JP6858499B2 - Optical film manufacturing method - Google Patents

Description

The present invention relates to a method for producing an optical film capable of obtaining an optical film having a low shrinkage force.

Optical films such as polarizing films used for polarizing plates having a light transmitting and shielding function and retardation films having a function of correcting birefringence are basic components of a liquid crystal display (LCD). Many polarizing plates have a structure in which a protective film such as a cellulose triacetate (TAC) film is bonded to the surface of the polarizing film, and the polarizing film constituting the polarizing plate is a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol"). "to" may be referred to as PVA ") matrix produced by uniaxially stretching a (uniaxial stretched oriented film was oriented) to iodine dye (I ₃ ^- and I ₅ ^-, etc.), such as a dichroic organic dye The ones on which bicolor dyes are adsorbed are the mainstream.

LCDs are used in a wide range of applications such as small devices such as calculators and watches, mobile phones, laptop computers, LCD monitors, LCD color projectors, LCD TVs, in-vehicle navigation systems, and measuring devices used indoors and outdoors. In recent years, there has been a particular demand for larger screens and thinner screens. Along with this, it is required to reduce the shrinkage force of the polarizing film and reduce the warp of the polarizing plate.

As a method for reducing the shrinkage force of the polarizing film, Patent Document 1 describes a first drying step of drying at 25 ° C. or higher and lower than 65 ° C. to a weight moisture content of 20% or less, and a weight moisture content of 14 at 65 ° C. or higher and 115 ° C. or lower. A method having a second drying step of drying to% or less is described. However, with respect to the polarizing film produced by stretching in the length direction of the film by this method, shrinkage in the width direction perpendicular to the stretching direction is suppressed, so that the appearance of the polarizing film is improved, but the polarizing film It was difficult to significantly reduce the contraction force in the length direction (stretching direction) of the film.

Japanese Unexamined Patent Publication No. 2012-47799

The present invention has been made to solve the above problems, and an object of the present invention is to provide a manufacturing method capable of obtaining an optical film having a low shrinkage force, particularly a shrinkage force in the length direction (stretching direction).

As a result of diligent studies to achieve the above object, the present inventors subjected a PVA film having a water content of 25 to 80% by mass to a drying step in producing an optical film, and in the drying step, PVA was subjected to a drying step. We have found that the shrinkage force of the obtained optical film can be significantly reduced by maintaining the drying temperature from 25% by mass to 15% by mass of the film at 80 ° C. or higher, and completed the present invention. I let you.

That is, the present invention
[1] A method for producing an optical film in which at least a swelling step, a stretching step, and a drying step are performed on the PVA film in this order.
A PVA film having a moisture content of 25 to 80% by mass is subjected to the drying step, and in the drying step, the drying temperature from 25% by mass to 15% by mass of the PVA film is maintained at 80 ° C. or higher. A method for producing an optical film, which is characterized by the above;
[2] A PVA film having a moisture content of 40 to 80% by mass is subjected to the drying step, and in the drying step, the drying temperature from 40% by mass to 25% by mass of the film is set to 80 ° C. to 25% by mass. The production method of the above [1], which is characterized by maintaining the temperature at 120 ° C.;
[3] The production method of the above [1] or [2], which comprises drying a polyvinyl alcohol film having a moisture content of 15% by mass or less at 95 ° C. or higher in the drying step;
[4] The production method of the above [1] to [3], wherein the degree of polymerization of PVA exceeds 3600;
[5] The production methods of [1] to [4] above, wherein a polarizing film made of an optical film is obtained by subjecting a PVA film film to at least a swelling step, a dyeing step, a stretching step and a drying step in this order;
Regarding.

According to the manufacturing method of the present invention, an optical film such as a polarizing film having extremely low shrinkage force, particularly shrinkage force in the length direction (stretching direction) can be obtained. By using the polarizing film, a polarizing film having extremely small warpage and excellent polarizing performance can be obtained.

Examples of the PVA used for producing the raw PVA film include those obtained by saponifying a polyvinyl ester obtained by polymerizing a vinyl ester.

The types of the above vinyl esters are not particularly limited, and examples thereof include vinyl acetate, vinyl formate, vinyl propionate, vinyl pivalate, vinyl versatic acid, vinyl laurate, vinyl stearate, vinyl benzoate, isopropenyl acetate and the like. Can be mentioned. These vinyl esters may be used alone or in combination of two or more. Among these vinyl esters, vinyl acetate is preferable from the viewpoints of ease of production, availability, cost and the like of PVA.

In the production of the polyvinyl ester, the vinyl ester may be subjected to a polymerization reaction as it is, or may be mixed with a solvent or other components and then subjected to a polymerization reaction.

The polymerization method of the vinyl ester may be any of batch polymerization, semi-batch polymerization, continuous polymerization, semi-continuous polymerization and the like, and the polymerization method includes a massive polymerization method, a solution polymerization method, a suspension polymerization method, an emulsion polymerization method and the like. Known methods can be applied. A massive polymerization method or a solution polymerization method in which the polymerization proceeds in a solvent such as solvent-free or alcohol is usually adopted. When obtaining a polyvinyl ester having a high degree of polymerization, an emulsion polymerization method is also preferable. The solvent of the solution polymerization method is not particularly limited, but is, for example, alcohol. The alcohol used as the solvent in the solution polymerization method is a lower alcohol such as methanol, ethanol or propanol. The amount of the solvent used in the polymerization system may be selected in consideration of the chain transfer of the solvent according to the degree of polymerization of the target PVA. For example, when the solvent is methanol, the solvent and the total vinyl ester contained in the polymerization system are used. The mass ratio of {= (solvent) / (total vinyl ester)} is preferably selected from the range of 0.01 to 10, and more preferably from the range of 0.05 to 3.

