TW202242457A - Optical film, production method therefor, and polarizing plate - Google Patents

Abstract

Provided is an optical film containing a crystalline polymer having a negative intrinsic birefringence, wherein: the refractive index nx1 of the optical film in a direction which is perpendicular to the thickness direction and in which the maximum refractive index is applied, the refractive index ny1 of the optical film in a direction which is perpendicular to the thickness direction and perpendicular to the direction of nx1, and the refractive index nz1 of the optical film in the thickness direction satisfy expression (1); and the refractive index Re/d of the optical film in the in-plane direction satisfies expression (2). Expression (1): nx1 > nz1 > ny1; expression (2): Re/d ≥ 3*10<SP>-3</SP>.

Description

Optical film, manufacturing method thereof, and polarizing plate

The present invention relates to an optical film, its manufacturing method and a polarizing plate.

Conventionally, a production technique of a thin film using a resin has been proposed (Patent Document 1).

"Patent Documents" "Patent Document 1": Japanese Patent No. 4486854

Sometimes resins are used to produce optical films with anisotropic refractive index. Such an optical film having anisotropy in refractive index can be provided in a display device as a film such as a reflection suppressing film, a viewing angle compensation film, or the like. To give a specific example, with an optical film whose three-dimensional refractive indices nx, ny, and nz satisfy nx>nz>ny, it becomes possible to improve the display quality when viewing the display surface from an oblique direction.

A method for producing an optical film having three-dimensional refractive indices nx, ny, and nz satisfying nx>nz>ny is conventionally known. For example, there is known a method of producing an optical film satisfying nx>nz>ny by subjecting a resin film to an appropriate stretching process. However, it has been difficult to fabricate such optical films using polymers with negative intrinsic birefringence. Moreover, because polymers with negative intrinsic birefringence generally have low mechanical strength, it is difficult to stretch them at large stretching ratios. Accordingly, it is extremely difficult to obtain an optical film having a large birefringence in conventional methods.

The present invention is made in view of the aforementioned problems, and its object is to provide an optical film comprising a crystalline polymer having negative intrinsic birefringence, having a refractive index satisfying nx>nz>ny and having a large birefringence; A crystalline polymer with inherent birefringence, a method for manufacturing an optical film having a refractive index satisfying nx>nz>ny; and a polarizing plate provided with the aforementioned optical film.

The inventors of the present invention have devoted themselves to research to solve the aforementioned problems. As a result, the present inventors found that the aforementioned problems can be solved by using a method including stretching a film before stretching as a negative C plate comprising a crystalline polymer having negative intrinsic birefringence, and completed the present invention .

That is, the present invention includes the following.

[1] An optical film comprising a crystalline polymer having negative intrinsic birefringence, wherein the optical film has a refractive index nx ₁ in a direction perpendicular to a thickness direction and in a direction that imparts a maximum refractive index, The refractive index ny ₁ in the direction perpendicular to the thickness direction and the direction perpendicular to the direction nx ₁ and the refractive index nz ₁ in the thickness direction satisfy the following formula (1), the birefringence Re/ in the in-plane direction of the aforementioned optical film d satisfies the following formula (2). nx ₁ ＞nz ₁ ＞ny ₁ (1) Re/d≧3×10 ⁻³ (2)

[2] The optical film as described in [1], wherein the optical film is a stretched film.

[3] The optical film as described in [1] or [2], wherein the optical film has an elongated shape.

[4] A polarizing plate comprising the optical film and the polarizing film according to any one of [1] to [3].

[5] The polarizing plate according to [4], wherein the slow axis of the optical film and the absorption axis of the polarizing film form an angle of 80° to 100°.

[6] A method of manufacturing an optical thin film, which is a direction perpendicular to the thickness direction and a refractive index nx ₁ of the direction giving the maximum refractive index, a direction perpendicular to the thickness direction and a direction perpendicular to the direction nx ₁ The method of manufacturing an optical film in which the refractive index ny ₁ and the refractive index nz ₁ in the thickness direction satisfy the formula (1), the aforementioned manufacturing method includes: a step of preparing a pre-stretched film comprising a crystalline polymer having negative intrinsic birefringence ( i), and the step (ii) of stretching the aforementioned unstretched film, the refractive index nx ₂ of the aforementioned unstretched film prepared in the step (i) is the direction perpendicular to the thickness direction and the direction that imparts the maximum refractive index, The refractive index ny ₂ in the direction perpendicular to the thickness direction and the direction perpendicular to the nx ₂ direction and the refractive index nz ₂ in the thickness direction satisfy the following formula (3). nx ₁ >nz ₁ >ny ₁ (1) nz ₂ <nx ₂ ≒ny ₂ (3)

[7] The method for producing an optical film as described in [6], wherein step (i) includes: preparing a resin film comprising a crystalline polymer having negative intrinsic birefringence, and The aforementioned resin film is brought into contact with a solvent to obtain the aforementioned unstretched film.

[8] The method for producing an optical film according to [6] or [7], wherein the optical film has a single-layer structure.

[9] The method for producing an optical film according to any one of [6] to [8], wherein the crystalline polymer having negative intrinsic birefringence is a polystyrene polymer.

According to the present invention, it is possible to provide an optical film comprising a crystalline polymer having a negative intrinsic birefringence, having a refractive index satisfying nx>nz>ny and having a large birefringence; including a crystalline polymer having a negative intrinsic birefringence An object, a method for manufacturing an optical film having a refractive index satisfying nx>nz>ny; and a polarizing plate provided with the aforementioned optical film.

Embodiments and examples are disclosed below to describe the present invention in detail. However, the present invention is not limited to the implementation forms and examples shown below, and can be implemented with arbitrary changes within the scope not departing from the scope of the patent application and its equivalent scope of the present invention.

In the following description, the in-plane retardation Re of the film is a value represented by Re=(nx-ny)×d unless otherwise noted. Also, the birefringence in the in-plane direction of the film is the value shown by (nx-ny) unless otherwise noted, and is accordingly shown by Re/d. Furthermore, the retardation Rth in the thickness direction of the film is a value represented by Rth={[(nx+ny)/2]-nz}×d unless otherwise noted. Also, unless otherwise noted, the birefringence in the thickness direction of the film is represented by {[(nx+ny)/2]-nz}, and accordingly represented by Rth/d. Furthermore, the NZ coefficient of the film is the value shown by (nx-nz)/(nx-ny) unless otherwise noted. Here, nx represents the refractive index which is the direction (in-plane direction) perpendicular to the thickness direction of a film and the direction which gives a maximum refractive index. ny represents the refractive index in the direction perpendicular to the nx direction which is the aforementioned in-plane direction of the film. nz represents the refractive index in the thickness direction of the film. d represents the thickness of the film. Measurement wavelength, unless otherwise noted, is 550 nm.

In the following description, a material having positive intrinsic birefringence means a material whose refractive index in the extending direction becomes larger than that in a direction perpendicular thereto, unless otherwise noted. Accordingly, a polymer having positive intrinsic birefringence means, unless otherwise noted, a polymer in which the refractive index in the extending direction becomes larger than that in a direction perpendicular thereto. Also, a material having negative intrinsic birefringence means a material in which the refractive index in the extending direction becomes smaller than the refractive index in a direction perpendicular thereto, unless otherwise noted. Accordingly, a polymer having negative intrinsic birefringence means, unless otherwise noted, a polymer in which the refractive index in the extending direction becomes smaller than that in a direction perpendicular thereto.

In the following description, the so-called "long strip" shape refers to a shape that has a length of 5 times or more relative to the width, preferably 10 times or more, and specifically refers to a shape that can be rolled. The shape of a film that is long enough to be stored or transported in roll form. The upper limit of the length is not particularly limited, but is usually 100,000 times or less relative to the width.

In the following description, the directions of the so-called elements are "parallel", "perpendicular" and "orthogonal". Unless otherwise noted, within the range that does not impair the effect of the present invention, it can also include, for example, within ±5°. error within the range.

The angle formed by the optical axes (absorption axis, transmission axis, slow axis, etc.) of each film in a member including multiple films means the angle when the film is viewed from the thickness direction, unless otherwise noted.

"Polarizing plate", "circular polarizing plate", "wavelength plate" and "negative C plate", unless otherwise noted, include not only rigid parts but also flexible parts such as resin films.

