Introduction and Origin of Silk Fibre
Silk is another ancient and most opulent textile fibres ever discovered by human beings. Silk fibres are long and continuous, filaments formed of protein as it comes as a result of silkworm secretion. The fibres are highly valued not only due to its softness, and natural shine but also its strength, elasticity and thermo-regulation. Silk is a specialised product in the textile industry being a natural filament fibre -in comparison to cotton (staple fibre) or wool (staple fibre).
Fig: for demonstration purpose only
The importance of silk goes way beyond fashion. It has influenced economics, shaped the global trade and still plays roles in sustaining the livelihood of the rural people in places such as India and China. Silk is also under investigation in the 21 st century, used in technical fields of biomedicine, cosmetics and even aerospace.
Historical Overview and Global Significance
The earliest record of silk production is over 5000 years ago with the very earliest production traced back to ancient China. China even kept secrets of sericulture (rearing of sericulture) to preserve the monopoly on silk trade over the centuries. The knowledge later found its way to Asia and Europe and started shifting the culture and economy with the so-called Silk Road.
India ranks as the second largest producer of silk in the world and it is the only country producing all the five varieties of silks that are commercially important. Indian silk industry is closely connected with the handloom culture, rural jobs and market prospects in export.
Silk Fibres Types
Silk has various types and each type has a different characteristic. They are distinguished according to the type of silkworm and plants that they cater.
1. Mulberry Silk
- The silkworm Bombyx mori produces it by feeding entirely on the mulberry leaves.
- Produces more than 70 percent of the world supply of silk.
- It has a reputation of being soft, uniform and of a brilliant white shine.
2. Tussar Silk
- Is the production of the wild silkworm, the Antheraea mylitta, an Indian species.
- coarsely woven, and hard and nonlustrous as compared to mulberry silk.
- Golden/beige coloured naturally; commonly employed in the tribal and rural handloom weaving industries.
3. Eri Silk
- Obtained by Samia ricini silkworms, fed on castor leaves.
- They are referred as to peace silk as the moth abandons the cocoon prior to obtaining the fibre.
- It is very thin, soft and cottony and provides great thermal insulation making it ideal in shawls and winter wear.
4. Muga Silk
- Native in Assam, made by Antheraea assamensis.
- Very rare and famed by its golden colour in nature.
- Very long-lasting and becomes shiny with every wash.
5. Spider Silk (Lab-synthesised / Natural Harvesting)
- Not grown commercially as other silks.
- It is structurally notable due to its high tensile strength as well as its elasticity.
- Applied in military research and development, as well as biomedical R&D; production currently is at laboratory scale only.
All varieties of silk fibres have different physical, chemical and aesthetic properties, which determines how to use them in various final applications.
The Special Role of India in World Silk Production
The climatic diversity in India combined with the variety of host plants offers the country a chance to re-ar to many silkworm species in many conditions. Silkworm rearing and silk weaving clusters are the leaders in Karnataka, Andhra Pradesh, Assam, Jharkhand, and West Bengal states.
The Government of India also promotes silk production with an aid of the institutions such as Central Silk Board in India, which is instrumental in fundamental research, outreach services, and popularization of sericulture as a source of sustainable livelihood.
Morphology and Structure of Silk Fibre
Silk fibre can be differentiated with other natural fibres in terms of morphology and structure. It is the only natural continuous filament fibre, which implies that it can be spun into long uniform yarns without having to twist short fibres to together. This gives it its soft characteristic and the natural shine.
Silk is a protein fibre, mostly made up of fibroin which makes the structural core and the gummy sericin that coats the outside of the filaments upon cocoon production.
Cross-Sectional and Surface Construction
Raw silk when viewed under microscope can form a triangular or an oval cross section, depending on the manner in which the silkworm secreted the filament. The smooth surfaces and oblique geometry cause light to reflect on a variety of directions and this is what gives the silky effect of the silk cloth.
