Interpenetrating Polymer Network Adhesive
Weber, Gary Robert
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Recent products in commercial aerospace such as the Boeing 787 and the Airbus A350 have featured an increased use of thermoset adhesive bonded primary structure assemblies. In addition to carbon fiber laminates and fiber metal laminates like GLARE and TiGr, one potential growth area is selective fiber reinforcement of metallic structures. The thermal expansion differential between metals and composite laminates leads to thermal residual stresses and thermal distortions when elevated temperature cure adhesives are used. Room temperature cure paste adhesives offer a potential solution, but they are messy to use, have limited working lives and have unsatisfactory environmental durability. Radiation cured adhesives also offer room temperature curing capability, but they suffer from low toughness since traditional phase separated, elastomeric or thermoplastic toughening mechanisms are not compatible with the rapid cure rates of radiation curing. The proposed solution explored in this research was the use of interpenetrating polymer networks (IPNs) to toughen electron beam cured acrylate adhesives. The proposed solution involves an initially cured stiff acrylate network to provide the strength of the adhesive system combined with the subsequent cure of a flexible epoxy network to provide the toughness of the adhesive system. Previous IPN research has explored combinations of flexible acrylates with stiff epoxies and stiff acrylates with stiff epoxies, but a literature review did not reveal any combinations of stiff acrylates combined with flexible epoxies. Electron beam curing was used to explore combinations of different acrylate and epoxy network formulations. To explore the effect of the ratio of the acrylate and epoxy networks, a model IPN material system was explored. IPN material samples were characterized to determine degree of cure, glass transition temperature, density, strength, modulus and fracture toughness. The IPN was also used as the matrix adhesive in a carbon fiber laminate. A parabolic response was found in IPN properties, resulting in deviations from a linear rule of mixtures relationship that can be related to the product of the acrylate and the epoxy phase fractions. IPN single edge notched beam fracture toughness values in excess of 2.50 MPa m½ were observed whereas the values for pure acrylate were on the order of 1.00 MPa m½. Sources of IPN synergy were linked to IPN morphology, to chemical synergy between the acrylate and epoxy monomers and to enhanced shear yielding due to a mismatch between the component networks.