Holzforschung, Vol. 58, pp. 205–208, 2004 • Copyright  by Walter de Gruyter • Berlin • New York 02003 Wood/plastic ratio: Effect on performance of borate biocides against a brown rot fungus John Simonsen*, Camille M. Freitag, Antonio Silva and Jeffrey J. Morrell Department of Wood Science and Engineering, Oregon State University, Corvallis, Oregon, USA *Corresponding author: Department of Wood Science and Engineering, Oregon State University, Corvallis, Oregon 97331, USA; Fax: q1-541-737-3385 E-mail: john.simonsen@oregonstate.edu Abstract The effect of wood/plastic ratio and the presence of a boron compound on resistance to biodegradation of wood plastic composites (WPC’s) by the brown rot fun- gus Gloeophyllum trabeum was investigated in a soil block exposure. Weight losses of all WPC’s were gen- erally lower than those of solid wood, even when only the wood component of the WPC was used in calculating weight loss. Higher wood content was associated with greater weight losses, suggesting that the plastic encap- sulated wood at lower wood levels. Borates markedly reduced weight losses at all wood/plastic ratios. Weight losses tended to be slightly lower with a Na/Ca borate than with similar levels of zinc borate. Mechanical prop- erties did not correlate well with weight losses under the conditions evaluated, but these effects may have been masked by moisture sorption. The causes and implica- tions of these differences are discussed. Keywords: borate; brown rot; Gloeophyllum trabeum; wood/plastic composites. Introduction The use of wood plastic composites (WPC’s) continues to grow as producers seek to blend the unique material properties of these two polymers (Sellers et al. 2000). Recently, the use of WPC’s as a substitute for wood decking has been increasing (Wolcott and Englund 1999). The decking market has long been dominated by west- ern red cedar, coastal redwood and pressure-treated wood. While these materials provide excellent perform- ance, they require some level of maintenance by the homeowner. WPC’s have been marketed for these appli- cations as decay-resistant materials that require little or no maintenance. The plastic component in the original WPC’s was pre- sumed to protect the wood against biological attack, but several reports have suggested that the wood remains susceptible to fungal degradation despite its close asso- ciation with the plastic (Schmidt 1993; Naghipour 1996; Morris and Cooper 1998; Verhey et al. 2001). These stud- ies have shown that the wood component sorbs water and eventually decays, albeit more slowly initially than does solid wood of the same species. The industry has responded by incorporating zinc borate (Zn B) into the WPC. Borates are excellent fungicides and insecticides, but the primary borate used for wood preservation, sodi- um octaborate tetrahydrate, is highly water soluble and tends to leach from wet wood (Murphy et al. 1995). Zinc borate (Zn B) is far less soluble, and field tests suggest that composites incorporating this material perform rea- sonably well under harsh sub-tropical conditions. While these results are promising, there are relatively few data on the ability of Zn B and other borate compounds to protect the wood component in a WPC against fungal attack, nor is there a substantial literature on the effects of other variables, such as wood/plastic ratio, on the per- formance of these systems. In this report, we describe the ability of zinc borate and a sodium/calcium borate salt (Na/Ca B) to protect WPC’s containing various ratios of wood and plastic against fun- gal attack in a soil block test. Materials and methods Materials Wood Wood flour (pine wood flour ground to pass a 60-mesh screen) was supplied by Professor M. Wolcott, Wash- ington State University, Pullman. Plastics Injection grade polypropylene (PP) (Fortilene HB 1602) was donated by Solvay Polymers, Houston, TX, USA. High-den- sity polyethylene (HDPE) was contributed by Phillips Petrochem- ical Company (Houston, TX, USA) as Marlex EHM 6007. The molecular weight was 120,000 (by gel permeation chromatog- raphy); the density, 0.964; and the melt index, 0.65 g 10 min y1 (1908C 2 kg y1 ). Preservatives Zinc borate (Zn B) was donated by U.S. Borax Inc., Valencia, CA, USA, as their product Borogard B. A blend of calcium and sodium borates (Na/Ca B) was donated by the Quality Borate Co., Cleveland, OH, USA, as their product QB- 13. Composite preparation The test materials were prepared in a Brabender Plasticorder with mixing bowl and cam blades attached. The temperature was controlled at 1908C. The components of each sample were added to the mixing bowl in the order plastic, filler (wood flour) and preservative. The wood/plastic ratios were 0:100, 20:80, 40:60, and 60:40 (wt wt y1 ). The preservatives were added to the mixture as powders at 0.0, 0.5 or 1.0% (wt chemical wt wood y1 ). The mixture was blended for at least 10 min after addi- tion of all the components to ensure thorough mixing. The sam- ples were then removed from the mixing bowl, cooled, and ground to a coarse powder in a Wiley mill before compression