Characterization of the property changes of extruded wood–plastic composites during year round subtropical weathering

• Increased degradation rates were characterized in subtropical outdoor WPCs.
• Color shaded at the first 90 days and cracking occurred after 90 or 180 days.
• Carbonyl indexes increased continuously to 50–100% in 1 year.
• Morphology of fracture areas confirmed with the experiment results.

Wood–plastic composites (WPCs) are an emerging material which has been widely applied in outdoor application, but their long-term decomposition behaviors under subtropical weather have not been completely characterized. In this study, WPCs manufactured using recycled wood flour and polypropylene resin were tested for the changes in surface characteristics, physical properties, and mechanical strengths after different time of weathering exposure for one year. Color differential tests showed that significant increase in lightness occurred at the first 90 days of weathering exposure, while notable cracking and mass loss of the components occurred after 90 or 180 days. Carbonyl indexes increased continuously during the testing period for all the tested WPCs. Moisture contents increased most significantly at the initial 90 days of the weathering tests and reached steady states afterward. Increases of water absorption and thickness swelling, and decreases in mechanical properties, were not dramatic except for the samples with excessive-dosed wood contents (i.e. 60% wood flour). The degradation rates of all the parameters were higher than the literature values, which is most likely due to higher sunlight intensity and heavier rainfall in the tested subtropical regions. Changes in morphology of the WPCs surfaces and fracture areas confirmed with the experiment results, and more “pulled-out” fibers were observed for the WPCs after 1 year weathering exposure.