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

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, polarizing film and polarizing plate using the same

Technical Field

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

Background Art

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

Polarizing plates are generally manufactured as follows: a polyvinyl alcohol film (hereinafter sometimes referred to as "PVA") is dyed and uniaxially stretched, and a polarizing film is further fixed using a boron compound or the like as needed to manufacture the polarizing film, and then a protective film such as a triacetyl cellulose (TAC) film is attached to the surface of the polarizing film. In recent years, energy conservation of LCDs has been required, and polarizing plates have also been required to have high transmittance. With the high transmittance of polarizing plates, unevenness of polarizing plates caused by uneven film thickness of PVA films, which was not observed in the past, will also be observed, requiring PVA films with better flatness than previous products.

Patent Document 1 describes a method for producing a PVA film having better flatness than conventional films by suppressing fluctuations during casting using an air knife.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2017-008298

Summary of the invention

Problems to be solved by the invention

However, although Patent Document 1 can achieve better film thickness flatness in the MD direction than before, there is the following problem: since the thin film thickness portions are concentrated in the width direction, stress is concentrated in the thin film thickness portions, and although the observability is low, regular polarizing plate unevenness can still be observed, and the requirement for further higher transmittance of LCDs cannot be met.

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

Means for solving problems

The present inventors have repeatedly conducted in-depth research and found that the above-mentioned problem can be achieved by adjusting the phase deviation of the film thickness unevenness at a specific position in the TD direction (width direction) to a specific range on the basis of further reducing the film thickness unevenness in the MD direction (flow direction) of the PVA film. Based on this finding, further repeated research has been conducted, thereby completing the present invention.

That is, the present invention is:

[1] A polyvinyl alcohol film, wherein a film thickness profile at an arbitrary point _F0 on the film surface is defined as _F0 (x), a film thickness profile at a point -100 mm from _F0 in the TD direction is defined as F- ₁₀₀ (x), a film thickness profile at a point -200 mm from _F0 in the TD direction is defined as F- ₂₀₀ (x), a film thickness profile at a point +100 mm from _F0 in the TD direction is defined as F ₊₁₀₀ (x), and a film thickness profile at a point +200 mm from _F0 in the TD direction is defined as F ₊₂₀₀ (x), an average inclination value represented by the average of the inclination values in the MD direction of each film thickness profile, i.e., _F′0 (x), F′ _{-100 (} x), F′-200(x), F′ ₊₁₀₀ (x) and F′ ₊₂₀₀ (x) in the MD direction is 0.02 or less, and |F′ when the point where _F′0 (x) shows a minimum value is defined as _Cn (n=1, 2, 3, ...) The phase parameter defined by the average value _{of -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 )| is greater than 0.015;

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

[3] The polyvinyl alcohol film according to [1] or [2], wherein the degree of swelling is 170 to 220%.

[4] The polyvinyl alcohol film according to any one of [1] to [3] above, which has a thickness of 30 μm to 65 μm;

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

[6] The polyvinyl alcohol film according to [5] above, wherein the optical film is a polarizing film;

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

[8] A polarizing plate produced by attaching a protective film to at least one side of the polarizing film described in [7] above;

[9] A method for producing a polyvinyl alcohol film, comprising: forming the polyvinyl alcohol film using a polyvinyl alcohol aqueous solution having a concentration of 32% by mass or less, and stretching the polyvinyl alcohol film 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.

Effects of the Invention

According to the present invention, it is possible to provide a PVA film in which unevenness in a polarizing plate after stretching is less likely to be observed than in the related art, and a polarizing film and a polarizing plate using the PVA film.

Hereinafter, the present invention will be described in detail.

<Measurement of PVA film thickness>

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 the film thickness in the width direction, it is necessary to use a two-dimensional film thickness meter for measurement.

<Device used> Line scanning film thickness meter (TI-750HR-1: manufactured by Otsuka Electronics Co., Ltd.)

<Measurement interval> MD: 1mm TD: 1mm

<Measured MD length> 1024mm

<Measurement TD length> 750mm/1 scan

(Average tilt value in MD direction)

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. The detailed calculation method of the MD average tilt value is described below.

According to the two-dimensional film thickness profile obtained by using a line scanning film thickness meter, when an arbitrary point of the PVA film is set to position 0mm, the value obtained by performing 8ptFFT smoothing processing on the film thickness profiles of the positions of +100mm, +200mm, -100mm, and -200mm (positive and negative directions) from the arbitrary point toward the TD direction is set to F ₊₁₀₀ (x), F ₊₂₀₀ (x), F _-100 (x), and F _-200 (x). In addition, the film thickness profile of the point at position 0mm is set to F ₀ (x). Here, the subscripts "0", "+100", "+200", "-100", and "-200" represent the position in the TD direction, and x represents the position in the MD direction. In other words, about x, when a certain point is set to 0mm, along the MD direction, the film thickness profiles of 1024 points are measured every 1mm from 1mm to 1024mm. The film thickness profile of the 1024 points is measured for the position of 0 mm, +100 mm, +200 mm, -100 mm, and -200 mm in the TD direction from the aforementioned arbitrary point. It should be noted that the film thickness profile of the 1024 points is measured every 1 mm, and the measurement interval in the MD direction and the number of points for measuring the film thickness profile along the MD direction can be appropriately designed.

