TWI848916B - Polarizing plate, liquid crystal display device, manufacturing method thereof, and polyvinyl alcohol-based polarizing film - Google Patents

Abstract

The present invention provides a high-quality polyvinyl alcohol-based polarizing plate with an excellent balance between heat resistance, durability and polarization performance, a liquid crystal display device having the polarizing plate, a manufacturing method thereof, and a polyvinyl alcohol-based polarizing film used in the polarizing plate. The polarizing plate has a polyvinyl alcohol-based polarizing film having a glass transition temperature of 95°C or more, and a protective film disposed on at least one side of the polyvinyl alcohol-based polarizing film. In the liquid crystal display device, the polarizing plate and the image display device are stacked. The manufacturing method of the liquid crystal display device is to control the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate to be above 95°C by drying a stacked body of a polarizing plate having a polyvinyl alcohol-based polarizing film having a glass transition temperature of less than 95°C and an image display device.

Description

Polarizing plate, liquid crystal display device, manufacturing method thereof, and polyvinyl alcohol-based polarizing film

The present invention relates to a polarizing plate, a liquid crystal display device, a method for manufacturing the same, and a polyvinyl alcohol-based polarizing film. More specifically, the present invention relates to a polarizing plate having a polyvinyl alcohol-based polarizing film with an excellent balance between optical properties and heat resistance and durability (polyvinyl alcohol-based polarizing plate).

In recent years, the development of liquid crystal display devices has been amazing, and they are widely used in smart phones, tablet computers, personal computers, LCD TVs, projectors, car panels, etc. Polarizing plates are used in the above-mentioned liquid crystal display devices. As polarizing plates, they are mainly used in which a protective film is laminated on a polarizing film obtained by adsorbing and aligning iodine or dichroic dyes on a polyvinyl alcohol-based film. In recent years, with the expansion of the use of liquid crystal display devices, the temperature range and humidity range of the use environment have been expanded compared to the previous ranges. Therefore, polarizing plates with better heat resistance and durability and high polarization degree than before are necessary.

As a method for improving the heat resistance and durability of polarizing plates, some have proposed methods such as using a polarizing film using a specific azo dye (for example, see Patent Document 1), using a polarizing film with iodine adsorption orientation and further containing a dichroic organic dye (for example, see Patent Document 2). In addition, some have proposed a polarizing plate with specific optical properties containing a polarizer with iodine adsorption orientation and an iodine phase difference value Ri of 160nm or more (for example, see Patent Document 3), or a polarizing plate with a limited weight ratio of iodine to potassium and a limited amount of boron element (for example, see Patent Document 4) as a polarizing plate with excellent heat resistance and durability. [Prior Art Documents] [Patent Documents]

[Patent Document 1] International Publication No. 2016/186196 [Patent Document 2] Japanese Patent Application Publication No. 2012-3172 [Patent Document 3] Japanese Patent Application Publication No. 2016-139151 [Patent Document 4] International Publication No. 2016/060087

[The problem that the invention wants to solve]

However, in the technology disclosed in the above-mentioned patent document 1, a polarizing film with high heat resistance and durability can be obtained by using a specific dye, but its polarization degree is as low as 99.9%, which has the problem of insufficient contrast.

In the above-mentioned patent document 2, a polarizing film with high polarization performance can be obtained, but in its manufacturing process, it is necessary to set up two or more dyeing tanks for dyeing with dichroic dyes and iodine. Furthermore, in order to adjust the color tone of the polarizing film, it is also necessary to precisely manage the concentration of two or more types of dyeing tanks, so there is a problem of low productivity.

Furthermore, the disclosed technology of the above-mentioned patent document 3 can suppress the degradation of optical characteristics in the heat resistance test at about 85°C, but there is a problem that the heat resistance durability under more severe temperature conditions cannot be improved. The disclosed technology of the above-mentioned patent document 4 can maintain the absorbance at 700nm above 2.3 when placed at 105°C for 30 minutes, but there is a problem that the heat resistance durability cannot be improved when exposed to a high temperature environment for a longer period of time.

Therefore, in this invention, under such background, a high-quality polyvinyl alcohol-based polarizing plate with excellent balance between heat resistance and polarization performance, a liquid crystal display device having the polarizing plate and a manufacturing method thereof, and a polyvinyl alcohol-based polarizing film used in the polarizing plate are provided. [Means for Solving the Problem]

In view of such a situation, the inventors of this case have devoted themselves to continuous research and found that by using a polarizing plate with a polyvinyl alcohol-based polarizing film having a higher glass transition temperature than conventional polyvinyl alcohol-based polarizing plates, a polyvinyl alcohol-based polarizing plate with an excellent balance between heat resistance and polarization performance can be obtained.

