Kinetics of three-dimensional polymerization and the effects of monofunctional molecules on the properties of crosslinked polyether urethane elastomers

Abstract

The polyether and polyester diols are known to contain normally some monofunctional molecules (MFM) as well as non-functional groups as admixtures. Their amounts are not the same in technical grade polyether(ester)diols, which could be one of the reasons for the non-reproducibility of the physico-chemical and mechanical characteristics of the sterically crosslinked polyurethane elastomers produced from them. A clarification of the effects of metered amounts of MFM on the mechanical properties of such elastomers is thus of great practical importance. The study of the effects of such admixtures on the steric polymerization kinetics would possibly clarify the applicability of the known theories about gel formation [3, 4] to polyurethane cast-moulding systems. Existing theories on gel formation were developed on the basis that all the terminal functional groups present in the reaction mixture have identical reactivities. The kinetics of urethane formation, established by us on model and true systems using high molecular weight (mol.wt.) diols and low mol.wt. triols, showed evidence that the hydroxyl-containing reagents used to synthesize the sterically crosslinked elastomers had different reactivities [5]. The reactivity difference of identical type functional groups means that the experimentally determined % conversions at the “gel point” disagreed with the calculated in parallel and consecutive reactions [6]. These results must therefore be assessed after consideration of these reactivity differences and their effects on the formation of crosslinks, and on the physico-mechanical properties of crosslinked polyurethane elastomers.