Use of acoustic levitation to examine the drying behavior of microdroplets of polymer latex dispersions

Abstract

An acoustic levitator was used to determine the drying kinetics and behavior of microdroplets of colloidal polymer latex dispersions. It was possible to measure the kinetic behavior of evaporation rate, droplet surface temperature, and droplet/particle aspect ratio for two polymers at two different volume fractions of dispersed phase, ϕint. A drying air temperature of 60 °C, a relative humidity of 1 %, and a flow rate of 0.5 L/min through the levitator chamber were used. Under these conditions Eudragit NE 30 D was dried above its minimum film-forming temperature and a critical point of drying could be detected from sharp changes in surface temperature and droplet/particle shape. The drying rate depended inversely on ϕint which increases continually during drying. Aquacoat was dried below its minimum film-formation temperature, and a two-step change in particle/droplet shape and surface temperature was observed. Although the nanoparticles of Aquacoat form an interfacial film at the first step, this does not substantially reduce drying rate. Only the second step change in surface temperature was accompanied by a decrease in drying rate as the surface film is now sufficiently condensed but not fused. The levitator detects therefore a differentiated picture of the changes taking place in drying rate, surface temperature, and droplet/particle.