Effects of surface characteristics of wood on bonding performance of low-molar ratio urea–formaldehyde resin

Low-molar ratio UF resin was used in bonding veneers of five wood species. Significantly different bonding strengths were observed among the used wood species. Much higher bonding strengths were found for cherry, sapele mahogany and black walnut, which would be attributed to their natural roughness that allows formation of more mechanical interlockings. After 60s Gliding Arc Discharge (GAD) plasma treatment of wood surface, the wet bonding performance was significantly improved for most of the wood species. The SEM images of wood surface indicated that the roughening, micrometer-scale peeling or etching effects of GAD plasma treatment would be related to the improved bonding performance. XPS characterizations showed that plasma treatment resulted in remarkable increase of O/C ratio for all wood species. Detailed analysis of C1s spectra revealed that drastic oxidation induced by plasma generated high content of oxygen-containing groups, while degradation led to exposure of more oxygen-containing components like cellulose and hemicellulose. In contrast to untreated surface, stronger interactions can be formed between highly polar wood surface and UF resin. GAD plasma appeared to be an efficient tool in both physical and chemical modifications of wood surface.