TWI801599B - Polarizing film and method for manufacturing polarizing film - Google Patents

Abstract

The present invention provides a polarizing film capable of suppressing uneven streaks. The thickness of the polarizing film of the present invention is 8 μm or less, and the average value of the difference between the maximum thickness and the minimum thickness per 50 mm area from one end to the other end in the direction perpendicular to the absorption axis is 70 nm or less.

Description

Polarizing film and method for manufacturing polarizing film

The invention relates to a polarizing film and a manufacturing method of the polarizing film.

Background of the invention In a liquid crystal display device which is a typical image display device, polarizing films are arranged on both sides of the liquid crystal cell according to the image forming method. Moreover, 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 also been proposed, and polarizing films can also be applied to these images. display device. A method for producing a polarizing film, for example, has been proposed in which a laminate having a resin substrate and a polyvinyl alcohol (PVA) resin layer is extended, and then dyed to obtain a polarizing film on the resin substrate (for example, Patent Document 1). A thinner polarizing film can be obtained by this method, so it can contribute to the thinning of image display devices in recent years and has attracted attention. However, when the above-mentioned conventional thin polarizing film is applied to an image display device, streaks and unevenness may be visually recognized.

prior art literature patent documents Patent Document 1: Japanese Patent Laid-Open No. 2001-343521

Summary of the invention The problem to be solved by the invention The present invention is made to solve the above-mentioned conventional problems, and its main purpose is to provide a polarizing film capable of suppressing the occurrence of uneven streaks and a method for manufacturing the polarizing film.

method to solve the problem The thickness of the polarizing film of the present invention is 8 μm or less, and the average value of the difference between the maximum thickness and the minimum thickness per 50 mm area from one end to the other end in the direction perpendicular to the absorption axis is 70 nm or less. In one embodiment, the single transmittance is 44.5% or more, and the degree of polarization is 99.0% or more. According to another aspect of the present invention, a polarizer is provided. The polarizing plate has: the above-mentioned polarizing film and a protective layer disposed on at least one side of the above-mentioned polarizing film. According to another aspect of the present invention, a method for manufacturing a polarizing film is provided. The manufacturing method of the polarizing film comprises the following steps: forming a polyvinyl alcohol-based resin layer containing a polyvinyl alcohol-based resin on one side of a thermoplastic resin substrate to form a laminate; and sequentially performing aerial stretching on the above-mentioned laminate. Treatment and dyeing treatment: The above-mentioned polyvinyl alcohol-based resin layer after the above-mentioned aerial stretching treatment has a crystallization index calculated by attenuated total reflection spectroscopy of 1.55 to 1.7, and an orientation function of 0.22 to 0.31. In one embodiment, after the above-mentioned aerial stretching treatment, further includes performing underwater stretching treatment on the above-mentioned laminate; the stretching ratio of the above-mentioned aerial stretching treatment is 3.0 times or more, and the stretching ratio of the above-mentioned underwater stretching treatment is 1.8 times or less.

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

A. Polarizing film The thickness of the polarizing film according to one embodiment of the present invention is 8 μm or less, and the average value of the difference between the maximum thickness and the minimum thickness per 50 mm area from one end to the other end along the direction perpendicular to the absorption axis (sometimes below Called thickness variation) is 70nm or less. Thickness variation, for example, can measure the thickness of the polarizing film at intervals of 2mm from the above-mentioned one end to the other end, calculate the difference between the maximum thickness and the minimum thickness in each 50mm area, and then calculate the average value of the above differences in each area to obtain have to. The thickness of the polarizing film can typically be measured using an optical interference film thickness gauge. Conventional thin polarizing films have thickness variations along the direction perpendicular to the absorption axis due to their manufacturing methods. As a result, streaks along the absorption axis may be uneven when applied to image display devices. Regardless of whether the thickness of the polarizing film of this embodiment is very thin or not, the variation in thickness is small. When the polarizing film is applied to an image display device, it can suppress uneven streaks.

The thickness of the polarizing film is preferably 1 μm to 8 μm, preferably 1 μm to 7 μm, and more preferably 2 μm to 5 μm. The thickness variation is preferably less than 50nm, preferably less than 40nm, and most preferably less than 30nm. The thickness variation should be small, but actually the lower limit is, for example, 5 nm.

