KR20230121742A - Optical film and its manufacturing method, and polarizing plate - Google Patents

Abstract

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 the thickness direction giving the maximum refractive index, and a direction perpendicular to the thickness direction nx ₁ An optical film in which the refractive index ny ₁ in the direction perpendicular to 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 optical film satisfies the following formula (2) .
nx ₁ > nz ₁ > ny ₁ (1)
Re/d ≥ 3 × 10 ^-3 (2)

Description

Optical film and its manufacturing method, and polarizing plate

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

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

Japanese Patent No. 4486854

An optical film having refractive index anisotropy may be produced using a resin. The optical film having refractive index anisotropy may be installed in a display device as a film such as an antireflection film or a viewing angle compensation film, for example. For example, an optical film in which three-dimensional refractive indices nx, ny, and nz satisfy nx > nz > ny can improve display quality when the display surface is viewed from an oblique direction.

A manufacturing method of an optical film in which the three-dimensional refractive indices nx, ny, and nz satisfy nx > nz > ny has been conventionally known. For example, a method for producing an optical film satisfying nx>nz>ny has been known by subjecting a resin film to appropriate stretching treatment. However, conventionally, it has been difficult to manufacture such an optical film using a polymer having negative intrinsic birefringence. In addition, since polymers having negative intrinsic birefringence generally have low mechanical strength, it is difficult to stretch them at a large draw ratio. Therefore, in the conventional method, it was particularly difficult to obtain an optical film having large birefringence.

The present invention was devised in view of the above problems, and includes an optical film comprising a crystalline polymer having negative intrinsic birefringence, having a refractive index satisfying nx > nz > ny, and having large birefringence; a method for producing an optical film comprising a crystalline polymer having negative intrinsic birefringence and having a refractive index satisfying nx > nz > ny; And, it is an object to provide; a polarizing plate provided with the above optical film.

The present inventors conducted intensive studies in order to solve the above problems. As a result, the present inventors have found that the above problems can be solved by using a method including stretching a film before stretching as a negative C plate containing a crystalline polymer having negative intrinsic birefringence, and the present invention has completed

That is, the present invention includes the following.

[1] An optical film comprising a crystalline polymer having negative intrinsic birefringence,

The refractive index nx ₁ of the optical film in a direction giving the maximum refractive index as a direction perpendicular to the thickness direction, a refractive index ny ₁ in a direction perpendicular to the direction of nx ₁ as a direction perpendicular to the thickness direction, and a refractive index nz in the thickness direction ₁ satisfies the following formula (1),

An optical film in which 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)

[2] The optical film according to [1], wherein the optical film is a stretched film.

[3] The optical film according to [1] or [2], wherein the optical film has a long shape.

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

[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] The refractive index nx ₁ in the direction perpendicular to the thickness direction giving the maximum refractive index, the refractive index ny ₁ in the direction perpendicular to the direction of nx ₁ as the direction perpendicular to the thickness direction, and the refractive index nz ₁ in the thickness direction , as a method for producing an optical film satisfying formula (1);

The manufacturing method,

Step (i) of preparing a pre-stretching film comprising a crystalline polymer having negative intrinsic birefringence;

a step (ii) of stretching the film before the stretching;

The refractive index nx ₂ of the pre-stretching film prepared in step (i) in the direction giving the maximum refractive index as a direction perpendicular to the thickness direction, and the refractive index ny in the direction perpendicular to the nx ₂ direction as a direction perpendicular to the thickness direction ₂ , and the refractive index nz ₂ in the thickness direction satisfy the following formula (3), the manufacturing method of the optical film.

nx ₁ > nz ₁ > ny ₁ (1)

nz ₂ < nx ₂ ≒ ny ₂ (3)

[7] step (i),

preparing a resin film comprising a crystalline polymer having negative intrinsic birefringence;

The manufacturing method of the optical film as described in [6] which includes making the said resin film and a solvent contact, and obtaining the said pre-stretching film.

[8] The manufacturing method of the 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-based polymer.

According to the present invention, an optical film comprising a crystalline polymer having negative intrinsic birefringence, having a refractive index satisfying nx > nz > ny, and having large birefringence; a method for producing an optical film comprising a crystalline polymer having negative intrinsic birefringence and having a refractive index satisfying nx > nz > ny; And, a polarizing plate provided with the above optical film; can be provided.

Hereinafter, the present invention will be described in detail by showing embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and can be implemented with arbitrary changes within a range not departing from the scope of the claims of the present invention and their equivalents.

In the following description, the in-plane retardation Re of the film is a value represented by Re = (nx - ny) x d unless otherwise indicated. Incidentally, birefringence in the in-plane direction of the film is a value represented by (nx - ny) unless otherwise specified, and therefore is represented by Re/d. The retardation Rth in the thickness direction of the film is a value represented by Rth = [{(nx + ny)/2} - nz] x d unless otherwise specified. In addition, the birefringence of the film in the thickness direction is a value represented by [{(nx + ny)/2} - nz] unless otherwise specified, and therefore is represented by Rth/d. In addition, the NZ coefficient of a film is a value represented by (nx - nz)/(nx - ny) unless otherwise indicated. Here, nx represents the refractive index in the direction giving the maximum refractive index as a direction perpendicular to the thickness direction of the film (in-plane direction). ny represents the refractive index in the direction perpendicular to the nx direction as the in-plane direction of the film. nz represents the refractive index of the thickness direction of a film. d represents the thickness of the film. The measurement wavelength is 550 nm unless otherwise noted.

In the following description, unless otherwise specified, a material having positive intrinsic birefringence means a material whose refractive index in the stretching direction is greater than the refractive index in the direction perpendicular thereto. Accordingly, a polymer having positive intrinsic birefringence means a polymer whose refractive index in the stretching direction is greater than the refractive index in the direction perpendicular thereto, unless otherwise specified. Note that, unless otherwise specified, a material having negative intrinsic birefringence means a material whose refractive index in the stretching direction is smaller than the refractive index in the direction perpendicular thereto. Accordingly, a polymer having negative intrinsic birefringence means a polymer whose refractive index in the stretching direction is smaller than the refractive index in the direction perpendicular thereto, unless otherwise specified.

In the following description, the shape of "long" refers to a shape having a length of 5 times or more of the width, preferably having a length of 10 times or more, specifically, the extent to which it is wound up in a roll shape and stored or transported. refers to the shape of a film having a length of There is no particular limitation on the upper limit of the length, but it is usually less than 100,000 times the width.

In the following description, unless otherwise specified, the directions of elements are "parallel", "perpendicular" and "orthogonal" within a range that does not impair the effect of the present invention, for example, within a range of ±5°. may contain an error of

Unless otherwise specified, the angle formed by the optical axis (absorption axis, transmission axis, slow axis, etc.) of each film in a member provided with a plurality of films represents the angle when the above-mentioned film is viewed from the thickness direction. .

"Polarizing plate", "circular polarizing plate", "wavelength plate" and "negative C plate" include not only rigid members but also members having flexibility such as, for example, resin films, unless otherwise specified. .

[One. Overview of the optical film according to the first embodiment]

The optical film according to the first embodiment of the present invention satisfies the following requirements (A) to (C) in combination.

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): 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 is the direction perpendicular to the thickness direction of the optical film and represents the refractive index in the direction giving the maximum refractive index. ny ₁ is the direction perpendicular to the thickness direction of the optical film and the direction perpendicular to the nx ₁ direction Represents the refractive index. nz ₁ represents the refractive index in the thickness direction of the optical film.)