That is, the present invention is based on the following [1] to [6]. [1] A polarizing plate comprising a polyvinyl alcohol-based polarizing film and a protective film provided on at least one side of the polyvinyl alcohol-based polarizing film, wherein the glass transition temperature of the polyvinyl alcohol-based polarizing film is above 95°C. [2] A polarizing plate as described in [1] above, wherein the moisture content is below 2% by weight. [3] A polarizing plate as described in [1] or [2] above, wherein the boric acid content of the polyvinyl alcohol-based polarizing film is above 20% by weight. [4] A liquid crystal display device, wherein the polarizing plate as described in any one of [1] to [3] above and an image display device are stacked. [5] A method for manufacturing a liquid crystal display device, which is a method for manufacturing a liquid crystal display device as described in [4] above, characterized in that: by drying a laminate of a polarizing plate having a polyvinyl alcohol-based polarizing film with a glass transition temperature of less than 95°C and an image display device, the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate is controlled to be above 95°C. [6] A polyvinyl alcohol-based polarizing film, characterized in that: the glass transition temperature is above 95°C. [Effects of the invention]

The polarizing plate of the present invention has a polyvinyl alcohol-based polarizing film with a glass transition temperature of 95°C or higher, so it has an excellent balance between heat resistance and polarization performance, and can stably perform its performance even under extremely high temperature conditions of 95°C or higher.

The following describes the embodiments of the present invention in detail. However, the present invention is not limited to these embodiments.

One embodiment of the polarizing plate of the present invention comprises a polyvinyl alcohol-based polarizing film having a glass transition temperature (Tg) of 95°C or higher, and a protective film disposed on at least one side of the polyvinyl alcohol-based polarizing film. This is the greatest feature of the present invention.

The glass transition temperature (Tg) of the polyvinyl alcohol-based polarizing film must be above 95°C. Considering the heat resistance and durability, it is preferably above 100°C, and more preferably above 105°C. The upper limit is usually 125°C. That is, if the glass transition temperature (Tg) does not reach the lower limit, the heat resistance and durability of the polarizing plate will be insufficient and the purpose of the present invention cannot be achieved.

The glass transition temperature (Tg) is a value measured by heating a polyvinyl alcohol-based polarizing film (polarizing plate) from -80°C to 155°C at a heating rate of 5°C/min using DSC (Q2000 manufactured by TA Instruments, 5 mg sample).

The production of the polyvinyl alcohol polarizing film is firstly to obtain a polyvinyl alcohol film using a polyvinyl alcohol resin as a raw material, and then to roll the polyvinyl alcohol film as a raw material, and to produce the film through the steps of swelling, dyeing, boric acid crosslinking, stretching, washing, drying, etc. Here, an example of the production method of the polyvinyl alcohol polarizing film is described in detail in the order of steps.

[Materials for forming polyvinyl alcohol-based films] First, the polyvinyl alcohol-based resins as materials for forming polyvinyl alcohol-based films are described. As the polyvinyl alcohol-based resins, unmodified polyvinyl alcohol-based resins are usually used, that is, resins produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. If necessary, resins obtained by saponifying a copolymer of vinyl acetate and a small amount (usually less than 10 mol %, preferably less than 5 mol %) of a component copolymerizable with vinyl acetate may also be used. As components copolymerizable with vinyl acetate, there may be mentioned unsaturated carboxylic acids (e.g., including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutylene, etc.), vinyl ethers, unsaturated sulfonic acid salts, and the like. Furthermore, a modified polyvinyl alcohol-based resin obtained by chemically modifying the saponified hydroxyl group may also be used.

Furthermore, as the polyvinyl alcohol resin, a polyvinyl alcohol resin having a 1,2-diol structure in the side chain may be used. The polyvinyl alcohol resin having a 1,2-diol structure in the side chain may be obtained, for example, by: (i) a method of saponifying a copolymer of vinyl acetate and 3,4-diethoxy-1-butene, (ii) a method of saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethyl carbonate, (iii) a method of saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, (iv) a method of saponifying a copolymer of vinyl acetate and glycerol monoallyl ether, etc.

