Penetration of urea-formaldehyde adhesives in wood tissue, part I: Radial penetration of UF adhesives into beech

Adhesive penetration plays an important role in wood adhesion, since wood is a porous material. The degree of penetration mostly depends on the wood factors, resin type and formulation and processing parameters. Tangentially cut 5 mm thick beech wood (Fagus moesiaca) plies, 100 mm long (parallel to grain) and 30 mm wide, were prepared for this study. The urea-formaldehyde (UF) adhesive was applied to the surface of one ply. Two plies were assembled into sample so that the grains of two plies were parallel. Samples were pressed in a hydraulic press at 120?C and 0,7 MPa for 15 min. Microtome test-specimens were cut of each sample. 20 ?m thick microtomes were cut by sliding microtome apparatus, exposing a bondline with a cross-sectional surface. The lack of more exhausting research on the penetration of urea-formaldehyde adhesives in wood is evident. Since ureaformaldehyde (UF) glue resins were the most important type of adhesives in the wood industry in the last 60 years (Dunky, 2000), the objective of this research was microscopic detection of UF adhesive penetration in wood tissue. Four types of UF resins with different levels of polycondensation were used in this research. Safranin was added in resins, since epi-fluorescence microscope was used in this research for measuring the adhesive penetration.