29 Fibre Testing – Quantitative tests

S. Sundaresan

INTRODUCTION

Textile industry is the second largest industry after the agriculture industry. The textile industry is making products for apparel end use to high end application like bullet proof vests. For making the product different kind of fibres used according to their physical and chemical characteristics. At this juncture the identification fibres are an important phenomenon. The fibres can be identified using quantitative test like dissolving test, burning test, density measurement etc…This module will give an overall idea on the method of identifying the fibre by doing various tests.

Learning objectives

At the end of 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.

Range of tests

The following technical test can be used to identify the 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 isused. 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 chemically modified 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 polybutyleneterephthalate), 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 fibres, 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

iv) Element identification

v) Density measurement

vi) Determination of melting point, and

vii) Feeling test.

The non-technical used to identify the fibres are

i) Feeling test :The feeling test requires perception if it is to be of any value. Skilled perception is acquired only after handling many different fabrics over a period of time. Limitations of this test become apparent when examining and comparing fabrics of different fiber content.

ii) Burning tests: To recognize the composition of fabrics by the burning test, the sample of fibre, yarn of fabric should be moved slowly towards a small flame and the reaction to heat carefully observed. One end of the sample should be put directly into flame to determine its burning rate and characteristics. The burning odour should be noted and the characteristics of the ash such as amount, form, hardness and colour should be examined.

Technical Tests: There are certain technical tests performed for identifying various fibers. These tests require high technology laboratory equipment and are much more reliable than the non-technical fibre tests. Technical tests require high skilled personnel and technical knowhow of handling chemicals and their accurate analysis. These tests are very valuable for those fabrics that are a blend of different yarns and also have certain special properties including flame retardance etc..,

Types of Technical Tests

Microscopic test : Microscopic test is a technical test that involves identifying the fabric with the help of a microscope with a magnification of minimum 100 power. The test can easily distinguish between fibers. The test identifies the natural fibers more easily as compared to manmade ones. Synthetic fibers are very similar in appearance and the increase in the number of varieties, makes it a little tough to distinguish the fibers even under a microscope. 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 cross section examined on the microscope, the shape of the cross-section and the macroscopic features in the cross-section can help identifying some fibres.
Chemical test: Chemical tests are another technical means of identifying fibers. But chemical tests are not intended for the general consumers. Different types of chemical tests are undertaken to establish the identity of the fibers used. These tests give accurate and precise analysis. The tests are conducted in research laboratory

For doing the fibre identification test the following are necessary

Preparation of test specimen
Apparatus for microscopic examination
Reagents used for chemical tests

Identification of fibre using feeling test

The feeling test requires perception if it is to be of any value. Skilled perception is acquired only after handling many different fabrics over a period of time. Limitations of this test become apparent when examining and comparing fabrics of different fiber content.

Identification of fibre using burning test

To recognize the composition of fabrics by the burning test, the sample of fibre, yarn of fabric should be moved slowly towards a small flame and the reaction to heat carefully observed. One end of the sample should be put directly into flame to determine its burning rate and characteristics. The burning odour should be noted and the characteristics of the ash such as amount, form, hardness and colour should be examined.

(a) when approaching the flame

(b) on the burning behaviour inside the flame

(d) relating to the smell emitted

(e) on the residue left behind after the fibre has burnt out. Identification of fibre using microscopic test

Microscopic test is a technical test that involves identifying the fabric with the help of a microscope with a magnification of minimum 100 power. The test can easily distinguish between fibers. The test identifies the natural fibers more easily as compared to manmade ones. Synthetic fibers are very similar in appearance and the increase in the number of varieties, makes it a little tough to distinguish the fibers even under a microscope. 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 cello tape 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 cello tape. 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 and it is interesting to observe that the characteristic features of these fibres are quite different and thus can assist in their identification. 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.

Identification of fibre using chemical test

Chemical tests are another technical means of identifying fibers. But chemical tests are not intended for the general consumers. Different types of chemical tests are undertaken to establish the identity of the fibers used. These tests give accurate and precise analysis.The tests are conducted in research laboratory

Fibre Identification-Solvent Test

The solubility of a fibre in a particular chemical agent is a means of identification. The fibre can be placed in a chemical at a particular temperature and the solubility will confirm the type of fibre.

Methods available for characterization of the structural, physical, and chemical properties of fibers Optical and Electron Microscopy: Optical microscopy (OM) has been used for many years as a reliable method to determine the gross morphology of a fiber in longitudinal as well as cross-sectional views.

Elemental and End-Group Analysis: The qualitative and quantitative analysis of the chemical elements and groups in a fiber may aid in identification and characterization of a fiber

Infrared Spectroscopy: Infrared spectroscopy is a valuable tool in determination of functional groups within a fiber

Ultraviolet-Visible Spectroscopy: The ultraviolet-visible spectra of fibers, dyes, and finishes can provide clues concerning the structure of these materials, as well as show the nature of electronic transitions that occur within the material as light is absorbed at various wavelengths by unsaturated groups giving an electronically excited molecule.

Nuclear Magnetic Resonance Spectroscopy: Nuclear magnetic resonance (NMR) spectroscopy measures the relative magnitude and direction (moment) of spin orientation of the nucleus of the individual atoms within a polymer from a fiber in solution in a highintensity magnetic field

X-Ray Diffraction: X-rays diffracted from or reflected off of crystalline or semicrystalline polymeric materials will give patterns related to the crystalline and amorphous areas within a fiber

Thermal Analysis: Physical and chemical changes in fibers may be investigated by measuring changes in selected properties as small samples of fiber are heated at a steady rate over a given temperature range in an inert atmosphere such as nitrogen.

Molecular Weight Determination: Molecular weight determination methods provide information concerning the average size and distribution of individual polymer molecules making up a fiber Mechanical and Tensile Property Measurements: Mechanical and tensile measurements for fibers include tenacity or tensile strength, elongation at break, recovery from limited elongation, stiffness (relative force required to bend the fiber), and recovery from bending.

Specific Gravity: The specific gravity of a fiber is a measure of its density in relation to the ensity of the same volume of water, and provides a method to relate the mass per unit volume of a given fiber to that of other fibers.

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,

(d) relating to the smell emitted,

(e) on the residue left behind after the fibre has burnt out. Solubility Tests

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 218º C, nylon 66 at 265ºC). 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.

i) Nylon 66 and Nylon 6: Soluble in formic acid (85%) and m-cresol

ii) Cellulose triacetate: Soluble in chloroform and methylene dichloride

iii) Wool: soluble in 5% NaOH at room temperature

iv) Silk: Soluble in 5% NaOH (hot)

v) Viscose rayon: dissolves in sodium zincate solution

vi) PET: Dissolves in ortho chlorophenol at room temperature

Density Measurement

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

Determination of Melting Point

The melting temperature of a fibre can be very accurately determined with the help of a differential calorimeter.

Conclusion

From this module the various method of identifying the fibres has been studied. The chemicals which are used to dissolve the fibres are discussed. The microscopic appearance of the fibre is also dealt in detail. The fibre melting point also used to identify the types of fibre. The techniques discussed in the module is very much useful for blend analysis in case of the textile product made up of synthetic and natural fibre blend.

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References

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.
Kothari, V.K. (Ed.) Quality Control. (Sen, K. Chapter 4 Textile Fibres: Classification and Identification, Pp.46-54.) Textile Dept. IIT, New Delhi.
Sreenivasa Murthy, H.V. 1987. Introduction to Textile Fibres. The Textile Association (India), Mumbai.