WO2008111702A1

WO2008111702A1 - Preparation method of polyvinyl alcohol polarizer film and polyvinyl alcohol polarizer film prepared by using the same

Info

Publication number: WO2008111702A1
Application number: PCT/KR2007/002337
Authority: WO
Inventors: In-Seok Oh; Yoo-Seong Hong; Bong-Tae Kim; Kyoung-Sik Choi; Hwa-Sub Shim; Jun-Wuk Park
Original assignee: Lg Chem, Ltd.
Priority date: 2007-03-15
Filing date: 2007-05-11
Publication date: 2008-09-18
Also published as: TW200844503A

Abstract

Disclosed are a method for preparing a polyvinyl alcohol polarizer film and a polyvinyl alcohol polarizer film prepared by using the same. The method of a polyvinyl alcohol polarizer film includes: (a) dissolving a polyvinyl alcohol polymer, a water-soluble cross-linking agent, a plasticizer and a surfactant in water to prepare an aqueous polyvinyl alcohol polymer solution; (b) solution-casting, drying and heat-treating the aqueous polyvinyl alcohol polymer solution to prepare a polyvinyl alcohol film; and (c) dyeing the polyvinyl alcohol film. The method of the present invention may be useful to reduce the transfer of polyvinyl alcohol polymer into a skin layer of the film because the polyvinyl alcohol polymer is cross-linked by a water-soluble cross-linking agent in the solution-casting and drying steps. Accordingly, a dyeing property and stretchability are improved in the subsequent dyeing and stretching processes, and the polarizer film has improved mechanical property and polarization degree.

Description

PREPARATION METHOD OF POLYVINYL ALCOHOL POLARIZER FILM AND POLYVINYL ALCOHOL POLARIZER FILM PREPARED BY USING

THE SAME

TECHNICAL FIELD

The present invention relates to a method for preparing a polyvinyl alcohol polarizer film and a polyvinyl alcohol polarizer film prepared by using the same, and more particularly to a method for preparing a polyvinyl alcohol polarizer film having a high polarization degree by preparing a polyvinyl alcohol film having a good uniformity toward a thickness direction by using a solution casting process and dyeing the polyvinyl alcohol film.

BACKGROUND ART

In the recent years, there has been significant advance in the field of information technology (IT) industries, and therefore a liquid crystal display (LCD), which is one of advanced parts in the IT industries, has also been developed as a medium for transmitting a variety of information as the modern industrial society advances into a highly information-oriented society.

A polarizer sheet having a polarizer film is adhered to both sides of a flat panel display liquid crystal cell such as LCD or organic EL, and therefore functions as a part of a liquid crystal display panel. The liquid crystal display panel has a configuration where the polarizer sheet is adhered to both sides of the liquid crystal cell. FIG. 1 schematically shows a conventional polarizer sheet. Referring to FIG. 1, the polarizer sheet 1 includes a polarizer 6 arranged in its central region; and protective films 2,3 adhered to both sides of the polarizer 6 by means of pressure-sensitive adhesive layers 4,5. The protective films 2,3 are arranged in both sides of the polarizer 6 to play a role in preventing damage of the polarizer 6. The upper protective film 2 (a separator film) is peeled off and discarded before it is attached to the liquid crystal cell, and therefore the exposed pressure-sensitive adhesive layer 4 acts as a medium for adhesion between the polarizer 6 and the liquid crystal cell. The lower protective film 3 is also peeled off and discarded after it is adhered to the liquid crystal cell. At this tune, the pressure-sensitive adhesive layer 5 is also peeled off together with its being adhered to the protective film 3. The protective films 2,3 may be formed of ester resin such as PET.

The polarizer 6 arranged between the protective films 2,3 includes a polarizer film 10 and protective layers 7,8 adhered to both sides of the polarizer film 10 using adhesive layers 9 interposed between the protective layers 7,8 and the polarizer film 10. Here, the polarizer film 10 is generally made of a polyvinyl alcohol polymer dyed with iodine or dichromic dye, and the protective layers 7,8 are made of triacetyl cellulose (TAC), etc.

