JP2003335711A - Method for purifying liquid crystalline compound - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for purifying a liquid crystalline compound by heat treatment to obtain the liquid crystalline compound which does not reduce voltage retention after exposed to high temperature conditions in a manufacturing process or the like of a liquid crystal display element. <P>SOLUTION: The method for purifying the liquid crystalline compound comprises heating the liquid crystalline compound in a specific temperature range to give 0.5-5% decomposition of the liquid crystalline compound under an oxygen-containing atmosphere, subsequently bringing the liquid crystalline compound into contact with a nitrogen-containing basic compound and purifying the liquid crystalline compound with an adsorbent. The liquid crystalline compound comprises elements selected from a group consisting of carbon, hydrogen, fluorine, chlorine and oxygen and has no aliphatic unsaturation. The specific temperature range is from 20°C less than a temperature A (°C) to 150°C higher than the temperature A, wherein the temperature A express an extrapolated heat generation-beginning temperature of a peak in the side of the lowest temperature in a temperature range higher than a nematic phase-isotropic phase transition temperature of the liquid crystalline compound obtained by differential scanning calorimetry of the liquid crystalline compound under the oxygen-containing atmosphere. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying a liquid crystal compound, and more particularly to a method for purifying a liquid crystal compound which is less likely to have a decrease in voltage holding ratio due to heating during manufacture of the liquid crystal display or outdoor use of the liquid crystal display.

[0002]

2. Description of the Related Art Liquid crystal displays have expanded their display capacity from twisted nematic liquid crystal displays to super twisted nematic liquid crystal displays in line with the development of computers since they appeared as calculator displays. In particular, an active matrix liquid crystal display (hereinafter referred to as AM
-LCD is abbreviated. ) Has high image quality that can replace the cathode ray tubes that have been used for a long time, and continues to grow as the most promising display at the moment, supported by flattening and energy saving.

In the AM-LCD, in order to increase the contrast, a switching element such as a thin film transistor or a diode is attached to each pixel and a voltage is applied to the pixel. This is different from the conventional nematic liquid crystal display passive drive method, in which each pixel is tens of meters through a switching element.
It is driven by applying a voltage every s. For this reason,
If the applied voltage cannot be completely held until the next writing time, which is several tens of ms after the voltage is applied,
This will cause deterioration of the display. When the voltage between the electrodes is lowered, the transmitted light intensity is changed and the contrast is lowered.

Therefore, in order to maintain the voltage holding ratio, the liquid crystal compound for AM-LCD is usually purified by a method such as recrystallization, distillation, liquid chromatography, etc. for impurities causing the decrease in the voltage holding ratio. I have been using it since then. Also,
As a method for removing water and metal ions in a liquid crystal compound, JP-A-62-210420 discloses a method of contacting a liquid crystal compound with silica gel, which is disclosed in JP-A-58-1774.
The method disclosed in Japanese Patent Laid-Open Publication No. Sho 5 is a method of contacting with activated alumina.
No. 2-59081 discloses a method of treating with an ion exchange resin, and JP-A No. 63-261224 discloses a method of contacting with zeolite. Furthermore, JP-A-50-108186 and JP-A-51-1106.
In Japanese Patent Laid-Open No. 9-86812 and Japanese Patent Laid-Open No. 4-86812, a liquid crystal compound is put between a pair of electrodes facing each other, and an electric field is applied, whereby metal ions such as Na + and K + having a relatively large mobility due to the electric field and SO. A method for removing ionic impurities such as ₄ ²⁻ , NO ₃ ⁻ , Cl ^{− and the} like are disclosed.

[0005]

However, even when a liquid crystal compound purified by the above method and having a voltage holding ratio that can withstand practical use is used, the liquid crystal compound is still used in the process of manufacturing a liquid crystal display. Is exposed to high temperatures,
The voltage holding ratio sometimes decreased.

The problem to be solved by the present invention is to provide a purification method for obtaining a liquid crystal compound which does not cause a reduction in voltage holding ratio even when exposed to high temperatures.

[0007]

Even when a liquid crystal compound which has been purified by the above-mentioned conventional method and has a voltage holding ratio that can withstand practical use is used, the liquid crystal compound is still produced in the manufacturing process of the liquid crystal display. It is speculated that the reason why the voltage holding ratio is lowered when exposed to a high temperature is that trace amounts of impurities that cannot be completely removed by the above-mentioned purification method are present in the liquid crystal compound and decomposed by heating. In the present invention, a liquid crystal compound is heated in the presence of oxygen at a temperature at which impurities can be decomposed to decompose a trace amount of impurities which are easily decomposed by heating, and then contacted with a basic compound containing nitrogen, The above problems have been solved by providing a method for purifying a liquid crystal compound, which is purified with an adsorbent.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a liquid crystal compound is heat-treated in an atmosphere containing oxygen within a range such that the decomposition rate of the liquid crystal compound is 0.5 to 5%, and then the liquid crystal compound is added with nitrogen. It is carried out by bringing it into contact with a basic compound, and then purifying with an adsorbent. Specifically, chemical changes such as oxidation and decomposition by heat treatment of impurities that are contained in the liquid crystal compound and cannot be removed by ordinary purification methods such as recrystallization, distillation, and liquid chromatography, which are particularly susceptible to thermal decomposition. Is generated, and the chemical structure is changed so that it is easily adsorbed by the adsorbent, and then purified.

