JP4739070B2 - Optically isotropic cellulose acylate film and method for producing retardation film - Google Patents

Description

  The present invention relates to a method for producing a cellulose acylate film having optical isotropy. More specifically, the present invention relates to an optically isotropic cellulose acylate film that can be used for optical applications such as a liquid crystal display, and a method for producing a retardation film.

Cellulose acylate typified by cellulose acetate and the like is widely used for photographic base films and the like because of its high transparency. Furthermore, since the optical anisotropy is small, it is widely used as a polarizer protective film for polarizing plates in liquid crystal display devices. However, even cellulose acylate with small optical anisotropy tends to cause in-plane orientation during the drying process by the solution casting method, and this contribution causes optical anisotropy in the thickness direction. there were. Due to this optical anisotropy in the thickness direction, birefringence occurs when the film is viewed obliquely, and light leakage occurs, so the influence cannot be ignored especially in the viewing angle characteristics of liquid crystal display devices that have achieved high contrast. ing. Therefore, in order to obtain a polarizer protective film that can be applied without substantially impairing the contrast of the liquid crystal cell when viewed from an oblique direction, not only in-plane but also optical anisotropy in the film thickness direction is controlled. There is a need for an improved film. In order to solve such a problem, a cellulose acylate film containing a substituent other than an acetyl group has been proposed (see, for example, Patent Document 1 and Patent Document 2). In addition, methods for suppressing the expression of birefringence have been proposed depending on the type or amount of plasticizer or additive (see, for example, Patent Document 3, Patent Document 4, Patent Document 5, and Patent Document 6). Moreover, the method of heating a film more than a glass transition temperature is proposed in a drying process (for example, refer patent document 7).
JP-A-9-90101 JP 2005-68314 A Japanese Patent Laid-Open No. 2002-14230 Japanese Patent Laid-Open No. 11-246704 Japanese Patent Laid-Open No. 11-92574 JP 2000-63560 A JP 2005-138375 A

  However, there is a problem that sufficient optical isotropy cannot be obtained only by a method of replacing the substituent of cellulose acylate with another as in Patent Document 1. In addition, when the kind and amount of the plasticizer and additive are changed as in Patent Document 2 to Patent Document 6, another problem such as reduced transparency of the film or insufficient heat resistance. was there. Furthermore, although the method of controlling the drying conditions as in Patent Document 7 can reduce the optical anisotropy, the film expands and contracts during high-temperature heating, which makes it difficult to transport the film or loosens the film. It was easy and there was a problem in the mass productivity.

  In view of such a viewpoint, an object of the present invention is to provide a method for producing an optically isotropic cellulose acylate film that is small in birefringence in the thickness direction as well as in the film plane and excellent in mass productivity. Furthermore, it aims at providing the method of manufacturing the cellulose acylate phase difference film excellent in the viewing angle characteristic by extending | stretching these optically isotropic cellulose acylate films.

  As a result of intensive studies, the present inventor has found that birefringence in the thickness direction can be controlled by selecting the type of support during the production of the cellulose acylate film.

That is, the present invention has an in-plane birefringence (ΔNh) of 3.0 × 10 ⁻⁵ or less and a thickness direction birefringence (ΔNv) of 1.1, which are characterized by satisfying all of the following A to C. The present invention relates to a method for producing an optically isotropic cellulose acylate film of × 10 ⁻³ or less.
A: The film is formed by the solvent cast method. B: The support used in the solvent cast method is a plastic film having a wet tension of 25 mN or more and 50 mN or less. C: The total acylation degree of cellulose acylate is 2. 4 or more and 3.0 or less

  Furthermore, in the method for producing an optically isotropic cellulose acylate film of the present invention, the support is preferably a biaxially stretched polyester film.

  Furthermore, in the method for producing an optically isotropic cellulose acylate film of the present invention, it is preferable that the cellulose acylate film does not substantially contain fine particles.

  Furthermore, this invention relates to the manufacturing method of the retardation film which extends | stretches the optically isotropic cellulose acylate film manufactured by one of the said methods at least to a uniaxial direction.

  According to the present invention, a cellulose acylate film having optical isotropy can be mass-produced. When such a cellulose acylate film is used for a polarizer protective film of a liquid crystal display device, even in a liquid crystal display device realizing high contrast, the birefringence when the film is viewed from an oblique direction is small, and a good viewing angle characteristic is obtained. Obtainable. Furthermore, by stretching a cellulose acylate film having optical isotropy, a retardation film having small optical anisotropy in the thickness direction can be obtained, and the same effect can be obtained.