The polymerization initiator used when the vinyl ester is subjected to the polymerization reaction may be selected from known polymerization initiators such as azo-based initiators, peroxide-based initiators, and redox-based initiators according to the polymerization method. .. The azo-based initiator is, for example, 2,2'-azobisisobutyronitrile, 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (4-methoxy-2,4-). Dimethylvaleronitrile). Peroxide-based initiators are percarbonate-based compounds such as diisopropylperoxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethylperoxydicarbonate; t-butylperoxyneodecanate, α- Perester compounds such as cumyl peroxyneodecanate; acetylcyclohexylsulfonyl peroxide; 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; acetyl peroxide. Potassium persulfate, ammonium persulfate, hydrogen peroxide and the like may be combined with the above-mentioned initiator to obtain a polymerization initiator. The redox-based initiator is, for example, a polymerization initiator that is a combination of the above-mentioned peroxide-based initiator and a reducing agent such as sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, L-ascorbic acid, and longalit. The amount of the polymerization initiator used varies depending on the type of the polymerization initiator and cannot be unconditionally determined, but it may be selected according to the polymerization rate. For example, when 2,2'-azobisisobutyronitrile or acetyl peroxide is used as the polymerization initiator, 0.01 to 0.2 mol% is preferable with respect to vinyl ester, and 0.02 to 0.15 mol%. Is more preferable. The polymerization temperature is not particularly limited, but is appropriately about room temperature to 150 ° C., preferably 40 ° C. or higher and lower than the boiling point of the solvent used.

The above polymerization reaction may be carried out in the presence of a chain transfer agent. The chain transfer agent is, for example, aldehydes such as acetaldehyde and propionaldehyde; ketones such as acetone and methyl ethyl ketone; mercaptans such as 2-hydroxyethanethiol; and phosphinates such as sodium phosphite monohydrate. Of these, aldehydes and ketones are preferably used. The amount of the chain transfer agent used can be determined according to the chain transfer coefficient of the chain transfer agent used and the degree of polymerization of the target PVA, but is generally 0.1 to 10 parts by mass with respect to 100 parts by mass of the vinyl ester. Part is preferable.

PVA can be obtained by saponifying the polyvinyl ester obtained as described above. By saponifying the polyvinyl ester, the vinyl ester unit in the polyvinyl ester is converted into the vinyl alcohol unit.

The saponification of the polyvinyl ester can be carried out, for example, in a state where the polyvinyl ester is dissolved in an alcohol or a hydrous alcohol. Examples of the alcohol used for saponification include lower alcohols such as methanol and ethanol, and methanol is preferable. The alcohol used for saponification may contain other solvents such as acetone, methyl acetate, ethyl acetate and benzene in a proportion of 40% by mass or less based on the mass thereof. The catalyst used for saponification is, for example, an alkali metal hydroxide such as potassium hydroxide or sodium hydroxide, an alkali catalyst such as sodium methylate, or an acid catalyst such as mineral acid. The temperature at which saponification is carried out is not limited, but is preferably in the range of 20 to 60 ° C. Since a gel-like product is usually precipitated as the saponification progresses, the product can be pulverized and then washed, dried, or the like to obtain PVA in the form of chips. The saponification method is not limited to the above-mentioned method, and a known method can be applied, and the method for preparing the PVA chip is not particularly limited to the above-mentioned method.

The PVA may be composed only of vinyl alcohol units, but may further contain structural units derived from other monomers copolymerizable with the vinyl ester. Examples of the other monomer include α-olefin having 2 to 30 carbon atoms such as ethylene, propylene, 1-butyl, and isobutene; (meth) acrylic acid or a salt thereof; methyl (meth) acrylate, (meth). ) Ethyl acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) acrylate, t-butyl (meth) acrylate, (Meta) acrylate esters such as 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, octadecyl (meth) acrylate; (meth) acrylamide; N-methyl (meth) acrylamide, N-ethyl ( Meta) acrylamide, N, N-dimethyl (meth) acrylamide, diacetone (meth) acrylamide, (meth) acrylamide propanesulfonic acid or a salt thereof, (meth) acrylamide propyldimethylamine or a salt thereof, N-methylol (meth) acrylamide or (Meta) acrylamide derivatives such as the derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; methylvinyl ether, ethylvinyl ether, n-propylvinyl ether, i-propylvinyl ether, n-butylvinyl ether Vinyl ethers such as i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether; vinyl cyanide such as (meth) acrylonitrile; vinyl halide such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride; acetic acid. Allyl compounds such as allyl and allyl chloride; maleic acid or salts thereof, esters or acid anhydrides; itaconic acid or salts thereof, esters or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; unsaturated sulfonic acids and the like. Can be done. The above polyvinyl ester can have a structural unit derived from one or more of the other monomers described above. The other monomer may be present in the polymerization system in advance when the vinyl ester is subjected to the polymerization reaction, or it may be added to the system during the polymerization reaction. Can be used.

The above PVA may be modified with one or more graft-copolymerizable monomers as long as the effects of the present invention are not impaired. Examples of the graft copolymerizable monomer include unsaturated carboxylic acids or derivatives thereof; unsaturated sulfonic acids or derivatives thereof; α-olefins having 2 to 30 carbon atoms. The proportion of structural units derived from the graft copolymerizable monomer in PVA is preferably 5 mol% or less based on the number of moles of all structural units constituting PVA.

