CN111095055B

CN111095055B - Polarizing film, polarizing plate, and method for producing polarizing film

Info

Publication number: CN111095055B
Application number: CN201880059538.6A
Authority: CN
Inventors: 后藤周作; 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: 2021-09-10
Anticipated expiration: 2038-09-06
Also published as: CN111095055A; WO2019054275A1; TW201915065A; TWI749242B; KR20200054191A; KR102296079B1

Abstract

The present invention provides a polarizing film having excellent optical characteristics. The polarizing film of the present invention comprises a polyvinyl alcohol resin film containing iodine and has a thickness of 8 μm or less, and has an orthogonal absorbance A at a wavelength of 550nm₅₅₀Orthogonal absorbance A at a wavelength of 210nm₂₁₀Ratio of (A)₅₅₀/A₂₁₀) 1.4 to 3.5, and the orthogonal absorbance A of the polarizing film at a wavelength of 470nm₄₇₀With orthogonal absorbance A at a wavelength of 600nm₆₀₀Ratio of (A)₄₇₀/A₆₀₀) 0.7 to 2.00, and the orthogonal b value of the polarizing film is more than-10 and less than + 10. Such a polarizing film can be obtained, for example, by a production 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 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.

Description

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

Technical Field

The invention relates to a polarizing film, a polarizing plate, and a method for producing 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. 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 film which is thin and has excellent optical characteristics, a polarizing plate, and a method for producing such a polarizing film.

Means for solving the problems

The polarizing film of the present invention comprises a polyvinyl alcohol resin film containing iodine and has a thickness of 8 μm or less, and has an orthogonal absorbance A at a wavelength of 550nm₅₅₀Orthogonal absorbance A at a wavelength of 210nm₂₁₀Ratio of (A)₅₅₀/A₂₁₀) 1.4 to 3.5, and an orthogonal absorbance A of the polarizing film at a wavelength of 470nm₄₇₀With orthogonal absorbance A at a wavelength of 600nm₆₀₀Ratio of (A)₄₇₀/A₆₀₀) 0.7 to 2.00, and the orthogonal b value of the polarizing film is more than-10 and less than + 10.

In one embodiment, the polarizing film has a thickness of 5 μm or less.

In one embodiment, the ratio (A) is₅₅₀/A₂₁₀) Is 1.8 or more.

In one embodiment, the polarizing film has a cell transmittance of 42.5% or more.

According to another aspect of the present invention, there is provided a polarizing plate having the above polarizing film and a protective layer disposed on at least one side of the polarizing film.

According to another aspect of the present invention, there is provided a method for manufacturing the above polarizing film, 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 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 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 iodide is potassium iodide.

In one embodiment, the content of potassium iodide 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 drying shrinkage treatment is performed using a heated roller.

In one embodiment, the temperature of the heated roller is 60 ℃ to 120 ℃.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a polarizing film having a thin shape and an orthogonal absorbance a at a wavelength of 550nm can be realized by a combination of 2-stage stretching including auxiliary stretching in a gas atmosphere and stretching in an aqueous solution, and drying and shrinking by a heating roller, which are performed by adding a halide (typically potassium iodide) to a polyvinyl alcohol (PVA) resin₅₅₀Orthogonal absorbance A at a wavelength of 210nm₂₁₀Ratio of (A)₅₅₀/A₂₁₀) Very large, orthogonal absorbance A at a wavelength of 470nm₄₇₀With orthogonal absorbance A at a wavelength of 600nm₆₀₀Ratio of (A)₄₇₀/A₆₀₀) Is a given value or more, and the orthogonal b value of the polarizing film is larger than the given value.

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 transmittance of the polarizing films obtained in examples and comparative examples in comparison with A₅₅₀/A₂₁₀A graph of the relationship of (a).

Fig. 4 is a graph showing the relationship between the wavelength and the orthogonal absorbance of the polarizing films obtained in example 1, example 2, and comparative example 1 in comparison.

