Biaxially oriented polyester film for metal lamination

1. A multilayer, biaxially oriented polyester film for metal sheet lamination, comprising a base layer B, an outer layer A and an outer layer C, wherein the outer layer A is amorphous, the base layer B and the outer layer C are crystalline, and where the crystalline base layer B comprises, based on the mass of polyester in the layer, at least 85% by weight of ethylene-terephthalate-derived units and from 2 to 15% by weight of ethylene-isophthalate-derived units and where the amorphous outer layer A comprises, based on the mass of polyester in the layer, more than 19% by weight of ethylene-isophthalate-derived units and no cycloaliphatic diol, and

wherein, said outer layer A has an adhesion-promoting silane-based coating that is adhered to a metal sheet.

2. Polyester film according to claim 1 , wherein the polyester of the base layer B is comprised of at least 90% by weight of ethylene-terephthalate-derived units, based on the total mass of polyester.

3. Polyester film according to claim 1 , wherein the polyester of the base layer B is comprised of from 3 to 10% by weight of ethylene-isophthalate-derived units.

4. Polyester film according to claim 1 , wherein the polyester of the base layer B is comprised of from 4 to 8% by weight of ethylene-isophthalate-derived units.

5. Polyester film according to claim 1 , wherein (i) the film comprises ethylene-terephthalate units and (ii) the ethylene-terephthalate-derived units and ethylene-isophthalate-derived units together amount to at least 95% by weight of the polyester based on the mass of polyester in layer B.

6. Polyester film according to claim 1 , wherein the amorphous outer layer A is comprised of at least 19% by weight of ethylene-isophthalate-derived units and of up to 81% by weight of ethylene-terephthalate-derived units, based in each case on the total mass of polyester in the outer.

7. Polyester film according to claim 1 , wherein the outer layer C comprises at least 0.05% by weight of inert particles.

8. Polyester film according to claim 1 , wherein the polyesters of the film were produced via titanium-based catalysts.

9. Polyester film according to claim 1 , wherein the film comprises from 50 to 10000 ppm of a free-radical scavenger and the outer layer C comprises at least 300 ppm of free-radical scavenger.

10. Polyester film according to claim 9 , wherein the free-radical scavenger is selected from one or more of the compounds with the CAS numbers 1709-70-2, 3135-18-0, 6683-19-8 and 57569-40-1.

11. Polyester film according to claim 1 , wherein said silane-based coating of the outer layer (A) is formed of amino-functional silane.

12. Polyester film according to claim 11 , wherein the amino-functional silane in an unhydrolyzed state is a compound of the formula

(R 1 ) a Si(R 2 ) b (R 3 ) c ,

in which

R 1 is a non-hydrolysable functional group having at least one primary amino group,

R 2 is a hydrolysable group, wherein the hydrolysable group is selected from the group consisting of a lower alkoxy group, an acetoxy group or a halide, and

R 3 is an unreactive, non-hydrolysable group, wherein the non-hydrolysable group is selected from the group consisting of a lower alkyl group or a phenyl group;

a is greater than or equal to 1;

b is greater than or equal to 1;

c is greater than or equal to zero and

a+b+c=4 and

lower alkyl or alkoxy groups are those having from 1 to 4 carbon atoms and halides are fluoro, chloro, bromo or iodo.

13. Polyester film according to claim 1 , wherein said film has a thickness of from 5 to 20 μm.

14. Process for the production of a polyester film according to claim 1 , comprising compressing and liquefying the polyester mixtures of the layers A, B and C in a plurality of extruders to form melts, shaping the melts in a coextrusion die to give a flat melt film, drawing the flat melt film off on a chill roll and on one or more take-off rolls to form a prefilm, cooling, solidifying and then biaxially orienting the prefilm, optionally coating the outer layer side A, wherein the base layer B and the outer layer C are crystalline and the base layer B comprises, based on the mass of polyester, from 2 to 15% by weight of isophthalate-derived units and the amorphous outer layer A comprises, based on the mass of polyester, more than 19% by weight of isophthalate-derived units, and where the outer layer A has a silane-based coating.

15. Metal sheet laminate comprising a film according to claim 1 .

16. Metal sheet laminate according to claim 15 , wherein said metal sheet laminate is a cap for a drink can.

17. Metal sheet laminate according to claim 15 , wherein the metal sheet is an aluminium sheet.

18. The polyester film according to claim 1 , wherein the silane-based coating consists of amino-functional silane.

19. The polyester film according to claim 1 , wherein the outer layer A and the outer layer C both comprise polyethylene terephthalate made of ethylene glycol and dimethyl terephthalate.

20. A multilayer, biaxially oriented polyester film for metal sheet lamination comprising a base layer B, an outer layer A and an outer layer C,

wherein the outer layer A is amorphous, the base layer B and the outer layer C are crystalline, and

wherein said layer A and layer C are placed on opposite sides of layer B, and

wherein, the crystalline base layer B comprises, based on the mass of polyester in the layer, from 2 to 15% by weight of ethylene-isophthalate-derived units and at least 85% by weight of ethylene-terephthalate-derived units, and

wherein, the amorphous outer layer A comprises, based on the mass of polyester in the layer, more than 27% by weight of ethylene-isophthalate-derived units and up to 73% by weight of ethylene-terephthalate-derived units

wherein, said outer layer A has a coating consisting of functional silane rendering the film no longer heat sealable, and

wherein, said coating is adhered to a metal sheet.

21. Polyester film according to claim 20 , wherein the ethylene-terephthalate-derived units and ethylene-isophthalate-derived units together amount to at least 95% by weight of the polyester in said film; and

wherein said functional silane is selected from the group consisting of N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 4-aminobutyltriethoxysilane, 4-aminobutyldimethylmethoxysilane or p-aminophenyltrimethoxysilane.

22. A multilayer, biaxially oriented polyester film for metal sheet lamination comprising a base layer B, an outer layer A and an outer layer C, wherein the outer layer A is amorphous, the base layer B and the outer layer C are crystalline, and where the crystalline base layer B comprises 70 to 85% by weight of ethylene-terephthalate-derived units, based on the mass of polyester in the layer, from 15 to 30% by weight of butylene-terephthalate-derived units and the amorphous outer layer A comprises, based on the mass of polyester in the layer, more than 19% by weight of ethylene-isophthalate-derived units, and

wherein, said outer layer A has a silane-based coating consisting of functional silane, and

wherein, said coating is adhered to a metal sheet.

23. The polyester film according to claim 22 , wherein the ethylene-terephthalate-derived units and butylene-terephthalate-derived units amount to at least 95% by weight based on the mass of the polyester in layer B.