JPH0438789B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a liquid crystal composition for dual-frequency driving, which performs optical ON-OFF control by applying signals of a frequency component lower than fc and a frequency component higher than fc of the sandwiched liquid crystal material to a liquid crystal panel. More specifically, the present invention relates to a liquid crystal composition for a liquid crystal light valve that has a very high light response speed. In recent years, information processing has become larger in capacity, faster in calculations, and
OA (office automation) equipment is rapidly becoming popular. Printers, which are one of the terminal devices for office automation, are also required to be high speed, high print quality, and low cost.
Since there is currently no product on the market that satisfies these requirements, the emergence of one is strongly desired. A printing device that uses a liquid crystal light valve as the optical signal generator and applies electrophotographic technology (Japanese Patent Application Laid-Open No. 1983-93568)
has reached a practical level in the above points,
Attention has been paid. First, FIG. 1 shows an outline of the configuration of the above-mentioned liquid crystal light valve type printing apparatus. Light is written onto the photosensitive drum 102 by an optical signal generating section 101 using a liquid crystal light valve. At this time, the photosensitive drum 102 is charged in advance by the corona charger 110. At this time, when printing characters, the optical signal is usually such that light is generated corresponding to the character part. This forms an electrostatic latent image, which is developed with toner by a magnetic brush developer 103. The developing method at this time is usually reversal development. Thereafter, the toner is transferred to the plain paper 104 by the transfer corona discharger 105.
The image is transferred to and fixed by the fixing device 106.
The toner remaining on the photosensitive drum after transfer is removed by a blade 108, and the electrostatic latent image is removed by a static elimination lamp 109.
The static electricity is removed and the process ends. FIG. 2 shows the configuration of the optical signal generator. light source 20
The light emitted from 1 is transmitted to the liquid crystal panel sections 211 to 218.
ON-OFF control is performed, and the transmitted light is transmitted through the condensing lens 221
The image is formed on the photoreceptor by . The LCD panel is
Narrowed by two polarizing plates 211, the glass plate 2
A thin metal film 214 used as an optical mask, an insulating film 215, and a liquid crystal alignment control film 21 are coated on 12 by vapor deposition, sputtering, dipping, etc., and further formed by photo-etching.
6, and a liquid crystal 218 is held between a seal 217 and a liquid crystal panel. FIG. 3 is a front view of the liquid crystal panel, which is composed of a microshutter group 302 that transmits and blocks light, and a light mask section 301 that constantly blocks light. Next, the driving principle of the liquid crystal light valve will be explained. FIG. 4 shows signals applied to the liquid crystal panel and transmitted light characteristics. However, what is shown here is the case of dynamic driving. When a signal having a frequency component higher than fc is applied to the liquid crystal during the aperture period _T1 [Fig. 4b], the panel transmits light [Fig. 4a]. Next, when a signal having a frequency component lower than fc is applied to the liquid crystal for a period of _T2 [Fig. 4b], the panel blocks light. T ₃ is a so-called selection period, and the duty in this case is T ₃ /T ₄ .
It is already known that the alignment state of liquid crystal molecules can be controlled by the frequency of the applied signal, so the details are omitted, but this phenomenon is that the sign of the dielectric anisotropy of the liquid crystal is reversed by the frequency of the applied signal. Based on nature. FIG. 5 shows the frequency dependence of the dielectric constant ε in the long axis direction and the dielectric constant ε⊥ in the short axis direction of liquid crystal molecules.
Bordering the crossover frequency fc (frequency where ε = ε⊥),
The sign of the dielectric anisotropy Δε is reversed. The driving principle of the liquid crystal light valve has been explained above.
When using this liquid crystal light valve in the optical writing section of a printing device, many constraints must be met in order to achieve high speed, high print quality, and even high reliability of the printing device. First, high light energy must be supplied onto the photosensitive drum during the writing time (T ₃ in FIG. 4). In order to satisfy this requirement, it is of course possible to improve the characteristics to some extent by changing the light source optical system around the panel and the driving method, but it largely depends on the basic characteristics of the liquid crystal. The following items are listed as basic properties required of liquid crystal materials in order to realize an excellent liquid crystal light valve. 1. High-speed response with low voltage. 2. Wide temperature and voltage margins. 3. Must be able to ensure high reliability. Liquid crystal materials for dual-frequency drive have been researched and excellent products have been produced for displays, etc., but they have extremely high-speed response (several milliseconds or less), such as those used in liquid crystal light valves used in printing devices. There is still little research into what is required, and liquid crystal materials that satisfy all of the above-mentioned characteristics are not well known. The present invention improves the characteristics of a liquid crystal material for liquid crystal light valves used in printing devices, etc., and has the following objectives: 1. High-speed response at low voltage. 2. Wide temperature and voltage margins. 3. Must be able to ensure high reliability. The object of the present invention is to create a liquid crystal composition for dual-frequency driving that satisfies the above requirements in a well-balanced manner. Below, we will explain these requirements. 1. High-speed response with low voltage. If an IC or the like is used in the drive circuit to reduce the size and cost of the finished product, the upper limit of the drive voltage and drive frequency will be limited. Furthermore, in order to reduce power consumption, low voltage and low frequency are desirable. The response speed τ of the liquid crystal is

