Biaxially oriented polyester film and laminate

Abstract

To provide a biaxially oriented polyester film limited in thickness unevenness and defect, and a laminate.SOLUTION: This biaxially oriented polyester film is configured such that the number of foreign objects satisfying the following (a) and (b) is equal to or smaller than 0.1 / m2, and melting specific resistance is 2.0×106 Ω cm-9.0×106 Ω cm: (a) observed as bright spots when polarization microscopic observation is carried out under cross-nicol; and (b) a long diameter required for carrying out polarization microscopic observation under cross-nicol is 50-100 μm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to biaxially oriented polyester films and laminates. [Background technology]

[0002] Polyester film is used for various release applications, including the manufacture of ceramic green sheets, release agents for liquid crystal polarizing plates, and photoresists.

[0003] When forming polyester resin into film, electrostatic casting is a commonly used method in which a high voltage is applied to the upper surface of an unsolidified sheet-like material, causing the sheet to adhere tightly to a rotating cooling drum. However, increasing the speed of the rotating cooling drum to increase the film formation rate in electrostatic casting reduces the adhesion between the sheet-like material and the rotating cooling drum, resulting in reduced uniformity of film thickness and transparency, as well as defects on the film surface due to uneven voltage application.

[0004] To address these challenges, Patent Documents 1 and 2 describe the consideration of adding substances that improve electrostatic application. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2000-44703 [Patent Document 2] Japanese Patent Application Publication No. 10-36495 [Overview of the project] [Problems that the invention aims to solve]

[0006] Patent Document 1 proposes a method of adding a quaternary phosphonium sulfonic acid salt as a way to improve electrostatic application, but there are concerns that this method will generate metallic impurities because it contains a large amount of alkali metal or alkaline earth metal.

[0007] Patent Document 2 proposes a method for producing a polyester resin composition in which a sulfonic acid compound is added and no metal element other than a polymerization catalyst is added. However, the amount of phosphoric acid added is large, and it is insufficient to obtain electrostatic application properties for film forming.

[0008] In addition, as a method for improving the electrostatic application property, a method of adding a large amount of an alkali metal compound, an alkaline earth metal compound, or a phosphorus compound to polyester has been proposed. Although the electrostatic application property can be improved to some extent by this method, there is a problem that metal foreign matters are generated due to the use of a large amount of the metal compound, deteriorating the transparency.

[0009] Recently, when another layer is provided on a polyester film and used, for example, when used for releasing a polarizing plate, the polyester film and the polarizing plate are subjected to a cross Nicol inspection while being laminated. Therefore, it is highly required to reduce film defects such as foreign matters in the polyester film so as not to inhibit the cross Nicol inspection.

[0010] In the above-described conventional technology, it is insufficient to make the thickness unevenness small and to prevent the film from significantly inhibiting the cross Nicol inspection. Therefore, the problem to be solved by the present invention is to provide a biaxially oriented polyester film and a laminate with small thickness unevenness and few defects.

Means for Solving the Problems

[0011] In order to solve the above problems, the present invention has the following configuration. That is, [I] A biaxially oriented polyester film, wherein the number of foreign matters satisfying the following (a) and (b) is 0.1 piece / m 2 or less, and the melt specific resistance is 2.0 × 10 6 Ω·cm or more and 9.0 × 10 6 Ω·cm or less, biaxially oriented polyester film. (a) When observed with a polarizing microscope under cross Nicol, it is observed as a bright spot. (b) The major axis determined by polarized light microscopy under crossed nicols is 50 μm or more and less than 100 μm. [II] A biaxially oriented polyester film containing sulfur and phosphorus, wherein the sulfur content relative to the total mass of the biaxially oriented polyester film is 2 to 20 ppm by mass, the phosphorus content relative to the total mass of the biaxially oriented polyester film is 2 to 16 ppm by mass, and the total content of calcium, magnesium, and manganese relative to the total mass of the biaxially oriented polyester film is less than 5 ppm by mass. [III] A biaxially oriented polyester film according to [I] or [II], wherein the amount of linear oligomers generated is 100 μg / g or more and 250 μg / g or less. [IV] Number of foreign objects generated at the end: 20 pieces / m 2 More than 90 pieces / m 2 A biaxially oriented polyester film as described in any of the following [I] to [III]. [V] A biaxially oriented polyester film used for release purposes, as described in any of [I] to [IV]. [VI] A biaxially oriented polyester film according to any one of [I] to [V], used as a base film when manufacturing polarizing plates. A laminate having a biaxially oriented polyester film and a functional layer as described in any of [VII], [I], or [VI]. [Effects of the Invention]

[0012] The present invention provides a biaxially oriented polyester film and laminate with minimal thickness variations and few defects. This makes it possible to prevent the film from significantly hindering cross-nicol inspection. [Modes for carrying out the invention]

[0013] The polyester in the biaxially oriented polyester film of the present invention comprises a dicarboxylic acid component and a diol component. In this specification, a component refers to the smallest unit that can be obtained by hydrolysis of polyester.

[0014] In order to carry out the present invention, it is preferable to use terephthalic acid as a dicarboxylic acid component constituting the polyester at an amount of 30 mol% or more relative to the total dicarboxylic acid components. Other dicarboxylic acid components besides terephthalic acid include, but are not limited to, aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedionic acid, dimer acid, eicosanedionic acid, pimelic acid, azelaic acid, methylmalonic acid, and ethylmalonic acid; alicyclic dicarboxylic acids such as adamantanedicarboxylic acid, norbornenedicarboxylic acid, cyclohexanedicarboxylic acid, and decalindicarboxylic acid; and dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodium sulfoisophthalic acid, phenylendanedicarboxylic acid, anthracenedicarboxylic acid, phenantradiocarboxylic acid, and aromatic dicarboxylic acids such as 9,9'-bis(4-carboxyphenyl)fluorenic acid, or their ester derivatives.

[0015] Furthermore, examples of diol components constituting such polyesters include, but are not limited to, aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, and 1,3-butanediol; alicyclic diols such as cyclohexanedimethanol, spiroglycol, and isosorbide; diols such as bisphenol A, 1,3-benzenedimethanol, 1,4-benzenedimethanol, 9,9'-bis(4-hydroxyphenyl)fluorene, and aromatic diols; and diols formed by the linking of multiple diols mentioned above.

[0016] The polyester of the present invention can be produced by known methods. Specifically, the esterification step can be carried out using one or more esterification reactors under stirring. For example, when using a single esterification reactor, the reaction temperature is usually 240-280°C, the relative pressure to atmospheric pressure is usually 0-400 kPa, and the reaction time is usually 1-10 hours. The esterification reaction rate of the esterification reaction product obtained in the esterification step is usually 95% or more.

