Post-Fabrication Lamination with PP and PET Films for Improved Mechanical Performance of Injection-Molded Wood Fiber/PP Composites

This study investigates the effect of polymer film lamination on the tensile performance of wood fiber-reinforced polypropylene (WP) composites. Neat polypropylene (PP) and WP containing 25 wt% wood fiber were injection-molded and laminated with 0.1 mm PP or polyethylene terephthalate (PET) films using a compatible adhesive. Four configurations were examined: unlaminated (0S), single-sided half-length (1S-H), single-sided full-length (1S-F), and double-sided full-length (2S-F). Mechanical properties and fracture morphology were characterized by uniaxial tensile tests and scanning electron microscopy (SEM), alongside measurements of surface roughness. PET lamination produced the greatest strength enhancements, with 2S-F specimens achieving gains of 12% for PP and 21% for WP, whereas PP lamination gave minimal or negative effects, except for a 5% increase in WP. Strength improvements were attributed to surface smoothing and suppression of crack initiation, as confirmed by roughness measurements and SEM observations. PET’s higher stiffness and strength accounted for its superior reinforcement relative to PP. Fractographic analysis revealed flat regions near specimen corners—interpreted as crack initiation sites—indicating that lamination delayed crack propagation. The results demonstrate that PET film lamination is an effective and practical post-processing strategy for enhancing the mechanical performance of wood–plastic composites.