JP2012256765A - White laminate polyester film for sealing backside of solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a brilliant white laminate polyester film for sealing the backside of a solar cell having a hydrolysis resistance and an ultraviolet light resistance.SOLUTION: In the white laminate polyester film for sealing the backside of a solar cell, a polyester layer (B) containing a white pigment is a base layer, one polyester layer (A) of both surface layers contains 0.2-5.0 wt% of ultraviolet absorber, and the content of white pigment of the polyester layer (A) is smaller than that of the polyester layer (B). The limit viscosity of the laminate polyester film is 0.65 dl/g or higher, and the amount of terminal carboxyl groups is 26 equivalents/t or less.

Description

  The present invention relates to a white laminated polyester film for sealing a back surface of a solar cell, which has hydrolysis resistance and ultraviolet resistance and has good gloss.

  Photovoltaic power generation that converts light energy into electrical energy using the photoelectric conversion effect is widely used as a means for obtaining clean energy. And with the improvement of the photoelectric conversion efficiency of a photovoltaic cell, the photovoltaic power generation system has come to be provided also in many private houses. In order to use such a solar power generation system as an actual energy source, a solar cell module having a configuration in which a plurality of solar cells are electrically connected in series is used.

  Solar cell modules are mainly solar cell front sheet (mainly glass) / encapsulant (mainly EVA) / photoelectric conversion layer (called cell part) / encapsulant (mainly EVA) / solar cell back surface protection. A typical sheet is a structural example. A fluorine-based film is often used for the solar cell protection sheet, but a polyester-based film is often used because of its high cost.

  A technique in which a polyester film is used for the solar cell back surface protection sheet is disclosed. It is known that when a polyester film is used in a high-temperature and high-humidity environment, hydrolysis of an ester bond site in a molecular chain occurs and mechanical properties deteriorate. Therefore, various studies have been made to suppress hydrolysis assuming that the polyester film is used outdoors for a long period (for example, 20 years) or used in a high humidity environment.

  It is known that the hydrolysis of polyester is faster as the amount of terminal carboxyl groups in the polyester molecular chain is higher. Therefore, Patent Document 1 and Patent Document 2 disclose a technique for improving hydrolysis resistance by adding a compound that reacts with carboxylic acid to reduce the amount of carboxyl groups at the molecular chain terminals. However, these compounds have a high possibility of inducing gelation and generating foreign substances in the melt extrusion step or the material recycling step in the film forming process, and are not preferable from an environmental and cost viewpoint.

  Patent Document 3 and Patent Document 4 describe that the durability of polyester is improved by using a titanium compound and a phosphorus compound. By the said invention, since the destruction resulting from the degradation of the polyester polymer chain itself is suppressed, it can be utilized as a solar cell back surface protection sheet. However, UV resistance when exposed outdoors is not considered.

  Patent Document 5 and Patent Document 6 describe a polyester film containing a white pigment and having hydrolysis resistance. By the said invention, it becomes a film which improved not only hydrolysis resistance but ultraviolet-ray resistance, and can utilize as a sheet | seat for solar cell back surface protection. However, glossiness is not taken into consideration, and a satisfactory appearance is not obtained when printed.

JP-A-9-227767 JP-A-8-73719 JP 2007-204538 A JP 2010-163613 A JP 2010-189558 A JP 2010-192743 A

  This invention is made | formed in view of the said actual condition, Comprising: The solution subject is providing the white laminated polyester film for solar cell backside sealing which has hydrolysis resistance and ultraviolet resistance, and has favorable glossiness. It is in.

  As a result of intensive studies in view of the above circumstances, the present inventors have found that the above-mentioned problems can be solved by using a polyester film having a specific configuration, and have completed the present invention.

  That is, the gist of the present invention is that the polyester layer (B) containing a white pigment is a base layer, and one polyester layer (A) of both surface layers contains 0.2 to 5.0 wt% of an ultraviolet absorber, The polyester layer (A) is a laminated polyester film having a white pigment content less than the white pigment content of the polyester layer (B), the intrinsic viscosity of the laminated polyester film is 0.65 dl / g or more, and terminal carboxyl It exists in the white lamination | stacking polyester film for solar cell backside sealing characterized by the base amount being 26 equivalent / t or less.

