JP2024145394A - Manufacturing method of polarizing film - Google Patents

Abstract

To provide a method for manufacturing a polarizing film from polyvinyl alcohol resin films, which method is capable of suppressing a reduction in manufacturing efficiency.SOLUTION: Provided is a polarizing film manufacturing method including a step for selecting a specific polyvinyl alcohol resin film, and a step for immersing the specific polyvinyl alcohol resin film in a swelling process liquid. The selection step includes: a step for immersing a film piece cut out from one or more polyvinyl alcohol resin films including the specific polyvinyl alcohol resin film in a swelling process liquid; a step for irradiating the film piece immersed in the swelling process liquid with inspection light and receiving light passing through or being reflected by the film piece so as to acquire an image of the film piece; and a step for selecting the specific polyvinyl alcohol resin film on the basis of the luminance information of the image.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a polarizing film.

A conventionally known polarizing film is one in which a dichroic pigment such as iodine or a dichroic dye is adsorbed and oriented on a uniaxially stretched polyvinyl alcohol resin film. The polarizing film can be produced by continuously transporting a long length of polyvinyl alcohol resin film, sequentially subjecting the film to a swelling process, dyeing process, crosslinking process, cleaning process, drying process, etc., and also subjecting the film to a uniaxial stretching process during the production process (for example, JP 2001-141926 A (Patent Document 1)).

JP 2001-141926 A

In the continuous production of polarizing films using long polyvinyl alcohol-based resin films, the production efficiency of polarizing films can depend on the properties of the polyvinyl alcohol-based resin film used as the raw material. For example, if the polyvinyl alcohol-based resin film is a film that is prone to significant curling (warping) when immersed in a processing liquid, the curled film ends may fold when passing through guide rolls or nip rolls after immersion in the processing liquid, or the folded parts may break during the stretching process. These problems reduce the production efficiency of polarizing films.

The object of the present invention is to provide a method for producing a polarizing film from a polyvinyl alcohol-based resin film, which can suppress a decrease in production efficiency.

The present invention provides the following method for producing a polarizing film.
[1] A step of selecting a specific polyvinyl alcohol-based resin film;
A step of immersing the specific polyvinyl alcohol-based resin film in a swelling treatment liquid;
A method for producing a polarizing film, comprising:
The selecting step includes:
a step of immersing a film piece cut out from one or more polyvinyl alcohol-based resin films including the specific polyvinyl alcohol-based resin film in a swelling treatment liquid;
a step of irradiating the film piece immersed in the swelling treatment liquid with inspection light and receiving the light transmitted through or reflected by the film piece, thereby acquiring an image of the film piece;
selecting the specific polyvinyl alcohol-based resin film based on brightness information of the image;
A method for producing a polarizing film comprising the steps of:
[2] The method for producing a polarizing film according to [1], wherein the specific polyvinyl alcohol-based resin film is selected by determining the degree of curling of the film piece based on the luminance information.
[3] The method for producing a polarizing film according to [1] or [2], wherein the luminance information is information about a time change in luminance distribution obtained from a plurality of the images acquired at different times.
[4] The method for producing a polarizing film according to any one of [1] to [3], wherein the image is at least one image selected from the group consisting of an image obtained by irradiating an inspection light toward a main surface of the film piece and an image obtained by irradiating an inspection light toward a side surface of the film piece.
[5] The method for manufacturing a polarizing film described in any one of [1] to [4], wherein in the image acquisition step, inspection light is irradiated onto the film piece via a first polarizing filter, and light transmitted through or reflected by the film piece is received via a second polarizing filter arranged in a crossed Nicol state with respect to the first polarizing filter.
[6] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising the steps of:
The method includes a step of immersing the polyvinyl alcohol-based resin film in a swelling treatment liquid,
The immersion step includes a step of irradiating an inspection light onto the polyvinyl alcohol-based resin film in a state where the polyvinyl alcohol-based resin film is immersed in a swelling treatment liquid, and receiving the light transmitted through or reflected by the polyvinyl alcohol-based resin film, thereby obtaining an image of the polyvinyl alcohol-based resin film.
[7] The method for producing a polarizing film according to [6], further comprising determining a degree of curling of the polyvinyl alcohol-based resin film based on luminance information of the image.
[8] The method for producing a polarizing film according to [6] or [7], wherein the luminance information of the image is information about a time change in luminance distribution obtained from a plurality of the images acquired at different times.
[9] The method for producing a polarizing film according to any one of [6] to [8], wherein the image is at least one image selected from the group consisting of an image obtained by irradiating an inspection light toward a main surface of the polyvinyl alcohol-based resin film and an image obtained by irradiating an inspection light toward a side surface of the polyvinyl alcohol-based resin film.
[10] The method for producing a polarizing film according to any one of [6] to [9], wherein in the image acquisition step, inspection light is irradiated onto the polyvinyl alcohol-based resin film via a first polarizing filter, and light transmitted through or reflected by the polyvinyl alcohol-based resin film is received via a second polarizing filter arranged in a crossed Nicol state with respect to the first polarizing filter.
[11] The method for producing a polarizing film according to any one of [6] to [10], further comprising a step of determining whether or not to change the conditions for the step of immersing in the swelling treatment liquid, based on luminance information of the image.

A method for producing a polarizing film from a polyvinyl alcohol-based resin film can be provided that can suppress a decrease in production efficiency.

