阅读说明：本技术 偏光膜、偏光板以及该偏光膜的制造方法 (Polarizing film, polarizing plate and method for producing the same ) 是由 森崎真由美 后藤周作 岛津亮 于 2020-02-20 设计创作，主要内容包括：提供具有高的单体透过率、且在高温高湿环境下的耐久性优异的偏光膜。本发明的偏光膜由包含碘的聚乙烯醇系树脂薄膜构成,且在至少一个面具有包含钛化合物的层。在一个实施方式中,钛化合物为水溶性的有机钛化合物。本发明的偏光板具有上述的偏光膜、和配置在偏光膜的至少一侧的保护层。(Provided is a polarizing film having a high monomer transmittance and excellent durability in a high-temperature, high-humidity environment. The polarizing film of the present invention is composed of a polyvinyl alcohol resin film containing iodine, and has a layer containing a titanium compound on at least one side. In one embodiment, the titanium compound is a water-soluble organotitanium compound. The polarizing plate of the present invention includes the above polarizing film and a protective layer disposed on at least one side of the polarizing film.)

1. A polarizing film comprising a polyvinyl alcohol resin film containing iodine and having a layer containing a titanium compound on at least one side.

2. The polarizing film according to claim 1, wherein the titanium compound is a water-soluble organic titanium compound.

3. The polarizing film according to claim 2, wherein the water-soluble organotitanium compound is represented by the following formula:

(HO)₂Ti[OCH(CH₃)COOR]₂

wherein R is a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms.

4. The polarizing film according to any one of claims 1 to 3, having a thickness of 8 μm or less.

5. The polarizing film according to any one of claims 1 to 4, having an iodine concentration of 3% by weight or more.

6. A polarizing plate, comprising: the polarizing film according to any one of claims 1 to 5, and a protective layer disposed on at least one side of the polarizing film.

7. A method for producing the polarizing film according to any one of claims 1 to 5, comprising the steps of:

forming a polyvinyl alcohol resin layer on one side of a long thermoplastic resin base material to form a laminate;

stretching and dyeing the laminate to form a polarizing film from the polyvinyl alcohol resin layer; and

an aqueous solution containing a titanium compound at a concentration of 4 to 50 wt% is applied to at least one surface of the polarizing film.

8. The production method according to claim 7, wherein a polyvinyl alcohol resin layer containing an iodide and a polyvinyl alcohol resin is formed on one side of the thermoplastic resin substrate.

9. The production method according to claim 8, comprising a step of subjecting the laminate to an in-air auxiliary stretching treatment, a dyeing treatment, an in-water stretching treatment, and a drying shrinkage treatment in this order, wherein the drying shrinkage treatment is performed by heating while conveying the laminate in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction.

10. The manufacturing method according to claim 9, wherein the drying shrinkage treatment is performed using a heated roller.

11. The manufacturing method according to claim 10, wherein the temperature of the heating roller is 60 to 120 ℃.

Technical Field

The present invention relates to a polarizing film, a polarizing plate, and a method for producing the polarizing film.

Background

In a liquid crystal display device, which is a typical image display device, polarizing films are arranged on both sides of a liquid crystal cell depending on an image forming method. As a method for producing a polarizing film, for example, the following methods are proposed: a polarizing film is obtained on a resin substrate by stretching a laminate having the resin substrate and a polyvinyl alcohol (PVA) -based resin layer, and then performing a dyeing treatment (for example, patent document 1). Since a polarizing film having a small thickness can be obtained by such a method, attention has been paid to a method contributing to thinning of an image display device in recent years. However, a thin polarizing film is required to have further improved durability under a high-temperature and high-humidity environment.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2001-343521

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above-described problems of the conventional techniques, and a main object thereof is to provide a polarizing film having a high monomer transmittance and excellent durability under a high-temperature and high-humidity environment, a polarizing plate, and a method for producing such a polarizing film.

Means for solving the problems

The polarizing film of the present invention is composed of a polyvinyl alcohol resin film containing iodine, and has a layer containing a titanium compound on at least one side.

In one embodiment, the titanium compound is a water-soluble organic titanium compound. In one embodiment, the water-soluble organic titanium compound is represented by the following formula:

(HO)₂Ti[OCH(CH₃)COOR]₂

wherein R is a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms.

In one embodiment, the polarizing film has a thickness of 8 μm or less.

In one embodiment, the iodine concentration of the polarizing film is 3% by weight or more.

According to another aspect of the present invention, a polarizing plate is provided. The polarizing plate comprises the above polarizing film and a protective layer disposed on at least one side of the polarizing film.

According to another aspect of the present invention, there is provided a method for producing the polarizing film described above. The method comprises the following steps: forming a polyvinyl alcohol resin layer on one side of a long thermoplastic resin base material to form a laminate; stretching and dyeing the laminate to form a polarizing film from the polyvinyl alcohol resin layer; and applying an aqueous solution containing a titanium compound at a concentration of 4 to 50 wt% to at least one surface of the polarizing film.

