JP6784142B2 - Method for manufacturing polyvinyl alcohol-based film for manufacturing polarizing film - Google Patents

Description

The present invention is a polyvinyl alcohol-based film for producing a polarizing film (hereinafter, may be simply referred to as "polyvinyl alcohol-based film"). In particular, it has excellent dyeability, a high degree of polarization, and little color unevenness. a method of manufacturing a polarizing film producing polyvinyl alcohol-based film that can be obtained polarizing film.

Conventionally, polyvinyl alcohol-based films have been used for many purposes as films having excellent transparency, and one of the useful uses is a polarizing film. Such a polarizing film is used as a basic component of a liquid crystal display, and in recent years, its use has been expanded to devices requiring high quality and high reliability.

Under these circumstances, a polarizing film having excellent polarization performance is required as the screens of liquid crystal televisions, multifunctional mobile terminals and the like have higher brightness, higher definition, larger area and thinner screen. Specifically, it is to further improve the degree of polarization and eliminate color unevenness.

Generally, a polyvinyl alcohol-based film for producing a polarizing film is produced from an aqueous solution of a polyvinyl alcohol-based resin by a continuous casting method. Specifically, an aqueous solution of a polyvinyl alcohol-based resin is cast on a cast mold such as a cast drum or an endless belt to form a film, and the film-formed film is peeled off from the cast mold and then flowed using a nip roll or the like. It is manufactured by drying with a heat roll or a floating dryer while transporting in the direction (MD direction).

In the above-mentioned transport step, the film-formed film is pulled in the flow direction (MD direction), so that the polyvinyl alcohol-based polymer is generally easily oriented in the MD direction, and the optical axis (slow phase axis) of the film. Often faces the MD direction. Inevitably, a polyvinyl alcohol-based film having a large refractive index Ny in the flow direction (MD direction, y-axis direction) and a small refractive index Nx in the width direction (TD direction, x-axis direction) can be obtained. The birefringence ΔNxy (defined by Nx−Ny) of such a polyvinyl alcohol-based film will have a negative value.

Further, in the transport process, shrinkage stress depending on the Poisson's ratio and shrinkage stress due to dehydration are generated in the width direction (TD direction) of the film-formed film. Therefore, if the stress in the TD direction is used, polyvinyl It is also possible to orient the alcohol-based polymer in the TD direction to some extent. In this case, the optical axis is oriented between the MD direction and the TD direction, and the birefringence ΔNxy tends to approach zero.

On the other hand, a polarizing film is generally produced by swelling a polyvinyl alcohol-based film, which is the raw material thereof, with water (including warm water), dyeing the film with a dichroic dye such as iodine, and stretching the film. To. In such a swelling step, it is necessary to quickly swell the polyvinyl alcohol-based film in the thickness direction. Further, in the dyeing step, it is necessary to uniformly swell the film so that the dye penetrates smoothly into the film.
The stretching step is a step of stretching the dyed film in the flow direction (MD direction) to highly orient the dichroic dye in the film. In order to improve the polarization performance of the polarizing film, The polyvinyl alcohol-based film used as the raw material needs to have good stretchability in the flow direction (MD direction).

In the production of the polarizing film, a case where the order of the stretching step and the dyeing step is reversed from the above is also carried out. That is, in this case, the polyvinyl alcohol-based film, which is the raw material, is swollen with water (including warm water), stretched, and dyed with a dichroic dye such as iodine. In order to improve the polarization performance, the polyvinyl alcohol-based film needs to have good swellability in the thickness direction and good stretchability in the flow direction (MD direction).

Further, in recent years, in order to reduce the thickness of the polarizing film, the polyvinyl alcohol-based film for producing the polarizing film has also been thinned, and films having a thickness of 20 μm or less are sometimes found. Such a thin film has a problem of productivity such as being broken by stretching during the production of a polarizing film.

As a method for improving the swelling property, for example, a method of adding a polyhydric alcohol as a water swelling aid to a polyvinyl alcohol-based resin (see, for example, Patent Document 1) has been proposed. As a method for improving the stretchability, for example, a method for specifying the ratio between the speed of the cast drum when forming a film and the final film winding speed (see, for example, Patent Document 2), and for forming a film with a cast drum. Later, a method of suspending and drying the film (see, for example, Patent Document 3) and a method of controlling the tension in the drying step of the film-formed film (see, for example, Patent Document 4) have been proposed. Further, polyvinyl alcohol-based films having reduced in-plane retardation have been proposed (see, for example, Patent Documents 5 and 6). Further, a polyvinyl alcohol-based retardation film having an in-plane retardation of 70 to 400 nm has been proposed (see, for example, Patent Document 7).

Japanese Unexamined Patent Publication No. 2001-302867 Japanese Unexamined Patent Publication No. 2001-315141 Japanese Unexamined Patent Publication No. 2001-315142 JP-A-2002-79531 Japanese Unexamined Patent Publication No. 2006-291173 JP-A-2007-137042 Japanese Unexamined Patent Publication No. 2013-61502

However, even the method of the above patent document is insufficient to improve the swellability and stretchability during the production of the polarizing film.

In the technique disclosed in Patent Document 1, even if the swellability of the entire polyvinyl alcohol-based film can be improved, the orientation state of the polyvinyl alcohol-based polymer is not taken into consideration, and the flow direction (MD direction) during the production of the polarizing film is not taken into consideration. It is difficult to efficiently improve the stretchability to. On the contrary, the addition of the water swelling agent tends to disturb the orientation state of the polymer, making it difficult to uniformly stretch the polymer in the flow direction (MD direction).

