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WALL SLIPPAGE IN HIGH-PRESSURE CAPILLARY VISCOMETRY: EFFECTS OF MOLECULAR WEIGHT, COMPOUND COMPOSITION, AND CAPILLARY SURFACE COATING


 Flow behavior is of major importance in the extrusion processing of rubber compounds. It is evaluated by means of a series of tests on a high-pressure capillary viscometer (HCV). Adhesion between the polymer melt and the capillary wall is assumed in all current calculation models, although such adhesion does not always pertain to the case of rubber compounds. To date, no uniform model discussed in the literature on the topic extensively describes the wall slippage behavior of rubber compounds. The phenomenon of wall slippage is analyzed by determining the power-law parameters n (flow exponent) and K (consistency factor) from the flow curve in the subcritical flow range. This makes it possible to explicitly calculate first the slip velocity and then the slippage ratio relative to the total volume flow as a function of the given shear rate and temperature. The work is based on the testing of EPDM raw polymers of different molecular weights in the HCV. In addition, EPDM compounds containing either a carbon black or a softener were analyzed with regard to their flow behavior. The rheological analysis was carried out on three variously coated flow channels. It was observed that with attainment of a critical wall shear stress, the wall slippage effect becomes more pronounced; thus, occurrences of flow anomalies such as slip-stick or shark-skin significantly influence processing and flow behavior. Wall slippage effects are noticeable, however, even before the critical wall shear stress is attained.