  ADVANTAGE OF THE INVENTION According to this invention, the biaxially-oriented polyester film for solar cell back surface protection with favorable glossiness which has hydrolysis resistance and ultraviolet-ray resistance can be provided, and the industrial value of this invention is high.

  The polyester film referred to in the present invention is a film stretched after cooling a molten polyester sheet extruded by a so-called extrusion method, which is melt-extruded from an extrusion die.

  In the present invention, the polyester used for the polyester film refers to a polyester obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. Examples of the aromatic dicarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid. Examples of the aliphatic glycol include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Representative polyesters include polyethylene terephthalate (PET), polyethylene-2,6-naphthalenedicarboxylate (PEN), and the like. Among these, polyethylene terephthalate (PET) is preferable.

  The production catalyst for the polyester raw material used for the polyester film of the present invention is preferably selected from a titanium-containing compound, an antimony-containing compound element, and an aluminum-containing compound. A particularly preferred catalyst is a catalyst comprising a titanium-containing compound.

  The titanium element content of the film of the present invention is usually 20 ppm or less, preferably 15 ppm or less, more preferably 9 ppm or less. Although there is no particular lower limit, in practice, about 2 ppm is the lower limit in the current technology. When there is too much content of a titanium compound, an oligomer tends to be by-produced in the step of melt-extruding polyester, and there is a tendency that the oligomer tends to be a polyester film deposited on the surface. The surface oligomer of the polyester film may cause causative substances such as contamination of the roll by causing the oligomer to be transferred to the roll used during film formation, and generation of film foreign matter. On the other hand, when the titanium element is not contained at all, the productivity at the time of production of the polyester raw material is inferior, and the polyester raw material reaching the target degree of polymerization may not be obtained.

  The phosphorus element of the film of the present invention is usually derived from a phosphoric acid compound, and is added during the production of the polyester. In the present invention, the amount of phosphorus element in the polyester component is usually in the range of 170 ppm or less, preferably 100 ppm or less, more preferably 50 ppm or less, and most preferably 15 ppm or less. Although there is no particular lower limit, in practice, about 3 ppm is the lower limit in the current technology. If the amount of phosphorus element is too large, gelation may occur during film formation, resulting in a foreign matter, which may cause deterioration in film quality.

  Examples of phosphoric acid compounds include known ones such as phosphoric acid, phosphorous acid or ester phosphonic acid compounds, phosphinic acid compounds, phosphonous acid compounds, phosphinic acid compounds, and specific examples include regular phosphoric acid. , Dimethyl phosphate, trimethyl phosphate, diethyl phosphate, triethyl phosphate, dipropyl phosphate, tripropyl phosphate, dibutyl phosphate, tributyl phosphate, diamyl phosphate, triamyl phosphate, dihexyl phosphate, tri Examples include hexyl phosphate, diphenyl phosphate, triphenyl phosphate, and ethyl acid phosphate.

  The amount of terminal carboxyl groups of the polyester component constituting the polyester film of the present invention can be measured by the method described later for the component from which the white pigment has been removed when the polyester film contains a white pigment.

  The amount of terminal carboxyl groups of the polyester film is 26 equivalent / t or less, preferably 20 equivalent / t or less. When the amount of terminal carboxyl groups of the polyester component constituting the polyester film exceeds 26 equivalents / t, the hydrolysis resistance of the polyester component is poor. Although there is no particular lower limit, it is usually about 5 equivalents / t in terms of the efficiency of the polycondensation reaction, thermal decomposition in the melt extrusion process, and the like.

  When the polyester film contains a white pigment, the intrinsic viscosity of the polyester film of the present invention can be measured by the method described below with respect to the component from which the white pigment has been removed.

  The intrinsic viscosity of the polyester film of the present invention is 0.65 dl / g or more, preferably 0.68 dl / g or more. When the intrinsic viscosity of the polyester component constituting the polyester film is 0.65 dl / g or more, a film having good long-term durability and hydrolysis resistance can be obtained. On the other hand, although there is no upper limit of the intrinsic viscosity of the polyester film, it is about 0.90 dl / g from the viewpoint of the efficiency of the polycondensation reaction and the prevention of pressure increase in the melt extrusion process.

