TW202346438A - Method for manufacturing polarizing film and manufacturing device - Google Patents

Abstract

This invention provides a method for manufacturing polarizing film with neutral hue and excellent optical properties and manufacturing device. This invention provides a method for manufacturing a polarizing film from a polyvinyl alcohol resin film, including a dyeing step of dyeing the polyvinyl alcohol resin film with a dichroic dye; and a cross-linking step of sequentially immersing the polyvinyl alcohol-based resin film after the dyeing step in n (n is an integer of 4 or more) cross-linking baths for cross-linking, wherein the temperature of the second and third upstream crosslinking baths is higher than the temperature of the first upstream crosslinking bath.

Description

Polarizing film manufacturing method and manufacturing device

The present invention relates to a method and manufacturing device for manufacturing a polarizing film from a polyvinyl alcohol resin film.

Polarizing plates are widely used as polarizing elements in image display devices such as liquid crystal display devices. The polarizing plate generally has a structure in which a transparent resin film (protective film, etc.) is bonded to one or both sides of the polarizing film using an adhesive or the like.

Polarizing films are mainly produced by immersing a raw material film containing polyvinyl alcohol-based resin in a dyeing bath containing dichroic dyes such as iodine, and then immersing it in a cross-linking bath containing a cross-linking agent such as boric acid. And the film is manufactured by uniaxially stretching it at any stage. Among uniaxial stretching, there are dry stretching, which stretches in the air, and wet stretching, which stretches in liquids such as the above-mentioned dyeing bath and cross-linking bath.

If the cross-linked polarizing film is exposed to a high temperature environment, it will easily shrink and may have insufficient durability. Japanese Patent Application Publication No. 2013-148806 (Patent Document 1) describes the provision of a polarizing film with excellent durability by reducing the boron content rate of the polarizing film to 1 to 3.5% by weight.

[Prior technical literature]

[Patent Document 1: Patent Document]

[Patent Document 1] Japanese Patent Application Publication No. 2013-148806

The object of the present invention is to provide a method and device for manufacturing a polarizing film with a neutral hue and excellent optical properties.

The present invention provides a manufacturing method and a manufacturing device for a polarizing film shown below.

[1] A method of manufacturing a polarizing film, which manufactures a polarizing film from a polyvinyl alcohol-based resin film, including

The dyeing step of dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment, and

A cross-linking step in which the polyvinyl alcohol-based resin film after the dyeing step is sequentially immersed in n cross-linking baths (n is an integer of 4 or more) to perform a cross-linking treatment,

Among them, the temperature of the second and third cross-linking baths arranged from the upstream side is higher than the temperature of the first cross-linking bath arranged from the upstream side.

[2] The method for manufacturing a polarizing film as described in [1], wherein the concentration of the boron compound in each cross-linking bath is 2.5% by mass or more.

[3] The method for manufacturing a polarizing film as described in [1] or [2], wherein the temperature of each cross-linking bath is 65°C or lower.

[4] A polarizing film manufacturing device that manufactures a polarizing film from a polyvinyl alcohol-based resin film, including

The dyed portion of the polyvinyl alcohol-based resin film dyed with a dichroic dye, and

The cross-linked portion of the polyvinyl alcohol-based resin film after the dyeing treatment is sequentially immersed in n cross-linking baths (n is an integer of 4 or more) to perform cross-linking treatment,

Among them, the temperature of the second and third cross-linking baths arranged from the upstream side is higher than the temperature of the first cross-linking bath arranged from the upstream side.

According to the present invention, it is possible to provide a manufacturing method and a manufacturing device for a polarizing film with a neutral hue and excellent optical properties.

10: Embryonic membrane composed of polyvinyl alcohol resin

11: embryonic membrane roll

13: Swelling bath

15:Dyeing bath

17a: 1st cross-linking bath

17b: 2nd cross-linking bath

17c: The third cross-linking bath

17d: The 4th cross-linking bath

19: Washing bath

21: Drying oven

23:Polarizing film

30~49, 56~61: Guide roller

50~52, 53a, 53b, 53c, 53d, 54, 55: Nip roller

FIG. 1 is a cross-sectional view schematically showing an example of a polarizing film manufacturing method according to the first embodiment of the present invention and a polarizing film manufacturing apparatus suitable for the method.

[Manufacturing method of polarizing film]

In the present invention, the polarizing film is one in which a dichroic pigment (iodine or dichroic dye) is adsorbed and aligned on a uniaxially stretched polyvinyl alcohol-based resin film. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying polyvinyl acetate-based resin. The degree of saponification is usually 85 mol% or more, preferably 90 mol% or more, and more preferably 99 mol% or more. The polyvinyl acetate resin may be, for example, in addition to polyvinyl acetate which is a homopolymer of vinyl acetate, or a copolymer of vinyl acetate and other monomers copolymerizable therewith. Examples of other copolymerizable monomers include: unsaturated carboxylic acids, olefins Hydrocarbons, vinyl ethers, unsaturated sulfonic acids, etc. The degree of polymerization of polyvinyl alcohol-based resin is usually 1,000 to 10,000, preferably about 1,500 to 5,000.

