CN108780172B

CN108780172B - Polarizing plate

Info

Publication number: CN108780172B
Application number: CN201680083155.3A
Authority: CN
Inventors: 滨本大介
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2016-03-04
Filing date: 2016-12-21
Publication date: 2021-07-20
Anticipated expiration: 2036-12-21
Also published as: WO2017149909A1; CN108780172A; TWI645223B; TW201732332A; JP6893762B2; KR20180107221A; KR102219988B1; SG11201807351UA; JP2017156663A; KR20200022539A

Abstract

The invention provides a polarizing plate which can be directly used in a state that a polarizing film is laminated on a resin substrate and has improved durability. The polarizing plate of the present invention comprises a polyester resin substrate and a polarizing film laminated on one side of the polyester resin substrate and having a thickness of 10 [ mu ] m or less, wherein the crystallinity of the polyester resin substrate calculated by attenuated total reflection spectroscopy is 0.55 to 0.80, and the boric acid concentration in the polarizing film is 10 to 20 wt%.

Description

Polarizing plate

Technical Field

The present invention relates to a polarizing plate.

Background

A method of forming a polyvinyl alcohol resin layer on a resin substrate, and stretching and dyeing the laminate to obtain a polarizing film has been proposed (for example, patent document 1). According to such a method, a polarizing film having a small thickness can be obtained, and thus, for example, the method can contribute to the reduction in thickness of an image display device and is attracting attention.

The polarizing film can be used as it is in a state of being laminated on the resin substrate (patent document 2). However, in such an embodiment, cracks may occur in the polarizer, and thus improvement in durability is required.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2000-338329

Patent document 2: japanese patent No. 4979833

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above problems, and a main object thereof is to provide a polarizing plate which can be used as it is in a state where a polarizing film is laminated on a resin substrate and has improved durability.

Means for solving the problems

The present invention provides a polarizing plate comprising a polyester resin substrate and a polarizing film laminated on one side of the polyester resin substrate and having a thickness of 10 μm or less. In the polarizing plate of the present invention, the crystallinity of the polyester-based resin substrate calculated by attenuated total reflection spectroscopy (ATR) measurement is 0.55 to 0.80, and the boric acid concentration in the polarizing film is 10 to 20 wt%.

In one embodiment, the polyester-based resin is polyethylene terephthalate or a copolymer thereof.

In one embodiment, the polarizing film is laminated on one side of the polyester resin substrate without interposing an adhesive layer.

In one embodiment, the polarizing plate does not have a protective film on a side of the polarizing film opposite to a side where the polyester resin base material is laminated.

In one embodiment, the polarizing plate has an easy-adhesion layer between the polyester resin substrate and the polarizing film.

According to another aspect of the present invention, there may be provided a method of manufacturing the polarizing plate described above. The manufacturing method comprises the following steps: forming a polyvinyl alcohol resin film on a polyester resin substrate to produce a laminate; stretching the laminate; dyeing the polyvinyl alcohol resin film; and crystallizing the polyester resin substrate.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a polarizing plate obtained by stretching and dyeing a laminate in which a polyvinyl alcohol resin film is formed on a resin substrate can be obtained which has excellent durability and can be used as it is in a state in which the polarizing film is laminated on the resin substrate by adjusting the crystallinity of the resin substrate and the boric acid concentration in the polarizing film to specific ranges.

Drawings

Fig. 1(a) and (b) are schematic cross-sectional views of a polarizing plate according to an embodiment of the present invention.

Detailed Description

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

A. Polarizing plate

The polarizing plate of the present invention comprises a polyester resin substrate and a polarizing film laminated on one side of the polyester resin substrate and having a thickness of 10 [ mu ] m or less. Fig. 1(a) is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. The polarizing plate 10a includes a polyester resin substrate 11 and a polarizing film 12, and the polarizing film 12 is laminated in close contact with one surface of the polyester resin substrate 11 (in other words, without an adhesive layer interposed therebetween). Fig. 1(b) is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention. The polarizing plate 10b further has a protective film 13. The protective film 13 is disposed on the side of the polarizing film 12 opposite to the side on which the polyester resin substrate 11 is disposed. The protective film 13 may be laminated on the polarizing film 12 with an adhesive layer interposed therebetween, or may be laminated in close contact therewith (without an adhesive layer interposed therebetween). In the polarizing

plates

10a and 10b, the polyester resin substrate 11 functions as a protective film. In the present invention, the resin base material used for stretching and dyeing in the process of producing the polarizing film can function as a protective film without being peeled off, and the occurrence of cracks can be suppressed even in a structure (the structure of fig. 1 (a)) having the resin base material (protective film) only on one side of the polarizing film. The polarizing

plates

10a and 10b may have an easy-adhesion layer (not shown) between the polyester resin substrate 11 and the polarizing film 12.

