JP2011155110A - Biaxially-oriented polyester film for solar cell rear-surface sealing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially-oriented polyester film for solar cell rear-surface sealing having hydrolysis resistance and UV resistance, as well as, low haze and low shrinkage. <P>SOLUTION: The biaxially-oriented polyester film for solar cell rear-surface sealing is a biaxially-oriented polyester film containing an ultraviolet absorbent, wherein the average value of light transmittance in a light wavelength range of 400-650 nm (Tave) is 75.0% or higher; as well as, the light transmittance at a light wavelength of 380 nm (Tuv) is 60.0% or lower; the content of antimony element in the polyester film is 10 ppm or less; the content of titanium element is 20 ppm or less; the content of phosphorus element is 170 ppm or less; the amount of terminal carboxyl group is 26 equivalent weight/t or less; the limiting viscosity is 0.65 dl/g or higher; and the heat shrinkage in a longitudinal direction of the film after a treatment for 30 minutes at 150 °C is 0.8% or lower. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a biaxially oriented polyester film for solar cell backside sealing that has hydrolysis resistance and UV resistance, low haze, and low heat shrinkage.

  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.

  Since the solar cell module is used for a long time in a high temperature and high humidity environment, the solar cell back surface sealing film is also required to have long-term durability. For example, Patent Document 1 discloses a technique using a fluorine-based film as a solar cell back surface sealing film. In this document, it is possible to reduce the heat shrinkage rate of the fluorine-based film by performing heat treatment on the fluorine-based film in advance, and at the time of vacuum laminating with ethylene vinyl acetate (hereinafter EVA) as a sealing material, It is described that it is effective in preventing deterioration of physical properties such as weather resistance and water resistance and improving yield. However, since the fluorine-based film is expensive, there is a problem that the solar cell module is also expensive.

  As a film for sealing the back side of solar cells, polyester films are being studied. However, when polyester films are used in a high-temperature and high-humidity environment, hydrolysis of ester bond sites in the molecular chain occurs, resulting in mechanical properties. Is known to deteriorate. Therefore, various studies have been made to suppress hydrolysis assuming that the polyester film is used outdoors for a long period (20 years) or 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 2 and Patent Document 3 disclose a technique for improving hydrolysis resistance by adding a compound that reacts with a 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. In addition, since solar cells are used to irradiate the sun, the film for this application must not only be resistant to hydrolysis but also prevent yellowing of the film due to sunlight. This requirement is not taken into account.

  Conventionally, solar cells have been installed based on the idea that they are installed on the roof, but demand for “see-through type solar cells” that look like window glass is also growing. This is a solar cell that can see the outside scenery from the inside of the window and can also generate electricity. This solar cell-type solar cell back surface sealing film is required to have low haze in addition to hydrolysis resistance.

  As a catalyst for polyester polycondensation, which is a raw material for polyester film, antimony trioxide is widely used due to its low cost and excellent catalytic activity, but this is the main component of polycondensation catalyst, that is, practical. When used in such an amount that a sufficient polymerization catalyst is exhibited, antimony trioxide is reduced during polycondensation to produce metal antimony particles. Then, the metal antimony particles are aggregated in the melt extrusion process at the time of film production, and are present in the film as black foreign matters of 20 to 50 μm. That is, these metal antimony particle aggregates have a problem of shielding light transmission and increasing haze.

  Even if a filter is used at the time of melt extrusion to remove these metal antimony particle aggregates, it has been difficult to completely remove the metal antimony particle aggregates through the filter while being deformed.

  Moreover, germanium compounds are known as polymerization catalysts for the polycondensation of polyester, which is a raw material for the base polyester film, but germanium compounds are very expensive and difficult to use for general purposes.

  In order to solve the above-mentioned problem, a proposal has been made to reduce the internal foreign matter of the film by specifying the contents of titanium element and phosphorus element (Patent Document 4), but an oligomer generated in the process of melt polymerization of polyester. It is not designed to take into account. That is, oligomers were generated inside or on the surface of the film, and it was difficult to obtain a film having a low haze.

