Tailored Capsule Based Self-Healing for Epoxy Matrix Composites

Self-healing materials are a novel class of materials able to repair structural or, more generally, functional damage to a part. They hence have the potential to improve reliability and cost effectiveness, because failure or loss of functionality may be postponed or completely avoided. This concept has been applied to all classes of materials, but is particularly suited to polymers and their composites. Indeed, it was first demonstrated for epoxies containing capsules of dicyclopentadiene and dispersed Grubbs’ catalyst. However, the high cost of the catalyst precludes commercial use of this self-healing system. In the present thesis, two alternative approaches to self-healing of an epoxy matrix were investigated: (1) a solvent based self-healing epoxy; (2) a two-component epoxy/amine self-healing system using a novel encapsulation technique. The solvent-based system showed efficient healing in an under-cured epoxy matrix. The solvent in question was ethyl phenylacetate (EPA) encapsulated with urea-formaldehyde (UF) or UF-polyurethane. It was demonstrated that during a crack event, the solvent diffused into the epoxy matrix following a non-Fickian case II profile, with a geometrical swelling of the surface layer of the crack. For a healing time of 24h, a critical crack gap of 30μm was identified, beyond which the swelling was insufficient to bridge the crack faces and the healing efficiency was limited to around 25%. For smaller crack gaps, solvent swelling led to intimate contact of the crack faces and reaction of residual functional groups through a reduction of the glass transition temperature Tg of the epoxy, resulting in improved healing efficiency. The use of shape memory alloys (SMA) that are strained during the fracture event and subsequently heated to reduce the crack opening was also investigated. High healing efficiencies were observed and it was found that longer SMA wire activation times further improved the healing efficiency up to almost 80% through improved crack closure and an increased degree of polymerization. The behavior of this material during aging was then investigated in partially cracked samples, guaranteeing small crack gaps. It was found that the premature diffusion of the solvent through the microcapsule membrane into the epoxy matrix led to a decrease in healing efficiency from around 75% in fresh samples, to about 15% in samples aged for 75 days. Increasing the thickness of the polymeric shell of the capsules did not significantly improve performance. Moreover, moisture uptake by the epoxy also contributed to the reduced healing efficiency. Because the healing agent is hydrophobic, a higher water content in the epoxy matrix limited solvent diffusion into the epoxy and hence the extent of healing. A potential solution to the problem of premature diffusion of solvent into the epoxy is suggested to be the use of other shell materials or additional coatings on the capsules. The response of the solvent based self-healing