Hydrolysis resistant polyester film

Abstract

The use of titanium dioxide particles coated by an organic coating for increasing the hydrolysis resistance of an oriented polyester film, particularly wherein the organic coating does not comprise or is not derived from a silane, and particularly wherein the organic coating is selected from an organophosphorus compound and a polymeric organic coating; and oriented polyester films comprising such titanium dioxide particles coated by an organic coating; and photovoltaic cells comprising such films.

Claims

1. A photovoltaic cell comprising a front-plane, electrode layer(s), photovoltaic-active layer(s), and a back-plane, wherein the front-plane and/or the back-plane comprises an oriented polyester film, wherein the oriented polyester film comprises titanium dioxide particles wherein said particles are coated by an organic coating which is an alkylphosphonic acid or an ester of the alkylphosphonic acid wherein the alkylphosphonic acid contains from 6 to 22 carbon atoms; and wherein the oriented polyester film comprises said titanium dioxide particles in an amount of from about 5 wt % by total weight of the film, and wherein the oriented polyester film does not comprise an additional organic hydrolysis stabiliser.

2. A photovoltaic cell according to claim 1, wherein the oriented polyester film comprises from about 5 wt % to about 40 wt %, preferably from about 5 wt % to about 20 wt %, of said titanium dioxide particles coated by said organic coating, by total weight of the film.

3. A photovoltaic cell according to claim 1, wherein said polyester film is a layer (A) in a multilayer film further comprising a second layer (B).

4. A photovoltaic cell according to claim 3, wherein said second layer (B) is a polyester film layer.

5. A photovoltaic cell according to claim 3, wherein the intrinsic viscosity of the polyester film is at least about 0.60, preferably at least about 0.64.

6. A photovoltaic cell according to claim 4, wherein the polyester of said second layer (B) is or comprises a polyester which is the same as the polyester of layer (A).

7. A photovoltaic cell according to claim 4, wherein polyester layer (A) comprises from about 5 wt % to about 40 wt %, preferably from about 5 wt % to about 20 wt %, preferably from about 10 wt % to about 20 wt %, of said titanium dioxide particles coated by said organic coating, by total weight of the layer.

8. A photovoltaic cell according to claim 4, wherein said second layer (B) comprises said titanium dioxide particles coated by said organic coating, preferably in an amount of from about 1 wt % to about 10 wt %, preferably from about 1 wt % to about 5 wt %, by total weight of the layer.

9. A photovoltaic cell according to claim 8, wherein the amount of said titanium dioxide particles coated by said organic coating in layer (A) is greater than the amount of titanium dioxide particles coated by said organic coating in layer (B).

10. A photovoltaic cell according to claim 4, wherein said second layer (B) comprises reclaimed waste film derived from the manufacture of said multilayer film.

11. A photovoltaic cell according to claim 4, wherein the thickness of layer (A) is less than the thickness of layer (B), preferably wherein the thickness of layer (A) is from about 10% to about 40%, preferably from about 20% to about 30%, of the thickness of layer (B).

12. A photovoltaic cell according to claim 1, wherein the total thickness of the oriented polyester film is from about 12 μm to about 500 μm, preferably from about 20 μm to about 100 μm.

13. A photovoltaic cell according to claim 1, wherein the oriented polyester film is a biaxially oriented polyester film.

14. A photovoltaic cell according to claim 1, wherein said polyester is selected from polyethylene terephthalate and polyethylene naphthalate, and is preferably from polyethylene terephthalate.

15. A photovoltaic cell according to claim 1, wherein the titanium dioxide is in the rutile crystal form.

16. A photovoltaic cell according to claim 1, wherein said titanium dioxide particles are uniformly and discretely coated by said organic coating.

17. A photovoltaic cell according to claim 1, wherein said coated titanium dioxide particles coated by an organic coating have a particle size of from about 0.01 μm to about 5.0 μm, preferably from about 0.10 μm to 0.40 μm.

18. A photovoltaic cell according to claim 1, wherein the film does not comprise an organic UV absorber.

19. A photovoltaic cell according to claim 1, wherein the film is white.

20. A photovoltaic cell according to claim 1, wherein the hydrolysis resistance of the polyester film is such that the Elongation To Break is at least 10%, preferably at least 20%, preferably at least 30%, after accelerated ageing at 121° C. and 100% relative humidity for at least 80 hours.

