Correlation Between Morphology and Notched Impact Strength of Microcellular Foamed Polycarbonate

Abstract

Polycarbonate has the reputation of having a tough breaking behavior, but it is widely unknown that this applies only to special conditions. The impact strength of polycarbonate depends on the temperature, thickness (with a tough brittle transition as thickness increases), contribution of notch tip radius, impact speed, physical blowing agent, molecular weight of the polymer, and processing parameters. Research results indicate that microcellular foams produced by injection molding with physical blowing agent (MuCell^TM Technology by Trexel) show a significantly higher notched impact strength than compact polycarbonate if the compact material is brittle under the same testing parameters. However, if the compact polycarbonate breaks toughly, the notched impact strength of the foamed material is always lower. Therefore, it is highly important to pay attention to the testing parameters and conditions when comparing the toughness of the foam with that of the compact material. The toughness of microcellular foams has similar properties like PC/ABS and PC/PP blend systems, which provides the possibility to combine the higher impact strength with the advantages of microcellular foaming such as weight reduction, lower shrinkage, shorter cycle times, lower clamp forces, and reduced melt viscosity. In order to use technologies and conditions, which are applied in the polymer industry as well, all materials were produced by an injection molding process. Special processing technologies such as gas counter pressure and precision mold opening were used in order to reach microcellular foam structures with cell diameters around 10 μm. These technologies yield exactly adjustable foam morphologies. Special morphologies are required to improve the notched impact strength of the foamed material. Two different equivalent models were extracted from the analyses, which indicate a significantly higher notched impact strength than the compact material under the same testing conditions. The knowledge of the ideal foam morphologies enables the industry to produce foamed materials with improved mechanical properties.

Keywords

microcellular foam injection moulding polycarbonate notched impact strength.

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