Common fiber identification methods

With the continuous progress of textile production technology, more and more new fibers have become raw materials for textiles, which has brought about the problem of identifying fibers used in textiles. The following is an introduction to commonly used fiber identification methods, including microscopy, combustion, reagent colorimetry, staining, dissolution, and so on.

1. Microscopic observation method

Make longitudinal and cross-sectional sections of fibers and observe their longitudinal and transverse morphology under a microscope. Identify fibers based on differences in morphological characteristics, as shown in Table 1.

Table 1 Morphological Characteristics of Various Fibers

Fiber type	Longitudinal morphology	Cross section shape
Lyocell fiber	Smooth	Regular circular or elliptical shape with skin core layer
Modal fiber	There are 1-2 grooves in the longitudinal direction	Irregular, similar to a round waist, relatively smooth, with a leather core
Soybean fiber	Irregular grooves and island shaped bumps on the surface	Flat dumbbell shaped and waist circular
Bamboo fiber	Surface has grooves	Sawtooth shaped, with a leather core layer
Chitin fiber	There are obvious grooves on the surface	The edge is serrated, and there are obvious small gaps in the core layer
Viscose fiber	Surface has grooves	Sawtooth shaped, with a leather core layer
Cotton fiber	With natural curls	Round waist with a central cavity
Ramie fiber	The fiber is relatively thick, with long stripes and bamboo shaped transverse joints	Round waist with a central cavity
Flax	Fine fibers with bamboo shaped transverse joints	Polygon with middle cavity
Silk fiber	Smooth surface	Irregular Triangle
Wool fiber	There are scales on the surface	Circular
Copper ammonia fiber	Smooth and glossy surface	Circular or nearly circular
Polyester fiber	Smooth surface, rod-shaped	Circular or nearly circular and various irregular cross-sections
Acetate fiber	Smooth surface with grooves	Trilobed or irregularly serrated
Acrylic fiber	Smooth surface with grooves or stripes	Circular, dumbbell shaped or leaf shaped
Nylon fiber	Smooth surface with small black spots	Circular or nearly circular and various irregular cross-sections
Vinylon fiber	Flat strip with grooves	Waist or dumbbell shaped
Spandex fiber	Smooth surface	Circular or cocoon shaped
Chloroprene fiber	Smooth surface, some with bony stripes	Circular or nearly circular
Polypropylene fiber	Smooth surface with some scars	Circular or nearly circular

2, combustion method

During the combustion process, different fibers may have different flame, smoke, odor, residue, etc. Therefore, fiber types can be distinguished based on the characteristics of fiber combustion and after combustion, as shown in Table 2.

Table 2 Characteristics of Various Fibers During and After Combustion

Fiber type	Approaching flames	In flames	Leave the flame	Burning odor	Residue morphology
Lyocell fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey, black, gray
Modal fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey, black, gray
Soybean fiber	Contraction	Burning does not melt, with black smoke	Not easily ignited	Barbecued and flavored	Crispy black gray
Bamboo fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey, black, gray
Chitin fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey, black, gray
Viscose fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey White Grey
Cotton fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey White Grey
Hemp fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey White Grey
Silk fiber	Contraction	Gradually burning	Not easily ignited	Barbecued and flavored	Crispy black gray
Wool fiber	Contraction	Gradually burning	Not easily ignited	Barbecued and flavored	Crispy black gray
Copper ammonia fiber	Non melting and non shrinkage	Rapid combustion	Continue to burn	Burnt paper flavor	Grey White Grey
Polyester fiber	Shrinkage melting	Melt first and then burn, with solution	Capable of delaying combustion	Aromatic taste	Glass shaped black brown hard ball
Acetate fiber	Shrinkage melting	Melt first and then burn, with solution	Capable of delaying combustion	Acetic acid flavor	Irregular black brown hard ball
Acrylic fiber	Shrinkage micro melting	Melting combustion with small sparks	Continue to burn	Spicy taste	Black Crispy Hard Block
Nylon fiber	Shrinkage melting	Melt first and then burn, with solution	Capable of delaying combustion	Ammonia odor	Glass shaped black brown hard ball
Vinylon fiber	Shrinkage melting	Burning	Continue to burn	Special sweet taste	Yellowish brown hard ball
Spandex fiber	Shrinkage melting	Molten combustion	Self extinction	Special odor	White adhesive block
Chloroprene fiber	Shrinkage melting	Melt combustion, large amount of black smoke	Unable to delay combustion	Hydrogen chloride odor	Dark brown hard lump
Polypropylene fiber	Slow contraction	Molten combustion	Continue to burn	Mild asphalt smell	Yellowish brown hard ball

3. Reagent Colorimetry

Due to the different structures of various fibers, the coloring reactions to iodine and potassium iodide solutions are different. The fibers can be identified by observing the color and swelling of the fibers after the action of reagents. This identification is only applicable to white fibers, and colored fibers need to undergo fading before being tested. Reagent preparation: Dissolve 20 g of iodine in 100 ml of saturated potassium iodide solution, immerse the fibers in the prepared solution for 1 minute, wash with sufficient water, and judge based on their color, as shown in Table 3.

