Silk Fiber (Natural Protein / Fibroin Filament)
Silk is a natural protein filament produced by silkworms — primarily the domesticated Bombyx mori (mulberry silkworm) — as a cocoon-building material. It is the only natural continuous-filament fiber: a single cocoon yields 800–1,500 meters of usable filament. Its unique triangular cross-section, combined with a layered fibroin-sericin structure, produces a luster and drape no other natural fiber matches. Global raw silk production is approximately 90,000 tons annually — a niche luxury fiber representing less than 0.1% of global fiber output by weight, but commanding premium prices and a market value of $18–22 billion.
Quick Facts
| Type | Natural protein continuous filament (animal secretion fiber) |
| Chemical composition | Fibroin (~75–80%): crystalline beta-sheet protein core; Sericin (~20–25%): amorphous gummy coating; wax, pigments, minerals (~1–3%) |
| Domestication | Bombyx mori domesticated ~5,000 years ago in China; sericulture spread via the Silk Road |
| Global raw silk production | ~90,000 tons/year; China ~63% (declining from ~158,000 t in 2016 to ~50,000 t in 2023); India ~40% and growing |
| Single filament fineness | Mulberry (B. mori): 2.6–3.7 dtex (2.4–3.4 denier); Tussah: 6.0–6.5 dtex — about twice as coarse |
| Filament length per cocoon | 800–1,500 m usable (total up to 3,000 m); 1 kg of raw silk requires ~5,500 cocoons |
| Density | 1.33–1.37 g/cm³ (degummed); 1.30–1.35 g/cm³ (raw with sericin) |
| Moisture regain | 9–11% (standard); similar to cotton in comfort but with superior drape |
| Cross-section | Triangular with rounded corners (桑蚕丝); sharp triangular (柞蚕丝); origin of silk's characteristic luster |
| Market value | $18–22 billion (2024); projected to reach $28–39 billion by 2030 (CAGR 6–10%) |
Fiber Structure: What Makes Silk Unique
| Structural feature | Description | Consequence for textile performance |
|---|
| Twin-filament (brin) structure | Each silk strand = two parallel fibroin filaments (brins) cemented together by sericin gum | After degumming, each brin is ~1.3–1.8 dtex; total filament ~2.6–3.7 dtex; enables microdenier-level fineness per brin |
| Triangular cross-section | Mulberry silk: triangular with rounded corners; Tussah silk: sharp wedge-shaped triangle | Acts as a natural prism — scatters and reflects light → silk's signature luster; sharper triangle = more glittering appearance |
| Fibroin beta-sheet crystalline core | Highly ordered anti-parallel beta-pleated sheet structure; Gly-Ala-Gly-Ala-Gly-Ser repeating sequences | Source of silk's remarkable strength: 3.0–4.0 cN/dtex (comparable to nylon); high crystallinity (~60–70%) |
| Sericin coating | Amorphous, water-soluble globular protein gum; ~20–25% of raw silk weight | Protects fibroin during cocoon formation and reeling; removed by degumming (boiling in soap/alkali) → weight loss ~22–25% → soft, lustrous "boiled-off" silk |
| No surface scales | Smooth, uniform surface without the overlapping scales of wool | No felting tendency; smooth against skin; low friction → excellent drape; but can be slippery during processing |
Classification
By Species (Primary Commercial Types)
| Type | Silkworm species | Diet | Filament (dtex) | Characteristics |
|---|
| Mulberry (桑蚕丝 / Cultivated) | Bombyx mori | Mulberry leaves exclusively | 2.6–3.7 | >90% of world silk; whitest, finest, most uniform; highest luster; the gold standard |
| Tussah / Tasar (柞蚕丝 / Wild) | Antheraea pernyi (China), A. mylitta (India) | Oak leaves (柞树/栎树) | 6.0–6.5 | Natural tan/beige color; coarser; sharper triangular section → more glitter; harder to degum and dye |
| Eri (蓖麻蚕丝 / "Peace Silk") | Samia ricini | Castor leaves | 2.5–3.5 | Moth emerges naturally from cocoon (non-violent harvesting); wool-like texture; spun rather than reeled |
| Muga (琥珀蚕丝) | Antheraea assama | Som and Soalu leaves | 4.0–5.5 | Endemic to Assam, India; natural golden-amber luster that intensifies with age and washing; most expensive silk |
| Spider silk (蜘蛛丝) | Various (Nephila, Araneus); recombinant production | N/A | 0.5–1.0 dtex | Highest strength and toughness of any natural fiber; cannot be farmed at scale; recombinant production (Bolt Threads, Spiber) emerging |
By Processing Form
| Form | How produced | Typical use |
|---|
| Raw silk / Filament silk (生丝) | Reeled directly from cocoons; sericin retained; 3–8 cocoon filaments reeled together into one yarn | Weaving (organzine for warp, tram for weft); further degumming and dyeing at fabric stage |
| Degummed / Boiled-off silk (熟丝 / 脱胶丝) | Sericin removed by boiling in soap/soda solution; weight loss ~22–25% | Soft, lustrous fabric; dyeing at yarn or fabric stage; knitting yarns |
| Spun silk / Schappe (绢丝) | From waste silk: damaged cocoons, brushing waste, pierced cocoons; cut into staple, carded, combed, and spun like cotton | Medium-quality yarn; shantung, pongee, knitwear; more affordable than reeled silk |
| Noil silk / Bourette (紬丝) | Short fibers from silk combing waste (<25 mm); carded and woollen-spun | Rustic, slubby texture; low-cost silk; craft yarns, textured fabrics |
