On the Networking Mechanisms of Additives-Accelerated Phenol–Formaldehyde Polycondensates

Determination by thermomechanical analysis (TMA) of the average number of degrees of freedom of polymer segments between crosslinking nodes of phenolformaldehyde (PF) resin-hardened networks indicate that additive-accelerated PF resin polycondensations and hardening presented several different acceleration mechanisms. Some additives such as sodium carbonate appear to present purely an acceleration effect on the polycondensation reaction. Other additives such as propylene carbonate 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 accelerator itself does participate, leading to a tighter final network. These alternate crosslinking 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 hydrolysis 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 liquidand solid-phase C-NMR supporting evidence of the mechanisms proposed is also presented. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 255–266, 1997 INTRODUCTION esters, anhydrides, amides, and others. Some of the mechanisms which have been proposed are based on circumstantial, indirect eviAlkaline phenol–formaldehyde (PF) resins have dence rather than on direct evidence, mainly bebeen used successfully for many decades as extecause the chemistry of phenolic resins which is rior wood adhesives for bonded wood products. characterized by the presence of an almost infinite While the influence of many additives on their variety of oligomeric positional isomerides prohardening has been studied for a long time, the duced with the progressing of the reaction is parmechanisms of hardening acceleration induced by ticularly difficult and tedious to unravel. Some these additives, especially in the presence of the of these mechanisms are not supported by any wood substrate, has only started to be studied in direct evidence and are often only assumptions recent years. Thus, mechanisms of PF hardenbased on unacceptable interpretations of the oring acceleration by catalytic surface activation by ganic chemistry involved. lignocellulosic materials have been reported, as Recently, a method was presented which well as a variety of mechanisms based on PF allows one to determine the average number of dehardening acceleration induced by carboxylic acid grees of freedom of the polymer segments between crosslinking nodes both during hardening of and of hardened polycondensation networks as a funcCorrespondence to: A. Pizzi. tion of the relative deflection of thermomechanical Journal of Applied Polymer Science, Vol. 66, 255–266 (1997) q 1997 John Wiley & Sons, Inc. CCC 0021-8995/97/020255-12 analysis (TMA) measurements in bending. The