Influence of flow channel surface on melt fracture and wall sliding behavior of a polypropylene melt

Polymer extrusion is widely used in the manufacture of plates or profiles. The quality of the extrudate is not only influenced by surface defect but even more by flow instabilities. A number of possible mechanisms and causes have been identified in literature. Wall slippage is one of those influencing factors. But it also plays an important role for the extrudate output. A special rheological slit die was developed to realize different slit heights and surfaces. It consists of a die body split into two halves that are mounted on a high pressure capillary rheometer. The key elements of this special die are special inserts that allow to realize different flow channel surfaces and slit heights. The die was used for measurements with rough and polished steel and inserts coated with titanium nitride, titanium aluminum nitride and PTFE. A polypropylene block copolymer that exhibits a high melt flow rate (MFR) was exposed to melt temperatures of 185, 195 and 210°C. The heights of die flow channel were 1 and 2 mm and the width was 10 mm. The extrudate showed surface defects like sharkskin at the two lower temperatures with a critical wall shear stress of about 0.08-0.09 MPa. So called volume instabilities or melt fracture could be identified at all temperatures. The critical wall shear stress for melt fracture is higher (0.12 MPa) for 210 than for 185 and 195°C (0.09 MPa) and increases slightly with roughness. But it shows some dependence on the polar portion of surface energy. Wall slippage was determined using the Mooney method for 1 and 2 mm die heights: The reduced volume flow rate was drawn against the reciprocal die height at constant wall shear stress. The slope of this plot gives the wall slipping velocity. First results indicate that a minimum wall shear stress is needed in order to get wall slippage. Its value increases with roughness. The wall shear velocity itself is thus higher for polished surfaces and increases with lower steepness at higher temperatures.