CN117001908A - Polyvinyl alcohol-based polymer film and its manufacturing method - Google Patents

Abstract

The present invention relates to a polyvinyl alcohol-based polymer film and a method for producing the same. The present invention relates to a PVA film, which is a PVA film with a thickness of 55 μm or less. The irregularities are in a period of 0.01 to 10 mm and the dotted lines arranged in an approximately straight line spanning more than 10 cm in the flow direction of the film are an average of 1 m of the film. 5 or less in the width direction; and, a manufacturing method, which is a method of manufacturing a PVA film with a thickness of 55 μm or less, using a film forming device equipped with a plurality of drying rollers with rotation axes parallel to each other, and the first drying roller of the film forming device When the film-forming stock solution containing PVA is discharged into a film shape and dried, and is further dried by the drying rollers subsequent to the second drying roller to produce a PVA film, the discharge speed of the film-forming stock solution is set to (S ₀ ) Reach 2.5～5.0m/minute.

Description

Polyvinyl alcohol-based polymer film and method for producing the same

This invention application is a divisional application of PCT patent application PCT/JP2016/064933, filed on May 20, 2016, and entitled “Polyvinyl alcohol-based polymer film and method for producing the same”. The application number of the parent application entering China is 201680031048.6.

Technical Field

The present invention relates to a thin polyvinyl alcohol polymer film (hereinafter sometimes "polyvinyl alcohol polymer" is abbreviated as "PVA") capable of producing optical films such as polarizing films with few optical defects and a method for producing the same, optical films such as polarizing films produced from the PVA film, and a method for producing the optical film.

Background Art

Polarizing plates with the function of transmitting and shielding light, together with liquid crystals with the function of switching light, are one of the important components of liquid crystal displays (LCDs). LCDs are gradually used in a wide range of small devices such as calculators and watches, laptops, LCD monitors, LCD color projectors, LCD TVs, car navigation systems, mobile phones, tablet terminals, and measuring equipment used indoors and outdoors. Among the application fields of these LCDs, LCD TVs, LCD monitors, etc. have promoted thinning in addition to promoting large screens. In addition, in recent years, thinning has also been promoted in tablet terminals that have become significantly popular. As a means for achieving thinning of LCDs, thinning of the glass used in LCDs can be cited, but from the perspective of solving the problem of glass warping caused by the shrinkage stress of the polarizing plate, the polarizing plate is also required to be thin.

Polarizing plates are usually manufactured by dyeing and uniaxially stretching a PVA film to produce a polarizing film, and then laminating a protective film such as a triacetyl cellulose (TAC) film on the surface of the polarizing film. Therefore, in order to achieve thinner polarizing plates, it is required to use a thinner PVA film to produce a thin polarizing film. Specifically, the thickness of the PVA film is required to be less than 55 μm, and further required to be less than 30 μm.

To date, various methods for manufacturing PVA films are known. For example, it is known that when a film-making raw material containing PVA is ejected onto a rotating drum of a drum film-making machine or onto a traveling belt of a belt film-making machine and dried to manufacture a PVA film, by making the speed ratio of the speed of the drum or belt to the ejection speed of the film-making raw material 1 to 5, a PVA film with uniform thickness and excellent smoothness can be manufactured over a large area (see Patent Document 1). Specifically, Patent Document 1 states that a PVA film with a thickness of about 75 μm and a thickness deviation of about 1.6 to 3.0 μm was manufactured by this method.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2002-79530.

Summary of the invention

Problems to be solved by the invention

As described above, a thinner PVA film is required, but when a thin PVA film is simply manufactured by a conventional film-making method, a new problem is that a large number of dotted line streaks (dotted line streaks) obtained by arranging a plurality of nanometer-level concave and convex surfaces are generated along the flow direction of the film. The inventors of the present invention have recognized that if such a conventional thin PVA film having a large number of dotted line streaks is used, a large number of optical defects are generated in the obtained optical film.

Therefore, an object of the present invention is to provide a thin PVA film capable of producing an optical film such as a polarizing film having few optical defects. Another object of the present invention is to provide an optical film having few optical defects produced from the PVA film.

Means for solving problems

In order to achieve the above-mentioned purpose, the inventors of the present invention have conducted in-depth research and found that when using a thin PVA film to manufacture an optical film such as a polarizing film, by making the number of dotted line marks in the PVA film reach a specific value or less, an optical film with few optical defects can be easily obtained. In addition, at the same time, it was found that when a film-making device having a plurality of drying rollers whose rotating axes are parallel to each other is used, a film-making stock solution containing PVA is ejected into a film shape and dried on the first drying roller located on the most upstream side of the film-making device, and further dried by a drying roller after the second drying roller connected to the downstream side of the first drying roller to manufacture a thin PVA film, by making the ejection speed (S ₀ ) of the film-making stock solution reach a specific range, a thin PVA film with a small number of dotted line marks that is different from the past can be manufactured smoothly and continuously. Based on these insights, the inventors of the present invention have further repeatedly conducted research and completed the present invention.

That is, the present invention relates to:

[1] A PVA film having a thickness of 55 μm or less, wherein the number of dotted line scratches is 5 or less per m of the film width, wherein the dotted line scratches are arranged in a substantially straight line with a period of 0.01 to 10 mm and a span of more than 10 cm in the film flow direction;

[2] The PVA film according to [1] above, wherein the width is 2 m or more;

[3] A method for producing a PVA film having a thickness of 55 μm or less, wherein a film-forming device having a plurality of drying rollers whose rotation axes are parallel to each other is used, and a film-forming stock solution containing PVA is ejected into a film shape and dried on a first drying roller of the film-forming device, and further dried by a second drying roller and subsequent drying rollers to produce the PVA film, wherein the ejection speed (S ₀ ) of the film-forming stock solution is 2.5 to 5.0 m/min;

[4] The production method according to [3] above, wherein the ratio (S ₁ /S ₀ ) of the peripheral speed (S ₁ ) of the first drying roller to the ejection speed (S ₀ ) of the film-forming stock solution is set to 7 or less;

[5] The method according to [3] or [4], which is a method for producing a PVA film having a width of 2 m or more;

[6] An optical film made from the PVA film described in [1] or [2] above;

[7] The optical film according to [6] above, which is a polarizing film;

[8] A method for producing an optical film, comprising: a step of uniaxially stretching the PVA film described in [1] or [2] above;

[9] The production method according to [8] above, which is a method for producing a polarizing film.

