JP2011208008A - Polyester film and back sheet for solar cell using the same and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film having high wet heat resistance over a long period, a back sheet for a solar cell which has high durability by using the polyester film and the solar cell using the same.SOLUTION: The polyester film contains a polyester layer (P-layer) satisfying following conditions (1) and (2). (1) The product of a crystal size of (100) plane of the polyester in the P-layer, a crystal size of (-105) plane and a crystal size of (0-11) plane is ≥10 nmand ≤110 nm. (2) The glass transition temperature of the polyester of the P-layer measured by a temperature modulated differential scanning calorimetry is ≥103°C.

Description

  The present invention relates to a polyester film having good moisture and heat resistance. In particular, the present invention relates to a polyester film that can be suitably used as a back sheet for a solar cell, and also relates to a back sheet for a solar cell and a solar cell using the film.

  Polyester (especially polyethylene terephthalate, polyethylene-2, 6-naphthalenedicarboxylate, etc.) resins are excellent in mechanical properties, thermal properties, chemical resistance, electrical properties, and moldability, and are used in various applications. Polyester films made from the polyester resin, especially biaxially oriented polyester films, are laminated with copper, solar cell backsheet, adhesive tape, flexible printed circuit board, membrane switch, surface due to its mechanical and electrical properties. Used as various industrial materials such as electrical insulation materials such as flat heating elements or flat cables, magnetic recording materials, capacitor materials, packaging materials, automotive materials, building materials, photographic applications, graphic applications, thermal transfer applications, etc. Yes.

  Of these applications, electrical insulation materials (such as solar cell backsheets), automotive materials, building materials, etc. used outdoors are often used in harsh environments for a long time. The molecular weight decreases due to hydrolysis, and the mechanical properties and the like deteriorate due to progress of embrittlement, so when used in a harsh environment for a long period of time or in an application where it is used in a damp state, High moisture and heat resistance is required. For example, in solar cell backsheet applications, in order to improve the service life of solar cells and reduce power generation costs, it is required to improve the heat and moisture resistance of polyester films.

  Therefore, various studies have been made to suppress hydrolysis of the polyester resin. For example, a technique for adding an epoxy compound (Patent Documents 1 and 2) and a polycarbodiimide (Patent Document 3) to improve the moisture and heat resistance of the polyester resin itself has been studied. In addition, with regard to the biaxially oriented polyester film, studies have been made to improve the moisture and heat resistance by setting the film to a high IV (high intrinsic viscosity) and controlling the degree of plane orientation (Patent Document 4).

  However, the above-described technique of Patent Document 4 is not sufficient in improving the heat and moisture resistance. Further, in the techniques of Patent Documents 1 and 2, there is a high possibility that gelation occurs during melt-molding to cause molding defects or foreign matters, and it is necessary to remove foreign matters. In addition, the effect of improving heat and humidity resistance is insufficient. In addition, although the technology of Patent Document 3 has a high effect of improving heat and humidity resistance, the compound produced by reaction with polycarbodiimide and polyester resin has low heat resistance, and therefore generates a decomposition gas harmful to the human body during melt molding. Explosion-proof measures are necessary due to safety issues. Therefore, a highly safe technique for improving heat and humidity resistance has been demanded.

JP-A-9-227767 Japanese Patent Laid-Open No. 2007-302878 Japanese National Patent Publication No. 11-506487 JP 2007-70430 A

  It is an object of the polyester film of the present invention to have high moisture and heat resistance.

In order to solve the above problems, the present invention has the following configuration. That is,
1. A polyester film comprising a polyester layer (P layer) that satisfies the following conditions (1) and (2).
(1) The product of the (100) plane crystallite size, the (−105) plane crystallite size, and the (0-11) plane crystallite size of the polyester of the P layer is from 10 nm ^{3 to} 110 nm ³ (2) Glass transition temperature determined by temperature-modulated differential scanning calorimetry (hereinafter also referred to as temperature-modulated DSC or m-DSC) of polyester in the P layer (this glass transition temperature is distinguished from glass transition temperature that is not temperature-modulated) Therefore, hereinafter, the glass transition temperature (m-Tg) or simply referred to as m-Tg) is 103 ° C. or higher. 2. The polyester film according to 1, wherein the product of the crystal size of the polyester of the P layer is 30 nm ³ or more.
3. 3. The polyester film according to 1 or 2, wherein the carboxyl group terminal amount change ΔC defined by the following formula (3) is 110 equivalent / t or less after heat treatment at 125 ° C. and 100% RH.
Carboxyl group terminal amount change ΔC (equivalent / t) = C1-C0 (3)
C0: Initial carboxyl group terminal amount of P layer polyester C1: Carboxyl group terminal amount of P layer polyester after heat treatment at 125 ° C. and 100% RH for 72 hours The alkali metal element content W1 in the polyester of the P layer is 2.5 ppm or more and 125 ppm or less, and the ratio W1 / W2 of the alkali metal element content W1 and the phosphorus element content W2 is 0.01 or more and 1 or less 1 The polyester film in any one of -3.
5. The main constituent components of the polyester of the P layer are a dicarboxylic acid constituent component and a diol constituent component, and a constituent component having a tri- or higher functional carboxylic acid group and / or hydroxyl group as a copolymer component is 0.005 with respect to all constituent components. The polyester film according to any one of 1 to 4 containing from mol% to 2.5 mol%.
6). The polyester film according to any one of 1 to 5, wherein the degree of crystallinity of the polyester in 1st RUN of differential scanning calorimetry (hereinafter also referred to as DSC) of the P layer is 20% or more and 38% or less.
The back sheet for solar cells using the polyester film in any one of 7.1-6.
A solar cell using the solar cell backsheet described in 8.7.
A method for producing a polyester film according to any one of 9.1 to 6, wherein the alkali metal element content W1 is 2.5 ppm or more and 125 ppm or less, and the alkali metal element content W1 and the phosphorus element content W2 A method for producing a polyester film, comprising: biaxially stretching a sheet containing a layer having a polyester having a ratio W1 / W2 of 0.01 or more and 1 or less under the following condition (4) or (5).
(4) The polyester film is stretched by a simultaneous biaxial stretching method at a glass transition temperature of Tg or higher and Tg + 10 ° C. or lower to an area magnification of 12 times or more. (5) The first biaxial stretching method is used to stretch the first axis of the polyester. 9. Stretch the second axis at a temperature of Tg + 15 ° C. at a glass transition temperature Tg to Tg + 10 ° C. The main constituent components of the polyester are a dicarboxylic acid constituent component and a diol constituent component, and a constituent component having a tri- or higher functional carboxylic acid group and / or hydroxyl group as a copolymer component is 0.005 mol% or more based on the total constituent components. The manufacturing method of the polyester film of 9 containing 2.5 mol%.
11. The method for producing a polyester film according to 9 or 10, wherein the heat treatment is performed within a range satisfying the following formula (6) after biaxial stretching.
35 ° C. ≦ Tm−Th ≦ 90 ° C. (6)
Tm: Melting point (° C) of polyester resin obtained by differential scanning calorimetry
Th: Heat treatment temperature (° C)
12 The method for producing a polyester film according to any one of 9 to 11, wherein the biaxial stretching method is a simultaneous biaxial stretching method.
Is the essence of this.

  ADVANTAGE OF THE INVENTION According to this invention, the polyester resin composition and polyester film which are excellent in heat-and-moisture resistance can be provided. Such polyester films include copper-clad laminates, solar cell back sheets, adhesive tapes, flexible printed boards, membrane switches, planar heating elements, flat cables, and other electrically insulating materials, capacitor materials, automotive materials, and building materials. It can be suitably used for applications in which moisture and heat resistance is important.

It is sectional drawing which shows typically an example of a structure of the solar cell using the polyester film of this invention.

The polyester film of the present invention is a polyester film including a polyester layer (P layer) that satisfies the following conditions (1) and (2).
(1) The product of the (100) plane crystallite size, the (−105) plane crystallite size, and the (0-11) plane crystallite size of the polyester of the P layer is from 10 nm ^{3 to} 110 nm ³ (2) The glass transition temperature calculated | required by the temperature modulation differential scanning calorimetry of the polyester of P layer is 103 degreeC or more.

  By satisfying the above requirements, it is possible to provide a polyester film that satisfies a high level of moisture and heat resistance over a long period of time.

  Polyester includes crystalline polyester and amorphous polyester, and crystalline polyester also has a crystal part and an amorphous part. When the crystalline polyester is stretched, a portion where the polyester is pseudo-crystallized by orientation (hereinafter referred to as an orientation crystallized portion) is generated in the amorphous portion, but not all of the amorphous portion is pseudo-crystallized. Here, the amorphous part has a lower density than the crystal part and the oriented crystallized part, has a large average intermolecular distance, has high molecular mobility, and is said to contribute to the flexibility of the polyester. ing. It is thought that wet heat degradation of polyester occurs by the following mechanism. When the polyester film is exposed to a moist and hot atmosphere, moisture (water vapor) enters the interior through the molecules of this amorphous part, which is low in density, and uses molecular protons as a reaction catalyst with the proton at the carboxyl group terminal of the polyester. The amorphous part having a high molecular weight is hydrolyzed to lower the molecular weight of the amorphous part. The low molecular weight amorphous part further increases the molecular mobility and is taken into the crystal part as a result of trying to adopt a stable structure, the crystal becomes enlarged, and the amorphous part contributing to flexibility and elongation decreases. As a result, as the density difference in the film increases, the film becomes more brittle, and eventually, even with a slight impact, stress tends to concentrate near the interface where the density difference is large, and it easily breaks. It becomes.

  In the present invention, it is possible to obtain a polyester film having a high heat-and-moisture resistance unprecedented by increasing the restraint of the molecular chain of the amorphous part and making it in a microcrystalline state. The mechanism is as follows. First, by setting the glass transition temperature (m-Tg) of the polyester of the polyester layer (P layer) to 103 ° C. or higher, the molecular chain of the amorphous part in the P layer can be made highly constrained. Thereby, the ingress of moisture into the amorphous part can be reduced. Even when water enters between molecules, the hydrolysis reaction probability can be reduced by suppressing the plasticization of the amorphous part or by suppressing the movement of the carboxyl group terminal. Furthermore, the progress of crystallization can be suppressed by reducing the molecular mobility after the hydrolysis reaction.

Next, the polyester crystals of the P layer are distributed in a microcrystalline state (specifically, the product of the (100) plane crystallite size, the (−105) plane crystallite size, and the (0-11) plane crystallite size. , 110 nm ³ or less), it is possible to suppress the enlargement even if the crystallization due to the hydrolysis reaction proceeds. As a result, the increase in the density difference of the film internal structure can be suppressed even when exposed to a humid heat atmosphere. As a result, the embrittlement in the humid heat atmosphere is suppressed, and even when stress is applied from the outside, the stress can be dispersed. .

  By giving all of the above effects, it is possible to suppress both the hydrolysis reaction of the polyester in a moist heat atmosphere and the embrittlement after the reaction. As a result, it is possible to obtain high heat and heat resistance that cannot be obtained with a conventional polyester film. It has been made.

  Hereinafter, the present invention will be described in detail with specific examples.

The polyester film of the present invention is a product of the (100) plane crystallite size, the (−105) plane crystallite size, and the (0-11) plane crystallite size of the polyester of the polyester layer (P layer) (hereinafter referred to as crystal size). the referred to as the product) is required to be 10 nm ³ or more 110 nm ³ or less. The product of crystallite size here means the (100) plane crystallite size, (−105) plane crystallite size, and (0-11) plane crystal of polyester of P layer measured by wide angle X-ray diffraction method. It is a value obtained by the product of each value of the child size, and is a value correlated with the volume of the crystal in the polyester (hereinafter, the expression of the crystal size may be used as the product of the crystal size). More preferably 20 nm ³ or more 105 nm ³ or less, more preferably 20 nm ³ or more 100 nm ³ or less, particularly preferably 30 nm ³ or more 90 nm ³ or less, and most preferably 40 nm ³ or more 80 nm ³ or less. In the polyester film of the present invention, when the product of the polyester crystal size of the polyester layer P layer exceeds 110 nm ³ , the crystal enlargement easily proceeds in a humid and hot atmosphere. Is unfavorable because it tends to be large and easily breaks when subjected to external stress. Moreover, if the product of the crystal size is less than 10 nm ³ , the hydrolysis resistance of the polyester constituting the P layer may be lowered and the heat and humidity resistance may be lowered, which is not preferable. In the polyester film of the present invention, the product of the (100) plane crystallite size, the (−105) plane crystallite size, and the (0-11) plane crystallite size of the polyester of the polyester layer (P layer) is 10 nm ³ or more and 110 nm ^3. By making the following, there is no decrease in hydrolysis resistance under a moist heat atmosphere, and it becomes possible to suppress enlargement due to the progress of crystallization, and as a result, embrittlement of the film is suppressed and high heat resistance to heat is obtained. Things will be possible.