The optical film described above has conventionally been difficult to manufacture, but can be easily manufactured when the manufacturing method according to the second embodiment described later is used.

[2. Crystalline Polymer with Negative Inherent Birefringence]

An optical film according to the first embodiment of the present invention includes a crystalline polymer having negative intrinsic birefringence. A crystalline polymer represents a polymer having crystallinity. A polymer having crystallinity indicates a polymer having a melting point Tm. That is, a polymer having crystallinity indicates a polymer whose melting point Tm can be observed with a differential scanning calorimeter (DSC).

The above crystalline polymer included in the optical film has negative intrinsic birefringence. When a crystalline polymer having negative intrinsic birefringence is stretched, it can develop a large refractive index in a direction perpendicular to the stretching direction. When the manufacturing method according to the second embodiment described later is performed using the crystalline polymer having negative intrinsic birefringence, refractive index satisfying Expression (1) and birefringence satisfying Expression (2) are easily achieved. can do.

As the crystalline polymer having negative intrinsic birefringence, a polymer containing an aromatic ring is preferable, and polystyrene-based polymers are exemplified. In the following description, a polystyrene-based polymer having crystallinity may be referred to as a "crystalline polystyrene-based polymer".

The crystalline polystyrene-based polymer may be a polymer of styrenic monomers. Therefore, the crystalline polystyrene-based polymer may be a polymer containing structural units having a structure formed by polymerizing styrenic monomers (hereinafter, arbitrarily referred to as "styrene-based units").

The styrenic monomer may be an aromatic vinyl compound such as styrene or a styrene derivative. Examples of the styrene derivative include compounds in which a substituent has been substituted at the benzene ring or the α- or β-position of styrene.

Examples of the styrenic monomer include styrene, alkyl styrene, halogenated styrene, halogenated alkyl styrene, alkoxy styrene, vinyl benzoic acid esters, and hydrogenated polymers thereof.

Examples of alkylstyrene include methylstyrene, ethylstyrene, isopropylstyrene, t-butylstyrene, 2,4-dimethylstyrene, phenylstyrene, vinylnaphthalene, and vinylstyrene. Examples of halogenated styrene include chlorostyrene, bromostyrene, and fluorostyrene. An example of halogenated alkyl styrene is chloromethyl styrene. Examples of alkoxystyrene include methoxystyrene and ethoxystyrene. Among the styrenic monomers, styrene, methyl styrene, ethyl styrene, and 2,4-dimethyl styrene are preferred. In addition, 1 type may be used for a styrenic monomer, and you may use it in combination of 2 or more types.

The crystalline polystyrene-based polymer preferably has an isotactic structure or a syndiotactic structure, and more preferably has a syndiotactic structure. That the crystalline polystyrene-based polymer has a syndiotactic structure means that the stereochemical structure of the crystalline polystyrene-based polymer has a syndiotactic structure. The syndiotactic structure refers to a three-dimensional structure in which phenyl groups, which are side chains, are alternately positioned in opposite directions in the Fischer projection equation with respect to the main chain formed by carbon-carbon bonds. Compared with polystyrene-based polymers having an atactic structure, polystyrene-based polymers having a syndiotactic structure usually have a low specific gravity and are excellent in hydrolysis resistance, heat resistance and chemical resistance.

The tacticity (stereoregularity) of the crystalline polystyrene-based polymer can be quantified by a nuclear magnetic resonance method ( ¹³ C-NMR method) using isotopic carbon. The tacticity measured by the ¹³ C-NMR method can be expressed by the abundance ratio of a plurality of consecutive constituent units. In general, for example, a dyad is a case of 2 consecutive units, a triad is a case of 3 units, and a pentad is a case of 5 units. In this case, the crystalline polystyrene-based polymer having a syndiotactic structure may have a syndiotacticity of usually 75% or more, preferably 85% or more, in the dyad (racemic diaad), or pentad (called semipentad) may have a syndiotacticity of usually 30% or more, preferably 50% or more.

The crystalline polystyrene-based polymer may be a homopolymer or a copolymer. Therefore, the crystalline polystyrene-based polymer may be a homopolymer of one type of styrenic monomer or a copolymer of two or more types of styrenic monomers. When the crystalline polystyrene-based polymer is a copolymer of two or more types of styrenic monomers, the ratio of each styrenic unit to the total 100% by weight of the crystalline polystyrene-based polymer is preferably 5% by weight or more, more preferably 10% by weight or more, preferably 95% by weight or less, more preferably 90% by weight or less.

Further, the crystalline polystyrene-based polymer may be a copolymer of one or two or more types of styrenic monomers and monomers other than the styrenic monomers. The ratio of the styrenic units included in the crystalline polystyrene-based polymer is preferably 80% by weight or more, more preferably 83% by weight or more, still more preferably 85% by weight, from the viewpoint of obtaining an optical film having desired optical properties. more than %

Usually, the ratio of any structural unit contained in the crystalline polystyrene-based polymer may be the same as the ratio of monomers corresponding to the structural unit to all monomers of the crystalline polystyrene-based polymer. Therefore, the ratio of styrenic units included in the crystalline polystyrene-based polymer may be equal to the ratio of styrenic monomers to all monomers of the crystalline polystyrene-based polymer.

The crystalline polystyrene-based polymer can be produced, for example, by polymerizing styrenic monomers in an inert hydrocarbon solvent or in the absence of a solvent using a titanium compound and a condensation product of water and trialkylaluminum as a catalyst (Japan See Patent Publication No. 62-187708).

The crystalline polymer having negative intrinsic birefringence may be used singly or in combination of two or more at an arbitrary ratio.

The weight average molecular weight Mw of the crystalline polymer having negative intrinsic birefringence is preferably 130,000 or more, more preferably 140,000 or more, particularly preferably 150,000 or more, preferably 500,000 or less, more preferably 450,000 or less, Especially preferably, it is 400,000 or less. Since the crystalline polymer having such a weight average molecular weight Mw can have a high glass transition temperature Tg, it can improve the heat resistance of an optical film.

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 a developing solvent.

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 particularly preferably 95°C or higher. When a crystalline polymer having such a high glass transition temperature Tg is used, the heat resistance of the optical film can be improved. The glass transition temperature of the crystalline polymer is preferably 160°C or lower, more preferably 155°C or lower, and particularly preferably 150°C or lower, from the viewpoint of smooth stretching in the manufacturing process of the optical film.

The melting point Tm of the crystalline polymer having negative intrinsic birefringence is preferably 200°C or higher, more preferably 210°C or higher, particularly preferably 220°C or higher, preferably 300°C or lower, more preferably 290°C or higher. °C or lower, particularly preferably 280 °C or lower. When the melting point Tm is within the above range, unintentional progress of 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. Therefore, an optical film having good appearance and optical properties can be easily obtained.

The glass transition temperature Tg and melting point Tm of a polymer can be measured by the following methods. First, a polymer is melted by heating, and the melted polymer is quenched with dry ice. Subsequently, using this polymer as a test body, the glass transition temperature Tg and melting point Tm of the polymer can be measured using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min (heating mode).

