JP7528663B2 - Biaxially oriented polyester film - Google Patents

Description

The present invention relates to a biaxially oriented polyester film that suppresses the aggregation and coarse formation of calcium carbonate particles and has excellent inspectability and transportability.

Release films based on polyester are used for various purposes such as multilayer ceramic capacitors (hereinafter sometimes written as MLCC), liquid crystal polarizing plate release, and photoresist. A typical release film is required to reduce the lamination misalignment and defects during lamination when a release object is placed on the release film, and further to have releasability between the release film and the release object placed on the release film. In addition, in polarizing plate release, the release film undergoes a crossed Nicol test while it is attached to the polarizing plate, so it is required to reduce foreign matter in the release film so as not to interfere with the crossed Nicol test (easy inspection in the crossed Nicol test). As a method for improving the transportability and releasability of release films, a technique for blending an appropriate amount of particles into a polyester film to form fine protrusions on the film surface has been known. In Patent Document 1, inorganic particles such as aggregated silica, spherical silica, and calcium carbonate are used. In addition, a technique for blending an appropriate amount of particles into a polyester film to increase the opacity of the film is known as a method for adjusting the haze of the film, and for example, Patent Document 2 uses a calcium compound.

Japanese Patent Application Publication No. 8-124150 Special Publication No. 6-86537

However, when the inorganic particle content in the polyester composition is increased to improve the transportability and releasability of the release film, coarse particles are more likely to be generated due to aggregation, which causes a problem of poor inspectability.

The object of the present invention is to eliminate the drawbacks of the conventional technology. In other words, the object is to provide a biaxially oriented polyester film that reduces the amount of agglomerated foreign matter originating from inorganic particles added to the film and has excellent inspectability and transportability.

The object of the present invention is achieved by the following means.
(1) A biaxially oriented polyester film containing calcium carbonate particles treated with a buffer solution containing phosphoric acid and an alkali metal phosphate , and satisfying the following formula (I):
Free calcium content/total calcium content < 1.0E-2 (I)
(The free calcium content was determined by dissolving a polyester film in o-chlorophenol at 150°C for 1 hour, centrifuging the resulting solution at a centrifugal acceleration of 40,900G at 20°C for 1 hour, decanting the resulting supernatant, adding acetone to cause precipitation, collecting the precipitate by filtration, and drying in vacuum at 150°C for 12 hours to obtain a polyester composition containing calcium.)
(2) The biaxially oriented polyester film according to (1), having a total calcium content of 4,500 ppm or less.
(3) The biaxially oriented polyester film according to (1) or (2), wherein the volume average diameter of the calcium carbonate particles is 3.0 μm or less.
(4) The biaxially oriented polyester film according to any one of (1) to (3), wherein the free calcium content and the total calcium content after heat treatment at 300° C. for 10 hours in a nitrogen atmosphere satisfy the following formula (II):
Free calcium content after heat treatment/total calcium content < 1.5E-2 (II)

The biaxially oriented polyester film of the present invention suppresses aggregation and coarse foreign matter formation of calcium carbonate particles, and has excellent inspectability and transportability.