  Moreover, the polyester film of this invention needs that the glass transition temperature calculated | required by the temperature modulation DSC of the polyester of a polyester layer (P layer) is 103 degreeC or more. The glass transition temperature (m-Tg) here is a temperature indicating the glass transition of the amorphous part of the polyester of the polyester layer (P layer) obtained by molding, and the amorphous of the polyester layer (P layer). This value correlates with the restraint property of the molecular chain of the part. Specifically, by temperature modulation DSC, the polyester layer (P layer) is 2 ° C./min in the range of 0 ° C. to the melting point Tm-75 ° C. of the polyester under a nitrogen flow atmosphere, and the temperature modulation period is 60 seconds. In the step-like change portion of the reversible component temperature modulation DSC chart obtained by measuring in a sine wave shape at 1 ° C., the glass transition temperature is determined according to “9.3 Determination of glass transition temperature (1) in JIS K7121 (1987)”. It is the temperature obtained by the same method as the method described in “Glass Transition Temperature Tmg”. More preferably, m-Tg is 104 ° C. or higher, further preferably 105 ° C. or higher, and particularly preferably 107 ° C. or higher. In the polyester film of the present invention, when the glass transition temperature (m-Tg) is less than 103 ° C., the molecular mobility of the amorphous part in the polyester (P layer) is high, and it is exposed to a humid heat atmosphere. It is not preferable because the hydrolysis reaction easily proceeds or the crystallization proceeds after the hydrolysis reaction, and the film becomes brittle easily. In the polyester film of the present invention, by setting the glass transition temperature (m-Tg) of the polyester layer (P layer) to 103 ° C. or higher, embrittlement of the film in a humid heat atmosphere can be suppressed, and high heat and heat resistance can be obtained. Is possible.

In the polyester film of the present invention, examples of a method that satisfies the above requirements (1) and (2) include a method that simultaneously satisfies the following (A) to (C).
(A) The polyester contains 0.1 to 5.0 mol / t of a buffering agent with respect to the entire P layer. As one specific example, when an alkali metal phosphate is used as a buffer, the alkali metal element content W1 in the polyester P layer is 2.5 ppm or more and 125 ppm or less, and the alkali metal element content. The ratio W1 / W2 of W1 and phosphorus element content W2 is in the range of 0.01 or more and 1 or less.
(B) In the case of the simultaneous biaxial stretching method, a sheet having a P layer containing the polyester is stretched to a glass transition temperature Tg of (Tg + 10 ° C.) or less to an area magnification of 12 times or more, and sequentially biaxially. In the case of the stretching method, the first axis should be stretched to a glass transition temperature Tg or higher (Tg + 10 ° C.) or lower and the second axis (Tg + 15 ° C.) or lower to an area magnification of 13 times or higher.
(C) After biaxial stretching, heat treatment is performed in the temperature range of the following formula (6).
35 ° C. ≦ Tm−Th ≦ 90 ° C. (6)
Tm: Melting point (° C) of polyester resin obtained by differential scanning calorimetry
Th: Heat treatment temperature (° C)
Furthermore, in addition to the above (A) to (C), it is preferable to satisfy the following (D).
(D) The main constituent components of polyester are a dicarboxylic acid constituent component and a diol constituent component, and a constituent component having a tri- or higher functional carboxylic acid group and / or hydroxyl group as a copolymerization component is 0.005 with respect to all constituent components. Containing at least 2.5% by mole.

Details will be described below.
(A) The buffering agent of the present invention is a substance that is soluble in the diol constituents constituting the polyester of the present invention, such as ethylene glycol, and dissociates after dissolution and exhibits ionicity. In the polyester film of the present invention, by containing a buffering agent in the P layer, the hydrolysis reaction can be suppressed by neutralizing protons derived from carboxyl end groups newly generated in the polyester and by the hydrolysis reaction (hereinafter referred to as the following). Not only the hydrolysis-inhibiting effect) but also interacts with the carbonyl group of the polyester to restrain the molecular chain. As a result, the molecular mobility of the amorphous part when subjected to thermal history such as preheating, stretching, heat treatment, etc. in the film forming process can be suppressed, and the crystal volume of the obtained film is compared with the conventional polyester film This can be reduced (hereinafter also referred to as a crystal volume reduction effect). Moreover, the progress of crystallization can be suppressed by reducing the molecular mobility when exposed to a moist heat atmosphere.

  As specific examples of the buffer, the buffer is preferably an alkali metal salt because of its high polyester polymerization reactivity, hydrolysis inhibiting effect, and crystal volume reducing effect. For example, phthalic acid, citric acid, carbonic acid And alkali metal salts with compounds such as lactic acid, tartaric acid, phosphoric acid, phosphorous acid, hypophosphorous acid and polyacrylic acid. Among them, potassium and sodium are preferable as alkali metal elements from the viewpoint of hardly generating precipitates due to catalyst residues. Specifically, potassium hydrogen phthalate, sodium dihydrogen citrate, disodium hydrogen citrate, Potassium dihydrogen acid, dipotassium hydrogen citrate, sodium carbonate, sodium tartrate, potassium tartrate, sodium lactate, potassium lactate, sodium bicarbonate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, phosphoric acid Examples thereof include sodium dihydrogen, sodium hydrogen phosphite, potassium hydrogen phosphite, sodium hypophosphite, potassium hypophosphite, and sodium polyacrylate.

In addition, the alkali metal salt represented by the following formula (I) is particularly preferable in terms of polymerization reactivity of the polyester resin and heat resistance at the time of melt molding, and further, the alkali metal salt is a polymerization reaction. From the viewpoint of polymerization reactivity, heat resistance, and moist heat resistance, it is more preferable that the alkali metal is sodium and / or potassium from the viewpoint of polymerization reactivity, heat resistance, and heat resistance.
POxHyMz (I)
(Here, x is an integer of 2 to 4, y is 1 or 2, z is 1 or 2, and M is an alkali metal.)
The content of the buffering agent is preferably 0.1 mol / t or more and 5.0 mol / t or less with respect to the P layer. More preferably, it is 0.7 mol / t or more and 3.0 mol / t. When it is less than 0.1 mol / t, the hydrolysis inhibiting effect and the crystal volume reducing effect are insufficient, and the obtained polyester film may not have sufficient wet heat resistance. On the other hand, if it exceeds 5.0 mol / t, the decomposition reaction is promoted by an excessive alkali metal, so that the molecular weight is lowered, which causes the heat-and-moisture resistance and the mechanical properties to be lowered.

  When the alkali metal salt represented by the above formula (I) is used as the buffer, it is preferable to use phosphoric acid in combination. Thereby, it becomes possible to improve a hydrolysis inhibitory effect, maintaining the crystal volume reduction effect, and can improve the heat-and-moisture resistance of the obtained polyester film more. In that case, the alkali metal element content W1 in the polyester P layer is 2.5 ppm or more and 125 ppm or less, and the ratio W1 / W2 of the alkali metal element content W1 and the phosphorus element content W2 is 0.01 or more and 1 or less. It is preferable to be in the range. By setting it as this range, it becomes possible to raise the hydrolysis-resistant suppression effect more, maintaining the high crystal volume reduction effect. More preferably, the alkali metal element W1 is 15 ppm or more and 75 ppm or less, and the ratio W1 / W2 of the alkali metal element content W1 and W2 is 0.1 or more and 0.5 or less. In the polyester film of the present invention, when the alkali metal element content W1 is less than 2.5 ppm, the hydrolysis inhibiting effect and the crystal volume reducing effect are insufficient, and the obtained polyester film cannot obtain sufficient moisture and heat resistance. There is a case. On the other hand, if it exceeds 125 ppm, the excessively present alkali metal promotes the thermal decomposition reaction at the time of melt-extrusion and the molecular weight is lowered, which may cause the moist heat resistance and mechanical properties to be lowered. Further, if the ratio W1 / W2 between the alkali metal element content W1 and the phosphorus element content W2 is less than 0.1, the hydrolysis inhibiting effect and the crystal volume reducing effect are insufficient, and if it exceeds 125 ppm, excess phosphoric acid. Reacts with the polyester during the polymerization reaction, and the phosphate ester skeleton is formed in the molecular chain, and the portion promotes the hydrolysis reaction, which may reduce the hydrolysis resistance. In the polyester film of the present invention, the alkali metal element W1 is 15 ppm or more and 75 ppm or less, and the ratio W1 / W2 of the alkali metal element content W1 and the phosphorus element content W2 is 0.1 or more and 0.5 or less. While maintaining a high crystal volume reduction effect, it becomes possible to further enhance the hydrolysis resistance suppressing effect, and as a result, it becomes possible to obtain high wet heat resistance.

  (B) The simultaneous biaxial stretching method for producing the polyester film of the present invention is performed by stretching the polyester film of the present invention at an area magnification of 12 times or more at a temperature of the glass transition temperature of Tg or more and Tg + 10 ° C. or less. In the sequential biaxial stretching method for production, it is preferable that the first axis is stretched at a glass transition temperature of Tg to Tg + 10 ° C. and the second axis is stretched at a magnification of 13 times or more at a temperature of Tg + 15 ° C. The glass transition temperature Tg mentioned here is a temperature rising process (temperature rising rate: 20) when the higher order structure of the polyester of the polyester layer (P layer) is temporarily canceled to be in an amorphous state and the sample is subjected to differential scanning calorimetry. It is the glass transition temperature Tg at ° C / min). In detail, it calculates | requires by the procedure of (A1)-(A4) mentioned later based on JISK-7121 (1987).

  In the case of the simultaneous biaxial stretching method, the stretching temperature is more preferably Tg + 5 ° C. or less, and the stretching ratio is preferably 14 times or more, more preferably 16 times or more. In the case of the sequential biaxial stretching method, the stretching temperature of the first axis is more preferably Tg + 5 ° C. or less, more preferably Tg + 3 ° C. or less, and the stretching temperature of the second axis is Tg + 10 ° C. or less, more preferably Tg + 7 ° C. or less. Yes, the draw ratio is 16 times or more.

  In the case of the simultaneous biaxial stretching method, when the stretching temperature exceeds Tg + 10 ° C., or in the case of the sequential biaxial stretching method, the stretching temperature of the first axis exceeds Tg + 10 ° C. and / or the stretching temperature of the second axis exceeds Tg + 15 ° C. The glass transition temperature (m-Tg) decreases as a result of the thermal history in the stretching process, which tends to increase the crystal volume, or makes it difficult to constrain the molecular chains in the amorphous part due to orientation even after stretching. Sometimes. Further, when the thermal history in the subsequent heat treatment step (C) is received, the crystal volume is likely to be coarsened, and the restraining property of the amorphous part is likely to be lowered. Thereby, the hydrolysis resistance and heat-and-moisture resistance of the obtained film may deteriorate. In the polyester film of the present invention, by stretching at the above temperature condition, it is possible to increase the restraint property of the amorphous part while suppressing an increase in crystal volume due to the thermal history of the stretching process, and the glass transition temperature (m− Tg) can be increased. In addition, it is possible to suppress an increase in crystal volume due to receiving the thermal history of the subsequent heat treatment step (C) and a decrease in glass transition temperature (m-Tg) due to a decrease in the restraining property of the amorphous part.

  Further, in the case of the simultaneous biaxial stretching method, when the stretching ratio is less than 12 times, or in the case of the sequential biaxial stretching method, the area ratio is less than 13 times, the orientation imparting is insufficient even if stretched. In addition, as a result of difficulty in restraining the molecular chain in the amorphous part, the glass transition temperature (m-Tg) may be lowered. Further, when the thermal history in the subsequent heat treatment step (C) is received, the crystal volume is likely to be coarsened, and the restraining property of the amorphous part is likely to be lowered. Thereby, the hydrolysis resistance and heat-and-moisture resistance of the obtained film may deteriorate. In the polyester film of the present invention, by stretching at the above stretching ratio, it is possible to increase the restraint of the amorphous part while suppressing an increase in the crystal volume due to the thermal history of the stretching process, and the glass transition temperature (m− Tg) can be increased. In addition, it is possible to suppress an increase in crystal volume due to receiving the thermal history of the subsequent heat treatment step (C) and a decrease in glass transition temperature (m-Tg) due to a decrease in the restraining property of the amorphous part.

