POLYESTER FILM COMPRISING AMORPHOUS POLYESTER

Abstract

A photovoltaic cell comprising a transparent front-plane, an opaque back-plane and an encapsulant resin wherein said back-plane comprises a polyester film comprising a base layer (B) comprising a crystallisable polyester and a heat-sealable layer (A) comprising an amorphous copolyester wherein: (i) said amorphous copolyester is derived from an aliphatic diol and a cycloaliphatic diol and at least one aromatic dicarboxylic acid (ii) said polyester film is disposed in the photovoltaic cell such that layer (A) is in contact with the encapsulant resin.

Claims

1. A photovoltaic cell comprising a transparent front-plane, an opaque back-plane and an encapsulant resin wherein said back-plane comprises a polyester film comprising a base layer (B) comprising a crystallisable polyester and a heat-sealable layer (A) comprising an amorphous copolyester wherein: (i) said amorphous copolyester is derived from an aliphatic diol and a cycloaliphatic diol and at least one aromatic dicarboxylic acid (ii) said polyester film is disposed in the photovoltaic cell such that layer (A) is in contact with the encapsulant resin.

2. (canceled)

3. The photovoltaic cell according to claim 1 wherein said base layer (B) comprises at least one hydrolysis stabiliser and/or at least one UV-stabiliser.

4. The photovoltaic cell according to claim 1 wherein said layer (A) comprises at least one UV-stabiliser and optionally at least one hydrolysis stabiliser.

5. The photovoltaic cell according to claim 1 wherein said layer (B) comprises an opacifying agent.

6. The photovoltaic cell according to claim 1 wherein said base layer (B) is the outermost layer of the photovoltaic cell.

7. The photovoltaic cell according to claim 6 wherein said base layer (B) comprises at least one hydrolysis stabiliser and at least one UV-stabiliser.

8. The photovoltaic cell according to claim 6 wherein said layer (A) comprises no or substantially no hydrolysis stabiliser and/or no or substantially no UV-stabiliser.

9. The photovoltaic cell according to claim 6 wherein layer (A) is free of particulate filler or contains particulate filler in amounts of no more than 2.5% by weight based on the weight of the polyester in the layer, or contains titanium dioxide in an amount from 2.0 to 4.0% or from 0.3 to less than 3.0% by weight based on the weight of the polyester in the layer.

10. The photovoltaic cell according to claim 1 wherein said polyester film comprises a further polyester layer (C) which is disposed on the base layer (B) on the opposite side thereof to the heat-sealable layer (A).

11. The photovoltaic cell according to claim 10 wherein said base layer (B) comprises at least one hydrolysis stabiliser, wherein said layer (A) comprises no or substantially no hydrolysis stabiliser.

12. The photovoltaic cell according to claim 10 wherein said layer (C) comprises at least one UV-stabiliser.

13. The photovoltaic cell according to claim 10 wherein layer (A) is free of particulate filler or contains particulate filler in amounts of no more than 2.5% by weight based on the weight of the polyester in the layer, or contains titanium dioxide in an amount from 2.0 to 4.0% or from 0.3 to less than 3.0% by weight based on the weight of the polyester in the layer.

14. The photovoltaic cell according to claim 10 wherein layer (B) contains no or substantially no UV-stabiliser.

15. The photovoltaic cell according to claim 10 wherein said layer (C) comprises crystallisable polyester.

16. The photovoltaic cell according to claim 15 wherein said layer (B) or layer (C) comprises an opacifying agent.

17. The photovoltaic cell according to claim 10 wherein said layer (C) comprises amorphous polyester derived from an aliphatic diol and a cycloaliphatic diol and at least one aromatic dicarboxylic acid, and further wherein there is disposed on the outer surface of layer (C) an anti-blocking layer (D).

18. The photovoltaic cell according to claim 17 wherein said layer (B) comprises an opacifying agent.

19. The photovoltaic cell according to claim 17 wherein said layer (D) is an in-line coated layer.

20. The photovoltaic cell according to claim 17 wherein said layer (D) comprises an acrylic resin.

21. A polyester film comprising a base layer (B) comprising a crystallisable polyester and a heat-sealable layer (A) comprising an amorphous copolyester wherein: (i) said amorphous copolyester is derived from an aliphatic diol and a cycloaliphatic diol and at least one aromatic dicarboxylic acid; (ii) said polyester film further comprises a polyester layer (C) which is disposed on the base layer (B) on the opposite side thereof to the heat-sealable layer (A); (iii) an anti-blocking layer (D) is disposed on the outer surface of layer (C).