21. A photovoltaic cell according to claim 1, wherein said electrode layer(s) and photovoltaic-active layer(s) are coated in an encapsulant, and wherein the back-plane comprises the oriented polyester film.

22. A photovoltaic cell according to claim 1, wherein the organic coating comprises n-octylphosphonic acid, n-decylphosphonic acid, 2-ethylhexylphosphonic acid, camphyl phosphonic acid, and/or an ester thereof.

Description

EXAMPLES

Example 1

(1) A masterbatch was prepared by incorporating titanium dioxide coated with an alumina layer and an organic coating as defined hereinabove (Tioxide® TR28; average particle size 0.21 μm) into polyethylene terephthalate (PET; IV of 0.61). The TiO.sub.2 content of the masterbatch was 40 wt %, based on the total weight of the composition.

(2) A two-layer coextruded film was produced in which layer (A) was derived from the TiO.sub.2 masterbatch and polyethylene terephthalate to provide a concentration of about 14 wt % TiO.sub.2 by total weight of layer (A). The masterbatch was combined with the PET in the hopper of a twin-screw extruder (with vacuum to remove moisture) on a conventional film manufacturing line. Layer (B) was polyethylene terephthalate and 40% reclaim.

(3) The two-layer film was 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 285° C., and then quenched in temperature onto a rotating, chilled metal drum. The cast film was collected at a process speed of about 17.8 m/min and was approximately 2135 in width. The cast extrudate was stretched in the direction of extrusion to approximately 3 times its original dimensions at a temperature of 86.5° C. The cooled stretched film was then passed into a stenter oven at a temperature of 102° C. where the film was dried and stretched in the sideways direction to approximately 3.97 times its original dimensions. Side-ways draw temperature was 125° C. The biaxially stretched film was heat-set at temperatures in the range of from 215 to 220° C.

(4) The final thickness of the resulting white film was 50 μm, with layer (A) being 10 about μm and layer (B) being about 40 μm.

Example 2 (Comparative)

(5) Example 2 corresponded to Example 1 except that the titanium dioxide was an alumina-coated rutile TiO.sub.2 (Titanix® JR301 from Tayca Corporation; average particle size 0.30 μm) which did not comprise the organic coating described herein.

Example 3 (Comparative)

(6) Example 1 was repeated using a further grade of alumina-coated TiO.sub.2 which did not comprise the organic coating described herein.

(7) The hydrolysis resistance of each of the films was then assessed by measuring its elongation to break before and after accelerated ageing, as defined herein, and the results shown in FIG. 1 (in which the x-axis is time (hours), and the y-axis is the ETB (%)). The film of Example 1 exhibited surprisingly superior hydrolysis resistance, compared to the similar films in which the titanium dioxide was not coated with the organic coating described herein.

Examples 4, 5, 6 and 7

(8) A series of 50 μm mono-layer PET films was prepared with various grades of coated TiO.sub.2 and without TiO.sub.2, as follows:

(9) Example 4: no TiO.sub.2 (control)

(10) Example 5: Tioxide® TR28 (invention)

(11) Example 6: Ti-Puree R104 (DuPont; silanized alumina-coated rutile TiO.sub.2; comparative)

(12) Example 7: Ti-Puree R960 (DuPont; alumina- and silica-coated rutile TiO.sub.2 with no organic coating; comparative)

(13) Masterbatches containing each TiO.sub.2 grade were prepared with 60% TiO.sub.2 in PET. Film examples 5, 6 and 7 were made by combining the masterbatch with a PET base polymer in the hopper of a twin-screw extruder (with vacuum to remove moisture) on a conventional film manufacturing line to provide about 10.5 wt % TiO.sub.2 in the final film.

(14) SiO.sub.2 was added via a 10% masterbatch to provide 2.3 wt % SiO.sub.2 in the final film. A liquid hydrolysis stabiliser (Cardura® E10P; glycidyl ester of versatic acid; Hexion Speciality Chemicals) was metered into the extruder at about 8 ml per kg of the polymer and its inorganic additives. The addition of TiO.sub.2 to an hydrolysis stabiliser-containing film is expected to decrease the hydrolysis stability of the film.

(15) The films were then tested by measuring the elongation to break before and after accelerated ageing, as defined herein, and the results shown in FIG. 2 (in which the x-axis is time (hours), and the y-axis is the ETB (%)).The film of Example 5 exhibited surprisingly superior hydrolysis resistance, compared to the films of Examples 6 and 7 in which the titanium dioxide was not coated with an organic coating as defined in the present invention.