JP2018083873A - Polyester film, and solar cell back sheet and solar cell comprising the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film that achieves both of excellent output improvement effect and excellent workability, and a film for a solar cell back sheet, and a solar cell back sheet and a solar cell comprising the same.SOLUTION: A polyester film has an average spectral reflectance of 90% or more in a wavelength range of 400-1200 nm, and satisfies the following (1) and (2) in both of the film longitudinal direction and width direction: (1) the ratio between thermal shrinkage stress (MPa) Aat 150°C and thermal shrinkage stress (MPa) Aat 100°C (A/A) is 15 or less; (2) the tensile strength is 100 MPa or more in both of the longitudinal direction and width direction.SELECTED DRAWING: None

Description

  The present invention relates to a polyester film.

  In recent years, photovoltaic power generation, which is clean energy, has attracted attention as a semi-permanent and pollution-free next-generation energy source, and solar cells are rapidly spreading.

A typical configuration of a general solar cell is shown in FIG. In a solar cell, a power generating element 3 is sealed with a transparent filler 2 such as EVA (ethylene-vinyl acetate copolymer), and a transparent substrate 4 such as glass and a resin sheet called a back sheet 1 are bonded together. Composed. Sunlight is introduced into the solar cell through the transparent substrate 4. Sunlight introduced into the solar cell is absorbed by the power generation element 3, and the absorbed light energy is converted into electric energy. The converted electric energy is taken out by a lead wire (not shown in FIG. 1) connected to the power generation element 3 and used for various electric devices. Here, the back sheet 1 is a sheet member that is installed on the back side of the power generation element 3 with respect to the sun and is not in direct contact with the power generation element 3. Various proposals have been made for the solar cell system and each member. For the back sheet 1, polyethylene-based, polyester-based, and fluorine-based resin films are mainly used. (See Patent Documents 1 to 3)
In a conventional back sheet for a solar cell module, a technique has been developed that improves the efficiency of a solar cell module by reflecting light that has passed between solar cells with the back sheet and incorporating the light into the cells. Specifically, a technology for improving the module efficiency by forming a reflective layer with white beads and a white binder on the substrate surface, and a technology for providing a highly reflective back sheet by forming a layer containing cavities (Patent Document 4) Have been proposed).

JP-A-11-261085 JP-A-11-186575 JP 2006-270025 A JP 2012-84670 A

  However, when a solar battery cell is produced using the solar battery backsheet film described in Patent Documents 1 to 4 as a solar battery backsheet, an appearance defect such that the end part is peeled off due to the thermal shrinkage behavior of the polyester film in a high temperature laminating process occurs. There was a problem to do.

  Then, this invention makes it a subject to provide the polyester film which makes the outstanding output improvement effect and external appearance defect suppression compatible by using for a solar cell backsheet use in view of the background of this prior art.

In order to solve the above problems, the present invention has the following configuration. That is, the average spectral reflectance at a wavelength of 400 to 1200 nm is 90% or more, and the following (1) and (2) are satisfied in any of the longitudinal direction and the width direction of the film. It is a polyester film.
(1) 0.99 ° C. of NetsuOsamu stress _{(MPa) A 0.99} and 100 ° C. of NetsuOsamu stress _(MPa) ratio _{A 100 (A 150 / A 100} ) is 15 or less (2) Tensile strength longitudinal and transverse direction 100 MPa or more in any direction

According to the present invention, compared to a solar battery back sheet using a conventional polyester film, it becomes possible to suppress appearance defects in a high-temperature laminating process for producing a solar battery cell, and further, the polyester film of the present invention is mounted. Thus, it is possible to provide a solar cell with higher power generation efficiency (hereinafter referred to as output improvement) than the conventional one.

It is sectional drawing which shows typically an example of a structure of the solar cell using the polyester film of this invention. The thickness direction cross section of a polyester film is shown typically.

The polyester film of the present invention has an average spectral reflectance at a wavelength of 400 to 1200 nm of 90% or more, and satisfies the following (1) and (2) in both the longitudinal direction and the width direction of the film. It is characterized by that.
(1) 0.99 ° C. of NetsuOsamu stress _{(MPa) A 0.99} and 100 ° C. of NetsuOsamu stress _(MPa) ratio _{A 100 (A 150 / A 100} ) is 15 or less (2) Tensile strength longitudinal and transverse direction In any of these directions, the polyester film of the present invention will be described in the range of 100 MPa to 100 MPa.

The polyester film of the present invention contains a polyester resin as a main component. Here, the polyester resin as a main component means that the polyester resin is contained in an amount exceeding 50% by mass with respect to the resin constituting the polyester film of the present invention.
The polyester resin used in the present invention is: 1) polycondensation of dicarboxylic acid or its ester-forming derivative (hereinafter collectively referred to as “dicarboxylic acid component”) and diol component, 2) carboxylic acid or carboxylic acid derivative in one molecule And a polycondensation of a compound having a hydroxyl group and 1) 2). The polymerization of the polyester resin can be performed by a conventional method.

  In 1), as the dicarboxylic acid component, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosandioic acid, pimelic acid, azelaic acid, methylmalonic acid, Aliphatic dicarboxylic acids such as ethylmalonic acid, alicyclic dicarboxylic acids such as adamantane dicarboxylic acid, norbornene dicarboxylic acid, cyclohexane dicarboxylic acid, decalin dicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 4,4′-diphenyletherdicarboxylic acid, 4,4′-diphenylsulfonedicarboxylic acid Acid, 5-sodium Hoisofutaru acid, phenyl ene boys carboxylic acid, anthracene dicarboxylic acid, phenanthrene carboxylic acid, 9,9'-bis (4-carboxyphenyl) aromatic dicarboxylic acids such as fluorene acid, or the like its ester derivatives and the like as a typical example. These may be used alone or in combination.

  Further, a dicarboxy compound obtained by condensing oxyacids such as l-lactide, d-lactide, hydroxybenzoic acid and the like, or a combination of a plurality of such oxyacids, at least one carboxy terminus of the dicarboxylic acid component described above Can also be used.

  Next, as the diol component, aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, Fragrances such as cycloaliphatic diols such as cyclohexanedimethanol, spiroglycol and isosorbide, bisphenol A, 1,3-benzenedimethanol, 1,4-benzenedimethanol, 9,9'-bis (4-hydroxyphenyl) fluorene Group diols are typical examples. Moreover, these may be used independently or may be used in multiple types as needed. In addition, a dihydroxy compound formed by condensing a diol with at least one hydroxy terminal of the diol component described above can also be used.

  In 2), examples of compounds having a carboxylic acid or a carboxylic acid derivative and a hydroxyl group in one molecule include oxyacids such as l-lactide, d-lactide and hydroxybenzoic acid, and derivatives thereof, oligomers of oxyacids, dicarboxylic acids Examples thereof include those obtained by condensing an oxyacid with one carboxyl group of the acid.

  Polyester resins obtained from the above two components include polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, polypropylene terephthalate, polybutylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, and mixtures thereof. Polyethylene terephthalate and polyethylene-2,6-naphthalenedicarboxylate are preferred from the viewpoint that the film-forming property is good, more preferably polyethylene from the point that a film for a solar battery backsheet having better adhesion can be obtained. Most preferred is terephthalate.

  In the polyester film of the present invention, the polyester resin constituting the film preferably has a terminal carboxyl group amount of 25 equivalent / ton or less, more preferably 20 equivalent / ton or less, and further preferably 17 equivalent / ton or less. When the amount of the terminal carboxyl group exceeds 25 equivalents / ton, the moisture and heat resistance of the polyester film is lowered. Therefore, when it is mounted on a solar cell installed outdoors for a long period of time, the film deteriorates and the adhesiveness is lowered. The film may be discolored and the reflectivity may be impaired, so that the output improvement may be reduced. On the other hand, the lower limit of the terminal carboxyl group amount is not particularly limited as long as the effect of the present invention is not impaired, but from the viewpoint of adhesion strength, 7 equivalent / ton or more is preferable, and 11 amount / ton or more is further. preferable. When the amount of terminal carboxyl groups is less than 7 equivalents / ton, there are cases where the polar end groups on the surface are insufficient and the absolute value of the adhesion strength becomes small.

  In the polyester film of the present invention, the intrinsic viscosity IV of the polyester resin constituting the film is preferably 0.63 dl / g or more and 0.80 dl / g or less, more preferably 0.65 dl / g or more, and still more preferably 0.67 dl. / G or more. When the intrinsic viscosity IV is less than 0.63 dl / g, the dispersibility of the nucleating agent that forms the cavities may be lowered, and the moisture and heat resistance of the polyester film may be lowered. On the other hand, when the intrinsic viscosity IV exceeds 0.80 dl / g, the extrudability of the polyester resin may deteriorate.

