JP7793370B2 - Polarizing film, laminated polarizing film, image display panel, and image display device - Google Patents

Description

The present invention relates to a polarizing film, a laminated polarizing film, an image display panel, and an image display device.

Conventionally, dyed polyvinyl alcohol films (containing dichroic substances such as iodine or dichroic dyes) have been used as polarizing films in various image display devices, such as liquid crystal display devices and organic electroluminescent display devices, due to their high transmittance and high polarization. These polarizing films are produced by subjecting polyvinyl alcohol films to various processes, such as swelling, dyeing, crosslinking, and stretching, in a bath, followed by washing and drying. Furthermore, these polarizing films are typically used as polarizing films (polarizing plates) with a protective film such as triacetyl cellulose attached to one or both sides using an adhesive.

The polarizing film is used as a laminated polarizing film (optical laminate) by laminating other optical layers as needed, and the polarizing film or the laminated polarizing film (optical laminate) is bonded between an image display cell such as a liquid crystal cell or an organic EL element and a front transparent plate (window layer) on the viewing side or a front transparent member such as a touch panel via a pressure-sensitive adhesive layer or adhesive layer, and is used as the various image display devices mentioned above (Patent Document 1).

In recent years, the applications of these various image display devices have expanded, including use in mobile devices such as mobile phones and tablet computers, as well as in-vehicle image display devices such as car navigation systems and rearview monitors. Accordingly, these image display devices are required to be more durable in harsher environments (e.g., high-temperature environments) than previously required, and an image display device designed to ensure such durability has been proposed (Patent Document 2).

JP 2014-102353 A JP 2018-101117 A

When polarizing films and laminated polarizing films using iodine-based polarizing films are exposed to high-temperature environments, the polyvinyl alcohol that makes up the polarizing film undergoes a dehydration reaction to become polyenated, causing the polarizing film to become discolored and reducing its individual transmittance.

After extensive research, the inventors discovered that the iodine contained in iodine-based polarizing films promotes polyenation in high-temperature environments. Therefore, reducing the iodine concentration (content) in the polarizing film is effective in suppressing the decrease in single-unit transmittance due to coloration of the polarizing film in high-temperature environments. However, when such polarizing films are used, as image display device panels become larger, polarizing films and laminated polarizing films tend to peel off when heated.

In addition, polarizing films and laminated polarizing films have the problem that, when placed in high-temperature, high-humidity environments, the boric acid crosslinks in the polarizing film are released and the iodine complexes are destroyed, resulting in a decrease in polarization.

In light of the above circumstances, the present invention aims to provide a polarizing film having a polarizing film that has a good initial polarization degree, is excellent in suppressing the decrease in single-unit transmittance due to coloration of the polarizing film in high-temperature environments, and is also excellent in suppressing the decrease in polarization degree (humidification durability) in high-temperature, high-humidity environments.

The present invention also aims to provide a laminated polarizing film, an image display panel, and an image display device using the above-mentioned polarizing film.

Specifically, the present invention relates to a polarizing film having transparent protective films bonded to both sides of the polarizing film, wherein the polarizing film is formed by adsorbing and aligning iodine on a polyvinyl alcohol-based film and has an iodine concentration of 3% by weight or more and 10% by weight or less, at least one of the transparent protective films has a moisture permeability of 200 g/( ^m2 ·24 h) or less, and the polarizing film satisfies the condition that a change in single-piece transmittance (ΔTs) expressed by general formula (1): ΔTs(%)= _Ts750 − _Ts0 (in general formula (1), _Ts0 is the single-piece transmittance of a laminate in which a glass plate is bonded to one side of the polarizing film via a pressure-sensitive adhesive layer, and _Ts750 is the single-piece transmittance after the laminate has been heat-treated at 105°C for 750 hours).

The present invention also relates to a laminated polarizing film in which the polarizing film is bonded to an optical layer.

The present invention also relates to an image display panel in which the polarizing film or the laminated polarizing film is bonded to an image display cell.

The present invention also relates to an image display device having a front transparent member on the polarizing film or laminated polarizing film side of the image display panel.

Although the details of the mechanism of action of the polarizing film of the present invention are unclear, it is presumed as follows. However, the present invention need not be interpreted as being limited to this mechanism of action.

The polarizing film of the present invention is a polarizing film having transparent protective films bonded to both sides of a polarizing film, wherein the polarizing film is formed by adsorbing and aligning iodine on a polyvinyl alcohol-based film and has an iodine concentration of 3% by weight or more and 10% by weight or less, at least one of the transparent protective films has a moisture permeability of 200 g/( ^m2 ·24 h) or less, and the polarizing film satisfies the following conditions: a change in single-piece transmittance (ΔTs) expressed by general formula (1): ΔTs(%)= _Ts750 − _Ts0 (in general formula (1), _Ts0 is the single-piece transmittance of a laminate in which a glass plate is bonded to one side of the polarizing film via a pressure-sensitive adhesive layer, and _Ts750 is the single-piece transmittance after the laminate has been heat-treated at 105°C for 750 hours). The polarizing film of the present invention exhibits a small change in single-unit transmittance (ΔTs) after the heat treatment (in a high-temperature environment), thereby suppressing a decrease in single-unit transmittance due to coloration of the polarizing film. The reason for this small change in single-unit transmittance (ΔTs) before and after the heat treatment is presumably because the temperature at which the dehydration reaction due to polyenation occurs in the polarizing film of the present invention is higher than that of conventional polarizing films. In particular, the heat treatment is a heat durability test used as an indicator of the heat resistance of in-vehicle displays, and is more severe than conventional heat durability tests. Furthermore, by setting the iodine concentration in the polarizing film within a certain range, the polarizing film of the present invention exhibits a good initial polarization degree while being able to control the temperature at which the dehydration reaction due to polyenation occurs to a higher temperature.

Furthermore, in the polarizing film of the present invention, at least one of the transparent protective films has a moisture permeability of 200 g/( ^m² ·24 h) or less, thereby preventing the intrusion of water into the polarizing film, and thereby exhibiting excellent effect of suppressing the decrease in polarization degree (humidity durability) even in a high-temperature, high-humidity environment.

<Polarizing film>
The polarizing film of the present invention is an iodine-based polarizing film formed by adsorbing and aligning iodine in a polyvinyl alcohol-based film, and has an iodine concentration of 3% by weight or more and 10% by weight or less. The polyvinyl alcohol (PVA)-based film can be any polyvinyl alcohol (PVA)-based film that is translucent in the visible light region and disperses and adsorbs iodine. The PVA-based film typically used as a raw sheet preferably has a thickness of about 1 to 100 μm, more preferably about 1 to 50 μm, and a width of about 100 to 5,000 mm.

The polyvinyl alcohol film may be made of polyvinyl alcohol or a derivative thereof. Examples of polyvinyl alcohol derivatives include polyvinyl formal, polyvinyl acetal; olefins such as ethylene and propylene; unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid, and their alkyl esters, modified with acrylamide, etc. The polyvinyl alcohol preferably has an average degree of polymerization of approximately 100 to 10,000, more preferably approximately 1,000 to 10,000, and even more preferably approximately 1,500 to 4,500. The polyvinyl alcohol preferably has a degree of saponification of approximately 80 to 100 mol%, more preferably approximately 95 mol% to 99.95 mol%. The average degree of polymerization and the degree of saponification can be determined in accordance with JIS K 6726.

The polyvinyl alcohol film may contain additives such as plasticizers and surfactants. Examples of plasticizers include polyols such as glycerin, diglycerin, triglycerin, ethylene glycol, propylene glycol, and polyethylene glycol, as well as condensates thereof. There are no particular restrictions on the amount of additives used, but a preferred amount is approximately 20% by weight or less of the polyvinyl alcohol film.

The polarizing film has an iodine concentration (content) of 3% by weight or more and 10% by weight or less, from the viewpoints of improving the initial polarization degree of the polarizing film and increasing the temperature at which the dehydration reaction due to polyenation occurs. From the viewpoints of improving the initial polarization degree of the polarizing film, the polarizing film preferably has an iodine concentration (content) of 3.5% by weight or more, and more preferably 4% by weight or more. From the viewpoint of increasing the temperature at which the dehydration reaction due to polyenation occurs, the iodine concentration (content) is preferably 9% by weight or less, more preferably 7% by weight or less, even more preferably 6% by weight or less, and most preferably 5% by weight or less.

