LESSON 3
TESTS FOR IDENTIFICATION OF FIBRES
STRUCTURE 3.0
OBJECTIVES
3.1
INTRODUCTION
3.2
RANGE OF TESTS TO IDENTIFY A W IDE RANGE OF FIBRES
3.3
MICROSCOPIC TESTS
3.4
BURNING TESTS
3.5
SOLUBILITY TESTS
3.6
ELEMENT IDENTIFICATION TESTS
3.7
DENSITY MEASUREMENT
3.8
DETERMINATION OF MELTING POINT
3.9
FEELING TEST
3.10 ASSIGNMENTS 3.10.1 3.10.2
CLASS ASSIGNMENTS HOME ASSIGNMENTS
3.11
SUMMING UP
3.12
POSSIBLE ANSWERS TO SELF-CHECK QUESTIONS
3.13
TERMINAL QUESTIONS
3.14
REFERENCES
3.15
SUGGESTED FURTHER READING
3.16
GLOSSARY
3. TESTS FOR IDENTIFICATION OF FIBRES After classifying textile fibres in the first lesson of this unit, their properties were described in the second lesson. This third and final lesson of the unit tells you how the fibres can be identified.
3.0
Objectives After going through this lesson, you will be able to;
•
Identify fibres using a wide range of techniques.
•
Observe the longitudinal and cross sectional microscopic view of different fibres with their respective unique features.
•
Understand the different burning characteristics of fibres.
•
Distinguish between the fibre types using the criterion of solubility in a chemical.
•
Distinguish fibres on the basis of density and melting temperatures on the basis of data.
3.1
Introduction
In recounting the history of textiles, it is generally not appreciated that till about 100 years ago, it used to be an age of natural fibres dominated by cotton, linen, jute, wool and silk fibres. As shown later in Table 3.1, because of some unique features (like peanut like cross section of cotton, scaly structure of wool fibre, triangular cross section of silk, etc.) these fibres could be identified by examining their longitudinal and cross sections on a low-magnification microscope. The first man-made fibre, viscose rayon, dates back to the end of the 19th Century and it also had a distinctly different cross-sectional geometry compared to the natural fibres of that time. However, as synthetic fibres were discovered and commercially produced - the first being nylon 66 in 1938 followed by nylon 6 in 1939, acrylic fibre in 1949, polyester fibre in 1943, polypropylene in 1957 – fibre identification became more complex because most of these synthetic fibres were smooth with circular cross sections and a rather featureless geometry. It therefore became necessary to extend the range of tests so that given an unknown fibre, its exact identity should be specified. A number of interesting developments that have taken place in the textile industry have made the process of identification more complex. Only two of these will be pointed out here. Large quantities of fabrics are now made from blended yarns, e.g. the blend of cotton with polyester fibre represents a very popular 2
product in which cotton confers comfort mainly due to its high moisture absorbing capacity and polyester fibre gives the fabric a number of other desirable characteristics, e.g. the requisite mechanical properties like strength and durability. Also because of the ability of polyester fibre to dry quickly (drip-dry) and its resistance to creasing, the fabric containing polyester fibre acquires the well-known wash and wear characteristic. Yarns taken out from these fabrics will thus contain both cotton and polyester fibres and the tests must be able to identify both of them. The second development worth mentioning is that of bicomponent fibres in which a single filament may contain two different fibreforming polymers - a core of polypropylene encased in a sheath of polyester. Both there will be revealed during the test. These and other developments have thus added a degree of complexity in the attempts made to identify the fibres and one must take note of this.
