Isocyanate crosslinking in two-component waterborne coatings

摘要

Two-component isocyanate curing films (2C NCO) yield a unique combination of properties, such as excellent mechanical and chemical resistances and very good film formation. For waterborne 2C NCO curing applications, dedicated polyols and polyisocyanates have been developed. Film properties such as drying rate, hardness development and conversion of isocyanate groups were found to depend strongly on the hydroxyl group content of the polyol, and with that the amount of polyisocyanate crosslinker which is added to a formulation. Tack-free time of a film does not, however, correlate to the conversion of isocyanate groups and, hence, to crosslink density. The crosslink density and chemical resistances increase with hydroxyl group content, although not all chemical resistances increase (coffee resistance) along this trend. This is explained with hydrolysis of isocyanates resulting in more hydrophilic urea links. Due to early vitrification of the curing film, diffusion of crosslinker and, hence, mixing of polyol and crosslinker are impaired. As a result, with increasing hydroxyl group concentration, an increasing part of the crosslinker cannot react with hydroxyl groups and can only undergo hydrolysis. (c) 2018 Society of Chemical Industry