The above-mentioned polyvinyl alcohol polarizer film is prepared by dissolving a polyvinyl alcohol polymer, a plasticizer and a surfactant in water to prepare an aqueous polyvinyl alcohol polymer solution, followed by solution-casting, drying and heat-treating the aqueous polyvinyl alcohol polymer solution to form a polyvinyl alcohol film and dyeing the polyvinyl alcohol film. However, if the polyvinyl alcohol film is prepared by using the solution casting method, it is difficult to form a polarizer film whose surface and inner part have the uniform structure since the polarizer film has a skin-core structure toward a thickness direction during the drying and heat treatment process.

That is to say, the aqueous polyvinyl alcohol polymer solution discharged from the die by means of the solution casting allows a polyvinyl alcohol polymer to be transferred into a skin layer of film when the solvent, water, flows out of the film. Generally, polymers rarely flow in the film during the above-mentioned drying and heat treatment steps, but the polyvinyl alcohol polymer is transferred into a surface layer of the film while a low molecular-weight water is dried since it has a strong interaction with molecules of the solvent, water. Because of this phenomenon, a density of the polyvinyl alcohol polymer is increased in the surface layer of the film, and therefore the skin layer of the film may have a higher crystallization degree than the core layer when a crystal is formed during the heat treatment process.

The ununiformity in the structure toward a thickness direction prevents iodine or a dichromic dye from penetrating into the deep inner part of the film during the dyeing process for preparing a polarizer film, and therefore the film is not dyed uniformly toward a thickness direction and its polarization degree is also deteriorated. Also, the core layer of the film has a poor strength, and therefore, if a thin polarizer film having a thickness of 50 μm or less is manufacture, its mechanical properties are not good, and a blocking phenomenon may appear in the polarizer film, resulting in difficulty in stretching workability.

DISCLOSURE OF INVENTION Accordingly, the present invention is designed to solve the problems of the prior art, and therefore it is one object of the present invention to provide a method for preparing a polyvinyl alcohol polarizer film having a uniform polymer density toward a thickness direction. Also, it is another object of the present invention to provide a polyvinyl alcohol polarizer film prepared by using the preparation method.

In order to accomplish the above object, the present invention provides a method for preparing a polyvinyl alcohol polarizer film, the method including:

(a) dissolving a polyvinyl alcohol polymer, a water-soluble cross-linking agent, a plasticizer and a surfactant in water to prepare an aqueous polyvinyl alcohol polymer solution;

(b) solution-casting, drying and heat-treating the aqueous polyvinyl alcohol polymer solution to prepare a polyvinyl alcohol film; and

(c) dyeing the polyvinyl alcohol film. Also, the present invention provides a polyvinyl alcohol polarizer film prepared by using the preparation method.

As described above, the polyvinyl alcohol molecule has an excellent compatibility with the solvent, water. Accordingly, when the aqueous polyvinyl alcohol polymer solution is solution-cast, and then dried and heat-treated, a surface region of the polyvinyl alcohol film made from the polyvinyl alcohol polymer has a higher polymer density than its core region since a polyvinyl alcohol polymer is transferred into a surface of the film during the water evaporation. Accordingly, it is difficult to perform the subsequent processes due to difference in physical properties between the skin layer and the core layer of the film when the polyvinyl alcohol film is dyed and stretched to prepare a polarizer film.

Accordingly, in order to minimize the transfer of a polyvinyl alcohol polymer, the inventors found that a cross-linking reaction is induced in a drying step by adding a water-soluble cross-linking agent to an aqueous polyvinyl alcohol polymer solution prior to the step of solution-casting the aqueous polyvinyl alcohol polymer solution, wherein the cross-linking agent is able to cross-link the polyvinyl alcohol polymer to each other. As a result, the transfer of the polyvinyl alcohol polymer was minimized during drying and heat treatment steps so that a molecule density can not be increased in a skin layer of the film. Therefore, the prevent invention was designed on the basis of the above findings.

Accordingly, a dyeing property and stretchability are improved in the subsequent processes such a dyeing and stretching process since the polyvinyl alcohol film prepared according to the preparation method of the present invention has a uniform structure toward a thickness direction, and the polarizer film prepared by using the preparation method also has improved mechanical properties and polarization degree.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings:

FIG. 1 is a diagram showing a cross section of a conventional polarizer sheet provided with a polyvinyl alcohol polarizer film. FIG. 2 is an optical microscopic photograph showing a dyeing property of the polyvinyl alcohol polarizer film according to Example 1 of the present invention.

