JP4665988B2 - Vertical alignment type LCD - Google Patents

Description

The present invention relates to a vertical alignment type liquid crystal display device.

  A liquid crystal display device can be directly connected to an IC circuit with a low voltage and low power consumption, and can be particularly thinned, so that it is widely used as a display device for a word processor, a personal computer, or the like. The basic configuration of this liquid crystal display device is one in which polarizing plates are provided on both sides of a liquid crystal cell.

  Such a liquid crystal display device is a liquid crystal display device using a twisted nematic (TN) having a twist angle of 90 degrees and a liquid crystal using a super twisted nematic (STN) having a twist angle of 160 degrees or more from the viewpoint of contrast or the like. Transition to display devices.

  However, since the liquid crystal display device using STN uses the birefringence of the liquid crystal, there is a problem that the background that is normally white in the liquid crystal display device using TN is colored blue or yellow, For this reason, in monochrome display, there is a problem that the contrast and viewing angle are narrow and colorization is difficult.

  In order to solve this problem, that is, to compensate for the birefringence, a technique using a retardation plate under the polarizing plate has been proposed. According to this technique, although the coloring problem is solved, the viewing angle is hardly improved. In addition, a technique has been proposed in which a birefringent film having a refractive index in the thickness direction larger than the refractive index in the direction perpendicular to the birefringent optical axis is used as a retardation plate. Furthermore, a technique has been proposed in which one or a plurality of films having positive and negative intrinsic birefringence values are used as a retardation plate. Further, as shown in Patent Document 1, a polarizing plate is proposed in which at least one protective film of a polarizer is a plastic film made of acetylcellulose having a retardation value in a plane direction measured with light having a wavelength of 590 nm of 30 to 70 nm. It was done.

  With these proposed techniques, the change in contrast due to the viewing angle is reduced, and the viewing angle characteristics are improved.

  By the way, the demand for improvement of the biggest problem in a liquid crystal display device having a large feature that other display devices do not have in terms of low voltage, low power consumption, and thinning, that is, a problem with a narrow viewing angle, is getting stronger. Further research is underway.

  As one of such research and development, a liquid crystal of a type different from the TN or STN type has been proposed. In other words, TN and STN type liquid crystal cells are of a type in which liquid crystal molecules are parallel to the alignment plate when the voltage is off and liquid crystal molecules are aligned perpendicular to the alignment plate when the voltage is on, whereas the liquid crystal molecules are aligned when the voltage is off. A type perpendicular to the plate and parallel to the alignment plate when the voltage is on, for example, a so-called vertical alignment (VA) type using negative liquid crystal having negative dielectric anisotropy has been developed.

  This VA type liquid crystal display device is a vertical alignment mode liquid crystal cell in which the liquid crystal molecules are perpendicular to the alignment plate when the voltage is off and the liquid crystal molecules are aligned parallel to the alignment plate when the voltage is on. In addition, the contrast is high and the viewing angle is relatively wide compared to those of the TN or STN type. However, as the liquid crystal screen becomes larger, there is a growing demand for further widening the viewing angle.

  While the present inventor has been researching a polarizing plate protective film in order to further widen the viewing angle of the VA mode liquid crystal, the VA type liquid crystal display device has an in-plane structure as shown in Patent Document 2. It was noticed that the effect was low even when a film in which the direction retardation value was controlled was used.

  Therefore, as a result of further research on the polarizing plate protective film, the retardation value in the thickness direction (Rt value), which is a value representing the anisotropy in the plane direction and the thickness direction, not the in-plane retardation value so far. As a result of finding out that the viewing angle of the VA type liquid crystal display device becomes wider when using a film in which the above formula 1) is controlled with a positive value, and further examining the method for increasing the thickness direction retardation value, The present invention has been reached.

  In addition, the optical film, particularly the polarizing plate protective film, has high heat and humidity to prevent the polarizing plate and the glass substrate of the liquid crystal cell from peeling due to shrinkage or deterioration due to moisture absorption of the polarizer under high temperature and high humidity. Durability is particularly important. Furthermore, high transparency, strength, handling properties, etc. are required.

  In addition, when the cellulose ester film is dissolved to form a film, a portion that is insufficiently esterified tends to remain in the film as an insoluble foreign matter. When this is incorporated in a liquid crystal display element, the refractive index of the foreign substance which is an insoluble matter is different from that of cellulose ester, and therefore, the polarization state is inhibited and abnormal light emission occurs. In recent years, liquid crystal display devices have been improved in definition, and defects due to such foreign matters have become a problem, and such a small number of foreign matters is also an important characteristic.

Patent Document 3 discloses that a specific range of acetyl group or propionyl group can be introduced to widen the selection range of the solvent without using chlorinated hydrocarbon as a solvent, as in TFT type and gradation display FSTN type. A fatty acid cellulose ester having a low retardation value in the plane direction and in the thickness direction has been proposed for the purpose of preventing the high contrast of the liquid crystal display device realizing high contrast from being impaired.
JP-A-7-218724 JP-A-7-218724 JP-A-9-90101

An object of the present invention is to provide a vertical alignment type liquid crystal display device having excellent excellent wet heat durability, or One viewing angle characteristics.

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

1. In a liquid crystal display device having a vertical alignment type liquid crystal cell sandwiched between two polarizers, a retardation value in a thickness direction defined by the following formula 1 is provided between the vertical alignment type liquid crystal cell and at least one polarizer. (Rt value) is 60 to 300 nm, has a polarizing plate protective film made of a single fatty acid cellulose ester film having an acetyl group and a propionyl group, and the polarizer and the polarizing plate protective film are bonded together, The vertical alignment type liquid crystal display device, wherein the polarizing plate protective film is directly provided in the vertical alignment type liquid crystal cell.