[1. Outline of the optical film related to the first embodiment]

The optical film according to the first embodiment of the present invention satisfies the combination of the following requirements (A) to (C). Requirement (A): Contains a crystalline polymer having negative intrinsic birefringence. Requirement (B): The refractive indices nx ₁ , ny ₁ and nz ₁ of the optical film satisfy the following formula (1). Requirement (C): The birefringence Re/d in the in-plane direction of the optical film satisfies the following formula (2). nx ₁ ＞nz ₁ ＞ny ₁ (1) Re/d≧3×10 ⁻³ (2) (nx ₁ represents the refractive index of the optical film in the direction perpendicular to the thickness direction and giving the maximum refractive index. ny ₁ represents the refractive index of the optical film in the direction perpendicular to the thickness direction and perpendicular to the direction of nx _1. nz ₁ represents the refractive index of the optical film in the thickness direction.)

The aforementioned optical film has been difficult to manufacture conventionally, but it can be easily manufactured by using the manufacturing method related to the second embodiment described later.

[2. Crystalline polymers with negative intrinsic birefringence]

The optical film related to the first embodiment of the present invention includes a crystalline polymer having negative intrinsic birefringence. The term "crystalline polymer" refers to a polymer having crystallinity. A crystalline polymer means a polymer having a melting point Tm. That is, a crystalline polymer means a polymer whose melting point Tm can be observed by a differential scanning calorimeter (DSC).

The aforementioned crystalline polymer contained in the optical film has negative intrinsic birefringence. A crystalline polymer with negative intrinsic birefringence exhibits a large refractive index in a direction perpendicular to its stretching direction when stretched. In the case of using this crystalline polymer with negative intrinsic birefringence to carry out the production method related to the second embodiment described later, the refractive index satisfying formula (1) and satisfying formula (2) can be easily achieved. ) of birefringence.

The crystalline polymer having negative intrinsic birefringence is preferably a polymer containing an aromatic ring, and examples thereof include polystyrene-based polymers. In the following description, a crystalline polystyrene polymer may be referred to as a "crystalline polystyrene polymer".

Crystalline polystyrene polymers are obtained as polymers of styrenic monomers. Accordingly, the crystalline polystyrene polymer is a polymer including a structural unit having a structure formed by polymerizing a styrene monomer (hereinafter referred to as "styrene unit" as appropriate).

The styrene-based monomer refers to an aromatic vinyl compound such as styrene or a styrene derivative. Examples of styrene derivatives include compounds in which substituents are substituted at the benzene ring of styrene or at the α-position or β-position.

Examples of the styrene-based monomer include styrene, alkylstyrene, halogenated styrene, halogenated alkylstyrene, alkoxystyrene, vinylbenzoate, and hydrogenated polymers thereof.

Examples of alkylstyrenes include methylstyrene, ethylstyrene, isopropylstyrene, tertiary butylstyrene, 2,4-dimethylstyrene, phenylstyrene, ethylene Naphthalene and vinyl styrene. Examples of halogenated styrene include chlorostyrene, bromostyrene, and fluorostyrene. As an example of halogenated alkylstyrene, methylchlorostyrene is mentioned. Examples of alkoxystyrenes include methoxystyrene and ethoxystyrene. Among styrene-based monomers, styrene, methylstyrene, ethylstyrene, and 2,4-dimethylstyrene are preferable. In addition, the styrene-based monomer may be used alone or in combination of two or more.

The crystalline polystyrene polymer preferably has a parallel structure or a parallel structure, more preferably a parallel structure. The so-called crystalline polystyrene polymer has a parallel structure, which means that the stereochemical structure of the crystalline polystyrene polymer is a parallel structure. The so-called anti-parallel structure refers to a three-dimensional structure in which the phenyl groups that are side chains are alternately located in opposite directions in the Fisher projection formula with respect to the main chain formed by carbon-carbon bonds. Compared with polystyrene polymers having a heterogeneous structure, polystyrene polymers having a parallel structure generally have a low specific gravity and are excellent in hydrolysis resistance, heat resistance, and chemical resistance.

Stereoisomerism (tacticity: stereoisomerism) of a crystalline polystyrene polymer can be quantitatively analyzed by a nuclear magnetic resonance method ( ¹³ C-NMR method) using isotopic carbon. Stereoisomerism measured by ¹³ C-NMR method can be represented by the ratio of the presence of multiple consecutive structural units. Generally, for example, when there are two consecutive structural units, it is a diad, when there are three, it is a triad, and when there are five, it is a pentad. In this case, the crystalline polystyrene polymer having a diad (racemic diad) usually contains 75% or more of diads, so as to have a diad of 85% or more. Preferably, or usually more than 30% in terms of pentads (racemic pentads), preferably more than 50% of the anti-stereoconformity.

The crystalline polystyrene polymer may be a homopolymer or a copolymer. Accordingly, the crystalline polystyrene polymer may be a homopolymer of one type of styrene-based monomer, or may be a copolymer of two or more types of styrene-based monomers. When the crystalline polystyrene-based polymer is a copolymer of two or more styrene-based monomers, the ratio of each styrene-based unit to 100% by weight of the entire crystalline polystyrene-based polymer is given by It is preferably at least 5% by weight, more preferably at least 10% by weight, more preferably at most 95% by weight, more preferably at most 90% by weight.

Furthermore, the crystalline polystyrene polymer may be a copolymer of one or more types of styrene monomers and monomers other than styrene monomers. The ratio of the styrene unit contained in the crystalline polystyrene polymer is preferably at least 80% by weight, more preferably at least 83% by weight, from the viewpoint of obtaining an optical film having desired optical properties. More than 85% by weight is more preferable.

Usually, the ratio of a certain structural unit contained in the crystalline polystyrene polymer should be the same as the ratio of the monomer corresponding to the aforementioned structural unit to all the monomers in the crystalline polystyrene polymer. Accordingly, the ratio of the styrene-based units contained in the crystalline polystyrene-based polymer is equal to the ratio of the styrene-based monomers to all the monomers in the crystalline polystyrene-based polymer.

Crystalline polystyrene polymers obtained by, for example, polymerizing styrenic monomers in an inert hydrocarbon medium or in the absence of a solvent, using titanium compounds and condensation products of water and trialkylaluminum as catalysts (cf. Patent Publication No. S62-187708).

The crystalline polymers having negative intrinsic birefringence may be used alone or in combination of two or more in arbitrary ratios.

The weight average molecular weight Mw of the crystalline polymer having negative intrinsic birefringence is preferably at least 130,000, more preferably at least 140,000, particularly preferably at least 150,000, and is preferably at most 500,000, more preferably at most 450,000 , preferably below 400,000. A crystalline polymer having such a weight average molecular weight Mw can have a high glass transition temperature Tg, so that the heat resistance of the optical film can be improved.

The weight average molecular weight (Mw) of the polymer can be measured as a polystyrene-equivalent value by gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene as an eluent.

The glass transition temperature Tg of the crystalline polymer having negative intrinsic birefringence is preferably 85°C or higher, more preferably 90°C or higher, and most preferably 95°C or higher. In the case of using a crystalline polymer having such a high glass transition temperature Tg, the heat resistance of the optical film can be improved. The glass transition temperature of the crystalline polymer is preferably at most 160°C, more preferably at most 155°C, and most preferably at most 150°C, from the viewpoint of smooth stretching during the production process of the optical film.

The melting point Tm of the crystalline polymer with negative intrinsic birefringence is preferably above 200°C, more preferably above 210°C, especially above 220°C, preferably below 300°C, and below 290°C More preferably, it is especially preferably below 280°C. When the melting point Tm is within the above-mentioned range, progress of unintended crystallization of the crystalline polymer at the time of production of the resin film and generation of foreign matter due to thermal decomposition can be suppressed. Accordingly, an optical film having good appearance and optical properties can be easily obtained.

The glass transition temperature Tg and melting point Tm of the polymer can be measured by the following methods. First, the polymer is melted by heating, and the melted polymer is rapidly cooled by dry ice. Next, using this polymer as a sample, use a differential scanning calorimeter (DSC) to measure the glass transition temperature Tg and melting point Tm of the polymer at a heating rate of 10°C/min (heating mode).

The degree of crystallinity of a crystalline polymer having a negative intrinsic birefringence contained in an optical film is generally high to a certain degree or more. The specific range of crystallinity is preferably above 10%, preferably above 15%, and especially preferably above 30%. The crystallinity of crystalline polymers is measured by X-ray diffraction.

Typically, optical films are formed from resins comprising crystalline polymers with negative intrinsic birefringence. In the following description, a resin containing a crystalline polymer may be referred to as a "crystalline resin". Accordingly, an optical film usually contains a crystalline resin, preferably only a crystalline resin. The aforementioned crystalline resin including a crystalline polymer having negative intrinsic birefringence generally has negative intrinsic birefringence. Furthermore, the crystalline resin is preferably a thermoplastic resin.

