Polyvinyl alcohol film, polarizing film and polarizing plate using same, and method for producing polyvinyl alcohol film

Abstract

This polyvinyl alcohol film has a refractive index of 1.500600-1.500740 in the MD direction. In the polyvinyl alcohol film, the average inclination value of the film thicknesses in the MD direction is 0.0265 μm / mm or less. In such a polyvinyl alcohol film, film breakage is suppressed even when subjected to a stretch treatment at a higher temperature and a higher ratio than in conventional treatments, and color unevenness when used as a polarizing film is less likely to be visually recognized.

Description

Polyvinyl alcohol film, polarizing film and polarizing plate using the same, and method for producing polyvinyl alcohol film

[0001] The present invention relates to a polyvinyl alcohol film, a polarizing film and a polarizing plate using the same, and a method for producing the polyvinyl alcohol film.

[0002] Polarizing plates, which have the function of transmitting and blocking light, are the basic components of liquid crystal displays (LCDs), along with liquid crystals, which change the polarization state of light. LCDs are widely used in a wide range of applications, including small devices such as calculators and watches, laptops, LCD monitors, LCD color projectors, LCD televisions, car navigation systems, mobile phones, and measuring instruments used indoors and outdoors.

[0003] Polarizing plates are generally manufactured by dyeing a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA"), uniaxially stretching it, and optionally fixing it with a boron compound or the like to produce a polarizing film, followed by laminating a protective film such as a triacetate cellulose (TAC) film to the surface of the polarizing film. In recent years, the increasing demand for LCDs has created a demand for efficient production of polarizing films, one of the LCD components. To improve the production efficiency of polarizing films, it is considered to employ high stretching ratio conditions using a high-temperature solution during polarizing film production. With the increasing stretching ratio, conventional PVA films may break during stretching, and PVA films with better stretchability than conventional products are required. One solution to this problem is to reduce the draw during film formation, but there is a concern that uneven thickness of the PVA film may result in visible unevenness in the polarizing plate.

[0004] Patent Document 1 describes a PVA film in which unevenness in the polarizing plate is less visible when stretched by setting the draw (stretch ratio) at a moisture content of 20% by mass or more to 1.075 to 1.135 in the production of the PVA film, and a polarizing film and polarizing plate using this PVA film. However, in recent years, there has been an increase in cases in which polarizing films are produced at higher temperatures and at higher stretch ratios than before, and this requires a PVA film that can be stretched at a high ratio.

[0005] International Publication No. 2022 / 145489

[0006] Therefore, an object of the present invention is to provide a PVA film that is less likely to break even when stretched at a higher temperature and a higher ratio than conventional films, and that, when made into a polarized film, causes less visible color unevenness, and a method for producing the same.

[0007] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by adjusting the refractive index of a PVA film in a direction parallel to the machine direction (hereinafter sometimes abbreviated as "MD") to a specific range and by using a PVA film with reduced film thickness unevenness in the MD. Based on this finding, further research led to the completion of the present invention.

[0008] That is, the present invention relates to [1] to

[10] . [1] A PVA film having a refractive index in the MD direction of 1.500600 to 1.500740 and an average gradient of the film thickness in the MD direction of 0.0265 μm / mm or less. [2] The PVA film according to [1], wherein the difference between the maximum and minimum film thicknesses is 2.23 μm or less. [3] The PVA film according to [1] or [2], wherein the average film thickness is 10 to 100 μm. [4] The PVA film according to any one of [1] to [3], wherein the refractive index in the TD direction is 1.500165 to 1.500350. [5] A polarizing film produced using the PVA film according to any one of [1] to [4]. [6] A polarizing plate having a protective film attached to at least one surface of the polarizing film according to [5]. [7] A method for producing a PVA film, comprising a casting step of casting a film-forming solution containing polyvinyl alcohol onto a support and drying it to obtain a film, a drying step of drying the film on at least three supports, and a heat-treating step of heat-treating the film on the support, wherein in the drying step, the surface temperature of the support is 85°C to 99°C, the support has a minimum draw ratio of 0.985 or more and a maximum draw ratio of 1.000 or less, and the support has a cumulative stretch ratio of 0.980 to 0.983, and the film has a moisture content of 9% to 14% by mass after the drying step. [8] The method for producing a PVA film according to [7], wherein in the casting step, the support has a draw ratio of 1.00 to 1.05 and the film has a moisture content of 14% to 20% by mass after the casting step. [9] The method for producing a PVA film according to [7] or [8], wherein the film has a moisture content of 1% to 9% by mass after the heat-treating step.

[10] The method for producing a PVA film according to any one of [7] to [9], wherein the cumulative stretch ratio of the support in the heat treatment step is 1.00 or less.

[0009] According to the present invention, there are provided a PVA film which is prevented from breaking even when stretched at a higher temperature and a higher ratio than conventional films, and which, when formed into a polarized film, causes less visible color unevenness, and a method for producing the same.

[0010] [PVA Film] The PVA film of the present invention has a refractive index in the MD direction of 1.500600 to 1.500740 and an average film thickness gradient in the MD direction of 0.0265 μm / mm or less. The MD direction is the direction parallel to the machine direction, i.e., the film transport direction, and the TD direction is the direction perpendicular to the MD direction.

[0011] In the PVA film of the present invention, the refractive index in the MD direction (hereinafter sometimes abbreviated as "MD refractive index") is 1.500600 to 1.500740. If the MD refractive index is less than 1.500600, the tension decreases when the PVA film is stretched during production of the polarized film, preventing the orientation of the PVA in the film and potentially reducing the degree of polarization of the resulting polarized film. The MD refractive index is preferably 1.500620 or higher, more preferably 1.500650 or higher, and even more preferably 1.500680 or higher. On the other hand, if the MD refractive index exceeds 1.500740, the tension increases when the PVA film is stretched during production of the polarized film, making the PVA film more susceptible to breakage when stretched at a high stretch ratio. The MD refractive index is preferably 1.500720 or lower, more preferably 1.500700 or lower, and even more preferably 1.500695 or lower. The MD refractive index can be determined by the method described in the examples below.

[0012] The PVA film of the present invention is characterized in that the average gradient value of the film thickness in the MD direction (hereinafter sometimes abbreviated as "MD average gradient value") is 0.0265 μm / mm or less. The MD average gradient value is preferably 0.0262 μm / mm or less, and more preferably 0.0260 μm / mm or less. If the MD average gradient value is greater than 0.0265 μm / mm, color unevenness is likely to occur when the film is made into a polarized film. Furthermore, the PVA film is likely to break when stretched at a high ratio during the production of the polarized film. On the other hand, the MD average gradient value is preferably 0.0250 μm / mm or more, more preferably 0.0255 μm / mm or more, and even more preferably 0.0257 μm / mm or more. When the MD average gradient value is equal to or greater than the lower limit, breakage of the PVA film is further reduced when stretched at a high ratio during the production of the polarized film, although the reason is not clear. The MD average gradient value can be determined by the method described in the Examples below.

