TW201710335A - Method for manufacturing polarizing film - Google Patents

Abstract

The present invention provides a method for manufacturing a polarizing film, comprising a step of stretching, allowing to dip polyvinyl alcohol-based resin film in a treatment bath while carrying. In addition, the uniaxial stretching is performed by using a difference in peripheral speeds between upstream nip rollers and downstream nip rollers. The step of the stretching has a low width reduction rate R (0.55 %/sec.) or less. A width of the polyvinyl alcohol-based resin film passing through the upstream nip rollers is set as W1 (mm), and a width of the polyvinyl alcohol-based resin film passing through the downstream nip rollers is set as W2 (mm). Also, the time required for carrying from the upstream nip rollers to the downstream nip rollers is set as T (seconds). The width reduction rate R is calculated with the following equation (1).

Description

Method for manufacturing polarizing film

The present invention relates to a method of producing a polarizing film which can be used as a constituent member of a polarizing plate.

The polarizing film used in the past is a film in which a dichroic dye such as iodine and a dichroic dye is adsorbed to a uniaxially stretched polyvinyl alcohol resin film. The polarizing film is usually a polarizing plate by bonding a protective film to one surface or both surfaces thereof, and can be used as a liquid crystal display device such as a liquid crystal television, a personal computer screen, or a mobile phone.

In general, the polarizing film is subjected to a treatment in which a long-length polyvinyl alcohol-based resin film to be continuously conveyed is sequentially immersed in a treatment bath such as a swelling bath, a dye bath, or a cross-linking bath, and an extension treatment is performed between the series of treatments. It is manufactured (for example, Japanese Laid-Open Patent Publication No. 2001-315140).

The polarizing film is conveyed while being continuously wound from the film roll (wound product) of the polyvinyl alcohol resin film, and is conveyed along the film conveyance path through the various treatment baths as described above, and The immersion in one or more stages from the swelling bath to the drawing of the crosslinking bath can be continuously performed by performing the stretching treatment. But in continuous manufacturing, especially at extensions When the film is broken, it is required to be improved from the viewpoint of productivity and yield of the polarizing film.

Accordingly, an object of the present invention is to provide a method for producing a polarizing film capable of suppressing film breakage during elongation processing.

The present invention provides a method of producing a polarizing film disclosed below.

[1] A method for producing a polarizing film, comprising: an extending step of immersing a polyvinyl alcohol-based resin film while being immersed in a treatment bath and using a peripheral speed difference between the upstream side nip roll and the downstream side nip roll; The uniaxial stretching is performed; the extending step is a low width reduction rate extending step having a width reduction ratio R of 0.55 (%/sec) or less; and the width of the polyvinyl alcohol resin film passing through the upstream side nip is set as W1 (mm), when the width of the downstream side nip is set to W2 (mm), and the time required for transporting from the upstream nip to the downstream nip is set to T (second), the width is reduced. The rate R is calculated by the following formula (1).

[2] The method for producing a polarizing film according to [1], wherein the low-width reduction rate extending step has a cumulative stretching ratio of 4.5 times when the polyvinyl alcohol-based resin film is stretched to pass through the downstream side nip roll. The above steps.

[3] The method for producing a polarizing film according to [1] or [2], wherein The low-width reduction rate extending step includes a step of immersing the polyvinyl alcohol-based resin film in a cross-linking bath and performing uniaxial stretching.

[4] The method for producing a polarizing film according to [3], wherein the polyvinyl alcohol-based resin film is immersed in a plurality of crosslinking baths. The low-width reduction rate extending step includes a step of immersing in a cross-linking bath disposed in the second and subsequent transport directions and performing uniaxial stretching.

[5] The method for producing a polarizing film according to any one of [1], wherein the polyvinyl alcohol resin film is immersed in a plurality of crosslinking baths. The extending step has an extension step at the first first crosslinking accompanying the immersion in the conveying direction of the crosslinking bath. In the extension step at the time of the first crosslinking, the cumulative stretching ratio of the polyvinyl alcohol-based resin film when passing through the upstream side nip roll is 2.5 times or more.

[6] The method for producing a polarizing film according to any one of [1], wherein the polyvinyl alcohol resin film is immersed in a plurality of crosslinking baths. The extending step is a step of extending the second crosslinking accompanying the second stage of the immersion in the conveying direction of the crosslinking bath. In the extension step at the time of the second crosslinking, the cumulative stretching ratio of the polyvinyl alcohol-based resin film when passing the upstream side nip roll is 4.0 times or more.

[7] The method for producing a polarizing film according to any one of [1], wherein the thickness of the polyvinyl alcohol-based resin film is 65 μm or less when it is not extended.

10‧‧‧Green film made of polyvinyl alcohol resin

11‧‧‧ Blank film roll

13‧‧‧Swelling bath

15‧‧‧dye bath

16‧‧‧1st cross-linking bath

17‧‧‧2nd cross-linking bath

18‧‧‧ Washing bath

21‧‧‧ drying oven

23‧‧‧ polarizing film

30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 60, 61‧ ‧ guide rollers

50, 51, 52, 53, 54, 55, 56‧ ‧ nip rollers

Fig. 1 is a cross-sectional view schematically showing an example of a method for producing a polarizing film of the present invention and a polarizing film producing apparatus using the same.

