JP2021002061A - Polyvinyl alcohol-based film and polarizing film - Google Patents

Abstract

To provide a polyvinyl alcohol-based film that can be used to produce a polarizing film with extremely few scratches, and to provide a polarizing film with few display defects and superior polarizing performance using such polyvinyl alcohol-based film.SOLUTION: A polyvinyl alcohol-based film with a thickness of 5-60 μm is provided, the film exhibiting a Young's modulus of 15-40 MPa on a surface thereof when a nanoindentation test is performed using a scanning probe microscope in water at 25°C.SELECTED DRAWING: None

Description

The present invention relates to a polyvinyl alcohol-based film useful as a raw film when producing a polarizing film. More specifically, the present invention relates to a polyvinyl alcohol-based film suitable for producing a wide, long and thin polarizing film having few defects such as scratches, and a polarizing film obtained by using the polyvinyl alcohol-based film.

In recent years, the development of liquid crystal display devices has been remarkable, and they are widely used in smartphones, tablets, personal computers, liquid crystal televisions, projectors, in-vehicle panels, and the like. A polarizing film is used in such a liquid crystal display device, and as the polarizing film, a polyvinyl alcohol-based film in which iodine or a dichroic dye is adsorbed and oriented is mainly used. In recent years, as screens have become higher in definition, higher in brightness, larger in size, and thinner, there is a need for a wide, long, and thin polarizing film having fewer display defects than conventional products.

The polarizing film is, for example, a polyvinyl alcohol-based film that is swollen with water (including warm water), dyed with iodine, stretched to align iodine molecules, and boric acid or the like to maintain the stretched state. It is manufactured by cross-linking with the cross-linking agent of the above and drying. Such production is carried out in water while transporting the film in the horizontal direction using a drive roll or a nip roll. In order to reduce the display defects of the polarizing film, it is important not only to reduce the defects of the polyvinyl alcohol-based film but also to avoid the defects that occur during the production of the polarizing film. In particular, scratches that occur when the film comes into contact with the rolls reduce the degree of polarization and light transmittance of the polarizing film and cause polarization unevenness, and therefore need to be avoided.

As a countermeasure against scratches generated during the production of the polarizing film, for example, a method for manufacturing the polarizing film focusing on the coefficient of static friction of the roll in contact with the polyvinyl alcohol-based film has been proposed (see Patent Document 1). Further, the dynamic friction coefficient with respect to the stainless steel roll is 0.03 or less, the content of the surfactant is 0.01 to 3% by weight with respect to the polyvinyl alcohol-based resin, and the surface roughness (Ra) is 0.05 μm. The following polyvinyl alcohol-based films for polarizing films have been proposed (see Patent Document 2).

Japanese Unexamined Patent Publication No. 2004-17321 Japanese Unexamined Patent Publication No. 2006-188661

However, even with the techniques disclosed in Patent Documents 1 and 2, since each step of producing the polarizing film is performed in water as described above, it is conveyed to the surface of the polyvinyl alcohol-based film softened by containing water. Since fine scratches are likely to occur due to contact with the roll, the scratches during the production of the polarizing film could not be completely eliminated.
When a relatively large area of scratches was generated on the polarizing film, the degree of polarization was lowered and color unevenness occurred. In addition, the shavings generated due to the scratches adhere to the polarizing film as foreign matter, which not only increases display defects but also contaminates the polarizing film production line, causing a significant decrease in the production yield. This phenomenon is particularly remarkable in the production of long, wide and thin polarizing films, and further improvement has been required. Most of the above-mentioned scratches have a length of about several mm in the flow direction (MD direction) of the polarizing film, and usually have a width of several microns or more and a depth of submicron or more.

Therefore, in the present invention, under such a background, a polyvinyl alcohol-based film capable of obtaining a polarizing film having extremely few scratches, and a polarizing film using such a polyvinyl alcohol-based film with few display defects and excellent polarizing performance are provided. To do.

However, as a result of diligent research in view of such circumstances, the present inventors have focused on the young ratio of the film surface in water at 25 ° C. for a polyvinyl alcohol-based film having a thickness of 5 to 60 μm, and set the value as a specific range. By doing so, it has been found that when a polarizing film is produced using the polyvinyl alcohol-based film, it is difficult to be scratched, and it is possible to produce a polarizing film having few display defects and excellent polarizing performance.

