JP2013200435A - Polyester film for polarizing plate-protection - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film for polarizing plate-protection characterized by allowing suppression of light interference coloring when disposed in a cross-nicol state as a polarizing plate.SOLUTION: A polyester film for polarizing plate-protection is characterized in that the film is a biaxially orientated polyester film configured with a 3 or more layer-laminated structure with both outermost layers having a thickness of 3.5 μm or more, the film has an in-plane retardation of 800 nm or less, the shrinkage (MD) in the film longitudinal direction when heat-treated at 180°C for 5 minutes is 3.0% or less, the shrinkage (TD) in the direction orthogonal to the film longitudinal direction is 1.0% or less, and the absolute value of shrinkage difference between MD and TD is 2.0% or less.

Description

  The present invention relates to a polarizing plate protective film, that is, a polarizing plate protective polyester film suitably used for a film used for protecting a polarizing plate by being laminated on a polarizing film.

  A polarizing plate used for a liquid crystal display is generally composed of a protective film / polarizing film / protective film, or a protective film / polarizing film / retardation film. As a protective film for a conventional polarizing plate, its high transparency and Many TAC films have been used because of their characteristics such as optical isotropy and low foreign matter. However, since the TAC film is formed by the solution casting method, it is not sufficient in terms of chemical resistance, scratch resistance, etc. In addition, as the size of liquid crystal displays increases in recent years, Variations in heat resistance and mechanical strength within the display surface are a problem. In addition, while the demand for cost reduction for liquid crystal displays is increasing, the high cost of TAC films is a problem. While the demand for liquid crystal displays is growing significantly, the supply of TAC films tends to be insufficient, and there is concern about a stable supply in the future.

  Many studies have been conducted to replace the TAC film with other materials such as cycloolefin polymer for the above problems (Patent Documents 1 and 2). However, since films made of other materials do not use general-purpose resins, there is a problem that costs are high. On the other hand, since the biaxially oriented polyester film is a general-purpose resin, there is no problem in terms of cost, but the biaxially oriented polyester film has birefringence and its main orientation axis does not exist in a certain direction in the film plane. For this reason, depending on the orientation design, a light interference color may be generated when the polarizing plate is arranged in a crossed Nicol state, or sufficient luminance may not be obtained depending on the angle of the main orientation axis. On the other hand, the unstretched polyester film is non-oriented, so there is no problem of light interference color and brightness reduction. That is not preferable.

  In addition, a method has been proposed in which the occurrence of light interference color is suppressed by balancing the aspect ratio of the film stretch ratio (Patent Documents 3 and 4). For example, Patent Document 3 proposes a method for producing a film in which both longitudinal stretching and lateral stretching are stretched by 3.6 times. Patent Document 4 proposes a method for producing a film in which both longitudinal stretching and lateral stretching are stretched by 3.7 times. Although both can suppress the light interference color, since the temperature at the time of stretching is high, there is a problem that breakage at the time of film formation tends to occur.

  Moreover, the polarizing plate is normally provided with the adhesive layer for bonding on the single side | surface. Furthermore, surface protective films are provided on both sides for the purpose of preventing damage, moisture, or dust adhesion during handling or during the manufacturing process of the liquid crystal display device. A surface protective film (protective film) in which a pressure-sensitive adhesive layer is laminated is used on the side opposite to the pressure-sensitive adhesive layer side of the polarizing plate, and a release film in which a release layer is laminated on the pressure-sensitive adhesive layer side (hereinafter referred to as “the protective film”). Unless otherwise specified, the polarizing plate means a surface protective film and a release film disposed on both sides).

  A polarizing plate having a surface protective film and a release film on both sides is usually processed in a roll state, then cut into a sheet state, stacked in units of several tens of sheets, packed, and supplied to a panel manufacturer.

  Panel manufacturers have a process of peeling the release film from the polarizing plate and bonding it to the liquid crystal cell via an adhesive. In this process, it is desirable to shorten the time required for bonding from the viewpoint of improving productivity. ing.

  Usually, the polarizing plates are taken out one by one from the stacked packaging state using a suction device and transferred to the bonding step. The suction device is composed of a suction cup and a shaft that sticks to the release film side of the polarizing plate, and has a mechanism that moves up and down in the thickness direction of the polarizing plate. The polarizing plate is taken out and transferred one by one. The transferred polarizing plate is set on a base having a hole (hereinafter referred to as a polarizing plate setting base) and fixed to the laminating machine by suction from the hole portion. Thereafter, the release film is peeled off, and a polarizing plate is bonded to the liquid crystal cell.

