JP2012178518A - White polyester film for protecting rear surface of solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a white polyester film for protecting a rear surface of a solar cell including, as a hydrolysis resistance improver, a carbodiimide-based compound, the film having excellent hydrolysis resistance, suppressing serious yellowing caused by UV, preventing a carbodiimide-based compound from bleeding out, and suppressing degradation in mechanical properties and degradation in adhesion between the film and a filler or an adhesive of the solar cell even in use for a long period of time under severe natural environment such as high temperature and high humidity.SOLUTION: A white polyester film for protecting a rear surface of a solar cell is a biaxially oriented polyester film having a three-layered structure composed of a base material layer (B) and surface layers (A) disposed on both faces of the base material layer. The surface layers (A) are made of a thermoplastic polyester composition including a UV absorber and no carbodiimide-based compound, and each of the surface layers (A) has a thickness of 3.0 μm or more. The base material layer (B) includes a carbodiimide-based compound and a white pigment, and Δb*, the difference between values before and after irradiating the biaxially oriented polyester film with xenon for 14 days, is 7.0 or less.

Description

  The present invention relates to a white polyester film for protecting the back surface of a solar cell. More specifically, it is a white solar cell back surface protective film containing a carbodiimide compound as a hydrolysis resistance improver, has excellent hydrolysis resistance, improved bleedout of the carbodiimide compound, and is colored by ultraviolet rays. The present invention relates to a white polyester film for protecting the back surface of a solar cell in which the suppression is suppressed.

In recent years, a solar power generation system using a solar cell module has been spread as one of power generation means using clean energy. The back surface protective film of the solar cell is mainly required to have a protective function for the solar cell module, to have excellent mechanical properties, and to have various properties such as weather resistance, heat resistance and hydrolysis resistance. Yes.
As such a solar cell back surface protective film, a plastic sheet having excellent strength characteristics is used, and in particular, a fluorine-based resin film is widely used. Recently, a polyethylene terephthalate film is used as a back surface protective film of such a solar cell. Various studies have been made (for example, Patent Document 1).

  Further, it has been studied to improve the photoelectric conversion efficiency of the battery by converting the reflected light into electricity by enhancing the concealing property such as coloring the back surface protective film of the solar battery white. For example, Patent Document 2 proposes a thermoplastic resin sheet for solar cells having a thermoplastic resin layer having a number average molecular weight of 18500 to 40,000 and containing 5 to 40% by weight of titanium dioxide based on the entire layer. A three-layer film using a single layer film or a thermoplastic resin layer containing a large amount of titanium dioxide as both surface layers is disclosed.

  However, a film using a polyethylene terephthalate film is relatively inexpensive, but heat resistance and hydrolysis resistance are not sufficient among plastic films. The back protection film for solar cells is particularly demanded to improve hydrolysis resistance, and various proposals have been made so far. For example, in Patent Document 2, a thermoplastic resin containing polyester is increased in molecular weight. It is described that the hydrolysis resistance is improved.

Further, as another method for improving the hydrolysis resistance of a polyester film, it has been studied to add various hydrolysis resistance improvers such as a hydrolysis resistance improver having an epoxy group or a carbodiimide compound to the film. .
For example, Patent Document 3 discloses a polyester film to which a monomer or polymer of a carbodiimide compound is added, and includes a carbodiimide compound as a three-layer laminated film only in the base layer of the inner layer, and a carbodiimide compound as both outer layers. A film having a layer of at most 1.5 μm free of is described.

  However, when a carbodiimide-based compound is used as a hydrolysis resistance improver, although hydrolysis resistance is improved, in the film manufacturing process or processing process, gas due to isocyanate or other by-products and decomposition products may be generated. In particular, a molded product having a large surface area such as a film is more likely to generate gas than an injection molded product.

  In addition, while the inventors proceeded with the study of the carbodiimide-based compound for the solar cell back surface protective film, the low-molecular-weight component of the carbodiimide remaining in the carbodiimide-based compound without remaining completely high in molecular weight And the like bleed out from the film, and it was newly found that peeling occurs between the solar cell filler or adhesive and the back surface protective film.

  In addition, the solar cell module is likely to have a problem of weather resistance, in particular, deterioration due to ultraviolet rays, and various studies have been made on a solar cell back surface protective film using a polyester film. For example, an ultraviolet absorber is added to a single-layer polyester film (Patent Literature 1, Patent Literature 4 and the like), an ultraviolet absorber is added to each layer of a white polyester film (Patent Literature 2), and the polyester film surface absorbs ultraviolet rays. Providing a layer having a function (Patent Document 4) has been studied. In addition, in Patent Document 3 in which it is proposed to use a carbodiimide compound, an ultraviolet stabilizer such as hydroxybenzophenone is used instead of an ultraviolet absorber because an ultraviolet absorber-containing film has poor weather resistance and the film rapidly turns yellow. It has been proposed to add.

  However, when a carbodiimide compound is used to improve hydrolysis resistance, not only polyester but also carbodiimide compound is easily yellowed with respect to ultraviolet rays, and it is difficult to suppress coloring with an ultraviolet absorber.

JP 2009-188105 A JP 2006-270025 A JP 2002-187965 A JP 2006-261189 A

  The present invention has been made paying attention to the above-mentioned problems, and in a white solar cell back surface protective film containing a carbodiimide-based compound as a hydrolysis resistance improver, it has excellent hydrolysis resistance and is remarkable due to ultraviolet rays. Suppresses yellowing, improves bleed-out of carbodiimide compounds, reduces mechanical properties even when used for a long time in harsh natural environments such as high temperature and high humidity, and with solar cell fillers and adhesives It is providing the white polyester film for solar cell back surface protection in which the adhesive fall was suppressed.

  Another subject of the present invention is a white solar cell back surface protective film containing a carbodiimide-based compound as a hydrolysis resistance improver, which is excellent in hydrolysis resistance and suppresses significant yellowing due to ultraviolet rays. Improved bleed-out of chemical compounds, preventing deterioration of mechanical properties and adhesiveness with fillers and adhesives even when used for long periods in harsh natural environments such as high temperatures and high humidity. Alternatively, it is an object of the present invention to provide a white polyester film for protecting a back surface of a solar cell in which film cohesive failure due to an influence of a member or the like laminated when being modularized by being incorporated in a solar cell is improved.

  As a result of intensive studies to solve the above problems, the present inventors have used a carbodiimide compound and a white pigment when using a carbodiimide compound as a hydrolysis resistance improver in a polyester film for protecting a back surface of a solar cell containing a white pigment. By providing a polyester layer that contains substantially no carbodiimide compound on both sides of the polyester layer, and making the layer thickness thicker than before, it can be used as a solar cell back surface protection film in harsh natural environments such as high temperatures and humidity. Even if it is used for a long time, the deterioration of mechanical properties is suppressed, the deterioration of the adhesiveness with the filler and adhesive of the solar cell is suppressed, and further, the carbodiimide-containing polyester film is used by using an ultraviolet absorber for the surface layer. In order to complete the present invention. Was Tsu.

  That is, an object of the present invention is a biaxially oriented polyester film having a three-layer structure comprising a base material layer (B) and a surface layer (A) provided on both sides thereof, and the surface layer (A) contains an ultraviolet absorber. And a thermoplastic polyester composition not containing a carbodiimide compound, the surface layer (A) has a thickness of 3.0 μm or more, and the base layer (B) comprises a carbodiimide compound and a white pigment. And the biaxially oriented polyester film is achieved by a white polyester film for protecting a back surface of a solar cell having Δb * of 7.0 or less before and after irradiation for 14 days in an environment of xenon irradiation.

