JP2024090853A - Optical film and polarization film using the same - Google Patents

Abstract

To provide an optical film and a polarization film having polarization dependent scattering functions with relatively simple configuration, capable of improving brightness and efficiency for light utilization of a liquid crystal display device at low cost.SOLUTION: The optical film comprises polymer matrix containing at least one or more types of polymer, and a plurality of monofilament which includes at least one or more types of polymer and is arranged substantially parallel in a longer direction in the polymer matrix. Refractive index of the monofilament in the longer direction is different from refractive index of the monofilament of the polymer matrix in a direction parallel to the longer direction. A haze value when polarization light parallel to a direction perpendicular to the longer direction of the monofilament made incident is 20% or less, transmittance when polarization light parallel to the longer direction of the monofilament is made incident is 60% or less, a diameter of the monofilament is 500 nm or less, and volume fraction in the polymer matrix of the monofilament is 10% or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical film having polarization separation performance, a manufacturing method for the optical film, and a polarizing film having the optical film. In particular, the present invention relates to an optical film suitable for liquid crystal display devices.

In LCD TVs, smartphones and other LCD display devices, the theoretical limit for the amount of light that can pass through the polarizing film on the backlight side and be taken in from the backlight to the liquid crystal layer is 50%, with the remaining 50% being absorbed by the polarizing film. There is a growing demand to reduce this absorption to increase light utilization efficiency, improve brightness, and reduce power consumption.
In response to this issue, research has been conducted into brightness-enhancing optical films that reflect polarized light absorbed by the polarizing film before it enters the film and recycles it on the backlight side, thereby improving light utilization efficiency and brightness.

For example, 3M's product DBEF (registered trademark) is commercially available. DBEF (registered trademark) has a multi-layer structure of several hundred layers, and transmits one polarized light and reflects the other polarized light by optical interference. To realize a stable optical interference function, high technology is required to control the thickness of each layer and the refractive index of each layer.
Furthermore, Patent Document 1 proposes an anisotropic scattering element in which scattering particles having an aspect ratio of 1 or more are dispersed and aligned in approximately one direction in a support medium having a different refractive index from the scattering particles. However, it is difficult to align scattering particles in approximately one direction.

Patent document 2 proposes a configuration in which composite fibers in which scattering fibers are oriented and arranged in a filler are placed in a polymer matrix. However, with this configuration, the reflected light was large and the optical performance was insufficient.

Patent Document 3 proposes a polarizing plate with a brightness-increasing function in which thermoplastic resin fibers impregnated with a dichroic dye are arranged in a single direction and fibers with optical interference function are arranged on the same directional plane and encapsulated in an optical resin. However, this configuration requires arranging multiple fibers with optical interference function and aligning the direction of the optical interference function in the same direction, making it difficult to increase the area of the polarizing plate and reduce its cost.

Patent No. 3090890 Patent No. 5209326 JP2007-233243

As described above, in liquid crystal display devices, various brightness-enhancing optical films have been investigated for the purpose of improving the light utilization efficiency of the backlight and improving the brightness of the liquid crystal display device. However, although they are technically advanced, their optical performance is still insufficient.
An object of the present invention is to provide an optical film and a polarizing film that have a polarization-dependent scattering function with a relatively simple structure and that can inexpensively improve the brightness and light utilization efficiency of a liquid crystal display device.

As a result of investigations aimed at solving the above problems, the present inventors have completed the present invention as described below.
[1]
A polymer matrix comprising at least one polymer;
a plurality of monofilaments of at least one polymer arranged longitudinally and generally parallel within the polymer matrix;
The refractive index in the longitudinal direction of the single fiber body is different from the refractive index of the polymer matrix in a direction parallel to the longitudinal direction of the single fiber,
a haze value of 20% or less when polarized light parallel to a direction perpendicular to the longitudinal direction of the single fiber body is incident thereon;
The transmittance when polarized light parallel to the longitudinal direction of the single fiber is incident is 60% or less.
The diameter of the single fiber is 500 nm or less,
The volume fraction of the single fibers in the polymer matrix is 10% or more.

