JP2021157156A - Method for manufacturing polarization film - Google Patents

Abstract

To provide a method for manufacturing a polarization film, capable of suppressing dyeing unevenness, excellent in the intra-surface uniformity of polarization performance (polarization degree, single substance transmittance, etc.), excellent for economy and environment, and capable of performing stable production.SOLUTION: A method for manufacturing a polarization film includes: a swelling process having a swelling tank 1; a dyeing process having a dyeing tank 2; and a stretching process having a stretching tank 3. When a stretching rate in the longitudinal direction of the polyvinyl alcohol film which last contacts a nip roll A in a stretching liquid 12 in the stretching tank 3 in the stretching process, is 100%, a stretching rate in the longitudinal direction of the polyvinyl alcohol film which first contacts a nip roll B after pulling up the polyvinyl alcohol film from a dyeing liquid 11 in the dyeing tank 2 in the dyeing process, is controlled to be 40% or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a polarizing film, which suppresses dyeing unevenness and has excellent in-plane uniformity of polarization performance (polarization degree, single transmittance, etc.).

Light emitting diodes (LEDs) are often used in personal digital assistants, LCD TVs, desktop electronic computers, electronic clocks, personal computers, instruments of automobiles and various machines, and the above LEDs are usually used together with polarizing plates. There is.
As the polarizing plate, a polarizing film made of a polyvinyl alcohol-based film in which iodine or a dichroic dye is adsorbed and oriented is generally used by laminating a protective film made of triacetyl cellulose or the like on one side or both sides of the polarizing film. Therefore, in order to provide an LED that is bright and has high contrast, a polarizing plate having both high transmittance and a degree of polarization, and thus a polarizing film is required.

The above-mentioned polarizing film is, for example, a polyvinyl alcohol-based film that is swollen with a swelling treatment liquid (including warm water) such as water, dyed with iodine or the like, stretched to arrange iodine molecules or the like, and stretched. It is obtained by cross-linking with a cross-linking agent such as boron for holding and drying. Such a manufacturing process is usually carried out continuously while unwinding a polyvinyl alcohol-based film wound in a roll shape, passing it through a plurality of rolls (nip roll, guide roll, etc.) and conveying it in the horizontal direction.

In recent years, with the increasing size of screens of LCD TVs and the explosive spread of smartphones, a wider polarizing film and a thinner polarizing film than before have been required, and a brighter and clearer display is required. Has come to be required. For this reason, uneven transmittance (dyeing unevenness) of the polarizing film, which has not been a problem in the past, has become a problem.
In order to improve such dyeing unevenness, it is conceivable to reduce the thickness unevenness (thickness variation) of the polyvinyl alcohol-based film as much as possible. However, there is a limit to the reduction of dyeing unevenness no matter how small the thickness unevenness is. Further, the thinner the film thickness, the larger the fluctuation with respect to the average film thickness, and the more conspicuous the uneven dyeing becomes. Therefore, a method for reducing dyeing unevenness is being sought by a method other than reducing the thickness unevenness.

As such a method, for example, Patent Document 1 provides at least one wet stretching step after the swelling treatment and before the dyeing treatment of the polyvinyl alcohol-based film, and in this stretching step, the polyvinyl alcohol-based film is watered. A method of uniaxial stretching at a stretching ratio of 1.1 times or more and less than 3 times in an aqueous solution containing 0.01 to 2.0 parts by weight of boric acid with respect to 100 parts by weight has been proposed.
Further, in the method of Patent Document 2, since the plasticizer and the low molecular weight polyvinyl alcohol eluted from the film are accumulated in the liquid of the swelling treatment bath, when there is a problem in the swelling treatment bath step, the liquid of the swelling treatment bath is used. When discarding, the above-mentioned plasticizer, low molecular weight polyvinyl alcohol, chloride and other chemicals are also discarded, which poses a problem in terms of economy and environment.
Further, in Patent Document 3, the polyvinyl alcohol-based film is continuously treated by passing through the swelling step, the water immersion treatment step, the dyeing step and the boric acid treatment step in this order, and the dyeing step and the boric acid treatment step are performed. Among these, a method of producing a polarizing film by uniaxially stretching in at least one step. In the water immersion treatment step, the polyvinyl alcohol-based film is stretched at a stretching ratio of 1 time or more and 1.05 times or less with respect to the mechanical direction. A method for producing a polarizing film that suppresses the problem has been proposed.