  Furthermore, in the polyester film of the present invention, the number average molecular weight Mn of the polyester resin constituting the film is preferably from 8000 to 40000, more preferably from 9000 to 30000, still more preferably from 10000 to 20000. When the number average molecular weight Mn is less than 8000, durability such as heat and moisture resistance and heat resistance may be lowered. On the other hand, when the number average molecular weight Mn exceeds 40,000, the polymerization is difficult and the extrudability of the polyester resin may be deteriorated even if the polymerization is possible.

  Furthermore, in the polyester film of the present invention, the polyester resin constituting the film preferably contains Mn or Na as a metal element. It is preferable that Mn is contained in the range of 50 to 200 ppm and Na is contained in the range of 10 to 80 ppm. More preferably, Mn and Na are contained. When Mn or Na is contained in the above range in the polyester resin, the moisture and heat resistance of the film can be improved, and a polyester film having excellent moisture and heat resistance, adhesion, and output improvement can be obtained.

  The polyester film of the present invention has an average spectral reflectance at a wavelength of 400 to 1200 nm of 90% or more. The average spectral reflectance here is a value obtained by measuring a reflection spectrum of a polyester film with a spectrophotometer and calculating an average value, which will be described in detail later. Preferably, the average spectral reflectance is 93% or more, more preferably 95% or more, and even more preferably 97% or more. When the average spectral reflectance is less than 90%, the reflectivity is insufficient and the output improvement property is insufficient.

In the present invention, in order for the average spectral reflectance at a wavelength of 400 to 1200 nm to be 90% or more, a form having cavities inside the polyester film is preferable. The “cavity” in the present invention was obtained by using a microtome, cutting the film perpendicular to the film surface direction without crushing the film in the thickness direction, and observing the cut surface of the film using an electron microscope. A cross-sectional area observed in the observation image represents a void having a size of 0.1 μm ² or more.
In the present invention, the method for forming cavities in the polyester film is not particularly limited, but a method in which a hollow nucleating agent is contained in the raw material of the polyester film and stretched during film formation is preferred. .
Examples of the cavity nucleating agent include organic nucleating agents such as olefin resins that are incompatible with the polyester resin, and inorganic nucleating agents such as inorganic particles and glass beads. As the nucleating agent, an organic nucleating agent is preferable because it may promote crystallization and deteriorate the workability, or the formation of cavities may become uniform in the thickness direction, resulting in a decrease in adhesion.

  Examples of olefin resins that are organic nucleating agents include aliphatic polyolefin resins such as polypropylene, polyethylene, high density polyethylene, low density polypropylene, ethylene-propylene copolymer, and polymethylpentene, and cyclic polyolefins such as cycloolefin polymers and cycloolefin copolymers. Resins, etc. are mentioned. Among them, as an organic nucleating agent, an olefin having a Vicat softening point of 140 ° C. or higher is excellent in improving the output of a solar cell backsheet film by forming fine cavities and increasing the reflectivity. Resin is preferable, and olefin resin at 180 ° C. or higher is more preferable. When an olefin resin having a Vicat softening point of less than 140 ° C. is used as the organic nucleating agent, the shape of the cavity may become too coarse and the reflectivity may decrease.

The amount of the organic nucleating agent contained in the polyester film is preferably 4% by mass or more and 15% by mass or less, more preferably 8% by mass or more and 13% by mass or less with respect to the total mass of the polyester film.
Here, when the amount of the organic nucleating agent contained in the polyester film is less than 4% by mass, the reflectivity may be lowered. On the other hand, when the amount of the organic nucleating agent exceeds 15% by mass, the difference between the heat yield stress at the low temperature and the heat yield stress at the high temperature is increased due to the internal cavity, and the heat yield stress ratio may be increased. . Moreover, the tensile strength of a polyester film may become small with an internal cavity.
When an organic nucleating agent is used, it is preferable to use a dispersion aid at the same time. As the dispersion aid, a polyester elastomer or amorphous copolymerized with a polyether structure, a bent skeleton structure, a bulky cyclohexane skeleton structure, etc. The polyester resin is preferably used, and the amount of the dispersion aid contained in the polyester film is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more, 8 based on the total mass of the polyester film. It is not more than mass%, more preferably not less than 3 mass% and not more than 6 mass%, and two kinds of dispersion aids can be used in combination.
Here, when the amount of the dispersion aid contained in the polyester film is less than 1% by mass, the effect as the dispersion aid is insufficient, and the reflectivity may be lowered. On the other hand, when the amount of the dispersion aid exceeds 10% by mass, thermal shrinkage increases due to a decrease in crystallinity of the polyester film, and the processability may deteriorate. Further, since the dispersibility is excessively improved, the formation of the cavities becomes uniform in the thickness direction, so that the adhesion may be lowered.

In the polyester film of the present invention, the cavity in the observation image of the cross section in the thickness direction draws a line perpendicular to the surface direction from one surface of the film to the other surface, and connects the surface from one surface to the other surface. The film passes through three points (the film thickness direction center point (C1 point), the intermediate point between the film thickness direction center point and the film surface (C2-1 point), (C2-2 point)) divided into four equal parts in the thickness direction. In a line parallel to the surface direction (divided horizontal line), the average area per cavity existing on the divided horizontal line passing through the point C1 is Sc (μm ² ), and the cavity 1 existing on the divided horizontal line passing through the point C2-1 When the average area per unit is Scs (μm ² ) and the average area per cavity existing on the dividing horizontal line passing through the point C2-2 is Scs ′ (μm ² ), (Sc / Scs), (Sc / Scs') Preferably one of 1.1 or more 35 or less, more preferably 1.5 to 20, still more preferably 2.0 to 15. Details of how to obtain Sc (μm ² ), Scs (μm ² ), and Scs ′ (μm ² ) will be described later.
Here, when both of (Sc / Scs) and (Sc / Scs ′) are less than 1.1, the polyester film of the present invention is used as a solar cell even if a fine cavity is formed inside the polyester film. The adhesion of the solar cell backsheet film because the force to peel off the adhesion surface is applied evenly in the film surface when it is brought into close contact with the sealing material EVA or other member film to be laminated during the production of the backsheet May decrease. On the other hand, if both (Sc / Scs) and (Sc / Scs') exceed 35, the deviation of the cavity area in the thickness cross section becomes too large, and peeling tends to proceed from the coarse cavity portion, resulting in Adhesion may be reduced. In addition, the light reflectivity due to the cavity may be reduced.
In the present invention, (Sc / Scs) and (Sc / Scs') adjust the shape of the cavity according to the type of the above-described cavity nucleating agent, the amount of the cavity nucleating agent, the amount of the dispersing agent, or the cooling rate at the time of melt extrusion during production. can do. For example, an olefin resin having a Vicat softening point of 140 ° C. or higher is used as an organic nucleating agent, and by increasing the amount of the cavity nucleating agent and the amount of the dispersion aid within a preferable range, a more uniform cavity can be refined and The amount of cavities increases and (Sc / Scs) and (Sc / Scs') decrease. On the other hand, by reducing the amount of the cavity nucleating agent and the amount of the dispersion aid within a preferable range, the deviation of the cavity area in the thickness cross section increases, and (Sc / Scs) and (Sc / Scs ′) increase.
That is, the polyester film of the present invention can adjust the internal cavity nucleating agent, the amount of the cavity nucleating agent, the amount of the dispersing agent, or the cooling rate at the time of melt extrusion during production within a preferable range ( At least one of (Sc / Scs) and (Sc / Scs') can be 1.1 to 35.

  In addition, when the values of (Sc / Scs) and (Sc / Scs') are different and only one of them is in the range of 1.1 to 35, the effect of improving the adhesion is expected (Sc / Scs) or It is preferable to locate the surface side where (Sc / Scs ′) is in a preferred range. For example, when the polyester film of the present invention is used as a film for a solar battery backsheet and only (Sc / Scs) is 1.1 or more and 35 or less, the sealing material for the solar battery is on the film surface side close to Scs. By arranging so that it may be located, the effect of this invention and adhesiveness can be made compatible. If both (Sc / Scs) and (Sc / Scs') are 1.1 or more and 35 or less, the adhesion of both sides of the polyester film is excellent. For example, one side of the polyester film is bonded to another member film. In the configuration in which the other surface is directly bonded to the solar battery cell, it is more preferable because excellent adhesion can be obtained on both surfaces of the film.

