Study Unit 4 Natural Protein Fibres: Silk Flashcards
History
The Chinese started producing silk in about 2700 BC. Interesting legends are told about how silk was discovered. For 3000 years the Chinese held the secret of silk and monopolised the silk industry. They were willing to export the silk yarn and fabric, but not the knowledge of how these were made. About AD 300 some silkworm eggs were smuggled to Japan and eventually the knowledge as well as the silkworms spread to the rest of the Eastern world as well as to France and Italy. Today silk fabrics are still woven in England, France and Italy, but silk fibres are no longer produced there. The fibres are produced in China, India and Thailand. Silk has always been considered a luxury fibre with great prestige value.
The growth and production of silk is known as sericulture. The silkworm, which is the larva of the Bombyx mori moth produces silk. A moth lays 700 eggs; each egg is smaller than a pinhead. They hatch when new leaves appear on mulberry trees. The larvae (little worms) are very small when they emerge from the eggs. larvae live on mulberry leaves, which they consume in large quantities, and they rapidly increase in size. The larva sheds its outer skin four times during the 35-day eating period, before it is ready to spin its cocoon.
Inside the completed cocoon, it changes into dormant chrysalis. During commercial silk production chrysalis must be killed by heat, as continuous fibres are required, and the
moth cuts the fibres when making its way out of the cocoon. Only those moths to be used for breeding are allowed to hatch, or emerge from their cocoon.
As the silkworm spins its cocoon, the liquid silk fibre emerges from two openings called spinnerets, which are situated in the head of the silkworm. These fibres solidify into silk filaments when they come into contact with the air. The fibres consist of the protein fibroin, and are coated with a gummy substance, sericin. Each single stand of silk is called a brin, and the two threads together are known as a bave.
Wild silk or tussah silk
Wild silk or tussah silk comes from wild silkworms that feed on oak leaves and produce brown, orange, yellow or green fibres. As the moth cuts the filaments when it emerges from the cocoon the fibres cannot be reeled, but are used as staple fibres for spun silk. Wild silk is often used to weave shantung fabrics.
Dupion silk
Dupion silk comes from two silkworms that spin their cocoons together or very close to each other as a result of which the yarn becomes entangled. It is then impossible to separate the yarn into two separate strands. They are irregular in diameter and are used in linen-like silk fabrics. It is very popular for wedding gowns.
Wild African silk (Mopane silk)
Wild African silk (Mopane silk) is derived from the Gonometa postica wild moth species endemic to the Kalahari regions of Southern Africa. The cocoons are harvested on a sustainable basis, degummed and processed in Botswana, Namibia and South Africa. The silk is fawn coloured with a soft sheen.
Fibre production
Reeling
Throwing
Degumming
Weighting
Spun silk
Reeling
The cocoons are placed in hot water to soften the gum. The surfaces of the cocoons are then brushed lightly to find the ends of the filaments. Several of these ends are threaded simultaneously through a guide and wound onto a wheel called a reel. The usable length of reeled filament that can be obtained from one cocoon is 300 to 600 metres. Silk is the only natural filament yarn.
Throwing
The reeled silk filaments are twisted to hold them together. Several kinds of silk yarn or threads can be made, depending on the amount of twist and on the methods of combining single yarns. This process is known as throwing. The word thrown as used here comes from the word “thrawn” which means “to twist”.
Degumming
The thrown silk threads still contain some sericin, which is removed by boiling the silk in soap and water. This brings out the natural lustre and the soft feel of the silk. Twenty five per cent of the mass is lost by degumming. Degumming can take place after the silk thread has been thrown or even after the fabric has been woven.
From time to time raw silk fabrics become available on the South African market. These are fabrics from which the sericin has not been removed, and which have not been boiled, hence the name “raw” silk. These fabrics are often woven from staple silk fibres. These fabrics do not look like anything like the silk from which silk scarves are made; the fabric actually reminds one of unbleached calico, and often has a similar colour.
Weighting
In order to replace some of the mass lost during the degumming process, silk is sometimes weighted with metallic salts. A small quantity of metallic weighting is not considered injurious, but large quantities will damage the fibres.
