JP2012226071A - Polarizer protection film, polarizer, liquid crystal display device, and method for manufacturing polarizer protection film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarizer protection film having unprecedentedly high transmittance and low haze with excellent stability against environmental variation, a method for manufacturing the same, a polarizer having the polarizer protection film, and a liquid crystal display device having the polarizer.SOLUTION: A polarizer protection film is formed of a single-layer film which contains 80 mass% or more of cellulose ester and has a total light transmittance of 93% or more and a haze of 5% or less. For the film having a total light transmittance T1, and a total light transmittance T2 after one minute heat treatment at Tg+10°C of the film, T1 is higher than T2 by 0.5% or more.

Description

  The present invention relates to a polarizing plate protective film having unprecedented high transmittance and low haze and excellent environmental stability, a method for producing the same, a polarizing plate including the polarizing plate protective film, and a liquid crystal display including the polarizing plate. Relates to the device.

  In recent years, there has been a demand for a polarizing plate having a high transmittance in order to increase the luminance and power consumption of a liquid crystal display device.

  A typical polarizing plate has a structure in which three films of a polarizing plate protective film, an absorbing polarizer, and a polarizing plate protective film are laminated. As the polarizer, a PVA (polyvinyl alcohol) film in which iodine is arranged is used because of its high degree of polarization. As the polarizing plate protective film, a cellulose ester-based material is generally used because of its high light transmittance and easy adhesion to a polarizer.

  In the liquid crystal display device, a liquid crystal panel having a role of switching transmitted light and polarizing plates are arranged on both sides thereof.

  When light passes through a liquid crystal display device, it is known that surface Fresnel reflection due to a difference in refractive index occurs at the interface between air and a polarizing plate protective film. This is one factor that hinders the high transmittance of the liquid crystal display device.

  Therefore, it is expected that the transmittance can be improved by reducing the difference in refractive index between the polarizing plate protective film and the air interface.

  In Patent Document 1, as a method for reducing the refractive index, it is proposed to contain independent fine bubbles in the resin. There are two specific methods for achieving this low refractive index: a method of containing a foaming agent in the crosslinkable polymer material and a method of removing the organic material by heat from a state in which a fine organic material is contained in the inorganic material. In the polarizing plate protective film, it is difficult to use a crosslinkable resin from the viewpoint of adhesiveness with a polarizer. In addition, when an inorganic film having fine pores is produced using the latter method, in addition to the above-mentioned adhesive problem, there is a problem in flexibility and it is difficult to apply it to a polarizing plate protective film. It is. Moreover, the effect of lowering the refractive index using these methods can be expected to reduce the surface reflectance at the same time. However, the decrease in the refractive index described in Patent Document 1 is 0.1% or less. The decrease in reflectance at the refractive index of typical polymer materials (typically 1.5) was estimated to be about 0.02%, and the effect of increasing the transmittance was very limited.

  Patent Document 2 proposes a method of forming a high-permeability cellulose film by forming fine bubbles in a cellulose ester film by a wet phase separation method. The transmittance of a polarizing plate protective film using a cellulose ester resin usually requires a transmittance of 90% or more, but the upper limit of the total light transmittance of this method is 82%. The method disclosed in Patent Document 2 is based on the premise that the total light transmittance is lowered, and is merely described as to how to suppress the reduction of the total light transmittance. By reducing the surface reflectance, The effect of further increasing the transmittance is not seen. Further, a specific method for increasing the transmittance is not substantially shown.

  Another problem related to the liquid crystal display device is suppression of luminance unevenness that occurs when the environment changes. Specifically, after the liquid crystal display device is exposed to high temperature and high humidity conditions, when the light source is turned on while returning to the average use environment (23 ° C., 50% RH), luminance unevenness is observed. In particular, in a backlight unit equipped with an LED light source that has been adopted in recent years, the amount of heat generation is large, and this problem appears remarkably. However, at present, no effective solution to this problem has been shown.

Japanese Patent No. 3549108 Japanese Patent No. 4502412

  The present invention has been made in view of the above problems, and its purpose is to provide a polarizing plate protective film having an unprecedented high transmittance and low haze and excellent environmental stability, a method for producing the same, and the polarizing plate. It is providing the polarizing plate provided with a protective film, and a liquid crystal display device provided with this polarizing plate.

  The above object of the present invention is achieved by the following configurations.

  1. A single-layer film containing cellulose ester in an amount of 80% by mass or more, having a total light transmittance of 93% or more and a haze of 5% or less, wherein the total light transmittance of the film is T1, Tg + 10 ° C. of the film A polarizing plate protective film, wherein T1 is 0.5% or more higher than T2 when the total light transmittance of the film after heat treatment for 1 minute is T2.

  2. 2. The polarizing plate protective film according to 1 above, wherein the refractive index increase values on both surfaces (front surface and back surface) are different by the heat treatment.

  3. 3. The polarizing plate protective film as described in 1 or 2 above, wherein a difference in transmittance between both surfaces (front surface and back surface) is 1% or more.

  4). It is a manufacturing method of the polarizing plate protective film which manufactures the polarizing plate protective film of any one of said 1-3, Comprising: Dope containing a cellulose ester, the good solvent with respect to this cellulose ester, and a poor solvent is on a support body. After the casting, the polarizing plate is treated within 5 minutes from casting with a poor solvent having an SP value closer to that of the cellulose ester than water or having a contact angle lower than that of water with respect to the cellulose ester. A method for producing a protective film.

  5). 5. The polarized light according to 4 above, wherein the poor solvent poor solvent having an SP value closer to the cellulose ester than water or having a contact angle lower than that of water with respect to the cellulose ester contains ethanol or isopropyl alcohol. Manufacturing method of board protective film.

  6). The polarizing plate protective film obtained by the manufacturing method of the polarizing plate protective film of any one of said 1-3 or the said polarizing plate protective film of said 4 or 5 is provided in the at least one side of a polarizing plate, It is characterized by the above-mentioned. A polarizing plate.

  7). 7. A liquid crystal display device using the polarizing plate according to 6 above, wherein the polarizing plate protective film according to 1 is disposed outside the liquid crystal panel, and includes an LED light source.

  According to the present invention, a polarizing plate protective film having unprecedented high transmittance, low haze, and excellent environmental stability, a method for producing the same, a polarizing plate provided with the polarizing plate protective film, and the polarizing plate are provided. A liquid crystal display device can be provided.

  Hereinafter, although the form for implementing this invention is demonstrated in detail, this invention is not limited to these.

  The conventionally proposed method for lowering the refractive index is to add a fine bubble (refractive index = 1) having a diameter shorter than the wavelength in the material, so that the apparent refractive index can be adjusted according to the volume ratio. Based on the principle of lowering.

