JP2022136808A - Method of manufacturing polarizing film - Google Patents

Abstract

To provide a polarizing film manufacturing method which allows for manufacturing a polarizing film with less shrinkage force.SOLUTION: A method of manufacturing a polarizing film is provided, comprising a first drying step for drying polyvinyl alcohol-based resin film at a first temperature of 20°C or less after bringing the film into contact with a treatment liquid, and a second drying step for drying the polyvinyl alcohol-based resin film at a second temperature different from the first temperature after the first drying step.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a method for producing a polarizing film that can be used as a constituent member of a polarizing plate.

As a polarizing film, a uniaxially stretched polyvinyl alcohol resin film is conventionally used in which a dichroic dye such as iodine or a dichroic dye is adsorbed and oriented. A polarizing film is usually made into a polarizing plate by laminating a protective film on one or both sides thereof with an adhesive. used. In recent years, along with the thinning of liquid crystal display devices, thin polarizing films have been demanded.

In general, a polarizing film is produced by subjecting a continuously transported long polyvinyl alcohol resin film to various treatments such as swelling, dyeing, cross-linking, and stretching in a bath, followed by washing. It is produced by drying (see, for example, JP-A-2012-47799).

JP 2012-477999 A

Since the polyvinyl alcohol-based resin film subjected to the drying process is subjected to stretching treatment, shrinkage force is generated due to the removal of water in the drying process, and the dimensional stability of the obtained polarizing film tends to decrease. was there.

In Patent Document 1, a polarizer is manufactured through a first drying step of drying at a heating temperature of 25 ° C. or higher and lower than 65 ° C. and a second drying step of drying at a heating temperature of 65 ° C. or higher and 115 ° C. or lower. , that a polarizer having excellent dimensional stability in a high temperature environment can be obtained.

An object of the present invention is to provide a method for producing a polarizing film that can produce a polarizing film with a more suppressed shrinkage force.

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

According to the method of the present invention, a polarizing film with suppressed shrinkage force can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows typically an example of the manufacturing method of the polarizing film which concerns on this invention, and a polarizing film manufacturing apparatus used therewith.

<Method for producing polarizing film>
In the present invention, the polarizing film is a uniaxially stretched polyvinyl alcohol resin film in which a dichroic dye (iodine or dichroic dye) is adsorbed and oriented. The polyvinyl alcohol-based resin that constitutes the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. The degree of saponification is usually 85 mol% or more, preferably 90 mol% or more, more preferably 99 mol% or more. The polyvinyl acetate-based resin may be, for example, polyvinyl acetate, which is a homopolymer of vinyl acetate, or a copolymer of vinyl acetate and other monomers copolymerizable therewith. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids and the like. The degree of polymerization of the polyvinyl alcohol resin is generally 1,000-10,000, preferably 1,500-5,000.

These polyvinyl alcohol-based resins may be modified, and for example, aldehyde-modified polyvinyl formal, polyvinyl acetal, polyvinyl butyral and the like may be used.

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

The width of the raw film is not particularly limited, and can be, for example, 300 to 6000 mm.

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

After the polarizing film is unwound from the original roll, the long raw film is continuously conveyed along the film conveying path of the polarizing film manufacturing apparatus and brought into contact with the treatment liquid. Continuous production as a long polarizing film can be achieved by carrying out the drying process.

Examples of the treatment steps include a swelling treatment step in which the original film is immersed in a swelling bath and then drawn out, a dyeing treatment step in which the film after swelling treatment is immersed in a dyeing bath and then drawn out, and a film after dyeing is drawn in a cross-linking bath. It can include a cross-linking treatment step of drawing after immersing in. In addition, between these series of treatment steps (that is, before and after any one or more treatment steps and/or during any one or more treatment steps), wet or dry uniaxial stretching treatment is performed. Other processing steps may be added as desired. Each of the treatment steps described above may be a process of immersing the film in one bath, or a process of immersing the film in two or more baths sequentially.

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

The drying step is a step of drying the polyvinyl alcohol-based resin film to obtain a polarizing film. For example, a polarizing film having a moisture content of less than 15% by weight can be obtained.

