Toughening versus flexibilizing

The rubbery particles promote the absorption of strain energy by complex interactions, which are thought to involve both craze formation and shear deformations. The overall mechanism is influenced by the size of the microscopic rubbery particles. 

The toughening process generally renders the viscosity behavior non-Newtonian (i.e. dependent on the shear rate). Typically, these systems show a decrease in viscosity with an increase in shear rate, or pseudoplasticity. The sensitivity of the viscosity to the strain rate becomes greater as the resin system becomes more advanced. 

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Another approach for toughening UP and VE resins which has had success with epoxy resins is the use of liquid rubber (or elastomer) additives. The chief benefit of emplojdng liquid rubbers (LR) versus a flexibilized resin is that decreases in hardness, stiffness and heat-deflection properties can be minimized. During cure, the liquid rubber phase separates from the resin and is concentrated in a particulate phase. Very little of the rubber remains in solution with the cured resin so the resin s heat deflection temperature is for the most part unaffected. The toughness of the two-phase, or composite, material will be a function of the microstructure, which in turn will depend on processing and cure conditions. The subject of epoxy resin toughening is covered in separate articles in this book. 

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