Polymer blends for multi-extruded wood-thermoplastic composites

Viviane Louise Villechevrolle

Wood-plastic composites (WPC's) are usually composed of wood blended with a single polymer. To address the issue of WPC's recycling and appraise the potential offered by using polymer blends in the making of WPC's, composites made of wood flour and several blends of isotactic polypropylene (PP), high density polyethylene (HDPE) and polyvinyl-chloride (PVC) were extruded. Both polymer blends and WPC's require intensive mixing to sufficiently homogenize the composite, therefore the influence of additional mixing was evaluated by re-extruding the composites another three times. Torque rheometry, bending tests and water sorption tests were performed to determine the influence of the repeated extrusions and the use of polymer blends on the composites' performances. The results indicated that the subsequent re-extrusions increased the water resistance and the brittleness of the composites. The use of polymer blends did not interfere with the processability and the strength. Within the tests performed, no disadvantage related to the use of polymer blends was discovered. Composites containing PVC were found easier to process and stiffer but more brittle and less water resistant than the composites made of PP or HDPE. PP-based composites were the most difficult to process, primarily due to extruder temperatures, but had the best water resistance. The performances of the composites depended on the nature of their components, not on the eventual presence of a polymer blend. Thermal properties were also investigated. The transition temperatures and the crystallinity of the samples were determined through differential scanning calorimeters (DSC). Tests for oxidation induction times (OIT) were conducted utilizing DSC techniques as well. Wood increased the melting point of most blends and reduced the crystallization temperature of HDPE-based composites. On the contrary, PVC diminished the melting point of PP-based blends. The different components were deemed immiscible and seemed to melt and crystallize separately. Repeated extrusions on the WPC's apparently increased the crystallinity degree as well as the rate and temperature of crystallization, but decreased the melting temperature, indicating a potential structural degradation of the composites. PVC blends showed the best resistance to oxidation while the OIT of PP and HDPE blends were particularly short. The number of extrusions did not change the oxidation times of the composites tested.

Polymer blends for multi-extruded wood-thermoplastic composites

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