(C) The polyester film of the present invention is preferably subjected to heat treatment in the temperature range of the following formula (6) after biaxial stretching.
35 ° C. ≦ Tm−Th ≦ 90 ° C. (6)
Tm: Melting point (° C) of polyester resin obtained by differential scanning calorimetry
Th: Heat treatment temperature (° C)
More preferably, the difference Tm−Th is 40 ° C. or higher and 90 ° C. or lower, more preferably 45 ° C. or higher and 80 ° C. or lower, and still more preferably 50 ° C. or higher and 75 ° C. or lower. When the difference Tm−Th exceeds 35 ° C., the crystal volume tends to be coarse due to the thermal history in the heat treatment process, and the restraint property of the amorphous part tends to be lowered. If it is less than 90 ° C., the thermal shrinkage rate becomes too high. For example, when the polyester film of the present invention is used for a back sheet for a solar cell, the back sheet is warped or wrinkled easily. Sometimes. In the production of the polyester film of the present invention, by performing the heat treatment so as to satisfy the above (1), the glass transition temperature (m-Tg) due to the increase in crystal volume due to the thermal history of the heat treatment step and the decrease in the restraining property of the amorphous part. It is possible to improve the heat shrinkage characteristics while suppressing the decrease of

  (D) In the polyester film of the present invention, the polyester of the polyester layer (P layer) is a polyester in which main constituents include a dicarboxylic acid constituent and a diol constituent. In addition, in this specification, a structural component shows the minimum unit which can be obtained by hydrolyzing polyester.

  Examples of the dicarboxylic acid component constituting the polyester include malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosandioic acid, pimelic acid, azelaic acid, methylmalon. Aliphatic dicarboxylic acids such as acid, ethylmalonic acid and the like, adamantane dicarboxylic acid, norbornene dicarboxylic acid, isosorbide, cyclohexanedicarboxylic 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'-diphenylether dicarboxylic acid, 5-sodium Sulfoiso Examples include dicarboxylic acids such as taric acid, phenylendanedicarboxylic acid, anthracene dicarboxylic acid, phenanthrene dicarboxylic acid, aromatic dicarboxylic acid such as 9,9′-bis (4-carboxyphenyl) fluorenic acid, or ester derivatives thereof. It is not limited. In addition, there may be added oxyacids such as l-lactide, d-lactide, hydroxybenzoic acid, and derivatives thereof, or a combination of a plurality of oxyacids to the carboxyl group terminal of the carboxylic acid component. Preferably used. Moreover, these may be used independently or may be used in multiple types as needed.

  Examples of the diol component constituting the polyester include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, and 1,3-butanediol. Aliphatic diols such as cyclohexanedimethanol, spiroglycol and isosorbide, bisphenol A, 1,3-benzenedimethanol, 1,4-benzenedimethanol, 9,9′-bis (4 -Hydroxyphenyl) fluorene, diols such as aromatic diols, and a series of a plurality of the above-mentioned diols are exemplified, but the invention is not limited thereto. Moreover, these may be used independently or may be used in multiple types as needed.

  In the polyester film of the present invention, the proportion of the aromatic dicarboxylic acid constituent component in all the dicarboxylic acid constituent components in the polyester in the P layer is preferably 90 mol% or more and 100 mol% or less. More preferably, it is 95 mol% or more and 100 mol%. More preferably, it is 98 mol% or more and 100 mol% or less, particularly preferably 99 mol% or more and 100 mol% or less, and most preferably 100 mol%, that is, all of the dicarboxylic acid components should be aromatic carboxylic acid components. If it is less than 90 mol%, the heat and moisture resistance and heat resistance may decrease. In the polyester film of the present invention, the ratio of the aromatic dicarboxylic acid component in the total dicarboxylic acid component in the polyester in the P layer is 90 mol% or more and 100 mol% or less, so that both heat resistance and heat resistance are achieved. It becomes possible to do.

  In the polyester film of the present invention, the main repeating unit consisting of a dicarboxylic acid component and a diol component, mainly composed of the polyester of the P layer, is ethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate, propylene terephthalate, butylene. Those composed of terephthalate, 1,4-cyclohexylenedimethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate and mixtures thereof are preferably used. The main repeating unit as used herein means that the total of the above repeating units is 80 mol% or more, more preferably 90 mol% or more of all repeating units in the case of polyester contained in the polyester layer (P layer). Preferably it is 95 mol% or more.

  Furthermore, it is preferable that ethylene terephthalate, ethylene-2,6-naphthalenedicarboxylate, and a mixture thereof are main constituents in that they can be easily polymerized at low cost and have excellent heat resistance. . In this case, when more ethylene terephthalate is used as a structural unit, a cheap and versatile film having heat and heat resistance can be obtained, and more ethylene-2,6-naphthalenedicarboxylate is used as a structural unit. When used, the film can be made more excellent in heat and moisture resistance.

  Polyester can be obtained by appropriately combining the above-mentioned compounds and polycondensed, but the polyester film of the present invention has a trifunctional or higher functional carboxylic acid group and / or hydroxyl group as a copolymerization component of the polyester of the P layer. It is preferable that the constituent component is contained in an amount of 0.005 mol% or more and 2.5 mol% with respect to all copolymer components.

  The component having a trifunctional or higher functional carboxylic acid group and / or hydroxyl group is a component having a total (a + b) of the number of carboxylic acid groups (a) and the number of hydroxyl groups (b) of 3 or more. Specifically, as the carboxylic acid component having 3 or more carboxylic acid groups (a), as a trifunctional aromatic carboxylic acid component, trimesic acid, trimellitic acid, pyromellitic acid, naphthalenetricarboxylic acid, anthracentricarboxylic Acids etc. are trifunctional aliphatic carboxylic acid constituents, such as methanetricarboxylic acid, ethanetricarboxylic acid, propanetricarboxylic acid, butanetricarboxylic acid, etc., tetrafunctional aromatic carboxylic acid constituents are benzenetetracarboxylic acid, benzophenone tetra Carboxylic acid, naphthalene tetracarboxylic acid, anthracene tetracarboxylic acid, berylene tetracarboxylic acid, etc. are tetrafunctional aliphatic carboxylic acid constituents such as ethane tetracarboxylic acid, ethylene tetracarboxylic acid, butane tetracarboxylic acid, cyclopentane tetra Rubonic acid, cyclohexanetetracarboxylic acid, adamantanetetracarboxylic acid, etc. are pentafunctional or higher aromatic carboxylic acid constituents such as benzenepentacarboxylic acid, benzenehexacarboxylic acid, naphthalenepentacarboxylic acid, naphthalenehexacarboxylic acid, naphthaleneheptacarboxylic acid. Acid, naphthalene octacarboxylic acid, anthracene pentacarboxylic acid, anthracene hexacarboxylic acid, anthracene heptacarboxylic acid, anthracene octacarboxylic acid, etc. are pentafunctional or higher aliphatic carboxylic acid constituents, such as ethanepentacarboxylic acid, ethaneheptacarboxylic acid , Butanepentacarboxylic acid, butaneheptacarboxylic acid, cyclopentanepentacarboxylic acid, cyclohexanepentacarboxylic acid, cyclohexanehexacarboxylic acid, adamantanepentacarbonate Bon acid, and adamantane hexacarboxylic acid, and the like These ester derivatives and acid anhydrides thereof as examples without limitation. Also preferred are those obtained by adding oxyacids such as l-lactide, d-lactide, hydroxybenzoic acid and the like, or a combination of a plurality of the oxyacids to the carboxy terminal of the carboxylic acid component. Used. Moreover, these may be used independently or may be used in multiple types as needed.

  Examples of the component having 3 or more hydroxyl groups (b) include trifunctional benzene, trihydroxynaphthalene, trihydroxyanthracene, trihydroxychalcone, trihydroxyflavone, and trihydroxycoumarin as trifunctional aromatic components. A trifunctional aliphatic alcohol component (q) as glycerin, trimethylolpropane, propanetriol, a tetrafunctional aliphatic alcohol component as a compound such as pentaerythritol, or a diol at the hydroxyl terminal of the above compound A component to which is added is also preferably used. Moreover, these may be used independently or may be used in multiple types as needed.

  In addition, among other oxyacids having both a hydroxyl group and a carboxylic acid group in one molecule, such as hydroxyisophthalic acid, hydroxyterephthalic acid, dihydroxyterephthalic acid, dihydroxyterephthalic acid, and the number of carboxylic acid groups (a) The total (a + b) with the number of hydroxyl groups (b) is 3 or more. In addition, it is also preferable to use those obtained by adding oxyacids such as l-lactide, d-lactide, and hydroxybenzoic acid, derivatives thereof, and a combination of a plurality of the oxyacids to the carboxyl group terminal of the above-described components. It is done. Moreover, these may be used independently or may be used in multiple types as needed.

  In the polyester film of the present invention, the content of the trifunctional or higher functional carboxylic acid group and / or hydroxyl group-containing component contained as a polyester copolymerization component of the polyester layer (P layer) is 0.005 with respect to the total copolymerization component. It is preferable to contain from mol% to 2.5 mol%. More preferably, in the case of stretching by the simultaneous biaxial stretching method, 0.01 mol% or more and 0.5 mol% or less, more preferably 0.025 mol% or more and 0.35 mol% or less, further preferably 0.15 mol%. % Or more and 0.25 mol% or less. In the case of the simultaneous biaxial stretching method, if less than 0.005 mol%, it is difficult to reduce the crystal volume, and the enlargement of the crystal tends to proceed in a humid heat atmosphere. The density difference is likely to be large, and may be easily broken when subjected to external stress. Moreover, when it exceeds 2.5 mol%, the hydrolysis resistance of the polyester which comprises P layer may fall, and moisture-heat resistance may fall. In the polyester film of the present invention, in the case of the simultaneous biaxial stretching method, the content of the trifunctional or higher functional carboxylic acid group and / or hydroxyl group-containing component contained as a polyester copolymerization component is 0. By setting the content to 005 mol% or more and 2.5 mol% or less, it becomes possible to suppress the enlargement due to the progress of crystallization without lowering the hydrolysis resistance in a humid heat atmosphere, and as a result, the embrittlement of the film is suppressed. Therefore, it becomes possible to obtain higher moisture and heat resistance.

  Further, the content of the component having a trifunctional or higher functional carboxylic acid group and / or hydroxyl group in the case of stretching by the sequential biaxial stretching method is more preferably 0.01 mol% or more and 0.5 mol% or less, more preferably 0.02 mol% or more and 0.1 mol% or less. In the case of the sequential biaxial stretching method, if it is less than 0.005 mol%, it is difficult to reduce the crystal volume, and the enlargement of the crystal tends to proceed in a humid heat atmosphere. The density difference is likely to be large, and may be easily broken when subjected to external stress. Moreover, if it exceeds 2.5 mol%, even if extrusion or stretching becomes difficult or even a film can be formed, it becomes difficult to impart orientation by stretching the first axis, resulting in a heat history in the second axis stretching step. In some cases, the crystal volume is likely to be coarsened, or even if it is stretched, it becomes difficult to constrain the molecular chain of the amorphous part by imparting orientation, resulting in a decrease in the glass transition temperature (m-Tg). In addition, when the thermal history in the subsequent heat treatment history step (C) is received, the crystal volume is likely to be coarsened, or the restraint of the molecular chains in the amorphous part is likely to be reduced, and the resulting film is resistant to hydrolysis. And heat and humidity resistance may deteriorate. In the polyester film of the present invention, in the polyester film of the present invention, in the case of the sequential biaxial stretching method, the content of the component having a tri- or higher functional carboxylic acid group and / or hydroxyl group contained as a copolymerization component of the polyester is totally reduced. By making it 0.005 mol% or more and 2.5 mol% or less with respect to the polymerization component, it becomes possible to suppress the enlargement due to the progress of crystallization without lowering the hydrolysis resistance in a humid heat atmosphere. Further, embrittlement of the film can be suppressed and higher heat and humidity resistance can be obtained.

  In the polyester film of the present invention, the number of carboxylic acid terminal groups of the polyester constituting the polyester layer (P layer) is preferably 20 equivalents / t or less. More preferably, it is 18 equivalent / t or less, More preferably, it is 16 equivalent / t or less, Especially preferably, it is 14 equivalent / t or less. If it exceeds 20 equivalents / t, even if the structure is controlled, the catalytic action by the proton of the carboxylic acid end group is strong, hydrolysis is promoted, and deterioration tends to proceed. In order to set the number of carboxylic acid end groups to 20 equivalents / t or less, 1) an esterification reaction between a dicarboxylic acid component and a diol component is performed, and when a predetermined melt viscosity is obtained by melt polymerization, A method of solid-phase polymerization after stranding, cutting and chipping, 2) From the end of the ester exchange reaction or esterification reaction to the initial stage of the polycondensation reaction (intrinsic viscosity is less than 0.3). It can be obtained by a combination of methods of adding, and the like. Moreover, it can obtain also by adding a buffering agent and terminal blocker at the time of shaping | molding.

