CN111095052A

CN111095052A - Polarizing plate, polarizing plate roll, and method for producing polarizing film

Info

Publication number: CN111095052A
Application number: CN201880059533.3A
Authority: CN
Inventors: 后藤周作; 岛津亮; 高永幸佑
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2017-09-13
Filing date: 2018-09-06
Publication date: 2020-05-01
Also published as: TW201915062A; KR20200054184A; TWI801395B; WO2019054270A1

Abstract

The invention provides a polarizing plate having excellent optical characteristics and suppressed variation in optical characteristics. The polarizing plate of the present invention comprises a polarizing film and a protective layer disposed on at least one side of the polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, and the polarizing plate is disposed at 50cm²The difference between the maximum value and the minimum value of the single transmittance in the region is 0.2% or less. Another polarizing plate of the present invention comprises a polarizing film and a protective layer disposed on at least one side of the polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a cell transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, and the polarizing plate has a width of 1000mm or more, and a difference between a maximum value and a minimum value of the cell transmittance at a position along the width direction is 0.3% or less.

Description

Polarizing plate, polarizing plate roll, and method for producing polarizing film

Technical Field

The invention relates to a polarizing plate, a polarizing plate roll, and a method for manufacturing a polarizing film.

Background

In a liquid crystal display device, which is a typical image display device, polarizing films are disposed on both sides of a liquid crystal cell due to its image forming system. In addition, along with the spread of thin displays, a display (OLED) having an organic EL panel mounted thereon and a display (QLED) using a display panel using an inorganic light emitting material such as quantum dots have been proposed. These panels have a highly reflective metal layer, and are prone to problems such as reflection of external light and reflection of background. Therefore, it is known to prevent these problems by providing a circularly polarizing plate having a polarizing film and a λ/4 plate on the viewing side. As a method for producing a polarizing film, for example, the following methods are proposed: a laminate having a resin substrate and a polyvinyl alcohol (PVA) -based resin layer is stretched, and then subjected to a dyeing treatment to obtain a polarizing film on the resin substrate (for example, patent document 1). Since a polarizing film having a small thickness is obtained by such a method, attention has been paid to the reduction in thickness of an image display device in recent years. However, the optical characteristics of the conventional thin polarizing film as described above are insufficient, and further improvement of the optical characteristics of the thin polarizing film is required.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2001-343521

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above conventional problems, and a main object thereof is to provide a polarizing plate, a polarizing sheet roll, and a method for manufacturing a polarizing film, which have excellent optical characteristics and in which variations in optical characteristics are suppressed.

Means for solving the problems

The polarizing plate of the present invention comprises a polarizing film and a protective layer disposed on at least one side of the polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, and the polarizing plate is disposed at 50cm²The difference between the maximum value and the minimum value of the single transmittance in the region is 0.2% or less.

Another polarizing plate of the present invention comprises a polarizing film and a protective layer disposed on at least one side of the polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a cell transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, the polarizing film has a width of 1000mm or more, and a difference between a maximum value and a minimum value of the cell transmittance at a position along the width direction is 0.3% or less.

In one embodiment, the polarizing film has a cell transmittance of 44.5% or less and a degree of polarization of 99.95% or less.

According to another aspect of the present invention, there is provided a polarizer roll in which the polarizer is wound in a roll shape.

According to still another aspect of the present invention, there is provided a method for manufacturing a polarizing film having a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, the method comprising: forming a polyvinyl alcohol resin layer containing an iodide or sodium chloride and a polyvinyl alcohol resin on one side of a long thermoplastic resin base material to prepare a laminate; and sequentially subjecting the laminate to an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment, wherein in the drying shrinkage treatment, the laminate is heated while being conveyed in the longitudinal direction so as to be shrunk by 2% or more in the width direction.

In one embodiment, the content of the iodide or sodium chloride in the polyvinyl alcohol resin layer is 5 to 20 parts by weight based on 100 parts by weight of the polyvinyl alcohol resin.

In one embodiment, the stretching ratio in the auxiliary stretching treatment in the gas atmosphere is 2.0 times or more.

In one embodiment, the drying shrinkage treatment step is a step of heating with a heating roller.

In one embodiment, the temperature of the heating roll is 60 to 120 ℃, and the shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment is 2% or more.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a polarizing plate having a polarizing film with a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, which has excellent optical characteristics and in which variation in optical characteristics is suppressed, can be provided.

Drawings

Fig. 1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention.

Fig. 2 is a schematic diagram showing an example of the drying shrinkage process using a heating roller.

Fig. 3 is a graph showing the optical characteristics of the polarizing plates obtained in examples and comparative examples.

Description of the symbols

10 polarizing film

20 st protective layer

30 nd 2 protective layer

100 polarizing plate

Detailed Description

Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

A. Polarizing plate

Fig. 1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. The polarizing plate 100 includes a polarizing film 10, a 1 st protective layer 20 disposed on one side of the polarizing film 10, and a 2 nd protective layer 30 disposed on the other side of the polarizing film 10. The polarizing film has a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more. One of the 1 st protective layer 20 and the 2 nd protective layer 30 may be omitted. One of the 1 st protective layer and the 2 nd protective layer may be a resin base material (described later) used for the production of the polarizing film.

The polarizing plate may be in a long shape or in a single sheet shape. When the polarizing plate is in a long form, it is preferably wound in a roll form to form a polarizing plate roll. The polarizing plate has excellent optical characteristics and has small variations in optical characteristics. In one embodiment, the width of the polarizing plate is 1000mm or more, and the difference (D1) between the maximum value and the minimum value of the single transmittance at a position along the width direction is 0.3% or less. The upper limit of D1 is preferably 0.29%, more preferably 0.28%. The smaller the D1, the more preferable the lower limit is, for example, 0.01%. When D1 is within the above range, a polarizing plate having excellent optical characteristics can be industrially produced. In another embodiment, the polarizer is at 50cm²The difference (D2) between the maximum value and the minimum value of the single transmittance in the region is 0.2% or less. The upper limit of D2 is preferably 0.15%, more preferably 0.1%. The smaller the D2, the more preferable the lower limit is, for example, 0.01%. When D2 is within the above range, it is possible to suppress luminance unevenness on a display screen when the polarizing plate is used in an image display device.