That is, the gist of the present invention is a polyvinyl alcohol-based film having a thickness of 5 to 60 μm, and the Young's modulus of the film surface when a nanoindentation test is performed in water at 25 ° C. using a scanning probe microscope is 15. It is a polyvinyl alcohol-based film characterized by being ~ 40 MPa.

The present invention also provides a polarizing film obtained by using the above-mentioned polyvinyl alcohol-based film.

The polyvinyl alcohol-based film of the present invention can reduce scratches during the production of a polarizing film, can produce a polarizing film having few display defects and excellent polarizing performance.

The present invention will be described in detail below.

The most characteristic feature of the polyvinyl alcohol-based film of the present invention is that the Young's modulus of the film surface is 15 to 40 MPa when a nanoindentation test is performed in water at 25 ° C. using a scanning probe microscope.

First, the nanoindentation test will be described.
The nanoindentation test in the present invention is a test for measuring the amount of displacement when a minute probe (cantilever probe) is pushed into a minute region on the surface of a test piece.
At this time, the Young's modulus (E) of the film surface can be calculated from the relationship between the load and the displacement amount. The calculation formula is as shown in the following formula (1).
Equation (1): E = (2 / π) × (k × d / σ) × [(1-ν ² ) / a]
(Here, E: Young's modulus, k: spring constant, d: cantilever deflection amount, σ: pushing amount, ν: Poisson's ratio, a: contact area radius)

Such a nanoindentation test can be performed using a commercially available scanning probe microscope, and can be tested in water or in the atmosphere. Generally, the Young's modulus of the film surface is measured in the atmosphere, but the measured value of the polyvinyl alcohol-based film fluctuates greatly depending on the thickness and the temperature and humidity of the environment. On the other hand, in water, a relatively stable measured value can be obtained by sufficiently swelling the surface of the polyvinyl alcohol-based film with water and then performing the measurement, and the polarizing film can be actually produced using the polyvinyl alcohol-based film. Considering that it is performed in water, it can be said that the measurement result in water is a more realistic evaluation. In the present invention, the polyvinyl alcohol-based film is immersed in water for 2 hours or more to sufficiently swell, and then the nanoindentation test is performed. The immersion conditions are preferably 0.1 to 10 hours, more preferably 1 to 3 hours.

The polyvinyl alcohol-based film of the present invention reduces scratches on the roll, and the degree of occurrence of such scratches also depends on the surface roughness of the roll used for transporting and stretching. Such rolls are generally mirror-finished so that the surface roughness Rz is 1 μm or less, but even protrusions having a height of about submicron existing on the surface damage the film. Therefore, it is necessary to measure the characteristics of the film surface layer using a minute indenter and adjust the Young's modulus optimal for producing a polarizing film. Brinell, Rockwell, Vickers, etc., which push a relatively large indenter into a relatively large area with a relatively large load, are difficult to evaluate accurately, and the nanoindentation test performed in the present invention is a more effective test. The method.

The polyvinyl alcohol-based film of the present invention has a Young's modulus of 15 to 40 MPa, preferably 16 to 40 MPa, when a nanoindentation test is performed in water at 25 ° C. using the scanning probe microscope described above. It is 35 MPa, particularly preferably 17 to 30 MPa.
If the Young's modulus of the film surface in such 25 ° C. water is less than the lower limit value, scratches with the roll increase and the object of the present invention cannot be achieved, and even if the upper limit value is exceeded, the film and the roll adhere to each other. The object of the present invention cannot be achieved due to a decrease in sex and an increase in scratches.
A notable point of the present invention is that it is generally considered that the higher the Young's modulus of the film surface and the harder it is, the less likely it is to be scratched. However, in reality, the film must be soft enough to ensure adhesion to the roll. It is the point that I found that it must be done. If the surface is too hard, the film only slides on the roll, and the film is not smoothly conveyed in accordance with the rotation of the roll, and scratches tend to increase.

Further, a remarkable effect of the present invention is a wound self-repairing function.
The self-repairing function is a function in which the polymer chain of the polyvinyl alcohol-based resin moves to a dented portion and is flattened. If the dent is deep, it is difficult to completely repair it, but for example, if it is a shallow wound with a depth of about submicron, it can be repaired. If the Young's modulus of the film surface of the polyvinyl alcohol-based film in water at 25 ° C is less than 15 MPa, deep scratches are likely to occur and self-repair does not occur, but when the film surface has the Young's modulus of the above-mentioned specific range, effective self Repair function tends to develop.

