TWI881988B - Polyvinyl alcohol film, stretched film, polarizing film, and method for producing polyvinyl alcohol film - Google Patents

Abstract

A PVA film is provided, which is a PVA film with excellent stretching processability and productivity, and can obtain a stretched film and a polarizing film with excellent formability; a stretched film and a polarizing film made from such a PVA film; and a method for making such a PVA film. A polyvinyl alcohol film comprises polyvinyl alcohol (A) and a modified conjugated diene polymer (B) having a hydrophilic group.

Description

Polyvinyl alcohol film, stretched film, polarizing film, and method for producing polyvinyl alcohol film

The present invention relates to a polyvinyl alcohol film, a stretched film, a polarizing film, and a method for manufacturing the polyvinyl alcohol film.

Polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA") is used in a wide range of application fields such as packaging film, water-soluble film, agricultural film, release film, optical film, etc.

PVA film is harder than other plastic films when it does not contain plasticizers, and mechanical properties such as impact strength and the passability of secondary processing such as stretching become problems. In order to prevent these problems, most PVA films are usually used to improve flexibility by adding plasticizers. In particular, when using PVA film as a raw material for polarizing film, high stretchability is required during stretching, so plasticizers are added to improve stretching processability. However, this kind of PVA film containing plasticizers will reduce the plasticizer over time, and there is a problem of reduced stretching processability.

As a method for imparting good elongation processability other than adding a plasticizer, in the case of PVA fibers, a method of adding an emulsified dispersion of ethylene/vinyl acetate copolymer to the spinning stock solution has been proposed (see Patent Document 1). [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 47-42050

[Problems to be solved by the invention]

On the other hand, as the use of PVA film, there are those that use PVA film that has been subjected to various stretching treatments. In such uses, since high stretchability is required for the PVA film and the residual stress that causes deformation after processing is reduced, it is important to have low tensile stress.

However, in the method of adding the emulsified dispersion of ethylene/vinyl acetate copolymer, it is difficult to provide high elongation to the PVA film while reducing its tensile stress, and the elongation processability of the PVA film is not sufficient. In addition, the tensile stress of the PVA film tends to increase as the deformation speed of the PVA film increases, so a PVA film with a small tensile stress even if the deformation speed is high is required.

In addition, polarizing films obtained by stretching PVA films are used for optical purposes, mainly for polarizing plates and sunglasses for liquid crystal displays. However, in recent years, in the use of folding displays and highly designed sunglasses that further deform polarizing films, it is known that polarizing films will produce parallel cracks in the stretching direction. The faster the deformation speed of the polarizing film, the more obvious this problem will be. Therefore, a polarizing film with excellent formability that will not produce cracks even if it is deformed under the condition of a high deformation speed is required.

Here, the inventors learned that by adding a rubber component to PVA, the stretching processability of the resulting PVA film, and the formability of the stretched film and polarized film will be improved. However, in the case where the rubber component has been added, when the film-making stock solution is cast on the surface of a substrate such as a metal drum to produce the PVA film, a new problem of the rubber component adhering to the substrate such as the metal drum will occur. If the amount of rubber component adhering to the substrate such as the metal drum increases, it may become dirty and adhere to the PVA film. Therefore, it is necessary to remove the rubber component that has been attached to the substrate such as the metal drum by washing at any time. However, in the case of using, for example, isoprene rubber as the rubber component, it may be difficult to fully remove it by washing with water. In this way, the rubber component (adherent matter) attached to the base material such as the metal drum may be difficult to remove even by cleaning, and the productivity may be reduced due to the labor of cleaning, which is not preferable.

The present invention is completed based on the above situation, and its purpose is to provide a PVA film that is excellent in stretching processability and productivity, and can obtain a stretched film and a polarizing film with excellent formability; a stretched film and a polarizing film made from such a PVA film; and a method for manufacturing such a PVA film. [Means for solving the problem]

The above-mentioned object is achieved by providing the following: [1] A polyvinyl alcohol film comprising polyvinyl alcohol (A) and a modified conjugated diene polymer (B) having a hydrophilic group; [2] The polyvinyl alcohol film as in [1], wherein the content of the modified conjugated diene polymer (B) is not less than 10 parts by mass and not more than 80 parts by mass relative to 100 parts by mass of the polyvinyl alcohol (A); [3] The polyvinyl alcohol film as in [1] or [2], wherein The degree of polymerization of polyvinyl alcohol (A) is 1,000 or more and 10,000 or less, and the degree of saponification is 95 mol% or more; [4] The polyvinyl alcohol film of any one of [1] to [3], wherein the weight average molecular weight of the modified covalent diene polymer (B) is 5,000 or more and 80,000 or less; [5] The polyvinyl alcohol film of any one of [1] to [4], wherein the hydrophilic group is a non-ionic polar group. [6] A stretched film obtained from a polyvinyl alcohol film as described in any one of [1] to [5]; [7] A polarizing film obtained from a polyvinyl alcohol film as described in any one of [1] to [5] or a stretched film as described in [6]; [8] A method for producing a polyvinyl alcohol film, comprising the following steps: using a mixture of polyvinyl alcohol (A) and a modified conjugated diene polymer (B) having a hydrophilic group; [9] A method for producing a polyvinyl alcohol film as described in [8], wherein the content of the modified covalent diene polymer (B) in the film-forming solution is 10 parts by mass or more and 80 parts by mass or less relative to 100 parts by mass of polyvinyl alcohol (A); [10] A method for producing a polyvinyl alcohol film as described in [8] or [9], wherein the hydrophilic group is a grafted chain having a repeating structure containing a non-ionic polar group. [Effect of the invention]

According to the present invention, a PVA film can be provided, which is a PVA film with excellent stretching processability and productivity, and can obtain a stretched film and a polarizing film with excellent formability; a stretched film and a polarizing film manufactured from such a PVA film; and a manufacturing method of such a PVA film.

[Form used to implement the invention]

Hereinafter, the PVA film, its manufacturing method, stretched film, and polarizing film of the present invention are described in detail.

<PVA film> The PVA film of one embodiment of the present invention comprises PVA (A) and a modified conjugated diene polymer (B) having a hydrophilic group. The PVA film is usually a film that has not been stretched (unstretched film). As described in detail below, a stretched film is obtained by stretching the PVA film. In addition, a polarizing film is obtained by dyeing the PVA film or the stretched film.

The PVA film has excellent stretching processability due to the inclusion of the modified conjugated diene polymer (B), and can also obtain a stretched film and a polarized film with excellent formability. Furthermore, since the modified conjugated diene polymer (B) contained in the PVA film has a hydrophilic group, even if the modified conjugated diene polymer (B) is attached to a substrate such as a metal drum, the attached substance can be easily removed by water or the like. Therefore, the productivity of the PVA film is also excellent.

[PVA(A)] PVA (polyvinyl alcohol) (A) is usually the main component of the PVA film. PVA(A) is a polymer having a vinyl alcohol unit ( _-CH2 -CH(OH)-) as a main structural unit. In addition, the so-called main structural unit refers to the structural unit that accounts for the largest proportion of all structural units, preferably a proportion of 50 mol% or more of all structural units (hereinafter, the same applies to "main structural unit"). In addition to the vinyl alcohol unit, PVA(A) may also have a vinyl ester unit and other units.

As PVA (A), polyvinyl ester obtained by saponifying one or more kinds of polymerized vinyl esters can be used. Examples of vinyl esters include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl trimethylacetate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropylene acetate. Among the vinyl esters, compounds having a vinyloxycarbonyl group ( _H2C =CH-O-CO-) in the molecule are preferred from the viewpoints of ease of production, ease of acquisition, and cost, and vinyl acetate is more preferred.

The polyvinyl ester is preferably obtained using only one or two or more vinyl esters as monomers, and more preferably obtained using only one vinyl ester as monomer. As long as the effects of the present invention are not seriously impaired, it may also be a copolymer resin of one or two or more vinyl esters and other monomers copolymerizable therewith.

The upper limit of the proportion of structural units derived from other copolymerizable monomers is preferably 15 mol%, more preferably 10 mol%, more preferably 5 mol%, and more preferably 1 mol, based on the molar number of all structural units constituting the copolymer resin. The lower limit of the proportion of vinyl alcohol units in all structural units in PVA (A) obtained by saponifying polyvinyl ester is preferably 85 mol%, more preferably 90 mol%, more preferably 95 mol%, and more preferably 99 mol%.

Examples of other monomers copolymerizable with vinyl esters include α-olefins having 2 to 30 carbon atoms, such as ethylene, propylene, 1-butene, and isobutylene; (meth)acrylic acid or its salts; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tertiary butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide; N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, (meth)acrylamide propane sulfonic acid or its salts, (meth)acrylamide propyl (Meth)acrylamide derivatives such as dimethylamine or its salts, N-hydroxymethyl (meth)acrylamide or its derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tertiary butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether; vinyl cyanide such as (meth)acrylonitrile; halogenated vinyl such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid, or its salts, esters or anhydrides; itaconic acid, or its salts, esters or anhydrides; vinyl silyl compounds such as vinyltrimethoxysilane; unsaturated sulfonic acids or their salts, etc.

The polyvinyl ester may have one or more structural units derived from the above-mentioned monomers.

As PVA(A), it is preferred to use one that has not been graft copolymerized. However, as long as the effect of the present invention is not seriously impaired, PVA(A) may be modified by one or more graft copolymerizable monomers. Graft copolymerization can be performed on at least one of polyvinyl ester and PVA obtained by saponifying the polyvinyl ester. Examples of graft copolymerizable monomers include: unsaturated carboxylic acids or their derivatives; unsaturated sulfonic acids or their derivatives; α-olefins having 2 to 30 carbon atoms, etc. The proportion of structural units derived from graft copolymerizable monomers in polyvinyl ester or PVA(A) is preferably 5 mol% or less based on the molar number of all structural units constituting polyvinyl ester or PVA(A).

