CN117001908A

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

Info

Publication number: CN117001908A
Application number: CN202311059168.5A
Authority: CN
Inventors: 森保二郎; 井下翔平
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 2015-05-28
Filing date: 2016-05-20
Publication date: 2023-11-07
Also published as: KR20180013908A; JPWO2016190235A1; JP6716553B2; KR102038212B1; WO2016190235A1; TWI741984B; CN107614242A; TW201708268A

Abstract

本发明涉及聚乙烯醇系聚合物膜和其制造方法。本发明涉及：PVA膜，其是厚度为55μm以下的PVA膜，凹凸为0.01～10mm的周期且在膜的流动方向上跨10cm以上排列为近似直线状而成的虚线条痕为平均1m膜的宽度方向5根以下；和，制造方法，其是制造厚度为55μm以下的PVA膜的方法，使用具备旋转轴彼此平行的多个干燥辊的制膜装置，在该制膜装置的第1干燥辊上将包含PVA的制膜原液喷出为膜状并干燥、并通过接续于其的第2干燥辊以后的干燥辊进一步干燥从而制造PVA膜时，使制膜原液的喷出速度(S₀)达到2.5～5.0m/分钟。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 polymer film and method for producing same

The present invention is a PCT patent application PCT/JP2016/064933, the filing date of which is 5/20/2016, and the division of the invention patent application entitled "polyvinyl alcohol polymer film and method for producing the same", and the parent application is 201680031048.6 in china.

Technical Field

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

Background

A polarizing plate having a function of transmitting and shielding light is one of important constituent elements of a Liquid Crystal Display (LCD) together with a liquid crystal or the like having a function of switching light. LCDs are used in a wide range of small devices such as calculators and watches, notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, car navigation systems, mobile phones, tablet terminals, and measurement devices used indoors and outdoors. Among these fields of application of LCDs, liquid crystal televisions, liquid crystal monitors, and the like have been increasingly thinned in addition to increasing the size of the screen. In addition, in recent years, thinning is also advancing in tablet personal computer terminals that are remarkably popular. As a means for realizing the thinning of the LCD, there is a thinning of glass used in the LCD, but from the viewpoint of solving the problem of glass warpage due to shrinkage stress of the polarizing plate accompanying this, the thinning of the polarizing plate is also required.

The polarizing plate is generally manufactured by: after a PVA film is dyed and uniaxially stretched to produce a polarizing film, a protective film such as a cellulose Triacetate (TAC) film is attached to the surface of the polarizing film to produce the polarizing film. Therefore, in order to achieve a thinner polarizing plate, it has been demanded to manufacture a thinner polarizing film using a thinner PVA film, and the thickness of the PVA film is demanded to be 55 μm or less, and further demanded to be 30 μm or less.

Heretofore, various methods for producing PVA films are known. For example, it is known that: when a PVA film is produced by ejecting a PVA-containing film-forming raw material onto a rotating drum of a drum film-forming machine or onto a traveling belt of a belt film-forming machine and drying the PVA film, a PVA film having a uniform thickness and excellent smoothness can be produced over a large area by setting a speed ratio of the drum or belt to the ejection speed of the film-forming raw material to 1 to 5 (see patent document 1). Specifically, patent document 1 describes that a PVA film having a thickness of about 75 μm and a thickness deviation of about 1.6 to 3.0 μm is produced by this method.

Prior art literature

Patent literature

Patent document 1: japanese patent application laid-open No. 2002-79530.

Disclosure of Invention

Problems to be solved by the invention

As described above, a thinner PVA film is required, but when simply producing a thin PVA film by a conventional film-forming method, a new problem arises in that a large number of broken-line streaks (broken-line streaks) are formed in which a plurality of nano-order irregularities are arranged along the flow direction of the film. The present inventors have recognized that if such a conventional thin PVA film having a large number of broken lines and streaks is used, optical defects are generated in a large amount in the obtained optical film.

Accordingly, 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. The present invention also provides an optical film having few optical defects, which is produced from the PVA film.

Means for solving the problems

To achieve the above object, the present inventors have conducted intensive studiesAs a result, it has been found that when an optical film such as a polarizing film is produced using a thin PVA film, an optical film having few optical defects can be easily obtained by setting the number of broken line streaks in the PVA film to a specific value or less. Further, it has been found that, when a thin PVA film is produced by using a film forming apparatus having a plurality of drying rolls with rotation axes parallel to each other, a film forming stock solution containing PVA is discharged as a film on the 1 st drying roll located on the most upstream side of the film forming apparatus and dried, and further dried by the 2 nd drying roll following the 2 nd drying roll located on the downstream side of the 1 st drying roll, the discharge rate of the film forming stock solution is increased (S ₀ ) When the number of broken lines is small, a thin PVA film different from the conventional PVA film can be produced smoothly and continuously. Based on these findings, the present inventors have further studied repeatedly, and have completed the present invention.

Namely, the present invention relates to:

[1] a PVA film having a thickness of 55 [ mu ] m or less, wherein 5 or less lines of broken lines are formed in a width direction of the film of 1m on average, wherein the lines of broken lines are formed in a period of 0.01 to 10mm and are arranged in a nearly straight line across 10cm or more in a flow direction of the film;

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

[3]a production method for producing a PVA film having a thickness of 55 μm or less, wherein a film-forming apparatus having a plurality of drying rolls having rotation axes parallel to each other is used, and a film-forming stock solution containing PVA is discharged onto a 1 st drying roll of the film-forming apparatus to form a film and dried, and further dried by a 2 nd drying roll connected thereto to produce a PVA film, wherein the discharge rate of the film-forming stock solution is set to (S ₀ ) Reaching 2.5-5.0 m/min;

[4]according to [3] above]The manufacturing method comprises the steps of setting the circumferential speed (S ₁ ) Relative to the ejection speed of the film-forming stock solution (S ₀ ) Ratio (S) ₁ /S ₀ ) Reaching below 7;

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

[6] an optical film produced from the PVA film of [1] or [2] above;

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

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

[9] the method according to [8], wherein the polarizing film is produced by the method.