FIG. 3 is an optical microscopic photograph showing a dyeing property of the polyvinyl alcohol polarizer film according to Comparative example 1 of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described in detail. At first, the preparation method of the polyvinyl alcohol polarizer film according to the present invention is described. The method for preparing a polyvinyl alcohol film according to the present invention includes:

(c) dyeing the polyvinyl alcohol film.

Hereinafter, the preferred embodiments of the present invention will be described step by step in more detail.

(a) Preparation of Aqueous Polyvinyl Alcohol Polymer Solution

This step is to dissolve a polyvinyl alcohol, a water-soluble cross-linking agent, a plasticizer and a surfactant in water to prepare an aqueous solution.

In the preparation method of the polyvinyl alcohol polarizer film according to the present invention, the polyvinyl alcohol polymer including vinyl alcohol as a repeating unit may be prepared by hydrolyzing a vinyl ester polymer obtained by polymerizing a vinyl ester monomer, followed by saponifying a vinyl ester unit into a vinyl alcohol unit. The vinyl ester monomer includes, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivaliate, vinyl vasatate, etc. Among them, the vinyl acetate is desirable. It goes without saying that other known monomers such as ethylene and propylene may be also co-polymerized without departing from the sprite and scope of the present invention when the vinyl ester monomer is polymerized. The polyvinyl alcohol polymer preferably has a saponification degree of 95 % or more, more preferably 99 % or more, and the most preferably 99.5 % or more. Heat resistance of the film may be deteriorated if the saponification degree is less than 95 %. Also, a mean polymerization degree of the polyvinyl alcohol polymer preferably ranges from 2,000 to 7,000, and more preferably from 2,600 to 5,000, in consideration of a heat resistance, a solution casting property, etc. of the prepared film.

In this step, the polyvinyl alcohol polymer preferably has a content of 10 to 60 % by weight, more preferably 15 to 45 % by weight, and the most preferably 20 to 40 % by weight on the basis of the total weight of the aqueous polyvinyl alcohol polymer solution, in consideration of formability and uniformity of the prepared film.

Meanwhile, in the preparation method of the present invention, water-soluble cross-linking agents, widely known in the art to which the present invention belongs, may be used herein. Specific examples of the water-soluble cross-linking agent includes, but is not limited to, boric acid, borate and phosphoric acid, and they may be used alone or in combinations thereof.

In this step, a content of the water-soluble cross-linking agent preferably ranges from 0.001 to 2 parts by weight, and more preferably from 0.005 to 1 part by weight, based on 100 parts by weight of the polyvinyl alcohol polymer in the aqueous solution. A cross-linking effect on the polyvinyl alcohol polymer may be lowered if the content of the water-soluble cross-linking agent is less than 0.001 parts by weight, while stretchability may be deteriorated during the stretching process if the content exceeds 2 parts by weight.

In the preparation method of the present invention, the plasticizer in the aqueous polyvinyl alcohol polymer solution, which is added to improve a dyeing property of the polyvinyl alcohol film, may be generally used if it is used for preparing a polyvinyl alcohol polarizer film. Polyhydric alcohol may be used as the plasticizer, and glycerine is particularly preferably used due to its compatibility with the polyvinyl alcohol polymer and its low optical scattering property. A content of the plasticizer preferably ranges from 5 to 20 parts by weight, and more preferably from 7 to 15 parts by weight on the basis of 100 parts by weight of the polyvinyl alcohol polymer in the aqueous polyvinyl alcohol polymer solution, in consideration of a dyeing property of the polyvinyl alcohol film, a film strength, and adhesion between the films in a winding roll.

In the preparation method of the present invention, the surfactant is added to the aqueous solution so as to minimize a tensile force when a film is peeled off from a drum after the casting process. In the present invention, a non-ionic surfactant, a cationic surfactant, an anionic surfactant and the like may be all used without limit if the surfactant may be used for preparing a polyvinyl alcohol polarizer film, and it is particularly preferred to used the non-ionic surfactant. Specific examples of the surfactant, which may be used in the present invention, include a polyacrylate-modified compound, a polysiloxane-modified compound, a polyoxyethylene ether compound, etc.