The crystallinity of the crystalline polymer having negative intrinsic birefringence contained in the optical film is usually higher than a certain extent. The specific crystallinity range is preferably 10% or more, more preferably 15% or more, and particularly preferably 30% or more. The degree of crystallinity of a crystalline polymer can be measured by an X-ray diffraction method.

Usually, the optical film is formed of a resin containing a crystalline polymer having negative intrinsic birefringence. In the following description, a resin containing a crystalline polymer may be referred to as "crystalline resin". Therefore, an optical film usually contains crystalline resin, and preferably consists only of crystalline resin. The above-described crystalline resin including a crystalline polymer having negative intrinsic birefringence usually has negative intrinsic birefringence. In addition, this 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, more preferably 80% by weight or more, particularly from the viewpoint of obtaining an optical film having desired optical properties. It is preferably 90% by weight or more, and usually 100% by weight or less.

The crystalline resin may contain an arbitrary component in combination with a crystalline polymer having negative intrinsic birefringence. Examples of the optional component include arbitrary polymers such as a crystalline polymer having positive intrinsic birefringence and an amorphous polymer having no crystallinity; lubricant; layered crystal compounds; fine particles such as inorganic fine particles; stabilizers such as antioxidants, heat stabilizers, light stabilizers, weathering stabilizers, ultraviolet absorbers, and near-infrared absorbers; plasticizer; colorants such as dyes and pigments; antistatic agent; etc. can be mentioned. 1 type may be used for arbitrary components, and may be used in combination of 2 or more types. The amount of any component can be appropriately determined within a range that does not significantly impair the effects of the present invention. The amount of the optional component may be in a range capable of maintaining, for example, the total light transmittance of the optical film at 85% or more.

[3. Refractive index of optical film]

The refractive indices nx ₁ , ny ₁ , and nz ₁ of the optical film according to the first embodiment of the present invention satisfy Formula (1). A film containing a polymer having negative intrinsic birefringence generally tends to develop large birefringence in the thickness direction by stretching. Therefore, conventionally, it has been difficult to produce a film having a refractive index that satisfies Formula (1) while containing a polymer having negative intrinsic birefringence. In contrast, as in the optical film according to the present embodiment, refractive indices nx ₁ , ny ₁ and nz ₁ satisfying Formula (1) can be achieved while including a polymer having negative intrinsic birefringence (requirement (A)) What happened (requirement (B)) is surprising to those skilled in the art.

An optical film having refractive indices nx ₁ , ny _{1 ,} and nz ₁ satisfying Formula (1) has not only the polarization state of light passing through the optical film in the thickness direction, but also the polarization state of light passing in an oblique direction that is neither parallel nor perpendicular to the thickness direction. The polarization state can also be changed appropriately. Therefore, the optical film can exert its optical function not only with respect to light in the thickness direction but also with respect to light in an oblique direction. For example, when an optical film can impart a phase difference of 1/4 wavelength to light passing through a thickness direction, the optical film can impart a phase difference of 1/4 wavelength to light passing in an oblique direction. can Further, for example, when the optical film can impart a phase difference of 1/2 wavelength to light passing in the thickness direction, the optical film can impart a phase difference of 1/2 wavelength to light passing in an oblique direction. can be granted

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

Birefringence Re/d in the in-plane direction of the optical film according to the first embodiment of the present invention satisfies Expression (2). Specifically, the birefringence Re/d of the optical film in the in-plane direction is usually 3.0 × 10 ^-3 or more, more preferably 3.5 × 10 ^-3 or more, more preferably 5.0 × 10 ^-3 or more, particularly preferably It is 8.0 × 10 ^-3 or more. An optical film having such a large birefringence Re/d in the in-plane direction can have a thin thickness for obtaining a desired in-plane retardation. Therefore, a thin optical film having appropriate in-plane retardation can be realized. The upper limit of 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, still more preferably Preferably, it is 20 × 10 ^-3 or less.

In general, in order to obtain a film having a large birefringence Re/d in the in-plane direction by stretching, a large stretching ratio is required. However, a film made of a polymer having negative intrinsic birefringence tends to develop a large refractive index in the thickness direction by stretching. Therefore, when stretching is performed at a large stretching ratio, a large refractive index occurs in the thickness direction, and as a result, it is difficult to obtain refractive indices nx ₁ , ny ₁ , and nz ₁ satisfying the above formula (1). Therefore, it is difficult to achieve the requirement (C) within the constraints of satisfying the requirement (B), and has not been realized in the past. Since it was originally difficult to realize a film that satisfies the requirements (A) and (B) as described above, it is understandable that the difficulty of satisfying the requirements (A) to (C) in combination is high. The optical film of the present embodiment has a high industrial value in that it can provide an optical film that satisfies the requirements (A) to (C) that were particularly difficult to realize in this way.

[5. Layer composition of optical film]

The optical film may have a multilayer structure including a plurality of layers, but preferably has a single layer structure. A single-layer structure represents a structure having only a single layer having the same composition and not including a layer having a composition different from the above composition. Therefore, it is preferable that the optical film has a layer formed of the above crystalline resin alone.

[6. Characteristics of optical film]

The optical film preferably has an in-plane retardation within an appropriate range depending on its use.

For example, the specific in-plane retardation Re of the optical film may be preferably 100 nm or more, more preferably 110 nm or more, particularly preferably 120 nm or more at a measurement wavelength of 550 nm, and also preferably 180 nm or less, more preferably 170 nm or less, particularly preferably 160 nm or less. In this case, the optical film can function as a 1/4 wave plate.

For example, the specific in-plane retardation Re of the optical film may be preferably 245 nm or more, more preferably 265 nm or more, particularly preferably 270 nm or more at a measurement wavelength of 550 nm, and also preferably 320 nm or less, more preferably 300 nm or less, and particularly preferably 295 nm or less. In this case, the optical film can function as a 1/2 wave plate.

Usually, an optical film having refractive indices nx ₁ , ny ₁ , and nz ₁ satisfying Expression (1) exhibits an NZ coefficient 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, particularly preferably greater than 0.2, and usually less than 1.0, preferably 0.9. less than, more preferably less than 0.85, particularly preferably less than 0.8. An optical film having an NZ coefficient within this range can effectively improve display quality, such as a viewing angle, contrast, and image quality, when installed in a display device.

The value of retardation Rth in the thickness direction of the optical film can be set according to the use of the optical film. The specific thickness direction retardation Rth of the optical film is preferably -100 nm or more, more preferably -50 nm or more, particularly preferably -20 nm or more, preferably 100 nm or less, more preferably 50 nm or less, particularly preferably 20 nm or less.

The value of birefringence Rth/d in the thickness direction of the optical film can be set according to the purpose of the optical film. The absolute value |Rth/d| of birefringence in the specific thickness direction of the optical film is preferably 0.01 × 10 ^-3 or more, more preferably 0.05 × 10 ^-3 or more, and particularly preferably 0.1 × 10 ^-3 or more. , is preferably 2.0 × 10 ^-3 or less, more preferably 1.5 × 10 ^-3 or less, and particularly preferably 1.0 × 10 ^-3 or less.

Retardation and NZ coefficient of a film can be measured using a retardation meter (for example, "AxoScan OPMF-1" by AXOMETRICS). In addition, birefringence of a film can be obtained by dividing retardation by thickness.

