Polyvinyl alcohol film, polarizing film and polarizing plate using the same

Abstract

Provided are a PVA film whereby unevenness in a polarizing plate when the PVA film is stretched is hardly visible even when transmission of an LCD is increased, a polarizing film that uses such a PVA film, and a polarizing plate.　A polyvinyl alcohol film in which, when F0(x) is defined as a film thickness profile for arbitrary points F0 on the film surface, F-100(x) is defined as a film thickness profile for points -100 mm in the TD direction from F0, F-200(x) is defined as a film thickness profile for points -200 mm in the TD direction from F0, F+100(x) is defined as a film thickness profile for points +100 mm in the TD direction from F0, and F+200(x) is defined as a film thickness profile for points +200 mm in the TD direction from F0, the average slope value represented by the average in the MD direction of F'0(x), F'-100(x), F'-200(x), F'+100(x), and F'+200(x), which are slope values in the MD direction of the respective film thickness profiles, is 0.02 or less, and a phase parameter defined by the average value of |F'-100(Cn)-F'0(Cn)|, |F'-200(Cn)-F'0(Cn)|, |F'+100(Cn)-F'0(Cn)|, and |F'+200(Cn)-F'-0(Cn)|, where Cn (n = 1, 2, 3, . . .) is the point at which F'0(x) is at a minimum value, is 0.015 or greater.

Description

[Technical Field]

[0001] This invention relates to a polyvinyl alcohol film, a polarizing film using the same, and a polarizing plate. [Background technology]

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

[0003] Polarizing plates are generally manufactured by dyeing, uniaxially stretching, and, if necessary, further fixing treatment with boron compounds, etc., a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA") to produce a polarizing film, and then laminating a protective film such as a cellulose triacetate (TAC) film onto the surface of the polarizing film. In recent years, there has been a demand for energy saving in LCDs, and therefore a demand for higher transmittance in polarizing plates. With the increased transmittance of polarizing plates, inconsistencies in the thickness of the PVA film, which were not visible in the past, have become visible, and there is a demand for PVA films with even better flatness than conventional products.

[0004] Patent Document 1 describes a method for manufacturing a PVA film with superior flatness compared to conventional methods by using an air knife to suppress fluctuations during casting. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2017-008298 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, in Patent Document 1, although better flatness of the film thickness in the MD direction (flow direction) than in the prior art can be obtained, the thin portions of the film thickness are aligned in the width direction, resulting in stress concentration in the thin portions of the film thickness. Although the visibility is low, regular polarization plate unevenness can be visually recognized, and there is a problem that it cannot meet the requirement of further increasing the transmittance of the LCD.

[0007] Therefore, an object of the present invention is to provide a PVA film in which unevenness of a polarizing plate is difficult to be visually recognized during stretching, a polarizing film using such a PVA film, and a polarizing plate even in further increasing the transmittance of an LCD.

Means for Solving the Problems

[0008] As a result of intensive studies, the present inventors have found that, in addition to further reducing the unevenness of the film thickness in the MD direction (flow direction) of the PVA film, by adjusting the phase shift of the film thickness unevenness at specific positions in the TD direction (width direction) within a specific range, the above problems can be achieved. Based on this finding, further studies were repeated to complete the present invention.

[0009] That is, the present invention is [1] When the film thickness profile of an arbitrary point F0 on the film surface is F0(x), the film thickness profile of the point -100 mm in the TD direction from F0 is F -100 (x), the film thickness profile of the point -200 mm in the TD direction from F0 is F -200 (x), the film thickness profile of the point +100 mm in the TD direction from F0 is F +100 (x), the film thickness profile of the point +200 mm in the TD direction from F0 is F +200 (x), the average inclination value represented by the average in the MD direction of the inclination values in the MD direction of each film thickness profile, namely F’0(x), F’ -100 (x), F’ -200 (x), F’ +100 (x) and F’ +200 (x) is 0.02 or less, and the point where F’0(x) shows a minimum value is C n|F' when n=1,2,3·· -100 (C n )-F'0(C n )|,|F' -200 (C n )-F'0(C n )|,|F' +100 (C n )-F'0(C n )|, and |F' +200 (C n )-F'0(C n A polyvinyl alcohol film in which the phase parameter, defined by the average value of ), is 0.015 or greater; [2] The polyvinyl alcohol film described in [1] above, having a width of 4 m or more; [3] A polyvinyl alcohol film according to [1] or [2], having a degree of swelling of 170-220%; [4] A polyvinyl alcohol film according to any of [1] to [3] above, having a thickness of 30 μm to 65 μm; [5] A polyvinyl alcohol film according to any of [1] to [4] above, which is a film for manufacturing optical films; [6] The polyvinyl alcohol film according to [5] above, wherein the optical film is a polarizing film; [7] A polarizing film manufactured using the polyvinyl alcohol film described in any of [1] to [6] above; [8] A polarizing plate manufactured by attaching a protective film to at least one side of the polarizing film described in [7] above; [9] A method for producing a polyvinyl alcohol film, comprising the steps of forming a polyvinyl alcohol film using an aqueous solution of polyvinyl alcohol with a concentration of 32 mass percent or less, and stretching the polyvinyl alcohol film at a stretch ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20 mass percent or more. [Effects of the Invention]

[0010] According to the present invention, there are provided a PVA film in which unevenness of a polarizing plate after stretching is less likely to be visually recognized than in the conventional case, a polarizing film using such a PVA film, and a polarizing plate.

[0011] Hereinafter, the present invention will be specifically described.

