuracy as the optical scanner, the automatic system realizes the sorting process four times faster.

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Tab. 3 Technical characteristics of pine logs assessed during sorting with three alternative scenarios.

Number of wood log

unexperienced worker Top Length Volume diameter of log of log of log (cm) (m3) (cm)

1 410 26 3 409 24 13 409 23.5 14 411 25 16 410 22.5 17 412 24 19 411 25 20 412 32 21 410 25 22 415 24 25 409 31 28 410 27.5 29 410 24.5 30 410 21 35 412 26 38 410 28 39 409 30 41 410 22 43 410 29 46 414 28.5 47 412 22 51 413 25.5 53 411 28 54 411 26.5 55 410 26 59 412 26 63 411 26.5 64 408 26 65 410 22.5 68 408 23 69 412 25 72 408 22 77 411 24.5 78 411 23 79 412 24 81 411 24 84 412 22 85 411 22 87 409 27 89 409 29 90 408 27 92 409 26 93 409 23 95 408 29 98 409 27 99 411 23.5 101 412 26 103 412 21.5 105 410 26 111 409 32 112 410 25 114 413 23 115 411 22 117 411 29 119 411 32 124 409 24 135 412 22 137 410 23 139 412 26 140 411 26 sum of wood logs volume

0.218 0.185 0.177 0.202 0.163 0.186 0.202 0.331 0.201 0.188 0.309 0.244 0.193 0.142 0.219 0.252 0.289 0.156 0.271 0.264 0.157 0.211 0.253 0.227 0.218 0.219 0.227 0.217 0.163 0.170 0.202 0.155 0.194 0.171 0.186 0.186 0.157 0.156 0.234 0.270 0.234 0.217 0.170 0.269 0.234 0.178 0.219 0.150 0.218 0.329 0.201 0.172 0.156 0.271 0.331 0.185 0.157 0.170 0.219 0.218 12.660

grading expert Top Length diameter of log of log (cm) (cm) 410 409 409 410 410 409 410 411 409 415 407 409 409 407 412 410 409 410 409 413 411 413 410 411 410 411 410 409 411 410 411 407 410 410 410 411 410 411 409 409 409 408 408 407 408 409 412 411 410 409 409 411 410 410 410 409 412 409 412 410

25 24 23 25 22 24 24 32 24 23 31 27 24 20 25 27 29 22 29 27 22 25 28 25 25 25 26 26 21 22 25 22 24 23 23 24 21 22 27 27 26 25 22 28 27 23 25 22 26 31 24 23 22 29 27 21 22 23 25 26

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Volume of log (m3) 0.225 0.208 0.192 0.225 0.177 0.208 0.208 0.361 0.208 0.195 0.336 0.260 0.208 0.147 0.226 0.260 0.297 0.177 0.297 0.262 0.177 0.227 0.279 0.226 0.225 0.226 0.242 0.242 0.163 0.177 0.226 0.175 0.208 0.192 0.192 0.209 0.162 0.177 0.260 0.260 0.242 0.224 0.176 0.277 0.259 0.192 0.226 0.177 0.242 0.338 0.208 0.193 0.177 0.298 0.260 0.162 0.178 0.192 0.226 0.242 13.410

in-line scanner Top Length diameter of log of log (cm) (cm) 411 408 408 409 409 412 409 412 409 413 407 409 409 406 411 410 408 409 409 413 411 414 410 410 411 411 407 407 409 410 411 405 410 410 409 410 410 410 408 409 409 409 408 407 407 410 411 411 410 408 409 411 409 410 410 408 411 408 409 412

27 26 23 26 22 25 25 33 26 23 31 26 25 23 25 29 32 23 29 29 22 26 28 26 26 26 26 27 22 23 25 22 24 23 24 23 22 23 27 29 27 27 22 29 27 23 26 22 27 31 24 24 23 30 28 23 22 24 27 25

Volume of log (m3) 0.212 0.229 0.181 0.229 0.166 0.212 0.212 0.342 0.212 0.196 0.322 0.229 0.196 0.181 0.246 0.264 0.342 0.166 0.302 0.283 0.166 0.235 0.264 0.229 0.229 0.229 0.229 0.264 0.166 0.181 0.229 0.181 0.212 0.181 0.181 0.196 0.166 0.181 0.264 0.283 0.264 0.246 0.196 0.264 0.246 0.166 0.229 0.181 0.229 0.322 0.212 0.181 0.181 0.283 0.246 0.166 0.166 0.196 0.229 0.212 13.403

50,0

log sorting rate LSR, m3/h

43,3 40,0

30,0

20,0

16,9 10,7

10,0

0,0 unexperienced worker

grading expert

in-line scanner

Fig. 6 Log sorting rate LSR of pine logs assessed with three sorting scenarios.

The optimal sawing patterns for each log quality-dimension combination as determined for logs with diameter dtop in the range from 20.0 to 29.5 cm are summarized in Table 4. The upper part of the table includes recommended boards distribution for the best quality logs (class A), while bottom part to moderate quality logs (class B and C). The list of cross sections for sawn boards corresponds to those in the Sylva sawmill portfolio frequently used in down-stream production or direct sells to clients.

Tab. 4 Optimized sawing patterns for pine logs in relation to the log diameter and its quality class. cross section of board (mm × mm)

number of boards at the log cross section quality class A

27 × 78 27 × 105 27 × 131 27 × 157 27 × 178 27 × 190 27 × 205 27 × 210 27 × 215 27 × 240 27 × 260

2 2 2 2 2 2 quality class B and C 2 2 1 1 -

52 × 103 52 × 110 52 × 140 52 × 178 52 × 190 52 × 215 52 × 235 52 × 260

20.0<dtop<21.5

22.0<dtop<23.5

24.0<dtop<25.5

26.0<dtop<27.5

28.0<dtop<29.5

2 2 3 -

2 2 3 -

2 2 2 3

2 2 -

2 2 -

2 2

Even if the volume of logs estimated by the grading expert and in-line scanner are similar, the differences due to assigned quality class are noticeable (Table 5). The expert identified only 2 logs as fulfilling requirements for assignment to the superior quality class A. Conversely, in-line scanner graded 51 logs as belonging to the quality class A, though none to class C. The discrepancy is related to different grading rules used for the sorting 97

decision. These codified by standards are more rigorous than the expert system rules implemented in the optical scanner. Another source of divergence is inability for detailed identification of knots by the scanner, combined with subjective evaluation of the scanner operator that have very limited time to rectify suggestion of the automatic grading system.

Presence and excessive size of knots was the most frequent criteria that forced the expert to downgrade the log quality class.

Tab. 5 Quality classes of graded logs and forecasted total volume of produced timber.

A B C

27 mm 52 mm Σ

number of assigned logs (pcs) unexperienced worker 0 0 60 total volume of timber with specified thickness (m3) unexperienced worker 0.000 8.572 8.572

grading expert 2 19 39

in-line scanner 51 9 0

grading expert 0.392 7.756 8.148

in-line scanner 7.855 1.177 9.032

Both, the number of logs as well as the total volume of timber forecasted to be produced from logs sorted according to three studied scenarios differed noticeably (Table 5). The lack of quality information assessed by the unexperienced worker results in downgrading of all logs to the quality class C. As a consequence, all these logs should be processed in the sawmill to thick (52 mm) boards considering only the small-end diameter of log dtop as a criteria for the selection of optimal sawing pattern. Even that, the total volume of the produced timber was higher than of elements sawed according to sorting decisions provided by the grading expert. It was an unexpected result, as according to the adopted sawmill procedures, both quality class B and C are considered as an equivalent resource and follow the same transformation path. Therefore, 58 (of 60) logs defined by the grading expert as not superior class A should be processed same way as logs assessed by unexperienced worker. The observed discrepancy in the total volume may be therefore caused by the measurement faults introduced by the unexperienced worker. The erroneous measurement of the log’s small diameter substantially affects the selection of the sawing pattern and consequently total volume of derived timber.

Quality sorting of studied logs by the in-line scanner resulted in a high number of logs considered as superior quality. As a consequence, these were processed as more valuable timber components of the smaller board thickness (27 mm). It resulted in the overall yield increase of ~0.9 m3, compare to yield obtained from logs sorted by the grading expert. This difference was caused by a higher recovery of timber products of smaller thickness. The waste area, corresponding to material losses, is considerably smaller in case of sawing 27 mm thick boards than that of 52mm, as can be noticed in Figure 4. Another advantage of thinner boards is a higher variety of allowed board widths accepted by the sawmill managers for sawing. It was doubled compare to boards of 52 mm thickness (Table 4). It permits even better optimization of the sawing pattern and further reduction of the wasted wood, despite greater losses on kerfs of saw blades.

The material yield recovered from logs after sawing process is an important quantifier of the production efficiency. However, it is the monetary value that determines economic sustainability of the sawmill.

98

Tab. 6 Quality class, expected timber volume and its value simulated for all experimental logs and three alternative sorting scenarios. numbe r of log

unexperienced worker expected quality lumber class of volume log (m3)

1 C 0.160 3 C 0.135 13 C 0.105 14 C 0.136 16 C 0.106 17 C 0.136 19 C 0.136 20 C 0.188 21 C 0.136 22 C 0.137 25 C 0.186 28 C 0.160 29 C 0.136 30 C 0.087 35 C 0.161 38 C 0.187 39 C 0.186 41 C 0.106 43 C 0.187 46 C 0.189 47 C 0.106 51 C 0.137 53 C 0.187 54 C 0.160 55 C 0.160 59 C 0.161 63 C 0.160 64 C 0.159 65 C 0.106 68 C 0.105 69 C 0.136 72 C 0.105 77 C 0.136 78 C 0.106 79 C 0.136 81 C 0.136 84 C 0.106 85 C 0.106 87 C 0.160 89 C 0.186 90 C 0.159 92 C 0.160 93 C 0.105 95 C 0.186 98 C 0.160 99 C 0.106 101 C 0.161 103 C 0.088 105 C 0.160 111 C 0.186 112 C 0.136 114 C 0.106 115 C 0.106 117 C 0.187 119 C 0.187 124 C 0.135 135 C 0.106 137 C 0.106 139 C 0.161 140 C 0.160 total value of timber (PLN)

expected lumber value (PLN) 85.71 72.50 56.43 72.86 56.57 73.03 72.86 100.59 72.68 73.57 99.86 85.71 72.68 46.85 86.12 100.11 99.86 56.57 100.11 101.08 56.84 73.21 100.35 85.92 85.71 86.12 85.92 85.29 56.57 56.29 73.03 56.29 72.86 56.70 73.03 72.86 56.84 56.70 85.50 99.86 85.29 85.50 56.43 99.62 85.50 56.70 86.12 47.08 85.71 99.86 72.68 56.98 56.70 100.35 100.35 72.50 56.84 56.57 86.12 85.92 4594.43

grading expert expected quality lumber class of volume log (m3) C C C C C C B A C B C C B C B B B C B B C B B B B C C C C C C C C C C C C C B B C C C C C B B C C A B B C C B C C C C C

0.136 0.135 0.105 0.136 0.106 0.135 0.136 0.197 0.135 0.107 0.185 0.160 0.135 0.087 0.136 0.160 0.186 0.106 0.186 0.161 0.106 0.137 0.187 0.136 0.136 0.136 0.160 0.160 0.088 0.106 0.136 0.105 0.136 0.106 0.106 0.136 0.087 0.106 0.160 0.160 0.160 0.135 0.105 0.185 0.159 0.105 0.136 0.106 0.160 0.196 0.135 0.106 0.106 0.187 0.160 0.087 0.106 0.105 0.136 0.160

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expected lumber value (PLN) 72.68 72.50 56.43 72.68 56.57 72.50 72.68 126.44 72.50 57.26 99.37 85.50 72.50 46.51 73.03 85.71 99.86 56.57 99.86 86.33 56.70 73.21 100.11 72.86 72.68 72.86 85.71 85.50 46.97 56.57 72.86 56.15 72.68 56.57 56.57 72.86 46.85 56.70 85.50 85.50 85.50 72.32 56.29 99.37 85.29 56.43 73.03 56.70 85.71 125.82 72.50 56.70 56.57 100.11 85.71 46.74 56.84 56.43 73.03 85.71 4409.65

in-line scanner expected quality lumber class of volume log (m3) A A A A A A A A A A B A A B A A A A A A A A A A A A A B A A A A A B A A A B A A B B B A A A A A A A A A A A A A A A B A

0.154 0.153 0.137 0.154 0.138 0.135 0.134 0.197 0.154 0.138 0.185 0.154 0.134 0.087 0.134 0.196 0.195 0.138 0.196 0.198 0.138 0.156 0.196 0.154 0.154 0.154 0.153 0.159 0.138 0.138 0.134 0.136 0.134 0.106 0.134 0.138 0.138 0.106 0.153 0.196 0.160 0.135 0.105 0.195 0.153 0.138 0.154 0.138 0.154 0.195 0.134 0.134 0.138 0.196 0.196 0.137 0.138 0.133 0.135 0.135

expected lumber value (PLN) 99.32 98.60 88.40 98.84 88.62 86.62 85.99 126.75 98.84 88.62 99.37 98.84 85.99 46.40 86.41 126.13 125.52 88.62 125.82 127.05 89.05 100.05 126.13 99.08 99.32 99.32 98.36 85.08 88.62 88.83 86.41 87.75 86.20 56.57 85.99 88.83 88.83 56.57 98.60 125.82 85.50 72.50 56.29 125.21 98.36 88.83 99.32 89.05 99.08 125.52 85.99 86.41 88.62 126.13 126.13 88.40 89.05 85.78 72.50 86.62 5681.44

A detailed simulation of the hypothetical market value of timber products obtained from logs sorted according to three investigated scenario was performed within framework of this research.

As mentioned before, thinner boards have higher commercial value that corresponded to 643 PLN/m3. Conversely, the value of thicker boards (52 mm) was 536 PLN/m3. It is expected therefore that higher price of products combined with the greater predicted volume of timber derived from logs sorted by in-line scanner results in higher value of the produced timber. It is confirmed in Figure 7, where the predicted economic gain is ~30% higher for logs sorted by the in-line scanner, compare to logs graded manually by workers. This is in agreement with other technical reports where implementation of triangulation scanners resulted in considerable increase of the sawmill sorting capacity and improvement in supply of logs with quality properly adjusted for production of floorboards (SIEKANSKI et al. 2019).

The commercial value of timber obtained after sawing logs sorted by the grading expert was slightly less than of unexperienced worker. It is a consequence of both, the adopted sawmill strategy to not differentiate quality classes B or C, and the overestimation of overall logs volume as predicted by the worker. It is important to emphasize that this economic simulation reflects only a hypothetical case representing particular perspective of the Sylva sawmill. The actual value of timber can be only determined after proper log sawing and following quality grading of boards resulted by that process. The summary of quality, expected timber volume and estimated value of derived products for all simulated logs is presented in Table 6. value of the produced timber, €

1319

1066

1024

1000

500

0 unexperienced worker

grading expert

in-line scanner

Fig. 7 The market value of assortments produced by the sawmill when implementing three diverse log sorting scenarios. The exchange rate of Euro (1 EUR = 4.30 PLN ) according to the National Bank of Poland as at 26th November 2019.

CONCLUSIONS The log sorting strategy has a tremendous impact on the sawmill efficiency. Three diverse approaches were investigated here to determine optimal solution for upgrading the current log sorting routines in the middle size sawmill in Poland. It was clearly demonstrated that in-line scanner equipped with triangulation sensors for the external log geometry assessment allows most accurate prediction of the log volumes as well as improves overall reliability of the quality grading. The automatic sorting system increased productivity by a factor of four. An integration of the scanner with the production process in Sylva sawmill was identified as the most profitable solution recommended for advancing current manufacturing process.

The quality sorting of logs by unexperienced worker is not an optimal solution as it results in biased volume estimation as well as low sorting rate.

100

A full advantage of the grading expert involvement in the routine sorting of logs was not properly explored due to the simulation constrains. There was not defined in the Sylva company any specific path to explore a moderate quality logs (such as class B). That class was a frequent grade assigned by the expert to investigated logs. The direct comparison with the other sorting strategies was very limited as a set of grading rules defined in standards is very conservative and strict. As a consequence the sorting results are hardly comparable between three sorting strategies investigated.

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WEB SOURCE 1. 2019. https://je-gmbh.de/en/products/joro-3d#nav-c46 Accessed October. 2019 WIERUSZEWSKI, M., MIKOŁAJCZAK, E., WANAT, L. 2019. Dilemmas of technological innovations on the example of selected products based on oak wood. In: Digitalisation and circular economy: forestry and forestry based industry implications. In 12th WoodEMA Annual International Scientific Conference on Digitalisation and Circular Economy: Forestry and Forestry Based Industry Implications. Varna, SEP 1113. pp. 3944.

ACKNOWLEDGEMENTS The authors are grateful for the support of the Ministry of Science and Higher Education of Poland under the Implementation Doctorate program (Agreement No 0059/DW/2018).

Furthermore, the authors gratefully acknowledge the European Commission for funding the InnoRenew CoE project (Grant Agreement #739574) under the Horizon2020 Widespread-Teaming program and the Republic of Slovenia (Investment funding of the Republic of Slovenia and the European Union of the European Regional Development Fund).

AUTHORS ADDRESSES Kazimierz Orlowski (ORCID id: 0000-0003-1998-521X) Daniel Chuchala (ORCID id: 0000-0001-6368-6810) Gdansk University of Technology Faculty of Mechanical Engineering Narutowicza 11/12.

80-233 Gdansk Poland Piotr Taube SYLVA Ltd.. Co..

2 Koscierska Street.

83-441 Wiele Poland Jakub Sandak (ORCID id: 0000-0001-9190-677X) InnoRenew CoE Livade 6.

6310 Izola Slovenia University of Primorska Andrej Marušič Institute Titov trg 4.

6000 Koper Slovenia 102

ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 103−111, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.09

GRANULOMETRIC COMPOSITION OF CHIPS AND DUST PRODUCED FROM THE PROCESS OF WORKING THERMALLY MODIFIED WOOD Alena Očkajová - Martin Kučerka - Richard Kminiak - Tomasz Rogoziński ABSTRACT The granulometric composition of chips and dust from the longitudinal milling and sanding of thermally modified oak and spruce wood over a set of modification temperatures: 160, 180, 200 and 220 °C is determined and differences between the factors affecting the granulometry during these two woodworking processes are defined in the paper. Sieve analysis yielded percentages on individual sieves. Residual curves were used to emphasize the difference in these technologies in the granulometric composition, where a shift to the left signals smaller (finer particles) or dust particles. While the sanding dust residue curves shift to the right due to the elevated temperature of the wood treatment, i.e. towards larger fine particles and medium coarse particles, the chip residue curves from the milling process shift to the left, with a higher share of medium coarse, fine and dust fractions due as the wood modification temperature increases.

Key words: woodworking, thermally modified wood species, granulometry, residual curves.

INTRODUCTION Thermally modified wood is a material that is coming more and more to the forefront with efforts to make use of its physical and mechanical properties in areas where it is suitable. The major advantage offered is the ability to use less valuable wood from a temperate zone, which is modified through the application of high temperatures to deliver new properties, many of which approach those of tropical wood species, which they could potentially replace. In order to make adequate use of thermally modified wood, it is necessary to know its response in all potential areas of application. Simply knowing how its properties change as a result of thermal modification or the actual technologies for processing and potential risks are insufficient on their own. For the individual technologies for working thermally modified wood, it is important to know how this wood will respond in terms of workability, emissions and the resulting surface quality, which have been the focus of numerous authors (BUDAKCI et al. 2013, KVIETKOVÁ et al. 2015, KAPLAN et al.

2018, SANDAK et al. 2017, KUBŠ et al. 2016, KOLEDA et al. 2018). A specific area within the individual methods of mechanical working of this wood is the actual chip forming process, or the granulometry of the chips or dust that form, which remains a little explored area in terms of the modification of wood by applying high temperatures and it is necessary to determine if the secondary material (chips and dust) may be used, what 103

percentage of fine fractions are created by these individual technologies, and if these increase the health and safety risks associated therewith, especially the formation of dust particles with dimensions of ≤ 0.100 mm, which are characterized as airborne, difficult to settle in space, and are a problem for the operating staff (BARCÍK and GAŠPARÍK 2014, DZURENDA et al. 2010, DZURENDA and ORLOWSKI 2011, IGAZ et al. 2019, KMINIAK and DZURENDA 2019, KUČERKA and OČKAJOVÁ 2018, MIKUŠOVÁ et al. 2019, ROGOZINSKI 2016, ROGOZINSKI et al. 2017). The occurrence of this dust is problematic in terms of health because they can be inhaled and settle on the skin, in mucous membranes, etc. A safety risk is posed by the fire hazard or explosion hazard they pose, and the smaller the particles generated by a woodworking process and the greater the quantity, the greater the probability of such risks.

Given thermally modified wood is characterised by low strength (both bending and tensile strength) and lower toughness (REINPRECHT and VIDHOLDOVÁ 2008, KAČÍKOVÁ and KAČÍK 2011, THERMOWOOD HANDBOOK 2003, ČABALOVÁ et al. 2016), a higher percentage of smaller particles is expected during woodworking (REINPRECHT and VIDHOLDOVÁ 2008, KRÁL and HRÁZSKÝ 2005), as the above-specified strengths are dominant in the chip forming process (SIKLIENKA et al. 2017).

The objective of this paper is to compare the influence of milling and sanding technologies of thermally modified oak and spruce wood (modified at temperatures of 160°C, 180°C, 200°C and 220°C) on the granulometric composition of the formed chips and wood sanding dust with confirmation or refutation of the influence of woodworking technology, wood species and modification temperature on increases in the share of fine (particle size ≤ 0.125 mm) and dust fractions (particle size ≤ 0.08 mm).

MATERIALS AND METHODS Sample preparation Sessile oak (Quercus petraea) and Norway spruce (Picea Abies) for experiment were prepared by OČKAJOVÁ et al. (2019). The samples were dried to a residual moisture content of 8 %.

Thermally modification of oak and spruce wood is exactly described by KUČERKA and OČKAJOVÁ (2018).

Milling and sanding machines Lower spindle milling machine ZDS-2 (Liptov machine shop, Slovakia), feeding device Frommia ZMD 252/137 (Machinenfabrik Ferdinand Fromm, Fellbach, Germany), milling head FH 45 Staton SZT (Turany, Slovakia) with diameter of 125 mm and thickness of 45 mm, rake angle γ = 25 °, cutting speed of 40 m·s1, feed rate of 15 m·min1, depth of cut of 1 mm.

