JP4635399B2 - Method for producing cellulose ester film - Google Patents

Description

【表２】

表１および表２中、ＭＤは機械方向、ＴＤは機械方向に垂直な方向すなわち横方向、寸法変化率は１１０℃で３時間加熱したフィルムの加熱前後のフィルム寸法の変化率（％）をそれぞれ意味する。
【００４８】
フィルム張力の測定は生産工程における条件設定により行い、その設定値が実測値と等しいものとした。
【００４９】
弾性率の測定はＪＩＳ Ｋ ７１２７の方法に従って行った。
【００５０】
【発明の効果】
本発明のセルロースエステルフィルムの製造方法によれば、ウェブ中の溶媒残留率を１００％から１０％に低減させる乾燥初期段階において、その所要時間を６０秒以下とし、機械方向のフィルム張力を、溶媒残留率を１０％より更に低減させる乾燥後期段階における機械方向のフィルム張力よりも大きくすることによって、弾性率が高く、寸法変化率が小さくなり、フィルム厚が薄くても、しわや折れ、ゆがみが生じることのないセルロースエステルフィルムを提供し、寸法安定性に優れた偏光板を提供することができる。 [0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a cellulose ester film by a solution casting method in which a dope based on a cellulose ester is cast from a die onto a circulating metal support in an endless belt shape or a drum shape .
[0002]
[Prior art]
A liquid crystal image display device can be directly connected to an IC circuit with low voltage and low power consumption, and can be reduced in thickness, so that it is widely used as a display device for word processors, personal computers, and the like. The basic configuration of a liquid crystal image display device is one in which polarizing plates are provided on both sides of a liquid crystal cell. Since a polarizing plate allows only light of a polarization plane in a certain direction to pass, it plays an important role in visualizing changes in the orientation of the liquid crystal display device due to an electric field in a liquid crystal image display device. The performance of the display device is greatly affected.
[0003]
The protective film of the polarizing plate is made of a cellulose ester, and is bonded to one or both sides of a polarizing film made of a polyvinyl alcohol film uniaxially stretched by alkali saponification and stained with iodine through an adhesive such as polyvinyl alcohol. The
[0004]
Generally, a cellulose ester film is manufactured by the following solution casting film forming method. First, the cellulose ester is dissolved in a mixed solvent of a good solvent that dissolves the cellulose ester and a poor solvent that does not dissolve, and an ultraviolet absorber, a plasticizer, and fine particles for improving the slipperiness of the film are added to the cellulose triacetate solution A dope film is prepared by casting a dope from a die onto a rotating stainless steel endless support having a mirror-finished surface (referred to as “dope”). The obtained film is hereinafter referred to as “web”), dried on the support, then peeled off, sent to the drying process and further dried with warm air, and the resulting cellulose ester film was wound in the final process. It is wound up into a roll by a take-off machine.
[0005]
In recent years, the use of liquid crystal image display devices tends to increase in application and weight reduction (thinning), and a thinner film is also required for a cellulose ester film used for a polarizing plate. Conventional protective films for polarizing plates mainly have a thickness of about 80 μm, but are gradually becoming thinner.
[0006]
[Problems to be solved by the invention]
Thus, when the film thickness is reduced, the physical strength of the film is lowered, so that the dimensional change of the film is likely to occur, and the appearance as a polarizing plate is often impaired.
[0007]
Conventionally, such problems include the selection of the type and amount of plasticizer to be included in the film, the optimization of the moisture permeability of the film, the regulation of the film refractive index, the method of improving the film itself, the drying temperature conditions, etc. Methods to improve by optimizing production conditions were studied. However, in such a method, the effect is insufficient particularly in a thin film product, and further improvement is necessary. Further, if the obtained film is left as it is, wrinkles, creases, distortions may occur, and the appearance may be remarkably impaired. In particular, a polarizing plate using a thin film has been required to improve dimensional stability.
