JP2004142228A - Method for producing polyvinyl alcohol film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a polyvinyl alcohol film which is excellent in uniformity of thickness and optical properties and good in appearance, has excellent effects in long-run film making properties, and is useful as a rolled polarizing film. <P>SOLUTION: In the method, when a polyvinyl alcohol resin aqueous solution is cast on a drum type roll to make the film, the roll of 25-50 mN/m in wet surface tension is used. To adjust the wet surface tension, the mirror-finished chromium surface of the roll is treated with an acid aqueous solution of pH 1-3, or an anionic surfactant is added in advance into the polyvinyl alcohol resin aqueous solution. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a polyvinyl alcohol-based film having excellent film thickness and optical performance uniformity and excellent film appearance, and also having a good long-run film-forming property. The present invention relates to a method for producing a polyvinyl alcohol-based film useful as a raw film for obtaining a polarizing film having excellent appearance.
[0002]
[Prior art]
Conventionally, a polyvinyl alcohol-based film is prepared by dissolving a polyvinyl alcohol-based resin in a solvent and defoaming to prepare a stock solution, and then using a metal heating roll or the like to form a film formed by a solution casting method (casting method). It is manufactured by drying.
The polyvinyl alcohol-based film thus obtained is used in many applications as a film having excellent shape stability, and one of its useful applications is an optical film, particularly a polarizing film.
Such a polarizing film is a film obtained by uniaxially stretching and dyeing the polyvinyl alcohol-based film, and is used as a basic component of a liquid crystal display. In recent years, applications have been expanded to devices that require high quality and high reliability, especially large-screen liquid crystal displays, etc., and the accompanying improvements in the required physical properties, such as large size and in-plane uniformity, have been made. Is strongly required.
[0003]
In such a case, when manufacturing a polarizing film using a polyvinyl alcohol-based film as a raw film, in order to have excellent optical characteristics, for example, a polyvinyl alcohol-based film can be stretched uniformly, or the film thickness and inner surface can be adjusted. It is necessary that the retardation (Rd) value be uniform and that the appearance of the film be good.
In particular, as a method for obtaining the thickness uniformity of a polyvinyl alcohol-based film, a method has been proposed in which a stock solution for film formation is cast on a metal surface having a surface roughness of 3S or less to form a film (for example, Patent Reference 1).
[Patent Document 1]
JP 2001-315138 A
[0004]
[Problems to be solved by the invention]
However, in the technology disclosed in the publication, although the film thickness uniformity can be obtained in terms of physical properties, the film appearance is not yet satisfactory, and further improvement is required to obtain higher quality and high optical characteristics. Is being done.
Also, in terms of production, in order to improve productivity, it is also important to have a long-run film-forming property such that film formation can be continued continuously for at least 7 days.
[0005]
Under such circumstances, in the present invention, under such a background, the uniformity of the film thickness and optical performance and the film appearance are extremely excellent, and the long-run film-forming property of constantly producing a high-quality film stably is also required. An object of the present invention is to provide a method for producing an excellent polyvinyl alcohol-based film, particularly a method for producing a polyvinyl alcohol-based film useful as a raw film of a polarizing film.
[0006]
[Means for Solving the Problems]
However, as a result of diligent research conducted by the present inventors to solve the above-described problems, when the polyvinyl alcohol-based resin aqueous solution is cast on a drum-type roll to form a film, the wet tension on the surface of the drum-type roll is 25 to 50 mN. / M is a method for producing a polyvinyl alcohol-based film using a drum-type roll, is capable of producing a film having excellent uniformity of film thickness and optical properties, high quality film appearance properties, and long run film formation. The present invention was also found to be excellent in properties, and the present invention was completed.
Furthermore, in the present invention, when the drum-type roll has a mirror-finished chromium surface, and the surface is treated with an acid aqueous solution having a pH of 1 to 3, and the wet tension is specified in the above-described range, the drum-type roll is particularly excellent in film quality. Uniformity of thickness and optical characteristics and appearance characteristics of a film can be exhibited.