The MD average tilt value of the present invention is calculated using the following (Formula 1) to (Formula 6). The film thickness data at each point of 1 mm of 1024 mm in the MD direction is differentiated, and the absolute value of the differential value is averaged with 1024 points, and the average value of 5 line segments is calculated, and the value thus obtained is defined as the MD average tilt value. The MD average tilt value is a numerical value representing the intensity of the uneven film thickness in the MD direction, and the larger the numerical value, the larger the uneven film thickness. It should be noted that the differential values of F ₀ (x), F ₊₁₀₀ (x), F ₊₂₀₀ (x), F _-100 (x), and F _-200 (x) can be represented as F′ ₀ (x), F′ ₊₁₀₀ (x), F′ ₊₂₀₀ (x), F′ _-100 (x), and F′ _-200 (x), respectively.

[Mathematical formula 1]

It should be noted that k in (Formula 1) to (Formula 6) is a symbol indicating that ₁ to 1024 is substituted into x in the functions of x, namely, F'0(x), F' ₊₁₀₀ (x), F' ₊₂₀₀ (x), F' _-100 (x), and F' _-200 (x).

The differential values of F ₀ (x), F ₊₁₀₀ (x), F ₊₂₀₀ (x), F _-100 (x), and F _-200 (x) may be defined as these inclination values. For example, the value obtained by dividing the difference between F ₀ (x) and F ₀ (x+1) by the distance between point x and point (x+1) may be defined as the inclination value. In the case of F ₊₁₀₀ (x), F ₊₂₀₀ (x), F _-100 (x), and F _-200 (x), the value obtained by dividing the difference between the film thickness profiles at point x and point (x+1) by the distance between point x and point (x+1) may be defined as the inclination value.

The MD average tilt value of the PVA film of the present invention calculated by the above formula must be 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 greater than 0.02, polarization unevenness is strongly observed when processing the polarizing plate, which impairs the display quality of the LCD.

In addition, the MD average tilt value (c) of the PVA film of the present invention at the position +200mm calculated according to the above formula is preferably 0.019μm/mm or less, and more preferably 0.016μm/mm or less. Similarly, the MD average tilt value (e) at the position -200mm is preferably 0.019μm/mm or less, and more preferably 0.016μm/mm or less.

There is no particular limitation on the method for obtaining a PVA film having an MD average tilt value of less than 0.02, and examples thereof include: a method of forming a polyvinyl alcohol film using a polyvinyl alcohol aqueous solution having a concentration of less than 32% by mass, and stretching the polyvinyl alcohol film at a stretching ratio of 1.075 to 1.135 when the water content of the polyvinyl alcohol film is greater than 20% by mass to produce the polyvinyl alcohol film.

<Phase parameter (θ)>

The PVA film of the present invention is characterized in that the phase parameter (θ) is 0.015 μm/mm or more and the MD average tilt value is 0.02 or less, which satisfies the following definitions simultaneously. The phase parameter (θ) is more preferably 0.016 μm/mm or more.

The phase parameter (θ) in the present invention is a numerical value defined by the following formula. In the following formula, C _n means the point where |F′ ₀ (C _n )| reaches a minimum, and the numbers C ₁ , C ₂ , C _{3 ,} ..., C _n are sequentially numbered from the upstream in the MD direction. The number of points where |F′ ₀ (C _n )| reaches a minimum is reduced compared to 1024 points.

[Mathematical formula 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)

It should be noted that k in (Formula 12) is a symbol indicating that 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 refers to: a parameter that represents the positional consistency in the MD direction between the node that causes the uneven film thickness at the position of 0 mm in the TD direction and the node that causes the uneven film thickness at the four positions deviating from this position in the TD direction. The larger the value, the more inconsistent the nodes that cause the uneven film thickness at the five points in the width direction, and the less likely uneven film thickness will occur.

The phase parameter (θ) calculated according to the above formula of the PVA film of the present invention must be 0.015 or more, preferably 0.0153 or more. If the phase parameter (θ) is less than 0.015, the node positions in the width direction of the PVA film are concentrated, which becomes the cause of stress concentration during stretching and regular polarizing plate unevenness extending along the width direction after stretching.

There is no particular limitation on the method for obtaining a PVA film having a phase parameter (θ) of 0.015 or more, and examples thereof include: forming a polyvinyl alcohol film using a polyvinyl alcohol aqueous solution having a concentration of 32% by mass or less, and stretching the polyvinyl alcohol film 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 to produce the polyvinyl alcohol film.

The MD average tilt value of the film surface of the PVA film of the present invention is less than 0.02, and the phase parameter (θ) is greater than 0.014. However, the MD average tilt value of at least one surface of the PVA film can be less than 0.02, and the phase parameter (θ) can be greater than 0.014, or the MD average tilt value of at least two surfaces of the PVA film can be less than 0.02, and the phase parameter (θ) can be greater than 0.014.

From the viewpoint of being used for large liquid crystal televisions, the width of the PVA film of the present invention is preferably 4 m or more, more preferably 4.5 m or more. On the other hand, if the width of the PVA film is too large, when a polarizing film is manufactured using a practical device, uniaxial stretching itself tends to become difficult to be uniformly performed, and therefore, the width of the PVA film is preferably 7 m or less.

