JP4697098B2 - Retardation film - Google Patents

Description

  The present invention relates to a retardation film having excellent optical characteristics such as a large refractive index in the thickness direction of the film, a large in-plane retardation, and a small wavelength dependency.

  Liquid crystal displays are widely used as the most important display devices in the multimedia society, from mobile phones to computer monitors, notebook computers, and televisions. Many optical films are used in liquid crystal displays to improve display characteristics. In particular, the retardation film plays a large role in improving contrast and compensating for color tone when viewed from the front or obliquely. As a conventional retardation film, polycarbonate or cyclic polyolefin is used, and these polymers are polymers having positive birefringence. Here, the sign of birefringence is defined as shown below.

  The optical anisotropy of the polymer film molecularly oriented by stretching or the like can be represented by a refractive index ellipsoid shown in FIG. Here, the refractive index in the fast axis direction in the film plane when the film is stretched is denoted by nx, the refractive index in the direction perpendicular thereto is denoted by ny, and the refractive index in the thickness direction of the film is denoted by nz. The fast axis is an axial direction having a low refractive index in the film plane.

  Negative birefringence means that the stretching direction is the fast axis direction, and positive birefringence means that the direction perpendicular to the stretching direction is the fast axis direction.

  That is, the refractive index in the stretching axis direction is small in uniaxial stretching of a polymer having negative birefringence (fast axis: stretching direction), and the axis orthogonal to the stretching axis direction in uniaxial stretching of a polymer having positive birefringence. The refractive index in the direction is small (fast axis: direction perpendicular to the stretching direction).

  The in-plane retardation (Re) is expressed as a value obtained by multiplying the refractive index (ny) in the direction perpendicular to the fast axis direction−the refractive index (ny) in the fast axis direction by the film thickness.

  Many polymers have positive birefringence. As a polymer having negative birefringence, there are acrylic resin and polystyrene. However, acrylic resin has a small retardation, and the properties as a retardation film are not sufficient. Polystyrene has a large photoelastic coefficient at room temperature and a phase difference that changes with a slight stress, such as a problem of stability of the phase difference, a problem of optical properties such as a large wavelength dependency of the phase difference, and low heat resistance. It is not used at present because of practical problems.

  Here, the wavelength dependence of the phase difference means that the phase difference changes depending on the measurement wavelength, and the ratio of the phase difference (R450) measured at a wavelength of 450 nm to the phase difference (R550) measured at a wavelength of 550 nm. It can be expressed as R450 / R550. In general, a polymer having an aromatic structure has a strong tendency to increase R450 / R550, and the contrast and viewing angle characteristics in a low wavelength region are deteriorated.

  Since a stretched polymer film exhibiting negative birefringence has a high refractive index in the thickness direction of the film and becomes an unprecedented retardation film, for example, a super twist nematic liquid crystal display (STN-LCD) or a vertical alignment liquid crystal display (VA-LCD), In-plane alignment type liquid crystal display (IPS-LCD), Reflective type liquid crystal display (reflection type LCD), etc. There is a strong market demand for retardation films having negative birefringence. There has been proposed a film manufacturing method in which a polymer having positive birefringence is used to increase the refractive index in the thickness direction of the film. One is a treatment method in which a heat-shrinkable film is bonded to one or both sides of a polymer film, the laminate is heated and stretched, and a shrinkage force is applied in the film thickness direction of the polymer film (see, for example, Patent Documents 1 to 3). .) In addition, a method of uniaxially stretching in a plane while applying an electric field to a polymer film has been proposed (see, for example, Patent Document 4).

  In addition, a retardation film composed of fine particles having negative optical anisotropy and a transparent polymer has been proposed (see, for example, Patent Document 5).

