JP6348867B2 - Stretched white polyester film and method for producing the same, solar cell backsheet, and solar cell module - Google Patents

Description

  The present invention relates to a stretched white polyester film and a production method thereof, a solar cell backsheet, and a solar cell module.

  In recent years, photovoltaic power generation that converts sunlight into electricity has attracted attention from the viewpoint of protecting the global environment. A solar cell module used for photovoltaic power generation has a structure in which a sealant / solar cell element / sealant / back surface protective film is laminated in this order on glass on which sunlight is incident.

  Solar cell modules must have high weather resistance so that they can maintain battery performance such as power generation efficiency over a long period of several decades even in harsh usage environments exposed to wind and rain and direct sunlight. It is said. In order to provide such weather resistance performance, a back surface protective film (hereinafter sometimes referred to as “solar cell back sheet” or “back sheet”) constituting the solar cell module or a sealing element for sealing the element. Various materials such as materials are also required to have weather resistance.

  Generally, resin materials such as polyester are used for the back sheet for solar cells. The surface of the polyester film usually contains a large amount of carboxyl groups and hydroxyl groups, tends to undergo hydrolysis in an environment where moisture exists, and tends to deteriorate over time. Therefore, the polyester film used in the solar cell module placed in an environment that is constantly exposed to wind and rain such as outdoors is required to have a hydrolyzable property suppressed.

  Moreover, in order to improve the power generation efficiency by reflecting the light transmitted through the sealing material without being absorbed by the solar cell element by the solar cell backsheet, use a white backsheet to which white particles such as titanium oxide are added. Has been proposed.

  For example, Patent Document 1 has a light reflectance of 50% or more at a wavelength of 550 nm, contains 3 to 50% by mass of inorganic fine particles such as titanium oxide, has an acid value of 1 to 30 eq / ton, and the limit of the film. A solar cell backside sealing polyester film having a viscosity of 0.60 to 0.80 dL / g is disclosed.

  Patent Document 2 includes 0.1 to 10% by mass of titanium oxide fine particles, 0.1 to 10% by mass of end-capping agent, and the like with respect to polyester, and has voids around the fine particles. A line segment (Lb) excluding a portion overlapping the orthogonal projection of the void on the film cross-section of the void existing around one fine particle on the film cross-section of the fine particle and the film cross-section of one fine particle The polyester film which has a polyester layer whose average value of ratio (Lb / Lp) of the long side (Lp) of the orthogonal projection to is 0.5-2 is disclosed.

  Patent Document 3 has a boundary region and two or more constant content regions including a pair of constant content regions facing each other across the boundary region, and the thickness of the boundary region is 0.15 to 3 μm. Yes, at least one of the pair of constant content regions facing each other across the boundary region contains fine particles or voids such as titanium oxide, and the content of the particles or voids in the pair of constant content regions facing each other across the boundary region A white multilayer polyester film containing fine particles or voids having a content ratio between the content ratios of a pair of constant content ratio regions facing each other across the boundary area is disclosed. ing.

International Publication No. 12/008488 JP 2014-25052 A JP 2014-162107 A

In order for a solar cell module to maintain high power generation efficiency over a long period of time, the performance required for a solar cell backsheet includes weather resistance (hydrolysis resistance), adhesion to a sealing material (peeling resistance), and light. Reflective properties are mentioned.
When manufacturing a back sheet having light reflectivity using polyester, generally, white particles (titanium oxide, etc.) are kneaded into polyester for improving light reflectivity, and a sheet for improving weather resistance. The polyester film extruded into a shape is stretched. The interface between the white particles and the polyester peels off due to stretching, and voids are generated. However, when many voids are generated, the strength of the film is greatly reduced when exposed to the outdoors for a long time, and the film is cleaved. As a result, the adhesion to the sealing material is reduced.

For example, in the back sheet disclosed in Patent Document 1, the adhesiveness (peeling resistance) with the sealing material is not taken into consideration.
Further, in the back sheet disclosed in Patent Document 2, no consideration is given to the adhesiveness with the sealing material when exposed to the outside air for a long period of time.
Further, in the back sheet disclosed in Patent Document 3, it is necessary to stack layers having different void amounts, and it is necessary to introduce dedicated equipment.

  The present invention has been made in view of the above circumstances, and is intended to achieve a single layer of stretched white polyester film excellent in weather resistance, light reflectivity, and peel strength, a method for producing the same, and high power generation efficiency over a long period of use. It aims at providing the solar cell backsheet and solar cell module which contribute.

In order to achieve the above object, the following invention is provided.
<1> Polyester and white particles are included, the content of white particles is 2 to 10% by mass with respect to the total mass of the film, and the average area per piece is 0.010 to 0.050 μm in the cross section in the film thickness direction. A stretched white polyester film having ² / piece voids.
<2> The stretched white polyester film according to <1>, wherein the ratio of the total area occupied by voids is 0.5 to 3% in the cross section in the thickness direction of the film.
<3> The stretched white polyester film according to <1> or <2>, wherein P / t is 6.5 to 13.5 mN / μm, where P is the tear strength and t is the thickness of the film.
The unit of P is mN, and the unit of t is μm.
The stretched white polyester film as described in any one of <1>-<3> whose <4> film thickness is 280-500 micrometers.
<5> The stretched white polyester film according to any one of <1> to <4>, wherein the intrinsic viscosity of the film is 0.65 to 0.85 dL / g.
<6> The stretched white polyester according to any one of <1> to <5>, wherein the white particles are titanium oxide.
<7> A solar cell backsheet comprising the stretched white polyester film according to any one of <1> to <6>.
<8> The solar cell backsheet according to <7>, which has an application layer on at least one surface of the stretched white polyester film.
<9> a solar cell element;
A sealing material for sealing the solar cell element;
A front substrate disposed outside the sealing material on the light-receiving surface side of the solar cell element;
The solar cell backsheet according to <7> or <8>, disposed on the side opposite to the light receiving surface side of the solar cell element and outside the sealing material;
Including solar cell module.
<10> A method for producing a stretched white polyester film according to any one of <1> to <6>,
An extrusion process in which a mixture containing raw material polyester and white particles is melt-extruded and then cooled to form an unstretched white polyester film;
A longitudinal stretching step of stretching an unstretched white polyester film in the longitudinal direction and a transverse stretching step of stretching in the width direction,
The transverse stretching step includes first to Nth transverse stretching steps, the nth transverse stretching step is performed continuously to the n-1 transverse stretching step, and the nth transverse stretching step is the n-1 transverse stretching step. The stretching speed with respect to the width direction of the white polyester film is increased more than the length of the white polyester film in the width direction of the white polyester film before the first transverse stretching process is started. A method for producing a stretched white polyester film having a stretching speed that increases the length in the width direction by 8 to 25% / second.
N is an integer of 2 or more, and n is an integer of 2 to N.
<11> The method for producing a stretched white polyester film according to <10>, wherein a stretching speed in the width direction of the first lateral stretching step is 4 to 10% / second.
<12> When the stretching speed in the width direction of the first transverse stretching step is Sa and the stretching speed in the width direction of the Nth lateral stretching step is Sb, the value of the stretching speed ratio Sb / Sa is 1.5 to 6. <10> or <11>, wherein the stretched white polyester film is produced.

  According to the present invention, a single-layer stretched white polyester film having excellent weather resistance, light reflectivity, and peel resistance, a method for producing the same, and a solar cell backsheet that contributes to achieving high power generation efficiency over a long period of use, and A solar cell module can be provided.

It is the schematic which shows an example of the biaxial stretching machine used for manufacture of the extending | stretching white polyester film of this invention. It is the schematic which shows an example of the extending | stretching aspect of the white polyester film in a horizontal stretch process in the manufacturing process of the stretched white polyester film of this invention.

Hereinafter, although embodiment of this invention is described, the following embodiment is an example of this invention and does not limit this invention.
In the specification of the present application, “to” indicating a numerical range is used in a sense including numerical values described before and after the numerical value as a lower limit value and an upper limit value. In addition, when only the upper limit value is described in the numerical range, it means that the lower limit value is also in the same unit as the upper limit value.

[Stretched white polyester film]
The stretched white polyester film of the present invention (hereinafter sometimes referred to as “white polyester film”, “polyester film”, or “film”) contains polyester and white particles, and the content of white particles relative to the total mass of the film. 2 to 10% by mass, and in the cross section in the film thickness direction, the average area per unit is voids of 0.010 to 0.050 μm ² / piece.

The stretched white polyester film of the present invention is excellent in weather resistance, reflectivity, and peel resistance. The reason is presumed as follows.
In the stretched polyester film mixed with white particles, it is considered that voids generated by “stretching” particularly for imparting weather resistance are reducing the strength of the film. As a result of repeated studies by the present inventor, when the void is too small, that is, stretching is insufficient, sufficient weather resistance cannot be imparted, and conversely, when the void is too large, it is exposed to the outside air for a long period of time. It has been found that moisture easily penetrates into the void, hydrolysis is accelerated, and the film is easily cleaved.

On the other hand, the stretched white polyester film of the present invention can obtain high light reflectivity when the content of white particles is 2% by mass or more, and maintains high strength when it is 10% by mass or less. be able to.
Moreover, the void which exists in the extending | stretching white polyester film of this invention is fully extended | stretched because the average area per piece in the cross section of a film thickness direction is 0.010 micrometer < ² > / piece or more, and has high weather resistance. On the other hand, since it is 0.050 μm ² / piece or less, many large voids do not exist in the film, and it is considered that the film is not easily cleaved. Therefore, even when exposed to the outside air for a long period of time, the adhesion (separation resistance) with other layers such as a sealing material is unlikely to decrease.

  Furthermore, as a result of repeated research by the present inventors, the raw material polyester mixed with white particles is melt-extruded to form an unstretched white polyester film, and then stretched to produce a stretched white polyester film with proper stretching conditions. By controlling to the range, the size of voids formed in the film can be controlled, and the single-layer stretched white polyester film of the present invention can be easily produced.

(polyester)
The kind in particular of polyester contained in the extending | stretching white polyester film of this invention is not restrict | limited, A well-known polyester can be used.
For example, the linear saturated polyester synthesize | combined from an aromatic dibasic acid or its ester-forming derivative, and diol or its ester-forming derivative is mentioned. Specific examples of the linear saturated polyester include polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, poly (1,4-cyclohexylenedimethylene terephthalate), polyethylene-2,6-naphthalate, and the like. Of these, polyethylene terephthalate, polyethylene-2,6-naphthalate, poly (1,4-cyclohexylenedimethylene terephthalate) and the like are particularly preferable from the viewpoint of the balance between mechanical properties and cost.

  The polyester may be a homopolymer or a copolymer. Furthermore, the white polyester film of the present invention may be a resin component obtained by blending a small amount of other types of resins such as polyimide with polyester.

  The type of polyester is not limited to the above, and other polyesters may be used. For example, a polyester synthesized using a dicarboxylic acid component and a diol component may be used, or a commercially available polyester may be used.

  When the polyester is synthesized, for example, (a) a dicarboxylic acid component and (b) a diol component can be obtained by performing at least one of an esterification reaction and a transesterification reaction by a known method.

  (A) As the dicarboxylic acid component, for example, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosandioic acid, pimelic acid, azelaic acid, methylmalonic acid Aliphatic dicarboxylic acids such as ethylmalonic acid; Alicyclic dicarboxylic acids such as adamantane dicarboxylic acid, norbornene dicarboxylic acid, cyclohexane dicarboxylic acid, decalin dicarboxylic acid; terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 4,4′-diphenyletherdicarboxylic acid, 5-sodium sulfoisophthalic acid, Phenylindanedicarboxylic , Anthracene dicarboxylic acid, phenanthrene carboxylic acid, 9,9'-bis (4-carboxyphenyl) aromatic dicarboxylic acids such as fluorene acid; dicarboxylic acids or their ester derivatives, and the like.