The amount of the crystalline polymer having negative intrinsic birefringence in 100% by weight of the crystalline resin is preferably 70% by weight or more, and 80% by weight from the viewpoint of obtaining an optical film with desired optical properties. The above is preferable, more preferably 90% by weight or more, and usually 100% by weight or less.

The crystalline resin may also contain arbitrary components combined with a crystalline polymer having negative intrinsic birefringence. Examples of optional components include arbitrary polymers such as crystalline polymers with positive intrinsic birefringence and non-crystalline polymers without crystallinity; slip agents; layered crystalline compounds; fine particles such as inorganic fine particles; antioxidants , heat stabilizers, light stabilizers, weather stabilizers, ultraviolet absorbers, near infrared absorbers and other stabilizers; plasticizers; colorants such as dyes and pigments; antistatic agents; etc. Optional components may be used alone or in combination of two or more. The amount of the optional components can be appropriately determined within the range that does not significantly impair the effect of the present invention. The amount of the optional components is such that the total light transmittance of the optical film can be maintained at 85% or more, for example.

[3. Refractive index of optical film]

The refractive indices nx ₁ , ny ₁ and nz ₁ of the optical film related to the first embodiment of the present invention satisfy the formula (1). Films comprising polymers with negative intrinsic birefringence generally have a tendency to exhibit large birefringence in the thickness direction upon stretching. Accordingly, it has conventionally been difficult to manufacture a thin film comprising a polymer having negative intrinsic birefringence and having a refractive index satisfying the formula (1). In contrast, the optical film related to this embodiment includes a polymer with negative intrinsic birefringence (requirement (A)) and at the same time can achieve the refractive indices nx ₁ , ny ₁ and nz ₁ ( Requirement (B)) is rather surprising to one of ordinary skill in the art.

An optical film having a refractive index nx ₁ , ny ₁ and nz ₁ satisfying the formula (1), not only the polarization state of the light passing through the optical film along the thickness direction, but also the oblique direction that is neither parallel nor perpendicular to the thickness direction The polarization state of the passing light varies moderately. Accordingly, the optical film can exert its optical function not only to the light in the thickness direction, but also to the light in the oblique direction. For example, when an optical film is required to impart a phase difference of 1/4 wavelength to light passing in the thickness direction, the optical film must also impart a phase difference of 1/4 wavelength to light passing in an oblique direction. Furthermore, for example, in the case where the optical film imparts a retardation of 1/2 wavelength to light passing in the thickness direction, the optical film also imparts a retardation of 1/2 wavelength to light passing in an oblique direction.

[4. Birefringence in the in-plane direction of optical films]

The birefringence Re/d in the in-plane direction of the optical film related to the first embodiment of the present invention satisfies the formula (2). Specifically, the birefringence Re/d in the in-plane direction of the optical film is usually 3.0×10 ⁻³ or more, preferably 3.5×10 ⁻³ or more, more preferably 5.0×10 ⁻³ or more, and 8.0×10 −3 or more. ^-3 or more is preferred. Optical films with birefringence Re/d of such a large in-plane direction can be thinned to obtain the desired in-plane retardation. Accordingly, a thin optical film having moderate in-plane retardation can be realized. The upper limit of the birefringence Re/d in the in-plane direction of the optical film is not particularly limited, but from the viewpoint of smooth production, it is preferably 40×10 ^{− 3} or less, more preferably 30×10 ^{− 3} or less, It is better to be below 20×10 ^{− 3} .

In general, in order to obtain a thin film having a large birefringence Re/d in the in-plane direction by stretching, a large stretching ratio is required. However, films comprising polymers with negative intrinsic birefringence tend to exhibit a large refractive index in the thickness direction by stretching. Accordingly, if stretching is performed at a large stretching ratio, a large refractive index will be generated in the thickness direction, and as a result, it is difficult to obtain the refractive indices nx ₁ , ny ₁ and nz ₁ satisfying the above-mentioned formula (1). Therefore, it is difficult to achieve the requirement (C) within the scope of satisfying the limitation of the requirement (B), and it has not been realized in the past. Since it is inherently difficult to realize a film satisfying the requirements (A) and (B) as described above, it can be understood that the combination of satisfying the requirements (A) to (C) is highly difficult. The optical film of this embodiment has high industrial value in that it can provide an optical film that satisfies the combination of the requirements (A) to (C), which are particularly difficult to achieve.

[5. Layer structure of optical thin film]

The optical film may also have a multilayer structure including a plurality of layers, but preferably has a single layer structure. The so-called single-layer structure means a structure having only a single layer body having the same composition and not having a layer body having a composition different from the aforementioned composition. Accordingly, the optical film preferably has a layer body composed of the aforementioned crystalline resin alone.

[6. Characteristics of Optical Films]

The optical film preferably has an in-plane retardation within an appropriate range according to its application.

Specifically, the specific in-plane retardation Re of the optical film can be measured at a wavelength of 550 nm, preferably at least 100 nm, more preferably at least 110 nm, especially preferably at least 120 nm, and can be less than 180 nm. Best, preferably below 170 nm, especially preferably below 160 nm. In this case, the optical film can function as a 1/4 wavelength plate.

For example, the specific in-plane retardation Re of the optical film is measured at a wavelength of 550 nm, preferably above 245 nm, preferably above 265 nm, especially preferably above 270 nm, and below 320 nm. Best, preferably below 300 nm, especially preferably below 295 nm. In this case, the optical film can function as a 1/2 wavelength plate.

Generally, an optical film having refractive indices nx ₁ , ny ₁ and nz ₁ satisfying formula (1) means that the NZ coefficient is greater than 0 and less than 1. The NZ coefficient of the optical film is represented by (nx ₁ -nz ₁ )/(nx ₁ -ny ₁ ). More specifically, the NZ coefficient of the optical film is usually greater than 0.0, preferably greater than 0.1, more preferably greater than 0.15, especially greater than 0.2, and usually less than 1.0, preferably less than 0.9, and less than 0.9. It is better to reach 0.85, and it is more preferable to be less than 0.8. When an optical film having an NZ coefficient in such a range is installed in a display device, it can effectively improve the display quality of the display device such as viewing angle, contrast, and image quality.

The value of the retardation Rth in the thickness direction of the optical film is set according to the application of the optical film. The specific retardation Rth in the thickness direction of the optical film is preferably above −100 nm, preferably above −50 nm, especially preferably above −20 nm, preferably below 100 nm, and below 50 nm Preferably, the thickness below 20 nm is especially preferred.

The value of birefringence Rth/d in the thickness direction of the optical film is set according to the application of the optical film. The absolute value of the birefringence in the thickness direction of the optical film |Rth/d| is preferably 0.01×10 ^{− 3} or more, more preferably 0.05×10 ^{− 3} or more, and most preferably 0.1×10 ^{− 3} or more , and preferably below 2.0×10 ^{− 3} , preferably below 1.5×10 ^{− 3} , and most preferably below 1.0×10 ^{− 3} .

The retardation and NZ coefficient of the film should be measured using a phase difference meter (such as "AxoScan OPMF-1" manufactured by AXOMETRICS). In addition, the birefringence of a thin film can be obtained by dividing the retardation by the thickness.

Crystalline polymers with negative intrinsic birefringence generally have poor mechanical strength. On the other hand, the optical film related to this embodiment can have high mechanical strength. According to the study of the present inventors, it is clear that the crystalline resin containing the crystalline polymer having negative intrinsic birefringence can be improved by bringing the resin film into contact with the solvent in the production method related to the second embodiment described later. mechanical strength. The mechanical strength of an optical film can be represented by, for example, bendability. For example, optical films can be bent using a mandrel with a diameter of 2 mm without breaking.

Optical films preferably have high transparency. The specific total light transmittance of the optical film is preferably above 80%, preferably above 85%, and most preferably above 88%. The total light transmittance of the film can be measured with an ultraviolet/visible light spectrometer at a wavelength of 400 nm to 700 nm.

Optical films preferably have low haze. The haze of the optical film is preferably less than 1.0%, more preferably less than 0.8%, more preferably less than 0.5%, ideally 0.0%. When the optical film with such a small haze is installed in a display device, the sharpness of the image displayed by the display device can be improved. The haze of the film must be measured with a haze meter (such as "NDH5000" manufactured by Nippon Denshoku Kogyo Co., Ltd.).

