Determination of service life time of ethylene propylene diene monomer (EPDM) reinforced with graphite

Rubber materials have been widely used in many fields since the discovery of vulcanization. These materials have an important place in sectors such as automotive, white goods, aircraft and rocket industry, railway and construction. The extensive use of rubber parts increases the importance of service life and rubber diversity. Each type of rubber has its own unique properties, and these properties can be further improved with special material additives depending on their usage areas. Ethylene Propylene Diene Monomer (EPDM) rubber is one of the most commonly used rubbers due to its properties, and it is a type of rubber that can be improved by using special materials to address its shortcomings. The aim of this study is to investigate the effect of adding graphite powder to the EPDM mixture created using EPDM rubber on both its mechanical properties and service life. For this purpose, four different types of mixtures were created within the scope of the study. Graphite powder (EPDM GP) and modifications of graphite powder (phosphoric acid modification (EPDM GP-PA) and heat treatment modification (EPDM GP-HT)) were added to the mixture recipes in a quantity of 10 phr as special materials, and their characterization properties, mechanical values, and service lives were compared with the pure mixture recipe (EPDM 0). In this context, first, the characterization analyses of graphite and its modifications were performed. X-ray diffraction (XRD), scanning electron microscopy analysis (SEM) and nitrogen adsorption/desorption analysis (BET) were performed for characterization analyses. Then, the rheological values, viscosities, hardnesses, tensile strengths, modulus values, elongations, densities, ashes, and crosslink densities of rubber mixtures were analyzed. In addition, aging tests were performed on the mixtures in hot air at 100 °C, 130 °C, and 160 °C. Aging was performed for 7 different periods for 100 °C (72h, 144h, 288h, 384h, 624h, 1440h, and 2400h), 7 different periods for 130 °C (48h, 72h, 144h, 288h, 384h, 624h, and 960h), and 7 different periods for 160 °C (12h, 24h, 72h, 96h, 120h, 144h, and 168h). Changes in the mechanical values and crosslink densities of all mixtures were examined after aging. Equations were created using the Arrhenius equation based on the elongation data that showed a 50% change from the initial value after aging for the calculation of estimated service life. Service lives at 110 °C, 120 °C, 130 °C, 140 °C, and 150 °C were calculated from the equations created. As a result of the characterization analyses, graphite and its modifications were compared, and it was observed that the modifications did not affect the crystal structure of the material according to XRD analysis, their morphological structures were similar according to SEM analysis, and the modifications created changes in the surface area, pore size, and volume of the graphite according to BET analysis. In the mechanical value examination, it was observed that graphite and its modifications had a positive effect on the mechanical values of the rubber. It was observed that adding graphite to the mixture increased the tendency of the mixture to burn, but the addition of phosphoric acid-modified graphite to the mixture reduced this tendency and improved it. In addition, adding graphite to the mixture also increased the crosslink densities (CLD), but opening between the bonds was observed with the addition of graphite modified with phosphoric acid. The mixture with the highest service life was found to be EPDM GP-PA mixture from the estimated service life equations created based on the 50% change in elongation values after aging. When the service lives of all mixtures were compared with each other, it was shown that adding graphite or modifications to the mixture extended the service life of the mixture.