Ethylene-propylene-diene monomer (EPDM) is a kind of synthetic rubbers and has been widely used in high temperature-resistance conveyor belts, hoses and cable sheaths. However, EPDM based on ethylidene-2-norbornene (ENB) is very sensitive to thermal oxidation.  

Thermo-oxidative aging of EPDM initially results in the degradation of EPDM molecular chains, and thus the formation of large number of free radicals. These free radicals then attack the double bonds of EPDM chains, leading to the increase in crosslinking density namely post-crosslinking. So it's necessary to add some antioxidants to enhance thermo-oxidative aging resistance of EPDM for its application at high temperature, especially for EPDM with high ENB content. 

Lately, researchers use zinc dimethacrylate (ZDMA) and 2-sulfur-benzimidazole (MB)/N-4(phenyl–phenyl)-maleimide (MC) simultaneously as synergistic antioxidants. It aims to improve the thermo-oxidative aging resistance of peroxide vulcanized EPDM for its application at high temperature.  

For comparison purpose, the control sample without antioxidants, the traditional antioxidant package, the combined antioxidant MB/MC, and pure ZDMA are also investigated. The results show that the crosslinking density of EPDM is only slightly decreased before aging by using the combined antioxidant MB/MC/ZDMA. As well, the increase in crosslinking density is much less than other antioxidants at the same aging time, indicating the largely improved aging resistance of EPDM. 

As a result, the change in mechanical properties is the lowest for EPDM with the combined antioxidant MB/MC/ZDMA at high temperature. Meanwhile, the results from structure analysis show that the degree of oxidation of EPDM is much lower than other systems after aging at 180 °C for 216 h by using the combined antioxidant MB/MC/ZDMA.  

In addition, the surfaces of EPDM samples with MB/MC/ZDMA as synergistic antioxidants or the combined antioxidant MB/MC are still smooth after aging at 180 °C for 216 h, whereas severe cracks occur for other antioxidative systems. These results suggest the good thermo-oxidative aging resistance of the synergistic antioxidant MB/MC/ZDMA. Finally, the synergistic antioxidative mechanism of MB/MC/ZDMA is carefully discussed. 

References: 

NING Nanying, MA Qin, ZHANG Yunqiang, ZHANG Liqun, WU Hanguang and TIAN Ming. Enhanced Thermo-oxidative Aging Resistance of EPDM at High Temperature by Using Synergistic Antioxidants. Polymer Degradation and Stability (Vol.102, pp.1-8, April 2014).  DOI: 10.1016/j.polymdegradstab.2014.01.037