The monomer transmittance of the polarizing film should be above 44.5%, and the degree of polarization should be above 99.0%. The single transmittance of the polarizing film is more preferably above 45.0%. The polarization degree of the polarizing film is more preferably 99.5% or more, and more preferably 99.9% or more. The above-mentioned single transmittance is representatively the Y value obtained by measuring with an ultraviolet-visible spectrophotometer and correcting for optical performance. The above-mentioned degree of polarization is representatively obtained by the following formula based on the parallel transmittance Tp and the cross transmittance Tc obtained by measuring with an ultraviolet-visible spectrophotometer and performing light performance correction. Degree of polarization (%)={(Tp-Tc)/(Tp+Tc)} ^1/2 ×100

The manufacturing method of the polarizing film includes the following steps: forming a polyvinyl alcohol-based resin layer (PVA-based resin layer) containing a polyvinyl alcohol-based resin (PVA-based resin) on one side of a thermoplastic resin substrate to form a laminate; And, the aerial stretching treatment and the dyeing treatment are sequentially performed on the above-mentioned laminated body. The crystallization index of the PVA-based resin layer after the above-mentioned in-air stretching process measured by attenuated total reflection spectroscopy (ATR) is not less than 1.55 and not more than 1.7, and the orientation function is not less than 0.22 and not more than 0.31. By controlling the crystallization index and orientation function of the PVA-based resin layer after air-stretching treatment within the above-mentioned ranges, a polarizing film with thin thickness, small thickness variation and high optical properties can be produced.

The crystallization index of the PVA-based resin layer after air-stretching is obtained by ATR measurement with a Fourier transform infrared spectrophotometer (FT-IR) and polarized light as the measuring light. Specifically, the measurement was carried out with the measurement polarized light at 0° and 90° with respect to the extending direction, and the intensity at 1141 cm ^-1 and 1140 cm ^-1 of the obtained spectrum was used for calculation according to the following formula. In addition, the strength of 1141 cm ^-1 has a correlation with the amount of crystallization portion of the PVA-based resin layer. Crystallization index = ((I _C-0 +2×I _C-90 )/3)/((I _R-0 +2×I _R-90 )/3) However, I _C-0 : the measured light ( Intensity I of 1141 cm ^-1 when measuring light (polarized light) incident in a direction parallel to the extending direction and measured _C-90 : Intensity I of 1141 cm ^-1 when measuring light (polarized light) is incident in a direction perpendicular to the extending direction _R-0 : Intensity I of 1140 cm ^-1 when measurement light (polarized light) is incident in a direction parallel to the stretching direction and measured I _R-90 : Measurement light (polarized light) is incident in a direction perpendicular to the stretching direction and measured Intensity at 1140cm ^-1

The orientation function (f) of the PVA-based resin layer after the in-air stretching process is obtained by ATR measurement using FT-IR, for example, using polarized light as the measurement light. Specifically, the measurement was carried out in a state where the measurement polarization was 0° and 90° with respect to the extending direction, and the intensity at 2941 cm ^-1 of the obtained spectrum was used to calculate according to the following formula. Here, the intensity I is the value of 2941cm ^-1 /3330cm ^-1 with 3330cm ^-1 as the reference peak. In addition, when f=1, it is completely oriented, and when f=0, it is random. Also, we think that the peak at 2941 cm ^-1 is due to the absorption of the vibration of the PVA main chain (-CH2-). f＝(3>cos ² θ>-1)/2＝(1-D)/[c(2D+1)] However, c＝(3cos ² β-1)/2 β＝90deg⇒f＝-2 ×(1-D)/(2D+1) θ: Molecular chain and extension direction β: Molecular chain and dipole moment D=(I⊥)/(I//) (The more oriented the PVA molecule, the larger the D value) I⊥: Intensity when measuring light (polarized light) is incident in a direction perpendicular to the extending direction and measured I//: Intensity when measuring light (polarized light) is incident in a direction parallel to the extending direction and measured

B. Polarizer Fig. 1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. The polarizing plate 100 has a polarizing film 10 , a first protective layer 20 disposed on one side of the polarizing film 10 , and a second 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 item A above. One of the first protective layer 20 and the second protective layer 30 may also be omitted. In addition, one of the first protective layer and the second protective layer may be a resin base material used to manufacture the above-mentioned polarizing film.

The first and second protective films are formed of any appropriate film that can be used as a protective layer of a polarizing film. Specific examples of the material of the main component of the film include cellulose resins such as triacetyl cellulose (TAC), polyester-based, polyvinyl alcohol-based, polycarbonate-based, polyamide-based, polyamide-based resins, etc. Imine-based, polyether-based, polystyrene-based, polystyrene-based, polynorbornene-based, polyolefin-based, (meth)acrylic-based and acetate-based transparent resins, etc. Further, thermosetting resins such as (meth)acrylic, urethane, (meth)acrylate urethane, epoxy, and silicone, or ultraviolet curable resins are also mentioned. Other examples include glassy polymers such as siloxane polymers. Furthermore, a polymer film described in Japanese Patent Laid-Open No. 2001-343529 (WO01/37007) can also be used. As the material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in the side chain can be used, such as A resin composition having an alternating copolymer of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer may be mentioned. The polymer film can be, for example, an extruded product of the above-mentioned resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) arranged on the side opposite to the display panel is typically not more than 300 μm, preferably not more than 100 μm, more preferably 5 μm to 80 μm, and more preferably It is preferably 10 μm to 60 μm. In addition, when surface treatment is applied, the thickness of the outer protective layer includes 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 more preferably 10 μm to 60 μm. In one embodiment, the inner protective layer is a retardation layer having an arbitrary and appropriate retardation value. As the retardation layer, a retardation film having an in-plane retardation of 40 nm or more and/or a thickness direction retardation of 80 nm or more can be used. The in-plane retardation is usually controlled in the range of 40~200nm, and the retardation in the thickness direction is usually controlled in the range of 80~300nm. Examples of retardation films include birefringent films obtained by uniaxially or biaxially stretching polymer materials, alignment films of liquid crystal polymers, and alignment layers of liquid crystal polymers supported by films. The thickness of the retardation film is not particularly limited, and is generally about 20-150 μm.