The present invention will be described in detail below.
The biaxially oriented polyester film of the present invention must satisfy the following formula (I): It is more preferably less than 9.0E-3, and even more preferably less than 7.0E-3.
Free calcium content/total calcium content < 1.0E-2 (I)
However, the free calcium content refers to the calcium content in a polyester composition obtained by dissolving the biaxially oriented polyester film in o-chlorophenol, centrifuging the resulting supernatant at a centrifugal acceleration of 40,900 G at 20° C. for 1 hour, adding acetone to the resulting supernatant to precipitate, and then recovering the resulting particles by filtration, as described in detail in the Examples. In a film containing calcium carbonate particles, free calcium is generated from the calcium carbonate particles due to the heat history during film formation. This generated free calcium forms by-products with the polyester composition, causing particle aggregation and coarse foreign matter. By setting the amount of free calcium within the above range, it is possible to suppress aggregation of calcium carbonate particles, and a film with excellent inspectability and transportability can be obtained.
As a result of extensive investigation, it has been found that the biaxially oriented polyester film of the present invention is obtained by mixing calcium carbonate particles with a phosphorus compound and then adding the mixture to a polyester polymerization system, thereby suppressing the generation of free calcium from the calcium carbonate particles.
The total calcium content (mass ratio) contained in the biaxially oriented polyester film of the present invention is preferably 4500 ppm or less, more preferably 2000 ppm or less. The lower limit is preferably 50 ppm or more, more preferably 100 ppm or more. By setting it in the above range, particle aggregation can be suppressed, and the transportability of the film is improved. The total calcium content refers to the calcium content measured by the method of the following Example (1), and is the total amount of free calcium and non-free calcium contained in calcium carbonate.
The volume average diameter of the calcium carbonate particles in the present invention is preferably 3.0 μm or less. This volume average diameter is a value obtained by SEM observation. The volume average diameter is more preferably 2.0 μm or less, and even more preferably 1.5 μm or less. The lower limit is preferably 0.1 μm or more, and more preferably 0.5 μm or more. By setting the diameter within the above range, it is possible to prevent the particles from becoming coarse foreign matter, and the inspectability and transportability in the crossed Nicols inspection are improved.

The calcium content in the polyester of the biaxially oriented polyester film of the present invention after heat treatment at 300° C. for 10 hours in a nitrogen atmosphere preferably satisfies the following formula (II), and more preferably is less than 1.0E-2.
Free calcium content after heat treatment/total calcium content < 1.5E-2 (II)
By setting the ratio of the amount of free calcium after heat treatment to the total calcium content within the above range, it is possible to prevent the polyester composition that is melt extruded during film formation and thermally deteriorated from binding with calcium components and becoming a foreign matter.
The biaxially oriented polyester film of the present invention is suitable for use as a release film for polarizing plate release. This is necessary to prevent scratches during the processing of the polarizing plate and to hold the adhesive for attaching the polarizing plate to a display substrate. The polyester film of the present invention is subjected to a release treatment, and is attached in a rolled state to a polarizing plate coated with an adhesive, and is peeled off when the polarizing plate is punched out and attached to the display substrate.
Next, a method for producing the polyester composition in the biaxially oriented polyester film of the present invention will be described.
The biaxially oriented polyester film of the present invention is obtained from a polyester composition obtained by polycondensation of a dicarboxylic acid component and a diol component.
Examples of the dicarboxylic acid component constituting the polyester composition include terephthalic acid, 2,6-naphthalenedicarboxylic acid, isophthalic acid, diphenyl 4,4'-dicarboxylic acid, 5-sodium sulfoisophthalic acid, oxalic acid, succinic acid, adipic acid, sebacic acid, malonic acid, dimer acid, and esters thereof. Among these, aromatic dicarboxylic acids and their ester-forming derivative components are preferred from the viewpoints of mechanical properties, heat resistance, and hydrolysis resistance. In particular, terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, and ester-forming derivatives thereof are preferred in terms of polymerizability and mechanical properties. Examples of diol components constituting such polyester compositions include aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, butanediol, 2-methyl-1,3-propanediol, hexanediol, and neopentyl glycol; alicyclic diols such as cyclohexanedimethanol, cyclohexanediethanol, norbornanedimethanol, norbornanediethanol, tricyclodecanedimethanol, tricyclodecanediethanol, decalindimethanol, and decalindiethanol; and cyclic esters such as isosorbide. Examples of the diol include saturated heterocyclic primary diols containing ether, various alicyclic diols such as cyclohexanediol, bicyclohexyl-4,4'-diol, 2,2-bis(4-hydroxycyclohexylpropane), 2,2-bis(4-(2-hydroxyethoxy)cyclohexyl)propane, cyclopentanediol, 3-methyl-1,2-cyclopentadiol, 4-cyclopentene-1,3-diol, and adamantanediol, aromatic diols such as paraxylene glycol, bisphenol A, bisphenol S, styrene glycol, 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene, and 9,9'-bis(4-hydroxyphenyl)fluorene, and diols in which a plurality of the above diols are linked together. Among these, diols having a boiling point of 230°C or less are preferred because they can be easily distilled out of the reaction system, and aliphatic diols are more preferred because they are low cost and highly reactive. Furthermore, ethylene glycol is particularly preferred from the viewpoint of mechanical properties.
The polyester composition can be produced by a conventionally known method, but is not limited thereto. For example, the polyester composition can be produced by the following two-stage process using a dicarboxylic acid component or its ester and a diol component as main raw materials. That is, the first stage is (A) an esterification reaction or (B) an ester exchange reaction, and the second stage is (C) a polycondensation reaction.
In the first step, the esterification reaction step (A) is a step in which a dicarboxylic acid and a diol are esterified at a predetermined temperature until a predetermined amount of water is distilled off to obtain a low polymer, and the transesterification reaction step (B) is a step in which a dicarboxylic acid ester and a diol are esterified at a predetermined temperature until a predetermined amount of alcohol is distilled off to obtain a low polymer.
In the second step, the polycondensation reaction (C) is a step for obtaining a polyester composition from the low polymer obtained by the esterification reaction (A) or the transesterification reaction (B).

