CN114730034A

CN114730034A - Polarizing film, polarizing plate and image display device

Info

Publication number: CN114730034A
Application number: CN202080078496.8A
Authority: CN
Inventors: 南川善则; 黑原薫
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2019-11-11
Filing date: 2020-10-28
Publication date: 2022-07-08
Also published as: WO2021095526A1; JPWO2021095526A1; TW202125002A; KR20220098343A

Abstract

The invention provides a thin polarizing film with suppressed dimensional change while maintaining optical characteristics. The polarizing film of the present invention is composed of a polyvinyl alcohol resin film containing iodine, and has a monomer transmittance of 43.0% or more and an orientation characteristic index (P-98)/F of 7 or more, calculated from a polarization degree P and an orientation function F.

Description

Polarizing film, polarizing plate and image display device

Technical Field

The invention relates to a polarizing film, a polarizing plate and an image display device.

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 in accordance with an image forming method. As a method for producing a polarizing film, for example, the following methods are proposed: a laminate having a resin substrate and a polyvinyl alcohol (PVA) -based resin layer is stretched and then subjected to a dyeing treatment to obtain a polarizing film on the resin substrate (for example, patent document 1). According to this method, a polarizing film having a small thickness can be obtained, and therefore, attention is paid to a polarizing film that can contribute to the reduction in thickness of a display panel such as a liquid crystal panel or an organic EL panel (as a result, the reduction in thickness of an image display device). However, a display panel using a thin polarizing film may be warped due to a change in the size (typically, shrinkage) of the polarizing film.

In order to solve the problem of the warpage, the shrinkage can be alleviated by lowering the stretching ratio in the production of the polarizing film, but the optical characteristics are lowered by lowering the stretching ratio, and therefore, it is difficult to obtain a polarizing film having both dimensional stability and optical characteristics.

Documents of the prior art

Patent document

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

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above conventional problems, and a main object thereof is to provide a thin polarizing film in which dimensional change is suppressed while maintaining optical characteristics.

Means for solving the problems

According to one aspect of the present invention, there is provided a polarizing film comprising a polyvinyl alcohol resin film containing iodine, the polarizing film having a monomer transmittance of 43.0% or more and an orientation characteristic index (P-98)/F of 7 or more as calculated from a polarization degree P and an orientation function F.

In one embodiment, the polarizing film has a monomer transmittance of 44.5% or less.

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

In one embodiment, the polarizing film contains an alcohol.

In one embodiment, the above alcohol comprises at least 1 selected from the group consisting of ethanol and glycerol.

According to another aspect of the present invention, there is provided a polarizing plate comprising the above polarizing film.

According to another aspect of the present invention, there is provided an image display device including the polarizing plate.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a thin polarizing film in which dimensional change is suppressed while maintaining optical characteristics can be obtained.

Drawings

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

Fig. 2 is a schematic diagram showing an example of the drying shrinkage treatment 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 an embodiment of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing iodine, has a monomer transmittance of 43.0% or more, and has an orientation characteristic index (P-98)/F of 7 or more, which is calculated from a polarization degree P and an orientation function F. In a polarizing film made of a PVA-based resin film, generally, the higher the orientation of the PVA-based resin, the higher the polarization degree, and the orientation depends mainly on the stretching ratio of the PVA-based resin film. Therefore, in the conventional polarizing film, if the orientation is relaxed in order to suppress the dimensional change, the polarization degree is lowered. In contrast, the polarizing film according to the embodiment of the present invention can achieve both dimensional stability and optical characteristics by setting the orientation characteristic index (P-98)/F to 7 or more.

The polarizing film has an orientation characteristic index (P-98)/F (where P is the polarization degree (%) of the polarizing film and F is the orientation function of the polarizing film) of 7 or more, preferably 8 or more, more preferably 9 or more, and still more preferably 10 or more. The upper limit of the orientation characteristic index may be, for example, 20. The orientation characteristic index is a value that becomes smaller when the degree of polarization is low and/or the degree of orientation is high. Therefore, an orientation characteristic index of 7 or more means that both a high polarization degree and orientation relaxation can be achieved.