  When the polyester film of the present invention is used as a solar battery back sheet, it is possible to recycle light by diffusing the light that has passed between solar battery cells while diffusing the solar battery back sheet to increase the power generation output. is there. Here, from the viewpoint of increasing the light diffusibility, a form in which inorganic particles are contained in the polyester resin composition constituting the polyester film is preferable. Examples of the inorganic particles here include calcium carbonate, magnesium carbonate, zinc carbonate, titanium oxide, zinc oxide, cerium oxide, magnesium oxide, barium sulfate, zinc sulfide, calcium phosphate, alumina, mica, mica, talc, clay, kaolin. , Lithium fluoride, calcium fluoride, and the like. Among these, calcium carbonate, magnesium carbonate, titanium oxide, zinc oxide, and barium sulfate are preferable from the viewpoint of easy processing with a polyester resin, and titanium oxide is preferable from the viewpoint of simultaneously improving the ultraviolet resistance of the solar battery backsheet film. More preferred. Examples of titanium oxide include crystalline titanium oxide such as anatase type titanium oxide and rutile type titanium oxide. From the viewpoint of increasing the difference in refractive index from the polyester used, titanium oxide having a refractive index of 2.7 or more is preferable, and at the same time, rutile type titanium oxide is used from the viewpoint of superior ultraviolet resistance. Further preferred.

  Although it does not restrict | limit especially as a form which makes the resin composition which comprises the polyester film of this invention contain an inorganic particle, it has a laminated structure of 3 layers or more, and both surface layers (one surface layer is P2 layer). And the other surface layer is a P2 ′ layer) at least one resin composition contains inorganic particles and does not have a surface layer (this layer is a P1 layer) Is preferably a structure containing the above-described cavity nucleating agent, and more preferably a three-layer structure including P2 layer / P1 layer / P2 ′ layer.

In addition, when inorganic particles are contained in the P2 layer and the P2 ′ layer, they may become a cavity nucleating agent and the surface layer may contain a small amount of cavities. At this time, the porosity (Ps) of the P2 layer and the porosity (Ps ′) of the P2 ′ layer are preferably 5.0% or less, more preferably 4.0% or less, and still more preferably 3.5%. It is as follows. When either of the porosity (Ps) and (Ps ′) of the P2 layer and the P2 ′ layer exceeds 5.0%, the surface layer side may become unstable in the cavity area and the adhesion may be lowered. . Further, when only one of the porosity (Ps) of the P2 layer and the porosity (Ps ′) of the P2 ′ layer is a film satisfying 5.0% or less, the surface side on which the effect of improving the adhesion is expected. By positioning the P2 layer or the P2 ′ layer, the adhesion can be further enhanced.
The structure having the P2 layer or P2 ′ layer containing the inorganic particles having ultraviolet resistance on the surface layer of the polyester film of the present invention makes use of the high reflectivity of the polyester film, and the output improvement effect and the ultraviolet light hitting the solar battery cell It can be said that it is a more preferable form as a film for a solar battery back sheet because it can achieve both UV resistance such as suppressing discoloration due to. Moreover, the structure which has an inorganic particle in both the resin composition which comprises P2 layer and P2 'layer, the effect of the ultraviolet-ray resistance by the side of the said photovoltaic cell is also with respect to the reflected light of the ultraviolet ray which hits the back surface of a solar cell. It can be demonstrated and can be said to be a more preferable form.

When the polyester film of the present invention has a structure having a P2 layer and / or a P2 ′ layer (hereinafter sometimes expressed as a P2 layer), the main component of the P2 layer does not impair the effects of the present invention. Any range can be selected freely. For example, by using the same polyester resin as the P1 layer as the main component of the P2 layer, a film for a solar battery back sheet having excellent adhesion between the P1 layer and the P2 layer interface can be obtained. Further, by using an acrylic resin or the like as the main component of the P2 layer, it becomes possible to provide a P2 layer with a higher filling of inorganic particles on the P1 layer by a coating method, and to achieve both excellent adhesion and improved output. It can be set as the film for battery back sheets.
Further, the thickness of the P1 layer is T1 (μm), the thickness of the P2 layer is T2 (μm), the thickness of the P2 ′ layer is T2 ′ (μm), and the concentration of inorganic particles contained in the resin composition constituting the P2 layer is W2. (% By mass), when the concentration of inorganic particles contained in the resin composition constituting the P2 ′ layer is W2 ′ (% by mass), of (T2 / T1) × W2, (T2 ′ / T1) × W2 ′ It is preferable that at least one satisfies 0.35 or more and 1.50 or less. More preferably, they are 0.75 or more and 1.40 or less, More preferably, they are 0.90 or more and 1.20 or less.

  Here, if both (T2 / T1) × W2 and (T2 ′ / T1) × W2 ′ are less than 0.35, the diffusibility of the P2 layer and the P2 ′ layer is insufficient, and either layer is a solar cell. Even if installed on the side, the output improvement of the present invention may be reduced. On the other hand, when both (T2 / T1) × W2 and (T2 ′ / T1) × W2 ′ exceed 1.50, the thermal contraction stress of the P2 layer and the P2 ′ layer increases, and the workability is reduced. There is. In addition, when only one of (T2 / T1) × W2 and (T2 / T1 ′) × W2 ′ is a film satisfying 0.35 or more and 1.50 or less, a solar cell that expects the effect of the present invention By positioning the P2 layer or the P2 ′ layer that satisfies the above range on the cell side, it is possible to achieve both improved output and workability.

  When the P1 layer of the polyester film of the present invention contains inorganic particles as a cavity nucleating agent, the concentration of inorganic particles is preferably 10% by mass or less with respect to the total mass of the P1 layer from the viewpoint of maintaining workability and adhesion. % Or less is more preferable, and 3% by weight or less is still more preferable. When the content of inorganic particles in the P1 layer exceeds 10% by mass, the heat shrinkage stress of the P1 layer increases and the workability deteriorates, or the (Sc / Scs) or (Sc / Scs') of the polyester film decreases. Adhesion may be reduced.

  In the polyester film of the present invention, in addition to the above-described cavity nucleating agent and inorganic particles, a heat resistant stabilizer, an oxidation resistant stabilizer, an ultraviolet absorber, and an ultraviolet light stabilizer are added as necessary without departing from the effects of the present invention. Additives such as an agent, an organic / inorganic lubricant, an organic / inorganic fine particle, a filler, a nucleating agent, a dye, and a coupling agent may be blended. For example, when an ultraviolet absorber is selected as an additive, the ultraviolet resistance of the polyester film of the present invention can be further improved. In addition, the electrical insulation can be improved by adding an antistatic agent or the like.

In the polyester film of the present invention, the ratio (A ₁₅₀ / A ₁₀₀ ) of the heat yield stress A _{150 at} 150 ° C. and the heat yield stress A ₁₀₀ at 100 ° C. is 15 or less in both the longitudinal direction and the width direction of the film. is there. The heat absorption stress here can be measured using thermomechanical analysis (TMA). The heat stress ratio (A ₁₅₀ / A ₁₀₀ ) is preferably 10 or less, more preferably 3 or less, and still more preferably 1.5 or less. The lower limit is not particularly limited, but is preferably close to 1.

In the present invention, when the longitudinal direction and the width direction of the polyester film are unknown, the degree of orientation of the polyester film is measured using a microwave transmission type molecular orientation meter MOA-6004 (manufactured by Oji Scientific Instruments). The direction with the strongest orientation is the longitudinal direction, and the direction perpendicular to the longitudinal direction is the width direction.
When a biaxially stretched polyester film is used for solar battery backsheets, EVA (ethylene-vinyl acetate copolymer) often used as a sealing material when thermal lamination with the sealing material is performed during solar cell production ) The resin starts to fuse with the polyester film showing viscosity at around 100 ° C. Furthermore, in thermal lamination, when the temperature is raised to around 150 ° C. in order to cross-link the EVA resin, when it is cooled to room temperature, there is a difference in the heat shrinkage behavior between the polyester film and the EVA resin, and the solar cell module The appearance defect that the polyester film peels from EVA at the end portion occurs. At this time, the ratio (A ₁₅₀ / A ₁₀₀ ) of the heat yield stress A _{150 at 150} ° C. and the heat yield stress A ₁₀₀ at 100 ° C. is 15 or less in both the longitudinal direction and the width direction of the film. Thus, the present inventors have found that the difference in the heat shrinkage behavior with the resin as the adherend can be suppressed in the processing step receiving the heat load as described above.

