Retardation film, polarizer and liquid crystal display device

Abstract

To provide a retardation film which can suppress unevenness in the contrast in a liquid crystal display device immediately after being taken out from a high-humidity environment, as well as a polarizer including the retardation film and the liquid crystal display device.SOLUTION: A retardation film according to the present invention is the retardation film which comprises a first layer, a second layer, and a third layer in this order. Each of the first layer, the second layer, and the third layer contains acetyl cellulose and fine particles. The substitution degree of the acetyl group in the acetyl cellulose contained in the second layer is in the range of 2.0-2.6. The second layer further contains polyester having hydroxyl groups at both terminals. The percentage content (mass%) of the fine particles in the second layer is greater than the percentage content (mass%) of the fine particles in the first layer and the third layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a retardation film, a polarizing plate, and a liquid crystal display device. More specifically, the present invention relates to a retardation film capable of suppressing uneven contrast in a liquid crystal display device immediately after being taken out from a high-humidity environment, and a polarizing plate and a liquid crystal display device including the retardation film.

Background Art

[0002] In a cellulose acetate film having a laminated structure, a technique of adding a matting agent in a large amount to the surface layer or only to the surface layer is known (see Patent Document 1).

[0003] However, such a cellulose acetate film has uneven contrast immediately after being taken out from a high-humidity environment, which may cause problems during inspection after transportation.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention has been made in view of the above situation. The problem to be solved by the present invention is to provide a retardation film capable of suppressing uneven contrast in a liquid crystal display device immediately after being taken out from a high-humidity environment, and a polarizing plate and a liquid crystal display device including the retardation film.

Means for Solving the Problems

[0006] In order to solve the above problems, the present inventors studied the causes and the like of the above problems. As a result, the present inventors found that in a retardation film having a first layer, a second layer, and a third layer in this order, all of the first layer, the second layer, and the third layer contain acetyl cellulose and fine particles, and the degree of substitution of acetyl groups of the acetyl cellulose contained in the second layer is in the range of 2.0 to 2.6, the second layer further contains a polyester having hydroxy groups at both ends, and the content rate [mass%] of the fine particles in the second layer is made larger than the content rates [mass%] of the fine particles in the first layer and the third layer, thereby finding that the above problems can be solved, and arriving at the present invention. That is, the above problems according to the present invention are solved by the following means.

[0007] 1. A retardation film having a first layer, a second layer, and a third layer in this order, wherein the first layer, the second layer, and the third layer all contain acetyl cellulose and fine particles, the degree of substitution of acetyl groups of the acetyl cellulose contained in the second layer is in the range of 2.0 to 2.6, the second layer further contains a polyester having hydroxy groups at both ends, and the content rate [mass%] of the fine particles in the second layer is larger than the content rates [mass%] of the fine particles in the first layer and the third layer. A retardation film characterized by the above.

[0008] 2. The content rate [mass%] of the fine particles in the second layer is in the range of 2 to 5 times the average content rate [mass%] of the fine particles in the first layer and the third layer. The retardation film according to claim 1, characterized by the above.

[0009] 3. The fine particles are silica fine particles. The retardation film according to claim 1, characterized by the above.

[0010] 4. A polarizing plate comprising the retardation film according to any one of claims 1 to 3. Characterized by the above.

[0011] 5. A liquid crystal display device comprising the retardation film according to any one of claims 1 to 3. characterized in that.

Advantages of the Invention

[0012] By the above means of the present invention, it is possible to provide a retardation film capable of suppressing unevenness in contrast in a liquid crystal display device immediately after being taken out from a high-humidity environment, a polarizing plate including the retardation film, and a liquid crystal display device.

[0013] Although the mechanism of expression or the mechanism of action of the effects of the present invention is not clear, it is speculated as follows.

[0014] In a high-humidity environment, water enters the inside of the retardation film, and when the water coordinates with the hydrophilic groups of acetyl cellulose, the retardation value of that part decreases. It is considered that this causes unevenness in contrast in the liquid crystal display device immediately after being taken out from a high-humidity environment. In the present invention, the "high-humidity environment" refers to an environment where the relative humidity is 75% RH or more.

[0015] The retardation film of the present invention has a first layer, a second layer, and a third layer in this order, and the content rate of fine particles is larger in the second layer than in the first layer and the third layer. Since the retardation film contains fine particles together with acetyl cellulose, the retardation value is less likely to be affected by water. This is presumably because the fine particles form hydrogen bonds with the hydrophilic groups of acetyl cellulose, making it difficult for water to interact with acetyl cellulose. On the other hand, if the content rate of the fine particles near the surface of the retardation film is increased too much, the slipperiness of the surface of the retardation film becomes too high, and problems such as winding deviation may occur. The present invention can increase the content rate of the fine particles in the second layer after adjusting the content rate of the fine particles in the first layer and the third layer while emphasizing slipperiness. As a result, the present invention can suppress the retardation value from being affected by water while maintaining the slipperiness within a good range.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0017] The retardation film of the present invention is a retardation film having a first layer, a second layer, and a third layer in this order, wherein the first layer, the second layer, and the third layer all contain acetyl cellulose and fine particles, and the degree of substitution of the acetyl group of the acetyl cellulose contained in the second layer is in the range of 2.0 to 2.6, and the second layer further contains a polyester having hydroxy groups at both ends, and the content rate [mass%] of the fine particles in the second layer is larger than the content rates [mass%] of the fine particles in the first layer and the third layer. This feature is a technical feature common to or corresponding to the following embodiments.

[0018] The content rate [mass%] of the fine particles in the second layer is preferably in the range of 2 to 5 times the average content rate [mass%] of the fine particles in the first layer and the third layer. By being 2 times or more, while maintaining a good sliding property range, the retardation value is more suppressed from being affected by water. By being 5 times or less, the haze does not become too high, and it is difficult to lower the contrast of the liquid crystal display device.

[0019] The fine particles are preferably silica fine particles. Since the silica fine particles have a refractive index close to that of acetyl cellulose, the generation of haze is suppressed by the fine particles being silica fine particles.

[0020] The polarizing plate and the liquid crystal display device of the present invention include the retardation film of the present invention.

[0021] Hereinafter, the details of the present invention will be described. In the present application, "~" is used in the sense of including the numerical values described before and after it as a lower limit value and an upper limit value.

[0022] [1. Phase difference film] The phase difference film of the present invention has a first layer, a second layer, and a third layer in this order. The first layer, the second layer, and the third layer all contain acetyl cellulose and fine particles. The degree of substitution of the acetyl group of the acetyl cellulose contained in the second layer is within the range of 2.0 to 2.6. The second layer further contains a polyester having hydroxy groups at both ends. The content rate [mass%] of the fine particles in the second layer is larger than the content rates [mass%] of the fine particles in the first layer and the third layer.

