Impact of particle size and surface charge density on redispersibility of spray-dried powders

Abstract

To understand the impact of surface charge density on the redispersibility of spray dried powders, the redispersibility of styrene-butadiene (SB) powders prepared from latexes with various particle sizes and acid contents was investigated. The redispersibility of these powders showed a step function from fully redispersible to non-redispersible. The phase boundary between these two regimes is determined by the product of the latex particle size and acid content, which is strongly related to the surface charge density of the latex particle. Results indicate that to achieve fully redispersible polymer powder, a proper balance of latex particle size and surface charge is needed. For the class of SB latexes studied in this work, with a certain acid level, it is possible to predict the minimum latex particle size that enables redispersibility of its spray dried powder. This phenomenological observation is unique and it may help us to reveal the fundamentals which govern the redispersibility. To maintain the redispersibility of a spray dried powder, a colloidal stabilizer that adsorbs onto the surfaces of latex particles is required. When latexes are brought to close proximity, strong van der Waals forces may deplete the surface stabilizers, causing irreversible latex agglomeration at spray drying temperatures. A qualitative DLVO analysis was employed to study the impact of various colloidal interactions in this system, including van der Waals, electrostatic, and steric interactions. Zeta potential measurements of the mixture of SB latex and polyvinyl alcohol (PVOH, used as a colloidal stabilizer) suggest that the surface charge density plays a significant role on redispersibility, and strong electrostatic repulsive interactions are needed to overcome the attractive van der Waals interaction. Size exclusion chromatography of serum phase analysis indicated that PVOH adsorption onto latex particles did not change significantly at different acid levels, suggesting low impact of steric repulsion on redispersibility. Conclusions from this work provide additional means to control the performance of redispersible materials.