JPS6321717B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a liquid crystal composition, particularly a liquid crystal composition suitable for use in a time-division driving system. FIG. 1 shows an example of a twisted nematic type (TN type) liquid crystal display element, which is one type of field effect type liquid crystal display element. The liquid crystal display element shown in the figure has a first substrate 1 and a second substrate 2, each made of transparent glass or the like, separated by a predetermined distance, for example, 5 to 15 μm.
The peripheries thereof are sealed with a sealing member 3 made of, for example, fritted glass or organic adhesive, and a nematic phase liquid crystal 4 is sealed in the internal space formed by these members. The predetermined spacing is obtained by a spacer 5, for example of fiber glass, glass powder or the like. Note that the sealing member 3 may also be used as a spacer without using the spacer 5 in particular. The first and second substrates 1 and 2 each have a predetermined pattern of electrodes 6 formed on their opposing inner surfaces, and furthermore, on the surfaces in contact with the liquid crystal, liquid crystal molecules near the surfaces are aligned in a desired fixed direction. The liquid crystal alignment control surfaces 7 and 8 are used to control liquid crystal orientation. Such an orientation control surface can be formed by, for example, an oblique evaporation film in which SiO is deposited obliquely to the substrate surface on a substrate surface having electrodes, or an organic polymer thin film or an inorganic thin film is coated on the substrate surface. It is made by applying a so-called rubbing process in which the surface is rubbed in a certain direction with cotton or the like. Regarding the liquid crystal alignment direction, the liquid crystal alignment control surface 7 of the first substrate 1 has a first fixed direction, and the second substrate 2 has a first constant direction.
By selecting the second fixed direction and making each direction different for the liquid crystal alignment control surface 8,
The molecules of the nematic phase liquid crystal 4 sandwiched between the substrates 1 and 2 are twisted and oriented from the first direction to the second direction. The angle between the first direction and the second direction, that is, the twist angle of the liquid crystal molecules can be arbitrarily selected, but generally approximately 90 degrees is selected as shown in FIG. A first polarizing plate 9 and a second polarizing plate 10 are arranged on the outside of the substrates 1 and 2, respectively. In this case, the angle between the polarization axes of the two polarizing plates 9 and 10 is usually the same as the twist angle of the liquid crystal molecules (the angle between the first direction and the second direction), or the angle between (the polarization axes are parallel) are selected. Usually, the alignment direction of the liquid crystal alignment surface and the polarization axis of the polarizing plate are arranged to be parallel or orthogonal to each other. When such a display element performs a normal display when viewed from the first substrate side, it is a reflective display element in which a reflector 11 is arranged on the back surface of the second polarizing plate 9, or a reflective display element in which a reflector 11 is arranged on the back surface of the second polarizing plate 9, or a second polarizing plate A light guide such as an acrylic resin plate or a glass plate having a desired thickness is inserted between the reflector 9 and the reflector 11, and a light source is placed at an appropriate position on the side of the light guide, which is widely used as a nighttime display element. Here, the twist angle is 90 degrees, and the polarization axis crossing angle is 90 degrees.
The principle of display operation of a reflective liquid crystal display element will be explained. Now, when there is no electric field in the liquid crystal layer, external light (ambient light incident on the first polarizing plate 9 of this liquid crystal display element) first passes along the polarization axis of the first polarizing plate 9. The light becomes linearly polarized light and enters the liquid crystal layer 4, but since the liquid crystal molecules are twisted 90 degrees between the layers, when it passes through the liquid crystal layer, the plane of polarization of the polarized light is rotated 90 degrees, and the second polarizing plate 10
Transmit. This light is reflected by the reflecting plate 11, passes through the second polarizing plate 10, the liquid crystal layer 4, and the first polarizing plate 9 in the reverse order as described above, and is emitted to the outside of the liquid crystal display element.