In general, in a polarizing plate obtained by stretching and dyeing a laminate in which a polyvinyl alcohol (hereinafter, sometimes referred to as "PVA") resin film is formed on a resin substrate, dimensional changes occur in the resin substrate and the PVA resin film due to relaxation of orientation stress, generation of shrinkage stress, and the like. In this case, since the two are different in the amount of dimensional change, strain is generated at the interface, and it is estimated that the generation of cracks is caused. In this regard, in the present invention, a polyester-based resin substrate having a crystallinity in a specific range is used as the resin substrate, and the boric acid concentration in the polarizing film is adjusted to be in a specific range. Accordingly, the amount of dimensional change of the resin substrate and the amount of dimensional change of the polarizing film are balanced in both the absorption axis direction of the polarizing film and the direction orthogonal thereto, and therefore, strain is not easily generated at the interface of the resin substrate and the polarizing film, and the generated strain can be dispersed in both directions. As a result, the occurrence of cracks can be suppressed. In the present specification, the term "orthogonal direction" includes a case of 90 ° ± 5.0 °, preferably 90 ° ± 3.0 °, and more preferably 90 ° ± 1.0 °. In addition, the "parallel direction" includes the case of 0 ° ± 5.0 °, preferably 0 ° ± 3.0 °, and more preferably 0 ° ± 1.0 °.

A-1 polarizing film

The polarizing film is substantially a PVA type resin film in which iodine is adsorbed and oriented. The thickness of the polarizing film is 10 μm or less, preferably 7.5 μm or less, and more preferably 5 μm or less. On the other hand, the thickness of the polarizing film is preferably 0.5 μm or more, more preferably 1.5 μm or more. If the thickness is too small, the optical properties of the obtained polarizing film may be deteriorated. The polarizing film preferably exhibits absorption dichroism at any wavelength of 380nm to 780 nm. The single transmittance of the polarizing film is preferably 40.0% or more, more preferably 41.0% or more, and further preferably 42.0% or more. The polarization degree of the polarizing film is preferably 99.8% or more, more preferably 99.9% or more, and further preferably 99.95% or more.

As the PVA-based resin forming the PVA-based resin film, any and appropriate resin can be used. Examples thereof include: polyvinyl alcohol, ethylene-vinyl alcohol copolymer. Polyvinyl alcohol is 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-based 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. When the saponification degree is too high, gelation may occur.

The average polymerization degree of the PVA-based resin may be appropriately selected depending on the purpose. The average degree of polymerization 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-.

The polarizing film contains boric acid. The boric acid concentration in the polarizing film is 10 to 20 wt%, preferably 12 to 19 wt%. When the boric acid concentration is within this range, the dimensional change rate of the polarizing film in the direction of the absorption axis and the direction orthogonal thereto can be made to be a value close to the dimensional change rate of the polyester-based resin substrate, and the difference between the dimensional change rate of the polarizing film in the direction of the absorption axis and the dimensional change rate of the polyester-based resin substrate can be prevented from being excessively larger than the difference between the dimensional change rate of the polarizing film in the direction orthogonal to the absorption axis and the dimensional change rate of the polyester-based resin substrate. The boric acid concentration in the polarizing film can be adjusted by: for example, in the method for producing a polarizing plate described later, the boric acid concentration in a stretching bath, an insolubilizing bath, a crosslinking bath, and the like are changed, and the immersion time in these baths is changed. The boric acid concentration (% by weight) in the polarizing film can be determined using, for example, a boric acid quantity index calculated from Attenuated Total Reflection (ATR) spectroscopic measurement.

(boric acid amount index) ═ (boric acid peak 665 cm)^-1Intensity of (2)/(control Peak 2941 cm)^-1Strength of (2)

(boric acid concentration) ═ boric acid quantity index (boric acid quantity index) × 5.54+4.1

Here, "5.54" and "4.1" are constants obtained by measuring the fluorescence X-ray intensity of a sample having a known boric acid concentration and preparing a calibration curve.

A-2 polyester resin base Material

As the material for forming the polyester-based resin, for example: polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), copolymerized PET (PET-G) containing alicyclic dicarboxylic acids or alicyclic diols containing isophthalic acid, cyclohexane ring, or the like, other polyesters, and copolymers, blends, and the like thereof. Among them, PET or copolymerized PET is preferably used. These resins are amorphous in an unstretched state and have excellent stretchability suitable for high-ratio stretching, and are crystallized by stretching and heating to impart heat resistance and dimensional stability. In addition, the PVA-based resin can be applied and dried in an unstretched state with sufficient heat resistance.

The glass transition temperature (Tg) of the polyester-based resin substrate is preferably 170 ℃ or lower. By using such a resin substrate, the PVA-based resin film can be prevented from crystallizing, and the stretchability can be sufficiently ensured. In view of smooth progress of water-based resin substrate plasticization and stretching in an aqueous solution, it is more preferably 120 ℃ or lower. In one embodiment, the glass transition temperature of the polyester-based resin substrate is preferably 60 ℃ or higher. By using such a polyester-based resin substrate, when a coating liquid containing a PVA-based resin described later is applied and dried, defects such as deformation (for example, generation of unevenness, sagging, wrinkles, and the like) of the polyester-based resin substrate can be prevented. The laminate may be stretched at an appropriate temperature (for example, about 60 to 70 ℃). In another embodiment, the glass transition temperature of the coating liquid containing the PVA-based resin may be lower than 60 ℃ as long as the polyester-based resin substrate is not deformed when the coating liquid is applied and dried. The glass transition temperature (Tg) is a value determined in accordance with JIS K7121.