The above-mentioned Patent Document 2 specifies the degree of stretching (pulling condition) in the MD direction when manufacturing a polyvinyl alcohol-based film, but if stretching in the TD direction is not taken into consideration, stretching during manufacturing of a polarizing film is specified. Not enough to improve sex. In general, it is difficult to stretch a polyvinyl alcohol-based film oriented in the MD direction in the MD direction during the production of a polarizing film. That is, it is difficult to further pull the polyvinyl alcohol-based polymer oriented in the MD direction in the MD direction because the molecular chain is forcibly stretched. On the contrary, it is relatively easy to pull the polyvinyl alcohol-based polymer oriented in the TD direction in the MD direction. However, if the polymer orientation in the TD direction is not uniform, it cannot be uniformly stretched in the MD direction during the production of the polarizing film. Patent Document 2 includes an example in which the polyvinyl alcohol-based film is not stretched so much in the MD direction (an example in which it is not pulled), but the shrinkage stress depending on the Poisson's ratio and the shrinkage stress due to dehydration alone are high in the TD direction. There is a problem that the molecular orientation cannot be made uniform sufficiently. That is, unless the polymer is stretched to some extent in the TD direction or at least fixed in the width direction, a uniform orientation state of the polymer in the TD direction cannot be obtained.

Although the technique disclosed in Patent Document 3 can uniformly dry the film after film formation, the orientation of the polymer cannot be controlled, which is insufficient to improve the swelling property and stretchability during the production of the polarizing film. ..
Although the technique disclosed in Patent Document 4 can make the thickness of the polyvinyl alcohol-based film uniform, it cannot control the orientation of the polymer, and is insufficient to improve the swelling property and stretchability during the production of the polarizing film. is there.
The disclosed techniques of Patent Documents 5 and 6 reduce the in-plane retardation of the polyvinyl alcohol-based film, but in terms of swellability and stretchability during production of the polarizing film when the in-plane retardation is reduced. There is room for improvement.
Since the disclosed technique of Patent Document 7 relates to a retardation film and not to a polyvinyl alcohol-based film for producing a polarizing film, the required performance is naturally different.

Therefore, in the present invention, under such a background, a polyvinyl alcohol system for producing a polarizing film, which is excellent in swelling property and stretchability at the time of producing a polarizing film, has high polarization performance, and can obtain a polarizing film with less color unevenness. film, it is an especially aims to provide a method for producing a polarizing film producing polyvinyl alcohol-based fill-time does not occur even rupture during the production of thin polarizing film.

However, as a result of diligent research by the present inventors to solve the above problems, a polyvinyl alcohol-based film obtained by a manufacturing method in which the birefringence ΔNxy on the xy plane and the birefringence ΔNxz on the xx plane are controlled within a specific range has been obtained . It has been found that the polarizing film is excellent in swelling property and stretchability at the time of manufacturing the polarizing film, and the polarizing film obtained by using such a polyvinyl alcohol-based film is a polarizing film having high polarizing performance and less color unevenness. The present invention has been completed.

That is, the present invention is an aqueous solution of po polyvinyl alcohol resin (however, excluding those containing blocked polyisocyanate) to form a film by a continuous casting method, after release from the cast mold, the flow direction (MD direction, y While transporting in the axial direction), the film is continuously dried and stretched in the width direction (TD direction), and the water content of the film before stretching in the width direction (TD direction) is 0.5 to 15% by weight, and the polyvinyl alcohol is used. A method for producing a polyvinyl alcohol-based film for obtaining a based film. In a polyvinyl alcohol-based film, the birefringence in the width direction (TD direction, x-axis direction) is Nx, and the birefringence in the flow direction (MD direction, y-axis direction). When Ny and the birefringence in the thickness direction (z-axis direction) are Nz, the birefringence ΔNxy on the xy plane and the birefringence ΔNxz on the xz plane satisfy the following equations (1) and (2). the method for producing a polarizing film producing polyvinyl alcohol-based film to it is an Abstract.
(1) ΔNxy = Nx−Ny ≧ 0
(2) ΔNxz = Nx-Nz ≧ 0.003

The method for producing a polyvinyl alcohol-based film of the present invention provides a polarizing film having excellent stretchability during production of a polarizing film, no breakage even when producing a thin polarizing film, excellent polarization performance, and less color unevenness. can do.
Conventionally, it has been common general knowledge that a polyvinyl alcohol-based film having a small birefringence is suitable for producing a polarizing film. The present invention has been obtained by breaking the common general technical knowledge, and even if the birefringence of the polyvinyl alcohol-based film is large, the orientation axis of the polyvinyl alcohol-based polymer is appropriate, and the x-axis, y-axis, and Based on the technical idea that if the refractive index of the z-axis satisfies the above relationship, the swelling property and stretchability during the production of the polarizing film are improved, and a polarizing film having an excellent degree of polarization and less color unevenness can be obtained. It was made.

The present invention will be described in detail below.
Method for producing a polyvinyl alcohol film of the present invention, port polyvinyl to form a film by alcoholic aqueous solution of the resin continuous casting method, after peeling off of the being film formation film from the casting mold, the flow direction (MD direction, y This is a method for producing a polyvinyl alcohol-based film obtained by continuously drying and stretching in the width direction (TD direction) while transporting the film in the axial direction.