FIG. 2 is a schematic cross-sectional view showing an example of a polarizing film production apparatus. FIG. 13 is a schematic top view illustrating an example of a method for acquiring an image of a film piece immersed in a test swelling treatment liquid. FIG. 13 is a schematic top view illustrating another example of a method for acquiring an image of a film piece immersed in a test swelling treatment liquid. FIG. 13 is a schematic top view illustrating yet another example of a method for acquiring an image of a film piece immersed in a test swelling treatment liquid. 1A to 1C are diagrams showing examples of images of a film piece obtained in an image acquisition process. 10A to 10C are diagrams showing other examples of images of the film piece obtained in the image acquisition process. FIG. 4 is a schematic diagram illustrating a curl angle.

The present invention relates to a manufacturing method for producing a polarizing film from a polyvinyl alcohol-based resin film (hereinafter also referred to as a "PVA-based resin film"). A polarizing film can be produced by subjecting a long PVA-based resin film to a series of treatments including immersion treatment (wet treatment) in a treatment tank containing a predetermined treatment liquid, drying treatment, etc., and in the manufacturing method according to the present invention, the immersion treatment includes a step of immersing the PVA-based resin film in a swelling treatment liquid. The obtained polarizing film is one in which a dichroic dye is adsorbed and oriented on the stretched PVA-based resin film.

<Outline of polarizing film manufacturing>
FIG. 1 is a schematic cross-sectional view showing an example of a polarizing film manufacturing apparatus, and is a diagram for explaining the above-mentioned series of processes. The polarizing film manufacturing apparatus shown in FIG. 1 is an apparatus for continuously manufacturing a long polarizing film 25 from a long PVA-based resin film 10, which is a raw material film. In FIG. 1, the arrows attached to the film indicate the film transport direction. The polarizing film manufacturing apparatus shown in FIG. 1 includes, in order from the upstream side in the film transport direction, a swelling treatment tank 13, a dyeing treatment tank 15, a crosslinking treatment tank 17, a cleaning treatment tank 19, and a drying furnace 20. Note that FIG. 1 shows an example in which one tank each of the swelling treatment tank 13, the dyeing treatment tank 15, the crosslinking treatment tank 17, and the cleaning treatment tank 19 is provided, but two or more tanks each may be provided as necessary.

In the manufacture of polarizing film 25 using the polarizing film manufacturing apparatus shown in FIG. 1, the PVA-based resin film 10 is continuously unwound and conveyed from the unwinding roll 11 while being successively immersed in the swelling treatment tank 13, the dyeing treatment tank 15, the crosslinking treatment tank 17, and the cleaning treatment tank 19, and finally passed through the drying furnace 20 for drying treatment to obtain the polarizing film 25. The polarizing film 25 manufactured as a long object may be sequentially wound around the winding roll 27.

The polarizing film manufacturing equipment has a transport path for the PVA-based resin film that is constructed so that it passes through a treatment tank in which the PVA-based resin film is immersed and a drying oven. The PVA-based resin film is transported along this transport path, where it undergoes a series of processes to produce the polarizing film.

As shown in FIG. 1, the transport path is constructed by multiple rolls that support and guide the traveling film (PVA-based resin film 10 and polarizing film 25). The multiple rolls can include a guide roll, which is a free roll that supports one side of the film, and/or a pair of rolls (usually drive rolls) that sandwich or press the film from both sides. In the example shown in FIG. 1, the polarizing film manufacturing apparatus includes guide rolls 1a-1l and nip rolls 2a-2f. These rolls can be placed in appropriate positions, such as before or after each treatment tank and drying oven, or inside the treatment tank and drying oven.

The driving roll refers to 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 refers to a roll that simply plays the role of supporting the running film and cannot provide a driving force for film transport.

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 PVA-based resin film 10, and the polarizing film manufacturing method can include a stretching process step (wet stretching process step) for the PVA-based resin film 10.

The polarizing film according to the present invention can include the following steps.
a wet treatment step S101 in which the PVA-based resin film is immersed in one or more treatment tanks containing a treatment liquid while being transported along a transport path for the PVA-based resin film; and a drying treatment step S102 in which the PVA-based resin film after the wet treatment is dried while being transported along the transport path.

The wet treatment step S101 generally includes the following steps in the following order:
A step of immersing the PVA-based resin film in a swelling treatment liquid while transporting the film along a transport path (swelling treatment step);
A step of immersing the PVA-based resin film in a dyeing treatment liquid while transporting the film along a transport path (dyeing treatment step);
A step of immersing the PVA-based resin film in a crosslinking treatment liquid while transporting the film along a transport path (crosslinking treatment step);
A step of immersing the PVA-based resin film in a cleaning treatment liquid while transporting the film along a transport path (cleaning treatment step).

In the first embodiment, the method for producing a polarizing film according to the present invention includes a step of selecting a specific PVA-based resin film prior to the step of immersing the film in a swelling treatment liquid.
The selecting step comprises:
immersing a film piece cut out from one or more PVA type resin films including the specific PVA type resin film in a swelling treatment liquid;
a step of irradiating the film piece immersed in the swelling treatment liquid with inspection light and receiving the light transmitted through or reflected by the film piece, thereby acquiring an image of the film piece;
selecting the specific polyvinyl alcohol-based resin film based on the luminance information of the image;
Includes.

In a second embodiment, the method for producing a polarizing film according to the present invention includes a step of immersing the film in a swelling treatment liquid (swelling treatment step) that irradiates the PVA-based resin film immersed in the swelling treatment liquid with inspection light and receives the light transmitted through or reflected by the PVA-based resin film to obtain an image of the PVA-based resin film.

