JP6740857B2 - Method for producing polyvinyl alcohol film for producing polarizing film - Google Patents

Description

The present invention relates to a polyvinyl alcohol film for a polarizing film production, more particularly, polarized light produced through the step of stretching in the flow direction was formed as a film by casting a film (MD direction) rather than the width direction (TD direction) a method for producing a film for manufacturing polyvinyl alcohol-based film, and in particular relates to the production how suitable polyvinyl alcohol-based film to produce a less polarizing film having excellent and color unevenness in the polarizing performance.

Conventionally, polyvinyl alcohol films have been used in many applications as films having excellent transparency, and one of the useful applications thereof is a polarizing film. Such a polarizing film is used as a basic constituent element of a liquid crystal display, and in recent years, its use has been expanded to devices that require high quality and high reliability.

Under such circumstances, a polarizing film having excellent optical characteristics is required as the screens of liquid crystal televisions, multifunctional mobile terminals, and the like have higher brightness, higher definition, larger area, and thinner. Specifically, it is to further improve the degree of polarization and eliminate color unevenness.

Conventional polyvinyl alcohol-based films for producing a polarizing film are generally produced from an aqueous solution of polyvinyl alcohol-based resin by a continuous casting method. Specifically, an aqueous solution of polyvinyl alcohol resin is cast into a casting mold such as a cast drum or an endless belt to form a film, and the formed film is peeled from the casting mold and then flowed using a nip roll or the like. It is manufactured by drying with a hot roll or a floating dryer while being conveyed in the direction (MD direction). In such a drying step, tensile stress is applied to the formed film in the flow direction (MD direction), so that the polyvinyl alcohol-based polymer is easily oriented in the MD direction, and the optical axis of the obtained polyvinyl alcohol-based film is increased. (Slow axis) often faces the MD direction. On the other hand, in the width direction (TD direction) of the formed film, contraction stress depending on Poisson's ratio and contraction stress due to dehydration are generated. It is also possible to orient the molecules in the TD direction to some extent. In this case, the optical axis of the obtained polyvinyl alcohol-based film is oriented between the TD direction and the MD direction.

On the other hand, a polarizing film is generally manufactured by dyeing a raw material of a polyvinyl alcohol film with a dichroic dye such as iodine and stretching the film. The stretching step is a step in which the dyed film is stretched in the machine direction (MD direction) to highly orient the dichroic dye in the film. In order to improve the polarization performance of the polarizing film, it is necessary that the raw material polyvinyl alcohol-based film has good stretchability in the MD direction. On the contrary, the width direction (TD direction) of the film is necked in by stretching in the flow direction (MD direction), but in order to obtain a wide polarizing film, such dimensional shrinkage in the width direction is suppressed as much as possible. And, it is necessary to make it uniform.
Incidentally, as the order of manufacturing the polarizing film, there is a case where a polyvinyl alcohol-based film which is a raw material is stretched and dyed with a dichroic dye such as iodine, and even in such a case, in order to improve the polarizing performance of the polarizing film. Requires the polyvinyl alcohol film to have good stretchability in the MD direction.

Furthermore, in recent years, polyvinyl alcohol-based films have been thinned in order to thin the polarizing film. Such a thin film has a problem in productivity such as breakage due to stretching during the production of the polarizing film.

As a method of improving the stretchability, for example, a method of specifying the ratio between the speed of the cast drum at the time of forming the film and the final film winding speed (see, for example, Patent Document 1), film formation by the cast drum. After that, a method of suspending and drying the film (for example, refer to Patent Document 2) and a method of controlling the tension in the drying process of the formed film (for example, refer to Patent Document 3) have been proposed. Further, a polyvinyl alcohol-based film having a Poisson's ratio of 0.5 to 0.7 when stretched by 5% in the winding direction has been proposed (see, for example, Patent Document 4).