Japanese Patent No. 2818983 JP 05-297223 A JP 05-323120 A Japanese Patent Laid-Open No. 06-088909 JP 2005-156862 A

  However, the methods proposed in Patent Documents 1 to 4 have a problem that the manufacturing process is very complicated and the productivity is inferior. In addition, the control of the uniformity of the phase difference and the like is significantly more difficult than the conventional control by stretching. When polycarbonate is used as the base film, the photoelastic constant at room temperature is large, and there is a problem in the stability of the phase difference, such as the phase difference being changed by a slight stress. Furthermore, there are problems such as the large wavelength dependence of the phase difference.

  Further, the retardation film obtained in Patent Document 5 is a retardation film having negative birefringence by adding fine particles having negative optical anisotropy, and from the viewpoint of simplification of the production method and economical efficiency. There is a need for a retardation film that does not require the addition of fine particles.

  Therefore, an object of the present invention is to provide a retardation film made of a specific resin and having excellent optical characteristics.

  As a result of intensive studies in view of the above problems, the inventors of the present invention are films made of a specific resin, and a retardation film in which a three-dimensional refractive index when the film is stretched satisfies a specific relationship has the above problems. The present inventors have found that they are satisfied and have completed the present invention.

  That is, the present invention is made of a fumaric acid ester resin, and the refractive index in the fast axis direction in the film plane is nx, the refractive index in the direction perpendicular thereto is ny, and the refractive index in the thickness direction of the film is nz. Is related to a retardation film, wherein nx <ny ≦ nz.

  Hereinafter, the retardation film of the present invention will be described in detail.

Examples of the fumaric acid ester-based resin used in the present invention include fumaric acid ester polymers. Among them, fumaric acid ester-based resins composed of 50 mol% or more of fumaric acid diester residue units represented by the general formula (1) are included. preferable. Here, R ₁ and R ₂ which are ester substituents of the fumaric acid diester residue unit are each independently a branched alkyl group having 3 to 12 carbon atoms or a cyclic alkyl group, and halogen such as fluorine and chlorine. Group; ether group; may be substituted with ester group or amino group, for example, isopropyl group, s-butyl group, t-butyl group, s-pentyl group, t-pentyl group, s-hexyl group, t-hexyl group Group, cyclopropyl group, cyclopentyl group, cyclohexyl group, and the like. Among them, isopropyl group, s-butyl group, t-butyl group, cyclopentyl group, cyclohexyl group are obtained because it is a retardation film having excellent heat resistance and mechanical properties. An isopropyl group is preferable because a retardation film having an excellent balance between heat resistance and mechanical properties is obtained.

  Here, as the fumaric acid diester residue unit represented by the general formula (1), specifically, a diisopropyl fumarate residue, a di-s-butyl fumarate residue, a di-t-butyl fumarate residue, Di-s-pentyl fumarate residue, di-t-pentyl fumarate residue, di-s-hexyl fumarate residue, di-t-hexyl fumarate residue, dicyclopropyl fumarate residue, fumaric acid Examples include dicyclopentyl residue, dicyclohexyl fumarate residue, diisopropyl fumarate residue, di-s-butyl fumarate residue, di-t-butyl fumarate residue, dicyclopentyl fumarate residue, fumaric acid A dicyclohexyl residue and the like are preferable, and a diisopropyl fumarate residue is particularly preferable.

  The fumaric acid ester resin composed of 50 mol% or more of the fumaric acid diester residue unit represented by the general formula (1), which is preferably used as the fumaric acid ester resin used in the present invention, is represented by the general formula (1). Monomer capable of copolymerization with fumaric acid diesters, a resin comprising 50 mol% or more of fumaric acid diester residue units and 50 mol% or less of residue units composed of monomers copolymerizable with fumaric acid diesters Residue units comprising, for example, styrene residues such as styrene residues and α-methylstyrene residues; acrylic acid residues; methyl acrylate residues, ethyl acrylate residues, butyl acrylate residues and the like Acrylic acid ester residues; methacrylic acid residues; methacrylic acid esters such as methyl methacrylate residues, ethyl methacrylate residues, butyl methacrylate residues Vinyl ester residues such as vinyl acetate residues and vinyl propionate residues; acrylonitrile residues; methacrylonitrile residues; olefin residues such as ethylene residues and propylene residues; Or 2 or more types can be mentioned. Among them, the fumaric acid diester residue unit represented by the general formula (1) is preferably 70 mol% or more, and the fumaric acid diester residue unit is particularly excellent in heat resistance and mechanical properties. Is preferably 80 mol% or more, more preferably 90 mol% or more.

