JP2022048523A - Biaxially-oriented polyester film - Google Patents

Abstract

To provide a biaxially-oriented polyester film which is excellent in inspectability and transportation property.MEANS FOR ACHIEVING THE PURPOSE: A biaxially-oriented polyester film contains calcium carbonate particles and satisfies a formula (I) written below. (free calcium content)/(total calcium content)<1.0E-2 (I), (the free calcium content is a calcium content in a polyester composition obtained by dissolving a polyester film in o-chlorophenol at 150°C for 1 hour, subsequently being separated by centrifugation with centrifugal acceleration of 40,900 G at 20°C for 1 hour, collecting an obtained supernatant solution by decantation, adding acetone thereto for precipitation, collecting the precipitate by filtration, and vacuum drying it at 150°C for 12 hours).SELECTED DRAWING: None

Description

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

The release film based on polyester is used for various purposes such as a laminated ceramic capacitor (hereinafter sometimes referred to as MLCC), a liquid crystal polarizing plate release film, and a photoresist. For a general release film, it is necessary to reduce the stacking deviation when the release object is provided on the release film and the defects at the time of lamination, and further, the release film and the release film provided on the release film. Releasability from the mold object is required. In addition, in the case of mold release of a polarizing plate, the release film is subjected to a cross Nicol inspection while being bonded to the polarizing plate, so that the foreign matter in the release film is reduced so as not to interfere with the cross Nicol inspection (in the cross Nicol inspection). Easy inspection) is required. As a method for improving the transportability and releasability of a release film, a technique of blending an appropriate amount of particles in a polyester film to form fine protrusions on the film surface has been known so far. Patent Document 1 uses inorganic particles such as aggregated silica, spherical silica, and calcium carbonate. Further, as a method for adjusting the haze of a film, a technique of blending an appropriate amount of particles in a polyester film to increase the opacity of the film is known. For example, Patent Document 2 uses a calcium compound.

Japanese Unexamined Patent Publication No. 8-124150 Special Fair 6-86537 Gazette

However, when the content of inorganic particles in the polyester composition is increased in order to improve the transportability and releasability of the release film, coarse particles are likely to be generated due to aggregation, and the inspectability is deteriorated. there were.

An object of the present invention is to eliminate the drawbacks of the prior art. That is, it is an object of the present invention to provide a biaxially oriented polyester film having excellent inspectability and transportability by reducing agglomerated foreign substances derived from inorganic particles added to the film.

The object of the present invention is achieved by the following means.
(1) A biaxially oriented polyester film containing calcium carbonate particles and satisfying the following formula (I).
Free calcium content / total calcium content <1.0E-2 (I)
(For the free calcium content, dissolve the polyester film in o-chlorophenol at 150 ° C. for 1 hour, centrifuge at a centrifugal acceleration of 40900 G and 20 ° C. for 1 hour, and collect the obtained supernatant solution by decantation and acetone. (Calcium content in polyester composition obtained by centrifuging, collecting the precipitate by filtration, and vacuum drying at 150 ° C. for 12 hours).
(2) The biaxially oriented polyester film according to (1), which has a total calcium content of 4500 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) Biaxial orientation according to any one of (1) to (3), wherein the free calcium content and the total calcium content after heat treatment at 300 ° C. and 10 hr under a nitrogen atmosphere satisfy the following formula (II). Polyester film.
Free calcium content after heat treatment / total calcium content <1.5E-2 (II)

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

The present invention will be described in detail below.
The biaxially oriented polyester film of the present invention needs to 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 is the supernatant obtained by dissolving the biaxially oriented polyester film in o-chlorophenol and then centrifuging at 20 ° C. for 1 hr after dissolving the biaxially oriented polyester film in o-chlorophenol. It is the calcium content in the polyester composition from which the particles obtained by adding acetone to the solution and precipitating and recovering by filtration are removed. In a film containing calcium carbonate particles, free calcium is generated from the calcium carbonate particles due to the heat history at the time of film formation. The generated free calcium forms a by-product with the polyester composition, which causes agglomeration of particles and formation of coarse foreign substances. By setting the amount of free calcium in the above range, it is possible to suppress the aggregation of calcium carbonate particles, and it is possible to obtain a film having excellent inspectability and transportability.
As a result of diligent studies, the biaxially oriented polyester film of the present invention suppresses the generation of free calcium from calcium carbonate particles by mixing calcium carbonate particles and a phosphorus compound and then adding them to the polyester polymerization system. It is a thing.
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. Within the above range, agglomeration of particles 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 Example (1) below, 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 further preferably 1.5 μm or less. The lower limit is preferably 0.1 μm or more, and more preferably 0.5 μm or more. Within the above range, it is possible to suppress the formation of coarse foreign substances in the particles, and the inspectability and transportability in the cross Nicol inspection are improved.

