JP2024101531A - Optical laminate and image display unit using optical laminate - Google Patents

Abstract

To provide an optical laminate that includes a liquid crystal alignment solidification layer with high birefringence and also has phase difference change in a high-temperature environment suppressed.SOLUTION: An optical laminate includes a liquid crystal alignment solidification layer having a circular polarization function or elliptic polarization function, where 5≤X≤20 and 0<Y≤20 (1) or 5<X≤20 and 0≤Y≤20 (2) hold for the amount X (pts.wt.) of a polymerization initiator and the amount Y (pts.wt.) of a cross-linking agent which are included in the liquid crystal alignment solidification layer when the liquid crystal alignment solidification layer includes 100 pts.wt. of a liquid crystal compound.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical laminate and an image display device using the optical laminate.

In recent years, image display devices such as liquid crystal display devices and electroluminescence (EL) display devices (e.g., organic EL display devices, inorganic EL display devices) have rapidly become widespread. In many image display devices, an optical laminate including a retardation film (e.g., an anti-reflection film in which a polarizing plate and a retardation film are integrated) is used. In recent years, as the demand for thinner image display devices has increased, the demand for thinner optical laminates has also increased. In order to reduce the thickness of optical laminates, the retardation layer (retardation film), which contributes greatly to the thickness, has been made thinner. A typical example of a thin retardation film is a retardation film using a liquid crystal compound. Since the birefringence (Δn) of a liquid crystal compound is significantly larger than that of a resin, the thickness required to obtain a desired in-plane retardation can be made significantly smaller than that of a stretched resin film. On the other hand, a retardation film using a liquid crystal compound has a large retardation change in a high-temperature environment. As a result, if such a retardation film is used as an anti-reflection film, the anti-reflection function of the anti-reflection film may be impaired in a high-temperature environment.

International Publication No. 2019/160016

The present invention has been made to solve the above-mentioned problems of the conventional art, and its main objective is to provide an optical laminate that includes a liquid crystal alignment solidified layer having high birefringence and in which changes in retardation are suppressed in a high-temperature environment.

[1] An optical laminate according to an embodiment of the present invention includes a liquid crystal alignment solidified layer having a circular polarization function or an elliptically polarizing function, and when the amount of the liquid crystal compound contained in the liquid crystal alignment solidified layer is taken as 100 parts by weight, the amount X (parts by weight) of the polymerization initiator and the amount Y (parts by weight) of the crosslinking agent contained in the liquid crystal alignment solidified layer satisfy the following formula (1) or (2):
5≦X≦20 and 0<Y≦20 (1)
5<X≦20 and 0≦Y≦20...(2).
[2] In the above [1], the amount X (parts by weight) of the polymerization initiator and the amount Y (parts by weight) of the crosslinking agent satisfy the following formula (3):
X+Y≦35 (3).
[3] In the above [1] or [2], the amount X (parts by weight) of the polymerization initiator and the amount Y (parts by weight) of the crosslinking agent satisfy the following formula (4):
X+Y≦25 (4).
[4] In any one of the above [1] to [3], the optical laminate further includes a polarizing plate.
[5] In the above [4], the optical laminate further comprises another retardation layer, the refractive index characteristics of which satisfy the relationship nz>nx=ny, on the opposite side of the liquid crystal alignment solidified layer from the polarizing plate.
[6] In the above [4], the liquid crystal alignment solidified layer has a first liquid crystal alignment solidified layer and a second liquid crystal alignment solidified layer; the first liquid crystal alignment solidified layer has an Re(550) of 200 nm to 300 nm, and the angle between its slow axis and the absorption axis of the polarizer of the polarizing plate is 10° to 20°; the second liquid crystal alignment solidified layer has an Re(550) of 100 nm to 200 nm, and the angle between its slow axis and the absorption axis of the polarizer of the polarizing plate is 70° to 80°.
[7] In the above [6], the optical laminate further comprises another retardation layer, the refractive index characteristics of which satisfy the relationship nz>nx=ny, on the opposite side of the liquid crystal alignment solidified layer from the polarizing plate.
[8] According to another aspect of the present invention, there is provided an image display device, the image display device comprising the optical laminate according to any one of [1] to [7] above.

According to an embodiment of the present invention, it is possible to realize an optical laminate that includes a liquid crystal alignment solidified layer having high birefringence and suppresses changes in phase difference in a high-temperature environment.

1 is a schematic cross-sectional view of an optical laminate according to one embodiment of the present invention. 2 is a schematic cross-sectional view of an optical laminate according to another embodiment of the present invention.

Representative embodiments of the present invention are described below, but the present invention is not limited to these embodiments.

