The effect of cross-link density on the efficacy of rubber toughening in thermosetting resins has been studied. Increasing the cross-link density changes the fracture toughness at a given temperature by both shifting the glass transition temperature (and thus the proximity of the test temperature to the glass transition temperature) and limiting the total crack-tip strain that can be realized at any given temperature.The temperature at which peak toughness is observed is shifted 50-120 °C lower in rubber-toughened systems compared to untoughened systems, with the peak of the rubber-toughened systems being four times the value observed in nontoughened systems. High cross-link density systems were found to be unresponsive to rubber toughening at any temperature.INCREASING THE CROSS-LINK DENSITY may directly affect fracture toughness at a given temperature (e.g., ambient temperature) by limiting the degrees of freedom for shear deformation. Alternatively, an increase in cross-link density has a potentially indirect effect on the fracture toughness at a given temperature by changing the glass-transition temperature (Tg) and, thus, the proximity of the service or testing temperature to Tg.
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