JP7537145B2 - Polyester Film - Google Patents

Description

The present invention relates to a polyester film with excellent inspectability.

Due to its good workability, polyester films are used in various industrial fields. For example, films with a polyester layer as a substrate and a releasable resin layer (a layer formed by applying silicone resin or epoxy resin) are suitable for use in release applications. Release films with a polyester layer as a substrate are used for various release applications such as ceramic green sheet production, liquid crystal polarizing plate release, and photoresist. A general release film is required to reduce the lamination misalignment and defects during lamination when a release object is placed on the release film, and to have releasability between the release film and the release object placed on the release film. In addition, since the presence of foreign matter on the release film can cause uneven coating and coating omissions when applying a resin layer, there is a high demand for reducing foreign matter on the release film. In particular, for polarizing plate release, the release film undergoes a crossed Nicol test while still attached to the polarizing plate, so it is also 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). On the other hand, since inspections are also performed using reflected light and transmitted light, it is also necessary to adjust the film haze of the release film to a suitable range. To date, a technique for improving the transportability and releasability of release films is known in which an appropriate amount of particles is incorporated into a polyester film to form fine protrusions on the film surface. Patent Document 1 uses inorganic particles such as aggregated silica, spherical silica, and calcium carbonate. In addition, a technique for adjusting the haze of a film is known in which an appropriate amount of particles is incorporated into a polyester film to increase the opacity of the film; for example, Patent Document 2 uses a calcium compound.

Japanese Patent Application Publication No. 8-124150 Special Publication No. 6-86537 JP 2005-138538 A

However, when the content of inorganic particles in polyester is increased, coarse particles are easily generated due to aggregation of inorganic particles. For example, in the polyester film for magnetic tape described in Patent Document 3, the added inorganic particles aggregate to form coarse protrusions, which cause defects such as dropouts and cause deterioration of scraping properties due to the falling off of aggregated particles. Similarly, in the case of release films, there is a problem in that coarse foreign matter of the added inorganic particles falls off during transport in downstream processes and contaminates the downstream processes. Another problem is that when a large amount of inorganic particles are added to improve the transportability, releasability, or film haze of the film, the coarse foreign matter formed by the aggregation of inorganic particles deteriorates the inspectability.

The present invention aims to eliminate the drawbacks of the conventional technology. That is, the present invention aims to provide a polyester film in which the particles contained in the film are organic particles, thereby suppressing the deterioration of inspectability due to the aggregation of inorganic particles and the contamination of downstream processes due to the falling off of inorganic particles.

In order to solve the above problems, the present invention has the following configuration.
[I] A polyester film containing organic particles, which is substantially free of inorganic particles, and in which, when a volume-based particle size distribution of the organic particles contained in the polyester film is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, the polyester film has one or more maximum values in the particle diameter range of 0.2 μm or more and 0.4 μm or less and one in the particle diameter range of 0.8 μm or more and 1.0 μm or less, and the arithmetic average surface roughness Ra of the polyester film is 5 nm or more on both sides.
[II] A polyester film according to [I], which is composed of two or more layers, at least one of which contains organic particles (such layer is referred to as Layer A), and which is substantially free of inorganic particles, and in which, when a volume-based particle size distribution of the organic particles contained in Layer A is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the abundance ratio of the particles, the organic particles have one or more maximum values in the particle diameter range of 0.2 μm to 0.4 μm and in the range of 0.8 to 1.0 μm, respectively, and in the plot of the organic particles contained in Layer A, the ratio (a/b) of the abundance ratio a of the maximum peak top having the highest value in the particle diameter range of 0.2 μm to 0.4 μm to the abundance ratio b of the maximum peak top having the highest value in the particle diameter range of 0.8 μm to 1.0 μm is 0.5 to 6.0.
[III] The polyester film according to [II], wherein the layer A is present on at least one surface layer, and the arithmetic mean surface roughness Ra of the surface having the layer A is 20 nm or more and 30 nm or less.
[IV] A polyester film according to [I], which is a three-layer polyester film, comprising organic particles in both surface layers (such layers are referred to as Layer A and Layer A'), wherein neither Layer A nor Layer A' substantially contains inorganic particles, wherein when the volume-based particle size distribution of the organic particles contained in Layer A and Layer A' is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the abundance ratio of the particles, each of the organic particles has one or more maximum values in the particle diameter range of 0.2 μm to 0.4 μm and in the particle diameter range of 0.8 μm to 1.0 μm, respectively, and in the plot of the organic particles contained in Layer A and Layer A', the ratio (a/b) of the abundance ratio a of the maximum peak top having the highest value in the particle diameter range of 0.2 μm to 0.4 μm to the abundance ratio b of the maximum peak top having the highest value in the particle diameter range of 0.8 μm to 1.0 μm is 0.5 to 6.0.
[V] The polyester film according to [IV], wherein both the surface having the layer A and the surface having the layer A' have an arithmetic mean surface roughness Ra of 20 nm to 30 nm.
[VI] The polyester film according to [IV] or [V], wherein a layer having no surface (such layer is referred to as Layer B) substantially does not contain inorganic particles, and when a volume-based particle size distribution of organic particles contained in Layer B is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, the polyester film has one or more maximum values in a particle diameter range of 0.8 μm or more and 1.0 μm or less.
[VII] The polyester film according to any one of [I] to [VI], having a film haze of 7% or more and 13% or less.
[VIII] The polyester film according to any one of [I] to [VII], which is used for release purposes.
[IX] The polyester film according to any one of [I] to [VII], which is used for a polarizing plate release agent.

