JP7461710B2 - Method for producing polyvinyl alcohol-based resin film - Google Patents

Description

The present invention relates to a method for producing a polyvinyl alcohol-based resin film, for example, a method for producing a polarizing film that can be used as a component of a polarizing plate.

Polyvinyl alcohol-based resin films are industrially useful materials, being used as polarizing films for optical display devices, agricultural materials, separation materials, etc. For example, the polyvinyl alcohol-based resin films may be subjected to wet treatments such as cleaning treatments to remove dirt and dust from the surface.
In addition, a polarizing film obtained from a polyvinyl alcohol-based resin film has been conventionally obtained by adsorbing and orienting a dichroic dye such as iodine onto a uniaxially stretched polyvinyl alcohol-based resin film. Such a polarizing film is produced by sequentially subjecting a polyvinyl alcohol-based resin film to a dyeing process in which the polyvinyl alcohol-based resin film is dyed with a dichroic dye, a crosslinking process in which the polyvinyl alcohol-based resin film is treated with a crosslinking agent, and a film drying process, and by subjecting the polyvinyl alcohol-based resin film to a stretching process during the production process (see, for example, Patent Document 1). In the method described in Patent Document 1, the dyeing process and the crosslinking process are performed by wet processing.

JP 2001-141926 A

One method for manufacturing a polarizing film (polyvinyl alcohol-based resin film) such as the polarizing film described in Patent Document 1 includes a wet treatment process in which the film is immersed in a treatment liquid in a chemical tank. The industrial manufacturing method for polarizing film includes a wet treatment process in which a long raw material polyvinyl alcohol-based resin film is continuously transported along a film transport path of a polarizing film manufacturing device, and is sequentially immersed in a dyeing treatment tank for carrying out the above-mentioned dyeing treatment and a crosslinking treatment tank for carrying out a crosslinking treatment, which are located on the transport path.

When a polarizing film is continuously produced according to the above method, the polyvinyl alcohol-based resin gradually dissolves in the treatment liquid, and the dissolved polyvinyl alcohol-based resin may precipitate from the treatment liquid due to some factor. When precipitation occurs, the precipitate (a solid containing the polyvinyl alcohol-based resin) adheres to the raw material polyvinyl alcohol-based resin film during the wet treatment, and further to the surface of the polarizing film, which may adversely affect the appearance and quality of the polyvinyl alcohol-based resin film, for example, the polarizing film.
In addition, when the concentration of the polyvinyl alcohol-based resin dissolved in the treatment liquid gradually increases, a solid containing the polyvinyl alcohol-based resin may precipitate on the surface of the polyvinyl alcohol-based resin film that is immersed in the treatment liquid and then pulled out of the treatment liquid. Such precipitates may also adversely affect the appearance and quality of the polyvinyl alcohol-based resin film, for example, a polarizing film.

The object of the present invention is to provide a method for the stable continuous production of polyvinyl alcohol-based resin films, such as polarizing films, while suppressing adhesion of the above-mentioned precipitates to the film.

The present invention provides a method for producing a polyvinyl alcohol-based resin film as described below.
[1] A method for producing a polyvinyl alcohol-based resin film, comprising:
a step of transporting a raw material polyvinyl alcohol-based resin film along a transport path passing through a treatment tank containing a treatment liquid and immersing the raw material polyvinyl alcohol-based resin film in the treatment liquid;
separating at least a portion of the treatment liquid into a first portion including bubbles present on a liquid surface and a second portion other than the first portion;
A method comprising:
[2] The method according to [1], further comprising a step of generating or increasing the foam before the separating step.
[3] A step of precipitating a solid containing a polyvinyl alcohol-based resin from the foam contained in the first portion;
removing at least a portion of the solids;
The method according to [1] or [2], further comprising:
[4] The method according to [3], further comprising the step of returning at least a portion of the liquid recovered by the removing step to the treatment tank.
[5] The method according to any one of [1] to [4], further comprising the step of returning at least a portion of the second portion to the treatment tank.
[6] The method according to any one of [1] to [5], wherein the foam contained in the first portion has a polyvinyl alcohol-based resin concentration higher than a polyvinyl alcohol-based resin concentration in the second portion.
[7] The method according to any one of [1] to [6], wherein the polyvinyl alcohol-based resin film is a polarizing film.

It is possible to provide a method for producing a polyvinyl alcohol-based resin film while suppressing adhesion of solids (precipitates) containing polyvinyl alcohol-based resin to the film, and for stably and continuously producing the polyvinyl alcohol-based resin film, particularly a polarizing film.

FIG. 1 is a schematic diagram showing an example of a method for producing a polarizing film, as an example of a polyvinyl alcohol-based resin film, and an example of a polarizing film production apparatus used therein.

Below, we will explain a typical example of a method for manufacturing a polarizing film, showing an embodiment of the invention.

As described above, a typical example of the present invention (hereinafter referred to as "this typical example") relates to a manufacturing method for manufacturing a polarized film from a raw material polyvinyl alcohol-based resin film (hereinafter also referred to as "raw material PVA-based resin film"). The polarized film is manufactured by subjecting the raw material PVA-based resin film to a series of treatments including immersion treatment in a treatment tank (wet treatment), drying treatment, etc.
The polarizing film produced by this typical example has a dichroic dye adsorbed and oriented on a raw material PVA-based resin film that has been stretched.

An example of a polarizing film production apparatus is shown in Figure 1. The polarizing film production apparatus shown in Figure 1 is an apparatus for continuously producing a long polarizing film 25 from a long raw material PVA-based resin film 10, which is a raw material film. The arrows in Figure 1 indicate the transport direction of the film or the flow direction of liquid or bubbles.
1 , a raw material PVA-based resin film 10 is continuously unwound from a winding roll 11, and is successively immersed in a swelling treatment tank 13, a dyeing treatment tank 15, a crosslinking treatment tank 17, and a cleaning treatment tank 19, and finally passed through a drying oven 21 for drying treatment to obtain a polarizing film 25. The polarizing film 25 produced as a long product may be sequentially wound around a winding roll 27, or may be subjected to a polarizing plate production step 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 polarizing film production apparatus typically has a wet treatment section (a zone in which wet treatment is performed using a treatment tank containing a treatment liquid in which the film is immersed) including a swelling treatment tank 13, a dyeing treatment tank 15, a crosslinking treatment tank 17, a cleaning treatment tank 19, etc., and a drying treatment section such as a drying furnace 21 (a zone in which drying treatment is performed on the film after the wet treatment).
1 has a transport path including a wet treatment section and a dry treatment section for a raw material PVA type resin film 10. By transporting the raw material PVA type resin film 10 along this transport path, a series of processes including the wet treatment and the dry treatment are performed, and a polarized film 25 is obtained.
The conveying speed of the raw PVA-based resin film 10 conveyed along the conveying path is usually 1 to 50 m/min, and from the viewpoint of production efficiency, is preferably 5 m/min or more.

