Cure Terms Glossary

Equivalent/Equivalent Weight

The equivalent weight (equivalent, or gram equivalent) of a substance is the weight of the substance which contains one mole of a given functional group. The equivalent weight is specific to that functional group.

For example, in the case of an alcohol, the equivalent weight in hydroxyl is the weight of the substance which contains 17.01 grams of OH. For an isocyanate, the equivalent weight in isocyanate is the weight of a substance which contains 42.02 grams of NCO. Hence for polyurethane formation (—NCO + HO → —NHCOO—) one equivalent of hydroxyl component and one equivalent of isocyanate component gives a 1:1 stoichiometry for the NCO / OH reaction.

In the general case, for equal numbers of reacting groups, then the number of equivalents for each co-reactant must be equal also.