JP7601646B2 - Method for producing hard coat film and method for preventing curling - Google Patents

Description

The present invention relates to a method for producing a hard-coated film having a hard-coat layer on a film substrate, and to suppress curling of the hard-coated film.

In recent years, curved displays and foldable displays (flexible displays, foldable displays) have been developed, and flexible resin films are being used instead of glass as materials for substrates and cover windows. In order to give the resin film high hardness, hard coat films are used in which a hard coat layer is provided on the surface of the resin film.

High hardness is required for components placed on the display surface, such as a cover window. On the other hand, for foldable displays, the hard coat layer must not peel off or crack even when repeatedly bent. In order to achieve both hardness and bending resistance in the hard coat layer, it is necessary to increase the flexibility of the curable resin that constitutes the hard coat layer while increasing the thickness of the hard coat layer to ensure hardness.

The hard coat layer is formed by applying a photocurable coating liquid (hard coat composition) to the surface of a resin film substrate to form a coating film, removing the solvent as necessary, and then curing the composition. As the thickness of the hard coat layer increases, the cure shrinkage increases, and compressive stress occurs at the interface between the film substrate and the hard coat layer, causing the film to warp (curl) with the surface on which the hard coat layer is formed facing inward (see Figure 7).

Methods proposed to suppress or reduce curling of hard coat films include forming a curl suppression layer on the non-hard coat layer side of the film substrate to balance the stress on the front and back of the film substrate (Patent Document 1) and treating the hard coat film with water vapor (Patent Document 2).

International Publication No. 2015/029667 JP 2011-215268 A

In applications such as display cover windows, there is a demand for hard coat films with thicker hard coat layers. However, the thicker the hard coat layer, the more the hard coat film tends to curl, which can make handling of the film difficult. In view of the above, the present invention aims to provide a hard coat film in which curling is suppressed even when the hard coat layer is thick.

One aspect of the present invention relates to a method for producing a hard-coated film having a hard-coat layer on the first main surface of a film substrate having a first main surface and a second main surface. The thickness of the hard-coat layer may be 5 μm or more.

A hard coat composition containing a curable resin and a photopolymerization initiator is applied onto the first main surface of the film substrate to form a coating film, and the coating film of the hard coat composition is irradiated with active light rays to photocure the curable resin.

The hard coat composition may be photocationically polymerizable. The photocationically polymerizable composition contains, for example, a curable resin having a photocationically polymerizable functional group and a photocationic polymerization initiator (photoacid generator). Examples of photocationically polymerizable functional groups include epoxy groups and oxetane groups. Examples of functional groups containing an epoxy group include alicyclic epoxy groups and glycidyl groups.

After irradiating the coating of the hard coat composition with actinic rays to generate active species from the photopolymerization initiator, the laminate in which the coating is provided on the film substrate is heated in a curved state so that the coating surface faces outward. The coating after irradiation with actinic rays may be in a semi-cured state until heat treatment. In the semi-cured coating, active species generated from the photopolymerization initiator by irradiation with actinic rays may remain. By heating in this state, curing (polymerization reaction) progresses, and the hardness of the coating (hard coat layer) increases.

The above heat treatment is carried out, for example, with the laminate curved along a roll. If the laminate is curved so that the second main surface of the film substrate (the surface opposite to the surface on which the coating film is formed) comes into contact with the roll, the surface on which the coating film is formed will be on the outside. The roll to which the laminate is attached may be a heated roll. In this case, the heat treatment is carried out by heat from the heated roll. With the laminate in line with the roll, heating may be carried out from the side on which the coating film is formed by heat from a heat source such as a heater.

The radius of curvature of the laminate during heat treatment is preferably 50 mm or less. The heating temperature during heat treatment is preferably 80°C or higher.

After irradiating the coating of the hard coat composition with actinic rays, the coating is curved so that the surface on which the coating is formed faces outward and then heated, resulting in a hard coat film with reduced curl.

FIG. 1 is a cross-sectional view of a hard coat film according to an embodiment. FIG. 1 is a conceptual diagram showing an outline of a film-forming apparatus for forming a hard coat layer on a film substrate and the process thereof. FIG. 3 is an enlarged view of the periphery of a heating unit in FIG. 2 . FIG. 4 is a schematic diagram showing a configuration example of a heating unit. FIG. 4 is a schematic diagram showing a configuration example of a heating unit. FIG. 4 is a schematic diagram showing a configuration example of a heating unit. FIG. 2 is an explanatory diagram of the principle of curling in a hard coat film. FIG. 2 is a diagram for explaining the curl direction of a hard coat film.

One aspect of the present invention relates to suppressing/reducing curl in a hard-coated film having a hard-coat layer on one main surface of a film substrate. A further aspect of the present invention relates to a method for producing a hard-coated film using the curl suppression method.

[Structure and materials of hard coat film]
FIG. 1 is a cross-sectional view of a hard coat film having a hard coat layer 3 on a first main surface 1A of a film substrate 1.

<Film substrate>
For example, a transparent film is used as the film substrate 1 of the hard coat film. The visible light transmittance (total light transmittance) of the transparent film is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more.

The resin material constituting the transparent film is preferably one that has excellent transparency, mechanical strength, and thermal stability. Specific examples of resin materials include polyester-based resins, (meth)acrylic-based resins, polyethersulfone-based resins, polysulfone-based resins, polycarbonate-based resins, polyamide-based resins, polyimide-based resins, polyolefin-based resins, cyclic polyolefin-based resins, polyarylate-based resins, polystyrene-based resins, and cellulose-based resins.

When a hard coat film is used as a cover window for a display, the film substrate is required to have excellent heat resistance and mechanical strength, and a preferred example of the resin material is transparent polyimide. While typical fully aromatic polyimides are colored yellow or brown, transparent polyimides with high visible light transmittance can be obtained by introducing an alicyclic structure, a bent structure, or a fluorine substituent.

