JP7838420B2 - Biaxially oriented polyester film and laminate - Google Patents

Description

This invention relates to biaxially oriented polyester films and laminates.

Polyester film is used in various release applications, including ceramic green sheet manufacturing, liquid crystal polarizing plate release, and photoresist applications.

When forming polyester resin into films, electrostatic casting is a commonly used method that applies a high voltage to the upper surface of an unsolidified sheet, causing it to adhere tightly to a rotating cooling drum. However, increasing the speed of the rotating cooling drum to increase the film formation rate in this electrostatic casting method reduces the adhesion between the sheet and the drum, leading to decreased film thickness uniformity, reduced transparency, and surface defects due to uneven voltage application.

To address these challenges, Patent Documents 1 and 2 explore the addition of substances that improve electrostatic application.

Japanese Patent Publication No. 2000-44703 Japanese Patent Application Publication No. 10-36495

Patent Document 1 proposes a method of adding a quaternary phosphonium sulfonic acid salt to improve electrostatic application, but this method involves the inclusion of large amounts of alkali metals or alkaline earth metals, raising concerns about the generation of metallic impurities.

Patent Document 2 proposes a method for producing a polyester resin composition by adding a sulfonic acid compound but without adding any metal elements other than the polymerization catalyst. However, the amount of phosphoric acid added was too high, making it insufficient to obtain the electrostatic properties necessary for film molding.

Furthermore, methods have been proposed to improve electrostatic conductivity by adding large amounts of alkali metal compounds, alkaline earth metal compounds, and phosphorus compounds to polyester. While these methods can improve electrostatic conductivity to some extent, they have the problem of generating metallic impurities due to the large amount of metal compounds used, thus degrading transparency.

Furthermore, in recent years, when polyester films are used with an additional layer, for example, as a release agent for polarizing plates, the polyester film and polarizing plate undergo cross-nicol inspection while bonded together. Therefore, there is a high demand for reducing film defects such as foreign matter within the polyester film so as not to interfere with this cross-nicol inspection.

The conventional techniques described above were insufficient to achieve minimal thickness variation and prevent the film from significantly hindering cross-nicol inspection. Therefore, the problem that this invention aims to solve is to provide a biaxially oriented polyester film and laminate with minimal thickness variation and few defects.

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 matter satisfying (a) and (b) below is 0.1 or less per ^m² , and the melting resistivity is 2.0 × ^10⁶ Ω·cm or more and 9.0 × ^10⁶ 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.
[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] A biaxially oriented polyester film according to any one of [I] to [III], wherein the number of foreign matter generated at the edges is 20 or ^more and 90 or ^less .
[V] A biaxially oriented polyester film according to any of [I] to [IV], used for release purposes.
[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].

This invention provides a biaxially oriented polyester film and laminate with minimal thickness variations and few defects. This prevents the film from significantly hindering cross-nicol inspection during cross-nicol inspection.

The polyester in the biaxially oriented polyester film of the present invention comprises a dicarboxylic acid component and a diol component. In this specification, "component" refers to the smallest unit obtainable by hydrolysis of the polyester.

In order to carry out the present invention, it is preferable to use terephthalic acid at a concentration of 30 mol% or more relative to the total dicarboxylic acid components as a dicarboxylic acid component of the polyester. 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 aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodium sulfoisophthalic acid, phenylendanedicarboxylic acid, anthracenedicarboxylic acid, phenantradiocarboxylic acid, and 9,9'-bis(4-carboxyphenyl)fluorenic acid, or their ester derivatives.

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 composed of multiple of the aforementioned diols linked together.

The polyester of the present invention can be manufactured by known methods. Specifically, the esterification process 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 to 280°C, the relative pressure to atmospheric pressure is usually 0 to 400 kPa, and the reaction time is usually 1 to 10 hours. The esterification reaction rate of the esterification reaction product obtained in the esterification process is usually 95% or higher.

Following the esterification reaction, a melt polycondensation step is preferable. This melt polycondensation step can typically be carried out continuously or batch-wise using one or more polycondensation reactors, and can be performed 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-wise reaction using a single polycondensation reactor, the reaction temperature is typically 250-290°C, the final absolute pressure after gradually reducing the pressure from atmospheric pressure is typically 0.013-1.3 kPa (0.1-10 Torr), and the reaction time is typically 1-20 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.