A-1 polarizing film

As described above, the polarizing film has a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more. Generally, the monomer transmittance and the absorbance have a trade-off relationship with each other, and if the monomer transmittance is increased, the absorbance decreases, and if the absorbance is increased, the monomer transmittance decreases. Therefore, it has been difficult to practically use a thin polarizing film satisfying optical characteristics of a single transmittance of 44.0% or more and a degree of polarization of 99.50% or more. As described above, the polarizing film according to one embodiment of the present invention has excellent optical characteristics such as a single transmittance of 44.0% or more and a degree of polarization of 99.50% or more. Further, by using the polarizing film of the present embodiment, a polarizing plate in which variation in optical characteristics is suppressed can be realized. One of the achievements of the present invention is to realize such a thin polarizing film (polarizing plate). Such a polarizing film (polarizing plate) can be used for an image display device, and can be suitably used for a circularly polarizing plate for an organic EL display device in particular.

The thickness of the polarizing film is preferably 1 μm to 8 μm, more preferably 1 μm to 7 μm, and still more preferably 2 μm to 5 μm.

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380nm to 780 nm. The single transmittance of the polarizing film is preferably 44.5% or less. The polarization degree of the polarizing film is preferably 99.70% or more, and more preferably 99.80% or less. On the other hand, the upper limit of the degree of polarization is preferably 99.95%. The monomer transmittance is typically a Y value obtained by measuring with an ultraviolet-visible spectrophotometer and correcting the visibility. The degree of polarization is typically determined from the parallel transmittance Tp and the orthogonal transmittance Tc obtained by measuring with an ultraviolet-visible spectrophotometer and correcting the visibility, and is obtained by the following equation.

Degree of polarization (%) { (Tp-Tc)/(Tp + Tc) }^1/2×100

In one embodiment, the transmittance of a thin polarizing film having a thickness of 8 μm or less is typically measured using an ultraviolet-visible spectrophotometer with a laminate of a polarizing film (surface refractive index: 1.53) and a protective film (refractive index: 1.50) as a measurement target. The reflectance at the interface of each layer changes depending on the refractive index of the surface of the polarizing film and/or the refractive index of the surface in contact with the air interface of the protective film, and as a result, the light may be transmittedThe measured value of the rate was changed. Therefore, for example, in the case of using a protective film having a refractive index of not 1.50, the measured value of the transmittance can be corrected based on the refractive index of the surface in contact with the air interface of the protective film. Specifically, the correction value C of the transmittance is the reflectance R of polarized light parallel to the transmission axis at the interface between the protective film and the air layer₁(transmission axis reflectance) is represented by the following equation.

C＝R₁－R₀

R₀＝((1.50－1)²/(1.50+1)²)×(T₁/100)

R₁＝((n₁－1)²/(n₁+1)²)×(T₁/100)

Wherein R is₀Is a transmission axis reflectance, n, in the case of using a protective film having a refractive index of 1.50₁Is the refractive index, T, of the protective film used₁Is the transmittance of the polarizing film. For example, when a base material (a cycloolefin film, a film with a hard coat layer, or the like) having a surface refractive index of 1.53 is used as the protective film, the correction amount C is about 0.2%. In this case, 0.2% is added to the transmittance obtained by the measurement, and the transmittance can be converted to the transmittance when a protective film having a surface refractive index of 1.50 is used. The transmittance T of the polarizing film was calculated based on the above formula₁The amount of change in the correction value C at 2% change is 0.03% or less, and the influence of the transmittance of the polarizing film on the value of the correction value C is limited. In the case where the protective film has absorption other than surface reflection, appropriate correction can be made in accordance with the absorption amount.

As the polarizing film, any suitable polarizing film may be used. The polarizing film can be typically produced using a laminate of two or more layers.

Specific examples of the polarizing film obtained using the laminate include a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed on the resin substrate by coating. A polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed on the resin substrate by coating can be produced by the following method: for example, a laminate of a resin base and a PVA type resin layer is obtained by applying a PVA type resin solution to a resin base and drying the solution to form a PVA type resin layer on the resin base; the laminate was stretched and dyed to obtain a polarizing film from the PVA type resin layer. In the present embodiment, the stretching typically includes immersing the laminate in an aqueous boric acid solution to perform stretching. Further, the stretching may further include stretching the laminate in a gas atmosphere at a high temperature (for example, 95 ℃ or higher) before the stretching in the aqueous boric acid solution, as necessary. The obtained resin substrate/polarizing film laminate can be used as it is (that is, the resin substrate can be used as a protective layer for a polarizing film), and the resin substrate can be peeled off from the resin substrate/polarizing film laminate and any suitable protective layer according to the purpose can be laminated on the peeled surface. Details of such a method for producing a polarizing film are described in, for example, japanese patent laid-open No. 2012-73580. The entire disclosure of this publication is incorporated herein by reference.

The method for manufacturing a polarizing film of the present invention comprises: forming a polyvinyl alcohol resin layer containing a halide and a polyvinyl alcohol resin on one side of a long thermoplastic resin base material to form a laminate; and subjecting the laminate to an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment in this order, wherein the laminate is heated while being conveyed in the longitudinal direction so as to be shrunk by 2% or more in the width direction. Thus, a polarizing film having a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more can be obtained, which has excellent optical characteristics and in which variation in optical characteristics is suppressed. That is, by introducing the auxiliary stretching, even when the PVA is coated on the thermoplastic resin, the crystallinity of the PVA can be improved, and high optical characteristics can be realized. Further, by simultaneously improving the orientation of the PVA in advance, it is possible to prevent problems such as degradation of the orientation and dissolution of the PVA when immersed in water in the subsequent dyeing step and stretching step, and to realize high optical characteristics. In addition, when the PVA-based resin layer is immersed in a liquid, disturbance of orientation of polyvinyl alcohol molecules and reduction of orientation can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained in a treatment step of immersing the laminate in a liquid, such as a dyeing treatment or a stretching treatment in an aqueous solution. Further, the optical characteristics can be improved by shrinking the laminate in the width direction by the drying shrinkage treatment.

