Ultrasonic Twin-Screw Extruder for Rubber Devulcanization

Tire recycling is an important issue in rubber industry. Among various recycling methods, ultrasonically aided extrusion is a very effective method. Devulcanized rubber with good mechanical properties can be continuously produced. Although ultrasonically aided extrusion for devulcanization has been well studied, there are no studies available on comparison among tire powders of different mesh size. Therefore, the present study is devoted to ultrasonic devulcanization of tire rubber particles of 10 and 30 mesh by means of a new ultrasonic twin-screw extruder at different ultrasonic amplitudes and a frequency of 40 kHz. Die pressure and ultrasonic power consumption during devulcanization were recorded. Degree of devulcanization of devulcanized rubbers by measuring their crosslink density, gel fraction and its revulcanization behavior as a function of time were investigated. The measured die pressure, ultrasonic power consumption and degree of devulcanization indicated that 30 mesh rubber exhibits a lower die pressure, higher power consumption and higher degree of devulcanization than those for rubber of 10 mesh. Due to higher level of devulcanization and lower viscosity of devulcanized rubbers at higher amplitudes, the temperature of devulcanized rubber at the die was reduced with an increase of ultrasonic amplitude. Generally, minimum and maximum torque on curing curve measured during revulcanization is higher for 10 mesh rubber. Rheological properties of devulcanized rubber and mechanical properties of revulcanizates were measured. Complex viscosity of devulcanized rubbers as a function of frequency for both meshes exhibits a power-law behavior with the power-law index being almost similar and the consistency index being higher for devulcanized rubber of 10 mesh. Revulcanizates of 30 mesh rubber show consistently higher the elongation at break. Cole-Cole plots, crosslink density and gel fraction of devulcanized rubbers, revulcanization behavior, and modulus of revulcanizates fall in two distinct groups based on the level of devulcanization and molecular structure of devulcanized rubber. Revulcanizates corresponding to the group of higher degree of devulcanization exhibits a higher elongation at break, while those corresponding to the group of lower degree of devulcanization exhibit a higher strength and modulus. The normalized gel fraction versus the normalized crosslink density was found to be unique and independent on the processing condition and rubber particle size.