[0017] It is preferable to proceed to a melt polycondensation step after the esterification reaction. The melt polycondensation step can usually be carried out in a continuous or batch manner using one or more polycondensation reactors, and can be carried out while gradually reducing the pressure from atmospheric pressure and distilling the generated ethylene glycol out of the system under heating and stirring. For example, in the case of a batch manner using a single polycondensation reactor, the reaction temperature is usually 250 to 290°C, the final absolute pressure obtained by gradually reducing the pressure from atmospheric pressure is usually 0.013 to 1.3 kPa (0.1 to 10 Torr), and the reaction time is usually 1 to 20 hours. Furthermore, from the viewpoint of improving the electrostatic casting properties during polyester film molding, compounds containing sulfur, phosphorus, calcium, magnesium, and manganese elements can be added during the polycondensation reaction.

[0018] The intrinsic viscosity of polyester resin can be determined by the stirring torque of the polymer at the end of polymerization. When the stirring torque is high, the melt viscosity of the polymer is high, and the intrinsic viscosity (IV) is also high. The stirring torque for determining the end of polymerization should be set so that the target intrinsic viscosity is achieved. In this invention, it is preferable to set the stirring torque for determining the end of polymerization so that the IV of the film is between 0.60 and 0.70. This range is preferable because it makes it easier to control the surface roughness.

[0019] The polymerized polyester resin obtained can be extruded in strand form from the bottom of the polymerization apparatus and cut with a cutter while being cooled with water. Since the chip shape can be controlled by cutting, polyester chips with a desirable bulk density can be obtained in this invention.

[0020] As the polycondensation reaction catalyst used in the present invention, one or more of antimony trioxide, antimony pentoxide, antimony acetate, antimony glycolate, germanium dioxide, organic titanium compounds, etc. can be used. Among them, antimony trioxide is preferable in terms of the transparency and availability of the obtained polyester.

[0021] A preferred embodiment of the biaxially oriented polyester film of the present invention is that the number of foreign substances satisfying the following (a) and (b) is 0.1 piece / m 2 or less, and the melt specific resistance is 2.0×10 6 Ω·cm or more and 9.0×10 6 Ω·cm or less, which is a biaxially oriented polyester film. (a) When observed with a polarizing microscope under crossed nicols, it is observed as a bright spot. (b) The major axis length obtained when observed with a polarizing microscope under crossed nicols is 50 μm or more and less than 100 μm.

[0022] The number of foreign substances satisfying (a) and (b) is preferably 0.1 piece / m 2 or less. By adopting this embodiment, a film that does not significantly inhibit the crossed nicols inspection can be provided. From the viewpoint of achieving higher quality, the number of the foreign substances is more preferably 0.07 piece / m 2 or less. Also, the number of the foreign substances is preferably 0.01 piece / m 2 or more. By adopting this embodiment, it is possible to suppress the surface from becoming too flat and the slipperiness from deteriorating, and the handling property from decreasing. The number of the foreign substances shall be determined by the following method.

[0023] <Method for measuring the number of foreign substances> As a method for measuring the number of foreign substances, a 10 cm square marking is made on the film sample, and within the marking, 100 cm 2The area portion is observed using a 20x polarizing microscope ("Eclipse" (registered trademark) Ci POL, manufactured by Nikon) under crossed nicols. The measurement room where the polarizing microscope is placed is darkened, and the light intensity on the polarizing microscope is set to the maximum value. In addition, no light other than the light source attached to the device is used. The longest diameter connecting the ends that appear as bright spots under polarization is measured as the major axis, and the number of spots between 50 and 100 μm is counted and marked. This is repeated 10,000 times, for a total of 100 m. 2 Observe.

[0024] The biaxially oriented polyester film of the present invention has a melting resistivity of 2.0 × 10⁻⁶ 6 Ω cm or more 9.0×10 6 It is preferable that the resistivity is Ω·cm or less. 6 Having a density of Ω·cm or less improves adhesion with the rotating cooling drum in the electrostatic casting method, enhances the uniformity of the resulting film thickness, and reduces haze deterioration due to defects on the film surface caused by uneven application. From a similar perspective, 7.0 × 10 6 Ω·cm or less is more preferable, and 4.0 × 10 6 A resistivity of Ω·cm or less is even more preferable. Also, a melting resistivity of 2.0 × 10⁻⁶ is preferable. 6 By having a capacitance of Ω·cm or higher, it is possible to suppress the deterioration of haze in the biaxially oriented polyester film, which occurs when electrostatic discharge occurs, preventing the charge from being retained until it comes into close contact with the rotating cooling drum.

[0025] The molten resistivity can be determined by the following method.

[0026] <Method for measuring molten resistance> After melting the biaxially oriented polyester film at 290°C, an area of ​​0.5 cm² was used. 2 Two stainless steel electrodes are inserted parallel to each other with an 8mm gap between them, and after the temperature stabilizes, the resistance value (R) is measured using a resistance meter (HIOKI RM3545). Subsequently, the melting resistivity (ρ) is calculated using the formula ρ(Ω·cm) = R × 0.5 / 0.8.

[0027] The number of foreign objects satisfying the above (a) and (b) is 0.1 pieces / m 2 The following is true, and the melting resistivity is 2.0 × 10⁻⁶. 6 Ω cm or more 9.0×10 6 As a means of achieving a biaxially oriented polyester film with a density of Ω·cm or less, it is preferable to use a biaxially oriented polyester film containing sulfur and phosphorus elements, wherein the sulfur content relative to the total mass of the biaxially oriented polyester film is 2 to 20 ppm by mass, the phosphorus content relative to the total mass of the biaxially oriented polyester film is 2 to 16 ppm by mass, and the total content of calcium, magnesium, and manganese elements relative to the total mass of the biaxially oriented polyester film is less than 5 ppm by mass.

[0028] A preferred embodiment of the biaxially oriented polyester film of the present invention is a biaxially oriented polyester film containing sulfur and phosphorus elements, wherein the sulfur content relative to the total mass of the biaxially oriented polyester film is 2 to 20 ppm by mass, the phosphorus content relative to the total mass of the biaxially oriented polyester film is 2 to 16 ppm by mass, and the total content of calcium, magnesium, and manganese elements relative to the total mass of the biaxially oriented polyester film is less than 5 ppm by mass.

[0029] This embodiment provides a biaxially oriented polyester film that has good color tone, good transparency, and minimal thickness variation, and does not significantly hinder cross-nicol inspection when cross-nicol inspection is performed.

[0030] The sulfur content relative to the total mass is preferably in the range of 2 to 20 ppm by mass. As the sulfur content increases, good electrostatic application during film formation can be obtained. When the sulfur content is 2 ppm by mass or more, the volume resistivity when the polyester resin composition melts decreases, resulting in good electrostatic casting properties during film formation and reducing thickness unevenness. When it is 20 ppm by mass or less, deterioration of color tone can be suppressed. From a similar viewpoint, the sulfur content relative to the total mass is more preferably 9 to 17 ppm by mass.