A part of the hydroxyl groups of PVA(A) may be crosslinked or not crosslinked. In addition, a part of the hydroxyl groups of PVA(A) may react with an aldehyde compound such as acetaldehyde or butyraldehyde to form an acetal structure.

The lower limit of the degree of polymerization of PVA(A) is preferably 1,000, more preferably 1,500, and even more preferably 1,700. When the degree of polymerization of PVA(A) is greater than the above lower limit, the toughness of the PVA film, the resulting stretched film, and the polarizing film can be improved. On the other hand, the upper limit of this degree of polymerization is preferably 10,000, more preferably 8,000, and even more preferably 5,000. When the degree of polymerization of PVA(A) is less than the above upper limit, the increase in the manufacturing cost of PVA(A) and the occurrence of defects during film formation can be suppressed. In addition, the degree of polymerization of PVA(A) means the average degree of polymerization measured in accordance with the description of JIS K6726-1994.

The lower limit of the saponification degree of PVA (A) may be, for example, 80 mol%, but preferably 95 mol%, more preferably 98 mol%, and still more preferably 99 mol%. By setting the saponification degree to the above lower limit, the effect of the present invention can be more fully exerted. In the case of use as a water-soluble film, PVA with a lower saponification degree may also be used. On the other hand, the upper limit of this saponification degree may also be 100 mol%. The so-called saponification degree of PVA (A) refers to the ratio (mol %) of the molar number of vinyl alcohol units relative to the total molar number of structural units (typically vinyl ester units) and vinyl alcohol units that can be converted into vinyl alcohol units by saponification. The saponification degree can be measured in accordance with the description of JIS K6726-1994.

As the lower limit of the content of PVA(A) in the PVA film, it is also preferably 60% by mass, more preferably 65% by mass, and further preferably 70% by mass, 75% by mass, 80% by mass, or 85% by mass. By setting the content of PVA(A) to be above the above lower limit, the characteristics of PVA(A) can be fully exerted, and the transparency and smoothness of the resulting stretched film and polarizing film can be improved. On the other hand, as the upper limit of this content, it is also preferably 99% by mass, more preferably 95% by mass, further preferably 90% by mass, and further preferably 85% by mass, 80% by mass, 75% by mass, or 70% by mass. By setting the content of PVA(A) to be below the above upper limit, the stretching processability, the formability of the resulting stretched film and polarizing film, etc. can be improved.

[Modified covalent diene polymer (B)] The modified covalent diene polymer (B) is a covalent diene polymer having a hydrophilic group. The modified covalent diene polymer (B) may also be a covalent diene rubber having a hydrophilic group. For example, the modified covalent diene polymer (B) is obtained by modifying an unmodified covalent diene polymer (B') (unmodified covalent diene rubber) to introduce a hydrophilic group.

The modified covalent diene polymer (B) contains a covalent diene unit as a monomer unit constituting the polymer (usually the main chain of the polymer). That is, the modified covalent diene polymer (B) is a polymer using a covalent diene as a monomer. As the covalent diene, for example, butadiene, isoprene, 2,3-dimethylbutadiene, 2-phenylbutadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene, 1,3-hexadiene, 1,3-octadiene, 1,3-cyclohexadiene, 2-methyl-1,3-octadiene, 1,3,7-octatriene, laurylene, α-mycochlorophene, β-mycochlorophene, chloroprene, etc. Among them, butadiene and isoprene are preferred, and isoprene is more preferred. Furthermore, the preferred structure of the main chain of the modified conjugated diene polymer (B) (such as the composition of monomer units) may be the structure of the unmodified conjugated diene polymer (B') described below.

The hydrophilic group possessed by the modified conjugated diene polymer (B) may be, for example, a group containing at least one of an oxygen atom and a nitrogen atom, and may include a hydroxyl group, a carboxyl group, an ester group (-COO-), an ether group (-O-), an amide group, a sulfonic acid group, an acid anhydride group, or a group containing a plurality of one or more of these groups.

As the hydrophilic group, a graft chain having a repeating structure including a nonionic polar group is preferred. Examples of the nonionic polar group include hydroxyl groups, ester groups, ether groups, amide groups, and acid anhydride groups, and ether groups are preferred. The so-called graft chain does not specify its synthesis method, but refers to a chain directly connected to the main chain or connected via other groups.

Examples of the repeating structure containing a nonionic polar group include a polyalkylene glycol structure represented by -(CR ² R ³ -CR ⁴ R ⁵ -O)- and a poly(meth)acrylate structure represented by -(CH ₂ -CR ⁷ COOR ⁸ )-. The polyalkylene glycol structure is preferred.

^R2 , ^R3 , ^R4 and ^R5 are each independently a hydrogen atom or a monovalent hydrocarbon group having 1 to 6 carbon atoms. ^R7 is a hydrogen atom or a methyl group. ^R8 is a monovalent hydrocarbon group having 1 to 6 carbon atoms. A part or all of the hydrogen atoms of the hydrocarbon group represented by ^R8 may be substituted with a hydrophilic group.

Examples of the monovalent hydrocarbon group having 1 to 6 carbon atoms represented by ^R2 , ^R3 , ^R4 , ^R5 and ^R8 include alkyl groups such as methyl, ethyl and propyl, aliphatic hydrocarbon groups such as alkenyl groups such as vinyl, alicyclic groups such as cyclohexyl, and aromatic hydrocarbon groups such as phenyl. Among them, aliphatic hydrocarbon groups are preferred, and alkyl groups are more preferred. Each of ^R2 , ^R3 , ^R4 and ^R5 is preferably a hydrogen atom or a methyl group, and is more preferably a hydrogen atom.

The graft chain having a polyalkylene glycol structure is a hydrophilic group having a plurality of ether groups. The graft chain having a polyalkylene glycol structure can be represented by the following formula (A), for example.

-(CR ² R ³ -CR ⁴ R ⁵ -O) _n -R ⁶ ... (A) In formula (A), R ² , R ³ , R ⁴ and R ⁵ are each independently a hydrogen atom or a monovalent hydrocarbon group having 1 to 6 carbon atoms. ^{R 6} is a hydrogen atom, a monovalent hydrocarbon group having 1 to 6 carbon atoms or an acyl group having 1 to 6 carbon atoms. n is an integer of 3 to 600.

Each of R ² , R ³ , R ⁴ and R ⁵ is preferably a hydrogen atom or a methyl group, more preferably a hydrogen atom. R ⁶ is preferably a monovalent hydrocarbon group, more preferably an alkyl group, more preferably a methyl group and an ethyl group, and still more preferably a methyl group. The lower limit of n is preferably 5, more preferably 7. The upper limit of n is preferably 200, more preferably 100, and still more preferably 40.

The graft chain having a poly(meth)acrylate structure can be represented by, for example, the following formula (B).

-(CH ₂ -CR ⁷ COOR ⁸ ) _m -R ⁹ ... (B) In formula (B), R ⁷ is a hydrogen atom or a methyl group. R ⁸ is a monovalent hydrocarbon group having 1 to 6 carbon atoms. A part or all of the hydrogen atoms of the hydrocarbon group represented by R ⁸ may be substituted with a hydrophilic group. R ⁹ is a hydrogen atom or a monovalent organic group. m is an integer of 3 to 600. The so-called organic group refers to a group containing carbon atoms, and examples thereof include a hydrocarbon group, an alkoxy group, an acyl group, a carboxyl group, or a group composed of these groups.

Among the graft chains having a repeating structure containing a nonionic polar group, a graft chain having a polyalkylene glycol structure is more preferred.

The average number of hydrophilic groups per molecule of the modified conjugated diene polymer (B) is preferably 1 to 30, more preferably 1 to 25, and even more preferably 1 to 20. Within the above range, the washability of deposits from the PVA film, that is, the productivity, is further improved.

The average number of hydrophilic groups per molecule of the modified conjugated diene polymer (B) can be obtained from the equivalent weight (g/hydrophilic group-mol) of the hydrophilic group of the modified conjugated diene polymer (B) and the number average molecular weight Mn converted to polystyrene by the following formula. (Average number of hydrophilic groups per molecule) = [(number average molecular weight Mn)/(molecular weight of styrene unit) × (average molecular weight of other monomer units other than conjugated diene and conjugated diene contained as needed)]/(equivalent weight of hydrophilic group)

The equivalent weight of the hydrophilic group of the modified conjugated diene polymer (B) means the mass of isoprene, butadiene and other monomers contained as required per 1 mol of the hydrophilic group. The equivalent weight of the hydrophilic group can be calculated from the area ratio of the peak derived from the hydrophilic group to the peak derived from the polymer main chain using ¹ H-NMR or ¹³ C-NMR.

In the modified conjugated diene polymer (B), the position for introducing the hydrophilic group may be the polymerization terminal or the side chain of the polymer chain, but from the viewpoint of facilitating the introduction of multiple hydrophilic groups, the side chain of the polymer chain is preferred. In addition, the hydrophilic group may include only one type or two or more types. That is, the modified conjugated diene polymer (B) may be modified by one type of modifying compound or by two or more types of modifying compounds.

The melt viscosity of the modified covalent diene polymer (B) measured at 38°C is preferably 0.1 to 4,000 Pa･s, more preferably 1 to 3,500 Pa･s, and even more preferably 1 to 3,000 Pa･s. By setting the melt viscosity of the modified covalent diene polymer (B) within the above range, the stretching processability of the obtained PVA film, the formability of the stretched film and the polarizing film, etc. are improved. In addition, the melt viscosity of the modified covalent diene polymer (B) is a value measured by a Brookfield viscometer at 38°C.