The polyester film of the present invention is a film with favorable inspectability and releasability, while suppressing process contamination due to the particles contained therein. In particular, when used for polarizing plate release, the effects of coarse particles that hinder crossed Nicols inspection when inspecting polarizing plates for defects are suppressed.

FIG. 1 is a diagram showing a schematic diagram of a defect observation method using a reflection inspection instrument. FIG. 1 is a diagram showing a schematic diagram of a defect observation method using a transmission inspection device. FIG. 1 is a diagram showing a schematic diagram of a bright spot defect observation method using a crossed Nicol inspection tool.

The present invention is described in detail below.

The polyester in the polyester film of the present invention has a dicarboxylic acid component and a diol component. In this specification, the component refers to the smallest unit that can be obtained by hydrolyzing the polyester.

In order to implement the present invention, it is preferable to use terephthalic acid as the dicarboxylic acid component constituting the polyester in an amount of 30 mol% or more based on the total dicarboxylic acid components. Examples of dicarboxylic acid components other than terephthalic acid include aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic 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-sodiumsulfoisophthalic acid, phenylendanedicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, and 9,9'-bis(4-carboxyphenyl)fluorene acid, or ester derivatives thereof, but are not limited thereto.

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 in which multiple diols are linked together.

The polyester of the present invention can be produced by a known method. Specifically, the esterification step is carried out using one or more esterification reaction tanks under stirring. For example, when a single esterification reaction tank is used, 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 step is usually 95% or more.

The melt polycondensation process can usually be carried out in a continuous or batch system using one or more polycondensation reaction tanks, and is carried out while gradually reducing the pressure from normal pressure and distilling the ethylene glycol produced under heating and stirring out of the system. For example, in the case of a batch system using a single polycondensation reaction tank, the reaction temperature is usually 250 to 290°C, the final absolute pressure, which is gradually reduced from normal pressure, is usually 1.3 to 0.013 kPa (10 to 0.1 Torr), and the reaction time is usually 1 to 20 hours.

The intrinsic viscosity of polyester resin can be determined by the polymerization end point based on the stirring torque of the polymer. When the stirring torque is high, the melt viscosity of the polymer is high and the intrinsic viscosity is also high. The stirring torque for determining the end point of the polymerization apparatus can be set so that the target intrinsic viscosity is obtained. In the present invention, it is preferable to set the stirring torque for determining the end point so that the IV of the film is 0.60 or more and 0.70 or less. By setting it in this range, it is easy to control the surface roughness, which is preferable.

The resulting polyester resin after polymerization is discharged in the form of strands from the bottom of the polymerization apparatus and cut with a cutter while being cooled with water. The chip shape can be controlled by cutting, so polyester chips with the desired bulk density can be obtained in the present invention.

The polycondensation reaction catalyst used in the present invention may be one or more of antimony trioxide, antimony pentoxide, antimony acetate, antimony glycolate, germanium dioxide, and organic titanium compounds. Among these, antimony trioxide is preferred in terms of the transparency of the resulting polyester and its availability.

The polyester film of the present invention is characterized in that it does not substantially contain inorganic particles, but contains organic particles. When inorganic particles are contained in a polyester film, it is possible to improve the releasability and transportability of the film and adjust the film haze so that the inspectability is good, but there is a risk that the metal elements of the inorganic particles contained in the film will aggregate and become foreign matter inside the film, which may impair the cross-Nicol inspectability. In addition, if inorganic particles are contained in the surface layer of the film, the inorganic particles will fall off the film during transport in the downstream process, which will cause process contamination. On the other hand, organic particles have a higher affinity with polyester than inorganic particles, so they are less likely to form voids with the particles as nuclei during stretching, and since they do not contain metal elements, they are also less likely to form aggregates. Furthermore, by controlling the average particle size, it is preferable that the film will not fall off from the surface. In the present invention, "substantially not containing inorganic particles" means that the content of inorganic particles is 0.05% by weight or less in the measurement method described below. It is more preferable that the content of inorganic particles is 0.03% by weight or less.

The polyester film of the present invention is required to have at least one maximum value in the particle diameter range of 0.2 μm to 0.4 μm and in the range of 0.8 μm to 1.0 μm when the volume-based particle size distribution of the organic particles contained in the polyester film is measured by the measurement method described below and the horizontal axis is the particle diameter and the vertical axis is the particle abundance ratio. By containing organic particles having a maximum value in such a range, both transportability and inspectability can be improved. More preferably, the smallest maximum value is in the range of 0.2 μm to 0.4 μm and the largest maximum value is in the range of 0.8 μm to 1.0 μm. If the smallest particle diameter maximum value is in the range of less than 0.2 μm, the transportability and windability of the film are deteriorated, and when used as a release film, a favorable smoothness for coating a laminate is obtained, but the releasability between the release film and the laminate is deteriorated. Furthermore, if the maximum value of the largest particle diameter is in the range exceeding 1.0 μm, the transportability and winding properties of the film are improved, but coating defects due to particles may occur. Furthermore, when used as a release film, although the releasability from the laminate is improved, excessive unevenness is created in the laminate on the release film, and the functionality of the laminate may be impaired by the transfer. On the other hand, if there is not one or more maximum values in the particle diameter range of 0.2 μm to 0.4 μm and in the range of 0.8 μm to 1.0 μm, for example, if the maximum value of the particle diameter is only in the range of more than 0.4 μm and less than 0.8 μm, smoothness suitable for coating the laminate cannot be obtained.

The organic particles contained in the polyester film of the present invention may be different types of particles or the same type of particles. Examples of organic particles that can be used include crosslinked polystyrene resin particles, crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, crosslinked polyester resin particles, polyimide particles, melamine resin particles, etc. For all types of particles, it is preferable that the particle shape and particle size distribution are uniform, and it is particularly preferable that the particle shape is spherical.

