Effect of devulcanized reclaimed rubber content on structure, mechanical properties, and thermo-oxidative aging behavior of ethylene-propylene-dien-monomer (EPDM) rubber

Rubber recycling refers to the circular economy process of processing waste rubber products into reclaim or rubber crumb/granulates through methods such as chemical treatment (e.g., devulcanization) and grinding. Especially the devulcanization has the goal of restoring its plasticity for reuse and revulcanization. However, the recycling process degrades rubber quality and incurs high energy consumption and costs, limiting its application. This study investigates the effects of varying concentrations of devulcanized EPDM on the rheology, mechanical properties, and thermo-oxidative aging behavior of EPDM compounds. Devulcanized rubber obtained through selective cracking of sulfur crosslinks was incorporated into an EPDM compound at different ratios. The addition of devulcanized EPDM accelerates the curing, but also reduces the crosslink density and the network uniformity. Mechanical testing indicates that higher devulcanized EPDM content increases shore A hardness but decreases tensile strength and elongation, attributed to insufficient interaction to the new rubber matrix, residual crosslinks and filler content in the recycled rubber. Thermo-oxidative aging evaluations indicate that devulcanized EPDM does not accelerate degradation or embrittlement, generally matching or even lagging behind the reference material. Furthermore, recipe adjustment can moderately improve properties of devulcanized EPDM, such as crosslink density and flexibility. These findings provide valuable insights for optimizing recycled rubber formulations, balancing sustainability with performance requirements.