| dc.description.abstract | Controlled rheology polypropylenes (CRPP) have been produced industrially by peroxide induced degradation in post-polymerization reactive extrusion (REX) operations. This has been well developed for commodity Ziegler-Natta based polypropylene (ZN-PP) resins, resulting in materials with controlled rheological properties with accompanying narrower molecular weight distribution (MWD). In this work, this methodology has been tested on both metallocene based polypropylenes (mPP), which have a narrow MWD initially, and ZN-PP resins at various peroxide concentrations. The differences in their degradation process as well as the properties of the resulting CRPPs were compared.
A previously developed kinetic model was used to simulate the degradation process at various peroxide concentrations. Modelling results for a mPP and a ZN-PP with similar molecular weight indicated that the MWDs of both materials approached “most probable distribution” (PDI=2) as expected, due to the random nature of the scission reactions involved. However, unlike the ZN-PP resin for which the polydispersity was sharply decreased, the MWDs of CRPPs from mPP was almost unchanged with a PDI of about 2. This indicated that polypropylenes from the two types could be used to produce CRPPs with specified/targeted MW and MWD patterns and therefore special properties.
Two groups of tailor-made CRPPs were produced using mPP and ZN-PP. One group had similar MW but different MWDs and another group had similar melt flow rate (MFR). In particular, the zero-shear viscosity and rheological polydispersity, which were obtained through oscillatory
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shear rheometry, were successfully applied to monitor the MW level and the MWD of the products.
The two series of materials were also submitted to extrusion tests using a single screw extruder equipped with a capillary die. Based on the extrusion results, with respect to the CRPPs of similar MW, materials of broader MWD, thus stronger shear-thinning at high shear rate, resulted in slower increase of extrusion pressure with apparent shear rate. The shear rate of the onset of extrudate melt-fracture, which coincided with previous studies of polypropylenes extrusion, was higher for broader MWD material. With respect to the CRPPs with similar MFR, a metric commonly used for grade inspection in industry, their extrusion behaviours were nearly identical. | en |
