The microscopic failure processes and their relation to the structure of amine-cured bisphenol-A-diglycidyl ether epoxies

Abstract

Electron and optical microscopy are used to study the relation between the structure and the microscopic flow and failure processes of diethylene triamine-cured bisphenol-A-diglycidyl ether epoxies. By straining films directly in the electron microscope, these epoxies are found to consist of 6 to 9 nm diameter particles which remain intact when flow occurs. It is suggested that these particles are intramolecularly crosslinked molecular domains which can interconnect to form larger network morphological entities. Epoxy films, either strained directly in the electron microscope or strained on a metal substrate, deform and fail by a crazing process. The flow processes that occur during deformation are dependent on the network morphology in which regions of either high or low crosslink density are the continuous phase. The fracture topographies of the epoxies are interpreted in terms of a crazing process. The coarse fracture topography initiation regions result from void growth and coalescence through the centre of a simultaneously growing poorly developed craze which consists of coarse fibrils. The surrounding smooth slow-crack growth mirror-like region results from crack propagation either through the centre or along the craze—matrix boundary interface of a thick, well developed craze consisting of fine fibrils.