EDGE Effect Correction for the Mooney Rheometer

The Mooney rheometer has been used universally as a tester for quality control of raw elastomers and compounds. White and Tokita showed that the tester modified to operate at a range of speeds could be used to obtain fundamental viscoelastic properties: steady-state shear stress-shear rate relationship, normal stress effect, and elastic recoil. We have also made effective use of this rheometer to obtain steady-state viscosity as a function of shear rate and complex viscosity as a function of deformation rate. In these investigations, the observed torque was converted to shear stress with a theoretical factor derived by taking into account both the parallel plate section and the concentric cylinder section. It was assumed that the edge effect was negligible. The viscosities calculated in this manner were compared to those obtained with other instruments and reasonably good agreement was obtained. Some time ago, Mooney and Treloar expressed the opinion that the viscosity obtained from the Mooney rheometer is somewhat larger than the correct value, attributing the disagreement to neglect of the edge effect. More recently, White and Tokita have restated the problem. In our recent work, the torque measurements were made with both the small and the large rotor. The viscosities calculated without considering the edge effect tended to be somewhat higher for the small rotor than for the large rotor. This observation implies that an edge effect exists, because measurements obtained with the small rotor, which has larger “edge volume”, would tend to be more significantly affected. In this paper a mathematical expression is sought for the edge correction, so that the correction factor may be calculated from the known properties of the material.