chemical test of fiber

Chemical Tests
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 Method: 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.

Solvent Method: 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.