JP2011242733A - Polyester film for protecting polarizing plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film having characteristics preferable for protecting a polarizing plate which prevents optical interference color from being generated when disposed in a crossed-Nicols state as the polarizing plate and is suitable for cutting the polarizing plate.SOLUTION: A polyester film for protecting a polarizing plate is a laminated polyester film configured with at least three layers, the intrinsic viscosity of the film is in the range of 0.50-0.90dl/g, the difference in the intrinsic viscosities between each of the layers are equal to or less than 0.3dl/g and an in-plane retardation is equal to or less than 2500nm.

Description

  The present invention relates to a polarizing plate protective film used for a polarizing plate, that is, a polarizing plate protective polyester film suitably used for a film used for protecting a polarizing plate by laminating the polarizing film. is there.

  In recent years, polarizing plates used for liquid crystal displays widely used as display devices for televisions, personal computers, digital cameras, mobile phones and the like are generally protective films / polarizing films / protective films or protective films / polarizing films / A TAC film has been used as a protective film for a polarizing plate because of its high transparency, optical isotropy, and a small amount of foreign matter. However, while the cost reduction demand for liquid crystal displays is increasing, the high cost of TAC films has become a problem.

  In view of the above problems, many studies have been conducted to substitute TAC films with other materials such as cycloolefin polymers. 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. However, since the biaxially oriented polyester film has birefringence, it is a light interference color when placed in a crossed Nicols state as a polarizing plate. May occur.

  Further, since ordinary polyester does not have UV absorbing ability, there is a problem that the liquid crystal deteriorates when used as a protective film for a polarizing plate.

  Furthermore, in polarizing plate manufacturing, there is a step of cutting the polarizing plate into the size of each liquid crystal display after laminating the optical film, but if the rigidity of the film is difficult to cut strongly in this step, the cause of productivity decline If the film is fragile and generates chips when it is cut, it will be a cause of foreign matter, and this will also reduce productivity.

JP-A-6-511117 JP 2006-227090 A JP 2001-66432 A

  The present invention has been made in view of the above circumstances, and the object thereof is to produce a light interference color when arranged in a crossed nicols state as a polarizing plate, and to have an ultraviolet absorption characteristic as required. An object of the present invention is to provide a polyester film for protecting a polarizing plate suitable for the cutting of a polarizing plate.

  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 laminated polyester film comprising at least three layers, wherein the intrinsic viscosity of the film is 0.50 to 0.90 dl / g, and the difference in intrinsic viscosity of each layer is 0.3 dl / g or less. In-plane retardation is 2500 nm or less, and exists in the polyester film for polarizing plate protection characterized by the above-mentioned.

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

  The polyester film referred to in the present invention is a film obtained by stretching a sheet melt-extruded from an extrusion die according to a so-called extrusion method.

The polyester constituting the film refers to a polymer containing an ester group obtained by polycondensation from dicarboxylic acid and diol or from hydroxycarboxylic acid. Examples of dicarboxylic acids include terephthalic acid, isophthalic acid, adipic acid, azelaic acid, sebacic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and diols include ethylene glycol and 1,4-butane. Examples include diol, diethylene glycol, triethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol, polyethylene glycol and the like, and examples of hydroxycarboxylic acid include p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. be able to.
Typical examples of such polymers include polyethylene terephthalate and polyethylene-2, 6 naphthalate.

  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 diameter 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 wt%, preferably 0.01 to 10.0 wt%, 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.

  In the polarizing plate using the polyester film of the present invention as a protective film, in-plane retardation of the polyester film needs to be 2500 nm or less, preferably 2000 nm or less, more preferably 1500 nm, in order to prevent the occurrence of light interference color. It is as follows. When the in-plane retardation of the film is larger than 2500 nm, the interference color of light becomes strong, and the original color of the image cannot be obtained on the liquid crystal display.

