CN107850718B

CN107850718B - Method for producing polarizing film, and polarizing plate

Info

Publication number: CN107850718B
Application number: CN201680041968.6A
Authority: CN
Inventors: 枝泽敏行; 早川诚一郎
Original assignee: Mitsubishi Chemical Corp
Current assignee: Mitsubishi Kasei Corp
Priority date: 2015-09-30
Filing date: 2016-06-17
Publication date: 2021-01-12
Anticipated expiration: 2036-06-17
Also published as: CN107850718A; TW201726770A; WO2017056579A1; TWI693248B; JPWO2017056579A1; KR20180063205A; JP6733542B2; KR102400122B1

Abstract

The present invention aims to solve the problems of folding and creasing in all the steps of producing a polarizing film, and to produce a polarizing film with less polarization unevenness with good productivity. A method for producing a polarizing film, comprising: 1) a step of unwinding a polyvinyl alcohol-based film from a roll and transferring the film in a horizontal direction, 2) a water swelling step, 3) a dyeing step, 4) a stretching step, and 5) a boric acid crosslinking step, wherein a liquid is sprayed onto both widthwise end portions of the film before and/or after any one of the steps 2) to 4) to prevent curling of the film.

Description

Method for producing polarizing film, and polarizing plate

Technical Field

The present invention relates to a method for producing a polarizing film used for a liquid crystal display device (hereinafter, may be abbreviated as LCD) or the like, a polarizing film, and a polarizing plate, and more particularly, to a method for producing a polarizing film obtained by dyeing a polyvinyl alcohol-based film as a raw roll with iodine, a polarizing film produced by the production method, and a polarizing plate using the polarizing film.

Background

LCDs, which use polarizing plates, are used in personal digital assistants, liquid crystal televisions, desktop computers, electronic clocks, personal computers, word processors, automobiles, mechanical measuring instruments, and the like. As the polarizing plate, a polarizing plate in which a protective film such as triacetyl cellulose is laminated on one or both surfaces of a polarizing film formed of a polyvinyl alcohol-based film in which iodine or a dichroic dye is adsorbed and oriented is generally used, and a polarizing plate having both high transmittance and high polarization degree is required in order to provide a clear LCD having high contrast.

The polarizing film described above is produced as follows: the polarizing film is produced by swelling a polyvinyl alcohol film with water (including warm water), dyeing with iodine, stretching to orient iodine molecules, crosslinking with a crosslinking agent such as boric acid to maintain the stretched state, and drying. In the above-mentioned production process, the polyvinyl alcohol-based film is unwound from a roll, and is continuously carried out while being conveyed in the horizontal direction by using a winder and a nip roll.

However, in the production steps of swelling with water, dyeing, stretching, boric acid crosslinking, and drying, the film absorbs water or dehydrates to curl, and folds and wrinkles occur at the ends in the width direction of the film, which causes a problem that the polarizing film cannot be produced with good productivity. In particular, in the case of a film, there is a problem that the film is broken and production is interrupted due to the folding and wrinkling. Further, the above-mentioned problem of the end portion also affects the inside of the polarizing film, and causes a reduction in the polarizing performance of the entire polarizing film, or a difficulty in uniform attachment of the polarizing plate to a liquid crystal cell or color unevenness and white leakage of a liquid crystal display image.

In recent years, with the increase in size of screens of liquid crystal televisions and the like, polarizing films having a wider width and a thinner thickness than conventional ones have been required, and among them, folding and creasing of end portions have been a problem to be solved.

In order to reduce the above-mentioned folding and wrinkling, a method of applying a constant tension in the width direction of the film by a drying process has been proposed (for example, see patent document 1). Further, a method of spraying water from nozzles to both ends in the width direction of a film moving in water has been proposed (for example, see patent document 2).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2006 and 189560

Patent document 2: japanese laid-open patent publication No. 7-247378

Disclosure of Invention

Problems to be solved by the invention

However, in the case of the technique disclosed in patent document 1, although folding and creasing in the drying step can be avoided, there is a problem that the film cannot be conveyed to the drying step after all if folding and creasing occur in the film in any of the steps of swelling with water, dyeing, stretching, and boric acid crosslinking before the drying step.

In addition, although the technique disclosed in patent document 2 can avoid folding and creasing in water, it cannot prevent folding and creasing in the air.

Accordingly, an object of the present invention is to solve the problems of folding and creasing in all the steps of producing a polarizing film, and to produce a polarizing film with less polarization unevenness with good productivity.

Means for solving the problems

Accordingly, the present invention provides the following aspects.

[ method for producing polarizing film ]

A method for producing a polarizing film, comprising: 1) a step of unwinding the polyvinyl alcohol film from a roll and transferring the film in a horizontal direction, 2) a water swelling step, 3) a dyeing step, 4) a stretching step, and 5) a boric acid crosslinking step,

before and/or after any one of the steps 2) to 4), liquid is sprayed to both ends of the film in the width direction, thereby preventing curling of the film.

