JP7219017B2 - Method for manufacturing polarizing film - Google Patents

Description

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

Along with the popularization of thin displays, displays equipped with organic EL panels (OLED) and displays using display panels using inorganic light-emitting materials such as quantum dots (QLED) have been proposed. These panels have highly reflective metal layers, and tend to cause problems such as external light reflection and background reflection. 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, there is a method in which a laminate having a resin substrate and a polyvinyl alcohol (PVA) resin layer is stretched and then dyed to obtain a polarizing film on the resin substrate. It has been proposed (for example, Patent Document 1). According to such a method, since a thin polarizing film can be obtained, it has attracted attention as a potential contribution to the thinning of image display devices in recent years. In addition, as the reflectance of the panel becomes lower as the performance of the display panel improves, the required characteristic of the degree of polarization becomes lower, and a polarizing plate with a higher transmittance is required. However, when trying to increase the transmittance of conventional thin polarizing films, problems such as the dissolution of PVA-based resins occur, and it has not been possible to produce films that can withstand optical applications.

JP-A-2001-343521

The present invention has been made to solve the above-mentioned conventional problems, and its main purpose is to provide a polarizing film, a polarizing plate, a polarizing plate roll, and a method for producing such a polarizing film, which have excellent optical properties. to do.

The polarizing film of the present invention has a thickness of 8 μm or less, a single transmittance of 48% or more, and a polarization degree of 85% or more.
In one embodiment, the single transmittance is 50% or less and the degree of polarization is 92% or less.
According to another aspect of the invention, a polarizing plate is provided. This polarizing plate has a polarizing film and a protective layer disposed on at least one side of the polarizing film.
In one embodiment, the polarizing plate has a width of 1000 mm or more, and the difference between the maximum value and the minimum value of single transmittance at positions along the width direction is 1% or less.
In one embodiment, the polarizer has a difference of 0.5% or less between the maximum and minimum single transmittance values within an area of 50 cm ² .
According to another aspect of the invention, a polarizer roll is provided. The polarizing plate roll is obtained by winding the polarizing plate into a roll.
According to another aspect of the present invention, a method for manufacturing a polarizing film is provided. This manufacturing method is a method for manufacturing the above polarizing film, wherein a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of a long thermoplastic resin substrate to form a laminate. Then, the laminate is subjected to an auxiliary aerial stretching treatment with a stretching ratio of 2.0 times or more, a dyeing treatment, an underwater stretching treatment, and heating while being conveyed in the longitudinal direction, so that the width is 2%. and a drying shrinkage treatment for shrinking as described above.
In one embodiment, the content of the halide in the polyvinyl alcohol-based resin layer is 5 to 20 parts by weight with respect to 100 parts by weight of the polyvinyl alcohol-based resin.
In one embodiment, the drying shrinkage treatment step is a step of heating using a heating roll.
In one embodiment, the temperature of the heating roll is 60° C. to 120° C., and the widthwise shrinkage of the laminate due to the drying shrinkage treatment is 2% or more.

According to the present invention, it is possible to provide a polarizing film having excellent optical properties such as a thickness of 8 μm or less, a single transmittance of 48% or more, and a degree of polarization of 85% or more.

1 is a schematic cross-sectional view of a polarizer according to one embodiment of the invention; FIG. FIG. 4 is a schematic diagram showing an example of drying shrinkage treatment using a heating roll. 4 is a graph showing optical properties of polarizing plates obtained in Examples and Comparative Examples.

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 according to one embodiment of the present invention has a thickness of 8 μm or less, a single transmittance of 48% or more, and a degree of polarization of 85% or more. In general, there is a trade-off between the single transmittance and the degree of polarization. Increasing the single transmittance may decrease the degree of polarization, and increasing the degree of polarization may decrease the single transmittance. For this reason, conventionally, it has been difficult to provide a thin polarizing film that satisfies the optical characteristics of a single transmittance of 48% or more and a degree of polarization of 85% or more. As described above, the polarizing film according to one embodiment of the present invention has excellent optical properties such as a single transmittance of 48% or more and a degree of polarization of 85% or more. Furthermore, by using the polarizing film of this embodiment, it is possible to realize a polarizing plate in which variations in optical properties are suppressed. Realization of such a thin polarizing film (polarizing plate) is one of the achievements of the present invention. Such a polarizing film (polarizing plate) can be used in an image display device, and is particularly preferably used as a circularly polarizing plate for an organic EL display device.

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

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 50% or less. The degree of polarization of the polarizing film is preferably 86% or more, more preferably 87% or more, still more preferably 88% or more. On the other hand, the upper limit of the degree of polarization is preferably 92%. The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction. The degree of polarization is typically obtained by the following formula based on the parallel transmittance Tp and the orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction.
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 a laminate of a polarizing film (surface refractive index: 1.53) and a protective film (refractive index: 1.50) It is measured using an ultraviolet-visible spectrophotometer with the body as the measurement target. Depending on the refractive index of the surface of the polarizing film and/or the refractive index of the surface of the protective film in contact with the air interface, the reflectance at each layer interface may change, resulting in a change in the measured transmittance. . Thus, for example, if a protective film with a refractive index not equal to 1.50 is used, the transmittance measurements may be corrected according to the refractive index of the surface of the protective film in contact with the air interface. Specifically, the transmittance correction value C is expressed by the following formula using the reflectance R ₁ (transmission axis reflectance) of polarized light parallel to the transmission axis at the interface between the protective film and the air layer.
C=R ₁ -R ₀
R ₀ = ((1.50−1) ² /(1.50+1) ² )×(T ₁ /100)
R ₁ = ((n ₁ −1) ² /(n ₁ +1) ² )×(T ₁ /100)
Here, R ₀ is the transmission axis reflectance when a protective film having a refractive index of 1.50 is used, n ₁ is the refractive index of the protective film used, and T ₁ is the transmittance of the polarizing film. is. For example, when a substrate having a surface refractive index of 1.53 (a cycloolefin film, a film with a hard coat layer, etc.) is used as the protective film, the correction amount C is about 0.2%. In this case, by adding 0.2% to the transmittance obtained by the measurement, it is possible to convert the transmittance into the transmittance when using a protective film having a surface refractive index of 1.50. According to the calculation based on the above formula, the amount of change in the correction value C when the transmittance T1 of the polarizing film is changed by ₂ % is 0.03% or less, and the transmittance of the polarizing film is equal to the correction value C has a limited effect on the value of Moreover, when the protective film has absorption other than surface reflection, appropriate correction can be performed according to the amount of absorption.

