JP2014063084A - Polyester film for protection of polarizing plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical polyester film for protecting a polarizing plate, which can suppress the generation of a light interference color when arranged in a crossed Nicol state as a polarizing plate and can maintain high transparency when incorporated in a liquid crystal display.
SOLUTION: A polyester film containing at least one kind of fine particles having birefringence, wherein the polyester film has an in-plane retardation of 1000 nm or less and a haze of 4.5% or less. Optical polyester film.
[Selection figure] None

Description

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

  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 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. 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, the sequential biaxially stretched polyester film is a general-purpose resin, so there is no problem in terms of cost, but the optical distortion occurs in the width direction of the film in the stretching process, and the retardation largely fluctuates in the width direction. In the biaxially stretched polyester film, it is difficult to reduce retardation in the entire width direction.

  Further, since the unstretched polyester film is non-oriented, there is no problem of light interference color, but it is difficult to make the thickness less than 100 μm, and it is preferable to use it as a protective film for polarizing plates that are strongly demanded to be thin. Absent.

  There has also been proposed a method for suppressing the generation of a light interference color by balancing the ratio of film stretch ratios 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, there is a problem in that the retardation varies more in the width direction by balancing the draw ratio, which is not practical.

  In order to solve the above problems, Patent Document 5 discloses a method of adding fine particles having birefringence. However, in Patent Document 5, the relationship between the blending amount and the in-plane retardation or haze of the film is not clarified, and the light interference color is suppressed according to the blending amount, but the transparency is lowered. It does not take into account the possibility that

JP-A-6-511117 JP 2006-227090 A JP-A-6-297561 JP 2011-110718 A International Publication No. 2O01 / 025364

  The present invention has been made in view of the above circumstances, and the problem to be solved is that it is possible to suppress the generation of a light interference color when placed in a crossed Nicol state as a polarizing plate, and maintain high transparency when incorporated in a liquid crystal display. An object of the present invention is to provide an optical polyester film for protecting 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 a polyester film containing at least one kind of birefringent fine particles, the in-plane retardation of the polyester film is 1000 nm or less, and the haze is 4.5% or less. An optical polyester film characterized by

  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.

Further, in the present invention, in order to adjust the retardation of the optical film, it contains particles having birefringence, and birefringence having birefringence opposite in sign to the birefringence of the base film. Fine particles are preferred. As the birefringent fine particles, for example, those described in International Publication No. 2001/0253643 or JP-A-2004-109355 can be used. Specific examples include calcium carbonate, strontium carbonate, magnesium carbonate, manganese carbonate, cobalt carbonate, zinc carbonate, various carbonates such as barium carbonate, various oxides represented by titanium _{oxide, MgSO 4 · 5Mg (OH)} 2 · 3H 2 O, 6CaO · 6SiO 2 · H 2 O, 9Al 2 such as birefringence whiskers such as _{O 3} · _2B 2 O ₃ and the like. In particular, tetragonal, hexagonal and rhombohedral crystals are preferably uniaxial birefringent crystals, orthorhombic, monoclinic and triclinic crystals. These may be single crystals or polycrystals. Further, rod-like or short fiber-like particles of polystyrene or acrylic resin are also preferably used. For example, it may be a short fiber particle having a polystyrene resin or an acrylic resin and manufactured by finely cutting ultrafine fibers. These fibers are preferably stretched during the production process because they easily develop birefringence. The resin contained in these particles is preferably crosslinked. However, the present invention is not limited to these, and various types can be used as long as requirements such as size, shape, and needle ratio are satisfied.

  These birefringent fine particles preferably have a major axis of 10 to 500 nm and an acicular ratio of 1: 2 or more, particularly preferably an acicular ratio of 1: 2 to 1: 100, and 1: 3 to 1. : 30 is preferable. The acicular ratio is obtained by the following formula (1) from the absolute maximum length and the diagonal width of the particles. This can be determined from image data obtained by observing fine particles or fine particles contained in the film with an electron microscope. These birefringent fine particles can be used alone or in combination of two or more.

Needle-shaped ratio = absolute maximum length / diagonal width (1)
(Here, the diagonal width is the shortest distance between the two straight lines when the image of the particle projected by two straight lines parallel to the absolute maximum length is sandwiched.)

