Role of particle cavitation in rubber-toughened epoxies: II. Inter-particle distance

Two types of conventional rubber modifiers and a series of hollow plastic micro-spheres were employed as toughening agents in a diglycidyl ether of bisphenol A (DGEBA) epoxy in Part I (Bagheri R, Pearson RA. Polymer 1996;37:4529) of this study. It was found that the rubber modifiers with different cavitation resistance and hollow plastic micro-spheres which act as pre-existing microvoids toughen epoxies in the same manner. The current study is composed to further examine the previous results in terms of the role inter-particle distance in rubber/microvoid toughened epoxies. It is shown that the fracture toughness in toughened blends goes through a ductile-to-brittle transition with inter-particle distance. The source of this transition is found to be a stress state change provided by voided particles. The Ligament between neighboring particles, thus, experiences a transition from plane strain to plane stress state by decreasing the inter-particle distance. Interestingly, it is shown that the transition in toughened blends does not occur at a specific inter-particle distance as frequently proposed in literature, but varies with the size of the modifier. Therefore, there is an influence of particle size on yielding of the toughened epoxies that is responsible for the shift in transition. (C) 1999 Elsevier Science Ltd. All rights reserved.