  However, in this method, it is difficult to completely eliminate the decrease in transmittance due to the influence of light scattering during the film transmission, and a polarizing plate protective film that increases the transmittance has not been obtained. The present inventors believe that by causing phase separation under mild conditions, a low-density region is formed in the polarizing plate protective film, the influence of scattering is minimized, and moisture absorption equilibrium can be quickly performed during environmental changes. Invented.

  The low density region formed in the polarizing plate protective film means that the total light transmittance of the polarizing plate protective film is T1, the total light transmittance of the film after heat treatment for 1 minute at Tg + 10 ° C. of the polarizing plate protective film. When T2 is set, it can be confirmed that T1 is 0.5% or more higher than T2. When the low-density region is not formed on the polarizing plate protective film, the polarizing plate protective film returns to the same density distribution as before the heat treatment by cooling after the heat treatment, so the decrease in the total light transmittance due to the heat treatment is small and 0.5%. Less than.

  In order to form a low-density region on the polarizing plate protective film, after casting a dope containing a cellulose ester, a good solvent for the cellulose ester, and a poor solvent on the support, the SP value is higher than that of water. It can manufacture by processing within 5 minutes with the poor solvent which is near or has a contact angle lower than water with respect to this cellulose ester.

  The present inventors have not theoretically accurately grasped the mechanism that can obtain the stability of the polarizing plate protective film, which is the effect of the present invention, with high transmittance and low haze, and stability against environmental fluctuations. I think so.

  Compared to the conventional phase separation method, the phase separation of the present invention causes phase separation under mild conditions. Therefore, even when fine pores are formed, the pores are very small in diameter, and phase separation under mild conditions has both a phase separation region and a region that is not phase separated. In the region not phase-separated, a region having a lower density than that normally formed by volatilization of the solvent in the course of drying is formed. It is thought that scattering was suppressed by these two effects, and a higher transmittance and a lower haze were achieved than before.

  The environmental fluctuation stability of the polarizing plate protective film, which is another effect of the present invention, is considered as follows.

  The polarizing plate protective film placed under a high temperature and high humidity condition is in a high humidity and humidity equilibrium state. Thereafter, when returning to a normal environment, the polarizing plate protective film gently moves to a humidity equilibrium state of normal humidity. Paying attention to the difference in position in the plane of the polarizing plate, the polarizing plate edge has a large part in contact with air, so the moisture absorption rate is relatively fast, while the central part has a relatively small part in contact with air. In addition, the evaporation rate is relatively low. For this reason, at the stage of transition to the humidity equilibrium state of normal humidity, the transition to the humidity equilibrium state of normal humidity proceeds relatively quickly at the end in the plane of the polarizing plate, and compared at the center in the plane of the polarizing plate. The transition to a normal humidity equilibrium state progresses slowly. That is, the moisture absorption differs depending on the position in the plane of the polarizing plate.

  When the backlight is turned on under this condition, the temperature shifts rapidly to a humidity equilibrium at the center of the polarizing plate, while the shift to the humidity balance is relatively slow at the end of the polarizing plate. Therefore, a difference occurs in the contraction force depending on the position in the plane of the polarizing plate. This difference in shrinkage force causes localization of force in the polarizing plate protective film plane. Due to the localization of the force, a stress is generated in the polarizing plate protective film, which causes unevenness in retardation. As a result, luminance unevenness of the liquid crystal display device is observed.

  The polarizing plate protective film of the present invention has a low refractive index region. In the low refractive index region, since the resin is in a low density state, the transition to humidity equilibrium can be performed quickly. For this reason, compared with the conventional polarizing plate protective film, the difference of the moisture absorption state of the part which touches the air of an edge part and a center part becomes relatively small. By this action, a uniform moisture absorption state can be maintained in the plane at the stage of shifting to humidity equilibrium. As a result, localization of force in the polarizing plate protective film surface is suppressed, and it is considered that stability against environmental fluctuations is improved.

〔material〕
<Cellulose ester>
The cellulose ester used in the polarizing plate protective film of the present invention is a carboxylic acid ester having about 2 to 22 carbon atoms, and is particularly preferably a lower fatty acid ester of cellulose. The lower fatty acid in the lower fatty acid ester of cellulose means a fatty acid having 6 or less carbon atoms. For example, cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate phthalate, etc., JP-A-10-45804, Mixed fatty acid esters such as cellulose acetate propionate and cellulose acetate butyrate as described in US Pat. No. 8,231,761, US Pat. No. 2,319,052 can be used. Among the above descriptions, the lower fatty acid esters of cellulose particularly preferably used are cellulose triacetate and cellulose acetate propionate. These cellulose esters can also be mixed and used.

  The total acyl group substitution degree of the cellulose ester is preferably 2 to 3. In particular, in the case of cellulose triacetate, those having a total acyl group substitution degree (acetyl group substitution degree) of 2.6 to 2.9 are preferably used.

  These acyl group substitution degrees can be measured according to the method prescribed in ASTM-D817-96.

  The value of Mw / Mn of the cellulose ester used for the polarizing plate protective film of the present invention is 1 to 10, preferably 1 to 5, and particularly preferably 1.4 to 3.0. In addition, although it is preferable that a cellulose ester film contains the cellulose ester whose value of Mw / Mn is 1.4-3.0 as a material, the cellulose ester (preferably cellulose triacetate or preferably included in the protective film for polarizing plates). The value of Mw / Mn of cellulose acetate propionate) is more preferably in the range of 1.4 to 3.0. In the synthesis process of cellulose ester, it is difficult to make it less than 1.4, and cellulose ester having a uniform molecular weight can be obtained by fractionation by gel filtration or the like. Moreover, since it is excellent in planarity as it is 3.0 or less, it is preferable. In addition, More preferably, it is 1.7-2.2. Since the average molecular weight and molecular weight distribution of the cellulose ester can be measured using high performance liquid chromatography, the number average molecular weight and the weight average molecular weight can be calculated using this, and the ratio can be calculated.

  The measurement conditions are as follows.

Solvent: Methylene chloride Column: Shodex K806, K805, K803G (Used by connecting three Showa Denko Co., Ltd.)
Column temperature: 25 ° C
Sample concentration: 0.1% by mass
Detector: RI Model 504 (manufactured by GL Sciences)
Pump: L6000 (manufactured by Hitachi, Ltd.)
Flow rate: 1.0ml / min
Calibration curve: Standard polystyrene STK standard polystyrene (manufactured by Tosoh Corp.) Mw = 100000-500 calibration curves with 13 samples were used. It is preferable to obtain 13 samples at approximately equal intervals.

  Moreover, the polymerization degree in this invention is a viscosity average polymerization degree, and the viscosity average polymerization degree (DP) of the cellulose ester (for example, cellulose acetate) was measured as follows.

