WO2007086538A1

WO2007086538A1 - Retardation plate, polarizer plate, and liquid crystal display

Info

Publication number: WO2007086538A1
Application number: PCT/JP2007/051331
Authority: WO
Inventors: Akinobu Ushiyama; Hiromoto Haruta; Yutaka Nozoe; Kazuhiko Takeuchi
Original assignee: Fujifilm Corpotation
Priority date: 2006-01-24
Filing date: 2007-01-23
Publication date: 2007-08-02
Also published as: TW200734703A; KR101314376B1; JP2007199149A; KR20080097440A; JP4855081B2

Abstract

A novel retardation plate is disloced. The retardation plate of the present invention comprises a first retardation layer and a second retardation layer, wherein the first retardation layer has an in-plane retardation Re of 20 ran to 150 run, and an Nz value, defined as Nz=Rth/Re+0.5 using an in-plane retardation Re and thickness-direction retardation Rth, of -6.5 to 0.5, and the second retardation layer has Re which satisfies |Re|<30 nm, and Rth which ranges from 80 nm to 400 nm.

Description

DESCRIPTION

RETARDATION PLATE, POLARIZER PLATE, AND LIQUID CRYSTAL DISPLAY

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display, and in particular to an in-plane switching (IPS) mode liquid crystal display driven by a transverse electric field applied to horizontally-aligned liquid crystal molecules. The present invention also relates to a retardation plate contributing to improvement in viewing angle characteristics of a liquid crystal display, in particular of an IPS-mode liquid crystal display, and a polarizer plate comprising the same.

2. Related Art

Liquid crystal displays employing so-called TN mode have been used widely. The TN-mode liquid crystal display comprises two crossed polarizer plates and a nematic liquid crystal layer aligned in a twisted manner between the polarizer plates, and is driven by an electric field applied in the direction normal to the substrates to the liquid crystal layer. This mode suffers from birefringence of the liquid crystal layer in oblique directions, and leakage of light in the black state as a consequence, because the liquid crystal molecules are rise up from the substrates in the black state. In order to overcome such problems, a film, formed of fixed liquid crystal composition in a hybrid alignment, has been used for optically compensation of the liquid crystal cell and for prevention of light leakage in a black state; and a TN-mode liquid crystal display employing such a film has been already put into practical use. It is, however, very difficult to achieve complete optical compensation of the liquid crystal cell without any problems, even with the film formed of fixed liquid crystal, and there has been still a problem of incomplete suppression of grayscale inversion on the bottom side of the screen unsolved.

For the purpose of overcoming this problem, a liquid crystal display (LCD) employing in-plane switching (IPS) mode driven by a transverse electric field applied to the liquid crystal, and an LCD employing a vertical alignment (VA) mode, in which a liquid crystal molecules having a negative dielectric anisotropy are aligned vertically in a multi-domain structure formed of projections or slit electrodes in a liquid crystal cell, have been proposed and put into practical use. These panels employing an IPS or VA mode have been developed as being intended for use not only as monitors but also as TVs exhibiting high luminance. For this reason, even a slight light leakage in oblique directions in the black state, which had not been recognized as a problem before, has been considered as one of factors contributing to lowering displaying quality.

In order to improve viewing-angle properties^' of IPS-mode LCDs in the black state, a birefringent optical compensation material, to be disposed between a liquid crystal layer and a polarizer plate, has been propose.

For example, there has been proposed a transverse electric field type liquid crystal display comprising a double refractive medium having a phase difference for compensating the increase and decrease of the retardation at the time of inclination of the liquid crystal layer between the substrate and the polarizing plate (see Japanese Laid-Open Patent Publication, occasionally referred to as "JPA" hereinafter, No. H9-80424) .

Films formed of various materials such as styrene-base polymers having negative intrinsic birefringence and discotic liquid crystalline compounds have been also proposed as an optical compensation film for IPS-mode LCDs (see JPA Nos. H10-54982, Hll-202323 and H9-292522) have been also proposed.

For compensation of IPS-mode LCDs, use of a compensation layer having positive uniaxial optical anisotropy and an optical axis vertical to the surface of the substrate (see JPA No. Hll-133408) ; use of a biaxial optical compensation sheet showing a half-wave retardation (see JPA No. Hll-305217); and use of a combination of a film showing a negative retardation as a protective film of the polarizer plate and an optical compensation layer showing a positive retardation provided thereon (see JPA No. H10-307291) ; have been also proposed. However, the above mentioned films or the like are to be used for cancelling the anisotropy of birefringence of the liquid crystal cell, thereby improving the viewing angle dependence, and most of LCDs comprising such films still suffer from off-axis light leakages from crossed linear polarizers. Even employing films which have been proposed as a film capable of reducing the off-axis light leakages, it is very difficult to optically compensate the liquid crystal cell completely without raising any problem. Moreover, an optical compensation sheet, capable of compensating the IPS-mode liquid crystal cell, needs a plurality of stretched birefringent polymer films, and the thickness of such sheet consequently increases, which is disadvantageous in terms of thinning of the display device. On the other hand, a tacky layer, used for stacking the stretched films, may shrink as being affected by changes in temperature and humidity, and may result in failures such as separation and warping of the films.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a liquid crystal display, in particular an IPS-type liquid crystal display, improved not only in the display quality, but also in the viewing angle properties, with a simple configuration.

Another object of the present invention is to provide a novel retardation plate contributing to improvement in the viewing angle properties of the liquid crystal display, in particular of the IPS-type liquid crystal display, and a polarizer plate using the same.

In one aspect, the invention provides a retardation plate comprising a first retardation layer and a second retardation layer, the first retardation layer having an in-plane retardation Re of 20 nm to 150 nm, and an Nz value, defined as Nz=Rth/Re+0.5 using an in-plane retardation Re and thickness-direction retardation Rth, of -6.5 to 0.5, and the second retardation layer having Re which satisfies |Re|<30 nm, and Rth which ranges from 80 nm to 400 nm.

As an embodiment of the invention, there is provided the retardation plate wherein the first retardation layer is a cellulose-derivative film. The cellulose derivative film may comprise at least one cellulose acylate derivative satisfies the conditions (a) and (b) below; and the film may be produced by casting a dope containing a cellulose acylate derivative, which satisfies the conditions (a) and (b) below, according to a solvent casting method to form a film and stretching the film:

(a) the cellulose acylate derivative has at least one substituent having a polarizability anisotropy Δα, defined by a mathematical expression (1) below, equal to or larger than 2.5^χlO^~24, in the place of at least one of three hydroxyl groups in a glucose unit of cellulose:

(1) : Δα=α_x-(α_y+α_z) /2 where α_x is the largest component among eigen values obtained by diagonalizing polarizability tensor; α_y is the second largest component among eigen values obtained by diagonalizing polarizability tensor; and α_z is a smallest component among eigen values obtained by diagonalizing polarizability tensor; and

(b) substitution degrees P_A and P_B, respectively representing a substitution degree of a substituent having a Δα of 2.5χlO^~24 cm³ or larger, and a substitution degree of a substituent having a Δα of smaller than 2.5^χlCT²⁴ cm³, satisfy mathematical expressions (3) and (4) below:

(3) : 2P_A+P_B>3.0 (4) : P_A>0.2.

In one preferred embodiment, the substituent having a Δα equal to or larger than 2.5^χlO^~24 cm³ is an aromatic acyl group, and a substituent having a Δα smaller than 2.5^χlO^~24 cm³ is an aliphatic acyl group.

The dope may contain at least one retardation adjusting agent such as a compound represented by the formula (I) below: Formula (I)

where, Ar¹, Ar² and Ar³ respectively represent an aryl group or an aromatic heterocyclic group; L¹ and L² respectively represent a single bond or a divalent linking group; n is an integer equal to or larger than 3; and Ar² and L² may be same or may be different respectively. As an embodiment, there is provided the retardation plate wherein the second retardation layer is formed of a composition comprising a discotic liquid crystal compound; and the mean director of molecules of the discotic liquid crystal compound is normal to a layer plane of the second retardation layer.

In this embodiment, the second retardation layer may comprise at least one compound having at least one triazine ring group and/or at least one fluoro-aliphatic polymer such as a copolymer (polymer "A") comprising at least one repeating unit derived from a compound having a fluoro-aliphatic group, and at least one repeating unit represented by the formula (5) below:

Formula (5)

where, R¹, R² and R³ respectively represent a hydrogen atom or a substitution; L represents a divalent linking group selected from Linking Group shown below or a divalent linking group consisting of two or more selected from Linking Group shown below; (Linking Group)

A single bond, -0-, -CO-, -NR⁴- (R⁴ represents a hydrogen atom, an alkyl group, an aryl group, or an aralkyl group) , -S-, -SO2-, -P(=0) (OR⁵)- (R⁵ represents an alkyl group, an aryl group, or an aralkyl group) , an alkylene group, and an arylene group; and

Q represents a carboxyl group (-COOH) or salt thereof, a sulfo group (-SO₃H) or salt thereof, or a phosphoxy group {-0P(=0) (OH) ₂} or salt thereof.

In another aspect, the invention provides a polarizer plate comprising, at least, a polarizer film and the retardation plate of the invention.

In one embodiment of the polarizer plate, the polarizer film, the first retardation layer and the second retardation layer are disposed in this order; and the direction of slow axis of the first retardation layer and the direction of absorption axis of the polarizer film are substantially perpendicular to each other.

In another embodiment of the polarizer plate, the polarizer film, the first retardation layer and the second retardation layer are disposed in this order; and the direction of slow axis of the first retardation layer and the direction of absorption axis of the polarizer film are substantially parallel to each other.

In another aspect, the invention provides a liquid crystal display comprising, at least, a liquid crystal cell and the polarizer plate of the invention.

The liquid crystal display may further comprise a second polarizer film. In this embodiment, the liquid crystal cell is disposed between the first and second polarizer film; and the absorption axes of the first and the second polarizer films are perpendicular to each other. There may be disposed only a substantially isotropic adhesive layer and/or a substantially isotropic transparent protective film between the second polarizer film, and one of the pair of substrates placedmore closer to the second polarizer film. The transparent protective film may be a cellulose acetate film which satisfies the relational expressions (I) and (II) below: (I): 0 < Re(630) < 10, and |Rth(630)| ≤ 25 (II): |Re(400)-Re(700) I ≤ 10, and | Rth (400) -Rth (700) | ≤35 where, Re (λ) is an in-plane retardation value (unit: nm) at a wavelength of λ nm, and Rth(λ) is a thickness-direction retardation value (-unit: nm) at a wavelength of λ nm.

In one embodiment of the liquid crystal display, the second retardation layer, the first retardation layer, and the first polarizer film are disposed in this order as viewed from the liquid crystal cell side; liquid crystal molecules in the liquid crystal layer are aligned horizontally with respect to the pair of substrates, and a mean direction of their long axes is parallel to a slow axis of the first retardation layer in a black state; and an absorption axis of the first polarizer film and the slow axis of the first retardation layer are perpendicular to each other.

In another embodiment of the liquid crystal display, the second retardation layer, the first retardation layer, and the first polarizer film are disposed in this order as viewed from the liquid crystal cell side, liquid crystal molecules in the liquid crystal layer are horizontally with respect to the pair of substrates and a mean direction of their long axes is perpendicular to a slow axis of the first retardation layer in a black state; and the absorption axis of the first polarizer film and the slow axis of the first retardation layer are parallel to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing an exemplary liquid crystal display of the present invention;

FIG. 2 is a schematic drawing showing another exemplary liquid crystal display of the present invention; and

FIG. 3 is a schematic drawing showing an exemplary pixel area of a liquid crystal display of the present invention.

In these drawings, reference numerals mean as follows:

1 pixel area of liquid crystal element;

2 pixel electrode;

3 display electrode;

4 rubbing direction;

5a, 5b mean director of liquid crystal molecules in the black state;

6a, 6b mean director of liquid crystal molecules in the white state;

7a, 7b, 19a, 19b protective films for polarizer film;

8, 20 polarizer films;

9, 21 polarization absorption axes of polarizer film;

10 first retardation layer;

11 slow axis of first retardation layer;

12 second retardation layer;

13, 17 cell substrates;

14, 18 rubbing directions of cell substrates;

15 liquid crystal layer; and

16 direction of slow axis of liquid crystal layer.

PREFERRED EMBODIMENT OF THE INVENTION The present invention will be described in detail. It is to be noted, in this description, that the term "... to ..." is used as meaning a range inclusive of the lower and upper values disposed therebefore and thereafter.

In the description, Re(λ) and Rth(λ) respectively mean an in-plane retardation and a retardation in a thickness-direction at wavelength λ. The Re(λ) is measured by using KOBRA-21ADH (manufactured by Oj i Scientific Instruments) for an incoming light of a wavelength λnm in a direction normal to a film-surface. The Rth(λ) is calculated by using K0BRA-21ADH based on three retardation values; first one of which is the Re (λ) obtained above, second one of which is a retardation which is measured for an incoming light of a wavelength λnm in a direction rotated by +40° with respect to the normal direction of the film around an in-plane slow axis, which is decided by KOBRA 21ADH, as an a tilt axis (a rotation axis) , and third one of which is a retardation which is measured for an incoming light of a wavelength λnm in a direction rotated by -40° with respect to the normal direction of the film around an in-plane slow axis as an a inclining axis (a rotation axis) ; a hypothetical mean refractive index and an entered thickness value of the film. The mean refractive indexes of various materials are described in published documents such as "POLYMER HANDBOOK" (JOHN WILEY&SONS, INC) and catalogs. If the values are unknown, the values may be measured with an abbe refractometer or the like. The mean refractive indexes of major optical films are exemplified below: cellulose acylate (1.48), cyclo-olefin polymer (1.52), polycarbonate (1.59), polymethyl methacrylate (1.49), polystyrene (1.59).

