JP7624852B2 - Manufacturing method of polarizing film - Google Patents

Description

The present invention relates to a method for producing a polarizing film that can be used as a component of a polarizing plate.

Conventionally, polarizing films have been made by adsorbing and aligning dichroic pigments such as iodine or dichroic dyes onto a uniaxially stretched polyvinyl alcohol resin film. Polarizing films are usually made into polarizing plates by laminating a protective film to one or both sides of the film using an adhesive, and are used in image display devices such as liquid crystal display devices for liquid crystal televisions, personal computer monitors, and mobile phones. In recent years, as liquid crystal display devices have become thinner, there has been a demand for thinner polarizing films.

Polarizing films are generally produced by subjecting a continuously transported long polyvinyl alcohol resin film to various processes, such as swelling, dyeing, crosslinking, and stretching, in a bath, followed by a washing process and drying (see, for example, JP 2012-47799 A).

JP 2012-477999 A

Since the polyvinyl alcohol resin film subjected to the drying process has been stretched, the removal of moisture during the drying process generates a shrinkage force, which tends to reduce the dimensional stability of the resulting polarizing film.

Patent Document 1 describes that a polarizer having excellent dimensional stability in a high-temperature environment can be obtained by manufacturing the polarizer through a first drying process in which the film is dried at a heating temperature of 25°C or more and less than 65°C, and a second drying process in which the film is dried at a heating temperature of 65°C or more and 115°C or less.

The present invention aims to provide a method for producing a polarizing film that can produce a polarizing film with reduced shrinkage force.

The present invention provides the following method for producing a polarizing film.
[1] A first drying step of drying the polyvinyl alcohol-based resin film after contact with the treatment liquid at a first temperature of 20° C. or lower;
a second drying step of drying the polyvinyl alcohol-based resin film at a second temperature different from the first temperature after the first drying step.
[2] The method for producing a polarizing film according to [1], wherein the second temperature is 95° C. or higher.
[3] The method for producing a polarized film according to [1] or [2], wherein in the first drying step, a gas having a wind speed of 1.5 m/s or more is applied to the polyvinyl alcohol-based resin film.
[4] The method for producing a polarized film according to any one of [1] to [3], further comprising a third drying step between the first drying step and the second drying step, in which the polyvinyl alcohol-based resin film is dried at a temperature equal to or higher than the first temperature and equal to or lower than the second temperature.

The method of the present invention makes it possible to produce a polarizing film with reduced shrinkage force.

1 is a cross-sectional view that illustrates an example of a polarizing film manufacturing method and a polarizing film manufacturing apparatus used in the method according to the present invention.

<Method of manufacturing polarizing film>
In the present invention, the polarizing film is a uniaxially stretched polyvinyl alcohol resin film in which a dichroic pigment (iodine or a dichroic dye) is adsorbed and aligned. The polyvinyl alcohol resin constituting the polyvinyl alcohol resin film is usually obtained by saponifying a polyvinyl acetate 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, polyvinyl acetate, which is a homopolymer of vinyl acetate, or a copolymer of vinyl acetate and other monomers copolymerizable therewith. Examples of the other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol resin is usually 1,000 to 10,000, and preferably 1,500 to 5,000.

These polyvinyl alcohol resins may be modified, for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc., modified with aldehydes may also be used.

In the present invention, an unstretched polyvinyl alcohol-based resin film (raw film) having a thickness of 65 μm or less (for example, 60 μm or less), preferably 50 μm or less, is used as the starting material for manufacturing the polarizing film. This makes it possible to obtain a thin polarizing film, for which market demand is increasing. The thinner the raw film, the more likely it is that the film will break during the stretching process. However, according to the present invention, even if the raw film is thin, film breakage can be effectively suppressed. The raw film may be a polyvinyl alcohol-based resin film that has been stretched in advance in the gas phase.

There are no particular restrictions on the width of the raw film, and it can be, for example, 300 to 6,000 mm, but the wider the film, the more likely it is to break during the stretching process.

In the present invention, the raw film is prepared as a roll (raw roll) of a long unstretched polyvinyl alcohol-based resin film.

