CN116490335A

CN116490335A - Method for producing polarizing film and polarizing film

Info

Publication number: CN116490335A
Application number: CN202180079334.0A
Authority: CN
Inventors: 大桥亘; 中井慎二
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 2020-11-26
Filing date: 2021-11-22
Publication date: 2023-07-25
Also published as: TW202231448A; JPWO2022113958A1; KR20230109718A; WO2022113958A1

Abstract

A method of manufacturing a polarizing film, comprising: a dyeing step of dyeing the PVA film with a dichroic dye; a stretching step of uniaxially stretching in an aqueous solution containing boric acid; and a drying step in which the aqueous solution of the stretching step has a boric acid concentration of 1 to 3% by mass, a total stretching ratio of 5.5 to 7.4 times, a total necking rate (A) represented by formula (1) of 57.5 to 61.0%, a necking rate (B) represented by formula (2) in the stretching step of 31.0 to 38.0%, and a necking rate (C) represented by formula (3) in the drying step of 9.8 to 16.5%. (a) = ((X1-X2)/X1) ×100 (1) (B) = { (Y1-Y2)/Y1 } ×100 (2) (C) = { (Z1-X2)/Z1 } ×100 (3) X1) represents the width (m) before the dyeing process; x2 represents the width (m) after the drying step; y1 represents the width (m) before the stretching step; y2 represents the width (m) after the stretching step; z1 represents the width (m) before the drying step.

Description

Method for producing polarizing film and polarizing film

Technical Field

The present invention relates to a method for producing a polarizing film and a polarizing film.

Background

A polarizing plate having a function of transmitting and shielding polarized light is the same as a liquid crystal for changing the polarization state of light as a basic constituent of a Liquid Crystal Display (LCD). Various polarizing plates have a structure in which a protective film such as a cellulose Triacetate (TAC) film is adhered to the surface of a polarizing film. As a polarizing film, a substrate (stretched film obtained by uniaxially stretching and orienting a polyvinyl alcohol film (hereinafter, sometimes abbreviated as "PVA") obtained by uniaxially stretching a polyvinyl alcohol film was allowed to adsorb an iodine dye (I) ₃ ^- 、I ₅ ^- Etc.) has been the main stream. Such a polarizing film is produced by uniaxially stretching a PVA film containing a dichroic dye in advance, or by causing the PVA film to adsorb a dichroic dye at the same time as uniaxially stretching the PVA film, or by causing the PVA film to adsorb a dichroic dye after uniaxially stretching the PVA film, or the like.

LCDs are widely used in small devices such as calculators, wristwatches, and smartphones, notebook computers, liquid crystal displays, liquid crystal color projectors, liquid crystal televisions, car navigation systems, cellular phones, and indoor and outdoor metering devices. In order to cope with the recent increase in performance of displays, polarizing films having excellent optical properties are demanded. On the other hand, the display has also been thinned, and accordingly, a polarizing film having a small shrinkage stress has been demanded.

Patent document 1 describes a method of irradiating a predetermined electromagnetic wave in a process for producing a polarizing film as a method of producing a polarizing film excellent in polarization performance. Patent document 2 describes a method of stretching a PVA film to 1.8 to 3.0 times in an aqueous boric acid solution at 60 to 70 ℃ and setting the total stretching ratio to 6 to 8 times as a method of producing a polarizing film excellent in polarizing performance and small in shrinkage stress.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open publication No. 2018-032925

Patent document 2: international publication No. 2017/138551

Disclosure of Invention

Problems to be solved by the invention

However, in the method described in patent document 1, the conventional manufacturing process requires an additional process, or if the intensity of electromagnetic waves is not strictly controlled, the PVA orientation is excessively relaxed, and the polarizing performance of the polarizing film is lowered, so that it is difficult to realize industrial implementation. In addition, in the method described in patent document 2, since the stretching conditions are too severe, the PVA film may be dissolved during the stretching, and there is a problem that the yield of the polarizing film is lowered.

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

Means for solving the problems

The inventors of the present invention have found as a result of intensive studies that: for manufacturing a polarizing film excellent in polarizing performance and small in shrinkage stress, it is important that: the shrinkage phenomenon of the film width of the PVA film in the stretching step and the drying step in the polarizing film production step (hereinafter, this phenomenon is sometimes referred to as a "necking phenomenon") is appropriately controlled, and the shrinkage ratio of the film width (hereinafter, this shrinkage ratio is sometimes referred to as a "necking ratio") is controlled to be within a certain range.

Namely, the present invention relates to:

[1] a method of manufacturing a polarizing film, comprising: a dyeing step of dyeing the polyvinyl alcohol film with a dichroic dye; a stretching step of uniaxially stretching the dyed polyvinyl alcohol film in an aqueous solution containing boric acid; and a drying step of drying the stretched polyvinyl alcohol film, wherein the aqueous solution of the stretching step has a boric acid concentration of 1 to 3 mass%, a total stretching ratio of 5.5 to 7.4 times, a total necking rate (A) represented by the following formula (1) of 57.5 to 61.0%, a necking rate (B) represented by the following formula (2) in the stretching step of 31.0 to 38.0%, and a necking rate (C) represented by the following formula (3) in the drying step of 9.8 to 16.5%;

total necking rate (a) = { (X1-X2)/X1 } ×100 (1)

Necking rate (B) = { (Y1-Y2)/Y1 } ×100 (2)

Necking rate (C) = { (Z1-X2)/Z1 } ×100 (3)

[ X1 ] is the length (m) of the width of the polyvinyl alcohol film before the dyeing step; x2 represents the length (m) of the width of the polyvinyl alcohol film after the drying step; y1 represents the length (m) of the width of the polyvinyl alcohol film before the stretching step; y2 represents the length (m) of the width of the polyvinyl alcohol film after the stretching step; z1 represents the length (m) of the width of the polyvinyl alcohol film before the drying step. The method comprises the steps of carrying out a first treatment on the surface of the

[2] The method for producing a polarizing film according to the above [1], wherein the necking rate (D) represented by the following formula (4) up to the stretching step is 46.0% to 54.0%;

necking rate (D) = { (X1-Y2)/X1 } ×100 (4)

[3] The method for producing a polarizing film according to the above [2], wherein the difference ((A) - (D)), i.e., the difference (W) between the total necking rate (A) and the necking rate (D) is 8.0 to 11.0%;

[4] the method for producing a polarizing film according to the above [3], wherein the ratio ((W)/(A)) of the necking rate (W) to the total necking rate (A) is 0.14 to 0.19;

[5] the method for producing a polarizing film according to any one of the above [1] to [4], wherein the total content of boron elements in the polarizing film is 2.0 to 4.0 mass%;

[6] the method for producing a polarizing film according to any one of the above [1] to [5], wherein the stretching temperature in the stretching step is 53℃to 70 ℃;

[7] the method for producing a polarizing film according to any one of the above [1] to [6], wherein the drying temperature in the drying step is 60℃to 100 ℃;

[8]according to the above [1]]～[7]The polarizing film of any one ofWherein the polarization degree of the polarizing film at 44% of the single transmittance is 99.963% or more and the shrinkage stress is 100N/mm ² The following are set forth;

[9]a polarizing film formed of a polyvinyl alcohol film, having a degree of polarization of 99.963% or more at a monomer transmittance of 44% and a shrinkage stress of 100N/mm ² The following are set forth;

[10] the polarizing film according to the aforementioned [9], wherein the total content of boron elements in the polarizing film is 4.0 mass% or less.

Effects of the invention

According to the method for producing a polarizing film of the present invention, a polarizing film having excellent polarization performance and small shrinkage stress can be produced. Thus, the resulting polarizing film can be suitably used for high-performance liquid crystal displays, especially liquid crystal displays that are sometimes used at high temperatures.

Drawings

Fig. 1 is a graph plotting the polarization degree when the monomer transmittance with respect to the shrinkage stress of the polarizing films obtained in examples 1 to 5 and comparative examples 1 to 9 is 44%.

Fig. 2 is a graph plotting tensile force against shrinkage stress of the polarizing films obtained in examples 1 to 5 and comparative examples 1 to 9.