  0.2 g of completely dried cellulose acetate was precisely weighed and dissolved in 100 ml of a mixed solvent of methylene chloride: ethanol = 9: 1 (mass ratio). This was measured with an Ostwald viscometer for the number of seconds dropped at 25 ° C., and the degree of polymerization was determined by the following equation.

ηrel = T / T0
[Η] = (lnηrel) / C
DP = [η] / Km
T: Number of seconds dropped for measurement sample T0: Number of seconds dropped for solvent alone C: Concentration (g / l)
Km: 6 × 10 ⁻⁴
The 6% viscosity of cellulose triacetate was measured as follows.

  61.67 g of the mixed solution (methylene chloride: methanol = 91: 9) is collected in an Erlenmeyer flask, charged with 3.00 g of a dry sample, sealed, and shaken with a horizontal shaker for about 1.5 hours. Then, it is completely dissolved by shaking for about 1 hour on a rotary shaker. The obtained 6 mass / volume% solution is transferred to a predetermined Ostwald viscometer mark, and the temperature is adjusted for about 15 minutes in a thermostat at 25 ± 1 ° C. Measure the flow down time between the time marks.

  The 6% viscosity is calculated by the following formula.

6% viscosity (cP · s) = flowing time (s) × viscosity coefficient 1cP = 1 mPa · s.

  The viscometer coefficient is obtained by measuring the number of seconds flowing down by the same operation as described above using a standard solution for viscometer calibration.

  As the cellulose ester, a cellulose ester synthesized using cotton linter, wood pulp, kenaf or the like as a raw material can be used alone or in combination. In particular, it is preferable to use a cotton linter (hereinafter sometimes simply referred to as a linter) or a cellulose ester synthesized from wood pulp alone or in combination.

  Moreover, the cellulose ester obtained from these can be mixed and used in arbitrary ratios, respectively. These cellulose esters are prepared by using an organic acid such as acetic acid or an organic solvent such as methylene chloride when sulfuric acid is used as the acylating agent (acetic anhydride, propionic anhydride, butyric anhydride). It can obtain by making it react by a conventional method using a protic catalyst like.

  In the case of acetylcellulose, it is necessary to extend the time for the acetylation reaction in order to increase the acetylation rate. However, if the reaction time is too long, the decomposition proceeds simultaneously, and the polymer chain is broken and the acetyl group is decomposed. Therefore, it is necessary to set the reaction time within a certain range in order to increase the degree of acetylation and suppress decomposition to some extent. It is not appropriate to define the reaction time because the reaction conditions are various and greatly change depending on the reaction apparatus, equipment and other conditions. As the decomposition of the polymer progresses, the molecular weight distribution becomes wider. Therefore, in the case of cellulose ester, the degree of decomposition can be defined by the value of weight average molecular weight (Mw) / number average molecular weight (Mn) that is usually used. That is, in the process of acetylation of cellulose triacetate, the weight average molecular weight used as one index of the reaction degree for allowing the acetylation reaction to be carried out for a sufficient time for acetylation without being excessively decomposed due to being too long. The value of (Mw) / number average molecular weight (Mn) can be used.

  The synthesized cellulose ester is preferably purified to remove low molecular weight components or to remove unacetylated components by filtration.

  In the case of a mixed acid cellulose ester, it can be obtained by a reaction described in JP-A-10-45804. The measuring method of the substitution degree of an acyl group can be measured according to the rule of ASTM-D817-96.

  Cellulose esters are also affected by trace metal components in cellulose esters. These are considered to be related to water used in the production process, but it is preferable that there are few components that can become insoluble nuclei, and metal ions such as iron, calcium, and magnesium contain organic acidic groups. Insoluble matter may be formed by salt formation with a polymer degradation product or the like that may be present, and it is preferable that the amount is small. The iron (Fe) component is preferably 1 ppm or less. The calcium (Ca) component is abundant in groundwater, river water, etc., and if it is too much, it becomes unsuitable as hard water, but it is not suitable for acidic components such as carboxylic acids and sulfonic acids, and many ligands. Coordination compounds, that is, complexes are easily formed, and scum (insoluble starch, turbidity) derived from many insoluble calcium is formed.

  The calcium (Ca) component is 60 ppm or less, preferably 0 to 30 ppm. The magnesium (Mg) component is preferably in the range of 0 to 70 ppm, particularly preferably 0 to 20 ppm, since too much will cause insoluble matter. Metal components such as iron (Fe) content, calcium (Ca) content, magnesium (Mg) content, etc. are pre-processed by completely digesting cellulose ester with micro digest wet cracking equipment (sulfuric acid decomposition) and alkali melting. After performing, it can obtain | require by performing an analysis using ICP-AES (inductively coupled plasma optical emission spectrometry).

(Other additives)
Moreover, as a component contained in the polarizing plate protective film of this invention, additives, such as a plasticizer, a peeling accelerator, and an ultraviolet absorber, can be added suitably. Plasticizers include phosphate ester plasticizers such as triphenyl phosphate and triethyl phosphate, phthalate ester plasticizers such as dimethyl phthalate and diethyl phthalate, glycols such as methyl phthalyl ethyl glycolate and ethyl phthalyl ethyl glycolate. An acid ester plasticizer, a polymer plasticizer, or the like can be used. The peeling accelerator is for facilitating peeling of the film obtained by solidifying the dope from the support, and for example, metal soap is used. As the ultraviolet absorber, a benzophenone ultraviolet absorber, a benzotriazole ultraviolet absorber, an ultraviolet absorbing polymer, or the like is appropriately used. These are preferably added in the dope.

  It is preferable that content of a plasticizer is 1-20 mass% with respect to a film. More preferably, it is 5-15 mass%, Most preferably, it is 7-12 mass%.

(Matting agent)
The polarizing plate protective film of the present invention may contain fine particles as a matting agent. Examples of the fine particles used in the matting agent include silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, talc, clay, calcined kaolin, and aluminum silicate as examples of inorganic compounds. Fine particles containing silicon are preferable in terms of low turbidity, and silicon dioxide is particularly preferable. Silicon dioxide fine particles are commercially available, for example, under the trade names Aerosil R972, R972V, R974, R812, 200, 200V, 300, R202, OX50, TT600 (above, Nippon Aerosil Co., Ltd.). Can do. Examples of the organic fine particles include silicone resin, fluororesin and acrylic resin. Silicone resins are particularly preferable, and those having a three-dimensional network structure are particularly preferable. And can be used. Among these, Aerosil 200V and Aerosil R972V are particularly preferable because they have a large effect of reducing the friction coefficient while keeping the turbidity of the polarizing plate protective film low.

