# Surface Quality of Wood Plastic Composites as Function of Water Exposure
## 1 Introduction  
Wood plastic composite (WPC) is an engineered panel or lumber product manufactured from recycled plastic and fine wood particles or fibers. In the wood-based composite industry, WPC is a relatively new product as compared to traditional composites, such as particleboard and fiberboard. Wood plastic composites are extensively used for many applications, including exterior furniture, docking, playground equipment, fencing, and roofing [1; 2; 3; 4]. The wood plastic composite market in the US and Canada reached approximately USD 1.0 billion in sales in 2006 [5; 6; 7]. It is expected that the global wood plastic composite market will grow by around 13.2 % over the next decade, reaching approximately USD 9.7 billion by 2025 [8]. Pressure treated wood, specifically chromated copper arsenate (CCA), has been banned in the USA as of 2002 regarding its use where there is a direct human contact. Chemically treated wood products, such as decking material, have been losing market share due to health concerns, as well as due to environmental pollution. Such parameters force the exterior composite industry to develop new engineering products, including WPCs, within the perspective of sustainability. A typical WPC unit can be manufactured in a two-step process. The combination of wood material and thermoplastic, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), or polyvinyl chloride (PVC), is mixed together, resulting in compounding, which is a dough-like consistency [3; 4; 5]. At a commercial scale, a continuous production process is preferred due to its efficiencyas compared to that of a batch process [8; 9; 10; 11]. Wood particles with sizes ranging from 20 mesh to 60 mesh are a typical raw material, in addition to foaming, coupling agents, and dyes added into the mixture to enhance the overall properties of the final product [5; 12; 13]. The majority of WPCs are produced by an extrusion process which employs single or double screws to form the final shape of the product [14]. Eastern redcedar (_Juniperus virginiana_ L.) is one of the invasive species in Oklahoma, USA, creating a significant ecological problem with a substantial financial burden to the economy [15; 16]. In one study, it was found that 3.5 million and 6 million acres were invaded by eastern redcedar in Oklahoma in 1985 and 1994, respectively [16]. The magnitude of the eastern redcedar invasion in Oklahoma reached 12.6 million acres in 2013, which is 28% of the Oklahoma landscape area [17; 18]. It is also estimated by the Natural Resources Conservation Service (NRCS) that the eastern redcedar invasion has been increasing at a rate of around 425 hectares per day, covering over 150,000 hectares within a year and causing a cost of approximately USD47 million to the state [17; 19]. Unfortunately, the majority of the trees are not suitable for high quality lumber manufacturing, as prescribed fire in the field is one of the approaches to control the redcedar invasion [20]. In several previous studies, experimental particleboard and strand-type panels were manufactured in different configurations, namely single-layer and three-layer panels [18; 20]. Both the physical and mechanical properties of such products were found to be comparable to those of other wood species. In a previous study, WPCs using eastern redcedar particles and recycled plastic were also manufactured and their mechanical properties were evaluated [21]. In the same study, the distribution and interaction between wood particles and added clay was investigated and it was found that the penetration of clay into wood was validated by vibrational spectroscopy [21]. It is a fact that the majority of WPCs are used under environmental conditions exposing them to rain, snow, and overall harsh conditions. As is known, wood is a hygroscopic material, absorbing and desorbing moisture from its surrounding environment, resulting in dimensional instability.