Detailed Description

The method for manufacturing a polarizing film of the present invention comprises: a dyeing step of dyeing the PVA film with a dichroic dye; a stretching step of uniaxially stretching the dyed PVA film in an aqueous solution containing boric acid; and a drying step of drying the stretched PVA film, wherein the aqueous solution of the stretching step has a boric acid concentration of 1 to 3% by mass, a total stretching ratio of 5.5 to 7.4 times, a total necking rate (A) represented by the following formula (1) of 57.5 to 61.0%, a necking rate (B) represented by the following formula (2) in the stretching step of 31.0 to 38.0%, and a necking rate (C) represented by the following formula (3) in the drying step of 9.8 to 16.5%.

Total necking rate (a) = { (X1-X2)/X1 } ×100 (1)

Necking rate (B) = { (Y1-Y2)/Y1 } ×100 (2)

Necking rate (C) = { (Z1-X2)/Z1 } ×100 (3)

In the above formulae (1) to (3), X1 represents the length of the width of the PVA film before the dyeing step (the length of the width of the unstretched PVA film used for producing the polarizing film) (m); x2 represents the length (m) of the width of the PVA film after the drying step; y1 represents the length (m) of the width of the PVA film before the stretching step; y2 represents the length (m) of the width of the PVA film after the stretching step; z1 represents the length (m) of the width of the PVA film before the drying step. In the case where a heat treatment step to be described later is provided after the drying step, the length (m) of the width of the PVA film after heat treatment may be set to X2. When the crosslinking step is provided before the stretching step, the length (m) of the width of the PVA film after the crosslinking step and before the stretching step may be set to Y1. When the cleaning step described later is provided after the stretching step, the length (m) of the width of the PVA film after the stretching step and before the cleaning step may be set to Y2.

In order to produce a polarizing film having excellent polarization properties and small shrinkage stress, it is necessary to control the total necking rate (a), the necking rate (B) in the stretching step, and the necking rate (C) in the drying step at the same time. By controlling the total necking rate (a), it is possible to suppress a decrease in polarization performance and yield.

In the stretching step, PVA is highly oriented, whereby the iodine dye is highly oriented or residual stress is generated. In the drying step, residual stress is released by the orientation relaxation and crystallization of PVA, and the following may occur: the unwanted iodine dye is decomposed by heat, which results in an improvement in polarization performance; and, even if necessary iodine dyes are decomposed by heat, the polarization performance may be lowered. Accordingly, by appropriately controlling the necking rate (B) in the stretching step and the necking rate (C) in the drying step, a polarizing film excellent in polarizing performance and small in shrinkage stress can be manufactured.

The total necking rate (a) must be 57.5% or more, preferably 58.5% or more, more preferably 59.0% or more. The total necking rate (a) must be 61.0% or less, preferably 60.8% or less, more preferably 60.5% or less. If the total necking rate (a) is less than 57.5%, the necking phenomenon up to the stretching step is insufficient, and the film width becomes too wide, and wrinkles are often generated on the surface of the polarizing film, so that the polarizing performance is easily lowered or the yield of the polarizing film is easily lowered, which is not preferable. On the other hand, when the total necking rate (a) exceeds 61.0%, the necking phenomenon may progress excessively until the stretching step, and the PVA film may break during the stretching step and the drying step, resulting in a decrease in the yield, or even an unnecessary iodine dye may stabilize, resulting in a decrease in the polarization performance, which is not preferable. The method for adjusting the total necking rate (a) to these ranges is not particularly limited, and examples thereof include methods for appropriately adjusting the boric acid concentration and temperature of the aqueous solution in each of the swelling step, dyeing step, crosslinking step, stretching step and cleaning step, the stretching ratio in each of the swelling step, dyeing step, crosslinking step, stretching step and cleaning step, and the drying temperature and drying time in the drying step.

The necking rate (B) in the stretching step is necessarily 31.0% or more, preferably 32.0% or more, and more preferably 33.0% or more. The necking rate (B) in the stretching step is preferably 38.0% or less, more preferably 37.5% or less, still more preferably 37.0% or less, and particularly preferably 36.0% or less. If the necking rate (B) in the stretching step is less than 31.0%, the iodine dye cannot be highly oriented, and it is difficult to obtain a polarizing film having excellent polarizing performance, which is not preferable. On the other hand, if the necking rate (B) in the stretching step exceeds 38.0%, the reason is not yet specified, but since the interaction between PVA molecular chains and the crosslinking by boric acid become strong, the necking phenomenon cannot occur in the drying step, and the residual stress by the decomposition of the unnecessary iodine-based dye, the orientation relaxation and crystallization of PVA becomes insufficient, so that it is difficult to obtain a polarizing film excellent in polarizing performance and small in shrinkage stress, which is not preferable. The method of adjusting the necking rate (B) to these ranges is not particularly limited, and examples thereof include a method of appropriately adjusting the boric acid concentration, temperature, stretching ratio, and the like of the aqueous solution in the stretching step.

The necking rate (C) in the drying step is preferably 9.8% or more, more preferably 11.5% or more, still more preferably 12.0% or more, and particularly preferably 12.5% or more. The necking rate (C) in the drying step is required to be 16.5% or less, preferably 16.3% or less, and more preferably 16.1% or less. If the necking rate (C) in the drying step is less than 9.8%, the relaxation of residual stress by the orientation relaxation and crystallization of PVA becomes insufficient, and it is difficult to obtain a polarizing film having a small shrinkage stress, which is not preferable. On the other hand, if the necking rate (C) in the drying step exceeds 16.5%, the PVA tends to excessively undergo orientation relaxation, and even the necessary iodine dye tends to decompose, so that it is difficult to obtain a polarizing film excellent in polarizing performance, which is not preferable. The method of adjusting the necking rate (C) to these ranges is not particularly limited, and examples thereof include a method of appropriately adjusting the drying temperature, drying time, and the like in the drying step.

The total stretch ratio is required to be 5.5 times or more, preferably 5.8 times or more, more preferably 5.9 times or more, and particularly preferably 6.0 times or more. The total stretch ratio must be 7.4 times or less, preferably 7.3 times or less, more preferably 7.2 times or less, and particularly preferably 6.8 times or less. The total stretch ratio means: the ratio of the length of the polarizing film stretched through all the steps to the original length of the unstretched PVA film used for producing the polarizing film. If the total stretching ratio is less than 5.5 times, the necking phenomenon cannot be sufficiently generated and the iodine dye cannot be highly oriented, so that it is difficult to obtain a polarizing film excellent in polarizing performance, which is not preferable. On the other hand, if the total stretching ratio exceeds 7.4 times, the necking phenomenon excessively occurs, and the relaxation of residual stress by the orientation relaxation and crystallization of PVA cannot sufficiently occur, so that it is difficult to obtain a polarizing film having a small shrinkage stress, which is not preferable. In addition, stretching tends to be uneven, and the yield of the polarizing film is reduced, which is not preferable from the viewpoint of productivity. The method of adjusting the total stretching ratio to these ranges is not particularly limited, and examples thereof include a method of appropriately adjusting the stretching ratio in each of the swelling step, dyeing step, crosslinking step, stretching step, and cleaning step.

The boric acid concentration of the aqueous solution in the stretching step is preferably 1.0 mass% or more, more preferably 1.1 mass% or more, particularly preferably 1.2 mass% or more, and further preferably 1.4 mass% or more. The boric acid concentration of the aqueous solution in the stretching step is preferably 3.0 mass% or less, more preferably 2.9 mass% or less, particularly preferably 2.5 mass% or less, and further preferably 2.0 mass% or less. When the concentration of boric acid in the aqueous solution in the stretching step is less than 1.0 mass%, crosslinking by boric acid is insufficient, and the phenomenon of necking in the stretching step is insufficient, so that it is difficult to highly orient the iodine-based dye, and it is difficult to obtain a polarizing film excellent in polarizing performance, which is not preferable. On the other hand, when the boric acid concentration in the aqueous solution in the stretching step exceeds 3.0 mass%, crosslinking by boric acid is excessively formed, and an unnecessary iodine-based dye is formed in the stretching step, or the necking phenomenon in the drying step is insufficient, and release of residual stress by orientation relaxation and crystallization of PVA cannot sufficiently occur, or it is difficult to obtain a polarizing film excellent in polarizing performance and small in shrinkage stress, which is not preferable.

The necking rate (D) represented by the following formula (4) until the stretching step is preferably 46.0% or more, more preferably 48.5% or more, particularly preferably 49.0% or more, and further preferably 49.3% or more. The necking rate (D) represented by the following formula (4) until the stretching step is preferably 54.0% or less, more preferably 53.0% or less, particularly preferably 52.0% or less, and further preferably 51.2% or less.

