CN107406604B

CN107406604B - Stretched white polyester film, method for producing same, back sheet for solar cell, and solar cell module

Info

Publication number: CN107406604B
Application number: CN201680016333.0A
Authority: CN
Inventors: 浜田保弘
Original assignee: Fujifilm Corp
Current assignee: Fujifilm Corp
Priority date: 2015-03-31
Filing date: 2016-03-14
Publication date: 2020-07-24
Anticipated expiration: 2036-03-14
Also published as: CN107406604A; WO2016158357A1; JP2016194009A; KR102124240B1; JP6348867B2; KR20170118187A

Abstract

The invention provides a stretched white polyester film and its use, the stretched white polyester film contains polyester and white particles, the content of the white particles relative to the total mass of the film is 2-10 mass%, and the average area of each 1 particle on the cross section in the film thickness direction is 0.010-0.050 mu m²Pore of each. A method for producing a stretched white polyester film, comprising: the method comprises a longitudinal stretching step of stretching an unstretched white polyester film in a longitudinal direction and 1 st to Nth transverse stretching steps of stretching the white polyester film in a transverse direction, wherein the stretching speed of the white polyester film in the transverse direction is increased in the nth transverse stretching step as compared with the nth-1 transverse stretching step, and the stretching temperature is set to 140 to 180 ℃ in the nth transverse stretching step, and the length in the transverse direction is increased by 8 to 25%/second relative to the length in the transverse direction of the white polyester film before the 1 st stretching step is started. N is an integer of 2 or more, and N is an integer of 2 to N.

Description

Stretched white polyester film, method for producing same, back sheet for solar cell, and solar cell module

Technical Field

The present invention relates to a stretched white polyester film, a method for producing the same, a back sheet for a solar cell, and a solar cell module.

Background

In recent years, from the viewpoint of global environmental conservation, solar power generation that converts sunlight into electricity has attracted attention. A solar cell module for solar power generation has a structure in which a sealant/a solar cell element/a sealant/a back surface protection film are sequentially laminated on glass on which sunlight is incident.

The solar cell module is required to have high weather resistance so as to maintain battery performance such as power generation efficiency even in a severe use environment exposed to wind, rain and direct sunlight, for a long period of time such as several decades. In order to impart such weather resistance, various materials such as a back surface protective film (hereinafter, sometimes referred to as "back sheet for solar cell" or "back sheet") constituting a solar cell module and a sealing material for a sealing member are also required to have weather resistance.

Resin materials such as polyester are generally used for the back sheet for solar cells. In general, many carboxyl groups and hydroxyl groups are present on the surface of a polyester film, and hydrolysis is likely to occur in an environment where moisture is present, and deterioration with time tends to occur. Therefore, polyester films used for solar cell modules placed outdoors or the like are required to be inhibited from hydrolysis.

In order to reflect light that has passed through the sealing material without being absorbed by the solar cell element by the solar cell back sheet and improve the power generation efficiency, it has been proposed to use a white back sheet to which white particles such as titanium oxide are added.

For example, International publication No. 12/008488 discloses a polyester film for sealing the back surface of a solar cell, which has a light reflectance at a wavelength of 550nm of 50% or more, contains 3to 50 mass% of inorganic fine particles such as titanium oxide, has an acid value of 1to 30eq/ton, and has an intrinsic viscosity of 0.60 to 0.80d L/g.

Further, Japanese patent application laid-open No. 2014-25052 discloses a polyester film having a polyester layer, which contains fine particles of titanium oxide in an amount of 0.1to 10% by mass, a capping agent in an amount of 0.1to 10% by mass, and the like with respect to the polyester, and has pores around the fine particles, wherein the average value of the ratio (L b/L p) of the ratio of a line segment (L b) obtained by removing a portion overlapping with the orthographic projection of the fine particles in the film cross section from the long side of the orthographic projection of 1 fine particle in the film cross section to the long side (L p) of the orthographic projection of 1 fine particle in the film cross section is 0.5to 2.

Further, Japanese patent application laid-open No. 2014-162107 discloses a white multilayer polyester film having a boundary region and 2 or more constant content regions including a pair of constant content regions opposed to each other with the boundary region interposed therebetween, wherein the boundary region has a thickness of 0.15 to 3 μm, at least one of the pair of constant content regions opposed to each other with the boundary region interposed therebetween contains fine particles or voids such as titanium oxide, a difference in content of the fine particles or voids between the pair of constant content regions opposed to each other with the boundary region interposed therebetween is 1to 30%, and the boundary region contains fine particles or voids having a content between the contents of the pair of constant content regions opposed to each other with the boundary region interposed therebetween.

Disclosure of Invention

Technical problem to be solved by the invention

In order to maintain a high power generation efficiency of the solar cell module for a long period of time, the performances required for the back sheet for a solar cell include weather resistance (hydrolysis resistance), adhesiveness to a sealing material (peeling resistance), and light reflectivity.

When a back sheet having light reflectivity is produced using polyester, generally, white particles (titanium oxide or the like) are blended into polyester in order to improve the light reflectivity, and a polyester film extruded into a sheet shape is stretched in order to improve weather resistance. When the white particles are stretched, the interface between the white particles and the polyester is peeled off to generate voids (voids), but when a large number of voids are generated, the strength of the film is greatly reduced when the film is exposed to the outside for a long period of time, and cleavage and breakage of the film occur, resulting in a reduction in adhesion to the sealing material.

For example, in the back sheet disclosed in international publication No. 12/008488, no consideration is given to the adhesiveness (peeling resistance) with the sealing material.

Further, in the back sheet disclosed in japanese patent application laid-open No. 2014-25052, no consideration is given to the adhesion to the sealing material when exposed to the outside air for a long period of time.

Further, in the back sheet disclosed in japanese patent application laid-open No. 2014-162107, layers having different pore amounts need to be stacked, and a dedicated device needs to be introduced.

In view of the above circumstances, an object of the present invention is to provide a single-layer stretched white polyester film having excellent weather resistance, light reflectivity and peel strength, a method for producing the same, and a back sheet for a solar cell and a solar cell module which contribute to achieving a high power generation efficiency in long-term use.

Means for solving the technical problem

In order to achieve the above object, the following invention is provided.

< 1 > an oriented white polyester film comprising a polyester and white particles, wherein the content of the white particles is 2 to 10% by mass based on the total mass of the film, and the average area per 1 of the white particles is 0.010 to 0.050 μm in a cross section in the film thickness direction²Pore of each.

< 2 > the stretched white polyester film according to < 1 >, wherein the ratio of the total area occupied by the voids is 0.5to 3% in the cross section in the thickness direction of the film.

< 3 > the stretched white polyester film according to < 1 > or < 2 >, wherein when the tear strength is P and the thickness of the film is t, P/t is 6.5 to 13.5mN/μm.

P is expressed in mN and t is expressed in μm.

< 4 > the stretched white polyester film according to any one of < 1 > to < 3 >, wherein the film has a thickness of 280 to 500. mu.m.

< 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 0.85d L/g.

< 6 > the stretched white polyester film according to any one of < 1 > to < 5 >, wherein the white particles are titanium oxide.

< 7 > a back sheet for a solar cell, comprising the stretched white polyester film of any one of < 1 > to < 6 >.

< 8 > the back sheet for a solar cell according to < 7 > having a coating layer on at least one side of a stretched white polyester film.

< 9 > a solar cell module comprising:

a solar cell element;

a sealing material sealing 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 back sheet for a solar cell according to < 7 > or < 8 > is disposed outside the sealing material on the side of the solar cell element opposite to the light-receiving surface side.

< 10 > a method for producing a stretched white polyester film, which comprises:

an extrusion step of melt-extruding a mixture containing a raw material polyester and white particles, and then cooling the extruded mixture to form an unstretched white polyester film; and

a longitudinal stretching step of stretching an unstretched white polyester film in a longitudinal direction and a transverse stretching step of stretching the white polyester film in a transverse direction,

the transverse stretching step comprises 1 st to Nth transverse stretching steps, wherein the nth transverse stretching step is performed after the nth-1 transverse stretching step, the stretching speed of the white polyester film in the width direction is increased in the nth transverse stretching step compared with the nth-1 transverse stretching step, and the stretching temperature is set to be 140-180 ℃ in the Nth transverse stretching step, and the stretching speed is set to be 8-25% per second of the length of the white polyester film in the width direction before the 1 st transverse stretching step is started.

N is an integer of 2 or more, and N is an integer of 2 to N.

< 11 > the process for producing a stretched white polyester film according to < 10 >, wherein the stretching speed in the width direction in the 1 st transverse stretching step is 4 to 10%/second.

< 12 > the process for producing a stretched white polyester film, according to < 10 > or < 11 >, wherein the stretching speed ratio Sb/Sa is 1.5to 6, where Sa is the stretching speed in the width direction in the 1 st transverse stretching step and Sb is the stretching speed in the width direction in the Nth transverse stretching step.

Effects of the invention

The present invention can provide a single-layer stretched white polyester film having excellent weather resistance, light reflectance and peeling resistance, a method for producing the same, and a back sheet for a solar cell and a solar cell module that contribute to achieving high power generation efficiency over a long period of use.

Drawings

Fig. 1 is a schematic view showing an example of a biaxial stretching machine used for producing a stretched white polyester film of the present disclosure.

Fig. 2 is a schematic view showing an example of a stretching mode of the white polyester film in the transverse stretching step in the production process of the stretched white polyester film of the present disclosure.

Detailed Description

Embodiments of the present invention will be described below, but the following embodiments are examples of the present invention, and the present invention is not limited to the following embodiments.

In the present specification, "to" indicating a numerical range is used in a meaning including numerical values described before and after the range as a lower limit value and an upper limit value. When only the upper limit value is described as a unit in the numerical range, the lower limit value is also expressed as the same unit as the upper limit value.

[ stretched white polyester film ]

The stretched white polyester film (hereinafter sometimes referred to as "white polyester film", "polyester film" or "film") of the present disclosure contains polyester and white particles, the content of the white particles relative to the total mass of the film is 2 to 10% by mass, and the average area per 1 of the white particles in a cross section in the film thickness direction is 0.010 to 0.050 [ mu ] m²Pore of each.

The stretched white polyester film of the present disclosure is excellent in weather resistance, reflectivity, and peeling resistance. The reason for this is presumed as follows.

It is considered that in the stretched polyester film mixed with white particles, voids generated by "stretching" for imparting weather resistance in particular cause a decrease in strength of the film. As a result of repeated studies, the present inventors found the following: if the porosity is too small, i.e., if the stretching is insufficient, sufficient weather resistance cannot be provided, whereas if it is too large, moisture is likely to enter the porosity and promote hydrolysis when exposed to the outside air for a long period of time, and cleavage and breakage of the film are likely to occur.

In contrast, the stretched white polyester film of the present disclosure can obtain high light reflectivity by having a white particle content of 2 mass% or more, and can maintain high strength by having a white particle content of 10 mass% or less.

Also, it is considered that the voids present in the stretched white polyester film of the present disclosure have an average area of 0.010 μm per 1 in a cross section in the film thickness direction²Is sufficiently stretched at a rate of at least one thereof to have high weather resistance, and has a pass size of 0.050 μm²No more than one, no large voids are present in the film, and cleavage breakage of the film is difficult to occur. Therefore, even if exposed to the outside air for a long period of time, the adhesiveness (peeling resistance) to other layers such as the sealing material is not easily lowered.

As a result of repeated studies, the present inventors have found that when a 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, the size of voids formed in the film can be controlled by controlling the stretching conditions in an appropriate range, and thus the single-layer stretched white polyester film of the present disclosure can be easily produced.

