Wood Plastic Composites by Melt Impregnation: Polymer Retention and Hardness Yaolin Zhang, 1,2 S. Y. Zhang, 1,2 Ying Hei Chui, 2 Hui Wan, 1 Mosto Bousmina 3 1 Forintek Canada Corporation, 319 rue Franquet, Sainte-Foy, Quebec, Canada 2 Faculty of Forestry and Environmental Management, University of New Brunswick, New Brunswick, Canada 3 Canada Research Chair on Polymer Physics and Nanomaterials, Department of Chemical Engineering (CREPEC), Laval University, Quebec, Canada Received 16 May 2005; accepted 11 January 2006 DOI 10.1002/app.24120 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Wood plastic composites were prepared through impregnation of solid wood with polyethylene. The effects of impregnation parameters on polymer retention and hardness were investigated. A screening strategy of 16-run resolution IV design for seven factors at two levels was adopted. The seven factors were: ratio of maleated polyethylene in formulations, ratio of polyethylenes with different molecular weights, four process factors (vacuum, pressure, time, and temperature), and wood species (red maple and aspen). Polymer retention (PR) and Brinell hard- ness (H B ) were investigated and discussed on the basis of the impregnation parameters. The present work showed that process parameters (pressure and temperature), polymer impregnants (different molecular weight polyethylenes), and wood species contributed significantly to PR and H B . Increasing pressure and temperature resulted in a higher PR and H B , whereas increasing the molecular weight of poly- ethylene and switching wood species from aspen to red maple gave a lower PR and H B . This study was aimed at understanding how impregnation parameters affect the final properties of wood plastic composites and developing an optimal fabrication process for wood plastic composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1672–1680, 2006 Key words: wood plastic composite; impregnation; screen- ing design; hardness; polymer retention INTRODUCTION The shortage of high quality hardwoods has driven researchers and wood product manufacturers to seek alternatives, lower cost resources for value-added ap- plications. To reach this goal, proper technologies are needed to improve specific wood quality attributes (e.g., dimensional stability, durability, mechanical properties, and hardness) to meet end-use require- ments. One approach is to combine wood with poly- meric materials to create a new composite. There are two categories of wood plastic composites (WPC). One is prepared by impregnating solid wood with a monomer or prepolymer and then in situ polymeriza- tion. 1–5 The other is plastics reinforced with wood fiber or particles. Although the former produces stron- ger products than wood, the conversion rate of poly- merization hardly reaches 100%, and the residue monomers or prepolymers tend to leach from the product and have a negative impact on the environ- ment. Plastics reinforced with wood fiber have poor dimensional stability even though they are less expen- sive. These technological issues limit the acceptance of WPC by consumers. On the other hand, plastic waste disposal has been recognized worldwide as an environmental problem. Recycled plastics are readily available almost every- where. If one can develop new technologies for the cost-effective utilization of waste plastics with solid wood, it could solve the aforementioned problems of lack of quality wood and plastic waste disposal. Singh et al. 6 and Siau et al. 7 found that impregna- bility of chemicals into wood differs significantly, de- pending on the type of chemical and species of wood. Some chemicals can fill the empty lumens in wood, whereas others may be able to penetrate into the cell walls or react with the wood material. Costanza and Miyara 8 simulated the wood impregnation with vac- uum and pressure procedure, and validated that the impregnation process had a significant impact on chemical loading. Perng 9,10 found that the permeabil- ity of an impregnant in wood is related to its viscosity, and is dependent on wood species. However, up to now, no study has been undertaken to systematically quantify the permeability of melt thermoplastics into major commercial woods in eastern Canada. Such a study will help to assess the feasibility of developing high-performance WPC by impregnating solid wood with thermoplastics. Correspondence to: S. Y. Zhang (tony.zhang@qc.forintek.ca). Contract grant sponsor: Natural Sciences and Engineering Research Council of Canada (NSERC). Journal of Applied Polymer Science, Vol. 102, 1672–1680 (2006) © 2006 Wiley Periodicals, Inc.