(Definition of terms and symbols)
The definitions of terms and symbols used in this specification are as follows.
(1) Refractive index (nx, ny, nz)
"nx" is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis in the plane (i.e., the fast axis direction), and "nz" is the refractive index in the thickness direction.
(2) In-plane phase difference (Re)
"Re(λ)" is the in-plane retardation of a film measured with light having a wavelength of λ nm at 23° C. For example, "Re(550)" is the in-plane retardation of a film measured with light having a wavelength of 550 nm at 23° C. Re(λ) is calculated by the formula: Re=(nx-ny)×d, where d (nm) is the thickness of the film.
(3) Retardation in the thickness direction (Rth)
"Rth(λ)" is the retardation in the thickness direction of the film measured with light having a wavelength of λ nm at 23° C. For example, "Rth(550)" is the retardation in the thickness direction of the film measured with light having a wavelength of 550 nm at 23° C. Rth(λ) is calculated by the formula: Rth=(nx-nz)×d, where d (nm) is the thickness of the film.
(4) Nz Coefficient The Nz coefficient is calculated by Nz=Rth/Re.
(5) Angles When angles are referred to in this specification, unless otherwise specified, the angles include angles in both clockwise and counterclockwise directions.

A. Optical Laminate The optical laminate according to the embodiment of the present invention includes a liquid crystal alignment solidified layer having a circular polarization function or an elliptically polarizing function. The optical laminate may be, for example, a laminate in which a liquid crystal alignment solidified layer is formed on any suitable substrate, or a laminate of a liquid crystal alignment solidified layer and another optically functional film (e.g., a polarizing plate). The type, number, combination, lamination order, etc. of the other optically functional film laminated with the liquid crystal alignment solidified layer can be appropriately set according to the purpose. The laminate in which a liquid crystal alignment solidified layer is formed on a substrate may be laminated with another optically functional film as it is, or the liquid crystal alignment solidified layer may be transferred to another optically functional film and laminated. The liquid crystal alignment solidified layer may have a circular polarization function or an elliptically polarizing function by itself (e.g., a cholesteric liquid crystal layer), or may be combined with a polarizer to perform a circular polarization function or an elliptically polarizing function. The substrate preferably has an alignment restricting force. The alignment restricting force can be typically imparted by rubbing alignment, stretching substrate alignment, or photo-alignment. Preferably, it is stretching substrate alignment or photo-alignment. In this specification, the term "liquid crystal alignment solidified layer" refers to a layer in which liquid crystal compounds are aligned in a predetermined direction within the layer and the alignment state is fixed. The term "liquid crystal alignment solidified layer" is a concept that includes an alignment hardened layer obtained by hardening a liquid crystal monomer. In one embodiment of the liquid crystal alignment solidified layer, rod-shaped liquid crystal compounds are aligned in the slow axis direction of the liquid crystal alignment solidified layer (homogeneous alignment).

In an embodiment of the present invention, when the amount of the liquid crystal compound contained in the liquid crystal alignment solidified layer is 100 parts by weight, the amount X (parts by weight) of the polymerization initiator contained in the liquid crystal alignment solidified layer and the amount Y (parts by weight) of the crosslinking agent satisfy the following formula (1) or (2). With such a configuration, a liquid crystal alignment solidified layer having high birefringence and excellent heat durability can be obtained. As a result, it is possible to reduce the thickness of the liquid crystal alignment solidified layer (and thus the optical laminate), and to realize an optical laminate in which the phase difference change in a high-temperature environment is suppressed.
5≦X≦20 and 0<Y≦20 (1)
5<X≦20 and 0≦Y≦20 (2)
In either case of formula (1) or formula (2), the amount X of the polymerization initiator and the amount Y of the crosslinking agent can change while being related to each other. The amount X of the polymerization initiator and the amount Y of the crosslinking agent preferably satisfy the following formula (3), and more preferably satisfy the following formula (4). With such a configuration, it is possible to realize excellent heat durability while maintaining the Δn of the liquid crystal alignment solidified layer at a higher value. As a result, it is possible to reduce the thickness of the liquid crystal alignment solidified layer for obtaining a desired in-plane retardation, which can contribute to the thinning of the optical laminate (ultimately, the image display device to which the optical laminate is applied).
X+Y≦35...(3)
X+Y≦25...(4)
"X+Y" is more preferably 20 parts by weight or less, even more preferably 18 parts by weight or less, particularly preferably 15 parts by weight or less, and particularly preferably 13 parts by weight or less. The lower limit of "X+Y" may be, for example, 5.5 parts by weight, or may be, for example, 7.5 parts by weight. If "X+Y" is in such a range, a liquid crystal alignment solidified layer having a good balance between high Δn and excellent heat durability can be realized. The amount X of the polymerization initiator may vary depending on the amount Y of the crosslinking agent. The amount X of the polymerization initiator may be, for example, 6 parts by weight to 18 parts by weight, or may be, for example, 7 parts by weight to 15 parts by weight, or may be, for example, 8 parts by weight to 13 parts by weight. Similarly, the amount Y of the crosslinking agent may vary depending on the amount X of the polymerization initiator. The amount Y of the crosslinking agent may be, for example, 2.5 parts by weight to 20 parts by weight, or may be, for example, 4 parts by weight to 15 parts by weight, or may be, for example, 5 parts by weight to 13 parts by weight.

Below, as a representative example of an optical laminate, an optical laminate (polarizing plate with retardation layer) further including a polarizing plate will be described. First, an overview of the optical laminate (polarizing plate with retardation layer) will be described, and then the representative components (polarizing plate, liquid crystal alignment solidification layer, and another retardation layer) will be described in detail.

