Forced Air Plasma Treatment (FAPT) of Hybrid Wood Plastic Composite (WPC)–Fiber Reinforced Plastic (FRP) Surfaces

Forced atmospheric (air) plasma treatment (FAPT) was applied to wood plastic composite (WPC) and continuous glass fiber reinforced plastic (FRP) surfaces to improve their adhesive bonding properties. The FRP was composed of oriented continuous E-glass fibers in a polypropylene matrix, while the WPC was fabricated using wood flour, polypropylene and additives. The FAPT was applied using two levels of discharge length projected from the discharge head (2.5″ and 1″) to ionize the air, oxidize the surfaces and improve wettability. The treatment was performed by passing the electrode over either surface, five or ten times. Surface characterization consisted of thermodynamic (surface energy determination), chemical (X-ray photoelectron spectroscopy), mechanical (shear strength) and microscopic (atomic force microscopy (AFM)) analysis. The results indicate that the acid–base component of the surface energy for both WPC and FRP after FAPT correlates with an increase in wettability. X-ray photoelectron spectroscopy was performed on wood regions and non-wood regions of the WPC surfaces; the oxygen concentration increased to a larger extent in the non-wood regions. Bonding shear strength measurements indicated increases of 50% after FAPT on WPC surfaces (2.5″ discharge length, 1 pass) and up to 200% for the hybrid WPC–FRP. Atomic force microscopy measurements using a silicon tip probe showed increases in adhesive force interactions up to 56% on WPC surfaces post-FAPT.