ADVANTAGEOUS EFFECTS OF 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 same.

Detailed Description

The present invention will be described in detail below.

[ PVA film ]

The PVA film of the present invention has a thickness of 55 [ mu ] m or less, has a period of 0.01 to 10mm in the irregularities, and has an average of 5 or less lines of broken lines of 1m in the width direction of the film, the lines being arranged approximately linearly across 10cm or more in the flow direction of the film.

In the present invention, the predetermined number of broken lines are, as described above, obtained by arranging the irregularities in a substantially straight line across 10cm or more in the flow direction of the film with a period of 0.01 to 10 mm. In general, a broken line streak is generated along the flow direction of the film, and the length of 1 broken line streak is much longer than 10cm, but in this specification, streaks having a length of 10cm or more (streaks having the above-described period and arranged approximately linearly across 10cm or more in the flow direction of the film) are regarded as broken line streaks for the sake of convenience in distinguishing from defective points having a length of less than 10cm and being difficult to be called broken line streaks.

The type of the concave-convex is not particularly limited, and examples thereof include: as a result of arranging the convex portions protruding from the film surface, irregularities are formed; by arranging the concave portions recessed from the film surface, the concave-convex is formed as a result; the convex portions protruding from the film surface and the concave portions recessed from the film surface are alternately arranged, respectively, so that irregularities are formed.

The irregularities of the broken lines are arranged in a substantially straight line across 10cm or more in the flow direction of the film with a period of 0.01 to 10 mm. Here, the period is a length of the film in the flow direction for 1 set of irregularities (a set of 1 concave portion and 1 convex portion adjacent to each other), and among all the irregularities to be formed, the period is in the range of 0.01 to 10 mm. The period may be, for example, in the range of 0.1 to 5mm, and further in the range of 0.2 to 1 mm.

In the group of irregularities constituting the broken line streak, the difference in height between the portion having the highest height and the portion having the lowest height is usually in the range of 1 to 500nm, for example, in the range of 10 to 300nm, further in the range of 20 to 100nm, and particularly in the range of 30 to 90 nm.

The number of the broken streaks can be obtained as follows: the number of the broken lines intersecting the straight line is measured in the entire width direction on a straight line passing through any one portion in the flow direction in the width direction, and the number is divided by the width (unit: m) of the PVA film to obtain the number of the average 1m width. Here, the existence of the broken streak can be confirmed by, for example, a scanning white interference microscope or the like. Specifically, the number of the streaks of the broken line can be obtained by the method described later in the examples.

The number of the PVA film of the present invention in the width direction of the average 1m film of the broken line streaks is not more than 5, preferably not more than 3, more preferably not more than 2, but may be not more than 1.5, and further not more than 1. By setting the number of the broken lines to the above range, an optical film having few optical defects can be easily obtained. On the other hand, in the case of a thin PVA film of 55 μm or less, when the operating conditions are adjusted so that the number of streaks in the broken line becomes less than 0.05 in the width direction of 1m on average, there is a tendency that streak-like optical defects of the optical film are small, but the reason is not clear, but there is a tendency that problems such as thickness deviation easily occur in the film. Therefore, the number of the broken lines is preferably 0.05 or more, more preferably 0.10 or more, still more preferably 0.15 or more, and most preferably 0.20 or more in the width direction of 1m on average.

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

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

Examples of the vinyl ester used for producing PVA include vinyl acetate, vinyl formate, vinyl laurate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl stearate, and vinyl benzoate. These vinyl esters may be used alone or in combination. Among these vinyl esters, vinyl acetate is preferred from the viewpoint of productivity.

Examples of the comonomer include olefins having 2 to 30 carbon atoms (e.g., α -olefins) such as ethylene, propylene, 1-butene, and isobutylene; acrylic acid or a salt thereof; acrylic esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate (for example, alkyl esters having 1 to 18 carbon atoms of acrylic acid); methacrylic acid or a salt thereof; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate (for example, alkyl esters having 1 to 18 carbon atoms of methacrylic acid); acrylamide derivatives such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N-dimethylacrylamide, diacetone acrylamide, acrylamidopropane sulfonic acid or a salt thereof, acrylamidopropyl dimethylamine or a salt thereof, and N-methylolacrylamide or a derivative thereof; methacrylamide derivatives such as methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, methacrylamidopropane sulfonic acid or a salt thereof, methacrylamidopropyl dimethylamine or a salt thereof, N-hydroxymethyl methacrylamide or a derivative thereof; n-vinyl amides such as N-vinyl formamide, N-vinyl acetamide and N-vinyl pyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; halogenated vinyl groups 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 such as salts and esters thereof; vinyl silyl compounds such as vinyl trimethoxy silane; isopropenyl acetate; unsaturated sulfonic acid or its derivative, etc. Among these, preferred is an α -olefin, and particularly preferred is ethylene.

The average degree of polymerization of PVA is preferably 1,000 or more, more preferably 1,500 or more, and still more preferably 2,000 or more from the viewpoints of polarizing performance, durability, and the like of the obtained polarizing film. On the other hand, the upper limit of the average degree of polymerization of PVA is preferably 8,000 or less, particularly preferably 6,000 or less, from the viewpoints of ease of production of a homogeneous PVA film, stretchability, and the like.

The "average degree of polymerization" of PVA in the present specification means an average degree of polymerization measured in accordance with JIS K6726-1994, and is determined from the intrinsic viscosity measured in water at 30℃after the PVA is re-saponified and purified.

The saponification degree of PVA is preferably 95.0 mol% or more, more preferably 98.0 mol% or more, and still more preferably 99.0 mol% or more from the viewpoint of polarizing performance, durability, and the like of the obtained polarizing film.