The above-mentioned PVA may or may not have a part of its hydroxyl group crosslinked. Further, in the above-mentioned PVA, a part of its hydroxyl group may react with an aldehyde compound such as acetaldehyde or butyraldehyde to form an acetal structure, or may not react with these compounds to form an acetal structure. You may.

The content of vinyl alcohol units in PVA preferably exceeds 80 mol% based on the number of moles of all structural units constituting PVA, and is preferably 85 mol%, because the effect of the present invention is more pronounced. More preferably, it exceeds 90 mol%. In particular, when water resistance is required such as when the obtained PVA film is used as a raw film for producing an optical film, the content is preferably 95 mol% or more, preferably 97 mol% or more. Is more preferable, and 98 mol% or more is further preferable. When extremely high water resistance is required, the content is preferably 99 mol% or more, more preferably 99.5 mol% or more, and further preferably 99.8 mol% or more.

The degree of polymerization of PVA is preferably in the range of 1,500 to 6,000, more preferably in the range of 1,800 to 5,000, and in the range of 2,000 to 4,000. It is more preferable to have. When the degree of polymerization is 1,500 or more, the durability of the optical film can be further improved when the obtained PVA film is used as a raw film for producing an optical film. On the other hand, when the degree of polymerization is 6,000 or less, it is possible to suppress an increase in manufacturing cost and poor process passability during film formation. Further, from the viewpoint of further reducing the shrinkage force of the optical film, the degree of polymerization is preferably 3600 or more. The degree of polymerization of PVA in the present specification means the average degree of polymerization measured according to the description of JIS K6726-1994.

A film-forming stock solution can be obtained using the PVA chip described above. For example, it can be obtained by mixing one or more of a PVA chip and, if necessary, a surfactant, a plasticizer, an additive, etc., which will be described later, with a liquid medium. Here, a part or all of the liquid medium used may be impregnated with the PVA chip in advance. Further, in the film-forming stock solution, PVA may be in a state of being dissolved in a liquid medium or in a state of being melted. The above mixing is preferably carried out under heating.

Examples of the liquid medium used for preparing the membrane-forming stock solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. , Trimethylolpropane, ethylenediamine, diethylenetriamine and the like, and one or more of these can be used. Of these, water is preferable from the viewpoint of environmental load and recoverability.

The volatile content of the membrane-forming stock solution (the content ratio of volatile components such as liquid media removed by volatilization or evaporation during membrane-forming in the membrane-forming stock solution) varies depending on the membrane-forming method, membrane-forming conditions, etc., but is generally used. It is preferably in the range of 50 to 95% by mass, more preferably in the range of 55 to 90% by mass, and even more preferably in the range of 60 to 85% by mass. When the volatile content of the membrane-forming stock solution is 50% by mass or more, the viscosity of the membrane-forming stock solution does not become too high, filtration and defoaming during preparation of the membrane-forming stock solution are smoothly performed, and PVA with few foreign substances and defects. The production of the film becomes easy. On the other hand, when the volatile fraction of the film-forming stock solution is 95% by mass or less, the concentration of the film-forming stock solution does not become too low, and the production of an industrial PVA film becomes easy.

The film-forming stock solution preferably contains a surfactant. By containing the surfactant, the film-forming property is improved, the occurrence of thickness unevenness of the film is suppressed, and the film can be easily peeled off from the metal roll or belt used for the film-forming. When a PVA film is produced from a film-forming stock solution containing a surfactant, the PVA film may contain the surfactant. The type of the above-mentioned surfactant is not particularly limited, but an anionic surfactant or a nonionic surfactant is preferable from the viewpoint of peelability from a metal roll or a belt.

As the anionic surfactant, for example, a carboxylic acid type such as potassium laurate; a sulfate ester type such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and a sulfonic acid type such as dodecylbenzene sulfonate are suitable.

Examples of the nonionic surfactant include an alkyl ether type such as polyoxyethylene oleyl ether; an alkylphenyl ether type such as polyoxyethylene octylphenyl ether; an alkyl ester type such as polyoxyethylene laurate; and polyoxyethylene laurylamino. Alkylamine type such as ether; Alkylamide type such as polyoxyethylene lauric acid amide; Polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether; Alkanolamide type such as lauric acid diethanolamide and oleic acid diethanolamide; Polyoxy An allylphenyl ether type such as alkylene allylphenyl ether is suitable.

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

When the membrane-forming stock solution contains a surfactant, the content thereof is preferably in the range of 0.01 to 0.5 parts by mass with respect to 100 parts by mass of PVA contained in the membrane-forming stock solution, and is 0.02. It is more preferably in the range of ~ 0.3 parts by mass, and particularly preferably in the range of 0.05 to 0.1 parts by mass. When the content is 0.01 parts by mass or more, the film-forming property and the peelability are further improved. On the other hand, when the content is 0.5 parts by mass or less, it is possible to prevent the surfactant from bleeding out to the surface of the PVA film, causing blocking and deteriorating the handleability.

A PVA film is formed using the film-forming stock solution. Examples of the film forming method include a cast film forming method, an extrusion film forming method, a wet film forming method, and a gel film forming method. These film forming methods may adopt only one kind or a combination of two or more kinds. Among these film-forming methods, the cast film-forming method and the extrusion film-forming method are preferable because a PVA film having a uniform thickness and width and good physical characteristics can be obtained. The formed PVA film can be dried or heat-treated as needed.