Necking rate (D) = { (X1-Y2)/X1 } ×100 (4)

When the necking rate (D) up to the stretching step is less than 46.0%, the following tends to occur: among the necking phenomena occurring in all the steps, the necking phenomenon occurring until the stretching step is insufficient, and the iodine dye cannot be highly oriented, and it is difficult to obtain a polarizing film excellent in polarizing performance. On the other hand, when the necking rate (D) up to the stretching step exceeds 54.0%, the following tends to occur: among the necking phenomena occurring in all the steps, the necking phenomenon occurring up to the stretching step is excessive, and it is difficult to release residual stress by the orientation relaxation and crystallization of PVA by a method in which the yield of the polarizing film is not lowered after the stretching step, and it is difficult to obtain a polarizing film having a small shrinkage stress. The method for adjusting the necking rate (D) to these ranges is not particularly limited, and examples thereof include a method for appropriately adjusting the boric acid concentration and temperature of the aqueous solution in each of the dyeing step, the crosslinking step and the stretching step, the swelling step, the stretching ratio in each of the dyeing step, the crosslinking step and the stretching step, and the drying temperature and drying time in the drying step.

The difference ((a) - (D)) between the total necking rate (a) and the necking rate (D), that is, the necking rate difference (W), is preferably 8.0% or more, more preferably 8.5% or more, and particularly preferably 9.0% or more. The necking rate difference (W) is preferably 11.0% or less, more preferably 10.5% or less, and particularly preferably 10.0% or less. When the difference (W) between the total necking rate (a) and the necking rate (D) is smaller than 8.0%, the following tends to occur: in the necking phenomenon occurring in all the steps, the rate of the necking phenomenon occurring in the drying step is insufficient, and the residual stress by the decomposition of the unnecessary iodine-based dye, the orientation relaxation of PVA, and crystallization is easily insufficient, so that it is difficult to obtain a polarizing film excellent in polarizing performance and small in shrinkage stress. In particular, it tends to be difficult to reduce the shrinkage stress. On the other hand, when the necking rate difference (W) exceeds 11.0%, the following tends to occur: of the necking phenomena occurring in all the steps, the ratio of the necking phenomena occurring in the drying step is excessive, and the problem often occurs until the necessary decomposition of the iodine-based dye proceeds, and it is difficult to obtain a polarizing film excellent in polarizing performance.

The ratio ((W)/(a)) of the necking rate difference (W) to the total necking rate (a) is preferably 0.14 or more, more preferably 0.15 or more, particularly preferably 0.16 or more. The ratio ((W)/(a)) of the necking rate difference (W) to the total necking rate (a) is preferably 0.19 or less, more preferably 0.18 or less, and particularly preferably 0.17 or less. When the ratio ((W)/(a)) of the necking rate difference (W) to the total necking rate (a) is smaller than 0.14, the following tends to occur: in the necking phenomenon occurring in all the steps, the ratio of the necking phenomenon occurring in the drying step is insufficient, and the residual stress by the decomposition of the unnecessary iodine-based dye, the orientation relaxation of PVA, and crystallization is easily insufficient, and it is difficult to obtain a polarizing film excellent in polarizing performance and small in shrinkage stress. In particular, it tends to be difficult to reduce the shrinkage stress. On the other hand, when the ratio ((W)/(a)) exceeds 0.19, there is a tendency that: of the necking phenomena occurring in all the steps, the necking phenomena occurring in the drying step are often excessive in proportion, and the degradation of the necessary iodine dye proceeds until it becomes difficult to obtain a polarizing film excellent in polarizing performance.

The total content of boron elements in the polarizing film is preferably 2.0 mass% or more, more preferably 2.5 mass% or more, and particularly preferably 2.8 mass% or more. The total content of boron elements in the polarizing film is preferably 4.0 mass% or less, more preferably 3.8 mass% or less, and particularly preferably 3.7 mass% or less. When the total content of boron elements in the polarizing film is less than 2.0 mass%, crosslinking by boric acid is insufficient, and it is difficult to control the necking phenomenon in the stretching step and the drying step to highly orient the iodine-based dye, and it is difficult to obtain a polarizing film excellent in polarizing performance, which is not preferable. On the other hand, when the total amount of boron in the polarizing film exceeds 4.0 mass%, the crosslinking by boric acid is excessive, and it is difficult to control the necking phenomenon in the drying step, and thus it is difficult to obtain a polarizing film having a small shrinkage stress because of the relaxation of orientation of PVA and the release of residual stress by crystallization. The total boron content in the polarizing film can be determined by ICP emission analysis or the like. Specifically, the method described in the examples can be used to obtain the product.

In this way, the polarizing film obtained in the present invention is preferably: the degree of polarization at 44% of the monomer transmittance is 99.963% or more, and the shrinkage stress is 100N/mm ² Hereinafter, the present invention is applicable to a high-performance liquid crystal display, particularly a liquid crystal display which is used at a high temperature. The degree of polarization at 44% transmittance of the monomer is preferably 99.965% or more, more preferably 99.967% or more, particularly preferably 99.970% or more, and further preferably 99.975% or more.

The thickness of the polarizing film obtained by the production method of the present invention is preferably 5 μm or more, more preferably 10 μm or more, particularly preferably 12 μm or more, and further preferably 14 μm or more. The thickness of the polarizing film obtained by the production method of the present invention is preferably 60 μm or less, more preferably 45 μm or less, particularly preferably 30 μm or less, and further preferably 25 μm or less. If the thickness is less than 5 μm, the tensile fracture tends to occur during production, and the productivity may be lowered. On the other hand, if the thickness exceeds 60 μm, there is a possibility that performance required for the polarizing plate, such as film formation and weight reduction, is not satisfied.

<PVA>

The PVA film used in the production method of the present invention contains PVA. PVA is a polymer having a vinyl alcohol unit (-CH) ₂ -CH (OH) -) as the main structural unit.

The polymerization degree of PVA is preferably 1, 500 or more, more preferably 1,800 or more, and still more preferably 2,000 or more. The polymerization degree of PVA is preferably 6,000 or less, more preferably 5,000 or less, and still more preferably 4,000 or less. By setting the polymerization degree to 1 or 500 or more, the durability of the polarizing film obtained by uniaxially stretching the film can be improved. On the other hand, by setting the polymerization degree to 6,000 or less, it is possible to suppress an increase in manufacturing cost, a defective process qualification in film formation, and the like. The polymerization degree of PVA (A) in the present specification means an average polymerization degree measured in accordance with JIS K6726-1994.

The saponification degree of PVA is preferably 95 mol% or more, more preferably 96 mol% or more, and still more preferably 98 mol% or more from the viewpoint of water resistance of a polarizing film obtained by uniaxially stretching the film. The saponification degree of PVA in the present specification means: can be converted into a vinyl alcohol unit (-CH) by saponification relative to PVA ₂ -CH (OH) -) (typically vinyl ester units) to the total moles of vinyl alcohol units, the moles of vinyl alcohol units being the proportion (mole%). The saponification degree can be measured according to JIS K6726-1994.

The method for producing PVA is not particularly limited. Examples of the method include a method of converting vinyl ester units of a polyvinyl ester obtained by polymerizing a vinyl ester monomer into vinyl alcohol units. The vinyl ester monomer used for producing PVA (a) is not particularly limited, and examples thereof include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl tertiary carboxylate, vinyl caproate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, and vinyl benzoate. From an economical point of view, vinyl acetate is preferred.

The PVA may be obtained by converting vinyl ester units of a vinyl ester copolymer obtained by copolymerizing a vinyl ester monomer with other monomers copolymerizable therewith into vinyl alcohol units. Examples of the other monomer copolymerizable with the vinyl ester monomer include an α -olefin having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutylene; (meth) acrylic acid or a salt thereof; (meth) acrylic esters such as methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, octadecyl (meth) acrylate, and the like; (meth) acrylamide derivatives such as (meth) acrylamide, N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, N-dimethyl (meth) acrylamide, diacetone (meth) acrylamide, (meth) acrylamide propane sulfonic acid or a salt thereof, (meth) acrylamide propyldimethylamine or a salt thereof, and N-hydroxymethyl (meth) acrylamide or a derivative thereof; n-vinylamides such as N-vinylformamide, N-vinylacetamide and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether; ethylene cyanide such as (meth) acrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester or anhydride thereof; itaconic acid or a salt, ester or anhydride thereof; vinyl silyl compounds such as vinyl trimethoxy silane; unsaturated sulfonic acids, and the like. The vinyl ester copolymer may have a structural unit derived from 1 or 2 or more monomers among the other monomers. The other monomer may be used by making the vinyl ester monomer pre-existing in the reaction vessel when it is supplied to the polymerization reaction, or adding it to the reaction vessel while the polymerization reaction is in progress, or the like. From the viewpoint of optical properties, the content of the unit derived from the other monomer is preferably 10 mol% or less, more preferably 5 mol% or less, and still more preferably 2 mol% or less, relative to the number of moles of all the structural units constituting the PVA (a).

