Elastane, commonly known by the trade name Lycra or the generic term Spandex, is a synthetic fiber known for its exceptional elasticity and recovery properties. This fiber can stretch significantly past its original length and immediately return to its initial shape without lasting deformation. The unique performance characteristics of this fiber are a direct result of its chemical makeup and molecular architecture. Understanding the fiber’s identity, the precursors used, and the synthesis process reveals how this material is constructed at the molecular level.
Elastane’s Identity: A Block Copolymer
Elastane is chemically classified as a segmented polyurethane block copolymer, a long-chain synthetic polymer. This structure features alternating zones of distinct chemical properties, which enables the fiber’s remarkable ability to stretch and recover.
The polymer chain is composed of “soft segments” and “hard segments.” The soft segments are long, amorphous, and flexible chains that enable the fiber to elongate when a pulling force is applied. The hard segments are shorter, more rigid, and tend to crystallize, providing necessary strength and acting as physical crosslinks that anchor the structure.
The soft segment component typically accounts for 70 to 80 percent of the polymer’s mass. These flexible domains elongate under tension, while the interspersed hard domains ensure the fiber snaps back into place, maintaining the garment’s shape and fit. This ratio of soft to hard segments determines the final elasticity and tensile strength of the resulting fiber.
Essential Chemical Precursors
The production of elastane requires three chemical ingredients, each corresponding to the final segments in the polymer chain. The flexible soft segments are derived from high-molecular-weight polyols, which are long-chain compounds containing multiple hydroxyl groups. Polytetramethylene ether glycol (PTMEG) is a commonly used polyol.
The hard segments are formed using diisocyanates, which are compounds with two isocyanate functional groups. Methylene diphenyl diisocyanate (MDI) is a frequent choice for the aromatic diisocyanate component in elastane synthesis. The ratio of the polyol to the diisocyanate is precisely controlled during the initial reaction to produce fibers with a specific range of elasticity and durability.
The molecular weight of the polyol precursor profoundly influences the final physical properties of the finished fiber. Increasing the molecular weight of the PTMEG component can improve the phase separation between the soft and hard segments. This enhanced separation leads to better resilience, recovery, and improved flexibility at colder temperatures.
The Final Polymerization Process
The chemical synthesis of elastane is a step-growth polymerization process that begins with the formation of an intermediate compound called a prepolymer. The initial reaction involves mixing the macroglycol (polyol) with an excess molar amount of the diisocyanate. This reaction links the polyol chains with diisocyanate molecules, resulting in a prepolymer that is terminated on both ends with reactive isocyanate groups.
The second stage is chain extension, where the prepolymer molecules are connected into longer polymer chains. This is accomplished by reacting the prepolymer with a short-chain chain extender, typically a diamine such as ethylenediamine. The diamine reacts rapidly with the isocyanate end groups, linking the prepolymer units together to form the final segmented polyurethane-urea structure.
Once the polymer is formed in solution, it must be converted into a usable fiber, most commonly through a technique called solution dry spinning. The resulting polymer solution is pumped through a spinneret, which is a plate with tiny holes, aligning the liquid polymer into fine strands. These strands then pass through a heated cell where the solvent evaporates, leaving behind the solid elastane filaments that are bundled together to create the final yarn.