[0007]
In the present invention, the film thickness is determined by using an aqueous solution of a polyvinyl alcohol-based resin to which an anionic surfactant is added so that the film is formed under the condition that the wet tension of the drum-type roll surface is in the above range. In addition to uniformity and film appearance characteristics, it is possible to exert an effect that is particularly excellent in long-run film forming properties.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described specifically.
First, the polyvinyl alcohol-based resin used in producing the film of the present invention is usually produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate, but the present invention is not necessarily limited thereto. A small amount of unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. A modified polyvinyl alcohol-based resin containing a component copolymerizable with vinyl acetate may be used.
[0009]
In addition to the above modification, a polyvinyl alcohol-based resin containing a silyl group may be used. Polyvinyl alcohol may be post-modified with a silylating agent, or a silyl group-containing olefinically unsaturated monomer may be used in combination with vinyl acetate. Examples include a method of saponifying a copolymer obtained by polymerization. Examples of the silyl group-containing olefinically unsaturated monomer include vinyl silane and (meth) acrylamide-alkyl silane.
[0010]
The viscosity-average polymerization degree of the polyvinyl alcohol-based resin is not particularly limited, but is preferably 1,000 to 5,000, particularly preferably 1,200 to 5,000, and more preferably 1,400 to 4,500. If the viscosity average degree of polymerization is less than 1,000, sufficient optical performance cannot be obtained when an optical film is formed, and if it exceeds 5,000, stretching becomes difficult and industrial production becomes difficult, which is not preferable.
The viscosity average degree of polymerization is measured according to JIS K 6726.
[0011]
Further, the degree of saponification of the polyvinyl alcohol-based resin is preferably 80 mol% or more, particularly preferably 85 to 100 mol%, more preferably 98 to 100 mol%. If the degree of saponification is less than 80 mol%, sufficient optical performance cannot be obtained in the case of forming an optical film, which is not preferable.
[0012]
The polyvinyl alcohol-based resin may include one or more commonly used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, and polyethylene glycol as necessary. 30% by weight or less, preferably 3 to 25% by weight, more preferably 5 to 20% by weight. If the amount of the plasticizer exceeds 30% by weight, the film strength is poor, which is not preferable.
[0013]
Also, more preferably, in order to improve the releasability of the film, one or more of various release agents are used in an amount of 5% by weight or less, preferably 0.001 to 3% by weight, more preferably, based on the polyvinyl alcohol-based resin. It is also possible to contain 0.001 to 2% by weight. When the amount of the release agent exceeds 5% by weight, film surface appearance defects and blocking between films occur, which is not preferable.
[0014]
Thus, in the present invention, a polyvinyl alcohol-based film is produced by preparing an aqueous solution of a polyvinyl alcohol-based resin using the above-mentioned polyvinyl alcohol-based resin, casting the aqueous solution on a drum-type roll, forming a film, and drying.
[0015]
When preparing an aqueous solution of a polyvinyl alcohol-based resin, a solvent such as water, dimethylsulfoxide (DMSO), N-methylpyrrolidone, glycerin, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, or the like is used. Amine such as alcohol, ethylenediamine, diethylenetriamine and the like and mixtures thereof are used.
The practical concentration of the polyvinyl alcohol-based resin in the aqueous solution of the polyvinyl alcohol-based resin is 5 to 50% by weight.
[0016]
Next, the polyvinyl alcohol-based resin aqueous solution is cast from a T-shaped slit die onto a drum-type roll to form a film.
The material of the drum-type roll is not particularly limited, but stainless steel is usually preferably used, and the surface of the roll is preferably plated with metal to prevent damage. As the type of metal plating, for example, chromium plating, nickel plating, zinc plating and the like are suitably used, and they can be used alone or in combination of two or more kinds. It is preferable that the outermost surface is chromium plating from the viewpoint of properties.
[0017]
In the present invention, it is necessary to adjust the wetting tension of the drum-type roll surface to 25 to 50 mN / m during casting film formation, and if the wetting tension is less than 25 mN / m, cissing occurs in the aqueous polyvinyl alcohol-based resin solution. The appearance of the film becomes poor. On the other hand, if it exceeds 50 mN / m, the releasability of the film from the drum is reduced, and uneven and excessive stretching is applied, which adversely affects the optical performance. The preferred range of the wetting tension on the drum-type roll surface is 30 to 45 mN / m.