As shown in FIG. 1, the transport path is constructed by a plurality of rolls that support and guide the traveling film (raw PVA-based resin film 10 and polarizing film 25) through the wet processing section and the dry processing section. The plurality of rolls are a guide roll, which is a free roll that supports one side of the film, and/or a pair of rolls (usually including a drive roll), and the pair of rolls is, for example, a nip roll that sandwiches or sandwiches and presses the film from both sides. In the example shown in FIG. 1, the polarizing film manufacturing apparatus includes guide rolls 1a to 1l and nip rolls 2a to 2f. The plurality of rolls that define the transport path may include a suction roll, which is a type of drive roll. Usually, all of these rolls support the film in contact with one or both surfaces (main surfaces) of the film in the transport path. These rolls can be placed in appropriate positions, such as before or after each treatment tank and drying means (drying oven) or inside the treatment tank and drying means (drying oven).

A driving roll is a roll that can provide a driving force for film transport to the film in contact with it, and can be a roll to which a roll drive source such as a motor is directly or indirectly connected. A free roll is a roll that supports the running film and can rotate freely in response to the film transport.

The method for producing a polarizing film according to this exemplary embodiment includes the following steps:
a step of conveying the raw PVA type resin film along a conveying path passing through a treatment tank containing a treatment liquid and immersing the raw PVA type resin film in the treatment liquid (wet treatment step S101); and a step of separating at least a part of the treatment liquid into a first part including bubbles present on the liquid surface and a second part other than the first part (foam separation step S201).
including.

The resulting polarizing film 25 has been stretched (usually uniaxially stretched). For this reason, the polarizing film manufacturing apparatus can include a stretching means (wet stretching means) for the raw material PVA-based resin film 10, and the polarizing film manufacturing method can include a stretching process step (wet stretching process step) for the raw material PVA-based resin film 10.

(1) Raw PVA-based resin film The raw PVA-based resin film 10 provided to the wet treatment step S101 (introduced into the wet treatment section) is a film made of polyvinyl alcohol-based resin (hereinafter also referred to as "PVA-based resin"). The PVA-based resin refers to a resin containing 50% by weight or more of a structural unit derived from vinyl alcohol. As the PVA-based resin, a saponified polyvinyl acetate-based resin can be used. Examples of the polyvinyl acetate-based resin include polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate and other monomers copolymerizable therewith.
Examples of other monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, and (meth)acrylamides having an ammonium group.
The term "(meth)acrylic" refers to at least one selected from the group consisting of acrylic and methacrylic. The same applies to other terms with "(meth)".

The degree of saponification of the PVA-based resin can be in the range of 80.0 to 100.0 mol%, but is preferably in the range of 90.0 to 100.0 mol%, more preferably in the range of 94.0 to 100.0 mol%, and even more preferably in the range of 98.0 to 100.0 mol%. If the degree of saponification is less than 80.0 mol%, the water resistance and moist heat resistance of the resulting polarizing film 25 and the polarizing plate containing the same may decrease.

The degree of saponification is the ratio of acetate groups (acetoxy groups: -OCOCH ₃ ) contained in the polyvinyl acetate resin, which is the raw material of the PVA resin, that are converted to hydroxyl groups in the saponification process, expressed as a unit ratio (mol %). From the numbers of hydroxyl groups and acetate groups contained in the PVA resin after saponification, the following formula:
Saponification degree (mol %)=100×(number of hydroxyl groups)/(number of hydroxyl groups+number of acetate groups)
The degree of saponification can be determined in accordance with JIS K 6726 (1994).

The average degree of polymerization of the PVA-based resin is preferably 100 to 10,000, more preferably 1,500 to 8,000, and even more preferably 2,000 to 5,000. The average degree of polymerization of the PVA-based resin can also be determined in accordance with JIS K 6726 (1994). If the average degree of polymerization is less than 100, it is difficult to obtain a polarizing film 25 with favorable polarizing performance, and if it exceeds 10,000, the solubility in the solvent deteriorates, making it difficult to form (form) the raw material PVA-based resin film 10.

An example of the raw PVA-based resin film 10 is an unstretched film formed by forming the PVA-based resin into a film. The film-forming method is not particularly limited, and known methods such as melt extrusion and solvent casting can be used.
Another example of the raw PVA-based resin film 10 is a stretched film obtained by stretching the unstretched film. This stretching is usually uniaxial stretching, preferably longitudinal uniaxial stretching. Longitudinal stretching refers to stretching in the machine direction (MD) of the film, i.e., in the longitudinal direction of the film.
When the raw PVA-based resin film 10 to be subjected to the wet treatment step S101 (introduced into the wet treatment section) is a stretched film, the stretching is preferably dry stretching, which means stretching performed in air, and is usually longitudinal uniaxial stretching.

Examples of dry stretching include hot roll stretching, in which a film is passed between a hot roll with a heated surface and a guide roll (or a hot roll) having a different peripheral speed from that of the hot roll, and longitudinal stretching is performed under heating using the hot roll; inter-roll stretching, in which a film is passed through a heating means (such as an oven) between two nip rolls installed at a distance, and longitudinal stretching is performed by the difference in peripheral speed between the two nip rolls; tenter stretching; and compression stretching.
The stretching temperature (surface temperature of the heat roll, temperature inside the oven, etc.) is, for example, 80 to 150°C, and preferably 100 to 135°C.

The stretching ratio of the above stretching is usually 1.1 to 8 times, preferably 2.5 to 5 times, depending on whether wet stretching is performed in the wet treatment step S101 described below and the stretching ratio in the wet stretching.