Any appropriate polarizing film can be employed as the polarizing film. A polarizing film can typically be produced using a laminate of two or more layers.

A specific example of a polarizing film obtained using a laminate is a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate. A polarizing film obtained by using a laminate of a resin base material and a PVA-based resin layer formed by coating on the resin base material can be obtained, for example, by applying a PVA-based resin solution to the resin base material and drying the resin base material. forming a PVA-based resin layer thereon to obtain a laminate of a resin substrate and a PVA-based resin layer; stretching and dyeing the laminate to make the PVA-based resin layer a polarizing film; obtain. In this embodiment, stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching. Furthermore, stretching may further include stretching the laminate in air at a high temperature (eg, 95° C. or higher) before stretching in an aqueous boric acid solution, if necessary. The obtained resin substrate/polarizing film laminate may be used as it is (that is, the resin substrate may be used as a protective layer for the polarizing film), or the resin substrate may be peeled off from the resin substrate/polarizing film laminate. Then, any appropriate protective layer may be laminated on the release surface according to the purpose. Details of a method for manufacturing such a polarizing film are described in, for example, Japanese Patent Application Laid-Open No. 2012-73580. The publication is incorporated herein by reference in its entirety.

The method for producing a polarizing film of the present invention includes forming a laminate by forming a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate, and The above laminate is subjected to an auxiliary stretching process in the air with a stretching ratio of 2.0 times or more, a dyeing process, an underwater stretching process, and a drying shrinkage that shrinks by 2% or more in the width direction by heating while being transported in the longitudinal direction. and, in that order. As a result, a polarizing film having a thickness of 8 μm or less, a single transmittance of 48% or more, and a degree of polarization of 85% or more, and having excellent optical properties and suppressed variation in optical properties can be provided. . That is, by introducing auxiliary stretching, it is possible to improve the crystallinity of PVA even when PVA is coated on a thermoplastic resin, and to achieve high optical properties. At the same time, by increasing the orientation of PVA in advance, it is possible to prevent problems such as deterioration of orientation and dissolution of PVA when immersed in water in the subsequent dyeing process or stretching process, resulting in high optical properties. can be achieved. Furthermore, when the PVA-based resin layer is immersed in a liquid, disturbance of the orientation of the polyvinyl alcohol molecules and deterioration of 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 treatment steps such as dyeing treatment and underwater stretching treatment in which the laminate is immersed in a liquid. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by drying shrinkage treatment.

B. Polarizing Plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 has a polarizing film 10, a first protective layer 20 arranged on one side of the polarizing film 10, and a second protective layer 30 arranged on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in section A above. One of the first protective layer 20 and the second protective layer 30 may be omitted. As described above, one of the first protective layer and the second protective layer may be the resin base material used for manufacturing the polarizing film.

The polarizing plate may be elongated or sheet-shaped. When the polarizing plate is elongated, it is preferably wound into a roll to form a polarizing plate roll. A polarizing plate has excellent optical properties and small variations in optical properties. In one embodiment, the polarizing plate has a width of 1000 mm or more, and a difference (D1) between the maximum and minimum single transmittance values at positions along the width direction of 1% or less. The upper limit of D1 is preferably 0.8%, more preferably 0.6%. D1 is preferably as small as possible, but the lower limit is, for example, 0.01%. If D1 is within the above range, a polarizing plate having excellent optical properties can be industrially produced. In another embodiment, the polarizer has a difference (D2) between the maximum and minimum values of single transmittance in an area of 50 cm ² of 0.5% or less. The upper limit of D2 is preferably 0.25%, more preferably 0.15%. D2 is preferably as small as possible, but the lower limit is, for example, 0.01%. If D2 is within the above range, it is possible to suppress variations in luminance on a display screen when the polarizing plate is used in an image display device.

The first and second protective layers are formed of any suitable film that can be used as protective layers for polarizing films. Specific examples of the material that is the main component of the film include cellulose-based resins such as triacetyl cellulose (TAC), polyester-based, polyvinyl alcohol-based, polycarbonate-based, polyamide-based, polyimide-based, polyethersulfone-based, and polysulfone-based resins. , polystyrene-based, polynorbornene-based, polyolefin-based, (meth)acrylic-based, and acetate-based transparent resins. Thermosetting resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone, or ultraviolet curable resins may also be used. In addition, for example, a glassy polymer such as a siloxane-based polymer can also be used. Further, polymer films described in JP-A-2001-343529 (WO01/37007) can also be used. Materials for this film include, 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 nitrile group in a side chain. can be used, for example, a resin composition comprising an alternating copolymer of isobutene and N-methylmaleimide and an acrylonitrile-styrene copolymer. The polymer film can be, for example, an extrudate of the 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 100 μm or less. 5 μm to 80 μm, more preferably 10 μm to 60 μm. In addition, when the surface treatment is performed, the thickness of the outer protective layer is the thickness including the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the protective layer (inner protective layer) disposed on the display panel side preferably has a thickness of 5 μm to 200 μm, more preferably 10 μm to 100 μm, and still more preferably 10 μm to 60 μm. be. In one embodiment, the inner protective layer is a retardation layer with any suitable retardation value. In this case, the in-plane retardation Re(550) of the retardation layer is, for example, 110 nm to 150 nm. “Re(550)” is an in-plane retardation measured with light having a wavelength of 550 nm at 23° C., and is obtained by the formula: Re=(nx−ny)×d. Here, “nx” is the refractive index in the direction in which the in-plane refractive index is maximized (that is, the slow axis direction), and “ny” is the in-plane direction orthogonal to the slow axis (that is, the fast is the refractive index in the axial direction), 'nz' is the refractive index in the thickness direction, and 'd' is the layer (film) thickness (nm).

