TWI848216B - Polarizing plate and image display device using the same - Google Patents

Abstract

The present invention provides a polarizing plate, which, when applied to an image display device having a camera portion, can achieve excellent shooting function and face recognition function without providing a through hole or a transparent portion. The polarizing plate of the present invention has a polarizing element and a protective layer disposed on at least one side of the polarizing element, and the polarizing element is composed of a polyvinyl alcohol-based resin film containing a dichroic substance; and the reflected image contrast index of the polarizing plate is less than 15.

Description

Polarizing plate and image display device using the same

The present invention relates to a polarizing plate and an image display device using the polarizing plate.

In image display devices (such as liquid crystal display devices, organic EL display devices, quantum dot display devices), due to their image formation methods, a polarizing plate is usually arranged on at least one side of the display unit. In recent years, image display devices with camera units have rapidly become popular, and the camera unit can function not only as a shooting device, but also as a main component of a facial recognition system. In order to fully utilize the shooting function and facial recognition function, the polarizing plate is usually provided with a through hole or a transparent part (non-polarizing part) at the position corresponding to the camera unit. However, the through hole or the transparent part will often destroy the design, so it is desired to have a polarizing plate that can fully utilize the shooting function and facial recognition function without providing a through hole or a transparent part. Prior art literature Patent literature

Patent document 1: Japanese Patent Publication No. 2014-081482

Problem to be solved by the invention The present invention is made to solve the above-mentioned previous problems. Its main purpose is to provide a polarizing plate which, when used in an image display device having a camera part, can achieve excellent shooting function and face recognition function without providing a through hole or a transparent part.

Means for solving the problem The polarizing plate of the embodiment of the present invention has a polarizer and a protective layer arranged on at least one side of the polarizer, and the polarizer is composed of a polyvinyl alcohol-based resin film containing a dichroic substance; and the reflection image contrast index of the polarizing plate is 15 or less. In one embodiment, the thickness of the polarizer is 12µm or less. In one embodiment, the reflection image contrast index is 13 or less. In one embodiment, the polarizing plate has a protective layer only on one side of the polarizer. According to another aspect of the present invention, an image display device is provided. The image display device has a display unit and the polarizing plate arranged on at least one side of the display unit.

Effect of the invention According to the embodiment of the present invention, by controlling the reflected image contrast index to be below 15, a polarizing plate can be provided that can achieve excellent shooting function and face recognition function without providing a through hole or a transparent part when applied to an image display device having a camera part.

The following describes the embodiments of the present invention, but the present invention is not limited to these embodiments.

A. Overall structure of polarizing plate Figure 1 is a schematic cross-sectional view of a polarizing plate of an embodiment of the present invention. The polarizing plate 100 comprises: a polarizing element 10, a first protective layer (outer protective layer) 20 disposed on one side of the polarizing element 10 (for example, the side opposite to the display unit when the polarizing plate is applied to an image display device), and a second protective layer (inner protective layer) 30 disposed on the other side of the polarizing element 10 (for example, the display unit side when the polarizing plate is applied to an image display device). One of the first protective layer 20 or the second protective layer 30 may be omitted depending on the purpose. The polarizing element 10 is composed of a polyvinyl alcohol (PVA) resin film containing a dichroic substance (typically iodine or a dichroic dye).

In the embodiment of the present invention, the reflection image contrast index is less than 15, preferably less than 14.5, more preferably less than 13, and even more preferably less than 12. The smaller the reflection image contrast index is, the better, and its lower limit may be 5, for example. If the reflection image contrast index is within the range, the transmission wavefront aberration (optical strain of light transmitted through the polarizer, the details will be described later) can be reduced. As a result, when applied to an image display device with a camera part, excellent shooting function and face recognition function can be achieved without providing a through hole or a transparent part. The following explains the technical implications of specifying the reflection image contrast index. We speculate that one of the reasons why the transmission wavefront aberration becomes larger is related to the surface roughness of the polarizer (for example, the arithmetic mean roughness Ra). The inventors of the present invention have actively studied the relationship between the transmission wavefront aberration and the surface roughness of the polarizer, and found that even if the surface roughness of the polarizer is controlled, the transmission wavefront aberration cannot be properly controlled. Based on the new understanding, further trial and error were repeated, and it was found that by controlling the reflection image contrast index, which is a characteristic of the polarizer as a whole, the transmission wavefront aberration can be properly controlled, and thus the present invention was completed. The reason why the transmission wavefront aberration can be properly controlled by controlling the reflection image contrast index instead of controlling the surface roughness of the polarizer is not clear, but it is speculated that it may be because the optical factors of the components of the polarizer are optimized individually and/or mutually as a whole.

As mentioned above, the reflection image contrast index reflects the main optical factors of the components of the polarizer individually and/or mutually (such as the streaks of the polarizer, the refraction and/or scattering of the interface between the polarizer and the protective layer, and the unevenness of the protective layer). The reflection image contrast index can be measured in the following way. In a darkroom environment, light from a special lighting device for inspection (manufactured by Japan Technology Center, "S-Light") is irradiated to the polarizer at an angle of 45°; the image of the reflection image is read as digital data; image processing is performed on an area of 100mm×100mm, and the brightness variation is calculated as the standard deviation, which is used as the reflection image contrast index. In more detail, the brightness of the pixels of the image is digitized in a scale of 0~255, and its standard deviation is calculated, which is used as the reflection image contrast index.

