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.