- Fibroin: This is the main constituent of silk fibre; is a fibrous protein of high molecular orientation and crystallinity.
- Sericin: Water soluble protein, which is a layer of protection on fibroin. It is taken out in the process of degumming.
When the mixture has been degummed, only the fibroin is left as the textile fibre of use in the production of silk.
Crystallinity and Molecular Structure
Silk is made mostly of the amino acids glycine, alanine, and serine. These are repeated series which enable a close molecular packing into crystalline structures of beta-sheet.
- Crystallinity: Approximately 60–70%, depending on the silk type and treatment.
- Orientation: Molecules are aligned along the fibre axis, giving silk its strength and tensile properties.
Silk is so strong it can break steel on a weight-to-weight basis due to high crystallinity combined with mechanical order due to regular beta-sheet formation.
Comparative Notes
Compared to cotton and wool:
- Silk is more lustrous because of the smooth reflection on the surface.
- It is more homogeneous and flows because of a smaller diameter of its filaments, averaging 1013 microns.
- As compared to cellulose fibres, silk is more resistant to moisture as well as possess good dimensional stability.
Physical and Chemical Properties of Silk
Since processing, dyeing, finishing, and industrial purposes of silk are highly dependant on its physical and chemical behaviour, the need to know and understand how silk behaves has become important. Being a natural protein fibre, silk has some characteristics in common with wool, although it is much different in regard to surface structure, strength as well as reactivity towards chemicals.
Physical Properties of Silk
Tenacity (Strength)
- Dry strength: ~3.5–4.0 g/denier
- Wet strength: Retains about 85–90% of its dry strength
- Silk is considered one of the strongest natural fibres, especially when drawn and reeled properly.
Elongation and Elastic Recovery
- Elongation at break: ~15–20%
- Moderate elastic recovery, does not recover well from high strain
- Silk offers a comfortable stretch but can lose shape under prolonged tension.
Moisture Regain
- Around 11%, allowing good comfort and breathability.
- This makes silk suitable for both hot and cool climates.
Density of Silk Fibre: Before and After Degumming
Before Degumming (Raw Silk with Sericin):
- Density: Approximately 1.35 g/cm³
This includes both the fibroin core and the outer sericin coating, which is hydrophilic and adds bulk but not strength.
After Degumming (Pure Fibroin Silk):
- Density: Approximately 1.25 g/cm³
Degumming removes about 20–25% of the silk’s mass (mostly sericin), resulting in:
- A lighter fibre
- Increased luster and softness
- Improved flexibility and dye uptake
Luster
- High natural sheen due to its smooth, triangular cross-section
- This optical property enhances fabric brilliance without the need for added finishes.
Thermal Behavior
- Begins to yellow above 130–140°C
- Decomposes around 170°C
- Silk is sensitive to high temperatures, requiring caution during ironing and finishing.
Chemical Properties of Silk
Composition
- Silk is composed mainly of fibroin (70–75%) and sericin (20–25%). Fibroin is insoluble in water, while sericin is soluble in boiling water and weak alkalis.
Acid and Alkali Resistance
- Sensitive to strong acids (e.g. HCl), which hydrolyse peptide bonds
- Mild alkalis (like soap or dilute NaOH) can dissolve sericin for degumming
- Strong alkalis damage fibroin, reducing strength and gloss
Reaction to Oxidising Agents
- Moderate resistance to mild oxidising agents like hydrogen peroxide (used in bleaching)
- Sensitive to chlorine-based bleaches
Affinity for Dyes
- Excellent dye affinity due to presence of –NH₂ and –COOH groups
- Dyes used: Acid dyes, metal complex dyes, reactive dyes
- Silk fibres exhibit brilliant colours and fastness when dyed properly.
Combustion Behavior
- Burns slowly with the smell of burning hair (protein content)
- Leaves a brittle, black ash
Silk Production Process – From Sericulture to Filament Extraction
The Silk production is both a biological, as well as a mechanical process starting with raising the silkworms and finishing with the reeling of the silk out of the delicate cocoon. This is a sequential procedure that is called sericulture where both insect and the fibre are handled with great care to preserve the distinctive quality of silk.