In the present invention, the wetting tension is measured according to JIS K6768.
[0018]
The means for satisfying the wetting tension of the drum-type roll surface is not particularly limited, but a drum having a mirror-finished chromium surface is used, and the surface is treated with an acid aqueous solution having a pH of 1 to 3 (measured at 20 ° C., the same applies hereinafter). It should be processed.
Examples of the acid aqueous solution having a pH of 1 to 3 include acetic acid, sulfuric acid, hydrochloric acid, and nitric acid. Among them, an aqueous solution of acetic acid and nitric acid is particularly preferable. If the pH is outside the above range, it is difficult to obtain a wetting tension in the above range, which is not preferable.
[0019]
Further, in the present invention, as a method of adjusting the wetting tension in addition to treating the roll surface with an acid aqueous solution as described above, so that the film is formed under the condition that the surface wetting tension is in the above range, a polyvinyl alcohol-based resin aqueous solution is used. It is also possible to add an anionic surfactant in advance and cast the aqueous solution onto a drum-type roll to form a film. In this case, in addition to uniformity of film thickness and film appearance characteristics, long-run film forming properties can be improved. It is useful in that it improves more.
[0020]
The amount of the anionic surfactant to be added is not particularly limited, but is preferably 0.001 to 5 parts by weight, particularly 0.01 to 2 parts by weight, more preferably 0.01 to 2 parts by weight based on 100 parts by weight of the polyvinyl alcohol-based resin. It is preferably 1 part by weight. If the amount is less than 0.001 part by weight, it is difficult to adjust the wetting tension. If the amount exceeds 5 parts by weight, a problem of blocking occurs in the film, which is not preferable.
[0021]
Examples of anionic surfactants include fatty acid soaps, N-acyl amino acids and salts thereof, polyoxyethylene alkyl ether carboxylate, carboxylate types such as acylated peptides, alkylbenzene sulfonates, alkyl naphthalene sulfonates, and naphthalene. Sulfonic acid salt formalin polycondensate, melamine sulfonic acid salt formalin condensate, dialkyl sulfosuccinate ester salt, alkyl sulfosuccinate disalt, polyoxyethylene alkyl sulfosuccinate disalt, alkyl sulfoacetate, α-olefin sulfonate , Sulfonate type such as N-acylmethyltaurine salt, dimethyl-5-sulfoisophthalate sodium salt, sulfated oil, higher alkyl sulfate ester salt, polyoxyethylene alkyl ether sulfate salt, higher alcohol ethoxy sulfate , Sulfate type salts such as polyoxyethylene alkyl phenyl ether sulfate and monoglycol sulfate, and phosphate ester types such as polyoxyethylene alkyl ether phosphate, polyoxyethylene alkyl phenyl ether phosphate and alkyl phosphate And the like.
In carrying out the method of the present invention, it is of course possible to form a film by using both methods described above.
[0022]
The temperature of the drum type roll during film formation is practically 50 to 120 ° C., and the film is peeled off from the roll when the water content of the film reaches about 5 to 30% by weight. Subsequently, drying is continued using a single roll or a multi-stage roll, and after drying, an unstretched polyvinyl alcohol-based film is formed. After drying as necessary, heat treatment and humidity control are performed, and the polyvinyl alcohol-based film of the present invention is obtained.
[0023]
The polyvinyl alcohol-based film obtained above is usefully used as a raw film for optical use, particularly for a polarizing film.
Hereinafter, a method for manufacturing a polarizing film will be described.
[0024]
As a method for producing a polarizing film, such a polyvinyl alcohol-based film is stretched and dyed by immersing in a solution of iodine or a dichroic dye, or stretched and dyed simultaneously, or dyed with iodine or a dichroic dye. After stretching, a method of treating with a boron compound may be mentioned. Further, there is a method of stretching in a solution of a boron compound after dyeing, and the like can be appropriately selected and used.
