Production of micro and nanoparticles of thermolabile compounds using supercritical assisted atomization

2011 - 2012Supercritical Assisted Atomization (SAA) is a very efficient process to micronize several kind of compounds, such as catalysts, polymers and active principles, also for pharmaceutical applications. The process is based on the solubilization of a controlled amount of CO2 in the liquid mixture, containing the compound to micronize, to reduce the cohesive forces, related to viscosity and surface tension, in order to obtain smaller particles than those could be obtained by process such as spray drying. The solution, formed by the liquid feed and supercritical CO2, is injected through a nozzle in a precipitation chamber where fast and complete evaporation of the solvent from the droplets takes place. The particles are collected on a porous filter located at the bottom of the precipitator. The SAA process has many applications in the micronization field, for this reason it is important to have a better comprehension of the principal mechanisms involved in the atomization process, to have a better control on particle size and distribution. For this reason the first part of this thesis work has been devoted to the evaluation of vapor liquid equilibria, involved in the saturator, and the fluid-dynamic of the jet break up. To evaluate the composition of the vapor-liquid phases, in the equilibrium conditions, a new approach, based on Raman Scattering, has been used. This technique, compared with the traditional used in thermodynamical studies, has the advantage to avoid any disturbance during the measurements. The systems aceton-CO2 and acetone-water-CO2 have been studied and the obtained compositions have been compared with results reported in literatures. The obtained results revealed that the vapor liquid equilibria detected by Raman are comparable with those obtained by other techniques. This part of the work was made in collaboration with SAOT institute in Erlangen (Germany). It is also important to understand the role of supercritical CO2 in the atomization process, from fluiddynamic point of view. For this reason Supercritical Dissolved Gas Atomization (SGDA) plant is used to analyze the spray by laser diffraction technique. The SGDA experiments were performed using two solvents: water and ethanol. Supercritical CO2 shows different behaviours with these solvents, since it is not soluble in water but it has a good affinity with ethanol. The laser diffraction analysis reveals that the droplet mean diameter strongly depends on gas to liquid ratio (GLR), since at low value of GLR (lower than 2) bigger droplets were obtained, whereas at GLR higher than 2, smaller droplets were obtained and no variation in droplet mean diameter was observed at higher value of GLR. Droplets obtained using ethanol are smaller than those obtained using water, this because the solubilization of supercritical CO2 in ethanol allows the decrease of cohesive forces due to viscosity and surface tension. Finally, the analysis of spray by laser diffraction lets to make hypothesis on the regime of atomization: this is bubbly for low value of GLR1.5 the SMDs are considerably smaller than the diameter of the injector (2 order of magnitude), accordingly to the hypothesis of annular flow. On basis of the results obtained by the experiments on SGDA, a model compound was chosen for SAA micronization experiments, to enforce the theory previous discussed. Polyvinylpyrrolidone (PVP), that is a polymer that shows high affinity to polar solvents, was chosen as model compound. The solvents used were water, in which CO2 is not soluble, a water-acetone mixture, where acetone is a non-solvent for PVP and enhance the affinity of the liquid feed with supercritical CO2, and ethanol, that has a good affinity with CO2. The bigger particles (mean diameter ranged between 1.20 and 1.86 μm) were obtained using water as solvent, whereas using ethanol it was possible to produce the smallest particles (mean diameter ranged between 0.4 and 1.36 μm). Moreover, using ethanol as solvent, it was possible to change the operating conditions in the saturator in order to work in the one phase region of vapor liquid equilibria diagram. Keeping constant saturator temperature at 40°C, it was demonstrated that increasing saturator pressure (from 70 to 165 bar) smaller particles were obtained. When saturator pressure is higher than 100 bar, CO2 gives no contribution to a pneumatic effect, since, it is completely dissolved in the liquid. This fact could contribute to the demonstration that the main factor that influences the mean dimensions of the micro particles produced by SAA could be the amount of dissolved CO2; indeed, when SC-CO2 is completely solubilized in the liquid feed, smaller particles can be obtained... [edited by Author]XI n.s