  The birefringent fine particles are preferably surface-treated with a silane coupling agent, a titanate coupling agent, or the like.

  The birefringence of the birefringent fine particles is defined as follows. The refractive index for light polarized in the major axis direction of the birefringent fine particles is npr, and the average refractive index for light polarized in the direction orthogonal to the major axis direction is nvt. The birefringence Δn of the birefringent fine particles is defined by the following formula.

Δn = npr−nvt
That is, if the refractive index in the major axis direction of the birefringent fine particles is larger than the average refractive index in the direction orthogonal thereto, positive birefringence is obtained, and negative birefringence is obtained in the opposite case.

The absolute value of the birefringence of the birefringent fine particles used in the present invention is not particularly limited, but is preferably 0.01 to 0.3, more preferably 0.05 to 0.3. preferable. The birefringent crystal having positive _{birefringence, MgSO 4 · 5Mg (OH)} 2 · 3H 2 O, 6CaO · 6SiO 2 · H 2 O, the _{9Al 2 O 3 · 2B 2 O} 3 and the like. Examples of the birefringent crystal exhibiting negative birefringence include calcium carbonate and strontium carbonate. In the case of acicular crystals, it means a material whose refractive index in the long direction of the crystal is smaller than the refractive index in the direction perpendicular thereto.

  The carbonate fine particles can be produced by a uniform precipitation method or a carbon dioxide compounding method. For example, it can be produced by the methods described in JP-A-3-88714, JP-B-55-51852, JP-A-59-223225, and the like. The strontium carbonate crystal can be obtained by bringing strontium ions dissolved in water into contact with carbonate ions. Carbonate ions can be obtained by adding carbon dioxide gas to a solution containing a strontium compound by a method such as bubbling carbon dioxide, or by adding a substance that generates carbonate ions to cause reaction or decomposition.

  For example, strontium carbonate crystal fine particles can be produced by the method described in JP-A-2004-35347, and the strontium carbonate fine particles obtained by this method can be preferably used as the birefringent fine particles. Examples of the substance that generates carbon dioxide include urea, and strontium carbonate fine particles can be obtained by reacting carbon dioxide ions and strontium ions generated together with a hydrolyzing enzyme of urea. In order to obtain fine crystals, it is preferable to lower the temperature as much as possible. Cooling below the freezing point is preferable because fine crystal particles can be obtained. For example, it is also preferable to add an organic solvent such as ethylene glycol as a freezing point depressing substance, and it is preferable to add so that the freezing point is below 5 ° C below freezing point. Thereby, fine particles of strontium carbonate having an average particle size in the major axis direction of 500 nm or less can be obtained.

  Strontium carbonate is a biaxial birefringent crystal, and according to Japanese Patent Application Laid-Open No. 2004-35347, the refractive index in each optical axis direction is n (na, nb, nc) = (1.520,1. 666, 1.669), and it is reported that the major axis direction of the needle-like crystal substantially coincides with the optical axis direction having a refractive index of 1.520. Therefore, it has a negative birefringence effect with respect to the orientation direction of the acicular crystal. Since the strontium carbonate crystal particles are in a needle-like (rod-like) form, they can be statistically oriented in a predetermined direction by applying a stress in a state of being dispersed in a viscous medium.

  Examples of the method of blending the birefringent fine particles into the polyester film include a method of directly adding the birefringent fine particles to the extruder, and a method of adding a polyester resin kneaded with the birefringent fine particles to the extruder. Any one of these methods may be employed, or two methods may be used in combination.

  In the polyester film of the present invention, the haze is 4.5% or less, preferably 3.0% or less. If the haze is greater than 4.5%, the transparency of the polarizing plate is lowered and the visibility is lowered.

  In the polarizing plate using the polyester film of the present invention as a protective film, the in-plane retardation of the polyester film is preferably 1000 nm or less in order to prevent the occurrence of light interference color. When the in-plane retardation of the film is larger than 1000 nm, the interference color of light becomes strong, and the original color of the image may not 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 stretch ratio of the polyester film in the present invention is preferably unbalanced in the aspect ratio. For example, a film that has been stretched by 3.6 or 3.7 times in both longitudinal and transverse stretching has the problem that the retardation varies greatly in the width direction, which is not practical.

