On Thursday, May 27th, 2021, Mr. Rounak Ghosh has successfully defended his PDEng thesis on the topic: 'Design of a devulcanization process for tire material in a twin screw extruder'.
The design of a continuous devulcanization process for tire materials in twin screw extruder was the main goal of the project. In this study, sulphur-vulcanized natural rubber was thermo-mechano-chemically devulcanized, without any additives and also in presence of oil and a devulcanization aid (DA). The devulcanization process was designed in terms of temperature, residence time, fill factor, rotor speed, oil and DA concentration; these parameters were optimized for the best balance of degree of crosslink scission and mechanical properties.
The DA’s used in this study are conventional rubber additives: two silanes, which are generally used as coupling agents for silica (bis-(3-triethoxysilylpropyl)tetrasulfide, TESPT, and bis-(3-triethoxysilylpropyl)disulfide, TESPD), and an accelerator (N-cyclohexyl-2-benzothiazolesulfenamide, CBS). The sulphur moieties of these additives enable them to act as devulcanization aids, making the devulcanization process more effective and improving the final properties. Along with the characterization of the feed buffing, the following properties of the devulcanized, compounded as well as revulcanized rubber were evaluated:
§ Processability: viscosity, curing kinetics
§ Devulcanization efficiency: crosslink density, the ratio of crosslink to random scission
§ Filler-filler interaction: Payne effect
§ Mechanical properties: tensile strength, elongation at break, hardness, resilience
§ Optical properties: white rubber analysis, grey scale test, macrodispersion by microscopy
The viscosity as well as the stress strain properties of the final devulcanizate were given by the industrial partner, as requirements for processing and final properties of the envisaged application.
This devulcanization process design and optimization lead to a material which fulfilled the requirements in terms of degree of devulcanization and mechanical properties. However, the processing properties, in particular viscosity, were not satisfactory. Therefore, an additional post-treatment of the devulcanizate was designed, the compounding study. With the addition of oil, a lower viscosity and thus an easier processable material was achieved. Compounding of silica plus silane further improved the mechanical properties due to enhancement of filler-polymer interaction. Out of this process design study, TESPD as DA resulted in the highest quality of devulcanizate in terms of crosslink scission and mechanical properties.
The best devulcanized sample from the lab scale optimization was processed at a 80% fill factor, 160⁰C temperature, 6 minutes residence time with 4.5% TESPD and 5% TDAE oil: the tensile strength increased to 12.6 MPa, with 200% elongation at break, 84 MU viscosity, 60 Shore A hardness and 70% network breakdown.
The process was optimized in a lab scale internal mixer and upscaled in an extruder. The material devulcanized in the extruder and revulcanized showed a tensile strength of 7 MPa and up to 60% network breakdown was achieved. It turned out that the process was very sensitive to temperature within the experimental window.
Technical rubber goods like shock absorbers, floorings or insulation materials can be made from 100% devulcanizate using this high-quality devulcanized rubber. It can also be blended with virgin compounds in considerable quantities for high performance rubber products like tires, seals, conveyor belts etc..