1 is a schematic cross-sectional view of an optical laminate according to one embodiment of the present invention; FIG. 2 is a schematic cross-sectional view of an optical laminate according to another embodiment of the present invention. The illustrated optical laminates 100 and 102 each have a polarizing plate 10 and a liquid crystal alignment solidified layer (retardation layer) 20 in this order from the top of the drawing. The top of the drawing is the viewing side, and the bottom of the drawing is the image display panel side. The polarizing plate 10 has a polarizer 11, a protective layer (viewing side protective layer) 12 provided on one side of the polarizer 11, and a protective layer (inner protective layer) 13 provided on the other side of the polarizer 11. Either the protective layer 12 or 13 may be omitted. For example, the polarizing plate may be a so-called one-sided protective polarizing plate having a polarizer and a viewing side protective layer.

As described above, the liquid crystal alignment solidified layer 20 has a circular polarization function or an elliptically polarizing function. With such a configuration, an optical laminate having excellent anti-reflection properties can be obtained. The liquid crystal alignment solidified layer 20 may be a single layer as shown in FIG. 1, or may have a laminated structure of a first liquid crystal alignment solidified layer 21 and a second liquid crystal alignment solidified layer 22 as shown in FIG. 2.

In one embodiment, the optical laminate may further have another retardation layer (not shown) on the side of the liquid crystal alignment solidified layer 20 opposite the polarizing plate 10. The other retardation layer is typically a so-called positive C plate whose refractive index characteristics show the relationship nz>nx=ny. By providing such another retardation layer, it is possible to effectively prevent reflections in oblique directions, and it is possible to widen the viewing angle of the anti-reflection function.

For practical purposes, the optical laminate has an adhesive layer (not shown) as the outermost layer on the image display panel side, and can be attached to the image display panel. In this case, it is preferable that a release liner is temporarily attached to the surface of the adhesive layer until the optical laminate is used. By temporarily attaching the release liner, the adhesive layer is protected and the optical laminate can be formed into a roll.

Below, we will explain in detail the representative components of the optical laminate (polarizing plate, liquid crystal alignment solidification layer, and another retardation layer).

B. Polarizing Plate B-1. Polarizer Any appropriate polarizer can be adopted as the polarizer 11. The polarizer is typically composed of a polyvinyl alcohol (PVA)-based resin film containing a dichroic material (e.g., iodine). Examples of PVA-based resins include polyvinyl alcohol, partially formalized polyvinyl alcohol, ethylene-vinyl alcohol copolymer, and partially saponified ethylene-vinyl acetate copolymer. For example, the resin film forming the polarizer may be a single-layer resin film or a laminate of two or more layers.

Specific examples of polarizers made of a single-layer resin film include hydrophilic polymer films such as PVA-based films, partially formalized PVA-based films, and partially saponified ethylene-vinyl acetate copolymer films that have been dyed with iodine or a dichroic substance such as a dichroic dye and stretched, and polyene-based oriented films such as dehydrated PVA and dehydrochlorinated polyvinyl chloride. Preferably, a polarizer obtained by dyeing a PVA-based film with iodine and stretching it uniaxially is used because of its excellent optical properties.

The dyeing with iodine is carried out, for example, by immersing the PVA-based film in an aqueous iodine solution. The stretching ratio of the uniaxial stretching is preferably 3 to 7 times. The stretching may be carried out after the dyeing process or while dyeing. Alternatively, the film may be stretched and then dyed. If necessary, the PVA-based film may be subjected to a swelling process, a crosslinking process, a washing process, a drying process, or the like. For example, by immersing the PVA-based film in water and washing it with water before dyeing, it is possible to wash off dirt and antiblocking agents on the surface of the PVA-based film, and also to swell the PVA-based film and prevent uneven dyeing, etc.

Specific examples of polarizers obtained using a laminate include a laminate of a resin substrate and a PVA-based resin layer (PVA-based resin film) laminated on the resin substrate, or a polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by applying a PVA-based resin solution to a resin substrate and drying the substrate to form a PVA-based resin layer on the resin substrate to obtain a laminate of the resin substrate and the PVA-based resin layer; stretching and dyeing the laminate to make the PVA-based resin layer into a polarizer. In this embodiment, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is preferably formed on one side of the resin substrate. Stretching typically involves immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, the stretching may further include air-stretching the laminate at a high temperature (e.g., 95°C or higher) before stretching in the boric acid aqueous solution, if necessary. In addition, in this embodiment, the laminate is preferably subjected to a drying shrinkage treatment in which the laminate is heated while being conveyed in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction. Typically, the manufacturing method of this embodiment includes subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. By introducing the auxiliary stretching, it is possible to increase the crystallinity of PVA even when PVA is applied onto a thermoplastic resin, and it is possible to achieve high optical properties. At the same time, by increasing the orientation of PVA in advance, problems such as a decrease in the orientation of PVA or dissolution can be prevented when the PVA is immersed in water in the subsequent dyeing step or stretching step, and it is possible to achieve high optical properties. Furthermore, when the PVA-based resin layer is immersed in a liquid, the disorder of the orientation of polyvinyl alcohol molecules and the decrease in orientation can be suppressed compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizer obtained by immersing the laminate in a liquid in a treatment process such as a dyeing process and an underwater stretching process. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by a drying shrinkage process. The obtained resin substrate/polarizer laminate may be used as it is (i.e., the resin substrate may be used as a protective layer for the polarizer), or any suitable protective layer may be laminated on the peeled surface obtained by peeling the resin substrate from the resin substrate/polarizer laminate, or on the surface opposite to the peeled surface. Details of the manufacturing method of such a polarizer are described in, for example, JP 2012-73580 A and JP 6470455 A. The entire disclosures of these publications are incorporated herein by reference.