The polarizing film can be continuously manufactured as a long polarizing film by carrying out a processing step in which the long raw film is unwound from the raw film roll and continuously transported along the film transport path of the polarizing film manufacturing device to contact the film with a processing liquid, and then carrying out a drying step.

The above-mentioned processing steps may include, for example, a swelling processing step in which the raw film is immersed in a swelling bath and then pulled out, a dyeing processing step in which the film after the swelling processing is immersed in a dyeing bath and then pulled out, and a crosslinking processing step in which the film after the dyeing processing is immersed in a crosslinking bath and then pulled out. In addition, between these series of processing steps (i.e., before or after any one or more of the processing steps and/or during any one or more of the processing steps), a uniaxial stretching process is performed in a wet or dry manner. If necessary, other processing steps may be added. Each of the above processing steps may be a process in which the film is immersed in one bath, or a process in which the film is immersed in two or more baths in sequence.

The drying process includes at least a first drying process in which drying is performed at a first temperature, and a second drying process in which drying is performed at a second temperature, in this order. The first temperature is 20°C or lower. The second temperature is a temperature different from the first temperature. In the present invention, by providing such a drying process, a polarizing film with suppressed shrinkage force can be manufactured.

The drying process is a process for drying the polyvinyl alcohol-based resin film to obtain a polarizing film, and can obtain a polarizing film with a moisture content of, for example, less than 15% by weight.

The method for producing a polarizing film according to the present invention will be described in more detail below with reference to FIG. 1. FIG. 1 is a cross-sectional view that shows a schematic example of a method for producing a polarizing film according to the present invention and a polarizing film production apparatus used therein. The polarizing film production apparatus shown in FIG. 1 is configured to convey an original (unstretched) film 10 made of a polyvinyl alcohol resin along a film conveying path while continuously unwinding it from an original roll 11, so that the film passes through a swelling bath 13, a dyeing bath 15, a crosslinking bath 17, and a cleaning bath 19 provided on the film conveying path in sequence, then through a first drying oven 70, and finally through a second drying oven 90. The obtained polarizing film can be conveyed directly to the next polarizing plate production process (a process of laminating a protective film on one or both sides of the polarizing film 23). The arrow in FIG. 1 indicates the conveying direction of the film.

Although FIG. 1 shows an example in which one tank each of the swelling bath 13, dyeing bath 15, crosslinking bath 17, and cleaning bath 19 is provided, two or more tanks of one or more treatment baths (baths containing treatment liquids for treating a polyvinyl alcohol-based resin film provided on the film transport path, such as the swelling bath 13, dyeing bath 15, crosslinking bath 17, and cleaning bath 19, are collectively referred to as "treatment baths") may be provided as necessary.

The film transport path of the polarizing film manufacturing device can be constructed by arranging in appropriate positions, in addition to the above-mentioned treatment bath, first drying oven 70, and second drying oven 90, guide rolls 30-43, 60, and 61 that support the film being transported or can further change the film transport direction, and nip rolls 50-54, 82, and 83 that press and clamp the film being transported and can apply a driving force to the film by rotating it or can further change the film transport direction. The guide rolls and nip rolls can be arranged before and after each treatment bath and each drying oven or in the treatment bath, which allows the film to be introduced into and immersed in the treatment bath and withdrawn from the treatment bath (see Figure 1). For example, one or more guide rolls can be provided in each treatment bath, and the film can be immersed in each treatment bath by transporting the film along these guide rolls.

The polarizing film manufacturing apparatus shown in Figure 1 has nip rolls arranged before and after each treatment bath (nip rolls 50 to 54), which makes it possible to carry out inter-roll stretching in one or more treatment baths, in which longitudinal uniaxial stretching is performed by applying a peripheral speed difference between the nip rolls arranged before and after the treatment bath. Each process will be described below.

(Swelling treatment step)
The swelling treatment step is carried out for the purposes of removing foreign matter on the surface of the raw film 10, removing plasticizers in the raw film 10, imparting ease of dyeing, plasticizing the raw film 10, etc. The treatment conditions are determined within a range in which the purposes can be achieved and in which defects such as extreme dissolution or devitrification of the raw film 10 do not occur.