  In the polyester film of the present invention, the intrinsic viscosity of the film is 0.50 to 0.90 dl / g, preferably 0.60 to 0.80 dl / g. When the intrinsic viscosity of the film is smaller than 0.50 dl / g, it is not preferable because chips are generated when the polarizing plate using the polarizing plate using the polyester film of the present invention is cut. When the intrinsic viscosity of the film is larger than 0.80 dl / g, it is difficult to cut the polarizing plate using the polyester film of the present invention, which is not preferable.

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

  In the present invention, the polyester film preferably contains an ultraviolet absorber. The ultraviolet absorber is blended in order to prevent the deterioration of the liquid crystal of the liquid crystal display due to ultraviolet rays. Examples of the ultraviolet absorber contained in the polyester film include an organic ultraviolet absorber and an inorganic ultraviolet absorber.

  Examples of organic ultraviolet absorbers include salicylic acid-based compounds such as phenyl salicylate, pt-butylphenyl salicylate, p-octylphenyl salicylate, and benzophenone-based compounds such as 2-hydroxy-4-benzyloxy. Benzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-octoxybenzophenone, 2-hydroxy-4-dodecyloxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2 -2'-dihydroxy-4,4'-dimethoxybenzophenone and the like, benzotriazole-based, for example, 2- (2'-hydroxy-5'-t-octylphenyl) -benzotriazole, 2- (2'-hydroxy-5 '-T-octylphenyl) -benzotriazo 2- (2′-hydroxy-5′-methylphenyl) benzotriazole, 2- (2′-hydroxy-3′5′-di-t-butylphenyl) benzotriazole, 2- (2′-hydroxy) -3'-t-butyl-5'-methylphenyl) -5-chlorobenzotriazole, 2- (2'-hydroxy-3'5'-di-t-butylphenyl) 5-chlorobenzotriazole, and other natural products Examples include biological systems such as oryzanol, shea butter and baicalin, such as keratinocytes, melanin and urocanic acid. These organic ultraviolet absorbers can be used alone or in combination of two or more. These organic ultraviolet absorbers can be used in combination with a hindered amine compound as an ultraviolet stabilizer.

  Inorganic UV absorbers include titanium oxide, zinc oxide, indium oxide, tin oxide, talc, kaolin, calcium carbonate, titanium oxide-based composite oxide, zinc oxide-based composite oxide, ITO (tin-doped indium oxide), ATO (Antimony-doped tin oxide) and the like. Examples of the titanium oxide-based composite oxide include zinc oxide doped with silica and alumina. These inorganic ultraviolet absorbers can be used alone or in combination of two or more. Moreover, you may use together an organic type ultraviolet absorber and an inorganic type ultraviolet absorber.

  Examples of the method of blending the UV absorber into the polyester film include a method of directly adding the UV absorber to the extruder, a method of adding a polyester resin kneaded in advance to the extruder, and the like. Either one of the methods may be employed, or two methods may be used in combination.

  In any of the constituent members of the polarizing plate of the present invention, it is preferable that ultraviolet absorbing performance is imparted. When ultraviolet absorbing performance is not imparted to a member other than the polyester film, the light transmittance at a wavelength of 380 nm is preferably 10.0% or less in the polyester film of the present invention. More preferably, it is 5.0% or less. If the light transmittance at a wavelength of 380 nm is greater than 10.0%, the deterioration of the liquid crystal is promoted when a member other than the polyester film does not have ultraviolet absorption performance, which is not preferable.

  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 intrinsic viscosity of polyester 1 g of polyester from which other polymer components and pigments incompatible with polyester have been removed are precisely weighed, and 100 ml of a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio) is added. It was dissolved and measured at 30 ° C. When measuring the intrinsic viscosity of the surface layer and the intermediate layer, each laminated portion was physically scraped from the film, and each intrinsic viscosity was measured.

(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 light transmittance Using a spectrophotometer (Shimadzu Corporation UV-3100PC type), the scan speed is low, the sampling pitch is 2 nm, and the light transmittance is continuously measured in the wavelength range of 300 to 700 nm. The light transmittance at a wavelength of was detected.