A-2 protective layer

The 1 st protective layer and the 2 nd protective layer may be formed of any suitable film that can be used as a protective layer of a polarizing film. Specific examples of the material to be the main component of the film include cellulose resins such as Triacetylcellulose (TAC), polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth) acrylic acids, and transparent resins such as acetates. Further, there may be mentioned a heat-curable resin such as (meth) acrylic resins, carbamates, (meth) acrylic carbamates, epoxy resins, silicone resins, and ultraviolet-curable resins. In addition to these, for example, a glassy polymer such as a siloxane polymer can be cited. Further, the polymer film described in Japanese patent application laid-open No. 2001-343529 (WO01/37007) may be used. As a material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in a side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in a side chain can be used, and examples thereof include: a resin composition having an alternating copolymer of isobutylene and N-methylmaleimide, and an acrylonitrile-styrene copolymer. The polymer film may be, for example, an extrusion molded product of the above resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the side opposite to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 5 μm to 80 μm, and further preferably 10 μm to 60 μm. When the surface treatment is performed, the thickness of the outer protective layer is a thickness including the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (inner protective layer) disposed on the display panel side is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and still more preferably 10 μm to 60 μm. In one embodiment, the inner protective layer is a phase difference layer having any suitable phase difference value. In this case, the in-plane retardation Re (550) of the retardation layer is, for example, 110nm to 150 nm. "Re (550)" is an in-plane retardation measured at 23 ℃ with light of wavelength 550nm, and can be represented by the formula: re ═ x-ny) × d. Where "nx" is a refractive index in a direction in which the in-plane refractive index is maximized (i.e., the slow axis direction), "ny" is a refractive index in a direction orthogonal to the slow axis in the plane (i.e., the fast axis direction), "nz" is a refractive index in the thickness direction, and "d" is the thickness (nm) of the layer (film).

B. Method for producing polarizing film

A method for manufacturing a polarizing film according to an embodiment of the present invention includes: forming a polyvinyl alcohol resin layer (PVA-based resin layer) containing a halide and a polyvinyl alcohol resin (PVA-based resin) on one side of a long thermoplastic resin base material to form a laminate; and subjecting the laminate to an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment in this order, wherein the laminate is heated while being conveyed in the longitudinal direction so as to be shrunk by 2% or more in the width direction. The content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably carried out using a heated roll, and the temperature of the heated roll is preferably 60 to 120 ℃. The shrinkage rate of the laminate subjected to the drying shrinkage treatment in the width direction is preferably 2% or more. According to such a production method, the polarizing film described in the above item a can be obtained. In particular, a polarizing film having excellent optical characteristics (typically, monomer transmittance and polarization degree) and suppressed variations in optical characteristics can be obtained by producing a laminate including a halide-containing PVA-based resin layer, subjecting the laminate to stretching in multiple stages including auxiliary stretching in a gas atmosphere and stretching in an aqueous solution, and heating the stretched laminate with a heating roller. Specifically, by using a heating roller in the drying and shrinking process, the entire laminate can be uniformly shrunk while being conveyed. Thus, not only the optical characteristics of the obtained polarizing film can be improved, but also a polarizing film excellent in optical characteristics can be stably produced, and variations in optical characteristics (particularly, cell transmittance) of the polarizing film can be suppressed.

B-1 preparation of laminate

As a method for producing a laminate of the thermoplastic resin substrate and the PVA-based resin layer, any appropriate method can be adopted. Preferably, the PVA-based resin layer is formed on the thermoplastic resin substrate by applying a coating solution containing a halide and a PVA-based resin to the surface of the thermoplastic resin substrate and drying the coating solution. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin.

As a method for applying the coating liquid, any appropriate method can be adopted. Examples thereof include: roll coating, spin coating, wire-wound bar coating, dip coating, die coating, flow coating, spray coating, blade coating (doctor blade coating, etc.), and the like. The coating/drying temperature of the coating liquid is preferably 50 ℃ or higher.

The thickness of the PVA based resin layer is preferably 3 to 40 μm, more preferably 3 to 20 μm.

Before the PVA-based resin layer is formed, the thermoplastic resin substrate may be subjected to a surface treatment (for example, corona treatment), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved.

B-1-1. thermoplastic resin base Material

The thickness of the thermoplastic resin substrate is preferably 20 to 300. mu.m, more preferably 50 to 200. mu.m. If the thickness is less than 20 μm, the PVA-based resin layer may be difficult to form. When the thickness exceeds 300 μm, for example, in a stretching treatment in an aqueous solution described later, it takes a long time for the thermoplastic resin substrate to absorb water, and an excessive load may be required for stretching.

The water absorption of the thermoplastic resin substrate is preferably 0.2% or more, and more preferably 0.3% or more. The thermoplastic resin substrate absorbs water, and the water acts as a plasticizer to plasticize the thermoplastic resin substrate. As a result, the tensile stress can be greatly reduced, and the stretching can be performed at a high magnification. On the other hand, the water absorption of the thermoplastic resin substrate is preferably 3.0% or less, and more preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent a problem that the dimensional stability of the thermoplastic resin substrate is significantly lowered at the time of production, and the appearance of the obtained polarizing film is deteriorated. Further, the substrate can be prevented from being broken and the PVA based resin layer can be prevented from being peeled off from the thermoplastic resin substrate when stretched in an aqueous solution. The water absorption of the thermoplastic resin base material can be adjusted by, for example, introducing a modifying group into the constituent material. The water absorption is a value determined in accordance with JIS K7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 120 ℃ or lower. By using such a thermoplastic resin substrate, the crystallization of the PVA type resin layer can be suppressed, and the stretchability of the laminate can be sufficiently ensured. In addition, when plasticizing of the thermoplastic resin substrate with water and stretching in an aqueous solution are considered to be favorable, the temperature is more preferably 100 ℃ or lower, and still more preferably 90 ℃ or lower. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60 ℃ or higher. By using such a thermoplastic resin substrate, when the coating liquid containing the PVA-based resin is applied and dried, troubles such as deformation (for example, generation of unevenness, looseness, wrinkles, and the like) of the thermoplastic resin substrate can be prevented, and a laminate can be produced satisfactorily. Further, the PVA-based resin layer can be favorably stretched at an appropriate temperature (for example, about 60 ℃). The glass transition temperature of the thermoplastic resin substrate can be adjusted by, for example, using a crystallized material in which a modifying group is introduced into a constituent material and heating the crystallized material. The glass transition temperature (Tg) is a value determined in accordance with JIS K7121.

