Method for manufacturing phase difference films

Abstract

The present invention provides a phase difference film that can suppress the occurrence of display unevenness when applied to an image display device. [Solution] A solution containing a polymer and an organic solvent is applied to a support to form a coating film. The organic solvent of the coating film is dried and removed by heating to obtain a laminate in which a resin film is tightly laminated on the support. A phase difference film is obtained by stretching the laminate. During drying, a first heating is performed at 30-60°C, followed by a second heating at a higher temperature than the first heating and at least 40°C higher than the boiling point of the organic solvent. The viscosity of the solution at a temperature of 25°C is 10-40 poise.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a retardation film.

Background Art

[0002] In displays such as liquid crystal display devices, a retardation film is used for the purpose of optical compensation such as improving contrast and expanding the viewing angle. For example, in Patent Document 1, it is described that by combining a retardation film (negative B plate) having a refractive index anisotropy of nx > ny > nz and a retardation film (positive B plate) having a refractive index anisotropy of nz > nx > ny, light leakage when viewing an in-plane switching (IPS) type liquid crystal display device from an oblique direction can be reduced.

[0003] For a retardation film used for optical compensation, uniformity of film thickness and optical properties is required. Therefore, a solution coating method is widely used for forming the retardation film, and particularly for producing a film with a small thickness, the solution coating method is suitable. In the solution coating method, a resin solution (dope) in which a polymer is dissolved in a solvent is applied onto a support, and then the solvent is removed by heating and drying to form a laminate in which a resin film is closely laminated onto the support. Thereafter, if necessary, a retardation film can be obtained by stretching the resin film in at least one direction (for example, Patent Documents 2 and 3).

[0004] In Patent Document 3, it is described that by orienting the molecular chains of the polymer in the in-plane direction during solution coating, a resin film having a large birefringence in the thickness direction can be obtained, and by uniaxially stretching the laminate in which this resin film is closely laminated onto the support at the free end, a negative B plate having a refractive index anisotropy of nx > ny > nz or a positive B plate having a refractive index anisotropy of nz > nx > ny can be obtained.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-139747 [Patent Document 2] Japanese Patent Publication No. 2015-230415 [Patent Document 3] Japanese Patent Publication No. 2016-109924 [Overview of the project] [Problems that the invention aims to solve]

[0006] In image display devices that use a thin phase difference film (e.g., 20 μm or less) as an optical compensation film, display unevenness may be observed when the screen is viewed from an oblique angle. In recent years, screen resolution and brightness have increased, and even slight unevenness has become apparent as a quality issue.

[0007] Display inconsistencies in image display devices using thin phase difference films are mainly caused by in-plane variations in retardation (phase difference inconsistencies). The inventors' research revealed that inconsistencies already exist in the resin film before stretching, and these inconsistencies persist even after stretching, which is the cause of the inconsistencies in the phase difference film. It was also found that the greater the thickness of the resin film before stretching, the more pronounced the inconsistencies tend to be.

[0008] In view of the above, the present invention aims to provide a phase difference film that can form a resin film with minimal unevenness by solution deposition, thereby suppressing the occurrence of display unevenness when applied to an image display device. [Means for solving the problem]

[0009] In the present invention's method for manufacturing a phase difference film, a solution (dope) containing a polymer and an organic solvent is applied to a support to form a coating film, and the organic solvent of the coating film is dried and removed by heating to obtain a laminate in which a resin film is tightly laminated on the support. During drying, a first heating is performed at 30-60°C, followed by a second heating at a temperature higher than the first heating and higher than the boiling point of the organic solvent. The thickness of the resin film after drying is, for example, 6-20 μm.

[0010] The viscosity of the dope is 10 to 40 poise. The solid content concentration of the dope may be 10% by weight or more. The polymer may have negative intrinsic birefringence. The boiling point of the organic solvent is preferably 50 to 120°C. The support may be a resin film.

[0011] The resulting resin film may be stretched. The stretching may be performed in a laminated state in which the resin film is tightly laminated on a support. [Effects of the Invention]

[0012] The phase difference film of the present invention can suppress the occurrence of display unevenness when applied to an image display device. [Modes for carrying out the invention]

[0013] In the manufacturing process of a phase difference film, first, a solution containing a polymer and an organic solvent (dope) is applied to a support to form a coating film (coating film formation step). The laminate with the coating film formed on the support is heated to dry and remove the organic solvent from the coating film, thereby forming a laminate in which a polymer resin film is tightly laminated on the support (drying step). The resulting resin film may, if necessary, be peeled from the support and stretched in at least one direction, either as a standalone resin film or as a laminate integrated with the support (stretching step).

