Basic rheological investigations on the evaluation of the extrudability of highly reinforced rubber compounds

• Applicant: Professor Dr.-Ing. Christian Hopmann
• Subject Area: Plastics Engineering
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 536806107

Surface quality and dimensional accuracy represent central quality characteristics in the extrusion of semi-finished rubber products, since calibration downstream of the extrusion die is not possible. The quality characteristics are determined by the complex flow behavior of the rubber and depend on the formulation and the mechanical stress during compounding and subsequent processing. Frequent product changes and fluctuating flow properties of the discontinuously produced rubber compounds lead to high scrap rates and require constant extrusion process adjustment. Here, the production managers rely on empirical knowledge. Normal stress elasticity and relaxation behavior describe the swelling behavior and the occurrence of surface defects during rubber extrusion. These material parameters are measured using industrially established rheometers: The Rubber Process Analyzer and the high-pressure capillary rheometer. In the proposed research project, both types of rheometer will be used to identify fundamental relationships between the compound formulation of rubbers, compound production and swelling behavior, and the occurrence of surface defects in the extrusion process. In order to enable early and explicit conclusions from interactions between the compound formulation, the mixing process and the extrudability, the effects of all influencing variables on the viscoelastic properties, such as the relaxation time spectra and the normal stress elasticity, will be quantified. To achieve this, rubber compounds are prepared by varying the polymer structure, filler type and filler quantity. In the mixing process, the polymer and filler dispersion as well as oil incorporation are specifically varied in order to additionally change the rheological compound properties. Using the Rubber Process Analyzer, the relaxation time spectra is determined at constant shear deformation under variation of temperature and strain. High pressure capillary rheometry is applied to determine the first normal stress difference at steady shear under variation of temperature, deformation rate and die length. Furthermore, the true temperature of the extrudates is measured and the development of flow instabilities and swelling is evaluated. Extrusion tests of compounds with different relaxation behavior and normal stress elasticity under variation of melt temperature and throughput provide the data basis for the evaluation of the actual processing behavior. A specifically developed extrusion die in different capillary lengths is used for this purpose. The effect of compound formulation and compound production on relaxation time and normal stress elasticity is evaluated. Based on a comparison of the viscoelastic material behavior with phenomenological rheometry, a stress-dependent assessment of the swelling tendency and the occurrence of melt fracture is carried out using maps of the Deborah number and Weissenberg number.

• DFG Programme: Research Grants