Silk - Properties

In old English, silk was sioloc. The name is thought to have originated from the Greek seres, meaning the people from Eastern Asia, namely the Chinese. Silk is natural, protein filament. Its filament density is 1.34 g/cm3, which makes it a medium weight fiber. However, very light weight silk textile materials may be manufactured from silk filaments.

The raw silk strand consists of two silk filaments encased by a protein called sericin. The thickness of the raw silk strand and its uneven and irregular surface are due to the coating of sericin, which gives raw silk a coarse handle.The ability of a silk cocoon to withstand prolonged exposure to weather shows that sericin is very weather resistant. However, is sericin is readily soluble in mild alkaline solution, and when it is removed the two shiny silk filaments composing the raw silk strand are revealed.

Silk is very fine, regular, translucent filament. It may be up to 600 m long, but averages about 300 m in length. Depending upon the health, diet and state under which silk larvae extruded the silk filaments, their diameter may vary from 12 um to 30 um. This gives a fiber length to breadth ratio well in excess of 2000:1. The beauty and softness of silk's lustre is due to the triangular cross-section of the silk filament. As the silk filament is usually slightly twisted about itself, the angle of light reflection changes continuously. As a result, the intensity of the reflected light is broken, resulting in a soft, subdued lustre.

The silk-filament is a fine, coagulated stream of fibroin solution, and has no identifiable micro-structure. In this regard it resembles the man-made fibers. Fibroin is the chemical name for the protein which constitutes silk.

The Polymer System

The Silk Polymer

The silk polymer is a linear, fibroin polymer. It differs fron the wool polymers as follows :

1. Silk is composed of sixteen different amino acids compared with the twenty amino acids of the wool polymer. Three of these sixteen amino acids, namely alanine, glycine and serine, make up about four-fifths of the silk polymers composition.

2. The silk polymers are not composed of any amino acids containing sulphur. Hence, the polymer system of silk does not contain any disulphide bonds.

3. The silk polymer occurs only in the beta-configuration. It is thought that silk polymer is about as long as (140 nm), or only slightly longer than the wool polymer, and about 0.9 nm thick. Silk may be considered to have the same composition as that of wool except that the silk polymer system contains no disulphide bonds.

Physical properties
1. Tenacity - The silk filament is strong.This strength is due to its linear, beta-configuration polymers and very crystalline polymer system. These two factors permit many more hydrogen bonds to be formed in a much more regular manner. When wet, silk loses strength. This is due to water molecules hydrolysing a significant number of hydrogen bonds and in the process weakning the silk polymer.

2. Elastic-plastic nature - Silk is considered to be more plastic than elastic because its very crystalline polymer system does not permit the amount of polymer movement which could occur in a more amorphous system. Hence, if the silk material is stretched excessively, the silk polymers, which are already in a stretched state (they have a beta-configuration) will slide past each other. The process of stretching ruptures a significant number of hydrogen bonds. When stretching ceases, the polymers do not return to their original position, but remain in their new positions. This disorganises the polymer system of silk, which is seen as a distortion and wrinkling or creasing of the silk textile material. The handle of the silk is described as a medium, and its very crystalline polymer system imparts a certain amount of stiffness to the filaments. This is often misinterpreted, in that the handle is regarded as a soft, because of the smooth, even and regular surface of silk filaments.

3. Hygroscopic nature - Because silk has a very crystalline polymer system, it is less absorbent than wool. The greater crystallinity of silk's polymer system allows fewer water molecules to enter than does the amorphous polymer system of wool. The other hygroscopic properties of silk are rather similar to those of wool.

4. Thermal properties - Silk is more sensitive to heat than wool. This is considered to be partly due to the lack of any covalent cross links in the polymer system of silk, compared with the disulphide bonds which occur in the polymer system of wool. The existing peptide bonds, salt linkages and hydrogen bonds of the silk polymer system tend to break down once the temperature exceeds 100 degree C.

Chemical Properties
1. Effect of acids - Silk is degraded more readily by acids than is wool. This is because, unlike the wool polymer system with its disulphide bonds, there are no covalent cross-links between silk polymers. Thus perspiration, which is acidic, will cause immediate breakdown of the polymer system of silk. This is usually noticed as a distinct weakning of the silk textile material.

2. Effect of alkalis - Alkaline solutions cause the silk filament to swell. This is due to partial separation of the silk polymers by the molecules of alkali. Salt linkages, hydrogen bonds and van der Waals' forces hold the polymer system of silk together. Since these inter-polymer forces of attraction are all hydrolysed by the alkali, dissolution of the silk filament occurs readily in the alkaline solution. It is interesting to note that initially this dissolution means only a separation of the silk polymers from each other. However, prolonged exposure would result in peptide bond hydrolysis, resulting in a polymer degradation and complete destruction of the silk polymer.

3. Effect of sunlight and weather - The resistance of silk to the environment is not as good as that of wool. This lower resistance is due mainly to the lack of covalent crosslinks in the polymer system of silk.

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