Investigation of castor-oil foam adhesion on aluminium through single-lap tests

This work investigates the adhesion of a castor-oil-based polyurethane (PU) biofoam to the surface of 1200 aluminium. The foam is fabricated using water as the blowing agent. The apparent shear strength, joint toughness and adherent strength of single-lap joints are assessed following the ASTM standard D1002. 3D-printed moulds are developed for sample preparation. The study evaluates three different bonding strategies related to polymer curing: (1) Simple Cure, (2) Co-Cure with Adhesive Layer and (3) Direct Bonding. The Simple Curing method is conducted by expanding the PU biofoam in the mould cavity in contact with the aluminium sheets. In the second method, a layer of castor-oil PU resin is previously applied to the internal aluminium surfaces before pouring the water-PU foaming mixture. The foaming process thus occurs inside the mould as in (2), with the additional co-cure of the adhesive PU resin layer on the aluminium sheets. In the direct bonding method (3), a slab of previously cured foam is bonded to the aluminium faces using a layer of castor-oil PU resin previously applied to the internal metallic surfaces, as in (2). Compared to the co-curing strategy, the joint toughness provided by simple cure is 64% higher, even though the apparent shear strength is 26% lower. Direct bonding is the least effective strategy, providing joint toughness 39% lower than co-curing and apparent shear strength 26% lower than simple cure. This study is motivated by a subsequent study on foam-core sandwich panels, in which the best face-core bonding strategy will be used.