1. Sericulture: Rearing of Silkworms
The process begins with the cultivation of host plants like mulberry (Morus alba) and the rearing of Bombyx mori silkworms.
- Egg stage: Silkworm eggs are incubated under controlled temperature and humidity.
- Larval stage: Larvae are fed mulberry leaves for 25–30 days. They undergo several moults.
- Cocooning: The mature silkworm spins a cocoon using liquid silk secreted from its salivary glands. The process takes 2–3 days.
Each cocoon contains a single continuous silk filament that can be 600–900 metres long.
2. Harvesting and Stifling
Once cocooning is complete:
- Cocoons are harvested and sorted based on shape, colour, and shell ratio.
- Stifling is performed — the process of killing the pupa by exposing the cocoons to steam or dry heat. This prevents the moth from breaking the filament during emergence.
3. Reeling: Extraction of Silk Filament
Reeling is the mechanical unwinding of silk filament from the cocoon.
- Cocoons are softened in hot water to loosen sericin.
- The fibre is gently pulled and twisted to form a raw silk thread, typically combining 4–8 filaments.
- This raw silk is wound onto reels for further processing.
Fig: Silk reeling
4. Degumming (Desericinisation)
Sericin, the outer gum coating, is removed to enhance softness and sheen.
- Boiling in soap solution or mild alkali (Na₂CO₃) is used for degumming.
- Degumming removes 20–25% of cocoon weight, leaving behind pure fibroin.
Proper degumming is critical for:
- Improving dye uptake
- Preventing fibre stiffness
- Enhancing surface smoothness
5. Spinning and Weaving
- For spun silk, damaged or short filaments from pierced cocoons are carded and spun like staple fibres.
- Silk is woven using traditional handlooms, powerlooms, or shuttleless looms, depending on the end-use — from delicate chiffon sarees to durable upholstery.
Chemical Processing of Silk
1. Degumming (Desericinisation)
- Purpose: To eliminate the gum-like sericin, which covers the fibroin filaments.
- Process: Suspension of silk yarn/fabric in a weak soap solution or 0.5--1% sodium carbonate (Na₂CO₃) and subsequent boiling at 90–95°C for 30–60 minutes.
- Outcome: The filament gets gentler, shinier and is in a position to take in colours evenly.
- Weight Loss: The loss is usually 20 - 25% of original weight.
Note: A too high degrees of degumming can cause the fibre to become weak; a too low one will render the cloth dull and irregular in taking dyes.
2. Bleaching
- Bleaching is performed to give brightness since silk is usually off-white to yellowish.
- Reagent Employed: Hydrogen peroxide (H₂O₂) should be preferred as its oxidising properties are mild.
- Conditions The reaction is performed at 60–80°C and in a slightly alkaline environment (pH = 8.5-9).
- Caution: Chlorine bleaches should be avoided because they break down fibroin quickly.
- Naturally white mulberry silk may be omitted or bleaching to be shortened when dyeing with deep shades.
3. Dyeing
- Silk absorbs numerous different dyes because the fabric is amphoteric (it contains both the acidic and basic groups).
- Acid Dyes: Most favorite through brightness and the range of shade
- Metal Complex Dyes: The metal complex dyes are employed where high wash fastness is required
- Reactive Dyes: applies under controlled pH; it is more popular with blended fabrics
- Natural Dyes: They are popularly applied in handloom and heritage practices
During natural dyeing, Mordants such as alum or iron can be added to adjust shade and fastness.
4. Finishing
All sorts of chemical finishes may be used to enhance feel, durability, or functionality.