The intrinsic viscosity of polyester resin can be determined by the stirring torque of the polymer at the end of polymerization. Higher stirring torque results in higher melt viscosity and therefore higher intrinsic viscosity (IV). The stirring torque for determining the end of polymerization should be set to achieve the target intrinsic viscosity. 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 facilitates control of surface roughness and is therefore preferable.

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 water-cooled. Since the chip shape can be controlled by cutting, polyester chips with a desirable bulk density can be obtained in this invention.

In this invention, one or more polycondensation reaction catalysts can be used, such as antimony trioxide, antimony pentoxide, antimony acetate, antimony glycolate, germanium dioxide, and organotitanium compounds. Among these, antimony trioxide is preferred due to the transparency and availability of the resulting polyester.

A preferred embodiment of the biaxially oriented polyester film of the present invention is a biaxially oriented polyester film in which the number of foreign matter satisfying (a) and (b) below is 0.1 or less per ^m² , and the melting resistivity is 2.0 × ^10⁶ Ω·cm or more and 9.0 × ^10⁶ 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.

It is preferable that the number of foreign objects satisfying (a) and (b) is 0.1 or less per ^m² . This embodiment makes it possible to provide a film that does not significantly hinder cross-nicol inspection. From the viewpoint of achieving higher quality, it is more preferable that the number of foreign objects is 0.07 or less per ^m² . It is also preferable that the number of foreign objects is 0.01 or more per ^m² . This embodiment makes it possible to suppress the deterioration of slipperiness and handling performance due to the surface becoming too flat. The number of foreign objects shall be determined by the following method.

<Method for measuring the number of foreign objects>
To measure the number of foreign particles, a 10 cm square marking is placed on a film sample, and a 100 ^cm² area within the marking 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 a dark room, 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 edges that appear as bright spots under polarized light is measured as the major axis, and the number of particles between 50 and 100 μm is counted and marked. This is repeated 10,000 times, and a total of 100 ^m² is observed.

The biaxially oriented polyester film of the present invention preferably has a melt resistivity of 2.0 × ^10⁶ Ω·cm or more and 9.0 × ^10⁶ Ω·cm or less. A melt resistivity of 9.0 × ^10⁶ Ω·cm or less improves adhesion to the rotating cooling drum in the electrostatic casting method, improves the uniformity of the thickness of the resulting film, and reduces haze deterioration due to defects on the film surface caused by uneven application. From a similar viewpoint, 7.0 × ^10⁶ Ω·cm or less is more preferable, and 4.0 × ^10⁶ Ω·cm or less is even more preferable. Furthermore, a melt resistivity of 2.0 × ^10⁶ Ω·cm or more suppresses the deterioration of haze in the biaxially oriented polyester film, which occurs when frequent discharge from the application terminal to the rotating cooling drum occurs during electrostatic application, and the charge is not retained until it adheres to the rotating cooling drum.

Furthermore, the molten resistivity can be determined by the following method.

<Method for measuring molten resistivity>
After melting a biaxially oriented polyester film at 290°C, two stainless steel electrodes with an area of 0.5 ^cm² are inserted parallel to each other at an interval of 8 mm. Once the temperature stabilizes, the resistance (R) is measured using a resistance meter (HIOKI RM3545). Subsequently, the molten resistivity (ρ) is calculated using the formula ρ (Ω・cm) = R × 0.5 / 0.8.

As a means of achieving a biaxially oriented polyester film in which the number of foreign matter satisfying (a) and (b) above is 0.1 pieces/ ^m² or less, and the melting resistivity is 2.0 × ^10⁶ Ω·cm or more and 9.0 × ^10⁶ 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.

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.

This embodiment provides a biaxially oriented polyester film that exhibits good color tone, good transparency, minimal thickness variation, and does not significantly hinder cross-nicol inspection.

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, better electrostatic application during film molding can be obtained. When the sulfur content is 2 ppm by mass or more, the volume resistivity during melting of the polyester resin composition decreases, resulting in good electrostatic casting properties during film molding and reduced thickness unevenness. When the sulfur content is 20 ppm by mass or less, deterioration of color tone can be suppressed. From a similar viewpoint, a sulfur content of 9 to 17 ppm by mass relative to the total mass is more preferable.