The thickness of the polarizer is, for example, 12 μm or less, preferably 10 μm or less, more preferably 1 μm to 8 μm, and even more preferably 3 μm to 7 μm. By combining such a thin polarizer with a liquid crystal alignment solidification layer, it is possible to significantly reduce the thickness of the optical laminate. Furthermore, if the thickness of the polarizer is within the above range, curling during heating can be effectively suppressed, and good appearance durability during heating can be obtained.

The polarizer preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single transmittance of the polarizer is preferably 41.0% to 46.0%, and more preferably 42.0% to 45.0%. The degree of polarization of the polarizer is preferably 97.0% or more, more preferably 99.0% or more, and even more preferably 99.9% or more. According to an embodiment of the present invention, even if the single transmittance is in the above range, the degree of polarization can be maintained within such a range.

B-2. Protective layer The protective layers 12 and 13 are made of any suitable resin film. Representative materials for the resin film include cellulose resins such as triacetyl cellulose (TAC), cycloolefin resins such as polynorbornene, (meth)acrylic resins, polyester resins such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), polyolefin resins such as polyethylene, and polycarbonate resins. Representative examples of (meth)acrylic resins include (meth)acrylic resins having a lactone ring structure. (Meth)acrylic resins having a lactone ring structure are described in, for example, JP-A-2000-230016, JP-A-2001-151814, JP-A-2002-120326, JP-A-2002-254544, and JP-A-2005-146084. These publications are incorporated herein by reference. From the viewpoint of ease of processing into a modified shape, a cellulose-based resin is preferred, and TAC is more preferred. From the viewpoint of obtaining a polarizing plate having low moisture permeability and excellent durability, a cycloolefin-based resin and a (meth)acrylic resin are preferred.

The optical laminate is typically placed on the viewing side of an image display device, and the protective layer 12 is typically placed on the viewing side. Therefore, the protective layer 12 may be subjected to a surface treatment as necessary. Examples of surface treatments include hard coat treatment, anti-reflection treatment, anti-sticking treatment, and anti-glare treatment. Furthermore, the protective layer 12 may be subjected to a treatment (typically, imparting an (elliptical) polarizing function or imparting an ultra-high phase difference) to improve visibility when viewed through polarized sunglasses as necessary. By performing such treatments, excellent visibility can be achieved even when the display screen is viewed through polarized lenses such as polarized sunglasses. Therefore, the optical laminate can be suitably applied to image display devices that can be used outdoors.

In one embodiment, the protective layer 13 is preferably optically isotropic. In this specification, "optically isotropic" means that the in-plane retardation Re(550) is 0 nm to 10 nm, and the retardation in the thickness direction Rth(550) is -10 nm to +10 nm.

The thickness of each of the protective layers 12 and 13 is preferably 10 μm to 80 μm, more preferably 12 μm to 40 μm, and even more preferably 15 μm to 35 μm. If the protective layer 12 has been subjected to a surface treatment, the thickness of the protective layer 12 includes the thickness of the surface treatment layer.

C. Liquid crystal alignment solidified layer As described above, the liquid crystal alignment solidified layer 20 has a circular polarization function or an elliptical polarization function. Furthermore, as described above, the liquid crystal alignment solidified layer 20 may be a single layer as shown in FIG. 1, or may have a laminated structure of a first liquid crystal alignment solidified layer 21 and a second liquid crystal alignment solidified layer 22 as shown in FIG. 2. By using the liquid crystal alignment solidified layer as a retardation layer, a desired in-plane retardation can be realized with a thickness that is significantly thinner than that of a stretched film of a resin film. As a result, the optical laminate can be significantly thinner.

The birefringence Δn of the liquid crystal alignment solidified layer is preferably 0.06 or more, more preferably 0.08 or more, even more preferably 0.09 or more, and particularly preferably 0.10 or more. The upper limit of Δn may be, for example, 0.13, or may be, for example, 0.12. If Δn is in this range, the desired in-plane retardation can be achieved with a very thin thickness. As a result, it becomes possible to further thin the liquid crystal alignment solidified layer and the optical laminate, which can ultimately contribute to a significant reduction in the thickness of image display devices. According to an embodiment of the present invention, it is possible to realize a liquid crystal alignment solidified layer having such a high Δn and excellent heat durability.

Examples of liquid crystal compounds used in the liquid crystal alignment solidified layer include liquid crystal polymers and liquid crystal monomers. The liquid crystal compound is preferably polymerizable. If the liquid crystal compound is polymerizable, the alignment state of the liquid crystal compound can be fixed by aligning the liquid crystal compound and then polymerizing it. Here, the polymer formed by polymerization is non-liquid crystal. Therefore, the formed liquid crystal alignment solidified layer does not undergo transition to a liquid crystal phase, glass phase, or crystal phase due to temperature changes that are specific to liquid crystal compounds. As a result, the liquid crystal alignment solidified layer becomes a retardation layer that is not affected by temperature changes and has extremely excellent stability.