(polyester)

The kind of the polyester contained in the stretched white polyester film of the present disclosure is not particularly limited, and a known polyester can be used.

For example, a linear saturated polyester synthesized from an aromatic dibasic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof can be mentioned. Specific examples of the linear saturated polyester include polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, 1, 4-cyclohexanedimethylene terephthalate, and polyethylene-2, 6-naphthalenedicarboxylate. Among them, polyethylene terephthalate, polyethylene-2, 6-naphthalate, and 1, 4-cyclohexanedimethylene terephthalate are particularly preferable from the viewpoint of balance between mechanical properties and cost.

The polyester may be a homopolymer or a copolymer. The white polyester film of the present disclosure may contain, as a resin component, a resin obtained by mixing a small amount of another resin, for example, polyimide, with a polyester.

The kind of the polyester is not limited to the above polyester, 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.

The polyester can be obtained by, for example, subjecting the dicarboxylic acid component (a) and the diol component (b) to at least one of esterification and transesterification by a known method.

Examples of the dicarboxylic acid component (a) include aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosanedioic acid, pimelic acid, azelaic acid, methylmalonic acid and ethylmalonic acid; alicyclic dicarboxylic acids such as adamantane dicarboxylic acid, norbornene dicarboxylic acid, cyclohexane dicarboxylic acid, and decalin dicarboxylic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1, 4-naphthalenedicarboxylic acid, 1, 5-naphthalenedicarboxylic acid, 2, 6-naphthalenedicarboxylic acid, 1, 8-naphthalenedicarboxylic acid, 4 ' -diphenyldicarboxylic acid, 4 ' -diphenyletherdicarboxylic acid, 5-sodium isophthalate, phenylindanedicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, and 9,9 ' -bis (4-carboxyphenyl) fluorenic acid; and the like dicarboxylic acids or ester derivatives thereof.

Examples of the diol component (b) include aliphatic diols such as ethylene glycol, 1, 2-propanediol, 1, 3-propanediol, 1, 4-butanediol, 1, 2-butanediol, and 1, 3-butanediol; alicyclic diols such as cyclohexanedimethanol, spiroglycol and isosorbide; aromatic diols such as bisphenol a, 1, 3-benzenedimethanol, 1, 4-benzenedimethanol and 9, 9' -bis (4-hydroxyphenyl) fluorene; and the like.

As the dicarboxylic acid component (a), at least 1 kind of aromatic dicarboxylic acid is preferably used. More preferably, the aromatic dicarboxylic acid in the dicarboxylic acid component is contained as a main component. The "main component" herein means that the ratio of the aromatic dicarboxylic acid in the dicarboxylic acid component is 80% by mass or more. Dicarboxylic acid components other than aromatic dicarboxylic acids may also be contained. The dicarboxylic acid component is an ester derivative such as an aromatic dicarboxylic acid.

As the diol component (b), at least 1 of aliphatic diols is preferably used. The aliphatic diol may contain, for example, ethylene glycol, and preferably contains ethylene glycol as a main component. The main component herein means that the proportion of ethylene glycol in the glycol component is 80 mass% or more.

The amount of the aliphatic diol (e.g., ethylene glycol) to be used is preferably in the range of 1.015 to 1.50 mol based on 1mol of the aromatic dicarboxylic acid (e.g., terephthalic acid) and, if necessary, an ester derivative thereof. The amount of the aliphatic diol to be 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 to be used is in the range of 1.015 mol or more, the esterification reaction proceeds well, and when the amount is in the range of 1.50 mol or less, for example, by-production of diethylene glycol due to dimerization of ethylene glycol can be suppressed, and many properties such as melting point, glass transition temperature, crystallinity, heat resistance, hydrolysis resistance, and weather resistance can be maintained well.

A known reaction catalyst can be used for the esterification reaction or the transesterification reaction. 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, phosphorus compounds, and the like. It is generally 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 completion of the production of the polyester. In this method, for example, when a germanium compound is used as an example, it is preferable to directly add germanium compound powder.

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 the esterification reaction, it is preferable to use an organic chelate titanium complex having an organic acid as a ligand as a titanium compound serving as a catalyst, and to provide a process of adding at least the organic chelate titanium complex, a magnesium compound and a 5-valent phosphate having no aromatic ring as a substituent in this order in the step.

Specifically, in the esterification reaction step, first, before adding the magnesium compound and the phosphorus compound, the aromatic dicarboxylic acid and the aliphatic diol are mixed with the catalyst containing the organic chelate titanium complex as the titanium compound. Titanium compounds such as organic chelate titanium complexes also have high catalytic activity for esterification reactions, and thus can be favorably used for esterification reactions. In this case, the titanium compound may be added in the process of mixing the aromatic dicarboxylic acid component and the aliphatic diol component, or the aliphatic diol component (or the aromatic dicarboxylic acid component) may be mixed after mixing the aromatic dicarboxylic acid component (or the aliphatic diol component) and the titanium compound. Further, the aromatic dicarboxylic acid component, the aliphatic diol component and the titanium compound may be mixed at the same time. The mixing method is not particularly limited, and can be carried out by a known method.

Here, it is also preferable to add the following compound when the polymerization of the polyester is carried out.

As the 5-valent phosphorus compound, at least one of 5-valent phosphates having no aromatic ring as a substituent may be used. For example, a phosphate having a lower alkyl group having 2 OR less carbon atoms as a substituent [ (OR)₃-P ═ O; r ═ alkyl having 1 or 2 carbon atoms ], and specifically trimethyl phosphate, triethyl phosphate and the like are particularly preferable.

The amount of the phosphorus compound added is preferably an amount in the range of 50ppm to 90ppm in terms of the P element. The amount of the phosphorus compound is more preferably 60ppm to 80ppm in terms of the P element, and still more preferably 60ppm to 75 ppm.

By containing 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 50ppm or more in terms of Mg element, more preferably in the range of 50ppm to 100ppm, in order to impart high electrostatic applicability. The amount of the magnesium compound added is preferably in the range of 60ppm to 90ppm in terms of Mg element, and more preferably in the range of 70ppm to 80ppm, from the viewpoint of imparting electrostatic applicability.

In the esterification reaction step, it is particularly preferable to add a titanium compound as a catalyst component, a magnesium compound as an additive, and a phosphorus compound to melt-polymerize the titanium compound, the magnesium compound, and the phosphorus compound so that a value Z calculated from the following formula (i) satisfies the following relational formula (ii). Here, the P content is the amount of phosphorus derived from the entire phosphorus compound containing a 5-valent phosphate having no aromatic ring, and the Ti content is the amount of titanium derived from the entire Ti compound containing an organic chelate titanium complex. By selecting a catalyst system containing a titanium compound and using a magnesium compound and a phosphorus compound together and controlling the timing and the ratio of addition in this manner, a color tone with little yellowing can be obtained while maintaining a moderately high catalytic activity of the titanium compound, and heat resistance can be imparted to the catalyst system which is less likely to cause yellow coloration even when exposed to high temperatures during polymerization or during 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

Since the phosphorus compound not only acts on titanium but also interacts with the magnesium compound, it becomes an index quantitatively showing a balance of 3.

The formula (i) is a formula in which the amount of phosphorus that can act on titanium is subtracted from the total amount of phosphorus that can react. When the value Z is positive, phosphorus that inhibits titanium becomes excessive, and conversely, when the value Z is negative, phosphorus that inhibits titanium becomes insufficient. In the reaction, 1 atom of each of Ti, Mg, and P is not equivalent, and therefore, the molar number of each of the Ti, Mg, and P in the formula is multiplied by the valence number to be weighted.

In addition, the synthesis of polyester does not need to carry on special synthesis, and can use cheap and easily available titanium compound, phosphorus compound and magnesium compound to keep the reaction activity required for reaction, can obtain the color tone and heat resistance to the coloration excellent polyester.

In the formula (ii), from the viewpoint of further improving the color tone and the resistance to coloration against heat while maintaining the polymerization reactivity, it is preferable to satisfy 1.0. ltoreq. Z.ltoreq.4.0, and more preferably 1.5. ltoreq. Z.ltoreq.3.0.

As a preferred embodiment of the esterification reaction step, the following embodiment is preferred: preferably, 1to 30ppm of a chelate titanium complex having citric acid or a citrate as a ligand is added to an aromatic dicarboxylic acid and an aliphatic diol before the esterification reaction is completed; then, 60 to 90ppm (more preferably 70 to 80ppm) of a magnesium salt of a weak acid is added in the presence of a chelate titanium complex, and 60 to 80ppm (more preferably 65 to 75ppm) of a 5-valent phosphate having no aromatic ring as a substituent is added after the addition.

The esterification reaction step can be carried out while removing water or alcohol produced by the reaction out of the system under the condition of ethylene glycol reflux using a multistage apparatus in which at least 2 reactors are connected in series.

The esterification reaction step may be performed in one stage, or may be performed in a plurality of stages.

When the esterification reaction step is carried out in one stage, the esterification reaction temperature is preferably 230 to 260 ℃, more preferably 240 to 250 ℃.

When the esterification reaction step is carried out in a plurality of stages, the temperature of the esterification reaction in the first reaction tank is preferably 230 to 260 ℃, more preferably 240 to 250 ℃, and the pressure is preferably 1.0kg/cm²～5.0kg/cm²More preferably 2.0kg/cm²～3.0kg/cm². The temperature of the esterification reaction in the second reaction tank is preferably 230 to 260 ℃, more preferably 245 to 255 ℃ and the pressure is 0.5kg/cm²～5.0kg/cm²More preferably 1.0kgcm²～3.0kg/cm². In addition, when the esterification reaction is carried out in 3 stages or more, the conditions of the esterification reaction in the intermediate stage are preferably set to be the conditions between the first reaction tank and the final reaction tank.

On the other hand, the esterification reaction product produced in the esterification reaction is subjected to a polycondensation reaction to produce a polycondensate. The polycondensation reaction may be carried out in 1 stage, or may be carried out in a plurality of stages.

Next, the esterification reaction product such as an oligomer produced in the esterification reaction is subjected to polycondensation reaction. The polycondensation reaction can be preferably performed by supplying the esterification reaction product to the polycondensation reaction tanks in a plurality of stages.

For example, the polycondensation reaction conditions in the case of being carried out in 3-stage reaction vessels are preferably such that the reaction temperature in the first reaction vessel is 255 to 280 ℃, more preferably 265 to 275 ℃ and the pressure is 100to 10torr (13.3 × 10)^- ³MPa～1.3×10^-3MPa), more preferably 50to 20torr (6.67 × 10)^-3MPa～2.67×10^-3MPa), the reaction temperature of the second reaction vessel is 265 to 285 ℃, more preferably 270 to 280 ℃, and the pressure is 20to 1torr (2.67 × 10)^-3MPa～1.33×10^-4MPa), more preferably from 10to 3torr (1.33 × 10)^-3MPa～4.0×10^-4MPa), the reaction temperature of the third reaction vessel in the final reaction vessel is 270 to 290 ℃, more preferably 275 to 285 ℃, and the pressure is 10to 0.1torr (1.33 × 10)^-3MPa～1.33×10^-5MPa), more preferably 5to 0.5torr (6.67 × 10)^- ⁴MPa～6.67×10^-5MPa)。

The polyester synthesized as described above may further contain additives such as a light stabilizer, an antioxidant, an ultraviolet absorber, a flame retardant, a slipping agent (fine particles), a nucleating agent (crystallizing agent), and a crystallization inhibitor.

