WO2016031946A1 - Brightness enhancement film and liquid crystal display device - Google Patents

Abstract

This invention provides a brightness enhancement film having a λ/4 plate and a reflective polarizer, wherein the λ/4 plate includes a resin layer I, the resin layer I includes a polymer having a negative intrinsic birefringence value, the polymer includes a repeating unit derived from an acid anhydride monomer, the polymer includes 5-50% by weight of the repeating unit derived from the acid anhydride monomer, the reflective polarizer includes a light reflection layer formed by fixing a cholesteric liquid crystalline phase, and in which the light reflection layer is a laminated layer with an alignment layer in direct contact with the λ/4 plate interposed therebetween. The invention further provides a liquid crystal display device using the brightness enhancement film. When installed in the liquid crystal display device, the brightness enhancement film yields a high brightness improvement rate and is able to suppress diagonal color change.

Description

Brightness enhancement film and liquid crystal display device

The present invention relates to a brightness enhancement film and a liquid crystal display device. More specifically, the present invention relates to a brightness enhancement film that has high brightness and can suppress oblique color change when incorporated in a liquid crystal display device, and a liquid crystal display device using the brightness enhancement film.

Flat panel displays such as liquid crystal display devices (hereinafter also referred to as LCDs) have low power consumption and are increasingly used as space-saving image display devices year by year. The liquid crystal display device has a configuration in which a backlight (hereinafter also referred to as BL), a backlight side polarizing plate, a liquid crystal cell, and a viewing side polarizing plate are provided in this order.
In the flat panel display market in recent years, development for power saving, high definition, and color reproducibility is progressing as LCD performance improvement. These performance improvements are particularly noticeable in small-sized liquid crystal display devices such as tablet PCs and smartphones.
On the other hand, in the large size handling TV applications, the next-generation high-definition (4K2K, EBU ratio 100%) of the current TV standard (FHD, NTSC (National Television System Committee) ratio 72% ≒ (EBU European Broadcasting Union) ratio 100%) Development of the above is underway, and development for power saving, high definition, and improvement of color reproducibility is progressing as performance improvement similar to the small size. Therefore, power saving, high definition, and improvement in color reproducibility of liquid crystal display devices are increasingly required.

With the power saving of the backlight, it has been proposed to provide a reflective polarizer between the backlight and the backlight side polarizing plate. The reflective polarizer is an optical element that transmits only light oscillating in a specific polarization direction among light incident while oscillating in all directions, and reflects light oscillating in other polarization directions. This makes it possible to recycle the light that is reflected without being reflected by the reflective polarizer, and the light utilization rate in the LCD can be improved.
In this regard, by combining an optical sheet member (DBEF (registered trademark) (Dual Brightness Enhancement Film), etc.) between the backlight and the backlight-side polarizing plate, the light utilization rate of the BL is improved, and the backlight A technique for improving the luminance of a light while saving power is known (see Patent Document 1).
Patent Document 2 discloses a reflective polarizing plate having a structure in which a layer formed by fixing a λ / 4 plate and a cholesteric liquid crystal phase is laminated, and a layer formed by fixing three or more cholesteric liquid crystal phases having different pitches of cholesteric liquid crystal phases. A technology for improving the light utilization rate of BL by broadening the reflection wavelength region is described.

Here, when a reflective polarizing plate having a structure in which a λ / 4 plate and a layer in which a cholesteric liquid crystal phase is fixed is stacked is incorporated in a liquid crystal display device, the oblique polarization caused by the optical characteristics of the cholesteric liquid crystal phase and the λ / 4 plate. It is known that color changes are likely to occur when viewed from the direction. On the other hand, Patent Document 3 proposes a method of setting the pitch of the cholesteric liquid crystal phase to a short pitch on the light incident side, and providing a compensation layer having a refractive index larger in the vertical direction than the in-plane refractive index. . Patent Document 4 proposes a method in which the retardation in the thickness direction of the λ / 4 plate is less than zero.

Japanese Patent No. 3448626 JP-A-1-133003 Japanese Patent No. 3518660 WO2008 / 016056 JP 2010-181710 A JP 2009-288812 A

The techniques described in Patent Documents 1 and 2 have a problem that the manufacturing cost is high due to a complicated design that takes into account the multi-layer structure and the wavelength dispersion of the members in order to improve the light utilization rate in a wide band for white light. .
On the other hand, although the liquid crystal display device using the polarizing plate which combined the layer formed by fixing the cholesteric liquid crystal phase and the λ / 4 plate described in Patent Documents 3 and 4 contributes to the improvement of the light utilization rate of BL light, The improvement in color change when viewed obliquely was insufficient.
The problem to be solved by the present invention is to provide a brightness enhancement film capable of giving a high brightness improvement rate and suppressing oblique color change when incorporated in a liquid crystal display device.

The present inventors have intensively studied, and in general, a layer having a negative Rth including a resin having a negative intrinsic birefringence is compared to a layer formed by fixing a cholesteric liquid crystal phase in which Rth is positive. We focused on using it in combination as four plates. Patent Documents 5 and 6 describe the use of a λ / 4 plate including a layer made of a resin having a negative intrinsic birefringence. However, when the present inventors tried to form a brightness enhancement film as described in Patent Document 5 or Patent Document 6, the haze increased after the heat resistance test, and the intended brightness enhancement rate was not obtained.
Therefore, the present inventors have further studied a configuration using a λ / 4 plate including a layer made of a resin having a negative intrinsic birefringence, and have completed the present invention having the following configurations [1] to [12]. Completed.
That is, the said subject is solved by this invention of the following structures.

[1] A brightness enhancement film having a λ / 4 plate and a reflective polarizer,
The λ / 4 plate includes a resin layer I,
The resin layer I includes a polymer having a negative intrinsic birefringence value,
The polymer includes repeating units derived from acid anhydride monomers,
The polymer contains 5% by mass or more and 50% by mass or less of the repeating unit derived from an acid anhydride monomer,
The reflective polarizer includes a light reflecting layer formed by fixing a cholesteric liquid crystal phase,
The brightness enhancement film, wherein the light reflection layer is a layer laminated through an alignment layer in direct contact with the λ / 4 plate.
[2] The brightness enhancement film according to [1], wherein the light reflection layer is a pitch gradient layer in which the helical pitch of the cholesteric liquid crystal phase is gradually changed in the direction of the helical axis.
[3] The brightness enhancement film according to [1] or [2], wherein the light reflecting layer is a layer obtained by curing a polymerizable liquid crystal composition including a liquid crystal compound applied to the alignment film surface.
[4] The brightness enhancement film according to [3], wherein the intrinsic birefringence Δn of the liquid crystal compound is reverse wavelength dispersion.
[5] The brightness enhancement film according to any one of [1] to [4], wherein the repeating unit derived from the acid anhydride monomer is represented by the following formula I:

[6] The brightness enhancement film according to any one of [1] to [5], wherein the polymer includes a repeating unit represented by the following formula II.

[7] The λ / 4 plate includes a resin layer II containing a compound selected from the group consisting of a polymer having an alicyclic structure, a chain polyolefin, cellulose acylate, and polyester cellulose acylate. The brightness enhancement film according to any one of [6] to [6].
[8] The brightness enhancement film according to any one of [1] to [7], wherein the λ / 4 plate includes a resin layer II, a resin layer I, and a resin layer II in this order.
[9] The light reflecting layer is a layer formed by coating on the surface of the alignment film provided on the λ / 4 plate,
The brightness enhancement film according to [7] or [8], wherein the resin layer II and the alignment layer are in direct contact, and the alignment layer and the light reflection layer are in direct contact.
[10] The brightness enhancement film according to any one of [1] to [9], wherein the λ / 4 plate has a thickness of 10 to 300 μm.
[11] The brightness enhancement film according to any one of [1] to [10], further including a positive C plate.
[12] The brightness enhancement film according to any one of [1] to [11],
A liquid crystal display device including a backlight unit, the reflective polarizer, the λ / 4 plate, a backlight unit side polarizer, a liquid crystal cell, and a viewing side polarizer in this order.

