JP2016187001A - Rear surface protective sheet for solar cell module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rear surface protective sheet for a solar cell module excellent in abrasion resistance and lightfastness.SOLUTION: The rear surface protective sheet for a solar cell module is composed of at least a polyester film and an ultraviolet ray protective layer. The ultraviolet ray protective layer includes a white pigment. The Benard cell number of a surface of the ultraviolet ray protective layer is greater than or equal to 400 units/mmand less than or equal to 600 units/mm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a back surface protective sheet for a solar cell module, and specifically relates to a back surface protective sheet for a solar cell module excellent in long-term durability.

  Solar power generation is being put into practical use as an inexhaustible and non-polluting new energy source, and as a solar cell module for this purpose, a surface protection sheet, an adhesive resin layer, a solar cell, an adhesive resin layer, and a back surface protection sheet are used. Laminated and integrated products are widely known.

  Although the back surface protection sheet is not directly exposed to sunlight, depending on the installation method, the back surface protection sheet is exposed to sunlight due to wraparound or reflection. Therefore, it is important to impart light resistance to the back surface protection sheet.

  As a back surface protective sheet conventionally used, a white polyvinyl fluoride film (DuPont Co., Ltd. “Tedlar” etc.) can be exemplified, and a back surface protective sheet having a laminated structure in which a polyester film is sandwiched between the films is used for the purpose. Widely used. The polyvinyl fluoride film is excellent in light resistance, but is expensive and becomes an obstacle in terms of reducing the cost of the solar cell module. Moreover, since it contains fluorine, it has a problem of high disposal costs.

  In recent years, a back surface protective sheet in which an ultraviolet protective layer is laminated on a base film such as polyester has been proposed. Since the ultraviolet protective layer blocks the intrusion of light from the back surface, the base material is versatile and the cost can be reduced (Patent Document 1).

  By the way, in the case of a configuration in which an ultraviolet protective layer is provided, it is important that the function of the ultraviolet protective layer is developed over a long period of time. Recently, long-term quality assurance of 20 to 30 years has been required after installation, and the demand for long-term durability has been increasing year by year, but scratch resistance is important for the long-term durability of the UV protective layer. I understand that. That is, when the back surface protection sheet is processed into a solar cell module, the UV protection layer is subjected to various mechanical contacts by the handling and processing apparatus by human hands, and when it is applied as a solar cell module, the tool etc. May cause fine scratches on the UV protection layer due to the contact with the surface, and the damage will be enlarged from that part as a base point, the UV protection performance will be greatly deteriorated and the performance as a solar cell module will be deteriorated. There are things to do. Depending on the use conditions, the ultraviolet protective layer may be damaged by contact with sand or pebbles, and the scratch resistance of the ultraviolet protective layer needs to be further improved.

International Publication No. 2010/067780

  The problem to be solved by the present invention is that the ultraviolet protective layer has excellent scratch resistance, has light resistance that can withstand use in a harsh outdoor environment for a long period of time, and has a long-term durability. It is to provide a protective sheet.

  In order to solve the above problems, the present invention has the following configuration.

1st invention is a back surface protection sheet for solar cell modules which consists of a polyester film and the ultraviolet-ray protective layer provided in the surface, Comprising: This ultraviolet-ray protective layer contains a white pigment, and the number of Benard cells is 400 pieces / a back protective sheet for a solar cell module, wherein mm ² or more and 600 / mm ² or less.

  The second invention is characterized in that a coefficient of dynamic friction on the surface of the ultraviolet protective layer is 0.2 or less.

  The third invention is characterized in that the UV protective layer surface has a glossiness of 35% or more and 60% or less.

According to a fourth aspect of the invention, the ultraviolet protective layer has a color difference (ΔE ^* ) of 3 or less after being irradiated with ultraviolet rays having an irradiation intensity of 1000 W / m ² for 316 hours in an atmosphere of a temperature of 63 ° C. and a relative humidity of 30%. And the breaking elongation retention is 50% or more.

  The fifth invention is characterized in that the white pigment is titanium oxide.

  According to the present invention, there is provided a back surface protection sheet for a solar cell module that has an ultraviolet protective layer excellent in scratch resistance, has light resistance that can withstand use in a harsh outdoor environment for a long time, and withstands long-term use. be able to.

