What is Silk Fibre? | Types of Silk Fibers | Properties of Silk Fabrics

What is Silk Fibre?

Silk Fibre is a natural protein fiber, some forms of which can be woven into textiles. A type of silks, produced by caterpillars and mulberry silkworm, have been known and used in China, South Asia, and Europe since old times. The strands of raw silk as they are unwound from the cocoon consist of the two silk fibers mixed with sericin and other stuff. About 75 % of the strand is silk i.e. fibroin and 23 % is sericin; the remaining materials consist of fat and wax (1.5 %) and mineral salts (0.5 %). As a natural protein fiber silk has a notable attraction towards natural dyes.


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Types of Silk Fibers



Characteristics of Silk Fibre:


Properties of silk fabrics are discussed below:


  • Composition: The silk fiber is chiefly composed of 80% of fibroin, which is a protein in nature, and 20% of sericin, which is differently called silk gum.

  • Strength: Silk as a fibre, has excellent tensile strength, which enables it to withstand great pulling pressure. Silk is the strongest natural fibre and has moderate abrasion resistance. The strength of the fired yarns is mainly due to the continuous length of the fibre. Spun silk yarn though strong is weaker than thrown silk fiber yarns.

  • Elasticity: Silk fibre is an elastic fibre and may be stretched from 1/7 to 1/5 of its original length before breaking. It tends to return to its original size but gradually loses little of its elasticity. This would signify that the fabric would be less sagging and less binding resulting in the wearer's comfort.

  • Resilience: Silk fabrics hold their shape and have moderate resistance to wrinkling. Fabrics that are made from short-staple spun silk have less resilience.

  • Drapability: Silk has liability and suppleness that, supported by its elasticity and resilience, gives it excellent drapability.

  • Heat Conductivity: Silk is a protein fibre and is a non-conductor of heat similar to that of wool. This makes silk suitable for winter clothes.

  • Absorbency: Silk fabrics being protein in nature have good absorbency. The absorptive capacity of the silk fabric makes comfortable apparel even for a warmer atmosphere. Fabrics made from silk are comfortable in the summer and warm in the winter. Silk fiber can usually absorb about 11 percent of its weight in moisture, but the range modifies from 10 percent to as much as 30 percent. This property is also a significant factor in silk’s ability to be printed and dyed easily.


  • Cleanliness and Washability: Silk fabric does not attract dirt because of its smooth surface. The dirt, which gathers can be easily removed by wash or dry cleaning. It is often suggested for the silk garments to be dry-cleaned. Silk fabrics should always be washed with mild soap and strong agitation in the washing machine should be evaded. Silk water – spot easily, but after washing or dry cleaning will restore the surface of the fabric.

  • Reaction to Bleaches: Silk, like wool, is deteriorated with chlorine bleaches like sodium hypochlorite. However, mild bleach of hydrogen peroxide or sodium perborate may be used for silk.

  • Shrinkage: Silk fabrics are reduced only to normal shrinkage which can be restored by ironing. Crepe effect fabrics shrink considerably in the wash, but careful ironing with a moderately hot iron will restore the fabric to its original size.

  • Effect of Heat: Silk is sensitive to heat and starts to decompose at 330° F (165° C). The silk fabrics thus have to be ironed when damp.

  • Effect of Light: Silk fabric fades on exposure to sunlight. Raw silks are more resistant to light than degummed silk.

  • Resistance to Mildew: Silks will not mildew except left for some time in a damp state or under the extreme circumstances of tropical dampness.

  • Resistance to Insects: Silk may be charged by the larvae or clothe moths or carpet beetles.

  • Reaction to Alkalis: Silk is not as sensitive as wool to alkalis, but it can be damaged if the concentration and the temperature are high. A mild soap or detergent in lukewarm water is thus desirable.

  • Reaction to Acids: Concentrated mineral acids will dissolve silk quicker than wool. Organic acids do not harm silk.[

  • Affinity for Dyes: Silk has great absorbency and thus has a good affinity for dyes. Dyed silk is colorfast under most conditions, but its resistance to light is poor.

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  • Resistance to Perspiration: Perspiration and sunlight weaken and yellows silk fabrics. The silk itself deteriorates and the color is affected causing staining. Garments worn next to the skin should be washed or otherwise cleaned after each wearing.


Types of Silk


Raw silk:


Silk fiber as it originates from the cocoon is coated with a protecting layer called silk gum, or sericin. The silk gum is thick and stiff. Silk with all of its gum is termed raw silk.


Tussah silk:

Silk obtained from wild silkworms is called tussah silk. The natural appearance of tussah silk is usually not white, but shades of pale beige, grey, and brown. It is habitually coarser than cultivated silk.