When the hypothetical mean refractive index and a thickness value are put into KOBRA 21ADH, nx, ny and nz are calculated. And Nz values are calculated by assigning the values of Re and Rth, which are calculated according to the above described method, to the definition, Nz=Rth/Re + 0.5.

In the description, the term "parallel" or "perpendicular" means within a range less than ± 10° with respect to an exact angle. An error from the exact angle is preferably less than ± 5°, and more preferably less than ± 2°. The angle term "normal" means within a range less than ± 20° with respect to an exact angle. An error from the exact angle is preferably less than ± 15°, and more preferably less than + 10°. Also the term of "slow axis" means a direction where the refractive index becomes a maximum value. Also the refractive index is a value measured at λ = 550 nm within the visible light region, unless specified otherwise.

In the description, the term "polarizing plate" is used for both a polarizing plate in a continuous web form, and a polarizing plate which is cut into a size for incorporation in a liquid crystal apparatus (in the present description, "cutting" includes "punching", "cutout" and the like) , unless particularly specified. Also in the description, the terms of "polarizer film" and "polarizing plate" are used in different meanings, and a "polarizing plate" means a laminate member having, on at least a side of a "polarizer film", a transparent protective film for protecting the polarizer film.

Paragraphs below will detail embodiments of the present invention referring to the attached drawings. FIG. 1 and FIG. 2 are schematic drawings showing embodiments of the liquid crystal display of the present invention. FIG. 3 is a schematic drawing showing an exemplary pixel area of a liquid crystal display of the present invention.

[Liquid crystal display]

The liquid crystal display shown in FIG. 1 comprises polarizer films 8 and 20, a first retardation layer 10, a second retardation layer 12, a pair of substrates 13 and 17, and a liquid crystal layer 15 disposed between the substrates. The polarizer films 8 and 20 respectively have protective films 7a, 7b, and 19a, 19b thereon.

In the liquid crystal display shown in FIG. 1, the crystal cell comprises the substrates 13 and 17, and the liquid crystal layer 15 disposed therebetween. Product Δn*d of thickness d (μm) and refractive index anisotropy Δn of the liquid crystal layer in the transmission mode may have an optimum value within the range from 0.2 to 0.4 μm, for the IPS-type cell having no twisted structure . This range can ensure high luminance in the white state and small luminance in the black state, thereby makes the display device excellent in the brightness and contrast. Each of the substrates 13 and 17 has an alignment film (not shown in FIG. 1) as being formed on the surface thereof in contact with the liquid crystal layer 15, so that the liquid crystal molecules are aligned nearly horizontally with respect to the surface of the substrates along with the rubbing direction provided on the alignment film (rubbing directions 14 and 18 in FIG. 1) under no applied voltage or low applied voltage. On the inner surface of the substrate 13 or 17, there is formed an electrode (not shown in FIG. 2) capable of applying voltage to the liquid crystal molecules.

FIG. 3 schematically shows a state of alignment of the liquid crystal molecules in a single pixel area of the liquid crystal layer 15. FIG. 3 is a schematic drawing showing alignment of the liquid crystal molecules within a region having an area extremely as small as being equivalent to a single pixel of the liquid crystal layer 15, together with rubbing direction 4 of the alignment films formed on the inner surfaces of the substrates 13 and 17, and electrodes 2 and 3 formed on the inner surfaces of the substrates 13 and 17, and capable of applying voltage to the liquid crystal molecules. Under active operation using a nematic liquid crystal having a positive dielectric anisotropy as field-effect liquid crystal, the liquid crystal molecules are aligned along with the directors 5a and 5b under no applied voltage or under low applied voltage, thereby the black state is obtained. When voltage is applied between the electrodes 2 and 3, the liquid crystal molecules alter their direction of alignment as being indicated by directors 6a and 6b, depending on the voltage, thereby the white state is obtained.

Referring now back to FIG. 1, the polarizer films 8 and 20 are disposed, so as to cross the individual absorption axes 9 and 21 perpendicular to each other. Slow axis 11 of the first retardation layer 10 is perpendicular to absorption axis 9 of the polarizer film 8, and is parallel to mean slow axis 16 of the liquid crystal molecules in the liquid crystal layer 15 in the black state . In the configuration shown in FIG. 1, the same effect can be obtained if the slow axis 11 of the first retardation layer 10 is parallel to the absorption axis 9 of the polarizer film 8.

The first retardation layer 10 and second retardation layer 12 may be introduced into the liquid crystal display as a retardation plate comprising the two layers, or may be introduced into the liquid crystal display as a polarizer plate comprising the polarizer film 8, the protective film 7b, the first retardation layer 10 and the second retardation layer 12 stacked therein, .

In the liquid crystal display shown in FIG. 1, the polarizer film 8 has two protective films 7a and 7b thereon, however, the protective film 7b is omissible. The polarizer film 20 also has two protective films 19a and 19b thereon, however, the protective film 19a disposed closer to the liquid crystal layer 15 is also omissible. According to the invention, on the basis of position of the liquid crystal cell, the first retardation layer and the second retardation layer in the embodiment shown in FIG. 2 may be disposed between the liquid crystal cell and the polarizer film on the observer' s side, or may be disposed between the liquid crystal cell and the polarizer film on the back side. In either of these configurations, it is preferable in this embodiment that the second retardation layer is disposed more closer to the liquid crystal cell.

Another embodiment of the present invention is shown in FIG. 2. The liquid crystal display shown in FIG. 2 is an embodiment in which positions of the second retardation layer 12 and the first retardation layer 10 are exchanged, and the first retardation layer is disposed more closer to the liquid crystal cell.. In the liquid crystal display shown in FIG. 2, the protective film 7b or the protective film 19a may be omissible. In the embodiment shown in FIG. 2, the first retardation layer 10 is disposed so that the slow axis 11 is parallel to the absorption axis 9 of the polarizer film 8, and is perpendicular to the direction of slow axis 16 of the liquid crystal molecules in the liquid crystal layer 15 in the black state. In the configuration shown in FIG. 3, the same effect can be obtained, if the slow axis 11 of the first retardation layer 10 is perpendicular to the absorption axis 9 of the polarizer film.

In the embodiment shown in FIG. 2, the first retardation layer and the second retardation layer may be disposed, on the basis of the position of liquid crystal cell, between the liquid crystal cell and the polarizer film on the observer's side, or may be disposed between the liquid crystal cell and the polarizer film on the back side. Although FIG. 1 and FIG. 2 showed embodiments of the transmission-type display device having the upper polarizer plate and the lower polarizer plate, the present invention may relate to embodiment of the reflection-type display device having only one polarizer plate, wherein an optimum value of Δn ^• d is approximately halved, because the length of optical path in the liquid crystal cell is halved in this case.

The liquid crystal display of the present invention may comprise other components, without being limited to the configurations shown in FIG. 1 to FIG. 3. For example, a color filter may be disposed between the liquid crystal layer and the polarizer film. The surface of the protective film of the polarizer film may be subjected to anti-reflection finish or hard coating. Any members having electro-conductivity may be employed. The display device intended for use as the transmission-type one may be provided, on the back side thereof, with a back light using a cold-cathode or hot-cathode fluorescent tube, light emitting diode, field emission element, or electro-luminescent element. In this case, the back light may be disposed on the upper side or on the lower side in FIG. 1 and FIG. 2. A reflection-type polarizer plate, diffuser plate, prism sheet or light-guide plate may be disposed between the liquid crystal layer and the back light . As described in the above, the liquid crystal display of the present invention may be of the reflection type, wherein only one polarizer plate is disposed on the observer's side, and a reflective film is disposed on the back surface of the liquid crystal cell or on the inner surface of the lower substrate of the liquid crystal cell. Of course, a front light using the above-described light source may be disposed on the observer' s side of the liquid crystal cell.

It is however to be noted that, in the liquid crystal display shown in FIG. 1 and FIG. 2, it is preferable that there is no retardation region between the liquid crystal cell and the polarizer film 21, and instead that only an isotropic adhesive layer and/or a substantially isotropic transparent protective film are disposed therebetween. In the configurations shown in FIG. 1 and FIG. 2, the protective film 19a is preferably a substantially isotropic film. The substantially isotropic transparent protective film (a protective film of the polarizer film) is specifically a film having an in-plane retardation of 0 to 10 nm, and a thickness-direction retardation of -20 to 20 nm. A film containing cellulose acylate or cyclic polyolefin is preferable. A low-Re cellulose acylate film described later is also preferable. Examples of the adhesive capable of forming the isotropic adhesive layer include polyvinyl alcohol-base adhesive, and an adhesive composed of polyester-base polyurethane and an isocyanate-base crosslinking agent.

The liquid crystal display of the present invention includes those of direct image viewing type, image projection type, and light modulation type. In the present invention, embodiments in a form applied to active-matrix liquid crystal displays using three-terminal or two-terminal elements such as TFT and MIM are particularly preferable. Of course, an embodiment in a form applied to the passive-matrix liquid crystal display, also referred to as those based on time-division driving, is also effective.

Preferable optical characteristics of various components applicable to the liquid crystal display of the present invention, materials andmethods to be employed in producing these components . [First Retardation Layer]

In the present invention, the first retardation layer has an in-plane retardation Re of 20 nm to 150 nm, and has an Nz value, defined by Re and the thickness-direction retardation Rth as Nz=Rth/Re+0.5, of -6.5 to 0.5. In view of effectively reducing the leakage of light in oblique directions, the first retardation layer more preferably has an Re of 40 nm to 115 nm, and still more preferably 50 nm to 85 nm. Similarly, in view of effectively reducing the leakage of light in oblique directions, the first retardation layer more preferably has an Nz value of -5.5 to -0.5, and still more preferably -4.5 to -2.5.

Basically, materials and configurations of the first retardation layer are not specifically limited so far as the above-described optical characteristics are maintained. For example, the first retardation layer may be a retardation film composed of a birefringent polymer film, a retardation film formed by coating and successively heating a polymer solution or a polymer-melt fluid, and a retardation film comprising a retardation layer formed by coating a composition containing a low-molecular-weight or high-molecular-weight liquid crystalline compound. These films may be used in a stacked manner.

The birefringent polymer film is preferably those excellent in controllability of birefringence, transparency, and heat resistance. Polymer materials applicable herein are not specifically limited so far as they can be a uniform biaxial film. Among those, the polymer materials, which can be a film by a solvent cast method or an extrusion method, are preferable; and preferred examples of the polymer material include norbornene-base polymer, polycarbonate-base polymer, polyallylate-base polymer, polyester-base polymer, aromatic polymers such as polysulfone-base polymer, cellulose acylate, and blended polymers containing two, three or more species of these polymers. Among others, use of cellulose acylate is preferable.

Paragraphs below will detail the cellulose derivative film preferably applicable as the first retardation layer. (Cellulose Derivative Film)

The cellulose derivative film having the above-described optical characteristics required for the first retardation layer can be produced by appropriately selecting species of the cellulose derivative used as the source material, species and amounts of addition of additives, methods of forming the films, and conditions for the film formation. The cellulose derivative film having the optical characteristics can be obtained typically by the solvent cast process using a cellulose derivative-containing dope, which satisfies the conditions (a) and (b) below, and by successive stretching:

(a) the cellulose derivative has at least one substituent having a polarizability anisotropy Δα, defined by a mathematical expression (1) below, equal to or larger than 2.5^χlO^~24, in the place of at least one of three hydroxyl groups in a glucose unit of cellulose:

(1) : Δα=α_x-(α_y+α_z)/2 where Ot_x is the largest component among eigen values obtained by diagonalizing polarizability tensor; α_y is the second largest component among eigen values obtained by diagonalizing polarizability tensor; and α_z is a smallest component among eigen values obtained by diagonalizing polarizability tensor; and

(b) substitution degrees P_a and P_B, respectively representing a substitution degree of a substituent having a Δα of 2.5*10^~24 cm³ or larger, and a substitution degree of a substituent having a Δα of smaller than 2.5^χlO^~24 cm³, satisfy mathematical expressions (3) and (4) below: (3) : 2P_A+P_B>3.0 (4): P_A>0.2.

It is to be noted that the number of hydroxyl groups in a glucose unit of cellulose is three, and any cellulose derivatives naturally satisfies P_A+PB ≤3.

(Cellulose Derivative)

The cellulose derivative film is preferably produced using a cellulose derivative having, as the substitution biding to three hydroxyl groups on β-glucose ring, substitutions having a large polarizability anisotropy. The substitution having a large polarizability anisotropy on the β-glucose ring are oriented in the direction perpendicular to the principal chain of the β-glucose ring during stretching so as to maximize the polarizability anisotropy in the thickness-direction of the film. Use of this type of cellulose derivative allows stable production of the film having the optical characteristics described in the above.

In particular, by introducing the substitution having a large polarizability anisotropy, and by adjusting the degree of substitution thereof within the range which satisfies the formulae (3) and (4) , the film formed of such cellulose derivative have desired optical characteristics without degrading other various properties such as softening temperature of the film.

It is more preferable to use the cellulose derivative having the above-described P_A and P_B which satisfy the mathematical expressions (3') and (4') below: (3' ) : 2P_A+P_B>3.0 (4') : 0.2<P_A<2.0

It is still more preferable to use the cellulose derivative having the above-described P_A and P_B which satisfy the mathematical expressions (3") and (4") below: ( 3" ) : 2P_A+P_B>3 . 0 ( 4 " ) : 0 . 2<P_A<1 . 0 .