  Moreover, the white polyester film for solar cell back surface protection of this invention WHEREIN: As a preferable aspect, content of this ultraviolet absorber is 0.1 weight part or more and 10 weight part or more with respect to 100 weight part of thermoplastic polyester which comprises surface layer (A). Less than parts by weight, and the number average molecular weight of the ultraviolet absorber is 500 or more and 1500 or less, and the thermal weight decrease when the ultraviolet absorber is heated from room temperature to 250 ° C. at a temperature increase of 10 ° C./min. The ratio is 1% or less, and the content of the carbodiimide compound in the base material layer (B) is 0.3 parts by weight or more with respect to 100 parts by weight of the thermoplastic polyester constituting the base material layer (B). The content of the white pigment in the base material layer (B) is 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the thermoplastic polyester constituting the base material layer (B). The content of the white pigment in the surface layer (A) is 0 to 2 parts by weight with respect to 100 parts by weight of the thermoplastic polyester constituting the surface layer (A), and the white pigment is titanium oxide. The intrinsic viscosity of the thermoplastic polyester constituting the white polyester film for protecting the back side of the solar cell is in the range of 0.55 dl / g or more and 0.80 dl / g or less, and the terminal carboxyl group concentration of the thermoplastic polyester is 5 equivalents. Including those having at least one of the following: / ton or more and 25 equivalents / ton or less, and the carbodiimide compound is biscarbodiimide or aromatic polycarbodiimide.

  According to the present invention, even when used for a long time in a harsh natural environment such as high temperature and high humidity, deterioration in mechanical properties and coloration due to ultraviolet rays are suppressed, and at the same time, adhesion to solar cell fillers and adhesives The white polyester film for solar cell back surface protection which consists of thermoplastic polyester in which the fall of property was suppressed can be provided.

Hereinafter, the present invention will be described in detail.
[Thermoplastic polyester]
Obtained by polycondensation of an aromatic dicarboxylic acid component or an ester-forming derivative component thereof and a diol component or an ester-forming derivative component thereof as a thermoplastic polyester constituting each layer of the white polyester film for protecting a solar cell back surface of the present invention And thermoplastic polyesters.

Examples of the aromatic dicarboxylic acid component include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 4,4′-diphenyldicarboxylic acid. Examples of the diol component include ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and 1,6-hexanediol.
As the thermoplastic polyester, polyethylene terephthalate and polyethylene-2,6-naphthalenedicarboxylate are preferable. The thermoplastic polyester may be a homopolymer and may be a copolymer or a blend thereof, but a homopolymer is preferably used from the viewpoint of hydrolysis resistance. Further, the thermoplastic polyester of the surface layer and the base material layer may be of different types or the same.

The intrinsic viscosity of the thermoplastic polyester used as the raw material for the white polyester film for protecting the back surface of the solar cell of the present invention is preferably 0.50 dl / g to 0.90 dl / g, more preferably 0.55 dl / g for any layer. g to 0.85 dl / g. From the viewpoint of hydrolysis resistance, it is better that the thermoplastic polyester constituting the film has a higher intrinsic viscosity. For this purpose, it is preferable to produce a film using a thermoplastic polyester raw material having a higher intrinsic viscosity. However, if the intrinsic viscosity of the raw material is too high, the melt viscosity of the resin is high, so that melt extrusion becomes difficult and the polymerization time of the raw material may be long. Therefore, in consideration of the balance between the hydrolysis resistance and productivity of the film, it is preferable to use a thermoplastic polyester having an intrinsic viscosity in the above range. As a method for obtaining the thermoplastic polyester having such an intrinsic viscosity, it can be obtained by adjusting the polymerization time of the raw material, and further, in a solid phase state (solid phase polymerization), if necessary. Moreover, about the thermoplastic polyester which comprises a base material layer (B), polyester of this intrinsic viscosity number can be obtained also by using a carbodiimide compound.
The above intrinsic viscosity is a value calculated from a solution viscosity obtained by dissolving 0.6 g of a sample in 50 ml of orthochlorophenol by heating and then cooling the solution, and measuring the solution at 35 ° C. using an Ostwald viscosity tube. It is.

[Laminated film]
The white polyester film for protecting the back surface of the solar cell of the present invention is a biaxially oriented polyester film having a three-layer structure comprising a base layer (B) and a surface layer (A) provided on both sides thereof. Moreover, it is necessary to have the thickness of 3.0 micrometers or more per surface layer (A).
The carbodiimide compound used for imparting hydrolysis resistance partially contains a low molecular weight component even if it has a high molecular weight. Therefore, if the layer containing the carbodiimide compound is exposed on the film surface, the low molecular weight component bleeds out on the film surface. A film for protecting the back surface of a solar cell is usually provided with a filler layer such as EVA (ethylene vinyl acetate) and an adhesive layer for laminating a protective film, but there is a bleedout of a low molecular weight component of a carbodiimide compound. And the adhesiveness with these layers falls.
Furthermore, polyester resins and carbodiimide compounds are easily deteriorated by ultraviolet rays and easily turn yellow. From this point of view, if the layer containing the carbodiimide compound is exposed on the film surface, the film is likely to be colored.

In the present invention, in order to prevent such bleed out, a layer not containing a carbodiimide compound is provided on both sides of the base material layer (B) as a surface layer (A), and the thickness per surface layer is 3.0 μm or more, preferably 5 0.0μm or more suppresses bleed-out of low molecular weight components of carbodiimide compounds, and suppresses adhesive deterioration when bonded to a solar cell filler layer or adhesive layer as a solar cell back surface protective film. By doing so, the durability of the solar cell is enhanced. The upper limit value of the thickness per surface layer is not particularly limited, but is preferably 12.0 μm or less, and more preferably 10.0 μm or less, considering the thickness used as the solar cell back surface protective film. In addition, the thickness of the surface layer is the thickness after biaxial stretching.
In addition, when a polyester composition containing a carbodiimide compound is melt-extruded in a single layer, an isocyanate-based decomposition gas may be generated to irritate the mucous membrane. This suppresses the generation of irritating gas.
Further, in the present invention, by using a white pigment and further using an ultraviolet absorber for the surface layer, not only the deterioration of the thermoplastic polyester constituting the surface layer due to the ultraviolet rays is suppressed, but also the ultraviolet rays are transmitted to the base material layer. By preventing, yellowing due to deterioration of the thermoplastic polyester or carbodiimide compound constituting the base material layer can be effectively suppressed.

In order to impart hydrolysis resistance to the biaxially oriented polyester film of the present invention and to sufficiently suppress bleed out, the ratio of the thickness of the surface layer to the base material layer is determined as follows: surface layer / base material layer / surface layer = 1/6. / 1 to 1/12/1 is preferable. When the thickness of the base material layer with respect to the surface layer becomes relatively thin, the hydrolysis resistance by the base material layer may not be sufficiently exhibited.
The thickness of the biaxially oriented polyester film of the present invention is preferably 20 to 300 μm, more preferably 40 to 250 μm, and particularly preferably 50 to 200 μm.

[Surface (A)]
The surface layer (A) of the biaxially oriented polyester film of the present invention comprises a thermoplastic polyester composition containing an ultraviolet absorber and not containing a carbodiimide compound.
Here, “does not contain carbodiimide compounds” means that the surface layer contains no carbodiimide compounds or a tiny amount of carbodiimide compounds that bleed out to the surface even if they are contained. Means. For example, the carbodiimide compound is less than 0.1 parts by weight, and further 0.05 parts by weight or less with respect to 100 parts by weight of the thermoplastic polyester of the surface layer. Most preferably, the surface layer does not contain any carbodiimide compound.

