Wool Fiber (羊毛 / Keratin Protein Fiber)
Wool is a natural protein fiber grown by sheep — predominantly Merino — and is the most important animal fiber in textile use. Its defining structural features are a surface layer of overlapping scales (鳞片层), a bilateral cortex of ortho- and para-cortical cells that creates natural crimp, and a complex molecular architecture of alpha-helical keratin proteins. These features combine to deliver properties no synthetic fiber fully replicates: high elasticity, natural crimp and bulk, excellent moisture absorption without feeling wet, inherent flame resistance, and biodegradability. Global wool production is approximately 1.2 million tons (greasy basis) annually, making it a niche but premium fiber compared to cotton (~25 Mt) and polyester (~78 Mt).
Quick Facts
| Type | Natural protein (animal hair fiber); keratin-based |
| Chemical composition | Keratin protein: high-sulfur protein (matrix) + low-sulfur alpha-helical protein (fibrils) |
| Global production (greasy) | ~1.2 million tons/year; Australia ~25% of global output; China leads in processing |
| Australia fine-wool share | ~90% of world's apparel-grade fine wool; Merino dominant |
| Density | 1.30–1.32 g/cm³ (lighter than cotton at 1.54) |
| Moisture regain | 14–18% (standard); highest among common fibers; absorbs ~1/3 own weight without feeling wet |
| Fiber diameter range | Ultrafine: 11.5–16.5 μm; Fine (Merino): 17–24 μm; Medium: 25–37 μm; Coarse: >37 μm |
| Staple length | Fine wool: 50–120 mm; coarse wool: up to 300 mm |
| Crimp | Natural 3D wave; fine wool: 10–15 crimps/cm; coarse wool: 1–5 crimps/cm |
| Thermal behavior | Does not melt; chars at ~130°C; self-extinguishing; LOI ~25% (inherently flame-resistant) |
| Key trade names / types | Merino (Australia), Corriedale, Lincoln; worsted vs. woollen processing routes |
| Price (relative to cotton) | 5–20×, depending on fineness; ultrafine Merino (≤14.5 μm) can exceed gold by weight |
Fiber Structure
Wool's unique properties derive from a hierarchical structure unmatched by synthetic fibers:
| Layer (outside → in) | Composition | Function and significance |
|---|
| Cuticle / Scales (鳞片层) | Flattened, overlapping scale cells; ~10% of fiber mass | Protects the fiber core; directional friction (lower tip-to-root) enables felting/shrinkage; scale density and edge smoothness determine luster and hand feel |
| Cortex (皮质层) | Spindle-shaped cortical cells; ~90% of fiber mass | The main structural body; bilateral arrangement of ortho-cortex (softer, more reactive) and para-cortex (denser, more crystalline) creates differential swelling → natural crimp |
| Medulla (髓质层) | Loosely packed cells + air pockets; absent in fine wool | Present in coarse wool; reduces strength and elasticity; higher medullation = lower quality; interrupted medulla = "heterotypical" (两型毛) |
| Cell membrane complex | Lipid + protein intercellular cement | Binds cortical cells together; pathway for dye and chemical diffusion; removed in severe chemical treatments (damage) |
Classification
By Fiber Diameter (the primary commercial grade)
| Grade | Diameter (μm) | Staple length | Typical use |
|---|
| Ultrafine Merino | 11.5–16.5 | 50–80 mm | Luxury knitwear; next-to-skin; price comparable to cashmere or gold |
| Fine Merino | 16.5–20 | 60–100 mm | Premium suiting, high-quality knitwear, base layers |
| Medium Merino | 20–24 | 60–120 mm | General apparel; worsted and woollen systems |
| Fine crossbred | 24–30 | 80–150 mm | Mid-market suiting, knitwear, hand-knitting yarns |
| Medium crossbred | 30–37 | 100–200 mm | Carpets, blankets, upholstery, industrial felts |
| Coarse / carpet wool | >37 | 100–300 mm | Carpets, rugs, insulation, geotextiles |
By Processing Route
| System | Fiber requirements | Yarn characteristics |
|---|
| Worsted (精梳毛纺) | Longer, finer, more uniform; combed to remove short fibers (<30 mm) | Smooth, compact, parallel fibers; higher strength; clearer surface; suiting, fine knitwear |
| Woollen (粗梳毛纺) | Shorter, more variable; carded but not combed | Bulky, fuzzy, less parallel; softer hand; better insulation; tweed, blankets, knitwear |
