JP5325491B2 - Novel coating type optical compensation film and method for producing the same - Google Patents

Description

  The present invention has high transparency, high retardation (birefringence), low wavelength dispersion of retardation (birefringence), high organic solvent solubility (solution film formability), and sufficient film toughness. The present invention relates to an optical compensation film containing an alicyclic polyimide useful as an optical compensation (retardation) material and a method for producing the same.

  Conventionally, plasma displays have been the mainstream for large flat panel displays (FPDs) represented by television, but recently, demand for large liquid crystal televisions has rapidly expanded due to advances in the technology for increasing the size of liquid crystal displays (LCD). doing.

  Reduction of the viewing angle dependency of contrast is one of the most important required characteristics in the usage form of an LCD such as a large liquid crystal television. In recent years, LCDs in VA (vertical alignment) mode and IPS (in-plane alignment switching) mode, which are advantageous for widening the viewing angle, have been adopted in large liquid crystal televisions.

  Among LCD components, liquid crystal cells and polarizing films inherently have optical anisotropy, so an optical compensation film suitable for each of these members is used for the purpose of maintaining high contrast over a wide viewing angle. It has been.

  In general, a negative C plate and a positive A plate are used in combination as an optical compensation film for a liquid crystal layer in a VA mode LCD. Recently, a polymer film that is biaxially stretched at the same time is applied to the former.

  The retardation film is required not only to prevent a decrease in contrast when viewed obliquely but also to have a color compensation function by compensating for retardation (birefringence) wavelength dependency.

  In the liquid crystal molecular material used for the liquid crystal cell, the wavelength dependence of retardation (birefringence) often becomes normal wavelength dispersion in which the retardation (birefringence) value decreases as the wavelength increases. For example, in a STN mode LCD, since a liquid crystal material often has normal dispersion characteristics, a polycarbonate phase difference film having positive and large wavelength dispersion characteristics is used. On the other hand, in the VA mode liquid crystal cell, the retardation (birefringence) wavelength dependency of the liquid crystal layer may be considerably smaller than usual due to the liquid crystal material and color filter used. In order to compensate for this, the retardation (birefringence) Refraction) A retardation film having low wavelength dispersion characteristics is required.

  Recently, a cycloolefin polymer film stretched biaxially has been adopted as a retardation film for a VA mode liquid crystal cell for the above purpose. However, in order to insert the retardation film between the polarizing film and the liquid crystal cell, a step of laminating using an adhesive is necessary, which is disadvantageous for thinning the panel. On the other hand, a method of imparting a retardation function to a part of a polarizer protective film (for example, a triacetyl cellulose (TAC) film) of a polyvinyl alcohol / iodine polarizing film or using a retardation film having a polarizing film protective function Attempts have also been made to reduce the number of members in the LCD.

  As described above, in order to develop a large birefringence in order to apply the transparent polymer material to the retardation film, it is usually necessary to mechanically stretch the polymer film at a high magnification. If a transparent material with high retardation (birefringence) and low wavelength dispersion is obtained simply by applying and drying a polymer varnish on the polarizer protective film, the LCD manufacturing process will be greatly simplified and thinned. In addition, an effective material can be provided. For example, Patent Document 1 proposes the use of a coating layer using a polyimide resin, but does not describe wavelength dispersion.

Although Non-Patent Document 1 discloses alicyclic polyimide i-HPMDA-PI, the use as an alternative to a glass substrate is described, but it is unclear whether it can be used as an optical compensation film. It was.
Special table flat 8-511812 Polymer Preprints, Japan 268 Vol. 57, no. 1 (2008)

  The present invention has high transparency, high retardation (birefringence), low wavelength dispersion of retardation (birefringence), high organic solvent solubility (solution film formability), and sufficient film toughness. An object of the present invention is to provide an optical compensation film containing an alicyclic polyimide useful as an optical compensation (retardation) film material and a method for producing the same.