In the method for producing the polyester composition of the present invention, the catalyst used in the polycondensation reaction (C) may be a known polycondensation catalyst. Examples include compounds of antimony, titanium, aluminum, tin, germanium, etc. Among these, from the viewpoints of polymerization time and economic efficiency, it is preferable to use an antimony compound as the polycondensation reaction catalyst.

During the polycondensation reaction of the polyester composition in the biaxially oriented polyester film of the present invention, it is preferable to premix the calcium carbonate particles with a phosphorus compound and then add them to a polycondensation reactor. By premixing, the phosphorus compound protects the surface of the calcium carbonate particles and can suppress the elution of free calcium components from the calcium carbonate particles even when subjected to heat history, making it possible to provide a polyester film with little particle aggregation and coarse protrusions.
In addition, the calcium carbonate and the phosphorus compound are preferably added in a form in which they are thoroughly mixed, and it is preferable to add them as a mixed slurry. The solvent used in this case is preferably the same as that of the diol component of the polyester composition. For example, in the case of PET, it is particularly preferable to use ethylene glycol.
It is particularly preferable to use phosphoric acid and an alkali metal phosphate as the phosphorus compound to be added by mixing with calcium carbonate particles. Phosphoric acid alone is highly acidic, and calcium may be eluted when mixed with calcium carbonate particles, so it is preferable to mix it as a phosphate buffer. In addition, the addition of a phosphorus compound improves the hydrolysis resistance and heat resistance of the polyester composition, and makes it possible to suppress the amount of linear oligomers generated during heat treatment or heat processing of the film. Other phosphorus compounds that can be used include trimethyl phosphate, ethyl diethyl phosphonoacetate, phosphorous acid, disodium hydrogen phosphate, trisodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, and ethyl diethyl phosphonoacetate, and it is also possible to use a plurality of these phosphorus compounds in combination.
The amount of the phosphorus compound to be mixed in advance with the calcium carbonate particles is preferably 40 ppm or more relative to the weight of the polyester composition containing calcium carbonate. More preferably, it is 50 ppm or more. The upper limit is preferably 80 ppm or less, more preferably 70 ppm or less. By setting it within the above range, the calcium carbonate particles can be sufficiently protected by the phosphorus compound, and calcium elution can be suppressed. In addition, the polycondensation reaction of the polyester composition can be promoted without being inhibited.
Next, a method for producing the biaxially oriented polyester film of the present invention will be described. This is just an example, and the present invention is not limited to the product obtained by this example.