Rt value = ((n _x + _ny ) / 2−n _z ) × d (Formula 1)
(N _x is a refractive index of the largest direction of the refractive index in the film plane, n _y is the refractive index in the film plane in a direction perpendicular to n _x, n _z is the refractive index in the thickness direction of the film, d is film Represents the thickness (nm) of each).
2. Each of the polarizing plate protective films is provided between the polarizer and the vertical alignment type liquid crystal cell, each polarizing plate protective film is bonded to the polarizer, and each polarizing plate protective film is 2. The vertical alignment type liquid crystal display device according to 1, wherein the vertical alignment type liquid crystal cell is directly provided in the vertical alignment type liquid crystal cell.

3. 3. The vertical alignment type liquid crystal display device according to 1 or 2, wherein a retardation value in a thickness direction of the polarizing plate protective film is 90 to 200 nm.

4). 4. The vertical alignment type liquid crystal display device according to 3 above, wherein the retardation value in the thickness direction of the polarizing plate protective film is 100 to 175 nm.

5. The vertical alignment type liquid crystal display according to any one of 1 to 4, wherein the polarizing plate protective film has a retardation value (Ro value) in a plane direction defined by the following formula 2 of less than 30 nm. apparatus.
Ro value _{= (n x -n y) ×} d ( Equation 2)
(N _x is a refractive index of the largest direction of the refractive index in the film plane, n _y is the refractive index in the film plane in a direction perpendicular to n _x, d represents the thickness (nm) of the film.)

6). The vertical alignment type liquid crystal display device according to any one of 1 to 5, wherein the polarizing plate protective film has a thickness of 40 to 190 μm.

7). 7. The vertical alignment type liquid crystal display device according to 6, wherein the polarizing plate protective film has a thickness of 60 to 190 μm.

8). 8. The vertical alignment type liquid crystal display device according to 7, wherein the polarizing plate protective film has a thickness of 75 to 190 μm.

9. The vertical alignment according to any one of 1 to 8 above, wherein the total of the acetyl group substitution degree (DSac) and the propionyl group substitution degree (DSpr) of the fatty acid cellulose ester is 2.1 to 2.8. Type liquid crystal display device.

10. The vertical alignment type according to any one of 1 to 8, wherein the fatty acid cellulose ester has an acetyl group substitution degree of 1.5 to 2.3 and a propionyl group substitution degree of 0.6 to 1.2. Liquid crystal display device.

11. 11. The vertical alignment type liquid crystal display device as described in 9 or 10 above, wherein the fatty acid cellulose ester has an acetyl group substitution degree of less than 2.0.

12 The polarizing plate protective film is characterized in that the number of foreign matters having a size of 5 to 50 μm recognized in the polarization crossed Nicol state is 200 or less per 250 mm ^{2 and} 0 foreign matters exceeding 50 μm. 4. A vertical alignment type liquid crystal display device according to any one of the above.

13. 13. The vertical alignment type liquid crystal display device as described in 12 above, wherein the polarizing plate protective film has 100 or less foreign matters having a size of 5 to 50 μm per 250 mm ² recognized in a polarization crossed Nicol state .

The present invention has excellent wet heat durability, it is possible to provide a vertical alignment type liquid crystal display device having excellent or One viewing angle characteristics.

  Hereinafter, the present invention will be described in detail.

The fatty acid cellulose ester constituting the polarizing plate protective film used in the present invention is a mixed fatty acid ester in which a part of the hydroxyl group of cellulose is substituted with an acetyl group and a propionyl group. The total of the degree of acetyl group substitution and the degree of propionyl group substitution is preferably 2.1 to 2.8. By setting it as this range, the retardation value in the thickness direction can be increased, and a good viewing angle improvement effect can be obtained and high wet heat durability can be obtained. A more preferred range is 2.5 to 2.75, particularly 2.6 to 2.7. Moreover, the acetyl substitution degree is preferably 1.5 to 2.3, and the propionyl substitution degree is preferably 0.6 to 1.2. In the above, the degree of acetyl substitution is more preferably less than 2.0, and further preferably less than 1.5 to less than 2.0. Here, the degree of substitution refers to the number of bound fatty acids bonded to hydroxyl groups in cellulose in terms of glucopyranose units, and the degree of acetyl substitution refers to the degree of acetylation in ASTM-D817-91 (testing method for cellulose acetate, etc.). Follow measurement and calculation. The measuring method of propionyl group substitution degree can be measured according to ASTM-D817-96.

  The number average molecular weight of the fatty acid cellulose ester is 70,000 to 300,000, preferably 90,000 to 200,000.

  The fatty acid cellulose ester can be synthesized using an acid anhydride or acid chloride as an acylating agent. When the acylating agent is an acid anhydride, an organic acid (eg, acetic acid) or methylene chloride is used as a reaction solvent, and an acidic catalyst such as sulfuric acid is used as a catalyst. When the acylating agent is an acid chloride, a basic compound is used as a catalyst. In the industrially most common synthesis method, cellulose is mixed with an organic acid component containing an organic acid (acetic acid, propionic acid) corresponding to acetyl group and propionyl group or an acid anhydride (acetic anhydride, propionic anhydride). The cellulose ester is synthesized by esterification. The amount of the acetylating agent and propionylating agent used is adjusted so that the ester to be synthesized falls within the range of the substitution degree described above. It is preferable that the usage-amount of a reaction solvent is 100-1000 mass parts with respect to 100 mass parts of cellulose. It is preferable that the usage-amount of an acidic catalyst is 0.1-20 mass parts with respect to 100 mass parts of cellulose. The reaction temperature is preferably 10 to 120 ° C. Further, after the acylation reaction is completed, the degree of substitution may be adjusted by hydrolysis (saponification) as necessary. After completion of the reaction, the reaction mixture (cellulose ester dope) is separated using conventional means such as precipitation, washed and dried to obtain a fatty acid ester of cellulose (cellulose acetate propionate).

