This study provides a comprehensive assessment of the effects of carbonyl iron powder incorporation on ethylene-propylene-diene monomer composites cured via sulphur, peroxide and peroxide–co-agent systems following thermo-oxidative aging. After exposure to 155 °C for 72 h, sulphur-cured formulations containing carbonyl iron powder exhibited an approximately 70% increase in crosslink density relative to their non-aged counterparts, accompanied by pronounced decreases in elongation at break (≈88%) and tear strength (≈69%). In contrast, peroxide-cured systems demonstrated substantially higher resistance to thermo-oxidative degradation, a behaviour attributed to the superior stability of carbon–carbon crosslinks generated during peroxide vulcanisation. Notably, the composite cured with peroxide and a Type II co-agent displayed minimal formation of additional crosslinks (<14%), maintained tear resistance and showed only a moderate reduction in elongation (≈35%). Magnetic and thermal analyses confirmed that the incorporation of carbonyl iron powder did not significantly affect properties such as magnetisation or thermal stability of the elastomeric matrix. Among all systems evaluated, the formulation cured with peroxide and the Type II co-agent (Formulation 6) exhibited the highest thermo-oxidative stability, indicating its suitability for applications requiring moderate magnetic responsiveness combined with enhanced long-term durability.
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