[0014] The thickness of the resin film obtained by drying and removing the organic solvent from the coating film is preferably 6 to 20 μm. A coating film thickness of 6 μm or more yields a phase difference film with a large absolute value of thickness directional retardation (Rth). A resin film thickness of 20 μm or less yields a phase difference film with excellent film productivity and fewer appearance defects such as coating streaks. The thickness of the resin film may be 15 μm or less, 10 μm or less, or 8 μm or less.

[0015] Dope is a solution of resin material for forming a phase difference film, and contains polymers and organic solvents.

[0016] The polymer constituting the phase difference film may have positive intrinsic birefringence or negative intrinsic birefringence. For the production of phase difference films where the refractive index nx in the thickness direction is smaller than the refractive index nx in the slow phase axis direction within the plane of the phase difference film, i.e., positive A plates (nx>ny=nz), negative C plates (nx=ny>nz), and negative B plates (nx>ny>nz), a polymer with positive intrinsic birefringence is preferably used. On the other hand, for phase difference films where the refractive index nz in the thickness direction is larger than the refractive index ny in the fast phase axis direction within the plane of the phase difference film, i.e., negative A plates (nz=nx>ny), positive C plates (nx=ny<nz)、およびポジティブBプレート(nz> For the production of (nx>ny), polymers having negative intrinsic birefringence are preferably used. Here, nx and ny are the refractive indices in the in-plane slow axis direction and fast axis direction, respectively, and nz is the refractive index in the thickness direction.

[0017] In one embodiment, a polymer having negative intrinsic birefringence is used as the polymer constituting the phase difference film. Examples of polymers having negative intrinsic birefringence include those in which chemical bonds or functional groups with high polarization anisotropy, such as aromatic or carbonyl groups, are introduced into the side chains of the polymer. Specifically, examples include acrylic resins, styrene resins, maleimide resins, fumarate ester resins, and the like.

[0018] The organic solvent is not particularly limited as long as it dissolves the polymer mentioned above without dissolving the support, and various solvents commonly used in solution film formation can be used. The boiling point of the organic solvent is preferably 50 to 120°C, but may also be 60 to 100°C. If the boiling point is excessively low, the temperature of the coating surface may decrease due to the rapid evaporation of the organic solvent, and the film may become cloudy due to condensation. If the boiling point is excessively high, it may be difficult to dry and remove the solvent, and productivity may decrease due to the increased drying time.

[0019] Specific examples of the organic solvent include ketones such as acetone (boiling point: 56°C), methyl ethyl ketone (boiling point: 80°C), methyl isopropyl ketone (boiling point: 94°C), diethyl ketone (boiling point: 102°C), methyl isobutyl ketone (boiling point: 116°C), and methyl propyl ketone (boiling point: 102°C). Two or more organic solvents may be mixed. When using a mixed solvent, it is preferable that the boiling point of the mixed solvent is within the above range.

[0020] In addition to the polymer and the organic solvent, the dope may contain additives such as a leveling agent, a plasticizer, an ultraviolet absorber, and an anti-degradant as needed.

[0021] The viscosity of the dope at a temperature of 25°C is 10 to 40 poise. When the viscosity of the dope is 10 poise or more, unevenness in a resin film with a thickness of 6 μm or more tends to be reduced. When the thickness of the resin film after drying the organic solvent is 5 μm or less, almost no unevenness occurs even when using a low-viscosity dope. However, when increasing the coating thickness (wet thickness) to form a resin film with a thickness of 6 μm or more, unevenness is likely to occur. Increasing the solid content concentration of the dope and setting the viscosity to 10 poise or more reduces the coating thickness and is considered to contribute to reducing unevenness by suppressing the generation of convection at the drying interface when drying and removing the solvent. Also, when the viscosity of the dope is 40 poise or less, appearance defects such as coating streaks can be suppressed. The viscosity of the dope may be 12 poise or more, 35 poise or less, 30 poise or less, or 27 poise or less.

[0022] From the perspective of the removal efficiency of the solvent during drying and adjusting the dope viscosity within the above range, the solid content concentration of the dope is preferably 10% by weight or more, more preferably 13% by weight or more, and may be 15% by weight or more, 16% by weight or more, or 17% by weight or more. The solid content concentration of the dope is generally 50% by weight or less, and may be 40% by weight or less, 35% by weight or less, 30% by weight or less, 25% by weight or less, 20% by weight or less, or 18% by weight or less.

[0023] Examples of the support for applying the dope include a glass substrate, a metal substrate such as SUS, a metal drum, a metal belt, a resin film, etc. When stretching the resin film after drying the solvent together with the support, a resin film is used as the support.

