TW202233385A - Polyvinyl alcohol film, polarizing film using same, and polarizing plate - Google Patents

Abstract

Provided are a PVA film whereby unevenness in a polarizing plate when the PVA film is stretched is hardly visible even when transmission of an LCD is increased, a polarizing film that uses such a PVA film, and a polarizing plate. A polyvinyl alcohol film in which, when F0(x) is defined as a film thickness profile for arbitrary points F0 on the film surface, F-100(x) is defined as a film thickness profile for points -100 mm in the TD direction from F0, F-200(x) is defined as a film thickness profile for points -200 mm in the TD direction from F0, F+100(x) is defined as a film thickness profile for points +100 mm in the TD direction from F0, and F+200(x) is defined as a film thickness profile for points +200 mm in the TD direction from F0, the average slope value represented by the average in the MD direction of F'0(x), F'-100(x), F'-200(x), F'+100(x), and F'+200(x), which are slope values in the MD direction of the respective film thickness profiles, is 0.02 or less, and a phase parameter defined by the average value of |F'-100(Cn)-F'0(Cn)|, |F'-200(Cn)-F'0(Cn)|, |F'+100(Cn)-F'0(Cn)|, and |F'+200(Cn)-F'-0(Cn)|, where Cn (n = 1, 2, 3, . . .) is the point at which F'0(x) is at a minimum value, is 0.015 or greater.

Description

Polyvinyl alcohol film and polarizing film and polarizing plate using the same

The present invention relates to a polyvinyl alcohol film, a polarizing film and a polarizing plate using the same.

Polarizing plates with light transmission and shielding functions and liquid crystals that change the polarization state of light are the basic components of liquid crystal displays (LCDs). LCDs can be used in a wide range of small devices such as computers and watches, notebook computers, LCD monitors, LCD color projectors, LCD TVs, car navigation systems, mobile phones, and indoor and outdoor measuring devices.

Polarizing plates are generally manufactured by subjecting a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA"), by dyeing, uniaxially stretching, and, if necessary, further subjecting to a fixation treatment based on a boron compound or the like. After the polarizing film, a protective film such as a triacetate cellulose (TAC) film is bonded to the surface of the polarizing film to manufacture. In recent years, energy saving of LCDs has been demanded, and high transmittance of polarizers has also been demanded. With the high transmission of polarizing plates, the unevenness of the polarizing plate caused by the uneven thickness of the PVA film, which has not been recognized so far, has become visually recognized. PVA film.

Patent Document 1 describes a method for producing a PVA film with better flatness than conventional ones, which suppresses variation during casting by using an air knife. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2017-008298

[The problem to be solved by the invention]

However, in Patent Document 1, even better film thickness flatness in the MD direction can be obtained than the conventional one, but since the thin film thickness portions are uniform in the width direction, stress is concentrated on the thin film thickness portions, and the visibility is low, but Regular polarizing plate unevenness can be visually recognized, and there is such a problem that it cannot meet the requirements for further higher transmission of LCDs.

Then, the present invention aims to provide a PVA film in which unevenness of a polarizing plate is not easily recognized when stretched even under the further increase in transmittance of LCD, and a polarizing film and polarizing light using such a PVA film. plate. [means to solve the problem]

The inventors of the present invention have repeatedly and intensively investigated, and found that, in addition to further reducing the film thickness unevenness in the MD direction (flow direction) of the PVA thin film, by shifting the phase of the film thickness unevenness at a specific position in the TD direction (width direction) The above-mentioned subject can be achieved by adjusting to a specific range, and based on this knowledge, the present invention has been completed by further repeated investigations.

That is, the present invention is [1] a polyvinyl alcohol film, wherein when the film thickness profile (film thickness profile) of any point F ₀ on the film surface is set to F ₀ (x), from F ₀ to the point of TD direction -100mm The film thickness distribution of the film is set to F- ₁₀₀ (x), the film thickness distribution from F ₀ to the point of TD direction -200mm is set to F _-200 (x), and the film thickness distribution from F ₀ to the point of TD direction +100mm is set to When the film thickness distribution is F _{+ 100} (x) and the point of +200 mm in the TD direction from F ₀ is set to F _{+ 200} (x), F' ₀ (x ), F' _-100 (x), F' _-200 (x), F' _{+ 100} (x), and F' _{+ 200} (x) The mean slope value expressed by the average of the MD directions is 0.02 or less. F' ₀ (x) In the case where the point where the minimum value is displayed is set to C _n (n=1, 2, 3・・), use |F' _-100 (C _n )-F' ₀ (C _n )|, | F' _-200 ( _Cn ) _-F'0 ( _Cn )|, |F' _{+ 100} ( _Cn ) _-F'0 ( _Cn )|, and |F' _{+ 200} ( _Cn )-F' The phase parameter defined by the average value of ₀ (C _n )| is 0.015 or more; [2] The polyvinyl alcohol film according to the aforementioned [1], whose width is 4 m or more; [3] The aforementioned [1] or [2] ] The polyvinyl alcohol film according to the description, whose degree of swelling is 170-220%; [4] The polyvinyl alcohol film according to any one of the aforementioned [1] to [3], whose thickness is 30 μm-65 μm; [5] The polyvinyl alcohol film according to any one of the above [1] to [4], which is a film for producing an optical film; [6] The polyvinyl alcohol film according to the above [5], wherein the optical film A polarizing film; [7] A polarizing film produced by using the polyvinyl alcohol film according to any one of the above [1] to [6]; [8] A polarizing plate comprising a protective film attached Manufactured on at least one side of the polarizing film as described in the aforementioned [7]; [9] A method of manufacturing a polyvinyl alcohol film, comprising the steps of: using a polyvinyl alcohol aqueous solution with a concentration of 32 mass percent or less to form polyethylene When the water content of the polyvinyl alcohol film is 20% by mass or more, the alcohol film is stretched at a stretching ratio of 1.075 to 1.135. [Effect of invention]

According to the present invention, there is provided a PVA film in which unevenness of the polarizing plate after the stretching process is further difficult to be visually recognized, and a polarizing film and a polarizing plate using such a PVA film.

[Form for carrying out the invention]

The present invention will be specifically described below.