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380nm to 780 nm. The polarizing film has a monomer transmittance of 43.0% or more, preferably 43.5% or more, and more preferably 43.7% or more. On the other hand, the monomer transmittance is preferably 44.5% or less, and more preferably less than 44.5%. The polarization degree of the polarizing film is preferably 99.50% or more, more preferably 99.60% or more, and further preferably 99.70% or more. Typically, the monomer transmittance is a Y value obtained by measuring with an ultraviolet-visible spectrophotometer and correcting the photosensitivity. The single transmittance is a value obtained by converting the refractive index of one surface of the polarizing plate to 1.50 and converting the refractive index of the other surface to 1.53. Typically, the degree of polarization is determined from the parallel transmittance Tp and the orthogonal transmittance Tc obtained by measuring with an ultraviolet-visible spectrophotometer and correcting the visibility, by the following equations.

Polarization degree (%) { (Tp-Tc)/(Tp + Tc) }^1/2×100

The orientation function (F) of the polarizing film is, for example, 0.25 or less, preferably 0.23 or less, and more preferably 0.20 or less. The orientation function (F) may be, for example, 0.1 or more, and may be, for example, 0.15 or more. If the orientation function is too small, a practically acceptable monomer transmittance and/or degree of polarization may not be obtained in some cases.

The orientation function (F) of the polarizing film is determined by measuring the polarized light as measurement light by using, for example, a fourier transform infrared spectrometer (FT-IR) and by total reflection Attenuated Total Reflectance (ATR). Specifically, measurement was performed in a state where the stretching direction of the polarizing film was parallel and perpendicular to the polarization direction of the measurement light, and 2941cm of the obtained absorbance spectrum was used^-1The intensity of (d) is calculated by the following equation. Here, the intensity I is 3330cm^-12941cm as reference peak^-1/3330cm^-1The value of (c). When F is 1, the orientation is complete, and when F is 0, the orientation is randomThe machine is composed of a machine body and a machine frame. Further, it can be considered that: 2941cm^-1The peak of (A) is derived from the main chain (-CH) of PVA in the polarizing film₂-) absorption by vibration.

F＝(3<cos²θ>-1)/2

＝(1-D)/[c(2D+1)]

＝-2×(1-D)/(2D+1)

Wherein,

c＝(3cos²beta-1)/2 and 2941cm^-1In the case of the vibration of (3), β is 90 °.

θ: angle of molecular chain relative to stretching direction

Beta: angle of transition dipole moment relative to molecular chain axis

D＝(I_⊥)/(I_//) (in this case, the more the PVA molecules are oriented, the larger D becomes)

I_⊥: measurement of absorption intensity when the polarization direction of light is perpendicular to the stretching direction of the polarizing film

I_//: measurement of absorption intensity when the polarization direction of light was parallel to the stretching direction of the polarizing film

The polarizing film preferably contains an alcohol. The content ratio of the alcohol in the polarizing film may be, for example, 1.0 × 10^-3Weight percent to 1.0 weight percent. By forming the polarizing film with an alcohol, the orientation characteristic index can be easily set to the desired range, and as a result, a thin polarizing film in which dimensional change is suppressed while maintaining optical characteristics can be suitably obtained. The mechanism for obtaining such an effect is presumed as follows, but the present invention is not limited thereto. That is, a part of the PVA-based resin film constituting the polarizing film is crystallized, and the orientation of the molecular chain is strengthened by the interaction of the hydroxyl group (typically, hydrogen bond) in the crystal portion which has little influence on the optical characteristics, but the interaction is weakened by introducing alcohol into the polarizing film, whereby the orientation around the crystal portion is relaxed or the flexibility is improved, and as a result, the PVA-based resin film is made less elastic while maintaining the optical characteristics, and the dimensional change (shrinkage) of the polarizing film can be suppressed.