[0024] The resin film as the support is preferably excellent in thermal stability and mechanical strength. Examples of the resin material include polyester, polyolefin, polycycloolefin, polyamide, polycarbonate, vinyl chloride, vinylidene chloride, imide-based polymer, sulfone-based polymer, etc. Among them, a polyester resin is preferably used because it has high solvent resistance.

[0025] Examples of the polyester resin include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and polyesters in which part or all of the glycol component and / or dicarboxylic acid of the monomer units constituting these polyesters are replaced with other monomer components. From the viewpoint of enhancing the mechanical strength of the film, a crystalline polyester is preferred.

[0026] The thickness of the support is not particularly limited as long as it has both self-supporting property and flexibility. The thickness of the support is generally about 20 μm to 200 μm, preferably 30 μm to 150 μm, and more preferably 35 μm to 100 μm.

[0027] The method for applying the dope onto the support is not particularly limited, and various coating methods such as knife-over-roll coating, kiss-roll coating, gravure coating, reverse coating, spray coating, Meyer bar coating, air knife coating, curtain coating, lip coating, die coating, etc. can be applied. The coating thickness (wet thickness) may be adjusted so that the thickness of the resin film after drying falls within the above range.

[0028] After applying a dope on a support and removing the organic solvent by heating, a laminate in which a resin film is closely laminated on the support is obtained. By performing a first heating at a relatively low temperature and then a second heating at a temperature higher than the boiling point, the occurrence of unevenness is suppressed and the amount of residual solvent can be appropriately reduced.

[0029] The temperature of the first heating is preferably 30 to 60 °C, and may be 35 to 55 °C or 40 to 50 °C. Also, the temperature of the first heating is preferably below the boiling point of the organic solvent. By performing the first heating at 30 °C or higher, the occurrence of unevenness is suppressed, and the clouding of the film due to condensation or the like is suppressed, and a film with high transparency can be obtained. When the temperature of the first heating is 60 °C or lower, appearance defects such as the generation of bubbles due to the sudden boiling of the organic solvent are reduced.

[0030] The heating time (first heating) at 30 to 60 °C is preferably 20 to 100 seconds, and may be 25 to 80 seconds or 30 to 70 seconds.

[0031] After performing the first heating, the second heating is performed at a temperature higher than the heating temperature in the first heating. The heating temperature in the second heating is preferably a temperature higher than the boiling point of the organic solvent, and preferably boiling point + 40 °C or higher. By performing the second heating at a temperature higher than the boiling point of the organic solvent, the organic solvent can be efficiently removed from the coating film, and a resin film with a small amount of residual solvent can be obtained. The temperature of the second heating is preferably 230 °C or lower, more preferably 200 °C or lower, and even more preferably 180 °C or lower.

[0032] When the coating film on the support is dried, the molecular chains of the polymer tend to orient in the in-plane direction. For example, when a polymer having negative intrinsic birefringence is oriented in the plane, the refractive index nz in the thickness direction of the coating film becomes relatively large with respect to the refractive index in the plane, and a positive C-plate characteristic having a refractive index anisotropy of nx ≒ ny < nz (Rth is a negative value) is exhibited.

[0033] As described above, by applying a dope having a predetermined viscosity onto a support and performing a first heating at 30 to 60°C, a film with little unevenness and excellent appearance can be obtained. This is considered to be related to the reduction of the in-plane variation of the thickness-direction retardation Rth due to the reduction of the thickness unevenness of the resin film.

[0034] A laminate in which a resin film is closely laminated on a support, or the resin film after peeling the support from the laminate can be directly put into practical use as a retardation film. Further, the resin film may be stretched in at least one direction to impart optical anisotropy.

[0035] When the polymer constituting the resin film has negative intrinsic birefringence, as described above, the unstretched resin film has positive C-plate characteristics with refractive index anisotropy of nx≒ny<nz. When stretching is performed, the refractive index in the stretching direction becomes smaller, and a negative A-plate having refractive index anisotropy of nz>nx=ny or a positive B-plate having refractive index anisotropy of nz>nx>ny can be obtained. That is, by using a polymer having negative intrinsic birefringence, a retardation film having refractive index anisotropy of nz≧nx≧ny can be obtained.

[0036] At the time of stretching, the resin film may be stretched after peeling from the support, or the laminate in which the resin film is closely laminated on the support may be stretched. Since the thickness of the resin film is 20 μm or less and the handling property of the resin film alone is low, a method of stretching the laminate in which the resin film is closely laminated on the support without peeling the support is preferable.

