Method for producing hard coat film and method for preventing curling

Abstract

To provide a hard coat film which is slightly curled.SOLUTION: A hard coat film has a cured resin layer (8) on a first main surface (1A) of a film base material (1). A thickness of a hard coat layer may be 5 μm or more. A coated film is formed by coating a hard coat composition containing a curable resin and a photopolymerization initiator onto the first main surface of the film base material, the coated film is irradiated with active rays, and then a laminate of the film base material and the coated film is heated in a state where the laminate is curved so that a coated film formation surface is allowed to be outside. Heat treatment may be performed in a state where a second main surface of the film base material is brought into contact with a roll such as a heated roll.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for producing a hard coat film having a hard coat layer on a film substrate, and to suppress curling of the hard coat film. [Background technology]

[0002] In recent years, curved displays and foldable displays (flexible displays, foldable displays) have been developed, and flexible resin films have come to be used instead of glass as materials for substrates and cover windows. In order to impart high hardness to the resin film, a hard coat film having a hard coat layer provided on the surface of the resin film is used.

[0003] High hardness is required for members arranged on the display surface, such as a cover window. On the other hand, in a foldable display, it is required that the hard coat layer does not peel off or crack even when repeatedly bent. In order to achieve both hardness and bending resistance of the hard coat layer, it is necessary to increase the thickness of the hard coat layer while increasing the flexibility of the curable resin constituting the hard coat layer to ensure hardness.

[0004] 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. When 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).

[0005] As methods for suppressing or reducing curling of a hard coat film, a method of 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), a method of treating the hard coat film with water vapor (Patent Document 2), and the like have been proposed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2015 / 029667 [Patent Document 2] JP 2011-215268 A Summary of the Invention [Problem to be solved by the invention]

[0007] In applications such as display cover windows, hard coat films having a thicker hard coat layer are required, but the thicker the hard coat layer, the more the hard coat film tends to curl, which may 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. [Means for solving the problem]

[0008] One aspect of the present invention relates to a method for producing a hard coat film, comprising providing a film substrate having a first principal surface and a second principal surface with a hard coat layer on the first principal surface. The hard coat layer may have a thickness of 5 μm or more.

[0009] A hard coat composition containing a curable resin and a photopolymerization initiator is applied onto a first main surface of a film substrate to form a coating film, and the coating film of the hard coat composition is irradiated with actinic rays to photocure the curable resin.

[0010] The hard coat composition may be photocationically polymerizable. The photocationically polymerizable composition includes, for example, a curable resin having a photocationically polymerizable functional group and a photocationic polymerization initiator (photoacid generator). Examples of the photocationically polymerizable functional group include an epoxy group and an oxetane group. Examples of the functional group including an epoxy group include an alicyclic epoxy group and a glycidyl group.

[0011] After irradiating the coating film of the hard coat composition with active rays to generate active species from the photopolymerization initiator, the laminate in which the coating film is provided on the film substrate is heated in a curved state so that the coating film-forming surface faces outward. The coating film after irradiation with active rays may be in a semi-cured state until heat treatment. In the semi-cured coating film, active species generated from the photopolymerization initiator by irradiation with active rays may remain. By heating in this state, curing (polymerization reaction) proceeds, and the hardness of the coating film (hard coat layer) increases.

[0012] The 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 (the surface opposite to the surface on which the coating film is formed) of the film substrate 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 placed along 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.

[0013] The radius of curvature of the laminate during the heat treatment is preferably 50 mm or less.The heating temperature during the heat treatment is preferably 80° C. or more. Effect of the Invention

[0014] 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, thereby obtaining a hard coat film with reduced curling. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view of a hard coat film according to an embodiment. [Diagram 2] 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. [Diagram 3] FIG. 3 is an enlarged view of the periphery of a heating unit in FIG. 2. [Figure 4]FIG. 4 is a schematic diagram showing a configuration example of a heating unit. [Diagram 5] FIG. 4 is a schematic diagram showing a configuration example of a heating unit. [Figure 6] FIG. 4 is a schematic diagram showing a configuration example of a heating unit. [Figure 7] FIG. 2 is an explanatory diagram of the principle of curling in a hard coat film. [Figure 8] FIG. 2 is a diagram for explaining the curl direction of a hard coat film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] One aspect of the present invention relates to suppressing / reducing curl in a hard coat 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 coat film using the curl suppression method.