The surfactant is preferably added in a content of 0.001 to 2 parts by weight on the basis of 100 parts by weight of the polyvinyl alcohol polymer, in consideration of a peeling property and surface roughness, etc. of the film.

(b) Formation of Polyvinyl Alcohol Film This step is to cast, dry and heat-treat the aqueous solution, prepared in the above step of preparing an aqueous polyvinyl alcohol polymer solution, to form a polyvinyl alcohol film.

The solution casting and drying process of this step may be performed under the conditions widely known in the art to which the present invention belongs, but the present invention is not limited to the conditions. Referring to one embodiment of the solution casting and drying process, the aqueous polyvinyl alcohol polymer solution prepared in the step (a) is cast onto the drum at a temperature of 80 to 100 ^"C through a

T-die having a temperature of 80 to 100 ^°C, thereby to form a film. At this time, a distance between the T-die and the drum is preferably maintained in a range of 10 mm or less, and preferably 5 mm or less.

If the film is formed on the drum, the film is peeled off from the drum and passed through a plurality of drying rolls having a temperature of 70 to 90 ^°C to form a polyvinyl alcohol film whose both surfaces are uniform. That is to say, the transfer of polymers into the surface layer of the film is inhibited in the film during the drying process since the polyvinyl alcohol polymer is cross-linked with each other by the water-soluble cross-linking agent added when preparing the aqueous casting solution. Therefore, a film having a high uniformity without a difference in density of the polymer toward a thickness direction of the film is prepared.

The polyvinyl alcohol film formed through the above-mentioned step is later subject to the heat treatment process so as to ensure mechanical properties. The heat treatment is preferably performed at a temperature of 100 to 160 ^°C, and more preferably 120 to 140 ^°C, in consideration of the mechanical properties and stretchability of the film.

In this step, there is no limitation on the heat treatment method, but conventional heat treatment methods may be used herein if they are used in the art to which the present invention belongs. As one example of the heat treatment method, it is possible to use a method for heat-treating a film while the film is in contact with a high-temperature roll, or a method for heat-treating a film with a high-temperature air, for example a hot air having a temperature of 100 to 160 ^°C in an air floating manner. In order to heat-treat a film uniformly, the film is preferably heat-treated in the air floating manner.

The heat-treated polyvinyl alcohol film preferably further undergoes a step of controlling humidity. The humidity control step is performed to remove a thermal history caused during the drying and heat treatment process, thereby to improve stretchability and a polarization property. There is no particular limitation on the humidity control method, but the humidity control method may be performed under humidity control conditions widely used in the art to which the present invention belongs. For example, the humidity control step may be performed under conditions of a temperature of 80 to 150 ⁰C and a humidity of 50 to 80 %.

The polyvinyl alcohol film prepared according to the above-mentioned method may be wound using conventional techniques known in the art to which the present invention belongs. Particularly, the anti-blocking agent, which is added for the purpose of solving the problem of the adhesion between the films in a winding roll, should not be added in the case of the present invention. This is why the water-soluble cross-linking agent in the film has an effect on the inhibition of the adhesion between the films by adjusting reactivity of the film surface to a suitable level.

(c) Dyeing of Polyvinyl Alcohol Film

This step is to dye the polyvinyl alcohol film prepared according to the above-mentioned step.

There is no particular limitation on the dyeing process in the present invention, but dyeing processes, generally used in the art to which the present invention belongs, may be performed with iodine or a dichromic dye, and the polyvinyl alcohol film is preferably dyed in an aqueous iodine and boric acid solution, and potassium iodide is more preferably added to the aqueous solution. For example, it is possible to dye the polyvinyl alcohol film by immersing the polyvinyl alcohol film at 3 to 10 minutes in an aqueous solution containing 0.03 to 0.2 % iodine and 0.5 to 5 % potassium iodide.

The polyvinyl alcohol film prepared thus is completed into the polyvinyl alcohol polarizer film through the stretching process. The stretching process may be performed before/after the dyeing step of the polyvinyl alcohol film, or performed at the same time as the dyeing step. That is to say, the polyvinyl alcohol film may be dyed and then stretched, and stretched and then dyed, or the polyvinyl alcohol film may also be dyed and stretched at the same time. There is no particular limitation on the stretching process, but stretching processes, which is generally used in the art to which the present invention belongs, may be used herein. For example, the polyvinyl alcohol film is pre-heated at a temperature of 40 to 80 ^°C for 30 seconds to 5 minutes, and then stretched 1.1 to 7 times in the length at a temperature of 40 to 80 ^°C, and preferably 45 to 60 ⁰C.

