JPWO2017138551A1 - Polarizing film and method for producing the same - Google Patents

Abstract

  In the method for producing a polarizing film in which at least a swelling step, a dyeing step, a first crosslinking stretching step, and a second crosslinking stretching step are performed in this order on the polyvinyl alcohol film; the average degree of polymerization of polyvinyl alcohol contained in the polyvinyl alcohol film is In the first cross-linking stretching step, in the 40 to 55 ° C. aqueous solution containing 1 to 5% by mass of boric acid, the stretching ratio in the step is 1.1 to 1.3 times and the total In the second cross-linking stretching step, in the aqueous solution at 60 to 70 ° C. containing 1 to 5% by mass of boric acid, Uniaxial stretching is performed so that the stretching ratio is 1.8 to 3.0 times and the total stretching ratio is 6 to 8 times. Thereby, a polarizing film having excellent polarizing performance and low shrinkage stress can be produced.

Description

  The present invention relates to a polarizing film comprising a polyvinyl alcohol film containing an iodine dichroic dye and a method for producing the same.

A polarizing plate having a light transmission and shielding function is a basic component of a liquid crystal display (LCD) together with a liquid crystal that changes a polarization state of light. Many polarizing plates have a structure in which a protective film such as a cellulose triacetate (TAC) film is bonded to the surface of a polarizing film. As a polarizing film, an iodine pigment (I ₃ ) is applied to a matrix (a uniaxially stretched oriented film) obtained by uniaxially stretching a polyvinyl alcohol film (hereinafter, “polyvinyl alcohol” may be abbreviated as “PVA”). ^- and I ₅ ^-, etc.) has become mainstream those adsorbed. Such a polarizing film can be obtained by uniaxially stretching a PVA film preliminarily containing a dichroic dye, adsorbing a dichroic dye simultaneously with uniaxial stretching of the PVA film, or dichroic after uniaxially stretching the PVA film. Manufactured by adsorbing dyes.

  LCDs are used in a wide range of applications such as small devices such as calculators and watches, notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, in-vehicle navigation systems, mobile phones, and measuring devices used indoors and outdoors. In response to the recent high performance of displays, there is a need for polarizing films having excellent optical performance.

  In order to obtain a polarizing film having excellent optical performance, various manufacturing methods have been proposed. In Patent Documents 1 to 4, in the method for producing a polarizing film, the PVA film is immersed in water to swell, dyed with iodine-based dichroic dye, and then cross-linked and stretched in an aqueous boric acid solution. It is described that a polarizing film having excellent polarization characteristics, uniform optical characteristics, excellent appearance, and the like can be obtained by swelling treatment in a plurality of tanks having different conditions. These patent documents describe various techniques for the step of crosslinking in a boric acid aqueous solution and stretching treatment. The example of Patent Document 1 describes a method in which the film is immersed in a boric acid aqueous solution at 50 ° C. and stretched 1.5 times. The example of Patent Document 2 describes a method in which the film is immersed in a boric acid aqueous solution at 55 ° C. and stretched 2.5 times. The Example of patent document 3 describes the method of extending | stretching in boric-acid aqueous solution of 55 degreeC, after immersing in 40 degreeC boric-acid aqueous solution. Moreover, the Example of patent document 4 describes the method of extending | stretching 1.53 times in 60 degreeC boric-acid aqueous solution, after extending | stretching 1.33 times in 30 degreeC boric-acid aqueous solution.

  Further, Patent Document 5 describes a method for producing a polarizing film that can reduce the shrinkage rate in the TD direction (direction perpendicular to the longitudinal direction) when exposed to high temperature conditions. Specifically, the PVA film is immersed in water to swell, dyed with iodine-based dichroic dye, and then cross-linked in a boric acid aqueous solution and subjected to stretching treatment. It is said that the shrinkage rate in the TD direction can be reduced by stretching at a magnification of 50% or more of the magnification. Although the Example of patent document 5 has described the process immersed in the boric acid aqueous solution of 56.5 degreeC, extending | stretching is not given in the said aqueous solution. Moreover, it has not measured about the shrinkage | contraction rate of MD direction (longitudinal direction of a film).

  In recent years, LCDs are increasingly used for mobile applications such as notebook computers and mobile phones. Since such LCDs for mobile use are used in various environments, a polarizing film having excellent dimensional stability even at high temperatures is required. For that purpose, it is desirable that the polarizing film has a small shrinkage stress at a high temperature. However, the methods as described in Patent Documents 1 to 5 have failed to obtain a polarizing film that can sufficiently achieve both excellent polarization performance and small shrinkage stress. This is because if the polarization performance is increased, the contraction stress is increased, and if the contraction stress is decreased, the polarization performance is decreased. Therefore, producing a polarizing film having high polarization performance and low shrinkage stress has been a difficult problem to solve.

JP 2006-65309 A JP 2014-197050 A JP 2006-267153 A JP 2013-140324 A JP 2012-3173 A

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a polarizing film having excellent polarization performance and low shrinkage stress, and a method for producing the same.

The above-mentioned subject is a manufacturing method of a polarizing film which performs at least a swelling process, a dyeing process, a first crosslinking stretching process, and a second crosslinking stretching process in this order for a polyvinyl alcohol film,
The polyvinyl alcohol film has a thickness of 5 to 100 μm,
The average degree of polymerization of polyvinyl alcohol contained in the polyvinyl alcohol film is 2500-3500,
In the swelling step, the polyvinyl alcohol film is swollen by immersing in water at 10 to 50 ° C.,
In the dyeing step, the polyvinyl alcohol film is impregnated with an aqueous solution of 10 to 50 ° C. containing 0.5 to 3% by mass of iodine and potassium iodide in total to impregnate the polyvinyl alcohol film, and total stretching Uniaxially stretched so that the magnification is 2-3 times,
In the first cross-linking stretching step, in a 40 to 55 ° C. aqueous solution containing 1 to 5% by mass of boric acid, the stretching ratio in the step is 1.1 to 1.3 times and the total stretching ratio is 2.5. Uniaxially stretched to ~ 3.5 times,
Subsequently, in the second crosslinking stretching step, in a 60 to 70 ° C. aqueous solution containing 1 to 5% by mass of boric acid, the stretching ratio in the step is 1.8 to 3.0 times and the total stretching ratio is 6 to 6%. The problem is solved by providing a method for producing a polarizing film, characterized in that the film is uniaxially stretched to 8 times.

At this time, in the second cross-linking stretching step, the maximum stretching stress is preferably 15 N / mm ² or less. It is also preferable to obtain a polarizing film having a single transmittance of 42 to 45% and a polarization degree of 99.980% or more. It is also preferable to obtain a polarizing film having a shrinkage stress of 45 N / mm ² or less.

The above problem is a polarizing film comprising a polyvinyl alcohol film containing an iodine dichroic dye,
When the single transmittance is 43.5%, the degree of polarization is 99.990% or more, and the content (C _A ) of the structure factor A in the wide-angle X-ray diffraction measurement is 3 to 4.5%. The problem can also be solved by providing a polarizing film having a B content (C _B ) of 2.0 to 8.5% and a ratio (C _A / C _B ) of 0.4 or more. Is done.

The above-mentioned problem is a polarizing film made of a polyvinyl alcohol film containing an iodine dichroic dye,
The single transmittance is 42 to 45%,
The degree of polarization is 99.980% or more, the content (C _A ) of structure factor A in wide-angle X-ray diffraction measurement is 3 to 4.5%, and the content (C _B ) of structure factor B is 2 The problem can also be solved by providing a polarizing film having a ratio of 0.0 to 8.5% and a ratio (C _A / C _B ) of 0.4 or more.

In each stretched film, the structure factor A is a highly oriented structure factor derived from a polyvinyl alcohol-boron aggregate structure, and the structure factor B is a highly oriented structure factor derived from amorphous polyvinyl alcohol. Moreover, it is preferable that the polymerization degree of the polyvinyl alcohol contained in each said stretched film is 2500-3500. It is also preferred that the polarizing film has a shrinkage stress of 45 N / mm ² or less.

