Rheometer-independent description of the wall slip behaviour of rubber compounds

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Depending on the compound ingredients and the processing parameters, rubber compounds have an increased tendency to flow anomalies such as wall slip. As a result, rheological material measurements and the resulting material parameters are subject to errors, since the underlying analytical and numerical calculation approaches assume wall adhesion. Besides the influence on the extrusion process and thus the extrudate quality through wall slip effects, the non-consideration of wall slip in the context of flow simulations and the design of tools leads to errors. These are often only detected during production operation and result in time-consuming and cost-intensive corrections. The aim of this research project is to develop a methodology for the rheometer-independent description of the wall slip behaviour of rubber compounds. For this purpose, extensive rheological investigations of EPDM rubber compounds are carried out using high-pressure capillary rheometer (HKR), the Contifeed (combination of laboratory extruder with a screw diameter of 20 mm and HKR) as well as online extrusion rheometers on extruders with different screw diameters (32 mm, 60 mm). Three basic hypotheses are investigated in the research project. First, the wall slip behavior of rubber compounds depends on the size (geometry of the test channel) of the used rheometer. Secondly, the mirror relations according to Gleissle (I, II) are valid for both unfilled and filled rubber systems under wall-slip conditions. In addition, the wall slip behavior is influenced by the shear history in the extrusion process. By using the Rubber Process Analyzer (RPA), it is possible to determine the flow behavior of rubber compounds under the boundary condition of wall adhesion, since the RPA is considered to be wall-slip-free due to the test chamber design with the engagement flanks. For wall-adhering materials, the mirror relations according to Gleissle and the Cox-Merz rule have already been proven many times. The limited validity of these for filled elastomers is due to wall-slip effects. Considering the linear viscoelastic range and the correction of non-isothermal effects, deviations in the measured values between conventional rheometers and the RPA can be attributed to wall slip effects and can be corrected. A description is given of a procedure for correcting wall slip effects which is valid independent of the material and geometry. Due to the extrusion process, slip effects are apparent to a different extent than in HKR measurements, therefore correlations between the influencing variables and the slip behavior are established and combined in a map.