JP2012244019A - Polyester film for solar cell backside protection - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film for backside protection of a solar cell having high hydrolysis resistance, which can prevent brittle breakdown incident to film crystallization during long term outdoor exposure.SOLUTION: The polyester film for backside protection of a solar cell consists of a biaxially oriented polyester film where the difference of the recrystallization start temperature and the crystal fusion peak temperature is 45°C or higher, the amount of terminal carboxyl group is 26 equivalent/t or less, and the limit viscosity is 0.65 dl/g or higher.

Description

  The present invention relates to a polyester film for protecting a back surface of a solar cell having hydrolysis resistance, which can prevent brittle fracture accompanying film crystallization during long-term outdoor exposure.

  Photovoltaic power generation that converts light energy into electrical energy using the photoelectric conversion effect is widely used as a means of obtaining clean energy, and has been used in many private homes as the photoelectric conversion efficiency of solar cells increases. However, a solar power generation system has been installed. 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 protection 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 protective film. In Patent Document 1, 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 ethylene vinyl acetate (hereinafter referred to as EVA) as a sealing material may be abbreviated. )), It is described that it is effective in preventing deterioration of physical properties such as weather resistance and water resistance during vacuum laminating and improving yield. However, since the fluorine-based film is expensive, there is a problem that the solar cell module is also expensive.

  A technology using a polyester film is known as a film for protecting the back surface of a solar cell. However, 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, resulting in mechanical properties. to degrade. 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. For this reason, Patent Document 2 and Patent Document 3 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. Yes. 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 4 and Patent Document 5 describe that the durability of polyester is improved by using a titanium compound and a phosphorus compound. According to the inventions described in these documents, breakage caused by degradation of the polyester polymer chain itself is suppressed. However, since the device for controlling the orientation of the amorphous component of the polyester molecule has not been devised, the brittle fracture of the film caused by the accelerated crystallization of the polyester when the polyester film is exposed to the outdoors for a long time is improved. Not a satisfactory film.

JP 2002-83978 A JP-A-9-227767 JP-A-8-73719 JP 2007-204538 A JP 2010-163613 A

  The present invention has been made in view of the above circumstances, and its solution is to protect the back surface of a solar cell having high hydrolysis resistance, which can prevent brittle fracture associated with film crystallization during long-term outdoor exposure. An object of the present invention is to provide a biaxially oriented polyester film.

  As a result of intensive investigations in view of the above-mentioned actual situation, the present inventors have found that the above-described problems can be easily solved by using a polyester film having a specific configuration, and the present invention has been completed.

  That is, the gist of the present invention is that the difference between the recrystallization start temperature and the crystal melting peak temperature is 45 ° C. or more, the terminal carboxyl group amount is 26 equivalents / t or less, and the intrinsic viscosity is 0.65 dl / g or more. It consists of a certain biaxially oriented polyester film, and exists in the polyester film for solar cell back surface protection characterized by the above-mentioned.

  According to the present invention, it is possible to provide a biaxially oriented polyester film for protecting a back surface of a solar cell, which has hydrolysis resistance capable of preventing brittle fracture associated with film crystallization during long-term outdoor exposure. The industrial value of 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 in 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. 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 in 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 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, it is actually about 3 ppm. 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 the compound in the polyester film of the present invention needs to be within a specific range for the content of titanium element and phosphorus element detected by analysis using a fluorescent X-ray analyzer described later.

  In the polyester film of the present invention, the terminal carboxyl group content of the polyester component constituting the film is 26 equivalent / t or less, preferably 20 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 in terms 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 recrystallization start temperature and the crystal melting peak temperature in the present invention can be obtained by evaluating the polyester film by the temperature modulation DSC described later. That is, after the signal obtained by the temperature modulation DSC is separated into a heat capacity component (reversing signal) and a kinetic component (non reversing signal), the recrystallization start temperature and the crystal melting peak obtained from the kinetic component (non reversing signal) The temperature is defined as the recrystallization start temperature and the crystal melting peak temperature of the present invention.

