JP2007007885A - Polyester film for back protecting film of solar cell and back protecting film of solar cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyester film for the back protecting film of a solar cell capable of especially keeping strength over a long period of time while having excellent mechanical properties and heat resistance and having high durability capable of constructing a large-scaled solar cell generation system, and the back protecting film of the solar cell. <P>SOLUTION: The polyester film for the back protecting film of the solar cell contains 4-20 mol% of a 2,6-naphthalenediarboxylic acid component as the constituent of polyester with respect to all of dicarboxylic acids. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a polyester film for a solar cell back surface protective film. In detail, it is related with the polyester film for solar cell back surface protective films which shows the outstanding mechanical property and heat resistance, and is excellent in durability, and a solar cell back surface protective film using the same.

  In recent years, the photovoltaic power generation system has been spreading as one of power generation means using clean energy. The structure of the solar cell module is generally such that a plurality of plate-like solar cell elements are sandwiched between a light receiving side glass substrate and a back side protective film, as described in, for example, Japanese Utility Model Laid-Open No. 6-38264. It takes a structure in which the internal gap is filled with sealing resin.

  It is known to use a polyester film as a protective film on the back side of the solar cell (Japanese Patent Application Laid-Open Nos. 2001-148497, 2001-257372, and 2003-60218). These polyester films have been tried to be improved because durability in long-term use is still insufficient, and a high molecular weight polyethylene terephthalate film is used (Japanese Patent Laid-Open No. 2002-26354), and a polyethylene terephthalate having a low oligomer content. It has been proposed to use a film (Japanese Patent Laid-Open No. 2002-100788, Japanese Patent Laid-Open No. 2002-134770, Japanese Patent Laid-Open No. 2002-134771).

JP 2001-148497 A JP 2001-257372 A JP 2003-60218 A JP 2002-26354 A Japanese Patent Laid-Open No. 2002-100788 JP 2002-134770 A JP 2002-134771 A

  However, even with these improved techniques, it is still difficult to maintain practical strength as a solar cell back surface protective film for a long time, and it cannot be used for a large-scale photovoltaic power generation system.

  The present invention eliminates the problems of the prior art, has excellent mechanical properties and heat resistance, and can maintain strength particularly over a long period of time, and can build a large-scale photovoltaic power generation system. It is an object of the present invention to provide a durable polyester film for a solar cell back surface protective film and a solar cell back surface protective film.

  That is, this invention is a polyester film for solar cell back surface protective films which contains a 2, 6- naphthalene dicarboxylic acid component as a structural component of polyester 4 to 20 mol% per total dicarboxylic acid.

  According to the present invention, while having excellent mechanical properties and heat resistance, it is possible to maintain strength particularly for a long period of time, and it is possible to construct a large-scale photovoltaic power generation system with excellent durability. A polyester film for a film and a solar cell back surface protective film can be provided.

Hereinafter, the present invention will be described in detail.
[Polyester film]
The content of 2,6-naphthalenedicarboxylic acid component in the polyester film for solar cell back surface protective film of the present invention is 4 to 20 mol%, preferably 4.5 to 15 mol% of the total dicarboxylic acid component of the polyester constituting the film. % Is essential. If it is less than 4 mol%, the durability will not be improved, and if it exceeds 20 mol%, the crystallinity of the polyester will be significantly lost, resulting in a decrease in heat resistance.

  As another dicarboxylic acid component constituting this polyester, for example, terephthalic acid can be used. And as a diol component which comprises this polyester, for example, ethylene glycol, trimethylene glycol, tetramethylene glycol, and cyclohexane-1,4-dimethanol can be used, and ethylene glycol is preferable.

The polyester film for a solar cell back surface protective film of the present invention may be composed of a single layer of polyester or may be composed of multiple layers of polyester.
In the case of a constitution composed of a single layer of polyester, the polyester film is composed of, for example, a copolymerized polyalkylene terephthalate containing 4 to 20 mol% of 2,6-naphthalenedicarboxylic acid component per total dicarboxylic acid component. As the diol component constituting the copolymerized alkylene terephthalate, for example, ethylene glycol, trimethylene glycol, tetramethylene glycol, and cyclohexane-1,4-dimethanol can be used. That is, in the case of a single-layer polyester film, ethylene glycol or trimethylene is preferably used with 4 to 20 mol% 2,6-naphthalenedicarboxylic acid and 80 to 96 mol% terephthalic acid as the dicarboxylic acid component per total dicarboxylic acid component. It is comprised from the polyester which uses as a diol component at least 1 sort (s) of diol chosen from the group which consists of glycol, tetramethylene glycol, and cyclohexane- 1,4-dimethanol.

  In order to obtain this polyester, for example, a method of adding 2,6-naphthalenedicarboxylic acid during polymerization of the polyester can be used. In place of this, a polyester having 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component, for example, polyethylene 2,6-naphthalate, and other polyesters such as polyethylene terephthalate and polytetramethylene terephthalate in a molten state. A method of blending into a blend polymer may be used.

  When a multilayer polyester film is used as the polyester film for the solar cell back surface protective film of the present invention, the amount of the 2,6-naphthalenedicarboxylic acid component contained in each layer can be arbitrarily determined. That is, the 2,6-naphthalenedicarboxylic acid component may be contained in an amount of 4 to 20 mol% per 100 mol% of all the dicarboxylic acid components of the polyester constituting the film.

