Biodegradable polymer particle formation using supercritical carbon dioxide
Date
2006
Authors
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Publisher
University of Delaware
Abstract
We investigated a polymer particle production method, termed Polymer Liquefaction Using Supercritical Solvation (PLUSS), in this thesis. In PLUSS, high pressure, low temperature CO2 liquefies the polymer due to melting or glass transition temperature depression, and solid particles are formed when the pressure is released. ☐ An experimental and modeling study of PLUSS was conducted using the biodegradable polymer polycaprolactone (PCL). Spherical particles with an average size of 2 - 3 μm were made. We hypothesize that the particle shape is governed by the two – phase turbulent flow during the expansion, and that the particle properties are determined at the polymer freezing point. Based on these assumptions we established our PLUSS expansion modeling framework. ☐ The characteristic features of the Tmelt vs. P curve – a linear regime at modest pressure, followed by an abrupt transition to a second regime where Tmelt changes little with P – were interpreted quantitatively in this thesis using the Clapeyron equation. The slope of the lower pressure linear regime was quantitatively estimated from the normal Tmelt, the enthalpy of fusion of the polymer, and the Henry’s law coefficient for CO2 dissolved in the melt. The location of the abrupt change in slope was correlated with equivalence of the molar volumes of the compressed fluid and the Kuhn monomer unit of the polymer crystal. This method described not only PCL behavior in CO2, but also the behavior of other polymers, and certain non-polymeric compounds as well.PLUSS was investigated in this thesis. Spherical particles with an average size about 2 μm were obtained through this process. A homogeneous equilibrium, one-dimensional model was developed to describe the PLUSS two-phase expansion using the mass, momentum and energy balance equations. The Sanchez – Lacombe equation of state was selected to describe the polymer properties. The model was solved numerically in matrix form using the commercial software Matlab. This model used operating conditions – stagnation temperature T0, stagnation pressure P0, CO2 to polymer weight ratio (CTP) and backpressure BP – and design variables – capillary length L and diameter D – as inputs to describe the flow properties – temperature, pressure, velocity, viscosity, enthalpy – along the expansion capillary. For a typical PLUSS operating condition this model predicted a total flow rate of 50 g/s and a residence time of 0.5 millisecond. These values were comparable to experimental values. ☐ A turbulent droplet size correlation was used together with the flow model to predict a particle size of 2 μm, which was similar to experimental measurements with PCL. The model predicted that, at a fixed geometry the particle size is mostly controlled by P0 and that raising P0 can reduce the diameter by as much as 50%. Lowering T0 can also reduce the particle size, but only by about 10%. The particle size was predicted to be insensitive to CTP. Reducing either the capillary L or D would further reduce particle size by 50%. Based on our modeling, the key parameter which determines the particle size is the energy dissipation rate ε. Its typical value is 2x107 m2/s3. However the range of ε achievable experimentally is limited by choking flow when the CO2 vaporizes, so that the particle size cannot be varied by an order of magnitude.