(Polarizability anisotropy)

Polarizability anisotropy of the substitution can be calculated typically by using GaussianO3 (Revision B.03, a software from Gaussian, Inc., USA). More specifically, structure of the substitution is optimized by calculation based on the B3LYP/6-31G* level, and polarizability is then calculated using thus-obtained structure on the B3LYP/6-311+G** level. If the obtained polarizability tensor is diagonalized thereafter, polarizability anisotropy can be calculated using thus-obtained diagonal component, based on the mathematical expression (1) in the above .

The substitution having a large polarizability anisotropy is preferably aligned so that α_x and α_y are perpendicular and a_z is parallel to the principal chain of the cellulose derivative. Among others, those having α_x aligned in the thickness-direction of the film, and having α_y aligned in the in-plane direction of the film tend to give negative values of thickness-direction retardation Rth. The alignment of α_x and α_y may be largely ascribable to the positions of substitutions on the glucopyranose ring of the cellulose.

The substitution having a Δα of 2.5^χlO^"24 cm³ or larger is preferably selected from aromatic acyl groups. The substitution having an Δα of smaller than 2.5^χlO^~24 cm³ is preferably selected from aliphatic acyl group.

Exampled of the arylacyl group include the group represented by the formula (A) .

In the formula (A), X represents a substituent. Examples of the substituent include halogen atoms, cyano, alkyl, alkoxy, aryl, aryloxy, acyl, carbonamide, sulfonamide, ureido, aralkyl, nitro, alkoxycarbonyl, aryloxycarbonyl, aralkyloxycarbonyl, carbamoyl, sulfamoyl, acyloxy, alkenyl, alkynyl, alkylsulfonyl, arylsulfonyl, alkyloxysulfonyl, aryloxysulfonyl, alkylsulfonyloxy, aryloxysulfonyl, -S-R, -NH-CO-OR, -PH-R, -P(-R)₂, -PH-O-R, -P(-R) (-0-R), -P (-0-R) ₂, -PH (=0) -R-P (=0) (-R) ₂, -PH(=O)-O-R, -P(=0) (-R) (-0-R) , -P (=0) (-0-R) ₂, -O-PH(=O)-R, -0-P(=0) (-R)₂-0-PH(=0)-0-R, -0-P(=0) (-R) (-0-R), -0-P (=0) (-0-R)₂, -NH-PH (=0) -R, -NH-P (=0) (-R) (-0-R) , -NH-P (=0) (-0-R) ₂, -SiH₂-R, -SiH (-R)₂, -Si (-R) ₃, -0-SiH₂-R, -0-SiH (-R) ₂ and O-Si (-R) ₃. In the formulae, R represents an aliphatic, aromatic or heterocyclic group. In the formula, X preferably represents halogen, cyano, alkyl, alkoxy, aryl, aryloxy, acyl, carbonamide, sulfonamide or ureido; and more preferably represents an aliphatic, aromatic or heterocyclic group; more preferably halogen, cyano, alkyl, alkoxy, aryloxy, acyl or carbonamide; much more preferably halogen, cyano, alkyl, alkoxy or aryloxy; and further much more preferably halogen, alkyl or alkoxy.

Examples of halogen include fluorine, chlorine, bromine and iodine atoms. The alkyl group may have a cyclic or branched chain structure. The number of carbon atoms of the alkyl group is preferably from 1 to 20, more preferably from 1 to 12, much more preferably from 1 to 6 and further much more preferably from 1 to 4. Examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, t-butyl, hexyl, cyclohexyl, octyl and 2-ethylhexyl . The alkoxy group may have a cyclic or branched chain structure. The number of carbon atoms of the alkoxy group is preferably from 1 to 20, more preferably from 1 t 12, much more preferably from 1 to 6 and further much more preferably from 1 to 4. The alkoxy group may have at least one substituent. Examples of the alkoxy group include methoxy, ethoxy, 2-methoxyethoxy, 2-methoxy-2-ethoxy ethoxy, butyloxy, hexyloxy and octyloxy.

The number of carbon atoms of the aryl group is preferably from 6 to 20, and more preferably from 6 to 12. Examples of the aryl group include phenyl and naphthyl . The number of carbon atoms of the aryloxy group is preferably from 6 to 20 and more preferably from 6 to 12. Examples of the aryloxy group include phenoxy and naphthoxy. The number of carbon atoms of the acyl group is., preferably from 1 to 20, and more preferably from 1 to 12. Examples of the acyl group include formyl, acetyl and benzoyl. The number of carbon atoms of the carbonamide is preferably from 1 to 20 and more preferably from 1 to 12. Examples of the carbonamide include acetamide and benzamide. The number of carbon atoms of the sulfonamide is preferably from 1 to 20 and more preferably from 1 to 12. Examples of the sulfonamide include methane sulfonamide, benzene sulfonamide and p-toluene sulfonamide. The number of carbon atoms of the ureido group is preferably from 1 to 20, and more preferably from 1 to 12. Examples of the ureido group include non-substituted ureido.

The number of carbon atoms of the aralkyl group is preferably from 7 to 20, and more preferably from 7 to 12. Examples of the aralkyl group include benzyl, phenethyl and naphthyl. The number of carbon atoms of the alkoxycarbonyl is preferably from 1 to 20 and more preferably from 2 to 12. Examples of the alkoxycarbonyl group include methoxy carbonyl. The number of carbon atoms of the aryloxycarbonyl is preferably from 7 to 20 and more preferably from 7 to 12. Examples of the aryloxycarbonyl group include phenoxy carbonyl. The number of carbon atoms of the aralkyloxy carbonyl group is preferably from 8 to 20 and more preferably from 8 to 12. Examples of the aralkyloxy carbonyl include benzoyloxy carbonyl. The number of the carbamoyl group is preferably from 1 to 20 and more preferably from 1 to 12. Examples of the carbamoyl group include non-substituted carbamoyl and N-methyl carbamoyl. The number of carbon atoms of the sulfamoyl group is preferably not more than 20 and more preferably not more than 12. Examples of the sulfamoyl group include non-substituted sulfamoyl and N-methyl sulfamoyl. The number of carbon atoms of the acyloxy group is preferably from 1 to 20 and more preferably from 2 to 12. Examples of the acyloxy group include acetoxy and benzoyloxy.

The number of carbon atoms of the alkenyl group is preferably from 2 to 20 and more preferably from 2 to 12. Examples of the alkenyl group include vinyl, allyl and isopropenyl. The number of carbon atoms of the alkynyl group is preferably from 2 to 20 and more preferably from 2 to 12. Examples of the alkynyl group include thienyl. The number of carbon atoms of the alkylsulfonyl group is preferably from 1 to 20 and more preferably from 1 to 12. the number of carbon atoms of the arylsulfonyl group is preferably from 6 to 20 and more preferably from 6 to 12. The number of carbon atoms of the alkyloxysulfonyl group is preferably from 1 to 20 and more preferably from 1 to 12. The number of carbon atoms of the aryloxysulfonyl group is preferably from 6 to 20 and more preferably from 6 to 12.

In the formula (A) , the number "n" of the substituent X, which attaches to the aromatic ring, ranges from 0 to 5 and preferably from 1 to 3 and more preferably 1 or 2. When n is 2 or more, they may be same or different to each other, and may bind to each other to form a condensed multi ring such as naphthalene, indene, indan, phenanthrene, quinoline, isoquinoline, chromene, chromane, phthalazine, acridine, indole and indoline. The substituent is preferably selected from the group consisting of halogens, cyano, Ci_2o alkyls, Ci-20 alkoxys, C6-20 aryls, Ce-20 aryloxys, C1-.20 acyls, C1-20 carbonamides, Ci-20 sulfonamides and Ci-20 ureidos.

Examples of the group represented by the formula (A) include, but are not limited to, those shown below. Preferred examples include the group Nos . 1, 3, 5, 6, 8, 13, 18 and 28; and more preferred examples include the group Nos. 1, 3, 6 and 13.

As described in the above, the substitution having an Δα of smaller than 2.5^χlO^~24 cm³ is preferably selected from aliphatic acyl group, and more preferably selected from aliphatic acyl groups having the number of carbon atoms of 2 to 20, more specifically acetyl, propionyl, butylyl, isobutylyl, velelyl, pivaloyl, hexanoyl, octanoyl, lauroyl, stearoyl and so forth. Among these, acetyl, propyonyl and butylyl are more preferable, and acetyl is still more preferable. In the present invention, the above-described aliphatic acyl group means as including also those having additional substitutions, wherein such substitutions include those exemplified as X in the formula (A) shown in the above .

The cellulose derivative can be synthesized using acid anhydride or acid chloride as an acylation agent. Source cellulose of cellulose derivative applicable to the present invention includes cotton linter, and wood pulp (hard-wood pulp, soft-wood pulp) , wherein cellulose derivative obtained from either of both source celluloses can be used, allowing mixed use thereof on occasions. Detailed description of these source celluloses can be found, for example, in "Purasuchikku Zairyo Koza (17) Sen'iso-kei Jushi (Lecture Course of Plastic Materials (17) , Fiber-Forming Resins" (written by Marusawa and Uda, published by the Nikkan Kogyo Shimbun Co., Ltd., 1970) and JIII Journal of Technical Disclosure 2001-1745 (p.7-8), without any specific limitations .

Reaction solvents applicable for the case where the acylation agent is an acid anhydride include organic solvents (acetic acid, for example) and methylene chloride. As a catalyst, protic catalyst such as sulfuric acid can be used. For the case where the acylation agent is an acid chloride, applicable catalysts include basic compounds. One of most industrially general synthetic methods relates to esterification of cellulose using a mixed organic acid components which contains any of organic acids corresponded to acetyl group and other acyl groups (acetic acid, propionic acid, butyric acid) or acid anhydrides thereof (acetic anhydride, propionic anhydride, butylic anhydride) , to thereby synthesize the cellulose ester.

Celluloses such as cotton flower linter and wood pulp are generally esterified using a mixed solution of organic acid components as described in the above under the presence of sulfuric acid catalyst, after being activated using an organic acid such as acetic acid. The organic acid anhydride components are used generally in excess over the amount of hydroxyl group present in the cellulose. In this esterification, not only esterification but also hydrolysis (depolymerization rection) of the principal chain of cellulose (βl→4 glycoside bond) proceeds. Progress in the hydrolytic reaction of the principal chain results in lowering in the degree of polymerization of the cellulose derivative, and thereby degrades physical properties of the resultant cellulose derivative film. Reaction conditions such as reaction temperature can therefore be determined, while taking preferable ranges of the degree of polymerization and molecular weight of the resultant cellulose derivative into consideration.

To obtain the cellulose ester having a large degree of polymerization (large molecular weight) , it is essential to adjust the maximum temperature in the esterification reaction to 50⁰C or below. The maximum temperature is preferably adjusted to 35 to 50⁰C, and more preferably 37 to 47 °C. The reaction temperature of 35⁰C or higher is preferable in view of allowing the estrification reaction to proceed smoothly. The reaction temperature of 50⁰C or lower is preferable in view of avoiding non-conformities such as lowering in the degree of polymerization of cellulose ester.

In particular, substitution of the hydroxyl groups of cellulose with the aromatic acyl groups can generally be carried out according to a method employing a symmetric acid anhydride derived from an aromatic carboxylic acid chloride or an aromatic carboxylic acid, and a mixed acid anhydride. Particularly prefer able is a method of using an acid anhydride derived from an aromatic carboxylic acid (Journal of Applied Polymer . Science, Vol.29, 3981-3990 (1984)). Examples of the method for substitution of the hydroxyl groups with the aromatic acyl group typically include (1) a method comprsing once producing a cellulose fatty acid monoester or a diester, and then introducing the aromatic acyl group represented by the above-described formula (A) into the residual hydroxyl groups; and (2) a method comprising of carrying out the direct reaction of a mixed acid anhydride of an aliphatic carboxylic acid and an aromatic carboxylic acid with cellulose. In the former method (1) , the process of preparing the cellulose fatty acid ester or diester per se may be carried out according to the known conditions, but the succeeding reaction process of introducing the aromatic acyl group may be carried out according to the condisitons decided depending on the species of the aromatic acyl group, and, in general, may be carried out preferably at a reaction temperature of 0 to 100⁰C, more preferably 20 to 50⁰C, and preferably for a reaction time of 30 minutes or longer, and more preferably 30 to 300 minutes. In the latter method (2) , the conditions in the step of reaction of the mixed acid anhydride may be dicided depending on the species of the mixed acid anhydride, and, in general, the reaction temperature is preferably 0 to 100⁰C, more preferably 20 to 50⁰C, and the reaction time is preferably 30 to 300 minutes, and more preferably 60 to 200 minutes. Either of both reactions may be carried out in a solvent-less manner or in a solvent, and preferably in solvent. The solvent applicable herein may be dichloromethane, chloroform, dioxane and so forth. After the esterification, termination of the reaction while suppressing elevation in the temperature is successful in further suppressing lowering in the degree of polymerization, making it possible to synthesize a cellulose ester having a large degree of polymerization. More specifically, by adding a reaction terminating agent (for example, water, acetic acid) after completion of the reaction, an excessive portion of acid anhydride not involved in the esterification hydrolyzes to produce a correspondent organic acid. The hydrolytic reaction is accompanied by drastic heat generation, and raises the temperature inside the reaction apparatus. If the rate of addition of the reaction terminating agent is not too large, there is no anticipation of causing a problem such that the reaction system fiercely generates heat exceeding cooling capacity of the reaction apparatus, so as to drastically proceed the hydrolytic reaction of the principal chain of cellulose, and thereby the degree of polymerization of the resultant cellulose is undesirably decreased. A part of the catalyst is bound with cellulose during the esterification reaction, but most part of which releases from cellulose in the process of addition of the reaction terminating agent. If the rate of addition of the reaction terminating agent herein is not too large, a sufficient duration of time can be ensured for dissociation of the catalyst, making it less likely to cause a problem in that a part of the catalyst may remain as being bound to cellulose. Cellulose ester partially bound with a strong acidic catalyst is extremely low in the stability, and may readily decompose due to heat generated during drying, to thereby lower the degree of polymerization. For this reason, after the esterification reaction, it is preferable to terminate the reaction by adding the reaction terminating agent preferably over a duration of time of 4 minutes or longer, more preferably over 4 to 30 minutes . The duration of time of addition of 30 minutes or shorter is preferable in view of avoiding a problem of degradation in the industrial productivity.

