CN116783057A

CN116783057A - Polyvinyl alcohol film, and polarizing film and polarizing plate using same

Info

Publication number: CN116783057A
Application number: CN202180088168.0A
Authority: CN
Inventors: 下田康平; 森翔大朗; 大桥亘; 中井慎二
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 2020-12-28
Filing date: 2021-12-28
Publication date: 2023-09-19
Also published as: TW202233385A; KR20230121786A; WO2022145489A1; JPWO2022145489A1

Abstract

The invention provides a PVA film which is not easy to observe uneven polarization plate during stretching even though LCD high transmittance is realized, and a polarization film and a polarization plate using the PVA film. A polyvinyl alcohol film in which an arbitrary point F on the film surface ₀ The film thickness profile of (2) is set to F ₀ (x) Will be from F ₀ The film thickness profile from the point of-100 mm in the TD direction was set to F _‑100 (x) Will be from F ₀ The film thickness profile from the point of 200mm in the TD direction was set to F _‑200 (x) Will be from F ₀ The film thickness profile from the point +100mm toward TD was set to F ₊₁₀₀ (x) Will be from F ₀ The film thickness profile at the point of +200mm in TD direction was defined as F ₊₂₀₀ (x) In the case of using the inclination value in MD of each film thickness profile, namely F' ₀ (x)、F′ _‑100 (x)、F′ _‑200 (x)、F′ ₊₁₀₀ (x) And F' ₊₂₀₀ (x) The average inclination value expressed by the average of MD direction is less than 0.02, F 'is used' ₀ (x) The point showing the minimum value is set to C _n (n=1, 2,3 …) |f' _‑100 (C _n )‑F′ ₀ (C _n )|、|F′ _‑200 (C _n )‑F′ ₀ (C _n )|、|F′ ₊₁₀₀ (C _n )‑F′ ₀ (C _n ) I and F' ₊₂₀₀ (C _n )‑F′ ₀ (C _n ) The average value of i defines a phase parameter of 0.015 or more.

Description

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

Technical Field

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

Background

A polarizing plate having light transmitting and light blocking functions is the same as a liquid crystal for changing the polarization state of light as a basic constituent of a Liquid Crystal Display (LCD). LCDs are used in a wide range of small devices such as calculators and wristwatches, notebook computers, liquid crystal displays, liquid crystal color projectors, liquid crystal televisions, car navigation systems, cellular phones, and metering devices used indoors and outdoors.

The polarizing plate is generally manufactured as follows: a polarizing film is produced by dyeing a polyvinyl alcohol film (hereinafter, sometimes abbreviated as "PVA") and uniaxially stretching the film, and if necessary, further fixing the film with a boron compound or the like, and then a protective film such as a cellulose Triacetate (TAC) film is attached to the surface of the polarizing film. In recent years, energy saving of LCDs has been demanded, and high transmittance has been demanded for polarizing plates. With the increase in transmittance of the polarizing plate, unevenness of the polarizing plate due to film thickness unevenness of the PVA film, which has not been observed in the prior art, is also observed, and a PVA film having more excellent flatness than the prior art is demanded.

Patent document 1 describes a method for producing a PVA film having more excellent flatness than before by suppressing variation in flow delay by using an air knife.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2017-008298

Disclosure of Invention

Problems to be solved by the invention

However, patent document 1 has a problem that although film thickness flatness in the MD direction is better than conventional film thickness flatness can be obtained, the following problem is present: since the thin film portion concentrates in the width direction, stress concentrates in the thin film portion, and even though the visibility is low, regular unevenness of the polarizing plate can be observed, and the requirement for further higher transmittance of the LCD cannot be satisfied.

Accordingly, an object of the present invention is to provide a PVA film in which unevenness of a polarizing plate is less likely to be observed in stretching even when further higher transmittance of an LCD is achieved, and a polarizing film and a polarizing plate using such a PVA film.

Means for solving the problems

The present inventors have conducted intensive studies and as a result, found that: the present invention has been accomplished by further reducing the film thickness unevenness in the MD direction (flow direction) of a PVA film and adjusting the phase deviation of the film thickness unevenness in a specific position in the TD direction (width direction) to a specific range, thereby achieving the above-described object, and further conducting further research based on the finding.

Namely, the invention is as follows:

[1]a polyvinyl alcohol film in which an arbitrary point F on the film surface ₀ The film thickness profile of (2) is set to F ₀ (x) Will be from F ₀ The film thickness profile from the point of-100 mm in the TD direction was set to F _-100 (x) Will be from F ₀ The film thickness profile from the point of 200mm in the TD direction was set to F _-200 (x) Will be from F ₀ The film thickness profile from the point +100mm toward TD was set to F ₊₁₀₀ (x) Will be from F ₀ Film thickness profile starting from a point of +200mm in TD directionSet as F ₊₂₀₀ (x) In the case of using the inclination value in MD of each film thickness profile, namely F' ₀ (x)、F′ _-100 (x)、F′ _-200 (x)、F′ ₊₁₀₀ (x) And F' ₊₂₀₀ (x) The average inclination value expressed by the average of MD direction is less than 0.02, F 'is used' ₀ (x) The point showing the minimum value is set to C _n (n=1, 2,3 …) |f' _-100 (C _n )-F′ ₀ (C _n )|、|F′ _-200 (C _n )-F′ ₀ (C _n )|、|F′ ₊₁₀₀ (C _n )-F′ ₀ (C _n ) I and F' ₊₂₀₀ (C _n )-F′ ₀ (C _n ) The average value of the I defines a phase parameter of more than 0.015;

[2] the polyvinyl alcohol film according to the above [1], which has a width of 4m or more;

[3] the polyvinyl alcohol film according to the above-mentioned [1] or [2], which has a swelling degree of 170 to 220%;

[4] the polyvinyl alcohol film according to any one of the above [1] to [3], having a thickness of 30 μm to 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 film for optical use is a polarizing film;

[7] a polarizing film produced using the polyvinyl alcohol film described in any one of the above [1] to [6 ];

[8] a polarizing plate produced by adhering a protective film to at least one side of the polarizing film according to item [7 ];

[9] a method for producing a polyvinyl alcohol film, comprising: a step of forming a polyvinyl alcohol film using a polyvinyl alcohol aqueous solution having a concentration of 32 mass% or less, and stretching the polyvinyl alcohol film at a stretching ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20 mass% or more.