Referring to FIG. 1, the swelling process can be carried out by continuously unwinding the original film 10 from the original roll 11 while transporting it along the film transport path, immersing the original film 10 in the swelling bath 13 for a predetermined time, and then pulling it out. In the example of FIG. 1, from the time the original film 10 is unwound until it is immersed in the swelling bath 13, the original film 10 is transported along the film transport path established by the guide rolls 60, 61 and the nip roll 50. In the swelling process, the original film 10 is transported along the film transport path established by the guide rolls 30 to 32.

As the swelling liquid for the swelling bath 13, in addition to pure water, it is also possible to use an aqueous solution containing boric acid (JP Patent Publication No. 10-153709), chlorides (JP Patent Publication No. 06-281816), inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc., in an amount of 0.01 to 10% by weight.

The temperature of the swelling bath 13 is, for example, 10 to 50°C, preferably 10 to 40°C, and more preferably 15 to 30°C. The immersion time of the raw film 10 is preferably 10 to 300 seconds, and more preferably 20 to 200 seconds. Furthermore, when the raw film 10 is a polyvinyl alcohol resin film that has been stretched in advance in a gas, the temperature of the swelling bath 13 is, for example, 20 to 70°C, and preferably 30 to 60°C. The immersion time of the raw film 10 is preferably 30 to 300 seconds, and more preferably 60 to 240 seconds.

In the swelling process, the original film 10 tends to swell in the width direction, which can cause problems such as wrinkles in the film. One method for transporting the film while removing these wrinkles is to use rolls with a width-expanding function, such as expander rolls, spiral rolls, and crown rolls, as the guide rolls 30, 31, and/or 32, or to use other width-expanding devices such as cross guiders, bend bars, and tenter clips. Another method for suppressing the occurrence of wrinkles is to perform a stretching process. For example, a uniaxial stretching process can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between the nip rolls 50 and 51.

During the swelling process, the film also swells and expands in the film transport direction. Therefore, if the film is not actively stretched, it is preferable to take measures such as controlling the speed of the nip rolls 50, 51 placed before and after the swelling bath 13 to eliminate slack in the film in the transport direction. In addition, in order to stabilize the film transport in the swelling bath 13, it is also useful to control the water flow in the swelling bath 13 with an underwater shower or to use an EPC device (Edge Position Control device: a device that detects the edge of the film and prevents the film from meandering).

In the example shown in FIG. 1, the film pulled out from the swelling bath 13 passes through the guide roll 32 and the nip roll 51 in that order and is introduced into the dye bath 15.

(Dyeing process)
The dyeing process is carried out for the purpose of adsorbing and orienting a dichroic dye on the polyvinyl alcohol-based resin film after the swelling process. The treatment conditions are determined within a range in which the purpose can be achieved and in which defects such as extreme dissolution or devitrification of the film do not occur. With reference to FIG. 1, the dyeing process can be carried out by conveying the film along a film conveying path constructed by guide rolls 33 to 35 and nip rolls 51, immersing the film after the swelling process in a dye bath 15 (a treatment liquid contained in a dyeing tank) for a predetermined time, and then pulling it out. In order to enhance the dyeability of the dichroic dye, it is preferable that the film to be subjected to the dyeing process is a film that has been subjected to at least some degree of uniaxial stretching, or it is preferable to perform uniaxial stretching during the dyeing process instead of or in addition to the uniaxial stretching process before the dyeing process.