In one embodiment, the method of manufacturing a thermoplastic resin substrate includes forming a polyvinyl alcohol resin layer containing an iodide and a polyvinyl alcohol resin on one side of the thermoplastic resin substrate.

In one embodiment, the method includes the step of sequentially performing an in-air auxiliary stretching treatment, a dyeing treatment, an in-water stretching treatment, and a drying shrinkage treatment for shrinking the laminate in the width direction by 2% or more by heating while conveying the laminate in the longitudinal direction.

In one embodiment, the drying shrinkage treatment is performed using a heated roller. In this case, the temperature of the heating roller is, for example, 60 to 120 ℃.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a thin polarizing film having a high monomer transmittance and excellent durability in a high-temperature and high-humidity environment can be obtained. Such a thin polarizing film can be obtained by, for example, applying an aqueous solution containing a titanium compound at a predetermined concentration to the thin polarizing film.

Drawings

Fig. 1 is a schematic cross-sectional view of a polarizing plate according to 1 embodiment of the present invention.

Fig. 2 is a schematic diagram showing an example of the drying shrinkage process using a heating roller.

Detailed Description

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

A. Polarizing film

The polarizing film according to the embodiment of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing iodine, and has a layer containing a titanium compound on at least one surface. The layer is typically a coating layer. The coating layer may further include a water-soluble resin. By providing such a coating layer, the durability of the thin polarizing film in a high-temperature and high-humidity environment can be improved. This is presumably because the titanium compound functions as a crosslinking agent and crosslinks with PVA. Typically, such a coating layer can reduce Δ P described later. The effect of the coating layer is remarkable in a thin polarizing film. In the case where the thin polarizing film has a higher iodine concentration than the thick polarizing film, the stability of iodine is insufficient, and the humidification durability tends to be insufficient, the humidification durability can be significantly improved by the coating layer.

As the titanium compound, any suitable titanium compound can be used as long as it functions as a crosslinking agent for the water-soluble resin. Water-soluble organic titanium compounds are preferred. The water-soluble organic titanium compound is represented by, for example, the following formula (1):

(HO)₂Ti[OCH(CH₃)COOR]₂···(1)

wherein R is a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms, preferably a hydrogen atom. The water-soluble organic titanium compound may be a commercially available compound. Specific examples of commercially available products include product names "ORGATIX TC-315" and "ORGATIX TC-310" manufactured by Matsumoto Fine Chemical Co.Ltd.

As the water-soluble resin, any suitable water-soluble resin can be used as long as an aqueous coating layer-forming composition (aqueous coating layer solution) can be formed. Typical examples thereof include PVA resins and water-soluble acrylic resins. The PVA-based resin is preferred. The PVA-based resin has excellent adhesion to a polarizing film, is easy to form an aqueous solution having excellent handling properties, and can impart appropriate mechanical strength to the obtained coating layer. As the PVA-based resin, any suitable PVA-based resin can be used. Examples of the PVA-based resin include those described below in section C-1-2 relating to the method for producing a polarizing film.

The coating layer is formed by applying a coating layer-forming composition (aqueous solution for coating layer) containing a titanium compound, an aqueous medium, and if necessary, a water-soluble resin to a polarizing film and drying the coating layer. The concentration of the titanium compound and the water-soluble resin (when present) in the aqueous solution can be set in consideration of workability (coatability). The concentration can be adjusted so that the viscosity of the aqueous solution becomes, for example, 1 mPasec to 300 mPasec. The concentration of the titanium compound in the aqueous solution may be, for example, 4 to 50% by weight. When the water-soluble resin is present, the ratio of the water-soluble resin to the titanium compound in the aqueous solution (and as a result, the coating layer) is, for example, 1 to 200 parts by weight, preferably 5 to 200 parts by weight, and more preferably 10 to 100 parts by weight, based on 100 parts by weight of the water-soluble resin.

The thickness of the coating layer is preferably 10nm to 1000nm, more preferably 50nm to 800nm, and still more preferably 80nm to 500 nm. By using the coating layer in the specific thin polarizing film described in the present specification, the durability of the polarizing film under a high-temperature and high-humidity environment can be improved with a thickness much thinner than that of the conventional polarizing film. When the coating layer is too thin, the effect of the titanium compound may be insufficient. When the coating layer is too thick, drying may be difficult (as a result, formation of the layer itself may be difficult), and the coating layer may be of poor practicability.

A general structure of the coating layer is described in, for example, Japanese patent application laid-open No. 2008-257025. The description of this publication is incorporated herein by reference.

The polarizing film is composed of a PVA-based resin film containing iodine as described above. Preferably, the PVA-based resin constituting the PVA-based resin film (substantially a polarizing film) contains a PVA-based resin modified with an acetoacetyl group. With such a configuration, a polarizing film having a desired mechanical strength can be obtained. The amount of the acetoacetyl group-modified PVA resin is preferably 5 to 20 wt%, more preferably 8 to 12 wt%, based on 100 wt% of the entire PVA resin. When the blending amount is in such a range, a polarizing film having more excellent mechanical strength can be obtained.

The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, further preferably 5 μm or less, and particularly preferably 3 μm or less. The lower limit of the thickness of the polarizing film may be 1 μm in 1 embodiment, and may be 2 μm in another embodiment.