  (B) Examples of the diol component include fats such as 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) ) Aromatic diols such as fluorene;

  (A) It is preferable to use at least one aromatic dicarboxylic acid as the dicarboxylic acid component. More preferably, the dicarboxylic acid component contains an aromatic dicarboxylic acid as a main component. Here, the “main component” means that the proportion of aromatic dicarboxylic acid in the dicarboxylic acid component is 80% by mass or more. A dicarboxylic acid component other than the aromatic dicarboxylic acid may be included. Examples of such a dicarboxylic acid component include ester derivatives such as aromatic dicarboxylic acids.

  (B) It is preferable to use at least one aliphatic diol as the diol component. The aliphatic diol can contain ethylene glycol, and preferably contains ethylene glycol as a main component. Here, the main component means that the proportion of ethylene glycol in the diol component is 80% by mass or more.

  The amount of the aliphatic diol (for example, ethylene glycol) used may be in the range of 1.015 to 1.50 moles per mole of the aromatic dicarboxylic acid (for example, terephthalic acid) and, if necessary, its ester derivative. preferable. The amount of the aliphatic diol used is more preferably in the range of 1.02 to 1.30 mol, and still more preferably in the range of 1.025 to 1.10 mol. When the amount of the aliphatic diol used is in the range of 1.015 mol or more, the esterification reaction proceeds well, and in the range of 1.50 mol or less, for example, a by-product of diethylene glycol by dimerization of ethylene glycol. It is possible to keep a large number of properties such as melting point, glass transition temperature, crystallinity, heat resistance, hydrolysis resistance, weather resistance and the like.

  For the esterification reaction or transesterification reaction, a known reaction catalyst can be used. Examples of the reaction catalyst include alkali metal compounds, alkaline earth metal compounds, zinc compounds, lead compounds, manganese compounds, cobalt compounds, aluminum compounds, antimony compounds, titanium compounds, and phosphorus compounds. Usually, it is preferable to add an antimony compound, a germanium compound, a titanium compound or the like as a polymerization catalyst at an arbitrary stage before the production of the polyester is completed. As such a method, for example, when a germanium compound is taken as an example, it is preferable to add the germanium compound powder as it is.

  For example, in the esterification reaction step, an aromatic dicarboxylic acid and an aliphatic diol are polymerized in the presence of a catalyst containing a titanium compound. In this esterification reaction, an organic chelate titanium complex having an organic acid as a ligand is used as a catalyst titanium compound, and at least an organic chelate titanium complex, a magnesium compound, and an aromatic ring as a substituent are used in the process. It is preferable to provide a process of adding a pentavalent phosphate ester which is not included in this order.

  Specifically, in the esterification reaction step, first, an aromatic dicarboxylic acid and an aliphatic diol are mixed with a catalyst containing an organic chelate titanium complex, which is a titanium compound, prior to the addition of the magnesium compound and the phosphorus compound. Titanium compounds such as organic chelate titanium complexes have high catalytic activity for esterification reactions, so that esterification reactions can be performed satisfactorily. At this time, the titanium compound may be added to the mixture of the aromatic dicarboxylic acid component and the aliphatic diol component, or the aliphatic diol after mixing the aromatic dicarboxylic acid component (or aliphatic diol component) and the titanium compound. You may mix a component (or aromatic dicarboxylic acid component). Moreover, you may make it mix an aromatic dicarboxylic acid component, an aliphatic diol component, and a titanium compound simultaneously. There is no restriction | limiting in particular in a mixing method, It can carry out by a well-known method.

Here, in the polymerization of the polyester, it is also preferable to add the following compound.
As the pentavalent phosphorus compound, at least one pentavalent phosphate having no aromatic ring as a substituent is used. For example, a phosphoric acid ester having a lower alkyl group having 2 or less carbon atoms as a substituent [(OR) ₃ —P═O; R = an alkyl group having 1 or 2 carbon atoms], specifically, phosphoric acid Trimethyl, triethyl phosphate and the like are particularly preferable.

  As an addition amount of a phosphorus compound, the quantity from which P element conversion value becomes the range of 50 ppm-90 ppm is preferable. The amount of the phosphorus compound is more preferably 60 ppm to 80 ppm, and even more preferably 60 ppm to 75 ppm.

By including a magnesium compound in the polyester, the electrostatic applicability of the polyester is improved.
Examples of the magnesium compound include magnesium salts such as magnesium oxide, magnesium hydroxide, magnesium alkoxide, magnesium acetate, and magnesium carbonate. Among these, magnesium acetate is most preferable from the viewpoint of solubility in ethylene glycol.

  The amount of the magnesium compound added is preferably such that the Mg element conversion value is 50 ppm or more, and more preferably in the range of 50 ppm to 100 ppm, in order to impart high electrostatic applicability. The addition amount of the magnesium compound is preferably an amount that is in the range of 60 ppm to 90 ppm, more preferably an amount that is in the range of 70 ppm to 80 ppm, in terms of imparting electrostatic applicability.

In the esterification reaction step, the titanium compound as the catalyst component and the magnesium compound and phosphorus compound as the additive are so calculated that the value Z calculated from the following formula (i) satisfies the following relational expression (ii). It is particularly preferred to add and melt polymerize. Here, the P content is the amount of phosphorus derived from the entire phosphorus compound including the pentavalent phosphate ester having no aromatic ring, and the Ti content is the amount of titanium derived from the entire Ti compound including the organic chelate titanium complex. It is. Thus, by selecting the combined use of the magnesium compound and the phosphorus compound in the catalyst system containing the titanium compound, and controlling the timing and ratio of addition, the yellow color while maintaining the catalytic activity of the titanium compound moderately high A color tone with less taste can be obtained, and heat resistance that hardly causes yellowing can be imparted even when exposed to high temperatures during polymerization reaction or subsequent film formation (during melting).
(I) Z = 5 × (P content [ppm] / P atomic weight) −2 × (Mg content [ppm] / Mg atomic weight) −4 × (Ti content [ppm] / Ti atomic weight)
(Ii) 0 ≦ Z ≦ 5.0

This is an index for quantitatively expressing the balance between the three because the phosphorus compound not only acts on titanium but also interacts with the magnesium compound.
Formula (i) expresses the amount of phosphorus that can act on titanium by excluding the phosphorus content that acts on magnesium from the total amount of phosphorus that can be reacted. When the value Z is positive, it can be said that there is an excess of phosphorus that inhibits titanium, and conversely, when it is negative, there is a shortage of phosphorus necessary to inhibit titanium. In the reaction, since each atom of Ti, Mg, and P is not equivalent, each mole number in the formula is weighted by multiplying by a valence.

  Polyester synthesis does not require special synthesis, etc., and is inexpensive and easily available using titanium compounds, such phosphorus compounds, and magnesium compounds, while having the reaction activity required for the reaction. A polyester excellent in color tone and coloration resistance to heat can be obtained.

  In the formula (ii), it is preferable that 1.0 ≦ Z ≦ 4.0 is satisfied from the viewpoint of further enhancing the color tone and heat resistance with heat while maintaining the polymerization reactivity, and 1.5 ≦ Z ≦ 3. The case where 0 is satisfied is more preferable.

  As a preferred embodiment of the esterification reaction step, before the esterification reaction is completed, a chelate titanium complex having 1 ppm to 30 ppm of citric acid or citrate as a ligand is added to the aromatic dicarboxylic acid and the aliphatic diol. It is good to add. Thereafter, in the presence of the chelate titanium complex, 60 ppm to 90 ppm (more preferably 70 ppm to 80 ppm) of a weak acid magnesium salt is added, and after the addition, 60 ppm to 80 ppm (more preferably 65 ppm to 75 ppm) of fragrance. It is preferable to add a pentavalent phosphate that does not have a ring as a substituent.

  The esterification reaction step should be carried out using a multistage apparatus in which at least two reactors are connected in series under conditions where ethylene glycol is refluxed while removing water or alcohol produced by the reaction out of the system. Can do.

The esterification reaction process may be performed in one stage or may be performed in multiple stages.
When the esterification reaction step is performed in one step, the esterification reaction temperature is preferably 230 ° C to 260 ° C, more preferably 240 ° C to 250 ° C.
When the esterification reaction step is performed in multiple stages, the temperature of the esterification reaction in the first reaction tank is preferably 230 ° C to 260 ° C, more preferably 240 ° C to 250 ° C, and the pressure is 1.0 kg / cm. ^{2 ~5.0kg} / cm ^2, and more preferably from ^{2.0kg / cm 2 ~3.0kg / cm 2} . The temperature of the esterification reaction in the second reaction tank is preferably 230 ° C. to 260 ° C., more preferably 245 ° C. to 255 ° C., and the pressure is 0.5 kg / cm ^{2 to} 5.0 kg / cm ² , more preferably 1. 0.0 kg / cm ^{2 to} 3.0 kg / cm ² . Furthermore, when carrying out by dividing into three or more stages, it is preferable to set the conditions for the esterification reaction in the intermediate stage to the conditions between the first reaction tank and the final reaction tank.

  On the other hand, a polycondensation product is produced by subjecting the esterification reaction product produced by the esterification reaction to a polycondensation reaction. The polycondensation reaction may be performed in one stage or may be performed in multiple stages.

  The esterification reaction product such as an oligomer generated by the esterification reaction is subsequently subjected to a polycondensation reaction. This polycondensation reaction can be suitably performed by supplying it to a multistage polycondensation reaction tank.

For example, the polycondensation reaction conditions in the case of performing in a three-stage reaction tank are as follows: the first reaction tank has a reaction temperature of 255 ° C. to 280 ° C., more preferably 265 ° C. to 275 ° C., and a pressure of 100 to 10 torr (13 ^{.3 × 10 -3 MPa~1.3 × 10 -3} MPa), more preferably a ^{50torr~20torr (6.67 × 10 -3 MPa~2.67 ×} 10 -3 MPa), the second reaction The tank has a reaction temperature of 265 ° C. to 285 ° C., more preferably 270 ° C. to 280 ° C., and a pressure of 20 to 1 torr (2.67 × 10 ⁻³ MPa to 1.33 × 10 ⁻⁴ MPa), more preferably. is a ^{10tor~3torr (1.33 × 10 -3 MPa~4.0 ×} 10 -4 MPa), a third reaction vessel in the final reaction tank, the reaction temperature is 270 ° C. to 290 ° C. More preferably from 275 ° C. to 285 ° C., a pressure ^{10torr~0.1torr (1.33 × 10 -3 MPa~1.33 ×} 10 -5 MPa), more preferably 5torr~0.5torr (6.67 The aspect which is * 10 < ^-4 > MPa-6.67 * 10 < ^-5 > MPa) is preferable.

  Additives such as light stabilizers, antioxidants, UV absorbers, flame retardants, lubricants (fine particles), nucleating agents (crystallization agents), crystallization inhibitors, etc. to the polyester synthesized as described above May further be included.

In the synthesis of polyester, it is preferable to perform solid phase polymerization after polymerization by esterification reaction. By solid-phase polymerization, it is possible to control the moisture content of the polyester, the crystallinity, the acid value of the polyester, that is, the concentration of the terminal carboxyl group of the polyester, and the intrinsic viscosity.
In particular, the ethylene glycol (EG) gas concentration at the start of solid phase polymerization is preferably higher in the range of 200 ppm to 1000 ppm than the EG gas concentration at the end of solid phase polymerization, more preferably 250 ppm to 800 ppm, and even more preferably 300 ppm. It is preferable to carry out solid phase polymerization by increasing the concentration in the range of ˜700 ppm. At this time, AV (terminal COOH concentration) can be controlled by adding an average EG gas concentration (average gas concentration at the start and end of solid-phase polymerization). That is, AV can be reduced by reaction with terminal COOH by adding EG. EG is preferably 100 ppm to 500 ppm, more preferably 150 ppm to 450 ppm, and still more preferably 200 ppm to 400 ppm.

