JP2015044411A - Biaxially stretched polyethylene terephthalate film for releasing polarizing plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate having excellent contrast property, little haze variation due to heat treatment, excellent optical axis accuracy, and thermal dimensional stability.SOLUTION: A biaxially stretched polyethylene terephthalate film for releasing a polarizing plate is a laminated biaxially stretched polyethylene terephthalate film and satisfies the following constitutional requirements (1) to (4). (1) A laminate constitution of three or more layers is obtained by a co-extrusion method. (2) A polyester which constitutes both surface layers has the amount of the terminal hydroxyl (OH) groups of 70 eq/ton or less and a cyclic trimer content of 0.45 mass% or less. (3) An MOR value measured by a microwave transmission type molecular orientation meter is 1.80 to 2.10. and (4) An amount of variation of an orientation angle in a lateral direction of a film is 3.0 to 5.0° per 500 mm.

Description

  The present invention relates to a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate. More specifically, the present invention relates to a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate that has little change in haze due to heat treatment and has excellent inspection properties.

  A polarizing plate which is a constituent member of a liquid crystal display device is a rolled state in a state of a laminate in which an adhesive layer is provided on one surface, and a release film for protecting the polarizing plate is laminated on the adhesive layer. Wrapped around and transported or stored. A biaxially stretched polyethylene terephthalate film is widely used as a base material for such a polarizing plate release film from the viewpoint of strength function and cost. A biaxially stretched polyethylene terephthalate film having a structure in which linear polymers are oriented is a birefringent body that optically exhibits birefringence. Therefore, the biaxially stretched polyethylene terephthalate film has two optical axes perpendicular to the direction parallel to and perpendicular to the molecular orientation direction. Therefore, when laminated with the optical axis of the substrate of the release film inclined with respect to the optical axis of the polarizing plate, it exhibits transmitted light and interference color when placed under crossed Nicols, and visually inspects defects. It becomes a factor to inhibit. Accordingly, the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate is required to have excellent optical axis accuracy (Patent Document 1).

  Such a biaxially stretched polyethylene terephthalate resin film for releasing a polarizing plate is stretched in the longitudinal direction between rolls having a difference in rotational speed, and then in the width direction with the end of the film held in a tenter. Manufactured by drawing and heat setting. In this case, since the distortion of the optical axis, that is, the distortion of the orientation main axis, is larger at the end than at the center of the film due to the bowing phenomenon, only a limited product at the center can be used for this purpose. As a method for reducing the bowing of the film, a method of performing heat treatment after once cooling to below the glass transition temperature of the polyester after stretching in the width direction, a method of providing a nip roll after stretching in the width direction, and dividing the heat treatment chamber into a plurality of zones in stages. There have been proposed a method of raising the temperature, a method of providing a temperature distribution in the width direction to lead to the heat treatment zone, a method of increasing the draw ratio in the width direction, and the like. (Patent Documents 2 to 5)

  On the other hand, when the biaxially stretched polyethylene terephthalate resin film for releasing a polarizing plate is used as a protective film for a polarizing plate, a heat treatment such as a releasing process or an adhesive process may be performed in post-processing. However, when a polyester film is used as the base film, there is a problem that cyclic oligomers are precipitated on the surface of the base film due to the high temperature treatment and whitening occurs.

  Therefore, in order to reduce the oligomer in the polyester film, it has been proposed to propose the amount of the cyclic oligomer of the polyester raw material by a solid phase polymerization method (Patent Documents 6 to 9).

JP 2002-40249 A JP 2008-246665A JP 2008-163263 A JP 2005-14545 A Japanese Patent Laid-Open No. 2004-18588 JP-A-9-99530 JP 2000-141570 A JP 2003-191413 A Japanese Patent Laid-Open No. 2003-301057

  Currently, the film proposed in the above patent document is used for polarizing plate inspection. However, the increase in the screen size of displays has progressed dramatically, and 37-inch and 42-inch large-screen displays have penetrated the market. Furthermore, due to the demand for higher definition, it has become necessary to reliably recognize foreign matter and defects at a level that has not been regarded as a problem in the past. In order to satisfy such demands for the development of large-screen displays and the improvement of inspection accuracy, it has become necessary to reduce the distortion of the optical axis over a longer width.

  On the other hand, automation of inspection is progressing from the viewpoint of productivity, and automatic attachment of a release film to a polarizing plate, identification of fine foreign matters and defects by image analysis, and the like are performed. In this case, the conventional polarizing plate release film has a low degree of contrast, and fine foreign matters and defects may be overlooked during image processing.

  Furthermore, in the methods proposed in Patent Documents 6 to 9, although the amount of cyclic oligomer in the polyester can be reduced by solid-phase polymerization, the polycondensation reaction of the polyester proceeds at the same time, and the degree of polymerization of the obtained polyester increases. Furthermore, even if the amount of the cyclic oligomer in the film raw material is reduced, it is inevitable that the cyclic oligomer is generated as a by-product due to the thermal history in film melting and film formation. For this reason, efforts have been made to reduce the amount of cyclic oligomers in the film raw material as much as possible, but there is a limit to such measures from the viewpoint of productivity. Therefore, a sufficient low oligomer film has not been realized by regenerating the cyclic oligomer in the melt extrusion process during film formation.

  An object of the present invention is to release a polarizing plate having excellent inspection properties by eliminating the above-mentioned problems, having good contrast properties, excellent optical axis accuracy, and suppressing the amount of oligomers precipitated by heat treatment. It is to provide a biaxially stretched polyethylene terephthalate film.

1st invention which can solve the said subject is a laminated biaxially-stretched polyethylene terephthalate film, Comprising: The biaxially-stretched polyethylene terephthalate film for polarizing plate release which satisfy | fills the following structural requirements (1)-(4). is there.
(1) It has a laminated structure of three or more layers by coextrusion method. (2) The polyester constituting both surface layers has a hydroxyl (OH) terminal amount of 70 eq / ton or less and a cyclic trimer content of 0.00. 45% by mass or less (3) MOR value measured by microwave transmission type molecular orientation meter is 1.80 to 2.10
(4) The amount of change in the orientation angle in the film width direction is 3.0 ° to 5.0 ° per 500 mm.

  The biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention has good contrast, excellent optical axis accuracy, and low oligomer properties. Therefore, post-processing at a high temperature is possible, and it is suitable for high-precision inspection, particularly automatic inspection, of polarizing plates for large screen applications.

