Multiple local hydroxyl groups as a way to improve bond strength and durability in structural adhesives

Polymer chain-extenders with one (AE, i.e., aminoethanol), two (APD, i.e., 2-amino-1,3-propanediol), and three (tris, i.e., 2-amino-2-(hydroxymethyl)-1,3-propanediol) pendant ethanol groups were used in a model epoxy structural adhesive to investigate their influence on joint strength and durability. While AE and APD showed modest improvements in lap shear strength, the bond durability of adhesive joints was improved dramatically using tris without the need for a chemical surface treatment. Specifically, single lap shear measurements (ASTM D1002 10) using 2024-T3 adherends demonstrated that tris (0.28 - 1.4 wt%) incorporated into a model structural adhesive can improve the lap shear strengths by 50% under dry conditions and by 64% under hot/wet conditions (immersion in water at 63 degrees C for 14 days). The performance under hot/wet conditions is similar to that observed for adherends chemically surface treated with a commonly used silane coupling agent (i.e., 3-aminopropyltriethoxysilane). This enhanced performance of tris is likely a result of improved interfacial interactions because the small amounts of tris additive had an insignificant effect on the mechanical properties of the adhesive formulations themselves based on dynamic mechanical analysis. In addition, X-ray photoelectron spectroscopy measurements on fractured specimens showed that the adhesive remained on the substrate, implying largely cohesive failure in the specimens.