(4) Comprehensive judgment The performance as a polarizing plate protective film was evaluated in six stages from 1 to 6. 6 is the most excellent as a polarizing plate-protecting polyester film, and 1 is the least suitable as a polarizing plate-protecting polyester film. A film with an overall judgment of 2 or more was judged to be acceptable.

The polyester used in the following examples and comparative examples was prepared by the following production method.
(Method for producing polyester A)
Take 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and 0.07 part of calcium acetate monohydrate in a reactor, heat up and evaporate methanol to conduct transesterification, and take about 4 and a half hours after starting the reaction. The temperature was raised to 230 ° C. to substantially complete the transesterification reaction. 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. Two hours later, the reaction was completed, and a polyester A was obtained by converting into chips according to a conventional method. The solution viscosity IV of the obtained polyester chip was 0.45.

(Method for producing polyester B)
Take 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and 0.07 part of calcium acetate monohydrate in a reactor, heat up and evaporate methanol to conduct transesterification, and take about 4 and a half hours after starting the reaction. The temperature was raised to 230 ° C. to substantially complete the transesterification reaction. 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 3 hours, the reaction was completed, and a chip was obtained according to a conventional method to obtain polyester B. The solution viscosity IV of the obtained polyester chip was 0.58.

(Method for producing polyester C)
Take 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and 0.07 part of calcium acetate monohydrate in a reactor, heat up and evaporate methanol to conduct transesterification, and take about 4 and a half hours after starting the reaction. The temperature was raised to 230 ° C. to substantially complete the transesterification reaction. 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 polyester C was obtained by chipping according to a conventional method. The solution viscosity IV of the obtained polyester chip was 0.67.

(Polyester D manufacturing method)
The intrinsic viscosity of polyester C was increased by a solid phase polycondensation method. After treating in a preliminary crystallization tank under a nitrogen atmosphere at 170 ° C. for 0.5 hour, the moisture content becomes 0.005% at a temperature of 200 ° C. using a tower dryer in which an inert gas flows. Until dried. Thereafter, it was sent to a solid phase polymerization tank and subjected to solid phase polymerization at 240 ° C. for 4 hours to obtain a polyester D having an intrinsic viscosity of 0.77.

(Polyester E production method)
The intrinsic viscosity of polyester C was increased by a solid phase polycondensation method. After treating in a preliminary crystallization tank under a nitrogen atmosphere at 170 ° C. for 0.5 hour, the moisture content becomes 0.005% at a temperature of 200 ° C. using a tower dryer in which an inert gas flows. Until dried. Thereafter, it was sent to a solid phase polymerization tank and subjected to solid phase polymerization at 240 ° C. for 6 hours to obtain a polyester D having an intrinsic viscosity of 0.83.

(Polyester F manufacturing method)
The intrinsic viscosity of polyester C was increased by a solid phase polycondensation method. After treatment in a preliminary crystallization tank under a nitrogen atmosphere at 170 ° C. for 0.5 hour, the moisture content becomes 0.005% at a temperature of 200 ° C. using a tower type dryer in which an inert gas flows. Until dried. Thereafter, it was sent to a solid phase polymerization tank and subjected to solid phase polymerization at 240 ° C. for 8 hours to obtain polyester F having an intrinsic viscosity of 0.91.

(Polyester G production method)
The intrinsic viscosity of polyester C was increased by a solid phase polycondensation method. After treating in a preliminary crystallization tank under a nitrogen atmosphere at 170 ° C. for 0.5 hour, the moisture content becomes 0.005% at a temperature of 200 ° C. using a tower dryer in which an inert gas flows. Until dried. Thereafter, it was sent to a solid phase polymerization tank and subjected to solid phase polymerization at 240 ° C. for 12 hours to obtain polyester G having an intrinsic viscosity of 1.02.

(Method for producing polyester H)
When manufacturing the said polyester C, 2500 ppm of amorphous silica with an average primary particle diameter of 3.4 micrometers was added, and the polyester H was obtained. When the polyester C is produced, 2,2- (1,4-phenylene) bis [4H-3,1-benzoxazin-4-one] is added as an ultraviolet absorber so as to have a concentration of 10%. I was obtained.