In the polyester film of the present invention, the content of organic particles is preferably 0.09 to 0.42% by weight based on the total weight of the film. By setting the content in this range, it is possible to suppress particle-induced transfer while improving releasability and transportability.

The polyester film of the present invention must have an arithmetic mean roughness Ra of 5 nm or more on both sides of the film. It is preferably 20 nm or more and 30 nm or less. When the arithmetic mean roughness of the film surface is in this range, it is possible to achieve both favorable releasability and transportability. If the arithmetic mean roughness Ra is less than 5 nm, it is not preferable because the releasability from the laminate, transportability, and winding properties will deteriorate.

The polyester film of the present invention may have a laminated structure of two or more layers. By changing the amount of organic particles contained in at least one layer (layer A), the arithmetic mean roughness of the film surface can be adjusted, and releasability and transportability can be improved. When layer A is present on at least one surface layer, the arithmetic mean roughness of the surface having layer A is preferably 20 nm or more and 30 nm or less.

When the volume-based particle size distribution of the organic particles contained in layer A is measured and plotted with the particle diameter on the horizontal axis and the particle abundance ratio on the vertical axis, the polyester film of the present invention has one or more maximum values in the particle diameter range of 0.2 μm to 0.4 μm and in the range of 0.8 to 1.0 μm, and in the plot of the organic particles contained in layer A, the ratio (a/b) of the abundance ratio a of the maximum peak top having the highest value in the particle diameter range of 0.2 μm to 0.4 μm and the abundance ratio b of the maximum peak top having the highest value in the particle diameter range of 0.8 μm to 1.0 μm is preferably 0.5 to 6.0. If the abundance ratio a/b of the peak tops is less than 0.5, the transfer of the contained particles to the laminate may increase. If the abundance ratio a/b is greater than 6.0, the releasability and transportability may be impaired.

In the case of a three-layer structure, the polyester film of the present invention contains substantially no inorganic particles in both surface layers (Layers A and A'), and neither Layer A nor Layer A' contains substantially any inorganic particles. When the volume-based particle size distribution of the organic particles contained in Layers A and A' is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, each has one or more maximum values in the particle diameter range of 0.2 μm to 0.4 μm and in the particle diameter range of 0.8 μm to 1.0 μm, respectively, and it is preferable that in the plot of the organic particles contained in Layers A and A', the ratio (a/b) of the abundance ratio a of the maximum peak top having the highest value in the particle diameter range of 0.2 μm to 0.4 μm and the abundance ratio b of the maximum peak top having the highest value in the particle diameter range of 0.8 μm to 1.0 μm is 0.5 or more and 6.0 or less. In addition, it is preferable that the arithmetic mean roughness of both the surface having the layer A and the surface having the layer A' is 20 nm or more and 30 nm or less. The content of organic particles in the layer A and the layer A' is preferably 0.75 to 2.5 wt % based on the total weight of the layer A and the layer A'. By setting it in this range, it is possible to improve releasability and transportability while suppressing transfer caused by particles.

The polyester film of the present invention is preferably a three-layer structure. In the case of a three-layer structure, it is easy to mix and use recovered raw materials from edge portions generated in the film-forming process or recycled raw materials from other film-forming processes in the inner layer (layer B) that does not have a surface, to the extent that it does not adversely affect the film properties, and this is advantageous in terms of cost. In the case of a three-layer structure, the polyester film of the present invention can adjust the film haze by incorporating particles in the inner layer (layer B). In this case, it is preferable that the film does not substantially contain inorganic particles and contains organic particles from the viewpoint of ensuring crossed Nicol inspection properties.

In the polyester film of the present invention, when the volume-based particle size distribution of the organic particles contained in Layer B is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, it is preferable that the particle diameter has one or more maximum values in the range of 0.8 μm to 1.0 μm. If the maximum particle diameter value is less than 0.8 μm, a large amount of particles is required to adjust the film haze, and if the maximum particle diameter value is more than 1.0 μm, there is a risk of coarse protrusions affecting the surface shape of both outer layers. The content of organic particles in Layer B is preferably 0.03 to 0.2 wt % of the total weight of Layer B.

The polyester film of the present invention preferably has a film haze of 7% or more and 13% or less. When the polyester film of the present invention is used for polarizing plate release applications, an inspection using transmitted light or an inspection using reflected light may be performed. If the film haze of the polyester film exceeds 13%, the transmitted light may not be sufficiently transmitted during an inspection using transmitted light, which may cause problems in the inspection. On the other hand, if the film haze is less than 7%, the reflected light may be transmitted during an inspection using reflected light, which may cause problems in the inspection.

The method for producing the polyester film of the present invention will be explained below using a biaxially oriented polyester film as an example, but the polyester film of the present invention should not be interpreted as being limited to the film obtained by this example.

The polyester film of the present invention can be obtained by a method having the following steps.
(Step 1) A step of melt-extruding a polyester resin into a sheet, contacting the melt-extruded polyester resin into a sheet on a casting roll at 18 to 50°C for 1 to 15 seconds and cooling and solidifying the polyester resin to obtain an unstretched polyester film having a thickness of 180 to 1,400 μm.
(Step 2) The unstretched polyester film obtained in (Step 1) is stretched in the longitudinal direction at a stretching ratio of 2.5 to 5 times, and then cooled to obtain a uniaxially stretched polyester film.
(Step 3) A step of stretching the uniaxially stretched polyester film obtained in (Step 2) in the width direction at a stretching ratio of 3 to 6 times, which is higher than the stretching ratio in the longitudinal direction, and then cooling the film to obtain a biaxially stretched polyester film.
(Step 4) A step of heat-treating the biaxially stretched polyester film at a heat treatment temperature of 180 to 230° C. to obtain a biaxially oriented polyester film.

