High-performance epoxy-based adhesives modified with functionalized graphene nanoplatelets and triblock copolymers

Abstract

In this study, a high-performance epoxy-based adhesive was developed by adding graphene nanoplatelets (GNPs) and phase-separated triblock copolymers (BCPs) to enhance the lap shear strength of the adhesive joints. The GNPs were functionalized using an ultrasonicated ozonolysis (USO) method so as to achieve a homogeneous and stable dispersion in the epoxy matrix. The USO treated GNPs (OZ-GNPs) were incorporated into epoxy resin at different weight fractions using a 3-roll mill technique. Both poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) (MAM) and poly(styrene)-poly(butadiene)-poly(methyl methacrylate) (SBM) BCPs were mixed separately as toughening agents. Single lap shear joints were prepared by bonding aluminium sheets using the developed adhesives to study the lap shear strength of the joints. A significant improvement in the lap shear strength was observed for the hybrid epoxy system with GNPs and BCP. Adding 1.0 wt% OZ-GNPs into 10 wt% SBM modified epoxy adhesive resulted in the lap shear strength increase by 129%, compared to that of the unmodified epoxy. Raman spectroscopy and scanning electron microscope (SEM) studies found that the USO treatment facilitated the stable dispersions and improved epoxy bonding due to the oxygen functional groups introduced onto the GNP surfaces. Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA) studies showed that the GNPs and BCPs were well bonded with epoxy resin without compromising the thermal stability. To better understand the influence of GNPs and BCPs on the lap shear strength, the fracture surfaces of the joints were studied using SEM. The fracture images showed that the BCPs formed different nanostructures in the epoxy matrix and the resulting nanostructures activated different toughening mechanisms that were responsible for the improvement in the lap shear strength, while the OZ-GNPs increased the joint strength by stiffening the matrix.