Studies of the aging effect on the level of isocyanate residues in polyester-based can coating systems

A methyl ethyl ketone oxime-blocked isocyanate was used as a crosslinking agent in the curing of polyester–polyurethane (PEPU) can coatings. The completeness of cure was assessed from the amount of residual (un-reacted) isophorone diisocyanate (IPDI) trimer that was presented in the coating after curing. This amount of residual trimer depended strongly on the temperature at which the curing was undertaken and time given for curing. The degree of cure was also assessed, correlating with the reduced IPDI trimer migration potential. The amount of residual IPDI trimer that was extractable from a typical PEPU coating rose markedly (15- to 20-fold) if the wet coating was aged up to 6 months before use. The presence of a TiO₂ pigment increased the aging effect. An aluminum flake pigment had an even greater effect. The TiO₂ pigment increased aging by different degrees according to the type of surface modification that had been applied to the pigment. The grade having the greatest alumina content, as a result of surface modification, gave the greatest aging. The undesirable effects of aging were counteracted by the addition of more catalyst to the aged coating just before the coating was used to coat panels. Deactivation of the acidic polymerization catalyst system (based on organotin compounds) is thought to be the cause of this aging. These findings have allowed improvements to be made to the specifications of the pigments and the catalysts used, with respect to the consistent industrial production of low migration coatings.