As a result of intensive research in view of the above problems, the following general formula (1):
The following general formula (1):

(In formula (1), A represents a divalent aromatic group or an aromatic group.)
It was found that a polyimide varnish having a repeating unit represented by the formula (1) was coated on a polarizer protective film and dried to be a very useful retardation film in the industrial field, and the present invention was completed.

That is, the gist of the present invention is as follows.
1. An optical compensation film comprising a polyimide having a repeating unit represented by the above formula (1).

2. A method for producing an optical compensation film, which comprises applying and drying a polyimide varnish represented by the above formula (1) on a substrate.

  According to the present invention, an alicyclic compound having high transparency, high retardation (birefringence), low wavelength dispersion of retardation (birefringence), high organic solvent solubility (solution film-forming property), and sufficient film toughness. An optical compensation film material containing polyimide and a method for producing the same can be provided. This cycloaliphatic polyimide exhibits high retardation (birefringence) and low wavelength dispersion by applying and drying the varnish due to self-orientation, so it is extremely useful as a retardation film for VA mode LCDs. Useful.

    Embodiments of the present invention will be described in detail below, but these are examples of the embodiments of the present invention and the present invention is not limited to these contents.

  First, the polyimide constituting the optical compensation material of the present invention will be described. The alicyclic polyimide represented by the formula (1) is represented by the following formula (2):

Is used, and (1R, 2S, 4S, 5R) -cyclohexanetetracarboxylic dianhydride is used to form a polyimide precursor by equimolar polyaddition reaction with diamine. This can be produced by imidization.

  First, the transparency of polyimide constituting the optical compensation film according to the invention will be described. In general wholly aromatic polyimide systems obtained from aromatic tetracarboxylic dianhydrides and aromatic diamines, the diimide moiety acts as an electron acceptor and the diamine moiety acts as an electron donor, resulting in intramolecular and intermolecular charge transfer interactions. The polyimide film is markedly colored. On the other hand, since the alicyclic polyimide uses the alicyclic tetracarboxylic dianhydride, the charge transfer interaction is completely hindered, and the polyimide film can be made colorless and transparent.

Next, the birefringence of the optical compensation film according to the present invention will be described. Film surface direction X and Y-axis, if put the film thickness direction and Z-axis, equal X and Y directions of the refractive index _{(n x = n y = n} in) and the refractive index of the Z-direction _{(n z} ) In order to produce a higher negative C plate retardation film, a polymer film is usually biaxially stretched at a high magnification and orthogonally and the polymer chains are oriented as parallel as possible to the film surface (hereinafter referred to as surface). (Referred to as inner orientation). Without such stretching operations, to express a large birefringence (Δn = n in _{-n z)} only in a solution cast film process the varnish is usually very difficult, the formula (1) Alicyclic polyimides can achieve this.

  In order to induce a high birefringence by inducing in-plane orientation of the polyimide chain in the cast film forming process, it is preferable that the main chain skeleton of the polyimide has high linearity and rigidity. For this reason, it is preferable to use a diamine component used for the polymerization having a highly linear molecular structure.

  However, if a rigid and linear diamine is used alone, high birefringence is expected, but the resulting polyimide loses solubility in organic solvents, and the film formation process itself by applying and drying varnish is difficult. There is a fear.

  Even if rigid diamines are used, they are soluble in organic solvents at the polyimide precursor stage. Therefore, in order to form a polyimide film, a polyimide precursor varnish can be applied, dried, and thermally imidized instead of using a polyimide varnish. However, when the substrate is a TAC film having low heat resistance, a thermal imidization process that requires heating at 250 ° C. or higher cannot be employed.

  Moreover, when apply | coating and drying a polyimide varnish on a polarizer protective film, the solvent which does not erode a polarizer protective film layer is arbitrarily selected for the alicyclic polyimide represented by Formula (1). For example, when the polarizer protective film is a TAC film, a ketone solvent such as acetone, cyclopentanone, or cyclohexanone that does not erode the TAC film, an ester solvent such as ethyl acetate or γ-butyrolactone, or an aromatic solvent such as toluene or xylene It is necessary to form a stable varnish having high solubility in a solvent at room temperature.

