As a step studying mechanism of EPR processing in an internal mixer are investigated various processability indices. In this report, non oil-extended EPR is preferential subject. Several compound properties are plotted against mixing time in a Brabender Plastgraph, and it is found that most of graphs of compounds show their own bending points as is the case of SBR. The time consumption till the point may mean the minimum compounding time (t_min), as recognized by microscopic observation. The t_min by the extrusion shrinkage is characteristic of tough applicability on wide variation of recipes and mechanical conditions, though the t_min's by the resistivity and the maximum tensile stress fit to find the processability inherent to the EPR and independent of other conditions, if compounding is achieved with a “soft” recipe. With proceeding of mixing, the resistivity approches to an equilibrium value. The logarism of this equilibrium value is designated as PI value. Good corelation is shown by plotting the PI value against the t_min's. The measurement of the value is the simplest among these indices and suitable for quality control and general estimation of EPR processability.

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