In one embodiment, the water absorption rate of the polyester resin base material is preferably 0.2% or more, more preferably 0.3% or more. Such a polyester resin base material absorbs water, and the water functions as a plasticizer to plasticize the polyester resin base material. As a result, the tensile stress can be greatly reduced in the aqueous solution drawing, and the stretchability can be improved. On the other hand, the water absorption of the polyester resin base material is preferably 3.0% or less, more preferably 1.0% or less. By using such a polyester resin base material, it is possible to prevent problems such as a significant decrease in dimensional stability of the polyester resin base material and deterioration in appearance of the resulting laminate during production. Further, when the film is stretched in an aqueous solution, the film is prevented from being broken and the PVA based resin film is prevented from peeling from the polyester based resin substrate. The water absorption is a value obtained according to JIS K7209.

The thickness of the polyester resin base material is preferably 10 to 200. mu.m, more preferably 20 to 150. mu.m.

The crystallinity of the polyester resin substrate calculated by attenuated total reflectance spectroscopy (ATR) measurement is 0.55 to 0.80, preferably 0.58 to 0.80, and more preferably 0.60 to 0.75. When the crystallinity of the polyester-based resin substrate is within this range, the dimensional change rate of the polyester-based resin substrate in the absorption axis direction of the polarizing film and the direction orthogonal thereto can be made to be a value close to the dimensional change rate of the polarizing film, and the difference between the dimensional change rate of the polarizing film in the absorption axis direction and the dimensional change rate of the polyester-based resin substrate can be prevented from being excessively larger than the difference between the dimensional change rate of the polarizing film in the direction orthogonal to the absorption axis and the dimensional change rate of the polyester-based resin substrate. The crystallinity of the polyester-based resin substrate can be adjusted by changing the heating temperature and/or heating time at the time of crystallization, for example. The crystallinity of the polyester-based resin substrate can be calculated based on the following formula.

(degree of crystallization) ═ crystal peak 1340cm^-1Intensity)/(control Peak 1410cm^-1Strength)

A-3. protective film

Examples of the material for forming the protective film include: and (ii) cellulose resins such as (meth) acrylic resins, cellulose diacetate and cellulose triacetate, cycloolefin resins, olefin resins such as polypropylene, ester resins such as polyethylene terephthalate, polyamide resins, polycarbonate resins, and copolymer resins thereof. The thickness of the protective film is preferably 10 μm to 100 μm.

A-4. easy adhesive layer

The easy-adhesion layer may be a layer substantially composed of only the composition for forming an easy-adhesion layer, or may be a layer or a region in which the composition for forming an easy-adhesion layer is mixed (including compatible) with the material for forming the polarizing film. By forming the easy adhesion layer, excellent adhesion can be obtained. The thickness of the easy adhesion layer is preferably about 0.05 μm to 1 μm. The easy adhesion layer can be confirmed by, for example, observing the cross section of the polarizing plate with a Scanning Electron Microscope (SEM). The composition for forming an easy adhesion layer is described in detail in item B.

A-5. adhesive layer

The adhesive layer may be formed of any suitable adhesive or bonding agent. A representative adhesive layer is formed of an acrylic adhesive. A representative adhesive layer is formed of a vinyl alcohol adhesive.

B. Method for manufacturing polarizing plate

A representative manufacturing method of the polarizing plate of the present invention includes: forming a PVA-based resin film on a polyester-based resin substrate to produce a laminate; stretching the laminate; dyeing the PVA resin film; and crystallizing the polyester resin substrate.

B-1 preparation of laminate

As a method for forming the PVA-based resin film on the polyester-based resin substrate, any appropriate method can be adopted. The PVA-based resin film is preferably formed by applying a coating solution containing a PVA-based resin on a polyester-based resin substrate and drying the coating solution. In one embodiment, an easy adhesion layer is formed by applying and drying a composition for forming an easy adhesion layer on a polyester resin substrate, and a PVA-based resin film is formed on the easy adhesion layer.

The polyester resin base material is formed as described above. The thickness of the polyester resin base material (thickness before stretching described later) is preferably 20 to 300. mu.m, and more preferably 50 to 200. mu.m. When the thickness is less than 20 μm, the PVA based resin film may not be easily formed. If the thickness exceeds 300 μm, for example, in stretching in an aqueous solution, the polyester resin base material may take a long time to absorb water, and an excessive load may be required for stretching. The crystallinity of the polyester resin base material used for producing the laminate, which is calculated by Attenuated Total Reflection (ATR) spectrometry, may be, for example, 0.20 to 0.50.

The coating liquid is typically a solution obtained by dissolving 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 solvents may be used alone or in combination of two or more. Among them, 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. A uniform coating film adhering to the polyester resin substrate can be formed at such a resin concentration.