As an example of a specific film-forming method for PVA film, for example, a T-shaped slit die, a hopper plate, an I-die, a lip coater die, or the like is used to rotate the above-mentioned film-forming stock solution located on the most upstream side. Volatile components are uniformly discharged or spread on the peripheral surface of the heated first roll (or belt) and discharged or spread from one surface of the film discharged or spread on the peripheral surface of the first roll (or belt). Is evaporated and dried, followed by further drying on the peripheral surface of one or more rotating heated rolls placed downstream thereof, or after passing through a hot air drying device to further dry. , The method of winding by a winding device can be industrially preferably adopted. Drying with a heated roll and drying with a hot air drying device may be carried out in an appropriate combination. As another example, the above-mentioned film-forming stock solution is coated on a thermoplastic resin film using, for example, a T-shaped slit die, a hopper plate, an I-die, a lip coater die, etc., and then dried. Examples thereof include a method of obtaining a PVA film formed on a thermoplastic resin film.

The raw PVA film used in the present invention can contain a plasticizer. Preferred plasticizers include polyhydric alcohols, and specific examples include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, trimethylolpropane and the like. The PVA film can contain one or more of these plasticizers. Among these, glycerin is preferable from the viewpoint of improving stretchability.

The content of the plasticizer in the PVA film is preferably in the range of 1 to 20 parts by mass and more preferably in the range of 3 to 17 parts by mass with respect to 100 parts by mass of PVA contained therein. It is more preferably in the range of ~ 15 parts by mass. When the content is 1 part by mass or more, the stretchability of the PVA film is further improved. On the other hand, when the content is 20 parts by mass or less, it is possible to prevent the PVA film from becoming too flexible and the handleability from being lowered.

PVA films also include fillers, processing stabilizers such as copper compounds, weathering stabilizers, colorants, UV absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, and other thermoplastic resins. Additives such as lubricants, fragrances, defoamers, deodorants, bulking agents, release agents, mold release agents, reinforcing agents, cross-linking agents, fungicides, preservatives, crystallization rate retarders, etc., as needed Can be included.

The ratio of the total of PVA and the plasticizer in the PVA film is preferably 80% by mass or more, more preferably 90% by mass or more, and 95% by mass or more, based on the mass of the PVA film. Is even more preferable.

The thickness of the PVA film is not particularly limited, but is usually 1 to 100 μm, preferably 5 to 75 μm, and particularly preferably 10 to 60 μm. If the thickness is too thin, stretch breakage tends to occur easily during the uniaxial stretching process for producing an optical film. On the other hand, if the thickness is too thick, stretching spots tend to occur during the uniaxial stretching treatment for producing an optical film. The PVA film is preferably a single-layer film, but may be a laminate in which a PVA layer and another layer such as a thermoplastic resin film are laminated. In the case of a laminated body, the thickness of the PVA layer is preferably in the above range.

The width of the PVA film is not particularly limited and can be determined according to the intended use of the optical film to be manufactured. Since the screen size of liquid crystal televisions and liquid crystal monitors has been increasing in recent years, it is suitable for these applications if the width of the PVA film is set to 3 m or more. The width of the PVA film is 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 the uniaxial stretching itself when manufacturing the optical film with a practical device, so the width of the PVA film should be 7 m or less. Is preferable, and it is more preferably 6 m or less, and further preferably 5 m or less.

The shape of the PVA film is not particularly limited, but a more uniform PVA film can be continuously and smoothly produced, and the PVA film can be continuously used even when an optical film is produced using the PVA film. It is preferably a long film. The length of the long film (length in the length direction) is not particularly limited and can be appropriately set according to the intended use, for example, in the range of 5 to 30,000 m.

An optical film is produced by subjecting the raw PVA film thus obtained to at least a swelling step, a stretching step, and a drying step in this order. Usually, an optical film is obtained by first performing a swelling step on a raw PVA film, then performing various steps including a stretching step, and finally performing a drying step to dry the film.

As a method for producing a polarizing film made of the optical film, which is a preferred embodiment of the optical film of the present invention, at least a swelling step, a dyeing step, a stretching step and a drying step are performed in this order on the original PVA film. It is possible to adopt a method of applying at least a swelling step, a stretching step and a drying step in this order to the PVA film of the raw fabric containing the dye, and the former is preferable, at least the swelling step and the dyeing step. , The method of performing the cross-linking step, the stretching step and the drying step in this order is more preferable. It is also preferable that the PVA film is subjected to a stretching step, then a fixing treatment step, and then a drying step.

In the production method of the present invention, the PVA film may be appropriately subjected to a washing step before the drying step. Further, in the production method of the present invention, one type of step may be performed a plurality of times.

In the production method of the present invention, the swelling step can be performed by immersing the PVA film in water. The temperature of water at this time is preferably in the range of 20 to 40 ° C, more preferably in the range of 22 to 38 ° C, and further preferably in the range of 25 to 35 ° C. The time of immersion in water is preferably in the range of 0.1 to 5 minutes, and more preferably in the range of 0.5 to 3 minutes. The water when immersed in water is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and a water-soluble organic solvent.

The degree of swelling of the PVA film is preferably in the range of 160 to 240% by mass, more preferably in the range of 170 to 230% by mass, and particularly preferably in the range of 180 to 220% by mass. .. When the degree of swelling is 160% or more, it is possible to suppress the crystallization of PVA from progressing extremely, and it is possible to stably stretch to a higher magnification. On the other hand, when the degree of swelling is 240% by mass or less, dissolution of PVA during stretching is suppressed, and stretching can be performed even under higher temperature conditions. In the present invention, the degree of swelling of the PVA film is a value obtained by dividing the mass of the PVA film when immersed in distilled water at 30 ° C. for 30 minutes by the mass after drying at 105 ° C. for 16 hours after immersion. Means the percentage of.