The calcium content in the polyester of the biaxially oriented polyester film of the present invention after being heat-treated at 300 ° C. for 10 hours in a nitrogen atmosphere preferably satisfies the following formula (II) and is less than 1.0E-2. Is even more preferable.
Free calcium content after heat treatment / total calcium content <1.5E-2 (II)
By setting the ratio of the amount of free calcium to the total calcium content after the heat treatment within the above range, it is possible to prevent the heat-deteriorated polyester composition, which is melt-extruded during film formation, from binding to the calcium component and becoming a foreign substance. Can be done.
The biaxially oriented polyester film of the present invention is suitably used as a release film for polarizing plate release applications. The application is necessary to prevent scratches in the processing process of the polarizing plate and to hold a glue for attaching the polarizing plate to the display substrate. The polyester film of the present invention is subjected to a mold release treatment, and is attached to a polarizing plate coated with a glue in a rolled state, and is peeled off when the polarizing plate is punched out and attached to a display substrate.
Next, a method for producing a 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 polycondensing a dicarboxylic acid component and a diol component.
The dicarboxylic acid components constituting the polyester composition include terephthalic acid, 2,6-naphthalenedicarboxylic acid, isophthalic acid, diphenyl4,4'-dicarboxylic acid, 5-sophthalic sulfisophthalic acid, oxalic acid, succinic acid, and adipic acid. , Sevacinic acid, malonic acid, dimeric acid, or an ester thereof. Among them, aromatic dicarboxylic acids and ester-forming derivative components thereof are preferable from the viewpoints of mechanical properties, heat resistance, and hydrolysis resistance. In particular, terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid and their ester-forming derivative components are preferable from the viewpoint of polymerizable and mechanical properties. Further, as the diol component constituting the polyester composition, ethylene glycol and 1,2-propane are used. Alicyclic diols such as diols, 1,3-propanediols, butanediols, 2-methyl-1,3-propanediols, hexanediols, and neopentyl glycols, and alicyclic diols such as cyclohexanedimethanol, cyclohexanediethanol, and norbornandi. Saturated alicyclic primary diols such as methanol, norbornandiethanol, tricyclodecanedimethanol, tricyclodecanediethanol, decalindiethanol, decalindiethanol, saturated heterocyclic primary diols containing cyclic ethers such as isosorbide, and other cyclohexanediols. 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, Adamantanediol and other alicyclic diols, paraxylene glycol, bisphenol A, bisphenol S, styrene glycol, 9,9-bis (4-( Examples thereof include aromatic ring-type diols such as 2-hydroxyethoxy) phenyl) fluorene and 9,9'-bis (4-hydroxyphenyl) fluorene, and a series of the above-mentioned diols. Among these, diols having a boiling point of 230 ° C. or lower are preferable because they are easily distilled out of the reaction system, and aliphatic diols are more preferable because they are low in cost and have high reactivity. Further, ethylene glycol is particularly preferable from the viewpoint of mechanical properties.
The method for producing the polyester composition can be produced by a conventionally known method, but is not limited thereto. For example, it can be produced from the following two steps using a dicarboxylic acid component or an ester thereof and a diol component as main raw materials. That is, it is a first-step step consisting of (A) esterification reaction or (B) transesterification reaction, and a second-step step consisting of (C) polycondensation reaction following it.
In the step (A) esterification reaction of the first step, the dicarboxylic acid and the diol are esterified at a predetermined temperature and reacted until a predetermined amount of water is distilled off to obtain a low polymer. It is a process. (B) The step of transesterification reaction is a step of transesterifying a dicarboxylic acid ester and a diol and carrying out the reaction until a predetermined amount of alcohol is distilled off to obtain a low polymer.
Of the second-step steps, (C) polycondensation reaction is a step of obtaining a polyester composition from a low polymer obtained by (A) esterification reaction or (B) transesterification reaction.

