JP7719664B2 - Optical film manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing an optical film in which an easy-adhesion layer is formed on the surface of a resin film.

In recent years, applications for organic electroluminescence display devices, touch panels, and other devices have expanded. In these devices, various resin films are used for optical films such as protective films, retardation films, and front panels. Because optical films require high transparency, polyester-based films, cycloolefin-based films, acrylic-based films, and other resin films are commonly used.

Optical films are often used in a laminated state with other functional films, and therefore require good adhesion to other functional films. Therefore, it is known to provide an easy-adhesion layer on the surface of the optical film, whose main component is an adhesive-promoting resin (binder resin) such as polyester resin, acrylic resin, urethane resin, or siloxane resin, to impart easy adhesion to the optical film.

WO15/098539 WO16/088633

On the other hand, a phenolic antioxidant may be blended into the adhesive layer to improve durability, etc. Such an adhesive layer is formed by applying an adhesive composition containing a phenolic antioxidant to the surface of a resin film to form a coating film, and then drying and curing the coating film to form an adhesive layer on the resin film. However, adhesive compositions containing phenolic antioxidants can discolor the adhesive layer and take on a reddish tinge when heated during drying or curing of the coating film. Furthermore, optical films having adhesive layers containing phenolic antioxidants can discolor and take on a reddish tinge when heated, such as during drying when forming other coating layers or during drying after lamination with other functional films. When the adhesive layer takes on a reddish tinge, the transparency of the optical film is impaired, causing various problems, such as making it unusable in devices such as organic electroluminescence displays and touch panels.

The present invention has been made in consideration of these problems, and aims to provide a method for manufacturing an optical film that suppresses discoloration (redness) of the adhesive layer caused by the phenolic compound when an adhesive composition containing a phenolic compound is applied to a resin film and dried to form the adhesive layer.

The inventors of the present invention conducted extensive research into a method for manufacturing an optical film that suppresses discoloration of an adhesion layer caused by a phenolic compound when the adhesion layer is formed on a resin film using an adhesion composition containing a phenolic compound. As a result, they discovered that by irradiating the adhesion layer with active energy rays such as ultraviolet light after the adhesion layer is formed, discoloration of the adhesion layer is suppressed, making it possible to obtain an optical film with excellent transparency, and thus completed the present invention.

According to the present invention,
(1) There is provided a method for producing an optical film, which is characterized by comprising, in this order: a coating step of coating a surface of a resin film with a thermosetting easy-adhesion composition containing a binder resin and a phenolic compound to form a coating film on the surface of the resin film; a curing step of heating and drying the coating film to form an easy-adhesion layer on the surface of the resin film; and an irradiation step of irradiating the easy-adhesion layer with active energy rays,
(2) The method for producing an optical film according to (1), wherein the phenolic compound is a phenolic antioxidant,
(3) There is provided the method for producing an optical film according to (1) or (2), wherein the active energy rays are ultraviolet rays;
(4) The method for producing an optical film according to any one of (1) to (3), wherein the irradiation step is performed so that the cumulative light amount is 550 mJ/ ^cm2 or more;
(5) The method for producing an optical film according to any one of (1) to (4), wherein the irradiation step is carried out in the presence of oxygen,
(6) The method for producing an optical film according to any one of (1) to (5), wherein the curing step comprises heating the coating film so that the drying temperature of the coating film is 90° C. or higher,
(7) There is provided an optical film having an easy-adhesion layer made of a thermosetting easy-adhesion composition containing a binder resin and a phenolic compound on the surface of a resin film, wherein the optical film is characterized in that a change in the a ^* value of transmitted light in CIE1976 color space before and after heating at any temperature of 100°C or higher and 140°C or lower for 5 minutes satisfies the following formula (I):
(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.02...Equation (I)
(8) The optical film according to (7), wherein the optical film satisfies the following formula (II):
−0.15<(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.02...formula (II)

The optical film obtained by the manufacturing method of the present invention has excellent transparency suitable for use as an optical film, even when the adhesive layer contains a phenolic compound, as discoloration (redness) of the adhesive layer due to heating during drying or curing of the coating film is suppressed. Furthermore, adhesive layers irradiated with active energy rays such as ultraviolet rays do not discolor and can maintain their transparency even when heated again, such as during drying when forming other coating layers or during drying after lamination with other functional films.

