Maleated lignin coreaction with phenol-formaldehyde resins for improved wood adhesives performance

This research work investigated the physical and mechanical properties of particleboard panels bonded with maleated-lignin-phenol-formaldehyde (mLPF) resin. For this purpose, lignin was modified by maleic anhydride and then various contents of unmodified/modified lignin (30, 40 and 50 wt%) were added as a substitute for phenol in the phenol-formaldehyde (PF) resin synthesis. The properties of the synthesized resins were compared with those of an unmodified LPF and a control PF resin. The structure of the modified lignin was studied by FTIR, and the changes in curing behavior of the resins prepared were analyzed by DSC. The physicochemical properties of the synthesized resins (such as gel time, viscosity, solid content and density) and physical (water absorption) and mechanical properties (flexural properties, internal bond strength) of the prepared particleboard panels bonded with LPF resins were measured. The FTIR analysis indicated that, by treatment of lignin, the COOH, C–O and CC bonds increase while the content of O–H bond markedly decreases. The DSC analysis showed that curing of the mLPF resin occurred at a lower temperature than for the control PF resin and the unmodified lignin-phenol-formaldehyde (LPF) resins. The viscosity increased and the gel time was shortened by adding maleated lignin to the PF resin. The panels bonded with the modified lignin resins presented a lower formaldehyde emission, and a higher dimensional stability and mechanical strength than those bonded with the control PF resin and the unmodified lignin adhesive. Moreover, the mechanical strength and dimensional stability of the particleboard panels were significantly improved by increasing the resin content of maleated lignin from 0 to 50 wt%.