The greatest feature of the present invention is that in the polyvinyl alcohol-based film produced by the above method, the refractive index in the width direction (TD direction, x-axis direction) is Nx, and the refractive index in the flow direction (MD direction, y-axis direction) is Nx. When the refractive index in the thickness direction (z-axis direction) is Nz, the birefringence ΔNxy on the xy plane and the birefringence ΔNxz on the xz plane satisfy the following equations (1) and (2).
(1) ΔNxy = Nx−Ny ≧ 0
(2) ΔNxz = Nx-Nz ≧ 0.003

The above formulas (1) and (2) can be achieved, for example, by stretching the film-formed film in the width direction (TD direction), and the stretching ratio in that case is 1.05 to 1.5. It is preferably double, particularly preferably 1.1 to 1.4 times, and even more preferably 1.2 to 1.3 times. If the draw ratio in the width direction (TD direction) is too low or too high, it is difficult to control birefringence, and the swellability and stretchability during the production of the polarizing film tend to decrease.

The birefringence ΔNxy preferably satisfies the following formula (1'), and particularly preferably the following formula (1 ″).
(1') ΔNxy = Nx-Ny ≧ 0.001
(1'') ΔNxy = Nx−Ny ≧ 0.002

When the birefringence ΔNxy is less than the lower limit value, the stretchability in the flow direction (MD direction, y-axis direction) at the time of manufacturing the polarizing film is lowered, which is not preferable. That is, when the birefringence ΔNxy is less than the lower limit value (including the negative case), the polyvinyl alcohol-based polymer is considerably oriented in the flow direction (MD direction, y-axis direction), so that the molecular chain is oriented in this direction. It is difficult to stretch and stretch, which is not preferable.

The birefringence ΔNxz preferably satisfies the following formula (2'), and particularly preferably the following formula (2 ″).
(2') ΔNxz = Nx-Nz ≧ 0.004
(2'') ΔNxz = Nx-Nz ≧ 0.005

When the birefringence ΔNxz is less than the lower limit value, the swellability during the production of the polarizing film is lowered, which is not preferable. That is, when the birefringence ΔNxz is less than the lower limit value (including the negative case), the polyvinyl alcohol-based polymer is oriented in the thickness direction (z-axis direction), and it is difficult to swell in this direction. Not preferable.

In the present invention, the birefringence ΔNxy and the birefringence ΔNxz in the width direction (TD direction) are both preferably 0.001 or less, particularly preferably 0.0007 or less, still more preferably 0. It is less than 0005. If the contact is too large, color unevenness tends to occur in the polarizing film.

As a method for controlling the birefringence ΔNxy (nm) and the birefringence ΔNxz (nm), other than the method of stretching the film peeled from the cast mold in the width direction (TD direction) as in the present invention. , A method of adjusting the drying conditions of the aqueous solution, a method of adjusting the chemical structure of the polyvinyl alcohol-based resin, and the like.

Here, the method for producing a polyvinyl alcohol-based film for producing a polarizing film of the present invention will be described in more detail in step order.

[Film material]
First, the polyvinyl alcohol resin used in the present invention and its aqueous solution will be described.
In the present invention, as the polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based film, an unmodified polyvinyl alcohol-based resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate is usually used. Used. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol% or less, preferably 5 mol% or less) of vinyl acetate and a copolymerizable component may be used. it can. Examples of the component copolymerizable with vinyl acetate include unsaturated carboxylic acids (including, for example, salts, esters, amides, and nitriles) and olefins having 2 to 30 carbon atoms (for example, ethylene, propylene, and n-butene). , Isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. Further, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl group after saponification can also be used. These can be used alone or in combination of two or more.

Further, as the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain can also be used. The polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain is, for example, (i) a method for saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) acetate. A method for saponifying and decarbonizing a copolymer of vinyl and vinyl ethylene carbonate, (iii) saponifying and decarbonating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane. It can be obtained by a method of ketalization, (iv) a method of saponifying a copolymer of vinyl acetate and glycerin monoallyl ether, or the like.

The weight average molecular weight of the polyvinyl alcohol-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 polyvinyl alcohol-based resin is used as an optical film, and if it is too large, it is stretched when a polarizing film is produced using the polyvinyl alcohol-based film. Tends to be difficult. The weight average molecular weight of the polyvinyl alcohol-based resin is the weight average molecular weight measured by the GPC-MALS method.

The average degree of saponification of the polyvinyl alcohol-based resin used in the present invention is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, still more preferably 99.5 mol% or more, and particularly preferably 99. It is 8 mol% or more. If the average degree of saponification is too small, sufficient optical performance tends not to be obtained when a polyvinyl alcohol-based film is used as a polarizing film.
Here, the average degree of saponification in the present invention is measured according to JIS K 6726.

As the polyvinyl alcohol-based resin used in the present invention, two or more kinds having different modified species, modified amounts, weight average molecular weight, average saponification degree and the like may be used in combination.

In addition to the polyvinyl alcohol-based resin, the polyvinyl alcohol-based resin aqueous solution includes commonly used plastics such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylolpropane, if necessary. It is preferable to contain an agent and at least one nonionic, anionic, and cationic surfactant from the viewpoint of film-forming property. These can be used alone or in combination of two or more.

The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, and even more preferably 20 to 50% by weight. If the resin concentration of such an aqueous solution is too low, the drying load tends to be large, so that the production capacity tends to decrease, and if it is too high, the viscosity tends to be too high and uniform dissolution tends to be difficult.

Next, the obtained polyvinyl alcohol-based resin aqueous solution is defoamed. Examples of the defoaming method include static defoaming and defoaming with a multi-screw extruder. The multi-screw extruder may be a multi-screw extruder having a vent, and a twin-screw extruder having a vent is usually used.

