JP2011097012A - Polyester film for solar cell rear surface protective film - Google Patents

Abstract

The present invention provides a polyester film for a solar cell back surface protective film which is excellent in durability and adhesiveness under high temperature and high humidity.
SOLUTION: A white polyester film substrate containing 3 to 50% by mass of fine particles having an average particle diameter of 0.1 to 3 μm and having a thickness of 1.0 to 40 μm mainly composed of an amorphous polyester resin on at least one side. A polyester film for a solar cell back surface protective film having a thermal adhesive layer, having an acid value of 1 to 40 eq / ton, a whiteness of 50 or more, and a thickness of 38 to 1000 μm.
[Selection figure] None

Description

  The present invention relates to a polyester film for a solar cell back surface protective film. It is related with the polyester film for solar cell back surface protective films excellent in adhesiveness with EVA resin especially.

  The demand for solar cells capable of obtaining electric power by using energy not derived from petroleum fuel is increasing from the viewpoint of environmental protection. In general, a solar cell module has a plurality of plate-like solar cell elements sandwiched between a light receiving side glass substrate and a back surface protective film, and an ethylene-vinyl acetate copolymer (hereinafter referred to as EVA) in an internal gap. It has a structure filled with a sealing resin such as a resin.

  A plastic film having excellent mechanical properties, heat resistance, and moisture resistance is used as a substrate for the solar cell back surface protective film. For example, a back surface protective film using a polyethylene terephthalate film has been proposed (Patent Documents 1 and 2). In order to increase the power generation efficiency of the solar cell, it has also been proposed to use a white film as a back surface protective film (Patent Document 3).

JP 2002-26354 A JP 2003-60218 A JP 2009-147063 A JP 2008-4839 A

  By using a white polyester film as the base material of the solar cell back surface protective film, it is possible to reflect sunlight and increase power generation efficiency. The white polyester film needs to add a large amount of fine particles to the polyester resin. Therefore, in order to improve the dispersibility of the fine particles in the resin and the mixing state, it is possible to prepare a raw material in which two or more kinds of materials are premixed or to increase the melting time even in a normal extrusion process. Therefore, the resin tends to deteriorate easily. Therefore, when used as a solar cell under high temperature and high humidity, it has been a problem that durability is poor. Therefore, conventionally, as in Patent Document 4, it is common to create a laminated sheet as a functional layer separate from the white layer and the durable layer, and it has been difficult to achieve both of them. Furthermore, when using them as a solar cell module, it is necessary to have adhesiveness with EVA resin.

  An object of this invention is to provide the polyester film for solar cell back surface protective films excellent in durability under high temperature, high humidity, and adhesiveness with EVA resin.

This invention is made | formed in view of the above situations, Comprising: The polyester film for solar cell back surface protective films which was able to solve said subject consists of the following structures.
1. A white polyester film substrate having a thickness of 38 to 1000 μm, containing 3 to 50% by mass of fine particles having an average particle diameter of 0.1 to 3 μm, an acid value of 1 to 40 eq / ton, and a whiteness of 50 or more. A polyester film for a solar cell back surface protective film, 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 surface.
2. The white polyester film base material has a large number of fine cavities inside, and has an apparent specific gravity of 0.7 to 1.3.
3. The white polyester film base material contains a large number of cavities derived from an incompatible thermoplastic resin.
4). The said polyester film for solar cell back surface protective films characterized by using the polyester resin whose acid value is 0.1-30 eq / ton as a raw material of a white polyester film base material.
5. The said polyester film for solar cell back surface protective films characterized by using the white pigment masterbatch whose acid value is 1-40 eq / ton as a raw material of a white polyester film base material.

  The present invention has light reflection efficiency, excellent durability under high temperature and high humidity, and good adhesiveness with EVA resin. Therefore, it is useful in solar cells, particularly thin film silicon solar cells.

