The processing of wood-polymer composites (WPCs) plays a prominent role in the successful application of timber products in our everyday lives and economy. In this project, the role of methyl methacrylate (MMA) and 2-hydroxyl methacrylate (HEMA) in the formation of WPCs was investigated with an emphasis on the improvement in mechanical properties and dimensional stability of Hoop Pine. A series of WPCs were synthesed with various ratios of MMA to HEMA. The microstructure of the WPCs was examined, and related to the morphological model responsible for the observed physical behavior.
The formation of practical WPC flooring materials was also investigated. The associated changes in the hardness of the WPCs upon addition of solvent and applying an initial pressure to the wood were also studied.
Wood impregnation is important to produce wood-polymer composites and the diffusion process is a convenient starting point to investigate the impregnation characteristics of monomers into timber. The monomer uptake curves revealed that the initial rate of impregnation of HEMA exceeded that of MMA, however, at long times, the uptake was the same. A combination of anatomical structure and higher capillary tension led to lower uptakes under diffusion conditions. Sufficient monomer uptake was achieved using the immersion process after applying an initial vacuum, or using the pressure process.
The observation of the concentration of the monomer as a function of distance across a specimen provides complimentary information on the impregnation characteristics of the monomer. NMR imaging was used to obtain the concentration of monomer in the timber at various stages during the diffusion and immersion processes, as well as the pressure process. When an underlying Fickian profile was evident, the calculated diffusion coefficients indicated that HEMA diffused more rapidly in the radial direction than did MMA. When parallel zones occurred and high permeable longitudinal path was blocked, the impregnated treatment solution using the pressure impregnation technique was mainly concentrated on the surface.
The homopolymer, PMMA, is an important ingredient in the manufacture of wood-polymer composites. In this study, the physical and chemical properties of the composites were systematically modified by copolymerization of MMA with HEMA. The mechanical properties, hardness and compressive strength, of the WPCs were measured using "Australian methods for mechanically testing small clear specimens of timeber" [J.J. Mack, CSIRO DBR, Tec.Paper 31 (2nd series), 1979]. The dimensional stability was examined by immersion of the WPCs in distilled water for 264 hrs. The results for all WPCs showed that incorporation of pol>"mer into wood greatly increased the specific gravity of the wood resulting in an increase in the strength. The capacity of the WPCs to take up water was also reduced; however, the resistance to volumetric swelling was dependent on the HEMA concentration. The antiswell efficiency was found to increase with HEMA content up to 50 vol % and then decrease dramatically at higher HEMA concentrations. This arises from the swelling pressure exerted by the swelling of hydrophilic PHEMA.
The properties of WPCs are influenced by their microstructure. SEM micrographs of the WPCs with lower HEMA content showed that there were gaps between the polymer and the wood cell walls, suggesting that the molecular interactions are negligible. In samples with intermediate HEMA content, SEM micrographs showed that the gaps were less evident and cell-wall thickness increased. This indicates that the polymer binds closely to the cell walls and that polymer penetration into the cell walls increases.
However, for samples with high HEMA content, separation of the primary and secondary cell walls was observed, suggesting the existence of strong interactions in these samples. The analysis of the concentration gradient profiles within wood-PMMA and wood-PHEMA composites revealed that incorporation of polymer into wood changed the physical nature of the wood and that more polar monomers made more significant changes to the physical nature of the cell walls. FTIR measurements confirmed that only physical modification occurred during formation of wood-polymer composites and that chemical bonds between polymer and wood macromolecules were not formed.
An insight into the relationship between the structure, morphology and properties may be obtained from the viscoelastic properties and degradation behavior using DMA and TGA. The decrease in glass transition temperature of the polymer for samples containing HEMA arose from the presence of moisture and was more pronounced at higher HEMA content. However, a broad glass transition was correlated with a broad range of environments arising from deep adhesive penetration and intimate molecular association between HEMA units and wood macromolecules. These interactions also influenced the degradation behavior of the WPCs. While wood-PMMA composites with weak molecular interactions exhibited similar degradation behavior to that of virgin wood, wood-PHEMA with strong molecular interactions had enhanced thermal stability. The increased thermal stability at higher temperatures increased with HEMA concentration up to 30 vol %.
The formation of practice WPC flooring materials can be achieved with diluted monomer solutions using the immersion or pressure impregnation techniques. The hardness of the WPCs decreased with increasing methanol content in the solution, and up to 50-60 vol %, the hardness of the WPC achieved the required value of approximately 7 kN. The application of an initial pressure to the wood prior to impregnation also controlled the uptake of solution and the hardness of the WPCs was therefore controlled.
The collective data indicates that MMA and HEMA, as well as their monomer mixtures can be used for the preparation of wood-polymer composites with enhanced mechanical properties compared to virgin wood and moderately improved dimensional stability. The optimal monomer formulation is a ratio of 1:1 of MMA and HEMA. Practical WPC flooring materials can also be produced by addition of methanol at 30-60 vol % using the immersion or pressure impregnation techniques.