[2] The optical film according to [1] above, wherein the polymer matrix is substantially water-soluble.
[3] The optical film according to [1] or [2], wherein at least one of the polymers contained in the polymer matrix is a polymer containing polyvinyl alcohol as a main component.
[4] The optical film according to [3], wherein the polymer matrix contains 20 to 100 mass % of polyvinyl alcohol.
[5] The optical film according to any one of [1] to [4], wherein at least one type of polymer contained in the single fiber has birefringence.
[6] The optical film according to any one of [1] to [5], comprising a plurality of fiber structures each having a plurality of the single fibers as a group.
[7] A polarizing film in which the optical film according to any one of [1] to [6] above and a polyvinyl alcohol film are directly laminated together.

[8] A step of producing a composite fiber having a cross-sectional sea-island structure in which a sea phase containing at least one kind of polymer and an island phase containing at least one kind of polymer are continuously present in the longitudinal direction,
bundling the composite fibers to prepare a fiber bundle;
A step of fusing the composite fibers in the fiber bundle to each other and forming them into a film;
The method for producing an optical film according to any one of [1] to [6] above,
[9] The method for producing an optical film according to [8], further comprising a step of stretching the composite fiber prior to the step of preparing the fiber bundle.

The present invention provides optical films and polarizing films that have a polarization-dependent scattering function with a simple structure consisting of multiple single fibers and a polymer material, making it possible to inexpensively improve the brightness and efficiency of liquid crystal display devices.

1 is a schematic diagram of an optical film of the present invention. 2 is a cross-sectional view taken along line AA in FIG. 1. 3 is a cross-sectional view of another example of the optical film of the present invention. 1 is a cross-sectional view of a combination of the optical film of the present invention and a polarizer. FIG. 1 is a diagram showing the scattering state of transmitted light when a laser beam polarized in a direction perpendicular to the single fiber is incident on the film of Example 1 of the present invention. FIG. 1 is a diagram showing the scattering state of transmitted light when a laser beam polarized in a direction perpendicular to the single fiber is incident on the film of Comparative Example 1 of the present invention. FIG. 13 is a diagram showing the scattering state of transmitted light when a laser beam polarized in a direction perpendicular to the single fiber is incident on the film of Comparative Example 2 of the present invention. 1A to 1C are diagrams illustrating an example of a process for obtaining an optical film of the present invention. 13 shows the results of a simulation of the transmittance of an optical film versus film thickness when the refractive index of a single fiber is changed.

The optical film of the present invention comprises a polymer matrix containing at least one type of polymer, and a plurality of monofilaments made of at least one type of polymer and arranged in the polymer matrix in a state substantially parallel to each other in the longitudinal direction.
The polymer contained in the polymer matrix (hereinafter also referred to as "polymer material A") is a polymer having excellent transparency, such as polyvinyl alcohol (PVA), modified polyvinyl alcohol butyrate (PVB), ethylene-vinyl alcohol copolymer (EVOH), polymethyl methacrylate (PMMA), polycarbonate (PC), cycloolefin polymer (COP), cycloolefin copolymer (COC), etc. Polymer material A may be two or more kinds of polymers. In the case of two or more kinds of polymers, it is preferable that these polymers have excellent compatibility with each other from the viewpoint of transparency.

The polymer matrix may have birefringence, but it is preferable that the birefringence is small or that the birefringence is opposite to that of the material constituting the single fiber. Even if the polymer matrix has birefringence, when the birefringent material is stretched and oriented, the orientation of the birefringent material is generally relaxed when heat is applied, and the difference in refractive index between the stretching direction and the direction perpendicular thereto becomes small. When the polymer matrix is water-soluble, it can be made into a film at low temperatures, so that the orientation of the birefringent material can be made into a film without relaxing, and it is easy to maintain the difference in refractive index between the stretching direction and the direction perpendicular thereto.