It is a microscope observation photograph of an ultraviolet protective layer.

  The present invention will be described below.

The present invention is a back protective sheet for a solar cell module comprising at least a polyester film and an ultraviolet protective layer provided on the surface thereof, wherein the ultraviolet protective layer contains a white pigment, and the number of Benard cells is 400 / mm ^2. The back protective sheet for a solar cell module, characterized by being in the range of 600 / mm ² or less.

[Polyester film]
Since the polyester film used for this invention is used for the outermost layer exposed directly to external air, it is preferable that it is a polyester film excellent in hydrolysis resistance. In general, a polyester film contains 1.5 to 2% by weight of a low molecular weight polymer called an oligomer. Typical examples of polyester oligomers are cyclic trimers. Films with a high content of these materials have reduced mechanical strength during long-term exposure, such as outdoors, and cracks and material breaks due to the progress of hydrolysis due to rainwater, etc. Produce. On the other hand, by forming a polyester film from a polyester having a cyclic trimer content of 1.0% by weight or less obtained by polymerization by a solid phase polymerization method, hydrolysis under high temperature and high humidity In addition, a film excellent in heat resistance and weather resistance can be obtained. The content of the cyclic trimer is obtained, for example, by a method of measuring the content (% by weight) relative to the resin weight by measuring by liquid chromatography using a solution obtained by dissolving 100 mg of a polymer in 2 ml of orthochlorophenol. It is done.

  Moreover, as a polyester film, for example, additives such as an antistatic agent, an ultraviolet absorber, a stabilizer, an antioxidant, a plasticizer, a lubricant, a filler, a color pigment, and the like are impaired. A polyester film or the like added within the range can also be used.

  The thickness of the polyester film is not particularly limited, but is preferably in the range of 25 to 300 μm in view of the withstand voltage characteristics and cost of the back protective sheet.

[UV protective layer]
The ultraviolet protective layer in the present invention is selected in consideration of weather resistance, heat resistance, light resistance, adhesiveness to the base polyester film, etc. in addition to ultraviolet absorption for achieving the original function. The UV protection layer is a layer disposed on the outermost layer of the back surface protection sheet, and protects the back surface protection sheet laminated on the back surface side of the solar cell module from ultraviolet irradiation from the back surface side of the module. It has a function and is coated on the polyester film and laminated on the back protective sheet.

  It is important that the ultraviolet protective layer contains a white pigment as an ultraviolet absorber for achieving the above function. As the white pigment used in the UV protective layer, an inorganic pigment having high light stability is preferable because it is used outdoors for a long period of time. From the viewpoint of availability, price, and light reflectivity, titanium oxide is preferable. Preferably used. The average primary particle size of titanium oxide is preferably 0.15 μm or more and 0.5 μm or less from the viewpoints of color development, dispersibility with respect to the binder resin, and cost. These titanium oxides for pigments are produced industrially at low cost. Easy to use.

Furthermore, in order for the solar cell module to exhibit long-term durability for 20 to 30 years after installation, it is necessary that the ultraviolet protective layer be maintained for a long period of time, and the scratch resistance of the ultraviolet protective layer is important. Become. If the surface of the UV protection layer is rough, the UV protection layer is likely to be scraped off due to various mechanical contacts during processing and enforcement, sand and pebbles colliding and rubbing. In order to obtain an ultraviolet protective layer having excellent scratch resistance, it is important that the number of Benard cells on the coated surface of the ultraviolet protective layer is in the range of 400 cells / mm ^{2 to} 600 cells / mm ^2. mm and more preferably ² to 500 pieces / mm ² or less.

  The Benard cell referred to here is a tortoiseshell coating appearance formed on the coating surface of the UV protection layer as shown in FIG. 1, and as the solvent in the coating solution evaporates, the surface temperature of the coating solution decreases. Further, it is produced by further concentrating and generating a descending vertical flow and convection.