Optical films may contain solvents. This solvent may be incorporated into the film in the process of bringing the resin film into contact with the solvent in the manufacturing method related to the second embodiment described later. Specifically, all or part of the solvent incorporated into the film by contacting the resin film may enter the interior of the polymer. Therefore, even if drying is performed above the boiling point of the solvent, it is difficult to completely remove the solvent easily. Accordingly, the optical film may contain a solvent.

The optical film is preferably stretched film. The term "stretched film" refers to a film produced by stretching. Among them, the optical film is preferably a uniaxially stretched film. The so-called uniaxially stretched film means a film that is positively stretched in only one direction and is not positively stretched in other directions. Since the uniaxially stretched film can be produced by stretching in only one direction, the production process can be simplified, thereby enabling simple production.

The optical film may be a film cut into sheets, or may be an elongated film having an elongated shape. When the optical film has an elongated shape, the optical film and the elongated polarizing film can be bonded together to continuously manufacture a polarizing plate.

The thickness of the optical film can be appropriately set according to the application of the optical film. The specific thickness of the optical film is preferably above 5 μm, more preferably above 10 μm, especially above 15 μm, preferably below 50 μm, preferably below 40 μm, and below 30 μm For Yu Jia.

The optical thin film related to the first embodiment mentioned above can be manufactured by the manufacturing method related to the second embodiment described later.

[7. Outline of the manufacturing method of the optical film related to the second embodiment]

In the method for producing an optical film according to the second embodiment of the present invention, an optical film whose refractive indices nx ₁ , ny ₁ and nz ₁ satisfy the formula (1) is produced. According to the manufacturing method related to this second embodiment, the optical film related to the first embodiment can be manufactured. Furthermore, according to the manufacturing method related to the second embodiment, it is also possible to manufacture an optical film having birefringence Re/d in the in-plane direction that does not satisfy the formula (2).

The manufacturing method of the optical film related to the second embodiment includes the step (i) of preparing an unstretched film comprising a crystalline polymer having negative intrinsic birefringence, and Step (ii) of stretching the film before stretching.

In this production method, the refractive indices nx ₂ , ny ₂ and nz ₂ of the film before stretching prepared in step (i) satisfy the following formula (3). nz ₂ <nx ₂ ≒ny ₂ (3) (nx ₂ represents the refractive index in the direction perpendicular to the thickness direction of the film before stretching and gives the maximum refractive index. ny ₂ represents the direction perpendicular to the thickness direction of the film before stretching Refractive index in the direction perpendicular to the direction of nx _2. nz ₂ represents the refractive index in the thickness direction of the film before stretching.)

In the formula (3), "nx ₂ ≒ny ₂ " means that the birefringence "nx ₂ −ny ₂ " (=Re/d) in the in-plane direction is small. Specifically, the birefringence “nx ₂ −ny ₂ ” in the in-plane direction is larger than the absolute value of the birefringence in the thickness direction “|[(nx ₂ +ny ₂ )/2]−nz ₂ |” (=|Rth/d ｜) is still small, “nx ₂ ≒ny ₂ ” must be established. According to this, the birefringence Re/d(=nx ₂ -ny ₂ ) in the in-plane direction is usually higher than the absolute value of the birefringence in the thickness direction |Rth/d|(=|[(nx ₂ +ny ₂ )/2]-nz ₂ ｜) It can be judged as "nx ₂ ≒ny ₂ " for a film before stretching that is still small. Generally, when the birefringence Re/d in the in-plane direction is smaller than the absolute value |Rth/d| of the birefringence in the thickness direction, the NZ coefficient becomes smaller than −0.5 or larger than 1.5.

The film prior to stretching prepared in step (i) comprises a crystalline polymer having negative intrinsic birefringence. Accordingly, the pre-stretched film can generally have negative birefringent properties. The so-called film "has negative birefringence characteristics" means that when the film is stretched along a stretching direction, the increase in the refractive index in the direction perpendicular to the stretching direction is greater than the increase in the refractive index in the stretching direction Still big. Here, the aforementioned extending direction is generally perpendicular to the thickness direction, and accordingly, it is an in-plane direction. In terms of the amount of increase, when the refractive index becomes larger by stretching, the increase of the refractive index is positive, and when the refractive index becomes smaller by stretching, the increase of the refractive index is negative value. Therefore, if the unstretched film having the refractive indices nx ₂ , ny ₂ and nz ₂ satisfying formula (3) is stretched in step (ii), the refractive indices nx ₂ and nz ₂ will be relatively large, and the refractive index ny ₂ will be relatively large. get smaller. Therefore, according to the aforementioned manufacturing method, an optical film satisfying the formula (1) can be obtained. Usually, the optical film obtained by this operation also has a negative birefringence characteristic.

The method of preparing the film before stretching is not limited, but it is preferably prepared by a method including contacting a resin film including a crystalline polymer having negative intrinsic birefringence with a solvent. Accordingly, the step (i) preferably includes preparing a resin film of a crystalline polymer having negative intrinsic birefringence and contacting the resin film with a solvent to obtain a pre-stretched film.

The present inventors presume that the mechanism of obtaining a film before stretching by bringing a resin film into contact with a solvent is as follows. However, the technical scope of the present invention is not limited to the following mechanisms.

When a resin film containing a crystalline polymer is brought into contact with a solvent, the solvent will soak into the resin film. Through the action of the immersed solvent, the molecules of the crystalline polymer in the film will undergo micro-Brownian motion, and the molecular chains will be oriented. According to the study of the present inventors, it is conceivable that the solvent-induced crystallization of the crystalline polymer may proceed during the orientation of the molecular chains.

Incidentally, the surface area of the resin film is larger than that of the main surface on the front side and the back side. Accordingly, the penetration rate of the organic solvent is greater than the penetration rate in the thickness direction through the front surface or the rear surface. If so, the molecules of the aforementioned crystalline polymer are oriented in such a manner that the molecules of the polymer are oriented in the thickness direction.

If the molecules of a crystalline polymer with negative intrinsic birefringence are oriented along the thickness direction, the refractive index in the thickness direction of the film containing the molecules will be relatively smaller, and the refractive index in the in-plane direction perpendicular to the thickness direction will be relatively smaller. big. Accordingly, a pre-stretched film having a higher refractive index nx ₂ and ny ₂ in the in-plane direction than the refractive index nz ₂ in the thickness direction can be obtained.

[8. Step (i) of preparing the film before stretching]

The method for producing an optical film according to the second embodiment of the present invention includes the step (i) of preparing a film including a crystalline polymer having negative intrinsic birefringence before stretching. The step (i) preferably includes preparing a resin film comprising a crystalline polymer having negative intrinsic birefringence and contacting the resin film with a solvent to obtain a pre-stretched film.

The crystalline polymer with negative intrinsic birefringence has to be the same as that contained in the optical film. Accordingly, the resin film must contain the same crystalline resin as that contained in the optical film. Furthermore, the resin film is preferably composed of only the same crystalline resin as that contained in the optical film.

However, the crystallinity of the crystalline polymer contained in the resin film is preferably small. The specific crystallinity is preferably less than 10%, more preferably less than 5%, and most preferably less than 3%. If the crystallinity of the crystalline polymer contained in the resin film before being in contact with the solvent is low, most of the molecules of the crystalline polymer can be oriented in the thickness direction by contacting with the solvent, so it can be used in a wide range. Adjust the index of refraction in the range.

The resin film preferably has optical isotropy. Accordingly, the resin film preferably has a small birefringence Re/d in the in-plane direction, and preferably has a small absolute value |Rth/d| of birefringence in the thickness direction. Specifically, the birefringence Re/d in the in-plane direction of the resin film is preferably less than 0.7×10 ^{− 3} , more preferably less than 0.6×10 ^{− 3} , especially less than 0.5×10 ^{− 3} good. In addition, the absolute value of birefringence |Rth/d| in the thickness direction of the resin film is preferably less than 0.7×10 ^{− 3} , more preferably less than 0.6×10 ^{− 3} , and less than 0.5×10 ^{− 3} For Yu Jia. Having optical isotropy in this way means that the molecules of the crystalline polymer contained in the resin film have low orientation and are substantially non-oriented. In the case of using such an optically isotropic resin film, it is not necessary to precisely control the optical properties of the resin film, thereby simplifying the production of the optical film because it is not necessary to precisely control the orientation of the molecules of the crystalline polymer. method. Furthermore, when an optically isotropic resin film is used, an optical film with a small haze can generally be obtained.

