IntroductionWood plastic composites (WPCs) are increasingly used for exterior applications, which can lead to dimensional changes and fungal deterioration. WPCs have wide utilization in building construction and are used in siding and decks, windows, outdoor furniture, and other semistructural products (Mengeloğlu and Karakuş, 2008;Kord and Hosseinihashemi, 2014). However, WPCs are produced from various species of hardwoods, softwoods, and agricultural waste (up to 30%-70% by weight). Thus, WPCs have limited biological durability (Kim et al., 2008;Fabiyi and McDonald, 2010;Xu et al., 2013). Previous studies reported that wood-based panels such as particleboard and fiberboard prepared from durable species such as black locust and oak wood had a high biological durability and could be used in exterior expositions (Kamdem and Sean, 1994;Reinprecht and Zubková, 2010). WPCs for exteriors, with or without ground contact, should be adequately resistant against moisture and wood-destroying basidiomycetes. There is a commercial WPC lumber manufacturer in North America that uses western red cedar, which has biological durability, in the WPC formulation (http://www.choicedek.com/ clasbul2.htm).Currently, the most common preservative used in WPCs in North America is zinc borate due to environmental issues. Zinc borate is applied either as a powder or an emulsion/dispersion into the WPC (Laks, 1999;Badritala et al., 2013). Major problems with preservative chemicals used in WPCs include leachability and toxicity. Badritala et al. (2013) found that the flexural and tensile properties and impact strength of WPCs containing zinc borate were less than those of the untreated WPCs. They reported that these phenomena could be attributed to poor compatibility between the wood and polymer matrix due to the crystalline deposits of zinc borate and the dispersion and precipitation of zinc borate particles in the cavities of WPCs. The formation of agglomeration can cause a reduction in interfacial adhesion between the wood flour and polymer matrix (Ayrilmis et al., 2011(Ayrilmis et al., , 2012a(Ayrilmis et al., , 2012bKurt and Mengeloğlu, 2011).Black locust (Robinia pseudoacacia L.) is an important tree species due to its excellent durability and very good physical and mechanical properties (Stringer and Olson, 1987;Barrett et al., 1990;Molnár, 1995;Adamopoulos and Voulgaridis, 2003). However, some disadvantages of black locust wood such as crooked stems and smaller stem diameter, and irregular heartwood color (from yellow to green-olive hue), were introduced by the Tequbloc project (Inco-Copernicus, 2000). The increased resistance of black locust wood against biological attack is mainly due to its chemical composition (Adamopoulos et al., 2005).