JP 2001-315141 A JP, 2001-315142, A JP, 2002-79531, A JP, 2006-307058, A

However, even the method of the above patent document is not sufficient to improve the stretchability during the production of the polarizing film.
Specifically, the above-mentioned Patent Document 1 specifies the degree of stretching in the MD direction (the degree of pulling) when manufacturing a polyvinyl alcohol-based film, but if the stretching in the TD direction is not taken into consideration, polarized light It is insufficient to improve the stretchability during membrane production. In general, it is difficult to stretch a polyvinyl alcohol-based film stretched in the MD direction in the MD direction when manufacturing a polarizing film. That is, it is difficult to further pull the polyvinyl alcohol-based polymer oriented in the MD direction in the MD direction. On the contrary, it is relatively easy to pull the polyvinyl alcohol polymer oriented in the TD direction in the MD direction. However, if the polymer orientation in the TD direction is not uniform, it cannot be uniformly stretched in the MD direction during the production of the polarizing film. In Patent Document 1, there is an example in which the polyvinyl alcohol-based film is not stretched so much in the MD direction (an example in which it is not stretched). There is a problem that the molecular orientation cannot be made sufficiently uniform. That is, the orientation state of the polymer cannot be made uniform unless it is stretched to some extent also in the TD direction or at least fixed in the width direction.

In the technique disclosed in Patent Document 2, the film after film formation can be dried uniformly, but the orientation of the polymer cannot be controlled, and it is insufficient to improve the stretchability during the production of the polarizing film.
In the technique disclosed in Patent Document 3, although the film thickness of the polyvinyl alcohol-based film can be made uniform, the orientation of the polymer cannot be controlled, and it is insufficient to improve the stretchability during the production of the polarizing film.
In the technique disclosed in Patent Document 4, although the dimensional change in the TD direction can be made uniform in the stretching process at the time of manufacturing the polarizing film, there is room for improvement in the stretchability in the MD direction.

Thus, in the present invention, under such a background, a method for producing a polyvinyl alcohol-based film having excellent stretchability at the time of producing a polarizing film, which has a high polarizing performance and has little color unevenness, particularly a thin film method for producing a polyvinyl alcohol film even break when polarizing film production does not occur it is an object of the provide child a.

However, the inventors of the present invention have made extensive studies in view of such circumstances, and when the polyvinyl alcohol-based film is produced by the continuous casting method, after peeling the formed film from the casting mold, the flow direction (MD direction). By continuously stretching the film in the width direction (TD direction) while transporting it to (4), a polyvinyl alcohol-based film having excellent stretchability during the production of a polarizing film is obtained, and a polarized light obtained using the polyvinyl alcohol-based film is obtained. The present invention has been completed by finding that the film has a high polarization performance and has little color unevenness.

That is, the gist of the present invention is a method for producing a polyvinyl alcohol-based film for producing a polarizing film, which is obtained by forming an aqueous solution of a polyvinyl alcohol-based resin (excluding one containing a blocked polyisocyanate) by a continuous casting method. Then, after peeling the formed film from the casting mold, the film is continuously stretched in the width direction (TD direction) while being conveyed in the flow direction (MD direction), and before being stretched in the width direction (TD direction). the water content of the film is 0.5 to 15 wt%, a method for producing a polarizing film producing polyvinyl alcohol-based film, characterized in that to obtain a polyvinyl alcohol-based film.

Polyvinyl alcohol film for a polarizing film manufactured obtained by the production process of the polarizing film producing polyvinyl alcohol-based film of the present invention is excellent in stretchability at the time of the polarizing film production, cause breakage even when producing a thin polarizing film In other words, it is possible to obtain a polarizing film having high polarization performance and less color unevenness.

The present invention will be described in detail below.
Method for producing a polarizing film producing polyvinyl alcohol-based film of the present invention, an aqueous solution of polyvinyl alcohol resin (excluding those containing blocked polyisocyanate) polyvinyl for polarizing film production obtained by film by a continuous casting method a manufacturing method of an alcohol-based film, after removing the is film formation film from the casting mold, while conveying the flow direction (MD direction), and successively stretched in the transverse direction (TD direction), lateral direction ( the water content of the film before stretching in the TD direction) is 0.5 to 15% by weight, characterized in that to obtain a polyvinyl alcohol film for a polarizing film production.

That is, the greatest feature of the present invention is that the film separated from the casting mold is continuously stretched in the width direction (TD direction) while being conveyed in the flow direction (MD direction).
The flow direction (MD direction) in the present invention means the transport direction until the film is wound into a roll shape.

Here, the method for producing the polyvinyl alcohol film for producing a polarizing film of the present invention will be described in more detail in the order of steps.