[0013] In the PVA film of the present invention, the difference between the maximum and minimum film thickness in the MD direction is preferably 2.23 μm or less. This difference is more preferably 2.20 or less, and even more preferably 2.15 or less. Meanwhile, this difference is preferably 2.00 or more, more preferably 2.03 or more, even more preferably 2.08 or more, and even more preferably 2.10 or more. When this difference is equal to or greater than the lower limit or equal to or less than the upper limit, breakage of the PVA film is further reduced when stretched at a high ratio during the production of the polarizing film, although the reason is not clear. The difference between the maximum and minimum film thickness in the MD direction can be determined by the method described in the Examples below.

[0014] In the PVA film of the present invention, the average film thickness in the MD direction is preferably 10 to 100 μm. This average is more preferably 80 μm or less, even more preferably 60 μm or less, and even more preferably 50 μm or less. Having this average value equal to or less than the upper limit increases the drying rate during production of the PVA film or polarizing film. On the other hand, this average is more preferably 20 μm or more, even more preferably 30 μm or more. Having this average value equal to or greater than the lower limit reduces breakage of the PVA film when stretched at a high ratio during production of the polarizing film. The average film thickness in the MD direction can be determined by the method described in the Examples below.

[0015] In the PVA film of the present invention, the refractive index in the TD direction (hereinafter sometimes abbreviated as TD refractive index) is preferably 1.500165 to 1.500350. The TD refractive index is more preferably 1.500300 or less, even more preferably 1.500250 or less, and even more preferably 1.500195 or less. On the other hand, the TD refractive index is more preferably 1.500170 or more, even more preferably 1.500172 or more, and even more preferably 1.500174 or more. When the TD refractive index is equal to or greater than the above-mentioned lower limit or equal to or less than the above-mentioned upper limit, breakage of the PVA film when stretched at a high ratio during production of the polarizing film is further reduced, and a polarizing film with excellent polarization performance can be produced. The TD refractive index can be determined by the method described in the Examples below.

[0016] (PVA) In the PVA film of the present invention, a polymer produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer can be used as the PVA. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among these, vinyl acetate is preferred as the vinyl ester monomer.

[0017] The vinyl ester polymer is preferably a polymer obtained using only one or more vinyl ester monomers as the monomer, more preferably a polymer obtained using only one vinyl ester monomer as the monomer, and may also be a copolymer of one or more vinyl ester monomers with other monomers copolymerizable therewith.

[0018] Examples of other monomers include ethylene; olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or salts thereof; 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 salts thereof; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, methacrylic acid, and methacrylic acid. methacrylic acid esters such as i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamides such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, and 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; 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; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate.The vinyl ester polymer may have structural units derived from one or more of these other monomers.

[0019] The proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 mol% or less, more preferably 8 mol% or less, based on the number of moles of all structural units constituting the vinyl ester polymer. Generally, the crystallization of PVA tends to proceed more slowly as the proportion of structural units derived from other monomers in the vinyl ester polymer increases. Therefore, by appropriately copolymerizing these other monomers in the above ratio, the refractive index in each direction can be adjusted.

[0020] The degree of polymerization of PVA is preferably 1000 or more, more preferably 1500 or more, and even more preferably 1700 or more. By setting the degree of polymerization of PVA to the above lower limit or more, the flexibility of the PVA film can be improved. On the other hand, the degree of polymerization of PVA is preferably 8000 or less, more preferably 4000 or less, and even more preferably 3500 or less. By setting the degree of polymerization of PVA to the above upper limit or less, it is possible to suppress an increase in the production cost of PVA and the occurrence of defects during film formation. The degree of polymerization of PVA means the average degree of polymerization measured in accordance with the description of JIS K6726-1994.

[0021] The saponification degree of PVA is preferably 98 mol% or higher, more preferably 99 mol% or higher, and even more preferably 99.3 mol% or higher, because this improves the wet heat resistance of the resulting polarized film. Furthermore, the higher the saponification degree of PVA, the lower the water solubility of the PVA film, making it suitable for use in the manufacture of polarized films, including a stretching step in water. The saponification degree of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of structural units (typically vinyl ester units) that can be converted to vinyl alcohol units by saponification and vinyl alcohol units. The saponification degree can be measured in accordance with the description of JIS K6726-1994.

[0022] The PVA film of the present invention may contain one type of PVA alone, or may contain two or more types of PVAs that differ from each other in degree of polymerization, degree of saponification, degree of modification, etc.

[0023] The modified PVA may be, for example, ethylene-modified PVA. The degree of modification of the PVA is preferably 0.01 mol% or more, more preferably 0.1 mol% or more, and even more preferably 1 mol% or more. The degree of modification of the PVA is preferably 15 mol% or less, more preferably 8 mol% or less, and even more preferably 5 mol% or less.

[0024] The content of PVA in the PVA film of the present invention is not necessarily limited, but is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0025] (Plasticizer) The PVA film of the present invention preferably contains a plasticizer. By containing a plasticizer, the handleability and stretchability of the PVA film can be improved. Examples of plasticizers include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and sorbitol. These plasticizers may be used alone or in combination of two or more. Among these, ethylene glycol or glycerin is preferred as the plasticizer, and glycerin is more preferred, because it is less likely to bleed out onto the surface of the PVA film.

[0026] The content of the plasticizer in the PVA film of the present invention is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 4 parts by mass or more, per 100 parts by mass of PVA. By setting the content of the plasticizer to be equal to or greater than the above-mentioned lower limit, the stretchability of the PVA film is improved. On the other hand, the content of the plasticizer is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of PVA. By setting the content of the plasticizer to be equal to or less than the above-mentioned upper limit, it is possible to prevent the plasticizer from bleeding out onto the surface of the PVA film, which would reduce the handleability of the PVA film.

[0027] (Surfactant) The PVA film of the present invention preferably contains a surfactant. By including a surfactant, the handleability of the PVA film and the releasability of the PVA film from a film-forming device during production can be improved. The surfactant is not particularly limited, and for example, anionic surfactants and nonionic surfactants are preferably used.

