JP2006152013A - Easy-adhesive polyester film for solar cell back surface protective film and solar cell back surface protective film using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an easily adhesive polyester film for solar cell back surface-protecting films which exhibits excellent adhesive properties and adhering properties to EVA while exhibiting excellent mechanical properties, heat resistance and moisture resistance. <P>SOLUTION: The easily adhesive polyester film for solar cell back surface-protecting films comprises a polyester film and a resin film formed thereon by coating, wherein the resin film is formed by applying a coating liquid on the film and the coating liquid contains 10-100 wt% of a crosslinking agent (A) based on 100 wt% of the solid content thereof. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an easily adhesive film for a solar cell back surface protective film. More specifically, an ethylene-vinyl acetate copolymer (hereinafter abbreviated as EVA), which is a sealing resin for solar cells, is formed by forming an easy-adhesive coating film comprising a specific composition on at least one surface of a polyester film. It is related with the easily-adhesive polyester film which shows outstanding adhesion | attachment, and a solar cell back surface protective film using the same.

  In recent years, a solar power generation system has been spreading as one of power generation means using clean energy. The structure of a solar cell module is generally a plurality of plate-like solar cells between a glass substrate on the light receiving side and a protective film on the back side, as described in, for example, Japanese Utility Model Publication No. 6-38264. It has a structure in which an element is sandwiched and a sealing resin is filled in an internal gap. As the sealing resin, EVA is used because of its high transparency and excellent moisture resistance.

  A polyethylene resin, a polyester resin sheet, or a fluororesin film is used for the protective film on the back surface (Japanese Patent Laid-Open Nos. 11-261085 and 11-186575). However, such a protective film does not necessarily have sufficient adhesiveness with EVA, and there remains concern about long-term durability.

  On the other hand, a polyester film obtained by stretching a polyester sheet has excellent mechanical properties, heat resistance, and moisture resistance. However, polyester films, particularly polyester films that are biaxially stretched and highly oriented and crystallized, are inactive on the surface and have very poor adhesion to EVA. In order to improve the adhesiveness between such a polyester film and EVA, Japanese Patent Application Laid-Open No. 2003-60218 proposes laminating a thermal adhesive layer made of a styrene / olefin copolymer resin.

Japanese Utility Model Publication No. 6-38264 JP-A-11-261085 JP-A-11-186575 JP 2003-60218 A

However, the effect of laminating such a thermal adhesive layer is not yet sufficient, and it is difficult to obtain the adhesive strength necessary for constructing a large-scale photovoltaic power generation system.
The present invention eliminates the problems of the prior art and can build a large-scale photovoltaic power generation system, and has excellent mechanical properties, heat resistance, and moisture resistance, while maintaining adhesion and adhesion to EVA. It is an object of the present invention to provide an excellent easily-adhesive polyester film for a solar cell back surface protective film.

  That is, the present invention comprises a polyester film and a resin film applied thereon, and the resin film is formed by applying a coating liquid to the film. The coating liquid is a cross-linking agent (10 to 100% by weight per 100% by weight of solid content). It is an easily adhesive polyester film for solar cell back surface protective film characterized by containing A).

  ADVANTAGE OF THE INVENTION According to this invention, the easily adhesive polyester film for solar cell back surface protective films excellent in adhesiveness and adhesiveness with EVA can be provided, having excellent mechanical properties, heat resistance, and moisture resistance. .

Hereinafter, the present invention will be described in detail.
[Polyester film]
Examples of the polyester constituting the polyester film in the present invention include aromatic dicarboxylic acid components such as terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 4,4′-diphenyldicarboxylic acid, and ethylene glycol, 1, Aromatic polyesters composed of glycol components such as 4-butanediol, 1,4-cyclohexanedimethanol and 1,6-hexanediol are preferred, and polyethylene terephthalate and polyethylene-2,6-naphthalenedicarboxylate are more preferred. In particular, polyethylene-2,6-naphthalenedicarboxylate is preferred. The polyester may be a copolyester.

Polyester should contain organic or inorganic fine particles as a lubricant as necessary in order to improve the film winding property during film formation and the film transportability in the back surface protective film processing step for solar cells. Can do. Examples of such fine particles include calcium carbonate, calcium oxide, aluminum oxide, kaolin, silicon oxide, zinc oxide, crosslinked acrylic resin particles, crosslinked polystyrene resin particles, urea resin particles, melamine resin particles, and crosslinked silicone resin particles.
From the viewpoint of improving the surface reflectance as a solar cell back surface protective film and designability, it may be colored white, black, or other colors.