Narrow belt sanding machine JET JSG-96 (JPW Tool AG, Fälladen, Switzerland), cutting speed of 10 m·s1, grit size 80 of sanding belt HIOLIT XO P80 (KWH Group Ltd., Vaasa, Finland), individual pressure of wood sample on sanding belt.

Granular analysis Granular analysis was made in accordance with the standards STN 9096 (83 4610), STN 153105/STN ISO 3310-1 and steps by OČKAJOVÁ et al. (2019). A standard kit of several sieves ordered vertically (2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.125 mm, 0.080 mm, 0.063 mm, 0.032 mm, and bottom of the machinery – dust particles passed through all of the mesh screens) were placed on the vibrating stand of the sieving machine (Retsch AS 200c;

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Retsch GmbH, Haan, Germany) with an adjustable sieving interruption frequency (20 s) and a sieve deflection amplitude (2 mm/g).

RESULTS AND DISCUSSION Residue curves were used to evaluate the measured data, as they give a clear idea of the granulometric composition of the chips and sanding dust depending on the modification temperature as well as the mechanical woodworking technology itself, Fig. 1, 2. While the residue curves for the chips from the milling process move to the left towards fine particles as the wood modification temperature rises for both oak and spruce, the residue curves for sanding dust move to the right towards particles with a lower share of dust fractions with increasing wood modification temperature.

Fig. 1 Residual curves of chips and sanding dust depending on the modification temperature and the mechanical woodworking technology for oak.

Fig. 2 Residual curves of chips and sanding dust depending on the modification temperature and the mechanical woodworking technology for spruce.

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The influence of modification temperature and wood species in milling process In the case of chips produced from the longitudinal milling of oak, an increase was primarily noted as modification temperature increased among medium chips (0.5 mm and 0.25 mm sieves) along with the share of fine chips and particles size ≤ 0.125 mm as well as dust, particles size ≤ 0.08 mm. The largest share of dust was recorded at a thermal modification temperature of 220 °C – 3.63%. In the case of natural oak, chips in the sieves were predominant (2 mm, 1 mm and 0.5 mm) and the share of fine fractions and dust is approximately 1% and therefore it can be said that the decrease in mechanical properties of the wood manifested at higher modification temperatures, as the wood itself is more fragile, which was reflected in the formation of the dust fraction (REINPRECHT and VIDHOLDOVÁ 2008, KRÁL and HRÁZSKÝ 2005). This change was immediate at a modification temperature of 160 °C. The chips formed from the process of milling spruce, a coniferous tree species, show a different granulometric composition as compared to oak. Similar values in terms of the percentage share of chips from the milling process were obtained on the 2 mm and 1 mm sieves (i.e. the coarse fraction) for natural spruce and at modification temperatures of 160 and 180 °C, where the share of the coarse fraction was approximately 86.02 ÷ 95.03%. A significant difference was noted at a modification temperature of 200 °C, where the coarse fraction share fell to approximately half compared to natural wood and to about a third at a temperature of 220 °C. At modification temperatures of 200 and 220 °C, the percentage of medium coarse fraction on the 0.5 mm and 0.250 mm sieves increased. At a temperature of 220 °C this share was up to 59.67%, compared to a maximum share of this fraction of 13.19% (at a modification temperature of 180 °C).

The percentage share of the fine fraction, particles size ≤ 0.125 mm, for natural wood and wood modified at temperatures of 160 and 180 °C fluctuated in a range of 0.53% ÷ 0.70%, which rose to 5.44% for a modification temperature of 200 °C and doubled again at a modification temperature of 220 °C. While no dust particles with size of ≤ 0.08 mm were encountered with natural spruce and at modification temperatures of 160 and 180 °C, their percentage share fluctuated from 1.36% to 4.64% at modification temperatures of 200 and 220 °C and the same conclusion may be made as in the case of chips from the oak milling process, whereby at higher modification temperatures, the share of medium chips, fine fractions and wood dust increase, but significant changes only occur at temperatures of 200 °C and above, which correlates to the authors’ assertions that changes occurring as a result of increasing temperature occur later in coniferous species as compared to deciduous species due to their higher lignin content (REINPRECHT and VIDHOLDOVÁ 2008, KAČÍKOVÁ and KAČÍK 2009, 2011, GEFFERT et al. 2019). The decrease in hemicellulose content as a result of increased temperature is different between maple and oak too (GEFFERTOVÁ et al. 2018). ORLOWSKI et al. (2019) note that the sawing process on a frame saw produced a different granulometry in sawdust from beech wood and pine at working temperatures of around 105 °C. The influence of modification temperature and wood species in sanding process In the case of oak sanding dust, similar shares of particles were obtained on the individual sieves, with a significant change only appearing at a modification temperature of 220 °C. In the case of sanding dust and based on previous research, it may be said that the share of dust fractions, i.e. particles size ≤ 0.08 mm when sanding various types of wood is very high, ~ 85 ÷ 95% (MARKOVÁ et al. 2016, OČKAJOVÁ et al. 2018), which corresponds to the results from research conducted on natural oak and modification temperatures of 160, 180 and 200 °C, where shares of particles size ≤ 0.08 mm range from 92.10% to 94.72%, with a lower share of only 73.68% at a modification temperature of 220 °C (OČKAJOVÁ et 106

al. 2019). At this temperature, the share of larger (finer – 0.125 mm sieve and medium coarse – 0.250 mm sieve) particles captured on these ones increased dramatically compared to natural wood, specifically to 18.28% compared to 4.48% and to 6.42% compared to 0.6% for natural wood.

In the case of sanding dust from the longitudinal sanding of spruce, it may be said that the percentage shares of dust fractions (particles size ≤ 0.08 mm) were very high (84.44 ÷ 92.63%) and the values were similar for natural spruce and modification temperatures of 160 and 180 °C. A significant decrease in the value of dust fractions occured at modification temperatures of 200 and 220 °C (76.09 and 61.68%). At a temperature of 220°C, the percentage share of larger particles on the 0.125 mm sieve once again increased to 34.30% compared to natural wood, where this share was only 13.25%, and also on the 0.250 mm sieve (medium coarse particles), where this percentage share of particles is 3.87%, compared to 0.35% for natural wood. Based on these results, it may be said that sanding thermally modified wood does not generate a higher percentage of dust fractions, which was confirmed in experiments conducted by HLÁSKOVÁ et al. (2018) on beech wood. The working of such modified wood does not represent an elevated health or safety risk in such establishments, which correlates to the results produced by MIKUŠOVÁ et al. (2019), where no significant differences by modification temperature were identified in inhalable and respirable fractions when sanding thermally modified meranti wood.

The influence of technologies Granulometric analysis from the woodworking processes (milling and sanding) involving thermally modified wood showed a different stratification of chips or sanding dust on the individual sieves. Given that the effects of the thermal modification of the wood did not manifest themselves in a uniform manner in terms of granulometry, we supposed that in addition to the decrease in mechanical properties and decrease in density, a specific role plays the actual chip forming process, which varies significantly in the case of these 2 technologies.

In the case of milling, the geometry of the cutting wedge is defined precisely, and the chips that are created should have the same size, which is given by the cutting tooth feed rate (fz) and the depth of removal (e), Fig. 3. However, we know that each cutting wedge imparts a velocity to the resulting chip during milling equal to the cutting speed and at the same time the separated chip is deformed in some way from the face of the cutting wedge and receives a certain amount of potential energy. At the moment the chip is separated, this potential energy is transformed to kinetic energy in the form of the movement of the created chip and is therefore higher than the actual cutting speed, which results in the violent collision of the chip against the wall of the exhaust equipment or with other chips and their resulting breakdown. As such, the created chips do not have the same size and in the case of thermally modified wood with degraded mechanical properties and increased wood fragility, this effect may be further enhanced, which results in a higher share of fine chips and dust.

Fig. 3 Chip forming process during milling.

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In the case of sanding there is the tool geometry not defined. Each grain, with its varied shape resulting from crushing, with mostly negative geometry, functions as a separate chisel that detaches wood particles, which then melt in front of it in the form of wood dust filling the space between the grains (LISIČAN 1996), Fig. 4.

Fig. 4 Chip forming process during sanding (JOBBÁGYOVÁ 2008).

The length of a created chip was theoretically determined by the path of each cutting wedge over the workpiece; however, the cutting wedge deflected and then breaks out of the substrate differently, meaning every chip was different. Cutting speeds were fours of times lower compared to milling and therefore the same may be said of the speed of the particles that were generated. While mechanical properties, including bending strength and shear strength, were involved in the chip forming process and which were degraded by the thermal modification of wood, we supposed that the significant reduction in the density of the wood (OČKAJOVÁ et al. 2019) was reflected in the granulometry from the sanding process, which was indicated by previous experiments using various types of wood, with their densities and given that the individual sanding grains more easily penetrate into the wood with lower density and are able to scrape off larger particles (HAMMILÄ and USENIUS 1999, OČKAJOVÁ and BANSKI 2009).

CONCLUSION Based on the above, the conclusion from the viewpoint of granular analysis is that coniferous and deciduous tree species react differently to temperature increases and there are also differences among deciduous species: - when milling oak, the change in the granulometric composition of chips was immediate at a modification temperature of 160 °C, in spruce, the modification temperature was 200 °C when this change occurred, - when sanding oak, the change in the granulometric composition of chips occurred at a modification temperature of 220 °C, in spruce, the modification temperature was 200 °C when this change occurred.

The influence of woodworking technologies manifested itself in the granulometric composition of chips and dust. While a higher share of medium coarse chips, fine chips (particle size ≤ 0.125 mm) and dust too (particle size ≤ 0.08 mm) was produced during milling at higher modification temperatures as a result of the degraded mechanical properties of the thermally modified wood a lower percentage of dust fractions (particle size ≤ 0.08 mm) was produced during sanding and the share of larger fine fractions on the 0.125 mm and 0.250 mm sieves (medium coarse fraction) increased as a result of the lower density of the thermally modified wood.

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STN ISO 9096 (83 4610): 2004. Ochrana ovzdušia. Stacionárne zdroje znečisťovania. Manuálne stanovenie hmotnostnej koncentrácie tuhých znečisťujúcich látok.

STN 153105/ STN ISO 3310-1: 2000. Súbor sít na laboratórne účely.

ThermoWood Handbook [online]. © 2003. [cit. 2010-04-10]. Dostupné z: https://asiakas. kotisivukone.com/files/en.thermowood.palvelee.fi/downloads/ThermoWood_Handbuch.pdf ACKNOWLEDGEMENT This work was supported by the grant agency KEGA under the project No. 009TUZ-4/2017.

This experimental research was prepared within the grant project: APVV-17-0456 "Termická modifikácia dreva sýtou vodnou parou za účelom cielenej a stabilnej zmeny farby drevnej hmoty" as the result of work of author and the considerable assistance of the APVV agency.

AUTHORS´ADDRESSSES Doc. Ing. Alena Očkajová, PhD. Matej Bel University in Banská Bystrica Faculty of Natural Sciences Department of Technology Tajovského 40 974 01 Banská Bystrica Slovakia alena.ockajova@umb.sk 110

Ing. Martin Kučerka, PhD. Matej Bel University in Banská Bystrica Faculty of Natural Sciences Department of Technology Tajovského 40 974 01 Banská Bystrica Slovakia martin.kucerka@umb.sk Doc. Ing. Richard Kminiak, PhD. Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Woodworking T. G. Masaryka 2117/24 Slovakia richard.kminiak@tuzvo.sk Dr. hab. Tomasz Rogoziński Poznań University of Life Sciences Faculty of Wood Technology Wojska Polskiego 38/42 60-627 Poznań Poland trogoz@up.poznan.pl

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 113−123, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.10

NON-DESTRUCTIVE PENETRATION METHOD FOR DETERMINING THE QUALITY OF STRUCTURAL SPRUCE WOOD (Picea Abies KARST. L.) IN SITU Alena Rohanová ABSTRACT Structural timber is specific, because its quality is set by visual or mechanical grading.

Visual grading method by PILODYN 6J device is examined in the paper. Experiments were carried out using 5 spruce boards (Picea abies Karst. L) from Slovakia. Dimensions of boards were 40 × 200 × 2500 mm. Density of wood was set by gravimetric method according to EN 408 (2013). Depth of penetrations was measured using the device PILODYN 6J (hp).

Wood structure was described by the rate of growth (RoG) according to DIN 1052 (2004).

Three most used visual grading strength classes C30, C24 and C18 according to EN 338 (2016) were specified. Dependencies between measured characteristics were expressed by multistage parallel scale model (penetration depth ~ number of growth rings ~ rate of growth ~ strength class and wood density). It is possible to predict visual strength class of board and indicative density of wood (EN 338) by the proposed model in situ. Methods of model are easy to use, reliable and economically undemanding.

Key words: Spruce structural timber, board, density of wood, depth of penetration, rate of growth, visual strength class.

INTRODUCTION The wood is one of the basic construction materials, widely used as structural or additional elements. Efficiency and reliability of timber constructions is conditional by compliance at all levels of regulations. Quality of the structural timber and its diagnostics has an important position in the whole process. Quality of the structural timber is set by followed parameters: the modulus of rupture (MOR), the modulus of elasticity (MOE) and the density of wood. They can be detected by destructive (EN 408) or non-destructive method based on various principles (KRZOSEK et al. 2015, FRIDRICH and DENZLER 2010, KRZOSEK and BACHER 2011). Widely used key parameter for wood characteristics is the wood density. Currently used non-destructive methods to estimate the wood density are expensive, unavailable and statistically less significant (BOBADILL et al. 2013). Currently developed semi-destructive methods to estimate the wood density and the strength damage the wood only partially without weakening the material. All of these methods are marked as semi-destruction in situ. They are primarily used when the visual assessment of timber in situ is limited. Methods of drilling resistance are widely used e.g. conventional drill (ACUNA et al. 2013, BOBADILL et al. 2013) or core drill (KASAL 2003), resistograph or dynamical pin shooting as Pilodyn 6J (TEDER et al. 2011, ROHANOVÁ 2008, ROHANOVÁ and BAJZA 2017).

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Assessment shall take into account the tree species, its structure (width of growth rings) timber health (affected wood) and environmental conditions (temperature, humidity etc).

They also use PILODYN 6J or incremental drill to identify affected wood elements. TEDER et al. (2011) assessment of the timber health by PILODYN 6J device showed good correlation between the depth of penetration and the wood density r2 = 0.49. Shooting of pin or drilling gives only relative information about the wood density. However, these indirect methods give a good estimate of properties over the entire length or depth of the element, which is especially valuable if there is no direct access to the wood elements. For the wood density prediction can by also use non-destructive method by resitograph drilling. The drilling resistance testing is described in RINN et al. (1996), ACUNA et al. (2011), RIGGIO et al. (2014). The drilling resistance can detect also scale and location of inner wood defects, cracks or wood degradation. Authors report only observed dependence between the wood density and the depth of penetration/drilling resistance. The interaction with other parameters as the wood structure or the quality was not assessed.

HANSEN (2000), MÄKIPÄÄ and LINKOSALO (2011) state the universal use of the PILODYN 6J. At first, more than 20 years ago the technical manual (Technical Note NO. 55 – July 2000 by Ch. P. Hansen) defined PILODYN 6J as a device designed for living trees or electric poles applications. GÖRLACHER (1987) was the pioneer in the non-destructive testing of timber. The depth of penetration and the wood density detection as non-destructive testing of structural timber is considered as a very promising method. HANSEN (2000) describes the interaction between the depth of penetration and the angle of shooting. He confirmed no significant influence. The depth of penetration depends on the wood structure (spring and summer wood), its quality (healthy, old, reaction or degraded wood) (REINPRECHT 2016). The depth of penetration is also affected by the moisture content of wood according to GÖRLACHER (1987), HANSEN (2000), DUBOVSKÝ and ROHANOVÁ (2007), ROHANOVÁ (2013).

Application of the depth of penetration on timber declares TRIOMATIC industrially used equipment in machine-controlled systems. Local wood density and the moisture content of wood are determined by shooting two pins into the wood as non-destructive method. The compression load is measured in order to evaluate the wood density. The measured results are taken into account in machine sorting methods (SANDOZ and BENOIT 2007, Triomatic CBS-CBT).

Concentric layers – growth rings are located on the cross cut surface of tree. They reflect the time of grow during vegetation season. The significance of growth ring of coniferous wood is considerably higher than deciduous wood. Multiplicity of growth rings and their width influence the physical and mechanical properties of wood (POŽGAJ et al.

1993). Their dominant importance is in detecting of the wood density. The width of growth rings is determined by cross-oriented line length and number of annual growth rings.

Methods of testing and their limit values are stated by DIN 1052:2004, ČSN 73 2824-1/Z1 and STN 49 1531. They are considered an indicative criterion for visual quality assessment of the timber.

Wood characteristics determined by visual methods are criteria for the wood quality in strength classes. They can identify wood defects (nots, slope of grain, pith), biological degradation (decay, insect) and defects of wood growth (warping). They must be identified during machining of the wood. For example, cracks in structural timber (within, outer) could be repaired by gluing, what increases the wood quality. Limit values for each strength class are set in national regulations. Although the visual grading is less accurate, it is still used in practice. The results of the spruce timber experiments are stated by VEGA et al. (2011), STAPEL and DENZLER (2011), ROHANOVÁ et al. (2010), FRIEDRICH and DENZLER (2010).

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KRZOSEK et al. (2008), HERMOSO et al. (2016) assessed visually the pine (Pinus sylvestris L.) timber. They set a criterion for reducing the wood quality as rejects, knots, slope of grain and twist. Authors did not found out the rate of growth (RoG) significantly influenced the wood density and its quality.

This paper was focused on testing the quality of spruce structural timber by nondestructive penetration method. The wood structure characteristics (the width and number of annual rings) were determined by a visual method. The aim of the study is to design a multistage parallel scale model. The model allows mapping the visual class of strength and orientation density of the wood through the measured characteristics in situ (EN 338 -  mean ).

Quality of structural timber The quality of the structural timber is represented by the elasticity and strength properties but also by the density of wood. Euro standard EN 338 specifies their characteristic its values applied in both, visual and machine grading methods. Selected characteristic values and strength classes according to EN 338 are listed in Table 1. The classification of visual classes and species of the wood are stated by EN 1912. Only 3 classes of the strength are used in practice (yellow highlighted classes). They are listed in national regulations.

Tab. 1 Characteristic values and strength classes – standards in the countries.

Strength classes – characteristic values

Standards properties

EN 338

fm, k

(MPa)

C 14

C 16

C 18

14

16 8000

18 9000

290

310

320

340

350

360

380

390

400

430

350

370

380

410

420

430

460

470

480

520

-

-

S7

-

S10

-

S13

-

-

-

-

S7

-

-

S7K

-

S10K

-

S13K

-

-

-

-

-

KG

-

KS

-

KW

-

-

-

-

SII

-

-

SI

-

S0

-

-

-

E m, 0,mean (MPa) 7000

k  mean

(kgm−3)

C 22

C 24

C 27

C 30

C 35

C 40

C 50

22 24 27 30 35 40 50 10000 11000 11000 12000 13000 14000 16000

−3

(kgm )

DIN 4074-1 (Germany) ČSN 73 2824-1/Z1 (Czech Republic) ÖN DIN 4074-1 + A1 (Austria) PN–D -94021 (Poland) STN 49 1531 (Slovakia)

S10

S13

fm,k – 5-percentile characteristic value of bending strength, Em,0,mean - mean characteristic value of modulus of elasticity in bending parallel to grain, ρk – 5-percentile characteristic value of density, ρmean - mean characteristic value of density

PILODYN 6J The device is used for indicative testing of the wood density. The starting point is the dependence between the depth of penetration and the wood density according to tree species and the moisture content of wood. Operation and possibility of using are advantages of the device in situ. Two types of PILODYN 6J are used, one for structural timber and PILODYN 6J Forest for measurements of living trees or electric poles.

The device follows principles of shooting steel pin into the wood by differentiated energy. Shooting of pin is perpendicular to annual growth rings Fig. 1, Fig. 2).

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Fig. 1 PILODYN 6J - impact device with the scale and pin, loading rod, protective cap.

Fig. 2 System of penetration of working pin into wood in PILODYN

Rate of growth – RoG For structural timber the width of growth rings is expressed by the rate of growth (RoG) according to EN 14081-1 concerning methodology of measurements. RoG limits for softwoods and temperate hardwoods are 15 mm, 10 mm, 8 mm, 6 mm, 4 mm and 3 mm.

Similarly, to strength classes, the RoG commonly uses three limits in practice, 6 mm, 4 mm, and less (DIN 4074-1, ČSN 73 2824-1/Z1 and STN 49 1531).

Characteristic values of average wood density and RoG for three chosen strength classes according to DIN 4074-1and equivalents according to EN 338 are specified in Table 2.

Tab. 2 Characteristics of wood density and rate of growth in strength classes.

EN 338 C30 C24 C18

Strength classes according rules - characteristics Strength classes Density of wood DIN 4074-1 mean (kg·m3) S13, S13K 460 S10, S10K 420 S7, S7K 380

Rate of growth RoG (mm) less 4 4-6 unlimited

MATERIALS AND METHODS Material of specimens in this research came from the central part of Slovakia (region Žarnovica, altitude 230 m a.s.l., soil – cambisol). The board was cut out of spruce wood (Picea abies, Karst. L.) by random selection. Dimensions of boards: 40 × 200 × 2500 mm - 5 pcs.

Boards were divided into 3 segments and 9 test specimens (Figure 3). MÄKIPÄÄ - LINKOSALO (2011) described a similar process for both, dry and wet wood. Test specimens were conditioned under standard conditions, at the temperature of 20 ± 2 ° C and the relative air humidity of 65 ± 5%, at equilibrium humidity of 12% (reference humidity).

Fig. 3 Scheme of dividing the board into segments (1/3) and specimens (1/9).

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Density of wood ρ12 was determined according to EN 408. Number of measurements per board n = 18. The distribution of the density of wood along the board length was monitored on each board.

Depth of penetration hp was measured by device PILODYN 6J (pin diameter  = 2.5 mm).

Number of measurements per board n = 36.

Number of annual growth rings (Rn) was measured on the abscissa of penetration depth (hp).

The rate of growth RoG was calculated from measured data (Figure 4).

Fig. 4 Measurement and calculation of RoG.

