Shear strength of beech wood joints bonded with commercially produced PVAc D3 adhesives

Abstract

One of the most commonly used adhesive types in nonstructural applications of wood, with many advantageous characteristics, is polyvinyl acetate (PVAc), which is produced in four different classes (D1-D4), according to its durability to humidity and climatic conditions. In this study, six of the most commonly used and worldwide commercially available brands of PVAc wood adhesives of D3 durability class, of different cost and origin (locally produced or imported), were obtained from the market, aiming to study and compare the “wood to wood” adhesion ability of beech wood, which is one of the most important species used in furniture production in Europe, and assess the quality of the produced gluelines, according to standards [EN 204, EN 205]. Three different conditioning treatment processes were applied to these 6 D3-PVAc adhesives lap joint specimens the “tensile shear strength” of the specimens was determined, while after their destruction, the “wood failure” percentage was visually assessed on the bonding surfaces. According to the results, the tensile shear strength values of the joints ranged between 11.48 and 13.18 N/mm2, revealing satisfying adhesion strength for all the different cases of adhesive brand and process applied after adhesion, meeting the requirements of the standard for D3 durability class categorization. The factor of “adhesive brand type” affected significantly the shear strength values, whereas neither the “conditioning treatment” applied to the specimens, nor the interaction between the two factors of “adhesive brand type” and “treatment”, revealed a significant effect on the shear strength values of the specimens, indicating a good bonding performance, either under ambient conditions or under conditions of high humidity environments. The differences in the adhesives prices (euro/100 g), origin and other technical characteristics (viscosity, solids content), did not affect the shear strength of the joints. The satisfying bonding quality of the studied adhesives was verified by the high “wood failure” percentages recorded on almost all the specimens, as well as the stereoscopically observed bonding lines, suggesting the high potential of resistance to humidity and aging effect, which could extend even more the applications range of such environmentally friendly adhesives.