Synthesis and Characterization of Orange-Peel-Modified Particleboards Glued with Tannin-Based Resins

The increasing demand for sustainable materials is driving research into the valorization of agro-industrial waste as lignocellulosic alternatives in engineered wood product manufacturing. This study investigates the use of bitter orange (Citrus aurantium L.) peel as a bio-based filler in particleboards bonded with a bio-based tannin/hexamine adhesive. Panels were fabricated by partially substituting wood chips with orange peel powder at five levels (0, 2.5, 5, 10, and 15 wt%), hot-pressed at 180 °C for 8 min at a constant biomass-to-adhesive ratio of 9:1 (w/w). Firstly, the effect of temperature on orange peels and then the microscopic structure of the composite was observed through optical stereomicroscopy and SEM. Then, mechanical performance, water resistance, and thermal conductivity were also analyzed. Substitution of up to 10 wt% of wood chips with orange peel allowed for the maintenance of internal bond strength, modulus of rupture, and modulus of elasticity, while the 2.5 wt% formulation yielded a statistically significant improvement in axial thermal insulation. Conversely, when the amount of orange waste rises over 10 wt%, significant decreases in internal bond and modulus of rupture were recorded. These findings demonstrate that C. aurantium peels can be used as a viable filler for formaldehyde-free particleboards, offering a promising strategy for orange waste valorization in the wood panel industry.