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 phenolformaldehyde (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.

In Estonia, hardwoods form approximately 50% of all forest area, where the main species are birch (30%), gray alder (9%), aspen (6%) and black alder (4%). Birch has been extensively used by the veneer-based industry, but species like black alder, gray alder and aspen have not been commonly used by the veneer-based products industry due to the lower quality of this resource. The aim of this research is to determine the effect of different lay-up schemes and usages of gray alder, black alder and aspen on the mechanical properties of plywood, by replacing birch veneer in the plywood core with alternative wood species. The main veneer and plywood characteristics will be evaluated according to the current standards, e.g., veneer strength perpendicular to grain, plywood bonding and bending strength, and modulus of elasticity. All processing parameters will be kept similar to those used generally by birch plywood manufacturers. The results showed that birch and black alder plywood panels had generally the highest bending strength properties, followed by grey alder and aspen. It was also found that, for proper gluing, birch veneers had the lowest glue consumption, 152 g/m2, and aspen had the highest glue consumption, 179 g/m2. It was found that when lower density wood was used in the plywood, the product density increased. Low density wood veneers had higher glue consumption, and also higher compaction in thickness than birch veneers under the same pressure. Overall, it was shown that the wood species used in this study have slightly lower strength properties, but with proper lay-up schemes, these wood species could be successfully used by the veneer-based products industry.

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.

This study proposes an innovative model based on local grain angle measurements to predict the modulus of elasticity of LVL made from beech. It includes a veneers sorting method industrially compatible thanks to its low computational time. For this study 41 LVL panels were prepared from 123 beech sheets of veneers. Local grain angle was obtained with a two dimensional scanner and veneer density was measured. Several models based on these measurements have been developed and their ability to predict the modulus of elasticity of LVL panels have been compared. The model based only on local grain angle measurements have been proven more efficient than models taking into account the veneer density. The proposed method can be used to sort veneer during the peeling process and grade the production of LVL panels to optimize their mechanical properties even for low-quality veneer.

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aThe purpose of this work was to determine the optimal welding time for linear friction welding of birch (Betula pendula L.) wood while keeping the other parameters constant and at similar levels compared to other species in a similar density range. Specimens with dimensions of 20 × 5 × 150 mm 3 were welded together, and the influence of welding time (2.5, 3.0, 3.5, and 4.0 s) on the mechanical properties of the specimens was determined. The studies included a tensile-shear strength test as well as visual estimation of wood failure percentage (WFP). Additionally, X-ray microtomographic imaging was used to investigate and characterise the bond line properties as a non-destructive testing method. The highest mean tensile-shear strength, 7.9 MPa, was reached with a welding time of 3.5 s. Generally, all four result groups showed high, yet decreasing proportional standard deviations as the welding time increased. X-ray microtomographic images and analysis express the heterogeneity of the weld line clearly as well. According to the averaged group-wise results, WFP and tensile-shear strength correlated positively with an R 2 of 0.93. An extrapolation of WFP to 65% totals a tensile-shear strength of 10.6 MPa, corresponding to four common adhesive bonds determined for beech.

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