[0031] Examples of compounds containing the element sulfur include sulfide compounds, thiophene compounds, thiol compounds, and sulfonic acid compounds, among which sulfonic acid compounds are preferred from the viewpoint of obtaining good electrostatic properties when forming the film. Preferred sulfonic acid compounds are one or more sulfonium anions selected from methanesulfonic acid, butylsulfonic acid, trifluoromethanesulfonic acid, tetrafluoroethanesulfonic acid, nonafluorobutanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, 2,4,6-trimethylbenzenesulfonic acid, styrenesulfonic acid, perfluorooctanesulfonic acid, and heptadecafluorooctanesulfonic acid, and the cation paired with these is preferably one or more selected from alkali metal cations, quaternary phosphonium cations, and quaternary ammonium cations. In particular, from the viewpoint of suppressing the generation of oligomers when the polyester resin composition melts, sulfonic acid compounds that are copolymerized into the polyester molecular chain are more preferable, and as the sulfonic acid compound copolymerized into the polyester molecular chain, sulfonium anions having two or more ester-forming functional groups, such as 3,5-dicarboxybenzenesulfonic acid, are even more preferable.

[0032] The method for incorporating sulfur elements into the biaxially oriented polyester film of the present invention is not particularly limited. Sulfur elements can be incorporated into the polyester resin composition by known methods, such as adding one or more compounds containing sulfur elements during the polycondensation reaction of the polyester resin composition, or kneading the resulting polyester resin composition with a twin-screw kneading extruder.

[0033] The biaxially oriented polyester film of the present invention preferably has a phosphorus content in the range of 2 to 16 ppm by mass relative to the total mass. When the phosphorus content is 2 ppm by mass or more, a good color tone can be obtained in the biaxially oriented polyester film, and when it is 16 ppm by mass or less, the electrostatic casting properties during film formation are good and thickness variations can be reduced. From a similar viewpoint, it is more preferable that the phosphorus content is 2 to 10 ppm by mass relative to the total mass.

[0034] The method for incorporating phosphorus elements into the biaxially oriented polyester film of the present invention is not particularly limited, but one or more known phosphorus-containing compounds can be used, such as phosphoric acid, trimethyl phosphate, ethyldiethyl phosphonoacetate, phosphorous acid, alkali metal phosphates, and quaternary phosphonium cations. These phosphorus compounds can be incorporated into the polyester resin composition by known methods, such as adding them during the polycondensation reaction of the polyester resin composition or kneading them with the polyester resin composition after the polycondensation reaction using a twin-screw kneading extruder.

[0035] The biaxially oriented polyester film of the present invention preferably has a linear oligomer generation amount of 100 μg / g or more and 300 μg / g or less. In this embodiment, the linear oligomers act as a pseudo-lubricant during roll transport, and it is expected that the occurrence of surface scratches caused by friction between the film and the roll will be suppressed. On the other hand, by having an amount of 300 μg / g or less, defects caused by the linear oligomers themselves can be suppressed. From a similar viewpoint, a linear oligomer generation amount of 130 μg / g or more and 250 μg / g or less is more preferable.

[0036] In this invention, linear oligomers refer to straight-chain reaction products formed by the reaction of a carboxyl group of a dicarboxylic acid, such as terephthalic acid or mono-2-hydroxyethyl terephthalate, with a hydroxyl group of a diol, and are distinct from cyclic oligomers such as cyclic trimers. Furthermore, the degree of polymerization of the linear oligomers is 10 or less, regardless of whether the terminal group is carboxyl or hydroxyl. The amount of linear oligomers generated shall be determined by the method described in the examples.

[0037] As a means of achieving this embodiment, it is preferable to use an ionic compound in which both the cation and anion are composed of organic components, containing phosphorus and sulfur elements, during the polymerization of the polyester resin composition. This improves hydrolysis resistance and thermal decomposition resistance, suppresses the generation of linear oligomers that occur when the film is heat-treated or heat-processed, and suppresses the increase in oligomer defects. In addition, the improvement in various decomposition resistances suppresses the decrease in molecular weight of the polyester resin composition during film formation, improves the ductility of the film, reduces brittleness, and thus suppresses foreign matter at the edges.

[0038] The biaxially oriented polyester film of the present invention has a rate of 20 foreign matter particles per square meter at the edges. 2 More than 90 pieces / m 2 Preferably, the following is true: 25 pieces / m 2 More than 70 pieces / m 2The following is more preferable. By adopting the above configuration, it is possible to prevent defects when used as a process film (coating repelling, dents, process contamination), and by including a certain amount of edge foreign matter, blocking between films can be suppressed, resulting in a good winding appearance, and scratches caused by blocking can be suppressed. Furthermore, by improving the hydrolysis resistance and thermal decomposition resistance of the polyester resin composition and improving its ductility, a suitable number of edge foreign matter can be maintained. The number of edge foreign matter shall be determined by the method described in the examples. The type of foreign matter is not particularly limited, but it is preferable that it contains polyester powder and / or amide compounds, and more preferably that it contains polyester powder. Since the polyester powder is the same substance as the film, coating repelling is less likely to occur when applying release layers, etc., and it does not adversely affect the coatability.

[0039] In the biaxially oriented polyester film of the present invention, it is preferable that the total content of calcium, magnesium, and manganese elements relative to the total mass is less than 5 ppm by mass. By having a total content of calcium, magnesium, and manganese elements of less than 5 ppm by mass, a biaxially oriented polyester film with good color tone and haze and low levels of foreign matter can be obtained. In particular, when a sulfonic acid compound is used to contain sulfur elements in the biaxially oriented polyester film of the present invention, sulfonic acid and calcium ions, magnesium ions, and manganese ions tend to form foreign matter. Therefore, from the viewpoint of reducing foreign matter, it is preferable that the biaxially oriented polyester film of the present invention does not contain calcium, magnesium, and manganese elements.

[0040] When calcium, magnesium, and manganese elements are incorporated into the biaxially oriented polyester film of the present invention, the method is not particularly limited, but it is preferable to add the calcium compound, magnesium compound, and manganese compound during the polycondensation reaction of the polyester resin composition. The form of addition can be powder, slurry, or solution, but it is preferable to add it as a solution from the viewpoint of dispersibility. The solvent at this time is preferably the same as the diol component of the polyester composition. Examples of calcium compounds include calcium acetate, calcium propionate, calcium chloride, calcium bromide, calcium iodide, and calcium hydroxide. Examples of magnesium compounds include magnesium acetate, magnesium propionate, magnesium chloride, magnesium bromide, magnesium iodide, and magnesium hydroxide. Examples of manganese compounds include manganese acetate, manganese propionate, manganese chloride, manganese bromide, manganese iodide, and manganese hydroxide.

[0041] The amounts of each element will be determined by the following method.