Effects of the invention

According to the present invention, a PVA film in which unevenness of a polarizing plate after stretching is less likely to be observed than in the prior art, and a polarizing film and a polarizing plate using such a PVA film can be provided.

The present invention will be specifically described below.

< measurement of film thickness of PVA film >

The film thickness of the PVA film in the present invention is measured under the following conditions. From the viewpoint of performing phase contrast of film thickness in the width direction, measurement must be performed using a two-dimensional film thickness meter.

< apparatus for use > line scanning film thickness Meter (TI-750 HR-1: manufactured by Otsuka electronics Co., ltd.)

< measurement interval > MD:1mm TD:1mm of

< measurement MD Length >1024mm

< measurement TD Length >750mm/1 scan

(average inclination value in MD)

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

When an arbitrary point of the PVA film is set to a position of 0mm based on a two-dimensional film thickness profile obtained by line scanning of the film thickness meter, a value obtained by performing 8pt FFT smoothing processing on each film thickness profile of a position +100mm, a position +200mm, -a position-100 mm, and a position-200 mm (arbitrary positive and negative directions) in the TD direction from the arbitrary point is set to F ₊₁₀₀ (x)、F ₊₂₀₀ (x)、F _-100 (x) And F _-200 (x) A. The invention relates to a method for producing a fibre-reinforced plastic composite In addition, the film thickness profile of the point 0mm was defined as F ₀ (x) A. The invention relates to a method for producing a fibre-reinforced plastic composite Here, subscript words "0", "+100", "+200", "-100", "-200" denote positions in the TD direction, and x denotes positions in the MD direction. In other words, regarding x, when a certain point is 0mm, the film thickness profile of 1024 points is measured every 1mm from 1mm to 1024mm along the MD direction. The film thickness profile of 1024 points was measured for each of the positions of +100mm, +200mm, -100mm and-200 mm from any point in the TD direction. Here, measurement of film thickness profile of 1024 points was performed every 1mmThe measurement interval in the MD direction and the number of points for measuring the film thickness profile in the MD direction can be appropriately designed.

The MD average inclination value of the present invention is calculated by the following (equations 1) to (6). The film thickness data at each point of 1024mm in the MD direction was differentiated, the absolute value of the differentiated value was averaged over 1024 points, and then the average value of 5 line segments was calculated, and the value obtained by this was defined as the MD average inclination value. The MD average inclination value is a value indicating the strength of the film thickness unevenness in the MD direction, and the larger the value is, the larger the film thickness unevenness is. F is also described as ₀ (x)、F ₊₁₀₀ (x)、F ₊₂₀₀ (x)、F _-100 (x)、F _-200 (x) The differential values of (2) can be respectively shown as F' ₀ (x)、F′ ₊₁₀₀ (x)、F′ ₊₂₀₀ (x)、F′ _-100 (x)、F′ _-200 (x)。

[ mathematics 1]

It should be noted that the number of the substrates,k in the formulae 1 to 6 is F 'which is a function of x' ₀ (x)、F′ ₊₁₀₀ (x)、F′ ₊₂₀₀ (x)、F′ _-100 (x)、F′ _-200 (x) X is substituted into the symbols of 1 to 1024.

F can be set ₀ (x)、F ₊₁₀₀ (x)、F ₊₂₀₀ (x)、F _-100 (x)、F _-200 (x) Is defined as these tilt values, e.g. F can also be defined as ₀ (x) And F is equal to ₀ The value obtained by dividing the difference of (x+1) by the distance between the point x and the point (x+1) is defined as the inclination value. At F ₊₁₀₀ (x)、F ₊₂₀₀ (x)、F _-100 (x)、F _-200 (x) In the same way, the inclination value may be defined as a value obtained by dividing the difference between the film thickness profiles of the point x and the point (x+1) by the distance between the point x and the point (x+1).

The average MD tilt value of the PVA film of the present invention calculated according to the above formula is necessarily 0.02 μm/mm or less, preferably 0.019 μm/mm or less, and more preferably 0.016 or less. If the MD average tilt value is more than 0.02, uneven polarization is strongly observed when the polarizing plate is processed, and the display quality of the LCD is impaired.

The average MD inclination value (c) of the PVA film of the present invention at the position +200mm calculated by the above formula is preferably 0.019 μm/mm or less, more preferably 0.016 μm/mm or less. Similarly, the MD average tilt value (e) at the position of-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 having an MD average tilt value of 0.02 or less is not particularly limited, and examples thereof include: a method for producing a polyvinyl alcohol film, wherein a polyvinyl alcohol film is formed using a polyvinyl alcohol aqueous solution having a concentration of 32 mass% or less, and when the moisture content of the polyvinyl alcohol film is 20 mass% or more, the polyvinyl alcohol film is stretched at a stretching ratio of 1.075 to 1.135.

< phase parameter (θ) >

The PVA film of the present invention is characterized by satisfying the phase parameter (θ) defined below as 0.015 μm/mm or more and the MD average tilt value as described above as 0.02 or less at the same time. The phase parameter (. Theta.) is more preferably 0.016 μm/mm or more.

In the present inventionThe phase parameter (θ) of (a) is a numerical value defined by the following equation. C in the following formula _n Is as follows: for |F' ₀ (C _n ) The points where the values are extremely small are marked with C in order from upstream in the MD direction ₁ 、C ₂ 、C ₃ …C _n Is a number of (3). I F' ₀ (C _n ) Points where i reaches a minimum are fewer than 1024 points.

