Rubber devulcanization process

The need for a commercially viable devulcanization process has existed for years and continues to be a goal. Devulcanization without rubber chain degradation offers the potential for recycling used tires back into new products without sacrificing performance. 

Considerable effort has been directed to solving this problem. Although chemical probes have been developed that selectively cleave carbon-sulfur and sulfur-sulfur bonds but not carbon-carbon bonds, most of the effort on devulcanization processes has been focused on providing a usable form of rubber suitable for use as a reclaimed material in new articles. ... 

Radial tires (see later) do not use reclaimed rubber because they require higher abrasion resistance that cannot be attained by mixing reclaimed rubber. Better processes for the production of higher quality reclaimed rubber are needed in order to use it for radial tires. To improve the quality of reclaimed rubber, cross-links in vulcanizates should be severed selectively during a devulcanization process and no low-molecular-weight compound such as swelling solvent should remain in the reclaimed rubber after the devulcanization process. 

A devulcanization process that utilizes supercritical CO2 as a devulcanization reaction medium in the presence of diphenyl disulfide as a devulcanizing reagent has been reported [45]. The process devulcanizes unfilled NR vulcanizates effectively. Further, a comparison of measured sol/gel components as well as dynamic mechanical properties of the devulcanized rubber products of filled and unfilled NR vulcanizates has indicated that the presence of carbon black in the vulcamizate does not disturb the devulcanization in supercritical CO2. 

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. 

Also, attempts were made to add a certain amount of a fresh carbon black into the devulcanized rubber. It was shown that the vulcanizates containing a fresh carbon black exhibited better properties than the revulcanized rubber without an addition of fresh carbon black. However, in some cases, even carbon-filled devulcanized rubber shows mechanical properties similar to or better than the original rubber. In particular, this was shown for EPDM roofing membrane containing carbon black and significant amount of oil (Yun and Isayev, 2003b). Apparently, oil plays an important role in devulcanization process. Possibly, the presence of oil prevents a deactivation of the filler that was observed in vulcanizates not containing oil. But in order to prove this hypothesis further experiments are required. 

Another chemical method was recently proposed [43]. It is based on the use of 2-butanol solvent as a devulcanizing agent for sulfur-cured rubber under high temperature and pressure. It is claimed that the molecular weight of the rubber is retained, and its microstructure is not significantly altered during the devulcanization process. However, the process is extremely slow and requires separation of the devulcanized rubber from the solvent. 

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... 

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. 

Rubber Reclaim. For the traditional rubber "reclaim," crumb rubber is mixed with water, oil, and chemicals and heated under pressure, thus rupturing the carbon-sulfur bonds that cross-link the molecular matrix. The resulting partially devulcanized rubber may be formed into slabs or bales and shipped to manufacturers who process and vulcanize it for use as an alternative to virgin rubber to use in tires or to make mats and other rubber products. 

---