<Film thickness measurement of PVA film> In the present invention, the film thickness measurement of the PVA thin film is performed under the following conditions. From the viewpoint of comparing the film thickness phase in the width direction, measurement using a two-dimensional film thickness meter is required. <Apparatus used> Line scan film thickness gauge (TI-750HR-1: manufactured by Otsuka Electronics) <Measurement interval> MD: 1mm TD: 1mm <Measured MD length> 1024mm <Measurement of TD length> 750mm/1 scan

(average slope value in MD direction) The PVA film of the present invention is characterized in that the average slope value in the MD direction (hereinafter referred to as the MD average slope value) is 0.02 μm/mm or less. The MD average slope value is more preferably 0.018 μm/mm or less. Hereinafter, the detailed calculation method of the MD average slope value will be described.

A 2-dimensional film thickness distribution was obtained with a line scanning film thickness meter, and when an arbitrary point of the PVA film was set to a position of 0 mm from the 2-dimensional film thickness distribution, a position of +100 mm in the TD direction and a position of +200 mm from the arbitrary point in the TD direction were set. The film thickness distribution of each of the position, -100mm position, and -200mm position (the positive and negative directions are arbitrary) has been smoothed by 8ptFFT and set to F _{+ 100} (x), F _{+ 200} (x), F _-100 ( x) and F _-200 (x). In addition, the film thickness distribution at the point of the position 0 mm was made into F ₀ (x). Here, the subscript characters of "0", "+100", "+200", "-100", and "-200" indicate the position in the TD direction, and x indicates the position in the MD direction. That is, when a certain point is set to 0 mm, x is the film thickness distribution measured at 1024 points per 1 mm from 1 mm to 1024 mm in the MD direction. The measurement of the film thickness distribution at the 1024 points was performed for each of the position of 0 mm, the position of +100 mm, the position of +200 mm, the position of -100 mm, and the position of -200 mm in the TD direction from the aforementioned arbitrary points. In addition, although the film thickness distribution measurement of 1024 points per 1 mm is performed here, the measurement interval in the MD direction and the number of points where the film thickness distribution is measured along the MD direction can be appropriately designed.

The MD average slope value of the present invention is calculated using the following (Equation 1) to (Equation 6). The film thickness data at each point of 1024mm in the MD direction is differentiated, the absolute value of the differential value is averaged at 1024 points, and the average value of 5 lines is calculated and defined as the MD average slope. value. The MD average slope value is a numerical value indicating the strength of the film thickness unevenness in the MD direction, and the larger the numerical value is, the larger the film thickness unevenness is. In addition, the differential values of F ₀ (x), F _{+ 100} (x), F _{+ 200} (x), F _-100 (x), and F _-200 (x) can be expressed as F' ₀ (x), F ' _{+ 100} (x), F' _{+ 200} (x), F' _-100 (x), F' _-200 (x).

In addition, k in (Equation 1) to (Equation 6) indicates that F' ₀ (x), F' _{+ 100} (x), F' _{+ 200} (x), F' _-100 ( In x) and F' _{- 200} (x), 1 to 1024 are substituted into the symbols of x, respectively.

The differential value of F ₀ (x), F _{+ 100} (x), F _{+ 200} (x), F _{- 100} (x), F _{- 200} (x) can be defined as the slope value of these, but for example: also The slope value can be defined by dividing the difference between F ₀ (x) and F ₀ (x+1) by the distance between point x and point (x+1). Also in the case of F _{+ 100} (x), F _{+ 200} (x), F _-100 (x), and F _-200 (x), the difference between the film thickness distributions at point x and point (x+1) can also be calculated Divide by the distance between point x and point (x+1), and define the resulting value as the slope value.

In the PVA film of the present invention, the MD average slope value calculated from the above formula must be 0.02 μm/mm or less, preferably 0.019 μm/mm or less, and more preferably 0.016 μm/mm or less. If the MD average slope value is larger than 0.02, the polarization unevenness when the polarizing plate is processed will be strongly visually recognized, and the display quality of the LCD will be impaired.

Furthermore, the MD average slope value (c) of the PVA film of the present invention from the position + 200 mm calculated from the above formula is preferably 0.019 μm/mm or less, more preferably 0.016 μm/mm or less. Likewise, the MD average slope value (e) at the position -200 mm is preferably 0.019 μm/mm or less, more preferably 0.016 μm/mm or less.

The method for obtaining a PVA film with an MD average slope value of 0.02 or less is not particularly limited, but for example, a polyvinyl alcohol film is formed using an aqueous solution of polyvinyl alcohol with a concentration of 32% by mass or less, and a polyvinyl alcohol film is formed on polyethylene. When the water content of the alcohol film is 20 mass % or more, the method of producing a polyvinyl alcohol film by stretching at a stretching ratio of 1.075 to 1.135.

<Phase parameter (θ)> The characteristics of the PVA film of the present invention are that the phase parameter (θ) defined below is 0.015 μm/mm or more, and the above-mentioned MD average slope value is 0.02 or less. The phase parameter (θ) is more preferably 0.016 μm/mm or more.

In the present invention, the phase parameter (θ) is a numerical value defined by the following formula. In the following formula, C _n refers to a point where |F' ₀ (C _n )| is a minimum point, and numbers C ₁ , C ₂ , and C ₃ ... C _n in order from the upstream in the MD direction. |F' ₀ (C _n )| becomes a minimum point and becomes less than 1024 points.

In addition, k in (Formula 12) represents a notation in which C ₁ to C _n are substituted into C _n in λ(C _n ) which is a function of C _n .

The phase parameter (θ) in the present invention is a parameter indicating the consistency of the positions of the following nodes in the MD direction: the node at a position of 0 mm in the TD direction that causes the film thickness unevenness, and the position in the TD direction from there The nodes that cause the film thickness unevenness at the four positions shifted in the direction, and the larger the value, the more inconsistent the nodes that cause the film thickness unevenness at the 5 points in the width direction, which means that the film thickness is less likely to occur. uneven.

The phase parameter (θ) of the PVA film of the present invention calculated from the above formula needs to be 0.015 or more, preferably 0.0153 or more. If the phase parameter (θ) is less than 0.015, the positions of the nodes in the width direction of the PVA film are the same, so stress concentration occurs during stretching, and the regular polarizing plate is unevenly stretched in the width direction after stretching. Visually identifiable reasons.

The method for obtaining a PVA film having a phase parameter (θ) of 0.015 or more is not particularly limited, but for example, a polyvinyl alcohol film is formed using a polyvinyl alcohol aqueous solution with a concentration of 32% by mass or less, and the polyvinyl alcohol film is formed on the polymer film. When the water content of the vinyl alcohol film is 20 mass % or more, it is stretched at a draw ratio of 1.075 to 1.135 to produce a polyvinyl alcohol film.