The polarizing film preferably contains a compound with radical scavenging functionality. It is believed that the compound with radical scavenging functionality captures radicals generated by heating the polyvinyl alcohol of the polarizing film, thereby enabling the temperature at which the dehydration reaction of polyene formation occurs to be set to a higher temperature. Examples of compounds with radical scavenging functionality include hindered phenols, hindered amines, phosphorus compounds, sulfur compounds, benzotriazoles, benzophenones, hydroxylamines, salicylic acid esters, and triazines. From the viewpoint of easily enabling the temperature at which the dehydration reaction of polyene formation occurs to be set to a higher temperature, the compound with radical scavenging functionality is preferably, for example, a compound having a nitroxy radical or a nitroxide group. The compounds with radical scavenging functionality may be used alone or in combination of two or more types.

The compound having a nitroxy radical or a nitroxide group includes an N-oxyl compound (a compound having a C—N(—C) ^—O. functional group ( ^O. represents an oxy radical)) from the viewpoint of having a radical that is relatively stable in air at room temperature, and known compounds can be used. Examples of the compound having a nitroxy radical or a nitroxide group include compounds having an organic group with the following structure:
(In general formula (1), ^R1 represents an oxy radical, ^R2 to ^R5 each independently represent a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and n represents 0 or 1.) In general formula (1), the left side of the dotted line represents any organic group.

Examples of the compound having the organic group include compounds represented by the following general formulas (2) to (5).
(In general formula (2), R ¹ to R ⁵ and n are the same as above, R ⁶ represents a hydrogen atom, or an alkyl group, acyl group, or aryl group having 1 to 10 carbon atoms, and n represents 0 or 1.)
(In general formula (3), R ¹ to R ⁵ and n are the same as above, and R ⁷ and R ⁸ independently represent a hydrogen atom, or an alkyl group, acyl group, or aryl group having 1 to 10 carbon atoms.)
(In general formula (4), R ¹ to R ⁵ and n are the same as above, and R ⁹ to R ¹¹ independently represent a hydrogen atom, or an alkyl group having 1 to 10 carbon atoms, an acyl group, an amino group, an alkoxy group, a hydroxy group, or an aryl group.)
(In general formula (5), R ¹ to R ⁵ and n are the same as above, and R ¹² represents a hydrogen atom, or an alkyl group having 1 to 10 carbon atoms, an amino group, an alkoxy group, a hydroxy group, or an aryl group.)

In the general formulas (1) to (5), from the viewpoint of availability, ^R2 to ^R5 are preferably alkyl groups having 1 to 6 carbon atoms, and more preferably alkyl groups having 1 to 3 carbon atoms. Furthermore, in the general formula (2), from the viewpoint of availability, ^R6 is preferably a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and more preferably a hydrogen atom. Furthermore, in the general formula (3), from the viewpoint of availability, ^R7 and ^R8 are independently preferably a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and more preferably a hydrogen atom. Furthermore, in the general formula (4), from the viewpoint of availability, ^R9 to ^R11 are preferably a hydrogen atom or an alkyl group having 1 to 10 carbon atoms. Furthermore, in the general formula (5), from the viewpoint of availability, ^R12 is preferably a hydroxy group, an amino group, or an alkoxy group. Furthermore, in the general formulas (1) to (5), from the viewpoint of availability, n is preferably 1.

Examples of the compound having a nitroxy radical or a nitroxide group include the following compounds:
(In general formula (6), R represents a hydrogen atom, or an alkyl group, acyl group, or aryl group having 1 to 10 carbon atoms.)

In addition, from the viewpoint of being able to efficiently capture radicals generated in the polyenation reaction, the compound having radical scavenging function preferably has a molecular weight of 1,000 or less, more preferably 500 or less, and even more preferably 300 or less.

In addition, from the viewpoint of ease of migration into the moisture in the polarizing film, the compound having radical scavenging function is preferably a water-soluble compound that can dissolve at least 1 part by weight in 100 parts by weight of water at 25°C, more preferably a water-soluble compound that can dissolve at least 2 parts by weight in 100 parts by weight of water at 25°C, and even more preferably a water-soluble compound that can dissolve at least 5 parts by weight in 100 parts by weight of water at 25°C.

When the polarizing film contains the compound having the radical scavenging function, the content of the compound having the radical scavenging function in the polarizing film is preferably 0.01% by weight or more, more preferably 0.02% by weight or more, and even more preferably 0.05% by weight or more, from the viewpoint of suppressing a decrease in the single transmittance due to coloration of the polarizing film in a high-temperature environment; and from the viewpoint of appearance, it is preferably 10% by weight or less, more preferably 7% by weight or less, and even more preferably 3% by weight or less.

The polarizing film preferably contains chlorine ions (chloride ions). These chlorine ions are deliquescent and are thought to delay the dehydration reaction that occurs when the polyvinyl alcohol in the polarizing film is heated, allowing the temperature at which the dehydration reaction of polyenization occurs to be set higher.

If the polarizing film contains chlorine ions (chloride ions), the content of the chlorine ions in the polarizing film is preferably more than 2% by weight, more preferably 2.5% by weight or more, and even more preferably 3% by weight or more, from the viewpoint of suppressing a decrease in the single transmittance due to coloration of the polarizing film in a high-temperature environment; and from the viewpoint of preventing a change in the hue of the polarizing film in a high-temperature environment, the content of the chlorine ions is preferably 10% by weight or less, more preferably 7% by weight or less, and even more preferably 5% by weight or less.

<Method of manufacturing polarizing film>
The polarizing film is obtained by subjecting the polyvinyl alcohol-based film to any of a swelling step, a washing step, and at least a dyeing step, a crosslinking step, and a stretching step. The iodine content in the polarizing film can be controlled by the concentration of the iodine and iodides such as potassium iodide contained in any of the treatment baths in the swelling step, the dyeing step, the crosslinking step, the stretching step, and the washing step, and by the treatment temperature and treatment time in each of the treatment baths.

Furthermore, when producing a polarizing film containing the compound having radical scavenging functionality, the compound having radical scavenging functionality may be contained in one or more of the treatment baths in the swelling process, washing process, dyeing process, crosslinking process, and stretching process. The concentration of the compound having radical scavenging functionality contained in any of the treatment baths cannot be determined in general because it is affected by the number of treatments, treatment time, treatment temperature, etc., of each treatment. However, from the viewpoint of efficiently controlling the content of the compound having radical scavenging functionality in the polarizing film, it is typically preferably 0.03 wt% or more, more preferably 0.05 wt% or more, even more preferably 0.1 wt% or more, and preferably 15 wt% or less, more preferably 10 wt% or less, and even more preferably 5 wt% or less.

In particular, when a washing process is carried out after the dyeing, crosslinking, and stretching processes, the washing process makes it easy to adjust the content of iodine and the compound with radical scavenging function to the desired range, taking into account the processing conditions in the dyeing, crosslinking, and stretching processes, etc., from the perspective of being able to elute components such as iodine and the compound with radical scavenging function from or adsorb them onto the polyvinyl alcohol-based film.

When producing a polarizing film containing the compound having radical scavenging functionality, an aqueous solution containing the compound having radical scavenging functionality may be applied to the polarizing film to impregnate it. The application process may be performed before or after any of the swelling, dyeing, crosslinking, stretching, and washing processes described above. In particular, it is preferable to apply the coating after the washing process, as this makes it easier to adjust the content of the compound having radical scavenging functionality to the desired range. The application method is not particularly limited, and examples include wire bar coating, gravure coating, and spraying.

Furthermore, each treatment bath in the swelling, dyeing, crosslinking, stretching, and washing processes may contain additives such as zinc salts, pH adjusters, pH buffers, and other salts. Examples of zinc salts include zinc halides such as zinc chloride and zinc iodide; and inorganic zinc salts such as zinc sulfate and zinc acetate. Examples of pH adjusters include strong acids such as hydrochloric acid, sulfuric acid, and nitric acid, and strong bases such as sodium hydroxide and potassium hydroxide. Examples of pH buffers include carboxylic acids such as acetic acid, oxalic acid, and citric acid, and their salts; and inorganic weak acids such as phosphoric acid and carbonic acid, and their salts. Examples of other salts include chlorides such as sodium chloride, potassium chloride, and barium chloride; nitrates such as sodium nitrate and potassium nitrate; sulfates such as sodium sulfate and potassium sulfate; and salts of alkali metals and alkaline earth metals. In particular, from the viewpoint of obtaining a polarizing film that can suppress a decrease in the single-piece transmittance due to coloration of the polarizing film in high-temperature environments, it is preferable for one or more of the above treatment baths to contain a chloride.

The swelling process is a treatment process in which the polyvinyl alcohol film is immersed in a swelling bath. This process removes dirt and blocking agents from the surface of the polyvinyl alcohol film, and by swelling the polyvinyl alcohol film, it is possible to suppress uneven dyeing. The swelling bath typically uses a medium whose main component is water, distilled water, pure water, or other water. The swelling bath may contain surfactants, alcohol, and other additives as appropriate, according to conventional methods.

