Foaming of Polypropylene Blends: A Rheological Study
a Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, bInstitute of Hydrodynamics, Academy of Sciences of the Czech Republic
In order to obtain any desired morphological structure of polypropylene foams by extrusion process followed by foaming, a rheological study of the original materials and compounds is an inevitable prerequisite. Series of blends made from high molecular weight linear and high melt strength branched polypropylenes were prepared using a twin screw extruder, and foamed by the use of CO2. The shear-flow behaviour and strain hardening during elongation were identified as the key factors for controlling of the foaming process. The results of pressure dependent shear-flow studies with the aid of capillary rheometer equipped with backpressure chamber were used as input information for tailoring of compounds suitable for the foaming. The basic morphological parameters as the size of the bubbles and their density might be controlled by an interplay of material composition and processing conditions which both influence the degree of bubble coalescence and nucleation rate.
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