The iodine concentration in the polarizing film is preferably 3 wt% or more, more preferably 4 wt% to 10 wt%, and more preferably 4 wt% to 8 wt%. In the present specification, the "iodine concentration" refers to the amount of all iodine contained in the polarizing film. More specifically, iodine is represented by I in the polarizing film^-、I₂、I₃ ^-、PVA/I^3-Complex, PVA/I^5-When the complex or other form exists, the iodine concentration in the present specification means a concentration including all of the forms of iodine. The iodine concentration can be calculated from the fluorescent X-ray intensity and the film (polarizing film) thickness based on fluorescent X-ray analysis, for example.

The polarizing film preferably has a monomer transmittance of 42.0% or more, more preferably 42.5% or more, still more preferably 43.5% or more, and particularly preferably 45.0% or more. On the other hand, the monomer transmittance is preferably 48.0% or less, more preferably 46.0% or less. In the case where a thin polarizing film having a high monomer transmittance is sometimes reduced in durability under a high-temperature and high-humidity environment, according to the embodiment of the present invention, even when the thin polarizing film has such a high monomer transmittance, excellent durability under a high-temperature and high-humidity environment can be achieved. In the present specification, the simple descriptions of the monomer transmittance, the orthogonal transmittance and the polarization degree refer to the monomer transmittance, the orthogonal transmittance and the polarization degree before the durability test. The polarization degree of the polarizing film is preferably 99.95% or more, more preferably 99.99% or more. On the other hand, the degree of polarization is preferably 99.998% or less. According to the embodiment of the present invention, both high monomer transmittance and high polarization degree can be achieved, and excellent durability under a high-temperature and high-humidity environment can be achieved as described below. The monomer transmittance is typically a Y value measured with an ultraviolet-visible spectrophotometer and corrected for the photosensitivity. The monomer transmittance is a value obtained by converting the refractive index of one surface of the polarizing plate to 1.50 and the refractive index of the other surface to 1.53. The degree of polarization is typically determined based on the parallel transmittance Tp and the orthogonal transmittance Tc measured with an ultraviolet-visible spectrophotometer and corrected for the degree of visibility, and is calculated by the following formula.

Degree of polarization (%) { (Tp-Tc)/(Tp + Tc) }^1/2×100

In the embodiment of the present invention, the change amount Δ P of the polarization degree after the durability test at 60 ℃ and 95% relative humidity for 240 hours is-0.04% or more. Δ P is represented by the following formula.

ΔP＝P₂₄₀-P₀

In the above formula, P₂₄₀Degree of polarization after endurance test, P₀The polarization degree before the endurance test (the polarization degree explained above). That is, the polarizing film of the embodiment of the present invention has a small decrease in the degree of polarization under a high-temperature and high-humidity environment.

The polarizing film may be made of a single resin film or a laminate of two or more layers. Specific examples of the polarizing film obtained using the laminate include a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer applied to the resin substrate. A polarizing film obtained by using a laminate of a resin substrate and a PVA-based resin layer formed on the resin substrate by coating can be produced, for example, as follows: coating a PVA-based resin solution on a resin base material and drying the coating to form a PVA-based resin layer on the resin base material, thereby obtaining a laminate of the resin base material and the PVA-based resin layer; the laminate is stretched and dyed to form a polarizing film from the PVA-based resin layer. In an embodiment of the present invention, an aqueous solution containing a titanium compound at a concentration of 4 to 50 wt% is applied to at least one surface of a polarizing film. This can realize excellent durability under a high-temperature and high-humidity environment as described above. Preferably, a polyvinyl alcohol resin layer containing a halide and a polyvinyl alcohol resin is formed on one side of the resin substrate. The stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching. Further, the stretching may include, if necessary, in-air stretching of the laminate at a high temperature (for example, 95 ℃ or higher) before stretching in the aqueous boric acid solution. Furthermore, in the present embodiment, the laminate is preferably subjected to a drying shrinkage treatment in which the laminate is heated while being conveyed in the longitudinal direction to shrink the laminate in the width direction by 2% or more. Typically, the production method of the present embodiment includes a step of sequentially performing an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment on the laminate. By introducing the auxiliary stretching, even when the PVA is coated on the thermoplastic resin, the crystallinity of the PVA can be improved, and high optical properties can be achieved. Further, by simultaneously improving the orientation of the PVA in advance, it is possible to prevent problems such as reduction in orientation and dissolution of the PVA when the PVA is immersed in water in the subsequent dyeing step and stretching step, and to achieve high optical characteristics. Further, when the PVA-based resin layer is immersed in a liquid, disorder of the orientation of the polyvinyl alcohol molecules and reduction of the orientation can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This makes it possible to improve the optical properties of the polarizing film obtained through a treatment step of immersing the laminate in a liquid, such as dyeing treatment and underwater stretching treatment. Further, the optical properties can be improved by shrinking the laminate in the width direction by the drying shrinkage treatment. The obtained resin substrate/polarizing film laminate may be used as it is (that is, the resin substrate may be used as a protective layer for the polarizing film), or the resin substrate may be peeled from the resin substrate/polarizing film laminate and an arbitrary appropriate protective layer suitable for the purpose may be laminated on the peeled surface. Details of the method for producing the polarizing film will be described later in item C.