The polarizing plate preferably has a small transmission wavefront aberration as described above. Thus, when applied to an image display device having a camera portion, excellent shooting and facial recognition functions can be achieved without providing a through hole or a transparent portion. Transmission wavefront aberration is an indicator of the optical strain of light transmitted through the polarizing plate, and refers to the deviation of the light transmitted through the polarizing plate from the ideal wavefront (spherical surface). Therefore, when the transmission wavefront aberration becomes too large, the deviation of the light transmitted through the polarizing plate from the ideal wavefront (spherical surface) becomes larger, and the light beam emitted from one point of the object will not converge into one point, so there will be blurred and deformed images. The above-mentioned poor imaging condition may hinder correct recognition, especially in the facial recognition system. The transmission wavefront aberration is preferably less than 100nm, more preferably less than 50nm, more preferably less than 30nm, and particularly preferably less than 25nm. The smaller the transmission wavefront aberration, the better, and its lower limit may be, for example, 3nm. The transmission wavefront aberration can be achieved by controlling the reflection image contrast index below a predetermined value as described above. The transmission wavefront aberration can be represented by Pv·λ. Here, Pv represents the difference between the maximum and minimum values of the transmission wavefront aberration in the measurement range (Peak-Valley), which represents the ratio of the distance to the wavelength of the incident light. For example, when the transmission wavefront aberration is at a distance of 1/10 of the wavelength of the incident light, Pv=0.1. λ is the wavelength of the incident light (nm). In one embodiment, the transmission wavefront aberration can be measured using a HeNe laser with a wavelength of 632.8nm.

A-1. Polarizer As mentioned above, the polarizer is composed of a PVA-based resin film containing a dichroic substance (typically iodine or a dichroic dye). The dichroic substance is preferably iodine. The polarizer can be formed by a single layer of resin film or a laminate of two or more layers.

Specific examples of polarizers formed by a single layer of resin film include: hydrophilic polymer films such as polyvinyl alcohol (PVA) films, partially formalized PVA films, and partially saponified films of ethylene-vinyl acetate copolymers that have been dyed and stretched by dichroic substances such as iodine or dichroic dyes, and polyene-based oriented films such as dehydrated PVA or dehydrogenated polyvinyl chloride. It is advisable to use a polarizer obtained by dyeing a PVA film with iodine and uniaxially stretching it because of its excellent optical properties. The above-mentioned dyeing with iodine can be performed, for example, by immersing the PVA film in an iodine aqueous solution. The stretching ratio of the above-mentioned uniaxial stretching is preferably 3 to 7 times. Stretching can be performed after dyeing or while dyeing. In addition, it can also be dyed after stretching. If necessary, the PVA film can be subjected to swelling treatment, crosslinking treatment, cleaning treatment, drying treatment, etc. For example, by immersing the PVA film in water and washing it before dyeing, not only can the dirt or anti-adhesive agent on the surface of the PVA film be washed away, but the PVA film can also be swollen, thereby preventing uneven dyeing.

Specific examples of polarizers obtained using a laminate include a laminate using a resin substrate and a PVA-based resin layer (PVA-based resin film) laminated on the resin substrate, or a laminate using a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer obtained by using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by the following steps: coating a PVA-based resin solution on the resin substrate and drying it, forming a PVA-based resin layer on the resin substrate, and obtaining a laminate of the resin substrate and the PVA-based resin layer; and, stretching and dyeing the laminate to make the PVA-based resin layer into a polarizer. In this embodiment, stretching typically includes immersing the laminate in a boric acid aqueous solution and stretching it. And, if necessary, stretching can further include stretching the laminate in the air at a high temperature (e.g., above 95° C.) before stretching in the boric acid aqueous solution. The obtained resin substrate/polarizer laminate can be used directly (that is, the resin substrate can be used as a protective film for the polarizer), or the resin substrate can be peeled off from the resin substrate/polarizer laminate and used after any appropriate protective film is applied to the peeled off area layer according to the purpose. The details of the manufacturing method of the polarizer are described in, for example, Japanese Patent Publication No. 2012-73580 and Japanese Patent License No. 6470455. The entire contents of these publications are cited in this specification as reference.

Any appropriate resin can be used as the PVA resin for forming the above-mentioned PVA resin film. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer can be mentioned. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of PVA resin is usually 85 mol%~100 mol%, preferably 95.0 mol%~99.9 mol%, and more preferably 99.0 mol%~99.5 mol%. The saponification degree is obtained in accordance with JIS K 6726-1994. By using the PVA resin with the above-mentioned saponification degree, a polarizer with excellent durability can be obtained. When the saponification degree is too high, there is a risk of gelling.

The average degree of polymerization of the PVA resin can be appropriately selected according to the purpose. The average degree of polymerization is usually 1000~10000, preferably 1200~5000, and more preferably 1500~4500. In addition, the average degree of polymerization can be obtained according to JIS K 6726-1994.

The iodine concentration in the PVA resin film (polarizer) is, for example, 5.0 wt % to 12.0 wt %. Also, the boric acid concentration in the PVA resin film is, for example, 12 wt % to 25 wt %.

The thickness of the polarizer is, for example, less than 12 µm, preferably less than 8 µm, more preferably less than 7 µm, and more preferably less than 6 µm. On the other hand, the thickness of the polarizer is preferably greater than 1 µm, more preferably greater than 2 µm. The thicker the polarizer, the better the desired reflection image contrast index can be obtained. However, according to the embodiment of the present invention, even with a thin polarizer as described above, the desired reflection image contrast index can still be achieved.