The resin film preferably has a small solvent content, and more preferably does not contain a solvent. The ratio of the solvent contained in the resin film to 100% of the weight of the resin film (solvent content rate) is preferably 1% or less, more preferably 0.5% or less, particularly preferably 0.1% or less, ideally 0.0%. The amount of solvent contained in the resin film before contact with the solvent is small, and most of the molecules of the crystalline polymer can be oriented in the thickness direction by contact with the solvent, so it becomes possible to adjust the refractive index in a wide range . The solvent content of a resin film is measured by density.

The haze of the resin film is preferably less than 1.0%, more preferably less than 0.8%, more preferably less than 0.5%, and ideally 0.0%. The smaller the haze of the resin film, the easier it is to reduce the haze of the obtained optical film.

The thickness of the resin film is preferably set according to the thickness of the optical film to be produced. Usually by contact with a solvent, the thickness will increase. And, by stretching, the thickness becomes smaller. Therefore, the thickness of the resin film may also be set in consideration of changes in thickness due to contact with a solvent and elongation as described above.

The resin film can also be cut into sheets, but a strip-shaped film is preferred. By using the elongated resin film, since the optical film can be continuously produced by the roll-to-roll method, the productivity of the optical film can be effectively improved.

As the manufacturing method of the resin film, in terms of obtaining a solvent-free resin film, injection molding method, extrusion molding method, press molding method, inflation molding method, blow molding method, calender molding method, die casting Resin molding methods such as molding and compression molding are preferable. Among them, extrusion molding is preferable in terms of ease of thickness control.

The production conditions in the extrusion molding method are preferably as follows. The cylinder temperature (molten resin temperature) is preferably above Tm, preferably above "Tm+20°C", preferably below "Tm+100°C", and preferably below "Tm+50°C". Also, the cooling member that the molten resin extruded into a film form first contacts is not particularly limited, but casting rolls are generally used. The casting roll temperature is preferably above "Tg-50°C", preferably below "Tg+70°C", and preferably below "Tg+40°C". In the case of producing a resin film under such conditions, a resin film having a thickness of 1 μm to 1 mm can be easily produced. Here, "Tm" represents the melting point of the crystalline polymer having negative intrinsic birefringence, and "Tg" represents the glass transition temperature of the crystalline polymer having negative intrinsic birefringence.

The step (i) preferably includes exposing the resin film to a solvent after obtaining the resin film. The refractive index of the resin film changes through contact with the solvent, and a pre-stretched film having the refractive indices nx ₂ , ny ₂ and nz ₂ satisfying the formula (3) can be obtained.

As the solvent, an organic solvent is usually used. As for the specific type of solvent, a solvent that can be immersed in the resin film without dissolving the crystalline polymer contained in the resin film can be used, such as: cyclohexane, toluene, alkene, decahydronaphthalene and other hydrocarbon solvents; carbon disulfide. The kind of solvent may be 1 type, and may be 2 or more types.

The method of contacting the resin film and the solvent is not limited. Examples of the contact method include a spraying method of spraying a solvent on a resin film, a coating method of applying a solvent to a resin film, and a dipping method of immersing a resin film in a solvent. Among them, the dipping method is preferable because continuous contact can be easily performed.

The temperature of the solvent contacting the resin film is arbitrary within the range in which the solvent can maintain a liquid state, and accordingly, it can be set in a range of not less than the melting point of the solvent and not more than the boiling point.

The contact time is preferably at least 0.5 seconds, more preferably at least 1.0 seconds, and most preferably at least 5.0 seconds. The upper limit is not particularly limited, and may be, for example, 24 hours or less. However, since the degree of orientation tends not to change significantly even if the contact time is prolonged, it is preferable that the contact time be short within the range in which desired optical characteristics can be obtained.

The refractive index of the resin film changes by contacting with a solvent. This change in refractive index is usually expressed as a change in birefringence Rth/d in the thickness direction. The amount of change in birefringence Rth/d in the thickness direction due to contact with the solvent is preferably 0.1×10 ⁻³ or more, more preferably 0.5×10 ⁻³ or more, and most preferably 1.0×10 ⁻³ or more , and preferably below 50.0×10 ⁻³ , preferably below 10.0×10 ⁻³ , and most preferably below 5.0×10 ⁻³ . The change amount of the birefringence Rth/d in the thickness direction means the absolute value of the change in the birefringence Rth/d in the thickness direction. The specific change in the birefringence Rth/d in the thickness direction is the birefringence in the thickness direction of the resin film (film before solvent contact) is subtracted from the birefringence Rth/d in the thickness direction of the film before stretching (film after solvent contact) Rth/d is obtained in the form of its absolute value. Generally, the birefringence Rth/d in the thickness direction increases due to the contact between the resin film and the solvent.

The birefringence Re/d in the in-plane direction of the resin film may or may not change due to solvent contact. The amount of change in birefringence Re/d in the in-plane direction due to contact with the solvent is usually 0.0×10 ^{− 3} or more, preferably 0.1×10 ^{− 3} or more, and most preferably 0.2×10 ^{− 3} or more, and It is better to be less than 10×10 ^{− 3} , better to be less than 5×10 ^{− 3} , and especially better to be less than 2×10 ^{− 3} . The amount of change in birefringence Re/d in the in-plane direction means the absolute value of the change in birefringence Re/d in the in-plane direction. The specific change in the birefringence Re/d in the in-plane direction is subtracted from the birefringence Re/d in the in-plane direction of the film before stretching (film after solvent contact) in the in-plane direction of the resin film (film before solvent contact) The birefringence Re/d is obtained in the form of its absolute value.

By bringing the resin film into contact with the solvent as described above, a pre-stretched film having refractive indices nx ₂ , ny ₂ , and nz ₂ satisfying the formula (3) can be obtained. Equation (3) indicates that the film before stretching is a negative C plate. Accordingly, the refractive index nz ₂ in the thickness direction of the film before stretching is smaller than the refractive indices nx ₂ and ny ₂ in the in-plane direction. In addition, the refractive indices nx ₂ and ny ₂ in the in-plane direction of the film before stretching have the same value or close values. Therefore, the film before stretching usually has a relatively small difference between the refractive index nx ₂ and the refractive index ny ₂ , a relatively large difference between the refractive index nx ₂ and the refractive index nz ₂ , and a relatively large difference between the refractive index ny ₂ and the refractive index nz ₂ .

At this time, the difference between the refractive index nz ₂ in the thickness direction and the refractive indices nx ₂ and ny ₂ in the in-plane direction can be represented by the birefringence Rth/d in the thickness direction. That is, since the birefringence Rth/d in the thickness direction of the film before stretching is expressed by "Rth/d=[(nx ₂ +ny ₂ )/2]-nz ₂ ", the birefringence Rth/d in the thickness direction can be to represent the difference between the refractive index nz ₂ in the thickness direction and the refractive indices nx ₂ and ny ₂ in the in-plane direction. The birefringence Rth/d in the thickness direction of the film before stretching is preferably at least 0.2×10 ^{− 3} , more preferably at least 0.5×10 ^{− 3} , more preferably at least 1.0×10 ^{− 3} , and more than 10×10 ^{− 3} or less is better, 6.0×10 ^{− 3} or less is better, and 4.0×10 ^{− 3} or less is especially good.

Also, the difference between the refractive index nx ₂ and the refractive index ny ₂ in the in-plane direction can be represented by the birefringence Re/d in the in-plane direction. That is, since the birefringence Re/d in the in-plane direction of the film before stretching is represented by "Re/d=nx ₂ -ny ₂ ", the in-plane direction can be expressed by the birefringence Re/d in the in-plane direction The difference between the refractive index nx ₂ and the refractive index ny ₂ . Usually, the difference between the refractive index nx ₂ and the refractive index ny ₂ in the in-plane direction is smaller than the difference between the refractive index nz ₂ in the thickness direction and the refractive indices nx ₂ and ny ₂ in the in-plane direction. Accordingly, the birefringence Re/d in the in-plane direction of the film before stretching has a smaller value than the birefringence Rth/d in the thickness direction of the film before stretching. The specific range of birefringence Re/d in the in-plane direction of the film before stretching is preferably at least 0.1×10 ^{− 3} , more preferably at least 0.2×10 ^{− 3} , and most preferably at least 0.5×10 ^{− 3} , And preferably below 5.0×10 ^{− 3} , preferably below 4.0×10 ^{− 3} , and most preferably below 3.0×10 ^{− 3} .

It is good that the film has an NZ coefficient greater than 1.5 before stretching. The specific NZ coefficient of the film before stretching is preferably greater than 1.5, more preferably greater than 1.8, and most preferably greater than 2.0. The upper limit is not particularly limited, but it is preferably less than 200, more preferably less than 100, and particularly preferably less than 50.

