Micronization of Polyethylene Wax in an Extrusion Process using Supercritical Carbon Dioxide
Abedin, Nowrin Raihan
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Supercritical fluid technology is a well documented and emergent technology used in many industries today for the formation of micro- and nano- particles. The use of supercritical fluids allows synthesis of various types of particles since their properties can be varied with temperature or pressure, which sequentially can control the physical and chemical properties of the particles produced. Several different processes designed to generate powders and composites using supercritical fluids have been proposed in the past 20 years which can be used to synthesize materials with high performance specifications and unique functionality. In this research work, an extrusion micronization process using supercritical fluid has been proposed. This powder production technique could be a promising alternative to conventional techniques in terms of improvement in product quality as it provides a better control over particle size, morphology and particle size distribution, without degradation or contamination of the product. In addition, as extrusion is globally used for polymer production and processing, particle production by extrusion will allow production and processing in a single process step, eliminating the need for secondary particle production methods. The micronization process designed and described in this thesis involves a twin screw extruder equipped with a converging die and a high resistance spraying nozzle for particle production. A special CO2 injection device and polymer collection chamber was designed for CO2 supply and powder collection. To ensure complete dissolution of CO2 into the polymer matrix, stable injection of CO2, pressure generation and constant spray of micronized polymer particles, a special screw configuration was carefully designed for the extrusion process. The feasibility and the performance of this process have been demonstrated by experimental studies performed with low molecular weight polyethylene wax. Carbon dioxide at supercritical conditions was used as a solvent for processing the polymer. The generated polyethylene particles from the polyethylene wax/carbon dioxide solution system were analyzed and studied using an optical microscope, scanning electron microscope, capillary rheometer and differential scanning calorimeter. A detailed study on the effects of the processing parameters, such as temperature, pressure, flow rate and supercritical fluid on properties of polyethylene particle produced was carried out. The particle size data collected using an optical microscope indicate a significant impact of temperature and CO2 content on particle size. The obtained size data were utilized to generate particle size distribution plots and studied to analyze the effect of the processing variables. It was found that particle size distribution is affected by processing temperature and CO2 content. Studies of the SEM images reveal that the morphology of particles can be controlled by varying processing variables like temperature, polymer feed rate and CO2 content. The particles generated during this study indicate that particle production in an extrusion process using supercritical carbon dioxide is achievable and appears to be a promising alternative to conventional polymer particle production methods such as grinding, milling and other supercritical fluid-based precipitation methods. To validate and generalize the applicability of this process, micronization of other polymeric material should be performed. Commercialization of this technology will further require predictability and consistency of the characteristics of the product, for which a detailed understanding of the influence of all relevant process variables is necessary. In addition, development of theoretical models will further assist in the scale-up and commercialization of this supercritical fluid assisted micronization technology in the near future.