The alcohol may be an alcohol having a boiling point of less than 100 ℃ (hereinafter, sometimes referred to as a low-boiling-point alcohol), an alcohol having a boiling point of 100 ℃ or higher (hereinafter, sometimes referred to as a high-boiling-point alcohol), or a combination thereof. The boiling point of the high-boiling alcohol is preferably 150 ℃ or higher, more preferably 180 ℃ or higher, and still more preferably 250 ℃ or higher. The upper limit of the boiling point may be, for example, 310 ℃.

Typical examples of the low boiling point alcohol include lower monohydric alcohols having 1 to 4 carbon atoms. Specific examples thereof include methanol, ethanol, n-propanol, isopropanol and tert-butanol. The low-boiling alcohols may be used alone or in combination of two or more. Methanol, ethanol, n-propanol, and isopropanol are preferred.

The content of the low-boiling alcohol in the polarizing film is, for example, 10ppm to 300ppm, preferably 20ppm to 200ppm, more preferably 40ppm to 150ppm, and still more preferably 50ppm to 120 ppm. If the content ratio is too small, the orientation-relaxing effect may not be obtained. If the content ratio is too large, the amount of introduction at the time of production increases, and therefore the amount of volatilization into the working environment increases, and the risk in terms of safety may increase.

Typical examples of the high boiling point alcohol include higher alcohols, alcohols having a ring structure (for example, aromatic alcohols and alicyclic alcohols), and polyhydric alcohols. Specific examples thereof include glycerin, ethylene glycol, butanol, phenol, and pentanol. The high-boiling alcohols may be used alone or in combination of two or more. Preferred are glycerin and ethylene glycol.

The content of the high-boiling alcohol in the polarizing film is, for example, 0.1 to 1.0 wt%, preferably 0.1 to 0.9 wt%, more preferably 0.1 to 0.8 wt%, still more preferably 0.2 to 0.7 wt%, and particularly preferably 0.2 to 0.6 wt%. If the content ratio is too small, the orientation-relaxing effect may not be obtained. If the content ratio is too large, the reduction in polarization degree in a high-temperature and high-humidity environment may become large.

The thickness of the polarizing film is, for example, 8 μm or less, preferably 7 μm or less, more preferably 5 μm or less, and still more preferably 3 μm or less. The lower limit of the thickness of the polarizing film may be 1 μm in one embodiment, and may be 2 μm in other embodiments.

The shrinkage rate of the polarizing film in the absorption axis direction after heating at a temperature of 85 ℃ for 120 hours is preferably less than 1.10%, more preferably 1.05% or less, still more preferably 1.00% or less, and particularly preferably 0.95% or less. The shrinkage is the shrinkage of the sample having a size of 10cm in the absorption axis direction × 10cm in the transmission axis direction.

B. Polarizing plate

Fig. 1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. The polarizing plate 100 has 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 item a. One of the first protective layer 20 and the second protective layer 30 may be omitted. One of the first protective layer and the second protective layer may be a resin base material for manufacturing the above polarizing film.

The first protective layer and the second protective layer are formed of any appropriate film that can be used as a protective layer of a polarizing film. Specific examples of the material to be the main component of the film include cellulose resins such as Triacetylcellulose (TAC); transparent resins such as polyester, polyvinyl alcohol, polycarbonate, polyamide, polyimide, polyether sulfone, polysulfone, polystyrene, polynorbornene, polyolefin, (meth) acrylic, and acetate 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. In addition, 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) may 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 containing 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 above resin composition.

