JP6665512B2 - Optical film manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing an optical film used for a liquid crystal display device or the like.

  Various optical films (for example, a transparent protective film for protecting a polarizing element of a polarizing plate) are arranged in an image display area of the liquid crystal display device.

  As such an optical film, for example, a resin film having excellent transparency such as a cellulose ester film is used. Such an optical film is often manufactured as a long resin film by, for example, a solution casting (film forming) method.

  The solution casting method is, specifically, a method in which a resin solution (dope) in which a transparent resin as a raw material resin is dissolved in a solvent is cast on a running support and dried to an extent that it can be peeled off. The separated web (also referred to as a dope film or a casting film) is separated from the support, and the separated web is dried, stretched, and the like while being conveyed by a conveyance roller to form a long resin film. is there.

  In recent years, the demand for thin films of these optical films has been increased due to thinning of liquid crystal display devices. As the film becomes thinner, the physical properties of the film decrease, and even in the above-mentioned solution casting method, the risk of tearing and breaking due to the stretching clip increases in the transport, particularly in the stretching step.

  On the other hand, the optical performance required for the film is constant regardless of the film thickness. However, as the film thickness becomes thinner, the optical performance becomes more difficult to be exhibited, so that it is necessary to further increase the stretching ratio in order to achieve the target. Therefore, the risk of breaking the film in the stretching step is further increased, and at present, improvement is required.

Japanese Patent No. 367284 JP 2008-119866 A

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a manufacturing method for obtaining a high-quality thin film without causing breakage in a stretching step or the like of a solution casting method.

  In order to solve the above problem, the present inventors adjust the casting draw ratio to be large, thereby promoting the orientation of the stretched resin in the initial stage, and consequently lowering the stretching ratio required in the stretching step. I found that it became possible.

  By the way, as a similar technique, a technique of mainly improving the flatness of a film by adjusting the casting draw ratio (support speed / discharge flow rate) in the solution casting method to 1 to 3 has been reported. (Patent Document 1 mentioned above). In addition, a technique has been reported in which a horizontal stage failure is suppressed by adjusting the ratio of the tenter speed / moving speed of the cooling drum (Patent Document 2).

  However, the technique described in Patent Document 1 does not target a thin film and further considers improvement in flatness, and is not a technique achieved from the viewpoint of promoting orientation. Further, the technique described in Patent Document 2 is also intended to increase the film thickness at the time of peeling and prevent breakage, and to promote the orientation of the resin in the initial stage, and to reduce the stretching ratio in a subsequent stretching step. Absent.

  As a result of intensive studies, the present inventors have found that the above-mentioned problems can be solved by a method for producing an optical film having the following structure, and have further studied based on such findings, thereby completing the present invention.

  In the method for producing an optical film according to one embodiment of the present invention, in a solution casting method, a dope, which is a raw material solution for an optical film, is cast on a support, and a web (cast film) is formed on the support. Forming an optical film by peeling the web from the support, wherein a casting draw ratio represented by the following formula (1) at the time of casting the dope from the casting die to the support is 3: -6.

Formula (1) Casting draw ratio = support speed / discharge flow rate According to such a configuration, orientation is promoted by stretching the web immediately after casting, and failures such as breakage and display unevenness in the stretching process during production are caused. It is considered that a high-quality thin-film optical film can be efficiently and stably obtained without generating the problem.

  In the above-mentioned manufacturing method, it is preferable that the final thickness of the obtained optical film is 5 to 40 μm. The production method of the present invention is more effective in producing such a thin film.

  In the above-mentioned production method, in a state where the dope is cast on the support, the ratio of the non-casting area where the web (cast film) does not exist on the support to the entire circumference of the support is 3%. Preferably, the casting start position and the web peeling position are adjusted so as to be 50% or less. Thereby, the web can be oriented further before the stretching step as compared with the conventional method, and the stretching ratio to be achieved in the stretching step can be suppressed accordingly. In turn, it is believed that this can further reduce the risk of film breakage.

  Further, it is preferable to adjust the casting start position and the web peeling position so that the ratio of the non-casting range to the entire length of one support is 30 to 45%. It is thought that it can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, a high-quality thin-film optical film can be manufactured, without causing breakage in a manufacturing process.

