Aqueous Dispersion Comprising a Multistage Polymer and Process of Making the Same

A process of preparing the aqueous dispersion comprising a multistage polymer of by multistage free-radical polymerization, comprising:

• (i) preparing a first polymer in an aqueous medium by free-radical polymerization,
• (ii) preparing a second polymer in the presence of the first polymer obtained from step (i) by free-radical polymerization, and
• (iii) preparing a third polymer in the presence of the first polymer and the second polymer obtained from steps (i) and (ii) by free-radical polymerization,
• wherein the multistage polymer comprises the first polymer, the second polymer, and the third polymer,
• wherein the first polymer has a Tg less than 0° C., and comprises structural units of a carbonyl-containing functional monomer, and from zero to less than 0.1% by weight of the first polymer of structural units of a multifunctional monomer containing two or more different ethylenically unsaturated polymerizable groups;
• wherein the second polymer has a Tg less than 0° C. and comprises from 0.1% to 10% by weight of the second polymer of a multifunctional monomer containing two or more different ethylenically unsaturated polymerizable groups, and optionally structural units of a carbonyl-containing functional monomer;
• and
• wherein the third polymer has a Tg higher than 50° C. and comprises structural units of an ethylenically unsaturated nonionic monomer, and from zero to less than 0.1% by weight of the third polymer of structural units of a multifunctional monomer containing two or more different ethylenically unsaturated polymerizable groups;
• wherein the third polymer comprises, by weight based on the weight of the multistage polymer, from zero to less than 40% of structural units of methyl methacrylate.