Enhancement of strength of adhesive bond between wood and metal using atmospheric plasma treatment

Abstract Combinations of wood and metal are interesting hybrid composite materials, joining together the low density of wood with the stiffness and strength of metals. Different types of adhesives are used to connect wood and metal elements, but the compatibility between adhesives used and load-bearing materials must be sufficient, which often is challenging. In adhesive bonding technology, surface treatments are a crucial step in the process. In this study, an atmospheric plasma discharge was employed to enhance the adhesion strength of joints between common beech (Fagus sylvatica L.) wood, metals (steel and aluminum alloy), and four different types of adhesives. The optical properties of plasma discharges and its influence on treated substrates’ surface morphology depended on the inherent properties of the treated materials. X-ray photoelectron spectroscopy revealed the surface oxidation of all the materials after plasma treatment. Consequently, the surface free energy of all materials increased as well. The positive effect of the plasma treatment on the tensile shear strength of single-lap joints shows a high potential of atmospheric plasma treatment technology for enhancement of adhesives strength of joints combining wooden elements, wood and steel, or wood and aluminum alloys. In addition to that, expensive epoxy and polyurethane adhesives could be replaced by more affordable polyvinyl acetate and melamine-ureaformaldehyde adhesives, and still perform at equal levels if the plasma was applied prior to bonding.The authors acknowledge the financial support from the Slovenian Research Agency (Research Programmes Funding No. P4-0015, "Wood and lignocellulose composites" and No. P2-0082, "Thin-film structures and plasma surface engineering"). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 745936. The help of Tatjana Filipič from Jožef Stefan Institute for XPS analysis is gratefully acknowledged