[0025]
The thickness of the polyvinyl alcohol-based film used for the polarizing film is preferably from 30 to 100 μm, more preferably from 40 to 90 μm, particularly preferably from 50 to 80 μm. The thickness accuracy is undesirably reduced.
[0026]
In the stretching and dyeing and the boron compound treatment of the polyvinyl alcohol-based film (unstretched film), the stretching and the dyeing and the boron compound treatment may be performed separately or simultaneously. However, in the present invention, the dyeing step, the boron compound treatment It is desirable to carry out uniaxial stretching during at least one of the steps.
[0027]
The stretching is preferably performed 3 to 10 times, preferably 3.5 to 6 times in the uniaxial direction. At this time, slight stretching (extending to the extent of preventing shrinkage in the width direction or more) may also be performed in the direction perpendicular to the above. It is desirable to select the temperature condition during stretching from 40 to 170 ° C. Further, the stretching ratio may be finally set in the above range, and the stretching operation may be performed not only in one stage but also in any range of the manufacturing process.
[0028]
Dyeing to the film is performed by bringing a liquid containing iodine or a dichroic dye into contact with the film.
Usually, an aqueous solution of iodine-potassium iodide is used, the concentration of iodine is 0.1 to 20 g / l, the concentration of potassium iodide is 10 to 70 g / l, and the weight ratio of potassium iodide / iodine is 10 to 100. Is appropriate. The dyeing time is practically about 30 to 500 seconds. The temperature of the treatment bath is preferably 5 to 60C. A small amount of an organic solvent compatible with water other than the water solvent may be contained.
As the contact means, any means such as immersion, coating, spraying, etc. can be applied.
[0029]
The dyed film is then treated with a boron compound. Boric acid and borax are practical as boron compounds. The boron compound is used in the form of an aqueous solution or a mixed solution of water and an organic solvent at a concentration of about 0.3 to 2 mol / l, and it is practically desirable that a small amount of potassium iodide coexist in the liquid.
The treatment method is preferably an immersion method, but of course, a coating method and a spraying method can also be performed. The temperature at the time of the treatment is preferably about 40 to 70 ° C., and the treatment time is preferably about 2 to 20 minutes. If necessary, a stretching operation may be performed during the treatment.
[0030]
The polarizing film thus obtained can be used by laminating and adhering an optically isotropic polymer film or sheet as a protective film on one or both surfaces thereof.
Examples of such a protective film include films or sheets of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, polystyrene, polyether sulfone, polyarylene esters, poly-4-methylpentene, polyphenylene oxide, and the like.
[0031]
In addition, a curable resin such as a urethane resin, an acrylic resin, or a urea resin may be applied to one or both surfaces of the polarizing film instead of the protective film for the purpose of thinning, and the polarizing film may be laminated.
[0032]
Such a polarizing film (or a laminate of a protective film or a curable resin on at least one surface thereof) may have a transparent pressure-sensitive adhesive layer formed on one surface thereof by a generally known method, if necessary. In some cases, it is put to practical use. Examples of the pressure-sensitive adhesive layer include acrylates such as butyl acrylate, ethyl acrylate, methyl acrylate, 2-ethylhexyl acrylate, and α-monoolefin carboxylic acids such as acrylic acid, maleic acid, itaconic acid, and methacrylic acid. Those mainly composed of copolymers with acids, crotonic acid, etc. (including those to which vinyl monomers such as acrylonitrile, vinyl acetate, and styrene are added) do not inhibit the polarizing properties of the polarizing film. However, the present invention is not limited to this, and any transparent pressure-sensitive adhesive can be used. For example, a polyvinyl ether-based or rubber-based adhesive may be used.
[0033]
Further, it is also possible to provide various functional layers on one surface (the surface on which the pressure-sensitive adhesive is not provided) of the polarizing plate (the one provided with the pressure-sensitive adhesive). Examples include an anti-glare layer, a hard coat layer, an anti-reflection layer, a half-reflection layer, a reflection layer, a phosphorescent layer, a diffusion layer, an electroluminescence layer, a viewing angle widening layer, a brightness enhancement layer, and the like. It is also possible to, for example, an anti-glare layer and an anti-reflection layer, a light storage layer and a reflection layer, a light storage layer and a half reflection layer, a light storage layer and a light diffusion layer, a light storage layer and an electroluminescence layer, a half reflection layer and an electroluminescence layer, and the like. Combinations are included. However, it is not limited to these.