[ polarizing film ]

A polarizing film produced by the method for producing a polarizing film of the present invention.

[ polarizing plate ]

A polarizing plate is characterized in that a protective film is provided on at least one surface of a polarizing film of the present invention.

In the present invention, the "width direction of the film" generally refers to a direction substantially perpendicular to the longitudinal direction of the strip-shaped film. In particular, in a polyvinyl alcohol film unwound from a roll and transferred in a horizontal direction, the direction is substantially perpendicular to the transfer direction of the film, and typically refers to the direction in which the distance between both edges of the film is the shortest.

In the present invention, the width direction of the film is referred to as "TD direction", and the transfer direction of the film is referred to as "MD direction".

The polyvinyl alcohol film is hereinafter also referred to simply as "film".

According to the method for producing a polarizing film of the present invention, the both ends in the width direction of the polyvinyl alcohol film are sprayed with the liquid before and/or after any one of the steps of 2) the water swelling step, 3) the dyeing step, and 4) the stretching step, thereby preventing curling of the film, and therefore, the entire surface of the film can be kept flat, and folding and wrinkling can be avoided.

ADVANTAGEOUS EFFECTS OF INVENTION

The method of the present invention can produce a wide, long, thin polarizing film with good productivity, and can obtain a polarizing film having a uniform polarization degree from the center to the end of the polarizing film. Further, a wide, long, thin polarizing plate can be easily manufactured, and color unevenness and white leakage of a liquid crystal display image can be reduced.

Drawings

Fig. 1 is a view schematically showing liquid ejection locations in 1) a reel unwinding step, 2) a water swelling step, and 3) a dyeing step.

Fig. 2 is a view schematically showing a distance a between the liquid surface of the water swelling bath and the roller a and a distance B between the roller a and the roller B between the 2) water swelling step and the 3) dyeing step.

Fig. 3A is a view showing an ejection method when a liquid is ejected onto the surface of a polyvinyl alcohol film (PVA film).

Fig. 3B is a view showing an ejection method when a liquid is ejected onto the back surface of a polyvinyl alcohol film (PVA film).

Fig. 4 is a view showing an angle θ 1 formed between the ejection direction and the surface of the polyvinyl alcohol film when the liquid is ejected onto the front surface or the back surface of the polyvinyl alcohol film.

Fig. 5 is a view showing an angle θ 2 formed by a direction of a perpendicular projection of an ejection direction on a surface of the polyvinyl alcohol film and a transfer direction of the polyvinyl alcohol film when the liquid is ejected on the surface or the back surface of the polyvinyl alcohol film.

Fig. 6 is a view showing an angle θ 1 formed by the ejection direction and the surface of the polyvinyl alcohol film when the liquid is ejected onto the front surface or the back surface of the polyvinyl alcohol film, and an angle θ 2 formed by the direction of the ejection direction perpendicularly projected onto the surface of the polyvinyl alcohol film and the transfer direction of the polyvinyl alcohol film.

Fig. 7 is a view showing an example of an ejection method when a liquid is ejected onto a polyvinyl alcohol film.

Detailed Description

The technical features of the present invention will be described in detail below, but these represent examples of preferred embodiments, and the present invention is not limited to these.

The method for producing a polarizing film of the present invention comprises 1) a step of unwinding a polyvinyl alcohol film from a roll and transferring the film in a horizontal direction, 2) a step of swelling with water, 3) a step of dyeing, 4) a step of stretching, and 5) a step of crosslinking with boric acid. The step of 1) unwinding the polyvinyl alcohol film from the roll and transferring the film in the horizontal direction is hereinafter also referred to as 1) a reel unwinding step.

The polyvinyl alcohol film used in the above-described 1) reel-out step is formed by winding a polyvinyl alcohol resin in the form of a film.

As the polyvinyl alcohol resin used for the polyvinyl alcohol film, an unmodified polyvinyl alcohol resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate is generally used. The polyvinyl alcohol film used in the present invention is not necessarily limited thereto, and a polyvinyl alcohol resin obtained by saponifying a copolymer of vinyl acetate and a small amount (for example, 10 mol% or less, preferably 5 mol% or less) of a component copolymerizable with vinyl acetate may be used. Examples of the component copolymerizable with vinyl acetate include unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutylene, etc.), vinyl ethers, and unsaturated sulfonates. Further, a modified polyvinyl alcohol resin obtained by chemically modifying a saponified hydroxyl group may be used.

