Transient dynamics of polymer emulsification inside a twin-screw extruder: effect on process stability and particle size distribution

Solvent-free extrusion emulsification (SFEE) is a recently developed process for producing submicron particles of high viscosity polymers using a twin-screw extruder without hazardous solvents. Its rate dependency on mixing for catastrophic phase inversion makes the process knowingly sensitive to a variety of variables, included the manner by which it is started up. This study examined how transient dynamics at the beginning affects steady-state operations by varying the time for the surfactant-containing feed stream to reach its setpoint rate into the process, to better understand the mechanism of SFEE. The results showed that the path taken to reaching the setpoint conditions (i.e. transition time) determined process stability and whether successful emulsification occurred, not the actual setpoint conditions chosen. Combining particle size and residence time measurements with prior studies’ data related to the rate of reactions benefiting interfacial growth between the polymer/water phases and inline rheological findings, this unusual transient sensitivity was attributed to insufficient surface-active species present creating a persistent segregated regime of the flow field in the first half of the dispersion zone that exhibits an unstable phase morphology. Returning to the desired operating window was only possible once this segregated regime was purged from the process. The study offers a continuous processing perspective to this phenomenon previously observable in batch for high viscosity oil phase systems. Graphical abstract