22. The film according to claim 21 wherein said base layer (B) comprises at least one hydrolysis stabiliser.

23. The film according to claim 21 wherein said layer (C) comprises at least one UV-stabiliser, and optionally further comprises at least one hydrolysis stabiliser.

24. The film according to claim 21 wherein layer (A) is free of particulate filler or contains particulate filler in amounts of no more than 2.5% by weight based on the weight of the polyester in the layer or contains titanium dioxide in an amount from 2.0 to 4.0% or from 0.3 to less than 3.0% by weight based on the weight of the polyester in the layer.

25. The film according to claim 21 wherein layer (B) contains no or substantially no UV-stabiliser.

26. The film according to claim 21 wherein said base layer (B) comprises a crystallisable polyester selected from polyethylene terephthalate or polyethylene naphthalate.

27. The film according to claim 21 wherein said layer (D) is an in-line coated layer.

28. The film according to claim 21 wherein said layer (D) comprises an acrylic resin.

29. A polyester film comprising a base layer (B) comprising a crystallisable polyester and a heat-sealable layer (A) comprising an amorphous copolyester wherein: (i) said amorphous copolyester is derived from an aliphatic diol and a cycloaliphatic diol and at least one aromatic dicarboxylic acid; and (ii) said base layer (B) comprises at least one hydrolysis stabiliser.

30. The film according to claim 29 wherein said base layer (B) comprises at least one UV-stabiliser and is the outermost layer of the photovoltaic cell.

31. The film according to claim 29 wherein said polyester film further comprises a polyester layer (C) which is disposed on base layer (B) on the opposite side thereof to heat-sealable layer (A); wherein said layer (C) comprises amorphous polyester derived from an aliphatic diol and a cycloaliphatic diol and at least one aromatic dicarboxylic acid.

32. The film according to claim 31 wherein said layer (C) comprises at least one UV-stabiliser.

33. The photovoltaic cell according to claim 5 wherein said opacifying agent is selected from titanium dioxide and barium sulphate.

34. The photovoltaic cell according to claim 33 wherein said opacifying agent comprises titanium dioxide and is present in the layer in an amount of from about 3 wt % to about 15 wt % based on the total weight of the layer.

35. The photovoltaic cell according to claim 33 wherein said opacifying agent comprises barium sulphate.

36. The photovoltaic cell according to claim 3 wherein the hydrolysis stabiliser is present in a layer in an amount in the range of from 0.10 to 5.0 mol %, based on the amount of polyester in the layer.

37. The photovoltaic cell according to claim 3 wherein the UV-stabiliser in a layer is in the range from 0.2% to 1.5% by weight, relative to the total weight of the layer.

38. The photovoltaic cell according to claim 1 wherein the thickness of base layer (B) constitutes from about 70 to about 95% of the total thickness of the polyester film.

39. The photovoltaic cell according to claim 1 wherein the total thickness of the film is in the range of from about 100 to about 350 m, and/or the thickness of said layer (A) is from about 10 to about 25 m.

40. The photovoltaic cell according to claim 1 wherein the amorphous copolyester is derived from an aliphatic diol and a cycloaliphatic diol and one aromatic dicarboxylic acid.

41. The photovoltaic cell according to claim 1 wherein said aromatic dicarboxylic acid is terephthalic acid, and/or wherein said aliphatic diol ethylene glycol and/or said cycloaliphatic diol is 1,4-cyclohexanedimethanol.

42. The photovoltaic cell according to claim 1 wherein the molar ratio of the cycloaliphatic diol to the aliphatic diol is in the range from 10:90 to 70:30.

43. The photovoltaic cell according to claim 1 wherein said film is biaxially oriented.

44. The photovoltaic cell according to claim 1 wherein said film is white.

45. The photovoltaic cell according to claim 1 wherein said film exhibits an adhesion strength of the heat-sealable layer (A) of the film to an encapsulant material of at least 40N/cm, measured as the linear average load.

46. The photovoltaic cell according to claim 1 wherein said encapsulant resin is EVA.

Description

EXAMPLES

[0119] In the following discussion, intrinsic viscosity values are those measured on the polymer chip unless otherwise specified, and reference to PETG is to a copolyester of terephthalic acid, 1,4-cyclohexanedimethanol and ethylene glycol (33:67 CHDM:EG) (Skygreen S2008 (IV=0.79), optionally blended with Skygreen PN100 (IV=0.70)) unless otherwise specified.