  In addition, although heat is applied to the sheet at the time of vacuum laminating with the solar cell back surface sealing sheet and ethylene vinyl acetate (hereinafter sometimes abbreviated as EVA) as a sealing material, when the heat shrinkage rate of the sheet is high, There is a possibility that the solar battery cell may be destroyed due to stress caused by sheet contraction. Therefore, the low heat shrinkage rate is calculated | required also with respect to the polyester film for solar cell backside sealing used for a sheet | seat.

JP 2002-83978 A JP-A-9-227767 JP-A-8-73719 JP-A-6-170911

  An object of the present invention is to provide a biaxially oriented polyester film for sealing a back surface of a solar cell that has hydrolysis resistance and UV resistance, low haze, and low heat shrinkage.

  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 a biaxially oriented polyester film containing an ultraviolet absorber, the average value of light transmittance (Tave) at a light wavelength of 400 to 650 nm is 75.0% or more, and the light wavelength of 380 nm. The light transmittance (Tuv) in the film is 60.0% or less, the antimony element content in the polyester film is 10 ppm or less, the titanium element content is 20 ppm or less, and the phosphorus element content is 170 ppm or less. The terminal carboxyl group amount is 26 equivalent / t or less, and the intrinsic viscosity is 0.65 dl / g or more.

  According to the present invention, it is possible to provide a biaxially oriented polyester film for sealing a back surface of a solar cell that has hydrolysis resistance and UV resistance, low haze, and low heat shrinkage. The industrial value of the present invention is high.

Hereinafter, the present invention will be described in more detail.
The polyester film referred to in the present invention is a film stretched as necessary 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 amount of the compound in the polyester film of the present invention needs to be within a specific range for the content of antimony element, titanium element, and phosphorus element detected by analysis using a fluorescent X-ray analyzer described later.

  The titanium element content of the film of the present invention needs to be 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. If the content of the titanium compound is too large, oligomers are by-produced in the step of melt-extruding the polyester, resulting in a film having increased haze due to the oligomers. Further, when no titanium element is contained, 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 cannot 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 needs to be in the range of 170 ppm or less, preferably in the range of 160 ppm or less, and more preferably in the range of 150 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, which may be a cause of deterioration of film quality, and it is preferable because hydrolysis of the film after film formation is promoted. Absent. In addition to containing a specific amount of the above-described titanium compound and containing a phosphorus compound, a significant effect can be exerted on the reduction of the contained oligomer.

  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.

  In the polyester film obtained by the present invention, the amount of terminal carboxyl groups of the polyester component constituting the film is 26 equivalent / t or less, preferably 24 equivalent / t or less. When the amount of terminal carboxyl groups exceeds 26 equivalent / t, the hydrolysis resistance of the polyester component is poor. Although there is no particular lower limit, it is usually about 5 equivalents / t from the viewpoints of the efficiency of the polycondensation reaction, thermal decomposition in the melt extrusion process, and the like.

  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 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.

  The average value (Tave) of light transmittance at a light wavelength of 400 to 650 nm of the film of the present invention is 75.0% or more, preferably 78.0% or more, and more preferably 80.0% or more. If Tave is less than 75.0%, it is not preferable as a PET film for a back sheet for backside sealing for see-through solar cells. Although there is no particular upper limit for Tave, it is usually about 90.0%.

  The film of the present invention contains an ultraviolet absorber, and the content thereof is generally 0.01 to 10.0% by weight, preferably 0.1 to 1.8% by weight, based on the total thickness of the polyester film. When the content of the ultraviolet absorber is less than 0.01% by weight, the polyester compound may be deteriorated by ultraviolet rays transmitted through the polyester film. On the other hand, the effect of preventing the deterioration of the polyester is saturated even when the amount of the ultraviolet absorber exceeding 10.0% by weight is contained, and conversely, the ultraviolet absorber bleeds out on the surface and the adhesiveness is lowered. There is a possibility that the surface characteristics are deteriorated.