  In the polyester film of the present invention, an inorganic white pigment is added to the polyester in order to improve ultraviolet resistance. Examples of white pigments that can be used include titanium dioxide and barium sulfate. Although at least one selected from these may be used, in the present invention, it is more preferable to use titanium dioxide from the viewpoint of preventing a decrease in whiteness after wet heat treatment. Further, the white pigment may be in the form of a porous or hollow porous material, and further, a white pigment that has been subjected to a surface treatment to improve dispersibility in the resin may be used.

  The average particle diameter of the white pigment in the white polyester layer (B) containing the white pigment of the present invention is preferably from 0.05 to 5.0 μm, more preferably from 0.1 to 3.0 μm. If the average particle size is outside the above range, uniform dispersion becomes difficult, and the smoothness of the film surface may deteriorate.

  The amount of the white pigment added in the white polyester layer (B) is not particularly limited, but is preferably in the range of 1 to 16% by weight, more preferably in the range of 2 to 12% by weight, and 3 to 10% by weight. Those within the range are more preferred. If the addition amount is less than the above range, it is difficult to improve the characteristics such as optical density and whiteness of the film, and as a result, the ultraviolet resistance is poor. Conversely, above the above range, the hydrolysis resistance is poor.

  In the present invention, in order to improve the glossiness of the laminated polyester film, it is necessary to make the particle content in the polyester layer (A) smaller than that of the white polyester layer (B). If the content of the particles in the polyester layer (A) exceeds the above-mentioned white polyester layer (B), the glossiness is deteriorated and various inconveniences are caused. It is desirable that the particle content in the polyester layer (A) is 50% or less, preferably 20% or less of the content in the white polyester layer, since the gloss becomes better. The kind and size of the fine particles to be contained may be the same as or different from those of the white polyester layer (B).

  In the present invention, when the laminated polyester film of the present invention is used for an outdoor base material or the like, the polyester layer (A) contains an ultraviolet absorber in order to prevent deterioration, fading, discoloration, etc. of the base film layer due to ultraviolet rays. It is necessary.

  The ultraviolet absorber used in the present invention is preferably one that has excellent heat resistance, good compatibility with the above-described polyester, can be uniformly dispersed, and is less colored and does not adversely affect the resin. In particular, a film having a weight loss temperature of 5% by weight or more, which is a measure of heat resistance, of 250 ° C. or more is suitable in terms of film forming properties and film characteristics.

  As an ultraviolet absorber, various adaptations, such as a benzoxazine type or a triazine type, are possible, for example. In the case of a benzoxazine-based compound, a compound represented by 2,2 ′-(1,4-phenylene) bis [4H-3,1-benzoxazin-4-one]: CAS Number 18600-59-4 (for example, Cytec's CYASORB) UV-3638F), if triazine, 2- [4,6-bis (2,4-dimethylphenyl) 1,3,5-triazin-2-yl] -5- (octyloxy) phenol: CAS Number 2725- Compounds represented by 22-6 (for example, Cytec's CYASORB UV-1164) and 2- (4,6-diphenyl-1,3,5 triazin-2-yl) -5-[(hexyl) oxy] -phenol : Use CAS Number 147315-50-2 (eg Ciba Tinuvin 1577FF) It can be.

  The ultraviolet absorber is not particularly limited, and may be used alone or in combination of two or more in some cases.

  The content of the ultraviolet absorber in the polyester layer (A) is 0.2 to 5.0% by weight, and more preferably in the range of 0.3 to 3.0% by weight. If the content of the UV absorber is less than the above range, sufficient UV resistance cannot be obtained for the polyester layer (A), and even though the polyester layer (B) of the core layer has good UV resistance, It is inferior in UV resistance. Moreover, when content of a ultraviolet absorber exceeds the said range, it will be easy to cause a hydrolysis-resistant fall.

  The polyester layer (A) preferably contains fine particles other than white pigments such as porous silica particles and alumina particles in the surface layer to such an extent that the gloss of the film is not deteriorated. When there are no fine particles such as porous silica particles or alumina particles on the surface layer, the friction between the film and the roll is intense during film formation, and scratches are likely to occur on the film surface. Among these fine particles, porous silica particles that are aggregate particles of temporary particles are particularly preferable. Since the porous silica particles are less likely to generate voids around the particles when the film is stretched, the porous silica particles have a feature that does not reduce the gloss of the film.