[0042] <Method for measuring the amount of each element> The content of phosphorus, magnesium, manganese, and calcium is determined by finely cutting a biaxially oriented polyester film, forming it into a cylindrical shape using a melt press, and measuring the fluorescence X-ray intensity using a fluorescence X-ray analyzer (model: 3270) manufactured by Rigaku Denki Co., Ltd., and then calculating it from a pre-prepared calibration curve.

[0043] To determine the sulfur content, a biaxially oriented polyester film is finely shredded and then burned using an automated sample combustion device (model: AQF-2100) manufactured by Mitsubishi Chemical Analytec. The generated gas is absorbed into a solution, and a portion of the absorption solution is analyzed using an ion chromatograph (model: ICS1600) manufactured by DIONEX. The sample is weighed and measured in n=2 quantities, and the average of the measured values ​​is taken as the sulfur content.

[0044] The total thickness of the biaxially oriented polyester film of the present invention is preferably 15 μm to 50 μm, and more preferably 25 μm to 40 μm, even in the case of a single-layer or two-layer or more structure.

[0045] The biaxially oriented polyester film of the present invention can be obtained by a method comprising the following steps.

[0046] (Step 1) Using terephthalic acid or dimethyl terephthalate and ethylene glycol as raw materials, a low polymer such as BHT (bishydroxyethyl terephthalate) is obtained by reactions such as esterification and transesterification. Then, terephthalic acid and ethylene glycol, as well as compounds containing sulfur, phosphorus, calcium, magnesium, and manganese elements are added, and a polyester resin is obtained by polycondensation reaction.

[0047] (Step 2) A process to obtain an unstretched polyester film with a thickness of 180 to 1400 μm by melt-extruding polyester resin into a sheet, and then contacting the melt-extruded polyester resin into a sheet on a casting roll at 18 to 50°C for 1 to 15 seconds to cool and solidify it.

[0048] (Step 3) The unstretched polyester film obtained in (Step 2) is stretched in the longitudinal direction at a stretching ratio of 2.5 to 5 times, and then cooled to obtain a uniaxially oriented polyester film.

[0049] (Step 4) A step to obtain a biaxially oriented polyester film by stretching the uniaxially oriented polyester film obtained in (Step 3) at a stretching ratio of 3 to 6 times in the width direction, and at a stretching ratio in the width direction that is higher than the stretching ratio in the longitudinal direction, followed by cooling.

[0050] (Step 5) A step to obtain a biaxially oriented polyester film by heat-treating the biaxially oriented polyester film at a heat treatment temperature of 180 to 230°C.

[0051] The following provides a detailed explanation of each step.

[0052] (Step 1) Preparation of polyester resin A slurry consisting of terephthalic acid and ethylene glycol is gradually added to an esterification reactor containing BHT (bishydroxyethyl terephthalate) dissolved at 250°C, and the esterification reaction proceeds while water is distilled off. The temperature in the reaction system is controlled to 245-250°C, and the esterification reaction is terminated when the reaction rate reaches 95%, and the resulting esterified product is charged in molten state into a polymerization apparatus equipped with a distillation device.

[0053] Antimony trioxide and a compound containing sulfur and phosphorus are added as an ethylene glycol solution. Subsequently, the pressure in the polymerization reactor is gradually reduced to below 0.13 kPa in 35 minutes, and at the same time, the temperature is gradually increased to 279°C to carry out the polymerization reaction and obtain a polyester resin.

[0054] (Step 2) Preparation of unstretched film The polyester resin is dried as needed, supplied to the extruder, and melt-extruded. In order to ensure that the intrinsic viscosity of the film is within the above range, the average intrinsic viscosity of the polyester resin supplied to the extruder is preferably 0.55 to 0.64 dl / g, and more preferably 0.55 to 0.62 dl / g.

[0055] Next, the polyester resin extruded by the extruder is filtered. Since even small foreign matter can cause defects in the film, it is effective to use a high-precision filter that can capture 95% or more of foreign matter larger than 5 μm. The molten polyester resin undergoes thermal decomposition and hydrolysis, which breaks its molecular chains and reduces its intrinsic viscosity. For stable melt extrusion, the temperature and moisture content of the polyester resin during melt extrusion are preferably 5 to 40°C above the melting point of the polyester resin and 300 ppm or less.

[0056] Next, the polyester resin is formed into a sheet using a T-type die or the like, and the sheet-formed polyester resin is cooled and solidified on a casting roll to obtain an unstretched film. In this case, the temperature of the casting roll is preferably 18 to 50°C, and the cooling time during which the sheet-formed polyester resin is in contact with the casting roll is preferably 1 to 15 seconds. If the temperature of the casting roll is below 18°C, condensation is likely to occur on the casting drum, which may worsen the film-forming properties. If the temperature of the casting roll exceeds 50°C, the crystal plane orientation index xi value may decrease. This is because high temperatures on the casting roll worsen the cooling efficiency, causing microcrystals to form in the resulting unstretched film, which in turn promotes crystal orientation in the subsequent stretching process. Similarly, if the cooling time in contact with the casting roll is less than 1 second, the cooling efficiency worsens, and as a result of the formation of microcrystals in the resulting unstretched film, the crystal plane orientation index xi value tends to decrease. The cooling time by the casting roll can be extended by increasing the diameter of the casting roll or lowering the line speed, but considering equipment space and productivity, the upper limit is 15 seconds. More preferably, the temperature of the casting roll is 20-30°C, and the cooling time during which the sheet-shaped polyester resin contacts the casting roll is 3-12 seconds.

[0057] Furthermore, the thickness of the unstretched film obtained in step 2 is preferably 180 to 1400 μm. If the thickness of the unstretched film is less than 180 μm, the film thickness is insufficient to stretch it so that the orientation angle and thermal shrinkage rate are within the desired range, and film tearing may occur during stretching. On the other hand, if the thickness of the unstretched film exceeds 1400 μm, when the polyester resin sheet is cooled and solidified on the casting roll, uneven cooling occurs in the thickness direction, and the crystal plane orientation index χi tends to decrease. In addition, the final thickness of the biaxially oriented polyester film may fall outside the range suitable for polarizing plate release applications.

[0058] (Step 3) Preparation of uniaxially oriented film A uniaxially oriented polyester film is obtained by stretching the unstretched film obtained in step 2 above at a stretching ratio of 2.5 to 5.0 times in the longitudinal direction and then cooling it. Stretching in the longitudinal direction is preferably performed in one or multiple stages at a stretching temperature of 90 to 130°C. From the viewpoint of suppressing bowing phenomena and thickness unevenness in the longitudinal direction of the film, a stretching temperature of 100 to 120°C and a stretching ratio of 3 to 4 times are more preferable, and from the viewpoint of preventing stretching unevenness and scratches, stretching is preferably performed in two or more stages. Furthermore, although shrinkage in the width direction occurs due to longitudinal stretching, it is preferable that the width reduction of the film from this stretching process to the cooling process be 15% or less. If the width reduction of the film exceeds 15%, meandering or width fluctuations may occur in the film, or the uniformity of the plane orientation in the width direction of the film may deteriorate, making it difficult to achieve a crystal plane orientation index (χi) of 6.0 or higher over a width of 5m. The shrinkage of the film width can be controlled by adjusting the thickness profile of the film edge before longitudinal stretching, or by adjusting the stretching tension with a nip roll or similar device.

