CN109153797B

CN109153797B - Polyvinyl alcohol film, method for producing same, and polarizing film using same

Info

Publication number: CN109153797B
Application number: CN201780028573.7A
Authority: CN
Inventors: 北村秀一; 早川诚一郎
Original assignee: Mitsubishi Chemical Corp
Current assignee: Mitsubishi Chemical Corp
Priority date: 2016-05-12
Filing date: 2017-05-10
Publication date: 2023-04-11
Anticipated expiration: 2037-05-10
Also published as: KR20190008213A; WO2017195813A1; CN109153797A; TWI734782B; TW201819142A; JP6879205B2; JPWO2017195813A1; KR102365676B1

Abstract

The present invention provides: a polyvinyl alcohol film which has excellent stretchability in the production of a polarizing film, has a high degree of polarization, and can provide a polarizing film having little color unevenness, particularly a polyvinyl alcohol film which does not break in the production of a thin polarizing film, a method for producing the polyvinyl alcohol film, and a polarizing film using the polyvinyl alcohol film. The polyvinyl alcohol film of the invention is characterized in that the polyvinyl alcohol film has a length of 4km or more, and the tensile modulus X (MPa) in the longitudinal direction (MD) of the film in a state where the moisture content of the film is 9% by weight is 5-12 MPa.

Description

Polyvinyl alcohol film, method for producing same, and polarizing film using same

Technical Field

The present invention relates to a polyvinyl alcohol film, particularly a polyvinyl alcohol film having excellent stretchability, capable of providing a polarizing film having a high degree of polarization and little color unevenness, a method for producing the same, and a polarizing film using the same.

Background

In recent years, liquid crystal display devices have been developed remarkably, and are widely used in smart phones, tablet personal computers, liquid crystal televisions, projectors, vehicle-mounted panels, and the like. In the liquid crystal display device, a polarizing film is used, and a polarizing film in which iodine or a dichroic dye is adsorbed and oriented on a polyvinyl alcohol film is mainly used as the polarizing film. In recent years, with the increase in screen definition, brightness, size and thickness of a screen, a polarizing film having a higher polarization degree than conventional polarizing films, uneven color, and a wide width, a long length and a thin thickness has been required. The polarizing film in the present invention is also referred to as a polarizing film or a polarizing plate.

Generally, a polarizing film is produced as follows: a polyvinyl alcohol film as a material is unwound from a roll, swollen in water (including warm water) while being conveyed in the Machine Direction (MD), dyed with a dichroic dye such as iodine, and stretched to produce a polyvinyl alcohol film. The stretching step is as follows: the dyed film is stretched in the flow direction (MD) to highly orient the dichroic dye in the film, but in order to increase the polarization degree of the polarizing film, the polyvinyl alcohol-based film to be a blank must have good stretchability in the flow direction (MD).

In addition, as the order of producing the polarizing film, the reverse of stretching and dyeing is also performed. That is, in the case where a polyvinyl alcohol film as a material is swollen in water (including warm water) and then stretched, and is dyed with a dichroic dye such as iodine, in the above case, in order to increase the polarization degree of a polarizing film, the polyvinyl alcohol film must also have good stretchability in the flow direction (MD). For example, if the stretching tension is too high, stretching to a predetermined stretching magnification ratio is not performed, and the dichroic dye is not sufficiently oriented, so that the degree of polarization tends not to be increased. Conversely, when the stretching tension is too low, the orientation of the dichroic dye is not stable even when the stretching is performed up to a predetermined stretching magnification, and color unevenness tends to occur in the polarizing film.

On the other hand, a polyvinyl alcohol film as a preform is generally produced from an aqueous solution of a polyvinyl alcohol resin as a raw material by a continuous casting method. Specifically, the polyvinyl alcohol resin is produced by casting an aqueous solution of the polyvinyl alcohol resin on a casting die such as a casting drum or an endless belt to form a film, peeling the film from the casting die, drying the film by a hot roll while conveying the film in the flow direction (MD) by a nip roll or the like, and, if necessary, heat-treating the film by a suspension dryer (floating dryer) or the like. In the above-mentioned conveying step, the film formed into a film is stretched in the flow direction (MD), and therefore, the polyvinyl alcohol polymer is easily oriented in the flow direction (MD), and if the orientation is too large, the stretchability in the flow direction (MD) of the polyvinyl alcohol film is reduced, and finally the polarizing performance of the polarizing film is reduced.

Further, in recent years, a polyvinyl alcohol film has been thinned for thinning of a polarizing film, and conventionally, the thickness of the polyvinyl alcohol film is about 60 μm, but the thickness is about 45 μm at present, and is expected to be 30 μm in the near future. The thin polyvinyl alcohol film has a problem of productivity such as breakage due to stretching in the production of a polarizing film.

As a method for improving the stretchability of a polyvinyl alcohol-based film, for example, there have been proposed: a method in which the ratio of the speed of the casting drum to the final film take-up speed is set to a specific ratio in the production of a polyvinyl alcohol-based film (see, for example, patent document 1); a method in which a film is floated and dried after being formed on a casting drum (for example, see patent document 2); a method of controlling stretching in the drying step of a polyvinyl alcohol film (see, for example, patent document 3).