A crystalline polymer having negative intrinsic birefringence generally has poor mechanical strength. In contrast, the optical film according to the present embodiment can have high mechanical strength. According to the study of the present inventors, it is found that the mechanical strength of a crystalline resin containing a crystalline polymer having negative intrinsic birefringence is increased by bringing the resin film into contact with a solvent in the manufacturing method according to the second embodiment described later. there is. The mechanical strength of an optical film can be represented by bendability, for example. For example, the optical film may not be damaged even when it is bent using a mandrel having a diameter of 2 mm.

It is preferable that an optical film has high transparency. The specific total light transmittance of the optical film is preferably 80% or more, more preferably 85% or more, and particularly preferably 88% or more. The total light transmittance of the film can be measured in a wavelength range of 400 nm to 700 nm using an ultraviolet/visible spectrometer.

It is preferable that an optical film has a small haze. The haze of the optical film is preferably less than 1.0%, more preferably less than 0.8%, particularly preferably less than 0.5%, and ideally 0.0%. When such an optical film having a small haze is installed in a display device, the sharpness of an image displayed on the display device can be improved. The haze of a film can be measured using a haze meter (for example, "NDH5000" by Nippon Denshoku Kogyo).

The optical film may contain a solvent. This solvent may be absorbed into the film in the step of bringing the resin film into contact with the solvent in the manufacturing method according to the second embodiment described later. In detail, by contacting a resin film, all or part of the solvent absorbed in the film can penetrate into the inside 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. Therefore, the optical film may contain a solvent.

It is preferable that an optical film is a stretched film. A stretched film refers to a film manufactured through stretching treatment. Especially, it is preferable that an optical film is a uniaxially stretched film. A uniaxially stretched film refers to a film that is positively stretched in only one direction and is not subjected to active stretching treatment in other directions. Since the uniaxially stretched film can be manufactured by stretching in only one direction, the manufacturing process can be simplified, and thus simple manufacturing can be realized.

The optical film may be a single sheet film or a long film having a long shape. When an optical film has the shape of a long picture, it is possible to bond an optical film and a long polarizing film together, and to manufacture a polarizing plate continuously.

The thickness of the optical film can be appropriately set depending on the use of the optical film. The specific thickness of the optical film is preferably 5 μm or more, more preferably 10 μm or more, particularly preferably 15 μm or more, preferably 50 μm or less, more preferably 40 μm or less, particularly preferably is less than 30 μm.

The optical film according to the first embodiment described above can be manufactured by a manufacturing method according to a second embodiment described later.

[7. Overview of Manufacturing Method for Optical Film According to the Second Embodiment]

In the optical film manufacturing method according to the second embodiment of the present invention, an optical film having refractive indices nx ₁ , ny ₁ , and nz ₁ satisfying Expression (1) is manufactured. According to the manufacturing method according to the second embodiment, the optical film according to the first embodiment can be manufactured. Further, according to the manufacturing method according 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 Expression (2).

The manufacturing method of the optical film according to the second embodiment,

Step (i) of preparing a pre-stretching film comprising a crystalline polymer having negative intrinsic birefringence;

Process of stretching the film before stretching (ii)

includes

In this manufacturing 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 is a direction perpendicular to the thickness direction of the film before stretching and represents the refractive index in the direction giving the maximum refractive index. ny ₂ is a direction perpendicular to the thickness direction of the film before stretching and perpendicular to the direction of nx ₂ Represents the refractive index in the direction nz ₂ represents the refractive index in the thickness direction of the film before stretching.)

In Formula (3), “nx ₂ ≈ ny ₂ ” indicates that birefringence “nx ₂ - ny ₂ ” (= Re/d) in the in-plane direction is small. Specifically, birefringence in the in-plane direction “nx ₂ - ny ₂ ” is the absolute value of birefringence in the thickness direction “|{(nx ₂ + ny ₂ )/2} - nz ₂ |” (= |Rth/d|) If it is smaller than this, “nx ₂ ≒ ny ₂ ” can hold. Therefore, normally, the birefringence Re/d (= nx ₂ - ny ₂ ) in the in-plane direction is the absolute value of birefringence in the thickness direction |Rth/d|(= |{(nx ₂ + ny ₂ )/2} - nz ₂ | ), it can be judged that a pre-stretched film smaller than "nx ₂ ≈ ny ₂ " is satisfied. Normally, when birefringence Re/d in the in-plane direction is smaller than the absolute value |Rth/d| of birefringence in the thickness direction, the NZ coefficient is smaller than -0.5 or larger than 1.5.

The pre-stretching film prepared in step (i) contains a crystalline polymer having negative intrinsic birefringence. Therefore, this pre-stretching film can normally have negative birefringence characteristics. The fact that a film "has negative birefringence" means that when the film is stretched in one stretching direction, the increase in refractive index in the direction perpendicular to the stretching direction is greater than the increase in refractive index in the stretching direction. Here, the stretching direction is usually perpendicular to the thickness direction, and therefore may be an in-plane direction. Since it is an increase amount, when the refractive index increases by stretching, the increase in the refractive index becomes a positive value, and when the refractive index decreases by stretching, the increase in the refractive index becomes a negative value. Therefore, when the pre-stretching film having refractive indices nx ₂ , ny ₂ and nz ₂ satisfying Expression (3) is stretched in step (ii), the refractive indices nx ₂ and nz ₂ can be relatively large, and the refractive index ny ₂ is relatively can be reduced to Therefore, according to the manufacturing method described above, an optical film satisfying Expression (1) can be obtained. Usually, the optical film obtained in this way may also have negative birefringence characteristics.

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

The present inventor guesses that the structure in which the film before extending|stretching is obtained by contact of a resin film and a solvent is as follows. However, the technical scope of the present invention is not limited by the structure below.

When a resin film containing a crystalline polymer is brought into contact with a solvent, the solvent penetrates into the resin film. Due to the action of the infiltrating solvent, micro-Brownian motion occurs in the molecules of the crystalline polymer in the film, and the molecular chains are oriented. According to the study of the present inventors, it is considered that the solvent-induced crystallization phenomenon of the crystalline polymer may progress during orientation of the molecular chains.

By the way, the surface area of a resin film is large in the front surface and back surface which are main surfaces. Therefore, the permeation rate of the organic solvent is large in the thickness direction passing through the above surface or back surface. Then, the orientation of the molecules of the crystalline polymer may proceed so that the molecules of the polymer are oriented in the thickness direction.

When the molecules of the crystalline polymer having negative intrinsic birefringence are oriented in the thickness direction, the refractive index of the film containing the molecules in the thickness direction becomes relatively small, and the refractive index in the in-plane direction perpendicular to the thickness direction becomes relatively large. Thus, a pre-stretched film having a refractive index nx ₂ and ny ₂ larger in the in-plane direction than the refractive index nz ₂ in the thickness direction is obtained.

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

A method for manufacturing an optical film according to a second embodiment of the present invention includes step (i) of preparing a pre-stretch film containing a crystalline polymer having negative intrinsic birefringence. It is preferable that this step (i) includes preparing a resin film containing a crystalline polymer having negative intrinsic birefringence and contacting the resin film with a solvent to obtain a film before stretching.

A crystalline polymer having negative intrinsic birefringence may be the same as that included in the optical film. Therefore, the resin film may contain the same crystalline resin as that contained in the optical film. Furthermore, it is preferable that a resin film consists only of the same crystalline resin as what is contained in an 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 particularly preferably less than 3%. If the crystallinity of the crystalline polymer contained in the resin film before contact with the solvent is low, since many crystalline polymer molecules can be oriented in the thickness direction by contact with the solvent, the refractive index can be adjusted over a wide range. .