The thickness of the film substrate is not particularly limited, but from the viewpoints of strength, workability such as ease of handling, thin layer property, etc., it is preferably about 5 to 300 μm, more preferably 10 to 250 μm, and even more preferably 20 to 200 μm.

<Hard Coat Composition>
The hard coat layer is formed by applying a hard coat composition containing a curable resin and a photopolymerization initiator onto a film substrate and photocuring the resin component. That is, the hard coat layer is a cured resin layer made of a photocured product of the hard coat composition.

(Hardening Resin)
The curable resin (binder resin) has photocurability. The photocurable resin is a polyfunctional compound having two or more photopolymerizable (preferably ultraviolet-polymerizable) functional groups. The polyfunctional compound may be a monomer or oligomer. The photopolymerizable functional group may be radically polymerizable or cationic polymerizable. Examples of the radically polymerizable functional group include functional groups having an ethylenically unsaturated double bond, such as a vinyl group and a (meth)acryloyl group. Examples of the cationic polymerizable functional group include cyclic ether groups, such as an epoxy group and an oxetane group. Among these, epoxy groups and oxetane groups are preferred because they can be cured by photocationic polymerization and have small curing shrinkage. Examples of photocationically polymerizable functional groups containing an epoxy group include a glycidyl group and an alicyclic epoxy group. Among these, alicyclic epoxy groups are preferred because they have high reactivity in photocationic polymerization.

Specific examples of photoradical polymerizable curable resins include acrylic resins such as urethane acrylate, polyester acrylate, epoxy acrylate, and polyol acrylate. Specific examples of photocationically polymerizable curable resins include epoxy resins. As photocurable resins, polysiloxane compounds having epoxy groups as photopolymerizable functional groups, as disclosed in WO2018/096729, WO2014/204010, JP2017-8142A, etc., may be used.

(Photopolymerization initiator)
The photocurable hard coat composition contains a photopolymerization initiator in addition to the curable resin. As the photopolymerization initiator, a photoradical polymerization initiator, a photocationic polymerization initiator (photoacid generator), or the like may be used depending on the polymerizability of the curable resin. The content of the photopolymerization initiator in the hard coat composition is about 0.05 to 10 parts by weight, and may be 0.1 to 5 parts by weight, or 0.2 to 2 parts by weight, based on 100 parts by weight of the curable resin.

Photoradical polymerization initiators include acetophenones, benzophenones, Michler's benzoyl benzoate, α-aminoxime esters, thioxanthones, propiophenones, benzils, benzoins, and acylphosphine oxides.

Examples of photocationic polymerization initiators include strong acids such as toluenesulfonic acid or boron tetrafluoride; onium salts such as sulfonium salts, ammonium salts, phosphonium salts, iodonium salts, and selenium salts; iron-arene complexes; silanol-metal chelate complexes; sulfonic acid derivatives such as disulfones, disulfonyldiazomethanes, disulfonylmethanes, sulfonylbenzoylmethanes, imidosulfonates, and benzoin sulfonates; and organic halogen compounds.

The active species of the photocationic polymerization initiator is an acid generated by light irradiation. The active species has a longer life span than photoradicals, and the polymerization reaction continues for a long time even after light irradiation. In addition, since heating is required for the photocationic polymerization to proceed, if light irradiation is performed at a low temperature, a semi-cured state in which the acid as the active species remains in the composition is maintained even after light irradiation. Therefore, it is possible to proceed with the reaction of the uncured curable resin by heating after light irradiation.

(Photosensitizer)
The hard coat composition may contain a photosensitizer for the purpose of improving photosensitivity, etc. Examples of the photosensitizer include anthracene derivatives, benzophenone derivatives, thioxanthone derivatives, anthraquinone derivatives, and benzoin derivatives. Among them, anthracene derivatives, thioxanthone derivatives, and benzophenone derivatives are preferred from the viewpoint of photoinduced electron donative properties.

(Reactive Diluent)
The hard coat composition may contain a reactive diluent (photocurable monomer) in addition to the photocurable resin. By adding a reactive diluent to the composition, the density of the photopolymerization reaction points (crosslinking points) increases, and the curing speed may be increased.

(Additives)
The hard coat composition may contain additives such as fine particles, colorants, plasticizers, dispersants, wetting agents, thickeners, leveling agents, defoamers, flame retardants, antistatic agents, antioxidants, ultraviolet absorbers, polymerization inhibitors, etc. The hard coat composition may also contain a thermoplastic or thermosetting resin material in addition to the above-mentioned photocurable resin.

(solvent)
The hard coat composition may be solvent-free or may contain a solvent. The solvent is preferably one that does not dissolve the film substrate. On the other hand, by using a solvent that has a degree of solubility that swells the film substrate, the adhesion between the film substrate and the hard coat layer may be improved.

Examples of solvents include ketones such as acetone, methyl isobutyl ketone, diisobutyl ketone, and cyclohexanone; alcohols such as ethanol, isopropyl alcohol, butanol, and cyclohexanol; esters such as methyl acetate, ethyl acetate, butyl acetate, and isopropyl acetate; ethers such as dioxane, tetrahydrofuran, diethylene glycol methyl ether, and propylene glycol methyl ether; aliphatic hydrocarbons such as hexane; alicyclic hydrocarbons such as cyclohexane; aromatic hydrocarbons such as toluene and xylene; amides such as N,N-dimethylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone; alkyl halides such as chloroform, dichloromethane, and dichloroethane; sulfoxides such as dimethyl sulfoxide; and cellosolves such as methyl cellosolve and ethyl cellosolve.

(Preparation of Hard Coat Composition)
The method for preparing the hard coat composition is not particularly limited. For example, the above-mentioned components may be mixed and mixed with a hand mixer or a static mixer, or kneaded with a planetary mixer, disperser, roll, kneader, or the like.