  When the polyester film is evaluated by temperature modulation DSC, the difference between the recrystallization start temperature and the crystal melting peak temperature (hereinafter sometimes expressed as ΔTmc) in the signal separated by the kinetic component is 45 ° C. or more, preferably Is 48 ° C. or higher. A large ΔTmc means that the recrystallization start temperature is low. The reason why the recrystallization start temperature is low is that the number of tie molecules which are oriented amorphous polyester molecules is large. Holding the polyester film in a high temperature environment promotes crystallization of the polyester. In a polyester film in which crystallization is promoted, a film with less oriented tie molecules, which is an oriented amorphous component, is more likely to lead to brittle fracture of the film with a slight deformation than when there are many tie molecules. In the film of the present invention, if ΔTmc is 45 ° C. or higher, brittle fracture of the film due to crystallization promotion is unlikely to occur, which is preferable. On the other hand, although there is no upper limit of ΔTmc of the polyester film, about 60 ° C. is preferable from the viewpoint of preventing the heat shrinkage rate from increasing due to the excessive amount of the oriented amorphous component.

  The difference between the recrystallization start temperature separated by the kinetic component and the crystal melting peak temperature (ΔTmc) when evaluated by the temperature modulation DSC is the recrystallization start temperature (Tci) obtained by the temperature modulation DSC described later. Using a crystal melting peak temperature (Tmp), a calculation is performed. The crystal melting peak temperature (Tmp) is preferably 255 ° C. or higher.

  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 may be mentioned, but not limited to, precipitated particles obtained by making catalyst residuals 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 size 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 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 in an amount of usually 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. Also in the melt extrusion process, the amount of terminal carboxyl groups increases depending on the conditions. Therefore, in the present invention, the polyester residence time in the extruder in the extrusion process is shortened. Dry sufficiently so that the water content is 50 ppm or less, preferably 30 ppm or less. When a twin screw extruder is used, a vent port is provided and 40 hectopascals or less, preferably 30 hectopascals or less, more preferably 20 hectopascals or less. A method such as maintaining the reduced pressure 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 preferable range of the heat setting temperature varies depending on the thickness of the polyester film. That is, if the polyester film has a thickness of 50 μm, the heat setting temperature is usually 215 to 233 ° C., preferably 218 to 231 ° C., more preferably 220 to 227 ° C. If the heat setting temperature is lower than 215 ° C., the contraction rate in the longitudinal direction may be too high to be practically used. On the other hand, when the heat setting temperature is higher than 233 ° C., the recrystallization start temperature Tci is increased, and as a result, ΔTmc may be lower than 45 ° C. A high Tci, that is, a small ΔTmc means that the number of tie molecules that are oriented amorphous polyester molecules that contribute to bonding of crystals is reduced. Holding the polyester film in a high temperature environment promotes crystallization of the polyester. In a polyester film with accelerated crystallization, brittle fracture of the film with little deformation compared to when there are more tie molecules that are oriented amorphous polyester components when there are fewer tie molecules that are oriented amorphous polyester components. It becomes easy to connect to.

  If the thickness of the polyester film is 75 μm, the heat setting temperature is usually 212 to 227 ° C., preferably 215 to 226 ° C., more preferably 218 to 225 ° C.

  If the thickness of the polyester film is 125 μm, the heat setting temperature is usually 210 to 226 ° C., preferably 212 to 224 ° C., more preferably 215 to 222 ° C. If the thickness of the polyester film is 188 μm, the heat setting temperature is usually 208 to 225 ° C., preferably 210 to 224 ° C., more preferably 213 to 223 ° C.

  The setting idea of these heat setting temperature ranges is the same as the setting idea of the heat setting temperature range at 50 μm.

