WO2010079798A1

WO2010079798A1 - Polyester film for solar cell back surface protection film

Info

Publication number: WO2010079798A1
Application number: PCT/JP2010/050084
Authority: WO
Inventors: 勝也伊藤; 潤稲垣; 睦夫西; 史朗濱本
Original assignee: 東洋紡績株式会社
Priority date: 2009-01-07
Filing date: 2010-01-07
Publication date: 2010-07-15

Abstract

Provided is a polyester film for a solar cell back surface protection film for a thin film silicon solar cell, which has excellent durability under high temperature and high humidity. The polyester film for a solar cell back surface protection film is characterized in that: whiteness is 50 or greater; 3‑50 mass% of microparticles, the mean particle size of which is 0.1‑3 µm, are contained; and a film acid value is at least 1 (eq/ton) and not more than 30 (eq/ton).

Description

Polyester film for solar cell back surface protective film

This invention relates to the polyester film for solar cell back surface protective films.

The demand for solar cells capable of obtaining power using energy not derived from petroleum fuel is increasing from the viewpoint of environmental protection. As described in, for example, Japanese Utility Model Publication No. 6-38264, a solar cell module generally has a plurality of plate-like solar cell elements sandwiched between a glass substrate on a light receiving side and a back surface protective film, It takes a structure in which the internal gap is filled with sealing resin.

プラスチック A plastic film having excellent mechanical properties, heat resistance and moisture resistance is used for the back surface protective film. For example, Japanese Patent Laid-Open Nos. 2002-26354 and 2003-60218 propose a back surface protective film using a polyethylene terephthalate film. And a white back surface protective film may be used for the purpose of improving the power generation efficiency of a solar cell.

JP 2002-26354 A Japanese Patent Laid-Open No. 2003-60218 JP-A-60-250946 JP 2004-247390 A JP 2002-134771 A JP 2007-208179 A JP 2008-85270 A

By using a white polyester film, it is possible to reflect sunlight and increase power generation efficiency. A white polyester film needs to add a large amount of particles to a polyester substrate. Therefore, in order to improve their dispersibility and mixing state, the raw material is preliminarily mixed with two or more kinds of materials, and the resin is deteriorated because the melting time is increased even in a normal extrusion process. Easy to do. Therefore, when used as a solar cell under high temperature and high humidity, it has been a problem that durability is poor.

In addition, when using them as solar cell modules, it may be necessary to have adhesiveness with EVA resin.

Furthermore, with the progress of thinning of solar cell elements, sunlight easily passes through the silicon thin film and reaches the back protective film directly. Therefore, decomposition / deterioration of the back surface protective film due to light irradiation is becoming a problem. For this reason, in a large-scale photovoltaic power generation system, a vast solar cell module is installed, so that the back surface protective film may be decomposed and deteriorated due to reflection from the ground.

The present invention relates to the above-mentioned problem, that is, a polyester film for a solar cell back surface protective film having good durability under high temperature and high humidity. In addition, the present invention relates to a polyester film for a solar cell back surface protective film having good adhesion to EVA resin. In addition, the present invention relates to a polyester film for a solar cell back surface protective film having good durability under light irradiation.

The first invention contains 3 to 50% by mass of fine particles having a whiteness of 50 or more and an average particle size of 0.1 to 3 μm, and the acid value of the film is 1 (eq / ton) to 30 (eq / ton) It is a polyester film for solar cell back surface protective film characterized by the following.
2nd invention is the said polyester film for solar cell back surface protective films which has a coating layer which has at least 1 sort (s) of a polyester resin, a polyurethane resin, or a polyacryl resin as a main component at least on one side.
A third invention is the above-mentioned polyester film for a solar cell backside protective film, characterized by having a thermal adhesive layer having a thickness of 1.0 to 40 μm mainly composed of an amorphous polyester resin on at least one side.
A fourth invention is the polyester film for a solar cell back surface protective film, wherein the fine particles are titanium dioxide fine particles mainly composed of a rutile type and contain 3 to 50% by mass of the fine particles.
5th invention is the said polyester film for solar cell back surface protective films characterized by having an apparent specific gravity of 0.7 or more and 1.3 or less by having many fine cavities inside a film.
The sixth invention contains a polyester layer (skin layer) containing many cavities derived from fine particles having an average particle size of 0.1 to 3 μm, and contains many cavities derived from a thermoplastic resin incompatible with polyester. The polyester film for a solar cell back surface protective film, wherein a polyester layer (core layer) is laminated.
7th invention is the polyester film for solar cell back surface protective films characterized by the acid value of polyester used as a film raw material being 1 (eq / ton) or more and 30 (eq / ton) or less.

The present invention has light reflection efficiency and excellent durability under high temperature and high humidity. Furthermore, the preferable aspect of this invention has favorable adhesiveness with EVA resin in addition to the said effect. Furthermore, the preferable aspect of this invention has the outstanding durability under light irradiation in addition to the said effect. Therefore, it is useful in solar cells, particularly thin film silicon solar cells.

The polyester in the present invention is a polycondensation of an aromatic dicarboxylic acid or its ester such as terephthalic acid, isophthalic acid or naphthalenedicarboxylic acid with a glycol such as ethylene glycol, diethylene glycol, 1,4-butanediol or neopentyl glycol. Polyester produced. In addition to the method of directly reacting aromatic dicarboxylic acid and glycol, these polyesters can be polycondensed by transesterification of alkyl ester of aromatic dicarboxylic acid and glycol, or diglycol ester of aromatic dicarboxylic acid. Can be produced by a method such as polycondensation. Typical examples of such polyester include polyethylene terephthalate, polyethylene butylene terephthalate, polyethylene-2,6-naphthalate and the like. This polyester may be a homopolymer or a copolymer of a third component. In any case, in the present invention, a polyester having an ethylene terephthalate unit, a butylene terephthalate unit or an ethylene-2,6-naphthalate unit of 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more is preferable. .

The polyester used as the film raw material preferably has an acid value of 1 (eq / ton) to 30 (eq / ton), more preferably 2 (eq / ton) to 20 (eq / ton), Preferably they are 2 (eq / ton) or more and 16 (eq / ton) or less. If it exceeds 30 (eq / ton), a film having good hydrolysis resistance cannot be obtained. Polyesters of less than 1 (eq / ton) are difficult to produce industrially.

The white polyester film substrate used in the present invention is preferably a white polyester film having a whiteness of 50 or more. The higher the whiteness is, the higher the reflectance of sunlight is. Therefore, the whiteness is preferably 50 or more, more preferably 60 or more, and further preferably 80 or more for power generation efficiency when used as a solar cell module. .