  The diamine that can be used for the polyimide constituting the optical compensation material of the present invention to realize the above-mentioned required properties is not particularly limited, but among the diamine components, for example, the following formulas (3) to (5):

It is preferred to use a diamine having a rigid structure having a fluorine substituent, either alone or in part.

  In addition to the above diamine, the low wavelength dispersibility can be further improved by partially using an aliphatic diamine as a copolymerization component.

In addition to the above, as a diamine component, a diamine monomer such as 4,4′-oxydianiline having a flex bond may be used as a copolymer component to further improve the solubility and film toughness of polyimide.
<Method for producing polyimide precursor>
The method for producing the polyimide precursor constituting the optical compensation film according to the present invention is not particularly limited, and a known method can be applied. More specifically, it is obtained by the following method. First, diamine is dissolved in a polymerization solvent, and substantially equimolar amount of the alicyclic tetracarboxylic dianhydride powder is gradually added to the solution, and 0 to 100 ° C., preferably 20 using a mechanical stirrer. Stir at -60 ° C for 0.5-150 hours, preferably 1-48 hours. In this case, the monomer concentration is 5 to 50% by weight, preferably 10 to 40% by weight. By carrying out polymerization in this monomer concentration range, a polyimide precursor solution having a uniform and high degree of polymerization can be obtained. When the polymerization degree of the polyimide precursor increases too much and the polymerization solution becomes difficult to stir, it can be appropriately diluted with the same solvent.

  From the viewpoint of the toughness of the polyimide film, it is desirable that the degree of polymerization of the polyimide precursor is as high as possible. By performing the polymerization in the above monomer concentration range, the degree of polymerization of the polymer is sufficiently high, and the solubility of the monomer and the polymer can be sufficiently ensured. When polymerization is performed at a concentration lower than the above range, the degree of polymerization of the polyimide precursor may not be sufficiently high, and when polymerization is performed at a concentration higher than the above monomer concentration range, the monomer and the polymer to be generated are not sufficiently dissolved. It may become. In addition, when an aliphatic diamine is used, salt formation often occurs at the initial stage of polymerization and the polymerization is hindered. However, in order to increase the severity as much as possible while suppressing the salt formation, attention should be paid to the monomer concentration at the time of polymerization. It is.

  Further, from the viewpoint of polyimide film toughness and varnish handling, the intrinsic viscosity of the polyimide precursor is preferably in the range of 0.1 to 5.0 dL / g, and in the range of 0.3 to 3.0 dL / g. It is more preferable.

  The aromatic diamine that can be used in the polymerization is not particularly limited as long as the required characteristics of the polyimide constituting the optical compensation film according to the present invention and the polymerization reactivity of the polyimide precursor are not impaired, but p-phenylenediamine is not particularly limited. M-phenylenediamine, 2,4-diaminotoluene, 2,5-diaminotoluene, 2,4-diaminoxylene, 2,4-diaminodurene, 4,4′-diaminodiphenylmethane, 4,4′-methylenebis (2 -Methylaniline), 4,4'-methylenebis (2-ethylaniline), 4,4'-methylenebis (2,6-dimethylaniline), 4,4'-methylenebis (2,6-diethylaniline), 4, 4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether Ter, 2,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminobenzophenone, 3,3'-diaminobenzophenone, 4,4'-diamino Benzanilide, 4-aminophenyl-4′-aminobenzoate, benzidine, 3,3′-dihydroxybenzidine, 3,3′-dimethoxybenzidine, o-tolidine, m-tolidine, 2,2′-bis (trifluoromethyl) Benzidine, 1,4-bis (4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1,3-bis (3-aminophenoxy) benzene, 4,4′-bis (4 -Aminophenoxy) biphenyl, bis (4- (3-aminophenoxy) phenyl) sulfone, bis (4 -(4-aminophenoxy) phenyl) sulfone, 2,2-bis (4- (4-aminophenoxy) phenyl) propane, 2,2-bis (4- (4-aminophenoxy) phenyl) hexafluoropropane, 2 , 2-bis (4-aminophenyl) hexafluoropropane, p-terphenylenediamine and the like. Two or more of these may be used in combination.

