Bio-based elastane in swimwear

Swimwear is where elastane is asked to do the most and is treated the worst. It is stretched to high elongation while wet, held there for hours, and exposed simultaneously to hypochlorite, ultraviolet light, salt and sunscreen oils. Understanding what a bio-based version changes requires knowing which part of the molecule is doing the work.

The fibre is a two-phase solid

Elastane is a segmented polyurethane-urea, and its elasticity is not a property of a single chain but of a phase-separated architecture. Long, flexible soft segments — usually polytetramethylene ether glycol, PTMEG — are chain-extended with a diisocyanate and a diamine. The resulting urethane and urea groups are strongly polar and hydrogen-bond to each other, so they segregate out of the soft matrix into rigid hard domains. Those domains act as physical cross-links: they anchor the chain ends while the soft segments, well above their glass transition, coil and uncoil freely. Stretching lowers the conformational entropy of the soft segments and the fibre pulls back to recover it. Recovery is therefore only as good as the hard domains that hold the network together.

What the bio-based route replaces

The renewable-carbon route substitutes the soft segment. Fermentation-derived 1,4-butanediol is dehydrated to tetrahydrofuran and polymerised to PTMEG, giving a polyether chemically indistinguishable from the petrochemical one; the diisocyanate hard segment usually remains fossil. Two honest consequences follow. Bio-based content is a mass fraction of the polymer, verifiable by radiocarbon measurement under ASTM D6866, and it should be stated as such rather than as a property of the garment. And because the bio-derived soft segment is the same molecule, it inherits exactly the same degradation chemistry.

Chlorine attacks the hard domains

Hypochlorous acid in pool water attacks the N–H of the urethane and especially the urea groups. Chain scission there does not merely shorten molecules — it dismantles the physical cross-links, so the fibre loses its ability to snap back and the garment bags out at the seat and straps long before it looks worn. This is why chlorine-resistant swimwear is often built from polyester-based elastomers such as PBT, which have no urea linkages to attack, or from elastanes with modified hard segments and hindered-amine or zinc-oxide additives. Colourfastness to chlorinated pool water is assessed under ISO 105-E03; that test measures the dye, not the elastomer, so it is not a substitute for a stretch-recovery measurement after chlorine exposure.

Ultraviolet light attacks the soft segment

Polyethers are vulnerable at the carbon adjacent to the ether oxygen: hydrogen abstraction there gives a radical that forms a hydroperoxide, which decomposes and cuts the chain. Sunlight and chlorine therefore attack different parts of the same fibre, and swimwear gets both at once. Sunscreen makes it worse in a third way, since organic UV filters and oils absorb into the soft phase and plasticise it, lowering modulus and easing the diffusion of oxidants inward.

None of these mechanisms distinguish renewable carbon from fossil carbon. A bio-based swimsuit is a claim about where the atoms came from; durability in a chlorinated pool still has to be engineered into the hard segment and the finish.