Furthermore, the polyester film of the present invention has a tensile strength of 100 MPa or more in both the longitudinal direction and the width direction. The tensile strength is preferably 120 MPa or more, more preferably 130 MPa or more, and still more preferably 135 MPa or more. If the tensile strength is less than 100 MPa in one of the longitudinal direction and the width direction of the polyester film, the polyester film becomes weak and the appearance defect such that wrinkles occur following the shrinkage of the EVA resin during cooling occurs. . The upper limit of the tensile strength is not particularly limited, but is preferably 200 MPa or less from the viewpoint of achieving both reflectivity.
That is, according to the present invention, the ratio (A ₁₅₀ / A ₁₀₀ ) of the heat yield stress A _{150 at 150} ° C. to the heat yield stress A ₁₀₀ at 100 ° C. is 15 or less in both the longitudinal direction and the width direction of the polyester film. In addition, by setting the tensile strength to 100 MPa or more, it becomes possible to suppress the appearance defect at the time of thermal lamination, and both workability and output improvement can be achieved.
Furthermore 15 0.99 ° C. of NetsuOsamu stress _{A 0.99} and of 100 ° C. of NetsuOsamu stress _{A 100} ratio of _(A 150 _/ A ₁₀₀₎ or less, while the tensile strength above 100 MPa, said (Sc / Scs), (Sc / Scs') satisfying at least one of 1.1 and 35 or less suppresses the difference in thermal shrinkage behavior while enhancing the adhesion between the polyester film of the present invention and the solar cell sealing material. Thus, the effect of suppressing the appearance defect can be remarkably obtained. This effect is presumed that the appearance failure seen in the solar cell backsheet is due to the peeling of the module end caused by the difference in heat shrinkage behavior when the adhesion between the polyester film and the sealing material is reduced. Yes.

In the polyester film of the present invention, the heat yield stress A _{100 at} 100 ° C. is preferably 0.010 MPa or more and 0.4 MPa or less, more preferably 0.020 MPa or more, 0 in both the longitudinal direction and the width direction. .2 MPa or less, more preferably 0.030 MPa or more and 0.1 MPa or less. When the heat yield stress A ₁₀₀ at 100 ° C. is less than 0.010 MPa in either the longitudinal direction or the width direction of the polyester film, the polyester film cannot follow the thermal shrinkage behavior of the EVA resin, The peeling may be further deteriorated. On the other hand, when it exceeds 0.4 MPa, the polyester film may shrink before the EVA resin is fused, and wrinkles may be further deteriorated.
The heat-absorbing stress and tensile strength of the polyester film can be adjusted by controlling the hollow structure of the polyester film and / or the film-forming conditions of the polyester film, and further by off-line processing after film formation.
The thickness of the polyester film of the present invention is preferably from 25 μm to 350 μm, more preferably from 30 μm to 300 μm, still more preferably from 50 μm to 260 μm. When the thickness of the polyester film of the present invention is less than 25 μm, wrinkles may occur during lamination and workability may deteriorate. On the other hand, if the thickness is greater than 350 μm, the winding property may deteriorate.

(Production method of polyester film)
Next, an example is given and demonstrated about the manufacturing method of the polyester film of this invention. This is an example, and the present invention should not be construed as being limited to the product obtained by such an example.

  First, the polyester resin used as the raw material of the polyester film of the present invention can be obtained by subjecting a dicarboxylic acid or an ester derivative thereof and a diol to a transesterification reaction or an esterification reaction by a known method. Examples of conventionally known reaction catalysts include alkali metal compounds, alkaline earth metal compounds, zinc compounds, lead compounds, manganese compounds, cobalt compounds, aluminum compounds, antimony compounds, titanium compounds, and phosphorus compounds. Preferably, an alkali metal compound, a manganese compound, an antimony compound or a germanium compound, or a titanium compound is preferably added as a polymerization catalyst at any stage before the normal production method is completed, and the adhesion of the polyester film is further increased. From the viewpoint, it is more preferable to add a sodium compound or a manganese compound. As such a method, for example, when a manganese compound is taken as an example, it is preferable to add the manganese compound powder as it is.

  Moreover, the terminal carboxyl group amount of the polyester resin is heated at a polymerization temperature or after polymerization of the polyester resin at a temperature of 190 ° C. to less than the melting point of the polyester resin under reduced pressure or a flow of an inert gas such as nitrogen gas. It can be controlled by the so-called solid phase polymerization time. Specifically, the amount of terminal carboxyl groups increases as the polymerization temperature increases, and the amount of terminal carboxyl groups decreases as the time of solid phase polymerization increases.

  The polyester film of the present invention contains a cavity nucleating agent, inorganic particles, and the like by blending master pellets prepared by melting and kneading raw materials in advance using a vented biaxial kneading extruder or tandem extruder. preferable. At this time, since the master pellet receives a heat history, there is a concern that the heat deterioration is not a little progressed. Therefore, it is more preferable to prepare a master pellet containing the cavity nucleating agent and inorganic particles at a higher concentration, and mix and dilute them. Specifically, when adding the cavity nucleating agent to the polyester film of the present invention, a master pellet having a higher content than the target cavity nucleating agent content in the polyester film is prepared in advance, and this is used as the main component of the polyester film. It mixes with the polyester resin which is a component, and adjusts to the target content.

  Next, the polyester film forming method of the present invention is a method in which a raw material adjusted to have a composition of a polyester film is heated and melted in an extruder and extruded from a die cooled onto a cast drum (melting method). Cast method) can be used.

Subsequently, the obtained sheet is guided to a roll group heated to a temperature of 70 to 110 ° C., stretched in the longitudinal direction (longitudinal direction, that is, the traveling direction of the sheet), and cooled with a roll group having a temperature of 20 to 50 ° C. . Subsequently, the both ends of the sheet are guided to a tenter while being gripped by clips, and are stretched at a magnification of 3 to 5 times in a direction (width direction) perpendicular to the longitudinal direction in an atmosphere heated to a temperature of 80 to 130 ° C. . Under the present circumstances, in the polyester film of this invention, the draw ratio of a longitudinal direction has preferable 3.4 times or less, More preferably, it is 3.3 times or less. If the draw ratio in the longitudinal direction exceeds 3.4 times, the plane orientation of the polyester film becomes too strong, and the ratio of the heat yield stress A _{150 at 150} ° C. to the heat yield stress A ₁₀₀ at 100 ° C. (A ₁₅₀ / A ₁₀₀ ) May become larger.

Subsequently, heat setting is performed in the tenter after stretching. The set temperature at this time is preferably 210 ° C. or higher and 250 ° C. or lower, more preferably 215 ° C. or higher and 240 ° C. or lower, and further preferably 220 ° C. or higher and 230 ° C. or lower. When heat setting is performed at less than 210 ° C., the shrinkage stress remaining in the polyester film becomes too strong, and the heat yield stress ratio (A ₁₅₀ / A ₁₀₀ ) may increase.

In the polyester film of the present invention, as a means for adjusting the ratio (A ₁₅₀ / A ₁₀₀ ) of the heat yield stress A ₁₅₀ at 150 ° C. and the heat yield stress A ₁₀₀ at 100 ° C. to an annealing process after film formation. Can be used. Specifically, there is a method in which the film obtained above is heat-treated in a hot air oven installed between a film winding roll and a film winding roll. At this time, the oven set temperature is preferably 140 ° C. or higher, more preferably 145 ° C. or higher, and even more preferably 150 ° C. or higher. When the set temperature of the oven is lower than 140 ° C., the suppression of heat yield stress at 150 ° C. is insufficient, and as a result, the heat yield stress ratio (A ₁₅₀ / A ₁₀₀ ) may increase. Although the upper limit of the oven set temperature is not particularly limited, it is preferably 240 ° C. or lower. When the oven set temperature exceeds 240 ° C., the orientation of the film is lost, and the tensile strength and heat absorption stress at 100 ° C. May decrease.
Further, the time during which the film is heat-treated in the oven is preferably 10 seconds or more and 200 seconds or less, more preferably 13 seconds or more and 150 seconds or less, and further preferably 15 seconds or more and 100 seconds or less. preferable. If the time during which the film is heat-treated in the oven is less than 10 seconds, the annealing treatment may be insufficient and the heat yield stress ratio (A ₁₅₀ / A ₁₀₀ ) may increase. On the other hand, if the time exceeds 600 seconds, the film is excessively annealed, and the heat yield stress at 100 ° C. is lowered, resulting in an increase in the heat yield stress ratio (A ₁₅₀ / A ₁₀₀ ).

Further, the winding tension at the oven outlet during annealing is preferably 20 N or more and 300 N or less, more preferably 30 N or more and 250 N or less, and further preferably 40 N or more and 200 N or less. When the winding tension is less than 20N, the flatness of the film is deteriorated, and as a result, the workability may be lowered. On the other hand, if the winding tension exceeds 300 N, too much tension is applied in the longitudinal direction of the film, and as a result, the heat absorption stress ratio (A ₁₅₀ / A ₁₀₀ ) may increase.

That is, in the present invention, in order to make the ratio (A ₁₅₀ / A ₁₀₀ ) of the heat yield stress A ₁₅₀ at 150 ° C. and the heat yield stress A ₁₀₀ at 100 ° C. of the polyester film to 15 or less, the adjustment of the above film forming conditions, In addition, it is necessary to perform an annealing process after the film formation, and it is more preferable to perform the annealing process after the film formation in order to make the heat stress ratio (A ₁₅₀ / A ₁₀₀ ) a preferable range.

