Hollow polymer latex particles

A process for preparing a hollow polymer particle comprising

• A. emulsion polymerizing (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and one or more crosslinking monomers to form an acid-free hydrophilic core copolymer phase which is base-hydrolizable at temperatures greater than the T.sub.g of the copolymer;
• B. emulsion polymerizing (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.;
• and one or more vinyl acid monomers;
• and, optionally, one or more monovinyl aromatic monomers;
• and, optionally, one or more crosslinking monomers to form an intermediate shell copolymer phase which is compatible with the hydrophilic core copolymer phase;
• C. emulsion polymerizing one or more monovinyl aromatic monomers;
• and one or more vinyl acid monomers;
• and one or more crosslinking monomers;
• and, optionally, (meth)acrylate monomers, vinyl esters of carboxylic acid monomers or a mixture thereof having a Tg for their copolymer greater than 50.degree. C. to form an an outer rigid shell copolymer phase which is hydrophobic and crosslinked, the copolymer having a T.sub.g greater than 80.degree. C., the outer shell copolymer being compatible with the intermediate shell copolymer;
• and
• D. at a temperature from 100.degree. C. to 150.degree. C., expanding the particle produced in C. by exposing the particle to a strong alkaline solution in an amount from 0.75 to 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, to produce a hollow polymer particle;
• or at a temperature from 100.degree. C. to 150.degree. C., expanding and encapsulating the particle produced in C. by
• i. mixing the particle with an encapsulation mixture comprising a monovinyl aromatic monomer, a crosslinking monomer, an initiator and a strong alkaline solution containing from 0.75 to 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, and
• ii. subjecting the mixture resulting from i. to simultaneous expansion and polymerization conditions
• to produce a hollow polymer particle which is encapsulated in a hard shell;
• or at a temperature from 100.degree. C. to 150.degree. C., expanding the particle produced in C. by
• i. exposing the particle to a strong alkaline solution in an amount from 0.75 to 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, and then, at a temperature from 75.degree. C. to 150.degree. C., encapsulating the hollow polymer particle by
• ii. emulsion polymerizing an encapsulation mixture comprising a monovinyl aromatic monomer and a crosslinking monomer in the presence of the expanded particles to produce a hollow polymer particle which is encapsulated in a hard shell.