  If two polarizing plates are overlapped from the stacked polarizing plates and the polarizing plates are taken out, the overlapping polarizing plates will be peeled off in the step of peeling the release film in the subsequent step. In that case, since the adhesive is not exposed, a problem that the polarizing plate cannot be bonded to the liquid crystal cell occurs, and the bonding machine stops. This greatly increases production loss. In order to prevent this problem, it is important that the polarizing plates are reliably taken out one by one. If the polarizing plates are not curled, the ease of peeling between the stacked polarizing plates will not be stable, and it will not be possible to take out the polarizing plates one by one only by the vertical movement of the suction device. .

  In addition, if the amount of curl and the direction of curl of the polarizing plates are not aligned, there may be a deviation in the positional relationship with the suction device when the polarizing plates are taken out from the stacked polarizing plates one by one. For this reason, the positional relationship between the polarizing plate setting table and the polarizing plate is shifted, and adjustment may take time. Furthermore, if the surface of the polarizing plate curled inward is the release film surface or its opposite surface, the suction force is adjusted for each polarizing plate when fixing the polarizing plate to the polarizing plate set base. In some cases, adjustment time may be required, and the bonding production speed decreases.

  As a method for adjusting the curl of the polarizing plate, there has been reported a method in which a release film laminated on the pressure-sensitive adhesive layer side of the polarizing plate is tensioned and bonded to the polarizing plate (Patent Document 5). However, if the tension is not appropriate, flatness defects such as wrinkles may occur in the release film. Planarity defects such as wrinkles may cause air bubbles to enter the surface of the pressure-sensitive adhesive, and it is preferable that they be as few and stable as possible.

  It has been reported that curling is adjusted by adjusting the shrinkage rate of a polyester film (Patent Document 6). However, in Patent Document 5, optical interference occurs and it is not preferable to use it for a protective film for a polarizing plate.

Japanese Patent No. 3297450 JP 2006-227090 A JP-A-6-297561 JP 2011-110718 A JP 2005-49597 A JP 2007-183423 A

  The present invention has been made in view of the above circumstances, and the problem to be solved is to provide a polarizing plate protecting polyester film characterized in that generation of a light interference color can be suppressed when the polarizing plate is placed in a crossed Nicol state. There is to do.

  As a result of intensive studies in view of the above problems, the present inventors have found that the above problems can be easily solved by a specific polarizing plate protecting polyester film, and have completed the present invention.

  That is, the gist of the present invention is a biaxially oriented polyester film having a laminated structure of three or more layers having a thickness of both outermost layers of 3.5 μm or more, the in-plane retardation of the film is 800 nm or less, and 180 ° C. The shrinkage rate (MD) in the film length direction when heat-treated at 5 minutes is 3.0% or less, and the shrinkage rate (TD) in the direction perpendicular to the film length direction is 1.0% or less, The absolute value of the difference in shrinkage between MD and TD is 2.0% or less.

  According to the present invention, an inexpensive polyester film having excellent optical properties can be provided as a polarizing plate protective film, and the industrial value of the present invention is high.

  The polyester film of the present invention is a laminated film consisting of an inner layer (hereinafter sometimes referred to as B layer) and at least three layers, all of which are co-melt extruded from a die of an extruder. What is extruded by the so-called coextrusion method is obtained as a stretched and heat-treated product. Hereinafter, the coextruded three-layer film will be described. However, the present invention is not limited to the three-layer film as long as the gist of the present invention is not exceeded, and may be a multilayer of four layers or more.

  In the present invention, the polyethylene terephthalate copolymer constituting both outermost layers is usually a polyester comprising 85 to 95 mol% of ethylene terephthalate. In this polyester, the components that can be copolymerized as the third component include acid components such as dicarboxylic acid components such as 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, and the like. Mellitic acid, pyromellitic acid and the like can be mentioned, and among them, it is preferable to use isophthalic acid as a copolymerization component. Examples of the alcohol component that can be copolymerized include diol components such as butylene glycol, propylene glycol, polyethylene glycol, 1,4-cyclohexanedimethanol, diethylene glycol, neopentyl glycol, and polyalkylene glycol. . These can be used alone or in combination of two or more.