Typically, when the polarizing plate 100 is applied to an image display device, the protective layer (outer protective layer) disposed on the side opposite to the display panel has a thickness of 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 one embodiment, the inner protective layer is a phase difference layer having an arbitrary and appropriate 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, and is obtained by the formula Re ═ nx-ny × d. Here, "nx" is a refractive index in a direction in which the in-plane refractive index is maximized (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

A method for manufacturing a polarizing film according to an embodiment of the present invention includes: coating a PVA resin solution on one side of a long thermoplastic resin base material and drying the PVA resin solution to form a PVA resin layer to prepare a laminated body; stretching and dyeing the laminate to form a polarizing film from the PVA-based resin layer; and introducing an alcohol into the polarizing film. By introducing the alcohol, a polarizing film can be realized in which the dimensional change is suppressed while maintaining excellent optical characteristics. The PVA-based resin solution preferably further contains a halide. The above-mentioned production method preferably includes subjecting the laminate to an in-air auxiliary stretching treatment, a dyeing treatment, an underwater 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. In this case, the alcohol is preferably introduced between the stretching treatment in water and the drying shrinkage treatment. The PVA-based resin layer is subjected to dyeing treatment to form a complex with iodine, and after orientation and crystallinity are improved by stretching treatment, alcohol is introduced, whereby orientation by interaction of hydroxyl groups around the crystal portion of the PVA-based resin layer can be selectively relaxed or flexibility can be improved. 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 rate in the width direction of the laminate by the drying shrinkage treatment is preferably 2% or more. According to this production method, the polarizing film described in the above item a can be suitably obtained. In particular, a polarizing film having excellent optical characteristics (typically, monomer transmittance and polarization degree) can be obtained by producing a laminate including a halide-containing PVA-based resin layer, stretching the laminate in multiple stages including in-air auxiliary stretching and underwater stretching, and heating the stretched laminate with a heating roll.

Preparation of C-1. laminate

As a method for producing a laminate of the thermoplastic resin substrate and the PVA-based resin layer, any and appropriate method can be adopted. Preferably, the 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 and appropriate method can be adopted. Examples thereof 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 based resin layer is preferably 3 to 40 μm, and 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 and appropriate thermoplastic resin film can be used. Details of the thermoplastic resin substrate 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

As described above, the coating liquid may contain a halide and a PVA-based resin. Typically, the coating liquid is 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. When the resin concentration is such as described above, 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 into 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 and appropriate resin can be used. Examples thereof include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer can be 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 K6726-1994. By using the PVA-based resin having such a saponification degree, a polarizing film having excellent durability can be obtained. If the saponification degree 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 polymerization degree can be determined in accordance with JIS K6726-.

As the halide, any and suitable halide can be used. For example, iodide and sodium chloride may be cited. As the iodide, for example, potassium iodide, sodium iodide and lithium iodide are cited. 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. When the amount of the halide is more than 20 parts by weight based on 100 parts by weight of the PVA-based resin, the halide may bleed out, and the finally obtained polarizing film may become cloudy.

In general, the PVA resin layer is stretched to increase the orientation of polyvinyl alcohol molecules in the PVA resin, but when the stretched PVA resin layer is immersed in a liquid containing water, the orientation of polyvinyl alcohol molecules may be disturbed and the orientation may be reduced. In particular, when a laminate of a thermoplastic resin substrate and a PVA-based resin layer is subjected to boric acid underwater stretching, the degree of orientation 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 substrate. For example, while stretching of a PVA film monomer in boric acid water is generally performed at 60 ℃, 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, orientation of PVA at the beginning of stretching may be reduced in a stage before it is increased by underwater stretching. On the other hand, by preparing 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, when the PVA-based resin layer is immersed in a liquid, the alignment disorder and the decrease in alignment of the polyvinyl alcohol molecules can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This can 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 characteristics, a two-stage stretching method combining dry stretching (auxiliary stretching) and boric acid underwater stretching is selected. By introducing the auxiliary stretching as in the two-stage stretching, the thermoplastic resin base material can be stretched while being inhibited from being crystallized, the problem of the reduction in stretchability due to the excessive crystallization of the thermoplastic resin base material in the subsequent boric acid water stretching can be solved, and the laminate can be stretched to a higher magnification. Further, in the case of coating a PVA-based resin 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 of coating a PVA-based resin on a general metal drum, 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. On the other hand, even when the PVA-based resin is applied to the thermoplastic resin substrate by introducing the auxiliary stretching, the crystallinity of the PVA-based resin can be improved, and high optical characteristics can be realized. Further, by improving the orientation of the PVA-based resin in advance, it is possible to prevent problems such as a decrease in the orientation and dissolution of the PVA-based resin when the PVA-based resin is immersed in water in a subsequent dyeing step and stretching step, and to realize high optical characteristics.