The polarizer preferably exhibits absorption dichroism at any wavelength between 380nm and 780nm. The single body transmittance of the polarizer is preferably 40.0% to 46.0%, preferably 40.5% to 43.0%. The polarization degree of the polarizer is preferably above 99.9%, preferably above 99.95%, and even more preferably above 99.98%.

A-2. Protective layer The first and second protective layers are formed of any appropriate film that can be used as a protective layer for a polarizer. Specific examples of the material that is the main component of the film include cellulose resins such as triacetate cellulose (TAC), or transparent resins such as polyester, polyvinyl alcohol, polycarbonate, polyamide, polyimide, polyether sulfone, polysulfone, polystyrene, polynorbornene, polyolefin, (meth)acrylic, and acetate. In addition, thermosetting resins or ultraviolet curing resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone can also be used. Other examples include glassy polymers such as silicone polymers. In addition, the polymer film described in Japanese Patent Publication No. 2001-343529 (WO01/37007) can also be used. As the material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted amide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in the side chain can be used, for example, a resin composition containing an alternating copolymer composed of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer can be used. The polymer film can be, for example, an extruded product of the above resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the first protective layer (outer protective layer) 20 disposed on the side opposite to the display unit is typically 300µm or less, preferably 100µm or less, more preferably 5µm to 80µm, and more preferably 10µm to 60µm. In addition, when surface treatment is performed, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the thickness of the second protective layer (inner protective layer) 30 disposed on the display unit side is preferably 5µm~200µm, more preferably 10µm~100µm, and more preferably 10µm~60µm. In one embodiment, the inner protective layer is preferably optically isotropic. In this specification, "optically isotropic" means that the in-plane phase difference Re(550) is 0nm~10nm, and the phase difference Rth(550) in the thickness direction is -10nm~+10nm. In another embodiment, the inner protective layer is a phase difference layer having any appropriate phase difference value. At this time, the in-plane phase difference Re(550) of the phase difference layer is, for example, 110nm~150nm, and the angle formed by its slow axis and the absorption axis of the polarizer is, for example, 40°~50°. "Re(550)" is the in-plane phase difference measured at 23°C with light of a wavelength of 550nm, and can be obtained by the formula: Re=(nx-ny)×d. "Rth(550)" is the phase difference in the thickness direction measured at 23°C with light of a wavelength of 550nm, and can be obtained by the formula: Re=(nx-nz)×d. Here, "nx" is the refractive index in the direction where the in-plane refractive index reaches the maximum (that is, the slow axis direction), "ny" is the refractive index in the direction orthogonal to the slow axis in the plane (that is, the fast axis direction), "nz" is the refractive index in the thickness direction, and "d" is the thickness of the layer (film) (nm). Furthermore, as described above, the second protective layer (inner protective layer) 30 can ideally be omitted.

B. Manufacturing method of polarizer The polarizer can be obtained by a manufacturing method including the following steps: forming a polyvinyl alcohol resin layer (PVA resin layer) including a halogenated substance and a polyvinyl alcohol resin (PVA resin) on one side of a long strip of thermoplastic resin substrate to form a laminate; and sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment, wherein the drying shrinkage treatment is to heat the laminate while conveying the laminate in the long side direction so as to shrink the laminate by more than 2% in the width direction. The content of the halogenated substance in the PVA resin layer is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin. The drying and shrinking treatment is preferably carried out using a heating roller, and the temperature of the heating roller is preferably 60°C to 120°C. The shrinkage rate in the width direction obtained by the drying and shrinking treatment of the laminate is preferably above 2%. According to the manufacturing method, the polarizer described in the above item B can be obtained. In particular, after preparing a laminate comprising a PVA-based resin layer containing a halogenated substance, the laminate is stretched by a multi-stage stretching including air-assisted stretching and underwater stretching, and the stretched laminate is then heated by a heating roller, thereby obtaining a polarizer having excellent optical properties (represented by single body transmittance and polarization degree) and suppressed optical property variations. Specifically, by using a heated roller in the drying and shrinking step, the laminate can be uniformly shrunk while being transported. This not only improves the optical properties of the resulting polarizer, but also allows stable production of polarizers with excellent optical properties, while suppressing variations in the optical properties of the polarizer (especially single-unit transmittance).

B-1. Preparation of laminate The method of preparing the laminate of the thermoplastic resin substrate and the PVA resin layer can be any appropriate method. It is preferable to apply a coating liquid containing a halogenated substance and a PVA resin to the surface of the thermoplastic resin substrate and dry it, thereby forming a PVA resin layer on the thermoplastic resin substrate. As mentioned above, the content of the halogenated substance in the PVA resin layer is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin.

The coating liquid can be applied by any appropriate method, such as roller coating, spin coating, wire rod coating, dip coating, die coating, curtain coating, spray coating, scraper coating (corner wheel coating, etc.). The coating and drying temperature of the coating liquid is preferably above 50°C.

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

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 the above treatment, the adhesion between the thermoplastic resin substrate and the PVA resin layer may be improved.

B-1-1. Thermoplastic resin substrate Thermoplastic resin substrates may be any appropriate thermoplastic resin film. Details of thermoplastic resin film substrates are described, for example, in Japanese Patent Publication No. 2012-73580. The entire contents of the publication are cited as a reference in this specification.