As the multilayer polyester film, it is preferable to use one of the following multilayer polyester films from the viewpoint of obtaining high heat resistance.
(1) A multilayer polyester film comprising one polyester layer and another polyester layer disposed thereon and having a melting point higher by 5 ° C. or more.
(2) A multilayer polyester film comprising one polyester layer and another polyester layer disposed on both sides and having a melting point higher by 5 ° C. or more.

  In these cases, a polyester containing 0 to 10 mol% of 2,6-naphthalenedicarboxylic acid component is used as a layer having a high melting point, and a 10- to 30 mol% 2,6-naphthalenedicarboxylic acid component is used as a layer having a low melting point. Polyester containing can be used.

  These layers may be a copolymerized polyester containing a 2,6-naphthalenedicarboxylic acid component as a copolymerized component, such as a copolymerized polyethylene terephthalate, or a poly 2,6-naphthalenedicarboxylate and other polyesters such as polyethylene. It may be a mixture with terephthalate.

The melting point is the melting endothermic peak temperature when a sample obtained by melting the polyester once and then rapidly cooling and solidifying it is heated at a rate of 20 ° C./min with a differential calorimeter.
The thickness of the high melting point layer is preferably larger than the thickness of the low melting point layer. In the case of a multilayer film, the total thickness (a) of the total thickness of the high melting point layer and the total thickness of the low melting point layer is totaled. The ratio a / b with respect to the total thickness (b) is preferably 0.01 to 1, more preferably 0.05 to 0.5.

  The polyester film for a solar cell back surface protective film of the present invention is preferably manufactured by a coextrusion film forming method from the viewpoint of obtaining high adhesion to each layer when manufactured as a multilayer film, and more preferably from the viewpoint of obtaining high mechanical strength. Is produced by biaxial stretching.

  Whether the polyester film for solar cell back surface protective film of the present invention is a single layer or a multilayer, the shrinkage ratio in the film longitudinal direction when the polyester film is heat-treated at 150 ° C. for 30 minutes is preferably 0 to 1. 5%. If it exceeds 1.5%, it may be deformed during heat processing, which is not preferable.

[Fine particles]
In the polyester film for solar cell back surface protective film of the present invention, in order to improve the roll-up property of the film at the time of film formation and improve the transportability of the film in the solar cell back surface protective film processing step, as a lubricant It is preferable to contain organic fine particles and / or inorganic fine particles.

Examples of the inorganic fine particles include calcium carbonate, calcium oxide, aluminum oxide, kaolin, silicon oxide, and zinc oxide particles.
Examples of the organic fine particles include crosslinked acrylic resin particles, crosslinked polystyrene resin particles, urea resin particles, melamine resin particles, and crosslinked silicone resin particles.

  In addition to these fine particles, other resins such as colorants, antistatic agents, antioxidants, lubricants, catalysts, polyethylene, polypropylene, ethylene-propylene-polymers, olefinic ionomers, etc. are also films such as mechanical strength. You may make it contain arbitrarily in the range which does not impair a characteristic. From the viewpoint of improving the surface reflectivity and design properties, for example, white, black, or other colors may be used, and for that purpose, dyes and / or pigments may be included.

[Ultraviolet absorber]
In order to improve the weather resistance of the film, the polyester film for solar cell back surface protective film of the present invention can contain an ultraviolet absorber. Examples of the ultraviolet absorber include 2,2′-p-phenylenebis (3,1-benzoxazin-4-one), 2,2 ′-(4,4′-diphenylene) bis (3,1-benzoxazine- 4-one) and 2,2 ′-(2,6-naphthylene) bis (3,1-benzoxazin-4-one).

  The content of the ultraviolet absorber is preferably 0.1 to 5% by weight, more preferably 0.2 to 3% by weight, based on the weight of the polyester. If it is less than 0.1%, the effect of preventing UV deterioration is small, and if it exceeds 5% by weight, the film-forming properties of the polyester deteriorate, which is not preferable.

  Examples of methods for incorporating a UV absorber into the polyester film for solar cell backside protective film include, for example, a method of adding to a polyester in a polyester polymerization step, a method of kneading into a polyester in a melting step before film formation, and impregnation of a biaxially stretched film Can be used. Among these, the method of kneading into the polyester in the melting step before film formation is preferable from the viewpoint of preventing a decrease in the degree of polymerization of the polyester. The kneading in this case can be performed by, for example, a compound powder direct addition method or a master batch method.

[Easily adhesive layer]
In the polyester film for solar cell back surface protective film of the present invention, it is preferable to provide an easy-adhesion layer on the polyester film for the purpose of adhesion to the sealing resin of the solar cell element and improvement of adhesion durability. By providing the easy-adhesion layer, it is possible to obtain a strong and durable adhesive force with the sealing resin, prevent deterioration from the interface when the solar cell is formed, and further improve the durability.

  In order to obtain excellent adhesive strength and durability, the easy adhesion layer is preferably formed by applying a coating liquid containing a crosslinking agent. Examples of the crosslinking agent used include oxazoline group-containing polymers, urea resins, melamine resins, and epoxy resins. In addition, it is preferable that 10-100 weight% of crosslinking agents are contained in a coating liquid.

