Mechanical properties and microstructures formations of wood thermoplastic composites

In this study, four wood fibers, namely CTMP birch, CTMP aspen fiber, BCTMP aspen fiber and bacterial cellulose, were employed as reinforcement materials in polypropylene or polyethylene matrix. The most important problem encountered with wood composites is the inherent incompatibility between hydrophilic fiber materials and hydrophobic polymer matrices. Chemical coupling plays an important role in improving interfacial bonding strength in wood composites. In this study, the effects of coupling agent type and structure, graft polymerization of coupling agents and coupling process on chemical coupling were investigated. Coupling mechanisms were established both for maleated PP and maleated PE. In addition, the effect of the initiator on resulting strength was also investigated.;It was found that the composite could achieve optimum impact strength without weakening tensile strength after compounding 30-35min. In addition, the bulk mixing method (no-premixing method) could lead to superior impact and tensile strength compared to pre-mixing method with wood fiber pre-coated by the mixture of the maleated polymer and small amount of polymer matrix. The addition sequence of DCP at final-step is the best method to achieve higher impact strength without tensile strength decrease for wood composite due to the difference in the oxidation and grafting reaction time in presence of initiator.;It is well known that the mechanical properties of wood composites were significantly enhanced by employment of coupling agents and affected by their characteristics. However, the properties deteriorated with excess coupling agent employment due to its entanglements and slippages. The optimum concentration of coupling agent was found to be 3wt% for wood composites which was reinforced with 30wt% wood fiber and 0.2wt% DCP.;Wood fiber showed different strength behaviors in incompatibilized and compatibilized systems. Generally, both impact and tensile strength decreased with an increase of wood fiber content without compatibilizer presented. Oppositely, the strength was improved in the presence of a compatibilizer as wood fiber concentration increased due to the increased bonding on more reactive sites. The extent of interfacial interactions was evaluated with the addition of 1wt% bacterial cellulose exhibiting superior impact strength based on traditional wood composites. It was an interesting finding that the introduction of NC at low content (<2wt%) could increase the impact and tensile strength of both polymeric nanocomposites and wood composites, but weakens the strength at high content (up to 20wt%). Different organo-NC exhibited different behaviors due to the different nature of surfactants, and the differences would be magnified after the system was compatibilized by coupling agent and initiator. Moreover, natural NC led to better performance than concentrates-NC as well as other modified NC except I.34TCN which has reactive pendant groups --QH ready to react with coupling agent.;Surface treatments of wood fibers were carried out with two kinds of maleated polyolefins: MAPE and MAPP. The blending time as well as the addition sequence of wood fiber and initiator was optimized. Maleated coupling agents were compounded with wood fiber during melt blending. The composites were prepared in a standard mold equipped with rollers, an adjustable temperature oil bath and a changeable mixing rate. The composites were prepared at a variable temperature, with 170°C for PE and 190°C for PP, 60rpm mixing rate and an optimium blending time.;FQA and SEM were used to monitor the morphological changes of wood fiber and their fracture modes. In addition, FTIR and XPS analyses revealed the decrease in hydrophilicity of wood fiber with maleated polyolefin treatments and new bond formation with employment of coupling agent because of chemical bonding occurring at the interface.;For melt-blending process, driving and shear force were applied to wood composites. The interfacial morphology was illustrated with the break-off, cracking, curling, extruding, twisting, fibrillations, and pinwheels models.;The chemical reactions between maleated polyolefins and wood fibers in a blender process were also determined by XPS with the goal to evaluate the grafting efficiency and the formation of ester links.;There was also an interesting finding that the shake-up effect of surface lignin was eliminated after maleated polymer was employed regardless of the type and MA% of maleated polymer used as well as the fact that the nitrogen band was missing. As an engineering material, wood composites reinforced with CTMP wood fiber could achieve good performance due to rich surface coverage by lignin and extractives which acted as internal lubricant and natural antioxidant during high temperature process. (Abstract shortened by UMI.)...