In one embodiment, the liquid crystal alignment solidified layer can be formed using a composition containing a polymerizable liquid crystal compound (polymerizable liquid crystal compound). In this specification, the polymerizable liquid crystal compound contained in the composition refers to a compound that has a polymerizable group and has liquid crystal properties. The polymerizable group means a group that participates in a polymerization reaction, and is preferably a photopolymerizable group. Here, the photopolymerizable group refers to a group that can participate in a polymerization reaction by an active radical or acid generated from a photopolymerization initiator.

The mechanism by which liquid crystal compounds exhibit liquid crystallinity may be thermotropic or lyotropic. The liquid crystal phase may be nematic or smectic. From the standpoint of ease of production, the liquid crystallinity is preferably thermotropic nematic liquid crystal.

The temperature range in which the liquid crystal monomer exhibits liquid crystallinity varies depending on the type of the monomer. Specifically, the temperature range is preferably 40°C to 120°C, more preferably 50°C to 100°C, and most preferably 60°C to 90°C.

The single layer and laminated structures are explained below.

C-1. Single layer In this embodiment, the liquid crystal alignment solidified layer 20 can function as a so-called λ/4 plate. In this case, the Re(550) of the liquid crystal alignment solidified layer 20 is preferably 100 nm to 200 nm. Furthermore, the angle between the slow axis of the liquid crystal alignment solidified layer 20 and the absorption axis of the polarizer 11 is preferably 40° to 50°, more preferably 42° to 48°, even more preferably 44° to 46°, and particularly preferably about 45°.

The Re(550) of the liquid crystal alignment solidified layer is more preferably 110 nm to 180 nm, even more preferably 120 nm to 160 nm, and particularly preferably 130 nm to 150 nm.

Since the liquid crystal alignment solidified layer has an in-plane retardation as described above, it has a relationship of nx>ny. As long as the liquid crystal alignment solidified layer has the relationship of nx>ny, it exhibits any appropriate refractive index characteristic. The refractive index characteristic of the liquid crystal alignment solidified layer typically exhibits the relationship of nx>ny≧nz. Note that "ny=nz" here includes not only the case where ny and nz are completely equal, but also the case where they are substantially equal. Therefore, there may be cases where ny<nz within a range that does not impair the effects of the embodiment of the present invention. The Nz coefficient of the liquid crystal alignment solidified layer is preferably 0.9 to 2.0, more preferably 0.9 to 1.5, and even more preferably 0.9 to 1.2. By satisfying such a relationship, when the optical laminate is used in an image display device, a very excellent reflection hue can be achieved.

The thickness of the liquid crystal alignment solidified layer can be set so that it can function most appropriately as a λ/4 plate. In other words, the thickness can be set so that a desired in-plane retardation is obtained. Specifically, the thickness can be, for example, 1.0 μm to 5.0 μm, or, for example, 1.0 μm to 3.0 μm. In this way, by using the liquid crystal alignment solidified layer as a retardation layer, a desired in-plane retardation can be achieved with a thickness that is much thinner than that of a stretched resin film.

C-2. Laminated structure When the liquid crystal alignment solidified layer has a laminated structure of a first liquid crystal alignment solidified layer and a second liquid crystal alignment solidified layer from the polarizing plate side, the first liquid crystal alignment solidified layer can typically function as a λ/2 plate, and the second liquid crystal alignment solidified layer can typically function as a λ/4 plate. Specifically, the Re(550) of the first liquid crystal alignment solidified layer is preferably 200 nm to 300 nm, more preferably 230 nm to 290 nm, and even more preferably 250 nm to 280 nm; the Re(550) of the second liquid crystal alignment solidified layer is as described in section C-1 for a single layer. The thickness of the first liquid crystal alignment solidified layer can be adjusted so that a desired in-plane retardation of the λ/2 plate is obtained. Specifically, the thickness can be, for example, 2.0 μm to 4.0 μm. The thickness of the second liquid crystal alignment solidified layer can be adjusted so that a desired in-plane retardation of the λ/4 plate is obtained. Specifically, the thickness may be, for example, 1.0 μm to 2.5 μm. In this embodiment, the angle between the slow axis of the first liquid crystal alignment solidified layer and the absorption axis of the polarizer is preferably 10° to 20°, more preferably 12° to 18°, and even more preferably 14° to 16°; the angle between the slow axis of the second liquid crystal alignment solidified layer and the absorption axis of the polarizer is preferably 70° to 80°, more preferably 72° to 78°, and even more preferably 74° to 76°. The arrangement order of the first liquid crystal alignment solidified layer and the second liquid crystal alignment solidified layer may be reversed, and the angle between the slow axis of the first liquid crystal alignment solidified layer and the absorption axis of the polarizer and the angle between the slow axis of the second liquid crystal alignment solidified layer and the absorption axis of the polarizer may be reversed. The first liquid crystal alignment solidified layer and the second liquid crystal alignment solidified layer may exhibit an inverse dispersion wavelength characteristic in which the retardation value increases depending on the wavelength of the measurement light, may exhibit a positive wavelength dispersion characteristic in which the retardation value decreases depending on the wavelength of the measurement light, or may exhibit a flat wavelength dispersion characteristic in which the retardation value changes very little depending on the wavelength of the measurement light.