In the synthesis of the polyester, it is preferable to carry out solid-phase polymerization after polymerization by esterification reaction. By performing the solid-phase polymerization, the water content and crystallinity of the polyester, and the acid value of the polyester, that is, the concentration of the terminal carboxyl group and the intrinsic viscosity of the polyester can be controlled.

In particular, the Ethylene Glycol (EG) gas concentration at the start of solid-phase polymerization is preferably set to be higher in the range of 200ppm to 1000ppm, more preferably 250ppm to 800ppm, and further preferably 300ppm to 700ppm, than the EG gas concentration at the end of solid-phase polymerization, and solid-phase polymerization is performed. At this time, the terminal COOH concentration was controlled by adjusting the average EG gas concentration (average of gas concentrations at the start and end of solid-phase polymerization). That is, by adding EG to react with terminal COOH, the terminal COOH concentration can be reduced. EG is preferably 100ppm to 500ppm, more preferably 150ppm to 450ppm, and further preferably 200ppm to 400 ppm.

The temperature of the solid-phase polymerization is preferably 180 to 230 ℃, more preferably 190 to 215 ℃, and still more preferably 195 to 209 ℃.

The solid-phase polymerization time is preferably 10to 40 hours, more preferably 14 to 35 hours, and still more preferably 18 to 30 hours.

Here, the polyester preferably has high hydrolysis resistance. Therefore, the carboxyl group content in the polyester is preferably 50 equivalents/t or less (t represents ton, and ton represents 1000 kg.) more preferably 35 equivalents/t or less, and still more preferably 20 equivalents/t or less. When the carboxyl group content is 50 equivalents/t or less, hydrolysis resistance is maintained, and the decrease in strength with time due to moist heat can be suppressed to a small extent. The lower limit of the carboxyl group content is preferably 2 equivalents/t, more preferably 3 equivalents/t, from the viewpoint of maintaining adhesiveness to a layer (e.g., resin layer) formed of a polyester.

The carboxyl group content in the polyester can be adjusted by the kind of polymerization catalyst, film-forming conditions (film-forming temperature and time), solid-phase polymerization, additives (capping agent, etc.), and the like.

(blocking agent)

The white polyester film of the present disclosure can further improve hydrolysis resistance (weather resistance) by adding an end-capping agent.

The disclosed white polyester film can contain 0.1-10 mass% of an end-capping agent relative to the total mass of the polyester. The amount of the end-capping agent added is more preferably 0.2 to 5% by mass, and still more preferably 0.3 to 2% by mass, based on the total mass of the polyester contained in the polyester film.

Hydrolysis of polyester by H generated from carboxyl group at molecular terminal or the like⁺Since the catalytic effect of (a) is accelerated, it is effective to add a blocking agent which reacts with a terminal carboxyl group in order to improve hydrolysis resistance (weather resistance).

When the amount of the end-capping agent added is 0.1% by mass or more based on the total mass of the polyester, the effect of improving weather resistance is easily exhibited, and when the amount is 10% by mass or less, the effect of the end-capping agent as a plasticizer for the polyester is suppressed, and the decrease in mechanical strength and heat resistance can be suppressed.

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

The capping agent (especially a carbodiimide capping agent) is preferably of high molecular weight. Volatilization in melt film formation can be reduced by using a high molecular weight end-capping agent. The molecular weight of the blocking agent is preferably 200 to 10 ten thousand, more preferably 2000 to 8 ten thousand, and further preferably 1to 5 ten thousand. When the molecular weight of the end-capping agent (particularly, a carbodiimide end-capping agent) is in the range of 200 to 10 ten thousand, the end-capping agent is easily dispersed uniformly in the polyester, and the effect of improving weather resistance is easily exhibited. In addition, the end-capping agent is difficult to volatilize during extrusion and film formation, and the effect of improving weather resistance is easily exhibited.

The molecular weight of the blocking agent is a weight average molecular weight.

Carbodiimide-based capping agent:

the carbodiimide compound having a carbodiimide group includes monofunctional carbodiimide and polyfunctional carbodiimide, and examples of the monofunctional carbodiimide include dicyclohexylcarbodiimide, diisopropylcarbodiimide, dimethylcarbodiimide, diisobutylcarbodiimide, dioctylcarbodiimide, t-butylisopropylcarbodiimide, diphenylcarbodiimide, di-t-butylcarbodiimide, di- β -naphthylcarbodiimide, and the like.

As the polyfunctional carbodiimide, a carbodiimide having a polymerization degree of 3to 15 can be preferably used. Specifically, 1, 5-naphthalene carbodiimide, 4 ' -diphenylmethane carbodiimide, 4 ' -diphenyldimethylmethane carbodiimide, 1, 3-phenylene carbodiimide, 1, 4-phenylene diisocyanate, 2, 4-toluene carbodiimide, 2, 6-toluene carbodiimide, a mixture of 2, 4-toluene carbodiimide and 2, 6-toluene carbodiimide, hexamethylene carbodiimide, cyclohexane-1, 4-carbodiimide, xylylene carbodiimide, isophorone carbodiimide, dicyclohexylmethane-4, 4 ' -carbodiimide, methylcyclohexane carbodiimide, tetramethylxylylene carbodiimide, 2, 6-diisopropylphenyl carbodiimide and 1,3, 5-triisopropylbenzene-2, 4-carbodiimide, and the like.

Since the carbodiimide compound generates an isocyanate gas by thermal decomposition, a carbodiimide compound having high heat resistance is preferable. In order to improve the heat resistance, the higher the molecular weight (polymerization degree), the more preferable the terminal of the carbodiimide compound is to have a structure having high heat resistance. Further, when thermal decomposition occurs, further thermal decomposition is likely to occur, and therefore it is necessary to set the extrusion temperature of the polyester as low as possible.

The carbodiimide of the end-capping agent is also preferably a carbodiimide having a cyclic structure (for example, a carbodiimide having a cyclic structure described in Japanese patent application laid-open No. 2011-153209). The carbodiimide having a cyclic structure exhibits the same effect as that of the above-mentioned high-molecular-weight carbodiimide even when the molecular weight is low. This is because the terminal carboxyl group of the polyester and the cyclic carbodiimide undergo a ring-opening reaction, one of which reacts with the polyester and the other of which reacts with the other polyester to increase the molecular weight, and therefore generation of isocyanate gas can be suppressed.

Among carbodiimides having a cyclic structure, it is preferable in the present disclosure that the end-capping agent is a carbodiimide compound containing a cyclic structure having a carbodiimide group with a first nitrogen and a second nitrogen thereof bonded through a bonding group. The end-capping agent is more preferably a carbodiimide (also referred to as an aromatic cyclic carbodiimide) having a cyclic structure in which at least 1 carbodiimide group adjacent to an aromatic ring is present and a first nitrogen and a second nitrogen of the carbodiimide group adjacent to the aromatic ring are bonded to each other through a bonding group.

The aromatic cyclic carbodiimide may have a plurality of cyclic structures.

The aromatic cyclic carbodiimide can also preferably use an aromatic carbodiimide having a ring structure in which a first nitrogen and a second nitrogen which do not have 2 or more carbodiimide groups in the molecule are bonded to each other through a linking group, that is, a monocyclic aromatic carbodiimide.

The cyclic structure has 1 carbodiimide group (-N ═ C ═ N —) and its first nitrogen and second nitrogen are bonded through a bonding group. One cyclic structure has only 1 carbodiimide group, and when a plurality of cyclic structures such as spiro rings are present in the molecule, if 1 carbodiimide group is present in each cyclic structure bonded to a spiro atom, a plurality of carbodiimide groups may be present as a compound. The number of atoms in the cyclic structure is preferably 8 to 50, more preferably 10to 30, further preferably 10to 20, and particularly preferably 10to 15.

The number of atoms in the cyclic structure is the number of atoms directly constituting the cyclic structure, and is, for example, 8 in the case of an 8-membered ring and 50 in the case of a 50-membered ring. When the number of atoms in the cyclic structure is 8 or more, the stability of the cyclic carbodiimide compound is increased, and the storage and use are easy. From the viewpoint of reactivity, the upper limit of the number of ring members is not particularly limited, but a cyclic carbodiimide compound having an atomic number of 50 or less is less difficult to synthesize and can be kept at a low cost. From this viewpoint, the number of atoms in the cyclic structure is preferably 10to 30, more preferably 10to 20, and particularly preferably 10to 15.

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

[ chemical formula 1]

Epoxy-based end-capping agent:

preferable examples of the epoxy compound include glycidyl ester compounds and glycidyl ether compounds.

Specific examples of the glycidyl ester compound include glycidyl benzoate, glycidyl tert-butylbenzoate, glycidyl p-toluate, glycidyl cyclohexanecarboxylate, glycidyl pelargonate, glycidyl stearate, glycidyl laurate, glycidyl palmitate, glycidyl behenate, glycidyl versatate, glycidyl oleate, glycidyl linoleate, glycidyl linolenate, glycidyl behenate, glycidyl stearate, diglycidyl terephthalate, diglycidyl isophthalate, diglycidyl phthalate, diglycidyl naphthalenedicarboxylate, diglycidyl methylphthalate, diglycidyl hexahydrophthalate, diglycidyl tetrahydrophthalate, diglycidyl cyclohexanedicarboxylate, glycidyl p-toluate, glycidyl n-toluate, glycidyl ethyl toluate, glycidyl ether, glycidyl, 1 or 2 or more species of diglycidyl adipate, diglycidyl succinate, diglycidyl sebacate, diglycidyl dodecanedioate, diglycidyl octadecanedioate, triglycidyl trimellitate, and tetraglycidyl pyromellitate 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-benzoyloxyethane, and bisglycidyl polyethers obtained by the reaction of a bisphenol such as 2, 2-bis- [ p- (β, γ -epoxypropoxy) phenyl ] propane, 2-bis- (4-hydroxyphenyl) methane with epichlorohydrin, and 1 or 2 or more of these may be used.

Oxazoline-based blocking agent:

as the oxazoline compound, a bisoxazoline compound is preferable, and specifically, 2 '-bis (2-oxazoline), 2' -bis (4-methyl-2-oxazoline), 2 '-bis (4, 4-dimethyl-2-oxazoline), 2' -bis (4-ethyl-2-oxazoline), 2 '-bis (4, 4' -diethyl-2-oxazoline), 2 '-bis (4-propyl-2-oxazoline), 2' -bis (4-butyl-2-oxazoline), 2 '-bis (4-hexyl-2-oxazoline), 2' -bis (4-phenyl-2-oxazoline) and the like can be exemplified, 2,2 ' -bis (4-cyclohexyl-2-oxazoline), 2 ' -bis (4-benzyl-2-oxazoline), 2 ' -p-phenylenebis (2-oxazoline), 2 ' -m-phenylenebis (2-oxazoline), 2 ' -o-phenylenebis (2-oxazoline), 2 ' -p-phenylenebis (4-methyl-2-oxazoline), 2 ' -p-phenylenebis (4, 4-dimethyl-2-oxazoline), 2 ' -m-phenylenebis (4-methyl-2-oxazoline), 2 ' -m-phenylenebis (4, 4-dimethyl-2-oxazoline), 2,2 ' -ethylenebis (2-oxazoline), 2 ' -tetramethylenebis (2-oxazoline), 2 ' -hexamethylenebis (2-oxazoline), 2 ' -octamethylenebis (2-oxazoline), 2 ' -decamethylenebis (2-oxazoline), 2 ' -ethylenebis (4-methyl-2-oxazoline), 2 ' -tetramethylenebis (4, 4-dimethyl-2-oxazoline), 2 ' -9,9 ' -diphenoxyethanedibis (2-oxazoline), 2 ' -cyclohexylenebis (2-oxazoline), 2 ' -diphenylenebis (2-oxazoline), and the like. Among these, 2' -bis (2-oxazoline) may be most preferably used from the viewpoint of reactivity with a polyester. The bisoxazoline compounds mentioned above may be used alone or in combination of two or more kinds as long as the object of the present invention is achieved.