In the method for producing a polyester composition of the present invention, a known polycondensation catalyst can be used as the catalyst used for the (C) polycondensation reaction. For example, compounds such as antimony, titanium, aluminum, tin and germanium can be mentioned. Among these, it is preferable to use an antimony compound as a polycondensation reaction catalyst from the viewpoint of polymerization time and economy.

During the polycondensation reaction of the polyester composition in the biaxially oriented polyester film of the present invention, it is preferable that the calcium carbonate particles are mixed with the phosphorus compound in advance and then added into the polycondensation reactor. By mixing in advance, the phosphorus compound protects the surface of the calcium carbonate particles, and even if it receives a heat history, it is possible to suppress the elution of the calcium component free from the calcium carbonate particles, resulting in aggregation and coarseness of the particles. It becomes possible to provide a polyester film having few protrusions.
In addition, it is preferable that calcium carbonate and a phosphorus compound are sufficiently mixed in the form of addition, and it is preferable to add as a mixed slurry. The solvent at this time is preferably the same as 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 in combination as the phosphorus compound to be added by mixing with calcium carbonate particles. Phosphoric acid alone has a strong acidity, and calcium may elute when mixed with calcium carbonate particles, so it is preferable to mix as a phosphate buffer solution. Further, by adding the phosphorus compound, the hydrolysis resistance and heat resistance of the polyester composition are improved, and the amount of linear oligomer generated when the film is heat-treated or heat-processed can be suppressed. Other phosphorus compounds used include trimethylphosphoric acid, ethyldiethylphosphonoacetate, phosphite, disodium hydrogen phosphate, trisodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, Examples thereof include ethyl diethylphosphonoacetate, 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 with respect 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, and more preferably 70 ppm or less. Within the above range, calcium carbonate particles can be sufficiently protected with a phosphorus compound, and calcium elution can be suppressed. In addition, the reaction can proceed without inhibiting the polycondensation reaction of the polyester composition.
Next, a method for producing the biaxially oriented polyester film of the present invention will be described. This is an example, and the present invention is not construed as being limited to those obtained by such an example.

A method (melt casting method) in which the polyester composition is heated and melted in an extruder and extruded from a base onto a cooled cast drum to be processed into a sheet can be used. At this time, in order to remove the unmelted matter in the polyester composition, it may be filtered by a fiber sintered stainless metal filter. Further, various additives such as compatibilizers, plasticizers, weather resistant agents, antioxidants, heat stabilizers, lubricants, antistatic agents, conductive materials, and ultraviolet absorbers are used as long as they do not impair the effects of the present invention. Agents, flame retardants, flame retardants and the like may be added.

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

In the case where the base material of the laminated film has a laminated structure, two different polyester compositions are put into two extruders, melted, merged, and co-extruded from a base onto a cooled cast drum. A method of processing into a sheet shape (a method of melt-forming a film by a coextrusion method) can be preferably used. Hereinafter, this method will be described in detail.

First, the polyester composition is put into an extruder and heated and melted. Laminate using multiple extruders, multi-layer manifolds or confluence blocks (eg, confluence blocks with short confluences), extrude the sheet from the mouthpiece and cool with a casting roll to make an unstretched film. At this time, the polyester composition melted by the extruder is filtered by a filter. Since even a very small foreign substance becomes a coarse protrusion defect when it enters the film, it is effective to use a filter having a high accuracy of collecting 95% or more of a foreign substance having a size of, for example, 5 μm or more. Then, it is adhered and cooled and solidified by static electricity on a drum cooled to a surface temperature of 10 to 60 ° C. to prepare an unstretched sheet. Next, this unstretched sheet is guided to a roll group heated to a temperature of 70 to 140 ° C., stretched 3 to 4 times in the longitudinal direction (longitudinal direction, that is, the traveling direction of the sheet), and a temperature roll of 20 to 50 ° C. Cool in groups. Subsequently, the sheet is guided to the tenter while grasping both ends with clips, and is stretched 3 to 5 times in the 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 each of the longitudinal direction and the width direction, and the area ratio (longitudinal stretching ratio × lateral stretching ratio) is preferably 9 to 17 times. If the area magnification is less than 9 times, the durability of the obtained biaxially stretched sheet becomes insufficient, and conversely, if the area magnification exceeds 17 times, tearing tends to occur easily during stretching. Further, after stretching, a relaxation treatment of 0.1% or more and less than 5.0% may be performed in the longitudinal and / or width directions. As the biaxial stretching method, in addition to the sequential secondary stretching method in which the longitudinal direction and the width direction stretching are separated as described above, the simultaneous biaxial stretching in which the longitudinal direction and the width direction stretching are performed at the same time. It doesn't matter which method you use.

