Polarizing films and polarizing plates

Abstract

To provide a polarization film excellent in durability under a high-temperature and high-humidity environment.SOLUTION: A polarization film includes a polyvinyl alcohol resin film containing an iodine, and has a treatment layer on a surface thereof. The treatment layer is a solidification layer of a coating film of a process liquid at pH 3.0 or lower containing a polyvinyl alcohol resin. In the polarization film, an absorbancy Abs240 at a wavelength 600 nm after an endurance test for 240 hours at a temperature 60°C and a relative humidity 95% satisfies a relation below, with regard to an absorbancy Abs0 before the endurance test: 1.05≤Abs240 / Abs0≤1.44. In one embodiment, a thickness of the treatment layer is 0.2 to 1.7 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to polarizing films and polarizing plates. [Background technology]

[0002] Liquid crystal display devices, a typical image display device, have polarizing films arranged on both sides of the liquid crystal cell due to their image forming method. As a method for manufacturing polarizing films, for example, a method has been proposed 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 (for example, Patent Document 1). Such a method can produce a thin polarizing film, and has attracted attention as it can contribute to the thinning of image display devices in recent years. However, for thin polarizing films, further improvement in durability under high temperature and high humidity environments is required. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2001-343521 [Overview of the project] [Problems that the invention aims to solve]

[0004] The present invention was made to solve the above-mentioned conventional problems, and its main objective is to provide a polarizing film, a polarizing plate, and a method for manufacturing such a polarizing film that have excellent durability in high-temperature and high-humidity environments. [Means for solving the problem]

[0005] The polarizing film according to an embodiment of the present invention is composed of a polyvinyl alcohol-based resin film containing iodine, with a treatment layer formed on its surface. This treatment layer is a solidified layer of a coated film containing a polyvinyl alcohol-based resin and having a pH of 3.0 or less. The absorbance Abs at a wavelength of 600 nm after a 240-hour durability test of this polarizing film at a temperature of 60°C and a relative humidity of 95% is measured. 240 However, the following relationship is satisfied with respect to the absorbance Abs0 before the durability test. 1.05 ≤ Abs 240 / Abs0≦1.44 In one embodiment, the thickness of the treated layer is 0.2 μm to 1.7 μm. In one embodiment, the polarizing film has a single-layer transmittance of 43.0% or more. In one embodiment, the polarizing film has a thickness of 8 μm or less. According to another aspect of the present invention, a polarizing plate is provided. This polarizing plate comprises the polarizing film described above and a protective layer disposed on at least one side of the polarizing film. [Effects of the Invention]

[0006] According to the present invention, by contacting a polarizing film with a treatment solution having a pH of 3.0 or lower, a polarizing film with excellent durability in high-temperature and high-humidity environments can be obtained. Specifically, the polarizing film according to the embodiment of the present invention has an absorbance of Abs at a wavelength of 600 nm after a 240-hour durability test at a temperature of 60°C and a relative humidity of 95%. 240 However, the following relationship is satisfied with respect to the absorbance Abs0 before the durability test: Abs 240 / Abs0>1.00 In other words, the polarizing film according to the embodiment of the present invention shows an increase in absorbance at a wavelength of 600 nm during a heating and humidification durability test. This means that the polarizing performance of the polarizing film according to the embodiment of the present invention can be improved in high-temperature and high-humidity environments. While the polarizing performance of a polarizing film is normally expected to decrease in high-temperature and high-humidity environments, the improvement in durability of the polarizing film according to the embodiment of the present invention in such environments is an unexpectedly excellent result. [Brief explanation of the drawing]

[0007] [Figure 1] It is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. [Figure 2] It is a schematic view showing an example of a drying and shrinking process using a heating roll.

Embodiments for Carrying Out the Invention

[0008] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

[0009] A. Polarizing film The polarizing film according to an embodiment of the present invention is composed of a polyvinyl alcohol (PVA)-based resin film containing iodine, and the absorbance Abs at a wavelength of 600 nm after a durability test of 240 hours at a temperature of 60 °C and a relative humidity of 95% 240 satisfies the following relationship with respect to the absorbance Abs0 before the durability test. Abs 240 / Abs0 > 1.00 This indicates that in the polarizing film according to an embodiment of the present invention, the PVA-I5 - complex having absorption near 600 nm is not broken even in a heat and humidity durability test, but rather can increase. The PVA-I5 - complex is broken in a high temperature and high humidity environment, and the polarization performance of the polarizing film is usually expected to decrease in a high temperature and high humidity environment. However, such excellent durability of the polarizing film according to an embodiment of the present invention is unexpectedly excellent. Although not clear theoretically, such excellent durability can be realized by bringing the polarizing film into contact with a treatment liquid having a pH of 3.0 or less. Abs 240 / Abs0 is preferably 1.05 or more, more preferably 1.10 or more, still more preferably 1.15 or more, particularly preferably 1.20 or more, and especially preferably 1.25 or more. Abs 240 The upper limit of / Abs0 can be, for example, 2.00. The absorbance is typically the orthogonal absorbance. The orthogonal absorbance is obtained by the following formula based on the orthogonal transmittance Tc measured when obtaining the polarization degree described later. Orthogonal absorbance = log10(100 / Tc) Note that the absorbance Abs0 before the durability test is the absorbance in the normal state of the polarizing film. The Abs0 of the polarizing film at a wavelength of 600 nm is, for example, less than 5.0, preferably 4.3 or less, and more preferably 4.0 or less. The lower limit of Abs0 can be, for example, 2.0.