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 film

The polarizing film of the embodiment of the present invention comprises a polyvinyl alcohol (PVA) -based resin film containing iodine, has a thickness of 8 μm or less, and has an orthogonal absorbance A at a wavelength of 550nm₅₅₀Orthogonal absorbance A at a wavelength of 210nm₂₁₀Ratio of (A)₅₅₀/A₂₁₀) An orthogonal absorbance A at a wavelength of 470nm of 1.4 or more₄₇₀With orthogonal absorbance A at a wavelength of 600nm₆₀₀Ratio of (A)₄₇₀/A₆₀₀) Is 0.7 or more, and the orthogonal b value of the polarizing film is more than-10. The polarizing film according to the embodiment of the present invention has a ratio (A) higher than that of a normal thin polarizing film₅₅₀/A₂₁₀) And (A)₄₇₀/A₆₀₀) Is very large. This means that the polarizing film has a very small content ratio of iodide ions (having absorption in the ultraviolet region around 210 nm) that do not form a complex with PVA, and a very large content ratio of PVA-iodine complexes (having absorption in the visible light region). More specifically, in the polarizing film according to the embodiment of the present invention, PVA-I having absorption at around 600nm is used₅ ^-Complex compoundsHas a very large content ratio of PVA-I and has an absorption near 480nm₃ ^-The content ratio of the complex is maintained without being greatly reduced. Here, since the thickness of the polarizing film is the length of the optical path, simply reducing the thickness of the polarizing film also shortens the optical path length and lowers the polarization performance. Since the amount of iodine that can be contained in the polarizing film is also limited, it is necessary to efficiently use iodine contained in the polarizing film in order to achieve both high polarizing performance and reduction in thickness of the polarizing film. That is, by reducing iodide ions that absorb light in the ultraviolet region but do not contribute to polarization performance and increasing the ratio of PVA-iodine complex that absorbs light in the visible light region, both high polarization performance and thin polarizing film can be achieved. In other words, by increasing the ratio (A)₅₅₀/A₂₁₀) Thus, high optical characteristics can be realized with a thin profile. In addition, by comparing the ratio (A)₄₇₀/A₆₀₀) Keeping the polarization to a given value or more can realize good polarization performance over the entire visible light region. When the amount of iodine in the thin polarizing film is limited, it is difficult to increase the ratio (A) in the conventional technique₅₅₀/A₂₁₀) And ratio (A)₄₇₀/A₆₀₀) Both of these may be increased according to the embodiment of the present invention. Ratio (A)₅₅₀/A₂₁₀) Preferably 1.8 or more, more preferably 2.0 or more, and further preferably 2.2 or more, in the ratio (A)₅₅₀/A₂₁₀) The upper limit of (d) may be, for example, 3.5. Ratio (A)₄₇₀/A₆₀₀) Preferably 0.75 or more, more preferably 0.80 or more, and still more preferably 0.85 or more, as the ratio (A)₄₇₀/A₆₀₀) The upper limit of (b) is, for example, 2.00, preferably 1.33. The orthogonal absorbance is determined by the following equation based on the orthogonal transmittance Tc measured when the degree of polarization is determined as described later.

Orthogonal absorbance log10(100/Tc)

It is one of the achievements of the present invention to realize a thin polarizing film having such characteristics. Such a polarizing film can be used for an image display device, and in particular, can be suitably used for a back-side polarizing plate of a liquid crystal display device.

In addition, in the embodiment of the present invention, the orthogonal b value of the polarizing film is greater than-10, preferably-7 or more, more preferably-5 or more as described above, and the upper limit of the orthogonal b value is preferably +10 or less, more preferably +5 or less. According to the invention of the present application, such a range of orthogonal b values can be achieved. The orthogonal b value indicates a hue when the polarizing film (polarizing plate) is placed in an orthogonal state, and a larger absolute value of the value means that the hue is observed in an orthogonal hue (black display in an image display device). For example, when the value of the orthogonal b is as low as-10 or less, the black display looks bluish and the display performance is degraded. That is, according to the embodiment of the present invention, a polarizing film capable of realizing an excellent hue in black display can be obtained. The orthogonal b value can be measured by a spectrophotometer represented by V-7100.