Further, as the polyvinyl alcohol resin, a polyvinyl alcohol resin having a 1, 2-diol structure in a side chain may be used. The polyvinyl alcohol resin having a 1, 2-diol structure in the side chain is obtained, for example, by (i) a method of saponifying a copolymer of vinyl acetate and 3, 4-diacetoxy-1-butene, (ii) a method of saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethylene carbonate, (iii) a method of saponifying and deoxonizing a copolymer of vinyl acetate and 2, 2-dialkyl-4-vinyl-1, 3-dioxolane, and (iv) a method of saponifying a copolymer of vinyl acetate and glycerol monoallyl ether.

The weight average molecular weight of the polyvinyl alcohol resin is preferably 10 to 30 ten thousand, more preferably 11 to 28 ten thousand, and particularly preferably 12 to 26 ten thousand from the viewpoint of optical performance and stretchability.

The average saponification degree of the polyvinyl alcohol resin is usually preferably 98 mol% or more, more preferably 99 mol% or more, particularly preferably 99.5 mol% or more, and particularly preferably 99.8 mol% or more, from the viewpoint of optical performance.

The polyvinyl alcohol resin used in the present invention may be a combination of two or more kinds of polyvinyl alcohol resins different in modification type, modification amount, weight average molecular weight, average saponification degree, and the like.

The polyvinyl alcohol film used in the present invention is produced as follows: the polyvinyl alcohol film is produced by producing an aqueous solution of a polyvinyl alcohol resin using the polyvinyl alcohol resin, discharging the aqueous solution, casting the aqueous solution on a casting die such as a casting drum, a casting belt, or a casting resin film to form a film, drying the film, and winding the film on a roll.

The aqueous polyvinyl alcohol resin solution contains, in addition to the polyvinyl alcohol resin, a plasticizer generally used, such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, or trimethylolpropane, if necessary, and a nonionic, anionic, and/or cationic surfactant is preferred from the viewpoint of film-forming properties.

The resin concentration of the polyvinyl alcohol resin aqueous solution is preferably 10 to 60 wt%, more preferably 15 to 55 wt%, and particularly preferably 20 to 50 wt%.

The temperature of the aqueous polyvinyl alcohol resin solution when discharged to the casting mold is preferably 80 to 100 ℃, and particularly preferably 85 to 98 ℃. The discharge rate of the polyvinyl alcohol resin aqueous solution discharged to the casting mold is preferably 0.1 to 5 m/min, more preferably 0.2 to 4 m/min, and particularly preferably 0.3 to 3 m/min.

The surface temperature of the casting mold such as a casting drum is preferably 40 to 99 ℃, particularly preferably 60 to 95 ℃.

The drying of the film formed by the casting die is performed by conveying the film while alternately bringing the front and back surfaces of the film into contact with the outer peripheral portions of the plurality of heat rolls. After drying by means of a hot roll, the film can be heat treated. The heat treatment is preferably carried out at 60 to 150 ℃, particularly preferably 80 to 130 ℃.

Both ends of the film may be cut off by slitting before winding the film around the roll.

The thickness of the polyvinyl alcohol film is preferably 5 to 60 μm from the viewpoint of thinning of the polarizing film, more preferably 5 to 30 μm from the viewpoint of further thinning, and particularly preferably 10 to 30 μm from the viewpoint of avoiding breaking.

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

The length of the polyvinyl alcohol film is preferably 4km or more, and more preferably 4.5km or more, and particularly preferably 5km or more from the viewpoint of increasing the area.

The upper limit of the length of the film is preferably 50km or less, more preferably 40km or less, and particularly preferably 30km or less, from the viewpoint of avoiding breakage.

In the production method of the present invention, before and/or after any one of the above-described 2) water swelling step, 3) dyeing step, and 4) stretching step, the liquid is sprayed to both widthwise end portions of the conveyed film, thereby preventing curling of the film.

The liquid ejection may be performed after 5) the boric acid crosslinking step after 4) the stretching step and 6) the drying step thereafter, but in the above-mentioned steps, the film is crosslinked and dried to have high hardness, so that folding and wrinkles are not easily generated, and the effect by the ejection is not significant.

The timing for performing the liquid ejection is preferably 2) after the water swelling step and/or 3) after the dyeing step from the viewpoint of avoiding wrinkles among the above-mentioned 2) water swelling step, 3) dyeing step and 4) stretching step, and is more preferably 2) after the water swelling step and is particularly preferably within 10 seconds after the water swelling step from the viewpoint of flattening the polarizing film from the viewpoint of avoiding wrinkles after the water swelling step.

The surface of the film to be sprayed is not particularly limited, and the film can be sprayed on the front surface, the back surface, and both surfaces. In general, a polyvinyl alcohol film to be a raw roll is obtained by discharging an aqueous solution of a polyvinyl alcohol resin, casting the solution in a casting die such as a casting belt, forming a film, and continuously drying the film. In the above drying, the conditions of both surfaces are not always completely the same, and moisture on one surface side is often small or crystallinity is high. Of course, since the surface with a small amount of moisture tends to absorb water and to stretch, the film tends to curl on the opposite surface side in the 2) water swelling step and 3) dyeing step in the production of the polarizing film. In the above case, in the present invention, it is effective to eject the liquid to both end portions of the opposite surface side as the curling direction.

