Polymerization of resole resins with several formaldehyde/phenol molar ratios: Amine catalysts against sodium hydroxide catalysts

Abstract

AbstractTriethylamine and sodium hydroxide catalyzed phenol/formaldehyde resole resins were investigated in terms of their behavior during both addition and polymerization reactions. Amine‐catalyzed prepolymers were mainly ortho‐substituted structures, whereas the sodium hydroxide catalyst directed the addition reaction to para reactive sites. During polymerization, triethylamine led to dimethylene ether bridges as the principal linkages between aromatic structures, increasing their final concentration as the starting hydroxymethyl group concentration increased. In contrast, the use of sodium hydroxide reduced dramatically the dimethylene ether bridge concentration, favoring methylene bridge formation. The influence of hydroxyl ions on the stability of quinone methide intermediates could be the reason for those differences. Despite the formation of dimethylene ether bridges, at higher curing temperatures, more oxidized groups started to appear in cured resoles when the formaldehyde/phenol molar ratio was higher. The presence of infrared bands associated with quinones, aldehydes, and/or carbonyl groups, mainly in high‐formaldehyde‐content resins, could indicate a direct oxidation process from dimethylene ether bridges and/or residual hydroxymethyl groups, without the formation of methylene bridges. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2623–2631, 2006