<PVA-based resin film>
The PVA-based resin film 10, which is a raw material film, is a film composed of a polyvinyl alcohol-based resin (PVA-based resin). 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)acryl" refers to at least one selected from the group consisting of acrylic and methacrylic.

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 may be reduced.

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 %), and is represented by 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 PVA-based resin film 10.

An example of the PVA-based resin film 10 is an unstretched film formed by forming a film from the polyvinyl alcohol-based resin. 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 PVA-based resin film 10 is a stretched film formed 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, that is, in the longitudinal direction of the film. This stretching is preferably dry stretching. Dry stretching refers to stretching performed in the air, and is usually longitudinal uniaxial stretching. Examples of dry stretching include hot roll stretching, in which the film is passed between a hot roll with a heated surface and a guide roll (or a hot roll) with a different peripheral speed from the hot roll, and longitudinal stretching is performed under heating using the hot roll; inter-roll stretching, in which the film is passed through a heating means (oven, etc.) 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 hot 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 PVA-based resin film 10 may contain additives such as plasticizers. A preferred example of a plasticizer is a polyhydric alcohol, and specific examples include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, triglycerin, tetraethylene glycol, trimethylolpropane, polyethylene glycol, and the like. The 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 mass, and preferably 7 to 15 parts by mass, per 100 parts by mass of the PVA-based resin constituting the PVA-based resin film 10.

The thickness of the raw material film, the PVA-based resin film 10, depends on whether the PVA-based resin film 10 has been stretched or not, but is usually 10 to 150 μm. From the viewpoint of thinning the resulting polarizing film 25, the thickness 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.

The length in the longitudinal direction of the raw material film, the PVA-based resin film 10, is, for example, 1,000 m or more and 30,000 m or less, and preferably 20,000 m or less. An example of the length in the width direction (direction perpendicular to the longitudinal direction) of the PVA-based resin film 10 is, for example, 1,300 mm or more and 5,000 mm or less.

<Swelling Treatment Step>
The swelling treatment step is a step of immersing the PVA-based resin film in a swelling treatment liquid while transporting the film along a transport path.
The 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 alcohols. The swelling treatment liquid may also contain boric acid, chlorides, inorganic acids, inorganic salts, etc. The swelling treatment is performed by immersing the PVA-based resin film 10 in the swelling treatment liquid. The swelling treatment is performed for the purposes of removing foreign matter from the PVA-based resin film 10, removing plasticizers, imparting ease of dyeing, plasticizing the film, etc.

During the swelling treatment, the 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. The temperature of the swelling treatment liquid is usually 10 to 70°C, preferably 15 to 50°C. The immersion time of the PVA-based resin film 10 (residence time in the swelling treatment liquid) is usually 10 to 600 seconds, preferably 20 to 300 seconds.

<Dyeing process>
The dyeing treatment step is a step of immersing the PVA-based resin film in a dyeing treatment liquid while transporting the film along a transport path.
The dyeing treatment liquid contained in the dyeing treatment tank 15 contains a dichroic dye. The PVA-based resin film 10 is immersed in the dyeing treatment liquid to perform the dyeing treatment. As a result, the dichroic dye is adsorbed onto the PVA-based resin film 10. The dichroic dye may be iodine or a dichroic organic dye, and is preferably iodine. Only one type of dichroic dye may be used alone, or two or more types may be used in combination.

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, and cobalt chloride, may be coexisted. The content of iodine in the dyeing treatment liquid is usually 0.003 to 1 part by mass per 100 parts by mass of water. The content of iodides such as potassium iodide in the dyeing treatment liquid is usually 0.1 to 20 parts by mass per 100 parts by mass of water. The temperature of the dyeing treatment liquid 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 treatment liquid) of the PVA-based resin film 10 is usually 20 to 600 seconds, and preferably 30 to 300 seconds.

In order to improve the dyeability of the dichroic dye, 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 while the dyeing process is being performed. The cumulative stretching ratio up to the dyeing process (the stretching ratio during 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.

<Crosslinking Treatment Step>
The crosslinking treatment step is a step of immersing the PVA-based resin film in a crosslinking treatment liquid while transporting the film along a transport path.
The crosslinking treatment liquid contained in the crosslinking treatment tank 17 contains a crosslinking agent. The dyed PVA-based resin film 10 is immersed in the crosslinking treatment liquid to perform crosslinking treatment. This makes the PVA-based resin film 10 water-resistant and adjusts the hue (complementary color treatment) by crosslinking. A stretching treatment (usually a uniaxial stretching treatment) may be performed while the crosslinking treatment is being performed.

Examples of the crosslinking agent include boric acid, glyoxal, and glutaraldehyde, and boric acid is preferably used. Two or more crosslinking agents can be used in combination. The content of the crosslinking agent in the crosslinking treatment liquid is usually 0.1 to 15 parts by mass, preferably 1 to 12 parts by mass, per 100 parts by mass of water. When the dichroic dye is iodine, the crosslinking treatment liquid preferably contains an iodide in addition to the crosslinking agent. The content of the iodide in the crosslinking treatment liquid is usually 0.1 to 20 parts by mass, preferably 5 to 15 parts by mass, per 100 parts by mass of water. Examples of iodides include potassium iodide and zinc iodide. The crosslinking treatment liquid may also contain compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate. The temperature of the crosslinking treatment liquid is usually 50 to 85°C, and preferably 50 to 70°C. The immersion time of the PVA-based resin film 10 (residence time in the crosslinking treatment solution) is usually 10 to 600 seconds, preferably 20 to 300 seconds.