（式中、Ｘ¹は上記式に表わされたＸ¹からの２本の結合手が1位、２位の位置関係にある、２価の芳香族残基であり；ｎは１、２または３であり；Ｒ¹はｎ価の炭化水素残基で、これはさらにヘテロ原子を含有していてもよい、またはＲ¹はｎ＝２のとき直接結合であることができる。）
で表わされる環状イミノエステルおよび下記式（II）

（式中、Ａは下記式（II）-ａ

で表わされる基であるかまたは
下記式（II）-ｂ

で表わされる基であり；Ｒ²およびＲ³は同一もしくは異なり１価の炭化水素残基であり；Ｘ²は４価の芳香族残基で、これはさらにヘテロ原子を含有していてもよい。）
で表わされる環状イミノエステルから選ばれる少なくとも１種の化合物を、未反応の形態で用いるのが好ましい。 [Ultraviolet absorber]
In order to improve the weather resistance of the film, the polyester constituting the polyester film in the present invention preferably contains an ultraviolet absorber. As this ultraviolet absorber, the following formula (I)

(In the formula, X ¹ is a divalent aromatic residue in which two bonds from X ¹ represented by the above formula are in the 1-position and 2-position positional relationship; or be a 3; R ¹ is a n-valent hydrocarbon residue, which can further may contain a hetero atom, or R ¹ is a direct bond when n = 2).
And a cyclic imino ester represented by the following formula (II)

(In the formula, A represents the following formula (II) -a

Or a group represented by the following formula (II) -b

R ² and R ³ are the same or different monovalent hydrocarbon residues; X ² is a tetravalent aromatic residue, which may further contain a heteroatom. . )
It is preferable to use at least one compound selected from cyclic iminoesters represented by the formula in an unreacted form.

  Such a cyclic imino ester is a compound known as an ultraviolet absorber and is described, for example, in JP-A-59-12952.

In the general formula (I), X ¹ is a divalent aromatic residue in which two bonds from X ¹ represented by the formula (I) are in the 1-position and 2-position positional relationship; n is 1, 2 or 3; R ¹ is an n-valent hydrocarbon residue, which may further contain heteroatoms, or R ¹ may be a direct bond when n = 2 it can.

（式中、Ｒは−Ｏ−、−ＣＯ−、−Ｓ−、−ＳＯ₂−、−ＣＨ₂−、−（ＣＨ₂）−または−Ｃ（ＣＨ₃）₂−である。）
で表わされる基を挙げることができる。これらのうち、特に１，２−フェニレンが好ましい。 X ¹ is preferably 1,2-phenylene, 1,2-naphthylene, 2,3-naphthylene, or the following formula:

(In the formula, R is —O—, —CO—, —S—, —SO ₂ —, —CH ₂ —, — (CH ₂ ) — or —C (CH ₃ ) ₂ —).
The group represented by these can be mentioned. Of these, 1,2-phenylene is particularly preferable.

The aromatic residue exemplified for X ¹ is, for example, alkyl having 1 to 10 carbon atoms such as methyl, ethyl, propyl, hexyl, decyl, etc .; aryl having 6 to 12 carbon atoms such as phenyl, naphthyl, etc .; Cycloalkyl such as cyclopentyl, cyclohexyl, etc .; C 8-20 aralkyl such as phenylethyl; C 1-10 alkoxy such as methoxy, ethoxy, decyloxy, etc .; nitro; halogen such as chlorine, bromine, etc .; C 2-10 And may be substituted with a substituent such as acetyl, proponyl, zenzoyl, decanoyl, etc.

R ¹ can be an n-valent hydrocarbon residue (where n is 1, 2 or 3) or can be a direct bond only when n is 2.
As the monovalent hydrocarbon residue (when n = 1), first, for example, an unsubstituted aliphatic group having 1 to 10 carbon atoms, an unsubstituted aromatic group having 6 to 12 carbon atoms, or 5 to 5 carbon atoms. 12 unsubstituted alicyclic groups.

  Examples of the unsubstituted aliphatic group having 1 to 10 carbon atoms include methyl, ethyl, propyl, hexyl, and decyl. Examples of the unsubstituted aromatic group having 6 to 12 carbon atoms include phenyl, naphthyl, and biphenyl; Examples of the unsubstituted alicyclic group having 5 to 12 carbon atoms include cyclopentyl and cyclohexyl.

（式中、Ｒ⁴は炭素数２〜１０のアルキレン、フェニレンまたはナフチレンである。）
で表わされる基、下記式（ｄ） As the monovalent hydrocarbon residue, secondly, for example, the following formula (c)

(In the formula, R ⁴ is alkylene having 2 to 10 carbon atoms, phenylene or naphthylene.)
A group represented by the following formula (d):

（式中、Ｒ⁵は炭素数１〜１０のアルキル基、フェニル基またはナフチル基である。）
で表わされる基、下記式（ｅ）

(In the formula, R ⁵ is an alkyl group having 1 to 10 carbon atoms, a phenyl group, or a naphthyl group.)
A group represented by the following formula (e):

（式中、Ｒ⁴およびＲ⁵の定義は上記に同じであり、Ｒ⁶は水素原子またはＲ⁵に定義された基のいずれかである。）
で表わされる基、下記式（ｆ）

(In the formula, the definitions of R ⁴ and R ⁵ are the same as above, and R ⁶ is either a hydrogen atom or a group defined for R ^5. )
A group represented by formula (f)

（式中、Ｒ⁴およびＲ⁶の定義は上記に同じであり、Ｒ⁷は水素原子またはＲ⁵に定義された基のいずれかである。）
で表わされる置換された脂肪族残基または芳香族残基を挙げることができる。

(Wherein R ⁴ and R ⁶ have the same definitions as above, and R ⁷ is either a hydrogen atom or a group defined for R ^5. )
The substituted aliphatic residue or aromatic residue represented by these can be mentioned.