Effects of the Invention

According to the present invention, there are provided a thin PVA film capable of producing an optical film such as a polarizing film having few optical defects, a method for producing the same, an optical film having few optical defects produced from the PVA film, and a method for producing the optical film.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail.

[PVA film]

The PVA film of the present invention has a thickness of 55 μm or less, and has irregularities with a period of 0.01 to 10 mm and an average of 5 or less broken lines arranged in a substantially straight line over 10 cm or more in the film flow direction in the film width direction.

In the present invention, the dashed line marks with a predetermined number are obtained by having a period of 0.01 to 10 mm and being arranged in a nearly straight line over a span of more than 10 cm in the flow direction of the film as described above. Usually, dashed line marks are generated along the flow direction of the film, and the length of one dashed line mark is much greater than 10 cm, but in this specification, in order to distinguish from defect points with a length of less than 10 cm and difficult to be called dashed line marks, for convenience, the streaks with a length of more than 10 cm (the streaks with the above-mentioned period and arranged in a nearly straight line over a span of more than 10 cm in the flow direction of the film) are regarded as dashed line marks.

There is no particular restriction on the types of the above-mentioned projections and depressions, and examples thereof include: projections and depressions are formed by arranging projections protruding from the film surface; projections and depressions are formed by arranging depressions sunken from the film surface; projections and depressions are formed by alternately arranging projections protruding from the film surface and depressions sunken from the film surface, etc.

The concavoconvexity of the above-mentioned dotted line marks is obtained by being arranged in a nearly straight line over a span of more than 10 cm in the flow direction of the film with a period of 0.01 to 10 mm. Here, the period refers to the length of a group of concavoconvexity (a group of adjacent concave and convex portions) in the flow direction of the film, and in the above-mentioned dotted line marks, the period is within the range of 0.01 to 10 mm for all the concavoconvexities as the object. The period can be, for example, within the range of 0.1 to 5 mm, and further within the range of 0.2 to 1 mm.

In a group of projections and depressions constituting the above-mentioned dotted line marks, the difference in thickness between the highest height and the lowest height is mostly within the range of 1 to 500 nm, for example, within the range of 10 to 300 nm, further within the range of 20 to 100 nm, and particularly within the range of 30 to 90 nm.

The number of the above-mentioned dotted line marks can be obtained as follows: for the PVA film as the object, on a straight line in the width direction passing through any part of the flow direction, the number of the above-mentioned dotted line marks that cross the straight line is measured throughout the width direction, and the number is divided by the width of the PVA film (unit: m), thereby obtaining the number of roots per 1m width on average. Here, the dotted line marks can be confirmed to exist by, for example, a scanning white interference microscope. Specifically, the number of dotted line marks can be obtained by the method described later in the examples.

For the PVA film of the present invention, the number of roots in the width direction of the average 1m film of the above-mentioned dotted line marks must be 5 or less, preferably 3 or less, more preferably 2 or less, can be 1.5 or less, and further can be 1 or less. By making the number of dotted line marks within the above range, an optical film with few optical defects can be easily obtained. On the other hand, in a thin PVA film of less than 55μm, when the operating conditions are adjusted in a manner such that the number of dotted line marks reaches an average of less than 0.05 roots in the width direction of 1m, the optical film has few streak-shaped optical defects. However, although the reason is not clear, there is a tendency for problems such as thickness deviation to easily occur in the film. Therefore, the number of dotted line marks in the average 1m width direction is preferably 0.05 or more, more preferably 0.10 or more, further preferably 0.15 or more, and most preferably 0.20 or more.

Examples of PVA used to form the PVA film include: PVA (unmodified PVA) obtained by polymerizing vinyl ester to obtain polyvinyl ester and saponifying the obtained polyvinyl ester; modified PVA obtained by graft copolymerizing a comonomer onto the main chain of PVA; modified PVA obtained by copolymerizing vinyl ester and a comonomer to obtain a modified polyvinyl ester and saponifying the obtained modified polyvinyl ester; so-called polyvinyl acetal resin obtained by cross-linking a part of hydroxyl groups of unmodified PVA or modified PVA with aldehydes such as formalin, butyraldehyde, and benzaldehyde; and the like.

When the PVA forming the PVA film is a modified PVA, the modification amount in the PVA is preferably 15 mol % or less, more preferably 5 mol % or less.

As the aforementioned vinyl ester used in the manufacture of PVA, for example, vinyl acetate, vinyl formate, vinyl laurate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl stearate, vinyl benzoate, etc. can be cited. These vinyl esters can be used alone or in combination. Among these vinyl esters, vinyl acetate is preferred from the viewpoint of productivity.

In addition, examples of the comonomer include olefins having 2 to 30 carbon atoms (α-olefins, etc.), such as ethylene, propylene, 1-butene, and isobutylene; acrylic acid or its salts; acrylic acid esters (e.g., alkyl esters of methacrylic acid having 1 to 18 carbon atoms, etc.), such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methacrylic acid esters (e.g., alkyl esters of methacrylic acid having 1 to 18 carbon atoms, etc.), such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidopropanesulfonic acid or its salts, acrylamidopropanesulfonic acid or its salts, acrylamide derivatives such as dimethylamine or its salts, N-hydroxymethyl acrylamide or its derivatives; methacrylamide derivatives such as methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, methacrylamidopropanesulfonic acid or its salts, methacrylamidopropyl dimethylamine or its salts, N-hydroxymethyl methacrylamide or its derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, Vinyl ethers such as isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; halogenated vinyls such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids such as maleic acid and itaconic acid, and derivatives thereof such as salts or esters thereof; vinyl silyl compounds such as vinyltrimethoxysilane; isopropenyl acetate; unsaturated sulfonic acid or its derivatives, etc. Among these, α-olefins are preferred, and ethylene is particularly preferred.