1 is a cross-sectional view of an optical film according to one embodiment of the present invention. 1 is a schematic diagram of a production line for an optical film according to one embodiment of the present invention.

An optical film according to one embodiment of the present invention and a method for producing the same will now be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an optical film. As shown in FIG. 1, optical film 10 comprises a resin film 11 and an easy-adhesion layer 12 located on resin film 11. Easy-adhesion layer 12 is formed by applying a thermosetting easy-adhesion composition containing a binder resin and a phenolic compound, diluted with a solvent as necessary, onto the resin film, and then drying and curing the composition. Each component will be described in turn.

[Resin film]
The resin film is made of a material that is translucent to visible light, such as a plastic material. Examples of plastic materials include, but are not limited to, cellulose-based resins, polyester-based resins, polyimide-based resins, polyamideimide-based resins, polyamide-based resins, polyolefin-based resins such as polyethylene and polypropylene, cycloolefin-based resins such as cycloolefin polymers and cycloolefin copolymers, (meth)acrylic resins, polyvinyl chloride, polycarbonate-based resins, urethane-based resins, polyvinyl alcohol-based resins, and polyarylate-based resins. These may be used alone or in combination of two or more. Note that the term "(meth)acrylic resin" used herein refers to an acrylic resin or a methacrylic resin.

From the standpoint of productivity, a long resin film is preferred. There are no particular restrictions on the thickness of the resin film, but considering strength and flexibility, it is preferably 5 to 300 μm, more preferably 10 to 200 μm, and even more preferably 10 to 100 μm. If the thickness is less than 5 μm, sufficient strength may not be obtained. If the thickness exceeds 300 μm, flexibility may decrease and the film may not be suitable for use as an optical film.

The resin film may or may not have been stretched. The stretching may be uniaxial or biaxial. For example, the stretching ratio is preferably 1.5 times or more in area ratio, more preferably 2.5 times or more, and even more preferably 4.5 times or more.

The resin film may be a single layer or multiple layers. Furthermore, as long as the effects of the present invention are achieved, each of these layers may contain various additives as needed. Examples of additives include colorants such as pigments and dyes; plasticizers; fluorescent brighteners; dispersants; heat stabilizers; light stabilizers; ultraviolet absorbers; antistatic agents; antioxidants; fine particles; surfactants; and the like. These may be used alone or in combination of two or more.

The total light transmittance of the resin film, calculated as a 1 mm thickness, is preferably 85% or more, and more preferably 90% or more. Total light transmittance can be measured in accordance with JIS K0115 using a spectrophotometer (V-570 ultraviolet-visible-near-infrared spectrophotometer, manufactured by JASCO Corporation).

[Thermosetting easily adhesive composition]
The thermosetting adhesive composition forms an adhesive layer on an optical film by thermal curing, and contains a binder resin and a phenol compound. From the viewpoint of forming a coating film in the coating step described below, the thermosetting adhesive composition may be diluted with a solvent as needed. Examples of the solvent include water, alcohol (ethanol, etc.), ketone, ether, hydrocarbon, and aromatic solvent (benzene, toluene, xylene, etc.), and these can be used alone or in combination of two or more.

[Binder resin]
The binder resin is an adhesive-promoting resin that imparts easy adhesion to the resin film, and any conventionally known resin can be used without any particular limitation. Examples of binder resins include polyester resins, polycarbonate resins, (meth)acrylic resins, urethane resins, urethane acrylate resins, polyolefin resins, and siloxane resins. These can be used alone or in combination of two or more. The binder resin may also be a resin containing a thermosetting catalyst or a resin containing a thermosetting crosslinking agent.

[Phenol-based compounds]
The phenol-based compound imparts durability to the easy-adhesion layer, and examples thereof include resol-type phenolic resins, novolac-type phenolic resins, phenol-based antioxidants, etc. Among these, phenol-based antioxidants are preferred from the viewpoint of durability.

