JP7567514B2 - Optical film and method for producing composite optical film - Google Patents

Description

The present invention relates to an optical film, a composite film including the optical film, and a method for producing the optical film.

Resin films with specific optical properties have been used for optical purposes. For example, a film with an Nz coefficient that satisfies 0<Nz<1 is called a three-dimensional retardation film. It is known that when a three-dimensional retardation film is provided in a display device such as a liquid crystal display device, it can exert an effect of reducing coloring of the display surface when viewed from an oblique direction.

Three-dimensional retardation films have a greater retardation in the z-axis direction (i.e., thickness direction) than in the y-axis direction (i.e., in-plane direction perpendicular to the in-plane slow axis direction). Therefore, they cannot be manufactured by a normal retardation film manufacturing method, such as simply stretching a resin for optical films with a normal positive intrinsic birefringence. For this reason, various methods for manufacturing three-dimensional retardation films with different materials and manufacturing processes have been proposed (for example, Patent Documents 1 and 2).

International Publication No. 2019/188205 International Publication No. 2020/137409

Previously proposed methods for manufacturing three-dimensional retardation films that combine resins with positive and negative intrinsic birefringence have had problems such as requiring a complex stretching process and a lamination process after stretching, which requires a lot of time for positioning.

In addition, display devices are generally subject to degradation due to ultraviolet rays in external light, and there is a demand for functions that suppress such degradation.

The object of the present invention is therefore to provide an optical film that can exhibit good effects as a three-dimensional retardation film, can exhibit the effect of suppressing deterioration of a display device, and can be easily manufactured, as well as a manufacturing method for easily manufacturing such an optical film.

The present inventors have conducted research to solve the above problems. As a result, the present inventors have found that the above problems can be solved by using a specific material having a specific ultraviolet absorbing ability as a material for constituting the optical film. Based on this finding, the present inventors have completed the present invention.
That is, the present invention includes the following.

[1] An optical film comprising a layer of a resin (a) containing a crystalline polymer, the optical film having a light transmittance of 5% or less at a wavelength of 380 nm and an Nz coefficient of greater than 0 and less than 1.
[2] The optical film according to [1], which contains an ultraviolet absorbing agent.
[3] The optical film according to [1] or [2], wherein the resin (a) has a positive intrinsic birefringence value.
[4] The optical film according to any one of [1] to [3], comprising a plurality of layers, at least one of which is located on a surface of the optical film and is made of the resin (a).
[5] A layer structure of (A layer)/(B layer)/(A layer), the A layer being a layer of a resin (a) containing a crystalline polymer,
The optical film according to [4], wherein the layer B is a layer of a resin (aB) containing a crystalline polymer and an ultraviolet absorber.
[6] The optical film according to any one of [1] to [5], wherein the crystalline polymer contains an alicyclic structure.
[7] A composite optical film comprising the optical film according to any one of [1] to [6] and a polarizer layer,
a slow axis of the optical film and a transmission axis of the polarizer layer form an angle of 42° to 48°.
[8] A composite optical film comprising the optical film according to any one of [1] to [6] and a conductive layer.
[9] A method for producing the optical film according to any one of [1] to [6],
A step (I) of obtaining a film (I) having a layer of a resin (a) and containing an ultraviolet absorber;
a step (II) of contacting one or both surfaces of the film (I) with a solvent to change the refractive index in the thickness direction of the film (I) to obtain a film (II);
A method for producing an optical film, comprising:
[10] The method for producing an optical film according to [9], wherein the film (I) has a layer structure of (pA layer)/(pB layer)/(pA layer), the pA layer is a layer of a resin (a) containing a crystalline polymer, and the pB layer is a layer of a resin (aB) containing a crystalline polymer and an ultraviolet absorber.
[11] The method for producing an optical film according to [9] or [10], wherein the step (I) includes co-extrusion forming the film (I).
[12] The method for producing an optical film according to any one of [9] to [11], further comprising a step (III) of stretching the film (II) to form a film (III).

The present invention provides an optical film and a composite optical film that can exhibit good effects as a three-dimensional retardation film, can exhibit the effect of suppressing deterioration of a display device, and can be easily manufactured, as well as a manufacturing method for easily manufacturing such an optical film.

The present invention will be described in detail below with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and may be modified and implemented as desired without departing from the scope of the claims of the present invention and the scope of equivalents thereto.

In the following description, the in-plane retardation Re of a layered structure such as a film is a value expressed by Re = (nx - ny) x d unless otherwise specified. The retardation Rth in the thickness direction of a layered structure is a value expressed by Rth = [{(nx + ny)/2} - nz] x d unless otherwise specified. The Nz coefficient of a layered structure is a value expressed by (nx - nz)/(nx - ny) unless otherwise specified.

nx represents the refractive index in a direction perpendicular to the thickness direction of the layered structure (in-plane direction) that gives the maximum refractive index. ny represents the refractive index in the in-plane direction of the layered structure that is perpendicular to the nx direction. nz represents the refractive index in the thickness direction of the layered structure. d represents the thickness of the layered structure. The measurement wavelength is 590 nm unless otherwise specified.

In the following description, unless otherwise specified, a material with positive intrinsic birefringence means a material whose refractive index in the stretching direction is greater than the refractive index in the direction perpendicular to the stretching direction. Furthermore, a material with negative intrinsic birefringence means a material whose refractive index in the stretching direction is smaller than the refractive index in the direction perpendicular to the stretching direction, unless otherwise specified. The value of intrinsic birefringence can be calculated from the dielectric constant distribution.

In the following description, the slow axis of a layered structure is the in-plane slow axis unless otherwise specified.

In the following description, a "long" film refers to a film that is 5 times or more longer than its width, preferably 10 times or more longer, specifically a film that is long enough to be wound into a roll for storage or transportation. There is no particular upper limit on the length, but it is usually 100,000 times or less longer than the width.

[Optical Film]
The optical film of the present invention includes a layer of a resin (a) containing a crystalline polymer. The optical film of the present invention may contain an ultraviolet absorbing agent.

In a preferred embodiment, the optical film of the present invention comprises a plurality of layers, and one or more of the layers located on the surface of the optical film are made of resin (a). That is, one or both of the front and back surfaces of the optical film are made of a layer of crystalline resin (a). By having such a structure in which the layer of crystalline resin (a) is exposed on the surface, the manufacturing method described below can be employed to easily impart the optical film with the desired optical properties.

