Even if SN methodology is valid, one must bear in mind that the fatigue performance of natural rubber is compound dependent. The fatigue life for any given stress/strain combination (or tearing energy if you like that approach) will be affected by the cure system used to cure the compound, the...
Nitrile rubber has a characteristic pungent odor and the smell may be unrelated to any of the compounding ingredients. You might try the test on the raw polymer to determine if that is the source of your problem. If so, you might be able to find a cleaner polymer (less residual monomer and...