Drying rate variations of latex dispersions due to salt induced skin formation

Film formation from latex dispersions with varying concentrations of sodium dodecylsulfate (SDS) and sodium persulfate (NaPS) was studied with a sorption balance. The drying rate decreased significantly at a critical volume fraction of polymer (pc). Under constant drying conditions the pc varied due to differences in particle stabilization. In SDS containing samples, the droplets wetted larger areas, the film thicknesses decreased and, consequently, the initial evaporation rate was decreased. The decrease in the initial evaporation rate first continued with increasing SDS concentration but leveled off at an apparent critical micelle concentration (CMC). Samples containing NaPS had different types of film formation mechanisms with large variations in pc and the total drying time, which could be explained by differences in the electrostatic stabilization. For dialyzed dispersions containing no NaPS, pc was close to 0.7. In samples with medium high NaPS concentration a skin was formed at the air interface causing an early shift in the evaporation rate, resulting in 0.25<pc<0.7. At high NaPS concentration the particles coagulated and settled giving an apparent increase in pc, i.e., values above 0.7. Deviations from the skin formation behavior predicted by the Routh and Russel (RR) model were observed.