JP6743502B2 - Method for producing polyvinyl alcohol film - Google Patents

Description

The present invention relates to the production how the polyvinyl alcohol-based fill beam. More particularly, the present invention is a retardation in the film width direction is small, relates wide film, the thinner, manufacturing how the polyvinyl alcohol-based fill beam having excellent possibility homogeneity used until the end.

Conventionally, a polyvinyl alcohol film is prepared by dissolving a polyvinyl alcohol resin in a solvent such as water to prepare a stock solution, and then forming a film by a solution casting method (casting method) and drying it using a metal heating roll or the like. It is manufactured by The polyvinyl alcohol-based film thus obtained is used in many applications as a film excellent in transparency and dyeability, and one of its useful applications is a polarizing film. Such a polarizing film is used as a basic constituent element of a liquid crystal display, and in recent years, its use has been expanded to devices that require high quality and high reliability.

Under these circumstances, with the increase in the screen size of displays such as LCD TVs, there is a need for a polarizing film that is much wider, longer and thinner than conventional products, and that has excellent homogeneity. It has been demanded that the polyvinyl alcohol-based film as well be wide, long and thin, and have excellent homogeneity even in the end portion. If the polyvinyl alcohol film is not homogeneous to the edges, the polarization performance of the polarizing film obtained from such edges will not be uniform, and the polarizing film is likely to be deformed, reducing the durability of displays such as LCD TVs. Let
A specific demand for a polyvinyl alcohol-based film is a width of 4 m or more and a homogeneity that can be fully used up to the edges. Further, even in a thin film having a thickness of less than 50 μm and which is difficult to manufacture, there is a demand for a homogeneity that can be used up to the edges.

As a measure against such a problem, for example, in Patent Document 1, the retardation in the film surface is 15 nm or less, and the difference in retardation between the end portion and the central portion in the width direction (TD) is 5 nm or less. Alcohol-based films have been proposed. Further, for example, Patent Document 2 proposes a polyvinyl alcohol-based film having an in-plane retardation value of 30 nm or less and an uneven retardation value in the width direction (TD) of 15 nm or less.

JP, 2006-291173, A JP, 2007-137042, A

However, it is difficult for the technologies disclosed in Patent Documents 1 and 2 to cope with the recent widening and thinning. For example, the film of the example in Patent Document 1 is a thick film having a substantial width of less than 4 m and a thickness of 50 μm or 75 μm because the retardation is measured 5 cm inside from both ends. .. In addition, in the example of Patent Document 2, the width of the film is only 3 m (the measurement of the retardation is the whole width in 1 cm increments) and the thickness is 50 μm, which is a thick film. Difficult to do.

In addition, when the polyvinyl alcohol-based film has a float in the widthwise (TD) end and is not flat, it is difficult to wind it up on a roll, and particularly when the widthwise end is lifted, During the manufacturing process of the polarizing film, phenomena such as winding wrinkles and edge folds occur. Furthermore, when the film absorbs moisture during storage and transportation, there is a problem in that the film warps and waviness phenomenon increases, and the film edge must be discarded in the production of the polarizing film.

Therefore, provided this background under the present invention, a retardation in the film width direction is small, wide films, thinner, the production how the polyvinyl alcohol-based fill beam having excellent possibility homogeneity used until the end The purpose is to do.

Therefore, the present invention provides the following aspects of the invention.
[ Method for producing polyvinyl alcohol film]
A method for producing a polyvinyl alcohol-based film having a width of 4 m or more and a thickness of less than 50 μm by discharging an aqueous solution of a polyvinyl alcohol-based resin into a cast mold and sprinkling to form a film, and continuously drying the film.
A film having a water content adjusted to 1 to 20% by weight is tensioned outward in the width direction by at least one device selected from a cross guider and an expander roll, and the film width after drying is adjusted to 0. 92 to 1.0 times,
When the in-plane retardation at the center of the film in the width direction is Rc (nm) and the in-plane retardation at the end of the film in the width direction is Re (nm), the polyvinyl alcohol-based film has the following formula (1) and A method for producing a polyvinyl alcohol film , which satisfies (2).
Formula (1) Rc≦50 nm
Formula (2) 0.7≦Rc/Re≦1.0