  Examples of the oxazoline group-containing polymer include polymers described in JP-B-63-48884, JP-A-2-60941, JP-A-2-99537, and polymers based thereon. Specific examples include a polymer obtained by polymerizing an addition-polymerizable oxazoline (a) represented by the following formula (I) and, if necessary, another monomer (b).

（但し、式中のＲ^１、Ｒ^２、Ｒ^３およびＲ^４は、それぞれ、水素、ハロゲン、アルキル基、アラルキル基、フェニル基および置換フェニル基から選ばれる置換基を示し、Ｒ^５は付加重合性不飽和結合基を有する非環状有機基を示す。）

(However, R ¹ , R ² , R ³ and R ⁴ in the formula are each a substituent selected from hydrogen, halogen, alkyl group, aralkyl group, phenyl group and substituted phenyl group, and R ⁵ is addition polymerization. An acyclic organic group having a polymerizable unsaturated bond group.)

  Examples of the addition polymerizable oxazoline (a) represented by the formula (I) include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline. 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline. These can be used as one kind or a mixture of two or more kinds. Among these, 2-isopropenyl-2-oxazoline is preferred because it is easily available industrially.

  Next, the monomer (b) other than the addition polymerizable oxazoline is not particularly limited as long as it is a monomer copolymerizable with the addition polymerizable oxazoline (a). For example, methyl acrylate, methyl methacrylate, butyl acrylate, Acrylic esters such as butyl methacrylate, unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid, unsaturated nitriles such as acrylonitrile and methacrylonitrile, acrylamide, methacrylamide, N-methylolacrylamide and N-methylol Unsaturated amides such as methacrylamide, vinyl esters such as vinyl acetate and vinyl propionate, vinyl ethers such as methyl vinyl ether and ethyl vinyl ether, α-olefins such as ethylene and propylene, vinyl chloride, vinyl chloride , Mention may be made of halogen-containing -α such as vinyl fluoride, beta-unsaturated monomers, styrene, such as α- methylstyrene alpha, and beta-unsaturated aromatic monomers. These can be used as one kind or a mixture of two or more kinds.

  In order to obtain a polymer using the addition-polymerizable oxazoline (a) and, if necessary, at least one other monomer (b), polymerization can be carried out by a conventionally known polymerization method. For example, various methods such as an emulsion polymerization method (a method in which a polymerization catalyst, water, a surfactant and a monomer are mixed together for polymerization), a monomer dropping method, a multistage polymerization method, and a pre-emulsion method can be employed.

  A conventionally known polymerization catalyst can be used. For example, normal radical polymerization initiators such as hydrogen peroxide, potassium persulfate, and 2,2'-azobis (2-aminodipropane) dihydrochloride can be exemplified. The polymerization temperature is usually 0 to 100 ° C, preferably 50 to 80 ° C. The polymerization time is usually 1 to 10 hours. Examples of the surfactant include conventionally known anionic, nonionic, cationic, amphoteric surfactants and reactive surfactants.

  When the addition-polymerizable oxazoline (a) and at least one other monomer (b) are used to obtain a polymer, the amount of addition-polymerizable oxazoline (a) is 0.5% by weight based on the total monomers. It is preferable to determine appropriately within the above range. When the addition amount of the addition polymerizable oxazoline (a) is less than 0.5% by weight, it may be difficult to achieve the object of the present invention.

  Specific examples of the epoxy resin used as the crosslinking agent include polyepoxy compounds, diepoxy compounds, and monoepoxy compounds. Examples of the polyepoxy compounds include sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris (2-hydroxyethyl) isocyanate, glycerol polyglycidyl ether, trimethylol. Propane polyglycidyl ether, N, N, N ′, N′-tetraglycidylmetaxylylenediamine, N, N, N ′, N′-tetraglycidyl-4,4′-diaminodiphenylmethane, N, N, N ′, N'-tetraglycidyl-1,3-bisaminomethylcyclohexane can be exemplified. As the diepoxy compound, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, resorcin diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether Polytetramethylene glycol diglycidyl ether can be exemplified. Examples of the monoepoxy compound include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether. Among them, N, N, N ′, N′-tetraglycidylmetaxylylenediamine, N, N, N ′, N′-tetraglycidyl-4,4′-diaminodiphenylmethane, N, N, N ′, N′— Tetraglycidyl-1,3-bisaminomethylcyclohexane is preferred.

  Examples of the urea resin used as the crosslinking agent include dimethylol urea, dimethylol ethylene urea, dimethylol propylene urea, tetramethylol acetylene urea, and 4-methoxy 5-dimethylpropylene urea dimethylol.

  Examples of the melamine resin used as the crosslinking agent include compounds obtained by reacting methylol melamine derivatives obtained by condensing melamine and formaldehyde with ethers such as methyl alcohol, ethyl alcohol, isopropyl alcohol as lower alcohols. These may be used in a mixture.

Moreover, as a melamine resin used as a crosslinking agent, a monomethylol melamine, a dimethylol melamine, a trimethylol melamine, a tetramethylol melamine, a pentamethylol melamine, a hexamethylol melamine can be illustrated, for example.
Among these crosslinking agents, an oxazoline group-containing polymer is preferable because it exhibits particularly excellent easy adhesion. These crosslinking agents may be used independently and may use 2 or more types together.

