Physical Gelation of Melamine Formaldehyde Resin Solutions. II. A Combined Light-Scattering and Low-Resolution Relaxation Proton NMR Study

摘要

The kinetic of physical gelation in aqueous melamine formaldehyde(MF) resin solutions were studied with the aid of low-resolution ~1H NMR T_2 relaxation experiments in combination with both static and dynamic light-scattering measurements. The investigations were conducted on a series of MF resins with increasing degress of condensation. We show that MF aggregates (aided by hydrogen bonds) were immediately formed upon cooling from reaction to room temperature, that is, storage temperature. Surprisingly, the growth of these agregates, which eventually led to the formation of a physical gel, did not have a major effect on molecular mobility. By means of light-scattering experiments we were also to monitor the increase of the size of MF aggregates 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 aggregates were destroyed, depending on the degree of condensation, in the temperature range 35-60degC according to NMR, and 40-75degC as determined by light scattering. The process of physical gelation was reversible; upon subsequent cooling, the MF aggregates were formed anew.