Additives may be added to the coating liquid. Examples of the additives include plasticizers and surfactants. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol and glycerin. Examples of the surfactant include nonionic surfactants. These additives can be used for further improving the uniformity, dyeability and stretchability of the PVA-based resin film obtained. Further, as the additive, for example, an easily adhesive component can be cited. By using the easily adhesive component, the adhesion between the polyester resin substrate and the PVA resin film can be improved. As a result, for example, the PVA-based resin film can be prevented from peeling off from the substrate, and dyeing and stretching in an aqueous solution described later can be performed satisfactorily. As the easy-adhesion component, for example, a modified PVA such as acetoacetyl-modified PVA can be used.

As a method for applying the coating liquid, any appropriate method can be adopted. Examples thereof include: roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, doctor blade coating (such as comma coating), and the like.

The coating and drying temperature of the coating liquid is preferably 50 ℃ or higher.

The thickness of the PVA based resin film (thickness before stretching described later) is preferably 3 to 20 μm.

Before the PVA-based resin film is formed, the polyester-based resin substrate may be subjected to a surface treatment (e.g., corona treatment), or a composition for forming an easy adhesion layer may be applied (coating treatment) on the polyester-based resin substrate. By performing such treatment, the adhesion between the polyester resin substrate and the PVA-based resin film can be improved. As a result, for example, the PVA-based resin film can be prevented from peeling off from the substrate, and dyeing and stretching described later can be performed satisfactorily.

The composition for forming an easy adhesion layer preferably contains a polyvinyl alcohol component. As the polyvinyl alcohol-based component, any suitable PVA-based resin can be used. Specific examples thereof include: polyvinyl alcohol and modified polyvinyl alcohol. Examples of the modified polyvinyl alcohol include polyvinyl alcohols modified with an acetoacetyl group, a carboxylic acid group, an acrylic group, and/or a urethane group. Among them, acetoacetyl group-modified PVA is preferably used. As the acetoacetyl group-modified PVA, a polymer having at least a repeating unit represented by the following general formula (I) is preferably used.

[ chemical formula 1]

In the formula (I), the ratio of n to l + m + n is preferably 1% to 10%.

The average degree of polymerization of the acetoacetyl-modified PVA is preferably 1000 to 10000, more preferably 1200 to 5000. The saponification degree of the acetoacetyl group-modified PVA is preferably 97 mol% or more. The pH value of a 4 wt% aqueous solution of the acetoacetyl-modified PVA is preferably 3.5 to 5.5. The average degree of polymerization and the degree of saponification can be determined in accordance with JIS K6726-.

The composition for forming an easy-adhesion layer may further contain a polyolefin component, a polyester component, a polyacrylic component, and the like, depending on the purpose and the like. The easy adhesion layer-forming composition preferably further contains a polyolefin component.

As the polyolefin-based component, any suitable polyolefin-based resin can be used. Examples of the olefin component as the main component of the polyolefin-based resin include: an olefin hydrocarbon having 2 to 6 carbon atoms such as ethylene, propylene, isobutylene, 1-butene, 1-pentene, 1-hexene and the like. These components may be used alone, or two or more of them may be used in combination. Among these, olefinic hydrocarbons having 2 to 4 carbon atoms such as ethylene, propylene, isobutylene, and 1-butene are preferably used, and ethylene is more preferably used.

The proportion of the olefin component in the monomer components constituting the polyolefin-based resin is preferably 50 to 95% by weight.

The polyolefin-based resin preferably has a carboxyl group and/or an acid anhydride group thereof. Such a polyolefin resin can be dispersed in water to form an easy-adhesive layer satisfactorily. Examples of the monomer component having such a functional group include: unsaturated carboxylic acids and anhydrides thereof, half esters and half amides of unsaturated dicarboxylic acids. Specific examples thereof include: acrylic acid, methacrylic acid, maleic anhydride, itaconic acid, itaconic anhydride, fumaric acid, crotonic acid.

The polyolefin resin has a molecular weight of 5000 to 80000, for example.

In the composition for forming an easy adhesion layer, the blending ratio of the polyvinyl alcohol component to the polyolefin component (the former: the latter (solid component)) is preferably 5: 95 to 60: 40, and more preferably 20: 80 to 50: 50. When the polyvinyl alcohol component is too large, there is a risk that sufficient adhesion cannot be obtained. Specifically, the peeling force required for peeling the polarizing film from the resin substrate is reduced, and there is a possibility that sufficient adhesion cannot be obtained. On the other hand, when the polyvinyl alcohol component is too small, the appearance of the obtained polarizing plate may be impaired. Specifically, when the easy-adhesion layer is formed, a defect such as white turbidity of the coating film occurs, and there is a possibility that it is difficult to obtain a polarizing plate having excellent appearance.

The composition for forming an easy-adhesion layer is preferably aqueous. The easy-adhesion layer-forming composition may contain an organic solvent. Examples of the organic solvent include ethanol and isopropanol. The concentration of the solid content of the easy adhesion layer-forming composition is preferably 1.0 to 10% by weight.