Here, the "saponification degree" of PVA in the present specification means a ratio (mol%) of the number of moles of the vinyl alcohol unit relative to the total number of moles of the structural unit (typically, vinyl ester unit) and the vinyl alcohol unit which can be converted into the vinyl alcohol unit by saponification. The saponification degree of PVA can be measured according to JIS K6726-1994.

The PVA constituting the PVA film may be 1 kind of PVA, or may be 2 or more kinds of PVA different from each other in 1 or 2 or more kinds of average polymerization degree, saponification degree, modification degree, and the like. The content of PVA in the PVA film is preferably in the range of 50 to 100 mass%, more preferably in the range of 80 to 100 mass%, and even more preferably in the range of 85 to 100 mass%.

The PVA film preferably contains a plasticizer because it can improve mechanical properties such as impact strength, step passability during secondary processing, stretchability, and the like. Preferred plasticizers include polyhydric alcohols, specifically, ethylene glycol, glycerol, diglycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and the like. The PVA film may contain 1 or 2 or more of these plasticizers. Among these plasticizers, from the viewpoint of, for example, improving the effect of stretching the PVA film and the stretchability when used, glycerol, 1 or 2 or more of dipropylene glycol and ethylene glycol are preferably used, and glycerol is more preferably used.

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, further preferably 5 parts by mass or more, and further preferably 30 parts by mass or less, more preferably 20 parts by mass or less, further preferably 15 parts by mass or less, relative to 100 parts by mass of PVA contained in the PVA film. By setting the content to 1 part by mass or more, the stretchability of the PVA film can be further improved. On the other hand, when the content is 30 parts by mass or less, the PVA film is prevented from becoming excessively soft and the handleability is prevented from being lowered.

The PVA film preferably contains a surfactant from the viewpoints of improving its handling properties, and also from the viewpoint of peelability from a film forming apparatus at the time of producing 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 esters such as octyl sulfate; sulfonic acid type such as dodecylbenzenesulfonate.

Examples of the nonionic surfactant include alkyl ether type surfactants such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkylphenyl ethers such as polyoxyethylene octylphenyl ether; alkyl esters such as polyoxyethylene laurate; alkylamines such as polyoxyethylene lauryl amino ether; alkylamide type such as polyoxyethylene lauramide; polypropylene glycol ethers such as polyoxyethylene polyoxypropylene ether; alkanolamide type such as lauric acid diethanolamide and oleic acid diethanolamide; and allylphenyl ethers such as polyoxyalkylene allylphenyl ether.

The PVA film may contain 1 or 2 or more of these surfactants. Among these surfactants, nonionic surfactants are preferable, and alkanolamide surfactants are particularly preferable, and dialkanolamides (e.g., diethanolamide) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids having 8 to 30 carbon atoms) are more preferable, because of their excellent effect of reducing film surface abnormalities during film formation.

Since the handleability of the PVA film, the peelability from the film forming apparatus at the time of producing the PVA film, and the occurrence of blocking can be further improved, 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, still more preferably 0.05 parts by mass or more, and further preferably 1 part by mass or less, more preferably 0.5 parts by mass or less, still more preferably 0.3 parts by mass or less, relative to 100 parts by mass of PVA.

The PVA film may contain, if necessary, an antioxidant, an ultraviolet absorber, a lubricant, a colorant, a preservative, a mold inhibitor, a polymer compound other than the above components, and other components such as moisture. The PVA film may contain 1 or 2 or more of these other components.

Since a thin optical film (polarizing film or the like) can be produced, the thickness of the PVA film must be 55 μm or less, preferably 40 μm or less, and may be 30 μm or less, and further may be 20 μm or less. In the case of a thinner PVA film, the broken line streak becomes more problematic, and the effect of the present invention can be particularly remarkably achieved in the case of a PVA film having such a thickness. On the other hand, the lower limit of the thickness of the PVA film is not particularly limited, and the thickness is preferably 3 μm or more, more preferably 5 μm or more, still more preferably 10 μm or more in view of the handleability of the PVA film, step passability at the time of producing the optical film, optical properties of the obtained optical film (polarizing properties of the polarizing film, etc.), and the like. The thickness of the PVA film can be measured at any five positions and found as an average value thereof.

The shape of the PVA film is not particularly limited, and a long film is preferable because a uniform PVA film can be continuously and smoothly produced and can be continuously used even when an optical film such as a polarizing film is produced using the PVA film. The long film is preferably wound around a cylindrical core or the like to be formed into a film roll. In the case of a long film, the length (length in the flow direction) of the PVA film is not particularly limited and may be appropriately set according to the application, but in the case of continuous use from a film roll, the longer the length of the PVA film, the more the loss when switching the film roll can be reduced, and therefore, the length is preferably 500m or more, more preferably 1,000m or more, still more preferably 5,000m or more, and particularly preferably 8,000m or more. The upper limit of the length is not particularly limited, and the length may be, for example, 30,000m or less.

The form of the PVA film is not particularly limited, and may be a single layer (single layer film) or a laminate such as a PVA film formed on a thermoplastic resin film by a coating method or the like, and may be a single layer form from the viewpoint of achieving the effect of the present invention more remarkably, the complexity of the lamination (coating or the like), the cost of the thermoplastic resin film, or the like.