The weight average molecular weight (Mw) of the modified covalent diene polymer (B) is preferably 5,000 to 80,000, more preferably 6,000 to 50,000, and even more preferably 7,000 to 30,000. The Mw of the modified covalent diene polymer (B) is the weight average molecular weight converted to polystyrene obtained by gel permeation chromatography (GPC). By setting the Mw of the modified covalent diene polymer (B) within the above range, the processability during manufacturing is excellent and the economy becomes good. In addition, the stretching processability of the PVA film, the formability of the stretched film and the polarizing film, etc. will be improved. In addition, two or more modified covalent diene polymers (B) with different Mw can also be used in combination.

The molecular weight distribution (Mw/Mn) of the modified conjugated diene polymer (B) is preferably 1.0 to 20.0, more preferably 1.0 to 15.0, and even more preferably 1.0 to 10.0. By setting Mw/Mn to the above range, the viscosity deviation of the modified conjugated diene polymer (B) becomes smaller, which is preferred. In addition, the molecular weight distribution (Mw/Mn) means the ratio of the weight average molecular weight (Mw)/number average molecular weight (Mn) converted to standard polystyrene obtained by GPC measurement.

The vinyl content of the modified covalent diene polymer (B) is 0 mol% or more and less than 100 mol%, preferably 1 mol% or more and less than 70 mol%, and more preferably 3 mol% or more and less than 50 mol%. The so-called "vinyl content" means the total mol% of covalent diene units bonded by 1,2-bonding or 3,4-bonding (covalent diene units bonded by other than 1,4-bonding) in the total 100 mol% of the covalent diene units contained in the modified liquid diene polymer (B). The vinyl content can be calculated from the area ratio of the peak derived from the covalent diene unit bonded by 1,2-bonding or 3,4-bonding to the peak derived from the covalent diene unit bonded by 1,4-bonding using ¹ H-NMR. The vinyl content of the modified covalent diene polymer (B) can be set to a desired value by, for example, controlling the type of solvent used when producing the unmodified covalent diene polymer (B'), the polar compound used as needed, the polymerization temperature, and the like.

The glass transition temperature (Tg) of the modified covalent diene polymer (B) is preferably -150 to 50° C., more preferably -130 to 50° C., and even more preferably -130 to 30° C. By setting Tg within the above range, the stretching processability of the PVA film and the formability of the stretched film are further improved.

The modified covalent diene polymer (B) may be used alone or in combination of two or more.

The lower limit of the content of the modified covalent diene polymer (B) in the PVA film may be, for example, 5 parts by mass, relative to 100 parts by mass of PVA (A), but may preferably be 10 parts by mass, more preferably 15 parts by mass, and still more preferably 20 parts by mass, 25 parts by mass, 30 parts by mass, or 35 parts by mass. By setting the content of the modified covalent diene polymer (B) to be greater than the lower limit, the stretching processability of the PVA film and the formability of the stretched film and polarizing film are further improved.

The upper limit of the content of the modified conjugated diene polymer (B) in the PVA film may be, for example, 100 parts by mass relative to 100 parts by mass of PVA, but may also be preferably 80 parts by mass, more preferably 60 parts by mass, and still more preferably 40 parts by mass, 30 parts by mass, or 20 parts by mass. By setting the content of the modified conjugated diene polymer (B) to be below the upper limit, the stretching processability of the PVA film may be further optimized. In addition, by setting the content of the modified conjugated diene polymer (B) to be below the upper limit, transparency and other optical properties, surface characteristics, etc. may be improved.

(Synthesis method of modified conjugated diene polymer (B)) The synthesis method of the modified conjugated diene polymer (B) is not particularly limited, but it can be obtained, for example, by acid-modifying the unmodified conjugated diene polymer (B') and further grafting it with the compound (X) described later as needed.

In the unmodified covalent diene polymer (B') used as a raw material, it is preferred that 50% by mass or more of all monomer units constituting the polymer are monomer units of butadiene and/or isoprene. The total content of butadiene units and isoprene units relative to all monomer units of the unmodified covalent diene polymer (B') is preferably 60 to 100% by mass, and more preferably 70 to 100% by mass.

Examples of other monomer units other than butadiene units and isoprene units that may be contained in the unmodified conjugated diene polymer (B') include conjugated diene units other than butadiene and isoprene described above, and aromatic vinyl compound units.

Examples of the aromatic vinyl compound to be provided as the aromatic vinyl compound unit include styrene, α-methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 4-propylstyrene, 4-tert-butylstyrene, 4-cyclohexylstyrene, 4-dodecylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 2,4,6-trimethylstyrene, 2-ethyl-4-benzylstyrene, 4-(phenylbutyl)styrene, 1-vinylnaphthalene, 2-vinylnaphthalene, vinylanthracene, N,N-diethyl-4-aminoethylstyrene, vinylpyridine, 4-methoxystyrene, monochlorostyrene, dichlorostyrene, divinylbenzene, etc. Among these, styrene, α-methylstyrene and 4-methylstyrene are preferred.

The content of monomer units other than butadiene units and isoprene units in the unmodified co-polymerized diene polymer (B') is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less. For example, when the content of the vinyl aromatic compound unit is below the above range, the stretching processability of the PVA film and the formability of the stretched film and polarizing film tend to be improved.

The unmodified covalent diene polymer (B') is preferably a polymer obtained by polymerizing a covalent diene and other monomers contained as required, for example, by emulsion polymerization, solution polymerization or the like.

As the emulsion polymerization method, a known method can be applied or in accordance with a known method. For example, a monomer containing a predetermined amount of a covalent diene is emulsified and dispersed in the presence of an emulsifier, and emulsion polymerization is performed using a free radical polymerization initiator.

Examples of emulsifiers include long-chain fatty acid salts and rosin acid salts having a carbon number of 10 or more. Examples of long-chain fatty acid salts include potassium salts or sodium salts of fatty acids such as capric acid, lauric acid, myristic acid, palmitic acid, oleic acid, and stearic acid.

As a dispersant, water is usually used, and a water-soluble organic solvent such as methanol or ethanol may also be used as long as it does not hinder the stability during polymerization. As free radical polymerization initiators, for example, persulfates such as ammonium persulfate and potassium persulfate, organic peroxides, hydrogen peroxide, etc. can be listed.

In order to adjust the molecular weight of the unmodified conjugated diene polymer (B'), a chain transfer agent may be used. Examples of the chain transfer agent include thiols such as tert-dodecyl mercaptan and n-dodecyl mercaptan; carbon tetrachloride, thioglycolic acid, diterpene, terpinolene, γ-terpinene, α-methylstyrene dimer, and the like.

The temperature of the emulsion polymerization can be appropriately set according to the type of radical polymerization initiator used, but is usually in the range of 0 to 100° C., preferably in the range of 0 to 60° C. The polymerization mode may be either continuous polymerization or batch polymerization.

The polymerization reaction can be stopped by adding a polymerization terminator. Examples of the polymerization terminator include amine compounds such as isopropylhydroxylamine, diethylhydroxylamine, and hydroxylamine, quinone compounds such as hydroquinone and benzoquinone, and sodium nitrate.

After the polymerization reaction stops, an anti-aging agent may be added as needed. After the polymerization reaction stops, unreacted monomers are removed from the obtained latex as needed. Then, a salt such as sodium chloride, calcium chloride, potassium chloride, etc. is used as a coagulant, and an acid such as nitric acid and sulfuric acid is added as needed to adjust the pH of the coagulation system to a specified value while coagulating the unmodified conjugated diene polymer (B'). Thereafter, the polymer is recovered by separating the dispersion medium. Then, after washing with water and dehydration, the unmodified conjugated diene polymer (B') is obtained by drying. Furthermore, during solidification, the latex may be mixed with an extender oil that has become an emulsified dispersion as required, and the extended unmodified covalent diene polymer (B') may be recovered in the form of the oil-extended unmodified covalent diene polymer (B').

As the solution polymerization method, a known method or a method according to known methods can be applied. For example, a monomer including a conjugated diene can be polymerized in a solvent using a Ziegler catalyst, a metallocene catalyst, an active metal or an active metal compound capable of anionic polymerization, and in the presence of a polar compound as needed.

Examples of the solvent include aliphatic hydrocarbons such as n-butane, n-pentane, isopentane, n-hexane, n-heptane, isooctane, etc.; alicyclic hydrocarbons such as cyclopentane, cyclohexane, methylcyclopentane, etc.; and aromatic hydrocarbons such as benzene, toluene, xylene, etc.

Examples of active metals capable of anionic polymerization include alkaline metals such as lithium, sodium, and potassium; alkaline earth metals such as palladium, magnesium, calcium, strontium, and barium; and rare earth metals such as yttrium and neodymium. Among active metals capable of anionic polymerization, alkaline metals and alkaline earth metals are preferred, and alkaline metals are more preferred.

As the active metal compound capable of anionic polymerization, an organic alkali metal compound is preferred. Examples of the organic alkali metal compound include: organic monolithium compounds such as methyl lithium, ethyl lithium, n-butyl lithium, dibutyl lithium, tertiary butyl lithium, hexyl lithium, phenyl lithium, diphenyl lithium; polyfunctional organic lithium compounds such as dilithium methane, dilithium naphthalene, 1,4-dilithium butane, 1,4-dilithium-2-ethylcyclohexane, 1,3,5-trilithium benzene; sodium naphthalene, potassium naphthalene, etc. Among the organic alkali metal compounds, organic lithium compounds are preferred, and organic monolithium compounds are more preferred.