[0120] Several multi-layer films based on polyethylene terephthalate (PET) and comprising at least two layers were extruded and cast using a standard melt coextrusion system. The coextrusion system was assembled using two independently operated extruders which fed separate supplies of polymeric melt to a standard coextrusion block or junction at which these streams were joined. From the coextrusion block, the melt-streams were transported to a conventional, flat film extrusion die which allowed the melt curtain to be cast from the common coextrusion die at 275 C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 6.5 m/min and was approximately 1280 mm in width. The cast extrudate was stretched in the direction of extrusion to approximately 3 times its original dimensions at a temperature of 82 C. The cooled stretched film was then passed into a stenter oven at a temperature of 115 C. where the film was dried and stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched film was heat-set at temperatures in the range of from 220 to 230 C. The identities and final thicknesses of the layers in the resulting films are described in further detail below. This manufacturing method is applicable to Comparative Examples 1, 2 and 3 below.

Comparative Example 1

[0121] A 230 m three-layer film was manufactured in accordance with the above procedure to prepare a film having an ABA layer structure. The polyesters were fed at a rate of 130:1740:130 kg/hr (A:B:A). The PET polyester from which layer A was made comprised 18 wt % BaSO.sub.4 and exhibited an IV of 0.81. Hydrolysis stabiliser (Cardura E10P) was added to each layer A at a rate of 3.6 litres/hour during the film manufacturing process. In addition, UV-stabiliser (Tinuvin 1577) was added (via a 20 wt % PET-based masterbatch at a rate of 13 kg/hr) to provide a concentration of 1 wt % of the UV-stabiliser in each layer of the final film. Each Layer A was 15 m thick. The PET from which the Layer B was made comprised 41 wt % of PET polymer having an IV of 0.79; 22 wt % of a PET polymer having an IV of 0.81 and comprising 18 wt % BaSO.sub.4; and 37 wt % of reclaimed film. Hydrolysis stabiliser (Cardura E10P) was added to layer B at a rate of 6 litres/hour during the film manufacturing process. Layer B was 200 m thick.

Comparative Example 2

[0122] A 212 m three-layer film was manufactured in accordance with the above procedure to prepare a film having an ABA layer structure. The polyesters were fed at a rate of 130:1740:130 kg/hr (A:B:A). The PET polymer from which each layer A was made had an IV of 0.79. Prior to film formation, TiO.sub.2 was added at a level of 14 wt % (via a 60% PET-based masterbatch) and UV-stabiliser (Tinuvin 1577) was added at a level of 1 wt % (via a 20% PET-based masterbatch). Hydrolysis stabiliser (Cardura E10P) was added to each layer A at a rate of 1.2 litres/hour during the film manufacturing process. Each layer A was 14 m in thickness. The PET from which the layer B was made had an IV of 0.79. Prior to film formation, TiO.sub.2 was added at a level of 8 wt % TiO.sub.2 (via a 60 wt % PET-based masterbatch). Hydrolysis stabiliser (Cardura E10P) was added to layer B at a rate of 6 litres/hour during the film manufacturing process. Layer B comprised 35% reclaimed film. Layer B was 184 m in thickness.

Comparative Example 3

[0123] A 265 m three-layer film was manufactured in accordance with the above procedure to prepare a film having an ABA layer structure. The polyesters were fed at a rate of 130:1740:130 kg/hr (A:B:A). The PET polymer used to prepare each layer A had an IV of 0.79. Prior to film formation, TiO.sub.2 was added at a level of 14 wt % (via a 60% PETG-based masterbatch) and UV-stabiliser (Tinuvin 1577) was added at a level of 1 wt % (via a 20% PET-based masterbatch). Hydrolysis stabiliser (Cardura E10P) was added to each layer A at a rate of 1.2 litres/hour during the film manufacturing process. Each layer A was 17.5 m in thickness. Each layer A in the final film comprised PETG in an amount of 9.2 wt %. The PET from which the layer B was made had an IV of 0.79. Prior to film formation, TiO.sub.2 was added at a level of 8 wt % TiO.sub.2 (via a 60 wt % PETG-based masterbatch). Hydrolysis stabiliser (Cardura E10P) was added to layer B at a rate of 6 litres/hour during the film manufacturing process. Layer B comprised 35% reclaimed film. Layer B was 230 m in thickness.