(UV absorber)
One feature of the present invention is that the surface layer (A) constituting the biaxially oriented polyester film contains an ultraviolet absorber. By using an ultraviolet absorber for the surface layer, not only suppresses the deterioration of the thermoplastic polyester constituting the surface layer due to ultraviolet rays, but also prevents the ultraviolet rays from penetrating to the substrate layer, thereby forming the heat constituting the substrate layer. Yellowing due to deterioration of the plastic polyester or carbodiimide compound can be effectively suppressed.

The content of the ultraviolet absorber is preferably in the range of 0.1 to 10 parts by weight with respect to 100 parts by weight of the thermoplastic polyester constituting the surface layer (A). 5 parts by weight, more preferably the lower limit is 1 part by weight, the more preferred upper limit is 7 parts by weight, and the still more preferred upper limit is 5 parts by weight.
If the content of the ultraviolet absorber in the surface layer is less than the lower limit value, sunlight may hit the polyester resin on the surface layer and deteriorate the polyester resin on the surface layer. In some cases, ultraviolet rays are transmitted through the base material layer without being absorbed into the base material layer, and the polyester resin or carbodiimide compound in the base material layer may be deteriorated. On the other hand, if the content of the ultraviolet absorber in the surface layer exceeds the upper limit, the hydrolysis resistance of the biaxially oriented polyester film is reduced, or adhesion to EVA (ethylene vinyl acetate) which is a filler for solar cells. In some cases, the properties may deteriorate, or the film may turn yellow.

As the kind of the ultraviolet absorber, it is preferable to include one or both of a benzotriazole ultraviolet absorber and a triazine ultraviolet absorber.
Examples of the benzotriazole-based ultraviolet absorber include 2,2′-methylenebis [4- (1,1,3,3-tetramethylbutyl) -6-2H-benzotriazol-2-yl] phenol and 2,4-bis. And (1-methyl-1-phenylethyl) -6- (2H-benzotriazol-2-yl) phenol.
Examples of triazine ultraviolet absorbers include 2,4-diphenyl-6- (2-hydroxyphenyl-4-hexyloxyphenyl) -1,3,5-triazine, 2- (2-hydroxyphenyl-4- [2- Ethylhexyloxyphenyl])-4,6-bis (4-phenylphenyl) -1,3,5-triazine, 2- (2-hydroxy-4- [1-octyloxycarbonylethoxy] -4,6-bis ( 4-phenylphenyl) -1,3,5-triazine, and the like.

  The number average molecular weight of the ultraviolet absorber is preferably 500 or more and 1500 or less, and more preferably 600 is the lower limit. Such an ultraviolet absorber may have a high molecular weight by introducing a polymerizable group or a reactive group, or may be incorporated in a synthetic resin. By using an ultraviolet absorber having a molecular weight within this range, the amount of the ultraviolet absorber that is thermally decomposed or volatilized during production of the film can be kept low, and a sufficient effect of preventing deterioration due to ultraviolet rays can be obtained. it can. Further, the amount of bleeding out on the film surface can be suppressed to a small extent, and the adhesiveness is not affected.

  In the ultraviolet absorbent according to the present invention, the thermal weight reduction rate of the ultraviolet absorbent when heated at a temperature of 10 ° C./min from room temperature to 250 ° C. is preferably 1% or less, and 0.5% or less. More preferably it is. If the thermal weight loss rate measured under these conditions exceeds the upper limit, the UV absorber may be thermally decomposed or volatilized during film production, adversely affecting the manufacturing process, or obtaining the desired UV absorber concentration. May not be possible.

  The melting point of the ultraviolet absorber in the present invention is preferably 150 to 300 ° C. When the melting point is less than 150 ° C., the heat resistance is inferior and the polyester composition is likely to be thermally decomposed during melt kneading. On the other hand, when the melting point of the ultraviolet absorber exceeds 300 ° C., the solubility in polyester tends to be insufficient, and dispersion failure tends to occur.

(Other additives)
Further, a white pigment may be used for the surface layer (A) as long as it is a small amount as an additive, and the white pigment is added in an amount of 2 parts by weight or less based on 100 parts by weight of the thermoplastic polyester constituting the surface layer. It is preferable to contain. The upper limit of the white pigment is preferably 1.8 parts by weight, more preferably 1.5 parts by weight, still more preferably 1.0 parts by weight, particularly preferably 0.8 parts by weight, and most preferably 0.5 parts by weight. Part. On the other hand, when such a white pigment is used, the lower limit of the content is preferably 0.05 parts by weight or more, and more preferably 0.1 parts by weight or more.

  By using the white pigment also for the surface layer, it is possible to suppress deterioration of the surface layer polyester and the deterioration of the polyester or carbodiimide compound in the base layer due to ultraviolet rays together with the ultraviolet absorber. If the content of the white pigment is less than the lower limit, the synergistic effect on the ultraviolet rays by the white pigment may not be sufficiently exhibited. On the other hand, when the content of the white pigment exceeds the upper limit, the hydrolysis resistance of the polyester film is reduced, the adhesiveness with EVA that is a filler of the solar cell is reduced, the film is peeled off, or When incorporated into a solar cell and modularized, the influence of a member or the like laminated may cause an influence such as film cohesive failure.

  The white pigment is not particularly limited as long as it has an effect of blocking the transmission of sunlight, and examples thereof include particles of titanium oxide, barium sulfate, calcium carbonate, silicon dioxide, alumina, etc. Among these, titanium oxide, particularly By using rutile-type crystal-based titanium oxide, light can be efficiently absorbed with respect to the absorption wavelength that deteriorates the polyester film, and degradation of the polyester can be prevented and deterioration with time can be suppressed.

  The white polyester film for protecting the back surface of the solar cell of the present invention may contain various conventionally known additives in order to further improve the performance as necessary, for example, lubricants, antioxidants, antistatic agents, difficult A flame retardant and a coloring agent can be added. As an antioxidant, a hindered phenol compound can be illustrated, for example. These additives may be blended in at least one layer of the laminated film, or a coating layer may be provided on the laminated film and blended in the coating layer.

[Base material layer (B)]
The base material layer (B) of the biaxially oriented polyester film of the present invention is a layer comprising a thermoplastic polyester composition containing a carbodiimide compound and a white pigment. As the thermoplastic polyester of the thermoplastic polyester composition, the type of thermoplastic polyester described in the surface-layer thermoplastic polyester can be used. The thermoplastic polyester of the base material layer may be the same as or different from the thermoplastic polyester of the surface layer (A).

(Carbodiimide compounds)
In the present invention, the carbodiimide compound is contained in the base material layer (B). The carbodiimide compound has high reactivity with the thermoplastic polyester, and can impart high hydrolysis resistance required for the back surface protective film of the solar cell.
The carbodiimide-based compound is preferably contained in the range of 0.3 to 2.5 parts by weight, more preferably 0.6 to 100 parts by weight of the thermoplastic polyester constituting the base material layer (B). It is not less than 1.5 parts by weight.

  If the content of the carbodiimide compound is less than the lower limit, the effect as a hydrolysis resistance improver may not be sufficiently exhibited. On the other hand, using a carbodiimide compound exceeding the upper limit not only provides no further hydrolysis resistance effect, but also decreases the production efficiency due to thickening of the thermoplastic polyester, and the film is colored yellow. Or excessive carbodiimides may react with each other to form foreign matters in the film.

As the carbodiimide-based compound, biscarbodiimide or aromatic polycarbodiimide is preferably used, and a compound represented by R ₁ —N═C═N—R ₂ is preferably used. R ₁ and R ₂ are each independently a linear hydrocarbon group having 1 to 12 carbon atoms, an alicyclic hydrocarbon group having 6 to 20 carbon atoms, or an aromatic hydrocarbon group having 6 to 14 carbon atoms. The compound represented by the above formula may be a polymer linked via R ₁ and R ₂ .