  The polarizing film of the present invention has excellent polarizing performance and low shrinkage stress. Therefore, it can be suitably used for a high-performance liquid crystal display, particularly a liquid crystal display that may be used at high temperatures. Moreover, according to the manufacturing method of this invention, such a polarizing film can be manufactured.

It is a schematic diagram of a polarizing film manufacturing apparatus. It is the graph which plotted the degree of polarization at the time of the unit transmittance of 43.5% of the polarizing film obtained in Example 1 and comparative examples 1-4, 7, and 8 with respect to the contraction stress. FIG. 6 is a diagram in which a baseline straight line is drawn on an I (2θ) profile. FIG. 3 is a diagram in which the corrected I (2θ) profile is separated into “PVA amorphous”, “PVA crystal”, and “PVA-boric acid aggregated structure”. It is the figure which plotted the integrated intensity value (A) of "PVA amorphous", "PVA crystal", and "PVA-boric acid aggregation structure" obtained by the waveform separation analysis with respect to the azimuth. It is the figure which isolate | separated the integral intensity value (A) into the orientation component and the non-orientation component. It is the figure which plotted the content rate (C _A ) of the structure factor _A and the content rate (C _B ) of the structure factor B against the shrinkage stress. It is the figure which plotted ratio (C _A / C _B ) with respect to shrinkage stress.

  The polarizing film of the present invention is made of a polyvinyl alcohol film containing an iodine dichroic dye, has excellent polarizing performance, and has a small shrinkage stress. Such a polarizing film applies specific manufacturing conditions when a polyvinyl alcohol film (PVA film) is subjected to at least a swelling process, a dyeing process, a first crosslinking stretching process, and a second crosslinking stretching process in this order. Can be manufactured. Hereinafter, the manufacturing method of the polarizing film of this invention is demonstrated in detail.

  The PVA contained in the PVA film used for the production of the polarizing film of the present invention can be obtained by saponifying a polyvinyl ester obtained by polymerizing one or more vinyl esters. Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among these, vinyl acetate is preferable from the viewpoint of ease of production, availability, and cost of PVA.

  The polyvinyl ester may be obtained using only one or two or more kinds of vinyl esters as a monomer, but one or two as long as the effects of the present invention are not impaired. It may be a copolymer of at least one kind of vinyl ester and another monomer copolymerizable therewith.

  Examples of other monomers copolymerizable with vinyl ester include α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene and isobutene; (meth) acrylic acid or salts thereof; (meth) Methyl acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) acrylate, (meth) (Meth) acrylic acid esters such as t-butyl acrylate, 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, octadecyl (meth) acrylate; (meth) acrylamide, N-methyl (meth) Acrylamide, N-ethyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, diacetone (meth) acrylamide (Meth) acrylamide propanesulfonic acid or a salt thereof, (meth) acrylamidopropyldimethylamine or a salt thereof, (meth) acrylamide derivatives such as N-methylol (meth) acrylamide or a derivative thereof; N-vinylformamide, N-vinylacetamide, N-vinyl amides such as N-vinyl pyrrolidone; 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, stearyl vinyl ether; Vinyl cyanides such as (meth) acrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride; And allyl compounds such as allyl chloride; maleic acid or its salts, esters or acid anhydrides; itaconic acid or its salts, esters or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; unsaturated sulfonic acids, etc. Can do. Said polyvinyl ester can have a structural unit derived from 1 type, or 2 or more types of an above described other monomer.

  The proportion of structural units derived from other monomers in the polyvinyl ester is preferably 15 mol% or less, preferably 10 mol% or less, based on the number of moles of all structural units constituting the polyvinyl ester. More preferred is 5 mol% or less.

  In particular, when the other monomer is a monomer that may promote water solubility of the obtained PVA, such as (meth) acrylic acid or unsaturated sulfonic acid, the polarizing film In order to prevent PVA from being dissolved in the production process, the proportion of structural units derived from these monomers in the polyvinyl ester is 5 mol% or less based on the number of moles of all structural units constituting the polyvinyl ester. It is preferable that it is 3 mol% or less.

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

  PVA may have some of its hydroxyl groups crosslinked or uncrosslinked. Moreover, said PVA may react with aldehyde compounds, such as acetaldehyde and a butyraldehyde, etc. to form an acetal structure, and the said PVA does not react with these compounds and does not form an acetal structure. May be.

  It is preferable that the average degree of polymerization of PVA is 2500-3500. The average degree of polymerization is more preferably 2600 or more, and more preferably 3300 or less. When the average degree of polymerization is 2500 or more, a polarizing film having excellent polarizing performance can be easily obtained even when stretched at a high temperature in the second cross-linking stretching step. On the other hand, when the average degree of polymerization exceeds 3500, it may be difficult to reduce the shrinkage stress of the obtained polarizing film. Here, the average degree of polymerization of PVA means the average degree of polymerization measured according to the description of JIS K6726-1994. In addition, although PVA in a polarizing film contains the crosslinked structure by a boric acid, if a boric acid ester is hydrolyzed and removed, there will be no substantial change in the average degree of polymerization of PVA itself.

  The degree of saponification of PVA is preferably 98 mol% or more, more preferably 98.5 mol% or more, and further preferably 99 mol% or more from the viewpoint of the polarizing performance of the polarizing film. When the degree of saponification is less than 98 mol%, PVA tends to be eluted during the production process of the polarizing film, and the eluted PVA may adhere to the film and reduce the polarizing performance of the polarizing film. In this specification, the degree of saponification of PVA refers to the total number of moles of structural units (typically vinyl ester units) that can be converted into vinyl alcohol units by saponification and the vinyl alcohol units of PVA. The proportion (mol%) occupied by the number of moles of vinyl alcohol units. The degree of saponification can be measured according to the description of JIS K6726-1994.

  It is preferable that the content rate of PVA in the PVA film used by this invention exists in the range of 50-99 mass% from the ease of manufacture of the desired polarizing film. The content is more preferably 75% by mass or more, further preferably 80% by mass or more, and particularly preferably 85% by mass or more. Moreover, it is more preferable that it is 98 mass% or less, it is more preferable that it is 96 mass% or less, and it is especially preferable that it is 95 mass% or less.

  It is preferable that a PVA film contains a plasticizer from a viewpoint of the extending | stretching improvement at the time of extending | stretching it. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylol propane. One or more of the agents can be included. Among these, glycerin is preferable from the viewpoint of the effect of improving stretchability.

  It is preferable that content of the plasticizer in a PVA film exists in the range of 1-20 mass parts with respect to 100 mass parts of PVA contained in it. When the content is 1 part by mass or more, the stretchability of the PVA film can be further improved. On the other hand, when the content is 20 parts by mass or less, it is possible to prevent the PVA film from becoming too flexible and handling properties from being lowered. The content of the plasticizer in the PVA film is more preferably 2 parts by mass or more, further preferably 4 parts by mass or more, and particularly preferably 5 parts by mass or more with respect to 100 parts by mass of PVA. Further, the content of the plasticizer is more preferably 15 parts by mass or less, and further preferably 12 parts by mass or less. Although depending on the manufacturing conditions of the polarizing film, the plasticizer contained in the PVA film may be eluted when the polarizing film is manufactured, so that the total amount does not always remain in the polarizing film.

  The PVA film may further contain components such as an antioxidant, an antifreezing agent, a pH adjusting agent, a masking agent, an anti-coloring agent, an oil agent, and a surfactant as necessary.

  The thickness of the PVA film used in the production method of the present invention is 5 to 100 μm. When the thickness is 100 μm or less, a thin polarizing film can be easily obtained. The thickness of the PVA film is preferably 60 μm or less. On the other hand, when the thickness is less than 5 μm, it becomes difficult to produce a polarizing film, and uneven dyeing tends to occur. The thickness of the PVA film is preferably 7 μm or more. The thickness here refers to the thickness of the PVA layer in the case of a multilayer film.

  The PVA film may be a single layer film or a multilayer film having a PVA layer and a base resin layer. In the case of a single layer film, the film thickness is preferably 20 μm or more, and more preferably 30 μm or more, in order to ensure handling properties. On the other hand, in the case of a multilayer film, the thickness of the PVA layer can be 20 μm or less, or 15 μm or less. The thickness of the base resin layer in the multilayer film is usually 20 to 500 μm.