C. Method for producing polarizing film A method for producing a polarizing film according to one embodiment of the present invention comprises polyvinyl alcohol containing a halide and a polyvinyl alcohol resin (PVA resin) on one side of a long thermoplastic resin substrate. Forming a system resin layer (PVA-based resin layer) to form a laminate, and subjecting the laminate to an auxiliary air-stretching process with a draw ratio of 2.0 times or more, a dyeing process, and an underwater stretching process, and a drying shrinkage treatment for shrinking the film by 2% or more in the width direction by heating while transporting it in the longitudinal direction. The content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably performed using a heating roll, and the temperature of the heating roll is preferably 60°C to 120°C. The shrinkage ratio in the width direction of the laminate due to drying shrinkage treatment is preferably 2% or more. According to such a manufacturing method, the polarizing film described in the above section A can be obtained. In particular, a laminate containing a PVA-based resin layer containing a halide is produced, the laminate is stretched in multiple stages including auxiliary stretching in the air and stretching in water, and the laminate after stretching is heated with a heating roll. It is possible to obtain a polarizing film having excellent optical properties (typically, single transmittance and degree of polarization) and suppressed variations in optical properties. Specifically, by using a heating roll in the drying shrinkage treatment step, the entire laminate can be uniformly shrunk while being transported. As a result, it is possible not only to improve the optical properties of the obtained polarizing film, but also to stably produce a polarizing film with excellent optical properties, and to suppress variations in the optical properties of the polarizing film (in particular, the transmittance of a single unit). can do.

C-1. Production of Laminate Any appropriate method can be adopted as a method for producing a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating liquid containing a halide and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and dried to form a PVA-based resin layer on the thermoplastic resin substrate. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin.

Any appropriate method can be adopted as a method for applying the coating liquid. Examples thereof include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating, etc.). The coating/drying temperature of the coating liquid is preferably 50° C. or higher.

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

Before forming the PVA-based resin layer, the thermoplastic resin substrate may be surface-treated (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 Substrate The thickness of the thermoplastic resin substrate is preferably 20 μm to 300 μm, more preferably 50 μm to 200 μm. If the thickness is less than 20 μm, it may be difficult to form the PVA-based resin layer. If it exceeds 300 μm, for example, in the later-described underwater stretching treatment, it may take a long time for the thermoplastic resin substrate to absorb water, and an excessive load may be required for stretching.

The thermoplastic resin substrate preferably has a water absorption of 0.2% or more, more preferably 0.3% or more. Thermoplastic resin substrates can absorb water and be plasticized with the water acting like a plasticizer. As a result, the stretching stress can be greatly reduced, and the film can be stretched at a high draw ratio. On the other hand, the water absorption rate of the thermoplastic resin substrate is preferably 3.0% or less, more preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent problems such as deterioration in the appearance of the obtained polarizing film due to a marked decrease in the dimensional stability of the thermoplastic resin substrate during production. Moreover, it is possible to prevent breakage of the base material and separation of the PVA-based resin layer from the thermoplastic resin base material during stretching in water. The water absorption rate of the thermoplastic resin substrate can be adjusted, for example, by introducing a modifying group into the constituent material. The water absorption is a value determined according to JIS K7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 120°C or lower. By using such a thermoplastic resin substrate, it is possible to sufficiently secure the stretchability of the laminate while suppressing the crystallization of the PVA-based resin layer. Furthermore, considering the plasticization of the thermoplastic resin substrate with water and the satisfactory stretching in water, the temperature is preferably 100° C. or lower, more preferably 90° C. or lower. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60°C or higher. By using such a thermoplastic resin substrate, deformation of the thermoplastic resin substrate (for example, generation of unevenness, sagging, wrinkles, etc.) occurs when the coating liquid containing the PVA-based resin is applied and dried. It is possible to prevent the problem of and produce a good laminate. Moreover, the PVA-based resin layer can be satisfactorily stretched at a suitable temperature (for example, about 60°C). The glass transition temperature of the thermoplastic resin substrate can be adjusted, for example, by heating using a crystallization material that introduces a modifying group into the constituent material. The glass transition temperature (Tg) is a value determined according to JIS K7121.

Any appropriate thermoplastic resin may be employed as a constituent material of the thermoplastic resin base material. Examples of thermoplastic resins include ester resins such as polyethylene terephthalate resins, cycloolefin resins such as norbornene resins, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, and copolymer resins thereof. is mentioned. Among these, norbornene-based resins and amorphous polyethylene terephthalate-based resins are preferred.

In one embodiment, an amorphous (not crystallized) polyethylene terephthalate resin is preferably used. Among them, amorphous (difficult to crystallize) polyethylene terephthalate resin is particularly preferably used. Specific examples of amorphous polyethylene terephthalate resins include copolymers further containing isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers further containing cyclohexanedimethanol or diethylene glycol as glycols.

In a preferred embodiment, the thermoplastic resin base material is composed of a polyethylene terephthalate-based resin having an isophthalic acid unit. This is because such a thermoplastic resin substrate is extremely excellent in stretchability and can suppress crystallization during stretching. This is probably because the introduction of the isophthalic acid unit gives the main chain a large bend. A polyethylene terephthalate-based resin has a terephthalic acid unit and an ethylene glycol unit. The isophthalic acid unit content is preferably 0.1 mol % or more, more preferably 1.0 mol % or more, relative to the total of all repeating units. This is because a thermoplastic resin base material having extremely excellent stretchability can be obtained. On the other hand, the isophthalic acid unit content is preferably 20 mol % or less, more preferably 10 mol % or less, relative to the total of all repeating units. By setting such a content ratio, the degree of crystallinity can be favorably increased in the drying shrinkage treatment described later.