The MD average slope value of the film surface of the PVA film of the present invention is 0.02 or less, and the phase parameter (θ) is 0.014 or more, but the MD average slope value of at least one surface of the PVA film is 0.02 or less, and the phase parameter (θ) It should just be 0.014 or more, and the MD average slope value of at least both sides of the PVA film may be 0.02 or less, and the phase parameter (θ) may be 0.014 or more.

The width of the PVA film of the present invention is preferably 4 m or more, more preferably 4.5 m or more, from the viewpoint of being used for a large-scale liquid crystal television. On the other hand, if the width of the PVA film is too large, it is likely to become difficult to uniformly stretch itself evenly when a polarizing film is produced by a practical apparatus. Therefore, the width of the PVA film is preferably 7 m or less.

The swelling degree of the PVA film of the present invention is preferably more than 170%, more preferably more than 180%. The degree of swelling of the PVA film is preferably below 220%, more preferably below 210%. When the degree of swelling is less than 170%, uneven swelling tends to occur and uniform dyeing during dyeing tends to occur. If the degree of swelling is more than 220%, wrinkles will occur during the process, which tends to be unfavorable. As a method of adjusting the degree of swelling of the PVA film to these ranges, for example, a method of appropriately adjusting the temperature of the heat treatment after drying of the PVA film can be mentioned.

The degree of swelling is an index that shows the water retention capacity of the PVA film when it is immersed in water. It can be calculated by dividing the mass of the PVA film after immersing it in water at 30°C for 30 minutes by the weight after drying at 105°C for 16 hours after immersion. The quality is obtained as a percentage.

The volatile content ratio of the film-forming stock solution used in the film-forming of the PVA film of the present invention is preferably 70% by mass or more, more preferably 71% by mass or more. If the volatile content ratio of the PVA aqueous solution used for film formation is less than 70% by mass, the effect of reducing film thickness unevenness due to the leveling effect at the time of casting cannot be expected, and the MD average slope value becomes high, so it is relatively high. not good. If the ratio of the volatile content of the film-forming stock solution of the PVA aqueous solution is too large, the drying time increases and the productivity decreases, which is not preferable.

Here, in the present invention, the "volatile content ratio of the membrane-forming stock solution" refers to the volatile content ratio obtained by the following formula. Volatile content ratio (mass %) of membrane-forming stock solution = {(Wa-Wb)/Wa}×100 (In the formula, Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) when the film-forming stock solution of Wa (g) was dried in an electric heat dryer at 105° C. for 16 hours.)

As the conditions for the film formation of the PVA film of the present invention, when the water content of the PVA aqueous solution used for film formation is 20% by mass or more, the draw ratio, which is the draw ratio, is preferably 1.075 or more. The stretching is more preferably performed at 1.08 or more. When the water content of the PVA aqueous solution used for film formation is preferably 20% by mass or more, the stretching ratio is preferably 1.135 or less, which is the stretching ratio, and the stretching is more preferably less than 1.13. More preferably, it is set to less than 1.12 and stretched. In general, the stretching ratio is controlled by the speed ratio of the conveying rollers, and the larger the stretching ratio, the lower the limit stretching ratio during the stretching process, and the higher the probability of stretching fracture during the stretching process. Therefore, when the stretching ratio is 1.12 or more, a step of alleviating the influence of the stretching ratio may be required. Furthermore, when the stretch ratio is higher than 1.135, the limit stretch ratio during the stretching process is significantly lowered, which is not preferable. In addition, when the stretch ratio is less than 1.075, the phase parameter (θ) becomes small, which is unfavorable from the viewpoint of the visibility of the unevenness of the polarizing plate. In addition, regarding the conditions at the time of film-forming a PVA film, the moisture content at the time of stretching is preferably 20 mass % or more, more preferably 22 mass % or more. When the water content at the time of stretching is less than 20 mass %, the phase parameter (θ) is less likely to increase. For the above reasons, it is necessary to set the water content and the stretching ratio at the time of stretching within the above-described ranges.

(PVA) The PVA resin contained in the PVA film of the present invention can be obtained by saponifying polyvinyl ester, such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate. Polymerization of one or more of vinyl esters such as vinyl pivalate, vinyl pivalate, vinyl tertiary carbonate, vinyl laurate, vinyl stearate, vinyl benzoate, and propylene acetate obtained. Among the above-mentioned vinyl esters, vinyl acetate is also preferred from the viewpoints of easiness of production of PVA, easiness of acquisition, cost, and the like.

As long as the effect of the present invention is not impaired, PVA may be modified with one or two or more kinds of graft-copolymerizable monomers. The graft-copolymerizable monomers include, for example, unsaturated carboxylic acids or derivatives thereof; unsaturated sulfonic acids or derivatives thereof; α-olefins having 2 to 30 carbon atoms. The ratio of the structural unit derived from the graft-copolymerizable monomer in the PVA is preferably 5 mol % or less based on the molar number of all the structural units constituting the PVA.

A part of the hydroxyl group of PVA may be cross-linked, or it may not be cross-linked. Furthermore, the above-mentioned PVA may form an acetal structure by reacting a part of its hydroxyl groups with aldehyde compounds such as acetaldehyde and butyraldehyde, or may not react with these compounds and not form an acetal structure.

The degree of polymerization of PVA is preferably 2000 or more, more preferably 2200 or more, and still more preferably 2400 or more. When the degree of polymerization is lower than 2000, the durability of the obtained polarizing film tends to decrease. The degree of polymerization of PVA is preferably 2,700 or less, more preferably 2,650 or less, and still more preferably 2,600 or less. On the other hand, when the degree of polymerization exceeds 2,700, the production cost increases and the processability during film formation tends to deteriorate. In addition, the polymerization degree of PVA as used in this specification means the average degree of polymerization measured according to the description of JISK6726-1994.

From the viewpoint of the water resistance of the polarizing film, the saponification degree of PVA is preferably 98 mol % or more, more preferably 98.5 mol % or more, and further preferably 99 mol % or more. If the degree of saponification is less than 98 mol %, the water resistance of the polarizing film obtained tends to deteriorate. In addition, in this specification, the saponification degree of PVA refers to: PVA has it, relative to the total number of moles of the structural units (usually vinyl ester units) that can be converted into vinyl alcohol units by saponification and vinyl alcohol units. , the molar ratio of the vinyl alcohol unit (mol%). The saponification degree can be measured according to the description of JISK6726-1994.

