Natural fibers occur in a fiber or filament form in nature, distinguishing them from synthetic fibers manufactured through chemical processes. These filaments have been utilized for millennia for textiles and construction. Natural fibers are broadly categorized into three main groups based on their source of origin: plants, animals, or geological mineral deposits.
Fibers from Plant Sources
Fibers derived from plants are primarily composed of cellulose, the most abundant organic polymer on Earth. Cellulose is a polysaccharide, a long chain-like molecule made of repeating glucose units. This structure provides the plant cell wall with its stiffness and tensile strength. Plant fibers, often called lignocellulosic fibers, also contain varying amounts of hemicellulose, lignin, and pectin, which influence the fiber’s final characteristics.
These fibers are harvested from three distinct locations within the plant anatomy. Seed fibers, such as cotton, are hair-like outgrowths covering the seeds and are notable for their high cellulose purity. Bast or stem fibers, including linen, hemp, and jute, are bundled filaments found in the plant stem, requiring retting to separate them. Leaf fibers, such as sisal and abaca, are long, coarse filaments extracted directly from the leaves. The specific ratio of cellulose, lignin, and other components determines whether the fiber is soft and flexible, like cotton, or strong and coarse, like jute.
Fibers from Animal Sources
Fibers from animal sources are protein-based, unlike plant fibers which are cellulose-based. This protein structure gives them superior elasticity and warmth retention. These fibers fall into two main subcategories: those derived from hair or fur and those that are secreted filaments.
Hair and fur fibers, like wool, cashmere, and alpaca, are composed of the protein keratin. Keratin contains the sulfur-containing amino acid cysteine, which forms strong disulfide cross-links between protein chains. These chemical bonds allow the fiber to stretch and return to its original shape, contributing to elasticity and resilience. The crimped structure of these fibers also traps air, creating natural insulation and warmth.
The second type, filament fibers, is exemplified by silk, which is secreted by silkworms. Silk is composed mainly of the protein fibroin, structured into crystalline sheets. Unlike keratin, fibroin has a lower sulfur content, and its strength comes from the tight packing of its amino acid chains, giving silk its smooth texture and high luster. The continuous nature of the silk filament also contributes to the material’s strength.
Fibers from Mineral Sources
The third category of natural fibers is derived from geological sources, specifically mineral deposits. These fibers are inorganic in composition, contrasting sharply with the organic polymers of plant and animal origins. The chemical makeup of these geological fibers is based on the silicate family of minerals.
The most recognized example is asbestos, a term for a group of six fibrous silicate minerals. The fundamental structural unit of these minerals is the silicate tetrahedron, which arranges itself into long, flexible crystalline structures. Asbestos was valued for its unique properties, including its resistance to heat, fire, and chemical degradation. However, the microscopic, durable nature of these fibers makes them highly toxic when inhaled, leading to strict regulation of their use.