Bondability of Metal to Wood Joints II. Improvement of the water resistance of epoxy resin in metal-wood joints

To improve the water resistance of metal to wood glued joints, the effect o f a waterproofing treatment was investigated, using epoxy resin adhesives t hat cure at room temperature. In the case of metal to wood joints, water pe netrated the wood and the glue-line. It was impossible to inhibit water pen etration even for the specimens previously covered with sealant or staining oil, probably because the water-repellent cover was insufficient. When an isocyanate compound was used as a primer to retard the water penetration of the epoxy glue-line, not only the dry bond strength but also the wet bond strength improved to a certain extent. A multiple glue-line system, which tries to prevent the penetration of wate r into the epoxy glue-line, using a phenol formaldehyde (PF) resin sheet be tween the metal and the wood, was designed. In this case, a resorcinol form aldehyde adhesive was used for gluing the PF resin sheet and the wood, and an epoxy resin adhesive was used for gluing the PF resin sheet and the meta l. The results after water-soaking and cyclic vacuum pressure soaking-dryin g (VPSD) test showed approximately the same bond strength as compared with the normal state. Thickness of the PF resin sheet and the applied gluing pr essure had little effect on improvement of performance. On the basis of res ults obtained for water penetrability of PF resin sheet, it is concluded th at water absorption of the epoxy glue-line was delayed by the PF resin shee t layer.