  The concentration per solid content of the crosslinking agent in the coating solution is preferably 10 to 100% by weight, more preferably 20 to 100% by weight for an oxazoline-containing polymer, and it may be a urea resin, a melamine resin or an epoxy resin. 5 to 50% by weight. If the cross-linking agent is less than 10% by weight, the cohesive strength of the coating layer may be reduced, and the adhesion durability particularly under high humidity may be reduced.

  It is preferable that the easy-adhesion layer further contains a binder. The binder is preferably a polyester resin or an acrylic resin, and the glass transition point thereof is preferably 20 to 100 ° C, more preferably 30 to 90 ° C. If it is less than 20 ° C, blocking between the films may occur unfavorably, and if it exceeds 100 ° C, the coating layer may become brittle and adhesion may not be maintained.

  As the polyester resin having a glass transition point of 20 to 100 ° C., a polyester resin comprising a polybasic acid or an ester-forming derivative thereof and a polyol or an ester-forming derivative thereof can be used. That is, as the polybasic acid component, terephthalic acid, isophthalic acid, phthalic acid, phthalic anhydride, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, adipic acid, sebacic acid, trimellitic acid, pyromellitic acid , Dimer acid and 5-sodium sulfoisophthalic acid. A copolymer polyester resin is synthesized using two or more of these acid components. In addition, an unsaturated polybasic acid component such as maleic acid, itaconic acid or the like, or a hydroxycarboxylic acid such as p-hydroxybenzoic acid may be used in a slight amount. The polyol component includes ethylene glycol, 1,4-butanediol, diethylene glycol, dipropylene glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, xylene glycol, dimethylolpropane, poly (ethylene oxide) glycol, poly (Tetramethylene oxide) glycol and the like. Moreover, although these monomers are mentioned, it is not limited to these.

  As the acrylic resin having a glass transition point of 20 to 100 ° C., an acrylic resin obtained by polymerizing an acrylic monomer can be used. Examples of acrylic monomers include alkyl acrylates and alkyl methacrylates (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups. Hydroxyl group-containing monomers such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate and 2-hydroxypropyl methacrylate; epoxy group-containing monomers such as glycidyl acrylate, glycidyl methacrylate and allyl glycidyl ether; acrylic acid , Methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrene sulfonic acid and its salts (sodium salt, potassium salt, ammonium salt, tertiary amine salt, etc.) Monomers containing ruruboxy groups or salts thereof; acrylamide, methacrylamide, N-alkyl acrylamide, N-alkyl methacrylamide, N, N-dialkyl acrylamide, N, N-dialkyl methacrylate (alkyl groups include methyl group, ethyl group N-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, etc.), N-alkoxyacrylamide, N-alkoxymethacrylamide, N, N-dialkoxyacrylamide N, N-dialkoxymethacrylamide (alkoxy groups include methoxy, ethoxy, butoxy, isobutoxy, etc.), acryloylmorpholine, N-methylolacrylamide, N-methylolmethacrylamide, N- Monomers containing amide groups such as phenylacrylamide and N-phenylmethacrylamide; monomers of anhydrides such as maleic anhydride and itaconic anhydride; vinyl isocyanate, allyl isocyanate, styrene, α-methyl styrene, vinyl methyl ether, vinyl ethyl ether , Vinyl trialkoxysilane, alkylmaleic acid monoester, alkyl fumaric acid monoester, alkylitaconic acid monoester, acrylonitrile, methacrylonitrile, vinylidene chloride, ethylene, propylene, vinyl chloride, vinyl acetate, butadiene and other monomers .

  Among these, 2 to 20 moles of a hydroxyl group-containing monomer such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, N-methylol acrylamide, N-methylol methacrylamide and the like. %, Preferably 4 to 15 mol%.

To the easy-adhesion layer, inert fine particles can be added for the purpose of improving the handleability of the film or preventing blocking between the films. Such fine particles are organic fine particles or inorganic fine particles. Examples of inorganic fine particles include calcium carbonate, calcium oxide, aluminum oxide, kaolin, silicon oxide, and zinc oxide. Examples of the organic fine particles include crosslinked acrylic resin particles, crosslinked polystyrene resin particles, melamine resin particles, and crosslinked silicone resin particles.
A wax may be added to the easy adhesion layer for the purpose of obtaining excellent slipperiness, and an antistatic agent, a colorant, a surfactant, and an ultraviolet absorber may be further contained.

[Film Production Method]
In the present invention, when preparing a single-layer polyester film, a polyester resin copolymerized with 2,6-naphthalenedicarboxylic acid in advance or a resin containing a main polyester resin and 2,6-naphthalenedicarboxylic acid is used. The mixture is dried, melted in an extruder, extruded from a die, and cast on a cooling drum to obtain an unstretched film. In the case of a multi-layer film, two or more kinds of resins containing a predetermined 2,6-naphthalenedicarboxylic acid by copolymerization or blending as in the case of a single-layer film are dried separately and melted from separate extruders. Then, the molten resin is laminated before being extruded from the die, then extruded from the die, and cast on a cooling drum to obtain an unstretched film.