These polyvinyl alcohol-based resins can also be modified. For example, aldehyde-modified polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc. can also be used. In this specification, "polyvinyl alcohol-based resin" refers to a resin in which the structural unit of vinyl alcohol (-CH ₂ -CH(OH)-) accounts for 50 mol% or more of all the structural units contained in the resin. .

In the present invention, as a starting material for polarizing film production, an unstretched polyvinyl alcohol-based resin film (raw material film) with a thickness of 65 μm or less (for example, 60 μm or less), preferably 50 μm or less, and more preferably 45 μm or less is used. ). In this way, polarizing films for films that are increasingly required by the market can be obtained. The width of the raw film is not particularly limited, for example, it can be about 400mm~6000mm. The original film is prepared, for example, as a roll of a long unstretched polyvinyl alcohol-based resin film (original film roll).

The polarizing film can be continuously conveyed along the film conveying path of the polarizing film manufacturing device while unwinding the above-mentioned long raw film from the raw film roll, and immersed in the treatment liquid (hereinafter, "contained in the treatment tank") Also called "processing bath"), specific processing steps are performed after extraction and then a drying step is performed to continuously manufacture long polarizing films. In addition, the treatment step is not limited to the method of immersing the membrane in a treatment bath, as long as the treatment is performed by bringing the treatment liquid into contact with the membrane in addition to the cross-linking step described below, and may also be performed by spraying or flowing down. , dripping, etc. to make the treatment liquid adhere to the membrane surface to perform membrane treatment. When the treatment step is performed by immersing the membrane in a treatment bath, the number of treatment baths used to perform one treatment step is not limited to one, and the membrane may be immersed in two or more treatment baths in sequence. Complete a processing step.

Examples of the above-mentioned treatment liquid include swelling liquid, dyeing liquid, cross-linking liquid, washing liquid, and the like. Examples of the above-mentioned processing steps include a swelling step in which the swelling liquid is brought into contact with the original film and the swelling treatment is performed. The dyeing step of bringing the dyeing solution into contact with the swollen-treated membrane to perform the dyeing treatment; the cross-linking step of bringing the cross-linking liquid into contact with the dyed membrane to perform the cross-linking treatment; and the cross-linking step of making the washing liquid and the cross-linked membrane The membrane is contacted to perform the cleaning step of the cleaning process. Furthermore, between the series of processing steps (ie, before and after any one or more processing steps and/or during any one or more processing steps), the uniaxial stretching process is performed in a wet or dry manner. Other processing steps can be added as needed.

<Cross-linking step>

The cross-linking step is a treatment performed for the purpose of cross-linking the film. This treatment can improve water resistance or heat resistance and further adjust the hue (preventing the film from being bluish, etc.). In the cross-linking step of the present invention, the membrane is sequentially immersed in n (n is an integer of 4 or more) cross-linking baths to perform cross-linking treatment.

The temperature of the second and third cross-linking baths arranged from the upstream side is higher than the temperature of the first cross-linking bath arranged from the upstream side, preferably by 1°C or more, more preferably The temperature is 2°C or more, and more preferably 3°C or more. After the second one, by continuously arranging at least two cross-linking baths with a temperature higher than that of the first cross-linking bath, a polarizing film with a neutral hue, specifically a smaller absolute value of the orthogonal b value, can be obtained. , and then a polarizing film with excellent optical properties can be obtained. The reason for this is considered to be that the membrane can be cross-linked efficiently by performing the cross-linking treatment immediately after the cross-linking treatment in the first cross-linking bath using two or more cross-linking baths having a temperature higher than that of the first cross-linking bath. . Adding iodine complexes can improve the hue, relieve the stress caused by cross-linking, and increase the extension ratio (the extension ratio in the drying step is also increased). By increasing the stretching ratio, a polarizing film with excellent optical properties can be obtained.

Furthermore, the temperature of the cross-linking bath arranged in the fourth and subsequent places may be higher than the temperature of the cross-linking bath arranged in the first place, may be a lower temperature, or may be the same temperature. If the fourth cross-linking bath is arranged for the purpose of hue adjustment, it is preferably the same temperature as the first cross-linking bath or a lower temperature. The temperature of the cross-linking baths arranged after the fourth one is lower than that of the first cross-linking bath. In the case of the temperature of the bath, the difference between the temperature of the fourth and subsequent cross-linking baths and the temperature of the first cross-linking bath is preferably 5°C or less.

In this specification, the conveyance direction of the film is assumed to be conveyed from the upstream side to the downstream side. When comparing two locations, the upstream side refers to the upstream side along the film conveyance direction.