The raw PVA-based resin film 10 may contain additives such as plasticizers. Preferred examples of plasticizers are polyhydric alcohols, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, triglycerin, tetraethylene glycol, trimethylolpropane, and polyethylene glycol. The raw PVA-based resin film 10 may contain one or more types of plasticizers.
The content of the plasticizer is usually 5 to 20 parts by weight, and preferably 7 to 15 parts by weight, based on 100 parts by weight of the PVA-based resin constituting the raw PVA-based resin film 10 .

The thickness of the raw PVA-based resin film 10 that is subjected to the wet treatment step S101 (introduced into the wet treatment section) is usually 10 to 150 μm, depending on whether the raw PVA-based resin film 10 is a stretched film or not, and from the viewpoint of thinning the resulting polarizing film 25, is preferably 100 μm or less, more preferably 65 μm or less, even more preferably 50 μm or less, and particularly preferably 35 μm or less (for example, 30 μm or less, or even 20 μm or less).

(2) Wet treatment step S101
The wet treatment section where the wet treatment step S101 is carried out is a zone disposed on the transport path of the raw PVA type resin film 10, and includes a treatment tank containing a treatment liquid in which the raw PVA type resin film 10 is immersed. In this wet treatment section, the wet treatment step S101 is carried out in which the raw PVA type resin film 10 is immersed in the treatment liquid while being transported.

The wet treatment section normally includes, as the treatment tanks, a dyeing treatment tank 15 and a crosslinking treatment tank 17, and preferably further includes a swelling treatment tank 13 and a cleaning treatment tank 19. These treatment tanks are normally arranged in the following order from the upstream side of the transport path: swelling treatment tank 13, dyeing treatment tank 15, crosslinking treatment tank 17, and cleaning treatment tank 19 (see FIG. 1).
1 shows an example in which one tank each of the swelling treatment tank 13, dyeing treatment tank 15, crosslinking treatment tank 17, and cleaning treatment tank 19 is provided, but if necessary, two or more tanks of the dyeing treatment tank 15 may be provided, and two or more tanks of the crosslinking treatment tank 17 may be provided. The same applies to the swelling treatment tank 13 and the cleaning treatment tank 19, and two or more tanks each may be provided.

(2-1) Swelling Treatment Step The swelling treatment in the swelling treatment step is carried out as necessary for the purposes of removing foreign matter from the raw PVA-based resin film 10, removing plasticizers, imparting ease of dyeing, plasticizing the raw PVA-based resin film, etc.
Referring to FIG. 1, the swelling treatment step can be carried out by transporting a raw PVA type resin film 10 along a film transport path while continuously unwinding it from an unwinding roll 11, immersing the raw PVA type resin film 10 in a swelling treatment tank 13 containing a swelling treatment liquid for a predetermined period of time, and then withdrawing it.

The treatment liquid (swelling treatment liquid) contained in the swelling treatment tank 13 may be, for example, water (pure water, etc.), or may be an aqueous solution containing a water-soluble organic solvent such as an alcohol. The swelling treatment liquid may also contain boric acid, chlorides, inorganic acids, inorganic salts, etc.
The temperature of the swelling solution is usually 10 to 70° C., preferably 15 to 50° C., and more preferably 15 to 35° C. The immersion time of the raw PVA-based resin film 10 (residence time in the swelling solution) is usually 10 to 600 seconds, and preferably 15 to 300 seconds.

During the swelling treatment, the raw material PVA-based resin film 10 may be subjected to a wet stretching treatment (usually a uniaxial stretching treatment). In this case, the stretching ratio is usually 1.2 to 3 times, preferably 1.3 to 2.5 times. With reference to Fig. 1, for example, the uniaxial stretching treatment can be performed in a swelling treatment tank 13 by utilizing the difference in peripheral speed between nip rolls 2a and 2b.
In the example shown in FIG. 1, the film pulled out from the swelling treatment tank 13 passes through a guide roll 1 c and a nip roll 2 b in this order, and is introduced into a dyeing treatment tank 15 .

(2-2) Dyeing Treatment Step The dyeing treatment is carried out for the purpose of adsorbing and orienting a dichroic dye on the raw PVA-based resin film 10, etc.
1, the dyeing process can be carried out by transporting the raw PVA type resin film 10 along a film transport path, immersing the raw PVA type resin film 10 in a dyeing process tank 15 for a predetermined time, and then withdrawing it. The dyeing process tank 15 is a tank for immersing the raw PVA type resin film 10 in a dyeing process liquid contained therein. The raw PVA type resin film 10 immersed in the dyeing process liquid is preferably a film following the swelling process (immersed in a swelling process tank 13).
The dyeing solution contained in the dyeing tank 15 is a solution (usually an aqueous solution) containing a dichroic dye. The dichroic dye may be iodine or a dichroic organic dye, and is preferably iodine. The dichroic dye may be used alone or in combination of two or more kinds.

When iodine is used as the dichroic dye, the dyeing treatment liquid can be an aqueous solution containing iodine and potassium iodide. Instead of potassium iodide, other iodides such as zinc iodide may be used, or potassium iodide may be used in combination with other iodides. In addition, compounds other than iodides, such as boric acid, zinc chloride, cobalt chloride, etc., may be coexisted. When boric acid is added, it is distinguished from the crosslinking treatment liquid described below in that it contains iodine. For example, if it contains about 0.003 parts by weight or more of iodine per 100 parts by weight of water, it can be considered as a dyeing treatment liquid. The content of iodine in the dyeing treatment liquid is usually 0.003 to 1 part by weight per 100 parts by weight of water. When the dyeing treatment liquid contains an iodide such as potassium iodide, the content is usually preferably 0.1 to 20 parts by weight per 100 parts by weight of water.

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

As described above, the polarizing film manufacturing apparatus can include two or more dyeing treatment tanks 15. In this case, the composition and temperature of each dyeing treatment liquid may be the same or different.

In order to improve the dyeability of the dichroic dye, it is preferable that the raw 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, the film may be stretched while the dyeing process is being performed. The cumulative stretching ratio before the dyeing process (the stretching ratio during the dyeing process when there is no stretching step before the dyeing process) is usually 1.6 to 4.5 times, and preferably 1.8 to 4 times. With reference to FIG. 1, for example, the uniaxial stretching process can be performed in the dyeing process tank 15 by utilizing the difference in peripheral speed between the nip rolls 2b and 2c.
In this typical example shown in FIG. 1, the film pulled out from the dyeing treatment tank 15 passes through a guide roll 1f and a nip roll 2c in this order, and is introduced into a crosslinking treatment tank 17.