The amount of the organoalkali metal compound used can be appropriately set according to the melt viscosity, molecular weight, etc. of the target unmodified conjugated diene polymer (B') and the modified conjugated diene polymer (B), but is usually used in an amount of 0.01 to 3 parts by mass based on 100 parts by mass of all monomers including the conjugated diene.

The organoalkali metal compound can also be used in the form of an organoalkali metal amide by reacting with a diamine such as dibutylamine, dihexylamine, or dibenzylamine.

Polar compounds are usually used to adjust the microstructure of the conjugated diene site in anionic polymerization without deactivating the reaction. Examples of polar compounds include ether compounds such as dibutyl ether, tetrahydrofuran, and ethylene glycol diethyl ether; tertiary amines such as tetramethylethylenediamine and trimethylamine; alkali metal alkoxides, phosphine compounds, etc. Polar compounds are usually used in an amount of 0.01 to 1000 moles relative to the organic alkali metal compound.

The temperature of the solution polymerization is usually in the range of -80 to 150° C., preferably in the range of 0 to 100° C., and more preferably in the range of 10 to 90° C. The polymerization mode may be either batch mode or continuous mode.

The polymerization reaction can be stopped by adding a polymerization terminator. Examples of the polymerization terminator include alcohols such as methanol and isopropanol. The unmodified conjugated diene polymer (B') can be separated by injecting the obtained polymerization reaction liquid into a poor solvent such as methanol to precipitate the unmodified conjugated diene polymer (B'), or by washing the polymerization reaction liquid with water, separating and drying the separated polymer.

As a method for producing the unmodified covalent diene polymer (B'), among the above methods, the solution polymerization method is preferred.

The acid modification of the unmodified conjugated diene polymer (B') can be carried out by a conventionally known method, but preferably, a method of adding an unsaturated carboxylic acid or an unsaturated carboxylic acid derivative to the unmodified conjugated diene polymer (B') is used.

Examples of the unsaturated carboxylic acid include maleic acid, fumaric acid, itaconic acid, (meth)acrylic acid, and the like. Examples of the unsaturated carboxylic acid derivative include anhydrides, esters, amides, imides, and the like of the unsaturated carboxylic acid.

Among the unsaturated carboxylic acids and unsaturated carboxylic acid derivatives, maleic acid and its derivatives are preferred, and maleic anhydride is more preferred. That is, as the acid-modified conjugated diene polymer, maleic anhydride-modified conjugated diene polymer is preferred. In addition, the acid-modified conjugated diene polymer itself is also a modified conjugated diene polymer (B) having a hydrophilic group such as a carboxyl group and anhydride group.

The method of adding unsaturated carboxylic acid or unsaturated carboxylic acid derivative to the unmodified conjugated diene polymer (B') is not particularly limited. For example, the method of adding unsaturated carboxylic acid or unsaturated carboxylic acid derivative to the unmodified conjugated diene polymer (B') can be adopted, and then a free radical catalyst is added as needed, and the mixture is heated in the presence or absence of an organic solvent. In addition, the acid-modified conjugated diene polymer can be directly modified by the compound (X) described below, or it can be modified after hydrogenation.

As the compound (X) to be reacted with the acid-modified conjugated diene polymer, there can be mentioned compounds (alcohols or amines) having at least one hydroxyl group or amino group. By reacting such a compound (X) with the acid-modified conjugated diene polymer, an ester group or an amide group which is a hydrophilic group is formed into the conjugated diene polymer. As the compound (X), a compound having a hydrophilic group in addition to one hydroxyl group or amino group is preferred. Such preferred compounds also include compounds having two or more hydroxyl groups or amino groups. By reacting such a compound (X) with an acid-modified covalent diene polymer, in addition to forming an ester group or an amide group which is a hydrophilic group in the covalent diene polymer, a hydrophilic group further possessed by the compound (X) is introduced into the covalent diene polymer.

As compound (X), a compound having at least one hydroxyl group or amino group and a repeating structure containing a nonionic polar group is more preferred. As a more preferred compound (X), a compound represented by the following formula (1) can be cited. By using the compound represented by the following formula (1), a graft chain having a polyalkylene glycol structure, which is one form of a graft chain having a repeating structure containing a nonionic polar group, can be introduced into the conjugated diene polymer as a hydrophilic group.

In formula (1), ^R1 is a hydroxyl group or an amino group. ^R2 , ^R3 , ^R4 and ^R5 are each independently hydrogen or a alkyl group having 1 to 6 carbon atoms. ^R6 is a hydrogen atom, a alkyl group having 1 to 6 carbon atoms or an acyl group having 1 to 6 carbon atoms. n is an integer of 3 to 600.

R ¹ is preferably a hydroxy group. Preferred forms of R ² to R ⁶ and n in formula (1) may be the same as those of R ² to R ⁶ and n in formula (A) described above.

Examples of the compound represented by formula (1) include triethylene glycol, triethylene glycol monomethyl ether, polyethylene glycol, polyethylene glycol monoalkyl ether, polyethylene glycol monoalkyl alkyl ether, polyethylene glycol monoalkyl alkenyl ether, polyethylene glycol monoalkyl ester, propylene glycol, propylene glycol monoalkyl ether, polypropylene glycol, polypropylene glycol monoalkyl ether, methoxypolyethylene glycol amine, and monoalkyl ether of a copolymer of propylene oxide and ethylene oxide.

The lower limit of the weight average molecular weight of the compound represented by formula (1) is, for example, 100, preferably 200, and more preferably 300. On the other hand, the upper limit of the weight average molecular weight is, for example, 10,000, and more preferably 8,000, 6,000, 4,000, 2,000, or 1,000. The compound represented by formula (1) may be used alone or in combination of two or more.

The method for adding compound (X) to the acid-modified covalent diene polymer is not particularly limited, and the following method can be adopted: adding compound (X) to the acid-modified covalent diene polymer, and then adding an amine catalyst as needed, and heating in the presence or absence of an organic solvent.

Examples of the amine catalyst include tertiary amines such as tetramethylethylenediamine, trimethylamine, and dimethylbenzylamine.

In general, the organic solvent used in the above method includes hydrocarbon solvents, halogenated hydrocarbon solvents, etc. Among these organic solvents, hydrocarbon solvents such as n-butane, n-hexane, n-heptane, cyclohexane, benzene, toluene, and xylene are preferred.

During the reaction of the addition compound (X) by the above method, an anti-aging agent may be added from the viewpoint of suppressing side reactions.

As the aging inhibitor, for example, there are: 2,6-di-tributyl-4-methylphenol (BHT), 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 4,4'-thiobis(3-methyl-6-tert-butylphenol), 4,4'-butylenebis(3-methyl-6-tert-butylphenol) (AO-40), 3,9-bis[1,1-dimethyl-2-[3-(3-tri-butylphenol)] [(butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (AO-80), 2,4-bis[(octylthio)methyl]-6-methylphenol (Irganox1520L), 2,4-bis[(dodecylthio)methyl]-6-methylphenol (Irganox1726), 2-[1-(2-hydroxy -3,5-di-tri-pentylphenyl)ethyl]-4,6-di-tri-pentylphenyl acrylate (SumilizerGS), 2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate (SumilizerGM), 6-tert-butyl-4-[3-(2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxophosphinylcycloheptatrien-6-yloxy)propyl]-2-methylphenol (SumilizerGP), 2,4-di-tri-butylphenyl phosphite (Irgafos168), dioctadecyl 3,3'-dithiobispropionate, hydroquinone, p-methoxyphenol, N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine (NOCRAC 6C), bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (LA-77Y), N,N-bisoctadecylhydroxylamine (Irgastab FS 042), bis(4-tert-octylphenyl)amine (Irganox5057), etc. The aging inhibitor may be used alone or in combination of two or more.

The amount of the aging inhibitor added is preferably 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, based on 100 parts by mass of the unmodified covalent diene polymer (B').

The reaction conditions of the acid-modified covalent diene polymer and the compound (X) are not particularly limited, but for example, the temperature during the reaction is preferably 10 to 200° C., more preferably 30 to 180° C. The reaction time is preferably 1 to 200 hours, more preferably 1 to 100 hours, and even more preferably 1 to 50 hours.

The total amount of the unsaturated carboxylic acid, the unsaturated carboxylic acid derivative and the compound (X) added to the modified conjugated diene polymer (B) is preferably 1 to 400 parts by mass, more preferably 20 to 300 parts by mass, and even more preferably 50 to 250 parts by mass, relative to 100 parts by mass of the unmodified conjugated diene polymer (B'). The amount added can be determined using various analytical equipment such as nuclear magnetic resonance spectroscopy.

The modified covalent diene polymer (B) can be synthesized by the method described in, for example, International Publication No. 2010/038835, Japanese Patent Application Publication No. 2015-086283, etc., in addition to the above-mentioned synthesis method.

[Plasticizer] The PVA film may further contain a plasticizer. By containing a plasticizer in the PVA film, it is possible to improve the stretching processability, workability, roll quality, etc. As a preferred plasticizer, polyols can be listed, and as specific examples, ethylene glycol, glycerol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, trihydroxymethyl propane, etc. can be listed. Such plasticizers can be used alone or in combination. Among them, glycerol is preferred from the perspective of the effect of improving the stretching processability and roll quality.

The lower limit of the content of the plasticizer in the PVA film is preferably 1 part by mass, more preferably 5 parts by mass, relative to 100 parts by mass of PVA (A). By setting the content of the plasticizer to be above the above lower limit, processing elongation and the like are further improved. On the other hand, the upper limit of this content is preferably 20 parts by mass, more preferably 15 parts by mass. By setting the content of the plasticizer to be below the above upper limit, the PVA film can be prevented from becoming too soft or the plasticizer seeping out to the surface and reducing the workability.

