JP7469893B2 - Polarizing film manufacturing method and polarizing film manufacturing device - Google Patents

Description

The present invention relates to a method and an apparatus for manufacturing a polarizing film.

Conventionally, polarizing films have been made by adsorbing and aligning a dichroic dye such as iodine onto a uniaxially stretched polyvinyl alcohol resin film. In general, polarizing films are manufactured by sequentially dyeing a polyvinyl alcohol resin film with a dichroic dye, crosslinking a polyvinyl alcohol resin film with a crosslinking agent, and drying the film, and by stretching the polyvinyl alcohol resin film during the manufacturing process.

In the dyeing and crosslinking processes used in the manufacture of polarizing films, the polyvinyl alcohol resin film is brought into contact with a treatment liquid to carry out each process (see, for example, Patent Documents 1 to 3, etc.).

Patent No. 4070000 Japanese Patent No. 6054054 JP 2019-35956 A

The above-mentioned treatment liquid may be replenished while performing each of the above-mentioned processes, since the treatment liquid and the chemicals in the treatment liquid are consumed during the production of the polarizing film. Since the treatment liquid usually contains chemicals and water, the treatment liquid may be replenished by adding chemicals or water to the treatment liquid being used in each treatment step. It has been found that when water is replenished to the crosslinking liquid used in the crosslinking treatment step, the concentration of iodine ions (I ₃ ⁻ ) in the crosslinking liquid increases. It is believed that an increase in the concentration of iodine ions in the crosslinking liquid may cause a decrease in the optical performance of the resulting polarizing film.

The present invention aims to provide a method and apparatus for manufacturing a polarizing film that can produce a polarizing film with good optical performance.

The present invention provides a method and an apparatus for producing a polarizing film as described below.
[1] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising the steps of:
A step of treating the polyvinyl alcohol-based resin film with a dyeing treatment solution containing iodine;
A step of adding water having a dissolved oxygen content of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt to obtain a first crosslinking treatment liquid;
and immersing the polyvinyl alcohol-based resin film after the step of treating with the dyeing treatment liquid in a crosslinking treatment liquid containing the first crosslinking treatment liquid.
[2] The method for producing a polarizing film according to [1], wherein the water is pure water.
[3] The method for producing a polarizing film according to [1] or [2], wherein the water is deoxygenated water that has been subjected to a deoxygenation treatment.
[4] The method for producing a polarized film according to [3], wherein the deoxidation treatment is at least one selected from the group consisting of membrane deaeration treatment, reduced pressure treatment, bubbling treatment using an inert gas, heat treatment, and treatment of adding an oxygen scavenger.
[5] The method for producing a polarized film according to [3] or [4], wherein the deoxidation treatment includes at least a membrane deaeration treatment.
[6] The method for producing a polarized film according to any one of [1] to [5], wherein the temperature of the water added in the step of obtaining the first cross-linking treatment liquid is 5° C. or higher and 70° C. or lower.
[7] The immersion step is a step of immersing the polyvinyl alcohol-based resin film after the step of treating with the dyeing treatment liquid in a crosslinking tank containing the crosslinking treatment liquid,
The method for producing a polarized film according to any one of [1] to [6], wherein the step of obtaining the first crosslinking treatment liquid is carried out in an external tank located outside the crosslinking tank.
[8] The liquid containing the crosslinking agent and the iodide salt is at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank,
supplying at least a portion of the extracted crosslinking treatment liquid to the external tank;
The method for producing a polarized film according to [7], further comprising the step of supplying the first crosslinking treatment liquid from the external tank to the crosslinking tank.
[9] A polarizing film manufacturing apparatus for manufacturing a polarizing film using a polyvinyl alcohol-based resin film, comprising:
a dyeing processing section for treating the polyvinyl alcohol-based resin film with a dyeing processing solution containing iodine;
a first crosslinking treatment liquid preparation unit that obtains a first crosslinking treatment liquid by adding water having a dissolved oxygen amount of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt;
a crosslinking tank in which the polyvinyl alcohol-based resin film treated in the dyeing processing section is immersed in a crosslinking treatment liquid containing the first crosslinking treatment liquid.
[10] The apparatus for producing a polarizing film according to [9], further comprising a deoxidizing unit that performs a deoxidizing treatment to obtain the water.
[11] The apparatus for producing a polarizing film according to [9] or [10], further comprising a temperature adjusting unit for adjusting the temperature of the water.
[12] The apparatus for producing a polarized film according to any one of [9] to [11], wherein the first crosslinking treatment liquid preparation unit is provided outside the crosslinking tank.
[13] The liquid containing the crosslinking agent and the iodide salt is at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank,
a first supply section that supplies at least a portion of the extracted crosslinking treatment liquid to the first crosslinking treatment liquid preparation section;
The apparatus for producing a polarized film according to any one of [9] to [12], further comprising: a second supply unit that supplies the first crosslinking treatment liquid from the first crosslinking treatment liquid preparation unit to the crosslinking tank.

The present invention makes it possible to produce polarizing films with good optical performance.

1 is a schematic diagram illustrating an example of a method for producing a polarizing film of the present invention and an apparatus for producing a polarizing film used in the method.

Below, with reference to the drawings, a preferred embodiment of the polarizing film manufacturing method and polarizing film manufacturing apparatus of the present invention will be described. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

The polarizing film manufacturing method and polarizing film manufacturing apparatus according to this embodiment manufactures a polarizing film using a polyvinyl alcohol-based resin film (hereinafter sometimes referred to as a "PVA-based resin film"). The polarizing film can be manufactured as a long product continuously from a long PVA-based resin film, which is a raw material film, for example.

<Method of manufacturing polarizing film>
FIG. 1 is a schematic diagram showing an example of the polarizing film manufacturing method and the polarizing film manufacturing apparatus used therein according to the present embodiment. The arrows in FIG. 1 indicate the film transport direction or the liquid flow direction. In the manufacturing method of the polarizing film 25 shown in FIG. 1, a long polarizing film 25 can be continuously obtained from a long PVA-based resin film 10. Specifically, while the PVA-based resin film 10 is continuously unwound from a winding roll 11, a swelling process in a swelling tank 13, a dyeing process in a dyeing tank 15 (dyeing processing unit) containing a dyeing treatment liquid, a crosslinking process in a crosslinking tank 17 containing a crosslinking treatment liquid, and a cleaning process in a cleaning tank 19 are sequentially performed, and finally, the film is dried by passing through a drying oven 21 to obtain the polarizing film 25. The polarizing film 25 manufactured as a long object may be sequentially wound around a winding roll 27, or may be subjected to a polarizing plate manufacturing process in which a thermoplastic resin film such as a protective film is bonded to one or both sides of the polarizing film 25 without being wound.