[0059] The film width reduction shown here is calculated by dividing the difference between the film width immediately before the longitudinal stretching process and the film width after stretching and cooling by the film width immediately before the longitudinal stretching process.

[0060] A film temperature of 25-45°C during the cooling process in (Step 3) is preferable for stable stretching in the width direction in the next (Step 3) step.

[0061] (Step 4) Preparation of biaxially oriented film The uniaxially oriented polyester film obtained in step 3 is stretched in the width direction at a stretching ratio of 3.0 to 6.0 times, and the stretching ratio in the width direction is higher than the stretching ratio in the longitudinal direction. Stretching in the width direction is preferably performed at a stretching temperature of 90 to 130°C. When the stretching temperature is lower than 90°C and the stretching ratio is higher than 6.0 times, the orientation angle tends to decrease, but the film becomes more prone to breakage, and the crystal plane orientation index (χi) becomes lower. A stretching temperature of 100 to 120°C and a stretching ratio of 4.0 to 5.0 times are more preferable. Furthermore, to lower the orientation angle, it is preferable that the stretching ratio in the width direction is higher than the stretching ratio in the longitudinal direction. If the stretching ratio in the longitudinal direction is higher than the stretching ratio in the width direction, the molecular orientation within the film tilts towards the longitudinal direction, making it difficult to suppress variations in the orientation angle.

[0062] In manufacturing the biaxially oriented polyester film of the present invention, the process involves stretching in the longitudinal direction followed by stretching in the width direction. This is because, after the initial stretching in the width direction, the molecules are strongly oriented mainly in the width direction, but after the subsequent stretching in the longitudinal direction, they also become oriented in the longitudinal direction, resulting in a high orientation angle.

[0063] Next, the film stretched in the width direction is cooled at a film temperature of 25-45°C and a film width shrinkage rate of 0.1-20% / min to obtain a cooled biaxially oriented polyester film. A film temperature of 25-45°C during the cooling process is preferable because it suppresses the relaxation of orientation in the width direction due to width shrinkage and suppresses the bowing phenomenon. More preferably, it is 30-40°C. If the film temperature during the cooling process is higher than 45°C, the tension due to film width shrinkage affects the film-forming properties, and the effect of suppressing the relaxation of orientation in the width direction may not be sufficient. Cooling the film temperature to below 20°C during the cooling process may result in poor productivity.

[0064] Methods for cooling polyester film include air cooling using a tenter for heat treatment, air cooling using shielding plates such as aluminum plates above and below the heat treatment area to block hot air, and cooling using rolls. In air cooling using a tenter for heat treatment, all zones are connected in the longitudinal direction, so temperature differences can occur in the vertical and horizontal directions of the film due to the free flow of high-temperature air such as accompanying airflow, and the film temperature may not be sufficiently cooled. In such cases, this can be addressed by actively cooling the film by supplying compressed air or the like.

[0065] Furthermore, in the roll-based cooling method, there are no limitations on the number of rolls used or the set temperature, but it is preferable to use multiple rolls for cooling. In order to keep the film temperature within the above range in the roll-based cooling method, the roll temperature is preferably 20 to 45°C, and more preferably 30 to 40°C. In addition, in the roll-based cooling method, it is preferable to apply a load to the cooling roll using a nip roll to make the film adhere tightly, as this allows for stable cooling.

[0066] Furthermore, in this cooling process, the film width shrinkage rate is preferably 0.1 to 20% / min. If the width shrinkage rate is less than 0.1% / min, the film tension due to the suppression of film width shrinkage will affect the film formation, resulting in poor film formation and potentially causing film tearing. Also, if the width shrinkage rate is faster than 20% / min, the effect of suppressing orientation relaxation due to film width shrinkage is reduced, and the suppression of the bowing phenomenon may be insufficient. It is even more preferable that the film width shrinkage rate be 0.2 to 18% / min. As a method for controlling the width shrinkage rate, the shrinkage rate can be set from the cooling process length and film formation rate, and this can be achieved in various ways. Specifically, in the air cooling method in a tenter, the width shrinkage rate can be set to the desired value by gripping both ends with clips and adjusting the rail width.

[0067] The film width reduction rate during the cooling process shown here is calculated using equation (1), where W1 (mm) is the film width after the widthwise stretching process and immediately before entering the cooling process, W2 (mm) is the film width after the cooling process, and T1 (min) is the time taken during the cooling process.

[0068] Film width reduction speed = (W1 - W2) / W1 × 1 / T1 Equation (1) Furthermore, in the cooling process (step 3), it is preferable that the film remains at a reduced temperature for a certain period of time. The reason for this is presumed to be as follows: As mentioned above, it is thought that orientation relaxation occurs when the film shrinks in width during the cooling process, but it is presumed that a certain amount of time is required to stop orientation relaxation by cooling the film. Therefore, it is presumed that if the cooling process time is insufficient, orientation relaxation cannot be suppressed, and the effect of suppressing the bowing phenomenon will be reduced. When manufacturing the biaxially oriented polyester film of the present invention, the cooling process time is preferably 10 seconds or more, and more preferably 15 seconds or more. There is no particular upper limit to the cooling process time, but it is 60 seconds or less. This is desirable because it leads to better productivity.

[0069] (Step 5) Heat treatment of biaxially oriented film A biaxially oriented polyester film is obtained by heat-treating the biaxially oriented polyester film obtained in step 4. The heat treatment temperature is preferably 180 to 230°C, more preferably 180 to 215°C, and particularly preferably 185 to 210°C. If the heat treatment temperature is below 180°C, the heat treatment will be insufficient, and it may be difficult to keep the thermal shrinkage rate in the longitudinal direction of the film and the thermal shrinkage rate in the width direction of the film within the range of 2.5 to 7.0% after heat treatment at 150°C for 30 minutes. If the heat treatment temperature is higher than 230°C, bowing is more likely to occur, making it difficult to control the orientation angle within the above range, which is undesirable. Furthermore, in the above heat treatment, a relaxation treatment may be performed as needed. The relaxation treatment may be performed in either the width direction or the length direction, and may be performed simultaneously in both directions or separately. A relaxation rate of preferably 1 to 20%, and more preferably 1 to 15%, relative to the total width of the film is effective in obtaining a film with excellent thermal dimensional stability.