When the hard coat composition contains a solvent, the solids (non-volatiles) concentration of the composition is usually about 5 to 80% by weight, and may be about 10 to 70% by weight. In consideration of film-forming properties and the efficiency of drying and removing the solvent, the solids concentration of the hard coat composition is preferably 15% by weight or more, more preferably 20% by weight or more, and may be 25% by weight or more, 30% by weight or more, 35% by weight or more, or 40% by weight or more. As described above, the hard coat composition may be a solvent-free type.

The viscosity of the hard coat composition (coating liquid) is not particularly limited and may be set according to the coating method, etc., but is usually about 1 to 200 poise, and may be 5 to 150 poise or 10 to 100 poise.

[Formation of hard coat layer]
A hard coat composition is applied onto the first main surface 1A of the film substrate 1 to form a coating film, and the curable resin is photocured to form a hard coat layer 3. Fig. 2 is a conceptual diagram showing an outline of a film-forming apparatus and a film-forming process for forming the hard coat layer 3 on the first main surface 1A of the film substrate 1 while transporting the strip-shaped film substrate 1 by roll-to-roll.

In FIG. 2, a roll 10 in which a long film substrate is wound in a roll shape is wound around an unwind roll 11. The strip-shaped film substrate 1 unwound from the unwind roll 11 is continuously transported downstream of a transport path formed by transport rolls 41, 42, 43 and a pair of nip rolls 45. In the coating section 30, a coating of a hard coat composition is formed on the first main surface 1A of the film substrate 1. The laminate with the coating formed on the film substrate is transported to the light irradiation section 60 along a transport path formed by transport rolls 37, 38 and transport rolls 71, 72 arranged before and after the heating furnace, and light is irradiated from a light source 65 to the hard coat composition. It is then transported to the heating section 90 along a pair of nip rolls 75 and transport rolls 73, 74. After the heat treatment in the heating section 90, the laminate (hard coat film) 8 in which the hard coat layer 3 is formed on the film substrate 1 is transported along the transport rolls 83 and 84 and wound into a roll by the winding roll 81, to obtain the rolled hard coat film 80.

The nip roll 45 and the nip roll 75 sandwich the film between the two rolls and transport it downstream. At least one of the two rolls that make up the nip roll is a drive roll, and the film transport speed is kept constant by adjusting its rotation speed. The transport roll may be a drive roll or a free rotating roll. Although not shown, a transport roll may also be provided inside the heating furnace 50.

The conveying speed of the film substrate is not particularly limited and may be set appropriately taking into consideration the coating thickness, the time required for drying the solvent, the curing of the composition, and the heat treatment. The conveying speed of the film substrate is generally about 0.1 to 100 m/min, and may be 0.3 to 50 m/min, 0.5 to 30 m/min, or 1 to 20 m/min.

Below, each step in forming the hard coat layer is explained with reference to Figure 2.

<Formation of coating film>
A coating film of the hard coat composition is formed by applying the hard coat composition onto the first main surface 1A of the film substrate 1 transported to the coating section 30. In the embodiment shown in Fig. 2, the hard coat composition discharged from a die 31 is applied onto the first main surface 1A of the film substrate 1 in a state in which the second main surface 1B of the film substrate 1 is in contact with a backup roll 33.

The method for applying the hard coat composition onto the film substrate 1 is not particularly limited. In addition to die coating, examples of the application method include gravure coating, wire bar coating, knife roll coating, blade coating, and spray coating. The application thickness may be appropriately set according to the thickness of the hard coat layer.

<Drying>
When the hard coat composition contains a solvent, the solvent is dried. Drying may be performed at room temperature, but from the viewpoint of increasing the efficiency of removing the solvent, it is preferable to heat the laminate in which the coating film is formed on the film substrate in a heating furnace 50. The heating temperature is not particularly limited, but is about 30 to 200° C., and may be increased stepwise. The heating time is about 10 seconds to 10 minutes.

<Light irradiation>
In the light irradiation section 60, the coating film of the hard coat composition (if the composition contains a solvent, the coating film after removing the solvent) is irradiated with active energy rays from a light source 65 to perform photocuring. Fig. 2 illustrates a form in which the film substrate is irradiated with active light rays from a light source 65 while being transported on a backup roll 63.

Examples of active energy rays irradiated during photocuring include visible light, ultraviolet light, infrared light, X-rays, alpha rays, beta rays, gamma rays, and electron beams. Ultraviolet light is preferred as the active light ray because it has a high curing speed and excellent energy efficiency. Examples of light sources include low-pressure mercury lamps, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, xenon lamps, carbon arc lamps, LEDs, black lights, and chemical lamps.

The cumulative exposure dose of active energy rays is, for example, about 50 to 10,000 mJ/cm ² and may be set depending on the types and amounts of the curable resin and photopolymerization initiator contained in the hard coat composition, the thickness of the hard coat layer, etc.

At the stage of irradiation with light by the light irradiation unit 60, the coating film of the hard coat composition is preferably in a semi-cured state. By semi-curing the hard coat composition (not completely curing) at the stage of irradiation with light, yellowing due to overpolymerization or decomposition of the photopolymerization initiator is suppressed. In addition, at the stage of irradiation with light, there is little cure shrinkage, so curling is suppressed and roll-to-roll transportability is improved. Furthermore, if uncured components remain in the composition at the stage of irradiation with light, curing proceeds with the film curved during heating as described below, and curling of the hard coat film is more efficiently reduced.

To semi-cure the hard coat composition, for example, the amount of light irradiation may be adjusted. When the hard coat composition is photocationically polymerizable, the cured state can be adjusted by controlling the temperature during light irradiation in addition to the amount of light irradiation. When semi-cure the coating film of the hard coat composition, it is preferable to cure the composition to the extent that it does not adhere to the rolls (tack-free state) even if the coating film comes into contact with the transport roll or nip roll.