[0028] Examples of anionic surfactants include carboxylic acid surfactants such as potassium laurate; sulfate ester surfactants such as octyl sulfate; and sulfonic acid surfactants such as dodecylbenzenesulfonate.

[0029] Examples of nonionic surfactants include alkyl ether surfactants such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkyl phenyl ether surfactants such as polyoxyethylene octylphenyl ether; alkyl ester surfactants such as polyoxyethylene laurate; alkyl amine surfactants such as polyoxyethylene lauryl amino ether; alkyl amide surfactants such as polyoxyethylene lauric acid amide; polypropylene glycol ether surfactants such as polyoxyethylene polyoxypropylene ether; alkanolamide surfactants such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether surfactants such as polyoxyalkylene allyl phenyl ether.

[0030] Such surfactants may be used alone or in combination of two or more. As the surfactant, nonionic surfactants are preferred, alkanolamide surfactants are more preferred, and dialkanolamides (e.g., diethanolamides) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids having 8 to 30 carbon atoms) are even more preferred, due to their excellent effect of reducing surface abnormalities during film formation of the PVA film.

[0031] The surfactant content in the PVA film of the present invention is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, and even more preferably 0.05 parts by mass or more, relative to 100 parts by mass of PVA. On the other hand, the surfactant content is preferably 10 parts by mass or less, more preferably 1 part by mass or less, even more preferably 0.5 parts by mass or less, and particularly preferably 0.3 parts by mass or less, relative to 100 parts by mass of PVA. A surfactant content within the above range improves the releasability of the PVA film from the film-forming apparatus during production and prevents adhesion between PVA films (hereinafter sometimes referred to as "blocking"). It also prevents the surfactant from bleeding out onto the surface of the PVA film and deterioration of the appearance of the PVA film due to surfactant aggregation.

[0032] (Other Components) In addition to PVA, plasticizer, and surfactant, the PVA film of the present invention may contain other components such as a water-soluble polymer, moisture, antioxidant, UV absorber, lubricant, crosslinking agent, colorant, filler, preservative, antifungal agent, other polymer compound, etc., within a range that does not impair the effects of the present invention. The proportion of the total amount of PVA, surfactant, and plasticizer to the total mass of the PVA film is preferably 60 to 100 mass%, more preferably 70 to 100 mass%, and even more preferably 80 to 100 mass%.

[0033] (Shape of PVA Film) The shape of the PVA film of the present invention is not particularly limited, but a long film is preferred because it allows for continuous production of polarized films with good productivity. The length of the long film is not particularly limited and can be set appropriately depending on the application of the polarized film to be produced, and can be, for example, within a range of 5 m or more and 20,000 m or less. The width of the long film is not particularly limited and can be, for example, 50 cm or more. However, given the recent demand for wider polarized films, it is preferably 1 m or more, more preferably 3 m or more, and even more preferably 4 m or more. There is no particular upper limit to the width of the long film, but if the width is too wide, it tends to be difficult to uniformly stretch the film when producing polarized films using commercially available equipment, so the width of the PVA film is preferably 7 m or less.

[0034] The shape of the PVA film of the present invention is not particularly limited, and it may be a single-layer film or a multi-layer film (laminate). However, from the viewpoint of the complexity and cost of lamination (coating, etc.) work, a single-layer film is preferred.

[0035] The PVA film of the present invention can be produced by any method, including, for example, the following. Examples of such methods include a method in which a film-forming solution obtained by adding a solvent, additives, etc. to PVA to homogenize it is subjected to a casting film-forming method, a wet film-forming method (a method in which the solution is extruded into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method in which the film-forming solution is cooled to gel, and then the solvent is extracted and removed), or a combination of these methods; a melt-extrusion film-forming method in which a film-forming solution obtained using an extruder or the like is extruded through a T-die or the like to form a film; and an inflation molding method. Among these, the casting film-forming method and the melt-extrusion film-forming method are preferred because they can produce homogeneous films with good productivity.

[0036] In particular, by using a casting film-forming method and controlling the moisture content and draw ratio of the film in the casting and drying steps within specific ranges, the PVA film of the present invention can be produced more efficiently.

[0037] [PVA Film Manufacturing Method] The following method is preferred as a method for manufacturing a PVA film of the present invention: A method for manufacturing a PVA film, comprising: a casting step of casting a film-forming solution containing PVA onto a support and drying it to obtain a film; a drying step of drying the film on at least three supports; and a heat treatment step of heat-treating the film on the supports, wherein in the drying step, the surface temperature of the support is 85°C to 99°C, the minimum draw ratio of the support is 0.985 or more and the maximum draw ratio is 1.000 or less, and the cumulative stretch ratio of the support is 0.980 to 0.983, and the moisture content of the film after the drying step is 9% by mass or more and 14% by mass or less.

[0038] The method for calculating the moisture content of the film employed in the method for producing a PVA film of the present invention is as follows.

[0039] The film between the rolls during drawing (stretching) is sampled and its mass (Ww) is measured. The film is then placed in a hot air dryer and dried at 105°C for 16 hours, after which its mass (Wd) is measured. The moisture content is calculated using the following formula (I): Moisture content (mass%) = (Ww - Wd) / Ww x 100 (Formula I)

[0040] The draw ratio means the ratio of the winding speed (peripheral speed) of a support such as a drum or roll (peripheral speed of the rear support / peripheral speed of the front support), and is also used as a value representing the stretching ratio of a film.

[0041] The cumulative draw ratio is the product of the draw ratios between each support, such as a drum or roll, in a process. For example, when N supports (N is an integer of 2 or more) are used in one process, there are N-1 draw ratios, and the cumulative draw ratio is the product of all of these draw ratios.

[0042] The method for producing a PVA film of the present invention is divided into a casting step, a drying step, and a heat treatment step, which are carried out in this order. Each step will be described below.

[0043] (Casting process) In the casting process, first, a film-forming solution containing PVA, a solvent, and, if necessary, additives such as a plasticizer is prepared. Next, this film-forming solution is cast (supplied) in the form of a film onto a rotating support such as a metal roll or a metal belt. This forms a film of the film-forming solution on the support.

[0044] The film-forming solution generally passes through the die lip of a die such as a T-die and is cast in the form of a film onto a support such as a metal roll or a metal belt. On the support, the solvent evaporates from the surface of the cast film-like solution that is not in contact with the support (hereinafter sometimes referred to as the "free surface"), while it does not substantially evaporate from the surface that is in contact with the support (hereinafter sometimes referred to as the "touch surface"). This results in a distribution of the solvent concentration in the film thickness direction, where the solvent concentration is low on the free surface side and high on the touch surface side. Therefore, solidification of the PVA also proceeds first from the free surface.