From the viewpoint of efficiently performing cross-linking, the temperature of each cross-linking bath is preferably 40°C or higher, and more preferably 45°C or higher. Moreover, from the viewpoint of preventing dissolution of the polyvinyl alcohol-based resin film, the temperature of each crosslinking bath is preferably 65°C or lower, and more preferably 64°C or lower. The immersion time of the film in each cross-linking bath is about 3 to 600 seconds, preferably 4 to 300 seconds, more preferably 5 to 200 seconds.

The concentration of the boron compound in each cross-linking bath is preferably 2.5% by mass or more, more preferably 2.8% by mass or more. The concentration of the boron compound in each cross-linking bath is, for example, 4.0% by mass or less. The boron compound functions as a cross-linking agent, and examples thereof include boric acid, borax, and the like. In addition to the boron compound, the cross-linking bath may also contain cross-linking agents such as glyoxal and glutaraldehyde. As the solvent of the cross-linking bath, for example, water may be used, and an organic solvent that is compatible with water may also be included. From the viewpoint of promoting cross-linking of the polyvinyl alcohol-based resin film or balancing the amount of cross-linking in the polyvinyl alcohol film, the temperature of each cross-linking bath is preferably 30°C or higher, more preferably 35°C or higher. Furthermore, it is more preferable that it is 42 degreeC or more.

<First Embodiment>

One aspect of the manufacturing method of the polarizing film of the present invention will be described in detail with reference to FIG. 1 . FIG. 1 is a cross-sectional view schematically showing an example of a polarizing film manufacturing method of the present invention and a polarizing film manufacturing apparatus used therefor. The polarizing film manufacturing apparatus shown in FIG. 1 conveys the original (unstretched) film 10 made of polyvinyl alcohol-based resin along the film conveyance path while continuously unwinding it from the original film roll 11 . The swelling bath (the swelling liquid contained in the swelling tank) 13, the dyeing bath (the dyeing liquid contained in the dyeing tank) 15, and the first cross-linking bath (the first cross-linking bath contained in the cross-linking tank) are installed on the film transport path. Cross-linking liquid) 17a, second cross-linking bath (the second cross-linking liquid accommodated in the cross-linking tank) 17b, in order to pass through the third cross-linking bath (the third cross-linking liquid accommodated in the cross-linking tank) 17c, the fourth cross-linking bath (accommodated in the cross-linking tank) 17c, The fourth cross-linking liquid in the tank) 17d and the washing bath (the washing liquid contained in the washing tank) 19 are combined and finally passed through the drying furnace 21. The obtained polarizing film 23 can, for example, be directly transported to the next polarizing plate manufacturing step (the step of laminating a protective film on one or both sides of the polarizing film 23). The arrow in Figure 2 indicates the film conveyance direction.

In the description of Figure 1, "treatment tank" is a general term including swelling tank, dyeing tank, cross-linking tank and washing tank, and "processing liquid" is a general term including swelling liquid, dyeing liquid, cross-linking liquid and washing liquid, " "Processing bath" is a general term including swelling bath, dyeing bath, cross-linking bath and washing bath. The swelling bath, dyeing bath, cross-linking bath and washing bath respectively constitute the swelling part, dyeing part, cross-linking part and washing part in the manufacturing device of the present invention.

In addition to the above-mentioned treatment bath, the film conveying path of the polarizing film manufacturing device can also be conveyed by pressing or clamping the guide rollers 30 to 49 and 56 to 61 that can support the conveyed film or further change the film conveying direction. The nip rollers 50 to 55 are arranged at appropriate positions to impart driving force generated by the rotation of the film to the film, or to further change the film conveying direction. Guide rollers or nip rollers can be arranged before and after each treatment bath or in the treatment bath, so that the film can be introduced into the treatment bath and pulled out from the treatment bath [see Figure 1]. For example, the film can be immersed in each treatment bath by providing one or more guide rollers in each treatment bath and transporting the film along the guide rollers.

The polarizing film manufacturing apparatus shown in Fig. 1 has nip rollers (nip rollers 50 to 54) arranged before and after each treatment bath. This allows for any one or more treatment baths to be carried out between the nip rollers arranged before and after the treatment bath. Stretching between rollers by providing a circumferential speed difference to perform longitudinal uniaxial stretching.

In the polarizing film manufacturing device shown in Figure 1, there are four cross-linking baths, that is, n is 4. Each step is explained below.

(swelling step)

The swelling step is performed for the purpose of removing foreign matter on the surface of the original film 10, removing the plasticizer in the original film 10, imparting easy dyeing, and plasticizing the original film 10. The processing conditions are determined within a range that can achieve the purpose and does not cause adverse conditions such as extreme dissolution or devitrification of the original film 10 .

Referring to FIG. 1 , the swelling step can be performed by continuously rolling out the original film 10 from the embryonic film roll 11 while conveying it along the film transport path, immersing the original film 10 in the swelling bath 13 for a specific time, and then pulling it out. And implement. In the example of FIG. 1 , from unwinding the original film 10 to being immersed in the swelling bath 13 , the original film 10 is conveyed along the film conveyance path constructed by the guide rollers 60 and 61 and the nip roller 50 . In the swelling process, the film is transported along the film transport path constructed by the guide rollers 30 to 32 and the nip roller 51 .