  When using the multilayer film which has a PVA layer and a base-material resin layer as a PVA film, base-material resin must be what can be extended | stretched with PVA. Polyester, polyolefin resin, or the like can be used. Among these, an amorphous polyester resin is preferable, and an amorphous polyester resin obtained by copolymerizing polyethylene terephthalate or a copolymer component such as isophthalic acid or 1,4-cyclohexanedimethanol is preferably used. It is preferable to produce a multilayer film by applying the PVA solution to the base resin film. At this time, in order to improve the adhesiveness between the PVA layer and the base resin layer, the surface of the base resin film may be modified, or an adhesive layer may be formed between both layers.

  Although the shape in particular of a PVA film is not restrict | limited, Since it can supply continuously when manufacturing a polarizing film, it is preferable that it is a long PVA film. The length (length in the longitudinal direction) of the long PVA film is not particularly limited, and can be appropriately set according to the application of the polarizing film to be produced, for example, in the range of 5 to 20,000 m. It can be.

  The width | variety in particular of a PVA film is not restrict | limited, It can set suitably according to the use etc. of the polarizing film manufactured. In recent years, since the enlargement of screens of liquid crystal televisions and liquid crystal monitors has progressed, it is suitable for these applications when the width of the PVA film is 0.5 m or more, more preferably 1.0 m or more. On the other hand, if the width of the PVA film is too wide, it tends to be difficult to uniformly stretch the polarizing film when the polarizing film is produced by a device that has been put to practical use. Therefore, the width of the PVA film is 7 m or less. Is preferred.

  Using the PVA film described above as a raw material, the polarizing film of the present invention is produced. Specifically, a polarizing film is produced by performing at least a swelling process, a dyeing process, a first crosslinking stretching process, and a second crosslinking stretching process in this order. It is also preferable to perform a washing process and a drying process after the second cross-linking stretching process. Hereinafter, each step will be described in detail.

  In the production method of the present invention, first, the PVA film is subjected to a swelling step. In the swelling step, the PVA film is swollen by dipping in water at 10 to 50 ° C. The temperature of water is preferably 20 ° C. or higher, and preferably 40 ° C. or lower. By immersing in water in such a temperature range, the PVA film can be efficiently and uniformly swollen. The time for immersing the PVA film in water is preferably in the range of 0.1 to 5 minutes, and more preferably in the range of 0.5 to 3 minutes. By setting it as such immersion time, a PVA film can be efficiently swollen uniformly. The water in which the PVA film is immersed is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and a water-soluble organic solvent. In the swelling step, it is preferable to uniaxially stretch the PVA film. The draw ratio in that case is not particularly limited, but is preferably 1.2 to 2.8 times. The draw ratio is more preferably 1.5 times or more, and more preferably 2.5 times or less.

  In the manufacturing method of this invention, it uses for the dyeing | staining process after the said swelling process. In the dyeing step, the PVA film is impregnated with an aqueous solution of 10 to 50 ° C. containing 0.5 to 3 mass% of iodine and potassium iodide in total, and the total draw ratio is 2 It is uniaxially stretched to be 3 times. Thereby, while dyeing a PVA film with an iodine type dichroic dye, the molecular chain of PVA in a film is orientated and an iodine type dichroic dye is also orientated.

Dyeing is performed by immersing the PVA film in a dyeing bath containing an iodine-based dye. The dyebath is prepared by mixing iodine (I ₂ ) and potassium iodide (KI) with water. By mixing the iodine and potassium iodide and water, I ₃ ^- and I ₅ ^- it can generate iodine dye such. The total content of iodine and potassium iodide in the dyeing bath is 0.5 to 3% by mass in total. The total content of iodine and potassium iodide is preferably 0.8% by mass or more, and more preferably 2.5% by mass or less. By dyeing in such a concentration range, it is possible to dye efficiently and uniformly. The mass ratio of potassium iodide to iodine (KI / I ₂ ) is preferably 10 to 200, and more preferably 15 to 150. The dyeing bath may contain a boron compound such as boric acid such as boric acid and borax, but the content is usually less than 5% by mass in terms of boric acid, and preferably 1% by mass. It is as follows.

  The temperature of the dyeing bath is 10-50 ° C. The temperature is preferably 15 ° C. or higher, and more preferably 20 ° C. or higher. The temperature is preferably 40 ° C. or lower, and more preferably 30 ° C. or lower. By dyeing within such a temperature range, the PVA film can be dyed efficiently and uniformly. Moreover, as time to immerse a PVA film in a dyeing bath, it is preferable to exist in the range for 0.1 to 10 minutes, and it is more preferable to exist in the range for 0.2 to 5 minutes. By setting it as such a time range, the PVA film can be dyed without spots.

  In the dyeing process, the PVA film is dyed and uniaxially stretched so that the total draw ratio is 2 to 3 times. By subsequently subjecting the PVA film having such a total draw ratio to a two-stage cross-linking stretching process, a polarizing film having excellent polarizing performance and low shrinkage stress can be obtained. What is necessary is just to make it the total draw ratio which passed through the process so far including a swelling process and a dyeing process become 2-3 times. The draw ratio in the dyeing process should just exceed 1 time, and it is more preferable that it is 1.05 times or more.

  In the manufacturing method of this invention, it is provided to a 1st bridge | crosslinking extending process and a 2nd bridge | crosslinking extending process after the said dyeing | staining process. By performing a two-stage cross-linking stretching process under different conditions, the crystal state and orientation state of the obtained polarizing film can be controlled, and a polarizing film having excellent polarizing performance and low shrinkage stress can be obtained. it can. Hereinafter, these two cross-linking stretching steps will be described.

In a 1st bridge | crosslinking extending process, in 40-55 degreeC aqueous solution containing 1-5 mass% boric acid, the draw ratio in this process is 1.1-1.3 times, and the total draw ratio is 2.5- Uniaxial stretching is performed so as to be 3.5 times. The boric acid aqueous solution into which the PVA film is immersed contains 1 to 5% by mass of boric acid. The concentration of boric acid is preferably 1.5% by mass or more, and preferably 4% by mass or less. By setting it as such a density | concentration, the intermolecular crosslinking reaction by a boric acid can be advanced at a suitable speed | rate. The boric acid may be any boric acid or borate ion in an aqueous solution, and either boric acid or borate can be used, but boric acid is preferred. The concentration in the case of using a borate is calculated in terms of mass of boric acid (H ₃ BO ₃ ). The boric acid aqueous solution may contain potassium iodide, and the concentration in that case is preferably in the range of 0.01 to 10% by mass. By containing potassium iodide, the polarizing performance of the obtained polarizing film can be adjusted. Potassium iodide may be included in the first crosslinking stretching step, potassium iodide may be included in the second crosslinking stretching step described later, or both steps may be included.

  The temperature of the boric acid aqueous solution in the first crosslinking stretching step is 40 to 55 ° C. The temperature is preferably 42 ° C. or higher, and is preferably 53 ° C. or lower. When the temperature is too low, the progress of the crosslinking reaction with boric acid becomes insufficient, and the polarizing property of the obtained polarizing film is deteriorated. On the other hand, when the said temperature is too high, there exists a possibility that PVA may elute from a film. And under such temperature conditions, it is uniaxially stretched so that the draw ratio is 1.1 to 1.3 times and the total draw ratio is 2.5 to 3.5 times. The total draw ratio is preferably 2.6 times or more, and preferably 3.4 times or less. As described above, in the first cross-linking stretching step, the boric acid cross-linking reaction is advanced while slightly uniaxially stretching and appropriately oriented. Thus, even when immersed in a high-temperature boric acid aqueous solution in the subsequent second cross-linking stretching step, PVA does not elute into the boric acid aqueous solution from the film or the strength of the film is not greatly reduced. It can be stretched to a magnification.

  Subsequently, in the second cross-linking stretching step, in a 60-70 ° C. aqueous solution containing 1-5% by mass of boric acid, the stretching ratio in the step is 1.8-3.0 times and the total stretching ratio is 6- It is uniaxially stretched so that it becomes 8 times. The composition of the boric acid aqueous solution used can be the same as that used in the first crosslinking and stretching step.