Each process is explained in detail below.

(Step 1) Preparation of Unstretched Film The polyester resin is dried as necessary, fed to an extruder and melt-extruded. In order to make the intrinsic viscosity of the film fall within the above range, the average intrinsic viscosity of the polyester resin fed to the extruder is preferably 0.55 to 0.64 dl/g, more preferably 0.55 to 0.62 dl/g. The variation in the intrinsic viscosity of the polyester resin fed to the extruder is preferably 0.002 to 0.030 dl/g, more preferably 0.005 to 0.030. The variation in the intrinsic viscosity of the polyester resin fed to the extruder is determined by the measurement method described below, and is calculated as the standard deviation σ of the values obtained by measuring the intrinsic viscosity of 50 samples randomly extracted from the polyester resin fed to the extruder as a raw material. By making the intrinsic viscosity of the polyester resin fed as a raw material have a certain variation, it is possible to increase the crystal plane orientation index of the film while decreasing the orientation angle. If the variation in the intrinsic viscosity of the polyester resin supplied as a raw material is less than 0.002 dl/g, the crystal plane orientation index χi value is low, and the bright spot defects may not be reduced. On the other hand, if the variation in the intrinsic viscosity of the polyester resin supplied as a raw material is more than 0.030 dl/g, the properties of the obtained biaxially oriented polyester film are not stable, and the orientation angle is high, and the light leakage when inspecting the polarizing plate by the crossed Nicol method may become large and hinder the inspection. The method of making the variation of the polyester resin supplied as a raw material within the above range is not particularly limited. When the polymerization of the polyester resin supplied as a raw material is obtained by a batch polymerization method, the variation in the intrinsic viscosity is larger than when it is obtained by a continuous polymerization method.

The polyester resin melt-extruded by the extruder is then filtered through a filter. Since even small foreign objects can cause defects in the film, it is effective to use a high-precision filter that captures more than 95% of foreign objects larger than 5 μm. The molecular chains of the molten polyester resin are broken by thermal decomposition and hydrolysis, causing a decrease in intrinsic viscosity. The temperature and moisture content of the polyester resin during melt extrusion are not particularly limited as long as the intrinsic viscosity of the final film can be within the above-mentioned ranges, but in order to perform stable melt extrusion, it is preferable that the temperature be +5 to +40°C above the melting point of the polyester resin, and the moisture content be 300 ppm or less.

The polyester resin is then formed into a sheet using a T-shaped die or the like, and the polyester resin formed into a sheet is cooled and solidified on a casting roll to obtain an unstretched film. In this case, it is preferable that the temperature of the casting roll is 18 to 50°C, and the cooling time for the polyester resin formed into a sheet to come into contact with the casting roll is 1 to 15 seconds. If the temperature of the casting roll is less than 18°C, condensation is likely to occur on the casting drum, and film-forming properties may deteriorate. If the temperature of the casting roll exceeds 50°C, the crystal plane orientation index χi value may be low. This is because the cooling efficiency is deteriorated when the temperature of the casting roll is high, so that microcrystals are generated in the obtained unstretched film, and the crystal orientation in the subsequent stretching process tends to proceed easily. Similarly, if the time for cooling in contact with the casting roll is less than 1 second, the cooling efficiency is deteriorated, and microcrystals are generated in the obtained unstretched film, which tends to result in a low crystal plane orientation index χi value. The cooling time by the casting roll can be increased by increasing the diameter of the casting roll or decreasing the line speed, but the upper limit is 15 seconds in consideration of equipment space and productivity. More preferably, the temperature of the casting roll is 20 to 30°C, and the cooling time during which the polyester resin molded into a sheet contacts the casting roll is 3 to 12 seconds.

The thickness of the unstretched film obtained in step 1 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 the film so that the orientation angle and heat shrinkage rate are within the desired range, and film breakage may occur during stretching. On the other hand, if the thickness of the unstretched film exceeds 1400 μm, uneven cooling occurs in the thickness direction when the polyester resin sheet is cooled and solidified on the casting roll, and the crystal plane orientation index χi value tends to be low. In addition, the final thickness of the biaxially oriented polyester film may be outside the range suitable for polarizing plate release applications.

(Step 2) Preparation of uniaxially stretched film The unstretched film obtained in (Step 1) is stretched in the longitudinal direction at a stretch ratio of 2.5 to 5.0 times and cooled to obtain a uniaxially stretched polyester film. The stretching in the longitudinal direction is preferably performed in one step or in multiple steps at a stretching temperature of 90 to 130 ° C. From the viewpoint of suppressing the bowing phenomenon and uneven thickness in the longitudinal direction of the film, the stretching temperature is more preferably 100 to 120 ° C. and the stretching ratio is more preferably 3 to 4 times, and from the viewpoint of preventing uneven stretching and scratches, it is preferable to perform the stretching in two or more steps. In addition, although shrinkage in the width direction occurs due to the longitudinal stretching, it is preferable that the width shrinkage of the film from this stretching step to the cooling step is 15% or less. If the width shrinkage of the film exceeds 15%, the film may meander or vary in width, or the uniformity of the plane orientation in the width direction of the film may deteriorate, making it difficult to make the crystal plane orientation index χi value 6.0 or more over a width of 5 m. The width shrinkage of the film can be controlled by adjusting the thickness profile of the film ends before longitudinal stretching or by adjusting the stretching tension using nip rolls or the like.
The width shrinkage of the film 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.
It is preferable that the film temperature in the cooling step in (step 2) is 25 to 45° C. in order to stably perform the width direction stretching in the next (step 3).

