Rubber particles network cavitation

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

Figure 5. Rubber particles with crazes in HIPS. The rubber particles are strongly elongated by cavitation and fibrillation in the rubber network around PS inclusions (HVEM image). The deformation direction is vertical.

Fig. 19.39 Fracture mechanism and energy dissipation. The flocculated rubber network becomes redispersed by the impact energy the crack propagates at the interface between free matrix and adsorbed polymer matrix layer cavitation occurs at the point where redispersion begins. (A) Formation of flocculated and tubular structures from physically dispersed rubber particles. In contrast to carbon black particles, the rubber particles may fuse in tubular form. (B, C) Fracture mechanism and energy dissipation. The flocculated rubber network becomes redispersed by the impact energy the crack propagates at the interface between the free matrix and the adsorbed polymer matrix layer cavitation occurs at the point where redispersion begins.

This is a simplification of the process occurring in a curing resin-hardener system and a detailed discussion may be found in Pascault et al (2002), Williams et al (1997) and Inoue (1995). The main parameter that it is important to control in the reactive phase separation is the diameter of the elastomer particle. This is because the toughness of the resulting network is controlled by the energy-absorbing mechanisms such as particle cavitation and rubber bridging of cracks. Also of importance is the limitation of the effect of the rubber dispersed phase on the critical properties of the cured epoxy resin such as the stiffness and Tg. This will be affected by the extent to which the rubber dissolves in the matrix-rich phase. 

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