In photocationic polymerization, an acid is generated as an active species from a photocationic polymerization initiator by light irradiation, but the rate of polymerization reaction is low unless heating is performed. Therefore, with only light irradiation, a part of the curable resin is in an uncured state, and the acid as an active species also remains unreacted in the film. In order to make the hard coat composition semi-cured, it is preferable that the temperature during light irradiation is lower than the temperature during heating described below. The temperature during light irradiation is preferably 100°C or less, more preferably 80°C or less, and even more preferably 60°C or less, and may be 50°C or less or 40°C or less. In order to suppress the temperature rise caused by heat from the light source 65, etc., cooling may be performed by blowing cold air or adjusting the temperature of the backup roll, etc.

<Heat Treatment>
After the coating film on the film substrate 1 is irradiated with light in the light irradiation section, the laminate is heated in the heating section 90. In the heating section 90, the laminate is heated in a curved state (see FIG. 3) so that the coating film-forming surface (the surface on which the hard coat layer 3 is formed) faces outward, thereby obtaining a hard coat film with reduced curling. In other words, the heat treatment is a treatment for suppressing/reducing curling of the hard coat film.

Photocurable resins generally shrink when cured. Therefore, due to the cure shrinkage that occurs when the hard coat composition is cured on the film substrate 701, compressive stress occurs at the interface between the film substrate 701 and the hard coat layer 703, and as shown in FIG. 7, the hard coat film 708 warps (curls) with the surface on which the hard coat layer 703 is formed facing inward. The thicker the hard coat layer 703 is, the greater the compressive stress at the interface, and the greater the tendency for the hard coat film to curl.

Figure 3 is an enlarged view of the heating section 90 in Figure 2, in which the second main surface 1B (the surface on which the hard coat layer is not formed) of the film substrate 1 is in contact with the heated roll 91, and the laminate in which the coating film (hard coat layer 3) is formed on the film substrate 1 is curved along the heated roll so that the coating film-formed surface is on the outside. In the curved portion, compressive strain occurs on the second main surface 1B of the film substrate 1, and expansive strain occurs on the first main surface 1A (the surface on which the coating film is formed) of the film substrate 1. Therefore, expansive strain (tensile stress) also occurs in the coating film.

While in contact with the heated roll 91, the laminate is curved with a radius of curvature R (equal to half the diameter of the heated roll 91) and heated in a state in which tensile stress acts on the coating. When polymerization proceeds by heating in this state, at the interface between the first main surface 1A of the film substrate 1 and the hard coat layer 3, the compressive strain due to cure shrinkage is offset by the expansive strain due to the curvature.

In this way, by heating the film while it is curved with the coating surface facing outward, the distortion at the interface between the film substrate 1 and the hard coat layer 3 is offset, and the hard coat layer is fixed in a cured state, suppressing curling of the hard coat film. The smaller the radius of curvature when curved (i.e., the smaller the diameter of the heat roll 91), the greater the expansion distortion (tensile stress) of the hard coat layer due to curving, and the greater the curl suppression effect. Therefore, even when the hard coat layer is thick and the accompanying compression distortion is large, curling of the hard coat film tends to be suppressed/reduced.

Cure shrinkage due to photocuring (including curing that progresses due to heating after light irradiation, due to the action of active species generated by light irradiation) occurs isotropically within the film's surface. Therefore, if the laminate is heated without being curved, the film will tend to curl in both the machine direction (MD) and the width direction (TD).

As described above, when the film is curved along the roll and heat-treated, tensile stress acts on the surface on which the hard coat layer is formed in the transport direction, reducing curling caused by cure shrinkage, whereas cure shrinkage occurs in the width direction without tensile stress acting on the film. Therefore, a hard coat film that is heat-treated while curved along the transport direction is prone to curling in the width direction, but when the film is heated while curved with a smaller radius of curvature R along the transport direction, curling in not only the transport direction but also the width direction tends to be suppressed/reduced.

One presumed factor for suppressing curling along the width direction in addition to the conveying direction is that when the radius of curvature of the curve along the conveying direction is reduced, a force acts in the opposite direction to the conveying direction (with the hard coat layer-formed surface on the outside). In other words, it is believed that curling along not only the conveying direction but also the width direction is suppressed by balancing the force (tensile stress) that tries to curl with the hard coat layer-formed surface on the outside along the conveying direction and the force (compressive stress) that tries to curl with the hard coat layer-formed surface on the inside along the width direction.

From the viewpoint of reducing curling of the hard coat film, it is preferable that the radius of curvature R of the curvature during heating in the heating section is smaller. As described above, the thicker the hard coat layer, the greater the tendency for curling to occur. Therefore, when the hard coat layer is thick, it is preferable to make the radius of curvature R of the curvature during heating smaller in order to suppress curling.

The optimal value of the radius of curvature R varies depending on the thickness of the hard coat layer and the occurrence of curl, but is preferably 100 mm or less. From the viewpoint of effectively suppressing/reducing curl even when the thickness of the hard coat layer is 5 μm or more, the radius of curvature R is more preferably 50 mm or less, even more preferably 40 mm or less, and may be 30 mm or less, 25 mm or less, or 20 mm or less. When the heat treatment is performed while the laminate is curved along a roll, in order to set the radius of curvature R of the curve in the above range, the diameter of the roll is preferably 200 mm or less, more preferably 100 mm or less, even more preferably 80 mm or less, and may be 60 mm or less, 50 mm or less, or 40 mm or less.

As shown in FIG. 2, when light irradiation is performed while the film is curved along the backup roll 63, it is preferable that the radius of curvature R of the curve in the heating section 90 is smaller than the radius of curvature r of the curve during light irradiation. R is more preferably 0.5r or less, and may be 0.3r or less, 0.2r or less, or 0.1r or less.