[0023] Hereinafter, the first layer and the third layer are also each referred to as a "skin layer". The second layer is also referred to as a "core layer".

[0024] FIG. 1 is a cross-sectional view of an embodiment of the phase difference film.

[0025] The phase difference film 20 shown in FIG. 1 has a first layer 21, a second layer 22, and a third layer 23 in this order. The thicknesses of the first layer 21 and the third layer 23 are each, for example, within the range of 1 to 10 μm. The thickness of the second layer 22 is, for example, within the range of 5 to 50 μm. From the viewpoint of suppressing the influence of water on the phase difference value, the second layer 22 is preferably thicker than each of the first layer 21 and the third layer 23, and is preferably thicker than the total thickness of the first layer 21 and the third layer 23. The total thickness of the phase difference film 20 is, for example, within the range of 10 to 200 μm, preferably within the range of 10 to 60 μm, and more preferably within the range of 10 to 40 μm.

[0026] The width of the phase difference film 20 is preferably within the range of 1 to 4 m, more preferably within the range of 1.3 to 4 m, and even more preferably within the range of 2.2 to 3 m. When the width is within the range of 2.2 to 3 m, it is possible to achieve both high levels of stretching uniformity and productivity.

[0027] The phase difference film 20 may have layers other than the first layer 21, the second layer 22, and the third layer 23. Even in this case, it is preferable that the first layer 21 and the third layer 23 are the outermost layers on both sides of the phase difference film 20.

[0028] [Layer 2] The second layer contains acetylcellulose having a degree of substitution of acetyl groups in the range of 2.0 to 2.6, fine particles, and a polyester having hydroxy groups at both ends. The second layer may contain other components.

[0029] (Acetylcellulose) Acetylcellulose is cellulose in which some or all of the hydrogen atoms of the hydroxy groups (-OH) at the 2nd, 3rd, and 6th positions in one glucose unit are substituted with acetyl groups. By the degree of substitution of the acetyl groups of the acetylcellulose contained in the second layer being within the range of 2.0 to 2.6, a desired retardation value is easily obtained.

[0030] The degree of substitution of acetyl groups represents the average number of acetyl groups per glucose unit. Specifically, the degree of substitution of acetyl groups represents how many of the hydrogen atoms of the hydroxy groups at the 2nd, 3rd, and 6th positions in one glucose unit are substituted with acetyl groups. Therefore, the maximum value of the degree of substitution of acetyl groups is 3.0. When the degree of substitution of acetyl groups is 3.0, all of the hydrogen atoms of the hydroxy groups at the 2nd, 3rd, and 6th positions are substituted with acetyl groups.

[0031] The acetyl groups may be substituted on average at the 2nd, 3rd, and 6th positions in one glucose unit, or may be substituted with a distribution.

[0032] The degree of substitution of acetyl groups is determined by the method specified in ASTM-D817-96.

[0033] From the viewpoint of obtaining desired optical properties, acetylcelluloses having different degrees of substitution may be mixed and used. The mixing ratio of acetylcelluloses having different degrees of substitution is not particularly limited.

[0034] From the viewpoint of mechanical strength, the number average molecular weight (Mn) of the acetylcellulose is preferably in the range of 2×10 4 ~3×10 5 and more preferably 2×10 4~1.2×10 5 within the range is more preferable, and 4×10 4 ~8×10 4 within the range is more preferable.

[0035] From the viewpoint of mechanical strength, the weight-average molecular weight (Mw) of acetyl cellulose is 2×10 4 ~1×10 6 within the range is preferable, and 2×10 4 ~1.2×10 5 within the range is more preferable, and 4×10 4 ~8×10 4 within the range is more preferable.

[0036] The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of acetyl cellulose can be measured by gel permeation chromatography (GPC) under the following conditions. Solvent: Methylene chloride Column: Three columns of Shodex K806, K805, and K803G (manufactured by Showa Denko KK) are connected and used. Column temperature: 25°C Sample concentration: 0.1 mass% Detector: RI Model 504 (manufactured by GL Sciences Inc.) Pump: L6000 (manufactured by Hitachi, Ltd.) Flow rate: 1.0 mL / min Calibration curve: A calibration curve using 13 samples within the range of Mw = 500 to 2,800,000 of standard polystyrene STK standard polystyrene (manufactured by Tosoh Corporation) is used. It is preferable to use the 13 samples at approximately equal intervals.

[0037] Acetyl cellulose can be synthesized, for example, by the following procedure. The raw material cellulose for acetyl cellulose is not particularly limited, and examples include cotton linter, wood pulp, kenaf, and the like. The raw material cellulose, acetic acid, acetic anhydride, and a catalyst (such as sulfuric acid) are mixed to esterify the cellulose. The reaction is allowed to proceed until a cellulose triester is formed. In the triester, all the hydrogen atoms of the three hydroxy groups in one glucose unit are substituted with acetyl groups. Then, by hydrolyzing the cellulose triester, acetyl cellulose having a desired degree of acetyl group substitution is obtained. Thereafter, through steps such as filtration, precipitation, washing with water, dehydration, and drying, acetyl cellulose is finally obtained. Specifically, it can be synthesized with reference to the method described in JP-A-10-45804.

[0038] Commercially available acetyl celluloses include "LM80, L20, L30, L40, L50" (manufactured by Daicel Corporation), "Ca398-3, Ca398-6, Ca398-10, Ca398-30, Ca394-60S" (manufactured by Eastman Chemical Company), and the like.

[0039] The content of acetyl cellulose in the second layer is, for example, in the range of 60 to 95% by mass.

[0040] (Polyester having hydroxy groups at both ends) The second layer contains a polyester having hydroxy groups at both ends. The portion of the polyester excluding the carboxy group and the hydroxy group exhibits relatively hydrophobicity. Therefore, even if the polyester has hydroxy groups at both ends, the whole molecule exhibits hydrophobicity. By containing such a polyester, the degree of hydrophobicity of the retardation film is improved. Therefore, the retardation film becomes less likely to take in moisture by containing the polyester. Further, the hydroxy groups at both ends of the polyester act on the water molecules that have entered the retardation film. By acting on the hydroxy groups at both ends of the polyester before the water molecules act on the hydrophilic groups of the acetyl cellulose, it is possible to suppress the coordination of water to the hydrophilic groups of the acetyl cellulose. Thereby, fluctuations in the retardation value of the retardation film can be suppressed, and thus unevenness in contrast in a liquid crystal display device immediately after being taken out from a high humidity environment can be suppressed.

