Possible phase transformation toughening in polymers.

Abstract

The toughness of brittle thermoset resins that are non-toughenable by inclusion of rubber particles have been considerably enhanced by inclusion of crystalline polymers without loss of inherent properties of the resins. The crystalline polymers studied for inclusion in thermoset resins were poly(butylene terephthalate) (PBT), nylon 6, and poly(vinylidene fluoride). Of the three, PBT has been found to be the most effective, possibly due to a phase transformation of the PBT in the region ahead of the crack tip during crack growth. The concept of "phase transformation toughening" is well-known in partially stabilized zirconia. The normal $\alpha$-phase of PBT is analogous to the tetragonal phase of zirconia used in ceramics in that they both can be transformed to a less dense phase. Direct evidence for the phase transformation in the toughening of thermoset resins has been difficult to obtain because of a back-transformation of PBT when the stress is removed. Indirect evidence has been found from the inclusion of both PBT and rubber, however. The usual cavitation of rubber in rubber-epoxy blends is suppressed in the presence of PBT phase. This indicates that the stress field in front of the crack tip is changed, as if caused by a volume expansion of the PBT phase. The possibility of "phase transformation toughening" in the PBT-epoxy system is supported also by the dependencies of the toughness and the fracture mechanism on the micro-morphology of PBT which is affected by processing variables. Other energy absorbing mechanisms do seem to be active in each of the crystalline polymer-toughened epoxies studied. They are crack path alteration, primary and secondary crack bridging, and ductile fracture of the matrix and included polymers.