First, in-plane birefringence and thickness direction birefringence in this specification are defined below. In the film plane, the refractive index in the direction with the highest refractive index (hereinafter referred to as the slow axis direction) is nx, and in the film plane, the direction perpendicular to the slow axis direction (hereinafter referred to as the fast axis direction). In-plane birefringence ΔNh and thickness-direction birefringence ΔNv are respectively expressed by the following equations, where ny is the refractive index and nz is the refractive index in the thickness direction.
ΔNh = nx−ny
ΔNv = (nx + ny) / 2−nz

  The present invention relates to a method for producing a cellulose acylate film having a small ΔNh and ΔNv, that is, optically isotropic.

  In general, as a film production method, a polymer is dissolved in a solvent, then cast on a support, and the solvent is dried. It is divided roughly into the melting method which forms into a film. In the method for producing an optically isotropic cellulose acylate film of the present invention, the film is preferably formed by a solvent cast method. The solvent cast method can obtain a film with higher isotropic properties than the melting method, and can easily control the thickness pattern and obtain a film with a uniform thickness, so that it can be used for optical applications such as liquid crystal displays. This is a preferable production method in use.

  In the solvent cast method, a solution in which a polymer and additives are dissolved in a solvent (hereinafter referred to as a dope) is cast on a flat support. In the present invention, a support made of plastic is used. The effect of using a support made of plastic will be described below.

  In general, an endless belt or drum roll made of a metal such as stainless steel is often used as the support. After casting on such a support and evaporating the solvent at a constant temperature, the web is obtained by peeling from the support, but the metal support is strong in adhesion to cellulose acylate. , There is a tendency that peeling of the support and the web is heavy. Therefore, it is necessary to apply a large tension to peel the web from the support. However, due to the tension at that time, the molecules are oriented in the casting direction of the web, and the birefringence ΔNh in the film plane increases. . In order to suppress the occurrence of such in-plane birefringence, a method of further drying the web before peeling from the support to lighten the peeling is used. The occurrence of in-plane birefringence is suppressed by reducing the amount of residual solvent in the web and lightening the peeling from the support.

  However, as drying on the support proceeds, this time, the molecules are oriented in the film plane as compared to the thickness direction of the web, and the in-plane refractive indices nx and ny and the refractive index nz in the thickness direction There is a problem that the difference increases, that is, the birefringence in the thickness direction increases. In other words, in producing a cellulose acylate film using a metal support, if the in-plane birefringence is to be reduced due to the peelability of the web and the support, it is necessary to proceed with drying. However, since the birefringence in the thickness direction increases accordingly, it is difficult to obtain a material having high optical isotropy.

  On the other hand, surprisingly, when a plastic film is used for the support, the peelability from the cellulose acylate is lighter than that of the metal support, and the support and the web can be used even when the amount of residual solvent in the web is large. It was found that can be peeled off appropriately, and both in-plane and birefringence in the thickness direction can be lowered.

  Furthermore, a continuous body such as an endless belt or a drum roll cannot be replaced unless the process is stopped once a defect such as a scratch has occurred. The replacement requires a great deal of time and money, but is a discontinuous body. In the case of using a plastic film support, since it is sufficient to reuse a defect such as a scratch, it is not necessary to stop the process for maintenance work such as replacement of the support. Is also excellent.

  The type of plastic used as the support is not particularly limited as long as it does not dissolve in the solvent used in the solvent casting method, but for the purpose of preventing deformation in the drying process, a material having excellent heat resistance is used. It is preferable. Furthermore, the thing which can peel the web of a cellulose acylate suitably is preferable. In order to improve the releasability of the cellulose acylate from the web, the wetting tension of the support is preferably 25 mN / m or more and 50 mN / m or less, and 30 mN / m or more and 45 mN / m or less. Is more preferably 33 mN or more and 43 mN or less. If the wetting tension is below the above range, the polymer solution may not be uniformly cast on the support, or the web and the support may be peeled too lightly and may be peeled off during drying. On the other hand, if the wetting tension exceeds the above range, peeling of the web and the support becomes heavy, and an isotropic film may not be obtained. The wetting tension can be measured by a method described in JISK-6768 using a commercially available wettability test reagent.