| Douppioni (双宫丝) | From double cocoons (two silkworms spin together); irregular, slubby filament | Textured, irregular appearance; popular in bridal wear, dupioni fabric |
Properties
| Property | Rating | Engineering implication |
|---|
| Tensile strength | ★★★★★ Excellent for a natural fiber | 3.0–4.0 cN/dtex — comparable to nylon 6; strongest natural fiber; wet strength retains ~80% |
| Elongation at break | ★★★★☆ Good | 15–25%; higher than cotton (6–8%), lower than wool (25–35%); good elasticity for a natural fiber |
| Luster | ★★★★★ Unmatched | Triangular cross-section + smooth surface = natural prism effect; proprietary standard of "silk luster" referenced by all other fibers |
| Drape | ★★★★★ Excellent | Low bending rigidity + smooth surface + continuous filament → fluid, elegant drape; defining characteristic of silk fabrics |
| Moisture regain | ★★★★☆ Good (9–11%) | Absorbs moisture without clammy feel; comfortable in warm weather; similar absorbency to cotton with better hand feel |
| Elastic recovery | ★★★☆☆ Moderate | Better than cotton and viscose, but not as good as wool or nylon; moderate wrinkle recovery; often blended or finished for wrinkle resistance |
| Abrasion resistance | ★★☆☆☆ Poor | Continuous filaments abrade and fibrillate; wet abrasion resistance is particularly poor; not for high-wear applications |
| UV / light resistance | ★☆☆☆☆ Very poor | Worst among natural fibers; fibroin photodegrades rapidly; yellowing, strength loss, and embrittlement with sun exposure; must be lined or shaded |
| Chemical resistance | ★★★☆☆ Selective | Alkali-sensitive (degumming uses mild alkali; strong alkali dissolves silk); moderate acid resistance (better than cotton); oxidizers damage fibroin |
| Thermal behavior | ★★★☆☆ Moderate | Decomposes at ~170°C; does not melt (protein); self-extinguishing; chars on burning with characteristic burnt-hair smell |
| Biocompatibility | ★★★★★ Excellent | Hypoallergenic; used in surgical sutures and wound dressings; fibroin is FDA-approved for medical devices; naturally anti-microbial to some extent |
Advantages vs. Limitations
Advantages
- Only natural continuous-filament fiber (800–1,500 m per cocoon)
- Strongest natural fiber (3.0–4.0 cN/dtex — comparable to nylon)
- Unmatched natural luster from triangular cross-section
- Excellent drape and fluid hand feel
- Good moisture absorption (9–11%) with comfortable wear
- Hypoallergenic and biocompatible (medical-grade)
- Lightweight with excellent warmth-to-weight ratio
- Natural sheen intensifies with proper finishing
- Biodegradable and renewable
- Premium market positioning — commands highest fiber prices
Limitations
- Very poor UV/light resistance — degrades and yellows rapidly
- Poor wet abrasion resistance — not for high-wear use
- Expensive: raw silk $50–80/kg; finished fabric $15–100+/meter
- Labor-intensive production: ~5,500 cocoons → 1 kg raw silk
- Degumming required (22–25% weight loss) for soft luster
- Alkali-sensitive — requires careful laundering
- Moderate wrinkle recovery — often needs blending or finishing
- Limited supply (~90,000 t/yr) with declining Chinese production
- Water spotting can leave permanent marks on some finishes
- Slippery during cutting and sewing — requires skilled handling
Silk vs. Key Luxury / Natural Competitors
| Property | Silk (Mulberry) | Wool (Fine Merino) | Cotton (Fine) | Viscose (Filament) |
|---|
| Fiber type | Natural protein filament | Natural protein staple | Natural cellulose staple | Regenerated cellulose filament |
| Strength (cN/dtex) | 3.0–4.0 (best natural) | 1.0–1.7 (weakest) | 2.5–4.5 | 1.5–2.4 (dry); ↓50% wet |
| Moisture regain (%) | 9–11 | 14–18 (best) | 7–8.5 | 11–13 |
| Luster | ★★★★★ Unmatched | ★★☆☆☆ Matte | ★★☆☆☆ Matte | ★★★★☆ Bright (artificial) |
| Drape | ★★★★★ Best | ★★★☆☆ Good | ★★★☆☆ Good | ★★★★☆ Very good |
| UV resistance | ★☆☆☆☆ Worst | ★★★☆☆ Moderate | ★★★☆☆ Moderate | ★★☆☆☆ Poor |
| Abrasion resistance | ★★☆☆☆ Poor | ★★★☆☆ Moderate | ★★★☆☆ Moderate | ★★☆☆☆ Poor |
| Elastic recovery | ★★★☆☆ Moderate | ★★★★★ Excellent | ★★☆☆☆ Poor | ★★☆☆☆ Poor |
| Price (relative to cotton) | 10–50× | 5–20× | 1× (reference) | 1–2× (filament) |
| Global output (t/yr) | ~90,000 | ~1,200,000 | ~25,000,000 | ~6,000,000 |
Summary
Silk is the premium natural fiber — the strongest, the most lustrous, and the only natural continuous filament. Its triangular cross-section and smooth fibroin structure create a combination of luster, drape, and hand that defines the luxury textile category. Its limitations — extreme UV sensitivity, poor wet abrasion, and high cost — are accepted trade-offs in the luxury market segment where aesthetics, feel, and exclusivity matter more than durability or economy. For the yarn engineer, silk selection centers on species (Mulberry vs. Tussah vs. Eri), form (reeled filament vs. spun vs. noil), degumming state (raw vs. boiled-off), and twist level (organzine vs. tram). See the Textile Material framework for substance–form–performance logic.