History
Different Properties and Characteristics of Silk Fiber
Fine Structure and Appearance:
Silk is the only natural filament. It is a solid fiber. The filaments are 300-1800 yards long. Silk fiber has a double rod-like structure, covered with lumps of gum. Wild silk fiber is very irregular and resembles flattened, wavy ribbons with longitudinal markings. Cultivated silk is smooth, cylindrical, and generally uniform in thickness, like glass rods.
Silk Fiber
Silk is a natural protein filament that can be converted into textiles by either knitting or weaving techniques. The protein fiber/filament of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. For commercial use, it is almost entirely limited to filament from cocoons produced by the caterpillars of several moth species belonging to the genus Bombyx and commonly called silkworms reared in captivity (sericulture).
The silkworm, Bombyx mori, is a domesticated moth species that plays a crucial role in silk production, a practice known as sericulture.
Silk is a natural protein filament that can be converted into textiles by either knitting or weaving techniques. The protein fiber/filament of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. For commercial use, it is almost entirely limited to filament from cocoons produced by the caterpillars of several moth species belonging to the genus Bombyx and commonly called silkworms reared in captivity (sericulture).
Silk cocoon
- The shining appearance of silk is due to the triangular prism-like structure of the silk fiber, which allows silk cloth to refract incoming light at different angles, thus producing different colors. Silks are produced by several other insects, but generally, only the silk of moth caterpillars has been used for textile manufacturing.
- The raw silk strand from which a cocoon is built consists of two fine filaments cemented together by sericin gum. [Sericin gum is a protein derived from silkworms that is used in various applications, including medicine, cosmetics, and food.]
It has several beneficial properties, such as being moisturizing, promoting cell growth, and having antioxidant and anti-inflammatory effects. 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.
A single silk fiber cross-section
As seen in the cross-section, the strand of cocoon silk is of irregular shape. It is roughly oval with an 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 a tenacity usually of 30.9-44.1 cN/tex. Wet strength is 75-85 percent of the dry strength.
Elongation:Silk filaments have an elongation at a break of 20-25 percent under normal conditions. At 100 percent 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 percent 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 percent.
Effect of Heat:Silk will withstand higher temperatures than wool without decomposing when heated at 140 degrees Celsius. It will remain unaffected for prolonged periods. It decomposes quickly at 175 degrees 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.