[0010] The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, still more preferably 5 μm or less, and particularly preferably 3 μm or less. The lower limit of the thickness of the polarizing film can be 1 μm in one embodiment and 2 μm in another embodiment. Such a thickness can be realized, for example, by producing a polarizing film using a laminate of a resin substrate and a PVA-based resin layer coated and formed on the resin substrate as described later. When the polarizing film is produced from a single resin film, the thickness of the polarizing film can be, for example, 12 μm to 35 μm.

[0011] The polarizing film preferably exhibits absorption dichroism at any wavelength from 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 42.0% or more, more preferably 42.5% or more, still more preferably 43.0% or more, particularly preferably 43.5% or more, and especially preferably 44.0% or more. On the other hand, the single transmittance is preferably 47.0% or less, more preferably 46.0% or less. The degree of polarization of the polarizing film is preferably 99.95% or more, more preferably 99.99% or more. On the other hand, the degree of polarization is preferably 99.998% or less. According to the embodiment of the present invention, in this way, it is possible to achieve both a high single transmittance and a high degree of polarization, and to realize excellent durability in a high-temperature and high-humidity environment as described above. The above single transmittance is typically the Y value measured using an ultraviolet-visible spectrophotometer and corrected for visual sensitivity. Also, the single transmittance is the value when the refractive index of one surface of the polarizing plate is converted to 1.50 and the refractive index of the other surface is converted to 1.53. The above degree of polarization is typically determined by the following formula based on the parallel transmittance Tp and the orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer and corrected for visual sensitivity. Degree of polarization (%)={(Tp-Tc) / (Tp+Tc)} 1 / 2 ×100

[0012] In one embodiment, the transmittance (single-layer transmittance) of a thin polarizing film of 8 μm or less is typically measured using a UV-Vis spectrophotometer on a laminate of a polarizing film (surface refractive index: 1.53) and a protective layer (protective film) (refractive index: 1.50). Depending on the refractive index of the surface of the polarizing film and / or the refractive index of the surface in contact with the air interface of the protective layer, the reflectance at the interface of each layer changes, and as a result, the measured transmittance may change. Therefore, for example, when using a protective layer with a refractive index other than 1.50, the measured transmittance may be corrected according to the refractive index of the surface in contact with the air interface of the protective layer. Specifically, the corrected transmittance value C is expressed by the following formula, using the reflectance R1 (transmission axis reflectance) of polarization parallel to the transmission axis at the interface between the protective layer and the air layer. C = R1 - R0 R0 = ((1.50 - 1) 2 / (1.50+1) 2 ) × (T1 / 100) R1 = ((n1-1)) 2 / (n1+1) 2 ) × (T1 / 100) Here, R0 is the transmission axis reflectance when a protective layer with a refractive index of 1.50 is used, n1 is the refractive index of the protective layer used, and T1 is the transmittance of the polarizing film. For example, when a substrate with a surface refractive index of 1.53 (such as a cycloolefin film or a film with a hard coat layer) is used as the protective layer, the correction amount C is approximately 0.2%. In this case, by adding 0.2% to the transmittance obtained by measurement, it is possible to convert the transmittance of a polarizing film with a surface refractive index of 1.53 to that of a protective layer with a refractive index of 1.50. Furthermore, according to calculations based on the above formula, the change in the correction value C when the transmittance T1 of the polarizing film is changed by 2% is 0.03% or less, indicating that the influence of the polarizing film's transmittance on the correction value C is limited. In addition, if the protective layer has absorption other than surface reflection, appropriate correction can be made according to the amount of absorption.

[0013] The polarizing film may be made using a single resin film or 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 coated on the resin substrate. A polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be made, for example, by applying a PVA-based resin solution to a resin substrate, drying it to form a PVA-based resin layer on the resin substrate, and obtaining a laminate of the resin substrate and the PVA-based resin layer; or by stretching and dyeing the laminate to make the PVA-based resin layer a polarizing film. In the embodiment of the present invention, the polarizing film is brought into contact with a treatment solution with a pH of 3.0 or less. This makes it possible to achieve excellent durability in high temperature and high humidity environments as described above. Preferably, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of the resin substrate. Stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, the stretching process may, if necessary, further include air stretching of the laminate at a high temperature (e.g., 95°C or higher) before stretching in an aqueous boric acid solution. In addition, in this embodiment, preferably, the laminate is subjected to a drying shrinkage treatment in which it shrinks by 2% or more in the width direction by heating while being transported in the longitudinal direction. Typically, the manufacturing method of this embodiment includes applying an auxiliary air stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment to the laminate in this order. By introducing auxiliary stretching, it becomes possible to increase the crystallinity of PVA even when PVA is coated on a thermoplastic resin, making it possible 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 a decrease in the orientation of PVA and dissolution when immersed in water in the subsequent dyeing and stretching processes, making it possible to achieve high optical properties. Furthermore, when the PVA-based resin layer is immersed in a liquid, the disorder of the orientation of polyvinyl alcohol molecules and the decrease in orientation can be suppressed compared to when the PVA-based resin layer does not contain halides. This makes it possible to improve the optical properties of polarizing films obtained through processing steps that involve immersing the laminate in a liquid, such as dyeing and underwater stretching.Furthermore, the optical properties can be improved by shrinking the laminate in the width direction through a drying shrinkage treatment. The resulting resin substrate / polarizing film laminate may be used as is (i.e., 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, and any appropriate protective layer may be laminated onto the peeled surface according to the purpose. Details of the manufacturing method for the polarizing film will be described later in Section C.