Some of the hydroxyl groups in the PVA may or may not be crosslinked. In addition, a part of hydroxyl groups in the PVA may be reacted with an aldehyde compound such as acetaldehyde or butyraldehyde to form an acetal structure, or may not be reacted with these compounds to form an acetal structure.

Ethylene is preferred as the monomer copolymerizable with the vinyl ester monomer, since the stretchability is improved, the occurrence of failures such as stretching and stretch breaking at higher temperatures is reduced, and the productivity of the polarizing film is further improved. In the case where the PVA contains ethylene units, the content of ethylene units is preferably 1 mol% or more, more preferably 2 mol% or more, based on the number of moles of all the structural units constituting the PVA, from the viewpoints of the stretchability, the stretchable temperature, and the like described above. When the PVA contains ethylene units, the content of ethylene units is preferably 10 mol% or less, more preferably 6 mol% or less, based on the number of moles of all the structural units constituting the PVA, from the viewpoints of stretchability, stretchable temperature, and the like.

< PVA film >

The PVA film used in the production method of the present invention may contain a plasticizer in addition to the PVA described above. Preferred plasticizers include polyhydric alcohols, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Further, 1 or 2 or more of these plasticizers may be contained. Among these, glycerin is preferred from the viewpoint of improving the stretchability.

The content of the plasticizer in the PVA film is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and further preferably 5 parts by mass or more, per 100 parts by mass of PVA (a). The content of the plasticizer in the PVA film is preferably 20 parts by mass or less, more preferably 17 parts by mass or less, and further preferably 15 parts by mass or less, per 100 parts by mass of PVA (a). When the content is 1 part by mass or more, the stretchability of the film is improved. On the other hand, when the content is 20 parts by mass or less, the film becomes too soft and the handleability is prevented from being lowered.

The PVA film may be further blended with other additives than PVA (a) and plasticizers, such as a processing stabilizer, a weather-resistant stabilizer, a colorant, an ultraviolet absorber, a light stabilizer, an antioxidant, an antistatic agent, a flame retardant, other thermoplastic resins, a lubricant, a perfume, a defoaming agent, a deodorizing agent, an extender, a peeling agent, a mold release agent, a reinforcing agent, a crosslinking agent, a mold inhibitor, a preservative, and a crystallization rate retarder, as appropriate. The content of the other additives in the PVA film is generally preferably 10 mass% or less, more preferably 5 mass% or less.

The swelling degree of the PVA film is preferably 160% or more, more preferably 170% or more, particularly preferably 180% or more. The swelling degree of the PVA film is preferably 240% or less, more preferably 230% or less, particularly preferably 220% or less. By setting the swelling degree to 160% or more, the extremely occurrence of crystallization can be suppressed, and the stretching can be stably performed to a high magnification. On the other hand, by setting the swelling degree to 240% or less, dissolution during stretching is suppressed, and stretching can be performed even at a higher temperature.

The thickness of the PVA film is not particularly limited, but is usually 1 μm or more, preferably 5 μm or more, and particularly preferably 10 μm or more. The thickness of the PVA film is usually 100 μm or less, preferably 60 μm or less, particularly preferably 45 μm or less. If the PVA film is too thin, stretch breaking tends to occur easily in the stretching step for producing the polarizing film. If the PVA film is too thick, stretching unevenness tends to occur in the stretching step for producing the polarizing film, and the resulting polarizing film becomes thick. Therefore, it tends to be difficult to use for lightweight, slim, and small devices such as smart phones and notebook computers.

The width of the PVA film is not particularly limited and may be determined according to the use of the polarizing film to be produced, and the like. In recent years, from the viewpoint of increasing the screen size of liquid crystal televisions and liquid crystal displays, it is preferable to set the width of the PVA film used for producing the polarizing film to 3m or more. On the other hand, if the width of the PVA film used for producing the polarizing film is too large, it is easy to make it difficult to uniformly stretch the film when the polarizing film is produced by a device which has been put into practical use, and therefore, the width of the PVA film used for producing the polarizing film is preferably 10m or less.

The method for producing the PVA film is not particularly limited, and a production method in which the thickness and the width of the film after film formation are uniform is preferably used. The production can be performed using, for example, the following film-forming stock solutions: a film-forming stock solution obtained by dissolving PVA (a) and, if necessary, 1 or 2 or more of the plasticizer, the other additives, and the surfactant described later, etc., in a liquid medium; 1 or 2 or more kinds of plasticizer, other additives, surfactant, liquid medium, and the like are further included as needed, and the PVA (a) is melted to form a film-forming stock solution. When the film-forming stock solution contains at least 1 of a plasticizer, other additives, and a surfactant, these components are preferably uniformly mixed.

Examples of the liquid medium used for 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, and one or more of these may be used. Among them, water is preferable from the viewpoints of environmental burden and recyclability.

The volatile fraction of the film-forming stock solution (the content of volatile components such as a liquid medium removed by evaporation or evaporation during film formation) also varies depending on the film-forming method, film-forming conditions, etc., and is usually preferably 50 mass% or more, more preferably 55 mass% or more. The volatile fraction of the film forming stock solution varies depending on the film forming method, film forming conditions, and the like, and is usually preferably 95% by mass or less, more preferably 90% by mass or less. By setting the evaporation percentage of the film-forming stock solution to 50 mass% or more, the viscosity of the film-forming stock solution is not excessively high, and filtration and defoaming can be smoothly performed when the film-forming stock solution is prepared, thereby facilitating the production of a film having less foreign matters and defects. On the other hand, by setting the volatile content of the film-forming stock solution to 95 mass% or less, the concentration of the film-forming stock solution is not too low, and thus the film can be easily produced industrially.

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

The anionic surfactant is preferably a carboxylic acid type such as potassium laurate; sulfate esters such as polyoxyethylene lauryl ether sulfate, sodium alkyl sulfate, potassium alkyl sulfate, ammonium alkyl sulfate, triethanolamine alkyl sulfate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxypropylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, and octyl sulfate; sulfonic acid type such as sodium alkyl sulfonate, potassium alkyl sulfonate, ammonium alkyl sulfonate, triethanolamine alkyl sulfonate, sodium alkylbenzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkyl naphthalene sulfonate, disodium alkyl sulfosuccinate, disodium polyoxyethylene alkyl sulfosuccinate, and dodecylbenzene sulfonate; sodium alkyl phosphate, potassium alkyl phosphate, ammonium alkyl phosphate, triethanolamine alkyl phosphate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxypropylene alkyl ether phosphate, sodium polyoxyethylene alkyl phenyl ether phosphate, and the like.

As the nonionic surfactant, for example, alkyl ether type surfactants such as polyoxyethylene oleyl ether are suitable; alkylphenyl ethers such as polyoxyethylene octylphenyl ether; alkyl esters such as polyoxyethylene laurate; alkylamines such as polyoxyethylene lauryl amino ether; alkylamide type such as polyoxyethylene lauramide; polypropylene glycol ethers such as polyoxyethylene polyoxypropylene ether; alkanolamide type such as lauric acid diethanolamide and oleic acid diethanolamide; and allylphenyl ethers such as polyoxyalkylene allylphenyl ether.

These surfactants may be used alone or in combination of 1 or more than 2.

When the film-forming stock solution contains a surfactant, the content thereof is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, particularly preferably 0.05 parts by mass or more, relative to 100 parts by mass of PVA (a) contained in the film-forming stock solution. When the film-forming stock solution contains a surfactant, the content thereof is preferably 0.5 parts by mass or less, more preferably 0.3 parts by mass or less, particularly preferably 0.2 parts by mass or less, relative to 100 parts by mass of PVA (a) contained in the film-forming stock solution. When the content is 0.01 parts by mass or more, the film forming property and the peeling property are further improved. On the other hand, when the content is 0.5 parts by mass or less, bleeding of the surfactant to the surface of the PVA film can be suppressed to cause blocking, and the handleability is lowered.