Hereinafter, the method for producing a polarizing film according to the present invention will be described in more detail with reference to FIG. FIG. 1 is a cross-sectional view schematically showing an example of a polarizing film manufacturing method and a polarizing film manufacturing apparatus used therein according to the present invention. The polarizing film manufacturing apparatus shown in FIG. 1 continuously unwinds a raw (unstretched) film 10 made of a polyvinyl alcohol-based resin from a raw roll 11 and conveys it along a film conveying route, thereby producing a film. Swelling bath 13, dyeing bath 15, cross-linking bath 17, and cleaning bath 19 provided on the conveying path are sequentially passed, then first drying oven 70, and finally second drying oven 90. It is configured. The obtained polarizing film can be conveyed as it is to the next polarizing plate production step (step of laminating a protective film on one side or both sides of the polarizing film 23), for example. The arrows in FIG. 1 indicate the transport direction of the film.

FIG. 1 shows an example in which each of the swelling bath 13, the dyeing bath 15, the cross-linking bath 17 and the washing bath 19 is provided. , the dye bath 15, the cross-linking bath 17 and the cleaning bath 19, which are provided on the film conveying path and contain processing solutions for processing the polyvinyl alcohol resin film, are collectively referred to as "processing baths". ) may be provided in two or more tanks.

The film transport path of the polarizing film manufacturing apparatus includes the treatment bath, the first drying oven 70 and the second drying oven 90, as well as the guide roll 30 that supports the transported film or can change the film transport direction. 43, 60, 61, and nip rolls 50 to 54, 82, 83 that can press and hold the transported film and apply a driving force to the film by its rotation, or can change the film transport direction. can be constructed by placing the Guide rolls and nip rolls can be placed before and after each treatment bath and each drying furnace, and in the treatment bath, thereby allowing the introduction and immersion of the film into the treatment bath and the withdrawal from the treatment bath [Fig. 1]. For example, the film can be immersed in each treatment bath by providing one or more guide rolls in each treatment bath and transporting the film along these guide rolls.

In the polarizing film manufacturing apparatus shown in FIG. 1, nip rolls are arranged before and after each treatment bath (nip rolls 50 to 54). It is possible to perform inter-roll stretching in which longitudinal uniaxial stretching is performed with a peripheral speed difference between rolls. Each step will be described below.

(Swelling treatment step)
The swelling treatment process is performed for the purpose of removing foreign matter from the surface of the raw film 10, removing the plasticizer in the raw film 10, imparting easy dyeability, plasticizing the raw film 10, and the like. The processing conditions are determined within a range in which the object can be achieved and in which problems such as extreme dissolution or devitrification of the original film 10 do not occur.

Referring to FIG. 1, in the swelling treatment step, raw film 10 is continuously unwound from raw film roll 11 and conveyed along a film conveying path, and raw film 10 is immersed in swelling bath 13 for a predetermined time. and then withdraw. In the example of FIG. 1 , the raw film 10 is conveyed along the film conveying path constructed by the guide rolls 60 and 61 and the nip rolls 50 from when the raw film 10 is unwound until it is immersed in the swelling bath 13. be done. In the swelling process, the film is conveyed along a film conveying path constructed by guide rolls 30-32.

As the swelling liquid of the swelling bath 13, in addition to pure water, boric acid (JP-A-10-153709), chloride (JP-A-06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohol It is also possible to use an aqueous solution to which 0.01 to 10% by weight of a compound or the like is added.

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

In the swelling treatment, the raw film 10 tends to swell in the width direction, causing wrinkles in the film. As one means for transporting the film while removing the wrinkles, a roll having a widening function such as an expander roll, a spiral roll, or a crown roll is used as the guide rolls 30, 31 and/or 32, or a cross guider or bend bar is used. , using other widening devices such as tenter clips. Another means for suppressing the occurrence of wrinkles is to apply a stretching treatment. For example, the uniaxial stretching process can be performed in the swelling bath 13 by utilizing the peripheral speed difference between the nip rolls 50 and 51 .

