| Recycling of waste tires and vulcanized rubbers is a challenging problem because of the presence of a three dimensional crosslink network. The high power ultrasonic devulcanization process is the most recent approach. Though ultrasonic devulcanization is a fast and clean method of recycling, the process needs further improvement in order to be suitable for industrial applications. Therefore, the development of improved ultrasonic reactors and optimization of physical properties of recycled rubbers are required. In an attempt to resolving these issues, the present work describes the results of comparative studies of three ultrasonic devulcanization reactors, coaxial, barrel and grooved barrel reactor, built for recycling of waste tires. The degree of devulcanization of rubbers in these reactors was directly related to the residence time in the devulcanization zone and the level of ultrasonic power. Higher output and improved properties were achieved using a newly built barrel reactor. In addition, recyclability of unfilled and carbon black (CB) filled EPDM rubbers and EPDM based roofing membrane was investigated. The characterization of the materials before and after recycling was carried out via the measurement of gel fraction, crosslink density, cure behavior, dynamic and tensile properties. The tensile strength of revulcanized unfilled EPDM was found to be much higher than that of the virgin vulcanizates with the elongation at break being practically intact. However, due to the deactivation of filler under ultrasonic treatment, mechanical properties of revulcanized CB filled EPDM deteriorated with an increase of filler concentration. At the same time, the tensile strength of the revulcanized roofing membrane containing CB and oil was found to be close to that of the virgin one.; The Dobson-Gordon theory of rubber network statistics has been employed in order to interpret the experimental data on the dependence of gel fraction on crosslink density in the gel of devulcanized EPDM rubbers. A fairly good agreement between experimental and theoretical data was achieved under the assumption of spatial-temporal randomness of rubber degradation. |