[0037] The stretching method is not particularly limited, and includes longitudinal uniaxial stretching (free-end uniaxial stretching) in which the longitudinal direction is stretched without fixing both ends in the width direction of the film, transverse stretching in which both ends in the width direction of the film are gripped by a tenter clip or the like and stretched in the width direction, and simultaneous biaxial stretching in which both ends in the width direction of the film are gripped and stretched in the width direction while changing the moving speed of the gripping tool such as a tenter clip in the longitudinal direction to stretch in the longitudinal direction. Sequential biaxial stretching in which longitudinal stretching and transverse stretching are sequentially performed may also be performed.

[0038] In the case of free-end uniaxial stretching, as the film stretches longitudinally, shrinkage occurs in the width and thickness directions. Therefore, when the polymer constituting the resin film has negative intrinsic birefringence, the refractive index in the longitudinal direction (ny) decreases, while the refractive indices in the width direction (nx) and the thickness direction (nz) increase. In free-end uniaxial stretching, generally, the shrinkage rate in the width direction is equivalent to that in the thickness direction, and the reduction rate (or increase rate) of the refractive index in the width direction is equivalent to that in the thickness direction. Due to the orientation during drying on the support, when the resin film has a refractive index anisotropy of nx = ny < nz, the refractive index anisotropy of nz > nx is retained before and after stretching. Therefore, when the absolute value of Rth of the resin film is large or the stretching ratio is small, a positive B plate having a refractive index anisotropy of nz > nx > ny can be obtained by free-end uniaxial stretching.

[0039] The stretching temperature is not particularly limited, but it is preferably a temperature at which both the support and the resin film formed thereon can be stretched, and is set according to the type of polymer constituting the resin film (retardation film) and the thermal properties of the support, etc. The stretching temperature is generally about 100°C to 200°C, preferably about 120°C to 180°C.

[0040] The stretching ratio is preferably 1.01 times or more, more preferably 1.03 times or more. In free-end uniaxial stretching, the larger the stretching ratio, the greater the tendency for the front retardation Re to increase. The stretching ratio is generally 3 times or less, and may be 2.5 times or less or 2 times or less. From the perspective of suppressing display unevenness, it is preferable that the Re of the retardation film is smaller, and the stretching ratio may be 1.5 times or less, 1.3 times or less, or 1.2 times or less.

[0041] The retardation film is used for optical compensation of image display devices, etc. The retardation film obtained as described above has high in-plane uniformity and little unevenness, so it is possible to achieve a display with suppressed unevenness and improve the visibility of the image display device.

[0042] Phase difference films may be used in combination with other films such as polarizers. Examples of polarizers include hydrophilic polymer films such as polyvinyl alcohol films, partially formalized polyvinyl alcohol films, and partially saponified ethylene-vinyl acetate copolymer films, which are uniaxially stretched after adsorbing dichroic substances such as iodine or dichroic dyes, and polyene-based oriented films such as dehydrated polyvinyl alcohol or dehydrochlorinated polyvinyl chloride. Among these, polyvinyl alcohol (PVA) polarizers are preferred because they have a high degree of polarization, and are made by adsorbing dichroic substances such as iodine or dichroic dyes onto polyvinyl alcohol films and oriented in a predetermined direction.

[0043] The arrangement angle of the polarizer and the phase difference film is not particularly limited. For example, when using a phase difference film for optical compensation to suppress light loss when viewing a liquid crystal display device from an oblique direction, it is preferable to arrange them so that the absorption axis direction of the polarizer and the slow phase axis direction of the phase difference film are parallel or perpendicular. [Examples]

[0044] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0045] [Examples of polymer synthesis] In an autoclave equipped with a stirrer, condenser, nitrogen inlet tube, and thermometer, 48 parts by weight of hydroxypropyl methylcellulose (Shin-Etsu Chemical Co., Ltd., trade name Metroze 60SH-50), 15601 parts by weight of distilled water, 8161 parts by weight of diisopropyl fumarate, 240 parts by weight of 3-ethyl-3-oxetanylmethyl acrylate, and 45 parts by weight of t-butyl peroxypivalate, a polymerization initiator, were placed. After 1 hour of nitrogen bubbling, radical suspension polymerization was carried out by holding the mixture at 49°C for 24 hours while stirring. The mixture was then cooled to room temperature, and the suspension containing the resulting polymer particles was centrifuged. The obtained polymer was washed twice with distilled water and twice with methanol, and then dried under reduced pressure to obtain a white fumarate ester resin.

