Synergistic Effects of Impregnation Agents Used in Plywood Production Relative to the Shear Strength of Bonded Joints

The main objective of this research was to analyse the synergistic effects of impregnation agents that are most commonly used in plywood production on the strength of bonded joints. Two water-based impregnations (Teknol aqua 1410-01 and Cetol WV 885 BPD+) and one solvent-based impregnation (Gori 605) were used in this study. The tested adhesives were polyvinyl acetate (PVAC) ADHESIVE 3384 (1-component PVAC adhesive), EPI (emulsion polymer isocyanate) system 1920/1993, and polyurethane (PUR) 1968 (1-component PUR adhesive). Test specimens were made from European beech timber (Fagus sylvatica L.) for plywood production according to EN 205 (2017). Individual tests were conducted according to EN 204 (2001) for PVAC adhesives, and according to EN 12765 (2001) for PUR adhesives. Although the effect of impregnation on bond strength was demonstrated, it was different depending on the type of adhesive. A clear positive synergistic effect on the bond strength was shown for isocyanate-based adhesives in almost all of the tests. The synergistic effect of PVAC adhesive was very variable in each individual test and showed a negative influence of higher moisture content and dipropylene glycol content in solvent-based impregnation.