  The average particle diameter of the primary particles constituting the porous silica particles is preferably in the range of 0.001 to 0.1 μm. If the average particle size of the primary particles is less than 0.001 μm, ultrafine particles are generated by crushing at the slurry stage, which may form aggregates and cause haze to increase. On the other hand, when the average particle size of the primary particles exceeds 0.1 μm, the porosity of the particles is lost, and as a result, the feature of generating less voids may be lost.

  Furthermore, the pore volume of the aggregated particles is usually in the range of 0.5 to 2.0 ml / g, preferably 0.6 to 1.8 ml / g. When the pore volume is less than 0.5 ml / g, the porosity of the particles is lost, voids are easily generated, and the transparency of the film tends to be lowered. When the pore volume is larger than 2.0 ml / g, crushing and aggregation are likely to occur, and it may be difficult to adjust the particle size.

  The method for adding the porous silica particles to the polyester film in the present invention is not particularly limited, and a known method can be adopted. For example, it can be added at any stage for producing the polyester, but it is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification or before the start of the polycondensation reaction after completion of the transesterification reaction. The condensation reaction may proceed. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a blending of dried particles and a polyester raw material using a kneading extruder. It is done by methods.

  In addition to the above-mentioned particles, conventionally known antioxidants, heat stabilizers, lubricants, antistatic agents, and dyes can be added to the polyester film of the present invention as necessary.

  The thickness of the polyester film of the present invention is not particularly limited as long as it can be formed as a film, but is usually 10 μm to 500 μm, preferably 15 to 400 μm, and more preferably 20 to 300 μm.

  In the present invention, it is preferable to use two or three or more polyester melt extruders to form a laminated film of three or more layers by a so-called coextrusion method. As a structure of a layer, it is preferable to set it as an A / B / A structure using the raw material used for a polyester layer (A), and the raw material used for a polyester layer (B). The thickness of the polyester layer (A) is preferably in the range of 1 (μm) to 50 (μm).

  Hereinafter, although the manufacturing method of the polyester film of this invention is demonstrated concretely, as long as the summary of this invention is satisfied, this invention is not specifically limited to the following illustrations.

  That is, a dried or undried polyester chip (polyester component) by a known method is supplied to a kneading extruder, heated to a temperature equal to or higher than the melting point of the polyester component, and melted. Next, the melted polyester is extruded from a die, and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition temperature, thereby obtaining a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, the electrostatic application adhesion method and / or the liquid application adhesion method are preferably employed. Even in the melt extrusion process, the amount of terminal carboxyl groups increases depending on the conditions. Therefore, in the present invention, it is necessary to shorten the residence time of the polyester in the extruder in the extrusion process. Dry sufficiently so that the amount is not more than 50 (ppm), preferably not more than 30 (ppm). When using a twin screw extruder, a vent port is provided and not more than 40 hectopascals, preferably not more than 30 hectopascals, more preferably Adopt a method such as maintaining a reduced pressure of 20 hectopascals or less.

  In the present invention, the sheet thus obtained is stretched biaxially to form a film. Specifically describing the stretching conditions, the unstretched sheet is preferably stretched 2 to 6 times at 70 to 145 ° C. in the machine direction to form a uniaxially stretched film, and then 90 to 160 ° C. in the transverse direction. Then, the film is stretched 2 to 6 times and moved to a heat setting step at 180 to 245 ° C.

  Hydrolysis resistance and transparency of a polyester film are properties related to the entire film. Regarding the film having a laminated structure by co-extrusion according to the present invention, as a whole polyester component constituting the film, the structure of the layer containing the terminal carboxyl group amount, intrinsic viscosity, phosphorus element, titanium element, magnesium element, fine particles The above range is preferable.

  In the present invention, the polyester film can be obtained by shortening the residence time of the polyester component in the extruder in the extrusion process of the polyester chip, for example, in order to set the terminal carboxyl group amount of the polyester component in the polyester film in a specific range. . Moreover, you may obtain the polyester film whose terminal carboxyl group amount is a specific range by forming the polyester chip of low terminal carboxyl group amount into a film. In film production, if a recycled material that has undergone a melting step is blended, the amount of terminal carboxyl groups of the polyester component tends to increase outside a specific range. Therefore, it is preferable not to blend such a recycled material in the present invention. Moreover, even if it mix | blends a reproduction | regeneration raw material, it is preferable to use the flakes obtained by grind | pulverizing the film obtained on its own, and about 40 weight% or less normally about the quantity, Preferably it is 20 weight% or less.