3.2
Range of Tests to Identify a Wide Range of Fibres
There are around twenty fibres that must be considered whenever an identification exercise for an unknown fibre is on. These include the natural fibres, viz. cotton, wool, silk, linen, and jute (it may be mentioned that jute is being blended in small amounts with other firbes for some textile fabric production). Pineapple fibre is not included in this list as only very small amount of this fibre is used. The second category comprises man-made fibres based on natural feedstock, the major fibre in this category being viscose rayon. As noted earlier, this is made by regenerating pure cellulose fibre from cellulose xanthate. A direct route has now been found and Tencel and Lyocell are the trade names for manmade cellulose fibres through the direct route. The two important chemicallymodified cellulose fibres that are made in small quantities are cellulose diacetate and cellulose triacetate fibres. The third important category is that of synthetic fibres and includes polyamides (nylon 66 and nylon 6), polyester (mainly Polyethylene terephthalate or PET and small quantities of polybutylene terephthalate), acrylic (polyacrylonitrile) and modified acrylic (modacrylic), polyolefin (polypropylene and polyethylene) and polyurethane (Lycra, Spandex). Amongst inorganic fibres asbestos, glass, metallic and carbon fibres are worth mentioning. So given these twenty or so fibes, it is unlikely that a single test will lead to its identification – there is a need to have a range of tests and the following have been found to be particularly useful: i)
Microscopic examination of the longitudinal and cross sections of the fibre,
ii)
Burning test in a flame, and
iii) Solubility tests in chemical reagents. In addition to these three tests, the following four tests also provide useful information: iv) Element identification v)
Density measurement 3
vi) Determination of melting point, and vii) Feeling test. In practice, identification tests are used in combination. The various tests listed above will now be briefly described.
3.3
Microscopic Tests
The microscopic test reveals the macroscopic features of the fibre. When observed along the length (longitudinal section), the surface features are revealed. When a fibre is cut in the perpendicular direction and a thin crosssection examined on the microscope, the shape of the cross-section and the macroscopic features in the cross-section can help identifying some fibres. An optical microscope with a magnification of at least 100 is generally used. A projection microscope is however, preferred since it gives an enlarged view on the screen, which can be traced on a tracing paper. If the microscopes are of the polarizing type, the contrast is sharper and more information can be collected. Thus polarizing projection microscopes allow greater amount of detail and are therefore generally used. To examine the fibre in the longitudinal direction, a few fibres (or a few short lengths of cut filaments) are straightened and parallelized and placed on a glass slide. They may be secured with the help of cellotape on both ends. To reduce scattering of light, the fibre is immersed in a drop of inert liquid having a refractive index close to the refractive Index of the fibre and covered with a cover glass. The sample is then mounted on the microscope stage and its focussed image observed on the screen. The longitudinal texture may then be traced on tracing paper. The cross-section can be made as follows: A bundle of straight and parallel fibres is embedded in a cork with the help of a needle in which the yarn or filaments are threaded. A thin section of the cork is then carefully cut using a new blade and this thin section is then placed on a glass slide and secured with cellotape. The assembly is mounted on a microscope. The cross sectional view, when combined with the corresponding longitudinal view, may then assist in identifying the fibre. The cross-sectional (top) and longitudinal sectional (bottom) views of cotton, wool and silk fibres taken on a scanning electron microscope are shown in Fig. 3.1 and it is interesting to observe that the characteristic features of these fibres are quite different and thus can assist in their identification.
4
The characteristic features shown by some other fibres are shown schematically in Fig. 3.2.
Fig. 3.1 Cross section (top) and longitudinal section (bottom) of cotton (left), wool (middle) and silk (right) fibres.
Fig. 3.2 Cross- section (top) and longitudinal section (bottom) of some common fibres.
It may be added that important synthetic fibres like polyester and nylon are generally made with circular cross section and their longitudinal and cross sectional views are featureless and are not of great assistance in identifying them. However, it is worth pointing out that fibres with non-circular cross-sections (trilobal, triangular, octagonal, etc.) are also made in small quantities.
5
The common observations that may be made from an examination of the longitudinal and cross sectional views of a number of fibres are summarized in Table 3.1. Table 3.1 Microscopic Appearance of Some Common Fibres Fibre Cotton
Longitudinal Section: Appearance Flat, irregular convoluted ribbons which change direction with the twist (mercerised cotton is smoother and less irregular) Rough surface with scales protruding out
Wool
Silk (degummed) Viscose Rayon Nylon, polyester, polypropylene Acrylic
Smooth with distinct striations. Striated, smooth Smooth, rod-like
lengthwise
Flat, irregular striations
Cross-section: Appearance Peanut or bean shaped with lumen* running through the *length. Nearly round, medulla present in coarse fibres is concentric and irregular in size. Mostly triangular, irregular. Irregular, serrated Regular, round Irregular, dog-bone shape
*Lumen is an irregular hole running through the middle. While cutting the cross-section, it may sometimes get covered.
