Solvent-Based Self-Healing Epoxy Materials
Journal Article - Open Access
University of Illinois at Urbana-Champaign Urbana United States
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Epoxy resins have a variety of applications in the automobile, aerospace, and adhesives industries. Within the past decade, White and coworkers have designed self-healing materials employing a ring-opening metathesis polymerization ROMP reaction between embedded microcapsules containing dicyclopentadiene monomer and wax-protected Grubbs catalyst in an epoxy resin matrix.1 This system has many limitations such as catalyst availability and cost, environmental toxicity, and stability. To overcome some of these limitations, a simple, one-component system based on common organic solvents has recently been developed, eliminating the need for the catalyst which was previously required for self-healing to be observed. Early reports of crack healing in an epoxy resin required high temperature conditions for healing to occur.2 This observed healing after fracture of the virgin material was due to molecular diffusion and reaction of residual functionality during subsequent heating of the material above its glass transition temperature.3-5 Solvent addition is also responsible for some reports of healing, i.e. ethanol and methanol were used to seal the cracks of thermoplastic polymers under high temperature conditions.6 The invoked healing mechanism involved wetting of the polymer surface and swelling of the bulk polymer material, which led to chain interlocking across the crack plane and recovery of virgin mechanical properties. More recent research has explored the effects of tetrahydrofuran THF in epoxy-amine polymerizations.7-9 Thus far, there has been no record of using solvents to autonomically heal cracks in thermoset materials at ambient temperatures. Our objective in this work is to demonstrate that crack damage in epoxy-based thermosets can be healed autonomically with organic solvents, preventing further crack propagation, and recovering the materials original properties.