The liquid crystal compound used in the present invention is composed of an element selected from the group consisting of carbon, hydrogen, fluorine, chlorine and oxygen and is not particularly limited except that it has no aliphatic unsaturated group. It is preferable that the liquid crystal compound is resistant to chemical change or decomposition when heated to 120 ° C to 300 ° C. Those having an aliphatic unsaturated group or having a structure containing an element other than carbon, hydrogen, fluorine, chlorine, or oxygen such as a cyano group, the liquid crystal compound itself is inferior in heat resistance and is 120 ° C or less. However, it may be easily decomposed. In particular, the present invention is not suitable for purifying a liquid crystal compound having an aliphatic unsaturated bond in its own structure.

As the liquid crystal compound to which the present invention is applied, specifically, a liquid crystal display element, that is, a twisted nematic liquid crystal display element, a super twisted nematic type liquid crystal display element, a polymer dispersion type liquid crystal type liquid crystal display element, and optical self-compensation. Examples include nematic liquid crystal compounds generally used for birefringent liquid crystal display elements and the like, and chiral smectic liquid crystal compounds generally used for ferroelectric liquid crystal displays, and examples thereof include fluorine, chlorine, alkyl groups, alkoxy groups, and fluorine. Alkyl group, chlorinated alkyl group, fluorinated alkoxy group, biphenyl type which may have a substituent such as chlorinated alkoxy group, the same phenylcyclohexane type, the same benzoic acid phenyl ester type, the same cyclohexylcarboxylic acid phenyl ester type, Same naphthalene, same phenylnaphthalene, same cyclo Hexyl naphthalene, the phenyl naphthalene, the tetralin system, the phenyl tetraline type, the cyclohexyl tetralin system, the phenyl tetraline type, the decalin system,
The same phenyldecalin type, the same cyclohexyldecalin type,
The same cyclohexylphenyl decalin type, the same phenanthrene type,

The same phenylphenanthrene type, the same cyclohexylphenanthrene type, the same cyclohexylphenylphenanthrene type, the same tetrahydrophenanthrene type, the same phenyltoterahydrophenanthrene type, the same cyclohexyltetrahydrophenanthrene type, the same cyclohexylphenyltetrahydrophenanthrene type, the same octahydrophenanthrene type, The same phenyl octahydrophenanthrene system, the same cyclohexyl octahydrophenanthrene system, the same cyclohexyl phenyl octahydrophenanthrene system, the same perhydrophenanthrene system, the same phenyl perhydrophenanthrene system, the same cyclohexyl perhydrophenanthrene system, the same cyclohexyl phenyl perhydrophenanthrene system, Bicyclooctene system, Indan system, Ben It is a liquid crystal compound of indane system and the like.

In order to oxidatively decompose the impurities by the heat treatment, the presence of oxygen is essential in the heat treatment step, and specifically, in an atmosphere containing oxygen in the range of 5 to 100% by volume, especially 15 to It is desirable to perform the heat treatment in an atmosphere containing 70% by volume. As a component other than oxygen in this atmosphere, an inert gas such as nitrogen, argon or neon is preferable. For example, air is preferable because it has an oxygen concentration of 21% by volume and can be easily used.

In setting the heat treatment temperature, the extrapolation of the lowest temperature peak in the temperature range where the liquid crystal compound obtained by differential scanning calorimetry (hereinafter referred to as DSC) shows an isotropic phase in an atmosphere containing oxygen. The heat generation start temperature A (° C.) is preferably within a temperature range of 20 ° C. lower than A to 150 ° C. higher than A, and most preferably 10 ° C. lower than A to 70 ° C. higher than A.

In the thermal analysis of the liquid crystal compound using DSC in an atmosphere containing oxygen, heat generation due to oxidation or thermal decomposition is observed in a temperature range where the liquid crystal compound shows an isotropic phase. The temperature at the intersection of the baseline of this exothermic peak and the tangent line of the rising curve of the exothermic peak with the maximum gradient is the extrapolated exothermic onset temperature A. This extrapolation exothermic onset temperature A is often observed in the range of 100 ° C. to 300 ° C., but in the case of a liquid crystal compound in which a decrease in voltage holding ratio after a heat treatment step is a problem, it is 100 ° C. to 200 ° C. One peak is often observed within the range, and heat generation due to thermal decomposition of the liquid crystal compound itself is often observed on the higher temperature side.