[0045] Herein, the volatile content of a PVA film-forming solution refers to the value calculated by the following formula II: Volatile content of a film-forming solution (mass%) = {(Wc - Wd) / Wc} × 100 (formula II) In the formula II, Wc represents the mass (g) of a sample taken from the film-forming solution, and Wd represents the mass (g) of the sample Wc (g) when it 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.

[0046] The volatile content of the membrane-forming solution (the content in the membrane-forming solution of volatile components such as the liquid medium that are removed by volatilization or evaporation during membrane formation) varies depending on the membrane-forming method, membrane-forming conditions, etc., but is preferably in the range of 50 to 90 mass%, more preferably in the range of 55 to 80 mass%. When the membrane-forming solution has a volatile content of 50 mass% or more, the viscosity of the membrane-forming solution does not become too high, making membrane formation easier. On the other hand, when the membrane-forming solution has a volatile content of 90 mass% or less, the viscosity of the membrane-forming solution does not become too low, improving the thickness uniformity of the resulting PVA film.

[0047] To obtain the PVA film of the present invention, the surface temperature of the support onto which the film-forming solution is poured is preferably 65°C or higher, more preferably 70°C or higher, and even more preferably 90°C or higher. To obtain the PVA film of the present invention, the surface temperature of the support onto which the film-forming solution is poured is preferably 120°C or lower, more preferably 105°C or lower, and even more preferably 99°C or lower. When the surface temperature is equal to or lower than the upper limit, the film surface is excellent. When the surface temperature is equal to or higher than the lower limit, the film is easily dried sufficiently.

[0048] The crystallization or orientation of the PVA in the film cast on the support can be controlled in the subsequent drying and heat treatment processes by controlling the draw ratio of the support. The rate of crystallization and the degree of orientation are affected by the draw ratio, the proportion of structural units derived from other monomers in the PVA, the degree of polymerization of the PVA, the degree of saponification of the PVA, the content of the plasticizer, the volatile content of the film-forming solution, and the heating temperature.

[0049] While the film is heated on the support, hot air may be blown uniformly over the entire area of ​​the non-contact surface of the film at a speed of 1 to 10 m / sec. The lower limit of the temperature of the hot air blown over the non-contact surface is preferably 50°C or higher, more preferably 70°C or higher, while the upper limit of the temperature of the hot air is preferably 150°C or lower, more preferably 120°C or lower.

[0050] In the PVA film manufacturing method of the present invention, the moisture content of the film after peeling from the support in the casting step (the moisture content of the film after the casting step) is preferably 14% by mass or more and 20% by mass or less, and more preferably 14% by mass or more and 18% by mass or less. A moisture content of not more than the upper limit improves peeling from the support, making it easier to reduce thickness unevenness and retardation unevenness in the resulting PVA film. On the other hand, a moisture content of not less than the lower limit improves adhesion of the film to the support, making it easier to transport the film. Furthermore, the effect of draw during peeling is suppressed from becoming too strong, which would increase the refractive index in the MD direction, making it easier to obtain a PVA film with a high breaking ratio.

[0051] In the PVA film production method of the present invention, the draw ratio in the casting step (the draw ratio between the support in the casting step and the initial support in the drying step) is preferably 1.00 to 1.05, more preferably 1.01 to 1.05, and even more preferably 1.01 to 1.04. When the draw ratio is within the above range, it becomes easier to achieve both improved stretchability of the PVA film and reduced color unevenness in the polarizer.

[0052] In the PVA film manufacturing method of the present invention, the moisture content of the film after peeling from the support in the casting step can be adjusted by adjusting the PVA concentration of the film-forming solution. The PVA concentration of the film-forming solution is preferably 23% by mass or more, more preferably 25% by mass or more. The PVA concentration of the film-forming solution is preferably 37% by mass or less, more preferably 35% by mass or less.

[0053] The moisture content of the film after peeling from the support in the casting process can also be adjusted by changing the peripheral speed of the support in the casting process. For example, if the peripheral speed of the support is slow, the time the film is in contact with the support will be longer, thereby reducing the moisture content of the film when peeled from the support. The peripheral speed of the support is preferably 8 m / min or more, more preferably 11 m / min or more, and even more preferably 14 m / min or more. The peripheral speed of the support is preferably 30 m / min or less, more preferably 27 m / min or less, and even more preferably 24 m / min or less. When the peripheral speed of the support is within the above range, a PVA film with excellent in-plane uniformity and minimal thickness variation can be produced while maintaining the productivity of the PVA film.

[0054] (Drying Process) In the method for producing a PVA film of the present invention, the surface temperature of the support during the drying process is 85°C to 99°C, the minimum draw ratio of the support during the drying process is 0.985 or more and the maximum draw ratio is 1.000 or less, and the cumulative stretch ratio of the support during the drying process is 0.980 to 0.983. Methods for drying the film include heating the support itself with a heat medium or blowing hot air onto the surface of the film opposite the surface that is in contact with the support. Stretching the film during the drying process can be performed by using multiple supports and controlling the peripheral speed of each support. The moisture content of the film after the drying process is preferably 9% by mass to 14% by mass, more preferably 9% by mass to 12% by mass, and particularly preferably 9% by mass to 10% by mass. By keeping the moisture content of the film after the drying process within the above range, changes in the orientation of the PVA when the PVA film is stretched are suppressed, making it easier to suppress breakage of the PVA film during production of a polarized film.

[0055] In the drying step, the surface temperature of the support is preferably 87° C. or higher and 97° C. or lower, in order to efficiently produce the PVA film of the present invention. The cumulative stretch ratio of the support in the drying step is preferably 0.981 or higher and 0.982 or lower. The minimum value of the draw ratio of the support in the drying step is preferably 0.987 or higher. Meanwhile, the maximum value of the draw ratio is preferably 0.998 or lower.

[0056] (Heat Treatment Step) In the heat treatment step, the film peeled from the support such as the drying roll or metal belt in the drying step is transported to a support such as a heated roll and heat-treated on the support. Examples of methods for heat-treating the film include a method of heating the support itself with a heat medium or the like, a method of blowing hot air onto the surface of the film opposite to the surface in contact with the support, and a method using a drying oven.

[0057] The specific heat treatment method is not particularly limited, and examples include a method of blowing hot air using a floating dryer, a method of irradiating near-infrared rays using an infrared lamp, and a method of contacting the PVA film with a heat treatment roll. However, to obtain the PVA film of the present invention, a method of directly contacting the surface of the PVA film with a heat treatment roll is preferred. Furthermore, when heat treating the PVA film using multiple heat treatment rolls, it is preferable to alternately contact one surface and the other surface of the PVA film with the heat treatment roll. This allows the refractive index in each direction on both sides of the PVA film to be uniform. In this case, the number of heat treatment rolls is preferably two or more, and more preferably four or more.