The weight average molecular weight of the polyvinyl alcohol resin is preferably 100,000 to 300,000, more preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000, from the viewpoint of manufacturing the polyvinyl alcohol polarizing film. That is, if the weight average molecular weight is too small, it will be difficult to obtain sufficient optical performance when the polyvinyl alcohol resin is made into an optical film, and if it is too large, it will be difficult to stretch the polyvinyl alcohol polarizing film from the polyvinyl alcohol film. In addition, the weight average molecular weight of the polyvinyl alcohol resin is the weight average molecular weight measured by the GPC-MALS method.

The average saponification degree of the polyvinyl alcohol resin is usually preferably 98 mol% or more, more preferably 99 mol% or more, more preferably 99.5 mol% or more, and particularly preferably 99.8 mol% or more, from the viewpoint of the optical performance of the polyvinyl alcohol polarizing film. That is, if the average saponification degree is too low, the polyvinyl alcohol polarizing film made from the polyvinyl alcohol film tends to have insufficient optical performance. Here, the average saponification degree is measured in accordance with JIS K 6726.

As the polyvinyl alcohol-based resin, two or more types that are different in the modified substance, modification amount, weight average molecular weight, average saponification degree, etc. may be used in combination.

Then, the polyvinyl alcohol resin is used to prepare a polyvinyl alcohol resin aqueous solution. The polyvinyl alcohol resin aqueous solution is explained.

The polyvinyl alcohol resin aqueous solution is obtained by dissolving the polyvinyl alcohol resin in a solvent such as water. As the solvent, in addition to water, dimethyl sulfoxide (DMSO); N-methylpyrrolidone; polyols such as glycerol, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trihydroxymethylpropane; amines such as ethylenediamine and diethylenetriamine; and mixtures thereof may also be used.

The polyvinyl alcohol resin aqueous solution may contain, in addition to the polyvinyl alcohol resin, a commonly used plasticizer such as glycerol, diglycerol, triglycerol, ethylene glycol, triethylene glycol, polyethylene glycol, trihydroxymethylpropane, or at least one of a nonionic, anionic, and cationic surfactant as needed, and preferably contains them from the viewpoint of film forming properties. One of them may be selected and used alone, or two or more of them may be used in combination.

The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by weight, preferably 17 to 55% by weight, and more preferably 20 to 50% by weight, from the viewpoint of productivity. That is, if the resin concentration of the aqueous solution is too low, the drying load tends to increase and productivity tends to decrease, and if it is too high, the viscosity tends to become too high and it tends to be difficult to prepare a uniform solution.

The obtained polyvinyl alcohol resin aqueous solution is usually subjected to defoaming treatment. Examples of defoaming methods include static defoaming and defoaming by a multi-spindle extruder. As a multi-spindle extruder, any multi-spindle extruder with a vent hole is sufficient, and a double-spindle extruder with a vent hole is usually used.

[Manufacturing method of polyvinyl alcohol film] After the above defoaming treatment, the polyvinyl alcohol resin aqueous solution is introduced into the T-slit die in fixed amounts, and discharged and cast onto a rotating casting drum to form a film by a continuous casting method. Then, the film obtained by the film formation is dried, etc. to obtain a polyvinyl alcohol film. The following describes the manufacturing method of the polyvinyl alcohol film.

The above-mentioned continuous casting method is a method of making a film by, for example, discharging a polyvinyl alcohol-based resin aqueous solution from a T-slit die and pouring it onto a rotating casting drum, an endless belt, a resin film or other casting mold. The surface temperature of the casting drum or other casting mold is preferably 40 to 99°C, preferably 60 to 95°C. The film obtained by casting is dried by transporting the film while the surface and back of the film are alternately in contact with the outer periphery of a plurality of hot rollers. After drying by the hot roller, the film can also be heat-treated. For heat treatment, it is preferably carried out at 60 to 150°C, preferably 80 to 130°C. The film that has been dried and heat-treated as needed is cut and removed at both ends (slit) to form the polyvinyl alcohol film. The polyvinyl alcohol film is then wound around a core tube to form a film roll.

The thickness of the polyvinyl alcohol-based film is preferably 5 to 75 μm, and is preferably 10 to 60 μm in view of thinning the polarizing film, and is further preferably 15 to 60 μm in view of durability.

Furthermore, the width of the polyvinyl alcohol-based film is preferably 4 m or more, more preferably 4.5 m or more in view of large area, and particularly preferably 4.5 to 6 m in view of avoiding breakage.