The width of the PVA film is not particularly limited, and may be appropriately set according to the use of the PVA film, an optical film such as a polarizing film produced therefrom, and the like, and in recent years, it is suitable for such use if the width of the PVA film is 2m or more, more preferably 3m or more, and still more preferably 4m or more from the viewpoint of increasing the screen size of a liquid crystal television or a liquid crystal monitor. On the other hand, if the width of the PVA film is too large, it is easy to make it difficult to uniformly uniaxially stretch when the optical film is produced by a practical apparatus, and therefore 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, and 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 production method of the present invention for producing a PVA film having a thickness of 55 μm or less is the following production method: when a film forming apparatus having a plurality of drying rolls (hereinafter, referred to as a 1 st drying roll and a 2 nd drying roll … … in this order from the most upstream side to the downstream side) with rotation axes parallel to each other is used, a film forming stock solution containing PVA is discharged onto the 1 st drying roll of the film forming apparatus into a film shape and dried, and further dried by drying rolls following the 2 nd drying roll, thereby producing a PVA film, the discharge rate of the film forming stock solution is set to (S ₀ ) Reaching 2.5-5.0 m/min.

In the production method of the present invention, a film forming apparatus having a plurality of drying rolls with their rotation axes parallel to each other is used, a film forming stock solution containing PVA is discharged as a film on a 1 st drying roll of the film forming apparatus and dried, and the PVA film is produced by further drying the film by a 2 nd drying roll and subsequent drying rolls connected downstream of the 1 st drying roll. In this film forming apparatus, the number of drying rolls is preferably 3 or more, more preferably 4 or more, and still more preferably 5 to 30.

The plurality of drying rolls are preferably formed of a metal such as nickel, chromium, copper, iron, stainless steel, or the like, and in particular, the surfaces of the drying rolls are more preferably formed of a metal material which is hard to corrode and has specular gloss. In order to improve the durability of the dry roller, it is more preferable to use a dry roller plated with a single layer or a combination of 2 or more layers such as a nickel layer, a chromium layer, and a nickel/chromium alloy layer.

Since the film can be dried more uniformly in the heating direction when drying the film in the process from the 1 st drying roller to the final drying roller, it is preferable that the film be dried so that the film surface in contact with the 1 st drying roller (hereinafter, sometimes referred to as "1 st drying roller contact surface") and the film surface not in contact with the 1 st drying roller (hereinafter, sometimes referred to as "1 st drying roller non-contact surface") alternately face the drying rollers of the 1 st drying roller to the final drying roller at any portion of the film.

When the film-forming raw liquid containing PVA is discharged as a film on the 1 st drying roll of the film-forming apparatus, the film-forming raw liquid containing PVA may be discharged (cast) as a film on the 1 st drying roll using a known film-forming discharge apparatus (film casting apparatus) such as a T-slot die, a feeder plate (hopper plate), an I-die, or a lip coater die.

Ejection speed of film-forming stock solution (S) ₀ ) Must be 2.5 to 5.0 m/min. Theoretically, by decreasing the ejection speed of the film-forming stock solution (S ₀ ) On the other hand, the circumferential speed of the 1 st drying roller is increased (S ₁ ) Relative to the ejection speed of the film-forming stock solution (S ₀ ) Ratio (S) ₁ /S ₀ ) Or conversely by increasing the ejection speed of the film-forming stock solution (S ₀ ) While on the other hand the circumferential speed of the 1 st drying roller is reduced (S ₁ ) Relative to the ejection speed of the film-forming stock solution (S ₀ ) Ratio (S) ₁ /S ₀ ) Although it is possible to produce PVA films having the same thickness at a specific production rate, the present inventors have found that, when producing thin PVA films having a thickness of 55 μm or less, the production rate of the film-forming stock solution is controlled by the discharge rate (S ₀ ) When the number of broken lines is within the above range, a thin PVA film different from the conventional one, which is particularly problematic in the thin PVA film, can be smoothly and continuously produced. The ejection speed of the film-forming stock solution (S) ₀ ) The number of the broken lines is preferably 2.6 m/min or more, more preferably 2.7 m/min or more, and still more preferably 2.8 m/min or more, because the number of broken lines can be further reduced. On the other hand, if the ejection speed of the film-forming stock solution (S ₀ ) If the content is too high, it tends to be difficult to stably produce a PVA film, and therefore the ejection rate of the film-forming stock solution (S ₀ ) Preferably 4.8 m/min or less,More preferably not more than 4.5 m/min, still more preferably not more than 4.2 m/min, particularly preferably not more than 4.0 m/min. The ejection rate of the film-forming stock solution (S ₀ ) The linear velocity in the flow direction of the film-forming raw liquid is obtained by dividing the volume per unit time of the film-forming raw liquid discharged from the film-like discharge device by the opening area of the slit portion of the film-like discharge device (the product of the slit width of the film-like discharge device and the average value of the slit opening).

As a film-forming stock solution containing a PVA film, it can be prepared by mixing PVA with a liquid medium to prepare a solution, or by melting PVA pellets containing a liquid medium or the like to prepare a melt or the like.

Examples of the liquid medium used in this case include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, diethylenetriamine, and the like, and these liquid mediums may be used singly or in combination of 1 or more than 2. Among these, water, dimethyl sulfoxide, or a mixture of both is preferably used, and in particular, water is more preferably used.

If desired, the film-forming stock solution is blended with 1 or 2 or more kinds of plasticizers, surfactants, other components, and the like as described above in the description of the PVA film in the above amounts.

The volatile fraction of the film-forming stock solution in the production of the PVA film is preferably in the range of 50 to 90 mass%, more preferably in the range of 55 to 80 mass%, and even more preferably in the range of 60 to 75 mass%. If the evaporation fraction of the film-forming stock solution is too low, the viscosity of the film-forming stock solution may become too high, and filtration and defoaming may become difficult, or the film formation itself may become difficult. On the other hand, if the evaporation fraction of the film-forming stock solution is too high, the viscosity may become too low, and the uniformity of the thickness of the PVA film may be impaired.

The term "evaporation fraction of the film forming stock solution" in the present specification means an evaporation fraction obtained by the following formula [ I ].

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

(wherein Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of the film-forming stock solution obtained by drying Wa (g) in an electrothermal drier at 105℃for 16 hours).