  The film of the present invention is made of a polyethylene terephthalate resin. Here, the polyethylene terephthalate-based resin contains ethylene glycol and terephthalic acid as main components. Other dicarboxylic acid components and glycol components may be copolymerized as long as the object of the present invention is not impaired. Examples of the other dicarboxylic acid components include isophthalic acid, p-β-oxyethoxybenzoic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-dicarboxybenzophenone, bis- (4-carboxyphenylethane), adipine Examples include acid, sebacic acid, 5-sodium sulfoisophthalic acid, cyclohexane-1,4-dicarboxylic acid, and the like. Examples of the other glycol component include propylene glycol, butanediol, neopentyl glycol, diethylene glycol, bisphenol A and other ethylene oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and the like. In addition, oxycarboxylic acid components such as p-oxybenzoic acid can also be used.

  As a polymerization method of such polyethylene terephthalate (hereinafter simply referred to as PET), a direct polymerization method in which terephthalic acid and ethylene glycol and, if necessary, other dicarboxylic acid component and diol component are directly reacted, and dimethyl terephthalate are used. Any production method such as a transesterification method in which an ester (including a methyl ester of another dicarboxylic acid as necessary) and ethylene glycol (including another diol component as necessary) are transesterified can be used. .

  The intrinsic viscosity of the polyethylene terephthalate is preferably in the range of 0.45 to 0.70. If the intrinsic viscosity is lower than 0.45, the film is easily torn, and if it is higher than 0.70, the increase in filtration pressure is increased and high-precision filtration becomes difficult.

  In order to improve the visual accuracy at the time of foreign matter inspection of the polarizing plate, it is desirable to suppress oligomer precipitation on the film surface by a heat treatment step of film post-processing and to reduce haze change. Therefore, it is preferable that the amount of change in haze after heat treatment at 170 ° C. for 20 minutes is 0.1% or less. If the amount of change in haze after heat treatment at 170 ° C. for 20 minutes is greater than 0.1%, the ease of foreign matter inspection of the polarizing plate may be hindered.

  A method of using polyethylene terephthalate having a low cyclic trimer content is included in order to make the amount of change in haze after heat treatment within the above range. In order to reduce the oligomer content, if treatment as described below is performed, the polyethylene terephthalate resin may be yellowish and the visibility may be lowered. Therefore, in order to keep the amount of change in film haze before and after heating within the above range and maintain the visibility of the film, the surface layer is composed of three or more layers using a polyethylene terephthalate resin having a low cyclic trimer content. It is preferable to have a laminated structure having As the layer structure of the film, if layer A is made of polyethylene terephthalate resin with a low oligomer content, and other layers of polyethylene terephthalate resin are B layer and C layer, the layer structure in the film thickness direction is A / B / A, A / A configuration such as B / C / A or A / B / C / B / A is conceivable. Among these, in order to suppress the occurrence of curling due to the bimetal configuration, it is preferable to adopt the A / B / A configuration (two types and three layers configuration).

  In particular, the polyester constituting both surface layers of the present invention preferably has a hydroxyl (OH) terminal amount of 70 eq / ton or less and a cyclic trimer content of 0.45 mass% or less.

  It is important that the hydroxyl (OH) terminal amount of the polyester constituting both surface layers of the present invention is 70 eq / ton or less. Since the polyester constituting both surface layers of the present invention has a hydroxyl (OH) terminal amount reduced to the above range, the formation of a cyclic trimer can be suitably reduced. Therefore, regeneration of the cyclic trimer in the melting step at the time of film formation can be suppressed, and the low oligomer amount of the film raw material can be suitably maintained. The hydroxyl (OH) terminal amount is more preferably 68 eq / ton or less, and still more preferably 65 eq / ton or less. The hydroxyl (OH) terminal amount is preferably small, but if it is too small, hydrolysis of the polyester resin tends to occur. Therefore, from the viewpoint of the durability of the film, the lower limit of the hydroxyl (OH) terminal amount is preferably 40 eq / ton or more, and more preferably 50 eq / ton or more. It is important for the present invention to find out the influence of hydroxyl (OH) terminal cyclic oligomer regeneration in the polyester resin, and there is no particular method for controlling the amount of hydroxyl (OH) terminal of the polyester film. By performing heat treatment in a water atmosphere, the amount of hydroxyl (OH) end can be suitably controlled.

  It is considered as follows that the cyclic oligomer regenerated upon melting of the polyester depends on the amount of hydroxyl (OH) terminal of the polyester molecular chain. The cyclic trimer is generated by a decomposition reaction from the end of the polyester molecular chain, but at the hydroxyl (OH) end, the cyclic trimer is easily generated by a nucleophilic reaction by a hydroxyl (OH) group present in the vicinity. Thus, in the present invention, the hydroxyl (OH) terminal amount of the polyester resin is set to a predetermined amount or less, thereby suppressing the regeneration of the cyclic oligomer amount at the time of melting and suitably suppressing the heating whitening. Thereby, precipitation of the oligomer on the film surface can be suppressed also in the heat treatment at a high temperature in the post-processing, and a film with high inspectability can be obtained.

  The cyclic trimer content of the polyester constituting both surface layers of the present invention is preferably 0.45% by mass or less. When the cyclic trimer content is 0.45% by mass or more, oligomers may precipitate in the film-forming process, and heat whitening may occur easily, or the film-forming process and subsequent processes may be contaminated. . In order to make the amount of the cyclic trimer of the polyester within the above range, it is preferable to heat-treat the polyester raw material, but in the present invention, the polyester constituting both surface layers by reducing the amount of hydroxyl (OH) terminals. The amount of the cyclic trimer in the medium can be suitably reduced within the above range. The upper limit of the preferable cyclic trimer content in the present invention is 0.40% by mass. Although the amount of cyclic trimer is preferably small, in view of productivity, the lower limit of the amount of cyclic trimer is preferably 0.05% by mass, more preferably 0.10% by mass, and 0.20% by mass. % Is more preferable.

  Examples of a method for reducing the hydroxyl (OH) terminal amount of polyester include a method of increasing the molecular weight of polyester to reduce the terminal amount per unit, and a method of water-saturating a raw material chip. In particular, in the present invention, the crude polyester as a film raw material is subjected to a heat treatment in a water atmosphere, whereby the cyclic trimer content can be suitably reduced and the hydroxyl (OH) terminal amount can be suitably reduced. Therefore, it is preferable. The heat treatment is characterized by performing a high-temperature heat treatment at 190 to 260 ° C. under a flow rate of a moisture-containing inert gas containing water. By performing the heat treatment under an inert gas flow rate, the cyclic trimer can be suitably reduced without unnecessarily increasing the PET intrinsic viscosity. This point is different from the conventional solid phase polymerization. Further, unlike the solid phase polymerization method conventionally performed by the batch method, the productivity is excellent in that the treatment can be continuously performed under a flow rate.