As a method for controlling the young ratio of the film surface in water at 25 ° C., a method for adjusting the chemical structure and molecular weight of the polyvinyl alcohol-based resin, a method for adjusting the component and blending amount of the additive of the polyvinyl alcohol-based resin aqueous solution, and polyvinyl Examples include a method of adjusting the film forming conditions of the alcohol-based film and a method of adjusting the heat treatment conditions of the polyvinyl alcohol-based film.
Among these, in terms of the control range, a method of adjusting the component and blending amount of the additive of the polyvinyl alcohol-based resin aqueous solution (preferably a method of adjusting the blending amount of glycerin) and the heat treatment condition of the polyvinyl alcohol-based film are adjusted. This method is preferable, and more preferably, the heat treatment temperature is adjusted from the viewpoint of accurately controlling the Young ratio of the film surface.

When the young ratio of the film surface in water at 25 ° C. is controlled by the amount of glycerin blended in the above-mentioned aqueous solution of polyvinyl alcohol-based resin, the blending amount (ratio) of such glycerin is 5 to 15% by weight based on the polyvinyl alcohol-based resin. It is preferably 6 to 12% by weight. If the amount of glycerin is too small, the Young's modulus of the film surface in 25 ° C. water tends to increase, and if it is too large, the Young's modulus of the surface in 25 ° C. water tends to decrease.

The heat treatment temperature when controlling the Young's modulus of the film surface in 25 ° C. water under the above heat treatment conditions is preferably 80 to 150 ° C., particularly preferably 80 to 140 ° C., and even more preferably 90 to 140 ° C. If the heat treatment temperature is too low, the Young's modulus of the surface in 25 ° C. water tends to decrease, and if it is too high, the Young's modulus of the film surface in 25 ° C. water becomes too high, and the stretchability during the production of the polarizing film decreases. Tend to do. Examples of such a heat treatment method include a method of blowing hot air with a floating dryer and a method of irradiating near infrared rays with an infrared lamp. The heat treatment time is usually 0.1 to 60 minutes.

The polyvinyl alcohol-based film of the present invention preferably has a Young's modulus of 100 to 1,000 MPa on the film surface when a nanoindentation test is performed using a scanning probe microscope in the atmosphere of 25 ° C. and 60% RH. Especially preferably, it is 200 to 700 MPa. If the Young's modulus of the film surface in the atmosphere is too low or too high, it becomes difficult to control the Young's modulus of the film surface in water.

The polyvinyl alcohol-based film of the present invention preferably has a film surface roughness Ra of 3 nm or less, particularly preferably 2 nm or less, and further preferably 1 nm or less when measured in water at 25 ° C. using a scanning probe microscope. Is.
If the surface roughness Ra in the water is too large, the scratches during the production of the polarizing film tend to increase.
The surface roughness Ra is a value when a 10 μm square is measured in water at 25 ° C. using a scanning probe microscope.

The polyvinyl alcohol-based film of the present invention is produced by forming a film of a polyvinyl alcohol-based resin as a raw material.

As the polyvinyl alcohol-based resin used in the present invention, an unmodified polyvinyl alcohol-based resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate is usually 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 vinyl acetate and a copolymerizable component may be used. it can. Examples of the component copolymerizable with vinyl acetate include unsaturated carboxylic acids (including, for example, salts, esters, amides, and nitriles) and olefins having 2 to 30 carbon atoms (for example, ethylene, propylene, and n-butene). , Isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. Further, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl group after saponification can also be used.

Further, as the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain can also be used. The polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain is, for example, (i) a method for saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) acetate. A method for saponifying and decarbonizing a copolymer of vinyl and vinyl ethylene carbonate, (iii) saponifying and decarbonating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane. It can be obtained by a method of ketalization, (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-based resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000. If 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 if it is too large, it tends to be difficult to stretch the polyvinyl alcohol-based film during production of a polarizing film. There is. The weight average molecular weight of the polyvinyl alcohol-based 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, still more preferably 99.5 mol% or more, and 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 a polyvinyl alcohol-based 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 amounts, weight average molecular weight, average saponification degree and the like may be used in combination.