<Method of Manufacturing Polarized Film>

In the polarizing film of the present invention, a dichroic dye (iodine and a dichroic dye) is adsorbed and aligned on a uniaxially stretched polyvinyl alcohol resin film. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. The degree of saponification is usually about 85 mol% or more, preferably about 90 mol% or more, more preferably about 99 mol% or more. The polyvinyl acetate-based resin is, for example, a polyvinyl acetate of a homopolymer of vinyl acetate, and may be a copolymer of vinyl acetate and another monomer copolymerizable with vinyl acetate. Examples of other monomers which can be copolymerized include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is usually from about 1,000 to 10,000, preferably from about 1,500 to 5,000.

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

In the present invention, as a starting material for producing a polarizing film, an unstretched polyvinyl alcohol-based resin having a thickness of preferably 65 μm or less (for example, 60 μm or less), more preferably 50 μm or less, and still more preferably 35 μm or less is used. Film (blank film). Thereby, a polarizing film of a film which is increasingly required in the market can be obtained. The thinner the green film, the more likely the film breakage occurs during the stretching process. However, according to the present invention, even when the green film is thin (for example, 60 μm or less), film cracking can be effectively suppressed. Further, it is preferable to use a thickness of 10 μm or more for the green film. The width of the green film is not particularly limited and can be, for example, about 400 to 6000 mm. In general, the larger the film width, the easier it is to cause film cracking during the stretching treatment. However, according to the present invention, even if the film width is large, film cracking can be effectively suppressed.

In the present invention, the green film is a roll (blank roll) of a polyvinyl alcohol-based resin film which is not stretched.

In the polarizing film, the long strip film can be continuously wound and conveyed along the film transport path of the polarizing film manufacturing apparatus, and a predetermined processing step can be performed to continuously manufacture the long polarized film. membrane. The predetermined processing step may include: a swelling treatment step of immersing the green film in the swelling bath; a dyeing treatment step of immersing the film after the swelling treatment step in the dyeing bath; and immersing the dyed film in the crosslinking bath Pull out the cross-linking processing steps.

In the present invention, the film is immersed in a treatment bath (a swelling bath, a dye bath, a cross-linking bath, a washing bath, and the like are provided on the film transport path for accommodating the treatment of the polyvinyl alcohol-based resin film). The liquid bath, which is also collectively referred to as a "treatment bath", is transported as one or more of the above-described processing steps, and has a single-axis by using a peripheral speed difference between the upstream nip roller and the downstream nip roller. The extension step of the extension.

In the present invention, a nip roller and its peripheral speed are different The next nip rolls along the conveyance path are referred to as "upstream side nip rolls" and "downstream side nip rolls", respectively, and the steps from the upstream side nip rolls to the downstream side nip rolls are set as an extension step. The manufacturing method of the present invention has one or more such extension steps. Further, in addition to the above-described stretching step in which the film is immersed in the treatment bath, there may be an extension step in the gas phase after the film is immersed in the treatment bath.

The treatment baths as described above disposed on the transport path may be one or more of the same type of treatment baths. In addition, the polyvinyl alcohol-based resin film is immersed in the plurality of processing baths from the upstream side nip to the next downstream side nip roll even when it is immersed from the upstream side nip roll to the next downstream side nip roll. The interval is set as an extension step. On the other hand, even if it is immersed in a treatment bath (for example, a crosslinking bath) from the upstream side nip roll to the next downstream side nip roll, it is accompanied by the next upstream side nip roll to the next downstream side nip roll. In the case of a treatment bath (for example, a crosslinking bath) which is immersed in the same kind as the preceding stage, each of these lines is provided as an additional extension step. The downstream side nip roller of the extension step of the preceding stage has a case of doubling the upstream side of the stretching step of the next stage. In the present invention, other processing steps may be added as necessary.

In the present invention, as the above-described stretching step in which the film is immersed in the treatment bath, the step is a low-width reduction rate extending step having a width reduction ratio R of 0.55 (%/sec) or less. The inventors of the present invention have found that in order to suppress film breakage, it is effective to adjust the width reduction ratio R, and it is more effective to have a low width reduction rate extending step having a width reduction ratio R of 0.55 (%/sec) or less. From the viewpoint of suppressing cracking, it is particularly effective to perform a low-width reduction rate extension step for a film having a cumulative stretching ratio of 2.4 times or more, and more effective is to be tired. The film having a stretch ratio of 4.0 or more is subjected to a low width reduction rate extending step. Further, the width reduction rate R in the low width reduction rate extending step is preferably 0.50 (%/second) or less, more preferably 0.45 (%/second) or less. Further, the width reduction rate R is usually 0.01 (%/sec) or more. Here, the width reduction ratio R is set to W1 (mm) by the width of the polyvinyl alcohol-based resin film when passing through the upstream side nip, in the extension step of immersing in the treatment bath, and passes through the downstream side. The width of the polyvinyl alcohol-based resin film at the time of nip roll is set to W2 (mm), and the time required for the conveyance from the upstream side nip roll to the downstream side nip roll is set to T (second), The value calculated by the above formula (1).

