Physico-mechanical properties and long-term creep behavior of wood-plastic composites for construction materials: Effect of water immersion times

The optimal formulation described by Srivabut C. et al. (2019) of 51.8 wt% rPP, 35.9 wt% RWF, 7.2 wt% CC, 3.9 wt% MAPP, 0.2 wt% UV stabilizer, and 1.0 wt% Lub was used in the study, it was shown to possess the best mechanical and physical properties for wood-plastic composites (WPCs). This formulation was used to investigate the mechanical and physical properties and effects of stress, temperature, and time on flexural creep with 35% of the ultimate flexural strength, after water immersion for various times. In addition, the time-stress superposition (TSS) principle was used to predict the lifetime for WPCs. The results showed that the mechanical properties generally decreased up to 40% with immersion time in which samples absorb water quickly in the first week, after which these absorption increases more slowly until the saturation point. This is most likely due to the increase of free OH groups in the composites and the long-term absorption, resulting the decrease of mechanical resistance of the WPCs. All of the WPC specimens experienced increased lightness and total color change, indicative of fading or lightening. The L* of the control sample had lower lightness than WPCs immersed for 1, 5, and 10 weeks. Also, the ∆E* in all cases increased with immersion time until 10 weeks. Almost no color change was observed in first week of immersion, after which it gradually increased. The creep behavior of WPCs was dependent on stress, temperature, and water immersion time, increasing with all of these. The master curve from time-stress superposition principle was in a good agreement with R2 of 87.45% for long-term creep. It was found that the lifetime predictions of WPC products exceeded 10 years for 3 MPa stress at 25 °C (control sample). However, the WPCs after 10 weeks of water immersion showed poorer performance, with lifetimes of the composite products under 3 and 15 MPa stress at 25 °C estimated to not reach 5 years.