In the production method of the present invention, the dyeing step can be performed by contacting the PVA film with a dichroic dye. By subjecting the swollen PVA film to a dyeing step, a polarizing film, which is a preferred embodiment of the optical film of the present invention, is produced. Iodine-based pigments are generally used as the dichroic pigments. The dyeing step may be performed before the stretching step described later or after the stretching step, but the former is preferable. The dyeing step is generally performed by immersing the PVA film in a solution containing iodine-potassium iodide (particularly an aqueous solution) as a dyeing bath, and such a dyeing method is also preferably adopted in the present invention. To. The concentration of iodine in the dyeing bath is preferably in the range of 0.01 to 0.5% by mass, and the concentration of potassium iodide is preferably in the range of 0.01 to 10% by mass. The temperature of the dyeing bath is preferably 20 to 50 ° C, particularly preferably 25 to 40 ° C. The time for immersing the PVA film in the dyeing bath is preferably in the range of 0.1 to 10 minutes, more preferably in the range of 0.2 to 5 minutes.

In the production method of the present invention, it is preferable to carry out a crosslinking step on the PVA film. By performing a cross-linking step of cross-linking the PVA film, it is possible to more effectively prevent PVA from elution into water during wet stretching at a high temperature. From this point of view, it is preferable that the crosslinking step is performed before the stretching step. When performing the above dyeing step, it is preferable to carry out a cross-linking step after the dyeing step. The cross-linking treatment can be performed by immersing the PVA film in an aqueous solution containing a cross-linking agent. As the cross-linking agent, one or more kinds of boron compounds such as borate such as boric acid and borax can be used. The concentration of the cross-linking agent in the aqueous solution containing the cross-linking agent is preferably in the range of 1 to 15% by mass, more preferably in the range of 2 to 7% by mass, and in the range of 3 to 6% by mass. Is even more preferable. Sufficient stretchability can be maintained when the concentration of the cross-linking agent is in the range of 1 to 15% by mass. The aqueous solution containing the cross-linking agent may contain an auxiliary agent such as potassium iodide. The temperature of the aqueous solution containing the cross-linking agent is preferably in the range of 20 to 50 ° C., particularly preferably in the range of 25 to 40 ° C. By setting the temperature within the range of 20 to 50 ° C., cross-linking can be performed efficiently.

Apart from the stretching step described later, the PVA film may be stretched during or between the steps described above. By performing such stretching (pre-stretching), it is possible to prevent the PVA film from being wrinkled. The total stretching ratio of the pre-stretching (magnification obtained by multiplying the stretching ratio in each step) is 4 based on the original length of the PVA film of the original fabric before stretching from the viewpoint of polarization performance in the case of producing a polarizing film. It is preferably 2 times or less. The draw ratio in the swelling step is preferably 1.05 to 3 times, the draw ratio in the dyeing step is preferably 3 times or less, and the draw ratio in the cross-linking step is preferably 2 times or less.

In the production method of the present invention, the stretching step can be performed by uniaxially stretching the PVA film using a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be carried out in an aqueous solution containing boric acid, in the above-mentioned dyeing bath or in the fixing treatment bath described later. Further, in the case of the dry stretching method, stretching may be carried out at room temperature, stretching may be carried out while heating, or may be carried out in air using a PVA film after water absorption. Among these, the wet stretching method is preferable, and uniaxial stretching is more preferable in an aqueous solution containing boric acid. The concentration of boric acid in the boric acid aqueous solution is preferably in the range of 0.5 to 6.0% by mass, more preferably in the range of 1.0 to 5.0% by mass, and 1.5. It is particularly preferably in the range of ~ 4.5% by mass. Further, the boric acid aqueous solution may contain potassium iodide, and the concentration thereof is preferably in the range of 0.01 to 10% by mass.

The stretching temperature in the stretching step is preferably in the range of 50 to 90 ° C, more preferably in the range of 50 to 80 ° C, and particularly preferably in the range of 50 to 70 ° C. From the viewpoint that the shrinkage force of the obtained optical film in the stretching direction is significantly reduced and the dimensional stability is further improved, the PVA film is formed at such a stretching temperature (liquid temperature) by using a wet stretching method. Uniaxial stretching is particularly preferable. By performing the stretching step on the film under such conditions, the amount of water contained in the PVA film increases. Then, due to the plasticizing effect of a large amount of water in the PVA film, the amorphous portion of the PVA film stretched more than necessary is relaxed during the drying step, the PVA chains are easily rearranged, and the resulting optical film shrinks. It is considered that the force, particularly the contraction force in the stretching direction, is significantly reduced. By reducing the shrinkage force of the optical film in this way, the dimensional stability can be further improved, and the warp when the optical film is attached to the protective film can be further reduced. When the stretching temperature is out of the above range, or when the dry stretching method is adopted, it may be difficult to set the moisture content of the PVA film to be subjected to the drying step to 25% by mass or more.

From the viewpoint of polarization performance and the like when a polarizing film is manufactured, the draw ratio in the stretching step is preferably 1.2 times or more, more preferably 1.5 times or more, and more preferably 2 times or more. Is more preferable. Further, the total draw ratio including the draw ratio of the above-mentioned pre-stretch (the ratio obtained by multiplying the draw ratio in each step) is 6.8 times or more based on the original length of the PVA film of the original fabric before stretching. It is preferably 6.9 times or more, more preferably 7.0 times or more. The upper limit of the total draw ratio is not particularly limited, but the draw ratio is preferably 8 times or less in order to prevent stretch breakage.