Moreover, the temperature of solid state polymerization is preferably 180 ° C to 230 ° C, more preferably 190 ° C to 215 ° C, and further preferably 195 ° C to 209 ° C.
The solid phase polymerization time is preferably 10 hours to 40 hours, more preferably 14 hours to 35 hours, and still more preferably 18 hours to 30 hours.

Here, the polyester preferably has high hydrolysis resistance. Therefore, the carboxyl group content in the polyester is preferably 50 equivalent / t or less (where t means ton, where ton means 1000 kg), and more preferably 35 equivalent / t or less, More preferably, it is 20 equivalent / t or less. When the carboxyl group content is 50 equivalents / t or less, hydrolysis resistance can be maintained, and a decrease in strength when subjected to wet heat aging can be suppressed to be small. The lower limit of the carboxyl group content is 2 equivalents / t, more preferably 3 equivalents / t, and even more preferably 3 equivalents / t, from the viewpoint of maintaining adhesion between the layer formed on the polyester (for example, a resin layer). Is desirable.
The carboxyl group content in the polyester can be adjusted by polymerization catalyst species, film forming conditions (film forming temperature and time), solid phase polymerization, and additives (end-capping agent, etc.).

(End sealant)
The white polyester film of the present invention can be further improved in hydrolysis resistance (weather resistance) by adding a terminal blocking agent.

  The white polyester film of this invention can contain 0.1-10 mass% terminal blocker with respect to the total mass of polyester. The added amount of the terminal blocking agent with respect to the total mass of the polyester contained in the polyester film is more preferably 0.2 to 5% by mass, and still more preferably 0.3 to 2% by mass.

Since the hydrolysis of polyester is accelerated by the catalytic effect of H ⁺ generated from the carboxyl group at the end of the molecule, an end-capping agent that reacts with the terminal carboxyl group is used to improve hydrolysis resistance (weather resistance). It is effective to add.
If the added amount of the end-capping agent is 0.1% by mass or more with respect to the total mass of the polyester, the effect of improving the weather resistance is easily exhibited. Is suppressed, and the decrease in mechanical strength and heat resistance is suppressed.

  Examples of the end capping agent include an epoxy compound, a carbodiimide compound, an oxazoline compound, and a carbonate compound, and carbodiimide having high affinity with polyethylene terephthalate (PET) and high end capping ability is preferable.

It is preferable that terminal blocker (especially carbodiimide terminal blocker) is high molecular weight. This can reduce volatilization during melt film formation. The molecular weight of the end-capping agent is preferably 200 to 100,000, more preferably 2000 to 80,000, still more preferably 10,000 to 50,000. If the molecular weight of the end-capping agent (particularly carbodiimide end-capping agent) is in the range of 200 to 100,000, it is easy to uniformly disperse in the polyester, and the effect of improving weather resistance can be sufficiently exhibited. Moreover, it is difficult to evaporate during extrusion and film formation, and it becomes easy to express the effect of improving weather resistance.
In addition, the molecular weight of terminal blocker means a weight average molecular weight.

Carbodiimide end-capping agent:
The carbodiimide compound having a carbodiimide group includes a monofunctional carbodiimide and a polyfunctional carbodiimide. Examples of the monofunctional carbodiimide include dicyclohexylcarbodiimide, diisopropylcarbodiimide, dimethylcarbodiimide, diisobutylcarbodiimide, dioctylcarbodiimide, t-butylisopropylcarbodiimide, and diphenylcarbodiimide. , Di-t-butylcarbodiimide, di-β-naphthylcarbodiimide, and the like. Particularly preferred are dicyclohexylcarbodiimide and diisopropylcarbodiimide.

  As the polyfunctional carbodiimide, carbodiimide having a polymerization degree of 3 to 15 is preferably used. Specifically, 1,5-naphthalene carbodiimide, 4,4′-diphenylmethane carbodiimide, 4,4′-diphenyldimethylmethane carbodiimide, 1,3-phenylene carbodiimide, 1,4-phenylene diisocyanate, 2,4-tolylene Carbodiimide, 2,6-tolylene carbodiimide, mixture of 2,4-tolylene carbodiimide and 2,6-tolylene carbodiimide, hexamethylene carbodiimide, cyclohexane-1,4-carbodiimide, xylylene carbodiimide, isophorone carbodiimide, isophorone carbodiimide, Dicyclohexylmethane-4,4′-carbodiimide, methylcyclohexanecarbodiimide, tetramethylxylylene carbodiimide, 2,6-diisopropylphenylcarbodiimide and 1 It can be exemplified 3,5-triisopropyl-2,4-carbodiimide.

  The carbodiimide compound is preferably a carbodiimide compound having high heat resistance because an isocyanate gas is generated by thermal decomposition. In order to improve heat resistance, it is preferable that the molecular weight (degree of polymerization) is high, and it is more preferable that the terminal of the carbodiimide compound has a structure with high heat resistance. Further, once thermal decomposition occurs, further thermal decomposition is likely to occur. Therefore, it is necessary to devise measures such as setting the extrusion temperature of the polyester as low as possible.

  The terminal blocker carbodiimide is also preferably a carbodiimide having a cyclic structure (for example, a carbodiimide having a cyclic structure described in JP 2011-153209 A). Even if the carbodiimide having a cyclic structure has a low molecular weight, the same effect as that of the above high molecular weight carbodiimide is exhibited. This is because the terminal carboxyl group of the polyester and the cyclic carbodiimide undergo a ring-opening reaction, one reacts with this polyester, and the other with the ring-opening reacts with another polyester to increase the molecular weight, so that an isocyanate gas is generated. This is because it is suppressed.

Among carbodiimides having a cyclic structure, in the present invention, the terminal blocking agent is a carbodiimide compound having a carbodiimide group and a cyclic structure in which the first nitrogen and the second nitrogen are bonded by a bonding group. Is preferred. Further, the end capping agent has a cyclic structure in which at least one carbodiimide group adjacent to the aromatic ring is present, and the first nitrogen and the second nitrogen of the carbodiimide group adjacent to the aromatic ring are bonded by a bonding group. More preferred is carbodiimide (also referred to as aromatic cyclic carbodiimide).
The aromatic cyclic carbodiimide may have a plurality of cyclic structures.
The aromatic cyclic carbodiimide is preferably an aromatic carbodiimide having no ring structure in which the first nitrogen and the second nitrogen of two or more carbodiimide groups are bonded by a linking group in the molecule, that is, a monocyclic ring. Can be used.

  The cyclic structure has one carbodiimide group (—N═C═N—), and the first nitrogen and the second nitrogen are bonded by a bonding group. One cyclic structure has only one carbodiimide group. For example, when a molecule has a plurality of cyclic structures such as a spiro ring, one cyclic structure bonded to a spiro atom is included in each cyclic structure. As long as it has a carbodiimide group, the compound may have a plurality of carbodiimide groups. The number of atoms in the cyclic structure is preferably 8 to 50, more preferably 10 to 30, further preferably 10 to 20, and particularly preferably 10 to 15.

  Here, the number of atoms in the cyclic structure means the number of atoms that directly constitute the cyclic structure. For example, it is 8 for an 8-membered ring and 50 for a 50-membered ring. If the number of atoms in the cyclic structure is 8 or more, the stability of the cyclic carbodiimide compound increases, and storage and use become easy. Moreover, although there is no special restriction | limiting regarding the upper limit of the number of ring members from a reactive viewpoint, the cyclic carbodiimide compound of 50 or less atoms has little difficulty of synthesis | combination, and can suppress cost low. From this viewpoint, the number of atoms in the cyclic structure is preferably 10 to 30, more preferably 10 to 20, and particularly preferably 10 to 15.

  Specific examples of the carbodiimide-based end capping agent having a cyclic structure include the following compounds. However, the present invention is not limited to the following specific examples.

Epoxy end sealant:
Preferable examples of the epoxy compound include glycidyl ester compounds and glycidyl ether compounds.

  Specific examples of the glycidyl ester compound include benzoic acid glycidyl ester, t-Bu-benzoic acid glycidyl ester, P-toluic acid glycidyl ester, cyclohexanecarboxylic acid glycidyl ester, pelargonic acid glycidyl ester, stearic acid glycidyl ester, and lauric acid glycidyl ester. , Glycidyl palmitate, glycidyl behenate, glycidyl versatate, glycidyl oleate, glycidyl linoleate, glycidyl linolein, glycidyl behenol, glycidyl stearol, diglycidyl terephthalate, isophthalic acid Diglycidyl ester, phthalic acid diglycidyl ester, naphthalenedicarboxylic acid diglycidyl ester Ter, methylterephthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, cyclohexanedicarboxylic acid diglycidyl ester, adipic acid diglycidyl ester, succinic acid diglycidyl ester, sebacic acid diglycidyl ester, dodecane Examples thereof include diglycidyl diacid, octadecanedicarboxylic acid diglycidyl ester, trimellitic acid triglycidyl ester, pyromellitic acid tetraglycidyl ester, and the like. One or more of these can be used.

  Specific examples of the glycidyl ether compound include phenyl glycidyl ether, O-phenyl glycidyl ether, 1,4-bis (β, γ-epoxypropoxy) butane, 1,6-bis (β, γ-epoxypropoxy). ) Hexane, 1,4-bis (β, γ-epoxypropoxy) benzene, 1- (β, γ-epoxypropoxy) -2-ethoxyethane, 1- (β, γ-epoxypropoxy) -2-benzyloxyethane 2,2-bis- [р- (β, γ-epoxypropoxy) phenyl] propane, 2,2-bis- (4-hydroxyphenyl) propane, 2,2-bis- (4-hydroxyphenyl) methane, etc. Bisglycidyl polyether obtained by the reaction of bisphenol and epichlorohydrin, and the like, and these may be used alone or in combination of two or more. It kills.

Oxazoline-based end-capping agent:
As the oxazoline compound, a bisoxazoline compound is preferable, and specifically, 2,2′-bis (2-oxazoline), 2,2′-bis (4-methyl-2-oxazoline), and 2,2′-bis. (4,4-dimethyl-2-oxazoline), 2,2′-bis (4-ethyl-2-oxazoline), 2,2′-bis (4,4′-diethyl-2-oxazoline), 2,2 '-Bis (4-propyl-2-oxazoline), 2,2'-bis (4-butyl-2-oxazoline), 2,2'-bis (4-hexyl-2-oxazoline), 2,2'- Bis (4-phenyl-2-oxazoline), 2,2'-bis (4-cyclohexyl-2-oxazoline), 2,2'-bis (4-benzyl-2-oxazoline), 2,2'-p- Phenylenebis (2-oxazoline), 2,2'-m- Enylene bis (2-oxazoline), 2,2'-o-phenylene bis (2-oxazoline), 2,2'-p-phenylene bis (4-methyl-2-oxazoline), 2,2'-p-phenylene bis (4,4-dimethyl-2-oxazoline), 2,2′-m-phenylenebis (4-methyl-2-oxazoline), 2,2′-m-phenylenebis (4,4-dimethyl-2-oxazoline) ), 2,2′-ethylenebis (2-oxazoline), 2,2′-tetramethylenebis (2-oxazoline), 2,2′-hexamethylenebis (2-oxazoline), 2,2′-octamethylene Bis (2-oxazoline), 2,2′-decamethylenebis (2-oxazoline), 2,2′-ethylenebis (4-methyl-2-oxazoline), 2,2′-tetramethylenebis (4,4 - Methyl-2-oxazoline), 2,2′-9,9′-diphenoxyethanebis (2-oxazoline), 2,2′-cyclohexylenebis (2-oxazoline), 2,2′-diphenylenebis ( 2-oxazoline) and the like. Among these, 2,2′-bis (2-oxazoline) is most preferably used from the viewpoint of reactivity with polyester. Furthermore, as long as the objective of this invention is achieved, the bisoxazoline compound mentioned above may be used individually by 1 type, or may use 2 or more types together.

