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SILK • Silk is a natural protein fibre, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity (sericulture). The shimmering appearance of silk is due to the triangular prism-like structure of the silk fibre, which allows silk cloth to refract incoming light at different angles, thus producing different colors.
Wild silk • A variety of wild silks, produced by caterpillars other than the mulberry silkworm have been known and used in China, South Asia, and Europe since ancient times. However, the scale of production was always far smaller than that of cultivated silks. They differ from the domesticated varieties in color and texture, and cocoons gathered in the wild usually have been damaged by the emerging moth before the cocoons are gathered, so the silk thread that makes up the cocoon has been torn into shorter lengths. Commercially reared silkworm pupae are killed by dipping them in boiling water before the adult moths emerge, or by piercing them with a needle, allowing the whole cocoon to be unraveled as one continuous thread. This permits a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm.
Macro structure • Silk is a natural protein filament. It’s density is 1.34 g/cm cube which makes it a medium weight fibre. • raw silk strand consists of two silk filaments encased by a protein called sericin. This sericin gives raw silk a coarser handle. Sericin is also very weather resistant since it can withstand prolonged weather exposure. • Shiny silk filaments are revealed when sericin is removed in mild alkaline solution. • Silk is a fine, regular, translucent filament. It may be 600m long but average about 300 m in length. • Its cross-section may vary from 12µm to 30µm. this gives fibre length to breath ration in excess of 2000:1. • The beauty and softness of silk is due to the triangular cross-section of silk filament. As silk filament is tightly twisted and angle of reflection changes continuously. As a result, the intensity of reflected light is broken resulting in a soft subdued luster.
Micro-structure • The irregular, random, longitudinal view of silk filaments are not sufficiently distinctive to identify silk under the microscope. The triangular cross-section can be used to identify silk. This appearance is due to the slit-like opening of the silk secreting glands, one being located on either side within the mouth of the silk moth larvae. • Silk is a coagulating stream of fibroin solution, and has no identifiable micro-structure. In this regard it resembles the man-made fibres.
Silk polymer • Silk polymer is linear, fibroin polymer. fibroin is the name of protein which constitutes silk. • Silk polymer is composed of 16 different amino acids. Three amino acids called Alanine, glycine and serine make up about fourth-fifth of silk polymer. • Silk polymer is not composed of any group containing sulphur. Hence it does not contain any disulphide bonds. • The silk polymer occur only in beta configuration. Silk polymer is about 140 nm long and 0.9 nm thick. • Silk is considered as being composed of layers of folded, linear polymers. That explained the high crystallinity i.e. its crystalline regions are 65-70% and amorphous region are 30-35%.
Silk fibre • Silk polymer system contains no disulphide bonds. • Like wool the repeating unit of silk is amino acid. • Important chemical grouping is of silk polymer is peptide groups. • The major forces of attraction are thought to be hydrogen bonds. These are only effective across a distance less than 0.5 nm.
Properties • Tenacity: Silk is strong fibre. Its strength is due to its beta-configuration, linear structure and very crystalline polymer. these two factors permit many more hydrogen bonds to be formed in a much more regular manner. Usually 30.9 – 44.1 cN/tex. Wet strength is 75 to 85 prcnt of dry strength. When wet silk loses strength. This is due to water molecules hydrolyzing a significant number of hydrogen bonds. • Elastic nature: Silk is considered to be more plastic than elastic. Because it’s a crystalline polymer it does not allow polymer movement which could occur in amorphous region. Its handle is regarded as soft because of the smooth, even, and regular surface of silk filament. • Hygroscopic nature: Because it’s a crystalline polymer, its less absorbent than wool. • Thermal properties: Silk is more sensitive to heat than wool. This due to the lack of covalent linkages in the polymer system. The salt linkages, hydrogen bonds tend to break when temperature reaches 100°C.
Moisture regain is 11%. • Specific gravity is 1.25. • Effect of sunlight: Sunlight tends to encourage the decomposition of silk by atmospheric oxygen. • Electric properties:
Chemical properties • Acid causes immediate breakdown of silk polymer because there are no covalent linkages between silk polymers. • Alkaline solution cause silk filament to swell. Initially alkali will separate the silk polymers from each other. Prolonged exposure will hydrolyze peptide bond, resulting in polymer degradation and complete destruction of silk polymer. So yellowing of white or dulling of colored textile occurs during laundering. It is due to rearrangement of polymers. • Effect of bleach is same as wool. • The resistance to environment is not as good as that of wool. This low resistance is due to lack of covalent cross linkages.