Blocked isocyanates have been available for many years to produce room-temperature stable, one pack (1K), heat-curable coatings.1 As in all aspects of coating formulation, there is increasing demand for improved quality without increasing formulation costs. This latest development from the Speciality Chemicals Division of Baxenden Chemicals seeks to fulfill both these criteria, by building on the success of the 3,5-dimethyl pyrazole (DMP)-blocked isocyanates, such as Trixene BI 7982 and BI 7960, to incorporate a second crosslinking strategy that will improve the quality of coatings derived from this blocking agent.
The chemistry of blocking and unblocking is well understood,2-3 with the main blocking agents [e-capralactam (ECAP), methylethylketoxime (MEKO), and DMP] reacting by way of equilibrium between free isocyanate and blocked isocyanate (see Figure 1). Increasing temperature moves the equilibrium to the left, releasing isocyanate groups, which then react with active hydrogen species to generate urethane bonds.