On the Networking Mechanisms of Additives-Accelerated Phenol–Formaldehyde

ABSTRACT: Determination by thermomechanical analysis (TMA) of the average num-ber of degrees of freedom of polymer segments between crosslinking nodes of phenol-formaldehyde (PF) resin-hardened networks indicate that additive-accelerated PF resin polycondensations and hardening presented several different acceleration mecha-nisms. Some additives such as sodium carbonate appear to present purely an accelera-tion effect on the polycondensation reaction. Other additives such as propylene carbon-ate appear to present an acceleration effect by also inducing an increase in the average functionality of the system, due to alternate crosslinking reactions to which the acceler-ator itself does participate, leading to a tighter final network. These alternate crosslink-ing reactions can be of a different nature. In the propylene carbonate case, the reaction is related to a Kolbe–Schmitt reaction. In formamide, instead, it is related to its hydro-lysis to formic acid and ammonia with the subsequent rapid reaction of the latter with two or more hydroxybenzyl alcohol groups of PF resols. The rapid reaction of the {NH2 group of formamide with two hydroxybenzyl alcohol groups of PF resols, a reaction which is also characteristic of urea and methylamine, also appears likely to occur. Both liquid- and solid-phase 13C-NMR supporting evidence of the mechanisms proposed i