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Author's personal copy journal of dentistry 36 (2008) 618–625
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Evaporating solvents with a warm air-stream: Effects on adhesive layer properties and resin–dentin bond strengths Celso Afonso Klein-Ju´nior a, Christiana Zander-Grande b, Roberto Amaral b, Rodrigo Stanislawczuk b, Eugeˆnio Jose´ Garcia b, Ricardo Baumhardt-Neto c, Ma´rcia Margarete Meier d, Alessandro Dourado Loguercio b, Alessandra Reis b,* a
School of Dentistry, Department of Dentistry, University Luterana do Brasil, Cachoeira do Sul, Rio Grande do Sul, Brazil School of Dentistry, Department of Restorative Dentistry, University Estadual de Ponta Grossa, Ponta Grossa, Parana´, Brazil c School of Chemistry, Department of Chemistry and Materials Science, University Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil d FGM Dental Products, Department of Research and Development, Joinville, Santa Catarina, Brazil b
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Article history:
Objectives: This study evaluated the effect of a warm or cold air-dry stream for solvent
Received 27 November 2007
evaporation on the microtensile resin–dentin bond strength (mTBS), nanoleakage pattern
Received in revised form
(SEM), degree of conversion (DC) and solvent evaporation rates (SE) of an ethanol/water-
6 April 2008
(Adper Single Bond, [SB] 3MESPE) and an acetone-based (Prime & Bond 2.1, [PB] Dentsply),
Accepted 20 April 2008
two-step etch-and-rinse adhesive system. Materials and methods: Adhesives were applied on demineralized dentin surfaces. For SE, a warm or cold air-dry stream (10 s) was applied prior to light-activation (10 s). Bonded sticks
Keywords:
(0.8 mm2) were tested in tension (0.5 mm/min). Two bonded sticks from each tooth were
Adhesive systems
immersed in a 50% (w/v) solution of silver nitrate (24 h), photodeveloped (8 h) and analyzed by
Bond strength
SEM. The DC and solvent evaporation rate of the adhesives were evaluated under FTIR and
Solvent
analytical balance, respectively. Data were analyzed by two-way ANOVA and Tukey test
Evaporation
(a = 0.05).
Degree of conversion
Results: Higher mTBS and lower nanoleakage were observed when the SE step was per-
Dentin
formed with warm air-dry stream. However, the DC of the adhesives was not altered by the use of a warm air-dry. Conclusions: The use of a warm air-dry stream seems to be a clinical tool to improve the bond strength and the quality of the hybrid layer (less nanoleakage infiltration), since it might reduce the number of pores within the adhesive layer. # 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Etch-and-rinse adhesives require a separate step of etching, which is usually performed with 30–40% phosphoric acid. In their original configuration they were released to be applied in a three-step procedure, in which after etching, the surfaces
were primed and then bonded with a flow, non-solvated bonding resin.1 In an attempt to reduce clinical steps and save time manufacturers produced simplified etch-and-rinse adhesives by joining the components of the primer and the bonding resin into a single solution. If on one hand, this modification allowed
* Corresponding author at: Universidade Estadual de Ponta Grossa, Mestrado em Odontolgia Rua Carlos Cavalcanti, 4748- Bloco M, Sala 64A - Uvaranas, 84030-900 Ponta Grossa, PR, Brazil. Tel.: +55 42 3220 3741; fax: +55 49 3551 2004. E-mail address: reis_ale@hotmail.com (A. Reis). 0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2008.04.014
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the accomplishment of the bonding protocol in two steps,1 on the other hand, the hydrophilic features of these simplified adhesives were increased as the primer/bond solution should be compatible to the intrinsically moist, acid-etched dentin. Consequently, the adhesive solutions became more permeable to water from the oral environment and from the underlying bonded dentin,2–4 leading to incompatibility issues5–7 and faster degradation of resin–dentin bonds comparatively to their three-step version.8–10 This is somewhat true, that a recent systematic review of current clinical trials has reported that in general, the two-step etch-and-rinse adhesives perform clinically less favorable than the conventional three-step etch-and-rinse adhesives.11 While 79% of the two-step etch-and-rinse adhesives fulfilled the provisional acceptance ADA guidelines, only 51% fulfilled the full acceptance ADA guidelines.11 Resin–dentin bond strength and their durability seem to rely on the quality of the hybrid layer,12 i.e. on the proper impregnation of the dentin substrate and on the formation of a high cross-linking