Technology, devulcanization

Numerous publications are devoted in the past to the study of the effect of ultrasound under static conditions on polymer solutions (Basedow and Ebert, 1987 Price, 1990 Price etal., 1992 Schmid and Rommel, 1939 Schmid, 1940 Jellinek and White, 1951) and on polymer melts during extrusion (Isayev et al., 1987, 1990 Isayev, 1990 Isayev and Mandelbaum, 1991 Peshkovsky et al., 1983 Garcia and Isayev, 1991). Around the 1970s, the effect of vibration on polymer melts was studied by many researchers, as summarized in the review paper by Fridman and Peshkovsky (1990). Significant efforts have also been made to understand the mechanism of the effect of ultrasoimd on fluids (Suslick, 1989 Suslick et al., 1990) and degradation of polymer in solution (Gooberman, 1960). 

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. 

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 

FIGURE 15.3 Schematic diagram of coaxial reactor (a), barrel reactor (b), and grooved barrel reactor (c) built for devulcanization of rubbers. 

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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. 

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. 

A technology for the devulcanization of sulfur-cured scrap elastomers was developed, (Kohler and O Neill, 1997a,b) using a material termed Delink (Sekhar and Kormer, 1995), and this technique was designated as the Delink process. In this process, 100 parts of 40 mesh or finer mbber crumb is mixed with... 

In addition to use of organic chemicals, mbbers can be devulcanized by means of inorganic compounds. Discarded tires and tire factory waste were devulcanized by desulfurization of suspended mbber vulcanizate cmmb (typically 10 30 mesh) in a solvent such as toluene, naphtha, benzene, or cyclohexane in the presence of sodium (Myers et al., 1997). The alkali metal cleaves mono-, di-, and polysulfidic crosslinks of the swollen and suspended vulcanized mbber crumb at around 300°C in the absence of oxygen. However, this process may not be economical because the process involves swelling of the vulcanized mbber crumb in an organic solvent where the metallic sodium in molten condition should reach the sulfidic crosslink sites in the mbber cmmb. Also, solvent may cause pollution and be hazardous. A technology was also proposed to reclaim powder mbbers using an iron oxide phenyl hydrazine based catalyst (Kawabata et al., 1981) and copper(I) chloride-tributylamine catalyst (Kawabata et al., 1979). 

This technology (Benko and Beers, 2002a,b,c) utilizes a solvent to treat (devulcanize) the surface of rubber crumb particles of sizes within about 20-325 meshes. It is a variation of earlier disclosed technology (Hunt et al., 1999). The process is carried out at a temperature range between 150°C and 300 C at a pressure of at least 3.4 MPa in the presence of solvent selected from the group consisting alcohols and ketones. Among various solvents, the 2-butanol exhibited the best ability to devulcanize sulfur-cured SBR rubber. Duration of the process is above 20 min. 

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)... 

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