C. Manufacturing method of polarizing film The method for producing a polarizing film according to an embodiment of the present invention is as described above, including the following steps: forming a polyvinyl alcohol-based resin layer containing a polyvinyl alcohol-based resin on one side of a thermoplastic resin substrate to form a laminate; and, The above-mentioned laminated body is sequentially subjected to aerial stretching treatment and dyeing treatment. The above-mentioned polyvinyl alcohol-based resin layer after the above-mentioned in-air stretching treatment has a crystallization index calculated by attenuated total reflection spectroscopy of 1.55 to 1.7, and an orientation function of 0.22 to 0.31. Preferably, after the aerial stretching treatment, the laminate is subjected to underwater stretching treatment; the stretching ratio of the aerial stretching treatment is 3.0 times or more, and the stretching ratio of the underwater stretching treatment is 1.8 times or less. According to the above, compared with the previous manufacturing method, setting the stretching ratio of the aerial stretching treatment to be higher and setting the stretching ratio of the underwater stretching treatment to be lower can promote the crystallization of the PVA resin layer after the aerial stretching treatment and at the same time suppress excessive crystallization of the thermoplastic resin substrate. In this way, a polarizing film with small thickness variation and excellent optical properties can be produced.

C-1. Fabrication of laminated body Arbitrary and appropriate methods can be adopted for the method of producing the laminate of the thermoplastic resin base material and the PVA-based resin layer. Preferably, the coating solution containing PVA-based resin is coated on the surface of the elongated thermoplastic resin substrate and dried, thereby forming a PVA-based resin layer on the thermoplastic resin substrate.

Any appropriate method can be adopted for the coating method of the coating liquid. Examples include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, knife coating (comma coating, etc.) and the like. The coating and drying temperature of the above-mentioned coating solution 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 can be subjected to surface treatment (such as corona treatment, etc.), and an easy-adhesive layer can also be formed on the thermoplastic resin substrate. By performing the above 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 it is less than 20 μm, it may be difficult to form a PVA-based resin layer. If it is larger than 300 μm, it may take a long time for the thermoplastic resin base material to absorb water during elongation treatment in water as described later, and there is a possibility that excessive load will be imposed on the elongation.

The water absorption rate of the thermoplastic resin substrate is preferably above 0.2%, more preferably above 0.3%. The thermoplastic resin substrate absorbs water, and the water acts as a plasticizer for plasticization. As a result, elongation stress can be greatly reduced and high-magnification elongation can be achieved. On the other hand, the water absorption of the thermoplastic resin substrate is preferably 3.0% or less, more preferably 1.0% or less. By using the thermoplastic resin base material, it is possible to prevent the dimensional stability of the thermoplastic resin base material from being remarkably lowered during production, which would cause poor appearance of the obtained polarizing film, and the like. It can also prevent the substrate from breaking when it is stretched in water, or the PVA-based resin layer from peeling off from the thermoplastic resin substrate. In addition, 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 obtained in accordance with JIS K 7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably below 120°C. By using such a thermoplastic resin base material, the crystallization of the PVA-based resin layer can be suppressed, and the extensibility of the laminate can be sufficiently ensured. In addition, in consideration of plasticizing the thermoplastic resin substrate with water and good underwater stretching, it is more preferably below 100°C, more preferably below 90°C. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60° C. or higher. By using the thermoplastic resin substrate, it is possible to prevent defects such as deformation of the thermoplastic resin substrate (such as unevenness, drooping or wrinkling, etc.) when coating and drying the coating liquid containing the above-mentioned PVA-based resin, Thereby, a laminated body was produced satisfactorily. In addition, the stretching of the PVA-based resin layer can be favorably performed at an appropriate temperature (for example, about 60° C.). In addition, the glass transition temperature of the thermoplastic resin substrate can be adjusted by heating, for example, a crystallization material capable of introducing a modifying group into the constituent material. The glass transition temperature (Tg) is a value calculated based on JIS K 7121.