When iodine is used as the dichroic dye, the dyeing solution of the dye bath 15 may be, for example, an aqueous solution having a mass 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 may be used in combination with other iodides. Compounds other than iodides, such as boric acid, zinc chloride, cobalt chloride, etc., may also be allowed to coexist. When boric acid is added, it is distinguished from the crosslinking treatment described below in that it contains iodine, and any aqueous solution containing 0.003 parts by mass or more of iodine per 100 parts by mass of water can be considered as a dye bath 15. The temperature of the dye bath 15 when the film is immersed is usually 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C, and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

When a water-soluble dichroic dye is used as the dichroic pigment, the dyeing solution of the dye bath 15 may be, for example, an aqueous solution having a mass ratio of dichroic dye/water = 0.001 to 0.1/100. This dye bath 15 may contain a dyeing assistant, for example, an inorganic salt such as sodium sulfate or a surfactant. 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 dye bath 15 when the film is immersed is, for example, 20 to 80°C, preferably 30 to 70°C, and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

As described above, in the dyeing process, the film can be uniaxially stretched in the dye bath 15. The film can be uniaxially stretched by, for example, creating a difference in peripheral speed between the nip rolls 51 and 52 arranged before and after the dye bath 15.

In the dyeing process, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, as in the swelling process, the guide rolls 33, 34 and/or 35 may be rolls with a width-expanding function, such as expander rolls, spiral rolls, or crown rolls, or other width-expanding devices, such as cross guiders, bend bars, or tenter clips. Another means for suppressing the occurrence of wrinkles is to carry out a stretching process, as in the swelling process.

In the example shown in FIG. 1, the film pulled out from the dye bath 15 passes through the guide roll 35 and the nip roll 52 in order and is introduced into the crosslinking bath 17.

(Crosslinking Treatment Step)
The crosslinking process is a process carried out for the purpose of imparting water resistance through crosslinking, adjusting the hue (preventing the film from becoming bluish, etc.), etc. Referring to Fig. 1, the crosslinking process can be carried out by conveying the film along a film conveying path constructed by guide rolls 36 to 38 and nip rolls 52, immersing the dyed film in a crosslinking bath 17 (a crosslinking liquid contained in a crosslinking tank) for a predetermined time, and then pulling it out.

The crosslinking liquid of the crosslinking bath 17 can be an aqueous solution containing, for example, 1 to 10 parts by mass of boric acid per 100 parts by mass of water. When the dichroic dye used in the dyeing process is iodine, the crosslinking liquid preferably contains an iodide in addition to boric acid, and the amount can be, for example, 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodides include potassium iodide and zinc iodide. Compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present.

In the crosslinking treatment, the concentrations of boric acid and iodide, and the temperature of the crosslinking bath 17 can be appropriately changed depending on the purpose. For example, when the purpose of the crosslinking treatment is to make the film water-resistant by crosslinking, and the swelling treatment, dyeing treatment, and crosslinking treatment are performed on the polyvinyl alcohol resin film in this order, the crosslinking agent-containing liquid of the crosslinking bath can be an aqueous solution having a concentration of boric acid/iodide/water = 3 to 10/1 to 20/100 by mass ratio. If necessary, other crosslinking agents such as glyoxal or glutaraldehyde may be used instead of boric acid, or boric acid may be used in combination with other crosslinking agents. The temperature of the crosslinking bath when the film is immersed is usually 50 to 70 ° C, preferably 53 to 65 ° C, and the immersion time of the film is usually 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 200 seconds. Furthermore, when dyeing and crosslinking are performed in this order on a polyvinyl alcohol resin film that has been stretched before the swelling treatment, the temperature of the crosslinking bath 17 is usually 50 to 85°C, preferably 55 to 80°C.

In a crosslinking treatment aimed at adjusting the hue, for example, when iodine is used as the dichroic dye, a crosslinking agent-containing liquid with a mass ratio of boric acid/iodide/water = 1-5/3-30/100 can be used. The temperature of the crosslinking bath when the film is immersed is usually 10-45°C, and the immersion time of the film is usually 1-300 seconds, preferably 2-100 seconds.

The crosslinking treatment may be carried out multiple times, usually 2 to 5 times. In this case, the composition and temperature of each crosslinking bath used may be the same or different as long as they are within the above range. The crosslinking treatment for water resistance by crosslinking and the crosslinking treatment for color adjustment may each be carried out in multiple steps.

The difference in peripheral speed between nip roll 52 and nip roll 53 can be used to perform uniaxial stretching in crosslinking bath 17.