[0017] [Hard coat film composition and materials] 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.

[0018] <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.

[0019] The resin material constituting the transparent film is preferably one having excellent transparency, mechanical strength, and thermal stability. Specific examples of the resin material include polyester resin, (meth)acrylic resin, polyethersulfone resin, polysulfone resin, polycarbonate resin, polyamide resin, polyimide resin, polyolefin resin, cyclic polyolefin resin, polyarylate resin, polystyrene resin, and cellulose resin.

[0020] When the hard coat film is used as a cover window of 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 general wholly 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.

[0021] The thickness of the film substrate is not particularly limited, but from the viewpoints of strength, workability such as handleability, 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.

[0022] <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.

[0023] (curable 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.

[0024] Specific examples of the photo-radical polymerizable curable resin include acrylic resins such as urethane acrylate, polyester acrylate, epoxy acrylate, and polyol acrylate. Specific examples of the photo-cationically polymerizable curable resin include epoxy resins. As the photo-curable resin, a polysiloxane compound having an epoxy group as a photo-polymerizable functional group, as disclosed in WO2018 / 096729, WO2014 / 204010, JP2017-8142A, etc., may be used.

[0025] (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), etc. may be used depending on the polymerization property 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.

[0026] Examples of the photoradical polymerization initiator include acetophenones, benzophenones, Michler's benzoyl benzoate, α-aminoxime ester, thioxanthones, propiophenones, benzils, benzoins, and acylphosphine oxides.

[0027] Examples of the photocationic polymerization initiator 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-allene complexes; silanol-metal chelate complexes; sulfonic acid derivatives such as disulfones, disulfonyldiazomethanes, disulfonylmethanes, sulfonylbenzoylmethanes, imide sulfonates, and benzoin sulfonates; and organic halogen compounds.

[0028] The active species of the photocationic polymerization initiator is an acid generated by light irradiation, and the active species has a longer life than photoradicals, so the polymerization reaction continues for a long time even after light irradiation. Since heating is required for the progress of photocationic polymerization, 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.

[0029] (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.

[0030] (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.

[0031] (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.

[0032] (solvent) The hard coat composition may be a solvent-free type 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.

[0033] Examples of the solvent include ketones such as acetone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, etc.; alcohols such as ethanol, isopropyl alcohol, butanol, cyclohexanol, etc.; esters such as methyl acetate, ethyl acetate, butyl acetate, isopropyl acetate, etc.; ethers such as dioxane, tetrahydrofuran, diethylene glycol methyl ether, propylene glycol methyl ether, etc.; aliphatic hydrocarbons such as hexane, etc.; alicyclic hydrocarbons such as cyclohexane, etc.; aromatic hydrocarbons such as toluene and xylene, etc.; amides such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, etc.; alkyl halides such as chloroform, dichloromethane, dichloroethane, etc.; sulfoxides such as dimethyl sulfoxide, etc.; and cellosolves such as methyl cellosolve, ethyl cellosolve, etc.

[0034] (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.

[0035] When the hard coat composition contains a solvent, the solid content (non-volatile content) concentration of the composition is usually about 5 to 80% by weight, and may be about 10 to 70% by weight. Considering film-forming properties and the efficiency of drying and removing the solvent, the solid content 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.

[0036] The viscosity of the hard coat composition (coating liquid) is not particularly limited and may be set depending on the coating method, etc., but is usually about 1 to 200 poise, and may be 5 to 150 poise or 10 to 100 poise.

[0037] [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, 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 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.