[0012] <Measurement of film thickness of PVA film> The film thickness measurement of the PVA film in the present invention is performed under the following conditions. From the viewpoint of comparing the phases of the film thickness in the width direction, measurement using a two-dimensional film thickness meter is essential. <Device used> Line scan film thickness meter (TI-750HR-1: manufactured by Otsuka Electronics Co., Ltd.) <Measurement interval> MD: 1 mm TD: 1 mm <Measured MD length> 1024 mm <Measured TD length> 750 mm / 1 scan

[0013] (Average inclination value in MD direction) The PVA film of the present invention is characterized in that the average inclination value in the MD direction (hereinafter referred to as the MD average inclination value) is 0.02 μm / mm or less. The MD average inclination value is more preferably 0.018 μm / mm or less. The detailed calculation method of the MD average inclination value will be described below.

[0014] From the two-dimensional film thickness profile obtained by the line scan film thickness meter, when an arbitrary point of the PVA film is set as position 0 mm, the film thickness profiles at positions +100 mm, +200 mm, -100 mm, and -200 mm in the TD direction from this arbitrary point (the positive and negative directions are arbitrary) are each subjected to 8pt FFT smoothing processing, and are designated as F +100 (x), F +200 (x), F -100 (x) and F -200Let (x) be the film thickness profile at position 0 mm. Here, the subscripts "0", "+100", "+200", "-100", and "-200" represent positions in the TD direction, and x represents a position in the MD direction. In other words, with a certain point as 0 mm, 1024 film thickness profiles are measured at 1 mm intervals from 1 mm to 1024 mm along the MD direction. These 1024 film thickness profile measurements are performed at positions 0 mm, +100 mm, +200 mm, -100 mm, and -200 mm in the TD direction from the aforementioned arbitrary point. Note that although 1024 film thickness profile measurements were performed at 1 mm intervals here, the measurement interval in the MD direction and the number of points for measuring the film thickness profile along the MD direction can be designed as appropriate.

[0015] The MD average gradient value of this invention is calculated using the following equations (1) to (6). The film thickness data at each point every 1 mm along a 1024 mm MD direction is differentiated, the absolute value of the derivative is averaged over 1024 points, and the average value for 5 lines is calculated to determine the MD average gradient value. The MD average gradient value is a numerical value that represents the intensity of film thickness unevenness in the MD direction, and a larger value indicates greater film thickness unevenness. Note that F0(x), F +100 (x), F +200 (x), F -100 (x), F -200 The derivatives of (x) are F'0(x) and F' +100 (x), F' +200 (x), F' -100 (x), F' -200 It can be expressed as (x).

[0016]

number

[0017] Note that in equations (1) to (6), k is a function of x, F'0(x), F' +100 (x), F' +200 (x), F' -100 (x), F' -200This symbol indicates that in (x), each of the values ​​from 1 to 1024 can be substituted for x.

[0018] F0(x), F +100 (x), F +200 (x), F -100 (x), F -200 The derivative of (x) can be defined as these slope values, but for example, the difference between F0(x) and F0(x+1) divided by the distance between point x and point (x+1) can also be defined as the slope value. +100 (x), F +200 (x), F -100 (x), F -200 Similarly, in the case of (x), the slope value can also be defined as the difference in film thickness profiles between point x and point (x+1) divided by the distance between point x and point (x+1).

[0019] The PVA film of the present invention must have an average MD gradient value of 0.02 μm / mm or less, preferably 0.019 μm / mm or less, and more preferably 0.016 μm / mm or less, calculated from the above formula. If the average MD gradient value is greater than 0.02, the polarization unevenness when processed into a polarizing plate will be strongly visible, impairing the display quality of the LCD.

[0020] Furthermore, the PVA film of the present invention preferably has an average MD slope value (c) at position +200 mm calculated from the above formula of 0.019 μm / mm or less, and more preferably 0.016 μm / mm or less. Similarly, the average MD slope value (e) at position -200 mm is preferably 0.019 μm / mm or less, and more preferably 0.016 μm / mm or less.

[0021] There are no particular limitations on the method for obtaining a PVA film with an average MD gradient of 0.02 or less. For example, one method involves forming a polyvinyl alcohol film using an aqueous solution of polyvinyl alcohol with a concentration of 32% by mass or less, and then stretching the polyvinyl alcohol film at a stretching ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20% by mass or more.

[0022] <Phase parameter (θ)> The PVA film of the present invention is characterized by simultaneously satisfying the following conditions: a phase parameter (θ) defined below is 0.015 μm / mm or greater, and the above-mentioned average MD gradient value is 0.02 or less. A phase parameter (θ) of 0.016 μm / mm or greater is more preferable.

[0023] In this invention, the phase parameter (θ) is a numerical value defined by the following formula. In the following formula, C n That is, |F'0(C n The points where )| is minimized are C1, C2, C3...C, in order from upstream in the MD direction. n These are numbered as follows: |F'0(C n The number of points where )| is minimized will be less than 1024.

number

[0024] Note that k in (Equation 12) is C n λ(C) is a function of n ) in C n C1~C n These symbols indicate that each of them is to be substituted.

[0025] In this invention, the phase parameter (θ) is a parameter that represents the degree of agreement in the MD direction between the node causing film thickness unevenness at the 0 mm position in the TD direction and the nodes causing film thickness unevenness at four positions shifted in the TD direction from there. The larger this value, the more uneven the nodes causing film thickness unevenness at the five points in the width direction become, indicating that film thickness unevenness is less likely to occur.

[0026] The PVA film of the present invention requires that the phase parameter (θ) calculated from the above formula be 0.015 or greater, and preferably 0.0153 or greater. If the phase parameter (θ) is less than 0.015, the nodal positions in the width direction of the PVA film will be aligned, causing stress concentration during stretching, which results in the visible appearance of regular polarizing plate irregularities extending in the width direction after stretching.

[0027] There are no particular limitations on the method for obtaining a PVA film with a phase parameter (θ) of 0.015 or higher, but one example is to form a polyvinyl alcohol film using an aqueous solution of polyvinyl alcohol with a concentration of 32% by mass or less, and then, when the moisture content of the polyvinyl alcohol film is 20% by mass or higher, stretch it at a stretch ratio of 1.075 to 1.135 to produce the polyvinyl alcohol film.