[Film formation process]
After the defoaming treatment, the polyvinyl alcohol-based resin aqueous solution is introduced into the T-shaped slit die in a fixed amount, discharged and cast on a rotating cast drum, and formed by a continuous casting method.

The continuous casting method in the present invention is, for example, a method of ejecting and casting an aqueous solution of a polyvinyl alcohol-based resin from a T-type slit die into a casting mold such as a rotating cast drum, an endless belt, or a resin film to form a film. Is. After the film is peeled off from the cast mold, the film is continuously dried by a heat roll while being conveyed in the flow direction (MD direction, y-axis direction), and may be heat-treated by, for example, a floating dryer.

The resin temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-shaped slit die is preferably 80 to 100 ° C, particularly preferably 85 to 98 ° C.
If the resin temperature of the polyvinyl alcohol-based resin aqueous solution is too low, the flow tends to be poor, and if it is too high, foaming tends to occur.

The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 50 to 200 Pa · s, and particularly preferably 70 to 150 Pa · s at the time of discharge.
If the viscosity of the aqueous solution is too low, the flow tends to be poor, and if it is too high, casting tends to be difficult.

The discharge rate of the polyvinyl alcohol-based resin aqueous solution discharged from the T-type slit die to the cast drum is preferably 0.2 to 5 m / min, particularly preferably 0.4 to 4 m / min, and even more preferably 0. It is 6 to 3 m / min.
If the discharge rate is too slow, the productivity tends to decrease, and if it is too fast, the flow tends to be difficult.

The diameter of such a cast drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and even more preferably 2.8 to 4 m.
If the diameter is too small, the drying section on the cast drum becomes short, so that the speed tends to be difficult to increase, and if it is too large, the transportability tends to decrease.

The width of such a cast drum is preferably 4 m or more, particularly preferably 4.5 m or more, still more preferably 5 m or more, and particularly preferably 5 to 6 m.
If the width of the cast drum is too small, productivity tends to decrease.

The rotation speed of the cast drum is preferably 3 to 50 m / min, particularly preferably 4 to 40 m / min, and even more preferably 5 to 35 m / min.
If the rotation speed is too slow, the productivity tends to decrease, and if it is too fast, the drying tends to be insufficient.

The surface temperature of such a cast drum is preferably 40 to 99 ° C, particularly preferably 60 to 95 ° C.
If the surface temperature is too low, drying tends to be poor, and if it is too high, foaming tends to occur.

[Film formed]
The water content of [before stretching of the film in the width direction (TD direction)] above manner film are films, 0.5 to 15 wt% der is, particularly preferably 1 to 13 wt%, more preferably Is 2-12% by weight. If the water content is too low or too high, it is difficult to control birefringence, and the swelling property and stretchability during the production of the polarizing film tend to decrease.

In order to adjust the water content, if the water content of the film before stretching in the width direction (TD direction) is too high, it is preferable to dry the film before stretching in the width direction (TD direction), and conversely. If the water content of the film before stretching in the width direction (TD direction) is too low, it is preferable to adjust the humidity before stretching in the width direction (TD direction). More preferably, the conditions of the drying step are adjusted so that the moisture content is within the above range.

Such drying can be performed by a known method using a heating roll, an infrared heater, or the like, but in the present invention, it is preferably performed with a plurality of heating rolls, and particularly preferably the temperature of the heating roll is 40 to 150 ° C. More preferably, it is 50 to 140 ° C. Further, in order to adjust the water content, a humidity control area may be provided before stretching in the width direction (TD direction).

[Transportation / stretching process]
Then, the film formed as described above and having a water content adjusted is continuously or intermittently stretched in the width direction (TD direction) while being conveyed in the flow direction (MD direction).

In the present invention, it is not necessary to particularly stretch the film-formed film in the flow direction (MD direction), and it is sufficient to convey the film with a tensile tension that does not bend. As a matter of course, stretching in the width direction (TD direction) causes neck-in depending on the Poisson's ratio in the flow direction (MD direction), and dehydration shrinkage also occurs in the flow direction (MD direction) during drying. Therefore, the film shrinks slightly in the flow direction (MD direction).

Rather, stretching in the flow direction (MD direction) is not preferable so that the dimension in the flow direction (MD direction) is extended. Before and after stretching in the width direction (TD direction), the dimensional change rate in the flow direction (MD direction) is preferably 0.8 to 1.0, particularly preferably 0.9 to 1.0, and even more preferably 0. It is .95 to 1.0. If the dimensional change rate is too small or too large, it is difficult to control birefringence, and the swellability and stretchability during the production of the polarizing film tend to decrease.

The preferred range of the transport speed in the flow direction (MD direction) of the formed film is 5 to 30 m / min, particularly preferably 7 to 25 m / min, and even more preferably 8 to 20 m / min. If the transport speed is too slow, the productivity tends to decrease, and if it is too fast, the color unevenness of the polarizing film tends to increase.

The method of simultaneously transporting the formed film in the flow direction (MD direction) and stretching in the width direction (TD direction) is not particularly limited, but for example, both ends in the width direction of the film are clipped with a plurality of clips. It is preferable that the film is sandwiched and conveyed and stretched at the same time. In such a case, the arrangement of the clips at each end is preferably a pitch of 200 mm or less, particularly preferably a pitch of 100 mm or less, and even more preferably a pitch of 50 mm or less.
If the pitch of the clip is too wide, the stretched film tends to be bent, and the thickness unevenness and the phase difference unevenness at both ends of the film in the width direction tend to increase. Further, the clip holding position (tip portion of the clip) is preferably 100 mm or less from both ends in the width direction of the film-formed film. If the clip holding position (tip portion) is located too far in the center portion in the width direction of the film, the end portion of the film to be discarded tends to increase and the product width tends to be narrowed.