An ultraviolet protective layer having a large number of Benard cells per unit area has many irregularities at the boundary of the Benard cell, so that the ultraviolet protective layer is easily scraped off and has poor scratch resistance. Further, titanium oxide is reduced at the boundary of the Benard cell, and ultraviolet light is transmitted through the sparse part of the titanium oxide to deteriorate the polyester film of the base material, so that the light resistance is inferior. Therefore, when the number of Benard cells exceeds 600 cells / mm ² , these problems become significant. On the other hand, if the number of Benard cells is set to 400 cells / mm ² or less, the restriction on the process becomes large, which is not practical and leads to the appearance defect due to uneven gloss.

  In order to suppress the generation of Benard cells and reduce the number of Benard cells, it is effective to reduce the convection when the coating liquid dries. Specifically, the coating liquid is increased by increasing the solid content concentration. In order to increase the viscosity or slow down the evaporation rate, it is effective to add a high boiling point solvent to the coating liquid or to reduce the thickness of the coating liquid. It is also effective to avoid abrupt drying of the coating liquid by extending the time until the coating liquid is applied to the polyester film and then entering the drying oven. As described above, if the number of Benard cells is too small, the appearance becomes poor due to coating unevenness due to these coating conditions.

The Benard cell number of the coated surface of the UV protective layer in order to 600 / mm ² or less 400 / mm ² or more, it is preferable that the solid concentration of the coating solution of 20 wt% or more 35 wt% or less. Further, among the diluting solvents in the coating solution, a solvent having a boiling point of 140 ° C. or more and 160 ° C. or less, such as 1-methoxypropyl-2-acetate (abbreviation: PGM-Ac, boiling point 146 ° C.) or cyclohexanone (boiling point 156 ° C.) is used. It is preferable to blend it by weight% or more, and more preferably 8 weight% or more and 30 weight% or less.

  Moreover, after coating a coating liquid on a polyester film, the time until entering a drying oven is preferably 5 seconds or more, and more preferably 7 seconds or more. The time required to enter the drying oven after coating the coating liquid on the polyester film is determined by the distance from the coating part to the drying oven and the line speed. For example, the distance from the coating part to the drying oven is 5 m, When the line speed is 60 m / min, the time required to enter the drying oven after coating is 5 seconds.

  Moreover, it is preferable to make the dynamic friction coefficient of the surface of the ultraviolet protective layer of this invention lower than 0.2, More preferably, it is 0.15 or less. When the dynamic friction coefficient is higher than 0.2, the scratch resistance is inferior, and the ultraviolet protective layer is easily scraped off.

  Furthermore, the glossiness of the ultraviolet protective layer surface of the present invention is preferably 35% or more and 60% or less. When the glossiness is less than 35%, the surface of the ultraviolet protective layer becomes rough, the scratch resistance is inferior, and the ultraviolet protective layer tends to be scraped off. On the other hand, if the glossiness exceeds 60%, it may cause an appearance defect due to uneven glossiness.

  In the ultraviolet protective layer in the present invention, a curing agent having a functional group capable of reacting with a hydroxyl group in the acrylic resin is preferably blended for the purpose of improving the properties of the resin layer. By blending the curing agent, the effects of improving the adhesion between the polyester film and the ultraviolet protective layer and improving the durability of the ultraviolet protective layer accompanying the introduction of the crosslinked structure can be obtained. In particular, when designing the back surface protection sheet for solar cell modules so that the ultraviolet protection layer is located in the outermost layer, in the manufacturing process of the solar cell module, specifically in the glass laminating step (cell filling step), Since the ultraviolet protective layer is exposed to a heat treatment of 30 minutes or longer at a high temperature of about 150 ° C. at the maximum, particularly heat resistance is required, and it is preferable to use a crosslinking agent that can react with the hydroxyl group of the binder resin. Among them, a prescription that uses a polyisocyanate curing agent and promotes the formation of urethane bonds (crosslinked structure) is preferable. Examples of the polyisocyanate-based crosslinking agent used as the crosslinking agent include aromatic polyisocyanates, araliphatic polyisocyanates, alicyclic polyisocyanates, and aliphatic polyisocyanates.

  The method for coating and forming the ultraviolet protective layer in the present invention on the base polyester film is not particularly limited, and a known coating technique can be used. As the coating method, various methods can be applied. For example, a roll coating method, a dip coating method, a bar coating method, a die coating method, a gravure roll coating method, or a combination of these methods can be used. it can.