As the swelling liquid of the swelling bath 13, in addition to pure water, boric acid (Japanese Patent Application Laid-Open No. 10-153709) or chloride (Japanese Patent Application Laid-Open No. 1009) or chloride (Japanese Patent Application Laid-Open No. 1999) may be used, in addition to pure water, in the range of about 0.01 to 10% by weight. Publication No. 06-281816), aqueous solutions of inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc.

The temperature of the swelling bath 13 is, for example, about 10 to 50°C, preferably about 10 to 40°C, more preferably about 15 to 30°C. The immersion time of the original film 10 is preferably about 10 to 300 seconds, more preferably about 20 to 200 seconds. In addition, when the original film 10 is a polyvinyl alcohol-based resin film that has been stretched in a gas beforehand, the temperature of the swelling bath 13 is, for example, about 20 to 70°C, preferably about 30 to 60°C. The immersion time of the original film 10 is preferably about 30 to 300 seconds, more preferably about 60 to 240 seconds.

During the swelling treatment, problems such as swelling of the raw film 10 in the width direction and wrinkles in the film are likely to occur. As a means for conveying the film while obtaining the wrinkles, the guide rollers 30, 31 and/or 32 can be used to use rollers with a widening function such as expansion rollers, spiral rollers, and crown rollers, or use of transverse rollers such as Guides, bending rods, tenter clamps and other expansion devices. Another method used to suppress the occurrence of wrinkles is to implement Apply extension treatment. For example, the difference in peripheral speed between the nip roller 50 and the nip roller 51 can be used to perform the uniaxial stretching process in the swelling bath 13 .

During the swelling treatment, since the film also swells and expands in the conveying direction of the film, when the film is not actively stretched, in order to eliminate the relaxation of the film in the conveying direction, for example, it is better to use a control device arranged in the swelling bath 13 Methods such as the speed of the front and rear nip rollers 50 and 51. In addition, in order to stabilize the film transport in the swelling bath 13, a water shower is used to control the water flow in the swelling bath 13, or an EPC device (Edge Position) is used.

Control device: a device that detects the end of the film and prevents the film from snaking) is also useful.

In the example shown in FIG. 1 , the film pulled out from the swelling bath 13 is introduced into the dyeing bath 15 through the guide roller 32 , the nip roller 51 , and the guide roller 33 in sequence.

(staining step)

The dyeing step is performed for the purpose of adsorbing and aligning the dichroic pigment to the polyvinyl alcohol-based resin film after the swelling process. The treatment conditions are determined within a range that can achieve the purpose and within a range that does not cause adverse events such as extreme dissolution or devitrification of the film. Referring to Figure 1, the dyeing step can be carried out in the following manner: conveying along the film conveying path constructed by the nip roller 51, the guide rollers 33 to 36 and the nip roller 52, and placing the swollen film in the dyeing bath 15 (accommodated in the dyeing bath). It is immersed in the treatment liquid of the tank for a specific time and then pulled out. In order to improve the dyeability of the dichroic dye, the film used in the dyeing step is preferably a film that has been subjected to at least a certain degree of uniaxial stretching treatment, or it is preferable to replace the uniaxial stretching treatment before the dyeing treatment, or in addition to the dyeing In addition to the uniaxial stretching treatment before treatment, uniaxial stretching treatment is performed during the dyeing process.

When iodine is used as the dichroic pigment, the dyeing solution of the dyeing bath 15 may use, for example, an aqueous solution with a weight ratio of iodine/potassium iodide/water=0.003~0.3/0.1~10/100. Other iodides such as zinc iodide may be used instead of potassium iodide, or potassium iodide and other iodides may be used in combination. Also, iodine can also be used Compounds other than compounds, such as boric acid, zinc chloride, cobalt chloride, etc., coexist. When the dyeing solution of the dyeing bath contains boric acid, it is different from the cross-linking bath in that it contains iodine at a concentration higher than that of boric acid. The concentration of iodine in the dyeing bath is preferably 0.6mM or more, more preferably 1.0mM or more, further preferably 2.0mM or more. The temperature of the dyeing bath 15 when dipping the film is usually around 10~45°C, preferably 10~40°C, more preferably 20~35°C, and the immersion time of the film is usually around 30~600 seconds, preferably 60~ 300 seconds.

When a water-soluble dichroic dye is used as the dichroic pigment, the dyeing solution of the dyeing bath 15 may use, for example, an aqueous solution with a concentration by weight ratio of dichroic dye/water=0.001~0.1/100. In the dyeing bath 15 , dyeing auxiliaries and the like may coexist, and for example, inorganic salts such as sodium sulfate or surfactants may be included. Only one type of dichroic dye may be used alone, or two or more types of dichroic dyes may be used in combination. The temperature of the dyeing bath 15 when the film is immersed is, for example, about 20 to 80°C, preferably 30 to 70°C. The immersion time of the film is usually about 30 to 600 seconds, preferably about 60 to 300 seconds.