  Moreover, the light transmittance (Tuv) of the light wavelength 380 nm of the film of this invention is 60.0% or less, More preferably, it is 50.0% or less, More preferably, it is 40.0% or less. When Tuv is larger than 60.0%, it cannot be said that it is sufficient to prevent the polyester compound from being deteriorated by ultraviolet rays transmitted through the polyester film.

  The ultraviolet absorber used in the present invention may be an ultraviolet absorber that can be contained in the polyester. For example, a triazine compound, a benzophenone compound, a benzotriazole compound, a salicylate compound, a cyanoacrylate compound, Examples include benzoxazine compounds and cyclic imino ester compounds. Among these, triazine compounds, benzophenone compounds, and benzoxazine compounds are preferable because of their good compatibility with polyesters.

  In order to improve workability in the film winding process, coating process, vapor deposition process, and the like, it is desirable that the polyester film in the present invention contains fine particles. The fine particles include inorganic particles such as silica, calcium carbonate, magnesium carbonate, calcium sulfate, barium sulfate, lithium phosphate, magnesium phosphate, calcium phosphate, lithium fluoride, aluminum oxide, and kaolin, and organic particles such as acrylic resin and guanamine resin. In addition, there can be mentioned, but not limited to, precipitated particles in which the catalyst residual is made into particles. Among these 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 film when the film is stretched, the porous silica particles have the feature of improving the transparency of the film.

  The average particle diameter of 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, there is a possibility that the feature with less generation of voids may be lost.

  Furthermore, the pore volume of the aggregated particles is preferably in the range of 0.5 to 2.0 ml / g, more 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 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, it is preferable not to contain as much as possible a metal compound that can act as a catalyst in order to suppress thermal decomposition and hydrolysis, but in order to reduce the volume resistivity at the time of melting in order to improve the productivity of the film, magnesium, Metals such as calcium, lithium, and manganese can be contained in the polyester component, usually at 300 ppm or less, preferably 250 ppm or less.

  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 to 500 μm, preferably 15 to 400 μm, and more preferably 20 to 300 μm.

  In the present invention, two or three or more polyester melt extruders can be used to form a laminated film of two layers or three or more layers by a so-called coextrusion method. As the layer structure, an A / B structure using an A raw material and a B raw material, or an A / B / A structure, and further using a C raw material to form an A / B / C structure or other film. Can do.

  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 50 ppm or less, preferably 30 ppm or less. When a twin screw extruder is used, a vent port is provided, and a reduced pressure of 40 hectopascals, preferably 30 hectopascals, more preferably 20 hectopascals or less. A method such as maintaining the above is adopted.

  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 the heat setting process is started.

  The heat setting is preferably performed at 160 ° C. to 240 ° C. for 1 second to 600 seconds, and more preferably 170 ° C. to 230 ° C. When the heat setting temperature is less than 160 ° C., the shrinkage rate in the longitudinal direction is too high, the annealing treatment conditions become severe, and as a result, the distortion of the obtained film increases and cannot be put to practical use. On the other hand, when the heat setting temperature is 240 ° C. or higher, a polyester film having good hydrolysis resistance cannot be obtained.

  In the present invention, it is preferable to perform an annealing treatment. The annealing treatment referred to in the present invention means that the heat-fixed biaxially stretched polyester film is heat-treated in a state where substantially no tension is applied.

  The heat treatment temperature during the annealing treatment is preferably in the range of 40 ° C. or more lower than the glass transition temperature (Tg) to the melting point (Tm) of the biaxially oriented polyester film.

When the annealing treatment is performed, the biaxially stretched polyester film is stretched if a large tension is applied. Therefore, it is preferable that the biaxially stretched polyester film is annealed in a state where the tension is not substantially applied. The state where substantially no tension is applied specifically means that the film tension (g / mm ² ) during annealing is 100 or less, preferably 80 or less, more preferably 60 or less.

  As for the form of annealing treatment, in-line annealing recipe that performs annealing treatment in the film manufacturing process, offline annealing recipe that treats after film production, etc. can be considered, but offline annealing is not limited by the film production speed A prescription is preferred.

  The time for annealing is not particularly limited and varies depending on the thickness of the biaxially stretched polyester film and the annealing temperature, but is generally preferably 5 seconds or more, more preferably 10 seconds to 60 minutes, and even more preferably 30 seconds to 20 minutes. is there.