  In addition, when the polyester film of the present invention has a P2 layer, for example, the raw material constituting the P1 layer and the raw material constituting the P2 layer are put into two different extruders, melted and then merged, and cooled from the die A co-extrusion process on a cast drum (coextrusion method), or a roll coating method that forms a P2 layer dissolved in a solvent after a polyester film having a P1 layer is formed alone A method (coating method) in which a P2 layer is formed by applying a dip coating method, a bar coating method, a die coating method, a gravure roll coating method and the like and then drying the solvent is preferably used.

  The polyester film of the present invention is produced by the production method described above. Since the obtained polyester film has excellent reflectivity and excellent workability, it is suitably used for a film for a solar battery back sheet, a base film for a blood glucose level sensor, a base film for a magnetic card, etc. be able to. In particular, the use of the film for a solar battery back sheet makes it possible to suppress the output improvement of the solar battery back sheet and poor appearance during the production of the solar battery cell by making use of excellent reflectivity and workability.

(Solar cell back sheet)
Next, a solar battery back sheet using the polyester film of the present invention will be described. The solar battery backsheet using the polyester film of the present invention is a layer having other functions such as gas barrier properties and adhesion to a sealing material resin on one or both sides of the polyester film, using the polyester film alone. It is characterized by providing.
As a method of providing the other functional layer, a method for separately producing a polyester film and the other functional layer and thermocompression bonding with a heated group of rolls (thermal laminating method), a method of attaching them through an adhesive ( Bonding method), other methods of dissolving the material for forming the laminated material in a solvent and applying it on the film (coating method), applying a curable material on the film and then curing it by electromagnetic wave irradiation, heat treatment, etc., A method of depositing / sputtering a material to be laminated on a film and a combination of these can be used.
In particular, in the present invention, after the other lamination material is applied on the polyester film by a coating method, it is introduced into an oven to volatilize the solvent. More preferably, it is also performed.

  Since the solar battery backsheet using the polyester film of the present invention has both excellent reflectivity and processability compared to the conventional solar battery backsheet, while improving the power generation efficiency of the mounted solar battery, The appearance defect can be suppressed.

(Solar cell)
Next, a solar cell using the polyester film of the present invention will be described. The solar cell using the polyester film of the present invention is directly mounted with the polyester film as a solar cell backsheet film. Alternatively, the solar battery back sheet is mounted.
A structural example of the solar cell of the present invention is shown in FIG. As a transparent substrate 4 such as glass and a solar cell back sheet 1, a power generating element connected with a lead wire (not shown in FIG. 1) for taking out electricity is sealed with a transparent sealing material 2 such as EVA resin. However, the present invention is not limited to this and can be used for any configuration.

  Here, in the solar battery of the present invention, the solar battery backsheet 1 plays a role of protecting the power generation cell installed on the back surface of the sealing material 2 that seals the power generation element. Here, it is preferable that the solar battery backsheet is disposed so that the P2 layer is in contact with the sealing material 2 in terms of increasing the power generation efficiency of the solar battery. By setting it as this structure, it can be set as the solar cell which made the outstanding adhesiveness and power generation efficiency of the polyester film of this invention compatible.

  The power generating element 3 converts light energy of sunlight into electric energy, and is based on crystalline silicon, polycrystalline silicon, microcrystalline silicon, amorphous silicon, copper indium selenide, compound semiconductor, dye enhancement Arbitrary elements such as a sensitive system can be used in series or in parallel according to the desired voltage or current depending on the purpose. Since the transparent substrate 4 having translucency is located on the outermost surface layer of the solar cell, a transparent material having high weather resistance, high contamination resistance, and high mechanical strength characteristics in addition to high transmittance is used. In the solar cell of the present invention, the transparent substrate 4 having translucency can be made of any material as long as the above characteristics are satisfied. Examples thereof include glass, tetrafluoroethylene-ethylene copolymer (ETFE), polyfluoride. Vinyl fluoride resin (PVF), polyvinylidene fluoride resin (PVDF), polytetrafluoroethylene resin (TFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polytrifluoroethylene chloride resin (CTFE) ), Fluorinated resins such as polyvinylidene fluoride resin, olefinic resins, acrylic resins, and mixtures thereof. In the case of glass, it is more preferable to use a tempered glass. Moreover, when using the resin-made translucent base material, what extended | stretched the said resin uniaxially or biaxially from a viewpoint of mechanical strength is used preferably. Moreover, in order to give these substrates the adhesiveness with EVA resin etc. which are the sealing materials of an electric power generation element, it is also preferably performed to give the surface a corona treatment, a plasma treatment, an ozone treatment, and an easy adhesion treatment. .

  The sealing material 2 for sealing the power generating element covers the surface of the power generating element with resin and fixes it, protects the power generating element from the external environment, and has a light-transmitting base material for the purpose of electrical insulation. In addition, a material having high transparency, high weather resistance, high adhesion, and high heat resistance is used to adhere to the backsheet and the power generation element. Examples thereof include ethylene-vinyl acetate copolymer (EVA), ethylene-methyl acrylate copolymer (EMA), ethylene-ethyl acrylate copolymer (EEA) resin, ethylene-methacrylic acid copolymer (EMAA), Ionomer resins, polyvinyl butyral resins, and mixtures thereof are preferably used.

  As described above, by mounting the polyester film of the present invention on a solar battery as a solar battery back sheet, it becomes possible to suppress the appearance defect of the power generation cell while increasing the power generation efficiency. The solar cell of the present invention can be suitably used for various applications without being limited to outdoor use and indoor use such as a solar power generation system and a power source for small electronic components.

[Measurement method and evaluation method of characteristics]
(1) Polymer characteristics (1-1) Amount of terminal carboxyl groups (in the table, described as COOH amount)
The terminal carboxyl group amount was measured by the following method according to the method of Malice. (Reference: M. J. Malice, F. Huizinga, Anal. Chim. Acta, 22 363 (1960))
2 g of a measurement sample was dissolved in 50 mL of o-cresol / chloroform (weight ratio 7/3) at a temperature of 80 ° C., titrated with a 0.05 N KOH / methanol solution, and the terminal carboxyl group concentration was measured. The value of In addition, the indicator at the time of titration used phenol red, and the place where it changed from yellowish green to light red was set as the end point of titration. If there is insoluble matter such as inorganic particles in the solution in which the measurement sample is dissolved, the solution is filtered to measure the weight of the insoluble matter, and the value obtained by subtracting the weight of the insoluble matter from the measurement sample weight The following correction was made.

(1-2) Intrinsic viscosity IV
The measurement sample was dissolved in 100 ml of orthochlorophenol (solution concentration C (measurement sample weight / solution volume) = 1.2 g / ml), and the viscosity of the solution at 25 ° C. was measured using an Ostwald viscometer. Similarly, the viscosity of the solvent was measured. [Η] was calculated by the following formula (α) using the obtained solution viscosity and solvent viscosity, and the obtained value was defined as the intrinsic viscosity (IV).
ηsp / C = [η] + K [η] ² · C (α)
(Where ηsp = (solution viscosity / solvent viscosity) −1, K is the Huggins constant (assuming 0.343))
In addition, when there existed insoluble matters, such as inorganic particles, in the solution in which the measurement sample was dissolved, the measurement was performed using the following method.
(I) A measurement sample is dissolved in 100 mL of orthochlorophenol to prepare a solution having a solution concentration higher than 1.2 g / mL. Here, let the weight of the measurement sample used for orthochlorophenol be a measurement sample weight.
(Ii) Next, the solution containing insoluble matter is filtered, and the weight of the insoluble matter is measured and the volume of the filtrate after filtration is measured.
(Iii) Orthochlorophenol was added to the filtrate after filtration, and (measured sample weight (g) −weight of insoluble matter (g)) / (volume of filtrate after filtration (mL) + added orthochlorophenol) The volume (mL) is adjusted to 1.2 g / 100 mL.
(For example, when a concentrated solution having a measurement sample weight of 2.0 g / solution volume of 100 mL was prepared, the weight of insoluble matter when the solution was filtered was 0.2 g, and the filtrate volume after filtration was 99 mL Adjust to add 51 mL of orthochlorophenol ((2.0 g-0.2 g) / (99 mL + 51 mL) = 1.2 g / mL))
(Iv) Using the solution obtained in (iii), the viscosity at 25 ° C. is measured using an Ostwald viscometer, and using the obtained solution viscosity and solvent viscosity, η] is calculated, and the obtained value is defined as intrinsic viscosity (IV).

(1-3) Metal Element Content Regarding the Mg, Mn, and Sb metal element amounts, the X-ray fluorescence analysis method (X-ray fluorescence analyzer manufactured by Rigaku Corporation (model number: 3270)) is used. Quantification was carried out by atomic absorption spectrometry (manufactured by Tadate Seisakusho: Polarized Zeeman atomic absorption photometer 180-80. Frame: acetylene-air).