The polyvinyl alcohol-based film in the present invention has a rectangular shape when installed on a flat surface plate, a direction in which a relatively long side extends is called a longitudinal direction, and a length of a relatively short side is a width. ".
In the present invention, the "width direction of the film" is generally a direction substantially orthogonal to the longitudinal direction of the film. In particular, in a polyvinyl alcohol-based film formed by being discharged and drowned in a cast mold, it is a direction substantially orthogonal to the flow direction of the film at the time of film formation, and typically, it is between both edges of the film. The direction when the distance is the shortest.
The "center portion in the width direction of the film" refers to a region or a point in which the distance from both edges in the width direction of the film is substantially equal.
The “width direction end portion of the film” refers to a region 1 cm center side from the edge in the width direction of the film.
The “outer side in the width direction of the film” refers to the direction from the center portion in the width direction of the film to the end portion.
In the present invention, the width direction of the film is referred to as “TD” or “TD direction”, and the flow direction of the film is referred to as “MD” or “MD direction”.
In the following, the polyvinyl alcohol-based film is also simply referred to as "film".

In the present invention, in order to obtain a large-area and uniform polarizing film, it is important to reduce the in-plane retardation at both ends in the width direction of the polyvinyl alcohol film. In addition, considering that the film is stretched in the machine direction (MD direction) in the polarizing film manufacturing process, the in-plane retardation at both ends in the width direction is set to be equal to or larger than the in-plane retardation at the center in the width direction. It is also important. That is, in the manufacturing process of the polarizing film, the center portion in the width direction of the film is stretched in the MD direction and the in-plane retardation only increases, whereas the both ends in the width direction of the film are also stretched in the TD direction. Due to the so-called neck-in phenomenon (contraction), the in-plane retardation tends not to increase as much as the central portion. Therefore, in the step of preparing the polyvinyl alcohol-based film that is the original film of the polarizing film, by setting the in-plane retardation at both ends in the width direction to be equal to or greater than the value of the in-plane retardation at the center in the width direction, The in-plane retardation can be made substantially equal between the widthwise central portion and both widthwise end portions of the film.

Since the polyvinyl alcohol-based film of the present invention is uniform even in the widthwise end portion, it is preferably used as a raw material for a wide, long, and thin polarizing film, and a polarizing film having no optical color unevenness can be obtained. it can. Furthermore, since there is no floating at both ends in the film width direction, the polarizing film can be easily manufactured, and a large-area polarizing plate can be obtained.

FIG. 1 is a diagram schematically showing the relationship between the tension direction of the cross guider and the film flow direction. FIG. 2 is a diagram schematically showing an example of the installation position of the cross guider.

Hereinafter, the constitution of the present invention will be described in detail, but these show examples of preferred embodiments, and the present invention is not limited to these contents.
The polyvinyl alcohol-based film of the present invention has a width of 4 m or more and a thickness of less than 50 μm, has an in-plane retardation in the widthwise (TD) central portion of the film of Rc (nm), and a widthwise (TD) end portion of the film. When the in-plane retardation is Re (nm), the following expressions (1) and (2) are satisfied.
Formula (1) Rc≦50 nm
Formula (2) 0.7≦Rc/Re≦1.0

The in-plane retardation can be represented by (nx-ny)×d (nm). In the formula, nx represents the maximum (slow axis direction) refractive index in the plane of the film, ny represents the refractive index in the direction orthogonal to the slow axis (fast axis direction) in the plane of the film, and d Represents the thickness of the film.

Polyvinyl alcohol film of the present invention is to a film by discharging and salivation an aqueous solution of Po polyvinyl alcohol resin cast mold can be continuously dried. More specifically, a polyvinyl alcohol-based film is formed by discharging and casting a polyvinyl alcohol-based resin aqueous solution onto a cast mold such as a cast drum (drum type roll) or cast belt, and forming a film by a casting method and drying. It can be manufactured continuously . Among the casts type, widening and lengthening, cast drum in view of film thickness uniformity are preferred.

The polyvinyl alcohol-based resin used in the present invention is usually an unmodified polyvinyl alcohol-based resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. 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 a component copolymerizable with vinyl acetate may be used. it can. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (eg, ethylene, propylene, n-butene). , Isobutene, etc.), vinyl ethers, unsaturated sulfonates and the like. Further, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl group after saponification can also be used.