The liquid crystal alignment solidified layer of this embodiment is formed, for example, using a composition containing any suitable liquid crystal monomer. As the liquid crystal monomer, for example, polymerizable mesogen compounds described in JP-A-2002-533742 (WO00/37585), EP358208 (US5211877), EP66137 (US4388453), WO93/22397, EP0261712, DE19504224, DE4408171, GB2280445, etc. can be used. Specific examples of such polymerizable mesogen compounds include BASF's product name LC242, Merck's product name E7, and Wacker-Chem's product name LC-Sillicon-CC3767. As the liquid crystal monomer, for example, a nematic liquid crystal monomer is preferable.

C-3. Polymerization initiator and crosslinking agent The liquid crystal alignment solidified layer is typically formed using a liquid crystal polymer and/or a liquid crystal monomer as described above, so the liquid crystal alignment solidified layer contains a polymerization initiator. In addition, a crosslinking agent may be used to fix the alignment state of the liquid crystal compound, so the liquid crystal alignment solidified layer may contain a crosslinking agent. The amount X of the polymerization initiator and the amount Y of the crosslinking agent satisfy the above formula (1) or (2), preferably satisfy the above formula (3), and more preferably satisfy the above formula (4).

The polymerization initiator may be a photopolymerization initiator or a thermal polymerization initiator. A photopolymerization initiator is preferable. A photopolymerization initiator has the advantage that it is possible to generate radicals only by irradiation with light without thermal cleavage. Examples of the polymerization initiator include an alkylphenone-based photopolymerization initiator, a benzophenone-based photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator, an oxime ester-based photopolymerization initiator, and a cationic photopolymerization initiator. Specific examples of the compound include 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide. The polymerization initiator may be used alone or in combination of two or more kinds.

Examples of crosslinking agents include acrylic crosslinking agents and epoxy crosslinking agents. Examples of acrylic crosslinking agents include bifunctional (meth)acrylates such as 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 2-hydroxy-3-methacrylpropyl acrylate, 2-hydroxy-1,3-dimethacryloxypropane, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, and ethoxylated bisphenol A di(meth)acrylate; Examples of the polyfunctional (meth)acrylate include trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated glycerin tri(meth)acrylate, tris-(2-acryloxyethyl)isocyanurate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, ethoxylated dipentaerythritol poly(meth)acrylate, and polypentaerythritol poly(meth)acrylate. Note that (meth)acrylate means acrylate and/or methacrylate. The crosslinking agent may be used alone or in combination of two or more.

D. Another retardation layer The other retardation layer may be a so-called positive C plate, whose refractive index characteristics show the relationship nz>nx=ny, as described above. By using a positive C plate as the other retardation layer, it is possible to effectively prevent reflection in an oblique direction, and it is possible to widen the viewing angle of the anti-reflection function. In this case, the retardation Rth(550) in the thickness direction of the other retardation layer is preferably -50 nm to -300 nm, more preferably -70 nm to -250 nm, even more preferably -90 nm to -200 nm, and particularly preferably -100 nm to -180 nm. Here, "nx=ny" includes not only the case where nx and ny are strictly equal, but also the case where nx and ny are substantially equal. That is, the in-plane retardation Re(550) of the other retardation layer may be less than 10 nm.

The separate retardation layer may be formed of any suitable material. The separate retardation layer is preferably made of a film containing a liquid crystal material fixed in homeotropic alignment. The liquid crystal material (liquid crystal compound) that can be homeotropically aligned may be a liquid crystal monomer or a liquid crystal polymer. Specific examples of the liquid crystal compound and the method of forming the retardation layer include the liquid crystal compound and the method of forming the retardation layer described in [0020] to [0028] of JP-A-2002-333642. In this case, the thickness of the separate retardation layer is preferably 0.5 μm to 10 μm, more preferably 0.5 μm to 8 μm, and even more preferably 0.5 μm to 5 μm.

E. Image display device The optical laminate described in the above items A to D can be applied to an image display device. Therefore, the embodiment of the present invention also includes an image display device using such an optical laminate. Representative examples of image display devices include liquid crystal display devices and organic EL display devices. The image display device according to the embodiment of the present invention typically includes the optical laminate described in the above items A to D on the viewing side.

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

(1) Birefringence Δn
The liquid crystal alignment solidified layer of the optical laminate obtained in the examples and comparative examples was attached to an alkali-free glass plate (45 mm x 50 mm x 0.7 mm) via an adhesive (thickness 23 μm), and then the substrate was peeled off. In this way, a test sample having a configuration of liquid crystal alignment solidified layer/adhesive layer/glass plate was obtained. The thickness of the liquid crystal alignment solidified layer of the test sample was measured with an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., "MCPD9800"). Furthermore, the Re (550) of the test sample was measured using "AXO-Scan" manufactured by AXOMETRICS Co., Ltd. The obtained Re (550) was divided by the thickness of the liquid crystal alignment solidified layer to calculate Δn.

