Monday, March 1, 2010

Textiles review: natural protein fibers

This is part three of my textiles review, in which we go over the natural protein fibers.

Natural protein fibers come from animals. Wools are the hair and fur of animals, and silk is secreted by silkworms. The protein in wools is keratin, and the protein in silk is fibroin. While the word “wool” technically refers to any hair fiber from any animal, it is commonly understood to refer only to sheep hair. Specialty wools (fibers from other animals) are usually referred to by the animal’s name, and they cost more than sheep wool.

All natural protein fibers share certain properties because of their common chemical composition. These properties are:

Resiliency: Protein fibers are resilient. They resist wrinkling, and wrinkles may disappear between uses. Wool is more resilient than silk.

Hygroscopic: Protein fibers are highly absorbent. They shed water slowly, so they retain their insulation properties and remain dry to the touch while wet.

Weaker when wet: Protein fibers have lower tenacity when wet. Silk loses about 15% strength when wet, and wool loses about 40%.

Specific gravity: Protein fibers have a lower specific gravity than cellulosic fibers.

Harmed by alkalis, oxidizing agents, and dry heat: Detergents, bleaches, and sweat damage the fibers. Steam should be use when ironing. Most protein fiber garments require dry-cleaning.

Flame resistant: Protein fibers do not ignite easily and will self-extinguish. They emit a burnt-hair smell and leave behind a black, powdery ash.

This section is about sheep wool. Specialty wools will be covered later.
Wool is a staple fiber from sheep’s hair. Wool is produced primarily in Australia, New Zealand, China, and Eastern Europe. The U.S. produces less than 1% of the world’s wool.

Wool production
Sheep are sheared once per year, usually in the spring. A newly removed fleece (raw or grease wool) contains 30% to 70% by weight of impurities such as dirt, grease, and sweat. The fleece is cleaned to produce scoured wool. The fleece is graded to evaluate it for fineness and length, then sorted to separate the sections of different qualities. Wool fibers are laid parallel to one another and twisted together to make yarns. Felt is made by pressing wool fibers together without twisting them into yarns.

Physical structure of wool
Length: Long, fine wool fibers, with an average length of 2½ inches, are used for worsted yarns. Short, coarse fibers, with an average length of 1½ inches, are used for woolen yarns.

Longitudinal view: Wool is a naturally crimped fiber. The fiber twists and bends back and forth around its axis giving it spring like qualities. The crimp accounts for wool’s excellent resiliency, flexibility, elongation, and elastic recovery.

Cross-sectional view: Wool fibers have diameters ranging from 10 to 50 micrometers. The fibers are round. The cuticle, the outer layer of the fiber, contains a dense layer of scales. These scales make the fibers water repellent, and allow felting. The cortex is the main part of the fiber. The cells on either side of the cortex react differently to moisture and temperature, giving wool its crimp. The center of the wool fiber is the medulla, a microscopic honeycomb-like structure containing air spaces. These air spaces add to wool’s thermal retention. Most worsted fibers do not contain medullas.

Properties of wool:
Aesthetics: Wool varies in color from white to dark brown. It accepts and holds dyes well. Woolen wool has a matte appearance; worsted wool may be more lustrous. Texture and drape are determined by yarn and fabric structure and by finish.

Durability: Wool fabrics are durable. Wool has a low tenacity (1.5 g/d), but its excellent elongation and elastic recovery allow it to resist damage. Wool’s wet tenacity is 1.0 g/d.

Comfort: Wool is highly hygroscopic – it is highly absorbent and it releases moisture slowly. Wool absorbs small droplets of moisture such as sweat while repelling larger droplets such as rain. Wool is a poor conductor of heat which makes it an excellent insulator. Wool continues to provide insulation while wet. Wool may induce an allergic reaction in some people.

Appearance retention: Wool is a highly resilient and elastic fiber. It resists wrinkling, and recovers its shape. If properly cared for wool has good dimensional stability, but if improperly washed it will shrink, felt, and tear. Wool should be dry-cleaned. Wool does not soil or stain readily, and with proper cleaning techniques stains can be removed.

Care: I can’t say this enough: WOOL IS DRY-CLEAN ONLY! Wool is damaged by alkalis (most detergents) and chlorine bleach. Moths eat wool; store your wool carefully. Wool is slow to ignite and it self-extinguishes, but there is still no good reason for you to set your wool clothes on fire.