[Other additives, etc.] The PVA film may be further appropriately blended with fillers, processing stabilizers such as copper compounds, weathering stabilizers, colorants, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, defoamers, deodorants, extenders, strippers, release agents, reinforcing agents, crosslinkers, mildew inhibitors, preservatives, crystallization rate retarders, surfactants and other additives as needed.

Among other additives, it is preferred to include a surfactant from the viewpoint of film forming properties. By including a surfactant, the occurrence of uneven thickness of the PVA film is suppressed, or the film becomes easier to be peeled off from a substrate such as a metal roll or belt used for film forming. The type of surfactant is not particularly limited, but from the viewpoint of the peeling property of the metal roll or belt, anionic surfactants and cationic surfactants are preferred.

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

As the cationic surfactant, preferred are, for example: alkyl ether type such as polyoxyethylene oleyl ether; alkyl phenyl ether type such as polyoxyethylene octylphenyl ether; alkyl ester type such as polyoxyethylene laurate; alkylamine type such as polyoxyethylene laurylamine ether; alkylamide type such as polyoxyethylene laurate amide; polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether; alkanolamide type such as lauric acid diethanolamide and oleic acid diethanolamide; allyl phenyl ether type such as polyoxyalkylene allylphenyl ether, etc.

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

In the case where the PVA film contains a surfactant, the lower limit of the content is preferably 0.01 parts by mass, and more preferably 0.03 parts by mass, relative to 100 parts by mass of PVA (A). When the content of the surfactant is above the above lower limit, the peeling property, film-forming property, etc. are further improved. On the other hand, as the upper limit of this content, it is preferably 0.5 parts by mass, and more preferably 0.3 parts by mass. When the content of the surfactant is below the above upper limit, it is possible to suppress the surfactant from seeping into the surface of the PVA film, causing the films to adhere to each other and reducing the operability.

The upper limit of the content of other additives other than PVA (A), modified covalent diene polymer (B), plasticizer and surfactant in the PVA film is preferably 10% by mass, more preferably 5% by mass, more preferably 1% by mass, and still more preferably 0.2% by mass. When the content of other additives is greater than the above upper limit, the processing elongation of the PVA film, the formability and transparency of the resulting stretched film and polarizing film may be affected.

[Shape, etc.] The shape of the PVA film is not particularly limited, but a long strip-shaped film is preferred for continuous production with good productivity. The length of the long strip-shaped PVA film is not particularly limited, and can be appropriately set according to the application, etc. For example, the length can be set within the range of 5m or more and 20,000m or less. The width of the PVA film is not particularly limited, and for example, in the case of a water-soluble film, the lower limit can be set to 1cm. In addition, in various applications, since PVA films with a wide width have been required in recent years, the lower limit is preferably 1m, more preferably 2m, and even more preferably 4m. The upper limit of the width of the PVA film is not particularly limited, but for example, it can be set to 7m. If the width is too wide, it tends to become difficult to produce uniformly when using a practical device to manufacture a PVA film.

The PVA film may be a single-layer film or a multi-layer film (laminate).

The upper limit of the thickness (average thickness) of the PVA film is not particularly limited, but is, for example, 100 μm, preferably 60 μm, and more preferably 40 μm. On the other hand, as the lower limit of this thickness, it is preferably 1 μm, more preferably 5 μm, and more preferably 10 μm. When the thickness of the PVA film is within the above range, operability, stretching processability, etc. can be improved. In addition, the thickness (average thickness) is set to the average value of the measured values at any 5 locations.

[Applications] The PVA film can be used for various applications similar to conventional PVA films, such as packaging films, water-soluble films, agricultural films, release films, and optical films. In addition, the PVA film is suitable as a raw film for stretched films and polarizing films. In particular, the PVA film is suitable as a raw film for optical films because it has excellent stretching processability and can obtain stretched films and polarizing films with excellent formability. That is, by stretching the PVA film, an optical film can be preferably obtained. In addition, the PVA film of one embodiment of the present invention can be a packaging film, a water-soluble film, an agricultural film, a release film, a stretched film, an optical film, or a polarizing film.

Optical films are light-transmitting films that can be used in optical devices. Representative examples of optical devices include liquid crystal display devices and organic EL displays. Examples of optical films include polarizing films, polarizer protection films, color compensation films, brightness enhancement films, field of view expansion films, and phase difference films. In addition, the PVA film can be used as a gas barrier film for organic EL displays, etc., taking advantage of its excellent transparency and gas barrier properties.

In addition, the PVA film is suitable for use as a raw film for a stretched film and a polarizing film as described above, but can also be used for various purposes without being stretched.

<Production method of PVA film> The production method of the PVA film is not particularly limited, and a production method that makes the thickness and width of the PVA film after film formation more uniform can be preferably adopted. For example, the film can be obtained by using a film-forming stock solution in which PVA (A) and a modified covalent diene polymer (B) constituting the PVA film, and other components such as a plasticizer are dissolved in a liquid medium as needed. In addition, as needed, the film-forming stock solution obtained by melting PVA (A) and the like can also be used for production.

The method for producing a PVA film in one embodiment of the present invention comprises the step of using a film-forming stock solution prepared by mixing PVA (A) and a dispersion containing a modified covalent diene polymer (B) having a hydrophilic group to form a film. According to the method, a PVA film having excellent stretching processability and productivity can be produced, and a stretched film and a polarizing film having excellent formability can be obtained.

The specific forms and preferred forms of the PVA (A) and the modified conjugated diene polymer (B) used in the production method are the same as the PVA (A) and the modified conjugated diene polymer (B) as components of the PVA film in one embodiment of the present invention described above.

The lower limit of the content of the modified covalent diene polymer (B) in the film-forming stock solution may be, for example, 5 parts by mass, but may be preferably 10 parts by mass, more preferably 15 parts by mass, and even more preferably 20 parts by mass, 25 parts by mass, 30 parts by mass, or 35 parts by mass, relative to 100 parts by mass of PVA (A). On the other hand, the upper limit of this content may be, for example, 100 parts by mass, but may be preferably 80 parts by mass, more preferably 60 parts by mass, and even more preferably 40 parts by mass, 30 parts by mass, or 20 parts by mass.

In the film-forming stock solution, the modified conjugated diene polymer (B) is preferably uniformly mixed. In addition, by mixing the dispersion of the modified conjugated diene polymer (B) with the liquid medium, PVA (A) and other additives, a film-forming stock solution in which the modified conjugated diene polymer (B) is uniformly mixed can be effectively obtained. In addition, when the film-forming stock solution contains a plasticizer, other additives, etc., it is preferred that these components are uniformly mixed.

The method for producing the dispersion of the modified conjugated diene polymer (B) is not particularly limited, but it can be obtained, for example, by adding water after synthesizing the modified conjugated diene polymer (B), and stirring with an emulsifier or a stirrer, etc. At this time, the average particle size of the modified conjugated diene polymer (B) can be adjusted by adjusting the stirring intensity and stirring time, adjusting the pH by acid or alkali, selecting an emulsifier, etc.

The acid or base used for producing the dispersion of the modified conjugated diene polymer (B) is not particularly limited, but examples of the acid include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, citric acid, carbonic acid, acetic acid, and the like, and examples of the base include sodium hydroxide, potassium hydroxide, ammonia, and the like.

When an emulsifier is used in the preparation of the dispersion of the modified conjugated diene polymer (B), the emulsifier is not particularly limited, and general emulsifiers such as anionic, cationic, and cationic-anionic can be used. Specific examples of anionic emulsifiers include sodium, potassium, or ammonium salts of aliphatic carboxylic acids such as laurate, myristic, palmitic, stearic, and alkenyl succinate; sodium, potassium, or ammonium salts of inhomogenized or hydrogenated natural rosin; sodium, potassium, or ammonium salts of aliphatic sulfuric acid compounds such as lauryl sulfate, etc. In addition, examples of cationic-anionic emulsifiers include polyoxyethylene octylphenyl ether sulfonate, polyoxyethylene octylphenyl ether sulfate, polyoxyethylene alkyl ether sulfate, etc. Examples of countercations of these salts include sodium, potassium, or ammonium.

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

The volatile content of the film-forming stock solution (the proportion of volatile components such as liquid medium in the film-forming stock solution that are removed by volatility or evaporation during film-forming) also varies depending on the film-forming method, film-forming conditions, etc., but in general, as a lower limit, it is preferably 50% by mass, more preferably 55% by mass, and even more preferably 60% by mass. When the volatile content of the film-forming stock solution is above the lower limit, the viscosity of the film-forming stock solution will not become too high, and the filtration and defoaming when preparing the film-forming stock solution will proceed smoothly, making it easy to produce a PVA film with fewer foreign matter and defects. On the other hand, as an upper limit of this volatile content, it is preferably 95% by mass, and even more preferably 90% by mass. When the volatile content of the film-forming stock solution is below the above upper limit, the concentration of the film-forming stock solution will not become too low, and it becomes easy to produce an industrial PVA film.

When a PVA film is formed using a film-forming stock solution, a conventionally known method is used. As a film-forming method, a method of applying a film-forming stock solution to a substrate such as a metal drum to form a PVA film on the substrate may be used, or a method of directly forming a single-layer PVA film may be used. As a film-forming method, for example, a casting film-forming method, an extrusion film-forming method, a wet film-forming method, a gel film-forming method, etc., and only one of these film-forming methods may be used, or two or more of these film-forming methods may be used in combination. Among these film-forming methods, casting film-forming method and extrusion film-forming method are preferred in terms of obtaining a PVA film having uniform thickness and width and good physical properties.

