BACKSHEET FOR PHOTOVOLTAIC MODULES COMPRISING AN ALIPHATIC POLYAMIDE

Abstract

The present invention relates to a backsheet for photovoltaic modules comprising a polymeric layer comprising an aliphatic polyamide comprising 1,10-decanedioic acid. Examples of such aliphatic polyamides are polyamide 4,10, polyamide 5,10 or polyamide 6,10. Preferably polyamide 4,10 is present in the rear layer of the backsheet. A polyolefin layer is preferably present in the core layer of the backsheet. It is however also possible that the polyamide is present in the core layer and polyolefin is present in the rear layer of the backsheet. The polyolefin is preferably chosen from the group consisting of polyethylene, polypropylene or ethylene-propylene copolymers. More preferably the polyolefin is polypropylene. The backsheet preferably comprises at least a further polymeric layer comprising a polymer selected from the group consisting of an optionally functionalized polyolefin such as a maleic anhydride functionalized polypropylene homo or copolymer. The present invention further relates to a photovoltaic module containing essentially, in order of position from the front-sun facing side to the back non-sun-facing side, a transparent pane, a front encapsulant layer, a solar cell layer comprised of one or more electrically interconnected solar cells, a back encapsulant layer and the back-sheet according to the present invention.

Claims

1. Backsheet for photovoltaic modules comprising a core and/or rear layer comprising an aliphatic polyamide containing monomer units of an aliphatic linear dicarboxylic acid with at least 8 carbon atoms.

2. Backsheet according to claim 1 whereby the aliphatic linear dicarboxylic acid is chosen from the group of 1,10-decanedioic acid, 1,11-undecandioic acid, 1,12-dodecanedioic acid, 1,13-tridecanedioic acid, 1,14-tetradecanedioic acid, 1,15-pentadecanedioic acid, 1,16-hexadecanedioic acid, 1,17-heptadecanedioic acid and 1,18-octadecanedioic acid.

3. Backsheet according to claim 1 whereby the aliphatic linear dicarboxylic acid is 1,10-decanedioic acid.

4. Backsheet according to claim 1 whereby the aliphatic polyamide also contains at least a further monomer unit derived from a diamine alkane whereby the alkane comprises at least 4 carbon atoms.

5. Backsheet according to claim 4 whereby the diamine alkane is chosen from 1,4-diamine butane, 1,6-hexamethylene diamine or 1,5-pentamethylenediamine.

6. Backsheet according to claim 1 whereby the aliphatic polyamide is chosen from polyamide 4,10, polyamide 5,10 or polyamide 6,10.

7. Backsheet according to claim 1 whereby the aliphatic polyamide is impact modified polyamide.

8. Backsheet according to claim 1 whereby the backsheet comprises a further polymeric layer comprising a polyolefin.

9. Backsheet according to claim 1 whereby the aliphatic polyamide is present in the rear layer of the backsheet.

10. Backsheet according to claim 9 whereby the polyolefin layer is present in the core layer of the backsheet.

11. Backsheet according to claim 1 whereby the polyamide is present in the core layer of the backsheet.

12. Backsheet according to claim 11 whereby the polyolefin is present in the rear layer of the backsheet.

13. Backsheet according to claim 8 whereby the polyolefin is chosen from the group consisting of ethylene homo or copolymers, propylene homo or copolymers, ethylene-propylene copolymers, propylene-ethylene copolymers, ethylene-norbornene copolymers or polymethylpentene.

14. Backsheet according to claim 13 whereby the polyolefin is a polypropylene homopolymer, an ethylene-propylene copolymer, a propylene-ethylene copolymer or a mixture thereof.

15. Backsheet according to claim 1 whereby the backsheet comprises at least a further polymeric layer facing the cells comprising a functionalized polyolefin.

16. Backsheet according to claim 15 whereby the functionalized polyolefin is selected from the group consisting of ethylene vinylacetate, ethylene-maleic anhydride copolymer or ethylene alkyl (meth)acrylate copolymers.

17. Backsheet according to claim 1 further comprising at least a connecting or adhesive layer between the layer facing the cells and the core layer and/or the core layer and the rear layer.

18. Backsheet according to claim 17 whereby the adhesive layer comprises a polymer selected from the group consisting of maleic anhydride grafted polyethylene or maleic anhydrate grafted polypropylene.