[0034]
Such polarizing films include electronic desk calculators, electronic watches, word processors, personal computers, monitors, liquid crystal televisions, personal digital assistants, liquid crystal display devices such as instruments for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, display elements. (CRT, LCD, etc.), reflection reduction layer, medical equipment, building materials, toys, etc.
[0035]
【Example】
Hereinafter, the present invention will be described specifically with reference to examples.
In the examples, “parts” and “%” are based on weight unless otherwise specified.
[0036]
Example 1
Using a polyvinyl alcohol resin having a viscosity average degree of polymerization of 1700 and a saponification degree of 99.7 mol%, a 45% aqueous solution of a polyvinyl alcohol resin (using glycerin as a plasticizer and polyoxyethylene alkylamino ether as a release agent) Is prepared by casting the aqueous solution from a T-shaped slit die using a drum type roll at 100 ° C. as described below, drying to a water content of 10%, followed by heat treatment at 120 ° C. for 3 minutes, A polyvinyl alcohol-based film having a water content of 4% was obtained by wetting.
[0037]
[Drum type roll]
A total of 460 g / m3 of a nitric acid aqueous solution (concentration: 6%) with a pH of 1 is applied to the surface of a drum-type roll which has been subjected to nickel plating and further chrome plating on a stainless steel base material and mirror-finished. ² The wet tension on the roll surface was set to 32 mN / m.
Next, when the thickness uniformity of the obtained polyvinyl alcohol-based film having an average thickness of 75 µm was evaluated by the following method, the thickness variation per unit mm in the width direction was 0.2 µm / mm.
[0038]
(Measurement of thickness variation: based on JIS K 6900)
Using a film thickness nest tester (“K306C” manufactured by Anritsu Corporation), three points of the film are sampled in the width direction (TD direction) at 1 m intervals in the film flow direction (MD direction), and the thickness is measured. The maximum value of the thickness variation per mm was determined.
[0039]
Furthermore, after sandwiching a polyvinyl alcohol-based film at an angle of 45 ° between two polarizing plates in a crossed Nicols state, and observing in a transmission mode using a light box having a surface illuminance of 5000 lux, optical color unevenness ( Rd nonuniformity) was not observed.
After continuously producing the film for 14 days, the thickness and appearance of the film were measured and observed in the same manner. The average thickness was 75 μm, the thickness variation was 0.3 μm / mm, and the optical color unevenness was observed. No long run property was observed.
[0040]
Further, the obtained polyvinyl alcohol-based film was unwound at 1.25 m / min, swollen in a washing tank (24 ° C.), and then 1.3 times in an iodine tank (20 ° C., iodine concentration 0.05 g / l). In a boric acid bath (50 ° C., iodine concentration 0.0012 g / l, boric acid concentration 47 g / l), uniaxial stretching is performed 1.7 times, and a total of 6 times uniaxial stretching is performed at a winding speed of 7.5 m / min. Stretching was performed to obtain a polarizing film.
[0041]
After sandwiching the obtained polarizing film at an angle of 45 ° between two polarizing plates in a crossed Nicols state, when observed in a transmission mode using a light box having a surface illuminance of 5000 lux, it was attributed to the polyvinyl alcohol-based film. No optical color unevenness was observed.
[0042]
Example 2
The same experiment as in Example 1 was performed except that a polyvinyl alcohol-based film having a water content of 3.5% was obtained using a polyvinyl alcohol-based resin having a viscosity average degree of polymerization of 2600 and a saponification degree of 99.7 mol%. However, the concentration of the aqueous polyvinyl alcohol resin solution was 35%, and the following drum-type rolls were used.
[0043]
[Drum type roll]
A total of 460 g / m of an aqueous solution of acetic acid at pH 3 (concentration: 0.5%) was applied to the surface of a drum-type roll which had been subjected to nickel plating and then chrome plating on a stainless steel base material and mirror-finished. ² The wet tension on the roll surface was 35 mN / m.