In a more preferred embodiment, the optical film of the present invention has a layer structure of (A layer)/(B layer)/(A layer). That is, in this case, the optical film of the present invention has a B layer and an A layer provided on both the front and back surfaces of the B layer. Here, the A layer is a layer of resin (a), while the B layer is a layer of resin (aB) containing a crystalline polymer and an ultraviolet absorber. By having such a layer structure, it is possible to suppress bleeding out of the ultraviolet absorber. Therefore, even if a high concentration of ultraviolet absorber is blended into a thin film, it is possible to make an optical film with high practicality. In addition, it is possible to suppress contamination of the production line by the ultraviolet absorber during the film manufacturing process.

[Resin (a)]
Resin (a) may be a crystalline polymer, i.e., a resin containing a polymer having crystallinity. A polymer having crystallinity refers to a polymer having a melting point Tm. In other words, a polymer having crystallinity refers to a polymer whose melting point can be observed by a differential scanning calorimeter (DSC). The crystalline resin is preferably a thermoplastic resin.

The crystalline polymer preferably has a positive intrinsic birefringence. By using a crystalline polymer having a positive intrinsic birefringence, the resin (a) can be a resin having a positive intrinsic birefringence value. In this case, an optical film that satisfies the requirements of the present invention, particularly the requirements regarding the Nz coefficient, can be particularly easily produced.

The crystalline polymer may be, for example, polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN); polyolefins such as polyethylene (PE) and polypropylene (PP); and the like, but is not particularly limited, and preferably contains an alicyclic structure. By using a crystalline polymer containing an alicyclic structure, the mechanical properties, heat resistance, transparency, low moisture absorption, dimensional stability, and light weight of the resulting optical film can be improved. A polymer containing an alicyclic structure refers to a polymer having an alicyclic structure in the molecule. Such a polymer containing an alicyclic structure may be, for example, a polymer that can be obtained by a polymerization reaction using a cyclic olefin as a monomer, or a hydrogenated product thereof.

Examples of alicyclic structures include cycloalkane structures and cycloalkene structures. Among these, cycloalkane structures are preferred because they are more likely to produce optical films with excellent properties such as thermal stability. The number of carbon atoms contained in one alicyclic structure is preferably 4 or more, more preferably 5 or more, and preferably 30 or less, more preferably 20 or less, and particularly preferably 15 or less. When the number of carbon atoms contained in one alicyclic structure is within the above range, mechanical strength, heat resistance, and moldability are highly balanced.

In a crystalline polymer containing an alicyclic structure, the ratio of structural units having an alicyclic structure to all structural units is preferably 30% by weight or more, more preferably 50% by weight or more, and particularly preferably 70% by weight or more. By increasing the ratio of structural units having an alicyclic structure as described above, heat resistance can be improved. The ratio of structural units having an alicyclic structure to all structural units can be 100% by weight or less. In addition, in a crystalline polymer containing an alicyclic structure, the remainder other than the structural units having an alicyclic structure is not particularly limited and can be appropriately selected depending on the purpose of use.

Examples of the crystalline polymer containing an alicyclic structure include the following polymer (α) to polymer (δ). Among these, polymer (β) is preferred because it is easy to obtain an optical film having excellent heat resistance.
Polymer (α): A ring-opening polymer of a cyclic olefin monomer, which has crystallinity.
Polymer (β): A hydrogenated product of polymer (α) and has crystallinity.
Polymer (γ): An addition polymer of a cyclic olefin monomer, which has crystallinity.
Polymer (δ): A hydrogenated product of polymer (γ) and has crystallinity.

Specifically, as the crystalline polymer containing an alicyclic structure, a ring-opening polymer of dicyclopentadiene having crystallinity and a hydrogenated ring-opening polymer of dicyclopentadiene having crystallinity are more preferable. Among them, a hydrogenated ring-opening polymer of dicyclopentadiene having crystallinity is particularly preferable. Here, a ring-opening polymer of dicyclopentadiene refers to a polymer in which the ratio of structural units derived from dicyclopentadiene to all structural units is usually 50% by weight or more, preferably 70% by weight or more, more preferably 90% by weight or more, and even more preferably 100% by weight.

The hydrogenated ring-opening polymer of dicyclopentadiene preferably has a high ratio of racemo dyads. Specifically, the ratio of racemo dyads in the repeating units of the hydrogenated ring-opening polymer of dicyclopentadiene is preferably 51% or more, more preferably 70% or more, and particularly preferably 85% or more. A high ratio of racemo dyads indicates high syndiotactic stereoregularity. Therefore, the higher the ratio of racemo dyads, the higher the melting point of the hydrogenated ring-opening polymer of dicyclopentadiene tends to be.
The ratio of the racemo dyads can be determined based on the ¹³ C-NMR spectrum analysis described in the Examples below.

As the above polymer (α) to polymer (δ), the polymers obtained by the manufacturing method disclosed in WO 2018/062067 can be used.

The melting point Tm of the crystalline polymer is preferably 200°C or higher, more preferably 230°C or higher, and preferably 290°C or lower. By using a crystalline polymer having such a melting point Tm, an optical film with an even better balance between moldability and heat resistance can be obtained.

Typically, a crystalline polymer has a glass transition temperature Tg. The specific glass transition temperature Tg of a crystalline polymer is not particularly limited, but is typically 85°C or higher and typically 170°C or lower.

The glass transition temperature Tg and melting point Tm of a polymer can be measured by the following method. First, the polymer is melted by heating, and the molten polymer is rapidly cooled with dry ice. Next, using this polymer as a test specimen, 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 weight average molecular weight (Mw) of the crystalline polymer is preferably 1,000 or more, more preferably 2,000 or more, and preferably 1,000,000 or less, more preferably 500,000 or less. A crystalline polymer having such a weight average molecular weight has an excellent balance between moldability and heat resistance.

The molecular weight distribution (Mw/Mn) of the crystalline polymer is preferably 1.0 or more, more preferably 1.5 or more, and preferably 4.0 or less, more preferably 3.5 or less. Here, Mn represents the number average molecular weight. A crystalline polymer having such a molecular weight distribution has excellent moldability.

The weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of the polymer can be measured in polystyrene equivalent values by gel permeation chromatography (GPC) using tetrahydrofuran as the developing solvent.

The degree of crystallinity of the crystalline polymer contained in the optical film is not particularly limited, but is usually high to a certain degree. The specific range of the degree of crystallinity is preferably 10% or more, more preferably 15% or more, and particularly preferably 30% or more.
The crystallinity of a crystalline polymer can be measured by X-ray diffraction.

The crystalline polymer may be used alone or in any combination of two or more types in any ratio.

The proportion of the crystalline polymer in the crystalline resin (a) is preferably 50% by weight or more, more preferably 70% by weight or more, and particularly preferably 90% by weight or more. When the proportion of the crystalline polymer is equal to or more than the lower limit, the birefringence expression and heat resistance of the resulting optical film can be improved. The upper limit of the proportion of the crystalline polymer can be 100% by weight or less.

The crystalline resin (a) may contain optional components in addition to the crystalline polymer. Optional components include, for example, antioxidants such as phenol-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants; light stabilizers such as hindered amine-based light stabilizers; waxes such as petroleum wax, Fischer-Tropsch wax, and polyalkylene wax; nucleating agents such as sorbitol-based compounds, metal salts of organic phosphoric acids, metal salts of organic carboxylic acids, kaolin, and talc; diaminostilbene derivatives, coumarin derivatives, and azole derivatives (e.g., benzoxazole derivatives, benzotriazole derivatives, benzoimide derivatives, and benzoyl ester derivatives). Examples of the optional components include fluorescent brighteners such as benzothiazole derivatives, carbazole derivatives, pyridine derivatives, naphthalic acid derivatives, and imidazolone derivatives; ultraviolet absorbers such as benzophenone-based ultraviolet absorbers, salicylic acid-based ultraviolet absorbers, and benzotriazole-based ultraviolet absorbers; inorganic fillers such as talc, silica, calcium carbonate, and glass fiber; colorants; flame retardants; flame retardant assistants; antistatic agents; plasticizers; near-infrared absorbers; lubricants; fillers; and any polymer other than a crystalline polymer, such as a soft polymer. One type of optional component may be used alone, or two or more types may be used in any ratio.

[Organic solvent contained in resin (a)]
The resin (a) constituting the optical film of the present invention may contain an organic solvent, which is usually incorporated into the film in step (II) of the production method of the present invention.

All or part of the organic solvent incorporated into the film in step (II) may penetrate into the interior of the polymer. Therefore, even if drying is performed at or above the boiling point of the organic solvent, it is difficult to easily remove the solvent completely. Therefore, the optical film of the present invention usually contains an organic solvent.

The organic solvent may be one that does not dissolve the crystalline polymer. Preferred organic solvents include, for example, hydrocarbon solvents such as toluene, limonene, and decalin; and carbon disulfide. The type of organic solvent may be one type or two or more types.

The ratio of the organic solvent contained in the optical film of the present invention (solvent content) relative to 100% by weight is preferably 10% by weight or less, more preferably 5% by weight or less, and particularly preferably 0.1% by weight or less. The lower limit of the solvent content is not particularly limited, but may be 0.001% by weight or more, or 0.01% by weight or more. The solvent content in the film may be measured by thermogravimetric analysis.

[Resin (aB)]
The resin (aB) contains a crystalline polymer and an ultraviolet absorber. The resin (aB) may be a mixture of the above-mentioned resin (a) and an ultraviolet absorber.

Examples of ultraviolet absorbers include organic ultraviolet absorbers such as triazine-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, acrylonitrile-based ultraviolet absorbers, salicylate-based ultraviolet absorbers, cyanoacrylate-based ultraviolet absorbers, naphthalimide-based ultraviolet absorbers, and phthalocyanine-based ultraviolet absorbers. Among these, benzotriazole-based ultraviolet absorbers and triazine-based ultraviolet absorbers are preferred, and benzotriazole-based ultraviolet absorbers are more preferred.

Examples of benzotriazole-based UV absorbers include 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol], 2-(3,5-di-tert-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole, 2-(2H-benzotriazol-2-yl)-p-cresol, 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, 2-benzotriazol-2-yl-4,6-di-tert-butylphenol, 2-[5-chloro(2H)-benzotriazol-2-yl]-4-methyl-6-(tert-butyl )phenol, 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol, 2-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol, 2-(2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalimidylmethyl)phenol, reaction product of methyl 3-(3-(2H-benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl)propionate/polyethylene glycol 300, 2-(2H-benzotriazol-2-yl)-6-(straight-chain and side-chain dodecyl)-4-methylphenol, etc. Examples of commercially available triazole-based ultraviolet absorbers include "ADEKA STAB LA-31" manufactured by ADEKA Corporation.

Examples of triazine-based ultraviolet absorbers include 2-(2-hydroxy-4-hexyloxyphenyl)-4,6-diphenyl-s-triazine, 2-(2-hydroxy-4-propoxy-5-methylphenyl)-4,6-bis(2,4-dimethylphenyl)-s-triazine, 2-(2-hydroxy-4-hexyloxyphenyl)-4,6-diviphenyl-s-triazine, 2,4-diphenyl-6-(2-hydroxy-4-methoxyphenyl)-s-triazine, 2,4-diphenyl-6-(2-hydroxy-4-ethoxyphenyl)-s-triazine, 2,4-diphenyl-6-(2-hydroxy-4-propoxyphenyl)-s-triazine, Examples of such triazine-based ultraviolet absorbers include 2,4-diphenyl-6-(2-hydroxy-4-butoxyphenyl)-s-triazine, 2,4-bis(2-hydroxy-4-octoxyphenyl)-6-(2,4-dimethylphenyl)-s-triazine, 2,4,6-tris(2-hydroxy-4-hexyloxy-3-methylphenyl)-s-triazine, 2,4,6-tris(2-hydroxy-4-octoxyphenyl)-s-triazine, 2-(4-isooctyloxycarbonylethoxyphenyl)-4,6-diphenyl-s-triazine, and 2-(4,6-diphenyl-s-triazin-2-yl)-5-(2-(2-ethylhexanoyloxy)ethoxy)phenol. Commercially available triazine-based ultraviolet absorbers include, for example, "ADEKA STAB LA-F70" manufactured by ADEKA Corporation.