[0028] The PVA film of the present invention has an average MD slope value of 0.02 or less on the film surface and a phase parameter (θ) of 0.014 or more. However, at least one surface of the PVA film may have an average MD slope value of 0.02 or less and a phase parameter (θ) of 0.014 or more, or at least both sides of the PVA film may have an average MD slope value of 0.02 or less and a phase parameter (θ) of 0.014 or more.

[0029] From the viewpoint of use in large-screen LCD televisions, the width of the PVA film of the present invention is preferably 4 m or more, and more preferably 4.5 m or more. On the other hand, if the width of the PVA film is too large, it becomes difficult to perform uniform uniaxial stretching when manufacturing the polarizing film with commercially available equipment, so the width of the PVA film is preferably 7 m or less.

[0030] The PVA film of the present invention preferably has a swelling degree of 170% or more, and more preferably 180% or more. The swelling degree of the PVA film is preferably 220% or less, and more preferably 210% or less. If the swelling degree is less than 170%, uneven swelling tends to occur, and uniform dyeing is not possible during dyeing. If the swelling degree is greater than 220%, wrinkles tend to occur during the process, which is undesirable. A method for adjusting the swelling degree of the PVA film to these ranges is, for example, to appropriately adjust the temperature of the heat treatment after drying the PVA film.

[0031] The degree of swelling is an indicator of the water retention capacity of a PVA film when immersed in water. It can be calculated as a percentage by dividing the mass of the PVA film after immersion in 30°C water for 30 minutes by the mass after drying at 105°C for 16 hours.

[0032] The volatile content of the film-forming solution used for forming the PVA film of the present invention is preferably 70% by mass or more, and more preferably 71% by mass or more. If the volatile content of the PVA aqueous solution used for film formation is less than 70% by mass, the effect of reducing film thickness unevenness due to the leveling effect during casting cannot be expected, and the average MD slope value becomes high, which is undesirable. If the volatile content of the PVA aqueous solution used for film formation is too high, the drying time increases and productivity decreases, which is undesirable.

[0033] Here, the "volatile content of the film-forming stock solution" in this invention refers to the volatile content calculated by the following formula. Volatile fraction (mass%) of the film-forming solution = {(Wa-Wb) / Wa} × 100 (In the formula, Wa represents the mass (g) of the film-forming solution, and Wb represents the mass (g) of the film-forming solution after drying Wa (g) in an electric dryer at 105°C for 16 hours.)

[0034] As conditions for forming the PVA film of the present invention, it is preferable to stretch the film with a draw ratio of 1.075 or higher when the moisture content of the PVA aqueous solution used for film formation is 20% by mass or higher, and more preferably 1.08 or higher. When the moisture content of the PVA aqueous solution used for film formation is 20% by mass or higher, it is preferable to stretch the film with a draw ratio of 1.135 or lower, more preferably less than 1.13, and even more preferably less than 1.12. Generally, the draw ratio is controlled by the speed ratio of the conveyor rolls, and as the draw ratio increases, the limiting stretching ratio during the stretching process decreases, and the probability of stretching breakage during the stretching process increases. For this reason, when the draw ratio is 1.12 or higher, a process to mitigate the effect of the draw ratio may be necessary. Also, a draw ratio higher than 1.135 is undesirable because it significantly reduces the limiting stretching ratio during the stretching process. Furthermore, if the draw ratio is less than 1.075, the phase parameter (θ) becomes small, which is undesirable from the viewpoint of visibility of unevenness in the polarizing plate. In addition, as a condition for forming the PVA film, the moisture content during drawing is preferably 20% by mass or more, and more preferably 22% by mass or more. If the moisture content during drawing is lower than 20% by mass, the phase parameter (θ) does not tend to increase. For these reasons, it is necessary to set the moisture content during drawing and the draw ratio within the above range.

[0035] (PVA) The PVA resin contained in the PVA film of the present invention can be, for example, obtained by saponifying a polyvinyl ester obtained by polymerizing one or more vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versaticate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among the above vinyl esters, vinyl acetate is preferred in terms of ease of production, availability, and cost of PVA.

[0036] 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 such graft copolymerizable monomers include unsaturated carboxylic acids or their derivatives; unsaturated sulfonic acids or their derivatives; and α-olefins having 2 to 30 carbon atoms. The proportion of structural units derived from graft copolymerizable monomers in PVA is preferably 5 mol% or less, based on the total number of moles of structural units constituting PVA.

[0037] PVA may or may not have some of its hydroxyl groups cross-linked. Furthermore, some of the hydroxyl groups of the above-mentioned PVA may react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure, or they may not react with these compounds to form an acetal structure.

[0038] The degree of polymerization of PVA is preferably 2000 or higher, more preferably 2200 or higher, and even more preferably 2400 or higher. If the degree of polymerization is less than 2000, the durability of the resulting polarizing film tends to decrease. The degree of polymerization of PVA is preferably 2700 or lower, more preferably 2650 or lower, and even more preferably 2600 or lower. On the other hand, if the degree of polymerization exceeds 2700, the manufacturing cost tends to increase, and the process passability during film formation tends to deteriorate. In this specification, the degree of polymerization of PVA refers to the average degree of polymerization measured in accordance with the description in JIS K6726-1994.

[0039] The degree of saponification of PVA is preferably 98 mol% or higher, more preferably 98.5 mol% or higher, and even more preferably 99 mol% or higher, from the viewpoint of the water resistance of the polarizing film. If the degree of saponification is less than 98 mol%, the water resistance of the resulting polarizing film tends to be poor. In this specification, the degree of saponification 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) and vinyl alcohol units that can be converted into vinyl alcohol units by saponification. The degree of saponification can be measured in accordance with the description in JIS K6726-1994.

[0040] (Plasticizer) The PVA film of the present invention preferably contains a plasticizer. Examples of plasticizers include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane, and the PVA film of the present invention may contain one or more of these plasticizers. Among these, glycerin is preferred in terms of its effect on improving stretchability.