B-1-2. Coating liquid The coating liquid contains the halogenated substance and the PVA resin as described above. The coating liquid is typically a solution obtained by dissolving the halogenated substance and the PVA resin in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyols such as trihydroxymethylpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Of these, water is preferred. The PVA resin concentration of the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. If the resin concentration is the above, a uniform coating film can be formed that is closely attached to the thermoplastic resin substrate. The content of the halogenated compound in the coating solution is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin.

Additives may also be mixed into the coating liquid. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyols such as ethylene glycol and glycerol. Examples of surfactants include non-ionic surfactants. These may be used to further enhance the uniformity, dyeability, and elongation of the resulting PVA-based resin layer.

The above-mentioned PVA resin can adopt any appropriate resin. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer can be mentioned. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of PVA resin is usually 85 mol%~100 mol%, preferably 95.0 mol%~99.95 mol%, and more preferably 99.0 mol%~99.93 mol%. The saponification degree can be obtained in accordance with JIS K 6726-1994. By using PVA resin with the above-mentioned saponification degree, a polarizer with excellent durability can be obtained. When the saponification degree is too high, there is a risk of gelling. As mentioned above, the PVA-based resin preferably includes a PVA-based resin modified with an acetyl group.

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

The above-mentioned halides may be any appropriate halides. Examples thereof include iodides and sodium chloride. Examples of iodides include potassium iodide, sodium iodide and lithium iodide. Among these, potassium iodide is preferred.

The amount of the halogenated substance in the coating liquid is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin, and more preferably 10 to 15 parts by weight relative to 100 parts by weight of the PVA resin. If the amount of the halogenated substance is greater than 20 parts by weight relative to 100 parts by weight of the PVA resin, the halogenated substance may overflow and the resulting polarizer may become cloudy.

Generally speaking, after the PVA resin layer is stretched, the orientation of the polyvinyl alcohol molecules in the PVA resin layer will become higher. However, if the stretched PVA resin layer is immersed in a water-containing liquid, the orientation of the polyvinyl alcohol molecules will be disordered and the orientation will decrease. In particular, when the laminate of the thermoplastic resin and the PVA resin layer is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin, the tendency of the above orientation to decrease is very obvious. For example, the stretching of a PVA film monomer in boric acid water is generally carried out at 60°C. In contrast, the stretching of a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is carried out at a relatively high temperature of around 70°C. At this time, the orientation of the PVA at the beginning of the stretching decreases before it rises by stretching in water. In contrast, by preparing a laminate of a halogenated PVA-based resin layer and a thermoplastic resin substrate, and stretching the laminate at a high temperature in air (auxiliary stretching) before stretching in boric acid water, the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA resin layer is immersed in a liquid, the orientation disorder and the reduction of orientation of the polyvinyl alcohol molecules can be suppressed more than when the PVA resin layer does not contain halides. As a result, the optical properties of the polarizer obtained by immersing the laminate in a liquid through a dyeing process and an underwater stretching process can be improved.

B-2. Auxiliary stretching in the air In order to obtain high optical properties, a two-stage stretching method combining dry stretching (auxiliary stretching) and stretching in boric acid water is selected. In the two-stage stretching method, by introducing auxiliary stretching, the crystallization of the thermoplastic resin substrate can be suppressed while stretching, solving the problem of reduced stretchability due to excessive crystallization of the thermoplastic resin substrate in the subsequent boric acid water stretching, and the laminate can be stretched at a higher magnification. Furthermore, when coating PVA resin on a thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, the coating temperature must be lower than when coating PVA resin on a general metal roller, resulting in a problem that the crystallization of PVA resin becomes relatively low and sufficient optical properties cannot be obtained. In contrast, by introducing auxiliary stretching, the crystallization of PVA resin can be improved even when coating PVA resin on a thermoplastic resin, thereby achieving high optical properties. Furthermore, by improving the orientation of the PVA resin in advance, it is possible to prevent the orientation of the PVA resin from being reduced or dissolved when immersed in water in the subsequent dyeing step or stretching step, thereby achieving high optical properties.

The stretching method of the air-assisted stretching can be a fixed-end stretching (for example, a method of stretching using a tenter stretching machine) or a free-end stretching (for example, a method of uniaxial stretching by passing the laminate between rollers of different circumferential speeds). In one embodiment, the stretching method of the air-assisted stretching can be, for example, a biaxial stretching using a tenter stretching machine. By appropriately setting the stretching conditions of the biaxial stretching, the obtained polarizer can be given a predetermined biaxial property. As a result, a polarizer with a desired puncture strength can be achieved.

The stretching ratio of the air-assisted stretching in the long side direction should be 2.3 times or more, preferably 2.4 times to 3.5 times. In the embodiment of the present invention, as mentioned above, the width residual rate (the width after shrinkage relative to the original width: %) is controlled by adopting biaxial stretching. Specifically, the difference between the width residual rate of the air-assisted stretching (i.e. after the air-assisted stretching) and the free shrinkage width residual rate should be 2% or more, preferably 3% or more, and more preferably 5% or more. The maximum value of the difference can be, for example, 15%. Here, the free shrinkage width residual rate refers to the width residual rate when the free end is extended along the long side direction at the same stretching ratio. Specifically, the free shrinkage width residual rate when the stretching ratio is x times is calculated as (1/x ^1/2 ) × 100. For example, when stretched to 2.4 times, the free shrinkage width residual rate is (1/(2.4) ^1/2 ) × 100 = 64.5%. We believe that this is because when stretching along the long side during free shrinkage, the width direction and the thickness direction will shrink at the same ratio. In addition, the maximum stretching ratio (long side direction) when combining air-assisted stretching and underwater stretching is preferably 5.0 times or more, 5.5 times or more, and 6.0 times or more relative to the original length of the laminate. In this specification, the "maximum elongation ratio" means the elongation ratio before the laminate breaks, which is 0.2 lower than the elongation ratio obtained by separately confirming the occurrence of fracture of the laminate.