[0014] B. Polarizing plate Figure 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 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 as described in Section A above. One of the protective layers, 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 a resin substrate used in the manufacture of the above polarizing film.

[0015] The first and second protective layers are formed from any suitable film that can be used as a protective layer for the polarizing film. Specific examples of materials that make up the main component of the film include cellulosic resins such as triacetylcellulose (TAC), and transparent resins such as polyester, polyvinyl alcohol, polycarbonate, polyamide, polyimide, polyethersulfone, polysulfone, polystyrene, polynorbornene, polyolefin, (meth)acrylic, and acetate. Thermosetting resins or UV-curing resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone can also be used. In addition, glassy polymers such as siloxane polymers can also be used. Polymer films described in Japanese Patent Application Publication No. 2001-343529 (WO01 / 37007) can also be used. As materials for this film, for example, a resin composition containing a thermoplastic resin having substituted or unsubstituted imide groups in its side chains, and a thermoplastic resin having substituted or unsubstituted phenyl groups and nitrile groups in its side chains can be used. Examples include a resin composition having an alternating copolymer of isobutene and N-methylmaleimide, and an acrylonitrile-styrene copolymer. The polymer film may be, for example, an extruded product of the above resin composition.

[0016] When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) located on the opposite side of the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 5 μm to 80 μm, and even more preferably 10 μm to 60 μm. If a surface treatment is applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

[0017] When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (inner protective layer) placed on the display panel side is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and even more preferably 10 μm to 60 μm. In one embodiment, the inner protective layer is a phase difference layer having any appropriate phase difference value. In this case, the in-plane phase difference Re(550) of the phase difference layer is, for example, 110 nm to 150 nm. "Re(550)" is the in-plane phase difference measured with light of wavelength 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 maximum (i.e., the slow phase axis direction), "ny" is the refractive index in the direction perpendicular to the slow phase axis in the plane (i.e., the fast phase axis direction), "nz" is the refractive index in the thickness direction, and "d" is the thickness of the layer (film) (nm).

[0018] C. Method for manufacturing polarizing films A method for manufacturing a polarizing film according to one embodiment of the present invention includes: applying a PVA-based resin solution to one side of a long thermoplastic resin substrate and drying it to form a PVA-based resin layer to form a laminate; stretching and dyeing the laminate to make the PVA-based resin layer a polarizing film; and contacting the polarizing film with a treatment solution having a pH of 3.0 or less. By contacting the polarizing film with a treatment solution having a pH of 3.0 or less, a polarizing film with excellent durability in high temperature and high humidity environments can be realized. Preferably, the PVA-based resin solution further contains a halogen. Preferably, the above manufacturing method includes subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, a water-based stretching treatment, and a drying shrinkage treatment in which the laminate shrinks by 2% or more in the width direction by heating while being conveyed in the longitudinal direction. The halogen content in the PVA-based resin solution (resulting in a PVA-based resin layer) is preferably 5 to 20 parts by weight per 100 parts by weight of PVA-based resin. The drying shrinkage treatment is preferably carried out using a heated roll, and the temperature of the heated roll is preferably 60°C to 120°C. The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment is preferably 2% or more. By such a manufacturing method, the polarizing film described in Section A above can be obtained. In particular, by preparing a laminate containing a PVA-based resin layer containing a halide, performing multi-stage stretching of the laminate including air-assisted stretching and underwater stretching, and heating the stretched laminate with a heated roll, a polarizing film with excellent optical properties (typically, single-layer transmittance and unit absorbance) can be obtained.

[0019] C-1. Fabrication of the laminate Any suitable method can be used to produce a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating solution containing a halogenated compound 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 halogenated compound content in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

[0020] Any suitable method can be used to apply the coating solution. Examples include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, knife coating (comma coating, etc.). The application and drying temperature of the above coating solution is preferably 50°C or higher.

[0021] The thickness of the PVA resin layer is preferably 3 μm to 40 μm, and more preferably 3 μm to 20 μm.

[0022] Before forming the PVA resin layer, the thermoplastic resin substrate may be subjected to surface treatment (e.g., corona treatment), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such treatments, the adhesion between the thermoplastic resin substrate and the PVA resin layer can be improved.

[0023] C-1-1. Thermoplastic resin base material Any suitable thermoplastic resin film can be used as the thermoplastic resin substrate. Details of the thermoplastic resin substrate are described, for example, in Japanese Patent Application Publication No. 2012-73580. The entire description of that publication is incorporated herein by reference.

[0024] C-1-2. Application Solution The coating solution contains a halogenated compound and a PVA-based resin, as described above. Typically, the coating solution is a solution obtained by dissolving the halogenated compound and the PVA-based resin in a solvent. Examples of solvents include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used individually 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 per 100 parts by weight of the solvent. Such a resin concentration allows for the formation of a uniform coating film that adheres closely to the thermoplastic resin substrate. The halogenated compound content in the coating solution is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

[0025] Additives may be added to the coating solution. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyhydric alcohols such as ethylene glycol and glycerin. Examples of surfactants include nonionic surfactants. These may be used to further improve the uniformity, dyeability, and stretchability of the resulting PVA-based resin layer.