[0070] The biaxially oriented polyester film of the present invention is preferably used for release applications. The biaxially oriented polyester film of the present invention has a melting resistivity of 2.0 × 10⁻⁶ 6 Ω cm or more 9.0×10 6 When the density is Ω·cm or less, the adhesion to the rotating cooling drum in the electrostatic casting method is improved, resulting in improved thickness uniformity of the resulting film. Furthermore, when the total content of calcium, magnesium, and manganese is less than 5 ppm by mass, a biaxially oriented polyester film with good color tone and haze, and low levels of metallic foreign matter can be obtained. As a result, when cross-nicol inspection is performed after laminating the biaxially oriented polyester film to a polarizing plate, defects such as thickness unevenness and metallic foreign matter in the biaxially oriented polyester film, which can cause polarization unevenness, can be reduced. Therefore, when used for release applications, a biaxially oriented polyester film that does not significantly hinder cross-nicol inspection can be provided.

[0071] The biaxially oriented polyester film of the present invention is preferably used as a base film when manufacturing polarizing plates. Because the biaxially oriented polyester film of the present invention has a small number of large bright spot defects, it is less likely to cause problems during cross-nicol inspection when manufacturing polarizing plates, and can be used more preferably when manufacturing polarizing plates of 32 inches or larger, where a high level of quality is required.

[0072] The biaxially oriented polyester film of the present invention is suitably used as a release film for large polarizing plates. Specifically, it is preferably used in the following embodiments.

[0073] First, the biaxially oriented polyester film of the present invention is provided on at least one side of the polarizer to form a laminate. Here, the laminate is preferably arranged in the order of "biaxially oriented polyester film of the present invention / adhesive layer A / polarizer / adhesive layer B / protective sheet," and more preferably in the order of "biaxially oriented polyester film of the present invention / silicone layer / adhesive layer A / polarizer / adhesive layer B / protective sheet." Known materials can be used for the silicone layer, adhesive layer, polarizer, and protective sheet.

[0074] Next, the laminate is inspected. Since the laminate uses the biaxially oriented polyester film of the present invention, good inspectability is obtained. After that, the biaxially oriented polyester film of the present invention is peeled off from the laminate to obtain a polarizing plate. Here, a polarizing plate refers to a member having at least a polarizer, but for example, when the biaxially oriented polyester film of the present invention is peeled off (removed) from a "biaxially oriented polyester film of the present invention / adhesive layer A / polarizer / adhesive layer B / protective sheet" or a "biaxially oriented polyester film of the present invention / silicone layer / adhesive layer A / polarizer / adhesive layer B / protective sheet" (laminated), a polarizing plate is obtained in which "adhesive layer A / polarizer / adhesive layer B / protective sheet" are provided in that order. It is preferable that the silicone layer is peeled off together with the biaxially oriented polyester film of the present invention.

[0075] A preferred embodiment of the laminate of the present invention comprises the biaxially oriented polyester film and a functional layer. The functional layer is a layer other than the biaxially oriented polyester, and may be a single layer or a multilayer. The laminate can be preferably obtained by coating or laminating the functional layer to the biaxially oriented polyester film. [Examples]

[0076] The methods for measuring and evaluating characteristic values ​​in the examples and comparative examples are as follows.

[0077] (1) Number of bright spots As a method for measuring the number of foreign objects, a 10cm square marking is placed on a film sample, and the area within the marking (100cm) is measured. 2 The area was observed using a 20x polarizing microscope ("Eclipse" (registered trademark) Ci POL, manufactured by Nikon) under crossed nicols. The measurement room where the polarizing microscope was placed was a dark room, and the light intensity of the polarizing microscope was set to the maximum value. In addition, no light other than the light source attached to the apparatus was used. The longest diameter connecting the ends that appeared as bright spots under polarization was measured as the major axis, and the number of spots between 50 and 100 μm was counted and marked. This was repeated 10,000 times, for a total of 100 m 2 I observed it.

[0078] (2) Melt resistivity of biaxially oriented polyester film (unit: Ω·cm) After melting the biaxially oriented polyester film at 290°C, an area of ​​0.5 cm² was used. 2 Two stainless steel electrodes were inserted parallel to each other at an 8mm interval, and after the temperature stabilized, the resistance value (R) was measured using a resistance meter (HIOKI RM3545). Subsequently, the melting resistivity (ρ) was calculated using the formula ρ(Ω·cm) = R × 0.5 / 0.8.

[0079] (3) Elemental content in biaxially oriented polyester film (unit: mass ppm) The content of phosphorus, magnesium, manganese, and calcium was determined by finely cutting a biaxially oriented polyester film, forming it into a cylindrical shape using a melt press, and measuring the X-ray fluorescence intensity using a Rigaku Denki Co., Ltd. X-ray fluorescence analyzer (model: 3270). The content was then calculated from a pre-prepared calibration curve.

[0080] To determine the sulfur content, a biaxially oriented polyester film was finely shredded and then burned using an automated sample combustion device (model: AQF-2100) manufactured by Mitsubishi Chemical Analytec. The generated gas was absorbed into a solution, and a portion of the absorption solution was analyzed using an ion chromatograph (model: ICS1600) manufactured by DIONEX. Samples were weighed and measured in n=2 quantities, and the average of the measured values ​​was taken as the sulfur content.

[0081] (4) Haze of biaxially oriented polyester film (unit: %) In accordance with JIS K7105-1981, a sample measuring 4.0 cm in length and 3.5 cm in width was cut from the center of the film's width direction, and the haze was measured using a haze meter (Suga Test Instruments HGM-2DP (for C light source)).

[0082] (5) Color tone of biaxially oriented polyester film After melting the biaxially oriented polyester film, it was extruded in strand form into cold water from a die, pelletized using an extruder cutter, and filled into a powder measurement cell. The b-value was measured using the reflection method with n=3 using a color difference meter (SM Color Meter SM-T) manufactured by Suga Test Instruments Co., Ltd. The arithmetic mean of the measured values ​​was used as the color tone b-value and as an evaluation index for color tone.

[0083] (6) σ value of variation of surface thickness relative to the average value within the plane (film thickness unevenness) A thickness measurement sensor installed outside the film-forming machine: The surface thickness was measured using the film thickness distribution measuring device "FiDiCa" (registered trademark) manufactured by JFE Techno Research Corporation. Measurements were taken over a product width of 1.5 m and a length of 1,000 m. The speed was set to 50 m / min. Based on this surface thickness data, the sigma value of the variation in surface thickness relative to the average value within the plane was calculated and defined as the film thickness unevenness.

[0084] (7) Amount of linear oligomers produced 0.1 g of film was freeze-dried and dried under reduced pressure at 160°C for 6 hours, then sealed in ampoules under an N2 atmosphere. The ampoules were then heated at 290°C for 20 minutes, and the amounts of terephthalic acid, mono-2-hydroxyethyl terephthalate, and bis-2-hydroxyethyl terephthalate as linear oligomers were measured using a high-performance liquid chromatograph LC20A (Shimadzu Corporation). The total amount was used to determine the amount of linear oligomers generated per 1 g of film (μg / g).