(2-3) Crosslinking Treatment Step The crosslinking treatment in the crosslinking treatment step is carried out for the purposes of imparting water resistance through crosslinking, adjusting hue (complementary color), and the like.
1, the crosslinking treatment can be carried out by transporting the raw PVA-based resin film 10 following the dyeing treatment step S101 (immersed in the dyeing treatment bath 15) along a film transport path, immersing the raw PVA-based resin film 10 in a crosslinking treatment bath 17 for a predetermined time, and then withdrawing it. The crosslinking treatment bath 17 is a bath for immersing the raw PVA-based resin film 10 in the crosslinking treatment liquid contained therein.
The crosslinking treatment liquid contained in the crosslinking treatment tank 17 is a liquid (usually an aqueous solution) containing a crosslinking agent. The raw PVA-based resin film 10 after the dyeing treatment step is immersed in the crosslinking treatment liquid to perform crosslinking treatment.

Examples of the crosslinking agent contained in the crosslinking treatment liquid include boric acid, glyoxal, glutaraldehyde, etc., and preferably boric acid. Two or more types of crosslinking agents can be used in combination.
The content of the crosslinking agent in the crosslinking treatment liquid is generally 0.1 to 15 parts by weight, and preferably 1 to 12 parts by weight, per 100 parts by weight of water.

When the dichroic dye is iodine, the crosslinking treatment liquid preferably contains an iodide in addition to a crosslinking agent.
Examples of iodides include potassium iodide and zinc iodide.
The content of the iodide in the crosslinking treatment liquid is generally 0.1 to 20 parts by weight, preferably 5 to 15 parts by weight, per 100 parts by weight of water.

The crosslinking treatment solution used in the crosslinking treatment step may contain compounds other than iodides. Examples of such compounds include zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate.

The temperature of the crosslinking treatment liquid is generally 20 to 85°C, and preferably 30 to 70°C. The immersion time (residence time in the crosslinking treatment liquid) of the raw material PVA-based resin film 10 is generally 10 to 600 seconds, and preferably 20 to 300 seconds.

As described above, the polarizing film manufacturing apparatus may include two or more crosslinking treatment tanks 17. In this case, the compositions and temperatures of the crosslinking treatment liquids may be the same or different. The crosslinking treatment liquid may have a concentration of a crosslinking agent, an iodide, etc., and a temperature according to the purpose of immersing the raw PVA-based resin film 10. The crosslinking treatment for water resistance by crosslinking and the crosslinking treatment for hue adjustment (complementary color) may each be performed in a plurality of steps.
In general, when both a crosslinking treatment for water resistance by crosslinking and a crosslinking treatment for hue adjustment (complementary color) are performed, a tank (complementary color tank) for performing the crosslinking treatment for hue adjustment (complementary color) is disposed in the latter stage. The temperature of the processing solution contained in the complementary color tank is, for example, 10 to 55°C, and preferably 20 to 50°C. The content of the crosslinking agent in the processing solution contained in the complementary color tank is, for example, 1 to 5 parts by weight per 100 parts by weight of water. The content of the iodide in the processing solution contained in the complementary color tank is, for example, 3 to 30 parts by weight per 100 parts by weight of water.

A stretching treatment (usually a uniaxial stretching treatment) may be performed while the crosslinking treatment is being performed. For example, referring to Fig. 1, the uniaxial stretching treatment can be performed in a crosslinking treatment tank 17 by utilizing the difference in peripheral speed between a nip roll 2c and a nip roll 2d.
In the example shown in FIG. 1, the film pulled out from the crosslinking treatment tank 17 passes through a guide roll 1 i and a nip roll 2 d in this order, and is introduced into a cleaning treatment tank 19 .

(2-4) Cleaning Step The polarizing film manufacturing method may further include a cleaning step after the crosslinking step, and for this purpose, the polarizing film manufacturing apparatus may further include a cleaning tank 19 disposed downstream of the crosslinking tank 17. The cleaning step is carried out for the purpose of removing excess chemicals adhering to the raw material PVA-based resin film 10 after the crosslinking step, etc.
1, the cleaning treatment can be carried out by conveying the raw PVA type resin film 10 after the crosslinking treatment step (immersed in the crosslinking treatment bath 17) along a film conveying path, immersing the raw PVA type resin film 10 in a cleaning treatment bath 19 for a predetermined time, and then pulling it out. Alternatively, the cleaning treatment may be a treatment in which a cleaning liquid is sprayed onto the raw PVA type resin film 10 after the crosslinking treatment step, for example as a shower, or immersion in the cleaning treatment bath 19 and spraying of the cleaning liquid may be combined. Fig. 1 shows an example of the case where the raw PVA type resin film 10 is immersed in the cleaning treatment bath 19 for cleaning treatment.

The cleaning liquid contained in the cleaning tank 19 or sprayed can be, for example, water (e.g., pure water) or an aqueous solution containing a water-soluble organic solvent such as 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 washing process is being performed. With reference to FIG. 1, for example, the uniaxial stretching process can be performed in the washing process tank 19 by utilizing the difference in peripheral speed between nip rolls 2d and 2e.

(2-5) Stretching Step In the wet treatment step, the raw material PVA-based resin film 10 may be subjected to wet stretching.
The wet stretching is usually uniaxial stretching, and can be carried out while carrying out any one of a swelling treatment, a dyeing treatment, a crosslinking treatment, and a washing treatment, or during two or more treatments selected from these.
The wet stretching is preferably performed in the crosslinking step or in one or more steps prior to the crosslinking step. As described above, in order to improve the dyeability of the dichroic dye and obtain a polarizing film 25 having good polarization properties, it is more preferable that the raw material PVA-based resin film 10 to be subjected to the dyeing step has been subjected to at least some degree of stretching.
From the viewpoint of the polarization properties of the resulting polarizing film 25, the stretching ratio in the wet stretching is preferably adjusted so that the final cumulative stretching ratio of the polarizing film 25 (when the raw material PVA-based resin film 10 to be subjected to the wet treatment is a stretched film, the cumulative stretching ratio including the stretching of this film) is 3 to 8 times.