[0008]
In view of the above points, an object of the present invention is to provide a cellulose ester film that does not cause wrinkling, bending, or distortion even when the film thickness is thin, and to provide a polarizing plate that is excellent in dimensional stability. .
[0009]
[Means for Solving the Problems]
As a result of repeated investigations to solve the above problems, the present inventors have determined that the dimensional stability of the polarizing plate is the dimensional stability of a polarizing film made of a polyvinyl alcohol film and a protective film made of a cellulose ester film protecting the polarizing film. It has been found that it is greatly related to the relationship with dimensional stability and depends on the dimensional change rate and elastic modulus of the cellulose ester film.
[0010]
In particular, the dimensions of the final polarizing plate due to the balance between the force to change the size of the cellulose ester film and the elastic modulus of the cellulose ester film (the force to suppress the change in size) against the change in size of the polyvinyl alcohol film. It turns out that change is dominated.
[0011]
The present inventor also shows that the cellulose ester film is in the drying process at the time of manufacture, in the initial stage of drying (that is, when the solvent residual ratio is high and the film is soft), and in the direction perpendicular to the machine direction. That is, it has been found that by applying a large tension in the lateral direction, the elastic modulus is high and the dimensional change rate is small.
[0012]
The present invention has been completed based on such findings.
[0013]
That is, in the present invention, a dope containing a cellulose ester is cast on a rotating support, and the obtained web is peeled from the support and then dried to produce a cellulose ester film having a thickness of 20 to 60 μm. In the initial stage of drying in which the solvent residual ratio in the web is reduced from 100% to 10%, the required time is 60 seconds or less (preferably 30 seconds or more), the film tension in the machine direction is 10%. %, The film tension in the machine direction in the late stage of drying, which is further reduced by more than% , is a method for producing a cellulose ester film .
[0014]
DETAILED DESCRIPTION OF THE INVENTION
First, the raw material used in the method for producing a cellulose ester Ruff Irumu according to the present invention, will be described.
[0015]
The cellulose ester that forms the base of the dope is selected from the group of linter pulp, wood pulp, and kenaf pulp, and may be obtained by reacting cellulose with acetic anhydride, propionic anhydride, or butyric anhydride by a conventional method. Of these, cellulose triacetate, cellulose acetate propionate, cellulose acetate butyrate and cellulose acetate propionate butyrate having a substitution degree of all acyl groups with respect to hydroxyl groups of cellulose of 2.5 to 3.0 are preferable. The substitution degree of the acetyl group of the cellulose ester is preferably at least 1.5. The acyl group substitution degree of the cellulose ester can be measured according to ASTM D-817-91. The molecular weight of the cellulose ester is preferably 70,000 to 300,000, particularly 80,000 to 200,000 as the number average molecular weight in view of mechanical strength when formed into a film. Usually, cellulose ester is in the form of flakes after treatment such as water washing after the reaction, and is used in that shape, but by making the particle size 0.05-2.0 mm granular, it can accelerate dissolution in a solvent. it can.
[0016]
In the cellulose ester film, an ultraviolet absorber is usually contained. As the ultraviolet absorber, one that 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 that absorbs visible light having a wavelength of 400 nm or more as much as possible from the viewpoint of good liquid crystal display properties is preferable. . The transmittance at a wavelength of 370 nm is preferably 10% or less, more preferably 5% or less, and even more preferably 2% or less. Specific examples of the ultraviolet absorber include oxybenzophenone compounds, benzotriazole compounds, salicylic acid ester compounds, benzophenone compounds, cyanoacrylate compounds, nickel complex compounds, and the like. Is particularly preferred. Examples of commercially available benzotriazole ultraviolet absorbers include Tinuvin 109, Tinuvin 171, Tinuvin 326, Tinuvin 327, Tinuvin 328, etc. manufactured by Ciba Specialty Chemicals. Two or more kinds of ultraviolet absorbers may be used. As a method for adding the ultraviolet absorber to the dope, it may be added after dissolving the ultraviolet absorber in an organic solvent such as alcohol, methylene chloride, methyl acetate, dioxolane, or directly into the dope composition. For an inorganic powder that does not dissolve in an organic solvent, this is dispersed in a mixture of an organic solvent and a cellulose ester with a dissolver or a sand mill, and this dispersion is added to the dope. The usage-amount of a ultraviolet absorber may be 0.5-20 mass% with respect to a cellulose ester, Preferably it is 0.6-5.0 mass%, More preferably, it is 0.6-2.0 mass%. .