As the reaction terminating agent, water and alcohol capable of decomposing the acid anhydride are generally used. It is to be noted, however, that a mixture of water and an organic acid is preferably used, in view of avoiding deposition of triester which shows only a low solubility to various organic solvents. By proceeding the esterification reaction under conditions described in the above, a high-molecular-weight cellulose ester having a mass average degree of polymerization of 500 or larger may readily be synthesized.

The cellulose derivative preferably has a mass average degree of polymerization of 10 to 800, more preferably 370 to 600. The cellulose derivative preferably has a number average molecular weight of 1,000 to 230,000, more preferably 75,000 to 230,000, and still more preferably 78,000 to 120,000. The cellulose derivative having a small mass average molecular weight can be used also as an additive, in a form of polymer blend with cellulose triacetate, from which it is expected to control wavelength dispersion in retardation of the retardation film.

The cellulose derivative film preferably has the refractive index maximized in the thickness-direction of the film, so that the thickness-direction retardation consequently has a negative value. More specifically, a preferable range of the in-plane retardation of the cellulose derivative film can be expressed as 20 nm≤|Re(630) |≤150 nm, a preferable range of Nz value defined by Nz=Rth/Re+0.5 is expressed as -6.5≤Nz≤0.5, more preferably 30 nm≤|Re(630) |≤120 nm and -4.5<Nz≤-0.5, and still more preferably 40 nm≤|Re(630) |≤90 nm and -4.0≤Nz≤-1.0.

For the purpose of adjusting the retardation within a desired range, a retardation adjusting agent may be added to the cellulose derivative film. It is to be noted herein that a term "retardation adjusting agent" is used for any of the agents capable of elevating, lowering and expressing retardation. For the case where the cellulose derivative film is produced by the solvent cast process, the retardation adjusting agent can be added to the dope. The retardation adjusting agent adoptable to the present invention is preferably such as having a large intrinsic birefringence, and is a compound readily alignable in the film, that is, a compound having a strong ability of expressing retardation. Compounds having this sort of property can be exemplified by rod-like compound and discotic compound. When the film is stretched after being added thereto with these compounds, the rod-like or discotic molecules are aligned, making it possible to control the retardation over a wide range. In particular, for the case where these compounds have liquid crystalline properties, alignment of the rod-like liquid crystal compound can increase the refractive index in the stretching direction, whereas alignment of the discotic liquid crystalline compound in parallel with the surface of the film can increase the refractive index in the in-plane direction of the film. For the case where the cellulose derivative film contains no such retardation adjusting agent, and in particular for the case where the cellulose acylate having a large substitution degree of aromatic cyclic acyl group, having a large polarizability anisotropy, is used, birefringence increases in the direction perpendicular to the stretching direction (including in-plane direction and thickness-direction) . When it is needed to increase the thickness-direction negative retardation without increasing the in-plane retardation, such liquid crystalline compound is preferably added to the film to increase birefringence in the stretching direction and to reduce the in-plane retardation. (Retardation adjusting agent)

The retardation adjusting agent is preferably selected from the compounds having a maximum end-to end molecular length equal to or more than 20 A, and have a ratio of long axis to short axis equal to or more than 2.0. It is possible to carry out molecular orbital calculation using a molecular orbital calculation software (e.g., MOPAC and WinMOPAC) to thereby determine the maximum end-to end molecular length and the ratio of long axis to short axis. Preferred examples of the retardation adjusting agent include the compounds represented by the formula (1) .

Formula (1)

In the formula (1) , Ar¹, Ar² and Ar³ respectively represent an aryl group (aromatic hydrocarbon group) , or an aromatic heterocyclic group; L¹ and L² respectively represent a single bond or a divalent linking group; n is an integer equal to or more than 3; and plural Ar² or L² may be same or different each other.

Te aryl group represented by Ar¹, Ar² or Ar³ is preferably selected from Cε-30 aryl groups. The aryl group may have a single-ring structure or a multi-condensed ring structure. The aryl group may also have at least one substituent if possible. The substituent may be selected from the Substituent Group T described later. The aryl group is more preferably selected from C6-20 aryl groups and much more preferably selected from Cε-12 aryl groups such as phenyl, p-methylphenyl and naphthyl .

The aromatic heterocyclic group represented by Ar¹, Ar² or Ar³ is preferably selected from the aromatic heterocyclic groups in which at least one atom selected from nitrogen, oxygen or sulfur atoms is embedded; and more preferably selected from the 5- or 6-membered aromatic heterocyclic groups in which at least one atom selected from nitrogen, oxygen or sulfur atoms is embedded. The aromatic heterocyclic group may have at least one substituent if possible. The substituent may be selected from the Substituent Group T described later.

Examples of the aromatic heterocyclic group include residues of furan, thiophene, pyrrole, imidazole, pyrazole, pyridine, pyrazine, pyridazine, triazole, triazine, indole, indazole, purine, thiazoline, thiazole, thiadiazole, oxazoline, oxazole, oxadiazole, quinoline, isoquinoline, phthalazine, naphthylidine, quinoxaline, quinazoline, cinnoline, pteridine, acridine, phenanthrene, phenazine, tetrazole, benzimidazole, benzoxazole, benzthiazole, benzotriazole, tetrazaindene, pyrrolotriazole and pyrazolotriazole. Preferred examples of the aromatic heterocyclic group include residues of benzimidazole, benzoxazole, benzthiazole and benzotriazole.

In the formula (1) , L¹ and L² respectively represent a single bond or a divalent linking group. Examples of the divalent linking group include -NR⁷- (R⁷ represents a hydrogen atom, or a substituted or non-substituted alkyl or aryl group) , -SO₂-, -CO-, alkylenes, substituted alkylenes, alkenylenes, substituted alkenylenes, alkynylenes, -0-, -S-, -SO- and any combinations thereof; among those, -O-, -CO-, -SO₂NR⁷-, -NR⁷SO₂-, -CONR⁷-, -NR⁷CO-, -COO- and -OCO- and alkynylenes are preferred; and -CONR⁷-, -NR⁷CO-, -COO-, -OCO- and alkynylenes are more preferred.

In the compound represented by the formula (1) , when Ar², biding to both of L¹ and L², is phenylene, it is preferred that the relation between the positions of L¹-Ar²-L² and L²-Ar²-L² is a para-position, 1,4- position.

In the formula, n is preferably an integer equal to or more than 3, more preferably ranges from 3 to 7; and much mnore preferably ranges from 3 to 5.

Among the compounds represented by the formula (1) , the compound represented by the formula (2) are preferred as a retardation adjusting agent.

Formula (2)

In the formula (2), R¹¹, R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R²¹, R²², R²³ and R²⁴ respectively represent a hydrogen atom or a substituent; Ar² represents an aryl group or an aromatic heterocyclic group; L² and L³ respectively represent a single bond or a divalent linking group; n is an integer equal to or more than 3; and plural Ar² and L² may be same or different each other.

In the formula (2) , the definitions of Ar², L², and n are same as those found in the formula (1) respectively; and their preferred scopes are same as those in the formula (1) respectively. L³ represents a single bond or a divalent linking group. The divalent linking group is preferably selected from -NR⁷- (R⁷ represents a hydrogen atom or a substituted or non-substituted alkyl or aryl group) , an alkylenes, substituted alkylenes, -0- and any combonations thereof; and among those, -0-, -NR⁷-, -NR⁷SO₂- and -NR⁷CO- are more preferred.

R¹¹, R¹², R¹³, R¹⁴, R¹⁵ and R¹⁶ respectively represent a hydrogen atom or a substituent; preferably a hydrogen atom, an alkyl group or an aryl group; more preferably a hydrogen atom, a C₁-₄ alkyl group such as methyl, ethyl, propyl and isopropyl, or a C₆-12 aryl group such as phenyl or naphthyl; and much more preferably a C₁-₄ alkyl group.

R²², R²³ and R²⁴ respectively represent a hydrogen atom or a substituent; more preferably a hydrogen atom, an alkyl group, an alkoxy group or hydroxy; and much more preferably a hydrogen atom or an alkyl group (further much more preferably C_3.-₄ alkyl group and most preferably methyl) .

The Substituent Group T mentioned above will be explained below.

Substituent Group T includes halogen atoms such as fluorine, chlorine, bromine and iodine atoms; alkyls (preferably Ci-₃₀ alkyls) such as methyl, ethyl, n-propyl, iso-propyl, tert-butyl, n-octyl, and 2-ethylhexyl; cylcoalkyls (preferably C₃-.₃₀ substitute non-substituted cycloalkyls) such as cyclohexyl, cyclopentyl and 4-n-dodecylcyclohexyl; bicycloalkyls (preferably C₅-₃₀ substitute or non-substituted bicycloalkyls, namely monovalent residues formed from C₅-₃₀ bicycloalkanes from which a hydrogen atom is removed) such as bicyclo [1, 2, 2] heptane-2-yl and bicyclo [2, 2, 2] octane-3-yl; alkenyls (preferably C2-30 alkenyls) such as vinyl and allyl; cycloalkenyls (preferably C₃-₃₀ substituted or non-substituted cycloalkenyls, namely monovalent residues formed from C₃-₃₀ cycloalkenes from which a hydrogen atom is removed) such as 2-cyclopentene-l-yl and 2-cyclohexene-l-yl; bicycloalkenyls (preferably C₅-₃₀ substituted or non-substituted bicycloalkenyls, namely monovalent residues formed from C₅-₃₀ bicycloalkenes from which a hydrogen atom is removed) such as bicyclo [2,2,1] hepto-2-en-l-yl and bicyclo [2,2,2] octo-2-en-4-yl; alkynyls (preferably C2-30 alkynyls) such as etynyl and propargyl; aryls (preferably C₆-₃₀ aryls) such as phenyl, p-tolyl and naphthyl; heterocyclic groups (preferably (more preferably C3-.₃₀ substituted or non-substituted, 5-membered or 6-membered, aromatic or non-aromatic heterocyclic monovalent residues) such as 2-furyl, 2-thienyl, 2-pyrimidinyl and 2-benzothiazolyl; cyano, hydroxyl, nitro, carboxyl, alkoxys (preferably Ci-₃₀ substituted or non-substituted alkoxys) such as methoxy, ethoxy, iso-propoxy, t-butoxy, n-octyloxy and 2-methoxyethoxy; aryloxys (preferably C₆-₃₀ substituted or non-substituted aryloxys) such as phenyloxy, 2-methylphenoxy, 4-t-butylphenoxy, 4-t-butylphenoxy, 3-nitrophenoxy and 2-tetradecanoyl phenoxy; silyloxys (preferably 03-20 silyloxys) such as trimethylsilyloxy and t-butyldimethylsilyloxy; hetero-cyclic-oxys (preferably C2-₃₀ substituted or non-substituted hetero-cyclic-oxys) such as 1-phenyltetrazole, -5-oxy and 2-tetrahydropyrenyloxy; acyloxys (preferably C₂-₃₀ substitute or non-substituted alkylcarbonyloxys and C₆-3o substituted or non-substituted arylcarbonyloxys) such as formyloxy, acetyloxy, pivaloyloxy, stearoyoxy, benzoyloxy and p-methoxyphenylcarbonyloxy; carbamoyloxys (preferably Cl-30 substituted or non-substituted carbamoyloxys) such as N,N-dimethyl carbamoyloxy, N,N-diethyl carbamoyloxy, morpholinocarbonyloxy, N,N-di-n-octylaminocarbonyloxy and N-n-octylcarbamyloxy; alkoxy carbonyloxys (preferably C₂-₃₀ substituted or non-substituted alkoxy carbonyloxys) such as methoxy carbonyloxy, ethoxy carbonyloxy, t-butoxy carbonyloxy and n-octyloxy carbonyloxy; aryloxy carbonyloxys (preferably C₇-₃o substituted or non-substituted aryloxy carbonyloxys) such as phenoxy carbonyloxy, p-methoxyphenoxy carbonyloxy and p-n-hexadecyloxyphenoxy carbonyloxy; aminos (preferably Co_3o substituted or non-substituted alkylaminos and C₆-₃0 substituted or non-substituted arylaminos) such as amino, methylamino, dimethylamino, anilino, N-methyl-anilino and diphenylamino; acylaminos (preferably Ci_₃₀ substituted or non-substituted alkylcarbonylaminos and C₆-₃₀ substituted or non-substituted arylcarbonylaminos) such as formylamino, acetylamino, pivaloylamino, lauroylamino and benzoylamino; aminocarbonylaminos (preferably C₃.-30 substituted or non-substituted aminocarbonylaminos) such as carbamoylamino, N,N-dimethylaminocarbonylamino, N,N-diethylamino carbonylamino and morpholino carbonylamino; alkoxycarbonylaminos (preferably C2-30 substituted or non-substituted alkoxycarbonylaminos) such as methoxycarbonylamino, ethoxycarbonylamino,t-butoxycarbonylamino, n-octadecyloxycarbonylamino and N-methyl-methoxy carbonylamino; aryloxycarbonylaminos (preferably C₇-₃o substituted or non-substituted aryloxycarbonylaminos) such asphenoxycarbonylamino, p-chloro phenoxycarbonylamino and m-n-octyloxy phenoxy carbonylamino; sulfamoylaminos (preferably Co-₃₀ substituted or non-substituted sulfamoylaminos) such as sulfamoylamino, N, N-dimethylamino sulfonylamino and N-n-octylamino sulfonylamino; alkyl- and aryl-sulfonylaminos (preferably C₁-.₃₀ substituted or non-substituted alkyl-sulfonylaminos and C6-₃₀ substituted or non-substituted aryl-sulfonylaminos) such as methyl-sulfonylamino, butyl-sulfonylamino, phenyl-sulfonylamino,

