Method for preparing an aqueous dispersion of biocide-free organic opacifying pigment particles

A method for preparing an aqueous dispersion of multistage polymer particles comprising the steps of:

• a) contacting under emulsion polymerization conditions in the presence of a free radical initiator an aqueous dispersion of core polymer particles with a first monomer to form an aqueous dispersion of core/first stage polymer particles;
• then
• b) contacting the dispersion of core/first stage polymer particles with a free radical chaser;
• then
• c) contacting the dispersion of core/first stage polymer particles from step b) with i) from 8 to 20 weight percent of a swelling monomer, based on the weight of the core/first stage polymer particles and the swelling monomer, and ii) a polymerization inhibitor to inhibit the polymerization of the swelling monomer;
• then
• d) contacting the dispersion from step c) with a neutralizing swelling agent to neutralize the core and induce swelling of the core with water;
• e) initiate polymerization of the swelling monomer with a redox coupling agent to form an aqueous dispersion of core/shell polymer particles with less than 1000 ppm of unreacted swelling monomer;
• and optionally
• f) adding hydrogen peroxide and a chelating agent that is not complexed with a multivalent metal cation;
• wherein, the core polymer particles comprise 20 to 60 weight percent structural units of a salt
• of a carboxylic acid monomer and from 40 to 80 weight percent structural units of a nonionic monoethylenically unsaturated monomer;
• the first monomer and the swelling monomer have a Tg in the range of from 60° C. and 120° C.;
• the weight-to-weight ratio of core polymer particles to the sum of the first monomers and swelling monomer is in the range of from 1:9 to 1:20;
• wherein the redox coupling agent comprises a t-C4-C10-alkyl hydroperoxide and a reductant;
• with the proviso that when step f) is not carried out, the mole-to-mole ratio of the t-C4-C10-alkyl hydroperoxide to the reductant is in the range of from 5:1 to 50:1;
• and when step f) is carried out, the mole-to-mole ratio of t-C4-C10-alkyl hydroperoxide to the reductant is in the range of from 2:1 to 3:1 and the added amount of hydrogen peroxide is in the range of 0.1 to 1 weight percent, based on the weight of the composition, and the added amount of chelating agent not complexed with a multivalent metal cation is in the range of from 50 ppm to 1000 ppm.