[0026] Any suitable resin can be used as the PVA-based resin mentioned above. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymers can be used. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymers are obtained by saponifying ethylene-vinyl acetate copolymers. The degree of saponification of PVA-based resins is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The degree of saponification can be determined in accordance with JIS K 6726-1994. By using a PVA-based resin with such a degree of saponification, a polarizing film with excellent durability can be obtained. If the degree of saponification is too high, there is a risk of gelation.

[0027] The average degree of polymerization of PVA resins can be appropriately selected depending on the purpose. The average degree of polymerization is typically 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. The average degree of polymerization can be determined in accordance with JIS K 6726-1994.

[0028] Any suitable halide can be used as the above-mentioned halide. Examples include iodide and sodium chloride. Examples of iodide include potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferred.

[0029] The amount of halogen in the coating solution is preferably 5 to 20 parts by weight per 100 parts by weight of PVA resin, and more preferably 10 to 15 parts by weight per 100 parts by weight of PVA resin. If the amount of halogen per 100 parts by weight of PVA resin exceeds 20 parts by weight, the halogen may bleed out, and the resulting polarizing film may become cloudy.

[0030] Generally, stretching a PVA-based resin layer increases the orientation of polyvinyl alcohol molecules within the PVA-based resin. However, immersing the stretched PVA-based resin layer in a water-containing liquid can disrupt the orientation of the polyvinyl alcohol molecules, potentially reducing their degree of orientation. This tendency to decrease orientation is particularly pronounced when stretching a laminate of a thermoplastic resin and a PVA-based resin layer in boric acid water, especially when stretching the laminate at a relatively high temperature in boric acid water to stabilize the stretching of the thermoplastic resin. For example, while stretching a PVA film alone in boric acid water is typically performed at 60°C, stretching a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a much higher temperature of around 70°C. In this case, the orientation of the PVA in the initial stages of stretching may decrease before it increases due to water stretching. In contrast, by fabricating a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate, and performing high-temperature stretching (auxiliary stretching) in air before stretching the laminate 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 disorder of the orientation of polyvinyl alcohol molecules and the decrease in orientation can be suppressed compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through processing steps that involve immersing the laminate in a liquid, such as dyeing and water stretching.

[0031] C-2. Aerial Auxiliary Extension Treatment In particular, to obtain high optical properties, a two-stage stretching method combining dry stretching (auxiliary stretching) and stretching in boric acid water is selected. By introducing auxiliary stretching, as in two-stage stretching, it is possible to stretch the thermoplastic resin substrate while suppressing crystallization, solving the problem of reduced stretchability due to excessive crystallization of the thermoplastic resin substrate during subsequent stretching in boric acid water, and enabling stretching of the laminate to a higher magnification. Furthermore, when coating a PVA-based resin onto a thermoplastic resin substrate, it is necessary to lower the coating temperature compared to coating a PVA-based resin onto a normal metal drum in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate. As a result, the crystallization of the PVA-based resin becomes relatively low, which can lead to the problem of not being able to obtain sufficient optical properties. In contrast, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when coating a thermoplastic resin, making it possible to achieve high optical properties. Furthermore, by simultaneously enhancing the orientation of the PVA resin beforehand, it is possible to prevent problems such as a decrease in the orientation or dissolution of the PVA resin when it is immersed in water during subsequent dyeing and stretching processes, thereby achieving high optical properties.

[0032] The stretching method for aerial assisted stretching may be fixed-end stretching (for example, stretching using a tenter stretcher) or free-end stretching (for example, uniaxial stretching by passing the laminate between rolls with different peripheral speeds). However, free-end stretching can be actively adopted to obtain high optical properties. In one embodiment, the aerial stretching process includes a heated roll stretching step in which the laminate is stretched by the difference in peripheral speed between heated rolls while being transported in its longitudinal direction. Typically, the aerial stretching process includes a zone stretching step and a heated roll stretching step. The order of the zone stretching step and the heated roll stretching step is not limited; the zone stretching step may be performed first, or the heated roll stretching step may be performed first. The zone stretching step may be omitted. In one embodiment, the zone stretching step and the heated roll stretching step are performed in this order. In another embodiment, the film end is gripped in a tenter stretcher and stretched by widening the distance between tenters in the flow direction (the widening of the distance between tenters becomes the stretching ratio). At this time, the distance of the tenters in the width direction (perpendicular to the flow direction) is set to be arbitrarily close. Preferably, it can be set to be closer to the free end stretching ratio with respect to the stretching ratio in the flow direction. In the case of free end stretching, the shrinkage ratio in the width direction = (1 / stretching ratio) 1 / 2 It is calculated as follows.

[0033] Aerial assisted stretching may be performed in one stage or in multiple stages. When performed in multiple stages, the stretching ratio is the product of the stretching ratios of each stage. Preferably, the stretching direction in aerial assisted stretching is substantially the same as the stretching direction in underwater stretching.