According to the present invention, it is possible to provide a brightness enhancement film capable of improving brightness and suppressing oblique color change when incorporated in a liquid crystal display device.

Hereinafter, the present invention will be described in detail.
The description of the constituent elements described below may be made based on typical embodiments of the present invention, but the present invention is not limited to such embodiments.
In this specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
In the present specification, the “half width” of a peak means the width of the peak at a peak height of 1/2.
The reflection center wavelength and half width of the light reflection layer can be obtained as follows.
When the transmission spectrum of the light reflection layer is measured using a spectrophotometer UV3150 (Shimadzu Corporation), a peak of decrease in transmittance is observed in the selective reflection region. Of the two wavelengths having a transmittance of 1/2 the maximum peak height, the wavelength value on the short wave side is λ1 (nm) and the wavelength value on the long wave side is λ2 (nm). The reflection center wavelength and the half width can be expressed by the following formula.
Reflection center wavelength = (λ1 + λ2) / 2
Half width = (λ2-λ1)

In this specification, Re (λ) and Rth (λ) represent in-plane retardation and retardation in the thickness direction at the wavelength λ, respectively. The unit is nm. Re (λ) is measured with KOBRA 21ADH or WR (manufactured by Oji Scientific Instruments Co., Ltd.) by making light having a wavelength of λ nm incident in the normal direction of the film. In selecting the measurement wavelength λnm, the wavelength selection filter can be exchanged manually, or the measurement value can be converted by a program or the like. When the film to be measured is represented by a uniaxial or biaxial refractive index ellipsoid, Rth (λ) is calculated by the following method. This measuring method is also partially used for measuring the average tilt angle on the alignment layer side of the discotic liquid crystal molecules in the optically anisotropic layer, which will be described later, and the average tilt angle on the opposite side.
Rth (λ) is Re (λ) with the in-plane slow axis (determined by KOBRA 21ADH or WR) as the tilt axis (rotation axis) (in the absence of the slow axis, in-film plane) Measure the light at a wavelength of λnm from each tilted direction in steps of 10 degrees from the normal direction to 50 ° on one side with respect to the film normal direction. KOBRA 21ADH or WR is calculated based on the measured retardation value, the assumed value of the average refractive index, and the input film thickness value. In the above case, in the case of a film having a direction in which the retardation value is zero at a certain tilt angle with the in-plane slow axis from the normal direction as the rotation axis, retardation at a tilt angle larger than the tilt angle. The value is calculated by KOBRA 21ADH or WR after changing its sign to negative. The retardation value is measured from two inclined directions with the slow axis as the tilt axis (rotation axis) (if there is no slow axis, the arbitrary direction in the film plane is the rotation axis). Rth can also be calculated from the following formula (A) and formula (B) based on the value, the assumed value of the average refractive index, and the input film thickness value.

Note that Re (θ) represents a retardation value in a direction inclined by an angle θ from the normal direction. In the formula (A), nx represents the refractive index in the slow axis direction in the plane, ny represents the refractive index in the direction orthogonal to nx in the plane, and nz is the direction orthogonal to nx and ny. Represents the refractive index. d is the film thickness.
Rth = ((nx + ny) / 2−nz) × d Expression (B)

When the film to be measured is a film that cannot be expressed by a uniaxial or biaxial refractive index ellipsoid, that is, a film having no so-called optical axis, Rth (λ) is calculated by the following method. Rth (λ) is −50 ° with respect to the normal direction of the film, using Re (λ) described above as the in-plane slow axis (determined by KOBRA 21ADH or WR) as the tilt axis (rotary axis). Then, 11 points of light having a wavelength of λ nm are incident in 10 ° steps from 1 ° to + 50 °, and the measured retardation value, average refractive index assumption and input film thickness value are used as the basis. Calculated by KOBRA 21ADH or WR. In the above measurement, as the assumed value of the average refractive index, the values in the polymer handbook (John Wiley & Sons, Inc.) and catalogs of various optical films can be used. If the average refractive index is not known, it can be measured with an Abbe refractometer. The average refractive index values of main optical films are exemplified below: cellulose acylate (1.48), cycloolefin polymer (1.52), polycarbonate (1.59), polymethyl methacrylate (1.49), Polystyrene (1.59). By inputting these assumed values of average refractive index and film thickness, KOBRA 21ADH or WR calculates nx, ny, and nz. Nz = (nx−nz) / (nx−ny) is further calculated from the calculated nx, ny, and nz.

In the light reflection layer formed by fixing the cholesteric liquid crystal phase, when the normal ordinary refractive index no and the extraordinary refractive index ne of the liquid crystal are used, the average value of the in-plane refractive index is (nx + ny) / 2 = (no + ne) / 2
It is represented by
Further, since the refractive index in the film thickness direction is no, Rth of the light reflecting layer formed by fixing the cholesteric liquid crystal phase can be expressed by the following formula. In the brightness enhancement film of the present invention, Rth of the first light reflection layer, the second light reflection layer, and the third light reflection layer adopts a value calculated using the following formula, and the first light reflection wavelength is λ nm. Rth of the light reflecting layer, the second light reflecting layer, and the third light reflecting layer is denoted as Rth (λ).
Rth = {(no + ne) / 2−no} × d = {(ne−no) / 2} × d
Note that ne and no can be measured with an Abbe refractometer.

In addition, as a method for obtaining Rth of a layer formed by fixing a cholesteric liquid crystal phase, a method using a polarization ellipso can be applied.
For example, M.M. Kimura et al. Jpn. J. et al. Appl. Phys. 48 (2009) When the ellipsometry method is used as described in 03B021, the thickness, pitch, twist angle, etc. of the layer formed by fixing the cholesteric liquid crystal phase can be obtained, and the value of Rth can be obtained therefrom. it can.

In this specification, “visible light” means light having a wavelength of 380 nm to 780 nm. Moreover, in this specification, when there is no special mention about a measurement wavelength, a measurement wavelength is 550 nm.
In this specification, an angle (for example, an angle such as “90 °”) and a relationship (for example, “orthogonal”, “parallel”, “intersection at 45 °”, etc.) are allowable in the technical field to which the present invention belongs. The range of errors to be included. For example, it means that the angle is within the range of strict angle ± 10 °, and the error from the strict angle is preferably 5 ° or less, and more preferably 3 ° or less.

In the present specification, the “absorption axis” of a polarizer or a polarizing plate means a direction having the highest absorbance.
In the present specification, the “slow axis” of a retardation film such as a λ / 4 plate means a direction in which the refractive index is maximum.
In the present specification, “polarizer” and “reflection polarizer” are distinguished from each other.
Further, in this specification, numerical values, numerical ranges, and qualitative expressions (for example, “equivalent”, “equal”, etc.) indicating optical characteristics of each member such as a retardation region, a retardation film, and a liquid crystal layer are used. ) Is interpreted to indicate numerical values, numerical ranges and properties including generally allowable errors for liquid crystal display devices and members used therefor.

[Brightness enhancement film]
The brightness enhancement film has a λ / 4 plate and a reflective polarizer. The reflective polarizer includes a light reflection layer formed by fixing a cholesteric liquid crystal phase. As will be described later, the brightness enhancement film is provided in the liquid crystal display device and has a function of improving the brightness of the liquid crystal display device.
The film thickness of the brightness enhancement film is preferably 11 to 400 μm, more preferably 15 to 200 μm, and particularly preferably 20 to 150 μm.
It is known that a light reflection layer formed by fixing a cholesteric liquid crystal phase used for a reflective polarizer of a brightness enhancement film generally has a positive Rth. In the present invention, when a light reflection layer having such a cholesteric liquid crystal phase fixed and a λ / 4 plate including a layer made of a resin having a negative intrinsic birefringence are used in combination, when viewed from an oblique direction, Suppression of the color change of the was realized.