Comparative Example 4

[0124] Commercially available 50 m Melinex D389 film (DuPont Teijin Films) was used for this examples. The film has a coextruded AB structure, wherein:

Layer A=copolyester of PET with isophthalic acid (IPA) (TA:IPA=82:12) with a thickness 10 m; and
Layer B=PET comprising 0.125 wt % China clay filler, and 1.5% Tinuvin 1577 UV-absorber, with a thickness 40 m.

Comparative Example 5

[0125] This comparative example is Melinex D387 film (DuPont) having a thickness of 30 m. The film has the same layer composition and structure as Melinex D389 except that the PET layer comprises 4% Tinuvin 1577 UV-stabiliser. The thickness of the B-layer is 24 m.

Comparative Example 6

[0126] This comparative example is Melinex M342 film (DuPont) having a thickness of 100 m. The film is clear with a coextruded ABA layer structure comprising an unfilled PET base layer (B). The outer layers (A) are made from the same copolyester of PET with isophthalic acid (IPA) (TA:IPA=82:12) used in Melinex D389 and D387 and further comprising 0.125 wt % China clay filler. The PET base layer (B) does not contain Tinuvin 1577 UV-stabiliser. The thickness of the base layer (B) is about 73 m with two approximately equal layers (A) on either side.

[0127] The comparative examples above were tested in the dry adhesion test described hereinabove and the results are presented in Table 1 below.

TABLE-US-00001 TABLE 1 Dry Adhesion Max Peak Linear Average Sample (N/cm) (N/cm) Comp. Ex. 1 3 2 Comp. Ex. 2 7 2 Comp. Ex. 3 12 6 Comp. Ex. 4 34 22

[0128] Comparative Examples 1 and 2 demonstrate that the filled PET film performs poorly in the dry adhesion test. Comparative Example 3 demonstrates that small amounts of PETG copolyester blended into the PET can increase the dry adhesion, albeit only by a relatively small amount, and below the target adhesion strength. Comparative Example 4 (the Melinex D389 film) demonstrates that the IPA-containing copolyester increased dry adhesion strength, but below the target. Comparative Example 5 (Melinex D387) exhibited an adhesion strength effectively the same as Comparative Example 4 because the D389 and D387 films use the same IPA-containing copolyester in the heat-sealable layer (A). The same is true of Comparative Example 6.

Example 1

[0129] From the coextrusion block, the melt-streams were transported to a conventional, flat film extrusion die which allowed the melt curtain to be cast from the common coextrusion die at 275 C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 7 m/min and was approximately 1250 mm in width. The cast extrudate was stretched in the direction of extrusion to approximately 3 times its original dimensions at a temperature of 82 C. The cooled stretched film was then passed into a stenter oven at a temperature of 115 C. where the film was dried and stretched in the sideways direction to approximately 3.3 times its original dimensions. The biaxially stretched film was heat-set at temperatures in the range of from 200 to 212 C. The final film was 120 m in thickness and comprised two layers, wherein Layer A (12 m) was PETG (100% Skygreen S2008); and layer B (108 m) was PET (IV=0.79) comprising 4.5 wt % TiO.sub.2 (R-KB-6; Sachtleben). The film was tested in the dry adhesion test. The film was also tested in the damp-heat test described herein, along with the best-performing comparative example from the dry adhesion test, i.e. Comparative Example 4. The results are collated in Table 2 below.

TABLE-US-00002 TABLE 2 24 h DHT 120 h DHT 500 h DHT Dry adhesion adhesion adhesion adhesion Max Linear Max Linear Max Linear Max Linear Peak Av. Peak Av. Peak Av. Peak Av. Sample (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) C. Ex. 4 34.19 22.59 33.31 18.26 28.21 Ex. 1 63.0 46.7 63.2 49.1 66.7 57.2

[0130] Comparative Example 4 retained some adhesion after 24 hrs in the damp-heat test but failed after about 120 hrs. In contrast, the film of Example 1 according to the present invention exhibited very strong adhesion, even after 500 hours in the damp-heat test. Example 1 also exhibited a very high maximum peak in the adhesion tests.