  Examples of the linear hydrocarbon group include an alkyl group having 1 to 12 carbon atoms and an alkenyl group having 1 to 12 carbon atoms. Examples thereof include a methyl group, an ethyl group, a propyl group, a butyl group, a hexyl group, a propenyl group, a butynyl group, and a hexynyl group. Examples of the alicyclic hydrocarbon group include cycloalkyl groups having 6 to 20 carbon atoms, such as a cyclohexyl group and a cyclooctyl group. Examples of the aromatic hydrocarbon group having 6 to 14 carbon atoms include a phenyl group and a naphthyl group. The aromatic hydrocarbon group may be substituted with an alkyl group having 1 to 8 carbon atoms such as a methyl group, an ethyl group, a propyl group, or a butyl group.

  Such carbodiimide compounds include N, N′-diisopropylcarbodiimide, N, N′-di-n-butylcarbodiimide, N, N′-di-n-hexylcarbodiimide, N, N′-dicyclohexylcarbodiimide, N, N ′. -Diphenylcarbodiimide, N, N'-bis (2-methylphenyl) carbodiimide, N, N'-bis (2-ethylphenyl) carbodiimide, N, N'-bis (2-isopropylphenyl) carbodiimide, N, N ' -Bis (2,6-dimethylphenyl) carbodiimide, N, N'-bis (2,6-diethylphenyl) carbodiimide, N, N'-bis (2,6-diisopropylphenyl) carbodiimide, N, N'-bis (2,6-dimethoxyphenyl) carbodiimide, N, N′-bis (2,4,6- Limethylphenyl) carbodiimide, carbodilite LA-1 (registered trademark), carbodilite HMV-8CA (registered trademark), stabuxol I (registered trademark), stabuxol P (registered trademark), benzene-2,4-diisocyanate-1, 3,5-tris (1-methylethyl) carbodiimide (trade name, Stavaxol P100 (registered trademark)) and the like can be mentioned.

  Among them, those having a molecular weight of 5000 or more are particularly preferable, because this is very good not only from the viewpoint of hydrolysis resistance under a humid environment but also from the viewpoint of hydrolysis resistance under a high temperature environment exceeding 100 ° C. is there. Among the preferred compounds, benzene-2,4-diisocyanate-1,3,5-tris (1-methylethyl) carbodiimide (trade name, Stavaxol P100 (registered trademark)) is particularly preferred.

  As a method of adding the carbodiimide compound to the base material layer, a masterbatch containing a high concentration of carbodiimide compound is produced, and this masterbatch and a thermoplastic polyester containing no carbodiimide compound are melt-kneaded to obtain a predetermined amount. A method for producing a composition adjusted to the content of the carbodiimide compound is preferred. The concentration of the carbodiimide compound in the master batch is preferably in the range of 5% by weight to 20% by weight of the carbodiimide compound with respect to the total weight of the master batch. If the carbodiimide compound concentration is higher than 20% by weight, the viscosity may increase in the production process of the masterbatch and the production efficiency may be reduced, or excessive carbodiimides may react with each other to become foreign substances. On the other hand, when the concentration of the carbodiimide compound is low, the concentration added to the target polyester film cannot be satisfied, and the effect as a hydrolysis resistance improver may not be sufficiently exhibited. The concentration of the carbodiimide compound in the masterbatch is more preferably in the range of 10 to 17% by weight, and most preferably 15% by weight, based on the total weight of the masterbatch. As another method, it is also possible to use a method in which a carbodiimide compound is heated and melted in a liquid state and added directly in the middle of the extruder of the base material layer (B).

(White pigment)
The base material layer (B) in the present invention is a layer containing a white pigment. By using a biaxially oriented polyester film containing a white pigment as a solar cell back surface protective film, it absorbs and / or reflects sunlight, and there is little discoloration over time or reduction in film strength due to sunlight irradiation. Strength retention can be increased. At the same time, the back reflection of sunlight can be increased, and the conversion efficiency of the solar cell can be increased. Moreover, by using such a white pigment for the base material layer (B), the amount of white pigment in the surface layer can be reduced, and film cohesive failure during film peeling can be suppressed.

  Examples of such white pigments include titanium oxide, barium sulfate, and calcium carbonate, and titanium oxide is particularly preferable. Since the solar cell back surface protective film is used outdoors for many years, it is preferably a material that enhances absorption and / or reflection of sunlight and is excellent in light resistance. Titanium oxide, particularly rutile crystalline titanium oxide is used. Thus, light can be efficiently absorbed with respect to an absorption wavelength that deteriorates the polyester film, decomposition of the polyester can be prevented, and deterioration with time can be effectively suppressed.

The content of the white pigment contained in the base material layer (B) is preferably 1 part by weight or more and 10 parts by weight or less, more preferably 100 parts by weight of the thermoplastic polyester constituting the base material layer (B). It is 2 to 7 parts by weight, more preferably 3 to 6 parts by weight.
If the content of the white pigment is less than the lower limit, the amount sufficient to absorb and / or reflect sunlight is not satisfied, the polyester film is deteriorated, and the strength of the film may be reduced or yellowing may be affected. In addition, the back surface reflection efficiency of sunlight may not be sufficient. On the other hand, when the content of the white pigment exceeds the upper limit, the strength of the film after deterioration with time may be reduced, leading to a decrease in hydrolysis resistance.

[Intrinsic viscosity and terminal carboxyl group concentration]
The intrinsic viscosity of the thermoplastic polyester constituting the white polyester film for protecting the back surface of the solar cell is preferably in the range of 0.55 dl / g to 0.80 dl / g, more preferably 0.60 dl / g to 0.80 dl / g. The following is preferred. Moreover, it is preferable that the terminal carboxyl group density | concentration of the thermoplastic polyester which comprises the white polyester film for solar cell back surface protections is the range of 5 equivalent / ton or more and 25 equivalent / ton or less, and also 5 equivalent / ton or more and 20 equivalent / ton or less. More preferably, it is the range. The thermoplastic polyester constituting the white polyester film for protecting the back surface of the solar cell has a higher intrinsic viscosity, and the lower the terminal carboxyl group concentration, the better the hydrolysis resistance. Such intrinsic viscosity and terminal carboxyl group concentration refer to the properties of the thermoplastic polyester of the entire film after film formation.

The intrinsic viscosity of the thermoplastic polyester after film formation is also measured using the method described in the above-mentioned thermoplastic polyester. Further, the terminal carboxyl group concentration mentioned here is obtained by dissolving 10 mg of a sample in 0.5 ml of a mixed solvent of HFIP (hexafluoroisopropanol): deuterated chloroform = 1: 3, adding a few drops of isopropylamine, and a ¹ H-NMR method. It is determined quantitatively by (50 ° C., 600 MHz).
Such a terminal carboxyl group concentration can be obtained by performing solid phase polymerization after polymerization of the thermoplastic polyester and further reacting with the carbodiimide compound of the present invention.

[Hydrolysis resistance]
The white polyester film for protecting the back surface of the solar cell of the present invention preferably has a half-life of 80 hours or more when the elongation at break before and after aging in an environment of a temperature of 121 ° C. and a humidity of 100% RH is reduced to 50%. . By having such a half-life, good mechanical strength can be maintained even when the film of the present invention is used outdoors for a long time. If the rupture elongation retention rate is less than the lower limit value, the hydrolysis resistance is insufficient, which may cause deterioration in long-term outdoor use and may not maintain sufficient strength as a solar cell back surface protective film. In order to make the breaking elongation retention of the polyester film within such a range, the base layer (B) contains a carbodiimide compound, and the limiting viscosity number and terminal carboxyl group concentration of the thermoplastic polyester of the base layer (B) are set. It is preferable to manufacture within the predetermined range and under the manufacturing conditions described in the present invention.

