The purpose of this article is to provide a better understanding of the complexities of urethane catalysis and to develop guidelines for catalyst selection, and to troubleshoot commonly encountered problems for catalyst selection.

The reaction of isocyanates with hydroxyl groups is very slow in the absence of a catalyst. Metal catalysts are often used to achieve sufficient reaction rates and cured properties for a variety of systems including coatings, elastomers, sealants and potting compounds. Systems crosslinked with aliphatic isocyanates are particularly dependent on catalysts to accelerate the reaction. Aromatic isocyanates are generally more reactive than aliphatic types and only require the use of catalysts under certain conditions.

Tin compounds have served the industry well. Dibutyltin dilaurate (DBTDL) can be considered the workhorse catalyst for urethane coatings. It is efficient, e.g., a very low level of catalyst will greatly increase the NCO/OH reaction rate. However, as with any catalyst, certain problems can be encountered with DBTDL, including issues of stability-reactivity, hydrolysis of ester groups, catalysis of the water/isocyanate reaction and environmental concerns.