Hereinafter, the present invention will be described in more detail with reference to examples. The physical property values in the examples were measured by the following methods.

(1) Quantification of total calcium atoms (elements) in polyester film (unit: ppm (mass ratio))
After finely cutting the polyester film, it was molded into a columnar shape with a melting press machine, and quantification was performed using a fluorescent X-ray analyzer (model number: 3270) manufactured by Rigaku Denki Co., Ltd.
(2) Quantification 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 40900 G and 20 ° C. for 1 hour. The obtained supernatant solution was collected by decantation, and 500 mL of acetone was added to precipitate the precipitate. The precipitate was collected by filtration and vacuum dried at 150 ° C. for 12 hours, and quantified by the analysis method (1). As a centrifuge, a himac CR20G (rotor: R19A) manufactured by Hitachi was used.
(3) Measurement of average particle volume in polyester film (unit: μm)
The polyester film was plasma-treated, and the volume average diameter of the particles was measured with a Hitachi field emission scanning electron microscope (model number S-4000) and a Nideco SEM-IMAGENALYZER (model number Ludex AP). In addition, when analyzing the particle size, measurements of 20 fields of view or more were performed at a magnification of 5000 times, the equivalent circle diameter was measured from at least 200 particles, and the volume average diameter was calculated by regarding it as a pseudo three-dimensional sphere. ..

(4) Quantification of calcium atoms (elements) in the polyester film after heat treatment (unit: ppm)
The polyester film was vacuum dried at 150 ° C. for 3 hours and then at 180 ° C. for 7.5 hours as a pretreatment, heated and melted at 300 ° C. for 10 hours under nitrogen flow, and then rapidly cooled in water. The content of particles of the obtained composition was calculated by the analysis method of (1) above.

(5) Quantification of free calcium atoms (elements) in the polyester film after heat treatment (unit: ppm)
After the polyester film was heat-melted by the method (4), calcium atoms (elements) were quantified by the method (2).
(6) Inspectability evaluation An LED light (HBLF-WSL1500, HBLF-WSL700 manufactured by Atto) and a first polarizing plate capable of adjusting the angle are provided as lighting means, and a CCD camera having a resolution of 50 μm (GMFMB3B80 manufactured by Hutec) is provided as a light receiving means. And a second polarizing plate capable of adjusting the angle were combined, and a plurality of cross Nicol testers were used. The film 100 m was inspected without changing the detection sensitivity of the camera of the Cross Nicol inspector, 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 measured and evaluated as an index of inspectability (each). 〇 or above was passed).
〇: The number of bright spots of 200 μm or more is 0.6 / m ² or less.
Δ: The number of bright spots of 200 μm or more is 0.6 to 1.0 / m ² or less.
X: The number of bright spots of 200 μm or more is 1.0 / m ² or more.