For the polyvinyl alcohol-based film of the present invention, a polyvinyl alcohol-based resin aqueous solution is prepared using the above-mentioned polyvinyl alcohol-based resin, and the polyvinyl alcohol-based resin aqueous solution is used to discharge and cast the aqueous solution into a rotating cast mold. It can be continuously produced by forming a film and drying it by a casting method, and for example, it can be produced by the following steps.
(A) A step of forming a film by a casting method.
(B) A step of heating and drying the formed film.
(C) A step of slitting both ends of the dried film and then winding it on a roll.

Here, examples of the cast type include a cast drum (drum type roll), an endless belt, and the like, but it is preferable to use a cast drum from the viewpoint of being excellent in width, length, and film thickness uniformity.
The case where the cast type is a cast drum will be described below.

First, the step (A) will be described.

In the step (A), first, the above-mentioned polyvinyl alcohol-based resin is washed with water and dehydrated using a centrifuge or the like to obtain a polyvinyl alcohol-based resin wet cake having a water content of 50% by weight or less. Is preferable. If the water content is too high, it tends to be difficult to obtain the desired aqueous solution concentration.
The polyvinyl alcohol-based resin wet cake is dissolved in warm water or hot water to prepare a polyvinyl alcohol-based resin aqueous solution.

The method for preparing the polyvinyl alcohol-based resin aqueous solution is not particularly limited, and for example, it may be prepared using a heated multi-screw extruder, and the dissolution can provided with the vertical circulation flow generation type stirring blade is described above. It is also possible to put in the polyvinyl alcohol-based resin wet cake and blow steam into the can to dissolve and prepare an aqueous solution having a desired concentration.

In addition to the polyvinyl alcohol-based resin, the polyvinyl alcohol-based resin aqueous solution contains a commonly used plasticizer such as glycerin and a surfactant consisting of at least one of nonionic, anionic, and cationic surfactants. However, it is preferable in terms of film-forming property of the polyvinyl alcohol-based film. The content of the plasticizer such as glycerin and the surfactant is preferably 1 to 20% by weight, more preferably 5 to 15% by weight, based on the polyvinyl alcohol-based resin.

The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, and even more preferably 20 to 50% by weight. If the resin concentration of such an aqueous solution is too low, the drying load tends to be large, so that the production capacity tends to decrease, and if it is too high, the viscosity tends to be too high and uniform dissolution tends to be difficult.

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 having a vent. As the multi-screw extruder having a vent, a twin-screw extruder having a vent is usually used.

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

The temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-shaped slit die is preferably 80 to 100 ° C, particularly preferably 85 to 98 ° C.
If the temperature of the polyvinyl alcohol-based resin aqueous solution is too low, the flow 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 at the time of discharge, and particularly preferably 70 to 150 Pa · s.
If the viscosity of the aqueous solution is too low, the flow 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 even more preferably 0. It is 6 to 3 m / min.
If the discharge rate is too slow, the productivity tends to decrease, and if it is too fast, the flow 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 even more preferably 2.8 to 4 m.
If the diameter is too small, the drying length tends to be insufficient and the speed tends to be low, 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, still more preferably 5 m or more, and particularly preferably 5 to 6 m.
If the width of the cast drum is too small, 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 even more 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 such a cast drum is preferably 40 to 99 ° C, particularly preferably 60 to 95 ° C.
If the surface temperature is too low, drying tends to be poor, and if it is too high, foaming tends to occur.

Next, the step (B) will be described. The step (B) is a step of heating and drying the formed film.

Drying of the film formed by the cast drum is performed by alternately contacting the front surface and the back surface of the film with a plurality of metal heating rolls. The surface temperature of the metal heating roll is usually 40 to 150 ° C., preferably 50 to 130 ° C., particularly preferably 60 to 110 ° C. If the surface temperature is too low, it tends to be poorly dried, and if it is too high, it tends to be too dry, resulting in poor appearance such as swelling.
The metal heating roll is, for example, a roll having a diameter of 0.2 to 2 m whose surface is hard chrome plated or mirror-treated, and is usually dried using 2 to 30 rolls, preferably 10 to 25 rolls. Is preferable.

In the present invention, it is preferable to perform the above-mentioned heat treatment in addition to drying.

The dried film becomes a product (polyvinyl alcohol-based film of the present invention) through the step (C). The step (C) is a step of slitting both ends of the film and winding the film on a roll.

