WO2020021847A1

WO2020021847A1 - Polarizing film and method for manufacturing polarizing film

Info

Publication number: WO2020021847A1
Application number: PCT/JP2019/021299
Authority: WO
Inventors: 幸佑 ▲高▼永; 後藤　周作; 亮嶋津
Original assignee: 日東電工株式会社
Priority date: 2018-07-25
Filing date: 2019-05-29
Publication date: 2020-01-30
Also published as: JP2020016743A; TWI801599B; JP7041017B2; KR102606109B1; CN112513694A; TW202007518A; KR20210035174A; CN112513694B

Abstract

The present invention provides a polarizing film capable of reducing streaking. The polarizing film according to the present invention has a thickness of 8 µm or less; the average difference between the maximum and minimum thicknesses of the polarizing film in each 50-mm region from one end to the other end in a direction perpendicular to the absorption axis is 70 nm or less.

Description

Polarizing film and method for manufacturing polarizing film

The present invention relates to a polarizing film and a method for manufacturing a polarizing film.

(4) In a liquid crystal display device, which is a typical image display device, polarizing films are disposed on both sides of a liquid crystal cell due to the image forming method. With the spread of thin displays, displays (OLEDs) equipped with an organic EL panel and displays (QLEDs) using a display panel using an inorganic light emitting material such as quantum dots have been proposed. A polarizing film can also be applied. As a method of manufacturing a polarizing film, for example, a method of stretching a laminate having a resin base material and a polyvinyl alcohol (PVA) -based resin layer and then performing a dyeing treatment to obtain a polarizing film on the resin base material is known. It has been proposed (for example, Patent Document 1). According to such a method, a polarizing film having a small thickness can be obtained, and thus, it is noted that it can contribute to a reduction in thickness of an image display device in recent years. However, when the conventional thin polarizing film as described above is applied to an image display device, uneven streaks may be visually recognized.

JP 2001-343521 A

The present invention has been made to solve the above-mentioned conventional problems, and a main object of the present invention is to provide a polarizing film capable of suppressing the occurrence of uneven streaks, and a method for manufacturing such a polarizing film.

The polarizing film of the present invention has a thickness of 8 μm or less, and the average value of the difference between the maximum thickness and the minimum thickness of each 50 mm region from one end to the other end along the direction perpendicular to the absorption axis is 70 nm. It is as follows.
In one embodiment, the single transmittance is 44.5% or more and the degree of polarization is 99.0% or more.
According to another aspect of the present invention, a polarizing plate is provided. The polarizing plate includes the polarizing film and a protective layer disposed on at least one side of the polarizing film.
According to another aspect of the present invention, a method for manufacturing a polarizing film is provided. The method for producing a polarizing film includes forming a laminate by forming a polyvinyl alcohol-based resin layer containing a polyvinyl alcohol-based resin on one side of a thermoplastic resin base, and applying an air stretching treatment and a dyeing treatment to the laminate. And the polyvinyl alcohol-based resin layer after the aerial stretching treatment has a crystallization index calculated by attenuated total reflection spectroscopy of 1.55 or more and 1.7 or less, and The orientation function is 0.22 or more and 0.31 or less.
In one embodiment, the method further includes performing an underwater stretching process on the laminate after the aerial stretching process, wherein the stretching ratio in the aerial stretching process is 3.0 times or more, and the stretching in the underwater stretching process is performed. Magnification is 1.8 times or less.

FIG. 1 is a schematic sectional view of a polarizing plate according to one embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

A. Polarizing Film The polarizing film according to one embodiment of the present invention has a thickness of 8 μm or less, and has a maximum thickness and a minimum thickness for each region of 50 mm from one end to the other end along a direction perpendicular to the absorption axis. Is 70 nm or less (hereinafter sometimes referred to as thickness variation). The thickness variation is, for example, measuring the thickness of the polarizing film from the one end to the other end at an interval of 2 mm, calculate the difference between the maximum thickness and the minimum thickness in an area of every 50 mm, in each area It is obtained by calculating the average value of the difference. The thickness of the polarizing film can be typically measured using an optical interference thickness meter. Conventional thin polarizing films may have thickness variations along a direction perpendicular to the absorption axis due to the manufacturing method and the like. As a result, when applied to an image display device, stripes along the absorption axis may occur. In some cases, unevenness in shape occurred. The thickness variation of the polarizing film of the present embodiment is small despite its very small thickness. Such a polarizing film can suppress occurrence of uneven streaks when applied to an image display device.

The thickness of the polarizing film is preferably 1 μm to 8 μm, more preferably 1 μm to 7 μm, and still more preferably 2 μm to 5 μm. The thickness variation is preferably 50 nm or less, more preferably 40 nm or less, and particularly preferably 30 nm or less. The thickness variation is preferably small, but a practical lower limit is, for example, 5 nm.

