Process for the preparation of an aqueous polymer dispersion

1. A process for the preparation of an aqueous polymer dispersion comprising polymerizing, by free radical aqueous emulsion polymerization, ethylenically unsaturated monomers in the presence of at least one dispersant and at least one initiator,

wherein the ethylenically unsaturated monomers comprise a monomer A and a monomer B,

the monomer A is included in an amount of from 0.1 to 10% by weight of a total amount of the ethylenically unsaturated monomers,

the monomer A is at least one ethylenically unsaturated monomer having a solubility of ≧200 g per 1000 g of demineralized water at 20° C. and 1 atm (absolute),

the monomer B is included in an amount of from 90 to 99.9% by weight of the total amount of the ethylenically unsaturated monomers,

the monomer B is at least one ethylenically unsaturated monomer having a solubility of ≦100 g per 1000 g of demineralized water at 20° C. and 1 atm (absolute),

the monomer A and the monomer B sum to 100% by weight,

the polymerizing includes polymerizing only from 0.1 to 10% by weight of the total amount of the monomer B in an aqueous polymerization medium at a polymerization stage 1, and thereafter

adding the total amount of the monomer A and the remaining amount of the monomer B to the aqueous polymerization medium to polymerize the monomer A and the monomer B under polymerization conditions at a polymerization stage 2.

2. The process according to claim 1 , wherein the monomer A and the monomer B are metered in continuously in the polymerization stage 2.

3. The process according to claim 1 , wherein the monomer A and the monomer B are metered in as a monomer mixture in the polymerization stage 2.

4. The process according to claim 1 , wherein the polymerizing at the polymerization stage 1 comprises polymerizing ≧1 and ≦8% by weight of the total amount of the monomer B.

5. The process according to claim 1 , wherein the polymerizing at the polymerization stage 1 includes adding the at least one dispersant in an amount of ≧2 mmol per 10 g of the monomer B.

6. The process according to claim 1 , wherein the monomer A is at least one ethylenically unsaturated monomer selected from the group consisting of acrylic acid, methacrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, acrylamide, methacrylamide, N-(2-methacryloyloxyethyl)ethyleneurea, N-(2-acryloyloxyethyl)ethyleneurea, 2-acetoacetoxyethyl methacrylate, 2-acetoacetoxyethyl methacrylate, diacetoneacrylamide, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate and hydroxypropyl methacrylate.

7. The process according to claim 1 , wherein the monomer B is at least one ethylenically unsaturated monomer selected from the group consisting of methyl acrylate, ethyl acrylate, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, 2-propylheptyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, 2-propylheptyl methacrylate, styrene, vinyltoluene, 2-methylstyrene, 4-methylstyrene, 2-n-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene, vinyl acetate, vinyl propionate, acrylonitrile and methacrylonitrile.

8. The process according to claim 1 , wherein the at least one dispersant comprises an emulsifier.

9. The process according to claim 1 , wherein the at least one dispersant comprises at least one of a nonionic emulsifier and an anionic emulsifier.

10. The process according to claim 1 , wherein the adding at the polymerization stage 2 includes adding one of the monomer A, the monomer B, and a combination thereof by a gradient or stage procedure of polymerization.

11. An aqueous polymer dispersion obtained by the process according to claim 1 .

12. A polymer powder obtained by drying the aqueous polymer dispersion according to claim 11 .

13. A method of applying a composition including the aqueous polymer dispersion according to claim 11 , wherein the composition is one of an adhesive, a sealing compound, a plastic render, a paper coating slip, a fiber web, a paint, a coating material for organic substrates and a coating material for modification of mineral binders.

14. A method of applying a composition including the polymer powder according to claim 12 , wherein the composition is one of an adhesive, a sealing compound, a plastic render, a paper coating slip, a fiber web, a paint, a coating material for organic substrates and a coating material for modification of mineral binders.

15. The process according to claim 1 , wherein the monomer A is at least one ethylenically unsaturated monomer having a solubility of ≧500 g per 1000 g of demineralized water at 20° C. and 1 atm (absolute).

16. The process according to claim 1 , wherein the monomer B is at least one ethylenically unsaturated monomer having a solubility of 20 g per 1000 g of demineralized water at 20° C. and 1 atm (absolute).

17. The process according to claim 1 , wherein the ethylenically unsaturated monomers consist of the monomer A and the monomer B.

18. The aqueous polymer dispersion according to claim wherein the dispersion includes polymer particles having a weight average diameter of ≧30 nm to ≦100 nm.

19. The aqueous polymer dispersion according to claim 11 , wherein the dispersion includes polymer particles having a particle size distribution D w50 /D N50 of ≦2.0, where D w50 represents a weight average particle diameter, and D N50 represents a number average particle diameter.

20. The aqueous polymer dispersion according to claim 19 , wherein the particle size distribution D w50 /D N50 is ≦1.1.