The temperature of the swelling bath is preferably about 10 to 60°C, more preferably about 15 to 45°C, and even more preferably about 18 to 30°C. The immersion time in the swelling bath cannot be determined in general because the degree of swelling of the polyvinyl alcohol film is affected by the temperature of the swelling bath, but is preferably about 5 to 300 seconds, more preferably about 10 to 200 seconds, and even more preferably about 20 to 100 seconds. The swelling process may be carried out only once, or multiple times as necessary.

The dyeing process is a treatment process in which the polyvinyl alcohol film is immersed in a dye bath (iodine solution), allowing iodine to be adsorbed and oriented in the polyvinyl alcohol film. The iodine solution is typically preferably an aqueous iodine solution, containing iodine and an iodide as a solubilizing agent. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Among these, potassium iodide is preferred from the viewpoint of controlling the potassium content in the polarizing film.

The concentration of iodine in the dye bath is preferably about 0.01 to 1% by weight, and more preferably about 0.02 to 0.5% by weight. The concentration of iodide in the dye bath is preferably about 0.01 to 20% by weight, more preferably about 0.05 to 10% by weight, and even more preferably about 0.1 to 5% by weight.

The temperature of the dye bath is preferably about 10 to 50°C, more preferably about 15 to 45°C, and even more preferably about 18 to 30°C. The immersion time in the dye bath cannot be determined in general because the degree of dyeing of the polyvinyl alcohol film is affected by the temperature of the dye bath, but is preferably about 10 to 300 seconds, and more preferably about 20 to 240 seconds. The dyeing process may be carried out only once, or multiple times as necessary.

The crosslinking process involves immersing the polyvinyl alcohol film in a treatment bath (crosslinking bath) containing a boron compound. The boron compound crosslinks the polyvinyl alcohol film, allowing iodine molecules or dye molecules to be adsorbed to the crosslinked structure. Examples of the boron compound include boric acid, borate salts, and borax. The crosslinking bath is typically an aqueous solution, but may also be a mixed solution of water and a water-miscible organic solvent. The crosslinking bath may also contain potassium iodide to control the potassium content in the polarizing film.

In the crosslinking bath, the concentration of the boron compound is preferably approximately 1 to 15% by weight, more preferably approximately 1.5 to 10% by weight, and even more preferably approximately 2 to 5% by weight. Furthermore, when potassium iodide is used in the crosslinking bath, the concentration of potassium iodide in the crosslinking bath is preferably approximately 1 to 15% by weight, more preferably approximately 1.5 to 10% by weight, and even more preferably approximately 2 to 5% by weight.

The temperature of the crosslinking bath is preferably about 20 to 70°C, and more preferably about 30 to 60°C. The immersion time in the crosslinking bath cannot be determined in general because the degree of crosslinking of the polyvinyl alcohol film is affected by the temperature of the crosslinking bath, but is preferably about 5 to 300 seconds, and more preferably about 10 to 200 seconds. The crosslinking process may be carried out only once, or multiple times as necessary.

The stretching process is a processing step in which a polyvinyl alcohol-based film is stretched to a predetermined magnification in at least one direction. Generally, the polyvinyl alcohol-based film is stretched uniaxially in the conveying direction (longitudinal direction). There are no particular restrictions on the stretching method, and either wet stretching or dry stretching can be used. The stretching process may be carried out only once, or may be carried out multiple times as necessary. The stretching process may be carried out at any stage in the production of a polarizing film.

The treatment bath (stretching bath) used in the wet stretching method can typically be water or a solvent such as a mixture of water and a water-miscible organic solvent. The stretching bath may contain potassium iodide to control the potassium content in the polarizing film. When potassium iodide is used in the stretching bath, the concentration of potassium iodide in the stretching bath is preferably approximately 1 to 15 wt%, more preferably approximately 2 to 10 wt%, and even more preferably approximately 3 to 6 wt%. The treatment bath (stretching bath) may also contain the boron compound to prevent film breakage during stretching. In this case, the concentration of the boron compound in the stretching bath is preferably approximately 1 to 15 wt%, more preferably approximately 1.5 to 10 wt%, and even more preferably approximately 2 to 5 wt%.

The temperature of the stretching bath is preferably about 25 to 80°C, more preferably about 40 to 75°C, and even more preferably about 50 to 70°C. The immersion time in the stretching bath cannot be determined in general because the degree of stretching of the polyvinyl alcohol film is affected by the temperature of the stretching bath, but is preferably about 10 to 800 seconds, and more preferably about 30 to 500 seconds. The stretching treatment in the wet stretching method may be carried out together with one or more of the swelling process, dyeing process, crosslinking process, and washing process.

Examples of the dry stretching method include roll-to-roll stretching, heated roll stretching, and compression stretching. The dry stretching method may be carried out together with the drying process.

The total stretching ratio (cumulative stretching ratio) applied to the polyvinyl alcohol film can be set appropriately depending on the purpose, but is preferably approximately 2 to 7 times, more preferably approximately 3 to 6.8 times, and even more preferably approximately 3.5 to 6.5 times.

The cleaning process is a treatment process in which the polyvinyl alcohol film is immersed in a cleaning bath, which can remove foreign matter remaining on the surface of the polyvinyl alcohol film. The cleaning bath typically uses a medium whose main component is water, such as distilled water or pure water. Furthermore, in order to control the potassium content in the polarizing film, the cleaning bath may contain potassium iodide. In this case, the concentration of potassium iodide in the cleaning bath is preferably approximately 1 to 10% by weight, more preferably approximately 1.5 to 4% by weight, and even more preferably approximately 1.8 to 3.8% by weight.

The temperature of the cleaning bath is preferably about 5 to 50°C, more preferably about 10 to 40°C, and even more preferably about 15 to 30°C. The immersion time in the cleaning bath cannot be determined in general because the degree of cleaning of the polyvinyl alcohol film is affected by the temperature of the cleaning bath, but is preferably about 1 to 100 seconds, more preferably about 2 to 50 seconds, and even more preferably about 3 to 20 seconds. The swelling process may be carried out only once, or multiple times as necessary.

The method for producing a polarizing film of the present invention may include a drying step. The drying step is a step of drying the polyvinyl alcohol-based film washed in the washing step to obtain a polarizing film, and a polarizing film having the desired moisture content is obtained by drying. The drying can be carried out by any appropriate method, such as natural drying, air drying, or heat drying.

The drying temperature is preferably about 20 to 150°C, and more preferably about 25 to 100°C. The drying time cannot be determined in general because the degree of drying of the polarizing film is affected by the drying temperature, but is preferably about 10 to 600 seconds, and more preferably about 30 to 300 seconds. The drying process may be carried out only once, or may be carried out multiple times as necessary.

From the viewpoint of improving the initial polarization degree of the polarizing film, the thickness of the polarizing film is preferably 1 μm or more, more preferably 2 μm or more, and from the viewpoint of preventing peeling due to heat, the thickness is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. In particular, to obtain a polarizing film with a thickness of approximately 8 μm or less, the following method for producing a thin polarizing film can be applied, in which the polyvinyl alcohol-based film is a laminate including a polyvinyl alcohol-based resin layer formed on a thermoplastic resin substrate.

<Method for manufacturing a thin polarizing film>
A method for producing a thin polarizing film includes 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 long thermoplastic resin substrate to form a laminate, and subjecting the laminate to an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. In particular, to obtain a polarizing film with excellent optical properties, a two-stage stretching method is selected, which combines an in-air auxiliary stretching treatment (dry stretching) with an underwater stretching treatment in a boric acid aqueous solution.

Any appropriate method can be used to prepare the laminate, including, for example, applying a coating liquid containing the PVA-based resin to the surface of the thermoplastic resin substrate and drying it. The thickness of the thermoplastic resin substrate is preferably approximately 20 to 300 μm, and more preferably approximately 50 to 200 μm. The thickness of the PVA-based resin layer is preferably approximately 3 to 40 μm, and more preferably approximately 3 to 20 μm.

The thermoplastic resin substrate preferably has a water absorption rate of approximately 0.2% or more, more preferably approximately 0.3% or more, from the viewpoint of absorbing water to significantly reduce the stretching stress and enabling high stretching ratios. On the other hand, the thermoplastic resin substrate preferably has a water absorption rate of approximately 3% or less, more preferably approximately 1% or less, from the viewpoint of preventing defects such as a significant decrease in the dimensional stability of the thermoplastic resin substrate and a deterioration in the appearance of the resulting polarizing film. The water absorption rate can be adjusted, for example, by introducing a modifying group into the constituent materials of the thermoplastic resin substrate. The water absorption rate is a value determined in accordance with JIS K 7209.