The surface temperature of the 1 st drying roll is not particularly limited, but is preferably in the range of 70 to 120 ℃, more preferably in the range of 80 to 105 ℃, and even more preferably in the range of 85 to 95 ℃ from the viewpoints of uniformity of drying of a film, productivity, and the like.

Drying of the film-forming stock solution discharged in the form of a film on the 1 st drying roller may be performed by heating only from the 1 st drying roller, and from the viewpoints of uniformity of drying, drying speed, and the like, it is preferable to dry the film by blowing hot air to the non-contact surface of the 1 st drying roller while heating the film by the 1 st drying roller, and applying heat from both surfaces of the film.

When hot air is blown onto the 1 st drying roll non-contact surface of the film on the 1 st drying roll, hot air having a wind speed of 1 to 10 m/sec, more preferably 2 to 8 m/sec, and still more preferably 3 to 8 m/sec is blown onto the entire area of the 1 st drying roll non-contact surface. If the air velocity of the hot air blown against the non-contact surface of the 1 st drying roll is too low, condensation of water vapor or the like occurs at the time of drying on the 1 st drying roll, and water droplets thereof drop down to the film, and there is a possibility that defective spots may occur in the PVA film finally obtained. On the other hand, if the air speed of the hot air blown against the non-contact surface of the 1 st drying roller is too high, the thickness variation occurs in the PVA film finally obtained, and with this, problems such as dyeing variation easily occur.

The temperature of the hot air blown onto the non-contact surface of the 1 st drying roll of the film is preferably 50 to 150 ℃, more preferably 70 to 120 ℃, still more preferably 80 to 95 ℃ from the viewpoints of drying efficiency, drying uniformity and the like. The dew point temperature of the hot air blown onto the non-contact surface of the 1 st drying roll of the film is preferably 5 to 20 ℃, more preferably 10 to 15 ℃.

The method for blowing hot air onto the 1 st drying roll non-contact surface of the film is not particularly limited, and a method in which hot air having a uniform air speed and a uniform temperature is blown onto the 1 st drying roll non-contact surface of the film, preferably onto the entire film, may be employed, and among these, a nozzle method, a rectifying plate method, a combination thereof, or the like is preferable. The blowing direction of the hot air to the 1 st drying roll non-contact surface of the film may be a direction opposite to the 1 st drying roll non-contact surface, may be a direction substantially along the circumferential shape of the 1 st drying roll non-contact surface of the film (a direction substantially along the circumference of the roll surface of the 1 st drying roll), or may be a direction other than the direction.

When the film is dried on the 1 st drying roll, it is preferable to exhaust the volatile components generated from the film by drying and the blown hot air. The method of exhausting is not particularly limited, and it is preferable to use an exhausting method that does not cause a deviation in wind speed and a deviation in temperature of hot air blown against the non-contact surface of the 1 st drying roll of the film.

Since the number of broken lines and streaks can be further reduced and the stability at the time of production is excellent, the circumferential speed of the 1 st drying roll (S ₁ ) Preferably in the range of 12 to 35 m/min, the circumferential speed (S) of the 1 st drying roll ₁ ) More preferably 15 m/min or more, still more preferably 30 m/min or less, still more preferably 28 m/min or less, particularly preferably 26 m/min or less.

Since the number of broken lines and streaks can be further reduced and the stability at the time of production is excellent, the circumferential speed of the 1 st drying roll (S ₁ ) Relative to the ejection speed of the film-forming stock solution (S ₀ ) Ratio (S) ₁ /S ₀ ) Preferably 7 or less, more preferably 6.8 or less, still more preferably 6.5 or less, particularly preferably 6.3 or less, and furthermore, preferably more than 3, more preferably more than 5, still more preferably more than 5.2, particularly preferably more than 5.5, most preferably more than 6.

When the film-forming stock solution discharged onto the 1 st drying roll in the form of a film is dried on the 1 st drying roll, and the film is peeled from the 1 st drying roll with a volatile fraction (volatile fraction of the film at the time of peeling from the 1 st drying roll) of preferably 5 to 30% by mass, more preferably 7 to 20% by mass, still more preferably 8 to 15% by mass. By setting the volatile fraction of the film at the time of peeling from the 1 st drying roller to 5 mass% or more, it is possible to suppress the difference in drying speed between the 1 st drying roller contact surface and the 1 st drying roller non-contact surface from becoming excessive, and the film is liable to curl. Further, by setting the film volatilization fraction at the time of peeling from the 1 st drying roller to 30 mass% or less, it is possible to suppress the increase in thickness variation.

The term "film volatility" as used herein refers to a volatility obtained by the following formula [ II ].

Volatile fraction (mass%) of film = { (Wc-Wd)/Wc } ×100[ ii ]

(wherein Wc represents the mass (g) of a sample collected from the film, wd represents the mass (g) of the sample obtained by putting Wc (g) into a vacuum drier having a temperature of 50 ℃ and a pressure of 0.1kPa or less and drying for 4 hours).

The film dried on the 1 st drying roll to a volatile fraction of 5 to 30 mass% is peeled off from the 1 st drying roll, and then the non-contact surface of the 1 st drying roll of the film is preferably opposed to the 2 nd drying roll, whereby the film is dried by the 2 nd drying roll.

The film dried by the 2 nd drying roll is peeled off from the 2 nd drying roll, and is dried by a plurality of drying rolls such as the 3 rd drying roll, the 4 th drying roll, and the 5 th drying roll … … according to the number of drying rolls or the like provided in the film forming apparatus.

The surface temperature of each drying roll from the 2 nd drying roll to the final drying roll is preferably 40 ℃ or higher, more preferably 45 ℃ or higher, still more preferably 50 ℃ or higher, and is preferably less than 100 ℃, more preferably less than 90 ℃, still more preferably less than 85 ℃, particularly preferably less than 80 ℃ from the viewpoint of uniform drying property, drying speed, and the like.