The fumaric acid ester resin used in the present invention has a number average molecular weight (Mn) of 1 × 10 in terms of standard polystyrene obtained from an elution curve measured by gel permeation chromatography (hereinafter referred to as GPC). It is preferably ⁴ or more. Particularly, it is preferably 2 × 10 ⁴ or more and 2 × 10 ⁵ or less because it is a retardation film having excellent mechanical properties and excellent moldability during film formation.

  As a method for producing the fumaric acid ester resin constituting the retardation film of the present invention, any method may be used as long as the fumaric acid ester resin is obtained. For example, fumaric acid diesters, and in some cases fumaric acid ester It can be produced by performing radical polymerization or radical copolymerization using a monomer copolymerizable with acid diesters. Examples of the fumaric acid diesters include diisopropyl fumarate, di-s-butyl fumarate, di-t-butyl fumarate, di-s-pentyl fumarate, di-t-pentyl fumarate, and di-fumarate. -S-hexyl, di-t-hexyl fumarate, dicyclopropyl fumarate, dicyclopentyl fumarate, dicyclohexyl fumarate and the like. Examples of monomers copolymerizable with fumarate diester include styrene, α -Styrenes such as methylstyrene; Acrylic acid; Acrylic esters such as methyl acrylate, ethyl acrylate and butyl acrylate; Methacrylic acid; Methacrylic esters such as methyl methacrylate, ethyl methacrylate and butyl methacrylate; Vinyl esters such as vinyl acetate and vinyl propionate; acrylonitrile Methacrylonitrile; can be exemplified one or two or more such, ethylene, olefins such as propylene.

  Further, as the radical polymerization method to be used, it can be carried out by a known polymerization method, for example, any of a bulk polymerization method, a solution polymerization method, a suspension polymerization method, a precipitation polymerization method, an emulsion polymerization method and the like can be adopted. is there.

  Examples of the polymerization initiator used in the radical polymerization method include benzoyl peroxide, lauryl peroxide, octanoyl peroxide, acetyl peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, and dicumyl peroxide. , Organic peroxides such as t-butylperoxyacetate and t-butylperoxybenzoate; 2,2′-azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (2-butyronitrile), 2 , 2′-azobisisobutyronitrile, dimethyl-2,2′-azobisisobutyrate, 1,1′-azobis (cyclohexane-1-carbonitrile) and the like.

  And there is no restriction | limiting in particular as a solvent which can be used in a solution polymerization method, a suspension polymerization method, a precipitation polymerization method, and an emulsion polymerization method, For example, aromatic solvents, such as benzene, toluene, xylene; Methanol, ethanol, propyl alcohol, butyl alcohol Examples include alcohol solvents such as cyclohexane, dioxane, tetrahydrofuran (THF), acetone, methyl ethyl ketone, dimethylformamide, isopropyl acetate, water, and the like, and mixed solvents thereof.

  Moreover, the polymerization temperature at the time of performing radical polymerization can be suitably set according to the decomposition temperature of a polymerization initiator, and generally it is preferable to carry out in the range of 40-150 degreeC.

  The retardation film of the present invention is a retardation film made of the fumarate ester resin and does not require addition of fine particles.

  The retardation film of the present invention has the following relationship when the refractive index in the fast axis direction in the film plane is nx, the refractive index in the vertical direction is ny, and the refractive index in the thickness direction of the film is nz. A retardation film characterized by nx <ny ≦ nz. By satisfying nx <ny ≦ nz, viewing angle compensation performance of STN-LCD, IPS-LCD, reflective LCD, transflective LCD, etc. It becomes an excellent retardation film.