In addition, it is proposed that: a polyvinyl alcohol film having a reduced in-plane retardation (see, for example, patent document 4); a polyvinyl alcohol film produced so as to have tensile elongations in the flow direction (MD) and the width direction (TD) equal to each other (see, for example, patent document 5); and a polyvinyl alcohol film having a young's modulus (tensile modulus) reduced to 20MPa or less by irradiation with electromagnetic waves (see, for example, patent document 6).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2001-315141

Patent document 2: japanese patent laid-open No. 2001-315142

Patent document 3: japanese patent laid-open publication No. 2002-79531

Patent document 4: japanese patent laid-open publication No. 2006-291173

Patent document 5: japanese patent laid-open No. 2002-30164

Patent document 6: japanese patent laid-open publication No. 2013-97232

Disclosure of Invention

Problems to be solved by the invention

However, even when the methods of the above patent documents are used, the stretchability of the polyvinyl alcohol-based film in the production of a polarizing film is not sufficiently improved.

The technique disclosed in patent document 1 specifies the degree of elongation (stretching) in the flow direction (MD) in the production of a polyvinyl alcohol film, but if drying conditions and heat treatment conditions are not taken into consideration, the stretchability of the polyvinyl alcohol film is not sufficiently improved.

In the technique disclosed in patent document 2, although the film after film formation can be uniformly dried, the tensile modulus cannot be controlled, and the stretchability of the polyvinyl alcohol-based film in the production of a polarizing film cannot be sufficiently improved.

In the technique disclosed in patent document 3, although the film thickness can be made uniform, the tensile modulus cannot be controlled, and the stretchability of the polyvinyl alcohol-based film in the production of a polarizing film cannot be sufficiently improved.

Although the technique disclosed in patent document 4 can reduce the in-plane retardation of the film, there is room for improvement in terms of controlling the tensile modulus and improving the stretchability of the polyvinyl alcohol film when producing a polarizing film. In the production method of the disclosed technology, the film after drying (70 ℃) and heat treatment (120 ℃) is once cooled to 50 ℃ or lower and then heated again at 50 to 100 ℃ before being wound up on a roll film, but the heating step is complicated and there is room for improvement in productivity.

In the technique disclosed in patent document 5, although the tensile elongation of the polyvinyl alcohol film can be stabilized, the tensile modulus itself cannot be controlled, and the stretchability of the polyvinyl alcohol film in the production of a polarizing film cannot be sufficiently improved.

Although the tensile modulus can be reduced in the technique disclosed in patent document 6, the tensile modulus in the examples is 13.8 to 17.8MPa, and the tensile modulus in the comparative examples in which electromagnetic wave irradiation is not performed is 27MPa, and further reduction in the tensile modulus is desired.

In the examples of patent documents 1 to 6, only polyvinyl alcohol films having a thickness of 50 μm or more are described, and further improvement is desired in order to meet the recent demand for reduction in thickness.

Accordingly, the present invention provides in such context: a polyvinyl alcohol film which has excellent stretchability in the production of a polarizing film, and which can provide a polarizing film having a high degree of polarization and little color unevenness, particularly a polyvinyl alcohol film which does not break in the production of a thin polarizing film when formed into a thin film having a thickness of 25 [ mu ] m or less, a method for producing the same, and a polarizing film using the same.

Means for solving the problems

Accordingly, the present inventors have made extensive studies in view of the above circumstances, and as a result, have found that: the polyvinyl alcohol film having a length in the flow direction (MD) of at least a specific length and having a tensile modulus in the flow direction (longitudinal direction, MD) controlled in a specific range has excellent stretchability in the production of a polarizing film, and a thin polarizing film can be produced with a high yield. Moreover, it was found that: the polarizing film obtained using the polyvinyl alcohol film has a high degree of polarization and little color unevenness.

That is, the gist of the present invention 1 is a polyvinyl alcohol film having a length of 4km or more, wherein a tensile modulus X (MPa) in a longitudinal direction (MD) of the film with a moisture percentage of 9% by weight is 5 to 12MPa.

The invention also provides a method for producing a polyvinyl alcohol film according to claim 1, comprising: a film-forming step of forming a film from an aqueous solution of a polyvinyl alcohol resin by a continuous casting method; a drying step of drying the film formed; and a heat treatment step of heat-treating the dried film, wherein the temperature for drying the film formed in the drying step is 100 ℃ or higher, and the heat treatment step is a step of cooling the dried film to 50 ℃ or lower and then heating the film at 60 to 99 ℃.

The invention also provides a polarizing film as defined in claim 3, which is obtained by using the polyvinyl alcohol film as defined in claim 1.

ADVANTAGEOUS EFFECTS OF INVENTION

The polyvinyl alcohol film as the gist of the invention 1 has an excellent stretchability in the production of a polarizing film and exhibits an effect that breakage does not occur in the production of a thin polarizing film because the polyvinyl alcohol film has a tensile modulus X in the longitudinal direction (MD) of 5 to 12MPa in a state where the moisture percentage is 9% by weight.

In particular, when the thickness D of the film is 5 to 25 μm and the thickness D (μm) and the tensile modulus X (MPa) satisfy the following formula (1), the effect of improving the fracture resistance in the production of the polarizing film is exhibited.

0.4≤X/D≤0.6···(1)

When the tensile modulus Y (MPa) in the width direction (TD) in the state where the moisture content of the film is 9% by weight and the tensile modulus X (MPa) in the longitudinal direction (MD) satisfy the following formula (2), the effect of improving the stretchability in the production of the polarizing film is exhibited.

0.7≤Y/X≤1.3···(2)

In addition, when the tensile strength F in the longitudinal direction (MD) is 100MPa or more, the effect of preventing the breakage in the production of a thin polarizing film is exhibited.