  There is no particular limitation on the infrared heating furnace to which the annealing treatment is performed. For example, it is preferable that a large number of infrared heaters wider than the running film width are installed in the upper part of the furnace at regular intervals so as to cover the entire width of the running film.

  As the infrared heater, both a near-infrared heater and a far-infrared heater including a sheathed heater can be used, but a near-infrared heater is preferable in terms of heat damage to the film.

  The heat treatment of the film is performed with the furnace atmosphere at a predetermined temperature, and this temperature can be adjusted, for example, by the following method. A thermocouple temperature detection end is installed between adjacent heaters / heaters in the furnace and on the traveling film at a film proximity position of about 5 cm, and the ambient temperature at each position is measured. This ambient temperature can be changed by the output of each installed heater, the number of heaters, the heater installation interval, the distance between the running film and the heater, the ventilation in the furnace, etc. While adjusting in the range of 0.5 to 1.2 kW / m and appropriately performing a constant air flow ventilation, the ambient temperature in the vicinity of the film can be set to a preferable region, that is, a range of 150 to 220 ° C.

  The infrared heating furnace is characterized in that a heating effect equal to or higher than that can be obtained even if the ambient temperature in the vicinity of the running film is lower than the ambient temperature in the same position of the hot air heating furnace. For this reason, in the hot air heating furnace, it is possible to shorten the processing time and increase the efficiency that could not be achieved, and to reduce the film distortion because of the short time processing.

  For example, by performing the annealing treatment as described above on the polyester film, the heat shrinkage rate in the longitudinal direction of the film after 30 minutes of treatment at 150 ° C. can be made 0.8% or less.

  The hydrolysis resistance of the polyester film is a property related to the entire film.In the present invention, in the case of a film having a laminated structure by coextrusion, as an entire polyester component constituting the film, an antimony element, a titanium element, It is necessary that the phosphorus element content, the terminal carboxyl group content, and the intrinsic viscosity are within the above ranges.

  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 addition, in the film production, when the recycled raw material that has undergone the melting step is blended, the amount of terminal carboxyl groups of the polyester component tends to increase out of a specific range, so in the present invention it is preferable not to blend such a recycled raw material Even so, it is 40% by weight or less, more preferably 20% by 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) Quantification of catalyst-derived elements Using a fluorescent X-ray analyzer (model “XRF-1800” manufactured by Shimadzu Corporation), the amount of elements in the film was determined by single-sheet measurement under the conditions shown in Table 1 below. . In the case of a laminated film, the content of antimony element (Sb), titanium element (Ti), and phosphorus element (P) with respect to the entire film was measured by melting the film and molding it into a disk shape.

(2) Amount of terminal carboxyl group (equivalent / t)
After pulverizing the polyester chip, the sample was dried at 140 ° C. for 15 minutes in a hot air dryer and cooled to room temperature in a desiccator. The solution was dissolved at 195 ° C. for 3 minutes while blowing dry nitrogen gas, and then 5 ml of chloroform was gradually added and cooled to room temperature. One to two drops of phenol red indicator were added to this solution, and titrated with 0.1 N sodium hydroxide in benzyl alcohol with stirring while blowing dry nitrogen gas, and the procedure was terminated when the color changed from yellow to red. Further, as a blank, the same operation was performed 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), B is the amount of benzyl alcohol solution of 0.1N caustic soda required for titration with blank (μl) , W is the amount (g) of the polyester resin sample, and f is the titer of the benzyl alcohol solution of 0.1N caustic soda]

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. (The above operation was performed under dry nitrogen gas blowing.) The titer (f) was calculated by the following equation.
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
The polyester film was dissolved in a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio), and using a capillary viscometer, the flow time of a solution having a concentration of 1.0 g / dl and the flow time of the solvent alone were reduced. From these time ratios, the intrinsic viscosity was calculated using the Huggins equation. At that time, the Huggins constant was assumed to be 0.33.