The method for producing a polyvinyl alcohol-based film by preparing a polyvinyl alcohol-based resin aqueous solution, casting the aqueous solution on a rotating cast drum (drum type roll), forming a film by a casting method, and drying the film will be described. However, it is also possible to cast a polyvinyl alcohol-based resin aqueous solution on a resin film or a metal belt to form a film and dry it.

The thickness of the polyvinyl alcohol-based film needs to be 5 to 60 μm from the viewpoint of thinning the polarizing film, and preferably 5 to 30 μm from the viewpoint of further thinning. From the viewpoint of the relationship between the characteristics of the present invention (Young's modulus of the film surface) and the thinning, it is particularly preferably 10 to 30 μm. The thickness of the polyvinyl alcohol-based film can be adjusted by adjusting the resin concentration in the polyvinyl alcohol-based resin aqueous solution, the discharge amount (discharge speed) of the polyvinyl alcohol-based resin aqueous solution to the cast drum, the rotation speed of the cast drum, and the like.

The width of the polyvinyl alcohol-based film is preferably 4 m or more, more preferably 4.5 m or more from the viewpoint of increasing the area, and particularly preferably 4.5 to 6 m from the viewpoint of avoiding breakage.

The length of the polyvinyl alcohol-based film is preferably 4 km or more, more preferably 4.5 km or more from the viewpoint of increasing the area, and particularly preferably 4.5 to 50 km from the viewpoint of transport weight. Then, preferably, a roll-shaped polyvinyl alcohol-based film can be obtained by winding a polyvinyl alcohol-based film having a width of 4 m or more and a length of 4 km or more on a roll.

Such a polyvinyl alcohol-based film has an appropriate Young's modulus on the surface, is preferably used as a polyvinyl alcohol-based film for optics, and is particularly preferably used as a raw material for a polarizing film.

Next, the polarizing film of the present invention will be described.

The polarizing film of the present invention is produced by unwinding the polyvinyl alcohol-based film from a roll, transferring it in the horizontal direction, and performing steps such as swelling, dyeing, boric acid cross-linking, stretching, washing, and drying.

The swelling step is performed before the dyeing step. In addition to being able to clean the dirt on the surface of the polyvinyl alcohol-based film by the swelling step, there is also an effect of preventing uneven dyeing by swelling the polyvinyl alcohol-based film. In the swelling step, water is usually used as the treatment liquid. If the main component of the treatment liquid is water, it may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol and the like. 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. Further, if necessary, a stretching operation may be performed during the treatment.

The dyeing step is performed by contacting the film with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide is used, and an iodine concentration of 0.1 to 2 g / L and a potassium iodide concentration of 1 to 100 g / L are suitable. It is practical that the 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. Further, if necessary, a stretching operation may be performed during the treatment.

The boric acid cross-linking step is carried out 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 mixed solution at a concentration of about 10 to 100 g / L, and it is preferable that potassium iodide coexists in the solution from the viewpoint of stabilizing the polarization performance. The temperature at the time of treatment is preferably about 30 to 70 ° C., the treatment time is preferably about 0.1 to 20 minutes, and if necessary, a stretching operation may be performed during the treatment.

In the stretching step, it is preferable to stretch 3 to 10 times, preferably 3.5 to 6 times in the uniaxial direction. At this time, a slight stretching (stretching to the extent of preventing shrinkage in the width direction or more) may be performed in the direction perpendicular to the stretching direction. The temperature at the time of stretching is preferably 30 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 step but also in any range of the manufacturing process.

The washing step is performed by immersing the polyvinyl alcohol-based film in, for example, an aqueous solution of iodide such as water or potassium iodide, and precipitates generated on the surface of the film can be removed. When an aqueous potassium iodide solution is used, the potassium iodide concentration may be about 1 to 80 g / L. The temperature during the cleaning 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. In addition, washing with water and washing with an aqueous solution of potassium iodide may be performed in an appropriate combination.

The drying step may be carried out in the air at 40 to 80 ° C. for 1 to 10 minutes.

The degree of polarization of the polarizing film is preferably 99.5% or more, more preferably 99.8% or more. If the degree of polarization is too low, it tends to be impossible to secure the contrast in the liquid crystal display.
The degree of polarization is generally the light transmittance (H ₁₁ ) measured at wavelength λ and the two polarized light in a state where two polarizing films are superposed so that their orientation directions are the same. It is calculated according to the following formula from the light transmittance (H ₁ ) measured at the wavelength λ in a state where the films are superposed so that the orientation directions are orthogonal to each other.
[(H ₁₁ −H ₁ ) / (H ₁₁ + H ₁ )] ^1/2

Further, the simple substance transmittance of the polarizing film of the present invention is preferably 42% 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 a single polarizing film using a spectrophotometer.

