Silk Fiber: Properties, Production Process and Uses
What is Silk Fiber?
Silk fiber is a natural protein fiber that has been highly valued for centuries due to its luxurious feel, shine, and strength. Produced by the Bombyx mori silkworm, silk is one of the most important and prestigious fibers used in the textile industry. Below is an overview of the properties and uses of silk fiber.
Properties of Silk Fiber:
Fine Structure and Appearance:
The raw silk strand from which a cocoon is built consists of two fine filaments cemented together by sericin gum. Seen under the microscope, raw silk has a rough and irregular surface and it is marked by lumps, folds and cracks in the sericin layer. Often the twin filaments of silk are separated for considerable distances, each with its own coating of sericin.
Seen in cross section, the strand of cocoon silk is of irregular shape. It is roughly oval with average diameter of 0.178mm. The individual filaments can be distinguished inside the sericin coating. They are triangular in cross section, with rounded angles. Usually, the filaments lie with one flat side of each facing the other.
The degummed filaments are smoothing surfaced and semitransparent. The diameter fluctuates from place to place, averaging 0.0127mm. The filaments become thinner towards the inside of the cocoon.
In the raw state, silk varies in color from cream to yellow. Most of this color lies in the sericin gum, and is lost when the filaments are degummed. The silky sheen develops after degumming.
Tensile Strength:
Silk is a strong fiber. It has tenacity usually of 30.9-44.1 cN/tex. Wet strength is 75-85 per cent of the dry strength.
Elongation:
Silk filaments have an elongation at break of 20-25 per cent under normal conditions. At 100 per cent R.H. the extension at break is 33 percent.
Elastic Properties:
The elastic recovery of silk after spinning is not as good as that of wool, but is superior to that of cotton or rayon. Once it has stretched by about 2 per cent of its original length, silk tends to remain permanently stretched. There is a slow elastic recovery or creep after extension, but the silk does not regain its original length.
Specific Gravity:
Degummed silk is less dense than cotton, flax, rayon or wool. It has a specific gravity of 1.25. Silk fabrics are often weighted by allowing the filaments to absorb heavy metallic salts; this increases the density of the material and affects its draping properties.
Effect of Moisture:
Like wool fiber, silk absorbs moisture readily. It can take up a third of its weight of water without feeling wet to the touch. Silk has a regain of 11 per cent.
Effect of Heat:
Silk will withstand higher temperatures than wool without decomposing heated at 140 degree Celsius. It will remain unaffected for prolonged periods. It decomposes quickly at 175 degree Celsius. Silk burns, emitting a characteristic smell like that of burning hair or horn.
Effect of Age:
Silk is attacked by atmospheric oxygen, and may suffer a gradual loss of strength if not carefully stored.
Effect of Sunlight:
Sunlight tends to encourage the decomposition of silk by atmospheric oxygen.
Production Process of Silk:
1. Sericulture:
- Silk Farming: The process begins with the cultivation of silkworms, specifically the Bombyx mori. The larvae feed on mulberry leaves and spin cocoons made of silk fibers.
2. Cocoon Harvesting:
- Cocoon Collection: Once the silkworms have completed spinning their cocoons, the cocoons are collected for processing.
3. Reeling:
- Silk Extraction: The cocoons are soaked in hot water to soften the sericin (a protein that holds the fibers together). This allows the long silk filaments to be unwound or “reeled” from the cocoon.
- Filament Length: Each cocoon yields a continuous filament that can be as long as 1,200 meters.
4. Spinning:
- Yarn Formation: The silk filaments are twisted together to form yarn. Depending on the desired thickness and texture, the filaments can be combined in different ways.
5. Weaving/Knitting:
- Fabric Production: The silk yarn is then woven or knitted into fabrics, which can be used in various applications, from fashion to home textiles.
Uses of Silk Fiber:
1. Fashion and Apparel:
- Luxury Clothing: Silk is widely used in high-end fashion for garments such as dresses, blouses, scarves, and ties. Its drape and luster make it a preferred choice for formal and evening wear.
- Lingerie: The softness and breathability of silk make it ideal for lingerie and sleepwear.
2. Home Textiles:
- Bedding: Silk is used in luxury bedding items like pillowcases, sheets, and comforters due to its smooth texture and temperature-regulating properties.
- Curtains and Upholstery: Silk fabrics are used in interior design for elegant curtains and upholstery.
3. Industrial Applications:
- Medical Uses: Silk’s biocompatibility makes it useful in medical applications such as sutures and wound dressings.
- Parachutes and Ropes: Historically, silk was used in the production of parachutes and ropes due to its strength and light weight.
4. Cultural and Traditional Uses:
- Textile Art: Silk has been used for centuries in traditional art forms, such as embroidery and painting, particularly in Asian cultures.
- Religious and Ceremonial Attire: Silk is often used in religious garments and ceremonial attire due to its symbolic significance and luxurious appearance.