  Hydrolysis resistance including brittle fracture of a 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 a whole polyester component constituting the film, ΔTmc The terminal carboxyl group amount, the intrinsic viscosity, the phosphorus element content, and the titanium element content must be in 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 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 by itself as it is, and about 40 weight% or less about an amount, More 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) Terminal carboxyl group amount equivalent / t
0.1 g of the crushed polyester chip or polyester film is dried at 140 ° C. for 15 minutes in a hot air dryer and cooled to room temperature in a desiccator, and then weighed into a test tube, and benzyl alcohol is collected. 3 ml was added and dissolved in 195 ° C. for 3 minutes while blowing dry nitrogen gas, 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 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)

(2) Intrinsic viscosity dl / g
The pulverized polyester chip or polyester film is dissolved in a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio), and the solution flows down at a concentration of 1.0 (g / dl) using a capillary viscometer. The flow time of only the solvent and the solvent was measured, and the intrinsic viscosity was calculated from the time ratio using the Huggins formula. At that time, the Huggins constant was assumed to be 0.33.

(3) Difference between recrystallization start temperature and crystal melting peak temperature of polyester film: ΔTmc
<Device>
TEA Instruments DSC Q2000 Type Differential Scanning Calorimeter Cooling device: RCS electric refrigerator (Operating range: -90 to 550 ° C)
<Measurement conditions>
Sample pan: TzeroPan + Tzero Lid (Al, crimp type, non-sealed)
Purge gas: N2 50ml / min Sample weight: 2.6mg (Punch out polyester film with 4.5φ and use with 2 sheets)
Temperature range: 0-300 ° C
Temperature rising rate: 2 K / min Modulation condition: Touch width 0.32 ° C., 1 period 60 seconds
The obtained signal is separated into a calorific value component (reversing signal) and a kinetic component (non-reversing signal). In the kinetic component (non-reversing signal), ΔTmc is determined from the difference between the crystal melting peak temperature: Tmp and the recrystallization start temperature: Tci, which is confirmed to be 190 ° C. or higher.

(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 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 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%: 84 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) 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 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.

<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 an 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. An ethylene glycol solution of magnesium acetate tetrahydrate is continuously added to the esterification reaction product in the transfer pipe through the addition pipe in such an amount that the content as magnesium atoms is 11 ppm by weight with respect to the produced polyester resin. Was added. Further, by using a separate addition pipe, an ethylene glycol solution of tetrabutyl titanate was continuously added to the produced polyester resin in an amount of 4 ppm by weight as titanium atoms.

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). Polyester (1) had an intrinsic viscosity of 0.62 dl / g and a terminal carboxyl group content of 15 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 0.78 dl / g at 215 ° C. in a nitrogen atmosphere continuously supplied to a tower-type solid phase polycondensation apparatus. Thus, polyester (2) was obtained by solid phase polycondensation. The amount of terminal carboxyl groups was 8 equivalent / t.

<Method for producing polyester (3)>
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 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)>
A slurry of 43 parts by weight of terephthalic acid and 19 parts by weight of ethylene glycol was charged in an esterification reaction vessel previously charged with 50 parts by weight of bis (hydroxyethyl) terephthalate and maintained at a temperature of 250 ° C. and a relative pressure of 1.2 × 105 Pa. The supply was carried out sequentially over 4 hours, and the esterification reaction was carried out over an additional hour after completion of the supply. This esterification reaction product was transferred to a polycondensation tank. Subsequently, orthophosphoric acid and germanium dioxide were successively added as ethylene glycol solutions at intervals of 5 minutes to the polycondensation tank to which the transesterification reaction product had been transferred. In addition, orthophosphoric acid was added so that it might contain 32.2 mol per 1 ton of resin as a phosphorus atom, and 0.6 mol per 1 ton of resin as a germanium atom about germanium dioxide. Thereafter, the reaction system was heated from 225 ° C. to 280 ° C. over 2 hours and 30 minutes, and reduced from normal pressure to 400 Pa (absolute pressure) in 85 minutes. The reaction was stopped at the time corresponding to 63 dl / g, and the polymer was discharged under nitrogen pressure to obtain polyester (4). The intrinsic viscosity of the polyester (4) was 0.63 dl / g, and the amount of terminal carboxyl groups was 51 equivalent / t.