A method (melt casting method) can be used in which the polyester composition is heated and melted in an extruder, extruded from a die onto a cooled casting drum, and processed into a sheet. At this time, in order to remove unmelted material in the polyester composition, it may be filtered through a fiber-sintered stainless steel metal filter. Furthermore, various additives, such as compatibilizers, plasticizers, weathering agents, antioxidants, heat stabilizers, lubricants, antistatic agents, conductive materials, ultraviolet absorbers, flame retardants, and flame retardant assistants, may be added within a range that does not impair the effects of the present invention.

As another method, a method (solution casting method) can be used in which the polyester composition is dissolved in a solvent, the solution is extruded from a die onto a support such as a casting drum or an endless belt to form a film, and the solvent is then dried and removed from the film layer to form a sheet.

In addition, when the substrate of the laminated film has a laminated structure, a method of feeding two different polyester compositions into two extruders, melting them, and then joining them together and processing them into a sheet by co-extrusion from a die onto a cooled casting drum (a method of melt-forming a film by co-extrusion) can be preferably used. This method is described in detail below.

First, the polyester composition is put into an extruder and heated to melt. The composition is laminated using multiple extruders, multiple-layer manifolds or merging blocks (e.g., merging blocks with a rectangular merging section), and a sheet is extruded from a die and cooled with a casting roll to produce an unstretched film. At this time, the polyester composition melted in the extruder is filtered through a filter. Even very small foreign objects can become large protrusion defects if they get into the film, so it is effective to use a high-precision filter that captures 95% or more of foreign objects of 5 μm or more. After that, the sheet is cooled to a surface temperature of 10 to 60°C and cooled and solidified by static electricity to produce an unstretched sheet. Next, the unstretched sheet is introduced into a group of rolls heated to a temperature of 70 to 140°C, stretched 3 to 4 times in the longitudinal direction (vertical direction, i.e., the direction in which the sheet travels), and cooled with a group of rolls heated to a temperature of 20 to 50°C. Next, the sheet is guided to a tenter while holding both ends with clips, and stretched 3 to 5 times in a direction perpendicular to the longitudinal direction (width direction) in an atmosphere heated to a temperature of 80 to 240°C. The stretching ratio is 3 to 5 times in both the longitudinal and width directions, and the area ratio (longitudinal stretching ratio x transverse stretching ratio) is preferably 9 to 17 times. If the area ratio is less than 9 times, the durability of the obtained biaxially stretched sheet becomes insufficient, and conversely, if the area ratio exceeds 17 times, it tends to be easily torn during stretching. In addition, after stretching, a relaxation treatment of 0.1% to less than 5.0% in the longitudinal and/or width directions may be performed. The biaxial stretching method may be either the sequential secondary stretching method in which the longitudinal and width directions are stretched separately as described above, or the simultaneous biaxial stretching method in which the longitudinal and width directions are stretched simultaneously.

The present invention will be described in more detail below with reference to examples. The physical properties in the examples were measured by the following methods.
Moreover, Examples 12 to 14 are to be read as Comparative Examples.

(1) Quantitative Determination of Total Calcium Atoms (Elements) in Polyester Film (Unit: ppm (mass ratio))
The polyester film was cut into small pieces, molded into a cylindrical shape using a melt press, and quantitative analysis was performed using a fluorescent X-ray analyzer (model number: 3270) manufactured by Rigaku Corporation.
(2) Quantitative analysis of free calcium atoms (elements) in polyester film 30 g of polyester film was added to 300 mL of o-chlorophenol, dissolved at 150°C for 1 hour, and then centrifuged at a centrifugal acceleration of 40,900 G at 20°C for 1 hour. The resulting supernatant solution was collected by decantation, and 500 mL of acetone was added to cause precipitation. The precipitate was collected by filtration and vacuum dried at 150°C for 12 hours, and quantified by the analytical method in (1). A HITACHI himacCR20G (rotor: R19A) centrifuge was used.
(3) Measurement of particle volume average diameter in polyester film (unit: μm)
The polyester film was plasma-treated, and the volume average diameter of the particles was measured using a Hitachi field emission scanning electron microscope (model S-4000) and a Nideco SEM-IMAGE ANALYZER (model Ludex AP). When analyzing the particle diameter, measurements were performed in 20 or more fields at a magnification of 5000 times, and the circle equivalent diameter was measured from at least 200 particles, which were regarded as pseudo-three-dimensional spheres to calculate the volume average diameter.

