Ethylene-propylene rubber compounds have been utilized in automotive coolant hoses for more than twenty-five years. Throughout this period, hose compositions have continually evolved to meet ever changing requirements of long-term serviceability, higher heat resistance, coolant fluid compatibility, and non-bloom features for the cover. Despite the commercialization of compounds with improved quality and consistency, radiator and heater hoses in certain vehicle models and designs develop cracks well before their expected service life capability. While all hoses including these failures are being exposed to higher levels of heat duress, physical property deterioration, via the usual auto-oxidation mechanism, does not alone seem to be the major cause.
This paper describes the nature of coolant hose cracking failures and laboratory studies to explain the observed striae, dendritic structures, and coolant exudation within the hose. The electrochemical influence on carbon black reinforced ethylene-propylene rubber hose compounds is demonstrated in a simple laboratory test. Under an applied low voltage, EPDM compounds are found to imbibe significant fluid, providing an ionic path of conductivity. Various compounding principles are presented to show the effects of enriched polymer content, carbon black level, inorganic fillers, and cure system variations on enhancing electrochemical resistance and minimizing coolant fluid ingression. Future direction for coolant hose compounds is proposed.