The degree of swelling of the PVA film of the present invention is preferably 170% or more, more preferably 180% or more. The degree of swelling of the PVA film is preferably 220% or less, more preferably 210% or less. If the degree of swelling is less than 170%, there is a tendency for uneven swelling to occur, and uniform dyeing cannot be achieved during dyeing. If the degree of swelling is greater than 220%, there is an undesirable tendency for wrinkles to occur during the process. As a method for adjusting the degree of swelling of the PVA film to these ranges, for example, a method of appropriately adjusting the heat treatment temperature of the PVA film after drying can be cited.

The swelling degree is an index showing the water retention capacity of a PVA film when immersed in water, and can be obtained as a percentage by dividing the mass of the PVA film after immersion in 30°C water for 30 minutes by the mass after drying at 105°C for 16 hours after immersion.

The volatility 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 volatility of the PVA aqueous solution used for film-forming is less than 70% by mass, the effect of reducing the uneven film thickness due to the leveling effect during casting cannot be expected, and the MD average tilt value becomes high, so it is not preferred. If the volatility of the film-forming stock solution of the PVA aqueous solution is too large, the drying time increases and the productivity decreases, so it is not preferred.

Here, the "volatile fraction of the membrane-forming stock solution" in the present invention means the volatile fraction determined by the following formula.

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

(Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of the film-forming stock solution after Wa (g) is dried in an electric drying machine at 105°C for 16 hours.)

As the conditions for the film-making of the PVA film of the present invention, it is preferred that: when the water content of the PVA aqueous solution used for film-making is 20% by mass or more, the draw ratio as the stretching ratio is set to 1.075 or more for stretching, and more preferably set to 1.08 or more for stretching. It is preferred that: when the water content of the PVA aqueous solution used for film-making is 20% by mass or more, the draw ratio as the stretching ratio is set to 1.135 or less for stretching, and more preferably set to less than 1.13 for stretching, and further preferably set to less than 1.12 for stretching. In general, the draw ratio is controlled by the speed ratio of the conveying roller. The larger the draw ratio, the lower the limit stretch ratio in the stretching process, and the higher the probability of stretching fracture in the stretching process. Therefore, when the draw ratio is 1.12 or more, it is sometimes necessary to ease the process of the influence of the draw ratio. In addition, if the draw ratio is higher than 1.135, the limit stretch ratio in the stretching process is significantly reduced, so it is not preferred. In addition, if the draft ratio is less than 1.075, the phase parameter (θ) becomes small, which is not preferred from the perspective of uneven observability of the polarizing plate. In addition, as a condition for manufacturing the PVA film, the moisture content during drafting is preferably 20% by mass or more, more preferably 22% by mass or more. If the moisture content during drafting is less than 20% by mass, the phase parameter (θ) is difficult to increase. For the above reasons, it is necessary to set the moisture content and the draft ratio during drafting to the above range.

(PVA)

As the PVA resin contained in the PVA film of the present invention, for example, resins obtained by saponifying polyvinyl esters obtained by polymerizing one or more of vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate can be cited. Among the above vinyl esters, vinyl acetate is preferred from the viewpoints of ease of manufacture, ease of acquisition, cost, etc. of PVA.

PVA can be modified with one or more graft copolymerizable monomers as long as the effect of the present invention is not impaired. Examples of such graft copolymerizable monomers include unsaturated carboxylic acids or their derivatives; unsaturated sulfonic acids or their derivatives; α-olefins having 2 to 30 carbon atoms, etc. The proportion of the structural units derived from the graft copolymerizable monomers in PVA is preferably 5 mol% or less based on the molar number of all structural units constituting PVA.

A part of the hydroxyl groups in the PVA may be cross-linked or uncross-linked. In addition, a part of the hydroxyl groups in the PVA may react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure or may not react with these compounds to form an acetal structure.

The degree of polymerization of PVA is preferably 2000 or more, more preferably 2200 or more, and further preferably 2400 or more. When the degree of polymerization is less than 2000, there is a tendency for the durability of the resulting polarizing film to decrease. The degree of polymerization of PVA is preferably 2700 or less, more preferably 2650 or less, and further preferably 2600 or less. On the other hand, when the degree of polymerization exceeds 2700, there is a tendency for the manufacturing cost to increase and the process passability during film formation to deteriorate. It should be noted that the degree of polymerization of PVA mentioned in this specification refers to the average degree of polymerization measured in accordance with the description of JIS K6726-1994.

From the viewpoint of 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 saponification degree is less than 98 mol%, the water resistance of the obtained polarizing film tends to deteriorate. It should be noted that the saponification degree of PVA in this specification refers to: the proportion (mol%) of the molar number of the vinyl alcohol unit relative to the total molar number of the structural unit (typically, vinyl ester unit) and vinyl alcohol unit that can be converted into the vinyl alcohol unit by saponification of the PVA. 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. As the plasticizer, polyols such as ethylene glycol, glycerol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, trimethylolpropane, etc. can be listed, and the PVA film of the present invention can contain one or more of these plasticizers. Among these, glycerol is preferably used from the viewpoint of improving the effect of stretchability.