The white polyester film substrate used in the present invention is preferably a white polyester film having a thickness of 38 to 1000 μm, more preferably 50 to 250 μm, and further preferably 75 to 188 μm. When the thickness of the substrate is less than 38 μm, the rigidity as a support becomes insufficient, and when it exceeds 1000 μm, it is not preferable because processing such as cutting becomes difficult.

The film of the present invention contains fine particles having an average particle diameter of 0.1 to 3 μm in an amount of 3 to 50% by mass, preferably 4 to 25% by mass, based on the total mass of the film. If it is 0.1 μm or less or exceeds 3 μm, it is difficult to make the whiteness of the film 50 or more even if the addition amount is increased. Moreover, if it is less than 3 mass%, it will become difficult to make whiteness into 50 or more. If it exceeds 50% by mass, the film weight increases, making it difficult to handle it during processing.

In addition, the average particle diameter of this invention is calculated | required by the electron microscope method. Specifically, the following method is used.
The fine particles are observed with a scanning electron microscope, the magnification is appropriately changed according to the size of the particles, and the photographed image is enlarged and copied. Next, the outer circumference of each particle is traced for at least 200 fine particles selected at random. The equivalent circle diameter of the particles is measured from these trace images with an image analysis apparatus, and the average value of these is taken as the average particle diameter.

As the fine particles of the present invention, inorganic or organic particles can be used. Examples of these fine particles include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, carbon black, acid value zinc, titanium oxide, zinc sulfide, and organic white pigment, but are not particularly limited. Absent. From the viewpoint of improving whiteness and productivity, titanium oxide or barium sulfate is preferable, and titanium oxide is more preferable. The titanium oxide may be either anatase type or rutile type. Further, the surface of the fine particles may be subjected to an inorganic treatment such as alumina or silica, or may be subjected to an organic treatment such as silicon or alcohol.

The addition of fine particles into the film is possible by using a known method, but a master batch method (MB method) in which a polyester resin and fine particles are mixed in an extruder in advance is preferable. Further, it is possible to adopt a method in which a polyester resin and fine particles which have not been dried in advance are put into an extruder and MB is produced while moisture and air are deaerated. Furthermore, it is preferable to prepare an MB using a polyester resin that has been slightly dried in advance to suppress an increase in the acid value of the polyester. In this case, a method of extruding while degassing, a method of extruding without deaeration with a sufficiently dried polyester resin, and the like can be mentioned.

As a preferred embodiment of the present invention, when excellent durability is required even under light irradiation, specifically, when the UV irradiation is performed at 63 ° C., 50% Rh, irradiation intensity of 100 mW / cm ² for 100 hours, the elongation at break is maintained. The rate is preferably 35% or more, more preferably 40% or more. Thus, in the preferable aspect of this invention, since photodecomposition and deterioration of a film are suppressed also by light irradiation, it is suitable as a back surface protective film of the solar cell used outdoors.

In order to control the durability under light irradiation within the above range, it is preferable to add fine particles of titanium dioxide mainly composed of rutile type as fine particles to the film of the present invention. Titanium oxide is mainly known in two crystalline forms, rutile and anatase. The anatase has a very high spectral reflectance of ultraviolet rays, whereas the rutile type has a high absorption rate of ultraviolet rays (spectral spectroscopy). (Reflectance is small). The present inventor paid attention to such a difference in spectral characteristics in the crystal form of titanium dioxide, and by using the rutile-type ultraviolet absorption performance, in the polyester film for protecting the back surface of the solar cell, the light resistance was improved. is there. As a result, the present invention is excellent in film durability under light irradiation even when other ultraviolet absorbers are not substantially added. For this reason, problems such as contamination due to bleeding out of the ultraviolet absorber and a decrease in adhesion are unlikely to occur.

In addition, as above-mentioned, the titanium dioxide particle in the preferable aspect of this invention has a rutile type as a main body. Here, “main body” means that the amount of rutile titanium dioxide in all titanium dioxide particles exceeds 50% by mass. Moreover, it is preferable that the amount of anatase type titanium dioxide in all the titanium dioxide particles is 10 mass% or less. More preferably, it is 5 mass% or less, Most preferably, it is 0 mass% or less. If the content of anatase type titanium dioxide exceeds the above upper limit, the amount of rutile type titanium dioxide in the total titanium dioxide particles may be reduced, resulting in insufficient ultraviolet absorption performance. Since the photocatalytic action is strong, the light resistance tends to be lowered by this action. Rutile titanium dioxide and anatase titanium dioxide can be distinguished by X-ray structure diffraction and spectral absorption characteristics.

When excellent durability is required even under light irradiation, titanium dioxide fine particles having an average particle diameter of 0.1 to 3 μm are contained in the film of the present invention in an amount of 3 to 50% by mass, preferably 4 to 4% by mass. 25% by mass is contained. If it is 0.1 μm or less or exceeds 3 μm, it is difficult to make the whiteness of the film 50 or more even if the addition amount is increased. Moreover, if it is less than 3 mass%, durability under light irradiation may fall. If it exceeds 50% by mass, the film weight increases, making it difficult to handle it during processing. Furthermore, when fine particles other than titanium dioxide are contained, even when the average particle size and the added amount of the fine particles are within the above ranges, durability under light irradiation is lowered. As fine particles other than rutile type titanium oxide, fine particles such as silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, carbon black, acid value zinc, zinc sulfide, organic white pigment, etc. in addition to anatase type titanium dioxide Is exemplified.

The rutile titanium dioxide fine particles described above may be subjected to inorganic treatment such as alumina or silica on the fine particle surface, or may be subjected to organic treatment such as silicon or alcohol. Rutile titanium dioxide may be subjected to particle size adjustment and coarse particle removal using a purification process before blending with the polyester composition. As industrial means of the purification process, for example, a jet mill or a ball mill can be applied as a pulverizing means, and as a classification means, for example, dry or wet centrifugation can be applied.

The film of the present invention may contain many fine cavities inside. In that case, the apparent specific gravity is 0.7 or more and 1.3 or less, preferably 0.9 or more and 1.3 or less, more preferably 1.05 or more and 1.2 or less. If it is less than 0.7, the film is not elastic and processing at the time of producing the solar cell module becomes difficult. Even if the film exceeds 1.3, it is within the range of the film of the present invention. However, if the film exceeds 1.3, the film weight is so large that it may become an obstacle when considering the reduction in weight of solar cells. There is.

The fine cavities can be formed from a thermoplastic resin that is incompatible with the fine particles and / or polyester described below. The term “cavity derived from a thermoplastic resin that is incompatible with fine particles or polyester” means that there are voids around the fine particles or the thermoplastic resin. For example, confirm with a cross-sectional photograph of the film by an electron microscope. Can do.