Japanese Unexamined Patent Publication No. 2005-114990 Japanese Unexamined Patent Publication No. 06-281816 Japanese Unexamined Patent Publication No. 2008-249766

However, although the method of Patent Document 1 can reduce color unevenness, since the cross-linking treatment with boric acid is performed before the dyeing treatment, only the vicinity of the surface of the polyvinyl alcohol-based film is dyed and the inside of the film is uniform. There is a problem that it becomes difficult to dye the film.
Further, in the method of Patent Document 2, when there is a problem that the plasticizer or low molecular weight polyvinyl alcohol eluted from the film is accumulated in the liquid of the swelling treatment bath, the liquid of the swelling treatment bath is replaced. There is an environmental problem because chemicals such as chloride are discarded at that time, and there is also an economic problem because a new chemical must be used again.
Further, in the method of Patent Document 3, there are cases where the effect is not sufficient even if the process is passed through a series of steps, and the film is often broken due to the swelling of the polyvinyl alcohol-based film during the water immersion treatment step. There is a problem that stable production is difficult and the yield is poor.

Therefore, in the present invention, under such a background, uneven dyeing is suppressed, the in-plane uniformity of polarization performance (polarization degree, single transmittance, etc.) is excellent, and economic and environmental aspects are also excellent, and stable production is achieved. It is an object of the present invention to provide a method for producing a polarizing film that can be carried out.

However, as a result of diligent research in view of such circumstances, the present inventor has not conventionally paid attention to dyeing in a dyeing step in a method for producing a polarizing film including a swelling step, a dyeing step, and a stretching step. Focusing on the stretching ratio of the polyvinyl alcohol-based film in the long direction on the nip roll B that comes into contact first after the polyvinyl alcohol-based film is pulled up from the treatment liquid, the stretching ratio up to this step is set to be relatively low for dyeing. The present invention has been completed by finding that it is possible to produce a polarizing film in which unevenness is suppressed and the in-plane uniformity of polarization performance (degree of polarization, single-unit transmittance, etc.) is excellent.

In order to achieve the above object, the gist of the present invention is the following [1] to [3].
[1] A method for producing a polarizing film by stretching a polyvinyl alcohol-based film in a long direction while transporting it via a plurality of nip rolls.
A swelling step of swelling the polyvinyl alcohol-based film with a swelling treatment liquid, a dyeing step of immersing the polyvinyl alcohol-based film in a dyeing treatment liquid in a dyeing tank for dyeing, and stretching the polyvinyl alcohol-based film in a stretching tank. It is provided with a stretching step of stretching in a long direction in a treatment liquid.
When the stretching ratio of the polyvinyl alcohol-based film in the long direction in the nip roll A that is last contacted in the stretching treatment liquid of the stretching step is 100%, the polyvinyl alcohol-based film is selected from the dyeing treatment liquid of the dyeing step. A method for producing a polarizing film, which controls the elongation ratio of the polyvinyl alcohol-based film in the long direction on the nip roll B, which is first contacted after the film is lifted, to 40% or less.
[2] The method for producing a polarizing film according to [1], further comprising a cross-linking step of immersing the polyvinyl alcohol-based film in a cross-linking treatment liquid to carry out a cross-linking treatment.
[3] The method for producing a polarizing film according to [1] or [2], further comprising a cleaning step of bringing the polyvinyl alcohol-based film into contact with a cleaning liquid for cleaning.

