The effect of UV radiation ageing on the structure, mechanical and gas permeability performances of ethylene–propylene–diene rubber

This work focused on the influence of UV radiation on the structures and performances of ethylene-propylene-diene rubber (EPDM). The exposure of EPDM to UV radiation was performed for various times (0, 7, 14, 21 and 28 days). The surface chemical structure and free volume of EPDM before and after UV ageing were determined by infrared spectroscopy (IR) and positron annihilation lifetime spectroscopy (PALS), respectively. The surface morphology, crosslink density, mechanical and gas permeability properties of EPDM were also analyzed. Under UV radiation, oxidation reactions occur on the EPDM surface and oxygenated species are formed. With prolonging ageing time, the oxidative degree increases, leading to more and denser microcracks on the surface. The plausible ageing mechanism of EPDM is suggested. At the initial 14 days of UV irradiation, the crosslink density of EPDM reduces due to the chain scission reaction, causing the increase of free volumes and thereby the enhancements of gases permeabilities. When the ageing time is longer than 14 days, crosslinking dominates and the free volumes decrease, bringing about the reductions of gases permeabilities. With prolonging ageing time, the tensile strength, glass transition temperature (T_g) and storage modulus of EPDM initially decrease and then enhance, in agreement with the change trends of crosslinking density.