Table 3 Coloring Reaction of Various Fibers on Iodine Solution during Combustion

Fiber type	Lyocell fiber	Modal fiber	Soybean fiber	Bamboo fiber	Chitin fiber
Iodine solution coloration	Black Blue Blue Blue	Blue Grey	Brown	Blue Grey	Black
Fiber type	Viscose fiber	Cotton fiber	Hemp fiber	Silk fiber	Wool fiber
Iodine solution coloration	Black Blue Blue Blue	Unstained	Unstained	Light yellow	Light yellow
Fiber type	Polyester fiber	Acetate fiber	Acrylic fiber	Nylon fiber	&Nbsp
Iodine solution coloration	Unstained	Unstained	Unstained	Black brown	&Nbsp

4. Staining Method

The use of dyeing method to distinguish fibers is mainly based on the different color reactions of various fibers to dyes. There are two dyeing methods: cold dyeing and boiling dyeing. The formula for dyeing method is shown in Table 4.

Table 4 Staining Formula

Cold dyeing method Boiling dyeing method Dyes Weight (g) Dyes Weight (g)

Direct Indigo 2B	2.5	Acid Fuchsin 6B	1
Acid Fuchsin 6B	3	Base light yellow (concentrated)	1
Picric acid	5	Disperse Blue GF	0.5
Tannic acid	5	Pancreatic bleaching T	0.5

Pour the formula in Table 4 into 50 ml of ethanol, stir and dissolve, and then dilute with distilled water to 500 ml for later use. The cold dyeing method directly puts the fibers into the solution for dyeing, then rinses the fibers with cold water, squeezes out water, and observes the color for identification; The boiling dyeing method involves adding fibers to a solution for dyeing, boiling for 3 minutes, rinsing 3 times, and then rinsing them in a 0.1% pancreatic bleaching T solution. The fibers are then squeezed out of water and the color is observed for identification. The dyeing reactions of various fibers are shown in Table 5.

Table 5 Dyeing Reactions of Various Fibers

Fiber type	Cotton fiber	Viscose fiber	Vinylon	Acrylic fiber	Nylon
Cold dyeing method	Purple	Red Purple	Light yellow green	Not colored	Light yellow
Boiling dyeing method	Earthy yellow	Light yellow	Dark Olive Green	Light Sky Blue	Dark grass green

5. Dissolution Method

Qualitative identification of plant fibers, animal fibers, mineral fibers, and chemical fibers can be achieved by using different chemical reagents to investigate the dissolution characteristics of different fibers at different temperatures. Can be used for qualitative identification of fibers. There are many chemical reagents that can be used for qualitative identification of fibers, such as sulfuric acid, hydrochloric acid, formic acid, nitric acid, sodium hydroxide, zinc oxide, N-dimethylformamide, sodium hypochlorite, cyclohexanone, copper ammonia solution, glacial acetic acid, acetone, dimethyl sulfoxide, etc. During the inspection, attention should be paid to the influence of reagent concentration and temperature on the test results. The test temperature is generally divided into room temperature (24-30 ℃) and boiling. The solubility test method can accurately identify various types of textile fibers.

6. Coloration reaction method with chlorine and nitrogen

The chlorine and nitrogen containing color reaction method can be used for rough classification of chemical fibers for further qualitative identification. Its experimental principle is that various chlorine and nitrogen containing fibers are detected by flame method or acid-base method, and will exhibit specific color reactions. Chlorine test: After touching the fibers with hot copper wire, move it to the oxidation flame of the flame and observe if the flame is green. If there is chlorine, a green flame will occur. Nitrogen test: Place a small amount of chopped fibers in a test tube, cover with an appropriate amount of sodium carbonate, heat to produce gas, and place a red litmus test paper at the mouth of the test tube to turn blue, indicating the presence of nitrogen. Among the existing textile fibers, chlorine containing fibers are mainly polyvinyl chloride and polyvinylidene chloride fibers, while nitrogen containing fibers include silk, wool fibers, polyacrylonitrile fibers, polyamide fibers, and polyurethane fibers.

7. Melting Point Method

Under the action of high temperature, thermoplastic fibers undergo a change in the bonding structure between macromolecules, resulting in a transition from solid to liquid state. Using a polarizing microscope or melting point microscope with a heating device, the fiber melting temperature (base melting point) can be measured by visual or photoelectric detection from changes in the appearance and morphology of the fiber. Different types of thermoplastic fibers have different melting points, which can be used to identify the types of fibers. The melting point method is generally not used solely for qualitative identification of fibers, but rather for verification and determination of the melting point of thermoplastic fibers. This is because some fibers have relatively close melting points, while others do not have obvious melting points.

8. Infrared Spectral Identification Method

The experimental principle for qualitative identification of various textile fibers using infrared spectroscopy is that when a beam of infrared light is irradiated on the tested sample, a portion of the light energy absorbed by the substance molecules is converted into the vibration and rotation energy of the molecules. By using an instrument to plot the absorption value with the corresponding wave number, the infrared absorption spectrum of the sample can be obtained. Each characteristic absorption band in the spectrum contains information about the functional groups in the sample molecules. Different substances have different infrared spectra. By comparing the standard infrared spectra of unknown fibers with known fibers, the category of fibers can be distinguished.