(Plasticizer) The PVA film of the present invention preferably contains a plasticizer. The plasticizers include, for example, polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, and trimethylolpropane. The PVA film can contain one or more of these plasticizers. Among these, glycerin is preferable from the viewpoint of the effect of improving stretchability.

The content of the plasticizer in the PVA film of the present invention is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and still more preferably 5 parts by mass or more, relative to 100 parts by mass of PVA contained therein. When the content of the plasticizer is 1 part by mass or more, the stretchability of the PVA film can be further improved. On the other hand, the content of the plasticizer in the PVA film is preferably 20 parts by mass or less, more preferably 17 parts by mass or less, and still more preferably 15 parts by mass or less, relative to 100 parts by mass of PVA. When the amount of the plasticizer is 20 parts by mass or less, it is possible to prevent the PVA film from becoming too soft and reducing the handleability.

(surfactant) The PVA film of the present invention preferably contains a surfactant. The film-forming properties of the PVA film are improved by producing a PVA film using a film-forming stock solution containing a surfactant. As a result, the occurrence of uneven thickness of the PVA film is suppressed, and the PVA film is easily peeled from the metal roll or belt used for film formation. When a PVA film is produced from a film-forming stock solution containing a surfactant, the obtained PVA film contains a surfactant.

The type of the surfactant is not particularly limited, but from the viewpoint of the releasability of the PVA film from a metal roll or a tape, and the like, an anionic surfactant and a nonionic surfactant are preferred.

For anionic surfactants, suitable ones are: carboxylic acid types such as potassium laurate; sulfate types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; sulfonic acids such as dodecylbenzenesulfonate type, etc.

As far as the nonionic surfactant is concerned, suitable ones are, for example, alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octyl phenyl ether; polyoxyethylene laurate, etc. Alkyl ester type; polyoxyethylene lauryl amino ether and other alkyl amine types; polyoxyethylene lauric acid amide and other alkyl amide types; polyoxyethylene polyoxypropylene ether and other polypropylene glycol ether types; oleic acid diethanol Alkyl amide type such as amide; allyl phenyl ether type such as polyoxyalkylene allyl phenyl ether, etc.

These surfactants may be used alone or in combination of two or more.

The lower limit of the content of the surfactant in the PVA film of the present invention is preferably 0.01 part by mass, more preferably 0.02 part by mass, and still more preferably 0.05 part by mass relative to 100 parts by mass of PVA. By setting the content of the surfactant to be more than the above-mentioned lower limit, the film formability and peelability of the PVA film are further improved. On the other hand, the upper limit of the content of the surfactant in the PVA film is preferably 0.5 parts by mass, more preferably 0.3 parts by mass, and still more preferably 0.2 parts by mass relative to 100 parts by mass of PVA. By making content of a surfactant below the said upper limit, it can suppress that a surfactant bleeds out (bleeding out) to the surface of a PVA film, and sticking occurs, and workability|operativity falls.

(other ingredients) The PVA film of the present invention may further contain components such as antioxidants, antifreeze agents, pH adjusters, concealing agents, anti-coloring agents, oil agents, and surfactants described later, as necessary.

(shape etc.) The shape of the PVA film of the present invention is not particularly limited, but is preferably a long film. Thereby, a more uniform PVA film can be manufactured continuously and easily, and at the same time, it can also be used continuously when manufacturing a polarizing film using this. The length (length in the longitudinal direction) of the elongated film is not particularly limited, and can be appropriately set according to the application and the like, for example, it can be set in the range of 5 to 30000 m.

The thickness of the PVA film of the present invention is desirably 30 to 65 μm. When the thickness of the PVA film is less than 30 μm, the workability during stretching is deteriorated, and when the thickness of the PVA film is more than 65 μm, it is not good as a polarizing plate for thin displays.

<Manufacturing method of PVA film> In the present invention, the manufacturing method of the PVA thin film is not particularly limited, but it is possible to adopt a film forming dope that has been homogenized by adding a solvent, additives, etc. Film production method (spray into poor solvent), dry and wet film production method, gel film production method (the method of temporarily cooling and gelling the film production solution, then extracting and removing the solvent to obtain a PVA film) , or a method of film-forming by a combination of these, or a melt extrusion that uses an extruder, etc. to obtain the above-mentioned film-making stock solution and extrudes it from a T-die (T-die), etc. for film-making Any method such as a film forming method or an inflation molding method. Among them, since a homogeneous film can be obtained with high productivity, a casting film forming method and a melt extrusion film forming method are also preferred. The casting method or the melt extrusion method of the PVA film will be described below.

The method for producing a polyvinyl alcohol film of the present invention includes a method for producing a polyvinyl alcohol film having the following steps: using a polyvinyl alcohol aqueous solution having a concentration of 32% by mass or less to form a polyvinyl alcohol film, and The step of stretching at a stretching ratio of 1.075 to 1.135 when the water content of the polyvinyl alcohol film is 20% by mass or more. By adopting this production method, a thin film without thickness unevenness can be produced.

When the ultimate draw ratio is lowered, a step of alleviating the influence of the above draw ratio may be added. As a specific method for alleviating the influence of the stretching ratio, a method of irradiating infrared rays or microwaves to the polyvinyl alcohol film, reducing the stretching ratio when the moisture content of the polyvinyl alcohol film is 15 to 5% by mass, or reducing the Method of small cumulative stretch ratio (product of stretch ratio when stretched multiple times), method of applying plasticizer after stretching, method of drying by superheated steam, drying at high temperature Any method can be used, such as a method of performing heat treatment by a floating dryer, or a combination of the methods, or a plurality of times.

In the case of forming a PVA film by a casting film forming method or a melt extrusion film forming method, the above-mentioned film forming stock solution is cast in a film form on a support such as a metal roll or a metal belt, and the solvent is removed by heating. cured and thinned. The cured film is peeled off from the support, dried by drying rollers, drying ovens, etc. as required, further subjected to heat treatment as required, and then wound up, thereby obtaining a roll-shaped long PVA film.

There is no particular limitation on the preparation method of the film-forming stock solution, for example, a method of dissolving PVA and additives such as plasticizers and surfactants in a dissolving tank or the like, or a method of dissolving the film using a uniaxial or biaxial extruder. When melt-kneading PVA in a water-containing state, it is a method of melt-kneading together with a plasticizer, a surfactant, and the like.