polymer inside the collagen mesh. As a result, different clinical approaches have been proposed to achieve this goal, such as increased application times of bonding agents,13 multiple adhesive coating,14 delayed polymerization,15,16 adhesive rubbing17,18 and longer exposure times of bonding systems.19 Most of these approaches favors solvent evaporation and contributes to the formation of a strong polymer. The use of a warm air-stream for solvent evaporation could theoretically improve solvent evaporation, but this approach has not been addressed yet. Therefore, the aim of this study was to compare the effects of the air stream temperature for solvent evaporation on the microtensile resin–dentin bond strength (mTBS) and nanoleakage pattern of an ethanol/water- (Adper Single Bond, [SB] 3M ESPE) and an acetone-based (Prime & Bond 2.1, [PB] Dentsply) two-step etch-and-rinse adhesive systems. The degree of conversion and solvent evaporation rates of the adhesives after solvent evaporation with both protocols was also investigated.
2.
Materials and methods
2.1.
Microtensile testing
Twenty extracted, caries-free human third molars were used. The teeth were collected after obtaining the patient’s informed
consent under a protocol approved by the University of Oeste of Santa Catarina Institutional Review Board. The teeth were disinfected in 0.5% chloramine, stored in distilled water and used within 6 months after extraction. A flat dentin surface was exposed after wet grinding the occlusal enamel on a # 180 grit SiC paper. The exposed dentin surfaces were further polished on wet #600-grit silicon-carbide paper for 60 s to standardize the smear layer. Two solvent-based, etch-and-rinse adhesive systems were tested: Adper Single Bond (SB-3M ESPE, St. Paul, MN, USA), an ethanol/water-based and Prime & Bond 2.1 (PB–Dentsply De Trey, Konstanz, Germany) an acetone-based system. The composition, application mode and batch number are described in Table 1. After acid etching with the respective etchants of each adhesive system, the surfaces were rinsed with distilled water for 15 s and air-dried for 15 s. The surfaces were, then, rewetted with water.20 Two coats of adhesive were slightly applied for 10 s. After each coat, the solvent evaporation was performed either with a warm (60 2 8C) or cold air (20 1 8C) for 10 s at a distance of 20 cm. In both cases, the air stream was generated by a commercially hair-dresser (SC831, Black & Decker, Uberaba, MG, Brazil). The speed of the air was 5.50 m/s and the air flow 0.0138 m3/s. The air emitted by the hairdresser in the cold condition was the same of the room temperature. The adhesives were light-cured for the respective recommended time using a quartz-tungsten halogen light set at 600 mW/cm2 (VIP, Bisco, Schaumburg, IL, USA) (Table 1). Resin composite build-ups (Filtek Z250, shade A2, 3M ESPE, St. Paul, MN, USA) were constructed on the bonded surfaces in 3 increments of 1 mm each that were individually light-cured for 30 s with the same light intensity. All the bonding procedures were carried out by a single operator at a room temperature of 20 8C and constant relative humidity. Five teeth were used for each combination of adhesive system and air temperature. After storage of the restored teeth in distilled water at 37 8C for 24 h, they were longitudinally sectioned in both a mesio-todistal and buccal-to-lingual directions across the bonded interface with a diamond saw in a Labcut 1010 machine (Extec Corp., Enfield, CT, USA) to obtain approximately 25 bonded sticks per tooth, each with a cross-sectional area of approximately 0.8 mm2. The number of premature debonded sticks (D) per tooth during specimen preparation was recorded. Specimens originated from the areas immediately above the
Table 1 – Adhesive systems: composition, application mode and batch number Adhesive systems Single Bond (3M ESPE)
Prime Bond 2.1 (Dentsply)
Composition 1. Scotchbond etchant–35% phosphoric acid 2. Adhesive–Bis-GMA, HEMA, dimethacrylates, polyalkenoic acid copolymer, initiators, water and ethanol 1. 32% phosphoric acid 2. Adhesive–UDMA, PENTA, Bis-GMA, butylated hydroxytoluene, 4-ethyl dimethyl aminobenzoate, cetylamine hydrofluoride, initiator and acetone
Application mode
Batch number
a, b, c, d, e, f, e, f, g
5FE
a, b, c, d, e, f, e, f, g
707608
(a) Acid-etching (15 s); (b) rinsing (15 s); (c) air-drying (30 s); (d) dentin rewetted with water; (e) one coat of adhesive; (f) air-dry for 10 s at 20 cm for solvent evaporation; (g) light-curing (10 s–600 mW/cm2). Phenyl-3,30-dicarboxylic acid; HEMA: 2-hydroxyethyl methacrylate; Bis-GMA: bisphenol A diglycidyl methacrylate; UDMA: urethane dimethacrylate; PENTA: dipentaerythritol pentaacrylate monophosphate.
