KR20240130071A - Polyvinyl alcohol film and method for producing polyvinyl alcohol film - Google Patents

Abstract

To provide a PVA film capable of being stretched at a high magnification even at high temperatures and a method for producing the PVA film.
A PVA film having a melting end temperature of 93°C or higher and less than 96°C when heat flow through the PVA film is measured in the presence of water using a differential scanning calorimeter.

Description

Polyvinyl alcohol film and method for producing polyvinyl alcohol film

The present invention relates to a polyvinyl alcohol film having good stretchability in high-temperature water and a method for producing the same.

Liquid crystal displays (LCDs) are used in notebook computers, LCD monitors, LCD televisions, smartphones, etc. As the basic components of the LCD, a liquid crystal with a light switching function and a polarizing plate with a light transmitting and shielding function are used.

A polarizing plate is composed of a protective film, such as a triacetic acid cellulose (TAC) film, an acrylic film, or a polyester film, bonded to both or one side of a polarizing film.

Polarizing films are mostly made by adsorbing a dichroic dye such as an iodine-based dye (such as I ₃ ^- or I ₅ ^- ) onto a matrix (a uniaxially stretched, oriented film) formed by uniaxially stretching a polyvinyl alcohol (hereinafter, "polyvinyl alcohol" is sometimes abbreviated as "PVA") film. Such polarizing films are manufactured by uniaxially stretching a PVA film containing a dichroic dye in advance, adsorbing a dichroic dye simultaneously with uniaxial stretching of a PVA film, or adsorbing a dichroic dye after uniaxially stretching a PVA film, etc.

Recently, with the expansion of the use of liquid crystal displays, improvement of polarization performance has been demanded. In order to improve polarization performance, improvement of the stretchability of the PVA film, which is the basis of the polarizing film, is important.

In order to improve the stretchability of a PVA film, raising the stretching temperature is one effective means. However, in the case of a conventional PVA film, stretching is difficult because breakage frequently occurs under conditions of high temperature and high magnification. The object of the present invention is to provide a PVA film that can be stretched at a high magnification, and a method for producing the PVA film.

The inventors of the present invention have conducted repeated thorough examinations to solve the above problems and completed the present invention. That is, the present invention is as follows [1] to [4].

[1] A polyvinyl alcohol film having a melting completion temperature of 93°C or more and less than 96°C when heat flow is measured using a differential scanning calorimeter in the presence of water;

[2] A polyvinyl alcohol film as described in [1], wherein when heat flow is measured for the polyvinyl alcohol film using a differential scanning calorimeter in the presence of water, the half-width of the dissolution peak is 26.5°C or more;

[3] A polarizing film obtained from a polyvinyl alcohol film described in [1] or [2];

[4] A method for manufacturing a polyvinyl alcohol film, comprising a film forming process, a drying process B, a drying process C, and a drying process D,

The film forming process is a process of forming a film by discharging a polyvinyl alcohol solution having a volatile content of 90 mass% or less into a film.

Drying process B is a process of drying a polyvinyl alcohol film having a volatile content of 20 mass% or more and 30 mass% or less at 90°C to 95°C.

Drying process C is a process of drying a polyvinyl alcohol film having a volatile content of 10 mass% or more and 20 mass% or less at 80°C to 90°C.

Drying process D is a process of drying a polyvinyl alcohol film having a volatile content of 5 mass% or more and 10 mass% or less at 70°C to 80°C.

A method for manufacturing a polyvinyl alcohol film, which comprises performing a film forming process, drying process B, drying process C, and drying process D in that order.

According to the present invention, a PVA film capable of being stretched at a high magnification and a method for producing the PVA film are provided.

Figure 1 is a drawing for explaining a specific method of the melting end temperature and the half-width of the melting peak.

In the analysis of a hydrous PVA film by a differential scanning calorimeter (hereinafter, “differential scanning calorimeter” is sometimes abbreviated as “DSC”), a DSC curve is obtained in which the horizontal axis represents temperature and the vertical axis represents heat flow (unit: W/g). Usually, in the analysis of a hydrous PVA film, a prominent dissolution peak appears accompanying the dissolution of PVA.

The dissolution end temperature in the present invention refers to the temperature of the high-temperature side intersection among the two intersections between the baseline of the obtained DSC curve and the tangent line at the inflection point of the DSC curve at the dissolution peak.

In addition, in the present invention, the function PVA film means a PVA film that is formed by punching a PVA film with a diameter of 4 mm, filling 15 mg into a high-capacity DSC pan made of stainless steel, further filling 75 mg of distilled water, covering the film with a lid, and maintaining the film at 30° C. for 10 minutes to allow absorption. The function PVA film contains 50 to 500 parts by mass of water per 100 parts by mass of the PVA film.

The PVA film of the present invention is a PVA film having a dissolution end temperature of 93°C or more and less than 96°C when measuring the heat flow of the PVA film in the presence of water. A specific method for determining the dissolution end temperature will be described later.

The reason why the PVA film of the present invention can be stretched at a high magnification is not necessarily clear, but when the dissolution end temperature is less than 93°C, the PVA film is likely to break during high-magnification stretching at high temperatures. This is presumed to be because the crystals in the PVA film dissolve at high temperatures, which easily reduces the strength of the film. The dissolution end temperature is preferably 93.5°C or higher, more preferably 94°C or higher, even more preferably 94.5°C or higher, and particularly preferably 95°C or higher. In addition, when the dissolution end temperature is 96°C or higher, it is presumed that the dissolution of the crystals in the PVA film becomes insufficient even during stretching at high temperatures, the tension becomes excessively high, and breakage easily occurs. The dissolution end temperature is preferably less than 95.8°C, more preferably less than 95.5°C, and even more preferably less than 95.3°C.

As a method for obtaining a PVA film having a melting end temperature of 93°C or more and less than 96°C when heat flow is measured with a differential scanning calorimeter in the presence of water, examples of the method include: discharging a PVA solution having a volatile fraction of 90 mass% or less into a film to form a film; drying at 90°C to 95°C when the volatile fraction of the PVA is 20 mass% or more and 30 mass% or less; drying at 80°C to 90°C when the volatile fraction of the PVA is 10 mass% or more and 20 mass% or less; and drying at 70°C to 80°C when the volatile fraction of the PVA is 5 mass% or more and 10 mass% or less.