[0017]
In the cellulose ester film, a matting agent and other fine particles are contained for improving slipperiness, blocking resistance and scratch resistance. Specific examples of such fine particles include, for example, silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, calcium carbonate, kaolin, talc, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, Examples include inorganic fine particles such as calcium phosphate and crosslinked polymer fine particles. Of these, silicon dioxide is preferable because it can reduce the haze of the film. The average particle size of the secondary particles of the fine particles is preferably 0.01 to 1.0 μm, and the content thereof is preferably 0.005 to 0.3% by mass with respect to the cellulose ester. In many cases, fine particles such as silicon dioxide are surface-treated with an organic material, but such a material is preferable because it can reduce the haze of the film. Examples of the organic substance for surface treatment include halosilanes, alkoxysilanes, silazane, and siloxane. The larger the average particle size of the fine particles, the larger the mat effect, and the smaller the average particle size, the better the transparency. The average particle size of the primary particles of the fine particles is preferably 5 to 50 nm, more preferably 7 to 14 nm. The fine particles are usually present as aggregates in the cellulose ester film, and preferably form irregularities of 0.01 to 1.0 μm on the surface of the cellulose ester film. Examples of commercially available silicon dioxide fine particles are Aerosil 200, 200V, 300, R972, R972V, R974, R202, R812, OX50, TT600, etc., manufactured by Aerosil Co., Ltd., particularly AEROSIL 200V, R972, R972V, R974, R202 and R812 are preferable. Two or more matting agents may be used in combination.
[0018]
The cellulose ester film contains a plasticizer such as a phthalate ester or a phosphate ester. Phosphate ester plasticizers include triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl phosphate, and the like. Phthalate ester types include diethyl phthalate, dimethoxy Ethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, etc., glycolic acid esters include triacetin, tributyrin, butyl phthalyl butyl glycolate, ethyl phthalyl ethyl glycolate, methyl phthalyl ethyl glycol And butyl phthalyl butyl glycolate. Phthalate ester or glycolate ester plasticizers are particularly preferred. Two or more plasticizers may be used in combination.
[0019]
Next, a method for preparing a dope containing the above raw materials will be described. A dope is formed by dissolving a flaky cellulose ester in an organic solvent mainly containing a good solvent for the cellulose ester while stirring in a dissolving kettle. As a dissolution method, a method performed at normal pressure, a method performed under heating below the boiling point of the main solvent, a method performed by heating and pressurizing above the boiling point of the main solvent, JP-A-9-95544, JP-A-9-95557 and There are a method of carrying out by a cooling dissolution method as disclosed in JP-A-9-95538 and a method of carrying out at a high pressure as disclosed in JP-A-11-21379. After dissolution, the dope is filtered with a filter medium, defoamed, and sent to the next process with a pump. The concentration of the cellulose ester in the dope is preferably 10 to 40% by mass, more preferably 15 to 30% by mass. In order to include the organic polymer in the dope among the additives described above, the polymer may be added after dissolving the polymer in an organic solvent in advance, or may be added directly to the dope. In this case, the polymer should not be clouded or phase separated in the dope.