Examples of the liquid to be sprayed include water (including warm water), methanol, ethanol, isopropanol, acid/alkali water, a dyeing solution, an aqueous glycerol solution, and aqueous solutions of various chemicals. Among them, from the viewpoint of not adversely affecting the performance of the polarizing film, a liquid such as water or a dyeing solution is preferable, and from the viewpoint of facility simplicity, water is more preferable. When water is used, the water temperature is preferably 10 to 50 ℃, and particularly preferably 20 to 40 ℃.

The liquid to be sprayed may be a liquid in a mist state, and water in a mist state is preferable from the viewpoint of not adversely affecting the performance of the polarizing film and from the viewpoint of facility convenience. Hereinafter, the liquid in the mist state is also referred to simply as "mist".

Here, the mist is generally formed by artificially atomizing a liquid, and means fine liquid droplets. The particle size of the mist is typically in the sub-micron to hundreds of microns. The particle size of the mist used in the present invention is preferably 1 to 500. mu.m, more preferably 2 to 100. mu.m, and particularly preferably 3 to 50 μm. If the particle size of the mist is too large, ejection unevenness tends to be generated easily, and if it is too small, control of ejection pressure becomes difficult, and the mist floats and adversely affects the environment.

The mist can be sprayed by using a commercially available mist sprayer. Examples of the mist sprayer include a one-fluid type sprayer which sprays a liquid compressed by a pump through a nozzle and a two-fluid type sprayer which sprays mist formed by colliding the compressed liquid with compressed air. The flow rate of mist at the time of spraying is preferably 0.01 to 1L/min, particularly preferably 0.1 to 0.5L/min, from the viewpoint of the spraying effect.

The discharge shape of the mist discharged from the nozzle may be a dot shape, a linear shape, a circular shape, an elliptical shape, or the like, but in the present invention, a circular shape is preferable from the viewpoint of the ejection effect. The ejection angle is not particularly limited, but is preferably 1 to 90 ° from the viewpoint of ejection effect. When a liquid other than water is used, a plastic nozzle having excellent corrosion resistance is preferable.

Hereinafter, as one of preferred embodiments of the present invention, the present invention will be described by taking a case where liquid ejection is performed after the 2) water swelling step as an example.

In general, the 2) water swelling step is carried out by immersing the polyvinyl alcohol film in water under conditions of preferably 10 to 45 ℃, particularly preferably 20 to 35 ℃, preferably 0.1 to 10 minutes, particularly preferably 0.5 to 5 minutes, from the viewpoint of controlling the degree of swelling of the film. In addition, a small amount of an additive such as an iodinated compound or a surfactant, alcohol, or the like may be added to the water.

For example, as shown in fig. 1, the polyvinyl alcohol film is passed through 1) a raw roll unwinding step of unwinding the polyvinyl alcohol film wound around a roll and transferring the film in a horizontal direction, and then the polyvinyl alcohol film charged into the water swelling tank is passed through 2) a water swelling step to swell the polyvinyl alcohol film with water. Then, the polyvinyl alcohol film lifted from the swelling bath is transferred to a dyeing bath through a roll, and is subjected to dyeing in 3) a dyeing step.

In the present invention, the liquid spraying to both ends of the film is preferably performed on the film between the steps after the 2) water swelling step and before the 3) dyeing step, as described above. Folding and creasing are most likely to occur between the above steps, and the films cannot be easily peeled off once they are stuck together due to folding and creasing. In addition, when the folding or the wrinkle occurs between the above steps, the steps 2) and 3) are an initial step of manufacturing the polarizing film, and thus have a great influence on a subsequent step. The specific position of the spray is preferably, for example, immediately after the water swelling tank of the 2) water swelling step and immediately after the first roll a. The position and direction of the jet are schematically indicated by arrows in fig. 1.

In the present invention, as shown in fig. 2, the distance a from the water swelling bath in the water swelling step of 2) to the contact with the first roll a is preferably 1m or less, and particularly preferably the distance B from the first roll a to the roll B before the dyeing bath is also 1m or less. If the distance a is too long, the folding and the folding tend to increase. The distance a from the water-swelling tank to the roller a represents the shortest distance from the liquid surface of the water-swelling tank to the roller a, and the distance B from the first roller a to the roller B before the dyeing tank represents the shortest distance between the roller a and the roller B.

The tension between the rolls in the series of steps is preferably 1 to 100N/m. If the tension between the rollers is too large, the rollers tend to be easily broken, and if the tension is too small, the rollers tend to be folded or wrinkled.

Further, the transfer speed of the film from the 2) water swelling step to the 3) dyeing step is preferably 1 m/min or more, more preferably 1.5 m/min or more, and particularly preferably 2 m/min or more, from the viewpoint of productivity.