As the constituent material of the thermoplastic resin substrate, any suitable thermoplastic resin can be used. Examples of the thermoplastic resin include: ester resins such as polyethylene terephthalate resins, cycloolefin resins such as norbornene resins, olefin resins such as polypropylene resins, polyamide resins, polycarbonate resins, and copolymer resins thereof. Of these, norbornene-based resins and amorphous polyethylene terephthalate-based resins are preferable.

In one embodiment, an amorphous (noncrystalline) polyethylene terephthalate-based resin is preferably used. Among them, amorphous (less likely to crystallize) polyethylene terephthalate resins are particularly preferably used. Specific examples of the amorphous polyethylene terephthalate resin include copolymers further containing isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers further containing cyclohexanedimethanol and diethylene glycol as diols.

In a preferred embodiment, the thermoplastic resin substrate is composed of a polyethylene terephthalate resin having an isophthalic acid unit. This is because such a thermoplastic resin substrate is excellent in stretchability and can be inhibited from crystallizing during stretching. This is considered to be because the introduction of the isophthalic acid unit can impart a large curve to the main chain. The polyethylene terephthalate resin has a terephthalic acid unit and an ethylene glycol unit. The content ratio of the isophthalic acid unit is preferably 0.1 mol% or more, more preferably 1.0 mol% or more, based on the total of all the repeating units. This is because a thermoplastic resin substrate having excellent stretchability can be obtained. On the other hand, the content ratio of the isophthalic acid unit is preferably 20 mol% or less, more preferably 10 mol% or less, relative to the total of all the repeating units. By setting such a content ratio, the crystallinity can be improved favorably in the drying shrinkage treatment described later.

The thermoplastic resin substrate may be stretched in advance (before the PVA-based resin layer is formed). In one embodiment, the elongated thermoplastic resin base material may be stretched in the transverse direction. The transverse direction is preferably a direction perpendicular to the stretching direction of the laminate described later. In the present specification, "orthogonal" means that the two are substantially orthogonal to each other. The term "substantially orthogonal" includes the case of 90 ° ± 5.0 °, preferably 90 ° ± 3.0 °, and more preferably 90 ° ± 1.0 °.

The stretching temperature of the thermoplastic resin substrate is preferably from Tg-10 ℃ to Tg +50 ℃ relative to the glass transition temperature (Tg). The stretch ratio of the thermoplastic resin base material is preferably 1.5 to 3.0 times.

As the method for stretching the thermoplastic resin substrate, any suitable method can be adopted. Specifically, the fixed end stretching may be performed, and the free end stretching may be performed. The stretching method may be dry or wet. The stretching of the thermoplastic resin substrate may be performed in one stage or may be performed in multiple stages. In the case of performing in multiple stages, the above-mentioned stretching ratio is the product of the stretching ratios in the respective stages.

B-1-2 coating liquid

The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid may be typically a solution obtained by dissolving the halide and the PVA-based resin in a solvent. Examples of the solvent include: water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These may be used alone, or two or more kinds may be used in combination. Of these, water is preferred. The concentration of the PVA based resin in the solution is preferably 3 to 20 parts by weight based on 100 parts by weight of the solvent. When the resin concentration is such as this, a uniform coating film can be formed in close contact with the thermoplastic resin substrate. The content of the halide in the coating liquid is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin.

Additives may be added to the coating liquid. Examples of additives include: plasticizers, surfactants, and the like. Examples of the plasticizer include: polyhydric alcohols such as ethylene glycol and glycerin. Examples of the surfactant include: a nonionic surfactant. These additives are used for the purpose of further improving the uniformity, dyeability and stretchability of the PVA-based resin layer obtained.

As the PVA-based resin, any suitable resin can be used. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer are cited. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying an ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The degree of saponification can be determined in accordance with JIS K6726-1994. By using the PVA-based resin having such a saponification degree, a polarizing film having excellent durability can be obtained. If the degree of saponification is too high, gelation may occur.

The average polymerization degree of the PVA-based resin may be appropriately selected according to the purpose. The average polymerization degree is usually 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. The average polymerization degree can be determined in accordance with JIS K6726-.

As the halide, any suitable halide can be used. Examples thereof include iodide and sodium chloride. Examples of the iodide include potassium iodide, sodium iodide, and lithium iodide. Of these, potassium iodide is preferred.

The amount of the halide in the coating liquid is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin, and more preferably 10 to 15 parts by weight based on 100 parts by weight of the PVA-based resin. When the amount of the halide exceeds 20 parts by weight based on 100 parts by weight of the PVA-based resin, the halide may bleed out, and the polarizing film finally obtained may be clouded.

Generally, the orientation of polyvinyl alcohol molecules in a PVA type resin is improved by stretching the PVA type resin layer, but if the stretched PVA type resin layer is immersed in a liquid containing water, the orientation of polyvinyl alcohol molecules may be disturbed, and the orientation may be degraded. In particular, when a laminate of a thermoplastic resin and a PVA-based resin layer is stretched in an aqueous boric acid solution, the orientation degree tends to be significantly reduced when the laminate is stretched in an aqueous boric acid solution at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin. For example, while stretching of a PVA film monomer in an aqueous boric acid solution is generally performed at 60 ℃, stretching of a laminate of a-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a high temperature such as a temperature of about 70 ℃, and in this case, the orientation of PVA at the initial stage of stretching is reduced in a stage before it is raised by stretching in an aqueous solution. On the other hand, by preparing a laminate of a halide-containing PVA type resin layer and a thermoplastic resin substrate and stretching the laminate at a high temperature in air (auxiliary stretching) before stretching the laminate in an aqueous boric acid solution, crystallization of the PVA type resin in the PVA type resin layer of the laminate after the auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, disturbance of the orientation of the polyvinyl alcohol molecules and reduction in the orientation can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through a treatment step of immersing the laminate in a liquid, such as a dyeing treatment or a stretching treatment in an aqueous solution.