[ math figure 2]

α(C _n )＝F′ ₊₁₀₀ (C _n )-F′ ₀ (C _n ) (formula 7)

β(C _n )＝F′ ₊₂₀₀ (C _n )-F′ ₀ (C _n ) (formula 8)

γ(C _n )＝F′ _-100 (C _n )-F′ ₀ (C _n ) (formula 9)

δ(C _n )＝F′ _-200 (C _n )-F′ ₀ (C _n ) (formula 10)

In the formula (12), k is represented by C _n The function of lambda (C _n ) Middle pair C _n Substituted into C ₁ ～C _n Is a symbol of (c).

The phase parameter (θ) in the present invention means: a parameter indicating the positional uniformity in the MD direction between a segment that causes film thickness unevenness at a position of 0mm in the TD direction and a segment that causes film thickness unevenness at 4 positions that deviate from the position in the TD direction, the greater the value, the less uniform the segment that causes film thickness unevenness at 5 points in the width direction, and the less likely the film thickness unevenness occurs.

The phase parameter (θ) of the PVA film of the present invention calculated according to the above equation is necessarily 0.015 or more, preferably 0.0153 or more. If the phase parameter (θ) is less than 0.015, the concentration of the node position in the width direction of the PVA film causes stress concentration during stretching, and unevenness of the regular polarizing plate extending in the width direction is observed after stretching.

The method for obtaining a PVA film having a phase parameter (θ) of 0.015 or more is not particularly limited, and examples thereof include: a method for producing a polyvinyl alcohol film, wherein a polyvinyl alcohol film is formed using a polyvinyl alcohol aqueous solution having a concentration of 32 mass% or less, and when the moisture content of the polyvinyl alcohol film is 20 mass% or more, the polyvinyl alcohol film is stretched at a stretching ratio of 1.075 to 1.135.

The average MD tilt 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 average MD tilt value of at least one surface of the PVA film may be 0.02 or less, the phase parameter (θ) is 0.014 or more, or the average MD tilt value of at least two surfaces of the PVA film is 0.02 or less and the phase parameter (θ) is 0.014 or more.

From the viewpoint of use in large-sized liquid crystal television applications, the width of the PVA film of the present invention is preferably 4m or more, more preferably 4.5m or more. On the other hand, if the width of the PVA film is too large, it is easy to make uniaxial stretching itself difficult to uniformly proceed in the case of producing a polarizing film by using a device which has been put into practical use, and therefore, the width of the PVA film is preferably 7m or less.

The swelling degree of the PVA film of the present invention is preferably 170% or more, more preferably 180% or more. The swelling degree of the PVA film is preferably 220% or less, more preferably 210% or less. If the swelling degree is less than 170%, uneven swelling tends to occur, and the dyeing tends to be uneven. If the swelling degree is more than 220%, wrinkles tend to occur in the process, which is not preferable. As a method of adjusting the swelling degree of the PVA film to these ranges, for example, a method of appropriately adjusting the heat treatment temperature of the PVA film after drying is cited.

The swelling degree is an index showing the water retention ability of the PVA film when immersed in water, and can be determined as a percentage by dividing the mass of the PVA film after being immersed in water at 30℃for 30 minutes by the mass of the PVA film after being dried at 105℃for 16 hours.

The volatile fraction of the film-forming stock solution used for forming the PVA film of the present invention is preferably 70 mass% or more, more preferably 71 mass% or more. If the volatile fraction of the PVA aqueous solution used for film formation is less than 70 mass%, the effect of reducing film thickness unevenness due to the leveling effect of casting cannot be expected, and the MD average tilt value becomes high, which is not preferable. If the volatile fraction 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, the term "volatile fraction of the film forming stock solution" in the present invention refers to a volatile fraction obtained by the following formula.

Volatile fraction (mass%) of film-forming stock solution = { (Wa-Wb)/Wa } ×100

(wherein Wa represents the mass (g) of the film-forming stock solution, wb represents the mass (g) of the film-forming stock solution when dried in an electrothermal drier at 105 ℃ for 16 hours.)

As the conditions for forming the PVA film of the present invention, preferable are: when the water content of the PVA aqueous solution used for film formation is 20 mass% or more, stretching is performed with a draft ratio of 1.075 or more, and more preferably, stretching is performed with a draft ratio of 1.08 or more. Preferably, it is: when the water content of the PVA aqueous solution used for film formation is 20 mass% or more, stretching is performed with a draft ratio of 1.135 or less, more preferably less than 1.13, and still more preferably less than 1.12. In general, the larger the draft ratio is, the lower the ultimate stretch ratio in the stretching process is, and the higher the stretch breaking probability in the stretching process is. Therefore, when the draft ratio is 1.12 or more, a step of reducing the influence of the draft ratio may be required. If the draft ratio is higher than 1.135, the ultimate stretch ratio in the stretch processing is remarkably reduced, which is not preferable. If the draft ratio is less than 1.075, the phase parameter (θ) becomes smaller, which is not preferable from the viewpoint of visibility of unevenness of the polarizing plate. The water content at the time of stretching is preferably 20 mass% or more, more preferably 22 mass% or more, as the conditions for producing the PVA film. If the water content at the time of drawing is less than 20 mass%, the phase parameter (θ) is less likely to become large. For the above reasons, it is necessary to set the water fraction and draft ratio at the time of drafting to the above ranges.

(PVA)

Examples of the PVA resin included in the PVA film of the present invention include resins obtained by saponifying polyvinyl esters obtained by polymerizing 1 or more of vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl tertiary carboxylate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among the vinyl esters, vinyl acetate is preferred from the viewpoints of ease of production, ease of acquisition, cost and the like of PVA.

The PVA may be modified with 1 or 2 or more graft copolymerizable monomers as long as the effect of the present invention is not impaired. Examples of the graft copolymerizable monomer include unsaturated carboxylic acids or derivatives thereof; unsaturated sulfonic acid or derivatives thereof; alpha-olefins having 2 to 30 carbon atoms, and the like. The proportion of the structural unit derived from the graft copolymerizable monomer in the PVA is preferably 5 mol% or less based on the number of moles of all the structural units constituting the PVA.

