Cure of secondary carbamate groups by melamine-formaldehyde resins

Melamine-formaldehyde (MF) resins have been used as crosslinkers for hydrox yl-functional co-reacants in thermoset coatings for about 60 years. Crossli nk densities of films prepared from oligomeric urethane polyols suggested t hat 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 group s have been prepared with several types of backbone structures. Cure of suc h co-reactants by MF resins has been studied using a gradient oven with det ermination 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 densi ties of cured films are consistent with complete conversion of secondary ca rbamate groups at temperatures only slightly higher than those used for cur e of hydroxyl groups. The -OH groups on certain acrylic polyols were conver ted to secondary carbamate groups. The original acrylic and the converted a crylic were both cured with MF resins. Acid resistance was much better for films prepared from the acrylic that contained secondary carbamate groups.