A systematic study was performed to understand the effects of the devulcanizing agent dibenzamido diphenyl disulfide (DBD) on the vulcanization and devulcanization process of a sulfur‐cured ethylene‐propylene‐diene monomer (EPDM) rubber. The influence of DBD on vulcanization was investigated by mixing DBD with virgin rubber and curative system. The devulcanization of rubber waste was achieved with varying amounts of DBD ranging from 0.4 to 13.8 wt% and temperatures from 150 to 200°C. The quality of vulcanizates and devulcanizates was evaluated by rheometer tests, temperature scanning stress relaxation measurements, and analysis of mechanical properties. During vulcanization, DBD acts as an accelerator in the presence of sulfur. When accelerators are added, the scorch time increases, and the cure rate decreases. Thus, DBD acts as a retarder. In the presence of activators, DBD leads to a significant reduction of crosslink density. This results in composites with high elongation at break and poor compression set values. The efficiency of the devulcanization of rubber waste depends strongly on DBD concentration and temperature. The monosulfidic crosslinks are cleaved by low concentrations of DBD, while polysulfidic crosslinks require higher concentrations. These results show that DBD is effective as a devulcanizing agent and degrades the network below 200°C.

The present work is a comparative study of the effects of mechanical shear, temperature, and concentration of a chemical agent on the devulcanization process of post-industrial ethylene propylene diene (EPDM) rubber waste. Devulcanization was carried out in a heating press (no shear), an internal mixer (low shear), and a co-rotating twin screw extruder (high shear) at temperatures ranging from 100 to 200 °C. The efficiency of pure dibenzamido diphenyl disulfide (DBD) and a commercial devulcanizing agent, Struktol A89®, containing DBD were studied. Based on the results, the devulcanization process was upscaled from 40 g per batch to a continuous process with a capacity of 270 g/h. The parameters were fine-tuned regarding flow rate, screw speed, and temperature. Blends of virgin rubber (VR) and 25, 50, and 75 wt% recyclates were compared with blends of VR and 25, 50, and 75 wt% of untreated RWP. The quality of the recyclate was determined by rheometer tests, SEM images, TGA, and mechanical properties. The best results were obtained with 2 wt% DBD in the extruder with a temperature profile of 120 to 80 °C, 50 rpm, and 4.5 g per minute (gpm). The tensile strength and strain at break of the recyclate already met the requirements of DIN EN 681-1:2006 for the production of sealing systems. The compression set and Shore A hardness were restored by mixing recyclate with 25 wt% VR.

Die Devulkanisierung – kombiniert mit der Aktivierung des Kautschuks in einem kontinuierlichen Verfahren – ist eine vielversprechende Methode, um ein Recyclingmaterial zu einem Rohstoffersatz zu verarbeiten. Ein neues Verfahren liefert chemisch aktive Gummimehlpartikel für Dämmstoffe.

Ethylene-propylene-diene rubber (EPDM) scrap was devulcanized in an internal mixer with varying amounts of dibenzamido diphenyl disulfide (DBD) at temperatures below 200 C. The devulcanization effect and sol-gel analyses of the devulcanizates, and the mechanical properties of the sulfur-cured revulcanizates were studied. Residual DBD was still present in the sol at 160 C and degraded DBD at 200 C. DBD affects the curing leading to poor properties. So, the temperature must be adjusted according to the DBD concentration to obtain a superior recyclate for sealing systems. At 0.4 wt% DBD, the degradation reaction was already complete at 120 C, but only 52% and 61% of the tensile strength σ and strain at break ε of the virgin material were achieved. At 160 C and 2 wt%, the degradation reaction was complete, and the DBD effect on properties was small; 65% and 86% of σ and ε were recovered, respectively. To prevent property degradation, 200 C was required at 3.9 wt% DBD, resulting in 97% and 95% of σ and ε, but only 70% of hardness.