  Even if such a terminal blocker is added to the resin layer on the polyester film, for example, the polyester and the terminal blocker do not react, so when the polyester film is produced, it is kneaded and directly reacted with the polyester molecule. is necessary.

(White particles)
The white polyester film of the present invention contains 2 to 10% by mass of white particles with respect to the total mass of the film. High light reflectivity can be obtained when the content of the white particles contained in the white polyester film of the present invention is 2% by mass or more, and high strength can be maintained when the content is 10% by mass or less.
From this viewpoint, the content of the white particles is preferably 2 to 10% by mass, more preferably 2.5 to 8.5% by mass with respect to the white polyester film.

The content of white particles contained in the white polyester film can be measured by the following method.
3 g of a film is taken as a measurement sample in a crucible and heated at 900 ° C. for 120 minutes in an electric oven. Then, after the electric oven has cooled, the crucible is taken out and the mass of ash remaining in the crucible is measured. This ash content is white particle content, and the mass obtained by dividing the mass of the ash by the mass of the measurement sample and multiplying by 100 is defined as the content of the white particles.
In addition, if it is before manufacture of a film, you may obtain | require content from the addition amount of the white particle (white pigment) used as a raw material.

The average particle size of the white particles is preferably 0.03 to 0.25 μm, more preferably 0.07 to 0.25 μm, and still more preferably 0.1 to 0.2 μm. If the average particle size of the particles is 0.03 to 0.25 μm, the whiteness of the film is improved, and the average area per cross section of the film derived from the white particles is 0.010 to 0.050 μm ^2. / Voids are easy to form.

The average particle diameter of the white particles contained in the white polyester film in the present invention is determined by an electron microscope method. Specifically, the following method is used.
The white particles in the cross section in the thickness direction of the film are observed with a scanning electron microscope, the magnification is appropriately changed according to the size of the particles, and the photographed image is enlarged and copied. The circumference of each particle is traced for at least 200 randomly selected particles. The equivalent circle diameter of the particles is measured from these trace images with an image analysis apparatus, and the average value of these is taken as the average particle diameter.
In addition, if it is before manufacture of a film, you may obtain | require an average particle diameter like the above about at least 200 particle | grains chosen at random from the white particle (white pigment) used as a raw material.

  Examples of the white particles contained in the white polyester film of the present invention include wet and dry silica, colloidal silica, calcium carbonate, aluminum silicate, calcium phosphate, alumina, magnesium carbonate, zinc carbonate, titanium oxide, zinc oxide (zinc white), Antimony oxide, cerium oxide, zirconium oxide, tin oxide, lanthanum oxide, magnesium oxide, barium carbonate, zinc carbonate, basic lead carbonate (lead white), barium sulfate, calcium sulfate, lead sulfate, zinc sulfide, mica, titanium mica, Talc, clay, kaolin, lithium fluoride, calcium fluoride and the like can be used. From the viewpoints of cost and availability, titanium oxide and barium sulfate are preferable. The particle surface may be subjected to an inorganic treatment such as alumina or silica, or may be subjected to an organic treatment such as silicone or alcohol.

  Among these, titanium oxide is preferable, and by using titanium oxide, excellent durability can be achieved even under light reflectivity and light irradiation.

  Titanium oxide includes rutile type and anatase type, but the white polyester film in the present invention is preferably whitened by adding titanium oxide particles mainly composed of rutile type. The anatase type has a very high spectral reflectance of ultraviolet rays, whereas the rutile type has a characteristic that the absorption rate of ultraviolet rays is large (spectral reflectance is small). By paying attention to the difference in spectral characteristics in the crystal form of titanium oxide and utilizing the rutile-type ultraviolet ray absorption performance, the light resistance (deterioration durability due to ultraviolet rays) can be improved in the solar cell backsheet. Thereby, it is excellent in the film durability under light irradiation, even if it does not add another ultraviolet absorber substantially. For this reason, it is difficult for the ultraviolet absorber to be contaminated by the bleed-out and to deteriorate the adhesiveness.

In addition, as described above, the titanium oxide particles according to the present invention are preferably mainly composed of rutile type. However, the term “main body” as used herein means that the amount of rutile type titanium oxide in all titanium oxide particles is 50% by mass. It means that it is over.
Moreover, it is preferable that the amount of anatase type titanium oxide in all the titanium oxide particles is 10 mass% or less. More preferably, it is 5 mass% or less, Most preferably, it is 0 mass%. Rutile titanium oxide and anatase titanium oxide can be distinguished by X-ray structural diffraction or spectral absorption characteristics.

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

  In the present invention, white particles of an organic material can also be used. As the white particles of the organic material, those that can withstand the heat during polyester film formation are preferable. For example, those made of a cross-linked resin are used, and specifically, polystyrene cross-linked with divinylbenzene is used.

  The white polyester film of the present invention may contain one type or two or more types of white particles. When 2 or more types of white particles are included, the total content of the white particles is 2 to 10% by mass.

(void)
The white polyester film of this invention has the void (cavity) whose average area per piece is 0.010-0.050micrometer < ² > / piece in the cross section of a film thickness direction. The voids in the white polyester film of the present invention are formed by exfoliation at the interface between the white particles and the polyester by stretching for imparting weather resistance, and the void area including the part of the white particles present in the cavity. And

When the average area per void contained in the film is 0.010 μm ² / piece or more, it can be sufficiently stretched to give high weather resistance, and when it is 0.050 μm ² / piece or less, it can be used for a long time. When exposed to the outside air, the amount of moisture intruded into the void can be reduced to suppress hydrolysis and to prevent the film from being cleaved.
From this viewpoint, the average area of voids per unit in the film thickness direction of the cross section is preferably 0.01 to 0.05 [mu] m ² / number, to be 0.02~0.05μm ² / Pieces More preferred.

In the cross section in the thickness direction of the film, the ratio of the total area occupied by voids (void occupation area) is preferably 0.5 to 3%. If the void ratio in the film is 0.5% or more, the film can be sufficiently stretched to give high weather resistance, and the light reflectivity is sufficient. On the other hand, if the void ratio in the film is 3% or less, hydrolysis can be suppressed and cleavage of the film can be prevented from occurring when exposed to the outside air for a long time.
From this viewpoint, the total area (occupied area) occupied by voids in the cross section in the thickness direction of the film is more preferably 0.6 to 3%, and further preferably 0.6 to 2.8%. .

  The fine cavities (voids) present in the white polyester film of the present invention can be mainly formed from white particles. The void derived from the white particles means that there are cavities around the white particles, and can be confirmed by, for example, a cross-sectional photograph of a white polyester film using an electron microscope. In addition, there are voids in which primary particles of two or more white particles aggregate and voids are formed around the aggregated particles, and voids in which no white particles are present in the voids during the observation work. To do.

  The method described in the Examples is used as a method for measuring the occupied area of voids contained in the white polyester film of the present invention and the average area per void.

  The white polyester film of the present invention preferably uses a fluorescent brightener such as thiofediyl to increase the whiteness. A preferable amount of the optical brightener added is 0.01% by mass or more and 1% by mass or less, more preferably 0.05% by mass or more and 0.5% by mass or less, and further preferably 0.1% by mass or more and 0.3% by mass or less. It is below mass%. Within this range, it is easy to obtain an effect of improving the light reflectivity, yellowing due to thermal decomposition during extrusion is suppressed, and a decrease in reflectivity is suppressed. As such a fluorescent whitening agent, for example, OB-1 manufactured by Eastman Kodak Company can be used.

(Thickness)
The thickness of the white polyester film of the present invention is preferably 280 to 500 μm. If the thickness of the film is 280 μm or more, it can have high voltage resistance. On the other hand, if the thickness of the film is 500 μm or less, a decrease in hydrolysis resistance due to a decrease in the heating / cooling ability of the film during film formation is suppressed, and the film is stretched without placing a high load on the stretching machine. It can be carried out.
From this viewpoint, the thickness of the film is more preferably 290 to 450 μm.
The method described in the Examples is used as the method for measuring the thickness of the white polyester film of the present invention.

(Intrinsic viscosity)
The white polyester film of the present invention preferably has an intrinsic viscosity (IV) of 0.65 to 0.85 dL / g.
If the IV of the film is 0.65 dL / g or more, sufficient weather resistance can be obtained. On the other hand, if the IV of the film is 0.85 dL / g or less, shear heat generation in the extrusion process is suppressed when the film is produced, and a decrease in hydrolysis resistance is suppressed.
From this viewpoint, the IV of the film is more preferably 0.67 to 0.80 dL / g, and further preferably 0.68 to 0.77 dL / g.
The method described in the Examples is used as a method for measuring IV of the white polyester film of the present invention.

(Tear strength)
The white polyester film of the present invention preferably has a P / t of 6.5 to 13.5 mN / μm when the tear strength is P (mN) and the thickness of the film is t (μm).
If the tear strength (P / t) per unit thickness of the film is 6.5 mN / μm or more, the film can be prevented from being cleaved when exposed to the outside air for a long period of time. On the other hand, if the tear strength (P / t) per unit thickness of the film is 13.5 mN / μm or less, sufficient weather resistance can be imparted.
From such a viewpoint, the tear strength (P / t) per unit thickness of the film is more preferably 7.0 to 13.5 mN / μm, and further preferably 7.0 to 13.0 mN / μm. .
The measuring method of the tear strength of the white polyester film of this invention uses the method described in the Example.

(Terminal carboxyl group concentration)
The white polyester film of the present invention preferably has a terminal carboxyl group concentration of 5 to 25 equivalent / ton. The terminal carboxyl group concentration is also referred to as an acid value (Acid value) and may be described as “AV”. In this specification, “equivalent / ton” represents a molar equivalent per ton and may be described as “eq / t”.

If the terminal carboxyl group concentration in the polyester film is 5 equivalents / ton or more, the surface carboxyl groups (COOH groups) do not decrease too much (polarity does not become too low), and are high with different materials such as other resin layers. It can have adhesiveness.
On the other hand, hydrolysis of the polyester molecule terminal CO ⁺ group is promoted using a catalyst. If the terminal carboxyl group density | concentration in a polyester film is 25 equivalent / tons or less, a hydrolysis-resistant fall can be suppressed.

  The terminal carboxyl group concentration is a value measured by the following method. That is, after dissolving 0.1 g of a resin measurement sample in 10 ml of benzyl alcohol, chloroform is further added to obtain a mixed solution, and phenol red indicator is added dropwise thereto. This solution is titrated with a standard solution (0.01 mol / L KOH-benzyl alcohol mixed solution), and the terminal carboxyl group concentration is determined from the amount dropped.

<Method for producing stretched white polyester film>
The method for producing the stretched white polyester film of the present invention is not particularly limited. For example, the stretched white polyester film of the present invention can be suitably produced by the following method.