  In the polyester film of the present invention, the intrinsic viscosity (IV) of the polyester constituting the polyester layer (P layer) is preferably 0.60 or more. More preferably, it is 0.65 or more, More preferably, it is 0.68 or more, Most preferably, it is 0.70 or more. If the IV is less than 0.60, the molecular weight is too low to obtain sufficient wet heat resistance and mechanical properties, or the entanglement between the molecules becomes too small, resulting in crystallization after hydrolysis in a humid heat atmosphere. May easily progress and may become brittle. In the polyester film of the present invention, by setting the IV of the polyester constituting the polyester layer (P layer) to 0.60 or more, high moist heat resistance and high mechanical properties can be obtained. Although the upper limit of IV is not particularly determined, it is preferably 1.0 or less, more preferably 0.9 from the viewpoint that the polymerization time is long, which is disadvantageous in cost and difficult to melt extrusion. It is as follows.

  In order to obtain the above-mentioned intrinsic viscosity, when a predetermined melt viscosity is obtained by melt polymerization, a chip is formed by discharging, forming a strand, and cutting, and a chip is formed once at an intrinsic viscosity lower than the target, and then solidified. There is a method of performing phase polymerization. Among these, in particular, when the IV of the polyester layer (P layer) is 0.65 or more, once the intrinsic viscosity lower than the target is reached, thermal degradation can be suppressed and the number of carboxylic acid end groups can be reduced. It is preferable to use a method of forming a chip and then performing solid-phase polymerization.

  Moreover, in the polyester film of the present invention, the polyester constituting the polyester layer (P layer) has a heat resistance and heat and humidity resistance that the content of diethylene glycol, which is a by-product during polymerization, is less than 2.0% by mass. It is preferable that it is less than 1.0 mass%.

  In the polyester film of the present invention, the polyester constituting the polyester layer (P layer) is preferably a highly crystalline resin. Specifically, in accordance with JIS K7122 (1987), the temperature rising rate is 20 ° C./min. The resin is heated from 25 ° C. to 300 ° C. at a rate of temperature increase of 20 ° C./min (1st RUN), held in that state for 5 minutes, then rapidly cooled to below 25 ° C., and then increased again from room temperature to 20 ° C./min. In the 2ndRUN differential scanning calorimetry chart obtained by heating up to 300 ° C. at a rate, it is preferable that the crystal melting calorie ΔCm obtained from the peak area of the melting peak is 15 J / g or more. It is preferable to use a resin having a heat of crystal melting of 20 J / g or more, more preferably 25 J / g or more, and even more preferably 30 J / g or more. By using a highly crystalline resin as the polyester constituting the P layer, high orientation is facilitated, and as a result, it is easy to increase the glass transition temperature (m-Tg) and further improve the heat and moisture resistance. be able to.

  In the polyester film of the present invention, the melting point Tm of the polyester constituting the polyester layer (P layer) is preferably 245 ° C. or higher and 290 ° C. or lower. The melting point Tm here is the melting point Tm in the temperature rising process (temperature rising rate: 20 ° C./min) obtained by DSC, and is 25 ° C. by the method based on JIS K-7121 (1987) as described above. To the melting point of the polyester + 50 ° C. at a heating rate of 20 ° C./min (1st RUN), held in that state for 5 minutes, then rapidly cooled to 25 ° C. or less, and again from room temperature to a heating rate of 20 ° C./min. The melting point Tm of the polyester is defined as the peak top temperature of the 2ndRun crystal melting peak obtained by raising the temperature to 300 ° C. More preferably, the melting point Tm is 247 ° C. or higher and 275 ° C. or lower, more preferably 250 ° C. or higher and 265 ° C. or lower. If the melting point Tm is less than 245 ° C, the heat resistance of the film may be inferior, and it is not preferable. If the melting point Tm exceeds 290 ° C, extrusion may be difficult. In the polyester film of the present invention, a polyester film having both heat resistance and workability can be obtained by setting the melting point Tm1 ¥ of the polyester constituting the polyester layer (P layer) to 245 ° C. or more and 290 ° C. or less.

  The polyester layer (P layer) of the polyester film of the present invention can be obtained by biaxially stretching the above-described polyester into a sheet shape. Here, in the polyester film of the present invention, the plane orientation coefficient of the polyester layer (P layer) is preferably 0.15 or more. More preferably, it is 0.155 or more, Furthermore, it is 0.1575 or more, More preferably, it is 0.1625 or more, Most preferably, it is 0.165 or more. When the plane orientation coefficient is less than 0.15, the restraint property of the amorphous chain is lowered, the glass transition temperature (m-Tg) is lowered, and the moisture and heat resistance may be lowered. In the polyester film of the present invention, high moisture and heat resistance can be obtained by setting the plane orientation coefficient to 0.15 or more.

  In the polyester film of the present invention, the polyester layer (P layer) has a difference Tm-Tmeta between 35 ° C. and 90 ° C. between the melting point Tm of the P layer obtained by differential scanning calorimetry and the slight endothermic peak temperature Tmeta of the P layer. It is preferable that The Tmeta and melting point Tm of the P layer here are values in the temperature raising process (temperature raising rate: 20 ° C./min) obtained by differential scanning calorimetry. Specifically, by a method based on JIS K-7121 (1987), heating from a temperature of 25 ° C. to the melting point of the polyester + 50 ° C. at a rate of temperature increase of 20 ° C./min (1stRUN), holding in that state for 5 minutes, Quenched to 25 ° C or less, and again raised from room temperature to 300 ° C at a rate of temperature increase of 20 ° C / min, with a minute endothermic peak temperature before the crystal melting peak in the 1stRUN differential scanning calorimetry chart. Therefore, Tmeta, and the Tm of the P layer with the temperature of the peak top in the crystal melting peak of 2ndRun.

  More preferably, Tm-Tmeta is 40 ° C. or higher and 90 ° C. or lower, more preferably 45 ° C. or higher and 80 ° C. or lower, and further preferably 50 ° C. or higher and 75 ° C. or lower. When Tm-Tmeta exceeds 90 ° C., the residual stress at the time of stretching is insufficient, and as a result, the thermal shrinkage of the film becomes too large. For example, this polyester film is used as a film for a solar cell backsheet. In some cases, in the bonding process when incorporating, even if the bonding becomes difficult, or even if the bonding can be performed, warpage of the solar cell may occur greatly when it is incorporated into the solar cell and used for a long period of time. Therefore, it is not preferable. Moreover, if Tm-Tmeta is less than 35 ° C., the crystal volume is likely to be coarsened due to the thermal history in the heat treatment process, or the restraint property of the amorphous part is liable to be reduced. Thermal crystallization is promoted and embrittlement may easily progress. In the polyester film of the present invention, when the difference Tm-Tmeta between the melting point Tm of the P layer and the slight endothermic peak temperature Tmeta of the P layer is 35 ° C. or more and 90 ° C. or less, both reduction of heat shrinkage rate and moisture and heat resistance are achieved. it can.

  Furthermore, in the polyester film of the present invention, it is preferable that Tmeta of the P layer constituting the P layer is 160 ° C. or higher and Tm−35 ° C. (where Tm−35 ° C.> 160 ° C.). More preferably, it is 170 ° C. or more and Tm−40 ° C. (however, Tm−40 ° C.> 170 ° C.), more preferably Tmeta is 180 ° C. or more and Tm−45 ° C. (however, Tm−45 ° C.> 180 ° C.), particularly preferably. Tmeta is 180 ° C. or higher and Tm−50 ° C. (where Tm−50 ° C.> 180 ° C.). If Tmeta is less than 160 ° C., the thermal shrinkage of the film becomes too large. For example, when this polyester film is used as a film for a back sheet for a solar cell, it is difficult to bond in the bonding step when incorporating. Even if it can be attached or bonded, it is not preferable because warpage of the solar cell may occur greatly when it is incorporated into the solar cell and used for a long time.

In addition, the P layer of the polyester film of the present invention preferably has a carbo end amount change ΔC defined by the following formula (3) of 110 ° C./t or less after performing 125 ° C. × 100% RH treatment. . Carboxyl group terminal amount change ΔC (equivalent / t) = C1-C0 (3)
C0: Initial carboxyl group terminal amount of the polyester in the P layer.
C1: The terminal amount of carboxyl groups of the polyester in the P layer after the treatment at 125 ° C. × 100% RH × 72 hr.

  More preferably, the carboxyl group terminal amount change ΔC is 100 equivalent / t or less, more preferably 95 equivalent / t or less. When ΔC exceeds 110 equivalent / t, even if the structure is controlled, the catalytic action by protons of the carboxylic acid end group is strong, and hydrolysis may be promoted over time, and deterioration may easily proceed.

  In the polyester film of the present invention, by setting the ΔC to 110 ° C. or less, it is possible to suppress the hydrolysis reaction over time, and to suppress the embrittlement of the film under a moist heat atmosphere. It becomes possible to raise more.

  The degree of crystallinity of the polyester of the polyester layer (P layer) contained in the polyester film of the present invention is preferably 20% or more and 38% or less. The crystallinity here refers to a differential scanning calorimetry chart obtained by heating the resin from 0 ° C. to the melting peak top temperature + 50 ° C. at a rate of temperature increase of 10 ° C./min according to JIS K7122 (1987). The heat of crystal fusion ΔCm is obtained from the peak area of the melting peak, and is obtained by dividing by ΔCm * when it is assumed that the polyester is 100% crystalline. More preferably, they are 25% or more and 38% or less, More preferably, they are 30% or more and 38% or less. In the polyester film of the present invention, when the degree of crystallinity of the polyester in the P layer is less than 20%, the ratio of the amorphous part is large, so the molecular mobility in the film is high, and the enlargement of crystals proceeds in a humid heat atmosphere. This is not preferable because it tends to cause embrittlement of the film. In addition, when the degree of crystallinity is higher than 38%, the amount of crystals is large, so that embrittlement due to the progress of crystallization in a wet heat atmosphere is likely to proceed, which is not preferable. In the polyester film of the present invention, by making the polyester crystallinity of the polyester layer (P layer) 20% or more and 38% or less, it becomes possible to suppress embrittlement due to the progress of crystallization in a humid heat atmosphere. It is possible to obtain high heat and humidity resistance.

  In the polyester film of the present invention, the P layer has a heat resistance stabilizer, an oxidation resistance stabilizer, an ultraviolet absorber, an ultraviolet stabilizer, an organic / inorganic type, as long as the effects of the present invention are not impaired as necessary. Additives such as lubricants, organic / inorganic fine particles, fillers, nucleating agents, dyes, dispersants, coupling agents, and bubbles may be blended.

  When the polyester film of the present invention is composed of only the P layer, it is preferably used in the case of a laminated structure with other layers (hereinafter, other layers may be abbreviated as P2 layers). In the case of a laminated structure, it is preferable that the ratio of the P layer is 40% or more of the entire polyester film in order to exhibit the high moisture and heat resistance effect of the polyester layer. More preferably, it is 50% or more, more preferably 70% or more, and particularly preferably 80% or more. If the ratio of the polyester layer (P layer) is less than 40%, the effect of improving the moisture and heat resistance by the polyester layer (P layer) may not be exhibited, which is not preferable. In the polyester film of the present invention, when the laminated structure is adopted, by setting the ratio of the polyester layer (P layer) to 40% or more, high moisture and heat resistance can be obtained as compared with the conventional polyester film.

  In the polyester film of the present invention, the thickness of the polyester layer (P layer) is preferably 10 μm or more and 500 μm or less, and more preferably 20 μm or more and 300 μm or less. More preferably, they are 25 micrometers or more and 250 micrometers or less. When thickness is less than 10 micrometers, the heat-and-moisture resistance of a film may fall too much. On the other hand, when it is thicker than 500 μm, for example, the thickness of the back sheet for a solar cell using this becomes too thick, and as a result, the thickness of the entire solar cell may become too large.

  Moreover, the thickness of the entire polyester film of the present invention is preferably 10 μm or more and 500 μm or less, and more preferably 20 μm or more and 300 μm or less. More preferably, they are 25 micrometers or more and 250 micrometers or less. When the thickness is less than 10 μm, it is difficult to ensure the flatness of the film. On the other hand, when it is thicker than 500 μm, for example, the thickness of the solar cell backsheet using this becomes too thick, and as a result, the thickness of the entire solar cell may become too large.

  In the polyester film of the present invention, examples of the P2 layer laminated on the polyester layer (P layer) include a polyester layer for imparting a function, an antistatic layer, an adhesion layer with other materials, and an ultraviolet resistance for having an ultraviolet resistance. Any layer such as a layer, a flame retardant layer for imparting flame retardancy, and a hard coat layer for enhancing impact resistance and scratch resistance can be formed. As a specific example, when the polyester film of the present invention is used as a film for a back sheet for a solar cell, easy adhesion for improving adhesion with other sheet materials and ethylene vinyl acetate embedded with a power generation element. In addition to the layer, the ultraviolet resistant layer, and the flame retardant layer, a conductive layer that improves the voltage at which a partial discharge phenomenon, which is an index of insulation, (hereinafter referred to as “partial discharge voltage”) is formed.