[0041] The plasticizer content 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 5 parts by mass or more, per 100 parts by mass of PVA contained therein. A plasticizer content of 1 part by mass or more can further improve the stretchability of the PVA film. On the other hand, the plasticizer content in the PVA film is preferably 20 parts by mass or less, more preferably 17 parts by mass or less, and even more preferably 15 parts by mass or less, per 100 parts by mass of PVA. A plasticizer content of 20 parts by mass or less can prevent the PVA film from becoming too flexible and thus reducing its handling properties.

[0042] (Surfactants) The PVA film of the present invention preferably contains a surfactant. By manufacturing a PVA film using a film-forming stock solution containing a surfactant, the film-forming properties of the PVA film are improved. As a result, the occurrence of thickness variations in the PVA film is suppressed, and the PVA film can be easily peeled off the metal rolls and belts used for film formation. When a PVA film is manufactured from a film-forming stock solution containing a surfactant, the resulting PVA film will contain the surfactant.

[0043] The type of surfactant is not particularly limited, but anionic and nonionic surfactants are preferred from the viewpoint of the release properties of the PVA film from metal rolls and belts.

[0044] Suitable anionic surfactants include, for example, carboxylic acid types such as potassium laurate; sulfate ester types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and sulfonic acid types such as dodecylbenzenesulfonate.

[0045] Suitable nonionic surfactants include, for example, alkyl ether types such as polyoxyethylene oleyl ether; alkylphenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkylamine types such as polyoxyethylene laurylamino ether; alkylamide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as oleic acid diethanolamide; and allylphenyl ether types such as polyoxyalkylene allylphenyl ether.

[0046] These surfactants may be used individually or in combination of two or more.

[0047] The lower limit of the surfactant content in the PVA film of the present invention is preferably 0.01 part by mass, more preferably 0.02 part by mass, and even more preferably 0.05 part by mass with respect to 100 parts by mass of PVA. By setting the surfactant content to be not less than the above lower limit, the film-forming property and peelability of the PVA film are further improved. On the other hand, the upper limit of the surfactant content in the PVA film is preferably 0.5 part by mass, more preferably 0.3 part by mass, and even more preferably 0.2 part by mass with respect to 100 parts by mass of PVA. By setting the surfactant content to be not more than the above upper limit, it is possible to suppress the surfactant from bleeding out to the surface of the PVA film and causing blocking, thereby reducing the handling property.

[0048] (Other components) The PVA film of the present invention may further contain components such as an antioxidant, an antifreezing agent, a pH adjuster, a masking agent, a coloring inhibitor, an oil agent, and a surfactant described later, if necessary.

[0049] (Shape, etc.) The shape of the PVA film of the present invention is not particularly limited, but a long film is preferred. Thereby, a more uniform PVA film can be continuously and easily produced, and it can also be continuously used when producing a polarizing film using the same. The length of the long film (the length in the length direction) is not particularly limited and can be appropriately set according to the application, etc. For example, it can be within the range of 5 to 30000 m.

[0050] The thickness of the PVA film of the present invention is preferably 30 to 65 μm. If the thickness of the PVA film is less than 30 μm, the handling property during stretching deteriorates, and if the thickness of the PVA film is greater than 65 μm, it is not preferable as a polarizing plate for a thin display.

[0051] <Manufacturing method of PVA film> In the present invention, the method for manufacturing PVA film is not particularly limited, but any method can be employed, such as using a film-forming stock solution obtained by adding a solvent and additives to PVA to homogenize it, and manufacturing the film using a casting method, a wet film-forming method (discharge into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method in which the film-forming stock solution is cooled and gelled, then the solvent is extracted and removed to obtain a PVA film), or a combination of these methods, or a melt extrusion film-forming method or inflation molding method in which the above-mentioned film-forming stock solution is obtained using an extruder and is extruded from a T-die to produce a film. Among these, the casting method and the melt extrusion film-forming method are preferred because they can produce a homogeneous film with high productivity. The casting method or the melt extrusion film-forming method for PVA film will be described below.

[0052] One method for producing a polyvinyl alcohol film according to the present invention is to form a polyvinyl alcohol film using an aqueous solution of polyvinyl alcohol with a concentration of 32% by mass or less, and then stretch the polyvinyl alcohol film at a stretching ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20% by mass or more. By adopting this manufacturing method, a film without thickness variations can be produced.

[0053] If the critical stretching ratio decreases, additional steps can be added to mitigate the effects of the stretching ratio. Specific methods for mitigating the effects of the stretching ratio include irradiating the polyvinyl alcohol film with infrared or microwave radiation, reducing the stretching ratio or cumulative stretching ratio (product of stretching ratios when multiple stretchings are performed) when the moisture content of the polyvinyl alcohol film is between 15% and 5% by mass, applying a plasticizer after stretching, drying with superheated steam, drying at high temperatures, and heat treatment with a floating dryer. Any of these methods can be adopted, combined, and performed multiple times.

[0054] When PVA film is manufactured by casting or melt extrusion, the above-mentioned film-forming solution is poured in a film-like manner onto a support such as a metal roll or metal belt, heated to remove the solvent, and solidified into a film. The solidified film is peeled from the support, dried using a drying roll or drying oven as needed, and further heat-treated as needed, and then wound up to obtain a long roll of PVA film.

[0055] There are no particular restrictions on the method of preparing the film-forming stock solution. For example, methods include dissolving PVA with additives such as plasticizers and surfactants in a dissolution tank, or melting and kneading hydrated PVA together with plasticizers and surfactants using a single-screw or twin-screw extruder.

[0056] Examples of liquid media used in preparing the film-forming stock solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine. One or more of these can be used. Of these, water is preferred due to its environmental impact and recoverability.