(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 for 72 hours. With Autograph AG-I (manufactured by Shimadzu Corp.), the elongation at break as a mechanical property of the film was measured at a speed of 200 mm / min with respect to the direction (TD direction) perpendicular to the film forming direction of the obtained film. It was measured. The maintenance ratio (%) of elongation at break before and after the treatment was calculated by the following formula and evaluated according to the following criteria.
Breaking elongation maintenance rate [%] = breaking elongation after treatment ÷ breaking elongation before treatment × 100
○: Maintenance rate is 50% or more △: Maintenance rate is 5-50%
X: Maintenance rate is less than 5%

(5) Light transmittance With a spectrophotometer (UV3100PC manufactured by Shimadzu Corporation), a halogen lamp light source is used to scan a 50 μm-thick film at a low scanning speed, a sampling pitch of 1 nm, and a light wavelength of 300 to 800 nm. From the measurement results, the light transmittance (Tuv) at a light wavelength of 380 nm was read, and the average value (Tave) of the light transmittance at a light wavelength of 400 to 650 nm was calculated as an arithmetic average. It was calculated from the above measurement results and was evaluated as follows.
◆ Light transmittance (A)
A: Tuv ≦ 40%
○: 40% <Tuv ≦ 50%
Δ: 50% <Tuv ≦ 60%
×: Tuv> 60%
◆ Light transmittance (B)
○: Tave ≧ 80%
Δ: 75% ≦ Tave <80%
×: Tave <75%

(6) Film haze Haze: H of a film having a thickness of 50 μm was measured with a haze meter (NDH-300A manufactured by Nippon Denshoku). Evaluation was made according to the following criteria.
○: H ≦ 0.004φ
×: H> 0.004φ
(Φ is weight fraction ppm of silica contained)

(7) Glass transition temperature (Tg)
Peak due to α dispersion of loss tangent tan δ measured with a dynamic viscoelasticity device (DVA-200 manufactured by IT Measurement Control Co., Ltd.) on a biaxially oriented polyester film under conditions of a frequency of 10 Hz and a heating rate of 10 ° C./min. Let it be temperature.

(8) Melting point (Tm)
10 ° C./min. With DSC7 manufactured by Perkin Elma. The endothermic peak temperature due to crystal melting obtained at the rate of temperature rise was defined as the melting point.

(9) Heat shrinkage rate It calculated by following Formula by heat-processing for 30 minutes in 150 degreeC atmosphere in a tension | tensile_strength state, and measuring the length of the sample before and behind that.
Heat shrinkage (%) = (L1-L0) / L0 × 100
(In the above formula, L1 (mm) means the sample length before heat treatment, and L0 (mm) means the sample length after heat treatment)
From the obtained results, the following criteria were evaluated.
○: Shrinkage rate in the longitudinal direction (MD) is less than 0.3% Δ: Shrinkage rate in the longitudinal direction (MD) is 0.3% or more and less than 0.8% ×: Shrinkage rate in the longitudinal direction (MD) is 0.00. 8% or more

(10) Comprehensive evaluation The evaluation of hydrolysis resistance, light transmittance A, light transmittance B, haze, and heat shrinkage is compared and judged with a low evaluation. (For example, if the hydrolysis resistance is ○, the light transmittance A ○, the light transmittance B ○, the haze x, and the heat shrinkage rate is ○, the overall evaluation is x.)

