Microwave-Treated Structural and Network Changes in Filled Industrial EPDM Waste: Effects of Particle Size and Exposure Time

Abstract Ethylene–propylene–diene monomer (EPDM) compounds used in automotive sealing systems contain permanent cross-links and high amounts of fillers and process oils, which complicate their recycling. This study examined the effects of particle-size fraction (≤125 and ≥125 μm) and microwave exposure time (60–180 s at 600 W) on the structural response of a highly filled industrial EPDM waste. Apparent sol fraction and apparent cross-link density were used as comparative indicators, supported by ATR-FTIR, XPS, TGA, and SEM analyses. The fine fraction treated for 60 s showed the lowest apparent cross-link density (0.72 × 10–4 mol/cm3), corresponding to an apparent reduction of 17.11% relative to untreated ground EPDM. Longer exposure is increasingly favored by network-forming responses, particularly in the coarse fraction, demonstrating competition between network loosening and secondary cross-linking. XPS showed changes in sulfur-containing surface environments, while ATR-FTIR revealed no appreciable shifts in the principal EPDM bands. TGA and SEM showed clear differences between the particle-size fractions. In revulcanized compounds containing 10 phr recycled EPDM, tensile strength remained within 7.71–8.32 MPa, and none of the recycled formulations differed significantly from the reference C-EPDM. Microwave treatment had a significant overall effect on tensile strength (p = 0.014), but only the treated fine fraction differed from its untreated counterpart (p = 0.046); elongation at break did not differ significantly. Surface roughness and Shore A hardness remained broadly comparable. Overall, microwave exposure produced competing particle-size- and time-dependent network responses, indicating a narrow processing window for the controlled modification of highly filled industrial EPDM waste.