Jens Stange

Rheological behavior of blends from a linear and a long-chain branched polypropylene

This paper investigates how the rheological behavior of a linear polypropylene is changed by blending with different amounts of a long-chain branched polypropylene. The zero shear-rate viscosities of the blends follows the logarithmic mixing rule between the two blend partners up to 50 wt % of the long-chain branched polypropylene. For the blend with 75 wt % long-chain branched polypropylene a deviation from the logarithmic mixing rule was found, which can be referred to a disentanglement of the long-chain branched fraction of molecules during the blend extrusion process similar to the findings for the extruded long-chain branched polypropylene. It is concluded that the linear polypropylene in the blends reduces the extrusion effect on the long-chain branched species in a way that for the blends with up to 50 wt % of the linear PP and no influence of the blend preparation process on the rheological behavior occurred. As the branching structure within the blends remains unchanged it could be shown that the...

Rheological properties and foaming behavior of polypropylenes with different molecular structures

This article investigates how rheological properties of polypropylenes with different molecular structures influence their foaming behavior. The molecular structure of the different polypropylenes is analyzed by size exclusion chromatography coupled with a light scattering detector, and by rheological means, such as the molar mass dependence of the zero shear-rate viscosity. The main focus of the rheological experiments is laid on the strain hardening and failure behavior of the melts in uniaxial elongational flow. For all linear polypropylenes investigated, a rupture of samples occurred before the maximum strain—accessible with the instrument used—was achieved. For the linear polypropylenes, a growth of the stress to rupture and the elongation at break were found with increasing molar mass, which go along with an increase of the expansion ratio in the foaming experiments. Besides the linear polypropylenes, which do not show strain hardening, several so-called high melt strength polypropylenes were invest...

Effect of long-chain branching on the foaming of polypropylene with azodicarbonamide

In this article the influence of long-chain branching on the foaming behavior of polypropylene (PP) is investigated. Different branching contents are achieved by blending a linear PP (L-PP) and a long-chain branched PP (LCB-PP). Whereas, the L-PP does not exhibit any strain hardening in laboratory stretching experiments, blends with amounts of the LCB-PP higher than 2 wt% already show a pronounced strain-hardening behavior. The strain hardening increased with a growing amount of the long-chain branched PP. A laboratory scale foaming apparatus based on a capillary rheometer is developed for the foaming experiments. In foaming tests with azodicarbonamide as chemical blowing agent, a significant improvement of the foaming behavior with respect to a higher expansion ratio, a lower amount of connected cells, and a more homogeneous cell size distribution is found with increasing content of the LCB-PP up to a concentration of 50 wt%. At this concentration, the foaming behavior of the LCB-PP is reached. The results demonstrate that low amounts of long-chain branching can significantly improve the optimal foaming process of PPs with a chemical blowing agent, and that additions of the linear material up to 50 wt% to the LCB-PP do not have any influence on the favorable foaming performance of the long-chain branched PP.