  The addition amount of the matting agent is preferably 0.005 to 0.5 mass%, more preferably 0.01 to 0.3 mass%, and most preferably 0.05 to 0.1 mass% with respect to the film.

(Good solvent)
The good solvent used for the manufacturing method of the polarizing plate protective film of this invention means the solvent whose solubility in 40 degreeC with respect to the cellulose ester used is 15 mass% or more.

  The good solvent is appropriately selected according to the type of cellulose ester used. For example, ketones (C3-5 dialkyl ketones such as acetone, methyl ethyl ketone, and methyl propyl ketone, cyclohexanone, etc.), esters ( Formic acid C1-4 alkyl esters such as ethyl formate, acetic acid C1-4 alkyl esters such as methyl acetate, ethyl acetate and butyl acetate, ethyl propionate and ethyl lactate), ethers (1,4-dioxane, tetrahydrofuran, tetrahydropyran , Diethyl ether, diisopropyl ether, dimethoxyethane and other cyclic or chain C4-6 ethers), cellosolves (C1-4 alkyl-cellosolves such as methyl cellosolve, ethyl cellosolve, butyl cellosolve), cellosolve acetates (methyl cellosolve) C1-4 alkyl-cellosolve acetate such as sorb acetate, ethyl cellosolve acetate), aromatic hydrocarbons (benzene, toluene, xylene, etc.), halogenated hydrocarbons (methylene chloride, ethylene chloride, etc.), amides (formamide, Acylamides such as acetamide, mono- or di-C1-4 acylamides such as N-methylformamide, N-methylacetamide, N, N-dimethylformamide, N, N-dimethylacetamide), sulfoxides (di-C1 such as dimethylsulfoxide) -3 alkyl sulfoxide), nitriles (acetonitrile, chloroacetonitrile, propionitrile, butyronitrile, etc. C1-6 alkyl nitrile, benzonitrile, etc.), organic acids (formic acid, acetic acid, propionic acid, etc.), organic acid anhydrides (anhydrous anhydride) Ynoic acid, acetic anhydride, etc.), and mixtures thereof.

  More specifically, halogenated hydrocarbons, C3-5 dialkyl ketones such as methyl ethyl ketone (especially acetone, methyl ethyl ketone), acetic acid C1-4 alkyl esters such as ethyl acetate (particularly methyl acetate, ethyl acetate), dioxane, dimethoxy Cyclic or linear C4-6 ethers such as ethane, C1-4 alkyl-cellosolves such as methyl cellosolve (especially methyl cellosolve, ethyl cellosolve), C1-4 alkyl-cellosolve acetates such as methyl cellosolve acetate (particularly methyl cellosolve) Acetate, ethyl cellosolve acetate) and mixed solvents thereof. Particularly preferred good solvents include solvents containing at least halogenated hydrocarbons (especially methylene chloride).

(Poor solvent)
The poor solvent used in the method for producing a polarizing plate protective film of the present invention means a solvent having no solubility in the cellulose ester used or having a solubility at 40 ° C. of less than 15% by mass.

  As a poor solvent, it will select suitably according to the kind of cellulose ester to be used, For example, ester (Formic acid C5-8 alkyl ester, such as amyl formate and isoamyl formate, butyl acetate, amyl acetate, hexyl acetate) C2-4 aliphatic carboxylic acid C3-10 which may have a C1-4 alkoxy group such as octyl acetate, 3-methoxybutyl acetate, 3-ethoxybutyl acetate, butyl propionate, 3-methoxybutyl propionate Alkyl esters (for example, C4-10 alkyl esters that may have C1-4 alkoxy groups), benzoic acid C1-4 alkyl esters such as methyl benzoate, ethyl benzoate, and propyl benzoate), alcohols (Ethanol, isopropyl alcohol, cyclopentanol, cyclohexanol C5-8 alcohols such as C4-8 cycloalkanol, amyl alcohol, isoamyl alcohol, hexyl alcohol, etc., which may be substituted by C1-4 alkyl groups such as methylcyclohexanol, dimethylcyclohexanol, cyclooctanol, 2-butoxyethanol , 3-butoxypropanol and other C2-6 alkoxy-C1-4 alcohols, furfuryl alcohol and other heterocyclic alcohols), ketones (methyl butyl ketone, methyl isobutyl ketone, methyl pentyl ketone, methyl isopentyl ketone, C3-10 dialkyl ketones (especially C6-10 dialkyl ketones) such as 2,6-dimethyl-4-heptanone, acetonyl acetone, acetophenone, etc., ethers (methyl phenyl ether, methoxy toluene, di Chirueteru, C7-10 ethers such as benzyl ethyl ether), aliphatic hydrocarbons (hexane, octane, nonane, C5-20 aliphatic hydrocarbons decane), and mixtures thereof can be exemplified.

  In the present invention, by causing phase separation under mild conditions, a low-density region is formed in the polarizing plate protective film, and the transmittance due to internal scattering is suppressed, so that high transmittance, low haze, and environmental fluctuation stability are achieved. I believe it was achieved. For this reason, it is important to select a poor solvent having a relatively high affinity for the cellulose ester. The high affinity between the cellulose ester and the poor solvent can be selected based on, for example, the difference in solubility parameter represented by the SP value, the low contact angle of the good and poor solvent with respect to the resin film, and the like. It is presumed that by using a poor solvent having a high affinity for cellulose ester, aggregation of cellulose ester molecules can be suppressed, and a low density region can be formed.

  In the present invention, the poor solvent having a higher affinity than water specifically means that the SP value difference between the cellulose ester film and the poor solvent or the contact angle of the poor solvent is smaller than that of water, or It means that the contact angle is low.

  The SP value, that is, the solubility parameter referred to in the present invention is a value obtained from the solubility parameter of Hildebrand, which is often used when evaluating the ease of dissolution of the non-electrolyte in an organic solvent. This solubility parameter is described in J.H. H. Hildebrand, J.M. M.M. Prausnitz. R. L. See “Regular and Related Solutions” by Scott, Van Nostrand-Reinhold, Princeton (1970), “Polymer Data Handbook Fundamentals”, Polymer Society of Japan. The solubility parameter values of various solvents are F. M.M. Barton, “Handbook of Solrity Parameters and Other Cohesion Parameters”, CRC Press, Boca Raton / Florida (1983), “Polymer Data Handbook Fundamentals”, Polymer Society.

The solubility parameter of the substance is
SP = (δE / V) 1/2
Where δE is the cohesive energy per mole and V is the molar volume.

  The solubility parameter can be obtained by several methods, such as a method of obtaining from the solubility, or a latent heat of vaporization method, a vapor pressure method, a swelling method, a surface tension method, a thermal swelling coefficient method, and a refractive index method. For example, water is 23.4, diacetylcellulose is 11.4, ethanol is 12.7, and isopropanol is 11.5.

