Silk Fiber Production, Types, Properties

The silk fiber is often hailed as the queen of textiles. It is a natural protein fiber produced by silkworms. Its rich history, unparalleled properties, and versatile applications make it a coveted material in various industries. Let’s dive into the fascinating world of silk fiber.

What is Silk Fiber?

Animal fiber and protein fiber are known as the queen of fibers (because it is the most among all textile fibers). It is renowned for its shimmering appearance and luxurious texture.

Consists of two types of proteins are:

1. Sericin: gummy coating which allows the two parallel filaments to stick together.
2. Fibroin: structural center.

The commercial silk industry is based on one insect: the silkworm, a caterpillar (Binomial name: Bambyx more).

Silk has captivated civilizations for millennia. Originating in ancient China around 2700 BCE, silk was once a closely guarded secret. It was reserved for royalty and the elite. Today, silk fabric remains a symbol of opulence and refinement, synonymous with elegance and sophistication.

Commercial Name of Silk Fabrics:

1. Crepe
2. Dupion
3. Organza
4. Satin
5. Matka

Most Slik Producing Countries: 

1. Turkey
2. Japan
3. China
4. Germany
5. South Korea

Chemical Composition:

• Fibroin: 72 – 75 %
• Sericin / gum: 22 – 23 %
• Ash: 0.1 – 1.5 %
• Wax & Fat: 1.4 – 2.7 %
• Salts: 0.3 – 1.6 %

Topography of Silk Fiber: (Microscopic View)

1. Cross-sectional view
2. Longitudinal view

• Fibroin
• Sericin

• Fibroin
• Sricin

Production Process of Silk:

The journey of silk fiber begins with sericulture. The plantation of silkworms is known as sericulture.

Farmers carefully nurture silkworms on mulberry leaves until they spin their cocoons. These cocoons are then harvested and subjected to a process called silk reeling, where the silk filament is unwound and spun into threads.

Finally, skilled artisans weave silk yarn into exquisite fabrics using traditional looms, preserving centuries-old craftsmanship.

The production flowchart of Slik are follows:

Sericulture
⬇️
Sorting the cocoons
⬇️
Softening the sericin
⬇️
Reeling
⬇️
Throwing
⬇️
Degumming
⬇️
Weighting

1. Sericulture: 

The breeding, nurturing, and care of the silkworm (Bombyx mori) is called sericulture.

Egg-hatching silkworm molting spinning the cocoon moth.

1. Egg production: A caterpillar lays 350 – 400 eggs.
2. Hatching: looz egg hatches 36000 silkworms.
3. Moulting: After 25 days from hatching, the silkworm becomes 10000 times heavier and filled with liquid silk.
4. Spinning the cocoon: Silk is produced in 2 glands in the head of the silkworm and then forced out in liquid form, 1 kg of silk can be produced from 5500 silkworms.

2. Soring the cocoons:

Cocoons are selected according to color, size, shape, and texture.

3. Softening the Sericin:

After sorting, the cocoons are passed through a series of hot and cold immersions to soften the sericin.

4. Reeling:

Unwinding the silk filament from the cocoon and then combining them together to make raw silk thread is the reeling process of silk.

5. Throwing:

The process of doubling and twisting reeled silk to produce the silk yarn is called throwing. 4 varieties of thrown silks are- crepe, tram, thrown, singles, and organize. 

6. Degumming:

A washing that is done to remove the sericin from hard silk fibroin is called degumming. By this method, 25 % of the total weight is lost and raw silk (hard) is converted into degummed silk (soft).

7. Weighting:

The process of restoring or increasing the weight loss during degumming by chemical treatment is called weighting. 

Process of Weighting: At first, the degummed silk is immersed in Sncl₄. Then it is washed with cold water. After that, the fiber is steeped in Na₂Hpo₄ solution and then soaked in Na₂Sio₃ solution. This three-step process is repeated several times till the expected weighting of the silk is reached. 

Par Weight: The amount of chemicals required to give the silk its original raw weight is called par weight.

• 100 kg raw silk; degumming; Weighting; 100gk: one par
• 100 kg degumming; weighting; 90 kg: 10% under par
• 100 kg degumming; weighting; 150 kg: 50% over par.

Effect of Weighting:

1. The density, volume, and weight of silk are increased.
2. The durability of the fabric is improved. 
3. Excessive weighting decreased durability. 