Thus, the polarizing film of the present invention can be obtained, and the polarizing film of the present invention is suitable for producing a polarizing plate having less polarization unevenness.
Hereinafter, the method for producing the polarizing plate of the present invention will be described.

The polarizing film of the present invention is formed into a polarizing plate by laminating an optically isotropic resin film as a protective film on one or both sides thereof via an adhesive. Examples of the protective film include films such as cellulose triacetate, cellulose diacetate, polycarbonate, polymethylmethacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methylpentene, and polyphenylene oxide. Or you can give a sheet.

The bonding method is a known method. For example, a liquid adhesive composition is uniformly applied to a polarizing film, a protective film, or both, and then the two are bonded and pressure-bonded to heat or activate. It is done by irradiating with energy rays.

Further, for the purpose of thinning the polarizing film, a curable resin such as a urethane resin, an acrylic resin, or a urea resin is applied to one or both sides of the protective film instead of the protective film, and the film is cured to form a polarizing plate. You can also do it.

The polarizing film and the polarizing plate obtained by the present invention have few display defects and color unevenness and are excellent in in-plane uniformity of polarization performance, and are excellent in in-plane uniformity of polarization performance. Liquid crystal displays such as watches, word processors, electronic paper, game machines, videos, cameras, photo albums, thermometers, audio, automobile and mechanical instruments, sunglasses, anti-glare glasses, three-dimensional glasses, wearable displays, display elements ( It is preferably used for antireflection layers for (CRT, LCD, 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 the example, "part" and "%" mean a weight standard.
Each physical property was measured as follows.

<Measurement conditions>
(1) Young's modulus (MPa) of the film surface in the underwater nanoindentation test
A 1 cm × 1 cm test piece is cut out from the obtained polyvinyl alcohol-based film, and the test piece is immersed in ion-exchanged water at 25 ° C. for 2 hours, and then a scanning probe microscope (“MFP-3D Standalone” manufactured by ASYLUM RESEARCH) is used. The Young's modulus (MPa) of the film surface in water at 25 ° C. was measured. The detailed measurement conditions are as follows.
Measurement mode: Force mode Cantilever: TR800PSA (S) (Resonance frequency: 73 kHz, spring constant: 0.57 N / m)
Scanning range: 1 μm x 1 μm
Number of captured data: 16 x 16
Pushing speed: 1.98 μm / s
Maximum pushing amount: 200 nm
The average value of the obtained 256-point data was defined as the Young's modulus (MPa) of the film surface in water. The preconditions for calculating the Young's modulus of the film surface are as follows.
Cantilever tip shape: Punch (cylinder), radius 9.00 nm
Poisson's ratio: 0.4

(2) Young's modulus (MPa) of the film surface in the atmospheric nanoindentation test
A 1 cm x 1 cm test piece was cut out from the same polyvinyl alcohol-based film as above, and the test piece was adjusted for one day in an environment of 25 ° C. and 60% RH. Scanning probe microscope (ASYLUM RESEARCH "MFP-3D stand-alone"" ) Was used to measure the Young's modulus (MPa) of the film surface in the air at 25 ° C. and 60% RH. The detailed measurement conditions are as follows.
Measurement mode: Force mode Cantilever: AC160TS (resonance frequency: 300 kHz, spring constant: 26 N / m)
Scanning range: 1 μm x 1 μm
Number of captured data: 16 x 16
Pushing speed: 1.98 μm / s
Maximum pushing amount: 20 nm
The average value of the obtained 256-point data was defined as the Young's modulus (MPa) of the film surface in the atmosphere. The preconditions for calculating the Young's modulus of the film surface are as follows.
Cantilever tip shape: Punch (cylinder), radius 9.00 nm
Poisson's ratio: 0.4