(Step 3) Preparation of biaxially stretched film The uniaxially stretched polyester film obtained in (Step 2) is stretched in the width direction at a stretch ratio of 3.0 to 6.0 times, and the stretch ratio in the width direction is higher than the stretch ratio in the longitudinal direction. The 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 stretch ratio is higher than 6.0 times, the orientation angle tends to decrease, but the film becomes more likely to break, and the crystal plane orientation index χi value becomes lower. It is more preferable that the stretching temperature is 100 to 120 ° C. and the stretch ratio is 4.0 to 5.0 times. In addition, in order to reduce the orientation angle, it is preferable that the stretch ratio in the width direction is higher than the stretch ratio in the longitudinal direction. If the stretch ratio in the longitudinal direction is higher than the stretch ratio in the width direction, the molecular orientation in the film will be inclined toward the longitudinal direction, making it difficult to suppress the orientation angle variation.
In producing the biaxially oriented polyester film of the present invention, the stretching in the longitudinal direction is followed by the stretching in the transverse direction, because if the stretching in the longitudinal direction is carried out after the stretching in the transverse direction, the molecules are strongly oriented mainly in the transverse direction after the first stretching in the transverse direction, but if the stretching in the longitudinal direction is carried out thereafter, the molecules are also oriented in the longitudinal direction, resulting in a high orientation angle.

Then, the film stretched in the width direction is cooled at a film temperature of 25 to 45°C and a film width shrinkage rate of 0.1 to 20%/min to obtain a cooled biaxially stretched polyester film. The film temperature in the cooling process is preferably 25 to 45°C, since this suppresses the relaxation of orientation in the width direction due to width shrinkage and suppresses the bowing phenomenon. It is more preferably 30 to 40°C. If the film temperature in the cooling process is higher than 45°C, the tension caused by the film width shrinkage may affect the film formability, and the effect of suppressing the relaxation of orientation in the width direction may not be fully achieved. If the film temperature in the cooling process is cooled to less than 20°C, productivity may be poor.

Methods for cooling polyester film include air-cooling using a tenter where heat treatment is performed, air-cooling using aluminum or other shielding plates above and below the heat treatment area to block hot air, and cooling using rolls. In the air-cooling method using a tenter where heat treatment is performed, all zones are connected in the longitudinal direction, so the free flow of hot air such as accompanying air currents can cause temperature differences above and below the film and across the width, making it impossible to sufficiently cool the film. In such cases, it is possible to actively cool the film by blowing in compressed air, etc.

In the cooling method using rolls, the number of rolls used and the set temperature are not limited, but it is preferable to use multiple rolls for cooling. In order to keep the film temperature in the above range in the cooling method using rolls, the roll temperature is preferably 20 to 45°C, and more preferably 30 to 40°C. In the cooling method using rolls, it is preferable to apply a load to the film and make it adhere closely to the cooling roll using a nip roll, as this allows for stable cooling.

In addition, in this cooling process, the width shrinkage rate of the film is preferably 0.1 to 20%/min. If the width shrinkage rate is less than 0.1%/min, the film tension caused by the suppression of the width shrinkage of the film may affect the film forming properties, which may cause film breakage. If the width shrinkage rate is faster than 20%/min, the effect of suppressing the orientation relaxation due to the width shrinkage of the film is small, and the suppression of the bowing phenomenon may be insufficient. It is more preferable that the width shrinkage rate of the film is 0.2 to 18%/min. The width shrinkage rate can be controlled by various methods, setting the width shrinkage rate from the cooling process length and the film forming speed. Specifically, in the air cooling method in a tenter, the width shrinkage rate can be set to the desired value by holding both ends with clips and adjusting the rail width.

The width shrinkage speed of the film in the cooling step shown here is calculated by formula (1) where W1 (mm) is the film width after the width direction stretching step and immediately before entering the cooling step, W2 (mm) is the film width after the cooling step, and T1 (min) is the passage time of the cooling step.
Film width shrinkage speed = (W1 - W2) / W1 x 1 / T1 Formula (1)
In addition, in the cooling step (step 3), it is preferable that the film is left in a state of reduced temperature for a certain period of time. The reason for this is presumed to be as follows. As described above, it is believed that orientation relaxation occurs during the width shrinkage in the cooling step, but it is presumed that a certain period of time is required to stop the orientation relaxation by cooling the film. Therefore, it is presumed that if the passing time of the cooling step is insufficient, orientation relaxation cannot be suppressed, and therefore the effect of suppressing the bowing phenomenon is small. When producing the biaxially oriented polyester film of the present invention, the passing time of the cooling step is preferably 10 seconds or more, more preferably 15 seconds or more. There is no particular upper limit to the passing time of the cooling step, but it is preferable that it is 60 seconds or less because productivity is good.
(Step 4) Heat Treatment of Biaxially Stretched Film The biaxially stretched polyester film obtained in (Step 3) above 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 heat shrinkage rate in the longitudinal direction of the film within the range of 2.5 to 7.0% and the heat shrinkage rate in the transverse direction of the film within the range of 2.5 to 8.0% after heat treatment at 150°C for 30 minutes. 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-mentioned range, which is not preferable.
In addition, in the above heat treatment, a relaxation treatment may be performed as necessary. The relaxation treatment may be performed in either the width direction or the length direction, and may be performed simultaneously or separately in the width direction and length direction. A relaxation rate of preferably 1 to 20%, more preferably 1 to 15%, based on the total width of the film is effective for obtaining a film with excellent thermal dimensional stability.

In the following, Examples 3 to 6 shall be read as Reference Examples 3 to 6. The measurement and evaluation methods for the property values in the examples and comparative examples are as follows.