B. Polarizing plate

Fig. 1 is a schematic cross-sectional view of a polarizing plate according to 1 embodiment of the present invention. The polarizing plate 100 includes: the liquid crystal display device includes a polarizing film 10, a 1 st protective layer 20 disposed on one side of the polarizing film 10, and a 2 nd 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 item a. One of the 1 st protective layer 20 and the 2 nd protective layer 30 may also be omitted. In the case where the coating layer is formed on the polarizing film, it is typical that the protective layer on the coating layer side may be omitted. As described above, one of the 1 st protective layer and the 2 nd protective layer may be a resin substrate used for producing the polarizing film.

The 1 st and 2 nd protective layers are formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of the material of the main component of the film include cellulose resins such as Triacetylcellulose (TAC), polyester resins, polyvinyl alcohol resins, polycarbonate resins, polyamide resins, polyimide resins, polyether sulfone resins, polysulfone resins, polystyrene resins, polynorbornene resins, polyolefin resins, (meth) acrylic resins, acetate resins, and the like transparent resins. Further, there may be mentioned thermosetting resins such as (meth) acrylic, urethane, (meth) acrylic urethane, epoxy, silicone and the like, ultraviolet-curable resins and the like. Further, for example, a glassy polymer such as a siloxane polymer can be cited. Further, the polymer film described in Japanese patent application laid-open No. 2001-343529 (WO01/37007) 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 can be used, and for example, a resin composition having an alternating copolymer of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer can be mentioned. The polymer film may be, for example, an extrusion molded product of the resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the 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 5 μm to 80 μm, and still 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. In 1 embodiment, the inner protective layer is a retardation layer having any suitable phase difference value. In this case, the in-plane retardation Re (550) of the retardation layer is, for example, 110nm to 150 nm. "Re (550)" is an in-plane retardation measured at 23 ℃ with light having a wavelength of 550nm, represented by the formula: re ═ x-ny) × d. Here, "nx" is a refractive index in a direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), "ny" is a refractive index in a direction orthogonal to the slow axis in the plane (i.e., the fast axis direction), "nz" is a refractive index in the thickness direction, and "d" is the thickness (nm) of the layer (film).

C. Method for producing polarizing film

The method for manufacturing a polarizing film of 1 embodiment of the present invention includes the steps of: coating a PVA-based resin solution on one side of a long thermoplastic resin base material and drying the PVA-based resin solution to form a PVA-based resin layer, thereby producing a laminate; stretching and dyeing the laminate to form a polarizing film from the PVA resin layer; and applying an aqueous solution containing a titanium compound at a concentration of 4 to 50 wt% to at least one surface of the polarizing film (the aqueous solution for the coating layer described in the above item a). By applying the aqueous solution (typically, by forming a coating layer), a polarizing film having excellent durability under a high-temperature and high-humidity environment can be obtained. Preferably, the PVA-based resin solution further contains a halide. Preferably, the production method includes a step of sequentially subjecting the laminate to an in-air auxiliary stretching treatment, a dyeing treatment, an in-water stretching treatment, and a drying shrinkage treatment of shrinking the laminate by 2% or more in the width direction by heating while conveying the laminate in the longitudinal direction. The content of the halide in the PVA-based resin solution (as a result, the PVA-based resin layer) is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably carried out using a heated roller, and the temperature of the heated roller is preferably 60 to 120 ℃. The shrinkage in the width direction of the laminate by the drying shrinkage treatment is preferably 2% or more. According to such a production method, the polarizing film described in the above item a can be obtained. In particular, a polarizing film having excellent optical characteristics (typically, monomer transmittance and unit absorbance) can be obtained by producing a laminate having a PVA-based resin layer containing a halide, stretching the laminate in multiple stages including air-assisted stretching and underwater stretching, and heating the stretched laminate with a heating roller.

Preparation of C-1. laminate

As a method for producing a laminate of the thermoplastic resin substrate and the PVA-based resin layer, any appropriate method can be adopted. Preferably, a PVA-based resin layer is formed on the thermoplastic resin substrate by applying a coating solution containing a halide and a PVA-based resin to the surface of the thermoplastic resin substrate and drying the coating solution. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin.

As a method for applying the coating liquid, any appropriate method can be adopted. Examples of the coating method include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating). The coating/drying temperature of the coating liquid is preferably 50 ℃ or higher.

The thickness of the PVA resin layer is preferably 3 to 40 μm, more preferably 3 to 20 μm.

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

C-1-1. thermoplastic resin base Material

As the thermoplastic resin substrate, any suitable thermoplastic resin film can be used. Details of the thermoplastic resin base material are described in, for example, japanese patent laid-open No. 2012-73580. The entire disclosure of this publication is incorporated herein by reference.