Such an end-capping agent is added to, for example, a resin layer on a polyester film, and the polyester and the end-capping agent do not react with each other, and therefore, it is necessary to incorporate the end-capping agent in the production of the polyester film so as to directly react with the polyester molecules.

(white particles)

The disclosed white polyester film contains 2-10 mass% of white particles relative to the total mass of the film. The white polyester film of the present disclosure can have a high light reflectance when the content of white particles contained therein is 2 mass% or more, and can maintain a high strength when the content is 10 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, based on the white polyester film.

The content of white particles contained in the white polyester film can be measured by the following method.

3g of the film was weighed in a crucible as a measurement sample, and heated at 900 ℃ for 120 minutes in an electric oven. Then, the crucible was taken out after the inside of the electric oven was cooled, and the mass of ash remaining in the crucible was measured. This ash content is a white particle component, and the mass of the ash content is divided by the mass of the measurement sample and multiplied by 100to obtain a white particle content (% by mass).

Before the production of the film, the content can be determined from the amount of the white particles (white pigment) used as a raw material.

The average particle diameter of the white particles is preferably 0.03 to 0.25. mu.m, more preferably 0.07 to 0.25. mu.m, and still more preferably 0.1to 0.2. mu.m. When the average particle diameter of the white particles is 0.03 to 0.25 μm, not only is the whiteness of the film improved, but also the average area per 1 film cross section easily formed from the white particles is 0.010 to 0.050 μm²Pore of each.

The average particle diameter of the white particles contained in the white polyester film in the present disclosure is determined by a method using an electron microscope. Specifically, the following method is used.

White particles in a cross section of the film in the thickness direction are observed by a scanning electron microscope, and the magnification is appropriately changed according to the size of the particles, and a photograph is taken and magnified and copied. For at least 200 particles selected randomly, the periphery of each particle is tracked. From these trace images, the equivalent circle diameters of the particles were measured by an image analyzer, and the average value of the diameters was defined as the average particle diameter.

In addition, before the production of the film, the average particle diameter can be determined in the same manner as described above for at least 200 particles randomly selected from white particles (white pigments) used as a raw material.

Examples of the white particles contained in the white polyester film of the present disclosure 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 dioxide, talc, clay, kaolin, lithium fluoride, calcium fluoride, and the like. Titanium oxide and barium sulfate are preferable from the viewpoint of cost and availability. The surface of the particles may be subjected to inorganic treatment such as alumina or silica, or may be subjected to organic treatment such as siloxane or alcohol.

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

Titanium oxide exists in rutile type and anatase type, and it is preferable that titanium oxide particles mainly containing rutile type are added to the white polyester film in the present disclosure to make the film white. While the spectral reflectance of the ultraviolet ray of rutile type is very high, anatase type has a characteristic of a large ultraviolet ray absorption rate (a small spectral reflectance). Focusing on the difference in spectral characteristics among the crystal forms of titanium oxide, the light resistance (durability of deterioration due to ultraviolet rays) can be improved in the back sheet for a solar cell by utilizing the ultraviolet absorption performance of rutile type. Thus, the film has excellent durability under light irradiation without substantially adding another ultraviolet absorber. Therefore, contamination due to the bleeding of the ultraviolet absorber and a decrease in adhesion are less likely to occur.

As described above, the titanium oxide particles according to the present disclosure preferably have a rutile type as a main component, and the term "main component" as used herein means that the amount of rutile type titanium oxide in the total titanium oxide particles exceeds 50 mass%.

Further, the amount of anatase titanium oxide in the total titanium oxide particles is preferably 10 mass% or less. More preferably 5% by mass or less, and particularly preferably 0% by mass. Rutile type titanium oxide and anatase type titanium oxide can be distinguished according to X-ray structure diffraction or spectral absorption characteristics.

The rutile titanium oxide particles may be surface-treated with an inorganic material such as alumina or silica, or may be surface-treated with an organic material such as siloxane or alcohol. The rutile type titanium oxide may be subjected to particle size adjustment or coarse particle removal using a purification process before blending with the polyester. As an industrial method of the purification process, for example, a jet mill or a ball mill can be applied as a pulverization method, and for example, dry or wet centrifugal separation can be applied as a classification method.

In the present disclosure, white particles of an organic material can also be used. As the white particles of the organic material, particles resistant to heat in polyester film formation are preferable, and for example, particles containing a crosslinking resin, specifically, polystyrene crosslinked with divinylbenzene, or the like can be used.

The white polyester film of the present disclosure may contain 1 or 2 or more kinds of white particles. When 2 or more kinds of white particles are contained, the total content of the white particles is set to 2 to 10% by mass.

(pores)

The disclosed white polyester film has an average area of 0.010-0.050 [ mu ] m per 1 piece in a cross section in the film thickness direction²Pores per (voids). The voids in the white polyester film of the present disclosure are formed by stretching to peel at the interface between the white particles and the polyester for imparting weather resistance, and the white particles present in the voids are included as the area of the voids.

The average area per 1 pore contained in the film was 0.010 μm²More than one, can be sufficiently stretched to impart high weather resistance, and has a pass rate of 0.050 μm²And/or less, the amount of moisture that enters into the pores when exposed to the outside air for a long period of time can be reduced, and the occurrence of cleavage and breakage of the film can be suppressed by suppressing hydrolysis.

From this viewpoint, the average area per 1 pore in the cross section in the film thickness direction is preferably 0.01 to 0.05 μm²A, more preferably 0.02 to 0.05 μm²A/one.

In a cross section of the film in the thickness direction, the ratio of the total area occupied by the pores (pore occupied area) is preferably 0.5to 3%. When the proportion of voids in the film is 0.5% or more, the film can be sufficiently stretched to impart high weather resistance and sufficient light reflectivity. On the other hand, if the proportion of pores in the film is 3% or less, hydrolysis can be suppressed when exposed to the outside air for a long period of time, and cleavage breakage of the film can be suppressed.

From this viewpoint, the total area (occupied area) occupied by the pores in the cross section of the film in the thickness direction is more preferably 0.6 to 3%, and still more preferably 0.6 to 2.8%.

The fine voids (pores) present in the white polyester film of the present disclosure can be mainly formed from white particles. The voids derived from the white particles mean that voids are present around the white particles, and can be confirmed, for example, on a cross-sectional photograph of a white polyester film obtained by an electron microscope. Further, there are also pores in which primary particles of 2 or more white particles are aggregated to form voids around the aggregated particles, or pores in which white particles are dropped off during the observation operation and no white particles are present in the pores.

The method for measuring the occupied area of the voids and the average area per 1 void contained in the white polyester film of the present disclosure was the method described in the examples.

In addition, in order to increase the whiteness, the white polyester film of the present disclosure preferably further uses a fluorescent whitening agent such as a thiophenediyl group. The amount of the fluorescent whitening agent added is preferably 0.01 to 1% by mass, more preferably 0.05 to 0.5% by mass, and still more preferably 0.1to 0.3% by mass. Within this range, the effect of improving the light reflectance is easily obtained, and yellowing due to thermal decomposition during extrusion can be suppressed, and a decrease in reflectance can be suppressed. As such a fluorescent whitening agent, OB-1 manufactured by Eastman Kodak Company, for example, can be used.

(thickness)

The thickness of the white polyester film is preferably 280 to 500 [ mu ] m. When the thickness of the film is 280 μm or more, the film can have high withstand voltage. On the other hand, when the film thickness is 500 μm or less, it is possible to suppress the deterioration of hydrolysis resistance due to the deterioration of the film temperature-raising cooling ability during film formation, and to stretch the film without applying a high load to the stretching machine during film stretching.

From this viewpoint, the thickness of the thin film is more preferably 290 to 450 μm.

The method for measuring the thickness of the white polyester film of the present disclosure was the method described in examples.

(intrinsic viscosity)

In the white polyester film of the present disclosure, the Intrinsic Viscosity (IV) of the film is preferably 0.65 to 0.85d L/g.

On the other hand, if the IV of the film is 0.85d L/g or less, shear heat generation in the extrusion step can be suppressed in the production of the film, and deterioration of hydrolysis resistance can be suppressed.

From this viewpoint, the IV of the film is more preferably 0.67 to 0.80d L/g, still more preferably 0.68 to 0.77d L/g.

The method for measuring IV of the white polyester film of the present disclosure was the method described in examples.

(tear Strength)

In the white polyester film of the present disclosure, when the tear strength is P (mN) and the thickness of the film is t (μm), P/t is preferably 6.5 to 13.5mN/μm.

When the tear strength (P/t) per unit thickness of the film is 6.5mN/μm or more, the occurrence of cleavage breakage of the film can be suppressed when exposed to the outside air for a long period of time. On the other hand, when the tear strength (P/t) per unit thickness of the film is 13.5mN/μm or less, sufficient weather resistance can be imparted.

From this viewpoint, the tear strength (P/t) per unit thickness of the film is more preferably 7.0 to 13.5mN/μm, and still more preferably 7.0 to 13.0mN/μm.

The method for measuring the tear strength of the white polyester film of the present disclosure was the method described in examples.

(terminal carboxyl group concentration)

In the white polyester film of the present disclosure, the terminal carboxyl group concentration is preferably 5to 25 equivalents/ton. The terminal carboxyl group concentration is also referred to as Acid value (Acid value), and is sometimes referred to as "AV". In the present specification, "equivalent/ton" means a molar equivalent per 1 ton, and may be referred to as "eq/t".

When the terminal carboxyl group concentration in the polyester film is 5 equivalents/ton or more, the carboxyl groups (COOH groups) on the surface are not too small (i.e., the polarity is not too low), and high adhesiveness to different materials such as other resin layers can be obtained.

On the other hand, H of COOH group at the end of polyester molecule⁺And acts as a catalyst to promote hydrolysis. When the terminal carboxyl group concentration in the polyester film is 25 equivalents/ton or less, the deterioration of hydrolysis resistance can be suppressed.

The terminal carboxyl group concentration was determined by dissolving 0.1g of a resin sample in 10ml of benzyl alcohol, adding chloroform to the solution to obtain a mixed solution, adding a phenol red indicator dropwise to the mixed solution, titrating the solution with a standard solution (0.01 mol/L KOH-benzyl alcohol mixed solution), and determining the terminal carboxyl group concentration from the amount of dropwise addition.

Method for producing < stretched white polyester film >

The method for producing the stretched white polyester film of the present disclosure is not particularly limited, and for example, the stretched white polyester film of the present disclosure can be preferably produced by the following method.

That is, the method for producing a stretched white polyester film of the present disclosure comprises: an extrusion step of melt-extruding a mixture containing a raw material polyester and white particles, and then cooling the extruded mixture to form an unstretched white polyester film; and a stretching step of stretching an unstretched white polyester film in at least one of a longitudinal direction and a width direction, wherein the stretching step includes 1 st to nth stretching steps of stretching the white polyester film in at least one of the longitudinal direction and the width direction, and the nth stretching step is performed subsequent to the N-1 st stretching step, and wherein a stretching speed of the white polyester film in one of the longitudinal direction and the width direction is increased in the nth stretching step as compared with the N-1 st stretching step, and wherein a stretching temperature is set to 140 to 180 ℃, and the stretching speed is set to be increased by 8 to 25%/sec per one-directional length with respect to the one-directional length of the white polyester film before the start of the 1 st stretching step. 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 disclosure, the stretching step is preferably followed by a heat setting step and a heat relaxing step.

