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A biaxially oriented polyester film comprising polyester and at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, wherein the monocarboxylic acid has from 5 to 50 carbon atoms, wherein said hydrolysis stabilizer is present in the film in the form of it

A biaxially oriented polyester film comprising polyester and at least one hydrolysis stabilizer selected from a glycidyl ester of a branched monocarboxylic acid, wherein the monocarboxylic acid has from 5 to 50 carbon atoms, wherein said hydrolysis stabilizer is present in the film in the form of its reaction product with at least some of the end-groups of said polyester, and wherein said reaction product is obtained by the reaction of the hydrolysis stabilizer with the end-groups of the polyester in the presence of a metal cation selected from the group consisting of Group I and Group II metal cations.

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1. A biaxially oriented polyester film comprising polyester and at least one hydrolysis stabiliser selected from a glycidyl ester of a branched monocarboxylic acid in the absence of glycidyl ether compound(s), wherein said polyester is polyethylene terephthalate, wherein the monocarboxylic acid has

1. A biaxially oriented polyester film comprising polyester and at least one hydrolysis stabiliser selected from a glycidyl ester of a branched monocarboxylic acid in the absence of glycidyl ether compound(s), wherein said polyester is polyethylene terephthalate, wherein the monocarboxylic acid has from 5 to 50 carbon atoms, wherein said hydrolysis stabiliser has formula (I): wherein:R1 and R2 are independently selected from alkyl;R3 is selected from hydrogen and alkyl; andwherein the total number of carbon atoms in the alkyl groups R1, R2 and R3 is from 3 to 48;wherein said hydrolysis stabiliser is present in the film in the form of its reaction product with at least some of the end-groups of said polyester, wherein the hydrolysis stabiliser is present in an amount in the range from 0.3% to 1.5%, relative to the total weight of the film and wherein said reaction product is obtained by the reaction of the hydrolysis stabiliser with the end-groups of the polyester in the presence of sodium cations, wherein the amount of the sodium cations present in the film, and/or present in the reaction mixture during the reaction of the hydrolysis stabiliser with the end-groups of the polyester, is at least 45 ppm by weight, relative to the amount of polyester, wherein the film exhibits an elongation to break, measured according to ASTM D882, of at least 10% after at least 76 hours when aged at 121° C. and 1.2 bar pressure. 2. The polyester film according to claim 1, wherein the intrinsic viscosity of the polyester in the polyester film is at least about 0.65 as measured according to ASTM D5225-98(2003). 3. The polyester film according to claim 1, wherein the hydrolysis stabiliser(s) in the polyester film consist(s) essentially of at least one glycidyl ester of a branched monocarboxylic acid. 4. The polyester film according to claim 1, wherein said branched monocarboxylic acid has from 5 to 15 carbon atoms, and/or wherein said branched monocarboxylic acid is saturated, and/or said branched monocarboxylic acid is a synthetic material. 5. The polyester film according to claim 1, wherein said hydrolysis stabiliser is manufactured by the reaction of epichlorohydrin with said branched monocarboxylic acid. 6. The polyester film according to claim 1 wherein R1 is selected from methyl, and R2 and R3 are independently selected from alkyl, wherein the total number of carbon atoms in the alkyl groups R2 and R3 is 7. 7. The polyester film according to claim 1, wherein said hydrolysis stabiliser is reacted with the polyester by injecting the additive into the molten polymer prior to the polymer being cast into a film. 8. The polyester film according to claim 1, wherein the polyester film further comprises a UV-absorber. 9. The polyester film according to claim 8 wherein the amount of UV-absorber is in the range from 0.1% to 10% by weight, relative to the total weight of the layer. 10. The polyester film according to claim 1, wherein the polyester film has been stabilised by heat-setting at a temperature within the range of from about 200 to about 225° C. 11. The polyester film according to claim 1, wherein the polyester of the polyester film exhibits an endothermic high temperature peak at a temperature of (A)° C. and an endothermic low temperature peak at a temperature of (B)° C., both peaks being measured by differential scanning calorimetry (DSC), wherein the value of (A-B) is in the range from 15° C. to 50° C. 12. The polyester film according to claim 1, wherein the polyester film exhibits a haze of no more than 30% and/or a TLT of at least 50%. 13. The polyester film according to claim 1, wherein the polyester film is selected from the group consisting of a white film, a black film and an opaque film. 14. The polyester film according to claim 1, further comprising an anti-oxidant. 15. The polyester film according to claim 1, wherein the polyester film has disposed on a first surface thereof an additional polymeric layer, wherein said additional polymeric layer is a polyester layer optionally comprising one or more additives independently selected from hydrolysis stabiliser(s), UV-absorber(s), anti-oxidant(s) and particulate inorganic filler(s). 16. The polyester film according to claim 1, further comprising one or more additional layer(s) disposed on one or both surfaces thereof to form a composite structure. 17. The polyester film according to claim 16, wherein the one or more additional layer(s) is selected from PET or PET-based polyesters. 18. The polyester film according to claim 16, wherein the one or more additional layer(s) comprises at least one hydrolysis stabiliser. 