<Λ / 4 plate>
The λ / 4 plate has an in-plane retardation Re (λ) at a specific wavelength λnm. Re (λ) = λ / 4
An optically anisotropic layer satisfying the above. In the brightness enhancement film, the λ / 4 plate functions as a layer for converting circularly polarized light obtained by passing through the reflective polarizer into linearly polarized light.
The λ / 4 plate preferably satisfies at least one of the following formulas (A) to (C), and more preferably satisfies all of the following formulas (A) to (C).
Formula (A) 450 nm / 4-35 nm <Re (450) <450 nm / 4 + 35 nm
Formula (B) 550 nm / 4-35 nm <Re (550) <550 nm / 4 + 35 nm
Formula (C) 630 nm / 4-35 nm <Re (630) <630 nm / 4 + 35 nm

The λ / 4 plate includes a resin layer I including a polymer having a negative intrinsic birefringence value. The resin layer I exhibits a negative Rth as a layer, particularly when formed through a stretching process. Therefore, a change in hue when viewed from an oblique direction can be suppressed by a combination of the resin layer I and a light reflection layer in which a cholesteric liquid crystal phase having positive Rth is fixed.

As the polymer having a negative intrinsic birefringence value, a polymer containing a repeating unit derived from an acid anhydride monomer is used. The present inventors derive from an acid anhydride monomer in a form in which a light reflecting layer is formed by coating a film on a λ / 4 plate and fixing a cholesteric liquid crystal phase using the λ / 4 plate as a support. By using the resin layer I containing a polymer containing 5% by mass or more and 50% by mass or less of repeating units, a brightness enhancement film having low haze even after a 30-second heat resistance test in an oven at 120 ° C. as a compulsory condition for the heat resistance test It was found that can be obtained.
The haze after the heat resistance test is preferably 0 to 1.0%, more preferably 0 to 0.7%, and still more preferably 0 to 0.5%.
The haze can be measured on a film sample of 40 mm × 80 mm at 25 ° C. and 60% RH with a haze meter (HGM-2DP, Suga Test Machine) according to JIS K-7136.

Although not bound by a specific theory, by using a resin layer containing a repeating unit derived from an acid anhydride monomer in the above amount, the thermal expansion coefficients of the λ / 4 plate and the light reflecting layer are matched, This is presumably because alignment defects and the like based on interlayer strain are less likely to occur. The resin layer I is a polymer having a negative intrinsic birefringence value of 90% by mass or more, 95% by mass, 98% by mass or more, 99% by mass or more, 99.5% by mass or more, 99.8% by mass or more, or 99 The resin layer I is also preferably made of a polymer having a substantially negative intrinsic birefringence value.

The acid anhydride monomer is not particularly limited, and examples thereof include maleic anhydride, citraconic anhydride, itaconic anhydride, 1,2,3,6-tetrahydrophthalic anhydride, 5-norbornene-2,3-dicarboxylic acid. Anhydrides, bicyclo [2.2.2] -5-octene-2,3-dicarboxylic anhydride, 3-methyl-1,2,6-tetrahydrophthalic anhydride, and 2-methyl-1,3, Examples include 6-tetrahydrophthalic anhydride. Of these, maleic anhydride is preferred.
The repeating unit derived from the acid anhydride monomer means a structure of a portion derived from the acid anhydride monomer in the polymer obtained by polymerization of the raw material containing the acid anhydride monomer, and the acid anhydride monomer is maleic anhydride. In the case of, it becomes the following part.

The resin layer I contains 5% by mass or more and 50% by mass or less of repeating units derived from the acid anhydride monomer with respect to the total mass of the resin layer I.
The content of the repeating unit derived from the acid anhydride monomer in the resin layer I is determined by the ATR measurement method of infrared spectroscopy. For example, in the case of a copolymer of maleic anhydride and styrene, refer to “Infrared absorption spectrum of maleic anhydride copolymer” (Fukui University Faculty of Engineering Research Report 18 (1), 173-181, 1970-03). Thus, a calibration curve using the peak intensity near 1780 cm ⁻¹ attributed to carbonyl of maleic anhydride and the peak intensity near 3050 cm ⁻¹ attributed to CH stretching of the benzene ring of styrene can be determined. Similarly, the case of using maleic anhydride other than the acid anhydride, by use of the peak in the vicinity of 1780 cm ^-1 vicinity or 1855Cm ^-1 attributable to the carbonyl of the acid anhydride to be used, and for the case of using a monomer other than styrene Similarly, by using a peak derived from the monomer to be used in a region that does not overlap with the peak attributed to the carbonyl, a calibration curve can be prepared to determine the content. For example, in the case of methyl methacrylate, it may be used α- near 1470 cm ^-1 attributable to the antisymmetric deformation methyl group, a peak near 1390 cm ^-1 attributable to symmetrical deformation.
About content of the repeating unit derived from the acid anhydride monomer in a polymer, it may be calculated | required as a ratio of the acid anhydride monomer in all the monomers at the time of polymer preparation.
The content of the repeating unit derived from the acid anhydride monomer in the polymer having a negative intrinsic birefringence value is 7% by mass to 45% by mass with respect to the total mass of the polymer having a negative intrinsic birefringence value. Is also preferable.

Other repeating units contained in the polymer having a negative intrinsic birefringence value are not particularly limited, but a vinyl aromatic monomer is used as a raw material as a repeating unit for realizing a polymer having a negative intrinsic birefringence value. The repeating unit formed is preferred. Examples of vinyl aromatic monomers include styrene, styrene derivatives such as 4-methylstyrene, 4-chlorostyrene, 3-methylstyrene, 4-methoxystyrene, 4-tert-butoxystyrene, and α-methylstyrene. Is mentioned. Of these, repeating units formed using styrene as a raw material shown below are preferred.

The polymer having a negative intrinsic birefringence value is preferably a copolymer of an acid anhydride monomer and a vinyl aromatic monomer, more preferably a copolymer of maleic anhydride and a vinyl aromatic monomer, More preferred is a copolymer of maleic anhydride and styrene.

The λ / 4 plate is also preferably a laminated film including a resin layer other than the resin layer I. The other resin layer may be a layer containing a polymer having a negative intrinsic birefringence value or a layer containing a polymer having a positive intrinsic birefringence value, but a laminated film, that is, a λ / 4 plate Preferably represents negative Rth. Examples of the other resin layer include a resin layer II containing a compound selected from the group consisting of a polymer having an alicyclic structure, a chain polyolefin, cellulose acylate, and polyester cellulose acylate.
It is also preferable that the λ / 4 plate includes the resin layer II, the resin layer I, and the resin layer II in this order.
The resin layer I and other resin layers other than the resin layer I may be bonded with an adhesive. Adhesives include acrylate, urethane, urethane acrylate, epoxy, epoxy acrylate, polyolefin, modified olefin, polypropylene, ethylene vinyl alcohol, vinyl chloride, chloroprene rubber, cyanoacrylate, polyamide A compound such as a system, a polyimide system, a polystyrene system, or a polyvinyl butyral system can be used.
Moreover, it is preferable that these layers are extended | stretched with the laminated | multilayer film after lamination | stacking.
As a specific method for producing the λ / 4 plate, reference can be made to the production method of the long wound body (A) described in JP-A-2009-288812.
The film thickness of the λ / 4 plate is preferably 10 to 300 μm, more preferably 30 to 200 μm.

<Reflective polarizer>
The reflective polarizer includes at least one light reflecting layer formed by fixing a cholesteric liquid crystal phase. The reflective polarizer preferably includes two or more light reflecting layers, more preferably includes two to four layers, and more preferably includes two or three layers. The reflective polarizer preferably includes two or more light reflecting layers having different reflection center wavelengths, and more preferably includes two or three light reflecting layers having different reflection center wavelengths.
The reflective polarizer preferably has a function of reflecting blue light, green light and red light.
The thickness of the reflective polarizer is preferably 1.5 to 60 μm, preferably 1.5 to 30 μm, more preferably 2 to 24 μm, and most preferably 2 to 18 μm.

(Light reflecting layer with fixed cholesteric liquid crystal phase)
A layer formed by fixing a cholesteric liquid crystal phase is known to selectively reflect either right-handed circularly polarized light or left-handed circularly-polarized light in a specific wavelength region and to exhibit selective reflection that transmits the other circularly polarized light. Yes. As a film including a layer in which a cholesteric liquid crystal phase exhibiting selective reflectivity is fixed, many films formed from a composition containing a polymerizable liquid crystal compound have been known (for example, Fuji Film Research Report No. 50 (2005 (Year) p.60-63), for the layer in which the cholesteric liquid crystal phase is fixed, those prior arts can be referred to.