Some of the hydroxyl groups in the PVA may be crosslinked or uncrosslinked. In addition, a part of hydroxyl groups in the PVA may be reacted with an aldehyde compound such as acetaldehyde or butyraldehyde to form an acetal structure, or may not be reacted with these compounds to form an acetal structure.

The polymerization degree of PVA is preferably 2000 or more, more preferably 2200 or more, and further preferably 2400 or more. In the case where the polymerization degree is less than 2000, the durability of the resulting polarizing film tends to be lowered. The polymerization degree of PVA is preferably 2700 or less, more preferably 2650 or less, and further preferably 2600 or less. On the other hand, when the polymerization degree exceeds 2700, the manufacturing cost increases and the process passability at the time of film formation tends to deteriorate. The polymerization degree of PVA referred to in the present specification means an average polymerization degree measured in accordance with the description of JISK 6726-1994.

The saponification degree of PVA is preferably 98 mol% or more, more preferably 98.5 mol% or more, and still more preferably 99 mol% or more from the viewpoint of water resistance of the polarizing film. If the saponification degree is less than 98 mol%, the water resistance of the resulting polarizing film tends to be poor. The saponification degree of PVA in the present specification means: the proportion of the number of moles of the vinyl alcohol unit (mol%) to the total number of moles of the structural unit (typically, vinyl ester unit) and the vinyl alcohol unit which are capable of being converted into the vinyl alcohol unit by saponification, which are possessed by PVA. The saponification degree can be measured as described in JISK 6726-1994.

(plasticizer)

The PVA film of the present invention preferably contains a plasticizer. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane, and the PVA film of the present invention may contain 1 or 2 or more of these plasticizers. Among these, glycerin is preferred from the viewpoint of improving the 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 further preferably 5 parts by mass or more, with respect to 100 parts by mass of PVA contained therein. By setting the content of the plasticizer to 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 further preferably 15 parts by mass or less, per 100 parts by mass of PVA. When the plasticizer is 20 parts by mass or less, the PVA film can be prevented from becoming too soft and the handleability is reduced.

(surfactant)

The PVA film of the present invention preferably comprises a surfactant. The film forming property of the PVA film is improved by producing the PVA film by using a film forming stock solution containing a surfactant. As a result, the PVA film can be easily peeled from the metal roll or belt used for film formation while suppressing occurrence of thickness unevenness of the PVA film. In the case of producing a PVA film from a film-forming stock solution containing a surfactant, the surfactant is contained in the resulting PVA film.

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

The anionic surfactant is preferably a carboxylic acid type such as potassium laurate; sulfate esters such as polyoxyethylene lauryl ether sulfate and octyl sulfate; sulfonic acid type such as dodecylbenzenesulfonate.

As the nonionic surfactant, for example, alkyl ether type surfactants such as polyoxyethylene oleyl ether are suitable; alkylphenyl ethers such as polyoxyethylene octylphenyl ether; alkyl esters such as polyoxyethylene laurate; alkylamines such as polyoxyethylene lauryl amino ether; alkylamide type such as polyoxyethylene lauramide; polypropylene glycol ethers such as polyoxyethylene polyoxypropylene ether; alkanolamide type such as diethanolamide oleate; and allylphenyl ethers such as polyoxyalkylene allylphenyl ether.

These surfactants may be used alone or in combination of 1 or more than 2.

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 further preferably 0.05 part by mass, relative to 100 parts by mass of PVA. When the content of the surfactant is not less than the lower limit, the film forming property and peeling property 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 further preferably 0.2 parts by mass, relative to 100 parts by mass of PVA. By setting the content of the surfactant to the above upper limit or less, bleeding of the surfactant to the surface of the PVA film can be suppressed, and blocking and a decrease in handleability can be prevented.

(other Components)

The PVA film of the present invention may further contain, if necessary, an antioxidant, an antifreeze agent, a pH adjuster, a masking agent, an anti-coloring agent, an oil agent, a surfactant described later, and the like.

(shape, etc.)

The shape of the PVA film of the present invention is not particularly limited, but is preferably a long film. Thus, a more uniform PVA film can be continuously and easily produced, and can be continuously used even when a polarizing film is produced using the same. The length (length in the longitudinal direction) of the long film is not particularly limited, and may be appropriately set according to the application or the like, and may be, for example, in the range of 5 to 30000 m.

The thickness of the PVA film of the present invention is preferably 30 to 65. Mu.m. If the thickness of the PVA film is less than 30 μm, the handling property upon stretching becomes poor, and if the thickness of the PVA film is more than 65 μm, it is not preferable as a polarizing plate for a thin display.

< method for producing PVA film >

In the present invention, the method for producing the PVA film is not particularly limited, and the following method can be employed: a method of forming a film by using a film-forming stock solution obtained by adding a solvent, an additive, or the like to PVA and homogenizing the same, and using a casting film-forming method, a wet film-forming method (spraying out the poor solvent), a dry-wet film-forming method, a gel film-forming method (a method of obtaining a PVA film by temporarily cooling and gelling the film-forming stock solution and then extracting and removing the solvent), or a combination thereof; a melt extrusion film-forming method in which the film-forming stock solution is obtained by using an extruder or the like, and is extruded from a T die or the like to form a film; any method such as inflation molding. Among these, the casting film forming method and the melt extrusion film forming method are preferable because they can obtain a homogeneous film with good productivity. Hereinafter, a casting film forming method or a melt extrusion film forming method of a PVA film will be described.

The method for producing a polyvinyl alcohol film of the present invention includes the following method for producing a polyvinyl alcohol film, which comprises: a step of forming a polyvinyl alcohol film using a polyvinyl alcohol aqueous solution having a concentration of 32 mass% or less, and stretching the polyvinyl alcohol film at a stretching ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20 mass% or more. By adopting the present manufacturing method, a film free from thickness unevenness can be manufactured.

