Wood-Polymer Composites

Abstract

Wood has been an essential material for human survival since the primitive state, for its wide abundance, renewable and environmentally benign nature, relative ease of working it, and outstanding mechanical properties. With the development of technology, wood came to be used for shelter, fuel, tools, boats, vehicles, bridges, furniture, engineering materials, weapons, and even raw materials for energy (Li et al., 2011a). Now, wood is widely used in various corners of human life. Wood possesses porous structures consisting of various cell walls, which are mainly composed of biopolymers, i.e., carbohydrate polymers of cellulose and hemicelluloses and phenolic polymers of lignin. The cellular structure of wood endows it with high strength-toweight ratio. Because of this, some high-quality wood can be used as structural materials. However, wood components are easy to be degraded by microorganisms, and susceptible to damage by fire (Fuller et al., 1997). Besides, as the most abundant functional group in wood cell walls is the hydroxyl group, the hygroscopic character of these groups can render wood with poor dimensional stability. In other words, wood will shrink as it dries, while conversely swell when it is wetted (Alfered, 1977). All these disadvantages limit the application of wood as high-quality materials. Furthermore, with the development of society, the consumption of wood has been rapidly increasing year by year. In contrast, however, the production of high-quality wood has been fleetly decreasing. The prominent contradiction has driven researchers to look for alternative low-quality resources for value-added applications. To achieve these goals, suitable technologies are needed to improve low-quality resources (especially specific wood quality) attributes (e.g., mechanical properties, dimensional stability, decay resistance and thermal stability) in order to meet end-use requirements (Wang et al. 2007; Zhang et al. 2006). As the above unfavorable behaviors of wood are fundamentally ascribed to the presence of numerous hydroxyl groups (reactive sites) in the wood major components and various cell cavities (major paths for moisture movement) within wood (Couturier et al. 1996; Yildiz et al. 2005), blocking these reactive sites or plugging the cavities could not only make the wood more resistant to moisture, but also improve its dimensional stability and physical as well as biodegradation properties. Consequently, treatment of wood to modify its structure and thus improve its physical and mechanical properties, as well as durability, has been carried out via chemical modification, chemical impregnation, compression during heating, and heating at high temperature (Handa et al.1976).