In the crosslinking process, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, as in the swelling process, the guide rolls 36, 37 and/or 38 may be rolls with a width-expanding function, such as expander rolls, spiral rolls, or crown rolls, or other width-expanding devices, such as cross guiders, bend bars, or tenter clips. Another means for suppressing the occurrence of wrinkles is to carry out a stretching process, as in the swelling process.

In the example shown in FIG. 1, the film pulled out from the crosslinking bath 17 passes through a guide roll 38 and a nip roll 53 in that order before being introduced into the film washing bath 19.

(Cleaning process)
The manufacturing method of the present invention may include a cleaning step after the crosslinking step. The cleaning step is performed for the purpose of removing excess chemicals such as boric acid and iodine attached to the polyvinyl alcohol-based resin film. The cleaning step may be performed, for example, by immersing the crosslinked polyvinyl alcohol-based resin film in a film cleaning bath 19, spraying the film with a film cleaning solution as a shower, or using both.

Figure 1 shows an example of a case where a polyvinyl alcohol resin film is immersed in a cleaning bath 19 for cleaning treatment. The temperature of the film cleaning bath 19 in the film cleaning treatment is usually 2 to 40°C, and the immersion time of the film is usually 2 to 120 seconds.

In addition, in the cleaning process, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles, the guide rolls 39, 40 and/or 41 may be rolls with a width-expanding function, such as expander rolls, spiral rolls, and crown rolls, or other width-expanding devices, such as cross guiders, bend bars, and tenter clips. In addition, in the film cleaning process, a stretching process may be performed to suppress the occurrence of wrinkles.

(Stretching process)
As described above, the raw film 10 is uniaxially stretched in a wet or dry manner during the above series of processing steps (i.e., before and after any one or more processing steps and/or during any one or more processing steps). Specific methods of uniaxial stretching include, for example, roll-to-roll stretching in which vertical uniaxial stretching is performed with a difference in peripheral speed between two nip rolls (e.g., two nip rolls arranged before and after a processing bath) constituting a film transport path, hot roll stretching as described in Japanese Patent No. 2731813, tenter stretching, etc., and preferably roll-to-roll stretching. The uniaxial stretching process can be performed multiple times from the raw film 10 to the polarizing film 23. As described above, the stretching process is also advantageous in suppressing the occurrence of wrinkles in the film.

The final cumulative stretch ratio of the polarizing film 23 based on the original film 10 is typically 4.5 to 7.0 times, and preferably 5.0 to 6.5 times.

The stretching process may be performed in any of the above-mentioned processing steps as long as it is performed before the drying process, and even if the stretching process is performed in two or more processing steps, the stretching process may be performed in any of the processing steps.

(First drying step)
After the cleaning process, a first drying process is performed in which the polyvinyl alcohol-based resin film is dried at a first temperature. Examples of the drying process include a method of introducing the film into a drying oven and a method of contacting the film with a heated roll. The first temperature in the first drying process is 20° C. or lower. In the case of a method of introducing the film into a drying oven and drying it, the first temperature refers to the temperature in the drying oven, and in the case of a method of contacting the film with a heated roll, the first temperature refers to the surface temperature of the heated roll. The first temperature is preferably 19° C. or lower, for example, 5° C. or higher, preferably 10° C. or higher, and more preferably 15° C. or higher.

The moisture content of the polyvinyl alcohol-based resin film when introduced into the first drying process is preferably 30% by mass or more. The moisture content of the polyvinyl alcohol-based resin film when discharged from the first drying process is, for example, less than 30% by mass.

The first drying step is not limited as long as it is a step that can reduce the moisture content of the polyvinyl alcohol-based resin film. A specific example is a step of blowing a gas at 20°C or less onto the polyvinyl alcohol-based resin film. Examples of the gas include air and nitrogen. The wind speed of the gas blown onto the polyvinyl alcohol-based resin film is preferably 0.1 m/s or more, more preferably 0.5 m/s or more, and even more preferably 1.5 m/s or more, for example, 5 m/s or less.