From the viewpoint of polarization performance and durability of the obtained polarizing film, the average degree of polymerization of PVA is preferably 1,000 or more, more preferably 1,500 or more, and further preferably 2,000 or more. On the other hand, with respect to the upper limit of the average degree of polymerization of PVA, from the viewpoint of ease of manufacture and stretchability of a homogeneous PVA film, the average degree of polymerization is preferably 8,000 or less, particularly preferably 6,000 or less.

Here, the "average degree of polymerization" of PVA in this specification means an average degree of polymerization measured in accordance with JIS K6726-1994, and is obtained from the intrinsic viscosity measured in water at 30°C after re-saponifying and purifying PVA.

From the viewpoint of polarization performance and durability of the obtained polarizing film, the saponification degree of PVA is preferably 95.0 mol % or more, more preferably 98.0 mol % or more, and even more preferably 99.0 mol % or more.

Here, the "saponification degree" of PVA in this specification refers to the ratio (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 the vinyl alcohol unit that can be converted into the vinyl alcohol unit by saponification. The saponification degree of PVA can be measured according to the description of JIS K6726-1994.

The PVA constituting the PVA film may be one type of PVA, or may be two or more types of PVA having different average polymerization degree, saponification degree, modification degree, etc., in one or more of them. The content of PVA in the PVA film is preferably in the range of 50 to 100% by mass, more preferably in the range of 80 to 100% by mass, and further preferably in the range of 85 to 100% by mass.

Because it is possible to improve mechanical properties such as impact strength, step passability during secondary processing, stretchability, etc., the PVA film preferably includes a plasticizer. As a preferred plasticizer, polyols can be cited, specifically, for example, ethylene glycol, glycerol, dipropylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, etc. can be cited. The PVA film can include one or more of these plasticizers. Among these plasticizers, from the viewpoints of the effect of stretchability when the PVA film is stretched and used, it is preferred to use one or more of glycerol, dipropylene glycol and ethylene glycol, and more preferably glycerol.

The content of the plasticizer in the PVA film 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 relative to 100 parts by mass of PVA contained in the PVA film, and is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and further preferably 15 parts by mass or less. By making the content 1 part by mass or more, the stretchability of the PVA film can be further improved. On the other hand, by making the content 30 parts by mass or less, the PVA film can be prevented from becoming too soft and reducing the handleability.

The PVA film preferably contains a surfactant from the viewpoint of improving its handling properties and peeling properties from a film-forming device when manufacturing the PVA film. The type of the surfactant is not particularly limited, and examples thereof include anionic surfactants, nonionic surfactants, and the like.

Examples of the anionic surfactant include carboxylic acid type surfactants such as potassium laurate; sulfate ester type surfactants such as octyl sulfate; and sulfonic acid type surfactants such as dodecylbenzenesulfonate.

Examples of the nonionic surfactant include alkyl ether types such as polyoxyethylene lauryl ether and 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 lauramide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; allyl phenyl ether types such as polyoxyalkylene allyl phenyl ether, and the like.

The PVA film may contain one or more of these surfactants. Among these surfactants, nonionic surfactants are preferred due to their excellent effect of reducing abnormalities on the film surface during film formation, and alkanolamide-type surfactants are particularly preferred, and dialkanolamides (e.g., diethanolamide, etc.) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids having 8 to 30 carbon atoms, etc.) are further preferred.

Since the handleability of the PVA film can be further improved, the peelability of the PVA film from the film-making device during the manufacture of the PVA film can also be improved, and the occurrence of agglomeration can be reduced, the content of the surfactant in the PVA film is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, and further preferably 0.05 parts by mass or more, relative to 100 parts by mass of PVA, and is preferably 1 part by mass or less, more preferably 0.5 parts by mass or less, and further preferably 0.3 parts by mass or less.

The PVA film may further contain other components such as antioxidants, ultraviolet absorbers, lubricants, colorants, preservatives, mildew preventers, other polymer compounds other than the above components, water, etc. The PVA film may contain one or more of these other components.

Since thin optical films (polarizing films, etc.) can be manufactured, the thickness of the PVA film must be less than 55 μm, preferably less than 40 μm, and can be less than 30 μm, and further can be less than 20 μm. In thinner PVA films, dotted line marks are more likely to become a problem, and the effect of the present invention can be particularly significantly achieved in PVA films with such a thickness. On the other hand, the lower limit of the thickness of the PVA film is not particularly limited. Considering the handleability of the PVA film, the passability of the steps during the manufacture of the optical film, the optical properties of the resulting optical film (polarizing properties of the polarizing film, etc.), the thickness is preferably more than 3 μm, more preferably more than 5 μm, and more preferably more than 10 μm. The thickness of the PVA film can be measured at any five positions and calculated as its average value.

The shape of the PVA film is not particularly limited. Since a uniform PVA film can be continuously and smoothly manufactured, and it can also be used continuously when using it to manufacture optical films such as polarizing films, it is preferably a long film. The long film is preferably wound on a cylindrical core and made into a film roll. When used as a long film, the length (length in the flow direction) of the PVA film is not particularly limited and can be appropriately set according to the purpose, etc., but when it is continuously rolled out from the film roll, the longer the length of the PVA film, the more it can reduce the loss when switching the film roll, so the length is preferably 500m or more, more preferably 1,000m or more, further preferably 5,000m or more, and particularly preferably 8,000m or more. There is no particular limit to the upper limit of the length, and the length can be, for example, 30,000m or less.

The form of the PVA film is not particularly limited, and may be in the form of a single layer (single-layer film), or may be in the form of a laminate, such as a PVA film formed by a coating method on a thermoplastic resin film. Any of the above is acceptable. From the viewpoint of more significantly achieving the effects of the present invention, the complexity of the lamination (coating, etc.) operation, and the cost of the thermoplastic resin film, etc., a single-layer form is preferred.

