Polypropylene-Based Wood-Plastic Composites from Recovered and Beetle-Infested Wood: Effect of Pre-Damaged Fibers on Mechanical Performance

The increasing scarcity of conventional feedstocks for WPCs requires alternative resources supporting circular material flows in the wood-based bioeconomy. This study investigated thermomechanical pulp (TMP) fibers from fresh, recovered, and beetle-infested spruce as well as fresh beech as reinforcements for polypropylene-based WPCs. Fiber geometry was characterized before and after injection molding, and composite performance was evaluated with and without a maleic-anhydride-grafted polypropylene coupling agent. After injection molding, the aspect ratio ranged from 7.1 to 7.9, with recovered spruce showing the highest value. The addition of 30 wt.-% TMP fibers increased the tensile modulus from 1.4 GPa for neat polypropylene to as much as 4.7 GPa for the coupled WPCs. Compatibilization markedly improved strength, resulting in tensile strengths of up to 54 MPa and flexural strengths of up to 80 MPa, compared with 33 MPa and 36 MPa for neat polypropylene, respectively. Recovered and beetle-infested spruce did not impair composite performance and achieved the highest tensile and flexural strength values. Beech-based WPCs showed lower tensile and flexural performance than spruce-based composites but still reached a technically relevant property level. Overall, recovered and beetle-infested spruce and fresh beech represent viable TMP raw materials for mechanically demanding WPC applications.