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

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

(Dyeing process)
The dyeing treatment step is performed for the purpose of adsorbing and orienting the dichroic dye on the polyvinyl alcohol-based resin film after the swelling treatment. The processing conditions are determined within a range in which the object can be achieved and in which problems such as extreme dissolution or devitrification of the film do not occur. Referring to FIG. 1, in the dyeing process, the film is conveyed along a film conveying path constructed by guide rolls 33 to 35 and nip rolls 51, and the film after the swelling treatment is dyed in a dyeing bath 15 (a treatment housed in a dyeing tank). liquid) for a predetermined period of time and then withdrawn. In order to enhance the dyeability of the dichroic dye, the film to be subjected to the dyeing process is preferably a film that has been uniaxially stretched to at least some extent, or instead of the uniaxial stretching process before the dyeing process, Alternatively, in addition to the uniaxial stretching treatment before the dyeing treatment, it is preferable to perform the uniaxial stretching treatment during the dyeing treatment.

When iodine is used as the dichroic dye, the dyeing solution in the dyeing bath 15 has a concentration by mass of iodine/potassium iodide/water=0.003 to 0.3/0.1 to 10/100. Certain aqueous solutions can be used. Instead of potassium iodide, other iodides such as zinc iodide may be used, or potassium iodide and other iodides may be used in combination. Compounds other than iodide, such as boric acid, zinc chloride, and cobalt chloride, may coexist. When boric acid is added, it is distinguished from the cross-linking treatment described later in that it contains iodine. can be regarded as The temperature of the dyeing bath 15 when the film is immersed is usually 10 to 45° C., preferably 10 to 40° C., more preferably 20 to 35° C., and the immersion time of the film is usually 30 to 600 seconds. It is preferably 60 to 300 seconds.

When a water-soluble dichroic dye is used as the dichroic dye, the dyeing liquid of the dyeing bath 15 includes, for example, an aqueous solution having a concentration by mass ratio of dichroic dye/water = 0.001 to 0.1/100. can be used. The dyeing bath 15 may contain a dyeing assistant or the like, and may contain, for example, an inorganic salt such as sodium sulfate or a surfactant. Only one type of dichroic dye may be used alone, or two or more types of dichroic dyes may be used in combination. The temperature of the dyeing bath 15 when the film is immersed is, for example, 20 to 80° C., preferably 30 to 70° C., and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

As described above, in the dyeing process, the film can be uniaxially stretched in the dyeing bath 15 . The uniaxial stretching of the film can be performed by a method such as creating a peripheral speed difference between the nip rolls 51 and 52 arranged before and after the dyeing bath 15 .

In the dyeing process as well as in the swelling process, the guide rolls 33, 34 and/or 35 are provided with widening functions such as expander rolls, spiral rolls and crown rolls in order to convey the polyvinyl alcohol resin film while removing the wrinkles of the film. can be used, or other widening devices such as cross guiders, bend bars, tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to apply stretching treatment, as in the case of swelling treatment.

In the example shown in FIG. 1, the film pulled out from the dyeing bath 15 passes through guide rolls 35 and nip rolls 52 in order and is introduced into the cross-linking bath 17 .

(Crosslinking treatment step)
The cross-linking treatment step is a treatment performed for the purpose of water resistance by cross-linking, hue adjustment (preventing the film from being bluish, etc.), and the like. Referring to FIG. 1, in the cross-linking treatment, the film is conveyed along a film conveying path constructed by guide rolls 36 to 38 and nip rolls 52, and is placed in a cross-linking bath 17 (a cross-linking liquid contained in a cross-linking tank) after dyeing treatment. It can be done by immersing the film for a period of time and then withdrawing it.

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

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

In the cross-linking treatment for the purpose of hue adjustment, for example, when iodine is used as a dichroic dye, a cross-linking agent with a concentration by mass of boric acid/iodide/water = 1 to 5/3 to 30/100 is included. liquid can be used. The temperature of the cross-linking bath when the film is immersed is usually 10 to 45° C., and the film immersion time is usually 1 to 300 seconds, preferably 2 to 100 seconds.

