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The combination of moderately slow crystal growth at large undercoolings together with the practical absence of sporadic nucleation makes isotactic polypropylene (iPP) an ideal material for controlled nucleation. In this review the different types of nucleating agents -inorganic and organic, particulate and soluble -for the different crystal modifications of iPP (, b and c) are presented together with their working mechanism and criteria for activity. The interaction between polymer type, nucleating agent and processing conditions in determining mechanical and optical properties conclude the survey.

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ABSTRACT:The influence of the molecular weight of the dispersed phase of ethylene-propylene rubber modified isotactic polypropylene (iPP/EPR) reactor blends was studied in a systematic way by varying their intrinsic viscosity (IV) from 1.7 to 6 dg/L while keeping the matrix melt flow rate (MFR) constant. Standard Charpy measurements were completed by a continuous analysis of the impact properties over a wide range of temperatures at fixed test speed. As expected, the higher the IV, the tougher the iPP/EPR blends. However, ductile-brittle transitions as key mechanical descriptors did not correlate linearly with M w , suggesting the macroscopic behavior of the blend to be controlled primarily by the morphology of the EPR particles. Moreover, strong correlations were found between impact mechanical properties, amount of stress-whitening, and strength of the molecular relaxations estimated from dynamical mechanical analysis.

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Potential and limits of dynamic mechanical analysis (DMA) as a tool for fracture resistance evaluation of isotactic polypropylenes and their polyolefin blends are presented. A minimum of information about the materials under investigation is a prerequisite to interpret the DMA traces in a right way. Although DMA is, in general, a powerful method to rank materials in term of toughness, care should be taken with (1) nucleated materials (where both intensity and strength of molecular relaxations need to be taken into account in material evaluation) and with (2) visbroken (i.e., peroxyde treated) grades. Except for these cases, the strengths of the principal or secondary molecular relaxation evaluated by DMA and the Charpy impact toughness correlate quantitatively when all the grades of a series exhibit unstable crack propagation. When changes in the macroscopic mode of fracture or in blend morphology occur, only qualitative correlations remain possible.