The ultraviolet absorber may be used alone or in any combination of two or more types in any ratio.

When resin (a) and an ultraviolet absorber are mixed to obtain resin (aB), the mixing ratio of these can be adjusted appropriately to obtain the desired optical properties. Specifically, the proportion of the ultraviolet absorber in the total of 100% by weight is preferably 1% by weight or more, more preferably 3% by weight or more, even more preferably 5% by weight or more, particularly preferably 7% by weight or more, and preferably 20% by weight or less, more preferably 18% by weight or less, particularly preferably 15% by weight or less. When two or more types of ultraviolet absorbers are used, the total amount thereof can be within the range.

The proportion of the ultraviolet absorber in the entire optical film of the present invention can be appropriately adjusted so as to obtain the desired optical characteristics. Specifically, the proportion of the ultraviolet absorber in 100% by weight of the optical film is preferably 0.28% by weight or more, more preferably 0.84% by weight or more, even more preferably 1.4% by weight or more, particularly preferably 1.97% by weight or more, and is preferably 5.6% by weight or less, more preferably 5.04% by weight or less, particularly preferably 4.2% by weight or less. When two or more types of ultraviolet absorbers are used, the total amount thereof can be within the range.

[Optical Properties of Optical Films]
The optical film of the present invention has a light transmittance of 5% or less at a wavelength of 380 nm, preferably 4% or less. The lower limit of the light transmittance at a wavelength of 380 nm is not particularly limited, but is ideally 0%. By having such a low light transmittance at 380 nm, when the optical film is used as a member constituting the display surface of a display device, the display device can be prevented from being deteriorated by ultraviolet rays from outside.

On the other hand, from the viewpoint of using the optical film of the present invention as an optical component, it is preferable that the light transmittance in the visible light region is high. Specifically, the light transmittance at a wavelength of 590 nm is preferably 80% or more, more preferably 85% or more, and ideally 100%, but can be less than 100%.

The Nz coefficient of the optical film of the present invention is greater than 0, preferably 0.2 or greater, and smaller than 1, preferably 0.8 or less. A film having such an Nz coefficient is called a three-dimensional retardation film. When a three-dimensional retardation film is provided in a display device such as a liquid crystal display device, it can exhibit an effect of reducing coloring of the display surface when viewed from an inclined direction. The optical film of the present invention can be easily manufactured by the manufacturing method of the present invention described below by providing a layer of crystalline resin (a).

The in-plane retardation Re of the optical film of the present invention can be adjusted to a value suitable for the application of the optical film. In one example, the preferred range of the in-plane retardation Re(590) at a wavelength of 590 nm is 137.5 nm or a value close to it, specifically, preferably 127.5 to 147.5 nm, and more preferably 130.5 to 144.5 nm. By setting the value of Re(590) within this range, the optical film can be used as a λ/4 waveplate. In another example, the preferred range of Re(590) is 275 nm or a value close to it, specifically, preferably 265 to 285 nm, and more preferably 268 to 282 nm. By setting the value of Re(590) within this range, the optical film can be used as a λ/2 waveplate.

In general, optical films used in devices such as display devices need to be at least a certain thickness to exhibit optical properties, but are also required to be thin in order to make the devices thinner. The thickness of the optical film of the present invention is not particularly limited, but by satisfying the requirements of the present invention, it is possible to obtain a film that satisfies the desired optical properties even if it is thin. Specifically, the thickness of the optical film of the present invention can be preferably 150 μm or less, more preferably 120 μm or less, and even more preferably 100 μm or less. Optical films having such a thin thickness can be easily manufactured by the manufacturing method of the present invention. The lower limit of the thickness of the optical film is not particularly limited, but can be, for example, 20 μm or more.

When the optical film of the present invention comprises the above-mentioned A layer and B layer, the thickness of each of these layers can be adjusted appropriately so as to obtain the desired optical characteristics. The thickness of the A layer is preferably 1 μm or more, more preferably 3 μm or more, while it is preferably 100 μm or less, more preferably 80 μm or less. The thickness of the B layer is preferably 1 μm or more, more preferably 3 μm or more, while it is preferably 120 μm or less, more preferably 100 μm or less. When the optical film comprises multiple A layers, the total thickness of the layers can be adjusted to the above-mentioned preferred range. Similarly, when the optical film comprises multiple B layers, the total thickness of the layers can be adjusted to the above-mentioned preferred range.

[Method for producing optical film]
The optical film of the present invention can be produced by a production method including the following steps (I) and (II). Such a production method will be described below as a method for producing the optical film of the present invention. The method for producing the optical film of the present invention can further include the following step (III) in addition to steps (I) and (II).

Step (I): A step of obtaining a film (I) having a layer of a resin (a) and containing an ultraviolet absorber.
Step (II): A step of contacting one or both surfaces of the film (I) with a solvent to change the refractive index in the thickness direction of the film (I) to obtain a film (II).
Step (III): A step of stretching the film (II) to obtain a film (III).

[Step (I)]
Step (I) can be performed by various known film-forming methods. In particular, extrusion film-forming is preferable from the viewpoint of production efficiency. When an optical film having a layer structure of (A layer)/(B layer)/(A layer) is produced, in step (I), a multi-layer film containing layers of materials corresponding to these is prepared as the film (I). Specifically, a film having a layer structure of (pA layer)/(pB layer)/(pA layer), in which the pA layer is a layer of resin (a) containing a crystalline polymer, and the pB layer is a layer of resin (aB) containing a crystalline polymer and an ultraviolet absorber, is prepared as the film (I). By preparing a film having such a layer structure as the film (I), an optical film having a layer structure of (A layer)/(B layer)/(A layer) can be easily produced. In addition, an optical film having good adhesion between the A layer and the B layer can be easily produced.

[Step (II)]
In step (II), one or both surfaces of the film (I) are contacted with a solvent. As the solvent, a solvent that can penetrate into the crystalline resin (a) without dissolving the resin can be appropriately selected. As the solvent, an organic solvent is usually used. Examples of the organic solvent include hydrocarbon solvents such as toluene, limonene, and decalin; and carbon disulfide. The type of solvent may be one type or two or more types.