Examples of the film forming method for producing a PVA film using the film forming stock solution include a casting film forming method, an extrusion film forming method, a wet film forming method, a gel film forming method, and the like. The number of these film forming methods may be 1 or 2 or more. Among these film forming methods, a casting film forming method and an extrusion film forming method are preferable from the viewpoint of obtaining a PVA film for producing a polarizing film having uniform thickness and film width and good physical properties. The PVA film thus formed may be dried and heat-treated as necessary.

As an example of a specific production method of the PVA film used in the production method of the present invention, the following method can be preferably used industrially: for example, the film-forming stock solution is uniformly sprayed or cast onto the circumferential surface of a first rotating heated roll (or belt) located on the most upstream side using a T-slot die, a hopper plate, an I-die, a lip coater die, etc., volatile components are volatilized from one surface of a film sprayed or cast onto the circumferential surface of the first roll (or belt) and dried, and then the film is further dried on the circumferential surface of 1 or more rotating heated rolls disposed on the downstream side thereof, or the film is passed through a hot air drying apparatus and further dried, and then wound up by a winding device. Drying by the heating roller and drying by the hot air drying device may be performed in an appropriate combination. Further, a layer containing PVA (a) is formed on one surface of a base film composed of a single resin layer, whereby a multilayer PVA film can be formed. The thickness of the base film in the multilayer film is usually 20 to 500 μm.

In the case of using a multilayer film as the PVA film, the base film must be stretchable together with PVA (a), and polyester, polyolefin resin, or the like may be used. Among them, amorphous polyester resins obtained by copolymerizing polyethylene terephthalate, isophthalic acid, 1, 4-cyclohexanedimethanol and the like are preferable, and amorphous polyester resins obtained by copolymerizing polyethylene terephthalate and isophthalic acid and 1, 4-cyclohexanedimethanol are preferable. The multilayer film is preferably produced by coating a PVA solution to a substrate film. In this case, in order to improve the adhesion between the PVA (a) layer and the base film, the surface of the base film may be modified or an adhesive layer may be formed between the two layers.

< method for producing polarizing film >

The method for producing a polarizing film of the present invention uses the PVA film described above as a raw material. Specifically, a method for producing a polarizing film, comprising: a method for producing a polarizing film, which includes a dyeing step of dyeing a PVA film with a dichroic dye, a stretching step of uniaxially stretching the dyed film, and a drying step of drying the stretched film, includes: and a stretching step of stretching the PVA film in an aqueous boric acid solution. In addition to the dyeing step, the stretching step, and the drying step, the PVA film may be further subjected to a swelling step, a crosslinking step, a cleaning step, a heat treatment step, and the like, as necessary. The order of the steps is not particularly limited, and 1 or 2 or more kinds of treatments may be simultaneously performed. In addition, the polarizing film may be produced by subjecting 1 or 2 or more of the steps to 2 or more times, preferably sequentially performing a swelling step, a dyeing step, a crosslinking step, a stretching step, and a drying step. It is also preferable that a cleaning step is further performed after the stretching step. Hereinafter, each step will be described in detail.

The swelling treatment may be performed by immersing the PVA film in water. The temperature of the water used for impregnating the film is preferably 20℃or higher, more preferably 22℃or higher, and still more preferably 25℃or higher. The temperature of the water used for impregnating the film is preferably 40℃or lower, more preferably 38℃or lower, and still more preferably 35℃or lower. The time for immersing in water is, for example, preferably 0.1 minutes or more, more preferably 0.2 minutes or more. The time for immersing in water is, for example, preferably 5 minutes or less, more preferably 3 minutes or less. The water used to impregnate the membrane is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or may be a mixture of water and a hydrophilic medium. By setting the immersion time as described above, the PVA film can be swollen efficiently and uniformly.

The dyeing process may be performed by contacting the dichroic dye with the PVA film. As the dichroic dye, an iodine-based dye or a dichroic dye is generally used, and in the production method of the present invention, an iodine-based dye is preferably used. The dyeing step may be performed at any stage of the stretching step before, during or after the stretching step, and is preferably performed before the stretching step in order to highly orient the iodine-based dye. The dyeing process is generally carried out by immersing the PVA film in a solution containing iodine-potassium iodide (particularly an aqueous solution) or a solution containing a plurality of dichroic dyes (particularly an aqueous solution) as a dyeing bath. The concentration of iodine in the dyeing bath is preferably in the range of 0.01 to 0.5 mass%. The concentration of potassium iodide in the dyeing bath is preferably in the range of 0.01 to 15 mass%. The temperature of the dyeing bath is preferably 20℃or higher, more preferably 25℃or higher. The temperature of the dyeing bath is preferably 50 ℃ or less, more preferably 40 ℃ or less. Suitable dyeing times are from 0.2 to 5 minutes.

In the case of using a dichroic dye, the dichroic dye is preferably an aqueous dye. The dye concentration in the dyeing bath is preferably 0.001 to 10 mass%. Dyeing auxiliaries may be used as desired. As the dyeing auxiliary, an inorganic salt such as sodium sulfate, a surfactant, and the like can be used. In the case of using sodium sulfate, the dye concentration in the dyeing bath is preferably 0.1 to 10 mass%. The dyeing temperature is preferably 30 to 80 ℃.

Specific dichroic dyes include c.i. direct yellow 28, c.i. direct orange 39, c.i. direct yellow 12, c.i. direct yellow 44, c.i. direct orange 26, c.i. direct orange 71, c.i. direct orange 107, c.i. direct red 2, c.i. direct red 31, c.i. direct red 79, c.i. direct red 81, c.i. direct red 247, c.i. direct green 80, c.i. direct green 59, and the like, and dichroic dyes developed for producing polarizing plates are preferable.

The dyeing bath may contain boric acid crosslinking agents such as boric acid, borax, etc., and the content of the boric acid crosslinking agents is usually less than 5 mass% in terms of boric acid conversion, and preferably 1 mass% or less.

The crosslinking step may be performed by immersing the PVA film in an aqueous solution containing a boric acid crosslinking agent. By performing the crosslinking step on the PVA film, the PVA molecular chains are crosslinked by boric acid, and the orientation of the PVA molecular chains is improved. As a result, the orientation of the dichroic dye adsorbed by the PVA film is improved, and thus the optical performance of the resulting polarizing film is improved. From this viewpoint, the crosslinking step is more preferably performed after the dyeing step and before the stretching step. As the boric acid crosslinking agent, 1 or 2 or more kinds of boron-containing inorganic compounds such as boric acid, borax and the like can be used, and boric acid is preferable as the boric acid crosslinking agent from the viewpoint of easiness of handling. From the viewpoint of maintaining sufficient stretchability, the concentration of the boric acid crosslinking agent in the aqueous solution containing the boric acid crosslinking agent is preferably 1 mass% or more, more preferably 2 mass% or more. Similarly, from the viewpoint of maintaining sufficient stretchability, the concentration of the boric acid crosslinking agent in the aqueous solution containing the boric acid crosslinking agent is preferably 10 mass% or less, more preferably 7 mass% or less. If the concentration of the boric acid crosslinking agent exceeds 10 mass%, crosslinking may be excessively performed, and stretchability may be reduced. In addition, when the concentration of the boric acid crosslinking agent is less than 1 mass%, the alignment property of the dichroic dye adsorbed by the PVA film may not be sufficiently improved, and the polarizing performance of the resulting polarizing film may not be sufficiently improved. The aqueous solution containing the boric acid crosslinking agent may contain an auxiliary agent for iodide such as potassium iodide. The temperature of the aqueous solution containing the boric acid crosslinking agent is preferably 20 ℃ or higher, particularly preferably 25 ℃ or higher, from the viewpoint of being capable of efficiently performing boric acid crosslinking. Similarly, the temperature of the aqueous solution containing the boric acid crosslinking agent is preferably 50 ℃ or less, particularly preferably 40 ℃ or less, from the viewpoint of being capable of efficiently performing boric acid crosslinking.