Hereinafter, a method of producing the polarizing film of the present invention will be described in more detail with reference to Fig. 1 . Fig. 1 is a cross-sectional view schematically showing an example of a method for producing a polarizing film of the present invention and a polarizing film producing apparatus using the same. The polarizing film manufacturing apparatus shown in Fig. 1 has a configuration in which a blank film 10 (not stretched) made of a polyvinyl alcohol-based resin is continuously wound from the green film roll 11 to be edged. The film transport path is transported, and the swelling bath 13 provided in the film transport path, the dye bath 15, the first cross-linking bath 16, the second cross-linking bath 17, and the washing bath 18 are sequentially passed through, and finally passed through. Drying furnace 21. The obtained polarizing film 23 can be directly conveyed to the next polarizing plate manufacturing step (step of bonding a protective film on one surface or both surfaces of the polarizing film 23). The arrow direction of the first figure shows the conveyance direction of the film.

The film transport path of the polarizing film manufacturing apparatus is in addition to the above In addition to the treatment bath, the guide rolls 30 to 44, 60, 61, and the nip rolls 50 to 56 can be constructed by being disposed at appropriate positions, wherein the guide rolls 30 to 44, 60, 61 support the film, or are further modified The film conveyance direction; the nip rollers 50 to 56 are capable of pressing and holding the film to be conveyed, supplying the film with a driving force obtained by the rotation thereof, or changing the film conveyance direction. The guide roller and the nip roller can be disposed in the front and rear of each treatment bath and in the treatment bath, whereby the introduction, the immersion film, and the extraction from the treatment bath can be performed [refer to Fig. 1]. For example, by providing one or more guide rolls in each treatment bath and transporting the film along the guide rolls, the film can be immersed in each treatment bath.

In the polarizing film manufacturing apparatus shown in Fig. 1, nip rollers (nip rollers 50 to 55) are disposed in front of and behind each of the processing baths, whereby they can be disposed in any one or more of the processing baths. The inter-roller extension (extension step) is performed between the nip rolls (between the upstream nip rolls and the downstream nip rolls) to impart a circumferential speed difference and to perform longitudinal uniaxial stretching. Hereinafter, each processing procedure will be described.

(swelling treatment)

The swelling treatment is carried out for the purpose of removing foreign matter on the surface of the green film 10, removing the plasticizer in the green film 10, imparting dyeability, and plasticizing the green film 10. The treatment conditions are determined within a range in which the object can be achieved without causing defects such as excessive dissolution of the green film 10 and loss of transparency.

Referring to Fig. 1, the swelling treatment is carried out by continuously ejecting the green film 10 from the blank roll 11 while transporting it along the film conveyance path, and the green film 10 is placed in the swelling bath 13 (contained in the swelling) The treatment liquid of the tank is immersed for a predetermined period of time and then pulled out and carried out. In the example of Figure 1, from the blank film 10 rolls up until it is immersed in the swelling bath 13, and the green film 10 is conveyed along the film conveyance path by the guide rolls 60 and 61 and the nip roll 50. The swelling treatment is carried along the film conveyance path constructed by the guide rolls 30 to 32.

In addition to pure water, the swell bath 13 can also be used by adding boric acid (JP-A No. 10-153709), chloride (JP-A-06-281816), inorganic acid, inorganic salt, water-soluble organic An aqueous solution in the range of about 0.01 to 10% by weight, such as a solvent or an alcohol.

The temperature of the swelling bath 13 is, for example, about 10 to 50 ° C, preferably about 10 to 40 ° C, more preferably about 15 to 30 ° C. The immersion time of the green film 10 is preferably from about 10 to 300 seconds, more preferably from about 20 to 200 seconds. Further, when the green film 10 is a polyvinyl alcohol-based resin film which is previously extended in a gas, the temperature of the swelling bath 13 is, for example, about 20 to 70 ° C, preferably about 30 to 60 ° C. The immersion time of the green film 10 is preferably from about 30 to 300 seconds, more preferably from about 60 to 240 seconds.

In the swelling treatment, the problem that the green film 10 is swollen in the width direction and wrinkles the film is likely to occur. As a means for removing the wrinkles and carrying the film, it is exemplified that the guide rolls 30, 31 and/or 32 have an expanded width function such as an expander roll, a spiral roll, and a crown roll. Rolls; or other spread width devices such as cross guides, bending rolls, and tenter clips.

In the swelling treatment, the stretching step may be performed in parallel, and the uniaxial stretching treatment is performed by the circumferential speed difference between the nip roller (upstream side nip roller) 50 and the nip roller (downstream side nip roller) 51. By performing the extension step in parallel, it is also possible Inhibits wrinkles.