  That is, the method for producing a stretched white polyester film of the present invention includes an extrusion process in which a mixture containing a raw material polyester and white particles is melt-extruded and then cooled to form an unstretched white polyester film, and an unstretched white polyester film Extending in at least one direction of the longitudinal direction and the width direction, and the stretching step includes first to Nth stretching for stretching the white polyester film in at least one direction of the longitudinal direction and the width direction. The n-th stretching step is performed continuously to the n-1 stretching step, and the n-th stretching step is stretched in one direction of the longitudinal direction or the width direction of the white polyester film rather than the n-1 stretching step. The speed is increased, and the N-th stretching step is performed at a stretching temperature of 140 to 180 ° C. and one of the white polyester films before the first stretching step is started. The length of the one direction and the stretching rate is increased from 8 to 25% / sec for the length of the. Here, N is an integer of 2 or more, and n is an integer of 2 to N.

In the method for producing a stretched white polyester film of the present invention, it is preferable to perform a heat setting step and a heat relaxation step after the stretching step.
In addition, after forming the unstretched white polyester film, before the stretching step, or after stretching in one direction, before performing stretching in the other direction, an in-line coat for forming an undercoat layer is formed. You may go.
Hereinafter, although each process is demonstrated concretely, the manufacturing method of the white polyester film of this invention is not limited to the following method.

(Extrusion process)
In the extrusion step, a mixture containing the raw material polyester and white particles is melt-extruded and then cooled to form an unstretched white polyester film (hereinafter sometimes referred to as an unstretched film).

  For example, white particles such as polyester and titanium oxide described above are used as a raw material, dried and melted, and the resulting melt (melt) is passed through a gear pump and a filter, and then a cooling roll ( A non-stretched white polyester film is obtained by extruding to a cast drum) and solidifying by cooling. Melting is performed using an extruder, and a single-screw extruder may be used, or a multi-screw extruder having two or more axes may be used.

  Various known methods can be used for blending the white particles into the polyester film. The following method is mentioned as the typical method.

(A) A method in which white particles are added before the end of the ester exchange reaction or esterification reaction during polyester synthesis, or white particles are added before the start of the polycondensation reaction.
(B) A method in which white particles are added to polyester and melt-kneaded.
(C) A master batch (also referred to as master pellet) in which a large amount of white particles is added by the method of (A) or (B) above is produced, and the master batch and white particles are contained or a small amount of white pigment is contained. A method of kneading polyester to contain a predetermined amount of white particles.
(D) A method of melt-kneading using the master pellet of (C) as it is.

  In this, the method (C), that is, a master batch (MB) in which a large amount of white particles are added is produced, and the master batch and a polyester containing no white particles or a small amount of white pigment are kneaded. And the method (masterbatch method) which contains a predetermined amount of white particles is preferable. Further, it is also possible to employ a method in which polyester and white particles that have not been dried in advance are put into an extruder and a master batch is produced while degassing moisture and air. Furthermore, it is preferable to prepare a masterbatch using a polyester that has been slightly dried in advance to suppress an increase in the acid value of the polyester. In this case, a method of extruding while degassing, a method of extruding without sufficiently degassing with sufficiently dried polyester, and the like can be mentioned.

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

The extrusion of the molten resin (melt) is preferably performed in an evacuated or inert gas atmosphere.
The melting temperature in the extruder is preferably from the melting point of the polyester used to the melting point + 80 ° C. or less, more preferably the melting point + 10 ° C. or more, the melting point + 70 ° C. or less, more preferably the melting point + 20 ° C. or more and the melting point + 60 ° C. or less. When the melting temperature in the extruder is a melting point + 10 ° C. or higher, the resin is sufficiently melted. On the other hand, when the melting temperature is 70 ° C. or lower, decomposition of polyester or the like is preferably suppressed. In addition, it is preferable to dry raw material polyester before throwing a raw material into an extruder, and a preferable moisture content is 10 ppm-300 ppm, More preferably, it is 20 ppm-150 ppm.

  For the purpose of further improving the hydrolysis resistance, an end-capping agent may be added when the raw material resin is melted.

  The end-capping agent may be added directly to the extruder together with the polyester or the like, but it is preferable from the viewpoint of extrusion stability that a polyester and a master batch are formed in advance and charged into the extruder.

  The extruded melt (melt) is poured onto a cast drum (cooling roll) through a gear pump, a filter, and a die. The shape of the die may be a T-die, a hanger coat die, or a fish tail. On the cast drum, the molten resin (melt) can be brought into close contact with the cooling roll using an electrostatic application method. The surface temperature of the cast drum can be approximately 10 ° C to 40 ° C. The diameter of the cast drum is preferably 0.5 m or more and 5 m or less, more preferably 1 m or more and 4 m or less. The driving speed of the cast drum (the linear velocity at the outermost periphery) is preferably 1 m / min to 50 m / min, more preferably 3 m / min to 30 m / min.

(Stretching process)
In the stretching process, an unstretched white polyester film (hereinafter, sometimes referred to as an unstretched polyester film) formed in the extrusion process is at least one of the longitudinal direction (MD: Machine Direction) and the width direction (TD: Transverse Direction). Stretch in the direction.
And a extending process has the 1st-Nth extending process which extends a white polyester film with respect to at least one direction of the longitudinal direction and the width direction, and the nth extending process continues to the n-1 extending process. The nth stretching step increases the stretching speed in one direction of the longitudinal direction or the width direction of the white polyester film more than the n-1 stretching step, and in the Nth stretching step, the stretching temperature is 140 to 180 ° C. And it is set as the extending | stretching speed which increases the length of one direction 8 to 25% / second with respect to the length of the one direction of the white polyester film before a 1st extending process start.

  When the unstretched polyester film is stretched in the longitudinal direction or width direction, the crystallization of the polyester is suppressed by stretching at a slow stretching speed at the initial stage of stretching, and the stretching speed is increased while the polyester is heated sufficiently at the end of stretching. By setting the desired draw ratio, peeling between the white particles and the polyester can be kept to a minimum, a minute space is formed around the white particles, and minute voids are easily formed.

In the stretching step, either uniaxial stretching for stretching the film in only one direction in the longitudinal direction or the width direction or biaxial stretching for stretching in both the longitudinal direction and the width direction may be performed, but from the viewpoint of improving weather resistance, biaxial stretching is performed. Preferably, from the viewpoint of facilitating stretching, the film may be stretched in the longitudinal direction (sometimes referred to as “longitudinal stretching”) and then stretched in the width direction (sometimes referred to as “lateral stretching”). More preferred. In the case of biaxial stretching, the first to Nth lateral stretching steps (N is 2 or more) as a stretching step (lateral stretching step) in which the unstretched white polyester film is longitudinally stretched in the longitudinal direction and then laterally stretched in the width direction. Integer). The nth lateral stretching step (n is an integer of 2 to N) is performed continuously to the n-1 lateral stretching step, and the nth lateral stretching step is the width direction of the white polyester film more than the n-1 lateral stretching step. Increase the stretching speed for. In the Nth lateral stretching step, the stretching temperature is 140 to 180 ° C., and the width in the width direction is 8 to 25% with respect to the width in the width direction of the white polyester film before the first lateral stretching step. It is preferable that the stretching speed is increased by 1 / sec.
Hereinafter, a case where stretching in the width direction is performed after stretching in the longitudinal direction will be described.

  FIG. 1 schematically shows an example of a biaxial stretching machine used for producing the stretched white polyester film of the present invention. FIG. 1 shows a biaxial stretching machine 100 and a polyester film 200 attached to the biaxial stretching machine 100. The biaxial stretching machine 100 includes a pair of annular rails 60a and 60b, and is arranged symmetrically with the polyester film 200 in between.

  The biaxial stretching machine 100 includes a preheating unit 10 that preheats the polyester film 200, a stretching unit 20 that stretches the polyester film 200 in an arrow TD that is a direction orthogonal to the arrow MD and applies tension to the polyester film, Heat fixing unit 30 that heats a given polyester film while applying tension, heat relaxation unit 40 that relaxes the heat-fixed polyester film by heating and fixing the polyester film, and polyester film that has undergone the heat relaxation unit And a cooling unit 50 for cooling the air.

The annular rail 60a includes at least gripping members 2a, 2b, 2e, 2f, 2i, and 2j that can move the edge of the annular rail 60a. The annular rail 60b is a gripping member 2c that can move the edge of the annular rail 60b. 2d, 2g, 2h, 2k, and 2l. The gripping members 2a, 2b, 2e, 2f, 2i, and 2j grip one end of the TD of the polyester film 200, and the gripping members 2c, 2d, 2g, 2h, 2k, and 2l are polyester films The other end of 200 TDs is gripped. The holding members 2a to 2l are generally referred to as chucks, clips, and the like.
The gripping members 2a, 2b, 2e, 2f, 2i, and 2j move counterclockwise along the edge of the annular rail 60a, and the gripping members 2c, 2d, 2g, 2h, 2k, and 2l are annular It moves clockwise along the edge of the rail 60b.

The holding members 2a to 2d hold the end portion of the polyester film 200 in the preheating unit 10 and move the edge of the annular rail 60a or 60b as it is, and the thermal relaxation unit in which the extending unit 20 and the holding members 2e to 2h are shown. After 40, the process proceeds to the cooling unit 50 where the gripping members 2i to 2l are shown. Thereafter, the gripping members 2a and 2b and the gripping members 2c and 2d are separated from each other at the end of the polyester film 200 at the end of the cooling unit 50 on the MD downstream side in the conveying direction, and the edge of the annular rail 60a or 60b is left as it is. It progresses along and returns to the preheating part 10. FIG.
As a result, the polyester film 200 moves to the arrow MD in FIG. 1, and is sequentially conveyed to the preheating unit 10, the stretching unit 20, the heat fixing unit 30, the heat relaxation unit 40, and the cooling unit 50.
The moving speed of the gripping members 2a to 2l is the transport speed at the gripping portion of the polyester film 200.

The gripping members 2a to 2l can change the moving speeds independently.
Accordingly, the biaxial stretching machine 100 enables lateral stretching in which the polyester film 200 is stretched to TD in the stretching unit 20, but the polyester film 200 is changed by changing the moving speed of the gripping members 2a to 2l. Can also be stretched to MD.
That is, simultaneous biaxial stretching can be performed using the biaxial stretching machine 100.

In FIG. 1, only twelve gripping members 2a to 2l are illustrated as gripping members that grip the end of the TD of the polyester film 200. However, in order to support the polyester film 200, the biaxial stretching machine 100 includes 2a to 2l. In addition, a gripping member (not shown) is included.
Hereinafter, the gripping members 2a to 2l may be collectively referred to as “grip member 2”.

(Preheating part)
In the preheating part 10, the polyester film 200 is preheated. Before the polyester film 200 is stretched, it is preheated to facilitate the lateral stretching of the polyester film 200.
The film surface temperature at the end point of the preheating part (hereinafter also referred to as “preheating temperature”) is preferably Tg−10 ° C. to Tg + 60 ° C., where Tg is the glass transition temperature of the polyester film 200, and Tg ° C. to Tg + 50. More preferably, it is ° C.
The end point of the preheating portion refers to the time when the preheating of the polyester film 200 is finished, that is, the position where the polyester film 200 is separated from the region of the preheating portion 10.

(Extension part)
In the stretching unit 20, the preheated polyester film 200 is laterally stretched at least in the direction (TD) perpendicular to the longitudinal direction (conveying direction, MD) of the polyester film 200 to give tension to the polyester film 200.
Stretching (transverse stretching) in the direction (TD) perpendicular to the longitudinal direction (conveying direction, MD) of the polyester film 200 is an angle direction perpendicular (90 °) to the longitudinal direction (conveying direction, MD) of the polyester film 200. It is intended to be stretched.

