Anti Rohumaa

The influence of lathe check depth and orientation on the bond quality of phenol-formaldehyde – bonded birch plywood

Abstract During the rotary peeling of veneer for plywood or the laminated veneer lumber manufacture, checks are formed in the veneer that are as deep as 70–80% of the veneer thickness. The results of this study show that, during adhesive bond testing, deep lathe checks in birch (Betula pendula Roth.) veneer significantly reduce the shear strength and the percent wood failure of phenol-formaldehyde (PF) – bonded plywood. The results also show that specimens tested with the checks pulled open or closed can fail by different mechanisms. Dried rotary peeled birch veneers were sanded to create uniform surfaces with lathe check depths varying from 30% to 90% of veneer thickness. Then, 7-ply plywood was manufactured with a commercial PF resin. After the preparation of the test specimens, the check depth of each specimen was measured microscopically. Subsequently, bond quality was measured according to EN 314. The results show that veneer checking alone can bring EN 314 specimens to the brink of failure even with an excellent adhesive. These findings stress the importance of measuring the depth of lathe checks and considering the orientations of checks during the testing to get a better understanding of bond quality in veneer-based products.

The influence of felling season and log-soaking temperature on the wetting and phenol formaldehyde adhesive bonding characteristics of birch veneer

Abstract Most adhesive studies employing wood veneer as the substrate assume that it is a relatively uniform material if wood species and veneer thickness are constant. In the present study, veneers from rotary cut birch (Betula pendula Roth) were produced from logs harvested in spring, autumn and winter, and soaked at 20°C and 70°C prior to peeling. Firstly, veneers produced from logs felled in autumn were dried at 103°C for 24 h and subsequently half of these veneers were heat-treated at 180°C for 3 h. In addition, veneers produced from logs felled in all three seasons were dried at 160°C for 3.5 min to simulate industrial drying. The wettability of veneers was evaluated goniometrically, and bonding strength was evaluated with an automated bonding evaluation system (ABES). The results show that soaking birch logs at 70°C rather than at 20°C before peeling, or harvesting trees in the spring rather than in the autumn or winter, gives rise to veneers with enhanced wettability and higher bond strengths with a phenol-formaldehyde adhesive. Changes in the preparation and history of a veneer surface may have a profound effect on the development of adhesive bonds.