Further, in-line coating (inline coat) for forming the undercoat layer may be performed after the formation of the unstretched white polyester film and before the stretching step, or after the stretching in one direction and before the stretching in the other direction.

Hereinafter, each step will be specifically described, but the method for producing the white polyester film of the present disclosure is not limited to the following method.

(extrusion Process)

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

For example, an unstretched white polyester film is obtained by drying and melting the above white particles such as polyester and titanium oxide as raw materials, passing the obtained melt (melt) through a gear pump and a filter, extruding the melt through a die onto a cooling roll (casting drum), and cooling and solidifying the melt. The melting may be performed using an extruder, a single screw extruder, or a multi-screw extruder having two or more screws.

When the white particles are blended into the polyester film, various known methods can be used. Typical examples thereof include the following methods.

(A) A method of adding white particles before finishing the transesterification or esterification reaction in the synthesis of the polyester, or adding white particles before starting the polycondensation reaction.

(B) A method of adding white particles to polyester and melt-kneading the mixture.

(C) A method of producing a master batch (also referred to as "master batch") containing a large amount of white particles added by the method (a) or (B), and a method of kneading the master batch and a polyester containing no white particles or a small amount of white pigment to contain a predetermined amount of white particles.

(D) A method of directly using the master batch of the above (C) and melt-kneading the same.

Among them, the method (C) is preferably a method of producing a master batch (hereinafter, sometimes referred to as "MB") to which a large amount of white particles are added, and a method of kneading the master batch and a polyester containing no white particles or a small amount of white pigment to contain a predetermined amount of white particles (hereinafter, sometimes referred to as "master batch method"). Further, a method of preparing a master batch by feeding polyester and white particles, which have not been dried in advance, into an extruder and degassing moisture, air, or the like may be employed. In addition, it is preferable to use a polyester which has been dried in advance to some extent to prepare a master batch, and in this case, the increase in the acid value of the polyester can be suppressed. In this case, there may be mentioned a method of extruding the polyester while degassing, a method of extruding the polyester sufficiently dried without degassing, and the like.

For example, the polyester resin to be put in the preparation of the Master Batch (MB) is preferably dried to reduce the moisture content. The drying conditions are preferably 100to 200 ℃, more preferably 120 to 180 ℃, for 1 hour or more, more preferably 3 hours or more, and still more preferably 6 hours or more. Thus, the polyester resin is sufficiently dried so that the moisture content of the polyester resin becomes preferably 50ppm or less, more preferably 30ppm or less.

The method of premixing is not particularly limited, and may be a batch method, or may be a premixing using a kneading extruder of a single screw or twin screws or more. When the master batch is prepared while degassing, the following methods are preferably employed: the polyester resin is melted at a temperature of 250 to 300 ℃, preferably 270 to 280 ℃,1, preferably 2 or more deaeration ports are provided in the preliminary mixer, continuous suction deaeration of 0.05MPa or more, more preferably 0.1MPa or more is performed, and the reduced pressure in the mixer is maintained.

The extrusion of the molten resin (melt) is preferably performed under vacuum exhaust or an inert gas atmosphere.

The melting temperature in the extruder is preferably from the melting point of the polyester used to the melting point +80 ℃ or lower, more preferably from the melting point +10 ℃ or higher to the melting point +70 ℃ or lower, and still more preferably from the melting point +20 ℃ or higher to the melting point +60 ℃ or lower. The melting temperature in the extruder is preferably +10 ℃ or higher because the resin is sufficiently melted, and +70 ℃ or lower because decomposition of the polyester or the like can be suppressed. The raw material polyester is preferably dried in advance before the raw material is fed into the extruder, and the water content is preferably 10ppm to 300ppm, more preferably 20ppm to 150 ppm.

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

The end-capping agent may be added directly to the extruder together with the polyester or the like, but from the viewpoint of extrusion stability, it is preferable to form the polyester and the master batch in advance and then feed the polyester and the master batch to the extruder.

The extruded melt (melt) was cast on a casting drum (chill roll) by a gear pump, a filter, and a die. The shape of the mould can be any one of a T-shaped mould, a clothes hanger type mould and a fishtail. On the casting roll, the molten resin (melt) can be made to adhere to the cooling roll using an electrostatic application method. The surface temperature of the casting drum can be set to approximately 10 ℃ to 40 ℃. The diameter of the casting drum is preferably 0.5m or more and 5m or less, more preferably 1m or more and 4m or less. The driving speed of the casting drum (the outermost peripheral linear speed) is preferably 1 m/min to 50 m/min, more preferably 3 m/min to 30 m/min.

(stretching Process)

In the stretching step, an unstretched white polyester film (hereinafter, may be referred to as an unstretched polyester film) formed in the extrusion step is stretched in at least one Direction of a longitudinal Direction (Machine Direction (MD)) and a Transverse Direction (TD).

The stretching step includes 1 st to nth stretching steps for stretching the white polyester film in at least one of the longitudinal direction and the width direction, and the nth stretching step is performed subsequent to the N-1 st stretching step, wherein the stretching speed of the white polyester film in one of the longitudinal direction and the width direction is increased in the nth stretching step as compared with the N-1 th stretching step, and the stretching temperature is set to 140 ℃ to 180 ℃ in the nth stretching step, and the stretching speed is set to be increased by 8% to 25%/sec in length relative to the length of the white polyester film in the one direction before the start of the 1 st stretching step.

When an unstretched polyester film is stretched in the longitudinal direction or the width direction, crystallization of the polyester is suppressed by stretching at a relatively slow stretching speed in the initial stage of stretching, and the stretching speed is increased to a desired stretching ratio in a state where the polyester is sufficiently heated in the final stage of stretching, whereby peeling between the white particles and the polyester can be maintained at the minimum, minute spaces are formed around the white particles, and minute voids are easily formed.

In the stretching step, the film may be stretched only in one of the longitudinal direction and the width direction by uniaxial stretching or biaxial stretching in both the longitudinal direction and the width direction, but from the viewpoint of improving weather resistance, biaxial stretching is preferable, and from the viewpoint of easier stretching, stretching in the longitudinal direction (sometimes referred to as "longitudinal stretching") and then stretching in the width direction (sometimes referred to as "transverse stretching") are more preferable. In the case of biaxial stretching, an unstretched white polyester film is longitudinally stretched in the longitudinal direction, and then, as a stretching step (transverse stretching step) of transversely stretching in the width direction, 1 st to nth transverse stretching steps (N is an integer of 2 or more) are performed. The nth transverse stretching step (N is an integer of 2 to N) is performed in succession to the nth-1 transverse stretching step, and the stretching speed of the white polyester film in the width direction is increased in the nth transverse stretching step as compared with the nth-1 transverse stretching step. In the Nth transverse stretching step, the stretching temperature is preferably 140 to 180 ℃, and the stretching speed is preferably 8 to 25% per second of the length of the white polyester film in the transverse direction before the 1 st transverse stretching step.

Hereinafter, a case where stretching is performed in the width direction 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 disclosure. Fig. 1 shows a biaxial stretching machine 100 and a polyester film 200 attached to the biaxial stretching machine 100. The biaxial stretcher 100 includes 1 pair of endless guides 60a and 60b, which are symmetrically arranged with a polyester film 200 interposed therebetween.

The biaxial stretcher 100 is divided into: a preheating part 10 for preheating the polyester film 200; a stretching unit 20 for stretching the polyester film 200 in a direction perpendicular to the arrow MD, that is, in the arrow TD to apply a stretching force to the polyester film; a heat-setting part 30 for heating the polyester film with the stretching force applied thereto; a thermal relaxation part 40 for heating the heat-set polyester film to relieve the stretching force of the heat-set polyester film; and a cooling part 50 for cooling the polyester film passing through the thermal relaxation part.

The ring-shaped guide 60a includes at least the grasping

members

2a, 2b, 2e, 2f, 2i, and 2j movable at the edge of the ring-shaped guide 60a, and the ring-shaped guide 60b includes at least the grasping

members

2c, 2d, 2g, 2h, 2k, and 2l movable at the edge of the ring-shaped guide 60 b. The grasping

members

2a, 2b, 2e, 2f, 2i, and 2j grasp one end portion of the TD of the polyester film 200, and the grasping

members

2c, 2d, 2g, 2h, 2k, and 2l grasp the other end portion of the TD of the polyester film 200. The gripping members 2a to 2l are generally called chucks, jigs, and the like.

In fig. 1, the grasping

members

2a, 2b, 2e, 2f, 2i and 2j move counterclockwise along the edge of the ring-shaped guide 60a, and the grasping

members

2c, 2d, 2g, 2h, 2k and 2l move clockwise along the edge of the ring-shaped guide 60 b.

The grasping members 2a to 2d grasp the end of the polyester film 200 in the preheating section 10, move along the edge of the endless guide 60a or 60b in this state, and advance to the cooling section 50 showing the grasping members 2i to 2l through the stretching section 20 and the thermal relaxation section 40 showing the grasping members 2e to 2 h. Then, the

gripper members

2a and 2b and the gripper members 2c and 2d release the end of the polyester film 200 at the end portion on the MD downstream side of the cooling section 50 in order in the conveying direction, and in this state, advance along the edge of the endless guide 60a or 60b, and return to the preheating section 10.

As a result, the polyester film 200 moves along an arrow MD in fig. 1, and is sequentially conveyed to the preheating section 10, the stretching section 20, the heat-setting section 30, the thermal relaxing section 40, and the cooling section 50.

The moving speed of the gripping members 2a to 2l is the transport speed of the polyester film 200 at the gripping portion.

The gripping members 2a to 2l can change the moving speed independently of each other.

Therefore, the biaxial stretcher 100 can stretch the polyester film 200 in the transverse direction in which the stretching unit 20 can stretch the polyester film 200 in the TD, and can also stretch the polyester film 200 in the MD by changing the moving speed of the grip members 2a to 2 l.

That is, simultaneous biaxial stretching can be performed using the biaxial stretcher 100.

In fig. 1, only 12 gripping members 2a to 2l that grip the end of the TD of the polyester film 200 are shown, but the biaxial stretcher 100 includes not shown gripping members in addition to 2a to 2l in order to support the polyester film 200.

Hereinafter, the grasping members 2a to 2l may be collectively referred to as "grasping member 2".

(preheating section)

The polyester film 200 is preheated in the preheating section 10. The polyester film 200 is previously heated before being stretched to facilitate the transverse stretching of the polyester film 200.

When the glass transition temperature of the polyester film 200 is Tg, the film surface temperature at the end point of the preheating section (hereinafter, also referred to as "preheating temperature") is preferably Tg-10 to Tg +60 ℃, more preferably Tg to Tg +50 ℃.

The preheating section end point is a point at which the preheating of the polyester film 200 is finished, that is, a position of the polyester film 200 away from the region of the preheating section 10.

(stretching part)

The preheated polyester film 200 is stretched at least in the Transverse Direction (TD) orthogonal to the longitudinal direction (conveyance direction, MD) of the polyester film 200 in the stretching section 20to impart a stretching force to the polyester film 200.

Stretching in The Direction (TD) orthogonal to the longitudinal direction (conveyance direction, MD) of the polyester film 200 (transverse stretching) means stretching in the direction perpendicular (90 °) to the longitudinal direction (conveyance direction, MD) of the polyester film 200.

Longitudinal stretching-

In the biaxial stretching, the white unstretched polyester film formed in the extrusion step is longitudinally stretched in the longitudinal direction of the polyester film with a tensile stress of, for example, 5MPa to 15MPa and a stretching ratio of 2.5 to 4.5.