  The thickness of the ultraviolet protective layer in the present invention is preferably 1 to 10 μm, more preferably 1.5 to 8 μm, and particularly preferably 2 to 5 μm. When this ultraviolet protective layer is formed by a coating method, if the thickness is less than 1 μm, a phenomenon such as repellency or film breakage tends to occur during coating, and it is difficult to form a uniform coating film. Adhesive strength, UV blocking properties, peeling resistance, and scratch resistance may not be sufficiently exhibited. On the other hand, when the thickness of the UV protective layer exceeds 10 μm, the UV blocking performance is fully expressed, but there are restrictions on the coating method (specific process capable of thick film coating, restrictions on equipment), and the production cost increases. There is a concern that the coating film sticks to the transport roll and the coating film peels off easily.

  In the ultraviolet protective layer of the present invention, a plasticizer, a heat stabilizer, an antioxidant, a reinforcing agent, a deterioration preventing agent, a weathering agent, a flame retardant, a release agent, etc. may be added as long as the characteristics are not impaired. Good.

[Back side protection sheet for solar cell module]
In the present invention, an ethylene / vinyl acetate copolymer resin (EVA), which is a sealing material for a solar cell element, a white film, a film having a metal oxide vapor deposition layer on the opposite surface of the film to the ultraviolet protective layer, if necessary. By stacking at least one film different from the base film among the films having thermal adhesive properties, a back protective sheet for a solar cell module that satisfies various required characteristics can be obtained.

  As the white film, “Lumirror” (registered trademark) E20F manufactured by Toray, which is a polyethylene terephthalate film, “Tedlar” (registered trademark) PV2001, manufactured by DuPont, which is a polyvinyl fluoride film, and “Kiner” manufactured by Arkema, which is a polyvinylidene fluoride film. "(Registered trademark) 302-PGM-TR" can be exemplified. An example of a film having an inorganic oxide vapor deposition layer is “Barrier Rocks” (registered trademark) 1011HG manufactured by Toray Film Processing Co., Ltd., in which a metal oxide vapor deposition layer made of aluminum oxide is formed on a polyethylene terephthalate film substrate. it can. Examples of the film having thermal adhesiveness with EVA include polyolefin films such as ZK93K, 4801, 4806, and B011W manufactured by Toray Film Processing Co., Ltd. When a white film is laminated, light reflectivity is imparted, when a film having a metal oxide vapor deposition layer is laminated, water vapor barrier property is imparted, and when an olefin film layer is laminated on the side facing the EVA Becomes excellent in thermal adhesiveness with EVA. Moreover, the film laminated | stacked on the back surface protection sheet for solar cell modules in this invention does not necessarily need to be 1 sheet, According to the characteristic to provide, each member film is combined suitably and the back surface protection sheet for solar cell modules should be designed. It ’s fine.

  A known dry laminating method can be used as a method of laminating films and processing into sheets. Bonding of the resin film using the dry laminating method includes a main component such as a polyether polyurethane resin, a polyester polyurethane resin, a polyester resin, a polyepoxy resin, and a polyisocyanate curing agent. A known dry laminating adhesive can be used. However, the adhesive layer formed using these adhesives does not cause peeling due to deterioration of the adhesive strength due to long-term outdoor use, which leads to deterioration in appearance and light reflectance. It is necessary not to cause yellowing. Moreover, as thickness of an adhesive bond layer, Preferably it is the range of 1-5 micrometers. If it is less than 1 μm, it may be difficult to obtain sufficient adhesive strength. On the other hand, if it exceeds 5 μm, the coating speed of the adhesive does not increase, and the amount of the adhesive used increases, leading to an increase in production cost.

