Content area
Full Text
Abstract
An extruded wood-polypropylene composite was dried at 103°C for approximately 13 months to investigate long-term drying weight changes. The material was also soaked in both fresh water and seawater until saturation and then redried at room temperature to study water immersion and redrying behaviors due to moisture cycling. The moisture content (MC) was approximately 1.5 percent and 1.1 percent for the face and core positions, respectively, when measured 11 months after the material was manufactured. Saturation MCs were approximately 20 percent and 19 percent when exposed to distilled water and seawater, respectively. The expansion of the material was orthotropic in character and non-recoverable. The volume change was not reversible with about a 2 percent increase after redrying. Meanwhile, the weight was also non-reversible when redried at room temperature at constant relative humidity. The changes in dimensions and weights of the specimens soaked in seawater were slightly smaller than those soaked in distilled water. The results of this study will be helpful in understanding the drying and immersion properties of wood-plastic composites.
Wood-plastic composites (WPCs) are intended for service outdoors where they can be exposed to different temperatures and relative humidity (RH). All composites may desorb water when exposed to lower humidity and/or higher temperature atmospheres and absorb moisture when exposed to humid atmospheres or when immersed in water. Polymers mixed with hydrophilic fillers, such as wood flour, will absorb more water compared to the unfilled polymer (Marcovich et al. 1999). The influence of moisture changes on the mechanical behavior of filled polymer systems can be significant (Lin et al. 2002, Yang et al. 2006, Cheng et al. 2009).
The moisture content (MC) of WPCs can be determined by drying in an oven at 1050C (Wang and Morrell 2004). During drying the wood weight loss is primarily due to loss of moisture, volatile organic compounds (VOCs), and other extraneous materials such as wood extractives. But, unknown weight variations due to differences in volatile losses and thermal degradation can also occur at 1030C (Keey et al. 2000). Furthermore, weight loss could occur at this temperature for pure polymers. For example, oxidized polypropylene (PP) films were found to exhibit a 2.2 percent weight loss after 24 hours at 1000C (Uzomah and Ugbolue 1997). Moisture can penetrate...