The polyester film of the present invention preferably has an elongation retention of 20% or more after standing for 84 hours under conditions of a temperature of 125 ° C. and a relative humidity of 100% RH. The elongation retention referred to here is measured based on ASTM-D882 (1999), and is based on the condition of the elongation at break E0 of the film before processing, temperature 125 ° C., and relative humidity 100% RH 84. This is a value obtained by the following formula (7) when the elongation at break after standing for a period of time is E1.
Elongation retention (%) = E1 / E0 × 100 (7)
In addition, E1 is the value measured using what cut | disconnected the sample in the shape of the measurement piece, and performed the process for 84 hours on the conditions of temperature 125 degreeC and relative humidity 100% RH. More preferably, the elongation retention determined by the above method is 20% or more, more preferably 30% or more, particularly preferably 40% or more, and most preferably 50% or more. In the polyester film of the present invention, when the elongation retention rate is less than 20%, for example, when used as a back sheet for a solar cell, the heat and heat resistance of the back sheet including the film becomes insufficient, and the back sheet is mounted. When the solar cell is used for a long period of time, the back sheet may break down when an external impact is applied to the solar cell (for example, when a falling stone hits the solar cell). Therefore, it is not preferable. In the polyester film of the present invention, by setting the elongation retention rate to 20% or more, for example, when used for a solar cell backsheet, the durability of the backsheet during long-term use can be enhanced.

  Next, although an example is demonstrated about the manufacturing method of the polyester film of this invention, this invention is not limited only to this example.

  In the method for producing a polyester film of the present invention, the resin used as a raw material is a mixture of the above-described dicarboxylic acid constituent component, diol constituent component, trifunctional or higher functional carboxylic acid group and / or a constituent having a hydroxyl group by a well-known method. It can be obtained by condensation. Here, for the dicarboxylic acid component and the component having a tri- or higher functional carboxylic acid group and / or hydroxyl group, the carboxyl group terminal amount of the resulting polyester group can be reduced by using an ester derivative of the carboxyl group. It is more preferable in that the heat and humidity resistance can be further improved.

  As an example of the polymerization step, it is produced by a first step for performing an esterification reaction or a transesterification reaction, a second step for adding an additive such as a polymerization catalyst and a buffer, and a third step for performing a polymerization reaction. A fourth step of performing a solid phase polymerization reaction may be added if necessary.

  In the first step, for example, an esterification reaction or a transesterification reaction is performed by a known method using a dicarboxylic acid component, a diol component, a trifunctional or higher functional carboxylic acid group and / or a component having a hydroxyl group. Can do. For example, when performing a transesterification reaction, a known transesterification reaction catalyst such as magnesium acetate, calcium acetate, manganese acetate, cobalt acetate, calcium acetate can be used, and antimony trioxide as a polymerization catalyst is added. May be. By adding several ppm of alkali metal such as potassium hydroxide during the esterification reaction, diethylene glycol by-product is suppressed and hydrolysis resistance is improved.

  The second step is a step of adding an additive such as a polymerization catalyst and a buffering agent after the esterification reaction or transesterification reaction is substantially completed until the intrinsic viscosity reaches 0.4. .

  As the polymerization catalyst, an ethylene glycol solution of germanium dioxide, antimony trioxide, titanium alkoxide, a titanium chelate compound, or the like can be used.

  Moreover, when adding a buffering agent, it melt | dissolves beforehand in diol components, such as ethylene glycol, it is preferable from the point of the dispersibility of polyester and the long-term hydrolysis resistance to add. In particular, the pH of the mixed solution at this time is preferably adjusted to an acidity of 2.0 or more and 6.0 or less from the viewpoint of suppressing foreign matter formation, and more preferably 4.0 or more and 6.0 or less. Moreover, it is preferable that the buffering agent is added with a polymerization catalyst added at intervals of 5 minutes or more from the viewpoint of polymerization reactivity, and the addition time may be after addition of the polymerization catalyst or before addition.

  When alkali metal phosphate is used as a buffer, phosphoric acid is 0.4 times or more and 1.5 times in terms of moles with respect to alkali metal phosphate from the standpoint of suppressing foreign matter formation and long-term hydrolysis resistance. It is preferable to add the following.

  Examples of other additives include magnesium acetate for the purpose of imparting electrostatic application characteristics, and calcium acetate as a co-catalyst, and can be added within a range that does not hinder the effects of the present invention. In particular, when an esterification reaction has been performed, the amount of carboxyl group end groups can be increased by adding the total diol component so that it is 1.5 times or more and 1.8 times or less in terms of moles with respect to the dicarboxylic acid component. Therefore, it is more preferable in that the hydrolysis resistance can be improved. Further, various particles may be added to add film slipperiness, or internal precipitation particles using a catalyst may be included.

  In the third step, the polymerization reaction can be carried out by a known method. In order to reduce the carboxyl group terminal group amount in the range of 20 equivalents / t or less, the polymerization reaction temperature is set to the melting point of polyester + 30 ° C. The following is preferable. Further, it is effective in that it is possible to reduce the amount of carboxyl group terminals further by forming chips once with an intrinsic viscosity of 0.5 or more and 0.6 or less and performing solid phase polymerization as the fourth step.

  In the fourth step, it is preferable to carry out the solid phase polymerization reaction at a solid phase polymerization temperature of melting point Tm-30 ° C. or lower of the polyester resin composition, melting point Tm-60 ° C. or higher and vacuum degree 0.3 Torr or lower.

  Next, in the production method of the polyester layer (P layer), when the polyester film of the present invention has a single film structure consisting of only the P layer, the raw material for the P layer is heated and melted in an extruder and cooled from the die. It is possible to use a method (melt cast method) for extruding into a sheet and processing it into a sheet. As another method, the raw material for the P layer is dissolved in a solvent, and the solution is extruded from a die onto a support such as a cast drum or an endless belt to form a film, and then the solvent is dried and removed from the film layer. A method of processing into a shape (solution casting method) or the like can also be used.

  When the P layer is produced by the melt casting method, the composition containing the dried polyester can be obtained by extruding with a normal extruder and a T die and biaxially stretching. At this time, it is better that the time from melting in a nitrogen atmosphere and feeding from the chip to the extruder to extrusion from the T-die is as short as possible. As a guideline, it is 30 minutes or less, more preferably 15 minutes or less, and even more preferably 5 It is preferable to set it to less than or equal to the minute in terms of suppressing increase in the number of carboxyl group terminals.

  Moreover, the manufacturing method in the case of the laminated structure in which the polyester film of this invention contains P layer and another layer (P2 layer) is as follows. When the material of each layer to be laminated is made of thermoplastic resin as the main constituent material, two different materials are put into two extruders, respectively, melted and co-extruded on a cast drum cooled from the die, into a sheet form Method of processing (coextrusion method), method of laminating a coating layer raw material on a sheet made of a single film into an extruder, melting and extruding and extruding from a die (melt laminating method), P2 layer laminated with P layer A method of making each separately, thermocompression bonding with a heated group of rolls (thermal laminating method), a method of bonding with an adhesive (adhesion method), and other materials for P2 layer are dissolved in a solvent, The method (coating method) which apply | coats the solution on P layer which produced previously, the method which combined these, etc. can be used.

  In addition, when the P2 layer is mainly composed of a material that is not a thermoplastic resin, the P2 layer and the P2 layer to be laminated are separately prepared and bonded together with an adhesive or the like (adhesion method) or curing. In the case of a conductive material, a method of curing by applying electromagnetic wave irradiation, heat treatment or the like after coating on the P layer can be used.

In order to set the crystal volume of the P layer to 10 nm ³ or more and 110 nm ³ and the glass transition temperature (m-Tg) to 103 ° C. or more, the sheet formed by the melt casting method or the melt casting method is biaxially stretched. Can be obtained. As a manufacturing method, in the case of the melt cast method, first, raw materials are put into an extruder (a plurality of extruders in the case of laminating P2 layers by a coextrusion method), melted and extruded from a die (laminated structure). In this case, it is co-extruded), and is cooled and solidified by static electricity on a cooled drum cooled to a surface temperature of 10 ° C. or more and 60 ° C. or less to produce an unstretched film.

The obtained unstretched film is biaxially stretched at a temperature equal to or higher than the glass transition temperature Tg. As a biaxial stretching method, 1) a simultaneous biaxial stretching method in which stretching in the longitudinal direction (hereinafter sometimes abbreviated as MD) and width direction (hereinafter sometimes abbreviated as TD) is performed simultaneously. 2) Longitudinal direction And sequential biaxial stretching method in which stretching in the width direction is separated. In the polyester film of the present invention, it undergoes a thermal history during the steps of stretching and heat treatment, but if an excessive thermal history is applied, the crystal volume will be enlarged, or even if stretched, the amorphous part due to orientation provision As a result of difficulty in constraining the molecular chain, the glass transition temperature (m-Tg) is likely to decrease. Therefore, in order to set the crystal volume to 10 nm ³ or more and 100 nm ³ or less and the glass transition temperature (m-Tg) to 103 ° C. or more, the following method is preferably exemplified.
1) In the case of simultaneous biaxial stretching When the polyester film of the present invention is stretched by simultaneous biaxial stretching, it is preferably stretched at a glass transition temperature of Tg to Tg + 10 ° C. or less. More preferably, it is Tg or more and Tg + 5 ° C. or less. In the case of the simultaneous biaxial stretching method, if the stretching temperature is less than Tg, stretching becomes difficult, and if the stretching temperature exceeds Tg + 10 ° C., the crystal volume tends to be coarsened due to the thermal history in the stretching process, Even if the film is stretched, it becomes difficult to constrain the molecular chain in the amorphous part by imparting the orientation. As a result, the glass transition temperature (m-Tg) may decrease. In addition, when receiving a thermal history in the subsequent heat treatment step, the crystal volume is likely to be coarsened, and the restraint of the molecular chain in the amorphous part is likely to be reduced. Thereby, the crystal | crystallization volume of the obtained film may coarsen, a glass transition temperature (m-Tg) may fall, and hydrolysis resistance and heat-and-moisture resistance may fall. In the case of simultaneous biaxial stretching in the polyester film of the present invention, stretching by stretching at a temperature not lower than the glass transition temperature Tg of the polyester and not higher than Tg + 10 ° C. suppresses the increase in crystal volume due to the thermal history of the stretching and heat treatment processes. However, the restraint property of the molecular chain of the amorphous part can be increased, the glass transition temperature (m-Tg) can be increased, and a polyester film having high moisture and heat resistance can be obtained.

  In the polyester film of the present invention, in the case of the simultaneous biaxial stretching method, the stretching ratio is preferably 12 times or more. More preferably, the draw ratio is 14 times or more, more preferably 16 times or more. In the case of the simultaneous biaxial stretching method, when the stretch ratio is less than 12 times, the orientation is not sufficiently imparted even if stretched, and it becomes difficult to restrain the molecular chains in the amorphous part. In addition, when receiving a thermal history in the subsequent heat treatment step, the crystal volume is likely to be coarsened, and the restraint of the molecular chain in the amorphous part is likely to be reduced. Thereby, the crystal | crystallization volume of the obtained film may coarsen, a glass transition temperature (m-Tg) may fall, and hydrolysis resistance and heat-and-moisture resistance may fall. In the polyester film of the present invention, by stretching at a draw ratio of 12 times or more, the restraint of the molecular chain in the amorphous part is enhanced while suppressing an increase in crystal volume due to the thermal history of the drawing and heat treatment steps. And the glass transition temperature (m-Tg) can be increased. In addition, it is possible to suppress an increase in crystal volume due to the thermal history of the subsequent heat treatment process and a decrease in glass transition temperature (m-Tg) due to a decrease in the restraining property of the amorphous part, and a polyester having high moisture and heat resistance. It can be a film.