  This unstretched film is stretched 2.5 times or more, preferably 3 to 5 times in the longitudinal direction at a temperature of Tg (glass transition temperature) -10 ° C. to Tg + 50 ° C. of the resin in the longitudinal direction, and then Tg + 10 to Tg + 50 ° C. At a temperature of 2.5 times or more, preferably 3 to 5 times in the transverse direction. This stretching is preferably 8 times or more, more preferably 9 times or more in terms of area magnification. In the case of a film having two or more layers, the stretching temperature is set based on the highest Tg among the two or more kinds of polyester resins constituting each layer.

  The stretched polyester film obtained as described above is further subjected to heat treatment. In the case of a single layer film, this heat treatment temperature is in the range of the melting point of the polyester resin from −100 ° C. to the melting point of −15 ° C. Preferably, the temperature is between -50 ° C. and -20 ° C., and further between -30 ° C. and -20 ° C., because thermal shrinkage during heat processing can be reduced. In the case of a multilayer film of two or more layers, the heat treatment temperature is set based on the highest melting point among two or more kinds of polyester resins constituting each layer. In particular, in the case of a two-layer film composed of layer A / layer B and a three-layer film composed of layer A / layer B / layer A, if heat treatment is performed at a temperature higher than the melting point of the polyester of layer B, the film shrinks. Since it can reduce, it is preferable. The heat treatment method is not particularly limited, and examples thereof include a method in which heat treatment is performed in the process immediately after stretching in film production, and a method in which heat treatment is performed after winding the film into a roll after completion of film production. The heat shrinkage rate of the film at 150 ° C. for 30 minutes is preferably 0 to 1.5%, more preferably 0 to 1%. If the thermal shrinkage rate exceeds 1.5%, defects such as wrinkling may occur in the heating process in solar cell production, and if it is less than 0%, deflection may occur in the heating process.

  Furthermore, when applying an easily bonding layer to a polyester film, after apply | coating the aqueous liquid containing the component which forms a membrane | film | coat to the stretchable polyester film, it is preferable to apply by drying and extending | stretching and heat-treating. The solid content concentration of the aqueous liquid is usually 30% by weight or less, and more preferably 10% by weight or less.

  The stretchable polyester film is an unstretched polyester film, a uniaxially stretched polyester film, or a biaxially stretched polyester film. Of these, a longitudinally stretched polyester film uniaxially stretched in the film extrusion direction (longitudinal direction) is particularly preferred.

  When applying an aqueous coating liquid to a film, as a pretreatment for improving the coating property, the film surface is subjected to physical treatment such as corona surface treatment, flame treatment, plasma treatment, etc., or chemically combined with the composition. It is preferable to use an inert surfactant in combination.

  Such a surfactant promotes the wettability of the aqueous coating liquid to the polyester film. For example, polyoxyethylene alkylphenyl ether, polyoxyethylene-fatty acid ester, sorbitan fatty acid ester, glycerin fatty acid ester, fatty acid metal soap, Examples include anionic and nonionic surfactants such as alkyl sulfates, alkyl sulfonates, and alkyl sulfosuccinates. The surfactant is preferably contained in an amount of 1 to 10% by weight in the composition forming the coating film. If it is this range, it can be set to 40 mN / m or less, and the repellency of a coating layer can be prevented.

  When applying an aqueous liquid to a polyester film, it is preferable to carry out a normal coating process, that is, a biaxially stretched and heat-fixed polyester film in a process separated from the film manufacturing process, so that dust, dust, etc. are easily involved. Absent. From such a viewpoint, application in a clean atmosphere, that is, application in a film manufacturing process is preferable. And according to this application | coating, the adhesiveness to the polyester film of a membrane | film | coat (coating film) further improves.

As the coating method, any known coating method can be applied. For example, a roll coating method, a gravure coating method, a roll brush method, a spray coating method, an air knife coating method, an impregnation method, and a curtain coating method can be used alone or in combination. The coating amount is preferably 0.5 to 20 g, more preferably 1 to 10 g, per 1 m ² of the running film. The aqueous liquid is preferably used as an aqueous dispersion or emulsion. In addition, a coating film may be formed only in the single side | surface of a film as needed, and may be formed in both surfaces.

A stretchable polyester film coated with an aqueous liquid is led to a drying and stretching process, which can be performed under conditions accumulated in the art. As preferable conditions, for example, the drying conditions are 90 to 130 ° C. × 2 to 10 seconds, the stretching temperature is 90 to 150 ° C., the stretching ratio is 3 to 5 times in the machine direction, and 3 to 5 times in the transverse direction. It is 1 to 3 times in the vertical direction, and 180 to 250 ° C. × 2 to 60 seconds when heat-fixed.
The thickness of the biaxially oriented polyester film after such treatment is preferably 50 to 300 μm, and the thickness of the coating film is preferably 0.01 to 1 μm.

[Production method of back surface protective film]
The polyester film for solar cell back surface protective film of the present invention can be suitably used as a solar cell back surface protective film, either alone or in combination of two or more. Under the present circumstances, it is preferable to laminate | stack the film and foil which have gas barrier property on the polyester film of this invention for the purpose of providing gas barrier property. Here, the gas barrier property refers to the water vapor barrier property, and the water vapor permeability measured in accordance with JIS Z0208-73 is preferably 5 g / (m ² · 24 h) or less. Examples of the film having a gas barrier property include a polyvinylidene chloride film, a polyvinylidene chloride coated film, a polyvinylidene fluoride coated film, a silicon oxide deposited film, an aluminum oxide deposited film, an aluminum deposited film, etc. A copper foil etc. can be illustrated. These films or foils can be laminated on the opposite side of the easy-adhesion surface of the polyester film of the present invention or can be sandwiched between two polyester films with the easy-adhesion side as the outside.