  In addition, since the retardation film of the present invention becomes a retardation film having more excellent optical properties, the in-plane retardation (Re) measured at a wavelength of 550 nm represented by the formula (a) is 50 to 2000 nm. Is particularly preferable, and it is preferably 50 to 1000 nm, more preferably 100 to 500 nm.

Re = (ny−nx) × d (a)
(Here, d indicates the thickness of the film.)
More specifically regarding the in-plane retardation (Re) measured at a wavelength of 550 nm represented by the above formula (a), the retardation film of the present invention is used for viewing angle compensation of a liquid crystal display in TN, VA, IPS, or OCB mode. When used as a retardation film, the in-plane retardation (Re) is preferably 50 nm or more, particularly preferably 100 nm or more, and more preferably 135 nm or more.

  Further, the in-plane retardation (Re) is preferably 100 to 200 nm when used as a circularly polarizing film obtained by laminating and integrating a retardation film and a polarizing plate. The circularly polarizing film is useful for an antireflection film such as an organic EL display, a brightness enhancement film, and the like in addition to a compensation film for a reflective liquid crystal display.

  Further, the in-plane retardation (Re) when the retardation film of the present invention is used as a half-wave film is preferably 200 to 400 nm, and is used for STN mode liquid crystal displays and for viewing angle compensation of brightness enhancement films. The in-plane retardation (Re) at the time is preferably 50 to 1000 nm.

  The wavelength dependence of the phase difference can be expressed as a ratio R450 / R550 of the phase difference (R450) measured at a wavelength of 450 nm and the phase difference (R550) measured at a wavelength of 550 nm. In the retardation film of the present invention, R450 / R550 is preferably 1.1 or less, particularly preferably 1.08 or less, and more preferably 1.05 or less.

  The thickness of the retardation film is preferably 10 to 400 μm, particularly preferably 20 to 150 μm, and further preferably 30 to 100 μm.

  There is no restriction | limiting in particular as a manufacturing method of the retardation film of this invention, For example, it can manufacture by forming into a film by methods, such as a solution cast method and a melt-cast method, and extending | stretching this film to a uniaxial or biaxial. .

  The solution casting method is a method of obtaining a film by casting a solution in which a fumaric ester resin is dissolved in a solvent (hereinafter referred to as a dope) on a support substrate and then removing the solvent by heating or the like. In this case, as a method for casting the dope on the support substrate, for example, a T-die method, a doctor blade method, a bar coater method, a roll coater method, a lip coater method or the like is used. Particularly, industrially, a method of continuously extruding a dope from a die onto a belt-like or drum-like support substrate is most common. Examples of the support substrate used include a glass substrate; a metal substrate such as stainless steel or ferrotype; and a plastic substrate such as polyethylene terephthalate. In the solution casting method, when forming a film having high transparency and excellent thickness accuracy and surface smoothness, the solution viscosity of the dope is a very important factor, and preferably 700 to 30000 cps. It is preferable that it is 1000-10000 cps. The melt casting method is a molding method in which a fumarate ester resin is melted in an extruder and extruded from a slit of a T die into a film shape, and then cooled and cooled with a roll or air.

  And the obtained film can be used as the retardation film of the present invention by controlling the retardation by stretching uniaxially or biaxially. Examples of the uniaxial stretching method include a free-width uniaxial stretching method, a stretching method using a tenter, and a stretching method between rolls. The biaxial stretching method includes, for example, a stretching method using a tenter or a tube-like expansion. There are methods. As the stretching conditions, unevenness in thickness is unlikely to occur, and since the retardation film has excellent mechanical properties and optical properties, the stretching temperature is preferably 80 to 250 ° C, particularly preferably 120 to 220 ° C. The draw ratio is preferably 1.01 to 5 times, particularly preferably 1.01 to 2 times.