Further, when the refractive index in the longitudinal direction (MD) is nx, the refractive index in the width direction (TD) is ny, the refractive index in the thickness direction is nz, and the thickness is D (μm), the effect that the swelling property in the thickness direction can be optimized and the stretchability in the production of the polarizing film can be optimized is exhibited when the phase difference Rth in the thickness direction calculated by the following formula (3) is 80 to 140nm.

Rth＝{(nx+ny)/2-nz}×1000×D···(3)

When the width of the film is 4m or more, the productivity can be improved.

The method for producing a polyvinyl alcohol film according to the second aspect of the present invention 2 is capable of producing: the polyvinyl alcohol film according to the above item 1, which is excellent in stretchability and fracture resistance when produced into a polarizing film.

The polarizing film according to claim 3 of the present invention uses the polyvinyl alcohol film according to claim 1, and thus exhibits an effect of exhibiting a high degree of polarization and reducing color unevenness.

Detailed Description

The present invention will be described in detail below.

The polyvinyl alcohol film of the invention is characterized in that the polyvinyl alcohol film has a length of 4km or more, and the tensile modulus X in the longitudinal direction (MD) measured in a state that the moisture percentage of the film is 9 wt% is 5-12 MPa.

The polyvinyl alcohol film is formed on a polarizing film by being conveyed in the longitudinal direction (MD) and subjected to a swelling step, a stretching step, and the like. Here, the polyvinyl alcohol film has a specific length (4 km or more) and a specific tensile modulus X (5 to 12 MPa) in the longitudinal direction (MD) of the polyvinyl alcohol film measured in a state where the moisture percentage is 9% by weight, and therefore, the polyvinyl alcohol film has excellent stretchability in the production of a polarizing film and is not broken in the production of a thin polarizing film. The obtained polarizing film exhibits high polarization degree and little color unevenness.

First, the above-mentioned tensile modulus of the polyvinyl alcohol film of the invention will be described.

The tensile modulus in the present invention means a tensile modulus in accordance with JIS K7127: 1999. values determined in an environment of 65% RH at 20 ℃. The tensile modulus is also referred to as young's modulus or tensile modulus.

The moisture content in the present invention is a value calculated from the weight of the polyvinyl alcohol film before and after drying at 105 ℃ for 16 hours in an atmosphere, and is calculated by the following formula, where a represents the weight before drying and B represents the weight after drying.

Water content (% by weight) =100 × (A-B)/A

As described above, in the process for producing a polarizing film, a polyvinyl alcohol film is stretched after swelling, but it is difficult to accurately and stably measure the tensile modulus after swelling. In the present invention, in view of the fact that the moisture content of a polyvinyl alcohol film generally used for producing a polarizing film is several%, the tensile modulus at a moisture content of 9 wt% which is easily controlled by humidity control is used as an index.

The method of the humidity control is not particularly limited, and for example, the moisture content can be stabilized at a moisture content close to 9% by weight of the equilibrium moisture content in the environment by humidity-controlling the relatively dry polyvinyl alcohol-based film immediately after production in the environment of 20 ℃ 65% rh for several hours to several tens of hours. In the present invention, the above-mentioned "moisture content of 9% by weight" means that the "moisture content is in the range of 9.0% by weight ± 0.5% by weight", and the tensile modulus is not substantially changed in this range. In addition, since the moisture content in the film changes with the equilibrium moisture content rate depending on the environmental humidity, the test piece whose moisture content is adjusted to 9 wt% is rapidly subjected to the measurement of the tensile modulus.

In the present invention, the in-plane fluctuation Δ X (MPa) of the tensile modulus X in the longitudinal direction (MD) in the state of a water content of 9 wt%, and the in-plane fluctuation Δ Y (MPa) of the tensile modulus Y (MPa) in the width direction (TD) in the state of a water content of 9 wt% are preferably 5MPa or less, particularly preferably 3MPa or less, and further preferably 2MPa or less.

If the in-plane variation Δ X and the in-plane variation Δ Y are too large, uneven stretching occurs in the production of the polarizing film, and color unevenness tends to occur in the polarizing film.

The method for producing the polyvinyl alcohol film of the invention will be explained. That is, the method for producing a polyvinyl alcohol film of the present invention comprises the steps of: a film-forming step (A) for forming a film from an aqueous solution of a polyvinyl alcohol resin by a continuous casting method; a drying step (B) for drying the film formed; and a heat treatment step (C) of heat-treating the dried film, wherein the temperature for drying the film formed in the drying step (B) is 100 ℃ or higher, and the heat treatment step (C) is performed by cooling the dried film to 50 ℃ or lower and then heating the film at 60 to 99 ℃.

[ film-forming Process (A) ]

First, the film forming step (a) will be described in detail.

As the polyvinyl alcohol resin used in the present invention, an unmodified polyvinyl alcohol resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate is generally used. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol% or less, preferably 5 mol% or less) of 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-based resin having a 1, 2-diol structure in a 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 dehydroketalizing 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. When the weight average molecular weight is too small, the polarization degree of the polarizing film tends to be lowered, and when it is too large, stretching in the production of the polarizing film using the polyvinyl alcohol film tends to be difficult. The weight average molecular weight of the polyvinyl alcohol resin is measured by the GPC-MALS method.

The average saponification degree of the polyvinyl alcohol resin used in the present invention is 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. If the average saponification degree is too small, the polarization degree of the polarizing film tends to decrease.

Here, the average degree of saponification in the present invention is measured according to JIS K6726.