Arbitrary and appropriate thermoplastic resins can be used as the constituent material of the thermoplastic resin base material. Examples of thermoplastic resins include ester-based resins such as polyethylene terephthalate-based resins, cycloolefin-based resins such as norcamphene-based resins, olefin-based resins such as polypropylene, polyamide-based resins, polycarbonate-based resins, and Other copolymer resins, etc. Among these, norcamphene-based resins and amorphous polyethylene terephthalate-based resins are preferable.

In one embodiment, an amorphous (uncrystallized) polyethylene terephthalate resin is preferably used. Among them, it is particularly preferable to use an amorphous (difficult to crystallize) polyethylene terephthalate resin. Specific examples of amorphous polyethylene terephthalate-based resins include copolymers that further contain isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, or copolymers that further contain Copolymers of cyclohexanedimethanol or diethylene glycol.

In a preferred embodiment, the thermoplastic resin substrate is composed of a polyethylene terephthalate resin having an isophthalic acid unit. This is because the thermoplastic resin base material has extremely excellent extensibility and can suppress crystallization at the time of elongation. We speculate that this is due to the large flexure imparted to the main chain by the introduction of isophthalic acid units. 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 repeating units. This is because a thermoplastic resin substrate with excellent extensibility 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, based on the total of all repeating units. By setting the above-mentioned content ratio, the degree of crystallization can be favorably increased in the drying shrinkage treatment described later.

The thermoplastic resin substrate may also have been pre-stretched (before forming the PVA-based resin layer). In one embodiment, it has been extended in the transverse direction of the elongated thermoplastic resin substrate. The lateral direction is preferably a direction perpendicular to the extending direction of the laminate described later. In addition, the term "orthogonal" in this specification also includes the case of being substantially orthogonal. Here, "substantially orthogonal" includes the case of 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~Tg+50°C relative to the glass transition temperature (Tg). The elongation ratio of the thermoplastic resin substrate should be 1.5 times to 3.0 times. An arbitrary and appropriate method can be adopted for the stretching method of the thermoplastic resin base material. Specifically, it can be extended from a fixed end, or can be extended from a free end. The extension method can be dry or wet. The stretching of the thermoplastic resin substrate can be carried out in one stage or in multiple stages. When it is carried out in multiple stages, the above-mentioned elongation ratio is the product of the elongation ratios of each stage.

C-1-2. Coating solution The above-mentioned coating solution refers to a solution obtained by dissolving the above-mentioned PVA-based resin in a solvent. Examples of the solvent include water, dimethylsulfide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, ethylene glycol, and the like. Amines such as diamine and diethylenetriamine. These may be used alone or in combination of two or more. Among them, water is the best. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. As long as it is the above-mentioned resin concentration, a uniform coating film can be formed in close contact with the thermoplastic resin substrate.

Additives may also be blended in the coating solution. Examples of additives include plasticizers and surfactants. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol and glycerin. The surfactant may, for example, be a nonionic surfactant. These can be used to further improve the uniformity, dyeability, and extensibility of the obtained PVA-based resin layer.

Arbitrary and appropriate resins can be used for the above-mentioned PVA-based resin. Examples thereof include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is usually 85 mol %-100 mol %, preferably 95.0 mol %-99.95 mol %, more preferably 99.0 mol %-99.93 mol %. The degree of saponification is obtained in accordance with JIS K 6726-1994. A polarizing film excellent in durability can be obtained by using a PVA-based resin having such a degree of saponification. When the degree of saponification is too high, gelation may occur.

The average degree of polymerization of the PVA-based resin can be appropriately selected according to the purpose. The average degree of polymerization is usually 1000-10000, preferably 1200-4500, more preferably 1500-4300. In addition, the average degree of polymerization can be obtained according to JIS K 6726-1994.

C-2. Air Assisted Extended Processing The elongation ratio of aerial elongation treatment should be 3.0 times to 4.0 times. In this way, the crystallization index and orientation function of the PVA-based resin layer after the air-stretching process can be controlled within a desired value range. In addition, as described above, by setting the stretching ratio of the in-air stretching process higher than the conventional manufacturing method, the stretching ratio for realizing desired optical characteristics in the underwater stretching process described later can be set lower. Low. Thereby, excessive crystallization of the thermoplastic resin base material caused by the underwater stretching treatment can be suppressed.

The stretching method of aerial auxiliary stretching can be fixed end stretching (such as stretching method using a tenter stretching machine), or free end stretching (such as the method of uniaxial stretching the laminated body through rollers with different peripheral speeds) , but in order to obtain high optical properties, free end extensions can be actively used. In one embodiment, the in-air stretching process includes a heating roll stretching step of conveying the elongated laminate in the direction of its long side and simultaneously stretching using the difference in peripheral speed between the heating rolls. In-air stretching typically includes a zone stretching step and a heating roll stretching step. In addition, the order of the zone stretching step and the heating roller stretching step is not limited, and the zone stretching step or the heating roller stretching step may be performed first. The region extension step can also be omitted. In one embodiment, the region stretching step and the heating roll stretching step are performed sequentially. In another embodiment, the end of the film is held in a tenter stretching machine, and the distance between the tenters is increased in the traveling direction to stretch (the increase in the distance between the tenters is the stretching ratio). At this time, the distance of the tenter in the width direction (vertical direction with respect to the traveling direction) is set so as to be arbitrarily close. Preferably, it can be set so that the extension ratio relative to the direction of travel can be approached by extending the free end. When the free end is extended, it is calculated as the shrinkage rate in the width direction = (1/extension ratio) ^1/2 .