The resin film preferably has optical isotropy. Therefore, 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 of the resin film in the in-plane direction is preferably less than 0.7 × 10 ^-3 , more preferably less than 0.6 × 10 ^-3 , and particularly preferably less than 0.5 × 10 ^-3 . The absolute value |Rth/d| of birefringence 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 particularly preferably less than 0.5 × 10 ^-3 . am. Having optical isotropy in this way indicates that the orientation of the molecules of the crystalline polymer contained in the resin film is low and is in a substantially non-orienting state. In the case of using such an optically isotropic resin film, precise control of the optical properties of the resin film is unnecessary, and therefore precise control of the orientation of the molecules of the crystalline polymer is unnecessary, so the manufacturing method of the optical film can be simplified. Moreover, when an optically isotropic resin film is used, an optical film with a small haze can usually be obtained.

It is preferable that content of a solvent is small, and, as for a resin film, it is more preferable that it 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) is preferably 1% or less, more preferably 0.5% or less, particularly preferably 0.1% or less, and ideally It is 0.0%. When the amount of the solvent contained in the resin film before contact with the solvent is small, since many crystalline polymer molecules can be oriented in the thickness direction by contact with the solvent, the refractive index can be adjusted in a wide range. The solvent content of a resin film can be measured by density.

The haze of the resin film is preferably less than 1.0%, preferably less than 0.8%, more preferably less than 0.5%, and ideally 0.0%. It is easy to make the haze of the optical film obtained so that the haze of a resin film is small.

It is preferable to set the thickness of the resin film according to the thickness of the optical film to be manufactured. Usually, the thickness increases by contacting with a solvent. Moreover, by extending|stretching, thickness becomes small. Therefore, the thickness of the resin film may be set in consideration of the change in thickness due to contact with the solvent and stretching as described above.

The resin film may be a single sheet film, but is preferably a long film. Since continuous manufacture of the optical film by the roll-to-roll method is possible by using a long resin film, productivity of the optical film can be effectively increased.

As a method for producing a resin film, resin molding methods such as injection molding, extrusion molding, press molding, inflation molding, blow molding, calender molding, cast molding, and compression molding are preferable since a resin film containing no solvent is obtained. . Among these, the extrusion molding method is preferable from the viewpoint of easy control of the thickness.

The manufacturing conditions in the extrusion molding method are preferably as follows. The cylinder temperature (molten resin temperature) is preferably Tm or higher, more preferably "Tm + 20°C" or higher, preferably "Tm + 100°C" or lower, more preferably "Tm + 50°C" or lower. am. In addition, although the cooling body which the molten resin extruded into the film form contacts initially is not specifically limited, Usually, a cast roll is used. The cast roll temperature is preferably "Tg - 50°C" or higher, preferably "Tg + 70°C" or lower, and more preferably "Tg + 40°C" or lower. When a resin film is manufactured under these conditions, a resin film having a thickness of 1 µm to 1 mm can be easily manufactured. 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.

It is preferable that process (i) includes contacting the resin film with a solvent after obtaining a resin film. The contact with the solvent changes the refractive index of the resin film, and a pre-stretched film having refractive indices nx ₂ , ny ₂ , and nz ₂ satisfying Formula (3) is obtained.

As a solvent, an organic solvent is usually used. As a specific type of solvent, a solvent that can penetrate into the resin film without dissolving the crystalline polymer contained in the resin film can be used, and examples thereof include hydrocarbon solvents such as cyclohexane, toluene, limonene, and decalin; carbon disulfide; can be mentioned. One type of solvent may be sufficient as it, and two or more types may be sufficient as it.

There is no restriction|limiting on the contact method of a resin film and a solvent. As a contact method, it is the spray method which sprays a solvent to a resin film, for example; a coating method of applying a solvent to a resin film; an immersion method in which a resin film is immersed in a solvent; etc. can be mentioned. Especially, the immersion method is preferable at the point which can perform continuous contact easily.

The temperature of the solvent brought into contact with the resin film is arbitrary within the range in which the solvent can maintain a liquid state, and therefore can be set within a range equal to or higher than the melting point and equal to or lower than the boiling point of the solvent.

The contact time is preferably 0.5 seconds or more, more preferably 1.0 seconds or more, and particularly preferably 5.0 seconds or more. The upper limit is not particularly limited, and may be, for example, 24 hours or less. However, since the degree of progress of orientation tends not to change significantly even if the contact time is lengthened, it is preferable that the contact time be short within the range where desired optical properties are obtained.

By being brought into contact with a solvent, the refractive index of the resin film changes. This change in refractive index usually appears as a change in birefringence Rth/d in the thickness direction. The amount of change in birefringence Rth/d in the thickness direction caused by contact with the solvent is preferably 0.1 × 10 ^-3 or more, more preferably 0.5 × 10 ^-3 or more, and particularly preferably 1.0 × 10 ^-3 or more. , and is preferably 50.0 × 10 ^-3 or less, more preferably 10.0 × 10 ^-3 or less, and particularly preferably 5.0 × 10 ^-3 or less. The change in birefringence Rth/d in the thickness direction is the absolute value of the change in birefringence Rth/d in the thickness direction. The specific amount of change in birefringence Rth/d in the thickness direction is obtained by subtracting the birefringence Rth/d in the thickness direction of the resin film (film before contact with the solvent) from the birefringence Rth/d in the thickness direction of the film before stretching (film after contact with the solvent). It is obtained as an absolute value. Usually, the birefringence Rth/d in the thickness direction increases due to contact between the resin film and the solvent.

The birefringence Re/d of the resin film in the in-plane direction may or may not change due to contact with a solvent. The amount of change in birefringence Re/d in the in-plane direction caused by contact with the solvent is usually 0.0 × 10 ^-3 or more, preferably 0.1 × 10 ^-3 or more, particularly preferably 0.2 × 10 ^-3 or more, preferably It is preferably 10 × 10 ^-3 or less, more preferably 5 × 10 ^-3 or less, and particularly preferably 2 × 10 ^-3 or less. The amount of change in birefringence Re/d in the in-plane direction represents the absolute value of the change in birefringence Re/d in the in-plane direction. The specific amount of change in birefringence Re/d in the in-plane direction is obtained by subtracting the birefringence Re/d in the in-plane direction of the resin film (film before contact with the solvent) from the birefringence Re/d in the in-plane direction of the film before stretching (film after contact with the solvent). It is obtained as an 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 Formula (3) is obtained. Formula (3) shows that the film before extending|stretching is a negative C plate. Therefore, 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 of the film before stretching in the in-plane direction are the same or close to each other. Therefore, in the film before stretching, the difference between the refractive index nx ₂ and the refractive index ny ₂ is usually relatively small, the difference between the refractive index nx ₂ and the refractive index nz ₂ is relatively large, and the difference between the refractive index ny ₂ and the refractive index nz ₂ is relatively large.

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 expressed by birefringence Rth/d in the thickness direction. That is, since birefringence Rth/d in the thickness direction of the film before stretching can be expressed as "Rth/d = {(nx ₂ + ny ₂ )/2} - nz ₂ ", the birefringence Rth/d in the thickness direction , 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 expressed. The birefringence Rth/d of the film before stretching in the thickness direction is preferably 0.2 × 10 ^-3 or more, more preferably 0.5 × 10 ^-3 or more, particularly preferably 1.0 × 10 ^-3 or more, and preferably 10 × 10 ^-3 or less, more preferably 6.0 × 10 ^-3 or less, particularly preferably 4.0 × 10 ^-3 or less.