Further, as the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in its side chain can be used. The polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain is, for example, (i) a method of saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) acetic acid. A method for saponifying and decarboxylating a copolymer of vinyl and vinyl ethylene carbonate, (iii) saponifying and decarboxylating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane. It can be obtained by a ketalization method, (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 resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and further 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 the weight average molecular weight is too large, it tends to be difficult to stretch the polyvinyl alcohol-based film during the production of a polarizing film. There is.
The weight average molecular weight of the polyvinyl alcohol 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, further preferably 99.5 mol% or more, particularly preferably 99. It is 8 mol% or more. When the average degree of saponification is too small, sufficient optical performance tends not to be obtained when the polyvinyl alcohol 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 amount, weight average molecular weight, average saponification degree and the like may be used in combination.

A polyvinyl alcohol-based resin aqueous solution is produced using the polyvinyl alcohol-based resin. It is preferable that the polyvinyl alcohol-based resin is washed with water and dehydrated using a centrifuge or the like to obtain a polyvinyl alcohol-based resin wet cake having a water content of 50% by weight or less. If the water content of such a wet cake is too high, it tends to be difficult to adjust it to a desired aqueous solution concentration. The polyvinyl alcohol-based resin wet cake is dissolved in warm water or hot water to prepare a polyvinyl alcohol-based resin aqueous solution.

The method for preparing the polyvinyl alcohol-based resin aqueous solution is not particularly limited, and may be prepared, for example, by using a heated multi-screw extruder. It is also possible to add the polyvinyl alcohol-based resin wet cake described above and blow water vapor into the can to dissolve it, thereby preparing an aqueous solution having a desired concentration.

In addition to the polyvinyl alcohol-based resin, the polyvinyl alcohol-based resin aqueous solution may be a commonly used plastic such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, or trimethylolpropane, if necessary. From the viewpoint of film-forming property, it is preferable to include an agent and a nonionic, anionic, and/or cationic surfactant.

The resin concentration of the aqueous polyvinyl alcohol resin solution thus obtained is preferably 10 to 60% by weight, particularly preferably 15 to 55% by weight, and further preferably 20 to 50% by weight.
If the resin concentration is too low, the drying load will be large and the production capacity will tend to be poor. If it is too high, the viscosity will be too high and uniform dissolution will be difficult to achieve.

Using the polyvinyl alcohol-based resin aqueous solution, the aqueous solution is discharged and cast onto a rotating cast drum, and the polyvinyl alcohol-based film of the present invention is continuously produced by, for example, forming a film by a casting method and drying. It can be manufactured by, for example, the following steps.
(A) A step of forming a film by a casting method.
(B) A step of heating and drying the formed film.
(C) A step of slitting both ends of the dried film and winding the film on a roll.
Hereinafter, the steps (A) to (C) will be sequentially described.

[Step (A)]
In the step (A), first, the polyvinyl alcohol-based resin aqueous solution is usually defoamed. Examples of the defoaming method include static defoaming and defoaming with a multi-screw extruder having a vent. As the multi-screw extruder having a vent, a twin-screw extruder having a vent is usually used.

After the defoaming treatment, the polyvinyl alcohol-based resin aqueous solution is introduced into the T-shaped slit die by a fixed amount, discharged and cast onto a rotating cast drum, and formed into a film by a casting method.

The temperature of the polyvinyl alcohol-based resin aqueous solution at the exit of the T-type slit die is preferably 80 to 100°C, and particularly preferably 85 to 98°C.
If the 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 aqueous solution of polyvinyl alcohol resin is preferably 50 to 200 Pa·s, and particularly preferably 70 to 150 Pa·s at the time of ejection.
If the viscosity of the aqueous solution is too low, the flow tends to be poor, and if it is too high, it is difficult to sprinkle.

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 further preferably 0. It is 6 to 3 m/min.
If the discharge speed is too low, the productivity tends to decrease, and if it is too high, the salinization 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 further preferably 2.8 to 4 m.
If the diameter is too small, the drying length tends to be insufficient and the speed tends to be difficult to obtain, and if it is too large, the transportability tends to decrease.

The width of the cast drum is preferably 4.5 m or more, particularly preferably 4.8 m or more, further preferably 5 m or more, and particularly preferably 5 to 7 m.
If the width of the cast drum is too small, the 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 further preferably 5 to 35 m/min.
If the rotation speed is too low, the productivity tends to decrease, and if it is too high, the drying tends to be insufficient.

