Cure Terms Glossary
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.