The method for producing the polarizing film 25 of this embodiment includes the following steps:
A method for producing a polarizing film from a PVA-based resin film, comprising the steps of:
A step of treating the PVA-based resin film 10 with a dyeing treatment solution containing iodine;
A step of adding water having a dissolved oxygen content of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt to obtain a first crosslinking treatment liquid;
and immersing the PVA-based resin film 10 after the step of treating with the dyeing treatment liquid in a crosslinking treatment liquid containing a first crosslinking treatment liquid.

The above steps produce a first crosslinking treatment liquid that is at least a part of the crosslinking treatment liquid used in the crosslinking treatment of the PVA-based resin film 10, and the crosslinking treatment of the dyed PVA-based resin film 10 can be suitably carried out using a crosslinking treatment liquid containing this first crosslinking treatment liquid. Each step will be described below.

(Process of treating with dyeing solution)
The step of treating with a dyeing treatment liquid is a dyeing treatment step in which the PVA-based resin film 10 is treated with a dyeing treatment liquid containing iodine, and is carried out for the purpose of, for example, adsorbing and orienting iodine in the PVA-based resin film 10. The PVA-based resin film 10 to be treated with the dyeing treatment liquid is preferably a film after a swelling treatment in a swelling tank 13 shown in FIG. 1. The method of treating with a dyeing treatment liquid containing iodine can be carried out, for example, by immersing the PVA-based resin film 10 in the dyeing treatment liquid for a predetermined time while transporting the PVA-based resin film 10 along a film transport path, and then pulling out the film from the dyeing treatment liquid, as shown in FIG. The method of treating with the dyeing treatment liquid may be carried out by applying the dyeing treatment liquid to the PVA-based resin film 10.

(Step of Obtaining First Cross-Linking Treatment Liquid)
The step of obtaining the first crosslinking treatment liquid includes a step of adding water having an oxygen dissolved amount of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt. The liquid containing a crosslinking agent and an iodide salt includes a crosslinking agent, an iodide salt, and a liquid. Examples of the liquid include water; alcohols such as ethanol; and mixtures thereof, and it is preferable that the liquid contains water. The step of obtaining the first crosslinking treatment liquid is performed to obtain a first crosslinking treatment liquid that forms a part of the crosslinking treatment liquid used in the crosslinking treatment in the crosslinking tank 17. The crosslinking treatment liquid used in the crosslinking treatment contains a crosslinking agent, an iodide salt, and water.

The step of obtaining the first crosslinking treatment liquid may be performed in order to reuse the crosslinking treatment liquid (hereinafter, sometimes referred to as "used crosslinking treatment liquid") after being used in the crosslinking treatment in the crosslinking tank 17 as a usable crosslinking treatment liquid. In this case, the liquid containing the crosslinking agent and the iodide salt to which the above-mentioned amount of dissolved oxygen in water is added can be a used crosslinking treatment liquid, and the addition of the above-mentioned amount of dissolved oxygen in water performed in the step of obtaining the first crosslinking treatment liquid can be a process for reusing the used crosslinking treatment liquid to prepare a crosslinking treatment liquid that can be used for crosslinking treatment. When performing the step of obtaining the first crosslinking treatment liquid using the used crosslinking treatment liquid, the liquid containing the crosslinking agent and the iodide salt may be, for example, at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank 17, and in this case, the step of obtaining the first crosslinking treatment liquid can be performed in an external tank 31 outside the crosslinking tank 17, as shown in FIG. 1. When a step of obtaining a first crosslinking treatment liquid is performed in the external tank 31, the method for producing the polarizing film 25 may include a step of supplying at least a portion of the crosslinking treatment liquid extracted from the crosslinking tank 17 to the external tank 31, and may also include a step of supplying the first crosslinking treatment liquid prepared in the external tank 31 to the crosslinking tank 17.

The amount of dissolved oxygen in the water added to the liquid containing the crosslinking agent and the iodide salt is 2.0 mg/L or less, and may be 1.5 mg/L or less, or may be 1.0 mg/L or less. The amount of dissolved oxygen is usually 0 mg/L or more, and may be 0.1 mg/L or more.

When water containing oxygen is added to a liquid containing a crosslinking agent and an iodide salt, the oxygen contained in the water oxidizes the iodide salt, and the iodine ion (I ₃ ⁻ ) concentration in the resulting first crosslinking treatment liquid may increase. In the step of obtaining the first crosslinking treatment liquid, by adding water with a dissolved oxygen content of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt, the oxidation of the iodide salt is suppressed, and therefore the increase in the iodine ion (I ₃ ⁻ ) concentration in the first crosslinking treatment liquid can be suppressed. It is believed that this makes it possible to obtain a polarizing film 25 with good optical performance.

The dyeing treatment liquid in the dyeing tank 15 shown in Fig. 1 contains iodine and usually contains an iodide salt to dissolve iodine in water. Since the PVA-based resin film 10 that has passed through the dyeing tank 15 is immersed in the crosslinking treatment liquid in the crosslinking tank 17, the crosslinking treatment liquid contains not only the crosslinking agent and iodide salt contained in the crosslinking treatment liquid, but also iodine and iodide salt carried in by the PVA-based resin film 10 that has passed through the dyeing tank 15. Therefore, when the crosslinking treatment of the PVA-based resin film 10 is repeatedly performed, the crosslinking treatment liquid in the crosslinking tank 17 tends to have an increased iodine ion (I ₃ ^- ) concentration due to the carryover of iodine from the dyeing treatment liquid and the oxidation of iodide salt. Therefore, when the liquid containing a crosslinking agent and an iodide salt is the used crosslinking treatment liquid described above, the iodine ion (I ₃ ⁻ ) concentration is likely to become a problem, so it is preferable to suppress the increase in the iodine ion (I ₃ ⁻ ) concentration in the first crosslinking treatment liquid by using water with the above-mentioned amount of dissolved oxygen.

The water with the above-mentioned dissolved oxygen amount may be pure water or tap water, but pure water is preferable. Pure water refers to water with high purity that does not contain or contains almost no impurities such as chlorine or salts. Examples of pure water include ion-exchanged water, distilled water, and reverse osmosis membrane water.

The water having the above-mentioned amount of dissolved oxygen is preferably deoxygenated water that has been subjected to a deoxygenation process. Any known deoxygenation process can be used as the deoxygenation process, but it is preferable that the deoxygenation process is one or more processes selected from the group consisting of membrane deaeration process, reduced pressure process, bubbling process using an inert gas, heat process, and process of adding an oxygen scavenger. Of these, it is preferable that the deoxygenation process includes at least a membrane deaeration process.