C-1-2 coating liquid

The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid is typically a solution obtained by dissolving the halide and the PVA-based resin in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These may be used alone or in combination of two or more. Among these, water is preferable. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight based on 100 parts by weight of the solvent. At such a resin concentration, a uniform coating film can be formed in close contact with the thermoplastic resin substrate. The content of the halide in the coating liquid is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin.

Additives may be compounded in the coating liquid. 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 nonionic surfactants. These can be used for the purpose of further improving the uniformity, dyeability and stretchability of the PVA-based resin layer obtained.

As the PVA-based resin, any suitable resin can be used. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer are listed. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying an ethylene-vinyl acetate copolymer. The saponification degree of the PVA resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The degree of saponification can be determined in accordance with JIS K6726-. By using the 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.

The average polymerization degree of the PVA-based resin can be appropriately selected according to the purpose. The average polymerization degree is usually 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. The average degree of polymerization can be determined in accordance with JIS K6726-.

As the halide, any suitable halide can be used. For example, iodide and sodium chloride are mentioned. Examples of the iodide include potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferable.

The amount of the halide in the coating liquid is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin, and more preferably 10 to 15 parts by weight based on 100 parts by weight of the PVA-based resin. If the amount of the halide exceeds 20 parts by weight based on 100 parts by weight of the PVA-based resin, the halide may bleed out, and the polarizing film finally obtained may be opaque.

In general, the orientation of polyvinyl alcohol molecules in a PVA-based resin is increased by stretching the PVA-based resin layer, but when the stretched PVA-based resin layer is immersed in a liquid containing water, the orientation of polyvinyl alcohol molecules may be disturbed and the orientation may be decreased. In particular, when a laminate of a thermoplastic resin and a PVA-based resin layer is stretched in boric acid water, the orientation degree tends to be significantly reduced when the laminate is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin. For example, stretching of a PVA film itself in boric acid water is generally performed at 60 ℃, while stretching of a laminate of a-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a high temperature of about 70 ℃, and in this case, the orientation of PVA at the initial stage of stretching is reduced at a stage before it rises due to underwater stretching. In contrast, by producing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate and stretching the laminate at a high temperature in air (auxiliary stretching) before stretching the laminate in boric acid water, crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after the auxiliary stretching can be promoted. As a result, in the case where the PVA-based resin layer is immersed in a liquid, disturbance of the orientation of the polyvinyl alcohol molecules and reduction of the orientation can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This makes it possible to improve the optical properties of the polarizing film obtained through a treatment step of immersing the laminate in a liquid, such as dyeing treatment and underwater stretching treatment.

C-2 auxiliary stretching treatment in air

In particular, in order to obtain high optical properties, a 2-stage stretching method combining dry stretching (auxiliary stretching) and boric acid underwater stretching is selected. By introducing the auxiliary stretching as in the 2-stage stretching, the thermoplastic resin substrate can be stretched while suppressing crystallization, and the laminate can be stretched to a higher magnification while solving the problem of the reduction in stretchability due to excessive crystallization of the thermoplastic resin substrate in the subsequent boric acid underwater stretching. Further, when a PVA-based resin is coated on a thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, the coating temperature needs to be lowered as compared with the case where the PVA-based resin is usually coated on a metal roll, and as a result, there is a problem that crystallization of the PVA-based resin is relatively lowered and sufficient optical characteristics cannot be obtained. In contrast, by introducing the auxiliary stretching, even when the PVA-based resin is applied to the thermoplastic resin, the crystallinity of the PVA-based resin can be improved, and high optical characteristics can be achieved. Further, by simultaneously improving the orientation of the PVA-based resin in advance, when the PVA-based resin is immersed in water in the subsequent dyeing step or stretching step, problems such as reduction in the orientation and dissolution of the PVA-based resin can be prevented, and high optical properties can be achieved.

The stretching method of the in-air auxiliary stretching may be fixed-end stretching (for example, a method of stretching using a tenter) or free-end stretching (for example, a method of uniaxially stretching a laminate by passing the laminate between rolls having different peripheral speeds), and the free-end stretching is actively employed for obtaining high optical characteristics. In 1 embodiment, the in-flight stretching treatment includes a heated roll stretching step of conveying the laminate in the longitudinal direction thereofThe stretching is performed by the difference in peripheral speed between the heated rolls. The in-air stretching process typically includes a zone stretching process and a heated roll stretching process. The order of the area stretching step and the heating roller stretching step is not limited, and the area stretching step may be performed first or the heating roller stretching step may be performed first. The zone stretching process may be omitted. In 1 embodiment, the zone stretching step and the heating roller stretching step are performed in this order. In another embodiment, stretching is performed in a tenter stretching machine by holding the film end and extending the distance between the tenters in the flow direction (the extension of the distance between the tenters is the stretching magnification). At this time, the distance of the tenter in the width direction (the direction perpendicular to the flow direction) is set so as to be arbitrarily close to each other. The draw ratio in the flow direction can be preferably set so as to draw closer to the free end. In the case of free end stretching, the shrinkage in the width direction (1/stretching ratio)^1/2To calculate.

