JP2011232660A - Polyester film for protecting polarizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film for protecting a polarizer, which is used as a protection film particularly for a polarizer of a small and medium-sized mobile such as a game machine, a personal digital assistance, an electronic diary and a mobile phone, does not generate optical interference color when being disposed in a crossed-Nicol state, and has the high ultraviolet absorption property and high light transmittance.SOLUTION: In a polyester film for protecting a polarizer, a main orientation axis with respect to an MD is 30 to 60 degrees and in-plane retardation of the film is equal to or lower than 2500 nm.

Description

  The present invention relates to a polarizing plate protective film used for a polarizing plate for mobile use, 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. It is about.

  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 / As a protective film for a conventional polarizing plate, a TAC film has been widely used because of its characteristics such as 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 addition, in particular for small and medium-sized polarizing plates for mobile devices such as game machines, personal digital assistants, electronic notebooks, mobile phones, etc., as the demand for thinning increases, there is a demand for reducing the thickness of the protective film of the polarizing plate, Further thinning is difficult with a TAC film.

  Many studies have been made to replace the TAC film with other materials such as cycloolefin polymer for the above problems.

  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. Therefore, 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. 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. On the other hand, the non-oriented polyester film is non-oriented, so there is no problem of light interference color and brightness reduction. However, it is difficult to make the thickness less than 100 μm, and there is a strong demand for thinning. It is not preferable to use it for a protective film.

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 problem to be solved is that, when arranged in a crossed Nicols state, particularly as a small and medium polarizing plate for mobile use, no light interference color is generated, ultraviolet absorption characteristics, light transmission It is providing the polyester film for polarizing plate protection which is excellent in rate.

  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 resides in a polarizing plate-protecting polyester film characterized in that the main orientation axis is 30 to 60 degrees with respect to MD and the in-plane retardation of the film is 2500 nm or less.

  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. Well, you may use two methods together.

  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 decrease. If the average particle size is less than 0.1 μm, the surface roughness may be too small, making it difficult to handle the film. 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.

  In a mobile small and medium polarizing plate using the polyester film of the present invention as a protective film, it is important that the angle of the main orientation axis in the polyester film with respect to MD is 30-60 degrees in order to prevent a decrease in luminance. Yes, preferably 35-55 degrees, particularly preferably 40-50 degrees. When the angle of the main alignment axis with respect to MD is larger than 60 degrees, the decrease in luminance is large. Even when the angle of the main alignment axis with respect to MD is smaller than 30 degrees, the luminance is greatly lowered, and the luminance becomes insufficient for a polarizing plate.

  In the mobile small and medium polarizing plate using the polyester film of the present invention as a protective film, in order to prevent the occurrence of light interference color, the in-plane retardation of the polyester film needs to be 2500 nm or less, more preferably. Is 2000 nm or less, particularly preferably 1500 nm or less. 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 in a mobile 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. If the thickness of the film is less than 4 μm, it tends to be difficult to form the film and to handle the film. 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 the polarizing plate of the present invention, it is necessary to impart ultraviolet absorption performance to any constituent member. 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.

  In addition, you may coat by the method called offline coating which coats after manufacture of a film as needed. 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) Visibility inspection Using polyvinyl alcohol (PVA) film (manufactured by Kuraray Co., Ltd., degree of polymerization 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 reduction is good with no light interference color ○: Brightness reduction or light interference color is a little but no problem △: Brightness reduction or light interference color is present but acceptable X: Decreased brightness or strong light interference color, cannot be used as a polarizing plate

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

(5) Comprehensive judgment ◎: Decrease in luminance No light interference color, good handleability, UV absorption characteristics, light transmittance and no problem as a polarizing plate ○: Light interference color, handleability, UV absorption characteristics, light transmittance Is slightly inferior, but it can be used as a polarizing plate. Δ: Light interference color, handling property, UV absorption property, light transmittance is inferior, but is an acceptable level as a polarizing plate. ΔΔ: Light interference Any of color, handleability, UV absorption characteristics, and light transmittance is inferior, but barely acceptable level as a polarizing plate ×: any of light interference color, handleability, UV absorption characteristics, light transmittance is inferior, Cannot be used as a polarizing plate.

Example 1:
(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 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 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), 1000 ppm of amorphous silica with an average particle diameter of 2 micrometers was added, and polyester (B) was created.

(Polyester C manufacturing method)
When the polyester (A) is produced, 2,2- (1,4-phenylene) bis [4H-3,1-benzoxazin-4-one] is added as a UV absorber so as to have a concentration of 10%. Thus, polyester (C) was prepared.

(Manufacture of polyester film)
The raw materials obtained by mixing the polyesters (A) and (C) in proportions of 85% and 15%, respectively, are used as the raw material for the B layer, the polyester (B) is used as the raw material for the A layer, and the raw materials for the A layer and the B layer are used. Amorphous sheets of two types and three layers (A / B / A) were obtained by melt extrusion using separate melt extruders. 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 3.4 times in the longitudinal direction at 90 ° C., further subjected to a preheating process in the tenter, stretched 4.5 times in the lateral direction at 90 ° C., and subjected to heat treatment at 230 ° C. for 10 seconds, A polyester film having a thickness of 38 μm was obtained from the end position of the roll wound up by. The evaluation results are shown in Table 1 below.

Example 2:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the longitudinal draw ratio was 4.0 times. The evaluation results are shown in Table 1.

Example 3:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the film was obtained from the position of the main orientation axis 55 degrees of the roll wound by the film forming machine. The evaluation results are shown in Table 1.

Example 4:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the film was obtained from the position of the main orientation axis 52 degrees of the roll wound up by the film forming machine. The evaluation results are shown in Table 1.

Example 5:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the longitudinal draw ratio was 3.6 times. The evaluation results are shown in Table 1.

Example 6:
In Example 1, a polyester film was obtained in the same manner as in Example 1, except that the film was obtained from the position of the main orientation axis 48 degrees of the roll wound up by the film forming machine. The evaluation results are shown in Table 1.

Example 7:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that polyester (A) was used as the raw material for the B layer. 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 thickness of the film was 48 μm. The evaluation results are shown in Table 1.

Example 9:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the thickness of the film was 25 μm. The evaluation results are shown in Table 1.

Example 10:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that a raw material in which polyesters (A) and (C) were mixed in proportions of 92% and 8%, respectively, was used as the raw material for the B layer. . The evaluation results are shown in Table 1.

Example 11:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the thickness of the film was 4.5 μm. The evaluation results are shown in Table 1.

Example 12:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the film thickness was 6 μm. The evaluation results are shown in Table 1.

Comparative Example 1:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the film was obtained from the position of the main orientation axis of 70 degrees of the roll wound by the film forming machine. The evaluation results are shown in Table 1.

Comparative Example 2:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that the film thickness was 60 μm. 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 polyester film for protecting a polarizing plate, wherein the main orientation axis is 30 to 60 degrees with respect to MD, and the in-plane retardation of the film is 2500 nm or less. 2. The polarizing plate according to claim 1, which is a laminated polyester film composed of at least three layers, containing an ultraviolet absorber in at least an intermediate layer, and having a light transmittance of 10.0% or less at a wavelength of 380 nm. Protective polyester film.