The thermoplastic resin substrate preferably has a glass transition temperature (Tg) of approximately 120°C or less, from the viewpoint of suppressing crystallization of the PVA-based resin layer while ensuring sufficient stretchability of the laminate. Furthermore, considering the plasticization of the thermoplastic resin substrate with water and the smooth underwater stretching, the glass transition temperature (Tg) is more preferably approximately 100°C or less, and even more preferably approximately 90°C or less. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably approximately 60°C or more, from the viewpoint of preventing problems such as deformation of the thermoplastic resin substrate during application and drying of the coating liquid and producing a good laminate. The glass transition temperature can be adjusted, for example, by introducing a modifying group into the constituent material of the thermoplastic resin substrate or by heating using a crystallizing material. The glass transition temperature (Tg) is a value determined in accordance with JIS K 7121.

Any suitable thermoplastic resin may be used as the constituent material of the thermoplastic resin substrate. Examples of such thermoplastic resins include ester resins such as polyethylene terephthalate resins, cycloolefin resins such as norbornene resins, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, and copolymer resins thereof. Among these, norbornene resins and amorphous polyethylene terephthalate resins are preferred. Furthermore, amorphous polyethylene terephthalate resins are preferred because they have excellent stretchability and can suppress crystallization during stretching. Examples of amorphous polyethylene terephthalate resins include copolymers containing isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers containing cyclohexanedimethanol or diethylene glycol as glycols.

The thermoplastic resin substrate may be subjected to a surface treatment (e.g., corona treatment) before forming the PVA-based resin layer, or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved. The thermoplastic resin substrate may also be stretched before forming the PVA-based resin layer.

The coating liquid is a solution in which a PVA-based resin is dissolved in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine, with water being preferred. These can be used alone or in combination. The concentration of the PVA-based resin in the coating liquid is preferably approximately 3 to 20 parts by weight per 100 parts by weight of the solvent, from the viewpoint of forming a uniform coating film that adheres closely to the thermoplastic resin substrate.

The coating solution preferably contains a halide to improve the orientation of polyvinyl alcohol molecules during stretching. Any appropriate halide can be used, including iodide and sodium chloride. Examples of iodides include potassium iodide, sodium iodide, and lithium iodide, with potassium iodide being preferred. The concentration of the halide in the coating solution is preferably approximately 5 to 20 parts by weight, and more preferably approximately 10 to 15 parts by weight, per 100 parts by weight of the PVA-based resin.

The coating liquid may also contain additives. Examples of such additives include plasticizers such as ethylene glycol and glycerin; and surfactants such as nonionic surfactants.

Any suitable method can be used to apply the coating liquid, including, for example, roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (such as comma coating). The drying temperature for the coating liquid is preferably about 50°C or higher.

The in-air auxiliary stretching process allows stretching while suppressing crystallization of the thermoplastic resin substrate, allowing the laminate to be stretched at a high magnification. The stretching method used in the in-air auxiliary stretching process may be fixed-end stretching (e.g., a method in which stretching is performed using a tenter stretching machine) or free-end stretching (e.g., a method in which the laminate is passed between rolls with different peripheral speeds for uniaxial stretching). However, free-end stretching is preferred from the perspective of obtaining high optical properties.

The stretching ratio in the auxiliary in-air stretching is preferably about 2 to 3.5 times. The auxiliary in-air stretching may be carried out in one stage or in multiple stages. When carried out in multiple stages, the stretching ratio is the product of the stretching ratios in each stage.

The stretching temperature in the auxiliary in-air stretching can be set to any appropriate value depending on the material forming the thermoplastic resin substrate, the stretching method, etc. For example, it is preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably equal to or higher than the glass transition temperature (Tg) + 10°C, and even more preferably equal to or higher than the glass transition temperature (Tg) + 15°C. On the other hand, the upper limit of the stretching temperature is preferably around 170°C, from the viewpoint of suppressing rapid crystallization of the PVA-based resin and suppressing problems caused by crystallization (for example, preventing the orientation of the PVA-based resin layer due to stretching).

If necessary, an insolubilization treatment may be performed after the auxiliary air-stretching treatment and before the dyeing treatment or underwater stretching treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The insolubilization treatment imparts water resistance to the PVA-based resin layer and prevents a decrease in the orientation of the PVA when immersed in water. The concentration of the boric acid aqueous solution is preferably approximately 1 to 5 parts by weight per 100 parts by weight of water. The liquid temperature of the insolubilization bath is preferably approximately 20 to 50°C.

The dyeing process is carried out by dyeing the PVA-based resin layer with iodine. Examples of the adsorption method include immersing the PVA-based resin layer (laminate) in a dye solution containing iodine, applying the dye solution to the PVA-based resin layer, and spraying the dye solution onto the PVA-based resin layer. Immersing the PVA-based resin layer (laminate) in a dye solution containing iodine is preferred.

The amount of iodine in the dye bath is preferably approximately 0.05 to 0.5 parts by weight per 100 parts by weight of water. To increase the solubility of iodine in water, it is preferable to incorporate the iodide into the iodine aqueous solution. The amount of iodide is preferably approximately 0.1 to 10 parts by weight, and more preferably approximately 0.3 to 5 parts by weight, per 100 parts by weight of water. The temperature of the dye bath is preferably approximately 20 to 50°C to suppress dissolution of the PVA-based resin. Furthermore, the immersion time is preferably approximately 5 seconds to 5 minutes, and more preferably approximately 30 to 90 seconds, from the viewpoint of ensuring the transmittance of the PVA-based resin layer. To obtain a polarizing film with good optical properties, the ratio of iodine to iodide in the iodine aqueous solution is preferably approximately 1:5 to 1:20, and more preferably approximately 1:5 to 1:10.

If necessary, a crosslinking treatment may be performed after the dyeing treatment and before the underwater stretching treatment. The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. This crosslinking treatment imparts water resistance to the PVA-based resin layer and prevents a decrease in PVA orientation when the layer is immersed in high-temperature water during the subsequent underwater stretching treatment. The boric acid concentration of the aqueous boric acid solution is preferably approximately 1 to 5 parts by weight per 100 parts by weight of water. Furthermore, when performing a crosslinking treatment, it is preferable to further incorporate the aforementioned iodide into the crosslinking bath. The incorporation of the iodide can suppress the elution of iodine adsorbed to the PVA-based resin layer. The amount of iodide incorporated is preferably approximately 1 to 5 parts by weight per 100 parts by weight of water. The temperature of the crosslinking bath (boric acid aqueous solution) is preferably approximately 20 to 50°C.

The underwater stretching process 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, allowing the PVA-based resin layer to be stretched at a high magnification while suppressing crystallization. The underwater stretching method may be fixed-end stretching (e.g., stretching using a tenter stretching machine) or free-end stretching (e.g., uniaxial stretching by passing the laminate between rolls with different peripheral speeds). However, free-end stretching is preferred from the perspective of achieving high optical properties.

The underwater stretching treatment is preferably carried out by immersing the laminate in an aqueous boric acid solution (stretching in boric acid water). Using an aqueous boric acid solution as the stretching bath can impart to the PVA-based resin layer rigidity that can withstand the tension applied during stretching and water resistance that prevents dissolution in water. The boric acid concentration of the aqueous boric acid solution is preferably 1 to 10 parts by weight, and more preferably 2.5 to 6 parts by weight, per 100 parts by weight of water. Iodide may also be blended into the stretching bath (aqueous boric acid solution). The liquid temperature of the stretching bath is preferably approximately 40 to 85°C, and more preferably approximately 60 to 75°C. The immersion time of the laminate in the stretching bath is preferably approximately 15 seconds to 5 minutes.

The stretching ratio in the underwater stretching is preferably at least 1.5 times, and more preferably at least 3 times.

The total stretching ratio of the laminate is preferably at least 5 times the original length of the laminate, and more preferably at least 5.5 times.

The drying shrinkage treatment may be carried out by zone heating, in which the entire zone is heated, or by heating the transport rolls (using so-called heated rolls). Preferably, both methods are used. Drying using heated rolls efficiently suppresses heat curling of the laminate, allowing for the production of a polarizing film with excellent appearance. Furthermore, since the laminate can be dried while maintaining a flat state, it is possible to suppress not only curling but also wrinkling. Furthermore, from the viewpoint that shrinking in the width direction during the drying shrinkage treatment can improve the optical properties of the resulting polarizing film, the shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment is preferably approximately 1 to 10%, and more preferably approximately 2 to 8%.