As described above, in terms of reducing curling due to cure shrinkage, the smaller the radius of curvature R during heating, the better. On the other hand, if the radius of curvature R is too small, the effect of tensile stress during curving is large, and curling may occur with the hard coat layer formed surface on the outside. In addition, if the radius of curvature R is too small, cracks or peeling may occur in the coating film (hard coat layer). Therefore, the radius of curvature R is preferably 3 mm or more, more preferably 5 mm or more, and may be 8 mm or more, 10 mm or more, or 15 mm or more. When the heat treatment is performed in contact with a roll such as a heat roll, in order to set the radius of curvature R of the curve in the above range, the diameter of the roll is preferably 6 mm or more, more preferably 10 mm or more, and may be 16 mm or more, 20 mm or more, or 30 mm or more.

The heating temperature in the heating section is not particularly limited, and may be adjusted so as to reduce curling of the hard coat film. From the viewpoint of ensuring the hardness of the hard coat layer by promoting the curing of the coating film in a semi-cured state by heating, and from the viewpoint of enhancing the curl suppression effect, it is preferable that the heating temperature when the film is heated in a curved state in the heating section 90 is higher than that during light irradiation. The heating temperature in the heating section is preferably 50°C or higher, more preferably 60°C or higher, and even more preferably 80°C or higher, and may be 100°C or higher, 120°C or higher, 140°C or higher, or 150°C or higher.

When heating is performed by contacting the laminate with a heated roll 91, the heating temperature is the surface temperature of the heated roll. When heating is performed from the side on which the coating film (hard coat layer) is formed as described below, the heating temperature is the higher of the atmospheric temperature directly above the coating film and the surface temperature of the roll in contact with the laminate (second main surface 1B of the film substrate 1).

When heating is performed by contacting the second main surface 1B of the film substrate 1 with the heat roll 91, short-term contact with the heat roll 91 may not transfer heat sufficiently from the heat roll, and a discrepancy may occur between the surface temperature of the heat roll and the temperature of the coating film (hard coat layer 3) formed on the first main surface 1A of the film substrate 1. From the viewpoint of sufficiently increasing the temperature of the coating film to promote curing, the surface temperature of the heat roll is preferably 80°C or higher, more preferably 100°C or higher, even more preferably 120°C or higher, and may be 140°C or higher or 150°C or higher.

As long as the film substrate 1 is in a range that has heat resistance, there is no particular upper limit to the heating temperature, but if the heating temperature is excessively high, yellowing of the hard coat layer due to overpolymerization or reverse curling of the hard coat film may occur. Therefore, the heating temperature is preferably 300°C or less, more preferably 250°C or less, and may be 230°C or less, 200°C or less, or 180°C or less.

From the viewpoint of enhancing the curl suppression effect, the time of the heat treatment in the curved state (contact time with the heat roll 91) is preferably 1 second or more, and may be 2 seconds or more, 3 seconds or more, 4 seconds or more, or 5 seconds or more. In addition, from the viewpoint of promoting the curing of the coating film in a semi-cured state, it is preferable that the heating temperature is in the above range.

The upper limit of the heat treatment time is not particularly limited, but in order to heat the laminate for a long time while it is curved along the roll while being transported roll-to-roll, it is necessary to reduce the transport speed, which causes a decrease in the productivity of the hard coat film. Therefore, when the heat treatment is performed while the laminate is transported roll-to-roll, the heat treatment time is preferably 100 seconds or less, more preferably 50 seconds or less, and may be 30 seconds or less, 20 seconds or less, 15 seconds or less, or 10 seconds or less. Even for such a short time, a hard coat film with suppressed curling can be obtained by heating the laminate in a curved state so that the hard coat layer forming surface is on the outside. From the viewpoint of efficiently reducing the curl of the hard coat film while promoting curing, it is preferable to reduce the radius of curvature when curved and perform a short heat treatment at a high temperature.

2 and 3 show a form in which the second main surface 1B of the film substrate 1 is brought into contact with a heat roll 91 in the heating section 90, and the film substrate 1 is curved so that the coating film forming surface faces outward, and then heated by heat from the heat roll, but the heating method is not limited to the heat roll. For example, in the heating section 490 shown in FIG. 4, a heat source 99 such as an infrared heater or a hot air blower is disposed opposite the transport roll 98, and heating is performed from the coating film forming surface side by heat from the heat source 99. The transport roll 98 may be a heat roll, and heating may be performed from both the heat roll and the heat source 99 .

The heat treatment with the laminate curved may be performed in multiple steps. For example, in FIG. 5, the heating section 590 is equipped with three heat rolls 92, 93, and 94, and heat treatment is performed with each of the heat rolls. As described above, in the heat treatment, the curl suppression effect tends to be greater as the radius of curvature R when curving with the coating film forming surface on the outside is smaller, but in order to reduce the radius of curvature R, it is necessary to reduce the diameter of the roll, which may make it difficult to ensure the contact time with the roll. As shown in FIG. 5, by performing the heat treatment on multiple rolls 92, 93, and 94, the heat treatment time can be ensured without reducing the conveying speed.

In FIG. 5, the rolls 92, 93, and 94 in contact with the second main surface 1B of the film substrate 1 are heated rolls, while the rolls 74, 78, 79, and 83 in contact with the coating film (hard coat layer 3) are preferably not heated. When the coating film is in contact with the rolls 74, 78, 79, and 83 and the laminate is curved with the coating film-formed surface on the inside, the heat treatment is not performed, and when the second main surface of the film substrate is in contact with the rolls 92, 93, and 94 and the laminate is curved with the coating film-formed surface on the outside, the curl of the hard coat film can be efficiently reduced by selectively performing the heat treatment. The rolls 74, 78, 79, and 83 in contact with the coating film (the rolls arranged on the upper side in FIG. 5) may be heated rolls, but their temperature is preferably lower than that of the rolls 92, 93, and 94 in contact with the second main surface of the film substrate (the rolls arranged on the lower side in FIG. 5). The temperature difference between the two (the temperature difference between the upper and lower rolls in FIG. 5) is preferably 30°C or more, more preferably 50°C or more, and may be 70°C or more, 80°C or more, 90°C or more, or 100°C or more.