  As a material for the adhesive layer, a known dry laminating adhesive can be used. In general, adhesives for dry laminating are prepared by diluting two components of the main agent and cross-linking agent with a diluting solvent. The cross-linking agent is highly reactive with active hydroxyl groups, its reaction rate and initial adhesion. A prescription using an isocyanate group-containing oligomer with a fast onset is preferred. In addition to these advantages, it is possible to form an adhesive resin layer that has high adhesive strength with the base film, and also has excellent constant temperature stability and long-term durability. Examples of the main resin used in combination with the isocyanate group-containing oligomer include polyether resins, polyester resins, polyol resins, and other urethane resins and epoxy resins, depending on detailed requirements and suitability for processing conditions. Can be appropriately selected and used. Moreover, depending on the structure of the back surface protection sheet for solar cell modules, it is also conceivable that ultraviolet rays reach the adhesive layer and induce photodegradation of the resin. From such a viewpoint, the resin used for forming the adhesive layer is preferably an aliphatic resin or an alicyclic resin that does not contain an aromatic ring or has a low content.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not necessarily limited thereto. The characteristic values in the examples and comparative examples of the present invention are based on the following measurement methods and evaluation criteria.
[Measurement of the number of Benard cells]
Using a Nikon Universal Industrial Microscope (UDM ECLIPSE LV100D-U) with a 50x objective and 10x eyepiece lens (total magnification of 500x), the number of Benard cells in the field of view of 146 μm in length × 201 μm in width The number of Benard cells cut at the four corners of the field of view was counted as ¼, the number of Benard cells cut at the four sides of the field of view was ½, and the number of Benard cells not cut at the field of view was counted as one. The visual field area was calculated, and the number of Benard cells per unit area (number / mm ² ) was calculated by dividing the number of Benard cells by the visual field area.

[Measurement of dynamic friction coefficient]
The dynamic friction coefficient of the UV protective layer is a friction player FPR-2100 manufactured by Resuka Co., Ltd., and the measurement conditions are a spiral measurement mode, a rotational speed of 6 rpm, a moving step of 0.1 mm / second, an applied load of 150 g, and a measurement indenter SUJ2 pin. The average value for 300 seconds was calculated as the dynamic friction coefficient. A dynamic friction coefficient of 0.2 or less was judged to be a preferable range.

[Glossiness measurement]
The glossiness of the UV protective layer was measured using a variable angle gloss meter UGV-6P manufactured by Suga Test Instruments Co., Ltd. with an incident angle and a light receiving angle of 60 degrees. A glossiness of 35% or more and 60% or less was judged to be a preferable range.

[Color tone change by UV irradiation]
Using Iwasaki Electric Co., Ltd. Eye Super UV Tester SUV-W161, an accelerated weathering tester, using an illuminometer, the UV intensity is adjusted to 1000 W / m ² in an atmosphere at a temperature of 63 ° C. and a relative humidity of 30%. To do. Under the conditions of the present apparatus, the back surface protective sheet for solar cell module of the present invention was irradiated with ultraviolet rays for 316 hours on the ultraviolet protective layer side.

The color difference (ΔE ^* ) on the UV absorbing protective layer side before and after UV irradiation was measured. The acceleration test and evaluation were performed at three sampling points in the width direction of the sheet. ΔE ^* is determined by measuring L ^* a ^* b ^* of the ultraviolet absorbing layer before and after ultraviolet irradiation using a color difference meter SP68 manufactured by X-rite, USA, and is obtained by ΔE ^* : {((L ^* after ultraviolet irradiation) ^. )-(L ^* before UV irradiation)) ² + ((a ^* after UV irradiation)-(a ^* before UV irradiation)) ² + ((b ^* after UV irradiation)-(b before UV irradiation) ^* )) ² } ^1/2 was calculated. A sodium metal halide lamp D65 was used as the standard light source. Evaluation was made with an average value of three points in the width direction. If ΔE ^* was 3 or less, the color difference was small, and it was judged to be a preferable range.

[Retention rate at break after UV irradiation]
Using Iwasaki Electric Co., Ltd. Eye Super UV Tester SUV-W161, an accelerated weathering tester, using an illuminometer, the UV intensity is adjusted to 1000 W / m ² in an atmosphere at a temperature of 63 ° C. and a relative humidity of 30%. To do. Under the conditions of the present apparatus, the back surface protective sheet for solar cell module of the present invention was irradiated with ultraviolet rays for 316 hours on the ultraviolet protective layer side.