The width of the PVA film is not particularly limited and can be appropriately set according to the use of the PVA film, the polarizing film and other optical films manufactured therefrom, etc. In recent years, from the perspective of promoting the large screen of liquid crystal televisions and liquid crystal monitors, if the width of the PVA film is 2m or more, more preferably 3m or more, and further preferably 4m or more, it is suitable for these uses. On the other hand, if the width of the PVA film is too large, it is easy to become difficult to uniformly perform uniaxial stretching when manufacturing the optical film by a practical device, so the width of the PVA film is preferably 7m or less.

[Method for producing PVA film]

The method for producing the PVA film of the present invention is not particularly limited, but the PVA film of the present invention can be produced smoothly and continuously according to the production method of the present invention described below.

That is, the manufacturing method of the present invention for manufacturing a PVA film with a thickness of less than 55 μm is the following manufacturing method: using a film-making device having a plurality of drying rollers (referred to as the first drying roller, the second drying roller, ... in sequence from the most upstream side to the downstream side) whose rotation axes are parallel to each other, a film-making solution containing PVA is ejected into a film shape and dried on the first drying roller of the film-making device, and further dried by drying rollers subsequent to the second drying roller to manufacture the PVA film, and the ejection speed (S ₀ ) of the film-making solution reaches 2.5 to 5.0 m/min.

In the manufacturing method of the present invention, a film-forming device having a plurality of drying rollers whose rotation axes are parallel to each other is used, a film-forming stock solution containing PVA is ejected into a film shape and dried on a first drying roller of the film-forming device, and further dried by drying rollers after the second drying roller connected to the downstream side of the first drying roller, thereby manufacturing a PVA film. In the film-forming device, the number of drying rollers is preferably 3 or more, more preferably 4 or more, and even more preferably 5 to 30.

The plurality of drying rollers are preferably formed of metal such as nickel, chromium, copper, iron, stainless steel, etc., and in particular, the surface of the drying roller is more preferably formed of a metal material that is hard to corrode and has a mirror finish. In addition, in order to improve the durability of the drying roller, it is more preferable to use a drying roller plated with a single layer or a combination of two or more layers such as a nickel layer, a chromium layer, or a nickel/chromium alloy layer.

With regard to the heating direction when drying the film in the process from the first drying roller to the final drying roller, since the film can be dried more evenly, at any portion of the film, it is preferred to dry the film in a manner such that the film surface in contact with the first drying roller (hereinafter sometimes referred to as the "first drying roller contact surface") and the film surface not in contact with the first drying roller (hereinafter sometimes referred to as the "first drying roller non-contact surface") are alternately opposed to the drying rollers from the first drying roller to the final drying roller.

When the film-forming stock solution containing PVA is ejected into a film shape on the first drying roller of the film-forming device, a known film-forming ejecting device (film-forming casting device) such as a T-type slit die, a hopper plate, an I-type die, a lip coater die, etc. is used to eject (cast) the film-forming stock solution containing PVA into a film shape on the first drying roller.

The ejection speed (S ₀ ) of the film-forming stock solution must be 2.5 to 5.0 m/min. Theoretically, a PVA film having the _{same thickness can be manufactured at a specific production speed by reducing the ejection speed (S 0 ) of the film-forming stock solution and increasing the ratio (S 1 /S 0 ) of the} peripheral speed (S ₁ ) of the first drying roller to the ejection speed (S ₀ ) of the film-forming stock solution, or conversely by increasing the ejection speed (S ₀ ) of the film-forming stock solution and decreasing the ratio (S ₁ /S ₀ ) of the peripheral speed (S ₁ ) of the first drying roller to the ejection speed (S 0 ) of the film-forming stock solution. However, the present inventors have found that when manufacturing a thin PVA film having a thickness of 55 μm or less, by making the ejection speed (S ₀ ) of the film-forming stock solution within the above range, a thin PVA film having fewer broken line marks, which are particularly problematic in thin PVA films, can be manufactured smoothly and continuously. The ejection speed (S ₀ ) of the film-forming stock solution is preferably 2.6 m/min or more, more preferably 2.7 m/min or more, and more preferably 2.8 m/min or more for reasons such as being able to further reduce the number of dotted line marks. On the other hand, if the ejection speed (S ₀ ) of the film-forming stock solution is too high, there is a tendency to be difficult to stably manufacture the PVA film, so the ejection speed (S ₀ ) of the film-forming stock solution is preferably 4.8 m/min or less, more preferably 4.5 m/min or less, more preferably 4.2 m/min or less, and particularly preferably 4.0 m/min or less. It should be noted that the ejection speed (S ₀ ) of the film-forming stock solution refers to the linear velocity of the flow direction of the film-forming stock solution, which can be obtained by dividing the volume per unit time of the film-forming stock solution ejected from the film-like ejection device by the opening area of the slit portion of the film-like ejection device (the product of the slit width of the film-like ejection device and the average value of the slit opening).

The film-forming stock solution containing a PVA film can be prepared by mixing PVA and a liquid medium to prepare a solution, or by melting PVA pellets containing a liquid medium to prepare a molten liquid.

As the liquid medium used at this time, for example, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, diethylenetriamine, etc. can be cited, and these liquid media can be used alone or in combination of two or more. Among these, water, dimethyl sulfoxide, or a mixture of the two is preferably used, and in particular, water is more preferably used.

The film-forming stock solution may contain one or more of the plasticizer, surfactant, and other components as described above in the description of the PVA film in the above-mentioned amounts, as desired.

The volatility of the film-forming stock solution in the manufacture of the PVA film is preferably in the range of 50 to 90% by mass, more preferably in the range of 55 to 80% by mass, and further preferably in the range of 60 to 75% by mass. If the volatility of the film-forming stock solution is too low, the viscosity of the film-forming stock solution may become too high, making it difficult to filter or degas, or making the film itself difficult. On the other hand, if the volatility of the film-forming stock solution is too high, the viscosity may become too low, thereby impairing the uniformity of the thickness of the PVA film.

Here, the "volatile fraction of the membrane-forming stock solution" in this specification refers to the volatility fraction calculated by the following formula [I].