In addition, when the optical film of the present invention is directly laminated to a polarizer, it is preferable to use a polyvinyl alcohol film as the polarizer from the viewpoint of screen quality due to high polarization selectivity. The polymer matrix is preferably water-soluble from the viewpoint of easy lamination with polyvinyl alcohol and water paste during lamination. In order to make the polymer matrix water-soluble, it is preferable to use at least one water-soluble polymer as the polymer material A.
An example of a water-soluble polymer is polyvinyl alcohol, and polyvinyl alcohol is preferably used as the polymer material A. From the viewpoint of water solubility, it is preferable that the polymer matrix contains 20 to 100% by mass of polyvinyl alcohol as the polymer material A, with the mass of the polymer matrix being 100% by mass.

The polymer contained in the single fiber (hereinafter also referred to as "polymer material B") may be polycarbonate (PC), polystyrenes (PS), aromatic polyesters, semi-aromatic polyesters, aromatic polyamides, semi-aromatic polyamides, polyimides, polymethyl methacrylate (PMMA), etc. Polymer material B may be two or more types of polymers. Since polymer material B is stretched when it is made into a fiber structure, a polymer having positive birefringence is preferred in order to obtain a refractive index difference with the polymer matrix during stretching. As a polymer having positive birefringence, aromatic polyesters, such as polyethylene terephthalate, polynaphthalene terephthalate, and polycarbonate are preferred.

As shown in Fig. 1, the optical film 1 of the present invention has single fibers 3 arranged in a polymer matrix 2 in a substantially parallel state in the longitudinal direction. The longitudinal direction is the fiber direction of the single fibers 3 as shown by the arrows in Fig. 1. The single fibers 3 are arranged in a substantially parallel state in this longitudinal direction. The substantially parallel state refers to a state in which the longitudinal directions of the single fibers 3 are oriented in substantially the same direction, and it is preferable that the longitudinal directions of all the single fibers 3 are within a range of ±5°.
The individual monofilaments 3 do not need to be aligned in a row, but may be arranged approximately parallel to one another within the plane of the polymer matrix (within the plane of the paper in FIG. 1).

Figure 2 is a cross-sectional view taken along line A-A in Figure 1. Each single fiber 3 does not need to be arranged substantially parallel to the thickness direction of the polymer matrix, and may be oriented in a random direction. From the viewpoint of polarization efficiency when the optical film of the present invention is used in combination with a polarizer, each single fiber 3 is preferably arranged substantially parallel in the thickness direction as well. The substantially parallel state is the same as described above.

In the optical film of the present invention, the refractive index of the single fiber in the longitudinal direction is different from the refractive index of the polymer matrix in the direction parallel to the longitudinal direction of the single fiber. The difference between the two refractive indices is preferably 0.05 to 0.35, and more preferably 0.1 to 0.25. As a combination of the polymer material A and the polymer material B that results in such a refractive index difference, it is preferable that the polymer material A is polyvinyl alcohol and the polymer material B is polyester terephthalate.
The refractive index in the longitudinal direction of the single fiber is the refractive index when polarized light parallel to the longitudinal direction of the single fiber is introduced, and the refractive index in the direction parallel to the longitudinal direction of the single fiber of the polymer matrix is the refractive index when polarized light parallel to the longitudinal direction of the single fiber of the polymer matrix is introduced.

In the optical film of the present invention, the haze value when polarized light is incident parallel to the direction perpendicular to the longitudinal direction of the single fiber is 20% or less. The direction perpendicular to the longitudinal direction means a direction perpendicular to the longitudinal direction in the plane of the polymer matrix in FIG. 1 (within the plane of FIG. 1).
The haze value when polarized light parallel to the direction perpendicular to the longitudinal direction of the single fiber is incident is preferably 17% or less, and more preferably 12% or less.

In the optical film of the present invention, the transmittance is 60% or less when polarized light is incident parallel to the longitudinal direction of the single fiber. The transmittance is preferably 50% or less, and more preferably 45% or less, when polarized light is incident parallel to the longitudinal direction of the single fiber.

The fiber diameter of the single fiber is 500 nm or less, and preferably 300 nm or less. If the single fiber diameter is greater than 500 nm, light scattering occurs, leading to a deterioration in optical performance. It is more preferable that the diameter is 200 nm or less, which is half the wavelength of visible light, and even more preferable that the diameter is 100 nm or less, which is 1/4 or less, which is sufficiently smaller than the wavelength of visible light.