The polarizing film preferably has a single transmittance of 44.5% or more and a degree of polarization of 99.0% or more. The single transmittance of the polarizing film is more preferably 45.0% or more. The degree of polarization of the polarizing film is more preferably 99.5% or more, and still more preferably 99.9% or more. The single transmittance is typically a Y value measured using a UV-visible spectrophotometer and subjected to visibility correction. The degree of polarization is typically determined by the following equation based on the parallel transmittance Tp and the orthogonal transmittance Tc, which have been measured using a UV-visible spectrophotometer and corrected for visibility.
Degree of polarization (%) = {(Tp−Tc) / (Tp + Tc)} ^1/2 × 100

The method for producing a polarizing film includes forming a polyvinyl alcohol-based resin layer (PVA-based resin layer) containing a polyvinyl alcohol-based resin (PVA-based resin) on one side of a thermoplastic resin base material to form a laminate; And subjecting the laminate to an air stretching process and a dyeing process in this order. The PVA-based resin layer after the air stretching treatment has a crystallization index calculated by attenuated total reflection spectroscopy (ATR) of 1.55 or more and 1.7 or less, and an orientation function of 0.22 or more and 0 or less. .31 or less. As described above, by controlling the crystallization index and the orientation function of the PVA-based resin layer after the aerial stretching treatment within the above ranges, the thickness is small, the thickness variation is small, and the polarizing film further has high optical characteristics. Can be manufactured.

The crystallization index of the PVA-based resin layer after the air stretching treatment is determined by ATR measurement using, for example, a Fourier transform infrared spectrophotometer (FT-IR) and polarized light as measurement light. Specifically, the measurement was carried out with the measurement polarized light at 0 ° and 90 ° with respect to the stretching direction, and the intensity was calculated at 1141 cm ⁻¹ and 1140 cm ⁻¹ of the obtained spectrum according to the following equation. You. Note that the intensity of 1141 cm ⁻¹ has a correlation with the amount of the crystal part of the PVA-based resin layer.
Crystallization index = ((I _C-0 + 2 × I _C-90 ) / 3) / ((I _R-0 + 2 × I _R-90 ) / 3)
However,
I _C-0 : Intensity of 1141 cm ⁻¹ when measuring with measuring light (polarized light) incident in a direction parallel to the stretching direction I _C-90 : Measurement with measuring light (polarized light) incident in a direction perpendicular to the stretching direction Intensity I _R-0 at 1141 cm ^-1 when the measurement light (polarized light) is incident in a direction parallel to the stretching direction and measured at an intensity I _R-90 at 1140 cm ^-1 : the measurement light (polarized light) in the stretching direction Of 1140 cm ^-1 when measured with vertical incidence

The orientation function (f) of the PVA-based resin layer after the air stretching treatment can be determined by, for example, ATR measurement using FT-IR and polarized light as measurement light. Specifically, the measurement is performed in a state where the measurement polarization is set to 0 ° and 90 ° with respect to the stretching direction, and the calculated value is calculated according to the following equation using the intensity of 2941 cm ⁻¹ of the obtained spectrum. Here, the intensity I, as a reference peak to 3330cm ^{^-1,} a value of 2941cm ^-1 / 3330cm -1. When f = 1, the orientation is perfect, and when f = 0, the orientation is random. The peak at 2941 cm ⁻¹ is considered to be absorption due to vibration of the main chain (—CH 2 —) of PVA.
f = (3 <cos ² θ> -1) / 2
= (1-D) / [c (2D + 1)]
However,
c = (3 cos ² β-1) / 2
β = 90deg⇒f = -2 × (1-D) / (2D + 1)
θ: molecular chain, stretching direction β: molecular chain, transition dipole moment D = (I⊥) / (I //)
(The value of D increases as the PVA molecules are oriented.)
I⊥: Intensity measured when measuring light (polarized light) is incident in the direction perpendicular to the stretching direction I //: Intensity measured when measuring light (polarized light) is incident in the direction parallel to the stretching direction

B. Polarizing Plate FIG. 1 is a schematic sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 includes a polarizing film 10, a first protective layer 20 disposed on one side of the polarizing film 10, and a second protective layer 30 disposed on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in the above section A. One of the first protective layer 20 and the second protective layer 30 may be omitted. Note that one of the first protective layer and the second protective layer may be a resin base material used for manufacturing the above-described polarizing film.

The first and second protective layers are formed of any appropriate film that can be used as a protective layer of a polarizing film. Specific examples of the material that is a main component of the film include a cellulosic resin such as triacetyl cellulose (TAC), polyester, polyvinyl alcohol, polycarbonate, polyamide, polyimide, polyethersulfone, and polysulfone. And a transparent resin such as polystyrene, polynorbornene, polyolefin, (meth) acrylic, and acetate. Further, a thermosetting resin such as (meth) acrylic, urethane, (meth) acrylic urethane, epoxy, silicone and the like, or an ultraviolet curable resin may also be used. In addition, for example, a glassy polymer such as a siloxane-based polymer may also be used. Further, a polymer film described in JP-A-2001-343529 (WO 01/37007) can also be used. As a material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in a side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in a side chain And, for example, a resin composition having an alternating copolymer of isobutene and N-methylmaleimide and an acrylonitrile / styrene copolymer. The polymer film may be, for example, an extruded product of the resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of a protective layer (outer protective layer) disposed on the side opposite to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably. The thickness is 5 μm to 80 μm, more preferably 10 μm to 60 μm. When the surface treatment is performed, the thickness of the outer protective layer is a thickness including the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (inner protective layer) disposed on the display panel side is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and still more preferably 10 μm to 60 μm. is there. In one embodiment, the inner protective layer is a retardation layer having any suitable retardation value. As the retardation layer, a retardation film having an in-plane retardation of 40 nm or more and / or a retardation having a thickness direction retardation of 80 nm or more can be used. The in-plane retardation is usually controlled in the range of 40 to 200 nm, and the thickness direction retardation is usually controlled in the range of 80 to 300 nm. Examples of the retardation film include a birefringent film obtained by uniaxially or biaxially stretching a polymer material, an alignment film of a liquid crystal polymer, and an alignment layer of a liquid crystal polymer supported by a film. The thickness of the retardation film is not particularly limited, but is generally about 20 to 150 μm.