(3) Surface roughness Ra (nm)
A 1 cm × 1 cm test piece is cut out from the obtained polyvinyl alcohol-based film, and the test piece is immersed in ion-exchanged water at 25 ° C. for 2 hours, and then a scanning probe microscope (“MFP-3D Standalone” manufactured by ASYLUM RESEARCH) is used. Using, the range of 10 μm × 10 μm was measured in water at 25 ° C. The detailed measurement conditions are as follows.
Measurement mode: Dynamic force mode (DFM)
Cantilever: TR800PSA (S) (resonance frequency: 73 kHz, spring constant: 0.57 N / m)
Scanning range: 10 μm x 10 μm
Number of captured data: 256 x 256
Scanning speed: 0.5Hz

(4) Scratches Ten 1 cm x 1 cm test pieces were cut out from the obtained polarizing film, and the presence or absence of scratches with a width of 10 μm or more was observed using a laser focus microscope VK-9700 (objective lens: 50 times) manufactured by KEYENCE Corporation. However, it was evaluated according to the following criteria.
(Evaluation criteria)
◯: All test pieces were not scratched.
Δ: One of the test pieces was scratched.
X: All test pieces were scratched.

(5) Polarization degree (%), single transmittance (%)
A test piece having a length of 4 cm and a width of 4 cm is cut out from the obtained polarizing film, and the degree of polarization (%) and the single transmittance (%) are measured using an automatic polarizing film measuring device (manufactured by JASCO Corporation: VAP7070). did.

(6) Polarization 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 two polarizing plates in a cross Nicol state (single transmittance 43.5%, polarization degree 99.9%) were formed. After sandwiching it at an angle of 45 °, optical color unevenness was observed in the transmission mode using a light box with a surface illuminance of 14,000 lux, and evaluated according to the following criteria.
(Evaluation criteria)
◯: No color unevenness.
Δ: There is slight color unevenness.
×: There is color unevenness.

<Example 1>
(Manufacturing of polyvinyl alcohol film)
Add 2,000 kg of polyvinyl alcohol-based resin having a weight average molecular weight of 142,000 and a saponification degree of 99.8 mol%, 4,500 kg of water, and 200 kg of glycerin as a plasticizer (10% by weight based on the polyvinyl alcohol-based resin), and stir. While raising the temperature to 140 ° C., the concentration was adjusted to a resin concentration of 25% to obtain a uniformly dissolved polyvinyl alcohol-based resin aqueous solution. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin-screw extruder having a vent to defoam, then the aqueous solution temperature is set to 90 ° C., and the discharge speed is 2.5 m / from the T-shaped slit die discharge port to the cast drum. The film was formed by casting in minutes. The film was peeled off from the cast drum, and the film was dried while the front surface and the back surface of the film were alternately brought into contact with a plurality of metal heating rolls. The temperature of the metal heating roll was 80 to 120 ° C. Further, using a floating dryer, hot air at 120 ° C. was blown from both sides of the film for 1 minute to perform heat treatment, and then both ends were slit and wound on a roll to have a width of 4.8 m, a thickness of 60 μm, and a length of 5 km. A polyvinyl alcohol-based film was obtained. The characteristics of the obtained polyvinyl alcohol-based film are shown in Table 1.

(Manufacturing of polarizing film)
The obtained polyvinyl alcohol-based film was unwound from a roll, transported in the horizontal direction, immersed in a water tank having a water temperature of 30 ° C. and swollen, and stretched 1.7 times in the flow direction. Next, while immersing in an aqueous solution of iodine (0.5 g / L and potassium iodide) at 30 ° C. for staining, the mixture was stretched 1.6 times in the flow direction, and then boric acid 40 g / L and potassium iodide were added. It was uniaxially stretched 2.1 times in the flow direction while being immersed in an aqueous solution (50 ° C.) having a composition of potassium iodide of 30 g / L and crosslinked with boric acid. Finally, it was washed with an aqueous potassium iodide solution and then dried at 50 ° C. for 2 minutes to obtain a polarizing film having a total draw ratio of 5.7 times. The characteristics of the obtained polarizing film were as shown in Table 1.