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

In the present invention, the water content of the film when peeled from the cast drum is preferably 10 to 30% by weight, particularly preferably 15 to 25% by weight.

[Step (B)]
Next, the step (B) will be described.
The step (B) is a step of heating and drying the formed film.

The film formed by the cast drum is dried by alternately contacting the front surface and the back surface of the film with a plurality of metal heating rolls (hereinafter, referred to as "heat rolls"). The surface temperature of the heat roll is usually 40 to 150°C, preferably 50 to 140°C. When the surface temperature is too low, poor drying tends to occur, and when the surface temperature is too high, excessive drying tends to result in poor appearance such as waviness.
The hot roll is, for example, a roll having a surface of hard chrome plating or a mirror finish and a diameter of 0.2 to 2 m, and usually 2 to 30 rolls, preferably 10 to 25 rolls can be used for drying. preferable.

In the present invention, a film adjusted to have a water content of 1 to 20% by weight, preferably 5 to 10% by weight is subjected to relatively weak tension on the outer side in the width direction (TD direction) of the film and dried. It is preferable that the film width is 0.92 to 1.0 times the film formation width from the viewpoint of controlling the in-plane retardation. Particularly preferably, the film width after drying is 0.93 to 0.97 times, more preferably 0.93 to 0.95 times the film forming width. Here, for example, when the film width is 5 m, the difference of 0.01 times corresponds to the difference of 5 cm.
Incidentally, the film-forming width here means the width of the film immediately after being peeled from the cast drum, and the film width after drying is the width of the polyvinyl alcohol-based film after being dried by a hot roll, or It is the width of the polyvinyl alcohol-based film after being heat-treated as necessary.

Further, it is particularly preferable from the viewpoint of tension control to generate tension outward in the film width direction (TD direction) by at least one device selected from a cross guider, an expander roll, a banana roll, and a mirabor roll. It is more preferable to generate the tension by using at least one device selected from expander rolls because the tension can be accurately controlled. The cross guider is particularly preferable because the surface of the film is not easily damaged. Hereinafter, the cross guider, expander roll, banana roll, and mirabeau roll may be collectively referred to as a cross guider.

The cross guider in the present invention is a widening roll installed between various rolls in the film flow direction, and two rolls paired up and down are in the width direction (TD direction) of the film being conveyed. It has the function of sandwiching the edges from the top and bottom (front and back of the film). As shown in FIG. 1, such roll pairs (cross guiders) are installed at both widthwise end portions of the film, and the film end portions form a predetermined angle (θ1) with respect to the film flow direction (MD direction). By sending the film to the film, it is possible to suppress shrinkage in the film width direction due to dehydration and neck-in. Such a cross guider can spread a running film from the center to the outside without damaging the film surface by friction or slip. Further, by sandwiching both ends of the film from above and below the film, it is possible to solve the problem that wrinkles, folds, shrinkage and the like occur.

The installation position of the cross guider or the like is not particularly limited, but is preferably before and after a plurality of heat rolls, and/or before and after the heat treatment described below, and particularly preferably in terms of controlling the in-plane retardation. It is before and after a certain heat roll, and more preferably immediately after the final heat roll in terms of reducing the waviness width of both end portions in the film width direction. FIG. 2 is a diagram schematically showing an example of the installation position of the cross guider and the like.
The “front and back” before and after the heat roll and before and after the heat treatment refer to the upstream side and the downstream side in the flow direction (MD direction) based on the heat roll or the heat treatment. Immediately after the final heat roll means a position within 1 m of each of the upstream side and the downstream side with respect to the nip roll arranged downstream of the most downstream heat roll (final heat roll) among the plurality of heat rolls. ..

When the cross guider is used, the angle θ1 (°) formed by the direction of tension by the cross guider and the film flow direction (MD direction) is 5 to 35° so that the flow of the conveyed film is not hindered. It is preferable to set to. The angle θ1 (°) is particularly preferably 5 to 30°, and more preferably 5 to 25° in terms of high speed transport of the film.