The thermoplastic resin substrate may be stretched in advance (before forming the PVA-based resin layer). In one embodiment, it is stretched in the transverse direction of the elongated thermoplastic resin substrate. The lateral direction is preferably a direction perpendicular to the stretching direction of the laminate described below. In addition, in this specification, "perpendicular" also includes the case of being substantially perpendicular. Here, "substantially orthogonal" includes 90°±5.0°, preferably 90°±3.0°, more preferably 90°±1.0°.

The stretching temperature of the thermoplastic resin substrate is preferably Tg-10°C to Tg+50°C with respect to the glass transition temperature (Tg). The draw ratio of the thermoplastic resin substrate is preferably 1.5 to 3.0 times.

Any appropriate method can be adopted as a method for stretching the thermoplastic resin substrate. Specifically, the drawing may be fixed end drawing or free end drawing. The stretching method may be a dry method or a wet method. The stretching of the thermoplastic resin substrate may be performed in one step or in multiple steps. When performing in multiple stages, the above-mentioned draw ratio is the product of the draw ratios in each step.

C-1-2. Coating Liquid The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid is typically a solution in which the halide and the PVA-based resin are dissolved in a solvent. Examples of solvents include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Among these, water is preferred. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight with respect to 100 parts by weight of the solvent. With such a resin concentration, it is possible to form a uniform coating film 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 with respect to 100 parts by weight of the PVA-based resin.

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

Any appropriate resin can be adopted as the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. An ethylene-vinyl alcohol copolymer is obtained by saponifying an ethylene-vinyl acetate copolymer. The degree of saponification of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, more preferably 99.0 mol% to 99.93 mol%. . The degree of saponification can be determined according to JIS K 6726-1994. By using a PVA-based resin having such a degree of saponification, a polarizing film having excellent durability can be obtained. If the degree of saponification is too high, gelation may occur.

The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the purpose. The average degree of polymerization is usually 1,000 to 10,000, preferably 1,200 to 4,500, more preferably 1,500 to 4,300. The average degree of polymerization can be determined according to JIS K 6726-1994.

Any appropriate halide can be employed as the halide. Examples include iodide and sodium chloride. Iodides include, for example, potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferred.

The amount of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight with respect to 100 parts by weight of the PVA resin, and more preferably 10 parts by weight to 15 parts by weight with respect to 100 parts by weight of the PVA resin. Department. If the amount of the halide exceeds 20 parts by weight with respect to 100 parts by weight of the PVA-based resin, the halide may bleed out and the finally obtained polarizing film may become cloudy.

In general, when the PVA-based resin layer is stretched, the orientation of the polyvinyl alcohol molecules in the PVA-based resin increases. Orientation may be disturbed and the orientation may be lowered. In particular, when stretching a laminate of a thermoplastic resin and a PVA-based resin layer in boric acid water, the laminate is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin. The tendency for the degree of orientation to decrease is remarkable. For example, the stretching of a single PVA film in boric acid water is generally carried out at 60° C., whereas the stretching of a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is It is carried out at a high temperature of about 70° C., and in this case, the orientation of PVA at the initial stage of stretching may be lowered before it is increased by stretching in water. On the other hand, by preparing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate and stretching the laminate at a high temperature (auxiliary stretching) in air before stretching in boric acid water, , the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, the disturbance of the orientation of the polyvinyl alcohol molecules and the deterioration of the orientation can be suppressed compared to 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 treatment steps such as dyeing treatment and underwater stretching treatment in which the laminate is immersed in a liquid.

C-2. Aerial Auxiliary Stretching In order to obtain particularly high optical properties, a two-stage stretching method combining dry stretching (auxiliary stretching) and stretching in boric acid solution is selected. By introducing auxiliary stretching, such as two-step stretching, it is possible to stretch while suppressing crystallization of the thermoplastic resin substrate, and excessive crystallization of the thermoplastic resin substrate in the subsequent stretching in boric acid water. It is possible to solve the problem that stretchability is reduced by stretching, and stretch the laminate at a higher magnification. Furthermore, when applying the PVA-based resin on the thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, compared to the case of applying the PVA-based resin on a normal metal drum As a result, the crystallization of the PVA-based resin becomes relatively low, which may cause a problem that sufficient optical properties cannot be obtained. On the other hand, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when the PVA-based resin is applied on the thermoplastic resin, and it is possible to achieve high optical properties. Become. At the same time, by increasing the orientation of the PVA-based resin in advance, it is possible to prevent problems such as deterioration of the orientation and dissolution of the PVA-based resin when it is immersed in water in the subsequent dyeing process or stretching process. , making it possible to achieve high optical properties.

The stretching method of the in-air auxiliary stretching may be fixed edge stretching (e.g., a method of stretching using a tenter stretching machine) or free edge stretching (e.g., a method of uniaxially stretching the laminate through rolls having different peripheral speeds). However, in order to obtain high optical properties, free end drawing can be positively employed. In one embodiment, the in-air stretching process includes a heating roll stretching step in which the laminate is stretched by a peripheral speed difference between heating rolls while being conveyed in the longitudinal direction. The air drawing process typically includes a zone drawing process and a hot roll drawing process. The order of the zone stretching process and the heating roll stretching process is not limited, and the zone stretching process may be carried out first, or the heating roll stretching process may be carried out first. The zone drawing step may be omitted. In one embodiment, the zone drawing step and the heated roll drawing step are performed in this order. In another embodiment, in a tenter stretching machine, the film is stretched by gripping the ends of the film and increasing the distance between the tenters in the machine direction (the extension of the distance between the tenters is the stretching ratio). At this time, the distance between the tenters in the width direction (perpendicular to the machine direction) is set to be arbitrarily close. Preferably, the draw ratio in the machine direction can be set to be closer to the free end draw. In the case of free end stretching, the shrinkage ratio in the width direction is calculated by (1/stretching ratio) ^1/2 .

