Plastic/wood-flour composites represent a new but fast growing class of materials in the United States. However, the low strength properties, high density and brittleness, and poor impact resistance of plastic/wood-flour composites prevent these products from capturing their full market potential, hi this dissertation, surface modification and extrusion foaming technologies were applied to address these drawbacks.

The chemical reactions between cellulosic materials (wood flour and cotton cellulose) and functionalized polyethylene coupling agents (acrylic acid-fimctionalized and maleic anhydride-functionalized) were studied by applying Fourier Transform Infrared (FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The experimental results indicate that chemical bonds between hydroxyl groups of cellulosic materials and functional groups of the coupling agents have occurred through esterification reactions. The effects of the coupling agent's functional monomer (acrylic acid vs. maleic anhydride) and base resin (polyethylene vs. polypropylene) types on the tensile, flexural, and Izod impact properties of high-density polyethylene (HDPE)/woodflour composites were investigated and the experimental results indicate that the types o f functional monomer and base resin are important factors determining the effectiveness of functionalized coupling agents for HDPE/wood-flour composites.

Although the tensile and flexural strength properties of polyolefin/wood-flour composites were enhanced with the addition of coupling agents, the impact resistance of the composites did not improve and the high density and brittleness of these composites were not reduced. Extrusion foaming technology was applied to address some of these problems.

The polyolefin/wood-flour composite foams were developed through a continuous extrusion process using factorial and central composite designs. The effects of material compositions and processing parameters including wood flour content, wood flour moisture content, chemical foaming agent (CFA) concentration, CFA form (with vs. without polymer carrier), CFA type (endothermic vs. exothermic), extruder die temperature, and extruder screw speed, together with the effect of coupling agent on the void fraction and average cell size of foamed polyolefin/wood-flour composites were examined. The experimental results indicate that a large amount of gas molecules available for cell growth is not the only requirement for the production of PP/wood-flour composite foams with a high void fraction. Processing at a high die temperature is also very important for the development of proper viscoelastic properties of the matrix for cell growth. The CFA type and form did not affect the void fractions of both neat HDPE and HDPE/wood-flour composites. However, a gas containment limit was observed for neat HDPE foams, whereas the average cell size achieved in the HDPE/wood-flour composite foams remained insensitive to the CFA content, irrespective of the CFA type. The experimental results also indicate that the use of coupling agent in the formulation is required to achieve HDPE/wood-flour composite foams with a high void fraction. When the wood flour contains 12% moisture, HDPE/wood-flour composite foams with the highest void fraction can be achieved at high extruder screw speed (120 rpm) and low extruder die temperature (170°C) without the use of chemical foaming agent.

Finally, the effect of void fraction on the mechanical properties of HDPE and HDPE/wood-flour composite foams was examined. The experimental results show that the presence of cellular structure reduced the notched Izod impact strength of both neat HDPE and HDPE/wood-flour composites. The elongation at break of foamed HDPE increased with void fraction, whereas the opposite trend was observed for HDPE/woodflour composites. In addition, the low density of the foamed HDPE and HDPE/woodflour composites was compromised by the loss in the tensile and flexural strengths, as well as the tensile and flexural moduli.