As a method for applying the composition for forming an easy adhesion layer, any appropriate method can be used. After the composition for forming an easy-adhesion layer is applied, the applied film may be dried. The drying temperature is, for example, 50 ℃ or higher.

B-2 stretching

As the method for stretching the laminate, any suitable method may be adopted. Specifically, the stretching may be fixed-end stretching (for example, a method using a tenter) or free-end stretching (for example, a method of passing the laminate between rolls having different peripheral speeds to perform uniaxial stretching). Further, simultaneous biaxial stretching (for example, a method using a simultaneous biaxial stretcher) or stepwise biaxial stretching may be used. The stretching of the laminate may be performed in one step or in multiple steps. When the multi-step process is performed, the stretch ratio (maximum stretch ratio) of the laminate described later is the product of the stretch ratios of the respective steps.

The stretching treatment may be a stretching method in which the laminate is immersed in an aqueous solution in a stretching bath, or a stretching method in a gas atmosphere. In one embodiment, the stretching treatment in aqueous solution is performed at least 1 time, preferably by combining the stretching treatment in aqueous solution with the stretching treatment in gas atmosphere. Stretching in an aqueous solution allows stretching at a temperature lower than the glass transition temperature (typically, about 80 ℃) of the polyester resin substrate or the PVA resin film, and allows stretching at a high magnification while suppressing crystallization of the PVA resin film. As a result, a polarizing film having excellent polarizing characteristics can be manufactured.

As the stretching direction of the laminate, any appropriate direction may be selected. In one embodiment, the elongated laminate is stretched in the longitudinal direction thereof. Specifically, the laminate is transported in the longitudinal direction and stretched in the transport direction (MD). In another embodiment, the stretching is performed along the width direction of the elongated laminate. Specifically, the laminate is transported in the longitudinal direction and stretched in The Direction (TD) orthogonal to the transport direction (MD).

The stretching temperature of the laminate can be set to any appropriate value depending on the material for forming the polyester resin base material, the stretching method, and the like. When the stretching method is employed in an air atmosphere, the stretching temperature is preferably not less than the glass transition temperature (Tg) of the polyester 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 stretching temperature of the laminate is preferably 170 ℃ or lower. By stretching at such a temperature, rapid progress of crystallization of the PVA type resin can be suppressed, and defects caused by the crystallization (for example, inhibition of orientation of the PVA type resin film by stretching) can be suppressed.

When the drawing method in an aqueous solution is employed as the drawing method, the liquid temperature of the drawing bath is preferably 40 to 85 ℃, more preferably 50 to 85 ℃. At such a temperature, the PVA-based resin film can be stretched to a high magnification while dissolution thereof is suppressed. Specifically, as described above, the glass transition temperature (Tg) of the polyester-based resin substrate is preferably 60 ℃ or higher in relation to the formation of the PVA-based resin film. In this case, when the stretching temperature is lower than 40 ℃, there is a possibility that the stretching cannot be performed well even when the plasticizing of the polyester resin base material with water is considered. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin film, and there is a possibility that excellent polarization characteristics cannot be obtained.

When the stretching in an aqueous solution is employed, the laminate is preferably stretched by immersing it in an aqueous boric acid solution (stretching in an aqueous boric acid solution). By using the aqueous boric acid solution as the stretching bath, the PVA-based resin film can be provided with rigidity capable of withstanding the tension applied during stretching and water resistance that does not dissolve in water. Specifically, boric acid can generate tetrahydroxyborate anions in an aqueous solution, and can be crosslinked with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin film can be provided with rigidity and water resistance, and can be stretched well, thereby producing a polarizing film having excellent polarizing properties.

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 with respect to 100 parts by weight of water. When the boric acid concentration is 1 part by weight or more, dissolution of the PVA-based resin film can be effectively suppressed, and a polarizing film having higher characteristics can be produced. In addition to boric acid or a borate, an aqueous solution obtained by dissolving a boron-containing compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent may be used.

Preferably, an iodide is added to the stretching bath (aqueous boric acid solution). The iodine compound can suppress elution of iodine adsorbed on the PVA-based resin film. Examples of the iodide include: potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, titanium iodide, and the like. Among them, potassium iodide is preferable. The concentration of the iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, relative to 100 parts by weight of water.

The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes. The stretching treatment in the aqueous solution is preferably performed after the dyeing treatment.

The stretch ratio (maximum stretch ratio) of the laminate is preferably 4.0 times or more, and more preferably 5.0 times or more, with respect to the original length of the laminate. Such a high stretch ratio can be realized by, for example, stretching in an aqueous solution (stretching in an aqueous boric acid solution). In the present specification, the "maximum stretching ratio" refers to the stretching ratio of the laminate immediately before the fracture, and is a value lower than the value at which the fracture of the laminate is separately observed by 0.2.

B-3. dyeing

The representative PVA-based resin film is dyed by adsorbing iodine to the PVA-based resin film. Examples of the adsorption method include: a method of immersing the PVA-based resin film (laminate) in a staining solution containing iodine, a method of applying the staining solution to the PVA-based resin film, and a method of spraying the staining solution on the PVA-based resin film. The PVA-based resin film (laminate) is preferably immersed in a dyeing solution. This is because iodine can be adsorbed well.