The present invention relates to a method for producing a polarizing film including a swelling step, a dyeing step, and a stretching step. When the stretching ratio of the polyvinyl alcohol-based film is 100%, the stretching ratio of the polyvinyl alcohol-based film in the long direction on the nip roll B which is first contacted after the polyvinyl alcohol-based film is withdrawn from the dyeing solution in the dyeing step is 40% or less. It is controlled so as to be. Therefore, the obtained polarizing film has excellent dyeing unevenness and excellent in-plane uniformity of polarization performance (polarization degree, single transmittance, etc.). Moreover, since it is not necessary to introduce a new device and the existing device can be utilized, it is economically excellent and stable production can be performed. Further, since it is not necessary to use a chloride solution as the treatment liquid in each step, it is also excellent in terms of environment.

It is a figure which shows typically the manufacturing apparatus of the polarizing film used in the manufacturing method of the polarizing film which is the object of one Embodiment of this invention. It is a figure which showed the modification of the said polarizing film manufacturing apparatus.

Hereinafter, embodiments for carrying out the present invention will be specifically described, but the present invention is not limited thereto.

The present invention relates to a production method for producing a polarizing film by stretching a polyvinyl alcohol-based film (hereinafter sometimes referred to as "PVA-based film") in a long direction thereof. First, the PVA-based film used in the production method of the present invention will be described, and then the production method of the present invention will be described.

[PVA-based film]
The PVA-based film used in the present invention is a film composed of a polyvinyl alcohol-based resin (hereinafter, may be referred to as "PVA-based resin"), and the PVA-based resin contains 50% by weight or more of a structural unit derived from vinyl alcohol. Refers to the resin contained. 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 a copolymer of vinyl acetate and another monomer 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.
In addition, "(meth) acrylic" represents at least one selected from the group consisting of acrylic and methacryl. The same applies to other terms with "(meta)".

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 94.0. It is in the range of 100.0 mol%, more preferably in the range of 98.0-100.0 mol%. When the degree of saponification is less than 80.0 mol%, the water resistance and moisture heat resistance of the obtained polarizing film and the polarizing plate containing the same tend to decrease.

The degree of saponification is expressed as a unit ratio (mol%) of the rate at which the _{acetic acid group (acetoxy group: −OCOCH 3} ) contained in the polyvinyl acetate resin, which is the raw material of the PVA resin, is changed to a hydroxyl group by the saponification step. It is defined by the following formula.
Degree of saponification (mol%) = 100 x (number of hydroxyl groups) / (number of hydroxyl groups + number of acetic acid groups)
The degree of saponification can be determined in accordance with JIS K 6726 (1994).

The weight average molecular weight of the PVA-based resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000. If the weight average molecular weight is too small, it tends to be difficult to obtain sufficient optical performance when the PVA-based resin is used as an optical film, and if it is too large, it tends to be difficult to stretch the PVA-based film during the production of the polarizing film. .. The weight average molecular weight of the PVA-based resin is the weight average molecular weight measured by the GPC-MALS method.

The PVA-based film is, for example, an unstretched film formed by forming the PVA-based resin. The film forming method is not particularly limited, and a known method such as a melt extrusion method or a solvent casting method can be adopted.
Another example of the PVA-based film is a stretched film obtained by stretching the unstretched film. This stretching is usually uniaxial stretching, preferably longitudinal uniaxial stretching. The longitudinal stretching means stretching of the film in the long direction (MD direction), that is, in the flow direction of the film.

When the PVA-based film is a stretched film, this stretching is preferably a dry stretching. Dry stretching refers to stretching performed in the air, and is usually longitudinal uniaxial stretching. In the dry stretching, a film was passed between a heat roll whose surface was heated and a guide roll (or a heat roll) having a peripheral speed different from that of the heat roll, and the heat roll was used. Thermal roll stretching that performs longitudinal stretching under heating; a roll that performs longitudinal stretching by the difference in peripheral speed between these two nip rolls while passing through a heating means (oven, etc.) between two nip rolls arranged at a distance. Inter-stretching; tenter stretching; compression stretching and the like can be mentioned.

The stretching temperature (surface temperature of the heat roll, temperature in the oven, etc.) is, for example, 80 to 150 ° C, preferably 100 to 135 ° C. The draw ratio of the above-mentioned stretching is usually 1.1 to 8 times, preferably 2.5 to 5 times.