[0041] The polyester having hydroxy groups at both ends preferably has a structure represented by the following general formula (I).

[0042] [Chemical formula]

[0043] In the formula, B represents a linear or branched alkylene having 2 to 6 carbon atoms, or a cycloalkylene group. A represents an aromatic ring having 6 to 14 carbon atoms. n represents a natural number of 1 or more.

[0044] The polyester having the structure represented by the general formula (I) is obtained from an aromatic dicarboxylic acid and a linear or branched alkylenediol having 2 to 6 carbon atoms, or a cycloalkylenediol. Both ends of the polyester are blocked with a monocarboxylic acid.

[0045] Examples of aromatic dicarboxylic acids having 8 to 16 carbon atoms include phthalic acid, isophthalic acid, terephthalic acid, 1,5-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,8-naphthalenedicarboxylic acid, 2,2'-biphenyldicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and the like. Among them, 2,6-naphthalenedicarboxylic acid or 4,4'-biphenyldicarboxylic acid is preferable.

[0046] Examples of linear or branched alkylene diols or cycloalkylene diols having 2 to 6 carbon atoms include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 2-methyl-1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, and the like. Among them, ethylene glycol, 1,2-propanediol, 1,3-propanediol, or 1,3-butanediol is preferable.

[0047] Among them, it is preferable for A to be a naphthalene ring or a biphenyl ring which may have a substituent in order to obtain the effects of the present invention. Here, the substituent is an alkyl group, an alkenyl group, or an alkoxyl group having 1 to 6 carbon atoms.

[0048] n is preferably in the range of 1 to 170.

[0049] The number average molecular weight of the polyester is preferably 20,000 or less, more preferably 10,000 or less. A polyester having a number average molecular weight in the range of 400 to 10,000 has good compatibility with acetyl cellulose. Further, a polyester having a number average molecular weight in the range of 400 to 10,000 is less likely to evaporate or volatilize during film formation, which is preferable. The number average molecular weight of the polyester can be measured by gel permeation chromatography (GPC). For low molecular weight compounds that cannot be measured by GPC, it can be determined by calculation from the structural formula.

[0050] The polyester according to the present invention can be synthesized by a thermal melt condensation method by a polyesterification reaction or a transesterification reaction of the above dicarboxylic acid and diol, or an interfacial condensation method of an acid chloride of the above dicarboxylic acid and glycols.

[0051] Hereinafter, polyesters having a structure represented by the general formula (I) will be exemplified.

[0052]

Chemical formula

[0053]

Chemical formula

[0054] The content of the polyester having hydroxy groups at both ends in the second layer is preferably in the range of 1 to 20% by mass, more preferably in the range of 5 to 15% by mass, relative to the content of acetyl acetate in the second layer.

[0055] (Fine particles) In the second layer, the fine particles suppress the interaction between water and acetyl cellulose by hydrogen bonding with the hydrophilic group portion of acetyl cellulose. As a result, the retardation film in which the second layer contains fine particles is less affected by water.

[0056] In the present invention, the "fine particles" refer to particles having an average particle diameter of primary particles in the range of 5 to 400 nm. The fine particles may be inorganic fine particles or organic fine particles.

[0057] Examples of the inorganic fine particles include fine particles containing silica (silicon dioxide), titanium dioxide, alumina (aluminum oxide), zirconium oxide, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, calcium phosphate, and the like. The inorganic fine particles are preferably silica fine particles. Since the refractive index of silica fine particles is close to that of acetyl cellulose, the generation of haze is suppressed by the inorganic fine particles being silica fine particles.

[0058] Examples of commercially available products of silica fine particles include Aerosil (registered trademark) R972, R972V, R974, R812, 200, 200V, 300, R202, OX50, TT600 (manufactured by Nippon Aerosil Co., Ltd.).

[0059] Examples of commercially available products of zirconium oxide fine particles include Aerosil (registered trademark) R976, R811 (manufactured by Nippon Aerosil Co., Ltd.).

[0060] Examples of the organic fine particles include fine particles containing silicone resin, fluororesin, acrylic resin, and the like. Among them, the organic fine particles are preferably fine particles containing silicone resin, and particularly preferably fine particles containing a silicone resin having a three-dimensional network structure.

[0061] Examples of commercially available products of silicone resin fine particles include Tospearl (registered trademark) 103, 105, 108, 120, 145, 3120, 240 (manufactured by Toshiba Silicone Co., Ltd.).

[0062] The average particle size of the primary particles of the fine particles is preferably in the range of 10 to 300 nm. The fine particles may be contained in the retardation film as secondary aggregates having a particle size in the range of 50 to 300 nm. Particles having an average particle size in the range of 100 to 400 nm are preferably contained as primary particles without aggregation.

[0063] The content rate of the fine particles in the second layer is preferably in the range of 0.01 to 5.0% by mass, more preferably in the range of 0.5 to 2.5% by mass, and still more preferably in the range of 0.7 to 2.0% by mass with respect to the total mass of the second layer.

[0064] (Other components) In addition to the above components, the second layer may contain sugar esters, acrylic polymers, plasticizers, and the like.

[0065] [1-2. The first layer and the third layer] The first layer and the third layer contain acetyl cellulose and fine particles. The first layer and the third layer may contain other components. The compositions of the first layer and the third layer may be the same as or different from each other.

[0066] (Acetyl cellulose) The acetyl cellulose contained in the first layer and the third layer may be the same as or different from the acetyl cellulose contained in the second layer. The acetyl cellulose contained in the first layer and the third layer may be the same as or different from each other.

[0067] The degree of substitution of the acetyl cellulose contained in the first layer and the third layer is not particularly limited, but is preferably in the range of 2.0 to 2.6. Thereby, a desired retardation value can be easily obtained.

[0068] The content rate of the acetyl cellulose in the first layer and the third layer is, for example, in the range of 60 to 95% by mass.

[0069] The content rate of the polyester having hydroxy groups at both ends in the first layer is preferably in the range of 1 to 20% by mass, more preferably in the range of 5 to 15% by mass, relative to the content rate of acetyl acetate in the first layer.

[0070] The content rate of the polyester having hydroxy groups at both ends in the third layer is preferably in the range of 1 to 20% by mass, more preferably in the range of 5 to 15% by mass, relative to the content rate of acetyl acetate in the third layer.

[0071] Other details of the acetyl cellulose are as described above.

[0072] (Fine particles) When the first layer and the second layer contain fine particles, the slipperiness of the surface of the retardation film is improved. Further, when the first layer and the second layer contain fine particles, when the retardation films are stacked on each other, it is possible to suppress damage and adhesion between the retardation films.