[Film material]
First, the polyvinyl alcohol resin, which is a material for the polyvinyl alcohol film, and the polyvinyl alcohol resin aqueous solution will be described.
In the present invention, the polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based film is usually an unmodified polyvinyl alcohol-based resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. Used. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol% or less, preferably 5 mol% or less) of a component copolymerizable with vinyl acetate may be used. it can. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (eg, ethylene, propylene, n-butene). , Isobutene), vinyl ethers, unsaturated sulfonates and the like. Further, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl group after saponification can also be used. These may be used alone or in combination of two or more.

Further, as the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in its side chain can be used. The polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain is, for example, (i) a method of saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) acetic acid. A method of saponifying and decarboxylating a copolymer of vinyl and vinyl ethylene carbonate, (iii) saponifying and decarboxylating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane. It can be obtained by a ketalization method, (iv) a method of saponifying a copolymer of vinyl acetate and glycerin monoallyl ether, or the like.

The weight average molecular weight of the polyvinyl alcohol resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and further preferably 120,000 to 260,000. When the weight average molecular weight is too small, it tends to be difficult to obtain sufficient optical performance when the polyvinyl alcohol-based resin is used as an optical film, and when it is too large, when a polarizing film is produced using the polyvinyl alcohol-based film, stretching is performed. Tends to be difficult. The weight average molecular weight of the polyvinyl alcohol resin is the weight average molecular weight measured by the GPC-MALS method.

The average degree of saponification of the polyvinyl alcohol-based resin used in the present invention is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.5 mol% or more, particularly preferably 99. It is 8 mol% or more. If the average degree of saponification is too small, sufficient optical performance tends not to be obtained when the polyvinyl alcohol film is used as a polarizing film.
Here, the average degree of saponification in the present invention is measured according to JIS K 6726.

As the polyvinyl alcohol-based resin used in the present invention, two or more kinds having different modified species, modified amount, weight average molecular weight, average saponification degree, etc. may be used in combination.

In addition to the polyvinyl alcohol-based resin, the polyvinyl alcohol-based resin aqueous solution may be a commonly used plastic such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, trimethylolpropane, etc. From the viewpoint of film-forming property, it is preferable to include an agent and at least one surfactant of nonionic, anionic and cationic. These may be used alone or in combination of two or more.

The polyvinyl alcohol-based resin aqueous solution thus obtained preferably has a resin concentration of 15 to 60% by weight, particularly preferably 17 to 55% by weight, and further preferably 20 to 50% by weight. If the resin concentration of the aqueous solution is too low, the drying load will be large and the production capacity will tend to be reduced. If it is too high, the viscosity will be too high and uniform dissolution will be difficult to achieve.

Next, the obtained polyvinyl alcohol-based resin aqueous solution is defoamed. Examples of the defoaming method include static defoaming and defoaming with a multi-screw extruder. As the multi-screw extruder, any multi-screw extruder having a vent may be used, and normally a twin-screw extruder having a vent is used.

[Film forming process]
After the defoaming treatment, the polyvinyl alcohol-based resin aqueous solution is introduced into the T-shaped slit die by a fixed amount, discharged and cast on a rotating cast drum, and formed into a film by a continuous casting method.

The continuous casting method in the present invention is, for example, a method of forming a film by discharging and casting an aqueous solution of polyvinyl alcohol resin from a T-shaped slit die to a casting mold such as a rotating cast drum, endless belt, or resin film. Is. Then, after peeling the formed film from the cast mold, the film may be continuously dried with a hot roll and heat-treated with, for example, a floating dryer.

The resin temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-type slit die is preferably 80 to 100°C, and particularly preferably 85 to 98°C.
If the resin temperature of the polyvinyl alcohol-based resin aqueous solution is too low, fluidity tends to be poor, and if it is too high, foaming tends to occur.

The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 50 to 200 Pa·s, and particularly preferably 70 to 150 Pa·s at the time of discharging.
If the viscosity of the aqueous solution is too low, flowability tends to be poor, and if it is too high, casting tends to be difficult.

The discharge rate of the polyvinyl alcohol-based resin aqueous solution discharged from the T-type slit die to the cast drum is preferably 0.2 to 5 m/min, particularly preferably 0.4 to 4 m/min, and further preferably 0. It is 6 to 3 m/min.
If the discharge speed is too slow, the productivity tends to decrease, and if it is too fast, casting tends to be difficult.

