Effects of Aging on the Viscoelastic Properties of NBR and HNBR Rubbers

Nitrile (NBR) and hydrogenated nitrile (HNBR) rubbers are widely used in the automotive and industrial applications due to their low temperature properties and resistance to various oils and fuels. It is an established fact, however, that at certain temperature conditions the polymeric component of the rubber may undergo scissoring and cross-linking when exposed to elevated temperature over time. The viscoelastic properties of such aged materials is therefore of interest to material and design engineers.

Rubber samples were aged in air and ASTM reference oil at 100°C for 168, 504 and 1000 hours. The linear viscoelastic behavior of these samples were investigated with a Dynamic Mechanical Analyzer (DMA) in the force controlled mode at frequencies between 0.1 and 100 hz and over a temperature range that included the glass transition of the material. Shift factors for generating the storage modulus (G’), loss modulus (G”) and tan delta mastercurves were calculated using WLF and SKF proprietary models. When significant aging occurred, the transition zone of the G’ mastercurve shifted to the lower frequencies. In such cases an increase was also seen in the glass transition temperature (measured via Differential Scanning Calorimetry - DSC).

The effect of aging on non-linear viscoelasticity was also studied for strains up to 0.25 and frequencies of 100 Hz. The data were analyzed using the MITLoas v2.2 software package. This allowed for identification of contributions from higher order (odd) harmonics and the use of Bowditch/Lissajous plots for evaluating the orthogonal outputs (elastic stress, viscous stress).