Hydrogen-Bond-Induced Crystallization in Low-Molar-Ratio Urea–Formaldehyde Resins during Synthesis

Low-molar-ratio urea–formaldehyde (UF) resins provide low formaldehyde emission at the expense of poor adhesion due to the formation of crystalline domains. We report for the first time the crystallization phenomenon during the synthesis stages of the resins. The molecular weights, thermal curing behaviors, chemical species, and crystallinities of the resins at each stage of the addition reaction, condensation reaction, condensation endpoint, and final stages were investigated with various techniques. As the synthesis proceeded, the molecular weight and peak temperature reached a maximum and decreased afterward. The X-ray diffraction (XRD) patterns also showed that crystalline structures in the addition reaction became amorphous in the condensation reaction and then formed crystalline domains in the final stage of resin synthesis. The crystalline domains were induced by hydrogen bonds between linear molecules resulting from the cleavage of oxymethylene and branched methylene ether linkages in the final stage after the second addition of urea. Thus, crystallization should be prevented during the synthesis of resins to improve their adhesion.