Physical and Chemical Properties of Silk Fiber

Properties of Silk Fiber

Rakibul Islam Khan
Department of Textile Engineering
Ahsanullah University of Science & Technology (AUST)

Physical Properties of Silk Fiber:

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 water molecules hydrolyzing a significant number of hydrogen bonds and in the process weakening the silk polymer.

Specific gravity - Degummed silk is less dense than cotton, flax, rayon or wool. It has a specific gravity of 1.25. Silk fibres are often weighted by allowing filaments to absorb heavy metallic salts; this increases the density of the material and increases its draping property.

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 betaconfiguration) will slide past each other. The process of stretching ruptures a significant number of hydrogen bonds.
Silk fiber
Elongation - Silk fibre has an elongation at break of 20-25% under normal condition. At 100% R.H. the extension at break is 33%.

Hygroscopic nature - Because silk has a very crystalline polymer system, it is less absorbent than wool but it is more absorbent than cotton. The greater crystallinity of silk's polymer system allows fewer water molecules to enter than do the amorphous polymer system of wool. It absorbs water well (M.R.11%), but it dries fairly quickly.

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 1000C.

Electrical properties - Silk is a poor conductor of electricity and tends to form static charge when it is handled. This causes difficulties during processing, particularly in dry atmosphere.

Hand feel - 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.

Drapes Property - Silk fibre is flexible enough and if silk fibre is used to make garments, then the fabric drapes well and this is why it can be tailored well too.

Abrasion resistance - Silk fabric possess good abrasion resistance as well as resistance to pilling.

Effect of sunlight - Silk is more sensitive light than any other natural fibre. Prolonged exposure to sunlight can cause partially spotted color change. Yellowing of silk fibre is generally occurred due to photo degradation by the action of UV radiation of sunlight. The mechanism of degradation is due to the breaking of hydrogen bonds followed by the oxidation and the eventual hydrolytic fission of the polypeptide chains.

Chemical Properties of Silk Fiber:

Action of water - The absorption of water molecules takes place in the amorphous regions of the fibre, where the water molecules compete with the free active side groups in the polymer system to form cross links with the fibroin chains. As a result, loosening of the total infrastructure takes place accompanied by a decrease in the force required to rupture the fibre and increase extensibility. Treatment of silk in boiling water for a short period of time does not cause any detrimental effect on the properties of silk fibre. But on prolonged boiling, silk fibre tends to loss its strength to some degree, which thought to occur because of hydrolysis action of water. Silk fibre withstands, however, the effect of boiling better than wool.

Effect of acids - Silk is degraded more readily by acids than wool. Concentrated sulfuric and hydrochloric acids, especially when hot, cause hydrolysis of peptide linkages and readily dissolve silk. Nitric acid turns the color of silk into yellow. Dilute organic acids show little effect on silk fibre at room temperature, but when concentrated, the dissolution of fibroin may take place. On treating of silk with formic acid of concentrated about 90% for a few minutes, a swelling and contraction of silk fibre occur. Like wool, silk is also amphoteric substance, which possesses the ability to appear as a function of the pH value either as an acid or as a base.

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 hydrolyzed by the alkali, dissolution of the silk filament occurs readily in the alkaline solution. 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. Whatever, silk can be treated with a 16-18% solution of sodium hydroxide at low temperature to produce crepe effects in mixed fabric containing cotton. Caustic soda, when it is hot and strong, dissolves the silk fibre.

Action of oxidizing agent - Silk fibre is highly sensitive to oxidizing agents. The attack of oxidizing agents may take place in three possible points of the protein 1. At the peptide bonds of adjacent amino groups,
  1. At the N-terminal residues and
  2. At the side chains
Though fibroin is not severely affected by hydrogen peroxide solution, nevertheless may suffer from the reduction of nitrogen and tyrosine content of silk indicate that hydrogen peroxide may cause breakage of peptide bonds at the tyrosine residues resulting in the weight loss of the fibre. The action of chlorine solution on the silk fibroin is more harmful than does the solution of hypochlorite. These solutions, even at their lower concentration, cause damage to fibroin.

Action of reducing agents - The action of reducing agents on silk fibre is still a little bit obscure. It is, however, reported that the reducing agents that are commonly found in use in textile processing such as hydrosulfite, sulfurous acids and their salts do not exercise any destructive action on the silk fibre.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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