[0073] The types of fine particles contained in the first layer and the third layer may be the same as or different from the types of fine particles contained in the second layer. The types of fine particles contained in the first layer and the third layer may be the same as or different from each other. The fine particles contained in the first layer and the third layer are preferably silica particles.

[0074] The degree of substitution of the acetyl cellulose contained in the first layer and the third layer is not particularly limited, but is preferably in the range of 2.0 to 2.6. Thereby, it becomes easy to obtain a desired retardation value.

[0075] The content rate of the fine particles in the first layer is preferably in the range of 0.01 to 5.0% by mass, more preferably in the range of 0.05 to 1.0% by mass, and still more preferably in the range of 0.1 to 0.5% by mass, based on the total mass of the first layer.

[0076] The content ratio of the fine particles in the third layer is preferably in the range of 0.01 to 5.0% by mass, more preferably in the range of 0.05 to 1.0% by mass, and still more preferably in the range of 0.1 to 0.5% by mass, based on the total mass of the third layer.

[0077] Other details of the fine particles are as described above.

[0078] (Polyester) The first layer and the third layer may contain polyester. The polyester contained in the first layer and the third layer may or may not be a polyester having hydroxy groups at both ends.

[0079] The types of polyester contained in the first layer and the third layer may be the same as or different from each other. Note that the polyester contained in the first layer and the third layer is preferably a polyester having hydroxy groups at both ends, similar to the polyester contained in the second layer. Further, the polyester preferably has a structure represented by the above general formula (I).

[0080] By containing a polyester having hydroxy groups at both ends in the first layer and the third layer, it is possible to suppress water from coordinating to the hydrophilic group of acetyl cellulose in each of the first layer to the third layer. Thereby, the fluctuation of the retardation value of the retardation film can be further suppressed, and thus the unevenness of contrast in the liquid crystal display device immediately after being taken out from a high humidity environment can be further suppressed.

[0081] The content ratio of the polyester in the first layer is preferably in the range of 1 to 20% by mass, more preferably in the range of 5 to 15% by mass, based on the content ratio of acetyl acetate in the first layer.

[0082] The content ratio of the polyester in the third layer is preferably in the range of 1 to 20% by mass, more preferably in the range of 5 to 15% by mass, based on the content ratio of acetyl acetate in the third layer.

[0083] (Other components) In addition to the above components, the first layer and the third layer may contain a sugar ester, an acrylic polymer, a plasticizer, etc.

[0084] [1-3. Relationship of the content rate of fine particles] In the retardation film of the present invention, the content rate [mass%] of the fine particles in the second layer is characterized by being larger than the content rate [mass%] of the fine particles in the first layer and the third layer. When the content rate of the fine particles is different between the first layer and the third layer, the content rate of the fine particles in the second layer is larger than the content rate of the fine particles in either the first layer or the third layer. Thereby, in the present invention, after adjusting the content rate of the fine particles in the first layer and the third layer with an emphasis on slipperiness, the content rate of the fine particles in the second layer can be increased. As a result, the present invention can suppress the retardation value from being affected by water while maintaining the slipperiness within a good range.

[0085] The content rate of the fine particles in the second layer is preferably in the range of 2 to 5 times the average content rate of the fine particles in the first layer and the third layer. By being 2 times or more, while the slipperiness is maintained within a good range, the retardation value is more suppressed from being affected by water. By being 5 times or less, the haze does not become too high, and it is difficult to lower the contrast of the liquid crystal display device.

[0086] [1-4. Retardation value of the retardation film] The in-plane retardation value Ro and the thickness-direction retardation value Rt of the retardation film are defined by the following formulas, respectively. Formula (i) Ro=(n x -n y )×d Formula (ii) Rt={(n x +n y ) / 2-n z}×d

[0087] In the above formulas (i) and (ii), n x represents the refractive index in the direction x where the refractive index is maximum in the in-plane direction of the film. n y represents the refractive index in the direction y orthogonal to the direction x in the in-plane direction of the film. n zrepresents the refractive index in the thickness direction z of the film. d [nm] represents the thickness of the retardation film.

[0088] Ro of the retardation film, when measured with light of wavelength 589 nm in an environment of 23°C and 55% RH, is preferably in the range of 30 to 90 nm. Rt of the retardation film, when measured with light of wavelength 589 nm in an environment of 23°C and 55% RH, is preferably in the range of 100 to 200 nm. When Ro and Rt are within the above ranges, light leakage when viewing a liquid crystal display device equipped with the retardation film of the present invention from an oblique direction is reduced.

[0089] Ro and Rt can be controlled by the composition of the retardation film, the stretching conditions during the production of the retardation film, etc.

[0090] Ro and Rt can be measured using an automatic birefringence meter. Examples of the automatic birefringence meter include "Axo Scan Mueller Matrix Polarimeter" (manufactured by Axometrics, Inc.), etc.

[0091] [1-5. Manufacturing method of retardation film] The manufacturing method of the retardation film may be a solution casting method or a melt casting method. Among them, the solution casting method is preferable.

[0092] The manufacturing method of the film by the solution casting method has, for example, the following steps. (1) Step of preparing the dope for each layer (2) Step of casting the dope for each layer onto a metal support (3) Step of drying the web (4) Step of peeling the film from the metal support (5) Step of stretching or holding the width of the film (6) Step of further drying the film (7) Step of winding up the film

[0093] (1) Step of preparing the dope for each layer In this step, necessary materials such as acetyl cellulose are dissolved or dispersed in a solvent to prepare a dope for each layer. A higher content of acetyl cellulose in the dope is preferable as it can reduce the drying load after casting on a metal support. Also, by not excessively increasing the content of acetyl cellulose, the pressure load during filtration can be suppressed, and good filtration accuracy can be obtained. From these viewpoints, the content of acetyl cellulose in the dope is preferably in the range of 10 to 35% by mass, more preferably in the range of 15 to 25% by mass, based on the total mass of the dope.

[0094] The solvent used for preparing the dope may be a single type or two or more types. From the viewpoint of production efficiency, it is preferable to mix a good solvent and a poor solvent for acetyl cellulose. From the viewpoint of the solubility of acetyl cellulose, a larger proportion of the good solvent is preferable. The mixing ratio of the good solvent and the poor solvent is preferably such that the good solvent is in the range of 70 to 98% by mass, and the poor solvent is in the range of 2 to 30% by mass. A solvent that can dissolve acetyl cellulose alone is defined as a "good solvent", and a solvent that swells or does not dissolve alone is defined as a "poor solvent".

[0095] The good solvent is not particularly limited, and examples include organic halogen compounds (such as methylene chloride), dioxolanes, acetone, methyl acetate, methyl acetoacetate, etc. Among them, methylene chloride or methyl acetate is preferable. The poor solvent is not particularly limited, and examples include methanol, ethanol, n-butanol, cyclohexane, cyclohexanone, etc.