The film dried by the final drying roll is preferably subjected to heat treatment in the manner described above. The heat treatment may 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 number of the heat treatment rollers may be 1 or more.

Since a PVA film having excellent hot water resistance and moderately crystallized can be obtained, the surface temperature of the heat treatment roller is preferably 90 ℃ or higher, more preferably 100 ℃ or higher, and still more preferably 110 ℃ or higher. In addition, from the viewpoint of improving the stretchability of the PVA film obtained, the surface temperature of the heat treatment roller is preferably 150 ℃ or less, more preferably 140 ℃ or less, and further preferably 130 ℃ or less.

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

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

The film obtained in the above manner may be further subjected to a humidity conditioning treatment, cutting of both end portions (edge portions) of the film, and the like, as necessary, and finally wound into a roll with a predetermined length to produce the PVA film of the present invention.

The volatile fraction of the PVA film finally obtained by the series of the above-described processes is preferably in the range of 1 to 5 mass%, more preferably in the range of 2 to 4 mass%.

[ use of PVA film ]

The use of the PVA film of the present invention is not particularly limited, and the PVA film of the present invention is preferably used as an initial film for producing an optical film such as a polarizing film or a retardation film because the PVA film of the present invention can produce an optical film having few optical defects. Such an optical film can be produced, for example, by subjecting the PVA film of the present invention to a treatment such as uniaxial stretching.

The method for producing a polarizing film using the PVA film of the present invention as a starting film is not particularly limited, and any conventionally employed method can be employed. Examples of such a method include dyeing and uniaxially stretching a PVA film, and uniaxially stretching a PVA film containing a dye. Specific methods for producing the polarizing film include a method of dyeing, uniaxially stretching, fixing, drying, and optionally further heat-treating the PVA film of the present invention. The order of dyeing and uniaxial stretching is not particularly limited, and dyeing may be performed before uniaxial stretching, dyeing may be performed simultaneously with uniaxial stretching, or dyeing may be performed after uniaxial stretching. In addition, the steps of uniaxial stretching, dyeing, etc. may be repeated a plurality of times. In particular, it is preferable to divide uniaxial stretching into 2 stages or more because uniform stretching is easy to carry out. When the PVA film has a laminate form as formed on a thermoplastic resin film, the uniaxially stretching is performed in a state in which the thermoplastic resin film is laminated, whereby the fracture during the uniaxial stretching can be further reduced.

As dyes for dyeing PVA films, 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) and the like can be used. These dyes may be used singly or in combination of 1 or more than 2. Dyeing can be usually performed by immersing the PVA film in a solution containing the dye, but the treatment conditions and treatment method thereof are not particularly limited.

The uniaxial stretching of the PVA film may be performed by either a wet stretching method or a dry heat stretching method, and from the viewpoint of the performance and stability of the quality of the obtained polarizing film, the wet stretching method is preferable. The wet stretching method includes a method in which a PVA film is stretched in an aqueous solution containing various components such as pure water, additives, and an aqueous medium, or in an aqueous dispersion in which various components are dispersed, and the uniaxial stretching method by the wet stretching method includes a method in which uniaxial stretching is performed in warm water containing boric acid, a method in which uniaxial stretching is performed in the dye-containing solution, a fixing treatment bath described later, and the like. The PVA film after water absorption may be uniaxially stretched in air, or may be uniaxially stretched by other methods. The uniaxial stretching is preferably performed along the flow direction of the PVA film.

The stretching temperature in the uniaxial stretching is not particularly limited, and a temperature in the range of 20 to 90 ℃, more preferably 25 to 70 ℃, still more preferably 30 to 65 ℃ is preferably used in the wet stretching, and a temperature in the range of 50 to 180 ℃ is preferably used in the dry stretching.

The stretching ratio of the uniaxial stretching (total stretching ratio in the case of performing the uniaxial stretching in a plurality of stages) is preferably as high as possible until the film is broken, more preferably 4 times or more, still more preferably 5 times or more, still more preferably 5.5 times or more, from the viewpoint of polarizing performance. The upper limit of the stretching ratio is not particularly limited as long as the film is not broken, and is preferably 8.0 times or less for uniform stretching.

The thickness of the uniaxially stretched film (polarizing film) is 1 to 30. Mu.m, particularly preferably 3 to 25. Mu.m. The thickness may be measured at any five positions and obtained as an average value thereof.

In the production of polarizing films, in order to firmly adsorb dyes to uniaxially stretched films, a fixing treatment is often performed. The fixing treatment is generally widely carried out by immersing the film in a fixing treatment bath to which boric acid and/or a boron compound is added. At this time, an iodine compound may be added to the treatment bath as needed.

Next, the film subjected to the uniaxial stretching treatment or the uniaxial stretching treatment and the fixing treatment is preferably subjected to a drying treatment (heat treatment). The temperature of the drying treatment (heat treatment) is preferably 30 to 150 ℃, particularly preferably 50 to 140 ℃. If the temperature of the drying treatment (heat treatment) is too low, the dimensional stability of the obtained polarizing film tends to be lowered, while if it is too high, deterioration of polarizing performance accompanied by decomposition of dye or the like tends to occur.

The polarizing plate can be produced by laminating an optically transparent protective film having mechanical strength on both surfaces or one surface of the polarizing film obtained in the above-described manner. As the protective film at this time, a cellulose Triacetate (TAC) film, a Cellulose Acetate Butyrate (CAB) film, an acrylic film, a polyester film, or the like can be used. As the adhesive for attaching the protective film, a PVA-based adhesive, a urethane-based adhesive, or the like is generally used, and among them, a PVA-based adhesive is preferably used.

The polarizing plate obtained in the above manner can be used as a member of a liquid crystal display device by being coated with an adhesive such as an acrylic adhesive and then bonded to a glass substrate. When the polarizing plate is bonded to the glass substrate, a retardation film, a viewing angle improving film, a brightness improving film, and the like can be bonded simultaneously.