[0038] 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, and 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 a light irradiation section 60 along a transport path formed by transport rolls 37 and 38 and transport rolls 71 and 72 arranged before and after a heating furnace, and light is irradiated from a light source 65 to the hard coat composition. Then, it is transported to a heating section 90 along a pair of nip rolls 75 and transport rolls 73 and 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, 84 and wound into a roll by the winding roll 81, thereby obtaining a rolled body 80 of the hard coat film.

[0039] Nip roll 45 and nip roll 75 sandwich the film between the two rolls and transport it downstream. At least one of the two rolls constituting the nip roll is a drive roll, and the transport speed of the film is kept constant by adjusting the 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 heating furnace 50.

[0040] The transport speed of the film substrate is not particularly limited and may be appropriately set in consideration of the coating thickness, the time required for drying the solvent, the curing of the composition, and the heat treatment, etc. The transport 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.

[0041] Each step of forming the hard coat layer will be described below with reference to FIG.

[0042] <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 with the second main surface 1B of the film substrate 1 in contact with a backup roll 33.

[0043] There is no particular limitation on the method of applying the hard coat composition onto the film substrate 1. Examples of the application method include die coating, gravure coating, wire bar coating, knife roll coating, blade coating, and spray coating. The application thickness may be appropriately set depending on the thickness of the hard coat layer.

[0044] <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.

[0045] <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 transported on a backup roll 63 while being irradiated with active light rays from a light source 65.

[0046] Examples of active energy rays irradiated during photocuring include visible light, ultraviolet light, infrared light, X-rays, α-rays, β-rays, γ-rays, and electron beams. As the active light rays, ultraviolet light is preferred because of its 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.

[0047] The cumulative dose of active energy rays is, for example, 50 to 10,000 mJ / cm 2 This 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.

[0048] At the stage of irradiation with light by the light irradiation section 60, the coating film of the hard coat composition is preferably in a semi-cured state. At the stage of irradiation with light, the hard coat composition is semi-cured (not completely cured), thereby suppressing yellowing due to overpolymerization or decomposition of the photopolymerization initiator. Furthermore, at the stage of irradiation with light, there is little cure shrinkage, so the occurrence of 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 in a state in which the film is curved during heating as described below, and curling of the hard coat film is more efficiently reduced.

[0049] In order 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 a degree that the composition does not adhere to the roll (tack-free state) even if the coating film contacts the transport roll or nip roll.

[0050] In photocationic polymerization, an acid as an active species is generated 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 in a semi-cured state, the temperature during light irradiation is preferably 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, cooling may be performed by blowing cold air or adjusting the temperature of a backup roll.

[0051] <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 surface on which the coating film is formed (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.

[0052] Photocurable resins generally shrink when cured. Therefore, due to the cure shrinkage 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.

[0053] Fig. 3 is an enlarged view of the heating section 90 in Fig. 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 a heated roll 91, and the laminate in which a 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 faces outward. In the curved portion, compressive strain occurs in the second main surface 1B of the film substrate 1, and expansive strain occurs in 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.

[0054] 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, compressive strain due to cure shrinkage is offset by expansive strain due to the curvature.

[0055] In this way, by heating the film in a curved state 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 heated 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.

[0056] Cure shrinkage due to photocuring (including curing that progresses due to heating after photoirradiation due to the action of active species generated by photoirradiation) occurs isotropically within the film plane. Therefore, if the laminate is heated without being curved, the film will tend to curl in both the machine direction (MD) and the cross direction (TD).

[0057] As described above, when the film is curved along the roll and the heat treatment is performed, tensile stress acts on the surface on which the hard coat layer is formed in the conveying direction, reducing curling due to cure shrinkage, whereas cure shrinkage occurs in the width direction without tensile stress acting. Therefore, a hard coat film that is curved along the conveying direction and heat treated is prone to curling along the width direction, but when the film is curved along the conveying direction with a smaller radius of curvature R and heated, curling in not only the conveying direction but also the width direction tends to be suppressed / reduced.

