JP2007268710A - Polyester film for protective film for solar cell rear face and protective film for solar cell rear face using polyester film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an easily adhesive polyester film for a protective film for a solar cell rear face which is excellent in mechanical properties, heat resistance, moisture resistance, and durability of adhesion to an EVA resin. <P>SOLUTION: The polyester film for the protective film is composed of a polyester film having ethylene terephthalate repeating units as main repeating units and an easily adhesive layer formed on the polyester film and has a specified X-ray diffraction intensity ratio. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a polyester film for a solar cell back surface protective film and a solar cell back surface protective film using the same.
More specifically, an ethylene-vinyl acetate copolymer (hereinafter abbreviated as EVA) resin, which is a sealing resin for solar cells, a polyester film for a solar cell back surface protective film exhibiting excellent adhesion durability, and a sun using the same The present invention relates to a battery back surface protective film.

  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 Laid-Open No. 6-38264. It has a structure in which an element is sandwiched and a sealing resin is filled in an internal gap. Here, as the sealing resin, EVA resin is used because of its high transparency and excellent moisture resistance.

  For example, JP-A-11-261085 and JP-A-11-186575 describe that a polyethylene-based resin, a polyester-based resin sheet, or a fluororesin film is used as the protective film on the back surface. However, such a protective film does not necessarily have sufficient adhesiveness with the EVA resin and remains uneasy about long-term durability.

  Polyester films, especially biaxially stretched films of polyethylene terephthalate and polyethylene naphthalate, have excellent mechanical properties, heat resistance, and moisture resistance, so they are used for magnetic tapes, ferromagnetic thin film tapes, photographic films, packaging films, and electronic parts. It is widely used as a film, an electrical insulating film, a metal plate laminating film, and a display member film, and has excellent performance as a solar cell back surface protective film. However, a polyester film, particularly a polyester film that has been biaxially stretched and highly oriented and crystallized, has an inactive surface, and has very poor adhesion to an EVA resin. Therefore, it has been studied to improve the adhesion between the polyester film and the EVA resin. For example, Japanese Patent Application Laid-Open No. 2003-60218 proposes to laminate a thermal adhesive layer made of a styrene / olefin copolymer resin. Has been.

JP-A-11-261085 JP-A-11-186575 JP 2003-60218 A

  However, the effect of improving the adhesion with EVA resin is not sufficient in the conventional technology, and it cannot be used particularly when constructing a large-scale photovoltaic power generation system. Also, biaxially stretched polyester films have molecular chains oriented in the stretching direction, that is, the in-plane direction of the film, and therefore tend to break down in a layered form because the cohesive force in the thickness direction of the film is weakened. For this reason, even if the adhesive force between the EVA resin and the film can be maintained, there is a problem that delamination of the film occurs and the durability deteriorates.

  The present invention provides a polyester film for a back surface protective film of a solar cell that eliminates the problems of the prior art and has excellent mechanical properties, heat resistance, and moisture resistance and is excellent in durability for adhesion to EVA resin. For the purpose.

  That is, the present invention comprises a polyester film having ethylene terephthalate units as a main repeating unit and an easy-adhesion layer provided thereon, wherein the polyester film exhibits an X-ray diffraction intensity ratio satisfying the following formula (I). It is a polyester film for solar cell back surface protective films.

  ADVANTAGE OF THE INVENTION According to this invention, the polyester film excellent in the adhesive durability with EVA resin can be provided, providing the outstanding mechanical property, heat resistance, and moisture resistance.

Hereinafter, the present invention will be described in detail.
[Polyester film]
The polyester film in this invention consists of polyester which has an ethylene terephthalate unit as a main repeating unit. In the present invention, the “main repeating unit” means 80 mol% or more per 100 mol% of all repeating units. The polyester of the polyester film may be a copolyester, and is preferably a polyester obtained by copolymerizing a copolymer component from the viewpoint of obtaining a film that does not easily cause delamination without impairing heat resistance. For this purpose, the amount of the copolymer component is 20 mol% or less, preferably 3 to 15 mol%. Examples of the copolymer component include aromatic dicarboxylic acid components such as isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 4,4′-diphenyldicarboxylic acid, Examples of the diol component include 4-butanediol, 1,4-cyclohexanedimethanol, and 1,6-hexanediol.

The polyester film in the present invention has an X-ray diffraction intensity ratio that satisfies the following formula (I).

  When the X-ray diffraction intensity ratio of the above formula (I) is less than 0.16, delamination tends to occur, and the durability of adhesion between the EVA resin and the film is lowered. If this X-ray diffraction intensity ratio exceeds 0.24, the heat resistance of the film may be lowered, and problems such as deformation in the sealing step of the solar cell may occur.

