Method for obtaining polybutadiene-based aqueous dispersions

1. A process for obtaining an aqueous dispersion of copolymers comprising polymerizing a blend of monomers comprising butadiene and at least one monomer selected from the group consisting of styrene, acrylic esters and vinyl nitriles, as an aqueous emulsion in the presence of one or more free-radical initiator(s), wherein said initiator(s) comprise an azocarboxylic acid ester of formula (I):

wherein:

R 1 , R 2 , R 3 and R 4 , which may be identical or different, are independently selected from the group consisting of:

linear or branched alkyls containing from 1 to 9 carbon atoms and preferably from 1 to 4 carbon atoms, optionally substituted with one or more substituents selected from hydroxyl, C 1 to C 6 alkoxy and halogen substituents;

C 3 to C 12 cycloalkyls, optionally substituted with one or more substituents selected from C 1 to C 6 alkyl, C 1 to C 6 alkoxy, hydroxyl and halogenated groups;

aralkyls optionally substituted with one or more C 1 to C 6 alkyl, C 1 to C 6 alkoxy, hydroxyl and halogenated groups;

aryls optionally substituted with one or more substituents selected from C 1 to C 6 alkyl, C 1 to C 6 alkoxy, hydroxyl and halogenated groups, wherein one or more of the combinations R 1 –R 2 and R 3 –R 4 optionally forms an aliphatic ring;

and wherein R″ and R′ are identical or different and are independently selected from the group consisting of unsubstituted linear or branched C 1 to C 10 aliphatic radicals.

2. The process as claimed in claim 1 wherein said blend of monomers further comprises up to 40% by weight of one or more other copolymerizable unsaturated monomers.

3. The process as claimed in claim 1 , characterized in that the initiator comprises 2,2′-diethyl azobisisobutyrate, or a mixture of 2,2′-diethyl azobisisobutyrate (DEAB) and 2,2′-dimethyl azobisisobutyrate (DMAB).

4. The process as claimed in claim 1 wherein the mass proportion of DEAB to DEAB is greater than 50%.

5. The process as claimed in claim 3 , characterized in that the initiator comprises a mixture of 2,2′-diethyl azobisisobutyrate, 2,2′-dimethyl azobisisobutyrate and 2-methyl 2′-ethyl azobisisobutyrate.

6. The process as claimed in claim 5 wherein the molar ratio of COOMe/COOEt is less than or equal to 10.

7. The process as claimed in claim 1 , characterized in that the free radical initiator(s) comprise said azocarboxylic acid ester in combination with other emulsion polymerization initiators.

8. The process as claimed in claim 7 wherein said other emulsion polymerization initiators comprise persulfates, hydroperoxides, or a mixture thereof.

9. The process as claimed in claim 8 , characterized in that the amount of azocarboxylic acid ester is between 5% and 95% by weight relative to the total weight of the initiators.

10. The process of claim 9 characterized in that the amount of azocarboxylic acid ester is between 30% and 70% by weight relative to the total weight of the initiators.

11. The process as claimed in claim 1 , characterized in that the total amount of initiators is between 0.05% and 4% by weight relative to the weight of monomers.

12. The process as claimed in claim 1 , characterized in that the polymerization is performed in the presence of a transfer agent.

13. The process as claimed in claim 1 wherein said R″ and R′ are identical or different and are independently selected from the group consisting of linear or branched C 1 to C 4 aliphatic radicals.

14. The process as claimed in claim 1 , characterized in that the polymerization temperature is between 40 and 100° C.

15. The process as claimed in claim 13 , characterized in that the polymerization temperature is between 50 and 90° C.