Ultrasonic Devulcanisation

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]. 

Figure 4.6 Schematic diagram of single-screw and twin-screw ultrasonic devulcanisation extruders. Reproduced with permission from Dr A.L Isayev, The University of Akron, Akron, OH, USA. 2014, Dr A.I. Isayev, The University of Akron...

Rubber Chemistry and Technology 70, No.l, March/April 1997, p. 120-8 VULCANISATION OF ULTRASONICALLY DEVULCANISED SBR ELASTOMERS... 

As a consequence, ultrasonically treated crosslinked rubbers and plastics become soft such that a possibility exists to reprocess and to shape waste rubbers and plastics into new useful products or to mix them with virgin polymers. The devulcanized rubber can also be revulcanized in a manner similar to that employed with a virgin rubber. The process of devulcanisation and decrosslinking is very short, on the order of a second or less, and can be controlled. The new process does not use any solvent and, therefore, is environmentally clean. 

In addition to the two reviews that are mentioned at the beginning of this section, a number of other relatively recent reviews of the processes and methods that have been developed and evaluated for the devulcanisation of waste rubber are available. For example, Majumdar published an overview in 2009 in the Chemical Weekly journal [4], which covered the challenges that face workers in this field and covered the main types of systems that have been developed (i.e., chemical, microwave, ultrasonic and so on). This article also covered the production and use of rubber crumb from waste rubber. In another article [5], Majumdar reviews the three main sources of reclaimed rubber that are available in the marketplace (rubber crumb, rubber powder and chemically digested reclaimed sheet) and describes their properties and uses. 

Sulphur cured SBR elastomers devulcanised by high power ultrasonic irradiation, were revulcanised using the same curing conditions as for virgin SBR. Crosslink density and gel fraction of virgin, devulcanised, and revulcanised SBR were investigated. 21 refs. 

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