(4) Quantitative analysis of calcium atoms (elements) in polyester film after heat treatment (unit: ppm)
The polyester film was pretreated by vacuum drying at 150° C. for 3 hours and then at 180° C. for 7.5 hours, followed by heating and melting at 300° C. for 10 hours under a nitrogen flow, and then quenching in water. The particle content of the obtained composition was calculated by the above analytical method (1).

(5) Quantitative analysis of free calcium atoms (elements) in polyester film after heat treatment (unit: ppm)
The polyester film was subjected to a heat melting treatment by the method of (4), and then the calcium atoms (element) were quantitatively determined by the method of (2).
(6) Inspectability evaluation A crossed Nicol inspection instrument was used, which was equipped with LED lights (HBLF-WSL1500, HBLF-WSL700 manufactured by Atto) and a first polarizing plate with adjustable angle as the illumination means, and a CCD camera with a resolution of 50 μm (GMFMB3B80 manufactured by Hutec) and a second polarizing plate with adjustable angle as the light receiving means, in which multiple combinations were arranged. 100 m of film was inspected without changing the detection sensitivity of the camera of the crossed Nicol inspection instrument, and the number of bright spot defects (light leakage due to defects in the film) with a detection size of 100 μm or more was counted and evaluated as an index of inspectability (◯ or more was considered to be pass).
◯: The number of bright spots of 200 μm or more is 0.6 or less per ^square meter.
△: The number of bright spots of 200 μm or more is 0.6 to 1.0 per ^square meter.
×: The number of bright spots of 200 μm or more is 1.0 or more per ^square meter.