B-2. auxiliary stretching treatment in gas atmosphere

In particular, in order to obtain high optical characteristics, a 2-stage stretching method combining dry stretching (auxiliary stretching) and stretching in an aqueous boric acid solution is selected. By introducing the auxiliary stretching as in the 2-stage stretching, the thermoplastic resin substrate can be stretched while suppressing crystallization, the problem of the reduction in stretchability due to excessive crystallization of the thermoplastic resin substrate at the time of subsequent stretching in an aqueous boric acid solution can be solved, and the laminate can be stretched at a higher magnification. Further, when the PVA type resin is coated on the thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, the coating temperature needs to be lowered as compared with the case where the PVA type resin is coated on a general metal drum, and as a result, there is a problem that crystallization of the PVA type resin is relatively lowered and sufficient optical characteristics cannot be obtained. On the other hand, by introducing the auxiliary stretching, even when the PVA type resin is coated on the thermoplastic resin, the crystallinity of the PVA type resin can be improved, and high optical characteristics can be realized. Further, by simultaneously improving the orientation of the PVA-based resin in advance, when the PVA-based resin is immersed in water in the subsequent dyeing step or stretching step, problems such as a decrease in the orientation and dissolution of the PVA-based resin can be prevented, and high optical characteristics can be realized.

The stretching method for assisting stretching in a gas atmosphere may be fixed-end stretching (for example, a method of stretching using a tenter) or free-end stretching (for example, a method of uniaxially stretching a laminate by passing the laminate between rolls having different peripheral speeds), and the free-end stretching is actively employed for obtaining high optical characteristics. In one embodiment, the stretching treatment in a gas atmosphere includes a heated roller stretching step of stretching the laminate by a difference in peripheral speed between heated rollers while conveying the laminate in the longitudinal direction thereof. The stretching treatment in a gas atmosphere typically includes a zone stretching step and a heated roller stretching step. The order of the area stretching step and the heating roller stretching step is not limited, and the area stretching step may be performed first or the heating roller stretching step may be performed first. The zone stretching process may also be omitted. In one embodiment, the zone stretching step and the heated roller stretching step are performed in this order. In another embodiment, the tenter stretching machine grips the film end portions, and stretches the film by expanding the distance between the tenters in the transport direction (the expansion of the distance between the tenters is the stretching magnification). At this time, the distance of the tenter in the width direction (the direction perpendicular to the conveying direction) is arbitrarily set close. Preferably, the stretching ratio in the transport direction may be set so as to stretch closer to the free end. In the case of free-end stretching, the degree of shrinkage in the width direction (1/stretch ratio) is determined by^1/2To calculate.

The auxiliary stretching in a gas atmosphere may be performed in one stage or in multiple stages. In the case of performing in multiple stages, the stretching magnification is the product of the stretching magnifications in each stage. The stretching direction in the auxiliary stretching in the gas atmosphere is preferably substantially the same as the stretching direction in the aqueous solution.

The stretching ratio in the auxiliary stretching in a gas atmosphere is preferably 2.0 to 3.5 times. The maximum stretching ratio in the case of auxiliary stretching in a combined gas atmosphere and stretching in an aqueous solution is preferably 5.0 times or more, more preferably 5.5 times or more, and even more preferably 6.0 times or more, with respect to the original length of the laminate. In the present specification, "maximum stretching ratio" means a stretching ratio immediately before the laminate is broken, and further, a stretching ratio at which the laminate is confirmed to be broken, and "maximum stretching ratio" means a value smaller than this value by 0.2.

The stretching temperature for assisting stretching in a gas atmosphere may be set to any appropriate value depending on the material for forming the thermoplastic resin substrate, the stretching method, and the like. The stretching temperature is preferably not less than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably not less than the glass transition temperature (Tg) +10 ℃ of the thermoplastic resin substrate, and particularly preferably not less than Tg +15 ℃. On the other hand, the upper limit of the stretching temperature is preferably 170 ℃. By stretching at such a temperature, rapid progress of crystallization of the PVA type resin can be suppressed, and defects caused by the crystallization (for example, inhibition of orientation of the PVA type resin layer by stretching) can be suppressed.

B-3. insolubilization treatment

If necessary, after the stretching treatment is assisted in a gas atmosphere, an insolubilization treatment is performed before the stretching treatment in an aqueous solution and the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. By performing insolubilization treatment, water resistance can be imparted to the PVA-based resin layer, and the PVA can be prevented from being degraded in orientation when immersed in water. The concentration of the aqueous boric acid solution is preferably 1 to 4 parts by weight with respect to 100 parts by weight of water. The liquid temperature of the insolubilization bath (aqueous boric acid solution) is preferably 20 to 50 ℃.

B-4. dyeing treatment

The dyeing treatment is typically performed by dyeing the PVA-based resin layer with iodine. Specifically, iodine is adsorbed to the PVA-based resin layer. Examples of the adsorption method include: a method of immersing the PVA-based resin layer (laminate) in a dyeing solution containing iodine; a method of applying the dyeing liquid to a PVA-based resin layer; a method of spraying the dyeing solution on the PVA-based resin layer, and the like. A method of immersing the laminate in a dyeing solution (dyeing bath) is preferable. This is because iodine can be adsorbed well.

The staining solution is preferably an aqueous iodine solution. The amount of iodine blended is preferably 0.05 to 0.5 parts by weight based on 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add iodide to the aqueous iodine solution. Examples of the iodide include: potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, titanium iodide, and the like. Of these, potassium iodide is preferred. The amount of the iodide is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, based on 100 parts by weight of water. In order to suppress the dissolution of the PVA based resin, the liquid temperature at the time of dyeing with the dyeing liquid is preferably 20 ℃ to 50 ℃. When the PVA-based resin layer is immersed in the dyeing liquid, the immersion time is preferably 5 seconds to 5 minutes, more preferably 30 seconds to 90 seconds, in order to ensure the transmittance of the PVA-based resin layer.

The dyeing conditions (concentration, liquid temperature, immersion time) can be set so that the monomer transmittance of the polarizing film finally obtained is 44.0% or more and the degree of polarization is 99.50% or more. As such dyeing conditions, it is preferable to use an aqueous iodine solution as the dyeing liquid, and to set the ratio of the contents of iodine and potassium iodide in the aqueous iodine solution to 1: 5-1: 20. the ratio of the content of iodine to potassium iodide in the aqueous iodine solution is preferably 1: 5-1: 10. thus, a polarizing film having the above-described optical characteristics can be obtained.

