Lignin has great potential for substituting phenol in the phenol formaldehyde adhesive. Three different lignins were used to synthesize lignin-phenol-formaldehyde resin, namely a commercial pine kraft lignin, a hardwood (sweet gum) and a non-wood (wheat straw) residual lignin isolated from a biorefinery process involving autohydrolysis and enzymatic hydrolysis (AER). These lignin sources were chosen based on their differences in structure, functionality and molecular weight. Lignins were characterized including chemical composition, molecular weight and reactive sites.  The effect of phenolation was also investigated, since phenolation can increase the lignin reactivity and also decrease the molecular weight. Phenolation is an economically viable method for lignin modification since the excess phenol used during phenolation can be used to develop crosslinks during the curing process for adhesives. The lignin-phenol-formaldehyde resins were applied on the yellow poplar plywood and evaluated in terms of shear strength. The results indicated that up to 25% phenol could be replace by pine kraft lignin without significantly sacrificing the adhesive strength. Phenolating the pine kraft lignin enabled for an increase in the substitution to approximately 45%.  Lignin-phenol-formaldehyde resin prepared using AER from both sweet gum and wheat straw exhibited comparable adhesive performance as phenol formaldehyde resin at substitutions levels up to 35%. However, phenolation did not improve the performance of AER based adhesives.  This research has demonstrated that lignin can be used to substitute for phenol in PF adhesives and the degree of substitution that can be achieved depends on the lignin source and reactivity.

Application of lignins in phenol formaldehyde adhesive

255th National Meeting (2018)

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