Examples of compounds containing the element sulfur include sulfide compounds, thiophene compounds, thiol compounds, and sulfonic acid compounds. Among these, 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. 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 during the melting of the polyester resin composition, sulfonic acid compounds that are copolymerized into the polyester molecular chain are more preferable. Even more preferable, the sulfonic acid compound copolymerized into the polyester molecular chain is a sulfonium anion having two or more ester-forming functional groups, such as 3,5-dicarboxybenzenesulfonic acid.

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 by kneading the resulting polyester resin composition with the compound using a twin-screw kneading extruder.

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. A phosphorus content of 2 ppm or more yields a good color tone in the biaxially oriented polyester film, while a content of 16 ppm or less results in good electrostatic casting properties during film formation and reduces thickness variations. From a similar viewpoint, a phosphorus content of 2 to 10 ppm by mass relative to the total mass is more preferable.

The method for incorporating phosphorus 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 by kneading them with the polyester resin composition after the polycondensation reaction using a twin-screw kneading extruder.

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 a linear oligomer generation 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.

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. They 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 a carboxyl or hydroxyl terminus. The amount of linear oligomers generated shall be determined by the method described in the examples.

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 when the film is heat-treated or heat-processed, and reduces 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, improving the ductility of the film and reducing brittleness, thus suppressing edge defects.

The biaxially oriented polyester film of the present invention preferably has an edge foreign matter generation rate ^of 20 to 90 pieces/ ^m² , and more preferably 25 to 70 pieces/ ^{m² .} By adopting the above configuration, it is possible to prevent defects (coating repelling, dents, process contamination) when used as a process film, and by including a certain amount of edge foreign matter, blocking between films can be suppressed, resulting in a good winding appearance, and defects caused by blocking can be suppressed. Furthermore, by improving the hydrolysis resistance and thermal decomposition resistance of the polyester resin composition and improving the ductility, the preferred number of edge foreign matter generation can be maintained. The number of edge foreign matter generation shall be determined by the method described in the examples. The type of foreign matter is not particularly limited, but it is preferably a polyester powder and/or an amide compound, and more preferably a 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.

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 less than 5 ppm by mass of calcium, magnesium, and manganese elements, a biaxially oriented polyester film with good color tone and haze, and low levels of impurities 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, magnesium, and manganese ions tend to form impurities. Therefore, from the viewpoint of reducing impurities, it is preferable that the biaxially oriented polyester film of the present invention does not contain calcium, magnesium, or manganese elements.

When incorporating calcium, magnesium, and manganese elements into the biaxially oriented polyester film of the present invention, the method is not particularly limited, but it is preferable to add the calcium, magnesium, and manganese compounds 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 them as a solution from the viewpoint of dispersibility. The solvent used 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.

The amounts of each element will be determined using the following method.

<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 number: 3270) manufactured by Rigaku Denki Co., Ltd., and then calculating it from a pre-prepared calibration curve.

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 resulting gas is absorbed into a solution, and a portion of the absorbent 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 considered the sulfur content.

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.

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

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

(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 on a casting roll at 18 to 50°C for 1 to 15 seconds to cool and solidify it.

(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.

(Step 4) The uniaxially oriented polyester film obtained in (Step 3) is stretched 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. After stretching, it is cooled to obtain a biaxially oriented polyester film.

(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.

The following provides a detailed explanation of each step.

(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 be between 245 and 250°C, and the esterification reaction is terminated when the reaction rate reaches 95%, and the resulting esterified product is charged in a molten state into a polymerization apparatus equipped with a distillation device.

Antimony trioxide and a compound containing sulfur and phosphorus are added as an ethylene glycol solution. The polymerization reaction is then gradually reduced in the polymerization reactor to below 0.13 kPa in 35 minutes, while simultaneously gradually increasing the temperature to 279°C to carry out the polymerization reaction and obtain a polyester resin.

(Step 2) Preparation of unstretched film The polyester resin is dried as needed and supplied to an extruder for melt extrusion. In order for the intrinsic viscosity of the film to be 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.

