Polyvinyl chloride (PVC)/wood-fiber composites are in growing demand as a substitute for solid wood components in the building industry because of the favorable performance and cost attributes of both wood fibers and PVC matrix. The higher density combined with their brittleness and lower impact resistance compared to unfilled PVC and/or natural solid wood may prevent this emerging class of materials from capturing their full market potential in applications such as decking, siding, and window and door frames. In this dissertation, different approaches including impact modification, batch microcellular and continuous extrusion foaming processes have been examined to overcome some of these drawbacks.

The effects of impact modifier types (crosslinked vs. uncrosslinked) and addition levels on the mechanical properties of rigid PVC/wood-fiber composites were examined. With the proper choice of modifier type and concentration, the impact strength of rigid PVC/wood-fiber composites can be significantly improved without degrading the tensile properties. Crosslinked modifiers were more effective and efficient in improving the impact resistance of rigid PVC/wood-fiber composites than the uncrosslinked modifier. However, impact modification of rigid PVC/wood-flour composites did not enhance their ductility or reduce the density of the products.

Foaming is an effective method for reducing the density and brittleness of polymers. Since most rigid PVC formulations includes impact modifiers, solid state microcellular foaming technology (batch process) was employed to investigate the influence of impact modification on the foamability of neat rigid PVC and rigid PVC/wood-flour composite samples. The experimental results indicated that impact modification (crosslinked and uncrosslinked modifiers) accelerated the rate of gas loss during foam ing process, which impeded the growth of nucleated cells. Consequently, impact modifiers are an unnecessary ingredient in the formulation of foamed neat rigid PVC and rigid PVC/wood-flour composites.

Since the batch foaming process used to generate cellular foamed structures in the composites is not likely to be implemented in the industrial production of foams because it is not cost-effective, the manufacture of PVC/wood-flour composite foams in an extrusion process needs to be investigated. The effects of chemical foaming agent (CFA) types (endothermic versus exothermic) and concentrations, as well as the influence of all-acrylic processing aid on the density and cell morphology of extrusion-foamed neat rigid PVC and rigid PVC/wood-flour composites were studied. Regardless of the CFA type, the density reduction of foamed rigid PVC/wood-flour composites was not influenced by the CFA content. The cell size, however, was affected by the CFA type, independent of CFA content. Exothermic foaming agent produced foamed samples with smaller average cell sizes compared to endothermic counterparts. The experimental results indicated that the addition of an all-acrylic processing aid in the formulation of rigid PVC/wood-flour composite foams provides not only the ability to achieve density comparable to that achieved in the neat rigid PVC foams, but also the potential of producing rigid PVC/wood-flour composite foams without using any chemical foaming agents.

The foamability of rigid PVC/wood-flour composites using moisture present in the wood flour as the foaming agent was investigated using a central composite design (CCD) experiment. It was determined that wood flour moisture could be used effectively permission of the copyright owner. Further reproduction prohibited without permission. as the foaming agent in the production of rigid PVC/wood-flour composite foams. Foam density as low as 0.4 g/cm³ was produced without the use of chemical foaming agents. However, successful foaming of rigid PVC/wood-flour composite with moisture strongly depends upon the presence of all-acrylic foam modifier in the formulation and the extrusion die temperature. The lowest densities were achieved when the all-acrylic foam modifier concentration was between 7-phr and 10-phr and extruder die temperature was as low as 170°C.

Finally, mechanical property characterizations of extrusion-foamed rigid PVC/wood-flour composites were done. Extrusion foaming reduced the density and the brittleness of the composites, but also caused a reduction in the tensile strength and modulus of the rigid PVC/wood-flour composites.

This study suggested that depending on the application, the problems associated with the rigid PVC/wood-flour composite products;​ high density, brittleness and low impact resistance can be overcome by adopting impact modification and/or extrusion foaming. Since impact modification improves the impact resistance of the material and extrusion foam ing reduces the brittleness and density of the material, a co-extrusion process, which utilizes separate extruders for the outside skin and inside core layer of the material, might be the most viable and cost-effective method for rigid PVC/wood-flour composites. The manufactu ring of impact modified skin and foamed core might provide overall low density, low brittleness and high impact resistance for rigid PVC/wood-flour composites.