  The thickness of the plastic film used as the support is not particularly limited, but is preferably 50 μm or more and 300 μm or less, more preferably 80 μm or more and 250 μm or less, and further preferably 100 μm or more and 200 μm or less. preferable. When the support is thinner than the above range, the self-supporting property is not lowered, so that slackening occurs when the solution is cast, and the resulting cellulose acylate film is also easily slackened. In addition, if the support is thicker than the above range, the flexibility of the support is lost, the handling property is inferior, and the cost of the support increases. The film used as the support is preferably biaxially stretched in order to maintain its strength.

  Specific examples of the support that satisfies the above requirements include films made of polyester such as polyethylene terephthalate and polyethylene naphthalate, films made of polyimide, and biaxially stretched products thereof. In particular, it is preferable to use a biaxially stretched polyethylene terephthalate film because a wide film suitable for the solvent casting method can be obtained at a low cost and has an appropriate peelability from cellulose acylate. Moreover, in order to make the adhesiveness of a web and a support body appropriate, what gave surface treatment can also be used. Here, the surface treatment includes discharge treatment such as corona discharge and plasma discharge, and a method of coating the surface with a hydrophilic or hydrophobic chemical substance.

  Furthermore, in the support used in the method for producing the optically isotropic cellulose acylate film of the present invention, the arithmetic average roughness (Ra) of the surface on which the polymer solution is cast is preferably 1 nm or more and 30 nm or less. More preferably, it is 2 nm or more and 20 nm or less. Since the surface roughness of the support is transferred to the web formed thereon, a film having a smooth surface is obtained when the centerline surface average roughness of the support is small, and the centerline surface average roughness of the support is obtained. When is large, a film having a rough surface can be obtained. If the surface of the film is excessively smooth, there is a problem that the slipperiness is lowered and scratches are easily generated on the transport roll in the step after peeling from the support. On the other hand, if the surface is excessively rough, unevenness is formed on the film surface, haze increases, or the unevenness acts like a lens, so a uniform display cannot be obtained in a liquid crystal display device. There is a case. Therefore, the center line average surface roughness of the support is preferably in the above-described range. For the same reason, the 10-point average roughness (Rz) of the surface of the support on which the polymer solution is cast is preferably 10 nm or more and 1000 nm or less, more preferably 20 nm or more and 500 nm or less. Ra and Rz can be measured by the method described in JIS-B0601 (1994 edition).

  Next, the cellulose acylate used in the method for producing the optically isotropic cellulose acylate film of the present invention will be described. The cellulose acylate is not particularly limited as long as the hydroxyl group of cellulose is substituted with an acyl group, but in particular, the cellulose acylate is a cellulose acylate substituted with an acyl group having 4 or less carbon atoms. Is preferred. As such a cellulose acylate, specifically, one in which the hydroxyl group of cellulose is substituted with any one of an acetyl group, a propionyl group, and a butyryl group is preferable. Specific examples of such cellulose acylate include those having a plurality of acyl groups such as cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, and cellulose acetate butyrate. Since these have high transparency and small birefringence, they are preferable for the use of the present invention. Among these, cellulose acetate or cellulose acetate propionate can be particularly preferably used because it can be generally produced or obtained at a low cost.

  Furthermore, in the present invention, the cellulose acylate preferably has a specific degree of acylation. Specifically, the total of acetyl substitution degree, propionyl substitution degree, and butyryl substitution degree is preferably 2.4 or more and 2.9 or less, and more preferably 2.5 or more and 2.8 or less.

  Here, the preferable range of the acylation degree of cellulose acylate will be described. Cellulose molecules are polysaccharides in which D-glucose, which is a basic unit, is linked in a straight chain by β-1,4 bonds. The acylation degree of cellulose acylate indicates how much the three hydroxyl groups present at positions 2, 3, and 6 in this D-glucose molecule are acylated on average in the cellulose molecule. The degree of conversion = 3 ”indicates that all the hydroxyl groups in the cellulose molecule are acylated. The degree of substitution at each position may be equal, or may be biased to any position. The acylation degree can be quantified by the method described in ASTM-D817-96. If the acyl group is less than the above range, birefringence is likely to develop, and it may be difficult to obtain an optically isotropic film even if a support that can be appropriately peeled is used. Moreover, when substitution degree is large, the solubility with respect to a solvent falls and it may become difficult to obtain a film by a solvent cast method. Therefore, it is preferable to use the cellulose acylate used in the present invention with the degree of substitution appropriately adjusted.