C. Method for Producing Polarizing Film The method for producing a polarizing film according to one embodiment of the present invention is, as described above, a method of forming a laminate by forming a polyvinyl alcohol-based resin layer containing a polyvinyl alcohol-based resin on one side of a thermoplastic resin base material. And subjecting the laminate to an air stretching process and a dyeing process in this order. The polyvinyl alcohol-based resin layer after the air stretching treatment has a crystallization index calculated by attenuated total reflection measurement of 1.55 or more and 1.7 or less, and an orientation function of 0.22 or more and 0.31 or less. It is as follows. Preferably, the method includes subjecting the laminate to an underwater stretching process after the aerial stretching process, wherein the stretching ratio in the aerial stretching process is 3.0 times or more, and the stretching ratio in the underwater stretching process is 1.8 times or less. . As described above, the crystallization of the PVA-based resin layer after the aerial stretching is promoted by setting the stretching ratio in the aerial stretching to be higher than that of the conventional manufacturing method and setting the stretching ratio to be lower in the underwater stretching. At the same time, excessive crystallization of the thermoplastic resin substrate can be suppressed. Thus, a polarizing film having small thickness variation and excellent optical characteristics can be obtained.

C-1. Production of Laminate Any appropriate method can be adopted as a method for producing a laminate of the thermoplastic resin substrate and the PVA-based resin layer. Preferably, a coating liquid containing a PVA-based resin is applied to the surface of the long thermoplastic resin base material and dried to form a PVA-based resin layer on the thermoplastic resin base material.

は Any appropriate method can be adopted as a method of applying the coating solution. For example, a roll coating method, a spin coating method, a wire bar coating method, a dip coating method, a die coating method, a curtain coating method, a spray coating method, a knife coating method (such as a comma coating method) and the like can be mentioned. The coating / drying temperature of the coating solution is preferably 50 ° C. or higher.

(4) The thickness of the PVA-based resin layer is preferably 3 μm to 40 μm, and more preferably 3 μm to 20 μm.

(4) Before forming the PVA-based resin layer, the thermoplastic resin substrate may be subjected to a surface treatment (for example, corona treatment), or the easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such a treatment, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved.

C-1-1. Thermoplastic resin base material The thickness of the thermoplastic resin base material is preferably from 20 μm to 300 μm, more preferably from 50 μm to 200 μm. If it is less than 20 μm, formation of a PVA-based resin layer may be difficult. If it exceeds 300 μm, for example, in the underwater stretching treatment described below, it takes a long time for the thermoplastic resin substrate to absorb water, and an excessive load may be required for stretching.

The thermoplastic resin substrate preferably has a water absorption of 0.2% or more, more preferably 0.3% or more. The plastic resin substrate absorbs water, and the water acts as a plasticizer and can be plasticized. As a result, the stretching stress can be greatly reduced, and stretching can be performed at a high magnification. On the other hand, the water absorption of the thermoplastic resin substrate is preferably 3.0% or less, more preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent problems such as the dimensional stability of the thermoplastic resin substrate being significantly reduced during production and the appearance of the obtained polarizing film being deteriorated. Further, it is possible to prevent the base material from being broken during the stretching in water and the PVA-based resin layer from peeling off from the thermoplastic resin base material. The water absorption of the thermoplastic resin substrate can be adjusted, for example, by introducing a modifying group into the constituent material. The water absorption is a value determined according to JIS K7209.

ガラス The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 120 ° C or lower. By using such a thermoplastic resin substrate, it is possible to sufficiently secure the stretchability of the laminate while suppressing crystallization of the PVA-based resin layer. Further, in consideration of plasticizing the thermoplastic resin substrate with water and performing good stretching in water, the temperature is more preferably 100 ° C. or lower, further preferably 90 ° C. or lower. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60 ° C. or higher. By using such a thermoplastic resin base material, the thermoplastic resin base material is deformed (for example, generation of irregularities, tarmi, wrinkles, and the like) when the coating liquid containing the PVA-based resin is applied and dried. The above problem can be prevented, and the laminate can be favorably manufactured. Further, the stretching of the PVA-based resin layer can be favorably performed at a suitable temperature (for example, about 60 ° C.). The glass transition temperature of the thermoplastic resin substrate can be adjusted, for example, by heating using a crystallization material that introduces a modifying group into the constituent material. The glass transition temperature (Tg) is a value obtained in accordance with JIS @ K # 7121.

