LOW TEMPERATURE SEALING BEHAVIOR EVALUATION OF ELASTOMERS IN AIRCRAFT HYDRAULIC SYSTEMS

ABSTRACT The low temperature sealing behaviors of five elastomer components used in aircraft hydraulic sealing systems were evaluated through a series of tests subsequent to exposure at 135 °C for a maximum of 28 days in air or hydraulic oil. These assessments encompassed standard material tests such as glass transition temperature (Tg), temperature retraction at 10% shrinkage, mechanical test, and compression set and a customized test to measure sealing force of a certain temperature. In addition, a low temperature gas leakage test was conducted. NBR stiffened upon aging with degradation of properties in an elongation loss, and Tg increased because chain mobility decreased due to increased crosslink density, and restoring force sharply decreased in cooling and in extracting plasticizer. In contrast, in the gas leakage test for both fluorocarbon rubber (FKM) and blends of epichlorohydrin with NBR (ECO), the Tg values showed good accordance, with minimal sealing temperature (Tseal), whether FKM or ECO were aged or not. However, the value of Tseal is significantly higher than that of Tg after NBR aging, indicating a complicated sealing mechanism. Thus, relying solely on standard material tests may not suffice for accurately forecasting the low temperature sealing performance in aircraft hydraulic sealing systems.