The tension P(N) applied to the outside in the film width direction by a cross guider or the like is preferably 1 to 1000 N, particularly preferably 2 to 500 N, and further preferably 2 to 200 N. In particular, the tension P(N) outward in the width direction when the cross guider is used is given by the following equation when the tension due to the cross guider is T(N) (see FIG. 1).
P(N)=T(N)×sin θ1

In the present invention, it is preferable to heat-treat the film after drying with a hot roll. The heat treatment temperature is preferably 60 to 150°C, particularly preferably 70 to 140°C. If the heat treatment temperature is too low, the water resistance of the polyvinyl alcohol-based film tends to be insufficient or phase difference unevenness tends to occur, and if it is too high, the stretchability during the production of the polarizing film tends to decrease. Examples of such a heat treatment method include a method in which a floating dryer is used, a method in which the film is once cooled to room temperature and then brought into contact with a high-temperature hot roll again, and a method in which both sides of the film are irradiated with near infrared rays using an infrared lamp Among these, the method of using a floating dryer is preferable from the viewpoint of uniform heat treatment.
In the present invention, before and after such heat treatment, a relatively weak tension, for example, 10 to 1000 N, is applied to the outside in the film width direction (TD direction) by at least one device selected from a cross guider, an expander roll, a banana roll, and a mirabor roll. You can also apply.

[Step (C)]
The raw material of the polyvinyl alcohol film, which has been dried in the step (B) and further heat-treated as necessary, becomes a product through the step (C). Step (C) is a step of slitting both ends of the dried polyvinyl alcohol-based film and winding the film on a roll.
As described above, if the in-plane retardation at both ends in the film width direction is significantly different from the in-plane retardation at the widthwise central part, or if there are wrinkles, folds, or shrinkage at both ends of the film, the area cut off with a slit. Will increase.
However, according to the present invention, it is possible to reduce the deviation of the in-plane phase difference between the both end portions and the central portion, and it is possible to suppress the occurrence of wrinkles and the like at both end portions of the film, so that the area cut off by the slit is reduced. And has the advantage of high manufacturing efficiency.

In the above, a method of preparing a polyvinyl alcohol-based resin aqueous solution, casting the aqueous solution on a rotating cast drum (drum type roll), forming a film by a casting method, and drying the polyvinyl alcohol-based film will be described. However, it is also possible to cast a polyvinyl alcohol-based resin aqueous solution on a resin film or on a metal belt, form a film, and dry it to produce a polyvinyl alcohol-based film.

[Polyvinyl alcohol film]
The polyvinyl alcohol film of the present invention thus obtained has a width of 4 m or more. It is preferably 4.5 m or more from the viewpoint of increasing the area, and more preferably 4.5 to 6 m from the viewpoint of avoiding breakage.
The thickness of the polyvinyl alcohol-based film is less than 50 μm, preferably 40 μm or less. It is particularly preferably 5 to 30 μm from the viewpoint of thinning, and more preferably 10 to 25 μm from the viewpoint of avoiding breakage.
The length of the polyvinyl alcohol film is preferably 4 km or more from the viewpoint of increasing the area, particularly preferably 4.5 km or more, and more preferably 5 km or more from the viewpoint of transport weight.
The upper limit of the length of the film is preferably 50 km or less, particularly preferably 40 km or less, and further preferably 30 km or less from the viewpoint of avoiding breakage.

The polyvinyl alcohol-based film of the present invention has the following formula (1), where Rc (nm) is the in-plane retardation at the center of the film in the width direction and Re (nm) is the in-plane retardation at the ends of the film in the width direction. ) And (2) are satisfied.
Formula (1) Rc≦50 nm
Formula (2) 0.7 (particularly 0.70)≦Rc/Re≦1.0 (particularly 1.00)

Preferably, the following expressions (1′) and (2′) are satisfied in terms of increasing the area of the polarizing film.
Formula (1′) Rc≦40 nm
Formula (2′) 0.8 (especially 0.80)≦Rc/Re≦1.0 (especially 1.00)

Particularly preferably, the following expressions (1″) and (2″) are satisfied in terms of increasing the area of the polarizing plate.
Formula (1″) Rc≦30 nm
Formula (2″) 0.9 (particularly 0.90)≦Rc/Re≦1.0 (particularly 1.00)

In the polyvinyl alcohol-based film of the present invention, when the film dried after the step (B) is placed on a flat surface plate, the waviness width at the film end is within 1 cm before the slit in the step (C). It is preferable. The waviness width is particularly preferably within 0.7 cm, more preferably within 0.5 cm. If the undulation width is too large, the amount of end discard at the time of slitting tends to increase. As a method for reducing the waviness width, a method of applying a relatively weak tension in the width direction (TD direction) of the film in the drying step, a method of reducing the water content when peeling the film from the cast drum, Examples include a method of reducing the film peeling strength from the drum by addition or polishing of the cast drum, and a method of reducing the amount of aqueous solution discharged from both ends of the T-shaped slit die.