[0096] As a method for dissolving acetyl cellulose when preparing the dope, known methods can be used. For example, by combining heating means and pressurizing means, the dope can be heated to a temperature above the boiling point at normal pressure. When acetyl cellulose is stirred and dissolved while heating the solvent at a temperature above the boiling point at normal pressure and within a range where the solvent does not boil under pressure, the generation of lumpy undissolved matter (gel or mama co) can be prevented. Also, after mixing acetyl cellulose with a poor solvent to wet or swell it, a good solvent may be further added and dissolved.

[0097] Examples of the pressurization method include a method of injecting an inert gas such as nitrogen gas into the dissolution vessel, a method of increasing the vapor pressure of the solvent by heating, etc. Heating is preferably performed from the outside. For example, a jacket type is preferable because temperature control is easy.

[0098] From the viewpoint of the solubility of acetyl cellulose, a higher heating temperature is preferable. Also, by not raising the heating temperature too much, the pressure load can be suppressed and good productivity can be obtained. From these viewpoints, the heating temperature is preferably in the range of 45 to 120°C, more preferably in the range of 60 to 110°C, and even more preferably in the range of 70°C to 105°C. The pressure is adjusted so that the solvent does not boil at the set temperature.

[0099] As another method for dissolving acetyl cellulose, a cooling dissolution method can be mentioned. By the cooling dissolution method, acetyl cellulose can be dissolved in a solvent such as methyl acetate.

[0100] The dope may be filtered using a suitable filter medium such as filter paper. From the viewpoint of removing insolubles etc., a smaller absolute filtration accuracy of the filter medium is preferable. Also, by not reducing the absolute filtration accuracy too much, clogging of the filter medium can be suppressed.

[0101] The preparation of the dope can be carried out using, for example, a static mixer, an in-line mixer, etc. Examples of the static mixer include those manufactured by Toray Engineering Co., Ltd. Examples of the in-line mixer include the Toray static in-pipe mixer ("Hi-Mixer SWJ", manufactured by Toray Engineering Co., Ltd.).

[0102] (2) Step of casting the dope of each layer onto a metal support In this step, the dopes of each layer are cast onto an endless metal support that moves infinitely. The method of casting the dopes of each layer is not particularly limited, and for example, a known co-casting method can be used. The metal support in the casting step is preferably one with a mirror-finished surface. The metal support is preferably a stainless-steel belt or a drum with a plated surface. The width of casting is preferably in the range of, for example, 1 to 4 m.

[0103] (3) Step of drying the web In this step, the dope cast on the metal support is dried as a web. The surface temperature of the metal support is preferably in the range of not less than -50°C and less than the boiling point of the solvent. A higher surface temperature can increase the drying rate of the web. Also, by not making the surface temperature too high, foaming of the web can be prevented and good film planarity can be obtained. From these viewpoints, the surface temperature is preferably in the range of 0 to 40°C, and more preferably in the range of 5 to 30°C. Also, by cooling the metal support, the web can be gelled, and the film may be peeled off from the drum in a state containing a large amount of residual solvent.

[0104] The method of controlling the temperature of the metal support is not particularly limited, and for example, a method of blowing warm air or cold air can be mentioned. Also, a method of bringing warm water into contact with the back side of the metal support can be mentioned. The method using warm water can shorten the time until the temperature of the metal support becomes constant because heat transfer is efficiently performed. When using warm air, air at a temperature higher than the target temperature of the metal support may be used.

[0105] (4) Step of peeling the film from the metal support In this step, the dried film is peeled off from the metal support. From the viewpoint of obtaining good planarity of the film, the amount of residual solvent when peeling the film (web) from the metal support is preferably in the range of 10 to 150% by mass. The amount of residual solvent is more preferably in the range of 10 to 40% by mass, and even more preferably in the range of 10 to 30% by mass. Here, the amount of residual solvent is defined by the following formula.

[0106] Residual solvent content [mass%] = {(M - N) / N} × 100 In the formula, M is the mass of the web or film at any given time. N is the mass of the web or film after heating at 115°C for 1 hour.

[0107] (5) Step of stretching or maintaining the width of the film In this step, the film with a high residual solvent content immediately after peeling is stretched or the width is maintained. It is preferable to use a tenter method in which the film is stretched in the transport direction (longitudinal direction) and the both ends of the film are gripped with clips or the like. Also, stretching may be simultaneously performed in the transport direction (longitudinal direction) and the width direction (lateral direction).

[0108] In the longitudinal stretching, the peeling tension is preferably 210 N / m or more, and more preferably in the range of 220 to 300 N / m.

[0109] By the stretching step, the refractive index of the film can be controlled, and the retardation values Ro and Rt can be controlled.

[0110] The final stretching ratio in the transport direction is preferably in the range of 1.0 to 2.0 times, and more preferably in the range of 1.01 to 1.5 times. The final stretching ratio in the width direction is preferably in the range of 1.01 to 2.5 times, and more preferably in the range of 1.05 to 2.0 times.

[0111] The method of stretching the film is not particularly limited. For example, as the stretching method, there is a method in which a peripheral speed difference is provided between a plurality of rollers, and the film is stretched in the longitudinal direction by utilizing the roller peripheral speed difference therebetween. As the stretching method, there is a method in which both ends of the film are fixed with clips or pins, and the interval between the clips or pins is widened in the transport direction to stretch the film in the longitudinal direction. Similarly, there is a method in which the interval between the clips or pins is widened in the width direction to stretch the film in the lateral direction. Similarly, there is a method in which the interval between the clips or pins is widened simultaneously in the transport direction and the width direction to stretch the film in both the longitudinal and lateral directions.

[0112] These stretching methods may be used in combination. Also, in the case of the tenter method, when the clip portion is driven by a linear drive method, smooth stretching can be achieved and the risk of film breakage or the like can be reduced.

[0113] These width retention or lateral stretching is preferably performed by the tenter method, and it may be a pin tenter or a clip tenter.

[0114] When the advance axis or retardation axis of the film exists in the film plane and the angle formed with the conveyance direction is θ1, θ1 is preferably in the range of -0.5 to +0.5°, more preferably in the range of -0.3 to +0.3°, and even more preferably in the range of -0.2 to +0.2°. This θ1 can be defined as the orientation angle. θ1 can be measured using an automatic birefringence meter "KOBRA-21ADH" (manufactured by Oji Scientific Instruments). When θ1 is within the above range, high brightness can be obtained in the display image. Also, light leakage can be suppressed or prevented, and in a color liquid crystal display device, colors can be faithfully reproduced.