Further, as the monovalent hydrocarbon residue, thirdly, the unsubstituted aromatic residue is substituted with the same substituent as exemplified as the substituent of the aromatic residue representing X ¹ , for example. You can list what you have. Therefore, examples of substitution with such a substituent include tolyl, methylnaphthyl, nitrophenyl, nitronaphthyl, chlorophenyl, benzoylphenyl, acetylphenyl, and acetylnaphthyl.

  As the monovalent hydrocarbon residue, a group represented by the above formula (c), (d), (e) or (f), that is, a substituted aliphatic residue or aromatic residue, in particular, a substituted one thereof. Aromatic residues are preferred.

  As the divalent hydrocarbon residue (when n = 2), first, for example, a divalent, unsubstituted aliphatic residue having 2 to 10 carbon atoms, or an unsubstituted aromatic having 6 to 12 carbon atoms. Group residues and unsubstituted alicyclic residue groups having 5 to 12 carbon atoms.

  Examples of the divalent unsubstituted aliphatic group having 2 to 10 carbon atoms include ethylene, trimethylene, tetramethylene, decamethylene and the like. Examples of the divalent unsubstituted aromatic residue having 6 to 12 carbon atoms include, for example, Examples of the divalent unsubstituted alicyclic residue having 5 to 12 carbon atoms include cyclopentylene and cyclohexylene.

（式中、Ｒ⁸はＲ⁴に定義された基のいずれかである。）
で表わされる基、または下記式（ｈ）

（式中、Ｒ⁸の定義は上記に同じであり、Ｒ⁹はＲ⁴に定義された基のいずれかであり、そしてＲ¹⁰はＲ⁶に定義された基のいずれかである。）
で表わされる置換された脂肪族残基または芳香族残基を挙げることができる。 As the above divalent hydrocarbon residue, secondly, for example, the following formula (g)

(Wherein R ⁸ is any of the groups defined for R ⁴ ).
Or a group represented by the following formula (h)

Wherein R ^{8 is} as defined above, R ⁹ is any of the groups defined for R ⁴ , and R ¹⁰ is any of the groups defined for R ⁶ .
The substituted aliphatic residue or aromatic residue represented by these can be mentioned.

In addition, as the divalent hydrocarbon residue, thirdly, the unsubstituted divalent aromatic residue is, for example, the same substituent as exemplified as the substituent of the aromatic group representing X ¹ above. Mention may be made of substituted ones.

In the case where n is 2, R ¹ is preferably a direct bond or an unsubstituted or substituted divalent aromatic hydrocarbon residue of the first to third groups, and particularly preferably two of them. Preferred are the first or third group of unsubstituted or substituted aromatic hydrocarbon residues with the bond from the furthest position, especially P-phenylene, P, P′-biphenylene or 2,6 Naphthylene is preferred.
Examples of the trivalent hydrocarbon residue (when n = 3) include a trivalent aromatic residue having 6 to 12 carbon atoms.

等を挙げることができる。
かかる芳香族残基は、上記１価の芳香族残基の置換基として例示したと同じ置換基で置換されていてもよい。 Examples of such aromatic residues include:

Etc.
Such an aromatic residue may be substituted with the same substituent as exemplified as the substituent of the monovalent aromatic residue.

In the general formula (I), R ² and R ³ are the same or different and are monovalent hydrocarbon residues, and X ² is a tetravalent aromatic hydrocarbon residue.
Examples of R ² and R ³ include the same groups as those exemplified for R ^{1 in} the case of n = 1 in the description of the above formula (I).

（ここで、Ｒの定義は式（ａ）に同じ。）
で表わされる基を挙げることができる。
上記４価の芳香族残基は、上記式（Ｉ）の説明において、Ｒ¹を表わす１価の芳香族残基の置換基として例示したと同じ置換基で置換されていてもよい。 Examples of tetravalent aromatic hydrocarbon residues include:

(Here, the definition of R is the same as in formula (a).)
The group represented by these can be mentioned.
The tetravalent aromatic residue may be substituted with the same substituent as exemplified as the substituent of the monovalent aromatic residue representing R ¹ in the description of the formula (I).

  Specific examples of the cyclic imino ester represented by the above formulas (I) and (II) used in the present invention include the following compounds.

Compound of formula (I) above
Compounds in the case of n = 1 2-methyl-3,1-benzoxazin-4-one, 2-butyl-3,1-benzoxazin-4-one, 2-phenyl-3,1-benzoxazine-4-one ON, 2- (1- or 2-naphthyl) -3,1-benzoxazin-4-one, 2- (4-biphenyl) -3,1-benzoxazin-4-one, 2-p-nitrophenyl- 3,1-benzoxazin-4-one, 2-m-nitrophenyl-3,1-benzoxazin-4-one, 2-p-benzoylphenyl-3,1-benzoxazin-4-one, 2-p -Methoxyphenyl-3,1-benzoxazin-4-one, 2-o-methoxyphenyl-3,1-benzoxazin-4-one, 2-cyclohexyl-3,1-benzoxazin-4-one, 2- p- Or m-) phthalimidophenyl-3,1-benzoxazin-4-one, N-phenyl-4- (3,1-benzoxazin-4-one-2-yl) phthalimide, N-benzoyl-4- (3 , 1-Benzoxazin-4-on-2-yl) aniline, N-benzoyl-N-methyl-4- (3,1-benzoxazin-4-on-2-yl) aniline, 2- (p- ( N-methylcarbonyl) phenyl) -3,1-benzoxazin-4-one.