The staining solution is preferably an aqueous iodine solution. The amount of iodine blended is preferably 0.1 to 0.5 parts by weight based on 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add an iodide to the aqueous iodine solution. Specific examples of the iodide are as described above. The amount of the iodide is preferably 0.02 to 20 parts by weight, more preferably 0.1 to 10 parts by weight, based on 100 parts by weight of water. In order to suppress the dissolution of the PVA based resin, the liquid temperature of the dyeing liquid at the time of dyeing is preferably 20 to 50 ℃. When the PVA-based resin film is immersed in the dyeing liquid, the immersion time is preferably 5 seconds to 5 minutes in order to ensure the transmittance of the PVA-based resin film. The dyeing conditions (concentration, liquid temperature, and immersion time) may be set so that the polarization degree or monomer transmittance of the polarizing film to be finally obtained is within a predetermined range. In one embodiment, the immersion time is set so that the polarization degree of the obtained polarizing film is 99.98% or more. In another embodiment, the immersion time is set so that the monomer transmittance of the obtained polarizing film is 40% to 44%.

The dyeing treatment may be carried out at any suitable time. When the stretching is performed in an aqueous solution, it is preferably performed before the stretching in an aqueous solution.

Crystallization of B-4

The crystallization of the polyester resin substrate can be performed, for example, by heating the polyester resin substrate (substantially, a laminate). The crystallization is preferably performed after the PVA-based resin film is dyed and stretched.

A representative heating temperature is a temperature exceeding the glass transition temperature (Tg) of the polyester resin substrate. The heating temperature is preferably 90 ℃ or higher, and more preferably 100 ℃ or higher. The heating temperature is preferably 125 ℃ or lower, and more preferably 120 ℃ or lower. By heating at such a temperature, the polyester resin base material can be made to have a desired crystallinity. The heating time may be set as appropriate depending on the heating temperature and the like. The heating time may be, for example, 3 seconds to 2 minutes.

The crystallization is preferably performed so that the haze value of the polyester resin substrate is 2% or less.

B-5. other treatment

The PVA-based resin film (laminate) may be appropriately subjected to a treatment for forming a polarizing film, in addition to stretching and dyeing. As the process for producing the polarizing film, for example: insolubilization, crosslinking, washing, drying, and the like. The number and order of these processes are not particularly limited.

The insolubilization is typically performed by immersing the PVA-based resin film (laminate) in an aqueous boric acid solution. The PVA-based resin film can be provided with water resistance by performing an insolubilization treatment. The concentration of the aqueous boric acid solution is preferably 1 to 4 parts by weight with respect to 100 parts by weight of water. The liquid temperature of the insolubilization bath (aqueous boric acid solution) is preferably 20 ℃ to 50 ℃. The insolubilization treatment is preferably performed before the stretching in the aqueous solution and the dyeing treatment.

The crosslinking treatment is typically performed by immersing the PVA-based resin film (laminate) in an aqueous boric acid solution. The PVA-based resin film can be provided with water resistance by performing the crosslinking treatment. The concentration of the aqueous boric acid solution is preferably 1 to 5 parts by weight with respect to 100 parts by weight of water. In addition, when the crosslinking treatment is performed after the dyeing treatment, it is preferable to further incorporate an iodide. The iodine compound can suppress elution of iodine adsorbed on the PVA-based resin film. The amount of the iodide is preferably 1 to 5 parts by weight based on 100 parts by weight of water. Specific examples of the iodide are as described above. The liquid temperature of the crosslinking bath (aqueous boric acid solution) is preferably 20 ℃ to 60 ℃. The crosslinking treatment is preferably performed before stretching in the above-mentioned aqueous solution. In a preferred embodiment, stretching in a gas atmosphere, dyeing treatment, and crosslinking treatment are sequentially performed.

The typical cleaning treatment is performed by immersing the PVA-based resin film (laminate) in an aqueous potassium iodide solution. The drying temperature of the drying treatment is preferably 30 to 100 ℃.

As described above, the polarizing plate of the present invention can be obtained by forming a polarizing film on a polyester resin substrate and crystallizing the polyester resin substrate.

C. Use of polarizing plate

The polarizing plate of the present invention can be mounted on, for example, a liquid crystal display device. In this case, the polarizing film is preferably mounted so as to be disposed closer to the liquid crystal cell than the polyester resin substrate. With such a configuration, the effect of the phase difference that the polyester resin base material may have on the image characteristics of the obtained liquid crystal display device can be eliminated.

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. In the following examples and comparative examples, "part" and "%" represent "part by weight" and "% by weight", respectively.

Thickness

The measurement was carried out using a digital micrometer (product name "KC-351C" manufactured by Anritsu Co.).

