Penetration and adhesion strength of phenol-tannin-formaldehyde resin adhesives for bonding three tropical woods

Phenol-tannin-formaldehyde (PTF) resins were prepared by adding bark tannin extracts from mangium (Acacia mangium) to phenol-formaldehyde (PF) resin adhesive. The resultant resin was used for bonding wood from three fast-growing tropical species, specifically, mangium, jabon (Anthocephalus cadamba), and mindi (Melia azedarach). The impact of the tannin extract content on the penetration and adhesion strength of PTF resin adhesives was studied for two different tannin contents (10% and 20%). The resin penetration into wood tissues of the three species was quantitatively measured at the bond line, using a light microscope. Confocal laser scanning microscopy was also used for detecting the resin penetration into wood tissues. The adhesion strength of the PTF resins was determined by measuring shear strength of three-ply parallel plywood constructed from the three tropical woods. As the amount of tannin increased, the depth of resin penetration increased for mangium and jabon wood but decreased for mindi wood. By contrast, the bond-line thickness decreased with the increasing tannin addition for all three wood species. The adhesion strength of plywood was improved as the amount of tannin increased. All plywood bonded with PTF resins using 20 percent tannin met the requirements of the EN 314-2 standard. These results suggest that the addition of tannin extract into PF resin improves the adhesion performance and can partially replace phenol in PTF resins.