(2) Cavity area ratio (2-1) Observation of film cross section Using a microtome, the polyester film of the present invention was cut perpendicular to the film surface direction without being crushed in the thickness direction. Subsequently, the image which observed the cut surface of the sample using the scanning electron microscope (SEM) (JEOL Co., Ltd. field emission scanning electron microscope "JSM-6700F") was obtained.
(2-2) Measurement of the porosity of the entire film The method of (2-1) was arbitrarily selected from a total of five locations in the film sample, and the cross section of the film was cut in the longitudinal and width directions of the film. About 10 places, the image observed by the maximum magnification which can observe the whole thickness direction of a film is prepared. Next, only the respective cavity portions are traced on a transparent film, and the ratio between the cavity area measured using an image analyzer (manufactured by Nireco Corporation: Luzex IID) and the entire film cross-sectional area in the observed image is calculated. The average value at 10 locations was defined as the porosity of the entire film.
(2-3) Measurement of porosity (Ps) and (Ps ′) of film surface layer With respect to a total of 10 observation cross-sections prepared in the same manner as (2-2), the total thickness from the surface layer of the P2 layer and the P2 ′ layer An image observed up to 5% at the maximum magnification that can be observed within the field of view was prepared. Similarly, the area ratio was calculated using an image analyzer, and the average value at 10 locations was defined as the porosity of the film surface layer.
(2-4) Measurement of cavity area of cavities existing on each horizontal line Images observed at the maximum magnification at which the entire thickness direction of the film can be observed with respect to a total of 10 observation cross sections produced in the same manner as (2-2) Prepare. Next, a line perpendicular to the film thickness direction is drawn for each observed image, and the line is divided into four equal points (film thickness direction center point (C1 point), intermediate point between the film thickness direction center point and the film surface) A line parallel to the thickness direction (divided horizontal line) is drawn on the film passing through (C2-1 point) and (C2-2 point). Subsequently, only the cavity portions present on the divided horizontal lines were traced on a transparent film, and the average area of the voids present on each horizontal line was determined using an image analyzer.
As for the number of cavities to be traced, if there are less than 20 cavities on the divided horizontal line in the observation image, all cavities are traced, and if there are 20 or more cavities, the center of gravity of the cavities is It is assumed that 20 cavities close to the points C1, C2-1, and C2-2 are selected and traced.
(2-5) For the average area obtained by calculating (2-4) the average cavity area ratio (Sc / Scs), (Sc / Scs ′) per cavity existing on the divided horizontal line passing through the C1 point Sc (μm ² ), the average area per cavity existing on the split horizontal line passing through the point C2-1 Scs (μm ² ), and the cavity 1 existing on the split horizontal line passing through the point C2-2 The cavity area ratio (Sc / Scs) or (Sc / Scs ′) is calculated with the average area per unit being Scs ′ (μm ² ), and the average value of 10 places in total is the average cavity area ratio (Sc / Scs) and (Sc / Scs').

(3) Average Spectral Reflectance Measurement Using a spectrophotometer U-3410 (manufactured by Hitachi, Ltd.), a φ60 mm integrating sphere (Model No. 130-0632) as a measurement unit was attached, and a 10 ° inclined spacer was attached to a standard white plate. Uses aluminum oxide (model number 210-0740) to measure the spectral reflectance of the polyester film in the wavelength range of 400 to 1200 nm at 10 nm intervals, and the average value is the average spectral reflectance at the wavelength of 400 to 1200 nm in the present invention. did.

  In the case where the polyester film has a P2 layer on one side, the measurement was performed such that the P2 layer was positioned on the incident surface side of the spectrophotometer.

(4) Tensile strength measurement A polyester film is cut into a size of 10 mm × 150 mm, and using a Tensilon universal testing machine RTG-1210 (A & D Co., Ltd.) at a chuck distance of 50 mm and a tensile speed of 300 mm / min. The maximum strength when the tensile test was performed was defined as the tensile strength in the present invention. The measurement was performed at n = 5, and the average value was obtained.

(5) Measurement of heat-absorbing stress A polyester film was cut into a size of 2 mm × 30 mm, and using a thermomechanical measuring device TMA / SS6000 (manufactured by Seiko Instruments Inc.), sample length (distance between chucks) 20 mm, initial The sample with the load adjusted to 12 mN is heated from room temperature to 180 ° C. at a temperature rising rate of 10 ° C./min while fixing the sample length, and the heat yield load (N) of the sample at each temperature (° C.) is measured. Next, the heat yield stress (MPa) was calculated from the obtained heat yield load (N) and the cross-sectional area of the polyester film. The measurement was performed at n = 5, and the average value was obtained.

(6) Evaluation of output improvement The flux “HOZAN H722” was applied to the front and back silver electrode portions of the polycrystalline silicon solar cell “G-Tech G156M3” with a dispenser. The wiring material “Hitachi Cable Co., Ltd. copper foil SSA-SPS0.2 × 1.5 (20)” cut to a length of 155 mm is placed on the end of the wiring material 10 mm away from one end of the cell on the surface side. Then, the back side was placed symmetrically with the front side, and using a soldering iron, the soldering iron was brought into contact from the back side of the cell, and the front and back sides were simultaneously soldered to produce one cell string.
Next, the longitudinal direction of the wiring material protruding from the cell of the produced 1 cell string and the longitudinal direction of the extraction electrode “copper foil A-SPS 0.23 × 6.0 manufactured by Hitachi Cable, Ltd.” cut into 180 mm are shown. Placed vertically, the flux was applied to the portion where the wiring material and the extraction electrode overlapped, and solder welding was performed, thereby producing strings with extraction electrodes. At this time, the short-circuit current was measured according to the standard state of JIS C8914: 2005, and the power generation performance of the cell alone was obtained.

Next, 190 mm × 190 mm glass (3.2 mm thick white plate heat-treated glass for solar cells manufactured by Asahi Glass Co., Ltd.) as a cover material, and 190 mm × 190 mm ethylene vinyl acetate (Sanvik sealing material 0.5 mm as a front side sealing material) Thickness), strings with extraction electrodes that have been evaluated for power generation performance of a single cell, 190 mm x 190 mm ethylene vinyl acetate (0.5 mm thick sealant manufactured by Sunvic) as the back side sealing material, and 190 mm x 190 mm The polyester film of the present invention was stacked and fixed in order, and the glass was set so as to be in contact with the hot plate of the vacuum laminator, and the hot plate temperature was 145 ° C., the vacuum was drawn for 4 minutes, the press was 1 minute, and the holding time was 10 minutes Then, vacuum lamination was performed to produce a solar cell for evaluation. At this time, the strings with extraction electrodes were set so that the glass surface was on the cell surface side. In the case where the polyester film of the present invention has a P2 layer on one side, the P2 layer side was set to face the power generation cell side.
The obtained solar cell module was subjected to measurement of a short-circuit current measured according to the standard state of JIS C8914: 2005, and the power generation performance of the solar cell in which the polyester film of the present invention was mounted as a solar cell backsheet was as follows. According to the formula (β), the output improvement property of the polyester film of the present invention was evaluated.
Power generation improvement rate by modularization (%) = (Power generation performance after modularization / Power generation performance of single cell-1) × 100 (%) (β)
From the power generation improvement rate obtained, the output improvement was determined as follows.
When the power generation improvement rate is 8.0% or more: A
When the power generation improvement rate is 7.5% or more and less than 8.0%: B
When the power generation improvement rate is 7.0% or more and less than 7.5%: C
When the power generation improvement rate is 6.5% or more and less than 7.0%: D
When the power generation improvement rate is less than 6.5%: E
As for output improvement, A to D are good, and among them, A is the best.

(7) Workability evaluation (7-1) End peeling evaluation at the time of lamination When a solar cell in which the polyester film of the present invention is mounted as a solar cell back sheet was prepared in the same procedure as (6), at the end The width of the polyester film peeled off from the EVA resin fixed to the glass was measured with calipers, and the processability was evaluated. In addition, the photovoltaic cell was produced by N = 5, and the length measurement of the width | variety was made into the average value which measured the center part of 4 sides, respectively.