構成 Any suitable thermoplastic resin can be adopted as a constituent material of the thermoplastic resin base material. Examples of the thermoplastic resin include, for example, ester resins such as polyethylene terephthalate resin, cycloolefin resins such as norbornene resin, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, and copolymer resins thereof. Is mentioned. Among these, a norbornene-based resin and an amorphous polyethylene terephthalate-based resin are preferred.

非晶質 In one embodiment, an amorphous (non-crystallized) polyethylene terephthalate resin is preferably used. Among them, an amorphous (hard to crystallize) polyethylene terephthalate resin is particularly preferably used. Specific examples of the amorphous polyethylene terephthalate-based resin include a copolymer further containing isophthalic acid and / or cyclohexanedicarboxylic acid as a dicarboxylic acid, and a copolymer further containing cyclohexanedimethanol or diethylene glycol as a glycol.

In a preferred embodiment, the thermoplastic resin substrate is made of a polyethylene terephthalate resin having an isophthalic acid unit. This is because such a thermoplastic resin substrate is extremely excellent in stretchability and can suppress crystallization during stretching. This is considered to be due to the fact that the introduction of the isophthalic acid unit gives a large bending to the main chain. The polyethylene terephthalate resin has a terephthalic acid unit and an ethylene glycol unit. The content ratio of the isophthalic acid unit is preferably at least 0.1 mol%, more preferably at least 1.0 mol%, based on the total of all repeating units. This is because a thermoplastic resin substrate having extremely excellent stretchability can be obtained. On the other hand, the content ratio of the isophthalic acid unit is preferably 20 mol% or less, more preferably 10 mol% or less, based on the total of all repeating units. By setting such a content ratio, the crystallinity can be favorably increased in the drying shrinkage treatment described below.

The thermoplastic resin substrate may be stretched in advance (before forming the PVA-based resin layer). In one embodiment, it is stretched in the transverse direction of a long thermoplastic resin substrate. The lateral direction is preferably a direction orthogonal to the stretching direction of the laminate described below. In addition, in this specification, "orthogonal" includes a case where they are substantially orthogonal. Here, “substantially orthogonal” includes 90 ° ± 5.0 °, preferably 90 ° ± 3.0 °, and more preferably 90 ° ± 1.0 °. The stretching temperature of the thermoplastic resin substrate is preferably from Tg−10 ° C. to Tg + 50 ° C. with respect to the glass transition temperature (Tg). The stretch ratio of the thermoplastic resin base material is preferably 1.5 to 3.0 times. Any appropriate method can be adopted as a method for stretching the thermoplastic resin substrate. Specifically, fixed-end stretching or free-end stretching may be used. The stretching method may be a dry method or a wet method. The stretching of the thermoplastic resin substrate may be performed in one step or may be performed in multiple steps. In the case of performing in multiple stages, the above-mentioned stretching ratio is a product of the stretching ratios in each stage.

C-1-2. Coating Liquid The coating liquid is typically a solution obtained by dissolving the PVA-based resin in a solvent. Examples of the solvent include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Among these, water is preferable. The concentration of the PVA resin in the solution is preferably 3 to 20 parts by weight based on 100 parts by weight of the solvent. With such a resin concentration, a uniform coating film adhered to the thermoplastic resin base material can be formed.

添加 Additives may be added to the coating solution. Examples of the additive include a plasticizer and a surfactant. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol and glycerin. Examples of the surfactant include a nonionic surfactant. These can be used for the purpose of further improving the uniformity, dyeability and stretchability of the obtained PVA-based resin layer.

任意 As the PVA-based resin, any appropriate resin can be adopted. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer can be mentioned. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying the ethylene-vinyl acetate copolymer. The degree of saponification of the PVA-based resin is usually from 85 mol% to 100 mol%, preferably from 95.0 mol% to 99.95 mol%, more preferably from 99.0 mol% to 99.93 mol%. . The degree of saponification can be determined according to JIS @ K-6726-1994. By using a PVA-based resin having such a saponification degree, a polarizing film having excellent durability can be obtained. If the degree of saponification is too high, gelation may occur.

(4) The average polymerization degree of the PVA-based resin can be appropriately selected according to the purpose. The average degree of polymerization is usually from 1,000 to 10,000, preferably from 1,200 to 4,500, and more preferably from 1,500 to 4,300. The average degree of polymerization can be determined according to JIS @ K-6726-1994.

C-2. Auxiliary aerial stretching process The stretching ratio in the aerial stretching process is preferably 3.0 times to 4.0 times. Thereby, the crystallization index and the orientation function of the PVA-based resin layer after the air stretching treatment can be controlled within a desired numerical range. Further, as described above, by setting the stretching ratio in the aerial stretching process higher than in the conventional manufacturing method, the stretching ratio for realizing desired optical characteristics in the underwater stretching process described below can be set lower. . Thereby, excessive crystallization of the thermoplastic resin substrate due to the underwater stretching treatment can be suppressed.