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

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

The surface temperature of the first drying roller is not particularly limited, but is preferably within a range of 70 to 120° C., more preferably 80 to 105° C., and even more preferably 85 to 95° C. from the viewpoint of uniformity of film drying and productivity.

The drying of the film-forming stock solution ejected in a film shape on the first drying roller can be performed only by heating from the first drying roller. From the viewpoints of uniform drying, drying speed, etc., it is preferred to blow hot air to the non-contact surface of the first drying roller while heating with the first drying roller, thereby applying heat from both sides of the film to perform drying.

When hot air is blown to the non-contact surface of the first drying roller of the film on the first drying roller, it is preferred to blow hot air at a wind speed of 1 to 10 m/s to the entire area of the non-contact surface of the first drying roller, more preferably at a wind speed of 2 to 8 m/s, and further preferably at a wind speed of 3 to 8 m/s. If the wind speed of the hot air blown to the non-contact surface of the first drying roller is too low, water vapor and the like will condense during drying on the first drying roller, and the water droplets will drip onto the film, which may cause defects in the final PVA film. On the other hand, if the wind speed of the hot air blown to the non-contact surface of the first drying roller is too high, the thickness of the final PVA film will vary, and with this, problems such as coloring deviation are likely to occur.

The temperature of the hot air blown onto the non-contact surface of the first drying roller of the film is preferably 50 to 150° C., more preferably 70 to 120° C., and even more preferably 80 to 95° C. from the viewpoints of drying efficiency, drying uniformity, etc. The dew point temperature of the hot air blown onto the non-contact surface of the first drying roller of the film is preferably 5 to 20° C., and more preferably 10 to 15° C.

There is no particular limitation on the method for blowing hot air onto the non-contact surface of the first drying roller of the film. Any method that can blow hot air with uniform wind speed and temperature onto the non-contact surface of the first drying roller of the film, preferably onto the entire surface, can be adopted, among which a nozzle method, a rectifying plate method or a combination thereof is preferably adopted. The blowing direction of the hot air onto the non-contact surface of the first drying roller of the film may be a direction opposite to the non-contact surface of the first drying roller, or may be a direction roughly along the circumferential shape of the non-contact surface of the first drying roller of the film (a direction roughly along the circumference of the roller surface of the first drying roller), or may be a direction other than this.

In addition, when the film is dried on the first drying roller, it is preferred to exhaust the volatile components generated by the film during drying and the hot air after blowing. The exhaust method is not particularly limited, but it is preferred to adopt an exhaust method that does not cause a deviation in wind speed and temperature of the hot air blown onto the non-contact surface of the first drying roller of the film.

The peripheral speed (S ₁ ) of the first drying roller is preferably within a range of 12 to 35 m/min, since the number of broken line marks can be further reduced and the stability during production is excellent. The peripheral speed (S ₁ ) of the first drying roller is more preferably 15 m/min or more, more preferably 30 m/min or less, further preferably 28 m/min or less, and particularly preferably 26 m/min or less.

Since the number of broken line marks can be further reduced and the stability during production is excellent, the ratio (S ₁ /S ₀ ) of the peripheral speed (S ₁ ) of the first drying roller to the discharge speed (S ₀ ) of the film-forming stock solution is preferably 7 or less, more preferably 6.8 or less, further preferably 6.5 or less, particularly preferably 6.3 or less, and is preferably greater than 3, more preferably greater than 5, further preferably greater than 5.2, particularly preferably greater than 5.5, and most preferably greater than 6.

The film-forming stock solution sprayed onto the first drying roller in a film shape is dried on the first drying roller, and the film is peeled off from the first drying roller when the volatility fraction (the volatility fraction of the film when peeled off from the first drying roller) is preferably 5 to 30 mass %, more preferably 7 to 20 mass %, and further preferably 8 to 15 mass %. By making the volatility fraction of the film when peeled off from the first drying roller 5 mass % or more, it is possible to prevent the difference in drying speed between the first drying roller contact surface and the first drying roller non-contact surface from becoming too large, thereby preventing the film from curling easily. In addition, by making the volatility fraction of the film when peeled off from the first drying roller 30 mass % or less, it is possible to prevent the thickness deviation from becoming large.

Here, the "volatile fraction of the film" in this specification means the volatility fraction calculated by the following formula [II].

Film volatile fraction (mass %) = {(Wc-Wd)/Wc} × 100 [II]

(Wc represents the mass (g) of the sample collected from the membrane, and Wd represents the mass (g) of the sample Wc (g) when placed in a vacuum dryer at a temperature of 50°C and a pressure of 0.1 kPa or less and dried for 4 hours.)

The film preferably dried to a volatility of 5 to 30% by mass on the first drying roll is peeled from the first drying roll, and then preferably dried by the second drying roll by placing the non-contact surface of the film with the first drying roll facing the second drying roll.

The film dried by the second drying roller is peeled off from the second drying roller and sequentially dried by a third drying roller, a fourth drying roller, a fifth drying roller, etc., depending on the number of drying rollers provided in the film forming apparatus.

From the viewpoints of uniform drying, drying speed, etc., the surface temperature of each drying roller from the second drying roller to the final drying roller is preferably 40°C or higher, more preferably 45°C or higher, further preferably 50°C or higher, and is preferably lower than 100°C, more preferably lower than 90°C, further preferably lower than 85°C, and particularly preferably lower than 80°C.

Preferably, the film dried by the final drying roller is subjected to heat treatment in the above-mentioned manner. The heat treatment can be performed using a heat treatment roller or other known heat treatment apparatus. When the heat treatment is performed by the heat treatment roller, the heat treatment roller may be one or more.

Since a PVA film having moderate crystallization and excellent hot water resistance can be obtained, the surface temperature of the heat treatment roll is preferably 90° C. or higher, more preferably 100° C. or higher, and further preferably 110° C. or higher. In addition, from the viewpoint of improving the stretchability of the obtained PVA film, the surface temperature of the heat treatment roll is preferably 150° C. or lower, more preferably 140° C. or lower, and further preferably 130° C. or lower.

