Fracture Toughness Healing in Epoxy Containing Both Epoxy and Amine Loaded Capsules

The feasibility of self-healing epoxy matrices using the same chemistry as the matrix material, namely a standard epoxy-amine system, is demonstrated. The two-component self-healing system is encapsulated in core-shell structures using standard in situ curing in a stirred emulsion and a novel microfluidic encapsulation technique for the epoxy monomer and the amine hardener, respectively. The recovery of fracture toughness after room temperature healing is above 50%, and increases to 100% when shape memory alloy wires are combined with a low temperature cure of the matrix to enhance crack closure. This healing ability is also preserved for higher curing temperatures, demonstrating the potential of this novel approach.

Tailored Capsule Based Self-Healing for Epoxy Matrix Composites

Sam Neuser

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 system to dynamic fatigue was also studied. While in static testing, matrix swelling governs crack healing, a different mechanism was observed in the case of dynamic loading. In contrast to the unmodified epoxy, which showed rapid crack propagation under all the conditions investigated, in the presence of the solvent, crack arrest was observed for stress intensities of up to 0.48 MPam1/2. This phenomenon was attributed to crack pinning owing to diffusion of the solvent into the matrix surrounding the crack tip. The elongation at break of the solvated epoxy increased threefold, and the Young’s modulus decreased by more than 50%. Using finite element analysis (FEM) to model the stress distribution around the solvent-infused crack tip, it was shown that the local crack tip stress was reduced by 43%, accounting for this novel toughening effect. While solvent healing performed well in an under-cured matrix and interesting phenomena were observed, a fully cured matrix with a two-component self-healing system may be preferable for real applications because it avoids issues pertaining to premature solvent diffusion. The possibility of encapsulating an amine curing agent in a core-shell structure was therefore investigated as an alternative. A microfluidic glass chip was used to encapsulate a hydrophilic amine/water based curing agent in an acrylate resin shell using ex-situ exposure to UV light. Low viscosity epoxy was encapsulated with UF using a modified agitated emulsion encapsulation technique. Successful self-healing was demonstrated for epoxy specimens containing these microcapsules, along with embedded SMA wires, with healing efficiencies of up to 100% and 40% for low and high temperature curing epoxy, respectively. Overall, this work represents a significant step towards affordable epoxy self-healing materials, the feasibility of solvent and epoxy/amine healing having been successfully demonstrated for an epoxy matrix. The epoxy/amine self-healing system is thought to be particularly promising, considering the early stage of development. Furthermore, the integration of SMA wires proved to improve healing efficiencies dramatically, especially with the epoxy/amine microcapsules. With further optimization and scale-up of the microfluidic encapsulation method, a new low cost self-healing system, and hence potentially compatible with a wide range of polymeric materials, including composites, is therefore within reach because it is based on a commonly used epoxy/amine adhesive.

EPFL2013

Active sensing and repair composites

Eva Kirkby

This thesis describes the development of a smart carbon fibre polymer composite that is capable of sensing its own damage and self-healing. Structural composites are used in the aerospace and marine industries, for example. During their lifetime impact events are inevitable, which can result in extensive internal damage. Once damaged, the only options to date are manual repair or replacement. If self-sensing and self-healing properties could be imparted to the next generation of structural composites, this would bring greater safety and reduced maintenance costs for aircraft, as well as an extended operational lifetime for equipment such as satellites. The concept of the new material is to embed three additional components into a fibre-reinforced composite material: (i) microcapsules containing a liquid healing agent, together with a solid catalyst in the matrix, (ii) optical fibre Bragg grating (FBG) strain sensors and (iii) woven shape memory alloy (SMA) wire actuators. An impact event causes a crack to propagate at the damage site, rupturing the microcapsules in its path. This releases the liquid healing agent into the crack, where it comes into contact with the catalyst and begins to polymerise. The strain shock pulse radiating out from the impact site is detected by a sparse array of FBG sensors, which locate the impact position by time-of-flight. With this information, the SMA wires in the impact region are thermally activated using resistive heating, causing them to contract and exert a compressive force, closing the crack. The SMA wires then remain activated during the polymerisation period of the healing agent. The new material is developed in several stages. In the first stage, SMA wires and microcapsules are combined in an epoxy matrix. The addition of SMA wires results in a significant improvement in healing efficiency; the healed fracture toughness approaches that of the virgin material. The improvement results both from crack-closure and also from the heating effect of the wires, which increases the degree of polymerisation of the healing agent. In the second stage, a liquid composite moulding cure schedule that allows straightforward integration of the SMA wires during composite fabrication is developed. With this process, the SMA wires do not need to be maintained in place with an external frame, even though the peak post-cure temperature exceeds the activation temperature of the wires. The wires remain bonded to the matrix both during processing and subsequent activation. In the third stage, impact sites are localised on composite plates to a precision of a few centimetres using three FBG sensors spaced several tens of centimetres apart. In the final stage, a prototype self-healing carbon-fibre composite with embedded microcapsules, Grubbs' catalyst and woven SMA wires is fabricated and tested. While the results presented in this thesis represent only a first step towards a fully-functional Active Sensing and Repair Composite, they successfully demonstrate the advantages of combining the three components in a single material, as well as validate the scientific concept of the system.

EPFL2009

Effect of aging on the performance of solvent-based self-healing materials

Véronique Michaud, Sam Neuser

Solvent healing of thermoset polymer matrices was suggested as an efficient and cost-effective alternative to other encapsulated self-healing systems. However, self-healing efficiency is not guaranteed over time. We report here on the autonomic self-healing of under-cured epoxy with ethyl phenylacetate-filled microcapsules under different aging conditions. We carried out quasi-static fracture tests and observed a decrease of healing efficiency from 77% for fresh samples down to 13% for samples aged at room temperature for 77 days. DSC results of pure epoxy matrix curing showed that the residual heat of reaction only decreased from 161 J g(-1) to about 135 J g(-1); this decrease was largely due to water absorption and could only partly explain the large decrease in healing efficiency. Furthermore, FTIR spectra did not indicate a change in conversion as observed by monitoring the associated oxirane peak in the pure epoxy matrix over time, confirming the above findings. While the microencapsulated solvent has good shelf life properties, evidence was found that solvent diffusion into the epoxy matrix prior to cracking was responsible for the reduced healing capability. DSC and FTIR results of aged self-healing epoxy confirmed the presence of the EPA solvent in the matrix even though no damage was introduced to the microcapsules. Therefore two phenomena responsible for the decreased solvent-healing capability in under-cured epoxy were identified: ambient moisture uptake and premature solvent diffusion through the microcapsule walls.