The PVA-based film can contain additives such as a plasticizer. The above additives may be used alone or in combination of two or more.
Preferable examples of the above plasticizer are polyhydric alcohols, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, triglycerin, tetraethylene glycol, trimethylolpropane, polyethylene glycol and the like. Can be mentioned. The content of the plasticizer is usually 5 to 20 parts by weight, preferably 7 to 15 parts by weight, based on 100 parts by weight of the PVA-based resin constituting the PVA-based film.

The thickness of the PVA-based film is usually 10 to 150 μm, although it depends on whether or not the PVA-based film is a stretched film, and is preferably 100 μm or less, more preferably 100 μm or less, from the viewpoint of thinning the obtained polarizing film. It is 65 μm or less, more preferably 50 μm or less, and particularly preferably 35 μm or less (for example, 30 μm or less, further 20 μm or less).

The present invention is a method for producing a polarizing film from such a PVA-based film, and FIG. 1 shows an example of a production apparatus used in the production method of the present invention. That is, the manufacturing apparatus continuously manufactures a long polarizing film from a long PVA-based film (hereinafter sometimes referred to as "raw fabric") which is a raw material film, and swells the PVA-based film. A swelling tank 1 that swells with the treatment liquid 10, a dyeing tank 2 that immerses the PVA-based film in the dyeing treatment liquid 11 for dyeing, and a stretching tank that stretches the PVA-based film in the stretching treatment liquid 12 in a long direction. 3 and a transport path having a plurality of rolls for transporting the PVA-based film to each of the treatment tanks.

The plurality of rolls include a plurality of types of rolls, for example, a guide roll 4 that supports one side of the PVA-based film, a nip roll 5 that sandwiches and presses the PVA-based film from both sides, and a roll that unwinds the PVA-based film. Examples include a take-out roll (not shown) and a take-up roll (not shown) for winding the obtained polarizing film. These are appropriately arranged before and after each treatment tank, in the treatment tank, and the like.

The rotation speed of each of the nip rolls 5 can be adjusted, and in the adjacent nip rolls 5, the rotation speed of the nip rolls 5 arranged on the traveling direction side can be increased or the diameter thereof can be increased. By doing so, the PVA-based film stretched during this period can be stretched in the elongated direction. The nip roll 5 is usually provided once in one step except for the stretching step described later. In the stretching step, a plurality of nip rolls 5 are usually provided.

A method for manufacturing a polarizing film according to an embodiment of the present invention will be described using the above manufacturing apparatus. The method for producing a polarizing film includes a swelling step, a dyeing step, and a stretching step, and each step will be described in detail below.

[Swelling process]
The swelling step is provided before the dyeing step in order to clean the surface of the PVA-based film and suppress dyeing unevenness by swelling the PVA-based film with the swelling treatment liquid 10. Is. Water is usually used as the swelling treatment liquid 10 in the swelling step, but if the main component is water, iodide, additives such as a surfactant, alcohol and the like may be contained. In the swelling step, the temperature of the swelling treatment liquid 10 is usually 10 to 45 ° C., and the time for immersing the PVA-based film in the swelling treatment liquid 10 is usually 0.1 to 10 minutes. The main component means a component that affects the characteristics of the material, and the content of the component is usually 50% by weight or more of the whole material.

[Dyeing process]
In the dyeing step, in order to dye the PVA-based film, the PVA-based film that has passed through the swelling step is immersed in the dyeing treatment liquid 11 containing iodine or a dichroic dye in the dyeing tank 2. As the dyeing solution 11, an iodine-potassium iodide aqueous solution is usually used, the iodine concentration is usually 0.1 to 2 g / L, and the potassium iodide concentration is usually 1 to 100 g / L. Is. The dyeing solution 11 may contain an organic solvent compatible with water. In the dyeing step, the temperature of the dyeing solution 11 is preferably 5 to 50 ° C., and the time for immersing the PVA-based film in the dyeing solution 11 is usually 30 to 500 seconds.