  In the production of an actual film, the above resin and additives are dissolved in a solvent to form a solution, which is then cast on a support. Examples of the additive include a plasticizer and a deterioration inhibitor.

  The plasticizer is used for the purpose of improving processing characteristics such as stretching and toughness. As the plasticizer, for example, phosphate ester or carboxylic acid ester can be used. Examples of the phosphoric acid ester include triphenyl phosphate and tricresyl phosphate. Representative examples of the carboxylic acid ester include phthalic acid esters and citric acid esters. Examples of the phthalic acid ester include dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, diphenyl phthalate and diethyl hexyl phthalate. Examples of the citrate ester include triethyl O-acetylcitrate and tributyl O-acetylcitrate. Examples of other carboxylic acid esters include butyl oleate, methylacetyl ricinoleate, dibutyl sebacate, various trimellitic acid esters, and the like.

  As the deterioration preventing agent, an antioxidant that suppresses deterioration due to oxidation, a heat stabilizer that imparts stability at high temperatures, and an ultraviolet absorber that prevents deterioration due to ultraviolet rays are used. An acid absorbent that absorbs free acid generated by decomposition can also be used for chlorinated resins and plasticizers. As the deterioration preventing agent, the above-mentioned phosphate ester compounds, phenol derivatives, epoxy compounds, amine derivatives and the like are used. Phenol derivatives include octylphenol, pentaphenone, diamylphenol and the like, and examples of amine derivatives include diphenylamine and the like.

  The amount of the additive added is preferably 0.01 to 5.0 parts by weight, more preferably 0.05 to 2.0 parts by weight, with respect to 100 parts by weight of cellulose acylate. Even if the addition amount is 5.0 parts by weight or more, the effect is hardly improved, and conversely, bleeding onto the film surface is observed or transparency tends to be lowered. When the addition amount is less than 0.01 parts by weight, the effect of the deterioration preventing agent is hardly recognized.

  Furthermore, by adding fine particles as a lubricant and forming irregularities on the film surface, the slipperiness can be improved, and the generation of scratches on the film surface during transportation in the production process can be prevented. However, in the production method of the present invention, as described above, the surface shape of the plastic film as the support can be transferred to the cellulose acylate web to improve its slipperiness. It can also be set as the structure which does not contain. Since the aggregate of the lubricant sometimes becomes a defect as a foreign substance, it is preferable that the lubricant does not contain fine particles in the use as an optical film. In the present invention, it is also useful in that a film with few scratches can be obtained without selecting a lubricant by selecting an appropriate surface shape of the plastic film used as the support.

  Further, in the present invention, there is no particular limitation regarding addition of components other than those described above, but in order not to lose the characteristics as a cellulose acylate film, cellulose acylate is added to 100 parts by weight of the total weight of the film. It is preferable to contain 80 parts by weight or more, and more preferably 90 parts by weight or more.

  The solvent used for dissolving the cellulose acylate and additives is not particularly limited as long as it dissolves the cellulose acylate, and is selected from known solvents such as ketones, esters, halogenated hydrocarbons and the like. . As the ketones, acetone, methyl ethyl ketone, methyl isobutyl ketone and the like can be used. As the esters, ethyl acetate, methyl acetate, butyl acetate, ethyl propionate, methyl propionate, or the like can be used. As the halogenated carbon, methylene chloride, chloroform and the like can be used. Among them, methylene chloride has an advantage that the cellulose acylate is easily dissolved and the productivity is high because the boiling point is low. Furthermore, since it is highly safe against a fire during drying, it is most suitably used when producing the retardation film of the present invention. Further, a poor solvent for cellulose acylate can be added to the above-mentioned solvent for the purpose of reducing the peeling of the web from the support. As the poor solvent, alcohols having 10 or less carbon atoms are preferable, and alcohols having 4 or less carbon atoms, that is, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol are more preferable. Preferably used. The amount of the poor solvent is preferably 1 part by weight or more and 20 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less when the total amount of the solvent is 100 parts by weight. More preferably, it is at least 10 parts by weight. If the amount of the poor solvent is small, the effect of lightening the peeling due to gelation may not be sufficient. In addition, when the amount of the poor solvent is too large, the solubility is decreased, the viscosity of the solution is increased, the productivity in the solvent cast method is inferior, or the gel component may be mixed into the film to become a bright spot. .