9 shows the results of calculations of the film thickness dependency of the transmittance for polarized light on the side to be reflected, using ray tracing software LightTools (Synopsys). The following assumptions were made in the calculations.
1. The optical film is composed of only two materials: a polymer matrix made of polymer material A, and a monofilament made of polymer material B.
2. Material absorption is zero for all materials, i.e. 1 - transmittance equals reflectance.
3. The refractive index of the polymer matrix is 1.5.
4. N is the refractive index of the single fiber for light polarized parallel to the longitudinal direction of the single fiber.
5. Single fibers with a fiber diameter of 250 nm are dispersed in a polymer matrix.
6. The volume fraction is 63% for the single fiber and 37% for the polymer matrix, with the total volume of the polymer matrix and single fiber being 100%.

The upper graph in Figure 9 shows the results when the optical film thickness is 0 to 100 μm, and the lower graph shows the results when the optical film thickness is 100 to 300 μm. As can be seen from Figure 9, as the thickness of the optical film increases, the amount of single fibers that scatter light increases, and the polarization selectivity increases. Therefore, from the viewpoint of optical performance, a thicker optical film is preferable, but a thicker optical film increases the cost. Furthermore, when the optical film of the present invention is used in a liquid crystal display, the weight and thickness of the liquid crystal display increase. For this reason, the thickness of the optical film is preferably 10 μm to 1000 μm, and more preferably 20 μm to 700 μm.

Furthermore, the larger the volume fraction of single fibers in the optical film, the greater the amount of light scattering bodies that scatter light even with the same thickness, and the higher the polarization selectivity. On the other hand, if the volume fraction of single fibers becomes too large, the distance between the single fibers becomes too close, and when the single fibers aggregate, the interface disappears and the scattering performance decreases. Therefore, the volume fraction of single fibers in the optical film of the present invention is 10% or more, preferably 10 to 80%, and more preferably 15 to 70%, assuming that the total volume of the polymer matrix and the single fibers is 100%.

The volume fraction can be calculated from the weight and specific gravity of polymer material A contained in the polymer matrix and polymer material B contained in the single fiber.

As shown in FIG. 3, the optical film of the present invention may further include a second polymer 4 (hereinafter, also referred to as "polymer material C") constituting the polymer matrix around the single fiber 3 in the polymer matrix 2. When the polymer material C is present, the refractive index difference between the polymer material A and the polymer material C is preferably 0.05 or less in order to suppress the reflection of light at the interface between the polymer material A and the polymer material C in FIG. 3. More preferably, it is 0.02 or less, and it is most preferable that the polymer material A and the polymer material C are the same polymer and the refractive index difference is 0. As shown in FIG. 3, when the cross section of the polymer matrix cut in the thickness direction is viewed, the polymer material C is preferably present so as to include a plurality of single fibers 3, and the polymer material C is preferably a fibrous fiber structure extending in the longitudinal direction of the plurality of single fibers 3. Note that when the polymer material C is fibrous extending in the longitudinal direction of the single fiber 3 and includes a plurality of single fibers 3, if the amount of polymer material A present between adjacent fibrous polymer materials C becomes too large, the area without the single fiber 3 increases, which is undesirable because it increases the scattering of light. When the polymer material C is in the form of fibers extending in the longitudinal direction of the single fibers 3 and includes a plurality of single fibers 3, the thickness between adjacent polymer materials C, i.e., the thickness between the fiber structures, is preferably 1 μm or less.
The polymer material C can be exemplified by the same materials as the polymer material A, and is preferably the same polymer.

The manufacturing method of the optical film of the present invention is shown in FIG. 8. As shown in FIG. 8(a), a plurality of composite fibers 7, each having a cross section of an island-in-sea structure, are arranged so as to be oriented approximately parallel to the same direction, and are contained in a polymer material C containing a plurality of single fibers 3 containing the polymer material B. At this time, the island phase of the island-in-sea structure contains the polymer material B, and the sea phase contains the polymer material C. The longitudinal direction of the island phase is arranged along the longitudinal direction of the composite fiber 7. Furthermore, as shown in FIG. 8(b), the composite fibers 7 are packed together so that the thickness between them is small, and the surroundings are covered with the polymer material A, which becomes the polymer matrix 2, as necessary. After that, the arrangement of the island-in-sea structure is broken by applying temperature and pressure, and a single fiber 3 arranged in a polymer matrix 2 containing the polymer material C is obtained, as shown in FIG. 8(c).