Drying conditions can be controlled by adjusting the heating temperature of the transport rolls (heating roll temperature), the number of heating rolls, and the contact time with the heating rolls. The temperature of the heating rolls is preferably approximately 60 to 120°C, more preferably approximately 65 to 100°C, and even more preferably 70 to 80°C. From the perspective of effectively increasing the crystallinity of the thermoplastic resin and effectively suppressing curling, approximately 2 to 40 transport rolls, preferably approximately 4 to 30, are typically provided. The contact time (total contact time) between the laminate and the heating rolls is preferably approximately 1 to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating rolls may be installed in a heating furnace or on a regular production line (in a room temperature environment). Preferably, they are installed in a heating furnace equipped with a blower. By using both heating roll drying and hot air drying, it is possible to suppress sudden temperature changes between the heating rolls, making it easy to control shrinkage in the width direction. The hot air drying temperature is preferably around 30 to 100°C. The hot air drying time is preferably around 1 to 300 seconds.

After the underwater stretching process, it is preferable to perform a washing process before the drying shrinkage process. The washing process is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

Furthermore, when producing a thin polarizing film containing the compound having radical scavenging functionality, the compound having radical scavenging functionality may be contained in one or more of the treatment baths for dyeing, underwater stretching, insolubilization, crosslinking, and washing. The concentration of the compound having radical scavenging functionality contained in any of the treatment baths cannot be determined in general because it is affected by the number of treatments, treatment time, treatment temperature, etc., of each treatment. However, from the viewpoint of efficiently controlling the content of the compound having radical scavenging functionality in the polarizing film, it is typically preferably 0.03 wt% or more, more preferably 0.05 wt% or more, even more preferably 0.1 wt% or more, and preferably 15 wt% or less, more preferably 10 wt% or less, and even more preferably 5 wt% or less.

In particular, when a washing process is performed, the washing process makes it easy to adjust the content of iodine and compounds with radical scavenging functionality within the desired range, taking into account the processing conditions for dyeing, underwater stretching, etc., from the perspective of being able to elute or adsorb components such as iodine and compounds with radical scavenging functionality from or onto the polyvinyl alcohol film.

When producing a thin polarizing film containing the compound having radical scavenging functionality, an aqueous solution containing the compound having radical scavenging functionality may be applied to the polarizing film to impregnate it. The application process may be carried out before or after any of the dyeing process, underwater stretching process, insolubilization process, crosslinking process, and washing process described above. In particular, it is preferable to apply the coating process after the washing process, as this makes it easier to adjust the content of the compound having radical scavenging functionality to the desired range. The application method is not particularly limited, and examples include wire bar coating, gravure coating, and spraying.

Each of the treatment baths used in the dyeing, underwater stretching, insolubilization, crosslinking, and washing processes may contain additives such as zinc salts, pH adjusters, pH buffers, and other salts. Examples of zinc salts include zinc halides such as zinc chloride and zinc iodide; and inorganic zinc salts such as zinc sulfate and zinc acetate. Examples of pH adjusters include strong acids such as hydrochloric acid, sulfuric acid, and nitric acid, and strong bases such as sodium hydroxide and potassium hydroxide. Examples of pH buffers include carboxylic acids such as acetic acid, oxalic acid, and citric acid, and their salts; and inorganic weak acids such as phosphoric acid and carbonic acid, and their salts. Examples of other salts include chlorides such as sodium chloride, potassium chloride, and barium chloride; nitrates such as sodium nitrate and potassium nitrate; sulfates such as sodium sulfate and potassium sulfate; and salts of alkali metals and alkaline earth metals. In particular, from the perspective of obtaining a polarizing film that can suppress a decrease in the single-piece transmittance due to coloration of the polarizing film in high-temperature environments, it is preferable for one or more of the treatment baths to contain a chloride.

<Polarizing film>
The polarizing film of the present invention is a polarizing film in which transparent protective films are bonded to both sides of the polarizing membrane, and at least one of the transparent protective films has a moisture permeability of 200 g/( ^m2 ·24 h) or less, and satisfies the condition that the change in single-piece transmittance (ΔTs) expressed by the general formula (1): ΔTs(%)=Ts ₇₅₀ −Ts ₀ (in the general formula (1), Ts ₀ is the single-piece transmittance of a laminate in which a glass plate is bonded to one side of the polarizing film via a pressure-sensitive adhesive layer, and Ts ₇₅₀ is the single-piece transmittance after the laminate has been heat-treated at 105°C for 750 hours).

<Transparent protective film>
The transparent protective film to be attached to at least one surface of the polarizing film is not particularly limited as long as the film has a moisture permeability of 200 g/( ^m² ·24 h) or less, and various transparent protective films used in polarizing films can be used. Examples of materials that can be used to form the transparent protective film include thermoplastic resins that are excellent in transparency, mechanical strength, thermal stability, moisture blocking properties, isotropy, etc. Examples of thermoplastic resins include cellulose ester resins such as triacetyl cellulose, polyester resins such as polyethylene terephthalate and polyethylene naphthalate, polyethersulfone resins, polysulfone resins, polycarbonate resins, polyamide resins such as nylon and aromatic polyamide, polyimide resins, polyolefin resins such as polyethylene, polypropylene, and ethylene-propylene copolymers, (meth)acrylic resins, cyclic polyolefin resins (norbornene resins) having a cyclo- or norbornene structure, polyarylate resins, polystyrene resins, polyvinyl alcohol resins, and mixtures thereof. The transparent protective film may have a cured layer formed from a thermosetting resin or an ultraviolet-curable resin, such as a (meth)acrylic, urethane, acrylic urethane, epoxy, or silicone resin, etc. Among these, cellulose ester resins, polycarbonate resins, (meth)acrylic resins, cyclic polyolefin resins, and polyester resins are preferred.

The thickness of the transparent protective film can be determined as appropriate, but generally, from the standpoint of strength, ease of handling, thinness, etc., it is preferably approximately 1 to 500 μm, more preferably approximately 1 to 300 μm, and even more preferably approximately 5 to 100 μm.

At least one of the transparent protective films (the first transparent protective film on one side of the polarizer) has a moisture permeability of 200 g/( ^m2 ·24 h) or less, and from the viewpoint of suppressing a decrease in the polarization degree of the polarizing film in a high-temperature, high-humidity environment, the moisture permeability is preferably 180 g/( ^m2 ·24 h) or less, and more preferably 150 g/( ^m2 ·24 h) or less. The other transparent protective film (the second transparent protective film on the other side of the polarizer) has a moisture permeability of preferably 200 g/( ^m2 ·24 h) or more, and more preferably 300 g/( ^m2 ·24 h) or more, from the viewpoint of production efficiency in a drying step after laminating the polarizing film and the transparent protective film. Furthermore, from the viewpoint of suppressing a decrease in the polarization degree of the polarizing film in a high-temperature, high-humidity environment, the moisture permeability is preferably 1200 g/( ^m2 ·24 h) or less, and more preferably 1000 g/( ^m2 ·24 h) or less. The moisture permeability can be calculated in accordance with the moisture permeability test (cup method) of JIS Z0208 by cutting a sample to a diameter of 60 mm, placing it in a moisture permeability cup containing about 15 g of calcium chloride, placing it in an incubator at a temperature of 40°C and a humidity of 90% RH, and measuring the increase in weight of calcium chloride before and after leaving it for 24 hours.

When the transparent protective film is attached to both sides of the polarizing film, the transparent protective films on both sides may be the same or different.

The transparent protective film can be a retardation plate having a front retardation of 40 nm or more and/or a thickness retardation of 80 nm or more. The front retardation is typically controlled to a range of 40 to 200 nm, and the thickness retardation is typically controlled to a range of 80 to 300 nm. When a retardation plate is used as the transparent protective film, the retardation plate also functions as a transparent protective film, allowing for a thinner film.

Examples of the retardation plate include birefringent films made by uniaxially or biaxially stretching polymer materials, oriented films of liquid crystal polymers, and films in which an oriented layer of liquid crystal polymer is supported by a film. There are no particular restrictions on the thickness of the retardation plate, but it is generally about 20 to 150 μm. The retardation plate may also be used by laminating it to a transparent protective film that does not have retardation.

The transparent protective film may contain any suitable additives such as ultraviolet absorbers, antioxidants, lubricants, plasticizers, release agents, color inhibitors, flame retardants, antistatic agents, pigments, and colorants.

A functional layer such as a hard coat layer, anti-reflection layer, anti-sticking layer, diffusion layer, or anti-glare layer can be provided on the surface of the transparent protective film that is not bonded to the polarizing film. The above-mentioned functional layers such as the hard coat layer, anti-reflection layer, anti-sticking layer, diffusion layer, or anti-glare layer can be provided on the protective film itself, or they can be provided separately from the protective film.

The polarizing film and the transparent protective film, or the polarizing film and the functional layer, are usually bonded together via an adhesive layer or a pressure-sensitive adhesive layer.

