Rheological behavior of controlled-rheology polypropylenes obtained by peroxide-promoted degradation during extrusion: Comparison between homopolymerand copolymer

In the present work, experimental studies of the free-radical-initiated mol ecular weight degradation of polypropylene in a modular self-wiping corotat ing twin-screw extruder are investigated. The control of the molecular weig ht distribution of polypropylene resins by peroxide degradation is widely u sed in the polymer industry. It allows one to adjust the viscosity of these resins to the level required for processing applications. The purpose of t his work was to characterize the influence of peroxide degradation on the r heological behavior of a polypropylene homopolymer and a block polypropylen e/polyethylene copolymer, which includes an addition of a low percentage of polyethylene (around 7%). The homopolymer exhibits a classical behavior: W hen the peroxide amount is increased, we observe a decrease in the viscosit y corresponding to a decreasing molecular weight and a pronounced shift tow ard more Newtonian behavior. The rheological behavior of the copolymer is i nfluenced by the presence of the polyethylene phase which greatly modifies the viscoelastic properties and increases the viscosity when the polypropyl ene matrix is highly degraded. (C) 2001 John Wiley & Sons, Inc.