1,4-Butanediol
1,4-Butanediol is the classic chain extender for MDI-based polyurethanes: small, symmetrical and able to build the hard segments that give elastomers their strength.
1,4-BDO is a linear four-carbon diol with a primary hydroxyl at each end (molecular weight 90.12 g/mol, equivalent weight about 45, OH number about 1,245 mg KOH/g). It is a colorless, viscous, hygroscopic liquid that freezes at about 20 °C and boils at about 230 °C.
BDO is used as a chain extender in polyurethane elastomers, cast and thermoplastic polyurethanes, and in some high-performance polyurethane coatings and binders. It is also a building block for polyester polyols such as butanediol adipates, and outside coatings it is a feedstock for THF, γ-butyrolactone and PBT.
Polyurethane elastomers are built from soft segments (long polyols) and hard segments (diisocyanate plus chain extender). With MDI, BDO forms well-ordered urethane hard segments that hydrogen-bond and phase-separate from the soft segments, giving high tensile strength, tear strength and resilience.
BDO has low volatility but is harmful if swallowed; in the body it is converted to gamma-hydroxybutyrate (GHB). Avoid ingestion and eye contact and follow the SDS. Store dry and closed, because it absorbs moisture. Since it freezes around room temperature, keep it in heated storage or gently remelt it and mix well before use.
MDI and BDO form symmetrical urethane hard segments that pack and hydrogen-bond well, producing strong, resilient elastomers with clear phase separation.
Its freezing point is about 20 °C, so it solidifies in cool storage. Warm it gently until fully liquid and mix before use.
Usually only a few percent by weight, but because its equivalent weight is about 45 it can account for a large fraction of the isocyanate demand. Always calculate equivalents rather than weights.