More specifically, the polyester film is guided to a roll group heated to a temperature of 70 ℃ or higher and 120 ℃ or lower, and is subjected to longitudinal stretching in the longitudinal direction (longitudinal direction, i.e., the direction of advance of the film) at a tensile stress of 5MPa or higher and 15MPa or lower and at a stretch ratio of 2.5 times or higher and 4.5 times or lower, more preferably at a tensile stress of 8MPa or higher and 14MPa or lower and at a stretch ratio of 3.0 times or higher and 4.0 times or lower. After the longitudinal stretching, the sheet is preferably cooled by a roll stack having a temperature of 20 ℃ or higher and 50 ℃ or lower.

Transverse stretching-

The longitudinal stretching is followed by the transverse stretching. The transverse stretching is preferably performed using a tenter. The longitudinally stretched white polyester film is guided to a tenter, and stretched in the width direction (transverse stretching) in an atmosphere heated to a temperature (stretching temperature) of 80 ℃ or higher and 180 ℃ or lower, for example. In the tenter, both ends of the polyester film are gripped by clips, the polyester film is conveyed in a heat treatment zone, and the clips are spread in the width direction, which is a direction perpendicular to the longitudinal direction, so that the polyester film can be transversely stretched.

The stretching in the width direction is performed in the 1 st to Nth transverse stretching steps (N is an integer of 2 or more), the nth transverse stretching step (N is an integer of 2 to N) is performed in succession to the nth-1 transverse stretching step, and the stretching speed of the white polyester film in the width direction is increased in the nth transverse stretching step as compared with the nth-1 transverse stretching step. In the Nth transverse stretching step, the stretching temperature is set to be 140-180 ℃, and the stretching speed is set to be 8-25% per second of the length of the white polyester film in the width direction before the 1 st transverse stretching step is started.

Fig. 2 schematically shows an example of a mode of stretching in the transverse direction in stages in the production of the white polyester film of the present disclosure.

In fig. 2, the position a is a position at one end in the width direction (TD) of the polyester film in a state where the polyester film is located in the preheating section and before the transverse stretching is started. The position B is a position at one end in the width direction (TD) of the polyester film in a state where the polyester film is positioned in the heat-set section and the transverse stretching is completed.

T1, T2 and T3 are positions at one end in the width direction (TD) of the polyester film at which the polyester film starts to be stretched in the transverse direction at the stretching speeds of the 1 st stage, the 2 nd stage and the 3 rd stage, respectively, in the stretching section. The widthwise (TD) end of the polyester film passes through T1, T2, and T3 from position a to position B.

In the present disclosure, when the transverse stretching is performed, the change in the stretching speed is not limited to 3 stages, but may be 2 or more stages, but is preferably changed in 3to 8 stages from the viewpoint of easy manufacturability.

In the transverse stretching, the film is stretched in the TD at each position of T1, T2, and T3 while the width of the film is gradually widened. The film is continuously conveyed in MD, stretching in TD is started at T1, and the film reaches T2 at a stretching angle theta with respect to TD₁Stretching is performed along the TD. The stretching speed in TD can be determined according to the conveying speed in MD and the stretching angle theta₁And (6) adjusting. That is, if the conveying speed in the MD is constant, the stretching angle θ is increased more₁The stretching speed along TD increases. The stretching speed can be determined from the length (width W) of the film TD before the start of transverse stretching₀) Is expressed as an increase in width per 1 second. For example, when the stretching speed in TD is changed in 3 stages as shown in FIG. 2, the width W of the film before the transverse stretching is started will be₀When the film is stretched along the TD so that the film width increases at a rate of 5 per 1 second from T1 to T2, the stretching speed in the TD is 5%/second, which is 100.

In the present embodiment, the stretching in the TD direction is startedIncreasing the stretching speed by increasing the stretching angle in stages or continuously while the film is conveyed in the MD from the start to the end of stretching, and increasing the stretching speed to the length W in the width direction before the stretching in the width direction is started at the stretching temperature of 140 to 180 ℃ in the Nth transverse stretching step₀Increasing the stretching speed of 8-25%/second width direction length to carry out Nth transverse stretching.

Here, the stretching temperature at the time of stretching the white polyester film 200 in the transverse direction represents the film surface temperature, and can be controlled by a temperature control mechanism provided in the tenter. The stretching temperature during the transverse stretching is increased together with the stretching speed, and the stretching temperature at the start of the transverse stretching is preferably 80 to 120 ℃, more preferably 85 to 115 ℃ from the viewpoint of suppressing crystallization of the polyester at the initial stage of the transverse stretching.

On the other hand, the drawing temperature in the nth transverse drawing step is preferably 140 to 180 ℃, more preferably 145 to 175 ℃ from the viewpoint of causing peeling at the interface between the white particles and the polyester to generate fine voids.

The stretching temperature is a value obtained by measuring the temperature of the film surface in the stretching step with a thermocouple.

By performing the transverse stretching by increasing the stretching speed in the width direction in stages in this manner, the size of the voids generated in the film starting from the white particles can be controlled, and the final stretching speed of the stretching section performing the transverse stretching step has the greatest influence on the control of the void size.

In the region of the film at a relatively low temperature in the first half of the stretching section, the stretching is slow, and interfacial separation between the white particles and the polyester is less likely to occur.

On the other hand, in the latter half of the film where the temperature is high, the film is stretched to a desired magnification to impart weather resistance. However, if the drawing speed is too high in the second half, the stress of the drawn polyester exceeds the interfacial adhesion between the white particles and the polyester, and peeling occurs between the white particles and the polyester. Conversely, if the stretching speed is too low, the orientation of the polyester molecules becomes insufficient, and the weather resistance becomes insufficient.

If the drawing speed is too high in the first half, the polyester tensile stress exceeds the interface adhesion 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 low, crystallization of the polyester proceeds, the polyester becomes hard, the tensile stress of the polyester increases, and peeling occurs between the polyester and the white particles first.

From this viewpoint, the stretching speed in the width direction in the 1 st transverse stretching step is preferably a stretching speed increased by 4 to 10% per second of the length in the width direction of the white polyester film before the 1 st transverse stretching step is started. For example, when the stretching speed in the transverse direction is changed in three stages as shown in fig. 2, the stretching speed from T1 to T2 is the stretching speed in the 1 st transverse direction stretching step.

Further, for the above reasons, the lower the stretching speed immediately after the start of stretching, the harder the polyester, so the tensile stress of the polyester immediately before the end of stretching tends to be high, and peeling from the white particles tends to occur. The stretching speed just before the end of stretching needs to be adjusted according to the stretching speed immediately after the start of stretching, and when the stretching speed in the width direction in the 1 st transverse stretching step is Sa and the stretching speed in the width direction in the nth transverse stretching step is Sb, the value of the stretching speed ratio Sb/Sa is preferably 1.5to 6.

In the transverse stretching step, transverse stretching is preferably performed with a tensile stress of 8MPa or more and 20MPa or less and a stretching ratio of 3.4 times or more and 5 times or less, and more preferably transverse stretching is performed with a tensile stress of 10MPa or more and 18MPa or less and a stretching ratio of 3.6 times or more and 4.5 times or less.

The stretching area magnification (longitudinal stretching magnification × transverse stretching magnification) by the above biaxial stretching is preferably 9 times or more and 20 times or less, and if the area magnification is 9 times or more and 20 times or less, a biaxially oriented polyester film having a thickness of 250 μm or more and 500 μm or less after stretching, a high degree of plane orientation, a crystallinity of 30% or more and 40% or less, and an equilibrium water content of 0.1 mass% or more and 0.25 mass% or less can be obtained, for example.

As the method of biaxial stretching, as described above, any of the sequential biaxial stretching methods in which the longitudinal stretching and the width stretching are separately performed and the simultaneous biaxial stretching method in which the longitudinal stretching and the width stretching are simultaneously performed may be used.

For example, when the 1 st to nth longitudinal stretching steps are performed in the longitudinal direction while the stretching speed in the width direction is kept constant, the stretching temperature, the stretching speed, and the stretching speed ratio in the 1 st to nth transverse stretching steps in the width direction described above can be similarly applied.

(Heat-setting step)

Subsequently, the biaxially stretched white polyester film was subjected to heat setting treatment.

In the heat-setting step, the film is subjected to a heat treatment at, for example, 160 ℃ to 230 ℃, preferably 170 ℃ to 220 ℃ (more preferably 180 ℃ to 210 ℃), for 1to 60 seconds (more preferably 5to 50 seconds).

When the heat-setting temperature is 160 ℃ or higher, the polyester is easily crystallized, and the polyester molecules can be immobilized in an extended state, whereby the hydrolysis resistance can be improved. When the heat-setting temperature is 230 ℃ or lower, slippage is less likely to occur in the entangled portions of the polyester molecules, shrinkage of the polyester molecules can be suppressed, and hydrolysis resistance can be improved.

The heat-setting temperature referred to herein is the surface temperature of the film during the heat-setting treatment.

In the heat-setting step provided after the stretching step, a part of the volatile basic compound having a boiling point of 200 ℃ or lower may be volatilized.

The heat setting step is preferably performed in a state where the heat setting step is held by the chuck in the tenter following the transverse stretching, and in this case, the chuck interval may be performed at the width at the end of the transverse stretching, or may be performed by further expanding or reducing the width. By forming fine crystals by heat setting treatment, mechanical properties and durability can be improved.

(thermal relaxation step)

The heat relaxation step is preferably performed after the heat setting step. The thermal relaxation step is a process of shrinking the film by applying heat to the film for stress relaxation. In the thermal relaxation step, relaxation is preferably performed in at least one of the longitudinal direction and the transverse direction, and the amount of relaxation is preferably 1% to 30% in both the longitudinal direction and the transverse direction (ratio to the width after stretching in the transverse direction), more preferably 2% to 20%, and still more preferably 3% to 15%. When the thermal relaxation temperature is Tr and the heat-setting temperature is Ts, the thermal relaxation temperature Tr is preferably in a temperature range of 100 ℃ or higher and 15 ℃ or higher lower than Ts, more preferably in a temperature range of 110 ℃ or higher and 25 ℃ or higher than Ts (110 ℃ or lower and Tr or lower than Ts), even more preferably in a temperature range of 120 ℃ or higher and 30 ℃ or lower than Ts (120 ℃ or lower and Tr or lower than Ts-30 ℃).

In the thermal relaxation step, the polyester film is thermally relaxed under the conditions within the above ranges to slightly release the stretching force of the polyester molecules, whereby the polyester film is excellent in dimensional stability while maintaining hydrolysis resistance, and troubles are less likely to occur in the downstream steps such as processing of the obtained polyester film.

The transverse relaxation can be performed by narrowing the width of the clips of the tenter. Also, the longitudinal relaxation can be performed by narrowing the interval between the adjacent clips of the tenter. This can be achieved by connecting adjacent clips in a pantograph shape and reducing the pantograph shape. Further, after the film is taken out from the tenter, the film can be relaxed by performing a heat treatment while being conveyed at a low tension. The tension is preferably 0N/mm per unit cross-sectional area of the film²～0.8N/mm²More preferably 0N/mm²～0.6N/mm²Further preferably 0N/mm²～0.4N/mm²。0N/mm²This can be performed by providing 2 or more pairs of nip rollers during conveyance and loosening the film (in a suspended state) therebetween.

(winding Process)

Both ends of the film carried out of the tenter and gripped by the clips are trimmed, and both ends are subjected to knurling (creasing) and then wound.

