Technical report | Copolymer Toughening of Epoxy Resin Systems for Low Temperature Cure Bonded Composite Repair

Abstract

Epoxies are the most widely used resins for bonded composite repair; however they possess low fracture toughness, impact strength and peel strength due to high cross linking densities. Toughening agents can be added to epoxy resins to improve these qualities; however these typically lead to an increase in resin viscosity and undesirable changes in mechanical performance, which can in turn make the resin unsuitable for use in certain bonded composite repair applications. In this work, the impact of two commercially available tri-block copolymers on the mechanical performance of two different low temperature cure epoxy resin systems was evaluated. It was found that with the addition of the copolymers to these resins in a wet lay-up scenario (typical of bonded composite repair applications), the resin fracture toughness could be improved by as much as 125%, with negligible impact to resin interlaminar shear strength, flexural strength and glass transition temperature. Use of these resins will improve standard bonded composite repairs and support development of indigenous composite multi-functional structures for aerospace applications.

Executive Summary

Epoxies are the most widely used polymer resin used in aircraft structures and for bonded composite repairs because of their high strength, low shrinkage and good compatibility with a range of different reinforcing fibres. Unfortunately, these resins exhibit poor fracture toughness, resistance to crack propagation, impact strength and peel strength. Toughening agents can be added to epoxies to improve these qualities; however this typically leads to an increase in resin viscosity and undesirable changes in mechanical performance. The use of copolymer additives has shown promise in this area, improving fracture toughness without impacting upon other properties desirable for bonded composite repair applications.

The aim of the research reported herein was to identify two toughened low temperature cure epoxy resin systems suitable for bonded composite repair applications within the Australian Defence Force; one resin for room temperature cure wet lay-up repairs; and the other for low temperature cure pre-preg repairs. The effect of two commercially available tri-block copolymers on the mechanical performance of these two different epoxy resin systems was evaluated. Mechanical performance was quantified by Short Beam Shear (SBS), four point bend flexural and Double Cantilever Beam (DCB) testing. This destructive analysis was complimented by thermal degradation studies, Dynamic Mechanical Thermal Analysis (DMTA) and Field Emission Scanning Electron Microscopy (FESEM).

It was found that the copolymer additives provided considerable fracture toughness improvements, as much as 125%, for both the wet lay-up and pre-preg epoxy resins considered, with a negligible impact on interlaminar shear, flexural strength or glass transition temperature. This improvement in fracture toughness is a highly desirable quality both for bonded composite repair, but also for new multifunctional composite structure applications.