The adhesive layer may be formed from any of the various adhesives used in polarizing films, including rubber-based adhesives, acrylic-based adhesives, silicone-based adhesives, urethane-based adhesives, vinyl alkyl ether-based adhesives, polyvinyl alcohol-based adhesives, polyvinyl pollidone-based adhesives, polyacrylamide-based adhesives, and cellulose-based adhesives. Among these, acrylic-based adhesives are preferred.

Examples of methods for forming the adhesive layer include a method in which the adhesive is applied to a release-treated separator or the like, dried to form an adhesive layer, and then transferred to a polarizing film or the like, or a method in which the adhesive is applied to a polarizing film or the like, and dried to form an adhesive layer. The thickness of the adhesive layer is not particularly limited, and is, for example, approximately 1 to 100 μm, preferably approximately 2 to 50 μm.

The adhesive used to form the adhesive layer can be any of the various adhesives used in polarizing films, including isocyanate-based adhesives, polyvinyl alcohol-based adhesives, gelatin-based adhesives, vinyl latex-based adhesives, and water-based polyesters. These adhesives are typically used as aqueous solutions, containing 0.5 to 60% by weight of solids.

In addition to the above, examples of the adhesive include active energy ray-curable adhesives such as ultraviolet-curable adhesives and electron beam-curable adhesives. Examples of the active energy ray-curable adhesives include (meth)acrylate adhesives. Examples of the curable components in the (meth)acrylate adhesives include compounds having a (meth)acryloyl group and compounds having a vinyl group. Furthermore, compounds having an epoxy group or an oxetanyl group can also be used as cationic polymerization-curable adhesives. The epoxy group-containing compound is not particularly limited as long as it has at least two epoxy groups in its molecule, and various commonly known curable epoxy compounds can be used.

The adhesive may be applied to either the transparent protective film side (or the functional layer side), the polarizing film side, or both. After lamination, a drying process is performed to form an adhesive layer consisting of the dried coated layer. After the drying process, ultraviolet light or electron beams can be irradiated as needed. The thickness of the adhesive layer is not particularly limited; when using a water-based adhesive, it is preferably approximately 30 to 5,000 nm, and more preferably approximately 100 to 1,000 nm. When using a UV-curable adhesive, an electron beam-curable adhesive, or the like, it is preferably approximately 0.1 to 100 μm, and more preferably approximately 0.5 to 10 μm.

The transparent protective film and the polarizing film, or the polarizing film and the functional layer, may be laminated via an intervening layer such as a surface modification treatment layer, an easy-adhesive layer, a blocking layer, or a refractive index adjustment layer.

Examples of surface modification treatments that form the surface modification layer include corona treatment, plasma treatment, primer treatment, and saponification treatment.

Examples of the adhesive that forms the easy-adhesion layer include forming materials containing various resins having a polyester skeleton, polyether skeleton, polycarbonate skeleton, polyurethane skeleton, silicone-based skeleton, polyamide skeleton, polyimide skeleton, polyvinyl alcohol skeleton, etc. The easy-adhesion layer is usually provided in advance on the protective film, and the easy-adhesion layer side of the protective film and the polarizing film are laminated together using the pressure-sensitive adhesive layer or adhesive layer.

The blocking layer functions to prevent impurities such as oligomers and ions eluted from transparent protective films, etc., from migrating (penetrating) into the polarizing film. The blocking layer may be transparent and capable of preventing impurities from eluting from transparent protective films, etc. Examples of materials that can be used to form the blocking layer include urethane prepolymer-based forming materials, cyanoacrylate-based forming materials, and epoxy-based forming materials.

The refractive index adjustment layer is a layer provided to suppress the decrease in transmittance that occurs due to reflection between the transparent protective film and layers with different refractive indices, such as a polarizing film. Examples of refractive index adjustment materials that form the refractive index adjustment layer include various resins, such as silica-based, acrylic-based, acrylic-styrene-based, and melamine-based resins, and forming agents containing additives.

The polarizing film of the present invention satisfies the condition that the change in single layer transmittance (ΔTs), represented by the general formula (1): ΔTs(%)=Ts ₇₅₀ −Ts ₀ (in the general formula (1), Ts ₀ represents the single layer transmittance of a laminate in which a glass plate is bonded to one surface of the polarizing film via a pressure-sensitive adhesive layer, and Ts ₇₅₀ represents the single layer transmittance after the laminate has been heat-treated at 105° C. for 750 hours), is 0% or more and 5% or less. The change in single layer transmittance (ΔTs) is preferably 0% or more and 3% or less, and more preferably 0% or more and 2% or less.

The polarizing film preferably has a polarization degree of 99.98% or more, and more preferably has a polarization degree of 99.99% or more.

<Laminated polarizing film>
The laminated polarizing film (optical laminate) of the present invention is one in which the polarizing film is bonded to an optical layer. The optical layer is not particularly limited, and may be, for example, one or more optical layers that are sometimes used in the formation of liquid crystal displays, such as a reflector, a semi-transmitting plate, a retardation plate (including half-wave or quarter-wave plates), or a viewing angle compensation film. Examples of the laminated polarizing film include a reflective polarizing film or a semi-transmitting polarizing film obtained by laminating a reflector or semi-transmitting reflector on the polarizing film, an elliptical polarizing film or a circular polarizing film obtained by laminating a retardation plate on the polarizing film, a wide-viewing angle polarizing film obtained by laminating a viewing angle compensation film on the polarizing film, and a polarizing film obtained by laminating a brightness enhancement film on the polarizing film.

An adhesive layer may be applied to one or both surfaces of the polarizing film or the laminated polarizing film to bond an image display cell, such as a liquid crystal cell or an organic EL element, to other components, such as a front transparent plate or a front transparent member, such as a touch panel, on the viewing side. A pressure-sensitive adhesive layer is preferred as the adhesive layer. The pressure-sensitive adhesive forming the pressure-sensitive adhesive layer is not particularly limited, but may be appropriately selected from those based on, for example, acrylic polymers, silicone polymers, polyesters, polyurethanes, polyamides, polyethers, fluorine-based polymers, or rubber-based polymers. In particular, pressure-sensitive adhesives containing acrylic polymers, which have excellent optical transparency, adequate wettability, cohesion, and adhesion, and excellent weather resistance and heat resistance, are preferred.

The adhesive layer can be applied to one or both surfaces of the polarizing film or laminate polarizing film by any suitable method. Examples of adhesive layer application include preparing an adhesive solution and applying it directly to the polarizing film or laminate polarizing film by a suitable application method such as casting or coating, or forming an adhesive layer on a separator and then transferring it onto the polarizing film or laminate polarizing film. The thickness of the adhesive layer can be determined appropriately depending on the intended use and adhesive strength, and is generally 1 to 500 μm, preferably 5 to 200 μm, and more preferably 10 to 100 μm. A polarizing film or laminate polarizing film having an adhesive layer on at least one surface thereof is referred to as an adhesive-layered polarizing film or adhesive-layered laminate polarizing film.

It is preferable to cover the exposed surface of the adhesive layer with a temporary separator to prevent contamination until the product is put into practical use. This prevents contamination of the adhesive layer during normal handling. Examples of suitable thin sheets, such as plastic film, rubber sheet, paper, cloth, nonwoven fabric, net, foam sheet, metal foil, or laminates thereof, can be used as the separator, and they can be coated with a suitable release agent, such as a silicone-based, long-chain alkyl-based, fluorine-based, or molybdenum sulfide-based release agent, as needed.

<Image display panel and image display device>
The image display panel of the present invention comprises an image display cell to which the polarizing film or the laminated polarizing film is attached, and the image display device of the present invention comprises a front transparent member on the polarizing film or laminated polarizing film side (viewing side) of the image display panel.

Examples of the image display cell include liquid crystal cells and organic EL cells. The liquid crystal cell may be, for example, a reflective liquid crystal cell that uses external light, a transmissive liquid crystal cell that uses light from a light source such as a backlight, or a semi-transmissive semi-reflective liquid crystal cell that uses both external and light from the light source. When the liquid crystal cell uses light from a light source, the image display device (liquid crystal display device) has a polarizing film on the side opposite the viewing side of the image display cell (liquid crystal cell), and also has a light source. The polarizing film on the light source side and the liquid crystal cell are preferably bonded together via an appropriate adhesive layer. The liquid crystal cell may be driven in any mode, such as VA mode, IPS mode, TN mode, STN mode, or bend alignment (π type).