  The molecular weight of the polyester is preferably higher in order to prevent delamination, but as the molecular weight increases, the stress applied during stretching increases, and as a result, the molecular orientation in the in-plane direction tends to increase. It is necessary to devise such as selecting a stretching condition that does not easily cause orientation, or using a copolyester that hardly causes molecular orientation.

  The polyester may contain fine particles as a lubricant as necessary in order to improve the film winding property during film formation, the film transportability in the back surface protective film processing step for solar cells, and the like. As such fine particles, either inorganic fine particles or organic fine particles may be used. 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, urea resin particles, melamine resin particles, and crosslinked silicone resin particles.

  For polyester films, resins such as colorants, antistatic agents, antioxidants, lubricants, catalysts, polyethylene, polypropylene, ethylene-propylene-polymers, and olefinic ionomers do not impair film properties such as mechanical strength. You may make it contain in the range. Moreover, in order to improve surface reflectivity or arrange a design when used as a solar cell back surface protective film, a dye or pigment may be contained in order to color white, black, or other colors.

[Ultraviolet absorber]
In order to improve the weather resistance of the film, the polyester constituting the polyester film in the present invention may 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). When the ultraviolet absorber is contained, the content thereof is preferably 0.1 to 5% by weight, more preferably 0.2 to 3% by weight per 100% by weight of the total of the polyester and the ultraviolet absorber. 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 film 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 used. In particular, kneading into the polymer in the melting step before film formation is preferable from the viewpoint of preventing a decrease in the degree of polymerization of the polyester. At that time, the kneading of the ultraviolet absorber can be carried out by employing a method such as a direct addition method of compound powder or a master batch method.

[Easily adhesive layer]
In this invention, an easy-adhesion layer is provided on a polyester film for the purpose of improving adhesiveness with EVA resin which is sealing resin of a solar cell. The easy adhesion layer may be provided on one side or on both sides. The easy-adhesion layer comprises (A) a polyester resin or acrylic resin having a glass transition point in the range of 20 to 100 ° C., or a mixture of these resins and polyvinyl alcohol in an amount of 0 to 90% by weight, and (B) a crosslinking agent of 10 to 100. It is preferably formed using a paint containing a solid content of% by weight. As the crosslinking agent (B), an oxazoline group-containing polymer, a urea resin, a melamine resin, and an epoxy resin are preferable, and an oxazoline group-containing polymer is particularly preferable.

  The easy adhesion layer can be provided using a known coating method. Preferably, the coating is performed by applying an aqueous coating solution containing a component constituting the easy-adhesion layer to a stretchable polyester film that has not been completely stretched, followed by drying, stretching, and heat treatment. The solid content concentration of the aqueous coating liquid is usually 30% by weight or less, preferably 10% by weight or less.

  The stretchable polyester film to be coated with the coating liquid is an unstretched film, a uniaxially stretched film, or a biaxially stretched film. Of these, a uniaxially stretched film uniaxially stretched in the film extrusion direction (longitudinal direction) is particularly preferred.

[Production method]
In order to obtain a polyester film satisfying the X-ray diffraction intensity ratio of the present invention, conditions during film production, particularly stretching conditions, are controlled according to the properties of the polyester used. For example, when homopolyethylene terephthalate is used, it is preferable to produce the film at a relatively low ratio of the longitudinal draw ratio and the transverse draw ratio. The draw ratio at this time is preferably 2 to 3.5 times, more preferably 2.2 to 3.0 times. Stretching in the longitudinal direction and the transverse direction may be performed simultaneously using a simultaneous biaxial film forming machine, and this method is preferable because a film with low orientation is easily obtained even at a high magnification. The glass transition temperature is abbreviated as Tg.

  Moreover, when using a copolyester, it is good to manufacture by a high draw ratio compared with the case where a homopolyethylene terephthalate is manufactured, Preferably a longitudinal draw ratio and a transverse draw ratio are 3.0-4.2 times, More preferably, it is good to manufacture by 3.2 to 3.8 times. When the copolymerization ratio is low, a relatively low draw ratio may be taken, and when the copolymerization ratio is high, a relatively high draw ratio may be taken.