Self-check Questions 1. Answer the following questions in terms of Yes/No. i) The cross sections of the following fibres show unique features: a. b. c. d. e.
Polyester Cotton Wool Nylon Jute
Yes / No Yes / No Yes / No Yes / No Yes / No
ii) The longitudinal sections of the following fibres show unique features. a. b. c. d. e.
Cotton Wool Polyester Polypropylene Silk
Yes / No Yes / No Yes / No Yes / No Yes / No
6
3.4
Burning Tests
The fibres being chemically different, they show different burning characteristics which can be used to identify them. The burning test is a relatively simple test as all that is needed is a flame and a keen observer who should carefully watch and note down the observations made (a) when approaching the flame, (b) on the burning behaviour inside the flame, (c) during removal from the flame, (d) relating to the smell emitted, and (e) on the residue left behind after the fibre has burnt out. The observations made on the burning behaviour of some common fibres are summarised in Table 3.2 Table 3.2 Burning Behaviour of Common Fibres Fibre
Approaching flame
In Flame
Cellulose Fibres (Cotton, viscose) Wool, silk
Do not shrink
Burn readily without melting
Curl away
Asbestos
Does not shrink Shrinks away from flame
Burn slowly sputter Does not burn, glows Melts, burns slowly, drips
Nylon
-do-
-do-
Polypropylene
-do-
-do-
Acrylic
-do-
Burns readily, sputters
Polyester
Behaviour outside the flame Continue to burn, afterglow
Smell
Residue
Burning hair
Small amount of light gray ash
Selfextinguishing Retains shape Burns, drips, may extinguish because of dripping -do-
Burning hair None
Easily crushable black bead Same as original
Sweet smell of ester
Hard, tough, gray bead
Pungent, burning beans Burning plastic Acrid
Hard, tough, light colour
Continues to burn -do-
Hard, tough, tan bead Irregular, hard, black bead
Activity 1.
Take a cotton fibre and a polyester fibre. Burn the two fibres separately and write down your observations.
7
3.5
Solubility Tests
The solubility of fibres in some specific chemical reagents (acid, alkali, bleaching agent, solvent) provides a definite means of identification, if not for a specific fibre, then for a generic group. When combined with the results of microscopic and burning tests, the results of solubility test make it possible to identify the fibres in most cases. There are different schemes for making solubility tests-of course they must be carried out in a prescribed order as they work on the principle of elimination. The following represents one such scheme meant to identify an unknown fibre: Step 1: Treat the fibre sample with 0.25-0.50% sodium hypochlorite solution. If soluble, they may be wool or silk. (To distinguish between the two, treat the fibre in cold 70% sulphuric acid- if soluble, it is silk, otherwise wool. Alternately, test the fibre for sulphur, which is present in wool). If the fibre is insoluble in sodium hypochlorite, go to Step 2. Step 2: Treat the fibre with cold acetic or glacial acetic acid. If soluble, the fibre could be cellulose diacetate or cellulose triacetate. (To distinguish between the two, treat the fibre with methylene chloride. If soluble, it is cellulose triacetate, if not cellulose diacetate). If the fibre is insoluble, go to Step 3. Step 3: Treat the fibre with cold (heat if necessary) formic acid. If soluble, the fibre is nylon 66 or nylon 6. (To distinguish between the two, treat the fibre with boiling dimethyl formamide (DMF). If soluble it is nylon 6, otherwise nylon 66. Alternately determine their melting points. Nylon 6 melts at 218ยบ C, nylon 66 at 265ยบC). If the fibre is insoluble, go to Step 4. Step 4: Treat the fibre in cold DMF. If soluble it is acrylic fibre, if insoluble, go to Step 5. Step 5: Boil the sample in chlorophenol. If soluble, it is poly (ethylene terephthalate) (polyester) fibre. If insoluble, go to Step 6. Step 6: Treat the fibre with 70% sulphuric acid. If soluble, it could be cotton or viscose rayon (To distinguish between the two, treat them with sodium Zincate. If soluble, it is viscose rayon). If insoluble in step 6, go to step 7. Step 7: Put the sample in water. If it floats, it could be polypropylene (PP) or polyethylene (PE). PP is soluble in boiling carbon tetrachloride, PP is soluble in boiling xylol. Some additional tests listed below may be performed for further confirmation: i) ii) iii) iv) v) vi)