Next, the polyester resin extruded by the extruder is filtered. Since even small foreign particles can cause film defects, it is effective to use a high-precision filter that captures 95% or more of foreign particles larger than, for example, 5 μm. The molten polyester resin undergoes thermal decomposition and hydrolysis, causing its molecular chains to break and its intrinsic viscosity to decrease. For stable melt extrusion, the temperature and moisture content of the polyester resin during melt extrusion are preferably between the melting point of the polyester resin and 300 ppm or less.

Next, the polyester resin is formed into a sheet using a T-shaped die or the like, and the sheet-formed polyester resin is cooled and solidified on a casting roll to obtain an unstretched film. At this time, the temperature of the casting roll is preferably 18 to 50°C, and the cooling time during which the sheet-formed polyester resin contacts 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 χi value may decrease. This is because high temperatures on the casting roll worsen the cooling efficiency, leading to the formation of microcrystals in the resulting unstretched film, which in turn facilitates crystal orientation in the subsequent stretching process. Similarly, if the cooling time during contact with the casting roll is less than 1 second, the cooling efficiency worsens, resulting in the formation of microcrystals in the resulting unstretched film, which tends to lower the crystal plane orientation index χi value. The cooling time by the casting roll can be extended by increasing the diameter of the casting roll or reducing 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 to 30°C, and the cooling time during which the sheet-shaped polyester resin contacts the casting roll is 3 to 12 seconds.

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 to achieve the desired orientation angle and thermal shrinkage rate, 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. Also, the final thickness of the biaxially oriented polyester film may fall outside the range suitable for polarizing plate release applications.

(Step 3) Preparation of Uniaxially Oriented Film The unstretched film obtained in (Step 2) is stretched in the longitudinal direction at a stretching ratio of 2.5 to 5.0 times and cooled to obtain a uniaxially oriented polyester film. 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. In addition, stretching in the longitudinal direction causes shrinkage in the width direction, but it is preferable that the width reduction of the film from this stretching step to the cooling step 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 value of 6.0 or more 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.

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.

In the cooling process in (Step 3), a film temperature of 25 to 45°C is preferable for stable stretching in the width direction in the following (Step 3).

(Step 4) Preparation of biaxially oriented film The uniaxially oriented polyester film obtained in (Step 3) is stretched at a stretching ratio of 3.0 to 6.0 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. 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 value decreases. A stretching temperature of 100 to 120°C and a stretching ratio of 4.0 to 5.0 times are more preferable. Furthermore, in order 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.

In manufacturing the biaxially oriented polyester film of the present invention, stretching is performed in the width direction after stretching in the longitudinal direction. This is because, after the initial width direction stretching, the molecules are strongly oriented primarily in the width direction. However, subsequent stretching in the longitudinal direction also causes orientation in the longitudinal direction, resulting in a high orientation angle.

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 inhibits 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 film formation, resulting in poor film-forming properties, and the effect of suppressing orientation relaxation 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.

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, since all zones are connected in the longitudinal direction, 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, which may prevent sufficient cooling of the film. In such cases, this can be addressed by actively cooling the film by supplying compressed air.

Furthermore, in the roll-based cooling method, while there are no limitations on the number of rolls used or the set temperature, it is preferable to use multiple rolls for cooling. To achieve the above-mentioned film temperature range in the roll-based cooling method, the roll temperature is preferably 20 to 45°C, and more preferably 30 to 40°C. Additionally, in the roll-based cooling method, applying pressure to the cooling rolls using nip rolls to ensure tight contact with the film allows for stable cooling, which is therefore preferable.

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, potentially leading to poor film formation and film tearing. 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. A film width shrinkage rate of 0.2 to 18%/min is even more preferable. The width shrinkage rate can be controlled by setting the rate based on the cooling process length and film formation rate, and this can be achieved in various ways. Specifically, in the air cooling method using a tenter, the width shrinkage rate can be set to the desired value by gripping both ends with clips and adjusting the rail width.

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

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 preferable to have a time of 60 seconds or less as this results in good productivity.

