Physical, Chemical and Mechanical Properties of Silk Fiber

Last Updated on 15/11/2021

Physical, Chemical and Mechanical Properties of Silk Fiber

Rakibul Islam Khan
Department of Textile Engineering
Ahsanullah University of Science & Technology (AUST)
Email: pl_20in@hotmail.com

Basic Concept on Silk Fiber:
Silk is at the luxury end of the market because it is expensive. Silk is a protein fiber of insect origin, being produced as a fine filament of long length from the body fluid of silkworm. The silkworms eat only the leaves of mulberry tree. Silk is a polypeptide, formed from four different amino acids. Silk comes in two varieties: wild or cultivated; the former is the most expensive because the cocoons have to be harvested from a wide area, which is an arduous process.

Silk fiber is relatively stiff and show good to excellent recovery from deformation depending on the temperature and humidity conditions. These fibers exhibit favorable heat-insulating properties but owing to their moderate electrical resistivity, they tend to build up static charge.

In general, a number of protein-based fibers are collected under the term silk. The threads can be spun by caterpillars, spiders and mussels. Among the most relevant silk producers are the larvae of the silk moth. Silk fibers are biodegradable and highly crystalline with a well-aligned structure. They have a higher tensile strength than glass fiber or synthetic organic fibers, good elasticity and excellent resilience. Silk fiber is normally stable up to 140°C, and the thermal decomposition temperature is greater than 1500°C. The densities of silk fibers are in the range of 1320–1400 kg/m³ with sericin and 1300–1380 kg/m³ without sericin.

The silk fiber is produced as filament in spinning glands. Raw silk actually consists of two major protein components: Fibroin (approximately 75–83 wt%) and sericin (25–17 wt%). Two filaments of fibroin are embedded into a layer of silk gum (sericin). The fiber is highly oriented and the protein chains are arranged in pleatedsheet conformation (β-keratin), which then forms microfibrils and macrofibrils.

The main countries that produce silk today are China, Japan, Vietnam, Korea, Thailand, India, Uzbekistan, Brazil and Romania.

Typical Properties of Silk Fiber:

• Parameter ————-Value
• Fineness ————->1–3.5 dtex
• Diameter ————>10–13 μm
• Fiber length ——–>700–1500 m
• Density ————->1.37 g / cm³
• Moisture regain —->9–11 %
• Breaking strength –>25–50 cN / tex
• Elongation ———->10–25 %
• Color —————–> Lustrous white

Physical Properties of Silk Fiber:

1. 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.

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

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 betaconfiguration) will slide past each other. The process of stretching ruptures a significant number of hydrogen bonds.

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

5. Hygroscopic nature/ Absorbency:
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.

6. 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 100°C.

7. 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.

8. 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.

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

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

11. Effect of sunlight:
Silk is more sensitive light than any other natural fiber. Prolonged exposure to sunlight can cause partially spotted color change. Yellowing of silk fiber 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:

1. Action of water:
The absorption of water molecules takes place in the amorphous regions of the fiber, 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 fiber 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 fiber. But on prolonged boiling, silk fiber tends to loss its strength to some degree, which thought to occur because of hydrolysis action of water. Silk fiber withstands, however, the effect of boiling better than wool.

2. 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 fiber 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 fiber 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.

3. 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 fiber.

4. Action of oxidizing agent:
Silk fiber 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 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.

5. Action of reducing agents:
The action of reducing agents on silk fiber 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 fiber.

Mechanical Properties of Silk Fiber:
Silk fibers are remarkable materials displaying unusual mechanical properties: strong, extensible, and mechanically compressible. They also display interesting thermal and electromagnetic responses, particularly in the ultraviolet (UV) range and form crystalline phases related to processing. The mechanical properties of silk fibers are a direct result of the size and orientation of the crystalline domains, the connectivity of these domains to the less crystalline domains, and the interfaces or transitions between less organized and crystalline domains. Other properties of silk such as good thermal stability, optical responses, dynamic mechanical behavior and time dependent responses have all been used in number of applications in various fields.

Table: Mechanical properties of different varieties of silk

1. Tensile properties:
The tensile properties of different varieties of silks in terms of tenacity, elongation-at-break and initial modulus have been determined by a number of workers.

Elongation-at-break, on the other hand, showed a higher value for all the non-mulberry silks compared to mulberry varieties. The values range between 31% and 35% for tasar, 34% and 35% for muga and 29% and 34% for eri silks, respectively. The elongation values for mulberry varieties ranged between 19% and 24%. Some of the mechanical properties of different varieties of silk are summarized in above Table.

2. Optical properties:
Silk fibroin extracted from silkworm cocoons is a unique biopolymer that combines biocompatibility, implantability and excellent optical properties. Silk may be used as an optical material for applications in biomedical engineering, photonics and nanophotonics. Silk can be nanopatterned with features smaller than 20 nanometre (nm). This allows manufacturing of structures such as (among others) holographic gratings, phase masks, beam diffusers and photonic crystals out of a pure protein film. The properties of silk allow these devices to be ‘biologically activated’ offering new opportunities for sensing and biophotonic components.

3. Visco-elastic behavior:
Silk fiber exhibits visco-elastic behavior. Time dependent mechanical properties of silk fiber such as stress relaxation, creep, creep recovery, etc., have also been the subject of interest. Creep is a phenomenon associated with time dependent extension under an applied load. The complementary effect is stress relaxation under a constant extension.

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Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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