Crosslinking chemistry and network structure in organic coatings. II. Effect of catalysts on cure of melamine formaldehyde/acrylic copolymer films

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Abstract
In a previous paper, the extents of reaction of various functional groups in different acrylic copolymer melamine‐formaldehyde crosslinked formulations were measured by infrared spectroscopy. From these data and a statistical model, various network structure parameters could be calculated. One of these, effective crosslink density, was found to correlate well with film solvent resistance. In this paper the effects of the addition of strong acid catalysts on cure chemistry and network structure are determined. Both solvent‐based and water‐based formulations have been studied. In the solvent‐based formulations studied, the addition of acid catalysts greatly enhanced the crosslinking reactions at low temperatures and dramatically lowered the cure temperature. Acid catalysts also lowered the cure temperature of a water‐based formulation based on a high (180°C) cure temperature melamine. In contrast, the addition of acid catalysts had a negligible effect on the cure chemistry of a water‐based formulation crosslinked with a low (130°C) cure temperature melamine. For this case the rate limiting process was found to be the rate of removal of neutralizing amine from the film. Cure temperatures in this system could only be lowered by replacing the amine used with a more volatile amine.