Silk

1. Introduction

Silk is one of expesive natural fibre and Silk is the queen of fibre,silk is the only natural protien filament.It is established wealth rank. Silk is one of the oldest fibres known to man. It is an animal fibre produced by certain insects to build their cocoons and webs. Although many insects produce silk, only the  filament produced by the mulberry silk moth Bombyx mori and a few others in the same genus is used by the commercial silk industry.



2. Structure
  • Cultivated silk fibres are triangular to oval in cross section ,while wild silk has to flat ribbon like shape.
  • The silk filament is self  slightly twisted,the angle of light reflection changes continuously,as a result of intensity of the reflected light is broken, resulting a soft subdued luster.
  • Silk consist of chiefly amino acids with low molecular weights.Peptide chain of silk may contain very few side chains. Raw silk strand consist of two filament covered by protien called sericin.
  • Thickness of raw silk have irregular  and uneven surface due to the coating of sericin which gives rough surface.
  • The amount of sericin  is 22 to 23 of weight of raw silk,sericin is weather resistance.
  • Silk is very fine,regular and translucent fibre, its length may be up to 600 meter (avg length 300 m) long  and diameter 12 micro meter to 30 micro meter.
  • Silk fibre is composed of  minute filaments that are often referred to as fibrils,they are 10 micro meter in diameter.




1. Polymer System

Functional Group: -COOH, -NH2, -CONH

  • The silk polymer is folded linear  fibroin polymer,it has no cystine linkage
  • The silk polymer is 140 am long and 9 urn thick,67 to 70% crystalline and have 16 anion acids.
2. Chemical Composition

The silk fibroin is composed of long chain amino acids unit joined by peptide link with hydrogen bonding between parallel of chains.

       Component                      Percentage %
          Fibroin                72-75%
          Sericin                22-23%
              Ash                 0.1-1.5
      Wax and fat                 1.4-2.7
            Salts                 0.3-1.6




3. Physical Properties
  • Tenacity-It is 4.3 gm/denier,silk filament is strong due to its linear and very crystalline polymer system,when wet ,silk losses is strength,this is due to hydrolyzing a significant no. of hydrogen bonds and in the process weakening the silk polymer.
  • Elastic Plastic Nature-Breaking extension is 23.4 and recovery 52,silk is more plastic than elastic because it is very crystalline.
  • Thermal Properties-Silk is more sensitive to heat than wool.,this is due to the lack of any covalent cross link in the polymer system of the silk.The existing peptide bonds,salt linkage and hydrogen bonds of the silk polymer tend to break down once when temperature exceeds 100 degree celcius.
  • Action of light-Silk is more sensitive to light can cause partially spotted colour changes(effective in presence of atmospheric oxygen).
  • Hygroscopic nature-Silk is absorbent fibre,like wool ,it absorb water well but dries fairly quick.
  • Resiliency-Moderate
  • Abrasion resistance-Good
  • Dimensional stability-Good




4. Chemical Properties
  • Effect of acids–
  • Silk is rapidly degraded by acids than wool,there is no covalent cross links between silk polymer.
  • Hot concentrated acids(sulfuric and hydrochloric acids) rapidly decomposes silk.
  • Weak (5 solution )and cold hydro fluorine (HF) acids  does not have any harmful action on silk,but it removes all inorganic material from the fibre.
  • Action of alkali-Silk is more resistance than wool fibre to alkali,alkaline solution cause silk fibre to swell,this is due to the partial separation of silk polymers by the molecules of alakli.salt linkage,hydrogen bonds and vander walls force holds the polymer system of the silk together.it dissolve in hot concentrated solution due to its polymer forces hydrolised by alkali.
  • Action of bleaches-It is called degumming. silk fibroin is highly sensitive to oxidizing agent. Attack of oxidizing agents may takes place in the peptide bonds,results on the weight and strength loss.Action of bleaches on silk is same as wool.

Cocoon  weight-2.2 to 1.5 g          
Raw silk%-65 to 84%          
Size of cocoon filament-200 to 300 mtr and dia 15 to 20 micron.       



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