19. Photovoltaic module comprising the backsheet according to claim 1.

20. Photovoltaic module according to claim 19 containing essentially, in order of position from the front-sun facing side to the back non-sun-facing side, a transparent pane, a front encapsulant, a solar cell layer comprised of one or more electrically interconnected solar cells, a back encapsulant and the back-sheet.

Description

EXAMPLES

Preparation Examples

[0042] The following preparation examples were obtained by mixing the specified weight % of each component, as shown in Table 1, and extruding at a rate of 20 Kg/h, with a screw speed of 250 rpm to produce pellets. Preparation Example 1 was extruded at 321° C. and 11 bar; Preparation Example 2 was extruded at 313° C. and 3 bar; and Preparation Example 3 was extruded at 316° C. and 4 bar. 100 kg of each sample was produced.

TABLE-US-00001 TABLE 1 Preparation Prep. Ex. 1 Prep. Ex. 2 Prep. Ex. 3 Example Number [wt. %] [wt. %] [wt. %] Polyamide 4,10 68.25% (MVR 26) Polyamide 4,10 68.25% (MVR 52) Akulon ® F128 68.25 (polyamide 6) Queo 8201 5.00 5.00 5.00 Plastomer 5.00 5.00 5.00 Irganox 1098 0.50 0.50 0.50 Tinivin 1577 1.00 1.00 1.00 Chimassorb 2020 0.25 0.25 0.25 TiO.sub.2 R105 20.00 20.00 20.00 Total 100.0 100.0 100.0

[0043] Akulon® F238 is a polyamide 6 from DSM. Queo™ 8201 is an ethylene plastomer from Borealis. Irganox® 1098 is a discolouring stabilizer for polymers form BASF. Tinuvin® 1577 is a UVA light absorber from BASF. Chimasorb® 2020 is a light stabilizer from BASF.

[0044] MVR is the melt viscosity rate of the polyamide 4,10. MVR is measured at 270° C. and 5 Kg, reported in mL/10 min.

Preparation of Material Stack

[0045] Material of a weathering layer (rear facing layer) of the Preparation Examples, a tie layer (adhesive layer), a structural layer (core layer) and a functional layer (layer facing cells) are respectively extruded and pelletized to obtain plastic pellets of each of the respective layers.

[0046] The weathering layer comprised granules of one material selected from Prep. Ex. 1, Prep. Ex. 2 and Prep. Ex. 3.

[0047] The tie layer comprised maleic anhydride grafted polypropylene and α-olefin block copolymer.

[0048] The core layer comprised copolymerised polypropylene.

[0049] The functional layer comprised polyethylene; ethylene copolymer; and copolymerized polypropylene.

[0050] For each example, the pellets were fed to one of multiple extruders, melt-extruded at a high temperature, passed through an adapter and a die, cooled by a cooling roller and shaped into a multi-layer film having a total thickness of 300 μm. Each example had, in order, the composition shown in Table 2.

TABLE-US-00002 TABLE 2 Weathering Weathering layer Tie layer Structural layer Tie layer Functional layer Example layer thickness thickness thickness thickness thickness No. material [μm] [μm] [μm] [μm] [μm] Ex. 1 Prep. Ex. 1 20 25 200 25 30 Ex. 2 Prep. Ex. 1 40 25 180 25 30 Ex. 3 Prep. Ex. 2 20 25 200 25 30 Ex. 4 Prep. Ex. 2 40 25 180 25 30 Comp. Prep. Ex. 3 20 25 200 25 30 Ex. A Comp. Prep. Ex. 3 40 25 180 25 30 Ex. B

Shrinkage

[0051] Samples were cut from materials of the Examples. The samples were heated to 150° C. for 30 minutes. Dimensions were measured by hand before and after treatment and % change calculated. Results are given in Table 3.

TABLE-US-00003 TABLE 3 Weathering Weathering Shrinkage Shrinkage layer layer thickness (machine (transverse Example No. material [μm] direction) [%] direction) [%] Ex. 2 Prep. Ex. 1 40 0.3 0.3 Ex. 4 Prep. Ex. 2 40 0.3 0.4 Comp. Ex. B Prep. Ex. 3 40 0.3 0.5

[0052] The results indicate that while shrinkage in the machine direction in all three examples is equivalent, shrinkage in the transverse direction is lower for the Examples 2 and 4 (PA4,10-containing samples) than for the Comparative Example B (PA6-containing sample. This indicates improved dimensional stability of a backsheet.