[0057] The PVA film can be further heat-treated after drying, if necessary. By heat-treating, the strength, degree of swelling, and birefringence of the PVA film can be adjusted. The surface temperature of the heat treatment roll for heat treatment is preferably 60°C or higher. Furthermore, the surface temperature of the heat treatment roll is preferably 135°C or lower, and more preferably 130°C or lower. If the surface temperature of the heat treatment roll is too high, too much heat will be applied, causing the size of the lamellar crystals in the PVA film to increase, making it difficult for the degree of swelling of the PVA film to satisfy the above range.

[0058] The PVA film manufactured in this way is then subjected to further treatments such as humidity control and trimming of both ends (edges) as needed. It is then wound into a roll on a paper or cylindrical core, packaged in a moisture-proof container, and becomes the final product.

[0059] The volatile content of the PVA film ultimately obtained by the series of processes described above is not necessarily limited. The volatile content of the ultimately obtained PVA film is preferably 1% by mass or more, and more preferably 2% by mass or more. The volatile content of the ultimately obtained PVA film is preferably 5% by mass or less, and more preferably 4% by mass or less.

[0060] The PVA film of the present invention has reduced film thickness unevenness in the MD direction and the phase shift of film thickness unevenness in the TD direction is adjusted to a specific range, making it suitable for use as a film for manufacturing optical films. Here, examples of optical films include polarizing films, viewing angle improving films, phase difference films, and brightness improving films, as will be described later, but polarizing films are preferred. The polarizing film of the present invention made using the PVA film of the present invention shows less visible unevenness in the polarizing plate after stretching compared to conventional films.

[0061] <Method for manufacturing optical films> In the following section, we will specifically explain a method for manufacturing polarizing films as an example of an optical film manufacturing method.

[0062] Polarizing films can typically be manufactured using PVA film as the base film, through processing steps such as swelling, dyeing, crosslinking, stretching, and fixing. Specific examples of processing solutions used in each step include swelling solution used in swelling, dyeing solution used in dyeing, crosslinking solution used in crosslinking, stretching solution used in stretching, fixing solution used in fixing, and washing solution used in washing.

[0063] The following describes the various processing steps that can be used in the manufacturing method for polarizing films. Note that in the manufacturing method for polarizing films, one or more of the following processes may be omitted, the same process may be performed multiple times, or different processes may be performed simultaneously.

[0064] (Cleaning process) It is preferable to perform a washing treatment on the PVA film before performing the swelling treatment on the PVA film. Such a washing treatment before the swelling treatment can remove anti-blocking agents and other substances adhering to the PVA film, and prevent contamination of each processing solution in the polarizing film manufacturing process with anti-blocking agents and other substances. The washing treatment is preferably performed by immersing the PVA film in the washing treatment solution, but it can also be performed by spraying the washing treatment solution onto the PVA film. For example, water can be used as the washing treatment solution. The temperature of the washing treatment solution is preferably 20°C or higher, more preferably 22°C or higher, even more preferably 24°C or higher, and particularly preferably 26°C or higher. A washing treatment solution temperature of 20°C or higher makes it easier to remove anti-blocking agents and other substances adhering to the PVA film. The temperature of the washing treatment solution is preferably 40°C or lower, more preferably 38°C or lower, even more preferably 36°C or lower, and particularly preferably 34°C or lower. Furthermore, a washing treatment solution temperature of 40°C or lower prevents a part of the surface of the PVA film from dissolving, causing the films to stick together and reducing their handling properties.

[0065] (Swelling treatment) The swelling treatment can be carried out by immersing the PVA film in a swelling treatment solution such as water. The temperature of the swelling treatment solution is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 24°C or higher. The temperature of the swelling treatment solution is preferably 40°C or lower, more preferably 38°C or lower, and even more preferably 36°C or lower. The immersion time in the swelling treatment solution is preferably, for example, 0.1 minutes or more, and more preferably 0.5 minutes or more. The immersion time in the swelling treatment solution is preferably, for example, 5 minutes or less, and even more preferably 3 minutes or less. The water used as the swelling treatment solution is not limited to pure water, but may be an aqueous solution in which various components such as boron-containing compounds are dissolved, or a mixture of water and an aqueous medium. The type of boron-containing compound is not particularly limited, but boric acid or borax is preferred from the viewpoint of ease of handling. When the swelling treatment solution contains a boron-containing compound, its concentration is preferably 6% by mass or less from the viewpoint of improving the stretchability of the PVA film.

[0066] (Dialysis treatment) The dyeing treatment is preferably carried out using an iodine-based dye as a dichroic dye, and the dyeing can be performed at any of the following stages: before, during, or after the stretching treatment. The dyeing treatment is preferably carried out by immersing the PVA film in a solution (preferably an aqueous solution) containing iodine-potassium iodide as the dyeing solution. The concentration of iodine in the dyeing solution is preferably in the range of 0.005 to 0.2% by mass, and the potassium iodide / iodine (mass) is preferably in the range of 20 to 100. The temperature of the dyeing solution is preferably 20°C or higher, and more preferably 25°C or higher. The temperature of the dyeing solution is preferably 50°C or lower, and more preferably 40°C or lower. The dyeing solution may also contain a boron-containing compound such as boric acid as a crosslinking agent. If the PVA film used as the base film is pre-containing a dichroic dye, the dyeing treatment can be omitted. Furthermore, the PVA film used as the base film can be pre-treated to contain boron-containing compounds such as boric acid and borax.

[0067] (Crosslinking treatment) In the manufacture of polarizing films, it is preferable to perform a crosslinking treatment after the dyeing treatment in order to strengthen the adsorption of dichroic dyes onto the PVA film. The crosslinking treatment can be carried out by using a solution (preferably an aqueous solution) containing a crosslinking agent as the crosslinking treatment solution and immersing the PVA film in the crosslinking treatment solution. As the crosslinking agent, one or more boron-containing compounds such as boric acid and borax can be used. If the concentration of the crosslinking agent in the crosslinking treatment solution is too high, the crosslinking reaction tends to proceed too much, making it difficult to perform sufficient stretching in the subsequent stretching treatment. Conversely, if the concentration is too low, the effect of the crosslinking treatment tends to be reduced. The concentration of the crosslinking agent in the crosslinking treatment solution is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. The concentration of the crosslinking agent in the crosslinking treatment solution is preferably 6% by mass or less, more preferably 5.5% by mass or less, and even more preferably 5% by mass or less.