[0085] (8) Surface scratches A 100 x 100 mm sample of film was taken. In a darkroom, one side of the sample was observed from the side illuminated by the light using a 2000 lx LED light (OHM EB-10KM), with the incident angle varied horizontally from 30° to 150° relative to the sample. Surface scratches that could be visually confirmed were counted, and the number of scratches per unit area was calculated. The same procedure was repeated 20 times, and the average was taken as the surface scratch count of the film. ◎:0 pieces / m 2 More than 2 pieces / m 2 below 〇:3 pieces / m 2 More than 5 pieces / m 2 below ×:6 pieces / m 2 That's all. (9) Number of defects in oligomers Each layer of the film is scraped away, leaving only layer A. This is then heated in an oven set to 150°C for 30 minutes. After being removed from the oven and cooled, silver is deposited onto the surface, and then platinum is coated by sputtering. The surface is observed using a scanning electron microscope at 2000x magnification in two fields, and the number of precipitated oligomers is counted in mm. 2 The values ​​were converted to per unit area. Oligomatous substances are characterized by their lumpy, hexagonal, or horseshoe-shaped forms. However, if it could not be determined that the substance was oligotous, it was confirmed and determined using a Fourier transform infrared microscope (FT-IR microscope). The measurement conditions for the FT-IR microscope were as follows. Manufacturer: Thermo Fisher Scientific Co., Ltd. Model names: NICOLET6700 and CONTINUμM infrared microscope Measurement conditions: Measurement range: 650cm -1 ~4000cm -1 , resolution: 8cm -1 Detector: MCT (10) Number of foreign objects generated at the end The number of foreign matter particles generated at the edges was evaluated using an adhesive roll. An adhesive roll (teknek p-type adhesive roll) was placed in contact with the edges of a polyester film from 0 to 50 mm, and contact was maintained for a length of 1000 m to collect attached edge foreign matter from the film surface. The adhesive roll was rotated 10 times back and forth on a transfer film (NTT-ATCR dust sampler) to transfer the attached foreign matter from the adhesive roll to the transfer film. The transfer film with the attached edge foreign matter transferred was then observed using a shape analysis laser microscope (KEYENCE VK-X250). The observation conditions were 81 fields of view automatically observed using a 20x objective lens with a measurement area of ​​700 μm × 530 μm. This was repeated 9 times, for a total of 729 fields of view (= 280.7 mm²). 2 The observations were made. The centerline of each field of view was calculated using the total number of measured pixels per field of view, 1024 × 768 pixels. Using a multi-file analysis application (KEYENCE VK-X series), a threshold (height of foreign object) of 0.5 μm or more and a size (area of ​​foreign object) of 75 μm were determined for each field of view. 2 Based on the above criteria, the detected protrusions were counted as foreign objects if the average vertical and horizontal ferret diameters were between 10 and 200 μm, and the number of such objects per unit area was defined as the number of foreign objects generated at the end. Measurements were taken once for each end, and the average value was used as the evaluation result.

[0086] (11)ΔIV The decrease in molecular weight of the polyester resin composition was evaluated by the difference in intrinsic viscosity ΔIV between the polyester resin composition before and after film formation. Intrinsic viscosity was measured at 25°C using an Ostwald viscometer after dissolving the polyester resin composition in orthochlorophenol. In the case of films, each layer of the film was scraped off, and the intrinsic viscosity of layer A was taken as the intrinsic viscosity after film formation. ΔIV = (Intrinsic viscosity of polyester resin composition) - (Intrinsic viscosity of film layer A).

[0087] (Example 1) A slurry consisting of 86 parts by mass of terephthalic acid and 37 parts by mass of ethylene glycol (1.15 times the molar amount of terephthalic acid) was gradually added to an esterification reactor containing 105 parts by mass of BHT (bishydroxyethyl terephthalate) dissolved at 250°C, and the esterification reaction was carried out while distilling off water. The temperature in the reaction system was controlled to 245-250°C, and the esterification reaction was terminated when the reaction rate reached 95%, and the resulting 105 parts by mass of esterified product (equivalent to 100 parts by mass of PET (polyethylene terephthalate)) was charged in molten state into a polymerization apparatus equipped with a distillation device.

[0088] 0.0084 parts by mass of antimony trioxide and 0.0175 parts by mass of tetrabutylphosphonium p-toluenesulfonate were added as an ethylene glycol solution. Subsequently, the pressure in the polymerization reactor was gradually reduced to 0.13 kPa or less over 35 minutes, and at the same time, the temperature was gradually increased to 279°C. The polymerization reaction was carried out until the intrinsic viscosity of the polyester resin composition reached 0.625. After that, the polycondensation reactor was returned to atmospheric pressure with nitrogen gas, and the mixture was discharged in strand form into cold water from a nozzle. The mixture was then pelletized into cylindrical shapes using an extruder to obtain the polyester resin composition.

[0089] Furthermore, a water slurry of divinylbenzene / styrene copolymer crosslinked particles with a volume-average particle size of 1.0 μm and a volume-shape coefficient of f=0.51, obtained by adsorbing monomers, was incorporated into the above-mentioned polyester resin composition pellets using a vented twin-screw kneader to obtain a master pellet containing 1 part by mass of divinylbenzene / styrene copolymer crosslinked particles with a volume-average particle size of 1.0 μm per 99 parts by mass of the polyester resin composition.

[0090] The volume shape coefficient f is expressed by the following formula. f = V / Dm 3 Here, V is the particle volume (μm). 3 ), where Dm is the maximum diameter (μm) of the particle on the projection plane. The volume shape coefficient f takes its maximum value of π / 6 when the particle is spherical.

[0091] These polyesters were each dried under reduced pressure at 160°C for 8 hours to achieve a moisture content of 100 ppm. They were then fed into separate extruders and melt-extruded at 275°C. After filtration through a high-precision filter with a collection efficiency of 95% and particles larger than 5 μm, the layers were combined and laminated using a rectangular three-layer confluence block to form a three-layer laminate consisting of polyester layer B, polyester layer A, and polyester layer B'. The above-mentioned master pellets were used for polyester layer B and polyester layer B', and the above-mentioned polyester resin composition was used for polyester layer A.

[0092] Subsequently, the film was cooled and solidified for 7 seconds on a casting roll with a surface temperature of 25°C using an electrostatic casting method via a slit die maintained at 285°C, yielding an unstretched film with a thickness of 570 μm. This unstretched film was first stretched 3.4 times in the longitudinal direction using a roll heated to 103°C and a radiation heater. The width reduction at this time was 14%. Next, it was stretched 4.4 times in the width direction at 110°C using a tenter. After that, the film was cooled to a temperature of 35°C at a width reduction rate of 18% / min during the cooling process. A roll method was used for this cooling process, with the roll temperature set to 30°C and the cooling process time set to 15 seconds. Then, heat treatment was performed at 195°C to create an intermediate product roll of biaxially oriented polyester film consisting of three layers with a total film thickness of 38 μm, a film lamination thickness of polyester layer B / polyester layer A / polyester layer B' = 2.0 μm / 34 μm / 2.0 μm, and a film width of 5.1 m. Samples were taken from the obtained intermediate product rolls and evaluated. The results are shown in Table 2.

