Aqueous binder composition

1. A process for preparing an aqueous binder composition, the process comprising at least the following steps A) and B):

A) preparing a carboxylic acid-functional vinyl polymer (polymer A) by emulsion polymerisation of a monomer composition A, wherein the monomer composition A comprises the following monomers:

Ai) at least one carboxylic acid functional olefinically unsaturated monomer; and

Aii) at least one olefinically unsaturated monomer different than the monomer Ai);

B) preparing a vinyl polymer (polymer B) by emulsion polymerization of a monomer composition B in the presence of the polymer A wherein the monomer composition B comprises the following monomer(s):

Bi) at least one olefinically unsaturated monomer;

Bii) optionally carboxylic acid functional olefinically unsaturated monomer different than the monomer Bi); wherein

the acid value of the polymer B is lower than the acid value of the polymer A; and wherein

a weight average molecular weight (Mw) of the polymer B is higher than a weight average molecular weight (Mw) of the polymer A as determined by Size Exclusion Chromatography analysis; and wherein

the polymers A and B are present in a weight ratio of the polymer A to the polymer B of from 5:95 to 95:5; and wherein

at least part of the carboxylic acid groups of the polymer A are deprotonated to obtain ionic water-dispersing groups; and wherein

the monomer composition A comprises at least two monomer compositions A1 and A2 which differ in chain-transfer agent concentration, wherein

the monomer composition A1 comprises at most 1 wt. % of chain-transfer agent relative to the monomers in the total monomer composition A1, and the monomer composition A2 comprises at least 1 wt. % of chain-transfer agent and at most 3.5 wt. % of chain-transfer agent relative to the monomers in the total monomer composition A2, and wherein

a difference in an amount of chain-transfer agent between the monomer composition A1 and the monomer composition A2 is at least 0.5 wt. %.

2. The process according to claim 1 , wherein the monomer compositions A1 and A2 are fed sequentially to a polymerisation reactor.

3. The process according to claim 2 , wherein the monomeric composition A1 is fed to the polymerisation reactor prior to the monomeric composition A2.

4. The process according to claim 1 , wherein the polymer A has a gradient polymeric chain morphology having a continually changing polymeric composition along the polymeric chain.

5. The process according to claim 1 , wherein the weight ratio of the monomer composition A1 to the monomer composition A2 is from 10:90 to 55:45.

6. The process according to claim 1 , wherein the difference in amount of chain-transfer agent between the monomer composition A1 and the monomer composition A2 is at least 1 wt. %.

7. The process according to claim 1 , wherein the chain-transfer agent is selected from the group consisting of mercaptans.

8. The process according to claim 1 , wherein the chain-transfer agent is selected from the group consisting of lauryl mercaptan, 3-mercaptopropionic acid, 2-mercaptoethanol, isooctyl thioglycolate and mixtures thereof.

9. The process according to claim 1 , wherein the monomer compositions A1 and A2 comprise the following monomers:

Ai) from 5 to 15 wt. % of at least one carboxylic acid functional olefinically unsaturated monomer;

Aii) from 85 to 95 wt. % of at least one olefinically unsaturated monomer different than Ai);

Aiii) optionally at least one wet adhesion promoting olefinically unsaturated monomer different than Ai and Aii), in an amount of at most 6 wt. %; and

Aiv) optionally at least one olefinically unsaturated monomer comprising crosslinkable groups, different than Ai), Aii) and Aiii), in an amount of at most 10 wt. %, wherein

the amounts are given relative to the total weight of monomers contained in Polymer A and wherein the summed amount of Ai), Aii), Aiii) and Aiv) is 100 wt. %.

10. The process according to claim 1 , wherein the monomer composition B comprises the following monomer(s):

Bi) from 87 to 100 wt. % of at least one olefinically unsaturated monomer; and

Bii) optionally carboxylic acid functional olefinically unsaturated monomer, different than Bi), in an amount of at most 3 wt. %; and

Biii) optionally olefinically unsaturated monomer with wet adhesion promoting groups, different than Bi) and Bii), in an amount of less than 2.5 wt. %; and

Biv) optionally olefinically unsaturated monomer comprising crosslinkable groups, different than Bi), Bii) and Biii), in an amount of less than 2.5 wt. %; wherein

the amounts are given relative to the total weight of monomers contained in Polymer B and wherein the summed amount of Bi), Bii), Biii) and Biv) is 100 wt. %.

11. The process according to claim 1 , wherein the polymer A has an acid value of from 45 to 65 mg KOH/g of solid polymer A and the polymer B has an acid value of 0 mg KOH/g of solid Polymer B.

12. The process according to claim 1 , wherein

a) the polymer A has a Tg from 70° C. to 125° C.; and

b) the polymer B has a Tg from −20° C. to 0° C.

13. The process according to claim 1 , wherein the polymer B has a weight average molecular weight (Mw) of at least 100,000 Daltons.

14. The process according to claim 1 , wherein the polymer A has a weight average molecular weight (Mw) less than 50,000 Daltons.

15. The process according to claim 1 , wherein the carboxylic acid functional olefinically unsaturated monomers Ai) and, if present Bii), are methacrylic acid.

16. The process according claim 1 , wherein the monomers Aii) and Bi) are selected from the group consisting of acrylates, methacrylates, arylalkylenes and mixtures thereof.

17. The process according to claim 1 , wherein the polymer A and the polymer B are present in a total amount which is at least 25 wt. % and at most 85 wt. %, relative to the total amount of binders in the aqueous binder composition.

18. An aqueous binder composition obtained by the process according to claim 1 .

19. An aqueous coating composition comprising the aqueous binder composition according to claim 18 .

20. A method of protective coating a substrate comprising the steps of:

(1) applying the aqueous coating composition from claim 19 onto a substrate comprising a lignocellulosic material and/or aged alkyd resin and/or plastic and/or metallic substrate and/or composite materials comprising glass, to form a coating thereon, and

(2) drying the coating.