Sunday, February 28, 2010

Textiles review: natural cellulosic fibers

This is part two of my textiles review in preparation for my upcoming test.

Plants contain cellulosic fibrous bundles in their roots, stems, leaves, and seed casings. These fibers may be easily removed from some plants to be used as textile fibers. Fibers are classified according to the part of the plant from which they are removed. Seed fibers grow in the seedpod of plants, bast fibers are from a plant’s roots or stems, and leaf fibers are obtained from leaves.

Examples of seed fibers are cotton, kapok, coir, and milkweed.

Examples of bast fibers are flax, ramie, hemp, and jute.

Examples of leaf fibers are piña, abaca, sisal, and henequen.

While there are great differences between various natural cellulosic fibers, there are some properties they all share because of their similar chemical make-up. Properties common to natural cellulosic fibers are:

Good absorbency – cellulose can take up a lot of moisture. Natural cellulosic fibers are good for summer wear, towels, diapers, and active sportswear.

Good conductor of heat (poor thermal retention) – Natural cellulosic fibers are bad insulators. Apparel made from these fabrics does not trap heat. It is a cool fabric suitable for use in warm environments.

Able to withstand high temperature – Natural cellulosic fibers can be washed and ironed at high temperatures. They may be boiled or autoclaved for sterilization.

Low resiliency – Fabrics wrinkle badly.

Poor loft – Fibers can form dense, high count yarns and fabrics. They may be used to make wind resistant fabrics.

Good electrical conductor – Fabrics do not build up a static charge.

Heavy fibers – Fiber density is approximately 1.5 g/cc. Natural cellulosic fibers are heavier than natural protein fibers and most manufactured fibers.

Damaged by mineral acids – Try to avoid acid stains. If you can’t avoid them, clean them quickly.

Resistant to alkalis – Fabrics may be washed with regular detergents.

Damaged by mildew, crickets, and silverfish – Store items in dry conditions.

Resistant to moths – Moths will not eat cellulosic fibers, but you still do not want moths living in your closet.

Inflammable – Fibers ignite quickly and continue to burn after being removed from the ignition source. Burnt fibers smell like burnt wood and leave behind a white or light grey powdery ash.

Poor to moderate sunlight resistance – The fibers are suitable for outdoor apparel, but window treatments should be lined to prevent too much exposure to sunlight.

Cotton is the most commonly used natural cellulosic fiber. Cotton fibers grow from the seeds in the boll (seedpod). Each boll contains seven or eight seeds, and each seed may have up to 20,000 fibers growing from it.

Cotton production
Ripe bolls are picked by machines. They are sent to a gin where the seeds are separated from the fibers. The fibers, called lint, are packed into bales and sent to spinning mills. Each bale weighs 480 pounds. The spinning mills make yarns.

Physical structure of cotton:
Color: Most cotton fibers are a creamy white or light tan, but some naturally colored cottons are available. Colored cottons produce less per acre than white cotton, but they sell for about twice as much. Brown, red, beige, and green are available. Unlike most dyed fabrics, colored cottons become darker with age and care. Cotton fibers take up dyes well.

Length: Cotton is a staple fiber. The fibers range in length from ½ to 2½ inches depending on the genetic variety of the plant. Long staple cottons, which are greater than 1 5/16 inch are finer and make stronger, higher quality yarns. They cost a lot more too. Sea Island, Egyptian, Pima, and Supima are types of long staple cottons. Upland cottons, the most commonly grown cottons in the U.S., are medium length cottons (7/8 to 1¼ inch). Short-stable cottons (less than ¾ inch) are produced primarily in Asia.

Cross-sectional view: Cotton fibers are 16 to 20 micrometers in diameter. The fiber has a thin primary cell wall surrounding a thicker secondary cell wall. The center of the fiber is the lumen, the central canal through which nutrients travel as the fiber grows. Immature fibers are U shaped, and mature fibers are kidney shaped.

Longitudinal view: Cotton fibers have a ribbon-like twist called convolutions. The convolutions allow the fibers to cling together which makes yarn spinning easy.

Properties of cotton:
Aesthetics: Cotton has a matte appearance and low luster. Cotton can be made slightly lustrous through mercerization, i.e. treating it with NaOH which causes the fibers to swell. The drape and texture is affected by the yarn size, fabric structure, and finish. Cotton wrinkles easily.

Durability: Cotton is a medium-strength fiber, with a dry breaking tenacity of 3.5 to 4.0 grams per denier. It is 30% stronger when wet. Cotton has good abrasion resistance, low elongation (3%), and moderate elasticity.

Comfort: Cotton’s good heat conductivity, good electrical conductivity, high absorbency, and soft hand make it a very comfortable apparel fabric, particularly for use in warm environments.

Appearance retention: Cotton has moderate appearance retention. Its low resiliency allows it to wrinkle easily, but wrinkles may be pressed out. Cotton will shrink unless it is given a durable-press or wrinkle-resistant finish. Cotton has moderate elastic recovery; it recovers 75% from 2% to 5% stretch. Stretched cotton garments stay stretched.