The polyvinyl alcohol polarizer film prepared according to the preparation method of the present invention has a uniform structure toward a thickness direction.

Also, the polyvinyl alcohol polarizer film has a very high polarization degree, which indicates that it has an improved polarization degree, compared to the conventional polyvinyl alcohol polarizer film.

Hereinafter, preferred embodiments of the present invention will be described for the purpose of better understanding, but the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention.

Example 1

27 % by weight of polyvinyl alcohol having a mean polymerization degree of 3200 and a saponification degree of 99.9 mol% was mixed and dissolved in water. Also, 0.1 parts by weight of boric acid, 12.4 parts by weight of glycerine and 0.1 parts by weight of a surfactant, polyoxyethylene ether compound, were added to the aqueous solution, based on 100 parts by weight of the polyvinyl alcohol in the aqueous solution, to prepare an aqueous solution containing 30.4 % by weight of a non-volatile compound.

The dissolution procedure was performed in a tank having a temperature of 130 ^°C or above in order to inhibit formation of undissolved materials. The prepared aqueous solution went through a defoaming process, and then was transferred to the T-die via a gear pump and a micro filter, and the aqueous solution was coated onto a drum having a temperature of 95 ^°C through the 95 ^°C T-die to form a film. At this time, a distance between the T-die and the drum was maintained in a range of 3 mm. The film was peeled off from the drum, and then passed through 11 drying rolls under a drying condition of 85 ^°C to form a polyvinyl alcohol film having a moisture content of 8 %. The film formed thus was passed through an air floating heating machine at a temperature of 120 ^°C to form a crystal. Then, the film went through a humidity control process to prepare a polyvinyl alcohol film having a thickness of 75 [M. The prepared polyvinyl alcohol film was swelled and dyed in an aqueous solution containing 0.05 % iodine and 0.3 % potassium iodide, and then stretched 5 times in a 50 ^°C bath in which 3 % boric acid was dissolved to prepare a polyvinyl alcohol polarizer film. Then, the polyvinyl alcohol polarizer film was dried at a temperature of 80 ^°C in the air floating manner to prepare a final polarizer film.

Example 2

30.0 % by weight of polyvinyl alcohol having a mean polymerization degree of

2600 and a saponification degree of 99.95 mol% was mixed and dissolved in water. Also, 0.01 parts by weight of boric acid, 12.4 parts by weight of glycerine and 0.1 parts by weight of a polyoxyethylene ester compound was added to the aqueous solution, based on 100 parts by weight of the polyvinyl alcohol in the aqueous solution, to prepare an aqueous solution containing 33.7 % by weight of a non-volatile compound. The subsequent processes were performed in the same manner as in Example 1 to prepare a polyvinyl alcohol polarizer film.

Example 3

This Example was repeated in the same manner as in Example 1 to prepare a polyvinyl alcohol polarizer film, except that 1.0 part by weight of boric acid was added in the preparation process of the aqueous solution, based on 100 parts by weight of the polyvinyl alcohol, to prepare an aqueous solution. However, the prepared polyvinyl alcohol polarizer film has a thickness of 50 /mi.

Comparative example 1

This Comparative example was repeated in the same manner as in Example 1 to prepare a polyvinyl alcohol polarizer film, except that the boric acid was not added in the preparation process of the aqueous solution to prepare an aqueous solution. However, the film was coated with starch prior to a winding process to prevent adhesion between the films in a winding roll. Such a film was cross-linked in an aqueous 3 % boric acid solution, dyed in an aqueous 0.05 % iodine and 0.3 % potassium iodide solution, and then stretched 5 times in a 50 ^°C bath in which 3 % boric acid was dissolved to prepare a polyvinyl alcohol polarizer film. Performance Evaluation

Dyeing and Stretchability

The polarizer films prepared in Example 1 and Comparative example 1 were evaluated for dyeing and stretching uniformity using an optical microscope (Leica DM digital microscopy). The results are shown in FIG. 2 and FIG. 3, respectively.

