Compounding and Mixing Methodology for Good Performance of EPDM in Tire Sidewalls

Abstract

ABSTRACT Ethylene propylene diene terpolymer (EPDM), which is a terpolymer of ethylene, propylene, and a diene, is characterized by having a fully saturated polymer backbone, resulting in excellent ozone and oxidative aging resistance. By contrast, diene elastomers such as natural rubber (NR) and polybutadiene rubber (BR) have high levels of unsaturation along their main polymer backbones, increasing their susceptibility to chemical degradation by main chain scission and therefore their need for a protection system. This is generally provided by adding combinations of primary antioxidants and migrating waxes. Protective systems based on paraphenylenediamines (PPDs) are very effective and are used extensively for the protection of tire sidewalls, but because they function by preferentially reacting with ozone, their activity becomes depleted over the service life of the tire. Other potential drawbacks are that PPDs form a reaction product that discolors the surface of the sidewall, and they are considered to be potentially hazardous to human health and the environment through contamination of water systems. By contrast, it has been shown that the addition of EPDM to a conventional sidewall compound can give a permanent high level of ozone protection that is nonhazardous and non-discoloring. However, differences in polarity, solubility, and levels of unsaturation create significant challenges for achieving successful blends of EPDM in typical NR/BR sidewall compounds, leading to inferior physical and dynamic properties. This study shows that by combining best practices for compound design and mixing methodology with optimum grades of EPDM, it has been possible to produce tire sidewall compounds having full ozone protection as well as physical and dynamic properties closely matching a typical sidewall compound. Additionally, this study shows that the presence of EPDM leads to improved properties after heat aging and vulcanized compounds having reduced energy dissipation behavior, which could lead to tires having reduced rolling resistance.