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

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.

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

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

UV-cured Epoxy for Adhesion in Steel Reinforced Thermoplastic Composites

Basile Golaz

This work addresses the development of composite materials that contain metallic load-bearing cords with the aim of combining outstanding mechanical performance with added functionality, including electrical conductivity, data transmission, and temperature and strain sensing. The corresponding elements are impregnated with a thermoplastic elastomer for protection, and to provide mechanical integrity and surface traction. However, in order to render the resulting smart multifunctional systems sufficiently robust and cost effective, it is necessary to find innovative solutions for the rapid establishment of a strong and durable interface between the metallic reinforcements and the surrounding elastomer. In order to ensure effective transfer of mechanical loads, the interface between metal oxides and polymers is typically modified using organosilane primers. These provide strong and durable bonds, but imply a sol-gel condensation at the interface characterized by relatively slow, temperature dependent reaction kinetics, which often precludes their application on-line. Additional processing and storage steps increase costs and may result in surface contamination or premature reaction of functional groups. It is to some extent possible to resolve these latter problems by surface activation with heat or solvent treatments prior to bonding, but such treatments may be unacceptable in terms of energy consumption and/or pollution. One of the main goals of the present work has therefore been to identify a suitable primer that can be deposited and cured on-line at high processing speeds. The first phase of the study focused on the adhesion phenomena associated with the interface of a thermoplastic polyurethane elastomer (TPU) co-moulded with galvanized steel wire, and their sensitivity to hydrothermal aging. Various surface treatments were investigated for the purposes of benchmarking and to assess their efficiency at the TPU-metal interface. γ-aminopropyltriethoxysilane (γ-APS) was deposited on the steel using water (pH5 or pH10), isopropanol or ethanol as a solvent and other surface treatments included degreasing, corona discharge, acid etching, gas flame and use of a zinc phosphate conversion coating. Adhesion was characterized using a single wire pull-out test based on the microbond test geometry on unaged specimens, after 2 months of dry aging or after 24h of hydrothermal aging in water at 50°C. γ-APS was found to increase adhesion between the TPU and the substrate, outperforming the other surface treatments in terms of strength and durability. The shear strength increased from 5-11 MPa to more than 20 MPa for the unaged specimens, and from ~5 MPa to more than 15 MPa after hydrothermal aging. However, effective treatment with γ-APS required careful optimization of the deposition parameters and a 1 hour thermal cure. At the same time, a finite element (FEM) approach was used to simulate an idealized wire-reinforced thermoplastic elastomer composite, taking into account the non-linear viscoelastic response of the matrix, and the results were compared with results from an analytical elastic shear-lag model for the single filament pull-out test. The analytical model was found to be sufficiently accurate for stress analysis, but was unsuitable for fracture properties. FEM analysis allowed evaluation of the build-up in internal stresses during processing, showing that these may reach levels of 3.3 MPa, with important consequences, particularly for treatments leading to low intrinsic interfacial shear strength. The next phase of the study involved the development of an innovative fast-curing coupling method using a cationic UV-curable primer with an optimized formulation based on an epoxy monomer. The approach was first to determine the curing kinetics and the final degree of cure of the epoxy resin by photo-DSC in order to optimize the processing conditions. The processing parameters studied were the curing time, temperature, UV intensity and thermal post-curing. The influence of the primer on the interfacial mechanical properties was then investigated using the single wire pull-out test in order to tune the formulation with respect to interfacial stress transfer through the primer before and after hydrothermal aging. The formulations comprised a hyperbranched monomer for cross-linking and flexibility, an oxetane monomer to increase curing speed and reduce viscosity, and an epoxysilane to improve the adhesion to the substrate. The proposed UV-cured epoxy primer outperformed the conventional methods in terms of processing time, adhesion and durability. The processing time was reduced from 1h for the silane primer to 5s for the UV-cured epoxy primer, and the interfacial shear strength reached more than 25 MPa before aging and more than 20 MPa after hydrothermal aging. In the final phase of the work, the fast-curing adhesion promotion system was applied to systems based on other metallic substrates and thermoplastic matrices (flame retardant TPU (FR-TPU), polyamide 12, high density polyethylene). The new primer increased adhesion to stainless steel and was also effective with FR-TPU and polyamide 12. Primer application was then successfully scaled up to the cord geometry and the interfacial adhesion tested by cord pull-out tests, as opposed to single wire pull-out tests, again before and after hydrothermal aging. In this configuration, the UV-cured primer increased the maximum pull-out force by 40% before aging and 20% after aging, even with non-optimized formulations. The primer was also found to reduce the fretting wear of steel cords, which is also a considerable advantage for long term composite properties. Finally, optical fibres sensors were successfully integrated in the TPU with sufficient interfacial stress transfer to provide the multifunctional coated transmission elements with accurate temperature and strain sensing capabilities. Use of the UV-cured epoxy primer was again shown to be highly beneficial in this case. Sensing accuracies of ±1 μstrain and ±0.2°C were achieved, with a spatial resolution of 0.10 mm.

EPFL2012