Plasma-assisted enhancement of adhesion strength between alder veneer and thermoplastic LDPE film in veneer-based composites

Abstract

The inherently weak adhesion between hydrophobic thermoplastics and hydrophilic wood remains a key limitation in thermoplastic‑bonded wood composites. This study investigates atmospheric air plasma treatment as a dry and scalable surface activation method to improve bonding between a low-density polyethylene (LDPE) film and alder veneer in plywood composites. To ensure a reliable basis for evaluating plasma effects, the thermoplastic-bonded plywood fabrication protocol was first optimised within this study. Plasma activation significantly improved interfacial properties, leading to enhanced mechanical performance of the composites. The modulus of rupture and the modulus of elasticity increased by up to 16% and 19%, respectively, while the wet shear strength improved by approximately 23%, allowing all treated panels to meet the requirements of EN 314 2. Surface analysis revealed a substantial increase in surface free energy (approximately twofold for LDPE and nearly threefold for veneer), accompanied by the introduction of oxygen‑containing functional groups (–OH, –C = O, –COOH). Microscopic observations confirmed an improved bondline integrity, with reduced delamination after water exposure. These findings demonstrate that plasma treatment effectively strengthens the wood-thermoplastic interfaces without chemical additives, offering a promising route for producing high-performance formaldehyde-free plywood.