  In the present invention, a coating layer is formed on one side or both sides of the film in order to impart adhesiveness, antistatic property, slipperiness, releasability, etc. to the film in or after the stretching step, or corona treatment. It is also possible to perform a discharge treatment such as.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples unless it exceeds the gist of the present invention. In addition, the measurement and evaluation method of various physical properties of a film are shown below.

(1) Titanium dioxide content (% by weight)
Using a fluorescent X-ray analyzer (model “XRF-1800”, manufactured by Shimadzu Corporation), the amount of titanium element in the film or the raw material polyester was determined under the conditions shown in Table 1 below. Regarding the titanium content of the surface layer and the core layer of the laminated film, after separating the surface layer and the core layer of the laminated film with a file, each sample is measured with a fluorescent X-ray analyzer, so that each layer of titanium element (Ti) Measure the content. The entire laminated film and the raw material polyester titanium (Ti) element content: C (% by weight) can be obtained by measuring a film obtained by melting the film and molding it into a disk shape. From the amount of elemental titanium, the weight percentage of titanium dioxide in each layer, the entire film, and the raw material polyester was determined.

(2) Amount of terminal carboxyl group (equivalent / t)
Using the titanium dioxide content C (% by weight) obtained above from a sample that was dried at 140 ° C. for 15 minutes in a hot air dryer and cooled to room temperature in a desiccator for the pulverized polyester raw material or polyester film Then, 1.0 g × 100 / (100-C) of the polyester film or crushed polyester raw material is precisely weighed and collected in a test tube, 30 ml of benzyl alcohol is added, and 195 ° C. for 3 minutes while blowing dry nitrogen gas. Then, 50 ml of chloroform was gradually added and cooled to room temperature. After only titanium dioxide was sedimented by centrifugation, the finished liquid was taken out. One to two drops of phenol red indicator were added to the supernatant, and titrated with a 0.1N sodium benzyl alcohol solution with stirring while blowing dry nitrogen gas, and the process was terminated when the color changed from yellow to red. It is assumed that 1.0 g of the polyester raw material is contained in the supernatant. Further, as a blank, the same operation was carried out without a polyester resin sample, and the acid value was calculated by the following formula.

Acid value (equivalent / t) = (A−B) × 0.1 × f / W
[Where A is the amount of benzyl alcohol solution of 0.1N caustic soda required for titration (μl) and B is the amount of benzyl alcohol solution of 0.1N caustic soda required for titration with a blank (μl) , W is the amount (g) of the polyester resin sample, and f is the titer of the benzyl alcohol solution of caustic soda of 0.1 (N)]
The titer (f) of 0.1N caustic soda benzyl alcohol solution was obtained by taking 5 ml of methanol into a test tube, adding 1 to 2 drops of phenol red ethanol solution as an indicator, and 0.1N caustic soda benzyl alcohol. Titrate to the color change point with 0.4 ml of solution, then add 0.2 ml of 0.1N hydrochloric acid aqueous solution of known titer as a standard solution and add it again to the color change point with 0.1N sodium hydroxide in benzyl alcohol. Titration was performed (the above operation was performed under blowing of dry nitrogen gas). The titer (f) was calculated by the following formula.

  Titer (f) = Titer of 0.1N hydrochloric acid aqueous solution × Sample volume of 0.1N hydrochloric acid aqueous solution (μl) / Titration volume of 0.1N sodium hydroxide in benzyl alcohol (μl)

(3) Intrinsic viscosity (dl / g)
Using the titanium dioxide content C (% by weight) obtained above in phenol / tetrachloroethane = 50/50 (weight ratio), the polyester film or the pulverized polyester raw material or polyester film was pulverized. A polyester raw material is precisely weighed and added so as to be 1.0 × 100 / (100-C) (g / dl). After dissolving at 120 ° C. for 10 minutes, the mixture was gradually cooled to room temperature. After sedimentation of titanium dioxide alone by centrifugation, the supernatant liquid is taken out, and the flow-down time of the supernatant liquid and the solvent-only flow time are measured. From these time ratios, the Huggins formula is used to determine the limit. Viscosity (dl / g) was calculated. At that time, the Huggins constant was assumed to be 0.33. The concentration of the polyester raw material in the finished liquid was 1.0 g / dl.