Polarization Degree

In the present invention, the polarization degree was measured according to the following Equation 1 by measuring a light transmission curve of two overlapped polarizer films. A used measuring machine was an n&kl280 Analyzer from N&K Technology.

Equation 1

Polarization Degree (%) X lOO

In the Equation 1, H₁ represents a light transmissivity obtained by measuring horizontally overlapped polarizer films, and H₂ represents a light transmissivity obtained by measuring vertically overlapped polarizer films. The light used in the measurement has a wavelength of 400 to 700 nm, and a mean value of the polarization degree was calculated for the same wavelength after the polarization degree was measured. The results according to the Examples 1 to 3 and Comparative example 1 are listed in the following Table 1.

Table 1

As seen from the results of the Table 1, it was revealed that the polyvinyl alcohol films prepared according to the Examples of the present invention have an excellent dyeing property and stretchability. The results of the dyeing property and stretchability might be confirmed from the optical microscopic photographic diagrams of FIG. 2 and FIG. 3 taken from the polarizer films prepared according to the Example 1 and the Comparative example 1. FIG. 2 is an optical microscopic photograph showing a dyeing property of the polyvinyl alcohol polarizer film according to Example 1 of the present invention, and FIG. 3 is an optical microscopic photograph showing a dyeing property of the polyvinyl alcohol polarizer film according to Comparative example 1 of the present invention. It was revealed that a central region of the film was uniformly dyed toward a thickness direction of the film as shown in FIG. 2, but some central region of the film was not dyed toward a thickness direction of the film as shown in FIG. 3. Accordingly, it was revealed that the polarizer film prepared according to the Example 1 of the present invention has an excellent dyeing property and stretchability, while the polarizer film of the Comparative example 1 has a poor dyeing property and stretchability.

From the results of the Table 1, it was also revealed that the polarizer films according to the present invention have a very excellent polarization degree, compared to that of the Comparative example 1.

INDUSTRIAL APPLICABILITY According to the preparation method of the present invention, a phenomenon in which a polyvinyl alcohol polymer is transferred into a skin layer of the film while evaporating water is improved in the steps of solution-casting and drying an aqueous polyvinyl alcohol polymer solution because the polyvinyl alcohol polymer is cross-linked by an added water-soluble cross-linking agent. Accordingly, a dyeing property and stretchability are improved in the subsequent steps such as dyeing and stretching processes, and the prepared polarizer film has improved mechanical property and polarization degree.

As described above, the preferred embodiments of the present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Claims

What is claimed is;

1. A method for preparing a polyvinyl alcohol polarizer film, the method comprising: (a) dissolving a polyvinyl alcohol polymer, a water-soluble cross-linking agent, a plasticizer and a surfactant in water to prepare an aqueous polyvinyl alcohol polymer solution;

(b) solution-casting, drying and heat-treating the aqueous polyvinyl alcohol polymer solution to prepare a polyvinyl alcohol film; and (c) dyeing the polyvinyl alcohol film.

2. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein the water-soluble cross-linking agent is one or a mixture of at least two selected from the group consisting of boric acid, borate and phosphoric acid.

3. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein a content of the water-soluble cross-linking agent ranges from 0.001 to 2 parts by weight, based on 100 parts by weight of the polyvinyl alcohol polymer.

4. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein the polyvinyl alcohol polymer has a saponification degree of 95 mol% or more.

5. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein the polyvinyl alcohol polymer has a mean polymerization degree of 2,000 to 7,000.

6. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein a content of the polyvinyl alcohol polymer ranges from 10 to 60 % by weight, based on the total weight of the aqueous polyvinyl alcohol polymer solution.

7. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, after the step (b), further comprising a step of controlling humidity of the polyvinyl alcohol film.

8. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein the step (c) is a step of dyeing a polyvinyl alcohol film in an aqueous iodine and boric acid solution.

9. The method for preparing a polyvinyl alcohol polarizer film according to claim 8, wherein the aqueous solution further includes potassium iodide.

10. The method for preparing a polyvinyl alcohol polarizer film according to claim 1, wherein the polarizer film has a thickness of 30 to 100 μm.

11. A polyvinyl alcohol polarizer film prepared by using the production method as defined in any one of claims 1 to 10.