As described in JP 2014-005578 A, the composite fiber 7 is produced by melting the polymer material C that constitutes the sea component and the polymer material B that constitutes the island components separately, and spinning them from a spinneret using a melt spinning device so that they become the sea component and the island component, respectively. The cross-sectional shape and diameter of the resulting composite fiber can be set as desired by changing the shape and size of the spinneret of the melt spinning device.

The optical film of the present invention can be laminated with a polarizer to form a polarizing film. Fig. 4 shows a schematic diagram of one embodiment of a polarizing film using the optical film of the present invention.
In FIG. 4, the optical film 1 of the present invention and a polarizer 5 are laminated together, and protective films 6 are provided on both surfaces of the laminate.
4 shows a laminated embodiment.
The optical film 1 and the polarizer 5 may be laminated together by heat fusion, an adhesive, etc. When the polymer matrix of the optical film 1 is water-soluble and the polarizer 5 is made of polyvinyl alcohol, they can be laminated together by using a water-based glue.

The obtained polarizing film can be suitably used in liquid crystal display devices such as liquid crystal televisions and smartphones.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The evaluation items and methods used in the following examples and comparative examples are as follows:

Example 1
According to the description of the examples in JP2014-005578A, an island-in-sea structure fiber was obtained in which the island component fibers that became single fibers were PET (specific gravity 1.38), the fiber diameter was 250 nm, and the number of fibers was 726 fibers were oriented in polyvinyl alcohol (EXCEVAL CP4104MI, specific gravity 1.20, manufactured by Kuraray Co., Ltd.). The sea component of this island-in-sea structure fiber was 80 wt %, and the island component was 20 wt %. The obtained island-in-sea structure fiber was wound around a stainless steel plate and oriented, and then pressed at 170°C for 60 seconds to integrate a plurality of composite fibers. Thereafter, the integrated film after pressing was cut out to obtain the optical film of Example 1. The volume ratio of the island component fibers in this film was 18% calculated from the weight ratio of the island-in-sea structure fiber, and the thickness of the film was 50 μm.

Comparative Example 1
According to the description of the examples in JP2014-005578A, an island-in-sea structure fiber was obtained in which the island components that became single fibers were PET (specific gravity 1.38), the fiber diameter was 250 nm, and the number of fibers was 726 fibers were oriented in polyvinyl alcohol (Exceval CP4104MI, manufactured by Kuraray Co., Ltd., specific gravity 1.20). The sea component of this island-in-sea structure fiber was 80 wt %, and the island component was 20 wt %. The obtained island-in-sea structure fiber was wound around a glass plate and oriented, and then placed in a PET container, which was filled with an aqueous solution of polyvinyl alcohol (Exceval CP4104MI, manufactured by Kuraray Co., Ltd.) to a thickness that completely buried the fibers. Thereafter, the water was evaporated at room temperature to produce a film in which the fiber structure was oriented, and an optical film of Comparative Example 1 was obtained. In this optical film, as shown in FIG. 8(a), the island-in-sea structure fibers remained as they were, and there was a lot of scattering. The thickness of this film was 120 μm, and the ratio of island components in the sea-island structure fiber was 7%, calculated from the film thickness and the weight ratio of the sea-island structure fiber.

Comparative Example 2
According to the description of the examples in JP2014-005578A, an island-sea structure fiber was obtained in which the island components that became single fibers were PET (specific gravity 1.38), had a fiber diameter of 1.5 μm, and 36 fibers were oriented in polyvinyl alcohol (Exeval CP4104MI, specific gravity 1.20, manufactured by Kuraray Co., Ltd.). The sea component of this island-sea structure fiber was 40 wt %, and the island component was 60 wt %. The obtained island-sea structure fiber was wound around a stainless steel plate and oriented, and then pressed at 170°C for 60 seconds to integrate the multiple fibers. Thereafter, the integrated film after pressing was cut out to obtain an optical film of Comparative Example 2. The volume ratio of the island components of the island-sea structure fiber in this film was 57%, calculated from the weight ratio of the island-sea structure fiber, and the thickness of the film was 50 μm.

