Examination of wall slippage effects in EPDM rubbers as a function of the molecular weight,the mixture composition and surface coatings in the high-pressure capillary viscosimeter(HCV)

摘要

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 in 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.In this study,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 slip ratio relative to total volume flow as a function of the given shear rate and temperature.The study involves the testing of EPDM raw polymers of different molecular weights in the HCV.In addition,carbon black-/plasticizer-containing EPDM compounds are analyzed with regard to flow behavior.Additionally,tests are carried out on three variously coated flow channels.With attainment of a critical wall shear stress,the wall slippage effect becomes more pronounced;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.