Model – interaction of characteristics Strength classes are expressed by class lines of C30 and C24. Class lines are set according to the formula (1), RoG = hp / Rn

(1)

Where hp is the depth of penetration in mm, Rn is the number of growth rings in pcs.

For example C30  RoG = 4 mm (e.g. 8/2, 12/3, 16/4), C24  RoG = 6 mm (e.g. 12/2, 18/3).

Procedure: PILODYN 6J measures the depth of penetration hp (mm). Rn is set on the abscissa of hp. Class line C30 or C24 is determined according to RoG value. Reliability of the model is verified trough the measured values ρ12 and by visual grading of the board into strength class (number of knots and RoG).

RESULTS AND DISCUSSION Results of experiments and basic statistical characteristics are summarized in Table 3 (average values 5 boards).

Tab. 3 The basic statistical characteristics of tested properties – wood density ρ12 (w = 12%), depth of penetration hp and rate of growth RoG (n - number of measurements, - mean value, xmax - maximum value, xmin – minimum value, Vx - coefficient of variation).

Parameters Density of wood ρ12 (kg·m3) Depth of penetration hp (mm) Rate of growth RoG (mm)

n 90 180 180

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Statistical characteristics x xmax xmin 392 15.6 5.5

438 23 8

341 7 2.5

Vx (%) 7 23 23

Selection of spruce boards was carried out randomly. Characteristics were analyzed separately for every single board (Table 4).

Tab. 4 Average values of characteristics for the board 1–5 (wood density ρ12, depth of penetration hp and rate of growth (RoG ) and visual strength class (EN 338, DIN 4074-1).

Density of wood Number board

1 2 3 4 5

Characteristics Depth of penetration + width of growth rings

Visual strength class

n

ρ12 (kg·m3)

n

hp (mm)

RoG (mm)

EN 338

DIN 4074 -1

18 18 18 18 18

363 369 400 400 426

36 36 36 36 36

18.6 18.3 12.9 14.4 14.1

6.2 6.1 5.9 5.5 3.7

C18 C18 C24 C24 C30

S7 S7 S 10 S 10 S 13

Figure 5 shows variability in the wood density in the board. No significant differences were found out between segments and test specimens (test ANOVA, p = 0,001). Location of segments and test specimens in the board do not affect measured values of the wood density.

Significant differences were determined in wood density values.

450 440 430

Density wood  12 (kg.m-3)

420 410 400 390 380 370

board 1 (363 kg.m-3)

360

board 2 (369 kg.m-3)

350

board 3 (400 kg.m-3)

340

board 4 (400 kg.m -3)

330 11

12

13

21

22

23

31

32

Part of board (segment 1/3, test specimen 11 - 33)

33

board 5 (426 kg.m -3)

Fig. 5 Variability of wood density in boards (segment 1/3, test specimen 11–33).

The dependence between the wood density and the depth of penetration (Figure 6) is only 12% (p = 0,001). If we compare our results with GÖRLACHER (1987) and HANSEN (2000), our results shows weaker dependence. The factors that could affect obtained dependence are for example various wood structures in the boards, not exact perpendicularity of shooting, location of the board in the prism and others. The same finding was also found between the wood density and RoG (Fig. 7).

118

460

Density of wood 12 (kg.m-3)

440

420

400

 12 = 429,23 - 2,668 * hp

380

r = - 0,34 360

340

320 6

8

10

12

14

16

18

20

22

24

Depth of penetration hp (mm)

Fig. 6 Relationship between density of wood (ρ12) and depth of penetration (hp).

460

Density of wood 12 (kg.m -3)

440

420

400

 12 = 422,35 - 6,039 * RoG 380

r = -0,34

360

340

320 2

3

4

5

6

7

8

9

10

11

Rate of growth RoG (mm)

Fig. 7 Relationship between density of wood (ρ12) and rate of growth (RoG).

Model and its application Interaction of model characteristics is shown in Figure 8.

- measured: depth of penetration (hp) ~ the number of growth rings - standard characteristics: rate of growth (RoG) ~ density of wood (ρ12) ~ strength class (C)

119

7 board 1 (363 kg.m -3) board 2 (369 kg.m -3) board 3 (400 kg.m -3) board 4 (400 kg.m -3) board 5 (426 kg.m -3)

6

Annual growth rings (pc)

5

oG (R -3 pth e *d m .

5 g 0,2 0k 46 0+ = 0 , 0 an 0=  me C3

=4

4

C24

3

00 = 0,

17 + 0,

pth

* de

 mean

G= ( Ro

mm

6m

)

m)

-3

.m 0 kg = 42

ed)

-3

2

C18

.m 0 kg - 38

t limi - un G (Ro

1

0 6

8

10

12

14

16

18

20

22

24

26

Depth of penetration hp (mm)

Fig. 8 Model interactions of characteristics: depth of penetration ~ number of growth rings ρ mean  rate of growth  strength class C30, C24 and C18 and wood density.

Description of results (Fig. 8):  The boards with low densities (board 1 and board 2 - red color) are added to zone of C18 eventually C24. The wood density variability along the board length means that a part of board has higher density than average C18 - 380 kg·m3. It is assumed that these values are ranked in the higher class (C24).  The boards with the middle density (board 3 and board 4 - blue color) have balanced the wood density along the length of board and are located around the line zone of C24.  The boards with the high density (board 5–green colour). Although there is a slight fluctuation in the density of the wood along the length of board, values are in the zone around the line C30 and above.

The boards were visually graded according to defects of the wood (knots, cracks, warping).

A match between the proposed model and the visual grading was confirmed. The results confirmed that it is not sufficient to evaluate only the ρ – hp dependence for the determination of the wood density. The dependence between the depth of penetration hp and the wood density was not confirmed. By expending research with parameters of the number and the width of growth rings, the level of reliability of methods in the model increases e.g. the same number of growth rings may vary, so the RoG value is different (even the strength class).

Model can predict the quality of structural timber based on the interaction of measured and modeled parameters in situ (visually, optically).

In practice, it is not important to define the wood density precisely, but to determine the strength class related to other characteristics (strength, flexibility) as accurately as possible.

The device MTG Timber Grader, for example, determines the wood density by the weighting method on the whole board as well as timber defects. Measuring is carried out using 1–2 boards of the timber stack, measured data are than representative for all boards in the stack.

120

Subsequently, the dynamic modulus of elasticity is determined by the vibration method and assigned to “C” strength class. E.g. the industrial machine Triomatic applies extra measurement module (two pins screwed). The compression load is measured in order to evaluate the wood’s density (SANDOZ and BENOIT, 2007). The wood density is a determining parameter of grading timber into strength classes. They are taken into account during design of timber elements according to EUROCODE 5.

CONCLUSIONS 1. The wood density along the board length does not have any significant differences (ANOVA, p = 0.001). Different densities of the wood were measured between single boards.

2. Weak linear correlation was found between the depth of penetration and the density of wood.

3. Calculation method of the width of growth rings according to the depth of penetration and number of growth rings is reliable and fast.

4. Linear correlations between the wood density and the width of growth rings were confirmed with all boards.

5. The linear dependence between the wood density ρ12 and the strength class “C” (EN 338) was confirmed experimentally.

6. Proposed multi-stage parallel scale model (penetration depth ~ number of annual rings ~ rate of growth ~strength class and wood density) can assign to the board the visual class of strength and the orientation density of the wood (EN 338) in situ. The wood structure characteristics can be detected both visually and optically.

7. The reliability of the model was verified by the measured densities of wood ρ12 along the board and the visual grading over knots, cracks, warping and a match was confirmed.

8. Methods for determination of parameters in the model are easy, reliable and economically undemanding.

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DUBOVSKÝ, J., ROHANOVÁ, A. 2007. Static and Dynamic Hardness of Chosen Wood Species. In WOODWORKING Technique, 2nd International Scientific Conference. Zalesina, Croatia, Faculty of Forestry, Zagreb, Croatia, 27 32, ISBN 953-6307-94-4.

FRIEDRICH, G., DENZLER, J. K. 2010. Comparison of Slovakian spruce from different regions. In Holz Forschung, Austria, Vienna, Timber Construction and Materials. Project GRADEWOOD, 2010, 6 p.

GÖRLACHER, R. 1987. Non-Destructive Testing of Wood: an in – situ Method for Determination of Density. In Holz as Roh- und Werkstoff. Vol. 45, 273–278.

HANSEN, CH. P. 2000. Application of the Pilodyn in Forest Tree Improvement. Replaces Technical Note No. 2. Forest Seed Centre, Humlebaek, Denmark.

HERMOSO, A., MATEO, R., ÍÑIGUEZ - GONZÁLEZ, G., MONTÓN, J., ARRIAGA, F. 2016. Visual Grading and Structural Properties Assessment of Large Cross-Section Pinus radiata D. Don Timber.

In BioResources 11(2), 53125321. DOI: 10.15376/biores.11.2. 5312-5321.

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KASAL, B. 2003. Semi-destructive Method for in-situ Evaluation of Compressive Strength of Wood Structural Members. In Forest Products Journal Vol. 53, No. 11/12 2003. 56–58.

KRZOSEK, S., GRZESKIEWICZ, M., BACHER, M. 2008. Mechanical properties of Polish-grow Pinus silvestris L. Structural sawn timber end user´s needs for wood material and products, In COST E53 Conference proceedings, 2008, TU Delft, The Netherlands, 2008, 253–260, ISBN/EAN: 978-905638-202-5.

KRZOSEK, S., MAŃKOWSKI, P. 2015. Machinelle Festigkeitssortierung erstmals in polnischem Sägewerk. In Annals of Warsaw University of Life Sciences – SGGW. Forestry and Wood Technology No 89, 2015: Ann. WULS-SGGW, For and Wood Technol. 89, 2015. p. 8388. ISSN 1898-5912.

KRZOSEK, S., BACHER, M. 2011. Aktueller Stand der machinellen Festigkeitssortierung von Schnittholz in Polen und in Europa. Annals of Warsaw University of Life Sciences – SGGW. In Forestry and Wood Technology No 74, 2011: Ann. WULS-SGGW, For and Wood Technol. 74, 2011. p. 254259. ISSN 1898- 5912.

MÄKIPÄÄ, R., LINKOSALO, T. 2011. A Non-Destructive Field Method for Measuring Wood Density of Decaying Logs. In Silva Fennica 45(5) Research notes. ISSN 0037-5330.

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Zvolen: Technical University in Zvolen, 2016. 69 s. ISBN 978-80-228-2921-2.

RIGGIO, M., ANTHONY, RW, AUGELLI, F., KASAL, B., LECHNER, T., MÜLLER, W., TANNERT, T. 2014. In situ assessment of structural timber using non-destructive techniques. In Materialy and struktury 47 (5), 749766. DOI: 10.1617 / s11527-013-0093-6.

RINN, F., SCHWEINGRUBER, FH, SCHÄR, E. 1996. Resistograph and X-ray density charts of wood comparative evaluation of drill resistance profiles and X-ray density charts of different wood species.

In Holzforschung 50 (4), 303311. DOI: 10.1515 / hfsg.1996.50.4.303.

ROHANOVÁ, A. 2008. Interaction of Density and Depth of the Pin Penetration into Spruce Wood Using the Pilodyn 6J Apparatus. In Interaction of Wood with Various Forms of Energy. Zvolen: Technical University in Zvolen 2008, ISBN 978-80-228-1927-5, 179-183.

ROHANOVÁ, A., LAGAŇA, R., DUBOVSKÝ, J. 2010. Grading characteristics of structural Slovak spruce timber determined by ultrasonic and bending methods. In The future of quality control for wood & wood products: the final conference of COST Action E 53: 4-7th May 2010, Edinburgh, UK. - Edinburgh: Edinburgh Napier University, 2010. 9 p.

ROHANOVÁ, A. 2013. Predikcia parametrov kvality smrekového konštrukčného dreva. Zvolen: Technical University in Zvolen, 2013. 79 pp. ISBN: 978-80-228-2631-0.

ROHANOVÁ, A., BAJZA, O. 2017. Semi-destruction Method Pilodyn 6J for Measuring Wood Density of Spruce Wood. In Acta Facultatis Technicae Zvolen. AFT XXII/1/2017. ISSN 1336-4472, 917.

SANDOZ, J., L. BENOIT, Y. 2007. Timber Grading Machine using Multivariate Parameters based on Ultrasonic and Density Measurement. In COST E 53 Conference - Quality Control for Wood and Wood Products. 15th – 17th October 2007, Warsaw, Poland.

STAPEL, P., DENZLER, J. K. 2011. Influence of the origin on specific properties of European spruce and pine. http://www.cte.napier.ac.uk/e53/47. In The future of quality control for wood & wood products: the final conference of COST Action E 53: 4-7th May 2010, Edinburgh, UK. - Edinburgh: Edinburgh Napier University, 2010.

TEDER, M., PILT, K., MILJAN, M., LAINURM, M., KRUUDA, R. 2011. Overview of some Nondestructive Methods for in situ Assessment of Structural Timber. In 3rd International Conference CIVIL ENGINEERING`11 Proceedings II MATERIALS AND STRUCTURES. VEGA, A., GUAITA, M., DIESTE, A., MAJADA, J., FERNÁNDEZ, I., BAÑO, V. 2011. Evaluation of the influence of visual parameters on wave transmission velocity in sawn chestnut timber. In 17th International nondestructive testing and evaluation of wood symposium: proceedings. Sopron: 2011, 311317. ISBN 978-963-9883-82-6.

EN 338: 2016, Structural timber. Strength classes. European Committee for Standardization (CEN), Brussels, Belgium.

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EN 408: 2013, Timber structures. Structural timber and glued laminated timber. Determination of some physical and mechanical properties. European Committee for Standardization (CEN), Brussels, Belgium.

EN 1912/AC: 2013, Structural timber. Strength classes. Assignment of visual grades and species.

European Committee for Standardization (CEN), Brussels, Belgium.

EN 14081-1: 2016, Timber structures – Strength graded structural timber with rectangular cross section. Part 1: General requirements. European Committee for Standardization (CEN), Brussels, Belgium.

Eurocode 5: 2011, Design of timber structures. Part 1-1: General. Common rules and rules for buildings. European Committee for Standardization (CEN), Brussels, Belgium. ČSN 73 2824 - 1: 2015, Strength grading of wood – Part1: Coniferous sawn timber. Czech Standards Institute, Praha, Czech Republic.

DIN 4074, Teil 1: 2003, Sortierung von Holz nach der Tragfähigkeit, Teil 1: Nadelschnittholz.

German Committee for Standardization, Berlin. ÖN DIN 4074-1 + A1: 2012, Sortierung von Holz nach der Tragfähigkeit, Teil 1: Nadelschnittholz.

PN–D- 94021: 2013, Tarcica iglasta konstrukcyjna sortowana metodami wytrzymałościowymi.

Polish Committee for Standardization, Warsaw, Poland.

STN 49 1531: 2001, Structural timber. Part 1: Visual strength grading. Slovak Institute of Technical Standardization Bratislava, Slovakia.

ACKNOWLEDGMENTS This work was supported by the Slovak Research and Development Agency under the contracts No. APVV 17-0206 and APVV -17-0583.

ADDRESSES OF AUTHORS doc. Ing. Alena Rohanová, PhD. Technical University in Zvolen T. G. Masaryka 24 960 01 Zvolen Slovakia rohanova@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 125−132, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.11

DEFORMATION COMPARISON OF UPHOLSTERED FURNITURE FRAMES WITH SIDE PLATES FROM PB, OSB AND PLY BY FEM Nelly Staneva - Yancho Genchev - Desislava Hristodorova ABSTRACT Deformation comparison by the method of finite elements (FEM) of one-seat upholstered furniture frames with rails of pine solid wood (Pinus Sylvestris L.) and side plates of different furniture boards (PB, OSB and PLY) was carried out. Three-dimensional (3D) geometric model of the upholstered furniture frame was created and linear static analyses with CAE system were carried out simulating light-service loading of the frames with different materials of the side plates. The orthotropic properties of the used materials were taken into account in the static analyses. FEAs were performed with regard to experimentally established coefficients of rotational stiffness of used corner joints with staples and PVA’c glue in the frames-case butt joints and end to face butt joints. The distribution of linear displacements and nodal rotations in 3D discrete models of upholstered furniture frames were analyzed. The comparative analysis determines side plates from PLY as the most suitable furniture boards for upholstered frames with side plates concerning their deformation behavior and side plates from PB (16 mm) as the most unfavorable. Results will serve for optimization of design of upholstered furniture frames with staple joints and different materials of side plates.

Key words: PB, OSB, PLY, staple joints, upholstered furniture frame, deformation, FEM.

INTRODUCTION The deformation behaviour of the upholstered furniture frames depends mainly on the physical and mechanical properties of the materials used in their construction. In the past years the wood boards as PB, OSB and PLY are widely used in the furniture practice. KASAL (2006) has recommended this especially in the frames of upholstered furniture.

Limited number of publications on studies of upholstered furniture frames made from PB, OSB or PLY with staple joints are available in the literature: SMARDZEWSKI (2001) has found the optimal solution for a single-seat arm-chair made of wood and chipboard with staples joints; Further, SMARDZEWSKI and PREKRAT (2009) have carried out experimental and numerical studies of two-person sofa frame with plates from PB and beam elements from pine and beech wood taking account of orthotropic properties of the materials.

Proposing new dimensions of the frame elements the authors have not established significant change of the construction rigidity; Three-seat sofa frame made entirely of OSB has been investigated by WANG (2007) using SAP 2000. She has modeled 3 different models by beam finite elements with two types of connections (rigid and semi-rigid) and two types of connectors (screws and metal plates; staples and metal plates). Wang has established the 125

most appropriate configuration of the sofa frame from OSB under light, medium and heavy loads and has established that the type of connectors does not change the displacements remarkably; ERDIL et al. (2008) have investigated 3-seat upholstered furniture frames made of OSB, yellow birch dowels and PVA, and also Douglas-fir and sweet gum plywood using the simplified methods of structural analysis. They have concluded that these materials may be used in upholstered furniture frames to meet specific design loads.

Preliminary investigations of the deformation behavior of upholstered frames with rails from pine solid wood and side plates of PB, OSB and plywood at light-service load have been carried out from STANEVA et al. (2018a, b, c) and GENCHEV et al. (2018) using FEM. Comparing the deformation behavior of upholstered frames with these materials STANEVA et al. (2018d) have established that in the field of assembling of the rear rail of the seat and in the base of the side plates the resultant linear displacement is greatest for the side plates form PB, 35% smaller for side plates from OSB and 62% smaller for PW in the field of the rear rail of the seat and 36% smaller for side plates from OSB and 59% smaller for side plates from PW in the base of the side plates. Next, STANEVA et al. (2019) have performed deformation study by FEM of upholstered frame with side plates from OSB (different producer) with elastic properties other than the previous ones. This necessitated a new comparative analysis of the deformation behavior of upholstered frames with side plates from PB, OSB and plywood.

The goal of this study was to compare the deformation characteristics of one-seat frames of upholstered furniture with side plates of PB, OSB and PLY and staple joints under light-service loading by the method of finite elements (FEM) using CAD/CAE.

MATERIAL AND METHODS Three-dimensional (3D) model of one-seat upholstered furniture frame with length 600 mm, width 725 mm and height 650 mm was created (Fig.1). The rails 25×50 mm are from pine solid wood (Pinus Sylvestris L.) and side plates from PB, OSB and PLY. Two 3D discrete models of the side frame with plate finite elements were created - without (model A) and with strengthening details under the rails of the seat (model B) – Fig.1. The generated Midplane mesh has 5130 plate finite elements and 33616 DOF's for model A and 5230 plate finite elements and 34096 DOF's for model B. A linear static analysis of each 3D discrete model (A and B) of the upholstered furniture frame with PB, OSB and PLY side plates was carried out with CAE system Autodesk Simulation Mechanical® by FEM. Orthotropic material type was introduced in the program: For rails and strengthening details: Scots pine (Pinus sylvestris L.) with measured density 431 kg/m³ (BDS EN 323:2001) and elastic characteristics: Ez = EL = 9000·106 N/m2, Ex=ET=593·106 N/m2, GLT =554·106 N/m2, LR=0.03, LT=0.027, TL=0.41, RL=0.49.

For side plates: Particleboards (PB) with thickness 16 mm and characteristics: measured density 678 kg/m³ (BDS EN 323:2001); Ex =E=2700·106 N/m2 and Ey = E//=1600·106 N/m2; Poisson ratios xy = 0.30 and yx = 0.18 according BODIG et al. (1982).

Oriented strandboard (OSB), type OSB2 (BDS EN 13986:2004) with thickness 15 mm and characteristics: measured density 596 kg/m³ (BDS EN 323:2001); Eх=E//= 3500·106N/m2; Eу=E=1400·106 N/m2; xy=0.30 according to THOMAS (2003) and yx= 0.24, calculated according to BODIG et al. (1982): xy yx =E , (1) E x

y

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Fig. 1. Discrete models A and B of upholstered frame.

Plywood (PLY) boards from birch (Betula) with thickness 15 mm, 11 layers (BDS EN 14279:2004) and characteristics: measured density 629 kg/m³ (BDS EN 323:2001); Ex=E//=7224·106 N/m2; Ey=E=5709·106 N/m2; xy=0.30 according to BODIG et al.(1982) and yx=0.237, calculated from equation 1.

Boundary conditions were set: bottom front rail – no translation on у direction and bottom rear rail no translation on х-, у- and z direction – Fig.1.

Semi-rigid connections between rails and side plates of the frame were simulated: narrow zones were modeled with established via tests by FEM lower modules of elasticity of the used materials in the place of joints according to the methodology of МARINOVA (1996); The experimentally established and calculated by HRISTODOROVA (2019) coefficients of rotational stiffness of the corner joints with two staples and PVA’c glue, loading under compression, were introduced in the nodes of the respective corner joints case butt joints (for pine-PB с = 1017 N·m/rad; for pine-OSB с = 1482 N·m/rad; for pinePW с = 1788 N·m/rad;) and end to face butt joints (for pine-PB с = 823 N·m/rad; for pineOSB с = 844 N·m/rad; for pine-PW с = 1433 N·m/rad;).

Each discrete frame model was loaded with a total load of 800 N, distributed as follows (Fig.1): 80% were set as a remote force, distributed between rails of the seat with application point of 100 mm in front of the rear rail; 16% were set as equal nodal forces, distributed on the edges of the two sides of the backrest.

More details and validity of this approach are given in STANEVA et al. (2018d).