The following examples further illustrate the present invention. Each characteristic value was measured by the following method.
(1) Melting point and glass transition point (Tg)
After the resin was melted once, the rapidly cooled and solidified sample was measured with a DuPont Thermal Analyst 2000 type differential calorimeter at a sample heating rate of 10 mg and 20 ° C./min. In the case of a multilayer film, the corresponding polyester layer was shaved while observing under a microscope, and 10 mg of a sample was obtained and measured in the same manner.

(2) Intrinsic viscosity The viscosity of the solution in an orthochlorophenol solvent was measured and determined at 35 ° C.

(3) Thermal contraction rate of polyester film According to JIS-C2318, the film is held in an oven set at 150 ° C. in a non-tensioned state for 30 minutes, and the dimensional change before and after the heat treatment in the longitudinal direction of the film The heat shrinkage rate was calculated by the following formula to obtain the heat shrinkage rate.
Thermal shrinkage% = ((L0−L) / L0) × 100
However, L0: Distance between gauge points before heat treatment, L: Distance between floating points after heat treatment

(4) Adhesiveness with ethylene-vinyl acetate copolymer The ethylene-vinyl acetate copolymer is hereinafter abbreviated as "EVA".
Two sheets of the film cut to 20 mm width × 100 mm length and one sheet of EVA sheet (High Sheet Industry Co., Ltd. SOLAR EVA (R) SC4) cut to 20 mm width × 50 mm length were prepared. Heat sealer (TPA manufactured by Tester Sangyo Co., Ltd.) is placed in the order of the film / EVA sheet / film so that the EVA sheet is located at the approximate center of the film and the surface to be evaluated for easy contact is on the EVA side. Press at -701-B). The pressure bonding conditions were 120 ° C. and 0.02 MPa for 5 minutes, then the temperature was raised to 150 ° C., the press pressure was increased to 0.1 MPa, and the pressure was bonded for 25 minutes. One end of the EVA bonded portion of the thermocompression-bonded sample was firmly fixed with a Hoffman clip. In an atmosphere of 23 ° C. and 50% RH, in accordance with JIS-Z0237, the upper and lower clips are sandwiched with the film of the non-adhered part opposite to the clip, and peeled at a peeling angle of 180 ° and a tensile speed of 100 mm / min. The load was measured until the clip reached the clip fixing part, and was used as the adhesive force.
EVA and film adhesion were evaluated as follows.
◎: Adhesive force 20 N / 20 mm or more: Adhesive property is very good ○: Adhesive force 10 N / 20 mm or more, less than 20 N / 20 mm ... Adhesive property △: Adhesive force 5 N / 20 mm or more and less than 10 N / 20 mm・ Slightly good adhesion ×: Adhesive force less than 5 N / 20 mm ・ ・ ・ Adhesive failure

(5) Durability of adhesion to EVA and durability of film The thermocompression-bonded sample prepared in (4) above was treated at 85 ° C. and 85% RH for 1000 hours in accordance with JIS-C8917-1998, and then ( 4) Clips were fixed in the same manner, and adhesive strength and film strength were measured.
Adhesion durability was evaluated as follows by using the ratio (β / α) of the adhesive strength (α) before the treatment and the adhesive strength (β) after the treatment measured in (4) as the adhesive retention rate.
◎: Adhesion retention rate 75% or more: Adhesive durability is very good ○: Adhesion retention rate 50% or more, less than 75% ... Adhesive durability is good △: Adhesion retention rate 25% or more, 50 Less than%: Adhesiveness slightly good X: Adhesiveness retention rate less than 25% ... Adhesive failure

(6) Durability of film Using a sample having a length of 100 mm and a width of 10 mm as a film sample, the elongation was measured at a gripping interval of 50 mm and a tensile speed of 100 mm / min according to JIS-C2318 5.3.3. The average of the measured values was taken as the film elongation (γ) before processing. Next, after the film sample was treated at 85 ° C. and 85% RH for 1000 hours in accordance with JIS-C8917-1998, the film elongation was measured by the same method as the measurement of the film elongation before the treatment, The average of the three measurements was taken as the film elongation (γ) after treatment. The ratio (δ / γ) of the film elongation before treatment (γ) and the film elongation after treatment (δ) (δ / γ) was evaluated as follows as the film elongation retention.
◎: Elongation retention 75% or more: film durability is very good ○: Elongation retention 50% or more, less than 75% ... Film durability is good △: Elongation retention 25% or more, 50% Less than ... Film durability is slightly good X: Elongation retention less than 25% ... Durability is poor

[Example 1]
Copolymer polyethylene terephthalate containing 350 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm and copolymerized 2,6-naphthalenedicarboxylic acid with 10 mol% based on the total dicarboxylic acid component (inherent viscosity: 0.80, DEG = 2 0.5 mol%, Tg = 81 ° C., Tm = 228 ° C.) was melt-extruded on a rotary cooling drum maintained at 20 ° C. to obtain an unstretched film. Next, the film was stretched 3.0 times in the longitudinal direction at 100 ° C., and then an aqueous coating solution (coating agent A) having a concentration of 8% of the composition for forming an easy-adhesion layer described below was uniformly applied on both sides thereof with a roll coater.

