Rubber-Thermoset Blends: Micro and Nano Structured

Highly cross-linked thermosets which are susceptible to brittle failure can be effectively toughened by blending them with rubbers. However, if the materials are already cross-linked, then blending with rubber as it is done in the conventional way with thermoplastics, is virtually impossible. Thus it asks for an altogether different method to accomplish successful blending. Initially, miscible liquid rubbers in small amounts or preformed rubber particles are incorporated in the matrix of curing agent incorporated precured thermosets resins and then the whole mass is subjected to curing. The phase separation, in case of liquid rubber toughening depends upon the formulation, processing and curing conditions and incomplete phase separation may occur resulting in unwanted lowering of glass transition temperature. The phase separation in case of liquid rubber is based upon nucleation and growth. In case of preformed rubber particles, these difficulties are not encountered and the resulting morphology can be better controlled. However, the problem of proper dispersion of these particles in the themoset resins limits the use of this method. The improvement in fracture resistance occurs in either case due to dissipation of mechanical energy by cavitation of rubber particles followed by shear yielding of the matrix. Rubber particle size plays an important role in improving toughening and very small or very large sizes are undesirable. The toughenability increases with increase in inherent ductility of the matrix.