The contact in step (II) can be achieved by any operation. Examples of the contact operation include a spray method in which a solvent is sprayed onto the surface of the film (I); a coating method in which a solvent is applied onto the surface of the film (I); and an immersion method in which the film (I) is immersed in a solvent. From the viewpoint of facilitating continuous contact, the immersion method is preferred.

The temperature of the solvent during contact in step (II) can be any temperature within the range in which the solvent can remain in a liquid state, and therefore can be set within the range of the melting point or higher and the boiling point or lower of the solvent.

When the film (I) is brought into contact with the solvent by immersion, the contact time is preferably 0.5 seconds or more, more preferably 1.0 second or more, particularly preferably 5.0 seconds or more, and preferably 120 seconds or less, more preferably 80 seconds or less, particularly preferably 60 seconds or less. When the contact time is equal to or more than the lower limit, the Nz coefficient of the film (I) can be effectively adjusted by contact with the solvent. On the other hand, even if the contact time is extended, the amount of adjustment of the Nz coefficient tends not to change significantly. Therefore, when the contact time is equal to or less than the upper limit, productivity can be increased without impairing the quality of the film.

As a result of contact with the solvent in step (II), the refractive index of film (I) changes to become film (II). Such a change brought about by contact with the solvent is difficult to obtain by a normal retardation film manufacturing method, such as simply stretching a resin for optical films. Therefore, as a result of such a change, the optical film of the present invention can be easily manufactured.

When the film (II) obtained as a result of step (II) has an Nz coefficient greater than 0 and less than 1, it can be used as is as the optical film of the present invention, which is the finished product. However, in many cases, the Nz coefficient of film (II) may be less than 0. In such cases, the optical film of the present invention can be easily manufactured by carrying out step (III).

[Step (III)]
In step (III), the film (II) obtained in step (II) is stretched. By stretching the film (II), the Nz coefficient of the film (II) can be changed and easily adjusted to a desired value greater than 0. Since the film (II) has been subjected to step (II), as a result of step (III), a film (III) having optical properties that are difficult to obtain by a normal retardation film manufacturing method such as simply stretching a resin for an optical film can be easily obtained.

The stretching in step (III) can be simple uniaxial stretching. The Nz coefficient can be easily adjusted by such simple uniaxial stretching. There is no limitation on the stretching direction in step (III), and examples include the longitudinal direction, the width direction, and the oblique direction. Here, the oblique direction refers to a direction perpendicular to the thickness direction, which forms an angle with the width direction that is neither 0° nor 90° (i.e., a direction that forms an angle with the width direction that is greater than 0° and less than 90°).

When attempting to produce a film having optical properties equivalent to those of the optical film of the present invention using a manufacturing method that does not include step (II), measures such as multiple complex stretching steps and blending a material with a negative intrinsic birefringence value into the material that constitutes the film are usually required. Implementing these measures is highly disadvantageous in terms of manufacturing efficiency. In contrast, the manufacturing method of the present invention is advantageous in terms of manufacturing efficiency, since the optical film of the present invention can be obtained through relatively easy steps.

The stretching ratio is preferably 1.1 times or more, more preferably 1.2 times or more, and is preferably 20.0 times or less, more preferably 10.0 times or less, even more preferably 5.0 times or less, even more preferably 2.0 times or less, and particularly preferably 1.9 times or less. It is desirable to set the specific stretching ratio appropriately depending on factors such as the optical properties, thickness, and strength of the optical film product. When the stretching ratio is equal to or greater than the lower limit, the birefringence can be significantly changed by stretching. Also, when the stretching ratio is equal to or less than the upper limit, the direction of the slow axis can be easily controlled and film breakage can be effectively suppressed.

The stretching temperature is preferably "Tg + 5°C" or higher, more preferably "Tg + 10°C" or higher, and preferably "Tg + 100°C" or lower, more preferably "Tg + 90°C" or lower. Here, "Tg" represents the glass transition temperature of the crystalline polymer. When the stretching temperature is equal to or higher than the lower limit, the film can be sufficiently softened and stretched uniformly. When the stretching temperature is equal to or lower than the upper limit, the film can be prevented from hardening due to the progress of crystallization of the crystalline polymer, so that the stretching can be performed smoothly and a large birefringence can be expressed by stretching. Furthermore, the haze of the obtained optical film can usually be reduced to increase transparency.

Since the birefringence can be changed by step (III), the Nz coefficient can be adjusted. Thus, film (III) having the desired optical properties is obtained by stretching in step (III). The obtained film (III) can be used as it is as the optical film of the present invention. Alternatively, film (III) can be further subjected to optional processing to form the optical film of the present invention. Examples of optional processing include adjustment of birefringence by heat treatment while maintaining the stretched dimensions or relaxation treatment by shrinking the stretched dimensions.

[Other steps]
The method for producing an optical film of the present invention may further include any step in combination with the above-mentioned steps. For example, after step (II), the method may include a step of removing the solvent attached to the film (II). Examples of the method for removing the solvent include drying and wiping.

When film (I) is prepared as a long film in step (I) and steps (II) and (III) are performed on the production line while the film is in a long state, a long optical film can be obtained. The long optical film can be wound into a roll as necessary to form a film roll. In addition, it can be cut into a desired shape as necessary.

[Optical film applications: composite optical film]
The optical film of the present invention can be processed into a desired shape such as a rectangle as necessary and used as a component of an optical device such as a display device. When the optical film of the present invention is used as a component of a display device, the display quality such as the viewing angle, contrast, and image quality of an image displayed on the display device can be improved. In addition, the optical film of the present invention can be combined with other layers by further bonding or the like to form a composite optical film.

In one example, the composite optical film comprises the optical film of the present invention and a polarizer layer. The polarizer layer can be a layer of a general linear polarizer such as one mainly composed of polyvinyl alcohol. In this case, it is preferable that the slow axis of the optical film and the transmission axis of the polarizer layer form an angle of 45° or close to it. Specifically, it is preferable that the angle is 42° to 48°. By having such an angular relationship, the optical film and the polarizer layer can function as an anti-reflection film that suppresses reflection of external light on the display surface of the display device. Such a composite optical film can be manufactured by bonding the optical film of the present invention and the polarizer layer together, if necessary, via an appropriate adhesive layer.

In another example, a composite optical film comprises the optical film of the present invention and a conductive layer. The conductive layer may be a metal foil film such as ITO, or a layer of nanowires. A composite optical film having such a conductive layer may be useful as a component of a touch panel, for example. Such a composite optical film may be manufactured by forming a conductive layer on the surface of the optical film of the present invention by a known forming method such as sputtering.