  In the retardation film of the present invention, it is preferable that an antioxidant is blended in order to increase the thermal stability of the retardation film itself or during film formation. Examples of the antioxidant include hindered phenol antioxidants, phosphorus antioxidants, and other antioxidants. These antioxidants may be used alone or in combination. And since an antioxidant effect | action improves synergistically, it is preferable to use together and use a hindered phenolic antioxidant and phosphorus antioxidant, for example, 100 weight part of hindered phenolic antioxidants in that case On the other hand, it is particularly preferable to use a phosphorus-based antioxidant mixed at 100 to 500 parts by weight. Moreover, as addition amount of antioxidant, 0.01-10 weight part is preferable with respect to 100 weight part of fumaric acid ester-type resin which comprises the retardation film of this invention, Especially 0.5-1 weight part is preferable. A range is preferable.

  Furthermore, as an ultraviolet absorber, for example, an ultraviolet absorber such as benzotriazole, benzophenone, triazine, or benzoate may be blended as necessary.

  The retardation film of the present invention is within the scope of the invention, and other polymers, surfactants, polymer electrolytes, conductive complexes, inorganic fillers, pigments, dyes, antistatic agents, antiblocking agents, lubricants, etc. May be blended.

  The retardation film of the present invention can be laminated with a polarizing plate to be used as a circular or elliptical polarizing plate, or further laminated with a polarizer made of polyvinyl alcohol / iodine or the like to form a polarizing plate. Moreover, it can also laminate | stack with the retardation films of this invention, or other retardation films.

  According to the present invention, it is useful as a compensation film or antireflection film for contrast and viewing angle characteristics of liquid crystal displays, and has optical characteristics such as a large refractive index in the thickness direction of the film, a large in-plane retardation, and a small wavelength dependency. An excellent retardation film can be provided.

  EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise noted, commercially available reagents were used.

~ Measurement of number average molecular weight ~
Using gel permeation chromatography (GPC) (manufactured by Tosoh Corporation, trade name HLC-802A), THF was used as a solvent and a standard polystyrene conversion value was obtained.

-Measurement of light transmittance and haze of film-
The light transmittance and haze of the produced film were measured using a haze meter (trade name NDH2000, manufactured by Nippon Denshoku Industries Co., Ltd.).

-Measurement method of refractive index-
Measurement was performed using an Abbe refractometer (manufactured by Atago) in accordance with JIS K 7142 (1981 version).

-Measurement of retardation and three-dimensional refractive index of film-
Measurement was performed using a fully automatic birefringence meter (trade name KOBRA-21ADH, manufactured by Oji Scientific Instruments).

~ Positive birefringence ~
Positive and negative birefringence was determined from the three-dimensional refractive index of the stretched film.

Synthesis Example 1 (Production Example of Diisopropyl Fumarate Homopolymer)
A 30 liter autoclave was charged with 18 kg of distilled water containing 0.2% by weight of partially saponified polyvinyl alcohol, 3 kg of diisopropyl fumarate, and 7 g of dimethyl-2,2′-azobisisobutyrate as a polymerization initiator, and a polymerization temperature. The suspension radical polymerization reaction was carried out under the conditions of 50 ° C. and a polymerization time of 24 hours. The obtained particles were filtered, sufficiently washed with methanol, and dried at 80 ° C. to obtain diisopropyl fumarate homopolymer. The number average molecular weight of the obtained diisopropyl fumarate homopolymer was 160,000.

Film creation example 1
The diisopropyl fumarate homopolymer obtained in Synthesis Example 1 was dissolved in a THF solution to give a 22% solution, and tris (2,4) as a hindered phenol-based antioxidant was added to 100 parts by weight of the diisopropyl fumarate homopolymer. -Di-tert-butylphenyl) phosphite 0.35 parts by weight and pentaerythritol-tetrakis (3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate) 0.15 as phosphorus antioxidant After adding 1 part by weight of 2- (2H-benzotriazol-2-yl) -p-cresol as an ultraviolet absorber, it was cast on a support substrate of a solution casting apparatus by T-die method, After drying at 80 ° C. and 120 ° C. for 15 minutes, a film having a width of 250 mm and a thickness of 120 μm was obtained.