<Method for producing polyester (1)>
A slurry is prepared by using a continuous polymerization apparatus comprising a slurry preparation tank, a two-stage esterification reaction tank connected in series to the slurry preparation tank, and a three-stage melt polycondensation tank connected in series to the second-stage esterification reaction tank. To the preparation tank, terephthalic acid and ethylene glycol were continuously supplied at 865 parts by weight and 485 parts by weight, respectively, and a 0.3 wt% ethylene glycol solution of ethyl acid phosphate was added to the phosphorus atom per 1 ton of the resulting polyester resin. Was added continuously in such an amount that the content became 0.129 mol / resin t, and a slurry was prepared by stirring and mixing. This slurry was subjected to a first stage esterification reaction tank set at 260 ° C. under a nitrogen atmosphere, a relative pressure of 50 kPa (0.5 kg / cm ² ), and an average residence time of 4 hours, and then at 260 ° C. under a nitrogen atmosphere. Then, the mixture was continuously transferred to a second stage esterification reaction tank set to a relative pressure of 5 kPa (0.05 kg / cm ² ) and an average residence time of 1.5 hours to cause an esterification reaction. Further, at that time, a 0.6 wt% ethylene glycol solution of magnesium acetate tetrahydrate is contained as magnesium atoms per 1 t of the obtained polyester resin through an upper pipe provided in the second stage esterification reaction tank. The amount was continuously added in an amount of 0.165 mol / resin t, and 60 parts by weight of ethylene glycol was continuously added through another upper pipe provided in the second stage esterification reaction tank. .

  Subsequently, when the esterification reaction product obtained above is continuously transferred to the melt polycondensation tank, tetra-n-butyl titanate is added to the esterification reaction product in the transfer pipe with a concentration of titanium atoms. As an ethylene glycol solution with 0.15% by weight and a water concentration of 0.5% by weight, it is continuously added in such an amount that the content as titanium atoms per 1 ton of the obtained polyester resin is 0.084 mol / resin t. However, the first stage melt polycondensation tank set at 270 ° C. and absolute pressure 2.6 kPa, then the second stage melt polycondensation tank set at 278 ° C. and absolute pressure 0.5 kPa, then Each polycondensation is carried out so that the intrinsic viscosity of the resulting polyester resin is 0.65 dl / g by continuously transferring to a third stage polycondensation tank set at 280 ° C. and an absolute pressure of 0.3 kPa. In the tank The melt residence time is adjusted and melt polycondensed. The polyester is continuously drawn into a strand from the outlet provided in the bottom of the polycondensation tank, cooled with water, and cut with a cutter to form a chip-like granule (1 ) Was manufactured. The amount of terminal carboxyl groups was 12 equivalent / t.

<Method for producing polyester (2)>
Polyester (1) is used as a starting material, and is continuously fed into a stirring crystallization machine maintained at about 160 ° C. in a nitrogen atmosphere so that the residence time is about 60 minutes. Continuously supplied to a solid phase polycondensation apparatus, and at 215 ° C. in a nitrogen atmosphere, the residence time was adjusted so that the intrinsic viscosity of the resulting polyester resin was 0.75 dl / g, and solid phase polycondensation was performed. 2) was obtained. The amount of terminal carboxyl groups was 8 equivalent / t.

<Method for producing polyester (3)>
Using 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol as starting materials, 0.09 parts by weight of calcium acetate as a catalyst is placed in the reactor, the reaction start temperature is 150 ° C., and the reaction temperature is gradually increased as methanol is distilled off. The temperature was raised to 230 ° C. after 3 hours.
After 4 hours, the transesterification reaction was substantially terminated. To this reaction mixture, 0.04 part of antimony trioxide and silica particles having an average particle size of 2.5 μm dispersed in ethylene glycol were added so that the content of the polyester with respect to the polyester was 0.06% by weight, and the polycondensation reaction was performed for 4 hours. Went. 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 40 pascals. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.60 dl / g due to a change in stirring power of the reaction vessel, and the polyester was discharged under nitrogen pressure. Subsequently, the obtained polyester was subjected to solid phase polymerization at 220 ° C. under vacuum to obtain a polyester (3). Polyester (3) had an intrinsic viscosity of 0.75 dl / g and a terminal carboxyl group content of 9 equivalents / 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. The reaction mixture was transferred to a polycondensation tank and the average particle size 2. An ethylene glycol slurry of 5 μm silica particles was added so that the content of the particles with respect to the polyester was 1.5% by weight, and a 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. 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 of the reaction tank, and the polymer was discharged under nitrogen pressure to obtain polyester (4). The intrinsic viscosity was 0.60 dl / g, and the amount of terminal carboxyl groups was 21 amounts / t.