[Stretching process]
In the stretching step, the PVA-based film that has passed through the dyeing step is stretched in the stretching treatment liquid 12 in the stretching tank 3 in the long direction (MD direction), and is usually 3 to 10 times as large as the original fabric. Although it is stretched, it is preferably stretched 3.5 to 6 times. The stretching ratio may be finally set in the above range, and the stretching operation can be performed not only in one step but also in a plurality of steps. Further, the temperature at the time of stretching is preferably 30 to 85 ° C. In the above stretching, a slight stretching (stretching to the extent of suppressing shrinkage in the width direction) may be added in a direction perpendicular to the stretching direction (width direction, TD direction).

As the stretching treatment liquid 12, an aqueous solution containing potassium iodide and boric acid can be used. When the aqueous solution contains boric acid, the boric acid aqueous solution is usually used at a concentration of about 10 to 100 g / L.

Then, in the method for producing a polarizing film of the present invention, when the stretching ratio of the PVA-based film in the long direction in the nip roll A which is last contacted in the stretching treatment 12 liquid of the stretching step is set to 100%, One of the features is to control the stretch ratio of the PVA-based film in the long direction in the nip roll B which is first contacted after the PVA-based film is pulled up from the dyeing treatment liquid 11 in the dyeing step to 40% or less. ..

In order to adjust the stretch ratio of the PVA-based film in the long direction, for example, it is possible to control the size of the diameter of the nip roll 5 and its rotation speed. The length of the PVA-based film when passing through the nip rolls A and B can be calculated from the outer peripheral lengths of the nip roll A and the nip roll B and the number of rotations thereof.

That is, the relationship when the transport speed of the nip roll A is v ₁ , the transport speed of the nip roll B is v _2, and the stretch ratio (%) of the PVA-based film at the point of the nip roll B in the long direction is m is as follows. It is represented by (1), and the present invention is characterized in that m in the following formula (1) is 40% or less at the nip roll B point. The transport speeds v ₁ and v ₂ can be calculated from the outer peripheral length and the number of revolutions of the nip rolls A or B.
m = v ₂ / v ₁ × 100 ... (1)

A polarizing film can be obtained by passing the PVA-based film that has passed through each step in this way through a drying step or the like.

[Drying process]
As the drying step, for example, the PVA-based film is transported into a hot-air oven capable of raising the temperature in the furnace by supplying hot air, or the PVA-based film is transferred to one or more heated bodies having a convex curved surface. The process of bringing the films into close contact with each other or the process of heating the PVA-based film using a heater may be performed.

Examples of the heating body include a roll (for example, a guide roll that also serves as a heat roll) that is provided with a heat source (for example, a heat medium such as hot water or an infrared heater) and can raise the surface temperature. Further, examples of the heater include an infrared heater, a halogen heater, a panel heater and the like.

The temperature of the drying process in the drying step (for example, the temperature inside the drying furnace, the surface temperature of the heat roll, etc.) is usually 30 to 100 ° C, preferably 50 to 90 ° C.

The polarizing film thus obtained may be sequentially wound on a winding roll to form a roll, or may be formed as it is without winding in a polarizing plate manufacturing step (a thermoplastic resin film (protective film) on one or both sides of the polarizing film). Etc.) can also be used in the step of laminating).

According to this configuration, when the PVA-based film is stretched in the long direction while being conveyed via the plurality of nip rolls 5 to produce a polarizing film, the nip roll A which comes into contact last in the stretching treatment liquid 12 in the stretching step When the stretch ratio of the PVA-based film in the long direction is 100%, the long-length direction of the PVA-based film in the nip roll B which is first contacted after the PVA-based film is pulled up from the dyeing treatment liquid 11 in the dyeing step. Since the stretch ratio of the film is controlled to 40% or less, uneven dyeing is suppressed, and the polarizing film has excellent in-plane uniformity of polarization performance (polarization degree, single transmission rate, etc.). In addition, since it is not necessary to introduce a new device and the existing device can be utilized, it is economically excellent. Further, since it is not necessary to use a chloride solution as the treatment liquid in each step, it is not necessary to dispose of the chloride solution, which is also excellent in terms of environment. Moreover, since the PVA-based film does not break due to swelling, stable production can be performed and the yield is excellent.