The heat treatment time is not particularly limited, but is preferably within a range of 3 to 60 seconds, more preferably within a range of 5 to 30 seconds, since the target PVA film can be produced more smoothly.

The film forming apparatus may include a hot air drying device, a humidity control device, and the like as necessary.

The film obtained as described above may be further subjected to humidity conditioning treatment, cutting of both ends (edges) of the film, etc. as necessary, and finally wound into a roll having a predetermined length to obtain the PVA film of the present invention.

The volatility of the PVA film finally obtained through the above series of treatments is preferably in the range of 1 to 5% by mass, more preferably in the range of 2 to 4% by mass.

[Application of PVA film]

The use of the PVA film of the present invention is not particularly limited. According to the PVA film of the present invention, since an optical film with few optical defects can be manufactured, the PVA film of the present invention is preferably used as an initial film for manufacturing optical films such as polarizing films and phase difference films. Such an optical film can be manufactured, for example, by using the PVA film of the present invention to perform uniaxial stretching and other treatments.

The method of using the PVA film of the present invention as an initial film to manufacture polarizing film is not particularly limited, and any method that has been used all along can be used. As such a method, for example, a method of dyeing and uniaxial stretching the PVA film or a method of uniaxial stretching the PVA film containing a dye can be cited. As a specific method for manufacturing polarizing film, a method of dyeing, uniaxial stretching, fixing treatment, drying treatment, and further heat treatment as needed can be cited for the PVA film of the present invention. The order of dyeing and uniaxial stretching is not particularly limited, and dyeing can be performed before uniaxial stretching, dyeing can be performed simultaneously with uniaxial stretching, or dyeing can be performed after uniaxial stretching. In addition, steps such as uniaxial stretching and dyeing can be repeated for many times. In particular, it is easy to stretch uniformly when uniaxial stretching is divided into more than 2 stages, so it is preferred. It should be noted that when the PVA film has the form of a laminate such as formed on a thermoplastic resin film, by performing uniaxial stretching in a state where the thermoplastic resin film is stacked, the fracture during uniaxial stretching can be further reduced.

As dyes for dyeing the PVA film, iodine or dichroic organic dyes (e.g., direct black 17, 19, 154; direct brown 44, 106, 195, 210, 223; direct red 2, 23, 28, 31, 37, 39, 79, 81, 240, 242, 247; direct blue 1, 15, 22, 78, 90, 98, 151, 168, 202, 236, 249, 270; direct violet 9, 12, 51, 98; direct green 1, 85; direct yellow 8, 12, 44, 86, 87; direct orange 26, 39, 106, 107 and other dichroic dyes) can be used. These dyes can be used alone or in combination of two or more. Dyeing can usually be carried out by immersing the PVA film in a solution containing the above-mentioned dyes, but the treatment conditions and treatment methods are not particularly limited.

The uniaxial stretching of the PVA film can be carried out by any one of the wet stretching method or the dry heat stretching method, and is preferably a wet stretching method from the viewpoint of the stability of the performance and quality of the obtained polarizing film. As a wet stretching method, a method for stretching the PVA film in an aqueous solution containing various components such as pure water, additives, and aqueous media, or an aqueous dispersion obtained by dispersing various components can be cited. As a specific example of the uniaxial stretching method carried out by the wet stretching method, a method for uniaxial stretching in warm water containing boric acid, a solution containing a dye, and a uniaxial stretching method in a fixed treatment bath described later can be cited. In addition, the PVA film after water absorption can be used to be uniaxially stretched in the air, or uniaxially stretched by other methods. Uniaxial stretching is preferably carried out along the flow direction of the PVA film.

The stretching temperature during uniaxial stretching is not particularly limited. It is preferably 20 to 90°C, more preferably 25 to 70°C, and further preferably 30 to 65°C during wet stretching, and preferably 50 to 180°C during dry heat stretching.

The stretching ratio of uniaxial stretching (the total stretching ratio when uniaxial stretching is performed in multiple stages) is preferably stretched as much as possible until the film is about to break from the viewpoint of polarization performance, specifically preferably 4 times or more, more preferably 5 times or more, and further preferably 5.5 times or more. The upper limit of the stretching ratio is not particularly limited as long as it is within the range that the film does not break, and in order to perform uniform stretching, it is preferably 8.0 times or less.

The thickness of the uniaxially stretched film (polarizing film) is 1 to 30 μm, particularly preferably 3 to 25 μm. The thickness can be determined by measuring the thickness at five arbitrary locations and taking the average value thereof.

When manufacturing polarizing film, in order to make the dye adsorb firmly to the film after uniaxial stretching, fixing treatment is usually performed. The fixing treatment is generally widely adopted by immersing the film in a fixing treatment bath added with boric acid and/or a boron compound. At this time, iodine compounds can be added to the treatment bath as needed.

Next, the film subjected to uniaxial stretching or uniaxial stretching and fixing is preferably dried (heat treated). The temperature of the drying (heat treatment) is preferably 30 to 150° C., particularly preferably 50 to 140° C. If the temperature of the drying (heat treatment) is too low, the dimensional stability of the resulting polarizing film is likely to decrease. On the other hand, if it is too high, the polarizing performance is likely to decrease, which is accompanied by the decomposition of the dye.

A protective film having optical transparency and mechanical strength can be attached to both sides or one side of the polarizing film obtained in the above manner to prepare a polarizing plate. As the protective film at this time, a triacetyl cellulose (TAC) film, a cellulose acetate butyrate (CAB) film, an acrylic film, a polyester film, etc. can be used. In addition, as an adhesive for attaching the protective film, a PVA-based adhesive, a urethane-based adhesive, etc. are generally used, among which a PVA-based adhesive is preferably used.

The polarizing plate obtained in the above manner can be coated with an acrylic adhesive and then bonded to a glass substrate to be used as a component of a liquid crystal display device. When bonding the polarizing plate to the glass substrate, a phase difference film, a viewing angle improvement film, a brightness improvement film, etc. can be bonded simultaneously.

Example

The present invention will be described in detail below with reference to Examples, but the present invention is not limited thereto. It should be noted that the measurement and evaluation methods used in the following Examples, Comparative Examples, and Reference Examples are as follows.