In the example shown in FIG. 1, the first drying step is a process in which the polyvinyl alcohol-based resin film is passed through a first drying oven 70 into which gas at 20° C. or less is blown. A blower 71 is provided in the first drying oven 70 to blow temperature-adjusted air toward the polyvinyl alcohol-based resin film being transported. From the viewpoint of reducing damage caused by wind pressure to the polyvinyl alcohol-based resin film, the angle between the blowing direction of the air blown from the blower 71 and the transport direction of the polyvinyl alcohol-based resin film is preferably 0° or more and 90° or less, and more preferably 0° or more and 60° or less.

The temperature of the air blown out from the blower 71 may be adjusted to be the same as the desired first temperature, and the difference from the desired first temperature is preferably 5°C or less, more preferably 3°C or less, and even more preferably 1°C or less. The temperature of the air blown out from the blower 71 may be the same as, lower than, or higher than the desired first temperature, but is preferably the same as or lower than the desired first temperature.

The duration of the first drying step is preferably 10 seconds or more, more preferably 30 seconds or more, for example, 100 seconds or less, preferably 50 seconds or less.

In the first drying step, the polyvinyl alcohol-based resin film is dried slowly, so that shrinkage is suppressed without applying a large load to the polyvinyl alcohol-based resin film, and a polarizing film with a small shrinkage force can be produced.

(Second drying step)
After the first drying step, a second drying step is performed in which the polyvinyl alcohol-based resin film is dried at a second temperature. The second temperature is not particularly limited as long as it is different from the first temperature, but is preferably a temperature higher than the first temperature. The second temperature refers to the temperature in the drying oven in the case of a method in which the film is introduced into a drying oven and dried, and refers to the surface temperature of the heated roll in the case of a method in which the film is brought into contact with a heated roll. The second temperature is, for example, 80°C or higher, preferably 85°C or higher, more preferably 90°C or higher, and even more preferably 95°C or higher. The second temperature is, for example, 120°C or lower.

The moisture content of the polyvinyl alcohol-based resin film when introduced into the second drying process is, for example, less than 30% by mass. In the second drying process, the moisture content is reduced from, for example, 15% by mass or more and less than 30% by mass to less than 15% by mass. The moisture content of the polyvinyl alcohol-based resin film when discharged from the second drying process is, for example, less than 15% by mass.

The second drying step is not limited as long as it is a step that can reduce the moisture content of the polyvinyl alcohol-based resin film. Examples of the drying method include a method using a hot air dryer, an infrared heater, or a combination of these. The time for the second drying step is preferably 10 seconds or more, more preferably 30 seconds or more, for example, 100 seconds or less, preferably 50 seconds or less.

By carrying out the first drying step and the second drying step as described above, a polarizing film with suppressed shrinkage force can be produced. The polarizing film 23 obtained in the above manner has a thickness of, for example, 5 to 30 μm. According to the method of the present invention, a polarizing film that simultaneously satisfies the following i) to iii) can be obtained.
i) Visibility-corrected single transmittance (Ty) is 42.0% or more;
ii) Visibility-corrected polarization degree (Py) is 99.970% or more;
iii) a contraction force of less than 2.73 N;
The luminous-corrected single transmittance (Ty), luminous-corrected polarization degree (Py), and shrinkage force are measured according to the description in the Examples section below.

(Third drying step)
After the first drying step and before the second drying step, a third drying step may be performed in which the polyvinyl alcohol-based resin film is dried at a third temperature that is equal to or higher than the first temperature and equal to or lower than the second temperature. The third drying step is a step of reducing the moisture content of the polyvinyl alcohol-based resin film. The third temperature is, for example, 15° C. or higher and 120° C. or lower.

When the third temperature is 20°C or lower, the drying method in the third drying step may be the same as that in the first drying step.

When the third temperature exceeds 20°C, the drying means in the third drying step is, for example, the same method as in the second drying step.

(Other treatment steps for polyvinyl alcohol-based resin film)
Treatments other than those described above may also be added. Examples of treatments that can be added include a treatment of immersion in an aqueous iodide solution not containing boric acid (complementary color treatment) and a treatment of immersion in an aqueous solution not containing boric acid but containing zinc chloride or the like (zinc treatment) performed after the crosslinking treatment step.