The membrane degassing process is a method of removing gas (oxygen) contained in water by reducing the pressure through a membrane (gas separation membrane) with fine pores that allow gas to pass but not water to pass. The decompression process is a method of removing oxygen contained in water by lowering the oxygen partial pressure of the gas in contact with the water. The bubbling process using an inert gas is a method of removing oxygen contained in water by supplying bubbles of an inert gas such as nitrogen or a rare gas (helium, neon, argon) to water. The heat treatment is a method of removing oxygen contained in water by heating the water. The temperature of the water when heated in the heat treatment is preferably 50°C or higher, may be 70°C or higher, or may be 90°C or higher, and the heat treatment may be a method of heating water to boiling. The process of adding an oxygen scavenger is a method of removing oxygen contained in water by adding a known oxygen scavenger such as hydrazine or sodium sulfite to water.

In the step of obtaining the first crosslinking treatment liquid, the temperature of the water containing the dissolved oxygen added to the liquid containing the crosslinking agent and the iodide salt is preferably 5°C or higher, more preferably 10°C or higher, and may be 20°C or higher, 30°C or higher, or 40°C or higher, and may be 70°C or lower, and may be 65°C or lower. The temperature of the water containing the dissolved oxygen is preferably ±3°C of the temperature of the crosslinking treatment liquid used in the immersion step described below, more preferably ±1°C, and even more preferably the same. When a heat treatment is performed in the membrane degassing treatment, the water containing the dissolved oxygen may be heated or cooled so that the temperature of the water containing the dissolved oxygen is in the above range, and the heat treatment as the membrane degassing treatment may be performed so that the temperature of the water containing the dissolved oxygen is the above temperature.

It is preferable that the amount of dissolved oxygen in the first crosslinking treatment liquid is the same as the amount of dissolved oxygen in the crosslinking treatment liquid described below (2.0 mg/L or less) or less than the amount of dissolved oxygen in the crosslinking treatment liquid. It is also preferable that the amount of dissolved oxygen in the water added to the liquid containing the crosslinking agent and iodide salt to obtain the first crosslinking treatment liquid is the same as the amount of dissolved oxygen in the crosslinking treatment liquid described below or less than the amount of dissolved oxygen in the crosslinking treatment liquid.

The step of obtaining the first crosslinking treatment liquid may include a step of supplying a chemical, such as a crosslinking agent, an iodide salt, or a compound other than the crosslinking agent and the iodide salt, to a liquid containing a crosslinking agent and an iodide salt. The step of supplying the chemical may be performed to adjust the liquid containing the crosslinking agent and the iodide salt to the concentration of the chemical contained in the crosslinking treatment liquid for performing the crosslinking treatment. The liquid containing the crosslinking agent and the iodide salt to which the chemical is supplied may be a used crosslinking treatment liquid. In this case, the step of supplying the chemical may be performed by replenishing the necessary chemical so that the used crosslinking treatment liquid can be reused as a crosslinking treatment liquid that can be used for the crosslinking treatment. When the step of obtaining the first crosslinking treatment liquid is performed in the external tank 31, the chemical may be supplied to the external tank 31.

When the process of obtaining the first crosslinking treatment liquid is carried out in an external tank 31 as shown in FIG. 1, the first crosslinking treatment liquid can be prepared in the external tank 31. In this case, the method of producing the polarizing film 25 may further include a process of supplying the obtained first crosslinking treatment liquid from the external tank 31 to the crosslinking tank 17.

(Immersion process)
The immersion step is a step of immersing the PVA-based resin film 10 after the step of treating with the dyeing treatment liquid in a crosslinking treatment liquid containing a first crosslinking treatment liquid, and can be a crosslinking treatment step of crosslinking the PVA-based resin film 10. The crosslinking treatment step is performed for the purpose of improving water resistance by crosslinking, adjusting hue, etc. An example of a method for immersing the PVA-based resin film 10 in a crosslinking treatment liquid is a method of immersing the PVA-based resin film in a crosslinking treatment liquid that contains the first crosslinking treatment liquid obtained in the step of obtaining the first crosslinking treatment liquid, which is contained in a crosslinking tank 17.

The first crosslinking treatment liquid obtained in the step of obtaining the first crosslinking treatment liquid has water added thereto with the above-mentioned amount of dissolved oxygen, and therefore an increase in the iodine ion (I ₃ ^- ) concentration is suppressed. Therefore, even in the crosslinking treatment liquid containing the first crosslinking treatment liquid, an increase in the iodine ion (I ₃ ^- ) concentration is suppressed. As a result, even if a PVA-based resin film 10 that has been dyed using this crosslinking treatment liquid is crosslinked, a polarizing film 25 with good optical performance can be obtained.

In the crosslinking treatment liquid, in order to suppress the deterioration of the optical properties of the polarizing film and to facilitate the adjustment of the transmittance of the polarizing film, it is preferable to keep the iodine ion (I ₃ ⁻ ) concentration low. Therefore, the iodine ion (I ₃ ⁻ ) concentration of the crosslinking treatment liquid containing the first crosslinking treatment liquid is usually preferably 0.5 mM or less, more preferably 0.3 mM or less, and even more preferably 0.1 mM or less.

The amount of dissolved oxygen in the crosslinking treatment solution is preferably 2.0 mg/L or less, may be 1.8 mg/L or less, and is usually 0 mg/L or more, and may be 0.1 mg/L or more.

The process for obtaining the first crosslinking treatment liquid in the manufacturing method of the polarizing film 25 of this embodiment may be performed in the crosslinking tank 17 shown in FIG. 1, or may be performed outside the crosslinking tank 17, for example, in the external tank 31, as shown in FIG. 1. The process for obtaining the first crosslinking treatment liquid may be performed in parallel with the immersion process (the process for performing the crosslinking process), or may be performed while circulating the crosslinking treatment liquid and the first crosslinking treatment liquid between the crosslinking tank 17 and the external tank 31. For example, as shown in FIG. 1, the crosslinking treatment liquid in the crosslinking tank 17 may be continuously extracted, the first crosslinking treatment liquid in the external tank 31 may be prepared, and the obtained first crosslinking treatment liquid may be continuously supplied to the crosslinking tank 17. In the crosslinking tank 17, the PVA-based resin film 10 after the process for treating with the dyeing treatment liquid may be immersed in the crosslinking treatment liquid containing the first crosslinking treatment liquid to perform the crosslinking treatment.

<Polarizing film manufacturing equipment>
The apparatus for producing the polarizing film 25 of this embodiment is an apparatus used in the above-mentioned method for producing the polarizing film 25. In the apparatus for producing the polarizing film 25 shown in Fig. 1, a long PVA-based resin film 10 is successively immersed in a swelling treatment liquid in a swelling tank 13, a dyeing treatment liquid in a dyeing tank 15 (dyeing processing unit), a crosslinking treatment liquid in a crosslinking tank 17, and a cleaning treatment liquid in a cleaning tank 19, and finally dried by passing through a drying furnace 21, thereby obtaining a long polarizing film 25.