<Cleaning process>
The cleaning treatment step is a step of immersing the PVA-based resin film in a cleaning treatment liquid while transporting the film along a transport path.
The cleaning solution contained in the cleaning tank 19 may be, for example, water (pure water, etc.), or may be an aqueous solution containing an added water-soluble organic solvent such as alcohols. The PVA-based resin film 10 after crosslinking is immersed in the cleaning solution to perform the cleaning process. The cleaning process is carried out as necessary for the purpose of removing excess crosslinking agent and chemicals such as dichroic dye attached to the PVA-based resin film 10. The temperature of the cleaning solution is, for example, 2 to 40° C. Stretching (usually uniaxial stretching) may be carried out while performing the cleaning process.

The cleaning treatment may be a treatment in which the cleaning treatment liquid is sprayed as a shower onto the PVA-based resin film 10 after the crosslinking treatment, or a combination of immersion in the above-mentioned cleaning treatment liquid and spraying of the cleaning treatment liquid. Figure 1 shows an example of a case in which the PVA-based resin film 10 is immersed in water in a cleaning treatment tank 19 to perform the cleaning treatment.

<Stretching process>
As described above, the PVA-based resin film 10 may be subjected to a wet stretching process in the wet treatment step S101. The wet stretching is usually uniaxial stretching, and may be performed while performing any one of swelling, dyeing, crosslinking, and washing, or during two or more selected from these processes. The wet stretching is preferably performed in the crosslinking process or in one or more steps prior to the crosslinking process. From the viewpoint of the polarization properties of the resulting polarizing film 25, the stretching ratio of the wet stretching is preferably adjusted so that the final cumulative stretching ratio of the polarizing film 25 (when the PVA-based resin film 10 to be subjected to the wet treatment has been stretched, the cumulative stretching ratio including this stretching) is 3 to 8 times.

When performing the wet stretching process, the polarizing film manufacturing apparatus includes a wet stretching means for the 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 crosslinking treatment 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 treatments, two nip rolls arranged at a distance from each other can be used as the wet stretching means.

<Drying process>
The drying process S102 is a process for drying the PVA-based resin film after the wet process. The PVA-based resin film 10 after the wet process S101 can be dried by introducing the film into a drying oven 20 while continuing to transport the film, thereby obtaining a polarizing film 25. 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 may be performed by a means other than a drying oven. Examples of drying processes other than a drying oven include a process in which the PVA-based resin film 10 is brought into close contact with one or more heating bodies having a convex curved surface, and a process in which the film is heated using a heater. Examples of the heating body include 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 include an infrared heater, a halogen heater, a panel heater, etc.

The temperature of the drying process (e.g., the temperature inside the drying furnace 20, the surface temperature of the heat roll, etc.) is usually 30 to 100°C, and preferably 50 to 90°C. The drying time is not particularly limited, but is, for example, 30 to 600 seconds.

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, and more preferably 15 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 obtained by using an absorptiometer with an integrating sphere to perform luminosity correction on the obtained transmittance and polarization degree 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 used in the polarizing plate production process (a process of laminating a protective film or the like on one or both sides of the polarizing film 25) without being wound up.

<First embodiment of the method for producing a polarizing film>
In the first embodiment, the method for producing a polarizing film according to the present invention includes a step of selecting a specific PVA-based resin film (specific film selection step) prior to the step of immersing in a swelling treatment liquid.

The specific film selection process includes the following steps.
a step of immersing a film piece cut out from one or more PVA type resin films including the specific PVA type resin film in a swelling treatment liquid (film piece immersion step);
a step of irradiating an inspection light onto the film piece immersed in a swelling treatment liquid and receiving the light transmitted through or reflected by the film piece to obtain an image of the film piece (image obtaining step); and a step of selecting the specific PVA-based resin film based on the luminance information of the image (selection step).

According to the method for producing a polarizing film including the specific film selection step, it is possible to confirm in advance how the PVA-based resin film from which the film piece is taken will behave when it is immersed in the swelling treatment liquid of a polarizing film production apparatus prior to the swelling treatment step based on a test using a film piece taken from the PVA-based resin film (referring to the above-mentioned film piece immersion step and image acquisition step), and therefore it is possible to specify a PVA-based resin film that is unlikely or unlikely to cause a decrease in production efficiency even when used in the swelling treatment step and thus in polarizing film production (hereinafter, such a PVA-based resin film is also referred to as a "PVA-based resin film suitable for polarizing film production"). By selecting and using such a specified PVA-based resin film in polarizing film production, it is possible to suppress a decrease in the production efficiency of polarizing films.
Each step of the specific film selection process will be described below.

(1) Film Piece Immersion Step In the film piece immersion step, first, a film piece is cut out from one or more PVA type resin films, which are long films or rolls thereof that are candidates for use in the production of a polarizing film, and which include the specific PVA type resin film to be selected for use in the production of a polarizing film.

The number of PVA-based resin films sampled and subjected to the above test may be one. For example, the above test may be performed on this film piece, and the PVA-based resin film may be determined to be suitable for producing polarized films. In this case, polarized films can be produced using the PVA-based resin film suitable for polarized film production without performing the above test on other PVA-based resin films.