  The obtained film had a light transmittance of 93%, a haze of 0.3%, and a refractive index of 1.470.

Film creation example 2
A polycarbonate resin (manufactured by Aldrich) is dissolved in a methylene chloride solution to make a 25% solution. Further, with respect to 100 parts by weight of the polycarbonate resin, 0.35 weight of tris (2,4-di-t-butylphenyl) phosphite as an antioxidant. Parts and pentaerythritol-tetrakis (3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate) 0.15 parts by weight, 2- (2H-benzotriazol-2-yl)-as UV absorber After adding 1 part by weight of p-cresol, it is cast on a support substrate of a solution casting apparatus by the T-die method, dried at 40 ° C., 80 ° C. and 120 ° C. for 15 minutes, respectively, and then a film having a width of 250 mm and a thickness of 100 μm Got.

  The obtained film had a light transmittance of 91%, a haze of 0.6%, and a refractive index of 1.583.

Film production example 3
A cyclic polyolefin resin (polynorbornene having an ester group, manufactured by Aldrich) is dissolved in a methylene chloride solution to form a 25% solution. Further, tris (2,4-di-t-) is used as an antioxidant for 100 parts by weight of the cyclic polyolefin resin. Butylphenyl) phosphite 0.35 parts by weight and pentaerythritol-tetrakis (3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate) 0.15 parts by weight, 2- (2H as UV absorber) After adding 1 part by weight of -benzotriazol-2-yl) -p-cresol, it was cast on a support substrate of a solution casting apparatus by the T-die method and dried at 40 ° C, 80 ° C and 120 ° C for 15 minutes each. Thereafter, a film having a width of 250 mm and a thickness of 100 μm was obtained.

  The obtained film had a light transmittance of 92%, a haze of 0.4%, and a refractive index of 1.510.

Example 1
The film obtained in Film Preparation Example 1 was cut into a square of 50 mm, and the temperature was 140 ° C. and the stretching speed was 10 mm / min. With a biaxial stretching apparatus (manufactured by Imoto Seisakusho). The film was subjected to free-width uniaxial stretching under the conditions of 1.125 times. As a result of measuring the three-dimensional refractive index of the stretched film, the refractive index in the stretching axis direction was small, and thus the obtained film had negative birefringence.

  From the results of the three-dimensional refractive index measurement (nx = 1.4681, ny = 1.4692, nz = 1.4727), the obtained film had a large refractive index in the thickness direction of nx <ny <nz. The in-plane retardation Re = (ny−nx) × d was as large as 131 nm. Furthermore, the phase difference ratio (R450 / R550) (wavelength dependence) was as low as 1.02.

  From these results, the obtained film has a negative birefringence, a large refractive index in the thickness direction, a large in-plane retardation, and a small wavelength dependence, so that it is suitable for a retardation film. there were.

Example 2
The film obtained in Film Preparation Example 1 was cut into a square of 50 mm, and the temperature was 140 ° C. and the stretching speed was 10 mm / min. With a biaxial stretching apparatus (manufactured by Imoto Seisakusho). The film was subjected to uniaxial stretching with a free width under the conditions of 1.25 times. As a result of measuring the three-dimensional refractive index of the stretched film, the refractive index in the stretching axis direction was small, and thus the obtained film had negative birefringence.

  From the results of the three-dimensional refractive index measurement (nx = 1.4667, ny = 1.4704, nz = 1.4729), the obtained film had a large refractive index in the thickness direction of nx <ny <nz. The in-plane retardation Re = (ny−nx) × d was as large as 418 nm. Furthermore, the phase difference ratio (R450 / R550) (wavelength dependence) was as low as 1.02.

  From these results, the obtained film has a negative birefringence, a large refractive index in the thickness direction, a large in-plane retardation, and a small wavelength dependence, so that it is suitable for a retardation film. there were.

