Textile fiber classification and identification: The underlying logic from essence to practical operation
I. The core definition of textile fibers: "Spinnability" is the soul
Textile fibers are not just any fibrous substances. In essence, they are "functional fibers" that can be processed into fabrics through processes such as spinning and weaving. The core requirement is "spinnability", which specifically includes four dimensions:
Length: It needs to reach more than several millimeters (fibers shorter than 5mm cannot be grasped and twisted by the spinning machine).
Fineness: The diameter is usually between 5 and 50 μm (too thick will result in rough yarn, while too thin will lead to insufficient strength).
Mechanical properties: It has sufficient tensile strength (able to withstand the tension during spinning) and flexibility (not easy to break).
Cohesion force: There must be frictional force or adhesive force between fibers (for example, the natural twists of cotton and the scales of wool can enable the fibers to aggregate into continuous yarns).
Simply put, textile fibers are "fibers that can be made into cloth" – they are the "raw material chips" of the textile industry and directly determine the core properties of fabrics such as sweat absorption, warmth retention, and wrinkle resistance.
II. Classification of textile fibers: From "naturally grown" to "artificially synthesized"
The classification logic of textile fibers is centered around "source + processing method", dividing them into two major systems: "natural fibers" and "chemical fibers". Specific types are further subdivided under each system.
(I) Natural fibers: The "original gifts" of nature
Natural fibers come directly from nature and do not require artificial synthesis. They can be used only after physical processing (picking and carding). According to different sources, they are divided into three categories: plant fibers, animal fibers, and mineral fibers.
1. Plant fiber: The plant form of cellulose
The core component of plant fibers is cellulose (a natural polysaccharide composed of glucose molecules), which comes from the phloem, seeds, stems, or leaves of plants. Its characteristics are "good hygroscopicity and easy degradation".
Cotton fiber: It comes from the seed hairs of cotton (formed by the extension of ovule epidermal cells). The longitudinal section has "natural twists" (like ropes twisted together), and the cross-section is "waist-round + central lumen" (the central lumen is a cavity for storing nutrients, and cotton with a smaller central lumen is denser). Cotton has excellent hygroscopicity (the moisture regain rate is about 8.5%), feels soft, and is the preferred raw material for underwear and T-shirts.
Hemp fiber: It comes from the phloem of hemp plants (for example, ramie comes from the phloem of the stem, and flax comes from the phloem of the stem). The longitudinal section has "transverse nodes + stripes" (similar to bamboo joints), and the cross - section is "polygonal + gaps". Hemp fiber has a high lignin content, so it is stiff and breathable, but has a rough feel. Ramie has less lignin and is softer, suitable for summer shirts; flax has more wax and is stiffer, suitable for mats or suit fabrics.
Bamboo fiber: Cellulose from bamboo stems (lignin and pectin are removed through chemical treatment). Its cross - section is in the shape of "ellipse + micropores". Its hygroscopicity is three times stronger than that of cotton, and it comes with the antibacterial component bamboo quinone. It is commonly used in towels and bedding.
2. Animal fibers: The "animal form" of protein
The core components of animal fibers are proteins (such as keratin and silk protein), which come from animal hair or secretions. They are characterized by "good warmth retention and luster".
Wool: It comes from the undercoat of sheep. The longitudinal section has a "scale layer" (covered like fish scales), and the cross - section is a "circle + medullary layer" (the medullary layer is an air layer, and its thermal insulation performance is three times higher than that of cotton). The "felting property" of wool (the scales become entangled after friction) enables it to be made into sweaters, but it is prone to pilling (the entanglement of scales causes the fiber ends to pop out).
Silk: It comes from the secretions of the silk glands of silkworms. The silk spun by domestic silkworms (mulberry silkworms) is "mulberry silk". Its longitudinal section is smooth, and its cross - section is "triangular", with a soft luster like pearls. The silk spun by wild silkworms (tussah silkworms) is "tussah silk". Its longitudinal section has nodules, and its cross - section is irregular. It is coarser and more yellowish, and is suitable for thick silk fabrics or carpets.