Types of Silk Fiber:

1. Raw silk: Silk containing sericin is called raw silk.
2. Tussah silk: Silk made from wild silkworms is known as Tussah silk.
3. Bombyx mori silk: Silk made from domesticated silkworms is called Bombyx mori silk. It is also known as mulberry silk.
4. Reeled silk/ thrown silk: Silk that is unwounded from the silkworm cocoon is called reeled-on thrown silk.
5. Spun silk: Silk made from broken cocoons is called spun silk.
6. Weighted silk: Silk that has been treated to restore or increase the weight lost during degumming is called weighted silk.
7. Pure silk: Silk containing no metallic weighting is called pure silk.

Diseases of Silkworm: 

1. Pebrine: Pebrine is caused by protozoan parasite. It causes black spots on the skin. It is the worst of all the silkworms.
2. Flacherie: Flacherie happens due to a digestive disorder or infective organism. It causes swelling and blackening of the insect body.
3. Gransserie: Grasserie is a virus disease. For this disease, worms become yellow and bloated.
4. Muscardine: Muscardine is a fungal disease. For this disease, worms become white and covered with spores. It kills the worm quickly. 
5. Tachina fly: worms are attacked by tachinea fly. These flies lay eggs on the worm’s body. The eggs hatch and maggots eat the body of the silkworm. 

Properties of Slik Fiber

Silk fiber’s inherent strength and durability ensure longevity. Meanwhile, its softness and smoothness provide unparalleled comfort against the skin. Additionally, silk’s natural luster and shine impart a radiant glow to any fabric, elevating it to a realm of timeless beauty.

Physical Properties: 

1. Length: 500 2000 m
2. Tenacity: 38 cN / tex
3. Extensibility: 10 – 30
4. Specific gravity: 1.34
5. Cross section: Triangular
6. Moisture regain: 11%
7. Crystallinity: 65 – 70 %
8. Thickness: 12 – 30 mm
9. Resiliency: Moderate
10. Color: White, yellow, golden yellow, yellowish green
11. Dimensional stability: Good
12. Abrasion resistance: Good

Chemical Properties:

1. The action of water: Silk belongs to highly absorbent fiber. It can absorb water up to 30% of its weight without feeling wet. 
2. The action of Acid: Silk is readily decomposed by hot concentrated acid. Acidic perspiration will cause the immediate breakdown of silk.
3. The action of alkali: Silk is not damaged by dilute alkali at room temperature. But hot and strong caustic soda dissolves the silk fiber. 
4. Effect of bleach: Bleaches containing chlorine compounds damage silk fiber.

Thermal Properties:

1. The action of Heat: Silk is sensitive to high temperatures. It can be treated at a temperature of 140º without any decommission. Above 170º C, it decomposes readily.
2. The action of light: Silk is more sensitive to light than any other natural fiber. Light tends to encourage the rotting of silk by atmospheric oxygen. Yellowing of fiber mainly occurs due to UV radiation of light.

Identification Test:

1. Burn test: Silk fiber burns, but does not melt. It curls away from the flame. It has the smell of burnt hair.
2. Solubility test: Silk is soluble in a concentrated solution of H₂So₄ and HNO₃ both at boiling & room temperature. 

Difference between wool and silk:

Wool	Silk
i) Produced from the fur of animals.	i) Produced from cocoons of silkworms.
ii) Helical configuration crosslink polymer.	ii) Linear fibroin polymer.
iii) Composed of 20 different amino acids.	iii) Composed of 16 different amino acids.
iv) contain disulfide bonds	iv) Do not contain disulfide bonds.
v) More absorbent	v) Less absorbent.
vi) More elastic.	vi) Less elastic.
vii) Polymer is shorter.	vii) Polymer is longer.

Application

• Apparel: Formal dress, wedding dress, luxury items.
• Accessories: Hats, ties, scarves, gloves, handbags, umbrellas.
• Household textiles: Carpets, bed cloths.
• Industrial textile: Sewing threads, racing bicycle tires.

Construction model of a Raw silk filament: 

• Leaflet structure
• Micro fibrils
• Fibril bundle
• Fibroin strands 
• Sericin
• Raw silk filament

Reference:

• https://www.silkpaintinggallery.com/silk.htm
• http://textiletuition.blogspot.com/2015/08/silk-its-type.html
• https://www.slideshare.net/Hossain2011/presentation-on-silk-58767828
• http://etheses.lse.ac.uk/1936/1/U222405.pdf
• https://www.linkedin.com/posts/textiletrainer_life-cycle-of-silkworm-with-4-easy-stage-activity-7111640611077705728-SBGQ
• https://www.meritnation.com/ask-answer/question/why-are-silk-fibres-called-filament-fibres/science/7859049
• https://www.britannica.com/topic/sericulture
• https://en.wikipedia.org/wiki/Weighted_silk
• https://www.laphilstore.com/square-felt-coaster-set-forest.html
• http://allabouttextiles.blogspot.com/2008/01/silk-properties.html