The stretching method of the aerial auxiliary stretching may be fixed-end stretching (for example, a method of stretching using a tenter stretcher) or free-end stretching (for example, a method of passing the laminate between rolls having different peripheral speeds to perform uniaxial stretching), and the free-end stretching is actively employed in order to obtain high optical characteristics. In one embodiment, the aerial stretch process comprises: and a heating roller stretching step of stretching the laminate by a circumferential speed difference between heating rollers while conveying the laminate in the longitudinal direction thereof. Typically, the in-air stretching process 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 one embodiment, the zone stretching step and the heated roller stretching step are performed in this order. In another embodiment, stretching is performed by gripping the film end portions in tenter stretching machines and widening the distance between the tenters in the moving direction (widening of the distance between the tenters becomes the stretching magnification). At this time, the distance of the tenter in the width direction (direction perpendicular to the moving direction) is set to be arbitrarily close. The stretch ratio in the moving direction can be preferably set so as to be closer to the free end stretch. In the case of free end stretching, the shrinkage in the width direction (1/stretching ratio)^1/2To calculate.

The aerial auxiliary stretching may be performed in one stage or may be performed in a plurality of stages. When the stretching is performed in a plurality of stages, the stretching ratio is the product of the stretching ratios of 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 case of combining the in-air auxiliary stretching and the underwater stretching is preferably 5.0 times or more, more preferably 5.5 times or more, and still more 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 refers to the stretching ratio at which the laminate is separately observed to break, and is a value smaller than this value by 0.2.

The stretching temperature of the in-air auxiliary stretching may be set to an arbitrary and 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 ℃, 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 by stretching) can be suppressed.

C-3 insolubilization treatment, dyeing treatment and crosslinking treatment

If necessary, after the in-air auxiliary stretching treatment and before the underwater stretching treatment and dyeing treatment, insolubilization treatment is performed. Typically, the insolubilization treatment is performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Typically, the dyeing treatment is performed by dyeing the PVA-based resin layer with a dichroic substance (typically, iodine). If necessary, after the dyeing treatment and before the stretching treatment in water, a crosslinking treatment is performed. Typically, the crosslinking treatment is 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. According to 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 and 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.

The laminate may be stretched by any suitable method. 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). Free end stretching is preferably chosen. The stretching of the laminate may be performed in one stage, or may be performed in a plurality of stages. When the stretching is performed in a plurality of stages, 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 an aqueous boric acid solution as a stretching bath, the PVA-based resin layer can be provided with rigidity capable of withstanding the tension applied during stretching and water-insoluble water resistance. Specifically, boric acid generates tetrahydroxyborate anions in an aqueous solution, and crosslinks the PVA-based resin via hydrogen bonds. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, and a polarizing film having excellent optical characteristics can be produced by stretching the PVA-based resin layer satisfactorily.

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.

The stretching bath (aqueous boric acid solution) is preferably 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 stretching temperature (liquid temperature of the stretching bath) is preferably 40 to 85 ℃, 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 in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is less 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 is higher, and thus excellent optical characteristics may not 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, and 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, relative to the original length of the laminate. By realizing such a high stretching ratio, a polarizing film having extremely excellent optical characteristics can be produced. Such a high stretch ratio can be achieved by using an underwater stretching method (boric acid underwater stretching).

