Catalytic activity of mineral montmorillonite on the reaction of phenol with formaldehyde

Abstract Phenolic resins of novolac type were synthesized in the presence of the mineral montmorillonite in its pristine (Na-Mt) or protonated form (H-Mt), with and without oxalic acid (OA) as a catalyst. The H-Mt acts as a strong catalyst regardless of the presence of OA. Contrary, when Na-Mt was added to the OA-catalysed reaction mixture the reaction yield decreased, however, a catalytic effect was observed in the absence of the OA catalyst. The structure of novolac resins is influenced by the addition of both, the Na-Mt and the H-Mt. While the H-Mt influences only a ratio between the o-o, o-p and p-p ethylene bridges, the Na-Mt, in the absence of OA, favours the formation of methylene ether bridges. The molar mass averages of novolac resins synthesized in OA catalysed reactions were always higher than in the absence of OA. Differences in the chemical structure influence the resins' glass transition temperature, which can be even below 0 °C.