Cure Terms Glossary

Isocyanate Prepolymer


Isocyanate prepolymers offer operational convenience in polyurethane formulation. By pre-reacting the isocyanate monomers with some of the polyol components, isocyanate-rich oligomers can be obtained which may be easier to handle on the factory floor than the original monomers (e.g., less prone to crystallization, lower volatility, etc.).

The correct stoichiometry is critical to avoid excessive molecular weight build up in the prepolymers which need to be sufficiently mobile for the final mixing and fabrication. The involvement of two separate mixing stages when using prepolymers causes this approach to be called the two-shot method.

The prepolymers themselves are mixed products comprising: isocyanate-capped polyurethanes, isocyanate-capped versions of the original polyols and residual unreacted monomeric isocyanate. They are characterised by their overall percentage isocyanate. As expected, viscosity increases as isocyanate content decreases. Full prepolymers are generally considered to be those with isocyanate levels below 16%. Quasi-prepolymers are those with isocyanate levels typically in the range 16-25%. Quasi-prepolymers have viscosities closer to those of the original polyols.