Chemical coupling plays an important role in improving interfacial bonding strength in wood-polymer composites. In this study, the effects of coupling agent type and structure, graft polymerization of coupling agents, interfacial wettability, coupling treatment and process, coupling agent distribution, and coupling agent performance on chemical coupling were investigated. Coupling mechanisms were established based on maleated polyethylene copolymers.
For maleated wood veneer, the relationship among graft rate, concentration, and retention of coupling agent followed three-dimensional parabloid models. Wettability of maleated wood surface was related to acid number, amount of free or ungrafted maleic anhydride groups, and coupling agent concentration. Dynamic contact angle of water droplets on maleated wood followed the natural decay process, whereas the spreading process of droplets fitted the Boltzmann sigmoid model. Compared with untreated composites, maleated composites had significant shifts in most TGA, DSC, and DMA spectra because of chemical coupling at the interface. For melt-blending process, the best interfacial bonding strength was achieved at short compounding time (e.g., 10 min), appropriate mixing temperature (e.g., 180oC), and moderate rotation speed (e.g., 90 rpm). With FTIR, ESCA, and SEM analyses, the evidence of chemical bridges at the interface was proved. The interfacial morphology was illustrated with the pinwheel models. For wood-plastic laminates, interfacial adhesion followed the monolayer models, while brush, switch, and amorphous structures applied to melt-blended composites. Therefore, the interface was strengthened with covalent bonding (such as esterification and carbon-carbon bonding), strong secondary bonding (e.g., hydrogen bonding), macromolecular chain entanglement, and mechanical interblocking. Coupling agent performance for maleated copolymers was mainly related to their acid number, molecular weight, backbone structure, and concentration. Coupling agents with large molecular weight, moderate acid number, and concentration were preferred to have better performance at the interface. Based on the experimental results, 226D, 100D, and C16 were the best coupling agents among seven maleated copolymers used in this study. Compared with untreated composites, maleated composites increased interfacial bonding strength by 140% and flexural modulus by 29% at the concentration level of 3%.