The polyester film for a solar cell back surface protective film of the present invention contains 3 to 50% by mass of fine particles having an average particle diameter of 0.1 to 3 μm, an acid value of 1 to 40 eq / ton, and a whiteness of 50 or more. A white polyester film substrate having a thickness of 38 to 1000 μm has 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 white polyester film substrate in the present invention is excellent in reflection efficiency and humidity resistance under high temperature and high humidity due to the above-described configuration. In addition, the thermal adhesive layer has excellent adhesion to the EVA resin due to the above configuration.

(White polyester film substrate)
The polyester in the white polyester film substrate used in the present invention is an aromatic dicarboxylic acid or ester thereof such as terephthalic acid, isophthalic acid or naphthalenedicarboxylic acid and ethylene glycol, diethylene glycol, 1,4-butanediol or neopentyl glycol. It is a polyester produced by polycondensation with glycol. 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 60 mol% or more, preferably 70 mol% or more, more preferably 90 mol% or more is preferable. .

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

  The white polyester film substrate used in the present invention needs to be a white polyester film having a thickness of 38 to 1000 μm, preferably 50 to 250 μm, and more 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 white polyester film substrate used in the present invention has 3 to 50 masses of fine particles having the above polyester as a main component and an average particle size of 0.1 to 3 μm to make the whiteness 50 or more. %, Preferably 4 to 25% by mass. If the average particle size of the fine particles is not in the range of 0.1 to 3 μm, it is difficult to set the whiteness of the film to 50 or more even if the addition amount is increased. Further, when the content of the fine particles is less than 3% by mass, it is difficult to set the whiteness to 50 or more. If it exceeds 50% by mass, the film weight increases, making it difficult to handle it during processing.

In the present invention, in order to improve the whiteness, the above-mentioned fine particles can be contained in the thermal adhesive layer. However, when adding fine particles to the thermal adhesive layer, it is important to achieve both whiteness improvement effect and thermal adhesiveness, and the fine particles added to the thermal adhesive layer should be 1 to 50% by mass of the layer. Is preferable, and it is more preferable to stop at 5 to 30% by mass.

  In the present invention, the average particle size of the fine particles is determined by electron microscopy. 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 100 or more randomly selected fine particles. 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 fine particles to be contained in the white polyester film substrate in the present invention, inorganic or organic fine particles can be used. Examples of these fine particles include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, titanium oxide, zinc sulfide, and organic white pigment, but are not particularly limited. . From the viewpoint of improvement in whiteness and productivity, a white pigment, that is, titanium oxide or barium sulfate is preferable, and titanium oxide is more preferable. The titanium oxide may be either anatase type or rutile type, but the rutile type is more preferable from the viewpoint of improving weather resistance. In addition, the surface of the fine particles may be subjected to an inorganic surface treatment such as alumina or silica, or may be subjected to an organic surface treatment such as silicone or alcohol.

  Fine particles can be added to the film 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. When manufacturing MB, in order to suppress hydrolysis at the time of manufacturing, a polyester resin and fine particles that have not been dried in advance are put into an extruder, and a method of manufacturing MB while degassing moisture and air is adopted. be able to. More preferably, an increase in the acid value of the polyester resin can be suppressed by preparing MB using a polyester resin dried in advance. When the polyester resin is used after being dried, the moisture content is preferably 1000 ppm or less, and more preferably 100 ppm or less.

  For example, when producing MB, it is preferable to reduce the moisture content of the polyester resin to be introduced in advance by drying. Drying conditions are preferably 100 to 200 ° C., more preferably 120 to 180 ° C., for 1 hour or longer, more preferably 3 hours or longer, and even more preferably 6 hours or longer. Thereby, it is sufficiently dried so that the moisture content of the polyester resin is preferably 50 ppm or less, more preferably 30 ppm or less. The premixing method is not particularly limited, and may be a batch method or a single-screw or twin-screw kneading extruder. When producing MB while degassing, melt the polyester resin at a temperature of 250 ° C to 300 ° C, preferably 270 ° C to 280 ° C, and provide one, preferably two or more degassing ports in the pre-kneader. It is preferable to adopt a method such as performing continuous suction deaeration of 0.05 MPa or more, more preferably 0.1 MPa or more, and maintaining the reduced pressure in the mixer.