Camel hair: It comes from the fine undercoat of camels (camel down). The fibers are finer (14 - 16μm), with a thicker medulla layer. Its thermal insulation is 50% higher than that of wool, and it is lighter in weight. It is often used in high - end down jackets.
3. Mineral fibers: The special existence in the inorganic world
Mineral fibers are natural inorganic fibers derived from silicates or oxides in the earth's crust. They are characterized by "heat resistance and non - combustibility", but they are niche products (some of them contain carcinogens).
Asbestos: A type of silicate fiber with extremely high heat resistance (it can withstand up to 1000°C). However, its fibers are very fine and can easily enter the lungs, and it has been banned in many countries.
Gypsum fiber: An oxide derived from gypsum. It is fire-resistant but highly brittle, and is used for fireproof curtains or building materials.
(II) Chemical fibers: The fiber revolution of "artificial manufacturing"
Chemical fibers are made through chemical processing using petrochemical raw materials or natural polymers as the basis. They are the core raw materials of modern textiles (accounting for over 70% of the global fiber production). According to the processing methods, they are divided into two categories: regenerated fibers and synthetic fibers.
1. Regenerated fibers: Rearrangement of natural raw materials
Regenerated fibers are made from natural polymers (cellulose, protein). They are dissolved into a solution through chemical treatment and then spun into fibers - in essence, it is an "industrial reconstruction of natural raw materials".
Viscose fiber: The most common regenerated cellulose fiber, with raw materials being wood pulp or cotton linters (short fibers after cotton processing). Processing flow: Alkalization (treatment with caustic soda) → Xanthation (treatment with carbon disulfide) → Spinning (acid bath coagulation). The longitudinal section of viscose has "grooves" (similar to sugarcane peel), and the cross - section is "serrated". Its hygroscopicity is comparable to that of cotton (moisture regain rate of 13%), but its wet strength is extremely low (only 50% of the dry strength), and it is prone to deformation after washing. It is commonly used in underwear and socks.
Modal: An upgraded version of "high wet modulus viscose", the raw material is European beech wood pulp. By optimizing the spinning process (increasing the concentration of the coagulation bath), the fiber molecules are arranged more closely - its wet strength is 30% higher than that of viscose, and it is softer and more lustrous. It is a commonly used raw material for high - end underwear and T - shirts (such as Uniqlo's modal series).
Soybean protein fiber: It uses soybean meal (the residue after oil extraction) as raw material. After extracting the protein, it is mixed with viscose solution for spinning. The fiber combines the luster of silk (due to the protein component) and the hygroscopicity of cotton (due to the cellulose component), but it has low strength (prone to breakage) and is often used for pajamas and scarves.
2. Synthetic fibers: The molecular creation of petrochemical raw materials
Synthetic fibers start from petroleum and natural gas. Through a "polymerization reaction", high - molecular resins (such as polyester and polyamide) are produced, and then they are spun into fibers. In essence, they are "completely artificially synthesized high - molecular materials".
Polyester (polyester fiber): The synthetic fiber with the highest global production (accounting for 60% of synthetic fibers), the raw materials are terephthalic acid + ethylene glycol, and 「polyethylene terephthalate (PET)」 is obtained after polymerization. The longitudinal section of polyester is smooth, and the cross-section is 「round or irregular (triangular, pentagonal)」. It is characterized by high strength (3 times that of cotton), good wrinkle resistance (no deformation when heated), and non - hygroscopicity (moisture regain of 0.4%). It is commonly used in suit fabrics and sportswear (quick - drying), but it is prone to pilling (the fiber surface is smooth, and the ends pop out after friction).