From the width of the edge peeling obtained by measurement, the workability was determined as follows.
When the width of the edge peeling is less than 1.0 mm: A
When the width of edge peeling is 1.0 mm or more and less than 1.5 mm: B
When the width of the edge peeling is 1.5 mm or more and less than 2.0 mm: C
When the width of edge peeling is 2.0 mm or more and less than 3.0 mm: D
When the edge peeling width is 3.0 mm or more: E
As for workability, A to D are good, and among them, A is the best.
(7-2) Five solar cells equipped with the polyester film of the present invention as a solar cell back sheet in the same procedure as (6) except that 380 mm × 380 mm glass was used as a wrinkle evaluation cover material during lamination. Produced. Workability was determined from the number of wrinkles visually confirmed on the backsheet side of the produced solar cell.
Wrinkles do not occur in all five solar cells: A
Wrinkles occur in one solar cell: B
Wrinkles occur in 2 to 3 solar cells: C
Wrinkles occur in four solar cells: D
Wrinkles occur in all five solar cells: E
As for workability, A to D are good, and among them, A is the best.

(8) Adhesion evaluation (8-1) Preparation of bonded sample A 125 μm thick biaxially stretched polyester film “Lumirror” (registered trademark) X10S (manufactured by Toray Industries, Inc.) was used as an adhesive for the polyester film of the present invention. Bonded with 90 parts by mass of “Takelac” (registered trademark) A310 (manufactured by Mitsui Takeda Chemical Co., Ltd.) and 10 parts by weight of “Takenate” (registered trademark) A3 (manufactured by Mitsui Takeda Chemical Co., Ltd.). After that, aging was performed for 48 hours in a thermostat adjusted to 40 ° C.
(8-2) Adhesion Strength Evaluation About the sample obtained in (8-1), using a highly accelerated life test apparatus pressure cooker (manufactured by Espec Corp.) under the conditions of a temperature of 120 ° C. and a relative humidity of 100%. After performing the treatment for 48 hours, the polyester film side of the present invention is horizontally fixed, and the adhesion is obtained from the peel strength when the peel test is performed at 180 ° peel at the bonded portion at a rate of 200 mm / min. Judgment was made as follows.
When peel strength is 4N / 15mm or more: A
When peel strength is 2N / 15mm or more and less than 4N / 15mm: B
When peel strength is 1N / 15mm or more and less than 2N / 15mm: C
When peel strength is 0.5 N / 15 mm or more and less than 1 N / 15 mm: D
When peel strength is less than 0.5 N / 15 mm: E
As for adhesion, A to D are good, and among them, A is the best.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated, this invention is not necessarily limited to these.

(Polyester resin material used for P1 layer)
1. PET raw material A (PET-a)
100 parts by mass of dimethyl terephthalate, 57.5 parts by mass of ethylene glycol, 0.03 parts by mass of manganese acetate tetrahydrate, 0. 03 parts by mass were melted at 150 ° C. in a nitrogen atmosphere. While stirring this melt, the temperature was raised to 230 ° C. over 3 hours to distill methanol, and the transesterification reaction was completed. After the transesterification reaction, an ethylene glycol solution (pH 5.0) in which 0.005 parts by mass of phosphoric acid and 0.021 parts by mass of sodium dihydrogen phosphate dihydrate were dissolved in 0.5 parts by mass of ethylene glycol was added. . The intrinsic viscosity of the polyester composition at this time was less than 0.2. Thereafter, the polymerization reaction was performed at a final temperature of 285 ° C. and a degree of vacuum of 0.1 Torr to obtain polyethylene terephthalate having an intrinsic viscosity of 0.52 and a terminal carboxyl group amount of 15 equivalents / ton. The obtained polyethylene terephthalate was dried and crystallized at 160 ° C. for 6 hours. Thereafter, solid-state polymerization was performed at 220 ° C. and a vacuum degree of 0.3 Torr for 8 hours to obtain polyethylene terephthalate (PET-a) having an intrinsic viscosity of 0.82 and a terminal carboxyl group amount of 10 equivalents / ton. The obtained polyethylene terephthalate composition had a glass transition temperature of 82 ° C. and a melting point of 255 ° C.

2. Hollow core master pellet A
Above 1. 42 parts by mass of PET resin A (PET-a) obtained by the above section, cycloolefin copolymer (COC) “TOPAS” (registered trademark) 6018 (Vicat softening point = 188 ° C.) manufactured by Polyplastics Co., Ltd., 40 parts by mass Then, 18 parts by mass of polyester elastomer (TPE) “Hytrel” (registered trademark) 7247 manufactured by Toray DuPont Co., Ltd. was melt-kneaded in a vented extruder at 290 ° C. to prepare a hollow core agent master pellet A.

3. Hollow nucleant master pellet B
Above 1. 42 parts by mass of PET resin A (PET-a) obtained according to the above, polymethylpentene (PMP) “TPX” (registered trademark) DX820 (Vicat softening point = 172 ° C.), 40 parts by mass, 18 parts by mass of a polyester elastomer (TPE) “Hytrel” (registered trademark) 7247 manufactured by Toray DuPont Co., Ltd. was melt-kneaded in a vented 290 ° C. extruder to prepare a hollow nucleating agent master pellet B.

4). Hollow nucleant master pellet C
Above 1. 26.3 parts by mass of the PET resin A (PET-a) obtained according to the paragraph, cycloolefin copolymer “TOPAS” (registered trademark) 6018 (Vicat softening point = 188 ° C.), 40 parts by mass, manufactured by Polyplastics Co., Ltd. Polyester elastomer (TPE) “Hytrel” (registered trademark) 7247 18 parts by mass produced by Toray DuPont Co., Ltd., 290 ° C. with vented 15.3 parts by mass of amorphous PET resin (PET-G) Copolyester GN071 produced by Eastman Chemical Co., Ltd. The mixture was melt-kneaded in an extruder of No. 1 to produce cavity core master pellets C.

5. Titanium oxide master pellet 100 parts by mass of PET resin A (PET-a) obtained according to the above and 100 parts by mass of rutile-type titanium oxide particles (TiO ₂ ) having an average particle diameter of 210 nm are melt-kneaded in a vented 290 ° C. extruder and oxidized. A titanium master pellet was prepared.

6). Barium sulfate master pellet 100 parts by mass of PET resin A (PET-a) obtained according to the above and 100 parts by mass of barium sulfate particles (BaSO ₄ ) having an average particle size of 1.5 μm are melt-kneaded in a vented 290 ° C. extruder, and sulfuric acid is added. Barium master pellets were prepared.

7). Coating agent for P2 layer (Coating agent a)
According to the formulation shown in the column of the main agent in Table 1, “Huls Hybrid” (registered trademark) polymer UV-G301 (solid content concentration: 40% by mass), an acrylic coating agent manufactured by Nippon Shokubai Co., Ltd. Titanium Co., Ltd. titanium oxide particles JR-709 and a solvent were mixed together and dispersed using a bead mill. Thereafter, a polyester plasticizer “Polysizer” (registered trademark) W-220EL manufactured by DIC Corporation is added as a plasticizer to obtain a base material of a coating material a for forming a resin layer having a solid content concentration of 51 mass%. It was.

  In the main agent obtained as described above, “Desmodur” (registered trademark) N3300 (solid content concentration: 100 mass%) manufactured by Sumika Bayer Urethane Co., Ltd., which is a nurate-type hexamethylene diisocyanate resin shown in Table 1. Is added in an amount calculated in advance so that the mass ratio of the resin layer-forming main agent is 100/4, and is further calculated in advance so that the solid content concentration is 20% by mass. Diluent: n-propyl acetate was weighed and stirred for 15 minutes to obtain a coating agent a having a solid content concentration of 20% by mass.

8). Polyester film A
77.5 parts by mass of PET raw material A (PET-a) vacuum-dried at 180 ° C. for 2 hours as a raw material constituting the P1 layer and 22.5 parts of the hollow nucleating agent master pellet A so as to have the composition shown in Table 2 On the other hand, 76 parts by mass of PET raw material A (PET-a) vacuum-dried at 180 ° C. for 2 hours as a raw material constituting the P2 layer and 24 parts by mass of titanium oxide master pellets were mixed, Each was melted and discharged in two different extruders heated to 280 ° C., merged so as to be laminated with P2 / P1 / P2 in a feed block, and then co-extruded from a T die. Next, the coextruded molten sheet was closely cooled and solidified by electrostatic application on a drum maintained at a surface temperature of 25 ° C. to obtain an unstretched sheet. Subsequently, after preheating the unstretched sheet with a group of rolls heated to a temperature of 80 ° C., a speed difference of 3.2 times is created between the roll heated to a temperature of 88 ° C. and the roll adjusted to a temperature of 25 ° C. Thus, the film was stretched 3.2 times in the longitudinal direction (longitudinal direction), and then cooled with a roll group having a temperature of 25 ° C. to obtain a uniaxially stretched sheet. Next, while holding both ends of the obtained uniaxially stretched sheet with clips, the tenter is guided to a preheating zone at a temperature of 80 ° C. in the tenter, and subsequently in a heating zone maintained at 90 ° C. in a direction perpendicular to the longitudinal direction (width) Direction) was stretched 3.4 times. Subsequently, a heat treatment was performed at 220 ° C. for 20 seconds in a heat treatment zone in the tenter, and further gradually cooled while performing a relaxation treatment in the 4% width direction, and the lamination ratio (P2: P1: P2) was 1: A polyester film A having a thickness of 13: 1 and an overall thickness of 160 μm was formed.

