Poly(acrylic acid) (PAA) is widely used as superabsorbent polymer, scale inhibitor and as dispersant, for example for the dispersion of CaCO3 slurries that are subsequently applied to paper levelling (around 32 billions lbs of CaCO3 per year are produced for this usage). The mechanism of CaCO3 dispersion relies on the electrostatically driven adsorption of the negatively charged PAA onto the cationic surface of mineral. As shown below, the average molecular weight and the polydispersity index (PDI = Mw/Mn) strongly influences the quality of the dispersion. By decreasing the PDI, less dispersant is needed, and the dispersion is less viscous. For this reason, the preparation of monodisperse PAA on a large scale is of high significance.
Preparation of Monodisperse PAA
We use RAFT technique, discovered by Rizzardo in 1998. Several chain transfer agents were screened. We found that the best ones are trithiocarbonate and phenoxy xanthates.
Adsorption Mechanism of PAA onto CaCO3 and other minerals
At neutral pH, CaCO3 solubility is low ( < 10-4 mol/l), and this compound is nearly entirely crystallized in water. Surface ions constituting the surface of these crystals are either Ca+ or CO3-. On certain crystal faces of calcite (100 and 104), there is an equimolar amount of calcium and carbonate : the surface is globally neutral, but on some other surfaces (012, eg), only Ca+ is present, conferring a positive charge to the surface. The ions are locally neutralized by solubilized carbonate ions in the Stern layer. In the presence of PAA, these solubilized carbonate ions are replaced by two polyacrylate ions through a thermodynamically favored process.
The adsorption mechanism of PAA onto CaCO3 surface first involves the adsorption of the shortest PAA chains, as these can diffuse first to the surface. These chains are then displaced by medium length PAA chains through an entropically favorable favorable. At this stage, surface Ca+ are complexed, and the crystal is protected by a globally negative PAA hairy layer. Longer PAA chains no longer approach the surface because of the very large electrostatic repulsion between the globally negative free polymer coil and the negatively charged crystal surface. Therefore, in polydisperse PAA samples, only chains with a precise molecular weight (around 3000 g/mol) are adsorbed, shorter and longer chains staying in solution.
Copolymers BA-AA : synthesis and
characterization
Copolymers of n-butyl acrylate and acrylic acid were prepared by extending the RAFT polyacrylic acid chains. These copolymers were found to be superior dispersnats for emulsion polymerization of acrylic monomers : with one percent, it is possible to prepare a latex with 50% solid content. More details can be founnd in the following publication:
Controlled Radical Polymerization of Acrylic Acid : Novel CaCO3 Dispersants