Figure 5 shows a form in which heat treatment is performed using multiple heated rolls, but the heating method on the multiple rolls is not limited to heating using heated rolls, and heating may be performed using a heat source such as an infrared heater or a hot air heater arranged opposite the rolls. In addition, heating from the film substrate side using the heated roll and heating from the coating side using a heat source may be performed simultaneously on the rolls.

In FIG. 6, the heating section 690 is equipped with multiple heat rolls 95, 96, and 97 of different diameters. In this embodiment, by selecting the heat roll to be used for the heat treatment, the radius of curvature of the curve during the heat treatment can be changed depending on the thickness of the hard coat layer and the occurrence of curling, etc., and can be applied to suppress/reduce curling of various types of hard coat films. In addition, the heat treatment may be performed on multiple rolls of different roll diameters.

[Use of hard coat film]
After the heat treatment in the heating section 90, the laminate (hard-coated film) 8 in which the hard-coating layer 3 is formed on the film substrate 1 is wound up by a winding roll 81 to obtain a long film roll 80. The hard-coated film may be attached to another film or the like via a pressure-sensitive adhesive layer or adhesive layer as necessary. A functional layer such as an anti-reflection layer, an anti-fouling layer, a gas barrier layer, or a transparent electrode may be provided on the surface of the hard-coating layer.

The hard coat film is cut into sheets of a specified size and attached to an image display device or the like for use. Because the hard coat film is prevented from curling, it is easy to work with when attaching it to an image display device or the like, and defects such as lifting and peeling can be prevented.

As described above, the hard coat composition is applied onto the strip-shaped film substrate 1 to form a coating film, and the solvent is removed as necessary. After that, light is irradiated, and the laminate of the film substrate and coating film is heated in a curved state, thereby obtaining a hard coat film with reduced curling.

The thickness of the hard coat layer 3 is about 1 to 100 μm. The thickness of the hard coat layer may be 3 μm or more, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The thickness of the hard coat layer 3 may be about 0.1 to 2 times the thickness of the film substrate 1, or may be 0.3 times or more, 0.5 times or more, or 0.7 times or more. Generally, the thicker the hard coat layer, the greater the tendency for curling to occur. However, by adjusting the radius of curvature during heat treatment, heating temperature, heating time, etc. according to the thickness of the hard coat layer, etc., as described above, a hard coat film with reduced curling can be obtained.

The curl amount is evaluated by placing a hard coat film cut into a square sheet of 150 mm x 150 mm on a horizontal table with the hard coat layer formed side on the upper side. The distance (lift amount) from the table is measured for each of the four vertices of the square, and the average value is the curl amount. When the sides AB and DC perpendicular to the conveying direction (MD) are lifted, it is considered to be a curl along the MD, and when the sides AD and BC perpendicular to the width direction (TD) are lifted, it is considered to be a curl along the TD (see Figure 8). When the center part of the film is lifted in the plane (when a curl in the opposite direction occurs), the film is placed on a table with the hard coat layer formed side on the lower side and the curl amount is measured. The curl amount (absolute value) of the hard coat film is preferably 40 mm or less, more preferably 30 mm or less, even more preferably 25 mm or less, and may be 20 mm or less, 15 mm or less, or 10 mm or less.

[Another embodiment of the curl suppression method]
In the above embodiment, while conveying the strip-shaped film substrate 1 by roll-to-roll, the hard coat composition is applied to form a coating film, and light irradiation and heating are performed as a series of steps, and then the hard coat film 8 is wound up to obtain a rolled body 80. The heat treatment for suppressing/reducing curling does not necessarily have to be performed in-line in series with the formation of the coating film, and after forming the coating film of the hard coat composition on the film substrate, the laminate of the film substrate and the coating film may be wound up in a roll, and then the heat treatment may be performed offline.

For example, a hard coat composition is applied onto a film substrate to form a coating film, and the solvent is removed as necessary. The composition is then semi-cured by irradiation with light, and the laminate is wound into a roll. The laminate is then unwound and transported roll-to-roll, and the heat treatment may be performed while the laminate is curved with the coating film-formed surface facing outward. In this way, when the heat treatment is performed offline, the transport speed of the film substrate during coating film formation and the transport speed of the film substrate (laminate) during heat treatment can be set separately, so that even if the radius of curvature of the curve is small (the diameter of the roll during heat treatment is small), it is possible to ensure the time for heat treatment without reducing the coating speed.

The heat treatment does not necessarily have to be performed by roll-to-roll. For example, the laminate cut to a predetermined size may be curved with the coating film surface facing outward and then heat treated. In this case, since there is no need to continuously transport the laminate, there is no need to bring it into contact with a roll to curve it. For example, the laminate may be rolled into a cylindrical shape and then heated in an oven or the like.

The present invention will be described in detail below by comparing examples and comparative examples, but the present invention is not limited to the following examples.

[Preparation of hard coat resin composition]
A reaction vessel was charged with 100 parts by weight of β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane ("SILQUEST A-186" manufactured by Momentive Performance Materials), 0.12 parts by weight of magnesium chloride, 11 parts by weight of water, and 11 parts by weight of propylene glycol monomethyl ether, and the mixture was stirred at 130° C. for 3 hours, and then devolatilized under reduced pressure at 60° C. to obtain a liquid compound (siloxane resin having an alicyclic epoxy group).

100 parts by weight of the above siloxane resin was mixed with 0.5 parts by weight of a photoacid generator ("CPI-101A" manufactured by San-Apro) to obtain a hard coat composition.