Compounds 2,2′-bis (3,1-benzoxazin-4-one), n = 2 = 2 , 2,2′-ethylenebis (3,1-benzoxazin-4-one), 2,2 ′ -Tetramethylenebis (3,1-benzoxazin-4-one), 2,2'-decamethylenebis (3,1-benzoxazin-4-one), 2,2'-p-phenylenebis (3 1-benzoxazin-4-one), 2,2′-m-phenylenebis (3,1-benzoxazin-4-one), 2,2 ′-(4,4′-diphenylene) bis (3,1 -Benzoxazin-4-one), 2,2 '-(2,6- or 1,5-naphthylene) bis (3,1-benzoxazin-4-one), 2,2'-(2-methyl- p-phenylene) bis (3,1-benzoxazin-4-one), 2,2 ′-(2-nitro-) p-phenylene) bis (3,1-benzoxazin-4-one), 2,2 ′-(2-chloro-p-phenylene) bis (3,1-benzoxazin-4-one), 2,2 ′ -(1,4-cyclohexylene) bis (3,1-benzoxazin-4-one), Np- (3,1-benzoxazin-4-one-2-yl) phenyl, 4- (3 1-benzoxazin-4-one-2-yl) phthalimide, Np- (3,1-benzoxazin-4-one-2-yl) benzoyl, 4- (3,1-benzoxazin-4-one) -2-yl) aniline.

Compound 1,3,5-tri (3,1-benzoxazin-4-on-2-yl) benzene, 1,3,5-tri (3,1-benzoxazin-4-one when n = 3 -2-yl) naphthalene, 2,4,6-tri (3,1-benzoxazin-4-one-2-yl) naphthalene

Compound of formula (II) 2,8-dimethyl-4H, 6H-benzo (1,2-d; 5,4-d ′) bis (1,3) -oxazine-4,6-dione, 2,7 -Dimethyl-4H, 9H-benzo (1,2-d; 4,5-d ') bis (1,3) -oxazine-4,9-dione, 2,8-diphenyl-4H, 8H-benzo (1 , 2-d; 5,4-d ′) bis (1,3) -oxazine-4,6-dione, 2,7-diphenyl-4H, 9H-benzo (1,2-d; 4,5-d ') Bis (1,3) -oxazine-4,6-dione, 6,6'-bis (2-methyl-4H, 3,1-benzoxazin-4-one), 6,6'-bis (2 -Ethyl-4H, 3,1-benzoxazin-4-one), 6,6'-bis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,6'-methylenebi (2-methyl-4H, 3,1-benzoxazin-4-one), 6,6′-methylenebis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,6′-ethylene Bis (2-methyl-4H, 3,1-benzoxazin-4-one), 6,6′-ethylenebis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,6 ′ -Butylenebis (2-methyl-4H, 3,1-benzoxazin-4-one), 6,6'-butylenebis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,6 ' -Oxybis (2-methyl-4H, 3,1-benzoxazin-4-one), 6,6'-oxybis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,6 ' -Sulfonylbis (2-methyl-4H, 3,1-benzoxazi -4-one), 6,6'-sulfonylbis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,6'-carbonylbis (2-methyl-4H, 3,1- Benzoxazin-4-one), 6,6′-carbonylbis (2-phenyl-4H, 3,1-benzoxazin-4-one), 7,7′-methylenebis (2-methyl-4H, 3,1) -Benzoxazin-4-one), 7,7'-methylenebis (2-phenyl-4H, 3,1-benzoxazin-4-one), 7,7'-bis (2-methyl-4H, 3,1) -Benzoxazin-4-one), 7,7'-ethylenebis (2-methyl-4H, 3,1-benzoxazin-4-one), 7,7'-oxybis (2-methyl-4H, 3, 1-benzoxazin-4-one), 7,7′-sulfonylbis (2-methyl) Til-4H, 3,1-benzoxazin-4-one), 7,7′-carbonylbis (2-methyl-4H, 3,1-benzoxazin-4-one), 6,7′-bis (2 -Methyl-4H, 3,1-benzoxazin-4-one), 6,7'-bis (2-phenyl-4H, 3,1-benzoxazin-4-one), 6,7'-methylenebis (2 -Methyl-4H, 3,1-benzoxazin-4-one), 6,7'-methylenebis (2-phenyl-4H, 3,1-benzoxazin-4-one).

（式中、Ｒ¹¹は２価の芳香族炭化水素残基である。）
で表わされる化合物が有利に用いられる。 Of the above exemplified compounds, the compound of the above formula (I), more preferably the compound of the above formula (I) when n = 2, particularly preferably the following formula (I) -1

(In the formula, R ¹¹ is a divalent aromatic hydrocarbon residue.)
Are preferably used.

  Among the compounds of formula (I) -1, 2,2′-p-phenylenebis (3,1-benzoxazin-4-one), 2,2 ′-(4,4′-diphenylene) bis ( 3,1-benzoxazin-4-one) and 2,2 ′-(2,6-naphthylene) bis (3,1-benzoxazin-4-one) are preferred.

