The effect of pH on surface activation of wood polymer composites (WPCs) with hydrogen peroxide for improved adhesion

Abstract

Wood Polymer Composites (WPCs) have become widely acceptable for use in different products, including furniture. This range of applications could be increased by improvement in effective bonding and jointing techniques. However adhesion between WPC components is problematic because of their low surface energy and the hydrophobic nature of the polyolefin matrices which are commonly used. Surface activation pretreatments using hydrogen peroxide solutions have been investigated, and shown an improvement in lap-joint shear when bonding using epoxy adhesives. The effect on surface energy, roughness and chemical composition was studied. The improvement in adhesion appeared to be governed by the pH of the treatment solution, with clear differences being identified between treatments in acidic and alkaline media.The best results were achieved with a mild alkaline hydrogen peroxide treatment at a pH of 7.5 (37% improvement); however a mild acidic hydrogen peroxide treatment also showed a suitable level of adhesion improvement (29%, pH 6). In the alkaline medium it was found that the hydroperoxy anion was efficient in altering both the lignin and the hemicellulose component of the wood flour. It was necessary to limit the extent of wood particle degradation by the hydroperoxy anion, explaining the poor adhesion for treatments with pH 9 peroxide solution. This study indicates that mild (pH 7.5) alkaline treatment solution could be employed. In the acidic medium, the free radical based reaction processes dominated, and were likely to have altered the polypropylene matrix of the WPC material.