In the swelling treatment, since the film is swollen and enlarged in the conveyance direction of the film, when the film is not actively stretched, it is preferable to control the nip roller 50 disposed before and after the swelling bath 13 in order to eliminate the film slack in the conveyance direction. , 51 speed and other means. Further, in order to stabilize the film transport in the swelling bath 13, the water flow in the swelling bath 13 is controlled by spraying in water, and an EPC apparatus (Edge Position Control) device is used to detect the end of the film, It is also useful to prevent membrane serpentine devices.

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

(dyeing treatment)

The dyeing treatment is carried out for the purpose of adsorbing and aligning the dichroic dye to the polyvinyl alcohol-based resin film after the swelling treatment. The treatment conditions are determined within a range in which the object can be achieved without causing defects such as excessive dissolution of the film and loss of transparency. Referring to Fig. 1, the dyeing process can be carried by the film transport path constructed by the guide rolls 33 to 35 and the nip rolls 51, and the swelled film is processed in the dyeing bath 15 (contained in the dyeing tank). The liquid is immersed for a predetermined period of time and then pulled out and carried out. In order to improve the dyeability of the dichroic dye, a film providing a dyeing treatment step is preferably a film which is subjected to at least a certain degree of uniaxial stretching treatment, or preferably a uniaxial stretching treatment prior to the substitution dyeing treatment, or in addition to dyeing. In addition to the uniaxial stretching process before the treatment, the stretching step of the uniaxial stretching process is performed in parallel during the dyeing process. The step of extending in parallel with the dyeing treatment is performed in the dye bath 15 The front and rear nip rollers (upstream side nip rolls) 51 and the nip rolls (downstream side nip rolls) 52 are provided with a circumferential speed difference.

When iodine is used as the dichroic dye, an aqueous solution having a concentration of iodine/potassium iodide/water = about 0.003 to 0.3 / about 0.1 to 10/100 in weight ratio can be used in the dyeing bath 15 . Other iodides such as zinc iodide may be used instead of potassium iodide, and potassium iodide may be used in combination with other iodides. Further, it may coexist with a compound other than iodide, for example, boric acid, zinc chloride, cobalt chloride or the like. When the boric acid is added, it is distinguished from the crosslinking treatment described later by the point of containing iodine, and the aqueous solution contains about 0.003 part by weight or more of iodine per 100 parts by weight of water, and is regarded as the dyeing bath 15. The temperature of the dyeing bath 15 when immersing the film is usually about 10 to 45 ° C, preferably 10 to 40 ° C, more preferably 20 to 35 ° C, and the immersion time of the film is usually about 30 to 600 seconds, preferably 60. Up to 300 seconds.

When a water-soluble dichroic dye is used as the dichroic dye, the dyeing bath 15 can use, for example, an aqueous solution having a concentration of dichroic dye/water = about 0.001 to 0.1/100 by weight. The dyeing bath 15 may coexist with a dyeing aid or the like, and may contain, for example, an inorganic salt such as sodium sulfate or a surfactant. The dichroic dye may be used alone or in combination of two or more kinds of dichroic dyes. The temperature of the dyeing bath 15 at the time of immersing the film is, for example, about 20 to 80 ° C, preferably 30 to 70 ° C, and the immersion time of the film is usually about 30 to 600 seconds, preferably about 60 to 300 seconds.

In the dyeing treatment, similarly to the swelling treatment, in order to remove the wrinkles of the film and transport the polyvinyl alcohol-based resin film, it is possible to use, for example, a spread roll, a spiral roll, or a convex roll on the guide roll guide rolls 33, 34 and/or 35. With expansion Rolls for width function; or other extended end devices such as cross guides, bending rolls, and tenter clips. Further, in order to suppress wrinkles, the stretching step performed in parallel during the dyeing treatment is also effective.

In the example shown in Fig. 1, the film drawn from the dyeing bath 15 is introduced into the first crosslinking bath 16 through the guide rolls 35 and the nip rolls 52 in this order.

(1st cross-linking process)

The first crosslinking treatment is a treatment for the purpose of preventing hydration, hue adjustment (preventing the film from bluing, etc.) by crosslinking. Referring to Fig. 1, the first crosslinking treatment system can be carried along the film conveyance path constructed by the guide rolls 36 to 38 and the nip rolls 52, and the dyed film is placed in the first crosslinking bath 16 (The treatment liquid contained in the crosslinking tank) is immersed for a predetermined period of time and then pulled out and carried out.

The first crosslinking bath 16 can contain, for example, about 1 to 10 parts by weight of an aqueous solution of boric acid, based on 100 parts by weight of water. When the dichroic dye used in the dyeing treatment is iodine, the first crosslinking bath 16 preferably contains an iodide in addition to boric acid, and the amount thereof can be, for example, 1 to 30 parts by weight based on 100 parts by weight of water. Examples of the iodide include potassium iodide, zinc iodide, and the like. Further, it may coexist with a compound other than iodide such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfonate, sodium sulfate or the like.

