The tendency of wood-plastic composites (WPC) to absorb moisture is very important in processing and also in mechanical properties, appearance, and dimensions. Swelling of natural fibers can lead to cracking of the material, degradation, and poor mechanical properties. The potential effect of water absorption on WPCs was discussed based on known results from a long experience in testing Enviroshake engineered roofing material. This presentation provided data obtained in the laboratory as well as in the field. The effect on composite properties of moisture content (MC) changes due to water absorption depended on the type of exposure (by immersion in water or by exposure to high humidity), shape of the part, and inherent properties of the material. With non-homogeneous materials such as Enviroshake, the rate of water absorption may be widely different through each edge and surface. An in-house method based on ASTM D570 was employed to study the effects of exposure to water or humid conditions for the WPC material. To verify the acceptability of the roofing product characteristics and process capability, some daily quality control test results were analyzed. Water absorption (24 hr.) average was 1.12 percent, while the maximum admissible absorption is 3 percent; values were between 0.50 and 2.17 percent. All of the data (approximately 5,500) have been summarized graphically. None of the samples showed any signs of cracking, spalling, or other forms of deterioration. But, the frequency and amplitude are not uniform. The curve has a random aspect, which makes it difficult to do estimations or find an empirical formula.
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