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

The stretching ratio in the air-assisted stretching is preferably 2.0 to 3.5 times. The maximum stretching ratio in the combination of the in-air auxiliary stretching and the underwater stretching is preferably 5.0 times or more, more preferably 5.5 times or more, and further preferably 6.0 times or more, with respect to the original length of the laminate. In the present specification, the "maximum stretching ratio" refers to the stretching ratio immediately before the laminate breaks, and is a value lower by 0.2 than the stretching ratio at which the laminate breaks.

The stretching temperature of the in-air auxiliary stretching may be set to any appropriate value depending on the material for forming the thermoplastic resin base material, the stretching method, and the like. The stretching temperature is preferably not less than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably not less than the glass transition temperature (Tg) +10 ℃ of the thermoplastic resin substrate, and particularly preferably not less than Tg +15 ℃. On the other hand, the upper limit of the stretching temperature is preferably 170 ℃. By stretching at such a temperature, rapid progress of crystallization of the PVA-based resin can be suppressed, and defects caused by the crystallization (for example, inhibition of orientation of the PVA-based resin layer due to stretching) can be suppressed.

C-3 insolubilization treatment, dyeing treatment and crosslinking treatment

If necessary, after the in-air auxiliary stretching treatment, an insolubilization treatment is performed between the stretching treatment in water and the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The dyeing treatment is typically performed by dyeing the PVA-based resin layer with a dichroic substance (typically, iodine). If necessary, a crosslinking treatment is performed after the dyeing treatment and before the stretching treatment in water. The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Details of the insolubilization treatment, the dyeing treatment and the crosslinking treatment are described in, for example, japanese patent laid-open No. 2012-73580 (described above).

C-4 stretching treatment in water

The underwater stretching treatment is performed by immersing the laminate in a stretching bath. By the underwater stretching treatment, the stretching can be performed at a temperature lower than the glass transition temperature (typically, about 80 ℃) of the thermoplastic resin substrate or the PVA-based resin layer, and the PVA-based resin layer can be stretched to a high magnification while suppressing crystallization thereof. As a result, a polarizing film having excellent optical characteristics can be produced.

Any suitable method may be used for stretching the laminate. Specifically, the stretching may be performed at a fixed end or at a free end (for example, a method of passing the laminate between rollers having different peripheral speeds to perform uniaxial stretching). It is preferred to select free end stretching. The stretching of the laminate may be performed in one stage or may be performed in multiple stages. In the case of performing the multi-stage process, the stretching ratio (maximum stretching ratio) of the laminate described later is the product of the stretching ratios of the respective stages.

The underwater stretching is preferably performed by immersing the laminate in an aqueous boric acid solution (boric acid underwater stretching). By using the aqueous boric acid solution as the stretching bath, rigidity that resists the tension applied during stretching and water resistance that does not dissolve in water can be imparted to the PVA-based resin layer. Specifically, boric acid generates tetrahydroxyborate anions in an aqueous solution and crosslinks with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin layer can be provided with rigidity and water resistance and can be stretched well, and a polarizing film having excellent optical characteristics can be produced.

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

Preferably, the stretching bath (aqueous boric acid solution) is mixed with an iodide. By adding an iodide, elution of iodine adsorbed to the PVA-based resin layer can be suppressed. Specific examples of the iodide are as described above. The concentration of the 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 drawing temperature (liquid temperature of the drawing bath) is preferably 40 to 85 ℃ and more preferably 60 to 75 ℃. At such a temperature, the PVA-based resin layer can be stretched to a high magnification while dissolution thereof is suppressed. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60 ℃ or higher from the viewpoint of the relationship with the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40 ℃, there is a possibility that the thermoplastic resin substrate cannot be satisfactorily stretched even when plasticization of the thermoplastic resin substrate by water is considered. On the other hand, as the temperature of the stretching bath is higher, the solubility of the PVA-based resin layer becomes higher, and there is a concern that excellent optical characteristics cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio by underwater stretching is preferably 1.5 times or more, more preferably 3.0 times or more. The total stretch ratio of the laminate is preferably 5.0 times or more, and more preferably 5.5 times or more, based on the original length of the laminate. By achieving such a high stretching ratio, a polarizing film having extremely excellent optical properties can be produced. Such a high stretch ratio can be achieved by using an underwater stretching method (boric acid underwater stretching).

C-5 drying shrinkage treatment

The drying shrinkage treatment may be performed by a zone heating method in which the entire zone is heated, or may be performed by heating a transport roller (using a so-called hot roller) (a hot roller drying method). Both are preferably used. By drying with a heating roller, the heating curl of the laminate can be efficiently suppressed, and a polarizing film having excellent appearance can be produced. Specifically, by drying the laminate in a state of being along the heating roller, the crystallization of the thermoplastic resin substrate can be efficiently promoted to increase the crystallinity, and the crystallinity of the thermoplastic resin substrate can be favorably increased even at a low drying temperature. As a result, the thermoplastic resin substrate has increased rigidity and is able to withstand shrinkage of the PVA-based resin layer due to drying, and curling is suppressed. Further, since the laminate can be dried while maintaining a flat state by using the heating roller, not only curling but also wrinkles can be suppressed. In this case, the laminate is shrunk in the width direction by the drying shrinkage treatment, whereby the optical properties can be improved. This is because the orientation of PVA and PVA/iodine complex can be effectively improved. The shrinkage in the width direction of the laminate by the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%.

