Reeling of Silk, Structure, Properties and Uses of Silk
Reeling of Silk
The unwinding of the fine silk filaments from the cocoons is called reeling. The cocoons are soaked in hot water to soften the sericin gum that is cementing the filaments. A revolving brush is used to find the end of the filaments. When the end of the filament has been picked up, it is drawn through a guide long with the filaments from several other cocoons. The filaments may be given slight twists to hold them together and reeled off the cocoons which are left floating in hot water to keep the gum softened.
Basin
A. Cocoons being reeled
B. Cocoons, after removal of knobs, with ends anchored to a hook, ready to replace spent cocoons.
C. Spent cocoons
Take-up motion
D. Porcelain button guide.
E. Crossing of the thread to remove the water.
F. Rotating six-armed winding reel.
G. Collapsible arm for removal of hank.
Winding
H. Drum rotated by belt from winding reel.
I. End of sliding rod causing to and fro movement across the direction of the thread.
J. Guide eye, set on sliding rod, to give width to the hank by spreading the ends.
Tussah cocoons are gummed more firmly and they are usually soaked in sodium carbonate before reeling. Silk is wound up by the reeler in the form of skeins. These are made up into bundles of about 2.7 Kg, called “books.” These are then packed into bales for shipment.
Structure of Silk
Silk fiber does not possess a cellular structure like all other natural fibers and it closely resembles to artificial fibers.
The raw silk strand from a cocoon is built composed of two fine filaments of fibroin cemented together by sericin gum. Fibroin is a protein material and at the inner side it is a beautiful silk fiber. It can be used as silk after removing the sericin, which is also a protein material. The raw silk has a dull structure due to presence of sericin and it is quite hard to touch.
The longitudinal view of raw silk exhibits a very irregular and rough surface structure, mostly in the sericin layer, which consists of fissures, creases, folds and uneven lumps. Under microscope, cultivated silk fibers are triangular to oval in cross-section, while wild silk has a flat ribbon-like shape. The degummed silk filaments are smooth surfaced and semi-transparent, while the raw silk is darker. Most of the color lies in the sericin gum and is lost when the filaments are degummed. Silk fiber is composed of minute filaments that are often referred to as fibrils. The fibrils are about 10µm in diameter.
Properties of Silk
Physical Properties
Tensile Strength:
Silk is a strong fiber. It has a tenacity of 30.9-44.1 eN/tex. Wet strength is 75-85 percent of dry strength.
Elongation:
Silk filaments have an elongation at break of 30-25% under normal condition. At 100% R.H., the extension at break is 33%.
Elastic Property:
The elastic recovery of silk after spinning is not so good as that of wool, but it is superior to that of cotton and rayon.
| Elastic Recovery | Japanese Silk | Tussah Silk |
|---|---|---|
| From 50% breaking load | 56% | 40% |
| From 50% breaking extension | 38% | 41% |
Specific Gravity:
Degummed silk is less dense than cotton, flax, rayon, or wool. It has a specific gravity of 1.25. Silk fibers are often weighted by allowing the filaments to absorb heavy metallic salts; this increases the density of the material and enhances its draping property.
Effects of Water and Moisture:
Silk is a highly absorbent fiber. Like wool, it absorbs water well (M.R. 11%), but it dries fairly quickly. The degummed silk dries up more quickly than raw silk and also gives a shorter dyeing period in textile processing. On average, it can absorb water up to 30% of its weight without feeling wet and increases in cross-sectional area by 46%. For this reason, garments made of silk take up perspiration from the human body rapidly. Even silk absorbs the dissolved substances present in water rather quickly compared to water itself.
Therefore, care should be taken in textile processing of silk fabrics. Water should be free from impurities, and possibly soft water should be used to get better silk in respect to soft-feeling as well as luster.
Treatment of silk in boiling water for a short period of time does not cause any detrimental effect. Silk fibroin withstands the effects of boiling water better than wool.
Effect of Heat:
Silk can withstand higher temperatures than wool without decomposition. Heated at 140°C, it will remain unaffected for prolonged periods. It decomposes quickly at 175°C.
Effect of Sunlight:
Silk is more sensitive to light than any other natural fiber. Prolonged exposure to sunlight can cause partial to complete color change. The effect of sunlight on silk is more pronounced in the presence of atmospheric oxygen. Yellowing of the silk fiber is generally due to photo degradation by the action of UV radiation of sunlight. The mechanism of degradation is due to the breaking of hydrogen bonds followed by oxidation and eventual hydrolytic fission of the polypeptide chains.