The content of the plasticizer in the PVA film of the present invention is preferably 1 mass part or more relative to 100 mass parts of PVA contained therein, more preferably 3 mass parts or more, and further preferably 5 mass parts or more. By making the content of the plasticizer be 1 mass part 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 mass parts or less relative to 100 mass parts of PVA, more preferably 17 mass parts or less, and further preferably 15 mass parts or less. By making the plasticizer be 20 mass parts or less, the PVA film can be prevented from becoming too soft and the handling property is reduced.

(Surfactant)

The PVA film of the present invention preferably contains a surfactant. By using a film-forming stock solution containing a surfactant to manufacture the PVA film, the film-forming property of the PVA film is improved. As a result, the occurrence of uneven thickness of the PVA film can be suppressed, and the PVA film can be easily peeled off from the metal roll or belt used for film-forming. In the case of manufacturing the PVA film from a film-forming stock solution containing a surfactant, the obtained PVA film contains a surfactant.

The type of surfactant is not particularly limited, but anionic surfactants and nonionic surfactants are preferred from the viewpoint of peeling properties of the PVA film from a metal roll or a belt.

Preferred anionic surfactants include, for example, carboxylic acid-type surfactants such as potassium laurate; sulfate-ester-type surfactants such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and sulfonic acid-type surfactants such as dodecylbenzenesulfonate.

As the nonionic surfactant, suitable ones include, for example, alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkylamine types such as polyoxyethylene lauryl amino ether; alkyl amide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as oleic acid diethanolamide; allyl phenyl ether types 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 mass parts relative to 100 mass parts of PVA, more preferably 0.02 mass parts, and further preferably 0.05 mass parts. By setting the content of the surfactant to more than the above lower limit, the film-making property 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 mass parts relative to 100 mass parts of PVA, more preferably 0.3 mass parts, and further preferably 0.2 mass parts. By setting the content of the surfactant to below the above upper limit, it is possible to suppress the surfactant from oozing out to the surface of the PVA film and causing adhesion and reduced handling.

(Other ingredients)

The PVA film of the present invention may further contain components such as an antioxidant, an antifreeze agent, a pH adjuster, a shielding agent, an anti-coloring agent, an oil agent, and a surfactant described below, as necessary.

(Shape, etc.)

The shape of the PVA film of the present invention is not particularly limited, and is preferably a long film. Thus, a more uniform PVA film can be manufactured continuously and easily, and can also be used continuously when it is used to manufacture a polarizing film. The length (length in the longitudinal direction) of the long film is not particularly limited, and can be appropriately set according to the purpose, etc., and can be set, for example, in the range of 5 to 30,000 m.

The thickness of the PVA film of the present invention is preferably 30 to 65 μm. If the thickness of the PVA film is less than 30 μm, the handling property during stretching is 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 manufacturing the PVA film is not particularly limited, and the following methods can be used: a method of using a film-making stock solution to which a solvent, additives, etc. are added and homogenized, and a film-making method, a wet film-making method (spraying into a poor solvent), a dry-wet film-making method, a gel film-making method (a method of temporarily cooling and gelling the film-making stock solution, extracting and removing the solvent to obtain a PVA film) or a combination thereof to make a film; a melt extrusion film-making method in which the above-mentioned film-making stock solution is obtained by using an extruder, etc., and the film is extruded from a T-die head, etc.; any method such as an inflation molding method. Among these, the cast film-making method and the melt extrusion film-making method are preferred because they can obtain a homogeneous film with good productivity. Below, the cast film-making method or the melt extrusion film-making method of the PVA film is described.

As a method for producing a polyvinyl alcohol film of the present invention, the following method for producing a polyvinyl alcohol film can be cited, which comprises the steps 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 water content of the polyvinyl alcohol film is 20 mass % or more. By adopting this production method, a film without uneven thickness can be produced.

When the ultimate stretch ratio is reduced, a step for alleviating the influence of the stretch ratio may be added. Specific methods for alleviating the influence of the stretch ratio include: irradiating the polyvinyl alcohol film with infrared rays or microwaves; reducing the stretch ratio and cumulative stretch ratio (the product of the stretch ratios when multiple stretchings are performed) when the moisture content of the polyvinyl alcohol film is 15 to 5% by mass; applying a plasticizer after stretching; drying with superheated steam; drying at high temperature; heat treatment with a suspension dryer, etc. Any method may be used, and each method may be combined or implemented multiple times.

When the PVA film is manufactured by the cast film method or the melt extrusion film method, the film-making stock solution is cast into a film on a support such as a metal roll or a metal belt and heated to remove the solvent, thereby solidifying to form a film. The solidified film is peeled off from the support, dried by a drying roll, a drying furnace, etc. as needed, further heat-treated as needed and wound up, thereby obtaining a long PVA film in a roll.

The method for adjusting the film-forming stock solution is not particularly limited, and examples thereof include: a method of dissolving PVA and additives such as plasticizers and surfactants in a dissolving tank, etc.; a method of melt-kneading the water-containing PVA with a plasticizer, surfactant, etc. using a single-screw or twin-screw extruder, etc.

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

The dried PVA film may be further heat treated as needed. The strength, swelling degree, birefringence, etc. of the PVA film can be adjusted by heat treatment. The surface temperature of the heat treatment roller used for heat treatment is preferably above 60°C. In addition, the surface temperature of the heat treatment roller is preferably below 135°C, more preferably below 130°C. If the surface temperature of the heat treatment roller is too high, too much heat is applied, the size of the lamellae in the PVA film becomes larger, and the swelling degree of the PVA film is difficult to meet the above range.