The organic EL cell preferably has a light-emitting body (organic electroluminescence light-emitting body) formed by sequentially stacking a transparent electrode, an organic light-emitting layer, and a metal electrode on a transparent substrate. The organic light-emitting layer is a laminate of various organic thin films, and various layer configurations can be used, such as a laminate of a hole-injection layer made of a triphenylamine derivative or the like and a light-emitting layer made of a fluorescent organic solid such as anthracene, a laminate of such a light-emitting layer and an electron-injection layer made of a perylene derivative or the like, or a laminate of a hole-injection layer, a light-emitting layer, and an electron-injection layer.

Examples of the front transparent member disposed on the viewing side of the image display cell include a front transparent plate (window layer) and a touch panel. The front transparent plate is a transparent plate with appropriate mechanical strength and thickness. Examples of such transparent plates include transparent resin plates such as acrylic resins and polycarbonate resins, and glass plates. Examples of the touch panel include various touch panels, such as resistive, capacitive, optical, and ultrasonic touch panels, as well as glass or transparent resin plates equipped with touch sensor functions. When a capacitive touch panel is used as the transparent plate, it is preferable to provide a front transparent plate made of glass or a transparent resin plate on the viewing side of the touch panel.

The polarizing film of the present invention has a good initial polarization degree, is excellent in suppressing the decrease in single-unit transmittance due to coloration of the polarizing film in high-temperature environments, and further has a polarizing film that is excellent in suppressing the decrease in polarization degree in high-temperature, high-humidity environments.Therefore, the polarizing film of the present invention, as well as laminated polarizing films, image display panels, and image display devices using the polarizing film, are suitable for use in in-vehicle image display devices such as car navigation systems and backup monitors.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
Example 1
<Preparation of polarizing film>
A polyvinyl alcohol film having an average degree of polymerization of 2,400, a degree of saponification of 99.9 mol%, and a thickness of 30 μm was prepared. The polyvinyl alcohol film was immersed in a swelling bath (water bath) at 20° C. for 30 seconds between rolls with different peripheral speed ratios, and stretched 2.2 times in the conveyance direction while swelling (swelling step). Subsequently, the polyvinyl alcohol film was immersed in a dye bath at 30° C. (iodine aqueous solution obtained by blending iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) while adjusting the iodine concentration so that the final polarizing film had an iodine concentration of 3.8 wt % for 30 seconds, and stretched 3.3 times in the conveyance direction relative to the original polyvinyl alcohol film (a polyvinyl alcohol film that was not stretched in the conveyance direction at all) (dyeing step). The dyed polyvinyl alcohol film was then immersed in a 40°C crosslinking bath (aqueous solution containing 3.5 wt% boric acid, 3.0 wt% potassium iodide, and 3.6 wt% zinc sulfate) for 28 seconds and stretched to 3.6 times the original polyvinyl alcohol film in the conveying direction (crosslinking step). The resulting polyvinyl alcohol film was then immersed in a 64°C stretching bath (aqueous solution containing 4.5 wt% boric acid, 5.0 wt% potassium iodide, and 5.0 wt% zinc sulfate) for 60 seconds and stretched to 6.0 times the original polyvinyl alcohol film in the conveying direction (stretching step). The resulting polyvinyl alcohol film was then immersed in a 27°C washing bath (aqueous solution containing 2.3 wt% potassium iodide and 1.0 wt% of a compound represented by the following general formula (9) as a radical-scavenging compound) for 10 seconds (washing step). The washed polyvinyl alcohol film was dried at 40°C for 30 seconds to produce a polarizing film. The iodine concentration of the polarizing film was determined by the following measurement method: The obtained polarizing film had a polyenation dehydration temperature (peak temperature of maximum water intensity detected by evolved gas analysis) of 210°C, a content of the compound represented by the following general formula (9) in the polarizing film of 0.3 wt%, and a thickness of 12 μm.

[Method for measuring iodine concentration (wt%) in polarizing film]
The iodine concentration (wt %) of the polarizing film was determined using a fluorescent X-ray analyzer (manufactured by Rigaku Corporation, trade name "ZSX-PRIMUS IV", measuring diameter: ψ20 mm) according to the following formula.
Iodine concentration (wt %)=14.474×(fluorescent X-ray intensity)/(film thickness)(kcps/μm) Note that the coefficient used to calculate the concentration differs depending on the measuring device, but the coefficient can be determined using an appropriate calibration curve.

[Method for measuring the dehydration temperature of polyenation]
The polarizing film was introduced into a heating furnace-type pyrolyzer (PY-2020iD, manufactured by Frontier Laboratories), and the generated gas was directly introduced into a TOFMS (MS-T100GCV, manufactured by JEOL) for measurement by evolved gas analysis (EGA/TOFMS). The peak temperature at which the detected water had the maximum intensity was determined, and this temperature was defined as the dehydration temperature for polyenization.
[Measurement conditions]
Temperature rise conditions: 40°C → +10°C/min → 350°C
Interface: Inert fused silica tubing, 2.5 m x 0.15 mm id
Carrier gas: He (1.0 mL/min)
Inlet temperature: 300℃
Inlet: Split ratio 20:1
Interface temperature: 300°C
Mass spectrometer: TOFMS
Ionization method: EI method Mass range: m/z=18

[Method for measuring the content (wt%) of the compound having radical scavenging function in the polarizing film]
Approximately 20 mg of polarizing film was collected, weighed, and dissolved by heating in 1 mL of water, and then diluted with 4.5 mL of methanol. The resulting extract was filtered through a membrane filter, and the concentration of the compound having a radical scavenging function in the filtrate was measured using HPLC (ACQUITY UPLC H-class Bio, manufactured by Waters Corporation).

<Preparation of Polarizing Film>
The adhesive used was an aqueous solution containing acetoacetyl group-containing polyvinyl alcohol resin (average polymerization degree 1,200, saponification degree 98.5 mol%, acetoacetylation degree 5 mol%) and methylolmelamine in a weight ratio of 3:1. Using this adhesive, a 47 μm-thick transparent protective film (moisture permeability: 380 g/(m²·24 h)) made of a triacetyl cellulose film (manufactured by Fujifilm Corporation, product name "TJ40UL") with HC formed thereon was laminated to one surface (viewing side) of the polarizing film obtained above, and a 30 μm-thick transparent protective film (manufactured by Nippon Shokubai Corporation, moisture permeability: 125 g/( ^m² ·24 h)) made of a (meth)acrylic resin (modified acrylic polymer having a lactone ring structure) was laminated to the other surface (image display ^cell side) of the polarizing film obtained above, as a first transparent protective film, using a roll laminator. The resulting film was then heated and dried in an oven (temperature: 90°C, time: 10 minutes), to produce a polarizing film in which transparent protective films were laminated to both surfaces of the polarizing film.

[Method for measuring single-piece transmittance under high temperature environment]
The polarizing film obtained above was cut into a size of 5.0 x 4.5 cm so that the absorption axis of the polarizing film was parallel to the long side. A glass plate (pseudo image display cell) was attached to the protective film surface of the polarizing film on the image display cell side via a 20 μm-thick acrylic adhesive layer, and the laminate was autoclaved at 50°C and 0.5 MPa for 15 minutes to produce a laminate. The resulting laminate was placed in a hot air oven at 105°C for 750 hours, and the single-unit transmittance (ΔTs) before and after heating was measured. The single-unit transmittance was measured using a spectrophotometer (manufactured by JASCO Corporation, product name "V7100") and evaluated according to the following criteria. The measurement wavelength was 380 to 700 nm (5 nm intervals). The results are shown in Table 1.
ΔTs (%) = Ts ₇₅₀ - Ts ₀
Here, Ts ₀ is the single layer transmittance of the laminate before heating, and Ts ₇₅₀ is the single layer transmittance of the laminate after heating for 750 hours.

[Evaluation of polarization degree]
The degree of polarization of a polarizing film can be measured using a spectrophotometer (manufactured by JASCO Corporation, product name "V7100"). A specific method for measuring the degree of polarization involves measuring the parallel transmittance (H0) and crossed transmittance (H90) of the polarizing film, and calculating the degree of polarization (%) from the formula: Polarization degree (%) = {(H0 - H90) / (H0 + H90)} 1/2 × 100. The parallel transmittance (H0) is the transmittance value of a parallel laminated polarizing film produced by stacking two identical polarizing films so that their absorption axes are parallel to each other. The crossed transmittance (H90) is the transmittance value of a crossed laminated polarizing film produced by stacking two identical polarizing films so that their absorption axes are perpendicular to each other. These transmittances are Y values corrected for luminosity using a 2-degree visual field (C light source) according to JIS Z 8701-1982. The results are shown in Table 1.