The liquid medium used for preparing the membrane-forming stock solution includes, for example, water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol , glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, diethylenetriamine, etc., one or two of these can be used above. Among them, water can be suitably used from the viewpoint of load on the environment and recyclability.

For the dried PVA film, further heat treatment can be performed as required. The strength, degree of swelling, birefringence, etc. of the PVA thin film can be adjusted by performing the heat treatment. The surface temperature of the heat treatment roll for heat treatment is preferably 60°C or higher. Furthermore, the surface temperature of the heat treatment roll is preferably 135°C or lower, more preferably 130°C or lower. If the surface temperature of the heat treatment roll is too high, the amount of heat supplied is too much, the size of the layered crystals in the PVA film increases, and it becomes difficult for the degree of swelling of the PVA film to satisfy the above range.

The PVA film produced in this way is rolled onto a cylindrical core in a roll shape, and is packaged in a moisture-proof manner, depending on the need for further running paper moisture conditioning treatment, cutting of both ends (ears) of the film, etc. become a product.

The volatile content ratio of the PVA film finally obtained by the above-mentioned series of treatments is not necessarily limited. The volatile content ratio of the finally obtained PVA film is preferably 1 mass % or more, more preferably 2 mass % or more. The volatile content ratio of the finally obtained PVA film is preferably 5 mass % or less, more preferably 4 mass % or less.

The film thickness unevenness in the MD direction of the PVA film of the present invention is reduced, and the phase shift of the film thickness unevenness in the TD direction is adjusted to a specific range, so it can be suitably used as a film for producing an optical film. Here, as will be described later, the optical film may be exemplified by a polarizing film, a viewing angle improvement film, a retardation film, a brightness improvement film, and the like, but a polarizing film is preferable. The polarizing film of the present invention produced by using the PVA film of the present invention is more difficult to visually recognize the unevenness of the polarizing plate after stretching than hitherto.

<Manufacturing method of optical film> Hereinafter, the manufacturing method of a polarizing film is specifically demonstrated as an example of the manufacturing method of an optical film.

The polarizing film can generally be produced by using a PVA film as an original film, and going through processing steps such as a swelling step, a dyeing step, a cross-linking step, a stretching step, and a fixing treatment step. Specific examples of the treatment liquid used in each step include swelling treatment liquid used in swelling treatment, dyeing treatment liquid (dyeing liquid) used in dyeing treatment, and crosslinking treatment liquid used in crosslinking treatment. The treatment liquid, the stretching treatment liquid used in the stretching treatment, the fixing treatment liquid used in the fixing treatment, the cleaning treatment liquid (washing liquid) used in the cleaning treatment, and the like.

Each processing step that can be employed in the production method for producing a polarizing film will be described below. In addition, in the manufacturing method of a polarizing film, one or two or more of each of the following treatments may be omitted, the same treatment may be performed a plurality of times, or other treatments may be performed simultaneously.

(cleaning treatment) Before the swelling treatment is performed on the PVA film, the PVA film is preferably washed. The anti-blocking agent and the like adhering to the PVA film can be removed by such a cleaning treatment before the swelling treatment, thereby preventing contamination of each treatment liquid with the anti-blocking agent and the like during the production process of the polarizing film. The cleaning treatment is preferably performed by immersing the PVA film in the cleaning treatment liquid, but may be performed by spraying the cleaning treatment liquid on the PVA film. As the cleaning treatment liquid, for example, water can be used. The temperature of the cleaning treatment liquid is preferably 20°C or higher, more preferably 22°C or higher, further preferably 24°C or higher, and particularly preferably 26°C or higher. When the temperature of the cleaning treatment liquid is 20° C. or higher, the removal of the anti-blocking agent and the like adhering to the PVA film becomes easy. The temperature of the cleaning treatment liquid is preferably 40°C or lower, more preferably 38°C or lower, further preferably 36°C or lower, and particularly preferably 34°C or lower. Furthermore, by setting the temperature of the cleaning treatment liquid to be 40° C. or lower, it is possible to prevent a part of the surface of the PVA film from being dissolved and the films from sticking to each other, thereby reducing the workability.

(swelling treatment) The swelling treatment system can be performed by immersing the PVA film in a swelling treatment liquid such as water. The temperature of the swelling treatment liquid is preferably 20°C or higher, more preferably 22°C or higher, and further preferably 24°C or higher. The temperature of the swelling treatment liquid is preferably 40°C or lower, more preferably 38°C or lower, and further preferably 36°C or lower. In addition, the time of immersion in the swelling treatment liquid is, for example, preferably 0.1 minute or more, more preferably 0.5 minute or more. In addition, the time for immersion in the swelling treatment liquid is, for example, preferably 5 minutes or less, and more preferably 3 minutes or less. In addition, the water used as a swelling process liquid is not limited to pure water, The aqueous solution which melt|dissolved various components, such as a boron-containing compound, may be a mixture of water and an aqueous medium. The type of the boron-containing compound is not particularly limited, but from the viewpoint of workability, boric acid or borax is preferred. When the swelling treatment liquid contains a boron-containing compound, the concentration thereof is preferably 6 mass % or less from the viewpoint of improving the stretchability of the PVA film.

(Dyeing treatment) The dyeing treatment may be performed using an iodine-based dye as a dichroic dye, and the dyeing period may be any stage before the stretching treatment, during the stretching treatment, and after the stretching treatment. The dyeing treatment is preferably performed by using a solution (preferably an aqueous solution) containing iodine-potassium iodide as the dyeing treatment liquid, and immersing the PVA film in the dyeing treatment liquid. The concentration of iodine in the dyeing treatment liquid is preferably in the range of 0.005 to 0.2 mass %, and the potassium iodide/iodine (mass) is preferably in the range of 20 to 100. The temperature of the dyeing treatment liquid is preferably 20°C or higher, more preferably 25°C or higher. The temperature of the dyeing treatment liquid is preferably 50°C or lower, more preferably 40°C or lower. In the dyeing treatment liquid, a boron-containing compound such as boric acid may be contained as a crosslinking agent. Further, if the PVA film used as the original film contains a dichroic dye in advance, the dyeing process can be omitted. Furthermore, the PVA film used as the original film may contain boron-containing compounds such as boric acid and borax in advance.