  For example, it can be synthesized by the following method. Add 907 g of acetic acid and 223 g of propionic acid to 299 g of cellulose and mix at 54 ° C. for 30 minutes. After the mixture is cooled, 318 g of acetic anhydride, 813 g of propionic anhydride, 10.6 g of sulfuric acid and 6.3 g of propionic acid cooled to about 20 ° C. are added for esterification. The maximum temperature in esterification is adjusted to 40 ° C. After carrying out the esterification reaction for 150 minutes, a mixed solution of 295 g of acetic acid and 98.5 g of water is added over 22 minutes as a reaction terminator to hydrolyze excess anhydride. While maintaining the temperature of the reaction liquid at 62 ° C., 886 g of acetic acid and 295 g of water are added. After 1 hour, an aqueous solution containing 18.0 g of magnesium acetate is added to neutralize sulfuric acid in the system. The obtained cellulose acetate propionate has an acetyl substitution degree of 2.0, a propionyl substitution degree of 0.8, and a number average molecular weight of about 100,000.

As the fatty acid cellulose ester constituting the polarizing plate protective film used in the present invention , either a fatty acid cellulose ester synthesized from cotton linter and a fatty acid cellulose ester synthesized from wood pulp can be used alone or in combination. It is preferable to use a large amount of cellulose triacetate synthesized from a cotton linter that has good releasability from a belt or drum because of high productivity. Since the peelability is remarkably improved when the ratio of cellulose triacetate synthesized from cotton linter is 60% or more, the use ratio is preferably 60% or more, more preferably 85% or more, and most preferably used alone. preferable.

Solvents for dissolving the fatty acid cellulose ester constituting the polarizing plate protective film used in the present invention to form a dope include methylene chloride, methyl acetate, ethyl acetate, amyl acetate, acetone, tetrahydrofuran, 1,3-dioxolane, 1 , 4 dioxolane, cyclohexanone, 2,2,2-trifluoroethanol, 2,2,3,3-tetrafluoro-1-propanol, 1,3-difluoro-2-propanol, and the like. Although a chlorinated solvent such as methylene chloride can be used technically without any problem, it is preferable that a chlorinated solvent is not contained as much as possible from the viewpoint of the global environment and working environment. Methyl acetate, ethyl acetate, acetone, etc. have few environmental problems. In particular, it is preferable that methyl acetate is contained in an amount of 50% by mass or more based on the total organic solvent, and it is preferable to use 5-30% acetone in combination with methyl acetate in combination with methyl acetate to reduce the dope viscosity. In the present invention, the phrase “containing no chlorinated solvent as much as possible” means that the chlorinated solvent is 10% or less based on the total amount of organic solvents. Preferably it is 5% or less, and most preferably not contained at all.

  The dope preferably contains 1 to 30% alcohol having 1 to 4 carbon atoms in addition to the organic solvent. When alcohol is contained, after the dope is cast on the support, the solvent starts to evaporate, and the web (the dope film after casting the dope on the support for casting) gels, and the web is strong. It can be easily peeled from the support, and the effect of promoting dissolution of the fatty acid cellulose ester can be obtained. Examples of the alcohol having 1 to 4 carbon atoms include methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, and tert-butanol. Of these, ethanol is preferred from the standpoint of dope stability, boiling point, drying property, non-toxicity, and the like.

  The solid content concentration of the dope is usually preferably 10 to 40%, and the dope viscosity is preferably adjusted to a range of 100 to 500 poise from the viewpoint of obtaining good film flatness. The dope prepared as described above is filtered with a filter medium, defoamed, and sent to the next process with a pump.

  In the dope, a plasticizer, a matting agent, an ultraviolet absorber, an antioxidant, a dye, or the like may be added.

  Since the fatty acid ester cellulose having an acetyl group and a propionyl group used in the present invention itself exhibits an effect as a plasticizer, sufficient film characteristics can be obtained without adding a plasticizer or with a slight addition amount. However, a plasticizer may be added for other purposes. For example, for the purpose of improving the moisture resistance of the film, alkylphthalylalkyl glycolates, phosphate esters, carboxylic acid esters and the like can be mentioned.

  Examples of alkyl phthalyl alkyl glycolates include methyl phthalyl methyl glycolate, ethyl phthalyl ethyl glycolate, propyl phthalyl propyl glycolate, butyl phthalyl butyl glycolate, octyl phthalyl octyl glycolate, methyl phthalyl ethyl Glycolate, ethyl phthalyl methyl glycolate, ethyl phthalyl propyl glycolate, methyl phthalyl butyl glycolate, ethyl phthalyl butyl glycolate, butyl phthalyl methyl glycolate, butyl phthalyl ethyl glycolate, propyl phthalyl butyl glycol Butyl phthalyl propyl glycolate, methyl phthalyl octyl glycolate, ethyl phthalyl octyl glycolate, octyl phthalyl methyl glycolate, octyl phthalate Ethyl glycolate, and the like.

  Examples of phosphate esters include triphenyl phosphate, tricresyl phosphate, phenyl diphenyl phosphate, and the like.