The length of the polyvinyl alcohol film is preferably 4 km or longer, more preferably 4.5 km or longer in view of large area, and particularly preferably 4.5 to 50 km in view of transportation weight.

[Method for Producing Polyvinyl Alcohol-Based Polarizing Film] Hereinafter, a method for producing a polyvinyl alcohol-based polarizing film according to an embodiment of the present invention using the polyvinyl alcohol-based film will be described.

The above-mentioned polyvinyl alcohol-based polarizing film is manufactured by pulling the polyvinyl alcohol-based film out from the above-mentioned film roll and transporting it in a horizontal direction, and then undergoing the steps of swelling, dyeing, boric acid crosslinking, stretching, washing, and drying. These steps are explained below.

The swelling step is performed before the dyeing step. The swelling step is usually performed by immersing the above-mentioned polyvinyl alcohol film in a treatment solution in a water tank. The swelling step can not only clean the dirt on the surface of the polyvinyl alcohol film, but also prevent uneven dyeing by swelling the polyvinyl alcohol film. Water is usually used as the above-mentioned treatment solution. The treatment solution only needs to have water as the main component, and a small amount of additives such as iodine compounds, surfactants, alcohol, etc. can also be added. The temperature of the treatment solution is usually around 10~45°C, and the time of immersion in the treatment solution is usually around 0.1~10 minutes.

The dyeing step is usually carried out by immersing the polyvinyl alcohol film in a liquid containing iodine or a dichroic dye. An iodine-potassium iodide aqueous solution is usually used as the liquid, and the iodine concentration is preferably 0.1-2 g/L, and the potassium iodide concentration is preferably 1-100 g/L. The dyeing time is about 30-500 seconds in practical terms. The temperature of the liquid is preferably 5-50°C. In addition to the aqueous solvent, the aqueous solution may also contain a small amount of an organic solvent compatible with water.

The boric acid crosslinking step is usually performed by immersing the polyvinyl alcohol film in a liquid containing a boron compound such as boric acid or borax. The liquid is an aqueous solution or a water-organic solvent mixture, and the concentration of the boron compound in the liquid is about 10 to 100 g/L. From the perspective of stabilizing polarization performance, potassium iodide is preferably coexisted in the liquid. The temperature of the liquid is preferably about 30 to 70°C, and the immersion time in the liquid is preferably about 0.1 to 20 minutes.

The stretching step can be performed independently or in at least one of the swelling step, dyeing step, and boric acid crosslinking step. In addition, the total stretching ratio is preferably set to 3 to 10 times in the uniaxial direction, more preferably 3.5 to 6 times. At this time, it is also OK to stretch slightly in the direction perpendicular to the stretching direction (to prevent shrinkage in the width direction, or stretching above). During stretching, the temperature around the stretched polyvinyl alcohol film is preferably 40 to 170°C.

The cleaning step is performed, for example, by immersing the polyvinyl alcohol film in water or an aqueous iodide solution such as potassium iodide. The precipitate generated on the surface of the polyvinyl alcohol film can be removed by the cleaning step. The concentration of potassium iodide in the case of using an aqueous potassium iodide solution is usually about 1~80g/L. The temperature of the above-mentioned aqueous iodide solution during cleaning is usually 5~50°C, preferably 10~45°C. The time of immersion in the above-mentioned water or iodide aqueous solution is usually 1~300 seconds, preferably 10~240 seconds. In addition, it is also possible to appropriately combine water cleaning and cleaning using an aqueous potassium iodide solution.

The drying step is usually performed by drying the polyvinyl alcohol film in air at an ambient temperature of 40-100°C, preferably 70-98°C, for 0.5-20 minutes.

Here, as control methods for increasing the glass transition temperature of the manufactured polyvinyl alcohol-based polarizing film, there can be cited, for example, (1) a method of increasing the boric acid content in the manufactured polyvinyl alcohol-based polarizing film by increasing the boric acid concentration of the liquid in which the polyvinyl alcohol-based film is immersed in the boric acid crosslinking step, (2) a method of increasing the crosslinking density of the polyvinyl alcohol-based polarizing film, (3) a method of reducing the moisture content of the polyvinyl alcohol-based polarizing film by using a desiccant such as phosphorus pentoxide, (4) a method of adjusting the ambient temperature and drying time in the above drying step, etc. Among these, the above methods (1) and (4) are more suitable from the viewpoint of the simplicity of adjustment.

