Abstract
De-vulcanization of rubber has been shown to be a viable process to reuse this valuable material. The purpose of the de-vulcanization is to release the crosslinked nature of the highly elastic tire rubber granulate. For present day passenger car tires containing the synthetic rubbers Styrene-Butadiene Rubber (SBR) and Butadiene Rubber (BR) and a high amount of silica as reinforcing filler, producing high quality devulcanizate is a major challenge. In previous research a thermo-chemical mechanical approach was developed, using a twin-screw extruder and diphenyldisulfide (DPDS) as de-vulcanization agent.The screw configuration was designed for low shear in order to protect the polymers from chain scission, or uncontrolled spontaneuous recombination which is the largest problem involved in de-vulcanization of passenger car tire rubber. Because of disadvantages of DPDS for commercial use, 2-2-dibenzamidodiphenyldisulfide (DBD) was used in the present study. Due to its high melting point of 140 °C the twin-screw extruder process needed to be redesigned. Subsequent milling of the devulcanizate at 60 °C with a narrow gap-width between the mill rolls greatly improved the quality of the devulcanizate in terms of coherence and tensile properties after renewed vulcanization. As the composition of passenger car tire granulate is very complex, the usefulness of the Horikx-Verbruggen analysis as optimization parameter for the de-vulcanization process was limited. Instead, stress-strain properties of re-vulcanized de-vulcanizates were used. The capacity of the twin-screw extruder was limited by the required residence time, implying a low screw speed. A best tensile strength of 8 MPa at a strain at break of 160% of the unblended renewed vulcanizate was found under optimal conditions.
Highlights
Rubber in general, and passenger car tire rubber in particular, heavily resist material degradation and survive for a long time in the environment
This study described the adaptations needed for screw configuration and settings of a twin-screw co-rotating extruder, as dictated by the change from DBDS to DBD as de-vulcanization agent, as well as additional optimizations for the de-vulcanization of passenger car tire rubber
This raised substantial problems due to the fact that this material resulted in a different crosslink structure than a carbon black reinforced rubber: The occurrence of remaining visible grains in White Rubber Analysis (WRA) of the de-vulcanizates had a direct relation with the ratio between size of the smallest and largest visible grains in the ground tire rubber (GTR) and the absolute size of the hereof, It was shown that thermo-chemical-mechanical de-vulcanization can be performed in two steps: a mainly thermo-chemical process at the de-vulcanization temperature in the twin-screw extruder, and the mechanical part of the process at a much lower temperature on a mill
Summary
Passenger car tire rubber in particular, heavily resist material degradation and survive for a long time in the environment. Due to the number of 109 tires/year produced worldwide (derived from [1,2,3]), wasted passenger car tires pose enormous environmental problems when dumped on a land-fill, because of their intrinsic resistance against decomposition, or when being burned because of the soot and fumes they produce. These problems are addressed by industry and academia by developing processes for re-plastcization like reclaiming, and for decomposition like pyrolysis, with the aim to reuse the valuable resulting materials for new products. The specific issues regarding de- and re-vulcanization of granulated passenger car tires, containing SBR and BR as synthetic polymers and a relatively high amount of silica as active filler, compared with carbon black, are not addressed
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