  Regarding the preparation of the solution, the method is not particularly limited, but a method in which the resin and the additive are dissolved in a solvent, a method in which the resin and the additive are separately dissolved, and mixed using a static mixer or the like are preferable. It can also be used. Furthermore, a method of dissolving cellulose acylates having different degrees of substitution individually and mixing them can also be suitably used.

  The preferable viscosity of the solution is preferably 1.0 Pa · s or more and 10.0 Pa · s or less, more preferably 1.5 Pa · s or more and 8.0 Pa · s at the liquid temperature when cast on the support. It is as follows. If the viscosity is smaller than the above range, a lot of energy is required to disperse the solvent, and the productivity may be inferior. Moreover, when larger than the said range, it may become difficult to obtain a uniform film.

  While the plastic film support is transported at a constant speed, the solution obtained above is cast on the support by a known coater such as a die system or a comma system, and then transported to a drying furnace. You can get an acylate web. If foreign matter adheres to the support before casting the solution, the foreign matter is embedded in the cellulose acylate web and becomes defective, so an adhesive roll or dust remover is provided in the support transport process to support it. It is preferable to cast the solution after removing foreign substances on the body. Moreover, after performing such dust removal by another process, it can also use for a casting process.

In order to obtain a uniform thickness of the web, it is important to prevent the solvent from abruptly scattering at the initial stage of drying, and the ambient temperature immediately after casting on the support should be lower than the boiling point of the solvent. preferable. Moreover, when the atmospheric temperature immediately after coating is lowered, the atmospheric temperature further decreases due to evaporation of the solvent, so that moisture in the drying furnace may condense and the film may be whitened. In order to prevent whitening due to such condensation, it is preferable to supply dry air or a gas having a low water content such as nitrogen gas into the furnace in the initial stage of drying, and keep the humidity in the furnace at 20% or less. From such a state, conditions are set so that the atmospheric temperature increases as drying proceeds, and it is preferable that the atmospheric temperature be (the boiling point of the solvent +50) ° C. or lower until the film surface is in a dry state. Drying conditions. After the obtained web is peeled from the support, it can be further dried to obtain an optically isotropic cellulose acylate film. Here, in the production method of the present invention, the residual solvent amount when peeling the web from the support is preferably 5% or more and 30% or less, more preferably 8% or more and 25% or less, more preferably 10% or more and 20%. More preferred are: If the residual solvent amount exceeds the above range, peeling marks may be formed on the web surface at the time of peeling, or the flatness of the film may be lost. Also, if the amount of residual solvent is below the above range, the web and the support will be peeled off naturally during drying, making it impossible to transport or increasing birefringence in the thickness direction, which may be inferior in optical isotropy. is there. Here, in the present specification, the residual solvent amount is represented by the following formula with respect to the weight before and after heating the web at 150 ° C. for 60 minutes.
Residual solvent amount (%) = 100 × [(weight before heating) − (weight after heating)] / (weight before heating)

  When the web and the support are peeled off, the web and the support may be wound together in a roll, and then peeled in a separate process, or the support and the web may be wound individually after peeling. You may take it. Moreover, only a support body may be wound up after peeling and a web may be continuously used for the next drying process. The wound support can be used again as a support. Such a plastic film support can be recycled and used any number of times as long as defects such as scratches do not occur.

  The web after peeling off from the support can be dried while being conveyed by a float method, a tenter method or a roll conveying method. Although the transport method is not limited, in the case of the float method, the film itself is subjected to complex stress, and optical characteristics are likely to be uneven. In the case of the tenter method, it is necessary to balance the film width shrinkage due to solvent drying and the tension to support its own weight depending on the distance between the pins or clips that support both ends of the film. If the birefringence in the direction is likely to occur and the tension is too low, the film may be slack without being able to support its own weight. On the other hand, in the case of the roll conveyance method, since the tension for stable film conveyance is in principle applied to the film flow direction (MD direction), it is easy to make the stress direction constant, and birefringence occurs in both the in-plane and thickness directions. Hateful. Therefore, it is most preferable to use a roll conveyance method for drying the film.

The cellulose acylate film after drying preferably has an in-plane birefringence (ΔNh) of 3.0 × 10 ⁻⁵ or less, more preferably 2.0 × 10 ⁻⁵ or less, and 1.0 × More preferably, it is 10 ⁻⁵ or less. The birefringence (ΔNv) in the thickness direction is preferably 1.1 × 10 ⁻³ or less, more preferably 1.0 × 10 ⁻³ or less, and 7.0 × 10 ⁻⁴ or less. More preferably. Further, the residual solvent amount is preferably 2% by weight or less, and more preferably 1% by weight or less.

