Ultrasonically devulcanized rubbers

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. 

Ultrasonically devulcanized rubbers consist of sol and gel. The gel portion is typically soft and has significantly lower crosslink density than that of the original vulcanizate. Due to the presence of sol the devulcanized rubber can flow and is subjected to shaping. Crosslink density and gel fraction of ultrasonically devulcanized rubbers were found to correlate by a universal master curve (Yushanov et al., 1996, 1998 Diao et al., 1999). This curve is unique for... 

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)... 

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. 

Zhu, S.H., Penbdis, A., Tzoganakis, C., and Ginzel, E. (2012) Ultrasonic properties and morphology of devulcanized rubber blends. J. Appl. 

Recycle Today there is a great emphasis on devulcanization methods to recycle the rubber in tires and other rubber products. Some of these new devulcanization methods use supercritical fluid technology, ultrasonic techniques, microwave energy, and biological modification.f These methods are explained in detail in the entry, which deals primarily with devulcanization. 

Isayev, A.I. Chen, J. Tukachinsky, A. Novel ultrasonic technology for devulcanization of waste rubbers. Rubber Chem. Technol. 1995, 68, 267. 

Isayev, A.I. Kim, S.H. Yushanov, P. Ultrasonic devulcanization of SBR rubber experimentation and modeling based on cavitation and percolation theories. Rubber Chem. Technol. 1998, 71, 168. 

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. 

Ultrasonic reclaiming of NR was reported by Okuda and Hatano in 1987, which was also patented. They subjected the NR vulcanizate to 50 kHz ultrasonic energy for 20 min to achieve devulcanization followed by revulcanization and obtained reclaimed rubber with similar properties to those of original rubber. Isayev and coworkers reported in a number of publications the phenomenon of devulcanization by ultrasound energy and they also patented their developments. ... 

Under the license from the University of Akron for the ultrasonic devulcanization technology, NFM Co. of Massillon, Ohio, has built a prototype of the single-screw extruder for ultrasonic devulcanization of tire and rubber products (Boron et al., 1996 Boron, 1999). It was reported that retreaded truck tires containing 15 wt.% and 30 wt.% of ultrasonically devulcanized carbon black-filled SBR had passed the preliminary dynamic endurance test (Boron, 1999). 

Extensive studies on the ultrasonic devulcanization of rubbers and some preliminary studies on ultrasonic decrosslinking of crosslinked plastics were... 

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). 

FIGURE 15.6 The stress-strain curves of unfilled virgin vidcanizates and revulcanized SBR obtained from rubbers devulcanized in coaxial reactor at various values of ultrasonic amplitude. 

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). 

Recently, ultrasonic devulcanization of tire rubbers was carried out in the presence of ozone (Lee et al., 2009a). This extruder was similar to the coaxial ultrasonic extruder depicted in Figure 15.3a. It was shown that by using treatment by ultrasound and ozone combination the mechanical properties of revulcanized rubbers were improved. 

FIGURE 15.11 Photographs of GRT powder, and devulcanized and revulcanized rubbers. Devulcanization conditions in ultrasonic twin-screw extruder are feed rate of 0.19 g/s, ultrasonic amplitude of 13 Jim, frequency of 40 kHz, ultrasonic zone gap of 2.5 mm temperature at various zones are from the feeding section to die 100/180/180/180/160/150°C. 

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