<Example 2>
A polyvinyl alcohol-based film and a polarizing film were obtained in the same manner as in Example 1 except that the amount of glycerin was 120 kg (6% by weight based on the polyvinyl alcohol-based resin). The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Example 3>
A polyvinyl alcohol-based film and a polarizing film were obtained in the same manner as in Example 1 except that the temperature of the hot air was 140 ° C. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Example 4>
Example 1 except that the discharge rate was 1.3 m / min, the film thickness of the polyvinyl alcohol film was 30 μm, the amount of glycerin was 120 kg (6% by weight based on the polyvinyl alcohol resin), and the temperature of the hot air was 100 ° C. In the same manner, a polyvinyl alcohol-based film and a polarizing film were obtained. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Example 5>
The same as in Example 1 except that the discharge rate was 1.3 m / min, the film thickness of the polyvinyl alcohol film was 30 μm, and the amount of glycerin was 120 kg (6% by weight based on the polyvinyl alcohol resin). A film and a polarizing film were obtained. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Example 6>
Example 1 except that the discharge rate was 1.3 m / min, the film thickness of the polyvinyl alcohol film was 30 μm, the amount of glycerin was 200 kg (10% by weight based on the polyvinyl alcohol resin), and the temperature of the hot air was 140 ° C. In the same manner, a polyvinyl alcohol-based film and a polarizing film were obtained. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Example 7>
Example 1 except that the discharge rate was 0.8 m / min, the film thickness of the polyvinyl alcohol film was 20 μm, the amount of glycerin was 160 kg (8% by weight based on the polyvinyl alcohol resin), and the temperature of the hot air was 120 ° C. In the same manner, a polyvinyl alcohol-based film and a polarizing film were obtained. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Comparative example 1>
A polyvinyl alcohol-based film and a polarizing film were obtained in the same manner as in Example 1 except that the amount of glycerin was 160 kg (8% by weight based on the polyvinyl alcohol-based resin) and the temperature of the hot air was 150 ° C. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

<Comparative example 2>
A polyvinyl alcohol-based film and a polarizing film were obtained in the same manner as in Example 1 except that the discharge rate was 1.3 m / min, the film thickness of the polyvinyl alcohol-based film was 30 μm, and the temperature of the hot air was 100 ° C. After manufacturing the polarizing film, when the dirt inside each bathtub was visually confirmed, shavings were confirmed. The characteristics of the obtained polyvinyl alcohol-based film and polarizing film are as shown in Table 1.

Since the Young's modulus of the film surface in water at 25 ° C. in the nanoindentation test in the polyvinyl alcohol-based films of Examples 1 to 7 is within the specific range of the present invention, when a polarizing film is prepared using the Young's modulus, it is damaged. A polarizing film with a small amount of water was obtained. On the other hand, since the Young's modulus of the polyvinyl alcohol-based films of Comparative Examples 1 and 2 was outside the specific range of the present invention, the polarizing film obtained by using the film had many scratches.
Then, it can be seen that the polarization characteristics of the polarizing film obtained from each polyvinyl alcohol-based film are better in Examples 1 to 7 than in Comparative Examples 1 and 2.

Although the specific embodiments of the present invention have been shown in the above examples, the above examples are merely examples and are not to be interpreted in a limited manner. Various variations apparent to those skilled in the art are intended to be within the scope of the present invention.

The polarizing film and the polarizing plate obtained by the present invention have few display defects and color unevenness and are excellent in in-plane uniformity of polarization performance, and are excellent in in-plane uniformity of polarization performance. Liquid crystal displays such as watches, word processors, electronic paper, game machines, videos, cameras, photo albums, thermometers, audio, automobile and mechanical instruments, sunglasses, anti-glare glasses, three-dimensional glasses, wearable displays, display elements ( It is preferably used for antireflection layers for (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

Claims (5)

It is a polyvinyl alcohol-based film having a thickness of 5 to 60 μm, and is characterized by having a Young's modulus of 15 to 40 MPa on the film surface when a nanoindentation test is performed in water at 25 ° C. using a scanning probe microscope. Polyvinyl alcohol-based film. The polyvinyl alcohol according to claim 1, wherein the Young's modulus of the film surface when a nanoindentation test is performed using a scanning probe microscope in the atmosphere of 25 ° C. and 60% RH is 100 to 1,000 MPa. System film. The polyvinyl alcohol-based film according to claim 1 or 2, wherein the surface roughness Ra of the film when measured in water at 25 ° C. using a scanning probe microscope is 3 nm or less. The polyvinyl alcohol-based film according to any one of claims 1 to 3, wherein the polyvinyl alcohol-based film is a roll-shaped polyvinyl alcohol-based film having a width of 4 m or more and a length of 4 km or more. A polarizing film obtained by using the polyvinyl alcohol-based film according to any one of claims 1 to 4.