  The ultraviolet absorption characteristics of these cyclic imino esters are described in, for example, Japanese Patent Application Laid-Open No. 59-12952 for representative compounds thereof, which are incorporated herein by reference.

The cyclic imino ester has excellent compatibility with the polyester, but reacts with the terminal hydroxyl group of the polyester as described in JP-A-59-12952 and US Pat. No. 4,291,152. Have the ability. Therefore, it is required to carefully mix the cyclic imino ester and the polyester so that the cyclic imino ester is contained in a substantially unreacted state. However, in the case of using a polyester in which the main proportion of the terminal group is a carboxyl group or a polyester in which the terminal hydroxyl group is blocked with a terminal blocking agent that is not reactive with the cyclic imino ester, the cyclic imino ester is unreacted. No special care needs to be taken to produce the composition containing in the state. When using a polyester in which the main proportion of terminal groups is a hydroxyl group, the melt mixing time is expressed by the following formula:
Logt ≦ −0.008T + 4.8
And Tm <T <320
(In the formula, t is a melt mixing time (second), T is a melt mixing temperature (° C.), and Tm is a polyester melting temperature (° C.).)
It is desirable to complete in a short time so as to satisfy the above. In this case, the cyclic imino ester and the polyester may react at a small ratio, but this reaction increases the molecular weight of the polyester. Depending on this ratio, the molecular weight reduction due to degradation of the polyester by the visible light absorber is prevented depending on this ratio. It is possible. When the cyclic imino ester reacts with the polyester, the ultraviolet absorption wavelength region generally tends to shift to a lower wavelength side than the unreacted ultraviolet absorption wavelength region, and therefore tends to transmit ultraviolet light on the higher wavelength side. It has.

  When the appropriate amount of the cyclic imino ester is added, there is almost no sublimation, so that the periphery of the die is less likely to be stained during film formation, and the film absorbs light in the vicinity of 380 nm from ultraviolet rays. And excellent properties to prevent film deterioration.

  The addition amount of the ultraviolet absorber is preferably 0.1 to 5% by weight, more preferably 0.2 to 3% by weight per 100% by weight of the polyester. If it is less than 0.1% by weight, the effect of preventing UV deterioration is small, and if it exceeds 5% by weight, the film-forming properties of the polyester are lowered, which is not preferable.

  As a method for adding the ultraviolet absorber to the polyester, for example, a method such as kneading into a polymer in a polyester polymerization step or a melting step before film formation, or impregnation into a biaxially stretched film can be employed. . From the viewpoint of preventing a decrease in the degree of polymerization of the polyester, a method of kneading into the polymer in the melting step before film formation is preferred. This kneading can be performed by, for example, a compound powder direct addition method or a master batch method.

[Coating]
In the present invention, the resin film coated on the polyester film is formed by applying a coating liquid containing the crosslinking agent (A). As a crosslinking agent (A), an oxazoline group containing polymer, an epoxy resin, a urea resin, and a melamine resin can be used, for example.

  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. Specifically, a polymer obtained by polymerizing an addition-polymerizable oxazoline (a) represented by the following formula and, if necessary, another monomer (b) can be mentioned.

（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.)

  Specific examples of the addition polymerizable oxazoline (a) represented by the above formula 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 and the like can be mentioned. 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.
Examples of the surfactant include conventionally known anionic, nonionic, cationic, amphoteric surfactants and reactive surfactants.
The polymerization temperature is usually 0 to 100 ° C, preferably 50 to 80 ° C. The polymerization time is usually 1 to 10 hours.

  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 (A) include polyepoxy compounds, diepoxy compounds, and monoepoxy compounds. Examples of the polyepoxy compound include sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris (2-hydroxyethyl) isocyanate, glycerol polyglycidyl ether, Methylolpropane polyglycidyl ether, N, N, N ′, N′-tetraglycidylmetaxylylenediamine, N, N, N ′, N′-tetraglycidyl-4,4′-diaminodiphenylmethane, N, N, N ′ , N′-tetraglycidyl-1,3-bisaminomethylcyclohexane and the like. Examples of the diepoxy compound include neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, resorcin diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, and polypropylene glycol diglycidyl. Examples include ether and polytetramethylene glycol diglycidyl ether. Examples of the monoepoxy compound include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether. Among these, N, N, N ′, N′-tetraglycidylmetaxylylenediamine, N, N, N ′, N′-tetraglycidyl-4,4′-diaminodiphenylmethane, N, N, N ′, N ′ Preferred examples include tetraglycidyl-1,3-bisaminomethylcyclohexane.

  Preferred examples of the urea resin used as the crosslinking agent (A) include dimethylol urea, dimethylol ethylene urea, dimethylol propylene urea, tetramethylol acetylene urea, 4-methoxy 5-dimethylpropylene urea dimethylol and the like.

  Examples of the melamine resin used as the crosslinking agent (A) 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, and mixtures thereof. Can be preferably mentioned.

  Examples of the methylol melamine derivative include monomethylol melamine, dimethylol melamine, trimethylol melamine, tetramethylol melamine, pentamethylol melamine, hexamethylol melamine and the like.