The PVA film produced in this manner may be subjected to further humidity conditioning treatment, cutting of both ends (sides) of the film, etc. as necessary, and then wound up in a roll on a cylindrical core and moisture-proof packaged to form a product.

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

The uneven film thickness in the MD direction of the PVA film of the present invention is reduced, and the phase deviation of the uneven film thickness in the TD direction is adjusted to a specific range, and therefore, it can be suitably used as a film for manufacturing an optical film. Here, the optical film can exemplify a polarizing film, a viewing angle improvement film, a phase difference film, a brightness enhancement film, etc. as described later, preferably a polarizing film. The polarizing film of the present invention made using the PVA film of the present invention is less likely to observe the unevenness of the polarizing plate after stretching than in the past.

<Method for producing optical film>

Hereinafter, as an example of the method for producing an optical film, a method for producing a polarizing film is given and described in detail.

Polarizing films are usually manufactured by using a PVA film as a raw material film and undergoing treatment processes such as a swelling process, a dyeing process, a cross-linking process, a stretching process, and a fixing process. Specific examples of the treatment liquid used in each process include a swelling treatment liquid used in the swelling process, a dyeing treatment liquid (dyeing liquid) used in the dyeing process, a cross-linking treatment liquid used in the cross-linking process, a stretching treatment liquid used in the stretching process, a fixing treatment liquid used in the fixing process, and a cleaning treatment liquid (cleaning liquid) used in the cleaning process.

The following is a description of each treatment process that can be used in the manufacturing method for manufacturing a polarizing film. It should be noted that in the manufacturing method of the polarizing film, one or more of the following treatments can be omitted, the same treatment can be performed multiple times, and other treatments can be performed simultaneously.

(Cleaning treatment)

Before the PVA film is subjected to a swelling treatment, it is preferred to perform a cleaning treatment on the PVA film. By performing such a cleaning treatment before the swelling treatment, the anti-blocking agent attached to the PVA film can be removed, and the various treatment liquids in the manufacturing process of the polarizing film can be prevented from being contaminated by the anti-blocking agent. The cleaning treatment is preferably performed by immersing the PVA film in a cleaning treatment liquid, or by blowing the cleaning treatment liquid onto the PVA film. As a cleaning treatment liquid, for example, water can be used. The temperature of the cleaning treatment liquid is preferably above 20°C, more preferably above 22°C, further preferably above 24°C, and particularly preferably above 26°C. By setting the temperature of the cleaning treatment liquid to above 20°C, it is easy to remove the anti-blocking agent attached to the PVA film. The temperature of the cleaning treatment liquid is preferably below 40°C, more preferably below 38°C, further preferably below 36°C, and particularly preferably below 34°C. In addition, by setting the temperature of the cleaning treatment liquid to below 40°C, it is possible to prevent a portion of the surface of the PVA film from dissolving, which causes the films to adhere to each other and reduce the handleability.

(Swelling treatment)

The swelling treatment can be carried out by immersing the PVA film in a swelling treatment solution such as water. The temperature of the swelling treatment solution is preferably above 20°C, more preferably above 22°C, and further preferably above 24°C. The temperature of the swelling treatment solution is preferably below 40°C, more preferably below 38°C, and further preferably below 36°C. In addition, the time of immersion in the swelling treatment solution is, for example, preferably above 0.1 minutes, and more preferably above 0.5 minutes. In addition, the time of immersion in the swelling treatment solution is, for example, preferably below 5 minutes, and more preferably below 3 minutes. It should be noted that the water used as the swelling treatment solution is not limited to pure water, and can be an aqueous solution in which various components such as boron-containing compounds are dissolved, or a mixture of water and an aqueous medium. The type of boron-containing compound is not particularly limited, and from the viewpoint of handleability, boric acid or borax is preferred. When the swelling treatment solution contains a boron-containing compound, its concentration is preferably below 6% by mass from the viewpoint of improving the stretchability of the PVA film.

(Dyeing treatment)

The dyeing treatment can be carried out using an iodine-based dye as a dichroic dye, and the dyeing period can be any stage before the stretching treatment, during the stretching treatment, or after the stretching treatment. The dyeing treatment is preferably carried out by using a solution containing iodine-potassium iodide (suitably an aqueous solution) as a 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% by mass, and potassium iodide/iodine (mass) is preferably in the range of 20 to 100. The temperature of the dyeing treatment liquid is preferably above 20°C, more preferably above 25°C. The temperature of the dyeing treatment liquid is preferably below 50°C, more preferably below 40°C. The dyeing treatment liquid may contain boron-containing compounds such as boric acid as a cross-linking agent. It should be noted that if the PVA film used as the original material film contains a dichroic dye in advance, the dyeing treatment can be omitted. In addition, the PVA film used as the original material film may also contain boron-containing compounds such as boric acid and borax in advance.

