Natural vs Synthetic Fibres

Fibres are a subclass of polymers characterized by strong intermolecular forces, high tensile strength, and a high length-to-diameter ratio. In polymer chemistry, fibres are distinguished by their crystalline nature, which arises from strong dipole-dipole interactions or hydrogen bonding between the macromolecular chains. Based on their origin and processing, they are categorized into natural, synthetic, and semisynthetic fibres.

The Polymer-Fibre Relationship

• Linear Alignment: Unlike elastomers which have loose, coiled structures, fibres consist of elongated, linear polymer chains aligned parallel to the fiber axis.
• Hydrogen Bonding: The presence of polar groups (such as amide or ester groups) enables intense intermolecular hydrogen bonding, imparting high modulus and low elasticity.

Natural Fibres

Natural fibres are obtained directly from geological, plant, or animal sources without fundamental chemical alteration of their polymer backbones.

Plant-Based (Cellulosic) Fibres

• Cotton: Consists of nearly 90% cellulose. Structurally, it is a polymer of β-D-glucose units. It exhibits high moisture absorption due to numerous free hydroxyl (-OH) groups that form hydrogen bonds with water molecules.
• Flax and Jute: Bast fibres derived from the phloem of plant stems. They contain a higher percentage of lignin and hemicellulose along with cellulose, making them coarser and structurally stronger than cotton.

Animal-Based (Protein) Fibres

• Silk: An animal polymer secreted by the silkworm (Bombyx mori). It is primarily composed of the protein fibroin, which is rich in the amino acids glycine, alanine, and serine. The chains form a beta-pleated sheet structure held together by hydrogen bonds, giving silk its characteristic tensile strength and luster.
• Wool: Obtained from sheep and other animals. It is composed of the complex protein keratin, which is rich in sulfur-containing amino acids like cystine. The presence of disulfide bridges (S-S bonds) creates a helical, resilient structure that gives wool its elasticity and crimp.

Synthetic Fibres

Synthetic fibres are entirely man-made macromolecules synthesized via polymerization of petrochemical-derived monomers. They are engineered for high durability, resistance to chemicals, and low moisture retention.

Polyamide Fibres

• Nylon 6,6: Manufactured via the condensation polymerization of hexamethylenediamine and adipic acid under high temperature and pressure.
  nH₂N(CH₂)₆NH₂ + nHOOC(CH₂)₄COOH → [ -NH(CH₂)₆NH-CO(CH₂)₄CO- ]_n + 2nH₂O
  The strong amide linkages (-CO-NH-) provide exceptional mechanical strength, elasticity, and abrasion resistance.
• Nylon 6: Synthesized by heating caprolactam with water at elevated temperatures via ring-opening polymerization. It is highly rugged and used extensively in industrial ropes, tyre cords, and heavy-duty fabrics.

Polyester Fibres

• Terylene (Dacron): Produced by the condensation polymerization of ethylene glycol (ethane-1,2-diol) and terephthalic acid (benzene-1,4-dicarboxylic acid). The repeating monomer units are linked by ester bonds (-CO-O-). It is highly resistant to wrinkling, moth damage, and chemical degradation, and is frequently blended with cotton (Polycot) or wool (Polywool).

Acrylic Fibres

• Orlon (Polyacrylonitrile / PAN): Formed by the addition polymerization of acrylonitrile monomers in the presence of a peroxide catalyst. It serves as a lightweight, warm, and wrinkle-resistant synthetic substitute for natural wool.

Semisynthetic Fibres

Semisynthetic fibres are produced by chemically treating natural polymers (primarily cellulose) to modify their physical properties and processability.

• Rayon (Viscose Rayon): Regenerated cellulose prepared by dissolving purified wood pulp in sodium hydroxide and carbon disulfide to form a viscous liquid, which is then extruded through spinnerets into a bath of sulfuric acid. While chemically identical to cellulose, its physical structure is altered to mimic the texture, drape, and luster of silk.
• Cellulose Acetate: Formed by acetylating cellulose with acetic anhydride and acetic acid, converting free hydroxyl groups into acetate esters. It is thermoplastic and used in specialty textiles and photographic film.

Comparative Analytical Framework

UPSC Prelims Facts and Applied Chemistry Trivia

The Burning Test for Fibre Identification

An analytical method used to distinguish fibre categories based on their combustion behavior:

• Cellulosic Fibres (Cotton, Rayon): Burn with a steady yellow flame, smell of burning paper, and leave a fine, gray ash residue.
• Protein Fibres (Silk, Wool): Burn slowly with a sputtering flame, smell of burning hair, and leave a dark, crushable irregular bead or ash.
• Synthetic Fibres (Nylon, Polyester): Melt and shrink away from the flame, emit a chemical odor, and leave a hard, uncrushable round plastic bead upon cooling.

Wrinkle Resistance Mechanism

Natural cotton fabrics wrinkle easily because water molecules break the internal hydrogen bonds between adjacent cellulose chains, allowing them to slip and reform bonds in a distorted state upon drying. Synthetic polyester lack these free hydroxyl groups, maintaining its polymer alignment and resisting deformation.

Microplastic Pollution

When synthetic fabrics like polyester and nylon are laundered, they shed microscopic plastic fragments known as microfibers (less than 5mm in size). Because they are non-biodegradable, these microfibers bypass water filtration systems, enter marine ecosystems, and bioaccumulate across the food chain, posing a significant ecological challenge.