Hydrolysis-resistant, multilayer polyester film with hydrolysis stabilizer

1. A multilayer polyester film comprising a hydrolysis stabilizer, wherein the hydrolysis stabilizer is an epoxidized alkyl ester of fatty acid or is a mixture of epoxidized alkyl esters of fatty acid or is an epoxidized fatty acid glyceride or is a mixture of epoxidized fatty acid glycerides, said film further comprising at least one external layer and at least one of said external layers does not comprise the hydrolysis stabilizer,

wherein the resulting film has an SV loss after autoclaving which is higher than said hydrolysis-stabilized film including said hydrolysis stabilizer within said external layer(s) up to a loss of 202 SV units after 96 hours at 110° C. in an autoclave with water saturation.

2. The polyester film as claimed in claim 1 , wherein the hydrolysis stabilizer is added to the polyester in the form of a dry liquid.

3. The polyester film as claimed in claim 1 , said film comprising a hydrolysis stabilizer, wherein the hydrolysis stabilizer is comprised of at least one of (i) one or more epoxidized alkyl esters of fatty acid and (ii) one or more epoxidized fatty acid glycerides, and is added to the polyester in the form of a dry liquid, said dry liquid comprising at least 50% of said hydrolysis stabilizer absorbed by a carrier material having an oil absorption, according to DIN 53601, of greater than 20 g/100 g.

4. A multilayer polyester film as claimed in claim 3 , wherein said carrier material comprises silicate, zeolite, aluminum phosphate, polymeric molecular sieve or carbon molecular sieve.

5. The polyester film as claimed in claim 1 , which comprises from 0.1 to 20% by weight of hydrolysis stabilizer.

6. The polyester film as claimed in claim 1 , wherein the epoxide oxygen content of the hydrolysis stabilizer is at least 0.5% by weight.

7. The polyester film as claimed in claim 1 , wherein the epoxidized fatty acid glycerides comprise epoxidized soybean oil, epoxidized linseed oil, epoxidized rapeseed oil, epoxidized sunflower oil, or epoxidized fish oil, or a mixture of these.

8. The polyester film as claimed in claim 1 , wherein the epoxidized alkyl esters of fatty acid comprise the 2-ethylhexyl esters of the unsaturated fatty acids or fatty acid mixtures derived from rapeseed oil, linseed oil, soybean oil, or fish oil.

9. The polyester film as claimed in claim 1 , wherein the epoxide oxygen content of the epoxidized alkyl esters of fatty acid is from 1.5 to 15% by weight.

10. The polyester film as claimed in claim 1 , which comprises a free-radical scavenger.

11. The polyester film as claimed in claim 1 , which comprises at least 0.1% by weight of carrier material derived from the addition of the hydrolysis stabilizer in the form of a dry liquid.

12. The polyester film as claimed in claim 11 , wherein the carrier material is a silicate.

13. The polyester film as claimed in claim 11 , wherein the carrier material comprises synthetic silica.

14. The polyester film as claimed in claim 1 , which comprises, as polyester, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or polytrimethylene terephthalate, or a mixture of these, or copolyesters of these.

15. The polyester film as claimed in claim 1 , wherein said film further comprises inorganic or organic particles.

16. The polyester film as claimed in claim 1 , wherein said film is a three-layer film.

17. The polyester film as claimed in claim 1 , wherein no external layers of said film comprises the hydrolysis stabilizer.

18. A process for production of a polyester film as claimed in claim 1 , said process comprising the steps of:

a) producing a multilayer film via coextrusion,

b) biaxial stretching of the film, and

c) heat-setting of the stretched film.

19. Solar modules comprising multilayer polyester film as claimed in claim 1 , said film comprising a hydrolysis stabilizer, wherein the hydrolysis stabilizer is an epoxidized alkyl ester of fatty acid or is a mixture of epoxidized alkyl esters of fatty acid or is an epoxidized fatty acid glyceride or is a mixture of epoxidized fatty acid glycerides, said film further comprising at least one external layer and at least one of said external layers does not comprise the hydrolysis stabilizer.

20. The polyester film as claimed in claim 1 , wherein said hydrolysis stabilizer-free external layer(s) has a thickness ranging from 0.1 to less than 1 micron.

21. The polyester film as claimed in claim 1 , wherein the hydrolysis stabilizer-free external layer(s) further comprise free radical scavenger.

22. A multilayer polyester film as claimed in claim 1 , said film comprising a hydrolysis stabilizer, wherein the hydrolysis stabilizer is an epoxidized alkyl ester of fatty acid or is a mixture of epoxidized alkyl esters of fatty acid or is an epoxidized fatty acid glyceride or is a mixture of epoxidized fatty acid glycerides, said film further comprising at least one external layer and at least one of said external layers does not comprise the hydrolysis stabilizer, said hydrolysis-stabilizer-free outer layer ranging in thickness from 0.1 microns to less than 5 microns.

23. A multilayer polyester film as claimed in claim 1 , said film comprising a hydrolysis stabilizer, wherein the hydrolysis stabilizer is an epoxidized alkyl ester of fatty acid or is a mixture of epoxidized alkyl esters of fatty acid or is an epoxidized fatty acid glyceride or is a mixture of epoxidized fatty acid glycerides, said film further comprising at least one external layer and at least one of said external layers does not comprise the hydrolysis stabilizer, said film exhibiting a longitudinal shrinkage of less than 4% and transverse shrinkage of less than 1% at 200° C.

24. A multilayer polyester film comprising from 0.1 to 20% by weight of a hydrolysis stabilizer, said hydrolysis stabilizer comprising an epoxidized alkyl ester of fatty acid or a mixture of epoxidized alkyl esters of fatty acid or an epoxidized fatty acid glyceride or a mixture of epoxidized fatty acid glycerides, said film further comprising at least one external layer and at least one of said external layers is a hydrolysis-stabilizer-free external layer having a thickness ranging from 0.1 to less than 5 microns,

wherein the resulting film has an SV loss after autoclaving which is higher than said hydrolysis-stabilized film including said hydrolysis stabilizer within said external layer(s) up to a loss of 26.0% of the film's initial SV, wherein said autoclaving is performed for 96 hours at 110° C. with water saturation.