The in-air auxiliary stretching may be performed in one step or in multiple steps. When it is carried out in multiple stages, the draw ratio is the product of the draw ratios in each step. The stretching direction in the in-air auxiliary stretching is preferably substantially the same as the stretching direction in the underwater stretching.

The draw ratio in the in-air auxiliary drawing is preferably 2.0 to 3.5 times. The maximum draw ratio when the auxiliary drawing in the air and the drawing in water are combined is preferably 5.0 times or more, more preferably 5.5 times or more, and still more preferably 6.0 times the original length of the laminate. That's it. As used herein, the term "maximum draw ratio" refers to the draw ratio immediately before the laminate breaks, and is 0.2 lower than the draw ratio at which the laminate breaks.

The stretching temperature for the in-air auxiliary stretching can be set to any appropriate value depending on the material for forming the thermoplastic resin base material, the stretching method, and the like. The stretching temperature is preferably the glass transition temperature (Tg) of the thermoplastic resin substrate or higher, more preferably the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C or higher, and particularly preferably Tg + 15°C or higher. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, it is possible to suppress rapid crystallization of the PVA-based resin and suppress problems caused by the crystallization (for example, hindrance of orientation of the PVA-based resin layer due to stretching). can. The crystallization index of the PVA-based resin after auxiliary stretching in air is preferably 1.3 to 1.8, more preferably 1.4 to 1.7. The crystallization index of the PVA-based resin can be measured by the ATR method using a Fourier transform infrared spectrophotometer. Specifically, measurement is performed using polarized light as measurement light, and the crystallization index is calculated according to the following formula using the intensities at 1141 cm ⁻¹ and 1440 cm ⁻¹ of the obtained spectrum.
_{Crystallization} index = (IC/ _IR )
however,
I _C : Intensity at 1141 cm ⁻¹ when measurement light is incident and measured I _R : Intensity at 1440 cm ⁻¹ when measurement light is incident and measured.

C-3. Insolubilization Treatment If necessary, an insolubilization treatment is performed after the auxiliary stretching treatment in the air and before the stretching treatment in water or the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The insolubilization treatment imparts water resistance to the PVA-based resin layer, and prevents deterioration of the orientation of the PVA when immersed in water. The concentration of the boric acid aqueous solution is preferably 1 to 4 parts by weight with respect to 100 parts by weight of water. The liquid temperature of the insolubilizing bath (boric acid aqueous solution) is preferably 20°C to 50°C.

C-4. Dyeing Treatment The dyeing treatment is typically performed by dyeing the PVA-based resin layer with iodine. Specifically, it is carried out by allowing the PVA-based resin layer to adsorb iodine. 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 coating the PVA-based resin layer with the dyeing solution, and a method of applying the dyeing solution to the PVA-based resin layer. A spraying method and the like can be mentioned. A preferred method is to immerse the laminate in a dyeing solution (dyeing bath). This is because iodine can be well adsorbed.

The staining solution is preferably an iodine aqueous solution. The amount of iodine compounded is preferably 0.05 to 0.5 parts by weight per 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add an iodide to the iodine aqueous solution. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. etc. Among these, potassium iodide is preferred. The amount of iodide compounded is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, per 100 parts by weight of water. The liquid temperature of the dyeing liquid during dyeing is preferably 20° C. to 50° C. in order to suppress the dissolution of the PVA-based resin. When the PVA-based resin layer is immersed in the staining solution, 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 finally obtained polarizing film has a single transmittance of 48% or more and a degree of polarization of 85% or more. As for such dyeing conditions, it is preferable to use an iodine aqueous solution as a dyeing solution and to set the content ratio of iodine and potassium iodide in the iodine aqueous solution to 1:5 to 1:20. The content ratio of iodine and potassium iodide in the iodine aqueous solution is 1:5 to 1:20. The content ratio of iodine and potassium iodide in the aqueous iodine solution is preferably 1:5 to 1:10. Thereby, a polarizing film having the optical properties as described above can be obtained.

When dyeing treatment is performed continuously after the treatment of immersing the laminate in a treatment bath containing boric acid (typically, insolubilization treatment), the boric acid contained in the treatment bath is mixed into the dyeing bath. The boric acid concentration in the dyeing bath may change over time, resulting in unstable dyeability. In order to suppress the destabilization of dyeability as described above, the upper limit of the boric acid concentration in the dyeing bath is preferably 4 parts by weight, more preferably 2 parts by weight with respect to 100 parts by weight of water. adjusted. On the other hand, the lower limit of the boric acid concentration in the dyeing bath is preferably 0.1 parts by weight, more preferably 0.2 parts by weight, and still more preferably 0.5 parts by weight with respect to 100 parts by weight of water. is. In one embodiment, the dyeing process is performed using a dyeing bath pre-blended with boric acid. This can reduce the rate of change in boric acid concentration when the boric acid in the treatment bath is mixed into the dyeing bath. The amount of boric acid blended in advance in the dyeing bath (that is, the content of boric acid not derived from the treatment bath) is preferably 0.1 to 2 parts by weight with respect to 100 parts by weight of water. , more preferably 0.5 to 1.5 parts by weight.

C-5. Crosslinking Treatment If necessary, a crosslinking treatment is applied after the dyeing treatment and before the underwater stretching treatment. The cross-linking treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. The cross-linking treatment imparts water resistance to the PVA-based resin layer, and prevents deterioration of the orientation of the PVA when immersed in high-temperature water in the subsequent underwater stretching. The concentration of the boric acid aqueous solution is preferably 1 to 5 parts by weight with respect to 100 parts by weight of water. Moreover, when cross-linking treatment is carried out after the dyeing treatment, it is preferable to further add an iodide. By blending iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. The amount of iodide compounded is preferably 1 to 5 parts by weight per 100 parts by weight of water. Specific examples of iodides are as described above. The liquid temperature of the cross-linking bath (boric acid aqueous solution) is preferably 20°C to 50°C.