  Furthermore, the amount of hydroxyl (OH) ends in the polyester resin can be suitably reduced by performing the heat treatment in a predetermined water atmosphere. The reason why the amount of hydroxyl (OH) ends is reduced in a water atmosphere is considered as follows. That is, the hydroxyl (OH) ends are bonded by a deethylene glycol reaction by heat treatment, and the hydroxyl (OH) ends are consumed. On the other hand, the reverse reaction of the ester reaction occurs in a water atmosphere, the polyester molecular weight is reduced, and a new molecular chain end is generated. Since these reactions occur as a mixture, it is believed that the resulting hydroxyl (OH) end will be reduced.

The heat treatment conditions for the crude polyester in the present invention are described in detail below.
In the heat treatment, the water content in the inert gas is preferably 3.5 to 30.0 g / Nm ³ , more preferably 4.0 to 20.0 g / Nm ³ . When the moisture content in the humidity control inert gas is less than 3.5 g / Nm ³ , the increase in intrinsic viscosity of the resulting polyester is remarkable. When the moisture content in the humidity control inert gas is excessive, a hydrolysis reaction may occur, and the intrinsic viscosity of the resulting polyester may be reduced.

  As the inert gas, a gas inert to polyester is used, and examples thereof include nitrogen gas, carbon dioxide gas, and helium gas. Nitrogen gas is particularly preferable because it is inexpensive.

  As the heat treatment apparatus, an apparatus that can uniformly contact the crude polyester and the inert gas is desirable. Examples of such a heat treatment apparatus include a stationary dryer, a rotary dryer, a fluidized bed dryer, and a dryer having a stirring blade.

  In heat processing, heat processing temperature becomes like this. Preferably it is 190 to 260 degreeC, More preferably, it is 200 to 250 degreeC. When the heat treatment temperature is less than 190 ° C., the decrease rate of the cyclic oligomer in the crude polyester may be small. When the heat treatment temperature exceeds the melting point of the polyester, the polyester melts and adhesion is likely to occur. Therefore, it is difficult to take out the obtained polyester from the heat treatment apparatus, and the molding operation may be difficult.

  The heat treatment time is usually preferably 1 to 70 hours, more preferably 2 to 60 hours, and further preferably 4 to 50 hours. If it is less than 1 hour, the cyclic oligomer in the crude polyester is not sufficiently reduced, and if it exceeds 70 hours, the rate of reduction of the cyclic oligomer in the crude polyester is small, and conversely problems such as thermal degradation occur. There is a fear, and the color tone is impaired.

  The flow rate of the inert gas is closely related to the intrinsic viscosity of the polyester. Further, the water content contained in the humidity control inert gas also affects the change in the intrinsic viscosity of the polyester. Therefore, the flow rate of the inert gas should be appropriately selected according to the water content, the desired intrinsic viscosity of the polyester, the heat treatment temperature, and the like.

  For example, when the moisture content of the humidity control inert gas is high, the flow rate is preferably increased in order to avoid adverse effects such as hydrolysis by water. Moreover, when making heat processing temperature high, in order to suppress the raise of the intrinsic viscosity of polyester, it is preferable to reduce the flow volume of an inert gas.

  The flow rate of the inert gas is preferably at least 1 liter per hour per kg of polyester, more preferably at least 5 liters. When the flow rate of the inert gas is less than 1 liter per hour per 1 kg of polyester, there is a risk of adverse effects such as yellowing of the resulting resin due to oxygen contamination. The upper limit of the flow rate of the inert gas is determined by the water content contained in the inert gas and the heat treatment temperature, but is 10,000 liters or less per kilogram of polyester, preferably 5,000 liters or less, more preferably 2 1,000 liters or less. Even if the flow rate of the inert gas is set to 10,000 liters or more, it does not deviate from the object of the present invention, but it is not necessary to unnecessarily increase the flow rate in consideration of the economical aspect.

  The heat treatment of the present invention is carried out by heat treatment while circulating an inert gas under normal pressure to slightly pressurized pressure.

In this case, since the purpose of pressurization is to prevent moisture and oxygen in the atmosphere from being mixed into the reactor during the heat treatment, the pressurization condition of 5.0 kg / cm ² or less is sufficient. Even when the pressurization condition exceeds 5.0 kg / cm ² , the object of the present invention is not deviated, but since the equipment is costly, it is meaningless to increase the pressure more than necessary.

  Furthermore, the oxygen concentration in the inert gas circulated from the viewpoint of color tone is required to be 50 ppm or less, preferably 25 ppm or less. When the oxygen concentration is 50 ppm or more, the color tone deteriorates due to the deterioration of the polyester of the present invention, specifically, yellowing is severe, resulting in a problem in product quality.

The polyester thus obtained preferably satisfies the following formula.
−0.05 dl / g ≦ Intrinsic viscosity before heat treatment −Intrinsic viscosity after heat treatment ≦ 0.05 dl / g

  By these treatments, the intrinsic viscosity (B) of the polyester composition after the heat treatment is preferably 0.50 dl / g or more and 0.70 dl / g or less, and further the intrinsic viscosity of the polyester composition (crude polyester) before the heat treatment. Between (A) and -0.05 dl / g≤ {(A)-(B)} ≤0.05 dl / g, and preferably -0.02 dl / g≤ {(A) -(B)} ≦ 0.02 dl / g is preferably satisfied. By setting the intrinsic viscosity (B) after the heat treatment to 0.50 dl / g or more, the occurrence of film breakage during film formation is reduced, which is advantageous. Moreover, by setting it as 0.70 dl / g or less, it can reduce that a temperature rises by the shear heat_generation | fever at the time of melt molding, and it is advantageous in suppressing the amount of cyclic oligomers in a product.

  Further, by satisfying −0.05 dl / g ≦ {(A) − (B)} ≦ 0.05 dl / g, there is no unnecessary temperature rise during film formation, and the amount of polyester cyclic oligomer is small and the color tone In addition to providing a good film, it is not necessary to newly provide a low-viscosity or high-viscosity brand for a polyester composition (crude polyester) before heat treatment, and an existing polymer that can be used as another product is used. It is possible to reduce the amount of cyclic oligomers used, which is economically advantageous.

  Further, the content of the cyclic trimer of the polyester composition after the heat treatment needs to be 0.4% by mass or less. More preferably, it is 0.35 mass%. When the content exceeds 0.4% by mass, the cyclic oligomer is regenerated at the time of film forming, and the cyclic oligomer is deposited on the film surface during film production or film processing process, causing the process to become a film product defect. There is.