It can be presumed that the liquid crystal compound from which such a thermal analysis result is obtained contains impurities that are easily decomposed by heat, and the extrapolated exothermic onset temperature A observed here is that the impurities are decomposed by heat. It can be estimated that it is the lowest temperature at which Therefore, in order to thermally decompose only the impurities that are easily thermally decomposed and the liquid crystal compound itself is not thermally decomposed as much as possible, the heat treatment temperature is equal to or higher than the minimum temperature at which the easily thermally decomposed impurities start thermal decomposition. It is preferable to set the temperature below the thermal decomposition temperature of the compound itself. Actually, a slight thermal decomposition occurs from a temperature lower than the extrapolation heat generation start temperature A observed by DSC.
The effect of the present invention can be obtained even if the heat treatment is performed at a temperature lower by 0 ° C. On the other hand, thermal decomposition of the liquid crystal compound itself can be considerably suppressed within a temperature range higher than A by 150 ° C.

When the temperature 20 ° C. lower than the extrapolation heat generation start temperature A falls below 120 ° C., the heat treatment temperature is 12
It is recommended to set it to 0 ° C. If the heat treatment is performed at a temperature lower than 120 ° C., even impurities that are relatively thermally decomposed are not oxidatively decomposed, and there is a possibility that impurities that are easily thermally decomposed cannot be removed. It is more preferable to set the heat treatment temperature at 140 ° C.

On the other hand, when the temperature 150 ° C. higher than the extrapolation heat generation start temperature A exceeds 300 ° C., the heat treatment temperature should be set to 300 ° C. If the heat treatment is performed at a temperature higher than 300 ° C, the liquid crystal compound itself is liable to undergo a rapid chemical change or decomposition, and a large amount of thermal decomposition products or oxides may be generated, so a large amount of adsorbent is required. And the yield of the liquid crystal compound itself may decrease. The heat treatment temperature is more preferably set to 260 ° C, and most preferably set to 200 ° C.

The rate of temperature rise in DSC is preferably 5 ° C./minute or less. If the rate of temperature increase is too high, an error such as the extrapolated heat generation start temperature A being shifted to a high temperature side becomes large.

In the purification method of the present invention, the heat treatment time is preferably 15 minutes to 24 hours. It is more preferably 30 minutes to 12 hours. The heat treatment time varies depending on the heat treatment temperature. As the heat treatment temperature is higher than the extrapolation heat generation start temperature A, the thermal decomposition of the impurities progresses more rapidly, so that the heat treatment time can be shortened. On the other hand, if the heat treatment temperature is not so high as compared with the extrapolation heat generation start temperature A, the progress of the thermal decomposition of impurities is slow, so the heat treatment time must be lengthened. Actually, the decomposition rate of the liquid crystal compound is set within the range of 0.5 to 5%. When the decomposition rate of the liquid crystal compound is less than 0.5%,
Since the heat treatment is insufficient, the impurities themselves cannot be thermally decomposed, and the impurities may not be completely removed. On the other hand, if the decomposition rate of the liquid crystal compound exceeds 5%, the yield of the liquid crystal compound obtained after purification decreases, which is not efficient.

When the impurities are brought into contact with oxygen more efficiently during the heat treatment, oxidative decomposition is promoted and the heat treatment time can be shortened. Therefore, it is preferable to heat the liquid crystal compound while stirring, or to heat the liquid crystal compound while blowing a gas containing oxygen into the liquid crystal compound. If necessary, the liquid crystal compound may be heated in an organic solvent solution and then heated. The use of microwaves as the heating source is more preferable because the heating efficiency is significantly increased.

After the liquid crystal compound is heat-treated under the above-mentioned heat treatment conditions, it is brought into contact with a basic compound containing nitrogen and then purified with an adsorbent. (Hereinafter, a nitrogen-containing basic compound is abbreviated as a nitrogen-containing compound.) By contacting with a nitrogen-containing compound, impurities generated by heating and decomposition products of a liquid crystal compound, which have particularly high acidity, are selected. Can be removed. Although the mechanism of this process is not clear, it is considered that the nitrogen-containing compound attracts highly acidic impurities due to its basicity, and the impurities and the lone electron pair possessed by the nitrogen atom of the nitrogen-containing compound form a complex. To be