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pulp chamber had their remaining dentin thickness (RDT) measured with a digital caliper and recorded (Absolute Digimatic, Mitutoyo, Tokyo, Japan). The cross-sectional area of each stick was measured with the digital caliper to the nearest 0.01 mm for calculation of the actual bond strength values (BS). Only half of the specimens, from each tooth, were tested in this study and they were randomly selected. Each bonded stick was attached to a modified device for microtensile testing with cyanoacrylate resin (Zapit, Dental Ventures of North America, Corona, CA, USA) and subjected to a tensile force in a universal testing machine (EMIC, Sa˜o Jose´ dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min. The failure modes were evaluated at 400 (HMV-2, Shimadzu, Tokyo, Japan) and classified as cohesive (failure exclusive within dentin or resin composite, C), adhesive (failure at resin/dentin interface–A), or adhesive/mixed (failure at resin/dentin interface that included cohesive failure of the neighboring substrates, A/M). The mean bond strength of all sticks from the same tooth was averaged for statistical purposes. The prematurely debonded specimens were included in the tooth mean. The average value attributed to specimens that failed prematurely during preparation was arbitrary and corresponded to approximately half of the minimum bond strength value that could be measured in this study (ca. 4.3 MPa).20 The BS mean for every testing group was expressed as the average of the five teeth used per group. The microtensile bond strength data was subjected to a two-way analysis of variance (adhesive/air temperature) and a post hoc test Tukey’s test at a = 0.05 for pair-wise comparisons.
2.2.
Degree of conversion
One drop (10 mL) of each adhesive solution was placed between acetate strips to achieve a thin film 8 mm in diameter. Before covering the adhesive with the upper acetate strip, they were gently air-dried either with a warm or dry stream (10 s) to allow for solvent evaporation. A FTIR spectrum of the uncured material was recorded and then, the specimens were photoactivated for 10 s. Each specimen was carefully removed with a narrow surgical knife and stored for 24 h in a dark, dry environment until the FTIR analysis of the degree of conversion (FTIR-8300, Shimadzu, Tokyo, Japan). The spectrum was obtained with 32 scans at 1 cm 1 resolution in transmission method. The percentage of unreacted carbon–carbon double bonds (% C C) was determined from the ratio of absorbance intensities of aliphatic C C (peak height at 1640 cm 1) against internal standard before and after curing of the specimen. The aromatic carbon–carbon bond (peak height at 1610 cm 1) absorbance was used as an internal standard. The degree of conversion (DC) was determined by subtracting the % C C from 100%. Three specimens were tested for each group. Degree of conversion results were evaluated statistically using two-way ANOVA and Tukey’s test at a pre-set significance level of 0.05.
2.3.
