Process of heat treating structured polymer particles

1. A process for producing an aqueous dispersion of hollow particles, the process comprising the steps of:

a. forming a plurality of polymer particles that are swellable upon neutralization or hydrolysis;

b. encapsulating the plurality of polymer particles by polymerizing monomer including styrene at a temperature of 50° C. to 80° C. to thereby form a plurality of core-shell particles; and

c. heating the core-shell particles to a temperature of 140° C. to 200° C.

d. cooling the polymer particles from the peak heat treatment temperature to a temperature at or below the Tg of the shell polymer at a rate of not greater than about 2° C. per minute.

2. The process of claim 1 , where said step of heating the core-shell particles is conducted at a temperature of at least 155° C.

3. The process of claim 1 , where said step of heating the core-shell particles is conducted at a temperature of at least 160° C.

4. The process of claim 1 , where said step of heating the core-shell particles is conducted at a temperature of at least 165° C.

5. The process of claim 1 , where said step of heating the core-shell particles is conducted at a temperature of at least 170° C.

6. The process of claim 2 , where said step of heating the core-shell particles takes place at a temperature of at most 190° C.

7. The process of claim 1 , wherein said step of encapsulating a plurality of polymer particles includes polymerizing monomer consisting essentially of styrene to form a polystyrene shell, and where said step of heating the core-shell particles takes place at a temperature at least 50° C. higher than the glass transition temperature of the polystyrene shell.

8. The process of claim 1 , wherein the core-shell particles are expanded during the heating step.

9. The process of claim 1 , where said step of heating takes place in the presence of a base or swelling agent.

10. The process of claim 1 , where the hollow particles have an average wet void fraction of at least about 0.30.

11. The process of claim 1 , wherein the plurality of polymer particles that are swellable upon neutralization or hydrolysis are prepared from monomer selected from the group consisting of (meth)acrylate monomers and vinyl carboxylic acid monomers.

12. The process of claim 1 , wherein the monomer includes at least 30 wt. % methyl methacrylate based on the total weight of the monomers.

13. The process of claim 1 , wherein said step of heating takes place in the presence of a base.

14. The process of claim 13 , wherein the base includes at least one of LiOH, KOH, and NaOH.

15. The process of claim 1 , wherein said step of encapsulating by polymerizing monomer including styrene takes place at a temperature of at least about 70° C. up to 80° C.