Novel Diels-Alder based self-healing epoxies for aerospace composites

Abstract

Epoxy resins containing Diels-Alder furan and maleimide moieties are presented with the capability to self-heal after exposure to an external heat source. A conventional epoxy amine system has been combined with furfuryl and maleimide functional groups in a two-step process, to avoid major sidereactions, and the concentration of a thermo-reversibly binding cross-linker was considered to balance thermoset and thermoplastic behaviours, and the subsequent self-healing performance.In the context of self-repair technologies an inbuilt 'intrinsic' self-healing system is deemed favourable as the healing agent can be placed in known 'hot spot' regions (i.e. skin-stringer run outs, ply drops and around drilled holes) where operational damage predominately occurs in load bearing aerospace structures. In this study, the mechanical and self-healing performance of furan functionalised epoxy resins containing varying amounts (10, 20, 30 or 40 pph) of bismaleimide were investigated using a bulk epoxy polymer tapered double cantilever beam (TDCB) test specimen geometry. Two forms, a thin film and a bulk material, were evaluated to account for future integration methods into fibre reinforced polymer (FRP) composites. The highest healing efficiency, with respect to the obtained initial load value, was observed from the 20 pph bulk material derivative. The polymers were successful in achieving consistent multiple (three) healing cycles when heated at 150 °C for 5 minutes.This novel investigated Diels-Alder material exhibits favourable processing characteristics for FRP composites as preliminary studies have shown successful coextrution with reinforcing fibres to form free standing films and dry fibre impregnation.

Healing of carbon fiber reinforced plastics by Diels–Alder based polymers: Effects of healing agent concentration and curing cycle

“…The healing mechanism extensively presented in Ref. is activated by heating the specimen above the melting temperature ( T m ) of the monomeric grains. In this approach, the SHAs do not need to be encapsulated because the rigid nature of the grains keeps the particles intact during dispersion in a resin host and subsequent processing.…”

“…The self‐healing material was interleaved as a layer of grains between two sheets of glass/epoxy composite and a qualitative mechanical study validate the feasibility of the concept in multiple healing cycles (heating to 150 °C for 5 min). Then, the mechanical characterization of the aforementioned DA‐based material in epoxy TDCB specimens was also conducted . In that study, the variable was the concentration of the BMI component.…”

“…In the current work, research work of Refs. is progressed a step forward. BMI pp resin blend grains were dispersed on the mid‐plane of a high performance prepreged CFRP to create an area with healing functionality.…”

“…BMI pp resin blend grains were dispersed on the mid‐plane of a high performance prepreged CFRP to create an area with healing functionality. Previous works studied the nature of the utilized BMI pp and preliminary attempts to impregnate it in lab‐scale polymer or composite coupons were achieved. Here, three individual studies were conducted.…”

Polyurethane adhesives containing Diels–Alder‐based thermoreversible bonds

“…The (retro‐)Diels–Alder reaction between furfuryl and maleimide groups is perhaps the best documented example of a thermoreversible reaction that has been used to introduce self‐healing properties in various polymeric host systems. Previously we have reported on such self‐healing systems based on polyacrylates and epoxy matrices. For the epoxy systems, we have developed an alternative self‐healing approach as well involving bismaleimides alone without presence of furfuryl functionalized components …”