In the PVA film of the present invention, the half-width of the dissolution peak is preferably 26.5°C or higher, more preferably 27.0°C or higher. It is thought that the half-width of the dissolution peak is somewhat correlated with the distribution of the crystal size, and it is thought that the wider the half-width of the dissolution peak, the wider the distribution of the crystal size. A specific method for determining the half-width of the dissolution peak will be described later. It is thought that the wider the half-width of the dissolution peak, the more likely it is that the tension during stretching will increase, and therefore the PVA molecules tend to be oriented easily during stretching, and the polarization performance tends to improve easily. On the other hand, if the half-width of the dissolution peak is too large, the dyeability of a dichroic dye may be reduced, and therefore the half-width of the dissolution peak is preferably 28°C or lower. As a method for obtaining a PVA film in which the half width of the dissolution peak is 26.5°C or higher when heat flow is measured with a differential scanning calorimeter in the presence of water, examples of the method include: discharging a PVA solution having a volatile fraction of 90 mass% or less into a film to form a film; drying at 90°C to 95°C when the volatile fraction of the PVA is 20 mass% or more and 30 mass% or less; drying at 80°C to 90°C when the volatile fraction of the PVA is 10 mass% or more and 20 mass% or less; and drying at 70°C to 80°C when the volatile fraction of the PVA is 5 mass% or more and 10 mass% or less.

In the present invention, as PVA, one manufactured by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer can be used. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versatate, isopropenyl acetate, and the like. Among these, vinyl acetate is preferable in terms of the ease of manufacturing PVA, ease of acquisition, cost, and the like.

The above vinyl ester polymer is preferably obtained using one or more types of vinyl ester monomers, but may also be a copolymer of one or more types of vinyl ester monomers and another monomer copolymerizable therewith.

Other monomers copolymerizable with such vinyl ester monomers include, for example, ethylene; C3-30 olefins such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; Acrylamide derivatives such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidepropanesulfonic acid or a salt thereof, acrylamidepropyldimethylamine or a salt thereof, N-methylolacrylamide or a derivative thereof; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or a salt thereof, N-methylolmethacrylamide or a derivative thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; Examples thereof include vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester, or acid anhydride thereof; itaconic acid or a salt, ester, or acid anhydride thereof; vinyl silyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. The vinyl ester polymer may have a structural unit derived from one or more of these other monomers.

The proportion of structural units derived from other copolymerizable monomers in the above vinyl ester polymer is not necessarily limited as long as it does not interfere with the effects of the present invention, but is preferably 15 mol% or less, and more preferably 5 mol% or less, based on the mole number of the total structural units constituting the vinyl ester polymer.

In the present invention, some of the hydroxyl groups of the PVA may be cross-linked or not. In addition, some of the hydroxyl groups of the above PVA may react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure, or they may not react with these compounds and form an acetal structure.

In the present invention, the degree of polymerization of PVA is not particularly limited, but, in terms of film strength and optical performance, the lower limit of the degree of polymerization is preferably 800 or more, more preferably 1000 or more, and even more preferably 1500 or more. On the other hand, in terms of productivity of PVA and productivity of PVA film, the upper limit of the degree of polymerization is preferably 10000 or less, more preferably 5000 or less, and even more preferably 4000 or less. Here, the degree of polymerization means the average degree of polymerization (P _A ) measured according to the description of JIS K 6726-1994. After completely saponifying the unsaponifiable portion (residual acetic acid group) in advance using sodium hydroxide, the relative viscosity with respect to water is determined using a viscometer, and the average degree of polymerization is calculated from the obtained limiting viscosity [η] by the following calculation formula.

In addition, the saponification degree of PVA is preferably 98 mol% or higher, and more preferably 99 mol% or higher. When the saponification degree is less than 98 mol%, there is a concern that the water resistance of the PVA film may become insufficient. Here, the saponification degree of PVA means the ratio (mol%) of the number of moles of the vinyl alcohol unit to the total number of moles of the vinyl alcohol unit and the structural unit (typically a vinyl ester monomer unit) of the PVA that can be converted into a vinyl alcohol unit by the degree of saponification. The saponification degree of PVA can be measured according to the description of JIS K 6726-1994.

In the present invention, the upper limit of the content of PVA in the PVA film is preferably 100 mass%, more preferably 98 mass%, and even more preferably 96 mass%. On the other hand, the lower limit of the content of PVA is preferably 50 mass%, more preferably 80 mass%, and even more preferably 85 mass%.

The PVA film of the present invention may contain any component as long as the effects of the present invention are not impaired. The optional components include surfactants, antioxidants, plasticizers, ultraviolet absorbers, lubricants, pH adjusters, colorants, preservatives, antifungals, other polymer compounds other than the above-mentioned components, moisture, and other components. The PVA film may contain one or more of these optional components.

In the PVA film of the present invention, it is one of the preferred embodiments to additionally blend a surfactant. By blending a surfactant, the handling properties thereof can be improved, or the peelability from a film forming device can be improved when producing a PVA film. There is no particular limitation on the type of surfactant to be used, but examples thereof include anionic surfactants and nonionic surfactants.

Examples of anionic surfactants include carboxylic acid types such as potassium laurate; sulfuric acid ester types such as octyl sulfate; and sulfonic acid types such as dodecylbenzene sulfonate.

Examples of the nonionic surfactants include alkyl ether types such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octyl phenyl ether; alkyl ester types such as polyoxyethylene laurate; alkyl amine types such as polyoxyethylene lauryl amino ether; alkyl amide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether types such as polyoxyalkylene allyl phenyl ether.

One type of surfactant may be used alone, or two or more types may be used in combination. Among these surfactants, nonionic surfactants are preferable because of their excellent effect of improving peelability during film formation, and in particular, alkanolamide type surfactants are more preferable, and dialkanolamides (e.g., diethanolamide, etc.) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids having 8 to 30 carbon atoms) are even more preferable.

The content of the surfactant blended in the PVA film of the present invention is preferably 0.01 part by mass or more, more preferably 0.02 part by mass or more, and even more preferably 0.05 part by mass or more, relative to 100 parts by mass of PVA. In addition, it is preferably 1 part by mass or less, more preferably 0.5 part by mass or less, and especially preferably 0.3 part by mass or less. If the content is less than 0.01 part by mass, problems such as poor peelability from a film forming device or blocking occurring between films when producing the PVA film tend to occur. On the other hand, if the content is more than 1 part by mass, problems such as bleeding out onto the film surface or deterioration of the film appearance due to aggregation of the surfactant tend to occur.

In the present invention, in order to suppress discoloration and deterioration due to oxidation of the surfactant, it is preferable to add an antioxidant. There is no particular limitation on the type of antioxidant, but organic antioxidants such as phenol, phosphite, thioester, benzotriazole, and hindered amine are preferable.

The content of the antioxidant in the PVA film of the present invention is preferably 0.01 mass% or more, and more preferably 0.05 mass% or more, based on the mass of the surfactant. The content of the antioxidant is preferably 3 mass% or less, and more preferably 1 mass% or less, based on the mass of the surfactant. If the content of the antioxidant is less than 0.01 mass%, and based on the mass of the surfactant, the oxidation inhibition effect may not be maintained for a long period of time, and if it exceeds 3 mass%, there is a risk that the antioxidant may aggregate and appear as a defect on the PVA film, thereby damaging the appearance.

