Structure‐property relationships in rubber‐toughened epoxies

Abstract

AbstractEpoxies toughened with two reactive liquid rubbers, an epoxy‐terminated butadiene acrylonitrile rubber (ETBN) and an amino‐terminated butadiene acrylonitrile rubber (ATBN), were prepared and studied in terms of their structure property relationships. A two‐phase structure was formed, consisting of spherical rubber particles dispersed in an epoxy matrix. A broad distribution of rubber particles was observed in all the materials with most of the particles ranging in size from 1 to 4 μm, but some particles exceeding 20 μm were also found. Impact strength, plane strain fracture toughness (KIC), and fracture energy (GIC) were increased, while Young's modulus and yield strength decreased slightly with increasing rubber content and volume fraction of the dispersed phase. Both GIC and KIC were found to increase with increasing apparent molecular weight between crosslinks and decreasing yield strength. The increased size of the plastic zone at the crack tip associated with decreasing yield strength could be the cause of the increased toughness. An ATBN‐toughened system containing the greatest amount of epoxy sub‐inclusion in the rubbery phase demonstrated the best fracture toughness in this series. In the present systems, rubber‐enhanced shear deformation of the matrix is considered to be the major toughening mechanism. Curing conditions and the miscibility between the liquid rubber and the epoxy resin determine the phase morphology of the resulting two‐phase systems. Kerner's equation successfully describes the modulus dependence on volume fraction for the two‐phase epoxy materials.