Modeling of melamine formaldehyde polymerization. II. Development of a simpler model

Abstract

The kinetic modeling of melamine-formaldehyde polymerization presents a relatively formidable mathematical challenge because of the simultaneous presence of several types of deviations from Flory's equal reactivity hypothesis. A molecule of melamine has six reactive amide hydrogens, which can react with the -CH₂OH groups of formaldehyde in solution. The reactivity of the secondary hydrogens on the melamine is about 61% of that of the primary hydrogens (induced asymmetry). There is a shielding effect present, i. e., the reactivity of the hydrogens near the outside of a multiringed polymer molecule is higher than that of the hydrogens inside the coiled molecules. Two bound -CH₂OH groups on the polymer molecules can self-condense to give methylene linkages, the reactivity depending upon the location of the two groups. And, to confound modeling effort still further, all these reactions are reversible. An earlier attempt at molding this system considered the reactions between 36 "basic" species. This model was far too detailed and failed to acount for the reverse reactions. In the present study, a simpler model has been proposed which involves fewer "basic" species. An improved model for intramolecular reactions is also developed. Several important characteristics of the polymerization have been obtained as a function of time. Results from this model have been compared with those obtained from the earlier model, and also compared with the short-time experimental results. The present model can be extended to account for the reverse reactions quite easily.