In the case of the method (1) above, the boric acid concentration of the liquid is preferably 20 to 90 g/L from the viewpoint of making the glass transition temperature above 95°C, and is more preferably 30 to 80 g/L from the viewpoint of taking both heat resistance, durability and elongation into consideration. By increasing the boric acid concentration of the liquid in this way, the boric acid content in the manufactured polyvinyl alcohol-based polarizing film can be increased. The boric acid content is preferably 20% by weight or more, preferably 20 to 30% by weight, and more preferably 21 to 29% by weight. If the boric acid content is too little, the heat resistance and durability of the polyvinyl alcohol-based polarizing film tend to be insufficient, and if it is too much, the polyvinyl alcohol-based polarizing film tends to be easily broken during the manufacture of the polyvinyl alcohol-based polarizing film.

In the case of the method (4) above, from the viewpoint of making the glass transition temperature above 95°C, the ambient temperature of the drying step is preferably 50-99°C, and from the viewpoint of heat resistance and durability, it is preferably 60-98°C, and the drying time is preferably 1-15 minutes, and more preferably 1.5-10 minutes. By this drying step, the moisture content of the polyvinyl alcohol-based polarizing film can be reduced to 2.0 wt% or less.

In this way, a polyvinyl alcohol-based polarizing film having a glass transition temperature of 95° C. or higher can be obtained.

The polarization degree of the polyvinyl alcohol-based polarizing film is preferably 99.9% or more, and preferably 99.92% or more. If the polarization degree is too low, the contrast in a liquid crystal display device such as a liquid crystal display may not be ensured.

In addition, the monomer transmittance of the polyvinyl alcohol-based polarizing film is preferably 42% or more. If the monomer transmittance is too low, it will tend to be impossible to achieve high brightness of liquid crystal display devices such as liquid crystal displays. The monomer transmittance is a value obtained by measuring the light transmittance of the polyvinyl alcohol-based polarizing film monomer using a spectrophotometer.

[Polarizing Plate Manufacturing Method] Then, a protective film having optical isotropy is attached to one or both sides of the obtained polyvinyl alcohol-based polarizing film via an adhesive to obtain a polarizing plate having at least one side provided with a protective film.

Examples of the protective film include resin films or sheets of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymers, cycloolefin copolymers, polystyrene, polyether sulfone, polyarylate, poly-4-methylpentene, polyphenylene ether, and the like.

The bonding of the polyvinyl alcohol-based polarizing film and the protective film can be performed by a known method, for example, by uniformly applying a liquid adhesive composition to the polyvinyl alcohol-based polarizing film, the protective film, or both, bonding the two together and pressing them together, and then heating and irradiating them with active energy rays.

In order to improve the heat resistance and durability of the polyvinyl alcohol-based polarizing film, the moisture content of the polarizing plate is preferably 2 wt % or less, more preferably 1.5 wt % or less, and even more preferably 1 wt % or less. The lower limit of the moisture content is usually 0.1 wt %. If the moisture content is too high, the heat resistance and durability of the polyvinyl alcohol-based polarizing film tend to be insufficient.

Examples of methods for adjusting the moisture content to a low level include (a) continuously drying the polarizing plate, (b) temporarily winding the polarizing plate and then performing heat treatment in a high-temperature storage, and (c) performing vacuum drying after wafer cutting of the polarizing plate. Among these, the method (a) is more suitable in order to uniformly adjust the moisture content in the width direction and the length direction.

In the case of the method (a) above, the ambient temperature during the drying process is usually 60-100°C, and preferably 70-95°C in consideration of optical characteristics. In addition, the drying time is usually 1-10 minutes, and preferably 2-8 minutes. In the drying process, it is preferable to perform two or more stages of drying processes with different ambient temperatures and drying times. Considering the appearance of the polarizing plate, it is particularly desirable to increase the ambient temperature of the latter stage of the drying process. Specifically, for example, the ambient temperature of the first stage of the drying process is preferably set to 60-80°C, and the ambient temperature of the second stage of the drying process is preferably set to 80°C-100°C.

In the case of the method (c) above, the vacuum drying treatment after the polarizing plate is cut into wafers is preferably carried out at an ambient temperature of 40 to 60° C. for about 15 minutes to 36 hours.

