Capillary rheometry of a binary mixture polymer/CO2 in a single screw extruder

• Hot-melt extrusion assisted by sc-CO2 was implemented with a biosourced polymer.
• Development of an on-line viscosity measurement based on capillary rheometry.
• The impact of CO2 on the rheology of the system was quantified.
• Limits of this simple technique have been evidenced.

Processing bio-sourced polymers with supercritical fluids is a promising route towards new green engineering processes. Supercritical carbon dioxide (sc-CO2), is soluble in large quantities in molten polymers, where it acts as plasticizer and swelling agent. It is used, in blending or foaming of polymers, particle formation and polymerisation process.The process of hot-melt extrusion assisted by sc-CO2 allowed the development of an on-line viscosity measurement based on capillary rheometry. Applied to a bio-sourced polyamide, it was validated by comparison with a classic capillary rheometer. Both data sets were in good agreement.A pseudoplastic fluid behaviour was observed with a 30 % viscosity decrease from 46 to 32 Pa s at 5000 s−1 and 220 °C, upon addition of CO2. However, viscosity decreased to a plateau before reaching the thermodynamic solubility. The comparison with a model coupling solubility and flow allowed to identify the method limitations, which were attributed to the kinetics of dissolution and mixing. The higher the shear stress, the higher the amount of CO2 at which the viscosity plateau is reached. These measurements may quantify the impact of the CO2 on the rheology of the system but also of the efficiency of the mixing process in our experimental setup.