2,3, 5-trichlorophenyl-sulfonylamino and p-methylphenyl-sulfonylamino; mercapto; alkylthios (preferably substituted or non-substituted alkylthios such as methylthio, ethylthio and n-hexadecylthio; arylthios (preferably C6-30 substituted or non-substituted arylthios) such as phenylthio, p-chlorophenylthio and m-methoxyphenylthio; heterocyclic-thios (preferably C₂-30 substituted or non-substituted heterocyclic-thios such as 2-benzothiazolyl thio and l-phenyltetrazol-5-yl-thio; sulfamoyls (preferably Co-30 substituted or non-substituted sulfamoyls) such as N-ethylsulfamoyl, N- (3-dodecyloxypropyl) sulfamoyl,

N,N-dimethylsulfamoyl, N-acetylsulfamoyl, N-benzoylsulfamoyl, N- (N' -phenylcarbamoyl) sulfamoyl; sulfo; alkyl- and aryl-sulfinyls (preferably Ci_₃o substituted or non-substituted alkyl- or C₆-3₀ substituted or non-substituted aryl-sulfinyls) such as methylsulfinyl, ethylsulfinyl, phenylsulfinyl and p-methylphenylsulfinyl; alkyl- and aryl-sulfonyls (preferably Ci_₃o substitute or non-substituted alkyl-sulfonyls and C₆-₃0 substituted or non-substituted arylsulfonyls) such as methylsulfonyl, ethylsulfonyl, phenylsulfonyl and p-methylphenylsulfonyl; acyls (preferably formyl, C2-30 substituted non-substituted alkylcarbonyls, and C₇_3o substituted or non-substituted arylcarbonyls) such as formyl, acetyl and pivaloyl benzyl; aryloxycarbonyls (preferably C₇-3o substituted or non-substituted aryloxycarbonyls) such as phenoxycarbonyl, o-chlorophenoxycarbonyl, m-nitrophenoxycarbonyl and p-t-butylphenoxycarbonyl; alkoxycarbonyls (preferably C2-30 substituted or non-substituted alkoxycarbonyls) methoxycarbonyl, ethoxycarbonyl, t-butoxycarbonyl and n-octadecyloxycarbonyl; carbamoyls (preferably Ci-30 substituted or non-substituted carbamoyls) such as carbamoyl, N-methylcarbamoyl, N, N-dimethylcarbamoyl, N,N-di-n-octylcarbamoyl and N- (methylsulfonyl) carbamoyl; aryl- or heterocyclic-azos (preferably Ce-₃o substituted or non-substituted arylazos and C3-30 substituted or non-substituted heterocyclicazos) such as phenylazo and p-chlorophenylazo, 5-ethylthio-l, 3, 4 -thiadiazol-2-yl-azo, imides such as N-succinimide andN-phthalimide; phosphinos (preferably C2-₃0 substituted or non-substituted phosphinos) such as dimethyl phosphino, diphenyl phosphino and methylphenoxy phosphino; phosphinyls (preferably C2-30 substituted or non-substituted phosphinyls) such as phosphinyl, dioctyloxy phosphinyl and diethoxy phosphinyl; phosphinyloxys (preferably C₂-₃0 substituted or non-substituted phosphinyloxys) such as diphenoxyphosphinyloxy and dioctyloxyphosphinyloxy; phosphinylaminos (preferably C₂-30 substituted or non-substituted phosphinylaminos) such as dimethoxy phosphinylamino and dimethylamino phosphinylamino; and silyls (preferably C₃-₃₀ substituted or non-substituted silyls) such as trimethylsilyl, t-butylmethylsilyl and phenyldimethylsilyl .

The substituents may be substituted by at least one substituent selected from these. Examples such substituent include alkylcarbonylaminosulfo, arylcarbonylaminosulfo, alkylsulfonylaminocarbonyl and arylsulfonylaminocarbonyl. More specifically, methylsulfonylaminocarbonyl, p-methylphenylsulfonylaminocarbonyl, acetylaminosulfonyl and benzoylaminosulfonyl are exemplified.

Plural substituents may be same are different each other. If possible, plural substituent may bind to each other to form a ring.

Examples of the compound represented by the formula (2) include, but are not limited to, those shown below.

The compound represented by the formula (3) is also preferred as a retardation adjusting agent.

In the formula, R³¹, R³², R³³, R³⁴, R³⁵ and R³⁶ respectively represent a substituent; L³¹ and L³² respectively represent a single bond or a divalent linking group; n and m respectively represent an integer from 0 to 4; andp and q respectively represent an integer from 0 to 3.

R³¹, R³², R³³, R³⁴, R³⁵ and R³⁶ may be same or different each other. The substituent represented by R³¹, R³², R³³, R³⁴, R³⁵ or R³⁶ is preferably selected from the Substituent Group T. Among those, an alkyl group, an alkoxy group, an alkoxycarbonyl group, an acyl group, an alkoxycarbonyloxy group, a cycloalkyl group, an acylamino group, cyano, and a halogen atom are more preferred. Plural substituent may be same or different each other, and, if possible, bind to each other to form a ring.

In the formula (3 ) , L³¹ and L³² respectively represent a single bond or a divalent linking group. L³¹ and L³² may be same or different each other. Preferred examples of the divalent linking group include -NR⁷- (R⁷ represents a hydrogen atom or a substituted or non-substituted alkyl or aryl group) , -SO₂-, -CO-, alkylene, substituted alkylene, alkenylene, substituted alkenylene, alkynylene, -0-, -S-, -SO- and any combinations thereof; among those, -0-, -CO-, -SO₂NR⁷-, -NR⁷SO₂-, -CONR⁷-, -NR⁷CO-, -C00-, -OCO- and alkynylene are more preferred; and -CONR⁷-, -NR⁷CO-, -C00-, -OCO- and alkynylene are much more preferably. Examples of the substituent include those above exemplified as a substituent of R³¹, R³², R³³, R³⁴, R³⁵ and R³⁶.

In the formula (3) , n and m respectively represent an integer from 0 to 4; and when n or m is equal to or more than 2, plural R³¹ or R³² may be same or different respectively. In the formula, p and q respectively represents an integer from 0 to 3; and when p or q is equal to or more than 2, plural R³³ or R³⁴ may be same ore different respectively. In the formula, R³³ and R³⁵ or R³⁴ and R³⁶ may bind to each other to form a ring. In terms of retardation adjusting effect, the retardation adjusting agent is preferably selected from symmetry compounds (for example, compounds represented by the formula (3) , in which 1, 4-substituents of the central cyclohexane ring have a same structure) .

Examples of the compound represented by the formula (3) include, but are not limited to, those shown below.

The compound represented by the formula ( 4 ) is also preferred as a retardation adjusting agent. Formula ( 4 )

In the formula (4) , R⁴¹, R⁴², R⁴³ and R⁴⁴ respectively represent a substituent; and E¹, E², E³ and E⁴ respectively represent an oxygen atom or a sulfur atom. In the formula, L⁴¹ and L⁴² respectively represent a divalent linking group; n and m respectively represent an integer from 0 to 4 ; and p and q respectively represent an integer from 1 to 10.

In the formula (4), the substituent represented by R⁴¹ or R⁴² is preferably selected from the Substituent Group T.

In the formula (4), preferred examples of the substituent represented by R⁴³ or R⁴⁴ are same as those exemplified as preferred examples of R⁴¹ or R⁴²; among those, an alkyl group, a cycloalkyl group, a bicycloalkyl group, an alkenyl group, a cycloalkenyl group, a bicycloalkenyl group, an alkynyl group, an aryl group, a heterocyclic group, a sulfamoyl group, an alkyl- or aryl-sulfonyl group, an acyl group, an aryloxycarbonyl group, an alkoxycarbonyl group and a carbamoyl group are preferred; and an alkyl group, a cycloalkyl group, an alkenyl group, an aryl group, an acyl group, an aryloxycarbonyl group, an alkoxycarbonyl group and carbamoyl group are more preferred.

In the formula (4) , L⁴¹ and L⁴² respectively represent a divalent linking group, and they may be same or different each other. The divalent linking group is preferably selected from the groups other than arylene; more preferably selected from the group consisting of alkylens, substituted alkylenes, alkenylens, substituted alkenylenes, alkynylenes and any combinations thereof. The combinations thereof may comprise other divalent linking group which links them. Examples of such other linking group include -NR⁷- (R⁷ represents a hydrogen atom or a substituted or non-substituted alkyl or aryl group) , -0-, -S-, -SO-, -SO2-, -CO-, -SO₂NR⁷-, -NR⁷SO₂-, -CONR⁷-, -NR⁷CO-, -COO- and -OCO- . Examples of the substituent include those above exemplified as examples of R⁴¹, R⁴², R⁴³ and R⁴⁴.

In the formula (4) , n andm respectively represent an integer from 0 to 4; n and m respectively represent an integer equal to or more than 2; an plural R⁴¹ or R⁴² may be same or different respectively. In the formula, p and q respectively represent an integer from 1 to 10; and when p or q is equal to or more than 2, plural E³, E⁴, L⁴¹ or L⁴² may be same or different respectively. In terms of retardation adjusting effect, the retardation adjusting agent is preferably selected from symmetry or pseudo-symmetry compounds (for example, compounds represented by the formula (4), in which 1, 4-substituents of the central cyclohexane ring have a same or similar structure) .

Examples of the compound represented by the formula (4) include, but are not limited to, those shown below.

The amount of addition of the retardation adjusting agent is preferably 0.01 to 20 parts by mass per 100 parts by mass of cellulose derivative, more preferably 0.1 to 15 parts by mass, and still more preferably 1 to 10 parts by mass. The retardation adjusting agent may be added to a dope of cellulose derivative used for preparing the film, wherein in view of ensuring stably mixing into the dope, it is preferable that the retardation adjusting agent is fully compatible with the cellulose derivative, and less likely to aggregate. For this purpose, a possible method is such as preliminarily preparing an adjusting agent solution by mixing and stirring a solvent and the retardation adjusting agent, adding the adjusting agent solution to a small amount of cellulose derivative solution separately prepared, followed by stirring, and further mixing the mixture with the main cellulose derivative dope solution.

The retardation adjusting agents may be used independently, or as a mixture of two or more species of compounds based on an arbitrary ratio of mixing. These retardation adjusting agents may be added at any timing in the process of preparation of the dope, or at the final stage of the dope preparation process.

[Second Retardation Layer]

In the present invention, the second retardation layer has an in-plane retardation Re of |Re|<30 nm, and an Rth of 80 nm to 400 nm. More preferable range of Rth of the second retardation layer varies depending on optical characteristics of other optical components, in particular largely depending on Rth of the protective film (triacetyl cellulose film, for example) of the polarizer film disposed more closer thereto in the liquid crystal display. For the purpose of effectively reducing leakage of light in oblique directions, the second retardation layer preferably has an Rth of 100 nm to 340 nm, and more preferably 180 nm to 300 nm. On the other hand, Re of the second retardation layer more preferably resides in a range of |Re|<10 nm.

Materials and configurations of the second retardation layer are not specifically limited, so far as they have the above- described optical characteristics. Any of a retardation film composed of a birefringent polymer film, a retardation film formed by coating and successively heating a polymer solution or a polymer-melt fluid, and a retardation film comprising a retardation layer formed by coating a composition containing a low-molecular-weight or high-molecular-weight liquid crystalline compound. These films may be used in a stacked manner.

The second retardation layer is preferably formed by using a composition containing at least one species of discotic liquid crystal compound, in which the discotic liquid crystal molecules are aligned while directing the director to the direction of the normal line on the layer plane, or in other words, the discotic liquid crystal molecules are aligned horizontally. (Discotic Liquid Crystal)

Examples of the discotic liquid crystalline compound adoptable to production of the second retardation layer include benzene derivatives described in a research report by C. Destrade et al. (MoI. Cryst., 71, p. Ill (1981)), torxene derivatives described in research reports by C. Destrade (MoI. Cryst., 122, p.141 (1985), Physics Lett. A, 78, p.82 (1990)), cyclohexane derivatives described in the research report by B. Kohne et al. (Angew. Chem. , 96, p.70 (1984)), and azacrown-base and phenylacetylene-base macrocycles described in research reports by J. M. Lehn et al. (J. Chem. Commun., p.1794 (1985)) and by J. Zhang et al. (J. Am. Chem. Soc, 116, p.2655 (1994)).

Examples of the discotic liquid crystalline compound also include compounds having a core at the center of the molecule and straight-chain alkyl groups, alkoxy groups, and substituted benzoyloxy groups radially substituted as side chains on the core. The compound is preferably such that the molecule thereof or aggregate of the molecules shows rotation symmetry, and thereby can ensure a certain alignment. It is not always necessary for an optically anisotropic layer composed of the discotic liquid crystalline compound that the compound finally contained in the optically anisotropic layer is a discotic liquid crystalline compound, but may also be a compound from which the liquid crystallinity is finally lost due to heat- or photo-assisted polymerization, crosslinking or polymerization, although originated from a low-molecular-weight discotic liquid crystalline molecule having heat- or photo-reactive groups. Preferable examples of the discotic liquid crystalline compound are described in JPANo. H8-50206. Polymerization of the discotic liquid crystalline compound is described in JPA' No. H8-27284.

