Fibre reinforced plastics, Adhesive bond, Wood, Hygrothermal compliance
Applications of fibre reinforced polymers (FRPs) for reinforcing timber structures are becoming increasingly common. Essential to the successful integration of these two materials is the quality of the bond between them. This paper describes a test program, which evaluates the bond performance of the FRP-Wood interface for a carefully selected range of adhesives and FRP types by means of the block-shear test. Adhesives examined comprised conventional wood laminating adhesives and structural epoxies. A five cycle vacuum-pressure-soak-drying procedure was used to assess the resistance of the bond interface to hygrothermal stresses. Two pultruded reinforcing materials were selected for the study comprising glass fibres aligned unidirectionally in a vinylester resin and an engineered thermoplastic polyurethane. The application of an adhesion promoter in order to increase the hygrothermal compliance was also investigated. It was seen that both adhesive and FRP type proved significant in ensuring the integrity of the bond. Strong durable bonds of high quality were formed using conventional wood laminating adhesives with specific FRP reinforcements. This provides an alternative to the generally accepted expensive structural epoxy adhesives, which are used to bond FRPs to wood. The application of the adhesion promoter increased the mean adherend failure percentage for moisture cycled FRP-wood specimens as well as reducing the variability in the results when bonding with Epoxy 1. Premature failure during the moisture cycling was also postponed for a number of the conventional wood laminating adhesives when bonding the engineered thermoplastic polyurethane with the use of the adhesion promoter. In general, the shear strengths of moisture cycled FRP-wood bonded specimens were lower in comparison to non-moisture cycled FRP-wood specimens, wood-wood bonded specimens and solid control specimens taken from the same board.