[0020]
As the organic solvent used for the preparation of the dope, a mixture of a good solvent and a poor solvent of cellulose ester is preferable in terms of production efficiency. A preferable range of the mixing ratio of the good solvent and the poor solvent is 70 to 98% by mass of the good solvent and 2 to 30% by mass of the poor solvent. A good solvent refers to a solvent that dissolves the cellulose ester used alone, and a poor solvent refers to a solvent that does not dissolve alone. Good solvents for cellulose esters include methyl acetate, ethyl acetate, amyl acetate, ethyl formate, acetone, cyclohexanone, methyl acetoacetate, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,4- Dioxane, 2,2,2-trifluoroethanol, 2,2,3,3-hexafluoro-1-propanol, 1,3-difluoro-2-propanol, 1,1,1,3,3,3-hexa Fluoro-2-methyl-2-propanol, 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,3,3-pentafluoro-1-propanol, nitroethane, 2- Pyrrolidone, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, methylene chloride, bromopropane, etc. And the like. Of these, methyl acetate, acetone or methylene chloride is preferred, but a non-chlorine organic solvent is more preferred in view of recent environmental problems. In addition, it is preferable to use a lower alcohol such as methanol, ethanol or butanol in combination with the above organic solvent because the solubility of the cellulose ester in the organic solvent can be improved or the dope viscosity can be reduced. Less ethanol is particularly preferred. Examples of the poor solvent include methanol, ethanol, n-butanol, cyclohexane, acetone, cyclohexanone, and the like.
[0021]
Next, in the cellulose ester film production process, the dope is cast on a circumferential metal endless support, the drying process on the endless support and the peeling process for peeling the web from the endless support and the web is dried. The process to do is described.
[0022]
The casting process is a process in which the dope is fed to a pressure die through a pressurized metering gear pump, and the dope is cast from the pressure die onto the endless support at the casting position. Other casting methods include a doctor blade method in which the film thickness of the cast web is adjusted with a blade, and a reverse roll coater method in which the thickness is adjusted with a reverse rotating roll. A pressure die is preferable because it is easy to make the film thickness uniform. The pressure die includes a coat hanger die and a T die, both of which are preferably used. In order to increase the film forming speed, two or more pressure dies may be provided on the endless support, and the dope amount may be divided and stacked. In adjusting the film thickness, it is preferable to control the dope concentration, the pumping amount, the slit gap of the die base, the die extrusion pressure, the speed of the endless support, and the like so as to obtain a desired thickness.
[0023]
In the drying process on the endless support, the web is heated on the endless support to evaporate the solvent. In order to evaporate the solvent, a method of sending warm air from the web side and the back side of the endless support, a method of heating with a heated liquid from the back side of the endless support, a method of heating from the web side and the back side of the endless support by radiant heat There is a method of combining these.
[0024]
In the peeling step, the organic solvent is evaporated on the endless support, and the web is peeled off by a peeling roll that assists peeling before the endless support goes around. After this step, the web is sent to a drying step. If the residual solvent amount of the web at the peeling point (the following formula) at the peeling point at which the web is peeled off from the endless support is too large, it will be difficult to peel off. May peel off, the web is peeled off with a residual solvent amount of 10 to 150% by mass. In the case of peeling at a time when the amount of residual solvent is large, if the web is too soft, the flatness at the time of peeling is impaired, or vertical streaks due to the peeling tension are likely to occur.
[0025]
The residual solvent amount can be expressed by the following formula.
[0026]
Residual solvent amount (% by mass) = {(MN) / N} × 100
Here, M is the mass of the web at an arbitrary point, and N is the mass when M is dried at 110 ° C. for 3 hours.
[0027]
In the present invention , when producing a cellulose ester film having a thickness of 20 to 60 μm by the solution casting film forming method, the required time is 60 in the initial stage of drying in which the solvent residual ratio in the web is reduced from 100% to 10%. The film tension in the machine direction is made greater than the film tension in the machine direction in the late stage of drying, which further reduces the solvent residual ratio from 10%.
[0028]
In preparing a cellulose ester film having a thickness of 20 to 60 μm by the solution casting film forming method, the required time is reduced in the drying stage in which the solvent residual ratio in the web is reduced from 20 to 10% to 10 to 5%. The film is stretched in the transverse direction within 30 seconds. Here, the draw ratio is preferably maximized at the initial stage of the stretching process and minimized at the later stage.
[0029]
After drying, the obtained cellulose ester film is wound into a roll by a winder in the final step.
