Competitive polycondensation of model compound (MUF) resin system by ¹³C NMR

Melamine-urea-formaldehyde (MUF) resin is an excellent adhesive in the field of wood adhesives, however the com-petition mechanism is questionable which affects the structure control and performance optimization of the resin. In this sduty, the competitive resin synthesis polycondensation reaction of MUF system under alkaline condition was studied based on the model compound 1, 3-dihydroxymethyl urea (UF₂) and melamine (M) system, and the competitive reaction mechanism in the system was deduced by ¹³C NMR quantitative analysis. The results show that the energy barrier of hydroxymethylation of melamine is lower than that of urea, and the priority of hydroxymethylation is lower; the addition of melamine results in a large amount of hydrolysis of UF₂, and the formed free formaldehyde, resulting in hydroxymethylation of melamine; there is obvious polycondensation reaction in UF₂+M system, mainly from the relationship between Hydroxymethylurea and melamine or hydroxymethylmelamine. The type I bridge bond structure of polycondensation mainly comes from the reaction of UF₂ and M, which is difficult to form the type II bridge bond. At low molar ratio, the formation of bridge bond is superior to that of ether bond. With the increase of molar ratio, the formation of ether bond shows advantages, but there is obvious competition between them. There may be competitive presence of the UF self-condensation products, melamine-formaldehyde (MF) self-condensation products and MUF co-condensed products after the polycondensation reaction.