Sources of toughness in modified epoxies.

Abstract

The sources of toughness improvement have been examined in epoxies modified with either soft rubber particles or rigid thermoplastic particles. Rubber-modified epoxies are toughened by a number of mechanisms including crack path defection, particle bridging, rubber particle cavitation, and plastic deformation in the epoxy matrix. The most potent of these toughening mechanisms involves cavitation of the rubber particles and concomitant shear banding in the matrix. This mechanism has been found to be influenced by three material parameters: first, the rubber particle size determines whether the particles will cavitate or merely bridge the two crack surfaces; second, the resistance to cavitation of the rubber particles determines the degree of toughness improvement provided by small rubber particles; and third, matrix ductility has a pronounced effect on toughenability when small rubber particles are used. Thermoplastic-modified epoxies are found to be toughened by the generation of microcracks in the epoxy matrix surrounding the crack tip. These microcracks are triggered by the presence of precipitated thermoplastic spheres. Two types of microcracks are observed. The first type are secondary microcracks which are not connected to the main crack and are oriented perpendicular to the maximum principal stress. The second type are bifurcations which appear to be connected to the main crack and are oriented at various angles, up to 30 degrees, away from the main crack plane. The type of microcrack generated by the various thermoplastic spheres appears to be influenced by the adhesion between the spheres and the matrix.