  The 80-hour aging in an environment of a temperature of 121 ° C. and a humidity of 100% RH is an accelerated test for examining hydrolysis resistance corresponding to an outdoor exposure state for about 40 years. The elongation retention before and after this aging is 50%. If it is less than the above, there is a possibility that the mechanical properties may deteriorate due to deterioration due to insufficient hydrolysis resistance and long-term outdoor use.

[Light resistance]
The white polyester film for protecting the back surface of the solar cell of the present invention has light resistance such that Δb * before and after irradiation for 14 days in an environment of xenon irradiation is 7.0 or less, preferably 5.0 or less. When Δb * is within such a range, yellow coloring over time when used outdoors is suppressed, and excellent light resistance can be maintained.
This Δb * is a xenon lamp (made by Atlas, trade name “Suntest CPS +”, irradiation intensity 550 W / m ² (irradiation wavelength: 300 to 800 nm)), and the film is irradiated with xenon for 14 days and irradiated with xenon. The amount of change in the b * value (Δb * value) of the film before and after is measured using an automatic color difference meter (SE-Σ90 type) manufactured by Nippon Denshoku Industries Co., Ltd.

  Such light resistance can be obtained by using the layer structure and the white pigment of the present invention, further containing a carbodiimide compound in the base material layer, and containing an ultraviolet absorber in the surface layer. Further, the effect is enhanced by increasing the amount of the ultraviolet absorber contained in the surface layer within the range of the present invention, and the effect is further enhanced by using titanium oxide as a white pigment.

[Adhesiveness]
The white polyester film for protecting the back surface of the solar cell of the present invention has an adhesive strength with EVA (ethylene vinyl acetate) in the range of 15 to 20 N / 15 mm, and further aging for 48 hours in an environment of temperature 121 ° C. and humidity 100% RH. After that, it is preferable to maintain an adhesive strength of 10 N / 15 mm or more. This adhesiveness can be achieved by providing a surface layer substantially free of a carbodiimide compound at a thickness of 3.0 μm or more per layer. On the other hand, if the thickness of the surface layer is less than 3.0 μm per layer, the low molecular weight component of the carbodiimide compound easily passes through the surface layer to the film surface from the base material layer containing the carbodiimide compound, and is initially In some cases, it may be difficult to maintain the adhesive strength after aging at 10 N / 15 mm or more. Further, the use of a carbodiimide compound for the surface layer substantially contributes to a decrease in adhesiveness.

[Solar cell back surface protective film]
The white biaxially oriented polyester film of the present invention is used for a film that protects the back surface of a solar cell, usually a member called a solar cell back surface protective film or a solar cell back sheet, and is used as a single film of the present invention. It may also be used as a composite with other films. On the biaxially oriented polyester film of the present invention, a sealing resin layer for a solar cell element such as EVA (ethylene vinyl acetate) is provided. The biaxially oriented polyester film of the present invention and the sealing resin layer may be bonded together via an adhesive layer. Moreover, you may provide an easily bonding layer in the film of this invention for the purpose of improving the adhesiveness of the biaxially-oriented polyester film of this invention, sealing resin, and an adhesive layer.

By using the white biaxially oriented polyester film of the present invention as a solar cell back surface protective film, it is excellent in hydrolysis resistance, and markedly yellowing and bleeding out due to ultraviolet rays are improved while using a carbodiimide-based compound. Even if it is used for a long time in a harsh natural environment such as the above, a decrease in mechanical properties and a decrease in adhesion with a filler or an adhesive can be suppressed, and the durability of the solar cell element can be improved.
In addition, in the range where the white pigment contained in the surface layer of the biaxially oriented polyester film is 2 parts by weight or less with respect to the thermoplastic polyester, after the terminal box or the like is pasted with an adhesive, the film is cohesively broken by its weight. The terminal box can be attached without any occurrence. In addition, it can improve the adhesion to EVA and other fillers and adhesives that are important as a solar cell back surface protective film, and can reduce the adhesion even when used in harsh natural environments such as high temperature and humidity for a long time. Since it can suppress, a solar cell element can be protected and durability can be improved.

[Production method]
The manufacturing method of the thermoplastic polyester used as a raw material of the white polyester film for solar cell back surface protection of this invention is demonstrated. Here, a method for producing a polyester having ethylene terephthalate as a main repeating unit will be described as an example. First, there is a method in which an aromatic dicarboxylic acid ester represented by dimethyl terephthalate and a diol represented by ethylene glycol and, if necessary, a copolymerization component are reacted by a transesterification reaction and then a polycondensation reaction is performed. In this production process, the transesterification is carried out while removing the generated alcohol, and then the phosphoric acid compound is added to substantially complete the transesterification, and then the reaction product obtained is added to an antimony compound and a titanium compound. A polycondensation reaction is performed by adding a metal catalyst. In order to obtain a higher hydrolysis-resistant polyester film, it is important to increase the intrinsic viscosity of the thermoplastic polyester and lower the terminal carboxyl group concentration. Is preferred.
In addition to the above ester exchange reaction, a method in which an aromatic dicarboxylic acid typified by terephthalic acid and a diol typified by ethylene glycol, and if necessary, a copolymerization component are esterified, followed by a polycondensation reaction May be used.

The biaxially oriented polyester film used for protecting the back surface of the solar cell of the present invention can be produced according to a conventionally known film forming method. As an example, the case where both the surface layer and the base material layer are composed of a polyethylene terephthalate composition is shown below. The melting point of the polymer may be expressed as Tm and the glass transition temperature as Tg.
First, a master material of an additive such as a polyester raw material, a carbodiimide compound, a powdery additive, and the like are separately prepared for the surface layer and the base material layer, blended to a predetermined blending ratio, and then dried as necessary. Subsequently, each raw material is melted and mixed at a temperature of 280 to 300 ° C. in separate extruders to form a multilayer structure by a simultaneous multilayer extrusion method using a feed block, and the polymer extruded from the slit die is cooled and solidified by a casting drum. And an unstretched laminated sheet is formed. That is, the polymer melt constituting the surface layer (A) and the polymer melt constituting the base material layer (B) are mixed into a surface layer (A) / base material layer (B) / surface layer (A) using a feed block. It laminates in 3 layers so that it may become, it develops on a slit die, and extrusion is carried out. At this time, the polymer laminated by the feed block maintains the laminated form. If the melt mixing temperature is less than 280 ° C., the resin is not sufficiently melted and the load on the extruder tends to be high. On the other hand, when the temperature for melting and mixing exceeds 300 ° C., the deterioration of the resin proceeds, and as a result, the hydrolysis resistance of the film may be reduced.

  Next, the obtained unstretched laminated sheet is stretched in the biaxial direction. The stretching method may be sequential biaxial stretching or simultaneous biaxial stretching. For example, when performing sequential biaxial stretching, this unstretched laminated sheet is heated by roll heating, infrared heating, or the like, and may be referred to as the longitudinal direction (hereinafter referred to as film continuous film forming direction, longitudinal direction, MD direction). ) To obtain a longitudinally stretched film. This stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. The stretching temperature is preferably a temperature equal to or higher than the glass transition point (Tg) of the polyester, and more preferably (Tg) ° C. to (Tg + 70) ° C. The film after longitudinal stretching is successively subjected to transverse stretching, heat fixing, and thermal relaxation treatment as necessary in a direction perpendicular to the longitudinal direction (hereinafter sometimes referred to as a transverse direction, a width direction, and a TD direction). The biaxially oriented film is applied, and these treatments are performed while the film is running. The transverse stretching starts from a temperature higher than the glass transition point (Tg) of the polyester and is performed while raising the temperature from (Tg + 5) ° C. to (Tg + 70) ° C. Although the temperature increase in the transverse stretching process may be continuous or stepwise (sequential), the temperature is generally increased sequentially. For example, the transverse stretching zone of the tenter is divided into a plurality along the film running direction, and the temperature is raised by flowing a heating medium having a predetermined temperature for each zone. The draw ratio is preferably 2.8 to 4.5 times, more preferably 3.0 to 4.0 times in both the longitudinal direction and the transverse direction. If the stretching ratio is less than the lower limit, film thickness unevenness may occur, or the contribution to hydrolysis resistance due to stretching orientation may not be sufficient. On the other hand, when the draw ratio exceeds the upper limit, breakage tends to occur during film formation.