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

Further, it is preferable that the polyvinyl alcohol resin is washed with a solvent such as water and then dehydrated by a centrifugal separator or the like to form a wet cake of the polyvinyl alcohol resin having a water content of 50 wt% or less. If the water content is too high, it tends to be difficult to obtain a desired concentration of the aqueous solution.

The wet cake of the polyvinyl alcohol resin is dissolved in warm water or hot water to prepare an aqueous polyvinyl alcohol resin solution.

The method for producing the aqueous solution of the polyvinyl alcohol resin is not particularly limited, and for example, the aqueous solution may be produced by a heated multi-screw extruder, or an aqueous solution having a desired concentration may be produced by introducing the wet cake of the polyvinyl alcohol resin into a dissolution tank provided with a stirring blade of a vertical circulation flow generation type, and blowing steam into the tank.

The aqueous solution of the polyvinyl alcohol resin preferably contains, in addition to the polyvinyl alcohol resin, a commonly used plasticizer such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, trimethylolpropane, or at least one surfactant selected from nonionic, anionic, and cationic surfactants, from the viewpoint of film forming properties of the polyvinyl alcohol film.

The plasticizer is preferably glycerin, and the content of the plasticizer in the aqueous solution of the polyvinyl alcohol resin is preferably 1 to 20% by weight.

The surfactant is preferably polyoxyethylene alkylamine or higher fatty acid alkanolamide, and the content of the surfactant in the aqueous solution of the polyvinyl alcohol resin is preferably 0.01 to 1% by weight.

The resin concentration of the thus obtained aqueous polyvinyl alcohol resin solution is preferably 15 to 60% by weight, more preferably 17 to 55% by weight, and particularly preferably 20 to 50% by weight. If the resin concentration of the aqueous solution is too low, the drying load tends to increase, and therefore, the productivity tends to decrease, and if it is too high, the viscosity tends to be too high and uniform dissolution tends to be difficult.

Next, the obtained polyvinyl alcohol resin aqueous solution is subjected to a defoaming treatment. Examples of the defoaming method include standing 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 generally used.

After the defoaming treatment, the polyvinyl alcohol resin aqueous solution was introduced into a T-slot die at a constant amount, discharged onto a rotating casting drum, cast, and formed into a film by a continuous casting method.

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

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

The discharge rate of the aqueous polyvinyl alcohol resin solution from the T-die to the casting drum is preferably 0.1 to 3.0 m/min, particularly preferably 0.2 to 2.5 m/min, and further preferably 0.3 to 2.0 m/min. If the discharge speed is too low, productivity tends to be reduced, and if it is too high, casting tends to be difficult.

The diameter of the casting drum is preferably 2 to 5m, particularly preferably 2.4 to 4.5m, and further preferably 2.8 to 4m. If the diameter is too small, the drying length tends to be short and the speed tends to be difficult to be generated, and if it is too large, the transportability tends to be lowered.

The width of the casting drum is preferably 4m or more, particularly preferably 4.5m or more, further preferably 5m or more, and particularly preferably 5 to 6m. If the width of the casting drum is too small, productivity tends to be reduced.

The rotational speed of the casting drum is preferably 1 to 30 m/min, particularly preferably 2 to 25 m/min, and further preferably 3 to 20 m/min. If the rotation speed is too slow, productivity tends to be reduced, and if it is too fast, dehydration tends to be insufficient.

The surface temperature of the casting drum is preferably 40 to 99 ℃, more preferably 50 to 90 ℃, and particularly preferably 55 to 80 ℃. If the surface temperature is too low, dehydration tends to be poor, and if it is too high, foaming tends to occur.

The film thus formed by performing the film-forming step (a) can be peeled from the casting drum.

[ drying step (B) ]

Next, the drying step (B) will be described in detail. The drying step (B) is a step of heating and drying the film formed as described above.

The film peeled from the casting drum (film formed as described above) is dried by being conveyed in the flow direction (MD) using a nip roll or the like, and by alternately contacting the front and back surfaces of the film with a plurality of heat rolls. The heat roll is preferably a roll having a surface subjected to hard chrome plating or mirror surface treatment and a diameter of 0.2 to 2m, and is dried by using usually 2 to 30 rolls, preferably 10 to 25 rolls.

The drying temperature (the temperature at which the film formed as described above is dried) is preferably 100 ℃ or higher, particularly preferably 100 to 120 ℃, further preferably 100 to 115 ℃, and particularly preferably 100 to 110 ℃. If the drying temperature is too low, drying tends to be poor, and if it is too high, appearance defects such as warpage tend to occur. The drying temperature in the present invention is the surface temperature of the heat roll having the highest temperature among the heat rolls used.

The drying time is not particularly limited, but is usually 1 to 60 seconds, preferably 2 to 50 seconds, particularly preferably 3 to 40 seconds, and further preferably 4 to 30 seconds. If the drying time is too short, drying tends to be poor, and if it is too long, appearance defects such as warping tend to occur.

The drying time in the present invention means a contact time between a heat roll having the highest temperature and the film formed as described above, and when there are a plurality of heat rolls having the highest temperature, an integrated value of the contact time between the heat rolls is obtained.

The moisture content of the dried film is preferably 5% by weight or less, particularly preferably 0.1 to 4% by weight, further preferably 0.2 to 3% by weight, particularly preferably 0.3 to 2.5% by weight. If the water content is too high, the finally obtained polyvinyl alcohol film tends to be poorly dried.

