This paper presents an experimental study on foam processing of PS and HIPS/wood-fiber composites in extrusion using moisture as a blowing agent. Wood-fiber inherently contains moisture that can potentially be used as a blowing agent. Undried wood-fiber was processed together with PS and HIPS materials in extrusion and wood-fiber composite foams were produced. The cellular morphology and volume expansion ratios of the foamed composites were characterized. Because of the high stiffness of styrenic materials, moisture condensation during cooling after expansion at high temperature did not cause much contraction of the foamed composite and a high volume expansion ratio up to 20 was successfully obtained. The experimental results showed that the expansion ratio could be controlled by varying the processing temperature and the moisture content in the wood-fiber. The effects of a small amount of a chemical blowing agent and mineral oil on the cell morphologies of plastic/wood-fiber composite foams were also investigated.
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