In some cases, multiple pieces of film are collected and subjected to the above test. For example, the first PVA-based resin film subjected to the above test is not a PVA-based resin film suitable for producing polarized films, and the above test is performed on other PVA-based resin films until a suitable PVA-based resin film is found.

The shape of the cut film piece is preferably a rectangular shape such as a rectangle or a square, and more preferably a rectangle. From the viewpoint of reproducibility, that is, reproducing as accurately as possible the behavior of the PVA-based resin film when immersed in the swelling treatment liquid of the polarizing film manufacturing device, it is preferable that the rectangular film piece is cut so that one pair of sides is parallel to the MD direction of the PVA-based resin film, and the other pair of sides is parallel to the TD direction of the PVA-based resin film. The size of the rectangular film piece is not particularly limited, but from the viewpoint of reproducibility, preferably, the length of the side parallel to the MD direction is 100 to 500 mm (e.g., 150 mm), and the length of the side parallel to the TD direction is 20 to 150 mm (e.g., 30 mm).

Next, the collected film piece is immersed in a swelling treatment liquid (hereinafter also referred to as "test swelling treatment liquid") and allowed to stand. The test swelling treatment liquid has the same composition as the swelling treatment liquid contained in the swelling treatment tank 13 that constitutes the polarizing film manufacturing apparatus. The temperature of the test swelling treatment liquid is also usually set to the same temperature as the swelling treatment liquid contained in the swelling treatment tank 13 that constitutes the polarizing film manufacturing apparatus.

For example, the film piece is entirely immersed in the test swelling treatment liquid contained in a container having a horizontal bottom. The container is preferably translucent, more preferably transparent, in order to enable the image acquisition process. Being transparent means that the transmittance in the visible light band is 90% or more. For example, the container may be made of a transparent resin, and a transparent container made of a (meth)acrylic resin is preferable because of its excellent transparency. In order to enable image acquisition using the first polarizing filter and the second polarizing filter described later, it is preferable to select a container with the above transmittance of about 90 to 95%. In addition, in order to enable image acquisition using the first and second polarizing filters described later, the container is preferably made of a non-oriented material, more preferably made of a non-oriented resin. The non-oriented material is, for example, a non-stretched material.

It is preferable that the main surface of the film piece immersed in the test swelling treatment liquid and the imaging unit (camera) 50 described below face each other directly. If the angle α of the main surface of the film piece relative to the imaging unit (camera) 50 when the main surface of the film piece faces the imaging unit (camera) 50 is set to 0°, from the viewpoint of improving the detectability and resolution of the obtained image, it is preferable that the angle be 0° to 45°, and more preferably 0° to 30°.

When a film piece is cut out so that one pair of sides is parallel to the MD direction of the PVA-based resin film and the other pair of sides is parallel to the TD direction of the PVA-based resin film, it is immersed in a test swelling treatment solution so that, for example, the sides parallel to the MD direction are parallel to the vertical or horizontal direction.

Specific procedures for immersing a piece of film in a test swelling treatment solution are, for example, as follows: [A] Prepare a jig that will set the angle α within the above range, place the prepared piece of film on this jig, fix its upper end (e.g., one TD side) with a clip, and attach a clip as a weight to the center of the width of the lower end (e.g., the other TD side). At this time, it is preferable that the weight of the clip as a weight is 3 to 5 g.

[B] The film piece prepared in [A] above, fixed to the jig and carrying a weight, is immersed in the test swelling treatment liquid at a predetermined temperature (e.g., 20 to 40°C) while maintaining the angle α constant. The immersion speed is preferably such that the film piece does not float away from the jig when it enters the test swelling treatment liquid.

(2) Image Acquisition Step Next, an image of the film piece immersed in the test swelling treatment solution is acquired. By acquiring the image, brightness information of the film piece can be obtained. From this brightness information, it is possible to grasp the degree of curling of the film piece.

Figure 2 is a schematic top view illustrating an example of a method for acquiring an image of a film piece immersed in a test swelling treatment liquid. In this example, a film piece 31 is immersed in a test swelling treatment liquid contained in a transparent container 30. A light source 40 that emits an inspection light 41, which is white light, is disposed outside the transparent container 30. An imaging unit (camera) 50 is disposed outside the transparent container 30, on the opposite side of the light source 40 relative to the film piece 31. In the imaging system shown in Figure 2, the inspection light 41 from the light source 40 is irradiated toward the main surface of the film piece 31 immersed in the test swelling treatment liquid, and the light transmitted through the film piece 31 is received by the imaging unit 50, which captures the image to acquire an image of the main surface of the film piece 31.

The light source 40 may be, for example, a lighting device such as a white LED. The light source 40 is preferably a surface light source.

Figure 3 is a schematic top view illustrating another example of a method for acquiring an image of a film piece immersed in a test swelling treatment liquid. As in this example, an image of the side of the film piece 31 may be acquired by irradiating the side of the film piece 31 immersed in the test swelling treatment liquid with inspection light 41 from a light source 40, and receiving and capturing the light transmitted through the film piece 31 by an imaging unit 50. In the image acquisition process, at least one of an image of the main surface and an image of the side of the film piece 31 is acquired, and both may be acquired. It is preferable that the image acquired in the image acquisition process includes at least an image of the main surface of the film piece 31.

The image of the main surface of the film piece 31 to be acquired may be an image of a portion of the main surface or an image of the entire main surface, but is preferably an image of the entire main surface. The same applies to images of the side surfaces.

