Revisiting the rheological characterization of NR using a Mooney Viscometer the need for low rotor speed

The standard Mooney measurement has been used for more than fifty years to characterize the rheological properties of natural rubber. According to international specifications, the Mooney viscosity of a material is the torque value read after subjecting the sample to shearing for 4 minutes, at 2 rpm, in a particular geometry. In this study, the Mooney viscosity of masticated and raw natural rubber was examined by using a multiple rotor speed Mooney viscometer. The double-logarithmic flow curve shows a decrease in the slope, which is characteristic of a transition from a proper flow regime to material failure, also observable by capillary rheometry. For all the samples studied, this transition takes place at rotor speeds lower than 2 rpm. These results suggest that standard Mooney measurements performed on natural rubber give misleading results. A Mooney characterization of natural rubber at lower speeds is recommended.