Melt fracture behavior of polypropylene-type resins with narrow molecular weight distribution. 1. Temperature dependence

Polypropylene (PP)-type resins with narrow molecular weight distribution, such as PP-type thermoplastic elastomer PER and controlled-rheology PP (CRPP) made by peroxide degradation of high molecular weight PP, have a problem of easy generation of skin roughness at extrusion. To examine the present state, the occurrence of skin roughness in PER and CRPP at extrusion was investigated with a capillary rheometer in a shear rate range of 12-6100 s(-1) and a temperature range of 180-280degreesC. A homo-PP (HPP) and a block-PP (BPP) with usual molecular weight distributions were used for comparison. HPP and BPP with usual molecular weight distributions show smooth extrudates at low shear rates and abruptly generate severe skin roughness "elastic failure" originating at the die entrance at a higher shear rate. PER and CRPP with narrow molecular weight distributions easily generate "sharkskin" melt fracture originating at the die exit, from a shear rate nearly one decade lower than rates of elastic failure of HPP and BPP. The sharkskin becomes more severe, with increasing shear rate, and attains to the elastic failure. The critical shear rate at which sharkskin occurs increases with increasing extrusion temperature. The critical shear rate is about 20 s(-1) at 180degreesC and about 120 s(-1) at 280degreesC, which is in the range encountered by the molten resin at extrusion processing. (C) 2002 Wiley Periodicals, Inc.