Surface properties and adhesion of wood fiber reinforced thermoplastic composites

The surface chemistry, wettability and topography of wood fiber reinforced thermoplastic composites were evaluated using Fourier transform infrared spectroscopy, contact angle analysis and profilometry. Wood plastic composites have a low surface energy (31 mJ/m2), a large surface roughness R (rms) (3.5–7.4 μm) and a large wetting hysteresis (63–90 mJ/m2) that stems from surface roughness, chemical heterogeneity and viscoelastic energy dissipation. Using a 180° peel test the adhesion of an acrylic coating on wood plastic composites was evaluated and a positive linear correlation (R2 = 0.89) between peel load and wetting hysteresis indicated that the wetting hysteresis was a good predictor of adhesion. Furthermore, strong linear correlations were found between peel load and surface roughness for formulations using polypropylene (R2 = 0.79) and formulations using high density polyethylene (R2 = 0.97). Formulations with polypropylene, which had higher surface wood index, also developed higher peel loads. Similarly formulations that did not use a maleic anhydride polypropylene coupling agent, which had higher surface roughness and have higher molecular mobility, also developed higher peel loads. Consequently surface roughness, polarity and viscoelastic energy dissipation mechanisms were proposed to be critical factors of adhesion on wood–plastic composites explaining the parallel between wetting hysteresis and peel load.