Background to Ultrasonic Process

These types of processes make use of the energy supplied by ultrasonic waves to preferentially break sulfur-sulfur bonds within crosslinks. They offer advantages over some of the other techniques in that they do not normally use solvents, or chemical agents, which helps to control cost and means that some health and safety issues do not have to be taken into consideration. They also have the potential advantage of being able to be operated continuously through the use of extruders. 

The technique has been found by workers such as Isayev to be suitable for use with a great variety of rubbery networks (Section 4.6.2), and in the presence of rigid filler particles. Whereas the ultrasound reactor and power settings to produce the desired devulcanisation performance vary significantly between the types of rubbers, the general nature of the network destruction and degradation are thought to be relatively similar. Post-process NMR studies on the different rubber systems have shown structural variations. 

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]. 

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