The reflection center wavelength that gives the peak of reflectance can be adjusted by changing the pitch or refractive index of the helical structure in the cholesteric liquid crystal phase of the light reflection layer formed by fixing the cholesteric liquid crystal phase. The pitch of the helical structure can be adjusted by changing the amount of chiral agent added. The pitch is the pitch length P of the helical structure in the cholesteric liquid crystal phase, and means the thickness of the molecular layer when the orientation direction of the molecular layer of the liquid crystal compound is rotated 360 degrees. The full width at half maximum Δλ (nm) of the selective reflection band indicating selective reflection depends on the relationship of Δλ = Δn × P, where Δλ depends on the birefringence Δn of the liquid crystal compound and the pitch length P. Therefore, the width of the selective reflection band can be controlled by adjusting Δn. Δn can be adjusted by adjusting the kind of the polymerizable liquid crystal compound and the mixing ratio thereof, or by controlling the temperature at the time of fixing the alignment.

The light reflecting layer is preferably a pitch gradient layer in which the pitch of the cholesteric liquid crystal phase is gradually changed in the direction of the helical axis. By using the pitch gradient layer, the reflection wavelength region of the light reflection layer can be broadened.
In the pitch gradient method, a wide half-value width can be realized by gradually changing the pitch in the spiral direction (normal film thickness direction) of the cholesteric liquid crystal phase. In the light reflection layer to which the pitch gradient method is applied, it is preferable that the pitch continuously changes in the film thickness direction. Moreover, in the light reflection layer to which the pitch gradient method is applied, it is preferable that the pitch continuously increases or decreases continuously from one surface of the layer to the other surface. In the pitch gradient method, the concentration of a compound that does not form a spiral in the thickness direction of the liquid crystal layer is continuously changed in the thickness direction of the liquid crystal layer, or the concentration of the chiral agent is continuously changed in the thickness direction of the liquid crystal layer. Alternatively, use a chiral agent with a photoisomerization moiety, and change the HTP (helical twisting power) of the chiral agent by isomerizing the photoisomerization part of the chiral agent with UV irradiation etc. when forming the light reflection layer. To achieve this. As this photoisomerization moiety, a vinylene group, an azo group, or the like is preferable.

Specifically, for example, a weak gradient of 0.01 to 50 mJ / cm ² and heating may be alternately repeated a plurality of times to obtain a pitch gradient layer. After the reflection wavelength region is expanded by the above-described weak ultraviolet irradiation or the like, for example, a relatively strong ultraviolet ray of, for example, 50 to 10,000 mJ / cm ² is irradiated to completely polymerize the liquid crystalline compound to form a cholesteric resin layer. be able to. The above-described weak ultraviolet irradiation and strong ultraviolet irradiation may be performed in the air, or a part or all of the process may be performed in an atmosphere in which the oxygen concentration is controlled (for example, in a nitrogen atmosphere). .

As the pitch gradient method, those described in others (Nature 378, 467-469 1995) and Japanese Patent No. 4990426 can be applied. Moreover, the compound which does not form a helix and has a fluorinated alkyl group as described in Japanese Patent No. 4570377 can also be used.

The thickness of the light reflecting layer is preferably 1.5 to 20 μm, more preferably 1.5 to 10 μm, and preferably 2 to 8 μm from the viewpoints of reflectivity, orientation disorder and prevention of transmittance reduction. More preferred is 2 to 7 μm.
The light reflecting layer formed by fixing the cholesteric liquid crystal phase can be formed as a coating cured layer obtained by curing a coating film after coating a polymerizable liquid crystal composition containing a liquid crystal compound on another layer. The light reflection layer closest to the λ / 4 plate in the brightness enhancement film is formed by coating and curing on the λ / 4 plate. It is also preferred that all light reflecting layers in the brightness enhancement film are formed by coating and curing on a λ / 4 plate.
In this specification, “on a λ / 4 plate” includes the meaning of “directly on the surface of the λ / 4 plate” or “directly on the surface of another layer such as an alignment layer provided on the surface of the λ / 4 plate”. . The other layers at this time may be one layer or two or more layers.

(Polymerizable liquid crystal composition)
In addition to the liquid crystal compound, the polymerizable liquid crystal composition may contain other components such as a chiral agent, an alignment controller, a polymerization initiator, and an alignment aid.
(Liquid crystal compound)
Examples of the liquid crystal compound include a rod-like liquid crystal compound and a disk-like liquid crystal compound. In order to form a light reflecting layer formed by fixing a cholesteric liquid crystal phase exhibiting positive Rth, it is preferable to use a rod-like liquid crystal compound.
Examples of the rod-like liquid crystal compound include azomethines, azoxys, cyanobiphenyls, cyanophenyl esters, benzoic acid esters, cyclohexanecarboxylic acid phenyl esters, cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines, alkoxy-substituted phenylpyrimidines, Phenyldioxanes, tolanes and alkenylcyclohexylbenzonitriles are preferably used. In addition to the above low-molecular liquid crystalline molecules, high-molecular liquid crystalline molecules can also be used.

It is more preferable to fix the orientation of the rod-like liquid crystal compound by polymerization, and examples of the polymerizable rod-like liquid crystal compound include those described in Makromol. Chem. 190, 2255 (1989), Advanced Materials 5, 107 (1993), US Pat. Nos. 4,683,327, 5,622,648, 5,770,107, WO 95/22586, 95/24455, 97/97 No. 0600, No. 98/23580, No. 98/52905, JP-A-1-272551, JP-A-6-16616, JP-A-7-110469, JP-A-11-80081, and JP-A-2001-328773. The described compounds can be used. Further, as the rod-like liquid crystal compound, for example, those described in JP-A-11-513019 and JP-A-2007-279688 can be preferably used.

The liquid crystal compound preferably has an intrinsic birefringence Δn having reverse wavelength dispersion, that is, wavelength dispersion satisfying the following formula.
Δn (450 nm) / Δn (550 nm) <1.0
The intrinsic birefringence Δn is calculated according to p. It can be measured according to the method described in 202.
By using a liquid crystal compound in which Δn is reverse wavelength dispersibility, the color tone when the brightness enhancement film is observed from an oblique direction can be further reduced. The liquid crystal compound in which Δn is reverse wavelength dispersive is preferably contained in an amount of 50 to 100% by mass, more preferably 70 to 100% by mass, based on the total amount of the liquid crystal compound in the polymerizable liquid crystal composition. preferable.

The addition amount of the liquid crystal compound in the polymerizable liquid crystal composition is preferably 80 to 99.9% by mass with respect to the solid content mass (mass excluding the solvent) of the polymerizable liquid crystal composition, and preferably 85 to 99. It is more preferably 5% by mass, particularly preferably 90 to 99% by mass.

(Chiral agent)
As the chiral agent, various known chiral agents (for example, described in Liquid Crystal Device Handbook, Chapter 3-4-3, TN, chiral agent for STN, page 199, edited by Japan Society for the Promotion of Science, 42nd Committee, 1989) You can choose from. A chiral agent generally contains an asymmetric carbon atom, but an axially asymmetric compound or a planar asymmetric compound containing no asymmetric carbon atom can also be used as the chiral agent. Examples of the axial asymmetric compound or the planar asymmetric compound include binaphthyl, helicene, paracyclophane, and derivatives thereof. The chiral agent may have a polymerizable group. When the chiral agent has a polymerizable group and the rod-shaped liquid crystal compound used in combination also has a polymerizable group, it is derived from the rod-shaped liquid crystal compound by a polymerization reaction between the chiral agent having a polymerizable group and the polymerizable rod-shaped liquid crystal compound. Polymers having repeating units and repeating units derived from chiral agents can be formed. In this embodiment, the polymerizable group possessed by the chiral agent having a polymerizable group is preferably the same group as the polymerizable group possessed by the polymerizable rod-like liquid crystal compound. Therefore, the polymerizable group of the chiral agent is also preferably an unsaturated polymerizable group, an epoxy group or an aziridinyl group, more preferably an unsaturated polymerizable group, and an ethylenically unsaturated polymerizable group. Particularly preferred.
The chiral agent may be a liquid crystal compound.
Examples of the chiral agent exhibiting a strong twisting force include, for example, JP 2010-181852 A, JP 2003-287623 A, JP 2002-80851 A, JP 2002-80478 A, and JP 2002-302487 A. The chiral agent described in the publication can be mentioned and can be preferably used in the present invention. Furthermore, isosorbide compounds having a corresponding structure can be used for the isosorbide compounds described in these publications, and isosorbide compounds having a corresponding structure can be used for the isomannide compounds described in these publications. It can also be used.