[ Heat treatment step (C) ]

Next, the heat treatment step (C) will be described in detail. The heat treatment step (C) is a step of heat-treating the dried thin film. The heat treatment step is generally any step that is performed as needed, and in the present invention, is a step necessary for controlling the tensile modulus. That is, in the heat treatment step (C), the film dried at 100 ℃ or higher in the drying step (B) is once cooled to 50 ℃ or lower (the temperature of the dried film itself), and is heated again at 60 to 99 ℃, whereby the tensile modulus is brought to a desired range and the stretchability in the production of a polarizing film is improved.

Examples of the cooling method include natural cooling in which the glass is left to stand and cooled naturally; a method of blowing cold air, and the like.

Examples of the heating method include a method of blowing hot air with a suspension dryer; a method of contacting with a hot roll, etc., in the present invention, a method using a suspension dryer is preferable in terms of stabilization of tensile modulus.

The heating temperature is preferably 65 to 95 ℃, particularly preferably 70 to 90 ℃, and further preferably 70 to 80 ℃. If the heating temperature is too low, the tensile modulus tends to decrease, and if it is too high, the tensile modulus tends to increase. In the case where the heating method is a method of blowing hot air through the suspension dryer, the heating temperature in the present invention refers to the temperature of the hot air, and in the case where the heating method is a method of contacting the heat roll, the heating temperature in the present invention refers to the surface temperature of the heat roll.

The heating time is not particularly limited, and when a suspension dryer is used, it is preferably 10 to 120 seconds, particularly preferably 20 to 90 seconds, and further preferably 30 to 60 seconds. If the heating time is too short, the fluctuation of the tensile modulus tends to increase, and if it is too long, the productivity tends to decrease.

The moisture percentage of the film after heating is preferably 0.1 to 2% by weight, particularly preferably 0.2 to 1.7% by weight, and further preferably 0.3 to 1.5% by weight. If the water content is too low, the water swelling during the production of the polarizing film tends to be difficult, and if it is too high, the finally obtained polyvinyl alcohol film tends to be poorly dried.

As described above, the drying step (B) and the heat treatment step (C) are described, but the temperature balance between both steps is important in the production method of the present invention, and the drying temperature in the drying step (B) is preferably 100 ℃ or higher, which is the boiling point of water, and in the above temperature range, a lower temperature is more preferably selected, and the heating temperature in the heat treatment step (C) is particularly preferably lower than 100 ℃ which is the boiling point of water.

[ polyvinyl alcohol-based film ]

The polyvinyl alcohol film of the invention which is long in the flow direction (MD) can be obtained by passing through the drying step (B) and the heat treatment step (C). The polyvinyl alcohol film is slit at both ends and wound up in a roll form on a core tube.

The width of the polyvinyl alcohol film of the present invention thus obtained is preferably 4m or more in terms of productivity improvement, and more preferably 4 to 6m in terms of prevention of breakage.

The polyvinyl alcohol film of the present invention has a length of 4km or more, preferably 5km or more in order to cope with a large area of a polarizing film and a liquid crystal screen, and particularly preferably 5 to 50km in terms of a transport weight.

The thickness D of the polyvinyl alcohol film is preferably 30 μm or less in terms of the thinning of the polarizing film, particularly preferably 25 μm or less in terms of the further thinning, and further preferably 5 to 25 μm, particularly preferably 5 to 20 μm in terms of the avoidance of breakage.

The thickness D (. Mu.m) can be adjusted depending on the resin concentration in the aqueous solution of the polyvinyl alcohol resin, the amount of discharge from the casting die (discharge rate), the draw ratio, and the like.

In the polyvinyl alcohol-based film of the present invention, when the thickness D of the film is 5 to 25 μm, the thickness D (μm) and the tensile modulus X (MPa) in the flow direction (MD) measured in a state where the moisture content of the film is 9% by weight preferably satisfy the following formula (1) in terms of improvement in fracture resistance in the production of a polarizing film.

0.4≤X/D≤0.6···(1)

In particular, the following formula (1') is preferably satisfied in terms of improvement of stretchability in the production of a polarizing film.

0.45≤X/D≤0.55···(1')

In the polyvinyl alcohol-based film of the present invention, the tensile modulus Y (MPa) in the width direction (TD) with a moisture content of 9 wt% preferably satisfies the following formula (2) in terms of improvement in stretchability in the production of a polarizing film.

0.7≤Y/X≤1.3···(2)

In particular, in terms of improvement in stretchability in the production of a polarizing film, it is preferable to satisfy the following formula (2').

0.75≤Y/X≤1.1···(2')

If the value of Y/X is too small, the stretchability in the Machine Direction (MD) in the production of a polarizing film tends to be reduced, whereas if it is too large, the stretchability tends to be unstable.

The polyvinyl alcohol film of the invention has a tensile strength F (MPa) in the flow direction (MD) with a moisture content of 9 wt% of preferably 100MPa or more, and particularly preferably 110MPa or more. If the tensile strength F (MPa) is too low, the thin polarizing film tends to be easily broken. The upper limit of the tensile strength F (MPa) is usually 200MPa.

In the polyvinyl alcohol film of the invention, when the refractive index in the longitudinal direction (MD) is nx, the refractive index in the width direction (TD) is ny, the refractive index in the thickness direction is nz, and the thickness is D (μm), the retardation Rth (nm) in the thickness direction calculated by the following formula (3) is preferably 80 to 140nm. More preferably 85 to 130nm, and still more preferably 90 to 120nm.

