Devolatilization of Molten Polymers During Multiphase Flow in a Double Screw Extruder

Devolatilization is a thermal separation process in order to remove low molecular solvents from mixtures of polymers. Extruders with partly filled devolatilization zones are often used for this process. The two-phase flow of the polymer and the evaporating monomers and solvents is complex due to free surfaces. In film flow and two-phase bubbly flow the polymer is heated by dissipation and cooled by evaporation of the low molecular solvent. Temperature and concentration fields are difficult to predict in extruders because of the complex flow field. Therefore the experimental investigations are carried out in special designed apparatus with a flow field similar to that in extruders and in a transparent double-screw extruder to investigate the different flow mechanisms. In order to nucleate bubbles of the volatile component the polymers must be supersaturated and some kind of deformation must exist. The bubble nucleation is shear induced. The changes in concentration during two-phase bubbly flow result in decreasing temperatures. The mass transfer rates are increased due to the large inner surfaces of the bubbles in the foam and so is the cooling by evaporation. The higher the foam expansion the better is the mass transfer.