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

The instability mechanism of aqueous melamine-formaldehyde (MF) resin solut ions is elucidated. In general, the plot of the storage stability, which is defined as the time from production of the resin till turbidity is observe d, against the reaction rime consists of an ascending and a descending bran ch. Investigations on fresh and aged (unstable) MF resin solutions, by vari ous analytical techniques Like HPLC and C-13 NMR, revealed that the storage stability is governed by a combination of three "different" physical proce sses. In aqueous MF solutions with a relatively low degree of condensation (at the start of the reaction) the precipitation of monomeric species contr ols the instability mechanism. As the reaction proceeds (as the degree of c ondensation increases) the apparent solubility of these monomers increases and their absolute concentration decreases. As a consequence, storage stabi lity develops. The dissolved monomeric and oligomeric species, i.e., water- soluble MF resins, can undergo cooperative secondary intermolecular interac tions (hydrogen bonding) as a result of which supramolecular aggregates are formed. This process will eventually lead to the formation of a physical g el. At still, longer reaction times the solubility of higher molecular weig ht species decreases and, as a result, the classical process of liquid-liqu id phase separation sets in. The proof for the existence of a gel was provi ded by frequency-dependent rheological experiments on the unstable MF sampl es. NMR investigation on fresh and aged MF resins indicated that the chemic al structure does not change noticeably during storage at room temperature. These studies indicated further that methylene bridges have a greater tend ency towards precipitation than ether bridges.