When the dyeing treatment is continuously performed after the treatment (typically, insolubilization treatment) of immersing the laminate in a treatment bath containing boric acid, the boric acid contained in the treatment bath is mixed into the dyeing bath, whereby the boric acid concentration in the dyeing bath changes with time, and as a result, the dyeing property may become unstable. In order to suppress the instability of dyeing properties as described above, the upper limit of the boric acid concentration in the dyeing bath is adjusted so that it is preferably 4 parts by weight, more preferably 2 parts by weight, per 100 parts by weight of water. On the other hand, the lower limit of the boric acid concentration of the dyeing bath is preferably 0.1 part by weight, more preferably 0.2 part by weight, and further preferably 0.5 part by weight with respect to 100 parts by weight of water. In one embodiment, the dyeing treatment is performed using a dyeing bath previously compounded with boric acid. This can reduce the rate of change in the boric acid concentration when boric acid in the treatment bath is mixed into the dyeing bath. The amount of boric acid to be blended in the dyeing bath in advance (i.e., the content of boric acid not derived from the treatment bath) is preferably 0.1 to 2 parts by weight, more preferably 0.5 to 1.5 parts by weight, based on 100 parts by weight of water.

B-5. Cross-linking treatment

If necessary, the crosslinking treatment is performed after the dyeing treatment and before the stretching treatment in an aqueous solution. The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. By performing the crosslinking treatment, water resistance can be imparted to the PVA-based resin layer, and the orientation of the PVA can be prevented from being lowered when the PVA is immersed in high-temperature water during subsequent stretching in an aqueous solution. The concentration of the aqueous boric acid solution is preferably 1 to 5 parts by weight with respect to 100 parts by weight of water. In addition, when the crosslinking treatment is performed after the dyeing treatment, it is preferable to further incorporate an iodide. The iodine compound can suppress elution of iodine adsorbed on the PVA-based resin layer. The amount of the iodide is preferably 1 to 5 parts by weight based on 100 parts by weight of water. Specific examples of the iodide are as described above. The liquid temperature of the crosslinking bath (aqueous boric acid solution) is preferably 20 ℃ to 50 ℃.

B-6 stretching treatment in aqueous solution

The stretching treatment in an aqueous solution is performed by immersing the laminate in a stretching bath. The stretching treatment in an aqueous solution allows stretching at a temperature lower than the glass transition temperature (typically, about 80 ℃) of the thermoplastic resin substrate or the PVA type resin layer, and allows stretching at a high magnification while suppressing crystallization of the PVA type resin layer. As a result, a polarizing film having excellent optical characteristics can be produced.

Any suitable method can be used for stretching the laminate. Specifically, the stretching may be fixed-end stretching or free-end stretching (for example, a method of passing the laminate between rolls having different peripheral speeds to perform uniaxial stretching), and the free-end stretching is preferably selected. The stretching of the laminate may be performed in one stage or in multiple stages. In the case of performing in multiple stages, the stretching ratio (maximum stretching ratio) of the laminate described later is the product of the stretching ratios in the respective stages.

The stretching in an aqueous solution is preferably performed by immersing the laminate in an aqueous solution of boric acid (stretching in an aqueous solution of boric acid). By using an aqueous boric acid solution as a stretching bath, the PVA-based resin layer can be provided with rigidity capable of withstanding the tension applied during stretching and water resistance insoluble in water. Specifically, boric acid can generate tetrahydroxyborate anions in an aqueous solution and can be crosslinked with the PVA-based resin by hydrogen bonds. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, and the PVA-based resin layer can be stretched well, whereby a polarizing film having excellent optical properties can be produced.

The aqueous boric acid solution is preferably obtained by dissolving boric acid and/or a borate in water as a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight, based on 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film having higher characteristics can be produced. An aqueous solution obtained by dissolving a boron compound such as borax other than boric acid or a borate, glyoxal, glutaraldehyde, or the like in a solvent may also be used.

Preferably, an iodide is added to the stretching bath (aqueous boric acid solution). The iodine compound can suppress elution of iodine adsorbed on the PVA-based resin layer. Specific examples of the iodide are as described above. The concentration of the iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, based on 100 parts by weight of water.

The drawing temperature (liquid temperature of the drawing bath) is preferably 40 to 85 ℃ and more preferably 60 to 75 ℃. At such a temperature, the PVA-based resin layer can be stretched at a high magnification while dissolution thereof is suppressed. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60 ℃ or higher in view of the relationship with the formation of the PVA-based resin layer. In this case, when the stretching temperature is lower than 40 ℃, there is a possibility that the thermoplastic resin substrate cannot be stretched well even when plasticization of the thermoplastic resin substrate by water is considered. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, and the less excellent optical characteristics may be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio by stretching in an aqueous solution is preferably 1.5 times or more, more preferably 3.0 times or more. The total stretch ratio of the laminate is preferably 5.0 times or more, and more preferably 5.5 times or more, based on the original length of the laminate. By realizing such a high stretch ratio, a polarizing film having very excellent optical characteristics can be produced. Such a high draw ratio can be achieved by employing a drawing method in an aqueous solution (drawing in an aqueous boric acid solution).

B-7. drying shrinkage treatment

The drying shrinkage treatment may be performed by heating the entire area to perform area heating, or may be performed by heating a transport roller (using a so-called hot roller) (hot roller drying method). Preferably both are utilized. By drying using a heating roller, the heating curl of the laminate can be efficiently suppressed, and a polarizing film having excellent appearance can be produced. Specifically, by drying the laminate in a state of being along the heating roller, the crystallization of the thermoplastic resin substrate can be efficiently promoted to increase the crystallinity, and the crystallinity of the thermoplastic resin substrate can be favorably increased even at a relatively low drying temperature. As a result, the thermoplastic resin substrate has increased rigidity and is able to withstand shrinkage of the PVA type resin layer due to drying, and curling can be suppressed. Further, since the laminate can be dried while being kept flat by using the heating roller, not only curling but also wrinkles can be suppressed. At this time, the laminate is shrunk in the width direction by the drying shrinkage treatment, whereby the optical characteristics can be improved. This is because the orientation of PVA and PVA/iodine complex can be effectively improved. The shrinkage rate of the laminate subjected to the drying shrinkage treatment in the width direction is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%. By using a heated roller, the laminate can be continuously shrunk in the width direction while being conveyed, and high productivity can be achieved.

Fig. 2 is a schematic diagram showing an example of the drying shrinkage process. In the drying shrinkage process, the laminate 200 is dried while being conveyed by the conveying rollers R1 to R6 and the guide rollers G1 to G4 which are heated to a predetermined temperature. In the illustrated example, the conveying rollers R1 to R6 are disposed so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but the conveying rollers R1 to R6 may be disposed so as to continuously heat only one surface (for example, the surface of the thermoplastic resin substrate) of the laminate 200.

