Structure and curing mechanism of resol phenol-formaldehyde prepolymer resins

Abstract

Prepolymer resol resins with different phenol/formaldehyde/NaOH molar ratios were synthesized and characterized by technical analysis as well as by 13C NMR spectra for samples in CD3OD solution. Larger amounts of NaOH promoted the binding of formaldehyde and para substitution including the formation of henzoquinone rings with two o,p′-methylene groups in the para position. The polycondensation between ortho-hydroxymethyl groups and one-substituted aromatic para positions as the main thermosetting curing process continued in the heat treatment of resol resins. The self-condensation between ortho-hydroxymethyl groups, the release and oxidation of formaldehyde, and the formation of hemiformals are processes of low significance in curing resol resins. The assignment of 13C NMR resonances depending on the presence of phenolic hydroxyl groups, phenoxide ions, and substituted quinone rings is discussed.