As shown in FIG. 1, the polarizing film 25 manufacturing apparatus includes, in order from the upstream side of the film transport path (path along which the PVA-based resin film 10 and the polarizing film 25 are transported), a swelling tank 13, a dyeing tank 15 (dyeing processing unit), a crosslinking tank 17, a cleaning tank 19, and a drying furnace 21. The polarizing film 25 manufacturing apparatus further includes a first crosslinking treatment liquid preparation unit 30 and a water preparation unit. The first crosslinking treatment liquid preparation unit 30 can include an external tank 31, a water supply unit 32, and a chemical supply unit 33. The water supply unit 32 is for supplying water having an oxygen dissolved amount of 2.0 mg/L or less, which is added to a liquid containing a crosslinking agent and an iodide salt. The chemical supply unit 33 is for replenishing the liquid containing a crosslinking agent and an iodide salt with chemicals such as a crosslinking agent, an iodide salt, and compounds other than the crosslinking agent and the iodide salt contained in the liquid. The water preparation unit may include a deoxidation unit that performs a deoxidation process to obtain water with the above amount of dissolved oxygen, and a temperature adjustment unit that adjusts the temperature of the water with the above amount of dissolved oxygen, and may be provided in the first crosslinking treatment liquid preparation unit 30.

The polarizing film 25 manufacturing apparatus may further include a first supply section 35 that supplies the crosslinking treatment liquid in the crosslinking tank 17 to the external tank 31, and a second supply section 36 that supplies the first crosslinking treatment liquid from the external tank 31 to the crosslinking tank 17. By providing these, the crosslinking treatment liquid and the first crosslinking treatment liquid can be circulated between the crosslinking tank 17 and the first crosslinking treatment liquid preparation section 30. The manufacturing apparatus may also have a polarizing plate preparation section that prepares a polarizing plate by laminating a protective film or the like to one or both sides of the obtained polarizing film 25.

As described above, the polarizing film 25 manufacturing device performs various processes while transporting the long PVA-based resin film 10, and the film transport path is provided with multiple rolls that support and guide the film during transport so that it passes through the above-mentioned tanks and parts. The multiple rolls include a guide roll that supports one side of the film and does not apply a driving force to the film, and a pair of nip rolls (usually drive rolls that can apply a driving force to the film) that are arranged on both sides of the film and sandwich the film. In the manufacturing device shown in FIG. 1, guide rolls 1a to 1k and nip rolls 2a to 2f are provided on the film transport path. Some of the guide rolls and some of the nip rolls may be suction rolls that can be used as drive rolls. These multiple rolls can be provided at any position on the film transport path.

The polarizing film manufacturing apparatus of the present embodiment is
An apparatus for producing a polarizing film (25) using a PVA-based resin film (10), comprising:
A dyeing tank 15 (dyeing processing section) for treating the PVA-based resin film 10 with a dyeing processing solution containing iodine;
a first crosslinking treatment liquid preparation unit 30 for obtaining a first crosslinking treatment liquid by adding water having a dissolved oxygen amount of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt;
and a crosslinking tank 17 in which the polyvinyl alcohol-based resin film treated in the dyeing tank 15 is immersed in a crosslinking treatment liquid containing a first crosslinking treatment liquid.

As described above, the polarizing film manufacturing apparatus may further include a water preparation section for preparing water with the above amount of dissolved oxygen.

The above-mentioned manufacturing apparatus can be used in the manufacturing method of the polarizing film 25 described above. According to the above-mentioned manufacturing apparatus, a first crosslinking treatment liquid that is at least a part of the crosslinking treatment liquid used in the crosslinking treatment of the PVA-based resin film 10 is prepared, and the crosslinking treatment of the dyed PVA-based resin film 10 can be suitably performed using a crosslinking treatment liquid containing this first crosslinking treatment liquid. Each part of the manufacturing apparatus of the polarizing film 25 is described below.

(dyeing tank)
The dyeing tank 15 shown in Fig. 1 can be used for carrying out a step of treating a PVA-based resin film 10 with a dyeing treatment liquid containing iodine in the manufacturing method of a polarizing film 25. The dyeing tank 15 contains the dyeing treatment liquid therein and is used, for example, to immerse the PVA-based resin film 10 swelled in the swelling tank 13 in the dyeing treatment liquid. The PVA-based resin film 10 is dyed with the dyeing treatment liquid while being supported and transported by guide rolls 1d and 1e provided in the dyeing treatment liquid in the dyeing tank 15. The PVA-based resin film 10 treated with the dyeing treatment liquid and drawn out of the dyeing tank 15 passes through a guide roll 1f and a nip roll 2c in this order and is introduced into a crosslinking tank 17.

When the process of treating the PVA-based resin film 10 with a dyeing treatment liquid containing iodine is carried out by applying the dyeing treatment liquid to the PVA-based resin film 10, an application device such as a spray can be used instead of the dyeing tank 15.

(First cross-linking treatment solution preparation section)
The first crosslinking treatment liquid preparation unit 30 can be used to perform a step of obtaining a first crosslinking treatment liquid in the manufacturing method of the polarizing film 25, and the first crosslinking treatment liquid preparation unit 30 can add the above-mentioned amount of water dissolved in oxygen to a liquid containing a crosslinking agent and an iodide salt. In this way, the first crosslinking treatment liquid preparation unit 30 prepares a first crosslinking treatment liquid that forms a part of the crosslinking treatment liquid used in the crosslinking treatment in the crosslinking tank 17. The first crosslinking treatment liquid preparation unit 30 can include a water supply unit for adding the above-mentioned amount of water dissolved in oxygen to the liquid containing a crosslinking agent and an iodide salt, and a chemical supply unit 33 for adding a chemical contained in the liquid containing a crosslinking agent and an iodide salt, such as a crosslinking agent or an iodide salt, to the liquid containing a crosslinking agent and an iodide salt.

The first crosslinking treatment liquid preparation unit 30 prepares the first crosslinking treatment liquid that becomes part of the crosslinking treatment liquid used in the crosslinking tank 17, and therefore may be provided in the crosslinking tank 17, or may be provided outside the crosslinking tank 17. For example, the first crosslinking treatment liquid preparation unit 30 shown in FIG. 1 is provided outside the crosslinking tank 17, and includes an external tank 31 that contains a liquid containing a crosslinking agent and an iodide salt, and a water supply unit 32 for adding the above-mentioned amount of water dissolved in oxygen to the liquid containing the crosslinking agent and the iodide salt in the external tank 31.