Specialty wools

Mohair comes from the Angora goat. It is a smooth, silky, fine fiber without crimp. Most fibers do not have a medulla. Mohair is highly resilient.

Cashmere is from the Cashmere goat. The fibers have very fine scales and no medullas. It is used to make fabrics with a warm, soft, buttery hand, lustrous appearance, and excellent draping characteristics. It is expensive.

Llama and Alpaca wools come from South American cousins of the camel. Alpaca fiber’s soft hand, good luster, and excellent draping characteristics make it good for apparel. Llama fiber is coarser than alpaca fiber. It is often used for coats and suitings.

Camel’s hair is from the two-humped beasties (Bactrian camels). The hair is shed naturally; camels do not need to be shorn. The fiber provides great insulation without weight. It is usually used in coats, scarves, and suits. It is a tan fiber that is usually used undyed.

Angora is from the Angora rabbit. It is a long, fine, fluffy, soft, and slippery fiber. The fiber does not dye well. It is difficult to spin into yarns because it is so sleek, so it is often blended with other wools. Lambs are cute, but the Angora rabbit is the most adorable wool critter.

Silk is the only natural filament fiber. Silk was first used as a fiber in Chine more than 4,000 years ago. It is excreted by the silkworm to spin its cocoon in an attempt to metamorphose into a moth.

Silk production
Sericulture is the production of cultivated silk. Silk moths lay eggs on specially prepared paper. After the larvae hatch they are fed mulberry leaves. After roughly five weeks the caterpillars begin to spin their cocoons. Cocoons are made from a single strand of silk, approximately one mile long. The strands of silk are coated with a gum, called sericin. After the cocoon is finished, the pupa is killed with heat. The cocoons are unwound to produce fibers, the sericin is removed, and the fibers are wound together to produce yarn.

Wild silk is produced by collecting empty chrysalises from wild silkworms. The adult moth tears through the fiber as it exits the cocoon, so wild silk is a staple fiber. The sericin is not removed from wild silks.

Physical structure of silk
Silk is a smooth, thin fiber. The diameter of silk fiber is approximately 11 micrometers. It is a solid fiber with a triangular cross shape. Slight striations may be present along the length of the fiber. Wild silk tends to be coarser than cultivated silk.

Properties of silk
Aesthetics: Silk is a luxury fiber. It has a soft luster with an occasional sparkle. It wrinkles more easily than wool, but it is more resilient than cotton. Its color, hand, and drape vary. Wild silks have duller luster and more texture than cultivated silks.

Durability: Silk is one of the strongest natural fibers with a dry tenacity of 4.5 g/d. It loses up to 20% strength when wet. Silk has medium elasticity; at 2% elongation it returns to only 90% of its original length. Silk is damaged by sweat, body oils, chlorine bleach, sunlight, mineral acids, metallic salts (deodorants), and carpet beetles. It is resistant to hydrogen peroxide.

Comfort: Silk is a poor conductor of heat, so it can provide insulation. Light weight silk fabrics are comfortable in warm weather. It has good absorbency, and it is hygroscopic. Silk may develop a static charge. Silk has a soft, smooth, silky hand.

Appearance retention: Silk has moderate resistance to wrinkling. With its low elasticity, if it gets stretched it will stay stretched. Silk has moderate abrasion resistance, but with its typical uses it is rarely subjected to harsh abrasions. Staple silk may pill.

Care: Silk should by dry-cleaned. Silk may be pressed at moderate heat (300oF) with a damp press cloth. Silk is flame retardant and self extinguishing, but you should still keep it away from open flames.

Types of silk
Tussah is the most common type of wild silk

Noil silk (waste silk) is a staple fiber made from the broken pieces and cocoon remnants of cultivated silk.

Duppioni silk is made if two silkworms spin their cocoons together. It is not possible to fully separate the fibers. The yarns have a thick-and-thin appearance. It is used to make shantung.

Spider silk is not currently commercially produced, but several researchers are attempting to develop cost effective ways to produce it. The protein in spider silk is spidroin. Spider silk has excellent tenacity and elasticity. Efforts have been made to collect the silk directly from spiders and to genetically enhance silkworms and goats to produce it. A piece of spider silk fabric is currently on display at the American Museum of Natural History in New York.

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