In the polyvinyl alcohol film of the present invention, when the film is placed on a flat platen, the maximum amount of floating from the platen surface at both ends in the film width direction is preferably 1 mm or less. The maximum value of the floating amount is particularly preferably 0.8 mm or less, more preferably 0.7 mm or less. If the maximum value of the floating amount is too large, it becomes difficult to wind the film and phenomena such as winding wrinkles and edge folds occur in the polarizing film manufacturing process, which tends to reduce the manufacturing yield. As a method of reducing the floating amount, a method of applying a relatively weak tension in the width direction (TD direction) of the film in the drying step, a method of reducing the moisture content when the film is peeled from the cast drum, Examples include a method of reducing the film peeling strength from the drum by addition or polishing of the cast drum, and a method of reducing the amount of aqueous solution discharged from both ends of the T-type slit die.

[Polarizing film]
The polyvinyl alcohol-based film of the present invention is excellent in homogeneity even up to the film end portion, and is suitably used as an optical polyvinyl alcohol-based film, and particularly preferably used as a raw material for a polarizing film.

Hereinafter, a method for manufacturing a polarizing film obtained using the polyvinyl alcohol film of the present invention will be described.

The polarizing film of the present invention is usually manufactured by unwinding the polyvinyl alcohol film from a roll and transferring it in the horizontal direction through steps such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying.

The swelling step is performed before the dyeing step. By the swelling step, stains on the surface of the polyvinyl alcohol-based film can be washed, and by swelling the polyvinyl alcohol-based film, uneven dyeing can be prevented. In the swelling process, water is usually used as the treatment liquid. The treatment liquid may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol or the like as long as the main component is water. 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 process is carried out by bringing the film into contact with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide is used, and it is suitable that the iodine concentration is 0.1 to 2 g/L and the potassium iodide concentration is 1 to 100 g/L. Practical 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 crosslinking step is performed 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 mixture at a concentration of about 10 to 100 g/L, and potassium iodide coexisting in the solution is preferable from the viewpoint of stabilizing the polarization performance. The temperature during the treatment is preferably about 30 to 70° C., the treatment time is preferably about 0.1 to 20 minutes, and a stretching operation may be performed during the treatment, if necessary.

In the stretching step, it is preferable to stretch in the uniaxial direction 3 to 10 times, particularly 3.5 to 6 times. At this time, some stretching (stretching to a degree that prevents shrinkage in the width direction or more) may be performed in the direction perpendicular to the stretching direction. The temperature during stretching is preferably 30 to 170°C. Furthermore, the stretching ratio may be finally set within the above range, and the stretching operation may be carried out not only in one stage but also in any stage of the manufacturing process.

The washing step is performed, for example, by immersing the polyvinyl alcohol film in water or an aqueous solution of iodide such as potassium iodide to remove the precipitate generated on the surface of the film. When using the potassium iodide aqueous solution, the potassium iodide concentration may be about 1 to 80 g/L. The temperature during the washing 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. The washing with water and the washing with an aqueous solution of potassium iodide may be appropriately combined.

The drying step may be performed in the air at 40 to 80° C. for 1 to 10 minutes.

The polarization degree of the polarizing film is preferably 99.5% or more, and particularly preferably 99.8% or more. If the degree of polarization is too low, it tends to be impossible to secure the contrast in the liquid crystal display.
The degree of polarization is generally calculated by measuring the light transmittance (H ₁₁ ) at a wavelength of λ in a state where two polarizing films are superposed so that their orientation directions are the same, and the two polarizing films. 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.
[(H ₁₁ −H ₁ )/(H ₁₁ +H ₁ )] ^1/2

Further, the single transmittance of the polarizing film of the present invention is preferably 42% 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 the polarizing film alone using a spectrophotometer.

Thus, the polarizing film of the present invention can be obtained. However, since the polarizing film of the present invention has no optical unevenness and excellent in-plane uniformity of polarization performance, it is suitable for producing a high-quality polarizing plate.
Hereinafter, a method for producing a polarizing plate from the polarizing film of the present invention will be described.