  Using the test method described in JIS C2151 (1996), using a test piece having a width of 10 mm and a length of 150 mm, the initial distance between the chucks was 55 mm, and the tensile strength at break of the back protective sheet was 300 mm / min. Was measured. The value of (B) / (A) × 100 (%) was determined as the breaking elongation retention rate from the breaking elongation (B) after ultraviolet irradiation with respect to the breaking elongation (A) before ultraviolet irradiation. The evaluation was performed with 5 samples each in the longitudinal direction of the back protective sheet, and the average value of these 5 samples was represented.

  It was judged that the breaking elongation retention after ultraviolet irradiation was 50% or more as a preferable range.

[Abrasion resistance of UV protective layer]
In order to evaluate the scratch resistance of the ultraviolet protective layer, a test was conducted under the following conditions. Make a measurement sample with a UV protective layer on the base polyester film, set the UV protective layer on the top to a Gakushin Abrasion Tester (JIS L0849 Friction Tester Type II), and dry the fabric with a dry cloth (dyeing test) Using a material "Kanakin" No. 3), 20 reciprocating frictions with a load of 200 g are performed. The state of the ultraviolet protective layer surface after the test was determined according to the following evaluation criteria. ○ and △ were accepted, and x was rejected.

  ○: No more than 5 scratches on the scratched surface.

  Δ: 6-20 scratches on the scratched surface.

  X: 21 or more scratches were generated on the scratched surface.

  Subsequently, the preparation of the ultraviolet protective layer-forming coating material in the present invention is as follows.

  The coating material A is 24 parts by mass of Titanium Oxide R-62N manufactured by Sakai Chemical Industry Co., Ltd., 33 parts by mass of acrylic resin “Acrynal” (registered trademark) TZ # 7501 manufactured by Toei Kasei Co., Ltd., manufactured by Sumika Bayer Urethane Co., Ltd. 3 parts by weight of “Sumijoule” (registered trademark) N3300, 50 parts by weight of methyl ethyl ketone, and 50 parts by weight of 1-methoxy-2-propanol were measured using a paint shaker (using glass beads with a diameter of 2 mm). By stirring for a minute, an ultraviolet protective layer-forming coating material having a solid content concentration of 25% by mass was obtained.

  The paints B to H were prepared in the same manner as paint A with the formulation according to Table 1.

  The adjustment of the adhesive for dry lamination in the present invention is as follows.

  12 parts by weight of dry laminate “Takelac” (registered trademark) A-310 (polyester polyurethane resin) manufactured by Mitsui Chemicals Polyurethanes Co., Ltd. “Takenate” which is an aromatic polyisocyanate compound manufactured by Mitsui Chemicals Polyurethanes Co., Ltd. ( (Registered Trademark) 1 part by weight of A-3 and 212 parts by weight of ethyl acetate were weighed and stirred for 15 minutes to obtain an adhesive for dry lamination having a solid content concentration of 3% by weight.

[Example 1]
As a polyester film, a hydrolysis-resistant polyethylene terephthalate film “Lumirror” (registered trademark) X10S (125 μm) manufactured by Toray Industries, Inc. was prepared. One side of this polyester film is subjected to corona treatment, and coating A is applied as a pre-drying coating amount of 14 g / m ² using a wire bar, and allowed to stand at room temperature for 5 seconds before drying. In an oven for 30 seconds, dried to provide an ultraviolet protective layer, and further subjected to aging at a temperature of 40 ° C. for 72 hours to promote the curing reaction of the ultraviolet protective layer, and to be used for a back protective sheet for solar cell modules. A processed film was obtained. The coating amount after drying of the ultraviolet protective layer was 3.5 g / m ² .

Furthermore, a dry laminating adhesive was applied to the opposite side of the UV protective layer of the coated film using a wire bar and dried at 150 ° C. for 30 seconds. After drying, the coating amount was 5 g / m ^2. Lamination with a white polypropylene film B011W (150 μm) manufactured by Film Processing Co., Ltd. was performed. The laminated film was aged at a temperature of 40 ° C. for 72 hours to promote the curing reaction of the adhesive layer, thereby obtaining a back surface protection sheet for a solar cell module.

  The evaluation results of the obtained back surface protective sheet for solar cell module are shown in Table 2.