  In the present invention, the copolyester constituting the B layer is a copolyester comprising a dicarboxylic acid component containing terephthalic acid and isophthalic acid and a glycol component containing ethylene glycol, and a polyester layer containing the copolyester It is.

  Moreover, B layer of the film of this invention consists of a polyester composition containing the said copolyester, and contains 1 or more types of polyester other than copolyester. The bifunctional acid component of the polyester other than the copolyester may be any one that mainly comprises an aromatic dicarboxylic acid or an ester-forming derivative thereof, such as terephthalic acid, 2,6-naphthalenedicarboxylic acid, or an ester-forming derivative thereof. Examples thereof include dimethyl terephthalate and dimethyl 2,6-naphthalenedicarboxylate. Among these, terephthalic acid and dimethyl terephthalate are preferable. Examples of the glycol component include ethylene glycol, butylene glycol, propylene glycol, polyethylene glycol, 1,4-cyclohexanedimethanol, and among these, ethylene glycol is preferable.

  Further, the polyester may be a polyester using one kind of aromatic dicarboxylic acid or an ester-forming derivative thereof and one kind of alkylene glycol as a starting material, and is a copolymer containing two or more kinds of components. Also good. As the components to be copolymerized, in addition to the above, for example, diol components such as diethylene glycol, neopentyl glycol, polyalkylene glycol, dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, trimellitic acid, pyromellitic An acid etc. are mentioned.

  In order to facilitate handling, the polyester film in the present invention may contain particles under conditions that do not impair transparency. Examples of particles used in the present invention include inorganic particles such as calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, and crosslinked polymers. Examples thereof include organic particles such as particles and calcium oxalate. Examples of the method of adding particles include a method of adding particles in a polyester as a raw material, a method of adding directly to an extruder, and the like. Two methods may be used in combination.

  The particle size of the particles used is usually 0.05 to 5.0 μm, preferably 0.1 to 4.0 μm. When the average particle size is larger than 5.0 μm, the haze of the film increases and the transparency of the film may be lowered. If the average particle size is smaller than 0.1 μm, the surface roughness becomes too small, and the film may be difficult to handle. The particle content is usually 0.001 to 30.0% by weight, preferably 0.01 to 10.0% by weight, based on the polyester. If the particle content is large, the haze increases and the transparency of the film may be lowered. If the particle content is small, the film may be difficult to handle.

  The method of adding particles to the polyester is not particularly limited, and a conventionally known method can be adopted. For example, it can be added at any stage for producing the polyester, but the polycondensation reaction may proceed preferably after the esterification stage or after the transesterification reaction. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a method of blending dried particles and a polyester raw material using a kneading extruder. It is performed by the method of blending.

  As a method for obtaining the B layer of the film of the present invention, a method in which a copolymerized polyester and another polyester are blended and melt-kneaded is preferably used. The content of the copolyester in the present invention is usually 40% by weight or more, preferably 50% by weight or more. If it is less than 40% by weight, the light interference color when incorporated into a liquid crystal panel is strong, and an image may be considerably different from the original color tone.

  In the polarizing plate using the polyester film of the present invention as a protective film, the in-plane retardation of the film in the polyester film needs to be 800 nm or less, preferably 500 nm or less, in order to prevent the occurrence of light interference color. When the in-plane retardation of the film is larger than 800 nm, the interference color of light becomes strong, and the original color of the image cannot be obtained in the liquid crystal display.

  The thickness of the polyester film of the present invention is preferably 4 to 50 μm, more preferably 4 to 38 μm. When the thickness of the film is less than 4 μm, it may be difficult to form the film, and it may be difficult to handle the film. When the film thickness is thicker than 50 μm, the polarizing plate becomes thick.

  The polyester film of the present invention is a biaxially oriented polyester film having a laminated structure of three or more layers, and the thickness of both outermost layers is 3.5 μm or more, preferably 5.0 μm or more. If the thickness of both outermost layers is less than 3.5 μm, there is a problem that breakage tends to occur during film formation.

