Engineering Plastics from Lignin. IX. Phenolic Resin Synthesis and Characterization

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Abstract
The performance of phenol-formaldehyde (PF) resins, formulated with lignin derivatives previously synthesized as phenolic resin prepolymers, was evaluated by thermal analysis of the curing process, and by a hard maple shear block test. At 54 and 60% phenol replacement levels, respectively, kraft (KL) and steam explosion lignin (SEL)-based resoles exhibited cure behavior very similar to a standard PF resin. Acid hydrolysis lignin gelled prematurely, and was found to be incompatible with the normal synthesis procedure. Differential scanning calorimetry (DSC) was used to compare kinetic parameters for the curing process of neat and lignin derived phenolic resins. Activation energies and cure rates determined by DSC showed no difference between adhesives. High lignin contents had no inhibitory effect on resin cure. Shear strength properties were evaluated in a compression test, and results illustrate that both lignin-based resins have acceptable strength properties, both in a dry and accelerated aging test. Of the lignins tested, kraft lignin consistently demonstrated superior performance as a pre-polymer in phenolic adhesives. This was attributed to differences in the chemical structure of the two lignins, which had been found to vary in terms of their reactivity with formaldehyde and phenol. KL had been noted to be more amenable to derivatization with formaldehyde and phenol, hence its ability to crosslink with a phenol-formaldehyde fraction during resin synthesis was increased. Positive structural features in KL are a high phenolic guaiacyl (3-methoxy, 4-hydroxy phenyl) content, low carbon-to-carbon bonding between aromatic rings, high solubility in alkali, and a higher number average molecular weight than SEL.