  Among these crosslinking agents (A), an oxazoline group-containing polymer is preferable because it exhibits particularly excellent easy adhesion. The crosslinking agent (A) may be used alone or in combination of two or more in some cases.

  The crosslinking agent (A) is contained in the coating solution in an amount of 10 to 100% by weight per 100% by weight of the solid content. The preferable content of the cross-linking agent (A) varies depending on the type of the cross-linking agent, and is preferably 20 to 100% by weight per 100% by weight of the solid content of the coating liquid if it is an oxazoline group-containing polymer. If it is an epoxy resin, it is preferably 10 to 50% by weight per 100% by weight of the solid content of the coating liquid. If the cross-linking agent (A) is less than this range, the cohesive strength of the coating layer is lowered, and particularly the adhesion durability under high humidity is lowered, which is not preferable.

  The coating liquid applied to the film preferably further contains a resin component (B) other than the crosslinking agent. That is, the coating liquid preferably contains 10 to 80% by weight of the crosslinking agent (A) and preferably 20 to 90% by weight of the resin component (B) other than the crosslinking agent per 100% by weight of the solid content. When the resin component (B) other than the crosslinking agent is less than 20% by weight, it is difficult to form a film, and as a result, adhesion with EVA may be insufficient, which is not preferable. If it exceeds 90% by weight, the blending amount of the crosslinking agent (A) becomes insufficient, so that the cohesive strength of the coating layer is lowered, and particularly the adhesion durability under high humidity is lowered, which is not preferable.

  The resin component (B) other than the crosslinking agent is preferably a polyester resin or an acrylic resin, and the glass transition points of these resins are preferably 20 to 100 ° C., and more preferably 30 to 90 ° C. for both. When the glass transition point is less than 20 ° C., blocking between the films may occur unfavorably, and when it exceeds 100 ° C., the coating layer becomes brittle and adhesion may not be maintained.

  As the polyester resin having a glass transition point in the range of 20 to 100 ° C., a polyester comprising the following polybasic acid or an ester-forming derivative thereof and 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, 5-sodium sulfoisophthalic acid and the like. 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, and p-hydroxybenzoic acid can 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, these monomers are mentioned.

  Moreover, as an acrylic resin whose glass transition point is the range of 20-100 degreeC, the acrylic resin formed by superposing | polymerizing the acrylic monomer which is illustrated below is mentioned. Examples of the acrylic monomer include alkyl acrylate, alkyl methacrylate (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl. 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; 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.) A monomer containing a carboxy group or a salt thereof such as acrylamide, methacrylamide, N-alkylacrylamide, N-alkylmethacrylamide, N, N-dialkylacrylamide, N, N-dialkylmethacrylate (the alkyl group includes a 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-di Alkoxyacrylamide, N, N-dialkoxymethacrylamide (alkoxy groups include methoxy, ethoxy, butoxy, isobutoxy, etc.), acryloylmorpholine, N-methylolacrylamide, N-methylolmethacrylamide Monomers containing amide groups such as N-phenylacrylamide and N-phenylmethacrylamide; monomers of acid anhydrides such as maleic anhydride and itaconic anhydride; vinyl isocyanate, allyl isocyanate, styrene, α-methyl styrene, vinyl methyl ether, vinyl Monomers such as 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, etc. Can be mentioned.

  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%.

  Inactive fine particles may be added to the resin film for the purpose of improving the handleability of the film or preventing blocking between the films. As such fine particles, organic or inorganic fine particles can be used, such as calcium carbonate, calcium oxide, aluminum oxide, kaolin, silicon oxide, zinc oxide, crosslinked acrylic resin particles, crosslinked polystyrene resin particles, melamine resin particles, crosslinked silicone resin. Particles can be exemplified.

A wax may be further added to the resin film for the purpose of obtaining better slipperiness. Specific examples of this wax include plant waxes such as carnauba wax, candelilla wax, rice wax, wood wax, jojoba oil, palm wax, rosin-modified wax, aucuric wax, sugar cane wax, esparto wax, bark wax, Animal waxes such as beeswax, lanolin, whale wax, ibota wax, shellac wax, mineral waxes such as montan wax, ozokerite, ceresin wax, petroleum waxes such as paraffin wax, microcrystalline wax, petrolactam, fishertro push wax, Synthetic hydrocarbon waxes such as polyethylene wax, oxidized polyethylene wax, polypropylene wax and oxidized polypropylene wax. Among these, carnauba wax, paraffin wax, and polyethylene wax are preferable because they are easy to adhere to hard coats and pressure-sensitive adhesives and have good lubricity. Wax is preferably used as an aqueous dispersion because of reduced environmental burden and ease of handling.
As a component that forms the resin film, for example, an antistatic agent, a colorant, a surfactant, or an ultraviolet absorber may be further blended.

[Production of film]
In the present invention, a resin film using the above components is applied to at least one surface of the polyester film. The coating of the resin film is preferably carried out by applying an aqueous liquid containing a component for forming a film to a stretchable polyester film, followed by drying, stretching, and heat treatment. 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 the resin film as a 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. The coating may be formed only on one side of the film or on both sides as necessary.