  The aliphatic diamine that can be used in the polymerization is not particularly limited as long as the required characteristics of the polyimide constituting the optical compensation film according to the present invention and the polymerization reactivity of the polyimide precursor are not impaired. 4'-methylenebis (cyclohexylamine), isophoronediamine, trans-1,4-diaminocyclohexane, cis-1,4-diaminocyclohexane, 1,4-cyclohexanebis (methylamine), 2,5-bis (aminomethyl) Bicyclo [2.2.1] heptane, 2,6-bis (aminomethyl) bicyclo [2.2.1] heptane, 3,8-bis (aminomethyl) tricyclo [5.2.1.0] decane, 1,3-diaminoadamantane, 2,2-bis (4-aminocyclohexyl) propane, 2,2-bis (4-aminocycline) Hexyl) hexafluoropropane, 1,3-propanediamine, 1,4-tetramethylenediamine, 1,5-pentamethylenediamine, 1,6-hexamethylenediamine, 1,7-heptamethylenediamine, 1,8-octa Examples include methylene diamine and 1,9-nonamethylene diamine. Two or more of these may be used in combination.

  From the viewpoint of developing birefringence of the polyimide film, a diamine having a highly rigid and highly linear structure effective for inducing in-plane orientation of the polyimide chain, that is, p-phenylenediamine, 2-trifluoromethyl-1,4- Phenylenediamine, 2,5-diaminotoluene, 3,4'-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 4-aminophenyl-4'-aminobenzoate, benzidine, 3,3'-dihydroxybenzidine, 3, 3'-dimethoxybenzidine, o-tolidine, m-tolidine, 2,2'-bis (trifluoromethyl) benzidine, p-terphenylenediamine, trans-1,4-diaminocyclohexane, etc. can be used as the diamine component. Yes, but the retardation (birefringence) of the polyimide film 2,2′-bis (trifluoromethyl) benzidine, 3,3′-bis (trifluoromethyl) benzidine, 2-trifluoromethyl-1,4-phenylenediamine, etc. from the viewpoint of low wavelength dispersibility and solvent solubility Is particularly preferably used. Further, the content of the rigid structure diamine is 10 to 100 mol%, preferably 30 to 95 mol% of the total amount of diamine used.

  The polyimide precursor is polymerized using an alicyclic tetracarboxylic dianhydride represented by the formula (2) and another alicyclic tetracarboxylic dianhydride as a copolymerization component. In this case, the copolymerizable alicyclic tetracarboxylic dianhydride is not particularly limited, but bicyclo [2.2.2] octane-2,3,5,6-tetracarboxylic dianhydride, bicyclo [ 2.2.2] Oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, 5- (dioxotetrahydrofuryl-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride 4- (2,5-dioxotetrahydrofuran-3-yl) -tetralin-1,2-dicarboxylic anhydride, tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride, bicyclo-3, Examples thereof include 3 ′, 4,4′-tetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, etc. Two or more of these may be used in combination. Alicyclic acid Among the water substances, bicyclo [2.2.2] octane-2,3,5,6-tetracarboxylic dianhydride is more preferably used.In the copolymerization, the acid represented by the formula (14) is used. The content of dianhydride is 10 to 99 mol% with respect to the total amount of acid dianhydride used.