When the ultimate stretch ratio is reduced, a step of alleviating the influence of the stretch ratio may be added. As a specific method for alleviating the influence of the stretch ratio, it is considered that: a method of irradiating a polyvinyl alcohol film with infrared rays and microwaves; a method of reducing the draw ratio (cumulative draw ratio) when the water content of the polyvinyl alcohol film is 15 to 5 mass%; a method of coating a plasticizer after stretching; a method of drying with superheated steam; a method of drying at high temperature; the method of heat treatment with a suspension dryer may be any method, and the methods may be combined or may be performed a plurality of times.

In the case of producing a PVA film by a casting film-forming method or a melt extrusion film-forming method, the film-forming stock solution is cast into a film on a support such as a metal roll or a metal belt, and is heated to remove the solvent, thereby solidifying the film-forming stock solution to form a film. The cured film is peeled from the support, dried by a drying roll, a drying furnace, or the like as necessary, and further heat-treated and wound up as necessary, whereby a long PVA film in a roll form can be obtained.

The method for adjusting the film-forming stock solution is not particularly limited, and examples thereof include: a method of dissolving PVA, a plasticizer, a surfactant and other additives in a dissolution tank or the like; and a method in which, when PVA in a water-containing state is melt-kneaded using a single-screw or twin-screw extruder, the PVA is melt-kneaded together with a plasticizer, a surfactant, or the like.

Examples of the liquid medium used for preparing the film-forming stock solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, diethylenetriamine, and the like, and one or more of these may be used. Among them, water is suitable from the viewpoint of environmental burden and recyclability.

The dried PVA film may be further heat treated as needed. The strength, swelling degree, birefringence, and the like of the PVA film can be adjusted by performing the heat treatment. The surface temperature of the heat treatment roller for performing the heat treatment is preferably 60 ℃ or higher. The surface temperature of the heat treatment roller is preferably 135 ℃ or less, more preferably 130 ℃ or less. If the surface temperature of the heat treatment roller is too high, the amount of heat applied becomes excessive, the size of the platelets in the PVA film becomes large, and the swelling degree of the PVA film is difficult to satisfy the above range.

The PVA film thus produced may be further subjected to a humidity conditioning treatment, cutting of both ends (edges) of the film, and the like, if necessary, and wound up into a roll on a cylindrical core, and then subjected to moisture-proof packaging to form a product.

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

The PVA film of the present invention is reduced in film thickness unevenness in the MD direction, and is adjusted to have a phase deviation in the TD direction within a specific range, and therefore can be suitably used as a film for producing an optical film. Here, as will be described later, examples of the optical film include a polarizing film, a viewing angle improving film, a retardation film, a brightness enhancing film, and the like, and a polarizing film is preferable. The polarizing film of the present invention produced using the PVA film of the present invention is less likely to cause unevenness of the polarizing plate after stretching than in the prior art.

< method for producing optical film >)

Hereinafter, a method for producing a polarizing film will be specifically described as an example of a method for producing an optical film.

The polarizing film is generally produced by using a PVA film as a raw material film and subjecting the film to a swelling step, a dyeing step, a crosslinking step, a stretching step, a fixing step, and other treatment steps. Specific examples of the treatment liquid used in each step include a swelling treatment liquid used in a swelling treatment, a dyeing treatment liquid (dyeing liquid) used in a dyeing treatment, a crosslinking treatment liquid used in a crosslinking treatment, a stretching treatment liquid used in a stretching treatment, a fixing treatment liquid used in a fixing treatment, and a cleaning treatment liquid (cleaning liquid) used in a cleaning treatment.

Each processing step that can be used in the manufacturing method for manufacturing a polarizing film is described below. In the method for producing a polarizing film, 1 or 2 or more 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)

The PVA film is preferably subjected to a washing treatment before the swelling treatment. By such a washing treatment before the swelling treatment, the anti-blocking agent or the like adhering to the PVA film can be removed, and contamination of each treatment liquid in the production process of the polarizing film by the anti-blocking agent or the like can be prevented. The cleaning treatment is preferably performed by immersing the PVA film in a cleaning treatment liquid, or by blowing a cleaning treatment liquid onto the PVA film. As the cleaning treatment liquid, for example, water can be used. The temperature of the cleaning liquid is preferably 20℃or higher, more preferably 22℃or higher, still more preferably 24℃or higher, and particularly preferably 26℃or higher. The removal of the anti-blocking agent or the like adhering to the PVA film is easily performed by setting the temperature of the cleaning treatment liquid to 20 ℃ or higher. The temperature of the cleaning liquid is preferably 40 ℃ or lower, more preferably 38 ℃ or lower, further preferably 36 ℃ or lower, particularly preferably 34 ℃ or lower. In addition, by setting the temperature of the cleaning treatment liquid to 40 ℃ or lower, it is possible to prevent the PVA film from being dissolved on a part of the surface thereof, thereby preventing the films from adhering to each other and reducing the handleability.

(swelling treatment)

The swelling treatment may 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℃or higher, more preferably 22℃or higher, and still more preferably 24℃or higher. The temperature of the swelling treatment liquid is preferably 40 ℃ or lower, more preferably 38 ℃ or lower, and further preferably 36 ℃ or lower. The time for immersing in the swelling treatment liquid is, for example, preferably 0.1 minutes or more, and more preferably 0.5 minutes or more. The time for immersing in the swelling treatment liquid is, for example, preferably 5 minutes or less, and more preferably 3 minutes or less. The water used as the swelling treatment liquid is not limited to pure water, and may be an aqueous solution in which various components such as a boron-containing compound are dissolved, or may be a mixture of water and an aqueous medium. The type of the boron-containing compound is not particularly limited, and boric acid or borax is preferable from the viewpoint of handling properties. 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 of before, during, and after the stretching treatment. The dyeing treatment is preferably performed by using a solution (suitably 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 potassium iodide/iodine (mass) is preferably in the range of 20 to 100. The temperature of the dyeing treatment liquid is preferably 20 ℃ or higher, more preferably 25 ℃ or higher. The temperature of the dyeing treatment liquid is preferably 50 ℃ or lower, more preferably 40 ℃ or lower. The dyeing treatment liquid may contain a boron-containing compound such as boric acid as a crosslinking agent. If a dichroic dye is previously contained in the PVA film used as the raw material film, the dyeing treatment can be omitted. The PVA film used as the raw material film may contain a boron-containing compound such as boric acid and borax.