<Coating agent A>
A four-necked flask was charged with 3 parts of sodium lauryl sulfonate as a surfactant and 181 parts of ion-exchanged water and heated to 60 ° C. in a nitrogen stream, and then 0.5 part of ammonium persulfate as a polymerization initiator, 0.2 part of sodium hydrogen nitrate was added, and further monomers 30.1 parts of methyl methacrylate, 21.9 parts of 2-isopropenyl-2-oxazoline, polyethylene oxide (n = 10) methacrylic acid 39.4 A mixture of 8.6 parts of acrylamide was added dropwise over 3 hours while adjusting the liquid temperature to 60 to 70 ° C. After completion of dropping, the reaction was continued with stirring while maintaining the same temperature range for 2 hours, and then cooled to obtain an acrylic aqueous dispersion having a solid content of 35% by weight. On the other hand, 0.2% by weight of silica filler (average particle size: 100 nm) (trade name Snowtex ZL manufactured by Nissan Chemical Co., Ltd.), polyoxyethylene (n = 7) lauryl ether (Sanyo Chemical Co., Ltd.) as a wetting agent An aqueous solution containing 0.3% by weight of the trade name NAROACTY N-70) was prepared. A coating agent A was prepared by mixing 15 parts by weight of an acrylic aqueous dispersion and 85 parts by weight of an aqueous solution.

Subsequently, this coated film was subsequently dried at 95 ° C., stretched 3.6 times at 110 ° C. in the transverse direction, and heat-set by shrinking 4% in the width direction at 205 ° C. to obtain a polyester film having a thickness of 50 μm. It was. The thickness of the coating film was 0.02 μm, and the thermal shrinkage in the longitudinal direction of the film was 1.3%.
The evaluation results are shown in Table 1.

[Comparative Example 1]
Copolymerized polyethylene terephthalate containing 600 ppm of bulk silicon oxide particles having an average particle size of 1.5 μm and 2 mol% of 2,6-naphthalenedicarboxylic acid based on the total dicarboxylic acid component (inherent viscosity: 0.65, DEG = 2 0.1 mol%, Tg = 78 ° C., Tm = 249 ° C.) was melt-extruded on a rotary cooling drum maintained at 20 ° C. to obtain an unstretched film. Next, the film was stretched 3.1 times at 100 ° C. in the longitudinal direction, then stretched 3.6 times at 110 ° C. in the transverse direction, and thermally fixed by shrinking 3% in the width direction at 228 ° C., and having a thickness of 50 μm. A polyester film for the back surface protective film was obtained. The heat shrinkage rate in the longitudinal direction of the film was 1.7%. The evaluation results are shown in Table 1.

[Example 2]
Copolymer polyethylene terephthalate A (inherent viscosity: 0.70, DEG = 96) containing 600 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm and copolymerizing 2,6-naphthalenedicarboxylic acid with 4 mol% of the total dicarboxylic acid component. 1.3 mol%, Tg = 80 ° C., Tm = 245 ° C.) and 20 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm, and 18 mol% of 2,6-naphthalenedicarboxylic acid with respect to all dicarboxylic acid components. Polymerized copolymer polyethylene terephthalate B (inherent viscosity: 0.70, DEG = 1.8 mol%, Tg = 81 ° C., Tm = 205 ° C.) is separately dried, melt-extruded, and copolymerized polyethylene terephthalate A inside the die. / Laminated polymer is laminated on two layers of copolymerized polyethylene terephthalate B, and is then placed on a rotating cooling drum maintained at 20 ° C. To obtain an unstretched film. Subsequently, after extending | stretching 3.0 times at 105 degreeC to the vertical direction, like Example 1, the coating agent A was apply | coated uniformly with the roll coater on both surfaces.

  Subsequently, this coated film was subsequently dried at 100 ° C., stretched 3.6 times at 115 ° C. in the transverse direction, heat-fixed by shrinking 2% in the width direction at 228 ° C., and a polyester film for a solar cell back surface protective film. Got. The thickness of this film was 50 μm in total, with the layer on the side formed of copolymerized polyethylene terephthalate A being 40 μm and the layer formed on the side of copolymerized polyethylene terephthalate B being 10 μm. The thickness of the coating film was 0.03 μm, and the heat shrinkage rate in the longitudinal direction of the film was 1.2%. The evaluation results are shown in Table 1.

[Example 3]
Copolymer polyethylene terephthalate C containing 600 ppm of bulk silicon oxide particles having an average particle size of 1.5 μm and 5 mol% of 2,6-naphthalenedicarboxylic acid based on the total dicarboxylic acid component (inherent viscosity: 0.65, DEG = 1.7 mol%, Tg = 80 ° C., Tm = 242 ° C.) and 20 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm, and 2,6-naphthalenedicarboxylic acid is 15 mol% with respect to the total dicarboxylic acid component. Polymerized copolymer polyethylene terephthalate D (inherent viscosity: 0.78, DEG = 2.1 mol%, Tg = 84 ° C., Tm = 214 ° C.) is separately dried, melt extruded, and copolymer polyethylene terephthalate C is formed inside the die. Laminated polymer is laminated on 3 layers of / copolymerized polyethylene terephthalate D / copolymerized polyethylene terephthalate C, An unstretched film was obtained by casting on a rotary cooling drum maintained at 20 ° C. Subsequently, after extending | stretching 3.0 times at 105 degreeC to the vertical direction, like Example 1, the coating agent A was apply | coated uniformly with the roll coater on both surfaces.