[0034] The stretching ratio in aerial assisted stretching is preferably 2.0 to 3.5 times. When aerial assisted stretching and underwater stretching are combined, the maximum stretching ratio is preferably 5.0 times or more, more preferably 5.5 times or more, and even more preferably 6.0 times or more, relative to the original length of the laminate. In this specification, "maximum stretching ratio" refers to the stretching ratio immediately before the laminate breaks, and is defined as a value 0.2 lower than the stretching ratio at which the laminate breaks, which is determined separately.

[0035] The stretching temperature in air-assisted stretching can be set to any appropriate value depending on the forming material of the thermoplastic resin substrate, the stretching method, etc. Preferably, the stretching temperature is above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg) + 10°C, and particularly preferably above Tg + 15°C. 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 the rapid crystallization of the PVA-based resin and suppress problems caused by such crystallization (for example, hindering the orientation of the PVA-based resin layer by stretching).

[0036] C-3. Immobilization treatment, dyeing treatment, and crosslinking treatment If necessary, an insolubilization treatment is performed after the air-assisted stretching treatment and before the water stretching treatment and dyeing treatment. Typically, the insolubilization treatment is performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Typically, the dyeing treatment is performed by dyeing the PVA-based resin layer with a dichroic substance (typically iodine). If necessary, a crosslinking treatment is performed after the dyeing treatment and before the water stretching treatment. Typically, the crosslinking treatment is performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Details of the insolubilization treatment, dyeing treatment and crosslinking treatment are described, for example, in Japanese Patent Publication No. 2012-73580 (above).

[0037] C-4. Underwater stretching treatment Underwater stretching is performed by immersing the laminate in a stretching bath. Underwater stretching allows stretching at a temperature lower than the glass transition temperature (typically around 80°C) of the thermoplastic resin substrate or PVA-based resin layer, enabling high-magnification stretching of the PVA-based resin layer while suppressing its crystallization. As a result, polarizing films with excellent optical properties can be manufactured.

[0038] Any suitable method can be used to stretch the laminate. Specifically, it may be fixed-end stretching or free-end stretching (for example, uniaxial stretching by passing the laminate between rolls with different peripheral speeds). Preferably, free-end stretching is selected. The stretching of the laminate may be carried out in one stage or in multiple stages. In the case of multiple stages, the stretching ratio of the laminate (maximum stretching ratio), which will be described later, is the product of the stretching ratios of each stage.

[0039] Stretching in water is preferably carried out by immersing the laminate in an aqueous boric acid solution (boric acid water stretching). By using an aqueous boric acid solution as the stretching bath, the PVA resin layer can be given rigidity to withstand the tension applied during stretching and water resistance that prevents it from dissolving in water. Specifically, boric acid can generate tetrahydroxyborate anions in the aqueous solution and crosslink with the PVA resin by hydrogen bonding. As a result, the PVA resin layer can be given rigidity and water resistance, allowing for good stretching and the production of a polarizing film with excellent optical properties.

[0040] The above-mentioned aqueous boric acid solution is preferably obtained by dissolving boric acid and / or a borate in water, which is the solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight, per 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA resin layer can be effectively suppressed, and a polarizing film with higher properties can be produced. In addition to boric acid or a borate, aqueous solutions obtained by dissolving boron compounds such as borax, glyoxal, glutaraldehyde, etc., in a solvent can also be used.

[0041] Preferably, iodide is added to the stretching bath (boric acid aqueous solution). By adding iodide, the elution of iodine adsorbed on the PVA 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, per 100 parts by weight of water.

[0042] 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 temperatures, high-magnification stretching is possible while suppressing the 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 falls below 40°C, even considering the plasticization of the thermoplastic resin substrate by water, good stretching may not be possible. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer becomes, which may prevent the acquisition of excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

[0043] The stretching ratio by underwater stretching is preferably 1.5 times or more, more preferably 3.0 times or more. The total stretching ratio of the laminate is preferably 5.0 times or more, and even more preferably 5.5 times or more, relative to the original length of the laminate. By achieving such high stretching ratios, it is possible to manufacture polarizing films with extremely excellent optical properties. Such high stretching ratios can be achieved by employing an underwater stretching method (boric acid underwater stretching).

[0044] C-5. Drying shrinkage treatment The above drying shrinkage treatment may be performed by zone heating, which involves heating the entire zone, or by heating the conveying rolls (using so-called heated rolls) (heated roll drying method). Preferably, both methods are used. By drying using heated rolls, heat curling of the laminate can be efficiently suppressed, and a polarizing film with excellent appearance can be manufactured. Specifically, by drying the laminate while it is aligned with the heated rolls, the crystallization of the thermoplastic resin substrate can be efficiently promoted, increasing the degree of crystallinity, and even at relatively low drying temperatures, the degree of crystallinity of the thermoplastic resin substrate can be increased well. As a result, the rigidity of the thermoplastic resin substrate increases, making it able to withstand the shrinkage of the PVA-based resin layer due to drying, and curling is suppressed. Furthermore, by using heated rolls, the laminate can be dried while maintaining a flat state, so not only curling but also wrinkles can be suppressed. At this time, the optical properties of the laminate can be improved by shrinking it in the width direction through the drying shrinkage treatment. This is because the orientation of PVA and PVA / iodine complex can be effectively increased. The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%.