Usually, the thickness of the resin film is increased by the impregnation of the solvent in contact with the resin film into the resin film. Accordingly, the thickness of the unstretched film obtained after contacting with the solvent is generally thicker than that of the resin film. In this case, the lower limit of the change rate of the thickness may be, for example, 1% or more, 5% or more, or 10% or more. In addition, the upper limit of the rate of change of the thickness is, for example, 80% or less, 50% or less, or 40% or less. The aforementioned change rate of thickness is a ratio obtained by dividing the thickness difference between the resin film (film before solvent contact) and the film before stretching (film after solvent contact) by the thickness of the resin film.

Generally, a film obtained by exposing a resin film to a solvent before stretching has a higher mechanical strength than a resin film before contact with a solvent. Accordingly, since the pre-stretching film can have excellent stretch resistance, even if severe stretching conditions are used in the step (ii), it is less likely to be broken. Accordingly, in the case of using an unstretched film obtained by contacting a resin film with a solvent, it can be stretched at a high temperature and at a high stretching ratio.

[9. Step (ii) of stretching the film before stretching]

The manufacturing method of the optical film according to the second embodiment of the present invention includes the step (ii) of stretching the pre-stretching film after the step (i). By stretching, the molecules of the polymer contained in the film before stretching can be oriented in a direction corresponding to the stretching direction. Accordingly, by stretching in the step (ii), since the refractive index of the film before stretching can be adjusted, an optical film having desired optical characteristics can be obtained.

The extending direction is not limited, and examples thereof include a longitudinal direction, a width direction, and an oblique direction. Here, the oblique direction means a direction perpendicular to the thickness direction and neither parallel nor perpendicular to the width direction. Furthermore, the extending direction may be a single direction, or may be two or more directions, but a single direction is preferable. Furthermore, among stretching in one direction, free uniaxial stretching in which no binding force is applied in directions other than the stretching direction is more preferable. By such extensions, optical films with desired optical properties can be easily manufactured.

Since a film before stretching usually has negative birefringence characteristics, when stretching in a single direction, an optical film having a slow axis in a direction perpendicular to the stretching direction can be obtained. Accordingly, since the direction of the slow axis of the optical film can be adjusted through the stretching direction, the stretching direction can also be selected according to the direction of the slow axis that the optical film is to display. Generally, a strip-shaped polarizing film has an absorption axis in its longitudinal direction and a transmission axis in its width direction. According to this, for example, in the case of obtaining an optical film having a slow axis perpendicular to the absorption axis of a polarizing film, the film before stretching can also be stretched along the long-side direction to obtain a slow axis in the width direction. axis of optical film. In this case, the polarizing plate can be efficiently produced by the roll-to-roll method using the long optical film and the long polarizing film.

The elongation ratio is preferably at least 1.1 times, more preferably at least 1.2 times, more preferably at most 20.0 times, more preferably at most 10.0 times, more preferably at most 5.0 times, and most preferably at most 2.0 times. The specific stretching ratio is appropriately set to meet expectations in accordance with factors such as optical properties, thickness, and mechanical strength of the optical film to be manufactured. When the stretching ratio is equal to or greater than the lower limit of the aforementioned range, the birefringence can be greatly changed by stretching. In addition, when the stretch ratio is not more than the upper limit of the above-mentioned range, the direction of the slow axis can be easily controlled, and film breakage can be effectively suppressed.

The stretching temperature is preferably above "Tg+5°C", preferably above "Tg+10°C", preferably below "Tg+100°C", and preferably below "Tg+90°C". Here, "Tg" represents the glass transition temperature of a crystalline polymer having negative intrinsic birefringence. When the stretching temperature is equal to or higher than the lower limit of the aforementioned range, the film before stretching can be sufficiently softened so as to be uniformly stretched. Also, at such a high temperature, the amount of solvent in the film can be reduced by drying. In particular, in the case of using a pre-stretched film obtained by contacting a resin film with a solvent, since the pre-stretched film has high mechanical strength, it is difficult to break even by stretching at a high temperature. Extending is preferred for efficient drying. In addition, when the stretching temperature is below the upper limit of the above-mentioned range, since the solidification of the film before stretching due to the progress of crystallization of the crystalline polymer can be suppressed, the stretching can be smoothly performed, and, by Stretching enables large birefringence to unfold. Furthermore, the haze of the obtained optical film can generally be reduced to improve transparency.

By applying the aforementioned stretching treatment, since the molecules in the film can be oriented along the stretching direction, the refractive index of the film before stretching changes. Accordingly, an optical film having refractive indices nx ₁ , ny ₁ and nz ₁ satisfying formula (1) can be obtained.

[10. Arbitrary process]

The method for producing an optical film may further include combining any process with the above-mentioned process (i) to process (ii). As an optional step, for example, a step of preheating the film before stretching before step (ii), a step of applying heat treatment to the optical film obtained in step (ii) to promote the crystallization of the crystalline polymer, an optical The process of drying the film to reduce the amount of solvent in the film, the process of heat-shrinking the optical film to remove the residual stress in the film, etc.

According to the aforementioned production method, a long optical film can be produced using a long film before stretching. The manufacturing method of an optical film may also include the process of winding up the elongated optical film manufactured in this way into a roll shape. In addition, the manufacturing method of an optical film may include the process of cutting out the elongated optical film into a desired shape.

[11. Optical film produced by the production method related to the second embodiment]

According to the manufacturing method related to the second embodiment mentioned above, an optical film having refractive indices nx ₁ , ny ₁ and nz ₁ satisfying the formula (1) can be obtained. The optical film may not have the in-plane birefringence Re/d satisfying the formula (2), but preferably has the in-plane birefringence Re/d satisfying the formula (2). According to the production method related to the second embodiment, the same optical film as that described in the first embodiment can be obtained except that it does not necessarily have the birefringence Re/d in the in-plane direction satisfying the formula (2).

[12. Polarizing plate]

A polarizing plate related to an embodiment of the present invention includes the above-mentioned optical film and polarizing film. Polarizing films generally function as linear polarizers. Accordingly, the polarizer can transmit some polarized light and shield other polarized light.

A polarizing film generally has an absorption axis and a transmission axis perpendicular to the absorption axis. Also, it is possible to absorb linearly polarized light having a vibration direction parallel to the absorption axis and to transmit linearly polarized light having a vibration direction parallel to the transmission axis. The vibration direction of the linearly polarized light means the vibration direction of the electric field of the linearly polarized light. At this time, the absorption axis of the polarizing film and the slow axis of the optical film preferably form a specific angle.

In one example, the angle between the absorption axis of the polarizing film and the slow axis of the optical film is preferably more than 80°, more preferably more than 85°, more preferably more than 88°, and preferably less than 100°. It is preferably below 95°, especially preferably below 92°. In this case, the optical film preferably has an in-plane retardation capable of functioning as a 1/2 wavelength plate. The polarizing plate related to this example can be used as a polarizing plate capable of compensating the viewing angle when it is installed in an image display device.

In another example, the angle between the absorption axis of the polarizing film and the slow axis of the optical film is preferably more than 40°, preferably more than 42°, more preferably more than 44°, and preferably less than 50° , preferably below 48°, especially preferably below 46°. In this case, the optical film preferably has an in-plane retardation capable of functioning as a 1/4 wavelength plate. The polarizing plate related to this example can be used as a circular polarizing plate that allows circularly polarized light in one rotation direction to pass through and shields circularly polarized light in the other rotation direction. The circular polarizing plate can suppress the reflection of external light by being arranged on the display surface of the display device.

As the polarizing film, any polarizing film may be used. As an example of a polarizing film, a film obtained by uniaxially stretching a polyvinyl alcohol film after absorbing iodine or a dichroic dye in a boric acid bath; Dye and extend the film obtained by further modifying a part of the polyvinyl alcohol unit in the molecular chain into a polyethylene extension unit. Among them, those containing polyvinyl alcohol are preferable as polarizing films.

If natural light is incident on the polarizing film, only one polarized light will pass through. The degree of polarization of the polarizing film is not particularly limited, but is preferably above 98%, preferably above 99%. Moreover, the thickness of the polarizing film is preferably 5 μm to 80 μm.

The above-mentioned polarizing plate may further include any layer. Examples of optional layers include: a protective film layer for polarizers; a bonding layer for laminating polarizing films and optical films; a hard coat layer such as an impact-resistant polymethacrylic resin layer; a base pad for optimizing the smoothness of the film layer; reflection suppression layer; antifouling layer; static suppression layer; etc. Such arbitrary layers may be provided in only one layer, or may be provided in two or more layers.