  Examples of the carboxylic acid ester include phthalic acid esters such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate and diethyl hexyl phthalate, and citrate esters such as acetyl trimethyl citrate, acetyl triethyl citrate and acetyl tributyl citrate. Can be mentioned. In addition, butyl oleate, methylacetyl ricinoleate, dibutyl sebacate, triacetin and the like are preferably used alone or in combination.

  Two or more plasticizers may be used in combination as required. A phosphoric ester plasticizer is preferable because a use ratio of 80% or less hardly causes hydrolysis of the cellulose ester film and is excellent in durability. Further, it is more preferable that the amount of phosphate ester plasticizer is small, and it is particularly preferable to use only a phthalate ester or glycolate ester plasticizer. Among them, methyl phthalyl methyl glycolate, ethyl phthalyl ethyl glycolate, propyl phthalyl propyl glycolate, butyl phthalyl butyl glycolate, octyl phthalyl octyl glycolate are preferable, and ethyl phthalyl ethyl glycolate is particularly preferable. It is done. These alkyl phthalyl alkyl glycolates may be used in combination of two or more. The amount of the plasticizer used for this purpose is preferably 1 to 30%, particularly 4 to 13%, based on the cellulose ester. These compounds may be added together with the cellulose ester and the solvent during the preparation of the cellulose ester solution, or may be added during or after the solution adjustment.

  A dye may be added for the purpose of improving the yellowness of the film. Since the cellulose ester has a slight yellow tint, it is preferable that the color can be colored gray as seen on a normal photographic support. Blue and purple dyes are preferably used. However, unlike the photographic support, there is no need to prevent light piping, so the content may be small, preferably from 1 to 100 ppm, more preferably from 2 to 50 ppm based on the cellulose ester. A plurality of dyes may be appropriately combined so as to become gray.

  If the film is difficult to slip, the films may block each other, resulting in poor handling. The film according to the present invention contains inorganic fine particles such as silicon dioxide, titanium dioxide, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, and calcium phosphate, and a matting agent such as a crosslinked polymer. Is preferred.

  In order to reduce the haze of the film, it is preferable that fine particles such as silicon dioxide are surface-treated with an organic substance. Preferred organic substances for the surface treatment include halosilanes, alkoxysilanes, silazanes, siloxanes and the like. The larger the average diameter of the fine particles is, the better the mat effect is, and the smaller the average diameter is, the better the transparency is. Therefore, the average primary particle diameter of the fine particles is 0.1 μm (100 nm) or less, preferably 5 to 50 nm, more preferably. 7 to 14 nm. Examples of the fine particles of silicon dioxide include AEROSIL 200, 200V, 300, R972, R972V, R974, R202, R812, OX50, TT600, etc. manufactured by Nippon Aerosil Co., Ltd., preferably AEROSIL 200V, R972, R972V, R974, R202. , R812 and the like. The matting agent is preferably blended so that the haze of the film is 0.6% or less and the dynamic friction coefficient is 0.5 or less. The amount of the matting agent used for this purpose is preferably 0.005 to 0.3% with respect to the fatty acid cellulose ester.

  Liquid crystal display devices are often used outdoors, and it is important to provide a polarizing plate protective film with a function of cutting ultraviolet rays. It is preferable to use an ultraviolet absorber for the film of the present invention, and the ultraviolet absorber is excellent in the ability to absorb ultraviolet rays having a wavelength of 370 nm or less from the viewpoint of preventing deterioration of the liquid crystal and has a wavelength of 400 nm from the viewpoint of good liquid crystal display properties. Those that absorb as little visible light as possible are preferred. In particular, the transmittance at a wavelength of 370 nm needs to be 10% or less.

  The ultraviolet absorber used for this purpose preferably has no absorption in the visible light region, and examples thereof include benzotriazole compounds, benzophenone compounds, and salicylic acid compounds. For example, 2- (2′-hydroxy-5-methylphenyl) benzotriazole, 2- (2′-hydroxy-3 ′, 5′-di-t-butylphenyl) benzotriazole, 2- (2′-hydroxy- 3'-di-t-butyl-methylphenyl) benzotriazole, 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-n-octoxybenzophenone, 4-dodecyloxy-2- Hydroxybenzophenone, 2,2 ', 4,4'-tetrahydroxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, phenyl salicylate, methyl salicylate and the like.

  In this invention, it is preferable to use 1 or more types of these ultraviolet absorbers, and you may contain 2 or more types of different ultraviolet absorbers.

  As a method for adding the ultraviolet absorber, it may be added to the dope after being dissolved in an organic solvent such as alcohol, methylene chloride, dioxolane, or directly into the dope. Those that do not dissolve in an organic solvent such as inorganic powder are added to the dope after being dispersed in an organic solvent and cellulose ester using a dissolver or a sand mill.

  The usage-amount of the ultraviolet absorber in this invention is 0.1-2.5% with respect to a cellulose ester, Preferably it is 0.5-2.0%, More preferably, it is 0.8-2.0%. When the amount of the ultraviolet absorber used is more than 2.5%, the transparency tends to deteriorate, which is not preferable.

  For the purpose of improving the heat resistance of the film, a hindered phenol compound is preferably used. The amount of these compounds added is preferably 1 ppm to 1.0%, more preferably 10 to 1000 ppm, based on the cellulose ester. In addition, a heat stabilizer such as an alkaline earth metal salt such as calcium or magnesium may be added.

  In addition to the above, an antistatic agent, a flame retardant, a slip agent, an oil agent and the like may be added as appropriate.