The stretching temperature of the air-assisted stretching can be set to any appropriate value according to the forming material of the thermoplastic resin substrate, the stretching method, etc. The stretching temperature is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate, and is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C, and is particularly preferably above Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at the above temperature, the rapid progress of crystallization of the PVA-based resin can be suppressed, thereby suppressing the undesirable conditions caused by the crystallization (for example, the orientation of the PVA-based resin layer is hindered by stretching). The crystallization index of the PVA-based resin after air-assisted stretching is preferably 1.3~1.8, and more preferably 1.4~1.7. The crystallization index of PVA resin can be measured by Fourier transform infrared spectrophotometer using ATR method. Specifically, polarized light is used as the measuring light for measurement, and the crystallization index is calculated by the following formula using the intensity of 1141cm ^-1 and 1440cm ^-1 of the obtained spectrum. Crystallization index = ( _IC / _IR ) However, _IC : the intensity of 1141cm ^-1 when the measuring light is incident and the measurement is carried out, _IR : the intensity of 1440cm ^-1 when the measuring light is incident and the measurement is carried out.

B-3. Insolubilization treatment, dyeing treatment and crosslinking treatment If necessary, an insolubilization treatment is performed after the air-assisted extension treatment and before the extension treatment or dyeing treatment in water. The above-mentioned insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. The above-mentioned dyeing treatment is typically 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 extension treatment in water. The above-mentioned crosslinking treatment can be performed typically by immersing the PVA-based resin layer in an aqueous solution of boric acid. The details of the insolubilization treatment, dyeing treatment and crosslinking treatment are described, for example, in Japanese Patent Publication No. 2012-73580 (mentioned above).

B-4. Underwater stretching treatment The underwater stretching treatment is performed by immersing the laminate in a stretching bath. Through the underwater stretching treatment, the laminate can be stretched at a temperature lower than the glass transition temperature of the thermoplastic resin substrate or the PVA resin layer (typically about 80°C), and high-ratio stretching can be performed while suppressing the crystallization of the PVA resin layer. As a result, a polarizer with excellent optical properties can be produced.

The lamination body may be stretched by any appropriate method. Specifically, it may be fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching the lamination body by passing it between rollers with different circumferential speeds). Free-end stretching is preferred. The lamination body may be stretched in one stage or in multiple stages. When stretched in multiple stages, the stretching ratio (maximum stretching ratio) of the lamination body described below is the product of the stretching ratios of each stage.

It is advisable to immerse the laminate in a boric acid aqueous solution for underwater stretching (boric acid underwater stretching). By using a boric acid aqueous solution as a stretching bath, the PVA resin layer can be given the rigidity to withstand the tension applied during stretching and the water resistance of being insoluble in water. Specifically, boric acid generates tetrahydroxyboric acid anions in an aqueous solution and can crosslink with the PVA resin through hydrogen bonds. As a result, the PVA resin layer can be given rigidity and water resistance, and good stretching can be performed, thereby producing a polarizer with excellent optical properties.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or boric acid salt in water which is a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight relative to 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 polarizer with higher characteristics can be manufactured. In addition to boric acid or boric acid salt, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, etc. in a solvent can also be used.

It is preferable to mix iodide in the stretching bath (boric acid aqueous solution). By mixing iodide, the dissolution of iodine adsorbed on the PVA resin layer can be suppressed. The specific example of iodide is as mentioned above. The concentration of iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, relative to 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) and the immersion time of the laminate in the stretching bath can be appropriately set according to the composition of the protective layer (represented by the material and whether it is arranged on one side or both sides of the polarizer). The stretching temperature can be, for example, below 70°C, and can be, for example, below 67°C, and can be, for example, below 66°C, and can be, for example, below 65°C. The lower limit of the stretching temperature can be, for example, 50°C, and can be, for example, 55°C. The immersion time of the laminate in the stretching bath can be, for example, more than 50 seconds, and can be, for example, more than 55 seconds, and can be, for example, more than 60 seconds. The upper limit of the immersion time can be, for example, 100 seconds. The combination of the stretching temperature and the immersion time can be, for example, 55°C to 66°C/55 seconds or more, and can be, for example, 60°C to 66°C/60 seconds to 80 seconds. The stretching in boric acid water is usually performed at around 70°C for about 50 seconds, but the inventors have found that by lowering the stretching temperature by several degrees and increasing the immersion time, the reflected image contrast index can be significantly reduced without reducing the optical properties of the polarizer. This is an unexpectedly excellent effect. It is also found that depending on the composition of the protective layer, the same effect can be obtained as by only lowering the stretching temperature or only increasing the immersion time.

The stretching ratio of the underwater stretching is preferably 1.5 times or more, preferably 3.0 times or more. The total stretching ratio of the laminate is preferably 5.0 times or more, more preferably 5.5 times or more relative to the original length of the laminate. By achieving the high stretching ratio, a polarizer with excellent optical properties can be manufactured. The high stretching ratio can be achieved by adopting an underwater stretching method (boric acid underwater stretching).