Polyamide (polyamide fiber): Commonly known as "nylon", the raw materials are adipic acid + hexamethylenediamine. After polymerization, "polyhexamethylene adipamide (PA66)" is obtained. The longitudinal section of polyamide is smooth, and the cross - section is circular. Its core advantage is wear resistance (10 times that of cotton and 2 times that of polyester) and good elasticity (elongation at break is 200%). It is commonly used in jeans stitching, stockings, and wind - proof mountaineering clothing. However, its thermal stability is poor (it deforms at temperatures above 150°C).
Spandex (polyurethane elastic fiber): The "king of elastic fibers", its raw materials are diisocyanate + polyol. After polymerization, "polyurethane" is obtained. The elongation of spandex can reach 500% - 700%, and it can rebound quickly. Just adding 1% - 5% can make the fabric elastic (such as the waistband of sports pants and the shoulder straps of underwear), but it has poor heat resistance (it cannot be ironed at high temperatures).
III. Identification of Textile Fibers: From "Empirical Judgment" to "Precise Analysis"
The core of fiber identification is to find the "unique features" (morphology, composition, thermal properties) of the fiber and verify them through corresponding methods. The commonly used methods are divided into four major categories: "empirical methods, microscopic methods, chemical methods, and instrumental methods".
(I) Sensory discrimination method: Use the "human senses" for preliminary screening
Sensory identification is the most basic method. It can quickly determine the fiber type through touching, visual inspection, listening, and smelling, and is suitable for daily scenarios.
Touch with hands: Cotton (soft with a rough feeling), linen (stiff and rough), silk (smooth and cool), wool (plump and elastic), polyester (smooth and stiff), nylon (soft and elastic), acrylic (light and fluffy).
Visually inspect: Cotton (lackluster, with short fuzz), Linen (dull in luster, with bamboo - like knots), Silk (with a soft pearly sheen), Wool (with a hazy luster, with scales), Polyester (bright in luster, with a glitter).
Hearing: Silk (it makes a "silk rustle" when rubbed - the fiber surface is smooth and the vibration sound is produced by friction), cotton/wool (no sound).
Smell by nose: Silk/wool (a slight smell of protein), cotton/linen (odorless), synthetic fibers (odorless).
(II) Combustion identification method: Examine chemical components through the "flame reaction"
Burning identification is the most commonly used method. The fiber composition is determined by the burning state, flame color, odor, and ash (the burning characteristics of different components vary greatly).
Combustion characteristics of common fibers:
Fiber type Combustion state Flame color Odor Ashes
Cotton/linen (cellulose) Burns quickly without melting Orange-yellow Smells like burning paper Grayish-white, soft and fine powder
Silk/Wool (Protein) Burns slowly and curls up first Yellow with a tinge of blue Smells like burnt hair (sulfur smell) Black brittle balls that can be crushed easily
Polyester (PET) First shrinks and melts, then burns Yellow with a tinge of blue Pungent ester smell Black hard balls, not easily broken
Polyamide (Nylon) Burns after melting and drips Yellow tip on a blue base Celery smell Light brown hard balls that can be crushed
Acrylic (Polyacrylonitrile) Melts and emits black smoke Red-orange Pungent odor Black brittle pieces
Practical operation skills: Take a 1-cm-long fiber, hold it with tweezers and ignite it. When wool burns, it will curl into a small ball (protein denaturation), with a small flame and light blue smoke. It smells like burning hair, and the ash is a black brittle ball (which can be crushed into powder with a press). When polyester burns, it first melts (melting point: 250°C), the flame has black smoke, it has a pungent smell, and the ash is a black hard ball (which cannot be crushed).
(III) Microscopic identification method: Make accurate judgments based on "morphological characteristics"
Microscope identification is the most intuitive morphological analysis. The type is determined by observing the longitudinal section (lengthwise) and cross-section (perpendicular direction) of the fibers - the morphology of each fiber is a "unique ID card".