The thickness of the polarizing film is preferably 1 μm to 8 μm, more preferably 1 μm to 7 μm, further preferably 2 μm to 5 μm, particularly preferably 2 μm to 4 μm, and particularly preferably 2 μm to 3 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 46.0% or less, more preferably 45.0% or less. On the other hand, the monomer transmittance is preferably 41.5% or more, more preferably 42.0% or more, and further preferably 42.5% or more. The polarization degree of the polarizing film is preferably 99.990% or more, and more preferably 99.998% or less. According to the embodiments of the present invention, both high monomer transmittance and high polarization degree can be achieved as described above. The monomer transmittance is typically a Y value obtained by measuring with an ultraviolet-visible spectrophotometer and correcting the visibility. The monomer transmittance is a value obtained by converting the refractive index of one surface of the polarizing plate to 1.50 and the refractive index of the other surface to 1.53. 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 of 8 μm or less is typically represented by a polarizing film (Table)Refractive index of face: 1.53) and protective film (refractive index: 1.50) was measured using an ultraviolet-visible spectrophotometer 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 measured value of the transmittance may change. 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, whereby the transmittance in the case of using a protective film having a refractive index of 1.50 can be converted into a polarizing film having a refractive index of 1.53 on the surface. 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 drying shrinkage treatment is performed by heating the laminate while conveying the laminate in the longitudinal direction. Thus, a polarizing film having a monomer transmittance of 43.0% or more and a unit absorbance of 0.85 or more, and excellent optical characteristics, despite its extremely thin thickness, can be provided. 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.

B. 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 10 is the polarizing film of the present invention described in the above item a. One of the 1 st protective layer 20 and the 2 nd protective layer 30 may be omitted. As described above, one of the 1 st protective layer and the 2 nd protective layer may be a resin base material used for producing the polarizing film.

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).

C. 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 unit absorbance) 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.

Preparation of C-1. 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, and 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.

C-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.

C-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-. 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.

C-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.

C-3. insolubilization

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 ℃.

C-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, and 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 43.0% or more and the degree of polarization is 99.980% or more. As such dyeing conditions, it is preferable to use an aqueous iodine solution as the dyeing liquid, and the ratio of the contents of iodine and potassium iodide in the aqueous iodine solution is 1:5 to 1: 20. The ratio of the iodine content to the potassium iodide content in the iodine aqueous solution is preferably 1:5 to 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.

C-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 ℃.

C-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, and 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, 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).

C-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), and both of them are preferably used. 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%.

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), and is preferably installed in a heating furnace provided with 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.

C-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) Monomer transmittance and orthogonal absorbance

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 by an ultraviolet-visible spectrophotometer (made by Nissan Spectroscopy V-7100) were respectively set as Ts, Tp, and Tc of the polarizing films. 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.

Further, the orthogonal absorbance was obtained by the following equation using Tc measured at each wavelength.

Orthogonal absorbance log10(100/Tc)

The orthogonal transmittance Tc at a wavelength of 210nm was measured using UV-3150 manufactured by Shimadzu corporation₂₁₀Determination of orthogonal Absorbance A₂₁₀And from the orthogonal transmittance Tc at a measurement wavelength of 550nm₅₅₀Determination of orthogonal Absorbance A₅₅₀. In addition, makeThe transmittance Tc at the wavelength of 470nm was measured by using the Japanese Spectroscopy system V-7100₄₇₀Determination of orthogonal Absorbance A₄₇₀And from the orthogonal transmittance Tc at a measurement wavelength of 600nm₆₀₀Determination of orthogonal Absorbance A₆₀₀。

Note that, for A₄₇₀And A₆₀₀The same measurement can be carried out using LPF-200 manufactured by Otsuka Denshi Co.

(3) Quadrature b value

The polarizing plates of examples and comparative examples were measured using an ultraviolet-visible spectrophotometer (product name "V7100" made by japan spectrophotometers) to determine the color in the cross nicol state. The results show that: the lower the orthogonal b value (a negative value and a larger absolute value), the more blue the color tone is, the less colorless the polarizing plate is.

[ example 1-1]

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 surface of the resin substrate was subjected to corona treatment (treatment condition: 55 W.min/m)²)。

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 became the value shown in table 1 (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 3.0 μm was formed on the resin substrate.

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. Next, the resin substrate was peeled off, and a polarizing plate having a protective film/polarizing film structure was obtained.

The obtained polarizing film and polarizing plate had a monomer transmittance of A₅₅₀/A₂₁₀、A₄₇₀/A₆₀₀And the orthogonal b values are shown in table 1.

[ examples 1-2]

Polarizing films and polarizing plates were produced in the same manner as in example 1-1, except that the concentration of the dyeing bath was adjusted so that the monomer transmittance (Ts) of the polarizing film became the value shown in table 1. The obtained polarizing film and polarizing plate had a monomer transmittance and A₅₅₀/A₂₁₀Shown in table 1. Further, the monomer transmittance is compared with A₅₅₀/A₂₁₀The results of comparison with examples 2 to 3 and comparative example 1 are shown in FIG. 3. Fig. 4 shows the relationship between the wavelength and the orthogonal absorbance, together with example 2 and comparative example 1. The comparative data in fig. 4 is data of a single transmittance of 43.5%.