The width of the wound film is preferably 0.8 to 10m, more preferably 1to 6m, and still more preferably 1.2 to 4 m. The thickness is preferably 30 to 500. mu.m, more preferably 40 to 480. mu.m, and still more preferably 45 to 450 μm. Such thickness adjustment can be achieved by adjusting the discharge amount of the extruder or adjusting the film forming speed (adjusting the speed of the cooling roll, the stretching speed in conjunction with the speed of the cooling roll, and the like).

The regenerated film such as the edge portion of the trimmed film is recovered as a resin mixture and reused. The film material of the regenerated film, which is the white polyester film of the next batch, is returned to the drying step as described above, and the production steps are sequentially repeated.

Through the above steps, the white polyester film of the present disclosure can be produced.

< Back sheet for solar cell >

The back sheet for a solar cell of the present disclosure includes the white polyester film of the present disclosure.

The back sheet for a solar cell of the present disclosure can be provided with a functional layer on at least one surface of the white polyester film of the present disclosure as needed. Examples thereof include an easy-adhesion layer, an ultraviolet absorbing layer, and a weather-resistant layer for improving adhesion to an adherend.

The back sheet for a solar cell of the present disclosure includes the white polyester film of the present disclosure, and thus exhibits stable weather resistance, adhesion, and light reflectance over a long period of time.

As a method for providing a functional layer on at least one surface of the white polyester film of the present disclosure, a known coating technique such as a roll coating method, a knife edge coating method, a gravure coating method, a curtain coating method, or the like can be used. The functional layer may be formed by in-line coating.

The back sheet for a solar cell has a functional layer (coating layer) formed by coating on at least one surface of the stretched white polyester film of the present disclosure, and thus can further improve any one of weather resistance, light reflectivity, and adhesiveness, or can impart other functions.

Further, surface treatment (flame treatment, corona treatment, plasma treatment, ultraviolet treatment, or the like) may be performed before the coating layer is applied.

Further, it is also preferable that another functional film is bonded to the white polyester film of the present disclosure via an adhesive layer.

< solar cell Module >

The solar cell module of the present disclosure includes: a solar cell element; a sealing material sealing 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 back sheet for a solar cell of the above embodiment, which is disposed outside the sealing material on the side opposite to the light-receiving surface side of the solar cell element.

That is, the solar cell module of the present disclosure is configured such that a solar cell element that converts light energy of sunlight into electric energy is disposed between a transparent front substrate (front surface protection member) on which sunlight is incident and the back sheet (back surface protection member) for a solar cell of the present disclosure described above, and the solar cell element disposed between the front substrate and the back sheet is sealed with a sealing material such as Ethylene Vinyl Acetate (EVA). By providing a solar cell module with a back sheet for a solar cell comprising the white polyester film of the present disclosure, peeling and cracking of the back sheet for a solar cell due to hydrolysis can be suppressed, and light can be reflected at a high reflectance to a solar cell element to improve power generation efficiency. Therefore, the solar cell module of the present disclosure can maintain high power generation efficiency outdoors for a long time.

Components other than the solar cell module and the back sheet are described in detail in, for example, "a material constituting a photovoltaic power generation system" (published by Kogyo Chosakai Publishing co., L td., 2008).

The transparent front substrate may be appropriately selected from the base materials that transmit light, as long as it has light transmittance that allows sunlight to transmit. From the viewpoint of power generation efficiency, a substrate having a higher light transmittance is more preferable, and as such a substrate, for example, a glass substrate, a substrate made of a transparent resin such as an acrylic resin, or the like can be preferably used.

As the solar cell element, various known solar cell elements such as silicon-based, e.g., single crystal silicon, polycrystalline silicon, amorphous silicon, etc., III-V or II-VI compound semiconductors, e.g., copper-indium-gallium-selenium, copper-indium-selenium, cadmium-tellurium, gallium-arsenic, etc., can be applied.

The white polyester film of the present disclosure is preferably used as a base film of a back sheet for a solar cell, but the application of the white polyester film of the present disclosure is not limited to a back sheet for a solar cell, and the white polyester film can be used as a film that is used outdoors for a long period of time. Specific examples of the film include a film for protecting a solar cell, a film for building materials, a film for outdoor advertising, and a heat insulating film.

Examples

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the following examples as long as the invention does not exceed the gist thereof. In addition, "part" is based on mass unless otherwise specified.

[ example 1]

< Synthesis of raw Material polyester resin 1 >

As shown below, a polyester resin (Ti catalyst-based PET) was obtained by a continuous polymerization apparatus using a direct esterification method in which terephthalic acid and ethylene glycol were directly reacted to distill off water and esterify them, and then polycondensed under reduced pressure.

(1) Esterification reaction

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

The obtained reaction product (oligomer) was transferred to a second esterification reaction tank, and reacted at an internal temperature of the reaction tank of 250 ℃ for an average residence time of 1.2 hours under stirring to obtain an oligomer having an acid value of 200 equivalents/ton. The second esterification reaction tank was partitioned into 3 zones, and an ethylene glycol solution of magnesium acetate was continuously supplied from the 2 nd zone so that the amount of Mg added was 75ppm in terms of element content, and an ethylene glycol solution of trimethyl phosphate was continuously supplied from the 3 rd zone so that the amount of P added was 65ppm in terms of element content.

(2) Polycondensation reaction

The esterification reaction product obtained above was continuously supplied to the first polycondensation reaction vessel, and stirred at a reaction temperature of 270 ℃ and a pressure in the reaction vessel of 20torr (2.67 × 10)^-3MPa), and an average residence time of about 1.8 hours.

The reaction product passed through the first polycondensation reaction vessel was transferred to the second polycondensation reaction vessel, and stirred in the second polycondensation reaction vessel at a reaction vessel internal temperature of 276 ℃ and a reaction vessel internal pressure of 5torr (6.67 × 10)^-4MPa) and a residence time of about 1.2 hours.

Then, the reaction product passed through the second polycondensation reaction vessel was transferred to the third polycondensation reaction vessel, and the temperature in the reaction vessel was 278 ℃ and the pressure in the reaction vessel was 1.5torr (2.0 × 10)^-4MPa) and a residence time of 1.5 hours to obtain polyethylene terephthalate (PET). The resulting PET (reaction product) was measured by high-resolution high-frequency inductively coupled plasma mass spectrometry (HR-ICP-MS; AtToM, manufactured by SII Nano Technology Inc.). As a result, Ti was 9ppm, Mg was 67ppm, and P was 58 ppm. P was slightly decreased from the initial amount added, and it was assumed that volatilization occurred during the polymerization.

Solid phase polymerization

The PET polymerized as described above was pelletized (diameter: 3mm, length: 7mm), and the obtained resin pellets (intrinsic viscosity IV: 0.60dl/g, terminal carboxyl group concentration: 16 eq/ton) were subjected to solid phase polymerization as follows.

In the solid phase polymerization, the polyester polymerized by the esterification reaction described above was heated at 140 ℃ for 7 minutes by using nitrogen at a dew point temperature of-30 ℃ to be precrystallized for the purpose of preventing fixation at the time of solid phase polymerization.

Subsequently, the resin was dried at 180 ℃ for 7 hours using heated nitrogen having a dew point of-30 ℃ to reduce the water content of the resin to 50ppm or less.

Subsequently, after the dried polyester resin was preheated to 210 ℃, nitrogen gas was circulated at 195 ℃ for 50 hours, thereby performing solid-phase polymerization. As the nitrogen gas circulation condition, a gas ratio (the amount of the circulated nitrogen gas relative to the amount of the discharged resin) was set to 1.3m³A nitrogen gas having a specific gravity of 0.08 m/sec, an ethylene glycol concentration of 240ppm, a water concentration of 12ppm, and a molar partial pressure ratio of ethylene glycol to water (molar partial pressure of ethylene glycol/molar partial pressure of water) of 20 was subjected to solid-phase polymerization. To make the above mixed gas composition, high-purity ethylene glycol having a water content of 100ppm was used in the ethylene glycol scrubber, and the temperature of the scrubber was set to 35 ℃. The pressure in the cleaner is set in the range of 0.1MPa to 0.11 MPa.

Subsequently, the resin (750kg/h) discharged from the reaction step was cooled to 60 ℃.

The obtained polyester resin after solid-phase polymerization had an Intrinsic Viscosity (IV) of 0.78d L/g and an amount of terminal COOH (AV) of 9 equivalents/ton.

< preparation of Master batch >

Titanium oxide was added to a part of the pellets before solid phase polymerization in such a manner that the content ratio thereof became 50 mass% of the whole pellets, and the mixture was kneaded to prepare a master batch (master batch).

Here, as the titanium oxide, ISHIHARA SANGYO KAISHA, &lttt transition = L "&gtt L &ltt/t & TD. (trade name: PF-739; average primary particle diameter ═ 0.25 μm) was used.

< formation of unstretched film >

After PET-1 and the master batch having been subjected to solid-phase polymerization as described above were dried to a water content of 100ppm or less, the resultant was mixed so that the titanium oxide content became 4 mass%, and the mixture was put into a hopper of a kneading extruder, melted at 290 ℃ and extruded. Further, as the extruder, a two-port type corotating intermeshing twin-screw extruder (diameter 110mm) having two vents was used. The melt was passed through a gear pump and a filter (pore size: 20 μm), and then extruded from a die onto a cooling casting drum (cooling roll). In addition, the extruded melt was adhered to a cooling casting drum by an electrostatic application method. Thus, an unstretched polyethylene terephthalate (PET) film having a thickness of about 3mm was formed.

< stretching of unstretched film >

Longitudinal stretching-

The unstretched film was passed between 2 pairs of nip rollers 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 represented by an increase ratio per 1 second with respect to the length of the unstretched film in the longitudinal direction.

Preheating temperature: 80 deg.C

Stretching temperature: 90 deg.C

Stretching ratio: 3.0 times of

Stretching speed: 300%/second

Transverse stretching-

The longitudinal stretching is followed by transverse stretching. The transverse stretching was performed by increasing the stretching temperature and the stretching speed in 3 stages. The specific conditions are as follows. Here, the stretching speed in the transverse stretching step is represented by an increase rate per 1 second with respect to the film width of the film after the longitudinal stretching before the 1 st transverse stretching step.

Preheating temperature: 110 deg.C

Stretching temperature immediately after the start of transverse stretching (1 st transverse stretching step): 110 deg.C

Stretching speed immediately after the start of transverse stretching (1 st transverse stretching step): 8%/second

Stretching temperature in the intermediate stage of transverse stretching (2 nd transverse stretching step): 130 deg.C

Stretching speed in the intermediate stage of transverse stretching (2 nd transverse stretching step): 11%/second

Stretching temperature immediately before completion of transverse stretching (3 rd transverse stretching step): 145 deg.C

Stretching speed immediately before completion of transverse stretching (3 rd transverse stretching step): 15%/second

Transverse stretching magnification (total): 4.2 times of

Heat-setting/heat-relaxation

The biaxially stretched film after the end of the longitudinal stretching and the transverse stretching was heat-set at 190 ℃ (heat-set time: 10 seconds).

After the heat-setting, the tenter width was reduced and heat relaxation was performed (heat relaxation temperature: 160 ℃ C.).

-take-up-

After heat setting and heat relaxation, the ends were trimmed by 10cm each. Then, both ends were subjected to indentation processing (knurling) with a width of 10mm, and then wound up with a tension of 25 kg/m. The film width was 1.5m and the roll length was 2000 m.

The biaxially stretched white polyester film of example 1 was obtained in the above manner.

< examples 2 to 11, comparative examples 1to 6 >

Stretched white polyester films of examples 2 to 11 and comparative examples 1to 6 were produced in the same manner as in example 1, except that the conditions for transverse stretching and the film properties were changed as shown in table 1.