[0058] One of the presumed factors 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 (with the hard coat layer formed surface on the outside) in the conveying direction. In other words, it is considered that curling along not only the conveying direction but also the width direction is suppressed by balancing the force (tensile stress) that causes curling with the hard coat layer formed surface on the outside along the conveying direction and the force (compressive stress) that causes curling with the hard coat layer formed surface on the inside along the width direction.

[0059] From the viewpoint of reducing curling of the hard coat film, the smaller the radius of curvature R of the curvature during heating in the heating section, the more preferable. 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.

[0060] The optimum value of the radius of curvature R varies depending on the thickness of the hard coat layer and the occurrence of curling, but is preferably 100 mm or less. Even when the thickness of the hard coat layer is 5 μm or more, from the viewpoint of effectively suppressing / reducing curling, the radius of curvature R is more preferably 50 mm or less, further preferably 40 mm or less, and may be 30 mm or less, 25 mm or less, or 20 mm or less. When the laminate is subjected to heat treatment in a state where it 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, further preferably 80 mm or less, and may be 60 mm or less, 50 mm or less, or 40 mm or less.

[0061] 2, when the light irradiation is performed while the film is curved along the backup roll 63, the radius of curvature R of the curve in the heating section 90 is preferably smaller than the radius of curvature r of the curve during the 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.

[0062] As described above, in terms of reducing curling caused by 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 influence 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, cracking 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 heated roll, in order to set the radius of curvature R of curvature to 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.

[0063] 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, the heating temperature when the film is heated in a curved state in the heating section 90 is preferably 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.

[0064] When heating is performed by bringing the laminate into contact with the 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 determined to be the higher of the atmospheric temperature directly above the coating film and the surface temperature of the roll in contact with the laminate (the second main surface 1B of the film substrate 1).

[0065] When heating is performed by contacting the second main surface 1B of the film substrate 1 with the heated roll 91, short-term contact with the heated roll 91 may not transfer heat sufficiently from the heated roll, which may cause a discrepancy between the surface temperature of the heated 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 heated 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.

[0066] The upper limit of the heating temperature is not particularly limited as long as the film substrate 1 has heat resistance, but if the heating temperature is too high, the hard coat layer may turn yellow due to overpolymerization, or the hard coat film may curl in the opposite direction. 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.

[0067] 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. Also, from the viewpoint of promoting the curing of the coating film in a semi-cured state, the heating temperature is preferably in the above range.

[0068] 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 conveyed roll-to-roll and curved along the roll, it is necessary to reduce the conveying speed, which causes a decrease in the productivity of the hard coat film. Therefore, when the heat treatment is performed while conveying the laminate 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 and promoting curing, it is preferable to reduce the radius of curvature when curving and perform a short heat treatment at a high temperature.

[0069] 2 and 3 show a configuration in which the second main surface 1B of the film substrate 1 is brought into contact with a heated roll 91 in the heating section 90 and is curved so that the coating film forming surface faces outward, and heating is performed by heat from the heated roll, but the heating method is not limited to a heated 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 a 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 heated roll, or heating may be performed from both the heated roll and the heat source 99. 。

[0070] The heat treatment in the curved state of the laminate may be performed in several separate steps. For example, in FIG. 5, the heating section 590 includes three heat rolls 92, 93, and 94, and the heat treatment is performed by 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 the laminate is curved 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 the multiple rolls 92, 93, and 94, the heat treatment time can be ensured without reducing the conveying speed.

[0071] In FIG. 5, the rolls 92, 93, 94 in contact with the second main surface 1B of the film substrate 1 are heated rolls, whereas the rolls 74, 78, 79, 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, 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, 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, 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, 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 them (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.

[0072] 5 shows a form in which heat treatment is performed using multiple heated rolls, 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. Also, heating from the film substrate side using the heated rolls and heating from the coating film side using a heat source may be performed simultaneously on the rolls.

