Assessment of solvent capsule-based healing for woven fibre-reinforced epoxies

Structural fibre-reinforced composites based on brittle matrices such as epoxy resins may be subject to short-term subcritical matrix damage that develops with time to the point where it may compromise the integrity of the structure. Currently, components that have been damaged at the microscale are usually inspected and, depending on the extent of the damage, repaired with patches or entirely replaced. Self-healing functionality would prevent development of such damage by repairing it as soon as it appears, thereby increasing the service life of the material. Among the many potential methods for introducing self-healing capacity to epoxy resins, incorporation of solvent-filled capsules is particularly promising. Cracks cause the capsules to break, releasing a solvent that is able to swell the crack faces and induce reaction of residual monomer and/or crosslinkable functional groups intentionally left by under-curing the matrix. Thus, a model system based on ethyl phenylacetate (EPA) has been shown to provide up to approximately 90% efficiency in recovering fracture toughness, and to effectively block fatigue crack propagation in pure epoxy resin. The present thesis focuses on the integration of a EPA solvent capsule-based healing system into a glass fibre-reinforced epoxy and the assessment of its effectiveness in repairing static interply damage. EPA-filled capsules with diameters in the range of 90-250 ÎŒm were produced using an oil-in-water emulsion technique and characterized in terms of their thermal stability and mechanical properties. Pure urea-formaldehyde (UF) as well as polyurethane/urea-formaldehyde (PU/UF) double wall capsule shell materials were analysed, showing that the wall thickness of pure UF capsules is independent of their diameter, whereas that of PU/UF capsules is 4 to 6 times greater and scales with the capsule diameter until a saturation value. The modulus of these shell materials was quantified using compression test and an analytical model, and found to range from 1 to 4 GPa depending on the amount of PU content in the shell composition. Bursting forces varied from 5 to 25 mN depending on the shell composition and diameter, thus providing a range of shell compositions to select from in order to tune the capsule behaviour. The integration of the capsules into the composite was carried out by dispersing them manually onto the reinforcement textile prior to processing. During this phase, the optimum capsule size range, position and concentration could be selected to fit in the interstices and survive textile packing. Textile packing properties were tested in compression; the presence of capsules led to a decrease of the reinforcement volume fraction at a given applied load value, in particular at low packing loads, and affected the usual power law behaviour of a textile in compression, up to the rupture of the capsules. Longitudinal permeability was also increased by a factor of 6 to 8 in presence of capsules for a given reinforcement volume fraction, whereas the transverse permeability was not greatly affected. A capsule-containing fibre-reinforced epoxy material was then produced using a suitably adapted processing protocol. Vacuum assisted resin infusion moulding (VARIM) was chosen as an industrially relevant, yet adaptable processing route. Priority was given to industrially relevant fibre volume fractions of approximately 50% and to optimizing processing parameters such as the lay-out and vacuum etc.

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

Processing of fibre and porosity gradients in cellular thermoplastic composites

Manuel Bühler

The aim of this study has been to develop processing techniques for novel bioresorbable thermoplastic cellular composites, reinforced with continuous inorganic fibres, for use as biodegradable scaffolds for bone tissue engineering. As the implanted composite scaffold starts to degrade, it should be progressively replaced by natural bone tissue, which should consequently support increasing proportions of the applied mechanical loads. In order to simulate natural high performance porous structures such as bone or light wood, composites with simultaneous gradients in fibre content and porosity were studied. In order to carry out investigations over wide ranges of fibre content and porosity and to ensure biocompatibility and bioresorbability, it was decided to focus on foamable poly(L-lactic acid) (PLA) and continuous glass fibres. As well as improved mechanical reinforcement due to the high aspect ratio, the use of continuous glass fibres offers the possibility of achieving higher loadings than with short fibres or particles, increasing the potential range of mechanical properties. For processing the composites, a fibre winding technique developed in house was used to prepare unidirectional preforms with gradient structures tailored at the fibre bundle level. PLA fibre bundles, containing 36 monofilaments were prepared on a spinning line, and mingled with glass fibre bundles comprising 250 or 1000 monofilaments. During the winding process, the PLA fibres were intimately mixed with the reinforcing fibres, to ensure a good impregnation during subsequent processing steps. Moreover it was possible to maintain the relative positions of the fibres during pre-consolidation of graded composites. The resulting preforms had a porosity of 25% and could be further consolidated to decrease the porosity, or foamed in an autoclave to obtain cellular composites (foams) with porosities of up to 90%. It was observed that, independently of the applied foaming conditions, the presence of continuous fibres reduced the porosity by an amount equivalent to twice the fibre volume fraction. This processing method provided composites with a simultaneous gradient in fibre content and porosity. In order to predict the Young's modulus and the collapse strength of the cellular composites a model was proposed, which combines the buckling of composite columns within a porous structure with the property predictions for neat polymer foams. In neat foams with a porosity of 60%, the Young's modulus was calculated to be 0.25 GPa. By adding a fibre volume fraction of 14% the modulus reached 1.2 GPa. The respective collapse strengths were 5 MPa and 16 MPa. Experimental results were consistent with the predictions and validated the model. The model determines the fibre content required for composites with a given porosity and stiffness. Gradient cellular composites were obtained with a range of porosity between 50% to 90% and a range of stiffness between 100 and 1500 MPa. Furthermore the processing parameters can be adjusted to maintain a desired porosity when fibres are added. Thus, composites that fulfill the mechanical requirements for bone scaffolds can be achieved in a rational manner. The long term behaviour of the foams was tested under physiological conditions. Viscoelasticity tests provided evidence for the important role of the unidirectionally oriented fibres. Whereas the cells in neat foams collapsed after at most two days when loaded at 0.69 MPa, the fibre reinforced foams resisted longer and their creep strain was well described by a power law. The fatigue properties were compared to reported values of natural trabecular bone. None of the neat foams with a porosity of 74% resisted to the first load cycle of 7.3 MPa. With a fibre volume fraction of 11% and a comparable porosity of 72%, the composite scaffolds resisted for at least 11'000 loading cycles. To compare, trabecular human bone failed after 1909 cycles, when loaded with 8.5 MPa. It was concluded that the reinforced composites have the potential to replace natural bone. Preliminary in vitro results with human foetal osteoblasts and in vivo tests on femurs of rats were promising because the cells proliferated well on the composite substrates. Further investigations for applications in bone tissue engineering can be planned. Furthermore, the studied process and the obtained results are envisaged for other material systems to prepare cellular composites for novel lightweight applications in transportation and building industry.

EPFL2008

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