B-5. Drying and shrinking treatment The above-mentioned drying and shrinking treatment can be performed by heating the area as a whole, or by heating the conveying roller (so-called using a heating roller) (heating roller drying method). It is preferred to use both. By using a heating roller to dry it, the heat curling of the laminate can be effectively suppressed, and a polarizer with excellent appearance can be manufactured. Specifically, by drying the laminate along the state of adding a hot roller, the crystallization of the above-mentioned thermoplastic resin substrate can be effectively promoted to increase the crystallinity. Even at a relatively low drying temperature, the crystallinity of the thermoplastic resin substrate can still be well increased. As a result, the rigidity of the thermoplastic resin substrate increases and becomes a state that can withstand the shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. In addition, by using a heating roller, the laminate can be dried while maintaining a flat state, so that not only curling but also wrinkling can be suppressed. At this time, the laminate can be shrunk in the width direction through a drying and shrinking treatment to improve the optical properties. This is because the orientation of PVA and PVA/iodine complex can be effectively improved. The shrinkage rate in the width direction obtained by the drying and shrinking treatment of the laminate is preferably 1%~10%, more preferably 2%~8%, and particularly preferably 4%~6%. By using heated rollers, the laminate can be continuously shrunk in the width direction while being conveyed, achieving high productivity.

FIG2 is a schematic diagram showing an example of a drying and shrinking process. In the drying and shrinking process, the laminate 200 is dried while being transported by using the conveying rollers R1 to R6 and the guide rollers G1 to G4 that have been heated to a predetermined temperature. In the example of the figure, the conveying rollers R1 to R6 are arranged to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but, for example, the conveying rollers R1 to R6 may also be arranged to continuously heat only one surface of the laminate 200 (for example, the surface of the thermoplastic resin substrate).

The drying conditions can be controlled by adjusting the heating temperature of the conveyor roller (temperature of the heating roller), the number of heating rollers, and the contact time with the heating roller. The temperature of the heating roller is preferably 60°C~120°C, more preferably 65°C~100°C, and particularly preferably 70°C~80°C. While the crystallinity of the thermoplastic resin can be increased well and the curling can be suppressed well, an optical laminate with excellent durability can be produced. In addition, the temperature of the heating roller can be measured by a contact thermometer. In the example of the figure, 6 conveyor rollers are provided, but there is no special limitation if there are multiple conveyor rollers. The number of conveyor rollers is usually 2~40, and it is preferably 4~30. The contact time (total contact time) between the laminate and the heating roller is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating roller can be placed in a heating furnace (such as an oven) or in a general manufacturing line (at room temperature). It is best to place it in a heating furnace equipped with an air supply mechanism. By combining drying with the heating roller and hot air drying, the rapid temperature change between the heating rollers can be suppressed, and the shrinkage in the width direction can be easily controlled. The temperature of hot air drying should be 30°C~100°C. In addition, the hot air drying time should be 1 second~300 seconds. The wind speed of the hot air should be around 10m/s~30m/s. In addition, the wind speed is the wind speed in the heating furnace, which can be measured with a mini fan-type digital anemometer.

B-6. Other treatments It is advisable to perform a cleaning treatment after the water stretching treatment and before the drying shrinkage treatment. The above cleaning treatment can be performed by immersing the PVA resin layer in a potassium iodide aqueous solution.

C. Image display device The polarizing plates described in the above items A and B can be applied to an image display device. Therefore, the image display device is also included in the embodiment of the present invention. The image display device has a display unit and the polarizing plates described in the above items A and B arranged on at least one side of the display unit. Examples of the image display device include a liquid crystal display device and an organic electroluminescent (EL) display device. The structure of the image display device is well known in the industry, so a detailed description is omitted.

Examples The present invention is described in detail below using examples, but the present invention is not limited to these examples. The evaluation items of the examples are as follows.

(1) Reflection image comparison index The polarizing plates obtained in the examples and comparative examples were placed on a horizontal plane in a small darkroom. A special lighting device for inspection (manufactured by Japan Technology Center Co., Ltd., "S-Light") was used as a light source, and the light from the light source was irradiated onto the polarizing plate at an angle of 45°. At this time, the polarizing plate was installed so that the absorption axis direction of the polarizing plate was perpendicular to the irradiation direction of the light source. The polarizing plate was attached to a light-shielding black acrylic plate through an acrylic adhesive (thickness 20µm). The polarizing plate having a protective layer on one side had the polarizer surface in contact with the adhesive, and the polarizing plate having a protective layer on both sides had the inner protective layer surface in contact with the adhesive. In addition, the contrast index of the reflected image of the black acrylic plate is smaller than that of the polarizing plate, so it does not affect the measurement results. The reflected electronic image projected onto the screen set on the wall in the dark room is photographed with a camera, and the image of the reflected image is read as digital data. Image processing is performed on the 100mm×100mm area of the read image, and the brightness difference is calculated as the standard deviation, which is used as the contrast index of the reflected image. More specifically, the brightness of the pixels of the image is digitized in a 0~255 scale, and its standard deviation is calculated and used as the contrast index of the reflected image. (2) Transmission wavefront aberration The polarizing plates obtained in the embodiment and the comparative example are placed in a measuring device (manufactured by ZYGO, Verifire interferometer system) for measurement. The light source is a HeNe laser with a wavelength of 632.8nm, and the spot diameter of the incident light is set to 1mm. The application program for spherical transmission wavefront measurement is used to obtain the transmission wavefront aberration calculated from the interference light.