Introduction of C-5. alcohol

In the embodiment of the present invention, alcohol is introduced after the stretching treatment in water (and typically, before the drying shrinkage treatment described later). The introduction of the alcohol may be carried out by any suitable means. For example, the laminate may be immersed in a treatment liquid containing alcohol, or the treatment liquid containing alcohol may be applied to the polarizing film surface of the laminate. Typically, the introduction of the alcohol may be carried out by impregnation. The impregnation can be carried out in any and suitable manner. For example, an alcohol may be added to the cleaning bath in the cleaning treatment to prepare a bath of the treatment liquid, a bath of the treatment liquid may be used instead of the cleaning bath, or a bath of the treatment liquid and the cleaning bath may be provided separately. Typically, an alcohol may be added to the cleaning bath (cleaning liquid) of the cleaning treatment. The alcohol concentration of the treatment liquid (cleaning liquid) is preferably 5 to 35 wt% in the case of a low boiling point alcohol, and preferably 0.03 to 1.0 wt% in the case of a high boiling point alcohol. The immersion time of the laminate in the treatment liquid (cleaning liquid) is preferably 1 to 20 seconds, more preferably 3 to 10 seconds.

C-6 drying shrinkage treatment

The drying shrinkage treatment may be performed by heating the entire region to heat the region, or may be performed by heating the transport roller (using a so-called hot roller) (hot roller drying method). Both are preferably used. By drying the laminate using a heating roller, the laminate can be effectively prevented from being warped by heating, and a polarizing film having excellent appearance can be produced. Specifically, by drying the laminate in a state where the laminate is along the heating roller, crystallization of the thermoplastic resin substrate can be effectively promoted, the crystallinity can be increased, 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 resistant to shrinkage of the PVA-based resin layer due to drying, and warpage is suppressed. Further, since the laminate can be dried while being maintained in a flat state by using the heating roller, not only warpage but also wrinkles can be suppressed. In this case, the laminate can be shrunk in the width direction by a drying shrinkage treatment to improve the optical characteristics. 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 laminate 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 that the surface of the PVA resin layer and the surface of the thermoplastic resin substrate are alternately and continuously heated, and for example, the conveying rollers R1 to R6 may be disposed so that only one surface (for example, the surface of the thermoplastic resin substrate) of the laminate 200 is continuously heated.

The drying conditions can be controlled by adjusting the heating temperature of the conveying roller (temperature of the heating roller), the number of heating rollers, the time of contact 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 ℃. At such a temperature, the crystallinity of the thermoplastic resin can be increased favorably, the warpage can be controlled favorably, and an optical laminate having extremely excellent durability can be produced. The temperature of the heating roller can be measured by a contact thermometer. In the illustrated example, 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 the drying by the hot roller in combination with the hot air drying, a rapid temperature change between the hot 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 fine blade type digital anemometer.

C-7. others

The thermoplastic resin substrate/polarizing film laminate obtained in the above-described manner can be used as a polarizing plate as it is (the thermoplastic resin substrate can be used as a protective layer); after the protective layer is laminated on the surface of the polarizing film of the laminate, the thermoplastic resin substrate may be peeled off and used as a polarizing plate having a protective layer/polarizing film structure; another protective layer may be bonded to the release surface of the thermoplastic resin substrate, and the polarizing plate having a protective layer/polarizing film/protective layer structure may be used.

D. Image display device

The polarizing film described in item a and/or the polarizing plate described in item B above can be applied to an image display device. Accordingly, the present invention includes an image display device using such a polarizing film and/or polarizing plate. Typical examples of the image display device include a liquid crystal display device and an Electroluminescence (EL) display device (for example, an organic EL display device and an inorganic EL display device).

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 characteristic is as follows. Unless otherwise specified, "parts" and "%" in examples and comparative examples are based on weight.

(1) Thickness of

The measurement was carried out using an interferometric film thickness meter (available under the name "MCPD-3000" available from Otsuka electronics Co., Ltd.).