Fig. 2 is a schematic diagram showing an example of the drying shrinkage treatment. In the drying shrinkage process, the stacked body 200 is dried while being conveyed by the conveying rollers R1 to R6 and the guide rollers G1 to G4 which are heated to a predetermined temperature. In the illustrated example, the conveying rollers R1 to R6 are disposed so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but for example, the conveying rollers R1 to R6 may be disposed so as to continuously heat only one surface (for example, the surface of the thermoplastic resin substrate) of the laminate 200.

The drying condition can be controlled by adjusting the heating temperature of the conveying roller (temperature of the heating roller), the number of heating rollers, the contact time with the heating roller, and the like. The temperature of the heating roller is preferably 60 to 120 ℃, more preferably 65 to 100 ℃, and particularly preferably 70 to 80 ℃. An optical laminate which can satisfactorily suppress curling by increasing the crystallinity of a thermoplastic resin and has extremely excellent durability can be produced. The temperature of the heating roller may be measured by a contact thermometer. In the example of the figure, 6 conveying rollers are provided, but there is no particular limitation as long as there are a plurality of conveying rollers. The number of the conveying rollers is usually 2 to 40, preferably 4 to 30. The contact time (total contact time) between the laminate and the heating roller is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, and still more preferably 1 to 10 seconds.

The heating roller may be disposed in a heating furnace (e.g., an oven) or may be disposed in a general production line (room temperature environment). Preferably, the heating furnace is provided with an air blowing means. By using drying by the heating roller and hot air drying in combination, a rapid temperature change between the heating rollers can be suppressed, and the shrinkage in the width direction can be easily controlled. The temperature of the hot air drying is preferably 30 to 100 ℃. The hot air drying time is preferably 1 to 300 seconds. The wind speed of the hot wind is preferably about 10m/s to 30 m/s. The wind speed is the wind speed in the heating furnace and can be measured by a digital wind speed meter of a miniature blade type.

C-6. other treatment

It is preferable to perform the washing treatment after the stretching treatment in water and before the drying shrinkage treatment. The cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

C-7. formation of coating layer

In this manner, a laminate (polarizing plate) of the thermoplastic resin substrate and the polarizing film can be obtained. In an embodiment of the present invention, an aqueous solution containing a titanium compound at a concentration of 4 to 50 wt% (the aqueous solution for the coating layer described in the above item a) is applied to at least one surface of the polarizing film. In one embodiment, the aqueous solution is applied to the polarizing film surface of the laminate. As a result, a laminate (polarizing plate) of the thermoplastic resin substrate/polarizing film/coating layer can be obtained. In this case, typically, the thermoplastic resin substrate can be used as a protective layer for the polarizing film as it is. In another embodiment, a resin film (to be a protective layer) is laminated on the polarizing film surface of the laminate to produce a protective layer/polarizing film/thermoplastic resin substrate laminate, and the thermoplastic resin substrate is peeled from the laminate to produce a protective layer/polarizing film laminate (polarizing plate). The aqueous solution was applied to the surface of the polarizing film of the obtained polarizing plate. As a result, a protective layer/polarizing film/coating layer laminate (polarizing plate) can be obtained.

As the method of applying the aqueous solution, any suitable method can be adopted. Specific examples thereof include the method described in the section C-1 as a method of applying a coating liquid for forming a PVA-based resin layer (polarizing film). The coating layer is formed by drying the coated aqueous solution. The drying temperature is, for example, 40 to 100 ℃ and the drying time is, for example, 1 to 20 minutes.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement method of each property is as follows. Unless otherwise specified, "parts" and "%" in examples and comparative examples are based on weight.

(1) Thickness of

The polarizing plates (protective layer/polarizing film/coating layer) of examples and comparative examples were cut, and the cross section of the polarizing plate was observed using a scanning electron microscope ("JSM 7100F", manufactured by japan electronics corporation), and the thickness of the coating layer was measured. The thickness of the polarizing film was measured by using an interference film thickness meter (manufactured by Otsuka electronics Co., Ltd., product name "MCPD-3000").

(2) Single body transmittance, orthogonal transmittance and degree of polarization

The polarizing plates (protective layer/polarizing film/coating layer) of examples and comparative examples were respectively designated as the polarizing film Ts, Tp and Tc, respectively, for the monomer transmittance Ts, the parallel transmittance Tp and the cross transmittance Tc measured by an ultraviolet-visible spectrophotometer (LPF 200, manufactured by ottaka electronics). These Ts, Tp and Tc are Y values measured with a 2-degree field of view (C light source) according to JIS Z8701 and corrected for visibility. The degree of polarization was determined from the Tp and Tc obtained by the following equation.

