Microstuctural Changes in Wood-Plastic Composites (WPC) Due to Extended Moisture Cycling and its Relationship to Mechanical Performance Changes

The use of wood plastic composite (WPC) materials has been popular as a durable material for applications such as residential decking. For shoreline or ground contact installations, WPCs may be exposed to water for extended periods. Therefore water cycling is an important factor that may affect the properties of WPCs. In this study, a WPC, specifically extruded wood polypropylene composite, was studied. The moisture content, difhsion dynamics, bulk mechanical properties, and microstructural changes in the material subjected to soaking-drying cycles were investigated. The microstructure of the material in dry, wet and redried states was examined in three dimensions using an Xray microtomography (XMT). The uni-dimensional (ID) moisture diffusion coefficient (Dm) of thin specimens exposed to a high humidity environment was about 8xl0-~cm 2/sec. It needs at least 660 days in osmotic (fresh) water for this WPC of 3.25cm in thickness to be saturated and more in seawater because the D,, was about 1.5 times larger in osmotic water than in seawater. For ID, 2D and 3D diffusion tests soaked in water, the Fickian\u27s models did not fit the test data well and the diffusion behaviors may be non-Fickian7s for both types of water. It was found that water immersion reduced the flexural, tensile and compressive properties of the composite, but not impact resistance strength. After about 13 months immersion, there were significant differences (alpha=0.05) in flexural strength and MOE soaked in osmotic water among dry, wet and redried treatments, but no significant differences in flexural strength and some for MOE in seawater were found. There was no significant effect of water type on the ultimate tensile strength. However, there was a significant effect of water type on tensile MOE. The ultimate compressive strength and chord MOE decreased after 13 months of soaking and increased somewhat after redried for about seven weeks for both types of water. Most mechanical properties except the flexural MOR had both the existence of non-reversible damage and a recoverable portion, presumably related to wood modulus change from dry to wet and then redried. XMT was used to study the microstructural changes in dry, wet and redried states to detect the relationship between microstructural changes and the mechanical property changes. The smallest volume of 1513 pixels cube may be used to present the specimen using medians and means. There were some differences for both median and mean in three states. The cumulative distribution function was used to analyze voids in these three states assumed 230-255 intensity values as void and a 713 pixels cube was used to analyze the void distributions in the three states. The void volume increased from dry to redry and the cube position chosen affected the results greatly