Here, when the polyester forming the polyester (P layer) contains a crosslinking component, the molecular chain has a branched structure, and the entanglement of the molecular chain becomes stronger. Although difficult to do, once the orientation is formed, the structure is fixed. Therefore, depending on the stretching method, the orientation tends to be biased in one direction, thereby increasing the dependence on the direction of moisture and heat resistance. Therefore, in order to make the above-mentioned range, after setting the area stretch ratio to 12 times or more, the stretch ratio E (MD) in the film longitudinal direction (in the case of sequential biaxial stretching, the first stretching direction) and In the case of sequential biaxial stretching, the ratio E (MD) / E (TD) of the stretching ratio E (TD) in the width direction (secondary stretching direction in the case of sequential biaxial stretching) is 0.95 or less (more The film is preferably stretched so that it is preferably 0.9 or less, more preferably 0.85 or less.
2) In the case of the sequential biaxial stretching method In the polyester film of the present invention, in the sequential biaxial stretching method, it is preferable to perform the first-axis stretching at a temperature of Tg or more and Tg + 10 ° C. or less. More preferably, it is Tg + 5 degrees C or less, More preferably, it is Tg + 3 degrees C or less. In the case of sequential biaxial stretching, if the stretching temperature of the first axis is less than Tg, stretching becomes difficult, and if the stretching temperature exceeds Tg + 10 ° C., the molecular chain in the amorphous part is restrained by orientation imparting even if stretched. As a result, the thermal history of the transverse stretching process tends to make the crystal volume coarse, and it becomes difficult to constrain the molecular chains in the amorphous part by imparting orientation in the transverse stretching. In addition, when receiving a thermal history in the subsequent heat treatment step, the crystal volume is likely to be coarsened, and the restraint property of the amorphous part is likely to be lowered. Thereby, the crystal | crystallization volume of the obtained film may coarsen, a glass transition temperature (m-Tg) may fall, and hydrolysis resistance and heat-and-moisture resistance may fall. In the polyester film of the present invention, in the case of sequential biaxial stretching, the first part is stretched at a temperature of Tg + 10 ° C. or lower, thereby suppressing an increase in crystal volume due to the thermal history of the stretching and heat treatment process, and the amorphous part. It is possible to increase the restraint property, and it is possible to increase the glass transition temperature (m-Tg), and it is possible to obtain a polyester film having high wet heat resistance.

  In the polyester film of the present invention, in the case of sequential biaxial stretching, it is preferable to carry out the second axial stretching at a temperature of Tg or more and Tg + 15 ° C. or less. More preferably, it is Tg + 10 degreeC, More preferably, it is Tg + 5 degreeC or less. In the case of sequential biaxial stretching, if the stretching temperature of the second axis is less than Tg, stretching becomes difficult, and if the stretching temperature exceeds Tg + 15 ° C., the crystal volume tends to become coarse or the orientation in the transverse stretching It becomes difficult to restrain the molecular chain in the amorphous part by the application. In addition, when receiving a thermal history in the subsequent heat treatment step, the crystal volume is likely to be coarsened, and the restraint property of the amorphous part is likely to be lowered. Thereby, the crystal | crystallization volume of the obtained film may coarsen, a glass transition temperature (m-Tg) may fall, and hydrolysis resistance and heat-and-moisture resistance may fall. In the polyester film of the present invention, in the case of sequential biaxial stretching, the second axial stretching is performed at a temperature of Tg + 15 ° C. or lower, thereby suppressing an increase in crystal volume due to the thermal history of the stretching and heat treatment process, and amorphous. The restraint property of the part can be increased, the glass transition temperature (m-Tg) can be increased, and a polyester film having high moist heat resistance can be obtained.

  In the polyester film of the present invention, in the case of sequential biaxial stretching, the stretching ratio is 3 to 5 times in each of the longitudinal direction and the width direction, but the area stretching ratio is preferably 13 or more. More preferably, the draw ratio is 14 times or more, more preferably 16 times or more. In the case of sequential biaxial stretching, if the draw ratio is less than 13 times the area, the orientation is not sufficiently imparted even if stretched, making it difficult to restrain the molecular chains in the amorphous part. In addition, when receiving a thermal history in the subsequent heat treatment step, the crystal volume is likely to be coarsened, and the restraint property of the amorphous part is likely to be lowered. Thereby, the crystal | crystallization volume of the obtained film may coarsen, a glass transition temperature (m-Tg) may fall, and hydrolysis resistance and heat-and-moisture resistance may fall. In the polyester film of the present invention, in the case of sequential biaxial stretching, the stretch ratio is stretched at an area of 13 times or more, thereby suppressing the increase in crystal volume due to the thermal history of the stretching process and enhancing the restraint of the amorphous part. The glass transition temperature (m-Tg) can be increased, and a polyester film having high moisture and heat resistance can be obtained.

  Here, when the polyester forming the polyester (P layer) contains a crosslinking component, the molecular chain has a branched structure, and the entanglement of the molecular chain becomes stronger. Although difficult to do, once the orientation is formed, the structure is fixed. Therefore, depending on the stretching method, the orientation tends to be biased in one direction, thereby increasing the dependence on the direction of moisture and heat resistance. Therefore, in order to make the above-mentioned range, after setting the area stretch ratio to 13 times or more, the stretch ratio E (MD) in the film longitudinal direction (in the case of sequential biaxial stretching, the first stretching direction) and In the case of sequential biaxial stretching, the ratio E (MD) / E (TD) of the stretching ratio E (TD) in the width direction (secondary stretching direction in the case of sequential biaxial stretching) is 0.95 or less (more It can be obtained by stretching so that it is preferably 0.9 or less, more preferably 0.85 or less.

  In any of the above stretching methods, it is easy to realize both miniaturization of crystal volume and glass transition temperature (m-Tg), and the simultaneous biaxiality is high in that the composition and the range of conditions that can be achieved are high. A stretching method is more preferable.

In addition, in order to complete the crystal orientation of the obtained biaxially stretched film, and to impart flatness and dimensional stability, heat treatment is performed for 1 second to 30 seconds at a temperature not lower than the Tg of the polyester and lower than the melting point, and uniform. After cooling slowly, cool to room temperature. Generally, when the heat treatment temperature Th is low, the thermal shrinkage of the film increases. On the other hand, if the heat treatment temperature is too high, the amorphous part is relaxed, the molecular mobility becomes high, and the glass lowering temperature is lowered or the crystal volume is increased. As a result, hydrolysis tends to occur in a moist and hot atmosphere, and since crystallization is likely to be enlarged when crystallization is caused by a hydrolysis reaction, embrittlement may easily occur. Therefore, it is not preferable. Therefore, in the method for producing a polyester film of the present invention, the heat treatment temperature Th is preferably such that the difference Tm-Th from the melting point Tm of the polyester satisfies the following formula (6).
35 ° C. ≦ Tm−Th ≦ 90 ° C. (6)
Tm: Melting point (° C) of polyester resin obtained by differential scanning calorimetry
Th: Heat treatment temperature (° C)
More preferably, the difference Tm−Th is 40 ° C. or higher and 80 ° C. or lower, and further preferably 50 ° C. or higher and 75 ° C. or lower. Note that the crystal volume, glass transition temperature (m-Tg), and Tmeta can be controlled by controlling Th. Moreover, in the said heat processing process, you may perform a 3-12% relaxation process in the width direction or a longitudinal direction as needed. Subsequently, if necessary, the P layer of the polyester film of the present invention can be formed by performing corona discharge treatment or the like to further improve the adhesion to other materials and winding.

  Next, as a method of forming the P2 layer on the P layer, in addition to the above-mentioned coextrusion method, melt lamination method, solution lamination method, thermal lamination method, etc., dry methods such as vapor deposition and sputtering, plating, etc. A wet method such as a method can also be suitably used. Examples of the method for forming the layer P2 made of different materials by the coating method include an in-line coating method applied during the formation of the polyester film of the present invention and an offline coating method applied to the polyester film after the film formation. Either of these methods can be used, but an in-line coating method is preferably used because it is efficient because it can be formed simultaneously with the formation of a polyester film and has high adhesion to the polyester film. Moreover, when coating, corona treatment etc. are preferably performed to the polyester film surface on the support body of a coating liquid.

  The polyester film of the present invention can be formed by the above-described process, and the obtained film has high moisture and heat resistance. The polyester of the present invention takes advantage of its features, such as copper-clad laminate, solar cell backsheet, adhesive tape, flexible printed circuit board, membrane switch, sheet heating element, flat cable, and other electrically insulating materials, capacitor materials, and automobiles. It can be suitably used for applications in which moisture and heat resistance such as construction materials and building materials are important. In these, it uses suitably as a film for solar cell backsheets.

  As long as the above-mentioned polyester film is used for the structure of the solar cell backsheet of the present invention, an arbitrary structure can be used. An ethylene-vinyl acetate copolymer that seals a power generating element on the polyester film of the present invention ( Hereinafter, it may be abbreviated as EVA.) EVA adhesion layer for improving the adhesion to the EVA, an anchor layer for increasing adhesion with the EVA adhesion layer, a water vapor barrier layer, an ultraviolet absorption layer for preventing ultraviolet degradation, power generation efficiency The back sheet for solar cells of the present invention is formed by forming a light reflecting layer for enhancing the appearance, a light absorbing layer for expressing the design, an adhesive layer for adhering each layer, a conductive layer for improving the partial discharge voltage, etc. Configure. Moreover, only the polyester film of this invention may be used as a solar cell backsheet.

  The EVA adhesion layer is a layer that improves adhesion to the EVA resin that seals the power generation element, and is disposed on the side closest to the power generation element, and contributes to adhesion between the backsheet and the system. The material is not particularly limited as long as adhesion with EVA resin is expressed. For example, EVA, EVA and ethylene-methyl acrylate copolymer (EMA), ethylene-ethyl acrylate copolymer (EEA), ethylene A mixture of butyl acrylate copolymer (EBA), ethylene-methacrylic acid copolymer (EMAA), ionomer resin, polyester resin, urethane resin, acrylic resin, polyethylene resin, polypropylene resin, polyamide resin or the like is preferably used. Moreover, in order to improve the adhesiveness to the back sheet | seat of an EVA contact bonding layer as needed, forming an anchor layer is also performed preferably. The material is not particularly limited as long as adhesion to the EVA adhesion layer is expressed, and for example, a mixture containing a resin as a main constituent such as an acrylic resin or a polyester resin is preferably used.

  The water vapor barrier layer is a layer for preventing water vapor from entering from the back sheet side in order to prevent water vapor deterioration of the power generation element when the solar cell is configured. It is formed by providing an oxide such as silicon oxide or aluminum oxide or a metal layer such as aluminum on the film surface by a known method such as vacuum deposition or sputtering. The thickness is preferably in the range of usually 100 angstroms or more and 200 angstroms or less. In this case, both the case where the gas barrier layer is directly provided on the polyester film of the present invention and the case where the gas barrier layer is provided on another film and this film is laminated on the film surface of the present invention are preferably used. Moreover, the method of laminating | stacking metal foil (for example, aluminum foil) on the film surface can also be used. In this case, the thickness of the metal foil is preferably in the range of 10 μm to 50 μm from the viewpoint of workability and gas barrier properties.

  The ultraviolet absorbing layer is a layer for blocking ultraviolet rays in order to prevent ultraviolet degradation of the resin in the inner layer, and any layer can be used as long as it has a function of blocking ultraviolet rays having a wavelength of 380 nm or less. The light reflecting layer is a layer that reflects light, and by forming this layer, the inner layer resin is prevented from UV degradation, or the light that reaches the back sheet without being absorbed by the solar cell system is reflected. This layer is used to increase the power generation efficiency by returning to the system side, and is a layer containing white pigments such as titanium oxide and barium sulfate, or bubbles. The light absorbing layer is a layer that absorbs light, and is a layer that is used to prevent UV degradation of the resin in the inner layer or improve the design of the solar cell by forming this layer.

  The back sheet for a solar cell of the present invention is formed by combining each of the above layers and the polyester film of the present invention. In the solar cell backsheet of the present invention, it is not necessary to form all of the above layers as independent layers, and it is also a preferred form to form a function integrated layer having a plurality of functions. Moreover, when the polyester film of this invention has a function already, it is also omissible. For example, when the polyester film of the present invention includes a layer containing a white pigment or bubbles and has light reflectivity, the light reflective layer is included, and the structure including a layer including a light absorber has light absorbability. In the case of a structure including an absorbing layer and a layer containing an ultraviolet absorber, the ultraviolet absorbing layer may be omitted.

  Since the polyester film of the present invention is superior in moisture and heat resistance compared to conventional polyester films, the solar cell backsheet including this film can have higher moisture and heat resistance than conventional backsheets. . Here, in the solar cell backsheet, in order for the backsheet to exhibit the high moisture and heat resistance effect of the polyester film of the present invention, the volume ratio of the P layer to the entire backsheet is preferably 10% or more. More preferably, it is 20% or more, more preferably 25% or more, still more preferably 30% or more, and particularly preferably 40% or more.