As described above, the liquid containing the crosslinking agent and the iodide salt used in the first crosslinking treatment liquid preparation unit 30 may be a used crosslinking treatment liquid or at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank 17. Therefore, for example, as shown in FIG. 1, when the first crosslinking treatment liquid preparation unit 30 is provided outside the crosslinking tank 17, the manufacturing apparatus for the polarizing film 25 can have a first supply unit 35 for supplying the crosslinking treatment liquid in the crosslinking tank 17 to the external tank 31, and a second supply unit 36 for supplying the first crosslinking treatment liquid from the external tank 31 to the crosslinking tank 17. The first supply unit 35 and the second supply unit 36 may be, for example, piping connecting the crosslinking tank 17 and the external tank 31. In addition, the first supply unit 35 may supply the crosslinking treatment liquid overflowing from the crosslinking tank 17 to the external tank 31 directly or via a pipe, and the second supply unit 36 may supply the first crosslinking treatment liquid overflowing from the external tank 31 to the crosslinking tank 17 directly or via a pipe.

The external tank 31 shown in FIG. 1 can contain at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank 17 through the first supply unit 35 as a liquid containing a crosslinking agent and an iodide salt. The external tank 31 can be used to add and mix the above-mentioned amount of dissolved oxygen in water to the liquid containing the crosslinking agent and the iodide salt contained therein. The external tank 31 may be provided with a stirring device such as a stirring blade or a stirrer for stirring the liquid inside. The external tank 31 may be covered with a heat insulating material so that the crosslinking treatment liquid can be maintained at a predetermined temperature, and may be provided with a temperature control device such as a heating unit or a cooling unit for setting the crosslinking treatment liquid to a predetermined temperature.

The water supply unit 32 shown in FIG. 1 can supply the above-mentioned dissolved oxygen amount of water to be added to the liquid containing the crosslinking agent and iodide salt in the external tank 31. The water supply unit 32 can supply the required amount of water containing the above-mentioned dissolved oxygen amount into the external tank 31. The supply amount of the above-mentioned dissolved oxygen amount of water from the water supply unit 32 may be set in advance so that the concentration of the crosslinking agent and iodide salt of the first crosslinking treatment liquid adjusted by the first crosslinking treatment liquid preparation unit 30 becomes the concentration of the crosslinking agent and iodide salt required for the crosslinking treatment liquid in the crosslinking tank 17, for example, or may be adjusted based on the concentration of the crosslinking agent and iodide salt measured for the liquid containing the crosslinking agent and iodide salt in the external tank 31. The water supply unit 32 may be covered with a heat insulating material so as to be able to supply the above-mentioned dissolved oxygen amount of water adjusted to a predetermined temperature, and may be equipped with a temperature adjustment device such as a heating unit or a cooling unit for adjusting the temperature of the above-mentioned dissolved oxygen amount of water to a predetermined temperature.

The drug supply unit 33 shown in FIG. 1 can supply drugs such as a crosslinking agent, an iodide salt, or a compound other than a crosslinking agent and an iodide salt to be added to the liquid containing the crosslinking agent and the iodide salt in the external tank 31. The supply amount of the drug added from the drug supply unit 33 may be preset so that the concentration of the drug in the first crosslinking treatment liquid adjusted by the first crosslinking treatment liquid preparation unit 30 is the concentration of the drug required for the crosslinking treatment liquid in the crosslinking tank 17, for example, or may be adjusted based on the concentration of the drug measured for the liquid containing the crosslinking agent and the iodide salt in the external tank 31.

(Water Preparation Section)
The water preparation unit can include a deoxygenation unit that performs a deoxygenation process to obtain water having the above amount of dissolved oxygen, and a temperature adjustment unit that adjusts the temperature of the water having the above amount of dissolved oxygen.

The deoxidizing section may be any known device for performing deoxidizing treatment, but is preferably one or more selected from the group consisting of a membrane degassing treatment section, a reduced pressure treatment section, a bubbling treatment section using an inert gas, a heating treatment section, and a deoxidizing agent supply section for adding a deoxidizing agent. Of these, it is preferable that the deoxidizing section includes at least a membrane degassing treatment section.

The membrane degassing treatment section preferably includes a gas separation membrane having micropores through which gas can pass but water cannot. The reduced pressure treatment section preferably includes a reduced pressure device capable of lowering the oxygen partial pressure of the gas in contact with the water. Examples of the bubbling treatment section include gas pipe tubes capable of supplying inert gas, air stones, diffusers (air diffusion tubes, air diffusion plates), and other air diffusion devices. Examples of the heating treatment section include known heating devices, such as those capable of heating water to a predetermined temperature or higher or boiling water. The oxygen scavenger supply section is not particularly limited as long as it is a device capable of supplying an oxygen scavenger to water.

The temperature adjustment unit may be a known device for adjusting the water with the above-mentioned dissolved oxygen amount to a predetermined temperature, and may be a heating unit, a cooling unit, or a unit equipped with both a heating unit and a cooling unit. It is preferable that the temperature adjustment unit adjusts the water with the above-mentioned dissolved oxygen amount to the temperature of the crosslinking treatment liquid contained in the crosslinking tank 17 described below. If the deoxygenation unit includes a heating treatment unit, the temperature of the water with the above-mentioned dissolved oxygen amount may be adjusted by the heating treatment unit.

The first cross-linking treatment liquid prepared in the first cross-linking treatment liquid preparation section 30 shown in FIG. 1 can be supplied from the external tank 31 to the cross-linking tank 17 through the second supply section 36. When the second supply section 36 is a pipe, the pipe may be covered with a heat insulating material so that the cross-linking treatment liquid adjusted to a predetermined temperature can be supplied to the cross-linking tank 17, and may be equipped with a temperature adjustment device such as a heating section or a cooling section for adjusting the cross-linking treatment liquid to a predetermined temperature.

(Cross-linking tank)
The crosslinking tank 17 can be used for carrying out a step of immersing the PVA-based resin film 10 after the step of treating with the dyeing treatment liquid in a crosslinking treatment liquid containing a first crosslinking treatment liquid in the manufacturing method of the polarizing film 25. The crosslinking tank 17 contains a crosslinking treatment liquid containing a first crosslinking treatment liquid therein, and is used for immersing the PVA-based resin film 10 dyed in the dyeing tank 15 in the crosslinking treatment liquid. The PVA-based resin film 10 is treated with the crosslinking treatment liquid while being supported and transported by guide rolls 1g and 1h provided in the crosslinking treatment liquid in the crosslinking tank 17. The PVA-based resin film 10 treated with the crosslinking treatment liquid and drawn out of the crosslinking tank 17 passes through a guide roll 1i and a nip roll 2d in this order, and is introduced into a cleaning tank 19.