(1) Volumetric Particle Size Distribution Measurement Using a microtome, a small piece was cut perpendicular to the surface of a polyester film, and its cross section was observed and photographed at 10,000 times magnification using a field emission scanning electron microscope JSM-6700F (manufactured by JEOL Ltd.). From the cross-sectional photograph, the particle size distribution of particles present in the film was obtained using image analysis software Image-Pro Plus (Nippon Roper Co., Ltd.). Cross-sectional photographs were selected from different arbitrary measurement fields, and the diameters (circle equivalent diameters) of 2,500 or more particles arbitrarily selected from the cross-sectional photographs were measured, and a volumetric particle size distribution was obtained by plotting the particle diameter on the horizontal axis and the particle abundance ratio on the vertical axis. In the volumetric particle size distribution, the abundance ratio of particles on the horizontal axis is expressed by the formula "abundance ratio = total volume of detected particles having the corresponding particle size / total volume of all detected particles x 100" according to a class at 10 nm intervals starting from 0 nm. In addition, the value of the peak top showing the maximum is set to 0.12 μm, which is the upper limit value of the class, when the particle diameter has a maximum in the range of more than 110 nm to 120 nm, for example. The particle diameter of the peak top showing the maximum was read from the particle abundance ratio chart obtained above, and the abundance ratio a of the maximum having the highest value in the particle diameter range of 0.2 μm to 0.4 μm, the abundance ratio b of the maximum having the highest value in the particle diameter range of 0.8 μm to 1.0 μm, and the ratio a/b were calculated.

(2) Film Haze According to JIS K7105 (1981), a sample was cut to a size of 4 cm in the longitudinal direction of the film and 3.5 cm in the width direction of the film, and the haze was measured using a haze meter (HGM-2DP (for C light) manufactured by Suga Test Instruments Co., Ltd.). Measurements were taken at three points evenly spaced in the width direction of the film, and the average value was taken as the measurement result.

(3) Arithmetic mean roughness Ra
The film was measured using a three-dimensional micro surface profiler (ET-350K manufactured by Kosaka Seisakusho) and the arithmetic mean roughness Ra was calculated from the linear profile curve of the film surface in accordance with JIS B0601-1994. The measurement was carried out under the following conditions.
Measurement length in X direction: 0.5 mm, feed speed in X direction: 0.1 mm/sec.
Y direction feed pitch: 5 μm, number of lines in Y direction: 40.
Cut-off: 0.25mm.
Stylus pressure: 0.02 mN.
Height (Z direction) magnification: 50,000 times.

(4) Inspectability in Reflection Inspection The equipment used to evaluate the inspectability with a reflection inspection device was an LED light (HBLF-L1400 manufactured by Atto) as an illumination means, a CCD camera (PCTME8040 manufactured by Hutec) with a resolution of 100 μm as a light receiving means, and a reflection inspection device set so that the irradiation angle and detection angle of the reflected light were 40°. When the film was inspected without changing the output of the LED light of the reflection inspection device and the detection sensitivity of the camera, the image signal detected by the camera was evaluated using an output value divided into 256 so that the maximum brightness was 255 and the minimum brightness was 0, and it was judged whether the base output value was within the range of 127±20. A schematic diagram of the device is shown in Figure 1.
◯: The base signal output value is within the range of 127±20 (both bright and dark defects can be normally inspected).
×: The base signal output value is outside the range of 127±20 (either bright or dark defects cannot be normally inspected).

(5) Inspectability in Transmission Inspection The equipment used to evaluate the ease of inspection in a transmission inspection device was an LED light (Hutech FLS-L3200) as a lighting means, a CCD camera (Hutech DCFMB1A80) with a resolution of 70 μm as a light receiving means, and a transmission inspection device installed so that the transmitted light directly enters the camera. When the film was inspected without changing the output of the LED light of the transmission inspection device and the detection sensitivity of the camera, the image signal detected by the camera was evaluated using an output value divided by 1024 so that the maximum brightness was 1023 and the minimum brightness was 0, and it was judged whether the base output value was within the range of 512 ± 40. A schematic diagram of the device is shown in Figure 2.
◯: The base signal output value is within the range of 512±40 (both bright and dark defects can be normally inspected).
×: The base signal output value is outside the range of 512±40 (either bright or dark defects cannot be normally inspected).

(6) Number of Bright Spot Defects Bright spot defects refer to light leakage due to defects in the film detected in the crossed Nicol inspection. As an instrument for evaluating bright spot defects, a crossed Nicol inspection instrument was used, which is equipped with an LED light (HBLF-WSL1500, HBLF-WSL700, manufactured by Atto) and a first polarizing plate with an adjustable angle as illumination means, and a CCD camera with a resolution of 50 μm (GMFMB3B80, manufactured by Hutec) and a second polarizing plate with an adjustable angle as light receiving means. 1000 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 with a detection size of 100 μm or more was counted. An overview of the instrument is shown in Figure 3.

(7) Inspectability in Crossed Nicols Inspection The number of bright spot defects described above was evaluated as an index of inspectability in the crossed Nicols inspection (◯ or higher was considered to be pass).
Good: 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.

(8) Particle shedding The shedding of added particles was evaluated by wiping the film off using a wiper cloth (Toray, Toray Industries, Inc., Toraysee) placed on a conveyor that unwound the film at a constant speed and wound it up on the winding side. The conveyor speed was 40 m/min, and 2000 m of wiping was performed. The surface of the wiper cloth after wiping was observed under a microscope (500x magnification). The shedding of particles was evaluated according to the following criteria.
A: A small amount of particle dropout was observed.
Δ: A small amount of particle dropout was observed.
×: A large amount of particles were observed to fall off.

