Ultrasonic devulcanization process

Ultrasonic devulcanization, therefore, causes significant degradation of polymer chains. A simple model based on a purely topological consideration was proposed and simulation of the process was carried out. In the model they have assumed a breakup... 

Chemical processes have been known for many years but are either ineffective in selectively cleaving cross-links or commercially not viable. Recent developments may eventually provide a true devulcanization process that is commercially viable. These include ultrasonic devulcanization, biotechnological devulcanization, and supercritical fluid devulcanization. 

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown (Isayev et al., 1996b) that the revulcanization process of devulcanized SBR was essentially different from those of the virgin SBR. The induction period is shorter or absent for revulcanization of the devulcanized SBR. This is also true for other unfilled and carbon black-filled mbbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules resulting in crosslinking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber (Levin et al., 1997a). 

In search for a more economical process for recycling used tires, the ultrasonic devulcanization technology was further investigated to develop a feasible process to minimize the stockpiles of waste tires. This process induces the cleavage of the chemical networks through the combination of ultrasonic and chemical devulcanization (Kim et al., 2003). According to authors, the products from devulcanization of tires are carbon black and extended oil that can be used in many applications. A claim was also made that the technology can separate sulfur. 

Recently, a novel continuous process has been developed for devulcanization of rubbers as a suitable way to recycle used tires and waste rubbers [83-119]. This technology is based on the use of high-power ultrasounds. The ultrasonic waves of certain levels, in the presence of pressure and heat, can quickly break up the three-dimensional network in crosslinked rubber. The process of ultrasonic devulcanization is very fast, simple, efficient, and solvent and chemical free. Devulcanization occurs at the order of a second and may... 

Extensive studies on the ultrasonic devulcanization of rubbers and some preliminary studies on ultrasonic decrosslinking of crosslinked plastics were carried out [83-122]. It was shown that this continuous process allows one to recycle various types of rubbers and thermosets. As a most desirable consequence, ultrasonically devulcanized rubber becomes soft, therefore making it possible for this material to be reprocessed, shaped, and revulcanized in very much the same way as the virgin rubber. This new technology has been used successfully in the laboratory to devulcanize a ground tire rubber (GRT)... 

After the microwave techniques, ultrasonic energy was used for the devulcanization of cross-linked rubber. The first work with ultrasonic energy was reported by Pelofsky in 1973, which was patented.In this process solid rubber articles such as tires are immersed into a liquid, which is then kept under a source of ultrasonic energy whereby the bulk rubber effectively disintegrates upon contact and dissolves into liquid. In this process ultrasonic irradiation is in the range of about 20 kHz and at a power intensity of greater than 100 W. 

The application of ultrasonic waves to the process of devulcanizing rubber is an attractive field of study. Most references indicate that rubber is vulcanized by ultrasound rather than devulcanized. Rubber devulcanization by using ultrasonic energy has been first discussed in Okuda and Hatano (1987). It was a batch process in which a vulcanized rubber was devulcanized at 50 kHz ultrasonic waves after treatment for 20 min under static conditions. The process claimed to break down carbon-sulfur bonds and sulfur-sulfur bonds, but not carbon-carbon bonds. The properties of the revulcanized rubber were found to be very similar to those of the original vulcanizates. 

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. 

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