The polyester used in the present invention can be optionally added with an incompatible thermoplastic resin, and is not particularly limited as long as it is incompatible with the polyester. Specific examples include polystyrene resins, polyolefin resins, polyacrylic resins, polycarbonate resins, polysulfone resins, and cellulose resins. In particular, polystyrene resins or polyolefin resins such as polymethylpentene and polypropylene are preferably used.

The mixing amount of these void forming agents, that is, the thermoplastic resin incompatible with the polyester, with respect to the polyester varies depending on the amount of the target void, but may be in the range of 3 to 20% by mass with respect to the entire film. More preferably, it is 5 to 18% by mass. And if it is less than 3 mass%, there exists a limit in increasing the production amount of a cavity. On the other hand, if it is 20% by mass or more, the stretchability of the film is remarkably impaired, and the heat resistance, strength, and waist strength are impaired.

The polyester film for a solar cell back surface protective film of the present invention can be a void-containing polyester film. The polyester film of the present invention may have a single layer or a laminated structure composed of two or more layers. The laminated structure includes a skin layer composed of a polyester layer containing many cavities derived from fine particles having an average particle diameter of 0.1 to 3 μm, and a polyester containing many cavities derived from a thermoplastic resin incompatible with polyester. It is also a preferred embodiment of the present invention to have a core layer composed of layers. Although the manufacturing method is arbitrary and is not particularly limited, for example, it can be manufactured as follows. First, as a method of joining the skin layer to the film surface, after supplying the polyester resin of the skin layer containing fine particles and the polyester resin of the core layer containing an incompatible thermoplastic resin to separate extruders, It is most preferable to employ a coextrusion method in which layers are laminated in a molten state and extruded from the same die.

Each raw material is mixed, put into an extruder, melted, extruded from a T-die, and adhered to a cooling roll to obtain an unstretched sheet. The unstretched sheet is further expanded by stretching between rolls having a speed difference (roll stretching), stretching by gripping and expanding by a clip (tenter stretching), stretching by expanding with air pressure (inflation stretching), and the like. Axial orientation treatment. By performing the orientation treatment, interfacial peeling occurs between the polyester / incompatible thermoplastic resin and between the polyester / fine particles, and many fine cavities appear. Therefore, the conditions for stretching / orienting the unstretched sheet are closely related to the formation of cavities.

First, the first-stage longitudinal stretching process is the most important process for forming many fine cavities inside the film. In the longitudinal stretching, stretching is performed between two or many rolls having different peripheral speeds. As a heating means at this time, a method using a heating roll or a method using a non-contact heating method may be used, or they may be used in combination. Among these, the most preferable stretching method is a method using both roll heating and non-contact heating. In this case, the film is first preheated to a temperature of 50 ° C. to the glass transition point of polyester using a heating roll, and then heated with an infrared heater.

Next, the uniaxially stretched film thus obtained is introduced into a tenter and stretched 2.5 to 5 times in the width direction. A preferred stretching temperature at this time is 100 ° C. to 200 ° C. The biaxially stretched film thus obtained is subjected to heat treatment as necessary. The heat treatment is preferably carried out in a tenter, preferably in the range of the melting point Tm-50 ° C. to Tm of the polyester.

(Coating layer)

In the present invention, in order to improve the adhesiveness to the EVA resin, it is preferable to have a coating layer containing at least one kind of polyester resin, polyurethane resin or polyacrylic resin as a main component on at least one side of the polyester film. Here, the “main component” refers to a component that is 50% by mass or more of the solid components constituting the coating layer. The coating solution used for forming the coating layer of the present invention is preferably an aqueous coating solution containing at least one of water-soluble or water-dispersible copolymerized polyester resin, acrylic resin and polyurethane resin. Examples of these coating liquids include water-soluble or water-dispersible copolyester resin solutions, acrylic resin solutions, polyurethane resin solutions disclosed in Japanese Patent No. 3567927, Japanese Patent No. 3589232, Japanese Patent No. 3589233, and the like. Etc.

The coating layer can be obtained by applying the coating liquid on one or both sides of a uniaxially stretched film in the longitudinal direction, drying at 100 to 150 ° C., and stretching in the transverse direction. The final coating amount of the coating layer is preferably controlled to 0.05 to 0.20 g / m ² . If the coating amount is less than 0.05 g / m ² , adhesion with the resulting EVA resin may be insufficient. On the other hand, when the coating amount exceeds 0.20 g / m ² , blocking resistance may be lowered. When coating layers are provided on both sides of the polyester film, the coating amounts of the coating layers on both sides may be the same or different, and can be independently set within the above range.

It is preferable to add particles to the coating layer in order to impart easy slipperiness. It is preferable to use particles having an average particle size of 2 μm or less. When the average particle diameter of the particles exceeds 2 μm, the particles easily fall off from the coating layer. The particles to be included in the coating layer include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass filler, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide, barium sulfate, calcium fluoride, and fluoride. Inorganic particles such as lithium, zeolite, molybdenum sulfide and mica, crosslinked polystyrene particles, crosslinked acrylic resin particles, crosslinked methyl methacrylate resin particles, benzoguanamine / formaldehyde condensate particles, melamine / formaldehyde condensate particles, polytetrafluoroethylene particles And heat resistant polymer particles. Among these particles, silica particles having a relatively close refractive index to the resin component of the coating layer are preferable.

Further, as a method for applying the coating solution, a known method can be used. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc. can be mentioned. Or it can carry out in combination.

(Thermal adhesive layer)
In the present invention, in order to improve the adhesiveness with the EVA resin, it is also preferable to laminate a thermal adhesive layer mainly composed of an amorphous polyester resin on at least one surface of the white polyester film substrate. The term “main component” as used herein means that the amorphous polyester resin is 50% by mass or more based on the mass of the entire thermal bonding layer. In addition, the thermal adhesive layer here is a layer that can be thermally bonded to the EVA resin under heating conditions. By laminating this thermal adhesive layer on the white polyester film substrate, it is possible to adhere to the EVA resin without providing an adhesive layer. It is important that the thickness of the thermal adhesive layer is 1 to 40 μm per layer. When the thickness of the thermal adhesive layer is less than 1 μm, the thermal adhesiveness and the surface strength are insufficient. On the other hand, when the thickness of the thermal adhesive layer exceeds 40 μm, the heat resistance decreases due to a large linear expansion coefficient or thermal contraction rate. The thickness of the thermal adhesive layer is preferably 3 to 30 μm, more preferably 5 to 25 μm, and particularly preferably 10 to 20 μm for the above reasons.

The means for providing the thermal adhesive layer on the surface of the white polyester film substrate is not particularly limited, but an unstretched sheet is produced using a method in which two kinds of resins are coextruded and laminated in a melt extrusion process, a so-called coextrusion method. It is preferable. Moreover, it is preferable to laminate | stack before a extending process also from a viewpoint of providing moderate heat resistance to a heat bonding layer, and to extend | stretch both a heat bonding layer and a base material layer.