(9) Scratch resistance: Run the SFT-700H back and forth 50 times in the MD longitudinal direction. (Width: 1/2 inch, Length: 30 cm (Measured length: 10 cm), Load: 100 g)
The treated sample is subjected to surface deposition in a bell jar vacuum deposition machine, and observed under a stereomicroscope. The number of scratches is counted at the angle at which the most scratches are visible.
◎: 15 or fewer scratches after 50 round trips.
◯: 16 to 20 or more scratches occurred after 50 round trips.
△: 21 to 29 scratches occurred after 50 round trips.
×: 30 or more scratches occurred after 50 round trips.

(10) Inorganic Particle Content Based on JIS K6813 (2002), the content of inorganic particles is determined as follows. A sample is weighed, and the weight is m1. The weighed sample is placed on a sample boat and heated in a cylindrical electric furnace preheated to 550°C under a nitrogen atmosphere for 45 minutes. The sample boat is allowed to cool for 10 minutes under a nitrogen atmosphere, then transferred to a desiccator and allowed to cool to room temperature, and the weight is weighed, and the weight is m2. The sample boat is then placed in a muffle furnace and incinerated at 900°C. The sample boat is cooled to room temperature in the desiccator, and the weight is weighed, and the weight is m3. The content (wt%) of inorganic particles is calculated using the following formula (a). This trial is carried out five times, and the arithmetic average is taken as the content (wt%). If the content is 0.05 wt% or less, it is determined that the sample does not substantially contain inorganic particles.
Inorganic particle content (wt%) = (m2-m3)/m1 ×100... (a).

Example 1:
A slurry of terephthalic acid and ethylene glycol (the molar ratio of ethylene glycol/terephthalic acid is 1.05 to 1.30) is subjected to an esterification reaction at 255° C. while distilling off water. After the esterification reaction, 0.0207% by weight (equivalent to 0.693 mol/t) of diantimony trioxide, 0.014% by weight (equivalent to 0.281 mol/t) of manganese acetate, 0.014% by weight (equivalent to 1.428 mol/t) of phosphoric acid, and 0.0005% by weight (equivalent to 0.09 mol/t) of an aqueous potassium hydroxide solution were added to the polyester composition, and the mixture was heated to 290° C. under vacuum to perform a polycondensation reaction, thereby obtaining polyester pellets A having an intrinsic viscosity of 0.70.

Next, a water slurry of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.3 μm was added to the above polyester pellets A that were substantially free of particles using a vented twin-screw kneader to obtain master pellets B that contained 2.0% by weight of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.3 μm relative to the polyester. Similarly, a water slurry of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.9 μm was added to the above polyester pellets A that were substantially free of particles using a vented twin-screw kneader to obtain master pellets C that contained 1.0% by weight of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.9 μm relative to the polyester.

Next, the master pellets B and C containing the divinylbenzene/styrene copolymer crosslinked particles were mixed to obtain polyester A (the polyester constituting layers A and A') containing 1.0% by weight of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.3 μm and 0.5% by weight of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.9 μm.

Furthermore, polyester raw material (recovered pellets) consisting of recycled film mainly consisting of edge parts generated in the film-forming process of the present invention, polyester pellets A containing substantially no particles, and master pellets C containing divinylbenzene/styrene copolymer crosslinked particles with an average particle size of 0.9 μm were mixed in the weight ratios shown in the table to obtain polyester B (polyester constituting layer B).

These polyesters were each dried under reduced pressure at 160°C for 8 hours to a moisture content of 100 ppm, and then fed to separate extruders, melt-extruded at 275°C, filtered through a high-precision filter with a collection efficiency of 95% for 5 μm or more, and then laminated together in a rectangular three-layer merging block to form a three-layer laminate consisting of polyester layer A/polyester layer B/polyester layer A'. Then, through a slit die maintained at 285°C, 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, to obtain an unstretched film with a thickness of 570 μm. This unstretched film was first stretched 3.4 times in the longitudinal direction by a roll heated to 103°C and a radiation heater. The width shrinkage at this time was 14%. Next, the film was stretched 4.4 times in the width direction at 110°C in a tenter. Then, the film was cooled to a film temperature of 35°C at a width shrinkage rate of 18%/min in the cooling process. In this cooling process, a roll method was adopted, the roll temperature was 30°C, and the passing time of the cooling process was 15 seconds. Then, heat treatment was performed at 195°C, and a biaxially oriented polyester film consisting of three layers, with a total film thickness of 38 μm, a film lamination thickness of polyester layer A/polyester layer B/polyester layer A'=2.0 μm/34 μm/2.0 μm, and a film width of 5.1 m, was wound up to prepare an intermediate product roll. Samples were taken from the obtained intermediate product roll, and particle diameter, film haze, arithmetic mean roughness, and number of bright spots were measured, and reflection inspection, transmission inspection, cross Nicol inspection, and particle shedding evaluation were performed. The peak top values in the volumetric particle size distribution were 0.27 μm and 0.84 μm, respectively, and the ratio a/b of their abundance ratios was 2.0. In addition, the film haze was 10%, the arithmetic mean roughness was 24 nm and 23 nm, and the number of bright spots, ease of inspection with a reflective inspection device, ease of inspection with a transmissive inspection device, ease of inspection with a crossed Nicol inspection device, particle shedding, and scratch resistance were all good, as shown in Table 1.

(Examples 2 and 3, Comparative Examples 3 and 4)
The same procedure as in Example 1 was carried out to obtain a biaxially oriented polyester film consisting of three layers, except that the amounts of polyester master pellets B and C added to form layer A were changed. The results are shown in Table 1.

