Method for preparing an aqueous dispersion of multistage polymer particles

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

• a) contacting under emulsion polymerization conditions an aqueous dispersion of core polymer particles with first monomers to form an aqueous dispersion of core/first stage polymer particles;
• and
• b) contacting under emulsion polymerization conditions the aqueous dispersion of core/first stage polymer particles with second monomers to form the aqueous dispersion multistage polymer particles;
• and
• c) neutralizing the aqueous dispersion of multistage polymer particles with a base;
• wherein
• the core polymer particles comprise from 25 to 60 weight percent structural units of a carboxylic acid monomer or a salt thereof, from 40 to 75 weight percent structural units of a nonionic ethylenically unsaturated monomer, and up to a total of 15 weight percent structural units of styrene and a high Tg hydrophobic monomer;
• wherein
• the high Tg hydrophobic monomer is one or more monomers selected from the group consisting of cyclohexyl methacrylate, isobornyl methacrylate, 4-t-butyl methacrylate, t-butylstyrene, and n-butyl methacrylate;
• the first monomers comprise from 5 to 15 weight percent of an carboxylic acid monomer or a salt thereof;
• 2) from 45 to 55 weight percent styrene;
• 3) from 37 to 47 weight percent methyl methacrylate or the high Tg hydrophobic monomer or a combination thereof;
• and
• the second monomers comprise from 80 to 99.9 weight percent styrene and from 0.1 to 0.5 weight percent of a multiethylenically unsaturated monomer;
• wherein,
• the weight-to-weight ratio of the core polymer particles to the sum of the first and second monomers is in the range of from 1:9 to 1:20;
• and the weight-to-weight ratio of the first monomers to the second monomers is in the range of from 1:3 to 1:8;
• with the proviso that the core/first stage polymer particles comprise, based on the weight of the core/first stage polymer particles, from 2 to 15 weight percent structural units of the high Tg hydrophobic monomer.