In the first crosslinking treatment, the concentration of boric acid and iodide and the temperature of the first crosslinking bath 16 can be appropriately changed in accordance with the purpose. For example, the purpose of the first crosslinking treatment is to cause water repellency by crosslinking, and the polyvinyl alcohol-based resin film is sequentially subjected to swelling treatment, dyeing treatment, and first crosslinking. At the time of the treatment, the crosslinking agent-containing liquid of the first crosslinking bath may be an aqueous solution having a concentration of boric acid/iodide/water = 3 to 10/1 to 20/100 by weight. If necessary, other crosslinking agents such as glyoxal or glutaraldehyde may be used instead of boric acid, and boric acid and other crosslinking agents may be used in combination. The temperature of the first crosslinking bath when immersing the film is usually about 50 to 70 ° C, preferably 53 to 65 ° C, and the immersion time of the film is usually about 10 to 600 seconds, preferably 20 to 300 seconds, more preferably It is 20 to 200 seconds. Further, when the pre-stretched polyvinyl alcohol-based resin film is sequentially subjected to the dyeing treatment and the first crosslinking treatment before the swelling treatment, the temperature of the crosslinking bath 17 is usually about 50 to 85 ° C, preferably 55 to 80. °C.

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

In the first cross-linking treatment, the circumferential speed difference between the nip roll (upstream side nip roll) 52 and the nip roll (downstream side nip roll) 53 can be used in the step of extending the parallel uniaxial stretching process. This extension step is also effective in order to suppress wrinkles.

In the first cross-linking treatment, in the same manner as the swelling treatment, in order to remove the wrinkles of the film and transport the polyvinyl alcohol-based resin film, it is possible to use, for example, a spread roll, a spiral roll, or a convex roll on the guide rolls 36, 37, and/or 38. Rollers with extended width function; or other extended width devices such as cross guides, bending rolls, tenter clips.

In the example shown in Fig. 1, it is pulled out from the first crosslinking bath 16. The film is introduced into the second crosslinking bath 17 through the guide rolls 38 and the nip rolls 53 in this order.

(2nd cross-linking process)

Referring to Fig. 1, the second crosslinking treatment can be carried by the film conveyance path constructed by the guide rolls 39 to 41 and the nip rolls 53, and the film after the first crosslinking treatment is applied to the second crosslinking bath. 17 (the treatment liquid contained in the crosslinking tank) is immersed for a predetermined period of time and then pulled out and carried out.

The second cross-linking process can be applied to the above description of the first cross-linking process. Therefore, in the second crosslinking treatment system, the circumferential speed difference between the nip roller (upstream side nip roller) 53 and the nip roller (downstream side nip roller) 54 can be used in the step of extending the parallel uniaxial stretching process.

The manufacturing apparatus of the present invention will be described with respect to the case where the two crosslinking baths 16 and 17 are provided, but the number of the crosslinking bath systems is not limited as long as it is one or more. The crosslinking treatment system is usually carried out 2 to 5 times. When there are three or more cross-linking baths, the composition and temperature of the third and subsequent cross-linking baths in the transport direction may be the same or different as long as they are within the above range of the description of the first cross-linking bath. The crosslinking treatment for water resistance by crosslinking and the crosslinking treatment for color adjustment can also be carried out in a plurality of steps.

(washing treatment)

The production method of the present invention can include a washing treatment step after the crosslinking treatment step. The washing treatment is carried out for the purpose of removing a drug such as excess boric acid or iodine adhering to the polyvinyl alcohol resin film. Washing treatment can for example It is sufficient to immerse the crosslinked polyvinyl alcohol resin film in the washing bath (water) 18, spray the water so that the water is sprayed, or use it in combination.

Fig. 1 shows an example in which a polyvinyl alcohol-based resin film is immersed in a washing bath 18 and washed. The temperature of the washing bath 18 in the washing treatment is usually about 2 to 40 ° C, and the immersion time of the film is usually about 2 to 120 seconds.

Further, in the washing treatment, for the purpose of removing wrinkles and transporting the polyvinyl alcohol-based resin film, rolls having an expanded width function such as a spread roll, a spiral roll, and a convex roll may be used for the guide rolls 42, 43 and/or 44. Or use other extended width devices such as cross guides, bending rolls, and tenter clips.

The washing treatment may be carried out by a step of extending the circumferential speed difference between the nip rolls (upstream side nip rolls) 54 and the nip rolls (downstream side nip rolls) 55 in parallel uniaxial stretching treatment. In order to suppress wrinkles, the stretching step is also effective.

(Extension processing)

The manufacturing method of the present invention has an extending step in which the film is uniaxially stretched in parallel with the above-described series of processing steps. The specific method of the uniaxial stretching process is such that the uniaxially extending inter-roller extension is performed by the circumferential speed difference between the upstream side nip roll and the downstream side nip roll as described above. Further, in addition to the step of extending the extension between the rolls, it may be carried out by a combination of heat roll extension, tenter stretching, and the like as described in Japanese Patent No. 2731813. The stretching treatment can be performed plural times from the time when the green film 10 is obtained until the polarizing film 23 is obtained. As described above, the extension treatment also has a wrinkle for suppressing the film. Lee.

