Physical gelation of melamine formaldehyde resin solutions. II. A combinedlight-scattering and low-resolution relaxation proton NMR study

The kinetics of physical gelation in aqueous melamine formaldehyde (MF) res in solutions were studied with the aid of low-resolution H-1 NMR T-2 relaxa tion experiments in combination with both static and dynamic light-scatteri ng measurements. The investigations were conducted on a series of MF resins with increasing degrees of condensation. We show that MF aggregates (aided by hydrogen bonds) were immediately formed upon cooling from reaction to r oom temperature, that is, storage temperature. Surprisingly, the growth of these aggregates, which eventually led to the formation of a physical gel, did not have a major effect on molecular mobility. By means of light-scatte ring experiments, we were able to monitor the increase of the size of MF ag gregates as a function of storage time. The physically gelled MF solutions were subjected to heating and subsequent cooling runs and again studied by light-scattering and nuclear magnetic resonance (NMR) experiments. MF aggre gates were destroyed, depending on the degree of condensation, in the tempe rature range 35-60 degrees C according to NMR, and 40-75 degrees C as deter mined by light scattering. The process of physical gelation was reversible; upon subsequent cooling, the MF aggregates were formed anew. (C) 1999 John Wiley & Sons, Inc.