(Cross-linking treatment)

When manufacturing a polarizing film, for the purpose of firmly adsorbing the dichroic dye to the PVA film, etc., it is preferred to perform a cross-linking treatment after the dyeing treatment. The cross-linking treatment can be performed by using a solution containing a cross-linking agent (suitably an aqueous solution) as a cross-linking treatment liquid, and immersing the PVA film in the cross-linking treatment liquid. As a cross-linking agent, one or more boron-containing compounds such as boric acid and borax can be used. If the concentration of the cross-linking agent in the cross-linking treatment liquid is too high, there is a tendency that the cross-linking reaction is excessively carried out, and it is difficult to fully stretch in the subsequent stretching treatment. In addition, if it is too little, there is a tendency that the effect of the cross-linking treatment is reduced. The concentration of the cross-linking agent in the cross-linking treatment liquid is preferably 1% by mass or more, more preferably 1.5% by mass or more, and more preferably 2% by mass or more. The concentration of the cross-linking agent in the cross-linking treatment liquid is preferably 6% by mass or less, more preferably 5.5% by mass or less, and more preferably 5% by mass or less.

In order to suppress the dissolution of dichroic dyes from the PVA film after dyeing, the cross-linking treatment liquid may contain iodine-containing compounds such as potassium iodide. If the concentration of the iodine-containing compound in the cross-linking treatment liquid is too high, although the reason is unclear, there is a tendency for the heat resistance of the resulting polarizing film to decrease. In addition, if it is too little, there is a tendency for the effect of suppressing the dissolution of dichroic dyes to decrease. The concentration of the iodine-containing compound in the cross-linking treatment liquid is preferably 1% by mass or more, more preferably 1.5% by mass or more, and further preferably 2% by mass or more. The concentration of the iodine-containing compound in the cross-linking 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, there is a tendency for the dichroic dye to be eluted and the obtained polarizing film to be easily unevenly dyed. In addition, if it is too low, the effect of the crosslinking treatment is sometimes reduced. The temperature of the crosslinking treatment liquid is preferably 20°C or more, more preferably 22°C or more, and further preferably 25°C or more. The temperature of the crosslinking treatment liquid is preferably 45°C or less, more preferably 40°C or less, and further preferably 35°C or less.

In addition to the stretching treatment described later, the PVA film can also be stretched separately in each of the above-mentioned treatments or between the treatments. By performing this stretching (pre-stretching), wrinkles can be prevented from forming on the surface of the PVA film. From the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio of the pre-stretching (the ratio obtained by multiplying the stretching ratios in each treatment) is preferably 4 times or less, and more preferably 3.5 times or less, based on the original length of the PVA film of the original material before stretching. From the viewpoint of the polarization performance of the resulting polarizing film, 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 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 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 stretching ratio in the dyeing treatment is preferably less than 2 times, more preferably less than 1.8 times, and further preferably less than 1.5 times, based on the original length of the PVA film. The stretching ratio in the dyeing treatment is further preferably more than 1.1 times, based on the original length of the PVA film. The stretching ratio in the cross-linking treatment is preferably less than 2 times, more preferably less than 1.5 times, and further preferably less than 1.3 times, based on the original length of the PVA film. The stretching ratio in the cross-linking treatment is further preferably more than 1.05 times, based on the original length of the PVA film.

(Stretching treatment)

The stretching process can be carried out by either a wet stretching method or a dry stretching method. In the case of a wet stretching method, a solution containing a boron-containing compound such as boric acid (suitable for an aqueous solution) can be used as a stretching treatment liquid, and it can be carried out in the stretching treatment liquid, or it can be carried out in a dyeing treatment liquid or a fixing treatment liquid described later. In addition, in the case of a dry stretching method, the PVA film after water absorption can be used to carry out in the air. Among these, a wet stretching method is preferred, and uniaxial stretching is more preferably carried out in an aqueous solution containing boric acid. In the case where the stretching treatment liquid contains a boron-containing compound, from the aspect of being able to improve the stretchability of the PVA film, the concentration of the boron-containing compound in the stretching treatment liquid is preferably 1.5% by mass or more, more preferably 2.0% by mass or more, and more preferably 2.5% by mass or more. From the aspect of being able to improve the stretchability of the PVA film, the concentration of the boron-containing compound in the stretching treatment liquid is preferably 7% by mass or less, more preferably 6.5% by mass or less, and more preferably 6% by mass or less.

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, there is a tendency for the hue of the resulting polarizing film to be significantly bluish. In addition, if it is too low, although the reason is unclear, there is a tendency for the heat resistance of the resulting polarizing film to decrease. 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 further 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, there is a tendency for the PVA film to dissolve and become soft and easy to break. In addition, if it is too low, there is a tendency for the stretchability to decrease. The temperature of the stretching treatment liquid is preferably 50°C or more, more preferably 52.5°C or more, and more preferably 55°C or more. The temperature of the stretching treatment liquid is preferably 70°C or less, more preferably 67.5°C or less, and more preferably 65°C or less. It should be noted that the preferred range of the stretching temperature when the stretching treatment is performed by the dry stretching method is also as described above.

When the stretching ratio in the stretching treatment is high, a polarizing film with better polarization performance can be obtained, so it is preferably 1.2 times or more, more preferably 1.5 times or more, and more preferably 2 times or more. In addition, from the perspective of the polarization performance of the obtained polarizing film, the total stretching ratio (the ratio obtained by multiplying the stretching ratios in each process) including the stretching ratio of the above-mentioned pre-stretching is preferably 5.5 times or more, more preferably 5.7 times or more, and more preferably 5.9 times or more, based on the original length of the raw material PVA film before stretching. The upper limit of the stretching ratio is not particularly limited. If the stretching ratio is too high, the PVA film is prone to tensile fracture, so 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 can be used. In the case of manufacturing a polarizing film, uniaxial stretching in the longitudinal direction is preferred from the viewpoint of obtaining a material with excellent polarization performance. Uniaxial stretching in the longitudinal direction can be performed by using a stretching device having a plurality of rollers parallel to each other and changing the circumferential speed between the rollers.