[Evaluation of humidity durability]
The laminate sample was left standing in a moist heat oven at 85°C and 85% RH for 500 hours, and then the polarization degree was evaluated using the spectrophotometer (manufactured by JASCO Corporation, product name "V7100") as described above. The results are shown in Table 1.
Good: Polarization degree of 90% or more Bad: Polarization degree of less than 90%

[Evaluation of heat peeling]
The polarizing film was cut to a size of 680 x 250 mm so that the absorption axis was parallel to the long side, and a glass plate (pseudo image display cell) was prepared by attaching a 20 μm thick acrylic adhesive layer to the protective film on the image display cell side of the polarizing film. The resulting laminate sample was left in a hot air oven at 105°C for 750 hours, and then the appearance was visually evaluated according to the following criteria. The results are shown in Table 1.
Good: There is slight peeling or bubbling at the edge, but no practical problems.
△: Peeling or bubbling is observed at the edge, but there is no problem in practical use unless it is for special applications (for example, narrow-frame displays in which the distance from the edge of the polarizing plate to the active area where the image is displayed is short).
x: Significant peeling at the edge, causing problems in practical use.

Example 2
<Preparation of polarizing membrane and polarizing film>
A polarizing film and a polarizing film were produced in the same manner as in Example 1, except that the iodine concentration of the dye bath was adjusted so that the iodine concentration of the final polarizing film would be 4.3 wt %. The polarizing film thus produced had a peak temperature of 208°C for maximum water intensity as detected by evolved gas analysis, a content of the compound represented by general formula (9) in the polarizing film of 0.3 wt %, and a thickness of 12 μm.

Example 3
<Preparation of polarizing membrane and polarizing film>
A polarizing film and a polarizing film were produced in the same manner as in Example 1, except that a 45 μm-thick polyvinyl alcohol film was used and the iodine concentration of the dye bath was adjusted so that the iodine concentration of the final polarizing film would be 3.5 wt %. The resulting polarizing film had a peak temperature of 210° C. at the maximum water intensity detected by evolved gas analysis, a content of the compound represented by general formula (9) in the polarizing film of 0.2 wt %, and a thickness of 18 μm.

Example 4
<Preparation of polarizing membrane and polarizing film>
A polarizing film and a polarizing film were produced in the same manner as in Example 1, except that the iodine concentration in the dye bath was adjusted so that the iodine concentration in the final polarizing film would be 4.8% by weight, and the potassium iodide concentration in the washing bath was adjusted to 3.6% by weight, and the compound represented by general formula (8) instead of general formula (9) was adjusted to 1.5% by weight. The resulting polarizing film had a peak temperature of maximum water intensity detected by evolved gas analysis of 210°C, a content of the compound represented by general formula (8) in the polarizing film of 0.4% by weight, and a thickness of 12 μm.

Example 5
<Preparation of polarizing membrane and polarizing film>
A polarizing film and a polarizing film were produced in the same manner as in Example 1, except that the iodine concentration in the dye bath was adjusted so that the iodine concentration in the final polarizing film would be 2.5% by weight, and the potassium iodide concentration in the washing bath was adjusted to 1.8% by weight, and 1.0% by weight of a compound represented by the following general formula (10) instead of the above general formula (9), and 15% by weight of sodium chloride. The resulting polarizing film had a peak temperature of 209°C for maximum water intensity as detected by evolved gas analysis, a content of the compound represented by the following general formula (10) in the polarizing film of 0.3% by weight, a chloride ion content of 3.1% by weight, and a thickness of 12 μm. The chloride ion content was determined by the following measurement method.

[Method for measuring chloride ions (chloride ions)]
5 mg of the sample was placed in a polypropylene container, 20 mL of ultrapure water was added, and the container was then capped. Next, extraction was carried out at 120°C for 1 hour, and the resulting extract was filtered using a membrane filter and an organic matter removal cartridge. The chloride ion content was calculated by quantitative analysis using ion chromatography.
[Measurement conditions]
Apparatus (anion): Thermo Fisher Scientific ICS-3000
Separation column: Dionex IonPac AS18-fast (4 mm x 150 mm)
Guard column: Dionex IonPac AG18-fast (4 mm x 30 mm)
Removal system: Dionex AERS-500 (external mode)
Detector: Electrical conductivity detector Eluent: KOH aqueous solution (using eluent generator EGCIII)
Eluent flow rate: 1.2mL/min
Sample injection volume: 250 μL

Example 6
<Preparation of polarizing membrane and polarizing film>
A polarizing membrane and a polarizing film were prepared in the same manner as in Example 5, except that the compound represented by general formula (10) was not added in the preparation of the polarizing membrane. The resulting polarizing membrane had a peak temperature of 209°C for maximum water intensity as detected by evolved gas analysis, a chloride ion content of 3.2 wt %, and a thickness of 12 μm.

<Comparative Example 1>
<Preparation of polarizing membrane and polarizing film>
A polarizing membrane and a polarizing film were prepared in the same manner as in Example 2, except that the compound represented by general formula (9) was not added to the cleaning bath as a compound having a radical scavenging function in the preparation of the polarizing membrane. The resulting polarizing membrane had a peak temperature of 197°C at the maximum water intensity detected by evolved gas analysis, a content of the compound represented by general formula (9) in the polarizing membrane of 0 wt %, and a thickness of 12 μm.

<Comparative Example 2>
<Preparation of polarizing membrane and polarizing film>
A polarizing film and a polarizing film were produced in the same manner as in Example 1, except that the iodine concentration in the dye bath was adjusted so that the iodine concentration in the final polarizing film would be 2.5% by weight, and that no compound represented by general formula (9) was added to the washing bath as a compound having a radical scavenging function. The resulting polarizing film had a peak temperature of 206°C of maximum water intensity as detected by evolved gas analysis, a content of the compound represented by general formula (9) in the polarizing film of 0% by weight, and a thickness of 12 μm.

<Comparative Example 3>
<Preparation of polarizing membrane and polarizing film>
A polarizing film was produced in the same manner as in Example 1, except that a triacetyl cellulose film (manufactured by Konica Minolta, product name "KC2CT") with a thickness of 20 μm and a moisture permeability of 1270 g/( ^m² ·24 h) was used as the first transparent protective film.

<Comparative Example 4>
<Preparation of polarizing membrane and polarizing film>
A polarizing film and a polarizing film were produced in the same manner as in Comparative Example 1, except that a 75 μm-thick polyvinyl alcohol film was immersed in a 35°C swelling bath (water bath) for 30 seconds between rolls with different peripheral speed ratios and stretched 2.2 times in the conveying direction while swelling (swelling step), the iodine concentration of the dye bath was adjusted so that the final polarizing film had an iodine concentration of 3.1 wt %, 3 wt % sodium chloride was added to the washing bath, and a 20 μm-thick triacetyl cellulose film (manufactured by Konica Minolta, product name "KC2CT") with a moisture permeability of 1270 g/m ² ·24 hr was used as the first transparent protective film. The resulting polarizing film had a peak temperature of 209°C for maximum water intensity as detected by evolved gas analysis, a chloride ion content of 1.1 wt %, and a thickness of 28 μm.

The polarizing films of each example and comparative example obtained above were used to carry out the above-mentioned [Method for measuring single-piece transmittance in a high-temperature environment], [Evaluation of polarization degree], [Evaluation of humidity durability], and [Evaluation of heat peeling]. The results are shown in Table 1.

Claims (7)

A polarizing film in which transparent protective films are attached to both sides of a polarizing film,
the polarizing film is formed by adsorbing and aligning iodine on a polyvinyl alcohol-based film, and has an iodine concentration of 3% by weight or more and 10% by weight or less;
the polarizing film contains more than 2% by weight of chloride ions;
At least one of the transparent protective films has a moisture permeability of 200 g/( ^m2 ·24 h) or less,
The polarizing film satisfies the general formula (1): ΔTs (%)=Ts ₇₅₀ −Ts ₀
(In the general formula (1), Ts ₀ represents the single transmittance of a laminate in which a glass plate is bonded to one surface of the polarizing film via a pressure-sensitive adhesive layer, and Ts ₇₅₀ represents the single transmittance of the laminate after being heat-treated at 105°C for 750 hours.) A polarizing film characterized by satisfying the condition that the change in single transmittance (ΔTs) represented by the general formula (1) is 0% or more and 5% or less. The polarizing film of claim 1, characterized in that the polarizing film has a thickness of 20 μm or less. 3. The polarizing film according to claim 1, wherein the polarization degree is 99.98% or more. A laminated polarizing film, comprising the polarizing film according to any one of claims 1 to 3 laminated to an optical layer. 10. An image display panel comprising an image display cell to which the polarizing film according to claim 1 or the laminated polarizing film according to claim 4 is attached. 6. An image display device comprising: a front transparent member on the polarizing film or laminated polarizing film side of the image display panel according to claim 5 . 7. The image display device according to claim 6, which is for vehicle use.