(Crosslinking treatment) When producing a polarizing film, it is preferable to perform a crosslinking treatment after the dyeing treatment for the purpose of making the dichroic dye adsorb firmly to the PVA film. The cross-linking treatment can be performed by using a solution (preferably an aqueous solution) containing a cross-linking agent as the cross-linking treatment liquid, and immersing the PVA film in the cross-linking treatment liquid. As a crosslinking agent, one type or two or more types of boron-containing compounds such as boric acid and borax can be used. When the concentration of the cross-linking agent in the cross-linking treatment liquid is too high, the cross-linking reaction tends to progress too much and sufficient stretching tends to be difficult in the subsequent stretching treatment, and when the concentration is too small, the There is a tendency for the effect of the crosslinking treatment to decrease. The concentration of the crosslinking agent in the crosslinking treatment liquid is preferably 1% by mass or more, more preferably 1.5% by mass or more, and still more preferably 2% by mass or more. The concentration of the crosslinking agent in the crosslinking treatment liquid is preferably 6 mass % or less, more preferably 5.5 mass % or less, and further preferably 5 mass % or less.

In order to suppress the elution of the dichroic dye from the dyed PVA film, an iodine-containing compound such as potassium iodide may be contained in the crosslinking treatment liquid. If the concentration of the iodine-containing compound in the crosslinking treatment liquid is too high, the reason is unclear, but the heat resistance of the polarizing film obtained tends to decrease. Furthermore, when the amount is too small, the effect of suppressing the elution of the dichroic dye tends to decrease. The concentration of the iodine-containing compound in the crosslinking treatment liquid is preferably 1 mass % or more, more preferably 1.5 mass % or more, and further preferably 2 mass % or more. The concentration of the iodine-containing compound in the crosslinking treatment liquid is preferably 6 mass % or less, more preferably 5.5 mass % or less, and further preferably 5 mass % or less.

If the temperature of the cross-linking treatment liquid is too high, the dichroic dye will be eluted, and the resulting polarizing film tends to be dyed unevenly, and if it is too low, the effect of the cross-linking treatment will be poor. will decrease. The temperature of the crosslinking treatment liquid is preferably 20°C or higher, more preferably 22°C or higher, and further preferably 25°C or higher. The temperature of the crosslinking treatment liquid is preferably 45°C or lower, more preferably 40°C or lower, and further preferably 35°C or lower.

In addition to the stretching treatment described later, the PVA film may be stretched during or between the above-mentioned treatments. By performing such stretching (pre-stretching), it is possible to prevent wrinkles from occurring on the surface of the PVA film. From the viewpoint of the polarizing properties of the obtained polarizing film, etc., the total stretching ratio before stretching (multiplying the stretching ratio in each treatment) is based on the original length of the PVA film of the original sheet before stretching , preferably 4 times or less, more preferably 3.5 times or less. From the viewpoint of the polarizing properties of the obtained polarizing film, etc., the total stretching ratio of the pre-stretching is more preferably 1.5 times or more based on the original length of the PVA film of the original sheet before stretching. The stretching ratio in the swelling treatment is preferably 1.1 times or more, more preferably 1.2 times or more, and further preferably 1.4 times or more based on the original length of the PVA film. The stretching ratio in the swelling treatment is preferably 3 times or less, more preferably 2.5 times or less, and further preferably 2.3 times or less, based on the original length of the PVA film. The draw ratio in the dyeing treatment is preferably 2 times or less, more preferably 1.8 times or less, and further preferably 1.5 times or less based on the original length of the PVA film. The stretching ratio in the dyeing process is more preferably 1.1 times or more based on the original length of the PVA film. The draw ratio in the crosslinking treatment is preferably 2 times or less, more preferably 1.5 times or less, and further preferably 1.3 times or less, based on the original length of the PVA film. The stretching ratio in the crosslinking treatment is more preferably 1.05 times or more based on the original length of the PVA film.

(stretching treatment) The stretching treatment may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, a solution (preferably an aqueous solution) containing a boron-containing compound such as boric acid can be used as the stretching treatment liquid, and it can be carried out in the stretching treatment liquid, or in the dyeing treatment liquid or described later. fixed in the treatment solution. In addition, in the case of a dry stretching method, it can be performed using the PVA film after water absorption in air. Among these, a wet stretching method is also preferable, and it is more preferable to perform uniaxial stretching in an aqueous solution containing boric acid. When the stretching treatment liquid contains a boron-containing compound, since the stretchability of the PVA film can be improved, the concentration of the boron-containing compound in the stretching treatment liquid is preferably 1.5 mass % or more, more preferably 2.0 mass % or more. , more preferably 2.5 mass % or more. Since the stretchability of the PVA film can be improved, the concentration of the boron-containing compound in the stretching liquid is preferably 7% by mass or less, more preferably 6.5% by mass or less, and still more preferably 6% by mass or less.

It is preferable that the stretching liquid contains an iodine-containing compound such as potassium iodide. If the concentration of the iodine-containing compound in the stretching liquid is too high, the hue of the obtained polarizing film tends to be blue-tinted, and the reason is unclear, but if it is too low, the obtained polarized light The heat resistance of the film tends to decrease. The concentration of the iodine-containing compound in the stretching liquid is preferably 2 mass % or more, more preferably 2.5 mass % or more, and further preferably 3 mass % or more. The concentration of the iodine-containing compound in the stretching liquid is preferably 8% by mass or less, more preferably 7.5% by mass or less, and still more preferably 7% by mass or less.

When the temperature of the stretching treatment liquid is too high, the PVA film tends to melt and becomes soft and tends to be easily broken, and when it is too low, the stretchability tends to decrease. The temperature of the stretching treatment liquid is preferably 50°C or higher, more preferably 52.5°C or higher, and further preferably 55°C or higher. The temperature of the stretching liquid is preferably 70°C or lower, more preferably 67.5°C or lower, and further preferably 65°C or lower. In addition, when the stretching treatment is performed by the dry stretching method, the preferred range of the stretching temperature is also as described above.

The stretching ratio in the stretching treatment is preferably 1.2 times or more, more preferably 1.5 times or more, and still more preferably 2 times or more, since the higher one can obtain a polarizing film with better polarizing performance. Furthermore, the total stretching ratio (the ratio of multiplying the stretching ratio in each step) of the above-mentioned pre-stretching ratio is also included. From the viewpoint of the polarizing performance of the obtained polarizing film, the ratio is based on the stretching ratio. The original length of the PVA film of the raw material before stretching is preferably 5.5 times or more, more preferably 5.7 times or more, and further preferably 5.9 times or more. The upper limit of the stretch ratio is not particularly limited, but if the stretch ratio is too high, stretch breakage of the PVA film tends to occur, so it is preferably 8 times or less.