[0068] To suppress the elution of dichroic dyes from the PVA film after dyeing, the crosslinking solution may contain an iodine-containing compound such as potassium iodide. If the concentration of the iodine-containing compound in the crosslinking solution is too high, the heat resistance of the resulting polarizing film tends to decrease for reasons unknown. Conversely, if the concentration is too low, the effect of suppressing the elution of dichroic dyes tends to decrease. The concentration of the iodine-containing compound in the crosslinking solution is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. The concentration of the iodine-containing compound in the crosslinking solution is preferably 6% by mass or less, more preferably 5.5% by mass or less, and even more preferably 5% by mass or less.

[0069] If the temperature of the crosslinking solution is too high, the dichroic dye tends to dissolve, leading to uneven dyeing in the resulting polarized film. Conversely, if the temperature is too low, the effect of the crosslinking treatment may be reduced. The temperature of the crosslinking solution is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 25°C or higher. The temperature of the crosslinking solution is preferably 45°C or lower, more preferably 40°C or lower, and even more preferably 35°C or lower.

[0070] In addition to the stretching process described later, the PVA film may be stretched during or between each of the above-mentioned processes. Such stretching (pre-stretching) can prevent wrinkles from forming on the surface of the PVA film. The total stretching ratio of the pre-stretching (the ratio obtained by multiplying the stretching ratios in each process) is preferably 4 times or less, and more preferably 3.5 times or less, based on the original length of the PVA film roll before stretching, from the viewpoint of the polarization performance of the resulting polarizing film. The total stretching ratio of the pre-stretching is more preferably 1.5 times or more, based on the original length of the PVA film roll before stretching, from the viewpoint of the polarization performance of the resulting polarizing film. The stretching ratio in the swelling process is preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.4 times or more, based on the original length of the PVA film. The stretching ratio in the swelling process is preferably 3 times or less, more preferably 2.5 times or less, and even more preferably 2.3 times or less, based on the original length of the PVA film. The stretching ratio in the dyeing treatment is preferably 2 times or less, more preferably 1.8 times or less, and even more preferably 1.5 times or less, based on the original length of the PVA film. The stretching ratio in the dyeing treatment is even more preferably 1.1 times or more, based on the original length of the PVA film. The stretching ratio in the crosslinking treatment is preferably 2 times or less, more preferably 1.5 times or less, and even more preferably 1.3 times or less, based on the original length of the PVA film. The stretching ratio in the crosslinking treatment is even more preferably 1.05 times or more, based on the original length of the PVA film.

[0071] (Stretching process) The stretching process may be carried out by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, a solution containing a boron-containing compound such as boric acid (preferably an aqueous solution) is used as the stretching solution, and the stretching can be carried out in the stretching solution, or in a dyeing solution or a fixation solution described later. In the case of the dry stretching method, the stretching can be carried out in air using the PVA film after water absorption. Among these, the wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. When the stretching solution contains a boron-containing compound, the concentration of the boron-containing compound in the stretching solution is preferably 1.5% by mass or more, more preferably 2.0% by mass or more, and even more preferably 2.5% by mass or more, in order to improve the stretchability of the PVA film. The concentration of the boron-containing compound in the stretching solution is preferably 7% by mass or less, more preferably 6.5% by mass or less, and even more preferably 6% by mass or less, in order to improve the stretchability of the PVA film.

[0072] It is preferable to include an iodine-containing compound such as potassium iodide in the stretching solution. If the concentration of the iodine-containing compound in the stretching solution is too high, the resulting polarizing film tends to have a strong bluish hue, and if it is too low, for reasons unknown, the heat resistance of the resulting polarizing film tends to decrease. The concentration of the iodine-containing compound in the stretching solution is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3% by mass or more. The concentration of the iodine-containing compound in the stretching solution is preferably 8% by mass or less, more preferably 7.5% by mass or less, and even more preferably 7% by mass or less.

[0073] If the temperature of the stretching solution is too high, the PVA film tends to melt, soften, and become prone to breakage. Conversely, if the temperature is too low, the stretchability tends to decrease. The temperature of the stretching solution is preferably 50°C or higher, more preferably 52.5°C or higher, and even more preferably 55°C or higher. The temperature of the stretching solution is preferably 70°C or lower, more preferably 67.5°C or lower, and even more preferably 65°C or lower. The preferred range of stretching temperature when the stretching process is performed by the dry stretching method is also as described above.

[0074] In the stretching process, a higher stretching ratio is preferable to obtain a polarizing film with superior polarization performance, so it is preferable to have a stretching ratio of 1.2 times or more, more preferably 1.5 times or more, and even more preferably 2 times or more. Furthermore, the total stretching ratio (the ratio obtained by multiplying the stretching ratios in each process), including the stretching ratio of the pre-stretching process described above, is preferable to have a stretching ratio of 5.5 times or more, more preferably 5.7 times or more, and even more preferably 5.9 times or more, based on the original length of the raw PVA film before stretching, from the viewpoint of the polarization performance of the obtained polarizing film. There is no particular upper limit to the stretching ratio, but if the stretching ratio is too high, stretching breakage of the PVA film is more likely to occur, so it is preferable to have a stretching ratio of 8 times or less.

[0075] There are no particular restrictions on the method of uniaxial stretching; uniaxial stretching in the longitudinal direction or transverse uniaxial stretching in the width direction can be employed. When manufacturing polarizing films, uniaxial stretching in the longitudinal direction is preferred because it yields films with superior polarization performance. Uniaxial stretching in the longitudinal direction can be performed by using a stretching device equipped with multiple rolls that are parallel to each other and changing the peripheral speed between each roll.

