Influence of amino-functionalized carbon nanotubes and acrylic triblock copolymer on lap shear and butt joint strength of high viscosity epoxy at room and elevated temperatures

Abstract

This study investigates the effectiveness of incorporating different amino-functionalized carbon nanotubes (CNTs) and acrylic triblock copolymer (BCP) into an aerospace-grade relatively high-viscosity epoxy (EA9396) resin to enhance the lap shear and tensile butt-joint strength at room and elevated temperatures (90 °C). To address the common issue of nanoparticle agglomeration in epoxy resin, which is exacerbated by its relatively high viscosity, a novel processing method was developed. This method involves ozone oxidation followed by functionalization with either polyethyleneimine (P + oz-CNTs) or a polyamine hardener (H + oz-CNTs). The functionalization not only ensures uniform dispersion of carbon nanotubes (CNTs) but also enhances their chemical reactivity with both the epoxy and block copolymer (BCP) matrix. The bonding performance of the epoxy, incorporating functionalized CNTs and BCP, was evaluated using single lap shear and tensile butt-joint tests. The results emphasize the substantial enhancement of both lap shear and butt-joint strength in the BCP-modified epoxy with the incorporation of functionalized CNTs (P + oz-CNTs and H + oz-CNTs), evident at both room temperature and 90 °C. At ambient conditions, the lap shear strength of the nanocomposite adhesives (P + oz-CNT + BCP/EA9396 and H + oz-CNT + BCP/EA9396) exhibited significant improvements of 40 % and 48 % respectively. At 90 °C, both formulations demonstrated a 20 % increase in lap shear strength. Furthermore, considerable advancements were observed in butt joint strength, with enhancements of 22 % (P + oz-CNT + BCP/EA9396) and 28 % (H + oz-CNT + BCP/EA9396) at room temperature. Importantly, compared to the unmodified epoxy, the butt joint strength displayed even more remarkable improvements of 49 % (P + oz-CNT + BCP/EA9396) and 42 % (H + oz-CNT + BCP/EA9396) at 90 °C. The analysis of fracture studies showed that introducing amine functionalization resulted in a more consistent dispersion of CNTs and enhanced the ability of the BCP/EA9396 to undergo plastic deformation. The study further demonstrates that functionalization with PEI and amine hardener reduces CNT pull-out, which enhances energy dissipation mechanisms and increases lap shear and butt joint strengths.