In the present invention, it is also preferable to provide a thermal adhesive layer on both sides of the white polyester film substrate from the viewpoint of suppressing curling of the film. The thermal adhesive layer in the present invention is mainly composed of an amorphous polyester resin, and has a thermal expansion coefficient greatly different from that of a base material layer mainly composed of a crystalline polyester resin. For this reason, when a thermal adhesive layer is provided only on one side of the substrate, it may curl like a bimetal depending on processing conditions and use conditions, and there is a concern about poor flatness and handling properties. When the thermal adhesive layers are provided on both surfaces of the substrate, the thickness ratio of the front and back thermal adhesive layers is preferably 0.5 to 2.0. When it is out of this range, curling may occur due to the above reason. Even when curling occurs, if the curl value after heating at 110 ° C. for 30 minutes in a no-load state is 5 mm or less, there will be no substantial hindrance to handling properties. It is preferably 3 mm or less, and more preferably 1 mm or less.

It is important for the thermal adhesive layer in the present invention to use an amorphous polyester resin as a main component occupying 50% by mass or more of the layer. The amorphous polyester resin here is a polyester resin having a heat of fusion of 20 J / g or less. The heat of fusion is measured by heating at a rate of 10 ° C./min in a nitrogen atmosphere using a DSC apparatus according to “Method for measuring the heat of transition of plastic” described in JIS − K7122. In the present invention, the heat of fusion is preferably 10 J / g or less, and more preferably 5 J / g or less. When the amount of heat of fusion exceeds 20 J / g, sufficient heat adhesion cannot be obtained.

The amorphous polyester resin preferably has a glass transition temperature of 50 ° C. or higher and 100 ° C. or lower. This glass transition temperature is a DSC curve obtained by heating at a rate of 10 ° C./min in a nitrogen atmosphere using a DSC apparatus according to “Method for measuring plastic transition temperature” described in JIS − K7121. Is the midpoint glass transition temperature (Tmg) determined based on The glass transition temperature of the amorphous polyester resin A is preferably 60 to 90 ° C, more preferably 70 to 85 ° C. When the glass transition temperature is less than 50 ° C., the heat adhesion is insufficient and deforms, or the thermal adhesive layer peels off again due to the temperature rise during use. On the other hand, when the glass transition temperature exceeds 100 ° C., it is necessary to heat the solar cell panel at a higher temperature, which increases the burden on the electric circuit and the like.

The kind of the amorphous polyester resin is not particularly limited, but from the viewpoint of versatility, cost, durability, or thermal adhesiveness, those in which various copolymerization components are introduced into the molecular skeleton of the aromatic polyester resin are preferable. Among the copolymer components to be introduced, the glycol components include ethylene glycol, diethylene glycol, neopentyl glycol (NPG), cyclohexanedimethanol (CHDM), propanediol, butanediol, and the acid components include terephthalic acid, isophthalic acid, and naphthalene. Dicarboxylic acid and the like are preferably used. In particular, a copolymer polyester resin in which isophthalic acid, CHDM, and / or NPG is introduced into the molecular skeleton of a polyethylene terephthalate resin is preferable from the viewpoint of processability, and one in which NPG is introduced is more preferable.

The thermal adhesive layer in the present invention preferably contains a thermoplastic resin that is incompatible with the amorphous polyester resin. As a result, the amorphous polyester resin and the thermoplastic resin form a phase separation structure in the thermal adhesive layer, and the slipperiness of the film can be improved by the film surface protrusions formed due to this structure.

The thermoplastic resin that is incompatible with the amorphous polyester resin is not particularly limited, but as a highly versatile resin, polystyrene, polycarbonate, acrylic, cyclic polyolefin and copolymers thereof, crystalline polyolefin such as polypropylene and polyethylene, etc. And copolymers thereof. Among these, from the viewpoint of excellent processability and thermal adhesiveness, polystyrene, polyolefin, or a copolymer thereof is preferable, and polystyrene, polypropylene, or polyethylene is more preferable.

In the present invention, the amount of the thermoplastic resin contained in the thermal adhesive layer is 1 to 30% by mass with respect to the material constituting the thermal adhesive layer. 3 to 25% by mass is preferable, and 5 to 20% by mass is more preferable. When the content of the thermoplastic resin is less than 1% by mass, the necessary slip properties cannot be obtained. When it exceeds 30% by mass, the thermal adhesiveness is inhibited.

In the present invention, it is one of the preferred embodiments that the above-mentioned fine particles are contained in the heat-adhesive layer as long as the heat-adhesive property is not impaired. Among the fine particles, white pigments, that is, titanium oxide or barium sulfate and composite particles thereof are preferable, and titanium oxide is more preferably used from the viewpoint of concealment effect. These white pigments are preferably contained in the heat bonding layer in an amount of 30% by mass or less, and more preferably 15% by mass or less. If added beyond the above range, the thermal adhesiveness may be inhibited.

(Film characteristics)
The film of the present invention has an acid value of 1 (eq / ton) to 30 (eq / ton), preferably 2 (eq / ton) to 20 (eq / ton), more preferably 2 (eq / ton). ton) to 16 (eq / ton). If it exceeds 30 (eq / ton), a film having good hydrolysis resistance cannot be obtained. A film of less than 1 (eq / ton) is difficult to produce industrially.

The film of the present invention has a breaking elongation retention ratio of 60% or more, preferably 70% or more, more preferably 80% or more, which is an evaluation of hydrolysis resistance. If it is less than 60%, the durability as a solar cell back surface protective film is low and cannot be used.

Next, examples and comparative examples of the present invention will be shown. The measurement / evaluation method used in the present invention is shown below.
1) Apparent specific gravity A film is accurately cut into a 10 cm × 10 cm square, and its thickness is measured at 50 points to obtain an average thickness t (unit: μm). Next, the mass of the sample is measured to 0.1 mg and is set to w (unit: g). And the apparent specific gravity was calculated by the following formula.
Apparent specific gravity (−) = (w / t) × 10000

2) Whiteness Whiteness was measured using Nippon Denshoku Industries Co., Ltd. Z-1001DP according to the JIS-L1015-1981-B method.

3) Acid value It measured with the following method about the film and raw material polyester resin.