When performing the wet stretching process, the polarizing film manufacturing apparatus includes a wet stretching means for the raw material PVA-based resin film 10. The wet stretching means is preferably a stretching means for performing inter-roll stretching. In the case of performing wet inter-roll stretching during the crosslinking process as an example, the stretching means for performing inter-roll stretching are two nip rolls 2c and 2d arranged before and after the crosslinking treatment tank 17. Similarly, when performing stretching during other wet processes, two nip rolls arranged at a distance from each other can be used as the wet stretching means.

(3) Drying Treatment Step The drying treatment section where the drying treatment step is carried out is a zone for drying the raw PVA type resin film 10 after the wet treatment step S101, which is located downstream of the wet treatment section on the transport path of the raw PVA type resin film 10. The raw PVA type resin film 10 after the wet treatment step S101 can be dried by introducing the film into the drying treatment section while continuing to transport the film, thereby obtaining a polarized film 25 (see FIG. 1).

The drying processing section includes a film drying means (heating means). A suitable example of the drying means is a drying oven. The drying oven is preferably capable of controlling the temperature inside the oven. The drying oven is, for example, a hot air oven that can increase the temperature inside the oven by supplying hot air or the like. The drying process by the drying means may be a process of closely contacting the raw material PVA-based resin film 10 after the wet processing step with one or more heating bodies having a convex curved surface, or a process of heating the film using a heater.

The heating body can be a roll (e.g., a guide roll that also serves as a heating roll) that has a heat source (e.g., a heat medium such as hot water or an infrared heater) inside and can increase the surface temperature. Examples of the heater can include an infrared heater, a halogen heater, a panel heater, etc. Figure 1 shows an example of introducing the raw material PVA-based resin film 10 after the wet treatment step S101 into a drying furnace 21 and performing the drying treatment.

The temperature of the drying process (e.g., 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 polarizing film 25 is a stretched (usually uniaxially stretched) PVA-based resin film to which a dichroic dye is adsorbed and oriented. The thickness of the polarizing film 25 is usually 2 to 40 μm. From the viewpoint of reducing the thickness of a polarizing plate including the polarizing film 25, the thickness of the polarizing film 25 is preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less.

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

The obtained polarizing film 25 may be wound up on a winding roll 27 in a roll form, or may be directly subjected to the polarizing plate production process (a process of laminating a thermoplastic resin film (such as a protective film) on one or both sides of the polarizing film 25) without being wound up.

(4) Foam Processing Step Hereinafter, the steps of performing some operation or treatment on bubbles contained in any of the processing liquids used in the wet processing step S101 or on a liquid containing such bubbles, and the steps that may be performed as a result of such steps, are collectively referred to as the "foam processing step".
In the method for producing a polarizing film according to this exemplary embodiment, the bubble treatment step includes:
A step of separating at least a part of the treatment liquid into a first part including bubbles present on the liquid surface and a second part other than the first part (a foam separating step S201).
including.
In one embodiment, the foam treatment step is performed before the foam separation step S201.
A step of generating or increasing foam (foam generating step S200)
Further includes:
In another embodiment, the foam treatment step includes a step of precipitating a solid (hereinafter referred to as "solid") containing a PVA-based resin from the foam contained in the first portion (solid precipitating step S202), and a step of removing at least a part of the solid (solid removing step S203).
Further includes:
In yet another embodiment, the foam treatment step comprises:
A step of returning at least a portion of the liquid recovered by the solid removal step S203 to the treatment tank (first reuse step S204).
Further includes:
In still yet another embodiment, the foam treatment step comprises:
A step of returning at least a part of the second portion to the treatment tank (second reuse step S205).
Further includes:
The present invention also includes combinations of two or more of the above-described embodiments.
Each step will be described below.

(4-1) Foam Separation Step S201 and Foam Generation Step S200
In this step, at least a portion of the treatment liquid used in the wet treatment step S101 is separated into a first portion including bubbles present on the liquid surface and a second portion other than the first portion.
The bubbles contained in the first portion are bubbles that can be used to generate solids therefrom, as will be described in detail later. The inventors' investigations have revealed that the bubbles contained in the first portion contain a more concentrated PVA-based resin than the treatment liquid itself (treatment liquid without bubbles). Therefore, by selectively removing the bubbles, the PVA-based resin dissolved in the treatment liquid can be efficiently removed, and the above-mentioned problem of solids adhering to the film can be effectively suppressed.
Furthermore, in the foam treatment process, the foam contained in the first portion is selectively separated, collected, and treated. This results in treating a liquid with a high concentration of PVA-based resin, and therefore the treatment efficiency can be improved compared to treating a liquid with a low concentration of PVA-based resin.
As described above, in the present invention, it is essential to separate and recover the bubbles containing the PVA-based resin in a concentrated state in the unruptured state in the foam treatment step, and to treat the bubbles.

The treatment liquid to be subjected to the foam separation step S201 is one or more treatment liquids selected from the group consisting of a swelling treatment liquid, a dyeing treatment liquid, a crosslinking treatment liquid, and a cleaning treatment liquid. For example, when there are two or more crosslinking treatment tanks 17, the foam separation step S201 may be performed for each of the crosslinking treatment liquids contained in two or more tanks selected from these. The same applies to the swelling treatment tank 13, the dyeing treatment tank 15, and the cleaning treatment tank 19.
Among the above-mentioned treatment liquids, since the PVA-based resin is likely to precipitate or the concentration of the PVA-based resin is likely to increase, it is preferable to perform the foam separation step S201 at least for the crosslinking treatment liquid, and it is more preferable to perform the foam separation step S201 at least for the crosslinking treatment liquid used in the color complementation tank.
The concentration of the PVA-based resin in the treatment liquid subjected to the foam separation step S201 may be, for example, about 5 ppm to 5000 ppm by weight.

The bubbles present on the liquid surface of the treatment liquid contained in the first portion may be bubbles unintentionally generated during the process of manufacturing the polarizing film, but preferably include bubbles actively (intentionally) generated. In the latter case, the bubble treatment process further includes a bubble generation process S200 for generating or increasing bubbles.
The bubble generating step S200 may be performed on the treatment liquid containing unintentionally generated bubbles. In this case, the bubble generating step S200 is a step of increasing the amount of bubbles.