In order to bring the liquid crystal compound after heating into contact with the nitrogen-containing compound, the liquid crystal compound may be cooled and mixed with the nitrogen-containing compound. Mixing is usually performed at room temperature, but if it is difficult to mix, it is preferable to perform heating. When the liquid crystal compound is in a liquid crystal state or an isotropic liquid and the nitrogen-containing compound is a liquid at the mixing temperature, they may be mixed as they are. Further, when the liquid crystal compound is a solid or when the nitrogen-containing compound is a gas or a solid, it is preferable to mix each of them as a solution in a solvent. Examples of the solvent used at that time include various organic solvents as the solvent of the liquid crystal compound, and examples of the solvent of the nitrogen-containing compound include water, alcohols, ethers, hydrocarbon solvents and the like. . When the liquid crystal compound solution and the nitrogen-containing compound solution are mixed,
It is preferable to select each solvent so that the solvent used for the liquid crystal compound and the solvent used for the nitrogen-containing compound are incompatible with each other. However, if the liquid crystal compound and the nitrogen-containing compound can be separated by performing extraction or the like after mixing, solvents that are compatible with each other may be used.

There is no particular limitation on the mixing method, and it is usually sufficient to stir for about 30 minutes to 3 hours. The use ratio of the nitrogen-containing compound is preferably 1 to 1000% by mass, and more preferably 5 to 200% by mass based on the liquid crystal compound. If the usage ratio is too low, impurities cannot be completely removed and the voltage holding ratio is lowered, and if the usage ratio is too high, stirring or separation from the liquid crystal compound may become difficult.

The nitrogen-containing compound used in the present invention is not particularly limited as long as it is basic, and examples thereof include ammonia, amine compounds, amide compounds, hydrazine compounds,
Examples thereof include a hydrazone compound, a hydroxylamine compound, a compound having a nitrogen-containing heterocycle, an imine compound, an azo compound, an amidine compound, a carbodiimide compound, a guanidine compound, and a cryptand compound. It is also possible to use salts of these compounds.

Ammonia used is usually an aqueous solution. The concentration is not particularly limited, but is preferably in the range of 1 to 50% by mass, and particularly preferably 5 to 30% by mass. Examples of amine compounds include primary amines such as methylamine, ethylamine, propylamine and cyclohexylamine, secondary amines such as dimethylamine and diethylamine, tertiary amines such as trimethylamine and triethylamine, and fats such as ethylenediamine and cyclopentanediamine. Group diamines, alkanolamines such as ethanolamine and diethanolamine, aromatic amines such as aniline and dimethylaniline, and salts of these amines (hydrochlorides, sulfates, acetates, etc.), among them aliphatic diamines and alkanols. Amines are preferred. These amine compounds may be used as they are or diluted with a solvent. Examples of the solvent used when diluting include water, alcohols, ethers, and hydrocarbon solvents. Examples of the amide compound (including the corresponding imidehydrin tautomer) include aromatic amide and aliphatic amide.

As the hydrazine compound, hydrazine, hydrazine hydrate, salts such as hydrazine hydrochloride and sulfate,
Examples thereof include alkylhydrazine such as methylhydrazine, ethylhydrazine and dimethylhydrazine, and aromatic hydrazine such as phenylhydrazine. Among them, hydrazine,
Hydrazine hydrate and hydrazine salts are preferred. Examples of the hydrazone compound include aldehyde hydrazone, ketone hydrazone, hydrates and salts thereof, and aromatic hydrazones such as hydrazobenzene.

Examples of the hydroxylamine compound include hydroxylamine, hydrates and salts thereof, N-alkylhydroxylamine and N-arylhydroxylamine, and among them, hydroxylamine and hydrates and salts thereof are particularly preferable. . As the compound having a nitrogen-containing heterocycle, pyridine, quinoline,
Examples include isoquinoline, pyridazine, pyrimidine, pyrazine, benzopyrrole, pyrazole, imidazole, and salts thereof. Examples of the imine compound include Schiff bases such as benzaldehyde phenylamine, N-substituted imines, and N-unsubstituted imines such as diphenylketimine.

Examples of the azo compound include azoalkanes such as azomethane and azoaryls such as azobenzene. Examples of the amidines include aliphatic amidines such as acetamidine, aromatic amidines such as N-benzamidine, and hydrates and salts thereof. Examples of carbodiimides include aromatic carbodiimides and aliphatic carbodiimides. As guanidines, guanidine,
Examples include guanidine acetic acid. Examples of the cryptand compound include macrocyclic polyamines such as 1,4,7,10,13,16-hexaazacyclooctadecane, polyamine-
Examples include ethers. These nitrogen-containing compounds are
They may be used alone or in combination of two or more. Among them, ammonia, amine compounds, amide compounds, hydrazine compounds, hydrazone compounds, hydroxylamine compounds, and compounds having a nitrogen-containing heterocycle are particularly preferable.