When a cellulose ester containing an insoluble foreign matter is formed into a film, the insoluble foreign matter causes irregular reflection. Therefore, when incorporated in a liquid crystal display device, the light of the liquid crystal cell is scattered and the display becomes difficult to see. Undissolved foreign matter is difficult to detect with ordinary light, but when two polarizing plates are placed in an orthogonal (crossed nicols) state and a cellulose ester film is placed between them and the light from the light source is applied from the opposite side, Since the foreign material appears to shine, its size and number can be easily measured. The number of foreign matters having a size of 5 to 50 μm is preferably 200 or less per 250 mm ² , and zero foreign matters having a size exceeding 50 μm. More preferably, the number of foreign matters of 5 to 50 μm is 100 or less per 250 mm ² . Foreign matter having a particle size of less than 5 μm does not cause much visual problems. In addition, foreign matters having a size exceeding 50 μm are hardly generated by a normal cellulose ester manufacturing method, and foreign matters such as metals and sealing materials having a size exceeding 50 μm are removed within the cellulose ester manufacturing process. For this reason, it is preferable to remove foreign substances with a filter from the dissolved dope. The filter to be used is not particularly limited as long as it is resistant to an organic solvent. For example, a sintered metal filter, a metal fiber filter, a resin filter (woven fabric, non-woven fabric), a ceramic filter, a glass filter, and filter paper can be used. The average opening of the filter can be appropriately changed depending on the size of the foreign matter to be removed, but is usually selected from a range of 0.1 to 100 μm. The filter may be used alone or a plurality of filters may be arranged in series. In particular, it is preferable to use a filter paper having a drainage time of 20 seconds or more and to filter at a filtration pressure of 1.6 MPa or less. More preferably, a filter paper having a drainage time of 30 seconds or more is used, and a filtration pressure is 1.2 MPa or less, more preferably a filter paper having a drainage time of 40 seconds or more is used, and the filtration pressure is 1.0 MPa or less. It is preferable to filter with. It is preferable to use two filter papers in piles. The filtration pressure can be adjusted by appropriately selecting the filtration flow rate and the filtration area.

  The obtained dope is cast on a support to form a film. As the film forming method, a band method or a drum method can be used. Next, the film thus obtained is peeled off from the support. Thereafter, it is dried while being conveyed through the drying zone under tension.

  Casting step: A dope is fed to a pressure die through a pressurized metering gear pump, and an endless metal belt for infinite transfer or a support for casting a rotating metal drum at the casting position (hereinafter simply referred to as a support). It may be a step of casting a dope from a pressure die on the top. The surface of the casting support is a mirror surface. Other casting methods include the doctor blade method in which the dope film is adjusted with a blade, or the reverse roll coater method in which the dope film is adjusted with a reverse-rotating roll. A pressure die that is easy to handle is preferred. The pressure die includes a coat hanger die and a T die, and any of them is preferably used. In order to increase the film-forming speed, two or more pressure dies may be provided, or the dope amount may be divided to overlap two or more layers.

  Solvent evaporation step: a step of evaporating the solvent by heating the web on a casting support. For evaporating the solvent, there are a method of blowing air from the web side and / or a method of heating with liquid from the back side of the support, and a method of heating from the front and back sides with radiant heat, but the back side liquid heating method is preferable because of good drying efficiency. . A method of combining them is also preferable.

  Peeling step: A step in which the web in which the solvent has evaporated on the support is peeled off at the peeling position. The peeled web is sent to the next process. If the residual solvent amount (the following formula) of the web at the time of peeling is too large, peeling will be difficult, or conversely if part of the web is peeled off after sufficiently drying on the support, part of the web will be peeled off. As a method for increasing the film forming speed, there is a gel casting method (gel casting method, which can be peeled off even if there is a large amount of residual solvent). Examples of the gel casting method include a method in which a poor solvent for cellulose ester is added to the dope to gel after dope casting, a method in which the temperature of the support is lowered and gelled. There is also a method of adding a metal salt in the dope. By gelling on the support and strengthening the film, peeling can be accelerated and the film forming speed can be increased. When peeling off when the amount of residual solvent is large, if the web is too soft, the flatness at the time of peeling will be impaired, and slippage and vertical stripes are likely to occur due to peeling tension, and the amount of residual solvent is determined by balancing productivity and quality. .

  Drying step: a step of drying the web using a drying device that alternately conveys the web through rolls arranged in a staggered manner and / or a tenter device that conveys the web while holding the width of both ends of the web with pins or clips. is there. Generally, the drying means blows hot air on both sides of the web, but there is also a means for heating by applying a microwave instead of the wind. Too much drying tends to impair the flatness of the finished film. Drying at a high temperature is preferably performed from a residual solvent amount of 8% or less. Throughout, the drying temperature is usually 40 to 250 ° C, preferably 70 to 180 ° C. The drying temperature, the amount of drying air, and the drying time differ depending on the solvent used, and the drying conditions may be appropriately selected according to the type and combination of the solvents used.

  In the drying process, the web tends to shrink in the width direction due to evaporation of the solvent. Shrinkage increases as the temperature rapidly dries. Drying while suppressing this shrinkage as much as possible is preferable for improving the flatness of the finished film. From this point of view, for example, a method of drying all or part of the drying process as shown in JP-A-62-46625 while holding both ends of the web with clips in the width direction (tenter method) Is preferred.

  Winding step: The step of winding the web as a film after the residual solvent amount becomes 2% or less. By setting the residual solvent amount to 0.4% or less, a film having good dimensional stability can be obtained. As a winding method, a generally used method may be used, and there are a constant torque method, a constant tension method, a Taber tension method, a program tension control method with a constant internal stress, and the like.