Nylon 66 and Nylon 6: Soluble in formic acid (85%) and m-cresol Cellulose triacetate: Soluble in chloroform and methylene dichloride Wool: soluble in 5% NaOH at room temperature Silk: Soluble in 5% NaOH (hot) Viscose rayon: dissolves in sodium zincate solution PET: Dissolves in orthochlorophenol at room temperature. 8
vii) Asbestos, glass: do not dissolve in common organic or inorganic solvents. (Lately the use of asbestos fibre in any product has been banned in many countries because of its suspected carcinogenic effect). The other identification tests listed earlier will now be briefly considered:
Activity 2.
Take some unknown fibres (both natural and synthetic). Perform burning tests on them and note down the observation made. Try to identify them on the basis of the observations made.
Self-check Questions 2. State whether the following statements are True / False. i) ii) iii) iv) v) vi) vii) viii) ix) x)
3.6
Element nitrogen is present in cotton fibre. True/False Nylons are the lightest fibres. True/False Viscose rayon is a protein fibre. True/False The presence of oxygen element in a textile fibre raises its density. True/False Out of the three major tests used to identify fibres, solubility test gives the most definite information. True/False Wool burns readily. True/False Asbestos is resistant to solvents. True/False It is safe to wash wool with ordinary washing soaps. True/False Acrylic fibre floats in water. True/False Silk fibre melts and drips on burning. True/False
Element Identification Tests
Tests for nitrogen and chlorine are sometimes performed for identifying nylon, Lycra, acrylic (nitrogen) and modacrylic (chlorine) fibres. 3.6.1 Test for detecting the presence of nitrogen (Soda lime test) Cover a few fibres in a small ignition tube with soda lime, plugging the mouth of the tube with glass wool to prevent spitting. When the tube in heated, ammonia will be found in the vapour if the fibre contains nitrogen. The vapour being strongly alkaline will turn litmus paper blue.
9
3.6.2 Test for detecting the presence of chlorine Heat a copper wire in a bunsen flame until the flame is no longer of green colour. Then take this wire out of the flame and while still hot, touch the fibres with the wire. Thermoplastic fibres will adhere to the wire. Place the wire with the fibres attached in the flame - a green colour will indicate the presence of chlorine.
3.7
Density Measurement
The definition of density is derived from the relationship volume X density = mass, i.e. density is the mass per unit volume of a substance and its units, are gm/cm3. It can be accurately measured using a density gradient column (described in suggested reading at no. 4). The density data on common fibres are shown in Table 3.3 Fibre Density Data Fibre Cotton Viscose rayon Linen Jute Polynosic Cellulose diacetate/Triacetate Wool Silk Nylon 6/Nylon 66 Polyester Acrylic Modacrylic Spandex Polyethylene (Low density) Polyethylene (High density) Polypropylene Glass
3.8
3 Density (gm/cm ) 1.52-1.55 1.49-1.52 1.53-1.55 1.49-1.50 1.50-1.52 1.30-1.35 1.30-1.33 1.25-1.34 1.14 1.38-1.40 1.16-1.19 1.29-1.34 1.20-1.25 0.92-0.94 0.95-0.97 0.91 2.50-2.54
Determination of Melting Point
The melting temperature of a fibre can be very accurately determined with the help of a differential calorimeter (described in suggested reading no. 4). The data on some common fibres is given in Table 3.4.