USE OF SUPERCRITICAL CO2 AS DISSOLVED GAS FOR THE ATOMIZATION OF ETHANOL

Supercritical dissolved gas atomization (SDGA) is an atomization process in which carbon dioxide at temperatures and pressures above its critical point is used as atomizing gas. In this work SDGA has been experimentally studied when ethanol is used as the liquid to be atomized. The spray characteristics in terms of droplets size and distribution have been investigated using a droplet size analyzer based on a laser diffraction method. Very narrow droplet size distributions were produced down to 2.5 μm. The main parameter that influences the droplets size is the gas to liquid mass ratio. From the point of view of the atomization mechanism, mean droplet diameter is mainly influenced by the two phase (gas liquid) flow formed within the atomization nozzle. The overall analysis of the experimental data confirms that dissolved gas atomization allows the formation of micrometric droplets that can be useful in fine-particles production processes

The beneficial effects of physical exercise on visuospatial working memory in preadolescent children

The relationship between physical exercise and improvement in specific cognitive domains in children and adolescents who play sport has been recently reported, although the effects on visuospatial abilities have not yet been well explored. This study is aimed at evaluating in school-age children practicing artistic gymnastics the visuospatial memory by using a table version of the Radial Arm Maze (table-RAM) and comparing their performances with those ones who do not play any sport. The visuospatial performances of 14 preadolescent girls practicing artistic gymnastics aged between 7 and 10 years and those of 14 preadolescent girls not playing any sport were evaluated in the table-RAM forced-choice paradigm that allows disentangling short-term memory from working memory abilities. Data showed that the gymnasts obtained better performances than control group mainly in the parameters evaluating working memory abilities, such as within-phase errors and spatial span. Our findings emphasizing the role of physical activity on cognitive performances impel to promote physical exercise in educational and recreational contexts as well as to analyse the impact of other sports besides gymnastics on cognitive functioning

Replication of micro-features on PLA: Effect of viscosity during injection molding with fast evolution of cavity surface temperature

Fast mold temperature evolution allows obtaining injection molded parts with lower filling pressure, better surface finishing and reduced frozen-in orientation. Recently proposed, thin heating devices, with very low thermal inertia, located just below the mold cavity surface, allow obtaining high surface temperature during filling and low surface temperature during cooling stage. The heating stage was limited to a very thin part below the cavity surface, this limits the increase of the processing time. The control of the surface temperature evolution allows controlling replication accuracy of micro-metric features during the injection molded objects. In this paper, the fast evolution of cavity surface temperature was adopted with the aim of obtaining accurate replication of microstructures on molded objects of two grades of poly-lactic acid. The use of two grades of poly-lactic acid allows establishing a correlation between the viscosity of the polymer and its replication ability. As far as the part quality is concerned, a relevant improvement of the replication accuracy of micro-features was attained by increasing cavity surface temperature. The viscosity of the polymer has also a great influence on the replication accuracy, since, the polymer with low viscosity brings to a better replication of the micro-features

Fast cavity surface temperature evolution in injection molding: Control of cooling stage and final morphology analysis

Fast mold surface temperature evolution in injection molding improves the surface finishing and replicability of the molded parts, and may significantly reduce frozen-in orientation. In this paper the effect of a fast control of cavity surface temperature evolution on the morphology and processing conditions of iPP molded parts has been characterized. Phenomena not previously encountered, such as a double pressure packing step when the cavity surface heating lasts longer than the packing step, have been pointed out. Significant effects on the samples frozen-in orientation have been observed by optical microscopy and confirmed by X-ray analysis. AFM analysis shows that it is possible to achieve isotropic morphology with cavity surface temperature kept constant at 150 °C for a long heating time and low holding pressure