The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples shown below, and can be modified and carried out as desired without departing from the scope of the claims of the present invention and the scope of equivalents thereto.
In the following description, the units "%" and "parts" are by weight unless otherwise specified. Furthermore, the operations described below were carried out at room temperature and pressure unless otherwise specified.

[Evaluation method]
(Method of measuring weight average molecular weight Mw and number average molecular weight Mn of polymer)
The weight average molecular weight Mw and number average molecular weight Mn of the polymer were measured as polystyrene equivalent values using a gel permeation chromatography (GPC) system ("HLC-8320" manufactured by Tosoh Corporation). In the measurement, an H-type column (manufactured by Tosoh Corporation) was used as the column, and tetrahydrofuran was used as the solvent. The temperature during the measurement was 40°C.

(Method of measuring hydrogenation rate of polymer)
The hydrogenation rate of the polymer was measured by ¹ H-NMR measurement at 145° C. using orthodichlorobenzene- ^d4 as a solvent.

(Method of 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, 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) using the polymer as a test specimen.

(Method for measuring the ratio of racemo-dyads in polymers)
The ratio of the racemo dyad in the polymer was measured as follows. The polymer was subjected to ¹³ C-NMR measurement by applying the inverse-gated decoupling method at 200°C using orthodichlorobenzene- ^d4 as a solvent. In the results of this ¹³ C-NMR measurement, a signal at 43.35 ppm derived from the meso dyad and a signal at 43.43 ppm derived from the racemo dyad were identified, with the peak at 127.5 ppm of orthodichlorobenzene- ^d4 as the reference shift. The ratio of the racemo dyad in the polymer was calculated based on the intensity ratio of these signals.

(Thickness Measurement)
The thickness of the film was measured using a contact thickness meter (Code No. 543-390, manufactured by MITUTOYO Corporation). The thickness of each layer in a film consisting of multiple layers was calculated from the thickness ratio obtained by observing the cross section of the film under a microscope and the ratio and the thickness of the entire film.

(Refractive Index)
The refractive index of the film was measured using a phase difference meter (AxoScan OPMF-1 manufactured by AXOMETRICS) at a wavelength of 590 nm.

(Measurement of Crystallinity)
The crystallinity of the film (III) was confirmed by X-ray diffraction in accordance with JIS K 0131. Specifically, the diffracted X-ray intensity from the crystallized portion was determined using a wide-angle X-ray diffractometer (Rigaku Corporation, "RINT 2000"), and the crystallinity was calculated from the ratio of the diffracted X-ray intensity to the entire diffracted X-ray intensity according to the following formula (i).
Xc=K・Ic/It Formula (i)
In the above formula (i), Xc represents the crystallinity of the test sample, Ic represents the diffracted X-ray intensity from the crystallized portion, It represents the total diffracted X-ray intensity, and K represents a correction term.

(Light transmittance at 380 nm and 590 nm)
The light transmittance of the laminate at wavelengths of 380 nm to 780 nm was measured using an ultraviolet-visible-near infrared spectrophotometer ("V-7200" manufactured by JASCO Corporation). The data acquisition interval during the measurement was 1 nm. From the obtained spectrum, the light transmittance at wavelengths of 380 nm and 590 nm was read.

(Suppression of deterioration of circularly polarizing plate: Maximum change rate of in-plane retardation ratio)
The film (III) obtained in each of the examples and comparative examples was attached to a circular polarizer (a layer made of a cured product of a cholesteric liquid crystal composition) using a polyvinyl alcohol-based pressure-sensitive adhesive to obtain a circular polarizing plate, and the in-plane retardation ratio (Re(450)/Re(550)) of the circular polarizing plate was measured. Here, Re(450) is the in-plane retardation (nm) at a measurement wavelength of 450 nm, and Re(550) is the in-plane retardation (nm) at a measurement wavelength of 550 nm. The in-plane retardation was measured using an "AxoScan" manufactured by Axometrics.

A circular polarizing plate was placed in a spectroscopic aging tester ("SPX" manufactured by Suga Test Instruments Co., Ltd.) so that light was incident from the side of film (III). A spectroscopic aging test was conducted on the circular polarizing plate by irradiating it with light of 100 W at 40°C for 24 hours. In the spectroscopic aging test, the irradiated light was split into wavelengths at 2 nm intervals, and the wavelength range of the irradiated light was 250 nm to 420 nm. Each of these split wavelengths of light was irradiated to a different location on the circular polarizing plate.

After the spectral aging test, the in-plane retardation ratio (Re(450)/Re(550)) corresponding to the light of each wavelength irradiated was measured for the circularly polarizing plate. The measurement was performed using "AxoScan" manufactured by Axometrics, in the same manner as before the spectral aging test.
The in-plane retardation ratio of the circular polarizer before the spectral aging test was set to 100%, and the in-plane retardation ratio of the circular polarizer after the spectral aging test was set to X%, and the change rate of Re(450)/Re(550) of the circular polarizer before and after the spectral aging test was measured. The absolute value of the change rate is given by |X-100| (%). The absolute value of the change rate corresponding to the light of each wavelength irradiated was obtained, and the maximum value of the absolute values of the change rates obtained in the irradiated wavelength range of 250 nm to 420 nm was taken as the maximum change rate for the circular polarizer. The smaller the maximum change rate, the more the deterioration of the circular polarizer is suppressed.
The suppression of deterioration of the circularly polarizing plate was evaluated based on the maximum rate of change according to the following criteria.
A: The maximum rate of change is less than 10%.
B: The maximum rate of change is 10% or more.

[Production Example 1: Crystalline resin (a)]
A metal pressure reactor was thoroughly dried and then purged with nitrogen. 154.5 parts of cyclohexane and a 70% cyclohexane solution of dicyclopentadiene (endo isomer content of 99% or more) were added to the metal pressure reactor. 42.8 parts (30 parts as dicyclopentadiene) and 1.9 parts of 1-hexene were added and heated to 53°C.