The difference between the refractive index nx ₂ and the refractive index ny ₂ in the in-plane direction can be expressed by the birefringence Re/d in the in-plane direction. That is, since birefringence Re/d in the in-plane direction of the film before stretching can be expressed as "Re/d = nx ₂ - ny ₂ ", by the birefringence Re/d in the in-plane direction, the refractive index nx ₂ in the in-plane direction and It can represent the difference in 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. Therefore, birefringence Re/d in the in-plane direction of the film before stretching may be smaller than 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 0.1 × 10 ^-3 or more, more preferably 0.2 × 10 ^-3 or more, particularly preferably 0.5 × 10 ^-3 or more, and preferably It is preferably 5.0 × 10 ^-3 or less, more preferably 4.0 × 10 ^-3 or less, and particularly preferably 3.0 × 10 ^-3 or less.

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

When the solvent in contact with the resin film penetrates into the resin film, the thickness of the resin film usually increases. Therefore, the thickness of the film before extending|stretching obtained after contact with a solvent is usually thicker than a resin film. At this time, 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 change rate of the thickness may be, for example, 80% or less, 50% or less, or 40% or less. The rate of change in thickness described above 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.

Usually, the film before extending|stretching obtained by making a solvent contact a resin film has high mechanical strength compared with the resin film before contact with a solvent. Therefore, since this pre-stretching film can have excellent stretching resistance, even if harsh stretching conditions are employed in step (ii), it is unlikely to cause breakage. Therefore, when using the pre-stretching film obtained by contacting a resin film with a solvent, stretching at a high temperature and stretching at a high draw ratio are possible.

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

The method for manufacturing an optical film according to the second embodiment of the present invention includes, after step (i), step (ii) of stretching the film before stretching. By stretching, the polymer molecules included in the film before stretching can be oriented in a direction along the stretching direction. Therefore, according to the stretching in step (ii), since the refractive index of the film before stretching can be adjusted, an optical film having desired optical properties can be obtained.

The stretching direction is not limited, and examples thereof include a longitudinal direction, a width direction, and an oblique direction. Here, the inclination direction is a direction perpendicular to the thickness direction, and indicates a direction that is neither parallel nor perpendicular to the width direction. Further, the stretching direction may be one direction or two or more directions, but one direction is preferable. Also, among stretching in one direction, free uniaxial stretching in which no restraining force is applied in directions other than the stretching direction is more preferable. According to these stretching, an optical film having desired optical properties can be easily manufactured.

Since the film before stretching usually has negative birefringence characteristics, when stretching in one direction is performed, an optical film having a slow axis in a direction perpendicular to the stretching direction is obtained. Therefore, since the direction of the slow axis of the optical film can be adjusted by the direction of stretching, the direction of stretching may be selected according to the direction of the slow axis to be expressed in the optical film. Usually, a long polarizing film has an absorption axis in its longitudinal direction and a transmission axis in its width direction. Therefore, for example, when it is desired to obtain an optical film having a slow axis perpendicular to the absorption axis of the polarizing film, the film before stretching may be stretched in the longitudinal direction to obtain an optical film having a slow axis in the width direction. In this case, roll-to-roll efficient production of a polarizing plate using a long optical film and a long polarizing film is possible.

The draw ratio is preferably 1.1 times or more, more preferably 1.2 times or more, preferably 20.0 times or less, more preferably 10.0 times or less, still more preferably 5.0 times or less, and particularly preferably 2.0 times or less. am. A specific draw ratio is preferably set appropriately according to factors such as optical properties, thickness, and mechanical strength of an optical film to be manufactured. When the stretching ratio is equal to or greater than the lower limit of the above range, the birefringence can be greatly changed by stretching. In addition, when the draw ratio is equal to or less than the upper limit of the above range, the direction of the slow axis can be easily controlled or film breakage can be effectively suppressed.

The stretching temperature is preferably "Tg + 5°C" or higher, more preferably "Tg + 10°C" or higher, preferably "Tg + 100°C" or lower, more preferably "Tg + 90°C" or lower. am. 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 above range, the film can be sufficiently softened before stretching so that stretching can be performed uniformly. In addition, at such a high temperature, the amount of solvent in the film can be reduced by drying. In particular, when using a pre-stretch film obtained by bringing a resin film into contact with a solvent, the pre-stretch film has high mechanical strength and is unlikely to break even when stretched at a high temperature. It is preferable to do Moreover, since hardening of the film before extending|stretching by advancing crystallization of a crystalline polymer can be suppressed when extending|stretching temperature is below the upper limit of the said range, extending|stretching can be performed smoothly. Further, large birefringence can be expressed by stretching. Moreover, transparency can be improved by making the haze of the optical film obtained small normally.

By carrying out the above stretching treatment, the molecules in the film can be oriented in the stretching direction, so the refractive index of the film before stretching is changed. Thus, an optical film having refractive indices nx ₁ , ny ₁ , and nz ₁ satisfying Expression (1) is obtained.

[10. random process]

The manufacturing method of an optical film may further include an arbitrary process in combination with the process (i) - process (ii) mentioned above. As an optional step, for example, a step of preheating the film before stretching before step (ii), a step of heat-treating the optical film obtained in step (ii) to promote crystallization of a crystalline polymer, and drying the optical film. a step of reducing the amount of solvent in the film by using the film, a step of heat-shrinking the optical film to remove residual stress in the film, and the like.

According to the said manufacturing method, a long optical film can be manufactured using the long film before extending|stretching. The manufacturing method of an optical film may include the process of winding up the long optical film manufactured in this way in roll shape. Moreover, the manufacturing method of an optical film may include the process of cutting out a long optical film into a desired shape.

[11. Optical film produced by the manufacturing method according to the second embodiment]

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

[12. polarizer]

A polarizing plate according to an embodiment of the present invention includes the above-described optical film and a polarizing film. A polarizing film can normally function as a linear polarizer. Therefore, the polarizing plate can transmit some polarized light and block other polarized light.

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

In one example, the angle formed by the absorption axis of the polarizing film and the slow axis of the optical film is preferably 80° or more, more preferably 85° or more, particularly preferably 88° or more, and preferably 100°. or less, more preferably 95° or less, particularly preferably 92° or less. In this case, the optical film preferably has in-plane retardation capable of functioning as a 1/2 wave plate. The polarizing plate according to this example can be used as a polarizing plate capable of compensating for a viewing angle when 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 40° or more, more preferably 42° or more, particularly preferably 44° or more, and preferably 50° or more. ° or less, more preferably 48 ° or less, particularly preferably 46 ° or less. In this case, the optical film preferably has in-plane retardation capable of functioning as a 1/4 wave plate. The polarizing plate according to this example can be used as a circular polarizing plate capable of transmitting circularly polarized light in one rotational direction and blocking circularly polarized light in the other rotational direction. When this circular polarizing plate is installed on the display surface of the display device, reflection of external light can be suppressed.