In the PVA film of the present invention, it is one of the preferable embodiments to further blend a plasticizer. By blending a plasticizer into the PVA film, flexibility is improved, and mechanical properties such as impact strength and process passability during secondary processing are improved. Preferred plasticizers include polyhydric alcohols, and examples thereof include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and trimethylolpropane. These plasticizers may be used singly or in combination of two or more. Among these plasticizers, glycerin is preferable from the viewpoint of an effect of improving stretchability when the PVA film of the present invention is stretched and used.

The content of the plasticizer contained in the PVA film is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of PVA. In addition, the content of the plasticizer is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 15 parts by mass or less, per 100 parts by mass of PVA. If the content of the plasticizer is less than 1 part by mass, there is a concern that the effect of improving mechanical properties such as impact strength or stretchability may be insufficient. On the other hand, if the content of the plasticizer exceeds 30 parts by mass, the film may become too flexible, which may result in reduced handleability, or problems such as the plasticizer bleeding out onto the film surface and causing a blocking phenomenon between films may occur.

There is no particular limitation on the thickness of the PVA film of the present invention, and it can be set to an appropriate thickness depending on the intended use. Specifically, when used as a raw material for a polarizing film, the average thickness is preferably within a range of 5 to 150 ㎛. In addition, the average thickness of the PVA film can be obtained by measuring the thickness at 10 arbitrary points (for example, 10 arbitrary points on a straight line drawn in the width direction of the PVA film) and taking the average value thereof.

There is no particular limitation on the width of the PVA film of the present invention, and it can be appropriately set depending on the purpose of the PVA film, the purpose of the optical film such as a polarizing film manufactured from it, etc. In view of the recent progress in larger screens of liquid crystal televisions and liquid crystal monitors, it is preferable for these purposes if the width of the PVA film is 2 m or more, more preferably 3 m or more, and even more preferably 4 m or more. On the other hand, if the width of the PVA film is too large, it tends to be difficult to uniformly perform uniaxial stretching when manufacturing a polarizing film, and therefore the width of the PVA film is preferably 7 m or less.

There is no particular limitation on the shape of the PVA film of the present invention, but since a more uniform PVA film can be continuously and smoothly manufactured, and since it can be used continuously when manufacturing an optical film such as a polarizing film using it, it is preferably a long film. The long film is preferably in the form of a film roll by winding it around a cylindrical core or the like. The length (length in the flow direction) of the PVA film is not particularly limited and can be appropriately set depending on the application, etc., but when continuously unwound from a film roll and used, etc., the longer the length of the PVA film, the less loss can be reduced when switching film rolls, and therefore the length is preferably 500 m or more, more preferably 1,000 m or more, still more preferably 5,000 m or more, and particularly preferably 8,000 m or more. There is no particular limitation on the upper limit of the length, but it can be, for example, 30,000 m or less.

There is no particular limitation on the form of the PVA film of the present invention. It may be in the form of a single layer (single-layer film), or it may be in the form of a laminate, such as a PVA film formed on a thermoplastic resin film by a coating method or the like.

The swelling degree of the PVA film of the present invention is preferably within the range of 180 to 220% from the viewpoints of productivity and performance of the polarizing film. The swelling degree of the PVA film is preferably 180% or more, more preferably 190% or more, and still more preferably 195% or more. In addition, the swelling degree of the PVA film is preferably 220% or less, more preferably 210% or less, and still more preferably 205% or less. The swelling degree of the PVA film can be adjusted to a smaller value, for example, by increasing the temperature of the heat treatment.

Here, the swelling degree of the PVA film in the present invention refers to the swelling degree obtained by the following formula [I].

Swelling (%) = N/M×100 [I]

(In the equation, N represents the mass (g) of the sample taken from the PVA film after immersing the sample in distilled water at 30°C for 30 minutes and removing the water attached to the surface of the sample, and M represents the mass (g) of the sample after drying the sample in a dryer at 105°C for 16 hours.)

The method for producing the PVA film of the present invention is not particularly limited, and can be produced by a known method. For example, any method can be adopted, such as a flexible film-forming method, a wet film-forming method (discharging into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method in which a PVA aqueous solution is once cooled to gel, and then the solvent is extracted and removed to obtain a PVA film), and a method by a combination of these, or a melt extrusion film-forming method in which PVA containing a solvent is melted and used as the film-forming solution. Of these, the flexible film-forming method and the melt extrusion film-forming method are preferable because a PVA film with high transparency and low coloring is obtained, and the flexible film-forming method is more preferable.

The method for manufacturing a PVA film of the present invention is a method for manufacturing a PVA film including a film forming process, a drying process B, a drying process C, and a drying process D.

(Unveiling process)

The film forming process in the method for manufacturing the PVA film of the present invention is a process of forming a film by discharging a PVA solution having a volatile content of 90 mass% or less into a film.

The PVA solution is a solution in which PVA and each component are uniformly mixed with a solvent, and is prepared as a film-forming stock solution for forming a PVA film. There is no limitation on the method for preparing the PVA solution as a film-forming stock solution, and any method may be adopted, such as a method in which PVA is dissolved in a solvent in a dissolution tank, and then other components are added to make it uniform, or a method in which a mixture of PVA and a solvent or plasticizer is supplied to an extruder, melted, and then other additives are added.

Examples of the above solvents include water, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, N-methyl pyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylol propane, ethylene diamine, diethylene triamine, etc. Among these, water is preferable in terms of small environmental load and recyclability. The above solvents may be used alone or in combination of two or more.

The volatile fraction of the PVA solution is 90 mass% or less. If the volatile fraction of the PVA solution is higher than 90 mass%, the viscosity becomes too low, and the uniformity of the thickness of the PVA film on the first drying roll is likely to be impaired. On the other hand, there is no particular limitation on the lower limit of the volatile fraction of the PVA solution, but if the volatile fraction of the PVA solution is too low, the viscosity of the PVA solution becomes too high, making filtration or defoaming difficult, or the film forming itself may become difficult, so it is preferably 50 mass% or more. From the above viewpoint, the volatile fraction of the PVA solution is preferably 50 mass% or more, more preferably 55 mass% or more, and even more preferably 60 mass% or more. In addition, from the above viewpoint, the volatile fraction of the PVA solution is preferably 90 mass% or less, more preferably 80 mass% or less, and even more preferably 75 mass% or less.

Here, the volatile fraction of the PVA solution refers to the volatile fraction calculated by the following formula [II].

Volatile fraction of PVA solution (mass%) = {(Wa－Wb)/Wa}×100 [II]

(In the formula, Wa represents the mass (g) of the PVA solution, and Wb represents the mass (g) after drying the PVA solution of Wa (g) in a dryer at 105°C for 16 hours.)