  The polyester film in the present invention has a shrinkage ratio (MD) in the film longitudinal direction of 3.0% or less when heat-treated at 180 ° C. for 5 minutes, and a shrinkage ratio (TD) in the direction perpendicular to the film longitudinal direction is 1.0. % Or less. More preferably, MD is 2.5% or less and TD is 0.5% or less. When MD is larger than 3.0% or TD is larger than 1.0%, the amount of curling becomes too large, and it is not preferable because it cannot be applied well to the polarizing plate. In addition, the absolute value of the difference in shrinkage between MD and TD needs to be 2.0% or less, and preferably 1.5% or less. If it is more than 2.0%, the curl amount becomes too large, and it is not preferable because it cannot be applied well to the polarizing plate.

  In the present invention, other additives may be added as necessary. Examples of such additives include stabilizers, lubricants, crosslinking agents, antiblocking agents, antioxidants, dyes, pigments, and the like.

  In the present invention, a polyester chip dried by a known method is supplied to a melt extrusion apparatus and heated to a temperature equal to or higher than the melting point of each polymer to be melted. Next, the molten polymer is extruded from a die and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition point to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed.

  In the present invention, the sheet thus obtained is preferably stretched in the biaxial direction to form a film. Specifically describing the stretching conditions, the unstretched sheet is preferably stretched 1.3 to 6 times at 80 to 130 ° C. in the longitudinal direction to form a longitudinal uniaxially stretched film, and then 90 to 160 ° C. in the lateral direction. And stretched 1.3 to 6 times. Heat treatment is preferably performed at 150 to 240 ° C. for 1 to 600 seconds. Further, at this time, a method of relaxing 0.1 to 20% in the longitudinal direction and / or the transverse direction in the maximum temperature zone of the heat treatment and / or the cooling zone at the heat treatment outlet is preferable.

  As a polarizing plate, it is preferable to provide a coating layer on at least one side in order to make it adhere to an adhesive of PVA or to improve adhesion to a hard coat.

  Further, the coating layer may contain an antistatic agent, an antifoaming agent, a coating property improving agent, a thickener, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment, and the like.

  As a coating method of the coating agent, a reverse roll coater, a gravure coater, a rod coater, an air doctor coater, or a coating apparatus other than these can be used.

  In addition, in order to improve the applicability | paintability and adhesiveness to the film of a coating agent, you may give a chemical process and an electrical discharge process to a film before application | coating. Further, in order to further improve the surface characteristics, a discharge treatment may be performed after the coating layer is formed.

The thickness of the coating layer is usually in the range of 0.02 to 0.5 μm, preferably 0.03 to 0.3 μm, as the final dry thickness. When the thickness of the coating layer is less than 0.02 μm, the effect of the present invention may not be sufficiently exhibited. When the thickness of the coating layer exceeds 0.5 μm, the films tend to stick to each other, and particularly when the coated film is re-stretched to increase the strength of the film, it adheres to the roll in the process. There is a tendency to become easy. The above problem of sticking appears particularly when the same coating layer is formed on both sides of the film.
If necessary, the film may be coated by a method called offline coating which is coated after the production of the film. The coating material may be either water-based and / or solvent-based in the case of off-line coating.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. Various physical properties and characteristics are measured or defined as follows. In the examples, “%” means “% by weight”.

(1) Measurement of main orientation axis Using a polarizing microscope manufactured by Carl Zeiss, the orientation of the polyester film is observed, and how many times the direction of the main orientation axis in the polyester film plane is inclined with respect to the MD of the polyester film. Asked. For measurement, when the main orientation axis exceeded 90 degrees, the complementary angle was defined as the angle of the main orientation axis with respect to the MD direction.

(2) Measurement of in-plane retardation The in-plane retardation of the film was measured using a cell gap inspection apparatus RETS-1100A manufactured by Otsuka Electronics Co., Ltd. The in-plane retardation of the film was measured at 23 ° C. using an optical interference method with an aperture diameter of 5 mm.

(3) Measurement of heat shrinkage rate A sample is cut out on a strip 15 mm wide × 150 mm long in the longitudinal direction from both ends of the film, heat treated in a 180 ° C. atmosphere for 5 minutes in a no-tension state, and the length of the sample before and after that is measured. Thus, the heat shrinkage rate (%) was calculated by the following formula.
Heat shrinkage rate (%) = [(ab) / a] × 100
(In the above formula, a is the sample length before heat treatment, and b is the sample length after heat treatment)
Further, the difference in shrinkage rate was calculated so as to be positive by subtracting a small value from a value larger than the measurement results at both ends.

