The flowability and hence the processability of polypropylene (PP) in injection molding processes increases with decreasing molecular weight. The toughness, however, strongly decreases with decreasing molecular weight. This leads to brittle fracture of injection molded parts. An increase of toughness can be obtained by controlling the crystal modification and the crystalline superstructures. However, no information can be found in literature regarding this toughness increase for PP with high flowability. The aim of the present work is therefore to influence the crystal modification and crystalline superstructures of an isotactic polypropylene with high flowability using nucleating agents and to increase the impact toughness by at least 50 % compared to the unnucleated iPP. Furthermore, the micromechanical mechanism of the beta modification in an iPP with high flowability will be explained. Different beta nucleating agents are therefore investigated regarding their influence on the crystallization behavior of iPP. The polymer is characterized regarding its morphology and the amount of beta modification as well as its (fracture)mechanical properties in order to correlate morphology and properties. Under quasistatic loading, it can be shown, that the results from literature for polypropylene types with lower MFR can be transferred to the polymer used in this study. The increased ductility can be explained by a phase transition from beta to alpha modification and a decreased mobility of the crystal lamellae in the alpha modification. It can be shown that a shish-kebab structure, is beneficial for the behavior of the polymer under quasistatic as well as dynamic load. This influence is less pronounced under short-time dynamic load. However, a deviation from a spherulitic morphology is necessary in order to improve the toughness. A shish-kebab structure alone (without beta modification), however, is not enough to increase the toughness of the material.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
|Subjects:||Materials science, Plastics|
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