After contact, the liquid crystal compound and the nitrogen-containing compound can be separated by using a separatory funnel or the like if the liquid crystal compound and the nitrogen-containing compound are not compatible with each other. When both are compatible, an organic solvent such as a hydrocarbon solvent and water are added to the compatible material, and the liquid crystal compound is transferred to the organic solvent layer and the nitrogen-containing compound is transferred to the aqueous layer. Then, the mixture is separated with a separating funnel or the like to obtain a solution of the liquid crystal compound in an organic solvent. The organic solvent solution of the liquid crystal compound is more preferably washed with an acid before being purified with an adsorbent.

The liquid crystal compound after being contacted with the nitrogen-containing compound by the above method is purified with an adsorbent. Contact with the nitrogen-containing compound may leave some amount of the nitrogen-containing compound in the liquid crystal compound, which can also be removed by an adsorbent. Purification with an adsorbent can be performed by a known method.

Examples of the adsorbent that can be used in the present invention include inorganic adsorbents and organic adsorbents. Examples of the inorganic adsorbent include silica gel, activated alumina, activated magnesia, magnesium silicate, zeolite, titanium oxide, and the like, and organic adsorbents include styrene-based, acrylic-based, phenol-based, and cellulose-based adsorbents. Examples include synthetic resin adsorbents. Of these, it is preferable to use one selected from silica gel, activated alumina, activated magnesia, and magnesium silicate or a combination thereof. Among them, it is preferable to use silica gel, which is preferable.

These adsorbents are dispersed in a liquid crystal compound or a solution containing the liquid crystal compound after being brought into contact with a nitrogen-containing compound, and the mixture is stirred and then filtered, or the adsorbent is packed in a column, It is preferable to pass the solution from the top of the column. In particular, it is preferable to use a column because it is possible to reliably remove impurities having low heat resistance that are chemically changed by heating. The amount of the adsorbent used is not particularly limited, but it is generally 1 to 30 times the amount of the liquid crystal compound. If the amount of adsorbent used is small, impurities may not be completely removed. On the other hand, if the amount used is too large, the time required for purification becomes long.

The developing solvent for the column may be a single solvent or a mixed solvent, but since the highly polar impurities once adsorbed by the adsorbent are not eluted again,
It is preferred to use a less polar solvent. Specifically, it is particularly preferable to use a hydrocarbon solvent such as n-hexane, n-heptane, benzene, cyclohexane or methylcyclohexane.

Usually, when a column is used, thermal decomposition products, oxides, etc. in the liquid crystal compound generated by heating remain on the upper part of the adsorbent layer. Therefore, it is not necessary to separate the liquid crystal compound in many cases, and the liquid crystal compound can be purified simply by column filtration.

The liquid crystal compound purified by the purification method of the present invention is further purified by washing with water or an aqueous alkali solution as necessary, distilling off the solvent and recrystallizing. Further, the purification method of the present invention may be repeated, or known and commonly used purification methods such as recrystallization, distillation and liquid chromatography may be combined. In this way, the liquid crystal compound purified by the purification method of the present invention exhibits a high voltage holding ratio even when exposed to high temperatures, and the voltage holding is maintained even after a heat treatment step or after being exposed outdoors. It shows excellent reliability with little decrease in the rate.

[0036]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. The evaluation was judged by the change in voltage holding ratio, and the change in voltage holding ratio after the heat treatment test was measured for the purified liquid crystal compound.

[DSC Measuring Method] "DSC822" manufactured by METTLER TOLEDO was used for measuring DSC. 4 mg of the liquid crystal compound to be measured was placed in an aluminum pan having a volume of 40 μl under an air atmosphere and pressure-bonded to make a sealed state. This was heated from -30 ° C to 300 ° C at a heating rate of 5 ° C per minute. The heat generation amount and heat absorption amount at this time are plotted against temperature, and the heat generation observed in the temperature range where the liquid crystal compound exhibits an isotropic phase is defined as the heat of thermal decomposition, and the extrapolation heat generation start temperature of the heat generation that occurs at the lowest temperature Was set as the thermal decomposition start temperature.

[Method of measuring decomposition rate of liquid crystal compound by heat treatment] 500 mg of liquid crystal compound before and after heat treatment
Was precisely weighed and dissolved in 100 mL of acetone. To this, 10 mL of a standard solution containing naphthalene as a standard substance was added to prepare a sample solution. This sample solution was analyzed by a gas chromatograph "GC-17A" manufactured by Shimadzu Corporation. The column is "DB-1" manufactured by J & W Scientific Co., Ltd.
Column length was 50 m. Helium was used as the carrier gas, and the column temperature was raised from 200 ° C. to 300 ° C. for analysis. Using naphthalene as a standard substance, change the concentration of the liquid crystal compound and analyze it to prepare a calibration curve, quantify the concentration of the liquid crystal compound in the sample solution before and after the heat treatment, and from the difference, decompose the liquid crystal compound by the heat treatment. I asked.

