Cavitation rubber network

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.

Figure 11.123. Fracture mechanism and energy dissipation the flocculated rubber network is becoming re-dispersed by impact energy the crack is propagating at the interface between free matrix and adsorbed polymer matrix layer cavitation occurs at the point where re-dispersion begins. [Reproduced from ref. 89 with kind permission of Huthig and Wepf publishers.]...

It is well known that some amounts of cavities or small bubbles are present in rubber during any type of mbber processing (Kasner and Meinecke, 1996). The formation of bubbles can be nucleated by precursor cavities of appropriate size (Gent and Tompkins, 1969). The proposed models (Isayev et al., 1996a,c,d Yashin and Isayev, 1999,2000) were based upon a mechanism of rubber network breakdown caused by cavitation, which is created by high intensity ultrasonic waves in the presence of pressure and heat. Driven by ultrasound, the cavities pulsate with amplitude depending mostly upon the ratio between ambient and ultrasonic pressures (acoustic cavitation). 

It is known that, in contrast to plastics, rubber chains break down only when they are fully stretched (Kinloch and Young, 1983 Kausch, 1987). An ultrasonic field creates high frequency extension-contraction stresses in crosslinked media. Therefore, the effects of rubber viscoelasticity have been incorporated into the description of dynamics of cavitation (Yashin and Isayev, 1999, 2000). The devulcanization of the rubber network can occur primarily around pulsating cavities due to the highest level of strain produced by the power ultrasound (Yashin and Isayev, 2000). 

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 13.6 Relative contributions (%) of the different toughening mechanisms in epoxy networks versus temperature ( ) rubber bridging ( ) shear yielding and (A) cavitation. (From the results of Huang et a ., 1993b.)... 

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. 

The exact mechanism by which the ultrasonic treatment causes devulcanisation is still under study. One theory is that acoustic cavitation occurs within the rubber and it is the collapse of these cavities that causes devulcanisation to occur. However, workers have also postulated that the collapse of these cavities was not the primary method of devulcanisation and that degradation of the network around the cavities should also be considered. The theories surrounding the mechanism by which devulcanisation is achieved by the use of ultrasound, and other important characteristics of the technique, are covered by a comprehensive review of ultrasonic devulcanisation written by Isayev and Ghose [62]. 

---