The first crosslinking treatment liquid preparation section 30 in the manufacturing apparatus for the polarizing film 25 of this embodiment may be provided in the crosslinking tank 17 shown in FIG. 1, or may be provided outside the crosslinking tank 17 as shown in FIG. 1. The preparation of the first crosslinking treatment liquid in the first crosslinking treatment liquid preparation section 30 may be performed in parallel with the crosslinking treatment in the crosslinking tank 17. When the first crosslinking treatment liquid preparation section 30 is outside the crosslinking tank 17, the crosslinking treatment liquid and the first crosslinking treatment liquid may be circulated between the crosslinking tank 17 and the first crosslinking treatment liquid preparation section 30. For example, as shown in FIG. 1, the crosslinking treatment liquid may be extracted from the crosslinking tank 17, the first crosslinking treatment liquid may be prepared in the first crosslinking treatment liquid preparation section 30, and the obtained first crosslinking treatment liquid may be continuously supplied to the crosslinking tank 17, while the PVA-based resin film 10 extracted from the dyeing tank 15 may be immersed in the crosslinking treatment liquid in the crosslinking tank 17 to perform the crosslinking treatment.

Below, we will explain in detail each step of the polarizing film manufacturing method and each component of the polarizing film manufacturing apparatus.

(PVA-based resin film)
The PVA-based resin film 10 used in the method for producing a polarizing film of this embodiment is a film formed using a polyvinyl alcohol-based resin. The polyvinyl alcohol-based resin refers to a resin containing 50% by mass or more of a structural unit derived from vinyl alcohol. As the polyvinyl alcohol-based resin, a saponified polyvinyl acetate-based resin can be used. The saponification degree of the polyvinyl acetate-based resin can be determined in accordance with JIS K 6727 (1994), and can be, for example, in the range of 80.0 to 100.0 mol%.

Examples of polyvinyl acetate resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate with other monomers that can be copolymerized with it. Examples of other monomers that can be copolymerized with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, and (meth)acrylamides having an ammonium group. In this specification, "(meth)acrylic" refers to at least one selected from the group consisting of acrylic and methacrylic. The same applies to other terms with "(meth)" added.

An example of the PVA-based resin film 10 may be an unstretched film made by forming a film from a polyvinyl alcohol-based resin, or a stretched film obtained by stretching this unstretched film. When the PVA-based resin film 10 is a stretched film, it is preferably a stretched film that has been uniaxially stretched longitudinally, and is preferably a stretched film that has been dry-stretched. When the PVA-based resin film 10 is a stretched film, the stretching ratio is usually 1.1 to 8 times.

The thickness of the PVA-based resin film 10 used in the polarizing film manufacturing method of this embodiment is usually 10 to 150 μm, and from the viewpoint of thinning the resulting polarizing film 25, it is preferably 100 μm or less, more preferably 75 μm or less, even more preferably 50 μm or less, and may be 30 μm or less.

(Polarizing film)
The polarizing film 25 obtained by the polarizing film manufacturing method of the present embodiment is a stretched PVA-based resin film having iodine adsorbed thereon and oriented therein. The thickness of the polarizing film 25 is usually 2 to 40 μm, and from the viewpoint of thinning the polarizing film, the thickness is preferably 30 μm or less, and more preferably 20 μm or less.

The luminosity-corrected single transmittance Ty of the resulting polarizing film 25 is preferably 40-47%, more preferably 41-45%, taking into consideration the balance with the luminosity-corrected polarization degree Py. The luminosity-corrected polarization degree Py is preferably 99.9% or more, more preferably 99.95% or more. Ty and Py can be determined by performing luminosity correction on the obtained transmittance and polarization degree using an integrating sphere spectrophotometer with a 2-degree visual field (C light source) according to JIS Z 8701.

(Swelling treatment step and swelling tank)
The swelling treatment step is a treatment that is carried out as necessary for the purposes of removing foreign matter from the PVA-based resin film 10, removing plasticizers, imparting dyeability, plasticizing the film, etc. As shown in Fig. 1, the swelling treatment step can be carried out by immersing the PVA-based resin film 10 in a swelling tank 13 containing a swelling treatment liquid for a predetermined time and then pulling it out.

The swelling treatment liquid contained in the swelling tank 13 may be, for example, water (e.g., pure water) or an aqueous solution to which a water-soluble organic solvent such as alcohol has been added. The swelling treatment liquid may contain boron compounds such as boric acid and borax, chlorides, inorganic acids, inorganic salts, etc. The temperature of the swelling treatment liquid is usually 10 to 70°C, preferably 15 to 50°C, and more preferably 15 to 35°C. The immersion time (residence time in the swelling treatment liquid) of the PVA-based resin film 10 is usually 10 to 600 seconds, and preferably 15 to 300 seconds. When the PVA-based resin film 10 is a stretched film, the temperature of the swelling treatment liquid is, for example, about 20 to 70°C, and may be 30 to 60°C.

During the swelling process, the PVA-based resin film 10 may be subjected to a wet stretching process (usually a uniaxial stretching process). In this case, the stretching ratio is usually 1.2 to 3 times, and preferably 1.3 to 2.5 times. In the manufacturing apparatus for the polarizing film 25 shown in Figure 1, for example, the uniaxial stretching process can be performed in the swelling tank 13 by utilizing the difference in peripheral speed between the nip rolls 2a and 2b.

In the manufacturing apparatus for polarizing film 25 shown in FIG. 1, the film pulled out from the swelling tank 13 passes through guide roll 1c and nip roll 2b in that order before being introduced into the dyeing tank 15.

(Dyeing process and dyeing tank)
Regarding the dyeing process and the dyeing tank 15, points other than those described above will be described. An aqueous solution containing iodine and an iodide salt can be used as the dyeing process liquid used in the dyeing process. Examples of the iodide salt include iodide salts of alkali metals, iodide salts of alkaline earth metals, and zinc iodide, and potassium iodide and zinc iodide are preferred, and potassium iodide is more preferred. Potassium iodide may be used in combination with other iodide salts.

The dyeing treatment liquid may contain boron compounds such as boric acid and borax, and compounds other than iodide salts such as zinc chloride and cobalt chloride. When a boron compound is added, it is distinguished from the crosslinking treatment liquid described below in that it contains iodine. For example, if an aqueous solution contains about 0.003 parts by mass or more of iodine per 100 parts by mass of water, it can be considered a dyeing treatment liquid. The iodine content in the dyeing treatment liquid is usually 0.003 to 1 part by mass per 100 parts by mass of water. The iodide salt content in the dyeing treatment liquid, such as potassium iodide, is usually 0.1 to 20 parts by mass per 100 parts by mass of water.