The cross-linking treatment may be performed multiple times, usually 2 to 5 times. In this case, the composition and temperature of each cross-linking bath used may be the same or different within the above ranges. The cross-linking treatment for water resistance by cross-linking and the cross-linking treatment for hue adjustment may be performed in a plurality of steps.

The uniaxial stretching treatment can also be performed in the cross-linking bath 17 by utilizing the peripheral speed difference between the nip rolls 52 and 53 .

In the cross-linking treatment, as in the swelling treatment, the guide rolls 36, 37 and/or 38 are provided with widening functions such as expander rolls, spiral rolls, and crown rolls in order to convey the polyvinyl alcohol-based resin film while removing wrinkles in the film. can be used, or other widening devices such as cross guiders, bend bars, tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to apply stretching treatment, as in the case of swelling treatment.

In the example shown in FIG. 1, the film pulled out from the cross-linking bath 17 passes through guide rolls 38 and nip rolls 53 in order and is introduced into the film washing bath 19 .

(Washing process)
The production method of the present invention can include a washing treatment step after the cross-linking treatment step. The cleaning treatment is performed for the purpose of removing excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The washing treatment can be carried out, for example, by immersing the crosslinked polyvinyl alcohol-based resin film in the film washing bath 19, by spraying the film with a film washing solution as a shower, or by using these together.

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

Also in the cleaning process, rolls having a widening function such as expander rolls, spiral rolls and crown rolls are used as the guide rolls 39, 40 and/or 41 for the purpose of conveying the polyvinyl alcohol resin film while removing wrinkles. Or other widening devices such as cross guiders, bend bars, tenter clips can be used. Further, in the film washing treatment, a stretching treatment may be applied to suppress the generation of wrinkles.

(Stretching process)
As described above, the raw film 10 is uniaxially stretched wet or dry during the series of treatment steps (that is, before and after any one or more treatment steps and/or during any one or more treatment steps). It is processed. A specific method of uniaxial stretching treatment is, for example, between two nip rolls (for example, two nip rolls arranged before and after the treatment bath) that constitute the film conveying path, and longitudinal uniaxial stretching is performed by applying a peripheral speed difference between the rolls. Stretching can be hot roll stretching or tenter stretching as described in Japanese Patent No. 2731813, preferably stretching between rolls. The uniaxial stretching process can be performed multiple times from raw film 10 to polarizing film 23 . As described above, the stretching treatment is also advantageous in suppressing the generation of wrinkles in the film.

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

The stretching treatment step may be performed in any of the above-described treatment steps as long as it is performed before the drying step. Even when the stretching treatment is performed in two or more treatment steps, the stretching treatment may be performed in any treatment step.

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

It is preferable that the polyvinyl alcohol resin film has a moisture content of 30% by mass or more when it is introduced into the first drying step. The moisture content of the polyvinyl alcohol resin film at the time of derivation from the first drying step is, for example, less than 30% by mass.

The first drying step is not limited as long as it can reduce the moisture content of the polyvinyl alcohol resin film. A specific example is a step of applying a gas of 20° C. or less to the polyvinyl alcohol resin film. Air, nitrogen, etc. are mentioned as gas. The wind speed of the gas applied to the polyvinyl alcohol resin film is preferably 0.1 m/s or more, more preferably 0.5 m/s or more, and still more preferably 1.5 m/s or more. /s or less.

In the example shown in FIG. 1, the first drying step is a process of passing the polyvinyl alcohol-based resin film through a first drying furnace 70 in which gas of 20° C. or less is blown out. A blower 71 is provided in the first drying oven 70 for blowing temperature-controlled air toward the polyvinyl alcohol resin film being conveyed. The angle between the blowing direction of the air blown from the blower 71 and the conveying direction of the polyvinyl alcohol resin film is 0° or more and 90° or less from the viewpoint of reducing the damage given to the polyvinyl alcohol resin film by wind pressure. 0° or more and 60° or less is more preferable.