<Method for producing polyester (5)>
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.
The polyester chip obtained above was solid-phase polymerized at 220 ° C. under vacuum to obtain polyester (5). The intrinsic viscosity was 0.75 dl / g, and the amount of terminal carboxyl groups was 25 equivalents / t.

<Method for producing polyester (6)>
In the production method of polyester (5), polyester (in the same manner as above) except that an amount of 15 g / resin t as a titanium atom per 1 t of the polyester resin from which tetra-n-butyl titanate is obtained is added. 6) was produced. The intrinsic viscosity was 0.73 dl / g, and the amount of terminal carboxyl groups was 25 equivalents / t.

<Method for producing polyester (7)>
In the production method of polyester (5), polyester (in the same manner as above) except that an amount of 40 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 0.71 dl / g and the amount of terminal carboxyl groups was 27 equivalents / t.

<Method for producing polyester (8)>
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. After adding 0.04 part of orthophosphoric acid to this reaction mixture, 0.04 part of antimony trioxide was added, 0.08 part by weight of silica particles having an average particle diameter of 2.6 μm dispersed in ethylene glycol 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 40 pascals. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.58 dl / g due to a change in stirring power in the reaction tank, and the polymer was discharged under nitrogen pressure to obtain a polyester chip. The obtained polyester chip was subjected to solid phase polymerization at 220 ° C. under vacuum to obtain a polyester (8) having an intrinsic viscosity of 0.78 dl / g and a terminal carboxyl group amount of 8 equivalent / t.

<Method for producing polyester (9)>
High-purity terephthalic acid and ethylene glycol were charged into a 2 liter stainless steel autoclave equipped with a stirrer, and an esterification reaction was performed according to a conventional method to obtain an oligomer mixture.
As a polycondensation catalyst for this oligomer mixture, (1) an ethylene glycol solution in which basic aluminum acetate was adjusted to have an aluminum compound content of 20 g / l; and (2) 3,5-di- 200 g of diethyl tert-butyl-4-hydroxybenzylphosphonate was added, and the mixture of an ethylene glycol solution of a phosphorus compound obtained by cooling after stirring at reflux at 185 ° C. for 60 minutes was used, and the residual amount of aluminum element was 20 ppm. The phosphorus element was added so that the residual amount of phosphorus was 80 ppm. Next, the mixture was stirred at 250 ° C. for 10 minutes under a nitrogen atmosphere at normal pressure. Thereafter, while raising the temperature to 280 ° C. over 60 minutes, the pressure of the reaction system is gradually lowered to 13.3 Pa (0.1 Torr), and further the intrinsic viscosity of the polyester is 0.55 dl / under 280 ° C. and 13.3 Pa. The polycondensation reaction was performed until g. The melt polycondensation reaction product taken out from the reaction vessel was extruded into a strand from a die, cooled and solidified, and cut by a cutter to form a polyester resin chip: polyester chip having an average weight of 24 mg. The intrinsic viscosity of the polyester chip was 0.56 dl / g, and the amount of terminal carboxyl groups was 13 equivalents / t. The polyester chip obtained by the above melt polymerization is subjected to solid phase polymerization at 220 ° C. under a reduced pressure of 0.5 mmHg, and a polyester (9) having an intrinsic viscosity of 0.78 dl / g and a terminal carboxyl group amount of 7 equivalent / t is obtained. Obtained.