As mentioned above, in the dyeing step, the dyeing bath 15 can be used to perform uniaxial stretching of the film. The uniaxial stretching of the film can be performed by a method such as providing a peripheral speed difference between the nip rollers 51 and 52 arranged before and after the dyeing bath 15 .

In the dyeing treatment, similarly to the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles on the film, guide rollers 33, 34, 35 and/or 36 can be used such as expansion rollers, spiral rollers, and crown rollers. Rollers with a widening function, or other widening devices using transverse guides, bending rods, and tenter clamps. Another means for suppressing the occurrence of wrinkles is to perform stretching treatment in the same manner as the swelling treatment.

In the example shown in FIG. 1 , the film pulled out from the dyeing bath 15 passes through the guide roller 36 , the nip roller 52 , and the guide roller 37 in order and is introduced into the first crosslinking bath 17 a .

(cross-linking step)

The cross-linking step is a treatment performed for the purpose of cross-linking the film. This treatment can improve water resistance or heat resistance and further adjust the hue (preventing the film from being bluish, etc.). In the cross-linking step, n (n is an integer of 4 or more) cross-linking baths are used. In the example shown in Figure 1, four cross-linking baths (n is 4) are arranged as the cross-linking baths for performing the cross-linking step. The first cross-linking bath 17a, the second cross-linking bath 17b and the third cross-linking bath are In 17c, a cross-linking process is performed for the purpose of water resistance, and in the fourth cross-linking bath 17d, a cross-linking process is performed for the purpose of hue adjustment.

Referring to Figure 1 , the first cross-linking step can be implemented in the following manner: conveying along the film conveying path constructed by the nip roller 52, the guide rollers 37 to 40 and the nip roller 53a, and immersing the dyed film in the first cross-linking step. The joint bath 17a (the first cross-linking bath) is heated for a specific time and then pulled out. The second cross-linking step can be implemented by conveying the film along the film conveying path constructed by the nip roller 53a, the guide rollers 41 to 44, and the nip roller 53b, and immersing the film after the first cross-linking step in the second cross-linking step. Bath 17b (the second cross-linking bath) is heated for a specific time and then pulled out. The third cross-linking step can be implemented by conveying the film along the film conveying path constructed by the nip roller 53b, the guide rollers 45 to 48, and the nip roller 53c, and immersing the film after the second cross-linking step in the third cross-linking step. Bath 17c (the third cross-linking bath) is set for a specific time, and then pulled out. The fourth cross-linking step can be implemented by conveying the film along the film conveying path constructed by the nip roller 53c, the guide rollers 49a to 49d, and the nip roller 53d, and immersing the film after the third cross-linking step in the fourth cross-linking step. Bath 17d (the fourth cross-linking bath) for a specific time, and then pull out.

When the dichroic dye used in the dyeing process is iodine, the cross-linking bath preferably contains iodide in addition to the boron compound. The concentration of iodide in the cross-linking bath can be, for example, 1 to 30% by mass. . Examples of the iodide include potassium iodide, zinc iodide, and the like. In addition, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate, etc., may also coexist.

The difference in peripheral speed between the nip roller 52 and the nip roller 53a can be used to perform the uniaxial stretching process in the first cross-linking bath 17a. The difference in circumferential speed between the nip roller 53a and the nip roller 53b can be used to perform single-step operation in the second cross-linking bath 17b. Shaft extension processing. The uniaxial stretching process can be performed in the third cross-linking bath 17c by utilizing the difference in peripheral speed between the nip roller 53b and the nip roller 53c. The uniaxial treatment can be performed in the fourth cross-linking bath 17d by utilizing the difference in peripheral speed between the nip roller 53c and the nip roller 53d.

In the cross-linking treatment, similarly to the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles on the film, guide rollers 38, 39, 40, 41, 42, 43, 44, 45, and 46 can be used. , 47, 48, 49a, 49b and/or 49d use rollers with a widening function such as expansion rollers, spiral rollers, crown rollers, or other widening devices such as transverse guides, bending rods, and tenter clamps. Another means for suppressing the occurrence of wrinkles is to perform stretching treatment in the same manner as the swelling treatment.

In the example shown in FIG. 1 , the film pulled out from the fourth crosslinking bath 17d is introduced into the washing bath 19 through the guide roller 49d and the nip roller 53d in order.

(washing step)

In the example shown in Figure 1, a washing step is included after the cross-linking step. The cleaning treatment is performed for the purpose of removing excess boric acid, iodine and other chemicals attached to the polyvinyl alcohol-based resin film. The washing step is performed, for example, by immersing the cross-linked polyvinyl alcohol-based resin film in the washing bath 19 . Furthermore, the cleaning step may be performed by spraying the membrane with a cleaning solution as a shower instead of immersing the membrane in the cleaning bath 19, or by combining immersing the membrane in the cleaning bath 19 with spraying the cleaning liquid. conduct.