The final cumulative stretching ratio of the polarizing film 23 is not limited, and is usually about 4.5 to 7 times, preferably 5 to 6.5 times, based on the green film 10.

As a result of intensive studies by the inventors of the present invention, it has been found that the film has a tendency to cause cracking in an extending step in which the film has a cumulative stretching ratio of 4.5 or more and an extending step in which the film is stretched after crosslinking. Further, the step of extending the film due to the elongation between the rolls is likely to be 0.55 (%/sec) or less, preferably 0.50 (%/sec) or less, more preferably 0.45 (%/). The low width reduction rate extension step of seconds) can suppress cracking. These findings, together with further investigation, are preferred in the production method of the present invention as having a low width reduction rate extending step of at least one of the following steps: [a] when the polyvinyl alcohol-based resin film is passed through the downstream side nip roll The step of extending the stretching ratio of 4.5 times or more and the step of immersing the mixture in the cross-linking bath and uniaxially stretching the film [b] can further enhance the film rupture suppressing effect. Moreover, it is also possible to have two steps at the same time, or the two steps may be the same extension step.

About [a]

When there are a plurality of extension steps in which the cumulative stretching ratio by the downstream side nip rolls is 4.5 times or more, at least one of such stretching steps is a low width reduction rate extending step. Further, even if the cumulative stretching ratio of the downstream side nip rolls is 4.5 times or more and the width reduction rate R is 0.55 (%/second) or less, the low width reduction rate has only one extension step, and the film rupture effect is suppressed. However, it is preferable that all of the extending steps of the cumulative stretching ratio of 4.5 times or more when passing through the downstream side nip rolls are low width reduction rate extending steps. Thereby, the suppression effect of film breakage can be further improved.

The stretching ratio in the stretching step in which the cumulative stretching ratio is 4.5 times or more when passing through the downstream side nip rolls is preferably 1.25 times or less, more preferably 1.21 times or less. By such a stretching ratio, film breakage can be further suppressed.

About [b]

When there are a plurality of extension steps in which the polyvinyl alcohol-based resin film is immersed in the crosslinking bath, at least one of such extension steps is a low-width reduction rate extending step. Further, in the case where a plurality of crosslinking baths are provided, the step of stretching the second and subsequent crosslinking baths (after the second crosslinking bath) is preferably a low width reduction rate extending step. Compared with the first cross-linking bath, the second and subsequent cross-linking bath systems are in a state in which the film cross-linking is further performed, and therefore, the step of stretching along the second and subsequent cross-linking baths is low. The width reduction rate extension step can further enhance the suppression effect of the film breakage.

About [a] and [b]

It is preferable that the cumulative stretching ratio of the polyvinyl alcohol-based resin film at the time of the upstream side nip roll is preferably the first stretching step (also referred to as "the first crosslinking-extension step") in the direction in which the crosslinking bath is carried. It is 2.5 times or more, more preferably 2.7 times or more. When the total stretching ratio is high, the stretching ratio in the crosslinking bath must be increased, and when the stretching ratio of the crosslinking bath is high, The film is prone to cracking. In addition, the step of extending the second stage of the conveyance direction of the immersion cross-linking bath (also referred to as "the second cross-linking extension step"), the cumulative stretching ratio of the polyvinyl alcohol-based resin film when passing the upstream side nip roll Preferably, it is 4.0 times or more, more preferably 4.2 times or more. Since the film is easily broken at the time of stretching as the crosslinking progresses, when the second crosslinking step is introduced, the cumulative stretching ratio is 4.0 times or more, and even when the total stretching ratio is high, the film can be prevented from being cracked.

The width reduction rate R in the extending step is, for example, the stretching ratio from the upstream side nip roller to the downstream side nip roller in the stretching step, the temperature of the treatment bath, and the composition of the treatment bath (for example, in a crosslinking bath) The boric acid concentration) or the cumulative stretching ratio, temperature, and the like up to the stretching step are adjusted.

(drying treatment)

After the washing treatment, it is preferred to carry out a treatment for drying the polyvinyl alcohol resin film. The drying of the film is not particularly limited, and can be carried out using a drying oven 21 as in the example shown in Fig. 1 . The drying temperature is, for example, about 30 to 100 ° C, and the drying time is, for example, about 30 to 600 seconds. The thickness of the polarizing film 23 obtained in the above manner is, for example, about 5 to 30 μm.

(other processing steps)

It is also possible to add processing other than the above processing. Examples of the treatment that can be added include an immersion treatment (complementing treatment) which does not contain boric acid in an aqueous solution of iodide, which is carried out after the crosslinking treatment, and an immersion treatment (zinc treatment) which does not contain boric acid and contains an aqueous solution such as zinc chloride.

<Polarizing plate>

A polarizing plate can be obtained by bonding a protective film to at least one surface of the polarizing film manufactured as described above through an adhesive. The protective film may, for example, be a film made of an ethyl ketone cellulose resin such as triethyl fluorenyl cellulose and diethyl fluorenyl cellulose; for example, polyethylene terephthalate or polynaphthalene a film made of a polyester resin of ethylene formate and polybutylene terephthalate; a polycarbonate resin film, a cycloolefin resin film; an acrylic resin film; and a chain olefin derived from a polypropylene resin. A film made of a resin.