As shown in the formula, it is inversely proportional to the square root of the dielectric anisotropy, so in order to obtain a high-speed response, it is necessary to increase the absolute value of the dielectric anisotropy at the driving frequency. It is also required to have low viscosity. Furthermore, since the drive frequency cannot be made very high, fc at the operating temperature must also be appropriately low. Since there is no liquid crystal material that satisfies these characteristics with a single component, multiple components must be mixed to satisfy each characteristic in a well-balanced manner. first,
fc is the adjustment of viscosity, but these two have contradictory properties. Among the ingredients in the composition provided by the present invention: (However, R ₁ has 3,4,5 carbon atoms, R ₂ has 1,2 carbon atoms, R ₃ has 3,5 carbon atoms, R ₄ has 4,5 carbon atoms,
7 represents an alkyl group or 4 represents a straight-chain alkyl group, and X represents CN or H. ) has the property that both fc and viscosity of the composition can be lowered to practical levels. Next, the component that adjusts the dielectric anisotropy at low frequencies, and the component that adjusts the dielectric anisotropy at high frequencies, are known liquid crystals or compounds with a molecular structure similar to liquid crystals. There are many materials that have a large absolute value of positive dielectric anisotropy, but there are very few materials that have a large absolute value of negative dielectric anisotropy. Furthermore, if restrictions are placed on both of them, such as low melting entropy, wide liquid crystal temperature range, and no adverse effects on fc, viscosity, etc., the types will be further limited. Among the components of the composition provided by the present invention: (However, R ₅ represents a straight chain alkyl group having 6, 7, or 8 carbon atoms.) has very large positive dielectric anisotropy and low fc. Furthermore, among the components of the composition, (However, R ₆ represents a linear alkyl group having 3.5 carbon atoms.) has a large absolute value of negative dielectric anisotropy and is also excellent in compatibility with other components. Next, regarding chiral nematic liquid crystal among the components of the composition provided by the present invention,
In the case of TN display panels, a small amount of this ingredient is added to prevent reverse tilt, it is used for guest-host display to improve contrast, and a display method that applies the memory effect of liquid crystals containing this ingredient is added. Many examples of uses of this component are known. Furthermore, some devices are well known in which the response speed is improved by utilizing the helical force of chiral nematic liquid crystals, that is, the force of returning to a helical structure. In the present invention, the reason why chiral nematic liquid crystal is added is that even in a two-frequency drive system, the same effect as described above can be obtained, namely,
This has the effect of improving response speed. The two-frequency drive system has the effect of increasing the speed particularly when driving using a signal having a higher frequency component than fc, that is, when aligning liquid crystal molecules square to the cell surface. The present inventors determined that by blending the above-mentioned components in appropriate proportions, the absolute values of Δε _L and Δε _H (particularly in conventional compositions, increasing the absolute value of Δε _H is not possible due to the balance with other properties). (It was difficult considering the
In addition, by adding chiral nematic liquid crystal, we succeeded in dramatically increasing the response speed at operating temperature and voltage. In the case of a liquid crystal composition, unlike a single component, the interaction between different components is expected to have a considerable influence on the viscosity, fc, and ultimately the response speed. Even in the composition provided by the present invention, this interaction seems to work in the direction of improving the properties of the composition, but the physical or quantum mechanical interpretation of this point remains to be seen. I look forward to the research. The above is the reason why the component according to the present invention was used to satisfy characteristic 1. 2. Wide temperature and voltage margins. First, regarding the temperature margin, FIG. 6 shows the temperature dependence of the dielectric anisotropy of liquid crystal. In the figure,
Δε _L is the dielectric anisotropy at low driving frequency, Δε _H
is the dielectric anisotropy at a high driving frequency. As can be seen from the figure, Δε _H decreases rapidly as the temperature rises. Although this tendency varies slightly depending on the liquid crystal, it is a basic characteristic of liquid crystals, so when actually used, a temperature range where the amount of change in Δε _H is small is selected, but in the case of the present invention When it is necessary to make the response very fast, as in the above case, it is necessary to lower the viscosity of the liquid crystal in a certain high temperature range. Based on the undeniable fact that Δε _H fluctuates quite a bit due to temperature changes, and in order to maintain a wide temperature margin, it is necessary to adopt a range in which the light transmittance characteristics of the liquid crystal are saturated. Become. That is, under a constant voltage, the response is fast and the light transmittance curve is saturated, and it is a prerequisite that the above-mentioned characteristic 1 is satisfied. Further, the voltage margin can be considered in the same way. 3. Must be able to ensure high reliability. Many studies and experiments have been conducted regarding the chemical, optical, or electrochemical stability of liquid crystals. Since all of the liquid crystals according to the present invention are ester-based liquid crystals, their optical reliability is high, and due to accelerated aging, etc., the reliability has been evaluated to be at a sufficient practical level. In order to satisfy the above-mentioned characteristics 1, 2, and 3 in a well-balanced manner at a practical level, the present invention has developed a liquid crystal material with excellent characteristics as a composition by appropriately selecting the blending ratio of each of the above-mentioned components. I was able to create. Next, details of the present invention will be explained with reference to examples. Example The liquid crystal compositions listed in Table 1 were prepared, and their basic characteristics and light transmittance characteristics were investigated.