FIG. 1 is a schematic diagram showing a basic configuration of an optical film manufacturing apparatus by a solution casting method using an endless belt support. FIG. 2 is a schematic diagram showing a non-casting area on a support.

  Hereinafter, embodiments according to the present invention will be described, but the present invention is not limited thereto.

(Production method of optical film)
FIG. 1 is an explanatory diagram illustrating a schematic configuration of an optical film manufacturing apparatus used in the present embodiment. The method for producing an optical film of this embodiment uses a solution casting method in which a dope containing a polymer and a solvent is cast from a casting die on a running support, and then peeled off as a film.

  The outline of the solution casting method will be described with reference to FIG. First, a resin such as a cellulose ester is dissolved in a mixed solvent of a good solvent and a poor solvent in a dissolving tank 1, and additives such as a plasticizer and an ultraviolet absorber are added thereto to prepare a resin solution (dope). I do. The good solvent and the poor solvent will be described later.

  Next, the dope prepared in the melting vessel 1 is fed to the casting die 3 by a conduit through a pressurized metering gear pump 2 through a conduit, and is cast on a support 6 composed of a rotary drive stainless steel endless belt for infinite transfer. A dope is cast from the casting die 3 to a position, thereby forming a web 9 as a casting film on the support 6. At this time, a decompression chamber (decompression chamber) 4 may be arranged on the upstream side of the casting die 3 in the moving direction of the traveling support 6, and the decompression chamber 4 is used to move the decompression chamber 4 upstream of the casting die 3. The dope is cast from the casting die 3 onto the support 6 while the pressure in the space (especially on the upstream side of the casting ribbon from the casting die 3 to the support 6) is reduced.

  For the casting of the dope by the casting die 3, a doctor blade method for adjusting the film thickness of the cast web with a blade, a method using a reverse roll coater for adjusting the film thickness with a roll that rotates the cast web in a reverse direction, There is a method using a pressure die. Among them, a method using a pressure die is preferable because the slit shape of the base can be adjusted and the film thickness can be easily made uniform. The pressure die includes a coat hanger die and a T die, and any of them can be preferably used.

  The support 6 is held by a pair of front and rear drums 5.5 and a plurality of intermediate rolls (not shown). A driving device (not shown) for applying tension to the support 6 is provided on one or both of the drums 5, whereby the support 6 is used under tension.

  The width of the support 6 is preferably 1000 to 3000 mm, and the width of the film after winding is preferably 1000 to 2500 mm. Thus, a wide optical film for a liquid crystal display device can be manufactured by a metal support method.

  The belt temperature at the time of film formation in the case of using an endless belt as the support 6 is 0 ° C. to a temperature lower than the boiling point of the solvent in a general temperature range, but a temperature lower than the boiling point of the solvent having the lowest boiling point in the mixed solvent. And more preferably in the range of 5 ° C. to the boiling point of the solvent −5 ° C. At this time, it is necessary to control the ambient humidity to be higher than the dew point. In addition, it is preferable that the moving speed of the support 6 under the production conditions is 80 m / min to 200 m / min.

  The dope cast on the support 6 in this manner also increases the gel film strength (film strength) by promoting drying until peeling.

  On the support 6, the web 9 is dried and solidified until it has a film strength capable of being peeled off from the support 6 by the peeling roll 8.

  The web temperature at the time of peeling the web 9 from the support 6 is preferably 0 to 30 ° C. Further, immediately after the web 9 is separated from the support 6, the temperature once drops rapidly due to the evaporation of the solvent from the contact surface side with the support 6, and volatile components such as water vapor and solvent vapor in the atmosphere tend to condense. Therefore, the web temperature at the time of peeling is more preferably 5 to 30C.

  The web 9 formed by the dope cast on the support 6 is heated on the support 6 and the solvent is evaporated from the support 6 until the web can be peeled off by the peeling roll 8.

  To evaporate the solvent, there are a method of blowing air from the web side, a method of transmitting heat from the back of the support 6 by liquid, a method of transmitting heat from the front and back by radiant heat, and the like, which is used alone or in combination as appropriate. I just need.