FIG. 1 shows an example in which a polyvinyl alcohol-based resin film is immersed in a cleaning bath 19 and subjected to cleaning treatment. The temperature of the washing bath 19 in the cleaning process is usually about 2 to 40°C, and the immersion time of the membrane is usually about 2 to 120 seconds.

Furthermore, in the cleaning process, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles, tools such as expansion rollers, spiral rollers, and crown rollers may be used in the guide rollers 56, 57, 58, and/or 59. Rollers with a widening function, or other widening devices using transverse guides, bending rods, and tenter clamps. In addition, during the film cleaning process, in order to suppress the occurrence of wrinkles, a stretching process may be performed.

(Extended steps)

As mentioned above, the original film 10 is subjected to uniaxial stretching in a wet or dry manner between the above-mentioned series of processing steps (that is, before, after and/or during any one or more processing steps). A specific method of the uniaxial stretching treatment may be, for example, inter-roller stretching in which a circumferential speed difference is provided between two nip rollers (for example, two nip rollers arranged before and after the treatment bath) constituting the film conveyance path and longitudinal uniaxial stretching is performed; For example, hot roll stretching, tenter stretching, etc. described in Japanese Patent No. 2731813, inter-roller stretching is preferred. The uniaxial stretching step can be performed a plurality of times from the original film 10 to obtaining the polarizing film 23 . As mentioned above, stretching treatment is also helpful in suppressing the occurrence of wrinkles in the film.

(drying step)

After the washing step, it is preferable to perform a process of drying the polyvinyl alcohol-based resin film. Drying of the film is not particularly limited, and can be performed using a drying oven 21 as shown in FIG. 2 . The drying furnace 21 may be provided with a hot air dryer, for example. The drying temperature is, for example, about 30 to 100°C, and the drying time is, for example, about 30 to 600 seconds. The process of drying the polyvinyl alcohol-based resin film can also be performed using a far-infrared heater. The thickness of the polarizing film 23 obtained in the above manner is, for example, about 5 to 30 μm.

The obtained polarizing film can be wound up sequentially on a winding roller to form a roll, or it can be directly used in the polarizing plate production step (the step of layering a protective film on one side or both sides of the polarizing film) without being rolled up.

The final cumulative extension ratio of the polarizing film 23 based on the original film 10 is usually about 4.5 to 8 times, preferably 6 to 8 times. The extension step can be performed in any processing step. When the extension processing is performed in two or more processing steps, the extension processing can also be performed in any processing step. In order to make the final cumulative stretching magnification of the polarizing film 23 be 6 to 8 times, it is ideal to also perform the drying step. Extended processing. The stretching ratio in the drying step is, for example, preferably 1.05 times or more, more preferably 1.10 times or more. In the drying step, by stretching more than 1.05 times, the optical properties of the obtained polarizing film can be improved. According to the present invention, since the tension generated in the cross-linking step is relaxed, it is possible to extend the film by 1.05 times or more without cutting it in the drying step.

(Other processing steps for polyvinyl alcohol-based resin films)

Processing other than the above-mentioned processing may also be added. Examples of additional treatments include immersion treatment (zinc treatment) in an aqueous solution that does not contain a boron compound and contains zinc chloride or the like.

<Polarizing film>

By using the above manufacturing method to produce a polarizing film, a polarizing film satisfying the following i) to iv is obtained:

i) The absolute value of the b value of the orthogonal hue is less than 0.9,

ii) The visual sensitivity correction unit penetration rate (Ty) is 42.0% or more,

iii) The visual sensitivity corrected polarization (Py) is 99.986% or above,

iv) The absorbance (A700) at a wavelength of 700 nm is 3.0 or more.

In the present invention, n cross-linking baths are used to perform the cross-linking step, and the temperature of the second and third cross-linking baths arranged from the upstream side is higher than that of the first cross-linking bath arranged from the upstream side. The temperature of the combined bath can be used to obtain the polarizing film of i) above, that is, a neutral hue polarizing film. The b value of the orthogonal hue is appropriately designed in combination with the hue of the color filter of the liquid crystal display device. However, by satisfying i) above, it can be set within the hue design range of the color filter of the general liquid crystal display device. .

The above-mentioned orthogonal hue means the hue of the light transmitted through the other side when light is irradiated from one side of the polarizing plate. The hue here can be expressed as a value and b value in the Lab colorimetric system, and measured using standard light. Furthermore, in the present invention, the actual measurement of the orthogonal hue of the polarizing film is carried out in a state where an adhesive layer is provided on one side of the polarizing film and the adhesive layer side is bonded to a glass plate. Lab color system such as JIS K5981: Hunter's brightness index L and hue a and b as recorded in "5.5 Accelerated Weathering Test" of 2006 "Synthetic Resin Powder Coating Film"

what is represented. As a concept similar to the Lab color system, there is the L*a*b* color system specified in JIS Z8781-4: 2013 "Color Measurement - Part 4: CIE1976 L*a*b* Space", but in the present invention Using Lab colorimetric system. The values of brightness index L and hue a and b are the three stimuli specified in JIS Z8722: 2009 "Measurement Method of Color - Reflected and Transmitted Object Color"

Based on the values X, Y and Z, it is calculated by the following formula.

