Mutual Interactions between Crystallization and Flow in Polypropylene

Main objective of the present work is the investigation of mutual interactions between flow and crystallization of an under-cooled polymer melt. Shearing of an under-cooled melt magnifies the number density of spherulites that grow after-wards. Depending on the crystallization conditions a layered structure, as typically found in injection moulded parts, is formed. However it should be clear that by further shearing during crystallization the flow properties will also be changed in some unknown way. For the purpose of this investigation a model experiment is used, in which an under-cooled polymer melt is forced to flow through a rectangular duct under a constant pressure drop. This sort of experiment has been used previously to study the influence of shearing conditions on the crystallization kinetics. The shearing times of this classical “short term shearing” experiments are extended here leading to an overlap of the shearing and the crystallization processes thus resulting in a coupling of flow and crystallization phenomena. The experimental studies are performed using as model material a polypropylene “Daplen DM 55 pharm” from Borealis. In the practical investigations the final extruded mass is determined and the intensity of linearly polarized red light passing through the melt during and after shearing are investigated. By use of an additional polariser (oriented parallel or perpendicular to the initial polarization direction) the polarization state of the transmitted light is recorded. In a series of experiments at 135°C and lower pressures of 170 to 180 bars a good correlation between shearing time and the time of reaching half of the initial intensity (“decay time”) is obtained. Shorter shearing times lead to longer decay times for the total intensity signal. At the same temperature of 135°C and higher pressures of 280 to 360 bars it turns out that small changes in the flow pattern in the entry region of the duct has a strong effect on the measurements, leading to poor correlations between shearing time, applied pressure and the intensity signals. Thus the extruded mass is used as a characteristic quantity in the experiments replacing the shearing time. In this experimental series the total intensity curves have similar shapes and the decay time shows a clear relation with the extruded mass. Higher extruded mass leads to a shorter decay time in a non-linear way. Additional evaluation of the polarization signal under perpendicular polars of the experiments at 135°C reveal further insight into the generated structures: The time for the polarization signal to approach full depolarization correlates strongly with the corresponding decay times from the total intensity signal. Using microtome cuts of the solidified material in the duct it is found, that small birefringent layers have been formed. At a melt temperature of 140°C two kinds of polarization signals can be distinguished: One group of experiments shows a small birefringent layer like those in the experiments at 135°C and the polarization signal approached full depolarization in a monotonous way. The other group can be associated with an oscillating polarization signal with decreasing amplitude that shows one minimum and in some cases two maxima instead of one, before reaching full depolarization due to dominance of scattering. ... Microtome cuts of the samples taken from the various experiments reveal a complex pattern with two transcrystalline layers close to the sample surfaces and bigger spherulites in the centre region. In many cases also fine crystalline layers are found which could be assigned to small differences in the temperature profiles due to the particular way of temperature control in both experiments. The experimental results in this work clearly show that the mutual interaction between crystallization and flow is decisive for the morphologies as obtained in practical processing routes of polymer melts.