[0030]
Cellulose ester Ruch Irumu obtained by the method of the present invention, there is no contamination excellent in optical isotropy, members of the liquid crystal display device, for example, protective film for polarizing plate, a retardation plate, a reflection plate, an optical compensation film It is used for a viewing angle improving film, an antiglare film, an antireflection film and an antistatic film. Especially, it is suitable for the protective film for polarizing plates.
[0031]
The polarizing plate can be produced by a conventionally known method. For example, a cellulose ester film is subjected to a surface saponification treatment with an aqueous sodium hydroxide solution, washed with water and dried to obtain a protective film for a polarizing plate having a surface saponified. Separately, a polyvinyl alcohol film is immersed in an aqueous solution containing iodine and boric acid and stretched to obtain a polarizing film. The protective film is bonded to one side or both sides of the polarizing film with an adhesive made of a completely saponified polyvinyl alcohol aqueous solution to form a polarizing plate.
[0032]
In the polarizing plate thus obtained, when the dimension change rate defined below of the polarizing film is a and the dimension change rate defined below of the protective film is b, b / a ≦ 0.20, preferably b / It is preferable that the relationship of a ≦ 0.05 is established, the thickness of the cellulose ester film is 20 to 60 μm, and the elastic modulus of the cellulose ester film is 3000 Mpa or more .
Dimensional change rate: Change rate of film size before and after heating of film heated at 110 ° C. for 3 hours (%)
Next, the present invention will be specifically described based on examples.
[0033]
Example 1
The following ingredients;
Cellulose triacetate with a degree of acetyl substitution of 2.88 (number average molecular weight 150,000) 100 parts by weight Plasticizer Triphenyl phosphate 10 parts by weight Solvent Methylene chloride 521 parts by weight Put 45 parts by weight of ethanol in a closed container and dissolve while stirring. A dope was prepared. This dope is cast from a die set to a predetermined slit width on an endless stainless steel endless support having a mirror-finished surface to obtain a dope film, which is dried on the support and then peeled off Then, it was sent to the drying process and further dried with warm air.
[0034]
In this drying step, in the initial drying stage in which the solvent residual ratio in the web is reduced from 100% to 10%, the required time is 50 seconds, the film tension in the machine direction is 200 N, and the solvent residual ratio is further higher than 10%. The film tension in the machine direction in the later stage of drying to be reduced was 130N. In the entire drying process, the film was stretched in the transverse direction by 103%.
[0035]
After drying, the obtained cellulose ester film was wound into a roll by a winder in the final step. Thus, a cellulose ester film having an elastic modulus in the machine direction of 3150 Mpa, an elastic modulus in the horizontal direction of 3050 Mpa, and a thickness of 40 μm was obtained.
[0036]
This cellulose ester film was surface saponified with a 2.5 mol / l sodium hydroxide aqueous solution at 40 ° C. for 60 seconds, washed with water for 3 minutes and dried to obtain a surface saponified protective film for a polarizing plate.
[0037]
Separately, 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 in the longitudinal direction at 50 ° C. to obtain a polarizing film.
[0038]
The polarizing plate was obtained by bonding the protective film to one surface of the polarizing film with a pressure-sensitive adhesive made of a completely saponified polyvinyl alcohol 5 mass% aqueous solution.
[0039]
The dimensional change rate (a) of the polarizing film made of a polyvinyl alcohol film was 3.00% in the machine direction and 1.00% in the lateral direction. The dimensional change rate (b) of the protective film was 0.14% in the machine direction and 0.04% in the horizontal direction. Therefore, the ratio of b / a was 0.047 in the machine direction and 0.040 in the lateral direction. The dimensional change rate of the obtained polarizing plate was 0.40% in the machine direction and 0.15% in the horizontal direction.