When the long PVA film is uniaxially stretched, the direction of uniaxial stretching is not particularly limited, and uniaxial stretching in the long direction and lateral uniaxial stretching in the width direction can be adopted. In the case of producing a polarizing film, uniaxial stretching in the long direction is preferable from the viewpoint of obtaining an excellent polarizing film. Uniaxial stretching in the long direction can be performed by using a stretching device including a plurality of rolls parallel to each other and changing the peripheral speed between the rolls. On the other hand, the lateral uniaxial stretching can be performed using a tenter type stretching machine. In the present invention, the shrinkage force in the stretching direction of the optical film is evaluated. Therefore, when the optical film is manufactured by uniaxially stretching the PVA film in the long direction, the shrinkage force in the long direction is evaluated. On the other hand, when an optical film is produced by laterally uniaxially stretching the PVA film in the width direction, the shrinkage force in the width direction is evaluated.

When producing a polarizing film, it is preferable to perform a fixing treatment step on the PVA film after the stretching step. As a result, the adsorption of the dichroic dye (iodine-based dye, etc.) on the PVA film becomes strong. As the fixing treatment bath used for the fixing treatment, an aqueous solution containing one or more kinds of boron compounds such as boric acid and borax can be used. Further, if necessary, an iodine compound or a metal compound may be added to the fixing treatment bath. The concentration of the boron compound in the fixing treatment bath is generally preferably about 2 to 15% by mass, particularly preferably about 3 to 10% by mass. By setting the concentration within the range of 2 to 15% by mass, the adsorption of the dichroic dye can be further strengthened. The temperature of the fixing treatment bath is preferably 15 to 60 ° C, particularly preferably 25 to 40 ° C.

In the production method of the present invention, the PVA film obtained by performing at least the swelling step and the stretching step in this order on the original PVA film is subjected to a drying step. At this time, it is necessary to subject the PVA film having a moisture content of 25 to 80% by mass to the drying step. When the moisture content before the drying step is less than 25% by mass, the shrinkage force of the obtained optical film becomes high. Therefore, the dimensional stability of the optical film is lowered, and warpage occurs when the optical film is attached to the protective film. The water content is preferably 30% by mass or more, more preferably 40% by mass or more. On the other hand, if the moisture content before the drying step exceeds 80% by mass, the PVA film may rapidly shrink and break in the drying step, or the polarizing performance may be significantly deteriorated when the polarizing film is manufactured. is there. The water content of the PVA film is preferably 70% by mass or less.

In the production method of the present invention, a PVA film having a water content of 25 to 80% by mass is subjected to a drying step. In addition, it is necessary to maintain the drying temperature of the PVA film from 25% by mass to 15% by mass at 80 ° C. or higher. By drying the PVA film at such a high temperature, the shrinkage force of the obtained optical film, particularly the shrinkage force in the stretching direction, is remarkably reduced. This improves the dimensional stability of the optical film. Further, the warp can be reduced when the optical film is attached to the protective film. The drying temperature from 25% by mass to 15% by mass of the PVA film is preferably maintained at 85 ° C. or higher, and more preferably 90 ° C. or higher. On the other hand, the upper limit of the drying temperature is not particularly limited, but it is preferably maintained at 140 ° C. or lower.

In the production method of the present invention, a PVA film having a moisture content of 40 to 80% by mass is subjected to the drying step, and in the drying step, the drying temperature until the moisture content of the PVA film becomes 40% by mass to 25% by mass. Is preferably maintained at 80 ° C to 120 ° C. As described above, even when the moisture content of the PVA film is higher, the shrinkage force of the obtained optical film is further reduced by drying at a high temperature. It is more preferable to maintain the drying temperature from 40% by mass to 25% by mass of the PVA film at 85 ° C. or higher, and even more preferably at 90 ° C. or higher. On the other hand, if the drying temperature exceeds 120 ° C., the polarizing performance may be insufficient when the polarizing film is manufactured. From this point of view, it is more preferable to maintain the drying temperature at 110 ° C. or lower.

In the drying step, it is preferable to dry the PVA film having a moisture content of 15% by mass or less at 95 ° C. or higher. As a result, the shrinkage force of the obtained optical film is further reduced. At this time, when further drying the PVA film having a moisture content of 15% by mass, the temperature may exceed 95 ° C. during drying, and it is not necessary to maintain the temperature exceeding 95 ° C. It is more preferable to dry the PVA film having a moisture content of 15% by mass or less at a temperature exceeding 120 ° C., and further preferably to dry at a temperature exceeding 135 ° C. Further, in the drying step, it is preferable to dry the PVA film until the moisture content becomes 10% by mass or less. When the moisture content of the optical film is in such a range, wrinkles are less likely to occur on the surface of the optical film when it is bonded to a protective film or the like. Further, when an ultraviolet curable adhesive is used for bonding with a protective film or the like, the adhesive force tends to be stable. In the drying step, it is more preferable to dry the PVA film until the moisture content becomes 5% by mass or less.

As described above, it is a major feature of the present invention that the PVA film having a high water content is dried at a high temperature in the drying step. As a result, the contraction force of the obtained optical film, particularly the contraction force in the stretching direction, is remarkably reduced. The dimensional stability is improved by reducing the shrinkage force of the optical film, and the warp of the laminated body can be reduced when the optical film is attached to the protective film. The reason for this effect is not clear, but it is relatively clear that drying a PVA film with a high water content at a high temperature relaxes and rearranges the amorphous portions that have been stretched more than necessary. It is presumed that the formation of PVA crystals with a large and stable structure contributes.