The diameter of such a cast drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and further preferably 2.8 to 4 m.
If the diameter is too small, the drying section on the cast drum becomes short, and thus the speed tends to be difficult to increase, and if it is too large, the transportability tends to decrease.

The width of such a cast drum is preferably 4 m or more, particularly preferably 4.5 m or more, further preferably 5 m or more, particularly preferably 5 to 6 m.
If the width of the cast drum is too small, the productivity tends to decrease.

The rotation speed of the cast drum is preferably 3 to 50 m/min, particularly preferably 4 to 40 m/min, and further preferably 5 to 35 m/min.
If the rotation speed is too slow, the productivity tends to decrease, and if it is too fast, the drying tends to be insufficient.

The surface temperature of the cast drum is preferably 40 to 99°C, and particularly preferably 60 to 95°C.
If the surface temperature is too low, poor drying tends to occur, and if it is too high, foaming tends to occur.

[Film formed]
The water content of the [film before performing the stretching in the width direction (TD direction)] above manner film are films, 0.5 to 15 wt% der is, particularly preferably 1 to 13 wt%, More preferably, it is 2 to 12% by weight.
If the water content is too low or too high, stretching in the width direction (TD direction) tends to be difficult.
In order to adjust the water content, if the water content of the film before stretching in the width direction (TD direction) is too high, it is preferable to dry the film before stretching in the width direction (TD direction). On the contrary, when the water content of the film before stretching in the width direction (TD direction) is too low, it is preferable to control the humidity before stretching in the width direction (TD direction). More preferably, the conditions of the drying step are adjusted so that the water content falls within the above range.

The drying can be performed by a known method such as a heating roll or an infrared heater, but in the present invention, it is preferable to perform the drying with a plurality of heating rolls, and particularly preferably, the temperature of the heating roll is 40 to 150°C. And more preferably 50 to 140°C. Further, in order to adjust the water content, a humidity adjustment area may be provided before stretching in the width direction (TD direction).

[Conveying and stretching process]
Then, the film which has been peeled from the cast mold and whose water content has been adjusted as described above is continuously stretched in the width direction (TD direction) while being conveyed in the flow direction (MD direction). This is the greatest feature of the present invention as described above.

In the present invention, it is not necessary to particularly stretch the formed film in the machine direction (MD direction), and it is sufficient to convey the film with a tensile tension such that the film does not bend. As a matter of course, due to the stretching in the width direction (TD direction), neck-in depending on the Poisson's ratio occurs in the flow direction (MD direction), and dehydration shrinkage also occurs in the flow direction (MD direction) during drying. .. Due to these contractions, even if the rotation speed of the transport roll or the heating roll is constant, an appropriate tension can be obtained in the flow direction (MD direction), and the complicated rotation speed control as in Patent Document 1 is unnecessary. ..
From the viewpoint of manufacturing, the dimension of the film in the machine direction (MD direction) is preferably constant, and particularly preferably, the dimensional change rate in the machine direction (MD direction) before and after stretching in the width direction (TD direction) is 0. It is 0.8-1.2, More preferably, it is 0.9-1.1.

The transport speed of the formed film in the flow direction (MD direction) is preferably 5 to 30 m/min, particularly preferably 7 to 25 m/min, and further preferably 8 to 20 m/min. If the transport speed is too slow, the productivity tends to decrease, and if it is too fast, in-plane unevenness tends to increase.

The method of simultaneously carrying the film formed film in the machine direction (MD direction) and stretching the film in the width direction (TD direction) is not particularly limited, but, for example, a plurality of width direction both ends of the film formed may be used. It is preferable to carry out the conveyance and the stretching at the same time by sandwiching with the clip. In such a case, the arrangement of the clips at each end is preferably 200 mm or less in pitch, particularly preferably 100 mm or less in pitch, and further preferably 50 mm or less in pitch. If the pitch of the clip is too wide, the film after stretching tends to bend, and the thickness unevenness and retardation unevenness at both widthwise end portions of the obtained polyvinyl alcohol-based film tend to increase. Further, it is preferable that the clip holding position (the tip of the clip) is 100 mm or less from both ends in the width direction of the formed film. If the clip holding position (tip portion) is located too much in the widthwise central portion of the film, the end portion of the film to be discarded increases, and the product width tends to narrow.