(Orientation control agent)
Examples of the alignment control agent include compounds exemplified in [0092] and [0093] of JP-A No. 2005-99248, and [0076] to [0078] and [0082] of JP-A No. 2002-129162. To [0085], the compounds exemplified in JP-A-2005-99248, [0094] and [0095], and JP-A-2005-99248, [0096]. Are included.
As the orientation control agent, compounds described in [0082] to [0090] of JP-A No. 2014-119605 can also be used.

(Photopolymerization initiator)
Examples of photopolymerization initiators include α-carbonyl compounds (described in US Pat. Nos. 2,367,661 and 2,367,670), acyloin ether (described in US Pat. No. 2,448,828), α-hydrocarbon substituted aromatics. Group acyloin compounds (described in US Pat. No. 2,722,512), polynuclear quinone compounds (described in US Pat. Nos. 3,046,127 and 2,951,758), a combination of triarylimidazole dimer and p-aminophenyl ketone (US patent) No. 3549367), acridine and phenazine compounds (JP-A-60-105667, US Pat. No. 4,239,850) and oxadiazole compounds (US Pat. No. 4,221,970), acylphosphine Oxide compounds (Japanese Patent Publication No. 63-40) No. 799, JP-B-5-29234, JP-A-10-95788, JP-A-10-29997) and the like.

(solvent)
The polymerizable liquid crystal composition may contain a solvent. As the solvent, an organic solvent is preferably used. Examples of organic solvents include amides (eg N, N-dimethylformamide), sulfoxides (eg dimethyl sulfoxide), heterocyclic compounds (eg pyridine), hydrocarbons (eg benzene, hexane), alkyl halides (eg , Chloroform, dichloromethane), esters (eg, methyl acetate, butyl acetate), ketones (eg, acetone, methyl ethyl ketone, cyclohexanone), ethers (eg, tetrahydrofuran, 1,2-dimethoxyethane). Alkyl halides and ketones are preferred. Two or more organic solvents may be used in combination.

(Application and curing of polymerizable liquid crystal composition)
The application of the polymerizable liquid crystal composition is carried out by using a suitable liquid crystal composition such as a roll coating method, a gravure printing method, a spin coating method, etc. It can be performed by a method of developing by a method. Furthermore, it can be performed by various methods such as a wire bar coating method, an extrusion coating method, a direct gravure coating method, a reverse gravure coating method, and a die coating method. In addition, a coating film can be formed by discharging a liquid crystal composition from a nozzle using an inkjet apparatus.

Thereafter, the polymerizable liquid crystal composition is cured to fix the alignment state of the molecules of the liquid crystal compound. Curing is preferably carried out by a polymerization reaction of a polymerizable group introduced into a liquid crystal molecule.
The coating film may be dried by a known method after the application of the polymerizable liquid crystal composition and before the polymerization reaction for curing. For example, it may be dried by standing or may be dried by heating.
The liquid crystal compound molecules in the polymerizable liquid crystal composition only need to be aligned in the steps of applying and drying the polymerizable liquid crystal composition.

For example, in an embodiment in which the polymerizable liquid crystal composition is prepared as a coating liquid containing a solvent, the coating film may be dried and the solvent may be removed to obtain a cholesteric liquid crystal phase. Further, heating at a transition temperature to the cholesteric liquid crystal phase may be performed. For example, the cholesteric liquid crystal phase can be stably formed by heating to the temperature of the isotropic phase and then cooling to the cholesteric liquid crystal phase transition temperature. The liquid crystal phase transition temperature of the aforementioned polymerizable liquid crystal composition is preferably in the range of 10 to 250 ° C., more preferably in the range of 10 to 150 ° C., from the viewpoint of production suitability and the like. When the temperature is lower than 10 ° C., a cooling step or the like may be required to lower the temperature to a temperature range exhibiting a liquid crystal phase. When the temperature exceeds 200 ° C., a high temperature is required to make the isotropic liquid state higher than the temperature range once exhibiting the liquid crystal phase, which is disadvantageous from waste of thermal energy, deformation of the substrate, and alteration.

The polymerization reaction includes a thermal polymerization reaction using a thermal polymerization initiator and a photopolymerization reaction using a photopolymerization initiator. A photopolymerization reaction is preferred. It is preferable to use ultraviolet rays for light irradiation for polymerization of liquid crystalline molecules. The irradiation energy is preferably 20 mJ / cm ² to 50 J / cm ² , and more preferably 100 to 800 mJ / cm ² . In order to accelerate the photopolymerization reaction, light irradiation may be performed under heating conditions.

In order to accelerate the curing reaction, ultraviolet irradiation may be performed under heating conditions. In particular, when forming the light reflecting layer, it is preferable to maintain the temperature at the time of ultraviolet irradiation within a temperature range exhibiting a cholesteric liquid crystal phase so that the cholesteric liquid crystal phase is not disturbed.
Also, since the oxygen concentration in the atmosphere is related to the degree of polymerization, if the desired degree of polymerization is not reached in the air and the film strength is insufficient, the oxygen concentration in the atmosphere is reduced by a method such as nitrogen substitution. It is preferable. A preferable oxygen concentration is preferably 10% or less, more preferably 7% or less, and most preferably 3% or less. The reaction rate of the curing reaction (for example, polymerization reaction) that proceeds by irradiation with ultraviolet rays is 70% or more from the viewpoint of maintaining the mechanical strength of the layer and suppressing unreacted substances from flowing out of the layer. Preferably, it is 80% or more, more preferably 90% or more. In order to improve the reaction rate, a method of increasing the irradiation amount of ultraviolet rays to be irradiated and polymerization under a nitrogen atmosphere or heating conditions are effective. Moreover, after superposing | polymerizing once, the method of hold | maintaining at a temperature higher than superposition | polymerization temperature, and pushing a reaction further by thermal polymerization reaction, and the method of irradiating an ultraviolet-ray again can also be used. The reaction rate can be measured by comparing the absorption intensity of the infrared vibration spectrum of a reactive group (for example, a polymerizable group) before and after the reaction proceeds.

It is sufficient that the optical properties based on the orientation of the liquid crystal compound molecules of the polymerizable liquid crystal composition, for example, the optical properties of the cholesteric liquid crystal phase are retained in the layer, and the cured λ / 4 plate or light reflection The liquid crystal composition of the layer no longer needs to exhibit liquid crystallinity. For example, the liquid crystal composition may have a high molecular weight due to a curing reaction and may no longer have liquid crystallinity.

In the formation of the light reflection layer, the cholesteric liquid crystal phase is fixed by the above-described curing, and the light reflection layer is formed. Here, the state in which the liquid crystal phase is “fixed” is the most typical and preferred mode in which the orientation of the liquid crystal compound in the cholesteric liquid crystal phase is maintained. However, it is not limited to this. Specifically, in a temperature range of 0 ° C. to 50 ° C., or -30 ° C. to 70 ° C. under severe conditions, this layer has no fluidity and is oriented by an external field or external force. It shall mean a state in which the fixed orientation form can be kept stable without causing a change in form.
Other methods for producing a light reflecting layer having a fixed cholesteric liquid crystal phase include, for example, the methods described in JP-A-1-133003, JP-A-3416302, JP-A-3363565, and JP-A-8-271731. You may refer to it.

