The importance of cavitation process in the toughening of epoxy and polyamide.

Abstract

CTBN-type reactive rubbers have long been known to be effective tougheners for certain 'ductile' epoxies. It is believed that the major energy absorbing mechanism in these systems is enhanced plastic deformation of the matrix in the form of localized shear banding and plastic void growth. However, there exists controversy regarding how this plastic deformation is enhanced by the addition of the rubber particles. Some investigators proposed that cavitation of rubber particles, which is a phenomenon consistently observed in fractured specimen of rubber-modified epoxies, relieves the tri-axial constraint created at the crack tip region which then allows the matrix to yield more readily. Others, however, have suggested that it is the stress concentration caused by the rubber particles which leads the enhanced plastic deformation of the matrix. Thus, according to this theory, the cavitation of rubber particles is not necessary because the stress concentration around a rubber particle and a void is essentially the same. It is the purpose of this research to demonstrate that the cavitation of CTBN-rubber is a very important step in the toughening of epoxies. To achieve this, several specifically designed mechanical tests were performed and combined with microscopy to study the deformation and fracture of unmodified and CTBN-rubber-modified epoxies. It has been shown that: (1) the cavitation of rubber particles is very effective in relieving transverse constraint in constrained condition, (2) the constraint-relieving effect changes the fracture mode from cracking in unmodified epoxy to plastic tearing in CTBN-rubber-modified epoxy, (3) compared with the enhanced shear plastic deformation in the epoxy matrix, plastic void growth after cavitation makes only a minor contribution to the increase in toughness, and (4) when the cavitation of CTBN-rubber is suppressed by superimposing hydrostatic pressure, the particles no longer toughen epoxy. It has also been demonstrated that the same cavitation mechanism is responsible for the impact toughening of polyamide-6, a thermoplastic material undergoes brittle failure by crazing-cracking mechanism in constrained conditions, by BRXP-elastomer.