  In any master batch produced by any method, when used for the white polyester film substrate of the present invention, the acid value is preferably 1 to 40 eq / ton, more preferably 3 to 35 eq / ton. 5 to 30 eq / ton is more preferable. MB having an acid value of less than 1 eq / ton is difficult to produce industrially, and using an acid value exceeding 40 eq / ton is not preferable because it increases the acid value of the polyester film substrate.

  The white polyester film substrate used in the present invention may contain many fine cavities inside. Thereby, higher whiteness can be suitably obtained. 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 has no stiffness and processing during the production of the solar cell module becomes difficult. A film exceeding 1.3 may be used, but if it exceeds 1.3, the film weight is large, which may be an obstacle when considering reducing the weight of the solar cell.

  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 a void exists around the fine particle or the thermoplastic resin, and is confirmed by, for example, a cross-sectional photograph of the film using an electron microscope. Can do.

  The incompatible thermoplastic resin is not particularly limited as long as it is incompatible with 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, a thermoplastic resin incompatible with the polyester resin, with respect to the polyester varies depending on the amount of the target void, but is in the range of 3 to 20% by mass with respect to the entire film. It is preferably 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 acid value of the white polyester film substrate used in the present invention is 1 to 40 eq / ton. Preferably it is 5-30 eq / ton, More preferably, it is 5-20 eq / ton. If it exceeds 40 (eq / ton), a film having good hydrolysis resistance cannot be obtained. A film of less than 1 (eq / ton) is difficult to produce industrially. In the present invention, both whiteness and durability, which have been difficult in the past, are achieved. Therefore, the present invention is characterized in that the acid value can be achieved while containing a high concentration of particles.

  In order to obtain a white polyester film substrate having an acid value in the above range, the acid value of the raw material polyester resin is preferably 0.1 to 30 eq / ton, more preferably 1 to 20 eq / ton, and still more preferably. 3 to 10 eq / ton. When it exceeds 30 eq / ton, it becomes difficult to obtain a film having good hydrolysis resistance. A polyester resin of less than 0.1 (eq / ton) is difficult to produce industrially.

  The white polyester film substrate used in the present invention has a breaking elongation retention ratio of 30% or more, preferably 50% or more, more preferably 70% or more, which is an index of hydrolysis resistance. If it is less than 30%, the durability as a solar cell back surface protective film is low, so it cannot be used.

  The white polyester film substrate used in 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 a large number of cavities derived from fine particles having an average particle diameter of 0.1 to 3 μm, and cavities derived from particles and / or incompatible thermoplastic resin in polyester. It is also a preferred embodiment of the present invention to have a core layer composed of a polyester layer containing a large number. The production method thereof is arbitrary and is not particularly limited. As a method of joining the skin layer to the film surface, the skin layer polyester resin containing fine particles and an incompatible thermoplastic resin are contained. A preferred method is to employ a co-extrusion method in which the polyester resin of the core layer is supplied to separate extruders, then laminated in a molten state and extruded from the same die.

  When adding particles to the core layer and the skin layer, increasing the particle content of the skin layer is preferable from the viewpoint of high light reflection on the film surface. In this case, the concentration of the particles in the skin layer is preferably 5 to 50% by mass, more preferably 8 to 30% by mass. The concentration of the particles in the skin layer is preferably 1 to 15% by mass, more preferably 3 to 10% by mass.