RESULTS AND DISCUSSION The results for linear displacements (resultant ures, x-, y- and z-displacement: ux, uy, uz), nodal rotations (resultant θres, x-, y- and z-nodal rotation: θx, θy, θz) and equivalent strains (maxPR, minPR) for the side plates of the frame for models A and B and for all investigated materials are shown in Table 1 and in Fig.2 to Fig.9. The main differences in the deformation behavior of the investigated frames are mainly expressed in the frame side plates from different materials, so the results for the side plates are only shown.

For both models (A and B) and for all investigated materials the maximum resultant linear displacement (ures) in the side plates was established in their base: in model B it is greater approximately 1.8 times for side plates from PB, 2.3 times for OSB and 1.6 times for PLY than the same in model A (Table 1). Dissolution in the base of side plates for model B was established due to the redistribution of the load, but differences of the linear displacements (in absolute values) are not significant: 0.35 mm for side plate from PB, 0.27 mm for OSB and 0.11 mm for PLY. The maximum resultant linear displacement in 127

Tab. 1 Maximal values of linear displacements and nodal rotations for side plates.

Parameter

Location

PB

OSB A 0.211 0.153 0.190 0.133 0.179 0.182

base front rail rear rail backrest front rail rear rail

B 0.767 0.088 0.127 0.152 0.080 0.110

θres, [°]

front rail rear rail

0.37 0.65

0.22 0.46

0.,44 0.,67

0.19 0.37

0.19 0.35

0.11 0.26

θy, [°]

rear rail

0.20

0.25

0.62

0.148

0.10

0.10

maxPR, [m/m]

front rail

0.00827

0.00514

0.01330

0.00197 0.00373

minPR, [m/m]

front rail rear rail

-0.00869

-0.00222

0.00740 -

0.00154 -0.00349

ures.10-3, [m]

uy.10-3, [m]

B 0.480 0.076 0.105 0.157 0.033 0.076

PLY A B 0.199 0.314 0.051 0.036 0.085 0.048 0.062 0.054 0.050 0.031 0.058 0.280

A 0.419 0.130 0.180 0.144 0.126 0.164

0.00176 0.00123 -

the side plate from PB is greater almost 2 times than that of side plate from OSB and PLY for model A and 1.6 times than that of side plate from OSB and 2.44 times than that of side plate from PLY for model B. The maximum resultant linear displacement in the side plates is determined mainly by the linear z-displacement (uz) – Fig.2 and Fig.3.

Another relatively high value for resultant linear displacement and z-displacement in the side plates was observed in model A in the field of rear rail of the seat for PB and PLY, as for OSB it is in the backrest; in model B relatively high value for resultant linear displacement and z-displacement is established in the field of the backrest for all materials Table 1, Fig.2 and Fig.3.

The maximum linear x-displacement (ux) in the side plates from PB was established upper in the backrest for both models A and B, as for side plates from OSB and PLY the maximum values for both models A and B were observed in the field of assembling of the front rail (Fig.4 and Fig.5). For model A the maximum linear x-displacement in the side plate from PB in the field of front rail is almost equal with that of side plate from OSB and 2.9 times greater than that of PLY. For model B the maximum linear x-displacement in the field of front rail is greatest in the side plate from PB, 1.5 times greater than that of side plate from OSB and 2.9 times than that of side plate from PLY. The maximum resultant nodal rotation θres in the side plates was received in the field of assembling of rear rail of the seat for both models A and B and for all materials (Table 1). It is determined mainly by x-nodal rotation (Fig.6 and Fig.7). In model A the maximum resultant nodal rotation is approximately equal for side plates from PB and OSB and 1.9 times greater than that of side plate from PLY. In model B it is greatest for side plate from PB and 1.2 times greater than that of side plate from OSB and 1.8 times than that of PLY. The maximum x-nodal rotation (θx) in the field of assembling of rear rail to the side plates from PB is equal with that of side plate from OSB and 1.8 times greater than that of PLY for model A (Fig.6). For model B the maximum x-nodal rotation in the side plates from PB is greatest, 1.3 times greater than that of side plate from OSB and 1.7 than that of PLY (Fig.7). The area with higher values of x-nodal rotation in the side plate is bigger in model B than that in the model A, due to the assembling of strengthening details under rails of the seat.

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Fig. 2 Distribution of linear z-displacement in side plates from PB, OSB and PLY (model A).

Fig. 3 Distribution of linear z-displacement in side plates from PB, OSB and PLY (model B).

Fig. 4 Distribution of linear x-displacement in side plates from PB, OSB and PLY (model A).

Fig. 5 Distribution of linear x-displacement in side plates from PB, OSB and PLY (model B).

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Fig. 6 Distribution of x-nodal rotation in side plates from PB, OSB and PLY (model A).

.

Fig. 7 Distribution of x-nodal rotation in side plates from PB, OSB and PLY (model B).

.

Fig. 8 Distribution of z-nodal rotation in side plates from PB, OSB and PLY (model A).

.

Fig. 9 Distribution of z-nodal rotation in side plates from PB, OSB and PLY (model B).

.

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The maximum z-nodal rotation (θz) in the side plates was received in the field of assembling of front rail of the seat for all materials in both models A and B (Fig. 8 and Fig. 9).

In model A the maximum z-nodal rotation in the side plates from OSB is greater 1.3 times than that of PB and 3.0 times than that of PLY. For model B the maximum value was received for side plate from PB and it is greater 1.8 times than that of side plate from OSB and 2.1 times than that of side plate from PLY. For all materials of the side plates the maximum value of maximum principal strain (maxPR) was received in the field of assembling of the front rail of the seat for both models A and B (Table 1). In model A for side plates from OSB it is 1.6 times greater than that of side plate from PB and 3.6 times than that of PLY. In model B the maximal value for side plate from PB is 2.6 times greater than that of side plate from OSB and 2.9 than that of PLY. In model A the maximum value of minimum principal strain (minPR) was established in the field of assembling of the front rail of the seat for side plates from PB and PLY, as in model B the maximum value was established in the field of assembling of the rear rail of the seat for side plate from OSB. For side plates from PB in model A it is 1.2 times greater than that of side plate from OSB and 2.5 times than that of PLY, in model B these relations are 1.4 and 1.8, respectively (Table 1).

It is established that the deformation characteristics of the side plates from used in this investigation OSB boards are better than the same of OSB boards, used in our previous investigations (STANEVA et al. 2018b, d). All the peculiarities of the deformation characteristics of investigated frame side plates are due to the material characteristics of the used furniture boards (PB, OSB and PLY) and especially to the elasticity modules and their orientation in the construction elements of the frame model.

CONCLUSIONS The maximum resultant linear displacement in the field of the base of the side plate is greatest for the side plates from PB for both models A and B, 50% smaller for OSB and 53% smaller for side plates from PLY in model A, 37% smaller for OSB and 58% smaller for side plates from PLY in model B. Concerning the deformation, the most unstable are the side plates in their base even for strengthened model B for all materials, which is why it is recommended to further strengthen the frame in this area.

In the field of assembling of the rear rail of the seat in the side plate, the maximum resultant linear displacement and the maximum resultant nodal rotation are almost equal (5% and 3% difference) for side plates from OSB and PB in model A. In the same field of the side plate in model B, the maximum resultant linear displacement and the maximum resultant nodal rotation of side plate from OSB are 17% and 20% smaller than that of side plate from PB, respectively.

The side plates from PLY have the best deformation characteristics, that is way PLY boards are recommended as the most suitable furniture boards for the upholstered frame with side plates: in the field of assembling of the rear rail of the seat the maximum resultant linear displacement and the maximum resultant nodal rotation are 5355% and 4648% smaller than that of side plates from PB and OSB for model A; for model B in the same field these characteristics are 5462% and 3043% smaller than that of side plates from OSB and PB. REFERENCES BODIG, J., JAYNE, B. 1982. Mechanics of Wood and Wood Composites. New York: Van Nostrand Reinhold Co. Inc, 1982. 711 pp. ISBN 0-442-00822-8.

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ERDIL, Y., KASAL, A., ECKELMAN, C. 2008. Theoretical analysis and design of joints in a representative sofa frame constructed of plywood and oriented strand board. In Forest Products J., 58 (7/8): 6267.

GENCHEV, Ya., STANEVA, N., HRISTODOROVA, D. 2018. FEM analysis of deformation and stresses of upholstered furniture skeleton made of Pinus Sylvestris and Plywood. In Sci. J. Management and Sustainable Development, 69, (2): 5661.

HRISTODOROVA, D. 2019. Stiffness coefficients in joints by staples of skeleton upholstered furniture.

In J. Innovation in Woodworking Industry and Engineering Design (INNO), 16 (2): 2733.

KASAL, A. 2006. Determination of the strength of various sofa frames with Finite Element Analysis, G.U. In Journal of Science,19 (4): 191203. МARINOVA, А. 1996. Methodology of stress and strain furniture structure analysis. In Proceeding of Intern. Sci. Conference "Mechanical Technology of Wood", Sofia, 257267.

SMARDZEWSKI, J. 2001. Construction optimisation of upholstred furniture. In Folia Forestalia Polonica, Seria B, zeszyt 32: 519.

SMARDZEWSKI, J., PREKRAT, S. 2009. Optimisation of a sofa frame in the integrated CAD-CAE environment. In Electronic J. of Polish Agricultural Universities, 12 (4) Wood Technology.

Available online http://www.ejpau.media.pl STANEVA, N., GENCHEV, YA., HRISTODOROVA, D. 2018a. Static analysis of an upholstered furniture skeleton with staple corner joints by FEM. In Innovation in Woodworking Industry and Engineering Design (INNO), 14(2): 7885.

STANEVA, N., GENCHEV, Ya., HRISTODOROVA, D.; ALEXANDROVA, G. 2018b. 3D modeling and FEM analysis of deformations of a furniture skeleton with OSB side plates. In Chip and Chipless Woodworking Processes. 11 (1): 165–170.

STANEVA, N., GENCHEV, Ya., HRISTODOROVA, D. 2018c. Approach to designing an upholstered furniture frame by the finite element method. In Acta Facultatis Xylologiae Zvolen, 60 (2): 61−69.

STANEVA, N., GENCHEV, Ya., HRISTODOROVA, D. 2018d. Comparative finite elements analysis of deformations in an upholstered furniture skeleton with side plates from PB, OSB and Plywood. In Proceedings of 29th International Conference on Wood Modification and Technology (ICSWT 2018) „Implementation of Wood Science in Woodworking Sector“, Zagreb: 171184.

STANEVA, N., GENCHEV, Ya., HRISTODOROVA, D. 2019. Analysis of deformations and stresses of an upholstered furniture frame with OSB side plates. In Proceeding of 30th Intern. Conference on Wood Modification and Technology – ICWST 2019 „Implementation of Wood Science in Woodworking Sector“, Zagreb, pp.177185.

THOMAS, W. 2003. Poisson’s ratio of an oriented strand board. In Wood Sci. Technology, 37 (3): 259268.

WANG, X. 2007. Designing, modeling and testing of joints and attachment systems for the use of OSB in upholstered furniture frames, PhD thesis, University Laval, Quebec. 264 pp. Available online: http://archimede. Bibl.unival.ca/archimede/fichiers/24743/24743.pdf

AUTHORS’ ADRESS Nelly Staneva, PhD. University of Forestry Faculty of Forest Industry Department of Furniture Production 1797 Sofia, Bulgaria nelly_staneva@yahoo.com

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 61(2): 133−149, 2019 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.12

ENVIRONMENTAL EVALUATION OF ALTERNATIVE WOODBASED EXTERNAL WALL ASSEMBLY Jozef Mitterpach  Rastislav Igaz  Jozef Štefko ABSTRACT During the past decades in the construction sector, the emphasis has been laid on the development of environmentally friendly materials and structures. Therefore, the environmental impact of composite wood-based materials is crucial for sustainable construction. The aim of this paper is to present the results of research on the environmental impact assessment of the external wall assembly of wood-based structures used in wood constructions. Eight wall assemblies usable in ultra-low energy structures and nearly-zero energy buildings (passive buildings) with U-values in the range from 0.0990.211 W/(m2K) were included in the analysed and evaluated group. The environmental analysis Life Cycle Impact Assessment (LCIA) was performed by SimaPro software, using the IMPACT2002+ method in terms of an impact on selected components (human health, ecosystem quality, climate change, resources). An assembly based on a cross-laminated timber (CLT) panel with insulation on the blown cellulose base was evaluated as a structure with the greatest environmental impact, closely followed by the assembly based on a box beam with glassbased mineral insulation. The impact assessments of both of these assemblies were more than 16 mPt. On the other hand, structures using wood fibre, straw and especially sheep wool as insulation were assemblies with the least negative impact on the environment. Their impact assessment was from 6.68.2 mPt. The research results also showed that the material assembly of the external wall significantly influences the LCIA analysis result, and the choice of insulating material is the most important, as the insulation is more than 80% of the volume of the wood-based structures on average. The results of the life cycle analysis show that the selection of structural and especially insulating materials plays an important role in the case of wood constructions.

Key words: LCA, LCIA, wood construction, green building.

INTRODUCTION At the time of the devastating environmental impact of today's civilization, the emphasis has been placed on the development of environmentally friendly materials, structures and technologies. Even in the construction sector, efforts are focused on minimizing the environmental impacts of structures, operating facilities or on producing and using the products along with the building energy efficiency trends.

Local material availability, its low cost, rapid construction, simple processing and a wide range of structural possibilities are the main benefits of wood-based structures.

Significantly, a lower negative impact on the environment from the point of view of life 133

cycle assessment is considered a significant contribution of wood in construction. Wood allows civil engineers to build light, standardized building structures with excellent thermal insulation properties (CORDUBAN et al. 2017). Therefore, wood constructions are popular all over the world. The progressive development in the field of timber construction results in the construction of multi-functional timber buildings accepted by a wide community of civil engineers and designers (MÜLLER et al. 2016).

A wide range of timber materials is used in the construction of wood-based structures.

Nowadays, a number of large scale wood-based sheathing materials (fibreboards, oriented strand boards, gypsum fibreboards, cement-bonded particle boards) also a number of woodbased thermal insulation materials (wood fibre, recycled cellulose, wood crust), as well as other materials (mineral insulation, polymer foam based insulation) are used in addition to solid wood (ŠTEFKO et al. 2010, KRIŠŤÁK et al. 2019). The range of insulating materials used is growing and the research on the use of wood waste, wood bark coconut fibre or sheep wool taking into account their lesser environmental impact compared to conventional materials is also conducted (CETINER and SHEA 2018, TUDOR et al. 2018, PANYAKAEW and FOTIOS 2011, ZACH et al. 2012, IGAZ et al. 2017). Despite worse insulation properties of these materials, their benefit is mainly due to their low cost and lesser environmental impact.

Due to the popularity of wood constructions, new materials, e.g. wood-concrete composites characterized by excellent fire resistance, good acoustic insulation properties, high heat capacity, and the possibility of prefabrication started to be used for low-energy construction (FADAI et al. 2016). Similar systems refer to as "hybrid building systems" (MÜLLER et al.

2016). Their higher negative impact in terms of the environment is the most significant disadvantage with respect to standard timber buildings. However, they provide better functional properties than wood constructions in many aspects.

Environmental impact assessment of buildings throughout their life cycle, from material manufacturing to construction disposal or another sub-period is becoming a part of a sustainable approach to the construction process in the world. Life cycle assessment methods can be applied in various scopes, partially in the case of building materials used and their production (ZABALZA et al. 2011, KRIŠŤÁK et al. 2014) or to buildings as a whole (BLENGINI, DI CARLO 2010, VILCHES et al. 2016). LCA analyses may include material production, their incorporation into construction, building usage, maintenance, dismantling and final waste disposal. Energy and material flows are defined in order to analyse and quantify environmental impacts in each life cycle phase. The LCA method was developed in the 1960s and now it is accepted worldwide. There is a detailed methodology, the application is internationally harmonized, standardized and used (BJORN et al. 2017). The requirements for the life cycle assessment (LCA) method are specified in the standards – namely EN ISO 14040:2006 and EN ISO 14044: 2006 entitled "Environmental Management. Life cycle assessment. Requirements and Guidelines."

In the life-cycle analytical methods, a number of input factors must be taken into account and their impact on various components must be assessed. The analyses show that the cumulative energy intensity of wood constructions can be up to 18% lower and the impact on climate change is up to 25% lesser in comparison to the massive constructions built by conventional construction technologies (HEEREN et al. 2015). The choice of materials used for construction and heating system selection is becoming increasingly important, particularly in the case of energy-efficient construction (DODOO et al. 2012).

In general, the LCA method is used to compare the environmental impacts of either products or services with respect to their life cycle (KOČÍ 2010 and BOGACKA et al. 2017).

In terms of buildings, the cycle starts after the raw material are extracted, goes through the production of building materials and units, their transport to construction, installation, continues in the stage of use including maintenance and eventually to the disposal of the 134

building after the end of its life, or recycling and energy recovery of construction waste.

Effective processing of LCA studies implies access to process, material, and energy flow databases. The method is one of the most important information tools for environmentally driven product policy.

The main benefits of the life cycle assessment method are: • Comparing the environmental impacts of products with regard to their function.

• Assessing the environmental impacts with respect to the product life cycle.

• Establishing system boundaries to clearly express the scope of the product system.

• Expressing environmental interventions not by calculating emission flows but by defined impact categories - converting weighted emission flows into specific values of impact category indicators.

• Identifying the transfer of environmental problems both in space and between different impact categories.

The aim of the paper is to introduce the results of the research in the area of environmental impacts assessment of the material assembly and construction type of the external walls of wooden buildings (LCA analysis). To conduct the assessment, eight types of commonly used external wall assemblies were chosen considering a wide range of used materials and constructional approaches. The software package of IMPACT 2002+ was used for the LCA analysis. The assessment considered the four areas of impacts: human health, ecosystem quality, climate change and resources).

MATERIALS AND METHODS Assembly of analysed structures Eight types of wall assembly were created in the research and analysis of the properties of wood-based external wall structures. Various types of structural load-bearing systems were taken into account when creating the assemblies (joist column systems, I-beams, composite cross-section beams, or cross-laminated panel), various materials used as insulating materials (mineral fibre insulation, polystyrene, PU-foam, straw, sheep wool, blown cellulose), various types of internal surfaces (gypsum plasterboard, gypsum fibreboard, clay plaster), ventilated and classic facade solutions and diffuse open and closed structures.

Moreover, selecting assemblies commonly used in real national conditions for wooden constructions was the most important step. The requirements of the current legislation in the field of thermal and technical requirements for wall structures were also taken into consideration. The structural and material construction of the individual S1– S8 assemblies used to determine the thermal properties and LCA analysis are shown in Figure 1 – Figure 8 (EPS - expanded polystyrene, CETRIS - cement particle board, PU – polyurethane, OSB - oriented strand board, HDF - high-density fibreboard, CLT - cross-laminated timber).

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Fig 1. Cross-section and materials in the assembly of the S1 structure.

Fig 2. Cross-section and materials in the assembly of the S2 structure.

Fig 3. Cross-section and materials in the assembly of the S3 structure.

Fig 4. Cross-section and materials in the assembly of the S4 structure.

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Fig 5. Cross-section and materials in the assembly of the S5 structure.

Fig 6. Cross-section and materials in the assembly of the S6 structure.

Fig 7. Cross-section and materials in the assembly of the S7 structure.

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Fig 8. Cross-section and materials in the assembly of the S8 structure.

Life cycle assessment The development in building regulations and standards is headed towards near-zero energy consumption (European Council for an Energy-Efficient Economy). However, many regulations refer to zero energy consumption focused only on operating energy and ignoring the energy stored in materials.

The aim of the research was to evaluate the environmental impacts of structures of wood-based external walls used in wood constructions. The studied assemblies were created from a wide range of materials used in timber constructions with the application of various structural approaches. The LCA method (Figure 9) (ISO 14040, ISO 14044, EN 15804:2012+A1:2013) was used in environmental impact analysis while input and emission outputs throughout the production chain from exploration, extraction of raw materials to processing, transport to the final use were taking into account energy.

Fig. 9 LCA analysis phases and their relations.

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The aim of the life cycle assessment was to identify the environmental impacts of the structures and components in the proposed S1 - S8 wall assemblies and to determine the magnitude of the negative impact of individual structures and their materials on selected components relating to the environment.

The scope of the study includes product stage (modules A1 to A3) according to ISO EN 15804. The results contain the overall environmental impact of all wall assemblies and their materials within the impact category, including the whole product stage, thus they are not subdivided into particular modules. 1m2 of the designed structures was determined as a functional unit. Therefore, the results of this study could be compared with other wall assemblies where parameters valid for 1 m2 are known. SimaPro 8.4 and the IMPACT 2002+ life cycle assessment method (PRÉ CONSULTANTS, 2016) were used to model and process the results of impacts of each wall assembly. Materials used in the assemblies and subsequently, in the assemblies of the external walls were analysed using the Life cycle analysis. An assessment of the impact on four components relating to the environment (human health, ecosystem quality, climate change, resources) was the result of the analysis.

IMPACT 2002+ evaluation method IMPACT 2002+ is an abbreviation for the IMPact Assessment of Chemical Toxics. It is an impact assessment methodology originally developed by the Swiss Federal Institute of Technology - Lausanne (EPFL) with the current development carried out by the same team of experts, called Ecointesys-life cycle systems now. The current methodology offers an acceptable performance by combining midpoints and endpoints (damage) approaches, linking all types of life cycle inventory results through 14 (15) midpoints and four damage (endpoints) impact categories (Figure 10). The base unit of the overall environmental impacts in the environmental assessment is Pt or mPt (point-standard eco-indicator normalized unit) (FRISCHKNECHT et al. 2007 and JOLLIET et al. 2003).

Fig. 10 Midpoints and endpoint category of impact indicators (JOLLIET et al. 2003).

Characterization The factors of toxicity and Eco toxicity followed the IMPACT 2002+ methodology.

Characterization factors of other categories are adapted from existing characterization

139

methods such as Eco-indicator 99, CML 2001, IPCC and Cumulative Energy Demand.

Characterization factors for nearly 1500 different LCI results were provided by the IMPACT 2002+ method (Figure 11). Environmental impacts at up to 15 midpoints (carcinogens, noncarcinogens, respiratory inorganics, ionizing radiation, ozone layer depletion, respiratory organics, aquatic Eco toxicity, terrestrial Eco toxicity, terrestrial acidification and nutrification, land occupation, aquatic acidification, aquatic eutrophication, global warming, non-renewable energy, and mineral extraction) that transforms into 4 endpoints: human health, ecosystem quality, climate change and resource consumption can be evaluated with the latest version of the IMPACT2002+ life cycle impact assessment (www.presustainability.com).