The temperature of the dyeing treatment solution is usually 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C. The immersion time of the PVA-based resin film 10 (residence time in the dyeing treatment solution) is usually 20 to 600 seconds, and preferably 30 to 300 seconds.

The manufacturing apparatus for the polarizing film 25 may include two or more dyeing tanks 15. In this case, the composition and temperature of each dyeing treatment liquid may be the same or different from each other independently.

In order to improve the dyeability of iodine, it is preferable that the PVA-based resin film 10 to be subjected to the dyeing process is stretched to at least some extent (usually uniaxially stretched). Instead of or in addition to the stretching process before the dyeing process, a stretching process may be performed during the dyeing process. The cumulative stretching ratio up to the dyeing process (the stretching ratio in the dyeing process if there is no stretching step before the dyeing process) is usually 1.6 to 4.5 times, and preferably 1.8 to 4 times. In the manufacturing apparatus for the polarizing film 25 shown in FIG. 1, for example, the uniaxial stretching process can be performed in the dyeing tank 15 by utilizing the difference in peripheral speed between the nip rolls 2b and 2c.

In the manufacturing apparatus for polarizing film 25 shown in FIG. 1, the film pulled out from the dyeing tank 15 passes through guide roll 1f and nip roll 2c in that order before being introduced into the crosslinking tank 17.

(Crosslinking process and crosslinking tank)
Regarding the dyeing process and the dyeing tank 15, points other than those described above will be described. The crosslinking process liquid used in the crosslinking process can be a liquid (usually an aqueous solution) containing an iodide salt and a crosslinking agent. Examples of iodide salts include iodide salts of alkali metals, iodide salts of alkaline earth metals, zinc iodide, etc., with potassium iodide and zinc iodide being preferred, and potassium iodide being more preferred. Potassium iodide may be used in combination with other iodide salts. Examples of crosslinking agents include boron compounds such as boric acid and borax, glyoxal, glutaraldehyde, etc., with boric acid being preferred. Two or more crosslinking agents can also be used in combination.

The crosslinking treatment solution may contain compounds other than the iodide salt and the crosslinking agent. Examples of such compounds include zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate.

The content of the iodide salt in the crosslinking treatment liquid is usually 0.1 to 20 parts by mass, and preferably 5 to 15 parts by mass, per 100 parts by mass of water. The content of the crosslinking agent in the crosslinking treatment liquid is usually 0.1 to 15 parts by mass, and preferably 1 to 12 parts by mass, per 100 parts by mass of water. The temperature of the crosslinking treatment liquid is usually 20 to 85°C, and preferably 30 to 70°C. The immersion time (residence time in the crosslinking treatment liquid) of the PVA-based resin film 10 is usually 10 to 600 seconds, and preferably 20 to 300 seconds.

The polarizing film 25 manufacturing apparatus may include two or more crosslinking tanks 17. In this case, the composition and temperature of each crosslinking treatment liquid may be the same as or different from each other independently. When multiple crosslinking tanks 17 are included, each crosslinking tank may be provided with a first crosslinking treatment liquid preparation section.

A stretching process (usually a uniaxial stretching process) may be performed while the crosslinking process is being performed. In the manufacturing apparatus for the polarizing film 25 shown in FIG. 1, for example, the uniaxial stretching process can be performed in the crosslinking tank 17 by utilizing the difference in peripheral speed between the nip rolls 2c and 2d.

In the manufacturing apparatus for polarizing film 25 shown in FIG. 1, the film pulled out from the crosslinking tank 17 passes through guide roll 1i and nip roll 2d in that order before being introduced into the cleaning tank 19.

(Cleaning process and cleaning tank)
The cleaning process is carried out for the purpose of removing excess chemicals adhering to the PVA-based resin film 10 after the crosslinking process. As shown in Fig. 1, the cleaning process can be carried out by transporting the PVA-based resin film 10 after the crosslinking process (immersed in a crosslinking bath 17) along a film transport path, immersing it in a cleaning bath 19 containing a cleaning solution for a predetermined period of time, and then pulling it out.

Instead of immersing the film in the cleaning tank 19 shown in FIG. 1, the cleaning process may involve spraying the PVA-based resin film 10 after the crosslinking process with a cleaning solution, for example as a shower. The cleaning process may also involve a combination of immersion in the cleaning tank 19 and spraying with the cleaning solution.

The cleaning treatment liquid can be, for example, water (e.g., pure water), or it can be an aqueous solution containing a water-soluble organic solvent such as an alcohol. The temperature of the cleaning bath is, for example, 2 to 40°C.

A stretching process (usually a uniaxial stretching process) may be performed while the cleaning process is being performed. In the manufacturing apparatus for polarizing film 25 shown in FIG. 1, for example, uniaxial stretching can be performed in cleaning tank 19 by utilizing the difference in peripheral speed between nip roll 2d and nip roll 2e.

In the manufacturing apparatus for polarizing film 25 shown in FIG. 1, the film pulled out of the cleaning tank 19 passes through guide roll 1l and nip roll 2e in that order before being introduced into the drying furnace 21.

(Stretching process and stretching processing section)
The stretching treatment may be performed while performing at least one of the above-mentioned swelling treatment step, dyeing treatment step, crosslinking treatment step, and washing treatment step. The stretching treatment performed in these steps is wet stretching, and is usually uniaxial stretching. The stretching treatment step is preferably performed in the crosslinking treatment step or in one or more stages prior to the crosslinking treatment step. In order to obtain a polarizing film 25 having good polarization properties by improving the dyeability of iodine, it is more preferable that the PVA-based resin film 10 to be subjected to the dyeing treatment step has been subjected to at least some stretching treatment.

The stretching ratio in the stretching process is adjusted from the viewpoint of the polarization properties of the resulting polarizing film 25 so that the final cumulative stretching ratio of the polarizing film 25 (if the PVA-based resin film 10 used as the original film is a stretched film, the cumulative stretching ratio including this stretching) is 3 to 8 times.

To carry out the stretching process, the polarizing film manufacturing apparatus includes a stretching processing section for stretching the PVA-based resin film 10. The stretching processing section preferably performs stretching between rolls, and examples of such stretching section include two nip rolls disposed before and after each tank as shown in FIG. 1.

(Drying process and drying oven)
The drying treatment section is located on the film transport path of the PVA type resin film 10, and in the production apparatus shown in Fig. 1, it is disposed downstream of the cleaning tank 19, and is a zone for drying the PVA type resin film 10 after the cleaning treatment step. In the drying treatment step, the PVA type resin film 10 after the cleaning treatment step can be dried by introducing the film into a drying oven 21 while continuing to transport the film, thereby obtaining a polarizing film 25.