0.014 parts of tetrachlorotungsten phenylimide (tetrahydrofuran) complex was dissolved in 0.70 parts of toluene to prepare a solution. 0.061 parts of a 19% diethylaluminum ethoxide/n-hexane solution was added to this solution and stirred for 10 minutes to prepare a catalyst solution. This catalyst solution was added to the mixture in the pressure-resistant reactor to start the ring-opening polymerization reaction. The mixture was then reacted for 4 hours while maintaining the temperature at 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) calculated from these was 3.21.

0.037 parts of 1,2-ethanediol was added as a terminator to 200 parts of the resulting solution of ring-opened dicyclopentadiene polymer, and the mixture was heated to 60°C and stirred for 1 hour to terminate the polymerization reaction. 1 part of a hydrotalcite-like compound (Kyowa Chemical Industry Co., Ltd.'s "Kyoward (registered trademark) 2000") was added to the mixture, which was heated to 60°C and stirred for 1 hour. 0.4 parts of a filter aid (Showa Chemical Industry Co., Ltd.'s "Radiolite (registered trademark) #1500") was then added, and the adsorbent and solution were filtered using a PP pleated cartridge filter (Advantec Toyo Co., Ltd.'s "TCP-HX").

100 parts of cyclohexane were added to 200 parts of the filtered ring-opened polymer solution (polymer amount 30 parts), and 0.0043 parts of chlorohydridocarbonyltris(triphenylphosphine)ruthenium was added, and a hydrogenation reaction was carried out at 6 MPa hydrogen pressure and 180°C for 4 hours. This produced a reaction liquid containing a hydride of the ring-opened polymer of dicyclopentadiene. The hydride precipitated from this reaction liquid, forming a slurry solution.

The hydride and the solution contained in the reaction liquid were separated using a centrifuge and dried under reduced pressure at 60°C for 24 hours to obtain 28.5 parts of a crystalline ring-opening polymer of dicyclopentadiene. 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 racemo-dyad ratio was 89%.

100 parts of the obtained hydrogenated ring-opening polymer of dicyclopentadiene was mixed with 1.1 parts of an antioxidant (tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane;"Irganox (registered trademark) 1010" manufactured by BASF Japan Ltd.), and then the mixture was charged into a twin-screw extruder (product name "TEM-37B", manufactured by Toshiba Machine Co., Ltd.) equipped with four die holes having an inner diameter of 3 mmΦ. The mixture of the hydrogenated ring-opening polymer of dicyclopentadiene and the antioxidant was molded into a strand shape by hot melt extrusion molding, and then chopped with a strand cutter to obtain pellets of crystalline resin (a). The operating conditions of the twin-screw extruder were as follows.
・Barrel temperature setting = 270-280℃
Die set temperature = 250°C
Screw rotation speed = 145 rpm

[Production Example 2: Production of amorphous resin (H2)]
(First step reaction: elongation of block A1)
A stainless steel reactor equipped with a stirrer, which had been thoroughly dried and purged with nitrogen, was charged with 320 parts of dehydrated cyclohexane, 55 parts of styrene, and 0.38 parts of dibutyl ether, and 0.41 parts of n-butyllithium solution (a hexane solution containing 15% by weight) was added with stirring at 60° C. to initiate the polymerization reaction, thereby carrying out the first-stage polymerization reaction. One hour after the start of the reaction, a sample was sampled from the reaction mixture and analyzed by gas chromatography (GC), revealing that the polymerization conversion rate was 99.5%.

(Second stage reaction: elongation of block B)
To the reaction mixture obtained in the first stage reaction, 40 parts of a monomer mixture consisting of 20 parts of styrene and 20 parts of isoprene was added, and then the second stage polymerization reaction was started. One hour after the start of the second stage polymerization reaction, a sample was taken from the reaction mixture and analyzed by GC, and the polymerization conversion rate was found to be 99.5%.

(Third stage reaction: elongation of block A2)
5 parts of styrene was added to the reaction mixture obtained in the second stage reaction, and the third stage polymerization reaction was then started. One hour after the start of the third stage polymerization reaction, a sample was taken from the reaction mixture, and the weight average molecular weight Mw and number average molecular weight Mn of the polymer were measured. The sample taken at this point was analyzed by GC, and the polymerization conversion rate was found to be almost 100%. Immediately thereafter, 0.2 parts of isopropyl alcohol was added to the reaction mixture to stop the reaction. This resulted in a mixture containing a polymer having a triblock molecular structure of A1-B-A2.

In the first and second stage reactions, the polymerization reaction was allowed to proceed sufficiently, so the polymerization conversion rate was approximately 100%, and therefore the weight ratio of St/Ip in block B is considered to be 20/20. From these values, it was found that the obtained polymer had a triblock molecular structure of St-(St/Ip)-St=55-(20/20)-5. The weight average molecular weight (Mw) of the polymer was 105,500, and the molecular weight distribution (Mw/Mn) was 1.04.

Next, the mixture containing the polymer was transferred to a pressure-resistant reactor equipped with a stirrer, and 8.0 parts of a diatomaceous earth-supported nickel catalyst ("E22U" manufactured by JGC Catalysts and Chemicals, nickel loading 60%) and 100 parts of dehydrated cyclohexane were added as a hydrogenation catalyst and mixed. The inside of the reactor was replaced with hydrogen gas, and hydrogen was further supplied while stirring the solution, and the hydrogenation reaction was carried out at a temperature of 190°C and a pressure of 4.5 MPa for 6 hours. The weight-average molecular weight (Mw) of the hydrogenated product of the polymer contained in the reaction solution obtained by the hydrogenation reaction was 111,800, and the molecular weight distribution (Mw/Mn) was 1.05.

After the hydrogenation reaction was completed, the reaction solution was filtered to remove the hydrogenation catalyst, and then 2.0 parts of a xylene solution containing 0.1 parts of pentaerythrityl tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] ("Songnox 1010" manufactured by Matsubara Sangyo Co., Ltd.), a phenolic antioxidant, was added and dissolved.

The solution was then dried at 260° C. and 0.001 MPa or less using a cylindrical concentrating dryer (Hitachi, Ltd.'s "CONTROL") to remove the solvent cyclohexane, xylene, and other volatile components from the solution. The molten polymer obtained after drying was extruded from a die into a strand shape, cooled, and molded with a pelletizer to produce 95 parts of pellets of an amorphous resin (H2) containing the amorphous hydrogenated polymer (X).
The resulting amorphous polymer (X) had a glass transition temperature Tg of 130° C., a weight average molecular weight (Mw) of 110,300, a molecular weight distribution (Mw/Mn) of 1.10, and a hydrogenation rate of approximately 100%.