Fig. 11 Characterization of S1- S8 structure assemblies, SimaPro, IMPACT2002+.

Fig. 12 Damage assessment of S1- S8 structures assemblies, SimaPro, IMPACT2002+.

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Damage assessment Damage assessment is identical to the environmental profile (Figure 12). It refers to impact categories, where damage corresponding to the selected impact categories is measured. It is expressed as a percentage of the impact of individual materials in the S1 - S8 structures on the environment at each endpoint (presustainability.com).

Normalization Damage factors are standardized (Figure 13) by dividing the impact into the emission unit by the overall effect of all substances of the different impact categories for which the characterization factors exist, e.g. per person per year. The unit of all standardized midpoints and endpoints is then (number of persons x year) / emission unit, thus the number of equivalent persons concerned in one year per emission unit (presustainability.com).

Fig. 13 Normalization of S1- S8 structure assemblies, SimaPro, IMPACT2002+.

Weighting Analysing the normalized results of four endpoint (damage) impact categories or fourteen midpoint indicators separately for the interpretation of the individual phases of the LCA is recommended by the authors of the IMPACT 2002+ method (Figure 14). Another step of weighting the data is added by PRé Consultant96s. For each damage category, a weighting factor of 1 is determined (JOLLIET et al. 2003).

Fig. 14 Weighting S1- S8 structure assemblies, SimaPro, IMPACT2002+.

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RESULTS AND DISCUSSION Analysis of thermal and technical properties of wall assemblies Selected external wall structures were assessed in terms of the required values of heat transfer coefficient U (U-value), thermal resistance R, minimum internal surface temperature θsi and the temperature factor fRsi with respect to the requirements of valid legislation – standards for thermal protection design STN 730540 as a national application document in the Slovak Republic. Following the values of the heat transfer coefficients U2D (Table 1) determined using the 2D models taking into account the two-dimensional heat conduction in volume of the constructions, the fact that all of the considered wall assemblies meet the required value [U = 0.22 W/(m2K)] according to STN 730540 can be stated. The maximum value of the heat transfer coefficient [U = 0.15 W/(m2K)] meets three of the considered assemblies (S3, S5 and S8). It is considered a reference value and according to the standard STN 730540 it is a normalized value from 1st January 2021 and a reference value for passive houses in some regions. Thus, all of these three assemblies can be used to design almost zero (passive) structures. A detailed analysis of the thermo-technical properties of these assemblies taking into account other parameters are mentioned in the work (MITTERPACH et al. 2018).

However, only the energy consumption during the operating stage of the life cycle is affected by the thermal protection quality of the sheathing fragment defined by the heat transfer coefficient U. U-value is a key variable in the calculation of specific heat loss, demonstrating the required specific heat and energy needs for heating or building classification in energy certification. However, in the current compulsory energy certification the overall energy consumption during the whole life cycle of the building, as well as the incorporated materials is not taken into account. Though, initiatives taking into account LCA analysis in the process of building certification in the world have already existed (ZABALZA BRIBIÁN et al. 2009).

Based on the above-mentioned evaluation of thermo-technical properties, it can be stated that the S4 assembly (I-beam with wood fibre) is the most effective in the structures complying with the normalized values [U ≤ 0.22 W/(m2K)]. In the group meeting target standard values [U ≤ 0.15 W/(m2K)] the S5 assembly is the most effective (I-beam with wood fibre). However, it is necessary to consider the fact that the insulation thickness used plays an important role, by which the monitored parameters can be changed in a relatively wide interval. In addition, an increase in the thickness of the insulation results in an increase in its volume in the assembly, and thus, the life cycle impact assessment results are affected significantly. A further important area, particularly for the investor is the financial intensity of the individual structures (e.g. price per 1 m2). This area was not a subject of the research, so it is not evaluated.

Tab. 1 Thermal transmittances L2D, heat transfer coefficients U2D, minimum internal surface temperature θsi, min and temperature factors fRsi for individual assemblies based on 2D models. label S1 S2 S3 S4 S5 S6 S7 S8

d (m) 0.2185 0.2325 0.4975 0.3115 0.4515 0.4270 0.2925 0.4795

L2D (W·m-1·K-1) 0.11487 0.13154 0.06170 0.10092 0.06446 0.11472 0.10611 0.09297

U2D (W·m-2·K-1) 0.1838 0.2105 0.0987 0.1615 0.1031 0.1978 0.1698 0.1488

R2D (m2·K·W-1) 5.271 4.581 9.922 6.022 9.529 4.846 5.719 6.510

θsi,min (°C) 18.47 18.18 18.90 18.56 19.10 18.97 18.37 18.81

fRsi (-) 0.956 0.948 0.969 0.959 0.974 0.971 0.953 0.966

Note: The thermal resistance value R2D was determined on the basis of U2D taking into account the heat transfer resistance values Rsi = 0.13 m2K/W and Rse = 0.04 m2K/W or Rse = 0.08 m2K/W for exterior side ventilated air gap assemblies

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Environmental impact assessment Numerous building LCA studies from various international stakeholders are published which compare conventional buildings and timber buildings to ascertain specific advantages and specificities of the latter. Moreover, most of them take into account only GHG emissions. These studies all differ in approach, system boundaries, database and scope, and therefore cannot be compared. However, they demonstrate that wood-based materials have advantages in terms of carbon storage capacity, therefore resulting in lower GHG emissions in the product stage. (HAFNER and SCHÄFER 2017).

The results of the LCIA analysis of the S1 - S8 structure assemblies are graphically shown in Figures 15 and 16. An exact quantitative assessment of the impacts of individual assemblies on selected environmental compartments is summarized in Table 2.

Fig. 15 LCIA of the S1- S8 structure assemblies, endpoints (%), SimaPro, IMPACT2002+.

The diagram in Figure 15 shows the comparison of the individual wall assemblies. The relative (percentage) impacts of the four categories of the assessed negative impacts of the individual structures within the life cycle can be seen there.

Summarized comparison of all considered assemblies is given in Fig. 16. Moreover, the relative effect ratio of individual assemblies in terms of impacts within the life cycle assessment was determined.

Fig. 16 LCIA of S1- S8 structure assemblies, endpoints (mPt), SimaPro, IMPACT2002+.

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The assessment of the materials used for the creation of 1m2 of a given wall assembly and its environmental impacts at endpoints is provided by the values calculated in Table 2 and Figure 16.

Tab. 2 Endpoint life cycle assessment for S1-S8 structures’ assemblies, IMPACT2002 +, mPt. Label

Human health (mPt)

Ecosystem quality (mPt)

Climate change (mPt)

Resources (mPt)

Total (mPt)

S1

4.569

0.501

3.576

4.393

13.039

S2

3.776

1.364

2.296

2.702

10.138

S3

7.585

2.161

3.182

3.400

16.328

S4

3.642

0.638

2.036

1.615

7.931

S5

3.664

0.698

2.049

1.628

8.038

S6

2.844

2.043

1.561

1.744

8.191

S7

2.366

1.867

1.220

1.143

6.596

S8 Total (mPt)

4.508 32.954

2.419 11.691

1.596 17.516

8.807 25.432

17.330 87.593

Total (%)

37.6

13.4

20.0

29.0

100.0

The "human health" category was the most affected category in assessing the environmental impact of wood-based building structures with conventional insulation materials. In this impact category, the effect of the S3 assembly at 7.59 mPt seemed to be the most negative due to the large volume of used glass fibre insulation (220·103 m3/m2).

In addition, the use of plasterboard construction profiles, gypsum plasterboards (7.8·103 m3/m2) and OSB boards (15.1·103 m3/m2) made a significant contribution. The effect of remaining structures, S1 and S8 assembly, was the greatest one. The S7 assembly comprising sheep wool had the lowest impact on the environment (2.37 mPt).

The "ecosystem quality" impact category was generally assessed as the category with the least relative negative impact in terms of the selected wood-based building structures.

Following the more detailed analysis, the impact of the S8 assembly consisting of the CLT panels was the highest in this category (93.8·10-3 m3/m2). This negative feature was caused by the use of a large volume of solid glued timber. The S1 assembly containing PU foam (0.50 mPt) was the best-rated structure in this category.

In terms of the impacts on the “climate change” category, the S1 structure with PU insulation was evaluated the worst one (3.59 mPt) due to the negative impact of the large volume of the polyurethane foam insulation used (92.8·103 m3/m2). The S7 structure using sheep wool as an insulation system (1.22 mPt) seemed to be the best option in this category.

The worst-ranked structure, in the "resource" impact category, was the S8 assembly (8.807 mPt). Due to the large volume of cellulose insulation used (118.4·103 m3/m2), it reached the negative primacy. The S1 and S3 structures also showed relatively high negative ratings in this category. In the case of the S1 assembly, it was because of PU foam and gypsum fibre boards. For the S3 assembly, the negative assessment is primarily due to mineral insulation, however, the impact of the use of OSB boards and gypsum plasterboards was also significant. The S7 sheep wool structure (1.143 mPt) achieved the best rating.

In a complex view of the proposed assemblies and their life cycle impact assessment, it is clear that the S8 assembly based on CLT panel and cellulose insulation had the highest negative impact with a total score of 17.33 mPt followed by the S3 structure (box beam with mineral insulation) with a total impact of 16.33 mPt and the S1 structure (I-beam with PU foam), with a total impact of 13.04 mPt. On the other hand, the S7 sheep wool structure (6.60 144

mPt), followed by the S4 and the S5 structure (both on the I-beam basis and wood fibre insulation) and the S6 structure (box beam with straw) was the best rated.

Based on the evaluation and analysis of the results it can be concluded that wall assemblies made using a large volume of foamed plastic materials such as PU foam, glass fibre and surprisingly CLT or cellulose-based insulation had the greatest negative impact.

For example, a study by DODOO et al. (2014) compared timber buildings using different construction methods (CLT panel, beam-and-column, and modular structures).

Hereby structure made of CLT panel offers lowest life cycle GHG emissions on contrary to the beam-and-column assembly as the worst wood-based structure in terms of produced GHG emissions.

However, the technology was not precisely specified in the available databases. The calculation can be significantly affected by the different representation of chemicals and raw materials (e.g. recyclate ratio), especially the cellulose-based insulation system evaluated in this study. In terms of large-scale materials, the greatest negative impact was performed by OSB and gypsum-based boards. Though, their volume representation in the assemblies was relatively small and so the negative impact was reflected particularly in the assemblies where insulation with good environmental assessment was used. On the other hand, the use of wood fibre materials (HDF board, wood fibre board, wood fibre insulation), straw and especially sheep wool appeared to be convenient. Therefore, taking into account environmental impacts, the use of different types of materials in the structure of wood-based assemblies must be discussed. A detailed analysis of the environmental impacts of the assemblies and used materials is mentioned in the work (MITTERPACH et al. 2018).

The research showed significant differences in the assessment of the environmental impacts of wood-based external wall assemblies. Insulation materials were identified as the most important materials for overall environmental assessment. Their significance relates to their proportional representation in the wood-based building structure where they account for more than 80% of the volume. Nevertheless, a case study done by PETROVIC et al. (2019) proved that cellulose-based insulation had dramatically low CO2e emissions in comparison with other materials mainly used in the building industry, such as glass wool and stone wool.

Looking at the current segment of the wood construction market, it is worth noting that the materials evaluated as relatively unsuitable from the environmental impact point of view are currently used most, especially because of their affordability and low price. Hence, it is rational that builders and investors prioritize such materials. Moreover, when there is a demand, manufacturers can make even more profit when producing these materials on a large scale. On the contrary, materials rated positively (with a low negative impact) have not been significantly applied to the market yet and their production is therefore burdened by relatively high fixed costs. However, they offer a number of advantageous properties (greater bulk density and heat capacity with a positive effect on thermal stability, good diffusion properties, ability to actively regulate and stabilize indoor humidity) not only in terms of environmental impacts. A major disadvantage and limiting criterion is their low availability (the investor generally has to search for a supplier) and a higher price.

Wood-based materials are currently employed for their significant structural, thermal, acoustical and environmental properties and, last but not least, for their aesthetic and formal features (ASDRUBALI et al. 2017).

Among the various environmental properties of wooden materials, embodied energy is one of the most important (ESTOKOVA et al. 2017, SHIRAZI and ASHURI 2018). Regulations on energy use and emissions of buildings have been mostly about operational energy, often overlooking other life cycle components such as embodied energy which can account for a significant portion of life cycle emissions. For example, the study by SHIRAZI and ASHURI (2018) showed that older buildings can have lower embodied environmental impact per 145

square meter and lower embodied energy than younger residential buildings, not considering a major renovation or retrofit over the entire life span.

The exact environmental impact of a new structure of wood construction materials is always linked to the specific material used. Some other environmental impact assessments (ONDOVA and ESTOKOVA 2016) have shown that the selection of materials for the construction of buildings throughout the life cycle is an important tool for sustainability when building a life cycle model for civil engineering.

CONCLUSIONS Eight types of wall assemblies representing a wide range of materials used and design approaches were created for the assessment of the environmental impacts of wood-based structures. From the thermo-technical characteristics point of view, 2D models of temperature fields of the considered assemblies were created. Subsequently, the thermal resistance and the heat transfer coefficient (U-value) were determined. In the evaluated group, there were five assemblies usable for low energy construction [U ≤ 0.22 W/(m2K)] and three assemblies for ultra-low energy (passive) construction [U ≤ 0.15 W/(m2K)].

The LCA analysis made it possible to identify the environmental impacts of the individual assemblies on the four environmental components and consequently the cumulative impact. A assembly based on a CLT panel with blown cellulose-based insulation was evaluated as a structure with the greatest environmental impact, closely followed by a assembly based on a box beam with glass-based mineral insulation. The impact assessments of both of these assemblies were more than 16 mPt. On the other hand, assemblies with the least negative impact on the environment were structures using wood fibre, straw and especially sheep wool as insulation. Their impact assessment was at 6.68.2 mPt. The analysis showed that the material assembly significantly influences the LCA analysis result and so the environmental assessment should be taken into account in the case of wood construction. The use of natural materials without high demands on energyintensive processing was proven to be advantageous. The choice of insulating material is the most important as the insulation is more than 80% of the volume of the wood-based structures on average.

Following the research results, the fact that there is the need to specify and complete the environmental assessment database data with respect to production technology or recycling used can be stated.

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Moderné drevostavby. Považská Bystrica : UNIPRINT, ISBN 80-967718-9-2 TUDOR, E. M., BARBU, M. C., PETUTSCHNIGG, A., REH, R. 2018. Added-value for wood bark as a coating layer for flooring tiles. In Journal of Cleaner Production, 2018, Vol. 170, pp. 13541360, DOI: 10.1016/j.jclepro.2017.09.156 VILCHES, A., GARCIA-MARTINEZ, A., SANCHEZ-MONTANES, B. 2016 Life cycle assessment (LCA) of building refurbishment: A literature review. In Energy and Buildings, Vol. 135, pp.

286301, DOI 10.1016/j.enbuild.2016.11.042 ZABALZA BRIBIÁN I, ARANDA USÓN A., SCARPELLINI, S. 2009. Life cycle assessment in buildings: state-of-the-art and simplified LCA methodology as a complement for building certification. In Building and Environment, Vol. 44(12), pp. 25102520, ISSN 0360-1323, DOI 10.1016/j.buildenv.2009.05.001 ZABALZA BRIBIÁN I, VALERO CAPILLA A, ARANDA USÓN A. 2011. Life cycle assessment of building materials: a comparative analysis of the energy and environmental impacts and evaluation of the eco-efficiency improvement potential. In Building and Environment, 2011, Vol. 46 (5), pp. 11331140, DOI 10.1016/j.buildenv.2010.12.002 ZACH, J., KORJENIC, A., PETRANEK, V., HROUDOVA, J., BEDNAR, T. 2012. Performance evaluation and research of alternative thermal insulations based on sheep wool. In Energy and Buildings, Vol. 49, pp. 246253, ISSN 0378-7788, DOI 10.1016/j.enbuild.2012.02.014 ACKNOWLEDGMENTS This study was funded by the Slovak Research and Development Agency: Grant No. APVV-17-0206 „Ultra-low Energy Green Building Based on Renewable Wood Material“.

This study was also funded by the Scientific Grant Agency of the Ministry of Education SR and the Slovak Academy of Sciences: Grand No. 1/0717/19 „Assessment of Environmental Impacts of Wood-Based Buildings Throughout the Whole Life Cycle“.

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AUTHORS’ADDRESS Ing. Jozef Mitterpach, PhD. Technical University in Zvolen Faculty of Wood Science and Technology T .G. Masaryka 24 960 01 Zvolen Slovakia jozef.mitterpach@gmail.com Ing. Rastislav Igaz, PhD. Technical University in Zvolen Faculty of Wood Science and Technology T .G. Masaryka 24 960 01 Zvolen Slovakia igaz@tuzvo.sk prof. Ing. Jozef Štefko, PhD. Technical University in Zvolen Faculty of Wood Science and Technology T .G. Masaryka 24 960 01 Zvolen Slovakia stefko@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 151−164, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.13

EMPLOYEES’ MOTIVATION PREFERENCES IN FOREST AND WOOD-PROCESSING ENTERPRISES Miloš Hitka – Martina Lipoldová – Jarmila Schmidtová ABSTRACT Employee performance and subsequently the enterprise performance is affected by human resource management. The impact of motivation and meeting employees’ needs on improving the performance and delivering it to required standard is fundamental. Motivation factors as a part of employee motivation in wood-processing and forest enterprises based on the average values of motivational needs are defined in the paper. Following the research results and preferences mentioned by respondents, motivation factors included in the motivational programme in forest and wood-processing enterprises with respect to existing significant differences can be defined. The motivational needs of employees in forest and wood-processing enterprises in the Slovak Republic are similar. In forest enterprises, motivation factors might be focused especially on finances and then on relationships and social status. In wood-processing enterprises motivation factors might be focused on relationships and social status primarily and after that on finances.

Keywords: motivation factors, motivational programme, forest enterprises, woodprocessing enterprises.

INTRODUCTION Market economy and social changes create the environment suitable for business activity across all economic sectors in Slovakia, including forestry and wood-processing.

Therefore, the forestry-wood sector is considered one of the most important in Slovak economy in terms of fulfilling social functions (LOUČANOVÁ et al. 2017, DUŠAK et al. 2017, BALÁŽOVÁ – LUPTÁKOVÁ 2016, FOREST EUROPE 2015). The contribution of the sector to GDP of the Slovak Republic is 0.33%. Due to historical development, it is male-dominated branch of the industry (average male to female ratio is 3:1) (ANKUDO-JANKOWSKA 2007). A slight increase in the number of highly educated employees can be observed (PALUŠ et al.

2011). It is specific physically demanding job affected by weather conditions. In terms of economic figures, the position of forest industry has been difficult for a long time (HAJDÚCHOVÁ et al. 2016). While social requirements for the wood production are met by running market, social requirements for other public functions of nonmarket character are not covered economically enough. Proceeds from timber distribution presenting more than 80% of total proceeds are the main source of income of the forest industry (LOUČANOVÁ et al. 2018).

Contribution of wood processing and furniture manufacturing industry to the GDP of the Slovak Republic is smaller with less than 1 per cent. It is manual and highly sophisticated work that need some sort of on-the-job training. The devaluation of technical and vocational education can result in a lack of skilled labour necessary for this industrial sector. As the

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number of small and medium sized enterprises in the forest industry is high, forest sector employment as a percentage of total employment in the country is not so high. However, in several Slovak regions, it provides a spectrum of employment opportunities (ZELENÁ SPRÁVA 2017). Despite sufficient quantity of raw material and independence from the need to import materials, amount of products completed is very low. It is especially due to losing purchasing power of inhabitants. In comparison to the European standard, the opportunity to sell cheaper products of poor quality imported from countries of central Europe is encouraged (NEDELIAKOVA – PANAK 2015).

Human resource management is a work with people including the analysis of employee performance, planning ahead the needs of workforce, selecting the candidates, training the employees, their development, evaluation and motivation (ANDREWS 2016, VETRÁKOVÁ et al. 2016). It is aimed at ensuring the human resources, subsequently human potential in order to achieve company goals (PEDRAZA MELO – BERNAL GONZÁLEZ 2018, LORINCOVÁ et al.

2016, SÁNCHEZ-SELLERO – SÁNCHEZ-SELLERO 2016, TOKARČÍKOVÁ – KUCHARČÍKOVÁ 2015, OLŠOVSKÁ et al. 2013). Due to the uniqueness of human potential as a creator of all the values, the potential of all employees must be used and developed deliberately to deliver new values (STACHO et al. 2017, BAJZIKOVÁ et al. 2013). The effective development of employee potential is based on systematic evaluation and motivation.

Employee motivation plays an important role in achieving great results in organisations (KUCHARČÍKOVÁ – MIČIAK 2018, DAVYDENKO et al. 2017, LUCAS et al. 2004, DUNFORD et al. 2001). Motivation encouraging an individual to work or to be interested in a specific branch of industry can be pragmatic and idealistic at the same time (ROSAKSZYROCKA 2014). Motivation factors included in a motivational programme are important.

Expectations and engagement are formed and the performance of an employee or the work groups is affected (JEONG – CHOI 2017, STACHO – STACHOVÁ 2017). Motives are parts of each personality inciting human activity to achieve specific goals (ARTZ 2008, STONE 2005).