  In the second cross-linking stretching step, the temperature of the boric acid aqueous solution is 60 to 70 ° C. The temperature is preferably 62 ° C. or higher, and preferably 68 ° C. or lower. If the temperature is too low, the shrinkage stress will increase. On the other hand, when the temperature is too high, PVA is eluted from the film into the boric acid aqueous solution, or the degree of polarization is reduced. And in the said temperature range, it uniaxially stretches so that it may become 1.8 to 3.0 times and a total draw ratio may be 6 to 8 times. The draw ratio in the second cross-linking stretching step is preferably 2 times or more, and preferably 2.8 times or less. The total draw ratio is preferably 6.2 times or more, and preferably 7.8 times or less. That is, the boric acid crosslinking reaction proceeds in a high-temperature boric acid aqueous solution while stretching at a relatively high magnification, and as a result, crystallization and immobilization of the oriented PVA in the drying process are promoted. Thereby, a polarizing film having high polarization performance and low shrinkage stress can be produced.

The maximum stretching stress in the second cross-linking stretching step is preferably 15 N / mm ² or less. Here, the maximum stretching stress is a value obtained by dividing the tensile force applied between adjacent rolls by the cross-sectional area of the raw material PVA film in the second cross-linking stretching step. When using three or more rolls, the maximum tensile force is adopted. By reducing the maximum stretching stress, a polarizing film having a small shrinkage stress can be obtained. The maximum stretching stress is more preferably 10 N / mm ² or less. Usually, the maximum stretching stress is 1 N / mm ² or more.

  In the first and second cross-linking stretching steps, when the PVA film is uniaxially stretched, it is performed by changing the peripheral speed between the rolls using a stretching apparatus having a plurality of rolls parallel to each other in the water bath. be able to.

  It is preferable to use for a washing | cleaning process after the said 2nd bridge | crosslinking extending | stretching process. In the cleaning process, unnecessary chemicals and foreign substances on the film surface are removed, and the optical performance of the finally obtained polarizing film is adjusted. The cleaning step can be performed by immersing the PVA film in a cleaning bath or by spraying a cleaning liquid on the PVA film. Water can be used as the cleaning solution, but these may contain potassium iodide. When potassium iodide is contained, the color tone of the polarizing film can be adjusted. The content of potassium iodide is preferably 0.1 to 10% by mass. The temperature of the cleaning liquid is usually 10 to 40 ° C, preferably 15 to 30 ° C. The cleaning bath may be used not only in one tank but also in a plurality of tanks. Moreover, the composition of the cleaning liquid in each tank when using a plurality of tanks can be adjusted according to the purpose.

  It is preferable to use for a drying process following the said washing | cleaning process. The temperature in the drying step is not particularly limited, but is preferably 30 to 150 ° C, and more preferably 50 to 130 ° C. A polarizing film excellent in dimensional stability is easily obtained by drying at a temperature within the above range.

  The thickness of the polarizing film of the present invention thus obtained is preferably 1 to 30 μm. If the thickness is less than 1 μm, it may be difficult to produce at a high speed, and more preferably 3 μm or more. On the other hand, when the thickness exceeds 30 μm, the stretching tension at the time of stretching may increase and the apparatus may be damaged, and more preferably 25 μm or less. The thickness here refers to the thickness of the PVA layer in the case of a multilayer film.

  When the obtained polarizing film is a single layer film of PVA, the thickness of the polarizing film is preferably 5 μm or more, and more preferably 7 μm or more in order to ensure handling properties. On the other hand, in the case of a polarizing film made of a multilayer film, the thickness of the PVA layer can be 5 μm or less, or 3 μm or less. The thickness of the base resin layer in the multilayer film is usually 10 to 250 μm.

  The single transmittance of the polarizing film of the present invention is preferably 42 to 45%. When the single transmittance is less than 42%, the brightness of the liquid crystal display decreases. The single transmittance is more preferably 42.5% or more. On the other hand, it is difficult to obtain a polarizing film having a high degree of polarization with a polarizing film having a single transmittance exceeding 45%, and the single transmittance is more preferably 44.5% or less. Moreover, it is preferable that the polarization degree of the polarizing film of this invention is 99.980% or more. When the degree of polarization is 99.980% or more, the image quality of the liquid crystal display is excellent. The degree of polarization is more preferably 99.982% or more.

The shrinkage stress of the polarizing film of the present invention is preferably 45 N / mm ² or less. Due to the small shrinkage stress, the dimensional stability is excellent even when used at high temperatures. The shrinkage stress is more preferably 40 N / mm ² or less. Here, the shrinkage stress means a value obtained by dividing a tension when a polarizing film as a sample is fixed and maintained at 80 ° C. for 4 hours by a cross-sectional area of the sample.

  The polarizing film of the present invention preferably has a “degree of polarization when the single transmittance is 43.5%” of 99.990% or more. This value is obtained by calculating the degree of polarization when it is assumed to be 43.5% when the single transmittance (T) of the polarizing film is not 43.5%. The polarization degree when the single transmittance is 43.5% is more preferably 99.991% or more, and further preferably 99.992% or more.

The calculation method of “degree of polarization when the single transmittance is 43.5%” is as follows. First, the relationship between the transmittance (T ′) excluding the surface reflection and the single transmittance (T) is expressed by Equation (1). At this time, the refractive index of PVA was 1.5, and the reflectance at the surface was 4%. The relationship among the transmittance (T ′), the degree of polarization (V), and the dichroic ratio (R) is expressed by Equation (2), and Equation (2) is modified from Equation (3). Here, the dichroic ratio (R) can be treated as a constant because it hardly varies depending on the dye concentration in a range where the single transmittance (T) does not vary greatly, for example, in the range of 42 to 45%. Accordingly, after measuring the single transmittance (T) and the degree of polarization (V), the dichroic ratio (R) of the polarizing film is determined as a constant by solving the equations (1) and (2) using these values. Can be calculated as The degree of polarization (V) when T = 43.5 (%) can be obtained from the equations (3) and (1) into which R is substituted.
T ′ = T / (1−0.04) ² (1)
R = {− ln [T ′ (1−V)]} / {− ln [T ′ (1 + V)]} (2)
T ′ = [1-V] ^{1 / (R−1)} / [1 + V] ^{R / (R−1)} (3)

  Moreover, when the polarizing film of this invention analyzed the structure by wide-angle X-ray diffraction (WAXD) measurement, it became clear that it had a structural characteristic different from the conventional polarizing film. This will be described below.

  A profile of the X-ray intensity with respect to the diffraction angle (2θ) is created by performing WAXD measurement on the polarizing film of the present invention. Then, the peak is divided according to the method described in the following examples. First, it is divided into three components of “PVA crystal”, “PVA amorphous”, and “PVA-boric acid aggregation structure”. Here, “PVA crystal” refers to a PVA chain in a crystalline state, and “PVA amorphous” refers to a disordered PVA chain that is not in a crystalline state. In addition, the peak derived from “PVA-boric acid aggregation structure” is a peak that is known to appear when boric acid is added to PVA. From the structure formed by the interaction of PVA and boric acid. It is thought that this is the diffraction signal.

  Next, an X-ray intensity profile with respect to the azimuth angle φ is created for the three components divided by the above method. This is further divided into three components, that is, a non-oriented component, a low-oriented component, and a highly-oriented component, respectively, so as to be divided into 9 components in total. In this way, the ratio (%) of each component can be determined when the total of the nine components is 100%. The high orientation component of “PVA-boric acid aggregation structure” was designated as structure factor A, and the high orientation component of “PVA amorphous” was designated as structure factor B. This time, it was found that the structure factor A is a structure necessary for enhancing the polarization performance, and the structure factor B is a causative structure of the shrinkage stress.