[Example 1] 1. Preparation of polarizer As a thermoplastic resin substrate, a long amorphous isophthalic acid copolymer polyethylene terephthalate film (thickness: 100µm) with a Tg of about 75°C was used, and one side of the resin substrate was subjected to a corona treatment. 13 parts by weight of potassium iodide was added to 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetyl acetyl modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER") in a ratio of 9:1, and the resulting mixture was dissolved in water to prepare a PVA aqueous solution (coating liquid). The PVA aqueous solution was applied to the corona treated surface of the resin substrate and dried at 60°C to form a PVA resin layer with a thickness of 13µm to produce a laminate. The obtained laminate was uniaxially stretched to 2.4 times in the longitudinal direction (long side direction) in an oven at 130°C (air-assisted stretching treatment). Then, the laminate was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (insolubilizing treatment). Next, the film was immersed in a dyeing bath (an iodine aqueous solution obtained by mixing iodine and potassium iodide at a weight ratio of 1:7 relative to 100 parts by weight of water) at a liquid temperature of 30°C for 60 seconds while adjusting the concentration so that the monomer transmittance (Ts) of the polarizer finally obtained becomes the desired value (dyeing treatment). Next, the film was immersed in a crosslinking bath (an aqueous boric acid solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid relative to 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (crosslinking treatment). Then, the laminate was immersed in a boric acid aqueous solution (boric acid concentration 4 weight%, potassium iodide concentration 5 weight%) at a liquid temperature of 64°C, and uniaxially stretched in the longitudinal direction (long side direction) between rollers with different peripheral speeds to achieve a total stretching ratio of 5.5 times (in-water stretching treatment). In addition, the immersion time of the laminate in the boric acid aqueous solution during the in-water stretching treatment was 75 seconds. Afterwards, the laminate was immersed in a cleaning bath (an aqueous solution obtained by mixing 4 weight parts of potassium iodide with 100 weight parts of water) at a liquid temperature of 20°C (cleaning treatment). Then, while drying in an oven maintained at about 90°C, it was brought into contact with a SUS heating roller maintained at a surface temperature of about 75°C (drying and shrinking treatment). In the above manner, a polarizer with a thickness of about 5µm was formed on the resin substrate.

2. Preparation of polarizing plate A polycarbonate resin film (40µm) is bonded as a protective layer to the surface of the polarizer obtained above (the side opposite to the resin substrate) through a UV curable adhesive. Specifically, the curable adhesive is applied to a total thickness of about 1.0µm and bonded using a roller. Then, UV rays are irradiated from the cycloolefin film side to cure the adhesive. Then, the resin substrate is peeled off to obtain a polarizing plate having a structure of a polycarbonate resin film (protective layer)/polarizer. The obtained polarizing plate is subjected to the evaluations of (1) and (2) above. The results are listed in Table 1.

In addition, a polycarbonate resin film was prepared in the following manner. 47.19 parts by mass of tricyclodecanedimethanol (hereinafter referred to as "TCDDM"), 175.1 parts by mass of diphenyl carbonate (hereinafter referred to as "DPC") and 0.979 parts by mass of a 0.2% by mass aqueous solution of cesium carbonate as a catalyst were added to a reaction vessel with respect to 81.98 parts by mass of isosorbide (hereinafter referred to as "ISB"), and the first step of the reaction was to heat the heating tank to 150°C and dissolve the raw materials while stirring as needed (approximately 15 minutes). Next, the pressure was set from normal pressure to 13.3 kPa, and the temperature of the heating tank was raised to 190°C in 1 hour, while the generated phenol was discharged from the reaction vessel. After the entire reaction vessel was kept at 190°C for 15 minutes, the pressure in the reaction vessel was set to 6.67 kPa as the second step, and the temperature of the heating tank was raised to 230°C in 15 minutes, and the generated phenol was discharged from the reaction vessel. Since the stirring torque of the stirrer will increase, the pressure in the reaction vessel is set to below 0.200 kPa in order to raise the temperature to 250°C in 8 minutes and further remove the generated phenol. After reaching the predetermined stirring torque, the reaction is terminated, and the generated reactants are extruded into water to obtain pellets of polycarbonate copolymer. The pellets are vacuum dried at 80°C for 5 hours, and then a film forming device equipped with a single-screw extruder (manufactured by Toshiba Machine Co., Ltd., cylinder setting temperature: 250°C), a T-die (width 200mm, setting temperature: 250°C), a cooling roller (setting temperature: 120~130°C) and a winder is used to produce a resin film. The obtained long strip of resin film is stretched obliquely at a stretching temperature of 135°C and a stretching ratio of 2.2 times to obtain a polycarbonate resin film with a thickness of 40µm.

[Example 2] A polarizing plate was produced in the same manner as in Example 1 except that the temperature of the boric acid aqueous solution in the underwater stretching treatment was set to 66°C, the immersion time in the boric acid aqueous solution was set to 60 seconds, and a cycloolefin film (manufactured by ZEON Corporation, Japan, 17µm) was used as a protective layer. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

[Example 3] A polarizing plate was prepared in the same manner as in Example 1 except that the temperature of the boric acid aqueous solution in the aqueous stretching treatment was set to 64°C, the immersion time in the boric acid aqueous solution was set to 50 seconds, and an acrylic resin film (thickness 40µm) was used as a protective layer. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. In addition, the acrylic resin film was prepared in the following manner. MS resin (copolymer of methyl methacrylate/styrene (molar ratio) = 80/20) was imidized with monomethylamine (imidization rate: 5%). The obtained imidized MS resin has pentamethyleneimide units, (meth)acrylate units and styrene units, and has an acid value of 0.5mmol/g. The imidized MS resin thus obtained was melt extruded to form a film. At this time, 0.66 parts by weight of the ultraviolet absorber was supplied to 100 parts by weight of the resin.