There is no particular limitation on the method of performing the stretching treatment by uniaxial stretching, and uniaxial stretching in the longitudinal direction or transverse uniaxial stretching in the width direction can be employed. In the case of producing a polarizing film, uniaxial stretching in the longitudinal direction is preferable from the viewpoint of obtaining one excellent in polarizing performance. The uniaxial stretching system in the longitudinal direction can be performed by using a stretching device provided with a plurality of rolls parallel to each other, and by changing the peripheral speed between the rolls.

In the present invention, the maximum stretching speed (%/min) when the stretching treatment is performed by uniaxial stretching is not particularly limited, but the maximum stretching speed is preferably 200%/min or more, more preferably 300% /min or more, more preferably 400%/min or more. Here, when the PVA film is stretched in two or more stages using three or more rolls with different peripheral speeds, the maximum stretching speed refers to the fastest stretching speed in this stage. In addition, in the case where the stretching treatment of the PVA film is performed in one stage without being divided into two or more stages, the stretching speed in this stage is the maximum stretching speed. In addition, the so-called stretching speed refers to the increase of the length of the PVA film increased by stretching relative to the length of the PVA film before stretching per unit time. For example, the so-called stretching speed of 100%/min refers to the speed at which the PVA film is deformed from the length before stretching to twice the length in 1 minute. The higher the maximum stretching speed is, the higher the stretching process (uniaxial stretching) of the PVA film can be performed, and as a result, the productivity of the polarizing film is improved, which is preferable. On the other hand, if the maximum stretching speed becomes too large, in the stretching process (uniaxial stretching) of the PVA film, excessive tension may be locally applied to the PVA film, and stretching fracture may easily occur. From such a viewpoint, the maximum stretching speed is preferably not more than 900%/min.

(washing treatment after dyeing treatment) After the dyeing treatment, it is preferable to wash the stretched PVA film. The cleaning treatment is preferably performed by immersing the PVA film in the cleaning treatment liquid, but may be performed by spraying the PVA film with the cleaning treatment liquid. As the cleaning treatment liquid, for example, water can be used. The water is not limited to pure water, and may contain, for example, an iodine-containing compound such as potassium iodide. In addition, the cleaning treatment liquid may contain a boron-containing compound, but in this case, the concentration of the boron-containing compound is preferably 2.0 mass % or less.

The temperature of the cleaning treatment liquid is preferably 5°C or higher, more preferably 7°C or higher, and further preferably 10°C or higher. Furthermore, the temperature of the cleaning treatment liquid is preferably 40°C or lower, more preferably 38°C or lower, and further preferably 35°C or lower. When the temperature of the cleaning treatment liquid is 5°C or higher, breakage of the PVA film due to freezing of moisture can be suppressed. Furthermore, when the temperature of the cleaning treatment liquid is 40° C. or lower, the optical properties of the obtained polarizing film are improved.

As a specific method at the time of producing a polarizing film, a method of subjecting a PVA film to dyeing treatment, stretching treatment, and also crosslinking treatment and/or fixing treatment can be mentioned. A preferable example includes a method of subjecting the PVA film to swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment (especially uniaxial stretching treatment), and washing treatment in this order. In addition, the stretching process may be performed in any of the above-mentioned processing steps, or may be performed in two or more stages.

A polarizing film can be obtained by drying the PVA film after each of the above-mentioned treatments. The method of drying treatment is not particularly limited, and examples thereof include a contact method in which the film is brought into contact with a heating roll, a method in which drying is performed in a hot air dryer, and a floating (floating) method in which drying is performed by hot air while suspending the film. ) method, etc.

<Polarizing film, polarizing plate> In order to make up for the mechanical strength, the polarizing film of the present invention is manufactured by attaching a protective film to at least one side. The polarizing film of the present invention is usually used as a polarizing plate by bonding an optically transparent and mechanically strong protective film. As the protective film, triacetate cellulose (TAC) film, cellulose acetate-butyrate (CAB) film, acrylic film, polyester film, etc. are used. In addition, the adhesive agent for bonding includes a PVA-based adhesive, a urethane-based adhesive, and the like, but among them, a PVA-based adhesive is also suitable.

The polarizing plate obtained in the above-described manner can be used as a component of an LCD by being bonded to a glass substrate after applying an adhesive such as acrylic. At the same time, it can also be laminated with retardation films, viewing angle enhancement films, brightness enhancement films, etc. [Example]

The present invention will be specifically described by the following examples, but the present invention is not limited by these examples at all.

<Calculate MD average slope value, phase parameter (θ), and average slope value at ±200mm position> The MD average slope value, the phase parameter (θ), and the average slope value at ±200 mm positions of the PVA films obtained in Examples or Comparative Examples were calculated by the above-described method. The results are shown in Table 1.

<Measurement of swelling degree of PVA film> A test piece of about 1.5 g was cut out from the PVA film obtained in the following Examples or Comparative Examples. Next, this test piece was immersed in 1000 g of distilled water at 30°C. After immersion for 30 minutes, the test piece was taken out, and the water on the surface was absorbed by a filter paper, and then the mass (We) was measured. Next, the test piece was put in a hot air dryer and dried at 105° C. for 16 hours, and then its mass (Wf) was measured. The degree of swelling of the PVA film was determined from the obtained masses We and Wf by the following formula. Degree of swelling (%)=(We/Wf)×100

<Ultimate stretching ratio at the time of evaluation of stretching> In the following Examples or Comparative Examples, the difficulty of tensile fracture at the time of stretching was evaluated by the fracture ratio of the PVA film at the time of producing the polarizing film. That is, the breaking ratio of the uniaxial stretching in the stretching treatment at the time of producing the polarizing film was measured 10 times, and the average value was set as the limit stretching ratio, and the following criteria were used for evaluation. A・・・The ultimate stretch ratio is 6.6 times or more B・・・The ultimate stretch ratio is 6.5 times or more and less than 6.6 times C・・・The ultimate stretch ratio is less than 6.5 times

<Evaluation of unevenness of polarizing plate> The polarizing plate sample obtained by the method described in Example 1 was cut into 30 cm × 30 cm, and two polarizing plates (single transmittance 43.5%, degree of polarization) were placed in a parallel Nicol state. 99.9%), the sample polarizers were sandwiched in a cross nicol state with respect to each polarizer, and the optical properties were visually observed in transmission mode using a backlight with a luminance of 40,000 cd/m ² . The polarization unevenness was evaluated by the following criteria. A・・・Color unevenness B・・・No color unevenness

