Improvement of a method for the correction of wall slip effects within the rheological measurements of filled rubber compounds

Abstract

Rubber compounds have an increased tendency to flow anomalies depending on the compound ingredients and the processing parameters. Due to wall slip effects, rheological material measurements and the material parameters derived from them are affected by errors, since the fundamental analytical and numerical calculation approaches assume wall adhesion. In the literature, various approaches for the description of wall slip effects exist, which partially lead to negative slip velocities as well as slip volume flows that exceed the total volume flow in the capillary. The purpose of this article is the improvement of an analytical model for the determination of slip velocities for both laboratory and near-process rheological investigations. A comparison shows the limitations of the established wall slip correction methods and the possibilities of the improved correction method.

Wall Slip-Free Viscosity Determination of Filled Rubber Compounds Using Steady-State Shear Measurements

“…In a previous study [2], the procedure as well as the evaluation routines of rheological investigations by means of HPCR analysis were described in detail. For this reason, this will not be a part of this work.…”

“…In addition to a rubber compound adjusted for wall adhesion by the use of silica, a rubber compound loaded with carbon black was also investigated. With this compound, pronounced wall slip effects were demonstrated in the HPCR measurements and in the extrusion process [2]. Similar to the silica-filled rubber compound, no LVE region could be detected for the carbon black-filled rubber compound (see Figure 11a), since the LVE region lies outside the measurement range in which no reproducible measurement data could be determined due to the insufficient noise-to-measurement signal ratio.…”

“…A shear rate range from 1 to 1000 s −1 was considered, and measurements were made from high to low shear rates. A detailed description of the rheological investigations by means of HPCR measurement as well as the associated corrections is given in [2], and is therefore omitted at this point. In the following, the fully corrected true viscosity data from the HPCR study series are considered.…”

“…Compared to commercial plastics, analytical and numerical process design approaches more often fail for rubber compounds. Due to non-isothermal effects and wall slip, rheological material measurements and the material parameters derived from them are affected by errors, since the fundamental analytical and numerical calculation approaches assume isothermal flow and wall adhesion [1][2][3][4]. Therefore, a better understanding of the rheological behavior of filled rubber compounds and the availability of suitable rheological characterization methods is essential.…”

“…With respect to slit capillaries, the Mooney and Geiger correction methods require data from at least two slit capillaries with the same W/H and L/H ratios, which significantly increases the time required for the measurement. For a review of the main studies on wall slip effects focusing on filled rubber compounds, refer to [2], while Hatzikiriakos [16] has given a comprehensive overview of wall slip mechanisms in complex fluids in general.…”