(2) Alcohol concentration of polarizing film

< ethanol concentration >

Cutting the polarizing film into 10cm²The sample was used as a measurement sample. The measurement sample was sealed in a 20mL headspace vial, and then heated at 175 ℃ for 30 minutes using a headspace sampler (HSS), and 1mL of the heated gas phase fraction was injected into a gas chromatograph (product name "6890N" manufactured by Agilent Technologies), and the amount of contained ethanol was calculated from the peak area corresponding to ethanol using the following standard curve.

y＝4.743E⁺⁰⁰x+3.105E^-02

< Glycerol concentration >

The amount of glycerin contained was calculated from the peak area corresponding to glycerin by freezing and pulverizing the polarizing film, collecting about 0.02g, placing in a screw tube, adding 0.5ml of methanol, extracting overnight, filtering the extract with a 0.45 μm membrane filter, and injecting 1 μ L of the filtrate into a gas chromatograph (product name "6890N" manufactured by Agilent Technologies), using the following standard curve.

y＝5.666E^-01x+1.833E^-00

(3) Monomer transmittance and degree of polarization

For the polarizing plates (protective layer/polarizing films) of examples and comparative examples, the monomer transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer (LPF-200, made by ottuka electronics) were respectively used as Ts, Tp, and Tc of the polarizing films. These Ts, Tp and Tc are Y values measured by a 2-degree visual field (C light source) of JIS Z8701 and corrected for visibility. The refractive index of the protective layer was 1.50, and the refractive index of the surface of the polarizing film opposite to the protective layer was 1.53. From the Tp and Tc thus obtained, the degree of polarization was determined by using the following formula.

Polarization degree (%) { (Tp-Tc)/(Tp + Tc) }^1/2×100

(4) Function of orientation

The polarizing films obtained in examples and comparative examples were subjected to total reflection attenuated spectroscopy (ATR) measurement on the surface of the polarizing film (the release surface of the PET substrate) using polarized infrared light as measurement light using a fourier transform infrared spectrometer (FT-IR) (product name: "Frontier" manufactured by Perkin electric corporation). Germanium was used for microcrystals that were bonded to the polarizing film, and the incident angle of the measurement light was set to 45 °. The calculation of the orientation function is performed by the following steps. The polarized infrared light (measurement light) to be incident is polarized light (s-polarized light) vibrating parallel to the surface to which the sample of germanium crystals is bonded, and the absorbance spectra of the polarized light films are measured in a state where the stretching direction of the polarized light films is arranged perpendicular (t) and parallel (/ /) to the polarization direction of the measurement light. From the obtained absorbance spectrum, the absorbance was calculated as (3330 cm)^-1Intensity) as a reference (2941 cm)^-1Strength) I. I is_⊥Is obtained from an absorbance spectrum obtained when the stretching direction of the polarizing film is arranged perpendicular (#) to the polarization direction of the measurement light (2941 cm)^-1Strength)/(3330 cm^-1Strength). In addition, I_//Obtained from an absorbance spectrum obtained when the stretching direction of the polarizing film was arranged parallel (/ /) to the polarization direction of the measurement light (2941 cm)^-1Strength)/(3330 cm^-1Strength). Here, (2941cm^-1Intensity) is measured at the bottom of the absorbance spectrum, i.e., 2770cm^-1And 2990cm^-12941cm at baseline^-1(3330 cm) of (D)^-1Strength) was 2990cm^-1And 3650cm^-13330cm at baseline^-1Absorbance of (b). Using the resultant I_⊥And I_//The orientation function F is calculated according to equation 1. When F is 1, the number isFully oriented, F is random when 0. Furthermore, 2941cm^-1The peak of (A) is the main chain (-CH) of PVA in the polarizing film₂-) absorption caused by vibrations. Furthermore, 3330cm^-1The peak of (2) is absorption by vibration of the hydroxyl group of PVA.

(formula 1) F ═ 3<cos²θ>-1)/2

＝(1-D)/[c(2D+1)]

Wherein,

c＝(3cos²beta-1)/2 and using 2941cm as described above^-1When the temperature of the water is higher than the set temperature,

θ: angle of molecular chain relative to stretching direction

Beta: angle of transition dipole moment relative to molecular chain axis

D＝(I_⊥)/(I_//)

(5) Dimensional change of polarizing film

The polarizing plates obtained in examples and comparative examples were cut into a size of 10cm in the absorption axis direction and 10cm in the transmission axis direction to prepare test pieces. The test piece thus obtained was put into an oven at 85 ℃ and the shrinkage in the absorption axis direction after heating for 120 hours was measured.

