JP2013054207A - Polyester film for protecting polarizing plate and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film for protecting a polarizing plate, which can reduce light interference coloring caused when disposed in a crossed Nicol state as a polarizing plate.SOLUTION: There are provided a polyester film for protecting a polarizing plate in which the film has an in-plane retardation of 500 nm or less and a plane orientation degree ΔP of 0.110 or less and a method for manufacturing the polyester film for protecting a polarizing plate in which a melt extrusion polyester sheet is biaxially stretched and heat-treated to form a film and a draw ratio in a lateral direction of the film is 4.0 times or more.

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. A TAC (triacetyl cellulose) film has been frequently used because of its characteristics such as high transparency, optical isotropy, and a small amount of 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 the liquid crystal display increases in recent years, the TAC film 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 has become a problem. On the other hand, while the demand for liquid crystal displays is growing remarkably, the supply of TAC films tends to be insufficient, and there is concern about a stable supply in the future.

  Many studies on replacing the TAC film with other materials such as a cycloolefin polymer have been made 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 is in a certain direction within 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.

  In addition, since the unstretched polyester film is non-oriented, there is no problem of light interference color and brightness reduction, but it is difficult to make the thickness less than 100 μm, and it is a protective film for a polarizing plate that is strongly demanded to be thin. It is not preferable to use it.

  There has also been proposed a method of suppressing the occurrence of light interference colors by balancing the ratio of the film stretch ratio in length and width (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, which is not practical.

JP-A-6-511117 JP 2006-227090 A JP-A-6-297561 JP 2011-110718 A

  This invention is made | formed in view of the said actual condition, Comprising: The solution subject is providing the polyester film for polarizing plate protection which can suppress generation | occurrence | production of a light interference color when arrange | positioning in a crossed Nicols state as a polarizing plate. .

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

  That is, the gist of the present invention is that a polarizing plate protecting polyester film and a melt-extruded polyester sheet are characterized in that the in-plane retardation of the film is 500 nm or less and the degree of plane orientation ΔP is 0.110 or less. The present invention resides in a method for producing a film for polarizing plate protection, characterized in that it is a method for producing a film that is biaxially stretched and heat-treated, and has a film transverse direction stretching ratio of 4.0 times or more.

  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 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 be lowered. 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 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 is 500 nm or less, preferably 200 nm or less, in order to prevent the occurrence of light interference color. When the in-plane retardation of the film is larger than 500 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. 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 greater than 50 μm, the polarizing plate becomes thick and may cause a problem in practice.

  The degree of plane orientation of the polyester film in the present invention needs to be 0.110 or less. When the degree of plane orientation is 0.110 or more and the in-plane retardation is 500 nm or less, there is a problem that breakage during film formation tends to occur.

  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) Degree of plane orientation (ΔP)
An Atago Abbe refractometer was used. Methylene iodide is mounted, the sample film is closely attached to the prism so that the measurement surface faces down, and the refractive index in the longitudinal direction, the width direction, and the thickness direction (Nx, respectively) using a monochromatic light sodium D line (589 nm) as a light source. Ny, Nz). From the obtained value, the plane orientation degree ΔP of each layer was determined by the following formula. In the case of different plane orientations in each layer of the film, the value converted by the thickness of each layer was used. The measurement sample was collected from the center part of the product master roll.
ΔP = (Nx + Ny) / 2−Nz

(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.

The raw materials used in the following examples and comparative examples were prepared as follows.
(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 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 3.4 times in the longitudinal direction at 90 ° C., and further subjected to a preheating step in the tenter, 4.0 times in the transverse direction at 90 ° C., and heat-treated at 230 ° C. for 10 seconds, A polyester film having a thickness of 38 μm (A layer: 3.2 μm, B layer: 31.6 μm) was obtained at a position where the main orientation axis of the roll taken up at 84 was 84 degrees. The evaluation results are shown in Table 1. The obtained polyester film was good with no reduction in luminance and no light interference color when incorporated in a liquid crystal panel.

Example 2:
In Example 1, as the raw material for the B layer, (A) and (C) were mixed at a ratio of 50% and 50%, respectively, and the main orientation axis of the roll wound up by the film forming machine was from a 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. The obtained polyester film was good with no reduction in luminance and no light interference color when incorporated in a liquid crystal panel.

Example 3:
In Example 2, a polyester film was obtained in the same manner as in Example 2 except that a 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. The obtained polyester film had a slight decrease in luminance when incorporated in a liquid crystal panel, but had no light interference color and was good.

Example 4:
In Example 2, the longitudinal draw ratio was set to 3.5 times, the transverse draw ratio was set to 4.5 times, and the film was obtained from the position of 89 degrees of the main orientation axis of the roll wound up by the film forming machine. In the same manner as in Example 1, a polyester film was obtained. The evaluation results are shown in Table 1. The obtained polyester film was good with no reduction in luminance and no light interference color when incorporated in a liquid crystal panel.

Example 5:
In Example 4, a polyester film was obtained in the same manner as in Example 1 except that the main orientation axis of the roll wound by the film forming machine was set to 52 degrees. The evaluation results are shown in Table 1. The obtained polyester film had a slight decrease in luminance when incorporated in a liquid crystal panel, but had no light interference color and was good.

Example 6:
In Example 2, (A) and (C) were mixed as raw materials for the B layer at a ratio of 15% and 85%, respectively, and the main orientation axis of the roll wound by the film forming machine was set to 88 degrees. Obtained a polyester film in the same manner as in Example 2. The evaluation results are shown in Table 1. The obtained polyester film was good with no reduction in luminance and no light interference color when incorporated in a liquid crystal panel.

Comparative Example 1:
In Example 1, polyester (A) was used as the raw material for the B layer at a rate of 100%, and the polyester was formed in the same manner as in Example 1 except that the main orientation axis of the roll wound by the film forming machine was 89 degrees. A film was obtained. The evaluation results are shown in Table 1. The obtained polyester film had a strong light interference color when incorporated in a liquid crystal panel, and the image was considerably different from the original color tone.

Comparative Example 2:
In Example 1, the temperature during longitudinal stretching was 110 ° C., the magnification was 3.6 times, the temperature for transverse stretching was 110 ° C., the magnification was 3.6 times, and the main orientation axis of the roll wound up by the film forming machine A polyester film was obtained in the same manner as in Example 1 except that the angle was set to 88 degrees. The evaluation results are shown in Table 1. The obtained polyester film had a light interference color and a light interference color when incorporated in a liquid crystal panel, resulting in an image different from the original color tone.

Comparative Example 3:
In Example 1, (A) and (C) were mixed as the raw material for the B layer at a ratio of 70% and 30%, respectively, and the main orientation axis of the roll wound up by the film forming machine was from the position of 88 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. The obtained polyester film had a light interference color and a light interference color when incorporated in a liquid crystal panel, resulting in an image different from the original color tone.

  The obtained 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 (2)

A polyester film for protecting a polarizing plate, wherein the in-plane retardation of the film is 500 nm or less, and the degree of plane orientation ΔP is 0.110 or less. The polyester film for protecting a polarizing plate according to claim 1, wherein the film is a method of producing a film obtained by biaxially stretching a melt-extruded polyester sheet and heat-treating the film, and the transverse stretching ratio is 4.0 times or more. Manufacturing method.