The interphase in phenol–formaldehyde and polymeric methylene di-phenyl-di-isocyanate glue lines in wood

It has been proposed in the literature that the penetration of resin compounds into the wood cell wall may enhance the durability of adhesive bonds. In the study presented here, the diffusion of resin compounds into the cell wall of two systems, phenol–formaldehyde (PF) and polymeric methylene di-phenyl-di-isocyanate (pMDI), respectively, was evaluated making use of the UV-absorbance typical for both resins. The effects of resin diffusion into the cell wall on cell-wall mechanical properties were assessed by nanoindentation. It could be shown that significant amounts of PF resin diffused into the cell wall, whereas no pMDI compounds could be detected. In both adhesive assemblies examined, cell walls at the immediate surface were damaged during machine planing. The diffusion of resin compounds into undamaged cell walls did not change their elastic indentation modulus significantly, but clearly increased the hardness of PF-glued cell walls. Cell walls damaged due to planing showed a significantly reduced hardness and indentation modulus. To provide good bond stability it is thus necessary that the glue penetrates deep enough into the wood structure to bond to undamaged cell walls.