[Example 2]
With coating material B instead of the coating A, 10.5 g / m ² was applied, the solar cell modules in the same way except that the drying after coating amount was 3.5 g / m ² and Example 1 as a pre-drying coating amount A back protective sheet was obtained. Compared with Example 1, since the solid content concentration of the paint was high, convection of the paint did not occur easily, the number of Benard cells in the UV protective layer was reduced, and the scratch resistance was good.

[Example 3]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that paint C was used in place of paint A. By blending a high boiling point solvent, the evaporation rate of the paint can be delayed, and the convection of the paint is less likely to occur, so the number of Benard cells is reduced and the scratch resistance is good.

[Example 4]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that paint D was used in place of paint A. By increasing the blending amount of the high boiling point solvent, the evaporation rate of the paint can be further delayed, and the convection of the paint becomes difficult to occur, so the number of Benard cells is reduced and the scratch resistance is good.

[Example 5]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that paint E was used in place of paint A. By blending a solvent having a higher boiling point, the evaporation rate of the coating material can be delayed, and the convection of the coating material is less likely to occur, so the number of Benard cells is reduced and the scratch resistance is good.

[Example 6]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that the time from application of the paint to introduction into the drying oven was 10 seconds. By elongating the time from application of the paint to the drying oven, the evaporation rate of the paint can be delayed, the temperature gradient in the paint becomes lower, and convection is less likely to occur, reducing the number of Benard cells, The scratch resistance was good.

[Comparative Example 1]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that the time from application of the paint to introduction into the drying oven was 2 seconds. Since the time from applying the paint to putting it into the drying oven has been shortened, the temperature gradient in the coating liquid has increased, and convection of the paint has become more likely to occur. The result was inferior to scratching.

[Comparative Example 2]
With paint F instead of the coating A, 19 g / m ² was applied as a pre-drying coating weight, except that the drying after coating amount was 3.5 g / m ² is the back side for a solar cell module in the same manner as in Example 1 A protective sheet was obtained. Since the coating amount before drying was large and convection of the paint was likely to occur, the number of Benard cells in the ultraviolet protective layer was increased, resulting in poor scratch resistance.

[Comparative Example 3]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that paint G was used in place of paint A. Since the low boiling point solvent was used as the diluent solvent, the evaporation rate was increased and the convection of the paint was liable to occur. As a result, the number of Benard cells in the UV protective layer increased, resulting in poor scratch resistance.

[Comparative Example 4]
A back protective sheet for a solar cell module was obtained in the same manner as in Example 1 except that paint H was used in place of paint A. Since a large amount of a high boiling point solvent having a low evaporation rate was blended in the diluting solvent, the glossiness was 45 to 70% and a large unevenness occurred depending on the location, and the appearance was poor.

As is apparent from the results of the above examples and comparative examples, according to the method of the present invention, it has excellent scratch resistance, has light resistance that can withstand use in a harsh outdoor environment for a long time, When used in the outer layer, it can be used as a back surface protective sheet for solar cell modules having an ultraviolet protective layer excellent in long-term durability.

Claims (5)

A back protective sheet for a solar cell module comprising at least a polyester film and an ultraviolet protective layer provided on the surface thereof, wherein the ultraviolet protective layer contains a white pigment, and the number of Benard cells is from 400 / mm ^{2 to} 600 / The back surface protection sheet for solar cell modules characterized by being in a range of mm ² or less.   The back surface protective sheet for a solar cell module according to claim 1, wherein a dynamic friction coefficient of the surface of the ultraviolet protective layer is 0.2 or less.   The back surface protective sheet for a solar cell module according to claim 1 or 2, wherein the glossiness of the surface of the ultraviolet protective layer is in the range of 35% to 60%. The ultraviolet protective layer has a color difference (ΔE ^* ) of 3 or less after being irradiated with ultraviolet rays having an irradiation intensity of 1000 W / m ² for 316 hours in an atmosphere of a temperature of 63 ° C. and a relative humidity of 30%, and the elongation at break is maintained. The back surface protection sheet for solar cell modules according to any one of claims 1 to 3, wherein the percentage is 50% or more. The said white pigment is a titanium oxide, The back surface protection sheet for solar cell modules in any one of Claim 1 to 4 characterized by the above-mentioned.