Examples 2 to 4

[0131] From the coextrusion block, the melt-streams were transported to a conventional, flat film extrusion die which allowed the melt curtain to be cast from the common coextrusion die at 275 C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 10.5 m/min and was approximately 300 mm in width. The cast extrudate was stretched in the direction of extrusion to approximately 3.2 times its original dimensions at a temperature of 78 C. The cooled stretched film was then passed into a stenter oven at a temperature of 110 C. where the film was dried and stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched film was heat-set at temperatures in the range of from 190-220 C. The film was a two layer film having an AB-layer structure. In all cases, the B layer was made from a PET homopolymer (IV=0.79), and the A-layer was made from PETG (100% Skygreen S2008). The compositions and final thicknesses of the layers in the resulting films are described in more in Tables 3 and 4 below. The UV-stabiliser used in these examples was Tinuvin 1577 (BASF). The results demonstrate that the film exhibits excellent performance in the damp-heat test.

TABLE-US-00003 TABLE 3 A Layer B Layer Ex. Polymer UV/wt % T/m TiO.sub.2/wt % T/m 2 PETG 1.5 20 0 83 3 PETG 0 23 4.5 82 4 PETG 1.5 18 4.5 94

TABLE-US-00004 TABLE 4 0 hours 24 hours DHT 100 hrs DHT 500 hrs DHT Max Lin. Max Lin. Max Lin. Max Lin. peak Av. peak Av. peak Av. peak Av. Ex. (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) (N/cm) 2 3 55.6 35.4 62.2 36.2 59.8 37.3 56.2 4 39.4 35.7 57.5 43.9 63.6 45.3 60.0 42.7

Examples 5 to 7

[0132] The film of Example 3 was coated on the PETG surface (layer A) with an anti-blocking layer (D) in order to demonstrate the technical effect thereof. The coating composition was a cross-linkable acrylic resin in the form of an aqueous dispersion of AC201 (Rohm & Haas) comprising an acrylic binder (MMA/EA/MA) and formaldehyde-melamine cross-linker, and had the following formulation: [0133] (i) 326 ml AC201 (46%); [0134] (ii) 40 ml Tween 20 (10%); [0135] (iii) 25 ml Ammonium Nitrate (10%); and [0136] (iv) 4209 ml deionised water

[0137] The coating was applied onto the surface of the finished film at a range of coat thicknesses by gravure coating (60 mesh (G10) gravure; offset arrangement). The adhesion strength of the coated Examples was tested using the dry adhesion test described herein and the results shown in Table 5 below.

TABLE-US-00005 TABLE 5 Acrylic coat Max peak Lin. Av. Example thickness (nm) (N/cm) (N/cm) 3 0 55.6 35.4 5 40 0.247 0.157 6 70 0.287 0.179 7 100 0.306 0.166

[0138] The data in Table 5 demonstrate the principle of an anti-blocking layer in the present invention. Such a layer can be used to overcoat a PETG layer, particularly in the symmetrical and more economic ABA film structure having both layers (A) as PETG, such that the adhesion strength of one of the PETG layers (A) to the encapsulant is retained, but the adhesion of the other PETG layer (A) is negated, thereby improving the overall processability and utility of the film.

Examples 8 to 11

[0139] From the coextrusion block, the melt-streams were transported to a conventional, flat film extrusion die which allowed the melt curtain to be cast from the common coextrusion die at 275 C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 7.4 m/min and was approximately 300 mm in width. The cast extrudate was stretched in the direction of extrusion to approximately 2.95 times its original dimensions at a temperature of 75 C. The cooled stretched film was then passed into a stenter oven at a temperature of 110 C. where the film was dried and stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched film was heat-set at temperatures in the range of from 190-220 C. The film was a two layer film having an AB-layer structure. The A-layer was made from PETG (75 wt % Skygreen S2008 blended with 25 wt % Skygreen PN100). Hydrolysis stabiliser (Cardura E10P) was optionally added to layer A at a rate of 2.5 ml/kg during the film manufacturing process. TiO.sub.2 was optionally added to the A layer to provide a level of 2 wt % TiO.sub.2 in the A-layer of the final film. In all cases, the B layer was made from a PET homopolymer (IV=0.79). Hydrolysis stabiliser (Cardura E10P) was added to layer B at a rate of 6.5 ml/kg during the film manufacturing process. In addition TiO.sub.2 was added to layer B as 60% PETG-based masterbatch to provide 4.5 wt % TiO.sub.2 in the B-layer of the final film. Layer B had a thickness of 140.7, 150.8, 145.8 and 157.6 m in Examples 8, 9, 10 and 11, respectively. The properties of the examples are described in Table 6 below.

