Recycling of rubber waste through thermomechanical devulcanization

Abstract

Approximately 70% of the rubber produced in the world is used in tires. The number of waste tires discarded worldwide each year is close to 330 million (approximately 4.4 million tons) and considering the average percent of natural and synthetic rubbers in tires is about 60%, 2.64 million tons of waste natural rubber are generated per year [1, 2]. In spite of all different ways of handling used tires, the most common is to deposit them in a landfill, creating a stock of tires. These stocks can cause fire danger and provide ambient for rodents, mosquitoes and other pests, causing health hazards and environmental problems [3]. Hence, rubber waste disposal is an environmental problem. Because of the three-dimensional structure of the rubbers and their specific composition that include several additives, their recycling is a current technological challenge. The technique which can break down the crosslink bonds in the rubber is called devulcanization. Strictly, devulcanization can be defined as a process where poly-, di-, and mono-sulfidic bonds, formed during vulcanization, are totally or partially broken. In the recent years super critical carbon dioxide (scCO2) was proposed as a green devulcanization atmosphere. This is because it is chemically inactive, nontoxic, nonflammable and inexpensive. Its critical point can be easily reach (31.1 °C and 7.38 MPa), and residual scCO2 in the devulcanized rubber can be easily and rapidly removed by releasing pressure [4]. In this study thermomechanical devulcanization of ground tire rubber (GTR) and ethylene propylene diene monomer rubber (EPDM) was performed in a twin screw extruder under diverse operation conditions. Supercritical CO2 was added in different quantities to promote the devulcanization. Temperature, screw speed and quantity of CO2 were the parameters that were varied during the process. The devulcanized rubber was characterized by its devulcanization percent and crosslink density by swelling in toluene. Results were analyzed using the Horikx model. Infrared spectroscopy (FTIR) and thermogravimetry (TGA) were also done, and the results were related with the Mooney viscosity. Regarding the GTR, the results showed that the crosslink density decreases as the extruder temperature and speed increases, and, as expected, the soluble fraction increase with both parameters. The Mooney viscosity of the devulcanized rubber decreases as the extruder temperature increases. The reached values were in good correlation (R= 0.96) with de the soluble fraction. In order to analyze if the devulcanization was caused by main chains or crosslink scission the Horikx's theory was used. Results showed that all experimental points falls between the theoretical curves, which means that the materials underwent the regeneration phenomenon. In the spectra obtained by FTIR it was observed that none of the characteristic peaks of the GTR were modified by the different devulcanization conditions. This was expected, because due to the low sulfur content (~1.4 phr) and the multiphasic composition of the GTR, it is very difficult to evaluate the devulcanization by this technique. The lowest crosslink density was reached with 1 cm3 /min of CO2, and the power consumed in that process was also near to the minimum. Regarding EPDM, all the experimental conditions caused the degradation of the polymer, without successful devulcanization. This reveals that it is necessary to find the correct devulcanization parameters for each type of rubber that would be devulcanized. The presented results encourage us to do further analyses to better understand the effect of the different conditions on the devulcanization process.