[Example 4] A polarizer with a thickness of about 5µm is formed on a resin substrate in the same manner as in Example 2. A cycloolefin film (manufactured by ZEON Corporation of Japan, 17µm) is bonded to the surface of the resulting polarizer (the surface on the opposite side to the resin substrate) as a protective layer using a UV-curing adhesive. Specifically, the curing adhesive is applied to a total thickness of about 1.0µm, and a roller machine is used for bonding. Then, UV rays are irradiated from the side of the cycloolefin film to cure the adhesive. Next, a polycarbonate resin film identical to that in Example 1 is bonded to the surface of the polarizer exposed by peeling off the resin substrate in the same manner as described above. According to the above method, a polarizing plate having a structure of cycloolefin film (inner protective layer)/polarizer/polycarbonate resin film (outer protective layer) is obtained. The obtained polarizing plate is subjected to the same evaluation as Example 1. The results are listed in Table 1. In addition, the cycloolefin film is prepared in the following manner. Prepare pellets of cycloolefin polymer (hydrogenated product of ring-opening polymer of norbornene monomer, trade name "ZEONOR1420R", manufactured by ZEON Corporation of Japan, glass transition temperature: 136°C), and dry them at 100.5 kPa and 100°C for 12 hours. 1.5 parts by weight of the pigment compound represented by the following formula was added to the resin weight (100 parts by weight), and a cycloolefin film was formed by using a uniaxial extruder at a mold temperature of 260°C using a T-type film melt extruder. [Chemical Formula 1]

[Example 5] A polarizing plate having a structure of acrylic resin film (inner protective layer)/polarizer/cycloolefin film (outer protective layer) was obtained in the same manner as in Example 4 except that the same acrylic resin film as in Example 3 was used as the inner protective layer and the same cycloolefin film as in Example 4 was used as the outer protective layer. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

[Example 6] A polarizing plate was produced in the same manner as in Example 5 except that the temperature of the boric acid aqueous solution in the underwater stretching treatment was set to 70°C and the immersion time in the boric acid aqueous solution was set to 60 seconds. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

[Comparative Example 1] A polarizing plate was produced in the same manner as in Example 2 except that the temperature of the boric acid aqueous solution in the water stretching treatment was set to 70°C and the immersion time was set to 50 seconds. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

[Comparative Example 2] A polarizer with a thickness of about 5µm was formed on a resin substrate in the same manner as in Example 1 except that the temperature of the boric acid aqueous solution in the water stretching treatment was set to 70°C and the immersion time was set to 50 seconds. A cycloolefin film (manufactured by ZEON Co., Ltd., Japan, 17µm) was bonded to the surface of the obtained polarizer (the side opposite to the resin substrate) as a protective layer using a UV-curable adhesive. Specifically, the curable adhesive was applied to a total thickness of about 1.0µm and bonded using a roller. Then, UV rays were irradiated from the cycloolefin film side to cure the adhesive. Next, an acrylic film (manufactured by Toyo Kohan Co., Ltd., 40 µm) was bonded to the surface of the polarizer exposed by peeling off the resin substrate in the same manner as described above. A polarizing plate having a structure of acrylic film (inner protective layer)/polarizer/cycloolefin film (outer protective layer) was obtained in the above manner. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

[Table 1]

As can be clearly seen from Table 1, the reflected image contrast index of the polarizing plate of the embodiment of the present invention is small, and as a result, the transmitted wavefront aberration is small. Therefore, the polarizing plate of the embodiment of the present invention can be understood as: when it is applied to an image display device with a camera part, it can achieve excellent shooting function and face recognition function without setting a through hole or a transparent part. The polarizing plate can be achieved by lowering the temperature of the boric acid aqueous solution and extending the immersion time in the water extension process when manufacturing the polarizer.

Industrial Applicability The polarizing plate of the present invention can be suitably used in image display devices (such as liquid crystal display devices, organic EL display devices, and quantum dot display devices).

10: Polarizer 20: 1st protective layer 30: 2nd protective layer 100: Polarizer 200: Laminated body G1~G4: Guide rollers R1~R6: Transport rollers

FIG1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. FIG2 is a schematic view showing an example of a drying and shrinking treatment using a heating roller in a method for manufacturing a polarizing element used in a polarizing plate according to an embodiment of the present invention.

10: Polarizer

20:1st protective layer

30: Second protective layer

100: Polarizing plate

Claims (4)

A polarizing plate having a polarizer and a protective layer disposed on at least one side of the polarizer, wherein the polarizer is composed of a polyvinyl alcohol resin film containing a dichroic substance; the protective layer is composed of a polycarbonate resin film containing isosorbide, tricyclodecane dimethanol and diphenyl carbonate as constituent units, or an acrylic resin film having pentamethyleneimide units, (meth)acrylate units and styrene units; and the reflected image contrast index of the polarizing plate is 13 or less. As in claim 1, the polarizing plate, wherein the thickness of the aforementioned polarizing element is less than 12 μm. A polarizing plate as claimed in claim 1 or 2, wherein only one side of the polarizing element has a protective layer. An image display device comprises a display unit and a polarizing plate as described in any one of claims 1 to 3 arranged on at least one side of the display unit.