The film after transverse stretching is dimensionally stable when heat treatment is performed at (Tm-55) ° C. to (Tm-15) ° C. with a constant width or a decrease in width of 10% or less, and the thermal shrinkage ratio is lowered while holding both ends. Sexuality is improved. When the heat treatment is performed at a temperature exceeding the upper limit, the flatness of the film is deteriorated, and the thickness unevenness may be increased. In addition, when heat treatment is performed at a temperature lower than the lower limit value, the effect of reducing the heat shrinkage rate may not be sufficiently exhibited, and when a terminal box or the like is attached to the solar cell back surface protective film of the present invention with an adhesive, etc. Cohesive failure may occur easily.
After heat setting, for example, as described in JP-A-57-57628, in the process of returning the film temperature to room temperature, both ends of the gripped film are cut off to adjust the film take-up speed in the vertical direction. Can be relaxed in the vertical direction. Examples of the means for relaxing include a method of adjusting the speed of the roll group on the tenter exit side. As the rate of relaxation, the speed of the roll group is reduced with respect to the film line speed of the tenter, preferably 1.0 to 4.0%, more preferably 1.2 to 3.5%. It is possible to adjust the heat shrinkage rate in the longitudinal direction by relaxing the film (this value of speed reduction is referred to as the relaxation rate) and controlling the relaxation rate. In addition, in order to increase the dimensional stability in the lateral direction, a desired heat shrinkage rate can be obtained by reducing the width of the film being gripped in the process until the both ends of the film are cut off.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example. Each characteristic value was measured by the following method.

(1) Composition analysis If necessary, the surface layer and the core layer are scraped off, and 5 mg of a sample is dissolved in 0.5 ml of heavy trifluoroacetic acid / deuterated chloroform = 1/1 mixed solvent, and ¹ H-NMR method (50 ° C., 600 MHz). Thus, the composition of the polymer and the organic additive in each layer was specified.

(2) Content of carbodiimide compound The content of the carbodiimide compound was calculated from the spectrum obtained by the ¹ H-NMR method described above.

(3) Layer configuration A sample was cut into a triangle, fixed in an embedded capsule, and then embedded in an epoxy resin. And after making the cross section parallel to the surface containing a vertical direction and a thickness direction into a 50-nm-thick thin-film section | slice with the microtome (ULTRACUT-S) for the embedded sample, the transmission electron microscope (S-4300 by Hitachi, Ltd.) was used. The film was observed and photographed at an accelerating voltage of 100 kv, and the thickness of each layer was measured 10 points from the photograph, and the average thickness of each layer was determined.

(4) Content of white pigment In the case of a single layer film, in the case of a laminated polyester film, about 10 g of each layer is scraped and sampled. A solvent is dissolved so that the polyester dissolves and the pigment does not dissolve, and the sample is dissolved. After that, the pigment was centrifuged from the polyester, and the content was calculated from the ratio of the pigment weight to the sample weight. Identification of the pigment species can be performed using SEM-XMA, quantitative analysis of metal elements by ICP, or the like.

(5) Film thickness Ten thicknesses of the film sample were measured with an electric micrometer (K-402B manufactured by Anritsu), and the average value was defined as the film thickness.

(6) Intrinsic viscosity number (IV)
The value calculated by the following formula was used from the solution viscosity measured at a temperature of 35 ° C. after being dissolved in a phenol: trichloroethane mixed solvent having a weight ratio of 6: 4.
ηsp / C = [η] + K [η] ² · C
Here, ηsp = (solution viscosity / solvent viscosity) −1, C is the weight of dissolved polymer per 100 ml of solvent (g / 100 ml), and K is the Huggins constant. The solution viscosity and solvent viscosity were measured using an Ostwald viscometer. The unit is indicated by [dl / g].

(7) Terminal carboxyl group concentration 10 mg of a sample was dissolved in 0.5 ml of a mixed solvent of HFIP (hexafluoroisopropanol): deuterated chloroform = 1: 3, a few drops of isopropylamine were added, and ¹ H-NMR method (50 ° C., (600 MHz).

(8) Thermal weight reduction rate of ultraviolet absorber As a method for measuring the thermal weight reduction rate, about 5 mg of a UV absorber sample was used, and a differential thermothermal weight simultaneous measurement device (manufactured by Seiko Denshi Kogyo Co., Ltd., trade name “TG” -DTA220 "), the temperature was increased from 25 ° C to 250 ° C at a rate of temperature increase of 10 ° C / min, and the thermal weight loss rate (unit:%) was measured.

(9) Hydrolysis resistance A strip-shaped sample piece cut to a length of 100 mm in the vertical direction and a width of 10 mm in the horizontal direction is left in an environmental test machine set at a temperature of 121 ° C. and a humidity of 100% RH for 72 hours. Aging was performed, and then a sample piece was taken out, and the elongation at break in the longitudinal direction of the sample was measured five times to obtain an average value. The tensile test was performed using Toyo Baldwin (trade name “Tensilon”), and the test was performed at a distance between chucks of 50 mm and a tensile speed of 50 mm / min. The value obtained by dividing the average value of the five points by the average value of the five points of elongation at break measured for the sample before being left in the environmental test machine is the elongation percentage at break [%], and the elongation at break after aging for 72 hours. The retention rate was evaluated as hydrolysis resistance according to the following criteria.
In addition, the hydrolysis resistance was judged to be good when the elongation at break elongation corresponding to ◎ or ○ was based on the following evaluation criteria.
<Evaluation criteria for breaking elongation retention (%) after aging at 121 ° C., 100% RH, 72 hours>
A: Breaking elongation retention is 80% or more B: Breaking elongation retention is 50% or more and less than 80% X: Breaking elongation retention is less than 50%

(10) Adhesive strength (initial value)
The film was sampled to a size of 200 mm × 200 mm, and a glass plate for solar cells (white glass made by AGC, 200 mm × 200 mm square, 3 mm thickness) and EVA sheet (Ultra Pearl PV Standard Cure Type, 170 mm × 170 mm square, thickness) 600 μm) and a film sample were laminated in this order, and laminated using a Nisshinbo small vacuum laminator (PVL0202S) to obtain a laminated sample. Subsequently, this laminated sample was aged in an oven at a temperature of 150 ° C. for 30 minutes. Then, a slit was made with a width of 15 mm from the film side, and the adhesive strength was measured by T-peeling at a tensile rate of 100 mm / min with a film of an unbonded part sandwiched using a tensile tester (unit: N / 15mm). Evaluation was made according to the following criteria.
A: Adhesive strength is 20 N / 15 mm or more, or a film piece is broken. O: Adhesive strength is 15 N / 15 mm or more and less than 20 N / 15 mm. X: Adhesive strength is less than 15 N / 15 mm.

(11) Adhesive strength (after wet heat treatment)
A sample was prepared by the same method as in (10) above, and the sample was held in an atmosphere of a temperature of 121 ° C. and a humidity of 100% RH for 48 hours. / 15 mm). Evaluation was made according to the following criteria.
A: Adhesive strength is 15 N / 15 mm or more, or a film piece is broken. O: Adhesive strength is 10 N / 15 mm or more and less than 15 N / 15 mm. X: Adhesive strength is less than 10 N / 15 mm.