  The amount of residual solvent can be expressed by the following formula.

Residual solvent amount (%) = (M−N) / N × 100
(Here, M is the mass of the web at any point in time, and N is the mass after drying M at 110 ° C. for 3 hours.)
The film thickness of the fatty acid cellulose ester film may be adjusted by controlling the dope concentration, the pumping amount, the slit gap of the die base, the extrusion pressure of the die, and the speed of the casting support. Further, as a means for making the film thickness uniform, it is preferable to adjust the information obtained by using the film thickness detection means by feeding it back to the apparatus.

  In the process from immediately after casting to drying, the atmosphere in the drying apparatus may be air, but may be performed in an inert gas atmosphere such as nitrogen gas or carbon dioxide gas. However, the danger of the explosion limit of the evaporating solvent in the dry atmosphere must always be considered.

In order to obtain a film having a retardation value (Rt value) in the thickness direction of the present invention, the following methods may be mentioned, but the invention is not limited thereto.
(1) In the process until the film is peeled off from the belt or drum, the Rt value increases when the residual solvent amount at the time of peeling is decreased, and decreases when it is increased. In this case, the residual solvent amount at the time of peeling is preferably 5 to 100%, more preferably 5 to 80%, and still more preferably 10 to 45%.
(2) If the peeling tension and the transport tension in the drying zone are increased, the Rt value decreases, and if it is decreased, it increases. A preferable peeling tension is 50 to 400 N / m, more preferably 100 to 300 N / m, and still more preferably 100 to 250 N / m. Moreover, the conveyance tension in a drying zone becomes like this. Preferably it is 50-200 N / m, More preferably, it is 80-150 N / m, More preferably, it is 80-120 N / m.
(3) There is a drying method such as a pin tenter method or a clip tenter method that dries while stretching the film. In this case, the Rt value increases as the draw ratio increases, and decreases as it decreases. As a preferable draw ratio, it is 2 to 50%, More preferably, it is 5 to 40%, More preferably, it is 10 to 30%.

The retardation film (Rt value) in the thickness direction of the polarizing plate protective film used in the present invention is a positive value and is 60 to 300 nm. Preferably it is 70-300 nm, More preferably, it is 90-200 nm, More preferably, it is 100-175 nm. The Rt value is determined by measuring the three-dimensional refractive index at 23O 0 C and 55% relative humidity at a wavelength of 590 nm using an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments). Refractive indices nx, ny, nz Is obtained.

Moreover, it is preferable that the thickness of the polarizing plate protective film used by this invention is 40-190 micrometers. 60-190 micrometers is more preferable, 100-175 micrometers is further preferable, and 120-160 micrometers is especially preferable.

It In the present invention, to suppress the fluctuation of the retardation value due to humidity, and in order to obtain good viewing angle characteristics, the equilibrium moisture content of the polarizing plate protective fill arm used in the present invention is less than 6% It is more preferable as the numerical value decreases to 5% or less, 4% or less, 3% or less, 2% or less, and 1% or less, but it is particularly preferable that the value is substantially zero. In the present invention, “substantially zero” represents 0.5% or less.

  The equilibrium moisture content was measured using a Karl Fischer moisture meter, specifically, a trace moisture measuring device CA-06 and a moisture vaporizer VA-06 (manufactured by Mitsubishi Kasei Co., Ltd.), and heated at 120 ° C. for 45 minutes. After that, the water content can be calculated by the following formula.

Moisture content (%) = residual moisture mass / film mass after heat treatment × 100
A conventionally well-known thing can be used for the polarizing film in which this polarizing plate protective film is provided in the surface. For example, a film made of a hydrophilic polymer such as polyvinyl alcohol is treated with a dichroic dye such as iodine and stretched, or a plastic film such as polyvinyl chloride is treated and oriented with polyene. . And a polarizing plate is comprised as what laminated | stacked the said polarizing plate protective film on the at least single side | surface of the polarizing film.

The polarizing plate thus obtained is provided on one side or both sides of the VA type liquid crystal cell. Thereby, the vertical alignment type liquid crystal display device of the present invention is obtained.

  Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

Example 1
(Production and evaluation of fatty acid cellulose ester film)
The following composition was put into a pressure sealed container, heated to 80 ° C. to make the pressure in the container 500 kPa, and completely dissolved while keeping warm and stirring.

Cellulose acetate propionate (CAP, substitution degree is listed in Table 1)
120 parts by mass 2- (2′-hydroxy-3 ′, 5′-di-t-butylphenyl) benzotriazole (ultraviolet absorber) 1 part by mass ethyl phthalyl ethyl glycolate (plasticizer) 4 parts by mass fine particle silica ( Nippon Aerosil Co., Ltd. AEROSIL200, 0.016 μm)
0.1 parts by mass Methyl acetate 300 parts by mass Ethanol 45 parts by mass The dope temperature was lowered to 40 ° C., returned to normal pressure, allowed to stand overnight, defoamed, and then the solution was produced by Azumi filter paper Co., Ltd. Filter paper No. Filtered using 244. Next, the dope was cooled and kept at 35 ° C., and was uniformly cast on a rotating endless stainless steel belt having a length of 6 m (effective length of 5.5 m) stretched between two drums. Hot water at 35 ° C. was brought into contact with the back surface of the stainless steel belt and dried on the stainless steel belt for 2 minutes, and then cold water at 15 ° C. was kept in contact with the back surface of the stainless steel belt. At the time when the solvent was evaporated until the amount of residual peeling solvent reached 20%, peeling was performed from the stainless steel belt at a peeling tension of 150 N / m. Next, after drying at 130 ° C. while fixing both ends of the peeled film, the film was further dried while being conveyed at a conveyance tension of 130 N / m with a large number of rolls. 1 was obtained.