The drying conditions can be controlled by adjusting the heating temperature of the transport roller (temperature of the heating roller), the number of heating rollers, the contact time with the heating roller, and the like. The temperature of the heating roller is preferably 60 to 120 ℃, more preferably 65 to 100 ℃, and particularly preferably 70 to 80 ℃. The crystallinity of the thermoplastic resin can be favorably increased, the curling can be favorably suppressed, and an optical laminate having extremely excellent durability can be produced. The temperature of the heating roller may be measured by a contact thermometer. In the illustrated example, 6 transport rollers are provided, but there is no particular limitation as long as there are a plurality of transport rollers. The number of the transport rollers is usually 2 to 40, preferably 4 to 30. The contact time (total contact time) between the laminate and the heating roller is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, and still more preferably 1 to 10 seconds.

The heating roller may be installed in a heating furnace (for example, an oven) or may be installed in a general manufacturing line (room temperature environment). Preferably, the air blowing mechanism is provided in a heating furnace having an air blowing mechanism. By using drying with a heating roller and hot air drying in combination, a rapid temperature change between the heating rollers can be suppressed, and shrinkage in the width direction can be easily controlled. The temperature of the hot air drying is preferably 30 to 100 ℃. The hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot wind is preferably about 10m/s to 30 m/s. The wind speed is the wind speed in the heating furnace and can be measured by a digital wind speed meter of a miniature blade type.

B-8 other treatment

It is preferable to perform the washing treatment after the stretching treatment in the aqueous solution and before the drying shrinkage treatment. The cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement method of each property is as follows. Unless otherwise specified, "parts" and "%" in examples and comparative examples are based on weight.

(1) Thickness of

The measurement was carried out by using an interferometric film thickness meter (product name "MCPD-3000" available from Otsuka Denshi Co., Ltd.).

(2) Transmittance and degree of polarization of monomer

For the polarizing plates (protective films/polarizing films) of examples and comparative examples, the single transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer (V-7100, manufactured by japan spectrographs) were used as Ts, Tp, and Tc of the polarizing films, respectively. These Ts, Tp and Tc are Y values obtained by measuring and correcting visibility with a 2-degree field of view (C light source) according to JIS Z8701. The refractive index of the protective film was 1.50, and the refractive index of the surface of the polarizing film opposite to the protective film was 1.53.

From the Tp and Tc thus obtained, the degree of polarization P was determined by the following equation.

Polarization degree P (%) { (Tp-Tc)/(Tp + Tc) }^1/2×100

The spectrophotometer can be measured similarly with LPF-200 manufactured by Otsuka electronics Co., Ltd. For example, the monomer transmittance Ts and the polarization degree P were obtained by measurement using V-7100 and LPF-200 for samples 1 to 3 of the polarizing plates having the same configurations as in the following examples, and the measured values thereof are shown in table 1. As shown in Table 1, it was found that the difference between the measured value of the monomer transmittance of V-7100 and the measured value of the monomer transmittance of LPF-200 was 0.1% or less, and that the same measurement results were obtained in all cases where any of the spectrophotometers was used.

[ Table 1]

For example, when a polarizing plate having an anti-glare (AG) surface-treated adhesive and having a diffusing property is used as a measurement target, different measurement results can be obtained depending on the spectrophotometer, and in this case, the difference between the measurement values depending on the spectrophotometer can be compensated by performing numerical conversion based on the measurement values obtained when the same polarizing plate is measured by each spectrophotometer.

(3) Variation in optical characteristics of long polarizing plate

From the long polarizing plates used in examples and reference examples, measurement samples were cut out at 5 positions at equal intervals in the width direction, and the single transmittance at the central portion of each of the 5 measurement samples was measured in the same manner as in the above (2). Next, the difference between the maximum value and the minimum value among the cell transmittances measured at the respective measurement positions was calculated, and this value was used as the variation in the optical characteristics of the long polarizing plate (the difference between the maximum value and the minimum value of the cell transmittance at the position of the long polarizing plate in the width direction).

(4) Variation in optical characteristics of single-sheet polarizing plate

From the long polarizing plates used in examples and reference examples, 100mm × 100mm measurement samples were cut out to obtain a single polarizing plate (50 cm)²) The deviation of the optical characteristics of (1). Specifically, the cell transmittances were measured at positions approximately 1.5cm to 2.0cm inward from the midpoint of each of the 4 sides of the measurement sample and at 5 positions in total in the central portion in the same manner as in the above (2). Then, the difference between the maximum value and the minimum value of the single transmittance measured at each measurement position was calculated, and the calculated value was used as the deviation (50 cm) of the optical characteristics of the single-sheet polarizing plate²The difference between the maximum and minimum of the monomer transmission within a region).

[ example 1]

1. Production of polarizing film

As the thermoplastic resin substrate, a long-sized amorphous isophthalic acid copolymerized polyethylene terephthalate film (thickness: 100 μm) having a water absorption of 0.75% and a Tg of about 75 ℃ was used. One side of the resin substrate was subjected to corona treatment.

To 100 parts by weight of a PVA resin obtained by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (trade name "GOHSEFIMER Z410" available from Nippon synthetic chemical Co., Ltd.) at a ratio of 9:1, 13 parts by weight of potassium iodide was added to prepare an aqueous PVA solution (coating solution).

The aqueous PVA solution was applied to the corona-treated surface of the resin substrate, and dried at 60 ℃ to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.

The free end of the obtained laminate was stretched in one direction in the longitudinal direction (longitudinal direction) by a factor of 2.4 in an oven at 130 ℃ between rolls having different peripheral speeds (auxiliary stretching treatment in a gas atmosphere).

Next, the laminate was immersed in an insolubilization bath (an aqueous boric acid solution prepared by adding 4 parts by weight of boric acid to 100 parts by weight of water) at a liquid temperature of 40 ℃ for 30 seconds (insolubilization treatment).

Next, the polarizing film was immersed in a dyeing bath (aqueous iodine solution prepared by mixing iodine and potassium iodide at a weight ratio of 1:7 with respect to 100 parts by weight of water) at a liquid temperature of 30 ℃ for 60 seconds while adjusting the concentration so that the monomer transmittance (Ts) of the finally obtained polarizing film becomes 44% or more (dyeing treatment).