The polarization degree of the above-mentioned polarizing plate is preferably 99.9% or more, preferably 99.92% or more. If the polarization degree is too low, there is a tendency that the contrast in liquid crystal display devices such as liquid crystal displays cannot be ensured. In addition, the polarization degree is generally calculated from the light transmittance ( _H11 ) measured at a wavelength λ when two polarizing plates are overlapped so that their orientation directions are in the same direction, and the light transmittance ( _H1 ) measured at a wavelength λ when two polarizing plates are overlapped so that their orientation directions are perpendicular to each other, according to the following formula. Polarization degree = [( _H11 - _H1 )/( _H11 + _H1 )] ^1/2

In addition, the single transmittance of the polarizing plate is preferably 42% or more. If the single transmittance is too low, it will tend to be impossible to achieve high brightness of liquid crystal display devices such as liquid crystal displays. The single transmittance is a value obtained by measuring the light transmittance of the single polyvinyl alcohol-based polarizing plate using a spectrophotometer.

[Liquid crystal display device] One embodiment of the liquid crystal display device of the present invention has the above-mentioned polarizing plate. That is, the liquid crystal display device is a stacked image display device such as the above-mentioned polarizing plate and a liquid crystal cell, etc., with an adhesive layer interposed therebetween. The above-mentioned liquid crystal display device can also be supplemented with components such as a lighting system as needed. Examples of the above-mentioned liquid crystal cell include TN type, STN type, VA type, IPS type, etc.

The above-mentioned liquid crystal display device has the above-mentioned polarizing plate having a polyvinyl alcohol-based polarizing film with a glass transition temperature of 95°C or above, so that images can be stably displayed even under extremely high temperature conditions of 95°C or above.

[Manufacturing method of liquid crystal display device] In the above-mentioned liquid crystal display device, if the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate is less than 95°C, the above-mentioned liquid crystal display device can be manufactured in the following manner. That is, by stacking a polarizing plate having a polyvinyl alcohol-based polarizing film with a glass transition temperature less than 95°C and the above-mentioned liquid crystal box with an adhesive layer interposed therebetween, and subjecting the stacked body to a drying treatment or a vacuum drying treatment, the glass transition temperature of the polyvinyl alcohol-based polarizing film of the above-mentioned polarizing plate is made to be above 95°C. In this case, the ambient temperature in the above-mentioned drying treatment is preferably 60~100°C, and the drying time is preferably 0.5~10 minutes. The ambient temperature in the above-mentioned vacuum drying treatment is preferably 50~80°C, and the drying time is preferably 10~120 minutes. The vacuum drying process refers to a drying process in a vacuum (absolute pressure of 1 kPa or less).

The above polarizing plate has excellent heat resistance, durability and polarization performance, and can be ideally used in portable information terminal equipment, computers, televisions, projectors, billboards, desktop computers, electronic clocks, word processors, electronic paper, game consoles, video recorders, cameras, photo frames, thermometers, stereos, liquid crystal display devices such as automobile or mechanical instruments, sunglasses, anti-glare glasses, stereo glasses, wearable displays, anti-reflection layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical fiber communication equipment, medical equipment, building materials, toys, etc. [Examples]

The present invention is further specifically described below with reference to the following examples, but the present invention is not limited to the following examples within the scope of the gist. The various physical properties were measured in the following manner.

<Measurement conditions> (1) Water content (wt%) Weigh 100 mg of the obtained polarizing plate and put it into a 2 ml glass bottle. Add anhydrous ethanol, seal it and heat it at 60°C for 24 hours. Then cool it to room temperature (25°C) and use a trace moisture measuring device CA-200 (Mitsubishi Chemical Analytech) to measure the water content of the polarizing plate.

(2) Glass transition temperature of polyvinyl alcohol polarizing film (°C) The glass transition temperature of the polyvinyl alcohol polarizing film was measured by heating the obtained polyvinyl alcohol polarizing film from -80°C to 155°C at a heating rate of 5°C/min using DSC (Q2000 manufactured by TA instruments, sample 5 mg).

(3) Boric acid content (weight %) After drying the obtained polyvinyl alcohol polarizing film at 105°C for 2 hours, weigh 100 mg, add 30 ml of water and heat to dissolve. After cooling the obtained solution to 25°C, add 5 ml of glycerin and stir, then titrate with 0.1 mol/L sodium hydroxide aqueous solution and calculate according to the following formula. Boric acid content (weight %) = 61.83 × 0.1 × f × V / weight of polyvinyl alcohol polarizing film × 100 f: valence of 0.1 mol/L sodium hydroxide aqueous solution (factor) V: titration amount (ml)

(4) Polarization degree (%), single body transmittance (%) A test piece of 4 cm in length × 4 cm in width was cut out from the center portion of the obtained polarizing plate in the width direction, and the polarization degree (%) and single body transmittance (%) were measured using an automatic polarizing film measuring device (manufactured by JASCO Corporation: VAP7070).