Examples of phenolic antioxidants include pentaerythrityl tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl}2,4,8,10-tetraoxaspiro[5,5]undecane, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 1,6-hexanediol-bis[3-(3,5-di-t- butyl-4-hydroxyphenyl)propionate], 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2,6-diphenyl-4-octadecyloxyphenol, stearyl (3,5-di-t-butyl-4-hydroxyphenyl)propionate, distearyl (3,5-di-t-butyl-4-hydroxybenzyl)phosphonate, thiodiethylene glycol Lithium ion bis[(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 4,4'-thiobis(6-t-butyl-m-cresol), 2-octylthio-4,6-di(3,5-di-t-butyl-4-hydroxyphenoxy)-s-triazine, 2,2'-methylenebis(4-methyl-6-t-butyl-6-butylphenol), 2,-2'-methylenebis(4-ethyl-6-t-butylphenol), bis[3,3-bis(4-hydroxy-3-t-butylphenyl)butyric acid]glycol ester, 4,4'-butylidenebis(6-t-butyl-m-cresol), 2,2'-ethylidenebis(4,6-di-t-butylphenol), 2,2'-ethylidenebis(4-s-butyl-6-t-butylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, bis[2-t-butyl-4-methyl-6-(2-hydroxy-3-t-butyl-5-methylbenzyl)phenyl]terephthalate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-t- butylbenzyl) isocyanurate, 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene, 1,3,5-tris[(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxyethyl]isocyanurate, tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, 2-t-butyl-4-methyl-6-(2-acryloyloxy-3-t-butyl-5-methylbenzyl)phenol, 3,9- Bis(1,1-dimethyl-2-hydroxyethyl)-2,4-8,10-tetraoxaspiro[5,5]undecane-bis[β-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate], triethylene glycol bis[β-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate], 1,1'-bis(4-hydroxyphenyl)cyclohexane, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), phenol), 2,2'-methylenebis(6-(1-methylcyclohexyl)-4-methylphenol), 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 3,9-bis(2-(3-t-butyl-4-hydroxy-5-methylphenylpropionyloxy)1,1-dimethylethyl)-2,4,8,10-tetraoxaspiro(5,5)undecane, 4,4'-thiobis(3-methyl-6-t-butylphenol), 4,4'-bis(3,5-di-t-butyl-4-hydroxybenzyl)sulfur amide, 4,4'-thiobis(6-t-butyl-2-methylphenol), 2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone, 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2,4-dimethyl-6-(1-methylcyclohexyl, styrenated phenol, 2,4-bis((octylthio)methyl)-5-methylphenol, etc. These can be used alone or in combination of two or more.

The content of the phenolic compound, calculated as solid content, is preferably 0.01 to 5 parts by weight, more preferably 0.01 to 3 parts by weight, and even more preferably 0.05 to 1 part by weight, per 100 parts by weight of the total binder resin. If resin components other than the binder resin are included, the phenolic compound should be blended with 100 parts by weight of the solid content including the other resin components.

The thermosetting adhesive composition may contain a crosslinking agent to improve its resistance to moist heat under high temperature and humidity conditions. Any appropriate crosslinking agent can be used, including, for example, urea compounds, epoxy compounds, melamine compounds, isocyanate compounds, oxazoline compounds, silanol compounds, and carbodiimide compounds. These can be used alone or in combination of two or more.

The content of the crosslinking agent, calculated as solid content, is preferably 0.1 to 15 parts by weight per 100 parts by weight of the total binder resin. It is more preferably 0.3 to 5 parts by weight, and even more preferably 0.5 to 3 parts by weight. If resin components other than the binder resin are included, the crosslinking agent should be blended per 100 parts by weight of the solid content including the other resin components.

The thermosetting adhesive composition may contain any appropriate fine particles depending on the function. Examples of fine particles include inorganic fine particles such as inorganic oxides such as silica, titania, alumina, and zirconia, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, and calcium phosphate.