As shown in FIG. 4, when acquiring an image of the film piece 31 immersed in the test swelling treatment liquid, it is preferable to irradiate the film piece 31 with the inspection light 41 through the first polarizing filter 61, and have the imaging unit 50 receive and capture the light transmitted through the film piece 31 through the second polarizing filter 62 arranged in a crossed Nicol state with respect to the first polarizing filter 61, thereby acquiring an image of the film piece 31. The above-mentioned imaging method using a polarizing filter makes it easy to acquire a clear image of the film piece 31 immersed in the swelling treatment liquid, which has transparency and may have orientation.

Linear polarizing plates can be used as the first polarizing filter 61 and the second polarizing filter 62. The linear polarizing plate can be a linear polarizer made of a PVA-based resin film with a protective film attached to one or both sides. The angle between the absorption axis of the first polarizing filter 61 and the absorption axis of the second polarizing filter 62 is, for example, 90 degrees ± 5 degrees.

In the image acquisition process, the image is acquired when a certain time T has elapsed from the time when the film piece 31 is immersed in the test swelling treatment liquid. The time when the film piece 31 is immersed in the test swelling treatment liquid refers to the time when the entire film piece 31 is immersed in the test swelling treatment liquid. T may be within a range of 1 second to 5 minutes, for example. The time T at which the image is acquired may be one time or multiple different times. By acquiring images at multiple different times, information on the change in luminance distribution over time can be obtained as luminance information of the film piece. The images acquired at multiple different times may be videos.

The inventor's research has revealed that the luminance information of an image obtained from the acquired image can be correlated with the degree of curl (warping) (or the state of curl) of the film piece 31. The correlation between the luminance information of an image and curl is presumably due to at least a difference in the optical path length to the imaging unit 50 between the curled and uncurled parts of the film piece 31. The greater the luminance, the greater the degree of curl tends to be.

Therefore, according to the manufacturing method including the specific film selection step, it is possible to confirm in advance, based on the test using a film piece taken from the PVA-based resin film (film piece immersion step and image acquisition step), what kind of curl behavior the film will exhibit when the PVA-based resin film from which the film piece was taken is immersed in the swelling treatment liquid of the polarizing film manufacturing device before the swelling treatment step. Based on this curl behavior, it is possible to determine whether the PVA-based resin film from which the film piece was taken is a PVA-based resin film suitable for polarizing film manufacturing (whether it is a film to be selected as a specific polyvinyl alcohol-based resin film).

5 and 6 show examples of images of a film piece obtained in the image acquisition process. FIG. 5 is an image of a film piece cut out from a PVA-based resin film having a thickness of 60 μm, and FIG. 6 is an image of a film piece cut out from a PVA-based resin film having a thickness of 20 μm. FIG. 5 (a) is an image when the above T is 14 seconds, (b) is an image when the above T is 23 seconds, and (c) is an image when the above T is 41 seconds. FIG. 6 (a) is an image when the above T is 11 seconds, (b) is an image when the above T is 35 seconds, and (c) is an image when the above T is 49 seconds. In each of FIG. 5 and FIG. 6 (a) to (c), the image on the left is an image of the main surface of the film piece, and the image on the right is an image of the side of the film piece. It can be seen that in each image, an image in which the luminance distribution can be clearly grasped is obtained. The images shown in FIG. 5 and FIG. 6 are images obtained by the method using the first and second polarizing filters described above. The swelling treatment liquid used for the test was pure water.

As shown in Figures 5 and 6, respectively, the degree (or state, shape) of curling of the film piece can be grasped and estimated from the brightness information of the acquired image. For example, in an image in which both ends of the film piece in the width direction have high brightness, it can be grasped and estimated that the film piece is curled at those ends. The degree of curling can be grasped and estimated in this way based on the knowledge that the greater the brightness, the greater the degree of curl, and on the knowledge regarding the relationship between brightness information acquired in advance using multiple film pieces and the actual degree (or state, shape) of curling.

The image acquisition process may be a process of acquiring an image of the film piece 31 by irradiating the film piece 31 immersed in the test swelling treatment liquid with inspection light 41 and receiving the light reflected from the film piece 31. In this case, the imaging unit (camera) 50 is disposed on the same side as the light source 40 with respect to the film piece 31. Even in an imaging system that receives reflected light, it is preferable that the film piece 31 is irradiated with the inspection light 41 via a first polarizing filter 61, and the imaging unit 50 receives and captures the reflected light from the film piece 31 via a second polarizing filter 62 disposed in a crossed Nicol state with respect to the first polarizing filter 61.

(3) Selection step This step is a step of identifying and selecting a PVA-based resin film suitable for producing a polarizing film as the specific PVA-based resin film based on the brightness information of the image acquired in the image acquisition step. The criterion for determining whether or not a PVA-based resin film is suitable for producing a polarizing film may be various criteria.

The above-mentioned criteria may be, for example, any of the following examples.
[a] When the brightness of an image obtained after a certain time T (one point) has elapsed since the film piece 31 was immersed in the test swelling treatment solution is sufficiently low, the PVA-based resin film from which the film piece 31 was taken can be determined to be a PVA-based resin film suitable for producing a polarizing film.
[b] After immersing the film piece 31 in the test swelling treatment liquid, images are obtained at multiple different times T, and information on the change in luminance distribution over time is obtained from these multiple images. If the luminance is sufficiently small at the time point when the time T1 has elapsed after immersion in the test swelling treatment liquid, it can be determined that the PVA-based resin film from which the film piece 31 was taken is a PVA-based resin film suitable for producing a polarizing film.

