1,4-Butanediol (BDO)

BDO is the dominant hard-segment chain extender for thermoplastic polyurethanes (TPU) because the four-carbon even-numbered chain enables close packing and hydrogen bonding between urethane groups, generating well-ordered hard-domain microstructure. This phase separation gives TPU coatings and films high tear strength, good abrasion resistance, and rubber elasticity. Compared to ethylene glycol, BDO gives softer, more flexible hard segments; compared to HDO, BDO produces higher hard-segment crystallinity and higher heat deflection temperature.