| Semi-worsted | Intermediate length and fineness | Compromise between worsted smoothness and woollen bulk; carpets, mid-grade yarns |
By Breed (selected major types)
| Breed | Diameter (μm) | Characteristics |
|---|
| Merino (美利奴) | 11.5–25 | Finest and most important apparel wool; >80% of Australian clip; high crimp, excellent elasticity |
| Corriedale (考力代) | 25–31 | Dual-purpose (meat + wool); medium fineness; good bulk; widely farmed in New Zealand and South America |
| Lincoln (林肯) | 34–41 | Long wool breed; lustrous; used for carpets, upholstery, hand-knitting yarns |
| Shetland | 20–30 | Soft, fine undercoat + coarser outer coat; traditional Fair Isle knitwear; natural color range |
| Cashmere (山羊绒) | 14–16 | Not sheep wool; from cashmere goat undercoat; ultra-fine, soft, ~1/10 of wool production by weight |
Properties
| Property | Rating | Engineering implication |
|---|
| Elasticity & recovery | ★★★★★ Excellent | Natural crimp + alpha-helical keratin → high elongation (25–35%) with near-complete recovery from small strains; garments resist wrinkling; carpets recover from compression |
| Moisture absorption | ★★★★★ Best of all fibers | 14–18% regain; absorbs ~33% of own weight without surface wetness; generates heat during absorption (heat of sorption) — natural climate buffer |
| Flame resistance | ★★★★★ Inherent | LOI ~25%; self-extinguishing; chars rather than melts (no melt-drip hazard); high ignition temperature (~570°C); high nitrogen + sulfur content inhibits combustion |
| Tensile strength | ★★☆☆☆ Low | 1.0–1.7 cN/dtex dry; drops to 0.7–1.3 cN/dtex wet (~30% reduction — opposite of cotton's gain); weakest of major apparel fibers |
| Abrasion resistance | ★★★☆☆ Moderate | Adequate for most apparel; inferior to nylon and polyester; felting under mechanical action + moisture can degrade surface |
| Felting / shrinkage | ★★☆☆☆ Problematic | Directional scale friction + elasticity + moisture → progressive, irreversible shrinkage (毡缩); requires anti-felting treatment (chlorine-Hercosett, plasma, enzyme) for machine-washable wool |
| Chemical resistance | ★★★☆☆ Selective | Acid-resistant (wool can be carbonized to remove vegetable matter with H₂SO₄); alkali-sensitive (NaOH dissolves wool — basis for burn test differentiation) |
| Dyeability | ★★★★☆ Good | Acid dyes, metal-complex dyes, reactive dyes; keratin side groups provide multiple dye sites; good color fastness |
| Thermal insulation | ★★★★★ Excellent | Natural crimp traps air; high moisture regain generates sorption heat; best cold-weather fiber |
| Microbial / odor resistance | ★★★★☆ Good | Natural antimicrobial properties; absorbs odor-causing bacteria; garments require less frequent washing than synthetics |
| UV resistance | ★★★☆☆ Moderate | Better than nylon and silk; worse than acrylic and polyester; prolonged sun exposure degrades keratin — weakens and yellows |
| Biodegradability | ★★★★★ Excellent | 100% biodegradable; decomposes in soil within months to years; releases nitrogen nutrients; no microplastic shedding |
Advantages vs. Limitations
Advantages
- Highest moisture regain of all fibers (14–18%); generates heat on absorption
- Excellent elasticity and recovery — resists wrinkling naturally
- Inherently flame-resistant (LOI ~25%); self-extinguishing; no melt-drip
- Superior thermal insulation — crimp traps air; cold-weather benchmark
- Natural antimicrobial and odor-resistant
- 100% biodegradable and renewable (annual fiber crop)
- Wide diameter range (11.5 → >50 μm) enables diverse product spectrum
- Acid-resistant (carbonizable for vegetable-matter removal)
- Good dyeability with multiple dye classes
- Soft, warm hand feel — irreplaceable in luxury and premium markets
Limitations
- Low tensile strength (1.0–1.7 cN/dtex); weakest major apparel fiber
- Strength drops ~30% when wet — opposite of cotton
- Felting and shrinkage: requires anti-felting treatment for machine washability
- Alkali-sensitive: NaOH dissolves wool (critical in laundering and finishing)