Solvent evaporation rate
Approximately 10 mL of each of the products, which corresponds to approximately one coat with saturated microbrush,
was obtained with a micropipette (Pipetman, Gilson, NY, USA) from the original container and transferred to small lightproof glass containers of known weight. They were immediately placed in an analytical balance (Mettler, type H6; Columbus, OH, USA; capacity to 160 g) and the baseline mass was recorded to the nearest 0.0001 mg. After 20 s, 1, 2, 3, 4 and 5 min, the mass was recorded again. No stopper that could prevent evaporation was used. The same procedure was repeated; however instead of leaving the adhesive undisturbed, a warm or cold air-stream was applied for 10 s before placing the adhesive into the analytical balance. The mass was measured after 20 s, 1, 2, 3, 4 and 5 min. Room temperature was set at 20 8C and the relative humidity approximately at 50%. Protection from light radiation was assured by covering the analytical balance with appropriate light filters. Five samples of each adhesive, in each experimental condition, were used. The percentage of loss of mass, based on the mean baseline recording, was calculated for each experimental condition. The data was subjected to a two-way ANOVA and Tukey’s test at a pre-set significance level of 0.05.
2.4. Scanning electron microscopy for nanoleakage evaluation Approximately three or four sticks from each tooth prepared for the microtensile testing were used for nanoleakage evaluation. Bonded sticks were coated with two layers of nail varnish applied up to within 1 mm of the bonded interfaces. The specimens were re-hydrated in distilled water for 10 min prior to immersion in the tracer solution. Ammoniacal silver nitrate was prepared according to the protocol previously described by Tay et al.21 The sticks were placed in the ammoniacal silver nitrate in darkness for 24 h, rinsed thoroughly in distilled water, and immersed in photo developing solution for 8 h under a fluorescent light to reduce silver ions into metallic silver grains within voids along the bonded interface. All sticks were wet-polished with 600-grit SiC paper to remove the nail varnish. Then, the specimens were placed inside an acrylic ring, which was attached to a double-sided adhesive tape, and embedded in epoxy resin. After the epoxy resin set, the thickness of the embedded specimens was reduced to approximately half by grinding with silicon carbide papers under running water. Specimens were polished with a 600-, 1000-, and 2000-grit SiC paper and 6, 3, 1 and 0.25 mm diamond paste (Buehler Ltd., Lake Bluff, IL, USA) using a polish cloth. They were ultrasonically cleaned, air dried, mounted on stubs, and coated with carbon-gold (MED 010, Balzers Union, Balzers, Liechtenstein). Resin–dentin interfaces were analyzed in a field-emission scanning electron microscope operated in the backscattered electron mode (JSM 6060, JEOL, Tokyo, Japan).
3.
Results
3.1.
Microtensile bond strength
Approximately 21–26 sticks could be obtained per tooth including those with premature debonding. The mean cross-sectional
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area ranged from 0.82 to 0.98 mm2 and no difference among groups was detected ( p > 0.05). The percentage of specimens with premature debonding and the frequency of each fracture pattern mode are shown in Table 2. Table 2 also depicts the overall means and the respective standard deviations of the resin–dentin bond strengths for all experimental groups. Neither the interaction adhesive vs. air temperature nor the main factor Adhesive was statistically significant ( p > 0.05). Only the main factor air temperature was statistically significant ( p = 0.001). Higher bond strength values were observed for both adhesives when the solvent evaporation step was performed with a warm air-stream. However, the means were only statistically significant for the SB system.
3.2.
Table 3 – Overall degree of conversion (%) and the respective standard deviations (MPa) obtained in each experimental conditiona Adhesive
Air temperature Cold
SB PB a
47.8 3.5 a 36.2 5.2 b
Warm 50.3 5.4 a 39.3 6.3 b
The same letters indicate statistically similar means ( p > 0.05).
Degree of conversion
The means and standard deviations of the degree of conversion for both adhesives under the experimental conditions of this study are shown in Table 3. The degree of conversion of the adhesives was not affected by the air temperature ( p = 0.36). Only the main factor adhesive was statistically significant ( p = 0.005). SB showed a higher DC than PB.
3.3.