[0073] In Fig. 6, the heating section 690 is equipped with multiple heated rolls 95, 96, 97 with different diameters. In this embodiment, by selecting the heated roll 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, the occurrence of curling, etc., and this 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 with different roll diameters.

[0074] [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.

[0075] The hard coat film is cut into a sheet of a predetermined size and used by laminating it to an image display device, etc. Since curling of the hard coat film is suppressed, the workability when laminating it to an image display device, etc. is excellent, and defects such as lifting and peeling can be suppressed.

[0076] As described above, the hard coat composition is applied onto a 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 the coating film is heated in a curved state, thereby obtaining a hard coat film with reduced curling.

[0077] 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, heating temperature, heating time, etc. during heat treatment according to the thickness of the hard coat layer, etc., as described above, a hard coat film with reduced curling can be obtained.

[0078] The curl amount is evaluated by placing the 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 in the plane of the film is lifted (when a curl in the opposite direction occurs), the film is placed on the 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.

[0079] [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 into a roll, and then the heat treatment may be performed offline.

[0080] For example, a hard coat composition is applied onto a film substrate to form a coating film, and the solvent is removed as necessary, and then the composition is semi-cured by light irradiation, and the laminate is wound into a roll. The laminate is then unwound and conveyed by 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 conveying speed of the film substrate during coating film formation and the conveying speed of the film substrate (laminate) during heat treatment can be set individually, 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.

[0081] The heat treatment does not necessarily have to be performed by roll-to-roll, and may be performed, for example, in a state where the laminate cut to a predetermined size is curved with the coating film-forming surface facing outward. In this case, since the laminate does not need to be continuously transported, it is not necessary to bring the laminate into contact with a roll to curve it, and for example, the laminate may be rolled into a cylindrical shape and heated in an oven or the like. EXAMPLES

[0082] The present invention will be specifically described below by comparing examples and comparative examples, but the present invention is not limited to the following examples.

[0083] [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 the mixture was subjected to devolatilization under reduced pressure at 60°C to obtain a liquid compound (siloxane resin having an alicyclic epoxy group).

[0084] A hard coat composition was obtained by blending 100 parts by weight of the above siloxane resin with 0.5 parts by weight of a photoacid generator ("CPI-101A" manufactured by San-Apro).

[0085] [Comparative Example 1] <Formation of semi-cured film> The above hard coat resin composition was applied to a biaxially oriented polyethylene terephthalate (PET) film substrate (thickness 50 μm, width 275 mm) with a width of 250 mm from a slot die coater while being transported by rolls at 0.5 m / min. The film on which the coating was formed was supported on a backup roll and irradiated with ultraviolet light (peak illuminance of the irradiated surface: 20 mW / cm ) from a mercury lamp with an output of 80 W. 2 , cumulative irradiation dose: 500mJ / cm 2 ) ultraviolet light to semi-cure the composition.

[0086] <Heat treatment> The laminate having a hard coat layer (semi-cured film) formed on the film substrate was heated for 4 minutes while being transported 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.

[0087] [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 circumferential direction being the conveying direction (MD) during coating film formation, heated in an oven at 120°C for 4 minutes, and then taken out to room temperature.

[0088] [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.

[0089] [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 at 180° along a heated roll (diameter 30 mm, temperature 180° C.) so as to be in contact with the heated roll, and 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.

[0090] [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.

[0091] [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 transported horizontally to obtain a hard coat film having a 35 μm hard coat layer on a 50 μm PET film substrate.

[0092] [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.

[0093] [Examples 7 and 8] After applying the hard coat composition and irradiating with ultraviolet light in the same manner as in Comparative Example 2, the film substrate was placed along the heated roll so as to be in contact with the film substrate, and the film substrate (the surface on which the hard coat layer was not formed) was curved so that the hard coat layer side was on the outside, and then the film was heated by the heated roll. The diameter and 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.

[0094] [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.

[0095] [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.

[0096] [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.