[0076] In the present invention, there are no particular restrictions on the maximum stretching speed (% / min) when the stretching process is performed by uniaxial stretching, however, the maximum stretching speed is preferably 200% / min or more, more preferably 300% / min or more, and even more preferably 400% / min or more. Here, the maximum stretching speed refers to the fastest stretching speed in a stage when the stretching process of the PVA film is performed in two or more stages using three or more rolls with different peripheral speeds. If the stretching process of the PVA film is performed in one stage without dividing it into two or more stages, the stretching speed in that stage becomes the maximum stretching speed. Furthermore, the stretching speed refers to the increase in the length of the PVA film per unit time compared to the length of the PVA film before stretching. For example, a stretching speed of 100% / min is the speed at which the PVA film is deformed to twice its length in one minute from its length before stretching. A higher maximum stretching speed is preferable because it allows for faster stretching (uniaxial stretching) of the PVA film, resulting in improved productivity of the polarizing film. On the other hand, if the maximum stretching speed becomes too high, excessive tension may be applied locally to the PVA film during the stretching process (uniaxial stretching), making stretching fracture more likely. From this perspective, it is preferable that the maximum stretching speed does not exceed 900% / min.

[0077] (Washing process after dyeing) It is preferable to perform a washing treatment on the PVA film after the dyeing treatment, preferably after the stretching treatment. The washing treatment is preferably carried out by immersing the PVA film in a washing solution, but it can also be carried out by spraying the washing solution onto the PVA film. For example, water can be used as the washing solution. The water is not limited to pure water and may contain an iodine-containing compound such as potassium iodide. The washing solution may also contain a boron-containing compound, in which case the concentration of the boron-containing compound is preferably 2.0% by mass or less.

[0078] The temperature of the cleaning solution is preferably 5°C or higher, more preferably 7°C or higher, and even more preferably 10°C or higher. Furthermore, the temperature of the cleaning solution is preferably 40°C or lower, more preferably 38°C or lower, and even more preferably 35°C or lower. A cleaning solution temperature of 5°C or higher can suppress the breakage of the PVA film due to the freezing of water. In addition, a cleaning solution temperature of 40°C or lower improves the optical properties of the resulting polarizing film.

[0079] Specific methods for manufacturing polarizing films include dyeing, stretching, and crosslinking and / or fixing treatments applied to a PVA film. A preferred example is a method in which the PVA film is subjected to swelling, dyeing, crosslinking, stretching (especially uniaxial stretching), and washing in this order. The stretching treatment may also be performed in any of the preceding treatment steps, or in two or more stages.

[0080] A polarizing film can be obtained by drying the PVA film after each of the above processes. There are no particular restrictions on the drying method; for example, a contact method in which the film is brought into contact with a heated roll, a method of drying in a hot air dryer, and a floating method in which the film is dried with hot air while suspended in the air can be used.

[0081] <Polarizing film, polarizing plate> The polarizing film of the present invention is manufactured by laminating a protective film to at least one side in order to enhance its mechanical strength. The polarizing film of the present invention is typically used as a polarizing plate by laminating a protective film that is optically transparent and has mechanical strength. Examples of protective films include cellulose triacetate (TAC) film, cellulose acetate-butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives for lamination include PVA-based adhesives and urethane-based adhesives, with PVA-based adhesives being particularly preferred.

[0082] The polarizing plate obtained as described above can be used as a component of an LCD by coating it with an adhesive such as an acrylic adhesive and then laminating it to a glass substrate. At the same time, it may be laminated with a retardation film, a viewing angle improvement film, a brightness improvement film, or the like.

Example

[0083] The present invention will be specifically described by the following examples, but the present invention is not limited to these examples in any way.

[0084] <Calculation of MD average tilt value, phase parameter (θ), and average tilt value at ±200 mm position> The MD average tilt value, phase parameter (θ), and average tilt value at the ±200 mm position of the PVA film obtained in the examples or comparative examples by the above method were calculated. The results are shown in Table 1.

[0085] <Measurement of swelling degree of PVA film> A test piece of about 1.5 g was cut out from the PVA film obtained in the following examples or comparative examples. Then, this test piece was immersed in 1000 g of distilled water at 30°C. After immersion for 30 minutes, the test piece was taken out, the water on the surface was blotted with filter paper, and then its mass (We) was measured. Subsequently, the test piece was placed in a hot air dryer and dried at 105°C for 16 hours, and then its mass (Wf) was measured. From the obtained masses We and Wf, the swelling degree of the PVA film was determined by the following formula. Swelling degree (%) = (We / Wf) × 100

[0086] <Evaluation of limit draw ratio during stretching> In the following examples or comparative examples, the ease of breakage during stretching was evaluated by the breakage ratio of the PVA film when producing a polarizing film. That is, the breakage ratio of uniaxial stretching in the stretching process when producing a polarizing film was measured 10 times, and its average value was used as the limit draw ratio and evaluated according to the following criteria. A ··· The limit draw ratio is 6.6 times or more B ··· The limit draw ratio is 6.5 times or more and less than 6.6 times C ··· The limit draw ratio is less than 6.5 times

[0087] <Evaluation of unevenness in polarizing plates> A polarizing plate sample obtained by the method described in Example 1 was cut to 30 cm x 30 cm, and the sample polarizing plate was placed between two polarizing plates in a parallel nicol state (single polarity transmittance 43.5%, polarization degree 99.9%) so that the sample polarizing plate was in a cross nicol state relative to each polarizing plate, resulting in a brightness of 40,000 cd / m². 2 Optical polarization unevenness was visually observed in transmission mode using the backlight and evaluated according to the following criteria. A... Uneven coloring B... No uneven coloring