(4) Hydrolysis resistance test Using a personal pressure cooker (manufactured by Hirayama Seisakusho Co., Ltd.), the polyester film is treated in an atmosphere of 120 ° C.-100% RH. With Autograph AG-I (manufactured by Shimadzu Corporation), the tensile elongation at break was measured as a mechanical property of the film at a speed of 200 mm / min with respect to the direction (TD) perpendicular to the film forming direction of the obtained film. Measure and obtain the tensile elongation at break rate. The tensile breaking elongation maintenance ratio is obtained from the following formula, that is, the quotient of the tensile breaking elongation before and after the personal pressure cooker treatment.

Tensile rupture elongation retention rate [%] = “tensile rupture elongation after personal pressure cooker treatment” ÷ “tensile rupture elongation before personal pressure cooker treatment” × 100
From the time when the tensile elongation at break maintenance rate reached less than 10%, the following criteria were used for evaluation.
A: Time at which the tensile fracture elongation maintenance ratio reached less than 10%: 96 hr or more O: Time at which the tensile fracture elongation maintenance ratio reached less than 10%: 60 hr or more and less than 96 hr Δ: Tensile fracture elongation maintenance ratio of 10 Time when it reached less than%: 72 hr or more and less than 84 hr x: Time when the tensile fracture elongation maintenance rate reached less than 10%: less than 72 hr

(5) UV resistance / UV irradiation equipment Suga Test Instruments Super Xenon Weather Meter: SX75
・ Ultraviolet irradiation device conditions Irradiance: 180 W / m ² (range of 300 to 400 nm)
Filter: Inner / Outer = Quartz / # 275
Black panel temperature: spraying / non-spraying = non-control / 63 ° C
Test bath temperature: spraying / non-spraying = 28 ° C./non-control Test bath humidity: spraying / non-spraying = 95% RH / 50% RH
Cycle time: spraying / non-spraying = 18 minutes / 102 minutes

-Evaluation method Super xenon weather meter: A polyester film is processed on the above-mentioned conditions using SX75 (made by Suga Test Instruments). With Autograph AG-I (manufactured by Shimadzu Corporation), the tensile elongation at break was measured as the mechanical properties of the film at a speed of 200 mm / min with respect to the film forming direction (MD). Measure and obtain the tensile elongation at break rate. The tensile breaking elongation retention rate is obtained from the following formula, that is, the quotient of the tensile breaking elongation before and after ultraviolet irradiation.

Tensile elongation at break [%] = “tensile elongation at break after super xenon weather meter treatment” ÷ “tensile elongation at break before super xenon weather meter treatment” × 100
From the time when the tensile elongation at break rate reached less than 40%, the following criteria were used for evaluation.
○: Time at which the tensile fracture elongation maintenance ratio reached less than 40%: 200 hr or more Δ: Time at which the tensile fracture elongation maintenance ratio reached less than 40%: 100 hr or more and less than 200 hr ×: Tensile fracture elongation maintenance ratio of 40 % Time reached: less than 100 hr

(6) Gloss Gloss meter manufactured by Nippon Denshoku Co., Ltd .: Glossiness was measured according to the method of JIS Z-8741 using VG-107 type. The reflectance of the sample was determined based on the reflectance of the black standard plate at an incident angle and a reflection angle of 60 degrees, and was used as the glossiness. The glossiness evaluation value was evaluated according to the following criteria.
○: Gloss value of 100 or more Δ: Gloss value of 80 or more and less than 100 ×: Gloss value of less than 80

<Method for producing polyester (1)>
Continuous polymerization apparatus comprising one slurry preparation tank, a two-stage esterification reaction tank connected in series thereto, and a three-stage melt polycondensation tank connected in series to the second esterification reaction tank And terephthalic acid and ethylene glycol are continuously fed to the slurry preparation tank at a weight ratio of 100: 45, and an ethylene glycol solution of ethyl acid phosphate is contained as a phosphorus atom in the produced polyester resin. A slurry is prepared by continuously adding, stirring and mixing in an amount of 4 ppm by weight, and this slurry is set at 267 ° C., a relative pressure of 100 kPa, and an average residence time of 4 hours under a nitrogen atmosphere. , Continuously fed to the first stage esterification reaction tank where the reaction product exists at a flow rate of 120 kg / hr, and then the first stage esterification The reaction product was continuously transferred to a second stage esterification reaction tank set at 265 ° C., a relative pressure of 5 kPa, and an average residence time of 2 hours under a nitrogen atmosphere, and further esterified. At that time, an amount of 322 mol / ton was continuously supplied to the polyester resin producing ethylene glycol through the upper pipe provided in the second stage esterification reaction tank. In this case, the esterification rate in the second stage esterification reaction tank was 97%.