(5) Heat resistance and durability test The polarizing plate was sandwiched between glass and placed in a 105°C dryer. After 150 hours, the appearance was visually observed and evaluated according to the following criteria. (Evaluation criteria) ○･･･No discoloration △･･･Some slight discoloration ×･･･Obvious discoloration

<Example 1> (Production of polyvinyl alcohol polarizing film and polarizing plate) A polyvinyl alcohol film with a thickness of 45 μm was drawn out from a film roll, transported in the horizontal direction, immersed in a water tank containing water at a temperature of 25°C, and stretched to 1.7 times in the flow direction (MD) while swelling. Then, it was immersed in a 30°C aqueous solution composed of 1.2 g/L iodine and 30 g/L potassium iodide, stretched to 1.6 times in the flow direction (MD) while dyeing, and then immersed in an aqueous solution composed of 50 g/L boric acid and 30 g/L potassium iodide (55°C), stretched to 2.1 times in the flow direction (MD) while performing boric acid crosslinking. Then, it was washed with potassium iodide and dried at an ambient temperature of 60°C for 2 minutes to obtain a polyvinyl alcohol polarizing film with a total stretching ratio of 5.7 times. The properties of the obtained polyvinyl alcohol polarizing film are shown in Table 1. Then, a triacetyl cellulose film with a film thickness of 40 μm was attached to both sides of the obtained polyvinyl alcohol polarizing film using a polyvinyl alcohol aqueous solution as an adhesive, and dried at an ambient temperature of 95°C for 2 minutes to obtain a polarizing plate with a single unit transmittance of 42.8% and a polarization degree of 99.95%. The properties of the polyvinyl alcohol polarizing film and the polarizing plate are shown in Table 1.

<Example 2> A polyvinyl alcohol film with a thickness of 45 μm was pulled out from a film roll, transported in a horizontal direction, immersed in a water tank containing water at a temperature of 25°C, and stretched to 1.7 times in the flow direction (MD) while swelling. Then, it was immersed in a 30°C aqueous solution composed of 1.2 g/L iodine and 30 g/L potassium iodide, and stretched to 1.6 times in the flow direction (MD) while dyeing. Then, it was immersed in an aqueous solution composed of 50 g/L boric acid and 30 g/L potassium iodide (55°C), and stretched to 2.2 times in the flow direction (MD) while crosslinking with boric acid. Then, it was washed with potassium iodide and dried at an ambient temperature of 60°C for 2 minutes to obtain a polyvinyl alcohol polarizing film with a total stretching ratio of 6.0 times. The properties of the obtained polyvinyl alcohol polarizing film are shown in Table 1. Then, a triacetyl cellulose film with a thickness of 40 μm was bonded to both sides of the obtained polyvinyl alcohol polarizing film using a polyvinyl alcohol aqueous solution as an adhesive, and dried at an ambient temperature of 80°C for 5 minutes, and then further dried at an ambient temperature of 95°C for 1 minute to obtain a polarizing plate with a single unit transmittance of 42.9% and a polarization degree of 99.96%. The properties of the polyvinyl alcohol polarizing film and the polarizing plate are shown in Table 1.

<Example 3> A polyvinyl alcohol-based polarizing plate was obtained in the same manner as in Example 2, except that the triacetyl cellulose film was dried at an ambient temperature of 80°C for 2 minutes after lamination. After an adhesive layer was provided on one side of the obtained polarizing plate, the wafer was cut to produce a polarizing plate wafer of 10 cm in length and 5 cm in width. The polarizing plate wafer was vacuum dried at an ambient temperature of 40°C for 16 hours. The properties of the obtained polarizing plate are shown in Table 1.

<Comparative Example 1> In Example 2, a polarizing plate was obtained in the same manner as in Example 2 except that the triacetyl cellulose film was dried at an ambient temperature of 80°C for 2 minutes after lamination. The properties of the polyvinyl alcohol-based polarizing film and the polarizing plate are shown in Table 1.

[Table 1]

It can be seen that the polarizing plates of Examples 1 to 3 have excellent heat resistance because the glass transition temperature of the polyvinyl alcohol-based polarizing film contained therein is higher than 95°C, so there is no discoloration during the heat resistance test. On the other hand, it can be seen that the polarizing plate of Comparative Example 1 has discoloration during the heat resistance test because the glass transition temperature of the polyvinyl alcohol-based polarizing film contained therein is lower than 95°C.