(crosslinking treatment)

In the production of the polarizing film, for the purpose of firmly adsorbing the dichroic dye to the PVA film or the like, it is preferable to perform a crosslinking treatment after the dyeing treatment. The crosslinking treatment may be performed by using a solution (suitably an aqueous solution) containing a crosslinking agent as the crosslinking treatment liquid, and immersing the PVA film in the crosslinking treatment liquid. As the crosslinking agent, 1 or 2 or more kinds of boron-containing compounds such as boric acid and borax can be used. If the concentration of the crosslinking agent in the crosslinking treatment liquid is too high, the following tends to occur: the crosslinking reaction proceeds excessively, and it is difficult to sufficiently stretch the film in the stretching treatment to be performed thereafter, and if it is too small, the effect of the crosslinking treatment tends to be lowered. 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% by mass or less, more preferably 5.5% by mass or less, and further preferably 5% by mass or less.

In order to inhibit the dichroic dye from eluting from the PVA film after the dyeing treatment, the crosslinking treatment liquid may contain an iodine-containing compound such as potassium iodide. If the concentration of the iodine-containing compound in the crosslinking treatment liquid is too high, the heat resistance of the resulting polarizing film tends to be lowered, although the reason is not clear. If the amount is too small, the effect of suppressing elution of the dichroic dye tends to be lowered. The concentration of the iodine-containing compound 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 iodine-containing compound in the crosslinking treatment liquid is preferably 6% by mass or less, more preferably 5.5% by mass or less, and further preferably 5% by mass or less.

If the temperature of the crosslinking treatment liquid is too high, the dichroic dye is eluted, and the resulting polarizing film tends to be uneven in dyeing, and if it is too low, the effect of the crosslinking treatment may be reduced. The temperature of the crosslinking treatment liquid is preferably 20℃or higher, more preferably 22℃or higher, and still more preferably 25℃or higher. The temperature of the crosslinking treatment liquid is preferably 45℃or lower, more preferably 40℃or lower, and still more preferably 35℃or lower.

In addition to the stretching treatment described later, the PVA film may be stretched separately in each of the above treatments or between the treatments. By performing such stretching (pre-stretching), wrinkles can be prevented from occurring on the surface of the PVA film. From the viewpoint of polarization performance of the obtained polarizing film, the total stretching magnification of the pre-stretching (magnification obtained by multiplying stretching magnification in each process) is preferably 4 times or less, more preferably 3.5 times or less, depending on the original length of the PVA film of the raw material before stretching. From the viewpoint of polarization performance of the obtained polarizing film, the total stretching magnification of the pre-stretching is more preferably 1.5 times or more depending on the original length of the PVA film of the raw material before stretching. The stretching ratio in the swelling treatment is preferably 1.1 times or more, more preferably 1.2 times or more, and still more preferably 1.4 times or more, depending 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 still more preferably 2.3 times or less, depending on the original length of the PVA film. The stretching ratio in the dyeing treatment is preferably 2 times or less, more preferably 1.8 times or less, and still more preferably 1.5 times or less, depending on the original length of the PVA film. The stretching ratio in the dyeing treatment is more preferably 1.1 times or more depending on the original length of the PVA film. The stretching ratio in the crosslinking treatment is preferably 2 times or less, more preferably 1.5 times or less, and still more preferably 1.3 times or less, depending on the original length of the PVA film. The stretching ratio in the crosslinking treatment is more preferably 1.05 times or more depending 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 (suitably an aqueous solution) containing a boron-containing compound such as boric acid may be used as the stretching treatment liquid, and the stretching treatment liquid may be used, or the stretching treatment liquid may be used in a dyeing treatment liquid or a fixing treatment liquid described later. In the case of the dry stretching method, the PVA film after water absorption may be used in air. Among these, the wet stretching method is preferable, and the uniaxial stretching is more preferable in an aqueous solution containing boric acid. When the stretching treatment liquid contains a boron-containing compound, 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, and even more preferably 2.5 mass% or more, from the viewpoint of improving the stretchability of the PVA film. The concentration of the boron-containing compound in the stretching treatment liquid is preferably 7 mass% or less, more preferably 6.5 mass% or less, and still more preferably 6 mass% or less, from the viewpoint of improving the stretchability of the PVA film.

The stretching treatment liquid preferably contains an iodine-containing compound such as potassium iodide. If the concentration of the iodine-containing compound in the stretching treatment liquid is too high, the resulting polarizing film tends to be significantly bluish in hue, and if it is too low, the heat resistance of the resulting polarizing film tends to be lowered although the reason is not clear. The concentration of the iodine-containing compound in the stretching treatment liquid is preferably 2% by mass or more, more preferably 2.5% by mass or more, and still more preferably 3% by mass or more. The concentration of the iodine-containing compound in the stretching treatment liquid is preferably 8% by mass or less, more preferably 7.5% by mass or less, and further preferably 7% by mass or less.

If the temperature of the stretching treatment liquid is too high, the PVA film tends to be dissolved and softened and easily broken, and if it is too low, the stretchability tends to be lowered. The temperature of the stretching treatment liquid is preferably 50℃or higher, more preferably 52.5℃or higher, and still more preferably 55℃or higher. The temperature of the stretching treatment liquid is preferably 70 ℃ or lower, more preferably 67.5 ℃ or lower, and even more preferably 65 ℃ or lower. The preferable range of the stretching temperature in the case of performing the stretching treatment by the dry stretching method is also as described above.