Microscopic characteristics of common fibers:
- Cotton: Longitudinal section (natural twist, like a twisted rope), cross-section (kidney-shaped + lumen);
- Hemp: Longitudinal section (horizontal nodal stripes, resembling bamboo joints), cross section (polygon + gaps);
- Silk: Longitudinal section (smooth without twists), cross-section (triangular/semi-elliptical);
- Wool: Longitudinal section (scale layer, resembling fish scales), cross-section (circular + medullary layer);
- Viscose: Longitudinal section (grooves), cross - section (zigzag).
- Polyester: Longitudinal section (smooth), cross-section (round/irregular).
Practical operation steps: Make the fiber into slices (longitudinal slice: cut longitudinal thin slices; transverse slice: cut transverse thin slices after paraffin embedding), and observe under a microscope. For example, to distinguish cotton from viscose, look at the cross-section: cotton is kidney-shaped with a lumen, while viscose is serrated without a lumen, which is very clear at a glance.
(IV) Chemical reagent identification method: Distinguish component differences through "chemical reactions"
Chemical identification utilizes the differences in the chemical stability of fibers - different fibers have different solubility and color - changing reactions to specific reagents, which is suitable for precise verification.
Common reagent reactions:
Iodine-potassium iodide solution: Cellulose fibers (cotton, linen, viscose) turn blue (cellulose forms a complex with iodine), while protein/synthetic fibers remain unchanged in color.
Concentrated nitric acid: Protein fibers (silk, wool) turn yellow (nitration reaction of benzene rings), while other fibers remain unchanged in color.
Hot phenol solution: Polyester (PET) dissolves (polyester is soluble in hot phenol), while nylon/cotton does not dissolve.
85% formic acid: Nylon dissolves (polyamide is soluble in formic acid), while polyester/cotton does not dissolve.
Practical example: To distinguish between wool and acrylic (both are fluffy), drop concentrated nitric acid. The wool turns yellow while the acrylic remains unchanged in color, allowing for an instant distinction.
(V) Instrumental analysis identification method: Use "modern technology" for final confirmation
Instrumental analysis is a precise laboratory-level identification method. By measuring the chemical structure, thermal properties, and morphology of fibers, it is suitable for complex or novel fibers (such as nanofibers).
Infrared spectroscopy (IR): Measure the absorption peaks of functional groups — Cellulose has O-H (3300 cm⁻¹) and C-O (1100 cm⁻¹) peaks; Protein has N-H (3300 cm⁻¹) and C=O (1650 cm⁻¹) peaks; Polyester has an ester group (1720 cm⁻¹) peak.
Scanning Electron Microscope (SEM): Observe the surface morphology with high resolution. For example, the surface of bamboo fibers has tiny cracks, and the surface of viscose has smooth grooves. All these can be clearly seen under the SEM.
Differential thermal analysis (DTA): Measure the thermal decomposition temperature - cotton (350°C), wool (230°C), polyester (400°C), with obvious temperature differences.
IV. The "Logical Chain" for Fiber Identification: A Judgment Process from Coarse to Fine
The core logic of fiber identification is "from experience to precision": first, make a preliminary judgment through sensory perception or combustion, then verify it with a microscope or chemical reagents, and finally confirm it through instrumental analysis.
For example, when encountering "unknown fibers", the steps are as follows:
1. Touch with hands (soft) → Observe with eyes (lustrous) → Burn (smell like burning hair) → Examine under a microscope (triangular cross-section) → Confirm it is "mulberry silk".
V. Only by understanding fibers can one understand fabrics
The essence of textile is "the arrangement and combination of fibers" - the sweat absorption of cotton, the luster of silk, and the wrinkle resistance of polyester are all extensions of fiber characteristics. By mastering classification and identification, one can shift from "seeing fabrics as mere cloth" to "seeing fabrics as a collection of fibers" and truly understand the performance logic of fabrics.
Simply put: fibers are the raw materials, fabrics are the end - products, and identification is the key to understanding the end - products.