In the present invention, the maximum stretching speed (%/min) when the stretching treatment is performed by uniaxial stretching is not particularly limited, and the maximum stretching speed is preferably 200%/min or more, more preferably 300%/min or more, and further preferably 400%/min or more. Here, the maximum stretching speed refers to: when the PVA film is stretched in two or more stages using three or more rollers with different circumferential speeds, the fastest stretching speed in the stage. It should be noted that when the PVA film is stretched in one stage without being divided into two or more stages, the stretching speed in the stage becomes the maximum stretching speed. In addition, the stretching speed refers to: the increase in the length of the PVA film increased by stretching relative to the length of the PVA film before stretching per unit time. For example, a 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 within 1 minute. The greater the maximum stretching speed, the faster the PVA film can be stretched (uniaxially stretched) at a high speed, and as a result, the productivity of the polarizing film will be improved, so it is preferred. On the other hand, if the maximum stretching speed becomes too high, excessive tension may be applied locally to the PVA film during the stretching process (uniaxial stretching) of the PVA film, and stretching breakage may occur easily. From this point of view, the maximum stretching speed is preferably not more than 900%/min.

(Cleaning after dyeing)

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 solution, or by blowing the cleaning treatment solution onto the PVA film. As the cleaning treatment solution, for example, water can be used. Water is not limited to pure water, and can contain iodine-containing compounds such as potassium iodide. It should be noted that the cleaning treatment solution can contain a boron-containing compound, in which case the concentration of the boron-containing compound is preferably 2.0 mass % or less.

The temperature of the cleaning solution is preferably 5°C or higher, more preferably 7°C or higher, and further preferably 10°C or higher. In addition, the temperature of the cleaning solution is preferably 40°C or lower, more preferably 38°C or lower, and further preferably 35°C or lower. By setting the temperature of the cleaning solution to 5°C or higher, the breakage of the PVA film caused by freezing of water can be suppressed. In addition, by setting the temperature of the cleaning solution to 40°C or lower, the optical properties of the obtained polarizing film are improved.

As a specific method for manufacturing a polarizing film, there can be cited a method of subjecting a PVA film to a dyeing treatment, a stretching treatment, a cross-linking treatment, and/or a fixing treatment. As a preferred example, there can be cited a method of subjecting a PVA film to a swelling treatment, a dyeing treatment, a cross-linking treatment, a stretching treatment (especially a uniaxial stretching treatment), and a cleaning treatment in sequence. In addition, the stretching treatment can be performed in any treatment process before the above treatments, or in multiple stages of two or more stages.

The polarizing film can be obtained by drying the PVA film after the above-mentioned treatments. The drying method 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, and a floating method in which the film is dried by hot air while floating.

<Polarizing film, polarizing plate>

In order to compensate for the mechanical strength, the polarizing film of the present invention is manufactured by pasting a protective film on at least one side. The polarizing film of the present invention is usually made into a polarizing plate by pasting an optically transparent and mechanically strong protective film. As the protective film, triacetyl cellulose (TAC) film, cellulose acetate-butyrate (CAB) film, acrylic film, polyester film, etc. are used. In addition, as the adhesive used for pasting, PVA adhesives, urethane adhesives, etc. can be listed, among which PVA adhesives are suitable.

The polarizing plate obtained as above can be used as a component of LCD by applying an acrylic adhesive or the like and pasting it on a glass substrate. At the same time, it can also be pasted with a phase difference film, a viewing angle improvement film, a brightness enhancement film, etc.

Example

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

<Calculation of MD average tilt value, phase parameter (θ), and average tilt value at ±200 mm position>

The MD average tilt value, phase parameter (θ), and average tilt value at ±200 mm positions of the PVA films obtained in Examples or Comparative Examples were calculated by the above method.

<Measurement of Swelling Degree of PVA Film>

A test piece of about 1.5 g is cut out from the PVA film obtained from the following examples or comparative examples. Next, the test piece is immersed in 1000 g of distilled water at 30°C. After immersion for 30 minutes, the test piece is taken out, and after absorbing the water on the surface with filter paper, its mass (We) is measured. Next, the test piece is placed in a hot air dryer, and after drying at 105°C for 16 hours, its mass (Wf) is measured. Based on the obtained masses We and Wf, the swelling degree of the PVA film is calculated using the following formula.

Swelling degree (%) = (We/Wf) × 100

<Evaluation of the Ultimate Stretching Ratio During Stretching>

The ease of stretching and breaking during stretching was evaluated based on the breaking ratio of the PVA film when manufacturing the polarizing film in the following examples or comparative examples. That is, the breaking ratio of the uniaxial stretching in the stretching process when manufacturing the polarizing film was measured 10 times, and the average value was taken as the limiting stretching ratio, and the evaluation was performed according to the following criteria.