(7) Evaluation of conveyability Using a slip tester (manufactured by Toyo Seiki Seisakusho Co., Ltd.), the A layer surface and the B layer surface of two laminated films were rubbed together and the value was measured three times according to JIS K 7125 (1999), and the static friction coefficient μs in the longitudinal direction was calculated from the average value. The static friction coefficient μs was used as an index of conveyability and evaluated according to the following criteria.
⊚: More than 0.1 and not more than 0.3 ◯: More than 0.3 and not more than 0.35 △: More than 0.35 and not more than 0.5 ×: More than 0.5
[Preparation of PET-A] A slurry consisting of 86 parts by weight of terephthalic acid and 37 parts by weight of ethylene glycol (1.15 times the molar amount relative to terephthalic acid) was gradually added to an esterification reactor containing 105 parts by weight of BHT (bishydroxyethyl terephthalate) melted at 250° C., and the esterification reaction was allowed to proceed. The temperature in the reaction system was controlled to be 245-250° C., and the esterification reaction was terminated when the reaction rate reached 95%, yielding BHT.
105 parts by weight (corresponding to 100 parts by weight of PET) of BHT was charged in a molten state from the esterification reactor into the polymerization apparatus, and the temperature was set to 255° C. An ethylene glycol solution of manganese acetate tetrahydrate (40 ppm as Mn atom (element) relative to the weight of the polyester composition), an ethylene glycol slurry of diantimony trioxide (100 ppm as Sb atom (element) relative to the weight of the polyester composition), and phosphoric acid (35 ppm as phosphorus atom (element) relative to the weight of the polyester composition) were added thereto. Thereafter, the temperature inside the polymerization apparatus was gradually raised to 290° C., and the pressure was reduced from normal pressure to 250 Pa or less, and polymerization reaction was carried out until a predetermined stirring torque was shown at 290° C. After the polymerization reaction was completed, the reaction system was returned to normal pressure with nitrogen gas, and the molten polyester in the polymerization apparatus was discharged into a water tank in the form of a strand, cooled, and then cut to obtain a pellet-shaped polyester composition (intrinsic viscosity 0.62).
[Preparation of MB-A] An esterification reaction was carried out in the same manner as in the preparation of PET-A to obtain BHT.
105 parts by weight (corresponding to 100 parts by weight of PET) of BHT was charged in a molten state from the esterification reactor into the polymerization apparatus, and the temperature was adjusted to 255° C. To this was added an ethylene glycol solution of manganese acetate tetrahydrate (23.5 ppm in terms of Mn atom (element) relative to the weight of the polyester composition) and an ethylene glycol slurry of antimony trioxide (250.5 ppm in terms of Sb atom (element) relative to the weight of the polyester composition). Thereafter, 5 parts by weight of ethylene glycol (0.15 times the molar ratio of the terephthalic component) was added to proceed with depolymerization, and then an ethylene glycol solution of phosphoric acid (40.8 ppm as P atom (element) relative to the weight of the polyester composition) and sodium dihydrogen phosphate dihydrate (14.0 ppm as Na atom (element) and 18.9 ppm as P atom (element) relative to the weight of the polyester composition) and an ethylene glycol slurry (volume average diameter 1.0 μm) with a concentration of calcium carbonate particles of 20 wt % were mixed together to form a homogeneous slurry, which was then added (1 part by weight as calcium carbonate). Thereafter, the temperature inside the polymerization apparatus was gradually raised to 290° C., and the pressure was reduced from normal pressure to 250 Pa or less, and polymerization reaction was carried out until a predetermined stirring torque was shown at 290° C. After the polymerization reaction was completed, the reaction system was returned to normal pressure with nitrogen gas, and the molten polyester in the polymerization apparatus was discharged into a water tank in the form of a strand, cooled, and then cut to obtain a pellet-shaped polyester composition (intrinsic viscosity 0.62).
Example 1
As the resin constituting the A layer, 50 parts by weight of PET-A and 50 parts by weight of MB-A were blended and dried under reduced pressure at 160°C for 2 hours, and then fed into the extruder for the A layer. As the resin constituting the B layer, 100 parts by weight of PET-A was dried under reduced pressure at 160°C for 2 hours, and then fed into the extruder for the B layer. In the extruder, each raw material was melted at 280°C, and laminated in a layer merging block to form a two-layer laminate consisting of the A layer and the B layer. Then, it was extruded onto a casting drum with a surface temperature of 25°C to produce a laminated sheet having a two-layer structure. Next, the sheet was preheated with a heated roll group, stretched 3.5 times in the longitudinal direction at a temperature of 90°C, and then cooled with a roll group at a temperature of 25°C to obtain a uniaxially stretched film. While both ends of the obtained uniaxially stretched film were held with clips, it was stretched 4.0 times in the width direction perpendicular to the longitudinal direction in a heating zone at a temperature of 110°C in a tenter. The film was then heat-set for 10 seconds at 230°C in the heat treatment zone of the tenter. The film was then uniformly cooled in the cooling zone and wound up to obtain a biaxially oriented polyester film with a thickness of 25 μm. The film's properties are shown in the table. The film had almost no free calcium content, no particle aggregation or coarse foreign matter formation, and excellent inspectability and transportability.
(Examples 2 to 5)
A biaxially oriented polyester film having a thickness of 25 μm was obtained in the same manner as in Example 1, except that the compounding ratio of MB-A (composition of A layer and B layer) was changed as shown in the table.
The biaxially oriented polyester film obtained in Example 2 had excellent inspectability.
The biaxially oriented polyester films obtained in Examples 3 to 5, like Example 1, had a low free calcium content and were excellent in inspectability and transportability.
Example 6
The film structure was a single layer of MB-A, and a biaxially oriented polyester film having a thickness of 25 μm was obtained by the method of Example 1. The obtained biaxially oriented polyester film had a low free calcium content and was excellent in inspectability and transportability, similar to Example 1.
(Example 7)
In preparing MB-A, the amount of calcium carbonate added was increased so that the content was as shown in the table, and a biaxially oriented polyester film having a thickness of 25 μm was obtained as a single film by the method of Example 6. The obtained biaxially oriented polyester film had no problems with inspectability or transportability.
(Comparative Example 1)
The composition of both the A layer and the B layer of the film was PET-A, and a biaxially oriented polyester film having a thickness of 25 μm was obtained by the method of Example 1. The obtained biaxially oriented polyester film had poor transportability because it did not contain calcium carbonate particles.