- Expensive: 5–20× cotton price; ultrafine grades exceed gold by weight
- Limited supply: ~1.2 Mt/year greasy; ~1% of total fiber production
- Fiber diameter over ~25 μm causes skin prickle (itch factor)
- Susceptible to moth and insect damage; requires protective treatment
- UV degradation with prolonged exposure; yellows with age
- Scaly surface creates directional friction effect — complicates processing
Yarn Engineering: Selection Guide
| Application | Recommended wool type | Processing route | Key criteria |
|---|
| Premium suiting (Super 120s–180s) | Ultrafine / fine Merino, ≤18.5 μm | Worsted; combed; high twist | Fiber diameter <19 μm; length >70 mm; low medullation; clean, uniform |
| Luxury knitwear (cashmere-like) | Ultrafine Merino, ≤16.5 μm; or Merino/cashmere blend | Worsted or woollen; low twist for softness | Fineness for softness; low prickle factor; good crimp for bulk |
| Performance base layers | Fine Merino, 17–19 μm; Superwash-treated | Worsted; compact or siro-spun | Machine-washable; moisture management; odor resistance; next-to-skin comfort |
| Outdoor / hiking socks | Medium Merino, 20–24 μm; often blended with nylon (15–25%) | Woollen or semi-worsted | Nylon for abrasion resistance; wool for insulation and moisture; anti-felting treatment |
| Traditional tweed / blankets | Medium crossbred, 28–35 μm | Woollen; carded; low twist | Bulk and insulation; fiber character and color variation; cost-tolerant |
| Carpet (premium) | Coarse crossbred, 30–40 μm; lustre wool (e.g., Lincoln) | Woollen or semi-worsted; BCF or staple | Resilience for crush recovery; natural stain resistance; flame resistance (safety) |
| Hand-knitting yarn | Medium Merino to Corriedale, 22–30 μm | Woollen or worsted; plied | Good bulk; elasticity; stitch definition; dye acceptance |
| Felting / craft wool | Medium crossbred, 25–35 μm; shorter staple | Carded web (not spun); needle-punch or wet-felt | Scale structure promotes felting; shorter fibers acceptable; lower cost |
| Blended wool/polyester suiting | Fine to medium Merino, 19–25 μm; blended 45/55 or 50/50 | Worsted; intimate blend at drawing | Wrinkle resistance from wool + dimensional stability from PET; cost reduction |
| Insulation / technical felt | Coarse wool; recycled wool fiber | Nonwoven; needle-punch or wet-lay | Thermal and acoustic insulation; fire resistance; biodegradable; sustainable building material |
Wool vs. Key Competitors
| Property | Wool (Merino) | Cotton | PET Polyester | Acrylic |
|---|
| Fiber type | Natural protein | Natural cellulose | Synthetic | Synthetic |
| Strength (cN/dtex) | 1.0–1.7 (↓30% wet) | 2.5–4.5 (↑10–20% wet) | 3.5–5.5 (stable wet) | 2.0–3.0 |
| Moisture regain (%) | 14–18 (best) | 7–8.5 | 0.4 (worst) | 1.5–2.0 |
| Elastic recovery | ★★★★★ Excellent | ★★☆☆☆ Poor | ★★★★☆ Good | ★★★☆☆ Moderate |
| Flame resistance | Self-extinguishing | Flammable | Melt-drip hazard | Flammable |
| Thermal insulation | ★★★★★ Best | ★★★☆☆ Good | ★★☆☆☆ Poor (unless hollow) | ★★★★☆ Good (wool-like) |
| Felting / shrinkage | Problematic (unless treated) | Minimal | None | None |
| Prickle / itch factor | >25 μm causes itch | None | None | None |
| Biodegradability | ★★★★★ Yes | ★★★★★ Yes | ★☆☆☆☆ No | ★☆☆☆☆ No |
| Cost (relative) | ★★★★★ High | ★★★☆☆ | ★☆☆☆☆ Lowest | ★★☆☆☆ |
| Global output (Mt/yr) | ~1.2 (greasy) | ~25 | ~78 | ~2 |
Summary
Wool is a low-volume, high-performance natural fiber whose value lies in properties that synthetics struggle to match: high elasticity with natural crimp, the highest moisture regain of any textile fiber, inherent flame resistance, and complete biodegradability. Its limitations — low strength, felting, and high cost — are accepted trade-offs in markets where comfort, warmth, and natural performance command a premium. For the yarn engineer, wool selection centers on fiber diameter (fineness), staple length, crimp, medullation level, and processing route (worsted vs. woollen) — each decision shaping the yarn's hand, insulation, strength, and surface character. See the Textile Material framework for substance–form–performance logic.