Example 3
The film obtained in Film Preparation Example 1 was cut into a square of 50 mm, and the temperature was 140 ° C. and the stretching speed was 10 mm / min. With a biaxial stretching apparatus (manufactured by Imoto Seisakusho). The film was subjected to free width uniaxial stretching under the conditions of 1.375 times. As a result of measuring the three-dimensional refractive index of the stretched film, the refractive index in the stretching axis direction was small, and thus the obtained film had negative birefringence.

  From the results of the three-dimensional refractive index measurement (nx = 1.4653, ny = 1.4714, nz = 1.4733), the obtained film had a large refractive index in the thickness direction of nx <ny <nz. The in-plane retardation Re = (ny−nx) × d was as large as 636 nm. Furthermore, the phase difference ratio (R450 / R550) (wavelength dependence) was as low as 1.02.

  From these results, the obtained film has a negative birefringence, a large refractive index in the thickness direction, a large in-plane retardation, and a small wavelength dependence, so that it is suitable for a retardation film. there were.

Comparative Example 1
The film obtained in Film Production Example 2 was cut into a square of 50 mm, and the temperature was 170 ° C. and the stretching speed was 10 mm / min. The film was subjected to uniaxial stretching with a free width under the conditions of 1.10 times. As a result of measuring the three-dimensional refractive index of the stretched film, the obtained film had positive birefringence because the refractive index in the axial direction perpendicular to the stretching axis direction was small.

  From the results of three-dimensional refractive index measurement (nx = 1.5844, ny = 1.5823, nz = 1.5823), the obtained film had a refractive index of nx> ny = nz and the thickness direction of the film was not large. .

  From these results, the obtained film was not suitable for the retardation film of the viewing angle compensation performance of STN-LCD and IPS-LCD.

Comparative Example 2
The film obtained in Film Preparation Example 3 was cut into a square of 50 mm, and the temperature was 180 ° C. and the stretching speed was 15 mm / min. The film was stretched 2.0 times by free-width uniaxial stretching under the following conditions. As a result of measuring the three-dimensional refractive index of the stretched film, the obtained film had positive birefringence because the refractive index in the axial direction perpendicular to the stretching axis direction was small.

  From the results of the three-dimensional refractive index measurement (1.5124, ny = 1.590, nz = 1.090), the obtained film had a refractive index in the thickness direction of nx> ny = nz and was not large.

  From these results, the obtained film was not suitable for the retardation film of the viewing angle compensation performance of STN-LCD and IPS-LCD.

Change in refractive index ellipsoid by stretching

Explanation of symbols

nx: Refractive index in the fast axis direction in the film plane.
ny: indicates the refractive index in the direction perpendicular thereto.
nz: The refractive index in the thickness direction of the film.

Claims (4)

A film made of a fumarate-based resin, wherein the refractive index in the fast axis direction in the film plane is nx, the refractive index in the direction perpendicular thereto is ny, and the refractive index in the thickness direction of the film is nz. The retardation film is characterized in that nx <ny ≦ nz (excluding a retardation film comprising at least one kind of fine optically negative particles and a fumaric acid ester resin) . 2. The retardation film according to claim 1, wherein an in-plane retardation (Re) measured at a wavelength of 550 nm represented by the formula (a) is 50 to 2000 nm.
Re = (ny−nx) × d (a)
(Here, d indicates the thickness of the film.) The retardation film according to claim 1 or 2, wherein the fumaric acid ester resin comprises 50 mol% or more of a fumaric acid diester residue unit represented by the general formula (1).

(Here, R ₁ and R ₂ each independently represents a branched or cyclic alkyl group having 3 to 12 carbon atoms.) The ratio (R450 / R550) of the phase difference (R450) measured at a wavelength of 450 nm and the phase difference (R550) measured at a wavelength of 550 nm is 1.1 or less, or any one of claims 1 to 3 Retardation film.

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