(Comparative Example 2)
In preparing MB-A, an ethylene glycol solution of phosphoric acid and sodium dihydrogen phosphate dihydrate was added, followed by the addition of an ethylene glycol slurry containing 20 wt % calcium carbonate particles (volume average diameter 1.0 μm) (1 part by weight as calcium carbonate), to obtain a biaxially oriented polyester film having a thickness of 25 μm in the same manner as in Example 1. The obtained biaxially oriented polyester film had poor inspectability due to an increase in the free calcium content, the formation of coarse foreign matter, and the aggregation of particles, due to insufficient protection of the calcium carbonate particle surfaces by the phosphorus compound.

(Examples 8 to 11)
A biaxially oriented polyester film having a thickness of 25 μm was obtained in the same manner as in Example 1, except that the volume average diameter of the calcium carbonate particles used in MB-A was changed. The biaxially oriented polyester film obtained in Example 7 had excellent inspectability. The biaxially oriented polyester films obtained in Examples 8 and 9 had low free calcium contents and excellent inspectability and transportability, similar to Example 1. The biaxially oriented polyester film obtained in Example 11 had excellent transportability.
(Examples 12 to 14)
In preparing MB-A, a biaxially oriented polyester film having a thickness of 25 μm was obtained in the same manner as in Example 1, except that the phosphorus species added was changed as shown in the table. The biaxially oriented polyester films obtained in Examples 12 and 13 were excellent in transportability. The biaxially oriented polyester film obtained in Example 14 was inferior to Example 1, but had no problems with inspectability and transportability.
(Examples 15 to 17)
In preparing MB-A, a biaxially oriented polyester film having a thickness of 25 μm was obtained in the same manner as in Example 1, except that the amount of phosphorus added was changed as shown in the table. The biaxially oriented polyester films obtained in Examples 15 and 16 had excellent transportability. The biaxially oriented polyester film obtained in Example 17 had a low free calcium content and excellent inspectability and transportability, similar to Example 1.

Claims (4)

A biaxially oriented polyester film containing calcium carbonate particles treated with a buffer solution containing phosphoric acid and an alkali metal phosphate , and satisfying the following formula (I):
Free calcium content/total calcium content < 1.0E-2 (I)
(The free calcium content was determined by dissolving a polyester film in o-chlorophenol at 150°C for 1 hour, centrifuging the resulting solution at a centrifugal acceleration of 40,900G at 20°C for 1 hour, decanting the resulting supernatant, adding acetone to cause precipitation, collecting the precipitate by filtration, and drying in vacuum at 150°C for 12 hours to obtain a polyester composition containing calcium.) The biaxially oriented polyester film according to claim 1, having a total calcium content of 4,500 ppm or less. The biaxially oriented polyester film according to claim 1 or claim 2, in which the volume average diameter of the calcium carbonate particles is 3.0 μm or less. The biaxially oriented polyester film according to any one of claims 1 to 3, wherein the free calcium content and the total calcium content after heat treatment at 300°C for 10 hours in a nitrogen atmosphere satisfy the following formula (II):
Free calcium content after heat treatment/total calcium content < 1.5E-2 (II)