Silk is more sensitive to the tendering action of sunlight in the presence of metallic weighting, which may cause due to the catalytic action of metallic salts used in weighting.
Chemical Properties
Effect of Acids:
Hot concentrated acids readily decompose silk. The degree of hydrolysis is much greater with acid than with alkali and is dependent on pH. In moderate concentration, acids cause a shrinkage in silk, and this shrinkage is used to make a crepe effect in silk fabrics.
Like wool, silk shows amphoteric behavior. In the pH value between 4 to 5, silk fiber shows minimum swelling and reactivity.
Inorganic acids like sulfuric acid and hydrochloric acid may cause hydrolysis of the peptide linkage and readily dissolve silk when it is concentrated and hot. Organic acids show little effect on silk compared to inorganic acids.
Action of Alkalis:
Similar to the action of acids, alkali causes hydrolysis of polypeptide linkage in silk and reduces its tensile strength. Dilute alkali at room temperature does not cause any considerable chemical and structural damage to silk, but the luster of the fiber may be softened during use. However, silk can be treated with a 16-18% solution of sodium hydroxide at low temperatures to produce crepe effects in mixed fabrics containing cotton. Caustic soda, when it is hot and strong, dissolves the silk fiber.
Action of Oxidizing Agents:
Silk fibroin is highly sensitive to oxidizing agents. The attack of oxidizing agents may take place at three possible points of the protein:
- At the peptide bonds of adjacent amino groups.
- At the side chains.
- At the N-terminal residues.
Therefore, bleaching of silk with hydrogen peroxide should be done with care. Hypochlorite bleaching must not be used, as it rapidly tenderizes silk.
Action of Reducing Agents
It is found that the reducing agents that are commonly used in textile processing such as hyposulfite, sulfurous acids and their salts do not have any destructive action on the silk fiber.
Electrical Properties
Silk is a poor conductor of electricity and tend to form a static charge when it is handled. This causes difficulties during processing, particular in a dry atmosphere.
Silk Weighting
The silk is very expensive fiber and its price depends on its weight. Therefore, the weighting process of silk is carried out with proper attention to increase its weight without hampering its appearance, luster, hand-feel and other related physical properties.
During degumming, silk may lose 25% of its weight. This weight loss can be restored or even increased by weighting process, where silk is treated with iron salts, stannic chloride, starch and sugar etc. The amount of chemicals required to give the silk its original raw weight is known as “par weight”. If 100 Kg of raw silk are degummed and then weighted in such a way that the weight of treated silk amounts to again 100 Kg, then it is said that the silk is weighted to one par. If results only 90 Kg, then silk is weighted 10% under par and with 150 Kg, it is 50% over par. Silk weight can be increased up to 400% by this process. Weighting process increases the drapability of silk fabrics, but it may cause the deterioration of silk fiber quality.
- Weighted silk is not as strong as pure silk.
- It decreases the durability of silk fiber.
- It may lead to produce the brittle surface to silk fiber.
- Reddish spots are frequently developed on weighted silk.
- Weighted silk is more sensitive to the tendering action of sunlight.
The degradation of weighted silk can be prevented to a considerable extent by treating with the solution containing ammonium sulfocyanide, glycerol and tannin. Treatment of weighted silk with thiourea and with hydrosulfite-formaldehyde compounds also decrease tendering action of weighting materials.
Silk in Use
For thousands of years, silk has reigned as the queen of fabrics. Yet until a few years ago, silk has been unchallenged in its position as the most desirable of all textile fibers.
Silk combines a high strength and flexibility with good moisture absorption, softness and warmth, excellent wear ability and a luxurious appearance.
Silk can be woven and knitted into a wide variety of fabrics. It provides all manner of materials from the sheerest chiffon to the heavy pile velvet. Silk is cool and comfortable in underwear or summer clothes. It is easy to clean. Silk garments have very smooth surface which does not collect dirt during wearing.
Laundering of Silk Garments
Despite its resistance to wear, silk is a delicate fabric. The filaments of silk are very fine and easily torn. It can be damaged by chemical action and therefore must be washed with care. Human perspiration can degrade the silk and silk garments should be washed regularly.
Silk should be laundered with washing powder or a mild detergent, rinsed thoroughly in soft water, dried gently and ironed while damp. Careless ironing may produce undesired spots in the silk garments. Moderate temperature should be used during ironing and a pressing cloth should be placed between the iron and the silk cloth.
Weighted silks are often affected drastically by washing or dry cleaning. This may cause the removal of the metallic salts from the fabric.