  When the polymer properties of the obtained polyester film A were measured, the intrinsic viscosity IV was 0.70 dl / g, the terminal carboxyl group amount was 14 equivalents / ton, and Mn contained 69 ppm, Sb contained 241 ppm, and Na contained 29 ppm. It was.

9. Polyester film B, E
As shown in Table 2, polyester films B and E were obtained in the same manner as polyester film A, except that the stretching conditions during film formation were changed.

10. Polyester film C
As shown in Table 2, a polyester film C was obtained in the same manner as the polyester film A except that the hollow master pellet B was used.

11. Polyester film D
As shown in Table 2, a polyester film D was obtained in the same manner as the polyester film A except that the concentration of the cavity nucleating agent was changed.

12 Polyester film F
As shown in Table 2, a polyester film F was obtained in the same manner as the polyester film A except that the concentration of the cavity nucleating agent was changed using the cavity master pellet B.
13. Polyester film G
As shown in Table 2, a polyester film G was obtained in the same manner as the polyester film A, except that the concentration of the cavity nucleating agent was changed using the cavity master pellet C.

14 Polyester film H
As shown in Table 2, a polyester film H was obtained in the same manner as the polyester film A except that barium sulfate master pellets were used instead of the hollow master pellets.

Example 1
The results of measuring the average spectral reflectance, tensile strength, thermal stress, and cavity area ratio of the polyester film A are as shown in Table 3. Corona treatment is performed on one side of the polyester film A as it is as a film for a solar battery back sheet. As a result of evaluating the output improvement property, workability, and adhesion, as shown in Table 4, the output improvement property and adhesion are all excellent, and in the workability, the end portion peeling occurs but there is no problem. I understood that.

(Example 2)
After applying paint a to one side of the polyester film A using a wire bar so that the paint thickness after drying becomes 3 μm, the polyester film is guided to an oven adjusted to the conditions described in Table 3, and the paint a is applied. Annealing was performed while drying.
The average spectral reflectance of the obtained polyester film, the tensile strength, and the results of the measurement of heat absorption stress are as shown in Table 3, and the output improvement property, workability, and adhesion of the polyester film are as shown in Table 4. Both proved to be very good.

(Example 3)
A polyester film was obtained in the same manner as in Example 2 except that the annealing conditions were changed as shown in Table 3. The average spectral reflectance, tensile strength, and heat yield stress measurement results of the obtained polyester film are as shown in Table 3, and the workability is as shown in Table 4, with the end peeling being less than that of Example 2. Although it is easy to generate | occur | produce, it turned out that it is favorable.

Example 4
A polyester film was obtained in the same manner as in Example 2 except that the polyester film B was used. The average spectral reflectance, tensile strength, and heat yield stress measurement results of the obtained polyester film are as shown in Table 3, and the workability is excellent as shown in Table 4, and the edge peeling is excellent. The generation of wrinkles was found to be good.

(Example 5)
A polyester film was obtained in the same manner as in Example 2 except that the polyester film C was used. The results of measuring the average spectral reflectance, tensile strength, thermal stress, and cavity area ratio of the obtained polyester film are as shown in Table 3, and the workability is very excellent as shown in Table 4. Although it was inferior in output improvement, it turned out that it is a range without a problem.

(Examples 6 and 7)
A polyester film was obtained in the same manner as in Example 2 except that the polyester films D and E were used. The results of measuring the average spectral reflectance, tensile strength, heat stress, and cavity area ratio of the obtained polyester film are as shown in Table 3, and the output improvement is very excellent as shown in Table 4. On the other hand, although it was easy to generate | occur | produce the wrinkle at the time of lamination, it turned out that it is a range without a problem.

(Example 8)
A polyester film was obtained in the same manner as in Example 2 except that the annealing conditions were changed as shown in Table 3. The results of measuring the average spectral reflectance, tensile strength, heat stress, and cavity area ratio of the obtained polyester film are as shown in Table 3, and the output improvement is inferior as shown in Table 4. It was found that there was no range and the workability was in a good range.

(Comparative Example 1)
The results of measuring the average spectral reflectance, tensile strength, heat stress, and cavity area ratio of the polyester film B are as shown in Table 3, and corona treatment is performed on one side of the polyester film B to improve output performance, workability, As a result of the evaluation of adhesion, as shown in Table 4, it was found that the output improvement was very good, but the end peeling at the time of lamination deteriorated and the workability was inferior.

(Comparative Example 2)
A polyester film was obtained in the same manner as in Example 2 except that the polyester film F was used. The results of measuring the average spectral reflectance, tensile strength, heat stress, and cavity area ratio of the obtained polyester film are as shown in Table 3, and the workability is very excellent as shown in Table 4. It turned out that it is inferior to output improvement.

(Comparative Example 3)
A polyester film was obtained in the same manner as in Example 2 except that the polyester film G was used. The results of measuring the average spectral reflectance, tensile strength, thermal stress, and cavity area ratio of the obtained polyester film are as shown in Table 3 and evaluated for output improvement, workability, and adhesion. As a result, it was found that while the output improvement was very excellent, the wrinkle generation at the time of lamination deteriorated and the processability was inferior.

(Comparative Example 4)
A polyester film was obtained in the same manner as in Example 2 except that the polyester film H was used. The results of measuring the average spectral reflectance, tensile strength, heat stress, and cavity area ratio of the obtained polyester film are as shown in Table 3, and the output improvement property and workability are inferior as shown in Table 4. I understood that.

The polyester film of the present invention can be suitably used for a film for a solar battery back sheet, a base film for a blood glucose level sensor, a base film for a magnetic card, etc., taking advantage of excellent reflectivity and workability. In particular, by using the film for a solar battery backsheet, it is possible to suppress the output improvement of the solar battery backsheet and the appearance defect at the time of solar cell production.

1: Solar cell back surface protection sheet 2: Sealing material 3: Power generation element 4: Transparent substrate 5: Surface on the sealing material 2 side of the solar cell backsheet 6: On the side opposite to the sealing material 2 of the solar cell backsheet Surface 7: Film thickness direction 8: Film surface direction 9: Intermediate point between film thickness direction center point and film surface (C2-1 point)
10: Film thickness direction center point (point C1)
11: Intermediate point between the film thickness direction center point and the film surface (point C2-2)
12: Split horizontal line passing through point C2-1 13: Split horizontal line passing through point C1 14: Split horizontal line passing through point C2-2 15: Cavity

Claims (7)

A polyester film having an average spectral reflectance of 90% or more at a wavelength of 400 to 1200 nm and satisfying the following (1) and (2) in any of the longitudinal direction and the width direction of the film.
(1) The ratio (A ₁₅₀ / A ₁₀₀ ) of the heat yield stress A _{150 at} 150 ° C. to the heat yield stress A ₁₀₀ at 100 ° C. is 15 or less. (2) The tensile strength is in either the longitudinal direction or the width direction. 100 MPa or more 2. The polyester film according to claim 1, wherein the heat absorption stress A _{100 at} 100 ° C. is 0.01 MPa or more and 0.4 MPa or less in both the longitudinal direction and the width direction of the film. The polyester film according to claim 1 or 2, which contains cavities inside the film. Three points (drawing a line perpendicular to the surface direction from one surface of the film to the other surface in the cross section in the thickness direction of the polyester film, and dividing the line connecting the one surface from the other surface into four equal parts in the thickness direction ( Lines parallel to the surface direction of the film passing through the center point in the film thickness direction (point C1), the intermediate point between the center point in the film thickness direction and the film surface (point C2-1), (point C2-2) (divided) In each of the horizontal lines)
Sc (μm 2) is the average area per cavity existing on the dividing horizontal line passing through C 1 point,
Scs (μm 2) is the average area per cavity existing on the dividing horizontal line passing through the C2-1 point,
When the average area per cavity existing on the divided horizontal line passing through the point C2-2 is Scs ′ (μm2), at least one of (Sc / Scs) and (Sc / Scs ′) is 1.1 or more. The polyester film according to claim 3, which is 35 or less.
(Note that (Sc / Scs) and (Sc / Scs ′) are parallel to the film thickness direction cross section and the film width direction at any five locations of the polyester film, the film being cut in parallel to the film longitudinal direction. The average value of the values obtained from the cross-section in the thickness direction of the film obtained by cutting the film. The polyester film in any one of Claims 1-4 used for a solar cell backsheet use. The solar cell backsheet using the polyester film in any one of Claims 1-5. A solar cell on which the solar cell back sheet according to claim 6 is mounted.

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