[0058] In the method for producing a PVA film of the present invention, the preferred range of moisture content of the film when it is brought into contact with the support in the initial heat treatment step and the reasons for this preference are the same as the preferred range of moisture content of the film after the drying step.

[0059] The lower limit of the surface temperature of the support for the heat treatment is preferably 80° C. or higher. On the other hand, the upper limit of the surface temperature is preferably 140° C. or lower, more preferably 130° C. or lower. By keeping the surface temperature of the support within the above range, it is possible to sufficiently promote crystal growth without impairing the stretchability.

[0060] In the method for producing a PVA film of the present invention, the cumulative stretch ratio in the heat treatment step is preferably 1.00 or less, and more preferably 0.995 or less, because this improves stretchability and makes it easier to reduce breakage during production of a polarizer.

[0061] The PVA film that has been subjected to the heat treatment step can be subjected to an additional heat treatment, if necessary.

[0062] The moisture content of the PVA film finally obtained by the series of treatments is not necessarily limited, but is preferably 1% by mass or more, more preferably 2% by mass or more. The moisture content of the PVA film finally obtained is not necessarily limited, but is preferably 9% by mass or less, more preferably 8% by mass or less.

[0063] The PVA film produced in this manner may be further subjected to humidity conditioning treatment, cutting of both ends (edges) of the film, etc., as necessary, and then wound into a roll on a cylindrical core and packaged in a moisture-proof manner to form a product.

[0064] [Polarizing Film] The PVA film of the present invention is usually an unstretched film. Since the PVA film of the present invention is an unstretched film, it can be suitably used as a film for producing optical films, and examples of optical films include polarizing films, viewing angle improving films, retardation films, and brightness improving films, but it is particularly suitable for producing polarizing films. Polarizing films produced using the PVA film of the present invention have little polarization unevenness, and such polarizing films are also an embodiment of the present invention. Note that stretched PVA films are also within the scope of the present invention.

[0065] The method for producing a polarized film is not particularly limited, and any conventionally used method may be employed. For example, a polarized film can be produced by subjecting the PVA film of the present invention to a swelling treatment, a dyeing treatment, a uniaxial stretching treatment, and, if necessary, a crosslinking treatment, a fixing treatment, a drying treatment, a heat treatment, or the like. In this case, the order of each treatment, such as the swelling treatment, the dyeing treatment, the uniaxial stretching treatment, and the fixing treatment, is not particularly limited, and one or more treatments may be performed simultaneously. Furthermore, one or more treatments may be performed twice or more times.

[0066] The swelling treatment can be carried out by immersing the PVA film in water. The lower limit of the water temperature during immersion is preferably 20°C, more preferably 22°C, and even more preferably 25°C. Meanwhile, the upper limit is preferably 40°C, more preferably 38°C, and even more preferably 35°C. The immersion time in water is preferably, for example, 0.1 minutes or more and 5 minutes or less. The water used for immersion 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 an aqueous medium.

[0067] The dyeing treatment is carried out using a dichroic dye such as an iodine-based pigment or dye, and the dyeing may be carried out at any stage before, during, or after uniaxial stretching. Dyeing is generally carried out by immersing the PVA film in a solution (particularly an aqueous solution) containing iodine and potassium iodide as a dye bath. The iodine concentration in the dye bath is preferably 0.01% by mass or more. The iodine concentration in the dye bath is preferably 0.5% by mass or less. The potassium iodide concentration is preferably 0.01% by mass or more. The potassium iodide concentration is preferably 10% by mass or less. The dye bath temperature is preferably 20°C or more. The dye bath temperature is preferably 60°C or less.

[0068] The crosslinking treatment can be carried out by immersing the PVA film in an aqueous solution containing a crosslinking agent. Crosslinking treatment introduces crosslinks into the PVA film, allowing uniaxial stretching at a relatively high temperature. The crosslinking agent used can be one or more boron compounds, such as boric acid and borax or other borate salts. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass or more, more preferably 2% by mass or more. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 15% by mass or less, more preferably 7% by mass or less. The temperature of the aqueous solution containing the crosslinking agent is preferably 20°C or higher. The temperature of the aqueous solution containing the crosslinking agent is preferably 60°C or lower.

[0069] The uniaxial stretching may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be performed in an aqueous solution containing boric acid, or in the dye bath described above or the fixing treatment bath described below. In the case of the dry stretching method, it can be performed in air. Among these, the wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. The concentration of boric acid in the aqueous boric acid solution is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more. The concentration of boric acid in the aqueous boric acid solution is preferably 6.0% by mass or less, more preferably 5.0% by mass or less, and even more preferably 4.0% by mass or less. The aqueous boric acid solution may also contain potassium iodide, and the concentration thereof is preferably 0.01% by mass or more. The concentration of potassium iodide is preferably 10% by mass or less.

[0070] In the wet stretching method, the stretching temperature in uniaxial stretching is preferably 30° C. or higher, more preferably 40° C. or higher, and even more preferably 50° C. or higher. The stretching temperature in uniaxial stretching is preferably 90° C. or lower, more preferably 80° C. or lower, and even more preferably 70° C. or lower.

[0071] The stretching ratio in uniaxial stretching is preferably 5 times or more, more preferably 5.5 times or more, from the viewpoint of the polarizing performance of the resulting polarizing film. There is no particular upper limit to the stretching ratio, but the stretching ratio is preferably 8 times or less.

[0072] When producing a polarizing film, a fixation treatment is preferably performed to strengthen the adsorption of the dichroic dye onto the PVA film. The fixation bath used for the fixation treatment can be an aqueous solution containing one or more boron compounds, such as boric acid and borax. If necessary, an iodine compound or a metal compound may be added to the fixation bath. The concentration of the boron compound in the fixation bath is generally preferably 2% by mass or more, more preferably 3% by mass or more. The concentration of the boron compound in the fixation bath is generally preferably 15% by mass or less, more preferably 10% by mass or less. The temperature of the fixation bath is preferably 15°C or more, more preferably 25°C or more. The temperature of the fixation bath is preferably 60°C or less, more preferably 40°C or less.

[0073] The drying treatment is preferably carried out at 30° C. or higher, and more preferably at 50° C. or higher. The drying treatment is preferably carried out at 150° C. or lower, and more preferably at 130° C. or lower. Drying at a temperature within the above range makes it easier to obtain a polarizing film with excellent dimensional stability.