The average particle size of the microparticles is not particularly limited, but from the viewpoint of maintaining the transparency of the adhesion layer, it is preferably 1 to 500 nm, more preferably 50 to 350 nm, and even more preferably 100 to 300 nm. By using microparticles with such a particle size, appropriate irregularities can be formed on the surface of the adhesion layer, effectively reducing the frictional force at the contact surface between the resin film and the adhesion layer and/or between the adhesion layers themselves, thereby suppressing blocking.

The content of the microparticles, calculated as solid content, is preferably 0.1 to 15 parts by weight per 100 parts by weight of the total binder resin. It is more preferably 0.3 to 5 parts by weight, and even more preferably 0.5 to 3 parts by weight. If resin components other than the binder resin are included, the microparticles should be blended with 100 parts by weight of the solid content including the other resin components.

The thermosetting adhesive composition may further contain any suitable additives. Examples of additives include dispersion stabilizers, thixotropic agents, ultraviolet absorbers, defoamers, thickeners, dispersants, surfactants, catalysts, lubricants, and antistatic agents.

Next, a method for manufacturing an optical film according to one embodiment of the present invention will be described. FIG. 2 is a schematic diagram of a roll-to-roll optical film manufacturing line 20. As shown in FIG. 2, the optical film manufacturing method includes a payout process in which a long resin film 11 is paid out in the longitudinal direction using a payout device 21; a coating process in which a thermosetting adhesive composition is applied to one side of the resin film using an applicator 22 to form a coating film on the resin film 11; a curing process in which the coating film is heated and dried in a drying oven 23 and cured to form an adhesive layer on the resin film 11; an irradiation process in which, while the resin film is supported on the other side by a back roll 25, an active energy ray irradiation device 24 irradiates the adhesive layer with active energy rays, such as ultraviolet rays, from one side of the resin film 11; and a winding process in which the optical film is wound up by a winding device 26. The resin film is guided and transported from the payout process to the winding process by multiple transport rolls 27.

[Feeding process]
The unwinding step is a step in which the rolled long resin film is rotated by a unwinding device to unwind the resin film in the longitudinal direction. Any known unwinding device can be used.

[Coating process]
The coating step is a step of continuously coating one side of a resin film with a thermosetting adhesive composition using a coating device to form a coating film. Examples of coating devices include spin coating, dip coating, bar coating, spray coating, blade coating, gravure coating, reverse coating, slot die coating, screen printing, and inkjet coating. Among these, slot die coating is preferred because it can apply the coating material uniformly.

The amount of the thermosetting adhesive composition applied in the application step may be appropriately determined taking into consideration the thickness of the adhesive layer after drying, and is not particularly limited, but is, for example, 1 g/m ² to 40 g/m ^2. The thickness of the adhesive layer after drying is, for example, 0.01 μm to 5 μm, preferably 0.03 μm to 3 μm, more preferably 0.05 μm to 1 μm, and particularly preferably 0.05 μm to 0.5 μm.

[Curing process]
The curing step is a step in which the resin film is transported into a drying oven or the like, the coating film on the resin film is heated and dried, and the coating film is thermally cured to form an easy-adhesion layer on the resin film. Any known drying oven can be used.

The drying temperature for the coating film during the curing process is preferably 90°C or higher, more preferably 100°C or higher, and even more preferably 120°C or higher. The upper limit of the drying temperature can be adjusted appropriately depending on the type of resin film and thermosetting adhesive composition, and is not particularly limited. For example, it is preferably 200°C or lower, and more preferably 150°C or lower. A drying temperature of 90°C or higher allows the coating film to dry and the thermosetting adhesive composition to cure. On the other hand, if a thermosetting adhesive composition containing a phenolic compound is heated to 90°C or higher for thermal curing, the adhesive layer after curing may discolor and take on a reddish tinge. The reason for this is unclear, but it is thought to be due to the phenolic compound oxidizing to form a colored quinone compound.