Example 1:
The polyester (2) and polyester (3) mixed at a ratio of 96: 4 are used as raw materials, melt extruded at 290 ° C. with a single vented twin-screw extruder, and surface temperature is applied using an electrostatic application adhesion method. Was rapidly cooled and solidified on a casting drum set at 40 ° C. to obtain an unstretched single layer 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 / transverse 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 average thickness of the obtained film was 50 μm. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 2:
In Example 1, with respect to the polyester raw material in the mixture, Example 1 and Example 1 were changed except that the polyester (2), polyester (3), and polyester (4) were changed to a polyester mixed at a ratio of 81: 4: 15. A film was obtained in the same manner. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 3:
In Example 1, a film was obtained in the same manner as in Example 1 except that the polyester raw material in the mixture was changed to a polyester in which the polyester (2) and the polyester (6) were mixed at a ratio of 50:50. . The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 4:
In Example 1, a film was obtained in the same manner as in Example 1 except that the temperature of heat setting 2 was changed to 226 ° C. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 5:
In Example 1, a film was obtained in the same manner as in Example 1 except that the temperature of heat setting 2 was changed to 231 ° C. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 6:
In Example 1, with respect to the polyester raw material in the mixture, the above-mentioned polyester (2), polyester (3), and flakes obtained by pulverizing the polyester film obtained in Example 1 were 76.8: 3.2: 20. A film was obtained in the same manner as in Example 1 except that mixing was performed at a ratio of 0.0. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 7:
In Example 1, the polyester raw material in the mixture was obtained in the same manner as in Example 1 except that the polyester (2) and the polyester (7) were changed to a polyester mixed at a ratio of 50:50. . Foreign matter was observed in the film, but the hydrolysis resistance was good. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 8:
In Example 1, a film was obtained in the same manner as in Example 1 except that the polyester raw material in the mixture was changed to 100% polyester. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 9:
In Example 1, a film was obtained in the same manner as in Example 1 except that the polyester raw material in the mixture was changed to a polyester in which the polyester (9) and the polyester (3) were mixed at a ratio of 96: 4. . The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 10:
In Example 9, a film was obtained in the same manner as in Example 9 except that the temperature of heat setting 2 was changed to 226 ° C. The properties and evaluation results of the obtained film are shown in Table 2 below.

Example 11:
In Example 1, with respect to the polyester raw material in the mixture, Example 1 and Example 1 were changed except that the polyester (2), polyester (3), and polyester (4) were changed to a polyester mixed in a ratio of 76: 4: 20. A film was obtained in the same manner. Gel foreign matter was observed in the film, but the hydrolysis resistance was good. The properties and evaluation results of the obtained film are shown in Table 2 below.
Example 12:

  In Example 1, a film was obtained in the same manner as in Example 1 except that the temperature of heat setting 2 was changed to 210 ° C. Although the heat shrinkage rate of the obtained film was high, a polyester film having good hydrolysis resistance could be obtained. The properties and evaluation results of the obtained film are shown in Table 2 below.

Comparative Example 1:
In Example 1, the polyester raw material in the mixture was obtained in the same manner as in Example 1 except that the polyester (1) and polyester (3) were changed to a polyester mixed at a ratio of 96: 4. . The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 2:
In Example 1, a film was obtained in the same manner as in Example 1 except that only the polyester (5) was changed with respect to the polyester raw material in the mixture. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 3:
In Example 1, a film was obtained in the same manner as in Example 1 except that the temperature of heat setting 2 was changed to 240 ° C. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 4:
In Example 8, a film was obtained in the same manner as in Example 8 except that the temperature of heat setting 2 was changed to 240 ° C. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 5:
In Example 9, a film was obtained in the same manner as in Example 9 except that the temperature of heat setting 2 was changed to 240 ° C. The properties and evaluation results of the obtained film are shown in Table 3 below.

Comparative Example 6:
In Example 6, a film was obtained in the same manner as in Example 6 except that the temperature of heat setting 2 was changed to 236 ° C. The properties and evaluation results of the obtained film are shown in Table 3 below.

The biaxially oriented polyester film obtained by the present invention can be suitably used as a biaxially oriented polyester film for protecting a solar cell back surface.

Claims (3)

It consists of a biaxially oriented polyester film in which the difference between the recrystallization start temperature and the crystal melting peak temperature is 45 ° C. or more, the terminal carboxyl group amount is 26 equivalents / t or less, and the intrinsic viscosity is 0.65 dl / g or more. The polyester film for solar cell back surface protection characterized by the above-mentioned. 2. The biaxially oriented polyester film for protecting a back surface of a solar cell according to claim 1, wherein the phosphorus element content in the polyester film is 170 ppm or less. The biaxially oriented polyester film for protecting a back surface of a solar cell according to claim 1 or 2, wherein the content of titanium element in the polyester film is 20 ppm or less.