- Softening: Hand feel is improved with non-ionic or cationic softeners
- Anti-yellowing Agents: No yellowing through oxidation or use of heat
- Weighting: Adding weight, lustre and fluff with metallic salts such as tin chloride, previously common but now unpopular since it is environmentally undesirable because of the salts used
- Anti-crease or Wash-and-Wear Finishes: available to blended silk garments
5. Key Challenges in Silk Processing
- Yellowing: Can occur due to high temperature, acidic residues, or exposure to nitrogen oxides.
- Fibre Weakening: Overexposure to alkalis or oxidisers can hydrolyse fibroin chains.
- Uneven Dye Uptake: Caused by incomplete degumming or poor scouring.
Applications of Silk Across Various Fields
Special qualities such as high strength, natural shine, biodegradability and biocompatibility of silk make it effective in a vast domains outside the fashion industry.
1. Clothing and Apparel
Silk is associated with luxury clothing. It is commonly used in:
- Dresses, sarees, ties and scarves
- Bridal wear and lingerie
- Formal and ceremonial clothing
Its drape, breathability, and affinity to the dyes makes it a fabric of choice in making high-end garments.
2. Home Furnishings
Silk finds use in:
- coverings of cushions, curtains, hangings on walls
- Luxury upholstery and bed linen
- It brings a beautiful flourish, but must be guarded against light and wear, because it is sensitive.
3. Medical and Bio-technological Applications
Silk fibroin, since it is biocompatible and biodegradable, can be used in:
- Sutures (used in ophthalmic and internal surgeries)
- Tissue engineering scaffolds
- Wound dressings and skin repair products
- Controlled drug delivery systems
4. Industrial and Technical Textiles
- Historically used in parachutes and aircraft tyres (before synthetic fibres)
- In optics and electronics, silk’s thin, stable films are being studied as biodegradable substrates for sensors
- Used in protective clothing, especially when blended with wool or synthetic fibres
5. Cosmetics and Personal Care
Silk protein hydrolysates are added to:
- Hair conditioners and skin creams
- Facial masks and soaps
- They enhance skin smoothness, reduce irritation, and retain moisture.
Innovations and Ongoing Research in Silk
Silk continues to evolve beyond tradition, with science pushing its boundaries into materials engineering, biotechnology, and sustainability.
1. Silk Fibroin-Based Biomaterials
Silk fibroin is being explored in:
- Artificial ligaments and cartilage regeneration
- Nano-fibre scaffolds for tissue regrowth
- 3D bioprinting of cellular structures
Its molecular stability and biodegradability offer advantages over synthetic polymers in the medical field.
2. Genetically Modified Silkworms
Scientists have successfully modified silkworms to:
- Produce fluorescent silk
- Spin spider silk proteins, which have much higher tensile strength
- Enhance disease resistance and production efficiency
This opens new possibilities for custom-engineered fibres.
3. Bio inspired Materials and Smart Textiles
Silk’s properties are being mimicked in the development of:
- Self-healing fibres
- Thermo-responsive fabrics
- Conductive silk composites for wearables
These are still under research but indicate silk’s relevance in future smart textile innovations.
4. Sustainability in Sericulture
Efforts are underway to:
- Reduce water and energy use in degumming and dyeing
- Spread peace-silk, non-violent reeling
- Plant-based enzymes should be used instead of synthetic chemicals in the processing
- Encourage organic mulberry farming and reuse of silk wastes
These are the efforts to eco-compliant silk to the modern sustainable markets.
Industry Relevance/Conclusion
Silk is a fibre that bridges the gap between ancient culture of handloom weaving and modern day science of materials. Whether it is luxury fashion or biomedical engineering, it has remained a pillar in cultural heritage as well as in scientific progress.
Silk industry in India is essential in:
- Livelihoods in the rural areas (more than 9 million people engage in sericulture)
- Handloom and khadi clusters
- Export earnings (silk and silk products have a world value)
- Other difficulties like disease outbreaks, price fluctuations and competition with synthetics however have to be driven by invention and policymaker backing.
Silk provides an excellent case study in natural fibre science, and processing technology to the textile students and to those in the textile industry.