C-6. Underwater Stretching Treatment Underwater stretching treatment is performed by immersing the laminate in a stretching bath. According to the underwater stretching treatment, the thermoplastic resin substrate and the PVA-based resin layer can be stretched at a temperature lower than the glass transition temperature (typically, about 80° C.), and the PVA-based resin layer undergoes its crystallization. can be stretched at a high magnification while suppressing the As a result, a polarizing film having excellent optical properties can be produced.

Any appropriate method can be adopted as a method for stretching the laminate. Specifically, fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching a laminate by passing it between rolls having different peripheral speeds) may be used. Free-end drawing is preferably chosen. The laminate may be stretched in one step or in multiple steps. When the stretching is performed in multiple stages, the draw ratio (maximum draw ratio) of the laminate described below is the product of the draw ratios in each step.

The stretching in water is preferably carried out by immersing the laminate in an aqueous boric acid solution (stretching in boric acid water). By using an aqueous boric acid solution as the stretching bath, the PVA-based resin layer can be imparted with rigidity to withstand tension applied during stretching and water resistance that does not dissolve in water. Specifically, boric acid can form a tetrahydroxyborate anion in an aqueous solution and crosslink with a PVA-based resin through hydrogen bonding. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, which can be satisfactorily stretched, and a polarizing film having excellent optical properties can be produced.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a borate salt in water as a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 3.5 to 7 parts by weight, and particularly preferably 4 to 6 parts by weight with respect to 100 parts by weight of water. is. 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 with higher properties can be produced. In addition to boric acid or borate salts, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent can also be used.

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

The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At such a temperature, the film can be stretched at a high magnification while suppressing dissolution of the PVA-based resin layer. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60° C. or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40° C., it may not be possible to stretch well even if the plasticization of the thermoplastic resin base material 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, which may make it impossible to obtain excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The draw ratio by underwater drawing is preferably 1.5 times or more, more preferably 3.0 times or more. The total draw ratio of the laminate is preferably 5.0 times or more, more preferably 5.5 times or more, relative to the original length of the laminate. By achieving such a high draw ratio, a polarizing film with extremely excellent optical properties can be produced. Such a high draw ratio can be achieved by adopting an underwater drawing method (boric acid solution drawing).

C-7. Drying Shrinkage Treatment The drying shrinkage treatment may be performed by zone heating performed by heating the entire zone, or by heating the transport roll (using a so-called heating roll) (heated roll drying method). Preferably both are used. By drying using a heating roll, it is possible to efficiently suppress heat curling of the laminate and produce a polarizing film having an excellent appearance. Specifically, by drying the laminated body along the heating roll, the crystallization of the thermoplastic resin substrate can be efficiently promoted to increase the degree of crystallinity, which is relatively low. Even at the drying temperature, the degree of crystallinity of the thermoplastic resin substrate can be favorably increased. As a result, the thermoplastic resin base material has increased rigidity and is in a state capable of withstanding shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. Moreover, by using a heating roll, the layered product can be dried while being maintained in a flat state, so that not only curling but also wrinkling can be suppressed. At this time, the laminate can be shrunk in the width direction by drying shrinkage treatment, thereby improving the optical properties. This is because the orientation of PVA and PVA/iodine complex can be effectively enhanced. The shrinkage ratio of the laminate in the width direction due to drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%. By using the heating roll, the laminate can be continuously shrunk in the width direction while being transported, and high productivity can be achieved.

FIG. 2 is a schematic diagram showing an example of drying shrinkage treatment. In the drying shrinkage process, the laminate 200 is dried while being transported by transport rolls R1 to R6 heated to a predetermined temperature and guide rolls G1 to G4. In the illustrated example, the transport rolls R1 to R6 are arranged so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate. The transport rolls R1 to R6 may be arranged so as to continuously heat only the surface of the resin base material.

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

The heating roll may be provided in a heating furnace (for example, oven), or may be provided in a normal production line (under room temperature environment). Preferably, it is provided in a heating furnace equipped with air blowing means. By using both heating roll drying and hot air drying, abrupt temperature changes between the heating rolls can be suppressed, and shrinkage in the width direction can be easily controlled. The temperature for hot air drying is preferably 30°C to 100°C. Moreover, the hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10m/s to 30m/s. The wind speed is the wind speed in the heating furnace and can be measured with a mini-vane digital anemometer.

C-8. Other Treatments Preferably, a washing treatment is performed after the underwater stretching treatment and before the drying shrinkage treatment. The cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of potassium iodide.