[0045] Figure 2 is a schematic diagram showing an example of a drying shrinkage process. 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 to continuously heat the PVA resin layer surface and the thermoplastic resin substrate surface alternately, but for example, the transport rolls R1 to R6 may be arranged to continuously heat only one side of the laminate 200 (for example, the thermoplastic resin substrate surface).

[0046] Drying conditions can be controlled by adjusting the heating temperature of the conveying rolls (temperature of the heating rolls), the number of heating rolls, and the contact time with the heating rolls. The heating roll temperature is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. This effectively increases the crystallinity of the thermoplastic resin, effectively suppresses curling, and produces an optical laminate with extremely high durability. The heating roll temperature can be measured using a contact thermometer. In the illustrated example, six conveying rolls are provided, but there are no particular restrictions on the number of conveying rolls as long as there are multiples. Typically, 2 to 40 conveying rolls are provided, preferably 4 to 30. The contact time between the laminate and the heating rolls (total contact time) is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

[0047] The heating rolls may be installed inside a heating furnace (e.g., an oven) or in a normal production line (at room temperature). Preferably, they are installed inside a heating furnace equipped with a blowing mechanism. By using heating roll drying in combination with 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. The hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10 m / s to 30 m / s. This wind speed is the wind speed inside the heating furnace and can be measured with a mini-vane type digital anemometer.

[0048] C-6. Contact with processing liquid As described above, a laminate of a thermoplastic resin substrate and a polarizing film can be obtained. In an embodiment of the present invention, the polarizing film is brought into contact with a treatment solution having a pH of 3.0 or less. In one embodiment, the polarizing film can be brought into contact with the treatment solution by bringing the laminate directly into contact with the treatment solution. In this case, typically, the thermoplastic resin substrate can be used as a protective layer for the polarizing film. Alternatively, a resin film (which will be a protective layer) may be bonded to the surface of the polarizing film of the laminate that has been brought into contact with the treatment solution to create a laminate of protective layer / polarizing film / thermoplastic resin substrate, and the thermoplastic resin substrate may be peeled off from the laminate to produce a polarizing plate having a protective layer / polarizing film configuration. In another embodiment, a resin film (which will be a protective layer) may be bonded to the surface of the polarizing film of the laminate to create a laminate of protective layer / polarizing film / thermoplastic resin substrate, and the thermoplastic resin substrate may be peeled off from the laminate to produce a laminate of protective layer / polarizing film (polarizing plate). By bringing the obtained polarizing plate into contact with the treatment solution, the polarizing film can be brought into contact with the treatment solution.

[0049] Contact between the polarizing film and the treatment solution can be carried out by any suitable method. Typical examples include coating the polarizing film with the treatment solution and immersing the polarizing film (substantially, a laminate or polarizing plate) in the treatment solution. Any suitable method can be used for coating. A specific example is the method described in Section C-1 for coating the solution. Immersion can also be carried out in any suitable manner. For example, the treatment solution may be added to the washing bath of the washing treatment, a bath of the treatment solution may be used instead of the washing bath, or the bath of the treatment solution may be provided separately from the washing bath. Typically, the washing treatment is carried out after the underwater stretching treatment and before the drying shrinkage treatment. If a separate bath of the treatment solution is provided, the bath of the treatment solution may be provided between the washing bath and the drying shrinkage treatment equipment (i.e., contact with the treatment solution may occur between the washing treatment and the drying shrinkage treatment), or it may be provided downstream of the means for peeling off the thermoplastic resin substrate (i.e., contact with the treatment solution may occur after peeling off the thermoplastic resin substrate).

[0050] Any suitable acidic liquid can be used as the treatment solution, as long as its pH is 3.0 or lower. Specific examples of treatment solutions include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and citric acid. The treatment solution is preferably a strong acid aqueous solution. Specific examples of strong acids include hydrochloric acid, sulfuric acid, and nitric acid. A lower pH (stronger acidity) of the treatment solution is preferable. Specifically, the pH is preferably 2.7 or lower, more preferably 2.5 or lower, even more preferably 2.0 or lower, and particularly preferably 1.5 or lower.

[0051] The acid concentration of the processing solution is preferably 0.02% to 3.0% by weight, more preferably 0.04% to 2.0% by weight, and even more preferably 0.1% to 1.0% by weight.

[0052] The processing solution may contain a water-soluble resin (e.g., a PVA-based resin). The water-soluble resin can function as a binder. The concentration of the water-soluble resin in the processing solution is preferably 3% to 5% by weight. In this case, a processing layer can be formed by coating and drying the processing solution. By forming such a processing layer, a polarizing film with the desired durability can also be obtained. The thickness of the processing layer is preferably 1.7 μm or less, and more preferably 0.2 μm to 1.4 μm.

[0053] After contact with the processing solution, drying may be performed as needed. The drying temperature is preferably 40°C to 90°C, and more preferably 50°C to 70°C.