The stretching method in the air-assisted stretching may be fixed-end stretching (for example, a method of stretching using a tenter stretching machine) or free-end stretching (for example, uniaxially stretching through a laminate between rolls having different peripheral speeds). Although good, free end stretching can be actively employed to obtain high optical properties. In one embodiment, the air stretching process includes a heating roll stretching step in which the long laminate is stretched by a peripheral speed difference between the heating rolls while being conveyed in the longitudinal direction. The aerial stretching process typically includes a zone stretching step and a heated roll stretching step. The order of the zone stretching step and the heating roll stretching step is not limited, and the zone stretching step may be performed first, or the heating roll stretching step may be performed first. The zone stretching step may be omitted. In one embodiment, the zone stretching step and the hot roll stretching step are performed in this order. In another embodiment, in a tenter stretching machine, the film is stretched by gripping the end of the film and increasing the distance between the tenters in the flow direction (the expansion of the distance between the tenters becomes the stretching ratio). At this time, the distance of the tenter in the width direction (perpendicular to the flow direction) is set to be arbitrarily small. Preferably, it can be set so as to be closer to the free-end stretching with respect to the stretching ratio in the flow direction. In the case of free end stretching, the shrinkage in the width direction is calculated as (1 / stretching ratio) ^1/2 .

補助 The in-air auxiliary stretching may be performed in one stage or may be performed in multiple stages. When the stretching is performed in multiple stages, the stretching ratio is a product of the stretching ratios of the respective stages. The stretching direction in the aerial auxiliary stretching is preferably substantially the same as the stretching direction in the underwater stretching.

The maximum stretching ratio in the case where the in-air assisted stretching and the underwater stretching are combined is preferably 5.0 times or more, more preferably 5.5 times or more, and still more preferably 6.0 times, with respect to the original length of the laminate. That is all. In the present specification, the “maximum stretch ratio” refers to a stretch ratio immediately before the laminate is broken, and separately refers to a stretch ratio at which the laminate is broken, and refers to a value 0.2 lower than the value.

延伸 The stretching temperature of the in-air auxiliary stretching can be set to any appropriate value according to the forming material of the thermoplastic resin base material, the stretching method, and the like. The stretching temperature is preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate + 10 ° C., and particularly preferably equal to or higher than Tg + 15 ° C. On the other hand, the upper limit of the stretching temperature is preferably 170 ° C. By stretching at such a temperature, it is possible to suppress the rapid progress of crystallization of the PVA-based resin, and to suppress problems caused by the crystallization (for example, hinder the orientation of the PVA-based resin layer by stretching). it can.

C-3. Insolubilization treatment If necessary, an insolubilization treatment is performed after the aerial auxiliary stretching treatment and before the underwater stretching treatment and the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in a boric acid aqueous solution. By performing the insolubilization treatment, it is possible to impart water resistance to the PVA-based resin layer and to prevent a decrease in the orientation of PVA when immersed in water. The concentration of the boric acid aqueous solution is preferably 1 part by weight to 4 parts by weight based on 100 parts by weight of water. The liquid temperature of the insolubilizing bath (boric acid aqueous solution) is preferably 20 ° C to 50 ° C.

C-4. Dyeing treatment The above-mentioned dyeing treatment is typically performed by dyeing a PVA-based resin layer with iodine. Specifically, it is performed by adsorbing iodine on the PVA-based resin layer. Examples of the adsorption method include a method of dipping a PVA-based resin layer (laminate) in a dyeing solution containing iodine, a method of coating the PVA-based resin layer with the dyeing solution, and a method of applying the dyeing solution to the PVA-based resin layer. A spraying method and the like can be mentioned. A preferred method is to immerse the laminate in a dye solution (dye bath). This is because iodine can be favorably adsorbed.

The staining solution is preferably an aqueous iodine solution. The amount of iodine is preferably 0.05 to 0.5 parts by weight based on 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add iodide to an aqueous iodine solution. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. And the like. Among these, potassium iodide is preferable. The amount of iodide is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, based on 100 parts by weight of water. The temperature of the dyeing solution at the time of dyeing is preferably 20 ° C. to 50 ° C. in order to suppress the dissolution of the PVA resin. When the PVA-based resin layer is immersed in the dyeing solution, the immersion time is preferably from 5 seconds to 5 minutes, more preferably from 30 seconds to 90 seconds, in order to secure the transmittance of the PVA-based resin layer.

Dyeing conditions (concentration, solution temperature, immersion time) can be set so that the degree of polarization or the single transmittance of the finally obtained polarizing film is within a predetermined range. In one embodiment, the immersion time is set so that the obtained polarizing film has a single transmittance of 44.5% to 45.0%. In another embodiment, the immersion time is set so that the degree of polarization of the obtained polarizing film is 99.0% or more.