(4) Inspection of visibility Using a polyvinyl alcohol (PVA) film (manufactured by Kuraray Co., Ltd., polymerization degree 2400), after stretching 3 times in the first bath (iodine, KI aqueous solution-30 ° C), the second bath ( The total draw ratio was stretched up to 6 times in boric acid and KI aqueous solution (55 ° C.) to obtain a polarizer. Then, using a PVA adhesive, a polarizing plate was prepared by laminating a TAC film having a thickness of 40 μm on one side and a polyester film on one side. The polarizing plate was mounted on a mobile liquid crystal panel as a polarizing plate on the backlight unit side so that the TAC film side was the liquid crystal side, and the visibility was confirmed.
◎: Brightness is good with no light interference color ○: Brightness is slightly reduced but there is no problem with light interference color △: There is a light interference color and the image is different from the original color tone ×: The light interference color is strong, and the image is considerably different from the original color tone.

(5) Evaluation of ease of breakage The ease of breakage in film formation was evaluated by the frequency of breakage.
<Evaluation criteria for ease of fracture>
(Difficult to break) ○> × (Easy to break)
In addition, among the above criteria, the ones with ◯ are levels that can be used without problems in production.

(6) Comprehensive evaluation Visibility and ease of breakage were comprehensively evaluated, and those that were excellent as polarizing plate protective films were evaluated as "good" and insufficient.

(Polyester chip manufacturing method)
Take 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and 0.07 part of calcium acetate monohydrate in a reactor, heat up the temperature and distill off methanol to conduct a transesterification reaction. In short, the temperature was raised to 230 ° C., and the transesterification reaction was substantially completed. Next, 0.04 part of phosphoric acid and 0.035 part of antimony trioxide were added and polymerized in accordance with a conventional method. That is, the reaction temperature was gradually raised to finally 280 ° C., while the pressure was gradually reduced to finally 0.05 mmHg. After 4 hours, the reaction was completed, and chipped into a polyester (A) according to a conventional method. The solution viscosity IV of the obtained polyester chip was 0.66.

(Method for producing polyester B)
When manufacturing the said polyester (A), 6000 ppm of amorphous silica with an average particle diameter of 3.2 micrometers was added, and polyester (B) was created.

(Method for producing polyester C)
80 parts of dimethyl terephthalate, 20 parts of dimethyl isophthalate, and 60 parts by weight of ethylene glycol are used as starting materials, 0.09 parts by weight of magnesium acetate tetrahydrate as a catalyst is used in the reactor, the reaction start temperature is 150 ° C., The reaction temperature was gradually increased as the solvent was distilled off, and the temperature was increased to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. After 0.04 part of ethyl acid phosphate was added to this reaction mixture, 0.03 part of antimony trioxide was added and a polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was terminated at a time corresponding to an intrinsic viscosity of 0.68 due to a change in the stirring power of the reaction vessel, and a chip was obtained according to a conventional method to obtain a polyester (C). The solution viscosity IV of the obtained polyester chip was 0.68.

Example 1:
The raw materials obtained by mixing the polyesters (A) and (C) at a ratio of 60% and 40% were used as the raw materials for the B layer, and the polyesters (A) and (B) were mixed at a ratio of 80% and 20%, respectively. The raw material was used as the raw material for the A layer, and the raw material for the A layer and the B layer was melt-extruded by separate melt extruders to obtain an amorphous sheet of two types and three layers (A / B / A). Subsequently, the sheet was coextruded on a cooled casting drum and solidified by cooling to obtain a non-oriented sheet. Next, the film was stretched 3.5 times in the longitudinal direction at 90 ° C., and further subjected to a preheating step in the tenter, 4.1 times in the transverse direction at 90 ° C., and heat-treated at 232 ° C. for 10 seconds, and then 180 ° C. In the width direction, 4.1% relaxation was applied in the width direction, and the roll was wound with a film forming machine, and the thickness was 38 μm (A layer: 4.0 μm, B layer: 30.0 μm) at a position where the main orientation axis was 84 degrees. A polyester film was obtained. The evaluation results are shown in Table 1.