[0044]
The film performance of the obtained polyvinyl alcohol-based film was evaluated in the same manner as in Example 1. As a result, the average thickness was 74 μm, the thickness variation was 0.3 μm / mm, and no optical color unevenness was observed. Even if continuous film formation was continued for 11 days, the thickness variation was 0.4 μm / mm.
[0045]
Further, the obtained polyvinyl alcohol-based film was unwound at 1.25 m / min, swollen in a washing tank (24 ° C.), and then 1.8 times in an iodine tank (20 ° C., iodine concentration 0.05 g / l). 1.9 times uniaxial stretching in a boric acid bath (50 ° C., iodine concentration 0.0012 g / l, boric acid concentration 47 g / l), and a total of 4.5 times at a winding speed of 5.6 m / min. Uniaxial stretching was performed to obtain a polarizing film.
When the appearance of the obtained polarizing film was observed in the same manner as in Example 1, no color unevenness due to the polyvinyl alcohol-based film was observed.
[0046]
Example 3
A polyvinyl alcohol-based resin having a viscosity average polymerization degree of 4000 and a saponification degree of 99.7 mol%, and containing 0.2% of sodium lauryl sulfate as an anionic surfactant. The same experiment as in Example 1 was carried out except that a drum type roll (no treatment with an acid aqueous solution, a wet tension of 57 mN / m), which was obtained by applying an aqueous solution to a stainless steel base material and applying a nickel plating and further a chrome plating to a mirror finish, was used. Was.
[0047]
One hour after the start of film formation, the wetting tension of the drum-type roll surface was measured and found to be 33 mN / m.
When the film performance of the obtained polyvinyl alcohol-based film was evaluated in the same manner as in Example 1, the average thickness was 75 μm, and the thickness variation was 0.4 μm / mm.
Further, in the production of the above film, even if the operation was continued for 27 days, the variation in the thickness of the film was 0.4 μm / mm, and there was no abnormality and good long-run property was obtained.
[0048]
Further, the polyvinyl alcohol-based film obtained in Example 3 was unwound at 1.25 m / min, swollen in a washing tank (24 ° C.), and then in an iodine tank (20 ° C., iodine concentration 0.17 g / l). The film was uniaxially stretched 1.7 times in a boric acid tank (50 ° C., iodine concentration 0.0012 g / l, boric acid concentration 47 g / l) 1.8 times, and further wound at a winding speed of 5.3 m / min for a total of 4 times. The film was uniaxially stretched by a factor of 2 to obtain a polarizing film.
When the appearance of the obtained polarizing film was observed in the same manner as in Example 1, no color unevenness due to the polyvinyl alcohol-based film was observed.
[0049]
Example 4
In Example 3, an experiment was performed using 0.1% of sodium polyoxyethylene lauryl ether sulfate as an anionic surfactant.
One hour after the start of film formation, the wetting tension of the drum-type roll surface was measured and found to be 32 mN / m.
When the film performance of the obtained polyvinyl alcohol-based film was similarly evaluated, the average thickness was 74 μm, and the thickness variation was 0.3 μm / mm.
In addition, in the production of the film, even if the operation was continued for 25 days, the thickness variation was 0.3 μm / mm, and there was no abnormality and good long-run property was obtained.
[0050]
Further, the obtained polyvinyl alcohol-based film was unwound at 1.25 m / min, swollen in a washing tank (24 ° C.), and then 1.7 times in an iodine tank (20 ° C., iodine concentration 0.17 g / l). In a boric acid tank (50 ° C., iodine concentration 0.0012 g / l, boric acid concentration 47 g / l), uniaxial stretching is performed 2.25 times, and further, a total of 5.0 times is performed at a winding speed of 6.4 m / min. Uniaxial stretching was performed to obtain a polarizing film.
When the appearance of the obtained polarizing film was similarly observed, no color unevenness due to the polyvinyl alcohol-based film was observed.
