Effect of ionic monomer concentration on latex and film properties for surfactant-free high solids content polymer dispersions

Sodium styrene sulfonate (NaSS) was used to stabilize waterborne poly(methyl methacrylate/butyl acrylate) dispersions obtained by surfactant-free seeded semicontinuous emulsion polymerization at 50 wt% solids content. The effect of NaSS concentration (0.175-3.6 wbm%) on the reaction kinetics, NaSS incorporation onto particles, colloidal stability and properties of emulsifier-free latexes and films was investigated. It was found that fraction of NaSS that was incorporated onto polymer particles increased with its concentration due to the increase of the ionic strength, which shifted the adsorption equilibrium of the NaSS containing oligoradicals towards the polymer particles. Properties of the latexes (freeze-thaw and salt stability) improved with the concentration of NaSS and its incorporation. The lack of migration and formation of aggregates of the stabilizing moieties in the polymer films was demonstrated by water contact angle measurements and AFM images. This resulted in improved properties of the films (gloss, water uptake and mechanical strength).