When the dyeing treatment is continuously performed after the treatment of immersing the laminate in a treatment bath containing boric acid (typically, the insolubilization treatment), the boric acid contained in the treatment bath is mixed into the dyeing bath. The boric acid concentration in the dye bath changes over time, and as a result, the dyeability may be unstable. In order to suppress the instability of the dyeing properties as described above, the upper limit of the concentration of boric acid in the dyeing bath is preferably 4 parts by weight, more preferably 2 parts by weight, based on 100 parts by weight of water. Adjusted. On the other hand, the lower limit of the concentration of boric acid in the dyeing bath is preferably 0.1 part by weight, more preferably 0.2 part by weight, and still more preferably 0.5 part by weight with respect to 100 parts by weight of water. It is. In one embodiment, the dyeing treatment is performed using a dyeing bath in which boric acid is previously mixed. This can reduce the rate of change in boric acid concentration when boric acid in the treatment bath is mixed into the dyeing bath. The amount of boric acid previously added to the dyeing bath (that is, the content of boric acid not derived from the treatment bath) is preferably 0.1 to 2 parts by weight with respect to 100 parts by weight of water. , More preferably 0.5 to 1.5 parts by weight.

C-5. Crosslinking treatment If necessary, crosslinking treatment is performed after the dyeing treatment and before the underwater stretching treatment. The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. By performing the cross-linking treatment, water resistance is imparted to the PVA-based resin layer, and it is possible to prevent a decrease in the orientation of PVA when immersed in high-temperature water during subsequent stretching in water. The concentration of the boric acid aqueous solution is preferably 1 part by weight to 5 parts by weight with respect to 100 parts by weight of water. When a crosslinking treatment is performed after the above-mentioned dyeing treatment, it is preferable to further add an iodide. By blending iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. The amount of iodide is preferably 1 to 5 parts by weight based on 100 parts by weight of water. Specific examples of iodide are as described above. The liquid temperature of the crosslinking bath (aqueous boric acid solution) is preferably 20 ° C. to 50 ° C.

C-6. Underwater stretching treatment The stretching ratio in the underwater stretching treatment is preferably 1.8 times or less, more preferably 1.5 times or less. Thereby, excessive crystallization of the thermoplastic resin substrate due to the underwater stretching treatment can be suppressed. Further, a high total stretching ratio in combination with the auxiliary stretching in the air is realized, and a polarizing film having extremely excellent optical properties can be manufactured.

The underwater stretching treatment is performed by immersing the laminate in a stretching bath. According to the in-water stretching treatment, the thermoplastic resin substrate and the PVA-based resin layer can be stretched at a temperature lower than the glass transition temperature (typically, about 80 ° C.), and the PVA-based resin layer is crystallized. And can be stretched at a high magnification. As a result, a polarizing film having excellent optical characteristics can be manufactured.

延伸 Any suitable method can be adopted as a method for stretching the laminate. Specifically, fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching through a laminate between rolls having different peripheral speeds) may be used. Preferably, free end stretching is selected. The stretching of the laminate may be performed in one stage or may be performed in multiple stages. In the case of performing in multiple stages, the stretching ratio (maximum stretching ratio) of the laminate described later is a product of the stretching ratios in each stage.

The underwater stretching is preferably performed by immersing the laminate in a boric acid aqueous solution (boric acid in water stretching). By using a boric acid aqueous solution as the stretching bath, it is possible to provide the PVA-based resin layer with rigidity to withstand the tension applied during stretching and water resistance that does not dissolve in water. Specifically, boric acid can generate a tetrahydroxyborate anion in an aqueous solution and crosslink with a PVA-based resin by hydrogen bonding. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, the film can be stretched favorably, and a polarizing film having excellent optical characteristics can be manufactured.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and / or borate in water as a solvent. The concentration of boric acid is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, particularly preferably 3 to 5 parts by weight, based on 100 parts by weight of water. It is. By setting the boric acid concentration to 1 part by weight or more, dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film having higher characteristics can be manufactured. In addition to the boric acid or the borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde or the like in a solvent can also be used.

Preferably, an iodide is blended in the above stretching bath (boric acid aqueous solution). By blending iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. Specific examples of iodide are as described above. The concentration of iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, based on 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40 ° C to 85 ° C, more preferably 60 ° C to 75 ° C. At such a temperature, the film can be stretched at a high magnification while suppressing the dissolution of the PVA-based resin layer. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin base material is preferably 60 ° C. or more in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40 ° C., there is a possibility that stretching may not be performed well even if plasticization of the thermoplastic resin substrate by water is considered. On the other hand, the higher the temperature of the stretching bath is, the higher the solubility of the PVA-based resin layer is, and there is a possibility that excellent optical properties cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

C-7. Drying Treatment The drying treatment may be performed by zone heating in which the entire zone is heated, or may be performed by heating the transport rolls (using a so-called heating roll) (heating roll drying method). Preferably, both are used. By drying using a heating roll, it is possible to efficiently suppress the heating curl of the laminate and produce a polarizing film having an excellent appearance. Specifically, by drying the laminate along the heating roll, the degree of crystallinity can be increased by efficiently promoting the crystallization of the thermoplastic resin base material, and is relatively low. Even at the drying temperature, the degree of crystallinity of the thermoplastic resin substrate can be favorably increased. As a result, the rigidity of the thermoplastic resin base material increases, and the thermoplastic resin base material becomes a state capable of withstanding shrinkage of the PVA-based resin layer due to drying, and curling is suppressed. In addition, by using a heating roll, the laminate can be dried while maintaining the flat state, so that not only curling but also generation of wrinkles can be suppressed. At this time, the optical characteristics can be improved by shrinking the laminate in the width direction by the drying shrinkage treatment. This is because the orientation of PVA and the PVA / iodine complex can be effectively increased.