  The structure factor A has a structure in which the oriented PVA chain is stabilized by intramolecular or intermolecular crosslinking of boric acid. D. Fujiwara et al., Polymer Preprints, Japan, 59, 2, 3043, 2010; D. Fujiwara et al., Polymer Preprints, Japan, 60, 2, 3393, 2011; K. Ohishi et al., Polymer, 51, 687-693, 2010, etc. To achieve high polarization performance, more polyiodine ions must be retained in the film. In the highly oriented “PVA-boric acid aggregated structure”, boric acid has the effect of stabilizing polyiodine ions in order to suppress thermal motion and orientation relaxation of the PVA chain. As a result, many polyiodine ions Can be held stably.

  The structure factor B is a structure in which the PVA chain is highly oriented by stretching and frozen as it is. This is due to non-crystallized PVA chains. At a temperature equal to or higher than the glass transition temperature of PVA, molecular mobility increases due to thermal motion, and since there is no restriction by crystals or boric acid, orientation is easily relaxed. Since the structure factor B is highly oriented, the force for relaxing the orientation is large.

  The structure factor A and the structure factor B can be controlled by various conditions when producing a polarizing film from a PVA film. For example, the structure factor A depends on the boric acid concentration of the crosslinking tank containing boric acid, and the higher the boric acid concentration, the larger the structural factor A. Further, for example, the structure factor B depends on the draw ratio, and by reducing the draw ratio, the orientation of the PVA chain can be suppressed and the structure factor B can be reduced.

  However, when the boric acid concentration is increased, there are problems such as a decrease in stretchability during processing and an increase in the structure factor B. On the other hand, when the draw ratio is reduced, the structure factor A decreases, and there is a problem that a film having high polarization performance cannot be obtained. That is, it has been difficult to obtain a polarizing film that simultaneously satisfies high polarization performance and low shrinkage stress.

This time, by adopting the production method as described above, it was possible to produce a polarizing film having a higher content of the structural factor A and a lower content of the structural factor B than the conventional polarizing film. As a result, it was possible to obtain a polarizing film having excellent polarization performance and low shrinkage stress, which had been difficult to produce until now. Specifically, the content (C _A ) of structure factor A in the wide-angle X-ray diffraction measurement is 3 to 4.5%, and the content (C _B ) of structure factor B is 2.0 to 8.5%. And a polarizing film having a ratio (C _A / C _B ) of 0.4 or more could be obtained.

The content (C _A ) of structure factor A is preferably 3 to 4.5%. When the content (C _A ) is 3% or more, excellent polarization performance can be obtained. On the other hand, the content (C _B ) of the structure factor B is preferably 2.0 to 8.5%, more preferably 2.5 to 8.5%, and 3.0 to 8.5%. It is still more preferable that it is 4.5 to 8.5%. When the content (C _B ) is 8.5% or less, the shrinkage stress can be reduced. The ratio (C _A / C _B ) between the two is preferably 0.4 or more. By taking a large value for the ratio (C _A / C _B ), a polarizing film having excellent polarizing performance and low shrinkage stress can be obtained.

A preferred embodiment of the polarizing film of the present invention is a polarizing film comprising a polyvinyl alcohol film containing an iodine dichroic dye; the degree of polarization when the single transmittance is 43.5% is 99.990% or more. And the content (C _A ) of the structure factor A in the wide-angle X-ray diffraction measurement is 3 to 4.5%, and the content (C _B ) of the structure factor B is 2.0 to 8.5%. And the ratio (C _A / C _B ) is 0.4 or more.

Another preferred embodiment of the polarizing film of the present invention is a polarizing film comprising a polyvinyl alcohol film containing an iodine-based dichroic dye; having a single transmittance of 42 to 45%; a degree of polarization of 99.980% In addition, the content (C _A ) of the structure factor A in the wide-angle X-ray diffraction measurement is 3 to 4.5%, and the content (C _B ) of the structure factor B is 2.0 to 8.5. %, And the ratio (C _A / C _B ) is 0.4 or more.

Another preferred embodiment of the polarizing film of the present invention is a polarizing film composed of a polyvinyl alcohol film containing an iodine dichroic dye; the degree of polarization when the single transmittance is 43.5% is 99.990% or more. And a polarizing film having a shrinkage stress of 45 N / mm ² or less.

Another preferred embodiment of the polarizing film of the present invention is a polarizing film comprising a polyvinyl alcohol film containing an iodine-based dichroic dye; having a single transmittance of 42 to 45%; a degree of polarization of 99.980% And a polarizing film having a shrinkage stress of 45 N / mm ² or less.

  The polarizing film of the present invention is usually used as a polarizing plate by bonding a protective film on both sides or one side. Examples of the protective film include those that are optically transparent and have mechanical strength. Specifically, for example, cellulose triacetate (TAC) film, acetic acid / cellulose butyrate (CAB) film, acrylic film, and polyester film. A film or the like can be used. In addition, examples of the adhesive for bonding include a PVA adhesive, a urethane adhesive, and an ultraviolet curable adhesive.

  The polarizing plate thus obtained can be used for a high-performance liquid crystal display (LCD). It is possible to provide a polarizing plate that is bright, has good polarization characteristics, and has excellent dimensional stability even when used under high temperature conditions. Therefore, it can be suitably used as a polarizing plate for various high-performance LCDs, particularly LCDs for mobile use.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Analytical methods and evaluation methods employed in the following examples and comparative examples were performed according to the following methods.

[Optical characteristics of polarizing film]
From the central part in the width direction of the polarizing film obtained in the following Examples and Comparative Examples, a rectangular sample of 3 cm in the longitudinal direction of the polarizing film and 1.5 cm in the vertical direction was taken, and a spectrophotometer with an integrating sphere (Nippon Bunko Co., Ltd. “V7100”), in accordance with JIS Z 8722 (object color measurement method), after correcting the visibility, the single transmittance (T) and the degree of polarization (V) Measured.

The dichroic ratio (R) of the polarizing film is calculated by solving the following equations (1) and (2) from the values of T and V thus obtained, and equations (3) and ( The degree of polarization when T = 43.5% was obtained by calculating 1). Here, the refractive index of PVA was assumed to be 1.5, and the reflectance at the surface was assumed to be 4%. The dichroic ratio (R) can be handled as a constant that does not vary depending on the dye concentration in a range where the single transmittance does not vary greatly, for example, in the range of 42 to 45%.
T ′ = T / (1−0.04) ² (1)
R = {− ln [T ′ (1−V)]} / {− ln [T ′ (1 + V)]} (2)
T ′ = [1-V] ^{1 / (R−1)} / [1 + V] ^{R / (R−1)} (3)

"Wide-angle X-ray diffraction (WAXD) measurement of polarizing film"
Wide angle X-ray diffraction (WAXD) measurement was performed using a D8 Discover apparatus manufactured by Bruker AXS. The incident X-ray wavelength was 0.154 nm (Cu target). Hi-STAR, a position sensitive 2D gas detector, was used as the detector, and the camera distance (distance between sample and detector) was set to approximately 150 mm. The X-ray generator used was a filament current of 110 mA, a voltage of 45 kV, and a collimator diameter of 0.5 mm.

  The polarizing film was cut into a rectangle with a short side of 5 mm and a long side of 20 mm. At this time, the long side direction was made to correspond with the extending | stretching direction of a polarizing film. A double-sided tape was affixed to the sample holder attached to the apparatus, and the cut polarizing film was fixed. In this measurement, a single polarizing film was attached to the sample holder, not a plurality of polarizing films. It was confirmed in advance that the signal of the polarizing film was sufficiently stronger than the background scattering and the electrical noise of the detector. However, when the intensity of the diffraction signal from the polarizing film is weak, a plurality of films may be stacked and measured. In that case, it is necessary to attach the polarizing film so that the stretching directions of the respective polarizing films completely coincide.

  The sample holder was attached to the stage of the X-ray apparatus so that the stretching direction of the polarizing film coincided with the Y-axis direction of the D8 Discover with GADDS apparatus. At this time, the normal to the polarizing film surface was made to coincide with the direction in which the X-rays were incident. The X-axis, Y-axis, and Z-axis of the device were adjusted so that X-rays were applied to the polarizing film from the incident direction.

