Radial Penetration of Urea-Formaldehyde Adhesive Resins into Beech ( Fagus Moesiaca )

Abstract

Penetration of adhesives plays an important role in wood adhesion, since wood is a porous material. Objectives of this work were (i) the evaluation of the influence of the degree of condensation of urea-formaldehyde (UF) resins on their penetration into beech, showing the distribution of the adhesive in the cell layers close to the bondline using microscopic detection, and (ii) the comparison of data for beech with results for fir obtained earlier. The degree of penetration mostly depends on the permeability and porosity of the wood surface as well as on the resin type and the size of the resin molecules. The process parameters, which also can affect penetration, were kept constant throughout all tests performed. The results show a significant correlation between the penetration behavior and the degree of condensation (molecular size, viscosity) of the resins and adhesive mixes based on these resins. The higher the degree of condensation, the lower the possibility for penetration, expressed as "Average penetration depth" (AP). AP into beech is higher than into fir. The portion of filled tracheids and vessels on the whole cross section of the interphase ("Filled interphase region" FIR) increases slightly with the degree of condensation, but this increase is not statistically significant. FIR in fir is higher than in beech.