[0046] [Film production] The fumarate ester resin obtained in the above synthesis example was dissolved in methyl ethyl ketone to prepare solutions with the solid content concentrations shown in Table 1. Furthermore, 5 parts by weight of tributyl trimellitate was added as a plasticizer to 100 parts by weight of the fumarate ester resin to prepare a dope. The viscosity of the dope was measured at 25°C using a cone-plate viscometer (VISCOMETER RE-85U, manufactured by Toki Sangyo, standard rotor (1°34'×R24), rotation speed 3.0 rpm).

[0047] A 75 μm thick polyester film (biaxially oriented film of polyethylene-terephthalate / isophthalate copolymer) was used as the support film. While the support was being transported in one direction, the coating thickness was set so that the film thickness after drying would be the value shown in Table 1, and the above dope was applied. After heating at the first heating temperature shown in Table 1 for 50 seconds, the film was further heated at the second heating temperature (150°C) for 30 seconds to obtain a laminate in which a resin film of fumarate ester-based resin was tightly laminated on the support.

[0048] [evaluation] <film thickness> The support was peeled off from the laminate of the support and the resin film, and the thickness of the resin film was measured using a digital microgauge.

[0049] <Village> A linear polarizing plate was bonded to the resin film side of a laminate consisting of a support and a resin film, using a roll-to-roll method with an adhesive layer in between. The support was then peeled off. This sample was cut into a 20cm x 20cm square and placed on a lightbox with the resin film side facing upwards. Another linear polarizing plate was placed on top of it to create crossed nicols, and the sample was visually inspected from an azimuth angle of 45° relative to the absorption axis of the polarizing plate and a polar angle (angle with the normal of the film) of 0-40° to check for unevenness. Samples without unevenness were marked OK, and those with unevenness were marked NG.

[0050] <Transparency> The resin film was visually inspected, and those showing cloudiness were deemed unacceptable, while those that were transparent were deemed acceptable.

[0051] Table 1 shows the manufacturing conditions and evaluation results for the phase difference films in each fabrication example.

[0052] [Table 1]

[0053] In Comparative Examples 1 and 2, where a low-viscosity dope was used to form a resin film with a dry thickness of approximately 5 μm, no unevenness was observed. However, in Comparative Examples 3 and 4, where the same dope was used but the coating thickness was increased to 6 μm or more after drying, unevenness occurred. On the other hand, in Examples 1-4, 7-11, where the viscosity of the dope was increased by increasing the solid content concentration, and the coating thickness was smaller than in Comparative Examples 3 and 4, no unevenness was observed. Furthermore, in Examples 5 and 6, where a high-viscosity dope was used and the coating thickness was larger than in Comparative Examples 3 and 4, no unevenness was observed.

[0054] In Comparative Example 5, which used the same dope as in Examples 3-7 and performed initial drying at room temperature without first heating, no unevenness was observed, but the entire surface of the film was cloudy due to condensation.

[0055] These results show that by using a high-viscosity dope within a range that does not produce defects such as coating streaks, and performing initial heating (first heating) at a temperature of 30°C or higher, a film with a smooth appearance and good appearance can be obtained.

Claims

1. A coating film formation step involves applying a solution containing a polymer and an organic solvent onto a support to form a coating film; A drying step to obtain a laminate in which the polymer resin film is tightly laminated on a support by drying and removing the organic solvent from the coating film by heating; and A stretching step of stretching the resin film in at least one direction, It has the following in order: The viscosity of the aforementioned solution at a temperature of 25°C is 10 to 40 poise. In the drying process, after performing a first heating at 30 to 60°C, a second heating is performed at a temperature higher than the first heating and at least 40°C higher than the boiling point of the organic solvent. In the stretching step, the laminate in which the resin film is tightly laminated on the support is stretched. A method for manufacturing a phase difference film.

2. The method for manufacturing a phase difference film according to claim 1, wherein the thickness of the resin film is 6 to 20 μm.

3. A method for manufacturing a phase difference film according to claim 1 or 2, wherein uniaxial stretching of the free end is performed in the stretching step.

4. A method for manufacturing a phase difference film according to any one of claims 1 to 3, wherein the support is a resin film.

5. A method for producing a phase difference film according to any one of claims 1 to 4, wherein the boiling point of the organic solvent is 50 to 120°C.

6. A method for producing a phase difference film according to any one of claims 1 to 5, wherein the solid content concentration of the solution is 10% by weight or more.

7. A method for manufacturing a phase difference film according to any one of claims 1 to 6, wherein the heating time in the first heating is 20 to 150 seconds.

8. A method for producing a phase difference film according to any one of claims 1 to 7, wherein the polymer has negative intrinsic birefringence.

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