  Furthermore, the hydroxyl (OH) terminal amount of the polyester composition after heat treatment needs to be 65 eq / ton or less. The lower the hydroxyl (OH) terminal amount, the more the oligomer regeneration during the melt extrusion in the film forming process of the polyester raw material can be suppressed. 60 eq / ton or less is more preferable.

  The three-dimensional center plane average surface roughness (SRa) on at least one surface, preferably both surfaces of the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention is preferably 0.020 μm or more and 0.035 μm or less. . In responding to automation and speeding up of the pasting work, when SRa is 0.020 μm or more, suitable workability (slidability) can be achieved. The lower limit of SRa is more preferably 0.021 μm or more, and further preferably 0.022 μm or more. On the other hand, it is preferable to provide surface irregularities from the viewpoint of workability. However, if the SRa exceeds 0.035 μm, light is diffused on the film surface due to the surface irregularities, and sufficient contrast is obtained in the polarizing plate inspection step. I can't. The upper limit of SRa is more preferably 0.032 μm or less, and further preferably 0.030 μm or less.

  In order to make SRa within the above range, it is preferable to add fine particles to the polyethylene terephthalate resin in the present invention. Thereby, workability | operativity (slidability) of a film becomes favorable and it is suitable for automation of a sticking process. Any fine particles can be selected, and examples include inorganic particles such as silica, calcium carbonate, barium sulfate, calcium sulfate, alumina, kaolinite, talc, and other organic particles. In particular, from the viewpoint of transparency and contrast, silica particles, particularly amorphous silica, having a refractive index relatively close to that of the resin component are suitable.

  On the other hand, in order to obtain high transparency and contrast, it is preferable that the polyethylene terephthalate constituting the film does not substantially contain particles. Here, “substantially contain no particles” means, for example, in the case of inorganic particles, when the inorganic element is quantified by fluorescent X-ray analysis, it is 50 ppm or less, preferably 10 ppm or less, particularly preferably the detection limit or less. Means content.

  As a preferred embodiment of the present invention, in order to obtain good transparency and stable workability (particularly surface friction characteristics), a film having a multilayer structure and a polyethylene terephthalate layer containing fine particles only in the surface layer is used. You can also. As such a film, polyethylene terephthalate having a multilayer structure (A / B / A) in which a surface layer (A layer) containing inert particles is laminated on both sides of a central layer (B layer) by a coextrusion method. It is preferable to use a film. The layers constituting the front and back surface layers may be the same or different, but in order to maintain the flatness of the film, the polyethylene terephthalate resin of the front and back surface layers may have the same configuration. desirable.

  The average particle size of the fine particles contained in the surface layer is preferably 1 to 10 μm, more preferably 1.5 to 7 μm, and still more preferably 2 to 5 μm. If the average particle diameter of the fine particles is 1.0 μm or more, it is preferable because the surface can be provided with a concavo-convex structure suitable for providing easy slipping. On the other hand, if the average particle size of the fine particles is 10 μm or less, it is preferable because high transparency and contrast are maintained. The content of inert particles in the surface layer is desirably 0.005 to 0.1% by mass, and preferably 0.008 to 0.07%. If the content of the fine particles is 0.005% by mass or more, it is preferable because the surface layer surface can be provided with a concavo-convex structure suitable for imparting slipperiness. On the other hand, if the content of fine particles is 0.1% by mass or less, high transparency and contrast properties are maintained, which is preferable.

In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method.
Take a picture of the particles with a scanning electron microscope (SEM) and at a magnification such that the size of one smallest particle is 2-5 mm, the maximum diameter of 300-500 particles (between the two most distant points) Distance) is measured, and the average value is taken as the average particle diameter.

  In the case of the (A / B / A) two-type three-layer structure containing particles in the surface layer, the thickness of the surface layer on one side is preferably 0.5 to 10 μm, and more preferably 1 to 5 μm. When the thickness of the surface layer exceeds the above range, the haze of the film may be lowered.

  The thickness of the film of the present invention is not particularly limited and is arbitrary, but is preferably 9 to 300 μm, more preferably 12 to 100 μm, and still more preferably 14 to 50 μm. If the thickness exceeds 300 μm, there is a problem in cost, retardation is increased, and visibility in crossed Nicol tends to be lowered. On the other hand, when the thickness is less than 9 μm, the mechanical properties are deteriorated and the function as a protective film cannot be performed.

  Corresponding to higher definition, the inspection accuracy of polarizing plate is improved. Therefore, it is desirable that the release film has high transparency. Therefore, the light transmittance in the biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention is preferably 85% or more, more preferably 87% or more, and further preferably 89% or more. In order to improve the inspection accuracy in the polarizing plate inspection process, the higher the light transmittance, the better, but in the polyethylene terephthalate film containing particles for slipperiness, 100% light transmittance is technically It is difficult to achieve, and the practical upper limit is 91%.

  It is desirable to obtain a high contrast in order to make the presence of foreign matter stand out in automatic inspection and to obtain more inspection accuracy. Therefore, the haze in the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention is preferably 5% or less, more preferably 4% or less, and further preferably 3% or less. More preferably, it is as follows. In order to obtain a high contrast, a lower haze is preferable, but 1% seems to be the lower limit in a polyethylene terephthalate film containing particles for slipperiness. In addition, the said haze and total light transmittance can be measured using a turbidimeter according to JIS-K7105.

  Conventionally, as in Patent Document 1, in order to improve the inspection performance by suppressing the occurrence of interference colors in the visual inspection performance under crossed Nicols, the distortion of the optical axis of the film has been reduced. However, even with such a film with improved optical axis accuracy, the contrast required for image inspection may not be obtained. Therefore, as a result of intensive studies, the present inventor has found that it is not sufficient to simply reduce the distortion of the optical axis from the mechanical axis, and the degree of orientation at the two mechanical axes is important for improving the contrast. Therefore, the MOR value in the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention is preferably 1.80 or more, more preferably 1.85 or more, and further preferably 1.90 or more. preferable. The MOR (Maximum Oriented Ratio) value is a ratio (maximum value / minimum value) between the maximum value and the minimum value of the transmission microwave intensity measured by the transmission type molecular orientation meter. The MOR value becomes smaller as the balance between the orientation difference between the longitudinal direction and the transverse direction becomes smaller, and becomes larger as the orientation strength in any one direction becomes stronger.

  The effect of the MOR value on the contrast is not well understood but is considered as follows. That is. Even when the orientation main axis coincides with the mechanical axis with high accuracy, if the orientation degree of the orientation main axis in the 90-degree direction is relatively strong, the path of the polarized light beam is blocked, resulting in a decrease in contrast. It is done. Therefore, when the MOR value is 1.80 or more, the orientation strength in the orientation main axis direction becomes strong, and the polarized light passes efficiently, which is considered to lead to improvement in contrast.