  The solvent used in the polymerization reaction is preferably an aprotic solvent such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, etc., but is formed with raw material monomers. There is no problem as long as the polyimide precursor is dissolved, and the structure is not particularly limited. Specifically, amide solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidone, γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-caprolactone, ε -Cyclic ester solvents such as caprolactone, α-methyl-γ-butyrolactone, carbonate solvents such as ethylene carbonate and propylene carbonate, glycol solvents such as triethylene glycol, m-cresol, p-cresol, 3-chlorophenol, 4-chloro Phenol solvents such as phenol, acetophenone, 1,3-dimethyl-2-imidazolidinone, sulfolane, dimethyl sulfoxide and the like are preferably used. In addition, other common organic solvents such as phenol, 0-cresol, butyl acetate, ethyl acetate, isobutyl acetate, propylene glycol methyl acetate, ethyl cellosolve, ptyl cellosolve, 2-methyl cellosolve acetate, ethyl cellosolve acetate, butyl cellosolve acetate, Tetrahydrofuran, dimethoxyethane, diethoxyethane, dibutyl ether, diethylene glycol dimethyl ether, methyl isobutyl ketone, diisobutyl ketone, cyclopentanone, cyclohexanone, methyl ethyl ketone, acetone, butanol, ethanol, xylene, toluene, chlorobenzene, terpene, mineral spirit, Petroleum naphtha solvents can also be added.

The polyimide precursor obtained as described above can be isolated as a powder by dropping, filtering and drying the polymerization solution into a large amount of poor solvent such as water or methanol.
<Method for producing alicyclic polyimide>
The alicyclic polyimide constituting the optical compensation material of the present invention can be produced by subjecting the polyimide precursor obtained by the above method to a dehydration ring-closing reaction (imidation reaction). The imidization method is not particularly limited, and a known method can be applied. Examples of usable forms of polyimide include films, metal foil / polyimide film laminates, plastic film / polyimide laminates, powders, molded articles, and varnishes.

  First, a method for producing a polyimide film will be described. When the substrate to be used is a heat-resistant substrate such as glass, copper, aluminum, stainless steel, silicon, etc., first, a polyimide precursor polymerization solution (varnish) is cast on these substrates, and 40 to 180 ° C. in an oven, preferably Dry at 50-150 ° C. Next, a polyimide film is produced by heating the obtained polyimide precursor film on a substrate in a vacuum, in an inert gas such as nitrogen, or in air at 200 to 400 ° C., more preferably 250 to 350 ° C. Can do. The heating temperature is preferably 200 ° C. or higher from the viewpoint of sufficiently carrying out the imidization ring-closing reaction, and 400 ° C. or lower from the viewpoint of the thermal stability of the produced polyimide film. Moreover, it is desirable to perform imidization in a vacuum or in an inert gas, but if the imidization temperature is not too high, imidation may be performed in air.

  The imidation reaction is performed by immersing the polyimide precursor film in a solution containing a dehydrating cyclization reagent such as acetic anhydride in the presence of a tertiary amine such as pyridine or triethylamine instead of the heat treatment as described above. Is also possible. Moreover, it is also possible to prepare a partially imidized polyimide precursor film by previously charging and stirring these dehydrating cyclization reagents in a polyimide precursor varnish, and casting and drying them on the substrate. This can be further heat-treated as described above to obtain a polyimide film.

  The dehydration cyclization reagent can be charged into the polyimide precursor varnish and stirred at −20 to 150 ° C., preferably 20 to 80 ° C. for 0.5 to 48 hours to complete the chemical imidization. When the polyimide is soluble in the reaction solution, the reaction solution can be dropped and filtered into a large amount of poor solvent such as water or methanol to isolate the polyimide as a powder. Moreover, a polyimide powder can be redissolved in the said polymerization solvent to make a polyimide varnish.

  When the polyimide precursor polymerization solution is diluted as it is or with the same solvent, the polyimide varnish of the present invention is used when the solution is heated to 150 to 250 ° C. and dissolved in the solvent used by the polyimide itself. It can be manufactured easily. When insoluble in the solvent, the polyimide powder can be obtained as a precipitate. At this time, toluene, xylene or the like may be added in order to azeotropically distill off water or the like which is a by-product of imidization. Further, a base such as γ-picoline can be added as a catalyst. The obtained varnish can be dropped, filtered and dried in a large amount of poor solvent such as water or methanol to isolate the polyimide as a powder.