Degree of polarization (%) { (Tp-Tc)/(Tp + Tc) }^1/2×100

(3) Optical durability

Glass (alkali-free glass) from which alkali components were removed was bonded to the polarizing film side of the polarizing plates of examples and comparative examples with an adhesive as a test sample. The test specimens were subjected to a durability test at a temperature of 60 ℃ and a relative humidity of 95% for 240 hours. The optical properties before and after the durability test were measured by the spectrophotometer described in (2) above, and Δ P was determined from the following equation.

ΔP＝P₂₄₀-P₀

In the above formula, P₂₄₀Degree of polarization after endurance test, P₀The degree of polarization before the endurance test.

[ example 1-1]

As the thermoplastic resin substrate, an amorphous ethylene terephthalate isophthalate copolymer film (thickness: 100 μm) having a long shape and a Tg of about 75 ℃ was used. One surface of the resin substrate is subjected to corona treatment.

In the following, with 9: 1 an aqueous PVA solution (coating solution) was prepared by adding 13 parts by weight of potassium iodide to 100 parts by weight of a PVA resin obtained by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl group-modified PVA (product name "GOHSEFIMER Z410" manufactured by Nippon synthetic chemical industries, Ltd.).

The above aqueous PVA solution was applied to the corona-treated surface of the resin substrate and dried at 60 ℃ to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.

The obtained laminate was uniaxially stretched 2.4 times in the longitudinal direction (longitudinal direction) at the free end between rolls having different peripheral speeds in an oven at 130 ℃ (air-assisted stretching treatment).

Next, the laminate was immersed in an insolubilization bath (an aqueous boric acid solution prepared by adding 4 parts by weight of boric acid to 100 parts by weight of water) at a liquid temperature of 40 ℃ for 30 seconds (insolubilization treatment).

Next, the polarizing film finally obtained was immersed in a dyeing bath (aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1: 7 with respect to 100 parts by weight of water) at a liquid temperature of 30 ℃ for 60 seconds while adjusting the concentration thereof so that the monomer transmittance (Ts) of the polarizing film finally obtained was 43.8% (dyeing treatment).

Next, the substrate was immersed for 30 seconds in a crosslinking bath (an aqueous boric acid solution prepared by adding 3 parts by weight of potassium iodide to 100 parts by weight of water and 5 parts by weight of boric acid) at a liquid temperature of 40 ℃ (crosslinking treatment).

Thereafter, the laminate was uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds while being immersed in an aqueous boric acid solution (boric acid concentration 4.0 wt%, potassium iodide concentration 5.0 wt%) having a liquid temperature of 70 ℃ until the total stretching ratio became 5.5 times (underwater stretching treatment).

Thereafter, the laminate was immersed in a cleaning bath (aqueous solution prepared by adding 4 parts by weight of potassium iodide to 100 parts by weight of water) at a liquid temperature of 20 ℃ (cleaning treatment).

Thereafter, the sheet was brought into contact with a heated roll made of SUS having a surface temperature of 75 ℃ for about 2 seconds while being dried in an oven maintained at 90 ℃ (drying shrinkage treatment). The shrinkage in the width direction of the laminate by the drying shrinkage treatment was 2%.

Thus, a polarizing film having a thickness of 5.0 μm was formed on the resin substrate. A cycloolefin Film (product name "G-Film" manufactured by ZEON corporation) as a protective layer (protective Film) was bonded to the surface of the polarizing Film with a UV curable adhesive (thickness 1.0 μm), and then the resin substrate was peeled off to obtain a polarizing plate having a protective layer/polarizing Film structure.

The surface of the polarizing film of the polarizing plate thus obtained was coated with an aqueous solution for a coating layer. The aqueous solution contained 4% by weight of a titanium compound (product name "ORGATIX TC-315" manufactured by Songbo pharmaceutical industries, Ltd.). The aqueous coating film was dried at 60 ℃ for 5 minutes to form a coating layer having a thickness of 100nm, thereby obtaining a polarizing plate having a protective layer/polarizing film/coating layer structure.

The obtained polarizing plate (substantially, polarizing film) was represented by Δ P in table 1.

[ examples 1-2]

A polarizing plate was produced in the same manner as in example 1-1, except that the titanium compound concentration of the aqueous solution for a coating layer was changed to 44% by weight. The obtained polarizing plate (substantially, polarizing film) was represented by Δ P in table 1.

[ example 2]

A polarizing plate was produced in the same manner as in example 1-1, except that the monomer transmittance was changed to 45%. The obtained polarizing plate (substantially, polarizing film) was represented by Δ P in table 1.

Comparative example 1

A polarizing plate was produced in the same manner as in example 1-1, except that the coating layer was not formed. The obtained polarizing plate (substantially, polarizing film) was represented by Δ P in table 1.

Comparative example 2

A polarizing plate was produced in the same manner as in example 2, except that the coating layer was not formed. The obtained polarizing plate (substantially, polarizing film) was represented by Δ P in table 1.

[ Table 1]

As is clear from table 1, the polarizing film of the example of the present invention is excellent in durability under a high-temperature and high-humidity environment.

Industrial applicability

The polarizing film and the polarizing plate of the present invention are suitably used for a liquid crystal display device.

Description of the reference numerals

10 polarizing film

20 st protective layer

30 nd 2 protective layer

100 polarizing plate