Lignins for phenol replacement in novolac-type phenolic formulations. II. Flexural and compressive mechanical properties

Several hexamethylenetetraamine-cured novolac-type lignophenolic resins have been subjected to mechanical analysis to check the influence of the addition of different types of lignins (kraft pine lignin, soda/anthraquinone flax lignin, and sulfonated kraft lignin from mixed softwoods) and the influence of different phenol substitution levels. Both flexural and compression tests have been performed to evaluate the influence of the curing temperature and pressure. The results have been compared with those from a commercial novolac system. Flexural tests show that lignin incorporation leads to systems with increased rigidity, whereas up to 82% of the maximum strength can be retained with respect to a commercial phenolic (phenol-formaldehyde) at a 45 wt % substitution degree. Compression analyses show that modified prepolymers are not significantly affected by the presence of lignin at a medium curing pressure (150 bar), but their deformability results considerably increase at high curing pressures (400 or 600 bar).