When the stretching ratio in the stretching treatment is high, a polarizing film or the like having more excellent polarizing performance can be obtained, and therefore, it is preferably 1.2 times or more, more preferably 1.5 times or more, and further preferably 2 times or more. From the viewpoint of the polarizing performance of the obtained polarizing film, the total stretching ratio (ratio obtained by multiplying the stretching ratios in the respective steps) including the stretching ratio before stretching is preferably 5.5 times or more, more preferably 5.7 times or more, and even more preferably 5.9 times or more, based on the original length of the PVA film of the raw material before stretching. The upper limit of the stretching ratio is not particularly limited, and if the stretching ratio is too high, stretching fracture of the PVA film tends to occur, so that it is preferably 8 times or less.

The method of stretching by uniaxial stretching is not particularly limited, and uniaxial stretching in the longitudinal direction and transverse uniaxial stretching in the width direction may be used. In the case of producing a polarizing film, uniaxial stretching in the longitudinal direction is preferable from the viewpoint of obtaining a substance excellent in polarization performance. The uniaxial stretching in the longitudinal direction can be performed by using a stretching device having a plurality of rolls parallel to each other and changing the peripheral speed between the rolls.

In the present invention, the maximum stretching speed (%/min) in the case of stretching treatment by uniaxial stretching is not particularly limited, but the maximum stretching speed is preferably 200%/min or more, more preferably 300%/min or more, and still more preferably 400%/min or more. Here, the maximum stretching speed means: when stretching the PVA film in two or more stages using 3 or more rolls having different peripheral speeds, the fastest stretching speed is obtained among the stages. When the stretching treatment of the PVA film is performed in 1 stage without dividing it into two or more stages, the stretching speed in the stage becomes the maximum stretching speed. The stretching speed means: an increase in the length of the PVA film increased by stretching per unit time relative to the length of the PVA film before stretching. For example, a stretch speed of 100%/min means: the PVA film was deformed from the length before stretching to a speed of 2 times the length in 1 minute. The larger the maximum stretching speed is, the more the stretching treatment (uniaxial stretching) of the PVA film can be performed at a high speed, 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 high, excessive tension may be locally applied to the PVA film during the stretching treatment (uniaxial stretching) of the PVA film, and stretch breaking may easily occur. From this point of view, the maximum stretching speed is preferably not more than 900%/min.

(washing treatment after dyeing treatment)

After the dyeing treatment, the PVA film after the stretching treatment is preferably subjected to a cleaning treatment. The cleaning treatment is preferably performed by immersing the PVA film in a cleaning treatment liquid, or by blowing a cleaning treatment liquid onto the PVA film. As the cleaning treatment liquid, for example, water can be used. The water is not limited to pure water, and may contain an iodine-containing compound such as potassium iodide. In this case, the concentration of the boron-containing compound is preferably 2.0 mass% or less.

The temperature of the cleaning liquid is preferably 5℃or higher, more preferably 7℃or higher, and still more preferably 10℃or higher. The temperature of the cleaning liquid is preferably 40 ℃ or lower, more preferably 38 ℃ or lower, and even more preferably 35 ℃ or lower. By setting the temperature of the cleaning liquid to 5 ℃ or higher, breakage of the PVA film due to freezing of moisture can be suppressed. In addition, the optical characteristics of the obtained polarizing film are improved by setting the temperature of the cleaning treatment liquid to 40 ℃ or lower.

Specific methods for producing the polarizing film include: and a method of applying dyeing treatment, stretching treatment, crosslinking treatment and/or fixing treatment to the PVA film. Preferable examples include: the PVA film is subjected to a swelling treatment, a dyeing treatment, a crosslinking treatment, a stretching treatment (in particular, a uniaxial stretching treatment) and a cleaning treatment in this order. The stretching treatment may be performed by any treatment step prior to the above treatment, or may be performed in a plurality of stages of 2 stages or more.

The PVA film subjected to each of the treatments described above is subjected to a drying treatment, whereby a polarizing film can be obtained. The method of the drying treatment is not particularly limited, and examples thereof include a contact method in which the film is brought into contact with a heating roller, a method in which the film is dried in a hot air dryer, a floating method in which the film is dried by hot air while floating, and the like.

< polarizing film, polarizing plate >)

In order to compensate for the mechanical strength, the polarizing film of the present invention is manufactured by sticking a protective film to at least one side. The polarizing film of the present invention is generally used by attaching a protective film that is optically transparent and has mechanical strength to a polarizing plate. As the protective film, a cellulose Triacetate (TAC) film, a Cellulose Acetate Butyrate (CAB) film, an acrylic film, a polyester film, or the like is used. Examples of the adhesive used for the adhesion include PVA-based adhesives and urethane-based adhesives, and among them, PVA-based adhesives are preferable.

The polarizing plate obtained in the above-described manner can be used as a component of an LCD by being adhered to a glass substrate after an adhesive such as an acrylic adhesive is applied. Meanwhile, the adhesive can be adhered to a phase difference film, a visual field angle improving film, a brightness enhancement film and the like.

Examples

The present invention is specifically illustrated by the following examples, but the present invention is not limited to these examples.

< calculation of average inclination value of MD, phase parameter (θ), + -200 mm position average inclination value >)

The average tilt value of the MD, the phase parameter (. Theta.) and the average tilt value at + -200 mm position of the PVA film obtained in the 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 >

About 1.5g of a test piece was cut out from the PVA film obtained in the following examples or comparative examples. Next, the test piece was immersed in 1000g of distilled water at 30 ℃. After dipping for 30 minutes, the test piece was taken out, and after sucking the surface water with a filter paper, the mass (We) thereof was measured. Subsequently, the test piece was put into a hot air dryer, dried at 105℃for 16 hours, and then the mass (Wf) thereof was measured. The swelling degree of the PVA film was determined from the obtained masses We and Wf by the following formula.

Swelling degree (%) = (We/Wf) ×100

< evaluation of ultimate draw ratio at the time of drawing >

The ease of stretch-break at the time of stretching was evaluated based on the breaking magnification of the PVA film at the time of producing a polarizing film in the following examples or comparative examples. That is, the breaking ratio of uniaxial stretching in the stretching treatment at the time of producing the polarizing film was measured 10 times, and the average value thereof was evaluated as the ultimate stretching ratio according to the following criteria.