The foam generating step S200 may be performed in the processing tank for the processing liquid contained in the processing tank, or may be performed on at least a part of the processing liquid extracted from the storage tank. When the foam generating step S200 is performed in the processing tank, it is preferable to perform the foam generating step S200 without bringing the foam into contact with the film in order to prevent adhesion of solids.
1 shows an example in which a first storage tank 30 is provided for storing at least a part of the crosslinking treatment liquid extracted from the crosslinking treatment liquid in the crosslinking treatment tank 17, and the at least a part of the crosslinking treatment liquid is introduced thereto through a connection path (connection line) 29, and a foam generating step S200 is performed on the crosslinking treatment liquid stored in the first storage tank 30. The connection path (connection line) 29 connects the crosslinking treatment tank 17 and the first storage tank 30.

The foam generating step S200 can be carried out, for example, by the following method, although it is not particularly limited thereto.
[a] A method in which bubbling is performed in the processing liquid in the processing tank or the first storage tank 30 using a pump or the like.
Examples of the gas used for bubbling include air and inert gases such as nitrogen, helium, argon, etc. The use of an inert gas is preferable because bubbling can be performed without oxidizing iodine ions.
The size (diameter) of the bubbles generated by bubbling is, for example, 1 μm to 5 cm, preferably 1 μm to 5 mm, and more preferably 1 μm to 1 mm.
[b] In addition to the above [a], a method of applying some kind of external impact to the processing liquid in the processing tank or the first storage tank 30.
[c] A combination of [a] and [b] above.

Specific examples of the above method (b) include the following methods.
[b-1] A method of dropping a liquid onto the surface of the treatment liquid in the treatment tank or the first storage tank 30.
The liquid may be fresh processing liquid or liquid returned to the processing tank in a first reuse step S204 and/or a second reuse step S205 described later in detail. The manner in which the liquid is dropped is not particularly limited, but from the viewpoint of efficiently generating bubbles, a shower-like liquid is preferably applied to the liquid surface of the processing liquid.
[b-2] A method of applying shear force by stirring the gas-liquid interface of the treatment liquid in the treatment tank or the first storage tank 30.
[b-3] A method of shaking the processing liquid in a state in which the processing liquid has a gas-liquid interface, for example by introducing the processing liquid into the first storage tank 30 to such an extent that there are spaces in the first storage tank 30 that are not filled with the processing liquid.
[b-4] A method in which dripping from the transport path of the polarizing film production equipment or from the raw material PVA-based resin film 10 being transported inside the polarizing film production equipment is utilized, and the dripping is allowed to fall onto the liquid surface of the processing liquid in the processing tank or first storage tank 30.
[b-5] A method of generating bubbles by shock or shaking of the processing liquid that occurs when the processing liquid in the processing tank is introduced into the first storage tank 30.

The foam generating step S200 may be performed in a connecting line for extracting the processing liquid from the processing tank. For example, referring to Fig. 1, a pump or the like may be used to suck at least a part of the crosslinking processing liquid together with gas from the crosslinking processing tank 17 through the connecting line 29, and bubbles may be generated in the connecting line 29 by shaking the crosslinking processing liquid caused by this. Alternatively, a means for stirring or shaking the crosslinking processing liquid flowing through the connecting line 29 may be provided in the connecting line 29, and bubbles may be generated in the connecting line 29 by this means.
If the bubble generating step S200 is performed after cooling the treatment liquid or while cooling the treatment liquid, the solubility of the PVA-based resin in water decreases, thereby increasing the efficiency of removing the PVA-based resin dissolved in the treatment liquid.
The cooling temperature of the treatment liquid is, for example, 0 to 25°C, and preferably 0 to 15°C.

In the foam separation step S201, the first portion can be separated from the foam-containing treatment liquid in the treatment tank or the first storage tank 30 by, for example, the following method.
[A] A method of separating a first portion consisting of bubbles present on the surface of the processing liquid and a portion of the liquid below it by overflowing the processing liquid from the processing tank or first storage tank 30.
[B] A method of sucking the foam in the treatment tank or the first storage tank 30 using a pump or the like. In this method as well, the first portion may contain a part of the liquid below the foam.
[C] A method of blowing gas onto bubbles on the surface of the treatment liquid in the treatment tank or the first storage tank 30 to remove the bubbles from above the liquid surface. In this method, the first portion may also include a portion of the liquid below the bubbles.
[D] A combination of two or more selected from the above [A] to [C].

1 shows an example in which a first portion of the crosslinking treatment liquid containing bubbles contained in a first storage tank 30 is introduced into a second storage tank 40 through a connecting path (connecting line) 31, thereby separating the first portion from a second portion. The connecting path (connecting line) 31 connects the first storage tank 30 and the second storage tank 40.
However, the present invention is not limited to the above example, and for example, when the treatment liquid in the treatment tank (e.g., the crosslinking treatment tank 17) unintentionally contains bubbles and the foam generating step S200 is not performed, or when the foam generating step S200 is performed in the treatment tank, the first storage tank 30 may be omitted, and the first part containing bubbles in the treatment tank may be directly introduced into the second storage tank 40 and separated into the first part and the second part. Of course, even in the above case, the treatment liquid containing bubbles may be temporarily stored in the first storage tank 30 and then separated into the first part and the second part.

As described above, the bubbles contained in the first portion contain concentrated PVA-based resin, and the bubbles contained in the first portion have a higher concentration of PVA-based resin than the treatment liquid before the bubbles were generated and the second portion. By using a method of precipitating solids from such bubbles and removing them, it is possible to increase the efficiency of removing the PVA-based resin from the treatment liquid before the bubbles were generated and the second portion, and also to increase the efficiency of removing the PVA-based resin.

(4-2) Solid Precipitation Step S202
This step is a step of precipitating a solid containing a PVA-based resin from the bubbles contained in the first portion. In the example shown in FIG. 1, the solid precipitating step S202 can be performed in the second storage tank 40.
The precipitated solid may further contain other components contained in the treatment liquid (for example, a crosslinking agent, an iodide, etc. in the case of a crosslinking treatment liquid) in addition to the PVA-based resin.