[0074] There are no particular limitations on the film thickness of the polarizing film, and it can be, for example, 30 μm or less, or even 25 μm or less. From the viewpoints of reducing the shrinkage stress of the polarizing film and even the polarizing plate using the polarizing film and preventing warping of the thin glass to be laminated, the film thickness of the polarizing film is preferably 20 μm or less, more preferably 15 μm or less. On the other hand, since a polarizing film that is too thin tends to be difficult to manufacture and handle, the film thickness of the polarizing film is preferably 3 μm or more.

[0075] [Polarizing Plate] The polarizing film of the present invention is used as a polarizing plate by laminating an optically transparent protective film having mechanical strength. Since the polarizing plate has high polarization performance derived from the polarizing film of the present invention, the polarizing plate is also an embodiment of the present invention. Examples of the protective film include cellulose triacetate (TAC) film, cellulose acetate butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives used for lamination include PVA-based adhesives and urethane-based adhesives, with PVA-based adhesives being particularly preferred.

[0076] The polarizing plate obtained as described above can be used as a part of an LCD by coating it with an acrylic or other adhesive and then laminating it to a glass substrate. At the same time, it may also be laminated with a retardation film, a viewing angle improving film, a brightness improving film, etc.

[0077] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The evaluation methods used in the following examples and comparative examples are shown below.

[0078] <Refractive index in MD or TD> From the PVA films obtained in the following examples or comparative examples, one rectangular sample measuring 5 cm in the TD direction and 9 cm in the MD direction was cut out so that the center of gravity of the sample coincided with the 50% position (the center of the PVA film in the width direction) relative to the entire TD direction. Next, using a retardation measurement device (manufactured by Otsuka Electronics Co., Ltd., "RETS-100nx"), the three-dimensional refractive index (Nx, Ny, and Nz) of the center of the width direction of the sample was measured at a measurement wavelength of 520 nm, and the obtained Nx was determined as the refractive index in the MD direction, and Ny was determined as the refractive index in the TD direction. During the measurement, the x-axis direction was the MD direction of the sample, the y-axis direction was the TD direction of the sample, and the z-axis direction was the film thickness direction of the sample, and the conditions were adjusted so that the average value of Nx, Ny, and Nz was 1.5. Detailed measurement conditions are as follows. Here, Nx was used as the refractive index in the MD direction because the MD direction coincided with the slow axis (x) direction of the sample, but if the MD direction coincides with the fast axis (y) direction of the sample, Ny must be used as the refractive index in the MD direction. Apparatus used: "RETS-100nx" manufactured by Otsuka Electronics Co., Ltd. Measurement method: Rotational grating method / low Re. mode Tilt angle: 45° Tilt direction: Fast axis tilt Re. dispersion formula: Quartic formula

[0079] <Film Thickness Measurement> The film thickness of the PVA film of the present invention was measured under the following conditions. Apparatus used: Film tester continuous thickness measuring machine (Model S-2268: manufactured by Fujiwork) Measurement interval: 0.5 mm interval in MD direction Measurement length: 4000 mm

[0080] <Average Film Thickness in MD> (1) Measurement of Film Thickness Profile In the PVA films obtained in the following Examples and Comparative Examples, a position at 50% of the entire area in the TD direction (the center in the width direction of the PVA film) was measured. ０ Next, P ０ From the position of each, the position of +50 mm in the TD direction is P ＋５０ , +300mm position P ＋３００ , -300mm position is P －３００ (The positive and negative directions are arbitrary in the TD direction.) Next, the center of gravity of the sample is P ＋５０A rectangular sample measuring 5 cm in the TD direction and 400 cm in the MD direction was cut out to coincide with the position of P ＋３００ and P －３００ Similarly, a rectangular sample measuring 5 cm in the TD direction and 400 cm in the MD direction was cut out from each position. ＋５０ , P ＋３００ and P －３００ A total of three samples were obtained, one for each position. ＋５０ For the samples obtained from the position, a film tester continuous thickness measuring machine (manufactured by Fujiwork, "S-2268") was used to measure the film thickness at 8,000 points every 0.5 mm from 0.5 mm to 4,000 mm in the MD direction, starting from a position 50% of the entire TD direction of the sample (the center in the width direction of the sample and the end in the machine direction). ＋５０ (x) was measured (x is an integer from 1 to 8000). For example, F ＋５０ (1) is P ＋５０ In the sample obtained from the position, the film thickness is measured at a position 0.5 mm in the MD direction, starting from a position 50% of the entire area in the TD direction. ＋３００ and P －３００ Similarly, for the samples obtained from the position, starting from a position that is 50% of the entire area of ​​the sample in the TD direction, the film thickness was measured at 8000 points every 0.5 mm from 0.5 mm to 4000 mm in the MD direction, and F ＋３００ (x) and F －３００ (x) was measured (x is an integer from 1 to 8000). ＋５０ (x), F ＋３００ (x) and F －３００ When measuring (x), the film thickness was measured at 8,000 points every 0.5 mm, but the film thickness measurement interval (0.5 mm) or the number of measurement points (8,000 points) can be set appropriately depending on the size of the sample, etc.

[0081] (2) Calculation of the average value of the film thickness profile F obtained in the above "(1) Measurement of the film thickness profile" ＋５０ (x), F ＋３００ (x) and F －３００(x) (x is an integer of 1 to 8000) according to the following formulas (1) to (3), ＋５０ Average value of (x) F ＋５０，Ａｖｅ , F ＋３００ Average value of (x) F ＋３００，Ａｖｅ , and F －３００ Average value of (x) F －３００，Ａｖｅ Then, the average of these average values ​​was calculated as the film thickness (nm) in the MD direction according to the following formula (4).