<Polarizing Plate>
A polarizing plate can be obtained by bonding a protective film to at least one side of the polarizing film produced as described above via an adhesive. Examples of the protective film include films made of acetylcellulose-based resins such as triacetylcellulose and diacetylcellulose; films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate-based resin films, cycloolefin-based resin films; acrylic-based resin films; and films made of linear olefin-based resins such as polypropylene-based resins.

In order to improve the adhesion between the polarizing film and the protective film, the bonding surface of the polarizing film and/or the protective film may be subjected to a surface treatment such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, or saponification treatment. Examples of adhesives used to bond the polarizing film and the protective film include active energy ray curable adhesives such as ultraviolet curable adhesives, aqueous solutions of polyvinyl alcohol resins or aqueous solutions containing crosslinking agents, and 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. It is also possible to use a combination of a cationic polymerizable epoxy compound and a radical polymerizable acrylic compound, and a combination of a photocationic polymerization initiator and a photoradical polymerization initiator as the initiator.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
1 , a polarizing film was produced using an apparatus further including a third drying oven between the first drying oven 70 and the second drying oven 90. Specifically, a long polyvinyl alcohol (PVA) raw film having a thickness of 45 μm [trade name "TS45" manufactured by Kuraray Co., Ltd., average polymerization degree 2400, saponification degree 99.9 mol % or more] was continuously transported while being unwound from a roll, and was immersed in a swelling bath of pure water at 25° C. for a residence time of 1 minute 20 seconds (swelling step).

Then, the film pulled out of the swelling bath was immersed in a dyeing bath 15 at 30°C containing 1.25 mmML of iodine, 1.25% by mass of potassium iodide, and 0.3% by mass of boric acid for a residence time of 2 minutes and 30 seconds (dyeing step). At this time, the film was stretched at a stretching ratio of 2.15 times and 1.56 times in the swelling step and dyeing step, respectively, and stretched so that the cumulative stretching ratio by the end of the dyeing step was 3.5 times. Next, the film pulled out of the dyeing bath was immersed in a first crosslinking bath at 59°C containing 8% by mass of potassium iodide and 4% by mass of boric acid for a residence time of 26 seconds, and stretched at a stretching ratio of 1.4 times while crosslinking (first crosslinking step). Next, the film was immersed in a second crosslinking bath at 59°C containing 8% by mass of potassium iodide and 4% by mass of boric acid for a residence time of 20 seconds, and stretched at a stretching ratio of 1.19 times while crosslinking (second crosslinking step).

Then, the film was immersed in a 43°C complementary bath containing 8% by mass of potassium iodide and 4% by mass of boric acid for a residence time of 10 seconds and stretched 1.00 times (complementary process). After the swelling process, dyeing process, first crosslinking process, second crosslinking process, and complementary process, the film was stretched so that the total stretch ratio based on the original film was 5.7 times.

The film removed from the complementary color bath was immersed in a cleaning bath of pure water at 13°C for a residence time of 2 seconds (cleaning step). The film removed from the cleaning bath was then passed through the first drying oven, the third drying oven, and the second drying oven in that order, and the first drying step, the second drying step, and the third drying step were carried out to obtain a polarizing film. The thickness of the obtained polarizing film was 12 μm.

In the first drying oven, a single fan blew air at a temperature of 18°C at a volume of 60 Hz and a velocity of 2.0-2.5 m/s to carry out the first drying process. In the third drying oven, a single fan blew air at a temperature of 18°C at a volume of 60 Hz and a velocity of 2.0-2.5 m/s to carry out the third drying process. In the second drying oven, a single fan blew air at a temperature of 100°C at a volume of 60 Hz and a velocity of 2.0-2.5 m/s to carry out the second drying process.

The polyvinyl alcohol resin film was controlled to remain in the drying oven for 36.6 seconds, 36.6 seconds, and 36.6 seconds in the first drying process, third drying process, and second drying process, respectively.