[Comparative Example 1]
<Formation of Semi-Cured Film>
The above hard coat resin composition was applied to a biaxially stretched polyethylene terephthalate (PET) film substrate (thickness 50 μm, width 275 mm) at a width of 250 mm from a slot die coater while being transported with a roll at 0.5 m/min. The film on which the coating was formed was supported on a backup roll and irradiated with ultraviolet rays (peak illuminance of irradiated surface: 20 mW/cm ² , cumulative irradiation amount: 500 mJ/cm ² ) from an 80 W mercury lamp to semi-cure the composition.

<Heat Treatment>
The laminate in which the hard coat layer (semi-cured film) was formed on the film substrate was heated for 4 minutes while being conveyed horizontally in a heating oven at 120°C to promote curing, thereby obtaining a hard coat film having a 20 μm hard coat layer on a 50 μm PET film substrate.

[Example 1]
After applying the hard coat composition and irradiating with ultraviolet light in the same manner as in Comparative Example 1, the laminate was once wound into a roll without heating. Then, the laminate in which the hard coat layer was formed on the film substrate was cut into a square of 150 mm x 150 mm, rolled into a cylindrical shape of 30 mm in diameter (15 mm radius) with the coating film-formed surface facing outward and the conveying direction (MD) during coating film formation as the circumferential direction, heated in an oven at 120°C for 4 minutes, and then taken out to room temperature.

[Examples 2 and 3]
A heat treatment for 4 minutes was carried out in the same manner as in Example 1, except that the radius of the cylinder when the laminate was rolled into a cylindrical shape was changed as shown in Table 1.

[Example 4]
After applying the hard coat composition and irradiating ultraviolet rays in the same manner as in Comparative Example 1, the film substrate was placed along a heated roll (diameter 30 mm, temperature 180° C.) at 180° so as to be in contact with the heated roll, and was heated by the heated roll in a state where the film substrate (non-hard coat layer side) was on the inside and the hard coat layer side was on the outside, and the film substrate was curved with a curvature radius of 15 mm. The contact time between the film substrate and the heated roll was 6 seconds.

[Example 5]
The diameter of the heated roll was changed to 100 mm, and the angle of the film along the heated roll (wrap angle) was changed to 60°. Otherwise, the laminate was subjected to heat treatment for 6 seconds on the heated roll in a curved state with a curvature radius of 50 mm in the same manner as in Example 4.

[Comparative Example 2]
The thickness of the hard coat layer (coating thickness of the composition) was changed to 35 μm. Otherwise, a semi-cured film was formed in the same manner as in Comparative Example 1, and heated at 120° C. for 4 minutes while being conveyed horizontally, to obtain a hard coat film having a 35 μm hard coat layer on a 50 μm PET film substrate.

[Example 6]
After applying the hard coat composition and irradiating with ultraviolet light in the same manner as in Comparative Example 2, the laminate was once wound into a roll without heating. Then, the laminate in which the hard coat layer was formed on the film substrate was cut into a square of 150 mm x 150 mm, rolled into a cylindrical shape with the radius shown in Table 1 with the coating film surface facing outward, heated in an oven at 120°C for 4 minutes, and then taken out to room temperature.

[Examples 7 and 8]
In the same manner as in Comparative Example 2, the hard coat composition was applied and the UV irradiation was performed, and the film substrate was placed along the heated roll so as to be in contact with the film substrate, and the film substrate (non-hard coat layer side) was curved so that the hard coat layer side was on the outside, and the film substrate was heated by the heated roll. The diameter and the embrace angle of the heated roll were adjusted so that the radius of curvature R of the curve and the heating time (contact time between the roll and the film) were the values shown in Table 1.

[Comparative Example 3]
The thickness of the hard coat layer (coating thickness of the composition) was changed to 50 μm. Otherwise, a semi-cured film was formed in the same manner as in Comparative Example 1, and heated at 120° C. for 4 minutes while being transported horizontally, to obtain a hard coat film having a 50 μm hard coat layer on a 50 μm PET film substrate.

[Comparative Example 4]
A hard coat film was obtained in the same manner as in Comparative Example 3, except that the heating time during horizontal conveyance was changed to 20 seconds.

[Examples 9 and 10]
After applying the hard coat composition and irradiating with ultraviolet light in the same manner as in Comparative Example 3, the laminate was once wound into a roll without heating. Then, the laminate in which the hard coat layer was formed on the film substrate was cut into a square of 150 mm x 150 mm, rolled into a cylindrical shape with the radius shown in Table 1 with the coating film surface facing outward, heated in an oven at 120°C for 4 minutes, and then taken out to room temperature.

[Examples 11 to 13]
In the same manner as in Comparative Example 3, the hard coat composition was applied and irradiated with ultraviolet light. Then, the film substrate was placed along the heated roll so as to be in contact with the film substrate, and the film substrate was curved so that the film substrate (the surface on which the hard coat layer was not formed) was on the inside and the hard coat layer side was on the outside, and then the film substrate was heated by the heated roll.

[evaluation]
<Carl>
The film was cut into a square of 150 mm x 150 mm, placed on a horizontal table with the hard coat layer formed on the top, and the distances (floating amounts) of the four vertices of the square from the table were measured, and the average value was taken as the curl amount. In Comparative Examples 1 to 3, the film was rolled up into a cylinder with the hard coat layer formed on the inside, and the curl amount could not be measured, so the curl strength was evaluated from the diameter of the cylinder. In Example 3 and Example 10, curling occurred along the MD with the hard coat layer formed on the outside, so the hard coat layer formed on the bottom was placed on a horizontal table, and the curl was measured, and the sign of the curl amount was taken as negative.

<Pencil hardness>
The pencil hardness of the surface of the hard coat layer of the hard coat film was measured according to "8.4.1 Pencil scratch test" of JIS K5400:1990.

[Evaluation Results]
Table 1 shows the thickness of the hard coat layer, the heat treatment conditions, the curl amount and the pencil hardness of the hard coat film in the examples and comparative examples.