  The peeling tension when the support 6 and the web 9 are peeled off by the peeling roll 8 is larger than the peeling force obtained by peeling force measurement such as JIS Z 0237. When the peeling force is equivalent to the peeling force obtained by the measurement method, the peeling position may be taken downstream, and therefore, the peeling force is set higher for stabilization. It has been confirmed that even if the film is formed with the same peeling tension in the process, when the peeling force according to the JIS measurement method is reduced, the variation in the cross Nicol transmittance (CNT) of the film is greatly reduced.

  The peeling tension value in the process is usually 20 to 400 N / m. However, in the case of an optical film that is made thinner than in the past, the amount of the residual solvent in the web 9 at the time of peeling is large, and the film is easily elongated in the transport direction. Therefore, the film is likely to shrink in the width direction, and when drying and shrinking overlap, the end is curled and folded, so that wrinkles are easily formed. For this reason, the peeling tension is preferably a minimum tension at which peeling can be performed to 300 N / m, and more preferably a minimum tension to 200 N / m.

  In FIG. 1, a belt-shaped support is illustrated as a support, but the support in the present embodiment is not limited to a belt-shaped support, and for example, a drum-shaped support may be used. .

  After the web 9 is dried and solidified on the support 6 until the web 9 has a releasable film strength, the web 9 is peeled off by a peeling roll 8, and then the web 9 is stretched in a tenter 10 in a stretching step.

  In the stretching step, a tenter method in which both sides of the web 9 are fixed with clips or the like and stretched is preferable as the film for a liquid crystal display device in order to improve the flatness and dimensional stability of the film.

  The residual solvent amount of the web 9 immediately before entering the tenter 10 in the stretching step is preferably 5 to 50% by mass, more preferably 10 to 35% by mass. The stretching ratio of the web in the tenter 10 in the stretching step is 3 to 100%, preferably 5 to 80%, and more preferably 5 to 60%.

  Moreover, the temperature of the hot air blown out from the hot air blowing slit in the tenter 10 is 70 to 200C, preferably 110 to 190C, and more preferably 115 to 185C.

  It is preferable to provide a drying device 11 after the tenter 10 in the stretching step. In the drying device 11, the web 9 is meandered by a plurality of transport rolls arranged in a staggered shape when viewed from the side, and the web 9 is dried during the meandering. Further, the film transport tension in the drying device 11 is affected by the physical properties of the dope, the amount of residual solvent in peeling and in the film transport step, the drying temperature, and the like. The film transport tension in drying is 10 to 400 N / m. And a width of 20 to 300 N / m is more preferable.

  The means for drying the web 9 is not particularly limited, and is generally performed using hot air, infrared rays, a heating roll, a microwave, or the like. Drying with hot air is preferred from the viewpoint of simplicity. For example, the web 9 can be dried by blowing the drying air 12 from the hot air inlet of the drying device 11 and discharging the exhaust air from the outlet of the drying device 11 to form the optical film F. The temperature of the drying air 12 is preferably from 40 to 160 ° C., and more preferably from 50 to 160 ° C. in order to improve flatness and dimensional stability.

  The steps from casting to transport and drying may be performed under an air atmosphere or under an inert gas atmosphere such as nitrogen gas. In this case, the drying atmosphere is, of course, carried out in consideration of the explosive limit concentration of the solvent.

  Before the transporting and drying step, the optical film F is applied to the end of the optical film F in order to prevent winding deviation and blocking (sticking between the films) in the winding step before being introduced into the winding step. It is preferable to form an embossed portion having many irregularities.

  Next, the film on which the forming process of the embossed portion has been completed is taken up by the take-up device 13 to obtain the original winding of the optical film F. By setting the residual solvent amount of the film at the end of drying to 0.5% by mass or less, preferably 0.1% by mass or less, a film having good dimensional stability can be obtained.

  The winding method of the film may be a commonly used winder, and there are methods for controlling the tension such as a constant torque method, a constant tension method, a taper tension method, and a program tension control method for constant internal stress. You can use them properly. The bonding of the film to the winding core (the winding core) may be performed using either a double-sided adhesive tape or a single-sided adhesive tape. The optical film F preferably has a width of 1000 to 2500 mm after winding.