  Particularly preferred examples of the poor solvent that causes phase separation under mild conditions include ethanol and isopropyl alcohol.

  The contact angle can be measured using a commercially available contact angle meter, for example, a contact angle meter CA-W manufactured by Kyowa Interface Science Co., Ltd.

[Production method of polarizing plate protective film]
(Phase separation method)
One preferred embodiment for obtaining the polarizing plate protective film of the present invention is a phase separation method. Phase separation methods include thermally induced phase separation method and poor solvent organic phase separation method, and poor solvent phase separation method includes dry process and wet process, but from the viewpoint of easy control of phase separation conditions, Solvent phase separation wet processes are preferred.

  The preferable aspect of the manufacturing method of the polarizing plate protective film of this invention is given as an example below.

  A dope solution comprising a mixed solvent of cellulose ester and a good solvent and a poor solvent for the cellulose ester is prepared. The dope is cast on a support, and the phase separation is performed by immersing the support in a poor solvent bath made of a poor solvent before the solvent contained in the dope is completely dried within 5 minutes from casting. . The film taken out from the poor solvent bath is peeled off from the support and dried to obtain a polarizing plate protective film.

(Dope)
The dope in the present invention means a solution comprising at least one cellulose ester and a good solvent and a poor solvent for the cellulose ester. In the dope, the above-mentioned other additives, matting agent and the like can be added as appropriate. In order to cause phase separation under mild conditions, it is preferable to appropriately select the ratio of the good solvent and the poor solvent in the dope.

  Moreover, in order to obtain a film having a smooth surface by casting the dope on a support, viscosity adjustment (solid content and solvent ratio) can be appropriately performed.

  Conditions may be changed depending on the type of good and poor solvents, the type of cellulose ester, and other additives.

  In one preferred embodiment, ethanol is selected as the poor solvent and methylene chloride is selected as the good solvent, and the ratio of the poor solvent and the good solvent is preferably adjusted between 1:99 and 40:60.

  In the present invention, from the viewpoint of causing phase separation under mild conditions, the ratio of the poor solvent and the good solvent is preferably in the range of 8:92 to 30:70. If the ratio of the poor solvent in the dope is too high, rapid phase separation tends to occur, and if the ratio of the poor solvent is too low, the phase separation itself tends not to occur. This is because the higher the initial poor solvent ratio, the more rapidly the poor solvent ratio is observed in the subsequent immersion treatment in the poor solvent bath.

  The concentration of the cellulose ester in the dope is preferably 17 to 35% by mass, particularly preferably 20 to 30% by mass. If the concentration is too high, the smoothness of the surface of the resulting film tends to deteriorate, and if it is too thin, the film thickness of the polarizing plate protective film cannot be obtained sufficiently, and it takes time to dry the solvent in production. May come out. In addition, the higher the ratio of cellulose ester to the solvent, the more gentle the tendency is to cause phase separation, and the lower the tendency is to cause rapid phase separation. This is considered to be due to the effect that the penetration rate of the solvent decreases when immersed in a poor solvent.

  The temperature when casting the polarizing plate protective film of the present invention is preferably adjusted in the range of 20 to 40 ° C. from the viewpoint of adjusting the viscosity suitable for casting film formation. The precursor web of the polarizing plate protective film formed by casting is a long polarizing plate whose temperature is controlled from the viewpoint of adjusting the solvent content, good solvent, and poor solvent ratio until impregnation in the poor solvent bath It is preferable for obtaining a protective film stably.

  Casting can be performed by a method such as casting on a smooth metal support or casting on a polymer film that is not affected by a solvent or heat. From the viewpoint of the difference between the front and back surfaces in the Abbe refractive index measurement of the polarizing plate protective film of the present invention and the difference in transmittance between the front and back surfaces, casting is performed on the polymer support and the entire support is immersed in a poor solvent bath. Alternatively, a method of casting in an endless belt-like metal support entering a poor solvent bath is preferable.

  This is considered to be because the poor solvent has a low refractive region having an asymmetric distribution in the thickness direction by selectively entering the poor solvent from one side rather than the penetration of the poor solvent from both sides of the polarizing plate protective film.

  In obtaining the polarizing plate protective film of the present invention, from the viewpoint of causing phase separation under mild conditions, it is possible to control the humidity of water as a powerful phase separation agent in the sense of controlling the intrusion from the air into the web. preferable. In the present invention, the less water that is taken in, the more likely to cause mild phase separation.

(Poor solvent bath)
As a method for producing the polarizing plate protective film of the present invention, treating with a poor solvent having a higher affinity than water, for example, immersing in the poor solvent bath or applying or spraying a solvent used in the poor solvent bath, etc. This means that a step of causing phase separation using the above method is included.

  The poor solvent bath comprises at least one kind of poor solvent to cause phase separation of the polarizing plate protective film of the present invention, and a mixture of the poor solvent and the good solvent can be used.

  The poor solvent used in the poor solvent bath is appropriately selected depending on the cellulose ester species. As a measure of the affinity between the poor solvent and the cellulose ester, a comparison by SP value and the contact angle of the poor solvent to be used are used. In order to form a low density phase in cellulose ester, it is very important in the present invention to cause phase separation under relatively mild conditions. Specifically, it can be appropriately selected from the above poor solvents. The poor solvent used in the poor solvent bath is preferably a solvent having a higher affinity for the cellulose ester than the poor solvent contained in the initial dope in order to cause phase separation under mild conditions. When the poor solvent contained in ethanol is ethanol, it is preferable to use isopropyl alcohol as the poor solvent used in the poor solvent bath.

  The temperature of the poor solvent bath is preferably controlled in order to stably obtain a long phase separation film. Lower temperatures in the antisolvent bath tend to cause phase separation under milder conditions. This is believed to be due to the penetration rate of the poor solvent used in the poor solvent bath into the film.

  The immersion time in the poor solvent bath is preferably performed for the purpose of adjusting the good solvent and the poor solvent amount in the film thickness direction and adjusting the respective ratios in the thickness direction. It can be set as appropriate in consideration of the influence of the composition of the dope and the drying to the poor solvent bath. The shorter the immersion time, the milder the condition because the amount of immersion of the poor solvent is smaller, and the longer the immersion time, the more the amount of penetration of the poor solvent, the more rapid phase separation tends to occur.

  As the time from casting to immersion in a poor solvent bath, a shorter time is preferable from the viewpoint of reducing the penetration of water that acts as a strong poor solvent into the cellulose ester film.

  A lower atmospheric humidity from casting to immersion in the poor solvent bath is preferable from the viewpoint of reducing introduction of water, which is a strong poor solvent, into the cellulose ester.