The stretchable polyester film coated with the aqueous liquid is guided to a drying step and a stretching treatment step as necessary. Such processing can be performed under conditions accumulated in the industry. As preferable conditions, 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 longitudinal direction, 3 to 5 times in the transverse direction, It is 1 to 3 times in the 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 250 μm, and the thickness of the coating film is preferably 0.01 to 1 μm.

[Back surface protection film]
The easily adhesive polyester film for solar cell back surface protective film of the present invention can be used as a solar cell back surface protective film, either alone or in combination.

It is preferable to laminate | stack the film and foil which have water vapor | steam barrier property in the easily-adhesive polyester film for solar cell back surface protective films of this invention in order to provide water vapor | steam barrier property. The easily adhesive polyester film for solar cell back surface protective film provided with a water vapor barrier layer preferably has a water vapor transmission rate of 5 g / (m ² · 24 h) or less measured according to JIS Z0208-73.

  Examples of the film having water vapor barrier properties include polyvinylidene chloride film, polyvinylidene chloride coated film, polyvinylidene fluoride coated film, silicon oxide deposited film, aluminum oxide deposited film, and aluminum deposited film. A copper foil can be exemplified.

  These films or foils are laminated on the opposite side of the EVA adhesive surface of the easily adhesive polyester film for the solar cell back surface protective film of the present invention, or two sheets of the solar cell back surface of the present invention with the EVA adhesive side facing outside. It can be used in a form that takes a structure sandwiched between easy-adhesive polyester films for protective films.

Hereinafter, the present invention will be further described by examples. Each characteristic value was measured by the following method.
(1) Glass transition point (Tg)
It measured with the temperature increase rate of 20 degree-C / min with the thermal analysis 2000 type | mold differential calorimeter made from DuPont.

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

(3) Adhesiveness with EVA 2 films cut to 20 mm width x 100 mm length, EVA sheet (SOLAR EVA (R) SC4 manufactured by High Sheet Industry Co., Ltd.) cut to 20 mm width x 50 mm length Prepare one each. 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 are 120 ° C. and 0.02 MPa for 5 minutes, then heated to 150 ° C., the press pressure is increased to 0.1 MPa, and the pressure is bonded for 25 minutes. Measure the adhesive strength of the thermocompression-bonded sample at 23 ° C and 50% RH in accordance with JIS-Z0237 with the unattached film sandwiched between the upper and lower clips at a peel angle of 180 ° and a tensile speed of 100 mm / min. did.
A: 20 N / 20 mm or more: Very good adhesion B: 10 N / 20 mm or more, less than 20 N / 20 mm ... Adhesion good Δ: 5 N / 20 mm or more and less than 10 N / 20 mm ... Adhesion slightly good × : Less than 5N / 20mm ... poor adhesion

(4) Durability of bonding with EVA The thermocompression bonding sample prepared in (3) above was treated at 85 ° C. and 85% RH for 1000 hours according to JIS-C8917-1998, and then bonded in the same manner as in (3) above. The strength was evaluated and compared with the adhesive strength before treatment.
◎: Adhesion retention 75% or more: Adhesion durability is very good ○: Adhesion retention 50% or more, less than 75% ... Adhesion durability △: Adhesion retention 25% or more, 50 Less than%: Adhesiveness slightly good X: Adhesiveness retention rate less than 25% ... Adhesive failure

(5) Weather resistance Using the sunshine weather meter (Suga Test Instruments Co., Ltd., WEL-SUN-HCL type), the sample prepared in the above (3) is 1000 hours according to JIS-K-6833b. (Equivalent to outdoor exposure for one year) An outdoor exposure promotion test was conducted by irradiation. The adhesiveness was measured in the same manner as in (3) above, and the evaluation was made by comparison with the adhesiveness before the acceleration test.
◎: Adhesion retention rate 75% or more: very good weather resistance ○: Adhesion retention rate 50% or more, less than 75% ... Weather resistance good △: Adhesion retention rate 25% or more, less than 50% ..Slightly good weather resistance ×: Adhesion retention rate of less than 25% ・ ・ ・ Poor weather resistance

[Examples 1 to 7 and Comparative Example 1]
Polyethylene terephthalate (intrinsic viscosity: 0.62) was melt extruded onto a rotating cooling drum maintained at 20 ° C. to obtain an unstretched film. Next, the film was stretched 3.3 times in the longitudinal direction at 100 ° C., and then an aqueous coating solution having a concentration of 8% by weight of the following coating film composition (Table 1) was uniformly applied on both sides thereof with a roll coater. Next, this coated film was subsequently dried at 95 ° C., stretched 3.6 times in the lateral direction at 110 ° C., and heat-set by shrinking 3% in the width direction at 220 ° C. to obtain a polyester film having a thickness of 50 μm. It was. The thickness of the coating film was 0.05 μm. These evaluation results are shown in Table 2.