[Preparation of Liquid Crystal Composition for Measuring Voltage Holding Ratio] A base liquid crystal containing an equal mass mixture of a liquid crystal compound 1 having a structure represented by the following formula (1) and a liquid crystal compound 2 having a structure represented by the following formula (2). It was a composition. It showed a nematic phase at 25 ° C. To 80 parts by mass of this base liquid crystal composition, 20 parts by mass of the liquid crystal compound purified in each of Examples and Comparative Examples was mixed to obtain a liquid crystal composition for measuring voltage holding ratio showing a nematic phase at 25 ° C.

[0040]

[Chemical 1] (1)

[0041]

[Chemical 2] (2)

[Measurement method of voltage holding ratio] Measuring cell: In the measuring cell of voltage holding ratio, ITO was formed as a transparent electrode on a glass substrate "1737" manufactured by Corning, and further polyimide manufactured by JSR. Alignment film "AL-1051"
A TN (twisted nematic) cell provided with was used. The electrode area was 0.64 cm ² and the cell thickness was 6 μm. Measurement condition: As shown in FIG. 1, a source voltage VS consisting of a rectangular wave of ± 5 V and 2.5 Hz is applied to a test cell for 64 μs by a high impedance FET (field effect transistor) switching by a gate pulse VG, and then cut off. To do. The area of the shaded area in the figure is obtained from the curve drawn by the voltage VL between the electrodes of the test cell in 1/2 cycle. The area when there was no VL attenuation was defined as 100%, and the area ratio to this was calculated as the voltage holding ratio. Measurement temperature is 80 ℃
And

[Heat Treatment Test] 2 g of the above liquid crystal composition for measuring voltage holding ratio was placed in a test tube with a stopper having a capacity of 10 mL, and 1
After degassing with a rotary pump for 5 minutes, it was sealed with nitrogen and sealed, and this was placed in an oven kept at 150 ° C. and heat-treated for 1 hour.

[Change in voltage holding ratio after heat treatment test of base liquid crystal composition] 2 g of the base liquid crystal composition was added to a capacity of 10 mL.
The mixture was placed in a test tube with a ground stopper and degassed by a rotary pump for 15 minutes, then sealed with nitrogen and tightly sealed, and this was placed in an oven kept at 150 ° C. and heat-treated for 1 hour. The voltage holding ratio before the test was 95.3% and after the test was 94.4%.

[Example 1] DSC under air atmosphere
As a result of the measurement, 5 g of the liquid crystal compound 3 represented by the following formula (3) in which thermal decomposition was observed at 160 ° C. was placed in a 50 ml eggplant-shaped flask and heated to 150 ° C. in an oven under an air atmosphere. After the heat treatment for 8 hours, the decomposition rate of the liquid crystal compound 3 was measured and found to be 1.5%. This was dissolved in 15 ml of n-hexane, 2 g of hydrazine monohydrate was added, and the mixture was stirred at room temperature for 2 hours. This was put in a separatory funnel, the n-hexane phase was washed 3 times with 20 ml of pure water, and then n-hexane was distilled off. The liquid crystal compound 3 thus obtained was purified with 5 ml of n
-Dissolved in hexane and used as an adsorbent, silica gel for column chromatography "BW-127Z" manufactured by Fuji Silysia Ltd.
It was passed through a glass column having an inner diameter of 20 mm filled with 25 g of “H” for 60 minutes for purification. N-Hexane was used as the mobile phase. After n-hexane was distilled off under reduced pressure from the eluate, it was recrystallized with a mixed solvent of methanol: ethanol = 4: 1 and dried under vacuum to obtain a purified liquid crystal compound 3. The liquid crystal compound 3 was mixed with the base liquid crystal composition according to the method for preparing the liquid crystal composition for measuring voltage holding ratio, and the voltage holding ratio was evaluated. As a result, the voltage holding ratio before the heat treatment test was 91.2%, and after the heat treatment test was 91.1%.

[0046]

[Chemical 3] (3)

Example 2 Liquid crystal compound 3 was purified in the same manner as in Example 1 except that 2 g of diethylamine was used as the nitrogen-containing compound instead of hydrazine monohydrate. This liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, 91.4% before the heat treatment test and 9 after the heat treatment test.
It was 1.3%.

Example 3 Liquid crystal compound 3 was purified in the same manner as in Example 1 except that 10 g of an aqueous ammonia solution having a concentration of 20% by mass was used instead of hydrazine monohydrate as the nitrogen-containing compound. This liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.0% before the heat treatment test and 90.8% after the heat treatment test.