As described above, the polyvinyl alcohol-based film specified in the present invention can be achieved by stretching the film formed in the width direction (TD direction), and the stretching ratio in that case is 1. It is preferably 05 to 1.5 times. It is particularly preferably 1.1 to 1.4 times, and even more preferably 1.2 to 1.3 times. If the draw ratio in the width direction (TD direction) is too low or too high, it is difficult to control birefringence, and the swellability and stretchability during the production of the polarizing film tend to decrease.

The continuous stretching step in the width direction (TD direction) may be one step (one time) or may be a plurality of steps (multiple times) so that the total stretching ratio is within the range of the stretching ratio (also referred to as sequential stretching). Called). For example, after the first step of stretching, a simple transfer with the width direction (TD direction) fixed may be performed, and the second and subsequent steps of stretching may be performed. Especially in the case of a thin film, the stress of the film is relaxed and breakage can be avoided by inserting such a simple transfer step. It is also possible to make the fixed width in the width direction (TD direction) narrower than the width after stretching.
When the width-fixed transfer step is inserted, the fixed width can be made narrower than the width after the first step of stretching. In addition, the film immediately after stretching tends to shrink due to stress relaxation, and shrinkage due to dehydration also occurs, so that the fixed width can be narrowed to these shrinkage widths. However, narrowing it beyond the shrinkage width is not preferable because the film bends.
Such a stretching step is preferably carried out after the film drying step, but may be carried out independently at least one of before and after the film drying step, or may be carried out during the drying step.

As a preferred embodiment of the present invention, the film is temporarily stretched by more than 1.3 times in the width direction (TD direction), and then the final stretch ratio in the width direction (TD direction) is 1.05-1. A method of shrinking the size by a factor of 5.5 may be used. In such a case, after temporarily stretching the film by more than 1.3 times, the film may be simply conveyed with a fixed width of 1.05 to 1.5 times. By such a technique, the stress of the film is relaxed, and it becomes possible to avoid breakage especially in the case of a thin film.

In the present invention, stretching in the width direction (TD direction) of the formed film is preferably performed at 50 to 150 ° C., particularly preferably 80 to 145 ° C., and even more preferably 100 to 140 ° C. If the stretching temperature is too low or too high, it is difficult to control birefringence, and the swellability and stretchability during the production of the polarizing film tend to decrease. When performing sequential stretching or intermittent stretching, the stretching temperature may be changed at each stretching step, or a temperature gradient may be provided during stretching.

In the present invention, the stretching time during stretching in the width direction (TD direction) of the film-formed film is preferably 2 to 60 seconds, particularly preferably 5 to 45 seconds, and even more preferably 10 to 30 seconds. If the stretching time is too short, the film tends to break, and conversely, if it is too long, the equipment load tends to increase. When performing sequential stretching, the stretching time may be changed at each stretching step.

In the present invention, the film may be stretched in the width direction (TD direction) and then heat-treated with a floating dryer or the like. The temperature of such heat treatment is preferably 60 to 200 ° C, particularly preferably 70 to 150 ° C, and even more preferably 100 to 140 ° C.
If the heat treatment temperature is too low, the dimensional stability tends to decrease, and conversely, if it is too high, the stretchability during the production of the polarizing film tends to decrease.
The heat treatment time is preferably 1 to 60 seconds, particularly preferably 5 to 30 seconds. If the heat treatment time is too short, the dimensional stability tends to decrease, and conversely, if the heat treatment time is too long, the swellability and stretchability during the production of the polarizing film tend to decrease.

[Polyvinyl alcohol-based film for manufacturing polarizing films]
Thus, the polyvinyl alcohol-based film for producing a polarizing film of the present invention is obtained, and is finally wound up on a roll to obtain a product. The thickness of the polyvinyl alcohol-based film is 5 to 60 μm in terms of birefringence, particularly preferably 5 to 45 μm in terms of thinning the polarizing film, more preferably 5 to 30 μm, and particularly preferably avoiding breakage. The point is 10 to 20 μm. The thickness of the polyvinyl alcohol-based film is adjusted by the resin concentration in the polyvinyl alcohol-based resin aqueous solution, the discharge amount to the cast mold (discharge rate), the draw ratio, and the like.

The width of the polyvinyl alcohol-based film is 2 m or more, particularly preferably 3 m or more from the viewpoint of increasing the area, and further preferably 4 to 6 m from the viewpoint of avoiding breakage.

The length of the polyvinyl alcohol-based film is 2 km or more, particularly preferably 3 km or more in terms of increasing the area, and further preferably 3 to 50 km in terms of transport weight.

The polyvinyl alcohol-based film of the present invention is very useful as a raw film for a polarizing film, and a polarizing film made of the polyvinyl alcohol-based film and a method for producing a polarizing plate will be described below.

[Manufacturing method of polarizing film]
The polarizing film of the present invention is produced by unwinding the polyvinyl alcohol-based film from a roll, transferring it in the horizontal direction, and performing steps such as swelling, dyeing, boric acid cross-linking, stretching, washing, and drying.

