Effect of third monomer type and content on the UV stability of EPDM

Various uncompounded ethylene–propylene–diene (EPDM) elastomers were photo-oxidized using standard Weather-O-Meter (WOM) aging and oxygen absorption measurements during UV irradiation. The influence of the type and amount of diene on the UV stability was investigated. FTIR (ATR) measurements of the carbonyl absorbance after WOM aging were comparable with the outcome of the oxygen absorption tests. Both techniques showed a decrease in UV stability with increasing diene content. In addition, EPDM containing 5-ethylidene-2-norbornene (ENB) as the third monomer showed a higher UV stability compared to EPDM containing dicyclopentadiene (DCPD) as the third monomer. Furthermore, a linear relation was found between the carbonyl absorbance after WOM aging and the third monomer content for DCPD- and ENB-containing EPDM samples with comparable ethylene/propylene (C2/C3) ratio. The microhardness (International Rubber Hardness Degree) and the Young's modulus of the various EPDMs as function of the WOM irradiation time showed a maximum. This maximum is the result of parallel photo-crosslinking and chain-scission reactions, the latter being dominant in the later stages of the oxidation process. Combining the results from the FTIR measurements with the Young's modulus showed that DCPD-containing EPDM has a higher propensity to crosslinking reactions compared to ENB-containing EPDM.