Aerial assisted extension can be performed in one phase or in multiple phases. When it is carried out in multiple stages, the extension ratio is the product of the extension ratios of each stage. The direction of extension in aerial auxiliary extension should be approximately the same as that of underwater extension.

The maximum elongation magnification when combining aerial auxiliary elongation and underwater elongation is preferably 5.0 times or more, more preferably 5.5 times or more, and more preferably 6.0 times or more relative to the original length of the laminate. The "maximum elongation ratio" in this specification means the elongation ratio immediately before the laminate is broken, and is a value 0.2 lower than the value obtained by separately confirming the elongation ratio of the laminate fracture.

The stretching temperature of the air-assisted stretching can be set to any and appropriate value according to the forming material of the thermoplastic resin substrate, the stretching method, and the like. The stretching temperature is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg)+10°C of the thermoplastic resin substrate, especially above Tg+15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. Stretching at such a temperature suppresses rapid progress of crystallization of the PVA-based resin, thereby suppressing disadvantages caused by the crystallization (for example, interruption of orientation of the PVA-based resin layer due to stretching).

C-3. Insoluble treatment Insolubilization treatment is performed after the air-assisted stretching treatment and before the underwater stretching treatment or dyeing treatment as necessary. The above-mentioned insolubilization treatment is typically carried out by immersing the PVA-based resin layer in a boric acid aqueous solution. By performing insolubilization treatment, water resistance can be imparted to the PVA-based resin layer, and the orientation of PVA can be prevented from being lowered when immersed in water. The concentration of the boric acid aqueous solution is preferably 1 to 4 parts by weight relative to 100 parts by weight of water. The liquid temperature of the insoluble bath (boric acid aqueous solution) should be 20°C~50°C.

C-4. Dyeing treatment The above-mentioned dyeing treatment is representatively carried out by dyeing the PVA-based resin layer with iodine. Specifically, it is performed by adsorbing iodine on the PVA-based resin layer. Examples of the adsorption method include: a method of immersing the PVA resin layer (laminate) in a dyeing solution containing iodine, a method of applying the dyeing solution to the PVA resin layer, and spraying the dyeing solution onto the PVA. The method on the resin layer, etc. A method of immersing the laminate in a dyeing solution (dyeing bath) is preferably employed. This is because iodine can be well adsorbed.

The above-mentioned dyeing solution is preferably iodine aqueous solution. The blending amount of iodine is preferably 0.05 to 0.5 parts by weight relative to 100 parts by weight of water. In order to increase the solubility of iodine in water, it is advisable to mix iodide in the iodine aqueous 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. Among these, potassium iodide is preferable. The blending amount of iodide is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, relative to 100 parts by weight of water. In order to inhibit the dissolution of PVA-based resin, the liquid temperature of the dyeing solution during dyeing should be 20°C~50°C. When immersing the PVA-based resin layer in the dyeing solution, in order to ensure the transmittance of the PVA-based resin layer, the immersion time is preferably 5 seconds to 5 minutes, and more preferably 30 seconds to 90 seconds.

Dyeing conditions (concentration, liquid temperature, and immersion time) can be set so that the degree of polarization or monomer transmittance of the finally obtained polarizing film falls within a predetermined range. In one embodiment, the immersion time is set so that the single-body transmittance of the obtained polarizing film becomes 44.5%-45.0%. In another embodiment, the immersion time is set so that the polarization degree of the obtained polarizing film may become 99.0% or more.

When the dyeing treatment is carried out after immersing the laminate in a treatment bath containing boric acid (typically insolubilization treatment), the boric acid contained in the treatment bath will be mixed into the dyeing bath, and the concentration of boric acid in the dyeing bath will vary with the dyeing bath. As time changes, the dyeing properties may become unstable as a result. In order to suppress destabilization of dyeability as described above, the upper limit of the concentration of boric acid in the dyeing bath is adjusted to preferably 4 parts by weight, more preferably 2 parts by weight, relative to 100 parts by weight of water. On the other hand, the lower limit of the concentration of boric acid in the dyeing bath is preferably 0.1 part by weight, more preferably 0.2 part by weight, and still more 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 premixed with boric acid. Thereby, the rate at which the boric acid concentration changes when the boric acid in the treatment bath mentioned above is mixed into the dyeing bath can be reduced. The amount of boric acid mixed in the dyeing bath in advance (that is, the content of boric acid not derived from the above-mentioned treatment bath) is preferably 0.1 to 2 parts by weight, more preferably 0.5 parts by weight, relative to 100 parts by weight of water ~1.5 parts by weight.