As a polarizing film, arbitrary polarizing films can be used. After adsorbing iodine or a dichroic dye to a polyvinyl alcohol film as an example of a polarizing film, Film obtained by uniaxially stretching in a boric-acid bath; A film obtained by adsorbing iodine or a dichroic dye to a polyvinyl alcohol film, stretching it, and further modifying a part of the polyvinyl alcohol units in the molecular chain to polyvinylene units; Among these, as a polarizing film, what contains polyvinyl alcohol is preferable.

When natural light is incident on a polarizing film, only one polarized light is transmitted. The degree of polarization of this polarizing film is not particularly limited, but is preferably 98% or more, more preferably 99% or more. In addition, the thickness of the polarizing film is preferably 5 μm to 80 μm.

The polarizing plate described above may further include an arbitrary layer. Arbitrary layers include, for example, a polarizer protective film layer; Adhesive layer for bonding a polarizing film and an optical film together; hard coat layers such as an impact-resistant polymethacrylate resin layer; A mat layer that improves the sliding properties of the film; antireflection layer; antifouling layer; antistatic layer; etc. can be mentioned. These arbitrary layers may form only one layer, or may form two or more layers.

[Example]

Hereinafter, the present invention will be described in detail by showing examples. However, the present invention is not limited to the examples shown below, and can be implemented with arbitrary changes within a range not departing from the scope of the claims of the present invention and their equivalents.

In the following description, "%" and "parts" representing amounts are based on weight unless otherwise specified. Incidentally, the operations described below were performed under atmospheric conditions at room temperature and normal pressure (23° C. and 1 atm) unless otherwise noted.

[Methods for Measuring Glass Transition Temperature Tg and Melting Point Tm]

The glass transition temperature Tg and melting point Tm of the polymer were measured as follows. First, the polymer was melted by heating, and the melted polymer was quenched with dry ice. Then, using this polymer as a test body, the glass transition temperature Tg and melting point Tm of the polymer were measured using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min (heating mode).

[Method of measuring retardation, NZ coefficient and slow axis direction of optical film]

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 retardometer (“AxoScan OPMF-1” manufactured by AXOMETRICS).

[Evaluation direction of bendability of optical film]

A mandrel bending test was performed using an optical film. Specifically, the optical film was bent according to the mandrel diameter, and whether or not it was broken was checked. When it broke, it was determined that the optical film could not withstand the mandrel diameter. This mandrel bending test was performed using mandrels with diameters of 6 mm, 3 mm and 2 mm, respectively. The diameter of the minimum mandrel at which the optical film does not break was determined as a measured value. It shows that an optical film is excellent in bendability, so that this measured value is small. When the optical film was not damaged at all even in the mandrel bending test using a mandrel having a diameter of 2 mm, the measured value of the bendability was set as "less than 2 mm".

[Production Example 1. Production of crystalline polystyrene]

17.8 g (71 mmol) of copper sulfate pentahydrate (CuSO ₄ 5H ₂ O), 200 ml of toluene, and 24 ml (250 mmol) of trimethylaluminum were placed in a glass container having an internal volume of 500 ml purged with argon, and 8 ml at 40 ° C. time reacted. 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. It was 610 as a result of measuring the molecular weight of this contact product by the freezing point depression method.

Then, to a reaction vessel, 5 mmol of the above contact product as an aluminum atom, 5 mmol of triisobutylaluminum, 0.025 mmol of pentamethylcyclopentadienyltitanium trimethoxide, and 1 mmol of purified styrene were added, and at 90 ° C. A polymerization reaction was performed for 5 hours. Thereafter, the product was washed repeatedly with methanol after decomposition of the catalyst components with a methanol solution of sodium hydroxide and dried to obtain 308 g of a polymer (crystalline polystyrene).

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

By repeating this operation, crystalline polystyrene having a syndiotactic structure required for evaluation was prepared.

[Preparation Example 2. Preparation of crystalline polymer having positive intrinsic birefringence]

The metal pressure-resistant reactor was sufficiently dried and then purged with nitrogen. In this metal pressure reactor, 154.5 parts of cyclohexane, 42.8 parts of a 70% concentration of dicyclopentadiene (endo body content of 99% or more) cyclohexane solution (30 parts as the amount of dicyclopentadiene), and 1.9 parts of 1-hexene added and warmed to 53 °C.

A solution was prepared by dissolving 0.014 part of tetrachlorotungstenphenylimide (tetrahydrofuran) complex in 0.70 part of toluene. To this solution, 0.061 part of a diethylaluminum ethoxide/n-hexane solution having a concentration of 19% was added and stirred for 10 minutes to prepare a catalyst solution. This catalyst solution was added to a pressure-resistant reactor to initiate a ring-opening polymerization reaction. Then, it was made to react for 4 hours, maintaining 53 degreeC, and the solution of the ring-opening polymer of dicyclopentadiene was obtained. The number average molecular weight (Mn) and weight average molecular weight (Mw) of the obtained ring-opening polymer of dicyclopentadiene were 8,750 and 28,100, respectively, and the molecular weight distribution (Mw/Mn) determined therefrom was 3.21.

To 200 parts of the obtained solution of the ring-opening polymer of dicyclopentadiene, 0.037 parts of 1,2-ethanediol was added as a terminating agent, heated to 60°C, and stirred for 1 hour to terminate the polymerization reaction. To this, one part of a hydrotalcite-type compound (“Kyoward (registered trademark) 2000” manufactured by Kyowa Chemical Industry Co., Ltd.) was added, heated to 60° C., and stirred for 1 hour. After that, 0.4 part of filter aid (“Radiolite (registered trademark) #1500” manufactured by Showa Chemical Industry Co., Ltd.) was added, and a PP pleat cartridge filter (“TCP-HX” manufactured by ADVANTEC Toyo Co., Ltd.) was used to filter the adsorbent and solution was separated by filtration.

To 200 parts of a ring-opening polymer solution of dicyclopentadiene after filtration (polymer weight: 30 parts), 100 parts of cyclohexane was added, 0.0043 part of chlorohydridecarbonyltris(triphenylphosphine)ruthenium was added, and hydrogen pressure was 6 MPa. , and a hydrogenation reaction was performed at 180°C for 4 hours. As a result, a reaction liquid containing a hydride of a ring-opening polymer of dicyclopentadiene was obtained. Hydride was precipitated in this reaction liquid, and it became a slurry solution.

The hydride and solution contained in the above reaction liquid are separated using a centrifugal separator, dried under reduced pressure at 60°C for 24 hours, and a crystalline polymer having positive intrinsic birefringence is obtained as a hydride of a ring-opening polymer of dicyclopentadiene. Got 28.5 copies. The hydrogenation rate of this hydride was 99% or more, the glass transition temperature Tg was 93°C, the melting point Tm was 262°C, and the ratio of racemo dyad was 89%.

[Example 1]

(1-1. Preparation of crystalline resin)

Crystalline polystyrene having a syndiotactic structure obtained in Production Example 1 was kneaded with a twin-screw extruder at 295°C to prepare pellets of a transparent crystalline resin.

(1-2. Film extrusion)

Pellets of the crystalline resin were melt-extruded using a hot-melting extrusion film molding machine equipped with a T die ("Measuring Extruder Type Me-20/2800V3" manufactured by Optical Control Systems), and rolled at a speed of 1.5 m/min. It was wound up to obtain a long raw film as a resin film before solvent contact with a width of approximately 120 mm. Operating conditions of the film forming machine are described below for each item.