<Method for producing polyester (5)>
Starting from 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol, magnesium acetate / tetrahydrate is added as a catalyst to the reactor, the reaction start temperature is 150 ° C., and the reaction temperature is gradually increased as methanol is distilled off. Was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. This reaction mixture was transferred to a polycondensation tank and orthophosphoric acid was added so that the amount of phosphorus was 1000 ppm, and then germanium dioxide was added to carry out 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 0.63 due to a change in stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure to obtain polyester (5). The intrinsic viscosity of the polyester (5) was 0.63 dl / g, and the amount of terminal carboxyl groups was 51 (equivalent / t). Met.

<Method for producing polyester (6)>
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. The reaction mixture was transferred to a polycondensation tank and the average particle size 2. An ethylene glycol slurry of 5 μm silica particles was added so that the content of the particles with respect to the polyester was 0.06 wt%, and a 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. 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. Subsequently, the obtained polyester chip was subjected to solid phase polymerization at 220 ° C. under vacuum to obtain polyester (6). The intrinsic viscosity was 0.75 dl / g, and the amount of terminal carboxyl groups was 25 equivalents / t.

<Method for producing polyester (7)>
In the production method of polyester (6), polyester (in the same manner as above) except that an amount of 15 g / resin t as a titanium atom per 1 t of polyester resin from which tetra-n-butyl titanate is obtained is added. 7) was produced. The intrinsic viscosity was 0.73 dl / g, and the amount of terminal carboxyl groups was 25 (equivalent / t).

<Method for producing polyester (8)>
In the production method of the polyester (6), the polyester (in the same manner as above) except that an amount of 40 g / resin t as a titanium atom per 1 t of the polyester resin from which tetra-n-butyl titanate is obtained is added. 8) was produced. The intrinsic viscosity was 0.71 dl / g and the amount of terminal carboxyl groups was 27 equivalents / t.

<Method for producing polyester (9)>
Polyester (9) was produced in the same manner except that silica particles were not added in the production method of polyester (6). The intrinsic viscosity was 0.77 dl / g, and the amount of terminal carboxyl groups was 23 equivalents / t.

<Method for producing polyester (10)>
The polyester (1) was subjected to a twin screw extruder with a vent, and 2,2- (1,4-phenylene) bis [4H-3,1-benzoxazin-4-one] was used as a UV absorber in a concentration of 10% by weight. Then, the mixture was melt kneaded to form chips, and an ultraviolet absorbent master batch polyester (10) was prepared. The intrinsic viscosity of the obtained polyester (10) was 0.59 dl / g, and the amount of terminal carboxyl groups was 25 equivalent / t.

<Method for producing polyester (11)>
Polyester (1) is supplied to a twin screw extruder with a vent, carbon black (oil furnace black average primary particle size 70 nm) is supplied so as to be 20% by weight, melt kneaded to form a chip, and carbon black Masterbatch polyester (11) was prepared. The obtained polyester (11) had an intrinsic viscosity of 0.56 dl / g and a terminal carboxyl group content of 30 equivalents / t.

Example 1:
The polyester (2), polyester (4), and polyester (10) mixed at a ratio of 94.0: 4.0: 2.0 is used as a raw material, and is 290 ° C. by one vented twin-screw extruder. Then, it was melt-extruded and rapidly cooled and solidified on a casting drum whose surface temperature was set to 40 ° C. using an electrostatic application adhesion method to obtain an unstretched single layer sheet. The obtained sheet was stretched 3.3 times in the longitudinal direction at 83 ° C., led to a tenter, stretched 3.7 times in the lateral direction at 110 ° C., and heat-set at 220 ° C. The average thickness of the obtained film was 50 μm. The obtained film was passed through an infrared heater direct heating furnace, treated in an atmosphere temperature zone of 150 to 210 ° C. for a treatment time of 10 seconds, and treated with a film tension of 40 g / mm ² for annealing treatment. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 2:
In Example 1, the polyester (2), the polyester (4), the polyester (5), and the polyester (10) were mixed at 78.0: 4.0: 15.0: 4.0 with respect to the polyester raw material in the mixture. A film was obtained in the same manner as in Example 1 except that the polyester was mixed at a ratio. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 3:
In Example 1, the polyester (2), the polyester (4), the polyester (9), and the polyester (10) are 29.5: 4.0: 65.0: 1.5 with respect to the polyester raw material in the mixture. A film was obtained in the same manner as in Example 1 except that the polyester was mixed at a ratio. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 4:
In Example 1, with respect to the polyester raw material in the mixture, the above polyester (2), polyester (7), and polyester (10) were changed to a polyester mixed at a ratio of 41.0: 50.0: 9.0. Obtained a film in the same manner as in Example 1. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 5:
In Example 1, the polyester (2), the polyester (3), the polyester (4), and the polyester (10) were mixed at 93.0: 1.0: 4.0: 2.0 with respect to the polyester raw material in the mixture. A film was obtained in the same manner as in Example 1 except that the polyester was mixed at a ratio. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 6:
In Example 1, a film was obtained in the same manner as in Example 1 except that the treatment time during annealing was 5 seconds. The properties and evaluation results of the obtained film are shown in Table 2 below.