In the production method of the present invention, when the drying step is performed on a PVA film having a moisture content of 40 to 80% by mass, the drying temperature until the moisture content of the PVA film becomes 50% by mass to 40% by mass is set. It is preferable to maintain the temperature below 120 ° C. If the temperature exceeds 120 ° C., the polarizing performance may be insufficient when a polarizing film is manufactured. From this point of view, it is more preferable to maintain the drying temperature at 110 ° C. or lower.

In the production method of the present invention, when a PVA film having a moisture content of 50 to 80% by mass is subjected to the drying step, the drying temperature until the moisture content of the PVA film reaches 50% by mass is maintained at 80 ° C. or lower. Is preferable. If the drying temperature exceeds 80 ° C., the polarizing performance may be insufficient when the polarizing film is manufactured. From this point of view, it is more preferable to maintain the drying temperature at 60 ° C. or lower, and it is further preferable to maintain the drying temperature at 40 ° C. or lower.

In the drying step, the PVA film can be dried using a hot air dryer, an infrared dryer, or the like, and it is particularly preferable to use a hot air dryer. When a hot air dryer is used, the temperature inside the dryer may be the above-mentioned drying temperature.

According to the manufacturing method of the present invention, an optical film such as a polarizing film, a retardation film, or a protective film having extremely low shrinkage force, particularly shrinkage force in the length direction (stretching direction) can be obtained. Among them, the polarizing film made of the optical film is used as a polarizing plate by laminating a protective film that is optically transparent and has high mechanical strength on both sides or one side thereof. As the protective film, a cellulose triacetate (TAC) film, a cellulose acetate / butyrate (CAB) film, an acrylic film, a polyester film, or the like is used. Examples of the adhesive used at this time include PVA-based adhesives and urethane-based adhesives, and among them, PVA-based adhesives are preferable.

The polarizing plate obtained as described above can be used as a component of an LCD by being coated with an adhesive such as acrylic and then bonded to a glass substrate. At the same time, it may be bonded to a retardation film, a viewing angle improving film, a brightness improving film, or the like.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto. The measurements and evaluations performed in the following examples and comparative examples are shown below.

[Film moisture content]
In Examples or Comparative Examples, the mass A (g) of the collected PVA film was measured, and then the PVA film was placed in a dryer at 105 ° C. After drying at 105 ° C. for 16 hours, the mass B (g) of the dried PVA film was measured. The moisture content (mass%) of the film was calculated using the following formula.
Moisture content of PVA film = (AB) ÷ A × 100

[Film shrinkage]
The shrinkage force of the polarizing film obtained in Examples or Comparative Examples was measured using an autograph "AG-X" with a constant temperature bath manufactured by Shimadzu Corporation and a video-type elongation meter "TR ViewX120S". A polarizing film whose humidity was adjusted at 20 ° C./20% RH for 18 hours was used for the measurement. After the constant temperature bath of the autograph "AG-X" was set to 20 ° C., a polarizing film [length direction (stretching direction) 15 cm, width direction 1.5 cm] was attached to the chuck (chuck interval 5 cm), and at the same time as the tension started, the tension was started. The temperature rise of the constant temperature bath to 80 ° C. was started. The polarizing film was pulled at a speed of 1 mm / min, the tension was stopped when the tension reached 2N, and the tension was measured up to 4 hours later in that state. At this time, since the distance between the chucks changes due to thermal expansion, a marked line sticker is attached to the chuck, and the distance between the chucks is increased by the amount of movement of the marked line sticker attached to the chuck using the video type extensometer "TR ViewX120S". Was measured so that it could be corrected. Since the minimum value of tension occurs at the initial stage of measurement (within 10 minutes from the start of measurement), the minimum value of tension was subtracted from the measured value of tension after 4 hours, and the difference was taken as the shrinkage force of the polarizing film.

[Polarization performance of film]
(1) Measurement of Transmittance Ts Two 2 cm samples were taken from the central portion of the polarizing film obtained in the Example or Comparative Example in the length direction (stretching direction) of the polarizing film, and a spectrophotometer with an integrating sphere was taken. Using (“V7100” manufactured by Nippon Spectroscopy Co., Ltd.), the visibility of the visible light region of the C light source and 2 ° field is corrected in accordance with JIS Z 8722 (measurement method of object color), and one sample is used. The light transmittance when tilted by + 45 ° and the light transmittance when tilted by −45 ° with respect to the length direction were measured, and their average value Ts1 (%) was obtained. In the same manner for the other sample, the light transmittance when tilted by + 45 ° and the light transmittance when tilted by −45 ° were measured, and their average value Ts2 (%) was obtained. Then, Ts1 and Ts2 were averaged by the following formula to obtain the transmittance Ts (%) of the polarizing film.
Ts = (Ts1 + Ts2) / 2

(2) Measurement of polarization degree V Light transmittance T‖ (%) when two samples collected in the measurement of transmittance Ts in (1) are stacked so that their length directions are parallel. The light transmittance T⊥ (%) when stacked so that the length directions are orthogonal to each other is measured in the same manner as in the case of “(1) Measurement of transmittance Ts” above, and the degree of polarization V ( %) Was asked.
V = {(T‖-T⊥) / (T‖ + T⊥)} ^1/2 x 100

[Example 1]
Contains 100 parts by mass of PVA (saponification degree 99.9 mol%, degree of polymerization 4000), 10 parts by mass of glycerin as a plasticizer, and 0.1 parts by mass of polyoxyethylene lauryl ether sodium sulfate as a surfactant, and the content of PVA. An aqueous solution having a content of 7% by mass was used as the film-forming stock solution. This was dried on a metal roll at 80 ° C., and the obtained film was heat-treated at a temperature of 119 ° C. for 10 minutes in a hot air dryer to produce a PVA film having a thickness of 30 μm. The degree of swelling of the PVA film was measured and found to be 215%.