[Irradiation process]
The irradiation process involves irradiating the surface of the adhesive layer with active energy rays, such as ultraviolet rays, from one side of the resin film while the resin film is supported by a back roll from the other side. In the present invention, this irradiation process can suppress discoloration (redness) of the adhesive layer that occurred during the curing process. Specifically, by irradiating the adhesive layer discolored during the curing process with active energy rays, such as ultraviolet rays, the redness of the adhesive layer disappears and the adhesive layer becomes transparent. While the reason for this is unclear, it is believed that the carbonyl group of the quinone compound is activated by active energy rays, such as ultraviolet rays, to form a radical, which then undergoes an addition reaction to form a hydroquinone compound, etc. During the irradiation process, the surface of the adhesive layer irradiated with active energy rays, such as ultraviolet rays, may be nitrogen-purged. However, from the viewpoint of suppressing discoloration of the adhesive layer, it is preferable to perform the irradiation process in a non-nitrogen-purged state (in the atmosphere) or in the presence of oxygen. The oxygen concentration on the surface of the adhesive layer irradiated with active energy rays, such as ultraviolet rays, is preferably 100 ppm or more, more preferably 1000 ppm or more.

Examples of active energy rays include ultraviolet rays and electron beams, and known active energy ray irradiation devices can be used. In the present invention, ultraviolet rays are preferably used as the active energy rays, and examples of ultraviolet irradiation devices include ultraviolet LEDs, low-pressure mercury lamps, high-pressure mercury lamps, metal halide lamps, and electrodeless ultraviolet lamps.

Preferably, the ultraviolet light has a main peak at a wavelength around 365 nm in its emission spectrum (here, a wavelength around 365 nm refers to a wavelength between 360 nm and 370 nm, and the main peak refers to the strongest emission intensity at wavelengths between 100 and 400 nm).

In the irradiation step, the cumulative light amount of ultraviolet light on the surface of the easy-adhesion layer is preferably 300 mJ/cm ² or more, more preferably 550 mJ/cm ² or more, even more preferably 700 mJ/cm ² or more, and particularly preferably 900 mJ/cm ² or more. If the cumulative light amount of ultraviolet light on the surface of the easy-adhesion layer is within the above range, discoloration (redness) of the easy-adhesion layer can be sufficiently improved.

A known back roll can be used. The set temperature of the back roll is preferably, for example, 25°C or higher and 75°C or lower, and more preferably 30°C or higher and 60°C or lower. If the set temperature of the back roll is within the above range, it is possible to effectively suppress temperature increases due to ultraviolet irradiation and to effectively suppress discoloration of the easy-adhesion layer.

[Winding process]
The winding step is a step of winding the produced optical film into a roll using a winding device. Any known winding device can be used.

The transport rolls guide and transport the long resin film under a predetermined tension from the unwinding device to the coating device, drying oven, active energy ray irradiation device (back roll), and winding device, in that order. They are composed of, for example, a drive roll, guide roll, and dancer roll. The transport speed of the resin film is not particularly limited, but is, for example, 1 to 50 m/min.

The optical film obtained by the manufacturing method of the present invention can maintain its transparency without discoloration even when the easy-adhesion layer is heated, for example, during drying when providing another coating layer or during drying after lamination with another functional film. Specifically, the optical film obtained by the manufacturing method of the present invention is characterized in that the easy-adhesion layer does not discolor (become reddish) even when heated again after ultraviolet irradiation, and it is preferable that the change in the a ^* value in the CIE 1976 color space before and after heating the optical film at a temperature of 90°C or higher and 140°C or lower for 5 minutes, preferably at 100°C for 5 minutes, satisfies the following formula (I) or (II). The a ^* value in the CIE 1976 color space refers to the average value measured at three points in the L ^* a ^* b ^* color system using a spectrocolorimeter (measurement conditions: transmitted light, measurement diameter φ15 mm, observation field of view 2°, light source C).
(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.02...Equation (I)
−0.15<(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.02...formula (II)
The lower limit of the change in the a ^* value in the CIE 1976 color space before and after heating the optical film at a temperature of 90°C or higher and 140°C or lower for 5 minutes, preferably at 100°C for 5 minutes, is preferably greater than -0.10, more preferably greater than -0.05.