  Therefore, in order to increase the inspection accuracy, it is preferable that the MOR value is high. However, if it exceeds 2.10, the orientation strength in the direction orthogonal to the orientation main axis is weakened, so that the film is easily torn in one direction and the tear resistance is lowered. To do. For this reason, problems such as film breakage and running failure are likely to occur during inspection. Therefore, the upper limit of the MOR value is 2.10 or less, preferably 2.08 or less, more preferably 2.05 or less.

  In the biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention, the amount of change in the orientation angle in the film width direction is in the range of 3.0 ° to 5.0 ° per 500 mm. If the angle exceeds 5.0 °, uniform optical axis accuracy cannot be obtained, and the inspection property is deteriorated due to the generation of interference colors, and cannot be used for high-precision inspection. The upper limit of the amount of change in the orientation angle per 500 mm in the film width direction is preferably 4.8 ° or less, and more preferably 4.5 ° or more. On the other hand, it is preferable that the amount of change in the orientation angle is low, but as described above, the orientation of the film is distorted from the center to the end due to the bowing phenomenon in film formation. Therefore, the inclination of the optical axis tends to occur along the film width direction. Therefore, in order to achieve both high thermal stability suitable for high-accuracy inspection of polarizing plates for large screen applications, the lower limit of the amount of change in orientation angle per 500 mm in the film width direction is substantially 3.0. °, preferably 3.5 °, more preferably 4.0 °.

  The biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention preferably has high thermal dimensional stability even in post-processing at a high temperature from the viewpoint of processability. The heat shrinkage rate of the film of the present invention when heated at 150 ° C. for 30 minutes is preferably 2.0% or less in both the longitudinal direction and the width direction, and more preferably 1.5% or less. When the heat shrinkage rate is 2.0% or less, high dimensional stability can be obtained even by high-temperature heat treatment at 150 ° C. or higher, which can significantly contribute to improvement of productivity. The heat shrinkage rate is preferably low, but about 0.5% is considered the lower limit from the viewpoint of production.

  In addition, the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate of the present invention has a difference between the heat shrinkage in the longitudinal direction and the heat shrinkage in the width direction of not more than 1.0% when heated at 150 ° C. for 30 minutes. Is preferable, more preferably 0.8% or less, still more preferably 0.5% or less. When the heat shrinkage rate is substantially the same in the longitudinal direction and the width direction and has a well-balanced heat shrinkage rate, wrinkles and thickness spots are not easily generated even by heat treatment, which is extremely suitable for defect inspection with a large-screen polarizing plate.

  The manufacturing method in the biaxially stretched polyethylene terephthalate film for polarizing plate release of this invention is demonstrated. A representative example using polyethylene terephthalate pellets will be described in detail, but the present invention is not limited thereto.

  First, it is dried by film raw material drying or hot air drying so that the moisture content is less than 100 ppm. Next, each raw material is weighed and mixed, supplied to an extruder, and melt extruded into a sheet. Furthermore, the melted sheet is brought into close contact with a rotating metal roll (casting roll) using an electrostatic application method and cooled and solidified to obtain an unstretched PET sheet.

  Further, high-precision filtration is performed at any place where the molten resin is kept at 280 ° C. in order to remove foreign substances contained in the resin. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but in the case of a stainless steel sintered filter medium, the removal performance of aggregates and high melting point organic substances mainly composed of Si, Ti, Sb, Ge, Cu Excellent and suitable.

  When the surface layer (A layer) and the intermediate layer (B layer) are coextruded and laminated, two or more extruders are used to extrude the raw materials of each layer, and a multi-layer feed block (for example, a confluence having a rectangular confluence) The two layers are joined together using a block), extruded into a sheet from a slit die, and cooled and solidified on a casting roll to form an unstretched film. Alternatively, a multi-manifold die may be used instead of the multilayer feed block.

  Next, the unstretched film obtained by the above method is subjected to sequential biaxial stretching or simultaneous biaxial stretching, followed by heat treatment.

  As disclosed in Patent Documents 2 to 5, methods for improving the optical axis accuracy have been proposed so far. However, as described above, with the method disclosed above, it has been difficult to achieve both contrast and tear resistance, and optical axial accuracy and thermal dimensional stability. In the present invention, by carrying out the following stretching method, the characteristics which are contradictory to each other have been made highly compatible.

(1) Control of stretch ratio in longitudinal direction and lateral direction In order to make the MOR value within the above range, it is preferable to control the stretch ratio in the longitudinal direction and the lateral direction.

  In order to obtain a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate in the present invention, it is preferable to perform longitudinal stretching in a range of 2.0 to 3.0 times, more preferably 2.6 to 2.9 times. It is. A longitudinal draw ratio of 2.0 times or more is preferable for achieving tear resistance, and in addition, thickness variation in the flow direction is reduced, which is preferable. Moreover, it is preferable that it is 3.0 times or less because the improvement in contrast and the amount of change in the orientation main axis become small.

  On the other hand, in order to obtain the biaxially stretched polyethylene terephthalate film for releasing the polarizing plate in the present invention, it is desirable to stretch in the width direction within a range of 4.2 to 4.8 times. A lateral stretch ratio of 4.2 times or more is preferable because the visual inspection property is good due to the improvement in contrast. When it is 4.8 times or less, the tear resistance is maintained, and the frequency of breakage is reduced, which is preferable.

  The tear resistance of the film can be evaluated by measuring the tensile elastic modulus. Specifically, the smaller one of the tensile modulus in the machine direction or the transverse direction of the film is preferably 80% or more, and more preferably 100% or more. If the tensile modulus is 80% or more, the film is hardly broken even in the rework work of the release film.

(2) Control of heat setting temperature So far, heat setting processing at a relatively low temperature has been recommended in order to maintain optical axis accuracy. However, in order to achieve both axial accuracy and thermal dimensional stability, the temperature of the heat setting process is preferably 220 ° C. or higher and 230 ° C. or lower in the present invention. When the temperature of the heat setting treatment is 220 ° C. or higher, the absolute value of the heat shrinkage ratio is preferably reduced. Moreover, it is preferable that the temperature of the heat setting treatment is 230 ° C. or less because the film is hardly opaque and the frequency of breakage is reduced.