In the solid precipitating step S202, a solid containing a PVA-based resin can be precipitated from the bubbles contained in the first portion by, for example, the following method.
[i] A method of bubbling in the first portion using a pump or the like.
Bubbling increases the surface area of the first portion, thereby promoting precipitation of solids.
Examples of the gas used for bubbling include air and inert gases such as nitrogen, helium, and argon.
The present inventors have found through their research that, in precipitating solids from bubbles by bubbling, the larger the area of the gas-liquid interface of the first portion is, the more efficient the solid precipitating process S202 is. Therefore, for example, when the first portion is accommodated in the second storage tank 40 and the solid precipitating step S202 is performed, it is preferable to increase the bottom area of the second storage tank 40 and increase the area of the gas-liquid interface of the first portion accommodated therein.
The inventors' investigations have revealed that the smaller the size of the bubbles generated by bubbling, the more efficient the solid precipitation. Therefore, the smaller the size of the bubbles, the better. The size (diameter) of the bubbles generated by bubbling is, for example, 1 μm to 5 cm, preferably 1 μm to 5 mm, and more preferably 1 μm to 1 mm.
[ii] A method of forming minute droplets, such as by spraying the first portion containing the bubbles.
By changing the form of the first portion including the bubbles into tiny droplets, the evaporation rate increases, the concentration of the PVA-based resin in the droplets increases, and the temperature of the droplets decreases due to the heat of vaporization, thereby accelerating the precipitation of solids.
[iii] A method for cooling the first portion.
Cooling increases the cohesive force between molecular chains of the PVA-based resin, promoting precipitation of a solid.
The cooling temperature of the first portion is, for example, 0 to 25°C, and preferably 0 to 15°C.
[iv] A method of applying a shear force to the first portion, such as by stirring the first portion.
[v] A combination of two or more selected from the above [i] to [iv].
In particular, the combination of the above [i] and [iii] is advantageous in terms of increasing the efficiency of solid precipitation.
Even if the first portion separated in the foam separation step S201 is only foam, the above methods [i] to [iv] can be applied by bursting the foam and turning it into liquid.

In the case where the first portion includes a foam portion and an associated liquid portion, the foam portion and the liquid portion may be separated and then the solid precipitation step S202 may be performed separately for each of them. By selectively collecting the foam portion containing concentrated PVA-based resin and performing the solid precipitation step S202 on this, it is possible to treat a material containing a higher concentration of PVA-based resin, thereby improving the processing efficiency.

(4-3) Solid Removal Step S203
This step is a step of removing at least a portion of the solid generated in the solid precipitating step S202.
The method for removing solids is not particularly limited, and examples thereof include a method for removing solids by utilizing physical adsorption, such as activated carbon treatment, and a method for removing solids by solid-liquid separation (filtration) using a solid-liquid separation device such as a filtration filter.
1 shows an example in which the solids are removed by introducing the first portion containing the precipitated solids into a filtration filter 50 through a connection path (connection line) 41. The connection path (connection line) 41 connects the second storage tank 40 and the filtration filter 50.

(4-4) First reuse process S204
The foam treatment step may further include a step of returning at least a portion of the liquid recovered in the solid removal step S203 (the first portion after the solids have been removed) to the treatment tank.
The liquid is returned to the treatment tank after temperature adjustment, etc., if necessary.
Referring to FIG. 1, the liquid can be returned to the treatment tank (crosslinking treatment tank 17) through a connection path 51 that connects the filtration filter 50 and the treatment tank.

The liquid returned to the processing tank has had its PVA-based resin concentration reduced through the solid removal step S203. Therefore, by continuously carrying out a series of steps, namely, the foam separation step S201 (and preferably the foam generation step S200), the solid precipitation step S202, the solid removal step S203, and the first reuse step S204, the concentration of the PVA-based resin in the processing liquid in the processing tank can be reduced or maintained to a level that can suppress the above-mentioned problem of deposits containing the PVA-based resin adhering to the film.

To adjust the amount of treatment liquid in the treatment tank, fresh treatment liquid may be added while returning at least a portion of the first portion from which solids have been removed to the treatment tank, or at a time separate from the step of returning at least a portion of the first portion from which solids have been removed to the treatment tank, and then the treatment tank may be replenished.

(4-5) Second reuse process S205
The foam treatment step may further include a step of returning at least a portion of the second fraction generated in the foam separation step S201 to the treatment tank.
The second portion is returned to the treatment tank after, as necessary, solid-liquid separation (filtration) using a solid-liquid separation device such as a filter and/or temperature adjustment.
Referring to FIG. 1, the second portion can be returned to the treatment tank through a connection path 32 connecting the first storage tank 30 and the treatment tank (crosslinking treatment tank 17).
When the foam separation step S201 is performed on the treatment liquid in the treatment tank, the second portion usually remains in the treatment tank, making the second reuse step S205 unnecessary.

The second portion returned to the processing tank is a fraction separated from the first portion, and therefore has a reduced concentration of PVA-based resin. Therefore, by continuously carrying out a series of steps, namely, the foam separation step S201 (and preferably the foam generation step S200), the solid precipitation step S202, the solid removal step S203, and the second reuse step S205, the concentration of the PVA-based resin in the processing liquid in the processing tank can be reduced or maintained to a level that can suppress the above-mentioned problem of solids (precipitations containing PVA-based resin) adhering to the film.

(4-6) Other embodiments of the foam treatment step The embodiment described above includes a solid precipitation step S202 in which a first portion containing foam is separated and recovered in the foam separation step S201, and then a solid containing a PVA-based resin is precipitated from the first portion, but is not limited thereto.
For example, the first portion recovered in the foam separation step S201 may be discarded rather than being subjected to the solid precipitation step S202. In this embodiment, at least a portion of the second portion generated in the foam separation step S201 may be returned to the treatment tank by the second reuse step S205 described above.
In such an embodiment, the fraction containing the PVA-based resin at a high concentration is removed as bubbles, so that the above-mentioned problem of deposits containing the PVA-based resin adhering to the film can be suppressed.

(5) Experimental Examples (5-1) Experimental Example 1
A polarizing film 25 was continuously produced from a long polyvinyl alcohol film (raw PVA-based resin film 10) using a polarizing film production apparatus similar to that shown in Fig. 1 except that it had two crosslinking treatment tanks (referred to as a first crosslinking treatment tank 17a and a second crosslinking treatment tank 17b). The polyvinyl alcohol film used was a polyvinyl alcohol film with a thickness of 60 µm, and the saponification degree of the polyvinyl alcohol constituting the film was 99.9 mol % or more and the average polymerization degree was 2,400.

[a] Swelling treatment step The raw PVA-based resin film 10 was transported while being continuously unwound from the unwinding roll 11, and was immersed in a swelling treatment tank 13 containing pure water at 30° C., and was stretched between the nip rolls 2a, 2b at a peripheral speed difference of 2.5 times so that the film did not slacken, thereby causing a swelling treatment.