  The cellulose material, the type and addition amount of the solvent, the plasticizer, the residual solvent amount and the film thickness of the film were changed as shown in Table 1, and film No. 1 and film No. 2 to 11 were produced.

  The film was measured and evaluated by the following method. The results are shown in Tables 1 and 2.

[Degree of substitution of fatty acid cellulose ester]
The degree of substitution is measured by a saponification method. The dried cellulose ester is precisely weighed and dissolved in a mixed solvent of acetone and dimethyl sulfoxide (volume ratio 4: 1), and then a predetermined 1 mol / L aqueous sodium hydroxide solution is added and saponified at 25 ° C. for 2 hours. Excess sodium hydroxide is titrated with 0.5 mol / L sulfuric acid using phenolphthalein as an indicator. A blank test is performed in the same manner as described above. Further, the supernatant of the solution after titration is diluted, and the composition of the organic acid is measured by an ordinary method using an ion chromatograph. Then, the degree of substitution (%) is calculated according to the following.

TA = (Q−P) × F / (1000 × W)
DSac = (162.14 × TA) / (1−42.14 × TA
+ (1-56.06 × TA) × (AL / AC))
DSpr = DSsc × (AL / AC)
(In the formula, P is 0.5 mol / L sulfuric acid amount (ml) required for the titration of the sample, Q is 0.5 mol / L sulfuric acid amount (ml) required for the blank test, and F is the titer of 0.5 mol / L sulfuric acid. , W is the sample mass (g), TA is the total organic acid amount (mol / g), AL / AC is the molar ratio of acetic acid (AC) and other organic acid (AL) measured by ion chromatography, DSsc is (The degree of substitution of the acetyl group, DSpr represents the degree of substitution of the propionyl group.)
[Number average molecular weight of fatty acid cellulose ester]
Measurement is performed by a high performance liquid chromatograph under the following conditions.
Solvent: Methylene chloride Column: MPW × 1 (manufactured by Tosoh Corporation)
Sample concentration: 0.2 W / V% Flow rate: 1.0 ml / min Sample injection amount: 300 ml
Standard sample: Polystyrene Temperature: 23 ° C
[Residual solvent amount]
Residual solvent amount (%) = (M−N) / N × 100
M is the mass of the film (web) before heating, and N is the mass after drying M at 110 ° C. for 3 hours.

[Thickness direction retardation value (Rt value)]
Using an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments (Inc.)), 23 ° C., 55% relative humidity, at wavelength 590 nm, for 3-D refractive index measurement, refractive indices _{n x, n} y, Find _nz . According to the following (Formula 1), the retardation value (Rt value) in the thickness direction is calculated.

Rt value = ((n _x + _ny ) / 2−n _z ) × d (Formula 1)
(N _x is a refractive index of the largest direction of the refractive index in the film plane, n _y is the refractive index in the film plane in a direction perpendicular to n _x, n _z is the refractive index in the thickness direction of the film, d is film Represents the thickness (nm) of each).
[Surface direction retardation value (Ro value)]
Using an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments (Inc.)), 23 ° C., 55% relative humidity, at wavelength 590 nm, for 3-D refractive index measurement, refractive index _n x, a _{n y} Ask. According to the following (Formula 2), the retardation value (Ro value) in the surface direction is calculated.

Ro value _{= (n x -n y) ×} d ( Equation 2)
(N _x is a refractive index of the largest direction of the refractive index in the film plane, n _y is the refractive index in the film plane in a direction perpendicular to n _x, d represents the thickness (nm) of the film.)
[Number of foreign objects]
Place a sample between two polarizing plates arranged in a polarization crossed Nicol state, shine light from one side of the polarizing plate, shine from the other polarizing plate side with a microscope, and appear to be white and have a size of 5 μm or more per 25 mm ² Measure the number of foreign objects arbitrarily at 10 locations. The total value is the number of foreign matters per 250 mm ² , and this measurement is repeated five times, and the average value is the foreign matter number. The microscope condition is 30 times with transmitted light.

(Production and evaluation of liquid crystal display devices)
Film No. obtained as described above was obtained. 1 was treated with an aqueous 2.5 mol / L sodium hydroxide solution at 40 ° C. for 60 seconds and washed with water for 3 minutes to obtain an alkali-treated film having a saponification layer.

  Next, a 120 μm-thick polyvinyl alcohol film was immersed in 100 parts by mass of an aqueous solution containing 1 part by mass of iodine and 4 parts by mass of boric acid, and stretched 4 times at 50 ° C. to produce a polarizing film. A polarizing plate was prepared by laminating the alkali-treated film with a 5% aqueous solution of a completely saponified polyvinyl alcohol as an adhesive on both sides of the polarizing film, and the polarizing plate was provided on both sides of the VA type liquid crystal cell. 1 was obtained.

  Similarly, film No. 2 to 11 are used for the liquid crystal display devices No. 2 to 11 were obtained.

  The film was measured and evaluated by the following method. The results are shown in Table 3.

[Viewing angle characteristics]
The obtained liquid crystal display device is subjected to white display, black display, and gray 8 gradation display using a VG365N video pattern generator manufactured by EMT Co., Ltd., and the contrast ratio at the time of white / black display is manufactured by Otsuka Electronics Co., Ltd. It measured with LCD-7000. The angle at which the contrast ratio ≧ 10 is defined as the viewing angle.