  The drying from casting to immersion in a poor solvent bath is preferably controlled in order to stably cause long phase separation. The drying time varies depending on the solid content ratio of the dope, the good solvent poor solvent species and ratio, and the cellulose ester species, and can be appropriately selected, but a normal film forming method that does not intentionally suppress the evaporation of the solvent. Is within 5 minutes. When it exceeds 5 minutes, it becomes difficult for the poor solvent to enter the web, and it becomes difficult to cause phase separation. From the viewpoint of suppressing the ingress of water as a phase separation agent, the shorter one tends to cause gentle phase separation.

(Drying process)
In order to obtain the polarizing plate protective film of this invention, it is the process of drying the film taken out from the poor solvent bath. The temperature of the drying step can be set as appropriate so that the solvent in the web is sufficiently volatilized. By applying a gradient of the solvent concentration in the film thickness direction by the drying process, the apparent refractive index in the thickness direction of the completed film can be adjusted. It is preferable that the temperature of a drying process shall be below Tg of a cellulose ester. It is considered that the low refractive region generated in the poor solvent bath can be maintained by drying at a temperature of Tg or lower.

  From the viewpoint of the flatness of the polarizing plate protective film, it is also preferable to dry the film by holding or stretching the film using a biaxial tenter or the like during the drying step.

[Polarizing plate protective film]
The polarizing plate protective film of the present invention is a single layer film containing 80% by mass or more of cellulose ester. The single layer film means that an antireflection film having a clear layer structure in which a material made of a low refractive material is applied on a substrate is excluded. This can be determined by a method of performing tomographic observation with an electron microscope or the like or by analyzing a film cross section with TOF-SIMS or the like.

  Moreover, since the polarizing plate protective film of the present invention is produced using gentle phase separation, the surface is substantially smooth and does not have a periodic fine structure.

(Total light transmittance)
The polarizing plate protective film of the present invention has a total light transmittance of 93% or more. The total light transmittance of the cellulose ester film is affected by the type of cellulose ester (more specifically, the total acyl substitution degree, the type of substituent, etc.). However, the total light transmittance of the cellulose ester film used as the present polarizing plate protective film is about 92%, and the polarizing plate protective film of the present invention has a film surface reflectance due to the influence of the low refractive index region. Since it is reduced, it is a polarizing plate protective film having an unprecedented high transmittance exceeding 93%.

(Total light transmittance change)
In the polarizing plate protective film of the present invention, when the total light transmittance of the film is T1, and the total light transmittance of the film after heat treatment at Tg + 10 ° C. for 1 minute is T2, T1 is 0.5% from T2. More expensive. By performing the heat treatment at Tg + 10 ° C. for 1 minute, the low density region in the film changes to a higher density normal density. Along with this, the refractive index increases and the reflectance at the air interface increases, and the value of the total light transmittance decreases. In addition, when there are fine bubbles inside the film, the difference in refractive index between the fine bubbles and the cellulose ester increases, so that internal scattering increases. As a result, the total light transmittance is reduced.

(Refractive index increase on front and back)
The polarizing plate protective film of the present invention has different refractive index increase values on the front and back surfaces by Abbe refractive index measurement by heat treatment at Tg + 10 ° C. for 1 minute. Tg means the glass transition point of the polarizing plate protective film, and can be measured by a differential scanning calorimeter. By the heat treatment at Tg + 10 ° C. for 1 minute, the low density region that exists asymmetrically in the film thickness direction changes to the normal density region. Along with this density change, the film has a more uniform density in the film thickness direction, and the refractive index increases on the front and back surfaces measured with an Abbe refractometer. The polarizing plate has a structure in which a PVA (polyvinyl alcohol film) polarizer in which iodine is arranged is sandwiched between two polarizing plate protective films. The polarizing plate protective film of the present invention reduces surface reflection at the air interface and simultaneously reduces the refractive index difference from the polarizer. For this reason, high transmittance can be achieved even when a polarizing plate is used.

(Front and back transmittance difference)
The polarizing plate protective film of the present invention has a front-back surface transmittance difference of 1% or more. The difference in transmittance between the front and back surfaces refers to a sample in which the interface between the film and glass is removed with a material having a higher refractive index than air such as grease, and the light incident surface is measured twice on the front and back surfaces of the film. It is a value obtained by taking the difference in total light transmittance.

  In the polarizing plate protective film of the present invention, the low refractive index region has an asymmetric distribution in the film thickness direction. The polarizing plate has a structure in which a PVA (polyvinyl alcohol film) polarizer in which iodine is arranged is sandwiched between two polarizing plate protective films. To achieve high transmittance of the polarizing plate, the polarizing plate protective film air The first is to reduce surface reflection on the interface side. Further, when the difference in refractive index between the polarizer and the polarizing plate protective film is reduced, it is possible to suppress a minute reflection occurring at this interface and achieve a high transmittance. Furthermore, if the ratio of the low refractive index region exists gently in the film thickness direction and the surface side with the large distribution of the low density region is arranged on the air interface side, humidity transfer becomes easy and the ability to follow environmental changes. Improved and contributes to the stability of environmental fluctuations of liquid crystal display devices. The difference in transmittance between the front and back surfaces is an index that not only defines a low refractive index region on the film surface in an Abbe refractometer, but also includes a gently low density region in the film thickness direction.

(Haze)
The polarizing plate protective film of the present invention has a haze of 5% or less. The polarizing plate protective film forms a low refractive index region by phase separation under mild conditions. For this reason, even when the film has a void structure that causes scattering, the polarizing plate protective film has a low haze because the refractive index difference between the void and the cellulose ester is small. In addition, this effect can simultaneously suppress a decrease in total light transmittance due to internal scattering, thereby achieving high transmittance.

〔Polarizer〕
The polarizing plate typically has a structure in which PVA (polyvinyl alcohol) in which iodine is arranged is sandwiched between two polarizing plate protective films. The polarizing plate of this invention is equipped with the polarizing plate protective film obtained with the manufacturing method of the polarizing plate protective film of any one of said 1-3 or the said polarizing plate protective film of said 4 at least on one side. . In the measurement of total light transmittance on both sides (front and back), the polarizing plate is made so that the incident surface with high total light transmittance of the polarizing plate protective film is located on the air interface side, and a polarizing plate is made. A board can be obtained.

  One factor that reduces the transmittance of the polarizing plate is surface reflection caused by the difference in refractive index between the polarizing plate protective film and air. The polarizing plate protective film of the present invention can achieve high transmittance in order to suppress this surface reflection.

  Furthermore, a polarizing plate having a high transmittance can be obtained by using a polarizing plate protective film having a transmittance difference of 1% or more on the front and back surfaces and arranging at least one incident surface having a large total light transmittance on the air interface side. In addition, since the reflection on the two air interfaces existing in the polarizing plate is suppressed by disposing on both sides, higher transmittance can be achieved.