They can be considered “an engine” of human activities. Moreover, they are a driving force of the human personality, psychological factors and reasons associated with the behaviour of individuals can be explained by motives (STACHOVÁ et al. 2019, FERRARO et al. 2018, MINÁROVÁ 2015, ALMOBAIREEK – MANOLOVA 2013). Needs that can be considered the sources of intrinsic motivation together with interests, values and ideals are the strongest motives of human behaviour and relate to the structure of human motivation (GOSSELIN et al. 2017, KERTÉSZ et al. 2017, LIŽBETINOVÁ 2017). The motives like aspiration and ambition follow the simpler motives only in the case they are fulfilled, i.e. a hierarchy in human motives can be observed (XU et al. 2017, ŽUPERKIENĖ – ŽILINSKAS 2008). Due to the fact that the structure of human motivation is complex combination of individual motives, the issue of motivation is included in the motivational programme of enterprises (FEJFAROVÁ – URBANCOVÁ 2016). Motivational programme is focused on ensuring the optimum use of manpower in the process of meeting the enterprise goals (standard performance and ability when employees do not use their spare energy) and at the same time, on meeting the needs and developing human personality at work (BRADY – KING 2018, DAUD 2015, KANFER et al. 2012, ROBBINS et al. 2007). Creating the environment to support employee motivation in an enterprise is the main goal of the motivational programme (MURA et al. 2017). Designing the effective motivational programme is based on the assumption that the enterprise is able to concentrate on factors reporting lower level of employee satisfaction or those important for employee performance because of any other reasons (CSEH PAPP et al. 2018). At the same time, observing the changes must be in the centre of the attention as the motivation factors are not stable (VETRÁKOVÁ et al. 2017). They are affected by the age, gender, education, experience, environment, etc. (LORINCOVÁ et al. 2018, KAMPF et al. 2017, JELAČIĆ et al. 2010).

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The aim of the paper is to compare the level of motivation of forest and woodprocessing enterprises in Slovakia. Defining the similarities or differences in motivational programmes in forest and wood-processing enterprises in Slovakia and possibilities to create a unified motivational programme taking into account specifics of the forest-wood complex will be considered the result of the research.

EXPERIMENTAL PART A questionnaire consisting of 30 closed-end questions was used to determine the level of motivation in the enterprise at the actual time (HITKA 2009). The questionnaire was divided into two parts. Socio-demographic and qualification characteristics of employees were investigated in the first part. Basic data on respondents about their age, gender, seniority, completed education and job position were gathered in this part. Individual motivation factors used to find out the characteristics of the work environment, working conditions, appraisal system and remuneration in the enterprise, personnel work in the company, social care system and employee benefits as well as information about employee satisfaction or dissatisfaction, their value orientation, attitude to work, to colleagues and to the enterprise were mentioned in the second part. Motivation factors were arranged in alphabetical order not to affect the respondents. Employees were asked to assign one point of five points of importance from the Likert scale to each question. The level of motivation of the desired and real state was determined by respondents. In the case of desired state, ideas of respondents are used determine the motivation in the future. On the other hand, in the case of the real state, satisfaction with the real-time motivation is presented and highlighted.

The data gathered were processed using the STATISTICA 12.0 software (Dell, Oklahoma City, Oklahoma). Firstly, descriptive statistics was used to define basic differences between selected sample sets consisting of employees in wood-processing and forest enterprises. The research was conducted in the course of four years, from 2016 to 2019. Representativeness was achieved with the number of respondents, 1,114 in total. 609 respondents were from wood-processing enterprises and 505 from forest enterprises.

Detailed description of respondents is given in Table 1.

Tab. 1 Description of the sample set.

Enterprises Gender Male Female Age Up to 30 31-40 41-50 51+ Completed education Primary Lower secondary Upper secondary Higher

Wood-processing enterprises Absolute Relative frequency frequency 378 62.07 231 37.93 Absolute Relative frequency frequency 154 25.29 242 39.74 157 25.78 56 9.20 Absolute Relative frequency frequency 82 13.46 161 26.44 229 37.60 137 22.50

153

Forest enterprises Absolute frequency 315 190 Absolute frequency 133 172 138 62 Absolute frequency 99 94 155 157

Relative frequency 62.38 37.62 Relative frequency 26.34 34.06 27.33 12.28 Relative frequency 19.60 18.61 30.69 31.09

Respondents’ participation in % 62.21 37.79

25.76 37.16 26.48 10.59

16.25 22.89 34.47 26.39

Years of working in the enterprise Less than 1 year 1-3 years 4-6 years 7-9 years 10 years and more Job position Manager Blue-collar worker White –collar worker

Absolute frequency 60 125 90 200 134 Absolute frequency 86 296 227

Relative frequency 9.85 20.53 14.78 32.84 22.00 Relative frequency 14.12 48.60 37.27

Absolute frequency 47 154 115 45 144 Absolute frequency 82 212 211

Relative frequency 9.31 30.50 22.77 8.91 28.51 Relative frequency 16.24 41.98 41.78

9.61 25.04 18.40 21.99 24.96

15.08 45.60 39.32

Subsequently, hypothesis were defined as follows: WH1 = It is assumed that in the employee motivation in wood-processing and forest enterprises in Slovakia, there are same motivation factors from the 30 given in top positions.

WH2 = It is assumed that the level of motivation factors in top positions mentioned by respondents in forest and wood-processing enterprises in Slovakia will be similar.

Due to the independence of sample sets, the significance of difference in the level of importance was evaluated using the two-sample t-test for independent samples when variances are equal or unequal.

Null hypotheses about the agreement of two means of two compared sets were tested.

The level of significance α = 0.05 was used in testing null hypothesis about the equality of average values of individual motivation factors (MASON – LIND 1990).

RESULTS AND DISCUSSION Following the results of perceiving the importance of motivation factors by the employees, the order of importance of motivation factors in wood-processing and forest enterprises could be determined (Table 2) and motivational programme tailored to the needs of employees could be created. When determining the motivational needs of the employees in wood-processing and forest enterprises in Slovakia, motivation factors atmosphere in the workplace, communication in the workplace, good work team, fringe benefits, work environment, supervisor’s approach, fair appraisal system, job security, basic salary and workload and type of work were preferred by the employees in wood processing enterprises.

The focus is put on factors related to relationship, finances and social needs. The employees of forest enterprises were motivated especially by the factors like fringe benefits, enough free time, basic salary, good work team, fair appraisal system, working hours, workload and type of work, job security, atmosphere in the workplace and individual decision making, i.e. mainly factors related to finances, relationship and social needs.

Tab. 2 The motivation factors as a part of employee motivation in wood-processing and forest enterprises based on average values of motivational needs.

WOOD-PROCESSING ENTERPRISES Motivation factor Atmosphere in the workplace Communication in the workplace Good work team Fringe benefits Work environment

FOREST ENTERPRISES

mean 4.33 4.20 4.19 4.18 4.17

Motivation factor Fringe benefits Free time Basic salary Good work team Fair appraisal system

154

mean 4.45 4.32 4.29 4.30 4.25

Supervisor’s approach Job security Basic salary Workload and type of work Fair appraisal system Prestige Job performance Selfactualisation Social benefits Information about performance result Opportunity to apply one’s own ability Region’s development Mental effort Competences Individual decision-making Working hours Job safety Career advancement Physical effort at work Company mission Name of the company Relation to the environment Education and personal growth Recognition Free time

4.14 4.06 4.06 4.04 4.00 4.00 4.00 4.00 3.99 3.98 3.98 3.98 3.97 3.97 3.96 3.95 3.94 3.93 3.93 3.88 3.87 3.64 3.63 3.59 3.54

Working hours Job security Atmosphere in the workplace Individual decision-making Workload and type of work Name of the company Communication in the workplace Work environment Supervisor’s approach Social benefits Information about performance result Selfactualisation Competences Relation to the environment Job performance Education and personal growth Physical effort at work Opportunity to apply one’s own ability Recognition Career advancement Prestige Company mission Job safety Region’s development Mental effort

4.28 4.26 4.23 4.23 4.27 4.21 4.19 4.14 4.13 4.12 4.12 4.12 4.11 4.11 4.10 4.09 4.09 4.06 4.04 3.99 3.92 3.87 3.87 3.84 3.82

Tab. 3 Frequency of evaluation and differences in significant motivation factors.

Fringe benefits 1 2 3 4 5 Total Work environment 1 2 3 4 5 Total Fair appraisal system 1 2 3 4 5 Total Basic salary

Absolute frequency

Relative frequency

Forest enterprises 4 0.79 15 2.97 45 8.91 128 25.35 313 61.98 505 100.00 Relative Absolute frequency frequency Forest enterprises 14 2.77 13 2.57 79 15.64 148 29.31 251 49.70 505 100.00 Relative Absolute frequency frequency Forest enterprises 3 0.59 12 2.38 71 14.06 178 35.25 241 47.72 505 100.00 Relative Absolute frequency frequency Forest enterprises

155

Absolute Relative frequency frequency Wood-processing enterprises 3 0.49 28 4.60 88 14.45 227 37.27 263 43.19 609 100.00 Absolute Relative frequency frequency Wood-processing enterprises 12 1.97 20 3.28 127 20.85 211 34.65 239 39.24 609 100.00 Absolute Relative frequency frequency Wood-processing enterprises 6 0.99 23 3.78 121 19.87 250 41.05 209 34.32 609 100.00 Absolute Relative frequency frequency Wood-processing enterprises

1 2 3 4 5 Total

9 16 68 206 206 505

Workload and type of work

Absolute frequency

1 2 3 4 5 Total

1.78 3.17 13.47 40.79 40.79 100.00 Relative frequency

10 1.64 29 4.76 126 20.69 242 39.74 202 33.17 609 100.00 Absolute Relative frequency frequency Wood-processing enterprises 11 1.81 35 5.75 143 23.48 204 33.50 216 35.47 609 100.00

Forest enterprises 3 0.59 13 2.57 69 13.66 175 34.65 245 48.51 505 100.00

Following descriptive statistics, the fact that the motivation factors corresponding with the needs of employees in forest and wood-processing enterprises are similar can be anticipated. It follows our hypothesis WH1. Further analysis is devoted to comparing the level of importance of ten most important motivation factors in wood-processing enterprises with identical motivation factors in forest enterprises using the t-test (Table 4).

Standard deviation wood-processing enterprises

t-test

p-level

4.23 4.16 4.30 4.45 4.13 4.14 4.26 4.27 4.25 4.14

Standard deviation forest enterprises

Atmosphere in the workplace 505 609 Communication in the workplace 505 609 Good work team 505 609 505 609 Fringe benefits Supervisor’s approach 505 609 505 609 Basic salary Job security 505 609 505 609 Workload and type of work 505 609 Fair appraisal system 505 609 Work environment Note: Statistically significant differences are in bold.

Mean wood-processing enterprises

Frequency wood-processing enterprises Mean forest enterprises

Motivation factor

Frequency forest enterprises

Tab. 4 Significant motivation factors in wood-processing and forest enterprises.

4.33 4.20 4.19 4.18 4.14 4.29 4.06 4.04 4.00 3.97

0.837 1.091 0.963 0.833 0.893 0.891 1.027 0.922 0.929 0.898

0.879 0.980 0.876 0.884 0.901 0.953 0.925 0.939 0.948 0.937

5.155 1.215 0.172 4.473 0.278 4.646 0.693 2.732 2.992 3.182

0.165 0.225 0.864 0.000 0.781 0.000 0.488 0.006 0.003 0.002

Following the results of the t-test, the fact that in the motivation factors fringe benefits, work environment, fair appraisal system, basic salary, workload and the type of work there are statistically significant differences in perceiving the importance of motivation between the employees in wood-processing and forest enterprises can be stated. Therefore, the hypothesis WH2 is refused.

The motivation factor fringe benefits was assigned with the highest number of points of importance most often (Table 3). Average values of the level of importance, we worked with, were dependent on the quantity of respondents’ responses at individual levels of the Likert scale. 61.98% was the highest level of importance assigned by the employees in forest enterprises. The second highest level of importance was assigned by the employees in woodprocessing enterprises – 37.27%, in comparison to 25.35% in forest enterprises.

156

4.40 4.55

4.35

4.50 4.45

4.30

4.40

Basic salary

Fringe benefits

4.25 4.35 4.30 4.25

4.20

4.15

4.20

4.10 4.15

4.05 4.10 4.05 Forest enterprises

Wood-processing enterprises

Mean Mean±StError Mean±1,96*StError

4.00 Forest enterprises

Wood-processing eneterprises

Mean Mean±StError Mean±1,96*StError

4.25 4,40

4.20 4,35

4.15

4,25

Work environment

Workload and type of work

4,30

4,20

4,15

4.10

4.05

4.00

4,10

3.95 4,05

3.90

4,00

3,95 Forest enterprises

Wood-processing enterprises

Mean Mean±StError Mean±1,96*StError

3.85 Forest enterprises

Wood-processing enterprises

Mean Mean±StError Mean±1,96*StError

4.35

4.30

4.25

Fair appraisal system

4.20

4.15

4.10

4.05

4.00

3.95

3.90 Forest enterprises

Wood-processing eneterprises

Mean Mean±StError Mean±1,96*StError

Fig. 1 Box-plots for significant differences in motivation factors in wood-processing and forest enterprises.

In the case of the motivation factor work environment, the fact that the average value of the level of motivation of investigated factor in forest enterprises was 4.18, whereby in wood-processing enterprises it was 4.17 was observed. Therefore, the fact that higher motivational power relating to the mentioned factor was in forest enterprises can be stated.

When investigating the dependence of the motivation factor name of the company, statistically significant differences between forest and wood-processing enterprises were observed.

157

The motivation factor fair appraisal system was evaluated by 47.72% of the respondents with the highest points. In wood-processing enterprises, mentioned factor was considered important by most respondents. It was assigned with four points by 41.05% of the respondents.

When evaluating the motivation factor basic salary, the fact that the mentioned factor was assigned with the two highest points of importance by the same number of respondents in forest enterprises can be stated. In the case of wood-processing enterprises, the factor was assigned with the four points by most of the employees, 39.74% of the respondents.

The analysis of the motivation factor work environment shows that there were statistically significant differences between the employees in forest and wood-processing enterprises. In forest enterprises, the mentioned factor was considered very important for the future motivation by higher number of respondents evaluating the level of motivation in comparison to the employees in wood-processing enterprises assigning the factor with 3-5 points.

Mean forest enterprises

Mean woodprocessing enterprises

Standard deviation forest enterprises

Standard deviation wood-processing enterprises

t-test

degrees of freedom

p-level

relating to mutual relationship relating to career aspiration relating to finance relating to work conditions relating to social needs

Frequency woodprocessing enterprises

Groups of motivation factors

Frequency forest enterprises

Tab. 5 Significant differences in groups of motivation factors in wood-processing and forest enterprises.

1,060

2,436

4.33

4.21

0.810

0.930

3.715

2,293.701

0.000

2,120

4,872

3.98

3.88

1.025

1.000

3.776

6,990.000

0.000

795

1,827

3.81

4.06

1.089

0.991

5.597

1,391.199

0.000

2,385

5,481

4.23

4.01

0.878

0.956

10.189

4,909.395

0.000

1,590

3,654

3.86

3.85

1.089

1.065

0.398

5,242.000

0.691

Note: Statistically significant differences are in bold.

Following the results of the research and quantity of respondents’ responses, the motivational programme for forest and wood-processing enterprises can be defined as follows: motivation factors in forest enterprises must be primarily focused on relationship and work conditions and only secondarily on the career advancement and finances. In woodprocessing enterprises the focus must be primarily put on relationship and finances and secondarily on work conditions and career advancement (Figure 2). Subsequently, specific factors affecting the level of motivation in individual branches of industry will be selected.

However, the fact that a lot of enterprises are not prepared to implement creative solutions of their managers and lots of managers are not familiar with the impact of motivation factors on the enterprise performance must be mentioned. When the managers’ skills improve and their performance optimises, trends in creating motivational programmes will be implemented in practice.

158

relating to mutual relationship relating to career aspiration relating to finance relating to work conditions relating to social needs 4.5 4.4 4.3

Values

4.2 4.1 4.0 3.9 3.8 3.7 Wood-processing enterprises

Forest enterprises

Fig. 2 Box-plots for significant differences in groups of motivation factors in wood-processing and forest enterprises.

CONCLUSION Essential part of the economy in each country is generated by small and medium-sized enterprises with their huge potential for growth and effect on stabilising the economy and regional development. They are one of the greatest driving power of regional development because of their importance in creating the jobs and business ambitions (KOVAĽOVÁ et al.

2018, BORISOV et al. 2018, NEMEC et al. 2017, LORINCOVÁ 2015, MUSOVÁ 2015, PALUŠ et al. 2015, HAJDUKOVÁ 2014, SCHULER 1992). Progress in economic growth is accompanied with the progress in human resource management and motivational programme (ZHU – WARNER 2019, LORINCOVÁ et al. 2018, STACHOVÁ et al. 2017, KAMPF et al. 2017, KRIŽANOVÁ et al. 2017, LIŽBETINOVÁ et al. 2016, POLIAČIKOVÁ 2016, GRENČAY et al. 2015, STACHOVÁ – STACHO 2015, SUCHOMEL et al. 2012, DEWETTINCK – REMUE 2011, DECI – RYAN 2008, RYAN et al. 2006, WRIGHT et al. 2001). Motivating employees in the right way is one of the prerequisite for the success of each enterprise (AYDIN – TIRYAKI 2018, MÉSZÁROS 2018).

Following the research, the fact that motivational needs associated with the most important motivation factors of the employees in forest and wood-processing enterprises are partially similar can be stated. The mentioned findings cannot be used in the case of the unified motivational programmes. The employees in wood-processing enterprises were motivated more by the motivation factors relating to relationship and finances. On the other hand, the motivation factors relating to finances and relationship were considered important by the employees in forest enterprises. At the same time, there were significant differences 159

between individual motivation factors evaluated by the employees in forest and woodprocessing enterprises. Following the results, it is clear that motivational programme in specific branches of industry must be created differently.

As creating the motivational programme is very hard and money consuming activity, it must be effective with positive impact on the enterprise economy. Therefore, systematic, thorough analysis of motivational needs of employees is necessary. At the present time, motivational programme can be created following the average importance of individual motivation factors in individual branches of industry in a unified way. In the future, employees’ requirements can change. Therefore, the motivational programme should be updated regularly according to the needs of an enterprise. At the same time, the fact that further research studies into motivational needs of employees in terms of gender, age, education, job position and seniority must be carried out can be stated, i.e. further research into this area focused on regional or international differences is expected.

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ACKNOWLEDGEMENT This research was supported by VEGA No. 1/0024/17 Computational model of motivation, VEGA No. 1/0115/20 Dependence of corporate culture type upon selected socio-demographic factors and industries in Slovak enterprises, and APVV-16-0297 Updating of anthropometric database of Slovak population.

ADDRESSES OF THE AUTHORS doc. Ing. Miloš Hitka, PhD. Ing. Martina Lipoldová Mgr. Jarmila Schmidtová, PhD. Technical University in Zvolen T. G. Masaryka 24 960 53 Zvolen hitka@tuzvo.sk xlipoldovam@tuzvo.sk jarmila.schmidtova@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 165−176, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.14

IMPACTS OF BEHAVIORAL ASPECTS ON FINANCIAL DECISIONMAKING OF OWNERS OF WOODWORKING AND FURNITURE MANUFACTURING AND TRADING ENTERPRISES Mariana Sedliačiková – Zuzana Stroková – Jarmila Klementová – Anna Šatanová – Mária Moresová ABSTRACT Behavioral finance is an area or sub-discipline of behavioral economics examining real financial behavior and decision-making of people, including the knowledge of psychology and sociology. The objective of the paper is to identify and investigate the impact of significant cognitive, psychological and emotional factors affecting the financial decisionmaking of owners of woodworking, furniture manufacturing and trading enterprises. The mapping of the addressed issue was carried out by means of an empirical survey in the practice of Slovak woodworking and furniture manufacturing and trading enterprises in the form of a questionnaire. The results of the survey were evaluated by descriptive, graphical and mathematical-statistical methods. Conclusions and recommendations were formulated based on the identification of key behavioral aspects (knowledge, security, freedom and sadness), their implementation could contribute to eliminate negative deviations and errors in the financial decision-making process of owners of woodworking and furniture manufacturing and trading enterprises.

Key words: behavioral finance, behavioral biases, woodworking enterprises, furniture manufacturing and trading enterprises, owners.

INTRODUCTION Decision making is one of the basic cognitive processes of human behavior by which a preferred option or a course of actions is chosen from among a set of alternatives based on certain criteria (WANG and RUHE 2007). Financial decisions are among the most important life-shaping decisions made by people. Because of cognitive constraints, many household decisions violate sound financial principles. Households typically have underdiversified stock holdings and low retirement savings rates. Investors overextrapolate from past returns and trade too often. Even top corporate managers, who are typically highly educated, make decisions that are affected by overconfidence and personal history. Many of these behaviors can be explained by well-known principles from cognitive science (FRYDMAN and CAMERER 2016).

Behavioral finance is an area or sub-discipline of behavioral economics that examines real behavior and decision-making of people and investors in the field of finance, including the knowledge of psychology and sociology (BALÁŽ 2009, BIKAS et al. 2013, AHMAD 2017,

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KAPOOR and PROSAD 2017, VALÁŠKOVÁ et al. 2019). According to ASAB et al. (2014), MADAAN and SINGH (2019), behavioral finance explains our action and behavior but modern finance is related to the explanation of actions of an economic man. According to MUSA et al. (2016), traditional finance is related to decisions in which full information are available for making investment decision.

In order to understand and explain individual decision making and investment behavior, it is necessary to study behavioral factors which impact it. Various scholars have studied factors of financial behavior and their impact on financial decision making, and in particular a special focus has been given to psychological biases. Usually investors are not aware of their behavioral biases. If investors become conscious of biases they can face, they can act more rationally (SEDAGHATI 2016, DERVISHAJ 2018). According to REHAN and UMER (2017) and BENČIKOVÁ et al. (2013), behavioral biases include both cognitive biases (such as anchoring, representativeness, mental accounting and availability) and emotional biases (such as risk aversion, overconfidence and regret aversion). TVERSKY and KAHNEMAN (1974) described in their article “Judgment under Uncertainty: Heuristics and Biases” the systematic errors in thinking of ordinary people, while analyzing the origin of such errors in the cognitive mechanism. They found out that emotional and psychological factors were the source of a change in the behavior of the subjects, but only when these were borderline situations of decision-making and getting to know something unknown. In that case, this could also be applied to the targeted subject of decision-making, i.e. owners of woodworking (WW) and furniture manufacturing and trading (FMT) enterprises.

According to POMPIAN (2006), standard economic theory is designed to offer mathematically structured solutions and to perceive the human being as an economically rational subject. They are based on idealized financial behavior. Behavioral finance, in turn, tries to emulate the phenomenon of the human psyche and is based on observed behavior.

AKERLOF and SHILLER (2010) and CHAFFAI and MEDHIOUB (2014) state that behavioral finance has originated as a new trend in economics and focuses on the economic aspects of deviations from the rational behavior of subjects, especially the impact of cognitive distortions, psychological and emotional condition of the subject. Factors of human behavior influence decision-making and disable to receive rationally new information through emotional action.

