Cure of secondary carbamate groups by melamine-formaldehyde resins

Melamine-formaldehyde (MF) resins have been used as crosslinkers for hydroxyl-functional coreactants in thermoset coatings for about 60 years. Crosslink densities of films prepared from oligomeric urethane polyols suggested that the methoxymethyl groups of MF resins could react with urethane groups (i.e., secondary carbamate groups) as well as reacting with hydroxyl groups. Co-reactants that contain secondary carbamate groups and no hydroxyl groups have been prepared with several types of backbone structures. Cure of such co-reactants by MF resins has been studied using a gradient oven with determination of impact resistance, solvent resistance, and hardness. Several formulations from these cure profile sets have been selected for crosslink density determinations by dynamic mechanical analysis (DMA). Crosslink densities of cured films are consistent with complete conversion of secondary carbamate groups at temperatures only slightly higher than those used for cure of hydroxyl groups. The -OH groups on certain acrylic polyols were converted to secondary carbamate groups. The original acrylic and the converted acrylic were both cured with MF resins. Acid resistance was much better for films prepared from the acrylic that contained secondary carbamate groups. Presented at the 25th International Waterborne, High-Solids, and Powder Coating Symposium, New Orleans, LA, Feb. 18–20, 1998. 730 Worcester Street, Springfield, MA 01151.