  From the viewpoint of suppressing the molecular weight reduction of the polyimide as much as possible, it is preferable to imidize the varnish without heating by adding and stirring a chemical imidizing reagent, rather than heating and imidizing the polyimide precursor varnish.

  Although it does not specifically limit as a solvent which can be used when melt | dissolving a polyimide powder in a solvent again to make a varnish, The said polymerization solvent can be used. Moreover, when a polyimide varnish is applied and dried on a polarizer protective film to form a polyimide film, a solvent that does not erode the polarizer protective film is preferably used. For example, when a typical TAC film is used as a polarizer protective film, γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-caprolactone, ε-caprolactone, α-methyl-γ-butyrolactone, butyl acetate, acetic acid Ester solvents such as ethyl and isobutyl acetate, ketone solvents such as methyl isobutyl ketone, diisobutyl ketone, cyclopentanone, cyclohexanone, methyl ethyl ketone and acetone, and aromatic solvents such as toluene and xylene are preferably used.

  In addition, the polyimide is polymerized in one step without once forming a polyimide precursor by reacting the alicyclic tetracarboxylic dianhydride represented by the formula (2) with a diamine in a solvent at a high temperature. be able to. At this time, the reaction solution may be maintained in a temperature range of 130 to 250 ° C., preferably 150 to 200 ° C. from the viewpoint of promoting the reaction. When the polyimide is insoluble in the solvent used, the polyimide is obtained as a precipitate, and when soluble, it is obtained as a polyimide varnish. The polymerization solvent is not particularly limited, and examples of usable solvents include aprotic solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide. However, a phenol solvent such as m-cresol or an amide solvent such as NMP is more preferably used. In order to azeotropically distill off water, which is a by-product of the imidization reaction, to these solvents, toluene, xylene, or the like can be added. A base such as γ-picoline can be added as an imidization catalyst. The resulting varnish can be dropped, filtered and dried in a large amount of poor solvent such as water or methanol to isolate the polyesterimide as a powder. When the polyimide is soluble in a solvent, the powder can be redissolved in the above solvent to obtain a polyimide varnish.

  A polyimide film can also be formed by applying the polyimide varnish on a substrate and drying at 40 to 400 ° C., preferably 100 to 350 ° C. In particular, when the polyimide film is formed on a TAC film, drying is performed at 40 to 150 ° C, preferably 50 to 140 ° C, from the viewpoint of heat resistance of the TAC film.

  The polyimide powder obtained as described above can be thermoformed into a film by heating and melting or compressing at 200 to 450 ° C., preferably 250 to 400 ° C.

  It is an isomer of polyimide by adding and stirring a dehydrating reagent such as N, N-dicyclohexylcarbodiimide or trifluoroacetic anhydride in the polyimide precursor solution and reacting at 0 to 150 ° C., preferably 20 to 100 ° C. Polyisoimide is formed. The isoimidization reaction can also be performed by immersing the polyesterimide precursor film in a solution containing the dehydrating reagent. After polyisoimide varnish is formed in the same procedure as described above, it can be easily converted to polyimide by heat treatment at 250 to 400 ° C., preferably 270 to 350 ° C.

Additives such as oxidation stabilizers, fillers, adhesion promoters, silane coupling agents, photosensitizers, photopolymerization initiators, sensitizers, end-capping agents, crosslinking agents, etc., as necessary, in the polyimide and its precursors Can be added.
<Required properties of polyimide>
The film characteristics required for applying the polyimide to the retardation film include high transparency, high retardation (birefringence), low wavelength dispersion characteristics of retardation (birefringence), high solubility, and sufficient film toughness. Can be mentioned.

