Toughening of thermoset polymers by rigid crystalline particles

The toughening of an aromatic amine-cured diglycidyl ether of bisphenol-A epoxy with particles of crystalline polymers was studied. The crystalline polymers were poly(butylene terephthalate), nylon 6, and poly(vinylidene fluoride). Nylon 6 and poly(vinylidene fluoride) were found to toughen the epoxy about as well as did an equivalent amount of CTBN rubber. Poly(butylene terephthalate) was found to toughen the epoxy about twice as well as did the rubber. The toughness of poly(butylene terephthalate)--epoxy blends was independent of particle size for sizes in the range of tens of mu m, but the toughness of the nylon 6--epoxy blends decreased with increasing particle size for sizes < approx 40 mu m. There was no loss of either Young's modulus or yield strength of the epoxy with the inclusion of either nylon 6 or poly(butylene terephthalate) and less loss of these with the inclusions of poly(vinylidene fluoride) than with the inclusions of rubber. Toughness seems to have arisen from a combination of mechanisms. The poly(butylene terephthalate)--epoxy blends alone seem to have gained toughness from phase-transformation toughening. Crack path alteration and the formation of steps and welts and secondary crack bridging seem to have accounted for an especially large part of the fracture energy of the poly(vinylidene fluoride)--epoxy blends. Secondary crack nucleation contributed to the toughness of the nylon 6--epoxy blends.