In order to improve the adhesion between the polarizing film and the protective film, corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer layer coating treatment, saponification treatment, etc. may be performed on the bonding surface of the polarizing film and/or the protective film. Surface treatment. The adhesive used for bonding the polarizing film and the protective film may, for example, be an active energy ray-curable adhesive such as an ultraviolet curable adhesive, an aqueous solution such as a polyvinyl alcohol-based resin, or a crosslinking agent may be blended therein. An aqueous solution of the agent, such as an aqueous binder of an urethane-based emulsion adhesive. The ultraviolet curable adhesive can be a mixture of an acrylic compound and a photoradical polymerization initiator, a mixture of an epoxy compound and a photocationic polymerization initiator, and the like. Further, a cationically polymerizable epoxy compound and a radically polymerizable acrylic compound may be used in combination, and a photocationic polymerization initiator and a photoradical polymerization initiator may be used in combination as a starting agent.

[Examples]

Hereinafter, the present invention will be more specifically described by way of examples, but the present invention is not limited by the examples. Also, in the following examples, The polarizing film was produced by the same apparatus as the polarizing film manufacturing apparatus shown in Fig. 1 . The guide rolls 30 to 44 all use a flat roll.

<Example 1>

(1) swelling treatment

An unstretched polyvinyl alcohol film (KURARAY POVALF1LM VF-PE #3000, manufactured by KURARAY Co., Ltd., degree of polymerization 2400, saponification degree: 99.9 mol% or more) having a thickness of 30 μm and a width of 450 mm was continuously continuously obtained from the green film. The roll 11 was taken out and transported, and the swell bath 13 containing pure water at 30 ° C was immersed for 100 seconds while the film was not slack and kept under tension.

(2) Dyeing treatment

Next, the film passed through the nip roller 51 was immersed in a dye bath 15 of 30 ° C of iodine/potassium iodide/water (weight ratio) of 0.05/2/100 for 120 seconds. In parallel with the dyeing process, a circumferential speed difference is applied between the nip rolls 51 and 52, and longitudinal uniaxial stretching between the rolls is performed (designed as "stretching step during dyeing").

(3) First crosslinking treatment

Next, in order to perform the first crosslinking treatment for the purpose of hydration resistance, the film passed through the nip roller 52 was immersed in a first crosslinking bath having a potassium iodide/boric acid/water (weight ratio) of 12/4.4/100 at 56 ° C. 16. Here, in parallel with the first crosslinking treatment, a longitudinal uniaxial extension between the rolls is carried out between the nip rolls 52 and 53 (the first cross-linking extension step), and the first cross-linking step is extended. At the beginning, that is, when the upstream stretching roller 52 passes through the cumulative stretching magnification (which can be regarded as the same as the stretching magnification at the end of the dyeing stretching step) and the film width (W1), and the first crosslinking step is completed, that is, The cumulative stretching ratio and film width (W2) at the downstream side nip roller 53 are shown in Table 1. Moreover, the time (T) required for the extension step at the time of the first crosslinking is shown in Table 1.

(4) Second crosslinking treatment

Next, the film after the first crosslinking treatment was immersed in a second crosslinking bath 17 having a potassium iodide/boric acid/water (weight ratio) of 40/4.4/100 at 40 °C. Here, in parallel with the second crosslinking treatment, a circumferential speed difference is applied between the nip rolls 53 and 54 to perform longitudinal uniaxial stretching between the rolls (the second cross-linking extension step). When the second cross-linking stretching step is started, that is, when the cumulative stretching ratio and the film width (W1) when passing through the upstream side nip roller 53, and the second cross-linking stretching step, that is, when passing through the downstream side nip roller 54, The cumulative stretching ratio and film width (W2) are shown in Table 1. Further, the time (T) required for the second crosslinking extension step is shown in Table 1.

(5) washing step, drying step

Subsequently, the film after completion of the second crosslinking treatment and the third stretching step was immersed in a washing bath 18 containing pure water at 6 ° C, and secondly, it was passed through a drying oven 21 and dried at 70 ° C for 3 minutes. A polarizing film 23 is produced.

(6) Calculation of width reduction rate

The step of extending at the time of the first crosslinking, the film which passes through the upstream side nip roller 52 The time required for the width to be W1 (mm), the film width when passing through the downstream side nip roller 53 to be W2 (mm), and the conveyance from the upstream side nip roller 52 to the downstream side nip roller 53 is set to T (second). The width reduction rate R calculated by the above formula (1) is calculated. The calculation results of the width reduction rate R (%/sec) are shown in Table 1.

Similarly, in the second cross-linking step, the film width when passing through the upstream side nip roller 53 is set to W1 (mm), and the film width when passing through the downstream side nip roller 54 is set to W2 (mm), from the upstream side. The time required for the nip roller 53 to be conveyed to the downstream side nip roller 54 is set to T (second), and the width reduction rate R calculated by the above formula (1) is calculated. The calculation results of the width reduction rate R (%/sec) are shown in Table 1.