If the retardation Rth (nm) in the thickness direction is too small, the swelling property in the thickness direction tends to be lowered, and if it is too large, the surface orientation of the polymer chains is strong, and therefore, the stretchability tends to be lowered in the production of a polarizing film.

Rth＝{(nx+ny)/2-nz}×1000×D···(3)

In the above embodiment, the method for producing a polyvinyl alcohol film was described by taking a case where a casting drum (drum roll) is used as a casting mold as an example, but the polyvinyl alcohol film may be produced by using a casting belt or a resin film as a casting mold.

The polyvinyl alcohol film of the present invention is excellent in stretchability, and therefore is particularly preferably used as a polarizing film material.

Here, a method for producing a polarizing film obtained by using the polyvinyl alcohol film of the present invention will be described.

[ method for producing polarizing film ]

The polarizing film of the present invention is produced by feeding the polyvinyl alcohol film from a roll and transferring the film in a horizontal direction, and then subjecting the film to swelling, dyeing, boric acid crosslinking, stretching, washing, drying, and the like.

The swelling step is performed before the dyeing step. The swelling step can clean dirt on the surface of the polyvinyl alcohol film, and also has an effect of preventing uneven dyeing by swelling the polyvinyl alcohol film. In the swelling step, water is generally used as the treatment liquid. When the main component of the treatment liquid is water, a small amount of an iodinated compound, an additive such as a surfactant, alcohol, or the like may be added. The temperature of the swelling bath is usually about 10 to 45 ℃ and the immersion time in the swelling bath is usually about 0.1 to 10 minutes.

The dyeing step is performed by contacting the polyvinyl alcohol film with a liquid containing iodine or a dichroic dye. An aqueous solution of iodine-potassium iodide is usually used, and a suitable concentration of iodine is 0.1 to 2g/L and a suitable concentration of potassium iodide is 1 to 100 g/L. The dyeing time is practically 30 to 500 seconds or so. 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.

The boric acid crosslinking step is carried out by using a boron compound such as boric acid or borax. The boron compound is used as an aqueous solution or a water-organic solvent mixture solution at a concentration of about 10 to 100g/L, and the coexistence of potassium iodide in the liquid is preferable from the viewpoint of stabilization of polarization performance. The temperature at the time of treatment is preferably about 30 to 70 ℃ and the treatment time is preferably about 0.1 to 20 minutes, and the stretching operation may be carried out during the treatment as needed.

The stretching step is preferably carried out 3 to 10 times, more preferably 3.5 to 6 times in the uniaxial direction [ the flow direction (MD) ]. In this case, the stretching may be performed slightly in a direction perpendicular to the stretching direction [ degree of preventing the shrinkage in the width direction (TD), or stretching more than that ]. The temperature during stretching is preferably 30 to 170 ℃. Further, the stretching magnification may be finally set within the above range, and the stretching operation may be performed only once or may be performed a plurality of times in the process of producing a polarizing film.

In the present invention, as described above, the stretching tension in the stretching step must be in an appropriate range. That is, when the stretching tension is too large, stretching cannot be performed up to a predetermined stretching magnification, and the dichroic dye is not sufficiently oriented, so that the degree of polarization tends not to be increased. Conversely, when the stretching tension is too low, the orientation of the dichroic dye is not stable even when the stretching is performed up to a predetermined stretching magnification, and color unevenness tends to occur in the polarizing film.

In general, in the process of producing a polarizing film, the stretching tension is set for each thickness, and therefore it is difficult to generalize the preferable range, but in comparison with the same thickness, the stretching tension is preferably reduced by at least one, and particularly preferably by at least three.

The cleaning step is performed by immersing the polyvinyl alcohol film in an aqueous iodide solution such as water or potassium iodide, for example, and can remove precipitates generated on the surface of the polyvinyl alcohol film. The potassium iodide aqueous solution has a potassium iodide concentration of about 1 to 80 g/L. The temperature for the washing treatment is usually 5 to 50 ℃ and preferably 10 to 45 ℃. The treatment time is usually 1 to 300 seconds, preferably 10 to 240 seconds. The water washing and the washing with the aqueous solution of potassium iodide may be combined as appropriate.

The drying step is performed, for example, by drying the polyvinyl alcohol film at 40 to 80 ℃ for 1 to 10 minutes in the air.

Thus, a polarizing film of the present invention was obtained. The polarization degree of the polarizing film of the present invention is preferably 99.5% or more, more preferably 99.8% or more. If the polarization degree is too low, the contrast ratio in the liquid crystal display tends to be not ensured.

In general, the degree of polarization was calculated by measuring the transmittance (H) at the wavelength λ in a state where 2 polarizing films were stacked so that the orientation directions thereof were the same direction ₁₁ ) The transmittance (H) was measured at a wavelength λ in a state where 2 polarizing films were stacked so that the orientation directions were orthogonal to each other ₁ ) Light transmittance (H) ₁₁ ) And light transmittance (H) ₁ ) The degree of polarization is calculated according to the following equation.

Degree of polarization = [ (H) ₁₁ -H ₁ )/(H ₁₁ +H ₁ )] ^1/2

Further, the polarizing film of the present invention has a single-sheet transmittance of preferably 42% or more, more preferably 43% or more. If the transmittance of the single plate is too low, the luminance of the liquid crystal display tends to be too high.

The single-sheet transmittance is a value obtained by measuring the transmittance of a polarizing film single sheet using a spectrophotometer.

The polarizing film of the present invention is suitable for producing a polarizing plate having excellent polarization degree and no color unevenness.