(Examples 4 and 5, Comparative Examples 5 and 6)
The same procedure as in Example 1 was carried out to obtain a three-layer biaxially oriented polyester film, except that the average particle size of the divinylbenzene/styrene copolymer crosslinked particles contained in the polyester master pellets B and C was changed. The results are shown in Table 1.

Example 6
The same procedure as in Example 1 was carried out to obtain a two-layer polyester film, except that polyesters A and B were melt-extruded and laminated in a rectangular two-layer merging block to form a two-layer laminate consisting of polyester layer A and polyester layer B. The results are shown in Table 1.

(Comparative Example 1)
Calcium carbonate with a true specific gravity of 2.71 g/cm ³ and an average particle size of 1.0 μm was prepared and made into a 10% by weight ethylene glycol slurry. This slurry was dispersed for one hour with a jet agitator, and then highly accurately filtered with a filter with a collection efficiency of 95% for particles of 5 μm or more. After the esterification reaction of this slurry was completed, diantimony trioxide was added to the polyester composition in an amount of 0.0105% by weight (equivalent to 0.352 mol/t), manganese acetate was added to the polyester composition in an amount of 0.030% by weight (equivalent to 0.602 mol/t), phosphoric acid was added to the polyester composition in an amount of 0.008% by weight (equivalent to 0.816 mol/t), and sodium dihydrogen phosphate was added to the polyester composition in an amount of 0.010% by weight (equivalent to 0.792 mol/t). The polycondensation reaction was then carried out in the same manner as above, to obtain calcium carbonate-containing master polyester pellets D containing 1.0% by weight of calcium carbonate with an average particle size of 1.0 μm and having an intrinsic viscosity of 0.62.

Master pellets B containing divinylbenzene/styrene copolymer crosslinked particles and master pellets D containing calcium carbonate were mixed to obtain polyester C (polyester constituting layers A and A') containing 1.0% by weight of divinylbenzene/styrene copolymer crosslinked particles having an average particle size of 0.3 μm and 0.50% by weight of calcium carbonate particles having an average particle size of 1.0 μm.

The same procedure as in Example 1 was carried out except that polyester C was used instead of polyester A, and a biaxially oriented polyester film consisting of three layers was obtained. The results are summarized in Table 1.

(Comparative Example 2)
A polyester raw material consisting of a recycled film mainly composed of edge portions generated in the film-forming process of the present invention, polyester pellets A containing substantially no particles, and master pellets D containing calcium carbonate particles having an average particle size of 1.0 μm were mixed in the weight ratio shown in the table to obtain polyester D (polyester constituting layer B).

The same procedure as in Example 1 was carried out except that polyester D was used instead of polyester B, and a biaxially oriented polyester film consisting of three layers was obtained. The results are summarized in Table 1.

1: Film to be inspected 2: Illumination means 3: Light receiving means 4: Signal processing means 5: Film to be inspected 6: Illumination means 7: Light receiving means 8: Signal processing means 9: Film to be inspected 10: First polarizing filter 11: Second polarizing filter 12: Illumination means 13: Light receiving means 14: Signal processing means

Claims (7)

A polyester film which is composed of three or more layers and contains organic particles, but which does not substantially contain inorganic particles, and in which, when a volume-based particle size distribution of the organic particles contained in the polyester film is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, the polyester film has one or more maximum values in a particle diameter range of 0.2 μm to 0.4 μm and one in a particle diameter range of 0.8 μm to 1.0 μm, and the arithmetic average surface roughness Ra of the polyester film on both sides is 23 nm to 28 nm . 2. The polyester film according to claim 1, wherein the polyester film is composed of three or more layers, at least one of which contains organic particles (such layer is referred to as Layer A), and Layer A is substantially free of inorganic particles, and when a volume-based particle size distribution of the organic particles contained in Layer A is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing particle abundance ratio, the organic particles have one or more maximum values in a particle diameter range of 0.2 μm to 0.4 μm and one in a particle diameter range of 0.8 to 1.0 μm, respectively, and in the plot of the organic particles contained in Layer A, a ratio (a/b) of the abundance ratio a of the maximum peak top having the highest value in the particle diameter range of 0.2 μm to 0.4 μm to the abundance ratio b of the maximum peak top having the highest value in the particle diameter range of 0.8 μm to 1.0 μm is 0.5 to 6.0. The polyester film according to claim 1, which is a three-layered polyester film, containing organic particles in both surface layers (such layers are referred to as Layer A and Layer A'), and neither Layer A nor Layer A' contains substantially inorganic particles, and when the volume-based particle size distribution of the organic particles contained in Layer A and Layer A' is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, each has one or more maximum values in the particle diameter range of 0.2 μm to 0.4 μm and in the particle diameter range of 0.8 μm to 1.0 μm, respectively, and in the plot of the organic particles contained in Layer A and Layer A', the ratio (a/b) of the abundance ratio a of the maximum peak top having the highest value in the particle diameter range of 0.2 μm to 0.4 μm and the abundance ratio b of the maximum peak top having the highest value in the particle diameter range of 0.8 μm to 1.0 μm is 0.5 to 6.0. 4. The polyester film according to claim 3, wherein the layer having no surface (such layer is referred to as Layer B) is substantially free of inorganic particles, and when the volume-based particle size distribution of organic particles contained in Layer B is measured and plotted with the horizontal axis representing particle diameter and the vertical axis representing the particle abundance ratio, the polyester film has one or more maximum values in the particle diameter range of 0.8 μm to 1.0 μm. 5. The polyester film according to claim 1 , having a film haze of 7% or more and 13% or less. The polyester film according to any one of claims 1 to 5 , which is used for release purposes. The polyester film according to any one of claims 1 to 5 , which is used for a release film of a polarizing plate.

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