Effects of Heat Treatment, Zn3BO6 Addition, and their Combination on the Mechanical Properties and Water Absorption Behavior of Beech Wood/Polypropylene Composites

Abstract

Individual and combined effects of thermal treatment and zinc borate (ZB) addition were studied relative to the mechanical properties and water resistance of oriental beech (Fagus orientalis Lipsky) wood-plastic composites (WPCs). Wood flour was thermally treated at 150 °C for 120 min under saturated steam before composite manufacturing. Flexural, tensile, impact, and water resistance properties were evaluated, and the data were analyzed using two-way ANOVA (p < 0.05). Thermal treatment increased the flexural strength from 57.2 MPa to 75.0 MPa, while the combined treatment produced the highest impact strength (7.56 J/m). Zinc borate-containing composites exhibited lower maximum water absorption (2.8 to 3.0%) than the control (8.6%), despite showing higher moisture diffusion coefficients. This indicated that the diffusion rate and equilibrium moisture uptake were driven by different mechanisms. SEM observations revealed that the combined treatment produced a more cohesive fracture morphology, although these changes were not accompanied by improved flexural or tensile strength. Overall, thermal treatment and zinc borate affected the performance characteristics through distinct mechanisms, providing an effective approach for improving the moisture resistance and durability of wood-plastic composites.