Devulcanization of EPDM Scrap Using an Ultrasonic Twin Screw Extruder: Effect of Screw Design

Waste ethylene propylene diene monomer (EPDM) rubber powder from postindustrial scrap is significant environmental problem. In attempt to solve this problem, a technology for its recycling and utilization is required. In the present study, the devulcanization of waste EPDM rubber was conducted by means of an ultrasonic twin screw extruder (TSE) operating at a frequency of 40 kHz and ultrasonic amplitudes varying from 0 to 13 μm. Two screw configurations with and without kneading elements were utilized in devulcanization. The die pressure and ultrasonic power consumption during devulcanization EPDM rubber significantly decreased and increased, respectively, with the ultrasonic amplitude. Curing behavior, dynamic properties and gel fraction of devulcanized EPDM and crosslink density, gel fraction, dynamic and mechanical properties of revulcanized EPDM were measured. A significant reduction of the gel fraction of the devulcanized rubber with the ultrasonic amplitude led to a reduction of its complex viscosity and storage modulus and an increase of the loss tangent. This effect also decreased the curing torque during revulcanization and the crosslink density of revulcanizates. It was found that an increase of the ultrasonic amplitude generally led to more devulcanization and continuous increase of the tensile strength and elongation at break and a reduction of the modulus at 100% elongation of revulcanizates. Specifically, the revulcanizate prepared from the devulcanized EPDM obtained at an ultrasonic amplitude of 13 μm using TSE containing kneading elements showed the tensile strength of 9 MPa and the elongation at break of 200%.