The liquid crystal film formed by fixing the cholesteric liquid crystal phase is formed using the λ / 4 plate itself included in the brightness enhancement film as a support. The polymerizable liquid crystal composition may be applied directly to the surface of the λ / 4 plate, or the polymerizable liquid crystal composition may be applied directly to the alignment layer formed on the λ / 4 plate. It is particularly preferable to apply to the alignment layer surface.
In the case of forming two or more light reflecting layers, the second and subsequent light reflecting layers may be formed by applying a polymerizable liquid crystal composition on the surface of the previously formed light reflecting layer. An alignment layer may be formed on the surface of the layer, and the polymerizable liquid crystal composition may be applied to the surface of the formed alignment layer. The second and subsequent light reflecting layers may be formed on another support and bonded to the previously formed reflecting layer.
When two or more light reflecting layers are formed, it is preferable to use a combination that reflects circularly polarized light in the same direction. Thereby, it is possible to align the phase states of the circularly polarized light reflected by the respective layers and prevent different polarization states in the respective wavelength ranges, thereby increasing the light use efficiency.

<Alignment layer>
The brightness enhancement film includes an alignment layer. The alignment layer is in direct contact with the λ / 4 plate. The alignment layer is used for aligning the molecules of the liquid crystal compound in the polymerizable composition when forming the light reflecting layer.
The alignment layer can be provided by means such as a rubbing treatment of an organic compound (preferably a polymer), oblique vapor deposition of an inorganic compound such as SiO, or formation of a layer having microgrooves. Furthermore, an alignment layer in which an alignment function is generated by application of an electric field, application of a magnetic field, or light irradiation is also used.
When the light reflecting layer is laminated directly on the light reflecting layer, the lower light reflecting layer may act as an alignment layer, and the liquid crystal compound for producing the upper light reflecting layer may be aligned. In such a case, the upper liquid crystal compound can be aligned without providing an alignment layer or without performing a special alignment process (for example, rubbing process).
Hereinafter, a rubbing alignment layer used by rubbing the surface as a preferred example will be described.

(Rubbing alignment layer)
Examples of the polymer that can be used for the rubbing treatment oriented layer include, for example, a methacrylate copolymer, a styrene copolymer, a polyolefin, polyvinyl alcohol, and the like described in paragraph No. [0022] of JP-A-8-338913. Examples include modified polyvinyl alcohol, poly (N-methylolacrylamide), polyester, polyimide, vinyl acetate copolymer, carboxymethylcellulose, and polycarbonate. Silane coupling agents can be used as the polymer. Water-soluble polymers (eg, poly (N-methylolacrylamide), carboxymethylcellulose, gelatin, polyvinyl alcohol, modified polyvinyl alcohol) are preferred, gelatin, polyvinyl alcohol and modified polyvinyl alcohol are more preferred, and polyvinyl alcohol and modified polyvinyl alcohol are most preferred. .

The aforementioned composition is applied to the rubbing-treated surface of the alignment layer to align the molecules of the liquid crystal compound. After that, if necessary, the alignment layer polymer and the polyfunctional monomer contained in the optically anisotropic layer are reacted, or the alignment layer polymer is crosslinked using a crosslinking agent, thereby the optical anisotropy described above. A layer can be formed.
The film thickness of the alignment layer is preferably in the range of 0.1 to 10 μm.

-Rubbing treatment-
The surface of the alignment layer, temporary support, λ / 4 plate, or light reflection layer to which the polymerizable liquid crystal composition is applied may be rubbed as necessary. The rubbing treatment can be generally performed by rubbing the surface of a film containing a polymer as a main component with paper or cloth in a certain direction. A general method of rubbing is described in, for example, “Liquid Crystal Handbook” (issued by Maruzen, October 30, 2000).

As a method for changing the rubbing density, a method described in “Liquid Crystal Handbook” (published by Maruzen) can be used. The rubbing density (L) is quantified by the following formula (A).
Formula (A) L = Nl (1 + 2πrn / 60v)
In the formula (A), N is the number of rubbing, l is the contact length of the rubbing roller, r is the radius of the roller, n is the number of rotations (rpm) of the roller, and v is the stage moving speed (second speed).

In order to increase the rubbing density, the rubbing frequency should be increased, the contact length of the rubbing roller should be increased, the radius of the roller should be increased, the rotation speed of the roller should be increased, and the stage moving speed should be decreased, while the rubbing density should be decreased. To do this, you can reverse this. In addition, the description in Japanese Patent No. 4052558 can also be referred to as conditions for the rubbing process.

<Positive C plate>
The brightness enhancement film may include a positive C plate in addition to the reflective polarizer and the λ / 4 plate.
The positive C plate is a film having a refractive index satisfying nz> nx = ny. The positive C plate can be produced by applying a homeotropic alignment liquid crystal composition to a substrate having vertical alignment ability and solidifying or curing the composition. By using a positive C plate in addition to the light reflecting layer having a positive Rth and the λ / 4 plate having a negative Rth, it is possible to further suppress the color change when viewed from an oblique direction.

[Application of brightness enhancement film]
When the brightness enhancement film is provided in the liquid crystal display device, the brightness enhancement film is provided between the backlight unit and the backlight unit side polarizer. At this time, the backlight unit, the reflective polarizer, the λ / 4 plate, the backlight unit side polarizer, the liquid crystal cell, and the viewing side polarizer may be arranged in this order.
When the brightness enhancement film is incorporated in a liquid crystal display device, the brightness enhancement film improves the brightness of the liquid crystal display device by the following mechanism.

The light reflecting layer formed by fixing the cholesteric liquid crystal phase contained in the reflective polarizer in the brightness enhancement film has at least one of right circularly polarized light and left circularly polarized light (circularly polarized light in the first polarization state) having a reflection center wavelength. Reflects in the nearby wavelength band and transmits the other (circularly polarized light in the second polarization state). The reflected circularly polarized light in the second polarization state is randomized in its direction and polarization state by a reflection member (also referred to as a light guide or an optical resonator), which will be described later, and is recycled. Again, part of the light is reflected as circularly polarized light in the first polarization state and the remaining part is transmitted as circularly polarized light in the second polarization state, thereby increasing the light utilization rate on the backlight side and increasing the brightness of the liquid crystal display device. Can be improved.
The light emitted from the reflective polarizer, that is, the polarization state of the transmitted light and the reflected light of the reflective polarizer can be measured, for example, by measuring the polarization with an Axoscan from Axometrics.

In the liquid crystal display device, the angle formed by the slow axis of the λ / 4 plate of the brightness enhancement film and the absorption axis of the backlight unit side polarizer may be 30 to 60 °. In addition to the known polarizing plate protective film, an adhesive layer may be disposed between the λ / 4 plate and the backlight unit side polarizer. For example, a polarizing plate having a polarizing plate protective film on both sides or one side of a polarizer and a λ / 4 plate of a brightness enhancement film may be bonded with an adhesive. Alternatively, the brightness enhancement film and the backlight unit side polarizer may be provided as an optical sheet member in which these are integrated, and in the optical sheet member, there is another between the λ / 4 plate and the polarizer. There may be a layer, but it may be in direct contact.

There are no particular limitations on the liquid crystal cell, backlight unit, polarizer (polarizing plate), polarizing plate protective film, etc. that constitute the liquid crystal display device, and any known or commercially available product can be used without any limitation. Can do. It is of course possible to provide a known intermediate layer such as an adhesive layer between the layers.
From the above mechanism, the liquid crystal display device converts and reflects the polarization state of the light emitted from the light source and reflected by the brightness enhancement film or the optical sheet member at the rear of the light source. It is preferable to include a reflecting member. In addition, the backlight unit preferably includes a known diffusion plate, diffusion sheet, prism sheet (for example, BEF), and a light guide. There is no restriction | limiting in particular as these members, A well-known thing can be used, For example, what is described in patent 3416302, patent 3363565, patent 4091978, patent 3448626 etc. can be used.