[Liquid Crystal Display]
The liquid crystal display device of the present invention uses the polarizing plate of the present invention, and the polarizing plate protective film of the present invention or the polarizing plate protective film obtained by the method for producing the polarizing plate protective film of the present invention is outside the liquid crystal panel. It is characterized by being arranged in.

  A liquid crystal display device typically has an arrangement in which a liquid crystal panel is sandwiched between two polarizing plates. When the polarizing plate provided with the polarizing plate protective film of the present invention is used to bond the polarizing plate and the liquid crystal panel, the surface having the high total light transmittance of the polarizing plate protective film is the air interface side (outside the liquid crystal panel). By disposing the liquid crystal display device as described above, a liquid crystal display device with high transmittance can be achieved by suppressing reflection at the air interface.

  It is also preferable from the viewpoint of increasing the transmittance to dispose the polarizing plate of the present invention on both the viewing side and the backlight side. From the viewpoint of suppressing the occurrence of uneven brightness when the environment of the liquid crystal display changes, it is preferable that the liquid crystal display device be disposed on the backlight side where the temperature change is larger. Moreover, since there exists an effect which suppresses the reflection of a display screen by arrange | positioning at the visual recognition side, it is also preferable to install the polarizing plate of this invention in the display surface side.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

Example 1
<< Production of polarizing plate protective film >>
The cellulose ester described in Table 1 was used, methylene chloride was used as a good solvent, ethanol was used as a poor solvent, and a dope was prepared according to the ratio described in Table 2. In addition, the poor solvent ratio described in Table 2 is the mass ratio of ethanol in all solvents, and the solid content ratio is the mass ratio of the cellulose ester contained in the dope.

  The dope controlled at 40 ° C. is coated on a PET (polyethylene terephthalate) film support with a knife knife coater, immersed in a first drying, poor solvent bath, and the polarizing plate described in Table 2 in the second drying procedure. A protective film was prepared. The drying time until immersion described in Table 2 is the time of the first drying, the immersion temperature is the temperature of the poor solvent bath, the immersion time is the time during which the film is immersed in the poor solvent bath, and the immersion solvent is the poor solvent bath. Solvent species. The environment during casting was 23 ° C. and 50% RH, and the second drying was at 100 ° C. for 15 minutes.

  In addition, the polarizing plate protective film samples 2-23 to 2-25 of the comparative examples were prepared by performing the second drying without performing the dipping operation in the poor solvent bath after the first drying.

<< Evaluation of polarizing plate protective film >>
The produced polarizing plate protective film was evaluated as follows.

(Total light transmittance)
Total light transmittance was measured with an integrating sphere using JASCO-V670 manufactured by JASCO Corporation. The film was conditioned for one day in an environment of 23 ° C. and 50% RH at the time of measurement, and then calculated as an average value of the measurement wavelength of 400 to 700 nm and transmittance to obtain the total light transmittance (%).

(Total light transmittance change)
The glass transition temperature Tg was measured using a differential scanning calorimeter DSC220 manufactured by Seiko Instruments Inc. according to JIS K7121. Based on the obtained Tg, heat treatment was performed at Tg + 10 ° C. for 1 minute with the four sides of the sample fixed. About the sample after heat processing, it measured on the same conditions as the said total light transmittance measuring method.

(Haze)
According to JIS K-6714, it measured using the haze meter (1001DP type, Nippon Denshoku Industries Co., Ltd.), and evaluated haze (%).

  The evaluation results are shown in Table 2.

  From the table | surface, the polarizing plate protective film 2-1 to 2-13 of this invention is high with respect to the polarizing plate protective film (Comparative Examples 2-23 to 2-25) produced with the manufacturing method of the conventional polarizing plate protective film. Transmittance. The haze was lower than 5%. This is thought to be because the surface reflectance was lowered due to the effect of lowering the refractive index of the polarizing plate protective film because the polarizing plate protective film of the present invention caused phase separation under mild conditions.

  From the table, the following tendencies were observed as conditions for producing a high transmittance film by the mild phase separation of the present invention.

  When the ratio of the poor solvent is low, a film having a high transmittance can be produced by mild phase separation. When too low, mild phase separation was not caused and the effect of the present invention was not seen.

  When the solid content ratio of the cellulose ester is low, a film having a high transmittance can be produced by mild phase separation. When it was too high, film formation was impossible and film formation was impossible. This is believed to be due to the effect of web viscosity during phase separation.

  As a kind of immersion solvent, a high transmittance film of the present invention could be obtained by using a solvent having high solvent affinity with cellulose ester. The polarizing plate protective film of the present invention could not be obtained with water that acts as a strong phase separator.

  The shorter the drying time until immersion, the higher the transmittance of the polarizing plate protective film could be produced. This is thought to be due to the effects of water intrusion from the atmosphere. Moreover, when time from casting to immersion was too long, the phase separation by a poor solvent process was not caused and the raise of the transmittance | permeability was not able to be confirmed.

  As the immersion temperature, a polarizing plate protective film having a higher transmittance was able to be produced at a lower temperature. I think this is because the speed of the poor solvent entering the web has decreased.

  The shorter the immersion time, the milder the condition because the amount of immersion of the poor solvent is smaller, and the longer the immersion time, the lower the transmittance because the amount of penetration of the poor solvent is large.

Example 2
<< Production of polarizing plate protective film >>
In the same manner as in Example 1, polarizing plate protective film samples 3-1 to 3-14 were produced under the conditions described in Table 3.

<< Evaluation of polarizing plate protective film >>
In the same manner as in Example 1, total light transmittance, change in total light transmittance and haze were measured, and the refractive index increase was measured as follows.

(Double-sided refractive index increase)
Using an Abbe refractometer, the refractive index of the front and back surfaces of the film was measured. Based on the Tg obtained by the differential scanning calorimeter (DSC) in the same manner as the total light transmittance change measurement, the sample was heat-treated at Tg + 10 ° C. for 1 minute, and then the refractive index of the film was measured using an Abbe refractometer. Evaluation was performed according to the following criteria.

A: The difference in the refractive index increase value on the front and back surfaces is 0.15 or more. ○: The difference in the refractive index increase value on the front and back surfaces is 0.05 to less than 0.15. X: The refractive index increase value on the front and back surfaces. Difference is less than 0.05 Table 3 shows the evaluation results.

  As is clear from the table, the polarizing plate protective films 3-1 to 3-8 of the present invention had high transmittance and low haze. Moreover, the difference of the refractive index rise value in the front and back by heat processing was large.

  As the poor solvent ratio, the solid content ratio, the time until dipping, the dipping temperature, and the dipping time were shorter, gentle phase separation occurred, and the same tendency as in Example 1 was observed.

