Tenacity and elastic plastic nature of silk

1. Silk:
Silk is a natural protein fiber excreted by the moth larva Bombyx mori,
better known as the common silkworm. It's amino acid composition is
close to that of the human skin. Silk is a fine continuous monofilament
fiber of high luster and strength and is highly valued as a prestige fiber.
Because of its high cost, it finds very 1imited use in textiles. A minor
amount of wild tussah silk is produced for specialty items. Attempts
have been made to commercialize silk from spiders over the years, but
all ventures have met with failure. Domestic and wild silks are
essentially uncross linked and relatively simple in amino acid
composition compared to the keratin fibers. The properties for silk 1isted
here are for silk formed by Bombyx mori moth larvae.
Silk fiber
This fiber is the result of caterpillar secretions of certain wild or domestic
butterflies which constitutes cocoons in which they lock themselves inside during
the metamorphosis.
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. Silk loses strength on wetting. This is due to

2. water molecules hydrolyzing a significant number of hydrogen bonds
and in the process weakening the silk polymer.
Tenacity of some common fibre:
Fiber Grams per denier
Raw cotton 3.0-4.9
Jute 3.0-5.8
Flax 2.6-7.7
Ramie 5.5
Silk 2.4-5.1
Wool 1.1-1.7
Hemp 5.8-6.8
Stress strain curve of silk fibre:
Fig: stress strain curve of silk fibre

3. Elastic-plastic nature –
Silk is considered to be more plastic than elastic because it’s 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 that 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.
Very crystalline polymer system does not permit the amount of
polymer movement which results in plastic nature of silk than elastic.
Hence, if silk textile material is stretched, the silk polymers which
have beta-configuration will slide past each other. This stretching
results in rupturing of hydrogen bonds. After stretching, the
polymers do not return to their original position, which leads to
distortion and wrinkling or creasing of the silk textile material.

4. Fig: Elastic-plastic nature of Silk
Conclusion:
Silk is a natural protein filament that can be converted into textile by
either knitting or weaving techniques. Silk is the only natural filament. It
is a solid fiber. The ratio of load required to break the specimen and the
linear density of that specimen is called tenacity. Because of crystalline
nature, the silk polymer cannot have much movement. Silk textile is in
stretched state, when stretching ceases; the polymers do not return to
their original position but remain in their new positions.