[0054] C-7. Variant Sections C-1 to C-6 describe a manufacturing method using a laminate of a resin substrate and a PVA-based resin layer applied to the resin substrate. However, the present invention can also be applied to a manufacturing method using a single PVA-based resin film. Typically, such a manufacturing method involves uniaxially stretching a long PVA-based resin film in the longitudinal direction using a roll stretcher, while subjecting it to swelling, dyeing, crosslinking, and washing treatments, and finally drying treatment. Contact with the treatment solution can typically be achieved by immersion in a washing bath to which the treatment solution has been added, immersion in a treatment bath after washing treatment, or application of the treatment solution after washing treatment. [Examples]

[0055] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement methods for each characteristic are as follows. Unless otherwise specified, "parts" and "%" in the examples and comparative examples are based on weight. (1) Thickness The measurement was performed using an interferometric film thickness gauge (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000"). (2) Single unit transmittance and orthogonal absorbance For the polarizing plates (protective layer / polarizing film) of the examples and comparative examples, the single-layer transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc measured using a UV-Vis spectrophotometer (Otsuka Electronics Co., Ltd., product name "LPF-200") were defined as the Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values ​​obtained by measuring with a 2-degree field of view (C light source) according to JIS Z8701 and correcting for luminous sensitivity. The refractive index of the protective film was 1.50, and the refractive index of the surface of the polarizing film opposite the protective film was 1.53. Furthermore, the orthogonal absorbance was determined using the Tc measured at each wavelength, according to the following formula. Orthogonal absorbance = log10(100 / Tc) The orthogonal absorbance Abs0 was determined from the orthogonal transmittance Tc at a measurement wavelength of 600 nm using the "LPF-200" manufactured by Otsuka Electronics Co., Ltd. Note that equivalent measurements of Abs0 can also be performed using instruments such as the "V-7100" manufactured by JASCO Corporation. Next, the polarizing plates were subjected to a 240-hour durability test at a temperature of 60°C and a relative humidity of 95%. The orthogonal absorbance (Abs) after the durability test was measured. 240 This was obtained in the same manner as described above.

[0056] [Example 1] As the thermoplastic resin substrate, an amorphous isophthalic copolymer polyethylene terephthalate film (thickness: 100 μm) in a long length, with a water absorption rate of 0.75% and a Tg of approximately 75°C was used. One side of the resin substrate was subjected to corona treatment (treatment conditions: 55 W·min / m²). 2 ) was applied. A PVA aqueous solution (coating solution) was prepared by adding 13 parts by weight of potassium iodide to 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "Gosephymer Z410") in a 9:1 ratio. A laminate was fabricated by applying the above PVA aqueous solution to the corona-treated surface of a resin substrate and drying it at 60°C to form a PVA-based resin layer with a thickness of 20 μm. The resulting laminate was uniaxially stretched 2.4 times in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 130°C (air-assisted stretching). Next, the laminate was immersed for 30 seconds in an insolubilization bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) (insolubilization treatment). Next, the polarizing plates were immersed for 60 seconds in a staining bath at a liquid temperature of 30°C (an iodine aqueous solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) while adjusting the concentration so that the final transmittance (Ts) of the polarizing plates obtained was 45.0% (staining treatment). Next, the material was immersed for 30 seconds in a crosslinking bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) (crosslinking treatment). Subsequently, the laminate was immersed in a boric acid aqueous solution (boric acid concentration 4.0 wt%, potassium iodide 5 wt%) at a liquid temperature of 70°C, and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to achieve a total stretch ratio of 5.5 times (underwater stretching treatment). Subsequently, the laminate was immersed in a washing bath at a liquid temperature of 20°C (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water, pH=6) (washing treatment). Subsequently, the laminate was dried in an oven maintained at 90°C while being brought into contact with a SUS (stainless steel) heated roll with a surface temperature maintained at 75°C for approximately 2 seconds (drying shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%. In this manner, a polarizing film with a thickness of 5.0 μm was formed on a resin substrate, and a cycloolefin-based film (manufactured by ZEON, product name "G-Film") as a protective layer (protective film) was bonded to the surface of the polarizing film using a UV-curing adhesive (thickness of 1.0 μm). Subsequently, the resin substrate was peeled off to obtain a laminate having a protective layer / polarizing film structure. The transmittance (Ts) of the obtained laminate was 45.0%, which is the value obtained by correcting the actual measured value by +0.2% to a state where the surface refractive index of the polarizing film / protective layer constituting the laminate is 1.53 / 1.53, and converting it to a state of 1.53 / 1.50. Next, a treatment solution (pH=1.3) obtained by dissolving 0.3 wt% hydrochloric acid and 3.5 wt% PVA (JC-25) in water was applied to the surface of the polarizing film of the laminate to a thickness of 0.6 μm, and dried at 60°C for 4 minutes to form a treatment layer. In this way, the polarizing plate of this embodiment was obtained.

[0057] For the obtained polarizing plate (essentially a polarizing film), the transmittance and Abs of the individual plate were determined. 240 Table 1 shows / Abs0.

[0058] [Example 2] A polarizing plate was prepared in the same manner as in Example 1, except that the concentration of the staining bath was adjusted to achieve a single-unit transmittance (Ts) of 44.0% for the polarizing film. The single-unit transmittance and Abs of the obtained polarizing plate (essentially a polarizing film) were examined. 240 Table 1 shows / Abs0.

[0059] [Examples 3-19] Polarizing plates were fabricated by adjusting the transmittance of the polarizing film, the contact method with the processing solution, the pH of the processing solution, the type of acid contained in the processing solution, and the thickness of the processing layer as shown in Table 1. For the obtained polarizing plates (essentially polarizing films), the transmittance and Abs were adjusted. 240 Table 1 shows / Abs0.