なお、例えば、アンチグレア（ＡＧ）の表面処理や拡散性能を有する粘着剤を備える偏光板を測定対象とする場合、分光光度計に依存して異なる測定結果が得られ得るが、この場合、同一の偏光板をそれぞれの分光光度計で測定したときの測定値に基づいて数値換算を行うことにより、分光光度計に依存する測定値の差を補償することが可能である。
（３）長尺状の偏光板の光学特性のバラつき
実施例および参考例の長尺状の偏光板から、幅方向に沿って等間隔に５か所の各位置で測定サンプルを切り出し、５つの各測定サンプルの中央部分の単体透過率を上記（２）と同様にして測定した。次いで、各測定位置において測定された単体透過率のうちの最大値と最小値との差を算出し、この値を長尺状の偏光板の光学特性のバラつき（長尺状偏光板の幅方向に沿った位置における単体透過率の最大値と最小値との差）とした。
（４）枚葉状の偏光板の光学特性のバラつき
実施例および参考例の長尺状の偏光板から、１００ｍｍ×１００ｍｍの測定サンプルを切り出し、枚葉状の偏光板（５０ｃｍ^２）の光学特性のバラつきを求めた。具体的には、測定サンプルの４辺の各辺の中点から内側に約１．５ｃｍ～２．０ｃｍ付近の位置および中央部分の計５か所の単体透過率を上記（２）と同様にして測定した。次いで、各測定位置において測定された単体透過率のうちの最大値と最小値との差を算出し、この値を枚葉状の偏光板の光学特性のバラつき（５０ｃｍ^２の領域内における単体透過率の最大値と最小値との差）とした。
（５）空中補助延伸後のＰＶＡ系樹脂層の結晶化指数
空中補助延伸後の積層体について、フーリエ変換赤外分光光度計（Ｐｅｒｋｉｎ Ｅｌｍｅｒ社製、製品名「ＦＴ－ＩＲ Ｆｒｏｎｔｉｅｒ」）を用い、偏光を測定光として、ＡＴＲ測定によりＰＶＡ系樹脂層表面の評価を行った。具体的には、測定を実施し、得られたスペクトルの１１４１ｃｍ^－１および１４４０ｃｍ^－１の強度を用いて、下記式に従って結晶化指数を算出した。
結晶化指数＝（Ｉ_Ｃ／Ｉ_Ｒ）
ただし、
Ｉ_Ｃ ：測定光を入射して測定したときの１１４１ｃｍ^－１の強度
Ｉ_Ｒ ：測定光を入射して測定したときの１４４０ｃｍ^－１の強度 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 characteristic is as follows. "Parts" and "%" in Examples and Comparative Examples are by weight unless otherwise specified.
(1) Thickness Measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name: "MCPD-3000").
(2) Single transmittance and degree of polarization For the polarizing plates (protective films/polarizing films) of Examples and Comparative Examples, single transmittance Ts measured using a UV-visible spectrophotometer (V-7100 manufactured by JASCO Corporation), The parallel transmittance Tp and the orthogonal transmittance Tc were defined as Ts, Tp and Tc of the polarizing film, respectively. These Ts, Tp and Tc are Y values measured with a 2-degree field of view (C light source) according to JIS Z8701 and subjected to visibility correction. 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 obtained Tp and Tc, the degree of polarization P was determined by the following formula.
Degree of polarization P (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100
As for the spectrophotometer, LPF-200 manufactured by Otsuka Electronics Co., Ltd. can be used for equivalent measurement. As an example, Table 1 shows the measured values of the single transmittance Ts and the degree of polarization P obtained by measurement using V-7100 and LPF-200 for Samples 1 to 3 of polarizing plates having the same configuration as in the following examples. show. As shown in Table 1, the difference between the measured value of the single transmittance of V-7100 and the measured value of the single transmittance of LPF-200 is 0.1% or less, and any spectrophotometer was used. It can be seen that equivalent measurement results can be obtained even in the case.

In addition, for example, when measuring a polarizing plate equipped with an anti-glare (AG) surface treatment or an adhesive having diffusion performance, different measurement results may be obtained depending on the spectrophotometer, but in this case, the same By performing numerical conversion based on the measured values obtained when the polarizing plate is measured by each spectrophotometer, it is possible to compensate for the difference in the measured values depending on the spectrophotometer.
(3) Variation in optical properties of long polarizing plates From the long polarizing plates of Examples and Reference Examples, measurement samples were cut out at five positions at equal intervals along the width direction, and five The single transmittance of the central portion of each measurement sample was measured in the same manner as in (2) above. Next, the difference between the maximum and minimum values of the single transmittance measured at each measurement position is calculated, and this value is used as the variation in the optical properties of the long polarizing plate (the width direction of the long polarizing plate difference between the maximum and minimum values of the single transmittance at the position along the
(4) Variation in optical properties of sheet-shaped polarizing plate A measurement sample of 100 mm × 100 mm was cut out from the long polarizing plate of Examples and Reference Examples, and the variation in optical properties of the sheet-shaped polarizing plate (50 cm ² ) was measured. asked for Specifically, the single transmittance at a total of 5 points in the vicinity of about 1.5 cm to 2.0 cm inside from the midpoint of each of the four sides of the measurement sample and the central portion is set in the same manner as in (2) above. measured by Next, the difference between the maximum and minimum single transmittance values measured at each measurement position is calculated, and this value is used as the variation in the optical properties of the sheet-shaped polarizing plate (single transmittance within a 50 cm ² area difference between the maximum and minimum values of ).
(5) Crystallization index of PVA-based resin layer after in-air auxiliary stretching The surface of the PVA-based resin layer was evaluated by ATR measurement using polarized light as measurement light. Specifically, the crystallization index was calculated according to the following formula using the intensities at 1141 cm ⁻¹ and 1440 cm ⁻¹ of the spectrum obtained by measurement.
_{Crystallization} index = (IC/ _IR )
however,
I _C : Intensity at 1141 cm ⁻¹ when measured with incident measurement light I _R : Intensity at 1440 cm ⁻¹ when measured with incident measurement light

[Example 1]
1. Preparation of Polarizing Film As a thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a water absorption of 0.75% and a Tg of about 75° C. was used. Corona treatment was applied to one side of the resin substrate.
Polyvinyl alcohol (degree of polymerization: 4,200, degree of saponification: 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOSEFIMER Z410") mixed at 9:1: 100 weight of PVA-based resin 13 parts by weight of potassium iodide was added to 10 parts by weight to prepare a PVA aqueous solution (coating liquid).
The above PVA aqueous solution was applied to the corona-treated surface of the resin base material and dried at 60° C. to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched 2.4 times at the free end in the machine direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 130° C. (in-air auxiliary stretching treatment).
Next, the laminate was immersed in an insolubilizing bath (an aqueous boric acid solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insolubilizing treatment).
Next, the finally obtained polarizing film is placed in a dyeing bath (iodine aqueous solution obtained by blending 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 ° C. It was immersed for 60 seconds while adjusting the concentration so that the single transmittance (Ts) was 48% or more (dyeing treatment).
Next, it was immersed for 30 seconds in a cross-linking bath at a liquid temperature of 40°C (an aqueous solution of boric acid obtained by blending 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water). (crosslinking treatment).
Thereafter, the laminate is immersed in an aqueous solution of boric acid (concentration of boric acid: 5.0% by weight) at a liquid temperature of 70° C., and stretched between rolls having different peripheral speeds in the longitudinal direction (longitudinal direction) at a total draw ratio of 5.5. It was uniaxially stretched so as to double (stretching in water).
After that, the laminate was immersed in a washing bath (aqueous solution obtained by blending 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20° C. (washing treatment).
After that, while drying in an oven kept at 90° C., it was brought into contact with a heating roll made of SUS whose surface temperature was kept at 75° C. for about 2 seconds (drying shrinkage treatment). The shrinkage ratio in the width direction of the laminate due to the drying shrinkage treatment was 5.2%.
Thus, a polarizing film having a thickness of 5 μm was formed on the resin substrate. Furthermore, the same procedure was repeated to produce a total of 10 polarizing films.
2. Preparation of polarizing plate On the surface of each polarizing film obtained above (the surface opposite to the resin base material), an acrylic film (surface refractive index: 1.50, 40 µm) was applied as a protective film, and an ultraviolet curable adhesive was applied. They were pasted together via an agent. Specifically, the curable adhesive was applied so as to have a total thickness of 1.0 μm, and was bonded using a roll machine. After that, UV rays were irradiated from the protective film side to cure the adhesive. Next, after slitting both ends, the resin substrate was peeled off to obtain 10 elongated polarizing plates (width: 1300 mm) having a structure of protective film/polarizing film.