In Comparative Examples 1 to 3, in which the film was heated while being transported horizontally, when the film was spread out flat and the center of the MD was pressed along the direction parallel to sides AB and DC in FIG. 8, it curled into a cylindrical shape along the MD, and when the center of the TD was pressed along the direction parallel to sides AD and BC in FIG. 8, it curled into a cylindrical shape along the TD. From these results, it is believed that in Comparative Examples 1 to 3, the shrinkage stress during hard coat layer curing acts in both the MD and TD directions. The thicker the hard coat layer, the smaller the diameter when curled into a cylindrical shape, so it is believed that the thicker the hard coat layer, the greater the effect of cure shrinkage.

In Comparative Example 4, where the heating temperature during horizontal conveyance was 120°C and the heating time was 20 seconds, curling was small, but the pencil hardness of the hard coat layer was 4B, which was insufficient. In Comparative Example 4, heating was insufficient, so the hard coat layer was not cured sufficiently, and it is believed that the curling was small because of the small cure shrinkage.

In Examples 1 to 5, in which the hard coat layer was curved and heated with the surface on which it was formed facing outward, the hard coat layer exhibited a hardness equivalent to that of Comparative Example 1, and curling was suppressed compared to Comparative Example 1. Similar results were obtained in comparing Comparative Example 2 with Examples 6 to 8, and in comparing Comparative Example 3 with Examples 9 to 13.

In Examples 1 and 2, in which the film cut into a sheet shape was rolled into a cylindrical shape and treated at 120°C for 4 minutes, there was no curl in the MD, but slight curl in the TD. In Example 3, in which the radius of curvature during rolling was reduced, curl occurred with the surface on which the hard coat layer was formed facing outward. A similar tendency was observed in comparing Examples 9 and 10.

These results show that the smaller the radius of curvature during heat treatment, the stronger the tendency for the film to curl outward; this tendency is balanced with the cure shrinkage of the hard coat layer, resulting in a hard coat film with minimal curl. It also shows that the thicker the hard coat layer is and the greater the original curl (when heated without curving, it curls into a cylindrical shape with a smaller diameter), the smaller the radius of curvature that must be used for heat treatment to reduce curling.

In Example 4, in which the film was heated while being transported on a heated roll at 180°C, a hard coat film with little curl was obtained, similar to Example 1, despite the short heat treatment time (contact time with the heated roll) of 6 seconds. Comparing Examples 1 and 4, it can be seen that when the film is heated on the roll while being transported roll-to-roll, a curl reduction effect similar to that achieved when the film is cut out and heated can be obtained by heating at a high temperature for a short period of time.

In Example 5, where the film was bent at a radius of curvature of 50 mm using a heated roll with a diameter of 100 mm and then heat-treated, a curl reduction effect was observed compared to Comparative Example 1, but the curl along the TD was greater compared to Example 4, where the film was bent at a radius of curvature of 15 mm and then heat-treated. A similar tendency was observed in comparing Examples 7 and 8, and in comparing Examples 11 to 13. These results show that when heat-treatment is performed while the film is bent along a roll, a hard-coated film with less curl can be obtained by reducing the diameter of the roll and bending the film at a smaller radius of curvature.

REFERENCE SIGNS LIST 1 substrate 3 hard coat layer 8 hard coat film 11 unwinding roll 81 winding roll 30 coating section 33 backup roll 31 die 50 heating furnace 60 light irradiation section 65 light source 63 backup roll 90, 490, 590, 690 heating section 91 to 97 heat roll 99 heat source

Claims (9)

A method for producing a hard coat film comprising the steps of: providing a film substrate having a first main surface and a second main surface with a hard coat layer on the first main surface;
a coating step of applying a hard coat composition containing a curable resin and a photopolymerization initiator onto a first main surface of the film substrate to form a coating film;
a light irradiation step of irradiating the coating film of the hard coat composition with actinic rays to photocure the curable resin; and a heating step of heating a laminate of the film substrate and the coating film after light irradiation,
In the heating step, heating is performed in a state in which the second main surface of the film substrate is in contact with a heated roll and the laminate is curved along the heated roll so that the coating film forming surface is on the outside.
A method for producing a hard coat film. The method for producing a hard coat film according to claim 1 , wherein heating is also performed from the coating film forming surface while the second main surface of the film substrate is in contact with a heated roll. The method for producing a hard coat film according to claim 1 or 2 , wherein the heating temperature in the heating step is 80° C. or higher. The method for producing a hard coat film according to any one of claims 1 to 3 , wherein the diameter of the heat roll is 100 mm or less. The method for producing a hard coat film according to any one of claims 1 to 4, wherein the coating film is in a semi-cured state when heating is performed in the heating step, and active species of the photopolymerization initiator generated by irradiation with active light in the light irradiation step remain in the coating film. The method for producing a hard coat film according to any one of claims 1 to 5, wherein the hard coat composition contains a curable resin having one or more polymerizable functional groups selected from the group consisting of an epoxy group and an oxetane group, and a photocationic polymerization initiator as the photopolymerization initiator. The method for producing a hard coat film according to any one of claims 1 to 6 , wherein the hard coat layer has a thickness of 5 µm or more. A method for suppressing curling of a hard coat film having a hard coat layer on a first main surface of a film substrate having a first main surface and a second main surface, comprising:
preparing a laminate having a hard coat layer made of a photocured product of a hard coat composition containing a curable resin and a photopolymerization initiator on a first main surface of the film substrate;
The method for suppressing curling includes heating the laminate in a curved state along a heated roll such that the second main surface of the film substrate is in contact with the heated roll and the surface on which the hard coat layer is formed is on the outside. The method for suppressing curl according to claim 8 , wherein the hard coat layer is in a semi-cured state before heating, and the hard coat layer is cured by the action of active species generated from the photopolymerization initiator during the heating.

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