C-5. Cross-linking treatment After the dyeing treatment and before the stretching treatment in water, a crosslinking treatment is performed as necessary. Typically, the above-mentioned crosslinking treatment can be performed by immersing the PVA-based resin layer in a boric acid aqueous solution. By performing cross-linking treatment, water resistance can be imparted to the PVA-based resin layer, and the orientation of PVA can be prevented from being lowered when immersed in high-temperature water during subsequent underwater stretching. The concentration of the boric acid aqueous solution is preferably 1 to 5 parts by weight relative to 100 parts by weight of water. In addition, when carrying out the crosslinking treatment after the above-mentioned dyeing treatment, it is preferable to further blend iodide. By mixing iodide, the elution of iodine adsorbed to the PVA-based resin layer can be suppressed. The blending amount of iodide is preferably 1 to 5 parts by weight relative to 100 parts by weight of water. Specific examples of iodide are as described above. The liquid temperature of the cross-linking bath (boric acid aqueous solution) should be 20°C~50°C.

C-6. Extended treatment in water The elongation ratio of the underwater elongation treatment is preferably less than 1.8 times, preferably less than 1.5 times. Thereby, excessive crystallization of the thermoplastic resin base material caused by the underwater stretching treatment can be suppressed. In addition, it is possible to realize a high total stretching ratio in combination with auxiliary stretching in the air, and to manufacture a polarizing film with extremely excellent optical characteristics.

The underwater stretching treatment is performed by immersing the laminate in a stretching bath. By stretching in water, it can be stretched at a temperature lower than the glass transition temperature (typically about 80°C) of the above-mentioned thermoplastic resin substrate or PVA-based resin layer, and can suppress the crystallization of the PVA-based resin layer at the same time Perform high-magnification extensions. As a result, a polarizing film with excellent optical properties can be produced.

An arbitrary and appropriate method can be used for the stretching method of the laminate. Specifically, it may be fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching a laminate through rolls having different circumferential speeds). Preferably a free end extension is selected. The extension of the laminate may be performed in one step or in multiple steps. When it is carried out in multiple stages, the elongation ratio (maximum elongation ratio) of the laminate described later is the product of the elongation ratios of each stage.

The underwater stretching is preferably carried out by immersing the laminate in an aqueous solution of boric acid (boric acid underwater stretching). By using an aqueous solution of boric acid as a stretching bath, the PVA-based resin layer can be given rigidity to withstand tension during stretching and water resistance that is insoluble in water. Specifically, boric acid generates tetrahydroxyboric acid anion in aqueous solution, which can cross-link with PVA-based resin through hydrogen bonding. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, and good stretching can be performed, thereby producing a polarizing film with excellent optical properties.

The above boric acid aqueous solution is preferably obtained by dissolving boric acid and/or borate in water as a solvent. On the other hand, 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 relative to 100 parts by weight of water. By setting the concentration of boric acid 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 characteristics can be produced. In addition, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, etc. in a solvent other than boric acid or borate can also be used.

It is convenient to mix iodide in the above-mentioned extension bath (boric acid aqueous solution). By mixing iodide, the elution of iodine adsorbed to the PVA-based resin layer can be suppressed. Specific examples of iodide 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, relative to 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C~85°C, more preferably 60°C~75°C. As long as it is the above-mentioned temperature, the dissolution of the PVA-based resin layer can be suppressed, and high-magnification stretching can be achieved. Specifically, as described above, considering the relationship with the formation of the PVA-based resin layer, the glass transition temperature (Tg) of the thermoplastic resin base material is preferably 60° C. or higher. At this time, if the stretching temperature is lower than 40° C., good stretching may not be possible even considering plasticizing the thermoplastic resin base material with water. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, and it may not be possible to obtain excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

C-7. Drying treatment The above-mentioned drying treatment may be carried out by heating the entire region by heating the region, or by heating the conveying roller (so-called use of a heating roller) (heating roller drying method). It is preferable to use both. By drying with a heating roll, heating curl of the laminate can be effectively suppressed, and a polarizing film having an excellent appearance can be produced. Specifically, by drying the laminated body along the heating roll, the crystallization of the above-mentioned thermoplastic resin substrate can be effectively promoted to increase the degree of crystallization, even at a relatively low drying temperature, It can still increase the crystallinity of the thermoplastic resin substrate well. As a result, the rigidity of the thermoplastic resin substrate is increased so that it can withstand shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. Moreover, since the layered body can be dried while maintaining the flat state by using the heating roller, not only the occurrence of curl but also the occurrence of wrinkling can be suppressed. At this time, the laminated body can be shrunk in the width direction through drying shrinkage treatment, so as to improve the optical properties. It is because it can effectively improve the orientation of PVA and PVA/iodine complexes.