Crosslinker A-1: Heavy weight having an oxazoline group composed of 30 mol% of methyl methacrylate / 2 mol% of 2-isopropenyl-2-oxazoline / 10 mol% of polyethylene oxide (n = 10) methacrylate / 30 mol% of acrylamide Combined (Tg = 50 ° C.).
Crosslinker A-2: Epoxy resin; N, N, N ′, N′-tetraglycidylmetaxylylenediamine Crosslinker A-3: Melamine resin; Trimethoxymethylmelamine (trimethylolmelamine etherified with methanol)
Crosslinking agent A-4: dimethylol ethylene urea.
Other resin component B-1: acid component is 2,6-naphthalenedicarboxylic acid 65 mol% / isophthalic acid 30 mol% / 5-sodium sulfoisophthalic acid 5 mol%, glycol component is ethylene glycol 90 mol% / diethylene glycol 10 mol % Copolymerized polyester (Tg = 80 ° C.).
Other resin component B-2: Acid component is composed of 80 mol% of terephthalic acid / 15 mol% of isophthalic acid / 5 mol% of 5-sodium sulfoisophthalic acid, and glycol component is composed of 60 mol% of ethylene glycol / 40 mol% of diethylene glycol. Copolyester (Tg = 43 ° C.).
Other resin component B-3: acrylic resin (Tg = 27 ° C.) composed of 55 mol% methyl methacrylate / 40 mol% ethyl acrylate / 3 mol% N-methylol acrylamide / 2 mol% 2-hydroxyethyl methacrylate
Filler: Silica filler (average particle size 60 nm)
Additive: Carnauba wax wetting agent: polyoxyethylene (n = 7) lauryl ether

以降は、実施例３と同様にフィルムを得た。このポリエステルフィルムの評価結果を表２に示す。 [Example 8]
Polyethylene terephthalate (inherent viscosity: 0.62) containing 1% by weight of an ultraviolet absorber represented by the following formula was melt-extruded on a rotating cooling drum maintained at 20 ° C. to obtain an unstretched film.

Thereafter, a film was obtained in the same manner as in Example 3. The evaluation results of this polyester film are shown in Table 2.

[Comparative Example 2]
A polyester film was obtained in the same manner as in Example 1 except that the aqueous liquid was not applied. The properties of the obtained polyester film are shown in Table 2.

[Example 9]
Polyethylene-2,6-naphthalate (intrinsic viscosity: 0.58) was melt extruded onto a rotating cooling drum maintained at 60 ° C. to obtain an unstretched film. Subsequently, after extending | stretching 3.3 times at 140 degreeC to the vertical direction, the aqueous coating liquid used in Example 3 was apply | coated uniformly with the roll coater on the single side | surface. Subsequently, this coated film was subsequently dried at 130 ° C., stretched 3.6 times in the transverse direction at 150 ° C., and heat-set by shrinking 3% in the width direction at 240 ° C. to obtain a polyester film having a thickness of 50 μm. It was. The thickness of the coating film was 0.05 μm. These evaluation results are shown in Table 2.

  As is clear from the results shown in Table 2, the polyester film for solar cell back surface protective film of the present invention is excellent in adhesion to EVA and is useful as a polyester film for solar cell back surface protective film.

  The polyester film for a solar cell back surface protective film of the present invention can be suitably used as a protective film on the back surface of a solar cell.

Claims (8)

  It consists of a polyester film and a resin film coated thereon, and the resin film is formed by applying a coating solution to the film, and the coating solution contains 10 to 100% by weight of the crosslinking agent (A) per 100% by weight of the solid content. An easy-adhesive polyester film for a solar cell back surface protective film.   The easily adhesive polyester film for a solar cell back surface protective film according to claim 1, wherein the crosslinking agent (A) is at least one selected from the group consisting of an oxazoline group-containing polymer, a urea resin, a melamine resin, and an epoxy resin.   The coating liquid applied to the film contains 10 to 80% by weight of a crosslinking agent (A) per 100% by weight of the solid content and a resin component (B) other than 20 to 90% by weight of the crosslinking agent, and a resin component other than the crosslinking agent (B) The easily adhesive polyester film for solar cell back surface protective films whose glass transition point is 20-100 degreeC polyester resin or acrylic resin.   The easily adhesive polyester film for solar cell back surface protective films according to claim 1, wherein the polyester film is a 2,6-naphthalenedicarboxylate film.   The easily adhesive polyester film for solar cell back surface protection films according to claim 1, wherein the polyester film contains 0.1 to 5 wt% of an ultraviolet absorber. The ultraviolet absorber is represented by the following formula (I)

(Where X ¹ is a divalent aromatic residue in which the two bonds from X ¹ represented by the above formula are in the 1-position and 2-position positional relationship; 2 or 3; R ¹ is an n-valent hydrocarbon residue, which may further contain a heteroatom, or R ¹ may be a direct bond when n = 2.)
And a cyclic imino ester represented by the following formula (II)

(Where A is the following formula (II) -a

Or a group represented by the following formula (II) -b

R ² and R ³ are the same or different monovalent hydrocarbon residues; X ² is a tetravalent aromatic residue, which may further contain a heteroatom. . )
The easily adhesive polyester film for solar cell back surface protective films of Claim 1 which is at least 1 sort (s) of compound chosen from the cyclic imino ester represented by these.   The solar battery back surface protective film according to claim 1, wherein the polyester film is a biaxially stretched polyester film, and the resin film is applied before the crystal orientation of the film is completed, and is applied by drying, stretching and heat treatment. Easy-adhesive polyester film.   The solar cell back surface protective film using the easily adhesive polyester film for solar cell back surface protective films in any one of Claims 1-7.