The swelling step is performed before the dyeing step. In addition to being able to clean the dirt on the surface of the polyvinyl alcohol-based film by the swelling step, there is also an effect of preventing uneven dyeing and the like by swelling the polyvinyl alcohol-based film. In the swelling step, water is usually used as the treatment liquid. If the main component of the treatment liquid is water, it may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol and the like. The temperature of the swelling bath is usually about 10 to 45 ° C., and the immersion time in the swelling bath is usually about 0.1 to 10 minutes.

The dyeing step is performed by contacting the film with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide is used, and an iodine concentration of 0.1 to 2 g / L and a potassium iodide concentration of 1 to 100 g / L are suitable. It is practical that the dyeing time is about 30 to 500 seconds. The temperature of the treatment bath is preferably 5 to 50 ° C. The aqueous solution may contain a small amount of an organic solvent compatible with water in addition to the water solvent.

The boric acid cross-linking step is carried out using a boron compound such as boric acid or borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixed solution at a concentration of about 10 to 100 g / L, and it is preferable that potassium iodide coexists in the solution from the viewpoint of stabilizing the polarization performance. The temperature at the time of treatment is preferably about 30 to 70 ° C., the treatment time is preferably about 0.1 to 20 minutes, and if necessary, a stretching operation may be performed during the treatment.

In the stretching step, the film is preferably stretched 3 to 10 times, preferably 3.5 to 6 times in the uniaxial direction. At this time, a slight stretching (stretching to the extent of preventing shrinkage in the width direction or more) may be performed in the direction perpendicular to the stretching direction. The temperature at the time of stretching is preferably 40 to 170 ° C. Further, the stretching ratio may be finally set in the above range, and the stretching operation may be performed not only once but also a plurality of times in the manufacturing process.

The washing step is performed by immersing the polyvinyl alcohol-based film in, for example, an aqueous solution of iodide such as water or potassium iodide, and precipitates generated on the surface of the film can be removed. When an aqueous potassium iodide solution is used, the potassium iodide concentration may be about 1 to 80 g / L. The temperature during the cleaning treatment is usually 5 to 50 ° C, preferably 10 to 45 ° C. The treatment time is usually 1 to 300 seconds, preferably 10 to 240 seconds. In addition, washing with water and washing with an aqueous solution of potassium iodide may be performed in an appropriate combination.

The drying step is, for example, drying the film in the air at 40-80 ° C. for 1-10 minutes.

A polarizing film is thus obtained, and the degree of polarization of such a polarizing film is preferably 99.5% or more, and particularly preferably 99.8% or more. If the degree of polarization is too low, the contrast in the liquid crystal display tends to decrease. In general, the degree of polarization is the light transmittance (H ₁₁ ) measured at wavelength λ and the two polarized light in a state where two polarizing films are superposed so that their orientation directions are the same. It is calculated according to the following formula from the light transmittance (H ₁ ) measured at the wavelength λ in a state where the films are superposed so that the orientation directions are orthogonal to each other.
Polarization = [(H ₁₁ −H ₁ ) / (H ₁₁ + H ₁ )] ^1/2

Further, the simple substance transmittance of the polarizing film of the present invention is preferably 44% or more. If the single transmittance is too low, it tends to be impossible to achieve high brightness of the liquid crystal display.
The simple substance transmittance is a value obtained by measuring the light transmittance of a single polarizing film using a spectrophotometer.

Next, a method for producing the polarizing plate of the present invention using the polarizing film of the present invention will be described.
The polarizing film of the present invention is suitable for producing a polarizing plate having less color unevenness and excellent polarization performance.

[Manufacturing method of polarizing plate]
The polarizing plate of the present invention is produced by laminating an optically isotropic resin film as a protective film on one or both sides of the polarizing film of the present invention via an adhesive. Examples of the protective film include films of cellulose triacetate, cellulose diacetate, polycarbonate, polymethylmethacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methylpentene, polyphenylene oxide and the like. Or you can give a sheet.

The bonding method is a known method. For example, a liquid adhesive composition is uniformly applied to a polarizing film, a protective film, or both, and then the two are bonded and pressure-bonded to be heated. It is performed by irradiating with active energy rays.

A curable resin such as a urethane resin, an acrylic resin, or a urea resin may be applied to one or both sides of the polarizing film and cured to form a cured layer to form a polarizing plate. In this way, the cured layer can replace the protective film and can be thinned.

The polarizing film and polarizing plate using the polyvinyl alcohol-based film of the present invention are excellent in polarization performance, and are portable information terminals, personal computers, televisions, projectors, signage, electronic desk computers, electronic clocks, word processors, electronic papers, and game machines. , Video, camera, photo album, thermometer, audio, liquid crystal display device such as instruments for automobiles and machinery, sunglasses, anti-glare glasses, three-dimensional glasses, wearable display, display element (CRT, LCD, organic EL, electronic paper) Etc.), preferably used for anti-reflection layers, optical communication equipment, medical equipment, building materials, toys, etc.

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 thereof is not exceeded.
In the example, the term "part" means a weight standard.

Then, the characteristics (birefringence, dimensional change rate) of the polyvinyl alcohol film and the characteristics of the polarizing film (polarization degree, single transmittance, color unevenness) in the following Examples and Comparative Examples are measured and evaluated as follows. went.
<Measurement conditions>
[Birrefringence ΔNxy (nm), Birefringence ΔNxz (nm)]
A test piece having a length of 4 cm and a width of 4 cm was cut out from the central portion in the width direction and the left and right end portions (10 cm inside from the film edge) of the obtained polyvinyl alcohol-based film, and a retardation measuring device (“KOBRA-WR”” was used. Birefringence ΔNxy (nm) and birefringence ΔNxz (nm) were measured using (manufactured by Oji Measuring Instruments Co., Ltd.).