Example 1:
The raw materials obtained by mixing the polyesters (B) and (H) at a ratio of 95% and 5%, respectively, are used as the raw material for the B layer, and the polyesters (B) and (I) are mixed at a ratio of 90% and 10%, respectively. The raw material for the B layer was used as the raw material for the B layer, and the raw material for the A layer and the B layer were 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 process in the tenter, 4.0 times in the transverse direction at 90 ° C., and heat-treated at 230 ° C. for 10 seconds to obtain a thickness of 38 μm. A polyester film was obtained. The evaluation results are shown in Table 1.

Example 2:
In Example 1, the raw materials obtained by mixing the polyesters (C) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (C) and (I) were 90% and 10%, respectively. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 3:
In Example 1, the raw materials obtained by mixing the polyesters (E) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (E) and (I) were 90% and 10%, respectively. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 4:
In Example 1, the raw materials obtained by mixing the polyesters (F) and (H) at a ratio of 95% and 5%, respectively, are used as the raw materials for the B layer, and the polyesters (F) and (I) are respectively 90% and 10%. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 5:
In Example 1, the raw materials obtained by mixing the polyesters (F) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (B) and (I) were respectively 90% and 10%. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 6:
In Example 1, the raw materials obtained by mixing the polyesters (C) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (B) and (I) were respectively 90% and 10%. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 7:
A polyester film was obtained in the same manner as in Example 1 except that the longitudinal draw ratio was 3.7 times and the transverse draw ratio was 5.2 times. The evaluation results are shown in Table 1.

Example 8:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the longitudinal draw ratio was 3.7 times and the transverse draw ratio was 3.8 times. The evaluation results are shown in Table 1.

Example 9:
A polyester film was obtained in the same manner as in Example 1 except that the raw materials in which the polyesters (B) and (I) were mixed in proportions of 92% and 8%, respectively, were used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 10:
A polyester film was obtained in the same manner as in Example 1 except that the raw materials obtained by mixing the polyesters (B) and (I) at a ratio of 94% and 6%, respectively, were used as the raw material for the B layer. The evaluation results are shown in Table 1.

Example 11:
A polyester film was obtained in the same manner as in Example 1 except that the raw materials in which the polyesters (B) and (I) were mixed at a ratio of 96% and 4%, respectively, were used as the raw materials for the B layer. The evaluation results are shown in Table 1.

Comparative Example 1:
In Example 1, the raw materials obtained by mixing the polyesters (A) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (A) and (I) were respectively 90% and 10%. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Comparative Example 2:
In Example 1, the raw materials obtained by mixing the polyesters (G) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (G) and (I) were 90% and 10%, respectively. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Comparative Example 3:
A polyester film was obtained in the same manner as in Example 1 except that the raw material obtained by mixing the polyesters (G) and (H) at a ratio of 95% and 5%, respectively, was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Comparative Example 4:
In Example 1, the raw materials obtained by mixing the polyesters (F) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (A) and (I) were respectively 90% and 10%. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

Comparative Example 5:
In Example 1, the raw materials obtained by mixing the polyesters (C) and (H) at a ratio of 95% and 5%, respectively, were used as the raw materials for the B layer, and the polyesters (C) and (I) were 90% and 10%, respectively. %, A polyester film was obtained in the same manner as in Example 1 except that the raw material mixed at a ratio of% was used as the raw material for the B layer. The evaluation results are shown in Table 1.

  The obtained results are summarized in Table 1 below.

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

Claims (2)

A laminated polyester film composed of at least three layers, wherein the intrinsic viscosity of the film is 0.50 to 0.90 dl / g, the difference in intrinsic viscosity of each layer is 0.3 dl / g or less, and the in-plane retardation is A polyester film for protecting a polarizing plate, having a thickness of 2500 nm or less. The polyester film for protecting a polarizing plate according to claim 1, wherein at least one of the intermediate layers contains an ultraviolet absorber and has a light transmittance of 10.0% or less at a wavelength of 380 nm.