  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 separated by stretching between rolls with different speeds (roll stretching), stretching by gripping and expanding by a clip (tenter stretching), stretching by inflation 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 in 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, first, the film is preheated to a temperature not higher than 50 ° C. to a 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 preferable 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 performed in a tenter, and is preferably performed in the range of the melting point Tm-50 ° C. to Tm of the polyester.

(Thermal adhesive layer)
In the present invention, in order to improve the adhesiveness to the EVA resin, a thermal adhesive layer mainly composed of an amorphous polyester resin is laminated 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 heat bonding layer is preferably 3 to 30 μm, more preferably 5 to 25 μm, and particularly preferably 10 to 20 μm from 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, 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.

  Moreover, in this invention, it is preferable from a point which suppresses the curl of a film to provide a heat bonding layer on both surfaces of a white polyester film base material. 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 providing a heat bonding layer on both surfaces of a base material, it is preferable that the thickness ratio of the heat bonding layer of the front and back is 0.5-2.0. 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 heat-bonding layer in the present invention that the amorphous polyester resin is a main constituent 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 amount of heat of fusion is measured by heating at a rate of 10 ° C./min in a nitrogen atmosphere using a DSC apparatus in accordance with “Method for measuring 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 measured according to “Method for measuring plastic transition temperature” described in JIS-K7121, and heated at a rate of 10 ° C./min in a nitrogen atmosphere using a DSC apparatus. The intermediate-point glass transition temperature (Tmg) required in the above. 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 type 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 component includes ethylene glycol, diethylene glycol, neopentyl glycol (NPG), cyclohexanedimethanol (CHDM), propanediol, butanediol, and the acid component includes 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. 3-25 mass% is preferable and 5-20 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 preferred embodiments to contain the fine particles in the thermal adhesive layer as long as the thermal adhesiveness 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.

(Coating layer)
In the present invention, in order to improve the adhesion to the EVA resin, it is also preferable to further laminate a coating layer on the surface of the thermal adhesive layer. The resin constituting the coating layer is preferably composed mainly of at least one of polyester resin, polyurethane resin and / or polyacrylic resin. 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 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 solution to one or both sides of a uniaxially stretched film in the longitudinal direction, drying at 100 to 150 ° C., and further 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 provide easy slipping. 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 oxide, 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.

  Moreover, a well-known method can be used as a method of apply | coating a coating liquid. 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.

  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) 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.

(2) 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.

(3) 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.

(4) Acid value of polyester resin Polyester resin was dissolved in deuterated chloroform / triethylamine solution, and inorganic particles were removed by centrifugation. Using this solution, the molar concentration of the carboxylic acid group at the end of the polyester molecular chain was determined by ¹ H-NMR.

(5) Apparent specific gravity The film is accurately cut into a 10 cm × 10 cm square, and its thickness is measured at 50 points to determine an average thickness t (unit: μm). Next, the mass of the sample is weighed with an accuracy of 0.01 mg to be w (unit: g). And the apparent specific gravity was calculated by the following formula.
Apparent specific gravity (g / cm ³ ) = (w / t) × 100

(6) Whiteness Whiteness JIS L 1015-1981 (Method B) was performed using Nippon Denshoku Industries Co., Ltd. Z-1001DP.

(7) Hydrolysis resistance HAST (Highly Accelerated Temperature and Humidity Stress Test) standardized in JIS C 60068-2-66 was performed. The equipment was EHS-221 manufactured by ESPEC CORP. 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 the treatment was measured according to JIS K 7161-1994, and the elongation at break was calculated according to the following formula.
Breaking elongation retention ratio (%) = (breaking elongation after treatment / breaking elongation before treatment) × 100

(8) 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. 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 on which the ease of contact of the film was to be evaluated 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.
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

[Example 1]
(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 diameter of 0. A mixture of 3 μm (electron microscopy) 50% by mass of rutile type titanium oxide is supplied to a vent type twin screw extruder, kneaded for about 20 minutes, continuously sucked at 0.1 MPa, and deaerated. While extruding at 275 ° C., a master batch pellet containing fine particles was prepared. Further, this master batch pellet was subjected to solid phase polymerization treatment under a vacuum of 10 Pa until the intrinsic viscosity became 0.79, thereby producing a fine particle-containing master batch (MB-A). The acid value of this MB-A was 23 (eq / ton).

