Development of a Dilatometer for Measurement of the PVT Properties of a Polymer/CO2 Solution Using a Foaming Extruder and a Gear Pump

Abstract

This paper presents an innovative dilatometer that can measure the pressure-volume-temperature (PVT) properties of polymer/CO2 solutions in a molten state. The basic rationale of the design is to determine the density (or equivalently, the specific volume) of a polymer/CO2 solution by separately measuring the mass and volume flow rates of the solution flowing in an extruder at each temperature and pressure. A positive-displacement gear pump mounted on an extruder is used to measure the volume flow rate of the solution. A single-phase polymer/CO2 solution is formed by injecting a metered amount of CO2 into a polymer melt and completely dissolving it in the melt using a foam extrusion line. The temperature of solution was precisely controlled and homogenized by using the second extruder in a tandem system and a heat exchanger with a static mixer. The pressure was controlled by the rotational speed of the screw in the second extruder. In order to reduce leakage across the gear pump, the difference between the upstream and downstream pressures was minimized using a variable resistance valve attached downstream of the gear pump. The mass flow rate was measured by directly collecting the extruded polymer melt for a fixed time after degassing CO2. A critical set of experiments was carried out to verify the functions of the system using pure polymer melts with known PVT data. Finally, the system was used to measure the specific volume of PS/CO2 solutions as a function of CO2 concentration, temperature, and pressure.