[ examples 2-1 to 2-4]

Polarizing films and polarizing plates were produced in the same manner as in example 1-1, except that the thickness of the obtained polarizing film was changed to 4.6 μm and the concentration of the dyeing bath was adjusted so that the monomer transmittance (Ts) of the polarizing film became the value shown in table 1. The obtained polarizing film and polarizing plate had a monomer transmittance and A₅₅₀/A₂₁₀Shown in table 1. For examples 2-3 (monomer transmission 43.5%), A₄₇₀/A₆₀₀And the orthogonal b values are also shown in table 1. Further, the monomer transmittance is compared with A₅₅₀/A₂₁₀The relationship of (a) is shown in FIG. 3. Fig. 4 shows the relationship between the wavelength and the orthogonal absorbance.

[ example 3-1]

Polarizing films and polarizing plates were produced in the same manner as in example 1-1, except that the thickness of the obtained polarizing film was changed to 5.0 μm by changing the PVA aqueous solution (coating solution), the concentration of the dyeing bath was adjusted so that the monomer transmittance (Ts) of the polarizing film became the value shown in table 1, and the conditions of the drying shrinkage treatment were changed so that the shrinkage ratio in the width direction became 5%. The obtained polarizing film and polarizing plate had a monomer transmittance and A₅₅₀/A₂₁₀Shown in table 1. Further, the monomer transmittance is compared with A₅₅₀/A₂₁₀The relationship of (a) is shown in FIG. 3.

Comparative examples 1-1 to 1-6

Polarizing films and polarizing plates were produced in the same manner as in example 1-1, except that potassium iodide was not added to the PVA aqueous solution (coating solution), the thickness of the obtained polarizing film was changed to 3.3 μm, the shrinkage rate in the width direction was set to 0.1% or less without using a heating roll in the drying shrinkage treatment, and the concentration of the dyeing bath was adjusted so that the monomer transmittance (Ts) of the polarizing film became the value shown in table 1. The obtained polarizing film and polarizing plate had a monomer transmittance and A₅₅₀/A₂₁₀Shown in table 1. For comparative examples 1-3 (monomer transmittance 43.4%), A₄₇₀/A₆₀₀And the orthogonal b values are also shown in table 1. Further, the monomer transmittance is compared with A₅₅₀/A₂₁₀The relationship of (a) is shown in FIG. 3. Fig. 4 shows the relationship between the wavelength and the orthogonal absorbance.

Comparative examples 1-7 to 1-8

Polarizing films and polarizing plates were produced in the same manner as in comparative example 1-1, except that the KI concentration in the cleaning bath was changed as shown in table 1. The obtained polarizing film and polarizing plate had a monomer transmittance of A₄₇₀/A₆₀₀And the orthogonal b values are shown in table 1. The polarizing plates obtained in these comparative examples were not subjected to a test because the display characteristics were significantly reduced₅₅₀/A₂₁₀The measurement of (1).

Comparative example 2-1

Polarizing films and polarizing plates were produced in the same manner as in example 1-1, except that the thickness of the obtained polarizing film was changed to 5.0 μm, the shrinkage rate in the width direction was set to 0.1% or less without using a heating roll in the drying shrinkage treatment, and the concentration of the dyeing bath was adjusted so that the monomer transmittance (Ts) of the polarizing film became the value shown in table 1. The obtained polarizing film and polarizing plate had a monomer transmittance and A₅₅₀/A₂₁₀Shown in table 1.

Comparative example 3-1

A PVA-based resin film (product name: PS-7500, thickness: 75 μm, average degree of polymerization: 2400, degree of saponification: 99.9 mol%, manufactured by Cola) was dipped in the resinThe film was immersed in a water bath at 30 ℃ for 1 minute, stretched 1.2 times in the carrying direction, then immersed in an aqueous solution at 30 ℃ containing 0.04 wt% of iodine and 0.3 wt% of potassium, dyed, and stretched 2 times based on the completely unstretched film (original length). Then, the stretched film was further stretched 3 times in terms of the original length while being immersed in an aqueous solution of 30 ℃ having a boric acid concentration of 4 wt% and a potassium iodide concentration of 5 wt%, and was further stretched 6 times in terms of the original length while being immersed in an aqueous solution of 60 ℃ having a boric acid concentration of 4 wt% and a potassium iodide concentration of 5 wt%, and was then immersed in a cleaning bath (an aqueous solution of 4 wt% of potassium iodide) of 30 ℃ and dried at 70 ℃ for 2 minutes, thereby obtaining a polarizer having a thickness of 27 μm. The polarizer obtained had a monomer transmittance of 43.0%. It is desired to determine A₂₁₀However, since the measurement limit is exceeded by excessively large, A cannot be obtained₅₅₀/A₂₁₀. The results are shown in Table 1.