Here, according to the method for producing a PVA film of an embodiment of the present invention, since the modified conjugated diene polymer (B) added to the film-forming stock solution has a hydrophilic group, the modified conjugated diene polymer (B) is not easily attached to a substrate such as a metal drum. Moreover, even if it is attached, it can be easily washed off by water or the like. Therefore, according to the production method, a PVA film having excellent stretching processability can be produced with high productivity.

The PVA film after filming can be heat treated as needed. The heat treatment temperature is not particularly limited, as long as it is appropriately adjusted. If the heat treatment temperature is too high, discoloration and degradation of the PVA film will be seen. Therefore, the upper limit of the heat treatment temperature is preferably 210°C, more preferably 180°C, and even more preferably 150°C. On the other hand, the lower limit of the heat treatment temperature is, for example, 60°C, preferably 90°C.

The heat treatment time is not particularly limited and can be adjusted appropriately, but from the perspective of efficiently manufacturing the PVA film, the upper limit is preferably 30 minutes, more preferably 15 minutes. On the other hand, the lower limit is, for example, preferably 10 seconds, more preferably 1 minute.

<Stretched film> A stretched film in one embodiment of the present invention is a stretched film obtained by stretching the PVA film in one embodiment of the present invention. In the stretched film, usually, the PVA is oriented in a specified direction (stretching direction). The stretched film can be stretched uniaxially or biaxially, but is preferably stretched uniaxially. The stretched film stretched uniaxially can be preferably used as an optical film such as a polarizing film. The stretched film can be a single-layer film or a multi-layer film, but is preferably a single-layer film. Since the stretched film contains a modified covalent diene polymer (B), it is not easy to cause problems such as rupture due to deformation, and is therefore suitable for foldable displays, sunglasses with high design, etc.

The upper limit of the thickness (average thickness) of the stretched film is, for example, 30 μm, preferably 16 μm. When the thickness of the stretched film is below the upper limit, sufficient thinning can be achieved. On the other hand, the lower limit of the thickness is preferably 5 μm, more preferably 8 μm. When the thickness of the stretched film is above the lower limit, it becomes less likely to break, and operability can be improved.

The stretched film can be used as a packaging film, a water-soluble film, an agricultural film, a release film, an optical film, etc., but is preferably used as an optical film.

As the optical film, there can be cited polarizing films, polarizer protection films, color compensation films, brightness enhancement films, etc., but among these, polarizing films are preferred.

<Method for producing stretched film> The stretched film can be obtained by a production method having the step of stretching the PVA film as described above. That is, in the production step of the stretched film, all steps except the stretching process are optional, and the stretched film can be produced by the same method as before except that the PVA film as described above is used. Therefore, in the production step of the stretched film, it is not necessary to include any treatment (dyeing treatment, fixing treatment, etc.) other than the stretching treatment in the production step of the polarizing film described later. That is, according to this production method, a stretched film can be obtained more easily without going through a special step.

By dyeing the PVA film or the stretched film, a polarizing film having excellent formability is provided.

<Polarizing film> The polarizing film of one embodiment of the present invention is obtained by dyeing the PVA film or stretched film of one embodiment of the present invention. The polarizing film has dichroic pigment and boric acid adsorbed on the PVA film or stretched film. The polarizing film is not prone to problems such as cracking due to deformation by containing a modified conjugated diene polymer (B), and is therefore suitable for foldable displays, sunglasses with high design, etc.

This polarizing film can also be used as a polarizing plate by laminating an optically transparent and mechanically strong protective film on both sides or one side thereof. As the protective film, triacetate cellulose (TAC) film, cellulose acetate-butyrate (CAB) film, acrylic film, polyester film, etc. are used. In addition, as the adhesive used for lamination, PVA adhesive, UV curing adhesive, etc. can be listed, and PVA adhesive is preferred.

The polarizing plate obtained as described above can also be further bonded with optical films such as a phase difference film, a viewing angle enhancement film, and a brightness enhancement film. In addition, as the viewing angle enhancement film, a stretched film of an embodiment of the present invention can also be used. The polarizing plate can be bonded to a glass substrate after being coated with an adhesive such as an acrylic adhesive and used as a component of a liquid crystal display device.

<Polarizing film manufacturing method> As a specific method for manufacturing the polarizing film, the PVA film is subjected to swelling treatment, dyeing treatment, uniaxial stretching treatment, and further subjected to crosslinking treatment, fixing treatment, cleaning treatment, drying treatment, heat treatment, etc. as needed. In this case, the order of each treatment such as swelling treatment, dyeing treatment, crosslinking treatment, uniaxial stretching, fixing treatment, etc. is not particularly limited, and two or more treatments may be performed at the same time. In addition, one or more of each treatment may be performed twice or more.

Furthermore, as another method for manufacturing the polarizing film, it is possible to list various treatments other than the swelling treatment on the stretched film. That is, the polarizing film can be obtained by subjecting the stretched film to dyeing treatment, uniaxial stretching treatment, and further subjecting the stretched film to crosslinking treatment, fixing treatment, cleaning treatment, drying treatment, heat treatment, etc. as needed. In this case, the order of the treatments such as dyeing treatment, crosslinking treatment, uniaxial stretching, fixing treatment, etc. is not particularly limited, and more than two treatments can be performed at the same time. Moreover, one or more of the treatments can be performed twice or more.

The manufacturing method of the polarizing film preferably comprises the following steps: after the PVA film is subjected to swelling treatment, dyeing treatment, uniaxial stretching treatment, and further subjected to crosslinking treatment, fixing treatment, cleaning treatment, drying treatment, heat treatment, etc. as needed, a uniaxial stretching step is performed using a dry stretching method.

The following describes in detail the case where the polarizing film is produced using the PVA film, but the conditions of the subsequent treatments such as the dyeing treatment are the same as those for the case where the stretched film is used for production.

The swelling treatment can be performed by immersing the PVA film in water. As the lower limit of the temperature of the water during immersion in water, 20°C is preferred, 22°C is more preferred, and 25°C is more preferred. On the other hand, as the upper limit of this temperature, 55°C is preferred, 40°C is more preferred, and 35°C is more preferred. Furthermore, as the lower limit of the time of immersion in water, 0.1 minute is preferred, and 0.5 minute is more preferred. On the other hand, as the upper limit of this time, 5 minutes is preferred, and 3 minutes is more preferred. In addition, the water during immersion in water is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or may be a mixture of water and an aqueous medium.

The dyeing process can be performed by bringing a dichroic pigment into contact with the PVA film. As the dichroic pigment, an iodine-based pigment or a dichroic dye is generally used.

In the case of iodine-based dyes, the period of dyeing treatment may be any stage before uniaxial stretching treatment, during uniaxial stretching treatment, and after uniaxial stretching treatment. Dyeing treatment is generally carried out by immersing the PVA film in a solution (especially an aqueous solution) containing iodine-potassium iodide as a dyeing bath. The concentration of iodine in the dyeing bath is preferably greater than 0.01 mass % and less than 0.5 mass %, and the concentration of potassium iodide is preferably greater than 0.01 mass % and less than 10 mass %. In addition, the lower limit of the temperature of the dyeing bath is preferably 20°C, and more preferably 25°C. On the other hand, the upper limit of this temperature is preferably 50°C, and more preferably 40°C.

In the case of dichroic dyes, the dyeing treatment can be performed at any stage of the PVA film production stage, before the uniaxial stretching treatment, during the uniaxial stretching treatment, and after the uniaxial stretching treatment. The dyeing treatment is to make the dichroic dye such as an azo compound adsorbed and impregnated on the PVA film and then perform a swelling treatment. In the case of omitting the swelling treatment, the swelling treatment can be performed simultaneously with the dyeing treatment.

In addition to using the azo compound in the form of a free acid, a salt of the compound may also be used. Such a salt is, for example, an alkaline metal salt such as a lithium salt, a sodium salt, and a potassium salt, or an organic salt such as an ammonium salt and an alkylamine salt, preferably a sodium salt.

Dyeing treatment is generally performed by immersing the PVA film in a solution (especially an aqueous solution) containing an azo compound as a dyeing bath. The concentration of each azo compound in the dyeing bath is preferably 0.00001 mass % or more and 10 mass % or less. In addition, the temperature of the dyeing bath is preferably 5 to 60°C, more preferably 20 to 50°C, and particularly preferably 35 to 50°C. The time of immersion in the solution can be appropriately adjusted, but it is preferably adjusted to 30 seconds to 20 minutes, and more preferably 1 to 10 minutes.

As a dyeing solution, in addition to the azo compound, a dyeing auxiliary may be further contained as needed. As dyeing auxiliary, sodium carbonate, sodium bicarbonate, sodium chloride, sodium sulfate, anhydrous sodium sulfate, and sodium tripolyphosphate can be listed. The content of the dyeing auxiliary can be adjusted to any concentration according to the time and temperature determined by the dyeing property of the dye, but as an individual content, it is preferably 0.01 to 5% by mass in the dyeing solution, and more preferably 0.1 to 2% by mass.

By subjecting the PVA film to a crosslinking treatment, PVA can be effectively prevented from dissolving into water during wet stretching at high temperature. From this point of view, the crosslinking treatment is preferably performed before the uniaxial stretching treatment. The crosslinking treatment can be performed by immersing the PVA film in an aqueous solution containing a crosslinking agent. As the above-mentioned crosslinking agent, one or more boron inorganic compounds such as borate salts such as boric acid and borax can be used. The lower limit of the concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass, more preferably 2% by mass, and even more preferably 3% by mass. On the other hand, the upper limit of this concentration is preferably 15% by mass, more preferably 7% by mass, and even more preferably 6% by mass. By keeping the concentration of the crosslinking agent within the above range, sufficient extensibility can be maintained. In the case of using an iodine-based dye, the aqueous solution containing a crosslinking agent may also contain an auxiliary agent such as potassium iodide. The lower limit of the temperature of the aqueous solution containing a crosslinking agent is preferably 20°C, more preferably 25°C. On the other hand, the upper limit of this temperature is preferably 50°C, more preferably 40°C. By setting this temperature within the above range, crosslinking can be performed efficiently.