Between the brightness enhancement film and the backlight unit, a layer that disturbs the polarization state of the light reflected from the light reflection layer (for example, a highly retardation film such as a stretched PET film) can be used to improve the brightness. preferable. More preferably, the relationship between the average refractive index of the layer disturbing the polarization state of the light reflected from the light reflecting layer and the average refractive index of the light reflecting layer closest to the backlight in the brightness enhancement film satisfies the following formula.
0 <average refractive index of layer disturbing polarization state of light reflected from light reflecting layer−average refractive index of third light reflecting layer <0.2

Hereinafter, the features of the present invention will be described more specifically with reference to examples and comparative examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

In the following examples, each numerical value was measured by the following method.
(Haze)
For the measurement of haze, a film sample of 40 mm × 80 mm was measured at 25 ° C. and 60% RH with a haze meter (HGM-2DP, Suga Test Instruments) in accordance with JIS K-7136.
(Re of film)
Using an automatic birefringence meter (manufactured by Oji Scientific Instruments, KOBRA-21ADH), measurement was performed at a wavelength of 550 nm at intervals of 50 mm in the width direction at a distance of 50 mm and in the flow direction at a length of 1000 mm. The in-plane direction retardation Re was determined by averaging all measurement results.

(Measurement of luminance improvement rate)
The front luminance of the liquid crystal display device was measured by the method described in JP 2009-93166 [0180]. That is, using the measuring device (EZ-Contrast 160D, manufactured by ELDIM), the measured front luminance during white display was measured. In addition, as a reference, the luminance was measured in the same manner even when there was no luminance enhancement film. The ratio of the brightness when the brightness enhancement film to be measured was placed to the brightness of the reference was determined as the brightness enhancement rate.

(Evaluation of oblique color change)
The oblique color change Δu′v ′ of the liquid crystal display device was evaluated by the following method. The hue color difference Δu′v ′ obtained by calculating the difference between the hue coordinates u ′ and v ′ in the front direction (polar angle 0 degree) and the polar angle direction 60 degrees is measured in the azimuth angle 0 to 360 degrees direction, and the average The value was used as an evaluation index of the diagonal color change Δu′v ′. A measuring machine (EZ-Contrast 160D, manufactured by ELDIM) was used for measuring the color coordinates u′v ′. Based on the results, evaluation was made according to the following criteria.
7: 40% or less less than the oblique color change of the liquid crystal display device of Comparative Example 101.
6: 35% or more and less than 40% less than the oblique color change of the liquid crystal display device of Comparative Example 101.
5: 30% or more and less than 35% less than the oblique color change of the liquid crystal display device of Comparative Example 101.
4: 20% or more and less than 30% less than the oblique color change of the liquid crystal display device of Comparative Example 101.
3: 10% or more and less than 20% less than the oblique color change of the liquid crystal display device of Comparative Example 101.
2: It is equal to or greater than the oblique color change of the liquid crystal display device of Comparative Example 101.

The content of the repeating unit derived from the acid anhydride monomer in the resin layer I was determined by the ATR measurement method of infrared spectroscopy. Using a ZnSe prism for the resin layer I, the peak intensity around 1780 cm ⁻¹ attributed to carbonyl of maleic anhydride and the peak near 3050 cm ⁻¹ attributed to CH stretching of the benzene ring of styrene by 45 ° incident light Calculated using intensity. In the following, “content of repeating unit derived from acid anhydride monomer” is described as “content of acid anhydride monomer” (specifically, “content of maleic anhydride”).

(Example 1)
In the quarter wave plate of Production Example 2 described in JP-A-2009-288812, a resin P1 (styrene maleic anhydride copolymer, maleic anhydride content 15 having a negative intrinsic refraction value instead of the resin “Daylark D332” 15 A laminated film 1 was obtained in the same manner except that (mass%) was used.
That is, a layer made of norbornene-based polymer 1 (layer II), a layer made of resin P1 having a negative intrinsic refraction value (styrene maleic anhydride copolymer, maleic anhydride content 15 mass%), and (layer I) An II layer (30 μm) -III layer having an adhesive layer (III layer) composed of a modified ethylene-vinyl acetate copolymer (Mitsubishi Chemical Corporation, trade name “Modic AP A543”, Vicat softening point 80 ° C.) 6 μm) -I layer (150 μm) -III layer (6 μm) -II layer (30 μm) of an unstretched laminate is obtained by coextrusion molding, and then this unstretched laminate is rolled Using a tenter stretching machine, the shaped body was stretched obliquely in the −13 ° direction with respect to the width direction at a stretching temperature of 138 ° C., a stretching ratio of 1.5 times, and a stretching speed of 115% / min. Rolled into a roll The laminated film 1 was obtained by scraping.
When Re of the obtained laminated film 1 was measured, it was 137 nm.

One side of the laminated film 1 was subjected to corona discharge treatment. A polyvinyl alcohol aqueous solution was applied to this corona discharge treated surface, dried at 120 ° C. for 5 minutes, and the resulting dry film was rubbed in one direction to obtain a long base material 1 having an alignment film. .
Subsequently, the cholesteric liquid crystal composition (X) obtained by mixing each component with the composition shown below was applied to the surface of the substrate 1 having the alignment film with a wire bar.
――――――――――――――――――――――――――――――――――
Cholesteric liquid crystal composition (X)
――――――――――――――――――――――――――――――――――
Compound B1 7.31 parts by mass Bar-shaped liquid crystal compound A2 30 parts by mass photopolymerization initiator (“IRG907”, manufactured by BASF) 1.20 parts by mass chiral agent (“LC756”, manufactured by BASF) 2.22 parts by mass of surface activity Agent (“KH40”, manufactured by Seimi Chemical Co., Ltd.) 0.04 parts by mass 2-butanone solvent 60.00 parts by mass ――――――――――――――――――――――――― ――――――――

The coating film was subjected to orientation treatment at 100 ° C. for 5 minutes and irradiated with ultraviolet rays in a nitrogen atmosphere. After applying a process of broadening the reflection band by repeating the process consisting of a 10 mJ / cm ² weak UV irradiation process followed by a heating process at 100 ° C. for 1 minute, it is cured by UV irradiation and dried. A long brightness enhancement film 1 having a cholesteric resin layer having a film thickness of 5.3 μm was obtained. About the obtained brightness improvement film 1, after heat-processing for 30 seconds in 120 degreeC oven, haze and the brightness improvement rate were measured. There was a 1.24 times increase in brightness over the reference.

(Example 2)
In place of the resin P1 of Example 1, resin P2 (styrene maleic anhydride copolymer, maleic anhydride content 8 mass%) was used, and the other operations were performed in the same manner as in Example 1 to improve luminance. Film 2 was obtained. About the obtained brightness improvement film 2, after heat-processing for 30 second in 120 degreeC oven, the haze and the brightness improvement rate were measured.
(Example 3)
Using resin P3 (styrene maleic anhydride copolymer, maleic anhydride content 5 mass%) instead of resin P1 of Example 1, the same operation as in Example 1 was performed, and the luminance was improved. Film 3 was obtained. About the obtained brightness improvement film 3, after heat-processing for 30 second in 120 degreeC oven, haze and the brightness improvement rate were measured.

Example 4
Using the styrene maleic anhydride copolymer (maleic anhydride content 40% by mass) instead of the resin P1 in Example 1, the same operation as in Example 1 above was performed, and the brightness enhancement film was obtained. 4 was obtained. About the obtained brightness improvement film 4, after heat-processing for 30 second in 120 degreeC oven, haze and the brightness improvement rate were measured.
(Example 5)
Instead of the rod-shaped liquid crystal compound of (X) of Example 1, the polymerizable liquid crystal compound (3) (the following structure) described in WO09 / 041512 was used, and the other operations were performed in the same manner as in Example 1, A brightness enhancement film 5 was obtained.

About the obtained brightness improvement film 5, after heat-processing for 30 seconds in 120 degreeC oven, haze and the brightness improvement rate were measured.
(Example 6)
Instead of the rod-like liquid crystal compound of (X) of Example 1, the polymerizable liquid crystal compound (5) (the following structure) described in WO09 / 041512 was used, and the other operations were performed in the same manner as in Example 1, A brightness enhancement film 6 was obtained.

