Kinetics of phenolic resol resin formation by HPLC

The kinetics of phenolic resol resins catalysed by sodium hydroxide and triethylamine have been studied by HPLC. Prepolymers were synthesized under controlled conditions: 80 degrees C, F/P = 1.8, and initial pH = 8.0. Resol catalysed by triethylamine showed higher addition of formaldehyde onto ortho positions of phenolic rings, whereas in the presence of sodium hydroxyde addition onto para sites was favoured. Although preferences existed towards addition of formaldehyde to phenol, both resins showed finally similar composition of compounds. Reaction products were identified by C-13 n.m.r. spectroscopy. Mainly, para,para-DPMs, and later on, and to a lesser extent ortho,para-DPMs were formed. No ortho,ortho-DPMs were detected. As a result of reheating the resin, an increase in high molecular weight compounds was observed, as well as one phenolic ring species which kept on decreasing. Para,para-DPMs were still formed while ortho,para-DPMs started disappearing due to the higher reactivity of the unbonded para position towards condensation reactions. Addition and condensation reactions can occur simultaneously due to the presence of free formaldehyde in the mixture arising from cleavage of the hemiformal species or condensation reactions between hydroxymethyl groups. Another resol synthesized under the same conditions but in the presence of triethanolamine was also analysed. It took much longer to reach the final prefixed point, and showed mainly low molecular species. (C) 1998 Elsevier Science Ltd. All rights reserved.