Example 2:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that a film was obtained from a position where the main orientation axis of the roll wound by the film forming machine was 49 degrees. The evaluation results are shown in Table 1.

Example 3:
In Example 1, the thickness of each layer was set to A layer: 5.0 μm, B layer: 28.0 μm, and the roll was wound with a film forming machine, and the roll was taken except for obtaining a film from a position of 89 degrees. A polyester film was obtained in the same manner as in Example 1. The evaluation results are shown in Table 1.

Example 4:
In Example 3, a polyester film was obtained in the same manner as in Example 3 except that the film was obtained from a position where the main orientation axis of the roll wound up by the film forming machine was 45 degrees. The evaluation results are shown in Table 1.

Example 5:
In Example 1, (A) and (C) were mixed as a raw material for the B layer at a ratio of 40% and 60%, respectively, and the main orientation axis of the roll wound up by the film forming machine was from the position of 89 degrees. A polyester film was obtained in the same manner as in Example 1 except that a film was obtained. The evaluation results are shown in Table 1.

Example 6:
In Example 5, a polyester film was obtained in the same manner as in Example 5 except that the film was obtained from a position where the main orientation axis of the roll taken up by the film forming machine was 46 degrees. The evaluation results are shown in Table 1.

Example 7:
In Example 1, the transverse stretching ratio was set to 4.0 times, the relaxation rate was set to 3.9%, and the main orientation axis of the roll wound by the film forming machine was obtained except that a film was obtained from a position of 88 degrees. A polyester film was obtained in the same manner as in Example 1.

Example 8:
In Example 7, a polyester film was obtained in the same manner as in Example 7 except that the main orientation axis of the roll wound by the film forming machine was 45 degrees. The evaluation results are shown in Table 1.

Comparative Example 1:
In Example 1, the thickness of each layer was set to A layer: 3.0 μm, B layer: 32.0 μm, and the roll was wound with a film forming machine, and the roll was taken except for obtaining a film from a position of 89 degrees. A polyester film was obtained in the same manner as in Example 1. The evaluation results are shown in Table 2.

Comparative Example 2:
In Example 3, as the raw material for the B layer, (A) and (C) were mixed at a ratio of 30% and 70%, respectively, and the main orientation axis of the roll wound up by the film forming machine was 45 degrees from the position. A polyester film was obtained in the same manner as in Example 3 except that a film was obtained. The evaluation results are shown in Table 2.

Comparative Example 3:
In Example 3, a polyester film was obtained in the same manner as in Example 3 except that the relaxation rate was 0.0% and the film was obtained from the position of the main orientation axis of the roll wound up by the film forming machine at 88 degrees. . The evaluation results are shown in Table 2.

Comparative Example 4:
In Example 3, a polyester film was obtained in the same manner as in Example 3 except that the heat setting temperature was set to 226 ° C., and the film was obtained from a position where the main orientation axis of the roll wound up by the film forming machine was 89 degrees. The evaluation results are shown in Table 2.

Comparative Example 5:
In Comparative Example 4, a polyester film was obtained in the same manner as in Comparative Example 4, except that the relaxation rate was 0.0% and the film was obtained from the position of the main orientation axis of the roll wound up by the film forming machine at a position of 87 degrees. . The evaluation results are shown in Table 2.

Comparative Example 6:
Comparative Example 5 is the same as Comparative Example 5 except that the heat setting temperature is 228 ° C., the relaxation rate is 6.5%, and the main orientation axis of the roll wound by the film forming machine is obtained from a position of 87 degrees. A polyester film was obtained in the same manner. The evaluation results are shown in Table 2.

  The results are summarized in Tables 1 and 2 below.

  The film of the present invention can be suitably used, for example, as a polarizing plate protective film.

Claims (1)

It is a biaxially oriented polyester film having a laminated structure of three or more layers having a thickness of both outermost layers of 3.5 μm or more, the in-plane retardation of the film is 800 nm or less, and when heat-treated at 180 ° C. for 5 minutes. The shrinkage rate (MD) in the film length direction is 3.0% or less, the shrinkage rate (TD) in the direction perpendicular to the film length direction is 1.0% or less, and the difference in shrinkage rate between MD and TD A polarizing plate-protecting polyester film characterized by having an absolute value of 2.0% or less.