[0115] (6) Step of further drying the film In this step, the peeled film is further dried. Drying may be performed after stretching or simultaneously. The residual solvent amount of the dried film is preferably 1% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.01% by mass or less.

[0116] The drying method is not particularly limited, and examples include a roller drying method in which the film is alternately passed through a large number of rollers arranged vertically for drying. Also, while stretching the film by the above tenter method, the film may be dried simultaneously.

[0117] The means for drying the film is not particularly limited, and examples include hot air, infrared rays, heating rollers, microwaves, etc. From the viewpoint of simplicity, the means for drying is preferably hot air.

[0118] The drying temperature is preferably increased stepwise within the range of 40 to 200°C. From the viewpoint of dimensional stability, the drying temperature is more preferably within the range of 50 to 140°C.

[0119] (7) Step of winding the film In this step, the finished film is wound, for example, in a roll shape.

[0120] [2. Polarizing plate and liquid crystal display device] The polarizing plate and liquid crystal display device of the present invention are characterized by including the above-described retardation film of the present invention.

[0121] FIG. 2 is a cross-sectional view of an embodiment of a liquid crystal display device.

[0122] The liquid crystal display device 500 shown in FIG. 2 includes a liquid crystal panel 100 and a backlight 200. The liquid crystal panel 100 has a first polarizing plate 50, a liquid crystal cell 60, and a second polarizing plate 70 in this order.

[0123] The first polarizing plate 50 is a polarizing plate located on the viewing side, which is the opposite side of the backlight 200 side, in the liquid crystal display device 500. The second polarizing plate 70 is a polarizing plate located on the backlight 200 side in the liquid crystal display device 500.

[0124] The first polarizing plate 50 and the second polarizing plate 70 each include a first optical film 10, a polarizer 30, and a second optical film 20 in this order. The first optical film 10 is located on the side farther from the liquid crystal cell 60 compared to the second optical film 20 in the first polarizing plate 50 and the second polarizing plate 70. The second optical film 20 is located on the side closer to the liquid crystal cell 60 compared to the first optical film 10 in the first polarizing plate 50 and the second polarizing plate 70.

[0125] In the first polarizing plate 50 and the second polarizing plate 70, the retardation film of the present invention is used as the second optical film 20. Similarly, in the liquid crystal display device 500, the retardation film of the present invention is used as the second optical film 20. Note that in the liquid crystal display device 500, at least one of the second optical film 20 included in the first polarizing plate 50 and the second optical film 20 included in the second polarizing plate 70 may be the retardation film of the present invention, and it is not necessary for both to be the retardation film of the present invention.

[0126] The first optical film 10 is an optical film that functions as a protective film, a retardation film, etc. in the first polarizing plate 50 and the second polarizing plate 70. The first optical film 10 contains, for example, polyester, an ultraviolet absorber, etc.

[0127] The polyester is preferably polyethylene terephthalate or polyethylene naphthalate. Polyethylene terephthalate and polyethylene naphthalate have a large intrinsic birefringence, and it is relatively easy to obtain a high retardation value even when the film is made thin. In particular, the effect of polyethylene naphthalate is remarkable.

[0128] The ultraviolet absorber protects the liquid crystal display device 500 (especially the alignment film included in the liquid crystal cell 60) from ultraviolet rays and improves the weather resistance of the liquid crystal display device 500. Examples of the ultraviolet absorber include cyclic iminoester-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, salicylic acid ester-based ultraviolet absorbers, cyanoacrylate-based ultraviolet absorbers, triazine-based ultraviolet absorbers, etc. Among them, the ultraviolet absorber is preferably a cyclic iminoester-based ultraviolet absorber or a benzotriazole-based ultraviolet absorber. The content of the ultraviolet absorber is preferably in the range of 0.1 to 10% by mass with respect to the total mass of the polyester.

[0129] In the environment of 23°C and 55% RH, the in-plane retardation value Ro of the first optical film 10 with respect to light of a wavelength of 589 nm is preferably in the range of 3,000 to 30,000 nm. When Ro is 3,000 nm or more, the interference color (rainbow unevenness depending on the observation angle) when the first optical film 10 is observed from an oblique direction can be reduced, and good visibility can be obtained. Further, when Ro is 30,000 nm or less, the thickness of the first optical film 10 can be reduced. Ro is preferably 5,000 nm or more, more preferably 8,000 nm or more, and still more preferably 10,000 nm or more.

[0130] The ratio (Ro / Rt) of the retardation values of the first optical film 10 is preferably in the range of 0.2 to 1.2, more preferably in the range of 0.5 to 1.0, and still more preferably in the range of 0.6 to 1.0. When the ratio (Ro / Rt) of the retardation values is within the above range, the rainbow unevenness depending on the observation angle of the first optical film 10 is reduced. The retardation values Ro and Rt can be controlled by the type of polyester, the stretching conditions during film production, and the like.

[0131] The thickness of the first optical film 10 is preferably 5 μm or more, more preferably 10 μm or more, still more preferably 15 μm or more, and particularly preferably 20 μm or more. When the thickness of the first optical film 10 is 5 μm or more, the first optical film 10 can obtain good water resistance and mechanical strength. The thickness of the first optical film 10 is preferably 300 μm or less, more preferably 200 μm or less, still more preferably 100 μm or less, and particularly preferably 40 μm or less. When the thickness of the first optical film 10 is 100 μm or less, the first optical film 10 can achieve both thin film properties and visibility.

[0132] The polarizer 30 is an element that passes only light with a polarization plane in a certain direction. Examples of the polarizer include polyvinyl alcohol-based polarizing films. The polyvinyl alcohol-based polarizing films include those obtained by dyeing a polyvinyl alcohol-based film with iodine and those obtained by dyeing with a dichroic dye.

[0133] As a method for manufacturing the polarizer 30, there is a method of forming a film from an aqueous polyvinyl alcohol solution, uniaxially stretching the obtained film, and dyeing it. After dyeing, it may be uniaxially stretched and subjected to a durability treatment with a boron compound or the like.

[0134] Examples of the polyvinyl alcohol include ethylene-modified polyvinyl alcohol described in JP-A-2003-248123 and JP-A-2003-342322. The ethylene-modified polyvinyl alcohol has an ethylene unit content in the range of 1 to 4 mol%, a degree of polymerization in the range of 2000 to 4000, and a saponification degree in the range of 99.0 to 99.99 mol%. The polyvinyl alcohol is preferably ethylene-modified polyvinyl alcohol having a hot water cutting temperature in the range of 66 to 73°C. The polarizer 30 containing this ethylene-modified polyvinyl alcohol is excellent in polarization performance and durability performance, has little color unevenness, and is particularly preferably used for a large liquid crystal display device 500.