【table】

【table】

[Table] ｜
H
It has the structure 〓
FIG. 7 shows the frequency dependence of the dielectric constant of the liquid crystal compositions shown in Table 1. However, the measurement temperature was 30℃. This liquid crystal was sealed in a test cell with a cell thickness of 4.5 μm,
Low frequency signal (30V, 1KHz), high frequency signal (30V,
When 1/2 dynamic drive was performed at 130KHz), the light transmittance characteristics shown in Figure 8 were obtained. From the figure, in 1/2 dynamic drive, a repetitive response is possible with a repetition period of 1.2 to 1.6 msec. However, the rubbing angle of the cells used at this time was 70°, and the polarization directions of the polarizing plates sandwiching the cells were perpendicular to each other. The measurement temperature was 40°C. Next, we conducted a printing experiment using this liquid crystal light valve. The same liquid crystal cell as in the above measurement was used, and the driving conditions were also the same. The light source has a structural formula
A high-intensity fluorescent lamp using a CeMgAl ₁₁ O ₁₉ :Tb ³⁺ phosphor was used, and Se sensitized with Te was used as the photoreceptor. Each component was arranged as shown in FIG. 1, and the surface movement speed of the photosensitive drum was 5 cm/sec. When we drove the liquid crystal light valve and performed optical writing, we were able to form a clear image depending on whether it was turned on or off. This example is an example of the use of the liquid crystal light valve provided by the present invention,
It can also be applied to optical writing devices other than printing devices, displays, etc. Furthermore, the liquid crystal composition used in this example is just an example, and even if each component is replaced with its homolog,
Furthermore, the inventor has conducted experiments to find that excellent characteristics similar to those of this example can be achieved even if the blending ratio is changed within the range of blending ratios in the table described in the scope of claims. Therefore, the present invention is not limited to this example. As mentioned above, by comprising compounds (1) to (4) and a chiral nematic liquid crystal compound, dual-frequency driving can be achieved by utilizing the leakage dispersion of the liquid crystal composition, for example, a liquid crystal light valve, etc. When used in applications, high-speed switching becomes possible, and the writing speed, temperature characteristics, voltage margin, etc. of printing devices can be dramatically improved.

[Brief explanation of drawings]

FIG. 1 is a structural diagram of a liquid crystal light valve type printing device. FIG. 2 is a schematic diagram of a liquid crystal light valve. FIG. 3 is a top view of a liquid crystal panel used in a liquid crystal light valve. FIG. 4 is a diagram showing the signals applied to the liquid crystal light valve and the optical response characteristics at that time. FIG. 5 is a frequency characteristic diagram of the dielectric constant of liquid crystal. FIG. 6 is a temperature characteristic diagram of dielectric anisotropy of liquid crystal. FIG. 7 is a frequency characteristic diagram of the dielectric constant of the liquid crystal composition used in this example. FIG. 8 is a diagram showing the optical response characteristics of the liquid crystal light valve according to this embodiment.

Claims (1)

[Claims] 1. The general formula is (However, R ₁ represents an alkyl group having 3, 4, or 5 carbon atoms, and R ₂ represents an alkyl group having 1 or 2 carbon atoms or a straight-chain alkyl group having 4 carbon atoms.) (However, R ₃ has 3.5 carbon atoms, R ₄ has 4 carbon atoms,
It represents a 5,7 straight chain alkyl group, and X represents CN or H. ) and the general formula is (However, R ₅ represents a straight chain alkyl group having 6, 7, or 8 carbon atoms.) A compound represented by (However, R ₆ represents a straight-chain alkyl group having 3.5 carbon atoms.) A liquid crystal composition for dual-frequency drive, characterized by comprising a compound represented by: and a chiral nematic liquid crystal compound.