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

The time of the first drying step is preferably 10 seconds or longer, more preferably 30 seconds or longer, for example, 100 seconds or shorter, preferably 50 seconds or shorter.

Since the polyvinyl alcohol resin film is gently dried in the first drying step, shrinkage is suppressed without applying a large load to the polyvinyl alcohol resin film, and a polarizing film with low shrinkage force can be produced.

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

The moisture content of the polyvinyl alcohol resin film when it is introduced into the second drying step is, for example, less than 30% by mass. In the second drying step, for example, the moisture content is decreased from 15% by mass or more to less than 30% by mass to less than 15% by mass. The moisture content of the polyvinyl alcohol resin film at the time of derivation from the second drying step is, for example, less than 15% by mass.

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

By having the first drying step and the second drying step as described above, it is possible to manufacture a polarizing film with suppressed shrinkage force. The thickness of the polarizing film 23 obtained as described above is, for example, 5 to 30 μm. According to the method of the present invention, it is possible to obtain a polarizing film that simultaneously satisfies i) to iii) below.
i) Visibility correction single transmittance (Ty) of 42.0% or more,
ii) visibility correction polarization degree (Py) of 99.970% or more,
iii) a contractile force of less than 2.73 N;
The visibility-corrected single transmittance (Ty), the visibility-corrected polarization degree (Py), and the contractile force are measured according to the description in the section of Examples described later.

(Third drying step)
After the first drying step and before the second drying step, the polyvinyl alcohol resin film is dried at a third temperature that is the same or higher than the first temperature and the same or lower than the second temperature. 3 You may have a drying process. A 3rd drying process is a process of reducing the moisture content of a polyvinyl-alcohol-type resin film. The third temperature is, for example, 15° C. or higher and 120° C. or lower.

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

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

(Other processing steps for polyvinyl alcohol-based resin film)
Processing other than the processing described above can also be added. Examples of treatments that can be added include immersion treatment in an iodide aqueous solution that does not contain boric acid (complementary color treatment), immersion treatment in an aqueous solution that does not contain boric acid and contains zinc chloride, etc. ( zinc treatment).

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

In order to improve the adhesion between the polarizing film and the protective film, the bonding surface of the polarizing film and/or the protective film is subjected to corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. may be treated. As the adhesive used for bonding the polarizing film and the protective film, an active energy ray-curable adhesive such as an ultraviolet-curable adhesive, an aqueous solution of a polyvinyl alcohol-based resin, or an aqueous solution in which a cross-linking agent is added thereto. and water-based adhesives such as urethane-based emulsion adhesives. The UV-curable adhesive may be a mixture of an acrylic compound and a photoradical polymerization initiator, a mixture of an epoxy compound and a photocationic polymerization initiator, or the like. Moreover, a cationic polymerizable epoxy compound and a radically polymerizable acrylic compound can be used together, and a photocationic polymerization initiator and a photoradical polymerization initiator can be used together as initiators.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

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

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

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

The film pulled out from the complementary color bath was immersed in a cleaning bath made of pure water at 13° C. for a residence time of 2 seconds (cleaning step). After that, the film pulled out from the washing bath is passed through the first drying oven, the third drying oven, and the second drying oven in that order, and the first drying process, the second drying process, and the third drying process are performed to obtain a polarizing film. got The thickness of the obtained polarizing film was 12 μm.

In the first drying furnace, air having a temperature of 18° C. was blown from one blower at an air volume of 60 Hz and an air velocity of 2.0 to 2.5 m/s to perform the first drying step. In the third drying furnace, air having a temperature of 18° C. was blown from one blower at an air volume of 60 Hz and an air velocity of 2.0 to 2.5 m/s to perform the third drying step. In the second drying furnace, the second drying step was carried out by blowing air having a flow rate of 60 Hz and a temperature of 100° C. from one blower at a wind speed of 2.0 to 2.5 m/s.

The polyvinyl alcohol resin film was kept in the drying oven for 36.6 seconds, 36.6 seconds, and 36.6 seconds in the order of the first drying process, the third drying process, and the second drying process, respectively.