The most significant psychological, emotional and cognitive factors are (DOLAN 2002): - love, hatred, sadness, happiness, powerlessness, panic, depression, desperation, anxiety (emotional); - knowledge, expertise, concentration, recognition ability, logical thinking, human character, short-term and long-term memory process (cognitive); - power, security, certainty, personality, shame, self-esteem, freedom, selfrealization, friendship, health, attractiveness (psychological).

According to HITKA et al. (2019) and LORINCOVÁ et al. (2019), the quality of human potential plays an important role and it is a key factor that affects the running of a company, its prosperity, as well as sustainable development. Currently, when advances in technology, information, and globalization occur most often, the human factor is becoming the biggest competitive advantage in woodworking enterprises.

The Slovak Republic is relatively independent of importing the natural resources inputs, being built on a domestic resource base of sustainable character, and therefore it is able to permanently show active balance of foreign trade. In relation to the positive situation related to natural resources, their suitable geographic location, and their acceptable energetic demands for processing wood, woodworking industry represents an important field of industry for the Slovak national economy, while thus enabling further development of small 166

and medium enterprises (HAJDÚCHOVÁ et al. 2016). Woodworking industry is composed of the wood, furniture, and cellulose-paper industries. These are based on processing wood, i.e. domestic ecological resource (POTKÁNY et al. 2018).

The objective of the paper is to identify and investigate the impact of significant cognitive, psychological and emotional factors affecting the financial decision-making of the owners of WW and FMT enterprises.

METHODOLOGY The research was focused on the analysis of the current situation of the issue concerning behavioral aspects that influence the financial decision-making of owners of WW and FMT enterprises. The data collection was carried out by means of a questionnaire survey focused on the WW and FMT enterprises operating in Slovakia. The first part of the questionnaire included demographic data, the aim of which was to differentiate the respondents according to the size of the enterprise (micro, small, medium and large), type of enterprise (production and non-production sector), length of time on the market (less than 1 year, less than 5 years, less than 15 years and more than 15 years) and the job position in the enterprise (employee, owner, manager). The second part of the questionnaire contained questions aimed at the expressing agreement or disagreement of the respondents with statements in the field of cognitive, psychological and emotional factors. Respondents expressed their opinion using five-step rating scale (-2 – very negative, -1 – negative, 0 – don´t know, 1 – positive, 2 – very positive) for each cognitive, psychological and emotional factor. The aim was to find out which behavioral factors have a significant impact on owners´financial decision-making.

The whole sample consisted of all organizations and entreprises operating in the Slovak Republic, i.e. 559,841 active economic subjects (Slovak Business Agency, 2019).

The random and purposive sampling was used for the selection of respondents into the selected sample. The purposive sampling was used for the selection of WW and FMT enterprises. Respondents were addressed through electronic forms (questionnaires) sent directly to their addresses. Subsequently, the sample size was defined using a mathematical relationship to calculate the minimum number of respondents to be involved in the survey (KOZEL et al. 2006):

𝑛≥

(𝑧 2 ×𝑝×𝑞) ∆2

→≥

(1.962 ×0.5×0.5) 0.052

→ 𝑛 = 384

(1)

n – minimum number of respondents; z – coefficient of reliability (z=1.96 =>the reliability of the research reaches 95.0%); p and q - the percentage of questioned respondents (the extent of knowledge of respondents with regard to the problem is unknown, the whole sample is divided in half, i.e. p and q = 50%); ∆ - maximum acceptable error (the value of maximum acceptable error was determined at 5%).

Out of the total number of 2.549 respondents, 453 respondents participated in the questionnaire survey. In order to keep the contextual framework of the paper, the evaluation of the survey results focused on the 412 owners of WW and FMT enterprises. Figure 1 presents the percentange of respondents according to the job position in the enterprise. Out of the total number of respondents, 91% were owners, 5% managers and 4% employees. On this basis, it was necessary to exclude 41 respondents from the sample, i.e. 23 managers and 18 employees.

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4% 5%

owner manager employee

91%

Fig. 1 Proportion of respondents according to the job position.

Two research questions (RQ) were formulated within the research area: RQ1 – What emotional factors are the source of cases when people change their behavior and deviate from rationality?

RQ2 – Which key cognitive, psychological and emotional factors influence the rational decision-making behavior of owners of WW and FMT enterprises?

Based on the research questions and the available literary sources (TVERSKY and KAHNEMAN 1974, DOLAN 2002, POMPIAN 2006, AKERLOF and SHILLER 2010, CHAFFAI and MEDHIOUB 2014) four hypothesis were formulated as follows: H1 = It is assumed that people are rational and their behavior corresponds to that. Emotions such as fear, love and hatred are the source of cases when people change their behavior and deviate from rationality.

H2 = It is assumed that the key factor that most influences the rational decision-making behavior of owners of WW and FMT enterprises is the expertise as a cognitive factor.

H3 = It is assumed that the key factor that most influences the rational decision-making behavior of owners of WW and FMT enterprises is the certainty as a psychological factor.

H4 = It is assumed that the key factor that most influences the rational decision-making behavior of owners of WW and FMT enterprises is the happiness as an emotional factor.

The results of the survey were processed and evaluated with statistical software STATISTICA 10. Testing was performed at the significance level α = 0.05. Graphical and descriptive methods were applied in order to evaluate the hypothesis H1. Pearson´s Chisquare test and contingency coefficients (Cramer´s V and Pearson´s contingency coefficient C) were used to evaluate the hypothesis H2, H3 and H4.

RESULTS AND DISCUSSION It terms of the enterprise size, the structure of the research sample consisted mainly of small enterprises (41%), medium enterprises (28%) and micro enterprises (25.1%). Large enterprises represented the lowest proportion (5.90%). Enterprises operating in the production sector constituted 65% and the rest was represented by the non-production sector (woodworking and furniture trading enterprises). With regard to the time on the market, the enterprises operating for less than 15 years (42%) and more than 15 years (34%) presented the biggest proportion. Enterprises operating on the market for less than 5 years constituted 168

17% and the rest 7% was represented by enterprises with less than 1 year on the market.

Figure 2 presents the findings concerning the change in behavior of owners of WW and FMT enterprises in borderline (unknown) situations by expressing their attitudes to the statement using five-step rating scale: Emotions such as fear, love and hatred are the source of cases when people change their behavior during borderline (unknown) situations. A very positive attitude with this statement was expressed by 18.3% of respondents and half (51.0%) respondents indicated a positive attitude. 22.9% of the respondents were not able to express themselves clearly and a total of 7.90% of respondents expressed negative or very negative attitudes. It follows that these owners do not consider specific emotions to be the cause of changes in their behavior. The hypothesis H1 has been confirmed by the graphical evaluation, i.e. the assumption that people are rational and their behavior corresponds to that. Emotions such as fear, love and hatred are the source of cases when people change their behavior and deviate from rationality.

60% 51.0% 50% 40% 30% 20%

22.9% 18.3%

10%

5.9% 2.0%

0% very positive

don´t know

positive

negative

very negative

Fig. 2 Emotions of fear, love and hatred as sources of behavioral change.

Another part of the results was focused on investigating the impact of cognitive, psychological and emotional factors on the behavior of owners in decision-making situations. Respondents could select several aspects belonging to groups of cognitive, psychological and emotional factors. Figure 3 presents the impact of cognitive factors on the rational behavior of owners. Out of cognitive factors, expertise has the biggest impact on rational behavior in the decision-making process of owners (72.7%). Other significant cognitive factors are knowledge (67.6%), logical thinking (56.7%) and character of the owners (54.3%).

67.6%

knowledge expertise

72.7%

concentration

30.2% 33.6%

recognition ability logical thinking

56.7% 54.3%

human character short-term and long-term memory processes

27.3% 0%

20%

40%

60%

80%

Fig. 3 Impact of cognitive factors on the rational behavior of owners of WW and FMT enterprises.

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Out of the psychological factors, it is the certainty (73.5%) that has the most significant impact on rational behavior of the owners. Security (67.6%), personality (49.1%) and self-realization (46.2%) have also reached a considerable impact. Figure 4 presents the significance of individual psychological factors.

power

38.0% 67.6%

security certainty

73.5% 49.1%

personality shame

22.9% 33.8%

selfesteem freedom

38.4% 46.2%

self-realisation friendship

30.4%

health

26.0%

attractiveness

23.0% 0%

20%

40%

60%

80%

Fig. 4 The impact of psychological factors on the rational behavior of owners of WW and FMT enterprises.

Happiness (76%) is the most important factor influencing the rational behavior of the owners in terms of emotional factors. Sadness (67.4%) as a counterpart of happiness is the second significant emotional factor. Love and hatred ranked among the third important factors. The significance of individual emotional factors for the rational behavior of owners is presented in Figure 5.

love

40.2%

hatred

39.0% 67.4%

sadness hapiness

76.0%

powerlessness

33.6%

panic

36.5%

depression

37.3%

desperation

34.8%

anxiety

26.2% 0%

20%

40%

60%

80%

Fig. 5 The impact of emotional factors on the rational behavior of owners of WW and FMT enterprises.

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In relation to the second research question, the graphical evaluation of the impact of behavioral (cognitive, psychological and emotional) factors has revealed that the key factors that most influence the rational decision-making behavior of owners of WW and FMT enterprises are the expertise as a cognitive factor, certainty as psychological and happiness as an emotional factor.

In the next part of the paper, the impact of cognitive, psychological and emotional factors on the financial decision-making process were pointed out through mathematicalstatistical analysis.

Based on the p level (0.00824), the null hypothesis in favor of the alternative hypothesis was rejected, implying a dependence between the influence of knowledge as a cognitive factor and owners of WW and FMT enterprises (Table 1). Less than 20% of theoretical frequencies were less than the value 5, thus the condition of good approximation was fulfilled. The value of the Cramer´s V reached the level 0.1526185, which pointed out the existence of a weak dependence between the respondent's job position and the impact of behavioral aspects on the financial decision-making process.

Tab. 1 Dependence between A4 (job position) and B7a (knowledge).

Statistics

Statistics: Owners x knowledge Chi-square

Degrees of freedom

p

Pearson´s Chi-square test

9.596477

df=2

p=.00824

Contingency coefficient

.1508716

Cramer´s V

.1526185

Based on the statistical data evaluation of the impact of security on internal interest groups, it was possible to confirm the interdependence between individual variables (Table 2). Due to the value of p level (0.00000), the null hypothesis was rejected in favor of the alternative hypothesis with confirmation of the existence of dependence between the owners of WW and FMT enterprises and security as a psychological factor. Also, in this case, the condition of good approximation was fulfilled (less than 20% of theoretical frequencies were less than 5). The contingency coefficient with the value of 0.2383203 and the Cramer´s V 0.453908 pointed out the existence of weak dependence between the job position of the respondents and behavioral aspects on financial decision-making process.

Tab. 2 Dependence between A4 (job position) and B7b (security).

Statistics: Owners x security Statistics

Chi-square

Degrees of freedom

p

Pearson´s Chi-square test

24.80926

df=2

p=.00000

Contingency coefficient

.2383203

Cramer´s V

.2453908

By comparing the p level (0.01817) with the selected significance level (α = 0.05), the null hypothesis was rejected (Table 3). The rejection of the null hypothesis in favor of the alternative hypothesis confirmed the existence of dependence between freedom as a psychological factor and owners of WW and FMT enterprises. The condition of good approximation was fulfilled also in this case. The contingency coefficient with the value of 0.381510 and the Cramer´s V 0.1394885 showed a weak dependence between the variables.

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Tab. 3 Dependence between A4 (job position) a B7b (freedom).

Statistics: Owners x freedom Statistics

Chi-square

Degrees of freedom

p

Pearson´s Chi-square test

8.016305

df=2

p=.01817

Contingency coefficient

.1381510

Cramer´s V

.1394885

Based on statistical observation, the p level was lower than the significance level (0.00000<0.5), which led to the rejection of the null hypothesis in favor of the alternative hypothesis, i.e. the existence of dependence between the sadness as an emotional factor and owners of WW and FMT enterprises (Table 4). Out of the statistical results, less than 20% of theoretical frequencies were less than 5, thus satisfying the condition of good approximation. The contingency coefficient with the value of 0.3087603 and the Cramer´s V 0.3246214 pointed out the existence of medium strong dependence between the variables.

Tab. 4 Dependence between A4 (job position) a B7c (sadness).

Statistics: Owners x sadness Statistics

Chi-square

Degrees of freedom

p

Pearson´s Chi-square test

43.41616

df=2

p=.00000

Contingency coefficient

.3087603

Cramer´s V

.3246214

Mathematical-statistical analysis of the impact of behavioral (cognitive, psychological and emotional) factors has confirmed the hypothesis H2, H3, and H4. These include knowledge (cognitive factor), security and freedom (psychological factor), sadness (emotional factor). The key behavioral aspects have contributed to the formulation of conclusions and recommendations aimed at the elimination of systematic errors in the financial decision-making of owners of WW and FMT enterprises.

Knowledge is the most valuable factor for the owners. It is considered the ability to use the expertise effectively to reach profit and growth. By applying, training and trying of expertise, it is possible to gain experience gradually and so increases the level of knowledge (BENČIKOVÁ et al. 2019). The lack of knowledge leads to subsequent errors in the first phase of decision making - lack of necessary information. OMARLI (2017) confirms in his study that intelligence, cognitive style, age, experience and level of knowledge play an important role in the decision-making process.

Security makes an everyday part of our lives. The owners of enterprises regularly make decisions based on subjective and objective factors to feel safe and that nothing would jeopardize the security of the enterprise. In the context of business activities, we encounter various security threats that negatively affect the functioning of the enterprise. As a result, fear and uncertainty prevail among most owners and entrepreneurs. With regard to fear and uncertainty, it is extremely important to set the objectives correctly at every decision-making and determine the likelihood of threats based on knowledge. Relevant information and sufficient knowledge and experience of the decision makers applied to solve the decisionmaking problem, help to specify the likelihood of occurrence of certain phenomenon and 172

also to identify the potential consequences of different decisions. The assessment of likelihood is extremely important since its task is to determine the extent, i.e. quantify the considered uncertainty. Under the conditions of uncertainty, the owners of enterprises can use a lot of historical data to determine likelihood of individual variants. In case they have no relevant information, the solution variants would not contribute to solving the occurred problem in the decision-making process. MERIGO (2015) confirms that our contemporary world is significantly influenced by various kinds of uncertainty. Some decisions are made intuitively and just with partially available information. There is no optimal decision-making model because the majority of decisions are made under the conditions of uncertainty.

Freedom of choice and decision-making are inseparable parts of a human being. Every entrepreneur wants to be own boss in the business, make decisions based on own convictions and experience, and so boldly realize the set goals. The owners (entrepreneurs) become often responsible for many other people who they manage and work with. Despite the fact that most owners (entrepreneurs) have a supportive network of people they can consult when making difficult and important decisions, the final decision is in the hands of enterprise owners. The meaning lies in the accepting responsibility for the decisions. LAU and HIEMISCH (2017) point out in their study that the freedom of making decisions has not been so far sufficiently described as a psychological variable. They presented a model of functional freedom of decision-making, i.e. inner ability to shape consciously complex decisions according to their own values and needs. Functional freedom is greatest when the decision-maker is rational, the structure of the decision is very vague and the decisionmaking process is based on conscious thinking and reflection.

Sadness as an emotional factor has an important impact on the behavior of owners of WW and FMT enterprises and in many cases, it hinders rational thinking. Whereas it invokes negative emotions in people, it absorbs the thinking. It this case it is possible to assume that the owners would make incorrect decisions in the first phase and the feedback can reveal that the choice of the variant was not correct. SHU et al. (2016) confirm the impact of sadness on the rational behavior in their study so that the sadness makes people (managers, owners) risk averse, less patient and more sensitive to negative experiences.

Many scholars (VIRLICS 2013, FRANCO and SANCHEZ 2016, KONSTANTINIDIS et al.

2018, TUR-PORCAR et al. 2018) confirmed that the effect of psychological, cognitive and emotional factors on an individual´s decisions is substantial and fundamental.

Owners of WW and FMT enterprises should accept individual behavioral aspects that have a significant impact on their decision-making. It is, therefore, necessary to understand the meaning of the impact of individual factors on their decision-making process in the enterprise and thus in time prevent the negative effect on their rational thinking. For each enterprise owner, victory means not just to defeat and dominate the market, but above all to win over themselves in terms of controlling their emotions, seeking independent thinking and resilience to the environment. According to KORTE (2003) and NIKOLIĆ (2018), important activities that could prevent systematic errors in financial decision-making may include to understand and avoid psychological deviations; to acquire sufficient experience and knowledge; not to overlap private life with the professional; to understand own deficiencies and avoid disturbances; early identification of potential risks and threats; backward look at strategy and reorganization; sufficient time for making important decisions; not making hasty decisions; and listen and be open to the opinions of others.

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CONCLUSION Entrepreneurship is a long-term, demanding process that requires a high level of involvement of enterprise owners. The performance of enterprise owners depends principally on their knowledge and experience. It is important to make the right decision and to have a sense of responsibility. It is essential to be able to communicate, feel empathetically, listen, be tolerant to others and have the ability of self-control at coping with stressful situations. Behavioral factors – knowledge, security, freedom and sadness inherently affect the behavior of owners at financial decisions in the enterprise.

Lack of experience and knowledge of business can lead to incorrect financial decisions that could jeopardize the existence of the business itself. Security is essential in terms of product quality and creates conditions for fulfilling the functions of the enterprise and achieving the set objectives in a stable environment. Freedom is the power of choice that is used to make important financial decisions. Sadness has a significant impact on owner's behavior change, and this effect can manifest itself in a positive way (positive thinking, avoiding negative and risk factors, talks) or in a negative way (failure to meet business objectives, worsening reputation, jeopardizing business existence).

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ACKNOWLEDGEMENT The paper has been written as a partial result of the projects APVV-18-0520, APVV-18-0378, APVV-17-0456 and APVV-17-0583.

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ADDRESSES OF THE AUTHORS doc. Ing. Mariana Sedliačiková, PhD. Ing. Zuzana Stroková, PhD. Ing. Jarmila Klementová, hD. Ing. Mária Moresová, PhD. et PhD. Technical University in Zvolen Department of Economics, Management and Business T. G. Masaryka 24 960 01 Zvolen Slovakia sedliacikova@tuzvo.sk strokova@tuzvo.sk klementova@tuzvo.sk moresova@tuzvo.sk prof. Ing. Anna Šatanová, CSc. The College of International Business ISM Slovakia in Prešov Duchnovičovo námestie 1 080 01 Prešov satanova@ismpo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(1): 177−188, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.1.15

IDENTIFICATION OF CUSTOMERS’ DRIVERS FOR THE WOOD BUILDING AS AN ECOLOGICAL INNOVATION IN BUILDING CONSTRUCTION IN SLOVAKIA Erika Loučanová – Miriam Olšiaková ABSTRACT Sustainable development became a discussed issue and its main idea is to find out how to satisfy present needs without compromising the capacity of future generations, guaranteeing the social well-being, balance among economic growth, and care for the environment. The building industry as one of the industries trying to fulfil the consumer demands connected with environmental and health requirements is presented in this paper.

It deals with the importance of innovation in the building construction materials for sustainability focusing on wood structures representing eco-innovation and an alternative to silicate building materials in this context. A comparison of conventional building materials and innovated alternative ecological option used in the building construction (wood-framed house) is given in the paper. The primary method for assessing the perception of these options was the Kano model. Subsequently, the customers´ drivers for wooden structures in Slovakia were identified by methods such as correlation matrix and force field of innovation.

Key words: ecological innovation, building materials, Kano model, sustainable growth.

INTRODUCTION The issue of sustainable development deals with the economic growth regarding the requirements of the society by creating the welfare conditions in short term, medium term as well as in long term period. Economic and demographic development naturally increases demand for natural resources. The approaches to the corporate social responsibility that are constantly innovated encourage the continual commitment to participate in the sustainable development by ecological innovations of individual key components adjusted to customers and trends in world markets when applying the principles of sustainable development (LOUČANOVÁ et al. 2015, ŠTERBOVÁ et al. 2016, PAROBEK et al. 2015, LOUČANOVÁ et al. 2017).

Current lifestyle trends, especially in the area of housing, point to a preferred return to a more natural, more personalized housing that is provided by family houses against the impersonal and often restrictive housing that is typical for living in a block of flats. Changes in preferences can be met through a wide range of options. The traditional and longestablished types of construction materials (e.g. brick and concrete) are supplemented by modern, innovative, and viable construction alternatives. One of the most popular alternatives is wood. Modern methods of construction promote the idea and application of environmentally and energetically efficient constructions. Just as the construction process itself significantly contributes to the depletion of natural resources, the production of 177

construction materials contributes to significant environmental pollution and greenhouse emissions (particularly CO2) (ŠVAJLENKA and KOZLOVSKÁ 2018).

Sustainable construction is a chance how the building industry can contribute to sustainable development. This idea lies in transforming the demand for sustainable development into an opportunity, creating and breaking into new markets, and innovating responses that satisfy traditional demands in the industry and the new societal demands for sustainable development (ŠTEFKO et al. 2013, MITTERPACH and ŠTEFKO 2016, BOURDEAU 1999). According to data in the European Union (EU) the building sector contributes to 42 % of final energy consumption, 35 % of total GHG emissions, 50 % of the utilization of extracted materials, and 30 % of water consumption (European Commission, 2011).

Following the mentioned figures, we can state that construction and housing play a fundamental role when aiming at enhancing societal goals for sustainable development. By developing the construction and utilization of buildings in the EU, it is claimed that the total final energy consumption could be decreased by approximately 40 %, total greenhouse gas (GHG) emissions by 35 %, and the use of building materials by 50 % (HERCZEG et al. 2014, OLŠIAKOVÁ et al. 2017).

The environmental requirements for innovative product management in relation to corporate social responsibility (CSR) based on the principles of sustainable development are implemented by the environmentally oriented management program (KOLLÁR and BROKEŠ 2005). Dealing with environmentally oriented management of product portfolio, it is necessary to apply activities that allow overcoming the conflicts among market, society and environment by ecological innovation. This is the reason why the companies try to improve the environmental performance of their products. The company regards also social and economic aspects when it considers environmental behaviour to ensure safer product for customers when creating the added value of the product (KALAMÁROVÁ et al. 2014, OLŠIAKOVÁ et al. 2016, PAROBEK et al. 2016, PALUŠ et al. 2018, HÄKKINEN 2007).

