Oxidation Effect on Silk

Reports regarding the oxidation of proteins are rather meagre since the reactions are very complex. Oxidizing agents may attack proteins in three possible points

• At the side chains

• At the N-terminal residues

• At the peptide bonds of adjacent amino groups.

Hydrogen peroxide is absorbed by silk and is thought to form complexes with amino acid groups and peptide bonds. It has been demonstrated that hydrogen peroxide diminishes the tyrosine content and further that the peptide bonds are broken at the tryosine residues. Peracetic acid causes more rapid scission and produces more acid groups than peroxide.

Degradation by Acids, Alkalis on Silk

Treatment of silk fibres with acid or alkaline substances causes hydrolysis of the peptide linkages. The degree of hydrolysis is based on the pH factor, which is at minimum between 4 and 8. Degradation of the fibre is exhibited by loss of tensile strength or change in the viscosity of the solution.

Hydrolysis by acid is more extensive than alkali, and it has been postulated that acid hydrolysis occurs at linkages widely distributed along the protein chain, whereas in the early stages of the alkaline treatment, hydrolysis happens at the end of the chain. Hydrochloric acid readily dissolves fibroin especially when heated - and this is used mainly in studies of hydrolysis. Hot concentrated sulphuric acid, while rapidly dissolving and hyrolyzing fibroin, also causes sulphation tyrosine.

Nitric acid readily decomposes fibroin, due to its powerful oxidizing properties and concurrently causes nitration of the benzene nuclei. Organic acids have few effects at room temperature when diluted, but in a concentrated form fibroin may be dissolved, along with a certain amount of decomposition.

Effect of Heat on Silk

If white silk is heated in an oven at 110ºC for 15 minutes, it begins to turn yellow. At 170ºC, silk disintegrates and at its burning points releases an empyreumatic odour.

Action of Water on Silk

Silk is a highly absorbent fibre, which readily becomes impregnated with water. Water, however, does not permanently affect silk fibre. Silk strength decreases about 20 percent when wet and regains its original strength after drying. The fibre expands but does not dissolve when steeped in warm water. Note that the fibre will also absorb dissolved substances present in water. This is the reason that special attention is given to the quality of the water utilized for reeling, washing, dyeing or finishing.

Electrical Properties of Slik

Silk is a poor conductor of electricity and accumulates a static charge from friction. This trait can render it difficult to handle in the manufacturing process. This static charge can be dissipated by high humidity or by maintaining a R.H. of 65 percent at 25ºC. Based on its insulating properties, silk is used extensively for covering wire in electrical equipment.

Effect of Light on Silk

Continuous exposure to light weakens silk faster than cotton or wool. Raw silk is more resistant to light than degummed silk. It is advised that silk drapery and upholstery fabrics be protected from direct exposure to the light.

Hygroscopic Nature of Silk

Denier=Fiber Weight(g)/Fiber Length(m) *9000

Moisture content and humidity are of critical importance to commercial silk production. Figure 4 illustrates the pattern of moisture regain where a hysteresis exists between the adsorption and desorption curves. Desorption measures a greater regain at a given relative humidity. For instance, given 65 percent RH, the adsorption regain value is 10 percent and the associated desorption value is 11.1 percent. Currently, 11 percent is the accepted moisture regain coefficient for silk; the mercantile weight of silk is derived based on this factor.

Tenacity and Elongation of Silk

Tenacity indicates the quantity of weight a given fibre can support before breaking. the typical tenacity of a bave is 3.6 to 4.8 g per denier . Degummed silk has greater tenacity than raw silk. Elongation defines the length to which a fibre may be stretched before breaking. Raw silk has an elongation of 18 to 23 percent of its original length. Excess moisture increases the elongation of silk, but decreases its tenacity.

Specific Gravity of Silk

The bave specific gravity on average of sericin and fibroin measures from 1.32 to 1.40. Generally, the specific gravity of sericin is slightly higher than that of fibroin

Features of Silk, Properties Silk, Bombyx Mori Silk

• The silk of Bombyx mori is composed of the proteins fibroin and sericin, matter such as fats, wax, sand pigments plus minerals.

• Fibroin in the Bombyx mori comprises a high content of the amino acids glycine and alanine, 42.8 g and 32.4 g respectively

• The key amino acids in sericin are serine (30.1 g), threonine (8.5 g), aspartic acid (16.8 g) and glutamic acid (10.1 g)

Sericin is a complex protein composed of three distinct components (I, II and III) of which sericin III is the interior layer directly adjacent to the fibroin core. The sericin I outer lay is the most soluble of the three constituents, while sericin III is difficult to dissolve. Viewed as a cross section, the brins have the appearance of equilateral triangles with rounded corners that face each other at their respective bases.
When the brin is crushed, it splinters into numerous minute fibrils revealing the actual structure of the brins. The thickness of each fibril is less than one micron and they are parallel to the axis of the fibre. A single fibril contains many microfibrils which, when examined with an electron microscope, have a diameter of approximately 100 Å per microfibril. Microfibrils contain micelles, which are separated into crystalline and amorphous segments.

Composition of Cocoon Shell

The silk filament forming the cocoon shell is composed of two brins (proteins) named fibroin and covered by silk gum or sericin. The amount of sericin ranges from 19 to 28 percent according to the type of cocoon.

The composition of the cocoon shell is given below:

• Fibroin -- 72-81 percent

• Sericin --  19-28 percent

• Fat and wax --  0.8-1.0 percent

• Colouring matter and ash -- 1.0-1.4 percent

Composition of a Whole Cocoon

The composition of the whole cocoon is defined as the cocoon shell, cocoon pupa and cast off skin . The cocoon pupa makes up the largest portion of its weight. Note that much of the cocoon content is water. Therefore it is necessary to remove the water to improve the cocoon filament for reeling and to better preserve the cocoon over a long period.