[0082] <Average gradient of film thickness in MD direction> F obtained in the above "(1) Measurement of film thickness profile" ＋５０ (x) (x is an integer between 1 and 8000) to calculate the moving average G ＋５０ (k) (k is an integer from 1 to 7989) was calculated. ＋５０ (k) is F ＋５０ (1), F ＋５０ (2), ..., F ＋５０ (8000) is a moving average of 10 points. Then, the obtained G ＋５０ G' obtained by differentiating (k) at intervals of two points ＋５０ The average value of the absolute value of (k) was calculated at the position +50 mm in the TD direction (P ＋５０ ) was calculated as the average gradient (a) in the MD direction. ＋３００ (x) or F －３００ (x) (x is an integer between 1 and 8000) to calculate the moving average G ＋３００ (k) or G －３００ (k) (k is an integer from 1 to 7989) was calculated. Then, the obtained G ＋３００ (k) or G －３００ G' obtained by differentiating (k) at intervals of two points ＋３００ (k) or G' －３００ The average value of the absolute value of (k) was calculated at the position +300 mm in the TD direction (P ＋３００ ) average inclination value in the MD direction (b), or at the position of -300 mm in the TD direction (P －３００ The average gradient value (c) of the film thickness in the MD direction was calculated from the average of the average gradient values, as shown in the following formula (8):

[0083]

[0084] <Difference between maximum and minimum film thickness in MD direction> F obtained in the above "(1) Measurement of film thickness profile" ＋５０ (x) (x is an integer between 1 and 8000), the maximum value is F ＋５０，Ｍａｘ , the minimum value is F ＋５０，ｍｉｎ That is, F ＋５０ (1), F ＋５０ (2), ..., F ＋５０ The maximum value of (8000) is F ＋５０，Ｍａｘ and the minimum value is F ＋５０，ｍｉｎ Similarly, the F obtained in the above "(1) Measurement of film thickness profile" ＋３００ The maximum value of (x) is F ＋３００，Ｍａｘ , the minimum value is F －３００，ｍｉｎ and F －３００ The maximum value of (x) is F －３００，Ｍａｘ , the minimum value is F －３００，ｍｉｎ Then, the difference between each maximum value and each minimum value (R ＋５０ , R ＋３００ , R －３００ ) average value (R ，Ａｖｅ ) was calculated as the difference (nm) between the maximum and minimum film thickness in the MD direction. ＋５０ = F ＋５０，Ｍａｘ -F ＋５０，ｍｉｎ (9) R ＋３００ = F ＋３００，Ｍａｘ -F ＋３００，ｍｉｎ (10) R －３００ = F －３００，Ｍａｘ -F －３００，ｍｉｎ (11) R ，Ａｖｅ = (R ＋５０ +R ＋３００ +R －３００ ) / 3 (12)

[0085] <Break Ratio> (1) Preparation of Measurement Samples Thirty-three rectangular samples measuring 50 mm in the TD direction and 90 mm in the MD direction were cut out of the PVA films obtained in the following Examples or Comparative Examples on an arbitrary line parallel to the TD direction from one end of the PVA film in the TD direction to a position 50% of the entire TD area (the center of the PVA film in the width direction). The 33 rectangular samples were cut out so that the center of gravity of the sample coincided with the 50% position of the entire TD area (the center of the PVA film in the width direction). Next, a benchmark line was drawn 20 mm inward from each end of the 90 mm MD point on each sample.

[0086] (2) Preparation of Calibration Curve A calibration curve was prepared using 12 of the 33 samples marked in the above "(1) Preparation of Measurement Samples." Specifically, the 12 samples were divided into four sets, each consisting of three samples. Next, the three samples in the first set were simultaneously clamped with a chuck at the outer edge of the marked lines on both ends of each sample, and a stretching test was performed using a laboratory stretching machine under the following conditions. First, while immersed in water at a temperature of 30°C, the three samples were uniaxially stretched in the MD direction to twice their original length (swelling treatment). Next, while immersed in an aqueous solution (temperature 32°C) containing 0.05% by mass of iodine and 1.2% by mass of potassium iodide, the three samples after the swelling treatment were uniaxially stretched in the MD direction to 2.5 times their original length (dyeing treatment). Next, three dyed samples were uniaxially stretched in the MD direction to three times their original length while immersed in an aqueous solution containing 2.6% by weight of boric acid (at a temperature of 32°C) (crosslinking treatment). Next, three crosslinked samples were uniaxially stretched in the MD direction to any stretching ratio between six times their original length and the stretching ratio at break while immersed in an aqueous solution containing 2.8% by weight of boric acid and 5% by weight of potassium iodide (at 55°C), and the gauge length and stretching time were measured for each of the three samples. The stretch ratio for each of the three samples was calculated by dividing the gauge length after the stretching test by the gauge length in the MD direction before the stretching test (50 mm). Similar measurements were performed on three samples from each of two to four sets, each stretching ratio being varied between six times their original length and the stretching ratio at break. The stretch ratio and stretching time were measured for a total of 12 samples. A calibration curve was then prepared from the relationship between the stretching ratio and stretching time for the obtained 12 samples.

[0087] (3) Evaluation of Breaking Ratio Of the 33 samples marked in "(1) Preparation of Samples for Measurement" above, the remaining 21 samples were divided into 7 sets, each consisting of 3 samples. A stretching test was performed on each of the 3 samples in sets 1 to 7, in the same manner as in "(2) Preparation of Calibration Curve" above, until all 3 samples broke. The stretching time for each of the 21 samples was measured. The stretching time was then used to determine the stretching ratio for each of the 21 samples from the calibration curve obtained in "(2) Preparation of Calibration Curve" above. The average value of the stretching ratios for 20 of the 21 samples was then calculated as the breaking ratio (times) of the PVA film, and the film was evaluated according to the following criteria. Samples with a breaking ratio rating of A were judged to have reduced breakage even when stretched under high-temperature conditions. A: Breaking ratio of 6.60 times or more; B: Breaking ratio of 6.45 times or more but less than 6.60 times; C: Breaking ratio of less than 6.45 times.

[0088] <Color Unevenness> (1) Production of Polarized Films PVA films obtained in the following Examples or Comparative Examples were slit into a width of 650 mm so that the center of gravity of the PVA film coincided with the center of gravity of the slit film. Polarized films were continuously produced by subjecting these films to swelling, dyeing, crosslinking, stretching, washing, and drying in this order. The swelling treatment was performed by uniaxially stretching the film 2.00 times in the longitudinal direction while immersed in pure water (swelling treatment solution) at 25°C. The dyeing treatment was performed by uniaxially stretching the film 1.26 times in the longitudinal direction (total stretching ratio 2.52 times) while immersed in a potassium iodide / iodine aqueous dyeing solution (dyeing treatment solution) (mass ratio (potassium iodide / iodine) = 23, iodine concentration 0.03 to 0.05% by mass) at 32°C. In this dyeing treatment, the iodine concentration in the dyeing treatment solution was adjusted to a range of 0.03 to 0.05% by mass so that the polarized film obtained after uniaxial stretching in the stretching treatment had a single transmittance of 43.5% ± 0.2%. The crosslinking treatment was performed by uniaxially stretching the film in the longitudinal direction to 1.19 times (total stretching ratio of 3.00 times) while immersed in a boric acid aqueous solution (crosslinking treatment solution) (boric acid concentration: 2.6% by mass) at 32°C. The stretching treatment was performed by uniaxially stretching the film in the longitudinal direction to 2.00 times (total stretching ratio of 6.00 times) while immersed in a boric acid / potassium iodide aqueous solution (stretching treatment solution) (boric acid concentration: 2.8% by mass, potassium iodide concentration: 5% by mass) at 55°C. The cleaning treatment was carried out by immersing the film in a potassium iodide / boric acid aqueous solution (cleaning treatment liquid) (potassium iodide concentration: 3 to 6% by mass, boric acid concentration: 1.5% by mass) at 22° C. for 12 seconds without stretching.

