Engineering Plastics from Lignin. VIII. Phenolic Resin Prepolymer Synthesis and Analysis

Abstract

A sequential derivatization of lignin with formaldehyde and phenol was investigated as a means of enhancing lignin's reactivity in phenol-formaldehyde resins. Kraft lignin (KL) and two novel biocon version lignins, steam explosion (SEL) and acid (H₂SO₄) hydrolysis lignin (AHL), were chemically modified by sequential reaction with formaldehyde and phenol. The results with regard to the chemical structure of the phenolic resin prepolymers showed that the ability to hydroxymethylate and phenolate is related to lignin structure. KL from pine proved to be more amenable to chemical modification with formaldehyde and phenol than was either SEL from aspen or AHL from pine. Degrees of substitution were determined by H–NMR spectroscopy and by oxidative analysis with alkaline permanganate. H–NMR spectroscopy revealed degrees of phenolation of 0.42, 0.29 and 0.21 per average C₉-unit for KL, AHL, and SEL, respectively; and permanganate oxidation illustrated that approximately 90%, 60%, and 60% of all available unsubstituted phenolic guaiacyl units in KL, AHL and SEL, but none of the corresponding non-phenolic or syringyl entities, had become derivatized.