  Although there is no restriction | limiting in the thickness of the polymer film which concerns on this invention, 10 micrometers-500 micrometers are preferable, 20 micrometers-300 micrometers are more preferable, 30 micrometers-150 micrometers are more preferable. When the thickness of the film exceeds the above range, productivity by the solvent cast method tends to be inferior. Moreover, when the thickness of the film is below the above range, the handling property of the film is inferior and may not be suitable for mass production.

  The cellulose acylate film thus obtained can be used as a polarizing plate by laminating a polarizer with an adhesive layer on one side or both sides, or by laminating with a polarizer via the adhesive layer. In particular, a polarizing plate obtained by laminating an optically isotropic cellulose acylate film obtained by the production method of the present invention with a polarizer can be applied without substantially impairing the contrast of a liquid crystal cell when viewed from an oblique direction. This is one of the preferred embodiments. Moreover, it can also be set as an optical compensation polarizing plate by the combination of the said polarizing plate and another optical compensation layer. Although the optical compensation layer is not particularly limited, for example, a stretched polymer film such as a uniaxial or biaxial polymer film, a liquid crystal alignment layer such as a discotic type or a nematic type, and a non-disclosure described in JP 2003-344856 A A birefringent layer made of a liquid crystalline polymer can be suitably used.

  Furthermore, a retardation plate can be obtained by stretching or shrinking the optically isotropic cellulose acylate film in at least a uniaxial direction. Such a method for producing a retardation film also belongs to the scope of the present invention.

For example, as described in JP-A No. 2000-137116 and the like, when a film made of cellulose acylate having a substitution degree adjusted to a predetermined range is stretched, the longer wavelength has a higher phase difference in the visible light region. A retardation film can be obtained and has excellent performance in optical compensation of a liquid crystal display device. However, on the other hand, such cellulose acylate tends to cause anisotropy in the thickness direction during film formation, and the anisotropy in the thickness direction remains even when a retardation film is formed. There is a problem that the phase difference is difficult to control. That is, when the refractive index in the slow axis direction in the film plane is nx, the refractive index in the fast axis direction is ny, and the refractive index in the thickness direction is nz, there is a problem that NZ represented by the following formula increases. there were.
NZ = (nx-nz) / (nx-ny)

  When NZ is large, the amount of change between the phase difference when the film is viewed from the front and the phase difference when the film is viewed from the oblique direction becomes large, and the contrast when the liquid crystal display device is viewed from the oblique direction is reduced. Although there is a problem that the change becomes large, a film made of cellulose acylate having a degree of substitution adjusted to a predetermined range is free as described in the examples of JP-A-2000-137116. In the case of end-uniaxial stretching, the value of NZ often exceeds 1.1. As a result of the study by the present inventors, it is estimated that the reason why NZ is increased in this way is that the birefringence in the thickness direction of the cellulose acylate film before stretching is large. In view of such a viewpoint, as a result of earnest studies, it was found that when the optically isotropic cellulose acylate film obtained by the above-described production method is stretched, a retardation film having a NZ of 1.1 or less can be obtained. .

  In the method for producing a retardation film of the present invention, the stretching method is not particularly limited, but for the purpose of reducing the above-mentioned NZ, the free end uniaxial stretching in the film transport direction due to the peripheral speed difference between the two pairs of nip rolls. It is preferable to use a stretching method. The free end stretching method can reduce the NZ coefficient as compared with the fixed end stretching method using a tenter or the like.

  Thus, the obtained retardation film can be used as what provided the adhesion layer in the single side | surface or both surfaces. Further, it can be laminated with a polarizing plate through an adhesive layer to form an optical compensation polarizing plate. Furthermore, the retardation film obtained by the present invention has a function as a retardation film and a function as a polarizer protective film, and is laminated with a polarizer without interposing another film, so that an optical compensation polarizing plate is obtained. It can also be. Furthermore, the optical isotropic cellulose acylate film can be used in combination with other optical compensation layers in the same manner as described for the use of the optically isotropic cellulose acylate film.

  Such an optical compensation polarizing plate can be applied without substantially impairing the contrast of the liquid crystal cell even when the liquid crystal display device is viewed from an oblique direction, and thus can contribute to the improvement of the image quality of the liquid crystal display device.