  Subsequently, this coated film was subsequently dried at 100 ° C., stretched 3.6 times at 115 ° C. in the transverse direction, and heat-set by shrinking 2% in the width direction at 235 ° C., and a polyester film for solar cell back surface protective film Got. The thickness constitution of this film was 50 μm in total, in which both outer layers formed of copolymerized polyethylene terephthalate C were 3 μm each, and the inner layer formed of copolymerized polyethylene terephthalate D was 44 μm. The thickness of the coating film was 0.06 μm, and the thermal shrinkage rate in the longitudinal direction of the film was 0.9%. The evaluation results are shown in Table 1.

[Comparative Example 2]
Copolymer polyethylene terephthalate E (inherent viscosity: 0.65, DEG = 2.3 mol%) containing 800 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm and 5 mol% of isophthalic acid copolymerized with respect to the total dicarboxylic acid component. Tg = 74 ° C., Tm = 241 ° C.), 50 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm, and copolymerized polyethylene terephthalate F (inherent viscosity) obtained by copolymerizing 12 mol% of isophthalic acid with respect to all dicarboxylic acid components. : 0.68, DEG = 2.4 mol%, Tg = 73 ° C., Tm = 224 ° C.), dried separately, melt extruded, and copolymerized polyethylene terephthalate E / copolymerized polyethylene terephthalate F / copolymerized polyethylene inside the die. Laminated polymer is layered on three layers of terephthalate E, and the key is placed on a rotating cooling drum maintained at 20 ° C. To obtain an unstretched film. Subsequently, after extending | stretching 3.0 times at 95 degreeC to the vertical direction, like Example 1, the coating material A was apply | coated uniformly with the roll coater on both surfaces.

  Subsequently, this coated film was subsequently dried at 100 ° C., stretched 3.6 times in the transverse direction at 110 ° C., and heat-set by shrinking 2% in the width direction at 232 ° C., and a polyester film for a solar cell back surface protective film Got. The thickness constitution of this film was 50 μm in total, in which both outer layers formed of copolymerized polyethylene terephthalate E were 3 μm each, and the inner layer formed of copolymerized polyethylene terephthalate F was 44 μm. The thickness of the coating film was 0.03 μm, and the heat shrinkage rate in the longitudinal direction of the film was 1.1%. The evaluation results are shown in Table 1.

[Example 4]
Copolymer polyethylene terephthalate G (inherent viscosity: 0.70, DEG = 96) containing 600 ppm of bulk silicon oxide particles having an average particle diameter of 1.5 μm and copolymerizing 2,6-naphthalenedicarboxylic acid with 4 mol% of the total dicarboxylic acid component. 2.2 mol%, Tg = 79 ° C., Tm = 244 ° C.), 20 ppm of bulk silicon oxide particles having an average particle size of 1.5 μm, and 1 wt% of an ultraviolet absorber represented by the following formula, Copolyethylene terephthalate H (inherent viscosity: 0.71, DEG = 2.2 mol%, Tg = 85 ° C., Tm = 211 ° C.) obtained by copolymerizing naphthalenedicarboxylic acid with 16 mol% of all dicarboxylic acid components is dried separately. , Melt extruded, and copolymerized polyethylene terephthalate G / copolymerized polyethylene terephthalate H / copolymerized polyethylene terephthalate inside the die A molten polymer was laminated on three layers of phthalate G, and cast on a rotating cooling drum maintained at 20 ° C. to obtain an unstretched film.

  Next, this coated film was subsequently dried at 100 ° C., stretched 3.6 times at 115 ° C. in the transverse direction, and heat-set by shrinking 2% in the width direction at 236 ° C., and a polyester film for solar cell back surface protective film Got. The thickness constitution of this film was 50 μm in total, in which both outer layers formed of copolymerized polyethylene terephthalate G were 4 μm each, and the inner layer formed of copolymerized polyethylene terephthalate H was 42 μm. The thickness of the coating film was 0.06 μm, and the heat shrinkage rate in the longitudinal direction of the film was 0.7%. The evaluation results are shown in Table 1.

  The polyester film for solar cell back surface protective film of the present invention is excellent in durability while having excellent mechanical properties and heat resistance, and is useful as a solar cell back surface protective film.

Claims (4)

  The polyester film for solar cell back surface protective films which contains a 2, 6- naphthalene dicarboxylic acid component as a structural component of polyester 4-20 mol% per total dicarboxylic acid.   The polyester for a solar cell back surface protective film according to claim 1, wherein the polyester film is a multilayer polyester film comprising one polyester layer and another polyester layer disposed thereon and having a melting point of 5 ° C or higher. the film.   The polyester film for a solar cell back surface protective film according to claim 1, comprising a polyalkylene terephthalate having 4 to 20 mol% of 2,6-naphthalenedicarboxylic acid as a copolymerization component per total dicarboxylic acid component.   The solar cell back surface protective film which consists of a polyester film for solar cell back surface protective films in any one of Claims 1-3.