  It is effective to control the heat shrinkage rate, particularly the heat shrinkage rate in the width direction, by narrowing the guide rail of the gripping tool by the heat setting process. The temperature for the relaxation treatment can be selected in the range from the heat setting treatment temperature to the glass transition temperature Tg of the polyethylene terephthalate film, but is preferably (heat setting treatment temperature) −10 ° C. to Tg + 10 ° C. The width relaxation rate is preferably 1 to 6%. If it is less than 1%, the effect is small, and if it is 6% or less, it is preferable in terms of the flatness of the film.

  The film of the present invention is produced by the above method, but is not limited to the above specifically disclosed method as long as it is within the scope of the technical idea. In producing the film of the present invention, it is important to control the longitudinal stretching, the lateral stretching, and the heat setting with high accuracy in a very narrow range based on the above technical idea.

  Still another manufacturing method will be described. As a method of stretching the film in the biaxial direction, simultaneous biaxial stretching or the obtained unstretched sheet is stretched in the longitudinal direction by a roll or a tenter type stretching machine, and then orthogonal to the first stretching direction. The method of extending | stretching to the width direction can be mentioned. The stretching temperature in the longitudinal direction needs to be 75 to 110 ° C. When the stretching temperature in the longitudinal direction is 75 ° C. or less, the film is broken and stable production cannot be performed. Further, at 110 ° C. or higher, thickness unevenness occurs in the obtained film, which can be used as a polarizing plate release application.

  Next, the effect of this invention is demonstrated using an Example and a comparative example. First, the evaluation method of the characteristic values used in the present invention is shown below.

[Evaluation method]

(1) Light transmittance, haze Measured according to JIS K 7105 “Testing method for optical properties of plastic” haze (cloudiness value). NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd. was used for the measuring instrument.

(2) Haze change (△ haze)
The haze value (A) before the heat treatment of the film and the haze value (B) after being heat-treated at 170 ± 3 ° C. for 20 minutes under no load at 170 ° C. in an atmosphere of 170 ° C. JIS K 7105 “Plastic Measured in accordance with “Testing method for optical properties of haze” (haze value). The amount of change in haze was determined from the following formula and evaluated according to the following classification.
Change in haze (Δhaze) (%) = (B) − (A)
<Haze change amount classification criteria>
○: Haze change amount is 0.1% or less ×: Haze change amount exceeds 0.1%

(3) Surface layer A, cyclic trimer content of polyester resin Finely pulverize film or resin, 0.1 g of mixed solvent of hexafluoroisopropanol (HFIP) / chloroform (2/3 (volume ratio)) Dissolved in 3 ml. To the resulting solution, 20 ml of chloroform was added and mixed uniformly. 10 ml of methanol was added to the resulting mixture to reprecipitate the linear polyester. Next, this mixed solution was filtered, and the precipitate was washed with 30 ml of a mixed solvent of chloroform / methanol (2/1 (volume ratio)) and further filtered. The obtained filtrate was concentrated to dryness on a rotary evaporator. 10 ml of dimethylformamide was added to the concentrated dried product to prepare a cyclic trimer measurement solution. This measurement solution was quantified using LC100 type high performance liquid chromatography manufactured by Yokogawa Electric Corporation.
In addition, the sample of the surface layer A produced the film sample extruded only by A layer, and used it as the sample.

(4) Surface layer A, hydroxyl (OH) terminal amount of polyester resin The film or resin was finely pulverized, and 15 mg was weighed. After completely dissolving in 0.1 ml of hexafluoroisopropanol (HFIP) -d2, it was diluted with 0.6 ml of deuterated chloroform. Furthermore, in order to shift the OH group peak of HFIP, 30 μl of pyridine-d5 was added, and measurement was performed by H-NMR (BBO-5 mm probe).
In addition, the sample of the surface layer A produced the film sample extruded only by A layer, and used it as the sample.

(5) Intrinsic viscosity 0.2 g of polyester was dissolved in 50 ml of a mixed solvent of phenol / 1,1,2,2-tetrachloroethane (60/40 (weight ratio)), and an Ostwald viscometer was used at 30 ° C. It was measured. In addition, the sample of the surface layer A produced the film sample extruded only by A layer, and used it as the sample.

(6) Heat shrinkage rate Measured according to JIS C 2318-1997 5.3.4 (dimensional change). In the direction to be measured, the film is cut to a width of 10 mm and a length of 250 mm, marked at intervals of 200 mm, and the distance (A) between the marks is measured under a constant tension of 5 gf. Next, the film is placed in an oven in an atmosphere of 150 ° C., subjected to heat treatment at 150 ± 3 ° C. for 30 minutes under no load, and then the mark interval (B) is measured under a constant tension of 5 gf. The thermal shrinkage rate was calculated from the following formula.
Thermal contraction rate (%) = (A−B) / A × 100

(7) Three-dimensional center plane average surface roughness (SRa)
Using a stylus type three-dimensional surface roughness meter (SE-3AK, manufactured by Kosaka Laboratory Ltd.), the surface of the film was subjected to the conditions of a needle radius of 2 μm, a load of 30 mg, and a needle speed of 0.1 mm / sec. , Measured in a longitudinal direction of the film with a cut-off value of 0.25 mm, measured over a measurement length of 1 mm, divided into 500 points at a 2 μm pitch, and the height of each point was measured with a three-dimensional roughness analyzer (manufactured by Kosaka Laboratory Ltd. , TDA-21). The same operation was performed 150 times continuously at intervals of 2 μm in the width direction of the film, that is, over 0.3 mm in the width direction of the film, and the data was taken into the analyzer. Next, the three-dimensional average surface roughness SRa was determined using the analyzer. The unit of SRa is μm. In addition, the measurement was performed 3 times and those average values were employ | adopted.

(8) MOR value The MOR value was measured using a MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments. The points to be measured are a total of three points: a central point in the width direction of the film and a 1/5 point on the end side connecting the central portion and both ends of the film. That is, the MOR value is measured at three points at a distance of 10%, 50%, and 90% from one end of the straight line in the width direction of the film.