  In the present invention, other additives may be added as necessary. Examples of such additives include stabilizers, lubricants, cross-linking agents, anti-blocking agents, antioxidants, dyes, pigments, and ultraviolet absorbers.

  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. It is preferable that the film is stretched 1.3 to 6 times 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 in-plane retardation Using a cell gap inspection apparatus RETS-1100A manufactured by Otsuka Electronics Co., Ltd., the in-plane retardation was measured at 20 mm intervals in the film width direction to obtain a maximum value and a minimum value. The in-plane retardation of the film was measured at 23 ° C. using an optical interference method with an aperture diameter of 5 mm.

(2) Measurement of haze Film haze was measured with an integrating sphere turbidimeter NDH-20D manufactured by Nippon Denshoku Industries Co., Ltd. according to JIS-K-7136.

(3) Evaluation 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. The best film for protecting a polarizing plate was rated as ◎, the excellent film as ◯, and the insufficient film as x.
◎: Excellent transparency and good with no light interference color ○: Transparency is slightly reduced, but there is no problem with light interference color ×: Transparency is reduced or light interference color is observed The image will be different from the tone of

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.

Example 1:
As said polyester (A), (B) and birefringent fine particles, the refractive index of each optical axis direction is n (na, nb, nc) = (1.520,1.666,1.669). When the needle-like crystal of strontium carbonate (average particle size in the major axis direction is 200 nm) is (C), the raw materials mixed in the proportions of 84.9%, 15% and 0.1% respectively are the raw materials for the A layer Polyester (A) was used as a raw material for the B layer. The raw materials for the A layer and the B layer were melt-extruded by separate melt extruders to obtain an amorphous sheet of (A / B / A) type 2 and 3 layers. 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 to obtain a thickness of 38 μm. A polyester film (A layer: 5.0 μm, B layer: 28.0 μm) was obtained. The evaluation results are shown in Table 1 below. The obtained polyester film was good without a decrease in transparency and a light interference color when incorporated in a liquid crystal panel.

Example 2:
In Example 1, (A), (B), and (C) were mixed in the proportions of 84.7%, 15%, and 0.3% as raw materials for the A layer, respectively, as in Example 1. A polyester film was obtained. The evaluation results are shown in Table 1. The obtained polyester film was good without a decrease in transparency and a light interference color when incorporated in a liquid crystal panel.

Example 3:
In Example 1, the same as in Example 1 except that (A), (B), and (C) were mixed in the proportions of 84.5%, 15%, and 0.5% as raw materials for the A layer, respectively. A polyester film was obtained. The evaluation results are shown in Table 1. The obtained polyester film had a slight decrease in transparency when incorporated in a liquid crystal panel, but was good without a light interference color.

Example 4:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that (A), (B), and (C) were mixed in the proportions of 84%, 15%, and 1%, respectively, as the raw material for the A layer. It was. The evaluation results are shown in Table 1. The obtained polyester film had a slight decrease in transparency when incorporated in a liquid crystal panel, but was good without a light interference color.

Comparative Example 1:
In Example 1, the needle-like crystal (C) of strontium carbonate, which is a birefringent fine particle, is not added, and (A) and (B) are mixed at a ratio of 85% and 15%, respectively, as the raw material for the A layer. Except for this, a polyester film was obtained in the same manner as in Example 1. The evaluation results are shown in Table 2 below. The obtained polyester film had a light interference color when incorporated in a liquid crystal panel, resulting in an image different from the original color tone.

Comparative Example 2:
In Example 1, (A), (B), and (C) were mixed in the proportions of 83.8%, 15%, and 1.2% as raw materials for the A layer, respectively, as in Example 1. A polyester film was obtained. The evaluation results are shown in Table 2. The obtained polyester film was remarkably reduced in transparency when incorporated in a liquid crystal panel, resulting in an image different from the original.

Comparative Example 3:
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 3.7 times. The evaluation results are shown in Table 2. The obtained polyester film had 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 Table 1 below.

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

Claims (1)

An optical polyester film comprising a polyester film containing at least one kind of birefringent fine particles, the polyester film having an in-plane retardation of 1000 nm or less and a haze of 4.5% or less.