In addition, since the change in hue of the easy-adhesion layer in the optical film is a thin film, it may be difficult to quantitatively evaluate it with a single optical film, so it may be evaluated by stacking multiple optical films. For example, when five optical films are stacked and the change in hue before and after heating is evaluated, it is preferable that the change in a ^* value in the CIE1976 color space before and after heating the five stacked optical films at any temperature of 90° C. or higher and 140° C. or lower for 5 minutes, preferably at 100° C. for 5 minutes, satisfies the following formula (III) or (IV):
(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.10...formula (III)
−0.25<(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.10...formula (IV)
The upper limit of the change in the a ^* value in the CIE 1976 color space before and after heating a five-ply laminated optical film at a temperature of 90°C or higher and 140°C or lower for 5 minutes, preferably at 100°C for 5 minutes, is preferably less than 0.07, more preferably less than 0.05, and the lower limit is preferably greater than -0.15, more preferably greater than -0.10.

Furthermore, when ten optical films are stacked and a change in hue before and after heating is evaluated, it is preferable that the change in the a ^* value in the CIE 1976 color space before and after heating the ten stacked optical films at a temperature of 90°C or higher and 140°C or lower for 5 minutes, preferably at 100°C for 5 minutes, satisfies the following formula (V) or (VI):
(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.40...Equation (V)
−0.35<(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.40 Formula (VI)
The upper limit of the change in the a ^* value in the CIE 1976 color space before and after heating a 10-ply laminated optical film at a temperature of 90°C or higher and 140°C or lower for 5 minutes, preferably at 100°C for 5 minutes, is preferably less than 0.20, more preferably less than 0.10, and the lower limit is preferably greater than -0.25, more preferably greater than -0.15.

From the viewpoint of ensuring stable performance as an optical component, the optical film obtained by the manufacturing method of the present invention preferably has a total light transmittance of 85% or more, and more preferably 90% or more. Total light transmittance can be measured in accordance with JIS K7361-1 using a haze meter (NDH 7000II manufactured by Nippon Denshoku Industries Co., Ltd.) or the like.

The haze of the optical film obtained by the manufacturing method of the present invention is not particularly limited, but is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.5% or less. Haze can be measured in accordance with JIS K7361-1997 using a haze meter (NDH 7000II manufactured by Nippon Denshoku Industries Co., Ltd.) or the like.

The total thickness of the optical film obtained by the manufacturing method of the present invention is preferably 5 μm or more, more preferably 7 μm or more, and particularly preferably 10 μm or more, and is preferably 300 μm or less, more preferably 200 μm or less, and particularly preferably 100 μm or less. By making the total thickness of the optical film equal to or greater than the lower limit of the above range, the mechanical strength of the optical film can be increased. Furthermore, by making it equal to or less than the upper limit, the overall thickness of the optical film can be reduced.

In the optical film obtained by the manufacturing method of the present invention, various functional layers may be formed, if necessary, on the surface opposite to the surface on which the easy-adhesion layer is formed. Examples of functional layers include an antistatic layer, a pressure-sensitive adhesive layer, an adhesive layer, a pressure-sensitive adhesive layer, an easy-adhesion layer, an anti-glare layer, an anti-fouling layer such as a photocatalyst layer, an anti-reflection layer, a hard coat layer, an ultraviolet shielding layer, a heat-ray shielding layer, an electromagnetic wave shielding layer, and a gas barrier layer.

Optical films obtained by the manufacturing method of the present invention include, for example, polarizer protective films, retardation films, viewing angle compensation films, light diffusion films, reflective films, anti-reflection films, anti-glare films, brightness enhancement films, and front panels for touch panels. The optical films obtained by the manufacturing method of the present invention may be optically isotropic films, or may be optically anisotropic films (for example, films that exhibit birefringence such as retardation).

The present invention will be explained in more detail below using examples. However, the present invention is not limited to the following examples.