  Hereinafter, the characteristic portions of the manufacturing method of the present embodiment will be described in more detail.

In the method for producing an optical film according to this embodiment, in the solution casting method as described above, a dope, which is a raw material solution for an optical film, is cast on a support, and a web (cast film) is formed on the support. ) And peeling the web from the support to produce an optical film, wherein the dope is cast from a casting die to the support by a casting draw ratio represented by the following formula (1): Is one of the major features.
Formula (1) Casting draw ratio = Support speed / Discharge flow rate

  According to such a configuration, it is considered that a high-quality thin film optical film can be efficiently and stably obtained without causing a failure such as breakage or display unevenness at the time of manufacturing.

  The inventors have scrutinized the results, especially when the film becomes thin, by setting the draw ratio in the specific range as described above, by orienting the web immediately after the draw in the transport direction, thereby improving the physical properties of the web. It is thought that the quality of the film can be improved. Therefore, even in the production of a thin film, it is considered that the stretching ratio in the stretching step can be reduced, the breakage of the film in the stretching step or the like can be prevented, and the optical performance of the film can be achieved.

  In the present embodiment, the dope is a resin solution serving as a raw material of a film, and a material that gels after casting on a support and has a hardness as a film is referred to as a web (cast film). That is, the film in the process of drying up to the finished optical film is referred to as a web. However, it should be noted that the boundary between the dome film formed by the dope, the web and the film is not exactly known. Further, as described above, the casting draw ratio is a ratio of the support speed to the discharge flow rate, that is, (support speed / discharge flow rate), and the discharge flow rate is determined by the die slit (hereinafter, die slit). Is the speed of the dope passing through the inside, and the speed of the support is the running speed of the support running endlessly.

  Further, in this embodiment, immediately after the drawing means immediately after the web is discharged from the die, and by adjusting the casting draw ratio as described above, the web is stretched until it lands on the support. It is believed that it can be oriented.

  A more preferred range of the casting draw ratio is 3 to 5. Thereby, it is considered that the above-described effects can be obtained more reliably.

  In a preferred embodiment, when casting the dope on the support, it is desirable that the web be cooled to further molecularly orient the dope.

  As a means for cooling such a web on the support, for example, a cooling device can be used, but in a state in which the dope is cast on the support, the web is cooled on the support with respect to the entire circumference of the support. It can also be implemented by adjusting the casting start position and the web peeling position so that the ratio of the non-casting range where the web (cast film) does not exist is 3 to 50%.

  More specifically, FIG. 2 is an enlarged schematic view showing a process in which the web 9 is peeled from the casting of the dope and transported to the tenter 10. 7 indicates the non-casting area, and if the non-casting area 7 is within the above range, the web can be oriented before the stretching step. In the above, the draw ratio to be achieved can be suppressed. In turn, it is believed that this can further reduce the risk of film breakage.

  More preferably, the non-casting range is desirably 30 to 45%.

  The non-casting range in the present embodiment can be controlled, for example, by adjusting the position of the casting die 3.

  The thickness (final thickness) of the optical film of the present embodiment after drying is preferably in the range of 5 to 40 μm as a finished film from the viewpoint of reducing the thickness of the liquid crystal display device. Here, the film thickness after drying refers to a film in which the residual solvent amount in the film is 0.5% by mass or less.

  According to the manufacturing method of the present embodiment, the above-described effects can be more exerted in manufacturing such a thin film, and a high-quality optical film can be provided.

  In the method for producing an optical film by the solution casting film forming method of the present embodiment, a resin solution (dope) containing a resin such as cellulose ester as a main material is added to a plasticizer, a retardation adjuster, an ultraviolet absorber, and fine particles (matte). ), At least one or more of low molecular weight substances, and a solvent. Hereinafter, these materials will be described.

  In the present embodiment, the resin used as the film material is not particularly limited, and a resin generally used in a solution casting method can be used. As a resin material for producing an optical film, preferable requirements include, for example, good adhesion to a polarizer, optical transparency, and the like. The visible light transmittance is 60% or more, preferably 80% or more, and particularly preferably 90% or more.