The uniaxial stretching treatment can be carried out by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be carried out in a boric acid aqueous solution, in the above-mentioned dyeing bath, or in the fixing treatment bath described later. In addition, in the case of the dry stretching method, the uniaxial stretching treatment can be carried out directly at room temperature, while heating, or by using the PVA film after absorbing water to carry out the uniaxial stretching treatment in the air. Among these, the wet stretching method is preferred, and the uniaxial stretching treatment in a boric acid aqueous solution is more preferred. The lower limit of the boric acid concentration of the boric acid aqueous solution is preferably 0.5% by mass, more preferably 1.0% by mass, and even more preferably 1.5% by mass. On the other hand, the upper limit of this boric acid concentration is preferably 6% by mass, and even more preferably 5% by mass. In the case of iodine-based dyes, the aqueous boric acid solution may contain potassium iodide, and the concentration thereof is preferably set to 0.01 mass % or more and 10 mass % or less.

The lower limit of the stretching ratio in the uniaxial stretching treatment is preferably 3 times, more preferably 5 times, from the viewpoint of the polarization performance of the obtained polarizing film. The upper limit of the stretching ratio is not particularly limited, but is preferably 10 times, more preferably 8 times, for example.

When manufacturing polarizing film, in order to stabilize the adsorption of dichroic pigments (iodine-based pigments, etc.) to PVA film, it is preferred to perform a fixing treatment after the uniaxial stretching treatment. As the fixing treatment bath used for the fixing treatment, an aqueous solution containing one or more boron inorganic compounds such as boric acid and borax can be used. In addition, iodine compounds and metal compounds can be added to the fixing treatment bath as needed. The lower limit of the concentration of the boron inorganic compound in the fixing treatment bath is preferably 0.5% by mass, and more preferably 1% by mass. On the other hand, the upper limit of this concentration is preferably 15% by mass, and more preferably 10% by mass. By setting this concentration within the above range, the adsorption of the dichroic pigment can be made more stable. The lower limit of the temperature of the fixing treatment bath is preferably 15°C. On the other hand, the upper limit of this temperature is preferably 60°C, and more preferably 40°C.

The cleaning process is generally performed by immersing the PVA film in water or the like. In the case of iodine-based pigments, from the viewpoint of improving polarization performance, the water or the like used in the cleaning process preferably contains an auxiliary agent such as potassium iodide. At this time, the concentration of iodides such as potassium iodide is preferably set to be greater than 0.5 mass % and less than 10 mass %. In addition, the lower limit of the temperature of the water or the like used in the cleaning process is generally 5°C, preferably 10°C, and more preferably 15°C. On the other hand, the upper limit of this temperature is generally 50°C, preferably 45°C, and more preferably 40°C. From the viewpoint of economy, it is not preferable that the temperature of the water or the like is too low. On the other hand, if the temperature of the water or the like is too high, the polarization performance may sometimes decrease.

The drying conditions are not particularly limited, but the lower limit of the drying temperature is preferably 30° C., more preferably 50° C. On the other hand, the upper limit of the drying temperature is preferably 150° C., more preferably 130° C. By drying at a temperature within the above range, a polarizing film having excellent dimensional stability can be easily obtained.

By performing heat treatment after drying treatment, a polarizing film with excellent dimensional stability can be obtained. Here, the so-called heat treatment refers to further heating the polarizing film having a moisture content of less than 5% after drying treatment to improve the dimensional stability of the polarizing film. The conditions of the heat treatment are not particularly limited, but it is preferably performed within the range of above 60°C and below 150°C. By performing heat treatment at above 60°C, the dimensional stabilization effect caused by the heat treatment can be enhanced. On the other hand, by performing heat treatment at below 150°C, the occurrence of yellowing of the polarizing film can be suppressed.

The polarizing film is not limited to a two-dimensional shape (planar shape), and also includes one processed into a three-dimensional shape. That is, the polarizing film may be formed by stretching or the like after dyeing. As methods for stretching the dyed film, there are various methods such as wet stretching, vacuum forming, and thermoforming, but are not limited to these. As for the stretching ratio, from the perspective of maintaining performance, it is preferably stretched 1.2 to 2 times, and more preferably 1.3 to 1.5 times. As for the stretching speed, from the perspective of productivity, it is preferably 100 to 10000%/minute, and more preferably 500 to 5000%/minute. As for the stretching temperature, it is preferably below 100°C when using iodine-based pigments, and it is preferably below 160°C when using dichroic dyes. In the case of iodine-based pigments, by forming at a temperature below 100°C, discoloration and cracking during stretching can be suppressed. [Example]

The present invention is specifically described by the following examples, but the present invention is not limited by these examples. In addition, each evaluation method adopted in the following examples and comparative examples is disclosed below.

[Method for determining the weight average molecular weight of modified conjugated diene polymer] The weight average molecular weight of modified conjugated diene polymer is determined by GPC (gel permeation chromatography) and converted to standard polystyrene molecular weight. The measuring apparatus and conditions are as follows. Apparatus          : GPC apparatus "GPC8020" manufactured by Tosoh Co., Ltd. Separation column     : "TSKgelG4000HXL" manufactured by Tosoh Co., Ltd. Detector        : "RI-8020" manufactured by Tosoh Co., Ltd. Eluent        : Tetrahydrofuran Eluent flow rate  : 1.0ml/min Sample concentration     : 5mg/10ml Column temperature     : 40℃

[Stretching processability (tensile stress) of PVA film] The PVA film was humidified at 23°C and 50% RH for 24 hours, and a film sheet with a length of 30 mm and a width of 10 mm was cut from the PVA film. Afterwards, the PVA film sheet was installed in a tensile testing device ("Single-column desktop testing machine: 5952") manufactured by INSTRON Corporation with the initial chuck spacing of 10 mm, and a tensile test was performed at a speed of 100 [mm/min]. The test force [N] when the chuck spacing at this time was 30 mm was divided by the original cross-sectional area [mm ² ] before stretching, and the resulting value was taken as the tensile stress [N/mm ² ]. At this time, the same measurement was repeated 10 times for each sample, and the average value was adopted as the data. When the tensile stress is less than 50 N/ ^mm2 , the film is easily elongated and is judged to have good elongation workability.

[Washability of conjugated diene polymer attached to metal drum during PVA film production (productivity)] During PVA film production, since the conjugated diene polymer on the surface of the PVA film will adhere to the metal drum, a washability test was conducted. Specifically, after the PVA film was produced, the metal drum was wiped with a cotton gauze wetted with water. Afterwards, the presence of residual conjugated diene polymer on the surface of the metal drum was visually confirmed. The case with residual conjugated diene polymer was designated as B, and the case without residual conjugated diene polymer was designated as A.

[Formability of polarizing film (TD fracture strain)] The polarizing film was humidified at 23°C and 50%RH for more than 24 hours, and a film sheet with a length direction (MD) of 10mm and a width direction (TD) of 30mm was cut from the polarizing film. After that, the polarizing film sheet was installed in a tensile testing device ("Autograph (AGS-H)") manufactured by Shimadzu Corporation in such a way that the initial chuck interval was 10mm and the stretching direction was consistent with the TD of the polarizing film. The tensile test was carried out at a speed of 100 [mm/min] (1000%/minute) in a heating environment of 140°C. At this time, the same measurement was repeated 10 times for each sample, and the one with the largest TD fracture strain was used as the data. When the TD fracture strain was 40% or more, it was easy to form, and the formability was judged to be good. Here, the TD fracture strain refers to the value (X/10)×100(%) when the polarizing film breaks after being stretched Xmm from the initial nip distance (10mm) in the above tensile test.

[Production Example 1] (1) Synthesis of a covalent diene polymer (B'-1) A 5L autoclave that had been fully dried was substituted with nitrogen, loaded with 1200g of hexane and 112g of dibutyl lithium (10.5 mass% hexane solution), and heated to 50°C. Thereafter, under stirring conditions, 1200g of isoprene was added successively while controlling the polymerization temperature to 50°C, and the polymerization was carried out for 1 hour. Thereafter, methanol was added to stop the polymerization reaction, and a polymer solution was obtained. Water was added to the obtained polymer solution and stirred, and the polymer solution was washed with water. After the stirring was terminated, the water was discharged after confirming that the polymer solution phase and the water phase were separated. The polymer solution after washing was vacuum dried at 70°C for 24 hours to obtain a covalent diene polymer (B'-1).

(2) Synthesis of aqueous dispersion of covalent diene polymer (B'-1) 15 g of an emulsifier ("Phosphanol RS-710" manufactured by Toho Chemical Industry Co., Ltd.) was added to 250 g of the above covalent diene polymer (B'-1) and stirred for 20 minutes. 177 g of water was added in small amounts while stirring was continued. After adding the specified amount of water, the mixture was stirred for 20 minutes to obtain an aqueous dispersion of covalent diene polymer (B'-1).

[Production Example 2] (1) Synthesis of a modified conjugated diene polymer (B-1) having a hydrophilic group In a reaction container, 200 g of the conjugated diene polymer (B'-1) obtained in Production Example 1 and 40 g of maleic anhydride were charged and reacted at 170°C for 24 hours. 408 g of polyethylene glycol monomethyl ether (polyethylene glycol monomethyl ether 1000 manufactured by Tokyo Chemical Industry Co., Ltd., average molecular weight: 1,000, average number of polyoxyethylene units: 22) and 0.7 g of N,N-dimethylbenzylamine were further charged and reacted for 6 hours to obtain a modified conjugated diene polymer (B-1) having a hydrophilic group. The weight average molecular weight of the obtained modified conjugated diene polymer (B-1) was 27,000.

