Flow and mixing efficiency characterisation in a CO2-assisted single-screw extrusion process by residence time distribution using Raman spectroscopy

Hot-melt extrusion of a bio-sourced polyamide has been implemented in a single-screw extruder with supercritical carbon dioxide injection. CO2 acts as a plasticiser in the extruder barrel and as a physical blowing agent at the die. To insure a better mixing and dissolution of the CO2 into the polymer melt, addition of a static mixer between the screw tip and the die was tested. The effect of both the static mixing element and the CO2 injection on the melt flow behaviour has been elucidated. A recent technique of in-line Raman spectroscopy was implemented to make a residence time distribution study, using titanium dioxide as a tracer. The use of a static mixer exerts a major modification on the flow behaviour: it improves mixing by enhancing dispersion. In addition, the structure of the manufactured products was studied: the static mixer led to more homogeneous porous structure. The broad range of CO2 incorporation (up to 25%, w/w) into the melt led to the manufacture of foams with adjustable porosity from 15 to 70%.