[0135] The thickness of the polarizer 30 is preferably in the range of 2 to 30 μm, and more preferably in the range of 2 to 20 μm.

[0136] The first polarizing plate 50 and the second polarizing plate 70 may have an adhesive layer (not shown) between the first optical film 10 and the polarizer 30, and between the polarizer 30 and the second optical film 20. The adhesive layer contains a cured product of an adhesive.

[0137] The first polarizing plate 50 and the second polarizing plate 70 can be manufactured by a general method for polarizing plates. The first polarizing plate 50 and the second polarizing plate 70 can be manufactured, for example, by the following procedure. The polarizer 30 is stretched. The polarizer 30 is immersed in an iodine solution. The surfaces of the first optical film 10 and the second optical film 20 to be bonded to the polarizer 30 are each appropriately surface-treated. The surface-treated surface of the first optical film 10 is bonded to at least one surface of the polarizer 30 using an adhesive. The surface-treated surface of the second optical film 20 is bonded to the other surface of the polarizer 30 using an adhesive. The bonding is preferably performed, for example, in a direction in which the absorption axis of the polarizer 30 and the slow axes of the first optical film 10 and the second optical film 20 are orthogonal. The adhesive is preferably an ultraviolet-curable adhesive.

[0138] The liquid crystal cell 60 is not particularly limited and may be a general one. The liquid crystal cell 60 has, for example, a layer structure of glass substrate / color filter / transparent electrode / orientation film / liquid crystal layer / orientation film / transparent electrode / TFT (Thin Film Transistor) / glass substrate in order from the first polarizing plate 50 side.

[0139] The first polarizing plate 50, the liquid crystal cell 60, and the second polarizing plate 70 may be bonded via an adhesive layer (not shown). The adhesive layer is a layer formed using, for example, a double-sided tape, an ultraviolet-curable adhesive, or the like. Examples of the double-sided tape include the substrate-less tape "MO-3005C" (thickness: 25 μm, manufactured by Lintec Corporation). The bonding method is not particularly limited, and a known method can be used.

[0140] The backlight 200 is not particularly limited and may be a general one. The backlight 200 can be, for example, an LED backlight using light-emitting diodes (LEDs: Light Emitting Diodes).

[0141] The driving methods of the liquid crystal display device 500 of the present invention can be a TN (Twisted Nematic) method, an STN (Super Twisted Nematic) method, an IPS (In-Plane Switching) method, an OCB (Optically Compensated Birefringence) method, a VA (Vertical Alignment) method, a HAN (Hybrid Aligned Nematic) method, etc. The VA method includes an MVA (Multi-domain Vertical Alignment) method and a PVA (Patterned Vertical Alignment) method. The retardation film 20 of the present invention is preferably used in the liquid crystal display device 500 particularly with the VA method.

Example

[0142] Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. In the following examples, unless otherwise specified, the operations were performed at room temperature (25°C). In the following examples, unless otherwise specified, “%” and “parts” mean “mass %” and “parts by mass”, respectively.

[0143] [Production of Retardation Film (Second Optical Film)] For the acetyl cellulose in the production of the retardation film, those having an acetyl group substitution degree of 1.8, 2.4, or 2.8 were used.

[0144] For the polyester in the production of the retardation film, a polyester having hydroxy groups at both ends and a polyester with both ends sealed were used. As the polyester having hydroxy groups at both ends, a polyester obtained by reacting terephthalic acid, 2,6-naphthalenedicarboxylic acid, and propylene glycol in a ratio of 1:1:3 was used. The molecular weight of the polyester having hydroxy groups at both ends is 556. As the polyester with both ends sealed, a polyester obtained by reacting terephthalic acid, propylene glycol, and benzoic acid in a ratio of 1:2:2 was used. The molecular weight of the polyester with both ends sealed is 490.

[0145] For the fine particles in the production of the retardation film, silica fine particles (Aerosil R812, manufactured by Nippon Aerosil Co., Ltd.) were used.

[0146] The following components were stirred and mixed with a dissolver for 50 minutes, and then dispersed with a Manton Gorin to prepare a fine particle dispersion (fine particle concentration: 11% by mass). Fine particles 11.0 parts by mass Ethanol 89.0 parts by mass

[0147] The following components were sufficiently mixed with an in-line mixer (Toray static in-line mixer, Hi-Mixer SWJ, manufactured by Toray Engineering Co., Ltd.) to prepare a dope for the skin layer. Methylene chloride 300.0 parts by mass Ethanol 30.0 parts by mass Acetyl cellulose (degree of substitution: 2.4) 79.7 parts by mass Polyester (both ends: hydroxy group) 19.9 parts by mass Fine particle dispersion (fine particle concentration: 11% by mass) 3.6 parts by mass

[0148] The following components were sufficiently mixed with an in-line mixer (Toray static in-line mixer, Hi-Mixer SWJ, manufactured by Toray Engineering Co., Ltd.) to prepare a dope for the core layer. Methylene chloride 300.0 parts by mass Ethanol 30.0 parts by mass Acetyl cellulose (degree of substitution: 2.4) 79.5 parts by mass Polyester (both ends: hydroxy group) 19.9 parts by mass Fine particle dispersion (fine particle concentration: 11% by mass) 5.5 parts by mass

[0149] Coextrusion was carried out simultaneously in multiple layers so that the dope for the skin layer, the dope for the core layer, and the dope for the skin layer were stacked in this order. The dope for the core layer formed the second layer, and the dopes for the skin layers on both sides thereof formed the first layer and the third layer, respectively. Here, the casting amounts of the respective dopes were adjusted so that the thickness of the second layer after stretching was 30 μm and the thicknesses of the first layer and the third layer were each 3 μm. On the casting band, the solvent of the web being cast was evaporated until the residual solvent amount reached about 30% by mass.

[0150] The film with a residual solvent amount of about 30% by mass was peeled off from the casting band. While applying hot air at 140 °C to the peeled film, stretching in the width direction by 35% was performed using a tenter.

[0151] Next, the transfer from tenter conveyance to roll conveyance was carried out, and the film was further dried at 120 °C. Then, the film was wound up.

[0152] By the above operations, the retardation film of Example 1 was produced.

[0153] In the production of the retardation film of Example 1, the retardation films of Examples 2 to 4 and Comparative Examples 1 to 5 were produced in the same manner except that the composition of each dope was appropriately changed so that the composition of the produced retardation film was as described in Tables I and II. In the column of "both ends" in Tables I and II, "OH" indicates that a polyester having hydroxy groups at both ends is used. "Sealed" indicates that a polyester with both ends sealed is used.