The stretching ratio in the width direction (TD direction) in the present invention is preferably 1.05 to 1.5 times, particularly preferably 1.1 to 1.4 times, and further preferably 1.1 to 1.3 times. Double. If the stretching ratio in the width direction (TD direction) is too low, the orientation of the polyvinyl alcohol polymer in the TD direction tends to decrease, and if it is too high, the stretchability during the production of the polarizing film tends to decrease.

The continuous stretching process in the width direction (TD direction) may be performed in one step (one time) or in a plurality of steps (several times) within the range of the draw ratio (also referred to as sequential drawing). The continuous stretching process is preferably performed after the film drying process, but may be performed at least one before the drying process, during the drying process, and after the drying process.
That is, after carrying out the first-stage stretching, simple conveyance with the width direction (TD direction) fixed may be carried out to carry out the second-stage and subsequent stretching. In particular, in the case of a thin film, the stress of the film is relaxed by inserting such a simple conveying process, and it becomes possible to avoid breakage.
When inserting a fixed width conveyance step, the fixed width can be made narrower than the width after the first stage stretching. The film immediately after stretching easily shrinks in the width direction (TD direction) due to stress relaxation, and shrinkage accompanying dehydration also occurs, so that the fixed width can be narrowed to these shrinking widths. However, if the shrinkage width is made narrower than the shrinkage width, the film is bent, which is not preferable.

As a preferred embodiment of the present invention, after the film formed is temporarily stretched in the width direction (TD direction) by more than 1.3 times, the final draw ratio in the width direction (TD direction) is 1 It is possible to use a method of shrinking the size so that the size becomes 0.05 to 1.5 times. In such a case, after the film is temporarily stretched over 1.3 times, the film may be simply conveyed at a fixed width of 1.05 to 1.5. By such a method, the stress of the film is relaxed, and it becomes possible to avoid breakage, especially in the case of a thin film.

In the present invention, the stretching in the width direction (TD direction) of the formed film is preferably performed at 50 to 150°C, particularly preferably 60 to 140°C, and further preferably 70 to 130°C.
If the stretching temperature is too low, the orientation of the polyvinyl alcohol-based polymer in the TD direction tends to decrease. On the contrary, if the stretching temperature is too high, the stretchability during the production of the polarizing film tends to decrease.

Further, the stretching time during stretching in the width direction (TD direction) of the formed film is preferably 2 to 60 seconds, particularly preferably 5 to 45 seconds, and further preferably 10 to 30 seconds.
If the stretching time is too short, the film tends to be broken, while if it is too long, the equipment load tends to increase.

In the present invention, after the film formed is stretched in the width direction (TD direction), it may be heat-treated with a floating dryer or the like, if necessary. The temperature of such heat treatment is preferably 60 to 200°C, particularly preferably 70 to 150°C, and further preferably 100 to 140°C.
If the heat treatment temperature is too low, the dimensional stability tends to decrease, and conversely, if it is too high, the stretchability during the production of the polarizing film tends to decrease.
The heat treatment time is preferably 1 to 60 seconds, particularly preferably 5 to 30 seconds. If the heat treatment time is too short, the dimensional stability tends to decrease, and conversely, if it is too long, the stretchability during the production of the polarizing film tends to decrease.

[Polyvinyl alcohol film]
Thus, a polyvinyl alcohol-based film (polyvinyl alcohol-based film after being stretched) is obtained by the production method of the present invention, and finally wound into a roll to obtain a product. The thickness of the polyvinyl alcohol-based film is preferably 60 μm or less from the viewpoint of thinning the polarizing film, particularly preferably 2 to 20 μm from the viewpoint of reducing shrinkage stress of the polarizing film during heat resistance, and further preferably 5 from the viewpoint of avoiding breakage. Is about 20 μm. The thickness of the polyvinyl alcohol-based film is adjusted by the resin concentration in the polyvinyl alcohol-based resin aqueous solution, the discharge amount (discharge speed) to the casting mold, the stretching ratio, and the like.

The width of the polyvinyl alcohol film is preferably 2 m or more, particularly preferably 3 m or more from the viewpoint of increasing the area, and further preferably 4 to 6 m from the viewpoint of avoiding breakage.