[Example 1] 100 parts by mass of PVA (degree of saponification 99.9 mol%, degree of polymerization 2400), 10 parts by mass of glycerol as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and 233 parts by mass of water And it melt-mixed by a melt extruder, and prepared the film-forming stock solution (volatile content ratio 70 mass %). Next, a PVA film was formed on the support (surface temperature of 98° C.) by ejecting the film from the T-shaped mold onto the support (surface temperature of 98° C.). When the water content of the sprayed PVA film became 24%, the stretching was performed by controlling the speed ratio of the rolls to make the stretching ratio 1.10. On the heat treatment roll in the latter half, contact heat treatment was performed at 105° C. to obtain a PVA thin film having a width of 5 m and a film thickness of 60 μm. In addition, the moisture content at the time of stretching was measured after tearing the conveyed PVA film between rolls applying stretching and measuring the mass (Ww), placing the film in a hot air dryer and drying at 105°C for 16 hours. its mass (Wd). The moisture content of the film was calculated by the following formula. Moisture content of film (%)=(Ww-Wd)/Ww×100

The obtained PVA film was slit into a width of 650 mm, and the film was subjected to swelling treatment, dyeing treatment, cross-linking treatment, stretching treatment, washing treatment, and drying treatment in this order to continuously produce polarized light. film. The swelling treatment was performed by being uniaxially stretched by 2.00 times in the longitudinal direction while being immersed in pure water (swelling treatment liquid) at 25°C. The dyeing treatment system is uniaxially stretched by 1.26 times in the longitudinal direction while being immersed in a potassium iodide/iodine aqueous dyeing solution (dyeing treatment solution) (potassium iodide/iodine (mass ratio) 23, iodine concentration 0.03 to 0.05 mass %) at a temperature of 32°C and proceed. For this dyeing treatment, the iodine concentration in the dyeing treatment liquid was adjusted in the range of 0.03 to 0.05 mass % so that the single transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment was 43.5% ±0.2% range. The crosslinking treatment was performed by uniaxial stretching in the longitudinal direction by 1.19 times while being immersed in a 32° C. boric acid aqueous solution (crosslinking treatment liquid) (boric acid concentration: 2.6 mass %). The stretching treatment was performed by uniaxial stretching 2.00 times in the longitudinal direction while being immersed in a 55° C. boric acid/potassium iodide aqueous solution (stretching treatment liquid) (boric acid concentration 2.8 mass %, potassium iodide concentration 5 mass %). The cleaning treatment was performed by immersing for 12 seconds in a potassium iodide/boric acid aqueous solution (cleaning treatment liquid) (potassium iodide concentration of 3 to 6 mass %, boric acid concentration of 1.5 mass %) at 22° C. without stretching.

[Examples 2 to 4, Comparative Examples 1 to 3] Except having changed the manufacturing conditions of the PVA film as shown in Table 1, it carried out similarly to Example 1, and performed manufacture and evaluation of the PVA film. In addition, the volatile content ratio of the film-forming stock solution was adjusted by adjusting the amount of water to be melt-mixed using a melt extruder. The results are shown in Table 1.

From the above-mentioned results, it is understood that the PVA film of the present invention is one with suppressed color unevenness during the stretching process.

[Table 1] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Manufacturing conditions Volatile content ratio (%) of membrane-forming stock solution 70.0 70.5 70.0 70.0 66.0 66.0 70.0 Moisture content of film during stretching (%) 24.0 23.0 24.5 24.0 19.0 19.0 24.0 extension ratio 1.100 1.080 1.100 1.130 1.100 1.050 1.070 PVA film Degree of swelling (%) 200 200 196 200 201 203 195 Film thickness (μm) 60 60 60 60 60 60 60 MD mean slope value 0.016 0.018 0.016 0.014 0.031 0.041 0.015 Phase parameter (θ) 0.016 0.016 0.015 0.016 0.027 0.011 0.014 Average slope value at ±200mm position 0.015 0.017 0.016 0.014 0.033 0.042 0.016 Evaluation uneven A A A A B B B Ultimate stretch ratio (55℃) 6.60 6.70 6.55 6.40 6.80 6.80 6.80 Limit magnification evaluation A A B C A A A

none.

none.

none.

Claims (9)

A polyvinyl alcohol film, wherein when the film thickness distribution (film thickness profile) of any point F ₀ on the film surface is set to F ₀ (x), the film thickness distribution of the point from F ₀ to the TD direction -100mm is set to F _{− 100} (x), the film thickness distribution from F ₀ to the point of -200mm in the TD direction is F _-200 (x), the film thickness distribution from F ₀ to the point of +100mm in the TD direction is F _{+ 100} (x), When the film thickness distribution at the point of +200 mm in the TD direction from F ₀ is set to F _{+ 200} (x), F' ₀ (x), F' _-100 (x) of the differential value in the MD direction of each film thickness distribution , F' _-200 (x), F' _{+ 100} (x) and F' _{+ 200} (x) in the MD direction, the mean slope value represented by the average value is less than 0.02, when F' ₀ (x) is displayed as the minimum value When the point is set to C _n (n=1, 2, 3・・), it is ｜F' _-100 (C _n )-F' ₀ (C _n )｜,｜F' _-200 (C _n )- F' ₀ (C _n )|, |F' _{+ 100} (C _n )-F' ₀ (C _n )| and |F' _{+ 200} (C _n )-F' ₀ (C _n )| The defined phase parameter (θ) is 0.015 or more. Such as the polyvinyl alcohol film of claim 1, its width is more than 4m. Such as the polyvinyl alcohol film of claim 1 or 2, its swelling degree is 170-220%. The polyvinyl alcohol film according to any one of claims 1 to 3, whose thickness is 30 μm to 65 μm. The polyvinyl alcohol film according to any one of claims 1 to 4, which is a film for producing optical films. The polyvinyl alcohol film of claim 5, wherein the optical film is a polarizing film. A polarizing film produced using the polyvinyl alcohol film according to any one of claims 1 to 6. A polarizing plate manufactured by attaching a protective film to at least one side of the polarizing film as claimed in claim 7. A method for producing a polyvinyl alcohol film, comprising the steps of: forming a polyvinyl alcohol film by using a polyvinyl alcohol aqueous solution with a concentration of 32% by mass or less, and when the water content of the polyvinyl alcohol film is 20% by mass or more, The step of stretching is carried out at a stretching ratio of 1.075 to 1.135.