In the above [a] and [b], "the brightness is sufficiently small" may mean, for example, that the brightness value is equal to or less than the threshold value S even at the highest point in the image of the film piece 31. The brightness threshold value S can be set in advance based on the above-mentioned knowledge regarding the relationship between the brightness value obtained using multiple film pieces and the actual degree (or state, shape) of curling, and on the knowledge regarding the relationship between the degree of curling and the tendency of the PVA-based resin film to fold or break during polarizing film production (the tendency to cause a decrease in polarizing film production efficiency). The points with the highest brightness may be both ends in the width direction of the film piece 31 that are prone to curling.

The time T for acquiring the image in [a] above and the time T1 after immersion in the test swelling treatment liquid in [b] above are not particularly limited, but are preferably shorter than the time from the time the PVA-based resin film is immersed in the swelling treatment liquid to the first time when the film is likely to fold or break in the actual polarizing film production using a PVA-based resin film. The first time can be, for example, the time when the film passes through the first guide roll 1c or the first nip roll 2b located downstream of the swelling treatment tank 13, as shown in Figure 1. This is because a film that has undergone strong curling is likely to fold or break when passing through the guide roll or nip roll.

In this process, when judging whether the PVA-based resin film from which the film piece 31 was taken is a PVA-based resin film suitable for polarizing film production based on the luminance information of the image, the judgment may be made based on the luminance value of the image as in [a] and [b] above, or the luminance value may be converted into the degree (or state, shape) of curl as shown in Figures 5 and 6, and the judgment may be made based on the result of this conversion. Such conversion may be performed by an arithmetic processing device (e.g., a computer) including a memory unit that stores the above-mentioned knowledge regarding the relationship between the luminance value obtained using multiple film pieces and the actual degree (or state, shape) of curl, and a calculation unit that converts the luminance value into the degree of curl based on the information stored in the memory unit.

The degree of curl can be quantified, for example, as a curl angle. Referring to FIG. 7, the curl angle θ is the angle between a horizontal reference plane 70 that is in contact with the highest convexity of the curled film 75 and a tangent line 71 of the curved surface of the curled portion.

The specific PVA-based resin film that is determined to be suitable for polarizing film production in the selection process is introduced into the swelling process for polarizing film production. This PVA-based resin film is a film that is unlikely to fold or break during the polarizing film production process, and by using such a PVA-based resin film, it is possible to suppress a decrease in the production efficiency of polarizing films.

It should be noted that PVA-based resin films that do not meet the above criteria can be judged not to be used in the manufacture of polarized films, or they can be subjected to a treatment, for example to reduce curling, and then subjected to the specific film selection process again.

<Second embodiment of the method for producing a polarizing film>
In the second embodiment, the method for producing a polarizing film according to the present invention includes a step of immersing the PVA type resin film in a swelling treatment liquid (swelling treatment step) including a step of irradiating the PVA type resin film immersed in the swelling treatment liquid with inspection light and receiving the light transmitted through or reflected by the PVA type resin film to obtain an image of the PVA type resin film.

In the first embodiment, before the swelling process of the polarizing film production, a film piece taken from a PVA-based resin film that is a candidate for use in the polarizing film production is immersed in a test swelling process liquid to perform an image acquisition process, whereas in the present embodiment, an image acquisition process is performed on the PVA-based resin film during the swelling process of the polarizing film production. That is, the image acquisition process in this embodiment is a process that is performed inline during the polarizing film production, and is a process of irradiating the PVA-based resin film immersed in the swelling process liquid contained in the swelling process tank 13 with inspection light and receiving the light transmitted through or reflected by the PVA-based resin film to acquire an image of the PVA-based resin film.

The manufacturing method of the polarizing film according to this embodiment can also suppress a decrease in the manufacturing efficiency of the polarizing film. In other words, if it can be determined by the in-line image acquisition process that the PVA-based resin film in the swelling process is not a PVA-based resin film suitable for manufacturing polarizing films, feedback can be provided to correct the polarizing film manufacturing conditions, thereby reducing factors that lead to a decrease in manufacturing efficiency, such as film folding or breakage.

The image acquisition process in this embodiment can be performed in the same manner as the image acquisition process in the first embodiment, and the above description of the image acquisition process in the first embodiment is cited for the image acquisition process in this embodiment. It is preferable that the image acquisition is performed early during the swelling process so that the above feedback can be performed at an appropriate timing.

In the image acquisition process in this embodiment, at least one of an image of the main surface and an image of the side surface of the PVA-based resin film is acquired, and both may be acquired. It is preferable that the image acquired in the image acquisition process includes at least an image of the main surface of the PVA-based resin film.

When acquiring an image of the PVA-based resin film, it is preferable to irradiate the PVA-based resin film with inspection light through a first polarizing filter, and have the imaging unit receive and capture the light that passes through the PVA-based resin film through a second polarizing filter that is arranged in a crossed Nicol state with respect to the first polarizing filter, thereby acquiring an image of the PVA-based resin film.

In the image acquisition process, the image is acquired when a certain time T has elapsed from the time when the PVA-based resin film is immersed in the swelling treatment liquid contained in the swelling treatment tank 13. T may be, for example, in the range of 1 second to 5 minutes. The time T at which the image is acquired may be one time or multiple different times. By acquiring images at multiple different times, information on the change in luminance distribution over time can be obtained as luminance information of the film piece. The images acquired at multiple different times may be videos.