(Step 5) Heat treatment of biaxially oriented film The biaxially oriented polyester film obtained in (Step 4) is heat-treated to obtain a biaxially oriented polyester film. 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 less than 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 within the range of 2.5 to 8.0%. If the heat treatment temperature is higher than 230°C, bowing is 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.

The biaxially oriented polyester film of the present invention is preferably used for release applications. When the melting resistivity of the biaxially oriented polyester film of the present invention is 2.0 × ^10⁶ Ω·cm or more and 9.0 × ^10⁶ 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 obtained film. Furthermore, when the total content of calcium, magnesium, and manganese elements is less than 5 ppm by mass, a biaxially oriented polyester film with good color tone and haze and low metallic foreign matter can be obtained. As described above, when the biaxially oriented polyester film is laminated to a polarizing plate and cross-nicol inspection is performed, defects such as thickness unevenness and metallic foreign matter in the biaxially oriented polyester film that 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.

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 interfere with cross-nicol inspection during polarizing plate manufacturing, and is particularly suitable for use when manufacturing polarizing plates of 32 inches or larger, where high quality is required.

The biaxially oriented polyester film of the present invention is suitably used as a release film for large-sized polarizing plates. Specifically, it is preferably used in the following manner.

First, a laminate is formed by providing the biaxially oriented polyester film of the present invention on at least one side of the polarizer. 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." Note that known materials can be used for the silicone layer, adhesive layer, polarizer, and protective sheet.

Next, the laminate is inspected. Since the laminate uses the biaxially oriented polyester film of the present invention, good inspection results are obtained. Then, the biaxially oriented polyester film of the present invention is peeled off the laminate to obtain a polarizing plate. Here, a polarizing plate refers to a component having at least a polarizer. For example, peeling (removing) the biaxially oriented polyester film of the present invention from a laminate consisting of "biaxially oriented polyester film of the present invention / adhesive layer A / polarizer / adhesive layer B / protective sheet" or "biaxially oriented polyester film of the present invention / silicone layer / adhesive layer A / polarizer / adhesive layer B / protective sheet" yields a polarizing plate in which "adhesive layer A / polarizer / adhesive layer B / protective sheet" are arranged in that order. It is preferable that the silicone layer be peeled off together with the biaxially oriented polyester film of the present invention.

A preferred embodiment of the laminate of the present invention comprises the biaxially oriented polyester film and a functional layer. The functional layer refers to 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 onto the biaxially oriented polyester film.

The methods for measuring and evaluating characteristic values in the examples and comparative examples are as follows:

(1) Number of bright spots To measure the number of foreign objects, a 10 cm square marking was placed on a film sample, and a 100 ^cm² area within the marking 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 device was used. The longest diameter connecting the edges of the bright spots visible 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, and a total of 100 ^m² was observed.

(2) Melt resistivity of biaxially oriented polyester film (unit: Ω・cm)
After melting a biaxially oriented polyester film at 290°C, two stainless steel electrodes with an area of 0.5 ^cm² were inserted parallel to each other at an 8 mm interval. Once the temperature stabilized, the resistance (R) was measured using a resistance meter (HIOKI RM3545). Subsequently, the molten resistivity (ρ) was calculated using the formula ρ (Ω・cm) = R × 0.5 / 0.8.

(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 fluorescence X-ray intensity using a fluorescence X-ray analyzer (model number: 3270) manufactured by Rigaku Denki Co., Ltd., and then calculating it from a pre-prepared calibration curve.

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 Co., Ltd. The resulting gas was absorbed into a solution, and a portion of the absorbent solution was analyzed using an ion chromatograph (model: ICS1600) manufactured by DIONEX Co., Ltd. Samples were weighed and measured in sets of n=2, and the average of the measured values was defined as the sulfur content.

(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 width direction, and the haze was measured using a haze meter (Suga Test Instruments HGM-2DP (for C light source)).

(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 was used as an evaluation index for color tone.

(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 "FiDiCa" (registered trademark) film thickness distribution measuring device manufactured by JFE Techno Research Corporation, was used to measure the surface thickness. 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.

(7) Amount of linear oligomers generated 0.1 g of film was freeze-dried and dried under reduced pressure at 160°C for 6 hours, then sealed in an ampoule under an _N2 atmosphere. The ampoule was 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).