(7) Rupture evaluation

The continuous production of the polarizing film was carried out, and it was confirmed that the continuous execution time of the film rupture or the continuous execution time of the film rupture did not occur. Further, when the film was broken, the cracking frequency of the film per hour was calculated from the continuous execution time until the film was broken. With respect to Example 1, the case where the polarizing film was continuously produced was carried out for 24 hours, and no film cracking occurred in any of the treatment steps during the operation for 24 hours. The results are shown in Table 1.

<Examples 2 to 5>

With respect to Examples 2 to 5, the production conditions different from those in Example 1 are shown in Table 1, and other production conditions were carried out in the same manner as in Example 1 to produce a polarizing film. Calculating the first cross-linking delay for the obtained polarizing film The width reduction rate of the stretching step in the stretching step and the second crosslinking step and the evaluation of the cracking were performed. The results are shown in Table 1.

As shown in Table 1, in the case of carrying out the continuous production of the polarizing film for 24 hours in Examples 2 and 3, no film cracking occurred in any of the processing steps in the 24 hours of operation. With respect to Example 4, the case where the polarizing film was continuously produced for 72 hours was carried out, and in the operation of 72 hours, no film breakage occurred in any of the treatment steps. With respect to Example 5, film breakage occurred at the time of performing the continuous production of the polarizing film for 60 hours.

<Comparative Examples 1 to 3>

With respect to Comparative Examples 1 to 3, the production conditions different from those in Example 1 are shown in Table 1, and other production conditions were carried out in the same manner as in Example 1 to produce a polarizing film. With respect to the obtained polarizing film, the width reduction rate of the first crosslinking extension step and the second crosslinking extension step and the crack evaluation were calculated. The results are shown in Table 1.

As shown in Table 1, for Comparative Example 1, film breakage occurred at the time of carrying out continuous production of the polarizing film for 3.5 hours. With respect to Comparative Example 2, film breakage occurred at the time when the continuous production of the polarizing film was carried out for 2.5 hours. With respect to Comparative Example 3, film breakage occurred at the time when the continuous production of the polarizing film was carried out for 4 hours.

10‧‧‧Green film made of polyvinyl alcohol resin

11‧‧‧ Blank film roll

13‧‧‧Swelling bath

15‧‧‧dye bath

16‧‧‧1st cross-linking bath

17‧‧‧2nd cross-linking bath

18‧‧‧ Washing bath

21‧‧‧ drying oven

23‧‧‧ polarizing film

30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 60, 61‧ ‧ guide rollers

50, 51, 52, 53, 54, 55, 56‧ ‧ nip rollers

Claims (7)

A method for producing a polarizing film, comprising: an extending step of immersing a polyvinyl alcohol-based resin film while being immersed in a processing bath and performing uniaxial use by a peripheral speed difference between the upstream side nip roll and the downstream side nip roll The extending step is a low-width reduction rate extending step having a width reduction ratio R of 0.55 (%/sec) or less; and the width of the polyvinyl alcohol-based resin film passing through the upstream side nip rolls is set to W1 (mm) When the width of the downstream side nip roll is set to W2 (mm), and the time required from the upstream side nip roll to the downstream side nip roll is set to T (second), the width reduction rate R is Calculated by the following formula (1): The method for producing a polarizing film according to claim 1, wherein the low-width reduction rate extending step has a cumulative stretching ratio of 4.5 times when the polyvinyl alcohol-based resin film is stretched to pass through the downstream side nip roll. The above steps. The method for producing a polarizing film according to the first aspect of the invention, wherein the low-width reduction rate extending step is performed by immersing the polyvinyl alcohol-based resin film in a cross-linking bath and performing uniaxial stretching. The steps. The method for producing a polarizing film according to claim 3, wherein the polyvinyl alcohol resin film is immersed in a plurality of crosslinking baths, and the low width reduction rate extending step has the polyethylene The alcohol-based resin film is immersed in a cross-linking bath disposed in the second and subsequent transport directions, and is uniaxially stretched. The method for producing a polarizing film according to claim 1 or 2, wherein the polyvinyl alcohol resin film is immersed in a plurality of crosslinking baths, and the stretching step is accompanied by immersion in a crosslinking bath. In the first first crosslinking-extension step, in the first crosslinking-extension step, the cumulative stretching ratio of the polyvinyl alcohol-based resin film when passing the upstream side nip roll is 2.5 times or more. The method for producing a polarizing film according to claim 1 or 2, wherein the polyvinyl alcohol resin film is immersed in a plurality of crosslinking baths, and the stretching step is accompanied by immersion in a crosslinking bath. In the second cross-linking step of the second step, in the second cross-linking step, the cumulative stretching ratio of the polyvinyl alcohol-based resin film when passing the upstream side nip roll is 4.0 times or more. The method for producing a polarizing film according to the first or second aspect of the invention, wherein the thickness of the polyvinyl alcohol-based resin film is 65 μm or less when it is not extended.