LAUSR

Abstract We propose a method to determine the limiting value of 1H spin–spin relaxation time (T2) for assessing the lifetime of thermally aged acrylonitrile butadiene rubber (NBR) parts. So far, it has been difficult to determine the limiting values of T2 without performing other experiments such as mechanical property measurements and air tightness tests. We exposed two types of NBR diaphragm sheets to air at 100 °C for up to 7540 h and 6000 h, respectively. After that, we tested their air-tightness and we measured T2 of the virgin and aged diaphragms. To evaluate the temperature dependence of T2, T2 measurements were performed at 20, 40, and 60 °C. Additionally, we observed the 13C Nuclear Magnetic Resonance (NMR) spectra, toluene swelling ratio, acetone soluble fraction, and glass transition temperature for only one type of the diaphragm sheets. The diaphragm aged for 7540 h had lost sealability, and all the results indicated that this was caused by the degradation of NBR. The 13C NMR spectra clearly revealed that the degradation of NBR proceeded in three stages: self-cross-linking reactions in the initial stage, concurrent reactions of cross-linking and molecular chain scission in the second stage, and an increased number of reactions of cross-linking and molecular chain scission in the final stage. The results of the T2 measurements showed that T2 is useful not only to evaluate the complicated degradation behavior quantitatively, but also to determine the different limiting values of T2 itself corresponding to large and small safety margins. Finally, we demonstrated that the temperature dependence of T2 is an effective parameter for quantitatively predicting the ultimate lifetime at each aging temperature with no safety margin for various thermally aged NBR rubber sheets.