When the liquid is ejected, the liquid is preferably ejected onto one or both of the ends in the width direction of the film using a nozzle. In this case, the liquid may be discharged to the front surface of the film as shown in fig. 3A, the liquid may be discharged to the back surface of the film as shown in fig. 3B, or the liquid may be discharged to both surfaces of the film. The nozzle is not particularly limited, and known nozzles such as a dot discharge nozzle, a line discharge nozzle, and a surface discharge nozzle may be used. The nozzle may be provided with a plurality of nozzles of the same kind, or may be a combination of a plurality of kinds.

The position of the nozzle is, as shown in fig. 2, preferably a distance of 1m or less from the outlet of the water swelling tank to the nozzle from the viewpoint of avoiding wrinkles of the film, more preferably a distance of 0.5m or less from the roller a for conveyance to the nozzle from the viewpoint of avoiding wrinkles of the film, between the film after coming out of the water swelling tank in the 2) water swelling step and before coming into contact with the roller a for conveyance. The distance from the outlet of the water-swelling tank to the nozzle and the distance from the conveying roller a to the nozzle each represent the shortest distance.

The distance between the nozzle and the film surface is preferably 1 to 100mm from the viewpoint of avoiding folding of the film, and particularly preferably 2 to 50mm from the viewpoint of avoiding folding of the film. The distance between the nozzle and the film surface indicates the shortest distance, and specifically, the distance between the nozzle and the film surface in a direction perpendicular to the film surface.

In the present invention, the flow rate and the ejection angle of ejection are important from the viewpoint of the curl prevention effect.

The flow rate of the spray is preferably 0.01 to 5 m/sec, more preferably 0.02 to 3 m/sec, and particularly preferably 0.03 to 2 m/sec. If the flow rate is too low, the curl prevention effect tends to be low, and if the flow rate is too high, the film tends to be easily bent.

When the liquid ejecting direction is projected perpendicularly to the surface of the film, that is, when the film surface is viewed in a direction perpendicular to the surface of the film, the liquid ejecting direction is preferably not parallel to the width direction of the film (TD direction) or the transfer direction of the film (MD direction). Therefore, it is preferable that the angle of ejection of the liquid includes not only the angle θ 1 formed by the ejection direction of the liquid and the surface of the film but also an angle θ 2 formed by the direction of the ejection direction of the liquid perpendicularly projected onto the surface of the film and the transfer direction (MD direction) of the film (see fig. 4 to 6).

In fig. 6, the film transfer direction is defined as an x-axis, the film width direction is defined as a y-axis, the direction perpendicular to the film surface is defined as a z-axis, the film surface is defined as an xy-plane, a plane parallel to the film width direction, which is a plane perpendicular to the film surface, is defined as a yz-plane, and a plane parallel to the film transfer direction, which is a plane perpendicular to the film surface, is defined as an xz-plane.

The angle θ 1 formed by the liquid ejection direction and the surface of the film is preferably 5 to 80 °, more preferably 10 to 70 °, and particularly preferably 20 to 60 °. If the ejection angle θ 1 is too low, the curl prevention effect tends to be low, and if it is too high, the film tends to be easily bent.

The angle θ 2 formed by the direction of the liquid jet projected perpendicularly to the plane of the film and the transfer direction (MD direction) of the film is preferably 5 to 175 °, more preferably 20 to 90 °, and particularly preferably 30 to 60 °. If the spray angle θ 2 is too low, the curl prevention effect tends to be lowered, and if it is too high, the film tends to wrinkle.

In the case of using a plurality of nozzles as shown in fig. 7, the flow rate and the ejection angle of the ejection may be the same for all the nozzles or may be different for each nozzle. As a method for making each nozzle different from each other, for example, a method of setting the jet flow speed to be gradually decreased or the jet angle θ 1 to be gradually lowered from the upstream side toward the downstream side in the film transfer direction (MD direction) can be cited.

In the case of the above method, the polyvinyl alcohol film is transferred to the following 3) dyeing step and 4) stretching step without folding or creasing.

The polarizing film of the present invention is produced by 1) unwinding a raw roll, followed by 2) swelling with water, 3) dyeing, 4) stretching, 5) crosslinking with boric acid, and optionally washing and drying. The above-mentioned production steps do not necessarily have to be performed in the order of 2) to 5), and a plurality of steps may be appropriately combined. For example, the 4) stretching step may be performed simultaneously with the 2) water swelling step, the 4) stretching step may be performed simultaneously with the 3) dyeing step after the 2) water swelling step, the 4) stretching step may be performed simultaneously with the 5) boric acid crosslinking step after the 2) water swelling step and the 3) dyeing step, or the 4) stretching step may be performed in any one of the 2) water swelling step, the 3) dyeing step and the 5) boric acid crosslinking step.

The following will explain the steps after swelling with water in 2).

