Untersuchungen zur Filmentgasung hochviskoser Polymere in Extrudern

Hirschfeld, Stefan

kassel university press, ISBN: 978-3-7376-0720-9, 2019, 200 Pages
(Berichte des Instituts für Mechanik 2/2019)

Zugl.: Kassel, Univ., Diss. 2019

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Content: The removal of undesired volatile compounds from highly viscous polymer melts is a critical and expensive task in plastics processing and manufacturing. Residual monomers, solvents and other impurities can impair the quality of the final goods and are often subject to strict environmental and health regulations. Rotating equipment is usually used for degassing polymers, as this results in good mixing and frequent surface renewal of the melt. At very low residual concentrations, the mass transport is diffusion-controlled and bubble-free. This mechanism of film degassing is the subject of few publications which conclude that the experimentally measured mass transport is significantly lower than predicted by theoretical models. In order to explain the reasons for these observations, systematic degassing experiments are carried out on two different experimental plants - a stirring vessel with a blade stirrer and a co-rotating twin-screw extruder. A highly viscous model material system consisting of polydimethylsiloxane as polymer and 1,1,2- trichloro-1,2,2-trifluoroethane as volatile component at ambient temperature is used for this purpose. The relevant material properties are determined by preliminary investigations. An inert gas flow of nitrogen is used to prevent foam formation of the polymer due to supersaturation. In order to estimate the influence of the gas-side mass transfer resistance, which in contrast to what is often assumed in the literature is not negligible in all cases, the gas flow rates are varied. A camera installed in the stirring tank and a transparent extruder housing allow the fluid flow to be monitored at any time. Video analysis and numerical methods are used to investigate the free surface flow in more detail. A systematic variation of rotational speed and filling degree is carried out at both experimental plants. The development of the mass flow was determined by temporal or local concentration measurements using thermogravimetric analysis. The results show detailed explanations for the deviations mentioned in the literature and give valuable recommendations for the design and layout of efficient degassing devices. The transferability of the degassing results
between the two test plants is proven.

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Polymerentgasung, Stofftransport, Oberflächenerneuerung, Kunststoffverarbeitung, Doppelschneckenextruder, Blattrührer

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