Examples

The present invention will be specifically described below by way of examples, but the present invention is not limited to these examples. The measurement and evaluation methods used in the following examples, comparative examples and reference examples are shown below.

[ number of streaks in dotted line ]

A region of 60cm in the flow direction was set at an arbitrary position of the PVA film to be measured, and the thickness profile near the streak-like defect point, which is considered to be a broken streak, was measured using a scanning white interference microscope ("New View"7300, manufactured by zygo corporation), to confirm whether or not the streak-like defect point was a broken streak defined in the present invention. In this way, the number of the broken lines of the straight line crossing the width direction of any one of the portions in the flow direction in the above-mentioned region portion is measured over the entire width direction of the straight line, and the number of the broken lines of the average 1m width is obtained by dividing the number by the width of the PVA film.

[ optical defect of polarizing film ]

After the polarizing film was properly divided in the width direction, it was placed between polarizing plates for observation (obtained by overlapping 2 parallel nicols and having a polarization degree of 99.99% or more) in the vertical direction, and the degree of optical defect was visually observed and evaluated by the following criteria.

O: optical defects are completely or hardly observed

X: can easily find optical defects

X×: optical defects can be found very easily.

Example 1

[ production of PVA film ]

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 part by mass of lauric acid diethanolamide and water were discharged from a T-slot die at a discharge rate of 2.8 m/min (S ₀ ) In a drying machine provided with a plurality of drying rolls and heat stations with their axes of rotation parallel to each otherThe 1 st drying roll (peripheral speed (S) ₁ ) 17 m/min), drying was performed on the 1 st drying roller while blowing hot air at 90 ℃ at a wind speed of 5 m/sec to the whole non-contact surface of the 1 st drying roller, and then, peeling was performed from the 1 st drying roller, further drying was performed between the 2 nd drying roller and the final drying roller immediately before the heat treatment roller so that the front surface and the rear surface at any portion of the film were alternately opposed to the respective drying rollers, and then, peeling was performed from the final drying roller. Subsequently, the film was heat-treated by a heat-treating roller having a surface temperature of 120℃to cut both end portions (edge portions), and then wound around a cylindrical core, thereby finally producing a long PVA film (single-layer film) having a thickness of 30.3 μm, a length of 2,000m, a width of 2.6m, and a volatile fraction (water fraction) of 2 mass%.

When the number of broken lines was measured by the above method, it was found that 6 broken lines were present in the PVA film, which were arranged approximately linearly across 10cm or more in the film flow direction (2.3 lines in the width direction of the average 1m film), with the height difference of 50 to 60nm, and the period of 0.3 to 0.7 mm. The results are shown in Table 1.

[ production of polarizing film ]

The PVA film was wound out and each treatment was performed simultaneously to continuously produce a polarizing film having a thickness of 12 μm.

Specifically, a PVA film was uniaxially stretched (stage 1 stretching) to 2.2 times the original length in the flow direction (MD) during immersion in water at 30 ℃, then uniaxially stretched (stage 2 stretching) to 3.3 times the original length in the flow direction (MD) during immersion in an aqueous solution of iodine/potassium iodide at 30 ℃ containing iodine at 0.03 mass% and potassium iodide at 3 mass%, then uniaxially stretched (stage 3 stretching) to 3.6 times the original length in the flow direction (MD) during immersion in an aqueous solution of boric acid/potassium iodide at 30 ℃ containing boric acid at 3 mass% and potassium iodide at 3 mass%, then, further, an aqueous solution of potassium iodide at 30 ℃ containing potassium at 3 mass% was dried for 4 minutes after immersion in an aqueous solution of boric acid/potassium iodide at 63 ℃ containing potassium iodide at 4 mass% and 5 mass% was dried, and then a polarizing film was produced by drying for 60 minutes.

For this polarizing film, the optical defect was evaluated by the above method. The results are shown in Table 1.

Example 2

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 part by mass of lauric acid diethanolamide and water were discharged from a T-slot die at a discharge rate of 2.8 m/min (S ₀ ) In a 1 st drying roll (peripheral speed (S) of a film forming apparatus having a plurality of drying rolls and heat treatment rolls with their rotation axes parallel to each other ₁ ) 17 m/min), drying was performed on the 1 st drying roller while blowing hot air at 90 ℃ at a wind speed of 5 m/sec to the whole non-contact surface of the 1 st drying roller, and then, peeling was performed from the 1 st drying roller, further drying was performed between the 2 nd drying roller and the final drying roller immediately before the heat treatment roller so that the front surface and the rear surface at any portion of the film were alternately opposed to the respective drying rollers, and then, peeling was performed from the final drying roller. Subsequently, both ends (edge portions) were cut by heat treatment with a heat treatment roller having a surface temperature of 115 ℃, and then wound around a cylindrical core, thereby finally producing a long PVA film (single-layer film) having a thickness of 40.6 μm, a length of 2,000m, a width of 2.6m, and a volatile fraction (water fraction) of 2 mass%.

When the number of broken lines was measured by the above method, it was found that 5 broken lines were present in the PVA film, which were arranged approximately linearly across 10cm or more in the film flow direction (1.9 broken lines in the width direction of the average 1m film), with the height difference of 50 to 60nm, and the period of 0.3 to 0.7 mm. Further, using the obtained PVA film, a polarizing film having a thickness of 17 μm was produced in the same manner as in example 1, and the optical defect was evaluated by the above-described method. The results are shown in Table 1.