  WAXD measurement was performed under the following conditions. 11 ° for the ω-axis of the sample (the axis set so that the angle between the normal to the polarizing film surface and the X-ray incident direction is ω, generally called the θ-axis), detector position 2 ° axis (the axis set so that the angle between the normal to the detector surface and the incident X-ray direction is 2θ) is 22 °, and the φ axis corresponding to rotation in the plane of the polarizing film is 90 ° or Set to 0 °. The φ axis coincides with the azimuthal direction in the plane of the polarizing film. When the stretching direction of the polarizing film is the meridian direction and the vertical direction in the plane is the equator line direction, if the φ axis is 90 °, diffraction information in the equator line direction is obtained, and if the φ axis is 0 °, the meridian direction diffraction information is obtained. Diffraction information can be acquired. Diffraction or scattering observed by the detector satisfies the Bragg condition. In the case of this measurement condition, for example, a 110 diffraction signal from the detected PVA crystal is diffraction due to (110) that approximately matches the thickness direction of the polarizing film. The measurement was performed with the φ axis at 90 ° or 0 ° and the X-ray exposure time of 60 minutes.

[WAXD measurement data analysis of polarizing film]
The obtained two-dimensional photograph of WAXD was converted into a one-dimensional profile of the X-ray intensity I (2θ) with respect to 2θ using GADDS (General Area Detector Diffraction System) software. The 2θ range was 5 ° to 35 °. Sampling steps were set at 0.05 ° intervals. The φ-axis range that coincides with the azimuth direction is -5 ° to 5 °, 5 ° to 15 °, 15 ° to 25 °, 25 ° to 35 °, 35 ° to 45 °, 45 ° to 55 °, 55 ° From 65 °, 65 ° to 75 °, 75 ° to 85 °, 85 ° to 95 °, 95 ° to 105 °, 105 ° to 115 °, 115 ° to 125 °, 125 ° to 135 °, 135 ° to 145 Divide in azimuth range of 10 ° in azimuth direction such as °, 145 ° to 155 °, 155 ° to 165 °, 165 ° to 175 °, 175 ° to 185 °, respectively (I (2θ) profile) Got. The stretching direction of the polarizing film corresponds to 0 °, and the vertical direction corresponds to 90 °. The same operation was applied to background scattering data (data measured under the same conditions without attaching a polarizing film).

  The analysis of I (2θ) profile was carried out by the following procedure. First, the I (2θ) profile in the same azimuth range obtained by background measurement was subtracted from the I (2θ) profile obtained by measurement of the polarizing film. For the I (2θ) profile minus background scattering, as shown in Fig. 3, the baseline connecting the intensity value I (6.5) when the 2θ position is 6.5 ° and the intensity value I (30.5) when the 2θ position is 30.5 ° A straight line was generated and further subtracted from the I (2θ) profile minus background scatter. The baseline straight line is a linear function passing through two points (6.5, I (6.5)) and (30.5, I (30.5)) on the I (2θ) -2θ coordinate. In order to suppress the influence of the variation of the measured data on the baseline straight line, I (6.5) is the arithmetic average value of I (6.0) to I (7.0), and I (30.5) is I (30.0) to I (31.0) The arithmetic average value of Here, in the I (2θ) profile in the meridian direction, that is, the azimuth angle near 0 °, a diffraction peak is observed due to polyiodine ions oriented in the stretching direction around 28 ° at 2θ position, so I (30.5) is It is affected by the diffraction peak due to polyiodine ions. Therefore, when a diffraction peak due to polyiodine ions was observed, the diffraction peak was removed in advance. Assuming that the diffraction peak shape can be expressed by a Gaussian function, the peak top position x, peak height h, and peak width (standard deviation σ of normal distribution) of the Gaussian function are eliminated so that the influence on the I (2θ) profile is eliminated. After adjustment, the Gaussian function was appropriately subtracted from the I (2θ) profile. These operations were performed on the I (2θ) profiles for all azimuth angles. The subtracted baseline straight line is hereinafter referred to as a corrected I (2θ) profile.

  FIG. 4 shows corrected I (2θ) profiles obtained by measuring the polarizing film and the poval film. The broad and diffuse scattering component observed from 2 ° to 10 ° to 30 ° mainly originates from the amorphous PVA. The peak component observed in the range of 19 ° to 21 ° at 2θ results from diffraction by (1-10) and (110) of the PVA crystal. On the other hand, it is known that the peak-like component observed from 21 ° to 23 ° in 2θ appears when boric acid is added to PVA, and formed by the interaction of PVA and boric acid. It is considered to be a diffraction signal from the structure. The structure formed by the interaction of PVA and boric acid is referred to as “PVA-boric acid aggregation structure”. That is, the corrected I (2θ) profile obtained by measuring the polarizing film can be separated into three components “PVA amorphous”, “PVA crystal”, and “PVA-boric acid aggregated structure”. Therefore, waveform separation analysis was applied to the corrected I (2θ) profile.

  It was assumed that scattering or diffraction signals from “PVA amorphous”, “PVA crystal”, and “PVA-boric acid aggregated structure” can be expressed by a Gaussian function. Gaussian function A, Gaussian function B, and Gaussian function C were used. Parameters defining the shape of the Gaussian function were a peak top position x, a peak height h, and a peak width (here, the standard deviation σ of a normal distribution is meant). The peak top position x, peak height h, and peak of the three Gaussian functions are matched so that the calculated-I (2θ) profile, which is the sum of the three Gaussian functions representing each component, matches the corrected I (2θ) profile. All parameters were optimized by least square fitting, with the width of the variable as a variable parameter. The result is shown in FIG. In this waveform separation analysis, the calculated-I (2θ) profile is expressed by three Gaussian functions so that it reflects the structure of the polarizing film without being affected by variations in measured data, errors due to analysis, and systematic errors in fitting. It must be properly expressed. Therefore, in this test, the following constraints (a) to (f) were introduced in the waveform separation analysis.

(A) Since signals in the range of 13 ° to 16 ° and 25 ° to 28 ° in 2θ can be regarded as scattering caused by “PVA amorphous”, I (2θ corrected only by the Gaussian function A in the 2θ range. ) Reproduce the profile.
(B) The signal in the range of about 17 ° to 21 ° at 2θ is scattering and diffraction generated from “PVA amorphous” and “PVA crystal”, and the contribution of “PVA crystal” is particularly large. The diffraction peak position by the crystal is known, and the peak top position x of the Gaussian function B is fixed.
(C) Signals in the range of approximately 20 ° to 23 ° at 2θ are scattering and diffraction resulting from “PVA amorphous” and “PVA-boric acid aggregated structure”. In particular, the contribution of “PVA-boric acid aggregation structure” is large. Therefore, the peak top position x of the Gaussian function C is fixed.
(D) In order to properly separate the “PVA crystal” and the “PVA-boric acid aggregation structure”, the Gaussian function B and the Gaussian function C have the same peak width. This is because the diffraction intensities at 17 ° to 21 ° and 20 ° to 23 ° are approximately equal regardless of the type of polarizing film, the diffraction peak due to “PVA crystal” and the diffraction peak due to “PVA-boric acid aggregate structure” This is because it is expected that there will be no significant difference in the shape.
(E) Based on the above (a) to (d), three Gausses such that a calculated-I (2θ) profile, which is the sum of three Gaussian functions, reproduces all corrected I (2θ) profiles well. The optimum values of peak top position x and peak width σ of each function were searched. As a result, x = 20.0 and σ = 4.0 for the Gaussian function A, x = 19.7 and σ = 1.3 for the Gaussian function B, and x = 22.0 and σ = 1.3 for the Gaussian function C. When adopted, it was found that all corrected I (2θ) profiles could be well separated without contradiction. The peak top position x and the peak width σ are fixed to the optimum values described above.
(F) For all corrected I (2θ) profiles, least square fitting was performed using only the peak height h of three Gaussian functions as a variable parameter. The fitting range was 6.5 ° to 30.5 °.

  In the I (2θ) profile in the equator line direction, that is, the azimuth angle near 90 °, 100 diffraction peaks are observed due to the PVA crystal oriented in the stretching direction around 11 ° at the 2θ position. When 100 diffraction peaks were observed, it was considered that the diffraction peak shape could be expressed by a Gaussian function and included in the waveform separation analysis into the I (2θ) profile. That is, the least square fitting was performed by appropriately adjusting the peak top position x, peak height h, and peak width σ of the Gaussian function.