The PVA film may be stretched (pre-stretched) in the above steps or between steps, in addition to the stretching step described later. In this way, from the viewpoint of the optical performance of the obtained polarizing film, the stretching ratio of the pre-stretching performed before the stretching step (the ratio obtained by multiplying the stretching ratios in the respective treatments) is preferably 1.5 times or more, more preferably 2.0 times or more, still more preferably 2.5 times or more, depending on the original length of the unstretched PVA film to be used for producing the polarizing film. On the other hand, the stretching ratio of the pre-stretching is preferably 3.6 times or less, more preferably 3.4 times or less. The stretching ratio in the swelling step is preferably 1.05 to 2.5 times. The stretching ratio in the dyeing step is preferably 1.1 to 2.5 times. The stretching ratio in the crosslinking step is preferably 1.1 to 2.5 times.

The stretching step is preferably performed in an aqueous solution containing boric acid. By performing the stretching step in an aqueous solution containing boric acid, the necking phenomenon in the stretching step and the drying step can be easily controlled. The concentration of boric acid in the aqueous solution containing boric acid is preferably 1% by mass or more, more preferably 1.1% by mass or more, particularly preferably 1.2% by mass or more, and further preferably 1.4% by mass or more. The concentration of boric acid in the aqueous solution containing boric acid is preferably 3% by mass or less, more preferably 3.0% by mass or less, particularly preferably 2.9% by mass or less, and further preferably 2.5% by mass or less. The aqueous solution containing boric acid may contain an auxiliary agent such as iodide, for example, potassium iodide, and the concentration thereof is preferably in the range of 0.01 to 10 mass%.

The stretching temperature in the stretching step is preferably 53℃or higher, more preferably 55℃or higher, and particularly preferably 55℃or higher. The stretching temperature in the stretching step is preferably 70℃or lower, 65℃or lower, and particularly preferably 60℃or lower. The stretching temperature herein refers to the temperature of an aqueous solution containing boric acid. By setting the temperature in this range, the moisture content of the polarizing film before crosslinking and drying by boric acid can be controlled to an appropriate range, and the necking phenomenon in the stretching step and drying step can be easily controlled. That is, when the stretching temperature exceeds 70 ℃, the following tends to occur: the reactivity of PVA with boric acid is lowered, and it is difficult to control the necking phenomenon in the stretching step and the drying step to highly orient the iodine dye due to insufficient crosslinking of boric acid, and it is difficult to obtain a polarizing film excellent in polarizing performance. In addition, there is also a risk that the PVA film dissolves in the stretching step and the yield of the polarizing film decreases, and therefore productivity tends to decrease. On the other hand, when the stretching temperature is less than 53 ℃, the following tends to occur: the moisture content of the polarizing film before drying is insufficient, and it is difficult to control the necking phenomenon in the drying step, and thus it is difficult to obtain a polarizing film having a small shrinkage stress because of the relaxation of the PVA orientation and the release of residual stress by crystallization.

The stretching ratio in the stretching step (in other words, the stretching ratio obtained from the length of the PVA film before and after the stretching step) is preferably 2.0 times or more, and more preferably 2.2 times or more from the viewpoint of the optical performance of the obtained polarizing film. The stretching ratio in the stretching step is preferably 4.0 times or less, more preferably 3.5 times or less.

The total stretching ratio obtained according to the original length of the unstretched PVA film used for producing the polarizing film must be 5.5 times or more, preferably 5.8 times or more, more preferably 5.9 times or more, and particularly preferably 6.0 times or more. The total stretch ratio must be 7.4 times or less, preferably 7.3 times or less, more preferably 7.2 times or less, and particularly preferably 6.8 times or less. By adjusting the total stretching ratio in this way, the necking phenomenon in all the steps can be easily controlled appropriately.

The tension in the stretching step can be obtained by measuring the tension carried between adjacent rollers with tension rollers provided between the adjacent rollers in the stretching step. The stretching tension in the stretching step is preferably 200N or more, more preferably 300N or more, and particularly preferably 450N or more. The stretching tension in the stretching step is preferably 1100N or less, more preferably 1000N or less, and particularly preferably 700N or less.

In the case of stretching a long PVA film, the stretching direction is not particularly limited, and uniaxial stretching in the long direction, transverse uniaxial stretching, so-called oblique stretching, may be employed, and uniaxial stretching in the long direction is preferable from the viewpoint of obtaining a polarizing film excellent in optical performance. The uniaxial stretching in the longitudinal direction can be performed by using a stretching device having a plurality of rolls parallel to each other and changing the peripheral speed between the rolls. On the other hand, the transverse uniaxial stretching may be performed using a tenter type stretching machine.

The stretching step is preferably followed by a cleaning step. Unwanted chemicals, foreign substances, or adjustment of optical characteristics of the polarizing film on the surface of the PVA film may be removed through a cleaning process. The washing process may be performed by immersing the PVA film in a washing bath or dispersing a washing liquid on the PVA film. As the cleaning liquid, water may be used, and these may contain an auxiliary agent for iodide such as potassium iodide, and a boric acid crosslinking agent. From the viewpoint of optical characteristics of the polarizing film, an auxiliary agent containing an iodide such as potassium iodide is preferable, and the content thereof is preferably 0.1 to 10 mass%. In addition, from the viewpoint of improving the appearance of the polarizing film, it is preferable to further contain a boric acid crosslinking agent, and when the boric acid crosslinking agent is contained, the content of the boric acid crosslinking agent is preferably 0.1 mass% or more, more preferably 0.5 mass% or more. When the polarizing film contains a boric acid crosslinking agent, the content of the boric acid crosslinking agent in the cleaning liquid is preferably 10 mass% or less, more preferably 5 mass% or less.

The temperature of the cleaning step is usually 10℃or higher, preferably 15℃or higher, and particularly preferably 20℃or higher. The temperature of the cleaning step is usually 50℃or lower, preferably 40℃or lower, and particularly preferably 35℃or lower. When the temperature of the cleaning step exceeds 50 ℃, the polarizing film may be wrinkled, and the appearance of the polarizing film may be deteriorated, which is not preferable. On the other hand, when the temperature of the cleaning step is less than 20 ℃, it is not preferable from the viewpoint of economy. The temperature of the cleaning step refers to the temperature of the cleaning liquid. The stretching ratio in the cleaning step is preferably 1.3 times or less, more preferably 1.2 times or less, and even more preferably less than 1.1 times.

However, in the crosslinking step, the stretching step, and the cleaning step, the PVA film may be immersed in an aqueous solution containing a boric acid crosslinking agent and stretched, and among these steps, the step having the highest stretching ratio among the steps may be defined as a stretching step, the step performed before the stretching step may be defined as a crosslinking step, and the step performed after the stretching step may be defined as a cleaning step. In the cleaning step, the PVA film is generally impregnated with an aqueous solution having a lower boric acid concentration than in the stretching step.

The method of the drying step is not particularly limited, and the drying temperature in the drying step is preferably 60℃or higher, more preferably 70℃or higher, and particularly preferably 75℃or higher. The drying temperature is preferably 100℃or less, more preferably 90℃or less, particularly preferably 85℃or less. By drying at a temperature within the above range, the necking phenomenon in the drying process can be easily and appropriately controlled. That is, when the drying temperature is less than 60 ℃, the following tends to occur: the necking phenomenon in the drying step is insufficient, and the residual stress due to the orientation relaxation and crystallization of PVA is not sufficiently released, so that it is difficult to obtain a polarizing film having a small shrinkage stress. On the other hand, when the drying temperature exceeds 100 ℃, there is a tendency that: in the drying step, a necking phenomenon tends to occur excessively, and sometimes even the necessary iodine dye is decomposed, and it is difficult to obtain a polarizing film excellent in polarizing performance. In addition, there is a case where the polarizing film changes red, and the yield of the polarizing film is lowered, and the productivity tends to be lowered.

The drying time in the drying step is not particularly limited, but is preferably 10 seconds or more, more preferably 25 seconds or more, and particularly preferably 40 seconds or more. The drying time in the drying step is preferably 120 seconds or less, more preferably 110 seconds or less, and particularly preferably 95 seconds or less. By setting the drying time to be within this range, the necking phenomenon in the drying process can be easily and appropriately controlled.

The stretching ratio in the drying step is preferably 1.3 times or less, more preferably 1.2 times or less, and still more preferably less than 1.1 times, from the viewpoint of easy obtaining of a polarizing film having a small shrinkage stress. In order not to hinder the release of residual stress, it is also preferable that the stretching is not substantially performed in the drying step.