  As described above, the object of the present invention is to provide a method for producing an optically isotropic cellulose acylate film using a plastic film support, and a method for producing a retardation film using an optically isotropic cellulose acylate film. There is.

  It should be noted that the specific embodiments such as the plastic type and thickness of the support, the film forming conditions of the film, the stretching conditions, and the following examples made in the section of the best mode for carrying out the invention are only the present invention. The technical contents of the present invention should be clarified and should not be construed in a narrow sense as being limited only to such specific examples, but those skilled in the art can make modifications within the spirit of the present invention and the scope of the appended claims. Can be implemented.

  Moreover, all the academic literatures and patent literatures described in this specification are incorporated herein by reference.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

  (Measuring method) The material characteristic values and the like described in the present specification are obtained by the following evaluation methods.

(1) Retardation and birefringence A 35 mm square sample was cut out from the center in the width direction of the film, and the thickness (d) was measured to the 0.1 μm unit using a thickness measuring machine manufactured by Anritsu Corporation. Furthermore, with an automatic birefringence meter KOBRA-WR manufactured by Oji Scientific Instruments, at a measurement wavelength of 586.7 nm, the phase difference in the film plane (R0) and the phase difference when tilted by 45 ° with the film slow axis as the rotation axis ( R45) was measured, and the thickness direction retardation (Rth) and NZ were calculated by the program attached to the apparatus. Values obtained by dividing the values of R0 and Rth by the film thickness were defined as in-plane birefringence (ΔNh) and birefringence in the thickness direction (ΔNv). These are expressed by the following relational expression.
ΔNh = R0 / d
ΔNv = Rth / d

(2) Residual solvent amount A sample of about 10 g was cut out from the vicinity of the center in the width direction of the film and the web, and after measuring the weight, the sample was heated in an oven at 150 ° C. for 60 minutes, and then the weight was measured again.
Residual solvent amount (%) = 100 × [(weight before heating) − (weight after heating)] / (weight before heating)

(3) Wetting tension Using a Wako Pure Chemicals wetting tension test mixture, the wetting tension was evaluated by the method described in JISK-6768.

(4) Surface roughness Arithmetic average roughness (Ra) and ten-point average roughness (Rz) were measured using a contact surface roughness meter (Surfcoder, SE-30C) manufactured by Kosaka Laboratory Ltd. and JIS-B0601. Based on the method described in (1994 edition), measurement was performed under the conditions of a stylus tip radius: 2 μm, a measurement pressure: 30 mg, a cut-off: 0.08 mm, and a measurement length: 1.0 mm.

(5) Evaluation of scratches A 50 cm square film was cut out from the central portion in the film width direction, a fluorescent lamp was reflected, and the presence or absence of scratches was visually confirmed. Using the flat illuminator (HF-SL-A312LC) manufactured by Dentsu Sangyo Co., Ltd., the presence or absence of a color change is confirmed by sandwiching the scratched part between two polarizing plates arranged in crossed Nicols. The number of confirmed scratches was counted.

(Preparation of cellulose acylate solution)
40 parts by weight of cellulose acetate propionate (cellulose ester CAP482-20 manufactured by Eastman Chemical) having an acetyl group substitution degree of 0.1, a propionyl group substitution degree of 2.6, and a number average molecular weight of 75,000, an acetyl group 60 parts by weight of diethyl acetate, cellulose acetate propionate (cellulose ester CAP482-0.5 manufactured by Eastman Chemical) having a substitution degree of 0.1, a substitution degree of propionyl group of 2.4, and a number average molecular weight of 25,000 A solution containing 2 parts by weight and 500 parts by weight of methylene chloride was prepared. This was designated as Solution 1.

(Examples 1-4 and Comparative Examples 1-3)
The solution 1 was supported by a comma coater on a support having a thickness of 125 μm and a width of 1650 mm in a state where a stress of 1.0 × 10 ⁶ N / m ² was applied in the flow direction in an environment of 23 ° C. and a humidity of 15%. The body and the comma roll were continuously cast so that the gap was 480 μm. The cast solution is continuously transported together with the support into a drying furnace adjusted to an ambient temperature of 35 ° C. by a far infrared heater, and then passes through the furnace in which the ambient temperature is gradually raised to 50 ° C. I let you. In addition, the drying oven continuously supplied dehumidified air having a dew point of −23 ° C. to keep the humidity at 15% or less and prevent whitening of the film due to moisture (dehumidification zone). Thereafter, it was further dried by passing it through a hot air oven whose temperature was raised stepwise from 60 ° C. to a maximum of 100 ° C. to obtain a web (hereinafter referred to as a hot air zone). In Comparative Example 1, since the support and the web peeled during the drying process, the tension could not be controlled, and the roll could not be wound.