(9) Molecular chain principal axis orientation angle (θ), optical principal axis tilt angle (ξ)
In the film width, the edge is 0%, and the other edge is 100%. From the region corresponding to 10% of the film width to the region corresponding to 90%, n 100 mm square film samples were cut out continuously at a pitch of 100 mm in the width direction. The square film sample was cut at a right angle with respect to either the longitudinal or width axis. For each film sample, using a MOA-6004 type molecular orientation meter manufactured by Oji Scientific Instruments Co., Ltd., the orientation angle (θi) of the molecular chain principal axis relative to the film longitudinal direction, and the machine axis direction (longitudinal direction) defined by the following formula , Or the width direction), the tilt angle (ξi) of the optical principal axis was measured.
When | θ | ≦ 45 degrees ξ = | θ |
When | θ |> 45 degrees ξ = | 90 degrees− | θ ||

(10) Amount of change in orientation angle Among the inclination angles of the optical main axis measured from the film sample, the maximum value was the maximum inclination angle (ξmax) of the optical main axis, and the minimum value was the minimum inclination angle (ξmin).
When the measurement position where the maximum inclination angle is obtained is Lmax (mm) and the measurement position where the minimum inclination angle is obtained is Lmin (mm), the amount of change in the orientation angle per 500 mm can be expressed by the following equation.
(Change in orientation angle) = (ξmax−ξmin) / (Lmax−Lmin) × 500

(11) Breaking resistance The obtained film was cut into a width of 10 mm, and in accordance with JIS-K-7127 (2000), using “Tensilon Universal Testing Machine RTA-T-4M” manufactured by Orientec Co., Ltd., the initial length A tensile test was performed at 50 mm and a tensile speed of 200 mm / min. Using the first linear portion of the stress-strain curve obtained by the tensile test, the tensile elastic modulus was obtained by dividing the difference in stress between two points on the straight line by the difference in strain between the same two points. . The measurement was performed in a standard environment adjusted to a temperature of 23 ± 2 ° C. and a relative humidity of 50 ± 15% RH, and the measurements were made in the vertical and horizontal directions. For the tensile modulus in the machine direction or the transverse direction, the smaller value of 80% or more was rated as ◯, and the value less than 80% was rated as x.

(12) Thermal wrinkle determination method The following silicone coating solution was applied to one side of the obtained film with a coating tension of 10 kg / m by a die coating method and dried in an oven at 120 ° C.
(Silicone coating solution)
Curing silicone (KS847H, Shin-Etsu Chemical) 100 parts by weight Curing agent (CAT PL-50T, Shin-Etsu Chemical) 2 parts by weight Diluent Methyl ethyl ketone / xylene / methyl isobutyl ketone 898 parts by weight -Cut from the surface, spread a length of 5 m on a flat table, reflect the light of the fluorescent lamp on the coated surface, and check for the presence of thermal wrinkles by the following evaluation method.
○: Thermal wrinkles are not seen at all and are good.
(Triangle | delta): A heat wrinkle is not seen on the whole surface, but a heat wrinkle is seen partially.
X: Thermal wrinkles can be confirmed on the entire surface.

(13) Inspectability The release film obtained in (12) was adhered to the polarizing film through an adhesive so that the width direction was parallel to the alignment axis of the polarizing film to obtain a polarizing plate. Between the white light source and the camera, two polarizing plates were placed in crossed Nicols, and a polarizing plate with a release film adhered between them was placed. A 5000-pixel line camera equipped with a 180 W Metahara transmission light as a light source and a 55 mm macro lens as a camera was used. Foreign matter inspection was performed on the inspection range (12 × 7 cm) while moving the sample at 5 m / min. The image signal from the camera was processed by an image processing apparatus, and the detected defects of 50 μm or more were measured. A similar sample was also measured for defects of 50 μm or more under crossed Nicols in the inspection range (12 × 7 cm). From the obtained results, the testability was evaluated as follows.
○: Contrast is good, and visual results and automatic inspection results are equivalent (98% of visual inspection defects)
(The above is automatically detected) and the slipperiness during the automatic inspection is good. △: The visual result and the automatic inspection result are almost equal (less than 98% of the number of defects of the visual inspection is automatically detected.

  The manufacturing method of the polyethylene terephthalate resin used in the following examples and comparative examples is as follows.

(Production of PET (B))
1,000 parts of dimethyl terephthalate, 700 parts of ethylene glycol, and 0.3 part of zinc acetate dihydrate were charged into a transesterification reaction can and subjected to a transesterification reaction at 120 to 210 ° C., and the produced methanol was distilled off. Further, 0.13 phosphoric acid and 0.3 part of antimony trioxide were added, and the pressure in the system was gradually reduced to 1 mmHg or less in 75 minutes. At the same time, the temperature was gradually raised to 280 ° C. A polycondensation reaction was carried out for 70 minutes under the same conditions, and the molten polymer was extruded into water from a discharge nozzle and formed into a cylindrical chip having a diameter of about 3 mm and a length of about 5 mm by a cutter. The obtained crude polyester had an intrinsic viscosity of 0.610 dl / g, a hydroxyl (OH) terminal amount of 70 eq / ton, and a cyclic trimer content of 1.05% by mass. In addition, all “parts” in the examples represent parts by mass. Let the obtained polyester be a polyester resin (PET (B)).

(Production of PET (0))
1,000 parts of dimethyl terephthalate, 700 parts of ethylene glycol, and 0.3 part of zinc acetate dihydrate were charged into a transesterification reaction can and subjected to a transesterification reaction at 120 to 210 ° C., and the produced methanol was distilled off. When the transesterification reaction is completed, silica particles (Fuji Silysia Chemical Co., Ltd., Silicia 310) are added to a final concentration of 1000 ppm, 0.13 phosphoric acid and 0.3 part antimony trioxide are added, The pressure inside the system was gradually reduced to 1 mmHg or less in 75 minutes. At the same time, the temperature was gradually raised to 280 ° C. A polycondensation reaction was carried out for 70 minutes under the same conditions, and the molten polymer was extruded into water from a discharge nozzle and formed into a cylindrical chip having a diameter of about 3 mm and a length of about 5 mm by a cutter. The obtained crude polyester had an intrinsic viscosity of 0.610 dl / g, a hydroxyl (OH) terminal amount of 70 eq / ton, and a cyclic trimer content of 1.05% by mass. Let the obtained crude polyester be a polyester resin (PET (0)).

Preparation of (PET (A) I) A polyester resin (PET (0)) was dried at 160 ° C. under reduced pressure, and then nitrogen gas conditioned to a moisture content of 6.4 g / Nm ³ was added per 1 kg of crude polyester. Circulated at a rate of 70 liters per hour and heat-treated at 207 ° C. for 48 hours. The obtained polyester had an intrinsic viscosity of 0.631 dl / g, a hydroxyl (OH) end amount of 59 eq / ton, and a cyclic trimer content of 0.27% by mass.

(Preparation of PET (A) II)
The polyester resin (PET (0)) was dried at 160 ° C. under reduced pressure, and then nitrogen gas conditioned to a moisture content of 15.3 g / Nm ³ was circulated at 300 liters per kg of crude polyester at 230 liters. Heat treatment was performed at ° C for 12 hours. The obtained polyester had an intrinsic viscosity of 0.617 dl / g, a hydroxyl (OH) terminal amount of 63 eq / ton, and a cyclic trimer content of 0.28% by mass.