The drying furnace 21 is, for example, a hot air oven that can increase the temperature inside the furnace by supplying hot air, etc. Instead of the drying furnace 21, the drying process may be performed using one or more heating bodies having a convex curved surface that adheres the PVA-based resin film 10 after the wet processing step, a heater, etc. Examples of the heating body include a roll (for example, a guide roll that also serves as a heat roll) that has a heat source (for example, a heat medium such as hot water or an infrared heater) inside and can increase the surface temperature. Examples of the heater include an infrared heater, a halogen heater, a panel heater, etc. The temperature of the drying process (for example, the temperature inside the drying furnace 21, the surface temperature of the heat roll, etc.) is usually 30 to 100°C, and preferably 50 to 90°C.

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

[Measurement of dissolved oxygen amount]
The amount of dissolved oxygen in the deoxygenated water used in each Reference Example was measured using a dissolved oxygen meter (B-506, manufactured by Iijima Electronics Co., Ltd.).

[Measurement of absorbance]
The absorbance at a wavelength of 380 nm was measured for the aqueous solution obtained in the Reference Example. The absorbance was measured using an absorptiometer (UV-2450, manufactured by Shimadzu Corporation) and a liquid cell with a sample filling section of 10 mm thickness. The absorbance at a wavelength of 380 nm is approximately proportional to the content (I ₃ ^- concentration) of iodine ions (I ₃ ^- ) in the aqueous solution, and a larger absorbance value at a wavelength of 380 nm indicates a higher iodine ion (I ₃ ^- ) concentration.

[Reference Example 1]
Deoxygenated water was obtained by bubbling nitrogen gas through ion-exchanged water. The amount of dissolved oxygen in the obtained deoxygenated water was measured at a temperature in the range of 39.5±1°C. The results are shown in Table 1. 400 g of boric acid and 1200 g of potassium iodide (KI) were added to 8 L of the obtained deoxygenated water, and the mixture was stirred for 4 minutes to obtain an aqueous solution. The absorbance of the obtained aqueous solution was measured. The results are shown in Table 1. The temperature of the liquid was maintained within the range of 39.5±1°C from the preparation of the aqueous solution to the measurement of absorbance.

[Reference Examples 2 to 4]
An aqueous solution was obtained in the same manner as in Reference Example 1, except that the amount of oxygen desorbed by the membrane degassing device was adjusted to use deoxygenated water having the amount of dissolved oxygen shown in Table 1. The absorbance of the aqueous solution obtained was measured. The results are shown in Table 1.

The results shown in Table 1 suggest that when water is added to a liquid containing a crosslinking agent and an iodide salt, the greater the amount of dissolved oxygen in the added water, the more likely it is that the iodine ion (I ₃ ⁻ ) concentration in the resulting first crosslinking treatment liquid will increase.

1a-1l guide rolls, 2a-2f nip rolls, 10 polyvinyl alcohol resin film (PVA resin film), 11 unwinding roll, 13 swelling tank, 15 dyeing tank, 17 crosslinking tank, 19 washing tank, 21 drying furnace, 25 polarizing film, 27 winding roll, 30 first crosslinking treatment liquid preparation section, 31 external tank, 32 water supply section, 33 chemical supply section, 35 first supply section, 36 second supply section.

Claims (13)

A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising the steps of:
A step of treating the polyvinyl alcohol-based resin film with a dyeing treatment solution containing iodine;
A step of adding water having a dissolved oxygen content of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt to obtain a first crosslinking treatment liquid;
and a step of immersing the polyvinyl alcohol-based resin film after the step of treating with the dyeing treatment liquid in a crosslinking treatment liquid containing the first crosslinking treatment liquid ,
The method for producing a polarizing film , wherein the water is pure water . The method for producing a polarizing film according to claim 1 , wherein the pure water is ion-exchanged water, distilled water, or reverse osmosis water . The method for producing a polarizing film according to claim 1 or 2, wherein the water is deoxygenated water that has been subjected to a deoxygenation process. The method for producing a polarizing film according to claim 3, wherein the deoxidizing treatment is at least one selected from the group consisting of membrane degassing treatment, reduced pressure treatment, bubbling treatment using an inert gas, heat treatment, and treatment of adding an oxygen scavenger. The method for producing a polarizing film according to claim 3 or 4, wherein the deoxidation treatment includes at least a membrane degassing treatment. The method for producing a polarizing film according to any one of claims 1 to 5, wherein the temperature of the water added in the step of obtaining the first crosslinking treatment liquid is 5°C or higher and 70°C or lower. The immersion step is a step of immersing the polyvinyl alcohol-based resin film after the step of treating with the dyeing treatment liquid in a crosslinking tank containing the crosslinking treatment liquid,
The method for producing a polarizing film according to any one of claims 1 to 6, wherein the step of obtaining the first crosslinking treatment liquid is carried out in an external tank located outside the crosslinking tank. the liquid containing the crosslinking agent and the iodide salt is at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank,
supplying at least a portion of the extracted crosslinking treatment liquid to the external tank;
The method for producing a polarized film according to claim 7 , further comprising the step of: supplying the first cross-linking treatment liquid from the external tank to the cross-linking tank. A polarizing film manufacturing apparatus for manufacturing a polarizing film using a polyvinyl alcohol-based resin film,
a dyeing processing section for treating the polyvinyl alcohol-based resin film with a dyeing processing solution containing iodine;
a first crosslinking treatment liquid preparation unit that obtains a first crosslinking treatment liquid by adding water having a dissolved oxygen amount of 2.0 mg/L or less to a liquid containing a crosslinking agent and an iodide salt;
a crosslinking tank in which the polyvinyl alcohol-based resin film treated in the dyeing processing unit is immersed in a crosslinking treatment liquid containing the first crosslinking treatment liquid ,
The apparatus for producing a polarizing film , wherein the water is pure water . The polarizing film manufacturing apparatus according to claim 9 further includes a deoxidizing unit that performs a deoxidizing process to obtain the water. The polarizing film manufacturing apparatus according to claim 9 or 10, further comprising a temperature adjustment unit that adjusts the temperature of the water. The polarizing film manufacturing apparatus according to any one of claims 9 to 11, wherein the first crosslinking treatment liquid preparation section is provided outside the crosslinking tank. the liquid containing the crosslinking agent and the iodide salt is at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking tank,
a first supply section that supplies at least a portion of the extracted crosslinking treatment liquid to the first crosslinking treatment liquid preparation section;
The apparatus for producing a polarized film according to any one of claims 9 to 12, further comprising: a second supply section that supplies the first crosslinking treatment liquid from the first crosslinking treatment liquid preparation section to the crosslinking tank.

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