The Fatigue and Fracture Properties of Nano-modified Epoxy Polymers

Abstract

The present Thesis investigates the quasi-static and cyclic-fatigue properties of nano-modified epoxy polymers. A diglycidyl ether of bisphenol-A was used as the base epoxy and polymer-based materials, block copolymers and core-shell rubber particles were used as the toughening agents. The morphologies formed by the tougheners were determined and the associated fracture and fatigue properties, including the toughening mechanisms, were investigated. The modified epoxy polymers with some of the best properties are described below.

The unmodified epoxy polymer had a fracture energy, GIc, of 494 J/m2 and this was increased to 2380 J/m2, when 10 wt.% of a polyurethane toughener was employed. This modified epoxy polymer possessed a second-phase morphology, which was a nano-sized ‘worm-like’ structure. This nano-phase debonded from the epoxy, which enabled plastic void growth to occur in the epoxy polymer. This mechanism also led to an increase in the cyclic-fatigue threshold fracture energy, Gth, from 153 J/m2 of the unmodified epoxy to 444 J/m2.

Nano-sized core-shell rubber particles (either 100 nm or 200 nm nominal diameters) were employed and they formed a well dispersed rubbery particulate phase. A maximum fracture energy of 2540 J/m2 was observed and the particles were found to undergo internal cavitation that induced plastic void growth. However, only the relatively large particles were able to cavitate under the cyclic-fatigue loading. Hence only these type of particles led to a meaningful increase in the fatigue threshold, Gth.

‘Hybrid’ epoxy polymers contained a polymer based toughener and core-shell rubber particles. Several of these hybrid formulations gave very significant improvements in both values of GIc and Gth. For example, one based upon a polyurethane and styrenebutadiene-core/functionalised polymethylmethacrylate-shell particles gave a value of 2570 J/m2 and 438 J/m2 for GIc and Gth, respectively.