(2) Change in Retardation The test sample obtained in the same manner as in (1) above was left in an oven at 85° C. for 500 hours, and the rate of change in retardation was calculated from the following formula: In the formula, Re(550) ₀ is the in-plane retardation before heating, and Re(550) ₅₀₀ is the in-plane retardation after heating.
Retardation change rate (%)=[{Re(550) ₅₀₀ −Re(550) ₀ }/Re(550) ₀ ]×100
In addition, when the retardation change rate is −3.5% or more (small absolute value), it is evaluated as “good”, and when it is smaller than −3.5% (large absolute value), it is evaluated as “poor”. In the vicinity of this change rate, the superiority or inferiority of the antireflection performance can become remarkable.

[Example 1: Preparation of an optical laminate having a liquid crystal alignment solidified layer/substrate structure]
1. Preparation of liquid crystal alignment solidified layer forming coating liquid A photopolymerizable liquid crystal compound exhibiting a nematic liquid crystal phase (BASF's "Paliocolor LC242", chemical formula below) was dissolved in cyclopentanone to prepare a solution with a solid content concentration of 20% by weight. A leveling agent (DIC's "Megafac F-563"), a photopolymerization initiator (IGM Resins's "Omnirad 907") and a crosslinking agent (Shin-Nakamura Chemical Co., Ltd.'s "A-DCP") were added to this solution to prepare a liquid crystal alignment solidified layer forming coating liquid. The amounts of the leveling agent, photopolymerization initiator and crosslinking agent added were 0.6 parts by weight, 7.5 parts by weight and 2.5 parts by weight, respectively, relative to 100 parts by weight of the photopolymerizable liquid crystal compound.

2. Preparation of optical laminate A stretched norbornene film (thickness: 23 μm, Re (550): 140 nm) was prepared as a substrate. The above-mentioned liquid crystal alignment solidified layer forming coating liquid was applied onto this substrate by a spin coater, and heated at 100 ° C. for 3 minutes to align the liquid crystal compound. After cooling to room temperature, the substrate was irradiated with ultraviolet light with an integrated light amount of 600 mJ / cm ² under a nitrogen atmosphere to perform photocuring, and the alignment state of the liquid crystal compound was fixed. In this way, an optical laminate having a substrate / liquid crystal alignment solidified layer (Re (550): 120 nm) configuration was prepared. The thickness of the liquid crystal alignment solidified layer was 1.5 μm, and the liquid crystal alignment solidified layer showed a refractive index distribution of nx > ny = nz. In addition, the liquid crystal alignment solidified layer showed a relationship of Re (450) > Re (550). The obtained optical laminate was subjected to the above evaluations (1) and (2). The results are shown in Table 1.

[Examples 2 to 10 and Comparative Examples 1 to 3: Preparation of optical laminates having a liquid crystal alignment solidified layer/substrate structure]
An optical laminate having a structure of substrate/liquid crystal alignment solidified layer (Re(550): 120 nm) was produced in the same manner as in Example 1, except that the amount X (parts by weight) of the photopolymerization initiator and the amount Y (parts by weight) of the crosslinking agent relative to 100 parts by weight of the liquid crystal compound were changed as shown in Table 1. The obtained optical laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. In Table 1, "-" means that no retardation layer was formed and measurement was not possible.

[Example 11: Preparation of a polarizing plate with a retardation layer]
1. Preparation of Polarizing Plate 1-1. Preparation of Polarizer As a thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used, and one side of the resin substrate was subjected to a corona treatment.
A PVA aqueous solution (coating solution) was prepared by adding 13 parts by weight of potassium iodide to 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (polymerization degree 4,200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., product name "GOHSEFFIMER") in a ratio of 9:1, and dissolving the resultant in water.
The above PVA aqueous solution was applied to the corona-treated surface of a resin substrate and dried at 60° C. to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched 2.4 times in the machine direction (longitudinal direction) in an oven at 130° C. (auxiliary in-air stretching treatment).
Next, the laminate was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insolubilizing treatment).
Next, the film was immersed in a dye bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) having a liquid temperature of 30° C. for 60 seconds while adjusting the concentration so that the single transmittance (Ts) of the finally obtained polarizer would have a desired value (dyeing treatment).
Next, the plate was immersed in a crosslinking bath (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (crosslinking treatment).
Thereafter, the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4 wt %, potassium iodide concentration: 5 wt %) at a liquid temperature of 70° C., and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 5.5 times (underwater stretching treatment).
Thereafter, the laminate was immersed in a cleaning bath (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20° C. (cleaning treatment).
Thereafter, the film was dried in an oven maintained at about 90° C., while being brought into contact with a SUS heated roll whose surface temperature was maintained at about 75° C. (drying shrinkage treatment).
In this manner, a polarizer having a thickness of about 5 μm was formed on the resin substrate, and a polarizing plate having a resin substrate/polarizer structure was obtained. The single transmittance Ts of the polarizer was 43.3%.