19. The polyester film according to claim 18, wherein the one or more additional layer(s) is selected from PET or PET-based polyesters, and wherein the hydrolysis stabiliser in the one or more additional layer(s) is selected from a glycidyl ester of a branched monocarboxylic acid, wherein the monocarboxylic acid has from 5 to 50 carbon atoms, wherein said hydrolysis stabiliser is present in the one or more additional layer(s) in the form of its reaction product with at least some of the end-groups of the polyester. 20. The polyester film according to claim 19, wherein the hydrolysis stabiliser in the one or more additional layer(s) has formula (I): wherein:R1 and R2 are independently selected from alkyl;R3 is selected from hydrogen and alkyl; andwherein the total number of carbon atoms in the alkyl groups R1, R2 and R3 is from 3 to 48. 21. The polyester film according to claim 16, wherein the polyester film is formed by co-extrusion of the respective film-forming layers. 22. The polyester film as described in claim 1 used as a layer in a photovoltaic cell, said photovoltaic cell comprising a front-plane, electrode layer(s), a photovoltaic-active layer, and a back-plane. 23. A photovoltaic cell comprising a front-plane, electrode layer(s), a photovoltaic-active layer, and a back-plane, wherein the front-plane and/or the back-plane comprises a polyester film as defined in claim 1. 24. A photovoltaic cell comprising a front-plane, electrode layer(s), a photovoltaic-active layer, and a back-plane, wherein said electrode layers and photovoltaic-active layer are encapsulated in an encapsulant, and wherein the back-plane and optionally the front-plane comprises a polyester film as defined in claim 1. 25. The photovoltaic cell according to claim 24, wherein said polyester film is an opaque or white film, wherein said polyester film has disposed on a first surface thereof an additional polymeric layer, wherein said additional polymeric layer exhibits a haze of no more than about 30%, and wherein said film is outermost in the multi-layer assembly. 26. A process for the manufacture of a biaxially oriented polyester film according to claim 1, wherein the process comprises: (i) extruding a layer of molten polyester and at least one hydrolysis stabiliser selected from a glycidyl ester of a branched monocarboxylic acid in the absence of glycidyl ether compound(s), wherein the monocarboxylic acid has from 5 to 50 carbon atoms, wherein said hydrolysis stabiliser has formula (I): wherein:R1 and R2 are independently selected from alkyl;R3 is selected from hydrogen and alkyl; andwherein the total number of carbon atoms in the alkyl groups R1, R2 and R3 is from 3 to 48;(ii) wherein said hydrolysis stabiliser is present in the extrudate in the form of its reaction product with at least some of the end-groups of said polyester, wherein the hydrolysis stabiliser is present in an amount in the range from 0.3% to 1.5%, relative to the total weight of the film and wherein said reaction product is obtained by the reaction of the hydrolysis stabiliser with the end-groups of the polyester in the presence of sodium cations, wherein the amount of the sodium cations present in the film, and/or present in the reaction mixture during the reaction of the hydrolysis stabiliser with the end-groups of the polyester, is at least 45 ppm by weight, relative to the amount of polyester;(iii) quenching the extrudate;(iv) stretching the quenched extrudate in two mutually perpendicular directions; and(v) heat-setting the film;wherein the film exhibits an elongation to break, measured according to ASTM D882, of at least 10% after at least 76 hours when aged at 121° C. and 1.2 bar pressure. 27. The process according to claim 26, wherein the process comprises the further step of manufacturing said hydrolysis stabiliser by the reaction of epichlorohydrin with said branched monocarboxylic acid. 28. The process according to claim 26, wherein the hydrolysis stabiliser is reacted with the polyester by injecting the additive into the molten polymer prior to extruding said layer. 29. The process according to claim 26, wherein the film has one or more additional layer(s) disposed on one or both surfaces thereof to form a composite structure and wherein the process comprises co-extrusion of the respective film-forming layers. 30. A method of improving the hydrolysis resistance of a biaxially oriented polyester film, said method comprising the step of reacting said polyester with at least one hydrolysis stabiliser selected from a glycidyl ester of a branched monocarboxylic acid in the absence of glycidyl ether compound(s), wherein the monocarboxylic acid has from 5 to 50 carbon atoms, wherein said hydrolysis stabiliser has formula (I): wherein:R1 and R2 are independently selected from alkyl;R3 is selected from hydrogen and alkyl; andwherein the total number of carbon atoms in the alkyl groups R1, R2 and R3 is from 3 to 48;wherein said hydrolysis stabiliser is present in the film in the form of its reaction product with at least some of the end-groups of said polyester, wherein the hydrolysis stabiliser is present in an amount in the range from 0.3% to 1.5%, relative to the total weight of the film and wherein said reaction product is obtained by the reaction of the hydrolysis stabiliser with the end-groups of the polyester in the presence of sodium cations, wherein the amount of the sodium cations present in the film, and/or present in the reaction mixture during the reaction of the hydrolysis stabiliser with the end-groups of the polyester, is at least 45 ppm by weight, relative to the amount of polyester, wherein the film exhibits an elongation to break, measured according to ASTM D882, of at least 10% after at least 76 hours when aged at 121° C. and 1.2 bar pressure.