Storage stability of melamine-formaldehyde resin solutions, 1. The mechanism of instability

Abstract

The instability mechanism of aqueous melamine-formaldehyde (MF) resin solutions is elucidated. In general, the plot of the storage stability, which is defined as the time from production of the resin till turbidity is observed, against the reaction time consists of an ascending and a descending branch. Investigations on fresh and aged (unstable) MF resin solutions, by various analytical techniques like HPLC and 13C NMR, revealed that the storage stability is governed by a combination of three “different” physical processes. In aqueous MF solutions with a relatively low degree of condensation (at the start of the reaction) the precipitation of monomeric species controls the instability mechanism. As the reaction proceeds (as the degree of condensation increases) the apparent solubility of these monomers increases and their absolute concentration decreases. As a consequence, storage stability develops. The dissolved monomeric and oligomeric species, i. e., water-soluble MF resins, can undergo cooperative secondary intermolecular interactions (hydrogen bonding) as a result of which supramolecular aggregates are formed. This process will eventually lead to the formation of a physical gel. At still longer reaction times the solubility of higher molecular weight species decreases and, as a result, the classical process of liquid-liquid phase separation sets in. The proof for the existence of a gel was provided by frequency-dependent rheological experiments on the unstable MF samples. NMR investigation on fresh and aged MF resins indicated that the chemical structure does not change noticeably during storage at room temperature. These studies indicated further that methylene bridges have a greater tendency towards precipitation than ether bridges.