| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 92 === The rheological behavior of PP/supercritical CO2 (SCCO2) mixture was investigated in this study. Both effects of shear thinning and viscous heating that seriously affected the melt viscosity were considered, and then the variation for PP viscosity with CO2 addition was examined. A traditional injection machine had been modified to generate the uniform CO2-containing PP melt. By measuring the pressure profile and flow rate of a slit die in front of the barrel during the injection procedure, and also monitoring the shear-induced temperature variation of the system, the dependence of apparent viscosity on both shear rate and temperature had been determined. The Rabinowitsch correction was used to further rectify the non-Newtonian behavior for the melts, and then true viscosities of both systems were obtained and compared. Due to the constraint of extruder that past studies adopted, the generated shear rates were limited below the order of 1000 (1/s). However, the shear rate experimented by this equipment had extended to an order of 10000 (1/s). Theoretical consideration based on Cross-Carreau models and combined with Eyring model was proposed to correlate the viscosity data. It was observed the pressure profile for pure PP melt within the slit die could be represented by a straight line at the region far above the die exit, and it showed a non-linear profile at the die exit as the consequence of exit-effect. A special deflection was found for PP/CO2 mixture at the region near the die exit, which exhibited the influence of gas separation when pressure reduced and was insufficient to keep the gas soluble. The regression result revealed the viscosity of PP/CO2 melt could be well correlated by Cross-Carreau/Eyring model. By increasing the system free volume and decreasing the density of molecular entanglement, the PP viscosity was effectively reduced by CO2 addition. Actually the viscosity of PP melt was dependent on CO2 content, temperature and shear rate. However, all three factors are competitive and the magnitude of viscosity reduction was determined by the dominant mechanism at different stages. The viscosity reduction was most apparent at low shear rates. As shear rate increased, the SCF contribution decreased, finally the viscosity reduction achieved a stable value at extremely high shear rates. Moreover, the viscosity reduction would be greater as the temperature was lower, but it also diminished when the temperature increased. The viscosity profiles exhibit similar shape between different temperatures. After scaling out the temperature and the CO2 effects, the master curve could well represent the distribution of all the experiment data.
By this thesis, the effect of CO2 content on viscosity variation of PP melt had been studied at highly non-Newtonian region. Both experiment data and theoretical correlation for PP and PP/CO2 mixture were proposed, which could be used as the fundamental of simulation for supercritical-fluid aided plastic injection molding process.
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