L=10Y ^1/2

a=17.5(10.2XY)/Y ^1/2

b=7.0(Y-0.847Z)/Y ^1/2

In the Lab color system, the hue a value and b value can represent the position equivalent to the chroma. When the hue a value increases, the hue changes to the red series, and when the hue b value increases, the hue changes to the yellow series. In addition, the closer to 0, the closer to achromatic color.

Furthermore, in the present invention, the cross-linking step is performed by using n cross-linking baths, and the temperatures of the second and third cross-linking baths arranged from the upstream side are higher than the temperatures of the cross-linking baths arranged from the upstream side. Starting from the temperature of the first cross-linking bath, the extension can be performed more than 1.05 times in the drying step, and a polarizing film satisfying the excellent optical properties of ii) and iii) above can be obtained. The sensitivity-corrected monomer transmittance (Ty) and the sensitivity-corrected polarization degree (Py) of the polarizing film here are measured according to the description of the following examples.

The absorbance (A700) of the polarizing film at a wavelength of 700nm should be above 3.0. The larger the absorbance value is, the more it can suppress the degradation of optical characteristics in a high-temperature environment.

<Polarizing plate>

A polarizing plate can be obtained by laminating a protective film on at least one side of the polarizing film produced in the above manner via an adhesive. As a protective film, for example, a film containing an acetyl cellulose-based resin such as triacetyl cellulose or diacetyl cellulose; polyethylene terephthalate, polyethylene naphthalate, and polyterephthalate Films composed of polyester resins of butylene glycol; polycarbonate resin films, cycloolefin resin films; acrylic resin films; and films of chain olefin resins including polypropylene resins.

In order to improve the adhesion between the polarizing film and the protective film, the laminating surface of the polarizing film and/or protective film can also be subjected to corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. handle. Examples of the adhesive used for bonding the polarizing film and the protective film include active energy ray-curable adhesives such as ultraviolet curable adhesives, aqueous solutions of polyvinyl alcohol-based resins, or adhesives containing a cross-linking agent. Aqueous solutions, such as water-based adhesives such as urethane emulsion adhesives. The ultraviolet curable adhesive may be a mixture of an acrylic compound and a photoradical polymerization initiator, or a mixture of an epoxy compound and a photocationic polymerization initiator. Alternatively, a cationically polymerizable epoxy compound and a radically polymerizable acrylic compound may be used together, and a photocationic polymerization initiator and a photoradical polymerization initiator may be used as initiators.

[Example]

Hereinafter, the present invention will be explained more specifically by showing test examples, but the present invention is not limited to these examples.

<Example 1>

Using the manufacturing apparatus shown in FIG. 1 and the conditions shown in Table 1, a polarizing film was produced from a polyvinyl alcohol-based resin film. Specifically, a long polyvinyl alcohol (PVA) embryonic membrane with a thickness of 45 μm [trade name "TS45" manufactured by Kuraray Co., Ltd., average degree of polymerization 2400, saponification degree 99.9 mol% or more] was rolled out from the roll. While being continuously transported, it is immersed in a swelling bath composed of 30°C water (deionized water) for 1 minute and 20 seconds to extend the time. Stretch 1.56 times (swelling step). Thereafter, the film taken out from the swelling bath was immersed in a 30° C. dyeing bath containing 1.0 mass % potassium iodide and 3.6 mM iodine for 100 seconds to extend 1.15 times (dying step). Next, the membrane taken out from the dyeing bath was immersed in a first cross-linking bath (the first cross-linking bath from the upstream side) at 59° C. containing 9.0 mass % potassium iodide and 3.0 mass % boric acid for 37 seconds to extend 1.46 times ( 1st cross-linking step). Next, it was immersed in a 63°C second cross-linking bath (the second cross-linking bath from the upstream side) containing 9.0 mass % potassium iodide and 3.0 mass % boric acid for 6 seconds to extend 1.70 times (second cross-linking step ). Then, it was immersed in a 63°C third cross-linking bath (the third cross-linking bath from the upstream side) containing 9.0 mass % potassium iodide and 3.0 mass % boric acid for 6 seconds to extend 1.34 times (third cross-linking steps). Thereafter, it was immersed in a 4th cross-linking bath (the 4th cross-linking bath from the upstream side) at 57°C containing 10.0 mass % potassium iodide and 3.0 mass % boric acid for 7 seconds to extend 1.00 times (4th cross-linking step) . By passing through the swelling step, the dyeing step and the cross-linking step, and stretching between rollers in the bath, the total stretch ratio based on the original film was set to 6.0 times.

