Continuous ultrasonic devulcanization comparison of carbon black filled synthetic isoprene and natural rubbers

Continuous ultrasonic devulcanization of carbon black (CB) filled isoprene rubber (IR) was carried out. Similar to unfilled IR, power consumption during the devulcanization of the filled IR increased with the increase of ultrasonic amplitude indicating a greater extent of devulcanization. This was in contrast with the devulcanization of CB filled natural rubber (NR) where power consumption showed a maximum at an intermediate amplitude suggesting a lesser extent of devulcanization at higher amplitude. Such a behavior evidently resulted from the difference in the amount of stereo-regular structures in the IR and NR rubbers. Gel fraction and crosslink density of the virgin vulcanizates, devulcanizates and revulcanizates were characterized. Kinetics of revulcanization, rheological properties of the devulcanizates and mechanical properties of revulcanizates were compared with those of virgin uncured IR and their vulcanizates. The effect of processing oil on the IR vulcanization, devulcanization and revulcanization was examined indicating a slight delay of the vulcanization and lower torque without affecting the degree of reversion. The addition of oil caused more devulcanization as indicated by an increase in the ultrasonic power consumption and a reduction of the gel fraction and crosslink density of devulcanized rubbers. The revulcanization of the IR occurred without the induction period, regardless of the presence of CB and processing oil. The retarder introduced in the recipe was effective in improving the scorch safety and minimizing the reversion in revulcanization. The experimental data on normalized gel fraction versus crosslink density of filled IR and NR at low CB loadings was found to fall into the master curve, which is probably determined by the main chain structure of both rubbers consisting of cis-1, 4 isoprene. At high CB loadings, some differences were observed. The simulation results predicted such a behavior.