As is clear from Table 1 and FIG. 3, the polarizing films of examples of the present invention are compared with the polarizing film of comparative example A₅₅₀/A₂₁₀Significantly larger, therefore, the content ratio of iodide ions in the polarizing film, which do not form a complex with PVA, is very small, and the content ratio of PVA-iodine complex, which has absorption in visible light, is very large. On the other hand, A₄₇₀/A₆₀₀It was not significantly reduced and was thus retained, and it was found that a PVA-iodine complex (PVA-I having an absorption near 600 nm)₅ ^-Complex and PVA-I having absorption near 470nm₃ ^-Complex) is well maintained, and has high polarization performance over the entire visible light region. It is assumed that the balance is greatly changed, as in A₄₇₀/A₆₀₀When the amount is less than 0.5 or more than 2.0, the polarization performance in a part of the visible light region is lowered, and there is a possibility that the hue greatly deviates from the colorless range and discoloration is observed. Polarization of comparative examples 1 to 3, compared with the polarizing film of examplesA of the film₄₇₀/A₆₀₀Larger, but A₅₅₀/A₂₁₀And is significantly smaller. In addition, polarizing films A of comparative examples 1 to 7 and 1 to 8₄₇₀/A₆₀₀If it is too small, the polarization performance in the short wavelength region is greatly reduced. Accordingly, it is found that the orthogonal transmittance at a short wavelength is selectively increased, the orthogonal b value is decreased, and the display color in the orthogonal state (black display) is largely deviated from the colorless range and turns blue. That is, the polarizing film according to the embodiment of the present invention can increase the ratio of the PVA-iodine complex having light absorption in the visible light region, and as a result, can achieve both high polarizing performance and a thinner polarizing film. In addition, the polarizing films of examples of the present invention had orthogonal b values very close to 0 (zero) as compared with the polarizing films of comparative examples 1 to 7 and 1 to 8. Therefore, a polarizing film having a colorless display hue in an orthogonal state (black display) can be obtained.

Industrial applicability

The polarizing plate having the polarizing film of the present invention can be suitably used for a liquid crystal display device.

Claims

1. A polarizing film comprising a polyvinyl alcohol-based resin film containing iodine and having a thickness of 8 μm or less,

the polarizing film has a monomer transmittance of 41.5% or more and 43.5% or less, a degree of polarization of 99.990% or more and 99.998% or less,

the orthogonal absorbance A of the polarizing film at a wavelength of 550nm₅₅₀Orthogonal absorbance A at a wavelength of 210nm₂₁₀Ratio of A₅₅₀/A₂₁₀1.4 to 3.5 of a metal oxide,

the orthogonal absorbance A of the polarizing film at a wavelength of 470nm₄₇₀With orthogonal absorbance A at a wavelength of 600nm₆₀₀Ratio of A₄₇₀/A₆₀₀Is in the range of 0.7 to 2.00,

the orthogonal b value of the polarizing film is greater than-10 and not more than + 10.

2. The polarizing film according to claim 1, having a thickness of 5 μm or less.

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

the ratio A₅₅₀/A₂₁₀Is 1.8 or more.

4. The polarizing film according to claim 1 or 2, having a monomer transmittance of 42.5% or more.

5. A polarizing plate comprising the polarizing film according to any one of claims 1 to 4 and a protective layer disposed on at least one side of the polarizing film.

6. A method for manufacturing a polarizing film, 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

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 polarizing film comprises a polyvinyl alcohol resin film containing iodine and has a thickness of 8 μm or less,

7. The manufacturing method according to claim 6,

the iodide is potassium iodide.

8. The manufacturing method according to claim 7,

the content of potassium iodide in the polyvinyl alcohol resin layer is 5 to 20 parts by weight per 100 parts by weight of the polyvinyl alcohol resin.

9. The production method according to any one of claims 6 to 8,

the drying shrinkage treatment is performed using a heated roller.

10. The manufacturing method according to claim 9,

the temperature of the heating roller is 60-120 ℃.