(8) Surface scratches A 100 x 100 mm sample of the film was taken, and 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 varying from 30° to 150° horizontally 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 scratches of the film.
◎: 0 pieces/ ^m² or more, 2 pieces/ ^m² or less ○: 3 pieces/ ^m² or more, 5 pieces/ ^m² or less ×: 6 pieces/ ^m² or more (9) Number of oligoma defects Each layer of the film was scraped off, leaving only layer A. This was heated in an oven set to 150°C for 30 minutes, then removed from the oven and cooled. Silver was deposited on the surface, and then platinum was coated by sputtering. The surface was observed in two fields at a magnification of 2000x using a scanning electron microscope, and the number of precipitated oligoma was counted and converted to per ^mm² . Oligoma are characterized by being lumpy, hexagonal, or horseshoe-shaped. If it could not be determined that the defect was due to oligoma, 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 name: NICOLET 6700 and CONTINU μM infrared microscope Measurement conditions: Measurement range: 650 ^cm⁻¹ to 4000 ^cm⁻¹ , Resolution: 8 ^cm⁻¹
Detector: MCT
(10) Number of foreign matter generated at the edges The number of foreign matter generated at the edges was evaluated using an adhesive roll. An adhesive roll (Teknek p-type adhesive roll) was placed so as to be in contact with the edges of the polyester film from 0 to 50 mm, and the contact was maintained for a length of 1000 m to collect the foreign matter attached to the edges of 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 on which the attached edge foreign matter had been transferred was then observed using a shape analysis laser microscope (KEYENCE VK-X250). The observation conditions were as follows: an automatic observation of 81 fields was performed using a 20x objective lens with a measurement area of 700 μm × 530 μm. This was repeated 9 times, and a total of 729 fields (= 280.7 ^mm² ) were observed. The center line of each field was calculated using the total number of measured pixels per field, 1024 × 768 pixels. Using a multi-file analysis application (VK-X series, manufactured by KEYENCE), a threshold (height of foreign object) of 0.5 μm or more and a size (area of foreign object) of 75 ^μm² or more were set for each field of view. Protrusions detected were counted as foreign objects if the average of the vertical and horizontal ferret diameters was between 10 and 200 μm, and the number of foreign objects per unit area was defined as the number of foreign objects at the end. Measurements were performed once for each end, and the average value was used as the evaluation result.

(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).

(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.

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 simultaneously, 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 through a nozzle. The mixture was then pelletized into cylindrical shapes using an extruder to obtain the polyester resin composition.

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.

The volume shape coefficient f is expressed by the following formula.
f = V/Dm ³
Here, V is the particle volume ( ^μm³ ) and Dm is the maximum diameter of the particle on the projection plane (μm). The volume-shape coefficient f takes its maximum value of π/6 when the particle is spherical.

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 layers B and B', while the above-mentioned polyester resin composition was used for polyester layer A.

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.

(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.

(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.

(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.

(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.

(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.

(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.

(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 content shown in Table 3. The results are shown in Table 4.

(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.

(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 content shown in Table 3. The results are shown in Table 4.

(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.

(Examples 14 and 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 achieve the phosphorus and sulfur content shown in Table 3. The results are shown in Table 4.

(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 amount 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, and 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.

(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.

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 simultaneously 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.

The biaxially oriented polyester film of the present invention exhibits minimal thickness variation and does not significantly hinder cross-nicol inspection. Therefore, it is suitable for release applications, particularly as a base film in the manufacture of polarizing plates; however, its applications are not limited to these.

Claims (6)

A biaxially oriented polyester film wherein the number of foreign matter satisfying (a) and (b) below is 0.1 or less per ^m² , and the melting resistivity is 2.0 × ^10⁶ Ω·cm or more and 9.0 × ^10⁶ 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. 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. The biaxially oriented polyester film according to claim 1 , wherein the number of foreign matter generated at the edges is 20 or ^more and 90 or ^less . A biaxially oriented polyester film according to claim 1 , used for mold release purposes. A biaxially oriented polyester film according to claim 1 , used as a base film when manufacturing polarizing plates. A laminate having a biaxially oriented polyester film and a functional layer as described in claim 1 .