Ultrasonic devulcanization of sulfur vulcanized natural rubber

Abstract

The high-energy ultrasound could be used to devulcanize rubber as it can focus energy into localized sites for selective bond rupture. The research work reported to date suggests that the ultrasonic technology is more suited to convert rubber waste to a usable material efficiently, effectively and environmental friendly. The ultrasonic devulcanization reactor consisted of three main sections, namely a power source, ultrasonic transducer with sample holding unit, and a monitoring system to measure the amplitude, frequency and power. N-cyclohexyl-2-benzthiazyl sulfenamide (CBS) accelerated unfilled natural rubber vulcanized with conventional sulfur vulcanizing system and with efficient sulfur vulcanizing system were used as the model rubber compounds in these experiments. 2 mm thick vulcanized rubber sheets were directly kept on the vibrating diaphragm of the ultrasonic transducer. The frequency of ultrasonic wave was varied in a range of 20 to 50 kHz and the power level was varied up to 800 watt. The treatment time was limited to 10 minutes when treated at high power levels. The vibrating amplitudes were measured at different power levels with the variation of ultrasonic frequency. Curing behaviour, gel content and cross-link density were studied for rubber samples devulcanized at different process conditions. The increase in cross-link density and gel content of the samples treated at lower amplitudes indicated the formation of additional cross-links. However, the higher vibrational energies associated with high amplitudes resulted in lower cross-link densities and gel contents indicating a breakdown of bonds. Cure curves of virgin and devulcanized NR samples suggested that the fast initial curing of devulcanized NR was due to the presence of active sufidized rubber molecules formed due to break down of some cross-links during devulcanization. The lower maximum torque values observed in the devulcanized samples were due to the partial breakdown of C-C bonds in the main chain. The tensile properties of the revulcanized samples gave comparable results with that of virgin rubber. A theoretical process model was developed to express the extent of devulcanization in terms of cross-link density. It was based on the vibrational energy transfer mechanism. The model treated the vulcanized rubber as a pure elastic solid containing void regions. Experimental and theoretical values lied within ± 10% error limits. The model showed that the media effect on the nature of void excitation was significant and the viscoelasticity was also considerable. However, the effect due to surface tension was negligible.