In the past ten years diesel engine performance has significantly increased in terms of kilowatts/litre (kW/L). These higher power density outputs create higher thermal loads on the cooling system and associated seals. While compatibility of elastomers in high temperature coolants has been studied and reported, the inevitable impact of thermal cycling on these elastomers is not well documented. This study examines the effects of thermal cycling in three general coolant categories on three different elastomers commonly considered for sealing hot engine coolants. The elastomers, by ASTM D1418 designation, are HNBR, FKM Type 2, and FEPM. The coolants are an organic acid technology (OAT) coolant, a propylene glycol premix coolant, and a corrosion inhibited de-ionized water. Normal service applications are characterized by an indefinite number of shutdowns and startups. Testing was designed to simulate such service. Aging periods incorporated ongoing 24 hour cycles: a 16 hour period to heat up and operate at 150°C, and an 8 hour period to cool off to ambient. O-rings, a common seal design, were subject to axial and radial deformation during testing. The o-rings’ sealing attributes were examined after four, ten, twenty, and forty cycles. Elastomeric properties were evaluated, before and after cyclical aging, in accordance with ASTM D1414-94 (“Standard Test Methods for Rubber O-rings”) and D412-06a (“Standard Test Methods for Vulcanized Rubber … - Tension”). Compressive stress relaxation (CSR) was evaluated using an in-house procedure, comporting with ASTM D6147-94.

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