(1) Preparation of sample A film or a raw material polyester resin is pulverized and vacuum-dried at 70 ° C. for 24 hours, and then weighed in a range of 0.20 ± 0.0005 g using a balance. The mass at that time is defined as W (g). A sample weighed with 10 ml of benzyl alcohol is added to a test tube, the test tube is immersed in a benzyl alcohol bath heated to 205 ° C., and the sample is dissolved while stirring with a glass rod. Samples with dissolution times of 3 minutes, 5 minutes, and 7 minutes are designated as A, B, and C, respectively. Next, a new test tube is prepared, and only benzyl alcohol is added and processed in the same procedure, and the samples when the dissolution time is 3 minutes, 5 minutes, and 7 minutes are designated as a, b, and c, respectively.
(2) Titration Titration is performed using a 0.04 mol / l potassium hydroxide solution (ethanol solution) whose factor is known in advance. The indicator is phenol red, and the titration (ml) of the potassium hydroxide solution is determined with the end point being changed from yellowish green to light red. The titration amounts of samples A, B, and C are XA, XB, and XC (ml). The titration amounts of samples a, b, and c are Xa, Xb, and Xc (ml).
(3) Calculation of acid value Using the titration amounts XA, XB, and XC for each dissolution time, the titration amount V (ml) at a dissolution time of 0 minutes is determined by the least square method. Similarly, titration volume V0 (ml) is obtained using Xa, Xb, and Xc. Subsequently, an acid value is calculated | required according to following Formula.
Carboxyl terminal concentration (eq / ton) = [(V−V0) × 0.04 × NF × 1000] / W
NF: factor W of 0.04 mol / l potassium hydroxide solution: sample mass (g)

4) Hydrolysis resistance HAST (Highly Accelerated Temperature and Humidity Stress Test) standardized in JIS-60068-2-66 was performed. The equipment was EHS-221 manufactured by ESPEC Co., Ltd. under the conditions of 105 ° C., 100% Rh, 0.03 MPa.
The film was cut into 70 mm × 190 mm, and the film was placed using a jig. Each film was placed at a distance where it did not touch. The treatment was performed for 200 hours under the conditions of 105 ° C., 100% Rh, 0.03 MPa. The elongation at break before and after treatment was measured according to JIS C 2318-1997 5.3.31 (tensile strength and elongation), and the elongation at break was calculated according to the following formula.
Breaking elongation retention ratio (%) = [(breaking elongation after treatment) × 100] / (breaking elongation before treatment)

5) Adhesiveness with EVA resin Two pieces of the film cut to 20 mm width × 100 mm length were cut, and EVA resin sheet (High Sheet Industry Co., Ltd. SOLAR EVA (R) SC4) was cut to 20 mm width × 50 mm length. I prepared one piece of each. The film was placed in the order of film / EVA resin sheet / film and pressed with a heat sealer so that the EVA resin sheet was positioned almost at the center of the film, and the surface to be evaluated for easy contact of the film was on the EVA side. . 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.
◎: 20 N / 20 mm or more ・・・ Adhesion is very good ◯: 10 N / 20 mm or more, less than 20 N / 20 mm ・・・ Good adhesion △: 5 N / 20 mm or more to less than 10 N / 20 mm ・・・ Adhesion slightly good × : Less than 5N / 20mm ... poor adhesion

6) Intrinsic viscosity of polyester resin The polyester resin was pulverized and dried, and then dissolved in a mixed solvent of phenol / tetrachloroethane = 60/40 (weight ratio). After removing inorganic particles by centrifuging this solution, the flow time of the solution having a concentration of 0.4 (g / dl) and the flow time of the solvent alone were measured at 30 ° C. using an Ubbelohde viscometer. From these time ratios, the intrinsic viscosity was calculated using the Huggins equation, assuming that the Huggins constant was 0.38.

7) Melting | fusing point and glass transition temperature of polyester resin DSC measurement was performed by the "method for measuring plastic transition temperature" described in JIS K7121. About 10 mg of resin side or film piece was sealed in an aluminum pan, melted at 300 ° C. for 3 minutes, and rapidly cooled and solidified with liquid nitrogen. A differential scanning calorimeter (manufactured by Seiko Instruments Inc., EXSTAR 6200DSC) was used as a measuring instrument, and the measurement was performed under a dry nitrogen atmosphere. After heating from room temperature at a rate of 10 ° C./min to determine the midpoint glass transition temperature, the melting peak temperature (melting point) was determined.

8) Heat of fusion of polyester resin
The amount of heat of fusion was determined by the “Method of measuring the transition heat of plastic” described in JIS K7122. The details of the DSC measurement were the same as those of the above melting point measurement.

9) Light resistance The accelerated light resistance test was performed after continuous UV irradiation treatment for 100 hours at 63 ° C., 50% Rh, irradiation intensity of 100 mW / cm ² using an I-super UV tester SUV-W151 manufactured by Iwasaki Electric Co., Ltd. This was done by evaluating the elongation at break. The elongation at break before and after each treatment was measured according to JIS C 2318-1997 5.3.31 (tensile strength and elongation), and the elongation at break was calculated according to the following formula.
Breaking elongation retention ratio (%) = [(breaking elongation after treatment) × 100] / (breaking elongation before treatment)

Example 1
(Preparation of fine particle-containing masterbatch)
The average particle size was 50% by mass of polyethylene terephthalate resin (PET-I) having an intrinsic viscosity of 0.64 and an acid value of 8.0 (eq / ton), which was dried at 120 ° C. under 10 ⁻³ torr for about 8 hours in advance. A mixture of 50% by mass of rutile titanium dioxide with a diameter of 0.3μm (electron microscopic method) is supplied to a vent type twin screw extruder, kneaded and extruded at 275 ° C while degassing to produce fine particles (titanium oxide) Containing masterbatch (MB-I) pellets were prepared. The acid value of this pellet was 8.6 (eq / ton).

(Production of film)
Next, the raw material of the layer (A) in which 50% by mass of polyethylene terephthalate resin (PET-I) and 50% by mass of the previously prepared MB-I were mixed, 90% by mass of PET-I and MB-I were 10% by mass is used as a raw material for the layer (B), put into separate extruders, mixed and melted at 280 ° C., and then, using a feed block, the layer B is joined to one side of the layer A in a molten state. . At this time, the discharge rate ratio of the A layer and the B layer was controlled using a gear pump. Next, the sheet was extruded onto a cooling drum adjusted to 30 ° C. using a T-die to prepare an unstretched sheet so as to be an A / B / A layer.

(Production of biaxially stretched film)
The obtained unstretched sheet was uniformly heated to 70 ° C. using a heating roll, and 3.3-fold roll stretching was performed at 90 ° C. The obtained uniaxially stretched film was led to a tenter, heated to 140 ° C. and transversely stretched 3.7 times, fixed in width and subjected to heat treatment at 220 ° C. for 5 seconds, and further at 220 ° C. in the width direction of 4%. By relaxing, a plastic film for a solar cell back surface protective film having a thickness of 188 μm (19/150/19) was obtained.

