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Surface treatments of wood–plastic composites (WPCs) to improve adhesion

In an effort to improve the adhesive bonding between wood–plastic composites (WPCs) formulated with polypropylene and a commercial epoxy adhesive, surface treatments were performed to chemically and/or physically modify the surface of WPCs. The treatments were performed on extruded WPC that had been planed and consisted of chromic acid treatment, flame treatment, water treatment, flame then water treatment and water then flame treatment. The strength of the adhesive bonds of the treated samples was tested following ASTM D 905 and the maximum shear stress was calculated for each treatment. The chromic acid and flame treatments increased their respective average shear strengths by 97% and 67% compared to an untreated control group. The increase in bond strength due to these two treatments is believed to be a result of their oxidative mechanisms. The water treatment, which consisted of covering the planed surface of a WPC with water for 10 min, resulted in an increase in shear strength of 31% relative to the control. Characterization of the water-treated WPC surface with profilometry and scanning electron microscopy indicated that the likely mechanism for the increase in bond strength was the absorption of water and subsequent swelling of the wood present in the WPC, creating greater surface area for bonding. The combination of flame and water treatments showed increased shear strength relative to the individual treatments alone, indicating that the two processes might act synergistically to facilitate the formation of stronger adhesive bonds.