The composition used for producing the second retardation layer may comprise a polymerization initiator. Among those, photo-polymerization initiator is preferable. Amount of use of the photo-polymerization initiator is preferably 0.01 to 20% by mass of the composition (which means solid component for those prepared as coating solution), and more preferably 0.5 to 5% by mass. Irradiation of light for polymerizing the discotic liquid crystalline molecules may be effected by ultraviolet radiation. Irradiation energy is preferably 20 mJ/ cm² to 50 J/ cm², and more preferably 100 to 800 mJ/ cm². The irradiation of light may be carried out under heating conditions, so as to promote the photo-polymerization reaction.

Thickness of the second retardation layer formed using a composition containing the above-described liquid crystal compound is preferably 0.1 to 10 μm, more preferably 0.5 to 5 μm, and still more preferably 1 to 5 μm.

The second retardation layer may be produced by using a compound containing a triazine ring, which promotes horizontal alignment of the discotic liquid crystal molecules, and/or a fluoro-aliphatic-group-containing polymer . (Triazine-Ring-Containing Compound)

The second retardation layer preferably contains at least one species of compounds having 1, 3, 5-triazine ring group. This compound contributes to reduction in the tilt angle of the liquid crystalline compound, in particular discotic liquid crystalline compound, or in other words, contributes to promotion of horizontal alignment. The compounds having 1, 3, 5-triazine ring group may be contained in the composition used for forming the second retardation layer. As the compounds having 1, 3, 5-triazine ring group applicable to the present invention, it is preferable to use the compounds described in paragraphs [0060] to [0078] of JPA No. 2005-134884, and paragraphs [0057] to [0081] of JPA No. 2005-99248. It is particularly preferable that the above-described compounds having 1, 3, 5-triazine ring group contains fluorine atom(s) . (Fluoro-aliphatic-group-containing Polymer)

The second retardation layer preferably contains at least one species of fluoro-aliphatic-group-containing polymer. The fluoro-aliphatic-group-containing polymer contributes, when included into the above-described composition, to improvement in coating performance of this composition prepared as a coating solution. It also has a function of promoting horizontal alignment of the discotic liquid crystalline molecules.

Preferable examples of the fluoro-aliphatic polymer include fluoro-aliphatic polymers described in paragraphs [0018] to

[0056] of JPANo. 2005-134884, paragraphs [0046] to [0054] of JPA

No.2004-331812, paragraphs [0057] to [0066] of JPANo.2004-333861, paragraphs [0084] to [0096] of JPA No. 2005-189285, paragraphs

[0076] to [0095] of JPA No. 2005-189286, paragraphs [0064] to

[0077] of JPA No. 2005-206638, and paragraphs [0099] to [0117] of JPA No. 2005-215398.

The polymer having fluoride- aliphatic group is preferably selected from the copolymers, hereinafter referred to as "Polymer A" occasionally, comprising at least one repeating unit represented by the formula (5) and at least one repeating unit derived from a compound having fluoride-aliphatic group.

In the formula (II) , R¹, R² and R³ respectively represent a hydrogen atom or a substituent group; L represents a divalent linking group selected from Linking Group shown below or a divalent linking group consisting of two or more selected from the Linking Group shown below; (Linking Group) a single bond, -0-, -CO-, -NR⁴- where R⁴ represents a hydrogen atom, an alkyl group, an aryl group or an aralkyl group) , -S-, -SO₂-, -P(=0) (OR⁵)- where R⁵ represents an alkyl group, an aryl group or an aralkyl group) , an alkylene group and arylene group;

Q represents a carboxyl group (-COOH) or a salt thereof, a sulfo group (-SO₃H) or a salt thereof, or a phosphonoxy group {-0P(=0) (OH)₂I or a salt thereof.

In the formula (5) , preferably, R¹, R² and R³ respectively represent a hydrogen atom, an alkyl group, a halogen atom such as fluorine, chlorine, bromine or iodine atom, or -L-Q describe hereinafter; more preferably, a hydrogen atom, a Ci_₆ alkyl group, a chlorine atom or -L-Q; much more preferably, a hydrogen atom or a Ci_₄ alkyl group; further much more preferably, a hydrogen atom or a C1-₂ alkyl group; and, most preferably, R² and R³ are hydrogen and R¹ is hydrogen or methyl. Examples of the alkyl group include methyl, ethyl, n-propyl, n-butyl and sec-butyl. The alkyl group may have at least one substituent. Examples of the substituent include halogen atoms, aryls, hetero cyclic groups, alkoxyls, aryloxys, alkylthios, arylthios, acyls, hydroxy, acyloxys, aminos, alkoxycarbonyls, acylaminos, oxycarbonyls, carbamoyls, sulfonyls, sulfamoyls, sulfonamides, sulfonyls and carboxyls. It is noted that when the alkyl group has any substituent, the carbon atom number of the alkyl group, described above, is the number of the carbon atoms included in the only alkyl group, and the carbon atoms included in the substituent are not counted. Numbers of carbon atoms included in the other groups described later are defined as same as that of the alkyl group.

In the formula, L is a divalent linking group selected from the above defined group or any combination of two or more selected from the above identified group. The R⁴ in -NR⁴- described above represents a hydrogen atom, an alkyl group, an aryl group or an aralkyl group, and desirably a hydrogen atom or an alkyl group. And the R⁵ in -PO(OR⁵)- represents an alkyl group, an aryl group or an aralkyl group, and desirably an alkyl group. When R⁴ or R⁵ is an alkyl group, an aryl group or an aralkyl group, the desired carbon numbers of them are same as those described in Substituent Group I. L desirably contains a single bond, -0-, -CO-, -NR⁴-, -S-, -SO2-, an alkylene group or arylene group; more desirably contains a single bond, -CO-, -0-, -NR⁴-, an alkylene group or an arylene group; and much more desirably represents a single bond. When L contains an alkylene group, the carbon atom number of the alkylene group is desirably from 1 to 10, more desirably from 1 to 8 and much more desirably from 1 to 6. Preferred examples of the alkylene group include methylene, ethylene, trimethylene, tetrabutylene and hexamethylene. When L contains an arylene group, the carbon atom number of the arylene group is desirably from 6 to 24, more desirably from 6 to 18 and much more desirably from 6 to 12. Preferred examples of the arylene group include phenylene and naphthalene. When L contains a divalent linking group consisting of a combination of an alkylene group and an arylene group, or in other words an aralkyl group, the carbon atom number in the aralkyl group is desirably from 7 to 36, more desirably from 7 to 26 and much more desirably from 7 to 16. Preferred examples of the aralkyl group include phenylene methylene, phenylene ethylene and methylene phenylene. L may have any substituent. Examples of the substituent are same as those exemplified for the substituent of R¹, R² or R³.

Examples of L include, however are not to be limited to, those shown below.

In the formula (5) , Q represents a carboxyl group or a carboxylate such as lithium carboxylate, sodium carboxylate, potassium carboxylate, ammonium carboxylate (for example, unsubstituted ammonium carboxylate, tetramethylammonium carboxylate, trimethyl-2-hydroxyethylammmonium carboxylate, tetrabutylammonium carboxylate, trimethylbenzylammonium carboxylate or dimethylphanylammmonium carboxylate) or pyridinium carboxylate; a sulfo group or a sulfate (examples of a counter cation are same as those exemplified for the carboxylate above) ; or a phosphonoxy group or a phosphonoxylate (examples of a counter cation are same as those exemplified for the carboxylate above) . Carboxyl, sulfo and phosphino groups are preferred, carboxyl and sulfo groups are more preferred, and carboxyl is most preferred.

The Polymer A may comprise a single or plural repeating represented by the formula (5) . The Polymer A may also comprise a single or plural repeating unit derived from a compound having a fluoride-aliphatic group. The Polymer A may also comprise a single or plural repeating unit other than the repeating units described above. Another repeating unit is not to be limited to a specific type, and any repeating unit derived from common monomers capable of radical-polymerization is preferably used.

The Polymer A preferably comprises a repeating unit derived from a compound represented by the formula [2] described in JPA No. 2004-333861.

The amount of the monomer containing a fluoride-aliphatic group is desirably not less than 5 mass %, more desirably not less than 10 mass %, and much more desirably not less than 30 mass % with respect to the total amount of all monomers constituting the Polymer A. The amount of the repeating unit represented by the formula (5) is desirably not less than 0.5 mass %, and more desirably from 5 to 50 mass % with respect to the total amount of all monomers constituting the Polymer A.

The weight-average molecular weight (Mw) of the Polymer A to be used in the invention is desirably less than or equal to 1,000,000, more desirably less than or equal to 500,000 and much more desirably ranges from 5, 000 to 50, 000. The Mw can be measured as a polystyrene (PS) equivalent molecular weight with gel permeation chromatography (GPC) .

The polymer A may have a polymerizable group as a substituent to fix the alignment state of discotic liquid crystal molecules.

Examples of the Polymer A include, but are not limited to, those shown below. Numerical values in formulae shown below mean mass % of each monomer, and Mw in formulae shown below mean PS-equivalent weight-average molecular weight measured by GPC with a column such as TSK Gel GMHxL, TSK Gel G4000 HxL, and TSK Gel G2000 HxL (manufactured by TOSOH) , THF as solvent and a differential refractometer as a detector. In the formulae, ^Nλa", "b", "c", ^ΛΛd" and the like mean weight ratios.

Preferable range of content of the above-described, fluoro-aliphatic-group-containing polymer (polymer "A") in the second retardation layer is 0.005 to 8% by mass in the composition

(composition excluding solvent, for the case of coating solution) , more preferably from 0.01 to 5% by mass, and still more preferably

0.01 to 1% by mass. The amount of addition of the polymer "A" less than 0.005% by mass may result in only a poor effect, whereas exceeding 8% by mass may result in insufficient drying of the coated film, or may adversely affect performance as the retardation layer

(uniformity in retardation, for example) .

(Alignment Film)

The second retardation layer containing the discotic liquid crystalline compound may be produced using an alignment film. The alignment film contributes not only to improvement in the alignment property of the discotic liquid crystalline molecules, but sometimes to improvement also in the adhesiveness between the first retardation layer composed of a cellulose derivative film or the like and the second retardation layer.

If the alignment film is formed using a polymer having a side chain having a crosslinking functional group bonded to the principal chain thereof, or a polymer having a crosslinking functional group in the side chain thereof having a function of aligning the liquid crystalline molecules, and the retardation film is formed thereon using a composition containing a multi-functional monomer, it is made possible to copolymerize the polymer in the alignment film and the multi-functional monomer in the retardation film formed thereon. As a consequence, covalent bonds can be formed not only between the multi-functional monomers, but also between the alignment film polymers, and between the multi-functional monomer and the alignment film polymer, and thereby the alignment film and the retardation film can be bound in a tight manner. Strength of the optical compensation sheet can therefore be improved to a considerable degree, by forming the alignment film using the polymer having the crosslinking functional groups. The crosslinking functional groups in the alignment film polymer preferably contains a polymerizable group, similarly to the multi-functional monomer. More specifically, those described in paragraphs [0080] to [0100] of JPA No. 2000-155216 can be exemplified.

The alignment film polymer may be crosslinked using a crosslinking agent, besides the crosslinking functional group described in the above. Examples of the crosslinking agent include aldehyde, N-methylol compounds, dioxiane derivatives, compounds which become functional by activating the carboxyl group thereof, active vinyl compounds, active halogen compounds, isoxazole and dialdehyde starch. Two or more crosslinking agents may be used in combination. More specifically, the compounds typically described in paragraphs [0023] to [0024] of JPA No. 2002-62426 can be exemplified. Highly active aldehyde, in particular, glutaraldehyde is preferable.

The amount of addition of the crosslinking agent is preferably in the range from 0.1 to 20% by mass of the polymer, and more preferably from 0.5 to 15% by mass. The amount of unreacted crosslinking agent remaining in the alignment film is preferably 1.0% by mass or below, and more preferably 0.5% by mass or below. This adjustment is successful in achieving a sufficient level of durability of the alignment film, without causing reticulation, even after a long-term use in the liquid crystal display, or after being allowed to stand under a high-temperature, high-humidity atmosphere.

The alignment film can be formed basically by coating a composition containing the above-described polymer, as a material for forming the alignment film, and a crosslinking agent onto a transparent support, followed by drying under heating (crosslinked) , and rubbed. The crosslinking reaction may be proceeded at an arbitrary timing, after the coating on the transparent support. For the case where a water-soluble polymer such as polyvinyl alcohol is used as a source material for forming the alignment film, the coating solution preferably uses a mixed solution of an organic solvent having a defoaming function (methanol, for example) and water. Ratio of mixing by mass of water:methanol is preferably 0:100 to 99:1, and more preferably 0:100 to 91:9. The adjustment can suppress foaming, and can considerably reduce defects on the surface of the alignment film, and consequently of the retardation layer. Examples of the coating method, which can be employed for forming the alignment film, include spin coating process, dip coating process, curtain coating process, extrusion coating process, rod coating process and roll coating process. The rod coating process is particularly preferable. Thickness of the alignment film after drying is preferably 0.1 to 10 μm. The drying under heating maybe proceeded at 20⁰C to 110⁰C. In view of forming a sufficient degree of crosslinking, the temperature is preferably 60⁰C to 100⁰C, and more preferably 80°C to 100°C. Drying time may be 1 minute to to 36 hours, and preferably 1 minute to 30 minutes. Also pH is preferably set to a value optimum to a crosslinking agent to be used, wherein the range from pH3.5 to 9.5 is preferable, and 3.5 to 5.5 is more preferable, for the case where glutaraldehyde is used.