C-8. Other Treatments Preferably, a washing treatment is performed after the underwater stretching treatment and before the drying treatment. The above-mentioned cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples. The measuring method and evaluation method of each characteristic are as follows. Unless otherwise specified, “parts” and “%” in Examples and Comparative Examples are based on weight.
(1) Crystallization index The PVA-based resin layer was subjected to ATR measurement using a Fourier-transform infrared spectrophotometer (Perkin Elmer, product name "SPECTRUM2000") for the laminate after the aerial stretching treatment, using polarized light as measurement light. The surface was evaluated. Specifically, the measurement was performed with the measured polarized light at 0 ° and 90 ° with respect to the stretching direction, and crystallization was performed according to the following formula using the intensity of the obtained spectrum at 1141 cm ⁻¹ and 1140 cm ⁻¹ . An index was calculated.
Crystallization index = ((I _C-0 + 2 × I _C-90 ) / 3) / ((I _R-0 + 2 × I _R-90 ) / 3)
However,
I _C-0 : Intensity of 1141 cm ⁻¹ when measuring with measuring light (polarized light) incident in a direction parallel to the stretching direction I _C-90 : Measurement with measuring light (polarized light) incident in a direction perpendicular to the stretching direction Intensity I _R-0 at 1141 cm ^-1 when the measurement light (polarized light) is incident in a direction parallel to the stretching direction and measured at an intensity I _R-90 at 1140 cm ^-1 : the measurement light (polarized light) in the stretching direction (1) Intensity of 1140 cm ^-1 when measured by incidence in the vertical direction (2) Orientation function For the laminate after the air stretching treatment, a Fourier transform infrared spectrophotometer (Perkin Elmer, product name “SPECTRUM2000”) was used. The surface of the PVA-based resin layer was evaluated by ATR measurement using polarized light as measurement light. Specifically, the measurement was performed with the measurement polarized light at 0 ° and 90 ° with respect to the stretching direction, and the orientation function was calculated according to the following equation using an intensity of 2941 cm ⁻¹ .
f = (3 <cos ² θ> -1) / 2
= (1-D) / [c (2D + 1)]
However,
c = (3 cos ² β-1) / 2
β = 90deg⇒f = -2 × (1-D) / (2D + 1)
θ: molecular chain, stretching direction β: molecular chain, transition dipole moment D = (I⊥) / (I //)
(The value of D increases as the PVA molecules are oriented.)
I⊥: intensity when measuring light (polarized light) is incident in the direction perpendicular to the stretching direction and measuring I //: intensity when measuring light (polarized light) is incident in the direction parallel to the stretching direction and measuring (3) Thickness variation The thickness of the polarizing film of each of Examples and Comparative Examples was measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name “MCPD-3700”) from one end to the other in the direction orthogonal to the absorption axis. The measurement was performed at 2 mm intervals up to the part.
Next, from the one end to the other end, a difference between a maximum thickness and a minimum thickness in a region of every 50 mm is calculated, an average value of the difference in each region is calculated, and the average value is calculated as a thickness. It was uneven.
(4) Optical properties (single transmittance and degree of polarization)
For the polarizing plates (protective film / polarizing film) of Examples and Comparative Examples, single transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer (V-7100 manufactured by JASCO Corporation). Are defined as Ts, Tp and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values measured with a two-degree visual field (C light source) according to JIS Z8701 and subjected to visibility correction. The refractive index of the protective film was 1.50, and the refractive index of the surface of the polarizing film opposite to the protective film was 1.53.
From the obtained Tp and Tc, the degree of polarization P was determined by the following equation.
Degree of polarization P (%) = {(Tp−Tc) / (Tp + Tc)} ^1/2 × 100
Based on the obtained values of the single transmittance and the degree of polarization, the optical characteristics were evaluated according to the following criteria.
Good: The degree of polarization is 99.0% or more. (Single transmittance = 44.5%)
X: The degree of polarization is less than 99.0%. (Single transmittance = 44.5%)

