Surface Layer of pMDI-Bonded Bamboo Particleboard with Soy Protein Adhesive: Mechanical Performance, Surface Characterization, and Pore Structure

Surface damage during machining can limit the finishing quality of bamboo particleboard bonded with highly reactive isocyanate adhesives. Rather than replacing pMDI throughout the panel, this study evaluated soy protein adhesive as a surface-layer modifier. Three formulations were compared, including 2.2% pMDI with 4% soy protein adhesive solids (G1), 1.1% pMDI with 4% soy protein adhesive solids (G2), and only 2.2% pMDI (G3). FTIR showed spectral changes after curing in the amide and C-O/C-O-C regions. DSC revealed a thermal event at approximately 115–118 °C, which may reflect contributions from multiple overlapping processes. G1 had the highest MOR (18.54 ± 1.95 MPa), internal bond strength (0.80 ± 0.04 MPa), and surface bond strength (1.59 ± 0.23 MPa). The 24 h thickness swelling was 3.79 ± 0.25%, and the surface bond strength was approximately 56% higher than that of G3. In the G1-G3 cut-edge comparison, Ra was 13.8 μm for G1 and 30.3 μm for G3. Micro-CT gave mean pore volume fractions of 15.15%, 16.23%, and 23.76% for G1, G2, and G3, respectively. Overall, the utilization of soy protein adhesive in the surface layers improved surface bond strength and cut-edge roughness. These findings provide a promising opportunity toward high-quality bamboo particleboard manufacturing.