Comparative example 1

Comprises PVA obtained by saponifying polyvinyl acetate (average degree of polymerization 2,400, degree of saponification 99.9 mol%) 100 parts by mass, 12 parts by mass of glycerol, 0.1 part by mass of lauric acid diethanolamide and water, and the evaporation fraction was 66% by mass, were discharged from the T-slit die at a discharge rate of 2.4 m/min (S ₀ ) In a 1 st drying roll (peripheral speed (S) of a film forming apparatus having a plurality of drying rolls and heat treatment rolls with their rotation axes parallel to each other ₁ ) 18 m/min), drying was performed on the 1 st drying roll while blowing hot air at 90 ℃ at a speed of 5 m/sec to the entire non-contact surface of the 1 st drying roll, followed by peeling from the 1 st drying roll, further drying between the 2 nd drying roll and the final drying roll immediately before the heat treatment roll so that the front and rear surfaces at any portion of the film alternately face each other with respect to the drying rolls, and then peeling from the final drying roll. Subsequently, both ends (edge portions) were cut by heat treatment with a heat treatment roller having a surface temperature of 115 ℃, and then wound around a cylindrical core, thereby finally producing a long PVA film (single-layer film) having a thickness of 29.6 μm, a length of 2,000m, a width of 2.6m, and a volatile fraction (water fraction) of 2 mass%.

When the number of broken lines was measured by the above method, it was found that 24 broken lines were present in the PVA film, the broken lines being arranged approximately straight across 10cm or more in the film flow direction (9.2 lines in the width direction of the average 1m film) with a period of 0.3 to 0.7mm for the irregularities having a height difference of 60 to 70 nm. Further, using the obtained PVA film, a polarizing film having a thickness of 12 μm was produced in the same manner as in example 1, and the optical defect was evaluated by the above-described method. The results are shown in Table 1.

Comparative example 2

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 part by mass of lauric acid diethanolamide and water were discharged from a T-slot die at a discharge rate of 2.0 m/min (S ₀ ) In a 1 st drying roll (peripheral speed (S) of a film forming apparatus having a plurality of drying rolls and heat treatment rolls with their rotation axes parallel to each other ₁ ) 15 m/min), and on the 1 st drying roller, heat at 90℃was blown at a speed of 5 m/sec to the whole non-contact surface of the 1 st drying rollerThe drying is performed while the air is running, and then the film is peeled off from the 1 st drying roll, further dried between the 2 nd drying roll and the final drying roll immediately before the heat treatment roll so that the front and rear surfaces at any portion of the film alternately face each other. Subsequently, both ends (edge portions) were cut by heat treatment with a heat treatment roller having a surface temperature of 115 ℃, and then wound around a cylindrical core, thereby finally producing a long PVA film (single-layer film) having a thickness of 30.3 μm, a length of 2,000m, a width of 2.6m, and a volatile fraction (water fraction) of 2 mass%.

When the number of broken lines was measured by the above method, it was found that 37 broken lines were present in the PVA film, the broken lines being arranged approximately straight across 10cm or more in the film flow direction (14.2 lines in the width direction of the average 1m film) with a period of 0.3 to 0.7mm for the irregularities having a height difference of 80 to 90 nm. Further, using the obtained PVA film, a polarizing film having a thickness of 12 μm was produced in the same manner as in example 1, and the optical defect was evaluated by the above-described method. The results are shown in Table 1.

Reference example

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 part by mass of lauric acid diethanolamide and water were discharged from a T-slot die at a discharge rate of 2.4 m/min (S ₀ ) In a 1 st drying roll (peripheral speed (S) of a film forming apparatus having a plurality of drying rolls and heat treatment rolls with their rotation axes parallel to each other ₁ ) 11 m/min), drying is performed on the 1 st drying roller while blowing hot air at 90 ℃ at a speed of 5 m/sec to the entire non-contact surface of the 1 st drying roller, and then the film is peeled off from the 1 st drying roller, further dried between the 2 nd drying roller and the final drying roller immediately before the heat treatment roller so that the front and rear surfaces at any portion of the film alternately face each other with respect to the drying rollers, and then peeled off from the final drying roller. Then, the sheet was heat-treated with a heat-treating roller having a surface temperature of 105℃to cut both end portions (edge portions), and then wound around a cylindrical core, thereby finally producing a sheet having a thickness of 74.5. Mu.m, a length of 2,000m, and a width 2.6m, and a 2 mass% volatile fraction (water fraction).

The number of the broken lines was measured by the above method for the PVA film, but the broken lines were not found. The results are shown in Table 1.

TABLE 1

Claims

1. The method for producing a polyvinyl alcohol polymer film having a thickness of 55 [ mu ] m or less comprises using a film-forming apparatus having a plurality of drying rolls having rotation axes parallel to each other, ejecting a film-forming stock solution containing a polyvinyl alcohol polymer onto a 1 st drying roll of the film-forming apparatus in a film form and drying the film-forming stock solution, and further drying the film-forming stock solution by a drying roll following a 2 nd drying roll of the film-forming apparatus to produce a polyvinyl alcohol polymer film, wherein the ejection speed S of the film-forming stock solution is set to ₀ Reaching 2.5-5.0 m/min, wherein the circumferential speed S of the 1 st drying roller is set ₁ Discharge speed S relative to film-forming stock solution ₀ Ratio S of ₁ /S ₀ Reaching a value of more than 5 and less than 7,

the polyvinyl alcohol polymer film has 3 or less lines of broken lines, on average, of 1m in the width direction of the film, wherein the lines of broken lines are arranged in a substantially straight line across 10cm or more in the flow direction of the film, with a period of 0.01 to 10mm of irregularities.

2. The method according to claim 1, wherein the polyvinyl alcohol polymer film having a width of 2m or more is produced.

3. The polyvinyl alcohol polymer film produced by the production method according to claim 1 or 2.

4. An optical film produced from the polyvinyl alcohol polymer film according to claim 3.

5. The optical film of claim 4 which is a polarizing film.

6. A method for producing an optical film, comprising: a step of monoaxially stretching the polyvinyl alcohol film according to claim 3.

7. The method according to claim 6, which is a method for producing a polarizing film.