Example 2
(Preparation of cavity forming agent)
As raw materials, 20% by mass of polystyrene with a melt flow rate of 1.5 (manufactured by Nippon Polystyrene Co., Ltd., G797N), 20% by mass of vapor phase polymerization polypropylene with a melt flow rate of 3.0 (F300SP, manufactured by Idemitsu Petrochemical), and melt flow 60% by mass pellet of polymethylpentene having a rate of 180 (Mitsui Chemicals Co., Ltd .: TPX DX-820) was supplied to a twin screw extruder and kneaded sufficiently to prepare a cavity forming agent (MB-II).
A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 1 except that PET-I: MB-I: MB-II was changed to 82: 10: 8 (mass%) as a raw material for the B layer. It was.

Example 3, Example 4, Comparative Example 1
Except that the acid value of polyethylene terephthalate resin as a film raw material was 10.1, 19.5, 30.2 (PET-II, PET-III, PET-IV, respectively) A plastic film for the battery back surface protective film was obtained.

Comparative Example 2
In the preparation of a master batch containing fine particles, a polyethylene terephthalate resin having an undried intrinsic viscosity of 0.64 and an acid value of 8.0 (eq / ton) stored in a paper bag as a raw material in a place where temperature and humidity are not controlled (PET-I) 50 mass% mixed with 50 mass% rutile type titanium dioxide having an average particle size of 0.3 μm (electron microscopic method) is supplied to a vent type twin screw extruder and kneaded to 305 ° C. A masterbatch (MB-III) pellet containing fine particles (titanium oxide) was prepared while degassing with a. The acid value of this pellet was 38.4 (eq / ton).
Other than that obtained the plastic film for solar cell back surface protective films by the method similar to Example 2. FIG.

Example 5
In the production of the master batch containing fine particles, barium sulfate having an average particle size of 0.6 μm was used instead of rutile titanium dioxide (MB-IV), and it was used instead of MB-I as a raw material for the A layer. A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 2.

Example 6
(Preparation of fine particle-containing masterbatch)
The average particle size was 50% by mass of polyethylene terephthalate resin (PET-I) having an intrinsic viscosity of 0.64 and an acid value of 8.0 (eq / ton), which was dried at 120 ° C. under 10 ⁻³ torr for about 8 hours in advance. A mixture of 50% by mass of rutile titanium dioxide with a diameter of 0.3μm (electron microscopic method) is supplied to a vent type twin screw extruder, kneaded and extruded at 275 ° C while degassing to produce fine particles (titanium oxide) Containing masterbatch (MB-I) pellets were prepared. The acid value of this pellet was 8.6 (eq / ton).

(Preparation of coating solution)
A transesterification reaction and a polycondensation reaction were carried out by a conventional method, and as a dicarboxylic acid component (based on the total dicarboxylic acid component) 46 mol% terephthalic acid, 46 mol% isophthalic acid and 8 mol% sodium 5-sulfonatoisophthalate, A water-dispersible sulfonic acid metal base-containing copolymer polyester resin having a composition of 50 mol% ethylene glycol and 50 mol% neopentyl glycol as a glycol component (based on the entire glycol component) was prepared. Next, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve, 0.06 parts by mass of a nonionic surfactant were mixed and then heated and stirred. After adding 5 parts by mass of a water-dispersible sulfonic acid metal base-containing copolymer polyester resin and continuing to stir until the resin is no longer agglomerated, the resin water dispersion is cooled to room temperature to obtain a solid content concentration of 5.0% by mass. A uniform water-dispersible copolymerized polyester resin liquid was obtained. Furthermore, after dispersing 3 parts by mass of aggregated silica particles (Silicia 310, manufactured by Fuji Silysia Co., Ltd.) in 50 parts by mass of water, 99.46 parts by mass of the water-dispersible copolyester resin solution was mixed with 99.46 parts by mass of the silicia 310. 0.54 parts by mass of the aqueous dispersion was added, and 20 parts by mass of water was added with stirring to obtain a coating solution.

(Production of biaxially stretched film)
The obtained unstretched sheet was uniformly heated to 70 ° C. using a heating roll, and 3.3-fold roll stretching was performed at 90 ° C. to obtain a uniaxially stretched polyester film. The coating solution was applied to one side of the obtained uniaxially stretched polyester film so that the final coating layer thickness was 0.08 g / m ^2, and then dried at 135 ° C.

By guiding the coated film to a tenter, heating it to 140 ° C. and transversely stretching it 3.7 times, fixing the width, applying heat treatment at 220 ° C. for 5 seconds, and further relaxing by 4% in the width direction at 220 ° C. A plastic film for a solar cell back surface protective film having a thickness of 188 μm (19/150/19) was obtained.

Example 7
(Preparation of cavity forming agent)
As raw materials, 20% by mass of polystyrene with a melt flow rate of 1.5 (manufactured by Nippon Polystyrene Co., Ltd., G797N), 20% by mass of vapor phase polymerization polypropylene with a melt flow rate of 3.0 (F300SP, manufactured by Idemitsu Petrochemical), and melt flow 60% by mass pellet of polymethylpentene having a rate of 180 (Mitsui Chemicals Co., Ltd .: TPX DX-820) was supplied to a twin screw extruder and kneaded sufficiently to prepare a cavity forming agent (MB-II).
A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 6 except that PET-I: MB-I: MB-II was changed to 82: 10: 8 (mass%) as a raw material for the B layer. It was.

Example 8, Example 9, Comparative Example 3
Except that the acid value of polyethylene terephthalate resin as a film raw material was 10.1, 19.5, 30.2 (PET-II, PET-III, PET-IV, respectively) A plastic film for the battery back surface protective film was obtained.

Comparative Example 4
In the preparation of a master batch containing fine particles, a polyethylene terephthalate resin having an undried intrinsic viscosity of 0.64 and an acid value of 8.0 (eq / ton) stored in a paper bag as a raw material in a place where temperature and humidity are not controlled (PET-I) 50 mass% mixed with 50 mass% rutile type titanium dioxide having an average particle size of 0.3 μm (electron microscopic method) is supplied to a vent type twin screw extruder and kneaded to 305 ° C. A masterbatch (MB-III) pellet containing fine particles (titanium oxide) was prepared while degassing with a. The acid value of this pellet was 38.4 (eq / ton).
Other than that obtained the plastic film for solar cell back surface protective films by the method similar to Example 7. FIG.

Example 10
In the production of the master batch containing fine particles, barium sulfate having an average particle size of 0.6 μm was used instead of rutile titanium dioxide (MB-IV), and it was used instead of MB-I as a raw material for the A layer. A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 7.

Example 11
A polyester film for a solar cell back surface protective film was obtained in the same manner as in Example 7 except that the coating layer was not provided.