3) 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 the concentration of iodine is preferably 0.1 to 2g/L and the concentration of potassium iodide is preferably 1 to 100 g/L. The dyeing time is about 30-500 seconds, which is practical. The temperature of the treatment bath is preferably 5 to 50 ℃. The aqueous solution may contain a small amount of an organic solvent having compatibility with water in addition to the aqueous solvent. As the contact means, any means such as dipping, coating, spraying, etc. can be applied.

4) The stretching step is preferably a step of stretching 3 to 10 times, particularly preferably 3.5 to 6 times in the uniaxial direction. In this case, the stretching may be performed in a direction perpendicular to the stretching direction by a certain amount (to prevent the shrinkage in the width direction, or to a greater extent). The temperature during stretching is preferably selected from the range of 30 to 170 ℃. Further, the draw ratio may be finally set within the above range, and the drawing operation may be performed not only in one stage but also in any range of stages in the production process.

5) The boric acid crosslinking 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 mixture solution at a concentration of about 10 to 100g/L, and a small amount of potassium iodide is preferably coexistent in the solution from the viewpoint of stabilizing polarization performance. Preferably, the temperature for the treatment is about 30 to 70 ℃ and the treatment time is about 0.1 to 20 minutes.

Then, the film may be subjected to a washing treatment. By the washing treatment, precipitates generated on the surface of the thin film can be removed. The washing treatment is performed by, for example, immersing the polyvinyl alcohol film in an iodide aqueous solution such as water or potassium iodide. The potassium iodide aqueous solution is preferably used in which the potassium iodide concentration is about 1 to 80 g/L. The temperature during the washing treatment is usually 5 to 50 ℃, preferably 10 to 45 ℃. 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 performed in an appropriate combination. Further, the film may be subjected to a drying step.

The drying process is carried out in the atmosphere at 40-80 ℃ for 1-10 minutes.

The polarization degree of the polarizing film thus obtained is preferably 99.8% or more, more preferably 99.9% or more. If the polarization degree is too low, the contrast ratio in the liquid crystal display tends to be not secured.

The degree of polarization is usually calculated as follows: the transmittance (H) measured at a wavelength λ was obtained in a state where 2 polarizing films were stacked so that the orientation directions thereof were aligned in the same direction₁₁) And a light transmittance (H) measured at a wavelength of λ in a state where 2 polarizing films are superposed such that the orientation directions thereof are orthogonal to each other₁) The calculation is performed according to the following equation.

[(H₁₁-H₁)/(H₁₁+H₁)]^1/2

Further, the polarizing film of the present invention preferably has a monomer transmittance of 42% or more. If the transmittance of the monomer is too low, the brightness of the liquid crystal display tends to be too high.

The monomer transmittance is a value obtained by measuring the transmittance of a polarizing film monomer using a spectrophotometer.

The width of the polarizing film of the present invention is preferably 1m or more, preferably 1.3m or more from the viewpoint of increasing the area, more preferably 1.5m or more from the viewpoint of further increasing the area, and particularly preferably 1.5 to 2.5m from the viewpoint of avoiding breakage.

The thickness of the polarizing film of the present invention is preferably 15 μm or less, more preferably 10 μm or less from the viewpoint of further thinning, and particularly preferably 2 to 9 μm, and particularly preferably 3 to 8 μm from the viewpoint of avoiding breakage.

The polarizing film of the present invention thus obtained has a small polarization unevenness, and is therefore suitable for producing a high-performance polarizing plate.

The following describes a method for producing a polarizing plate from the polarizing film of the present invention.

The polarizing film of the present invention is a polarizing plate formed by bonding an optically isotropic resin film as a protective film on one or both surfaces thereof via an adhesive. Examples of the protective film include films and sheets of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methylpentene, polyphenylene ether, and the like.

The bonding method is performed by a known method, for example, by uniformly applying a liquid adhesive composition to a polarizing film, a protective film, or both, bonding the polarizing film and the protective film to each other, pressure-bonding the polarizing film and the protective film, and heating and irradiating the polarizing film and the protective film with an active energy ray.

In addition, in order to make the polarizing film thinner, a curable resin such as a urethane resin, an acrylic resin, or a urea resin may be applied to one surface or both surfaces of the polarizing film instead of the protective film, and cured to form a polarizing plate.

The polarizing film and the polarizing plate obtained by the present invention have excellent polarizing performance, and are preferably used for liquid crystal display devices such as personal digital assistants, personal computers, televisions, projectors, signs, desktop calculators, electronic watches, word processors, electronic papers, game machines, video recorders, cameras, photo albums, thermometers, audio equipment, automobiles, and mechanical measuring instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, antireflection layers for display elements (CRT, LCD, organic EL, electronic papers, and the like), optical communication instruments, medical instruments, building materials, toys, and the like.

Examples

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples as long as the invention does not exceed the gist thereof.

