Effects of maleated polypropylene content on the extended creep behavior of wood‒polypropylene composites using the stepped isothermal method and the stepped isostress method

This study investigated the effectiveness of maleated polypropylene (MAPP) as a coupling agent for improving the physico-mechanical properties and creep resistances of wood‒polypropylene composites (WPCs). The results revealed that the composites with MAPP showed significantly decreased water absorption and thickness swelling, while the flexural properties and the wood screw holding strength increased as the MAPP content increased up to 3 wt%. Additionally, flexural creep tests were conducted at a series of elevated temperatures and stresses using the stepped isothermal method (SIM) and the stepped isostress method (SSM), respectively. The SSM-predicted creep compliance curves fit better with the experimental data than the SIM-predicted curves. On the other hand, the creep master curves for all of the WPCs were constructed from different SSM test conditions and were highly consistent with the long-term experimental creep data. The creep resistance values of the composites with MAPP were greater than those without MAPP, especially for the WPC with 5 wt% MAPP (WPCM5), due to the improvement in the interfacial compatibility between the wood fibers and PP matrix. Furthermore, according to the SSM-predicted creep behavior, the improvement in creep resistance (ICR) of WPCM5 reached 61% over a 20-year period compared to the WPC without MAPP.