Example 12
(Preparation of fine particle-containing masterbatch)
As a raw material, a polyethylene terephthalate resin (PET-A) having an intrinsic viscosity of 0.69 and an acid value of 8 (eq / ton) dried at 120 ° C. under a vacuum of 10 Pa for about 8 hours in advance has an average particle size of 0.1%. Mixing 50% by mass of 3μm (electron microscopy) rutile titanium oxide to a vent type twin screw extruder, kneading and extruding at 275 ° C while degassing, preparing fine particle-containing master batch pellets did. Further, this master batch pellet was subjected to solid phase polymerization under a vacuum of 10 Pa until the intrinsic viscosity became 0.79, to prepare a fine particle-containing master batch (MB-A). The acid value of MB-A was 23 (eq / ton).

(Preparation of amorphous polyester resin)
Transesterification and polycondensation reactions are carried out by conventional methods, and an amorphous polyester resin (Co-PET) comprising 100 mol% terephthalic acid as the dicarboxylic acid component, 70 mol% ethylene glycol and 30 mol% neopentyl glycol as the glycol component Was prepared. This resin had an intrinsic viscosity of 0.72 and an acid value of 19 eq / ton.

(Preparation of white polyester film with laminated thermal adhesive layer)
Next, A layer raw material in which amorphous polyester resin (Co-PET) dried to a moisture content of 30 ppm and polystyrene (Nippon Polystyrene Co., Ltd., G797N, melt flow rate 1.5) were mixed in 85% by mass and 15% by mass, respectively. Was fed into Extruder A, and B layer raw materials mixed with 60% by mass and 40% by mass of similarly dried PET-A and MB-A, respectively, were fed into Extruder B, mixed and melted at 280 ° C., and subsequently Using a feed block, the A layer was joined in a molten state on both sides of the B layer. At this time, the discharge rate ratio of the A layer and the B layer was controlled using a gear pump. Next, the sheet was extruded onto a cooling drum adjusted to 30 ° C. using a T-die to produce an unstretched sheet having an A / B / A layer structure.

The obtained unstretched sheet was uniformly heated to 70 ° C. using a heating roll, and 3.3-fold roll stretching was performed at 90 ° C. to obtain a uniaxially stretched polyester film. This was led to a tenter, heated to 140 ° C. and stretched to 3.7 times, fixed in width, subjected to heat treatment at 230 ° C. for 5 seconds, and further relaxed by 4% in the width direction at 220 ° C. The polyester film for solar cell back surface protective films which laminated | stacked the thermal adhesive layer of 188 micrometers (19/150/19) was obtained.

Example 13
(Preparation of coating solution)
Silica particles having a water-dispersed copolymer polyester resin (manufactured by Toyobo Co., Ltd., Vylonal) 3 mass%, a water-soluble urethane resin (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., Elastron), and an average particle size of 0.05 μm A water / isopropyl alcohol-based coating solution containing 1% by mass with respect to the solid content was prepared.

(Preparation of white polyester film with laminated thermal adhesive layer)
The raw material charged into the extruder B was changed to a mixture of PET-A / MB-A = 85/15 (mass%). In the biaxially stretched film production process, the coating solution was applied on both sides of the obtained uniaxially stretched polyester film so that the final coating layer thickness was 0.08 g / m ^2, and then dried at 135 ° C. Introduced to the tenter. Other than that was carried out similarly to Example 12, and obtained the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer.

Example 14
(Preparation of cavity forming agent master batch)
As raw materials, 20% by mass of polystyrene having a melt flow rate of 1.5 (manufactured by Nippon Polystyrene Co., Ltd., G797N), 20% by mass of vapor phase polymerization polypropylene having a melt flow rate of 3.0 (manufactured by Idemitsu Kosan Co., Ltd., F300SP), and 60% by mass of polymethylpentene having a melt flow rate of 180 (manufactured by Mitsui Chemicals: TPX DX-820) was mixed with pellets, supplied to a twin-screw extruder and sufficiently kneaded to prepare a cavity forming agent master batch (MB -B).

(Preparation of white polyester film with laminated thermal adhesive layer)
Solar cell back surface protection with a thermal adhesive layer laminated in the same manner as in Example 12 except that PET-A / MB-A / MB-B = 82/10/8 (mass%) was used as the raw material for the B layer. A polyester film for membrane was obtained.

Example 15
The polyethylene terephthalate resin used as a raw material was changed to one having an intrinsic viscosity of 0.69 and an acid value of 19 (eq / ton) (PET-B). Further, solid phase polymerization treatment was not performed in the production of the fine particle-containing master batch, and the master batch (MB-A2) having an acid value of 39 eq / ton was used. Except this, the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer by the method similar to Example 14 was obtained.

Comparative Example 5)
In the preparation of the master batch containing fine particles, PET-A (water content 3500 ppm) which was not subjected to drying treatment was used, and solid phase polymerization treatment was not conducted. The acid value of the pellet (MB-A3) was 57 (eq / ton). In place of PET-A, a polyethylene terephthalate resin (PET-C) having an acid value of 31 (eq / ton) was used. Except this, the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer by the method similar to Example 12 was obtained.

Comparative Example 6)
Regarding the raw material supplied to the extruder B, PET-C was used instead of PET-A, and MB-A3 was used instead of MB-A. Except this, the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer by the method similar to Example 14 was obtained.

Example 16)
A master batch (MB-A4) containing fine particles was prepared using anatase-type titanium oxide having an average particle size of 0.2 μm instead of rutile-type titanium oxide. A polyester film for a solar cell back surface protective film in which a thermal adhesive layer was laminated was obtained in the same manner as in Example 14 except that this was used in place of MB-A1 as a raw material for the B layer.

Example 17)
For the raw material supplied to the extruder A, PET-A was used instead of Co-PET. Except this, a polyester film for solar cell back surface protective film having no thermal adhesive layer was obtained in the same manner as in Example 14.

Example 18)
In Example 12, instead of the polystyrene resin supplied to the extruder A, a polyethylene resin (manufactured by Ube Industries, Umerit 2040F, melting point 116 ° C., density 0.918 g / cm ³ ) was used. The raw material ratio was changed to that shown in Table 5. This obtained the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer.

Example 19)
A transesterification reaction and a polycondensation reaction were carried out by a conventional method to prepare an amorphous polyester resin composed of 80 mol% terephthalic acid and 20 mol% isophthalic acid as the dicarboxylic acid component and 100 mol% ethylene glycol as the glycol component. This resin had an intrinsic viscosity of 0.67 and an acid value of 22 eq / ton. 95% by mass of this amorphous polyester resin and 5% by mass of polyethylene wax (manufactured by Mitsui Chemicals, NL500) were mixed and supplied to a twin screw extruder, and kneaded thoroughly to prepare a wax agent master batch (MB- C) was adjusted.
About the raw material supplied to Extruder A, except having changed into the ratio shown in Table 5, it carried out similarly to Example 12, and obtained the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer.