  After least square fitting, calculate the area of the three Gaussian functions and consider them as integral intensity values (A) of the signals resulting from “PVA amorphous”, “PVA crystal”, and “PVA-boric acid agglomerated structure”, respectively. It was. When a 100 diffraction peak was observed, the integrated intensity value of the 100 diffraction peak was included in the “PVA crystal”. The above waveform separation analysis is performed on the corrected I (2θ) profile for each azimuth, and the integrated intensity values for “PVA amorphous”, “PVA crystal”, and “PVA-boric acid aggregated structure” ( A) was calculated. For these analyses, Microsoft Excel software was used.

  FIG. 5 is a plot of integrated intensity values (A) of “PVA amorphous”, “PVA crystal”, and “PVA-boric acid aggregated structure” obtained by waveform separation analysis versus azimuth. Here, the azimuth angle was defined as follows. For example, the analysis result of the corrected I (2θ) profile in the azimuth range from -5 ° to 5 ° shows the corrected I (2θ) profile in the corrected azimuth range from 85 ° to 95 ° with respect to the azimuth angle of 0 °. The analysis results were plotted against an azimuth angle of 90 °.

  The azimuth dispersion plot A (φ) of the integrated intensity value of each component reflects the orientation state of each component with respect to the stretching direction of the polarizing film. When the scattered or diffracted signals observed in the analyzed 2θ range are considered to be mainly due to interference between PVA molecular chains, the proportion of signals observed at an azimuth angle of 90 ° is the PVA molecular chain in the stretching direction. Is approximately equal to the ratio of the components. The ratio of signals observed at an azimuth angle of 0 ° is approximately equal to the ratio of components in which PVA molecular chains are arranged in a direction perpendicular to the stretching direction. That is, FIG. 5 is approximately equal to the distribution function f (φ) of the orientation state of each component. When the PVA is completely unoriented, there is no azimuth dependency of the distribution function. On the other hand, if the PVA is oriented with a distribution in the stretching direction, the distribution function shows a peak shape with a maximum intensity of 90 °.

  Therefore, the azimuth dispersion plot A (φ) of the integrated intensity values was separated into the distribution function f1 (φ) of the orientation component and the distribution function f2 (φ) of the non-alignment component. Since the distribution function of the non-oriented component has no azimuth angle dependency, f2 (φ) = C (C is a constant) was used. As shown in FIG. 6, the component having a constant C at any azimuth is f2 (φ), and the component having a probability distribution at a specific azimuth is f1 (φ). For any polarizing film, it was found that the data processing should be performed according to the following procedure in order to obtain f1 (φ) with high accuracy from the azimuth dispersion plot A (φ). Assuming that the distribution function f1 (φ) of the orientation components of “PVA crystal” and “PVA-boric acid aggregate structure” can be expressed by the Lorentz function, the peak top position is 90 °, the peak height is h, and the peak half The value range was a variable parameter. Assuming that the distribution function f1 (φ) of the orientation component of “PVA amorphous” can be expressed as a linear sum of two Gaussian functions, the peak top position is 90 °, the peak height h of each function, The full width at half maximum was a variable parameter. A least square fitting was performed so that the sum of f1 (φ) and f2 (φ) was reproduced well for A (φ), and an optimal solution of constant C, peak height h, and peak half width was obtained. Fitting was performed in the azimuth range of 0 ° to 180 °.

  Here, of the orientation component distribution function f1 (φ) obtained by the fitting, the azimuth angle φ is in the range of 80 ° to 100 °, that is, the proportion of highly oriented components was determined as follows. First, in the azimuth angle range of 0 ° to 180 °, an integral value of the distribution function f1 (φ) of the orientation component was obtained. This was designated as F1. Next, an integral value in the range of the azimuth angle φ in the orientation component distribution function f1 (φ) in the range of 80 ° to 100 ° was calculated, and this was defined as F1a. F1-F1a is an integral value of 0 ° to 80 ° and 100 ° to 180 °, and this is defined as F1b. F1b is an orientation component having a small degree of orientation. In the azimuth angle range of 0 ° to 180 °, the integral value F2 of the distribution function f2 (φ) of the non-oriented component was calculated. Values of F1a, F1b, and F2 were obtained for “PVA amorphous”, “PVA crystal”, and “PVA-boric acid aggregated structure”, respectively. F1a is a high orientation component, F1b is a low orientation component, and F2 is an amount proportional to the non-alignment component.

  “PVA Amorphous”, “PVA Crystal”, “PVA-Boric Acid Aggregation Structure”, each of the high orientation component, low orientation component, non-orientation component, that is, the presence ratio of 9 components is as follows: It was regarded as the ratio of each component to the total sum of integral values. These are F1a-PVA amorphous, F1b-PVA amorphous, F2-PVA amorphous, F1a-PVA crystal, F1b-PVA crystal, F2-PVA crystal, F1a-PVA-boric acid aggregated structure, F1b-PVA-boric acid Aggregation structure, F2-PVA-boric acid aggregation structure.

  Among these, the highly oriented component F1a-PVA-boric acid aggregated structure of “PVA-boric acid aggregated structure” was found to have a high correlation with the polarization performance of the polarizing film as described above. This is because boric acid interacts and stabilizes the highly oriented and extended PVA chain by stretching to promote the formation and retention of polyiodine ions. A highly oriented component of “PVA-boric acid aggregated structure” is defined as structure factor A. On the other hand, it was found that the highly oriented component F1a-PVA amorphous of “PVA amorphous” has a high correlation with the shrinkage stress of the polarizing film. This is because a PVA chain that is highly oriented and stretched by stretching easily undergoes thermal motion by increasing the temperature and has a strong ability to relax the orientation, thereby causing shrinkage. The highly oriented component of “PVA amorphous” is designated as structure factor B.

FIG. 7 is a graph in which the content of the structural factor _A (C _A ) and the content of the structural factor B (C _B ) are plotted against the shrinkage stress according to the above method for the polarizing film samples manufactured under various conditions. Show. FIG. 8 shows a graph in which the ratio (C _A / C _B ) is plotted against the contraction stress. It can be seen that the shrinkage stress is well correlated with the content (C _A ) of the structure factor _A , the content (C _B ) and the ratio (C _A / C _B ) of the structure factor B.

[Shrinkage stress of polarizing film]
The shrinkage stress was measured using an autograph AG-X with a thermostatic bath manufactured by Shimadzu Corporation and a video extensometer TR ViewX120S. For the measurement, a polarizing film conditioned at 20 ° C./20% RH for 18 hours was used. After the temperature of the autograph AG-X is set to 20 ° C., a polarizing film (15 cm in the length direction and 1.5 cm in the width direction) is attached to the chuck (chuck interval 5 cm). Heating started. The polarizing film was pulled at a speed of 1 mm / min, and when the tension reached 2N, the tension was stopped, and the tension until 4 hours was measured in that state. At this time, because the distance between chucks changes due to thermal expansion, a gauge sticker is attached to the chuck, and the distance between chucks is corrected by the amount of movement of the gauge seal attached to the chuck using the video extensometer TR ViewX120S. Measurements were taken as possible. The value obtained by subtracting the initial tension 2N from the measured value of the tension after 4 hours was defined as the contraction force of the polarizing film, and the value obtained by dividing the value by the sample cross-sectional area was defined as the contraction stress (N / mm ² ).

[Maximum stretching stress in the second cross-linking stretching step]
The maximum stretching stress in the second cross-linking stretching step is a value obtained by measuring the stretching tension applied between adjacent rolls in the second cross-linking stretching step with a tension roll installed therebetween and dividing by the cross-sectional area of the raw PVA film. . When using three or more rolls, the maximum tensile force is adopted.

