The effect of surface properties on bond strength of birch, black alder, grey alder and aspen veneers

In this study, the effect of veneer preparation process on surface roughness, wettability and adhesive bond formation between birch (Betula pendula Roth), black alder (Alnus glutinosa L.), grey alder (Alnus incana L.) and aspen (Populus tremula L.) were evaluated. It is commonly known that adhesive bond is as strong as its weakest link. However, the results of present study show, that bond strength is not always directly related to properties of weakest wood material used in bonding. The roughness (Ra) for the birch was (11.13 μm) followed by black alder (13.32 μm), grey alder (17.25 μm) and aspen (23.28 μm). The best wettability was achieved on aspen (0.3°), which was followed by grey alder (5.0°), black alder (10.7°) and birch (43.6°). The results show that bonding lower quality and density wood species with birch does not significantly decrease the bond quality as it could be expected. This study shows that the bonds between birch and low quality and density wood species were significantly stronger than bonds formed solely between these low quality and density wood species. These results also imply that in utilization of low quality wood species by veneer-based industry does not immediately require adapting and changing the current processing parameters according to lower quality wood properties, if they are used in combination with higher density and quality wood.