Examples 20, 21)
The raw material supplied to the extruder A was a mixture of Co-PET, PET-A and polystyrene at the ratio shown in Table 3. Except this, a polyester film for a solar cell back surface protective film having no thermal adhesive layer was obtained in the same manner as in Example 12.

As shown in Tables 4 and 5, the polyester film for the solar cell back surface protective film of the examples within the scope of the present invention exhibited excellent durability and EVA resin adhesion. On the other hand, the films of Comparative Examples 1 and 2 that are outside the scope of the present invention have poor durability, and the films of Examples 17, 20, and 21 have poor adhesion to the EVA resin.

Example 22
(Preparation of fine particle-containing masterbatch)
The average particle size was 50% by mass of polyethylene terephthalate resin (PET-I) having an intrinsic viscosity of 0.64 and an acid value of 8.0 (eq / ton), which was dried at 120 ° C. under 10 ⁻³ torr for about 8 hours in advance. A mixture of 50% by mass of rutile type titanium dioxide having a diameter of 0.3 μm (electron microscopic method) is supplied to a vent type twin screw extruder, kneaded and extruded at 275 ° C. while degassing, and rutile type titanium dioxide fine particles. Containing masterbatch (MB-I) pellets were prepared. The acid value of this pellet was 8.6 (eq / ton).

(Production of film)
Next, the raw material of the layer (A) in which 50% by mass of polyethylene terephthalate resin (PET-I) and 50% by mass of the previously prepared MB-I, 70% by mass of PET-I and MB-I were mixed. 30% by mass is used as a raw material for the layer (B), and each is put into separate extruders, mixed and melted at 280 ° C., and subsequently, the layer B is joined in a molten state on one side of the layer A using a feed block. . At this time, the discharge rate ratio of the A layer and the B layer was controlled using a gear pump. Next, the sheet was extruded onto a cooling drum adjusted to 30 ° C. using a T-die to prepare an unstretched sheet so as to be an A / B / A layer.

Example 23
(Preparation of cavity forming agent)
As raw materials, 20% by mass of polystyrene with a melt flow rate of 1.5 (manufactured by Nippon Polystyrene Co., Ltd., G797N), 20% by mass of vapor phase polymerization polypropylene with a melt flow rate of 3.0 (F300SP, manufactured by Idemitsu Petrochemical), and melt flow 60% by mass pellet of polymethylpentene having a rate of 180 (Mitsui Chemicals Co., Ltd .: TPX DX-820) was supplied to a twin screw extruder and kneaded sufficiently to prepare a cavity forming agent (MB-II).
A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 22 except that PET-I: MB-I: MB-II was changed to 80: 12: 8 (mass%) as a raw material for the B layer. It was.

Example 24, Example 25, Comparative Example 7
A solar cell was produced in the same manner as in Example 23 except that the acid value of polyethylene terephthalate resin as a film raw material was 10.1, 19.5, 30.2 (PET-II, PET-III, and PET-IV, respectively). A plastic film for the battery back surface protective film was obtained.

Example 26
A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 24 except that the amount of MB-I added in Example 24 was changed as shown in the table.

Example 27
In the preparation of the fine particle-containing masterbatch, as a raw material, 50% by mass of polyethylene terephthalate resin (PET-I) and 50% by mass of anatase-type titanium dioxide having an average particle size of 0.3 μm (electron microscopic method) are bent. The mixture was supplied to a shaft extruder, kneaded and extruded at 275 ° C. while degassing to prepare fine particle-containing master batch (MB-III) pellets. The acid value of this pellet was 8.1 (eq / ton). A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 23 except that MB-III was used instead of the fine particle-containing master batch MB-I.

Comparative Example 8
In the preparation of a master batch containing fine particles, a polyethylene terephthalate resin having an undried intrinsic viscosity of 0.64 and an acid value of 8.0 (eq / ton) stored in a paper bag as a raw material in a place where temperature and humidity are not controlled (PET-I) 50 mass% mixed with 50 mass% rutile type titanium dioxide having an average particle size of 0.3 μm (electron microscopic method) is supplied to a vent type twin screw extruder and kneaded to 305 ° C. A masterbatch (MB-IV) pellet containing fine particles (titanium oxide) was prepared while degassing with a vacuum. The acid value of this pellet was 38.4 (eq / ton). A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 23 except that MB-IV was used instead of the fine particle-containing master batch MB-I.

Example 28
A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 22 except that MB-III was used instead of the fine particle-containing master batch MB-I.

Example 29
In the preparation of the master batch containing fine particles, barium sulfate having an average particle diameter of 0.6 μm was used instead of rutile titanium dioxide (MB-V), and it was used instead of MB-I as a raw material for the A layer. A plastic film for a solar cell back surface protective film was obtained in the same manner as in Example 23.

The polyester film for solar cell back surface protective film of the present invention is excellent in durability and light reflection efficiency under high temperature and high humidity, and is useful as a material constituting the solar cell back surface protective film.

Claims

3 to 50% by mass of fine particles having a whiteness of 50 or more and an average particle size of 0.1 to 3 μm, and the acid value of the film is 1 (eq / ton) to 30 (eq / ton). A polyester film for a solar cell back surface protective film.
2. The polyester film for a solar cell back surface protective film according to claim 1, comprising a coating layer mainly comprising at least one of polyester resin, polyurethane resin or polyacrylic resin on one side.
3. The polyester film for a solar cell back surface protective film according to claim 1 or 2, further comprising a thermal adhesive layer having a thickness of 1.0 to 40 μm mainly composed of an amorphous polyester resin on at least one side.
The polyester film for a solar cell back surface protective film according to any one of claims 1 to 3, wherein the fine particles are titanium dioxide fine particles mainly composed of a rutile type and contain 3 to 50% by mass of the fine particles. .
The polyester for solar cell back surface protective film according to any one of claims 1 to 4, wherein an apparent specific gravity of the film is 0.7 or more and 1.3 or less by having many fine cavities inside the film. the film.
Polyester layer (skin layer) containing many cavities derived from fine particles having an average particle size of 0.1 to 3 μm, and polyester layer (core layer) containing many cavities derived from a thermoplastic resin incompatible with polyester The polyester film for a solar cell back surface protective film according to any one of claims 1 to 5, which is laminated.
The polyester film for a solar cell back surface protective film according to any one of claims 1 to 6, wherein the acid value of the polyester as a film raw material is 1 (eq / ton) to 30 (eq / ton).