When discharging a PVA solution into a film, it is preferable to use a known film discharging device (film flexible device) such as a T-type slit die, a hopper plate, an I-type slit die, or a lip coater die to discharge (flex) the PVA solution into a film.

(drying process)

It is known that the crystal thickness of PVA in a PVA film is somewhat related to the drying temperature at the time of film formation, and the higher the drying temperature, the greater the crystal thickness tends to be. In the present invention, an attempt was made to adjust the drying temperature using the volatile content as an indicator at the time of film formation, and it was found that by appropriately adjusting the drying temperature according to the volatile content, it is possible to adjust the dissolution end temperature and the dissolution half-maximum width, leading to the present invention.

That is, the method for manufacturing a PVA film of the present invention is a method for manufacturing a PVA film, including a drying step B for drying a PVA film having a volatile content of 20 mass% or more and 30 mass% or less at 90°C to 95°C, a drying step C for drying a PVA film having a volatile content of 10 mass% or more and 20 mass% or less at 80°C to 90°C, and a drying step D for drying a PVA film having a volatile content of 5 mass% or more and 10 mass% or less at 70°C to 80°C, wherein the drying step B, the drying step C, and the drying step D are performed in this order after a film forming step.

In addition, when the volatile fraction of the PVA solution exceeds 30 mass%, in the manufacturing method of the present invention, it is preferable to include a drying process A for reducing the volatile fraction to 30 mass% or less between the film forming process and the drying process B. The drying temperature in the drying process A is preferably 90°C or higher, and more preferably 91°C or higher. The drying temperature in the drying process A is preferably 105°C or lower, and more preferably 98°C or lower. If the temperature of the drying process A is too high, defects on the film surface tend to increase, and if it is too low, the time required for drying becomes longer, which reduces production efficiency. In addition, the drying temperature in the drying process A refers to the surface temperature of the roll when the PVA film is dried on a roll.

In the drying process B, when the volatile fraction of the PVA film is 20 mass% or more and 30 mass% or less, the drying temperature is preferably 90°C or more, and more preferably 92°C or more. In the drying process B, when the volatile fraction of the PVA film is 20 mass% or more and 30 mass% or less, the drying temperature is preferably 95°C or less, and more preferably 94°C or less. However, it is not necessary that the drying temperature is always 90 to 95°C when in the above-mentioned volatile fraction range, and if the drying temperature is temporarily in the above-mentioned volatile fraction range, the PVA film of the present invention can be obtained. However, in the drying process B, when the volatile fraction of the PVA film is 20 mass% or more and 30 mass% or less, the longer the drying temperature is in the 90 to 95°C range, the easier it is to obtain the PVA film of the present invention. Specifically, for a time during which the volatile fraction of the PVA film is 20 mass% or more and 30 mass% or less, the time during which the drying temperature is 90 to 95°C is preferably 50% or more, more preferably 70% or more, and still more preferably 90% or more. In addition, the drying temperature in the drying process B refers to the surface temperature of the roll when the PVA film is dried on a roll.

In the drying process C, when the volatile fraction of the PVA film is 10 mass% or more and 20 mass% or less, the drying temperature is preferably 80°C or more, and more preferably 82°C or more. In the drying process C, when the volatile fraction of the PVA film is 10 mass% or more and 20 mass% or less, the drying temperature is preferably 90°C or less, and more preferably 88°C or less. However, it is not necessary that the drying temperature is always 80 to 90°C when in the above-mentioned volatile fraction range, and if the drying temperature is temporarily in the above-mentioned volatile fraction range, the PVA film of the present invention can be obtained. However, in the drying process C, when the volatile fraction of the PVA film is 10 mass% or more and 20 mass% or less, the longer the drying temperature is in the 80 to 90°C range, the easier it is to obtain the PVA film of the present invention. Specifically, for a time during which the volatile fraction of the PVA film is 10 mass% or more and 20 mass% or less, the time during which the drying temperature is 80 to 90°C is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more. In addition, the drying temperature in the drying process C refers to the surface temperature of the roll when the PVA film is dried on a roll.

In the drying process D, when the volatile fraction of the PVA film is 5 mass% or more and 10 mass% or less, the drying temperature is preferably 70°C or more, and more preferably 72°C or more. In the drying process D, when the volatile fraction of the PVA film is 5 mass% or more and 10 mass% or less, the drying temperature is preferably 80°C or less, and more preferably 78°C or less. However, it is not necessary that the drying temperature is always 70 to 80°C when in the above-mentioned volatile fraction range, and if the drying temperature is temporarily 70 to 80°C when in the above-mentioned volatile fraction range, the PVA film of the present invention can be obtained. However, in the drying process D, when the volatile fraction of the PVA film is 5 mass% or more and 10 mass% or less, the longer the drying temperature is 70 to 80°C, the easier it is to obtain the PVA film of the present invention. Specifically, for a time during which the volatile fraction of the PVA film is 5 mass% or more and 10 mass% or less, the time during which the drying temperature is 70 to 80°C is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more. In addition, the drying temperature in the drying process D refers to the surface temperature of the roll when the PVA film is dried on a roll.

(Drying method)

In the drying step of the method for manufacturing a PVA film of the present invention, there is no particular limitation on the method for drying the PVA solution discharged in a film shape, but from the viewpoint of making it easy to manufacture a uniform PVA film, a method is preferred in which a film forming device having a plurality of drying rolls whose rotation axes are parallel to each other is used, the PVA solution is discharged in a film shape onto the first roll of the film forming device to initiate drying, and the PVA film is further dried by a second roll or subsequent drying rolls that are connected downstream from the first roll. In the film forming device, the number of drying rolls is preferably 3 or more, more preferably 4 or more, and still more preferably 5 or more. In addition, the number of drying rolls is preferably 30 or less.

The drying roll is preferably formed of a metal such as iron or stainless steel, and in particular, it is more preferable that the surface of the drying roll is formed of a metal material that is difficult to corrode and has a mirror-like gloss. In order to increase the durability of the drying roll, it is more preferable to use a drying roll plated with a single layer or two or more layers of a nickel layer, a chrome layer, a nickel/chrome alloy layer, or the like.

When drying a film, in order to dry both sides of the film more evenly, it is preferable to dry the film surface that comes into contact with the first roll (hereinafter, sometimes abbreviated as the “first roll contact surface”) and the film surface that does not come into contact with the first roll (hereinafter, sometimes abbreviated as the “first roll non-contact surface”) alternately facing each drying roll.

Drying of the PVA solution discharged in the form of a film on the first roll may be performed only by heating from the first roll, but it is preferable to perform drying from both sides of the PVA film by simultaneously heating with the first roll and spraying hot air on the non-contact surface of the first roll, from the viewpoints of uniformity of drying, drying speed, etc.

