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

Production of waterborne dispersed polymers requires the application of surfactants to provide colloidal stability during synthesis and storage. Unfortunately, as they are only physically attached to the polymer particles they are prone to desorb from the particles and to migrate during film formation, which in turn translates to undesired effects on the film properties. This has thus triggered research to seek alternative ways for emulsion polymer production. Recently, the surfactant-free emulsion polymerization approaches using ionic initiators and monomers have been developed. Utilization of initiators as sole source of stabilization is not advisable since alterations in concentration of initiators significantly affects the reaction kinetics and molecular weight. On the other hand, use of ionic monomers for stabilization is challenging because they have a high solubility in water and therefore they tend to polymerize in the aqueous phase, which hinders their incorporation to the polymer particles.

Dr. Sevilay Bilgin, Professor Radmila Tomovska, and Professor José M. Asua from POLYMAT at the University of the Basque Country in Spain, investigated the applicability of sodium styrene sulfonate in stabilization of waterborne poly(methyl methacrylate/butyl acrylate) dispersions obtained by surfactant-free seeded semi-continuous emulsion polymerization at 50% by weight solids content. They studied the effects of sodium styrene sulfonate concentration on the reaction kinetics, colloidal stability and the properties of emulsifier-free latexes and films. Their research work is now published in European Polymer Journal.

Using a seed already stabilized with sodium styrene sulfonate, they synthesized high solids polymer latexes with different sodium styrene sulfonate concentrations. Post-polymerization was used to eliminate the residual monomer.

The study by POLYMAT researchers presented an in-depth investigation of the effects of sodium styrene sulfonate concentration on the reaction kinetics, particle size/distribution, its incorporation and stability of the latexes during synthesis and storage. The authors observed that the fraction of sodium styrene sulfonate that was incorporated onto the polymer particles increased with its concentration due to the increase of the ionic strength, which shifted the adsorption equilibrium of the sodium styrene sulfonate containing oligoradicals towards the polymer particles. Moreover the study has shown that the films cast from sodium styrene sulfonate containing latexes do not display any migration of the stabilizing sulfonate units, leading to coatings with improved performance (gloss, water uptake and mechanical strength) as compared to the control films.