<Example 2 and Comparative Example 1>
The polarized films of Example 2 and Comparative Example 1 were produced in the same manner as in Example 1, except that the temperatures of the air blown from the fans in the first drying oven, the third drying oven, and the second drying oven were set to the temperatures shown in Table 1.

In Examples 1 and 2 and Comparative Example 1, it was confirmed that the first temperature in the first drying process can be considered to be the same as the temperature of the air blown from the fan in the first drying oven, the third temperature in the third drying process can be considered to be the same as the temperature of the air blown from the fan in the third drying oven, and the second temperature in the second drying process can be considered to be the same as the temperature of the air blown from the fan in the second drying oven.

[Evaluation of Polarizing Film]
(a) Measurement of Single-unit Transmittance and Degree of Polarization A measurement sample having a size of 4 cm x 4 cm was cut out from the polarizing film obtained in each Example and Comparative Example. The MD transmittance and TD transmittance of this sample were measured in the wavelength range of 380 to 780 nm using a spectrophotometer equipped with an integrating sphere ("V7100" manufactured by JASCO Corporation), and the following formula was used:
Single piece transmittance (%) = (MD + TD) / 2
Degree of polarization (%) = {(MD-TD)/(MD+TD)}×100
The single transmittance and the degree of polarization at each wavelength were calculated based on the above.

The "MD transmittance" is the transmittance when the direction of the polarized light coming out of the Glan-Thompson prism is parallel to the transmission axis of the polarized film sample, and is represented as "MD" in the above formula. The "TD transmittance" is the transmittance when the direction of the polarized light coming out of the Glan-Thompson prism is perpendicular to the transmission axis of the polarized film sample, and is represented as "TD" in the above formula. The obtained single transmittance and polarization degree were subjected to luminosity correction using a 2-degree visual field (C light source) according to _JIS Z ₈₇₀₁ :1999 "Method of displaying color-XYZ color system and _X10Y10Z10 color system" to obtain the luminosity-corrected single transmittance (Ty) and luminosity-corrected polarization degree (Py). Table 1 shows the calculation results of the luminosity-corrected single transmittance (Ty), luminosity-corrected polarization degree (Py), and the b value of the orthogonal hue.

(b) MD shrinkage force A measurement sample with a width of 2 mm and a length of 30 mm, with the absorption axis direction (MD, stretching direction) as the long side, was cut out from the polarizing film obtained in each Example and Comparative Example. This sample was set in an analyzer (DMAQ800, Dynamic mechanical analyzer) manufactured by TA Co., Ltd., and the shrinkage force (MD shrinkage force) in the long side direction (absorption axis direction, MD) generated when the sample was held at 80° C. for 4 hours while keeping the dimensions constant was measured. Table 1 shows the measured shrinkage force values.

10: Raw film made of polyvinyl alcohol resin; 11: Raw roll; 13: Swelling bath; 15: Dyeing bath; 17: Crosslinking bath; 19: Washing bath; 21: Drying furnace; 23: Polarizing film; 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 60, 61: Guide roll; 50, 51, 52, 53, 54, 82, 83: Nip roll; 70: First drying furnace; 71: Blower; 90: Drying furnace.

Claims (4)

a first drying step of drying the polyvinyl alcohol-based resin film after contact with the treatment liquid at a first temperature of 20° C. or lower;
A second drying step of drying the polyvinyl alcohol-based resin film at a second temperature different from the first temperature after the first drying step,
The method for producing a polarizing film , wherein the moisture content of the polyvinyl alcohol-based resin film when discharged from the first drying step is less than 30 mass % . The method for producing a polarizing film according to claim 1, wherein the second temperature is 95°C or higher. The method for producing a polarizing film according to claim 1 or 2, wherein in the first drying step, a gas having a wind speed of 1.5 m/s or more is applied to the polyvinyl alcohol-based resin film. The method for producing a polarizing film according to any one of claims 1 to 3, further comprising a third drying step between the first drying step and the second drying step, in which the polyvinyl alcohol-based resin film is dried at a temperature equal to or higher than the first temperature and equal to or lower than the second temperature.

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