Structure formation in urea-formaldehyde resin synthesis

The structure formation in acid promoted polycondensation and particularly in alkaline post-treatment with urea was studied by ¹³C NMR spectroscopy in the three-step synthesis of urea- formaldehyde resins. Trishydroxymethyl urea was identified as the constituent of the mixture of hydroxylmethylated urea derivatives obtained in alkaline conditions. The formation of methylenes linked to secondary or tertiary amino groups occu rs only in acidic conditions by branching chains in the reaction with mono- and 1,3-bishydroxymethyl urea. Bishydroxymethyl groups do not take part in acid promoted polycondensation. The final structure of resins depends mostly on the migration of formaldehyde from bishydroxymethyl groups to urea with formation of mono- hydroxymethyl urea as the first preferred compound. A greater amount of oligomers with methylenes adjacent to secondary amino groups and containing singly bonded urea is a sign of a thorough heat treatment. A steady content of methylenes linked to tertiary amino groups is observed in heat treatment and ageing of resins.