  There is no particular limitation as long as the resin forms an optical film having the above properties, and examples thereof include cellulose diacetate resin, cellulose triacetate resin, cellulose acetate butyrate resin, and cellulose acetate propionate resin. Polyester resins such as ester resins, polyester resins, polycarbonate resins, polyarylate resins, polysulfone (including polyethersulfone) resins, polyethylene terephthalate resins, polyethylene naphthalate resins, polyethylene resins, polypropylene resins, cellophane, poly Vinylidene chloride resin, polyvinyl alcohol resin, ethylene vinyl alcohol resin, syndiotactic polystyrene resin, polycarbonate resin, cycloolefin resin, Methylpentene resins, polyether ketone resins, polyether ketone imide resin, polyamide resin, fluorine resin, nylon resin, polymethylmethacrylate resin, and acrylic resin. Among them, cellulose ester resins, cycloolefin resins, polycarbonate resins, and polysulfone (including polyethersulfone) resins are preferred. In the present invention, cellulose ester resins are particularly preferred in terms of production, cost, transparency, and adhesion. It is preferably used from the viewpoint of properties and the like.

  The cellulose ester is a cellulose ester in which a hydroxyl group derived from cellulose is substituted with an acyl group or the like. Examples thereof include cellulose acylate such as cellulose acetate, cellulose triacetate, cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate propionate butyrate, and cellulose acetate having an aliphatic polyester graft side chain. Among them, cellulose triacetate, cellulose acetate propionate, and cellulose acetate having an aliphatic polyester graft side chain are preferable. The cellulose ester used in the method of the present invention may contain other substituents.

  As an example of the cellulose triacetate, the degree of substitution of the acetyl group is preferably 2.0 or more and 3.0 or less. By setting the degree of substitution within this range, good moldability can be obtained, and desired in-plane retardation (Ro) and thickness direction retardation (Rt) can be obtained. If the degree of substitution of the acetyl group is lower than this range, the wet heat resistance as a retardation film, particularly the dimensional stability under wet heat may be inferior, and if the degree of substitution is too large, the required retardation characteristics will not be exhibited. There are cases.

  The cellulose as a raw material of the cellulose ester used in the present embodiment is not particularly limited, and examples thereof include cotton linter, wood pulp, and kenaf. Further, cellulose esters obtained therefrom can be mixed and used at an arbitrary ratio.

  The number average molecular weight of the cellulose ester is preferably in the range of 20,000 to 300,000 because the resulting film has high mechanical strength. Further, 40,000 to 200,000 is preferable. Various additives can be blended with the cellulose ester.

  Further, the dope used in the present embodiment may contain fine particles as a matting agent in addition to the cellulose ester resin as described above.

  At this time, the fine particles to be used are appropriately selected according to the purpose of use, but are preferably fine particles that can scatter visible light by being contained in a transparent resin. The fine particles may be inorganic fine particles such as silicon oxide or organic fine particles such as an acrylic resin.

  Further, the fine particles represented by silicon oxide are preferably surface-treated with an organic substance in that the haze of the manufactured optical film can be reduced. Preferred organic substances for the surface treatment include, for example, halosilanes, alkoxysilanes, silazane, siloxane, and the like. The larger the average particle size of the fine particles, the greater the matting effect and the smaller the average particle size, the better the transparency. Therefore, the average particle size of the primary particles of the fine particles is preferably from 5 nm to 50 nm, more preferably from 7 nm to 14 nm. .

  Examples of the fine particles of silicon oxide include AEROSIL 200, 200V, 300, R972, R972V, R974, R202, R812, OX50, and TT600 manufactured by Aerosil Co., Ltd., and preferably AEROSIL 200, 200V, R972, and R972V. , R974, R202, R812 and the like.

  As the solvent used in the present embodiment, a solvent containing a good solvent for the transparent resin can be used, and a poor solvent may be contained as long as the transparent resin does not precipitate. Examples of the good solvent for the cellulose ester-based resin include organic halogen compounds such as methylene chloride. Examples of the poor solvent for the cellulose ester resin include an alcohol having 1 to 8 carbon atoms such as methanol.