-Longitudinal stretching-
In biaxial stretching, with respect to the unstretched white polyester film formed in the extrusion process, in the longitudinal direction of the polyester film, the stretching stress is 5 MPa or more and 15 MPa or less, and the stretching ratio is 2.5 times or more and 4.5 times or less. The longitudinal stretching is performed.

  More specifically, the polyester film is led to a group of rolls heated to a temperature of 70 ° C. or more and 120 ° C. or less, and the stretching stress is 5 MPa or more and 15 MPa or less in the longitudinal direction (longitudinal direction, that is, the traveling direction of the film), and The longitudinal stretching is performed at a stretching ratio of 2.5 to 4.5 times, more preferably at a stretching stress of 8 to 14 MPa and a stretching ratio of 3.0 to 4.0 times. It is preferable to cool with the roll group of the temperature of 20 to 50 degreeC after longitudinal stretching.

-Transverse stretching-
After longitudinal stretching, lateral stretching is performed. The transverse stretching is preferably performed using a tenter. The vertically stretched white polyester film is guided to a tenter, and stretched in the width direction (lateral stretching) in an atmosphere heated to a temperature (stretching temperature) of 80 ° C. or higher and 180 ° C. or lower, for example. In the tenter, the polyester film can be stretched in the transverse direction by holding both ends of the polyester film with the clip and expanding the clip in the direction perpendicular to the longitudinal direction, that is, in the width direction while conveying the heat treatment zone.

  Stretching in the width direction is the first to Nth lateral stretching step (N is an integer of 2 or more), and the nth lateral stretching step (n is an integer of 2 to N) is continued to the n-1th lateral stretching step. The n-th lateral stretching step increases the stretching speed in the width direction of the white polyester film more than the n-1 lateral stretching step. The Nth transverse stretching step is performed at a stretching temperature of 140 to 180 ° C., and the length in the width direction is 8 to 25% with respect to the length in the width direction of the white polyester film before the start of the first lateral stretching step. The stretching speed is increased per second.

  FIG. 2 schematically shows an example of a mode in which transverse stretching is performed in stages in the production of the white polyester film of the present invention.

In FIG. 2, position A is the position of one end portion in the width direction (TD) of the polyester film in a state before the polyester film is positioned in the preheating portion and before the lateral stretching is started. The position B is a position of one end portion in the width direction (TD) of the polyester film in a state where the polyester film is located in the heat fixing portion and the lateral stretching is finished.
T1, T2, and T3 are polyester film positioned in the stretched portion, and each of the one end portions in the width direction (TD) of the polyester film where transverse stretching starts at the first, second, and third stretch speeds, respectively. Position. The end portion in the width direction (TD) of the polyester film reaches the position B from the position A through T1, T2, and T3.

  In addition, when performing transverse stretching in the present invention, the change in the stretching speed is not limited to three stages, and may be two or more stages. However, it is preferable to change in three to eight stages from the viewpoint of manufacturability.

  In lateral stretching, the film is stretched to TD at each position of T1, T2, and T3, and the width of the film gradually increases. The film is continuously conveyed to the MD, and stretching to TD starts at T1, and is stretched to TD at a stretching angle θ1 with respect to TD until T2. The stretching speed to TD can be adjusted by the transport speed to MD and the stretching angle θ1. That is, if the conveyance speed to MD is constant, the stretching speed to TD increases as the stretching angle θ1 is increased. The stretching speed can be represented by an increase in width per second with respect to the TD length (width W0) of the film before the lateral stretching is started. For example, when the stretching speed to TD is changed in three stages as shown in FIG. 2, the film width W0 before starting the transverse stretching is set to 100, and the film width is 5 per second from T1 to T2. If the film is stretched to TD so as to increase at a rate, the stretching speed with respect to TD is 5% / second.

  In the present embodiment, between the start of stretching to TD and the end of stretching, the stretching angle is increased stepwise or continuously while the film is conveyed to MD to increase the stretching speed. , At a stretching temperature of 140 to 180 ° C., and the stretching speed is increased to a stretching speed that increases the length in the width direction by 8 to 25% / second with respect to the length W0 in the width direction before starting stretching in the width direction. The Nth transverse stretching is performed.

Here, the stretching temperature at the time of lateral stretching of the white polyester film 200 represents the film surface temperature, and can be controlled by temperature control means provided in the tenter. The stretching temperature in the transverse stretching is increased with the stretching speed, and the stretching temperature at the start of the transverse stretching is preferably 80 to 120 ° C. from the viewpoint of suppressing crystallization of the polyester in the initial stage of the transverse stretching. It is more preferable that it is 85-115 degreeC.
On the other hand, the stretching temperature in the Nth lateral stretching step is preferably 140 to 180 ° C, and preferably 150 to 175 ° C, from the viewpoint of causing separation at the interface between the white particles and the polyester to generate minute voids. Is more preferable.
In addition, extending | stretching temperature is the value which measured the temperature of the film surface of the film in an extending | stretching process with the thermocouple.

In this way, the size of voids generated starting from white particles in the film can be controlled by performing transverse stretching by gradually increasing the stretching speed in the width direction. The influence of the final stretching speed of the stretched portion where the transverse stretching process is performed is the largest.
In the region where the film temperature in the first half of the stretched part is relatively low, the film is stretched gently so that interfacial peeling between the white particles and the polyester is difficult to occur.

On the other hand, in the latter half when the film temperature is high, the film is stretched to a desired magnification to give weather resistance. However, if the stretching speed is too high in the second half, the stress for stretching the polyester exceeds the interfacial adhesion between the white particles and the polyester, and separation occurs between the white particles and the polyester. Conversely, if the stretching speed is too slow, the orientation of the polyester molecules is insufficient and the weather resistance is insufficient.
On the other hand, if the stretching speed is too fast in the first half, the polyester stretching stress exceeds the interfacial adhesive force between the white particles and the polyester, and peeling occurs between the white particles and the polyester. On the other hand, if the stretching speed is too slow, the crystallization of the polyester proceeds and the polyester becomes harder, the polyester stretching stress increases, and separation between the polyester and the white particles occurs first.

  From this point of view, the stretching speed in the width direction of the first transverse stretching step increases the length in the width direction by 4 to 10% / second with respect to the length in the width direction of the white polyester film before the start of the first transverse stretching step. It is preferable that the drawing speed be adjusted. For example, when changing the stretching speed in the transverse direction in three stages as shown in FIG. 2, the stretching speed until stretching from T1 to T2 is the stretching speed of the first lateral stretching step.

  Moreover, since the polyester becomes harder as the stretching speed immediately after the start of stretching is slower for the above-described reason, the polyester stretching stress immediately before the termination of stretching tends to be high, and peeling from the white particles is likely to occur. The stretching speed immediately before the end of stretching needs to be adjusted according to the stretching speed immediately after the start of stretching, Sa is the stretching speed in the width direction of the first transverse stretching step, and Sb is the stretching speed in the width direction of the Nth lateral stretching step. Sometimes, the value of the drawing speed ratio Sb / Sa is preferably 1.5-6.

  In the transverse stretching step, it is preferable to perform transverse stretching in which the stretching stress is 8 MPa or more and 20 MPa or less, and the stretching ratio is 3.4 times or more and 4.5 times or less, and the stretching stress is 10 MPa or more and 18 MPa or less. It is more preferable to perform transverse stretching with a magnification of 3.6 times or more and 5 times or less.

  The stretching area ratio (longitudinal stretching ratio × lateral stretching ratio) by biaxial stretching is preferably 9 to 20 times. When the area magnification is 9 to 20 times, the thickness after stretching is 250 to 500 μm, the degree of plane orientation is high, the crystallinity is 30 to 40%, and the equilibrium moisture content is 0. A biaxially oriented polyester film having a content of 1% by mass or more and 0.25% by mass or less is obtained.

As the biaxial stretching method, as described above, in addition to the sequential biaxial stretching method in which the longitudinal direction and the width direction are separated separately, the simultaneous biaxial stretching method in which the longitudinal direction and the width direction are simultaneously stretched. Either may be sufficient.
Also, for example, when the stretching direction in the width direction is constant and the first to Nth longitudinal stretching steps are performed in the longitudinal direction, the stretching temperature, stretching speed, stretching in the first to Nth transverse stretching steps in the width direction described above. A speed ratio can be applied as well.

(Heat setting process)
Next, the biaxially stretched white polyester film is heat set.
In the heat setting step, for example, heat treatment at 160 ° C. to 230 ° C., preferably 170 ° C. to 220 ° C. (more preferably 180 ° C. to 210 ° C.) for 1 second to 60 seconds (more preferably 5 seconds to 50 seconds). Is applied to the film.
If the heat setting temperature is 160 ° C. or higher, the polyester is easily crystallized, and the polyester molecules can be fixed in an extended state, thereby improving the hydrolysis resistance. Further, when the heat setting temperature is 230 ° C. or less, slippage hardly occurs at a portion where the polyester molecules are entangled with each other, the polyester molecules can be prevented from shrinking, and the hydrolysis resistance can be improved.
In addition, the heat setting temperature here is the surface temperature of the film at the time of heat setting processing.

In the heat setting step provided after the stretching step, a part of the volatile basic compound having a boiling point of 200 ° C. or less may be volatilized.
The heat setting step is preferably performed in the state of being gripped by the chuck in the tenter following the transverse stretching. At this time, the chuck interval may be performed at the width at the end of the transverse stretching, further widened, or reduced in width. You may do it. By performing the heat setting treatment, microcrystals are generated, and mechanical properties and durability can be improved.

(Heat relaxation process)
It is preferable to perform a heat relaxation process following the heat setting process. A heat relaxation process is a process which shrinks a film by applying heat for stress relaxation to a film. In the thermal relaxation step, relaxation is preferably performed in at least one of length and width, and the amount of relaxation is preferably 1% to 30% (ratio to the width after lateral stretching), more preferably 2% to 20%. Preferably, it is 3% to 15%. When the heat relaxation temperature is Tr and the heat fixing temperature is Ts, the heat relaxation temperature Tr is 100 ° C. or higher and 15 ° C. or lower than Ts (100 ° C. ≦ Tr ≦ Ts−15 ° C.). It is more preferable that the temperature range is 110 ° C. or higher and 25 ° C. or lower than Ts (110 ° C. ≦ Tr ≦ Ts−25 ° C.), 120 ° C. or higher and 30 ° C. or lower than Ts. The region (120 ° C. ≦ Tr ≦ Ts−30 ° C.) is particularly preferable.

  In the thermal relaxation step, the polyester film is thermally relaxed under the conditions within the above range, and the tension of the polyester molecules is somewhat released, so that the dimensional stability is improved while maintaining hydrolysis resistance, and the obtained polyester film Failures in downstream processes such as machining are less likely to occur.

Lateral relaxation can be performed by reducing the width of the tenter clip. Moreover, longitudinal relaxation can be implemented by narrowing the interval between adjacent clips of the tenter. This can be achieved by connecting adjacent clips in a pantograph shape and shrinking the pantograph. Moreover, after taking out from a tenter, it can also heat-process and relieve | moderate, conveying with low tension. Tension is preferably cross-sectional area per ^0N / mm ^{2 ~0.8N} / mm 2 of film, more preferably ^{0N / mm 2 ~0.6N / mm 2} , more preferably ^0N / mm ^{2 ~0.4N} / mm 2 It is. 0 N / mm ² can be carried out by providing two or more pairs of nip rolls during conveyance and slacking them in a suspended manner.