C-8. Other processing It is preferable to carry out the washing treatment after the stretching treatment in water and before the drying treatment. Typically, the cleaning treatment described above can be performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution. Example

Hereinafter, the present invention will be specifically described with examples, but the present invention is not limited by these examples. The measurement method and evaluation method of each characteristic are as follows. In addition, "parts" and "%" in Examples and Comparative Examples are based on weight unless otherwise noted. (1) Crystallization index For the laminate after aerial extension treatment, use a Fourier transform infrared spectrophotometer (manufactured by Perkin Elmer, product name "SPECTRUM2000"), and use polarized light as the measurement light, and evaluate the PVA system by ATR measurement. resin layer surface. Specifically, the measurement was carried out with the measurement polarized light at 0° and 90° with respect to the extending direction, and the crystallization index was calculated according to the following formula using the intensities of 1141 cm ^-1 and 1140 cm ^-1 of the obtained spectrum. Crystallization index = ((I _C-0 +2×I _C-90 )/3)/((I _R-0 +2×I _R-90 )/3) However, I _C-0 : the measured light ( Intensity I of 1141 cm ^-1 when measuring light (polarized light) incident in a direction parallel to the extending direction and measured _C-90 : Intensity I of 1141 cm ^-1 when measuring light (polarized light) is incident in a direction perpendicular to the extending direction _R-0 : Intensity I of 1140 cm ^-1 when measurement light (polarized light) is incident in a direction parallel to the stretching direction and measured I _R-90 : Measurement light (polarized light) is incident in a direction perpendicular to the stretching direction and measured The intensity at 1140 cm ^-1 (2) Orientation function For the laminated body after the air extension process, use a Fourier transform infrared spectrophotometer (manufactured by Perkin Elmer, product name "SPECTRUM2000"), and use polarized light as the measurement light, by ATR measurement was used to evaluate the surface of the PVA-based resin layer. Specifically, the measurement was carried out in a state where the measurement polarization was 0° and 90° with respect to the extending direction, and the orientation function was calculated according to the following formula using the intensity at 2941 cm ⁻¹ . f＝(3>cos ² θ>-1)/2＝(1-D)/[c(2D+1)] However, c＝(3cos ² β-1)/2 β＝90deg⇒f＝-2 ×(1-D)/(2D+1) θ: Molecular chain and extension direction β: Molecular chain and dipole moment D=(I⊥)/(I//) (The more oriented the PVA molecule, the larger the D value) I⊥: Intensity when measuring light (polarized light) is incident in a direction perpendicular to the extending direction and measured I//: Intensity when measuring light (polarized light) is incident in a direction parallel to the extending direction and measured ( 3) Thickness variation Using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3700"), the polarized light of the examples and comparative examples was measured from one end to the other end in the direction perpendicular to the absorption axis at intervals of 2 mm. Thickness of the film: Next, calculate the difference between the maximum thickness and the minimum thickness in every 50mm area from the above-mentioned one end to the above-mentioned other end, and then calculate the average value of the above-mentioned differences in each area, and use the above-mentioned average value as the thickness variation. (4) Optical properties (single transmittance and degree of polarization) The polarizing plates (protective film/polarizing film) of Examples and Comparative Examples were measured using an ultraviolet-visible spectrophotometer (V-7100 manufactured by JASCO Corporation), and The measured single transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc are respectively used as Ts, Tp, and Tc of the polarizing film. These Ts, Tp, and Tc are based on the 2-degree field of view of JIS Z8701 (C light source) Carry out the Y value that carries out measurement and carry out optical performance correction gained.In addition, the refractive index of protective film is 1.50, and the refractive index of the polarizing film and the surface of the opposite side of protective film is 1.53. Utilize following formula from gained Tp and Tc Obtain the degree of polarization P. Degree of polarization P(%)={(Tp-Tc)/(Tp+Tc)} ^1/2 ×100 Based on the values of the obtained single transmittance and degree of polarization, evaluate the optical properties according to the following criteria. ○ : The polarization degree is above 99.0%. (Single transmittance = 44.5%) ×: The polarization degree is lower than 99.0%. (Single transmittance = 44.5%)