When spraying hot air onto the non-contact surface of the first roll of the PVA film on the first roll, it is preferable to spray hot air at a wind speed of 1 m/sec or more onto the entire area of the non-contact surface of the first roll, more preferably to spray hot air at a wind speed of 2 m/sec or more, and even more preferably to spray hot air at a wind speed of 3 m/sec or more. It is preferable to spray hot air at a wind speed of 10 m/sec or less onto the entire area of the non-contact surface of the first roll, and more preferably to spray hot air at a wind speed of 8 m/sec or less. If the wind speed of the hot air sprayed onto the non-contact surface of the first roll is too small, condensation such as water vapor may occur on the surrounding equipment, which may drip onto the PVA film, causing defects in the PVA film. On the other hand, if the wind speed of the hot air sprayed onto the non-contact surface of the first roll is too large, the PVA film is likely to have uneven thickness.

The temperature of the hot air sprayed onto the non-contact surface of the first roll of the PVA film is preferably 50°C or higher, more preferably 70°C or higher, and even more preferably 80°C or higher, in terms of drying efficiency, uniformity of drying, and the like. The temperature of the hot air sprayed onto the non-contact surface of the first roll of the PVA film is preferably 150°C or lower, more preferably 120°C or lower, and even more preferably 95°C or lower, in terms of drying efficiency, uniformity of drying, and the like. However, when the volatile content of the PVA film becomes 20 to 30 mass% on the first roll, that is, when transitioning from drying process A to drying process B, the surface temperature of the first roll and the hot air temperature are preferably lower than 95°C. In addition, with respect to the control of the dissolution completion temperature, which is the object of the present invention, it is thought to be strongly influenced by the temperature of the contact surface of the first roll.

The method for spraying hot air onto the non-contact surface of the first roll of the PVA film is not particularly limited, and any method capable of uniformly spraying hot air having a uniform wind speed or temperature onto the non-contact surface of the first roll of the PVA film, preferably onto the entire surface, may be adopted. Among these, a nozzle method, a baffle plate method, or a combination thereof is preferably adopted. The direction in which the hot air is sprayed onto the non-contact surface of the first roll of the PVA film may be a direction facing the non-contact surface of the first roll, a direction substantially following the circumference of the non-contact surface of the first roll of the PVA film (a direction substantially following the circumference of the roll surface of the first roll), or may be another direction.

In addition, when drying the PVA film on the first roll, it is preferable to exhaust the volatile matter generated from the PVA film and the hot air after spraying. The method of exhausting is not particularly limited, but it is preferable to adopt an exhausting method in which the hot air sprayed on the non-contact surface of the first roll of the PVA film does not cause uneven wind speed and temperature.

The PVA film dried on the first roll is peeled off from the first roll and then dried on the second roll. At this time, it is preferable that the non-contact surface of the PVA film of the first roll be brought into contact with and face the second roll, and the PVA film is dried with the second roll. Furthermore, the volatile fraction of the PVA film when peeled off from the first roll is preferably 30 mass% or less. If the volatile fraction exceeds 30 mass%, there is a risk that the peeling will become uneven, causing problems such as uneven thickness. The volatile fraction of the PVA film when peeled off from the first roll is more preferably 28 mass% or less, still more preferably 26 mass% or less, and particularly preferably 24 mass% or less.

The PVA film dried by the second roll is peeled from the second roll and sequentially dried by a plurality of drying rolls, such as the third roll, the fourth roll, the fifth roll, etc., depending on the number of drying rolls formed in the film forming device.

The surface temperature of each drying roll from the second roll to the final roll needs to be controlled to the temperature specified in the present invention up to a volatile content of 5 mass%, depending on the volatile content of the PVA film.

The PVA film dried to a volatile content of 5 mass% or less can be heat-treated appropriately according to the purpose. The heat treatment can be performed using a heat treatment roll or other known heat treatment device. When the heat treatment is performed using a heat treatment roll, the heat treatment roll may be one or more, but in order to uniformly heat-treat both sides of the film, it is preferable to heat-treat using two or more heat treatment rolls.

The surface temperature of the heat treatment roll is preferably 90°C or higher, more preferably 100°C or higher, and even more preferably 110°C or higher, from the viewpoint of appropriately progressing crystallization of PVA and obtaining a PVA film with excellent heat resistance. In addition, from the viewpoint of improving the stretchability of the obtained PVA film, the surface temperature of the heat treatment roll is preferably 150°C or lower, more preferably 140°C or lower, and even more preferably 130°C or lower.

There is no particular limitation on the heat treatment time, but from the viewpoint of smoothly manufacturing the target PVA film, it is preferably 1 second or longer, and more preferably 2 seconds or longer. There is no particular limitation on the heat treatment time, but from the viewpoint of smoothly manufacturing the target PVA film, it is preferably 60 seconds or shorter, and more preferably 10 seconds or shorter.

The above-mentioned film-making device may have a hot air drying device, a humidity control device, etc., as necessary.

The PVA film obtained as described above can be made into the PVA film of the present invention by, if necessary, performing hot air drying or humidity treatment, cutting both ends (edges) of the PVA film, etc., and finally winding the film into a roll with a predetermined length.

The volatile content of the PVA film finally obtained by the above series of treatments is preferably 1 mass% or more, more preferably 2 mass% or more. The volatile content of the PVA film finally obtained is preferably 5 mass% or less, more preferably 4 mass% or less.

By using the PVA film of the present invention, a polarizing film of the present invention is obtained. The polarizing film has excellent polarizing performance.

The method for manufacturing a polarizing film using the PVA film of the present invention is not particularly limited, and any known method may be adopted. For example, a method may be exemplified of dyeing a PVA film and performing uniaxial stretching, or of uniaxially stretching a PVA film containing a dye. A specific method for manufacturing a polarizing film may be a method of performing dyeing, uniaxial stretching, fixing treatment, drying treatment, and further heat treatment as necessary on the PVA film of the present invention. The order of dyeing and uniaxial stretching is not particularly limited, and dyeing may be performed before uniaxial stretching, dyeing may be performed simultaneously with uniaxial stretching, or dyeing may be performed after uniaxial stretching. Furthermore, the processes of uniaxial stretching, dyeing, etc. may be repeated multiple times. In particular, it is preferable to perform uniaxial stretching divided into two or more times, because it makes it easy to perform uniform stretching.

As dyes used for dyeing PVA films, iodine or dichroic organic dyes (e.g., dichroic dyes such as Direct Black 17, 19, 154; Direct Brown 44, 106, 195, 210, 223; Direct Red 2, 23, 28, 31, 37, 39, 79, 81, 240, 242, 247; Direct Blue 1, 15, 22, 78, 90, 98, 151, 168, 202, 236, 249, 270; Direct Violet 9, 12, 51, 98; Direct Green 1, 85; Direct Yellow 8, 12, 44, 86, 87; Direct Orange 26, 39, 106, 107) can be used. These dyes can be used alone or in combination of two or more. Dyeing can be performed by immersing the PVA film in a solution containing the dye, but the treatment conditions and treatment method are not particularly limited.