The alignment film is preferably provided on the transparent support. The alignment film is preferably such as being formed by crosslinking the polymer layer as described in the above. It is preferable to provide no rubbing for horizontal alignment of the discotic liquid crystalline compound. It is also allowable to align the liquid crystalline compound using the alignment film, to immobilize the liquid crystalline compound while keeping the state of alignment so as to form the retardation layer, and then to transfer only the retardation layer onto a polymer film (or transparent support) . [Retardation Plate]

The present invention relates to a retardation plate comprising the first and second retardation layer. The retardation plate of the present invention having the first and the second retardation layers contributes to improvement in the viewing angle characteristics of liquid crystal displays, in particular IPS-type liquid crystal displays. The retardation plate of the present invention can be disposed as a single optical component, in the liquid crystal display (preferably between the polarizer plate and the liquid crystal cell) . The retardation plate may also be bonded with the polarizer film, and may be incorporated as a polarizer plate into the liquid crystal display. Paragraphs below will explain the polarizer plate of the present invention, having the retardation plate and the polarizer film. [Polarizer Plate]

The polarizer plate of the present invention comprises the retardation plate of the present invention and a polarizer film.

According to the invention, known polarizer films such as iodine-containing polarizer film, dye-containing polarizer film using a dichroic dye, and polyene-base polarizer film may be employed as a polarizer film. The iodine-containing polarizer film and the dye-containing polarizer film are generally manufactured using polyvinyl alcohol-base film. The absorption axis of the polarizer film corresponds to the direction of stretching of the film. As a consequence, the polarizer film stretched in the longitudinal direction (direction of travel) has the absorption axis in parallel with the longitudinal direction, whereas the polarizer film stretched in the transverse direction (direction perpendicular to the direction of travel) has the absorption axis perpendicular to the longitudinal direction.

Preferable examples of the method of producing the polarizer plate of the present invention include a method comprising a step of continuously stacking the polarizer film and the retardation plate in their web form. The web-form polarizer plate is cut as being suited for the screen size of the liquid crystal display.

The polarizer film generally has a protective film. The retardation plate of the present invention can be made function as a protective film for the polarizer film, and in this case, there is no need of bonding an additional protective film to the polarizer film on the surface thereof to be bonded to the surface of the retardation plate. In the polarizer plate of the present invention, it is preferable that there is only disposed an isotropic adhesive layer, and/or a substantially isotropic transparent protective film between the polarizer film and the retardation plate of the present invention. The substantially isotropic transparent protective film (protective film for the polarizer film) is specifically a film having an in-plane retardation of 0 to 10 nm, and a thickness-direction retardation of -20 to 20 nm. A film containing cellulose acylate or cyclic polyolefin is preferable. Examples of the adhesive capable of forming the isotropic adhesive layer include polyvinyl alcohol-base adhesive, and an adhesive composed of polyester-base urethane and an isocyanate-base crosslinking agent.

The substantially isotropic low-Re cellulose acylate film which can be used as the transparent protective film preferably has an in-plane retardation Re (630) at 630 nm of 10 nm or smaller

(O≤Re (630) ≤IO) , and an absolute value of thickness-direction retardation Rth(630) of 25 nm or smaller (|Rth|≤25 nm), more preferably O≤Re ( 630)≤5 and |Rth|≤20 nm, and still more preferably

O≤Re (630) ≤2 and |Rth|≤15 nm. The low-Re cellulose acylate film preferably satisfies | Re (400) -Re (700) | ≤IO and

|Rth(400)-Rth(700) |≤35, more preferably | Re (400) -Re (700) |≤5 and

|Rth(400) -Rth(700) |≤25, and particularly preferably

|Re(400)-Re(700) |≤3 and | Rth (400) -Rth (700) | ≤15.

In a first embodiment of the polarizer plate of the present invention relates to a polarizer plate, the second retardation layer, the first retardation layer, and the polarizer film are disposed in this order, and the slow axis of the first retardation layer and the absorption axis of the polarizer film are substantially perpendicular to each other. In a second embodiment of the polarizer plate of the present invention, the second retardation layer, the first retardation layer, and the polarizer film are disposed in this order, and the slow axis of the first retardation layer and the absorption axis of the polarizer film are substantially parallel to each other. For the case where the first retardation layer is composed of a stretched polymer film, the direction of slow axis of the first retardation layer is adjustable typically by the direction of stretching. Although similar effect might be obtained even if the position of the first retardation layer and the second retardation layer is exchanged, it is preferable, for the case where the first retardation layer is composed of a polymer film (preferably a cellulose acylate film) , that the first retardation layer is disposed on the polarizer film side, in terms of enabling stable and simple production.

EXAMPLES

Paragraphs below will more specifically describe the features of the present invention, referring to Examples and Comparative Examples. Any materials, amount of use, ratio, details of processing, procedures of processing and the like may appropriately be modified, without departing from the spirit of the present invention. Therefore, the present invention should not limitedly be understood by the specific examples described below.

On a pair of glass substrates, as shown in FIG. 3, the electrodes (reference numerals 2 and 3 in FIG. 3) were provided so as to adjust the distance between the adjacent electrodes to 20 μm, thereon a polyimide film was provided as the alignment film, and rubbed. The rubbing treatment was carried out along with the direction 4 shown in FIG. 3. On one surface of another glass substrate obtained from elsewhere, a polyimide film is provided, and rubbed to obtain the alignment film. Two glass substrates were stacked and bonded, while opposing the individual alignment films with each other, keeping a 3.9-μm distance (gap; d) therebetween, and aligning the direction of rubbing of two glass substrates in parallel with each other, and a nematic liquid crystal composition having a refractive index anisotropy (Δn) of 0.0769 and a dielectric constant anisotropy (Δε) of 4.5 in positive was then filled with the space between the substrates. The liquid crystal layer was found to have a d*Δn value of 300 nm. <Preparation of First Retardation Layer> <1-1> Preparation of Cellulose Acylate Solution

A cellulose derivative having a degree of substitution of hydroxyl group of 1.2, a degree of substitution by acetyl group of 0.2, and a degree of substitution by the specific example No. 1 of the above-described aromatic acyl group of 1.6 and the composition below were put into a mixing tank, stirred for 6 hours so as to dissolve the individual components, to thereby prepare a cellulose derivative solution.

<l-2> Preparation of Matting Agent Dispersion

The composition below containing the cellulose derivative solution prepared by the above-described method was put into dispersion machine, to thereby prepare a matting agent dispersion.

Matting Agent Dispersion

Silica particle having a mean particle size of 16 nm (Aerosil R972 from Nippon Aerosil Co., Ltd.)

2.0 parts by mass

Methylene chloride 72.4 parts by mass

Methanol 10.8 parts by mass

Cellulose Derivative Solution described in the above

10.3 parts by mass

<l-3> Preparation of Retardation (Re) Adjusting Agent

The composition below containing the cellulose derivative solution prepared as described in the above was put into a mixing tank, dissolved by stirring under heating, to thereby prepare a Re adjusting agent solution.

Re Adjusting Agent Solution

Re adjusting agent (listed in Table 1)

20.0 parts by mass

Methylene chloride 58.3 parts by mass

Methanol 8.7 parts by mass

Cellulose acylate solution 12.8 parts by mass

The above-described solutions were mixed so as to adjust the matting agent to 0.15 parts by mass, and the Re adjusting agent to 15 parts by mass, per 100 parts by mass of cellulose acylate, and cotton was added into mixed solvent, dichloromethane/methanol (87/13 parts by mass) , under stirring, so as to adjust the mass concentration of cotton to 19% by mass, further stirred under heating, to thereby prepare a film-forming dope.

Thus-prepared dope was cast onto a metal support using a band casting machine, and the resultant self-supporting dope cast film was separated from the band. The separated dope film was held at the end thereof by a tenter, stretched 1.3 times in the film width, and dried thereafter while being held by the tenter, to thereby produce a cellulose acylate film of 80 μm thick, 100 m long in a longitudinal direction (direction of casting) , and 1.3 m wide in the width-wise direction. Through measurement of incident-angle-dependence of Re, using an automatic birefringence meter (KOBRA-21ADH, from Oji Scientific Instuments) , the cellulose acylate film No.l produced as described in the above was found to have an Re of 67 nm, an Rth of -200 nm, and consequently an Nz of -2.5. Thus-obtained film was used as the first retardation layer.

Cellulose acylate film Nos.2 to 5, and Nos.Hl and H2 were produced similarly to the method of producing the cellulose acylate film No.l, except that the materials shown in Table 1 were used respectively and stretching steps were carried out with stretching ratios listed in Table 1 respectively. Re, Rth and Nz values were calculated, and summarized in Table 1.

A coating liquid for preparing an alignment layer having a formulation below, was prepared and adjusted to pH3.5 using potassium hydroxide. The surface of the cellulose acylate film No.l produced as described in the above was saponified, and the coating liquid was applied to thus-saponified surface of the film, using a wire bar coater to an amount of 20 ml/m², and dried using a hot air at 60⁰C for 60 seconds, and further dried using a hot air at 100⁰C for 120 seconds, to thereby obtain an alignment film. (Formulation of Coating Liquid for Alignment Layer) Modified polyvinyl alcohol shown below

20 parts by mass

Potassium Hydroxide 0.05 parts by mass

Gultaraldehyde 0.5 parts by mass

Water 360 parts by mass

Methanol 120 parts by mass

Ethyl Citirate 0.35 parts by mass

(Preparation of Second Retardation Layer)

The ingredients shown below were dissolved in 102 parts by mass of methyl ethyl ketone, to thereby prepare a coating liquid for preparing a second retardation layer. Discotic Liquid Crystalline Compound shown below

41.01 parts by mass

Ethylene oxide-modified trimethylolpropane triacrylate (V#360, from Osaka Organic Chemical Industry, Ltd.)

4.06 parts by mass Triazine-ring-containing compound T-I shown below

0.04 parts by mass Fluoro-aliphatic polymer P-IOO shown below

0.18 parts by mass Photo-polymerization initiator (Irgacure 907, from Ciba Geigy)

1.35 parts by mass

Sensitizer (Kayacure DETX, from Nippon Kayaku Co., Ltd.) 0.45 parts by mass

Discotic liquid crystalline compound

Triazine-ring-containing compound T-I

Fluoro-aliphatic polymer P-IOO

The coating liquid was applied to the surface of the alignment film produced as described in the above, continuously using a #3.6 wire bar, heated at 130⁰C for 1 minute, to thereby align the molecules of the discotic liquid crystalline compound. The coating layer was then irradiated with UV at 100⁰C for 1 minute, using a high-pressure mercury lamp, to thereby polymerize the discotic liquid crystalline compound. The film was then cooled to room temperature, so as to obtain a retardation layer. Thickness of thus produced retardation plate was found to be 2.5 μm thick. In this way, the retardation film No.101 comprising the cellulose acylate film and the retardation layer was obtained. By measuring incident angle dependence of Re of thus-produced film No. 101 using an automatic birefringence meter (K0BRA-21ADH, from Oji Scientific Instuments) , and calculating optical characteristics ascribable only to the retardation layer, based on subtreaction of preliminarily-measured contribution by the cellulose acylate film No.l, Re was found to be 0.1 nm, and Rth was found to be 275 nm, proving that thus-produced retardation layer satisfies the optical characteristics necessary for the second retardation layer. It was also confirmed that the discotic liquid crystal molecules aligned near horizontally in the retardation layer.

On the other hand, retardation plate film Nos.102 to 107, and Nos.HlOl to 103 were also produced similarly to as described in the above, except that the triazine-ring compound T-I and/or fluoro-aliphatic polymer P-100, used in the previous production of the retardation layer, were not used, or replaced with the materials listed in Table 2, or that the cellulose acylate film No .1 used as a support was replaced respectively with the cellulose acylate films listed in Table 2.

It is understandable from the data shown in Table 1 and Table 2, in each of the retardation plate film Nos.102 to 107, the cellulose acylate film was found to satisfy the optical characteristics required for the first retardation layer, and the retardation layer formed thereon was found to satisfy the optical the optical characteristics required for the second retardation layer.

The ingredients shown below were put into a mixing tank, stirred under heating so as to dissolve the individual components, to thereby prepare a cellulose acetate solution "A". (Composition of Cellulose Acetate Solution "A") Cellulose acetate having a degree of substitution of 2.86

100 parts by mass Triphenyl phosphate (plasticizer)

7.8 parts by mass Biphenyl diphenyl phosphate (plasticizer)

3.9 parts by mass Methylene chloride (first solvent)

300 parts by mass

Methanol (second solvent) 54 parts by mass 1-Butanol 11 parts by mass

The ingredients shown below were put into another mixing tank, stirred under heating so as to dissolve the individual components, to thereby prepare an additive solution B-I. (Composition of Additive Solution B-I) Methylene chloride 80 parts by mass

Methanol 20 parts by mass

Optical anisotropy reducing agent

40 parts by mass

To 477 parts by mass of the cellulose acetate solution "A", 40 parts.by mass of the additive solution B-I was added, thoroughly stirred, to thereby prepare a dope. The dope was cast from a cast port onto a drum cooled to 0⁰C. The obtained film was peeled off while being kept at a solvent content of 70% by mass, held at both width-wise edges thereof with a pin tenter (a pin tenter shown in FIG. 3 of JPA No. H4-1009) , and dried while keeping a tenter width so as to keep a factor of stretching of 3% in the transverse direction (direction perpendicular to the mechanical direction) , under a state with a solvent content of 3 to 5% by mass . Thereafter, the film was further dried while allowing it to travel between rolls of an annealing apparatus, to thereby produce a low-Re cellulose acetate film of 80 μm thick.