[Example 1]
1. Preparation of Polarizing Film As a thermoplastic resin substrate, an amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm, width: 1,450 mm) having a long shape, a water absorption of 0.75%, and a Tg of about 75 ° C. was used. Using. One surface of the resin substrate was subjected to a corona treatment (treatment condition: 55 W · min / m ² ).
PVA aqueous solution containing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (trade name “Gosefimer Z410” manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) at a ratio of 9: 1 (coating) Liquid).
The PVA aqueous solution was applied to the corona-treated surface of the resin substrate and dried at 60 ° C. to form a PVA-based resin layer having a thickness of 8 μm, thereby producing a laminate.
The obtained laminate was stretched at a stretching temperature of 120 ° C. to 130 ° C. and uniaxially stretched 3.0 times in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds (auxiliary stretching in air).
Next, the laminate was immersed in an insolubilizing bath at a liquid temperature of 40 ° C. (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) for 30 seconds (insolubilization treatment).
Next, the polarizing film finally obtained is placed in a dyeing bath at a liquid temperature of 30 ° C. (an aqueous solution of iodine obtained by mixing iodine and potassium iodide in a weight ratio of 1: 7 with respect to 100 parts by weight of water). It was immersed for 60 seconds while adjusting the concentration so that the single transmittance (Ts) became 44.5% (dyeing treatment).
Next, it was immersed in a crosslinking bath of a liquid temperature of 40 ° C. (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water) for 30 seconds. (Crosslinking treatment).
Thereafter, while the laminate is immersed in a boric acid aqueous solution (boric acid concentration: 4.0% by weight) at a liquid temperature of 70 ° C, the total stretching ratio in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds is 5.5. The film was uniaxially stretched so as to be twice as large (underwater stretching treatment).
Thereafter, the laminate was immersed in a washing bath at a liquid temperature of 20 ° C. (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with respect to 100 parts by weight of water) (washing treatment).
Thereafter, by drying in an oven, a polarizing film having a width of 1500 mm and a thickness of 3.5 μm was formed on the resin substrate.
2. Preparation of Polarizing Plate On the surface of the polarizing film obtained above (the surface opposite to the resin substrate), an acrylic film (surface refractive index: 1.50, 40 μm) as a protective film was coated with an ultraviolet curable adhesive. And pasted together. Specifically, the adhesive was applied so that the total thickness of the curable adhesive became 1.0 μm, and was bonded using a roll machine. Thereafter, UV light was irradiated from the protective film side to cure the adhesive. Next, the resin substrate was peeled off to obtain a polarizing plate having a structure of protective film / polarizing film.

[Example 2]
A polarizing film and a polarizing plate were produced in the same manner as in Example 1 except that the stretching ratio was 3.5 times in the air-assisted stretching process.

[Comparative Example 1]
A polarizing film and a polarizing plate were produced in the same manner as in Example 1 except that the stretching ratio was 2.0 times and the stretching temperature was 140 ° C. in the auxiliary in-air stretching process.

[Comparative Example 2]
A polarizing film and a polarizing plate were produced in the same manner as in Example 1, except that the stretching ratio was set to 2.4 times in the auxiliary stretching in air.

[Comparative Example 3]
In the air-assisted stretching treatment, an attempt was made to produce a polarizing film in the same manner as in Example 1 except that the stretching ratio was 4.0 times and the stretching temperature was 140 ° C. The body was broken, and a polarizing film and a polarizing plate could not be produced.

[Comparative Example 4]
A polarizing film and a polarizing plate were produced in the same manner as in Example 1 except that the stretching ratio was set to 4.5 in the auxiliary in-air stretching process and that the in-water stretching process was not performed.

<Evaluation>
For Examples and Comparative Examples, the crystallization index and orientation function of the PVA-based resin layer after the air stretching treatment were calculated according to the above (1) and (2), and the thickness variation of the polarizing film was calculated according to the above (3), The optical properties of the polarizing plate were evaluated according to the above (4). Further, the presence or absence of streaks when the polarizing plates of Examples and Comparative Examples were applied to an image display device was confirmed. Table 1 shows the results.

よう As is clear from Table 1, the polarizing plates of Comparative Examples 1 and 4 had low optical characteristics, and the polarizing plates of Comparative Examples 1 and 2 showed uneven streaks. Under the production conditions of Comparative Example 3, the laminate was broken during the underwater stretching treatment, and it was not even possible to produce a polarizing film. On the other hand, the polarizing plates of Examples 1 and 2 had excellent optical characteristics, and no streaks were observed.

偏光 The polarizing film of the present invention is suitably used for an image display device.

Reference Signs List 10 polarizing film 20 first protective layer 30 second protective layer 100 polarizing plate

Claims

The thickness is 8 μm or less,
A polarizing film, wherein the average value of the difference between the maximum thickness and the minimum thickness for each 50 mm region from one end to the other end along a direction perpendicular to the absorption axis is 70 nm or less.
(2) The polarizing film according to claim 1, wherein the single film transmittance is 44.5% or more and the degree of polarization is 99.0% or more.
(4) A polarizing plate, comprising: the polarizing film according to claim 1; and a protective layer disposed on at least one side of the polarizing film.
On one side of the thermoplastic resin base material, a polyvinyl alcohol-based resin layer containing a polyvinyl alcohol-based resin is formed into a laminate, and the laminate is subjected to an air stretching process and a dyeing process in this order. Including
The polyvinyl alcohol-based resin layer after the air stretching treatment has a crystallization index calculated by attenuated total reflection spectroscopy of 1.55 or more and 1.7 or less, and an orientation function of 0.22 or more and 0.31 or less. The following is a method for producing a polarizing film.
After the aerial stretching process, further comprising performing an underwater stretching process on the laminate,
The method for producing a polarizing film according to claim 4, wherein the stretching ratio in the air stretching process is 3.0 times or more, and the stretching ratio in the underwater stretching process is 1.8 times or less.