(7) Transportability evaluation Using a slip tester (manufactured by Toyo Seiki Co., Ltd.), the value when the A layer surface and the B layer surface of the two laminated films are overlapped and rubbed according to JIS K 7125 (1999). Was measured three times, and the coefficient of static friction in the longitudinal direction μs was obtained from the average value. The static friction coefficient μs was evaluated according to the following criteria as an index of transportability.
⊚: More than 0.1 and 0.3 or less ◯: More than 0.3 and 0.35 or less Δ: More than 0.35 and 0.5 or less ×: More than 0.5.
[Preparation of PET-A] In an esterification reactor charged with 105 parts by weight of BHT (bishydroxyethyl terephthalate) melted at 250 ° C., 86 parts by weight of terephthalic acid and 37 parts by weight of ethylene glycol (1 part by weight with respect to terephthalic acid). A slurry consisting of .15 times mol) is gradually added to proceed the esterification reaction. The temperature in the reaction system was controlled to be 245 to 250 ° C., and the esterification reaction was terminated when the reaction rate reached 95% to obtain BHT.
105 parts by weight (equivalent to 100 parts by weight of PET) of BHT was charged from the esterification reactor into the polymerization apparatus in a molten state, and the temperature was adjusted to 255 ° C. Here, an ethylene glycol solution of manganese acetate tetrahydrate (40 ppm as Mn atom (element) with respect to the weight of the polyester composition) and an ethylene glycol slurry of diantimony trioxide (Sb atom (element) with respect to the weight of the polyester composition). 100 ppm) and phosphoric acid (35 ppm as a phosphorus atom (element) with respect to the weight of the polyester composition) were added. Then, the inside of the polymerization apparatus was gradually heated to 290 ° C., the pressure was reduced from normal pressure to 250 Pa or less, and the polymerization reaction was carried out at 290 ° C. until a predetermined stirring torque was exhibited. After completion of the polymerization reaction, the inside of the reaction system is made normal pressure with nitrogen gas, the molten polyester in the polymerization apparatus is discharged into a water tank in a strand shape, cooled, and then cut into a pellet-shaped polyester composition (intrinsic viscosity 0.62). ) Was obtained.
[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 (equivalent to 100 parts by weight of PET) of BHT was charged from the esterification reactor into the polymerization apparatus in a molten state, and the temperature was adjusted to 255 ° C. Here, an ethylene glycol solution of manganese acetate tetrahydrate (23.5 ppm as Mn atom (element) with respect to the weight of the polyester composition) and an ethylene glycol slurry of antimony trioxide (Sb atom (element) with respect to the weight of the polyester composition). ), 250.5 ppm) was added. Then, 5 parts by weight of ethylene glycol (0.15 times the molar amount of the terephthal component) was additionally added to proceed with depolymerization, and then phosphoric acid (40.8 ppm as a P atom (element) with respect to the weight of the polyester composition) and phosphorus. Ethylene glycol solution of sodium dihydrogen acid dihydrate (14.0 ppm as Na atom (element) and 18.9 ppm as P atom (element) with respect to the weight of polyester composition) and the concentration of calcium carbonate particles are 20 wt%. All ethylene glycol slurrys (volume average diameter 1.0 μm) were mixed to make a homogeneous slurry (1 part by weight as calcium carbonate). Then, the inside of the polymerization apparatus was gradually heated to 290 ° C., the pressure was reduced from normal pressure to 250 Pa or less, and the polymerization reaction was carried out at 290 ° C. until a predetermined stirring torque was exhibited. After completion of the polymerization reaction, the inside of the reaction system is made normal pressure with nitrogen gas, the molten polyester in the polymerization apparatus is discharged into a water tank in a strand shape, cooled, and then cut into a pellet-shaped polyester composition (intrinsic viscosity 0.62). ) Was obtained.
(Example 1)
PET-A was blended into 50 parts by weight and MB-A in 50 parts by weight as the resin constituting the A layer, dried under reduced pressure at 160 ° C. for 2 hours, and then put into an extruder for the A layer. Further, 100 parts by weight of PET-A as a resin constituting the B layer was dried under reduced pressure at 160 ° C. for 2 hours, and then put into an extruder for the B layer. Each raw material was melted at 280 ° C. in an extruder and merged and laminated by a merging block for layers to form a two-layer laminate consisting of an A layer and a B layer. Then, it was extruded onto a casting drum having a surface temperature of 25 ° C. to prepare a laminated sheet having a two-layer structure. Subsequently, the sheet was preheated with a heated roll group, then stretched 3.5 times in the longitudinal direction at a temperature of 90 ° C., and then cooled with a roll group having a temperature of 25 ° C. to obtain a uniaxially stretched film. .. While gripping both ends of the obtained uniaxially stretched film with clips, the film was stretched 4.0 times in the width direction perpendicular to the longitudinal direction in a heating zone having a temperature of 110 ° C. in the tenter. Further, subsequently, heat fixing was performed for 10 seconds at a temperature of 230 ° C. in the heat treatment zone in the tenter. Then, after slowly cooling uniformly in the cooling zone, the film was wound to obtain a biaxially oriented polyester film having a thickness of 25 μm. Each property of the film is shown in the table. There was almost no free calcium content, there was no agglutination of particles and no coarse foreign matter, and it was excellent in inspectability and transportability.
(Examples 2 to 5)
A biaxially oriented polyester film having a thickness of 25 μm was obtained by the same method as in Example 1 except that the compounding ratio of MB-A (composition of layer A and layer B) was changed as shown in the table.
The biaxially oriented polyester film obtained in Example 2 was excellent in inspectability.
The biaxially oriented polyester films obtained in Examples 3 to 5 had a low free calcium content and were excellent in inspectability and transportability, as in Example 1.
(Example 6)
A biaxially oriented polyester film having a thickness of 25 μm was obtained by the method of Example 1 with the film composition as a single film of MB-A only. Similar to Example 1, the obtained biaxially oriented polyester film had a low free calcium content and was excellent in inspectability and transportability.
(Example 7)
In the preparation of MB-A, the amount of calcium carbonate added was increased so that the content of calcium carbonate 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 problem in inspectability and transportability.
(Comparative Example 1)
A biaxially oriented polyester film having a thickness of 25 μm was obtained by the method of Example 1 with the composition of both the A layer and the B layer of the film as PET-A. Since the obtained biaxially oriented polyester film does not contain calcium carbonate particles, it has poor transportability.