[Dimensional change rate]
Before stretching in the width direction (TD direction), a mark with a distance of 1 m was made in the flow direction (MD direction) with a marker (2 points). After stretching in the width direction (TD direction), the distance L (m) between the two points was measured with a caliper, and the dimensional change rate was calculated according to the following formula.
Dimensional change rate = L (m) / 1 (m)

[Polarization (%), Elementary transmittance (%)]
A test piece 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 the degree of polarization (%) and single transmittance were transmitted using an automatic polarizing film measuring device (manufactured by Nippon Kogaku Co., Ltd .: VAP7070). The rate (%) was measured.

〔Color unevenness〕
A test piece having a length of 30 cm and a width of 30 cm was cut out from the obtained polarizing film, and 45 ° was formed between two polarizing plates in a cross Nicol state (single transmittance 43.5%, polarization degree 99.9%). After sandwiching at an angle, optical color unevenness was observed in the transmission mode using a light box with a surface illuminance of 14,000 lp, and evaluated according to the following criteria.
(Evaluation criteria)
○ ・ ・ ・ No color unevenness △ ・ ・ ・ Slight color unevenness × ・ ・ ・ Color unevenness

<Example 1>
(Preparation of polyvinyl alcohol film)
In a 5,000 L dissolution can, a polyvinyl alcohol resin with a weight average molecular weight of 142,000 and a saponification degree of 99.8 mol%, 2,500 kg of water, 105 kg of glycerin as a plasticizer, and polyoxy as a surfactant. 0.25 kg of ethylene laurylamine was added, and the temperature was raised to 150 ° C. with stirring to perform pressure dissolution, and the concentration was adjusted to obtain an aqueous solution of a polyvinyl alcohol-based resin having a resin concentration of 25% by weight. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin-screw extruder to defoam, then the aqueous solution temperature is set to 95 ° C., and the aqueous solution is discharged from a T-shaped slit die discharge port to a rotating cast drum (discharge speed 2. 5 m / min) and cast to form a film. The formed film was peeled off from the cast drum, and the film was dried while the front surface and the back surface of the film were alternately brought into contact with a total of 10 heat rolls. As a result, a film having a water content of 10% by weight (width 2 m, thickness 60 μm) was obtained. Next, the left and right ends of the film are sandwiched at a clip pitch of 45 mm, and while the film is conveyed in the flow direction (MD direction) at a speed of 8 m / min, 1 in the width direction (TD direction) at 120 ° C. using a stretching machine. The film was stretched 1-fold and finally heat-treated at 120 ° C. for 10 seconds to obtain a polyvinyl alcohol-based film (width 2.2 m, thickness 55 μm, length 2 km). The dimensional change rate in the flow direction (MD direction) before and after stretching in the width direction (TD direction) was 0.98. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.

(Preparation of polarizing film and polarizing plate)
The obtained polyvinyl alcohol-based film was unwound from the roll, transported in the horizontal direction, immersed in a water tank having a water temperature of 30 ° C. and swollen, and stretched 1.7 times in the flow direction (MD direction). Next, it was immersed in an aqueous solution of iodine (0.5 g / L) and potassium iodide (30 g / L) at 30 ° C. and stretched 1.6 times in the flow direction (MD direction) while dyeing, and then boric acid 40 g / L. It was immersed in an aqueous solution (50 ° C.) having a composition of L and potassium iodide of 30 g / L, and uniaxially stretched 2.1 times in the flow direction (MD direction) while boric acid cross-linking. Finally, the mixture was washed with an aqueous potassium iodide solution and dried at 50 ° C. for 2 minutes to obtain a polarizing film having a total draw ratio of 5.8 times. No breakage occurred during such production, and the characteristics of the obtained polarizing film were as shown in Table 2.
A triacetyl cellulose film having a thickness of 40 μm was attached to both sides of the polarizing film obtained above using an aqueous polyvinyl alcohol solution as an adhesive, and dried at 70 ° C. to obtain a polarizing plate.

<Example 2>
In Example 1, a polyvinyl alcohol-based film (width 2.) was formed in the same manner as in Example 1 except that the film-formed film was stretched 1.2 times in the width direction (TD direction) at 120 ° C. using a stretching machine. 4 m, thickness 50 μm, length 2 km) was obtained. The dimensional change rate in the flow direction (MD direction) before and after stretching in the width direction (TD direction) was 0.96. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. No breakage occurred during such production, and the characteristics of the obtained polarizing film were as shown in Table 2.

<Example 3>
In Example 1, the film-formed film was stretched 1.4 times in the width direction (TD direction) at 120 ° C. using a stretching machine, and then stress relaxation was performed to a fixed width of 2.4 m (equivalent to 1.2 times stretching). A polyvinyl alcohol-based film (width 2.4 m, thickness 50 μm, length 2 km) was obtained in the same manner as in Example 1 except for shrinking with. The dimensional change rate in the flow direction (MD direction) before and after stretching in the width direction (TD direction) was 0.96. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. No breakage occurred during such production, and the characteristics of the obtained polarizing film were as shown in Table 2.