A … limit stretch ratio of 6.6 times or more

The B … ultimate stretch ratio is more than 6.5 times and less than 6.6 times

C … limit stretch ratio of 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 30cm×30cm pieces, and between two polarizing plates in a parallel nicols state (the single transmittance was 43.5% and the polarization degree was 99.9%), the polarizing plates of the sample were sandwiched so as to be in a crossed nicols state with respect to the respective polarizing plates, and a luminance of 40000cd/m was used ² The optical polarization unevenness was visually observed in the transmission mode, and evaluated in accordance with the following criteria.

A … has uneven color

B … has no color unevenness

Example 1

A film-forming stock solution (volatile fraction: 70 mass%) was prepared by melt-mixing 100 parts by mass of PVA (having a saponification degree of 99.9 mol%, and a polymerization degree of 2400), 10 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and 233 parts by mass of water using a melt extruder. Then, the film was discharged from the T die onto a support (surface temperature: 98 ℃ C.) to form a PVA film on the support. At the time when the water content of the discharged PVA film became 24%, the draw ratio was set to 1.10 by controlling the speed ratio of the rolls, and stretching was performed. A PVA film having a width of 5m and a film thickness of 60 μm was obtained by contact heat treatment at 105℃using a heat treatment roller in the latter half. Regarding the water fraction at the time of stretching, the conveyed PVA film was torn off between rolls for stretching, after measuring the mass (Ww), the film was put into a hot air dryer, dried at 105 ℃ for 16 hours, and then the mass (Wd) thereof was measured. The water content of the film was calculated using the following formula.

Film moisture ratio (%) = (Ww-Wd)/ww×100

The obtained PVA film was cut into 650mm wide pieces, and the film was sequentially subjected to swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment, washing treatment, and drying treatment, to continuously produce a polarizing film. The swelling treatment was carried out by uniaxial stretching to 2.00 times in the longitudinal direction while immersing in pure water (swelling treatment liquid) at 25 ℃. The dyeing treatment is carried out by uniaxial stretching to 1.26 times in the longitudinal direction while immersing the dyeing solution (dyeing treatment solution) of potassium iodide/iodine (mass ratio) of 23 and iodine concentration of 0.03 to 0.05 mass%) in an aqueous solution of potassium iodide/iodine at a temperature of 32 ℃. In the dyeing treatment, the iodine concentration in the dyeing treatment liquid is adjusted to be in the range of 0.03 to 0.05 mass% so that the monomer transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment is in the range of 43.5% ± 0.2%. The crosslinking treatment was carried out by uniaxially stretching the impregnated side to 1.19 times in the longitudinal direction in an aqueous boric acid solution (crosslinking treatment liquid) at 32 ℃. The stretching treatment was carried out by uniaxial stretching to 2.00 times in the longitudinal direction while immersing the substrate in an aqueous boric acid/potassium iodide solution (stretching solution) (boric acid concentration: 2.8 mass% and potassium iodide concentration: 5 mass%) at 55 ℃. The washing treatment was performed by immersing in an aqueous solution of potassium iodide/boric acid (washing treatment liquid) (potassium iodide concentration 3 to 6 mass% and boric acid concentration 1.5 mass%) at 22 ℃ for 12 seconds without stretching.

Examples 2 to 4 and comparative examples 1 to 3

The PVA film was produced and evaluated in the same manner as in example 1, except that the production conditions of the PVA film were changed as shown in table 1. The volatile fraction of the film-forming stock solution can be adjusted by adjusting the amount of water to be melt-mixed in the melt extruder. The results are shown in Table 1.

From the above results, it can be seen that: the PVA film of the present invention is suppressed in color unevenness at the time of stretching processing.

TABLE 1

Claims

1. A polyvinyl alcohol film in which an arbitrary point F on the film surface ₀ The film thickness profile of (2) is set to F ₀ (x) Will be from F ₀ The film thickness profile from the point of-100 mm in the TD direction was set to F _-100 (x) Will be from F ₀ The film thickness profile from the point of 200mm in the TD direction was set to F _-200 (x) Will be from F ₀ The film thickness profile from the point +100mm toward TD was set to F ₊₁₀₀ (x) Will be from F ₀ The film thickness profile at the point of +200mm in TD direction was defined as F ₊₂₀₀ (x) In the case of using the differential value in MD direction of each film thickness profile, namely F' ₀ (x)、F′ _-100 (x)、F′ _-200 (x)、F′ ₊₁₀₀ (x) And F' ₊₂₀₀ (x) The average inclination value expressed by the average of MD direction is less than 0.02, F 'is used' ₀ (x) The point showing the minimum value is set to C _n (n=1, 2,3 …) |f' _-100 (C _n )-F′ ₀ (C _n )|、|F′ _-200 (C _n )-F′ ₀ (C _n )|、|F′ ₊₁₀₀ (C _n )-F′ ₀ (C _n ) I and F' ₊₂₀₀ (C _n )-F′ ₀ (C _n ) The average value of the values defines a phase parameter (θ) of 0.015 or more.

2. The polyvinyl alcohol film according to claim 1, having a width of 4m or more.

3. The polyvinyl alcohol film according to claim 1 or 2, having a swelling degree of 170 to 220%.

4. The polyvinyl alcohol film according to any one of claims 1 to 3, having a thickness of 30 μm to 65 μm.

5. The polyvinyl alcohol film according to any one of claims 1 to 4, which is a film for producing an optical film.

6. The polyvinyl alcohol film according to claim 5, wherein the optical film is a polarizing film.

7. A polarizing film produced using the polyvinyl alcohol film according to any one of claims 1 to 6.

8. A polarizing plate produced by attaching a protective film to at least one side of the polarizing film according to claim 7.

9. A method for producing a polyvinyl alcohol film, comprising: a step of forming a polyvinyl alcohol film using a polyvinyl alcohol aqueous solution having a concentration of 32 mass% or less, and stretching the polyvinyl alcohol film at a stretching ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20 mass% or more.