The length of the polyvinyl alcohol-based film is preferably 2 km or more, particularly preferably 3 km or more from the viewpoint of increasing the area, and further preferably 3 to 50 km from the viewpoint of transport weight.

Polyvinyl alcohol film obtained by the producing method of the present invention is a raw fill beam of the polarizing film, hereinafter, polarizing film made of the polyvinyl alcohol film, and a manufacturing method of the polarizing plate will be described.

[Production method of polarizing film]
The polarizing film of the present invention, the polyvinyl alcohol-based film obtained by the production method of the present invention, is rolled out from the roll and transported in the horizontal direction, swelling, dyeing, cross-linking boric acid, stretching, washing, drying, etc. Manufactured.

The swelling step is performed before the dyeing step. By the swelling step, stains on the surface of the polyvinyl alcohol film can be washed, and by swelling the polyvinyl alcohol film, there is an effect of preventing uneven dyeing. In the swelling process, water is usually used as the treatment liquid. The treatment liquid may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol or the like as long as the main component is water. The temperature of the swelling bath is usually about 10 to 45° C., and the immersion time in the swelling bath is usually about 0.1 to 10 minutes.

The dyeing process is carried out by bringing the film into contact with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide is used, and it is suitable that the iodine concentration is 0.1 to 2 g/L and the potassium iodide concentration is 1 to 100 g/L. Practical dyeing time is about 30 to 500 seconds. The temperature of the treatment bath is preferably 5 to 50°C. The aqueous solution may contain a small amount of an organic solvent compatible with water in addition to the water solvent.

The boric acid crosslinking step is performed using a boron compound such as boric acid or borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 10 to 100 g/L, and potassium iodide coexisting in the solution is preferable from the viewpoint of stabilizing the polarization performance. The temperature during the treatment is preferably about 30 to 70° C., the treatment time is preferably about 0.1 to 20 minutes, and a stretching operation may be performed during the treatment, if necessary.

In the stretching step, it is preferable to stretch the film uniaxially 3 to 10 times, preferably 3.5 to 6 times. At this time, some stretching (stretching to a degree that prevents shrinkage in the width direction or more) may be performed in the direction perpendicular to the stretching direction. The temperature during stretching is preferably 40 to 170°C. Further, the stretching ratio may be finally set within the above range, and the stretching operation may be carried out not only once but also plural times in the manufacturing process.

The washing step is performed, for example, by immersing the film in water or an aqueous iodide solution such as potassium iodide to remove the precipitate generated on the surface of the film. When using the potassium iodide aqueous solution, the potassium iodide concentration may be about 1 to 80 g/L. The temperature during the washing treatment is usually 5 to 50°C, preferably 10 to 45°C. The treatment time is usually 1 to 300 seconds, preferably 10 to 240 seconds. The washing with water and the washing with an aqueous solution of potassium iodide may be appropriately combined.

In the drying step, for example, the film is dried in the air at 40 to 80° C. for 1 to 10 minutes.

A polarizing film is thus obtained, and the polarization degree of such a polarizing film is preferably 99% or more, more preferably 99.5% or more. If the degree of polarization is too low, the contrast in the liquid crystal display tends to decrease.
The degree of polarization is generally calculated by measuring the light transmittance (H ₁₁ ) at a wavelength of λ with two polarizing films superposed so that their alignment directions are the same, and the two polarizing films. It is calculated according to the following formula from the light transmittance (H ₁ ) measured at the wavelength λ in a state in which the films are stacked so that the orientation directions are orthogonal to each other.
Polarization degree = _{[(H 11 -H 1) / (} H 11 + H 1) ] ^1/2

Further, the single transmittance of the polarizing film of the present invention is preferably 44% or more. If the single transmittance is too low, it tends to be impossible to achieve high brightness of the liquid crystal display.
The simple substance transmittance is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

Next, a method for manufacturing the polarizing plate of the present invention using the polarizing film of the present invention will be described.
INDUSTRIAL APPLICABILITY The polarizing film of the present invention is suitable for producing a polarizing plate having little color unevenness and excellent polarizing performance.

[Production method of polarizing plate]
The polarizing plate of the present invention is produced by laminating an optically isotropic resin film as a protective film on one surface or both surfaces of the polarizing film of the present invention via an adhesive. Examples of the protective film include films of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methylpentene, polyphenylene oxide and the like. Or give a sheet.