(rate of change in size)

The polarizing films and the resin substrates were peeled from the polarizing plates obtained in examples and comparative examples by applying starting points (starting points) to the ends, and the dimensional change rate was measured by a Thermomechanical Measuring Apparatus (TMA) after heating from 30 ℃ to 100 ℃ at 10 ℃/min and further holding at 100 ℃ for 60 minutes. In example 6 in which the easy adhesion layer was interposed, a sample obtained by separating the polarizing film and the resin substrate from a polarizing plate prepared in the same manner except that the easy adhesion layer was not formed, in the same procedure, was subjected to measurement to determine the dimensional change of the polarizing film and the resin substrate.

Dimension change ratio (%) (dimension after heat treatment-dimension before heat treatment)/dimension before heat treatment × 100

Degree of crystallinity

The polyester resin substrates obtained in examples and comparative examples were measured for the peak of crystallization (1340 cm) by attenuated total reflectance spectroscopy (ATR) measurement using a Fourier transform infrared spectrophotometer (FT-IR) (product name "SPECTRUM 2000" manufactured by Perkin Elmer Co., Ltd.)^-1) Intensity of (D) and control Peak (1410 cm)^-1) The strength of (2). From the obtained peak intensity of the crystal and the reference peak intensity, the crystallinity was calculated by the following formula.

Glass transition temperature: tg ` (Tg `)

Measured according to JIS K7121.

Concentration of boric acid

The polarizing films obtained in examples and comparative examples were measured for a boric acid peak (665 cm) by attenuated total reflection spectroscopy (ATR) measurement using polarized light as measurement light using a fourier transform infrared spectrophotometer (FT-IR) (product name "spectra 2000" manufactured by Perkin Elmer corporation)^-1) Is strongDensity and control peak (2941 cm)^-1) The strength of (2). From the obtained boric acid peak intensity and the control peak intensity, the boric acid amount index is calculated by the following formula, and further the boric acid concentration is determined from the calculated boric acid amount index by the following formula.

(boric acid quantity index) ═ boric acid peak value 665cm^-1Strength)/(control Peak 2941cm^-1Strength)

Evaluation of cracks

The polarizing plates obtained in examples and comparative examples were bonded to glass with an adhesive interposed therebetween such that the polyester resin substrate was located on the front surface side, and in this state, the polarizing plates were heated in an oven at 100 ℃ for 240 hours. The presence or absence of cracks in the heated polarizing plate was confirmed, and evaluated according to the following criteria.

Good: no generation of cracks

Poor: generation of cracks

[ example 1]

As the resin base material, a long-sized amorphous isophthalic acid copolymerized polyethylene terephthalate (IPA copolymerized PET) film (thickness: 100 μm) having a water absorption of 0.75% and a Tg of 75 ℃ was used.

After one surface of the resin substrate was subjected to corona treatment, an aqueous solution containing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (polymerization degree 1200, acetoacetyl-modified degree 4.6%, saponification degree 99.0 mol% or more, manufactured by japan synthetic chemical industries co., ltd., trade name "gohsefimetm Z200") at a ratio of 9: 1 was applied to the corona-treated surface at 25 ℃ and dried to form a PVA-based resin layer having a thickness of 11 μm. Thus, a laminate was produced.

The obtained laminate was uniaxially stretched at the free end in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds in an oven at 120 ℃ by a factor of 1.8 (auxiliary stretching in a gas atmosphere).

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 30 ℃ for 30 seconds (insolubilization treatment).

Subsequently, the substrate was immersed in a dyeing bath (aqueous iodine solution containing 0.2 parts by weight of iodine and 1.5 parts by weight of potassium iodide per 100 parts by weight of water) at a liquid temperature of 30 ℃ for 60 seconds (dyeing treatment).

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

Then, the laminate was immersed in an aqueous boric acid solution (aqueous solution obtained by blending 3 parts by weight of boric acid and 5 parts by weight of potassium iodide with respect to 100 parts by weight of water) at a liquid temperature of 70 ℃, and uniaxially stretched (stretched in an aqueous solution) in the longitudinal direction (longitudinal direction) between rolls different in peripheral speed so that the total stretching ratio was 5.5 times.

Then, 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) having a liquid temperature of 30 ℃ (cleaning treatment).

Next, the laminate was put into an oven at 100 ℃ for 30 seconds to crystallize the resin base material.

Thus, a polarizing plate in which a polarizing film having a thickness of 5 μm was laminated on a resin substrate was obtained.

[ example 2]

A polarizing plate was obtained in the same manner as in example 1, except that the laminate was put into an oven at 110 ℃ for 30 seconds to crystallize the resin base material.

[ example 3]

A polarizing plate was obtained in the same manner as in example 1, except that the laminate was put into an oven at 120 ℃ for 30 seconds to crystallize the resin base material.

[ example 4]

A polarizing plate was obtained in the same manner as in example 1, except that the amount of boric acid added to the stretching bath during stretching in the aqueous solution was 3.5 parts by weight, and the laminate was put into an oven at 110 ℃ for 30 seconds to crystallize the resin base material.

[ example 5]

A polarizing plate was obtained in the same manner as in example 1, except that the amount of boric acid added to the stretching bath during stretching in the aqueous solution was 2.5 parts by weight, and the laminate was put into an oven at 110 ℃ for 30 seconds to crystallize the resin base material.

[ example 6]

An easy adhesion layer was provided on one surface of a resin base material by the following method.

