Technical Tests for Fiber Identification
There are two types of methods that are used for identifying different fibers
- the nontechnical tests and the technical tests. The nontechnical tests include the feeling test and the burn test. The technical tests include microscope test and chemical test. The technical tests for fiber identification are carried out in laboratories and require technical knowledge and skills. As such, they are much more reliable methods for testing end product
as compared to the non technical tests.
The Nontechnical Tests- Feeling Test and Burning Test
Feeling test involves touching a fabric and feeling the fabric to know its component fibers. For example, wool fabrics
will feel warm when touched because the heat generated by wool, which is a nonconductor of heat, will remain in the touched area itself. On the other hand, the fabrics
made up of plant fibers
such as cotton fabrics
, linen fabrics
and even the rayon fabrics
, that are made from the cellulose of wood pulp or cotton fiber
, feel cool to touch. As they are conductors of heat, the heat generated by the finger passes off making the fabric cold. However, it requires a long experience of handling different fabrics over a period of time for such skillful perception. Also, it is difficult to examine and compare the fabrics made of different fiber contents with the feeling test.
The other nontechnical test for fiber identification
by the burn test- involves burning a sample of fabric and observing the various characteristics shown by it after burning in order to determine its fiber content. The burning test is more efficient than the feeling test but it also has its limitations. For example, fabrics made of biconstituent fibers, that are combination of two different textile polymers
, can not be identified with this test.
The Technical Tests- Microscope Test and Chemical Test
The technical tests for fiber identification done with the help of laboratory equipment are far more reliable than the nontechnical tests. However, technical knowledge and skill, particularly while handling chemicals, are the basic requirements for conducting these tests.
Microscopes having magnification of at least 100 power, can be successfully employed for testing and identifying the fiber contents of a fabric. Microscope test is very effective for testing the natural fabrics
. Difficulties can be faced while testing synthetic fabrics
as many of them have similar appearance. However, one must know, what the fibers look like under a microscope as many finishing processes like mercerizing and delustering, change the appearance of fibers under microscope. Apart from it, dark colored fabrics also cannot be tested with microscope as light cannot pass through dark substances. For such fabrics, either the textile dyes
be removed by stripping, bleaching etc. or they have to be chemically tested.
have their own peculiar structures, spots, lines and other marks that help in identifying them. Following are some examples of natural fibers and how they look like under a microscope:
The cotton fiber is a single elongated cell. Under a microscope, it looks like flat, spirally twisted ribbonlike tube with rough granular surface. However, mercerized cotton
doesn't have natural twist. The finishing process makes them swollen, straight, smooth and round with a shining surface.
, under a microscope, looks like having multiple sided cylindrical filaments with fine pointed edges. The filaments show nodes at intervals. It, in fact, looks like a bamboo stick having joints that results into a little unevenness.
has irregular, roughly cylindrical, multi cellular structure with tapered ends. Under a microscope, three basic layers are shown- epidermis (outer layer), cortex (middle layer) and medulla (inner layer). Medulla is seen only in coarse and medium wool fibers and that too under a highly powerful microscope.
Raw silk fiber
, composed of two filaments, has elliptical shape under the microscope. The two fine and lustrous filaments are shown clearly looking like transparent rods with triangular shape. Wild silk or tussah fiber has different appearance than the cultivated silk. It is flattened, coarse, thick and broader fiber having fine, wavy lines all across its surface whereas cultivated silk is narrower fiber with no marks on it.
Manmade fibers are difficult to identify through microscope because of similar appearance of many fibers. However, their certain distinguishable characteristics under a microscope have been mentioned below.
has uniform diameter with glass like shine. If delustered then rayon fiber shows marks similar to pepper, when viewed cross sectionally. Viscose fiber
of rayon looks irregular when viewed cross sectionally.
Acetate fiber looks lesser irregular than viscose rayon when viewed cross sectionally. It has indentations that look like occasional marks when viewed longitudinally.
There are many variants of nylon fiber
. However, generally it appears fine, round, smooth and translucent. Sometimes it has shiny appearance. If it looks dull, it will also be dotted under the microscope.
If viewed longitudinally, aramid fiber
looks smooth and straight. If viewed cross sectionally, it may be round or like peanut's shape.
Generally, polyester fiber
is smooth, straight. It looks round cross sectionally. However, with various finishing processes, its appearance changes in context of texture and luster.
Spandex: Spandex fiber
have the outstanding characteristic of appearing like groups of fibers fused together. However, different variants of spandex show different characteristics too. The Lycra fiber looks like fused multifilaments cross sectionally. Individual fibers are dotted and in shape like that of dog-bone. If viewed longitudinally, they appear straight.
When viewed cross sectionally, polypropylene fiber
looks somewhat round but it looks straight and smooth when viewed longitudinally.
The glass fiber
looks smooth, round, translucent, shiny and flexible.
Chemical tests for fiber identification can only be conducted in well equipped laboratories. There are two primary methods to conduct chemical testing- stain and solvent.
Stain technique uses acid and alkali on different fabrics to identify their fiber contents. Most of the fibers have two color reactions when treated with stain. A fiber stained with dilute acetic acid
turns to a specific color. The same fiber when stained with mild alkali like soda carbonate turns to a different color again specific to that fiber only. Acetate changes to light green color when acetic acid is used and turns orange when dilute carbonate of soda is used. Likewise, nylon turns beige in one and bright red in other. As double testing is done in this method, it is sometimes referred to as double-barreled stain identification.
Various solvents are used in this method to distinguish one kind of fiber from another. However, there is no single solvent or chemical that can be used on all fibers. Additionally, different solvent procedures are adopted to separate and identify the fibers that are combined together. It becomes very difficult to use solvent methods in view of fibers that have similar chemical characteristics. Also, when more fibers are mixed to produce blended fabric
, then also it becomes tough to identify the fibers with the help of solvent method. However, it is a very effective method for cross checking but in order to have accurate reports, the fabric has to be cleaned thoroughly and the finishing chemicals
should also be removed completely. The fabric has to be unraveled, yarns
have to be untwisted and the fibers have to be put in the solutions in as loose a condition as is possible.
As an example of solvent method, consider differentiating animal fibers
from plant fibers with alkali. If wool or silk fiber has to be eliminated from a blended fabric then strong alkalies can be used because animal particles are destroyed in it. Five percent of caustic soda or sodium hydroxide is used in water. The action of the chemical is hastened by boiling the solution before immersing the sample fabric in it. The wool or silk fiber gets completely dissolved in it. The plant fibers remain unaffected. For differentiating them, acid has to be used as the dilute acids destroy plant fibers. A drop of sulfuric acid has to be put on the sample fabric which, in turn, is placed between two blotters and pressed with hot iron. If it contains cotton, linen or rayon then the fabric gets charred at the spot.