<Example 4>
In Example 1, the discharge rate during film formation was 0.8 m / min, and a film having a water content of 5% by weight (width 2 m, thickness 20 μm) was stretched at 120 ° C. in the width direction (TD direction) using a stretching machine. A polyvinyl alcohol-based film (width 2.4 m, thickness 17 μm, length 2 km) was obtained in the same manner as in Example 1 except that the film was stretched 1.2 times. The dimensional change rate in the flow direction (MD direction) before and after stretching in the width direction (TD direction) was 0.98. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. Although the polyvinyl alcohol-based film of the original fabric was thin, no breakage occurred in the stretching process during the production of the polarizing film. The characteristics of the obtained polarizing film were as shown in Table 2.

<Example 5>
In Example 1, the discharge rate at the time of film formation was 0.8 m / min, and a film having a water content of 5% by weight (width 2 m, thickness 20 μm) was stretched at 120 ° C. in the width direction (TD direction) using a stretching machine. A polyvinyl alcohol-based film (width 2.6 m, thickness 15 μm,) similar to Example 1 except that it is stretched 1.4 times and then contracted to a fixed width of 2.6 m (equivalent to 1.3 times stretch) by stress relaxation. 2 km in length) was obtained. The dimensional change rate in the flow direction (MD direction) before and after stretching in the width direction (TD direction) was 0.98. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. Although the polyvinyl alcohol-based film of the original fabric was thin, no breakage occurred in the stretching process during the production of the polarizing film. The characteristics of the obtained polarizing film were as shown in Table 2.

<Comparative example 1>
A polyvinyl alcohol-based film (width 2 m, thickness 60 μm, length) in the same manner as in Example 1 except that the film-formed film was not stretched in the width direction (TD direction) using a stretching machine in Example 1. 2km) was obtained. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. The characteristics of the obtained polarizing film were as shown in Table 2.

<Comparative example 2>
In Example 1, instead of stretching the formed film 1.1 times in the width direction (TD direction) at 120 ° C. using a stretching machine, the film is fixed at 120 ° C. in the width direction (TD direction) to 2 m. Obtained a polyvinyl alcohol-based film (width 2 m, thickness 60 μm, length 2 km) in the same manner as in Example 1. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. The characteristics of the obtained polarizing film were as shown in Table 2.

<Comparative example 3>
A polyvinyl alcohol-based film (width 2 m, thickness 20 μm, length) in the same manner as in Example 4 except that the film-formed film was not stretched in the width direction (TD direction) using a stretching machine in Example 4. 2km) was obtained. The characteristics of the obtained polyvinyl alcohol-based film were as shown in Table 1.
Further, using the polyvinyl alcohol-based film, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1. The characteristics of the obtained polarizing film were as shown in Table 2.

From the results of the above Examples and Comparative Examples, Example 1 obtained from a polyvinyl alcohol-based film in which the birefringence ΔNxy on the xy plane and the birefringence ΔNxz on the xz plane satisfy the ranges specified by the above formulas (1) and (2). The polarizing films of to 5 have a high degree of polarization and no color unevenness, whereas the birefringence ΔNxy on the xy plane and the birefringence ΔNxz on the xy plane are the above equations (1) and (2). It can be seen that the polarizing films of Comparative Examples 1 to 3 obtained from the polyvinyl alcohol-based film outside the range specified in 1 are inferior in the degree of polarization and color unevenness is also observed.

The polarizing film and polarizing plate using the polyvinyl alcohol-based film of the present invention are excellent in polarization performance, and are portable information terminals, personal computers, televisions, projectors, signage, electronic desk computers, electronic clocks, word processors, electronic papers, and game machines. , Video, camera, photo album, thermometer, audio, liquid crystal display device such as instruments for automobiles and machinery, sunglasses, anti-glare glasses, three-dimensional glasses, wearable display, display element (CRT, LCD, organic EL, electronic paper) Etc.), preferably used for anti-reflection layers, optical communication equipment, medical equipment, building materials, toys, etc.

Claims (5)

An aqueous solution of a polyvinyl alcohol-based resin (excluding those containing block polyisocyanate) is formed by a continuous casting method, peeled from the cast mold, and then conveyed in the flow direction (MD direction, y-axis direction). A polyvinyl alcohol-based film obtained by continuously drying and stretching in the width direction (TD direction) and setting the water content of the film before stretching in the width direction (TD direction) to 0.5 to 15% by weight to obtain a polyvinyl alcohol-based film. In the polyvinyl alcohol-based film, the refractive index in the width direction (TD direction, x-axis direction) is Nx, the refractive index in the flow direction (MD direction, y-axis direction) is Ny, and the thickness direction (z-axis direction). When the refractive index in the direction) is Nz, the double refraction ΔNxy on the xy plane and the double refraction ΔNxz on the xz plane satisfy the following equations (1) and (2), which is a polyvinyl alcohol for producing a polarizing film. A method for manufacturing a system film.
(1) ΔNxy = Nx−Ny ≧ 0
(2) ΔNxz = Nx-Nz ≧ 0.003 The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to claim 1, wherein the film is stretched in the width direction (TD direction) at a stretching ratio of 1.05 to 1.5 times. The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to claim 1 or 2 , wherein stretching in the width direction (TD direction) is performed at 50 to 150 ° C. 3. Of claims 1 to 3 , wherein the film is temporarily stretched in the width direction (TD direction) by more than 1.3 times, and then dimensionally shrunk so that the stretch ratio becomes 1.05 to 1.5 times. The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to any one of the above. The invention according to any one of claims 1 to 4 , wherein the dimensional change rate in the flow direction (MD direction) of the film before and after stretching in the width direction (TD direction) is 0.8 to 1.0. A method for producing a polyvinyl alcohol-based film for producing a polarizing film.