Here, a method for producing a polarizing plate using the polarizing film of the present invention will be described.

[ method for producing polarizing plate ]

The polarizing plate is produced by laminating an optically isotropic resin film as a protective film on one or both surfaces of the polarizing film of the present invention 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 both to each other under pressure, and heating and irradiating the both with an active energy ray.

In addition, a curable resin such as a urethane resin, an acrylic resin, or a urea resin may be applied to one or both surfaces of the polarizing film and cured to form a cured layer, thereby forming a polarizing plate. In this way, the cured layer can be replaced by the protective film, and a thin film can be formed.

The polarizing film and the polarizing plate using the polyvinyl alcohol film of the invention are excellent in polarizing performance, and are preferably used for liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, billboards, desktop calculators, electronic watches, word processors, electronic papers, game machines, video cameras, photo albums, thermometers, audio equipment, automobiles, mechanical measuring instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, antireflection layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical fiber communication instruments, medical instruments, building materials, toys, and the like.

Examples

The present invention will be further specifically described below with reference to examples, but the present invention is not limited to the following examples unless the gist thereof is exceeded.

The properties (moisture content, tensile modulus, tensile strength, retardation in the thickness direction, and tensile tension) of the polyvinyl alcohol films and the properties (polarization degree, single-sheet transmittance, color unevenness) of the polarizing films in the following examples and comparative examples were measured and evaluated as follows.

< measurement Condition >

[ Water content (% by weight) ]

A test piece of 100 mm. Times.100 mm was cut out from the obtained polyvinyl alcohol film, and the water content (% by weight) was calculated from the initial weight A (g) and the weight B (g) obtained by drying the film for 16 hours at 105 ℃ in an atmosphere using a dryer according to the following formula.

Water percentage (wt%) =100 × (A-B)/A

[ tensile modulus (MPa), tensile strength F (MPa) ]

From the obtained polyvinyl alcohol film, test pieces of 120mm × 15mm were cut, humidity-adjusted so that the moisture content became 9 wt% ± 0.5 wt% in a constant temperature and humidity apparatus of 20 ℃ 65% rh, and then the film was subjected to a test in accordance with JIS K7127:1999 (tensile rate 1000 mm/min, inter-chuck distance 50 mm), and the tensile modulus X (MPa) in the flow direction (MD) and the tensile modulus Y (MPa) in the width direction (TD) were measured in an environment of 65% RH at 20 ℃. Then, the tensile strength F (MPa) was measured in the flow direction (MD).

[ retardation in the thickness direction Rth (nm) ]

A test piece having a length of 4 cm. Times.4 cm was cut out from the center and both ends (inside 10cm from the film end) in the width direction (TD) of the obtained polyvinyl alcohol film, and the retardation Rth (nm) in the thickness direction at 590nm was measured by a retardation measuring instrument ("KOBRA-WR" manufactured by Otsuki Kaisha).

< measurement Condition of Rth >

Incident angle: 50 degree

Tilting the central shaft: slow axis

Average refractive index: values determined using Abbe refractometer

[ tensile tension (N) ])

A test piece having a length [ longitudinal direction (MD) ] of 50mm and a width [ width direction (TD) ] of 35mm was cut out from the obtained polyvinyl alcohol-based film, both ends in the longitudinal direction (MD) were sandwiched by a chuck having a width of 35mm so that the distance between the chucks became 20mm, and then the test piece was immersed in warm water at 25 ℃ for 60 seconds and stretched 1.7 times (34 mm) in the longitudinal direction (MD), then stretched 1.6 times (54.4 mm) in the longitudinal direction (MD) in a dyeing solution at 28 ℃, finally stretched 2.05 times (111.5 mm) in the longitudinal direction (MD) in a boric acid solution at 55 ℃, and the tension (N) at that time was measured by a spring balance. The compositions of the staining solution and the boric acid solution used were as follows.

Dyeing liquid: iodine 0.9g/L, potassium iodide 30g/L

Boric acid solution: potassium iodide 30g/L and boric acid 25g/L

[ degree of polarization (%), monolithic transmittance (%) ]

Test pieces of 4cm in length × 4cm in width were cut from the central portion and both side end portions (inside 10cm from each of both side ends of the polarizing film) in the width direction (TD) of the obtained polarizing film, and the polarization degree (%) and the single-sheet transmittance (%) were measured using an automatic polarizing film measuring apparatus (manufactured by japan spectrographic corporation: VAP 7070). The above measurement was performed for the central portion and the front end portion/terminal end portion (inside 10m from each of the front end and the terminal end of the polarizing film) in the flow direction (MD) of the polarizing film.

[ uneven color ]

A test piece of 30cm × 30cm in length was cut out from the central portion and both side end portions (inside 10cm from each of both side ends of the polarizing film) in the width direction (TD) of the obtained polarizing film, and sandwiched between 2 polarizing plates (single sheet transmittance 43.5%, polarization degree 99.9%) in a cross nicol state at an angle of 45 °, and then optically color unevenness was observed in a transmission mode using a lamp box with surface illuminance 14000lx, and evaluated according to the following criteria.

(evaluation criteria)

823060 without color unevenness

X 8230that there is uneven color

The above evaluation was performed for the central portion and the front end portion/terminal end portion (inside 10m from each of the front end and the terminal end of the polarizing film) in the flow direction (MD) of the polarizing film.