Uniaxial stretching of a PVA film may be performed by either a wet stretching method or a dry stretching method, but from the viewpoint of stability of performance and quality of the resulting polarizing film, a wet stretching method is preferable. As a wet stretching method, a method may be exemplified in which a PVA film is stretched in an aqueous solution containing various components such as pure water, additives or aqueous media, or an aqueous dispersion in which various components are dispersed. Specific examples of a uniaxial stretching method by a wet stretching method include a method in which uniaxial stretching is performed in warm water containing boric acid, a method in which uniaxial stretching is performed in a solution containing the above-mentioned dye, or in a fixing treatment bath described later. Furthermore, a hydrous PVA film may be uniaxially stretched in the air, or uniaxially stretched by another method.

There is no particular limitation on the stretching temperature when stretching along one axis, but in the case of wet stretching, a temperature of preferably 20°C or higher, more preferably 25°C or higher, and even more preferably 30°C or higher is employed. In the case of wet stretching, a temperature of preferably 90°C or lower, more preferably 70°C or lower, and even more preferably 65°C or lower is employed. In the case of dry stretching, a temperature within a range of preferably 50 to 180°C is employed.

The stretching ratio of uniaxial stretching (the overall stretching ratio when uniaxial stretching is performed in multiple stages) is preferably stretched to the point just before the PVA film is cut in terms of polarization performance, and specifically, it is preferably 4 times or more, more preferably 5 times or more, and even more preferably 5.5 times or more. There is no particular limitation on the upper limit of the stretching ratio as long as the PVA film is not broken, but it is preferably 8.0 times or less in order to perform uniform stretching.

The thickness of the PVA film after uniaxial stretching is preferably 1 ㎛ or more, and particularly preferably 3 ㎛ or more. The thickness of the PVA film after uniaxial stretching is preferably 30 ㎛ or less, and particularly preferably 25 ㎛ or less. In addition, the thickness can be obtained by measuring the thickness at five arbitrary points and taking the average value.

In the production of polarizing films, fixation treatment is often performed in order to strengthen the adsorption of dye to a uniaxially stretched PVA film. The fixation treatment is generally and widely adopted by immersing the PVA film in a fixation treatment bath containing boric acid and/or a boron compound. At that time, an iodine compound may be added to the treatment bath as needed.

It is preferable to dry the PVA film that has been subjected to a uniaxial stretching treatment, or a uniaxial stretching treatment and a fixing treatment. The temperature of the drying treatment is preferably 30°C or higher, and particularly preferably 50°C or higher. The temperature of the drying treatment is preferably 150°C or lower, and particularly preferably 140°C or lower. If the temperature of the drying treatment is too low, the dimensional stability of the resulting polarizing film is likely to deteriorate, while if the temperature is too high, the polarizing performance is likely to deteriorate due to disturbance of the molecular orientation of PVA, etc.

A polarizing plate can be made by bonding a protective film that is optically transparent and also has mechanical strength to both or one side of the polarizing film obtained as described above. As the protective film in this case, a cellulose triacetate (TAC) film, a cellulose acetate-butyrate (CAB) film, an acrylic film, a polyester film, or the like is used.

There is no particular limitation on the method for bonding the polarizing film and the protective film, but examples thereof include a method in which an aqueous or solvent-based adhesive is applied between the polarizing film and the protective film, the bonding is performed and then dried, a method in which an adhesive is applied to one of the polarizing film and the protective film, the drying is performed and then the other film is pressed, and a method in which an active energy ray-curable adhesive is applied between the polarizing film and the protective film, the bonding is performed and then active energy rays are irradiated. From the viewpoints of handleability, environmental load, etc., a method using an aqueous adhesive or an active energy ray-curable adhesive is preferable.

It is preferable to modify the surface of the above protective film by a known method such as corona treatment, plasma treatment, UV treatment, or flame treatment, as necessary, in order to improve adhesion with the adhesive. In addition, for the same purpose, it is also preferable to perform adhesion-facilitating processing such as primer coating on the film surface.

The polarizing plate obtained in the above manner can be used as a component of a liquid crystal display device by bonding it to a glass substrate after coating it with an adhesive such as an acrylic adhesive. Before bonding the polarizing plate to the glass substrate, a phase difference film, a viewing angle improvement film, a brightness improvement film, etc. may be bonded thereto.

Example

The present invention is specifically described below by examples and the like, but the present invention is not limited in any way by these examples.

In addition, each evaluation method employed in the following examples and comparative examples is shown below.

[Measurement of Differential Scanning Calorimetry (DSC) of PVA Film]

A PVA film was punched out with a punch having a diameter of 4 mm, and 15 mg was charged into a high-capacity stainless steel DSC Pan. Further, 75 mg of distilled water was charged, followed by covering the lid and maintaining it at 30°C for 10 minutes to form a PVA film. Using a DSC Q2000 manufactured by TA Instruments, the temperature was increased to 110°C at a heating rate of 1°C/min, and differential scanning calorimetry (DSC) was measured to obtain a DSC curve in which the horizontal axis represents temperature and the vertical axis represents heat flow (W/g). The obtained DSC curve was baseline-corrected to obtain the dissolution peak temperature, dissolution end temperature, and half maximum width of the dissolution peak.

In addition, the melting end temperature and the half-width of the melting peak were obtained in the following order.

(1) Melting end temperature

(i) As shown in Fig. 1(a), draw a straight line connecting the points of 40°C and 100°C on the DSC curve and use this as the base line.

(ii) As shown in Fig. 1(b), the DSC curve is corrected so that the baseline is parallel to the X-axis (temperature) of the DSC chart (baseline correction).

(iii) As shown in Fig. 1(c), in the DSC curve on the high temperature side of the peak of the PVA melting peak of the baseline-corrected DSC curve, as the temperature increases, the point where the slope of the tangent line at each point on the DSC curve changes from increasing to decreasing, i.e., the inflection point, is obtained.

(iv) As shown in Fig. 1(c), a tangent line is drawn to the DSC curve at the inflection point, and the intersection point where the tangent line intersects the baseline is obtained.

(v) As shown in Fig. 1(c), the temperature of the intersection point is set as the melting end temperature.

(2) Half-width of the melting peak

(i) As shown in Fig. 1(d), a straight line is drawn that passes through the peak of the PVA melting peak of the baseline-corrected DSC curve and is perpendicular to the X-axis of the DSC chart, and the intersection point of this straight line and the baseline is obtained.

(ii) As shown in Figure 1(d), a straight line parallel to the X-axis of the DSC chart is drawn, passing through a point equidistant from the intersection of the melting peak and the baseline on the vertical straight line.