[Production Example 3: Resin (aB)]
A mixture was obtained by mixing 91.0 parts of the crystalline resin (a) obtained in Production Example 1 and 9.0 parts of a benzotriazole-based ultraviolet absorber ("LA-31" manufactured by ADEKA Corporation) in a twin-screw extruder. The mixture was then charged into a hopper connected to the extruder, fed to a single-screw extruder, melt-extruded, and molded into pellets to prepare pellets of resin (aB).

Example 1
(1-1. Process (I))
A film-making apparatus was prepared, which included a multi-manifold die capable of co-extrusion film-making of two kinds and three layers, two extruders connected to the multi-manifold die, and hoppers attached to each of the extruders.

The pellets of crystalline resin (a) obtained in Production Example 1 were placed in one of the hoppers and fed from the extruder to the multi-manifold die.

On the other hand, the pellets of resin (aB) obtained in Production Example 3 were placed in a separate hopper and fed from the extruder to the multi-manifold die.

Next, resins (a) and (aB) were extruded from the multi-manifold die in the form of a film and cast onto a cooling roll. This co-extrusion method produced a long film (I) having a pA layer made of resin (a) and a pB layer made of resin (aB) and a layer structure of (pA layer)/(pB layer)/(pA layer). The principal refractive indices nx(I), ny(I) and nz(I) of film (I) were measured. In addition, the thicknesses of the pA layer and pB layer in the obtained film (I) were measured.

(1-2. Step (II))
The film (I) obtained in (1-1) was conveyed at a speed of 5 m/min and passed through an immersion tank filled with toluene, thereby contacting the film (I) with toluene for 5 seconds. The film was then passed through an oven at 110° C. and dried for 60 seconds. This process changed the refractive index in the thickness direction of the film (I) to obtain a long film (II). The principal refractive indices nx(II), ny(II) and nz(II) of the film were measured.

(1-3. Step (III))
A tenter apparatus was prepared which was equipped with a pair of clip chains having a large number of clips capable of gripping both ends in the width direction of a long film, and a pair of guide rails capable of guiding the clip chains.

The film (II) obtained in (1-2) was fed to a tenter device and stretched. The track of the clip chain was set by a guide rail, and stretching was performed so that the stretching ratio in the longitudinal direction was 1.0 times (i.e., no change in the length in the longitudinal direction) and the stretching ratio in the transverse direction was 1.8 times. The stretching temperature was 130°C. By such stretching, a stretched film (III) was obtained as an optical film product.

The crystallinity, 380 nm light transmittance, 590 nm light transmittance, and polarizing plate deterioration suppression ability of the obtained optical film (film (III)) were evaluated. In addition, the principal refractive indices nx(III), ny(III), and nz(III) of film (III) were measured, and the NZ coefficient Nz(III) of film (III) was calculated.

Example 2
An optical film was obtained and evaluated in the same manner as in Example 1, except that the stretching ratio in the width direction in (1-3. Step (III)) was changed to 2 times.

Example 3
An optical film was obtained and evaluated in the same manner as in Example 1, except that the stretching ratio in the width direction in (1-3. Step (III)) was changed to 1.7 times.

Comparative Example 1
In (1-1. Step (I)), an optical film was obtained and evaluated by the same procedure as in Example 1, except that pellets of the resin (H2) obtained in Production Example 2 were used instead of the crystalline resin (a) obtained in Production Example 1 (thus, the resin (H2) and the resin (aB) were co-extruded).

Comparative Example 2
(C2-1. Single layer film)
The crystalline polymer (a) obtained in Production Example 1 was extruded into a single layer to obtain a single layer film (I') having a thickness of 39 μm.

(C2-2. Optical Film)
An optical film was obtained and evaluated by the same procedures as in Example 1 (1-2. Step (II)) onward, except that the monolayer film (I') was used instead of the film (I) obtained in (1-1. Step (I)).

Comparative Example 3
Except for the following changes, the same procedures as in Example 1 (1-2. Step (II)) and subsequent steps were carried out to obtain and evaluate an optical film.
Instead of the film (I) obtained in (1-1. Step (I)), the monolayer film (I') was used.
The stretching ratio in the width direction in (1-3. Step (III)) was changed to 1.7 times.

The overview and results of the examples and comparative examples are shown in Table 1.

As is clear from the results of the Examples and Comparative Examples, the manufacturing method of the present invention makes it possible to easily manufacture the optical film of the present invention, which can exhibit good effects as a three-dimensional retardation film and can exhibit the effect of suppressing deterioration of a display device.

Claims (7)

A method for producing an optical film comprising a layer of a resin (a) containing a crystalline polymer, the optical film having a light transmittance of 5% or less at a wavelength of 380 nm and an Nz coefficient of greater than 0 and less than 1, comprising the steps of:
A step (I) of obtaining a film (I);
A step (II) of contacting one or both surfaces of the film (I) with a solvent to change the refractive index in the thickness direction of the film (I) to obtain a film (II);
Including,
a layer structure of (pA layer)/(pB layer)/(pA layer), the pA layer being a layer of a resin (a) containing the crystalline polymer, and the pB layer being a layer of a resin (aB) containing a crystalline polymer and an ultraviolet absorber . The method for producing an optical film according to claim 1 , wherein the resin (a) has a positive intrinsic birefringence value. The method for producing an optical film according to claim 1 , wherein the crystalline polymer contains an alicyclic structure. The method for producing an optical film according to any one of claims 1 to 3, wherein the step (I) comprises co-extrusion producing the film (I). The method for producing an optical film according to any one of claims 1 to 4, further comprising a step (III) of stretching the film (II) to form a film (III). A method for producing a composite optical film, comprising the steps of:
A step of producing an optical film by the method for producing an optical film according to any one of claims 1 to 5 ;
and laminating the optical film and a polarizer layer such that a slow axis of the optical film and a transmission axis of the polarizer layer form an angle of 42° to 48°. A method for producing a composite optical film, comprising the steps of:
A step of producing an optical film by the method for producing an optical film according to any one of claims 1 to 5 ;
and forming a conductive layer on a surface of the optical film .