(12) Light resistance (b * value change after light resistance test)
The film was irradiated with xenon for 14 days using a xenon lamp (trade name “Suntest CPS +” manufactured by Atlas Co., Ltd., irradiation intensity of 550 W / m ² (irradiation wavelength: 300 to 800 nm)), and before and after the xenon irradiation. The amount of change in b * value (Δb * value) was measured using an automatic color difference meter (SE-Σ90 type) manufactured by Nippon Denshoku Industries Co., Ltd. The amount of change in the b * value was evaluated according to the following criteria.
◎: Δb * value is 5.0 or less ○: Δb * value exceeds 5.0, 7.0 or less ×: Δb * value exceeds 7.0

(13) Evaluation of cohesive fracture resistance of film The film was sampled to a size of 200 mm x 200 mm, and the film surfaces subjected to the evaluation of cohesive fracture resistance were bonded together with a dry laminate adhesive for a solar battery backsheet. As an adhesive for dry lamination, a urethane-based adhesive in which product name Takelac A315 (100 parts by weight) manufactured by Mitsui Chemicals Polyurethanes Co., Ltd. and product name Takenate A50 (10 parts by weight) is mixed has a solid coating amount. Adjustment and application were performed so as to be 3 g / m ² . As aging, it was left in a 50 ° C. environment for 4 days. Then, a slit with a width of 15 mm was inserted, the film of the unbonded part was sandwiched using a tensile tester, and the presence or absence of occurrence of cohesive failure was confirmed by T-peeling at a tensile rate of 500 mm / min. Evaluated by criteria.
A: There was no occurrence of cohesive failure in the film, and the film was broken due to high adhesive strength. O: There was no occurrence of cohesive failure in the film, but the film did not break. ×: The occurrence of cohesive failure was observed in the film. Was

Reference Example 1 Production of polyethylene terephthalate (PET-a)
A transesterification vessel was charged with 100 parts by weight of dimethyl terephthalate, 60 parts by weight of ethylene glycol, and 0.06 parts by weight of magnesium acetate tetrahydrate, heated to 150 ° C., melted and stirred. The reaction was advanced while the temperature inside the reaction vessel was slowly raised to 235 ° C., and the methanol produced was distilled out of the reaction vessel. When the distillation of methanol was completed, phenylphosphonic acid was added to complete the transesterification reaction. Thereafter, the reaction product was transferred to a polycondensation apparatus, and antimony oxide was added. Subsequently, the temperature in the polymerization apparatus was raised from 235 ° C. to 290 ° C. over 90 minutes, and at the same time, the pressure in the apparatus was reduced from atmospheric pressure to 100 Pa over 90 minutes. When the stirring torque of the contents of the polymerization apparatus reached a predetermined value, the interior of the apparatus was returned to atmospheric pressure with nitrogen gas to complete the polymerization. The valve at the bottom of the polymerization apparatus was opened and the inside of the polymerization apparatus was pressurized with nitrogen gas, and the polymerized polyethylene terephthalate was discharged into water in the form of a strand. The strand was chipped with a cutter. Thus, a polyethylene terephthalate polymer having an intrinsic viscosity of 0.60 dl / g and a terminal carboxyl group concentration of 17 equivalents / ton was obtained. This is referred to as PET-a.

Reference Example 2 Production of polyethylene terephthalate (PET-b)
The polymer (PET-a) obtained in Reference Example 1 was pre-dried at 150 to 160 ° C. for 3 hours, and then subjected to solid state polymerization at 210 ° C., 100 Torr, and a nitrogen gas atmosphere for 7 hours. The intrinsic viscosity after solid phase polymerization was 0.76 dl / g, and the terminal carboxyl group concentration was 10 equivalents / ton. This is referred to as PET-b.

Reference Example 3 Production of polyethylene terephthalate (PET-c)
85% by weight of the polymer (PET-b) obtained in Reference Example 2 and 15% by weight of an aromatic polycarbodiimide “Stabaxol P100” manufactured by Rhein Chemie are blended and fed to a twin-screw kneader and melted at 280 ° C. did. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and chipped with a cutter. This is referred to as PET-c.

Reference Example 4 Production of polyethylene terephthalate (PET-d)
60% by weight of the polymer (PET-b) obtained in Reference Example 2 and 40% by weight of rutile titanium oxide particles TCR-52 (average particle size 0.2 μm) manufactured by Sakai Chemical Co., Ltd. were blended, and a twin-screw kneader. And melted at 280 ° C. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and chipped with a cutter. This is referred to as PET-d.

Reference Example 5 Production of polyethylene terephthalate (PET-e)
60% by weight of the polymer (PET-b) obtained in Reference Example 2 and 40% by weight of precipitated barium sulfate particles 300R (average particle size 0.7 μm) manufactured by Sakai Chemical Industry Co., Ltd. were blended into a biaxial kneader. Feed and melt at 280 ° C. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and chipped with a cutter. This is referred to as PET-e.

Reference Example 6 Production of polyethylene terephthalate (PET-U1)
90% by weight of the polymer (PET-b) obtained in Reference Example 2 and a triazine ultraviolet absorber (manufactured by Ciba Specialty Chemicals, Inc., trade names: “Tinuvin 479”, 2- (2-hydroxy-4- [1- Octyloxycarbonylethoxy] -4,6-bis (4-phenylphenyl) -1,3,5-triazine, molecular weight 640, 250 ° C. heat reduction rate 0.2%) 10% by weight, and biaxial kneading The polyester composition was melted at 280 ° C. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and formed into chips by a cutter, which is referred to as PET-U1.

Reference Example 7 Production of polyethylene terephthalate (PET-U2)
90% by weight of the polymer (PET-b) obtained in Reference Example 2 and a benzotriazole ultraviolet absorber (manufactured by Adeka, trade name: “LA-31”, 2,2′-methylenebis [4- (1,1 , 3,3-tetramethylbutyl) -6-2H-benzotriazol-2-yl] phenol, molecular weight 695, 250 ° C. heat reduction rate 0.1%) 10% by weight and fed to a twin-screw kneader. And melted at 280 ° C. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and chipped with a cutter. This is referred to as PET-U2.

Reference Example 8 Production of polyethylene terephthalate (PET-U3)
90% by weight of the polymer (PET-b) obtained in Reference Example 2 and a triazine UV absorber (manufactured by BASF, trade name: “CGX-006UVA”, 2- (2-hydroxyphenyl-4- [2-ethyl) Ruhexyloxyphenyl])-4,6-bis (4-phenylphenyl) -1,3,5-triazine, molecular weight 640, 250 ° C. heat reduction rate 0.2%) 10 wt% It was supplied to a kneader and melted at 280 ° C. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and chipped with a cutter. This is referred to as PET-U3.

Reference Example 9 Production of polyethylene terephthalate (PET-U4)
90% by weight of the polymer (PET-b) obtained in Reference Example 2 and a benzotriazole ultraviolet absorber (manufactured by Ciba Specialty Chemicals, Inc., trade name: “Tinuvin 234”, 2,4-bis (1-methyl-1) -Phenylethyl) -6- (2H-benzotriazol-2-yl) phenol, molecular weight 467, 250 ° C. heat reduction rate 5%) 10 wt% blended, fed to twin screw kneader and melted at 280 ° C. did. The melt-kneaded polyester composition was made into a strand shape, discharged into water, and chipped with a cutter. This is referred to as PET-U4.