(Preparation of amorphous polyester resin)
A non-crystalline polyester resin (Co-PET) comprising 100 mol% of terephthalic acid as the dicarboxylic acid component, 70 mol% of ethylene glycol and 30 mol% of neopentyl glycol as the dicarboxylic acid component is carried out by a conventional method. 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) and polystyrene (manufactured by Nippon Polystyrene Co., Ltd., G797N, melt flow rate 1.5) dried to a moisture content of 30 ppm were mixed in 85% by mass and 15% by mass, respectively. Is fed into Extruder A, and similarly dried B-layer raw materials mixed with PET-A and MB-A at 60% by mass and 40% by mass are 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 a layer structure of A / B / A.

  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 2]
(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 put 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 this, it carried out similarly to Example 1, and obtained the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer.

Example 3
(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 in which a thermal adhesive layer was laminated in the same manner as in Example 1 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 4
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 3 was obtained.

[Comparative Example 1]
In the production of the master batch containing fine particles, PET-A (water content 3500 ppm) which is not subjected to drying treatment was used and solid phase polymerization treatment was not conducted. The acid value of this pellet (MB-A3) was 57 (eq / ton). Moreover, it replaced with PET-A and used the polyethylene terephthalate resin (PET-C) whose acid value is 31 (eq / ton). 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 1 was obtained.

[Comparative Example 2]
Regarding the raw material to be 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 3 was obtained.

Example 5
A fine particle-containing masterbatch (MB-A4) 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 in the same manner as in Example 3 was obtained except that this was used in place of MB-A1 as a raw material for the B layer.

[Comparative Example 3]
About the raw material supplied to the extruder A, it replaced with Co-PET and used PET-A. 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 3.

Example 6
In Example 1, 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 3. This obtained the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer.

Example 7
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 the extruder A, except having changed into the ratio shown in Table 3, it carried out similarly to Example 1, and obtained the polyester film for solar cell back surface protective films which laminated | stacked the heat bonding layer.

[Comparative Examples 4 and 5]
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 1.

  As shown in Tables 2 and 3, the polyester film for the solar cell back surface protective film of the examples within the scope of the present invention exhibited excellent moisture resistance and EVA resin adhesion. On the other hand, the films of Comparative Examples 1 and 2, which are outside the scope of the present invention, have poor moisture resistance, and the films of Comparative Examples 3 to 5 have poor adhesion to EVA resin, and are used as film bases for solar cell back surface protective films. Was unsuitable.

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

Claims (5)

  A thermal adhesive layer having a thickness of 1.0 to 40 μm mainly composed of an amorphous polyester resin is formed on at least one surface of a white polyester film substrate containing 3 to 50% by mass of fine particles having an average particle size of 0.1 to 3 μm. A polyester film for a solar cell back surface protective film, having an acid value of 1 to 40 eq / ton, a whiteness of 50 or more, and a thickness of 38 to 1000 μm.   The white polyester film base material has a large number of fine cavities inside, and has an apparent specific gravity of 0.7 to 1.3.   The polyester film for a solar cell back surface protective film according to claim 1 or 2, wherein the white polyester film base material contains a large number of cavities derived from incompatible thermoplastic resins.   The polyester resin for a solar cell back surface protective film according to any one of claims 1 to 3, wherein a polyester resin having an acid value of 0.1 to 30 eq / ton is used as a raw material for the white polyester film substrate.   The polyester film for a solar cell back surface protective film according to any one of claims 1 to 4, wherein a white pigment masterbatch having an acid value of 1 to 40 eq / ton is used as a raw material for the white polyester film substrate.