Example 4 Example 1 was repeated except that 10 g of a 20% by weight aqueous solution of hydroxylamine hydrochloride was used as the nitrogen-containing compound instead of hydrazine monohydrate.
Liquid Crystal Compound 3 was purified in the same manner as. This liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.2% before the heat treatment test and 90.9% after the heat treatment test.

Example 5 Liquid Crystal Compound 3 was purified in the same manner as in Example 1 except that 2 g of pyridine was used instead of hydrazine monohydrate as the nitrogen-containing compound. This liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.5% before the heat treatment test and 91.4% after the heat treatment test.

Example 6 Liquid crystal compound 3 was purified in the same manner as in Example 1 except that the amount of hydrazine monohydrate was 0.25 g. This liquid crystal compound 3 was used in Example 1.
It mixed with the base liquid crystal composition by the same method as above, and the voltage holding ratio was evaluated. As a result, 91.3% before the heat treatment test,
It was 91.2% after the heat treatment test.

Example 7 The amount of hydrazine monohydrate was set to 1
The liquid crystal compound 3 was purified in the same manner as in Example 1 except that the amount was 0 g. This liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.2% before the heat treatment test and 91.2% after the heat treatment test.

Example 8 The liquid crystal compound 3 was purified in the same manner as in Example 1 except that the heating temperature was 180 ° C. and the heat treatment time was 2 hours. The decomposition rate of the liquid crystal compound 3 after the heat treatment for 2 hours was 0.9%. The purified liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.4% before the heat treatment test and 91.4% after the heat treatment test.

Example 9 Liquid crystal compound 3 was purified in the same manner as in Example 1 except that the heating temperature was 260 ° C. and the heat treatment time was 15 minutes. The decomposition rate of liquid crystal compound 3 after the heat treatment for 15 minutes was 5.0%. The purified liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.0% before the heat treatment test and 90.8% after the heat treatment test.

[Example 10] Liquid crystal compound 3 was purified in the same manner as in example 1 except that oxygen gas was blown into liquid crystal compound 3 during heating and the heat treatment time was changed to 2 hours.
The decomposition rate of the liquid crystal compound 3 after the heat treatment for 2 hours is 2.1.
%Met. The purified liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.5% before the heat treatment test and 91.4% after the heat treatment test.

[Example 11] A liquid crystal compound 3 was purified in the same manner as in Example 1 except that a microwave oven was used to heat to 180 ° C by heating at the heating time of 20 minutes. . The decomposition rate of liquid crystal compound 3 after heat treatment for 20 minutes was 1.2%. The purified liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, it was 91.2% before the heat treatment test and 91.0% after the heat treatment test.

[Embodiment 12] DS in air atmosphere
As a result of measuring C, 5 g of liquid crystal compound 4 represented by the following formula (4) in which thermal decomposition was observed at 160 ° C. was placed in a 50 ml eggplant-shaped flask and heated to 180 ° C. in an oven under an air atmosphere. The decomposition rate of liquid crystal compound 4 after the heat treatment for 1 hour was 1.3%. This was dissolved in 15 ml of n-hexane, 2 g of hydrazine monohydrate was added, and the mixture was stirred at room temperature for 2 hours. This was put in a separatory funnel, the n-hexane phase was washed 3 times with 20 ml of pure water, and then n-hexane was distilled off. The obtained crude product of liquid crystal compound 4 was dissolved in 5 ml of n-hexane, and 25 g of silica gel "BW-127ZH" for column chromatography manufactured by Fuji Silysia Chemical Ltd. was used as an adsorbent.
It was passed through a glass column having an inner diameter of 20 mm filled with the mixture for 60 minutes for purification. N-Hexane was used as the mobile phase. The eluate was washed with ultrapure water, then n-hexane was distilled off under reduced pressure, recrystallized with a mixed solvent of methanol: ethanol = 4: 1, and vacuum dried to obtain a purified liquid crystal compound 4. The liquid crystal compound 4 was mixed with the base liquid crystal composition in the same manner as in Example 1, and the voltage holding ratio was evaluated. As a result, 94.8% before the heat treatment test and 90.
It was 6%.

[0058]

[Chemical 4] (4)

Comparative Example 1 Liquid crystal compound 4 was mixed with the base liquid crystal composition as it was without heat treatment and purification, and the voltage holding ratio was evaluated. As a result, it was 94.9% before the heat treatment test and 42.7% after the heat treatment test.