Moreover, the solar cell backsheet using the polyester film of the present invention preferably has an elongation retention of 15% or more after standing for 84 hours under the conditions of a temperature of 125 ° C. and a relative humidity of 100% RH. The elongation retention here is measured based on ASTM-D882 (1999), and is the condition of the breaking elongation E0 ′ of the backsheet before treatment, the temperature of 125 ° C., and the relative humidity of 100% RH. This is a value obtained by the following formula (2 ′) when the elongation at break after standing for 84 hours is defined as E1 ′.
Elongation retention (%) = E1 ′ / E0 ′ × 100 (2 ′)
E1 ′ is a value measured using a sample that was cut into the shape of a measurement piece and then treated for 84 hours under conditions of a temperature of 125 ° C. and a relative humidity of 100% RH. More preferably, the elongation retention obtained by the above method is 25% or more, more preferably 35% or more, and particularly preferably 45% or more. In the solar cell backsheet using the polyester film of the present invention, if the elongation retention rate is less than 15%, for example, when the solar cell equipped with the backsheet is used for a long period of time, the deterioration proceeds, and some externally When the impact is applied to the solar cell (for example, when a falling rock or the like hits the solar cell), the backsheet may break, which is not preferable. In the solar cell backsheet of the present invention, by setting the elongation retention rate to 15% or more, the durability of the solar cell during long-term use can be enhanced.

  The thickness of the solar cell backsheet of the present invention is preferably from 50 μm to 500 μm, and more preferably from 100 μm to 300 μm. More preferably, it is 125 μm or more and 200 μm or less. When the thickness is less than 10 μm, it is difficult to ensure the flatness of the film. On the other hand, when it is thicker than 500 μm, when it is mounted on a solar cell, the thickness of the entire solar cell may become too large.

  The solar cell of the present invention includes a back sheet for a solar cell using the polyester film of the present invention. The solar cell backsheet using the polyester film of the present invention can be made more durable or thinner than a conventional solar cell by taking advantage of its heat and moisture resistance higher than that of a conventional backsheet. An example of the configuration is shown in FIG. A power generating element connected with a lead wire (not shown in FIG. 1) for extracting electricity is sealed with a transparent filler 2 such as EVA resin, and is called a transparent substrate 4 such as glass and a back sheet 1. Although it is configured by bonding a resin sheet, it is not limited to this and can be used for any configuration.

  Here, in the solar cell of the present invention, the above-described solar cell backsheet 1 is installed on the back surface of the resin layer 2 in which the power generation element is sealed.

As described above, by incorporating the solar cell backsheet using the polyester film of the present invention into a solar cell system, it is possible to obtain a solar cell system that is more durable and / or thinner than conventional solar cells. It becomes. 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.
[Characteristic evaluation method]
A. Composition analysis of polyester Polyester was hydrolyzed with alkali, each component was analyzed by gas chromatography or high performance liquid chromatography, and the composition ratio was determined from the peak area of each component. An example is shown below. Dicarboxylic acid components and trifunctional or higher functional components having a carboxylic acid group were measured by high performance liquid chromatography. The measurement conditions can be analyzed by a known method, and an example of the measurement conditions is shown below.
Equipment: Shimadzu LC-10A
Column: YMC-Pack ODS-A 150 × 4.6 mm S-5 μm 120A
Column temperature: 40 ° C
Flow rate: 1.2ml / min
Detector: UV 240nm
The quantification of a diol component or a trifunctional or higher functional component having a hydroxyl group can be analyzed by a known method using gas chromatography. An example of measurement conditions is shown below.
Apparatus: Shimadzu 9A (manufactured by Shimadzu Corporation)
Column: SUPELCOWAX-10 capillary column 30m
Column temperature: 140 ° C. to 250 ° C. (heating rate 5 ° C./min)
Flow rate: Nitrogen 25ml / min
Detector: FID.

B. Intrinsic viscosity IV
A polyester film (polyester layer (P layer) in the case of a laminated film) is dissolved in 100 ml of orthochlorophenol (solution concentration C = 1.2 g / ml), and the viscosity of the solution at 25 ° C. is measured using an Ostwald viscometer. It was measured. Similarly, the viscosity of the solvent was measured. [Η] was calculated by the following formula (8) using the obtained solution viscosity and solvent viscosity, and the obtained value was used as the intrinsic viscosity (IV).
ηsp / C = [η] + K [η] ² · C (8)
(Where ηsp = (solution viscosity / solvent viscosity) −1, K is the Huggins constant (assuming 0.343))
C. Determination of Alkali Metal Element Content After dissolving a polyester film (in the case of a laminated film, a polyester layer (P layer)) in O-chlorophenol, an extraction operation was performed with 0.5 N hydrochloric acid. The extract was subjected to quantitative determination of Na, K, Li by atomic absorption spectrometry (manufactured by Hitachi, Ltd .: polarized Zeeman atomic absorption photometer 180-80, frame: acetylene-air). did. The quantification was performed using a calibration curve prepared in advance using a standard solution having a known concentration.

D. Determination of Phosphorus Element Content A polyester film (in the case of a laminated film, a polyester layer (P layer)) was measured for the intensity of fluorescent X-rays using a fluorescent X-ray analyzer (model number: 3270) manufactured by Rigaku Corporation. Using the obtained value, a phosphorus element content W2 was obtained using a calibration curve prepared in advance with a sample having a known phosphorus element content.

E. The number of carboxyl group terminal groups The polyester film (polyester layer (P layer) in the case of a laminated film) was measured by the method of Malice. (Reference M. J. Malice, F. Huizinga. Anal. Chim. Acta, 22 363 (1960)).

F. Carboxyl group terminal number change After the polyester film was treated for 72 hours at 125 ° C. and relative humidity of 100% RH in a pressure cooker, the carboxyl group terminal number C1 of the polyester layer of the treated sample was described in E. Measured according to the method of Further, the number of carboxyl group terminals C0 of the film before the treatment was measured according to the method described in the section E. Using the obtained C0 and C1, the carboxyl group terminal amount change ΔC was determined according to the following formula (3).
ΔC = C1-C0 (3)
G. (100) plane crystallite size, (−105) plane crystallite size, (0-11) plane crystallite size, and their product (crystal volume)
A plurality of polyester films were cut into strips having a longitudinal direction of 40 mm and a width of 1 mm, and a sample was prepared by superimposing them to a thickness of about 1 mm. Using this sample, a goniometer 2155D (slit 2 mmφ-1 ° − made by Rigaku Corporation) equipped with an X-ray generator 4036A2 type (X-ray source—CuKα ray (using Ni filter)) manufactured by Rigaku Corporation. 1 °) and wide-angle X-ray diffraction was performed by the transmission method under the following conditions.
2θ-θ step scan method: measurement range (2θ) 10 ° to 50 °, measurement step (2θ) 0.05 °, counting time 2 seconds.
Β step scan method: measurement range (β) 0 to 360 °, measurement step 0.5 ° (0-11, 100), 1.0 ° (−105), counting time 2 seconds.
Moreover, Rigaku Electric Co., Ltd. goniometer 2155S2 (slit diameter 1 ° -1 °) equipped with an X-ray generator RU-200R (X-ray source—CuKα ray (using Ni filter)) manufactured by Rigaku Corporation. -0.15 mm-0.45 mm), and wide-angle X-ray diffraction was performed by the reflection method under the following conditions.
2θ-θ continuous scan method: measurement range (2θ) 10 ° to 50 °, measurement step (2θ) 0.02 °, scan speed 3 ° / minute, and diffraction peak of transmitted light 2θ = about 16. (0-11) crystallite size from peak at 4 °, (θ) crystallite size from 2θ = about 43 °, and (100) crystallite from peak at 2θ = about 26 ° for diffraction peak of reflected light Each size was determined by the following formula.

  Using the obtained crystallite size values, the product of the (100) plane crystallite size, the (−105) plane crystallite size, and the crystal size in the (0-11) direction was determined.

H. Glass transition temperature by temperature modulation DSC (m-Tg)
Put 5 mg of polyester film (polyester layer (P layer) in the case of laminated film) into a standard aluminum container, and use polyester from TA Instrument, using Q100, from 0 ° under nitrogen flow (flow rate 50 mL / min). Measurement was carried out by sinusoidal temperature modulation with a temperature modulation amplitude of 1 ° C. with a temperature modulation period of 60 seconds in the range of melting point Tm-75 ° C. of 2 ° C./min. The specific heat calibration was performed with sapphire, and the data analysis was performed using “Universal Analysis 2000” manufactured by TA Instrument. The method described in “9.3 Determination of Glass Transition Temperature (1) Intermediate Glass Transition Temperature Tmg” in JISK7121 (1987) in the step-like change portion of the temperature-modulated DSC chart of the obtained reversible component The glass transition temperature (m-Tg) of the temperature-modulated DSC was determined in the same manner as above (the straight line equidistant in the vertical axis direction from the straight line extended from each baseline and the step-like change part of the glass transition). The point at which the curve intersects was defined as the glass transition temperature (m-Tg) of temperature-modulated DSC).

I. Crystallinity For polyester film (polyester layer (P layer) in the case of laminated film), according to JIS K7122 (1987), TA Instrument Q100 is used. For data analysis, “Universal Analysis 2000” is used. The measurement was carried out as follows. The resin sample is weighed in 5 mg each in a sample pan, and the resin is heated from 0 ° C. to 300 ° C. at a temperature rising rate of 10 ° C./min. ) Of “9. How to obtain transition heat”. Next, the crystallinity (%) was obtained by dividing the calorific value of the obtained crystal melting peak by the theoretical value of 100% polyester crystallinity. The amount of heat of crystal melting at 100% polyester crystallinity is 140.1 (J / g) when the resin constituting the polyester layer is mainly composed of ethylene terephthalate.

J. Glass transition temperature Tg, slight endothermic peak temperature Tmeta of P layer, melting point Tm
For a polyester film (polyester layer (P layer) in the case of a laminated film), according to JIS K7122 (1987), a differential scanning calorimeter “Robot DSC-RDC220” manufactured by Seiko Denshi Kogyo Co., Ltd. was used for data analysis. Measurements were performed using the disk session “SSC / 5200”.
It calculated | required in the procedure of following (A1)-(A4).
(A1) The sample enclosed in the sample pan is heated at a temperature increase rate of 20 ° C./min from a temperature of 25 ° C. to a melting point Tm + 50 ° C. at a temperature increase rate of 20 ° C./min (1stRUN).
(A2) Hold in that state for 5 minutes, and then rapidly cool to 25 ° C. or lower.
(A3) The temperature is increased again from room temperature to 300 ° C. at a temperature increase rate of 20 ° C./min (2ndRUN).
(A4) Tmeta was defined as the minute endothermic peak temperature before the crystal melting peak in the obtained 1stRUN differential scanning calorimetry chart. In addition, in the obtained 2ndRUN differential scanning calorimetry chart, in the stepwise change portion of the glass transition, JISK7121 (1987) describes “9.3 Determination of Glass Transition Temperature (1) Intermediate Glass Transition Temperature Tmg”. The glass transition temperature Tg of the polyester was obtained by the method described above (the point where the straight line equidistant from the straight line extending from each base line in the vertical axis direction and the curve of the stepwise change portion of the glass transition intersect) Transition temperature Tg). The peak top temperature in the 2ndRun crystal melting peak was defined as the melting point Tm of the polyester layer.

K. Elongation at break, heat and humidity resistance (elongation retention)
Based on ASTM-D882 (1997), the breaking elongation of the polyester film was measured by measuring the breaking elongation when the sample was cut into a size of 1 cm × 20 cm and pulled at a chucking distance of 5 cm and a pulling speed of 300 mm / min. In addition, the measurement was implemented about 10 samples each about the longitudinal direction and the width direction, and it was set as breaking elongation E0 by the average value.