[b] Dyeing process Next, the raw PVA-based resin film 10 that had passed through the nip rolls 2b was immersed for 120 seconds in a dyeing process tank 15 containing iodine/potassium iodide/water (weight ratio) of 0.05/2/100 at 30° C. In this dyeing process, a peripheral speed difference was provided between the nip rolls 2b and 2c, and inter-roll stretching (longitudinal uniaxial stretching) was performed at 1.1 times the original size.

[c] Crosslinking process Next, in order to carry out the first crosslinking process for the purpose of making the film water-resistant, the raw PVA-based resin film 10 that has passed through the nip rolls 2c was immersed in a first crosslinking process tank 17a at 56°C, in which the weight ratio of potassium iodide/boric acid/water was 12/4.4/100, for 30 seconds. In this first crosslinking process, a peripheral speed difference was also provided between the nip rolls, and inter-roll stretching (longitudinal uniaxial stretching) was carried out at 1.9 times. Next, the raw PVA-based resin film 10 after the first crosslinking process was immersed in a second crosslinking process tank (complementary color tank) 17b at 40°C, in which the weight ratio of potassium iodide/boric acid/water was 9/2.9/100, for 15 seconds (second crosslinking process). In this second crosslinking process, a peripheral speed difference was also provided between the nip rolls, and inter-roll stretching (longitudinal uniaxial stretching) was carried out at 1.05 times.
Thereafter, the raw material PVA-based resin film 10 after the second crosslinking treatment was immersed in a cleaning treatment tank 19 containing pure water at 15°C, and then passed through a drying oven 21 to be dried at 70°C for 3 minutes, thereby producing a polarizing film 25.

When the second crosslinking treatment tank 17b was observed after the continuous production of the polarizing film, bubbles were found to have formed on the surface of the crosslinking treatment liquid. Samples 1 (second part) and 2 (including the first and second parts) were taken from the crosslinking treatment liquid in the second crosslinking treatment tank 17b.
Sample 1: A sample taken from a portion of the crosslinking treatment liquid that had no bubbles. The polyvinyl alcohol concentration in Sample 1 was measured by GPC (gel permeation chromatography) and found to be about 1,300 ppm by weight.
Sample 2: A sample of bubbles present on the surface of the crosslinking treatment liquid together with the supernatant liquid below.

Sample 2 was also divided into two, and one of the two liquids was subjected to bubbling treatment (gas used: air) to produce sample 3.

The following evaluation tests were conducted on samples 1 to 3 and pure water as a reference sample. The evaluation tests were conducted at a temperature of 30°C. Samples 1 to 3 were left to stand for a while after sampling (sample 3 was left to stand after bubbling treatment) before the evaluation tests were conducted.

(1) Visual Observation The samples were visually observed to evaluate whether particles (solids) containing polyvinyl alcohol were observed in the liquid. The results are shown in Table 1. It was confirmed by GPC and infrared spectroscopy that the particles dispersed in the liquid contained polyvinyl alcohol.
The evaluation criteria are as follows:
A: No particles are visible.
B: A few particles are visible.
C: A large amount of particles are visible.
(2) Measurement of particles in liquid Using a liquid particle counter ("Light-shielding particle detector (KS-42D)" manufactured by Rion Co., Ltd.), the concentration of particles having each particle size in the liquid was measured. The results are shown in Table 1.

(5-2) Experimental Example 2
Sample 2 from Experimental Example 1 was stirred for 60 seconds to increase the amount of bubbles to prepare Sample 4, which was then filtered through a membrane filter with a pore size of 0.45 μm at room temperature. The filtrate was prepared as Sample 5. Samples 4 and 5 were subjected to the same submerged particle measurement as in Experimental Example 1. The results are shown in Table 2.

(5-3) Experimental Example 3
If the sample 5 obtained in Experimental Example 2 is returned to the crosslinking treatment liquid in the second crosslinking treatment tank 17b, the amount of solids (solids containing a PVA-based resin) in the crosslinking treatment liquid will be reduced, thereby making it possible to reduce the amount of the solids adhering to the continuously produced polarized film 25. The polarized film 25 obtained in this manner has excellent appearance and quality because the adhesion of the solids is suppressed.

1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1i, 1j, 1k, 1l guide rolls, 2a, 2b, 2c, 2d, 2e, 2f nip rolls, 10 raw polyvinyl alcohol resin film (raw PVA resin film), 11 unwinding roll, 13 swelling tank, 15 dyeing tank, 17 crosslinking tank, 19 cleaning tank, 21 drying oven, 25 polarizing film, 27 winding roll, 29 connection path, 30 first storage tank, 31 connection path, 32 connection path, 40 second storage tank, 41 connection path, 50 filtration filter, 51 connection path.

Claims (6)

A method for producing a polyvinyl alcohol-based resin film, comprising the steps of:
a step of transporting a raw polyvinyl alcohol-based resin film along a transport path passing through a treatment tank containing a treatment liquid that is a crosslinking treatment liquid having a temperature of 30 to 70° C. , and immersing the raw polyvinyl alcohol-based resin film in the treatment liquid;
separating at least a portion of the treatment liquid into a first portion including bubbles present on a liquid surface and a second portion other than the first portion;
Including,
Prior to the separating step,
removing at least a portion of the processing solution from the processing tank and storing it in a storage tank;
The method further includes a step of generating or increasing the foam in at least a portion of the treatment liquid contained in the storage tank,
the step of generating or increasing the foam is carried out after or while cooling the treatment liquid,
The cooling temperature of the treatment liquid in the cooling step is 0 to 15° C.,
A method according to claim 1, wherein in the separating step, the bubbles contained in the first portion are separated in a state of unpopped bubbles. A step of precipitating a solid containing a polyvinyl alcohol-based resin from the foam contained in the first portion;
removing at least a portion of the solid;
The method of claim 1 further comprising: The method of claim 2, further comprising returning at least a portion of the liquid recovered by the removing step to the treatment tank. The method according to any one of claims 1 to 3, further comprising returning at least a portion of the second portion to the treatment tank. The method according to any one of claims 1 to 4, wherein the foam contained in the first portion has a higher concentration of polyvinyl alcohol-based resin than the concentration of polyvinyl alcohol-based resin in the second portion. The method according to any one of claims 1 to 5, wherein the polyvinyl alcohol-based resin film is a polarizing film.

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