If the manufacturing method of the polyester film in this invention is demonstrated in more detail, it can manufacture as follows, for example. First, polyester or copolymer polyester is melt-extruded from a die into a film, cooled and solidified with a casting drum to form an unstretched film, and this unstretched film is totaled once or twice in the longitudinal direction at Tg to (Tg + 60) ° C. Is stretched to 2 to 5 times, preferably 2 to 3.5 times, and more preferably 2.2 to 3.0 times, and an aqueous coating liquid of an easy-adhesion layer is applied, and then Tg to ( Tg + 60) The film is stretched at a temperature of 2 to 5 times, preferably 2 to 3.5 times, more preferably 2.2 to 3.0 times in the width direction at a temperature. Specifically, the stretching temperature at this time is, for example, 90 to 150 ° C. The drying conditions for drying the film are preferably 90 to 130 ° C. and 2 to 10 seconds. And when further heat-fixing, it heat-sets, for example for 2 to 60 second at 180-250 degreeC. The thickness of the coating film when applying the aqueous coating liquid is preferably 0.01 to 1 μm, and the coating amount is, for example, 0.5 to 20 g, preferably 1 to 10 g per 1 m ² of the running film. The thickness of the easy adhesion layer finally obtained is, for example, 0.01 to 0.3 μm, and the thickness of the finally obtained film is, for example, 25 to 300 μm, preferably 30 to 250 μm.

  In addition, when applying the aqueous coating liquid to the film, it is not preferable to carry out a normal coating process, that is, a process separated from the film manufacturing process in the film that has been formed, because it is easy to entrap soot and dust. From this 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 film of an easily bonding layer further improves.

  As a coating method of the coating liquid, 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 aqueous coating liquid is preferably used as an aqueous dispersion or emulsion. The easy adhesion layer may be formed only on one side of the film or on both sides as required.

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

  In order to improve the dimensional stability during heating, for example, a method of shrinking in the vertical direction in a heat treatment step disclosed in JP-A-57-57628 or a film shown in JP-A-1-275031 in a suspended state, for example. A relaxation heat treatment method can be used.

[Solar cell back surface protective film]
The 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 of two or more. Furthermore, it is preferable to laminate a film or foil having gas barrier properties for the purpose of imparting gas barrier properties. Here, the gas barrier property means the water vapor barrier property, and means the property that the water vapor transmission rate measured in accordance with JIS Z0208-73 is 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 can be illustrated. These films or foils are laminated on the opposite side of the EVA adhesive surface of the polyester film for solar cell back surface protective film of the present invention, or are sandwiched between two polyester films with the EVA resin adhesive side facing outside. be able to.

Hereinafter, the present invention will be further described by examples.
Each characteristic value was measured by the following method.

(1) Intrinsic Viscosity The viscosity of a solution using an orthochlorophenol solvent was measured at 35 ° C.

(2) X-ray diffraction intensity ratio Using Cu-Kα ray as an X-ray source, conditions of divergence slit 1/2 °, scattering slit 1/2 °, light receiving slit 0.15 mm, scan speed 1.000 ° / min The X-ray diffraction intensity from the following polyethylene terephthalate crystal plane was measured by a multiple peak separation method using a Pseudo Voice peak model, and was determined from the following formula.

(3) Adhesiveness with EVA resin Two sheets of film cut to 20 mm width x 100 mm length, EVA resin sheet (SOLAR EVA (R) SC4 manufactured by High Sheet Industry Co., Ltd.) cut to 20 mm width x 50 mm length One piece of each was prepared. Heat sealer (Tester Sangyo Co., Ltd.) is placed in the order of film / EVA resin sheet / film so that the EVA resin sheet is positioned at the approximate center of the film and the surface to be evaluated for easy contact is on the EVA side. Pressing was performed using TP-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 pulling speed of 100 mm / min. did. EVA is an abbreviation for ethylene-vinyl acetate copolymer.
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% ... Adhesive durability is good △: Adhesion retention 25% or more, 50 Less than%: Adhesiveness slightly good X: Adhesiveness retention less than 25%: Adhesive failure

[Example 1]
Polyethylene terephthalate (inherent viscosity: 0.62) containing 10% by weight of white pigment A (rutile titanium oxide: JR-405 manufactured by Teika Co., Ltd.) as a colorant is melt extruded onto a rotating cooling drum maintained at 20 ° C. Thus, an unstretched film was obtained. Subsequently, after extending | stretching 2.7 times at 100 degreeC to the vertical direction, the following coating agent A was apply | coated uniformly with the roll coater on the both surfaces. This coated film was subsequently dried at 95 ° C., stretched 3.0 times at 110 ° C. in the transverse direction, and heat-set while shrinking 2% in the width direction at 235 ° C. to obtain a polyester film having a thickness of 50 μm. . The thickness of the coating film was 0.06 μm.