Example 1
An optical film was produced using the manufacturing method according to the embodiment described above. A 50 μm-thick polyethylene terephthalate film (product name: U48, manufactured by Toray Industries, Inc.) was prepared as the resin film. A thermosetting adhesive composition containing 100 parts by weight of an alkoxysilane hydrolysis polycondensate and 1 part by weight of a phenolic antioxidant was prepared and diluted with a solvent (a 3:1 mixture of IPA and BtOH) to a solids concentration of 1.0%. The resin film was conveyed at a speed of 2.5 m/min using a slot die coater. The coating amount was set to 9.1 g/ ^m² , resulting in an adhesive layer with a thickness of 0.08 μm after drying. The drying temperature in the drying oven was set to 100°C, and the drying time was 2 minutes. An electrodeless ultraviolet lamp (Heraeus, H bulb) was used as the ultraviolet irradiation device, and ultraviolet light having a main peak at a wavelength of 365 nm in the emission spectrum was irradiated so that the surface of the adhesive layer received an integrated light dose of 500 mJ/ ^cm² . The cumulative light intensity was estimated using an ultraviolet meter (Microcure, manufactured by Heraeus K.K.).

Example 2
An optical film was produced under the same conditions as in Example 1, except that the ultraviolet irradiation conditions were changed so that the cumulative light amount on the surface of the easy-adhesion layer was 670 mJ/cm ² .

Example 3
An optical film was produced under the same conditions as in Example 1, except that the ultraviolet irradiation conditions were changed so that the cumulative light amount on the surface of the easy-adhesion layer was 1070 mJ/cm ² .

Example 4
An optical film was produced under the same conditions as in Example 2, except that the drying temperature in the drying oven was changed to 140°C.

Example 5
An optical film was produced under the same conditions as in Example 3, except that the drying temperature in the drying oven was changed to 140°C.

Example 6
An optical film was produced under the same conditions as in Example 2, except that the surface of the adhesive layer to be irradiated with ultraviolet light was purged with nitrogen (under a nitrogen atmosphere, oxygen concentration 90 ppm).

(Comparative Example 1)
An optical film was produced under the same conditions as in Example 1, except that the surface of the easy-adhesion layer was not irradiated with ultraviolet light.

(Comparative Example 2)
An optical film was produced under the same conditions as in Comparative Example 1, except that the drying temperature in the drying oven was changed to 140°C.

The optical films produced in the examples and comparative examples were evaluated as follows. The evaluation results are shown in Table 1.

<HAZE>
The haze was measured with a haze meter (product name: NDH 7000II, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7136.

<Light transmittance>
The light transmittance was measured with a haze meter (product name: NDH 7000II, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7361-1.

<a ^* value in CIE 1976 color space>
Using a spectrophotometer manufactured by Nippon Denshoku Industries Co., Ltd. (model number: SE7700, measurement conditions: transmitted light, measurement diameter φ15 mm, observation field of view 2°, light source C), the L ^* a ^* b ^* color system was measured at three points, and the average was taken as the a ^* value.

<Color>
The prepared optical film sample (320 mm (TD direction) × 50 mm (MD direction)) was wound in the TD direction to form an optical film roll (diameter 10 mm), and the edges of the optical film were fixed with adhesive tape to prevent the optical film roll from unraveling. Next, the color of the optical film roll was evaluated visually. The evaluation criteria were as follows:
○: Colorless △: Slightly reddish ×: Reddish XX: Deeply reddish

As shown in Table 1, the optical films of Examples 1 to 6, in which an adhesive layer was formed on a resin film using a thermosetting adhesive composition containing a phenolic compound and the adhesive layer was then irradiated with active energy rays such as ultraviolet light after formation, showed results in which discoloration of the adhesive layer was suppressed and the layer exhibited excellent transparency. On the other hand, the optical films of Comparative Examples 1 and 2, in which an adhesive layer was formed on a resin film using a thermosetting adhesive composition containing a phenolic compound and the adhesive layer was not then irradiated with active energy rays such as ultraviolet light after formation, showed results in which the adhesive layer discolored and took on a reddish tinge.

Next, the optical films of Example 2 and Comparative Example 1 were evaluated as follows. The evaluation results are shown in Table 2.

