Penetration of urea formaldehyde adhesives in wood tissue, Part II: Radial penetration of UF adhesives into silver fir

Penetration is the ability of the adhesive to move into the voids on the substrate surface or into the substrate itself. Wood's cellular nature allows significant penetration of the adhesive into the substrate. Objective of this work was the evaluation of the penetration and hence the distribution of urea-formaldehyde (UF) resins by means of microscopic detection of the penetration of such UF adhesives into the wood tissue. Tangentially cut 5 mm thick silver fir (Abies alba Mill.) plies, 100 mm long (parallel to the grain) and 30 mm width, were prepared for this study. Four types of UF resins with different degree of condensation were investigated in this research. Safranin was added to the resins and epi-fluorescence microscope was used for measuring the adhesive penetration. The UF adhesive mixes, consisting of the various resins, extender and hardener were applied to the surface of one ply. Two plies, one with applied adhesive mix and one without adhesive mix, were assembled with parallel grain direction. Samples were pressed in a hydraulic press at 120°C and 1.0 MPa for 15 minutes. Test-specimens of 20 μm thickness were cut of each sample using a sliding microtome apparatus, exposing a bondline on a cross-sectional surface. The results show a significant correlation between the penetration behaviour and the degree of condensation (molecular sizes, viscosity) of the resins. The higher the degree of condensation, the lower the possibility for penetration, expresses as average penetration (AP) and as portion of filled tracheids on the whole cross section of interphase (filled interphase region FIR)