[0089] (2) Evaluation of Color Unevenness One sample measuring 20 cm in the MD direction and the full length in the TD direction was cut out from the polarizing film obtained in the above "(1) Production of Polarizing Film". Next, two polarizing plates (single transmittance 43.5%, polarization degree 99.9%) were separately prepared, and these two polarizing plates were superimposed so as to be in a parallel Nicol state. Next, the sample polarizing film was sandwiched between the two polarizing plates so that the sample polarizing film was in a cross Nicol state for each of the two polarizing plates, thereby obtaining three laminates. This laminate was then subjected to a luminance test at a brightness of 40,000 cd / m ２The laminate was illuminated from behind with a backlight and the polarizing film was visually observed for color unevenness, and evaluated according to the following criteria. A rating of A for color unevenness was determined to have suppressed color unevenness in the polarizing film. A: No color unevenness that would be a problem for a polarizing film was observed. B: Color unevenness that would be a problem for a polarizing film was observed.

[0090] [Example 1] 100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 2,500), 10 parts by mass of glycerin as a plasticizer, 0.1 parts by mass of lauric acid diethanolamide as a surfactant, and 233 parts by mass of water were melt-mixed in a melt extruder to prepare a film-forming solution with a volatile content of 70% by mass. The resulting film-forming solution was then extruded from a T-die onto the first support (surface temperature 94 ° C) of a film-forming apparatus equipped with multiple supports whose rotation axes are parallel to each other to form a film, and dried until the moisture content of the film reached 15% by mass (casting process). That is, the moisture content of the film after the casting process was 15% by mass. Furthermore, in the casting process, the draw ratio of the support (the draw ratio between the first support of the film-forming apparatus and the first drying roll in the drying process described below) was 1.02. Next, the film was peeled from the first support of the film-forming apparatus, and the drying or heat-treatment process was carried out so that the surface of the film that contacted the support and the surface that did not contact the support alternately faced each drying roll or heat-treatment roll. Specifically, the film obtained in the casting process was first dried on at least three supports (drying rolls after the first drying roll) (surface temperature 95°C) until the moisture content of the film reached 10% by mass (drying process). That is, the moisture content of the film after the drying process was 10% by mass. Furthermore, in the drying process, the minimum and maximum draw ratios of the support were 0.988 and 0.997, respectively, and the cumulative stretch ratio of the support was 0.982. Next, the film obtained in the drying process was heat-treated on at least two supports (heat-treatment rolls) (surface temperature 80 to 120°C) until the moisture content of the film reached 7% by mass (heat-treatment process), producing a PVA film (thickness 45 μm, width 80 cm, length 200 m). That is, the moisture content of the PVA film after the heat treatment step was 7% by mass. Furthermore, the cumulative stretch ratio of the support during the heat treatment step was 0.990. The refractive index in the MD direction, the refractive index in the TD direction, the thickness in the MD direction, and the average slope and difference between the maximum and minimum values ​​of the thickness in the MD direction were determined for the obtained PVA film using the methods described above, and the breaking ratio and color unevenness were evaluated. These results are shown in Table 2.

[0091] [Examples 2 to 3, Comparative Examples 1 to 6] PVA films were produced and evaluated in the same manner as in Example 1, except that the surface temperature of the support in the drying step, the minimum and maximum values ​​of the draw ratio, and the cumulative stretch ratio were changed as shown in Table 1. The results are shown in Table 2.

[0092]

[0093]

[0094] As shown in Table 2, the PVA films of Examples 1 to 3, which had specific ranges for the refractive index in the MD direction and the average gradient of the film thickness in the MD direction, had a break ratio of 6.60 or more even when uniaxially stretched at 55°C, and breakage was reduced even when stretched at high temperatures. Furthermore, the polarized films using the PVA films of Examples 1 to 3 did not exhibit color unevenness, which is problematic for polarized films. That is, it can be seen that the PVA films of Examples 1 to 3 reduced breakage even when stretched at high temperatures and high ratios, and suppressed color unevenness in the polarized film.

Claims

1. A polyvinyl alcohol film having a refractive index in the MD direction of 1.500600 to 1.500740 and an average gradient of film thickness in the MD direction of 0.0265 μm / mm or less.

2. The polyvinyl alcohol film according to claim 1, wherein the difference between the maximum and minimum film thicknesses is 2.23 μm or less.

3. The polyvinyl alcohol film according to claim 1 or 2, wherein the average thickness is 10 to 100 μm.

4. The polyvinyl alcohol film according to claim 1 or 2, wherein the refractive index in the TD direction is 1.500165 to 1.500350.

5. A polarizing film produced using the polyvinyl alcohol film according to claim 1 or 2.

6. A polarizing plate comprising the polarizing film according to claim 5 and a protective film attached to at least one surface of the polarizing film.

7. A method for producing a polyvinyl alcohol film, comprising a casting step of casting a film-forming solution containing polyvinyl alcohol onto a support and drying it to obtain a film, a drying step of drying the film on at least three supports, and a heat treatment step of heat treating the film on the support, wherein in the drying step, the surface temperature of the support is 85°C to 99°C, the minimum draw ratio of the support is 0.985 or more and the maximum draw ratio is 1.000 or less, the cumulative stretch ratio of the support is 0.980 to 0.983, and the moisture content of the film after the drying step is 9% by mass or more and 14% by mass or less.

8. The method for producing a polyvinyl alcohol film described in claim 7, wherein in the casting process, the draw ratio of the support is 1.00 or more and 1.05 or less, and the moisture content of the film after the casting process is 14% by mass or more and 20% by mass or less.

9. A method for producing a polyvinyl alcohol film described in claim 7 or 8, wherein the moisture content of the film after the heat treatment step is 1% by mass or more and 9% by mass or less.

10. The method for producing a polyvinyl alcohol film according to claim 7 or 8, wherein the cumulative stretch ratio of the support in the heat treatment step is 1.00 or less.

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