[0154] [Table 1]

[0155] [Table 2]

[0156] In each retardation film, the ratio (X / Y) of the content X [% by mass] of the fine particles in the second layer to the average content Y [% by mass] of the fine particles in the first and third layers is as shown in Table III. The retardation of each retardation film is as shown in Table III.

[0157] [Production of Polarizer] Iodine was adsorbed on the stretched polyvinyl alcohol film to produce a polarizer.

[0158] The following saponification treatment was performed on the retardation film. An aqueous sodium hydroxide solution of 1.5 mol / L was prepared and kept at 55°C. An aqueous dilute sulfuric acid solution of 0.005 mol / L was prepared and kept at 35°C. The retardation film was immersed in the above aqueous sodium hydroxide solution for 2 minutes, then immersed in water to thoroughly wash away the aqueous sodium hydroxide solution. Subsequently, it was immersed in the above aqueous dilute sulfuric acid solution for 1 minute, then immersed in water to thoroughly wash away the aqueous dilute sulfuric acid solution. Then, the retardation film was thoroughly dried at 120°C.

[0159] Next, using a polyvinyl alcohol-based adhesive, the saponified retardation film was used as the second optical film and attached to one side of the polarizer. The transmission axis of the polarizer and the slow axis of the retardation film were arranged to be parallel.

[0160] The same saponification treatment as above was performed on a commercially available cellulose triacetate film (Fujitac TD80UF, manufactured by Fujifilm Corporation).

[0161] Next, using a polyvinyl alcohol-based adhesive, the commercially available saponified cellulose triacetate film was used as the first optical film and attached to the opposite side of the side where the retardation film of the polarizer was attached. The transmission axis of the polarizer and the slow axis of the first optical film (commercially available cellulose triacetate film) were arranged to be orthogonal.

[0162] Next, a laminate of a retardation film, a polarizer, and a commercially available cellulose triacetate film was dried at 70°C for 10 minutes or more. Through the above operations, a polarizing plate was produced.

[0163] [Fabrication of Liquid Crystal Display Device] A 40-inch liquid crystal display (BRAVIA X1) manufactured by SONY was prepared. Then, the two polarizing plates attached to both sides of the liquid crystal cell of this liquid crystal display were peeled off. After that, the fabricated polarizing plates were respectively attached to both sides of the visual recognition side and the backlight side of the liquid crystal cell to fabricate a liquid crystal display device. At this time, in the polarizing plate arranged on the visual recognition side with respect to the liquid crystal cell, the second optical film was positioned on the liquid crystal cell side with respect to the polarizer, and the first optical film was positioned on the visual recognition side, and the polarizing plate on the visual recognition side was attached. Also, in the polarizing plate arranged on the backlight side with respect to the liquid crystal cell, the second optical film was positioned on the liquid crystal cell side with respect to the polarizer, and the first optical film was positioned on the backlight side, and the polarizing plate on the backlight side was attached.

[0164] [Evaluation] The fabricated liquid crystal display device was stored at 23°C and 95% RH for 48 hours. After storage, the backlight of the liquid crystal display device was continuously lit for 1 hour in an environment of 23°C and 55% RH, and then the front contrast was measured. The measurement of the front contrast was performed according to the following procedure.

[0165] (i) The front luminance of the display screen when the liquid crystal display device was white-displayed (the luminance measured from the normal direction of the display screen) was measured using EZ-Contrast160D manufactured by ELDIM. Similarly, the front luminance of the display screen when the liquid crystal display device was black-displayed was measured.

[0166] (ii) The ratio (B / A) of the front luminance B of the display screen when white-displayed to the front luminance A of the display screen when black-displayed was defined as the front contrast.

[0167] As described above, the front contrast at any 10 points on the display screen of the liquid crystal display device was measured.

[0168] The average value of the contrast was evaluated based on the average value of the obtained 10 front contrasts according to the following criteria. The evaluation results are as shown in Table III. A: The average value of the front contrast is 6000 or more. B: The average value of the front contrast is 5500 or more and less than 6000. C: The average value of the front contrast is 5000 or more and less than 5500. D: The average value of the front contrast is less than 5000.

[0169] Furthermore, among the obtained 10 front contrasts, the maximum value of the front contrast with the largest difference from the average value was determined. Then, the variation (%) of the front contrast was determined from the following formula.

[0170] Variation (%) of front contrast = {(Maximum value of front contrast) - (Average value of front contrast)} / (Average value of front contrast) × 100

[0171] The contrast unevenness was evaluated based on the variation of the front contrast according to the following criteria. The evaluation results are as shown in Table III. A: The variation of the front contrast is less than 1%, and there is no unevenness. B: The variation of the front contrast is 1% or more and less than 5%, and the unevenness is small. C: The variation of the front contrast is 5% or more and less than 10%, and there is some unevenness. D: The variation of the front contrast is 10% or more, and the unevenness is large.

[0172]

Table 3

[0173] In Comparative Example 4, the phase difference values Ro and Rt did not reach the required levels. In Comparative Example 5, when the phase difference film was wound up, the slipperiness of the skin layer was too high and winding deviation occurred.

[0174] From the above results, it can be confirmed that the retardation film of the present invention can suppress the unevenness of contrast in a liquid crystal display device immediately after being taken out from a high-humidity environment.

Explanation of Reference Numerals

[0175] 10 First optical film 20 Second optical film (retardation film) 21 First layer 22 Second layer 23 Third layer 30 Polarizer 50 First polarizing plate 60 Liquid crystal cell 70 Second polarizing plate 100 Liquid crystal panel 200 Backlight 500 Liquid crystal display device

Claims

1. A retardation film having a first layer, a second layer, and a third layer in this order, wherein the first layer, the second layer, and the third layer all contain acetyl cellulose and fine particles, the degree of substitution of the acetyl group of the acetyl cellulose contained in the second layer is in the range of 2.0 to 2.6, the second layer further contains a polyester having hydroxy groups at both ends, the content rate [mass%] of the fine particles in the second layer is larger than the content rates [mass%] of the fine particles in the first layer and the third layer, a retardation film characterized by the above.

2. The content rate [mass%] of the fine particles in the second layer is in the range of 2 to 5 times the average content rate [mass%] of the fine particles in the first layer and the third layer, the retardation film according to Claim 1, characterized by the above.

3. The fine particles are silica fine particles, the retardation film according to Claim 1, characterized by the above.

4. Comprising the retardation film according to any one of Claims 1 to 3, a polarizing plate characterized by the above.

5. Comprising the retardation film according to any one of Claims 1 to 3, a liquid crystal display device characterized by the above.

Images