Table 3.4 Melting Temperature Data of Some Fibres
Fibre
Melting temperature
10
Natural Cotton Wool Silk Man-Made Viscose rayon PET (Polyester) Nylon 6 Nylon 66 Acrylic Polypropylene Polyethylene (Low den.) Polyethylene (High den.)
3.9
Decomposes around 250ºC before softening/melting 220ºC Decomposes at 280ºC before it melts Decomposes before softening/melting 265ºC 218ºC 265ºC 320ºC (May decompose before melting) 165ºC 115ºC 135ºC
Feeling Test
It is a subjective test and can only be performed by one with skill in this art acquired after handling many different fabrics over a period of time-this refines the individual’s perception, e.g. when he feels fabrics through his fingers, the warmth in the finger is retained when the fibre is wool but is conducted away when touching fibres like cotton, linen or rayon, the fabrics thus feel cold to touch. Cotton is cool to touch and feels soft and inelastic. Linen is cold and smooth and has a leathery feel. However, the feeling test has its limitations and cannot always be relied upon.
Self-check Questions 3. Match the following. i) ii) iii) iv) v) vi)
Nylon 66 fibre Low density polyethylene Polypropylene fibre Wool fibre Silk fibre Jute fibre
a. Scaly surface b. Cellulosic fibre c. Lowest melting temp d. High melting temp. e. Triangular cross section f. The lightest fibre
3.10 Assignments 3.10.1 Class assignments i)
You may pick up a few numbered samples of fibres from out of those available and try to identify them using facilities made available to you. A report of the experiments performed and the conclusions reached should be submitted.
3.10.2 Home assignments i)
On a chart paper, draw the longitudinal section and cross sectional view of some common natural and synthetic fibres. 11
3.11 Summing Up In this lesson, the tests which are usually performed to identify fibres are described. These include microscopic tests, burning tests, solubility tests, density and melting point measurement, etc. On the basis of the information, an unknown fibre can be identified using data obtained from these tests individually or in combination. The investigation must be systematic and scientific.
3.12 Possible Answers to Self-check Questions 1.
2.
State whether the following statements are Yes / No. i)
a) no
b) yes
c) no
d) no
e) yes
ii)
a) yes
b) yes
c) no
d) no
e) yes
State whether the following statements are True / False. i) ii) iii) iv) v) vi) vii) viii) ix) x)
3.
False False False True False False True False True False
Match the following i) ii) iii) iv) v) vi)
d c f a e b
3.13 Terminal Questions 1.
Explain the scientific basis of the following questions. If necessary, consult relevant literature.
12
i) Though cotton burns readily, ladies are advised to wear cotton saris while working in kitchen or during Deepavali and not saris of synthetic material. Why? ii) Why is wool warm to the touch and cotton cool?
3.14 References and 1.
Gupta, V.B. and Kothari, V.K. (Eds.). 1997. Manufactured Fibre Technology. (Gupta, A.K. Chapter 10 Characterization of Polymers and Fibres. Pp. 203-247). Chapman & Hall. London.
2.
Kothari, V.K. (Ed.) Quality Control. (Sen, K. Chapter 4 Textile Fibres: Classification and Identification, Pp.46-54.) Textile Dept. IIT, New Delhi.
3.
Sreenivasa Murthy, H.V. 1987. Introduction to Textile Fibres. The Textile Association (India), Mumbai.
3.15 Suggested Further Reading 4.
British Standards Institute. British Standards Handbook No.11. 1963. Pp. 391-432.
5.
Dave, M.S. (Ed.). 1980. Textile Fibres. (Chapter 6 Chemical properties of fibres).Textile Association (India), Ahmedabad Unit, Ahmedabad.
3.16 Glossary 1.
Polarizing
Cause to vibrate in one direction only instead of all directions
2.
Refractive index
The ratio of the velocity of light in a vacuum to that in the medium
3.
Medium
the carrier
4.
Ignition
The act of starting a fire
5.
Bunsen flame
A gas burner used in laboratories; has an air valve to regulate the mixture of gas and air
13