HURMEKOSKI et al. (2015) states that environmental impacts of construction practices are considered in the context of material renewability and recyclability, as well as within the possibilities for choice of construction material with regard to the climate change mitigation.

Sustainable development is a much discussed issue in considering the acceptability and efficiency of solutions based on building and living preferences. There is a significant environmental change in societal values toward sustainability and sustainable development (e.g. AUTIO et al. 2009, PÄTÄRI et al. 2016). The values are reflected in the customer's purchasing behaviour. This idea can be also applied in the area of household preferences.

The use of wood in residential as well as in non-residential constructions has increased in previous years, but it is not used traditionally at all (TOPPINEN et al. 2016). There are different approaches from the perspective of architects and customers regarding the specific country.

ROOS et al. (2010) found out that architects and structural engineers in Sweden prefer wood because of its strength, environmental friendliness, easy handling, and appropriateness for use in conjunction with other materials. HEMSTRÖM et al. (2011) assessed the perceptions, attitudes, and interest of Swedish architects in wood using. They detected that architects and contract managers also associate it with several disadvantages and uncertainties, primarily with respect to fire safety, stability, durability, and acoustic properties.

The attitudes of architects towards wood were also reviewed in papers by O’CONNOR et al. 2004, BAYNE and TAYLOR 2006, BYSHEIM and NYRUD 2009, ROBICHAUD et al. 2009, KAPUTA and PALUŠ 2014, MAŤOVÁ and KAPUTA 2018.

Consumers and also construction material companies consider the environmental quality of wood to be important (TOIVONEN 2011, 2012). In the study by TOPPINEN et al.

(2013), elements related to the environmental sustainability of wooden products in housing, the social acceptability of products, and the aesthetic characteristics of wood can all be 178

associated with a distinct consumer lifestyle, consisting of a complex interplay among consumer backgrounds, values, and behaviour. According to TOIVONEN and HANSEN (2003), wood is additionally an attractive material compared to many other materials. However, environmental quality is typically not the main quality attribute driving consumers or organizational customers in construction materials choice.

Although the consumer perceptions of the environmental quality of wooden products can be identified and logical (TOIVONEN 2012), the practical meaning of environmental attributes can still be vague for the majority of consumers. In a study by HOIBO et al. (2015) from Norway, younger people with strong environmental values were found to be the best target for increasing wood-based urban housing. The domestic origin of wood materials has been found to associate with environmental quality in Europe (RAMETSTEINER 1998), and also in particular in Finland (TOIVONEN 2012). Also in other contexts, the environmental quality of wood has been found to connect with consumer willingness to buy and even to pay premiums for products of higher environmental quality (HANSMANN et al. 2006, O’BRIEN and TEISL 2004). Overall, consumer knowledge probably is yet likely to be relatively low when it comes to building materials impact on human health (KEITH 2011).

The results of this paper present growing differences in Slovak consumer behaviour considering the construction material. They mainly focus on chosen parameters representing their satisfaction but also the dissatisfaction in case their requirements are not met. This paper is aimed at identifying the customers´ drivers for wood buildings as an ecological innovation from the point of view of sustainable development of building construction in Slovakia.

MATERIALS AND METHODS The Kano model is the principal applied method of the research aimed at the identification of customers’ drivers of the wood building as an ecological innovation from the point of view of sustainable development of building construction in Slovakia is. The data were analysed according to the methodology of CHEN et al. (2010), LOUČANOVÁ (2016), DUCÁR et al. (2006), ULLAH and TAMAKI (2011) and LOUČANOVÁ et al. (2015).

GOODPASTURE (2003), TOMEK and VÁVROVÁ (2009) and TROMMSDORFF and STEINHOFF (2009) applied the Kano model for monitoring customers’ views regarding the requirements of the observed object which is elementary in a thorough understanding by customers.

To implement the Kano model, we took the following steps: 1. We identified the main requirements of the consumer: perception of wood housing, fire safety, lifetime, construction, thermal insulating properties, sound insulation, housing costs, price and quality of wood building.

2. We prepared a Kano questionnaire respecting the Kano model rules: according to prequeried customer requirements; a positive and negative question is formulated to each single requirement. The respondents could answer within the scope of the Likert scale.

3. We set a sample of respondents. The validity of the survey was determined by the methodology for respondents’ sample calculation: 𝑛 =

𝑍12 𝛼/2 ∗𝑆 2

where: Z12 α/2 - required confidence level H - margin of error s - standard deviation 179

𝐻2

(1)

The sample of respondents was determined at the confidence level of 99 %, with a tolerance error of +/- 5 % of the standard deviation of 0.5, which at the given data represented the value of 665.64, i.e., 666 respondents. Finally, 990 respondents were interviewed and the results according to a confidence level, standard deviation, and margin of error were relevant.

4. Evaluation of results and their interpretation: for each variable, individual responses to the positively and negatively asked question (statement) by the Kano cross rule (Table 1) were individually evaluated to specify the requirements for chosen types of building constructions. This approach classifies individual measured variables into requirements: mandatory (M), one-dimensional (O), attractive (A), irrelevant (I) or questionable (Q).

Tab. 1 Kano Model to evaluate customer requirements.

Positively formulated question

Strong agree Strong agree Partially agree Neutral attitude Partially disagree Strong disagree

Q R R R R

Negatively formulated question Partially Neutral Partially agree attitude disagree A A A I I I I I I I I I R R R

Strong disagree O M M M Q

Source: DUCÁK et al. 2006

Individual categories of product requirements affecting the customer satisfaction can be characterized by CHEN et al. (2010) as follows: Mandatory requirements (M) are obligatory requirements that customers consider as normal and are automatically expected. These requirements can be identified as primary or basic. Customers deal with them only in the case of non-compliance. Identifying them is an elementary importance mainly because even though their fulfilment is reflected in customers’ satisfaction, their deficit and failure is reflected in customers’ dissatisfaction immediately as they realize it.

One-dimensional requirements (O) are represented by those product attributes that lead to fulfilment and satisfaction or in the case of non-compliance to customers’ dissatisfaction, i.e., the higher degree of compliance with these requirements is, the more satisfied customers are, but compared to the mandatory requirements customers automatically do not expect them.

Attractive requirements (A) have a clear impact on customers’ satisfaction because they are requirements that customers did not expect. If attractive requirements are not met, it does not reflect customer dissatisfaction.

Reverse requirements (R), in some literature (DUCÁR et al. 2006, ULLAH and TAMAKI 2011) also called contradictory or exactly opposite, represent product attributes where customers react oppositely.

Irrelevant requirements (I) do not have any influence on customers. This category involves the attributes that are not critical for customers and their presence or absence does not affect their satisfaction or dissatisfaction (DUCÁR et al. 2006).

In addition to the above mentioned categories of product requirements, the Kano model also identifies the inconclusive, respectively questionable requirements (Q). Those represent a controversial outcome, which relates either to incorrectly formulated questions or it is caused by lack of understanding by customers.

Based on the identified customers´ requirements there were determined pro-innovative and anti-innovation forces for wood building that are presented in innovative force field of wood building.

180

Identified customers’ requirements were divided into groups and redistributed considering the shares of respondents’ sample in percentages. In order to generalize and determine individual dependencies among identified properties of houses and better knowledge of customer´s requirements, data from the database were evaluated by statistical methods. The degree of dependence among individual variables (identified characteristics of chosen types of building constructions) was determined through a correlation coefficient.

Its interpretation was carried out according to CHRÁSKA (2000), who describes the dependence among individual variables as positive from the limit from 0.20 to 1 (where the growth of the given variable causes the growth of the dependent variable), the opposite (negative) dependence from 0.2 to 1. COHEN (1988) presented these values for the interpretation of correlation coefficients in psychological research - a scale of correlation (in absolute value) below 0.1 was trivial, 0.10.3 small, 0.30.5 medium and above 0.5 large.

A correlation of 0.70.9 is often reported as very large and 0.91 as almost perfect.

RESULT AND DISCUSSION A database of gathered data was processed after the survey application by the Kano questionnaire. Within the demographic data, we focused mainly on gender, age and education level of respondents.

From the database of gathered data related to our survey, we evaluated the individual answers for each question by cross rule of the Kano model using the Kano table which is presented in the paper methodology. The determined properties were subsequently specified as one-dimensional (O), attractive (A), mandatory (M), questionable (Q), reverse (R) and indifferent (I) requirements. Their detailed specification is also described in the methodology.

Regarding the values presented in Table 2 which are based on the KANO model, we found out that the perception of wood buildings had no influence on respondents; actually they perceived this concept contradictory (44.44 %). It means that respondents had exactly opposite requirements representing the features of a competitive product of silicate building materials.

Respondents also perceived contradictory the requirements for the lifetime of these constructions, which were agreed by almost half of the respondents (49.49 %). Other requirements such as fire safety, wood building construction, thermal insulating properties, sound insulation, housing costs, price and wood building quality did not affect at all satisfaction, respectively customer dissatisfaction. It means that these were requirements that were not decisive for the customer and he is not interested in whether they are or they are not met.

On the contrary, respondents in Slovakia perceived silicate building materials much more positively. Especially the critical point of wood buildings - their lifetime - was perceived as an attractive requirement (40.4 %) in silicate building materials. It means that this requirement had clear impact on customers' satisfaction. The quality of these buildings was equally attractive for Slovak respondents (40.41 %). The price presented a mandatory requirement of silicate building materials and it was considered to be standard and automatically expected part of a product by customers (69.7 %). These requirements could be identified as primary or basic. Customers dealt with them only in the case of noncompliance. Their identification had an elementary importance because their fulfilment was reflected in customers' satisfaction, their deficit and failure was reflected in customers' dissatisfaction immediately as they realized it. Fire safety, construction, thermal and acoustic properties and the cost of living in a house made of silicate building materials were 181

requirements that did not affect customers and so they wee considered to be insignificant.

Based on the above findings, we can present these results by an innovative force field for wood building, which shows a significant prevalence of anti-innovation forces over for innovation force for the wood building.

Attributes

Requirements OneAttractive Mandatory Irrelevant Questionable Reverse dimensional Multiplicity Abs. Relat. Abs. Relat. Abs. Relat. Abs. Relat. Abs. Relat. Abs. Relat.

Silicate building materials

Wood building

Perception of 60 6.06 wood houses Fire safety 20 2.02 Lifetime 70 7.07 Construction 30 3.03 Thermal insulating 160 16.16 properties Sound 40 4.04 insulation Housing costs 130 13.13 Price 120 12.12 Quality 130 13.13 Fire safety 160 16.16 Lifetime 400 40.4 Construction 270 27.27 Thermal insulating 110 11.11 properties Sound 250 25.25 insulation Housing costs 80 8.08 Price 70 7.07 Quality 410 41.41 Abs. = Absolute, Relat. = Relative

Identified

Tab. 2 Results of surveys of customers’ requirements for wood building versus silicate building materials.

10

1.01

440

44.44

30

3.03

10

1.01

440 44.44 I/R

10 10 20

1.01 1.01 2.02

490 400 510

49.49 40.4 51.52

0 10 20

0 1.01 2.02

10 10 20

1.01 1.01 2.02

460 46.46 490 49.49 390 39.39

I R I

20

2.02

650

65.66

20

2.02

10

1.01

130 13.13

I

0

0

570

57.58

10

1.01

20

2.02

350 35.35

I

20 40 0 80 10 10

2.02 4.04 0 8.08 1.01 1.01

660 690 410 490 320 510

66.67 69.7 41.41 49.49 32.32 51.52

60 6.06 10 1.01 40 4.04 220 22.22 160 16.16 110 11.11

30 30 70 10 10 20

3.03 3.03 7.07 1.01 1.01 2.02

90 9.09 100 10.1 340 34.34 30 3.03 90 9.09 70 7.07

I I I I A I

0

0

650

65.66

20

2.02

10

1.01

200

20.2

I

20

2.02

570

57.58

80

8.08

20

2.02

50

5.05

I

0 690 30

0 69.7 3.03

660 20 310

66.67 2.02 31.31

10 10 0

1.01 1.01 0

30 30 70

3.03 3.03 7.07

Fig. 1 Innovative force field of wood building.

182

210 21.21 I 170 17.17 M 170 17.17 A

For a deeper analysis of the gathered data, we carried out further analysis to find out how respondents perceived individual types of buildings. A correlation matrix was calculated to generalize the monitored wood building and building made of silicate materials parameters. It refers to the relationship among monitored variables (Table 3).

Based on the correlation matrix, we could see that respondents were more or less aware (dependence power) of the relationships between the individual parameters of wood or silicate building materials.

Within wood buildings, they were aware of the strong dependence between the construction and the lifetime (0.66) of these buildings, which was also related to their quality.

Strong correlation was demonstrated in quality and fire safety (0.5). Medium correlation of quality was shown with heat-insulating and sound-insulating properties. Similarly, the medium correlation was proved between the price and lifetime of wood buildings, the housing costs and lifetime, and the construction and thermal insulating and acoustic properties, as well as the housing costs. Other parameters of wood building presented low correlation among themselves.

To identify the customers’ drivers of wood buildings as an ecological innovation from the perspective of sustainable development of building construction in Slovakia, we had to monitor a positive correlation of demographic data and parameters of wood buildings, as well as a negative correlation of wood and silicate building materials data.

The mean correlation was between university-educated respondents and wood building construction. Low correlation was identified between both genders and construction, age category from 15 to 25 years and the fire safety, lifetime and wood buildings construction.

A negative correlation was shown between the 2645 age category and lifetime.

However, the positive perceiving of living costs in wood buildings was connected with age category 4660 years. University-educated respondent perceived equally positively safety, lifetime, thermal insulation properties and quality of wood buildings (low correlation).

All negative correlations of silicate building materials and wood buildings pointed to the fact that silicate building materials are a strong competition for this innovative building alternative in all parameters in lower or stronger correlation. So they represent an antiinnovation forces for wood building and not customers´ drivers for wood building in the market. HEMSTRÖM et al. (2011) presented comparison of wood and silicate building materials from the point of view of respondents' attitudes. From the study follows that wood buildings are associated with disadvantages, respectively uncertainties, especially in the area of fire safety, lifetime and acoustic properties.

Based on data from the correlation matrix we could identify the basic customers´ drivers for wood buildings in Slovakia. They were their construction, fire safety, housing costs, quality, thermal insulating properties and lifetime, which are positively evaluated only by university educated respondents and some respondents of the youngest age category.

Finally, we can conclude that wood buildings in Slovakia have strong competition in buildings of silicate materials. TOPPINEN et al. (2013) state that they present an alternative in the building industry, but they are not traditionally used.

The main customer driver of wood buildings in Slovakia is their construction. As ROOS et al. state (2010) this construction is preferred by engineers because of wood strenght and environmental friendliness as well as its easy handling and connection with other materials.

As it is presented by HEMSTRÖM et al. (2011) and given correlation matrix, lower or stronger correlation od wood buildings is related to fire safety, housing costs, quality, thermal insulating properties and lifetime.

183

+

Demographics Gender Age categories Woman Men 15-25 26-45 46-65 65 + EL Woman 1.00 Men -1.00 1.00 15-25 0.02 -0.02 1.00 26-45 0.06 -0.06 -0.51 1.00 46-65 0.06 -0.06 -0.47 -0.38 1.00 65 + -0.25 0.25 -0.20 -0.16 -0.15 1.00 Elementary (EL) -0.24 0.24 -0.14 -0.11 -0.11 0.70 Secondary (SE) -0.25 0.25 -0.10 -0.09 0.20 0.01 University (UN) 0.34 -0.34 0.15 0.13 -0.17 -0.25 Definition (D) 0.15 -0.15 0.14 -0.06 0.04 -0.25 Fire safety (FS) 0.18 -0.18 0.14 -0.16 0.14 -0.24 Life 0.12 -0.12 0.19 -0.20 0.10 -0.19 Construction (CS) 0.21 -0.21 0.15 -0.06 0.03 -0.23 Thermal insulating properties (TIP) 0.02 -0.02 0.05 -0.19 0.15 -0.02 Sound insulation (SI) -0.03 0.03 0.16 -0.03 -0.03 -0.22 Housing costs (HC) 0.03 -0.03 0.01 -0.10 0.20 -0.20 Price 0.10 -0.10 -0.02 0.01 0.09 -0.15 Quality (Q) 0.06 -0.06 0.18 -0.14 0.04 -0.15 Fire safety (FS) -0.14 0.14 -0.04 0.03 -0.10 0.21 Life 0.03 -0.03 -0.09 0.19 -0.04 -0.09 Construction (CS) -0.02 0.02 -0.16 0.11 0.06 0.00 Thermal insulating properties (TIP) -0.03 0.03 -0.10 0.20 -0.10 0.01 Sound insulation (SI) -0.05 0.05 -0.08 0.00 -0.01 0.18 Housing costs (HC) -0.02 0.02 0.00 0.04 -0.06 0.04 Price -0.09 0.09 0.04 -0.05 -0.10 0.19 Quality (Q) -0.02 0.02 -0.11 0.08 0.01 0.05 Cohen correlation coefficient interpretation Color The value of the correlation coefficient Interpretation 0,0-0,1 trivial correlation 0,1-0,3 low correlation 0,3-0,5 medium correlation 0,5-0,7 strong correlation 0,7-0,9 very strong correlation 0,9-1,0 almost perfect correlation

Tab. 3 Correlation matrix.

Demographics

Wood building

Silicate building materials

184

1.00 -0.17 -0.17 0.04 -0.17 -0.20 -0.13 0.08 -0.20 -0.17 -0.11 -0.13 0.15 -0.13 -0.16 -0.07 0.18 -0.10 0.18 -0.05 1.00 -0.94 -0.09 -0.16 -0.07 -0.26 -0.19 0.08 -0.02 -0.03 -0.15 0.15 0.09 0.22 0.23 0.05 0.08 0.06 0.28

Education SE UN

1.00 0.15 0.22 0.14 0.30 0.16 -0.01 0.08 0.07 0.20 -0.20 -0.05 -0.17 -0.21 -0.11 -0.04 -0.13 -0.26 1.00 0.41 0.50 0.66 0.42 0.43 0.26 0.29 0.56 -0.43 -0.37 -0.53 -0.32 -0.40 -0.17 -0.22 -0.61

D

FS

1.00 0.54 0.53 0.16 0.40 0.47 0.36 0.50 -0.76 -0.39 -0.38 -0.19 -0.24 -0.35 -0.19 -0.25

Life

1.00 0.61 0.34 0.49 0.32 0.18 0.60 -0.33 -0.85 -0.56 -0.26 -0.44 -0.14 -0.22 -0.44

CS

1.00 0.42 0.38 0.34 0.21 0.65 -0.40 -0.44 -0.73 -0.32 -0.41 -0.15 -0.22 -0.49

TIP

1.00 0.16 0.36 0.11 0.34 -0.13 -0.37 -0.44 -0.85 -0.15 -0.34 -0.03 -0.57

SI

Wood building

1.00 0.25 0.25 0.34 -0.35 -0.33 -0.40 -0.10 -0.71 -0.15 -0.08 -0.33

HC

1.00 0.46 0.28 -0.27 -0.13 -0.20 -0.36 -0.17 -0.59 -0.33 -0.29

1.00 0.19 -0.28 -0.04 -0.18 -0.10 0.05 -0.36 -0.61 -0.09

Price

Q

1.00 -0.34 -0.48 -0.44 -0.29 -0.30 -0.15 -0.12 -0.52

FS

1.00 0.24 0.32 0.18 0.20 0.11 0.04 0.27

Life

1.00 0.64 0.30 0.34 0.22 0.08 0.46

CS

1.00 0.35 0.44 0.27 0.13 0.60

TIP

1.00 0.03 0.31 -0.01 0.52

SI

1.00 0.08 0.27 0.28

Brick building

HC

1.00 0.20 0.30

Price

1.00 0.00

Q

1.00

From the Slovak customers´ point of view wood buildings are important (positive correlation to perception of wood buildings), but they still icreasingly prefer buildings made of silicate materials. As it is described by TOIVONEN (2011, 2012, 2013), consumers and companies more likely consider wood buildings to be environmentally friendly and important, they associate them with sustainability in building industry. Consumers like them, mainly their aesthetic qualities, but like TOIVONEN and HANSEN (2003) present, these are not the key attributes in decision making of consumer and companies.

Demand for wood framed houses, as our study points out, is influenced by decisions of university educated consumers and younger people. This finding is also confirmed by HOIBO et al. (2015), who found out that young people are the best target for increasing woodbased housing, as well as a study by ROOS et al. (2010), which states that engineers prefer wood buildings to buildings made of silicate materials because of their construction. As it is reported by TOIVONEN (2012) and HOIBO et al. (2015), the main target group of these constructions is created mainly by people with strong environmental values and with willingness to buy and even pay for products of higher environmental quality (HANSMANN et al. 2006, O'BRIEN and TEISL 2004). As it is reported by WANG et al. (2014) the ascending trend of wood building as an ecological invention of buildings, , is possible through hybrid structures. It means combinations of wood or wood composite materials with other materials.

CONCLUSION Innovation together with sustainable growth has a substantial place in mainstream market economy. Sustainable growth is a persistently developing and essential factor in a globalized world which is constructed on three pillars - economic, social and environmental.

In order to support sustainable growth a lot of improvements have become an essential part of the building industry where the consumers challenge the question which type of the building is more suitable for them – a wood building or abuilding made of silicate materials.

This idea is also the part of many studies focusing on attitudes towards determined attributes of individual types of buildings.

Based on our findings, we can state that Slovak consumers are rather traditional what is reflected in their preferred type of building construction. Only university educated consumers and respondents from the lowest age category present higher interest in wood houses, that is connected with their properties such as type of construction, fire safety, housing costs, quality, thermal insulating properties and lifetime. These are the requirements that must be taken into account in marketing strategies focusing on wood buildings.

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ACKNOWLEDGEMENTS The authors are grateful for the support of the Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic, and the Slovak Academy of Sciences, Grant 11/0674/19, “Proposal of a model for the eco-innovation integration into the innovation process of companies in Slovakia in order to increase their performance".

AUTHORS ADDRESSES Ing. Erika Loučanová, PhD. Ing. Miriam Olšiaková, PhD. Technická univerzita vo Zvolene Drevárska fakulta Katedra marketingu, obchodu a svetového lesníctva T. G. Masaryka 24 960 01 Zvolen Slovenská republika loucanova@tuzvo.sk olsiakova@tuzvo.sk

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