Example 3
With respect to the polarizing plate protective films 2-2 and 3-4 of the present invention prepared in Example 1 and Example 2 and the polarizing plate protective films 2-23 and 3-14 of the comparative examples, both surfaces (front and back surfaces) are obtained by the following method. The total light transmittance difference was evaluated. As a result, in the polarizing plate protective films 2-2 and 3-4 of the present invention, the difference in transmittance between the front and back surfaces was 1.1% and 1.8%. On the other hand, in the polarizing plate protective films 2-2 and 3-4 of the comparative examples, the difference in transmittance between the front and back surfaces was 0.1% and 0.1%.

(Double-sided (front and back) transmittance difference)
A measurement sample in which a glass substrate (refractive index of 1.51) and a sample were adhered with glycerin (refractive index of 1.47) was prepared. The transmittance was measured by the same method as the total light transmittance measurement. At this time, the light incident surface was the sample side. Each sample was measured twice such that the front and back surfaces were on the air side, and the difference in transmittance was evaluated.

Example 4
<Production of polarizing plate>
One side of the polarizing plate protective film of a commercially available polarizing plate (G1220DUN made by Nitto Denko Corporation) was peeled off, and the polarizing plate protective films 2-2 and 3-4 prepared in Example 1 and Example 2 using a PVA aqueous solution. The polarizing plate protective films 2-23 and 3-14 of the comparative example were adhered as polarizing plate protective films, and the polarizing plates 4-2-2 and 4-3-4 of the present invention and the polarizing plate 4-2 of the comparative example, respectively. -23 and 4-3-14 were produced.

  In addition, the adhesive surface with the polarizer of polarizing plate protective film 2-2 and 3-4 was made into the incident surface used as the low transmittance from the data obtained by front and back total light transmittance change. That is, the polarizing plate protective film of the present invention was arranged such that the incident surface with high transmittance was on the air surface side.

<< Evaluation of polarizing plate >>
About the obtained polarizing plate, the total light transmittance was measured. In addition, in the case of a total light transmittance measurement, it measured in the state by which the polarizing plate protective film was arrange | positioned at the light source side.

  As a result, the polarizing plate 4-2-2 of the present invention had a 0.7% higher transmittance than the polarizing plate 4-2-23 of the comparative example. In addition, the polarizing plate 4-3-4 of the present invention had a 0.7% higher transmittance than the polarizing plate 4-3-14 of the comparative example.

  It is considered that this is because the polarizing plate protective film of the present invention has suppressed surface reflection, and thus the polarizing plate protective film can have a high transmittance.

Example 5
<Production of liquid crystal display device>
Instead of the polarizing plate on the backlight side of a commercially available LED backlight-equipped liquid crystal display device (REGZA 37V, manufactured by Toshiba Corporation), the polarizing plates 4-2-2 and 4-3-4 of the present invention, and the polarization of the comparative example The plates 4-2-23 and 4-3-14 are bonded together, and the liquid crystal display devices 5-2-2 and 5-3-4 of the present invention, and the liquid crystal display devices 5-2-23 and 5-3 of the comparative example, respectively. -14 was produced.

  In addition, arrangement | positioning at the time of adhere | attaching a polarizing plate and a liquid crystal panel was made for the polarizing plate protective film used as this invention and a comparative example to become an air surface side (outside with respect to a panel).

<Evaluation of liquid crystal display device>
As a result of evaluating the luminance unevenness of the manufactured liquid crystal display device by the following method, the liquid crystal display devices 5-2-2 and 5-3-4 of the present invention are ○, and the liquid crystal display devices 5-2-23 and 5- 3-14 resulted in x.

(Luminance unevenness)
The liquid crystal display device was conditioned for 100 hours in an environment of 60 ° C. and 90% RH using a constant temperature and high humidity bath. After taking out the liquid crystal display and adjusting the humidity at 23 ° C. and 50% RH for 1 hour, the backlight unit is turned on and visually observed for 24 hours every hour, and whether or not the luminance unevenness can be visually recognized according to the following criteria. evaluated.

○: Brightness unevenness was not visible in any observation. ×: Brightness unevenness was visible in any observation.

  This is because the liquid crystal display device of the present invention quickly follows the environmental fluctuations due to the effect of forming the low density region, and thus it is considered that luminance unevenness did not occur.

  Further, when the front luminance during white display of the liquid crystal display device was measured by the following method, the liquid crystal display devices 5-2-2 and 5-3-4 of the present invention were compared with the liquid crystal display device 5-2-23 of the comparative example. And 5-3-24, 0.7% and 0.7% improvement in front luminance was observed, respectively.

(Front brightness)
The liquid crystal display device was measured using a spectral radiance meter CS-2000 (manufactured by Konica Minolta Sensing Co., Ltd.). In the measurement, the value after 1 hour was used under the condition of 23 ° C. and 55% RH after the backlight of the liquid crystal display device was turned on to stabilize the light source.

  This is thought to be because the incident light from the backlight side has reduced surface reflection due to the refractive index difference between the polarizing plate protective film and air.

Claims (7)

  A single-layer film containing cellulose ester in an amount of 80% by mass or more, having a total light transmittance of 93% or more and a haze of 5% or less, wherein the total light transmittance of the film is T1, Tg + 10 ° C. of the film A polarizing plate protective film, wherein T1 is 0.5% or more higher than T2 when the total light transmittance of the film after heat treatment for 1 minute is T2.   The polarizing plate protective film according to claim 1, wherein the refractive index increase values of both surfaces (front surface and back surface) are different by the heat treatment.   The polarizing plate protective film according to claim 1 or 2, wherein a difference in transmittance between both surfaces (front surface and back surface) is 1% or more.   It is a manufacturing method of the polarizing plate protective film which manufactures the polarizing plate protective film of any one of Claims 1-3, Comprising: Dope containing a cellulose ester, the good solvent with respect to this cellulose ester, and a poor solvent is a support body Polarized light characterized by being treated with a poor solvent having an SP value closer to the cellulose ester than water or having a lower contact angle than water with respect to the cellulose ester within 5 minutes after casting. Manufacturing method of board protective film.   5. The poor solvent poor solvent having an SP value closer to the cellulose ester than water or having a contact angle lower than that of water relative to the cellulose ester contains ethanol or isopropyl alcohol. Manufacturing method of polarizing plate protective film.   The polarizing plate protective film obtained by the manufacturing method of the polarizing plate protective film of any one of Claims 1-3 or the polarizing plate protective film of Claim 4 or 5 is provided in the at least one side of a polarizing plate. A polarizing plate characterized by.   A polarizing plate according to claim 6, wherein the polarizing plate protective film according to claim 1 is disposed outside the liquid crystal panel, and comprises an LED light source.