Solvent evaporation rate
In Fig. 1A and B it can be see the mean percentages values of loss of mass for both adhesives during 5 min. Table 4 depicts the mean percentages values of loss of mass for both adhesives 20 s after being dispensed. The interaction adhesive vs. air temperature was statistically significant as well as the main factors Adhesive and Air Temperature ( p < 0.0001). As one can observe the percentage of mass of both adhesives after 20 s was significantly improved by the application of an air-dry stream. The use of a cold or warm air-dry was not significant for PB. However, the application of a warm air-dry significantly favored the evaporation rate of SB compared to the use of a cold air-stream. Nonetheless, in none of the conditions, the evaporation rate of SB was similar to PB.
3.4.
Scanning electron microscopy
Representative SEM images at the resin–dentin interfaces for the experimental conditions are depicted in Fig. 2. Single Bond, after solvent evaporation with a cold air-dry stream showed a poor seal, as many dentinal tubules were filled with silver
Fig. 1 – Loss of mass (%) in function of different solvent evaporation methods during 300 s in: (a) Prime & Bond 2.1; (b) Single Bond.
(Fig. 2a). Besides that, the entire thickness of the hybrid and adhesive layers, formed under this condition, was throughout impregnated with silver nitrate. This situation was not observed in the adhesive layer formed by Single Bond airdried with a warm stream (Fig. 2b). Although silver impregnation can still be observed in base of the hybrid layer (Fig. 2b) the magnitude of the silver nitrate penetration was not as evident as in Fig. 1a. Similarly, Prime & Bond 2.1 showed a very dense deposition of silver nitrate when the solvent was evaporated with a cold air-dry (Fig. 2c). However, contrary to
Table 2 – Number of specimens and their respective percentages (%) distributed according to the fracture pattern mode as well as the percentage of premature debonded specimens for each experimental condition as well as the overall microtensile bond strength values and the respective standard deviations (MPa) obtained in each experimental conditiona Air temperature
A/Mb
C
Debonded
mTBS
SB
Cold Warm
39 (79.6) 31 (66)
6 (12.2) 9 (19.1)
4 (8.2) 7 (14.9)
34.9 8.5 b 48.7 6.3 a
PB
Cold Warm
32 (80) 27 (65.9)
3 (7.5) 5 (12.1)
5 (12.5) 9 (22)
37.3 5.7 ab 44.7 5.2 ab
Adhesive
a b
Statistically similar means are represented by the same letters ( p > 0.05). A/M: adhesive/mixed fracture mode; C: dentin or resin cohesive fracture mode.
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Table 4 – Mean percentages of mass (%) and the respective standard deviations obtained in each experimental condition after dispensea Adhesive
Without air
Air temperature Cold
SB PB a
95.4 1.5 a 87.0 2.1 b
90.0 2.2 b 33.6 2.3 d
Warm 69.4 2.0 c 32.2 3.0 d
The same letters indicate statistically similar means ( p > 0.05).
Single Bond, this intense deposition did not occur in the entire thickness of the adhesive layer but only at the hybrid layer. This deposition was significantly reduced when the solvent evaporation of PB was performed with a warm air-stream (Fig. 2d).
4.
Discussion
Current adhesive systems are generally formulated with hydrophilic and hydrophobic resin monomers dissolved in acetone, ethanol and water or in solvent combinations.22 Solvents act as a transport medium and lower resin viscosity. This allows greater penetration of resins into the microporosites of the prepared tooth surface23 as well as enhances the mobility of radicals and growing polymer chains.24 The resin surface wetting capabilities are also improved and help to displace surface moisture without collapsing the demineralized collagen network.22 On the other hand, the presence of residual solvent might have an adverse effect on the performance of the resin–dentin bonds. It was already demonstrated that high solvent concentration within the adhesive polymer prior to light-curing
Fig. 2 – Representative backscattered SEM images of the interface bonded with Single Bond (a and b) and Prime & Bond 2.1 (c and d) to demineralized dentin. In (a and c), the solvent was evaporated with a cold air-dry stream, while in (b and d), a warm air-dry stream was employed. (a) Silver deposition occurred almost throughout the entire thickness of the hybrid layer. Intense penetration of silver nitrate can also bee seen into the tubules. (b) It can be seen that the amount of silver penetration was lower and practically occurred at the base of the hybrid layer. Only few dentin tubules were infiltrated by silver nitrate. (c) A higher amount of silver penetration can be observed at the base of the hybrid for PB. (d) The amount of silver nitrate penetration seems to be quite low and it was restricted to the base of the hybrid layer.