The polarizing film of the present invention is laminated on one side or both sides thereof with an optically isotropic resin film as a protective film via an adhesive to form a polarizing plate. Examples of the protective film include films of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methylpentene, polyphenylene oxide, and the like. Alternatively, a sheet may be used.

The laminating method is carried out by a known method, for example, a liquid adhesive composition is uniformly applied to a polarizing film, a protective film, or both, and then the both are laminated and pressure-bonded to each other to heat or activate energy. It is performed by irradiating a line.

Further, for the purpose of thinning the polarizing film, a urethane resin, an acrylic resin, a urea resin or other curable resin is applied to one or both surfaces of the protective film instead of the protective film, and cured to form a polarizing plate. You can also do it.

The polarizing film of the present invention has no optical unevenness and is excellent in the in-plane uniformity of the polarization performance, and is therefore a personal digital assistant, a personal computer, a television, a projector, a signage, an electronic desk calculator, an electronic watch, a word processor, an electronic paper, a game Machines, video, cameras, photo albums, thermometers, audio, liquid crystal display devices such as instruments for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic EL, electronic) It is preferably used for an antireflection layer for papers), optical communication devices, medical devices, building materials, toys and the like.

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.

The following characteristics of the polarizing films obtained in the examples and comparative examples were measured.
(1) In-plane retardation (nm)
A retardation measuring device (“KOBRA-WFD” manufactured by Oji Scientific Instruments Co., Ltd., measurement wavelength: 590 nm) was used to measure the retardation values at the center and both ends in the width direction of the polyvinyl alcohol-based film.
(2) Waviness width (cm)
The waviness width of the film end portion after the drying process was measured.
(3) Floating amount (mm)
A polyvinyl alcohol film of 1 m x 1 m was sampled and left on a flat surface plate for 1 hour in a constant temperature and humidity room at 23°C and 50% RH. The value was measured.
(4) Degree of polarization (%) and single transmittance (%)
A strip sample of 200 mm in the stretching direction×40 mm in the width direction was cut out from the center and both ends in the width direction of the obtained polarizing film, and 10 points were arranged at a pitch of 10 mm in the stretching direction using RETS-1100A manufactured by Otsuka Electronics Co., Ltd. The polarization degree and the single transmittance were measured, and the average value was taken.
(5) Optical unevenness A test piece having a length of 30 cm and a width of 30 cm was taken from the center and both ends in the width direction of the obtained polarizing film, and two polarizing plates in a crossed Nicol state (single transmittance 43.5%, After being sandwiched at an angle of 45° between the polarization degrees (99.9%), optical unevenness was visually observed in the transmission mode using a light box with a surface illuminance of 14,000 lx, and evaluated according to the following criteria.
(Evaluation criteria)
○: No optical unevenness ×: Optical unevenness

[Example 1]
(Production of polyvinyl alcohol film)
A weight average molecular weight of 142,000, a saponification degree of 99.8 mol% of polyvinyl alcohol resin 1,000 kg, water 2,500 kg, glycerin 100 kg as a plasticizer were added, and the temperature was raised to 140° C. with stirring to obtain a resin concentration. The concentration was adjusted to 30% by weight to obtain a uniformly dissolved polyvinyl alcohol-based resin aqueous solution.
Then, the polyvinyl alcohol-based resin aqueous solution was supplied to a twin-screw extruder having a vent to defoam, then the temperature of the aqueous solution was set to 95° C., and a discharge speed of 1.7 m from a T-type slit die discharge port to a cast drum. The film was formed by casting at a flow rate of 5 minutes. The film width after film formation was 5.5 m. The film having a moisture content of 15% by weight was peeled from the cast drum, and the film was dried while alternately contacting the front surface and the back surface of the film with a total of 10 hot rolls.
While the obtained film having a moisture content of 5% by weight was conveyed at a speed of 8 m/min, the film was widened in the width direction using a cross guider (ICO10U, manufactured by Yamada Denki Kogyo KK) and passed through a nip roll. An angle θ1 formed by the cross direction and the MD direction of the cross guider is 20°, and the tension in the width direction is 100N.
Then, heat treatment was performed by blowing warm air from both sides of the film. The film width after drying was 5.1 m, and the film width after drying/film-forming width was 0.93. The undulation width of the end portion after the heat treatment was 1 cm.
Finally, both ends of the film were slit to obtain a polyvinyl alcohol film having a width of 5 m, a thickness of 40 μm and a length of 5 km, which was wound in a roll shape. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1.