  The dope used in the present embodiment may contain other components (additives) other than the transparent resin, the fine particles and the solvent as long as the effects of the present invention are not impaired. Examples of the additive include a plasticizer, an antioxidant, an ultraviolet absorber, a heat stabilizer, a conductive substance, a flame retardant, a lubricant, and a matting agent.

  The plasticizer that can be used in the present embodiment is not particularly limited. For example, a phosphate ester plasticizer, a phthalate ester plasticizer, a trimellitate ester plasticizer, a pyromellitic acid plasticizer, Rate plasticizers, citrate plasticizers, polyester plasticizers, and the like can be preferably used.

  Phosphate esters include, for example, triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl phosphate and the like.Phthalate esters include, for example, diethyl phthalate. , Dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, butyl benzyl phthalate, and the like. Examples of the trimellitic acid-based plasticizer include tributyl trimellitate, triphenyl trimellitate, and triethyltrimellitate. Examples of pyromellitic ester plasticizers such as melitate include tetrabutyl pyromellitate and tetraphenyl pyromellitate. And glycolate esters such as tetraethyl pyromellitate, for example, triacetin, tributyrin, ethylphthalylethyl glycolate, methylphthalylethyl glycolate, butylphthalylbutyl glycolate, etc. For example, triethyl citrate, tri-n-butyl citrate, acetyl triethyl citrate, acetyl tri-n-butyl citrate, acetyl tri-n- (2-ethylhexyl) citrate and the like can be preferably used.

  Examples of the polyester-based plasticizer include copolymers of dibasic acids such as aliphatic dibasic acids, alicyclic dibasic acids, and aromatic dibasic acids with glycols. Although it does not specifically limit as a basic acid, For example, adipic acid, sebacic acid, phthalic acid, terephthalic acid, 1,4-cyclohexyl dicarboxylic acid, etc. can be used.

  As the glycol, for example, ethylene glycol, diethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 1,4-butylene glycol, 1,3-butylene glycol, 1,2-butylene glycol, or the like is used. I can do it. These dibasic acids and glycols may be used alone or in combination of two or more. The molecular weight of the polyester is preferably in the range of 500 to 2,000 as a weight average molecular weight from the viewpoint of compatibility with the cellulose resin.

  Further, in the cellulose ester film of the present embodiment, it is preferable to use an ultraviolet absorber for protecting the liquid crystal material and the like, and the ultraviolet absorber absorbs ultraviolet light having a wavelength of 370 nm or less from the viewpoint of preventing deterioration of the liquid crystal. From the standpoint of excellent liquid crystal display properties and excellent performance, absorption of visible light having a wavelength of 400 nm or more as little as possible is preferably used.

  In the present embodiment, a preferable polarizing plate is provided without deteriorating the durability of the polarizing plate by reducing the transmittance at a wavelength of 370 nm to 10% or less in a cellulose ester film having a thickness of 20 μm to 200 μm. Can be. The transmittance at a wavelength of 370 nm is more preferably 5% or less, and particularly preferably 2% or less.

  Further, a solution of the cellulose ester-based resin is obtained by mixing the above components. Further, the obtained solution of the cellulose ester-based resin is preferably filtered using a suitable filter material such as filter paper.

  The optical film manufactured by the manufacturing method of the present embodiment is a functional film used for various displays such as a liquid crystal display, a plasma display, and an organic EL display, particularly a liquid crystal display, and includes a polarizing plate protective film, a retardation film, and an anti-reflection film. It includes a film, a brightness enhancement film, and an optical compensation film for widening the viewing angle.

  By using the protective film for a polarizing plate composed of the optical film of the present embodiment, it is possible to provide a polarizing plate which is thinner and has excellent durability, dimensional stability, and optical isotropy.

  Further, by using a polarizing plate provided with the optical film of the present embodiment, a high-quality liquid crystal display device or the like can be realized. In particular, since the optical film of the present embodiment is a thin film, it is preferably used for applications such as smartphones and tablets.

  Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the examples.

[Example 1]
An optical film was manufactured by the following method.

