Dynamic Permittivity Measurement of Ground-Tire Rubber (GTR) during Microwave-Assisted Devulcanization

Abstract

Many efforts are being made to find innovative ways of recycling rubber from end-of-life tires (ELTs), also called ground tire Rubber (GTR). Recycling through devulcanization allows the reintroduction of rubber back into the manufacturing industry. Such a process requires providing enough energy to break the sulfur links, while preventing damage to the polymeric chain. Microwave heating is controllable, efficient, and it does not rely on conventional heating mechanisms (conduction, convection) which may involve high heating losses, but rather on direct dielectric heating. However, to adequately control the microwave-assisted devulcanization performance, a thorough knowledge of the GTR permittivity versus temperature is required. In this work, GTR permittivity was monitored during its devulcanization. A resonant technique based on a dual-mode cylindrical cavity was used to simultaneously heat rubber and measure its permittivity at around 2 GHz. The results show sharp changes in the GTR permittivity at 160 and 190 °C. After the GTR cooled down, a shifted permittivity evidences a change in the GTR structure caused by the devulcanization process. Microwave-assisted devulcanization effectiveness is proven through time-domain nuclear magnetic resonance (NMR) measurements, by verifying the decrease in the cross-link density of processed GTR samples compared to the original sample.

Real-Time Monitoring of Ground-Tire Rubber Microwave Devulcanization with Thermal and Electrochemical Sensors

“…The GTR had an apparent bulk density of 400 kg/m 3 , with granule sizes ranging from 0.5 to 1 mm. The MUT's primary components are as follows [27]: natural rubber (36 %), carbon black (27 %), synthetic rubber (22 %), inorganic components (8.5 %), additives and oil (4.6 %), and sulphur (1.9 %).…”

“…Samad et al [29] investigated the electrochemical gas sensor response under various conditions and concluded that it could provide a great opportunity to collect high-quality data on air pollutants. Additionally, in [26], three electrochemical sensors were employed to track the VSCs generated during the microwave-assisted devulcanization of GTR in the presence of nitrogen and a small oxygen percentage. When these VSCs were generated during the microwave irradiation, the experimental trials in this study demonstrated that the pyrometer was severely blinded.…”

“…The use of pyrometers or infrared cameras for monitoring the GTR surface temperature from a zenithal point is not reliable during the effective process of devulcanization, as described in [26], since fumes produced during chemical reactions blind both types of sensors.…”

“…According to several recent studies, the gaseous by-products of devulcanization could be used as control and monitoring variables [25,26]. Song et al [17], for instance, came to the conclusion that the desulphurization reaction and the sulphur generated during the microwave procedure are related, and according to [25], some major chains are broken during the microwave processing of waste rubber, releasing volatile organic compounds (VOCs) and VSCs, including hydrogen sulphide (H 2 S), sulphur dioxide (SO 2 ), and carbon disulphide (CS 2 ).…”

“…Despite the fact that permittivity is seldom employed to evaluate the processed rubber, in this contribution, it was evaluated by measuring its permittivity since it is directly related to its chemical composition and structure, as explained in our previous work [27]. Permittivity controls a material's reactivity to electromagnetic fields.…”