One surface of the resin substrate was subjected to corona treatment, and a mixed solution (solid content concentration 4.0%) obtained by mixing a 4.0% aqueous solution of acetoacetyl group-modified PVA (product name "GOHSEFIMERTM Z200", polymerization degree 1200, saponification degree 99.0 mol% or more, acetoacetyl group modification degree 4.6%, manufactured by japan synthetic chemical industries ltd.), a modified polyolefin resin aqueous dispersion (product name "aroaswbese 1030N", solid content concentration 22%) and pure water was applied to the corona-treated surface so that the thickness after drying was 2000nm, and dried at 60 ℃ for 3 minutes to form an easy-adhesion layer. Here, the solid content ratio of the acetoacetyl group-modified PVA to the modified polyolefin in the liquid mixture was 30: 70.

A polarizing plate was obtained in the same manner as in example 1, except that the surface of the easy-adhesion layer was subjected to corona treatment and a PVA-based resin layer was formed on the corona-treated surface.

Comparative example 1

A polarizing plate was obtained in the same manner as in example 1, except that the laminate was put into an oven at 85 ℃ for 30 seconds to crystallize the resin base material.

Comparative example 2

A polarizing plate was obtained in the same manner as in example 1, except that the amount of boric acid added to the stretching bath during stretching in the aqueous solution was set to 4.0 parts by weight, and the laminate was put into an oven at 110 ℃ for 30 seconds to crystallize the resin base material.

Comparative example 3

A polarizing plate was obtained in the same manner as in example 1, except that the laminate was put into an oven at 95 ℃ for 30 seconds to crystallize the resin base material.

The conditions for producing the polarizing plates in examples and comparative examples and the properties of the obtained polarizing plates are shown in table 1. In the table, MD is the absorption axis direction of the polarizer, and TD is the direction orthogonal to the absorption axis.

As shown in table 1, the polarizing plate of the example having the resin substrate having the specific crystallinity and the polarizing film satisfying the specific boric acid concentration suppressed the occurrence of cracks. On the other hand, the polarizing plate of the comparative example was found to have cracks and to have inferior durability to the polarizing plate of the example. In addition, the polarizing plate of example 6 is superior in adhesion between the resin base material and the polarizing film (PVA-based resin layer) as compared with the polarizing plates of other examples or comparative examples, and thus the occurrence of undesired peeling or floating of the polarizing film (PVA-based resin layer) or the resin base material during the production of the polarizing film or during the processing (e.g., punching) of the polarizing plate is appropriately prevented.

The reason why the generation of cracks was suppressed in the polarizing plates of examples is presumed as follows. That is, in the polarizing plate of the example, the difference between the dimensional change rate of the resin substrate in the MD direction and the dimensional change rate of the polarizing film (the dimensional change rate of the resin substrate-the dimensional change rate of the polarizing film) and the difference between the dimensional change rate of the resin substrate in the TD direction and the dimensional change rate of the polarizing film (the dimensional change rate of the resin substrate-the dimensional change rate of the polarizing film) are both within 5%, and the difference between the dimensional change rates in the TD direction (the absolute value) is a value close to the difference between the dimensional change rates in the MD direction (the absolute value) (within ± 1.5%). Since the polarizing film is oriented in the stretching direction (MD), mechanical properties tend to be weak in The Direction (TD) orthogonal to the stretching direction. Therefore, it is considered that by making the difference (absolute value) in the dimension change rate in the TD direction approximate to the difference (absolute value) in the dimension change rate in the MD direction, the strain is not biased in the TD direction, and the occurrence of cracks is suppressed.

Industrial applicability

The polarizing plate of the present invention is suitably used for, for example, an image display device.

Claims

1. A polarizing plate comprising a polyester resin substrate and a polarizing film laminated on one side of the polyester resin substrate and having a thickness of 10 μm or less,

the crystallinity of the polyester resin base material calculated by attenuated total reflection spectrometry is 0.60 to 0.80,

the boric acid concentration in the polarizing film is 10 to 20 wt%.

2. The polarizing plate according to claim 1, wherein the polyester-based resin is polyethylene terephthalate or a copolymer thereof.

3. The polarizing plate according to claim 1 or 2, wherein the polarizing film is laminated on one side of the polyester-based resin substrate without interposing an adhesive layer.

4. The polarizing plate according to claim 1 or 2, wherein a protective film is not provided on a side of the polarizing film opposite to a side where the polyester-based resin substrates are laminated.

5. The polarizing plate according to claim 1 or 2, wherein an easy adhesion layer is provided between the polyester-based resin substrate and the polarizing film.

6. A method for producing a polarizing plate as defined in any one of claims 1 to 5, comprising:

forming a polyvinyl alcohol resin film on a polyester resin substrate to produce a laminate;

stretching the laminate at a stretch ratio of 5.0 times or more;

dyeing the polyvinyl alcohol resin film; and

the stretched and dyed laminate is heated to crystallize the polyester resin base material, and the crystallinity calculated by attenuated total reflection spectroscopy is 0.60 to 0.80.