[0088] [Example 1] A film-forming stock solution (volatile content 70% by mass) was prepared by melt-mixing 100 parts by mass of PVA (saponification degree 99.9 mol%, degree of polymerization 2400), 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 in a melt extruder. Next, the stock solution was extruded in a film-like manner from a T-die onto a support (surface temperature 98°C) to form a PVA film on the support. When the moisture content of the extruded PVA film reached 24%, the draw ratio was set to 1.10 by controlling the speed ratio of the rolls, and the film was stretched. Contact heat treatment was performed at 105°C on the heat treatment rolls in the latter half of the process to obtain a PVA film with a width of 5 m and a thickness of 60 μm. The moisture content during drawing was determined by tearing the PVA film between the rolls during transport and measuring its mass (Ww), then placing the film in a hot air dryer and drying it at 105°C for 16 hours, after which its mass (Wd) was measured. The moisture content of the film was calculated using the following formula. Film moisture content (%) = (Ww - Wd) / Ww × 100

[0089] The obtained PVA film was slit to a width of 650 mm, and polarizing films were continuously manufactured by performing swelling, dyeing, crosslinking, stretching, washing, and drying treatments in this order. The swelling treatment was performed by uniaxial stretching to 2.00 times its length while immersed in pure water (swelling treatment solution) at 25°C. The dyeing treatment was performed by uniaxial stretching to 1.26 times its length while immersed in potassium iodide / iodine aqueous dyeing solution (dyeing treatment solution) at a temperature of 32°C (potassium iodide / iodine (mass ratio) 23, iodine concentration 0.03~0.05 mass%). In this dyeing treatment, the iodine concentration in the dyeing treatment solution was adjusted within the range of 0.03~0.05 mass% so that the transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment was in the range of 43.5% ± 0.2%. The crosslinking treatment was performed by uniaxially stretching the material 1.19 times in length while immersing it in a 32°C boric acid aqueous solution (crosslinking treatment solution) (boric acid concentration 2.6% by mass). The stretching treatment was performed by uniaxially stretching the material 2.00 times in length while immersing it in a 55°C boric acid / potassium iodide aqueous solution (stretching treatment solution) (boric acid concentration 2.8% by mass, potassium iodide concentration 5% by mass). The washing treatment was performed by immersing the material for 12 seconds in a 22°C potassium iodide / boric acid aqueous solution (washing treatment solution) (potassium iodide concentration 3-6% by mass, boric acid concentration 1.5% by mass) without stretching.

[0090] [Examples 2-4, Comparative Examples 1-3] Except for changing the manufacturing conditions of the PVA film as shown in Table 1, the PVA film was manufactured and evaluated in the same manner as in Example 1. The volatile content of the film-forming solution was adjusted by adjusting the amount of water mixed in the melt extruder. The results are shown in Table 1.

[0091] From the above results, it can be seen that the PVA film of the present invention has suppressed color unevenness during stretching.

[0092] [Table 1]

Claims

1. Let the film thickness profile at the position x in the MD direction with respect to an arbitrary point on the film surface be F ｎ , ｎ , ｎ , ｎ , ｎ , ｎ , ｎ , ｎ , ｎ , ０ , ０ , －２００ , ＋２００ , ０ , ０ , －１００ , ＋１００ (x), let the film thickness profile at the position x in the MD direction with respect to the point -100 mm in the TD direction from the arbitrary point be F －１００ (x), let the film thickness profile at the position x in the MD direction with respect to the point -200 mm in the TD direction from the arbitrary point be F －２００ (x), let the film thickness profile at the position x in the MD direction with respect to the point +100 mm in the TD direction from the arbitrary point be F ＋１００ (x), let the film thickness profile at the position x in the MD direction with respect to the point +200 mm in the TD direction from the arbitrary point be F ＋２００ (x), and let the derivative value in the MD direction of each film thickness profile be F’ ０ (x), F’ －１００ (x), F’ －２００ (x), F’ ＋１００ (x) and F’ ＋２００ (x), when |F’ 0(x)|, |F’ -100(x)|, |F’ -200(x)|, |F’ +100(x)| and |F’ +200(x)| are represented by the average in the MD direction, the average inclination value is 0.02 μm / mm or less, and the point where F’ ０ (x) shows a minimum value is C ｎ (n = 1, 2, 3 ···), when |F’ －１００ (C ｎ ) - F’ ０ (C ｎ ), |F’ －２００ (C ｎ ) - F’ ０ (C ｎ ), |F’ ＋１００ (C ｎ ) - F’ ０ (C ｎ ) and |F’ ＋２００ (C ｎ ) - F’ ０ (C ｎ ) are defined by the average value of the phase parameter (θ) being 0.015 μm / mm or more, a polyvinyl alcohol film.

2. The polyvinyl alcohol film according to claim 1, wherein the width is 4 m or more.

3. A polyvinyl alcohol film according to claim 1 or 2, wherein the degree of swelling is 170 to 220%.

4. A polyvinyl alcohol film according to any one of claims 1 to 3, having a thickness of 30 μm to 65 μm.

5. A polyvinyl alcohol film according to any one of claims 1 to 4, which is a film for manufacturing optical films.

6. The polyvinyl alcohol film according to claim 5, wherein the optical film is a polarizing film.

7. A polarizing film manufactured using the polyvinyl alcohol film described in any one of claims 1 to 6.

8. A polarizing plate manufactured by attaching a protective film to at least one side of the polarizing film described in claim 7.

9. A method for producing a polyvinyl alcohol film according to any one of claims 1 to 6, The process includes a step of forming a polyvinyl alcohol film using an aqueous solution of polyvinyl alcohol with a concentration of 32% by mass or less, and stretching the polyvinyl alcohol film at a stretch ratio of 1.075 to 1.135 when the moisture content of the polyvinyl alcohol film is 20% by mass or more. A method for producing a polyvinyl alcohol film, wherein the volatile content of the above-mentioned aqueous polyvinyl alcohol solution used as a film-forming stock solution is 70% by mass or more.

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