<Coating agent A>
48 parts of dimethyl 2,6-naphthalenedicarboxylate, 14 parts of dimethyl isophthalate, 4 parts of dimethyl 5-sodiumsulfoisophthalate, 31 parts of ethylene glycol and 2 parts of diethylene glycol are charged into a reactor, and 0.05 part of tetrabutoxy titanium Was added and heated under a nitrogen atmosphere while controlling the temperature at 230 ° C., and the produced methanol was distilled off to conduct a transesterification reaction. Subsequently, the temperature of the reaction system was gradually raised to 255 ° C., and the pressure inside the system was reduced to 1 mmHg to carry out a polycondensation reaction to obtain a polyester. 25 parts of this polyester was dissolved in 75 parts of tetrahydrofuran, and 75 parts of water was dropped into the resulting solution under high-speed stirring at 10,000 rpm to obtain a milky white dispersion. Then, this dispersion was subjected to a reduced pressure of 20 mmHg. Distillation was performed, and tetrahydrofuran was distilled off to obtain an aqueous dispersion of polyester having a solid content of 25% by weight.

  Next, 3 parts of sodium lauryl sulfonate as a surfactant and 181 parts of ion-exchanged water are charged into a four-necked flask and the temperature is raised to 60 ° C. in a nitrogen stream, and then 0.5% ammonium persulfate is used as a polymerization initiator. Part, 0.2 part of sodium hydrogen nitrite was added, and further 30.1 parts of methyl methacrylate as a monomer, 21.9 parts of 2-isopropenyl-2-oxazoline, polyethylene oxide (n = 10) methacrylic acid 39. A mixture of 4 parts and 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.

  Coating agent A was prepared by mixing 10 parts by weight of the above polyester aqueous dispersion, 5 parts by weight of the acrylic water dispersion and 85 parts by weight of the aqueous solution. The X-ray diffraction intensity ratio of the obtained polyester film was 0.17. Using this polyester film, the adhesiveness to EVA and the adhesive durability were evaluated, and both the adhesiveness ◎ and the adhesive durability ○ were good.

[Example 2]
Copolymer polyethylene terephthalate (inherent viscosity: 0) containing 10% by weight of white pigment A (rutile titanium oxide: JR-405 manufactured by Teika Co., Ltd.) as a colorant and copolymerized with 10 mol% of 2,6-naphthalenedicarboxylic acid .75) was melt extruded onto a rotating cooling drum maintained at 20 ° C. to obtain an unstretched film. Next, the film was stretched 3.4 times in the longitudinal direction at 100 ° C., and then the following coating agent A was uniformly applied on both sides thereof with a roll coater. This coated film was subsequently dried at 95 ° C., stretched 3.6 times in the transverse direction at 110 ° C., and heat-set while shrinking 2% in the width direction at 200 ° C. to obtain a polyester film having a thickness of 50 μm. . The thickness of the coating film was 0.06 μm. The X-ray diffraction intensity ratio of the obtained film was 0.23. When this polyester film was used to evaluate the adhesion to EVA and the adhesion durability, both the adhesion 接着 and the adhesion durability 耐久 were very good.

[Comparative Example 1]
Polyethylene terephthalate (inherent viscosity: 0.66) containing 10% by weight of white pigment A (rutile titanium oxide: JR-405 manufactured by Teika Co., Ltd.) as a colorant is melt extruded onto a rotating cooling drum maintained at 20 ° C. Thus, an unstretched film was obtained. Subsequently, after extending | stretching 3.0 times at 100 degreeC to the vertical direction, the following coating agent A was apply | coated uniformly with the roll coater on both surfaces. The coated film was subsequently dried at 95 ° C., stretched 3.1 times in the transverse direction at 110 ° C., and heat-set while shrinking 4% in the width direction at 225 ° C. to obtain a polyester film having a thickness of 50 μm. . The thickness of the coating film was 0.06 μm. The X-ray diffraction intensity ratio of the obtained polyester film was 0.13. When this polyester film was used to evaluate the adhesion and durability with EVA, the adhesion was very good with で, but the adhesion durability was x, and delamination of the polyester film was observed. It was.

[Comparative Example 2]
A polyester film having a thickness of 50 μm was prepared in the same manner as in Example 1 except that the coating agent A was not applied. The X-ray diffraction intensity ratio of the polyester film was 0.17.
When this polyester film was used to evaluate the adhesion to EVA and the adhesion durability, the adhesion was poor at x. The adhesion durability evaluation was not performed.

  The polyester film for solar cell back surface protective film of the present invention has excellent mechanical properties, heat resistance, and moisture resistance, and has excellent adhesion to EVA resin, and is a material constituting the solar cell back surface protective film. Useful.

Claims (2)

A solar cell back surface protection comprising a polyester film comprising ethylene terephthalate units as a main repeating unit and an easy-adhesion layer provided thereon, wherein the polyester film exhibits an X-ray diffraction intensity ratio satisfying the following formula (I): Polyester film for membrane.

  A solar cell back surface protective film comprising the polyester film for solar cell back surface protective film according to claim 1.