"Example"

Examples are disclosed below to specifically illustrate the present invention. However, the present invention is not limited to the embodiments shown below, and can be implemented with arbitrary changes within the scope not departing from the patent application scope of the present invention and its equivalent scope.

In the following descriptions, "%" and "part" of the amount are based on weight unless otherwise noted. In addition, the operations described below were carried out under the conditions of normal temperature and pressure (23°C, 1 atmosphere) in the atmosphere unless otherwise noted.

[Measuring method of glass transition temperature Tg and melting point Tm]

The measurement of the glass transition temperature Tg and the melting point Tm of a polymer is performed as follows. First, the polymer is melted by heating, and the melted polymer is rapidly cooled by dry ice. Next, using this polymer as a sample, a differential scanning calorimeter (DSC) was used to measure the glass transition temperature Tg and melting point Tm of the polymer at a heating rate of 10°C/min (heating mode).

[Measurement methods of retardation, NZ coefficient and slow axis direction of optical films]

The in-plane retardation Re of the optical film, the retardation Rth in the thickness direction, the NZ coefficient, and the slow axis direction were measured with a phase difference meter ("AxoScan OPMF-1" manufactured by AXOMETRICS Corporation).

[Evaluation method of bendability of optical film]

Mandrel tortuosity tests were performed using optical films. Specifically, the optical film was bent according to the diameter of the mandrel, and whether or not it was broken was checked. In the case of breakage, it was determined that the optical film could not withstand the diameter of the mandrel. The mandrel bending test was carried out using mandrels with diameters of 6 mm, 3 mm and 2 mm, respectively. The diameter of the smallest central axis at which the optical film will not break is obtained as a measurement value. The smaller the measurement value, the better the bendability of the optical film. When the optical film was not damaged at all in the mandrel bending test using a mandrel with a diameter of 2 mm, the measured value of the bendability was "less than 2 mm".

[Production example 1. Production of crystalline polystyrene]

Put 17.8 g (71 mmol) of copper sulfate pentahydrate (CuSO ₄ .5H ₂ O), 200 mL of toluene and 24 mL of trimethylaluminum (250 mm mol), and allowed to react at 40°C for 8 hours. After that, the solid portion was removed to obtain a solution. From the obtained solution, toluene was further distilled off under reduced pressure at room temperature to obtain 6.7 g of a contact product. The molecular weight of the contact product was measured by the freezing point depression method, and the result was 610.

Subsequently, 5 millimoles of the aforementioned contact product were added to the reaction vessel as aluminum atoms, 5 millimoles of triisobutylaluminum, 0.025 millimoles of pentamethylcyclopentadiene trimethoxide and 1 millimole of high-purity styrene ear, the polymerization was carried out at 90°C for 5 hours. After that, the catalyst component of the product was decomposed with a methanol solution of sodium hydroxide, washed with methanol repeatedly, and dried to obtain 308 g of a polymer (crystalline polystyrene).

The weight average molecular weight of the polymer was measured by gel permeation chromatography at 135° C. using 1,2,4-trichlorobenzene as a solvent. As a result, the polymer had a weight average molecular weight Mw of 350,000. Furthermore, it was confirmed that the obtained polymer was crystalline polystyrene having a parallel structure by measuring the melting point Tm and ¹³ C-NMR. Crystalline polystyrene has a melting point Tm of 270°C and a glass transition temperature of 100°C.

This operation was repeated to prepare crystalline polystyrene having a paired structure required for evaluation.

[Production Example 2. Production of a crystalline polymer with positive intrinsic birefringence]

The metal pressure-resistant reactor was fully dried and replaced with nitrogen. 154.5 parts of cyclohexane and 42.8 parts of a 70% cyclohexane solution of dicyclopentadiene (with an endo isomer content of 99% or more) were charged to this metal pressure-resistant reactor (the amount as dicyclopentadiene was 30 parts) and 1.9 parts of 1-hexene, heated to 53°C.

A solution was prepared by dissolving 0.014 parts of phenylimide tetrachloride (tetrahydrofuran) tungsten complex in 0.70 parts of toluene. Add 0.061 part of diethylethoxyaluminum/n-hexane solution with a concentration of 19% to this solution, and stir for 10 minutes to prepare a catalyst solution. Add the catalyst solution into the pressure-resistant reactor to start the ring-opening polymerization reaction. Thereafter, it was allowed to react for 4 hours while maintaining 53° C. to obtain a solution of a ring-opened polymer of dicyclopentadiene. The number average molecular weight (Mn) and weight average molecular weight (Mw) of the obtained ring-opened polymer of dicyclopentadiene were 8,750 and 28,100, respectively, and the molecular weight distribution (Mw/Mn) obtained from these was 3.21.

0.037 parts of 1,2-ethylene glycol was added as a terminator to 200 parts of the obtained solution of the ring-opened polymer of dicyclopentadiene, heated to 60° C., and stirred for 1 hour to terminate the polymerization reaction. One part of a hydrotalcite-like compound (“KYOWAAD (registered trademark) 2000” manufactured by Kyowa Chemical Industry Co., Ltd.) was added thereto, heated to 60° C., and stirred for 1 hour. Thereafter, 0.4 part of filter aid ("RADIOLITE (registered trademark) #1500" manufactured by Showa Chemical Industry Co., Ltd.) was added, and the adsorbent and the solution were separated by filtration using a PP pleated cartridge filter ("TCP-HX" manufactured by Advantec Toyo Co., Ltd.).

Add 100 parts of cyclohexane to 200 parts of the filtered ring-opening polymer solution of dicyclopentadiene (30 parts of polymer), add 0.0043 parts of hydrochlorinated carbonyl ginseng (triphenylphosphine) ruthenium, and press the hydrogen pressure of 6 MPa , 180 ° C for 4 hours hydrogenation reaction. Thereby, a reaction liquid containing a hydrogenated product of the ring-opening polymer of dicyclopentadiene can be obtained. The hydride in the reaction solution was precipitated to form a slurry solution.

The hydride contained in the aforementioned reaction solution was separated from the solution using a centrifugal separator, and dried under reduced pressure at 60° C. for 24 hours to obtain 28.5 parts of the hydride of the ring-opened polymer of dicyclopentadiene as a compound having positive intrinsic birefringence. crystalline polymer. The hydrogenation rate of this hydride is over 99%, the glass transition temperature Tg is 93°C, the melting point Tm is 262°C, and the ratio of racemic dyads is 89%.

[Example 1]

(1-1. Preparation of crystalline resin)

The crystalline polystyrene having a parallel structure obtained in Production Example 1 was kneaded at 295° C. with a biaxial extruder to produce transparent crystalline resin pellets.

(1-2. Extrusion into film)

The pellets of the crystalline resin were melt-extruded using a hot-melt extrusion film forming machine equipped with a T-die ("Measuring Extruder Type Me-20/2800V3" manufactured by Optical Control Systems Co., Ltd.), and wound at a speed of 1.5 m/min. roll, and obtain a strip-shaped raw film as a resin film before contact with a solvent with a width of about 120 mm. The operating conditions of the film forming machine are listed below. ． Barrel temperature setting = 280 ° C ~ 300 ° C ． Mold temperature = 300°C ． Screw revolutions = 30 rpm ． Casting roll temperature = 80°C

The thickness of the raw film was 25 μm. The retardation of the raw film was measured at a measurement wavelength of 550 nm, and the result was that the retardation in the plane was Re=7 nm and the retardation in the thickness direction Rth=-10 nm.

(1-3. Solvent contact)

The aforementioned raw material film was cut into a rectangle of 120 mm×120 mm. This rectangular raw film was immersed in cyclohexane stored in a tank as a solvent until the in-plane retardation Re = 58 nm and the retardation in the thickness direction Rth = 111 nm were obtained to obtain a film before stretching as a resin film after contact with the solvent. . The film before stretching was taken out from the cyclohexane, and the cyclohexane adhering to the surface of the film was wiped off, and then allowed to dry naturally in the air. The thickness of the obtained film before stretching was 28 μm.

(1-4. Extension)

A batch-type biaxial extension device (manufactured by ETO Corporation) was prepared. This stretching device has an oven unit and a jig for stretching that can fix the film. If this stretching device is used, the film can be stretched through the clips in an oven to stretch the film.

Cut the pre-stretched film into a rectangle of 100 mm × 100 mm. Both ends of the rectangular pre-stretched film were respectively clamped by the 5 clamps of the aforementioned stretching device. Stretch the pre-stretched film with a jig, and perform free uniaxial stretching along the long side direction of the elongated raw material film obtained in the extrusion film process. The stretching temperature was 130° C., and the stretching ratio was 1.3 times. By this stretching, an optical film as a uniaxially stretched film is obtained.