<Example 4> On both sides of the polarizing film obtained in Example 1, a triacetyl cellulose film with a film thickness of 40 μm is bonded using a polyvinyl alcohol aqueous solution as an adhesive, and dried at an ambient temperature of 80°C for 2 minutes to obtain a polarizing plate with a single body transmittance of 42.8% and a polarization degree of 99.94%. After an adhesive layer is provided on one side of the obtained polarizing plate, it is bonded to a liquid crystal display device to form a stack of the polarizing plates and the liquid crystal display device. Then, the stack is vacuum dried in a vacuum dryer at an ambient temperature of 60°C for 45 minutes. Then, a glass substrate is bonded to the polarizing plate of the above-mentioned stack, and a heat resistance durability test is performed in a dryer at an ambient temperature of 105°C for 150 hours. The results are shown in Table 2. Furthermore, the polarizing plate was peeled off from the above-mentioned laminated body after vacuum drying which was prepared separately, and the moisture content and the glass transition temperature of the polarizing film were measured. The results showed that the moisture content was 2.0% by weight and the glass transition temperature of the polarizing film was 98°C.

<Example 5> The laminate obtained in Example 4 was vacuum dried in a vacuum dryer at an ambient temperature of 70°C for 60 minutes. Then, a glass substrate was bonded to the polarizing plate of the laminate, and a heat resistance durability test was performed in a dryer at an ambient temperature of 105°C for 150 hours. The results are shown in Table 2. In addition, the polarizing plate was peeled off from the laminate after vacuum drying treatment prepared separately, and the moisture content and the glass transition temperature of the polarizing film were measured. The results showed that the moisture content was 0.9% by weight, and the glass transition temperature of the polarizing film was 114°C.

<Comparative Example 2> The laminate obtained in Example 4 was vacuum dried in a vacuum dryer at an ambient temperature of 70°C for 15 minutes. Then, a glass substrate was bonded to the polarizing plate of the laminate, and a heat resistance durability test was performed in a dryer at an ambient temperature of 105°C for 150 hours. The results are shown in Table 2. In addition, the polarizing plate was peeled off from the laminate after vacuum drying treatment prepared separately, and the moisture content and the glass transition temperature of the polarizing film were measured. The results showed that the moisture content was 2.3% by weight, and the glass transition temperature of the polarizing film was 93°C.

In the above-mentioned Examples 4 and 5 and Comparative Example 2, the evaluation criteria of the heat resistance durability test of the above-mentioned laminated body are as follows. (Evaluation Criteria) ○･･･No discoloration at all △･･･Some slight discoloration ×･･･Obvious discoloration

[Table 2]

In Examples 4 and 5, it can be seen that because the glass transition temperature of the polyvinyl alcohol-based polarizing film in the polarizing plate is above 95°C, the laminated body using the polarizing plate does not change color at all during the heat resistance test, and is excellent in heat resistance. In contrast, in Comparative Example 2, it can be seen that because the glass transition temperature of the polyvinyl alcohol-based polarizing film in the polarizing plate does not reach 95°C, the laminated body using the polarizing plate changes color during the heat resistance test.

The above embodiments present specific forms of the present invention, but the above embodiments are only illustrative and are not to be construed as limiting. Various modifications that are obvious to a person skilled in the art are intended to be included in the scope of the present invention. [Industrial Applicability]

The polarizing plate of the present invention has an excellent balance between heat resistance, durability and polarization performance, and can be ideally used in various liquid crystal display devices, automotive applications requiring high durability and polarization performance, and industrial instrument display applications required in various environments.

Claims (3)

A polarizing plate having a polyvinyl alcohol polarizing film and a protective film disposed on at least one side of the polyvinyl alcohol polarizing film, wherein the glass transition temperature of the polyvinyl alcohol polarizing film is above 95°C, the boric acid content of the polyvinyl alcohol polarizing film is above 20% by weight, and the moisture content of the polarizing plate is 0.1-0.9% by weight. A liquid crystal display device, characterized in that: the polarizing plate and the image display device as in item 1 of the patent application scope are stacked. A method for manufacturing a liquid crystal display device is a method for manufacturing a liquid crystal display device as described in item 2 of the patent application, characterized in that: by drying a laminate of a polarizing plate having a polyvinyl alcohol-based polarizing film with a glass transition temperature lower than 95°C and an image display device, the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate is controlled to be above 95°C.