[measurement]
(1) Polarized Light Transmittance A slit having an opening width of 1 mm was provided in a spectrophotometer (U4100 manufactured by Hitachi High-Technologies Corporation) and polarized light parallel to the fibrous structure was made incident thereon to measure the transmittance.

(2) Total light transmittance and haze Standard calibration was performed with an aperture of 2 mm x 2 mm attached to a haze meter (NDH5000 manufactured by Nippon Denshoku). After this, a polarizing film was attached and set so that the transmission axis direction of the polarizing film and the orientation direction of the fiber structure were perpendicular to each other, and the total light transmittance and haze were measured. Note that the haze of the polarizing film is slight, and its effect on the measurement results of the haze value of the sample can be ignored. On the other hand, since one polarized light is absorbed by the polarizing film, the transmittance in this measurement method is a maximum of 50%, unlike the polarized light transmittance in (1).

(3) Scattering state A polarizing film was inserted between a He-Ne laser (manufactured by Melles Griot) with a wavelength of 633 nm and the fiber structure so that light was incident in a direction perpendicular to the orientation direction of the fiber structure. The light was projected onto a screen at a position 600 mm away from the incident position, and the scattering state was photographed.

In Example 1, polarized light parallel to the fiber structure is reflected, and polarized light perpendicular to the fiber structure is transmitted. There is little scattering of polarized light perpendicular to the fiber structure, and the light is transmitted as is. By arranging the fibers perpendicular to the transmission axis of the polarizing plate of the liquid crystal display, a display with high brightness can be obtained.

In Comparative Examples 1 and 2, light is scattered for light polarized perpendicular to the fiber structure. When used in an LCD display, the light is scattered and brightness is reduced. Because light is scattered for light polarized perpendicular to the fiber, when used in an LCD display, the light is scattered and brightness is reduced.

1 Optical film 2 Polymer matrix 3 Single fiber 4 Second polymer constituting fiber structure 5 Polarizer 6 Protective film 7 Composite fiber

Claims (9)

A polymer matrix comprising at least one polymer;
a plurality of monofilaments of at least one polymer arranged longitudinally and generally parallel within the polymer matrix;
The refractive index in the longitudinal direction of the single fiber is different from the refractive index of the polymer matrix in a direction parallel to the longitudinal direction of the single fiber,
a haze value of 20% or less when polarized light is incident in a direction perpendicular to the longitudinal direction of the single fiber;
The transmittance when polarized light parallel to the longitudinal direction of the single fiber is incident is 60% or less.
The diameter of the single fiber is 500 nm or less,
The volume fraction of the single fibers in the polymer matrix is 10% or more. The optical film of claim 1, wherein the polymer matrix is substantially water-soluble. The optical film according to claim 1 or 2, wherein at least one of the polymers contained in the polymer matrix is a polymer whose main component is polyvinyl alcohol. The optical film according to claim 3, wherein the polymer matrix contains 20 to 100% by mass of polyvinyl alcohol. The optical film according to any one of claims 1 to 4, wherein at least one type of polymer contained in the single fiber has birefringence. An optical film according to any one of claims 1 to 5, comprising a plurality of fiber structures each having a group of a plurality of the single fibers. A polarizing film in which the optical film according to any one of claims 1 to 6 and a polyvinyl alcohol film are directly laminated together. A step of producing a composite fiber having a cross-sectional sea-island structure in which a sea phase containing at least one polymer and an island phase containing at least one polymer are continuously present in the longitudinal direction;
A step of bundling a plurality of the composite fibers to prepare a fiber bundle;
A step of fusing the composite fibers in the fiber bundle to each other and forming them into a film;
A method for producing the optical film according to claim 1 , comprising: The method for producing an optical film according to claim 8, further comprising a step of stretching the composite fiber prior to the step of producing the fiber bundle.