[Example 1]
100 parts by mass of PVA (saponified vinyl acetate polymer, degree of polymerization 3000, degree of saponification 99.9 mol%), 10 parts by mass of glycerin as a plasticizer, sodium polyoxyethylene lauryl ether sulfate as a surfactant: 0.1 mass A roll of PVA film having a thickness of 45 μm was obtained by casting a film using a film-forming stock solution composed of a part and water. A polarizing film was produced by sequentially performing a swelling process, a dyeing process, a first crosslinking stretching process, a second crosslinking stretching process, a washing process, and a drying process on the PVA film. The schematic diagram of a polarizing film manufacturing apparatus is shown in FIG.

Specifically, a polarizing film was produced as follows. First, in the swelling step, the PVA film was uniaxially stretched (first-stage stretch) in the length direction (MD) to twice the original length while being immersed in water at a temperature of 30 ° C. for 1 minute. . Subsequently, in the dyeing process, while immersed in an aqueous solution containing 0.06% by mass of iodine and 1.4% by mass of potassium iodide at a temperature of 30 ° C. for 1 minute, the length direction is increased up to 2.4 times the original length. (MD) was uniaxially stretched (second-stage stretching). Subsequently, in the first cross-linking stretching step, while immersed in an aqueous solution containing boric acid at a concentration of 2.6% by mass at a temperature of 50 ° C. for 2 minutes, the length is increased to 3 times the original length (MD). Uniaxial stretching (third stage stretching) was performed. Subsequently, in the second cross-linking stretching step, up to 7 times the original length while immersed in an aqueous solution at a temperature of 65 ° C. containing 2.8% by weight of boric acid and 5% by weight of potassium iodide. Uniaxial stretching (fourth stage stretching) was performed in the length direction (MD). The maximum stretching stress in the second cross-linking stretching process was 5.5 N / mm ² . Subsequently, in the washing step, the film was washed by dipping for 10 seconds in an aqueous solution containing boric acid at a concentration of 1.5% by mass and potassium iodide at a concentration of 5% by mass at a temperature of 22 ° C. Subsequently, in the drying step, a polarizing film having a thickness of 13.9 μm was produced by drying with a dryer at 80 ° C. for 90 seconds.

Using the obtained polarizing film, the degree of polarization and shrinkage stress when the structure factors A and B, the single transmittance, the polarization degree, and the single transmittance 43.5% were measured by the above-described methods. The contents of the high orientation component, low orientation component, and non-orientation component of the PVA crystal were 5.4%, 2.8%, and 15.1%, respectively. The contents of the highly oriented component, low oriented component, and non-oriented component of the PVA-boric acid aggregated structure were 3.2%, 1.4%, and 4.6%, respectively. The PVA amorphous high-orientation component, low-orientation component, and non-orientation component content was 8.0%, 10.3%, and 49.3%, respectively. That is, the content (C _A ) of structure factor A is 3.2%, the content (C _B ) of structure factor B is 8.0%, and the ratio (C _A / C _B ) is 0.4. Met. The single transmittance is 43.73%, the polarization degree is 99.982%, the polarization degree when the single transmittance is 43.5% is 99.993%, and the shrinkage stress is 34.5 N / mm ^2. Met. The evaluation results are shown in Table 2.

[Comparative Examples 1-8]
The thickness and degree of polymerization of the PVA film, the temperature of the boric acid aqueous solution in the first crosslinking stretching step, the total stretching ratio after the first crosslinking stretching step, the temperature of the boric acid aqueous solution in the second crosslinking stretching step, and after the second crosslinking stretching step A polarizing film was produced in the same manner as in Example 1 except that the total draw ratio was changed as shown in Table 1. Table 1 shows the value of the maximum stretching stress in the second crosslinking stretching step.

Using the obtained polarizing film, as in Example 1, the single transmittance, the degree of polarization, the degree of polarization when the single transmittance is 43.5%, the shrinkage stress, the content of structural factor A (C _A ) and the structure The content of factor B (C _B ) was measured. These evaluation results are summarized in Table 2. Moreover, the graph which plotted the polarization degree at the time of the single transmittance | permeability of 43.5% of the polarizing film obtained in Example 1 and Comparative Examples 1-4, 7, and 8 with respect to shrinkage stress is shown in FIG. As can be seen from FIG. That is, in Comparative Examples 1 to 8, if the degree of polarization is increased when the single transmittance is 43.5%, the shrinkage stress increases. On the other hand, in Example 1, it was remarkably different from Comparative Examples 1-8, and even if the polarization degree at the time of the single transmittance of 43.5% was made high, the shrinkage stress could be reduced. In Comparative Examples 5 and 6, the degree of polarization when the single transmittance is 43.5% is less than 99.950%, which is below the display range of the graph.

DESCRIPTION OF SYMBOLS 1 Schematic diagram of polarizing film manufacturing apparatus 2 PVA film roll 3 Swelling process 4 Dyeing process 5 First cross-linking stretching process 6 Second cross-linking stretching process 7 Washing process 8 Drying process 9 Polarizing film roll

Claims (8)

A method for producing a polarizing film, wherein at least a swelling process, a dyeing process, a first crosslinking stretching process, and a second crosslinking stretching process are performed in this order on a polyvinyl alcohol film,
The polyvinyl alcohol film has a thickness of 5 to 100 μm,
The average degree of polymerization of polyvinyl alcohol contained in the polyvinyl alcohol film is 2500-3500,
In the swelling step, the polyvinyl alcohol film is swollen by immersing in water at 10 to 50 ° C.,
In the dyeing step, the polyvinyl alcohol film is impregnated with an aqueous solution of 10 to 50 ° C. containing 0.5 to 3% by mass of iodine and potassium iodide in total to impregnate the polyvinyl alcohol film, and total stretching Uniaxially stretched so that the magnification is 2-3 times,
In the first cross-linking stretching step, in a 40 to 55 ° C. aqueous solution containing 1 to 5% by mass of boric acid, the stretching ratio in the step is 1.1 to 1.3 times and the total stretching ratio is 2.5. Uniaxially stretched to ~ 3.5 times,
Subsequently, in the second crosslinking stretching step, in a 60 to 70 ° C. aqueous solution containing 1 to 5% by mass of boric acid, the stretching ratio in the step is 1.8 to 3.0 times and the total stretching ratio is 6 to 6%. A method for producing a polarizing film, wherein the film is uniaxially stretched to be 8 times. The method for producing a polarizing film according to claim 1, wherein in the second cross-linking stretching step, the maximum stretching stress is 15 N / mm ² or less.   The method for producing a polarizing film according to claim 1 or 2, wherein a polarizing film having a single transmittance of 42 to 45% and a degree of polarization of 99.980% or more is obtained. The manufacturing method of the polarizing film in any one of Claims 1-3 which obtains the polarizing film whose shrinkage stress is 45 N / mm < ² > or less. A polarizing film comprising a polyvinyl alcohol film containing an iodine-based dichroic dye,
When the single transmittance is 43.5%, the degree of polarization is 99.990% or more, and the content (C _A ) of the structure factor A in the wide-angle X-ray diffraction measurement is 3 to 4.5%. _A polarizing film, wherein the _B content (C _B ) is 2.0 to 8.5% and the ratio (C _A / C _B ) is 0.4 or more.
Here, the structure factor A is a highly oriented structure factor derived from a polyvinyl alcohol-boron aggregate structure, and the structure factor B is a highly oriented structure factor derived from amorphous polyvinyl alcohol. A polarizing film comprising a polyvinyl alcohol film containing an iodine-based dichroic dye,
The single transmittance is 42 to 45%,
The degree of polarization is 99.980% or more, the content (C _A ) of structure factor A in wide-angle X-ray diffraction measurement is 3 to 4.5%, and the content (C _B ) of structure factor B is 2 _A polarizing film having a ratio of 0.0 to 8.5% and a ratio (C _A / C _B ) of 0.4 or more.
Here, the structure factor A is a highly oriented structure factor derived from a polyvinyl alcohol-boron aggregate structure, and the structure factor B is a highly oriented structure factor derived from amorphous polyvinyl alcohol.   The polarizing film of Claim 5 or 6 whose polymerization degree of the polyvinyl alcohol contained in the said polarizing film is 2500-3500. The polarizing film in any one of Claims 5-7 whose shrinkage stress is 45 N / mm < ² > or less.

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