[Examples 1 to 9]
After drying the polyester raw materials of the surface layer (A) and base material layer (B) shown in Table 1 at 180 ° C. for 3 hours in separate rotary vacuum dryers, they are supplied to separate extruders at 280 ° C. After melt extrusion and merging using a three-layer feed block, the sheet was molded into a sheet form from a slit die while maintaining the laminated state. The ratio of the thickness of each layer was adjusted by the amount of raw material supplied to each extruder, and the ratios shown in Table 1 were (A) layer / (B) layer / (A) layer. Further, an unstretched film obtained by cooling and solidifying the sheet with a cooling drum having a surface temperature of 20 ° C. was stretched 3.5 times in the longitudinal direction (longitudinal direction) at 100 ° C. and cooled by a roll group at 25 ° C. Subsequently, while holding both ends of the longitudinally stretched film with clips, the film was drawn to a tenter and stretched 3.7 times in a direction perpendicular to the longitudinal direction (lateral direction) in an atmosphere heated to 130 ° C. Then, heat setting is performed in an atmosphere heated to 220 ° C in a tenter for about 10 seconds, a width of 4% is inserted in the transverse direction, then both ends are cut off and relaxed at a relaxation rate of 2.5% in the longitudinal direction. After cooling to room temperature, a laminated film having the thickness described in Table 1 was obtained. The obtained film is excellent in discoloration resistance and hydrolysis resistance, and has good initial adhesive strength with EVA and adhesive strength after wet heat treatment, and is excellent in cohesive fracture resistance. It was suitable as a film.

[Comparative Example 1]
It was set as the single layer film only of a base material layer (B), and the film of thickness 50 micrometers was obtained on the conditions similar to Example 1 after extending | stretching. The characteristics of the obtained film were as shown in Table 1. Since there is no layer that prevents the low molecular weight component from bleeding out from the base material layer (B), the adhesive strength with EVA after the wet heat treatment is lowered, which is unsuitable as a laminated film for solar cells.

[Comparative Example 2]
Except for changing the blending ratio of the raw materials, the same operation as in Example 1 was performed to obtain a laminated film having a thickness of 50 μm. The characteristics of the obtained film were as shown in Table 1. Since carbodiimide was not added, the effect of improving hydrolysis resistance was not observed.

[Comparative Example 3]
The thickness ratio of each layer was changed to (A) layer / (B) layer / (A) layer = 40% / 20% / 40%, and the thickness ratio of 50 μm was the same as in Example 1 except that the mixing ratio of the raw materials was changed. A laminated film was obtained. The characteristics of the obtained film were as shown in Table 1. Since the ultraviolet absorber was not added to the surface layer (A), discoloration resistance could not be suppressed. Moreover, the thickness of the base material layer (B) in the film was thin, the thickness of the surface layer (A) not having high hydrolysis resistance was large, and the amount of the carbodiimide compound in the base material layer (B) was small. For this reason, the film as a whole was inferior in hydrolysis resistance and unsuitable as a laminated film for solar cells.

[Comparative Example 4]
A laminated film having a thickness of 50 μm was obtained in the same manner as in Example 1 except that the mixing ratio of the raw materials was changed. The characteristics of the obtained film were as shown in Table 1. The surface layer (A) contained a carbodiimide compound, and its low molecular weight component bleeded out from the surface layer (A) under wet heat, and the adhesive strength with EVA after wet heat treatment was reduced. Moreover, since the amount of the carbodiimide compound in the film decreased due to bleed-out, the film as a whole was inferior in hydrolysis resistance and unsuitable as a laminated film for solar cells.

[Comparative Example 5]
A laminated film having a thickness of 50 μm was obtained in the same manner as in Example 1 except that the mixing ratio of the raw materials was changed. The characteristics of the obtained film were as shown in Table 1. The white pigment was not added to any layer, and it was not a white film, and an ultraviolet absorber alone was not sufficient in the discoloration resistance effect and was unsuitable as a laminated film for solar cells.

[Comparative Example 6]
A laminated film was obtained in the same manner as in Example 2 except that the thickness of the film was changed to 25 μm. The characteristics of the obtained film were as shown in Table 1. Since the thickness of the surface layer (A) was thin, low molecular weight components bleed out from the base material layer (B) under wet heat, the adhesive strength with EVA after wet heat treatment was lowered, and it was not suitable as a laminated film for solar cells. there were.

[Comparative Example 7]
A laminated film having a thickness of 50 μm was obtained in the same manner as in Example 1 except that the mixing ratio of the raw materials was changed. The characteristics of the obtained film were as shown in Table 1. It is a white film that contains no white pigment in the base layer (B) and contains a large amount of white pigment exceeding 2 parts by weight only in the surface layer (A), which reduces the adhesive strength with EVA and aggregates in the film surface layer. Since destruction occurred, it was unsuitable as a laminated film for solar cells.

  The biaxially oriented polyester film of the present invention suppresses deterioration in mechanical properties and coloration due to ultraviolet rays even when used for a long time in a harsh natural environment such as high temperature and high humidity, and at the same time, the filler and adhesion of solar cells. Since the decrease in adhesiveness with the agent is suppressed, it is particularly useful as a white solar cell back surface protective film having long-term hydrolysis resistance and light resistance, or as a film constituting a white solar cell back surface protective film. is there.

Claims (9)

  A biaxially oriented polyester film having a three-layer structure comprising a base layer (B) and surface layers (A) provided on both sides thereof, wherein the surface layer (A) contains an ultraviolet absorber and contains a carbodiimide compound. It is composed of a thermoplastic polyester composition not included, and the thickness per layer of the surface layer (A) is 3.0 μm or more. The base material layer (B) contains a carbodiimide compound and a white pigment, and is biaxially oriented. The polyester film is a white polyester film for protecting a back surface of a solar cell, wherein Δb * before and after irradiation for 14 days in an environment of xenon irradiation is 7.0 or less.   The white polyester for protecting a back surface of a solar cell according to claim 1, wherein the content of the ultraviolet absorber is 0.1 parts by weight or more and 10 parts by weight or less based on 100 parts by weight of the thermoplastic polyester constituting the surface layer (A). the film.   The number average molecular weight of the ultraviolet absorber is 500 or more and 1500 or less, and the thermal weight loss rate when the ultraviolet absorber is heated from room temperature to 250 ° C. at a temperature increase of 10 ° C./min is 1% or less. The white polyester film for solar cell back surface protection of Claim 1 or 2.   The content of the carbodiimide compound in the base material layer (B) is 0.3 parts by weight or more and 2.5 parts by weight or less with respect to 100 parts by weight of the thermoplastic polyester constituting the base material layer (B). The white polyester film for solar cell back surface protection in any one of -3.   The content of the white pigment in the base material layer (B) is 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the thermoplastic polyester constituting the base material layer (B). The white polyester film for solar cell back surface protection of description.   The sun according to any one of claims 1 to 5, wherein the content of the white pigment in the surface layer (A) is 0 to 2 parts by weight with respect to 100 parts by weight of the thermoplastic polyester constituting the surface layer (A). White polyester film for battery back protection.   The white polyester film for protecting a back surface of a solar cell according to any one of claims 1 to 6, wherein the white pigment is titanium oxide.   The intrinsic viscosity of the thermoplastic polyester constituting the white polyester film for protecting the back surface of the solar cell is in the range of 0.55 dl / g or more and 0.80 dl / g or less, and the terminal carboxyl group concentration of the thermoplastic polyester is 5 eq / ton. The white polyester film for protecting a back surface of a solar cell according to any one of claims 1 to 7, which is in a range of 25 equivalents / ton or less.   The white polyester film for protecting a back surface of a solar cell according to any one of claims 1 to 8, wherein the carbodiimide compound is biscarbodiimide or aromatic polycarbodiimide.