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Influence of Morphology on Strength and Fracture Mechanisms of Isotactic Polypropylene

In semi-crystalline Polypropylene (PP), the right choice of the molecular composition (atactic content, molecular weight) and the focused adjustment of the microstructure (morphology) can reduce the sensitivity against crack propagation and optimize the yield stress of the material. Slowly crystallized, coarse-spherulitic morphologies possess the poorest values of yield stress, strain to failure, and fracture toughness, due to the concentration of plastic deformation in the small volume of the weak spherulite boundary regions. A further degradation can be caused by impurities and a corresponding “embrittlement” of the boundaries. On the other hand, the worsening of the properties of the coarse-spherulitic morphological areas can be highly reduced when the amount of atactic, non-crystallizable polymer chain segments in PP is enhanced. This occurs, however, at the cost of a slight reduction in hardness and elastic modulus.

Better values of “strength”, here defined as a product of yield stress and fracture toughness, are achieved by a quenching of the PP melt and a resulting fine-spherulitic morphology, especially when a higher molecular weight PP is used. In such a micro-structure, a pronounced craze formation or a homogeneous plastic deformation within a larger material volume can e.g. take place in front of a crack tip, which leads to a high energy dissipation and a hindering of the crack propagation under static loading conditions. This can even result in a complete crack stopping under cyclic deformation with a constant load amplitude. The reason is based on the very good “strain hardening” behavior of semi-crystalline polymers. The latter is especially pronounced in PP due to its helical chain conformation and its cross-hatched structure of the crystal lamellae within the fine spherulites.

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Author
 Klaus Friedrich

Klaus Friedrich
Institut für Werkstoffe
Ruhr-Universität Bochum

Information

Free keywords: Polypropylene, Morphology, Fracture Toughness, Fatigue, Atactic, Spherulite, Crack Propagation, Strengtht
Institute / chair: Fakultät für Maschinenbau der Ruhr-Universität Bochum
Language: German
Technical consultant for expert services: Prof. Dr.-Ing. E. Hornbogen (Betreuer), Prof. Dr. rer. nat. H. H. Kausch
Publication year: 1978
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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