[ example 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.

To 100 parts by weight of a PVA-based resin obtained by mixing polyvinyl alcohol (having a polymerization degree of 4200 and a saponification degree of 99.2 mol%) and acetoacetyl-modified PVA (trade name "GOHSEFIMER Z410" manufactured by Nippon synthetic chemical industries, Ltd.) at a ratio of 9:1, 13 parts by weight of potassium iodide was added to prepare an aqueous PVA solution (coating solution).

The PVA aqueous 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 resultant laminate was uniaxially stretched to 2.4 times along the longitudinal (lengthwise) free end in an oven at 130 ℃ between rolls having different peripheral speeds (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) having a liquid temperature of 40 ℃ for 30 seconds (insolubilization treatment).

Next, the polarizing film obtained finally 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 so that the monomer transmittance (Ts) of the polarizing film obtained finally became about 44% to 45% (dyeing treatment).

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

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

Thereafter, the laminate was immersed in a treatment bath (aqueous solution containing 4 wt% of potassium iodide and 6 wt% of ethanol) at a liquid temperature of 20 ℃ for 5 seconds, and ethanol was introduced into the PVA-based resin layer (polarizing film) while cleaning the laminate (cleaning treatment and introduction of ethanol).

Thereafter, while drying in an oven maintained at 90 ℃, it was brought into contact with a heated roll made of SUS whose surface temperature was maintained at 75 ℃ for about 2 seconds (drying treatment).

In this manner, 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 was laminated on 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 ethanol concentration in the polarizing film of the polarizing plate obtained was 15 ppm.

The obtained polarizing plate (substantially, polarizing film) was evaluated for each characteristic. The results are shown in Table 1.

[ example 2]

A polarizing plate was produced in the same manner as in example 1. The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in example 1. The results are shown in Table 1.

[ example 3]

A polarizing plate was produced in the same manner as in example 1, except that 0.2 wt% of glycerin was added instead of ethanol to the treatment bath. The glycerin concentration in the polarizing film of the polarizing plate obtained was 0.3 wt%. In addition, the obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in example 1. The results are shown in Table 1.

[ comparative examples 1 to 3]

A polarizing plate was produced in the same manner as in example 1, except that no alcohol was added to the cleaning bath (cleaning solution). The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in example 1. The results are shown in Table 1.

Comparative example 4

A polarizing plate was produced in the same manner as in example 1, except that no alcohol was added to the cleaning bath (cleaning solution) and that the stretching treatment was performed so that the total stretching ratio became 3.5 times. The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in example 1. The results are shown in Table 1.

[ Table 1]

As is clear from table 1: the polarizing plate (polarizing film) of the example having an orientation characteristic index of 7 or more maintained excellent optical characteristics, and dimensional change was suppressed.

Industrial applicability

The polarizing film and the polarizing plate of the present invention can be suitably used for image display devices such as liquid crystal display devices and organic EL display devices.

Description of the reference numerals

10 polarizing film

20 first protective layer

30 second protective layer

100 polarizing plate

Claims

1. A polarizing film comprising a polyvinyl alcohol resin film containing iodine,

the polarizing film has a monomer transmittance of 43.0% or more,

the orientation characteristic index (P-98)/F calculated from the polarization degree P and the orientation function F is 7 or more.

2. The polarized film according to claim 1 having a monomer transmittance of 44.5% or less.

3. The polarizing film according to claim 1 or 2, which has a thickness of 8 μm or less.

4. A polarized film according to any one of claims 1 to 3 which contains an alcohol.

5. The polarized film according to claim 4, wherein the alcohol comprises at least 1 selected from ethanol and glycerin.

6. A polarizing plate comprising the polarizing film according to any one of claims 1 to 5.

7. An image display device comprising the polarizing plate according to claim 6.