Comparative Example 1:
In Example 1, with respect to the polyester raw material in the mixture, the polyester (1), the polyester (4), and the polyester (10) were changed to a polyester mixed at a ratio of 94.0: 4.0: 2.0. Obtained a film in the same manner as in Example 1. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 2:
In Example 1, with respect to the polyester raw material in the mixture, the same method as in Example 1 except that the polyester (6) and polyester (10) were mixed in a ratio of 98.0: 2.0. A film was obtained. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 3:
In Example 1, with respect to the polyester raw material in the mixture, the polyester (3), polyester (5), and polyester (10) were changed to a polyester mixed at a ratio of 94.0: 4.0: 2.0. Obtained a film in the same manner as in Example 1. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 4:
In Example 1, the polyester (2), the polyester (4), the polyester (5), and the polyester (10) were mixed in a ratio of 74.0: 4.0: 20.0: 2.0 with respect to the polyester raw material in the mixture. A film was obtained in the same manner as in Example 1 except that the polyester was mixed at a ratio. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 5:
In Example 1, with respect to the polyester raw material in the mixture, the polyester (2), polyester (8), and polyester (10) were changed to a polyester mixed at a ratio of 48.0: 50.0: 2.0. Obtained a film in the same manner as in Example 1. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 6:
In Example 1, the polyester (2), the polyester (3), the polyester (4), and the polyester (10) are 84.0: 10.0: 4.0: 2.0 with respect to the polyester raw material in the mixture. A film was obtained in the same manner as in Example 1 except that the polyester was mixed at a ratio. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 7:
In Example 1, with respect to the polyester raw material in the mixture, the polyester (2), polypolyester (4), and polyester (10) were changed to a polyester mixed at a ratio of 95.5: 4.0: 0.5. Except for the above, a film was obtained in the same manner as in Example 1. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 8:
In Example 1, with respect to the polyester raw material in the mixture, the above polyester (2), polypolyester (4), polyester (10), and polyester (11) were 95.3: 4.0: 0.6: 0.1. A film was obtained in the same manner as in Example 1 except that the polyester was mixed at a ratio of. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 9:
In Example 1, a film was obtained in the same manner as in Example 1 except that the processing time during annealing was 3 seconds. The properties and evaluation results of the obtained film are shown in Table 2 below.

The biaxially oriented polyester film of the present invention can be suitably used as, for example, a biaxially oriented polyester film for solar cell back surface sealing.

Claims (1)

It is a biaxially oriented polyester film containing an ultraviolet absorber, the average value (Tave) of the light transmittance at a light wavelength of 400 to 650 nm is 75.0% or more, and the light transmittance (Tuv) at a light wavelength of 380 nm. 60.0% or less, antimony element content in the polyester film is 10 ppm or less, titanium element content is 20 ppm or less, phosphorus element content is 170 ppm or less, and terminal carboxyl group content is 26 equivalents / t or less. The intrinsic viscosity is 0.65 dl / g or more, and the heat shrinkage in the film longitudinal direction after treatment at 150 ° C. for 30 minutes is 0.8% or less. Axial-oriented polyester film.