(2) Synthesis of aqueous dispersion of modified conjugated diene polymer (B-1) To 250 g of modified conjugated diene polymer (B-1), 13 g of an aqueous sodium hydroxide solution (sodium hydroxide concentration: 50 mass %) was added, and the mixture was stirred at 50°C for 60 minutes. While stirring was continued, 451 g of water was gradually added in small amounts to obtain an aqueous dispersion of modified conjugated diene polymer (B-1).

[Production Example 3] (1) Synthesis of a modified conjugated diene polymer (B-2) having a hydrophilic group In a reaction vessel, 250 g of the conjugated diene polymer (B'-1) obtained in Production Example 1 and 50 g of maleic anhydride were charged and reacted at 170°C for 24 hours. 204 g of polyethylene glycol monomethyl ether (polyethylene glycol monomethyl ether 400 manufactured by Tokyo Chemical Industry Co., Ltd., average molecular weight: 400, average number of polyoxyethylene units: 8) and 0.9 g of N,N-dimethylbenzylamine were further charged and reacted for 6 hours to obtain a modified liquid diene polymer (B-2) having a hydrophilic group. The weight average molecular weight of the obtained modified conjugated diene polymer (B-2) was 18,000.

(2) Preparation of aqueous dispersion of modified conjugated diene polymer (B-2) To 250 g of modified conjugated diene polymer (B-2), 20 g of an aqueous sodium hydroxide solution (sodium hydroxide concentration: 50% by mass) was added, and the mixture was stirred at 50°C for 60 minutes. While stirring was continued, 443 g of water was gradually added in small amounts to obtain an aqueous dispersion of modified conjugated diene polymer (B-2).

[Example 1] (Preparation of PVA film) 100 parts by mass of PVA (saponified product of homopolymer of vinyl acetate, degree of polymerization 2,400, degree of saponification 99.5 mol%), 15 parts by mass of modified conjugated diene polymer (B-1), 10 parts by mass of glycerin as a plasticizer, 0.1 parts by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, and water were mixed to prepare a film-forming stock solution having a volatile content of 85% by mass. Here, the modified conjugated diene polymer (B-1) was mixed with the other components in the form of an aqueous dispersion obtained in the above-mentioned Preparation Example 2. The film-making stock solution was cast on a metal drum with a surface temperature of 80°C and dried until the volatile matter content (water content) reached 5% by mass, thereby obtaining a long PVA film (PVA film before heat treatment) with a thickness of 30 μm, a length of 1.5 m, and a width of 30 cm. The PVA film was heat-treated at a temperature of 110°C for 10 minutes to obtain the PVA film of Example 1.

(Production of polarizing film) A sample of 5 cm in width and 10 cm in length was cut from the center of the width direction of the obtained PVA film in a manner that allows uniaxial stretching within a range of 5 cm in width and 5 cm in length. This sample was immersed in pure water at 40°C for 120 seconds and uniaxially stretched to 1.3 times in the length direction for swelling treatment (uniaxial stretching in the first stage). Next, the sample was immersed in a dyeing treatment bath (temperature 48°C) containing an aqueous solution containing 0.00002% by mass of Direct blue 15 dye, 0.1% by mass of sodium tripolyphosphate, and 0.1% by mass of sodium sulfate for 300 seconds and uniaxially stretched to 2.4 times in the length direction as a whole to allow the dye to be adsorbed (uniaxial stretching in the second stage). Furthermore, while being immersed in a crosslinking treatment bath (temperature 40°C) containing an aqueous solution of 2% by mass of boric acid for 60 seconds, the entire film is uniaxially stretched to 2.7 times in the length direction to allow boric acid adsorption (uniaxial stretching in the third stage). Then, while being immersed in a stretching treatment bath (temperature 58°C) containing an aqueous solution of 3.9% by mass of boric acid, the film is uniaxially stretched in the length direction to 4.0 times the initial film length to allow alignment (uniaxial stretching in the fourth stage). After stretching, the film is immediately immersed in a water tank (temperature 25°C) serving as a cleaning tank for 5 seconds. Finally, it is dried at 70°C for 3 minutes to obtain the polarizing film of Example 1.

[Example 2] In the preparation of the PVA film of Example 1, except for adding 30 parts by weight of the modified conjugated diene polymer (B-1), the PVA film and polarizing film were obtained in the same manner.

[Example 3] In the preparation of the PVA film of Example 1, except for adding 45 parts by weight of the modified conjugated diene polymer (B-1), the PVA film and polarizing film were obtained in the same manner.

[Example 4] In the preparation of the PVA film of Example 3, except that the modified conjugated diene polymer (B-1) is replaced with the modified conjugated diene polymer (B-2), the PVA film and the polarizing film are obtained in the same manner.

[Comparative Example 1] In the preparation of the PVA film of Example 1, the PVA film and the polarizing film were obtained in the same manner except that the modified conjugated diene polymer (B-1) was not added.

[Comparative Example 2] In the preparation of the PVA film of Example 1, except that the modified conjugated diene polymer (B-1) is replaced with an unmodified conjugated diene polymer (B'-1), a PVA film and a polarizing film are obtained in the same manner.

The stretching processability of each PVA film obtained from each embodiment and comparative example, the cleaning property (productivity) of the metal drum attached when the PVA film is made, and the formability of the polarizing film are evaluated by the above method. The evaluation results are shown in Table 1. In addition, for Comparative Example 1, since the co-dyne polymer was not added to the film-making solution of the PVA film, the evaluation of the cleaning property (productivity) was not carried out. In addition, for Comparative Example 2, since the co-dyne polymer remained in the metal drum during the evaluation of the cleaning property (productivity), the evaluation of the stretching processability and formability was not carried out.

[Table 1] Composition Reviews Covalent Diene Polymer Stretchability Cleaning (Productivity) Formability Kind Content [weight] Tensile stress [N/mm ² ] - TD fracture strain[%] Embodiment 1 B-1 15 48 A 200 Embodiment 2 B-1 30 43 A 300 Embodiment 3 B-1 45 43 A 1520 Embodiment 4 B-2 45 41 A 440 Comparative example 1 without 0 52 - 13 Comparative example 2 B'-1 15 - B -

As shown in Table 1, the PVA films obtained from Examples 1 to 4 have low tensile stress and excellent stretching processability. In addition, in Examples 1 to 4, the covalent diene polymer attached to the metal drum during the production of the PVA film can be easily removed by washing, and the washing property is excellent. That is, in the PVA films of Examples 1 to 4, the time for washing the metal drum can be shortened, which can be said to have excellent productivity. Furthermore, the polarizing films obtained from Examples 1 to 4 have large TD fracture strain and are not prone to cracking during forming. That is, the polarizing films obtained from Examples 1 to 4 have excellent formability.

On the other hand, in Comparative Example 1 in which no covalent diene polymer was added, the resulting PVA film had low stretchability and the resulting polarizing film had poor formability. Furthermore, in Comparative Example 2 in which unmodified covalent diene polymer was added, the covalent diene polymer attached to the metal drum during the production of the PVA film could not be easily removed by washing, and the washing performance was poor. That is, the PVA film of Comparative Example 2 was time-consuming to clean the metal drum, and thus the productivity was low. [Possibility of industrial use]

The PVA film of the present invention can be used for packaging films, water-soluble films, agricultural films, release films, optical films, etc., and is particularly suitable for use as polarizing films of optical films.

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Claims (9)

A polyvinyl alcohol film comprising polyvinyl alcohol (A) and a modified conjugated diene polymer (B) having a hydrophilic group, wherein the polyvinyl alcohol (A) is obtained by saponifying polyvinyl ester, the degree of polymerization of the polyvinyl alcohol (A) is greater than 1,000 and less than 10,000, and the degree of saponification is greater than 95 mol %. The polyvinyl alcohol film of claim 1, wherein the content of the modified covalent diene polymer (B) is 10 parts by mass or more and 80 parts by mass or less relative to 100 parts by mass of the polyvinyl alcohol (A). The polyvinyl alcohol film of claim 1, wherein the weight average molecular weight of the modified covalent diene polymer (B) is greater than 5,000 and less than 80,000. The polyvinyl alcohol film of claim 1, wherein the hydrophilic group is a grafted chain having a repeating structure containing a non-ionic polar group. A stretched film obtained from the polyvinyl alcohol film of any one of claims 1 to 4. A polarizing film obtained from a polyvinyl alcohol film as described in any one of claims 1 to 4 or a stretched film as described in claim 5. A method for producing a polyvinyl alcohol film comprises the following steps: using a film-forming stock solution obtained by mixing polyvinyl alcohol (A) and a dispersion containing a modified conjugated diene polymer (B) having a hydrophilic group to form a film; The polyvinyl alcohol (A) is obtained by saponifying polyvinyl ester, the degree of polymerization of the polyvinyl alcohol (A) is greater than 1,000 and less than 10,000, and the degree of saponification is greater than 95 mol %. A method for producing a polyvinyl alcohol film as claimed in claim 7, wherein the content of the modified covalent diene polymer (B) in the above-mentioned film-forming stock solution is 10 parts by mass or more and 80 parts by mass or less relative to 100 parts by mass of polyvinyl alcohol (A). A method for producing a polyvinyl alcohol film as claimed in claim 7 or 8, wherein the hydrophilic group is a grafted chain having a repeating structure containing a non-ionic polar group.