Subsequently, the substrate was immersed in a crosslinking bath (an aqueous boric acid solution containing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid per 100 parts by weight of water) at a liquid temperature of 40 ℃ for 30 seconds (crosslinking treatment).

Then, while immersing the laminate in an aqueous boric acid solution (boric acid concentration 4.0 wt%) having a liquid temperature of 70 ℃, uniaxial stretching (stretching treatment in an aqueous solution) was performed between rolls having different peripheral speeds so that the total stretching ratio in the longitudinal direction (longitudinal direction) was 5.5 times.

Then, the laminate was immersed in a cleaning bath (aqueous solution containing 4 parts by weight of potassium iodide per 100 parts by weight of water) at a liquid temperature of 20 ℃.

Then, while drying in an oven maintained at 90 ℃, the sheet was contacted with a SUS heating roll maintained at a surface temperature of 75 ℃ for about 2 seconds (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction by the drying shrinkage treatment was 5.2%.

Thus, a polarizing film having a thickness of 5 μm was formed on the resin substrate. Further, the same process was repeated to produce 9 polarizing films in total.

2. Preparation of polarizing plate

An acrylic film (surface refractive index 1.50, 40 μm) as a protective film was bonded to the surface (the surface opposite to the resin substrate) of each of the polarizing films obtained above with an ultraviolet-curable adhesive. Specifically, the curable adhesive was applied so that the total thickness thereof became 1.0 μm, and was bonded using a roll machine. Then, UV light is irradiated from the protective film side to cure the adhesive. Subsequently, both ends were cut, and the resin substrate was peeled off, thereby obtaining 9 long-strip polarizers (width: 1300mm) having a structure of a protective film/polarizing film.

[ example 2]

18 polarizing films and polarizing plates were produced in the same manner as in example 1, except that the oven temperature and the heating roller temperature were set to 70 ℃ and 70 ℃ respectively in the drying shrinkage treatment. The shrinkage of the laminate in the width direction due to the drying shrinkage treatment was 2.5%.

Comparative example 1

5 polarizing films and polarizing plates were produced in the same manner as in example 1, except that potassium iodide was not added to the PVA aqueous solution (coating solution), the stretching ratio in the auxiliary stretching treatment in a gas atmosphere was 1.8 times, and a heating roll was not used in the drying and shrinking treatment.

Comparative example 2

6 polarizing films and polarizing plates were produced in the same manner as in example 1, except that the stretching ratio in the auxiliary stretching treatment in the gas atmosphere was 1.8 times and no heating roll was used in the drying and shrinking treatment.

[ reference example 1]

The polarizing film obtained in the same manner as in comparative example 2 was held for 30 minutes in a constant temperature and humidity region set at a temperature of 60 ℃ and a humidity of 90% RH. Then, a polarizing plate was produced in the same manner as in example 1.

The single transmittance and the degree of polarization were measured for each of the polarizing plates of examples and comparative examples. The results are shown in table 2 and fig. 3.

[ Table 2]

The polarizing film obtained by the manufacturing method of the comparative example cannot satisfy both the monomer transmittance of 44.0% or more and the polarization degree of 99.50% or more. In contrast, the polarizing film obtained by the production method of the example had excellent optical characteristics such as a single transmittance of 44.0% or more and a degree of polarization of 99.50% or more.

The polarizing plates of example 1 and reference example 1 were each measured for variations in optical characteristics of the long and single polarizing plates. The results are shown in Table 3.

[ Table 3]

	Deviation of transmission of monomer (strip shape)	Deviation of transmission of monomer (monomer)
			Example 1	0.27％	0.10％
Reference example 1	0.53％	0.34％

The variation in the individual transmittance of the long polarizing plate obtained by the manufacturing method of example was 0.3% or less, and the variation in the individual transmittance of the single polarizing plate obtained by the manufacturing method of example was 0.2% or less, and the variation in the optical characteristics was suppressed to such an extent that there was no problem. On the other hand, in the polarizing plate of the reference example obtained through the step of humidifying the polarizing film, the variation in optical characteristics of both the long and single polarizing plates was large.

Industrial applicability

The polarizing plate having the polarizing film of the present invention can be suitably used as a circularly polarizing plate for a liquid crystal display device, an organic EL display device, or an inorganic EL display device.

Claims

1. A polarizing plate comprising a polarizing film and a protective layer disposed on at least one side of the polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more,

the polarizer is 50cm²The difference between the maximum value and the minimum value of the single transmittance in the region is 0.2% or less.

2. A polarizing plate comprising a polarizing film and a protective layer disposed on at least one side of the polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more,

the width of the polarizing plate is 1000mm or more,

the difference between the maximum value and the minimum value of the single transmittance of the polarizing plate at the position along the width direction is 0.3% or less.

3. The polarizing plate according to claim 1 or 2,

the polarizing film has a monomer transmittance of 44.5% or less and a degree of polarization of 99.95% or less.

4. A polarizer roll obtained by winding the polarizer according to any one of claims 1 to 3 in a roll form.

5. A method for producing a polarizing film having a thickness of 8 μm or less, a monomer transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, comprising:

forming a polyvinyl alcohol resin layer containing an iodide or sodium chloride and a polyvinyl alcohol resin on one side of a long thermoplastic resin base material to prepare a laminate; and

the laminate is subjected to an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment in this order, and in the drying shrinkage treatment, the laminate is heated while being conveyed in the longitudinal direction so as to be shrunk by 2% or more in the width direction.

6. The method according to claim 5, wherein the polarizing film has a single-body transmittance of 44.5% or less and a degree of polarization of 99.95% or less.

7. The manufacturing method according to claim 5 or 6,

in the polyvinyl alcohol resin layer, the content of the iodide or sodium chloride is 5 to 20 parts by weight based on 100 parts by weight of the polyvinyl alcohol resin.

8. The production method according to any one of claims 5 to 7,

the stretching ratio in the auxiliary stretching treatment in the gas atmosphere is 2.0 times or more.

9. The production method according to any one of claims 5 to 8,

the drying shrinkage treatment step is a step of heating using a heating roller.

10. The manufacturing method according to claim 9,

the temperature of the heating roller is 60 to 120 ℃, and the shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment is 2% or more.