(Winding process)
The film coming out of the tenter is trimmed at both ends held by the clip, and after being subjected to knurling (embossing) at both ends, the film is wound up.
The preferred width of the film to be wound is 0.8 m to 10 m, more preferably 1 m to 6 m, and still more preferably 1.2 m to 4 m. The thickness is preferably 30 μm to 500 μm, more preferably 40 μm to 480 μm, and still more preferably 45 μm to 450 μm. Such adjustment of the thickness can be achieved by adjusting the discharge amount of the extruder and adjusting the film forming speed (adjusting the speed of the cooling roll, the stretching speed linked with this).

  In addition, the film for reproduction | regeneration, such as the edge part of the trimmed film, is collect | recovered and recycled as a resin mixture. The film for reproduction becomes a film raw material for the white polyester film of the next lot, and returns to the drying process as described above, and the manufacturing process is sequentially repeated.

  The white polyester film of this invention can be manufactured through the above process.

<Back sheet for solar cell>
The solar cell backsheet of the present invention comprises the white polyester film of the present invention.
A functional layer can be provided on at least one surface of the solar cell backsheet of the present invention and the white polyester film of the present invention as required. For example, an easy-adhesive layer, an ultraviolet absorbing layer, a weather-resistant layer and the like that increase the adhesion to the adherend can be used.
Since the solar cell backsheet of the present invention includes the white polyester film of the present invention, it exhibits stable weather resistance, adhesiveness, and light reflectivity during long-term use.

  As a method for providing the functional layer on at least one surface of the white polyester film of the present invention, a known coating technique such as a roll coating method, a knife edge coating method, a gravure coating method, or a curtain coating method can be used. Moreover, you may form a functional layer by the in-line coat mentioned above.

  The solar cell backsheet has a functional layer (coating layer) formed by coating on at least one surface of the stretched white polyester film of the present invention, so that any of weather resistance, light reflectivity, and adhesiveness can be obtained. Further improvement or other functions can be added.

Further, surface treatment (flame treatment, corona treatment, plasma treatment, ultraviolet treatment, etc.) may be performed before the coating layer is applied.
Moreover, it is also preferable to bond another functional film to the white polyester film of this invention through an adhesive layer.

<Solar cell module>
The solar cell module of the present invention includes a solar cell element, a sealing material that seals the solar cell element, a front substrate disposed outside the sealing material on the light receiving surface side of the solar cell element, and the solar cell element The solar cell backsheet of the above-described embodiment, which is disposed on the side opposite to the light receiving surface side and outside the sealing material.
That is, the solar cell module of the present invention includes a solar cell element that converts light energy of sunlight into electric energy, a transparent front substrate (surface protection member) on which sunlight is incident, and the solar cell of the present invention described above. The solar cell element disposed between the back sheet (back surface protection member) and the front substrate and the back sheet is sealed with a sealing material such as ethylene-vinyl acetate (EVA). The Since the solar cell module includes the solar cell backsheet including the white polyester film of the present invention, the occurrence of peeling and cracking due to hydrolysis of the solar cell backsheet is suppressed. Therefore, light can be reflected with high reflectivity to increase power generation efficiency. Therefore, the solar cell module of the present invention can maintain high power generation efficiency over a long period outdoors.

  Members other than the solar cell module and the back sheet are described in detail in, for example, “Solar Power Generation System Constituent Materials” (supervised by Eiichi Sugimoto, Kogyo Kenkyukai, 2008).

  The transparent substrate only needs to have a light-transmitting property through which sunlight can pass, and can be appropriately selected from base materials that transmit light. From the viewpoint of power generation efficiency, a higher light transmittance is preferable, and as such a substrate, for example, a transparent resin such as a glass substrate or an acrylic resin can be suitably used.

  Solar cell elements include silicon-based materials such as single crystal silicon, polycrystalline silicon and amorphous silicon, III-V groups such as copper-indium-gallium-selenium, copper-indium-selenium, cadmium-tellurium, gallium-arsenide, and II Various known solar cell elements such as -VI group compound semiconductor systems can be applied.

  Although the white polyester film of this invention is suitable as a base film of a solar cell backsheet, the use of the white polyester film of this invention is not limited to a solar cell backsheet, and is used outdoors for a long time. It can be used as a film. Specific examples include a protective film for solar cells, a film for building materials, a film for outdoor advertising, a heat shield film, and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist thereof. Unless otherwise specified, “part” is based on mass.

[Example 1]
<Synthesis of raw material polyester resin 1>
As shown below, terephthalic acid and ethylene glycol are directly reacted to distill off water, esterify, and then use a direct esterification method in which polycondensation is performed under reduced pressure, and a polyester resin (Ti Catalyst system PET) was obtained.

(1) Esterification reaction In the first esterification reaction tank, 4.7 tons of high-purity terephthalic acid and 1.8 tons of ethylene glycol are mixed over 90 minutes to form a slurry, and continuously at a flow rate of 3800 kg / h. To the first esterification reactor. Further, an ethylene glycol solution of a citrate chelate titanium complex (VERTEC AC-420, manufactured by Johnson Matthey) in which citric acid is coordinated to Ti metal is continuously supplied, and the average residence time is maintained at 250 ° C. in the reaction vessel with stirring. The reaction was carried out for about 4.3 hours. At this time, the citric acid chelate titanium complex was continuously added so that the amount of Ti added was 9 ppm in terms of element. The acid value of the obtained oligomer was 600 equivalents / ton.

  The obtained reaction product (oligomer) was transferred to the second esterification reaction tank, and reacted with stirring at a reaction tank temperature of 250 ° C. and an average residence time of 1.2 hours, and an acid value of 200 equivalents / ton. Got. The inside of the second esterification reaction tank is partitioned into three zones, and an ethylene glycol solution of magnesium acetate is continuously supplied from the second zone so that the amount of Mg added is 75 ppm in terms of element, From the third zone, an ethylene glycol solution of trimethyl phosphate was continuously supplied so that the added amount of P was 65 ppm in terms of element.

(2) the polycondensation reaction above-obtained esterification reaction product supplied to the first polycondensation reaction vessel continuously stirring, the reaction temperature 270 ° C., the reaction vessel pressure 20 torr (2.67 × 10 ^{- 3} MPa), and the polycondensation was conducted with an average residence time of about 1.8 hours.

The reaction product that has passed through the first polycondensation reaction tank is further transferred to the second double condensation reaction tank. While stirring in this reaction tank, the reaction tank temperature is 276 ° C., the reaction tank pressure is 5 torr (6.67 × 10 6). ^-4 MPa) at a residence time of about 1.2 hours (polycondensation).

Next, the reaction product that passed through the second double condensation reaction tank was further transferred to the third triple condensation reaction tank. In this reaction tank, the reaction vessel internal temperature was 278 ° C., the reaction vessel internal pressure was 1.5 torr (2.0 × 10 ⁻⁴ MPa) and a residence time of 1.5 hours (polycondensation) to obtain polyethylene terephthalate (PET). The obtained PET (reaction product) was measured using high resolution high frequency inductively coupled plasma mass spectrometry (HR-ICP-MS; AttoM manufactured by SII Nanotechnology). As a result, Ti = 9 ppm, Mg = 67 ppm, and P = 58 ppm. P is slightly reduced with respect to the initial addition amount, but is estimated to have volatilized during the polymerization process.

-Solid state polymerization-
The above polymerized PET was pelletized (diameter 3 mm, length 7 mm), and the resulting resin pellet (inherent viscosity IV = 0.60 dl / g, terminal carboxy group concentration = 16 equivalents / ton) was obtained as follows. The solid phase polymerization was carried out.

In the solid phase polymerization, the polyester polymerized by the above-described esterification reaction was heated at 140 ° C. for 7 minutes with nitrogen having a dew point temperature of −30 ° C., and pre-crystallization was performed for the purpose of preventing sticking during the solid phase polymerization.
Next, it was dried at 180 ° C. for 7 hours using heated nitrogen having a dew point temperature of −30 ° C., and the moisture content in the resin was adjusted to 50 ppm or less.

Next, the dried polyester resin was preheated to 210 ° C., and then solid-state polymerization was advanced by circulating nitrogen at 195 ° C. for 50 hours. As nitrogen circulation conditions, the gas ratio (the amount of nitrogen gas circulated with respect to the amount of resin discharged) is 1.3 m ³ / kg, the superficial velocity is 0.08 m / sec, the ethylene glycol concentration is 240 ppm, the water concentration is 12 ppm, the ethylene glycol and water The solid phase polymerization was allowed to proceed by using nitrogen having a molar partial pressure ratio of 20 (methylene partial pressure of ethylene glycol / molar partial pressure of water) of 20. In order to obtain the above mixed gas composition, high purity ethylene glycol having a water content of 100 ppm was used for the ethylene glycol scrubber, and the scrubber temperature was set to 35 ° C. The pressure in the scrubber was in the range of 0.1 MPa to 0.11 MPa.
Next, the resin (750 kg / h) discharged from the reaction step was cooled to 60 ° C.
The obtained polyester resin after solid phase polymerization had an intrinsic viscosity (IV) = 0.78 dL / g and a terminal COOH amount (AV) = 9 equivalents / ton.

<Preparation of master pellet>
Titanium oxide was added to a part of the pellet before solid phase polymerization so that the content ratio was 50% by mass of the whole pellet and kneaded to prepare a master pellet (master batch).
Here, Ishihara Sangyo (trade name: PF-739; average primary particle size = 0.25 μm) was used as titanium oxide.

<Formation of unstretched film>
After drying the solid phase polymerization as described above, the PET-1 and the master pellet are each dried to a moisture content of 100 ppm or less, and then mixed so that the amount of titanium oxide is 4% by mass, and put into a hopper of a kneading extruder. And melted and extruded at 290 ° C. As the extruder, a double vent type co-rotating mesh type twin screw extruder (diameter: 110 mm) provided with two vents was used. The melt (melt) was passed through a gear pump and a filter (pore diameter: 20 μm), and then extruded from a die to a cooling cast drum (cooling roll). The extruded melt was brought into close contact with the cooling cast drum by an electrostatic application method. Thereby, an unstretched polyethylene terephthalate (PET) film having a thickness of about 3 mm was formed.

<Stretching of unstretched film>
-Longitudinal stretching-
The unstretched film was passed between two pairs of nip rolls having different peripheral speeds, and stretched in the longitudinal direction (conveying direction) under the following conditions. Here, the stretching speed in the longitudinal stretching step is expressed as an increase rate per second with respect to the length in the longitudinal direction of the unstretched film.
・ Preheating temperature: 80 ℃
-Stretching temperature: 90 ° C
-Stretch ratio: 3.0 times-Stretch speed: 300% / second

-Transverse stretching-
After longitudinal stretching, transverse stretching was performed. The transverse stretching was performed by increasing the stretching temperature and the stretching speed in three stages. Specific conditions are as follows. Here, the stretching speed in the transverse stretching step is expressed as an increase rate per second with respect to the film width before the first transverse stretching step of the film after longitudinal stretching.
Preheating temperature: 110 ° C
Stretching temperature immediately after the start of lateral stretching (first lateral stretching step): 110 ° C.
Stretching speed immediately after the start of transverse stretching (first transverse stretching step): 8% / second Stretching temperature in the middle of transverse stretching (second transverse stretching step): 130 ° C
Stretching speed in the middle of the transverse stretching (second transverse stretching step): 11% / second Stretching temperature immediately before the end of the transverse stretching (third transverse stretching step): 145 ° C
Stretching speed immediately before the end of transverse stretching (third transverse stretching step): 15% / second Transverse stretching ratio (total): 4.2 times

-Heat fixation / thermal relaxation-
The biaxially stretched film after finishing the longitudinal stretching and the transverse stretching was heat-set at 190 ° C. (heat setting time: 10 seconds).
After heat setting, the tenter width was reduced to relax the heat (thermal relaxation temperature: 185 ° C.).