[0060] [Example 20] A polarizing plate was prepared in the same manner as in Example 2, except that the treatment solution did not contain a PVA-based resin (i.e., no treatment layer was formed) and the pH of the treatment solution was set to 0.9. The single-unit transmittance and Abs of the obtained polarizing plate (essentially a polarizing film) were measured. 240 Table 1 shows / Abs0.

[0061] [Example 21] In the same manner as in Example 2, laminates of thermoplastic resin substrate / PVA-based resin layer were subjected to air-assisted stretching, insolubilization, dyeing, crosslinking, and underwater stretching. The underwater stretched laminates were immersed in a treatment bath (pH=1.6) at a liquid temperature of 20°C (contact with the treatment solution). The treatment bath was prepared by adding hydrochloric acid to a normal washing bath (an aqueous solution obtained by adding 4 parts by weight of potassium iodide to 100 parts by weight of water). Subsequently, the laminate was dried in an oven maintained at 90°C while being brought into contact with a SUS (stainless steel) heated roll with a surface temperature maintained at 75°C for approximately 2 seconds (drying shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%. Next, a cycloolefin film (manufactured by ZEON, product name "G-Film") as a protective layer (protective film) was bonded to the surface of the polarizing film using a UV-curing adhesive (thickness 1.0 μm). After that, the resin substrate was peeled off to obtain a polarizing plate having a protective layer / polarizing film structure. The transmittance and Abs of the obtained polarizing plate (essentially a polarizing film) were determined. 240 Table 1 shows / Abs0.

[0062] [Comparative Examples 1-2] Polarizing plates were prepared in the same manner as in Examples 1 and 2, except that contact with the processing solution was not performed. The transmittance and Abs of the obtained polarizing plates (essentially polarizing films) were determined. 240 Table 1 shows / Abs0.

[0063] [Comparative Example 3] A polarizing plate was prepared in the same manner as in Comparative Example 1, except that the transmittance of the polarizing film alone was set to 43.0%. The transmittance and Abs of the obtained polarizing plate (essentially a polarizing film) were measured. 240 Table 1 shows / Abs0.

[0064] [Comparative Examples 4-9] Polarizing plates were fabricated by adjusting the transmittance of the polarizing film, the contact method with the processing solution, the pH of the processing solution, the type of acid contained in the processing solution, and the thickness of the processed layer (if formed) as shown in Table 1. For the obtained polarizing plates (essentially polarizing films), the transmittance and Abs were adjusted. 240 Table 1 shows / Abs0.

[0065] [Example 22] A 23 μm thick polarizing film was prepared by uniaxially stretching a 55 μm thick PVA resin film (manufactured by Nippon Gosei Co., Ltd., product name "PS7500") in the longitudinal direction using a roll stretching machine to achieve a total stretching ratio of 6.0, while simultaneously subjecting it to swelling, dyeing, crosslinking, and washing treatments, and finally drying treatment. After the washing treatment and before the drying treatment, one side of the PVA resin film (polarizing film) was coated with the same treatment solution as in Example 2, in the same manner as in Example 2. The transmittance and Abs of the obtained polarizing film were examined. 240 Table 1 shows / Abs0.

[0066] [Example 23] A polarizing film with a thickness of 23 μm was prepared in the same manner as in Example 22, except that the PVA resin film (polarizing film) was passed through a treatment bath similar to that in Example 21 instead of a washing bath (therefore, no treatment solution was applied after the washing treatment). The transmittance and Abs of the obtained polarizing film were examined. 240 Table 1 shows / Abs0.

[0067] [Table 1]

[0068] As is clear from Table 1, the polarizing film of the embodiment of the present invention is Abs after durability testing. 240 The Abs0 is greater than 1.00, indicating excellent durability in high-temperature and high-humidity environments. The polarizing films of Comparative Examples 1-3, which were not in contact with the processing solution, and the polarizing films of Comparative Examples 4-9, which were in contact with a processing solution with a pH greater than 3.0, all showed Abs0. 240 The / Abs0 value was less than 1.00. In Comparative Example 7, where boric acid was used as the treatment solution, the treatment solution gelled, making contact impossible. [Industrial applicability]

[0069] The polarizing film and polarizing plate of the present invention are suitably used in liquid crystal display devices. [Explanation of Symbols]

[0070] 10 Polarizing film 20. First protective layer 30. Second protective layer 100 polarizing plates

Claims

1. It is made of a polyvinyl alcohol-based resin film containing iodine. A treatment layer is formed on the surface. The treated layer is a solidified layer of a coating film of a treatment solution containing a polyvinyl alcohol-based resin with a pH of 3.0 or less. Absorbance Abs at a wavelength of 600 nm after a 240-hour endurance test at a temperature of 60°C and relative humidity of 95% ２４０ However, the absorbance Abs before the durability test ０ A polarizing film that satisfies the following relationship: 1.05≦Abs ２４０ / Abs ０ ≦1.44

2. The polarizing film according to claim 1, wherein the thickness of the treated layer is 0.2 μm to 1.7 μm.

3. A polarizing film according to claim 1 or 2, wherein the single-layer transmittance is 43.0% or more.

4. A polarizing film according to any one of claims 1 to 3, wherein the thickness is 8 μm or less.

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

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