The above esterification reaction product was continuously supplied to the first stage polycondensation reaction tank via the transfer pipe. At this time, the discharge pressure of the transfer pump provided in the transfer pipe was 500 kPa. A 0.6 wt% solution of magnesium acetate tetrahydrate in ethylene glycol is continuously added to the esterification reaction product in the transfer pipe in such an amount that the content as magnesium atoms is 7 ppm by weight with respect to the produced polyester resin. Added to. Using an addition pipe, an ethylene glycol solution of tetra-n-butyl titanate was continuously added to the resulting polyester resin in an amount such that the content as titanium atoms was 4 ppm by weight.

The melt polycondensation reaction conditions were 269 ° C. in the first stage polycondensation reaction tank, absolute pressure 4 kPa, average residence time 1 hour, 274 ° C. in the second stage polycondensation reaction tank, 0.4 kPa absolute pressure, average residence time The time was 0.9 hours, the third stage polycondensation reaction tank was 277 ° C., the absolute pressure was 0.2 kPa, and the average residence time was 1 hour. The melt polycondensation reaction product taken out from the third stage polycondensation reaction tank is extruded from a die into a strand, cooled and solidified, cut with a cutter, and one piece of polyester resin chip having an average particle weight of 24 mg: polyester (1). The intrinsic viscosity of the polyester (1) was 0.64 dl / g, and the amount of terminal carboxyl groups was 14 equivalents / t.

<Method for producing polyester (2)>
Polyester (1) is used as a starting material, and continuously fed in a stirrer crystallizer maintained at about 160 ° C. in a nitrogen atmosphere with the chips not overlapping so that the residence time is about 60 minutes. After crystallizing, the residence time is adjusted so that the intrinsic viscosity of the resulting polyester resin is 0.83 dl / g under a nitrogen atmosphere at 215 ° C. continuously by feeding to a tower-type solid phase polycondensation device. Thus, polyester (2) was obtained by solid phase polycondensation. The amount of terminal carboxyl groups was 6 equivalent / t.

<Method for producing polyester (3)>
100 parts by weight of dimethyl terephthalate and 200 parts by weight of ethylene glycol are used as starting materials, and as a transesterification catalyst, the magnesium content per 1 t of polyester resin from which magnesium acetate tetrahydrate can be obtained is 46 g / resin t. In addition, the reaction start temperature was 150 ° C., and the reaction temperature was gradually increased as methanol was distilled off. After 4 hours, the transesterification reaction was substantially terminated.

  This reaction mixture was transferred to a polycondensation tank, and an ethylene glycol slurry solution consisting of a mixture of silica particles having an average particle diameter of 2.5 μm, ethyl acid phosphate, magnesium acetate tetrahydrate, and tetra-n-butyl titanate was added. The polycondensation reaction was performed for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg.

  The amount of each compound in the ethylene glycol slurry solution is 74 g / resin t as the phosphorus element amount for ethyl acid phosphate so that the silica particles are 3.0% by weight with respect to the content of the obtained polyester. Thus, for magnesium acetate tetrahydrate, the amount of magnesium element is 46 g / resin t (including the magnesium added at the time of transesterification, the total amount of magnesium element is 92 g / resin t). -N-Butyl titanate is adjusted so that the amount of titanium element is 5 g / resin t.

  After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.60 due to a change in stirring power in the reaction tank, and the polymer was discharged under nitrogen pressure to obtain polyester (3). The intrinsic viscosity was 0.60 dl / g, and the amount of terminal carboxyl groups was 21 equivalent / t.

<Method for producing polyester (4)>
100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol are used as starting materials, and tetra-n-butyl titanate is obtained as a catalyst. The amount of titanium atoms per 1 ton of polyester resin is 5 g / resin t. In the reactor, the reaction start temperature was 150 ° C., and the reaction temperature was gradually increased as methanol was distilled off. After 4 hours, the transesterification reaction was substantially terminated. This reaction mixture was transferred to a polycondensation tank and subjected to a polycondensation reaction for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.55 dl / g due to a change in stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure to obtain a polyester chip. The polyester chip obtained above was solid-phase polymerized at 220 ° C. under vacuum to obtain polyester (4). The intrinsic viscosity was 0.75 dl / g, and the amount of terminal carboxyl groups was 25 equivalents / t.