-Winding-
After heat setting and heat relaxation, both ends were trimmed by 10 cm. Then, after extruding (knurling) with a width of 10 mm at both ends, it was wound up with a tension of 25 kg / m. The film width was 1.5 m and the winding length was 2000 m.
The biaxially stretched white polyester film of Example 1 was obtained as described above.

<Examples 2 to 11 and Comparative Examples 1 to 6>
Stretched white polyester films of Examples 2 to 11 and Comparative Examples 1 to 6 were produced in the same manner as in Example 1 except that the conditions for lateral stretching and film properties were changed as shown in Table 1.

[Evaluation of film]
The following evaluation was performed about the white polyester film after extending | stretching obtained in the Example and the comparative example. Each measurement result and evaluation result are shown in Table 1 below.

<Void area>
The proportion of voids in the film (void occupancy) and the area per void were evaluated by the following methods.

(Measurement of void occupancy)
1. The produced white polyester film is cleaved in the thickness direction along the TD and MD of the film with a microtome.
2. The fractured surfaces in each direction were observed with a scanning electron microscope at 3000 times. 9 or more pictures are taken at random in one direction to obtain a cross-sectional image of the white polyester film.
3. With the image analysis software (ImageJ), from the obtained image, a part where a gap is formed between the white particles and the polyester by peeling from the white pigment is found and traced along the outline of the void. At this time, the voids including the white particle portions present in the voids are formed as voids, and the portions in which the white particles fall out of the voids and become only cavities during the above steps 1 and 2 are similarly traced. If voids overlap, trace them together.
4). Next, fill the traced frame.
5. Trace the void, binarize the filled image, and separate the void part and the polyester part.
6). In the area calculation mode, the ratio of voids in one image is measured by dividing the number of pixels in the void portion by the number of pixels in the entire image.
7). The above operations 3 to 6 are also performed on other images, and the average is taken to obtain the ratio of voids in each direction.
8). Finally, the average of TD / MD is taken, and the ratio occupied by voids (void occupation ratio) is taken.

(Measurement of average area per void)
i. The above 1-5 are performed.
ii. In the area calculation mode, the number of void pixels is obtained and converted into an area.
iii. On the other hand, the number of voids is counted from the image obtained in 2.
iv. Ii to iii are also performed on other images.
v. By dividing the total area obtained in iv by the number of voids obtained in iv, the area per void in each direction is obtained.
vi. Finally, the average of TD and MD is taken as the average area per void.

<Thickness>
The thickness of the polyester film is the average thickness of the film measured using a contact-type film thickness meter (ID-F125, manufactured by Mitutoyo Corporation). Specifically, with a contact-type film thickness meter, 50 points were sampled at equal intervals over the length of 0.5 m in the length direction of the polyester film, and were equally spaced over the entire width of the film in the width direction (divided into 50 equal parts in the width direction). 50 points are sampled at point), and the thicknesses of these 100 points are measured. The average value of the obtained 100 points of thickness is calculated | required, and let this be the thickness of a polyester film.

<Intrinsic viscosity>
The produced polyester film is dissolved in a 1,1,2,2-tetrachloroethane / phenol (= 2/3 [mass ratio]) mixed solvent, and the intrinsic viscosity is determined from the solution viscosity at 25 ° C. in the mixed solvent. Asked.
ηsp / C = [η] + K [η] 2 · C
Here, ηsp = (solution viscosity / solvent viscosity) −1, C is the dissolved polymer mass per 100 ml of solvent (1 g / 100 ml in this measurement), and K is the Huggins constant (0.343) ). The solution viscosity and the solvent viscosity were each measured using an Ostwald viscometer.

<Weather resistance>
The weather resistance (hydrolysis resistance) of the polyester film obtained in each example was evaluated by the half life of elongation at break. The elongation at break elongation half-life is evaluated by subjecting the polyester film obtained in each example to a storage treatment (heat treatment) under conditions of 120 ° C. and a relative humidity of 100%, and the fracture exhibited by the polyester film after storage. The elongation (%) was evaluated by measuring a storage time (h) at which the elongation (%) is 50% with respect to the breaking elongation (%) exhibited by the polyester film before storage.
It shows that the weather resistance (hydrolysis resistance) of a polyester film is excellent, so that break elongation retention half-life is long. Here, it can be said that it is excellent in weather resistance practically if the half life of elongation at break is 95 hours or more.

<Peel test>
(EVA adhesion before wet heat treatment)
The polyester film obtained in each example was cut into a width of 20 mm × 150 mm to prepare two sample pieces. An EVA sheet (EVA sheet manufactured by Mitsui Chemicals Fabro Co., Ltd .: SC50B) is sandwiched between the two sample pieces, and a vacuum laminator (vacuum laminator manufactured by Nisshinbo Co., Ltd.) is used. It was bonded to the EVA sheet by hot pressing. The bonding conditions at this time were as follows.

Using a vacuum laminator, evacuation was performed at 128 ° C. for 3 minutes, and then pressure was applied for 2 minutes to temporarily bond. Thereafter, the main adhesion treatment was performed in a dry oven at 150 ° C. for 30 minutes. In this way, a sample for adhesion evaluation was obtained in which the 20 mm portion from one end of the two sample pieces adhered to each other was not bonded to EVA, and the EVA sheet was bonded to the remaining 100 mm portion.
The EVA non-adhered portion of the obtained adhesion evaluation sample was sandwiched between upper and lower clips with Tensilon (RTC-1210A manufactured by ORIENTEC), and a tensile test (peeling test) was performed at a peeling angle of 180 ° and a pulling speed of 300 mm / min. The peel strength was measured as the average of the forces detected during peeling. The peel strength here was also taken as the peel strength when cleaving occurred near the adhesion interface of the white polyester film adhered to the EVA sheet and separation from the EVA sheet. Further, when the film sample broke while peeling 100 mm, the detected force was not adopted and the result was “break”.

(EVA adhesion after wet heat treatment)
The EVA sheet was bonded between two sample pieces obtained by cutting the polyester film obtained in each example in the same manner as in the evaluation before wet heat aging. After bonding, heat treatment (wet heat treatment) was performed for 30 hours in an environment of 120 ° C. and 100% RH.
After the wet heat treatment, a tensile test (peel test) was performed in the same manner as the evaluation before the wet heat aging, and the peel strength was measured.
If both before and after the wet heat treatment are 4 N / mm or more, it can be said that the adhesiveness to the EVA (peeling resistance) is excellent.

<Tear strength>
The tear strength of the polyester film obtained in each example was measured as follows.
-A sample film is cut into MD and TD in a 2 cm width (short side) x 10 cm length (long side).
-A notch with a length of 5 cm is put in the center of the short side in parallel with the long side direction, and the stress is measured by the following method using a tensile tester. The measurement is performed at 25 ° C. and a relative humidity of 50%.
(1-1) One end of the cut portion is held by one chuck of the tensile tester and the other end is held by the other chuck.
(1-2) The tensile stress of the chuck is measured at 30 mm / min. As the distance between chucks increases, the stress increases and a flat portion appears. The stress at the flat end portion is taken as the tear strength, and the average value is obtained by measuring with the number of repetitions n = 3.
(1-3) This measurement is measured by MD and TD, and the average value is taken as the tear strength.

<Withstand voltage>
The withstand voltage was measured according to JIS T8010. Specifically, the withstand voltage of the polyester film of the present invention is evaluated by determining the partial discharge voltage using a partial discharge tester KPD2050 (manufactured by Kikusui Electronics Co., Ltd.).
The polyester film used for the solar cell module is also required to withstand voltage.

<Reflectance>
The reflectance of each example polyester film formed as described above was measured according to the method of paragraph [0084] of Japanese Patent No. 4766192.
Specifically, an integrating sphere is attached to a spectrophotometer (Shimadzu Corp. self-recording spectrophotometer "UV-3150"), and a standard white plate (SuperOptics white standard plate "ZRS-99-010-W") The reflectance of 100% was calibrated, and the spectral reflectances of the polyester films of the examples and comparative examples were measured. The measurement was performed in 1 nm increments in the wavelength range of 400 to 1200 nm, and the average value was obtained. At the time of measurement, a non-reflective black mount was placed on the back of the sample film and measured.
Here, if the reflectance is 85% or more, it can be said that the weather resistance is practically excellent.

  The results are shown in Table 1 below.

  The white polyester film produced in the examples was excellent in weather resistance, light reflectivity, and peel strength. In particular, Example 1-10 having a thickness of 280 μm or more has a withstand voltage of 1 kV or more and is suitable for a solar cell backsheet. In particular, the white polyester film of Example 1-9 has a thickness of 500 μm or less and has an advantage that it is easy to produce.

2 Gripping member
DESCRIPTION OF SYMBOLS 10 Preheating part 20 Stretching part 30 Heat fixing part 40 Thermal relaxation part 50 Cooling part 60 Annular rail 100 Biaxial stretching machine 200 White polyester film

Claims (12)

Including polyester and white particles,
The content of the white particles relative to the total mass of the film is 2 to 10% by mass,
The stretched white polyester film which has a void whose average area per piece is 0.010-0.050micrometer < ² > / piece in the cross section of a film thickness direction.   The stretched white polyester film according to claim 1, wherein a ratio of the total area occupied by the voids is 0.5 to 3% in a cross section in the thickness direction of the film.   The stretched white polyester film according to claim 1 or 2, wherein P / t is 6.5 to 13.5 mN / µm, where P is the tear strength and t is the thickness of the film.   The stretched white polyester film according to any one of claims 1 to 3, wherein the film has a thickness of 280 to 500 µm.   The stretched white polyester film according to any one of claims 1 to 4, wherein the intrinsic viscosity of the film is 0.65 to 0.85 dL / g.   The stretched white polyester film according to any one of claims 1 to 5, wherein the white particles are titanium oxide.   The solar cell backsheet containing the stretched white polyester film of any one of Claims 1-6.   The solar cell backsheet according to claim 7, which has a coating layer on at least one surface of the stretched white polyester film. A solar cell element;
A sealing material for sealing the solar cell element;
A front substrate disposed outside the sealing material on the light receiving surface side of the solar cell element;
The solar cell backsheet according to claim 7 or 8, wherein the solar cell backsheet is disposed outside the sealing material on the side opposite to the light receiving surface side of the solar cell element.
Including solar cell module. A method for producing a stretched white polyester film according to any one of claims 1 to 6,
An extrusion process in which a mixture containing raw material polyester and white particles is melt-extruded and then cooled to form an unstretched white polyester film;
A longitudinal stretching step of stretching the unstretched white polyester film in the longitudinal direction and a lateral stretching step of stretching in the width direction,
The transverse stretching step includes first to Nth transverse stretching steps, the nth transverse stretching step is performed continuously to the (n-1) th transverse stretching step, and the nth transverse stretching step is the n-1st transverse stretching step. The stretching speed of the white polyester film in the width direction is increased more than the transverse stretching step, and the Nth transverse stretching step is performed at a stretching temperature of 140 to 180 ° C., and the white color before the first transverse stretching step is started. The manufacturing method of the extending | stretching white polyester film made into the extending | stretching speed which makes the length of the width direction increase 8 to 25% / second with respect to the length of the width direction of a polyester film.
N is an integer of 2 or more, and n is an integer of 2 to N.   The method for producing a stretched white polyester film according to claim 10, wherein a stretching speed in the width direction in the first transverse stretching step is 4 to 10% / second.   When the stretching speed with respect to the width direction in the first transverse stretching step is Sa and the stretching speed with respect to the width direction in the Nth lateral stretching step is Sb, the value of the stretching speed ratio Sb / Sa is 1.5 to It is 6, The manufacturing method of the stretched white polyester film of Claim 10 or Claim 11.