(Preparation of PET (A) III)
Similar to the polyester resin (PET (0)) except that the polymerization time was adjusted, the intrinsic viscosity was 0.648 dl / g, the hydroxyl (OH) terminal amount was 64 eq / ton, and the cyclic trimer was contained. A crude polyester having an amount of 1.2% by mass was obtained. The obtained crude polyester was dried at 160 ° C. under reduced pressure, and then nitrogen gas conditioned to a water content of 15.3 g / Nm ³ was circulated at 300 liters per kg of the crude polyester at 220 ° C. Heat treatment was performed for 24 hours. The obtained polyester had an intrinsic viscosity of 0.623 dl / g, a hydroxyl (OH) terminal amount of 57 eq / ton, and a cyclic trimer content of 0.27% by mass.

(Preparation of PET (A) IV)
The polyester resin (PET (0)) was dried at 160 ° C. under reduced pressure, adjusted to a slight pressure of 0.1 kg / cm ² under a nitrogen atmosphere, and heat-treated at 215 ° C. for 24 hours. The obtained polyester had an intrinsic viscosity of 0.622 dl / g, a hydroxyl (OH) terminal amount of 73 eq / ton, and a cyclic trimer content of 0.30% by mass.

Example 1
(1) Production of biaxially stretched polyethylene terephthalate film for releasing polarizing plate PET (A) I was dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours as a raw material for the surface layer (A), and then supplied to the extruder 1. . Further, PET (B) as a raw material for the intermediate layer (B) was dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and then supplied to the extruder 2. The raw material supplied to the extruder 2 and the extruder 1 is set to a resin temperature of 280.degree. The layers were laminated using a layered confluence block so as to be A / B / A, and melt-extruded into a sheet form from the die. In addition, the thickness ratio of A layer and B layer was controlled using the gear pump of each layer so that it might become A / B / A = 8/84/8. In addition, a stainless steel sintered filter medium (nominal filtration accuracy: 95 μm of 10 μm particles) was used for each of the filters. The temperature of the die was controlled so that the temperature of the extruded resin was 275 ° C.

  Then, the extruded resin was cast on a cooling drum having a surface temperature of 30 ° C. and brought into close contact with the cooling drum surface using an electrostatic application method to be cooled and solidified to form an unstretched film having a thickness of 480 μm.

  The obtained unstretched sheet was heated to 78 ° C. with a roll group heated to 78 ° C., then heated to 105 ° C. with an infrared heater, and the roll group with a difference in peripheral speed was 2 in the longitudinal direction. Stretched 8 times.

  Subsequently, the obtained uniaxially stretched film was gripped with a clip and subjected to transverse stretching. The transverse stretching temperature of TD1 was 120 ° C., and the transverse stretching ratio was 4.5 times. Next, heat treatment was performed at 228 ° C. for 15 seconds, and 3% relaxation treatment was performed at 200 ° C. to obtain a biaxially stretched polyethylene terephthalate film for releasing a polarizing plate. The obtained film physical properties are shown in Table 3.

  The biaxially stretched polyethylene terephthalate film obtained in this example not only has excellent thermal dimensional stability at high temperatures, but also has excellent polarizing plate inspection properties. It was a film that can be suitably used for inspection.

Examples 2-3
A film was obtained in the same manner as described in Example 1 except that the raw material used for the surface layer (A) was changed as shown in Table 2. The obtained film physical properties are shown in Table 3.

Example 4
A film was obtained in the same manner as described in Example 1 except that the longitudinal draw ratio was changed to 2.6 times. The obtained film physical properties are shown in Table 3. The biaxially stretched polyethylene terephthalate film obtained in this example suppressed the change in the orientation angle and achieved a further excellent polarizing plate inspection property by having a large MOR value.

Example 5
A film was obtained in the same manner as described in Example 1 except that the longitudinal draw ratio was changed to 2.9 times. The obtained film physical properties are shown in Table 3. The biaxially stretched polyethylene terephthalate film obtained in this example was a film that could be used suitably for high-accuracy inspection, although the orientation angle fluctuation and the MOR value were slightly deteriorated.

Examples 6-8
A film was obtained in the same manner as in Example 1 except that the film forming conditions in Example 1 were changed as shown in Table 2. The obtained film physical properties are shown in Table 3.

Comparative Examples 1 and 2
A film was obtained in the same manner as described in Example 1 except that the PET of the surface layer A was changed as shown in Table 2. Table 3 shows the physical properties of the obtained film. The biaxially stretched polyethylene terephthalate film obtained in this example is excellent in dimensional stability at high temperatures, but has a large amount of change in haze after heat treatment and is inferior in polarizing plate inspection.

Comparative Example 3
A film was obtained in the same manner as described in Example 1, except that the changes were made as shown in Table 2. Table 3 shows the physical properties of the obtained film. The biaxially stretched polyethylene terephthalate film obtained in this example was inferior in dimensional stability at high temperatures, and could not be used for high-precision inspection in a polarizing plate manufacturing process for large screen applications.

Comparative Example 4
A film was obtained in the same manner as described in Example 1, except that the changes were made as shown in Table 2. Table 3 shows the physical properties of the obtained film. The biaxially stretched polyethylene terephthalate film obtained in this example had a low MOR value and a large change in the orientation angle, which was inferior in polarizing plate inspection.

Comparative Example 5
A film was obtained in the same manner as described in Example 1, except that the changes were made as shown in Table 2. Table 3 shows the physical properties of the obtained film. The biaxially stretched polyethylene terephthalate film obtained in this example was inferior in dimensional stability at high temperatures and inferior in polarizing plate inspection properties because of its small MOR value.

Comparative Example 6
The same method as in Example 1 was used, except for changing the conditions described in Table 2. The obtained film physical properties are shown in Table 3. The biaxially stretched polyethylene terephthalate film obtained in this example was easy to tear in one direction and had low tear resistance.

  The biaxially stretched polyethylene terephthalate film for releasing a polarizing plate of the present invention can be used by being attached to a polarizing plate which is a constituent member of a large liquid crystal display device, and is also suitable for defect inspection under crossed Nicols.

Claims (2)

A biaxially stretched polyethylene terephthalate film that satisfies the following requirements (1) and (2).
(1) It has a laminated structure of three or more layers by a coextrusion method. (2) The polyester constituting both surface layers has a hydroxyl (OH) terminal amount of 70 eq / ton or less. Furthermore, the biaxially-stretched polyethylene terephthalate film of Claim 1 which satisfy | fills the following requirements (3).
(3) The cyclic trimer content of the polyester constituting both surface layers is 0.45% by mass or less.