Cure Terms Glossary

Stoichiometry

As a general chemical term, stoichiometry covers the numerical relationships between the quantitities of reacting species. The term can be used to describe the proportions by which reagents combine. It can also be used to cover the relative proportions present in the mix (i.e. available for combination).

In polyurethane elastomer terminology, it takes the latter meaning - i.e. it is the ratio of available hydroxyl (total of the polyol and chain extender) to that of the available isocyanate, expressed in percentage terms (i.e. 1:1 = 100). Thus,

Stoichiometry = 100x(No. of equivalents of OH)/(No. of equivalents of NCO)

The alternative convention (e.g. for PU foams) is the isocyanate index.