<Example 2, Comparative Example 1>
In the first drying furnace, the third drying furnace, and the second drying furnace, except that the temperature of the air blown from the blower was set to the temperature shown in Table 1, Example 2 and comparison were performed in the same manner as in Example 1. A polarizing film of Example 1 was produced.

In Examples 1 and 2 and Comparative Example 1, the first temperature in the first drying step can be regarded as the same temperature as the temperature of the air blown from the blower in the first drying furnace, and the third temperature in the third drying step The temperature can be considered to be the same temperature as the temperature of the air blown from the blower in the third drying furnace, and the second temperature in the second drying step is the temperature of the air blown from the blower in the second drying furnace. It was confirmed that it can be regarded as the same temperature as

[Evaluation of polarizing film]
(a) Measurement of Single Transmittance and Degree of Polarization From the polarizing films obtained in each example and comparative example, a measurement sample of 4 cm×4 cm was cut out. This sample was measured for MD transmittance and TD transmittance in the wavelength range of 380 to 780 nm using a spectrophotometer with an integrating sphere ["V7100" manufactured by JASCO Corporation], using the following formula:
Single transmittance (%) = (MD + TD) / 2
Degree of polarization (%) = {(MD-TD) / (MD + TD)} x 100
Single transmittance and degree of polarization at each wavelength were calculated based on.

"MD transmittance" is the transmittance when the direction of polarized light emitted from the Glan-Thompson prism is parallel to the transmission axis of the polarizing film sample, and is expressed as "MD" in the above formula. The "TD transmittance" is the transmittance when the direction of the polarized light emitted from the Glan-Thompson prism is perpendicular to the transmission axis of the polarizing film sample, and is represented by "TD" in the above formula. Regarding the obtained single transmittance and degree of polarization, visibility correction was performed using a 2-degree field of view (C light source) of JIS Z 8701: 1999 “Color display method-XYZ color system and X ₁₀ Y ₁₀ Z ₁₀ color system”. Then, the luminosity-corrected single transmittance (Ty) and the luminosity-corrected polarization degree (Py) were obtained. Table 1 shows the calculation results of the luminosity correction single transmittance (Ty), the luminosity correction polarization degree (Py), and the b value of the orthogonal hue.

(b) MD Shrinkage Force From the polarizing film obtained in each example and comparative example, a measurement sample having a width of 2 mm and a length of 30 mm was cut out with the absorption axis direction (MD, stretching direction) as the long side. This sample was set in an analysis device (DMAQ800, Dynamic mechanical analyzer) manufactured by TA Corporation and held at 80 ° C. for 4 hours while keeping the dimensions constant. Contraction force (MD contraction force) was measured. Table 1 shows the measured contractile force values.

10 raw film made of polyvinyl alcohol-based resin, 11 raw roll, 13 swelling bath, 15 dyeing bath, 17 cross-linking bath, 19 cleaning bath, 21 drying oven, 23 polarizing film, 30, 31, 32, 33, 34, 35,36,37,38,39,40,41,42,43,60,61 guide roll, 50,51,52,53,54,82,83 nip roll, 70 first drying furnace, 71 blower, 90 drying Furnace.

Claims (4)

A first drying step of drying the polyvinyl alcohol-based resin film after being brought into contact with the treatment liquid at a first temperature of 20° C. or less;
A method for producing a polarizing film, comprising a second drying step of drying the polyvinyl alcohol-based resin film at a second temperature different from the first temperature after the first drying step. The method for producing a polarizing film according to claim 1, wherein the second temperature is 95°C or higher. Said 1st drying process WHEREIN: The manufacturing method of the polarizing film of Claim 1 or 2 which hits the gas of 1.5 m/s or more of wind speeds on the said polyvinyl-alcohol-type resin film. Between the first drying step and the second drying step, the polyvinyl alcohol resin film is dried at a temperature that is the same as or higher than the first temperature and at a temperature that is the same as or lower than the second temperature. Third drying The method for producing a polarizing film according to any one of claims 1 to 3, further comprising steps.

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