(Preparation of dope)
100 parts by mass of cellulose acetate propionate (acetyl group substitution degree + propionyl group substitution degree = 2.45, number average molecular weight (Mn) = 60000, weight average molecular weight (Mw) = 180000, Mw / Mn = 3.00)
Triphenyl phosphate 8 parts by mass Ethylphthalylethyl glycolate 2 parts by mass Methylene chloride 360 parts by mass Ethanol 60 parts by mass Tinuvin 109 (manufactured by Ciba Japan KK) 0.5 parts by mass Tinuvin 171 (manufactured by Ciba Japan KK) 0.5 parts by mass Aerosil 972V (manufactured by Nippon Aerosil Co., Ltd.) 0.2 parts by mass The above-mentioned material of the dope composition 1 was charged into a closed vessel, heated, stirred, completely dissolved, and filtered. After filtration by a filter press, the mixture was passed through a metal sintered filter (captured particle size = 10 microns). The silicon dioxide fine particles (Aerosil R972V) were added after being dispersed in ethanol.

(Manufacture of optical films)
A cellulose acetate propionate film was produced using the solution casting film forming apparatus shown in FIG. As the support (6) for casting the dope, an endless belt made of SUS316 and having a three-dimensional surface roughness (Ra) of 1.0 nm on average by a scanning atomic force microscope (AFM) was used. Was used.

  The filtered dope was uniformly cast on a SUS316 endless belt support (6) at a dope temperature of 35 ° C. and a temperature of 20 ° C. by a casting die (3) composed of a coat hanger die. The distance between the support and the tip of the die was set to 1 mm. When forming the web (9) on the support (6), the web (9) was opened downward as a means for reducing the pressure from the upstream of the casting so that the web (9) was formed in close contact with the support (6). A decompression chamber (4) was provided (average pressure of the decompression chamber-400 Pa).

  In Example 1, the discharge flow rate from the die was 23 m / min, the support speed was 80 m / min, and the casting draw ratio was 3.5. Further, the ratio of the non-casting range was 2%.

  The web (9) thus formed on the support (6) is dried by a constant drying air at a temperature of 25 ° C. while being conveyed on the support (6), and then peeled from the support (6). The film is peeled off by the roll (8) and then stretched 1.28 times (28%) in the width direction in a 100 ° C. atmosphere at a residual solvent amount of 10% by a tenter (10). The drying was completed by a drying device (11) at 125 ° C. while transporting the roll, and the film was wound by a winding device (13).

  The final thickness of the obtained cellulose triacetate propionate film (F) was 20 μm, the film width was 1300 mm, and the roll length of the film was 4000 m.

[Examples 2 to 5 and Comparative Examples 1 to 3]
A cellulose acetate propionate film was obtained in the same manner as in Example 1 except that the discharge flow rate, the support speed, the casting draw ratio, the non-casting range, the stretching ratio, and the final film thickness were adjusted as shown in Table 2. Was. The non-casting range was adjusted by changing the die position.

(Evaluation)
The following evaluation tests were performed on the optical films (Examples 1 to 5 and Comparative Examples 1 to 3) obtained as described above.

[Process suitability]
The process was evaluated according to the following criteria:
：: A film can be produced without any problem and can be wound up with a winder.

[Display unevenness]
The produced film sample was sandwiched between a polarizing plate in a crossed Nicols state, and the crossed Nicols were shifted until the light passed under the transmitted light, and the density of the transmitted light was visually observed.

  In addition, the polarizing plate produced as follows was used.

(Preparation of polarizing film)
In order to produce a liquid crystal display using the cellulose ester films produced in the above Examples and Comparative Examples, first, a polarizing film was produced. That is, a polyvinyl alcohol film having a thickness of 120 μm was uniaxially stretched at a temperature of 110 ° C. and a stretching magnification of 5 times. This was immersed in an aqueous solution consisting of 0.075 g of iodine, 5 g of potassium iodide and 100 g of water for 60 seconds, and then immersed in a 68 ° C. aqueous solution consisting of 6 g of potassium iodide, 7.5 g of boric acid and 100 g of water. This was washed with water and dried to obtain a polarizing film.

(Preparation of polarizing plate)
Then, according to the following steps 1 to 5, the Konica Minolta KC4UY 40 μm cellulose triacetate film (polarizing plate protective film: T-1) and the cellulose acetate propionate film prepared in each example were applied to the polarizing film described above. (Retardation film: T-2) was laminated to produce a polarizing plate.