  After the roll-shaped webs obtained in Examples 1 to 3 and Comparative Examples 1 and 3 were peeled from the support film, and further dried in a roll-suspended dryer with an atmospheric temperature of 100 ° C. for 15 minutes. Both ends of the film were cut off to obtain a long cellulose acylate film having a thickness of about 80 μm. Table 1 shows the types of the supports used, Table 2 shows the drying conditions and web characteristics, and Table 3 shows the characteristics of the obtained cellulose acylate film. In Comparative Example 2, the support and the web did not peel off, and the web could not be dried.

(Comparative Example 4 and Comparative Example 5)
An endless belt made of SUS304 having a width of 1650 mm was used as a support, and continuous casting was performed by a comma coater so that the gap between the support and the comma roll was 480 μm. The support and the cast solution are continuously transported into a drying furnace adjusted to an ambient temperature of 35 ° C. by a far infrared heater, and thereafter pass through the furnace in which the ambient temperature is gradually raised to 50 ° C. I let you. In addition, the drying oven continuously supplied dehumidified air having a dew point of −23 ° C. to keep the humidity at 15% or less and prevent whitening of the film due to moisture (dehumidification zone). After that, it was further dried by passing it through a hot air oven whose temperature was increased stepwise from 60 ° C. to 100 ° C. at maximum (hereinafter referred to as “hot air zone”). After peeling this web from the support, it was transported for 3 minutes in a pin tenter dryer having an atmospheric temperature of 80 ° C., and further dried for 13 minutes in a roll-suspended dryer having an atmospheric temperature of 100 ° C. The part was cut off to obtain a long cellulose acylate film having a thickness of about 80 μm. Table 2 shows the drying conditions and the characteristics of the web, and Table 3 shows the characteristics of the obtained cellulose acylate film.

  As shown in Tables 2 and 3, it can be seen that even when dried under the same temperature conditions depending on the type of the support, the peelability of the support and the web is different, and as a result, the optical isotropy of the cellulose acylate film is different. Further, it can be seen that when the surface roughness of the support is increased, a film with few scratches, which is a defect in appearance, can be obtained.

  In addition, when a plastic film support is used, the peelability between the support and the web can be controlled depending on the type of surface treatment, so even when the type of cellulose acylate and the solvent used are changed, the same production It has the advantage that it can be manufactured in a device.

(Example 5)
The cellulose acylate film obtained in Example 2 was transported in the film transport direction by a difference in peripheral speed between the inlet side nip roll and the outlet side nip roll using a roll stretching machine having a furnace length of 4 m and an atmospheric temperature of 144 ° C. in the furnace. The film was stretched uniaxially at 67% to obtain a retardation film.

(Example 6)
Using the cellulose acylate film obtained in Example 3, a retardation film was obtained in the same manner as in Example 5.

(Comparative Example 6)
A retardation film was obtained in the same manner as in Example 5 using the cellulose acylate film obtained in Comparative Example 5.
Table 4 shows the properties of the retardation films obtained in Examples 5 and 6 and Comparative Example 6.

  As shown in Example 5 and Example 6, by stretching an optically isotropic cellulose acylate film, a retardation film having small NZ and excellent viewing angle characteristics can be obtained.

Claims (4)

The in-plane birefringence (ΔNh) is 3.0 × 10 ⁻⁵ or less and the birefringence (ΔNv) in the thickness direction is 1.1 × 10 ⁻³ or less, which is characterized by satisfying all of the following A to C. A process for producing an optically isotropic cellulose acylate film.
A: The film is formed by the solvent cast method. B: The support used in the solvent cast method is a plastic film having a wet tension of 25 mN or more and 50 mN or less. C: The total acylation degree of cellulose acylate is 2. 4 or more and 3.0 or less   The method for producing an optically isotropic cellulose acylate film according to claim 1, wherein the support is a biaxially stretched polyester film.   The method for producing an optically isotropic cellulose acylate film according to claim 1 or 2, wherein the cellulose acylate film contains substantially no fine particles.   A method for producing a retardation film, comprising: stretching an optically isotropic cellulose acylate film produced by the method according to any one of claims 1 to 3 at least in a uniaxial direction.