Method for the surface crosslinking of polymer particles

1. A process for the surface crosslinking of a polymer, in particle form, comprising at least one amide function, comprising a step of using a crosslinking agent comprising at least two functional groups which each react with a labile hydrogen function of the amide functions of the polymer thereby effecting the crosslinking process, the crosslinking process being carried out at a temperature lower than the melting point of the polymer,

wherein the polymer particles have an average diameter which ranges from 1 to 200 μm, and

wherein the crosslinking agent is selected from the group consisting of a) compounds bearing at least two of the following functional groups: isocyanate, carbodiimide, epoxy, acyllactam, oxazoline and its isomers, oxazine and its isomers, and b) PCl 3 , and the crosslinking agent is in a content of from 0.1 to 15 mol %, relative to the total number of moles of the monomers constituting the polymer having at least one amide function.

2. The process as claimed in claim 1 , wherein the polymer is selected from the group consisting of polyamides and polyamide-imides and blends thereof.

3. The process as claimed in claim 2 , wherein the polymer is a polyamide.

4. The process as claimed in claim 3 , wherein the polymer is selected from the group consisting of PA6, PA11, PA12, PA6/12, PA6.12, PA6.10, PA10.10, PA10.12, PA6.6, PA8, PA4 and PA4.6, and blends of these polymers.

5. The process as claimed in claim 4 , wherein the polymer is PA12.

6. The process as claimed in claim 1 , wherein the crosslinking agent is a polyisocyanate.

7. A process for the polymerization of a polymer, comprising the following successive steps:

a polymerization step,

a step of crosslinking, in accordance with the process as claimed in claim 1 , of the polymer in powder form obtained in the previous step, and

an optional step of neutralization of the reaction medium.

8. The process as claimed in claim 7 , wherein the polymerization step is a step of anionic polymerization in solution in a solvent, in the presence of a catalyst and of an activator.

9. A particle of polymer powder comprising:

one or more areas consisting of polymer having one or more amide function(s), and

one or more area(s) comprising said polymer crosslinked with a crosslinking agent in accordance with the process as claimed in claim 1 .

10. The particle as claimed in claim 9 , wherein the particle comprises:

a core consisting of polymer having one or more amide function(s), and

a layer comprising said polymer crosslinked with a crosslinking agent in accordance with the process as claimed in claim 1 .

11. A composition comprising the particle of powder as claimed in claim 9 .

12. A method for producing objects by agglomeration of a powder by fusion brought about by radiation chosen from a laser beam, infrared radiation or UV radiation, wherein the powder is comprised of particles of powder as claimed in claim 9 .

13. A method of making cosmetic compositions, comprising using a particle of powder as claimed in claim 9 as an additive.

14. The process as claimed in claim 1 , wherein the polymer is a polyamide or blend of polyamides.

15. The process as claimed in claim 1 , wherein the crosslinking agent is in a content of from 0.5 to 10 mol %, relative to the total number of moles of the monomers constituting the polymer having one or more amide function(s).

16. The process as claimed in claim 1 , wherein the polymer is a polyamide and the crosslinking agent is a polyisocyanate.

17. A method of making an article selected from the group consisting of composites, structural adhesives, substrate coatings and transfer papers, comprising using particles of polymer powder as claimed in claim 9 in the article.