About the obtained brightness improvement film 6, after heat-processing for 30 second in 120 degreeC oven, the haze and the brightness improvement rate were measured.
(Comparative Example 101)
The laminate 1 of Example 1-1 described in JP 2010-181710 A was used as the brightness enhancement film 101. About the brightness improvement film 101, after heat-processing for 30 seconds in 120 degreeC oven, the haze and the brightness improvement rate were measured.
(Comparative Example 102)
The brightness enhancement film of Example 1 described in JP2011-118137A was designated as brightness enhancement film 102. About the brightness improvement film 102, after heat-processing for 30 second in 120 degreeC oven, the haze and the brightness improvement rate were measured.

(Comparative Example 103)
Using the styrene maleic anhydride copolymer (maleic anhydride content 2 mass%) instead of the resin P1 of Example 1, the same operation as in Example 1 was performed, and the brightness enhancement film 103 was obtained. Produced. About the brightness improvement film 103, after heat-processing for 30 second in 120 degreeC oven, the haze and the brightness improvement rate were measured.
(Comparative Example 104)
A brightness enhancement film 104 was produced in the same manner as in Example 1 except that the laminated film 1 was replaced with the λ / 4 retardation plate of the example described in Japanese Patent Application Laid-Open No. 2014-0774729. About the brightness improvement film 104, after heat-processing for 30 seconds in 120 degreeC oven, haze and the brightness improvement rate were measured.

(Comparative Example 105)
A brightness enhancement film 105 was produced in the same manner as in Example 1 except that the stretched retardation film 1 of Example 1 described in JP2012-198282A was used instead of the laminated film 1. About the brightness improvement film 105, after heat-processing for 30 second in 120 degreeC oven, the haze and the brightness improvement rate were measured.
(Comparative Example 106)
Using the laminated film (FAB-1) of Example 1 described in Japanese Patent Application Laid-Open No. 2011-242723 instead of the laminated film 1, the same operation as in Example 1 was performed to produce the brightness enhancement film 106. did. About the brightness improvement film 106, after heat-processing for 30 seconds in 120 degreeC oven, the haze and the brightness improvement rate were measured.

(Example 7)
A commercially available vertical alignment film (JALS-204R, manufactured by Nippon Synthetic Rubber Co., Ltd.) was diluted 1: 1 with methyl ethyl ketone on the laminated film 1 obtained in Example 1, and then 2.4 ml / m ² with a wire bar coater. Applied. Immediately, it was dried with 120 ° C. hot air for 120 seconds to form a rod-like liquid crystal homeotropic alignment layer.
On the alignment film, 1.8 g of the following rod-shaped liquid crystal compound, 0.06 g of photopolymerization initiator (Irgacure 907, manufactured by Ciba Geigy), 0.02 g of sensitizer (Kayacure DETX, manufactured by Nippon Kayaku Co., Ltd.), A solution prepared by dissolving 0.002 g of the following air interface side vertical alignment agent in 9.2 g of cyclohexane / cyclopentanone (= 65/35 (mass%)) was applied with a # 2 wire bar. This was affixed to a metal frame and heated in a constant temperature bath at 100 ° C. for 2 minutes to align the rod-like liquid crystal compound. Next, using a 120 W / cm high-pressure mercury lamp at 100 ° C., UV irradiation was performed for 30 seconds to crosslink the rod-like liquid crystal compound. Then, it stood to cool to room temperature.
Next, the crosslinked film was rubbed, and then the cholesteric liquid crystal composition (X) obtained in Production Example 1 was applied to the surface having the rubbed film with a wire bar. The coating film is oriented at 100 ° C. for 5 minutes, irradiated with ultraviolet rays in a nitrogen atmosphere to broaden the reflection band, then cured by ultraviolet irradiation, and has a cholesteric resin layer having a dry film thickness of 5.3 μm. A scale brightness enhancement film 7 was obtained.
About the obtained brightness improvement film 7, after heat-processing for 30 second in 120 degreeC oven, haze and the brightness improvement rate were measured. When the color from an oblique direction shifted from the normal direction of the reflective polarizer was compared with Example 1, the color was less than that of Example 1.

(Example 8)
Using the polymerizable liquid crystal compound (A5-1) described in JP-A-2011-207765 instead of the rod-shaped liquid crystal compound of (X) of Example 1, the same operations as in Example 1 were performed, except that A brightness enhancement film 8 was obtained.

The obtained brightness enhancement film 8 was heat-treated in an oven at 120 ° C. for 30 seconds, and then measured for haze and brightness enhancement rate. When the color from an oblique direction shifted from the normal direction of the reflective polarizer was compared with Example 1, the color was less than that of Example 1.

Example 9
In place of the rod-like liquid crystal compound (X) of Example 1, the polymerizable liquid crystal compound (Compound 9) described in WO2012 / 147904 pamphlet was used, and the other operations were performed in the same manner as in Example 1 to improve the luminance. Film 9 was obtained.

About the obtained brightness improvement film 9, after heat-processing for 30 seconds in 120 degreeC oven, haze and the brightness improvement rate were measured. When the color from an oblique direction shifted from the normal direction of the reflective polarizer was compared with Example 1, the color was less than that of Example 1.
Example 10
In place of the rod-like liquid crystal compound (X) of Example 1, the polymerizable liquid crystal compound (Compound 14) described in the pamphlet of WO2012 / 147904 was used, and the other operations were performed in the same manner as in Example 1 to improve the luminance. Film 10 was obtained.

About the obtained brightness improvement film 10, after heat-processing for 30 seconds in 120 degreeC oven, haze and the brightness improvement rate were measured. When the color from an oblique direction shifted from the normal direction of the reflective polarizer was compared with Example 1, the color was less than that of Example 1.
The Re of the λ / 4 plate in each brightness enhancement film, the haze of each brightness enhancement film, the brightness enhancement rate, and the oblique color change are shown in the table below.

Claims (12)

1. A brightness enhancement film having a λ / 4 plate and a reflective polarizer,
   The λ / 4 plate includes a resin layer I,
   The resin layer I includes a polymer having a negative intrinsic birefringence value,
   The polymer includes repeating units derived from acid anhydride monomers,
   The polymer contains 5% by mass to 50% by mass of the repeating unit derived from an acid anhydride monomer,
   The reflective polarizer includes a light reflecting layer formed by fixing a cholesteric liquid crystal phase,
   The brightness enhancement film, wherein the light reflecting layer is a layer laminated through an alignment layer in direct contact with the λ / 4 plate.
2. The brightness enhancement film according to claim 1, wherein the light reflecting layer is a pitch gradient layer in which a helical pitch of a cholesteric liquid crystal phase is gradually changed in a helical axis direction.
3. The brightness enhancement film according to claim 1 or 2, wherein the light reflection layer is a layer obtained by curing a polymerizable liquid crystal composition including a liquid crystal compound applied to the surface of the alignment film.
4. The brightness enhancement film according to claim 3, wherein the intrinsic birefringence Δn of the liquid crystal compound is reverse wavelength dispersion.
5. The brightness enhancement film according to any one of claims 1 to 4, wherein the repeating unit derived from the acid anhydride monomer is represented by the following formula I:
   

   
6. The brightness enhancement film according to any one of claims 1 to 5, wherein the polymer comprises a repeating unit represented by the following formula II.
   

   
7. The λ / 4 plate includes a resin layer II containing a compound selected from the group consisting of a polymer having an alicyclic structure, a chain polyolefin, cellulose acylate, and polyester cellulose acylate. The brightness enhancement film according to any one of the above.
8. The brightness enhancement film according to any one of claims 1 to 7, wherein the λ / 4 plate includes a resin layer II, a resin layer I, and a resin layer II in this order.
9. The light reflecting layer is a layer formed by coating on the surface of the alignment film provided on the λ / 4 plate;
   The brightness enhancement film according to claim 7 or 8, wherein the resin layer II and the alignment layer are in direct contact, and the alignment layer and the light reflection layer are in direct contact.
10. The brightness enhancement film according to any one of claims 1 to 9, wherein the λ / 4 plate has a thickness of 10 to 300 µm.
11. The brightness enhancement film according to any one of claims 1 to 10, further comprising a positive C plate.
12. The brightness enhancement film according to any one of claims 1 to 11,
    A liquid crystal display device including a backlight unit, the reflective polarizer, the λ / 4 plate, a backlight unit side polarizer, a liquid crystal cell, and a viewing side polarizer in this order.