Structural changes in urea-formaldehyde resins during storage

The changes in structure of fresh and stored commercial UF resins were characterised by high resolution C-13 NMR spectroscopy. The three-step synthesis produces the resin consisting of two parts: the polymeric part from condensation of formaldehyde with first amount of urea, and monomeric part formed after addition of second part of urea. Some differences in structure depending on details of synthesis are of temporary character and level mainly in transhydroxymethylation from polymeric to monomeric part of resin. The reactions occuring after addition of second urea and following during storage are considered as one continuous process. Methylenes adjacent to secondary and tertiary amino groups form only during acid condensation. The main reaction during storage is the formation of methylenes adjacent to secondary amino groups. This reaction occurs between free terminal hydroxymethyl and amino groups of both resin parts. The physical and mechanical properties of particle boards confirm the close performance characteristics of studied resins. Particle boards satisfy requirements for general purpose boards P2 and emission standard E1.