The laminating method is performed by a known method, for example, a liquid adhesive composition, evenly applied to the polarizing film, the protective film, or both, after bonding the two by pressure bonding, heating or It is performed by irradiating with active energy rays.

It is also possible to apply a curable resin such as a urethane resin, an acrylic resin, or a urea resin to one side or both sides of the polarizing film and cure the resin to form a cured layer to form a polarizing plate. In this case, the cured layer serves as a substitute for the protective film, and can be thinned.

The polarizing film and the polarizing plate using the polyvinyl alcohol-based film obtained by the production method of the present invention are excellent in polarization performance, portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic watches, word processors, Liquid crystal display devices such as electronic paper, game consoles, videos, cameras, photo albums, thermometers, audio, measuring instruments for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, It is preferably used for an antireflection layer for organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys and the like.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded.
In addition, "part" in the examples means weight basis.

<Measurement conditions>
Then, the characteristics (polarization degree, simple substance transmittance, color unevenness) of the polarizing films in the following Examples and Comparative Examples were measured and evaluated as follows.
[Polarization (%), single transmittance (%)]
A sample with a length of 4 cm and a width of 4 cm was cut out from the obtained polarizing film, and the degree of polarization (%) and the single transmittance (%) were measured by using an automatic polarizing film measuring device (VAP7070 manufactured by JASCO Corporation). ..

〔Color unevenness〕
A test piece having a length of 30 cm and a width of 30 cm was cut out from the obtained polarizing film, and was placed at 45° between two polarizing plates in the crossed Nicols state (single transmittance 43.5%, polarization degree 99.9%). After being sandwiched at an angle, a light box with a surface illuminance of 14,000 lx was used to observe optical color unevenness in the transmission mode, and evaluation was made according to the following criteria.
(Evaluation criteria)
○: no color unevenness △: slight color unevenness ×: color unevenness

<Example 1>
(Production of polyvinyl alcohol film)
In a 5,000 L dissolution can, a weight average molecular weight of 142,000, a saponification degree of 99.8 mol% of polyvinyl alcohol resin 1,000 kg, water 2,500 kg, glycerin 105 kg as a plasticizer, and polyoxy as a surfactant. Ethylene laurylamine (0.25 kg) was added thereto, the temperature was raised to 150° C. with stirring to dissolve under pressure, and an aqueous solution of a polyvinyl alcohol resin having a resin concentration of 25% by weight was obtained by adjusting the concentration. Next, the aqueous solution of polyvinyl alcohol resin is supplied to a twin-screw extruder for defoaming, the temperature of the aqueous solution is set to 95° C., and the solution is discharged from a T-shaped slit die discharge port to a rotating cast drum (discharge speed 2. 5 m/min) and casting to form a film. The film thus formed was peeled off from the cast drum, and the film was dried while alternately contacting a total of 10 hot rolls with the front surface and the back surface of the film. As a result, a film having a water content of 10% by weight (width 2 m, thickness 60 μm) was obtained. Then, both ends of the film in the width direction were clamped at a clip pitch of 45 mm, and the film was conveyed in the machine direction (MD direction) at a speed of 5 m/min while using a stretching machine in the width direction (TD direction) to 75 The film was stretched 1.2 times in the width direction (TD direction) at 0° C. and finally heat-treated at 120° C. for 10 seconds to obtain a polyvinyl alcohol-based film (width 2.4 m, thickness 50 μm). In stretching in the width direction (TD direction), the dimensional change rate in the flow direction (MD direction) before and after stretching was 1.0.

(Production of polarizing film and polarizing plate)
The obtained polyvinyl alcohol-based film was unrolled from a roll, and while being conveyed in the horizontal direction, it was immersed in a water tank at a water temperature of 30° C. to be swollen and stretched 1.7 times in the flow direction (MD direction). Next, while being immersed in an aqueous solution of 0.5 g/L of iodine and 30 g/L of potassium iodide at 30° C., it is stretched 1.6 times in the machine direction (MD direction) while dyeing, and then 40 g/borate L and potassium iodide were immersed in an aqueous solution (50° C.) having a composition of 30 g/L, and uniaxially stretched 2.1 times in the flow direction (MD direction) while boric acid crosslinking was performed. Finally, it was washed with an aqueous solution of potassium iodide and dried at 50° C. for 2 minutes to obtain a polarizing film with a total draw ratio of 5.8 times. No breakage occurred during the production, and the properties of the obtained polarizing film were as shown in Table 1.
Using a polyvinyl alcohol aqueous solution as an adhesive, a triacetyl cellulose film having a thickness of 40 μm was laminated on both surfaces of the polarizing film obtained above, and dried at 70° C. to obtain a polarizing plate.