< measurement conditions >

The polarization degree (%) of the polarizing films obtained in examples and comparative examples was measured as follows.

The polarization degree of the central portion and both end portions (portions 10cm from each end) in the width direction of the obtained polarizing film was measured using an Otsuka Denshi Kogyo: RETS-1100A assay.

< example 1>

(production of polyvinyl alcohol film)

1000kg of a polyvinyl alcohol resin having a weight average molecular weight of 142000 and a degree of saponification of 99.8 mol%, 2500kg of water, and 100kg of glycerol as a plasticizer were added thereto, and the mixture was heated to 140 ℃ while stirring, and the concentration was adjusted to 25% by weight of the resin concentration, thereby obtaining a uniformly dissolved polyvinyl alcohol resin aqueous solution.

Then, the aqueous polyvinyl alcohol resin solution was fed to a twin-screw extruder having a vent hole to be defoamed, and then the aqueous solution was cast at a temperature of 95 ℃ from a T-slot die outlet at a discharge speed of 0.6 m/min to a casting drum having a surface temperature of 90 ℃ to form a film.

The resulting film was then dried by means of a plurality of hot rolls, heat-treated at 120 ℃ for 3 minutes using a float dryer, and cut into 4m width by slitting. Finally, the film was wound around an aluminum core tube having an outer diameter of 17cm, an inner diameter of 16cm and a length of 4.4m to obtain a film roll in which a band-shaped polyvinyl alcohol film having a water content of 2% by weight (width 4m, length 5km and thickness 15 μm) was wound around the core tube.

(production of polarizing film)

The obtained polyvinyl alcohol film was unwound from a roll and transferred in the horizontal direction [1) a reel unwinding step ], swollen with warm water immersed in a water swelling bath, and stretched in the transfer direction (1 minute at 25 ℃ C., stretching ratio 1.7 times) [2) a water swelling step ].

The film was lifted from the water swelling tank, and water (23 ℃) was sprayed from above to both ends of the film, which was conveyed in the horizontal direction at a transfer speed of 2 m/min between 2 rollers arranged horizontally (distance between rollers 1m, tension between rollers 20N/m), through a nozzle, to prevent curling. The above-mentioned water spraying conditions are as follows.

A nozzle: flexible hose made by MISUMI Corporation

Nozzle position: 1m away from the outlet of the water-soluble expansion tank

Nozzle position (height): 5mm from the upper surface of the film

Jet flow speed: 0.1 m/sec

Injection angle θ 1: 30 degree

Injection angle θ 2: 45 degree

The obtained film was immersed in an aqueous solution containing 0.9g/L of iodine and 30g/L of potassium iodide to dye the film, and simultaneously stretched in the transfer direction (0.5 minute at 28 ℃ C., stretching ratio 1.6 times) [3 ] dyeing step ].

Then, the sheet was immersed in an aqueous solution of 25g/L boric acid and 30g/L potassium iodide, and uniaxially stretched in the transfer direction while carrying out boric acid crosslinking (1 minute at 55 ℃ C., stretching ratio 2.0 times) [5 ] boric acid crosslinking step ].

Then, the film was washed with an aqueous potassium iodide solution and dried to obtain a polarizing film (width 1.8m, thickness 7 μm) having a total stretching ratio of 5.4. The obtained polarizing film was not folded or wrinkled, and was not broken for a length of 12 km. The polarization characteristics of the obtained polarizing film are shown in table 1.

(production of polarizing plate)

Both surfaces of the obtained polarizing film were bonded with a triacetyl cellulose film (TAC film) having a thickness of 40 μm using a polyvinyl alcohol aqueous solution as an adhesive, and dried at 50 ℃. The polarizing plate obtained showed no polarization unevenness.

< examples 2 to 4>

A polarizing film was obtained in the same manner as in example 1 except that the spraying conditions in example 1 were the conditions shown in table 1. The obtained polarizing film was not folded or wrinkled, and was not broken for a length of 12 km. The polarizing film obtained was evaluated in the same manner as in example 1. The evaluation results are shown in table 1. Further, a polarizing plate was obtained in the same manner as in example 1. The polarizing plate obtained showed no polarization unevenness.

< example 5>

A polarizing film was obtained in the same manner as in example 1 except that water was in a mist state in example 1. The conditions for spraying the mist are as follows.

A nozzle: spraying Systems Co. Collision jet (HITJET) nozzle

Nozzle position: 1m away from the outlet of the water-soluble expansion tank

Nozzle position (height): 5mm from the upper surface of the film

Jet flow speed: 0.1 m/sec

Injection angle θ 1: 30 degree

Injection angle θ 2: 45 degree

Mist particle size: 20 μm

Mist flow rate: 0.2L/min

The polarizing film obtained was evaluated in the same manner as in example 1. The evaluation results are shown in table 1. Further, a polarizing plate was obtained in the same manner as in example 1. The polarizing plate obtained showed no polarization unevenness.