<Method for producing polyester (5)>
While melting and kneading the polyester (2) with a vented twin-screw extruder, titanium dioxide particles having an average particle diameter of 0.45 μm were added so that the titanium dioxide concentration was 50% by weight. The polyester (5) was obtained as a magnesium master batch by extruding into a strand form from a die, cooling and solidifying, and cutting with a cutter. For the polyester component, the intrinsic viscosity was 0.50 dl / g, and the amount of terminal carboxyl groups was 80 equivalents / t.

<Method for producing polyester (6)>
While melting and kneading polyester (2) with a vented twin-screw extruder, Cytec's CYASORB UV-3638F, which is a benzoxazine-based ultraviolet absorber, was added so as to be 10% by weight. The polyester (6) was obtained as a magnesium master batch by extruding into a strand form from a die, solidifying by cooling, and cutting with a cutter. For the polyester component, the intrinsic viscosity was 0.67 dl / g, and the amount of terminal carboxyl groups was 20 equivalents / t.

<Method for producing polyester (7)>
While melting and kneading the polyester (2) with a vented twin-screw extruder, Ciba Tinuvin 1577FF, which is a triazine-based ultraviolet absorber, was added to 10 wt%. A polyester (7) was obtained as a magnesium master batch by extruding into a strand from a die and solidifying by cooling and cutting with a cutter. For the polyester component, the intrinsic viscosity was 0.67 dl / g, and the amount of terminal carboxyl groups was 20 equivalents / t.

Example 1:
A polyester mixture prepared by mixing the polyester (2), the polyester (3) and the polyester (6) in a ratio of 90: 4: 6 is charged into the vented twin-screw extruder A (sub) and the polyester (2). And a polyester mixture prepared by mixing polyester (5) at a ratio of 93: 7 was charged into a twin screw extruder B (main) with a vent. Both raw materials are melted and kneaded in a twin-screw extruder at 290 ° C., and the resulting melt is joined in a multilayer T die so as to have a composition ratio of A / B / A = 5/90/5. Extrude into a shape. Using an electrostatic application adhesion method, the sheet was rapidly cooled and solidified on a casting drum set at a surface temperature of 40 ° C. to obtain an unstretched two-kind three-layered sheet. After the obtained sheet was stretched 3.3 times in the machine direction at 83 ° C., the temperature [° C.] in each zone of preheating / lateral stretching / heat setting 1 / heat setting 2 / heat setting 3 / cooling was set to 95/110 / The film was formed by guiding to a tenter set to 200/221/180/125 ° C. The film forming speed at this time was 16 m / min. The average thickness of the obtained film was 125 μm, and the surface layer / core layer / surface layer thickness (μm) was 6.25 / 112.50 / 6.25. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 2 to Example 7:
A film was prepared in the same manner as in Example 1 except that the polyester raw material charged into the vented twin screw extruder A (sub) and vented twin screw extruder B (main) was changed to the formulation shown in Table 2. Obtained. The properties and evaluation results of the obtained film are shown in Table 2 below.

Comparative Examples 1 to 6:
A film was prepared in the same manner as in Example 1 except that the polyester raw material charged into the vented twin-screw extruder A (sub) and vented twin-screw extruder B (main) was changed to the formulation shown in Table 3. Obtained. The properties and evaluation results of the obtained film are shown in Table 3 below.

  The polyester film of this invention can be utilized suitably as a white film for solar cell back surface sealing.

Claims (1)

The polyester layer (B) containing a white pigment is used as a base layer, and one polyester layer (A) on both surface layers contains an ultraviolet absorber in an amount of 0.2 to 5.0% by weight, and the white pigment of the polyester layer (A) It is a laminated polyester film having a content less than the white pigment content of the polyester layer (B), the intrinsic viscosity of the laminated polyester film is 0.65 dl / g or more, and the terminal carboxyl group content is 26 equivalents / t or less. A white laminated polyester film for sealing a back surface of a solar cell, characterized in that: