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Title:Solvent-based self-healing approaches for fiber-reinforced composites
Author(s):Jones, Amanda Rose
Director of Research:Sottos, Nancy R.
Doctoral Committee Chair(s):Sottos, Nancy R.
Doctoral Committee Member(s):White, Scott R.; Geubelle, Philippe H.; Ewoldt, Randy H.
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechans
Degree Granting Institution:University of Illinois at Urbana-Champaign
polymer matrix composites
Abstract:Damage in composite materials spans many length scales and is often difficult to detect or costly to repair. The incorporation of self-healing functionality in composite materials has the potential to greatly extend material lifetime and reliability. Although there has been remarkable progress in self-healing polymers over the the past decade, self-repair in fiber-reinforced composite materials presents significant technical challenges due to stringent manufacturing and performance requirements. For high performance, fiber-reinforced composites, the self-healing components need to survive high temperature processing, reside in matrix interstitial regions to retain a high fiber volume fraction, and have minimal impact on the mechanical properties of the host material. This dissertation explores several microencapsulated solvent-based self-healing approaches for fiber-reinforced composites at the fiber/ matrix interface size scale as well as matrix cracking. Systems are initially developed for room temperature cured epoxies/ glass fiber interfaces and successfully transitioned to carbon fibers and high temperature-cured, thermoplastic-toughened matrices. Full recovery of interfacial bond strength after complete fiber/matrix debonding is achieved with a microencapsulated solvent-based healing chemistry. The surface of a glass fiber is functionalized with microcapsules containing varying concentrations of reactive epoxy resin and ethyl phenyl acetate (EPA) solvent. Microbond specimens consisting of a single fiber and a microdroplet of epoxy are cured at 35C, tested, and the interfacial shear strengths (IFSS) during the initial (virgin) debonding and subsequent healing events are measured. Debonding of the fiber/matrix interface ruptures the capsules, releasing resin and solvent into the crack plane. The solvent swells the matrix, initiating transport of residual amine functionality for further curing with the epoxy resin delivered to the crack plane. Using a resin-solvent ratio of 3:97, a maximum of 100% IFSS recovery is achieved-- a significant enhancement over prior work that reported 44% average recovery of IFSS with microencapsulated dicyclopentadiene (DCPD) monomer and Grubbs' 1st Generation catalyst healing agents. The effects of capsule coverage, resin-solvent ratio, and capsule size on recovery of IFSS are also determined, providing guidelines for integration of this healing system into high fiber volume fraction structural composites. High healing efficiencies are achieved with capsules as small as 0.6 µm average diameter. The resin-solvent healing system is then extended to repair of a carbon fiber/epoxy interfacial bond. A binder is necessary to improve the retention of capsules on the carbon fiber surface. Two different methods for applying a binder to a carbon fiber surface are investigated. Healing efficiency is assessed by recovery of IFSS of a single functionalized fiber embedded in an epoxy microbond specimen. The two binder protocols produce comparable results, both yielding higher recovery of IFSS than samples prepared without a binder. A maximum of 91% recovery of IFSS is achieved. In the next study, the resin-solvent healing system is applied to both interfacial damage and matrix cracking in a model composite specimen, consisting of discrete fiber tows embedded in a room temperature cured epoxy. Glass fiber tows are precisely placed in a compact tension specimen for controlled crack growth. The progression of matrix cracking and fiber debonding is observed optically during testing. Healing potential is assessed by injection of the healing agents into reference specimens (no capsules). The area under the load-displacement curve recovered during the healing event serves as a metric for evaluation of healing performance. Though full recovery is achieved in neat epoxy specimens, healing efficiency in multi-tow specimens is limited to 50%, due to the larger crack separations and energy lost during fiber fracture. In the case of only a singular embedded fiber tow, healing efficiency increases to an average of 83% recovery with full recovery in several samples. Additionally, microcapsules are incorporated into the compact tension specimen and along the fiber tow interface to evaluate in situ healing. Several strategies to improve microcapsule thermal stability are investigated in order to transition solvent-based healing to high temperature cured material systems. A double shell wall technique is adopted for several different size scales of microcapsules. First, the effect of the inner polyurethane (PU) shell wall thickness on thermal stability is evaluated. Though high thermal stability at 180C is achieved for large (ca. 150 µm in diameter) capsules, smaller capsules (>2 µm in diameter) suffer from increased core loss. The addition of certain core thickeners improves thermal stability for small capsules (ca. 20% increase in core retention) when compared to capsules with solvent alone. However, an additional poly(dopamine) coating leads to the greatest improvement in thermal stability, with nearly full retention of the core solvent for all capsule size scales. Finally, a thermoplastic resin poly(bisphenol A-co-epichlorohydrin), PBAE, is blended with a high glass transition temperature (Tg) epoxy matrix to simultaneously toughen and act as a healing agent in combination with encapsulated solvents. Microcapsules are coated with poly(dopamine) to improve the thermal stability and retain the core solvent during a cure cycle at 180C. The fracture toughness of the high Tg epoxy (EPON~828: diamino diphenyl sulfone) is doubled by the addition of 20 wt % PBAE alone and tripled by the addition of both microcapsules and the thermoplastic phase. Self-healing is achieved with up to 57% recovery of fracture toughness of the toughened epoxy. Healing performance and fracture toughness of the microcapsule containing material remain stable after aging 30 days. The relative amounts of thermoplastic phase and the presence of solvent-filled microcapsules influence the storage modulus, Tg, and healing performance of the polymer.
Issue Date:2015-04-15
Rights Information:Copyright 2015 Amanda Jones
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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