From measurement of incident angle dependence of Re using an automatic birefringence meter (KOBRA-21ADH, from Oj i Scientific Instruments) , and calculation of the optical characteristics, Re was confirmed as 1 nm, and Rth was confirmed as 2.5 nm. <Production of Polarizer Plate "A">

Next, the stretched polyvinyl alcohol film was adsorbed with iodine to thereby produce the polarizer film. On the other hand, a commercially-available cellulose acetate film (Fujitack TD80UF, from FUJIFILM Corporation, Re=3 nm, Rth=45 nm) was saponified, and bonded to one surface of the polarizer film using a polyvinyl alcohol-base adhesive, to thereby produce a polarizer plate "A". <Production of Polarizer Plate B>

The stretched polyvinyl alcohol film was adsorbed with iodine to thereby produce the polarizer film. On the other hand, commercially-available cellulose acetate films (Fujitack TD80UF, from FUJIFILM Corporation) were saponified, and respectively bonded to both surfaces of the polarizer film using a polyvinyl alcohol-base adhesive, to thereby produce a polarizer plate B. <Production of Polarizer Plate C>

The polarizer film was similarly produced, the commercially-available cellulose acetate film (Fujitack TD80UF, from FUJIFILM Corporation) was saponified, and bonded to one surface of the polarizer film, using a polyvinyl alcohol-base adhesive. The low-Re cellulose acetate film produced in the above was bonded to the other surface of the polarizer film in the same manner, to thereby produce a polarizer plate C. <Production of Polarizer Plate No.l>

Using a polyvinyl alcohol-base adhesive, the retardation film No.101 was bonded to the surface of the polarizer plate "A" (the surface having no commercially-available cellulose acetate film bonded thereto) , so that the surface of the first retardation layer was disposed at the polarizer film side, and so that the absorption axis of the polarizer film and the slow axis of the first retardation layer are perpendicular to each other, to thereby produce a polarizer plate No.l.

The polarizer plate No. 1 was then bonded to one surface of the IPS-mode liquid crystal cell 1 produced in the above, so that the slow axis of the first retardation layer was parallel to the rubbing direction of the liquid crystal cell (that is, so that the slow axis of the first retardation layer was parallel to the mean slow axis of the liquid crystal molecules of the liquid crystal cell in the black state) , and so that the surface of the second retardation layer was disposed at the liquid crystal cell side. Next, on the other side of the IPS-mode liquid crystal cell, the polarizer plate C was bonded so that the low-Re cellulose acetate film was disposed at the liquid crystal cell side, and so that the polarizer plate 1 and the polarizer plate C were in a cross-Nicole arrangement in terms of absorption axes, to thereby produce a liquid crystal display No .201. Display characteristics of thus-produced liquid crystal display were evaluated as below. Results are shown in Table 2. (Evaluation of Display Characteristics)

Viewing angle dependence of light transmissivity of the fabricated liquid crystal display was measured. Measurement was carried out while varying the elevation angle from the front to oblique directions up to 80° at 10° intervals, and varying the azimuth angle as started from the right horizontal direction (0°) up to 360° at 10° intexvals. Differences in viewing-angle-dependent display characteristics (viewing-angle-dependent differences in hue and contrast, may occasionally be referred to as "viewing-angle-dependent characteristic difference", hereinafter) were evaluated, by observing hue and transmissivity at the individual viewing angles. Evaluation of Display Characteristics:

O : Good, showing only a slight viewing-angle-dependent characteristic difference;

Δ : Almost no viewing-angle-dependent characteristic difference; and

X : Large viewing-angle-dependent characteristic difference.

Other liquid crystal displays were also fabricated in the same manner as described above, except that the retardation film No.101 in the fabrication of the liquid crystal display No.201 was replaced respectively with the retardation film Nos.102 to 107 and Nos.HlOl to H103, and display characteristics were similarly measured. Results of evaluation are shown in Table 2.

[Liquid crystal display Nos.202 to 207]

Using the retardation film No.101, but according to the arrangements shown in Table 3, liquid crystal display Nos.202 to 208 were fabricated in the same manner as the liquid crystal display No.201, and the display characteristics were evaluated in the same manner as described above. [Liquid crystal display NO.H201 (Comparative Example)]

A liquid crystal display was fabricated by bonding the polarizer plate C, on both sides of the IPS-mode liquid crystal cell 1 fabricated in the above, according to the cross-Nicole arrangement. The polarizer plate was bonded so that the absorption axis of the first polarizer film was perpendicular to the rubbing direction of the liquid crystal cell, in the same manner as fabrication of the^' liquid crystal display No.201. Leakage of light from thus-fabricated liquid crystal display was measured. The display characteristic was evaluated as "x".

[Liquid crystal display No.H202 (Comparative Example)]

A polarizer plate was fabricated by bonding, using a polyvinyl alcohol-base adhesive, the cellulose acylate film No.1 produced in the above, to the polarizer plate "A" on the side thereof having no cellulose acetate film as the polarizer film bonded thereto, so that the film was disposed at the liquid crystal cell side, and so that the transmission axis of the polarizer film and the slow axis of the film were perpendicular to each other. In short, a polarizer plate 5 comprising only the first retardation layer but comprsing no second retardation layer was produced.

The polarizer plate 5 was bonded to one surface of the IPS-mode liquid crystal cell 1 fabricated in the above, so that the slow axis of the first retardation layer (cellulose acylate film No.1) was perpendicular to the rubbing direction of the liquid crystal cell (that is, so that the slow axis of the first retardation layer was perpendicular to the mean slow axis of the liquid crystal molecules of the liquid crystal cell in the black state) . Next, on the other surface of the IPS-mode liquid crystal cell 1, the polarizer plate "A" was bonded so that the polarizer plate protective film was disposed at the liquid crystal cell 1 side, and so that the polarizer plate "A" and the polarizer plate 5 were in the cross-Nicole arrangement, to thereby produce a liquid crystal display. Leakage of light from thus-produced liquid crystal display was measured. The display characteristic was evaluated as "Δ".

Table 3-3

* "Horizontal" and "Vertical" correspond to left-right direction and top-bottom direction of display, respectively.

* "Arbitrary" means "Horizontal" or "Vertical" direction.

* "None" means no retardation plate introduced.

Industrial Applicability

According to the invention, it is possible to improve the off-axis, especially in a 45° oblique-direction, contrast without degradation in the normal-direction displaying quality. Namely, according to the invention, it is possible to provide a liquid crystal display, in particular an IPS-type liquid crystal display, improved not only in the display quality, but also in the viewing angle properties, with a simple configuration. And it is also possible to provide a novel retardation plate contributing to improvement in the viewing angle properties of the liquid crystal display, in particular of the IPS-type liquid crystal display, and a polarizer plate using the same.

Cross-Reference to Related Applications

This application claims benefit of priority under 35 USC 119 to Japanese Patent Application No. 2006-014832 filed January 24, 2006.

Claims

1. A retardation plate comprising a first retardation layer and a second retardation layer, the first retardation layer having an in-plane retardation Re of 20 run to 150 nm, and an Nz value, defined as Nz=Rth/Re+0.5 using an in-plane retardation Re and thickness-direction retardation Rth, of -6.5 to 0.5, and the second retardation layer having Re which satisfies |Re|<30 run, and Rth which ranges from 80 nm to 400 ran.

2. The retardation plate of claim 1, wherein the first retardation layer is a cellulose-derivative film.

3. The retardation plate of claim 2, wherein the cellulose derivative film is a film obtained by casting a dope containing a cellulose derivative, which satisfies the conditions (a) and (b) below, according to a solvent casting method to form a film and stretching the film:

(a) the cellulose acylate derivative has at least one substituent having a polarizability anisotropy Δα, defined by a mathematical expression (1) below, equal to or larger than 2.5^χlO^~24, in the place of at least one hydrogen of three hydroxyl groups in a glucose unit of cellulose:

(1) : Δα=α_x- (α_y+α_z) /2 where α_x is the largest component among eigen values obtained by diagonalizing polarizability tensor; α_y is the second largest component among eigen values obtained by diagonalizing polarizability tensor; and α_z is a smallest component among eigen values obtained by diagonalizing polarizability tensor; and

(b) substitution degrees P_A and PB, respectively representing a substitution degree of a substituent having a Δα equal to or larger than 2.5^χlO^~24 cm³, and a substitution degree of a substituent having a Δα smaller than 2.5^χlO^~24 cm³, satisfy mathematical expressions (3) and (4) below:

(3) : 2P_A+P_B>3.0

(4) : P_A>0.2.

4. The retardation plate of claim 3, wherein the substituent having a Δα equal to or larger than 2.5x10²⁴ cm³ is an aromatic acyl group, and a substituent having a Δα smaller than 2.5^χlO^~24 cm³ is an aliphatic acyl group.

5. The retardation plate of claim 3, wherein the dope contains at least one retardation adjusting agent.

6. The retardation plate of claim 5, wherein the retardation adjusting agent is a compound represented by the formula (I) below:

Formula (I)

where, Ar¹, Ar² and Ar³ respectively represent an aryl group or an aromatic heterocyclic group; L¹ and L² respectively represent a single bond or a divalent linking group; n is an integer equal to or larger than 3; and Ar² and L² may be same or may be different respectively.

7. The retardation plate of claim 1, wherein the second retardation layer is formed of a composition comprising a discotic liquid crystal compound; and the mean director of molecules of the discotic liquid crystal compound is normal to a layer plane of the second retardation layer.

8. The retardation plate of claim 1, wherein the second retardation layer comprises at least one compound having at least one triazine ring group.

9. The retardation plate of claim 1, wherein the second retardation layer comprises at least one fluoro-aliphatic polymer .

10. The retardation plate of claim 9, wherein the fluoro-aliphatic polymer is a copolymer (polymer "A") comprising at least one repeating unit derived from a compound having a fluoro-aliphatic group, and at least one repeating unit represented by the formula (5) below: Formula (5)

where, R¹, R² and R³ respectively represent a hydrogen atom or a substitution; L represents a divalent linking group selected from Linking Group shown below or a divalent linking group consisting of two or more selected from Linking Group shown below;

(Linking Group)

A single bond, -0-, -CO-, -NR⁴- (R⁴ represents a hydrogen atom, an alkyl group, an aryl group, or an aralkyl group) , -S-, -SO₂-, -P(=0) (OR⁵)- (R⁵ represents an alkyl group, an aryl group, or an aralkyl group) , an alkylene group, and an arylene group; and

Q represents a carboxyl group ( -COOH) or salt thereof, a sulfo group (-SO₃H) or salt thereof, or a phosphoxy group {-OP(=0) (OH)₂I or salt thereof.

11. A polarizer plate comprising, at least, a polarizer film and a retardation plate comprising: a first retardation layer and a second retardation layer, wherein the first retardation layer has an in-plane retardation Re of 20 nm to 150 nm, and an Nz value, defined as Nz=Rth/Re+0.5 using an in-plane retardation Re and thickness-direction retardation Rth, of -6.5 to 0.5, and the second retardation layer has Re which satisfies |Re|<30 nm, and Rth which ranges from 80 nm to 400 nm.

12. The polarizer plate of claim 11, wherein the polarizer film, the first retardation layer and the second retardation layer are disposed in this order; and the direction of slow axis of the first retardation layer and the direction of absorption axis of the polarizer film are substantially perpendicular to each other.

13. The polarizer plate of claim 11, wherein the polarizer film, the first retardation layer and the second retardation layer are disposed in this order; and the direction of slow axis of the first retardation layer and the direction of absorption axis of the polarizer film are substantially parallel to each other.

14. A liquid crystal display comprising, at least, a liquid crystal cell and a polarizer plate as set forth in claim 11.

15. A liquid crystal display comprising, at least, a first polarizer film, a first retardation layer, a second retardation layer, and a liquid crystal cell comprising a pair of substrates and a liquid crystal layer disposed between the pair of substrates, wherein the first retardation layer has an in-plane retardation Re of 20 nm to 150 nm, and an Nz value, defined as Nz=Rth/Re+0.5 using an in-plane retardation Re and thickness-direction retardation Rth, of -6.5 to 0.5, and the second retardation layer hs Re which satisfies |Re|<30 nm, and Rth which ranges from 80 nm to 400 nm.

16. The liquid crystal display of claim 15, further comprising a second polarizer film, wherein the liquid crystal cell is disposed between the first and second polarizer film; and the absorption axes of the first and the second polarizer films are perpendicular to each other.

17. The liquid crystal display of claim 16, wherein there is disposed only a substantially isotropic adhesive layer and/or a substantially isotropic transparent protective film between the second polarizer film, and one of the pair of substrates placed more closer to the second polarizer film.

18. The liquid crystal display of claim 17, wherein the transparent protective film is a cellulose acetate film which satisfies the relational expressions (I) and (II) below: ( I ) : 0 < Re ( 630 ) ≤ 10 , and | Rth ( 630 ) | < 25

( II ) : | Re ( 400 ) -Re ( 700 ) I < 10 , and | Rth ( 400 ) -Rth (700 ) | <35 where, Re (λ) is an in-plane retardation value (unit: ran) at a wavelength of λ ran, and Rth(λ) is a thickness-direction retardation value (unit: ran) at a wavelength of λ ran.

19. The liquid crystal display of claims 15, wherein the second retardation layer, the first retardation layer, and the first polarizer film are disposed in this order as viewed from the liquid crystal cell side; liquid crystal molecules in the liquid crystal layer are aligned horizontally with respect to the pair of substrates, and a mean direction of their long axes is parallel to a slow axis of the first retardation layer in a black state; and an absorption axis of the first polarizer film and the slow axis of the first retardation layer are perpendicular to each other.

20. The liquid crystal display of claims 15, wherein the second retardation layer, the first retardation layer, and the first polarizer film are disposed in this order as viewed from the liquid crystal cell side, liquid crystal molecules in the liquid crystal layer are horizontally with respect to the pair of substrates and a mean direction of their long axes is perpendicular to a slow axis of the first retardation layer in a black state; and the absorption axis of the first polarizer film and the slow axis of the first retardation layer are parallel to each other.