[ evaluation of film ]

The white polyester films obtained in examples and comparative examples after stretching were evaluated as follows. The measurement results and evaluation results are shown in table 1 below.

< area of pores >

The ratio of pores in the film (pore occupancy) and the area per 1 pore were evaluated by the following methods.

(measurement of porosity)

1. The produced white polyester film was cut in the thickness direction along the TD and MD of the film by a cutter.

2. The cut section in each direction was observed 3000 times by a scanning electron microscope. The cross-sectional image of the white polyester film was obtained by randomly taking 9 or more images in one direction.

3. The obtained image was observed by image analysis software (ImageJ) for a portion which was peeled off from the white pigment to form a gap between the white particles and the polyester to form a void, and followed along the contour of the void. In this case, the white particles present in the pores are also included as pores, and the white particles are removed from the pores in the above steps 1to 2 and only the hollow portion is followed in the same manner. When the pores overlap with each other, the pores are regarded as one and tracked.

4. The tracked frame is then painted.

5. And tracking pores, and binarizing the smeared image and dividing the smeared image into a pore part and a polyester part.

6. In the area calculation mode, the ratio of pores in 1 image was determined by dividing the number of pixels in the pore portion by the number of pixels in the entire image.

7. The operations 3to 6 are also performed on other images, and the average is taken as the ratio of pores in each direction.

8. Finally, the average TD/MD is taken as the ratio of pores (pore occupancy).

(determination of average area per 1 pore)

i. The above-mentioned steps are carried out in the range of 1to 5.

in the area calculation mode, the number of pixels of the aperture is found and converted into the area.

in another aspect, the number of pores is counted from the image obtained in 2.

And ii to iii are also performed on other images.

v. as the area of each 1 pore in each direction by dividing the sum of the areas obtained in iv by the number of pores obtained in iv.

Finally, the average of TD and MD is taken as the average area per 1 pore.

< thickness >

The thickness of the polyester film was an average thickness of the film measured by using a contact type film thickness meter (manufactured by Mitutoyo Corporation, ID-F125). Specifically, the thickness of the polyester film was measured at 50 points at 0.5 m-intermediate intervals in the longitudinal direction of the film and at 50 points at total film formation width-intermediate intervals (50 points equally divided in the width direction) in the width direction by a contact film thickness meter at 100 points. The average of the obtained thicknesses at 100 points was determined and used as the thickness of the polyester film.

< intrinsic viscosity >

The polyester film thus produced was dissolved in a mixed solvent of 1,1,2, 2-tetrachloroethane and phenol (2/3 [ mass ratio ]), and the intrinsic viscosity was determined from the solution viscosity at 25 ℃ in the mixed solvent.

ηsp/C＝[η]+K[η]2·C

Here, η sp is (solution viscosity/solvent viscosity) -1, C is the dissolved polymer mass per 100ml of solvent (1 g/100ml in this measurement), K is the hagus constant (0.343), and the solution viscosity and the solvent viscosity were measured using an ostwald viscometer, respectively.

< weather resistance >

The weather resistance (hydrolysis resistance) of the polyester film obtained in each example was evaluated in terms of the half-life of the elongation at break retention. The evaluation of the half-life of the retention rate of elongation at break was carried out by subjecting the polyester films obtained in the respective examples to a storage treatment (heat treatment) at 120 ℃ under a relative humidity of 100%, and measuring the storage time (h) at which the elongation at break (%) exhibited by the polyester film after storage was 50% relative to the elongation at break (%) exhibited by the polyester film before storage.

The longer the half-life of the elongation at break retention rate, the more excellent the weather resistance (hydrolysis resistance) of the polyester film. Here, if the half-life of the elongation at break retention is 95 hours or more, it can be said that the weather resistance is excellent in practical use.

< Peel test >

(EVA adhesiveness before Heat-moisture treatment)

The polyester films obtained in the respective examples were cut into pieces having a width of 20mm and a length of × 150mm to prepare 2 sample sheets, and an EVA sheet (EVA sheet manufactured by Mitsui Chemicals Fabro, inc.: SC50B) cut into a width of 20mm and a length of × 100mm was sandwiched between the 2 sample sheets, and was bonded to the EVA sheet by hot pressing using a vacuum laminator (vacuum laminator manufactured by Nisshinbo co., L td.).

After evacuation was performed at 128 ℃ for 3 minutes using a vacuum laminator, pressure was applied for 2 minutes to temporarily bond. Then, the final adhesion treatment was performed in a drying oven at 150 ℃ for 30 minutes. In this way, a sample for adhesion evaluation was obtained in which a portion of 20mm from one end of 2 sample sheets adhered to each other was not adhered to EVA and an EVA sheet was adhered to the remaining portion of 100 mm.

The EVA unbonded portion of the obtained adhesive evaluation sample was held between upper and lower clamps by Tensilon (RTC-1210A manufactured by ORIENTEC), and a tensile test (peel test) was performed at a peel angle of 180 ° and a tensile speed of 300 mm/min, and the average peel strength of the force detected during 100mm peeling was measured. In addition, the peel strength here is also referred to as a peel strength when cleavage occurs in the vicinity of the bonding interface of the white polyester film bonded to the EVA sheet and the film is separated from the EVA sheet. When the film sample broke during the peeling of 100mm, the detected force was not used as a result of "breaking".

(EVA adhesiveness after Heat-moisture treatment)

The EVA sheet was bonded between 2 sample sheets obtained by cutting the polyester film obtained in each example in the same manner as the evaluation before wet heat exposure. After the bonding, heat treatment (wet heat treatment) was performed at 120 ℃ and 100% RH for 30 hours.

After the wet heat treatment, a tensile test (peel test) was performed in the same manner as in the evaluation before the wet heat treatment with time, and the peel strength was measured.

When the heat and moisture treatment is carried out before and after 4N/mm or more, the adhesiveness (peeling resistance) with EVA is excellent.

< tear Strength >

The tear strength of the polyester film obtained in each example was measured in the following manner.

The sample films were cut in the MD and TD at a width (short side) of 2cm and a length (long side) of × 10 cm.

A slit having a length of 5cm was formed in the center of the short side in parallel with the longitudinal direction, and the stress was measured by the following method using a tensile tester. The measurement was carried out at 25 ℃ and a relative humidity of 50%.

(1-1) one end of the notch is held by a chuck on one side of the tensile testing machine, and the other end is held by a chuck on the other side.

(1-2) the chuck was pulled at 30 mm/min, and the stress was measured. As the distance between the chucks increases, the stress increases, and a flat portion occurs. The stress at the flat portion was measured as the tear strength with the number of repetitions n being 3, and the average value was obtained.

(1-3) for the measurement, the measurement was carried out in MD and TD, and the average value was taken as the tear strength.

< Voltage resistance >

Specifically, the withstand voltage of the polyester film of the present disclosure was evaluated by obtaining a partial discharge voltage using a partial discharge tester KPd2050 (manufactured by KIKUSUE L ECTRONICS CORP.).

The polyester film used in the solar cell module is also required to have voltage resistance.

< reflectance >

The reflectance of the polyester film of each example formed as described above was measured by the method of paragraph [0084] of Japanese patent No. 4766192.

Specifically, an integrating sphere was attached to a spectrophotometer ("UV-3150" manufactured by Shimadzu Corporation), and the reflectance of a standard white plate ("ZRS-99-010-W" white standard plate manufactured by Spere Optics) was corrected to 100%, and the spectral reflectance of the polyester films of the examples and comparative examples was measured. The measurement was performed in a range of wavelengths of 400 to 1200nm in 1nm scale, and the average value was obtained. In the measurement, a black cardboard having no reflection was disposed on the back surface of the sample film.

Here, if the reflectance is 85% or more, the weather resistance is excellent in practical use.

The results are shown in table 1 below.

The white polyester films produced in the examples were excellent in weather resistance, light reflectivity and peel strength. In particular, examples 1to 10, which had a thickness of 280 μm or more, had a voltage resistance of 1kV or more, and were suitable for a back sheet for a solar cell. In particular, the white polyester films of examples 1to 9 had a thickness of 500 μm or less, and also had an advantage of easy production.

The entire disclosure of japanese patent application 2015-074614, filed 3/31/2015, is incorporated by reference into this specification.

All documents, patents, patent applications, and technical specifications cited in the present specification are incorporated by reference into the present specification to the same extent as if each document, patent application, and technical specification were specifically and individually indicated to be incorporated by reference.

Claims

1. A stretched white polyester film comprising a polyester and white particles,

the content of the white particles is 2 to 10% by mass based on the total mass of the film,

the average area of each 1 film in the cross section in the film thickness direction is 0.010 to 0.050 mu m²Pore of each.

2. The stretched white polyester film according to claim 1,

on the section of the film in the thickness direction, the proportion of the total area occupied by the pores is 0.5-3%.

3. The stretched white polyester film according to claim 1,

p/t is 6.5 to 13.5mN/μm when the tear strength is P and the thickness of the film is t.

4. The stretched white polyester film according to claim 1,

the film has a thickness of 280 to 500 μm.

5. The stretched white polyester film according to claim 1,

the intrinsic viscosity of the film is 0.65 to 0.85d L/g.

6. The stretched white polyester film according to claim 1,

the white particles are titanium oxide.

7. The stretched white polyester film according to claim 1,

the ratio of the total area occupied by the pores is 0.5to 3% in the cross section of the film in the thickness direction,

p/t is 6.5 to 13.5mN/μm when the tear strength is P and the thickness of the film is t,

the thickness of the film is 280 to 500 μm,

the intrinsic viscosity of the film is 0.65 to 0.85d L/g,

the white particles are titanium oxide.

8. A back sheet for a solar cell comprising the stretched white polyester film according to any one of claims 1to 7.

9. The back sheet for a solar cell according to claim 8,

the stretched white polyester film has a coating layer on at least one side.

10. A solar cell module, comprising:

a solar cell element;

a sealing material sealing the solar cell element;

a front substrate disposed outside the sealing material on a light-receiving surface side of the solar cell element; and

the back sheet for a solar cell according to claim 8, wherein the solar cell element is disposed on the side opposite to the light-receiving surface side, outside the sealing material.

11. A method for producing a stretched white polyester film according to any one of claims 1to 7, comprising:

a longitudinal stretching step of stretching the unstretched white polyester film in a longitudinal direction and a transverse stretching step of stretching the unstretched white polyester film in a transverse direction,

the transverse stretching step includes 1 st to Nth transverse stretching steps, the nth transverse stretching step is performed sequentially to the N-1 th transverse stretching step, the stretching speed of the white polyester film in the width direction is increased in the nth transverse stretching step compared with the N-1 th transverse stretching step, the stretching temperature is set to be 140 ℃ to 180 ℃ in the Nth transverse stretching step, and the stretching speed is set to be as follows in the Nth transverse stretching step: increasing the length of the white polyester film in the width direction by 8 to 25%/second relative to the length of the white polyester film in the width direction before the 1 st transverse stretching step,

n is an integer of 2 or more, and N is an integer of 2 to N.

12. The process for producing a stretched white polyester film according to claim 11,

the stretching speed in the width direction in the 1 st transverse stretching step is 4-10%/second.

13. The process for producing a stretched white polyester film according to claim 11,

the value of the stretching speed ratio Sb/Sa is 1.5to 6, where Sa is the stretching speed in the width direction in the 1 st transverse stretching step and Sb is the stretching speed in the width direction in the Nth transverse stretching step.

14. The process for producing a stretched white polyester film according to claim 11,

the stretching speed in the width direction in the 1 st transverse stretching step is 4 to 10%/second,