Failure Processes Governing High Rate Impact Resistance of Epoxy Resins Filled with Core Shell Rubber Nanoparticles
Epoxy resins are classically toughened by rubber additives, but the effectiveness of rubber toughening tends to diminish with increasing strain rate, decreasing temperature, and decreasing matrix ductility. In this study we demonstrate that low loadings o
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Failure Processes Governing High Rate Impact Resistance of Epoxy Resins Filled with Core Shell Rubber Nanoparticles Erich D. Bain, Daniel B. Knorr Jr., Adam D. Richardson, Kevin A. Masser, Jian Yu, and Joseph L. Lenhart Abstract Epoxy resins are classically toughened by rubber additives, but the effectiveness of rubber toughening tends to diminish with increasing strain rate, decreasing temperature, and decreasing matrix ductility. In this study we demonstrate that low loadings of 100–200 nm core-shell rubber (CSR) particulate additives can improve high strain rate (104– 105 s1) impact resistance by nearly 200 % for epoxy resins with glass transition temperatures Tg in a range between 60 and 110 C, without large reductions in Tg or stiffness. Size and surface chemistry of the CSR particles influence the ballistic response, with 200 nm diameter, weakly bound, poorly dispersed CSR particles providing the greatest toughening performance at low filler loadings and high rates. Impact resistance for a systematic series of CSR modified epoxies covers a transition from brittle to tough behavior, where the failure mechanism changes with effective fracture resistance. For brittle resins, failure is dominated by initiation of Hertzian cone fracture which depends strongly on fracture toughness KIC, while for tough resins, failure is dominated by plastic yield at the impact site and is independent of fracture toughness above a minimum KIC value of approximately 1.2–1.5 MPa-m1/2. Interestingly, quasistatic mechanical properties are reasonably effective qualitative predictors of high rate impact resistance, suggesting that the toughening mechanisms of CSR particles are similar over the rates studied here. The insights gained from this study are valuable for design of next generation adhesives, polymers, and polymer composite matrices for lightweight protective applications. Keywords Rubber toughened epoxies • High rate impact resistance • Failure mechanisms • Composites • Fracture
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Introduction
Cross-linked epoxy polymers are major components in technologically important material packages such as fiber reinforced polymer composites (FRPCs), adhesives, coatings, and bulk plaques for protective systems servicing a wide range of military, aerospace, transportation, and construction industries [1–3]. In FRPCs, where epoxy resin acts as the matrix, the relationship between epoxy toughness and composite high rate impact resistance is complex. Tensile failure of primary yarns and interlaminar delamination tend to be two primary dissipative mechanisms, where the former is dominated by the fiber properties [4], while the latter is at least partially dependent on the matrix [5]. For example, Kinloch, Sprenger, and coworkers have shown that interlaminar fracture energy Gc for a range of fiber and weave types systematically increases with epoxy matrix fracture toughness, when the matrix is toughened with rubbery particles, silica nanoparticles, or both [6, 7]. These authors additionally demonstrated improved high rate impact resist
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