TABLE-US-00006 TABLE 6 Adhesion A Layer 0 hours 100 hours TiO.sub.2 Hydrolysis T Max peak Lin. Av. Max peak Lin. Av. Ex. (wt %) stabiliser (m) (N/cm) (N/cm) (N/cm) (N/cm) 8 0 no 21.3 76.97 60.17 74.67 60.31 9 0 yes 22.2 74.46 64.22 77.06 62.34 10 2 yes 22.2 69.73 63.44 75.63 61.80 11 2 no 20.4 75.94 60.60 73.86 56.92

[0140] Adhesion was also tested after 500 hours damp heat treatment. The best performing film was Example 11, which exhibited a maximum peak of 46.52 N/cm and a linear average peak of 31.70 N/cm, which was significantly better than Examples 8 to 10.

Examples 12 to 15

[0141] From the coextrusion block, the melt-streams were transported to a conventional, flat film extrusion die which allowed the melt curtain to be cast from the common coextrusion die at 275 C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 6.7 m/min and was approximately 300 mm in width. The cast extrudate was stretched in the direction of extrusion to approximately 2.95 times its original dimensions at a temperature of 76 C. The cooled stretched film was then passed into a stenter oven at a temperature of 110 C. where the film was dried and stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched film was heat-set at temperatures in the range of from 190-220 C. The film was a two layer film having an AB-layer structure. The A-layer was made from PETG (75 wt % Skygreen S2008 blended with 25 wt % Skygreen PN100). In all cases, the B layer was made from a PET homopolymer (IV=0.79). Hydrolysis stabiliser (Cardura E10P) was added to layer B at a rate of 6.5 ml/kg during the film manufacturing process. In addition, TiO.sub.2 was added to layer B as 60% PETG-based masterbatch to provide 4.5 wt % TiO.sub.2 in the B-layer of the final film. The properties of the examples are described in Table 7 below.

TABLE-US-00007 TABLE 7 Adhesion Layer Thickness 0 hours 500 hours (m) Max peak Lin. Av. Max peak Lin. Av. Ex. A B (N/cm) (N/cm) (N/cm) (N/cm) 12 15.4 160.9 67.13 61.11 73.65 59.32 13 21.2 170.6 73.94 61.18 77.33 63.16 14 24.8 123.9 70.33 61.09 79.45 75.14 15 29.0 117.7 67.11 59.53 75.35 48.87

Examples 16 to 24

[0142] From the coextrusion block, the melt-streams were transported to a conventional, flat film extrusion die which allowed the melt curtain to be cast from the common coextrusion die at 275 C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 8 m/min and was approximately 300 mm in width. The cast extrudate was stretched in the direction of extrusion to approximately 2.95 times its original dimensions at a temperature of 76 C. The cooled stretched film was then passed into a stenter oven at a temperature of 110 C. where the film was dried and stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched film was heat-set at temperatures in the range of from 190-220 C. The film was a two layer film having an AB-layer structure. The A-layer was made from PETG (Skygreen S2008 optionally blended with Skygreen PN100). Hydrolysis stabiliser (Cardura E10P) was optionally added to layer A during the film manufacturing process. In all cases, the B layer was made from a PET homopolymer (IV=0.79). Hydrolysis stabiliser (Cardura E10P) was added to layer B at a rate of 6.5 ml/kg during the film manufacturing process. In addition, TiO.sub.2 was added to layer B as 60% PETG-based masterbatch to provide 4.5 wt % TiO.sub.2 in the B-layer of the final film. The properties of the examples are described in Table 8 below.

TABLE-US-00008 TABLE 8 Adhesion Layer Thickness 0 hours 500 hours (m) Max peak Lin. Av. Max peak Lin. Av. Ex. A B (N/cm) (N/cm) (N/cm) (N/cm) Layer A: copolyester was 85% S2008 & 15% PN100; hydrolysis stabiliser added at 2.4 ml/kg 16 11.2 145.3 62.50 53.34 60.03 65.86 17 17.9 130.3 66.08 57.23 74.99 66.62 18 21.1 137.5 71.32 59.86 78.10 68.81 19 22.0 124.5 64.92 60.52 84.52 66.52 20 27.8 119.3 73.07 61.63 70.38 46.78 Layer A: copolyester was 100% S2008; hydrolysis stabiliser added at 3.0 ml/kg 21 11.7 151.0 64.40 54.08 74.59 73.87 22 18.0 144.7 67.82 58.48 73.87 65.28 23 21.7 150.3 66.08 58.64 75.21 65.96 24 24.6 145.1 62.62 56.76 77.58 59.73