  The polyimide film needs to be uncolored. If the light transmittance at a wavelength of 400 nm of a film having a thickness of 20 μm is 75% or more as an index of transparency, there is no problem, preferably 80% or more, and more preferably 85% or more.

The polyimide film needs to have a high retardation (birefringence) value. No hindrance if the birefringence value Δn _(n in -n _z) is 0.01 or more at a wavelength of 550nm as an index of the retardation is preferably 0.02 or more, more preferably 0.03 or more. From the relationship of retardation Re = Δn × d (d is film thickness), the higher the birefringence value of the polyimide film, the more advantageous it is that the film thickness of the retardation film can be designed thinner.

The polyimide film needs to exhibit low wavelength dispersion characteristics of retardation (birefringence). If the ratio of retardation or birefringence at 450 nm and 550 nm (Re ₄₅₀ / Re ₅₅₀ = Δn ₄₅₀ / Δn ₅₅₀ ) is 1.06 or less, there is no problem, preferably 1.04 or less, more preferably 1. 03 or less.

  The polyimide film needs to exhibit sufficient film toughness. As an index, it is necessary that the film does not break by a 180 ° bending test.

  In order to form the polyimide film on a substrate by a solution casting method, solubility in an organic solvent is required. In particular, in order to form a film on a TAC film, solubility in a ketone solvent, an ester solvent, and an aromatic solvent that does not erode the TAC film is required. Further, from the viewpoint of preventing the heat resistance of the TAC film and the relaxation of orientation of the polyimide itself, it is desirable that the cast film formation (drying) temperature is as low as possible. For this reason, it is desirable that the solvent used in the solution casting film has a lower boiling point, but if it is 180 ° C. or lower, there is no problem, preferably 150 ° C. or lower, more preferably 130 ° C. or lower.

EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, it is not limited to these Examples. The physical property values in the following examples were measured by the following methods.
<Intrinsic viscosity>
A 0.5 wt% polyimide precursor solution was measured at 30 ° C. using an Ostwald viscometer.
<Cutoff wavelength (transparency)>
Using a UV-visible spectrophotometer (V-530) manufactured by JASCO Corporation, the visible / ultraviolet transmittance from 200 nm to 900 nm was measured. The wavelength (cutoff wavelength) at which the transmittance was 0.5% or less was used as an index of transparency. The shorter the cutoff wavelength, the better the transparency of the polyimide film.
<Light transmittance (transparency)>
The light transmittance at 400 nm was measured using an ultraviolet-visible spectrophotometer (V-530) manufactured by JASCO Corporation. The higher the transmittance, the better the transparency of the polyimide film.
<Birefringence: Δn and its wavelength dispersion>
Using an Abbe refractometer with polarizer (Abbe 4T) manufactured by Atago Co., Ltd., a refractive index (sodium lamp) at a constant wavelength in a direction (n _in ) parallel to a polyimide film (film thickness of about 20 μm) and a direction (n _out ) The wavelength was 589 nm or a xenon lamp spectral light source was used, and birefringence (Δn = n _in −n _out ) was determined from the difference in refractive index. For birefringence wavelength dispersion, monochromatic light obtained by separating light from a xenon lamp light source with a diffraction grating (bandpass 10 nm) was introduced into the refractometer through an optical fiber cable, and birefringence at each wavelength was measured.

