Plasma Treatment of Polypropylene-Based Wood-Plastic Composites (WPC): Influences of Working Gas

In this study, a polypropylene (PP)-based wood-plastic composite with maleic anhydride-grafted polypropylene (MAPP) as a coupling agent and a wood content of 60% was extruded and specimens were injection molded. The samples were plasma treated utilizing a dielectric barrier discharge (DBD) setup with three different working gases: Ar/O₂ (90%/10%), Ar/N₂ (90%/10%), and synthetic air. This process aims to improve the coating and gluing properties of the otherwise challenging apolar surface of PP based wood-plastic composites (WPC). Chemical analysis with X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) showed the formation of oxygen-based functional groups on the surface, independently from the working gas used for the treatment. Laser scanning microscopy (LSM) examined the surface roughness and revealed that the two argon-containing working gases roughened the surface more than synthetic air. However, the contact angle for water was reduced significantly after treatment, revealing measurement artifacts for water and diiodomethane due to the severe changes in surface morphology. The adhesion of acrylic dispersion coating was significantly increased, resulting in a pull-off strength of approximately 4 N/mm², and cross-cut tests assigned the best adhesion class (0), on a scale from 0 to 5, after plasma treatment with any working gas.