The image acquisition process may be a process of acquiring an image of the PVA-based resin film by irradiating the PVA-based resin film immersed in the swelling treatment liquid with inspection light and receiving the light reflected from the PVA-based resin film. In the imaging system that receives the reflected light, it is also preferable that the PVA-based resin film is irradiated with inspection light via a first polarizing filter, and the imaging unit receives and captures the reflected light from the PVA-based resin film via a second polarizing filter arranged in a crossed Nicol state with respect to the first polarizing filter.

The method for producing a polarizing film according to this embodiment preferably includes a step of determining whether or not to change the polarizing film production conditions based on the brightness information of the acquired image, and more preferably includes a step of determining whether or not to change the conditions of the swelling treatment step based on the brightness information of the acquired image. The criteria for this determination step may be the same as those in [a] and [b] in the first embodiment.

If the PVA-based resin film subjected to the swelling treatment process is judged to be a PVA-based resin film suitable for polarized film production based on the above judgment criteria [a] or [b], it can be judged that the polarized film production conditions may be continued without being changed and that polarized film production may be continued under the same conditions.

On the other hand, if it is determined that the PVA-based resin film subjected to the swelling treatment step is not a PVA-based resin film suitable for producing polarized films based on the above criteria [a] or [b], it is preferable to carry out a step of changing the polarized film production conditions, particularly the conditions of the swelling treatment step (feedback step). The conditions to be changed include the temperature of the swelling treatment liquid, the immersion time in the swelling treatment liquid, the tension of the film, and the stretching ratio when stretching is performed in the swelling treatment step. Even when using a PVA-based resin film that is prone to folding or breaking under the initial conditions, it is possible to correct the conditions to make the film less likely to fold or break by carrying out the feedback step, and this makes it possible to suppress a decrease in the production efficiency of polarized films.

1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1i, 1j, 1k, 1l guide rolls, 2a, 2b, 2c, 2d, 2e, 2f nip rolls, 10 polyvinyl alcohol resin film (PVA resin film), 11 unwinding roll, 13 swelling tank, 15 dyeing tank, 17 crosslinking tank, 19 cleaning tank, 20 drying oven, 25 polarizing film, 27 winding roll, 30 transparent container, 31 film piece, 40 light source, 41 inspection light, 50 imaging unit, 61 first polarizing filter, 62 second polarizing filter, 70 reference surface, 71 tangent to curved surface of curled portion, 75 curled film.

Claims (11)

Selecting a specific polyvinyl alcohol-based resin film;
A step of immersing the specific polyvinyl alcohol-based resin film in a swelling treatment liquid;
A method for producing a polarizing film, comprising:
The selecting step includes:
a step of immersing a film piece cut out from one or more polyvinyl alcohol-based resin films including the specific polyvinyl alcohol-based resin film in a swelling treatment liquid;
a step of irradiating the film piece immersed in the swelling treatment liquid with inspection light and receiving the light transmitted through or reflected by the film piece, thereby acquiring an image of the film piece;
selecting the specific polyvinyl alcohol-based resin film based on brightness information of the image;
A method for producing a polarizing film comprising the steps of: The method for producing a polarizing film according to claim 1, wherein the specific polyvinyl alcohol-based resin film is selected by determining the degree of curl of the film piece based on the brightness information. The method for producing a polarizing film according to claim 1, wherein the luminance information is information about the time change in luminance distribution obtained from a plurality of the images acquired at different times. The method for producing a polarizing film according to claim 1, wherein the image is at least one image selected from the group consisting of an image obtained by irradiating an inspection light toward a main surface of the film piece and an image obtained by irradiating an inspection light toward a side surface of the film piece. The method for manufacturing a polarizing film according to any one of claims 1 to 4, wherein in the step of acquiring the image, inspection light is irradiated onto the film piece through a first polarizing filter, and light transmitted through or reflected by the film piece is received through a second polarizing filter arranged in a crossed Nicol state with respect to the first polarizing filter. A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising the steps of:
The method includes a step of immersing the polyvinyl alcohol-based resin film in a swelling treatment liquid,
The immersion step includes a step of irradiating an inspection light onto the polyvinyl alcohol-based resin film in a state where the polyvinyl alcohol-based resin film is immersed in a swelling treatment liquid, and receiving the light transmitted through or reflected by the polyvinyl alcohol-based resin film, thereby obtaining an image of the polyvinyl alcohol-based resin film. The method for producing a polarizing film according to claim 6, wherein the degree of curling of the polyvinyl alcohol-based resin film is determined based on the brightness information of the image. The method for producing a polarizing film according to claim 6, wherein the luminance information of the image is information about the time change in luminance distribution obtained from a plurality of the images acquired at different times. The method for producing a polarizing film according to claim 6, wherein the image is at least one image selected from the group consisting of an image obtained by irradiating an inspection light toward a main surface of the polyvinyl alcohol-based resin film and an image obtained by irradiating an inspection light toward a side surface of the polyvinyl alcohol-based resin film. The method for manufacturing a polarizing film according to claim 6, wherein in the step of acquiring the image, inspection light is irradiated onto the polyvinyl alcohol-based resin film through a first polarizing filter, and light transmitted through or reflected by the polyvinyl alcohol-based resin film is received through a second polarizing filter arranged in a crossed Nicol state with respect to the first polarizing filter. The method for producing a polarizing film according to any one of claims 6 to 10, further comprising a step of determining whether or not to change the conditions of the step of immersing in the swelling treatment liquid based on the brightness information of the image.

Images