[Example 1]
<Polymerization of polyimide precursor, imidization and evaluation of polyimide film characteristics>
N, N-dimethylacetamide (DMAc) which was sufficiently dehydrated with Molecular Sieves 4A was charged with 5 mmol of 2,2′-bis (trifluoromethyl) benzidine (hereinafter referred to as TFMB) in a well-closed sealed reaction vessel with a stirrer. Then, 5 mmol of alicyclic tetracarboxylic dianhydride powder (manufactured by Iwatani Gas Co., Ltd.) represented by the formula (2) was added to the solution at once. At this time, the total monomer concentration is 30% by weight. Since the viscosity of the solution increased and it became difficult to stir, DMAc was added as appropriate, and finally diluted to a total monomer concentration of 17.8 wt%. Finally, the mixture was stirred for 72 hours to obtain a transparent, uniform and viscous polyimide precursor solution. This polyimide precursor solution did not precipitate or gel at all even when allowed to stand at room temperature and −20 ° C. for one month, and showed high solution storage stability. The intrinsic viscosity of this polyimide precursor measured in DMAc was 1.25 dL / g and was a high polymer. To this polyimide precursor solution, an excessive amount of acetic anhydride / pyridine (volume ratio 7/3) was added dropwise with stirring, and the mixture was stirred at room temperature for 24 hours for chemical imidization. At this time, the reaction solution did not gel. After the chemical imidization was completed, the reaction solution was dropped into a large amount of methanol, and the polyimide was precipitated and filtered, sufficiently washed with methanol, and then vacuum dried at 100 ° C. to obtain a polyimide powder. Using this part, the intrinsic viscosity of the polyimide measured at 30 ° C. in cyclopentanone was 1.32 dL / g. The polyimide powder was redissolved in cyclopentanone (10 wt%), applied to a glass substrate, and vacuum dried at 150 ° C. for 2 hours to obtain a transparent polyimide film having a thickness of about 10 μm. This polyimide film did not break even in the 180 ° bending test and showed sufficient flexibility. Similarly, when a thin film having a thickness of 5 μm was prepared and an infrared absorption spectrum was measured, it was confirmed that chemical imidization was almost completed.

Table 1 shows the physical property values of the obtained polyimide film. When this transparency was evaluated, the light transmittance at 400 nm was 87.4%, and the cutoff wavelength was 291 nm, indicating extremely high transparency. The birefringence measured using a sodium lamp as the light source was 0.0279, which was relatively high for an alicyclic polyimide. In addition, Re ₄₅₀ / Re ₅₅₀ = 1.039, which showed low wavelength dispersion. As an example, FIG. 1 shows a plot of Δn against a wide range of wavelengths.
[Example 2]
As a diamine component, instead of using TFMB alone, a polyimide precursor was polymerized in the same manner as described in Example 1 except that this and isophorone diamine (hereinafter referred to as IPDA) were used in combination. A film was cast to prepare a polyimide film, and the film properties were evaluated. The molar ratio of the diamine component is TFMB: IPDA = 80: 20. As shown in Table 1, this polyimide film exhibited high transparency, high birefringence and low wavelength dispersion of birefringence (Re ₄₅₀ / Re ₅₅₀ = 1.057) in the same manner as the polyimide described in Example 1.
[Comparative Example 1]
A polyimide precursor was polymerized from TFMB and 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride (hereinafter referred to as DMCBDA) according to the method described in Example 1. However, when a chemical imidizing agent was added thereto, the reaction solution gelled and it was difficult to complete imidization. Instead of chemical imidization, this polyimide precursor varnish was cast on a glass substrate and then heat imidized to form a polyimide film. Then, the polyimide varnish was re-dissolved in a solvent and tried to produce a polyimide varnish. It was hardly soluble in other solvents such as system solvents and ester solvents. Further, instead of DMCBDA, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and cis, cis, cis, cis-1,2,4,5- represented by the following formula (6) The system using cyclohexanetetracarboxylic dianhydride is also poor in solubility as described above, and it was impossible to apply the step of forming the optical compensation film of the present invention on the TAC by the solution casting method.

The relationship between the wavelength in Example 1 and (DELTA) n is represented.

Claims (3)

The following general formula (1):

An optical compensation film comprising a polyimide having a repeating unit represented by:
(In formula (1), A represents a divalent aromatic group or an aromatic group.) The following general formula (1):

A method for producing an optical compensation film, wherein a polyimide varnish having a repeating unit represented by formula (1) is applied to a substrate and dried (wherein A is a divalent aromatic group or aromatic group) Represents.)   The method for producing an optical compensation film according to claim 2, wherein the substrate is a polarizer protective film.

Images