[0093] (Examples 2 and 3) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that instead of adding tetrabutylphosphonium p-toluenesulfonate, sodium p-toluenesulfonate and trimethyl phosphate were added, resulting in the sulfur and phosphorus element content shown in Table 1. The results are shown in Table 2.

[0094] (Comparative Example 1) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that trimethyl phosphate was added instead of tetrabutylphosphonium p-toluenesulfonate, resulting in the phosphorus element content shown in Table 1. The results are shown in Table 2.

[0095] (Examples 4-6, Comparative Example 2) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that sodium p-toluenesulfonate was added to antimony trioxide and tetrabutylphosphonium p-toluenesulfonate, resulting in the sulfur and phosphorus element content shown in Table 1. The results are shown in Table 2.

[0096] (Examples 7-9) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that trimethyl phosphate was added to antimony trioxide and tetrabutylphosphonium p-toluenesulfonate, resulting in the sulfur and phosphorus element content shown in Table 1. The results are shown in Table 2.

[0097] (Comparative Example 3) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that sodium p-toluenesulfonate was added instead of tetrabutylphosphonium p-toluenesulfonate, resulting in the sulfur element content shown in Table 1. The results are shown in Table 2.

[0098] (Comparative Example 4) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that trimethyl phosphate was added to antimony trioxide and tetrabutylphosphonium p-toluenesulfonate, resulting in the sulfur and phosphorus element content shown in Table 1. The results are shown in Table 2.

[0099] (Example 10, Comparative Example 5) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that calcium acetate monohydrate was added to antimony trioxide and tetrabutylphosphonium p-toluenesulfonate to achieve the sulfur, phosphorus, and calcium element content shown in Table 3. The results are shown in Table 4.

[0100] (Example 11, Comparative Example 6) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that magnesium acetate tetrahydrate was added to antimony trioxide and tetrabutylphosphonium p-toluenesulfonate to achieve the sulfur, phosphorus, and magnesium content shown in Table 3. The results are shown in Table 4.

[0101] (Example 12, Comparative Example 7) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that manganese acetate tetrahydrate was added to antimony trioxide and tetrabutylphosphonium p-toluenesulfonate to achieve the sulfur, phosphorus, and manganese element content shown in Table 3. The results are shown in Table 4.

[0102] (Example 13) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that instead of adding tetrabutylphosphonium p-toluenesulfonate, the copolymer component tetrabutylphosphonium 3,5-dicarboxybenzenesulfonate was added, resulting in the sulfur and phosphorus element content shown in Table 3. The results are shown in Table 4.

[0103] (Examples 14, 15) A polyester resin composition and a biaxially oriented polyester film were obtained in the same manner as in Example 1, except that tetrabutylphosphonium p-toluenesulfonate was added to the phosphorus and sulfur content shown in Table 3. The results are shown in Table 4.

[0104] (Comparative Example 8) Instead of an esterification reaction, 100 parts by mass of dimethyl terephthalate, 70 parts by mass of ethylene glycol, and manganese acetate tetrahydrate were charged into the reactor in a manganese element content of 85 ppm by mass relative to the polyester resin composition. The contents were dissolved at 150°C, and the temperature of the reaction contents was gradually raised to 230°C while stirring. Methanol was distilled off to a predetermined amount to carry out a transesterification reaction and obtain low polymer BHT. From the polycondensation reaction onward, the polyester resin composition and biaxially oriented polyester film were obtained in the same manner as in Example 1. The results are shown in Table 4.

[0105] (Comparative Example 9) A slurry consisting of 86 parts by mass of terephthalic acid and 37 parts by mass of ethylene glycol (1.15 times the molar amount of terephthalic acid) was gradually added to an esterification reactor containing 105 parts by mass of BHT dissolved at 250°C, and the esterification reaction was carried out while distilling off water. The temperature in the reaction system was controlled to 245-250°C, and the esterification reaction was terminated when the reaction rate reached 95%, and the resulting 105 parts by mass of esterified product (equivalent to 100 parts by mass of PET) was charged in molten state into a polymerization apparatus equipped with a distillation device.

[0106] 0.007 parts by mass of tetra-n-butoxytitanium, 0.007 parts by mass of trimethyl phosphate, and 0.01 parts by mass of magnesium acetate tetrahydrate were added and stirred for 5 minutes. Next, 0.01 parts by mass of tetrabutylammonium methanesulfonate were added and stirred for 5 minutes. Subsequently, the pressure in the polymerization reactor was gradually reduced to 0.13 kPa or less in 35 minutes, and at the same time, the temperature was gradually increased to 279°C, and the polymerization reaction was carried out until the intrinsic viscosity of the polyester resin composition reached 0.625. After that, the polycondensation reactor was returned to atmospheric pressure with nitrogen gas, and the material was discharged in strand form into cold water from a nozzle and pelletized into cylindrical shapes using an extruder to obtain the polyester resin composition. A biaxially oriented polyester film was also obtained by the same method as in Example 1. The results are shown in Table 4.

[0107] [Table 1]

[0108] [Table 2]

[0109] [Table 3]

[0110] [Table 4] [Industrial applicability]

[0111] The biaxially oriented polyester film of the present invention has minimal thickness variation and does not significantly hinder cross-nicol inspection, making it suitable for release applications, particularly as a base film in the manufacture of polarizing plates, although its applications are not limited to these.

Claims

1. A biaxially oriented polyester film in which the number of foreign matter satisfying (a) and (b) below is 0.1 pieces / m ２ The following is true, and the melting resistivity is 2.0 × 10⁻⁶. ６ Ω・cm or more 9.0×10 ６ A biaxially oriented polyester film with a density of Ω·cm or less. (a) Observed as bright spots when observed under a polarizing microscope under crossed nicols. (b) The major axis determined by polarized light microscopy under crossed nicols is 50 μm or more and less than 100 μm.

2. The biaxially oriented polyester film according to claim 1, wherein the amount of linear oligomers generated is 100 μg / g or more and 250 μg / g or less.

3. Number of foreign objects generated at the end: 20 pieces / m ２ More than 90 pieces / m ２ The biaxially oriented polyester film according to claim 1, which is as follows:

4. A biaxially oriented polyester film according to claim 1, used for mold release purposes.

5. A biaxially oriented polyester film according to claim 1, used as a base film when manufacturing polarizing plates.

6. A laminate having a biaxially oriented polyester film and a functional layer as described in claim 1.

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