<Example 2>
In Example 1, the formed film was stretched 1.2 times in the width direction (TD direction) at 75° C., fixed at 75° C. (fixed width 2.4 m) and conveyed, and further 75° C. A polyvinyl alcohol film (width 2.9 m, thickness 42 μm) was obtained in the same manner as in Example 1, except that the film was stretched 1.2 times in the width direction (TD direction).
Further, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol film. The properties of the obtained polarizing film were as shown in Table 1.

<Example 3>
In Example 1, except that the discharge rate at the time of film formation was 1.0 m/min and the film having a water content of 5 wt% (width 2 m, thickness 25 μm) was stretched in the width direction (TD direction). Similarly, a polyvinyl alcohol-based film (width 2.4 m, thickness 20 μm) was obtained.
Further, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol film. The properties of the obtained polarizing film were as shown in Table 1.

<Comparative Example 1>
A polyvinyl alcohol-based film (width 2 m, thickness 60 μm) was obtained in the same manner as in Example 1 except that the film formed in Example 1 was not stretched in the width direction (TD direction).
Further, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol film. The properties of the obtained polarizing film were as shown in Table 1.

From the results of the above Examples and Comparative Examples, for the example products, a polarizing film having a high degree of polarization was obtained, while for the comparative example products, the obtained polarizing film was inferior in the degree of polarization, Mura was also observed.

The polarizing film using the polyvinyl alcohol-based film obtained by the production method of the present invention is excellent in polarizing performance, and is a personal digital assistant, personal computer, television, projector, signage, electronic desk calculator, electronic timepiece, word processor, electronic paper, Liquid crystal display devices such as game consoles, videos, cameras, photo albums, thermometers, audios, measuring instruments for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic EL, It is preferably used for a reflection reducing layer for electronic paper, etc., optical communication equipment, medical equipment, building materials, toys and the like.

Claims (8)

A method for producing a polyvinyl alcohol-based film for producing a polarizing film, which is obtained by forming an aqueous solution of a polyvinyl alcohol-based resin (excluding those containing a blocked polyisocyanate) by a continuous casting method, wherein the formed film is After peeling from the casting mold, the film was continuously stretched in the width direction (TD direction) while being conveyed in the flow direction (MD direction), and the water content of the film before stretching in the width direction (TD direction) was 0.5. as a 15 wt%, the production method of the polarizing film producing polyvinyl alcohol-based film, characterized in that to obtain a polyvinyl alcohol-based film. The method for producing a polyvinyl alcohol film for producing a polarizing film according to claim 1, wherein the stretching ratio in the width direction (TD direction) is 1.05 to 1.5 times. The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to claim 1, wherein the stretching in the width direction (TD direction) is performed in a plurality of stages. Stretching in the width direction (TD direction) is performed at 50 to 150° C. 4. The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to claim 1, wherein: Dimensional change rate in the transverse direction stretching the film flow direction before and after (TD direction) (MD direction), according to any one of claims 1 to 4, characterized in that 0.8 to 1.2 1. A method for producing a polyvinyl alcohol-based film for producing a polarizing film . The conveyance in the machine direction (MD direction) and the stretching in the width direction (TD direction) are carried out by sandwiching both widthwise end portions of the formed film with a plurality of clips, and arrangement, method for producing a polarizing film producing polyvinyl alcohol-based film according to any one of claims 1 to 5, characterized in that less than the pitch 200 mm. Polyvinyl alcohol film thickness is 5 to 20 [mu] m, a width of more than 2m, the length of the polarizing film producing polyvinyl alcohol-based film according to any one of claims 1 to 6, characterized in that at least 2km Production method. Method for producing a polarizing film producing polyvinyl alcohol-based film according to any one of claims 1 to 7, wherein the polyvinyl alcohol film is optical.