Barrel temperature setting = 280℃~300℃

Die temperature = 300°C

·Screw rotation rate = 30 rpm

・Cast roll temperature = 80℃

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

(1-3. Solvent contact)

The above raw film was cut into a rectangle of 120 mm × 120 mm. This rectangular raw film is immersed in cyclohexane as a solvent collected in a vat until in-plane retardation Re = 58 nm and retardation in the thickness direction Rth = 111 nm, the film before stretching as a resin film after solvent contact Got it. Before stretching, the film was taken out of cyclohexane, and after wiping off the cyclohexane adhering to the film surface, it was naturally dried in the air. The thickness of the obtained film before extending|stretching was 28 micrometers.

(1-4. Stretching)

A batch type biaxial stretching device (manufactured by Etosha) was prepared. This stretching device was equipped with an oven unit and a clip for stretching capable of fixing the film. If this stretching device is used, it is possible to stretch the film by pulling the film with a clip in an oven.

The film before stretching was cut into a rectangle of 100 mm x 100 mm. Both ends of this rectangular film before stretching were respectively held by five clips of the above stretching device. The pre-stretching film was pulled with a clip, and free uniaxial stretching was performed in the longitudinal direction of the long raw film obtained in the extrusion film forming step. The stretching temperature was 130°C and the magnification was 1.3 times. By this stretching, an optical film as a uniaxially stretched film was obtained.

As a result of measuring the retardation of the obtained optical film, in-plane retardation Re = 280 nm and thickness direction retardation Rth = 5 nm, and the direction of the slow axis of the optical film was a direction perpendicular to the stretching direction. In addition, the NZ coefficient at a measurement wavelength of 550 nm was 0.52. About the obtained optical film, the bendability was evaluated by the method mentioned above.

[Example 2]

The line speed in step (1-2) was changed, and the thickness of the long raw film was changed to 13 µm.

Further, in the step (1-3), the immersion time was adjusted so that a pre-stretch film having in-plane retardation Re = 33 nm and thickness direction retardation Rth = 56 nm was immersed in the solvent of the raw film. .

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

[Example 3]

The line speed in step (1-2) was changed, and the thickness of the long raw film was changed to 40 µm.

Further, in step (1-3), immersion in the solvent of the raw film was performed by adjusting the immersion time so that a pre-stretch film having in-plane retardation Re = 24 nm and thickness direction retardation Rth = 45 nm was obtained. .

Moreover, in the process (1-4), the draw ratio was changed to 1.1 times.

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

[Comparative Example 1]

To 100 parts of the hydride of the ring-opening polymer of dicyclopentadiene obtained in Production Example 2, an antioxidant (tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)pro Cionate] methane; After mixing 1.1 parts of “Irganox (registered trademark) 1010” manufactured by BASF Japan, a twin-screw extruder equipped with four die holes with an inner diameter of 3 mmφ (“TEM-37B” manufactured by Toshiba Machinery Co., Ltd.) was put into. A mixture of a hydride of a ring-opening polymer of dicyclopentadiene and an antioxidant was molded into a strand shape by hot-melt extrusion molding, and then shredded with a strand cutter to obtain crystalline resin pellets.

This pellet was used in step (1-2). Moreover, the line speed in process (1-2) was changed and the thickness of the long raw film was changed to 13 micrometers.

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

In addition, the stretching temperature in 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 in the same manner as in Example 1 except for the above matters.

[Comparative Example 2]

Immersion in the solvent of the raw film in process (1-3) was not performed.

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

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

[result]

The results of the above Examples and Comparative Examples are shown in the tables below. In the table below, the meaning of the abbreviation is as follows.

SPS: crystalline polystyrene.

Cy: cyclohexane.

Tl: Toluene.

[examine]

In the examples, an optical film having an NZ coefficient greater than 0 and less than 1 is obtained. Therefore, in the Example, it turns out that the optical film which has the refractive index nx ₁ , ny ₁ , and nz ₁ which satisfy Formula (1) is obtained. In addition, the optical films obtained in Examples achieve birefringence Re/d in the in-plane direction that satisfies Expression (2) by using crystalline polystyrene as a crystalline polymer having negative intrinsic birefringence. Therefore, according to the present invention, it was confirmed that an optical film satisfying the requirements (A) to (C) was obtained.

In the examples, since the birefringence Rth/d in the thickness direction of the raw film and the birefringence Rth/d in the thickness direction of the film before stretching are different, it can be seen that the refractive index of the raw film changed due to contact with the solvent. Further, in the examples, the film before stretching has a sufficiently small in-plane retardation Re compared to the in-plane retardation Rth in the thickness direction, and thus satisfies equation (3), so it can be seen that it is a negative C plate. there is. Since the pre-stretch film obtained in this way has negative birefringence properties, the aforementioned optical film is obtained by stretching.

In Examples, stretching is performed at a higher temperature than in Comparative Example 2 using the same material. This is for efficiently removing the solvent contained in the film before stretching according to the examples at a high temperature. Since the mechanical strength of the film before stretching was improved by contact with the solvent, breakage of the film did not occur even when the film was stretched at such a high temperature.

Claims (9)

An optical film comprising a crystalline polymer having negative intrinsic birefringence,
The refractive index nx ₁ of the optical film in a direction giving the maximum refractive index as a direction perpendicular to the thickness direction, a refractive index ny ₁ in a direction perpendicular to the direction of nx ₁ as a direction perpendicular to the thickness direction, and a refractive index nz in the thickness direction ₁ satisfies the following formula (1),
An optical film in which 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) According to claim 1,
The optical film, wherein the optical film is a stretched film. According to claim 1 or 2,
The optical film in which the said optical film has the shape of a long picture. A polarizing plate provided with the optical film according to any one of claims 1 to 3 and a polarizing film. According to claim 4,
The polarizing plate, wherein the slow axis of the optical film and the absorption axis of the polarizing film form an angle of 80 ° to 100 °. The refractive index nx ₁ in the direction giving the maximum refractive index as a direction perpendicular to the thickness direction, the refractive index _{ny 1} in the direction perpendicular to the direction of nx ₁ as the direction perpendicular to the thickness direction, and the refractive index nz ₁ in the thickness direction are expressed by the formula (1 ) as a method for producing an optical film that satisfies;
The manufacturing method,
Step (i) of preparing a pre-stretching film comprising a crystalline polymer having negative intrinsic birefringence;
including a step (ii) of stretching the film before the stretching;
The refractive index nx ₂ of the pre-stretching film prepared in step (i) in the direction giving the maximum refractive index as a direction perpendicular to the thickness direction, and the refractive index ny in the direction perpendicular to the nx ₂ direction as a direction perpendicular to the thickness direction ₂ , and the refractive index nz ₂ in the thickness direction satisfy the following formula (3), the manufacturing method of the optical film.
nx ₁ > nz ₁ > ny ₁ (1)
nz ₂ < nx ₂ ≒ ny ₂ (3) According to claim 6,
Step (i) is,
preparing a resin film comprising a crystalline polymer having negative intrinsic birefringence;
A method for producing an optical film, comprising contacting the resin film with a solvent to obtain the pre-stretch film. According to claim 6 or 7,
A method for producing an optical film, wherein the optical film has a single-layer structure. According to any one of claims 6 to 8,
A method for producing an optical film, wherein the crystalline polymer having negative intrinsic birefringence is a polystyrene-based polymer.