[0040]
Examples 2-3 and Comparative Examples 1-2
Except for changing the film tension in the machine direction in the initial stage of drying and the film tension in the machine direction in the latter stage of drying as shown in Table 1, the same operation as in Example 1 was performed to obtain a cellulose ester film. A polarizing plate was obtained in the same manner as in Example 1. Table 1 shows the measured elastic modulus and dimensional change rate of the cellulose ester film, the ratio of the dimensional change rate, the initial drying time, and the dimensional change rate of the polarizing plate.
[0041]
Reference example 1
The same film material components as in Example 1 were put in a closed container and dissolved with stirring to prepare a dope. This dope is cast from a die set to a predetermined slit width on an endless stainless steel endless support having a mirror-finished surface to obtain a dope film, which is dried on the support and then peeled off Then, it was sent to the drying process and further dried with warm air.
[0042]
In this drying step, in the drying stage in which the residual solvent ratio in the web was reduced from 20 to 10% to 10 to 5%, the required time was 24 seconds, and the film was stretched in the transverse direction within this time. The draw ratio was the maximum (105%) in the initial stage of the drawing process and the minimum (102%) in the late stage .
Thus, a roll of a cellulose ester film having an elastic modulus in the machine direction and the transverse direction of 3150 Mpa and a thickness of 40 μm was obtained.
[0043]
Using this cellulose ester film as a protective film, this was bonded to the same polarizing film as that of Example 1 to obtain a polarizing plate.
[0044]
The dimensional change rate (a) of the polarizing film made of a polyvinyl alcohol film was 3.00% in the machine direction and 1.00% in the lateral direction. The dimensional change rate (b) of the protective film was 0.14% in the machine direction and 0.04% in the horizontal direction. Therefore, the ratio of b / a was 0.047 in the machine direction and 0.040 in the lateral direction. The dimensional change rate of the obtained polarizing plate was 0.43% in the machine direction and 0.15% in the horizontal direction.
[0045]
Reference Examples 2-3, Comparative Reference Examples 1-2
Except for changing the draw ratio in the initial drawing stage and the draw ratio in the latter drawing stage as shown in Table 2, the same operation as in Reference Example 1 was performed to obtain a cellulose ester film, which was used in the same manner as in Example 1. A polarizing plate was obtained by the operation. Table 2 shows the measured elastic modulus and dimensional change rate of the cellulose ester film, the ratio of the dimensional change rate, the time of the drying stage, and the dimensional change rate of the polarizing plate.
[0046]
The results of Examples , Comparative Examples, Reference Examples, and Comparative Reference Examples are summarized in Tables 1 and 2.
[0047]
[Table 1]

[Table 2]

In Tables 1 and 2, MD is the machine direction, TD is the direction perpendicular to the machine direction, that is, the transverse direction, and the dimensional change rate is the change rate (%) of the film dimension before and after heating at 110 ° C. for 3 hours. means.
[0048]
The film tension was measured by setting conditions in the production process, and the set value was assumed to be equal to the actually measured value.
[0049]
The elastic modulus was measured according to the method of JIS K 7127.
[0050]
【The invention's effect】
According to the method for producing a cellulose ester film of the present invention , in the initial drying stage in which the solvent residual ratio in the web is reduced from 100% to 10%, the required time is set to 60 seconds or less, and the film tension in the machine direction is set to the solvent By making the residual rate higher than the film tension in the machine direction in the late stage of drying, which further reduces the residual rate by 10%, the elastic modulus is high, the dimensional change rate is small, and even if the film thickness is thin, wrinkles, creases, and distortions occur. A cellulose ester film that does not occur can be provided, and a polarizing plate excellent in dimensional stability can be provided .

Claims (1)

The dope containing the cellulose ester is cast on a rotating support, and the resulting web is peeled from the support and then dried to form a cellulose ester film having a thickness of 20 to 60 μm. In the initial stage of drying in which the residual rate is reduced from 100% to 10%, the required time is 60 seconds or less, and the film tension in the machine direction is the film in the machine direction in the late stage of drying in which the solvent residual rate is further reduced from 10%. The manufacturing method of the cellulose-ester film characterized by making it larger than tension | tensile_strength.