<Change in a ^* value in CIE 1976 color space (comparison before and after heating)>
The optical films of Example 2 and Comparative Example 1 were aged for one month in a constant temperature and humidity environment (23°C, 50% RH), and the a ^* values of the aged optical films were measured in the CIE 1976 color space using the above-mentioned measurement method. Next, the aged optical films were heated in a hot air dryer at 100°C for 5 minutes, and the a ^* values of the heated optical films were measured in the CIE 1976 color space using the above-mentioned measurement method. The difference between the a ^* values of the heated optical films and the a ^* values of the aged optical films was calculated. The a ^* values of the optical films of Example 2 and Comparative Example 1 were measured for the single optical films as well as for the 5-ply and 10-ply laminated optical films using the following method.
(Method for measuring laminated optical film)
A sample prepared by laminating a predetermined number of optical films cut to 50 mm × 50 mm was sandwiched between two fixing jigs (fixing jig/laminated film/fixing jig) with a hole cut out at the center (outer dimensions: 60 mm × 60 mm, hole cutout portion: 35 mm × 35 mm), and the sample was set in the measuring device so that the measurement light from the spectrocolorimeter was incident on the hole cut out portion, and the a ^* value was measured.

As shown in Table 2, the optical film of Comparative Example 1 aged for one month in a constant temperature and humidity environment (23°C, 50% RH) showed a fading discoloration (redness) in the adhesive layer, while the optical film of Example 2 showed almost no change in the discoloration of the adhesive layer. However, when these optical films were heated in a hot air dryer at 100°C for 5 minutes, the adhesive layer of the optical film of Comparative Example 1 discolored and took on a reddish tinge. As can be seen from these results, the adhesive layer made of a thermosetting adhesive composition containing a phenolic compound reversibly discolors when heated. However, by irradiating the cured adhesive layer with active energy rays such as ultraviolet light, discoloration of the adhesive layer can be irreversibly suppressed. Therefore, the optical film of the present invention can maintain its transparency without discoloration even when the adhesive layer is heated, such as during drying when forming other coating layers or during drying after lamination with other functional films.

10: Optical film 11: Resin film 12: Easy-adhesion layer 20: Production line 21: Paying device 22: Coating device 23: Drying furnace 24: Active energy ray irradiation device 25: Back roll 26: Winding device 27: Conveying roll

Claims (7)

a coating step of coating a surface of a resin film with a thermosetting adhesive composition containing a binder resin and a phenolic compound (excluding cases where the thermosetting adhesive composition contains a photopolymerization initiator that generates radicals when irradiated with ultraviolet light) , to form a coating film on the surface of the resin film;
a curing step of heating and drying the coating film to form an easy-adhesion layer on the surface of the resin film;
an irradiation step of irradiating the adhesive layer with ultraviolet light;
In this order ,
The method for producing an optical film, wherein the irradiation step comprises irradiating in the presence of oxygen at an oxygen concentration of 100 ppm or more so that the cumulative light amount is 550 mJ/cm ² or more. The method for producing an optical film according to claim 1, wherein the phenolic compound is a phenolic antioxidant. 3. The method for producing an optical film according to claim 1, wherein the curing step comprises heating the coating film so that the drying temperature of the coating film reaches 90° C. or higher. 4. The method for producing an optical film according to claim 1, wherein the ultraviolet light has a main peak in the range of 360 to 370 nm in the emission spectrum. 5. The method for producing an optical film according to claim 1, wherein the easy-adhesion layer has a thickness of 0.01 [mu]m or more and 5 [mu]m or less. 6. The method for producing an optical film according to claim 1, wherein the change in a ^* value of transmitted light in CIE 1976 color space before and after heating the optical film at a temperature of 100°C or higher and 140° C or lower for 5 minutes satisfies the following formula (I):
(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.02...Equation (I) The method for producing an optical film according to claim 6 , wherein the optical film satisfies the following formula (II):
−0.15<(a ^* value of the optical film after heating)−(a ^* value of the optical film before heating)<0.02...formula (II)