Adhesive Characterization of Acacia mangium Bark Extract and Its Effects on Properties and Formaldehyde Emissions of Rubberwood Particleboard

This study evaluated the feasibility of using Acacia mangium bark extract (BE) as a bio-based, formaldehyde-free adhesive for rubberwood particleboard, focusing on the effects of two catalysts: sodium hydroxide (alkaline) and boric acid (acidic). Gel time, viscosity, thermal behavior, mechanical performance, and formaldehyde emissions were assessed. The uncatalyzed BE adhesive showed a gel time of 29.59 min, which decreased to 6.44 min with sodium hydroxide (BE–NaOH adhesive) and 4.22 min with boric acid (BE–Boric adhesive), highlighting the catalytic effect. Differential scanning calorimetry revealed higher curing enthalpy for BE–NaOH adhesive, while thermogravimetric analysis indicated enhanced thermal stability under alkaline conditions. The viscosity of BE–NaOH adhesive was comparable to commercial urea formaldehyde (UF) adhesive. Particleboards (density 0.65 g/cm3) bonded with BE–NaOH, BE, or BE–Boric adhesive met the JIS A 5908:2015 Type 8 internal bond (IB ≥ 0.15 N/mm2) requirement. Boards bonded with BE achieved a modulus of rupture of 8.25 N/mm2, meeting the Type 8 threshold (≥ 8 N/mm2), while those with BE–NaOH adhesive showed 7.61 N/mm2, slightly below it. Formaldehyde emissions were very low (0.006–0.058 mg/L), achieving the F**** rating under JIS A 5908:2015. However, high thickness swelling and water absorption indicated limited water resistance, and overall mechanical properties remained lower than with UF/melamine urea formaldehyde (MUF) adhesives. Direct mechanical performance comparison with UF/MUF was not possible due to the lack of molar ratio data. These findings suggest BE adhesives are a promising sustainable alternative, though further improvements in water resistance and mechanical performance are needed.