WE CLAIM:
1. Masterbatch comprising by weight:
• from 1 to 50% of a blend of at least one fluoropolymer (A) and at least one interfacial agent (B);
• from 99 to 50% of a polyolefin (C);
• such that the two constituents (A) and (B) are intimately blended beforehand at a temperature such that at least one is in the melt state throughout its mass or on its surface, and are then blended with the polyolefin (C);
wherein the respective proportions of (A) and (B) by weight are such that (A)/(B) is between 30/70 and 70/30.
2. Masterbatch as claimed in claim 1, in which the blend of (A) and (B) is produced at between 10 and 120°C, and advantageously between 20 and 100°C, preferably between 40 and 100°C, and more preferentially still between 60 and 100°C.
3. Masterbatch as claimed in either of claims 1 and 2, in which the blend is produced at a temperature selected such that the interfacial agent (B) is in the melt state throughout its mass or on its surface and the fluoropolymer (A) is in the solid state.
4. Masterbatch as claimed in any one of claims 1 to 3, in which the fluoropolymer is a polyvinylidene fluoride (PVDF) homopolymer or copolymer.
5. Masterbatch as claimed in claim 4, in which the PVDF contains, by weight, at least 50% VDF.
6. Masterbatch as claimed in claim 5, in which the PVDF contains, by weight, at least 75% VDF.
7. Masterbatch as claimed in claim 6, in which the PVDF contains, by weight, at least 85% VDF.
8. Masterbatch as claimed in any one of claims 4 to 7, in which the comonomer is HFP.

9. Masterbatch as claimed in any one of the preceding claims, in which the interfacial agent (B)
is polyoxyethylene glycol.
10. Masterbatch as claimed in any one of the preceding claims, in which the proportion of (A) and (B) is from 1 to 30% per 99 to 70% of (C) respectively.
11. Masterbatch as claimed in claim 10, in which the proportion of (A) and (B) is from 1 to 10% per 99 to 90% of (C) respectively.
12. Masterbatch as claimed in any one of the preceding claims, in which the respective
proportions of (A) and (B) by weight are such that (A)/(B) is between 10/90 and 90/10.
13. Masterbatch as claimed in claim 1, in which the respective proportions of (A) and (B) by weight are such that (A)/(B) is between 40/60 and 60/40.
14. Masterbatch as claimed in any one of the preceding claims, used as a processing aid for extruding a polyolefin (D).
15.Masterbatch as claimed in claim 14, in which the polyolefin is extruded in film form.
16. Masterbach as claimed in either of claims 14 and 15, in which the proportion of masterbatch to be introduced into the polyolefin (D) to be extruded is such that the quantity of (A)+(B) with respect to (D) is from 100 ppm to 3000 ppm.
17. Process for preparing a masterbatch as claimed in claim 1, comprising the following steps:
1) at least one fluoropolymer (A) and at least one interfacial agent (B) are mixed
intimately at a temperature between 10 and 120°C, advantageously between 20 and 100°C, and preferably between 40 and 100°C, and more preferentially still between 60 and 100°C such that at least one is in the melt state throughout its mass or on its surface;
2) the mixture obtained in step 1) is subsequently incorporated into a polyolefin (C), to

form the masterbatch in granule form;
3). optionally, the masterbatch granules of step 2 are reground to give a powder of the masterbatch.
18. Process as claimed in claim 17, in which the temperature is chosen such that the interfacial agent (B) is in the melt state throughout its mass or on its surface and the fluoropolymer (A) is in the solid state.