[Example 1] 1. Making a polarizing film The thermoplastic resin substrate is an amorphous isophthalic acid copolymerized polyethylene terephthalate film (thickness: 100 μm , Width: 1450mm). And perform corona treatment on one side of the resin substrate (treatment condition: 55W·min/m ² ). Prepared PVA containing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mole%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z410") in a ratio of 9:1 Aqueous solution (coating solution). The above-mentioned PVA aqueous solution was applied to the corona-treated surface of the resin substrate and dried at 60° C. to form a PVA-based resin layer with a thickness of 8 μm, thereby producing a laminate. The obtained laminate was uniaxially stretched 3.0 times at the free end in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds at a stretching temperature of 120°C to 130°C (in-air assisted stretching treatment). Next, the laminate was immersed in an insoluble bath (an aqueous solution of boric acid obtained by mixing 4 parts by weight of boric acid with respect to 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insoluble treatment). Next, adjust the concentration of the dyeing bath (with respect to 100 parts by weight of water, an iodine aqueous solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7) with a liquid temperature of 30° C. so that the monomer of the polarizing film finally obtained The transmittance (Ts) becomes 44.5% while being immersed therein for 60 seconds (dyeing treatment). Next, it was immersed in a crosslinking bath (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (crosslinking treatment). ). Then, while the laminate was immersed in a boric acid aqueous solution (boric acid concentration: 4.0% by weight) at a liquid temperature of 70°C, it was uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different circumferential speeds so that the total stretching ratio Up to 5.5 times (extended treatment in water). Thereafter, the laminate was immersed in a cleaning bath (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with respect to 100 parts by weight of water) at a liquid temperature of 20° C. (washing treatment). Thereafter, by drying in an oven, a polarizing film having a width of 1500 mm and a thickness of 3.5 μm was formed on the resin substrate. 2. Make a polarizing plate. On the surface of the polarizing film prepared above (the side opposite to the resin substrate), stick an acrylic film (surface refractive index 1.50, 40 μm) through a UV-curable adhesive as a protective film. Specifically, it was applied so that the total thickness of the hardening adhesive was 1.0 μm, and bonded using a rolling mill. Thereafter, UV rays are irradiated from the protective film side to cure the adhesive. Next, the resin base material was peeled off, and the polarizing plate which has the structure of a protective film/polarizing film was manufactured.

[Example 2] A polarizing film and a polarizing plate were produced in the same manner as in Example 1, except that the stretching ratio was set to 3.5 times during the auxiliary stretching process in the air.

[Comparative example 1] A polarizing film and a polarizing plate were produced in the same manner as in Example 1 except that the stretching ratio was set to 2.0 times and the stretching temperature was set to 140° C. during the in-air assisted stretching process.

[Comparative example 2] A polarizing film and a polarizing plate were produced in the same manner as in Example 1 except that the stretching ratio was set to 2.4 times during the auxiliary stretching process in the air.

[Comparative example 3] In the air-assisted stretching process, the stretching ratio was set to 4.0 times, and the stretching temperature was set to 140°C. In addition, it was tried to produce a polarizing film in the same manner as in Example 1, but the laminate was broken during the stretching process in water. , and cannot produce polarizing film and polarizing plate.

[Comparative example 4] A polarizing film and a polarizing plate were produced in the same manner as in Example 1, except that the stretching ratio was set to 4.5 times during the aerial auxiliary stretching process, and no underwater stretching process was performed.

>Evaluation> For the examples and comparative examples, calculate the crystallization index and orientation function of the PVA-based resin layer after the aerial stretching process according to the above-mentioned (1) and (2), calculate the thickness variation of the polarizing film according to the above-mentioned (3), and calculate according to the above-mentioned ( 4) Calculate and evaluate the optical properties of the polarizer. In addition, it was confirmed whether the polarizing plates of Examples and Comparative Examples were applied to image display devices without streaks or unevenness. List the results in Table 1.

[Table 1]

It can be seen from Table 1 that the optical properties of the polarizing plates of Comparative Examples 1 and 4 are low, and the polarizing plates of Comparative Examples 1 and 2 have visible streaks and unevenness. Under the manufacturing conditions of Comparative Example 3, the laminate would be broken during stretching in water, and even a polarizing film could not be produced. In contrast, the polarizing plates of Examples 1 and 2 had excellent optical characteristics, and no streaks were observed.

Industrial availability The polarizing film of the present invention can be applied to image display devices.

10‧‧‧Polarizing film 20‧‧‧1st protective layer 30‧‧‧Second protective layer 100‧‧‧polarizer

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

10‧‧‧Polarizing film

20‧‧‧1st protective layer

30‧‧‧Second protective layer

100‧‧‧polarizer

Claims (1)

A method for manufacturing a polarizing film, comprising the steps of: forming a polyvinyl alcohol-based resin layer containing a polyvinyl alcohol-based resin on one side of a thermoplastic resin substrate to form a laminate; and sequentially performing aerial stretching on the aforementioned laminate Treatment and dyeing treatment; after the above-mentioned aerial stretching treatment, it further includes performing underwater stretching treatment on the aforementioned laminate; the stretching ratio of the aforementioned aerial stretching treatment is more than 3.0 times, and the aforementioned underwater stretching treatment The stretching ratio is less than 1.8 times; the aforementioned aerial stretching The polyvinyl alcohol-based resin layer after the treatment has a crystallization index calculated by attenuated total reflection spectroscopy of 1.55 to 1.7, and an orientation function of 0.22 to 0.31.

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