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The purpose of the current research was to investigate water/moisture absorption and associated thickness swelling and their rates in hybrid wood flour/E-glass fiber/propylene composite immersed in water, exposed to steam, and the influence of application of the glass fiber as well as maleic-anhydride-polypropylene (MAPP 2%) as coupling agent on those properties. The sample strips with 10 mm thickness and 70 mm width were made by using a laboratory twin-screw extruder. The specimens were immersed in water and exposed to steam for 4320 min to determine the water/moisture absorption and the thickness swelling as well as their rates in the composites. Results revealed that application of the glass fibers increased in the water/moisture absorption significantly in the hybrid composites due to formed micro-gaps in interfaces between wood flour and the glass fibers. However, application of the coupling agent (MAPP 2%) significantly reduced the micro-gaps and diminished the water/moisture absorption as well as the related thickness swelling in the hybrid composites.

In this study, creep/recovery behavior of wood flour/polypropylene (PP) composite was compared with solid wood and PP. Wood flour–PP composite samples (strips with 10 mm thickness and 70 mm width) were made using a laboratory twin-screw extruder. Short-term flexural creep tests at 30% of ultimate bending load were performed using flexural creep equipment. Total time to complete every test was 120 min (60 min creep and 60 min recovery). Creep parameters (relative creep, fractional deflection, creep rate, etc.) were measured for all creep test samples. The result showed that the relative creep in wood–plastic composite (WPCs, with or without coupling agent) is significantly higher than solid wood and lower than PP. Also, it was observed that the creep rate in WPC (with or without coupling agent) is significantly lower than plastic, but is approximately equal to solid wood. The use of coupling agent decreased relative creep and fractional deflection in wood flour–PP composite panels. Findley creep model predicted creep strain with a high acceptable accuracy for studied materials.

The influence of highly degraded high-density polyethylene (HDPE) on physical, rheological, and mechanical properties of HDPE-wood flour composites was studied. For this purpose, the virgin HDPE was subjected to accelerated weathering under controlled conditions for 200 and 400 h. The virgin and exposed HDPE and pine wood flour were compounded to produce wood flour-HFPE composites. The results showed that the accelerated weathering highly degraded HDPE. Degradation created polar functional carbonyl groups and also produced extensive cross-linking in HDPE and consequently poor processibility. The interruptions in the flow characteristics of the degraded HDPE potentially caused processing hurdles when using them for extrusion or injection molding manufacturing as only small part (10%) of virgin HDPE could be replaced by highly degraded HDPE for wood flour-HDPE composite manufacturing. The mechanical properties of composites containing highly degraded HDPEs were similar to the composites with virgin HDPE and in some cases they exhibited superior properties, with the exception being with the impact strength.

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