Yellowing Index after Damp Heat

[0053] The Examples were subjected to damp heat ageing in a \kitsch VC4200 climate chamber at a temperature of 85° C. and a relative humidity of 85% for 1000 hours. After this time, the samples were removed and colour was measured on a Minolta CM3700d spectrophotometer using D65 as illuminant, d/8 geometry, 10° viewing angle, specular included and UV included. These measurements were done using a white calibration tile as background. The change in yellowing index was calculated according to ASTM E313-96. Yellowing was measured directly on the weathering layer side of the multilayer sheet. Results are given in Table 4.

TABLE-US-00004 TABLE 4 Weathering layer Weathering layer Yellowing Index Example material thickness [μm] after 1000 hours Ex. 3 Prep. Ex. 2 20 8.1 Comp Prep. Ex. 3 20 9.7 Ex. A Ex. 4 Prep. Ex. 2 40 7.2 Comp. Prep. Ex. 3 40 8.6 Ex. B

[0054] The results indicate that after long exposure to high temperature and humidity, a coextruded stack of Examples 3 and 4 (comprising PA 4,10) displays less yellowing than a coextruded stack of Comparative Examples A and B (comprising PA 6). This is an improvement in high temperature stability of a backsheet of the present invention.

Breakdown Voltage

[0055] Samples were tested for breakdown DC voltage according to IEC TS62788-2. Results are given in Table 5.

TABLE-US-00005 TABLE 5 Weathering layer Weathering layer Breakdown voltage Example No. material thickness [μm] [kV] Ex. 1 Prep. Ex. 1 20 26.4 Ex. 3 Prep. Ex. 2 20 27.9 Comp. Ex. A Prep. Ex. 3 20 25.9

[0056] The results indicate that a backsheet of Examples 1 and 4 (comprising PA4,10) has a higher breakdown voltage than a backsheet of Comparative Example A (comprising PA6). This indicates increased electrical resistance of a backsheet of the present invention.

Water Vapour Transmission Rate (WVTR)

[0057] A sample of polymer sheet of 210*297 mm (A4) size and specified thickness was produced by a standard film extrusion process. Akulon® F136E1 is a polyamide 6 and was obtained from DSM. Ecopaxx® Q150 is a polyamide 4,10 and was obtained from DSM.

[0058] Each sample was subjected to water vapour transmission rate analysis in a Mocon Aquatran water vapor permeation instrument according to DIN 53122 part 2. The temperature was 23° C. and relative humidity was 0/85%+/−3%. Results are given in Table 6.

TABLE-US-00006 TABLE 6 WVTR WVTR Example Thickness [mg/m2 .Math. [mg .Math. mm/ No. Material [μm] day] m2 .Math. day] Ex. 5 Ecopaxx ® Q150 65 19197 1248 (PA4,10) Comp. Akulon ® F136E1 69.6 29291 2039 Ex. C (PA6)

[0059] The results show that Example 5 (PA4,10) has a lower water vapour transmission rate than Comparative Example C (PA6). This indicates that a backsheet comprising PA4,10 would have improved water barrier properties compared with one having a PA6 weathering layer.

Hydrolytic Stability

[0060] A sample of unstabilized polymer sheet of polyamide (PA) thickness 1 mm was produced by injection molding into a tensile bar according to ISO 527-1 BA. Each sheet was subjected to damp and heat by boiling in tap water at 135° C. and 3.1 bar for a specified time. Samples were then removed, allowed to cool to room temperature and subjected to a tensile strength test, while still damp. The sample was subjected to extension along its major axis at 50 mm/min until break. Results are shown in Table 7.

TABLE-US-00007 TABLE 7 Tensile strength [MPa] Time of Comp. Ex. D Comp Ex. E Ex. 6 treatment [h] PA6 PA6,6 PA4,10 24 38.00 46.00 48.00 200 36.00 37.00 47.00 500 — — 29.00 1000 — — 8.00

[0061] At 500 hours and above, the samples of Comparative Example D (PA6) and Comparative Example E (PA6,6) had lost structural integrity such that they could not be tested for tensile strength. Example 6 (PA4,10) still provided adequate results after heating for 1000 hours.

[0062] The results show that a PA4,10 has a higher tensile strength after damp heat treatment than a PA6 or PA6,6. This indicates that a backsheet comprising a PA4,10 would have improved structural properties in damp and warm environmental conditions compared with one having PA6 or PA6,6. This is an indication of an improvement in dimensional stability of a backsheet according to the present invention.