[Reference example 1]
Twelve polarizing films and polarizing plates were produced in the same manner as in Example 1, except that the dyeing treatment was performed so that the single transmittance (Ts) of the finally obtained polarizing film was 43% or more and less than 48%. bottom.

[Comparative Example 1]
Example 1 except that potassium iodide was not added to the PVA aqueous solution (coating solution), the stretching ratio in the air auxiliary stretching treatment was set to 1.8 times, and the heating roll was not used in the drying shrinkage treatment. However, the PVA-based resin layer dissolved in the dyeing treatment and the stretching treatment in water, and a polarizing film with a single transmittance of 48% or more could not be produced.

[Comparative Example 2]
An attempt was made to prepare 17 polarizing films and polarizing plates in the same manner as in Example 1, except that the stretch ratio in the air auxiliary stretching treatment was set to 1.8 times and the heating roll was not used in the drying shrinkage treatment. However, similarly to Comparative Example 1, a polarizing film having a single transmittance of 48% or more could not be produced.

[Reference example 2]
A polarizing film obtained in the same manner as in Comparative Example 2 was held in a constant temperature and constant humidity zone set at a temperature of 60° C. and a humidity of 90% RH for 30 minutes. Thereafter, a polarizing plate was produced in the same manner as in Example 1.

Single transmittance and degree of polarization were measured for each polarizing plate of Examples and Comparative Examples. Results are shown in Table 2 and FIG. In FIG. 3, the approximate curves of the plot of Example 1 and the plot of Reference Example 1, and the approximate curve of the plot of Comparative Example 2 are illustrated. The approximated curve of Comparative Example 2 is an approximated curve by cubic polynomial approximation, and the dashed line indicates an extrapolated portion of the approximated curve.

The polarizing film obtained by the manufacturing method of the comparative example did not satisfy both the single transmittance of 48% or more and the degree of polarization of 85% or more. In addition, as shown by the approximate curve of the plot of Comparative Example 2, in the manufacturing method of Comparative Example 2, when the dyeing treatment is performed so that the single transmittance is 48% or more, the degree of polarization is less than 85%. is expected. On the other hand, the polarizing films obtained by the production methods of Examples had excellent optical properties such as a single transmittance of 48% or more and a degree of polarization of 85% or more.

For each of the polarizing plates of Example 1 and Reference Example 2, variations in optical properties of the long polarizing plate and the sheet-like polarizing plate were measured. Table 3 shows the results.

The long polarizing plate obtained by the manufacturing method of the example has a variation in single transmittance of 1% or less, and the sheet-shaped polarizing plate obtained by the manufacturing method of the example has a single transmittance variation. is 0.5% or less, and variations in optical properties are suppressed to the extent that there is no problem. On the other hand, the polarizing plate of the reference example obtained through the step of humidifying the polarizing film had large variations in optical properties both in the long shape and in the sheet shape.

Further, for the polarizing films of Examples and Comparative Examples, the crystallization index of the PVA-based resin in the laminate after auxiliary stretching in the air was measured based on the above (5). In addition, it was confirmed whether or not the PVA-based resin dissolved during the dyeing treatment or the underwater stretching treatment. Table 4 shows the results.

A polarizing plate having the polarizing film of the present invention is suitably used as a circularly polarizing plate for an organic EL display device and an inorganic EL display device.

REFERENCE SIGNS LIST 10 polarizing film 20 first protective layer 30 second protective layer 100 polarizing plate

Claims (6)

A method for producing a polarizing film having a thickness of 8 μm or less, a single transmittance of 48% or more, and a polarization degree of 85% or more,
A coating liquid containing iodide or sodium chloride and a polyvinyl alcohol resin is applied to one side of a long thermoplastic resin substrate and dried to obtain polyvinyl alcohol containing the iodide or sodium chloride and the polyvinyl alcohol resin. Forming a system resin layer to form a laminate, and subjecting the laminate to an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and heating while conveying in the longitudinal direction to 2% or more in the width direction a drying shrinkage treatment for shrinking, and a manufacturing method including applying in this order. 2. The manufacturing method according to claim 1, which is a method for manufacturing a polarizing film having a single transmittance of 50% or less and a degree of polarization of 92% or less. 3. The production method according to claim 1, wherein the content of said iodide or sodium chloride in said polyvinyl alcohol-based resin layer is 5 to 20 parts by weight with respect to 100 parts by weight of said polyvinyl alcohol-based resin. The manufacturing method according to any one of claims 1 to 3, wherein the stretching ratio in the in-air auxiliary stretching treatment is 2.0 times or more. The manufacturing method according to any one of claims 1 to 4, wherein the drying shrinkage treatment step is a step of heating using a heating roll. 6. The manufacturing method according to claim 5, wherein the temperature of the heating roll is 60° C. to 120° C., and the widthwise shrinkage of the laminate due to the drying shrinkage treatment is 2% or more.

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