Block Copolymer and Silica Modified Epoxy Structural Adhesives

Abstract

Block copolymers have been studied extensively as toughening agents for epoxies over the past two decades, where most work has been focused on improving the bulk properties of epoxies. However, epoxies are commonly utilized in applications where bulk properties are only part of the story. We begin to address this by studying block copolymer modified epoxies as structural adhesives and exploring the impacts of these additives on adhesion strength. Poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) block copolymers were synthesized and dispersed in epoxy formulations, forming spherical micelle and bilayer vesicle structures. Both morphologies led to significant increases in critical strain energy release rate, GIc, over the neat epoxy with no reduction in the elastic modulus. Spherical micelle modified epoxy adhesives showed up to a 46% enhancement to single-lap-joint shear adhesion strength. Electron micrographs of the fracture surfaces indicated that micelle cavitation plays a role in both the toughening and adhesion strength enhancements of epoxies. In contrast, a 28% reduction in adhesion strength was observed in the bilayer vesicle modified epoxies.Recently, there has also been a growing interest in studying epoxy composites with both rubbery and rigid particle modifiers. We explore this by extending our understanding of block copolymer toughening to composites containing both block copolymer and silica nanoparticles. Nanosilica and spherical micelle-forming PEP-PEO modifiers were dispersed both individually and together in epoxy formulations. The nanosilica and PEP-PEO modifiers formed uniform dispersions when added individually in the matrix but led to limited aggregation of nanosilica particles when added together. Incorporating nanosilica in neat and block copolymer modified epoxies led to increases to both fracture toughness and elastic modulus. Combining both additives led to additive toughening beyond that obtained with the individually modified epoxies. The extent of these improvements also increased with nanosilica loading up to 25 wt%. Electron micrographs of the failure surfaces revealed both micelle cavitation and nanosilica debonding occurring in concert. These findings were taken further in a final study on how silica and block copolymer modified epoxy adhesives impact toughness and adhesion strength. Micelle-forming PEP-PEO and various types of silica particle modifiers were again dispersed in epoxy formulations. All modifiers were well dispersed when added individually to the matrix. When added together, microsilica particles remained well-dispersed, in contrast to the partial aggregation observed with nanosilica. Increases to both fracture toughness and elastic modulus were observed when all types of silica were incorporated, without significant effects from silica particle size or surface functionalization. Adhesion strength was increased by approximately 50% when block copolymer was added and was not affected by the silica. Overall, the incorporation of silica improved both elastic modulus and fracture toughness without compromising the adhesion strength enhancement from block copolymer micelles.