Mechanical Properties and Thermo‐Oxidative Aging of Hybrid EPDM Formulations With Bio‐Based Oils

Ethylene‐propylene‐diene monomer (EPDM) rubber relies on plasticizers for flexibility and processability; however, conventional petrochemical oils pose significant environmental challenges. This study investigates a commercially available bio‐based oil as a partial or full replacement for fossil‐based paraffinic oil in EPDM formulations. Compounds containing purely bio‐based or purely conventional oil exhibited similar mechanical properties, although the bio‐based variant showed slightly reduced thermo‐oxidative aging resistance. Blends with 25% and 75% bio‐based oil fractions yielded modest increases in tensile strength (5% and 16%, respectively), as well as increased stiffness and crosslink density, indicating good compatibility. In contrast, the 50% blend initially displayed poorer mechanical performance. Atomic force microscopy (AFM) linked this behavior to a specific phase morphology, even though the compound retained notable tear resistance. After tempering, particularly at 70°C, the mechanical properties of the 50% blend improved to levels comparable or better than the other variants. Temperature Scanning Stress Relaxation (TSSR) confirmed that these changes were likely rooted in rearrangements of the sulfur network. Notably, the tensile strength of this blend reached its peak after aging, showing a 35% increase compared to the unaged reference. These results suggest that phase morphology influences the distribution of curing chemicals.