Evaluation of bondability of different wood species using adhesives for wood-based panels

Due to climate change, European forests are expected to shift towards mixed stands, reducing softwood availability. As the wood-based panel industry relies heavily on softwoods, changes in raw material supply may affect adhesive bonding—an essential factor in panel production. This study investigates the bondability of eight wood species (beech, birch, oak, poplar, Douglas fir, larch, pine and spruce) using four adhesives commonly applied in panel manufacturing: polymeric methylene diphenyl diisocyanate (pMDI), modified MDI and two melamine-urea-formaldehyde (MUF) adhesives. Bond strength development was evaluated using the Automated Bonding Evaluation System (ABES). Results were modelled with the Hill function to extract kinetic parameters, and microscopy was used to assess adhesive distribution and bond line failure. Among the adhesives, pMDI showed the highest sensitivity to species variation, while modified MDI was less species dependent. MUF1 showed the most consistent performance and outperformed MUF2 in bond strength development and final strength, likely due to its higher formaldehyde-to-amino group ratio. Beech bonded with pMDI required increased pressing pressure, while larch exhibited the slowest strength development with all adhesives except MUF1. pMDI achieved the highest final strengths, especially with diffuse-porous species. Good compatibility was observed between pine and pMDI, while oak heartwood bonded nearly twice as fast as sapwood when using pMDI. Kinetic modelling revealed distinct curing profiles: MUF adhesives followed a sigmoidal trend, whereas isocyanates showed a hyperbolic pattern. ABES proved effective for evaluating bonding behaviour, though limitations such as adhesive squeeze-out and moisture variation must be considered when translating results to industrial applications.