< examples 6 to 8>

A polarizing film was obtained in the same manner as in example 5 except that the spraying conditions in example 5 were the conditions shown in table 1. The obtained polarizing film was not folded or wrinkled, and was not broken for a length of 12 km. The polarizing film obtained was evaluated in the same manner as in example 1. The evaluation results are shown in table 1. Further, a polarizing plate was obtained in the same manner as in example 1. The polarizing plate obtained showed no polarization unevenness.

< comparative example 1>

The production of a polarizing film was started in the same manner as in example 1 except that no spraying was performed in example 1. However, after the 2) water swelling step, folding and creasing were observed at both ends of the film, and as a result, a polarizing film having a length of 1km was produced, and as a result, breakage occurred in the dyeing bath. The polarization characteristics of the resulting portions are shown in table 1. Since a fracture occurred, the polarizing plate could not be manufactured.

[ Table 1]

As shown in table 1, according to the method for manufacturing a polarizing film of the present invention, a long polarizing film without folding or creasing can be obtained. Further, since the polarization degree is high not only in the central portion of the polarizing film but also in both end portions in the width direction, a polarizing film having a uniform polarization degree from the central portion to the end portions of the polarizing film can be obtained. Further, a wide, long, thin polarizing plate can be easily manufactured, and color unevenness and white leakage of a liquid crystal display image can be reduced.

Industrial applicability

The polarizing film of the present invention is used for liquid crystal display devices such as personal digital assistants, personal computers, televisions, projectors, signs, desktop calculators, electronic watches, word processors, electronic papers, game machines, video recorders, cameras, photo albums, thermometers, audio equipment, automobiles, and mechanical measuring instruments, sunglasses, antiglare glasses, stereoscopic glasses, wearable displays, antireflection layers for display elements (CRT, LCD, organic EL, electronic paper, and the like), optical communication instruments, medical instruments, building materials, toys, and the like.

Description of the reference numerals

PVA film: polyvinyl alcohol film

θ 1(°): angle formed by jetting direction and surface of film

θ 2(°): an angle formed by a direction of the jetting direction perpendicularly projected on the surface of the polyvinyl alcohol film and a transfer direction of the polyvinyl alcohol film

Distance A: distance between liquid level of water-soluble expansion tank and roller a

Distance B: distance between roller a and roller b

Claims

1. A method for producing a polarizing film, comprising: 1) a step of unwinding the polyvinyl alcohol film from a roll and transferring the film in a horizontal direction, 2) a water swelling step, 3) a dyeing step, 4) a stretching step, and 5) a boric acid crosslinking step,

ejecting a liquid in the air onto both ends in the width direction of the film before or after any one of the steps 2) to 4); or, any one or more of the steps of 2), 3), and 5) of spraying a liquid in the air onto both widthwise end portions of the film before and after any one of the steps of 2) to 4) may be performed in combination with the step of 4) to prevent curling of the film,

the angle formed by the spraying direction of the liquid and the surface of the film is 5-80 degrees.

2. The method for producing a polarizing film according to claim 1, wherein the spraying of the liquid is performed after 2) the water swelling process.

3. The method for producing a polarizing film according to claim 1 or 2, wherein an angle formed by a direction in which the liquid is jetted in a direction perpendicular to a surface of the film and a transfer direction of the film is 5 to 175 °.

4. The method for producing a polarizing film according to claim 1 or 2, wherein the spraying of the liquid is performed at a flow rate of 0.01 to 5 m/sec.

5. The method for producing a polarizing film according to claim 1 or 2, wherein the liquid is water or a dyeing liquid.

6. The method for manufacturing a polarizing film according to claim 1 or 2, wherein the liquid is a liquid in a mist state.

7. The method for manufacturing a polarizing film according to claim 6, wherein the liquid in a mist state is formed by spraying a compressed liquid from a nozzle.

8. The method for producing a polarizing film according to claim 6, wherein a particle diameter of mist in the liquid in the mist state is 1 to 500 μm.

9. The method for producing a polarizing film according to claim 6, wherein a flow rate of the liquid in the mist state is 0.01 to 1L/min.

10. The method for producing a polarizing film according to claim 1 or 2, wherein a transfer speed of the film from the 2) water swelling step to the 3) dyeing step is 1 m/min or more.

11. The method for producing a polarizing film according to claim 1 or 2, wherein the polarizing film has a thickness of 15 μm or less.

12. The method for producing a polarizing film according to claim 1 or 2, wherein the width of the polarizing film is 1m or more.

13. A polarizing film produced by the method for producing a polarizing film according to any one of claims 1 to 12, the polarizing film having a thickness of 2 to 9 μm.

14. A polarizing plate comprising the polarizing film according to claim 13 and a protective film provided on at least one surface of the polarizing film.