Mechanical properties of epoxy/alumina trihydrate-filled compositions
The mechanical properties of alumina trihydrate (ATH)-filled epoxy resin at loadings of up to 100 parts by weight ATH per hundred of resin (epoxy and hardener) (pphr) have been investigated. A low peak exotherm, increased Young's modulus and increased critical strain energy release rate (G[sub]IC) and critical stress intensity factor (K[sub]IC) can be achieved by incorporating a dispersion of ATH into an epoxy resin. However, the high filler loadings required for effective fire resistance reduce tensile strength and elongation. Tensile modulus increases with filler loading in line with previous studies and theoretical equations. However, the tensile strength is higher and the ultimate elongation lower than current theories predict. The tensile and fracture process in ATH-filled epoxy follows linear elastic fracture mechanics, but can be considered in two parts. The initiation of a crack occurs from a large critical flaw, either as a large particle or agglomerations of particles. A flaw can also be formed on the application of a tensile load, when large stress concentrations cause localised microcracking of the matrix. The propagation of a flaw requires more energy and is dependent on several possible mechanisms. Shear yielding and associated crack blunting are shown to be the most important mechanisms, whilst minor contributions from matrix microcracking and debonding of ATH particles are possible. The absence of crack pinning in this study is believed to be due to the inherently weak nature of ATH particles. The presence of a 10pphr rubber dispersion in ATH-filled epoxy only increases the values of G[sub]IC and K[sub]IC at low filler loadings. Amine-terminated butadiene acrylonitrile rubber (ATBN)-modified epoxy matrix exhibits little adhesion to ATH and therefore the efficiency of stress transfer between particle and matrix is reduced, diminishing shear yielding.