[0097] [Examples 11 to 13] In the same manner as in Comparative Example 3, the hard coat composition was applied and irradiated with ultraviolet light. The film substrate was then placed along a heated roll so that the film substrate was in contact with the heated roll, and the film substrate was curved so that 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. The film substrate was then heated by the heated roll.

[0098] [Evaluation] <Curl> 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 into a cylindrical shape 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 Examples 3 and 10, curling occurred along the MD with the hard coat layer formed on the outside, so the film was placed on a horizontal table with the hard coat layer formed on the bottom, and the curl was measured, and the sign of the curl amount was taken as negative.

[0099] <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.

[0100] [Evaluation Results] Table 1 shows the thickness of the hard coat layer and the heat treatment conditions, as well as the curl amount and pencil hardness of the hard coat film in the examples and comparative examples.

[0101] [Table 1]

[0102] In Comparative Examples 1 to 3, in which the film was heated while being conveyed horizontally, when the center of the MD was pressed along the direction parallel to sides AB and DC in FIG. 8 while the film was spread out flat, 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 considered that in Comparative Examples 1 to 3, the shrinkage stress during curing of the hard coat layer acts in both the MD and TD directions. Since the diameter of the cylindrical shape is smaller as the hard coat layer becomes thicker, it is considered that the effect of cure shrinkage is greater as the hard coat layer becomes thicker.

[0103] 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, it is believed that the hard coat layer was not cured sufficiently due to insufficient heating, and the curling was small due to the small curing shrinkage.

[0104] In Examples 1 to 5, in which heating was performed while the hard coat layer was curved with the surface on which the hard coat layer 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 the comparison of Comparative Example 2 with Examples 6 to 8, and in the comparison of Comparative Example 3 with Examples 9 to 13.

[0105] 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 hard coat layer formed surface on the outside. A similar tendency was observed in the comparison between Examples 9 and 10.

[0106] From these results, it can be seen that the smaller the radius of curvature during heat treatment, the stronger the action of curling outward, and this action is balanced with the cure shrinkage of the hard coat layer, resulting in a hard coat film with less curl. It can also be seen that the thicker the hard coat layer is and the larger the original curl is (when heated without curving, it curls into a cylindrical shape with a smaller diameter), the smaller the radius of curvature that is required for heat treatment to reduce curling.

[0107] In Example 4, in which the heat treatment was performed while the film was transported on a heated roll at 180°C, even though the heat treatment time (contact time with the heated roll) was short at 6 seconds, a hard coat film with little curl was obtained, similar to Example 1. Comparing Examples 1 and 4, it can be seen that when the film is transported roll-to-roll and heating is performed on the roll, heating at a high temperature for a short period of time can provide a curl reduction effect similar to that obtained when the film is cut out and then heat-treated.

[0108] In Example 5, in which the film was bent at a radius of curvature of 50 mm using a heated roll with a diameter of 100 mm and then subjected to heat treatment, a curl reduction effect was observed compared to Comparative Example 1, but the curl along the TD was greater compared to Example 4, in which the film was bent at a radius of curvature of 15 mm and then subjected to heat treatment. A similar tendency was observed in the comparison between Example 7 and Example 8, and also in the comparison between Examples 11 to 13. From these results, it can be seen that when the film is bent along a roll and then subjected to heat treatment, a hard coat film with less curl can be obtained by reducing the diameter of the roll and bending the film at a smaller radius of curvature. [Explanation of symbols]

[0109] 1 Base material 3 Hard coat layer 8. Hard Coat Film 11 Unwinding roll 81 Winding roll 30 Coating Section 33 Backup Role 31 Dice 50 Furnace 60 Light irradiation unit 65 Light source 63 Backup Roll 90,490,590,690 Heating section 91~97 Heat roll 99 Heat source

Claims

1. 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 order, 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.

2. 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.

3. 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.

4. A method for producing a hard coat film described in any one of claims 1 to 3, wherein the diameter of the heat roll is 100 mm or less.

5. 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.

6. 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 contains a photocationic polymerization initiator as the photopolymerization initiator.

7. 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.

8. 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.

9. 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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