[0051]
Example 5
The same experiment as in Example 1 was performed except that a 45% aqueous polyvinyl alcohol resin solution containing 0.1% of polyoxyethylene lauryl ether sodium sulfate was used.
[0052]
One hour after the start of film formation, the wetting tension of the drum-type roll surface was measured and found to be 32 mN / m.
When the film performance of the obtained polyvinyl alcohol-based film was evaluated in the same manner as in Example 1, the average thickness was 75 μm, and the thickness variation was 0.2 μm / mm.
Further, in the production of the above film, the thickness variation was 0.3 μm / mm even after the operation was continued for 28 days, and there was no abnormality and good long-run property was obtained.
[0053]
Further, the obtained polyvinyl alcohol-based film was unwound at 1.25 m / min, swollen in a washing tank (24 ° C.), and then 1.7 times in an iodine tank (20 ° C., iodine concentration 0.17 g / l). In a boric acid tank (50 ° C., iodine concentration 0.0012 g / l, boric acid concentration 47 g / l), a 2.0-fold uniaxial stretching is performed, and a winding speed is 5.3 m / min, for a total of 4.2-fold. Uniaxial stretching was performed to obtain a polarizing film.
When the appearance of the obtained polarizing film was observed in the same manner as in Example 1, no color unevenness due to the polyvinyl alcohol-based film was observed.
[0054]
Comparative Example 1
The same experiment as in Example 1 was carried out except that a stainless steel base material was subjected to nickel plating, further chromium plating, and a mirror-finished drum type roll having a wettability of 57 mN / m was used to obtain a polyvinyl alcohol-based film.
The film appearance of the obtained polyvinyl alcohol-based film was evaluated in the same manner as in Example 1, and a polarizing film was obtained in the same manner as in Example 1, and the physical properties were measured.
[0055]
As a result, the average thickness was 75 μm, the thickness variation was 2.2 μm / mm, and a lot of optical color unevenness was observed on the entire surface. Further, in the production of the film, when the operation was continued for 3 days, the thickness variation of the film was 2.5 μm / mm.
In the polarizing film manufactured in the same manner as in Example 1, color unevenness caused by the polyvinyl alcohol-based film was observed on the entire surface.
[0056]
Comparative Example 2
The same experiment as in Example 1 was performed except that a drum-type roll having a wet tension of 23 mN / m with a mirror finish finished by nickel plating, chrome plating and fluorine treatment on a stainless steel base material was used, and a polyvinyl alcohol-based film was obtained. Was.
The film appearance of the obtained polyvinyl alcohol-based film was evaluated in the same manner as in Example 1, and a polarizing film was obtained and the physical properties were measured in the same manner as in Example 1.
[0057]
As a result, a film having an average thickness of 75 μm was obtained, but a repelling phenomenon occurred and holes were formed throughout the film.
[0058]
【The invention's effect】
In the present invention, when a polyvinyl alcohol-based resin aqueous solution is cast on a drum-type roll to form a film, the wetting tension on the surface of the drum-type roll is adjusted to 25 to 50 mN / m, so that the film thickness and optical characteristics are uniform. It has excellent properties, has a very good film appearance, and has an excellent effect on long-run film-forming properties. In particular, it is possible to obtain a polyvinyl alcohol-based film useful as a raw film of a polarizing film.

Claims (4)

A method for producing a polyvinyl alcohol-based film, comprising: using a drum-type roll having a wet tension of 25 to 50 mN / m on the surface of the drum-type roll when casting the aqueous solution of the polyvinyl alcohol-based resin onto the drum-type roll to form a film. Method. The method for producing a polyvinyl alcohol-based film according to claim 1, wherein the drum type roll has a mirror-finished chromium surface, and the surface is treated with an acid aqueous solution having a pH of 1 to 3. 2. The aqueous solution of a polyvinyl alcohol-based resin to which an anionic surfactant is added so that the film is formed under the condition that the wet tension of the surface of the drum-type roll is 25 to 50 mN / m. A method for producing a polyvinyl alcohol-based film. The method for producing a polyvinyl alcohol-based film according to any one of claims 1 to 3, wherein the method is used for an original film of a polarizing film.