[Number of dotted lines]

At an arbitrary position of the PVA film to be measured, an area of 60 cm in the flow direction × the entire width direction is set, and a scanning white interferometer microscope ("New View" 7300, manufactured by ZYGO) is used to measure the thickness profile near the streak-like defect point considered to be a dotted line mark to confirm whether the streak-like defect point is a dotted line mark specified in the present invention. In this way, in the above-mentioned area, the number of dotted line marks of a straight line in the width direction that crosses any position in the flow direction is measured over the entire width direction of the straight line, and the number of dotted line marks with an average width of 1 m is calculated by dividing the number of dotted line marks by the width of the PVA film.

[Optical defects of polarizing film]

The polarizing film was appropriately divided in the width direction and placed between polarizing plates for observation (two stacked parallel Nicol prisms, polarization degree 99.99% or more) in the vertical direction. The degree of optical defects was visually observed and evaluated according to the following criteria.

○: Optical defects are completely or almost unobservable

×: Optical defects can be easily found

××: Optical defects can be found very easily.

[Example 1]

[Manufacturing of PVA film]

A film-forming stock solution containing 100 parts by mass of PVA (average degree of polymerization 2,400, degree of saponification 99.9 mol%) obtained by saponifying polyvinyl acetate, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide and water and having a volatility of 70% by mass was ejected into a film form from a T-slit die at an ejection speed (S ₀ ) of 2.8 m/min onto a first drying roll (circumferential speed (S ₁ ) of 17 m/min) of a film-forming apparatus including a plurality of drying rolls and a heat treatment roll whose rotation axes were parallel to each other, and was dried on the first drying roll while hot air at 90° C. was blown at a wind speed of 5 m/sec to the entire non-contact surface of the first drying roll, then peeled from the first drying roll, further dried between the second drying roll and the final drying roll immediately before the heat treatment roll in such a manner that the front and back sides of any portion of the film alternately faced the drying rolls, and then peeled from the final drying roll. Next, it was heat treated by a heat treatment roller with a surface temperature of 120°C, and after cutting the two ends (edges), it was wound on a cylindrical core, thereby finally producing a long PVA film (single-layer film) with a thickness of 30.3 μm, a length of 2,000 m, a width of 2.6 m, and a volatile fraction (water fraction) of 2 mass%.

When the number of dotted line marks on the PVA film was measured by the above method, it was found that there were 6 dotted line marks (2.3 in the width direction of 1 m film) with a height difference of 50 to 60 nm and a period of 0.3 to 0.7 mm and arranged in a nearly straight line over 10 cm in the flow direction of the film. The above results are shown in Table 1.

[Manufacturing of polarizing film]

Each treatment was performed while unwinding the obtained PVA film, thereby continuously producing a polarizing film having a thickness of 12 μm.

That is, the PVA film is uniaxially stretched (first stage stretching) in the flow direction (MD) to 2.2 times of the original length while immersed in water at 30°C, and then uniaxially stretched (second stage stretching) in the flow direction (MD) to 3.3 times of the original length while immersed in an iodine/potassium iodide aqueous solution at 30°C containing iodine at a concentration of 0.03% by mass and potassium iodide at a concentration of 3% by mass, and then immersed in a boric acid/potassium iodide solution at 30°C containing boric acid at a concentration of 3% by mass and potassium iodide at a concentration of 3% by mass. The film is uniaxially stretched (third stage stretching) in the flow direction (MD) to 3.6 times of the original length while in an aqueous solution, and then, while immersed in a 63°C boric acid/potassium iodide aqueous solution containing potassium iodide at a concentration of 4 mass% and at a concentration of 5 mass%, it is uniaxially stretched (fourth stage stretching) in the flow direction (MD) to 6.7 times of the original length, and further, after being immersed in a 30°C potassium iodide aqueous solution containing potassium iodide at a concentration of 3 mass%, it is dried in a 60°C dryer for 4 minutes to produce a polarizing film.

The polarizing film was evaluated for optical defects by the above method. The results are shown in Table 1.

[Example 2]

A film-forming stock solution containing 100 parts by mass of PVA (average degree of polymerization 2,400, degree of saponification 99.9 mol%) obtained by saponifying polyvinyl acetate, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide and water and having a volatility of 66 mass% was ejected into a film form from a T-slit die at an ejection speed (S ₀ ) of 2.8 m/min onto a first drying roll (circumferential speed (S ₁ ) of 17 m/min) of a film-forming apparatus including a plurality of drying rolls and a heat treatment roll whose rotation axes were parallel to each other, and was dried on the first drying roll while hot air at 90° C. was blown at a wind speed of 5 m/sec to the entire non-contact surface of the first drying roll, then peeled from the first drying roll, further dried between the second drying roll and the final drying roll immediately before the heat treatment roll in such a manner that the front and back sides of any portion of the film alternately faced the drying rolls, and then peeled from the final drying roll. Next, it was heat treated by a heat treatment roller with a surface temperature of 115°C, and after cutting the two ends (edges), it was wound on a cylindrical core, thereby finally producing a long PVA film (single-layer film) with a thickness of 40.6 μm, a length of 2,000 m, a width of 2.6 m, and a volatile fraction (water fraction) of 2 mass%.

When the number of dotted line marks was measured by the above method for the PVA film, it was found that there were 5 dotted line marks (1.9 in the width direction of 1 m film) arranged in a nearly straight line with a height difference of 50 to 60 nm and a period of 0.3 to 0.7 mm and spanning more than 10 cm in the flow direction of the film. In addition, a polarizing film with a thickness of 17 μm was manufactured in the same manner as in Example 1 using the obtained PVA film, and optical defects were evaluated by the above method. The above results are shown in Table 1.