The moisture resistance of the polyester film was determined by treating the sample in the shape of a measurement piece (1 cm × 20 cm) and then treating it with a pressure cooker manufactured by Hirayama Seisakusho at 125 ° C. and a relative humidity of 100% RH for 84 hours. After the treatment, the breaking elongation of the treated sample was measured based on ASTM-D882 (1997) when it was pulled at a chuck distance of 5 cm and a pulling speed of 300 mm / min. In addition, the measurement was implemented about 10 samples, respectively, and it was set as the breaking elongation E1 with the average value. Using the obtained breaking elongations E0 and E1, the elongation retention was calculated by the following formula (7).
Elongation retention (%) = E1 / E0 × 100 (7)
The obtained elongation retention was determined as follows.
When the elongation retention is 50% or more: S
When the elongation retention is 40% or more and less than 50%: A
Elongation retention is 30% or more and less than 40%: B
Elongation retention is 20% or more and less than 30%: C
Elongation retention is less than 20%: E
S or A or B or C is good, and S is the best.
Also, the breaking elongation and heat and humidity resistance of the backsheet are the same as described above, and the breaking elongation E1 after standing for 84 hours under the conditions of the breaking elongation E0 ′, temperature 125 ° C., and relative humidity 100% RH of the backsheet before treatment. 'Was obtained, and the elongation retention was calculated by the following formula (7').
Elongation retention (%) = E1 ′ / E0 ′ × 100 (7 ′)
In addition, each measurement was implemented about 10 samples about the back sheet longitudinal direction and the width direction, and it was set as E0 'and E1' by the average value.
The obtained elongation retention was determined as follows.
When elongation retention is 45% or more: S
When the elongation retention is 35% or more and less than 45%: A
Elongation retention is 25% or more and less than 35%: B
Elongation retention is 15% or more and less than 25%: C
Elongation retention is less than 15%: E
S or A or B or C is good, and S is the best.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated, this invention is not necessarily limited to these.
(Example 1-1)
As a first step, 100 parts by mass of dimethyl terephthalate, trimethyl trimellitic acid (added so as to have a molar ratio of dimethyl terephthalate / trimethyl trimellitic acid = 99.7 / 0.3), 57.5 parts by mass of ethylene glycol Then, 0.06 parts by mass of manganese acetate and 0.03 parts by mass of antimony trioxide were melted in a nitrogen atmosphere at 150 ° C., and then heated to 230 ° C. with stirring over 3 hours to distill methanol and transesterify The reaction was terminated. As the second step, 0.020 parts by mass of phosphoric acid (equivalent to 2 mol / t) and 0.031 parts by mass of sodium dihydrogen phosphate dihydrate (equivalent to 1.7 mol / t) after the transesterification reaction are completed. An ethylene glycol solution (PH 5.0) dissolved in 0.5 part by mass of ethylene glycol was added.
As a third step, the polymerization reaction was carried out at a final temperature of 285 ° C. and a degree of vacuum of 0.1 Torr to obtain a polyester having an intrinsic viscosity of 0.53 and a carboxyl group terminal group number of 12 equivalents / t. As the fourth step, the obtained polyethylene terephthalate was dried and crystallized at 160 ° C. for 6 hours, and then subjected to solid-phase polymerization at 220 ° C. and a vacuum degree of 0.3 Torr for 8 hours. A polyester having 10 equivalents / t of terminal groups was obtained. The results of evaluating the properties of the obtained polyester are shown in Table 1. It was found that 0.15 mol% of trimellitic acid was contained as a constituent component having a trifunctional or higher functional carboxylic acid group and / or hydroxyl group in all the constituent components of the polyester.

  The obtained polyester was vacuum dried at a temperature of 180 ° C. for 3 hours, then supplied to an extruder, melted at a temperature of 280 ° C. in a nitrogen atmosphere, and introduced into a T die die. At this time, a 400-mesh wire mesh was used as the filter of the extruder. Next, the molten single layer sheet is extruded from the inside of the T die die into a molten single layer sheet, and the molten single layer sheet is closely cooled and solidified by an electrostatic application method on a drum maintained at a surface temperature of 25 ° C. A layer film was obtained.

  Subsequently, while holding both ends of the unstretched monolayer film with clips, preheat at a temperature of 85 ° C. in a simultaneous biaxial stretching machine, and continuously 4.3 times in the longitudinal direction (longitudinal direction) at 85 ° C. The film was stretched 4.3 times in the direction perpendicular to the longitudinal direction (width direction). Subsequently, heat treatment was performed in the heat treatment zone 1 at a temperature of 200 ° C. for 20 seconds. Subsequently, after uniform cooling, the film was wound up to obtain a biaxially stretched film having a thickness of 75 μm.

The results of evaluating the properties of the obtained film are shown in Table 1. The film was found to be excellent in heat and humidity resistance. Next, this film was used as the first layer, and as a bonding layer, 90 parts by mass of “Takelac (registered trademark)” A310 (manufactured by Mitsui Takeda Chemical Co., Ltd.), “Takenate (registered trademark)” A3 (Mitsui Takeda Chemical Co., Ltd.) Manufactured) and a polyethylene film having a thickness of 100 μm was laminated thereon as a second layer to form a back sheet having a thickness of 180 μm. The obtained back sheet was evaluated for heat and moisture resistance. The results are shown in Table 1. It was found to have high resistance to moisture and heat.

(Examples 1-2 to 1-30)
A polyester film having a thickness of 75 μm was obtained in the same manner as in Example 1-1 except that the polyester was polymerized so as to have the composition and characteristics shown in Table 1 and the film was formed under the conditions shown in Table 1. Table 1 shows the results of evaluating the properties of the obtained polyester and film. It was found to be excellent in heat and humidity resistance. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. The results of evaluating the characteristics of the obtained back sheet are shown in Table 1. It was found to be excellent in heat and humidity resistance.

(Examples 1-31 to 1-36)
Except for the alkali metal element amount W1 and the phosphorus element amount W2 in Table 1, Example 1 except that the polyester having the characteristics shown in Table 1 was polymerized in the same manner as in Example 1-1, and a film was formed under the conditions in Table 1. As in Example 1, a 75 μm thick polyester film was obtained. Table 1 shows the results of evaluating the properties of the obtained polyester and film. It was found to be excellent in heat and humidity resistance. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. The results of evaluating the characteristics of the obtained back sheet are shown in Table 1. It was found to be excellent in heat and humidity resistance.

(Example 2-1)
An unstretched monolayer film was obtained in the same manner as in Example 1-1 except that the polyester was polymerized so as to have the composition shown in Table 2. Subsequently, the unstretched single layer film was preheated with a roll group heated to a temperature of 80 ° C., and then stretched 3.9 times in the longitudinal direction (longitudinal direction) using a heating roll having a temperature of 84 ° C. A uniaxially stretched film was obtained by cooling with a roll group at a temperature of ° C. While holding both ends of the obtained uniaxially stretched film with clips, it is led to a preheating zone at a temperature of 75 ° C. in the tenter, and continuously in a heating zone at a temperature of 85 ° C. in a direction perpendicular to the longitudinal direction (width direction). The film was stretched 4.5 times. Subsequently, a heat treatment was performed at a temperature of 200 ° C. for 20 seconds in the heat treatment zone 1 in the tenter, a heat treatment of 150 ° C. was further performed in the heat treatment zone 2, and a heat treatment was performed at a temperature of 100 ° C. in the heat treatment zone 3. During the heat treatment, a relaxation treatment of 4% was performed between the heat treatment zone 1 and the heat treatment zone 2. Subsequently, after uniform cooling, the film was wound up to obtain a biaxially stretched film having a thickness of 75 μm. The results of evaluating the properties of the obtained polyester and film are shown in Table 2. It was found to be excellent in heat and humidity resistance. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. Table 2 shows the results of evaluating the characteristics of the obtained backsheet. It was found to be excellent in heat and humidity resistance.

(Examples 2-2 to 2-27)
Polyester was polymerized in the same manner as in Example 1-1 except that the composition and properties shown in Table 2 were used, and a polyester film having a thickness of 75 μm was used in the same manner as in Example 2-1, except that the film was formed under the conditions in Table 2. Got. The results of evaluating the properties of the obtained polyester and film are shown in Table 2. It was found to be excellent in heat and humidity resistance. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. Table 2 shows the results of evaluating the characteristics of the obtained backsheet. It was found to be excellent in heat and humidity resistance.

(Comparative Examples 1-1 to 1-3)
A polyester film having a thickness of 75 μm was obtained in the same manner as in Example 1-1 except that the polyester was polymerized so as to have the composition and characteristics shown in Table 1, and the film was formed under the conditions shown in Table 1. The results of evaluating the properties of the obtained polyester and film are shown in Table 1. It turned out that it is inferior to heat-and-moisture resistance compared with an Example. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. The results of evaluating the characteristics of the obtained back sheet are shown in Table 1. It turned out that it is inferior to heat-and-moisture resistance compared with an Example.

(Comparative Examples 1-4 to 1-5)
A polyester film having a thickness of 75 μm was obtained in the same manner as in Example 1-1 except that the heat treatment temperature was formed under the conditions shown in Table 1. Table 1 shows the results of evaluating the properties of the obtained polyester and film. It turned out that it is inferior to heat-and-moisture resistance compared with an Example. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. The results of evaluating the characteristics of the obtained back sheet are shown in Table 1. It turned out that it is inferior to heat-and-moisture resistance compared with an Example.

(Comparative Examples 2-1 to 2-8, 14, 15)
A polyester was polymerized in the same manner as in Example 1-1 except that the polyester was polymerized to have the composition and characteristics shown in Table 2, and a thickness of 75 μm was obtained in the same manner as in Example 2-1, except that the film was formed under the conditions in Table 2. A polyester film was obtained. The results of evaluating the properties of the obtained film are shown in Table 2. It turned out that it is inferior to heat-and-moisture resistance compared with an Example. Moreover, the solar cell backsheet was obtained by the method similar to Example 1-1. Table 2 shows the results of evaluating the characteristics of the obtained backsheet. It turned out that it is inferior to heat-and-moisture resistance compared with an Example.

(Comparative Examples 2-9 to 2-13)
The polyester was polymerized in the same manner as in Example 1-1 except that the polyester was polymerized so as to have the composition and characteristics shown in Table 2, and 75 μm was obtained in the same manner as in Example 2-1, except that the conditions in Table 2 were used. An attempt was made to obtain an axially stretched polyester film, but film tearing occurred frequently in the transverse stretching process, and could not be obtained.

  The polyester film of the present invention is a polyester film excellent in moisture and heat resistance over a long period of time, taking advantage of its characteristics, such as a solar cell backsheet, a planar heating element, or a flat cable, an electrically insulating material, a capacitor material, It can be suitably used for applications such as automobile materials and building materials.

1: Back sheet for solar cell 2: Transparent filler 3: Power generation element 4: Transparent substrate

Claims (12)

A polyester film comprising a polyester layer (P layer) that satisfies the following conditions (1) and (2):
(1) The product of the (100) plane crystallite size, the (−105) plane crystallite size, and the (0-11) plane crystallite size of the polyester of the P layer is from 10 nm ^{3 to} 110 nm ³ (2) The glass transition temperature obtained by temperature-modulated differential scanning calorimetry of the polyester in the P layer is 103 ° C. or higher. The polyester film according to claim 1, wherein the product of the crystal size of the polyester of the P layer is 30 nm ³ or more. 3. The polyester film according to claim 1, wherein a carboxyl group terminal amount change ΔC after heat treatment at 125 ° C. and 100% RH defined by the following formula (3) is 110 equivalent / t or less.
Carboxyl group terminal amount change ΔC (equivalent / t) = C1-C0 (3)
C0: Initial carboxyl group terminal amount of polyester in P layer C1: Carboxyl group terminal amount of polyester in P layer after heat treatment at 125 ° C. and 100% RH for 72 hours The alkali metal element content W1 in the polyester of the P layer is 2.5 ppm or more and 125 ppm or less, and the ratio W1 / W2 of the alkali metal element content W1 and the phosphorus element content W2 is 0.01 or more and 1 or less. Item 4. The polyester film according to any one of Items 1 to 3. The main constituent components of the polyester of the P layer are a dicarboxylic acid constituent component and a diol constituent component, and a constituent component having a tri- or higher functional carboxylic acid group and / or hydroxyl group as a copolymer component is 0.005 with respect to all constituent components. The polyester film according to any one of claims 1 to 4, comprising from mol% to 2.5 mol%. The polyester film according to any one of claims 1 to 5, wherein the crystallinity of the polyester in 1st RUN of differential scanning calorimetry of the P layer is 20% or more and 38% or less. The solar cell backsheet using the polyester film in any one of Claims 1-6. A solar cell using the solar cell backsheet according to claim 7. It is a manufacturing method of the polyester film in any one of Claims 1-6, Comprising: Alkali metal element content W1 is 2.5 ppm or more and 125 ppm or less, and alkali metal element content W1 and phosphorus element content W2 A method for producing a polyester film, comprising: biaxially stretching a sheet containing a layer having a polyester having a ratio W1 / W2 of 0.01 or more and 1 or less under the following condition (4) or (5).
(4) The polyester film is stretched by a simultaneous biaxial stretching method at a glass transition temperature of Tg or higher and Tg + 10 ° C. or lower to an area magnification of 12 times or more. (5) The first biaxial stretching method is used to stretch the first axis of the polyester. Stretching the second axis at a temperature of Tg + 15 ° C. at a glass transition temperature Tg to Tg + 10 ° C., and extending to an area magnification of 13 times or more. The main constituent components of the polyester are a dicarboxylic acid constituent component and a diol constituent component, and a constituent component having a tri- or higher functional carboxylic acid group and / or hydroxyl group as a copolymer component is 0.005 mol% or more based on the total constituent components. The manufacturing method of the polyester film of Claim 9 containing 2.5 mol%. The method for producing a polyester film according to claim 9 or 10, wherein the heat treatment is performed in a range satisfying the following formula (6) after biaxial stretching.
35 ° C. ≦ Tm−Th ≦ 90 ° C. (6)
Tm: Melting point (° C) of polyester resin obtained by differential scanning calorimetry
Th: Heat treatment temperature (° C) The method for producing a polyester film according to any one of claims 9 to 11, wherein the biaxial stretching method is a simultaneous biaxial stretching method.

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