[Comparative Example 2] Liquid crystal compound 4 was purified in the same manner as in Example 12 except that after heat treatment, it was not contacted with hydrazine monohydrate and immediately purified by a silica gel column. This liquid crystal compound was mixed with the base liquid crystal composition in the same manner as in Example 1 to evaluate the voltage holding ratio. As a result, it was 94.6% before the heat treatment test and 87.6% after the heat treatment test.

Comparative Example 3 The liquid crystal compound 3 was mixed with the base liquid crystal composition as it was without any heat treatment and purification, and the voltage holding ratio was evaluated. As a result, it was 90.1% before the heat treatment test and 75.6% after the heat treatment test.

[Comparative Example 4] The liquid crystal compound 3 was purified by the same procedure as in Example 1 except that the heat treatment was not performed. The purified product of the obtained liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1 to evaluate the voltage holding ratio. As a result, it was 91.5% before the heat treatment test and 79.6% after the heat treatment test.

[Comparative Example 5] The liquid crystal compound 3 was purified in the same manner as in Example 1 except that the heat treatment was conducted in a nitrogen gas atmosphere instead of in an air atmosphere. The purified product of the obtained liquid crystal compound 3 was mixed with the base liquid crystal composition in the same manner as in Example 1 to evaluate the voltage holding ratio. As a result, it was 90.7% before the heat treatment test and 78.2% after the heat treatment test.

Comparative Example 6 After distilling the liquid crystal compound 4 under reduced pressure, 5 g of the obtained main fraction was dissolved in 5 ml of n-hexane and used as an adsorbent, silica gel "BW-127ZH" for column chromatography manufactured by Fuji Silysia Chemical Ltd. It was passed through a glass column having an inner diameter of 50 mm filled with 250 g for 4 hours for purification. N-Hexane was used as the mobile phase. The eluate was collected in 100 ml portions, and only the fractions having a gas chromatography purity of the liquid crystal compound 4 of 99.99% or more were collected and washed with ultrapure water, and then n-hexane was distilled off to obtain methanol: ethanol = The crystals were recrystallized from a mixed solvent of 4: 1 and dried in vacuum to obtain a purified liquid crystal compound 4.
This liquid crystal compound 4 was mixed with the base liquid crystal composition in the same manner as in Example 12, and the voltage holding ratio was evaluated. As a result, 95.1% before the heat treatment test and 63.6% after the heat treatment test.
%Met.

[0065]

INDUSTRIAL APPLICABILITY The purification method of the present invention is a liquid crystal compound having the lowest temperature in the temperature range in which the liquid crystal compound shows an isotropic phase among the peaks obtained by differential scanning calorimetry carried out in an atmosphere containing oxygen. Side peak extrapolation heat generation start temperature A
Within a temperature range of 20 ° C. lower than (° C.) to 150 ° C. higher than A and under an atmosphere containing oxygen,
By subjecting the liquid crystal compound to a heat treatment in the range of 0.5 to 5% to oxidize or decompose the thermally-decomposable impurities contained in the liquid crystal compound, a nitrogen-containing basic compound Contact with compound and then purification with adsorbent. By bringing into contact with the basic compound, it is possible to remove impurities generated by heating and those decomposed from the liquid crystal compound, which have particularly high acidity. According to the purification method of the present invention, a liquid crystal compound that does not cause a decrease in voltage holding ratio after a heat treatment test can be obtained.

[0066]

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing an example of a voltage waveform when a test cell in which a liquid crystal compound according to the present invention is injected is actively driven.

[Explanation of symbols]

VS source voltage VG gate voltage VL Voltage applied between both electrodes

Claims (3)

[Claims] 1. A differential scanning calorimetry measurement of a liquid crystal compound, which is composed of an element selected from the group consisting of carbon, hydrogen, fluorine, chlorine and oxygen, and has no aliphatic unsaturated group, in an atmosphere containing oxygen. 20 than the extrapolation exothermic onset temperature A (° C.) of the peak on the lowest temperature side in the temperature range higher than the nematic phase-isotropic phase transition temperature of the liquid crystal compound obtained in
After heating within a range from a temperature lower than C to a temperature higher than A by 150 ° C. and in a range where the decomposition rate of the liquid crystal compound is 0.5 to 5% in an atmosphere containing oxygen, the liquid crystal contains nitrogen. A method for purifying a liquid crystal compound, which comprises contacting with a basic compound to be purified, followed by purification with an adsorbent. 2. The nitrogen-containing basic compound is ammonia, an amine compound, an amide compound, a hydrazine compound, a hydrazone compound, a hydroxylamine compound,
The method for purifying a liquid crystal compound according to claim 1, which is a compound having a heterocycle containing nitrogen. 3. The nitrogen-containing basic compound,
The method for purifying a liquid crystal compound according to claim 1, which is used in an amount of 5 to 200% by mass based on the liquid crystal compound.