Elastane (Spandex): Synthesis, Properties, Uses, and Environmental Impact
Introduction
Elastane, commonly known as spandex or Lycra, is a synthetic fibre known for its strength and exceptional elasticity capable of stretching up to 500–800% of its original length and immediately recovering. It is widely used in textiles requiring flexibility, comfort, and stretchability.
Chemical Composition and Structure
Elastane is chemically a polyether-polyurea copolymer or segmented polyurethane. Its unique elastic properties are derived from its molecular structure, which features alternating hard and soft segments.
The Two-Segment Structure
| Segment Type | Composition | Chemical Role | Physical Property Imparted |
|---|---|---|---|
| Soft (Amorphous/Flexible) Segment | Long-chain flexible Macroglycol (typically polyether or polyester polyol, e.g., poly(tetramethylene ether) glycol) | Provides the ability to stretch easily and return to its original state. This segment has a low glass transition temperature (Tg). | Elasticity and Flexibility |
| Hard (Crystalline/Rigid) Segment | Short-chain rigid Diisocyanate and Chain Extender (forms urethane and/or urea linkages) | Creates strong, secondary (e.g., hydrogen) bonds with adjacent hard segments, acting as physical cross-links that hold the polymer network together. | Tensile Strength and Recovery Power |
Key Chemical Linkages
The two primary chemical groups are formed via polyaddition polymerization:
- Urethane Linkage: Formed by the reaction between a Diisocyanate (contains -NCO groups) and a Polyol (contains -OH groups).
- Urea Linkage: Formed when a Diisocyanate reacts with a Diamine chain extender. Urea linkages are often more resistant to heat and chemicals.
Synthesis
The production of elastane involves polymerization reactions to produce segmented polyurethane. The process typically includes the following steps:
- Step 1: A diisocyanate reacts with a macroglycol to form a prepolymer containing urethane linkages.
- Step 2: The prepolymer is chain-extended using a diamine or diol to form long-chain polyurethane-urea copolymers.
- Step 3: The polymer is dissolved in a solvent (such as dimethylformamide) and extruded through spinnerets to produce fibers.
- Step 4: The fibers are cured, stretched, and wound onto spools for textile applications.
n(HO-Polyol-OH) + (n+1)(OCN-R-NCO) → OCN-R-[NHCO-Polyol-OCONH-R]n-NCO
Prepolymer-NCO + NH2-R-NH2 + OCN-Prepolymer → Prepolymer-NHCONH-R'-NHCONH-Prepolymer
OR,Prepolymer-NCO + HO-R-OH + OCN-Prepolymer → Prepolymer-NHCONH-R'-NHCONH-Prepolymer
Manufacturing Process: Solution Dry Spinning
- Dry Spinning (most common): Polymer dissolved in DMF or DMAc, extruded into hot air, solvent evaporates.
- Wet Spinning: Extruded into coagulation bath.
- Melt Spinning: Used for thermoplastic elastane variants.
- Reaction Spinning: Chemical reaction during fiber formation (e.g., Lycra®).
The Solution Dry Spinning method is the most commonly used, accounting for approximately 95% of worldwide production.
Step 1: Prepolymer Formation (Polymerization)
Reactants: A flexible Macroglycol (long-chain polyol) and a stiff Diisocyanate (short-chain monomer, usually MDI: Methylene diphenyl diisocyanate) are mixed in a reaction vessel.
Ratio: The macroglycol to diisocyanate ratio is carefully controlled, often near 1:2 molar ratio, to ensure isocyanate groups are at the chain ends, forming a prepolymer (a low molecular weight polymer).
n(Glycol) + (n+1)(Diisocyanate) → Prepolymer
Step 2: Chain Extension Reaction
The prepolymer is reacted with a Chain Extender (e.g., a diamine like hydrazine or ethylene diamine) to significantly increase the molecular weight and form the final polyurethane-urea polymer.
This reaction creates the urea (hard) segments that provide the fiber's high tensile strength and recovery power.
Step 3: Solution Preparation (Dissolution)
The polymer is dissolved in a solvent, typically Dimethylacetamide (DMAC), to create a viscous, spinnable solution. This solvent must be removed later.