Spun silk
All silk that cannot be reeled into filament fibres is called waste silk. It can be used as staple fibre. Short lengths of silk fibres are carded and combed and then spun together. These staple lengths of silk come from pierced cocoons, double cocoons, the uneven silk fibres at the beginning of each cocoon and scrap or machine waste.
Mopane silk is also spun because it is a wild silk and the moth pierces the cocoon when exiting the cocoon. The cocoons are harvested after the moths have exited.
Fibre properties
Silk is an expensive fibre and it remains a luxury fabric. It will burn in a flame but is self- extinguishing when one removes the flame. Perspiration tends to weaken the fibre and silk blouses and dress tend to tear or disintegrate beneath the arms.
Physical structure
Silk is a natural filament fibre. It is usually between 600 to 1500 metres in length. The fibre surface is smooth (Refer to figure 3.1). Tussah silk may have slight striations along the length of the fibre. The filaments brins are triangular in cross-section with rounded corners. They are irregular in diameter, with a diameter of between 9–12 micrometres. The fibres are off-white to cream in colour and lustrous.
Properties relating to durability
- Abrasion resistance
- Strength
- Flexibility
- Elongation
- Abrasion resistance
Silk has moderate abrasion resistance but because of its cost and end-use, it is seldom subjected to harsh environments.
- Strength
Silk is one of the strongest natural fibres, with a tenacity of 31–44 cN/tex. Its strength is excellent in relation to its fineness. Also, its strength is about 20% lower when wet.
- Flexibility
Generally, flexibility is good.
- Elongation
Silk is not as elastic as wool (20% elongation at break).
Properties relating to comfort
- Moisture absorption
2. Heat conductivity
- Moisture absorption
Silk has good absorbency with a moisture regain of 11%. This contributes to its comfort in warm conditions. Silk may develop static cling.
- Heat conductivity
Silk is a poor conductor of heat which means it is warm to wear. However, it can be woven into very fine fabrics that can be cool to wear.
Properties relating to appearance retention
- Resiliency
- Dimensional stability
- Elasticity
- Resiliency
Silk has medium resiliency. Creases will hang out relatively well but not as well as in wool.
- Dimensional stability
Silk fabrics have good resistance to stretch or shrinkage when laundered. Raw silk fabrics are inclined to show more relaxation shrinkage than other silk fabrics.
- Elasticity
Silk has moderate elasticity. When it is stretched by 3%, its elastic recovery is only 90%. This poor elasticity may be seen in baggy elbows in silk blouses.
Properties relating to care
- Effect of alkalis
- Effect of acids
- Effect of sunlight
- Biological properties
- Effect of alkalis
Strong alkalis will damage silk.
- Effect of acids
Strong mineral acids will destroy silk, but organic acids do not damage silk and are used in some finishing process.
- Effect of sunlight
Sunlight exposure will weaken silk. Silk must be stored in carefully selected containers; otherwise the fibres will lose strength and eventually be destroyed.
- Biological properties
Silk is resistant to many micro-organisms but will be attacked by rot-producing bacteria. Silk is susceptible to attack from carpet beetles.
Durability
High
Abrasion resistance
Moderate
Tenacity
High for natural fibres
Elongation
Moderate
Comfort
High
Absorbency
High
Heat conductivity
Low
Appearance retention
Moderate
Resiliency
Moderate
Dimensional stability
High
Elastic recovery
Moderate
Care
Dry clean
May use mild detergents and gentle agitation, avoiding wringing and rubbing
Do not use detergents with stain removers containing protein-digesting enzymes
Silks may water spot easily
Do not hang in direct sunlight
Pure dye silks should be ironed damp with a press cloth
Wild silks should be ironed dry. Safe ironing temperature 149 oC Chlorine bleach will damage the fibres
For long-time storage seal to keep out light, air and insects
End Uses
It is used primarily for apparel, (eg dresses, shirts, blouses – suitable for warm or cold weather, neckties and scarves).
It is used for home furnishing items (eg upholstery and curtaining) because of its appearance and cost.
It is also used in Oriental rugs (eg Persian carpets).