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Bombyx Mori silk:

It is also known as mulberry silk which is generated by domesticated silkworm raised on a diet of mulberry leaves almost exclusively softer, finer, and more bright than tussah silk. This silk produces shades of white products.


Reeled silk or Thrown silk:

It is a term for silk fiber that is separated from the silkworm cocoon. It is the finest silk, the fibers are very long, bright, and of great strength.


Spun silk:

Silk made from the broken cocoon (from which the moths have already risen) and short fibers, feels more like cotton.


Weighted silk:

When yarns are prepared for weaving, the skeins of yarn are boiled in a soap solution to eliminate the natural silk gum or sericin. The silk may lose from 20 to 30 % of its original weight as a result of boiling. As silk has a great affinity for metallic salts such as those of tin and iron, the loss of weight is replaced through the absorption of metals. Thus a more complex fabric can be performed at a lower price than that of pure silk, which is known as weighted silk.


Pure silk:

If the natural gum or sericin is eliminated from the silk and not further material is added to enhance the weight of the fiber, i.e. silk containing no clanging weighting is called pure silk. Pure silk is exclusively soft and holds fine luster.


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Physical Properties of Silk Fiber:


Tenacity - The silk fiber is strong. This strength is due to its linear, beta form polymers, and very crystalline polymer system. These two factors allow many more hydrogen bonds to be formed in a greatly more regular manner. Silk loses strength in wetting. This is due to water molecules hydrolyzing a notable number of hydrogen bonds and in the process reducing the silk polymer.


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


Elastic-plastic nature - Silk is supposed to be more plastic than elastic because it’s very crystalline polymer system does not allow the amount of polymer movement which could happen in a more amorphous system. Hence, if the silk material is stretched extremely, the silk polymers that are now in a stretched state (They have a beta configuration) will slide past each other. The process of stretching tears a significant number of hydrogen bonds.


Elongation - Silk fiber has an elongation at break of 20-25% under normal condition. At 100% R.H., the extension at rest is 33%.


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


Thermal properties - Silk is extra sensitive to heat than wool. This is examined to be partly due to the lack of any covalent cross-links in the polymer system of silk, compared with the disulfide bonds which happen in the polymer system of wool. The existing peptide bonds, salt linkages, and hydrogen bonds of the silk polymer system lead to break down once the temperature overshoots 1000C.


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


The hand feels - The name of the silk is described as a medium and its very crystalline polymer method imparts a certain amount of stiffness to the fibers. This is often misunderstood, in that the handle is regarded as a soft, because of the smooth, even and the regular surface of silk fibers

.

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


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


Effect of sunlight - Silk is a more sensitive light than any other natural fibre. Extended exposure to sunlight can cause somewhat spotted color change. The yellowing of silk fiber is usually occurred due to photodegradation by the action of UV radiation of sunlight. The mechanism of degradation is due to the breaking of hydrogen bonds ensured by the oxidation and the eventual hydrolytic fission of the polypeptide chains.



Chemical Properties of Silk Fiber:


The action of water - The penetration of water molecules takes place in the amorphous regions of the fibre, wherever 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 support takes place followed by a decrease in the force required to rupture the fiber and improve extensibility. The method of silk in boiling water for a short period does not cause any harmful effect on the properties of silk fibre. But on continued boiling, silk fiber tends to lose its strength to an unusual degree, which is thought to occur because of the hydrolysis action of water. Silk fiber withstands, however, the effect of boiling more useful than wool.


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


Effect of alkalis - Alkaline solutions cause the silk fiber to swell. This is due to the partial division 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 powers of attraction are all hydrolyzed by the alkali, dissolution of the silk fiber occurs readily in the alkaline solution. Initially, this dissolution involves only a separation of the silk polymers from each other. However, continued exposure would result in peptide bond hydrolysis, resulting in polymer degradation and destruction of the silk polymer. Whatever silk can be managed with a 16-18% solution of sodium hydroxide at low temperatures to produce crepe results in the mixed fabric containing cotton. Caustic soda, when it is hot and strong, dissolves the silk fiber.


The action of the oxidizing agent - Silk fiber is extremely sensitive to oxidizing agents. 

The attack of oxidizing agents may take place in three possible points of protein 


  1. At the peptide bonds of nearby amino groups,
  2. At the N-terminal residues
  3. side chains

Though fibroin is not seriously influenced by hydrogen peroxide solution, nevertheless may suffer from the loss of nitrogen, and tyrosine content of silk indicates that hydrogen peroxide may create a breakage of peptide bonds at the tyrosine residues resulting in the weight loss of the fiber. 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.


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

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