(iii) As shown in Fig. 1(d), the distance (unit: ℃) between the two intersection points where the straight line parallel to the X-axis intersects the DSC curve is taken as the half-width of the melting peak.

[Volatile fraction of PVA solution, the original solution for the film]

After collecting about 10 g of PVA solution in a heat-resistant glass container, the heat-resistant container was sealed, and the mass Wa of the PVA solution excluding the air bag was measured to four decimal places. Next, the PVA solution, together with the heat-resistant container, was placed in a 105°C electric heating dryer, and the heat-resistant container was dried for 16 hours with the lid open, and the mass Wb of the dried PVA solution excluding the air bag was measured to four decimal places. From the obtained masses Wa and Wb, the volatile fraction (mass%) of the PVA solution, which is the original film-forming solution, was obtained by the following formula [III].

Volatile fraction of PVA solution (mass%) = {(Wa－Wb)/Wa}×100 [III]

[In the formula, Wa represents the mass (g) of the PVA solution, and Wb represents the mass (g) after drying the PVA solution of Wa (g) in an electric dryer at 105°C for 16 hours.]

[Roll surface temperature]

Before the unveiling, the roll surface temperature was checked. A total of five measurement points were set in the width direction of the roll, with the center position in the width direction as the central measurement point and two points on each side at 10 cm intervals as measurement points, so that a total of five measurement points were set in the width direction of the roll so that the measurement points were aligned on a straight line. The roll surface temperature of the five measurement points was measured to one decimal place using a non-contact surface thermometer. The average value of the roll surface temperature of the five measurement points was taken as the roll surface temperature.

[Volatile fraction of PVA film]

About 5 g of the film was sampled from the center of the width direction (TD) of the PVA film, placed in a heat-resistant glass container, and sealed. Subsequently, the mass Wc of the film excluding the air gap (heat-resistant container) was measured to four decimal places. Subsequently, the film, together with the heat-resistant container, was placed in a vacuum dryer at a temperature of 50°C and a pressure of 0.1 kPa or less, and dried for 4 hours with the lid of the heat-resistant container open, and the mass Wd after drying was measured to four decimal places. From the obtained masses Wc and Wd, the volatile fraction V of the PVA film was obtained by the following formula [IV].

V (mass%) = {(Wc－Wd)/Wc}×100 [IV]

[In the formula, V represents the volatile fraction (mass%) of the PVA film, Wc represents the mass of the PVA film sample (g), and Wd represents the mass (g) when the PVA film sample is placed in a vacuum dryer at a temperature of 50°C and a pressure of 0.1 kPa or less and dried for 4 hours.]

[Number of breaks when elongated 7 times]

From the center of the width direction of a PVA film roll, 10 samples measuring 5 cm in width × 9 cm in length were cut. In order to enable a stretching test over a range of 5 cm in width × 5 cm in length, each sample was mounted on a stretching jig, the sample was maintained in high-temperature water at 70°C for 1 minute, and then stretched up to 7 times at a speed of 120 mm/min. The stretching test was repeated 10 times, and the number of broken samples was determined.

[Example 1]

A PVA solution having a volatile content of 75 mass%, composed of 100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 4000), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first roll having a surface temperature of 94°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 94°C was blown at a speed of 5 m/sec onto the entire film surface of the non-contact surface with the first roll to promote drying. Next, the PVA film was peeled from the first roll, the non-contact surface of the first roll was faced against the second roll having a surface temperature of 85°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled from the second roll, the non-contact surface of the second roll was faced against the third roll having a surface temperature of 75°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed using a heat treatment roll having a surface temperature of 110°C, and the film was wound into a roll shape to obtain a PVA film (thickness 30 μm, width 3 m).

A sample was cut from the central portion of the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained dissolution curve, the dissolution peak temperature, the dissolution end temperature, and the half maximum width of the dissolution peak were obtained.

Next, the number of breaks when the PVA film was stretched 7 times was evaluated, and the number of breaks was 0. It was determined that the film after stretching had elasticity and that the tension required for orientation was generated.

The above results are summarized in Table 1.

[Example 2]

A PVA solution having a volatile content of 70 mass%, composed of 100 parts by mass of PVA (saponification degree of 99.3 mol%, degree of polymerization 2400, ethylene 2 mol% modified), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first roll having a surface temperature of 94°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 94°C was blown at a speed of 5 m/s onto the entire film surface of the non-contact surface with the first roll to promote drying. Next, the PVA film was peeled from the first roll, the non-contact surface of the first roll was faced against the second roll having a surface temperature of 85°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled from the second roll, the non-contact surface of the second roll was faced against the third roll having a surface temperature of 75°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed using a heat treatment roll having a surface temperature of 110°C, and the film was wound into a roll shape to obtain a PVA film (thickness 60 μm, width 3 m).

A sample was cut from the central portion in the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained DSC curve, the melting peak temperature, the melting end temperature, and the half maximum width of the melting peak were obtained.

Next, the number of breaks when the PVA film was stretched 7 times was evaluated, and the number of breaks was 0. It was determined that the film after stretching had elasticity and that the tension required for orientation was generated.

The above results are summarized in Table 1.

[Example 3]

A PVA solution having a volatile content of 75 mass%, composed of 100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 2400), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first roll having a surface temperature of 92°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 92°C was blown at a speed of 5 m/sec onto the entire film surface of the non-contact surface with the first roll to promote drying. Next, the PVA film was peeled from the first roll, the non-contact surface of the first roll was faced against the second roll having a surface temperature of 80°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled from the second roll, the non-contact surface of the second roll was faced against the third roll having a surface temperature of 75°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed using a heat treatment roll having a surface temperature of 110°C, and the film was wound into a roll shape to obtain a PVA film (thickness 60 μm, width 3 m).

A sample was cut from the central portion in the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained DSC curve, the melting peak temperature, the melting end temperature, and the half maximum width of the melting peak were obtained.

Next, the number of breaks when the PVA film was stretched 7 times was evaluated, and the number of breaks was 0. It was determined that the film after stretching had elasticity and that the tension required for orientation was generated.

The above results are summarized in Table 1.

[Example 4]

A PVA solution having a volatile content of 75 mass%, composed of 100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 2400), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first roll having a surface temperature of 90°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 90°C was blown at a speed of 5 m/sec onto the entire film surface on the non-contact surface with the first roll to promote drying. Next, the PVA film was peeled off from the first roll, the non-contact surface of the first roll was faced against the second roll having a surface temperature of 80°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled off from the second roll, the non-contact surface of the second roll was faced against the third roll having a surface temperature of 70°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed using a heat treatment roll having a surface temperature of 110°C, and the film was wound into a roll shape to obtain a PVA film (thickness 60 μm, width 3 m).