The retardation of the obtained optical film was measured, and the in-plane retardation Re=280 nm, the retardation in the thickness direction Rth=5 nm, and the direction of the slow axis of the optical film was perpendicular to the extending direction. Furthermore, the NZ coefficient at the measurement wavelength of 550 nm is 0.52. The bendability of the obtained optical film was evaluated by the above-mentioned method.

[Example 2]

The line speed in the step (1-2) was changed so that the thickness of the elongated raw film was changed to 13 μm.

Then, in step (1-3), the immersion time of the raw material film in the solvent was adjusted so that a pre-stretched film with in-plane retardation Re=33 nm and thickness direction retardation Rth=56 nm was obtained.

Production and evaluation of the optical film were performed by the same method as in Example 1 except for the above matters.

[Example 3]

The line speed in the step (1-2) was changed so that the thickness of the elongated raw film was changed to 40 μm.

Then, in step (1-3), the immersion time of the raw material film in the solvent was adjusted so that a film before stretching with in-plane retardation Re=24 nm and thickness direction retardation Rth=45 nm was obtained.

In addition, in the process (1-4), the elongation ratio was changed to 1.1 times.

Production and evaluation of the optical film were performed by the same method as in Example 1 except for the above matters.

[Comparative Example 1]

In 100 parts of the hydrogenated product of the ring-opening polymer of dicyclopentadiene obtained in Manufacturing Example 2, a mixed antioxidant (tetra{3-[3′,5′-bis(tertiary butyl)-4′-hydroxyphenyl ] methylene propionate} methane; "Irganox (registered trademark) 1010" manufactured by BASF Japan Co., Ltd.) 1.1 parts were put into a twin-screw extruder (manufactured by Toshiba Machine Co., Ltd.) "TEM-37B"). The mixture of hydrogenated ring-opening polymer of dicyclopentadiene and antioxidant is formed into strands by hot-melt extrusion, and then finely cut with a strand pelletizer to obtain crystalline resin particles.

This pellet is used in the step (1-2). Furthermore, the line speed in the step (1-2) was changed so that the thickness of the elongated raw material film was changed to 13 μm.

In addition, in the step (1-3), the type of solvent was changed to toluene. Furthermore, the immersion time of the raw material film in the solvent was adjusted so that a film before stretching with an in-plane retardation Re=10 nm and a retardation in the thickness direction Rth=−142 nm was obtained.

In addition, the stretching temperature in the step (1-4) was changed to 110° C., and the stretching ratio was changed to 1.2 times.

Production and evaluation of the optical film were performed by the same method as in Example 1 except for the above matters.

[Comparative Example 2]

The immersion of the raw material film in the solvent in the step (1-3) was not carried out.

In addition, in the step (1-4), the raw film was stretched instead of the unstretched film, and the stretching temperature was changed to 110°C.

Production and evaluation of the optical film were performed by the same method as in Example 1 except for the above matters.

[result]

The results of the foregoing examples and comparative examples are disclosed in the table below. In the table below, the meanings of the abbreviations are as follows. SPS: Crystalline polystyrene. Cy: cyclohexane. Tl: toluene.

[Table 1] [Table 1. Results of Examples and Comparative Examples] Example 1 Example 2 Example 3 Comparative example 1 Comparative example 2 resin SPS SPS SPS crystalline COP SPS raw film Film thickness (μm) 25 13 40 13 25 Re(nm) 7 5 7 3 7 Rth(nm) −10 −7 −14 6 −10 NZ coefficient −0.93 −0.90 −1.50 2.50 −0.93 solvent exposure solvent Cy Cy Cy Tl — Film before stretching Film thickness (μm) 28 15 44 17 — Re(nm) 58 33 twenty four 10 — Rth(nm) 111 56 45 −142 — NZ coefficient 2.41 2.20 2.38 −13.70 — The direction of the slow axis relative to the long-side direction of the elongated raw material film vertical vertical vertical vertical — extend Extension temperature (℃) 130°C 130°C 130°C 110°C 110°C Extension ratio (times) 1.3 1.3 1.1 1.2 1.3 Optical film Film thickness (μm) 17 27 41 16 twenty two Re(nm) 280 138 143 275 95 Rth(nm) 5 2 −5 10 −48 Re/d(×10 ^{− 3} ) 16.5 5.1 3.5 17.2 4.3 NZ coefficient 0.52 0.51 0.47 0.54 −0.01 Orientation of the slow axis relative to the direction of extension vertical vertical vertical parallel vertical Bending properties (mandrel test) Less than 2mm Less than 2mm Less than 2mm Less than 2mm 3mm

[discuss]

Optical films having an NZ coefficient greater than 0 and less than 1 can be obtained in the examples. Accordingly, it can be seen that in the embodiment, an optical film having the refractive indices nx ₁ , ny ₁ and nz ₁ satisfying the formula (1) can be obtained. Furthermore, the optical film obtained in the example uses crystalline polystyrene as a crystalline polymer having negative intrinsic birefringence, and achieves birefringence Re/d in the in-plane direction satisfying the formula (2). Accordingly, according to the present invention, it was confirmed that an optical film satisfying requirements (A) to (C) can be obtained.

It can be seen that in the examples, since the birefringence Rth/d in the thickness direction of the raw film is different from the birefringence Rth/d in the thickness direction of the film before stretching, the refractive index of the raw film changes due to contact with the solvent. Furthermore, it can be seen that in the examples, the in-plane retardation Re of the film before stretching is sufficiently smaller than the in-plane retardation Rth in the thickness direction, and thus satisfies formula (3), so the film before stretching becomes a negative C plate. The pre-stretched film obtained in this way has negative birefringence characteristics, so the aforementioned optical film can be obtained by stretching.

In Examples, stretching was performed at a higher temperature than in Comparative Example 2 using the same material. This is because the solvent contained in the pre-stretched film related to the embodiment is efficiently removed by high temperature. Since the mechanical strength of the film before stretching will be improved by contact with the solvent, even if it is stretched at such a high temperature, the film will not be damaged.

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Claims (9)

An optical thin film comprising a crystalline polymer having negative intrinsic birefringence, wherein the optical thin film has a refractive index nx ₁ in a direction perpendicular to the thickness direction and a direction that imparts a maximum refractive index, and is equal to The refractive index ny ₁ in the direction perpendicular to the thickness direction and the direction perpendicular to the aforementioned nx ₁ direction and the refractive index nz ₁ in the thickness direction satisfy the following formula (1), and the birefringence Re/d in the in-plane direction of the aforementioned optical film satisfies In the following formula (2), nx ₁ >nz ₁ >ny ₁ (1) Re/d≧3×10 ⁻³ (2). The optical film according to claim 1, wherein the optical film is a stretched film. The optical film according to claim 1, wherein the optical film has a strip shape. A polarizing plate comprising the optical film and polarizing film according to any one of claims 1 to 3. The polarizing plate according to claim 4, wherein the slow axis of the optical film and the absorption axis of the polarizing film form an angle of 80° to 100°. A method of manufacturing an optical thin film, which is the refractive index nx ₁ in the direction perpendicular to the thickness direction and the direction giving the maximum refractive index, and the refraction in the direction perpendicular to the thickness direction and perpendicular to the direction nx ₁ A method for producing an optical film having a ratio ny ₁ and a refractive index nz ₁ in the thickness direction satisfying formula (1), the production method comprising: a step (i) of preparing a pre-stretched film comprising a crystalline polymer having negative intrinsic birefringence , and the step (ii) of stretching the aforementioned pre-stretched film, the refractive index nx 2 of the aforementioned pre-stretched film prepared in the step (i) is the direction perpendicular to the thickness direction and the direction giving the maximum refractive index nx ₂ , is The refractive index ny ₂ in the direction perpendicular to the thickness direction and the direction perpendicular to the aforementioned nx ₂ direction and the refractive index nz ₂ in the thickness direction satisfy the following formula (3), nx ₁ >nz ₁ >ny ₁ (1) nz ₂ <nx ₂ ≒ny ₂ (3). The method for producing an optical film according to Claim 6, wherein step (i) includes: preparing a resin film comprising a crystalline polymer having negative intrinsic birefringence, and contacting the resin film with a solvent to obtain the film. The method for manufacturing an optical film as claimed in claim 6, wherein the optical film has a single-layer structure. The method for producing an optical film according to any one of claims 6 to 8, wherein the crystalline polymer having negative intrinsic birefringence is a polystyrene polymer.