In the above embodiment, the swelling step, the dyeing step, and the stretching step are performed in this order, but the order of each of the above steps does not have to be the same. However, it is more preferable to carry out the swelling step, the dyeing step, and the stretching step in this order in that uniform dyeing becomes possible. Further, other steps may be provided between or before and after each of the above steps. Examples of the other steps include a cross-linking treatment step and a cleaning step. That is, in the above-described embodiment, in the stretching step, the stretching and the cross-linking treatment are performed at the same time, but instead of performing these at the same time, the step of the cross-linking treatment may be provided as a step different from the stretching step.

Further, in the above embodiment, one treatment tank is provided for each step, but a plurality of treatment tanks may be provided in the same step and each treatment may be performed a plurality of times.

The number and arrangement of each roll provided in the transport path is not limited to this example. It can be appropriately arranged according to changes in the arrangement of each process and each treatment tank.

FIG. 2 is a schematic view of a manufacturing apparatus used in another embodiment. In this form, a cross-linking step is performed separately from the stretching step, and a cross-linking treatment tank 8 is independently provided between the dyeing tank 2 and the stretching tank 3. Further, in order to perform a cleaning step of cleaning the PVA-based film that has completed the stretching step, a cleaning tank 9 is provided after the stretching tank 3. The swelling step by the swelling tank 1, the dyeing step by the dyeing tank 2, and the stretching step by the stretching tank 3 are the same as those in the above-described embodiment shown in FIG. 1, and the description thereof will be omitted. Hereinafter, the crosslinking step and the cleaning step will be described.

[Crosslinking process]
In the above-mentioned cross-linking step, the cross-linking treatment liquid 13 is used, and examples of the cross-linking agent contained in the above-mentioned cross-linking treatment liquid 13 include boric acid, glyoxal, glutaraldehyde and the like, and boric acid is particularly preferably used. .. These can be used alone or in combination of two or more. The content of the cross-linking agent in the cross-linking treatment liquid 13 is usually 0.1 to 15 parts by weight, preferably 1 to 12 parts by weight, per 100 parts by weight of water.

In the dyeing step prior to this cross-linking step, when the dyeing treatment liquid 11 contains iodine, the cross-linking treatment liquid 13 preferably contains iodide in addition to the cross-linking agent. Examples of the iodide include potassium iodide and zinc iodide. These can be used alone or in combination of two or more. The content of iodide in the cross-linking treatment liquid 13 is usually 0.1 to 20 parts by weight, preferably 5 to 15 parts by weight, per 100 parts by weight of water.

The cross-linking treatment liquid 13 may contain a compound other than the cross-linking agent and iodide. Examples of such a compound include zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate and the like. These can be used alone or in combination of two or more.

The temperature of the cross-linking treatment liquid 13 is usually 20 to 85 ° C, preferably 30 to 70 ° C. The immersion time of the PVA-based film (residence time in the cross-linking treatment liquid 13) is usually 10 to 600 seconds, preferably 20 to 300 seconds.

As described above, it is preferable that the cross-linking treatment tank 8 is independently provided as the cross-linking step in that the finally obtained polarizing film is less likely to be wrinkled.

[Washing process]
In the cleaning step, the PVA-based film is brought into contact with the cleaning liquid for cleaning for the purpose of removing excess chemicals adhering to the PVA-based film immersed in the stretching treatment liquid 12. The cleaning step can be performed, for example, by immersing the PVA-based film in the cleaning liquid 14 in the cleaning tank 9 for a predetermined time and then pulling it out. Further, the cleaning liquid 14 may be sprayed on the PVA-based film, or a combination of dipping and spraying may be used.

For the cleaning liquid 14, for example, water (pure water or the like) can be used. Further, it may be an aqueous solution to which a water-soluble organic solvent such as alcohol is added, or may contain an organic solvent. The cleaning liquid 14 is usually used at 2 to 40 ° C. In the above-mentioned washing step, a stretching treatment (usually a uniaxial stretching treatment) may be performed while washing the PVA-based film.

