A STUDY OF EXTRUDATE DISTORTION IN CONTROLLED-RHEOLOGY POLYPROPYLENES

The melt fracture characteristics of controlled-theology polypropylene s (CRPP) were studied by means of capillary rheometry experiments. CRP Ps were produced through reactive extrusion of a commodity polypropyle ne resin using various peroxide concentrations. These materials exhibi ted lower molecular weights and narrower molecular weight distribution s than those of the starting commodity resin. The CRPP materials studi ed were found to exhibit only gross melt fracture. At extremely high s hear rates and relatively low temperatures, a sigmoidal flexure was ob served in the now curve of certain CRPPs. Generally, it was found that the severity of melt fracture decreased with increasing shear rate fo r a given material and temperature and in some cases, the extrudates e xhibited completely smooth surfaces. Also, the severity of surface dis tortions was reduced when high LID dies were employed at a given shear rate. The critical shear stress for the melt fracture onset was found to increase with decreasing molecular weight and polydispersity, and correlations have been developed between the critical stress values an d the polymer polydispersity and shear compliance.