A...Ultimate stretching 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…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 pieces and clamped between two polarizing plates in a parallel Nicol prism state (single polarity transmittance is 43.5%, polarization degree is 99.9%) in such a way that the polarizing plate of the sample presents an orthogonal Nicol prism state relative to each polarizing plate. Using a backlight with a brightness of 40,000 cd/ ^m2 , the optical polarization unevenness was visually observed in the transmission mode and evaluated according to the following criteria.

A…There is color unevenness

B…No color unevenness

[Example 1]

Use PVA (saponification degree of 99.9 mol%, polymerization degree of 2400) 100 parts by mass, glycerol as a plasticizer 10 parts by mass, lauric acid diethanolamide 0.1 parts by mass and water 233 parts by mass, melt extruder is used for melt mixing, and film-making stock solution (volatile fraction is 70% by mass) is prepared. Then, it is sprayed into a film shape on a support (surface temperature is 98°C) from a T die head to form a PVA film on the support. When the moisture content of the sprayed PVA film becomes 24%, the draft ratio is set to 1.10 by the speed ratio control of the roller, and stretched. Utilize the heat treatment roller of the second half, contact heat treatment is carried out at 105°C to obtain a PVA film with a width of 5m and a film thickness of 60μm. It should be noted that regarding the moisture content during stretching, the conveyed PVA film was torn off between the stretching rollers, and after measuring the mass (Ww), the film was placed in a hot air dryer and dried at 105°C for 16 hours, and then its mass (Wd) was measured. The moisture content of the film was calculated using the following formula.

Membrane moisture content (%) = (Ww-Wd)/Ww×100

The obtained PVA film was cut into 650 mm widths, and the film was subjected to swelling treatment, dyeing treatment, cross-linking treatment, stretching treatment, cleaning treatment, and drying treatment in sequence to continuously manufacture a polarizing film. The swelling treatment was performed by uniaxially stretching the film to 2.00 times in the length direction while immersing the film in pure water (swelling treatment liquid) at 25°C. The dyeing treatment was performed by uniaxially stretching the film to 1.26 times in the length direction while immersing the film in a potassium iodide/iodine aqueous dyeing solution (dyeing treatment liquid) at a temperature of 32°C (potassium iodide/iodine (mass ratio) of 23, iodine concentration of 0.03-0.05 mass%). In the dyeing treatment, the iodine concentration in the dyeing treatment liquid was adjusted in the range of 0.03-0.05 mass% so that the monomer transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment was in the range of 43.5%±0.2%. The cross-linking treatment was performed by uniaxially stretching the film to 1.19 times in the length direction while immersing the film in a 32°C boric acid aqueous solution (cross-linking treatment solution) (boric acid concentration of 2.6% by mass). The stretching treatment was performed by uniaxially stretching the film to 2.00 times in the length direction while immersing the film in a 55°C boric acid/potassium iodide aqueous solution (stretching treatment solution) (boric acid concentration of 2.8% by mass, potassium iodide concentration of 5% by mass). The cleaning treatment was performed by immersing the film in a 22°C potassium iodide/boric acid aqueous solution (cleaning treatment solution) (potassium iodide concentration of 3-6% by mass, boric acid concentration of 1.5% by mass) for 12 seconds without stretching.

[Examples 2 to 4, 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. It should be noted that the volatility of the film-forming stock solution can be adjusted by adjusting the amount of water used for melt mixing in the melt extruder. The results are shown in Table 1.

The above results show that the color unevenness of the PVA film of the present invention is suppressed during the stretching process.

[Table 1]

Claims (9)

1. A polyvinyl alcohol film, wherein the film thickness profile at an arbitrary point F ₀ on the film surface is F ₀ (x), and the film thickness profile at a point -100 mm in the TD direction from F ₀ is F _-100 (x), set the film thickness profile at the point -200mm from F ₀ towards the TD direction to F _-200 (x), set the film thickness profile at the point +100mm from F ₀ towards the TD direction is F ₊₁₀₀ (x), and when the film thickness profile at a point +200mm in the TD direction from F ₀ is set to F ₊₂₀₀ (x), use the differential value of each film thickness profile in the MD direction, which is F′ ₀ ( x), F′ _-100 (x), F′ _-200 (x), F′ ₊₁₀₀ (x), and F′ ₊₂₀₀ (x), the average tilt value expressed as an average in the MD direction is 0.02 or less. The point where F′ ₀ (x) shows the minimum value is set to |F′ _-100 (C _n )-F′ ₀ (C _n )|, |F′ when C _n (n=1, 2, 3...) _-200 (C _n )-F′ ₀ (C _n )|, |F′ ₊₁₀₀ (C _n )-F′ ₀ (C _n )| and |F′ ₊₂₀₀ (C _n )-F′ ₀ (C The phase parameter (θ) defined by the average value of _n )| is above 0.015. 2. The polyvinyl alcohol film according to claim 1, having a width of 4 m or more. 3. The polyvinyl alcohol film according to claim 1 or 2, with 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 optical film production. 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 manufactured by pasting a protective film on at least one side of the polarizing film according to claim 7. 9. A method for manufacturing a polyvinyl alcohol film, which includes: using a polyvinyl alcohol aqueous solution with a concentration of 32 mass% or less to form a polyvinyl alcohol film, and when the moisture content of the polyvinyl alcohol film is 20 mass% or more, use 1.075 The stretching process is carried out at a draw ratio of ~1.135.