1-2. Preparation of Polarizing Plate An HC-TAC film was attached to the surface of the obtained polarizer (the surface opposite to the resin substrate) via an ultraviolet-curable adhesive. The HC-TAC film was a film in which a HC layer (thickness 7 μm) was formed on a triacetyl cellulose (TAC) film (thickness 25 μm), and the TAC film was attached to the polarizer side. Next, the resin substrate was peeled off to obtain a polarizing plate having a configuration of HC layer/TAC film (protective layer)/polarizer.

2. Preparation of another retardation layer 20 parts by weight of a side-chain liquid crystal polymer represented by the following chemical formula (IV) (the numbers 65 and 35 in the formula indicate the mol% of the monomer unit, and are conveniently expressed as a block polymer: weight average molecular weight 5000), 80 parts by weight of a polymerizable liquid crystal exhibiting a nematic liquid crystal phase (manufactured by BASF: trade name Paliocolor LC242), and 5 parts by weight of a photopolymerization initiator (manufactured by Ciba Specialty Chemicals: trade name Irgacure 907) were dissolved in 200 parts by weight of cyclopentanone to prepare a liquid crystal coating liquid. Then, the coating liquid was applied to a PET substrate that had been subjected to a vertical alignment treatment by a bar coater, and then the liquid crystal was aligned by heating and drying at 80 ° C. for 4 minutes. This liquid crystal layer was irradiated with ultraviolet light to harden the liquid crystal layer, and another retardation layer (thickness 3 μm) exhibiting a refractive index characteristic of nz > nx = ny was formed on the substrate.

3. Preparation of polarizing plate with retardation layer The liquid crystal alignment solidified layer of the optical laminate of Example 1 was attached to the polarizer surface of the polarizing plate obtained above via an adhesive, and then the substrate was peeled off. That is, the liquid crystal alignment solidified layer was transferred to the polarizer surface of the polarizing plate. Furthermore, another retardation layer obtained above was transferred to the liquid crystal alignment solidified layer surface via an adhesive. In this way, an optical laminate (polarizing plate with retardation layer) was obtained. When the obtained polarizing plate with retardation layer was subjected to the same evaluation as in Example 1, it was confirmed that the retardation change was small.

[Comparative Example 4: Preparation of a polarizing plate with a retardation layer]
A polarizing plate with a retardation layer was obtained in the same manner as in Example 11, except that the optical laminate (substantially a liquid crystal alignment solidified layer) of Comparative Example 1 was used. The obtained polarizing plate with a retardation layer was subjected to the same evaluation as in Example 1, and it was confirmed that the retardation change was large beyond the allowable range.

As is clear from the above examples and comparative examples, the examples of the present invention provide a liquid crystal alignment solidified layer (and, as a result, an optical laminate) that has a high Δn and excellent heat durability.

The optical laminate according to the embodiment of the present invention can be suitably used in image display devices (typically, liquid crystal display devices and organic EL display devices).

REFERENCE SIGNS LIST 10 Polarizing plate 11 Polarizer 12 Protective layer 13 Protective layer 20 Liquid crystal alignment solidified layer 21 First liquid crystal alignment solidified layer 22 Second liquid crystal alignment solidified layer 100 Optical laminate 102 Optical laminate

Claims (8)

An optical laminate comprising a liquid crystal alignment solidified layer having a circular polarization function or an elliptically polarizing function,
When the amount of the liquid crystal compound contained in the liquid crystal alignment solidified layer is taken as 100 parts by weight, the amount X (parts by weight) of the polymerization initiator and the amount Y (parts by weight) of the crosslinking agent contained in the liquid crystal alignment solidified layer satisfy the following formula (1) or (2):
5≦X≦20 and 0<Y≦20 (1)
5<X≦20 and 0≦Y≦20...(2). 2. The optical laminate according to claim 1, wherein the amount X (parts by weight) of the polymerization initiator and the amount Y (parts by weight) of the crosslinking agent satisfy the following formula (3):
X+Y≦35 (3). 3. The optical laminate according to claim 2, wherein the amount X (parts by weight) of the polymerization initiator and the amount Y (parts by weight) of the crosslinking agent satisfy the following formula (4):
X+Y≦25 (4). The optical laminate of claim 1 further comprising a polarizing plate. The optical laminate according to claim 4, further comprising another retardation layer on the opposite side of the liquid crystal alignment solidified layer from the polarizing plate, the refractive index characteristics of which satisfy the relationship nz>nx=ny. The liquid crystal alignment solidified layer has a first liquid crystal alignment solidified layer and a second liquid crystal alignment solidified layer,
The first liquid crystal alignment solidified layer has an Re(550) of 200 nm to 300 nm, and the angle between the slow axis of the first liquid crystal alignment solidified layer and the absorption axis of the polarizer of the polarizing plate is 10° to 20°,
The second liquid crystal alignment solidified layer has an Re(550) of 100 nm to 200 nm, and the angle between the slow axis of the second liquid crystal alignment solidified layer and the absorption axis of the polarizer of the polarizing plate is 70° to 80°.
The optical laminate according to claim 4. The optical laminate according to claim 6, further comprising another retardation layer on the opposite side of the liquid crystal alignment solidified layer from the polarizing plate, the refractive index characteristics of which satisfy the relationship nz>nx=ny. An image display device comprising the optical laminate according to claim 1 .

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