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prevents the attainment of a high cross-linking polymer inside the hybrid layer25,26 and leads to pores and interfacial layers,27 affecting the overall performance of resin–dentin bonds.28 Ideally, solvents and water (from the moist demineralized dentin) should be completely eliminated from the dentin surface before light-curing. On this basis, there is often an airdrying process recommended as part of the clinical regimen for dentin bonding while using adhesives that contain solvents. However, the removal of solvents with a simple air-drying stream is not an easy task to be accomplished under clinical application. As water/solvent evaporates from the adhesive, the monomer density is found to increase sharply, creating a monomer concentration gradient which acts as a barrier for further solvent evaporation and thus, reduces the ability of water and solvents to evaporate from the adhesive.29 This situation is even worse for simplified adhesives such as the two-step etch-and-rinse adhesives evaluated in the present investigation, since the extent of solvent and water retention in polymer networks seems to be directly correlated with the hydrophilicity of the resin blends.30 In addition to that, the recommended clinical time for solvent evaporation is rather short as some studies have demonstrated that only periods of time longer than 12–20 min can ensure an almost complete solvent evaporation.15,31 In face of that some alternative methods to maximize solvent evaporation should be investigated, such as the one evaluated in the present study. One way to accelerate solvent evaporation, at least for water/ethanol-based systems is the use of a warm dry set at approximately 60 8C. Although a previous study has not observed any beneficial effect of warm air-dry on solvent evaporation rate, the temperature of the air was half of that employed in the present investigation.32 The use of a warm air-stream allowed an increase of 20 and 40% in the resin–dentin bonds for PB and SB, respectively. This could be attributed to the fact that when heat is delivered to a substance, energy comes in. That energy can be used either to increase the kinetic energy of the molecules, which causes an increase in temperature or that heat can be used to increase the potential energy of the molecules causing a change in the state.33 One could hypothesize that under the conditions of the study the heat delivered by the warm air-dry could have altered the manner molecules bond to one another. Consequently, this increased the evaporation rate of solvents from bonding interface allowing the achievement of higher resin– dentin bonds, as observed in the present investigation. However, the adhesives did not respond homogeneously to the delivered heat. Although a numerical increase in the resin– dentin bond strengths was observed for both adhesives, this increase was not statistically significant for the acetone-based system (PB). Different molecules differ in the amount of attraction that exists between them. For instance, the mutual attraction between water molecules and ethanol molecules are stronger than that of acetone, because it involves hydrogen bonding forces. As a result, the boiling temperature and the vapor pressure of ethanol and water are higher than that of acetone, which turns their evaporation more difficult. A recent study that examined the effect of organic solvent and water retention in comonomer blends with different hydrophilicity demonstrated that significantly more solvent and water were retained in ethanol-based adhesives when compared to
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acetone-based mixtures.30 This could be the reason of why the acetone-based system (PB) was less affected by the increase in the air temperature. Interestingly, the increase in bond strengths was not accompanied by an increase in the degree of conversion of the adhesive system as observed in the present study. Previous studies evaluating the effect of solvent concentration on the degree of conversion of adhesive films have observed that increasing amounts of solvents led to a reduction of their degree of conversion.27,28 However, one may consider that a wide range of solvent concentration was investigated being them not representative of the amount of solvent presented in the adhesive layer before and after application of a cold or warm air-drying procedures. There is a solvent concentration at which maximum conversion is reached; more or less solvent than this amount would decrease monomer conversion,24 and this seems to be related to the viscosity of the adhesive film.28 It is likely that non-solvated versions of adhesive systems might present a lower degree of conversion due to the increased viscosity of the solution. An increased viscosity restricts the mobility of reactive components during polymerization.28 On the other extreme, excess of solvents would cause a dilution of the components preventing the collision