STUDY UNIT 5: Natural cellulose fibres: Cotton Flashcards
History
Although the origin of cotton is unknown, there is some evidence that it was grown and used in India in about 3500 BC and in Peru in 3000 BC. Fragments of cotton fabrics dating back to about 500 BC have been found in American Indian caves and burial sites.
Cotton is still by far one of the most widely used fibres, although world consumption has dropped from 75% of all fibres to less than 50%. World cotton production was expected to be about 25.1 million tons in 2010/11, up 15% from 2009/10. China is the world’s largest producer of cotton fibre, and along with the United States of America, India and Pakistan produces the bulk of world cotton. Burkina Faso, Egypt, Tanzania, Nigeria and Zimbabwe are Africa’s largest cotton producers. However, African producers account for only about 2–3 % of total world production.
Although South Africa produces cotton fibre, it does not produce enough for local use. During 1995/96 South Africa produced 24 050 tons of cotton, but had to import an additional 46 245 tons. Since then production has fluctuated. South Africa’s 2010 cotton crop was expected to be the lowest for 50 years, with a projected 39 500 bales (200 kg/ bale) representing a 12% drop in production from 2009.
Fibre production
Cotton is a seed fibre, which means that the fibres that are used are seed hairs attached to, or surrounding, the seed of the plant. It is a natural cellulose fibre obtained from the seed hairs of the cotton plant. Cotton needs a warm, humid climate with enough water for irrigation. It requires a long frost-free period of 6 to 7 months, with 12 hours of sunlight daily.
Harvesting
The blossom appears 100 days after planting. After 2 days the flower falls and the young ball or seedpod remains. seedpod takes 50 to 80 days to ripen and burst open. The cotton can be picked by hand which gives a product better quality, or it can be picked by machine.
Ginning
The harvested raw cotton contains seeds, leaf fragments, dirt and other foreign matter, which must be removed before the cotton can be baled. The seeds are removed by the cotton gin. After entering the cotton gin, the cotton balls are thrown against rapidly turning saws. The dirt falls through the openings in the bottom of the machine. The saw teeth take up the cotton still attached to the seeds, pressing it against bars that are situated close to the saws. These teeth let the lint pass through but holds back the seeds. Eli Whitney invented the cotton gin in 1794.
Cotton goes through several cleaning processes and the by-products are used not only in the textile industry, but also in other industries. The short fuzzy fibres that remain on the seeds after they have been separated from the fibre in the cotton gin are called cotton linters. The longer fibres are known as cotton lint. The linters are removed and used in the manufacture of manufactured cellulosic fibres, plastics, photographic film and other purposes. The outer portion of the cotton seed, called the hull, is rich in nitrogen. These hulls are used in fertilizers, cattle feed, paper, plastics and explosives. The inner seeds are compressed to remove the cottonseed oil, which is used in cooking oils and soap, while the residue becomes cattle feed. It is estimated that as much as one-sixth of the income derived from cotton plant is obtained from these by-products.
Fibre properties
Cotton is very comfortable to wear next to the skin because of its fineness and softness. A number of finishing treatments can be applied to improve properties such as strength and lustre (mercerisation), reduce wrinkling (cross-linking/easy-care finish) and shrinkage (anti-shrink finish). Depending on the end-use, a flame retardant finish may also be applied; cotton burns easily.
Physical structure
Cotton fibres range from 12.5–63.5 mm in length. The fibres are in the shape of a collapsed, spirally twisted tube or ribbon (Refer to figure 3.1 and figure 5.1) with a rough, granular surface. A central lumen is visible in cross-sections. The diameter of the fibres
varies between 12 and 25 micrometres. A mercerisation treatment can cause the fibres to swell and straighten, resulting in a circular cross-section. Mercerisation increases the strength, absorbency and lustre of the fibre. The fibres are, generally, white to creamy.
Properties relating to durability
- Abrasion resistance
- Strength
- Flexibility
- Elongation
- Abrasion resistance
Abrasion resistance is moderate.
- Strength
Cotton has reasonable strength, with tenacity values ranging between 22.5–27 cN/tex. It is stronger when wet.
- Flexibility
Cotton is less flexible than wool but more flexible than flax.
- Elongation
Cotton has a low elongation (3–7% elongation at break).
Properties relating to comfort
- Moisture absorption
2. Heat conductivity
- Moisture absorption
Cotton has a moisture regain of 7% so is generally comfortable to wear. With high levels of water uptake cotton fabrics feel wet. Absorbency can be increase by mercerisation.
- Heat conductivity
Cotton is a good conductor of heat and is therefore cool to wear.
Properties relating to appearance retention
- Resiliency
- Dimensional stability
- Elasticity