The drying tension in the drying process can be obtained by measuring the tension carried between adjacent rollers with tension rollers provided between the adjacent rollers in the drying process. The drying tension in the drying step is preferably 100N or more, more preferably 200N or more, and particularly preferably 260N or more. The drying tension in the drying step is preferably 600N or less, more preferably 500N or less, and particularly preferably 400N or less.

By performing the heat treatment after the drying step, a polarizing film excellent in dimensional stability can be further obtained. Here, the heat treatment means: and a treatment for further heating the polarizing film having a moisture content of 5% or less after the drying treatment to improve the dimensional stability of the polarizing film. The heat treatment conditions are not particularly limited, and the heat treatment is preferably performed at 60℃or higher, particularly preferably at 70℃or higher. The heat treatment is preferably carried out at 150℃or less, particularly preferably at 100℃or less. When the heat treatment is performed at a temperature lower than 60 ℃, the dimensional stabilization effect by the heat treatment tends to be insufficient. When the heat treatment is performed at a temperature higher than 150 ℃, there is a case where the polarizing film is strongly red-changed. The time for performing the heat treatment is preferably 5 seconds or more, more preferably 15 seconds or more. The time for performing the heat treatment is preferably 100 seconds or less, more preferably 60 seconds or less. If the heat treatment is performed for a period of time shorter than 5 seconds, the dimensional stabilization effect by the heat treatment may not be sufficiently obtained. If the heat treatment is performed for a period of time longer than 100 seconds, the polarizing film may be changed to red.

< polarizing film >

The polarization degree of the polarization film produced by the production method of the present invention is preferably 99.963% or more when the single transmittance is 44%. If the degree of polarization is less than 99.963% when the monomer transmittance is 44%, there is a possibility that a high-quality LCD panel cannot be obtained. The degree of polarization at 44% transmittance of the monomer is preferably 99.965% or more, more preferably 99.967% or more, particularly preferably 99.970% or more, and further preferably 99.975% or more. Examples of the method for setting the degree of polarization to 99.963% or more when the single body transmittance of the polarizing film is 44% include a method for setting the total necking rate (a) to 57.5% or more and 61.0% or less, setting the necking rate (B) to 31.0% or more and 38.0% or less, and setting the necking rate (C) to 16.5% or less. In this case, the boric acid concentration of the aqueous solution in the stretching step is preferably 1.0 mass% or more and 3.0 mass% or less. In this case, the total stretch ratio is preferably set to 5.5 times or more.

The shrinkage stress of the polarizing film produced by the production method of the present invention is preferably 100N/mm ² Hereinafter, more preferably 90N/mm ² Hereinafter, it is particularly preferably 85N/mm ² The following is given. As a result of setting the shrinkage stress of the polarizing film to 100N/mm ² The following methods include, for example, a method in which the necking rate (B) is 38.0% or less and the necking rate (C) is 9.8% or more. In this case, the boric acid concentration of the aqueous solution in the stretching step is preferably 3.0 mass% or less. In this case, the total stretch ratio is preferably 7.4 times or less.

The polarizing film produced by the production method of the present invention is usually used as a polarizing plate by adhering optically transparent protective films having mechanical strength to both sides or one side thereof. As the protective film, a cellulose Triacetate (TAC) film, a Cellulose Acetate Butyrate (CAB) film, an acrylic film, a polyester film, or the like is used. Examples of the adhesive used for adhesion include PVA-based adhesives and UV-curable adhesives.

The polarizing plate obtained in the above-described manner may be attached to a retardation film, a viewing angle improving film, a brightness enhancement film, or the like. In addition, the polarizing plate may be applied with an acrylic adhesive or the like, and then attached to a glass substrate to be used as a member of an LCD.

Examples

The present invention will be specifically described with reference to the following examples, but the present invention is not limited to these examples at all. The respective evaluation methods used in the following examples and comparative examples are shown below.

[ calculation of the Total necking Rate (A) ]

In the following examples or comparative examples, the length X1 (m) of the width of the unstretched PVA film used for producing the polarizing film and the length X2 (m) of the width of the PVA film after the drying step were measured. The total necking rate (a) was calculated by substituting the obtained measurement value into the following formula (1).

Total necking rate (a) = { (X1-X2)/X1 } ×100 (1)

[ calculation of the necking Rate (B) in the stretching step ]

In the following examples or comparative examples, the length Y1 (m) of the width of the PVA film after the crosslinking step and before the stretching step and the length Y2 (m) of the width of the PVA film after the stretching step and before the cleaning step were measured. The necking rate (B) in the stretching step was calculated by substituting the obtained measurement value into the following formula (2).

Necking rate (B) = { (Y1-Y2)/Y1 } ×100 (2)

[ calculation of the necking Rate (C) in the drying Process ]

In the following examples or comparative examples, the length Z1 (m) of the width of the PVA film after the washing step and before the drying step and the length X2 (m) of the width of the PVA film after the drying step were measured. The necking rate (C) in the drying step was calculated by substituting the obtained measurement value into the following formula (3).

Necking rate (C) = { (Z1-X2)/Z1 } ×100 (3)

[ calculation of the necking Rate (D) until the stretching step ]

In the following examples or comparative examples, the length X1 (m) of the width of the unstretched PVA film used for producing the polarizing film and the length Y2 (m) of the width of the PVA film before and after the washing step were measured. The necking rate (D) until the stretching step was calculated by substituting the obtained measurement value into the following formula (4).

Necking rate (D) = { (X1-Y2)/X1 } ×100 (4)

The difference between the total necking rate (A) and the necking rate (D) was calculated ((A) - (D)), and the difference was defined as a necking rate difference (W). The ratio ((W)/(a)) of the necking rate (W) to the total necking rate (a) was calculated by dividing the necking rate (W) by the total necking rate (a).

[ optical Properties of polarizing film ]

In the following examples and comparative examples, rectangular samples of 4cm in the longitudinal direction by 2cm in the longitudinal direction of the polarizing film were taken from the central portions in the width direction and the longitudinal direction of the obtained polarizing film, and the parallel transmittance and the cross transmittance of the polarizing film were measured using a spectrophotometer V-7100 (manufactured by japan spectroscopy) with an integrating sphere and an automatic polarizing film measuring device VAP-7070S (manufactured by japan spectroscopy) provided with a Glan Taylor polarizing element. Here, the measurement wavelength range is set to 380 to 780nm, the transmittance when the vibration direction of polarized light incident to the polarizing film through the Glan Taylor polarizer is parallel to the transmission axis of the polarizing film is set to the parallel transmittance, and the case perpendicular to the transmission axis of the polarizing film is set to the cross transmittance. Thereafter, using the "polarizing film evaluation program" (manufactured by japan spectroscopic corporation), the visual sensitivity correction in the visible light range of the C light source and the 2 ° field of view was performed using the parallel transmittance and the cross transmittance in accordance with JIS Z8722 (measurement method of object color), and the individual transmittance and the polarization degree of the polarizing film were calculated to obtain the values of both as the optical characteristics of the polarizing film. More specifically, the degree of polarization was calculated when the monomer transmittance was 44%.

[ shrinkage stress of polarizing film ]

In the following examples and comparative examples, the shrinkage stress of the polarizing film was measured using an Autograph AG-X with a constant temperature bath and a camera stretcher TRViewX120S manufactured by Shimadzu corporation. In the measurement, a polarizing film was used which was conditioned at 20℃and 20% RH for 18 hours. After the autoclave of Autograph AG-X was set at 20 ℃, the polarizing film (15 cm in the longitudinal direction and 1.5cm in the width direction) was attached to a jig (5 cm in the jig interval), and the temperature rise of the autoclave to 80℃was started at the same time as the stretching was started. The polarizing film was stretched at a speed of 1mm/min, and the stretching was stopped at a point when the tension reached 2N, and the tension was measured in this state until 4 hours later. At this time, the distance between the jigs changes due to thermal expansionTherefore, the reticle label is attached to the jig, and the distance between the jigs is measured by using the camera stretcher TRViewX120S so that the amount of movement of the reticle label attached to the jigs can be corrected exactly. The value obtained by subtracting the initial tension 2N from the measured value of the tension after 4 hours was used as the shrinkage force of the polarizing film, and the value obtained by dividing the value by the cross-sectional area of the polarizing film was defined as the shrinkage stress (N/mm ² )。

[ calculation of the total boron content in the polarizing film ]

In the following examples and comparative examples, the obtained polarizing film was subjected to humidity control at 23℃and 50% RH for 16 hours, and after measuring the mass [ E (g) ] of the polarizing film, the polarizing film was dissolved in 20mL of distilled water so that the polarizing film became 0.005 mass%. The mass [ F (g) ] of the aqueous solution in which the polarizing film was dissolved was measured as a measurement sample. The boron concentration [ G (ppm) ] of the measurement sample was measured using a multifunctional ICP emission analyzer (ICP) manufactured by Shimadzu corporation. Thereafter, the total boron content (mass%) in the polarizing film was defined as a value calculated by taking the measured value into the following formula.

