Aerobic Composting Biodegradability of Wood–Plastic Composites Made from Recycled HDPE

A controlled composting biodegradation system was implemented to evaluate a wood–plastic composite (WPC) composed of wood fibers and recycled HDPE (rHDPE), in accordance with ASTM D5338, by measuring CO2 capture over 45 days. This evaluation was complemented with mechanical and physicochemical characterization, including stereomicroscopy/SEM, mass loss, water absorption, contact angle, tensile strength, FTIR, TGA, and DSC. The results showed 6.12% biodegradation, classifying the material as neither biodegradable nor compostable. SEM analysis revealed increased surface roughness, cracks, and microbial-like structures, together with a 10% decrease in contact angle. The mechanical properties declined by 33% (tensile strength), despite only 1.26% mass loss, which was attributed to weakening of the matrix–fiber interfacial adhesion due to water absorption. TGA, DSC, and FTIR supported the interpretation that degradation occurred preferentially in the wood fibers.