As described above, it is preferable that the cleaning tank 9 is provided as the cleaning step in that a polarizing film having few defects of foreign matter can be obtained.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.

First, the following PVA films were prepared.
[PVA-based film]
Weight average molecular weight 142,000, PVA resin with a saponification degree of 99.8 mol% 2,000 kg, water 5,000 kg, glycerin 240 kg as a plasticizer (total amount of 12 parts by weight with respect to 100 parts by weight of PVA resin) Was added and the temperature was raised to 140 ° C. with stirring to adjust the resin concentration to 25% by weight to prepare a uniformly dissolved PVA-based resin aqueous solution.
Next, the PVA-based resin aqueous solution is supplied to a twin-screw extruder having a vent to defoam, and then the aqueous solution temperature is set to 95 ° C. and discharged from a T-shaped slit die discharge port to a rotating cast drum (discharge speed). 1.9 m / min) and casting to form a film. The film-formed film was peeled off from the cast drum, and the front surface and the back surface of the film were dried while being alternately contacted with 20 metal heating rolls, and then the moisture content was adjusted.
Then, both ends in the width direction (TD direction) were slit to obtain a PVA-based film (moisture content 2.5%) having a thickness of 60 μm, a width of 5 m, and a length of 10 km. Finally, the PVA-based film was wound around the core tube in a roll shape to obtain a PVA-based film wound around the unwinding roll.

<Example 1>
Using the apparatus of FIG. 1, the prepared PVA-based film is unwound from the unwinding roll, and while being conveyed in the horizontal direction, it is immersed in water (30 ° C.) in the swelling tank 1 to swell the PVA-based film. Stretched in the elongated direction (MD direction) (swelling step). At this time, the stretch ratio of the PVA-based film in the long direction on the nip roll C that comes into contact first after the PVA-based film is pulled up from the swelling treatment liquid 10 in the swelling tank 1 in the swelling step is 24.0%. rice field.
Next, the amount of iodine is adjusted so that the single transmittance of the finally obtained polarizing film in the dyeing tank 2 is 43.7%, and the dyeing treatment liquid 11 having a composition containing 30 g / L of potassium iodide ( The PVA-based film was immersed in (30 ° C.) and stretched in the long direction (MD direction) while being dyed (dyeing step). At this time, the stretch ratio of the PVA-based film in the long direction on the nip roll B which is first contacted after the PVA-based film is pulled up from the dyeing solution 11 in the dyeing tank 2 in the dyeing step is 30.0%. rice field.
Then, the PVA-based film was stretched in the stretching treatment liquid 12 in the stretching tank 3 in the stretching step in three times in the elongated direction (MD direction) (stretching step). At this time, the stretching ratio of the PVA-based film in the long direction on the nip roll D that first came into contact with the stretching treatment liquid 12 in the stretching tank 3 was 60.0%. The stretch ratio (%) is calculated by assuming that the stretch ratio of the PVA-based film in the nip roll A that is last contacted in the stretch treatment liquid 12 in the long direction is 100%.
Finally, using an apparatus not shown in FIG. 1, the PVA-based film was washed with an aqueous potassium iodide solution (cleaning solution) (cleaning step), dried at 60 ° C. for 2 minutes (drying step), and the desired polarized light was obtained. I got a film.

<Example 2>
In Example 1, the same applies except that the stretching ratio of the PVA-based film in the long direction on the nip roll D that first comes into contact with the stretching treatment liquid 12 in the stretching tank 3 in the stretching step is 53.4%. Obtained a polarizing film.

<Comparative example 1>
In Example 1, the stretch ratio of the PVA-based film in the long direction on the nip roll C, which is first contacted after the PVA-based film is pulled up from the swelling treatment liquid 10 in the swelling tank 1 in the swelling step, is set to 32%. The stretching ratio of the PVA-based film in the nip roll B, which is first contacted after the PVA-based film is pulled up from the dyeing treatment liquid 11 in the dyeing tank 2 of the dyeing step, is set to 45%, and the stretching of the stretching step is performed. A polarizing film was obtained in the same manner except that the stretching ratio of the PVA-based film in the long direction on the nip roll D which was first contacted in the stretching treatment liquid 12 in the tank 3 was 63.3%.