[Comparative Example 1]

A film-forming stock solution containing 100 parts by mass of PVA (average degree of polymerization 2,400, degree of saponification 99.9 mol%) obtained by saponifying polyvinyl acetate, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide and water and having a volatility of 66 mass% was ejected into a film form from a T-slit die at an ejection speed (S ₀ ) of 2.4 m/min onto a first drying roll (circumferential speed (S ₁ ) of 18 m/min) of a film-forming apparatus including a plurality of drying rolls and a heat treatment roll whose rotation axes were parallel to each other, and was dried on the first drying roll while hot air at 90° C. was blown at a wind speed of 5 m/sec to the entire non-contact surface of the first drying roll, then peeled from the first drying roll, further dried between the second drying roll and the final drying roll immediately before the heat treatment roll in such a manner that the front and back sides of any portion of the film alternately faced the drying rolls, and then peeled from the final drying roll. Next, it was heat treated by a heat treatment roller with a surface temperature of 115°C, and after cutting the two ends (edges), it was wound on a cylindrical core, thereby finally producing a long PVA film (single-layer film) with a thickness of 29.6 μm, a length of 2,000 m, a width of 2.6 m, and a volatile fraction (water fraction) of 2 mass%.

When the number of dotted line marks was measured by the above method for the PVA film, it was found that there were 24 dotted line marks (9.2 in the width direction of 1 m film) arranged in a nearly straight line with a height difference of 60 to 70 nm and a period of 0.3 to 0.7 mm and spanning more than 10 cm in the flow direction of the film. In addition, a polarizing film with a thickness of 12 μm was manufactured in the same manner as in Example 1 using the obtained PVA film, and optical defects were evaluated by the above method. The above results are shown in Table 1.

[Comparative Example 2]

A film-forming stock solution containing 100 parts by mass of PVA (average degree of polymerization 2,400, degree of saponification 99.9 mol%) obtained by saponifying polyvinyl acetate, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide and water and having a volatility of 66 mass% was ejected into a film form from a T-slit die at an ejection speed (S ₀ ) of 2.0 m/min onto a first drying roll (circumferential speed (S ₁ ) of 15 m/min) of a film-forming apparatus including a plurality of drying rolls and a heat treatment roll whose rotation axes were parallel to each other, and was dried on the first drying roll while hot air at 90° C. was blown at a wind speed of 5 m/sec to the entire non-contact surface of the first drying roll, then peeled from the first drying roll, further dried between the second drying roll and the final drying roll immediately before the heat treatment roll in such a manner that the front and back sides of any portion of the film alternately faced the drying rolls, and then peeled from the final drying roll. Next, it was heat treated by a heat treatment roller with a surface temperature of 115°C, and after cutting the two ends (edges), it was wound on a cylindrical core, thereby finally producing a long PVA film (single-layer film) with a thickness of 30.3 μm, a length of 2,000 m, a width of 2.6 m, and a volatile fraction (water fraction) of 2 mass%.

When the number of dotted line marks was measured by the above method for the PVA film, it was found that there were 37 dotted line marks (14.2 in the width direction of 1 m film) arranged in a nearly straight line with a height difference of 80 to 90 nm and a period of 0.3 to 0.7 mm and spanning more than 10 cm in the flow direction of the film. In addition, a polarizing film with a thickness of 12 μm was manufactured in the same manner as in Example 1 using the obtained PVA film, and optical defects were evaluated by the above method. The above results are shown in Table 1.

[Reference example]

A film-forming stock solution containing 100 parts by mass of PVA (average degree of polymerization 2,400, degree of saponification 99.9 mol%) obtained by saponifying polyvinyl acetate, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide and water and having a volatility of 66 mass% was ejected into a film form from a T-slit die at an ejection speed (S ₀ ) of 2.4 m/min onto a first drying roll (circumferential speed (S ₁ ) of 11 m/min) of a film-forming apparatus including a plurality of drying rolls and a heat treatment roll whose rotation axes were parallel to each other, and was dried on the first drying roll while hot air at 90° C. was blown at a wind speed of 5 m/sec to the entire non-contact surface of the first drying roll, then peeled from the first drying roll, further dried between the second drying roll and the final drying roll immediately before the heat treatment roll in such a manner that the front and back sides of any portion of the film alternately faced the drying rolls, and then peeled from the final drying roll. Next, it was heat treated by a heat treatment roller with a surface temperature of 105°C, and after cutting the two ends (edges), it was wound on a cylindrical core, thereby finally producing a long PVA film (single-layer film) with a thickness of 74.5 μm, a length of 2,000 m, a width of 2.6 m, and a volatile fraction (water fraction) of 2 mass%.

The number of dotted line marks on the PVA film was measured by the above method, but no dotted line marks were found. The above results are shown in Table 1.

[Table 1]

Claims (7)

1. A manufacturing method, which is a method of manufacturing a polyvinyl alcohol-based polymer film with a thickness of 55 μm or less, using a film forming apparatus equipped with a plurality of drying rollers with rotation axes parallel to each other, and on the first drying roller of the film forming apparatus. When the film-forming stock solution containing the polyvinyl alcohol-based polymer is ejected into a film shape and dried, and is further dried by a drying roll subsequent to the second drying roll to produce a polyvinyl alcohol-based polymer film, the film forming solution is The discharge speed S ₀ of the original solution is 2.5 to 5.0 m/min, and the ratio S ₁ /S ₀ of the peripheral speed S ₁ of the first drying roller to the discharge speed S ₀ of the film-forming original solution is greater than 5 and 7 the following, The polyvinyl alcohol-based polymer film has an average of 3 or less dotted lines in the width direction of the film per meter, and the dotted line marks have a periodicity of 0.01 to 10 mm and are arranged approximately over 10 cm in the flow direction of the film. Made of straight lines. 2. The manufacturing method according to claim 1, which is a method of manufacturing a polyvinyl alcohol-based polymer film having a width of 2 m or more. 3. A polyvinyl alcohol-based polymer film produced by the production method according to claim 1 or 2. 4. An optical film produced from the polyvinyl alcohol-based polymer film according to claim 3. 5. The optical film according to claim 4, which is a polarizing film. 6. A method for producing an optical film, comprising the step of uniaxially stretching the polyvinyl alcohol-based polymer film according to claim 3. 7. The manufacturing method according to claim 6, which is a manufacturing method of a polarizing film.