Long-term water absorption and dimensional stability of composites from recycled polypropylene and rubberwood flour

The present study aims to investigate the moisture absorption of polypropylene (PP)/rubberwood flour (RWF) composites and its effects on dimensional stability. The compositions included different grades of plastic, and the amounts of wood flour, maleic anhydride-grafted polypropylene (MAPP), and ultraviolet (UV) stabilizer were varied. The composite materials were manufactured into panels by a twin-screw extruder. Long-term water absorption (WA), long-term thickness swelling (TS), and degradation of flexural properties of the composites were studied for a range of water immersion times. The WA and TS of the samples increased with RWF content and immersion time. Recycled PP gave higher WA and TS than virgin PP, for the composites with 45 wt% RWF. Increasing MAPP content from 3 to 5 wt% had no significant effect on WA and TS, whereas the addition of 1 wt% UV stabilizer increased them. A MAPP content of 3 wt% is recommended for moisture resistance, while the amount of UV stabilizer should be kept as low as possible. Flexural strength and modulus of composites also decreased with moisture uptake; however, < 3% WA did not significantly affect the flexural strength. In contrast, the maximum strain of composites consistently increased with WA.