(Comparative Example 2)
In the preparation of MB-A, after adding an ethylene glycol solution of phosphoric acid and sodium dihydrogen phosphate dihydrate, an ethylene glycol slurry (volume average diameter 1.0 μm) having a calcium carbonate particle concentration of 20 wt% was added. A biaxially oriented polyester film having a thickness of 25 μm was obtained by the same method as in Example 1 except (1 part by weight as calcium carbonate). In the obtained biaxially oriented polyester film, the surface of the calcium carbonate particles was not sufficiently protected by the phosphorus compound, so that the free calcium content increased, coarse foreign matter and particle agglomeration occurred, and the inspectability was poor. ..

(Examples 8 to 11)
A biaxially oriented polyester film having a thickness of 25 μm was obtained by the same method as in Example 1 except that the volume average diameter of the calcium carbonate particles used for MB-A was changed. The biaxially oriented polyester film obtained in Example 7 was excellent in inspectability. Further, the biaxially oriented polyester films obtained in Examples 8 to 9 had a low free calcium content and were excellent in inspectability and transportability, as in Example 1. The biaxially oriented polyester film obtained in Example 11 was excellent in transportability.
(Examples 12 to 14)
A biaxially oriented polyester film having a thickness of 25 μm was obtained by the same method as in Example 1 except that the phosphorus species to be added was changed as shown in the table in the preparation of MB-A. 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 that in Example 1, but had no problem in inspection and transportability.
(Examples 15 to 17)
A biaxially oriented polyester film having a thickness of 25 μm was obtained by the same method as in Example 1 except that the amount of phosphorus added was changed as shown in the table when producing MB-A. 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 was excellent in inspectability and transportability, as in Example 1.

Claims (4)

A biaxially oriented polyester film containing calcium carbonate particles and satisfying the following formula (I).
Free calcium content / total calcium content <1.0E-2 (I)
(For the free calcium content, dissolve the polyester film in o-chlorophenol at 150 ° C. for 1 hour, centrifuge at a centrifugal acceleration of 40900 G and 20 ° C. for 1 hour, and collect the obtained supernatant solution by decantation and acetone. (Calcium content in polyester composition obtained by centrifuging, collecting the precipitate by filtration, and vacuum drying at 150 ° C. for 12 hours). The biaxially oriented polyester film according to claim 1, wherein the total calcium content is 4500 ppm or less. The biaxially oriented polyester film according to claim 1 or 2, wherein 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. and 10 hr under a nitrogen atmosphere satisfy the following formula (II).
Free calcium content after heat treatment / total calcium content <1.5E-2 (II)