of reactive components. Unfortunately, no attempt was made in the present investigation to determine the amount of residual solvent in the adhesive films after using the two different modes of airdrying and this deserves further investigations. Based on the results of the present investigation we cannot assume that the increase in resin–dentin bond strength is due to an increase in the degree of conversion of the adhesives. It is likely that the increase in the resin–dentin bonds is due to an increase in the mechanical properties of the adhesive layer due to more solvent evaporation rates. An earlier study observed that although the solvent content did not affect the degree of conversion of bulk adhesive specimens, the flexural strength of these specimens, which is a mechanical property of the adhesive layer, were significantly reduced, since residual solvent might leave more pores in the specimens.27 This correlation between mechanical properties and resin– dentin bonds was also observed in other studies. For instance, a significant and positive correlation was observed between resin–dentin bond strength values and the ultimate strength of the adhesives.34,35 The presence of solvent-rich pores can be reinforced by the SEM findings of the present study. The amount of silver nitrate penetration was significantly higher in the specimens that were cold air-dried, as this caused a higher amount of water/solvent retention within the adhesive layer. It is accepted that one of the sources of nanoleakage expression within adhesive interfaces are the remnant water/solvent and the water flux from the underlying dentin.7 They represent areas within the adhesive layer in which water or solvent are incompletely removed resulting in regions of incomplete polymerization and/or hydrogel formation.7,36 They are therefore highly prone for deposition of silver nitrate as can be seen in the micrographs of the present investigation. One important issue that should be mentioned is the potential effects of high temperature in the W-air dry group on pulp as well as on dentinal fluid flow. The most widely
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accepted mechanism of dentin hypersensitivity is the hydrodynamic theory proposed by Bra¨nnstro¨m et al.,37 whereby fluid flow within dentinal tubules is altered (increased or changed directionally) by thermal, tactile or chemical stimuli near the exposed surface of the tubules. This alteration would lead to stimulation of the A-d fibres surrounding the odontoblasts. Therefore, the use of the warm temperature either in superficial, medium and deep cavities should be matter of further investigation to determine the clinical viability of the studied clinical approach. The use of a warm air-stream seems to be a useful tool to help clinicians to improve the quality of the resin–dentin bonds. However, further studies are still required in order to elucidate some of the hypothesis raised in this study and evaluate the effects of a warm air-dry stream in the long-term resin–dentin bonds.
5.
Conclusions
The resin–dentin bond strength and the quality of the hybrid layer (less nanoleakage infiltration) of adhesives can be improved by the use of a warm air-stream for solvent evaporation, mainly for water/ethanol-based systems. This seems to be mainly attributed to more solvent evaporation rather than improvement in the degree of conversion of the adhesive layer.
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Acknowledgements We would like to thank the help provided by the under´ tila Panta (School of graduate students Rafael Santos and A Dentistry, University Luterana do Brasil, Porto Alegre, RS, Brazil) and the engineer Endrigo Dourado Loguercio. This study was partially supported by CNPq grants 473101/2006-8 and 305870/2004-1 and FAPERGS.
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references 19. 1. Van Meerbeek B, De Munck J, Yoshida Y, Inoue S, Vargas M, Vijay P. Buonocore memorial lecture. Adhesion to enamel and dentin: current status and future challenges. Operative Dentistry 2003;28:215–35. 2. Tay FR, Pashley DH. Dentin adhesives: have they become too hydrophilic? Journal of Canadian Dental Association 2003;69:724–31. 3. Tay FR, Pashley DH. Water treeing—a potential mechanism for degradation of dentin adhesives. American Journal of Dentistry 2003;16:6–12. 4. Tay FR, Frankenberger R, Krejci I, Bouillaguet S, Pashley DH, Carvalho RM, et al. Single-bottle adhesives behave as permeable membranes after polymerization. I. In vivo evidence. Journal of Dentistry 2004;32:611–21. 5. Tay FR, Suh BI, Pashley DH, Prati C, Chuang SF, Li F. Factors contributing to the incompatibility between simplified-step adhesives and self-cured or dual-cured composites. Part II. Single-bottle, total-etch adhesive. Journal of Adhesive Dentistry 2003;5:91–105. 6. Suh BI, Feng L, Pashley DH, Tay FR. Factors contributing to the incompatibility between simplified-step adhesives and
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