A sample having a width of 5 cm and a length of 9 cm is cut from the central portion of the obtained PVA film in the width direction, and a uniaxial stretching jig is used so that the range of the width of 5 cm and the length of 5 cm can be uniaxially stretched in the flow direction during film formation. The sample was fixed. As a swelling step, this sample was immersed in pure water at 30 ° C., and during that time, the sample was uniaxially stretched 1.1 times in the length direction. Subsequently, iodine was adsorbed by immersing it in an aqueous solution (dyeing bath, temperature 30 ° C.) containing iodine and potassium iodide at a mass ratio of 1: 100 for 60 seconds, and during that time, it was 2.2 times (2.4 in total). It was uniaxially stretched in the length direction. At this time, the iodine concentration in the dyeing bath was adjusted so that the transmittance of the polarizing film after drying was 44%. Next, it was immersed in an aqueous solution (crosslinked bath, temperature 30 ° C.) containing 3% by mass of boric acid and 3% by mass of potassium iodide, and in the meantime, the length was 1.1 times (2.7 times in total). It was uniaxially stretched in the longitudinal direction. Further, it was immersed in an aqueous solution (stretching bath, temperature 62 ° C.) containing 4% by mass of boric acid and 6% by mass of potassium iodide, and the length was 2.1 times (5.8 times in total) during that time. It was uniaxially stretched in the direction. Then, it was immersed in an aqueous solution (washing bath, temperature 30 ° C.) containing potassium iodide at a ratio of 2.2% by mass for 5 seconds.

The PVA film (moisture content 50% by mass) taken out from the washing bath and wiped off the aqueous solution adhering to the surface was pre-dried at 20 ° C. for 1 minute under a constant length to adjust the water content of the PVA film to 40% by mass. Then, using a hot air dryer, the PVA film is dried at 100 ° C. (first drying step) until the moisture content of the PVA film reaches 15% by mass under a fixed length, and then dried at 140 ° C. for 2 minutes (second drying). Step) was carried out to obtain a polarizing film. The above temperature is the temperature inside the dryer. Table 1 shows the moisture content of each film, the polarization performance of the obtained polarizing film, the transmittance, and the shrinkage force.

[Example 2]
The PVA film was dried at 86 ° C. (first drying step) until the moisture content of the PVA film changed from 40% by mass to 15% by mass, and then dried at 125 ° C. for 2 minutes (second drying step). A polarizing film was produced in the same manner as in Example 1. Table 1 shows the moisture content of each film, the polarization performance of the obtained polarizing film, the transmittance, and the shrinkage force.

[Example 3]
The PVA film was dried at 100 ° C. (first drying step) until the moisture content of the PVA film changed from 40% by mass to 15% by mass, and then dried at 100 ° C. for 3 minutes (second drying step). The polarizing film 3 was produced in the same manner as in Example 1. Table 1 shows the moisture content of each film, the polarization performance of the obtained polarizing film, the transmittance, and the shrinkage force.

[Comparative Example 1]
The PVA film was dried at 100 ° C. (first drying step) until the moisture content of the PVA film changed from 40% by mass to 25% by mass, and then dried at 20 ° C. for 4 minutes (second drying step). The polarizing film 4 was produced in the same manner as in Example 1. Table 1 shows the moisture content of each film, the polarization performance of the obtained polarizing film, the transmittance, and the shrinkage force.

[Comparative Example 2]
The PVA film was dried at 70 ° C. (first drying step) until the moisture content of the PVA film changed from 40% by mass to 15% by mass, and then further dried at 70 ° C. for 4 minutes (second drying step). A polarizing film was produced in the same manner as in Example 1. Table 1 shows the moisture content of each film, the polarization performance of the obtained polarizing film, the transmittance, and the shrinkage force.

[Comparative Example 3]
The PVA film (moisture content 50% by mass) was pre-dried at 20 ° C. for 7 minutes under a constant length to set the film moisture content to 13% by mass, and the film moisture content changed from 13% to 8% by mass. A polarizing film was produced in the same manner as in Example 1 except that the film was dried at 100 ° C. (first drying step) and then dried at 100 ° C. for 3 minutes (second drying step). Table 1 shows the moisture content of each film, the polarization performance of the obtained polarizing film, the transmittance, and the shrinkage force.

As described above, according to the present invention, since a polarizing film having a low shrinkage force can be manufactured, it is possible to reduce the warp of the polarizing plate when a polarizing film is manufactured by adhering a protective film to the surface of the polarizing film. Become.

Claims (3)

A method for producing an optical film in which at least a swelling step, a stretching step, and a drying step are performed in this order on a polyvinyl alcohol film.
A polyvinyl alcohol film having a water content of 40 to 80% by mass is subjected to the drying step, and in the drying step, the drying temperature from 40% by mass to 25% by mass of the polyvinyl alcohol film is set to 80 ° C. to 120%. After maintaining the temperature, the drying temperature from 25% by mass to 15% by mass of the polyvinyl alcohol film is maintained at 80 ° C. or higher, and the polyvinyl alcohol film having a moisture content of 15% by mass or less is 95 ° C. A method for producing an optical film, which comprises drying as described above. The method for producing an optical film according to claim 1, wherein the degree of polymerization of polyvinyl alcohol exceeds 3600. The production method according to claim 1 or 2 , wherein a polarizing film made of an optical film is obtained by subjecting a polyvinyl alcohol film to at least a swelling step, a dyeing step, a stretching step, and a drying step in this order.