Synthesis of urea‐formaldehyde resins: NMR studies on reaction mechanisms

The structural changes which occur during the two step condensation of the liquid urea‐formaldehyde resins (UF) have been studied by NMR spectroscopy combined with chemical analyses. In the first step of the condensation (at 95°C and mildly acid reaction), the reaction takes place quickly with the formation of methylene linkages (NCH2N) and, to a minor extent, of methylol groups (NCH2OH). In the second step (at room temperature and mildly alkaline reaction) the reaction goes on very slowly, with the formation only of methylol groups. At the end the oxymethylene formaldehyde is only present as NCH2OCH2N groups.