The following items (polarization degree, single transmittance, dyeing unevenness) were evaluated for Examples 1 and 2 and Comparative Example 1, respectively. The results are also shown in Table 1 below.

[Polarization and single transmittance]
A polarizing film having a length of 4 cm and a width of 4 cm was cut out from the central portion of the obtained polarizing film in the width direction and used as a test piece. With respect to this test piece, the degree of polarization (%) and the single transmittance (%) were measured using an automatic polarizing film measuring device (VAP7070 manufactured by JASCO Corporation).

[Dyeing unevenness]
A polarizing film having a length of 30 cm × 30 cm was cut out from the central portion in the width direction of the obtained polarizing film and used as a test piece. This test piece is sandwiched between two polarizing plates in a cross Nicol state (polarization rate 99.9%, single transmittance 43.6%) at an angle of 90 °, and ^{a backlight having a luminous intensity of 30,000 Cd / m 2} is provided. The brightness was measured using a two-dimensional color difference luminance meter (manufactured by Konica Minolta Japan, CA-2500).
The measured luminance was obtained by averaging 600 points evenly in both the longitudinal direction and the width direction, and each luminance coefficient of variation was calculated based on the following equation (2).
Luminance coefficient of variation = standard deviation / average value ... (2)
Then, the dyeing unevenness was evaluated based on the following evaluation criteria with respect to the calculated value.
-Evaluation criteria 〇 (Good): The coefficient of variation of brightness is less than 0.08 in the long direction and less than 0.15 in the width direction.
× (bad): The coefficient of variation of brightness is 0.08 or more in the long direction and 0.15 or more in the width direction.

From the above results, it can be seen that Examples 1 and 2 have a high degree of polarization, suppress the occurrence of dyeing unevenness, and have a low luminance coefficient of variation.
On the other hand, in Comparative Example 1, it can be seen that the occurrence of dyeing unevenness is not suppressed, the luminance variation coefficient is high, and the variation coefficient in the long direction is particularly high.

It is suitable for producing a polarizing film that suppresses uneven dyeing and has excellent in-plane uniformity of polarization performance (polarization degree, single transmittance, etc.).

The production method of the present invention can produce a polarizing film having suppressed dyeing unevenness and excellent in-plane uniformity of polarization performance (polarization degree, single transmittance, etc.).

1 Swelling tank 2 Dyeing tank 3 Stretching tank 11 Dyeing treatment liquid 12 Stretching treatment liquid A Nip roll B Nip roll

Claims (3)

A method for producing a polarizing film by stretching a polyvinyl alcohol-based film in a long direction while transporting it via a plurality of nip rolls.
A swelling step of swelling the polyvinyl alcohol-based film with a swelling treatment liquid, a dyeing step of immersing the polyvinyl alcohol-based film in a dyeing treatment liquid in a dyeing tank for dyeing, and stretching the polyvinyl alcohol-based film in a stretching tank. It is provided with a stretching step of stretching in a long direction in a treatment liquid.
When the stretching ratio of the polyvinyl alcohol-based film in the long direction in the nip roll A that is last contacted in the stretching treatment liquid of the stretching step is 100%, the polyvinyl alcohol-based film is selected from the dyeing treatment liquid of the dyeing step. A method for producing a polarizing film, which comprises controlling the stretch ratio of the polyvinyl alcohol-based film in the long direction on the nip roll B which is first contacted after the film is lifted to 40% or less. The method for producing a polarizing film according to claim 1, further comprising a cross-linking step of immersing the polyvinyl alcohol-based film in a cross-linking treatment liquid to carry out a cross-linking treatment. The method for producing a polarizing film according to claim 1 or 2, further comprising a cleaning step of bringing the polyvinyl alcohol-based film into contact with a cleaning liquid for cleaning.

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