< example 1 >

(preparation of polyvinyl alcohol film)

1000kg of a polyvinyl alcohol resin having a weight average molecular weight of 142000 and a saponification degree of 99.8 mol%, 2500kg of water, and 120kg of glycerol as a plasticizer were placed in the autoclave, and the temperature was raised to 140 ℃ while stirring, so that the concentration was adjusted to 25% by weight, 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 port for deaeration, and then the aqueous solution was discharged from a T-die discharge port (discharge speed 0.8 m/min) to a rotating casting drum at a temperature of 95 ℃. The film thus formed was peeled off from the casting drum, and the front and back surfaces of the film were dried while being alternately brought into contact with 20 hot rolls. The drying temperature (surface temperature of the hot roll having the highest temperature) was 120 ℃ and the drying time (cumulative time of contact between the hot roll having the highest temperature and the film) was 12 seconds. Subsequently, the dried film was naturally cooled to temporarily cool the film itself to 40 ℃, and then heated by blowing hot air of 75 ℃ from both sides of the film for 60 seconds using a suspension dryer to obtain a polyvinyl alcohol film (thickness: 20 μm, width: 5m, length: 5 km) having a moisture content of 1 wt%. Finally, both ends of the polyvinyl alcohol film are slit and wound around a core tube in a roll form. The properties of the obtained polyvinyl alcohol film are shown in table 1 below.

(production of polarizing film)

The obtained polyvinyl alcohol film was unwound from a roll and conveyed in a horizontal direction by a conveying roller, and first, the film was stretched to 1.7 times in the flow direction (MD) within 1 minute while being immersed in a water bath with a water temperature of 25 ℃. Then, while immersing in an aqueous solution at 28 ℃ containing 0.9g/L iodine and 30g/L potassium iodide for dyeing, the resulting fabric was stretched 1.6 times in the flow direction (MD), and while immersing in an aqueous solution at 55 ℃ containing 25g/L boric acid and 30g/L potassium iodide for boric acid crosslinking, the resulting fabric was uniaxially stretched 2.0 times in the flow direction (MD). Finally, the film was washed with an aqueous potassium iodide solution and dried at 50 ℃ for 2 minutes to obtain a polarizing film having a total stretching ratio of 5.6 times. The polarizing film was produced without causing a break. The properties of the obtained polarizing film are shown in table 2 below.

< example 2, comparative example 1 >

Polyvinyl alcohol films and polarizing films were obtained in the same manner as in example 1, except that the production was performed under the conditions shown in table 1 below. The properties of the obtained polyvinyl alcohol film and the properties of the obtained polarizing film are shown in tables 1 and 2, respectively.

In comparative example 1, although the production of the polarizing film was attempted in the same manner as in example 1, the film was broken during stretching in the boric acid crosslinking step. The characteristics of table 2 are only those of the front end portion of the obtained polarizing film.

[ Table 1]

[ Table 2]

The polyvinyl alcohol films of examples 1 and 2 had a tensile modulus value within the specific range of the present invention, and therefore the obtained polarizing films had high polarization degree and no color unevenness, while the polyvinyl alcohol film of comparative example 1 had a tensile modulus outside the specific range of the present invention, and therefore, it was found that the films were easily broken, and the obtained polarizing films had poor polarization degree and color unevenness.

In addition, polyvinyl alcohol films having lengths of 4km and 10km were produced in the same manner as in example 1. The polyvinyl alcohol films having these lengths also exhibited the same tendency as in examples 1 and 2.

The above embodiments are merely illustrative and are not to be construed as limiting the present invention. Variations that are obvious to those skilled in the art are intended to be within the scope of the invention.

Industrial applicability

Claims

1. A polyvinyl alcohol film having a length of 4km or more, wherein a tensile modulus X in a longitudinal direction (MD) of the film with a water content of 9 wt% is 5 to 12MPa, a thickness D of the film is 5 to 25 μm, and the thickness D (μm) and the tensile modulus X (MPa) satisfy the following formula (1),

0.4≤X/D≤0.6 d···（1）。

2. the polyvinyl alcohol film according to claim 1, wherein a tensile modulus Y (MPa) in a width direction (TD) and a tensile modulus X (MPa) in a length direction (MD) in a state where a moisture content of the film is 9% by weight satisfy the following formula (2),

0.7≤Y/X≤1.3···（2）。

3. the polyvinyl alcohol film according to claim 1, wherein the tensile strength F in the longitudinal direction (MD) is 100MPa or more.

4. The polyvinyl alcohol film according to claim 1, wherein when the refractive index in the longitudinal direction (MD) is nx, the refractive index in the width direction (TD) is ny, the refractive index in the thickness direction is nz, and the thickness is D (μm), the retardation in the thickness direction Rth calculated by the following formula (3) is 80 to 140nm,

Rth=｛（nx+ny）/2-nz｝×1000×D···（3）。

5. the polyvinyl alcohol-based film according to claim 1, wherein the width of the film is 4m or more.

6. A method for producing a polyvinyl alcohol film according to any one of claims 1 to 5, comprising: a film-forming step of forming a film from an aqueous solution of a polyvinyl alcohol resin by a continuous casting method; a drying step of drying the film formed; and a heat treatment step of heat-treating the dried film, wherein in the drying step, the temperature for drying the film formed is 100 ℃ or more and the drying time is 1 to 60 seconds, and the heat treatment step is a step of heating the dried film at 60 to 99 ℃ after cooling the film to 50 ℃ or less, wherein the heating is performed using a suspension dryer, and the heating time is 10 to 120 seconds.

7. A polarizing film obtained by using the polyvinyl alcohol film according to any one of claims 1 to 5.