Step 4: Extrusion and Spinning (Dry Spinning)
The polymer solution is pumped through a spinneret (a metal plate with tiny holes) into a chamber where hot nitrogen and solvent vapor are passed over the extruded filaments. This heat causes the solvent to quickly evaporate and solidify the liquid polymer strands into solid fibers.
Step 5: Finishing and Winding
The solidified filaments are bundled, sometimes treated with a finishing agent (like magnesium stearate) to prevent them from sticking together, and then wound onto spools for use in textile production.
Properties
- High elasticity and extensibility (can stretch up to five times its original length).
- Excellent recovery after stretching (returns to original shape easily).
- Lightweight and soft texture.
- Good resistance to perspiration, body oils, and detergents.
- Low moisture absorption but breathable when blended with other fibers.
Uses
Elastane is used in a wide range of textile applications due to its stretch and resilience:
- Sportswear and activewear (leggings, swimsuits, cycling shorts).
- Undergarments and lingerie.
- Compression garments and medical textiles.
- Fashion garments requiring comfort and flexibility (jeans, dresses, and socks).
- Industrial applications where flexibility is important, such as elastic tapes and supports.
Performance Blends Applications
Elastane is nearly always used in a blend to combine its elasticity with the best properties of other fibers (e.g., breathability of cotton, durability of nylon). The percentage is critical for function:
| Blend Ratio | Application | Engineering Purpose |
|---|---|---|
| 2% to 5% Elastane with Cotton or Denim | Stretch Jeans, Casual Shirts, Basic T-shirts | Adds comfort, flexibility, and anti-bagging shape retention without sacrificing the fabric's soft hand-feel or breathability. |
| 10% to 20% Elastane with Nylon or Polyester | Swimwear, Cycling Gear, High-End Activewear | Provides maximum four-way stretch, recovery, and compression. The nylon/poly blend offers quick-drying and wicking properties. |
| 20% to 40% Elastane with Nylon | Shaping Garments, Compression Hosiery | Used where maximum power and recovery force are needed to apply pressure or maintain a firm structure. |
Fiber Degradation: Why Elastane Loses Stretch
Elastane fibers lose their elasticity—often called bagging out —due to the chemical and physical degradation of the soft segment and the disruption of the hard segment's hydrogen bonds.
Primary Causes of Degradation:
1. Hydrolysis (Moisture and Heat)
- Mechanism: High heat (especially in hot washing or drying) combined with moisture can break the urethane and urea linkages in both the soft and hard segments.
- Result: This leads to chain scission, permanently weakening the polymer backbone and reducing the fiber's ability to stretch and recover.
2. Oxidation and Chemical Damage
- Chlorine: Exposure to chlorine bleach (and chlorine in swimming pools) is highly destructive. Chlorine breaks the chemical bonds, causing irreversible loss of elasticity.
- UV Radiation: Prolonged exposure to sunlight (UV rays) causes oxidation, especially in the polyether soft segment, leading to chain damage and yellowing.
- Body Oils and Detergent Residue: These residues can chemically soften the hard segments, interfering with the critical hydrogen bonding that provides the recovery force.
3. Mechanical Fatigue
- Continuous, excessive stretching over a long period can eventually lead to permanent deformation as the soft chains get pushed beyond their elastic limit, similar to stretching a rubber band until it no longer snaps back entirely.
Environmental Impact
- Elastane is not biodegradable and contributes to microplastic pollution during washing.
- Its production consumes non-renewable resources such as petroleum-based precursors.
- Solvent-based spinning processes release volatile organic compounds (VOCs), affecting air quality.
- Waste elastane fabrics are difficult to recycle due to their composite nature.
Sustainability Efforts
- Development of bio-based polyurethane precursors to replace petroleum sources.
- Use of cleaner solvent-free or water-based synthesis methods.
- Recycling blends through chemical recovery processes to reduce textile waste.
- Designing garments for longer life and improved recyclability.
Conclusion
Elastane revolutionized the textile industry by introducing comfort and flexibility into fabrics. However, addressing its environmental challenges through sustainable production and recycling technologies is essential for future eco-friendly textile manufacturing.