Care: Cotton soils and stains easily. Cotton withstands high heat, it is stronger when wet, and it is resistant to alkalis, so it may be machine washed with hot water and normal detergent. Cotton may be ironed at high temperatures. Cotton should be stored in dry conditions to prevent damage from mildew.

Other seed fibers
Coir is from the fibrous mass between the outer shell and husk of coconuts. It is a stiff fiber. It is usually used to make highly durable indoor and outdoor mats, rugs, and tiles.

Kapok fiber is from the seed of the Java or Indian kapok tree. The fiber is soft, lightweight, and hollow. It breaks down easily and it is difficult to spin into yarns. It is used as fiberfill and as the stuffing for pillows. It used to be used as a stuffing for lifejackets and the mattresses on cruise ships because it is very buoyant.

Milkweed has properties similar to those of kapok.

Flax is one of the oldest textile fibers, but its use has declined since the invention of power spinning for cotton. Flax fabric is linen, although the word linen is now often used to refer to table, bed, and bath fabrics made from other materials. Flax is a bast fiber.

Production of flax
Flax fibers are from the stems and roots of the flax plant. The plant is cut or pulled out of the ground to keep the fibers as long as possible. The fibers are found just under the bark or outer covering of the plant. They are sealed together by pectins, gums, and waxes. There are four steps to obtain fibers from the plant:

Rippling – the plant is pulled through a machine to remove the seeds.

Retting – bacteria break down the pectin that binds the fibers together.

Scutching – the stalks are passed through rollers to crush and remove the outer covering so the fibers may be removed.

Hackling – the fibers are combed to separate them into individual strands.

Physical structure of flax
Length: Flax is a staple fiber. Flax fibers range in length from 2 to 36 inches. Short fibers are called tow; long fibers are called line.

Cross-sectional view: Flax fibers are 12 to 16 micrometers in diameter. They have a polygonal shape. There is a small central canal similar to cotton’s lumen.

Longitudinal view: Flax fibers are straight. There are crosswise markings, called nodes, along the length of the fiber.

Properties of flax
Aesthetics: Flax has a high luster that can be further increased with finishes. Flax has a stiffer drape and harsher hand than cotton. Flax takes up dyes well.

Durability: Flax is strong for a natural fiber. Its dry tenacity is 3.5 to 5.0 g/d, and its wet tenacity is 6.0 g/d. Flax has very low elongation (7%) and poor elasticity (65% recovery at 2% elongation). Flax has good flat abrasion resistance, but poor flex abrasion resistance. Repeatedly folding a piece of linen in the same place will cause the fibers to break.

Comfort: Flax is a good fabric for summer apparel. Its high absorbency (better than cotton), wicking properties, and poor thermal retention make it very comfortable in warm weather. It will not build up a static charge.

Appearance retention: Flax has poor appearance retention. It wrinkles very easily and does not recover from being stretched.

Care: Flax withstands high heat, it is stronger when wet, and it is resistant to alkalis, so it may be machine washed with hot water and normal detergent. Flax may be ironed at high temperatures, and it will require pressing often. Flax should be stored in dry conditions to prevent damage from mildew.

Other bast fibers
Ramie: Ramie fibers are 4 to 6 inches long. The fibers are whiter and softer than flax. Ramie does not retain dyes well unless it is dry-cleaned. Ramie is strong for a natural fiber, but it lacks resiliency, elasticity, and elongation potential. It is resistant to mildew, insects, and shrinkage. It is used for apparel, window treatments, ropes, paper, and table and bed linens.

Hemp: Hemp is similar to flax. The fibers range in length from 3 to 15 feet. Hemp production is illegal in the U.S. Hemp has a low environmental impact; it does not require pesticides. It produces 250% more fiber than cotton and 600% more fiber than flax on the same amount of land. Hemp plants can be used to extract zinc and mercury pollutants from soil. Hemp is used for ropes, apparel, and paper. Potheads are willing to pay inflated prices for hemp apparel because it is related to the marijuana plant.

Jute: Jute is one of the cheapest textile fibers, and one of the weakest cellulosic fibers. Jute has poor elasticity, elongation, sunlight resistance, mildew resistance, and colorfastness. It is used to produce sugar and coffee bagging, carpet backing, rope, and wall coverings. Burlap is made from jute.

Leaf fibers
Piña fibers are from the leaves of the pineapple plant. It is used to make lightweight, sheer, stiff fabrics for apparel, bags, and table linens. It is also used to make mats.

Abaca is from a member of the banana tree family. The fibers are coarse and very long (up to 15 feet). It is a strong, durable, and flexible fiber used for ropes, floor mats, table linens, apparel, and wicker furniture.

Sisal and henequen are similar plants. The fibers can be used to make strong ropes, but they are damaged to salt water so they are not suitable for maritime ropes. The fibers are used for upholstery, carpets, and wall coverings.

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