Thereafter, the film was immersed in a washing bath at 13° C. for 2 seconds to remove foreign matter adhering to the surface of the film (washing step). After the washing step, the film was dried in a drying oven at a temperature of 90° C. for 300 seconds and extended 1.039 times (drying step). In this way, a polarizing film is obtained. The thickness of the obtained polarizing film was 18 μm.

<Examples 2, 3, Comparative Example 1>

The iodine concentration of the dyeing bath, the boric acid concentration, the temperature of the second cross-linking bath, the iodine addition amount of the second cross-linking bath, and the extension ratio in the drying step were as shown in Table 1. Other than this, the A polarizing film was obtained under the same conditions as Example 1. The thickness of the polarizing films obtained was all 18 μm.

[Evaluation of polarizing film]

(a) Measurement of monomer transmittance and polarization degree

For the obtained polarizing film, the MD transmittance and TD transmittance in the wavelength range of 380 to 780 nm were measured using a spectrophotometer with an integrating sphere ["V7100" manufactured by JASCO Corporation], and the following formulas are as follows:

Monomer penetration rate (%)=(MD TD)/2

Polarization degree (%)={(MD-TD)/(MD TD)}×100

Calculate the monomer transmittance and polarization degree at each wavelength.

The so-called "MD transmittance" refers to the transmittance when the direction of the polarized light emitted from the Glan-Thomson lens is parallel to the transmission axis of the polarizing film sample, and is expressed as "MD" in the above formula. In addition, the so-called "TD transmittance" refers to the transmittance when the direction of the polarized light emitted from the Glan-Thomson lens is orthogonal to the transmission axis of the polarizing film sample, and is expressed as "TD" in the above formula. ”. For the obtained monomer transmittance and polarization, the visual sensitivity was measured using the 2-degree visual field (C light source) of JIS Z8701: 1999 "Color Display Method-XYZ Color System and X ₁₀ Y ₁₀ Z ₁₀ Color System" Correction is performed to obtain the visual sensitivity corrected single transmittance (Ty) and the visual sensitivity corrected polarization (Py). Table 1 shows the calculation results of the visual sensitivity corrected single unit transmittance (Ty) and the visual sensitivity corrected polarization degree (Py).

(b) Orthogonal hue b value

For the obtained polarizing film, the b value of the orthogonal hue was calculated according to the above method. Table 1 shows the calculation results of the b value of the orthogonal hue.

(c) Determination of A700

The absorbance (A700) at a wavelength of 700nm can be calculated from the orthogonal transmittance (Tc) of the polarizing film. The MD transmittance and TD transmittance of the polarizing film can be measured and calculated by the following formula.

Tc(700nm)(%)=[MD(700nm)/100]×[TD(700nm)/100]×100

A700=-log[Tc(700nm)/100]

Table 1 shows the measured values of A700.

[Table 1]

In Comparative Example 1, the temperature of the second cross-linking bath is lower than the temperature of the first cross-linking bath. From the results shown in Table 1, it can be seen that compared with the polarizing films obtained in Examples 1 to 3, the polarizing film obtained in Comparative Example 1 has a large absolute value of the orthogonal hue b value and a low degree of polarization.

10: Raw material film composed of polyvinyl alcohol resin

11: Raw film roll

13: Swelling bath

15:Dyeing bath

17a: 1st cross-linking bath

17b: 2nd cross-linking bath

17c: The third cross-linking bath

17d: The 4th cross-linking bath

19: Washing bath

21: Drying oven

23:Polarizing film

30~49, 56~61: Guide roller

50~52, 53a, 53b, 53c, 53d, 54, 55: pinch roller

Claims (4)

A method for manufacturing a polarizing film, which manufactures a polarizing film from a polyvinyl alcohol resin film, including The dyeing step of dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment, and A cross-linking step in which the polyvinyl alcohol-based resin film is sequentially immersed in n (n is an integer of 4 or more) cross-linking baths after the dyeing step to perform a cross-linking treatment, Among them, the temperature of the second and third cross-linking baths arranged from the upstream side is higher than the temperature of the first cross-linking bath arranged from the upstream side. The method for manufacturing a polarizing film according to claim 1, wherein the concentration of the boron compound in each cross-linking bath is 2.5% by mass or more. The method for manufacturing a polarizing film according to claim 1 or 2, wherein the temperature of each cross-linking bath is below 65°C. A polarizing film manufacturing device, which manufactures polarizing films from polyvinyl alcohol resin films, including The dyed portion of the polyvinyl alcohol-based resin film dyed with a dichroic dye, and The cross-linked portion of the polyvinyl alcohol-based resin film after the dyeing treatment is sequentially immersed in n cross-linking baths (n is an integer of 4 or more) to perform cross-linking treatment, Among them, the temperature of the second and third cross-linking baths arranged from the upstream side is higher than the temperature of the first cross-linking bath arranged from the upstream side.

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