Total content (mass%) of boron element in polarizing film= [ (g×10) ^-6 ×F)/E]×100

[ stretching tension in stretching Process ]

In the following examples and comparative examples, regarding the tension in the stretching process, the tension carried between the adjacent rollers was measured in the stretching process by using the tension roller provided between the adjacent rollers. When 3 or more rolls are used, the maximum tensile force thereof is used.

[ drying tension in drying Process ]

In the following examples and comparative examples, regarding the drying tension in the drying process, the tension carried between adjacent rollers was measured with a tension roller provided between the adjacent rollers in the drying process. When 3 or more rolls are used, the maximum drying tension thereof is used.

Example 1

A film was cast using a film-forming stock solution containing 100 parts by mass of PVA (saponified product of a vinyl acetate polymer, 2400 degrees of polymerization, 99.9 mol% of saponification degree), 10 parts by mass of glycerin as a plasticizer, 0.1 part by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, and water, to obtain a roll of PVA film having a thickness of 45. Mu.m. The PVA film is sequentially subjected to a swelling process, a dyeing process, a crosslinking process, a stretching process, a cleaning process, and a drying process, thereby producing a polarizing film. The length X1 (m) of the width of the unstretched PVA film used for producing the polarizing film was 0.65m.

Specifically, the polarizing film was manufactured as follows. First, in the swelling step, the PVA film is uniaxially stretched (first-stage stretching) in the longitudinal direction (MD direction) to 2 times the original length of the PVA film in a period of 90 seconds immersed in water at 25 ℃. Next, in the dyeing step, the PVA film was uniaxially stretched (second-stage stretching) to 2.4 times the original length of the PVA film in the longitudinal direction (MD direction) during the period of immersing for 163 seconds in an aqueous solution (weight ratio of iodine to potassium iodide is 1:23) containing 0.093 mass% of iodine and 2.14 mass% of potassium iodide at a temperature of 32 ℃. Next, in the crosslinking step, the PVA film was uniaxially stretched (third-stage stretched) along the longitudinal direction (MD direction) to 3 times the original length of the PVA film in a period of 135 seconds immersed in an aqueous solution containing boric acid at a concentration of 2.6 mass% and a temperature of 32 ℃. Next, in the stretching step, the PVA film was uniaxially stretched (stretched in the fourth stage) to 6.8 times the original length of the PVA film in the longitudinal direction (MD direction) during the period of immersion in an aqueous solution containing boric acid at a concentration of 1.5 mass% and potassium iodide at a concentration of 5 mass% at 56 ℃. The maximum tensile force in the stretching step was 679N. Next, in the cleaning step, the film was cleaned by immersing in an aqueous solution containing boric acid at a concentration of 1.5 mass% and potassium iodide at a concentration of 5.4 mass% for 10 seconds at a temperature of 22 ℃. Then, in the drying step, the polarizing film having a thickness of 15.6 μm was produced by drying for 90 seconds in a dryer at 80 ℃. The maximum drying tension in the drying step was 382.5N.

Using the resulting polarizing film, the monomer transmittance, the degree of polarization, the shrinkage stress, and the total boron element amount in the polarizing film were measured using the methods described above. The evaluation results are shown in table 1, and the relationship between the shrinkage stress and the polarization degree is shown in fig. 1. Fig. 2 shows the relationship between the maximum tensile force and the shrinkage stress.

Examples 2 to 5 and comparative examples 1 to 9

The stretching tension and drying tension were measured in the same manner as in example 1, and a polarizing film was produced in the same manner as in example 1, except that the concentration of the aqueous boric acid solution, the total stretching ratio, and the temperature of the aqueous boric acid solution in the stretching step were changed as in table 1, and the drying temperature and drying time in the drying step were changed as in table 1, and the iodine concentration in the dye tank was appropriately adjusted so that the monomer transmittance of the polarizing film became 44%. In examples 2 to 5 and comparative examples 1 to 9, the stretch ratio from the first stage stretch to the third stage stretch was set to be the same as that of example 1, and the fourth stage stretch was changed as compared with example 1, so that the total stretch ratio was adjusted to the values shown in table 1. Thereafter, the transmittance of the monomer, the degree of polarization, the shrinkage stress, and the total boron element amount in the polarizing film were evaluated by the methods described above. The results are shown in table 1, and the relationship between the shrinkage stress and the polarization degree is shown in fig. 1. Fig. 2 shows the relationship between the maximum tensile force and the shrinkage stress.

TABLE 1

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As shown in fig. 1, for examples 1 to 5 satisfying the regulations of the present invention, the polarizing films have small shrinkage force at high temperature and excellent optical properties.

As shown in fig. 2, in examples 1 to 3 in which the total amount of boron in the polarizing film was small, the relationship between the maximum tensile force and the shrinkage stress significantly changed to the low shrinkage stress side.

Claims

1. A method of manufacturing a polarizing film, comprising: a dyeing step of dyeing the polyvinyl alcohol film with a dichroic dye; a stretching step of uniaxially stretching the dyed polyvinyl alcohol film in an aqueous solution containing boric acid; a drying step of drying the stretched polyvinyl alcohol film,

the aqueous solution of the stretching step has a boric acid concentration of 1 to 3% by mass, a total stretching ratio of 5.5 to 7.4 times, a total necking rate (A) represented by the following formula (1) of 57.5 to 61.0%, a necking rate (B) represented by the following formula (2) of 31.0 to 38.0% in the stretching step, a necking rate (C) represented by the following formula (3) of 9.8 to 16.5% in the drying step,

total necking rate (a) = { (X1-X2)/X1 } ×100 (1)

Necking rate (B) = { (Y1-Y2)/Y1 } ×100 (2)

Necking rate (C) = { (Z1-X2)/Z1 } ×100 (3)

X1 represents the length (m) of the width of the polyvinyl alcohol film before the dyeing step; x2 represents the length (m) of the width of the polyvinyl alcohol film after the drying step; y1 represents the length (m) of the width of the polyvinyl alcohol film before the stretching step; y2 represents the length (m) of the width of the polyvinyl alcohol film after the stretching step; z1 represents the length (m) of the width of the polyvinyl alcohol film before the drying step.

2. The method for producing a polarizing film according to claim 1, wherein the necking rate (D) represented by the following formula (4) up to the stretching step is 46.0 to 54.0%,

necking rate (D) = { (X1-Y2)/X1 } ×100 (4).

3. The method for producing a polarizing film according to claim 2, wherein the difference ((a) - (D)), i.e., the necking rate difference (W) between the total necking rate (a) and the necking rate (D) is 8.0 to 11.0%.

4. The method for producing a polarizing film according to claim 3, wherein the ratio ((W)/(a)) of the necking rate (W) to the total necking rate (a) is 0.14 to 0.19.

5. The method for producing a polarizing film according to any one of claims 1 to 4, wherein the total content of boron elements in the polarizing film is 2.0 to 4.0 mass%.

6. The method for producing a polarizing film according to any one of claims 1 to 5, wherein the stretching temperature in the stretching step is 53 ℃ to 70 ℃.

7. The method for producing a polarizing film according to any one of claims 1 to 6, wherein a drying temperature in the drying step is 60 ℃ to 100 ℃.

8. The method for producing a polarizing film according to any one of claims 1 to 7, wherein the polarizing film has a degree of polarization of 99.963% or more and a shrinkage stress of 100N/mm when the single transmittance of the polarizing film is 44% ² The following is given.

9. A polarizing film formed of a polyvinyl alcohol film, having a degree of polarization of 99.963% or more at a monomer transmittance of 44% and a shrinkage stress of 100N/mm ² The following is given.

10. The polarizing film according to claim 9, wherein the total content of boron elements in the polarizing film is 4.0 mass% or less.