(Production of polarizing film)
The obtained polyvinyl alcohol-based film was unwound from a roll, conveyed horizontally using a conveyance roll, immersed in a water tank at a water temperature of 25° C. and swollen, and stretched 1.7 times in the flow direction. Next, it was stretched 1.6 times in the flow direction while being dipped and dyed in an aqueous solution of iodine at 0.5 g/L and potassium iodide at 30 g/L at 28°C. Next, it was immersed in an aqueous solution (55° C.) having a composition of boric acid 40 g/L and potassium iodide 30 g/L, and uniaxially stretched 2.1 times in the flow direction while boric acid crosslinking was performed. Then, it 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 5.7 times. Table 2 shows the polarization characteristics of the obtained polarizing film.

[Example 2]
In Example 1, a polarizing film was manufactured in the same manner as in Example 1 using a polyvinyl alcohol-based film in which the discharge speed was 0.8 m/min and the thickness was changed to 20 μm, and various characteristics were measured. The results are summarized in Table 1 and Table 2.

[Comparative Example 1]
A polarizing film was manufactured in the same manner as in Example 1 except that the widthwise expansion in the drying step was not performed, and various characteristics were measured. The results are summarized in Table 1 and Table 2.

As shown in Tables 1 and 2, the polyvinyl alcohol-based films of Examples 1 and 2 in which the value of Rc/Re is within the range defined by the present invention, the value of Rc/Re is outside the range defined by the present invention. Since the maximum values of the waviness width and the floating amount are low as compared with the polyvinyl alcohol-based film of Comparative Example 1, it can be seen that the film is a polyvinyl alcohol-based film that can be used up to the widthwise end portion and has excellent homogeneity. In addition, the polarizing film obtained from the polyvinyl alcohol-based film of Examples 1 and 2 has a uniform degree of polarization and a single transmittance as compared with the polarizing film obtained from the polyvinyl alcohol-based film of Comparative Example 1. Moreover, it can be seen that there is no optical unevenness in the widthwise central portion or both end portions, and the polarization characteristics are excellent.

The polarizing film produced from the polyvinyl alcohol film targeted by the present invention is a portable information terminal, a personal computer, a television, a projector, a signage, an electronic desk calculator, an electronic clock, a word processor, an electronic paper, a game machine, a video, a camera, Anti-reflection for photo albums, thermometers, audio devices, liquid crystal display devices for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic EL, electronic paper, etc.) It is preferably used for layers, optical communication devices, medical devices, building materials, toys and the like.

PVA film: Polyvinyl alcohol film

Claims (5)

A method for producing a polyvinyl alcohol-based film having a width of 4 m or more and a thickness of less than 50 μm by discharging an aqueous solution of a polyvinyl alcohol-based resin into a cast mold and sprinkling to form a film, and continuously drying the film.
A film having a water content adjusted to 1 to 20% by weight is tensioned outward in the width direction by at least one device selected from a cross guider and an expander roll, and the film width after drying is adjusted to 0. 92 to 1.0 times,
When the in-plane retardation at the center of the film in the width direction is Rc (nm) and the in-plane retardation at the end of the film in the width direction is Re (nm), the polyvinyl alcohol-based film has the following formula (1) and A method for producing a polyvinyl alcohol film , which satisfies (2).
Formula (1) Rc≦50 nm
Formula (2) 0.7≦Rc/Re≦1.0 The method for producing a polyvinyl alcohol-based film according to claim 1, wherein, when the film is placed on a flat platen, the maximum amount of floating from the platen surface at both ends in the film width direction is 1 mm or less. The tension in the film width direction is more occurs at least Kurosugaida chromatography method for producing a polyvinyl alcohol film of claim 1 or 2, wherein. The method for producing a polyvinyl alcohol-based film according to claim 3 , wherein the angle formed by the direction of tension by the cross guider and the flow direction of the film is 5 to 35°. The method for producing a polyvinyl alcohol-based film according to claim 3 , wherein the tension applied to the outer side in the film width direction by the cross guider is 10 to 1000 N.

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