[Damp heat durability]
The obtained liquid crystal display device was allowed to stand for 1000 hours at 80 ° C. and 90% RH, and the presence or absence of peeling of the film from the glass plate was observed.

◎: No peeling ○: Slight peeling is observed, but there is no practical problem ×: There is peeling [Overall evaluation]
The viewing angle characteristics and wet heat durability were comprehensively evaluated.

◎: Very good ○: Good ×: Unusable

From Table 3, cellulose ester film No. which does not contain a propionyl group. No. 6 liquid crystal display device No. 6 No. 6 was inferior in wet heat durability and inferior in viewing angle characteristics. However, the liquid crystal display device of the present invention, good viewing angle characteristics are obtained, and wet heat durability was good.

(Evaluation of fatty acid cellulose ester film)
Film No. 1, 5 and 6 were measured and evaluated by the following methods. The results are shown in Table 4.

[Transmissivity]
The light transmittance at a wavelength of 550 nm is measured using a spectrophotometer (U-3400, manufactured by Hitachi, Ltd.).

[Tear strength of film]
Measured according to ASTM D-1992. Using a digital tearing tester manufactured by Toyo Seiki Seisakusho Co., Ltd., a 12.7 mm cut is made in the center of the width direction of a film having a width of 50 mm and a length of 63.5 mm, and the tear strength is measured at 23 ° C. and a relative humidity of 55%. To do.

(Dynamic friction coefficient)
Measured according to JIS-K-7125 (1987). A film having a width of 120 mm and a length of 300 mm is fixed to a support base, and a film having a width of 80 mm and a length of 200 mm is placed thereon so that the front and back surfaces are in contact with each other. Further, a weight of 200 g is placed thereon and pulled horizontally at a speed of 100 mm / min, and the force F when moving is measured.

      Coefficient of dynamic friction = F (N) / weight of weight (N)

  From Table 4, the sample no. It can be seen that 1 and 5 have good characteristics as optical films.

Claims (13)

In a liquid crystal display device having a vertical alignment type liquid crystal cell sandwiched between two polarizers, a retardation value in a thickness direction defined by the following formula 1 is provided between the vertical alignment type liquid crystal cell and at least one polarizer. (Rt value) is 60 to 300 nm, has a polarizing plate protective film made of a single fatty acid cellulose ester film having an acetyl group and a propionyl group, and the polarizer and the polarizing plate protective film are bonded together, The vertical alignment type liquid crystal display device, wherein the polarizing plate protective film is directly provided in the vertical alignment type liquid crystal cell.
    Rt value = ((n _x + N _y ) / 2-n _z ) × d (Formula 1)
(N _x Is the refractive index in the direction with the largest refractive index in the film plane, n _y Is n _x Refractive index in the plane of the film in a direction perpendicular to n, n _z Represents the refractive index in the thickness direction of the film, and d represents the thickness (nm) of the film. ) Each of the polarizing plate protective films is provided between the polarizer and the vertical alignment type liquid crystal cell, each polarizing plate protective film is bonded to the polarizer, and each polarizing plate protective film is 2. The vertical alignment type liquid crystal display device according to claim 1, wherein the vertical alignment type liquid crystal display device is provided directly on the vertical alignment type liquid crystal cell. The vertical alignment type liquid crystal display device according to claim 1, wherein a retardation value in a thickness direction of the polarizing plate protective film is 90 to 200 nm. The vertical alignment type liquid crystal display device according to claim 3, wherein a retardation value in a thickness direction of the polarizing plate protective film is 100 to 175 nm. The vertical alignment type liquid crystal according to any one of claims 1 to 4, wherein the polarizing plate protective film has a retardation value (Ro value) in a plane direction defined by the following formula 2 of less than 30 nm. Display device.
Ro value _{= (n x -n y) ×} d ( Equation 2)
(N _x is a refractive index of the largest direction of the refractive index in the film plane, n _y is the refractive index in the film plane in a direction perpendicular to n _x, d represents the thickness (nm) of the film.) The vertical alignment liquid crystal display device according to claim 1, wherein the polarizing plate protective film has a thickness of 40 to 190 μm. The vertical alignment type liquid crystal display device according to claim 6, wherein the polarizing plate protective film has a thickness of 60 to 190 μm. The vertical alignment type liquid crystal display device according to claim 7, wherein the polarizing plate protective film has a thickness of 75 to 190 μm. 9. The vertical according to claim 1, wherein the total of the acetyl group substitution degree (DSac) and the propionyl group substitution degree (DSpr) of the fatty acid cellulose ester is 2.1 to 2.8. Alignment type liquid crystal display device. The vertical alignment according to any one of claims 1 to 8, wherein the fatty acid cellulose ester has an acetyl group substitution degree of 1.5 to 2.3 and a propionyl group substitution degree of 0.6 to 1.2. Type liquid crystal display device. The vertical alignment type liquid crystal display device according to claim 9 or 10, wherein the fatty acid cellulose ester has an acetyl group substitution degree of less than 2.0. The polarizing plate protective film has a size of a foreign material having a size of 5 to 50 μm recognized in a polarized crossed Nicol state of 250 mm. ² The vertical alignment type liquid crystal display device according to any one of claims 1 to 11, wherein there are no more than 200 foreign substances exceeding 50 µm per unit. The polarizing plate protective film has a size of a foreign material having a size of 5 to 50 μm recognized in a polarized crossed Nicol state of 250 mm. ² The vertical alignment type liquid crystal display device according to claim 12, wherein the number is 100 or less per unit.