  Step 1: Dipped in a 2 mol / L sodium hydroxide solution at 60 ° C. for 90 seconds, then washed with water and dried to obtain a polarizing plate protective film in which the side to be bonded to the polarizing film was saponified, and a retardation film. .

  Step 2: The polarizing film was immersed in a polyvinyl alcohol adhesive tank having a solid content of 2% by mass for 1 to 2 seconds.

  Step 3: Excess adhesive adhered to the polarizing film in step 2 was lightly wiped off, and the polarizing plate protective film and the retardation film treated in step 1 were laminated and arranged on both sides of the polarizing film.

Step 4: The retardation film, the polarizing film, and the back-side polarizing plate protective film laminated in step 3 were bonded together at a pressure of 20 N / cm ² to 30 N / cm ^{2 at} a transport speed of about 2 m / min.

  Step 5: The sample obtained by laminating the polarizing film, the retardation film and the polarizing plate protective film prepared in Step 4 was dried in a dryer at 80 ° C. for 5 minutes to prepare a polarizing plate.

  Samples in which unevenness was not confirmed in the evaluation of the polarizing plate were confirmed by panel evaluation prepared as follows.

The polarizing plate on the viewing side, which has been previously bonded, of the 40-inch display KLV-40J3000 made by Sony, which is a VA mode liquid crystal display device, is peeled off, and the above-prepared polarizing plate is aligned with the liquid crystal cell so that the absorption axis of the polarizing plate matches. To produce a VA mode liquid crystal display device. At that time, they were bonded so that the retardation film T-2 was on the liquid crystal cell side. The evaluation criteria for unevenness are as follows:
◎: No unevenness was observed even in the evaluation by the panel. ：: No shading of transmitted light was observed. △: Shading of transmitted light was slightly observed. ×: Shading of transmitted light was observed.

  Table 1 shows the results.

[Discussion]
As can be seen from Table 1, in Examples 1 to 5 obtained by the production method of the present invention, a phase difference can be obtained even if the stretching ratio is lowered, and thus the process suitability is satisfied without breaking during the production process. Was. Further, the optical films obtained in these examples were all of high quality with no display unevenness.

  In particular, in Example 4 where the ratio of the non-casting range was relatively large, a very high quality film could be obtained. On the other hand, in Example 1 in which the ratio of the non-casting range was relatively small and in Example 5 in which the final film thickness was increased, the result of the display unevenness was slightly inferior.

  In Comparative Example 1 in which the casting draw ratio did not satisfy the range of the present invention, a phase difference could be obtained by increasing the stretching ratio, but a break occurred during stretching. Further, when the draw ratio was lowered at the same casting draw ratio as in Comparative Example 1 (Comparative Example 2), a sufficient phase difference could not be obtained and display unevenness occurred. Further, in Comparative Example 3 in which the casting draw ratio was larger than the range of the present invention, lip coating adhered at the time of casting, and a die streak was generated, resulting in a judgment.

1: dissolving pot 2: pump 3: casting die 4: decompression chamber 5: front and rear winding drum 6: endless belt for casting (support)
7: non-casting range 8: peeling roll 9: web 10: tenter 11: roll transport dryer 12: warm air (dry air)
13: Winding machine F: Film

Claims (3)

In the solution casting method, a dope, which is a raw material solution for an optical film, is cast on a support, a web (cast film) is formed on the support, and the web is peeled from the support to form an optical film. In the method for producing
When the dope is cast from a casting die onto a support, the casting draw ratio represented by the following formula (1) is 3 to 6, and the final thickness of the obtained optical film is 5 to 40 μm. A method for producing an optical film, characterized by:
Formula (1) Casting draw ratio = Support speed / Discharge flow rate In the state where the dope is cast on the support, the ratio of the non-casting area where the web (cast film) does not exist on the support to the entire circumference of the support is 3 to 50%. The method for producing an optical film according to claim 1, wherein the casting start position and the web peeling position are adjusted. 3. The optical film according to claim 2 , wherein the casting start position and the web peeling position are adjusted so that the ratio of the non-casting range to the entire length of one support is 30 to 45%. Production method.

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