Aging of EPDM rubber

Abstract

AbstractThe effect of oxidative and thermal aging of EPDM raw rubber and rubber vulcanizate on gelation, network structure, and techical properties has been studied over a wide range of temperatures and times of aging. Three grades of EPDM having different types of diene have been considered. The kinetics of aging of both raw rubber and vulcanizates has also been reported.Different network structures have been produced due to different reactivity of temonomers. Cross‐link density, cross‐link efficiency, polysulfidic linkage, zinc sulfide sulfur, combined sulfur, free sulfur, and sulfur inefficiency change with aging time and temperature. The number of polysulfidic cross‐links decreases continuously with increase in time or temperature of aging. There is no definite trend in other structural features. It is quite evident that postcuring reactions as well as destruction of cross‐links are taking place in the aging process. The predominance of the former or latter depends on the structure of the diene.Tensile and tear strength of the compounds decreases or increases with time or temperature of aging. At high temperature, it always decreases with time. Changes in these properties can be approximately correlated with changes in structure only at high temperature. At lower temperature, relations between network structure and technical properties are complicated owing to the competing reactions discussed above. In general, retention of properties of ENB‐EPDM vulcanizate is the best. The properties after aging do not exclusively depend on the network structure, nature of cross‐link, rank of sulfur, or pendant or cyclic sulfide. The backbone structure is important in all these cases. Changes are less drastic on aging in nitrogen, but a similar trend is found. Raw EPDM rubber forms a gel, which increases with temperature of aging. The activation energy for gelation has been found to be 98 kJ/mol, similar to the oxidation of diene‐containing rubbers.The activation energy for desulfurization of various grades of EPDM vulcanizates is 60–85 kJ/mol. The activation energy for fracture calculated from aging data is, however, 30–36 kJ/mol from tensile measurement and 32–46 kJ/mol from tear strength measurement.