A sample was cut from the central portion in the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained DSC curve, the melting peak temperature, the melting end temperature, and the half maximum width of the melting peak were obtained.

Next, the number of breaks when the PVA film was stretched 7 times was evaluated, and the number of breaks was 1. It was determined that the film after stretching had elasticity and that the tension required for orientation was generated.

The above results are summarized in Table 1.

[Comparative Example 1]

A PVA solution having a volatile content of 70 mass%, composed of 100 parts by mass of PVA (saponification degree of 99.3 mol%, degree of polymerization 2400, ethylene 2 mol% modified), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first roll having a surface temperature of 80°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 80°C was blown at a speed of 5 m/sec onto the entire film surface of the non-contact surface with the first roll to promote drying. Next, the PVA film was peeled from the first roll, the non-contact surface of the first roll was faced against the second roll having a surface temperature of 80°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled from the second roll, the non-contact surface of the second roll was faced against the third roll having a surface temperature of 80°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed using a heat treatment roll having a surface temperature of 110°C, and the film was wound into a roll shape to obtain a PVA film (thickness 30 μm, width 3 m).

A sample was cut from the central portion in the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained DSC curve, the melting peak temperature, the melting end temperature, and the half maximum width of the melting peak were obtained.

Next, the number of breaks when the PVA film was stretched 7 times was evaluated, and the number of breaks was 9. The broken film was sticky and was judged to be partially dissolved.

The above results are summarized in Table 1.

[Comparative Example 2]

A PVA solution having a volatile content of 65 mass%, composed of 100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 2400), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first drying roll having a surface temperature of 80°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 80°C was blown at a speed of 5 m/sec onto the entire film surface on the non-contact surface with the first drying roll to promote drying. Next, the PVA film was peeled off from the first drying roll, the non-contact side of the first roll was faced against the second drying roll having a surface temperature of 85°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled off from the second drying roll, the non-contact side of the second roll was faced against the third drying roll having a surface temperature of 75°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed with a heat treatment roll having a surface temperature of 110°C, and then the film was wound into a roll shape to obtain a PVA film (thickness 30 μm, width 3 m).

A sample was cut from the central portion in the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained DSC curve, the melting peak temperature, the melting end temperature, and the half maximum width of the melting peak were obtained.

Next, the number of breaks when the PVA film obtained by the above method was stretched 7 times was evaluated, and the number of breaks was 8. The broken film was sticky and was judged to be partially dissolved.

The above results are summarized in Table 1.

[Comparative Example 3]

A PVA solution having a volatile content of 65 mass%, composed of 100 parts by mass of PVA (degree of saponification 99.9 mol%, degree of polymerization 2400), 9 parts by mass of glycerin as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and water, was prepared by heating and dissolving. This PVA solution was filtered, and the resulting film was discharged from a T die onto a first roll having a surface temperature of 98°C, and dried to a volatile content of 20 mass% (drying process A and drying process B). In addition, hot air at 98°C was blown at a speed of 5 m/sec onto the entire film surface of the non-contact surface with the first roll to promote drying. Next, the PVA film was peeled from the first roll, the non-contact surface of the first roll was faced against the second roll having a surface temperature of 93°C, and dried until the volatile content became 10 mass% (drying process C). Furthermore, the PVA film was peeled from the second roll, the non-contact surface of the second roll was faced against the third roll having a surface temperature of 85°C, and dried until the volatile content became 5 mass% (drying process D). Thereafter, heat treatment was performed using a heat treatment roll having a surface temperature of 110°C, and the film was wound into a roll shape to obtain a PVA film (thickness 30 μm, width 3 m).

A sample was cut from the central portion in the width direction of the obtained PVA film, and differential scanning calorimetry (DSC) of the PVA film was measured in the presence of water. From the obtained DSC curve, the melting peak temperature, the melting end temperature, and the half maximum width of the melting peak were obtained.

Next, the number of breaks when the PVA film was stretched 7 times was evaluated, and the number of breaks was 7. The adhesiveness of the broken film was low, but it was judged to have insufficient elasticity compared to the films of Examples 1 to 3.

The above results are summarized in Table 1.

As is clear from the examples in Table 1, the PVA film of the present invention enabled high-magnification stretching in high-temperature water. More specifically, in Examples 1 to 4 in which the dissolution end temperature was 93.2 to 95.3°C, the number of breaks when stretching 7 times in hot water at 70°C was 0 to 1, meaning that the occurrence of breaks when stretching at a high magnification was suppressed. Furthermore, in Examples 1 to 4, by setting the drying temperature in drying process B to 90 to 94°C, setting the drying temperature in drying process C to 80 to 85°C, and setting the drying temperature in drying process D to 70 to 75°C, it was possible to obtain a PVA film having a dissolution end temperature of 93°C or more and less than 96°C. Meanwhile, in the PVA films of the comparative examples whose dissolution completion temperature was either too low or too high, high-ratio stretching in high-temperature water was difficult. More specifically, in Comparative Examples 1 to 3 whose dissolution completion temperature was less than 93°C and 96°C or higher, the number of breaks at 7-fold stretching in hot water at 70°C was 7 to 9. Furthermore, in Comparative Examples 1 and 2, since the drying temperature in the drying process B was less than 90°C, the dissolution completion temperature was less than 93°C. In Comparative Example 3, since the drying temperature in the drying process B was 95°C or higher, the drying temperature in the drying process C was 90°C or higher, and the drying temperature in the drying process D was 80°C or higher, the dissolution completion temperature was higher than 96°C.

Claims (4)

A polyvinyl alcohol film having a melting completion temperature of 93°C or more and less than 96°C when heat flow is measured for the polyvinyl alcohol film by a differential scanning calorimeter in the presence of water. In paragraph 1,
A polyvinyl alcohol film, wherein the half-width of a dissolution peak is 26.5°C or more when heat flow is measured for the polyvinyl alcohol film using a differential scanning calorimeter in the presence of water. A polarizing film obtained from the polyvinyl alcohol film described in claim 1 or 2. A method for manufacturing a polyvinyl alcohol film, comprising a film forming process, a drying process B, a drying process C, and a drying process D,
The film forming process is a process of forming a film by discharging a polyvinyl alcohol solution having a volatile content of 90 mass% or less into a film.
Drying process B is a process of drying a polyvinyl alcohol film having a volatile content of 20 mass% or more and 30 mass% or less at 90°C to 95°C.
Drying process C is a process of drying a polyvinyl alcohol film having a volatile content of 10 mass% or more and 20 mass% or less at 80°C to 90°C.
Drying process D is a process of drying a polyvinyl alcohol film having a volatile content of 5 mass% or more and 10 mass% or less at 70°C to 80°C.
A method for manufacturing a polyvinyl alcohol film, which comprises performing a film forming process, drying process B, drying process C, and drying process D in that order.

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