Therefore, the viewer can observe the polarized light that is incident on the liquid crystal display element, reflected by the reflector, and then outputted from the liquid crystal display element again. In such a display element, when a predetermined voltage is applied to a predetermined selected electrode 6 and an electric field is applied to a predetermined region of the liquid crystal layer, the liquid crystal molecules in that region are aligned along the direction of the electric field. As a result, the optical rotation power of the plane of polarization is lost in that region, and the plane of polarization does not rotate in that region, so that the light polarized by the first polarizing plate 9 is blocked by the second polarizing plate 10. Therefore, the area appears dark to the observer. Therefore, by applying voltage to desired selected electrodes, desired display can be performed. In the field-effect liquid crystal display device as described above, it is particularly desirable that the liquid crystal composition used has the following properties. That is, (1) it has good adaptability to the orientation control surface, (2) it can operate in a wide temperature range, and (3) it has good responsiveness even in a wide temperature range, especially at low temperatures. Regarding the first requirement, it is extremely important to control the molecules of the liquid crystal 4 so that they are aligned parallel to each other and in one direction at the interface between the upper and lower plates, and this has been the case in the past. Controlled by SiO
This is accomplished by forming an obliquely evaporated film or by performing a rubbing treatment. In addition, for the second requirement, the minimum requirement is that it be liquid crystal at around normal temperature of 25℃, but in practical terms it should be liquid crystal in the temperature range of -10℃ to +60℃ or higher. A liquid crystal is required. The transition temperatures of solid and liquid crystals described in the present invention are determined and defined based on the following measurements. Individual liquid crystal single substances and mixed compositions composed of them often exhibit a supercooling phenomenon. In that case, the temperature is sufficiently low (for example -40℃)
The crystallization was measured by cooling the mixture to a temperature of 100.degree. C., and then the transition temperature at the time of temperature rise was measured using a micro-melting point measuring device, and this was taken as the solid-liquid crystal transition temperature. This second requirement has an extremely important meaning not only in ordinary static driving but also in driving by the so-called time-division driving system. That is, in recent years, it has become essential for liquid crystal display devices, particularly devices that require a large amount of information, such as calculators or matrix displays, to adopt a time-division driving method using a voltage averaging method or the like. Low voltage drive is desirable for calculators, etc., especially 4.5V drive (3 batteries for 1.5V batteries), which can be directly driven by connecting batteries in series, and 3V drive (battery).
Low voltage drive such as 1.5V (2 pcs.) is adopted. Since this low-voltage drive connects batteries in series, it does not require a booster circuit, and when combined with C-MOS, the battery life can be maintained at 500 to 2000 hours. However, if such a time-division drive system is adopted, operational constraints that do not occur in the static drive system will exist in principle. In other words, in a time-division display device, it is necessary to prevent crosstalk between picture elements at half-selected points and non-selected points. The voltage averaging method is the most commonly used method for preventing this crosstalk. This method was devised for the purpose of increasing the margin of operation by averaging the crosstalk voltage and increasing the difference from the selection voltage. This method will be explained below using specific examples. Average the crosstalk voltage to 1/3 of the selected voltage,
An example of applying the voltage averaging method using an alternating current drive waveform will be presented. The driving waveform of this method is as shown in the state diagram of FIG. In Figure 3, in the selected state ±
A voltage of V _p is applied to the liquid crystal, and a voltage of ±(1/3) V _p is applied to the liquid crystal in the half-selected state and non-selected state. At this time, the effective value ν _S1 of the applied voltage applied to the point at which the liquid crystal enters the display state, that is, the display point, is expressed by the following equation. However, N: Duty number. On the other hand, the effective value ν _S2 of the applied voltage applied to the non-display point of the liquid crystal is given by the following equation. ν _S2 = 1/3V _p ...(2) Here, in order to put the display point into the display state, ν _S1 ≧Vth with respect to the threshold voltage Vth of the liquid crystal, and crosstalk occurs at the non-display point. In order to avoid this, ν _S2 ≦
Must be Vth. That is, the conditions for displaying with crosstalk prevented by this driving method are as follows. ν _S2 ≦Vth≦ν _S1 ...(3) Substituting equations (1) and (2) into equation (3) and rearranging for V _p results in the following. Therefore, when V _p is changed and the brightness of display points and non-display points is measured, the results are as shown in FIG. 8. The threshold voltage of the liquid crystal converted to V _p at the display point and non-display point,
Vth ₁ and Vth ₂ exist, and when Vth ₁ ≦V _p ≦ Vth ₂ (5), display that prevents crosstalk is possible. Note that, according to equation (4), Vth ₁ and Vth ₂ can be expressed by the following equations. Vth ₂ = 3Vth... (7) Regarding formula (5), to be more precise, the lower limit of the voltage that can be displayed should be the saturation voltage Vsat ₁ shown in Figure 8, not Vth ₁ . . In other words, the following equation (8) is the equation that determines the voltage range where Vsat ₁ ≦V _p ≦Vth ₂ . It can be said that the device with a larger movable range of V _p in equation (8) has a wider operating margin. In the induction of the above equation, ν _S1 ,
ν _S2 Therefore, Vth ₁ , Vth ₂ , Vsat _1, etc. have been considered as constant values, but these can change depending on the temperature (T), viewing angle (φ, θ) for the element (Figure 4), etc. It is. In the explanation of equations (1) to (8), we have considered the viewing angle φ=0 as defined in FIG. 4, but in reality, φ takes values within a certain finite range. There are various factors that determine the operating margin, which will be explained below in order.
When considering the factors that determine the operating margin, it is convenient to take up the following three factors in order to see the essence of the problem. (a) Fluctuation of threshold voltage due to temperature (b) Fluctuation of threshold voltage due to angle (c) Sharpness of voltage-luminance characteristics Now, let us explain the relationship between (a) to (c) and the operating margin in detail. Clarify quantitatively using examples. The electro-optical characteristics of the time-division drive method were measured by the method shown in FIG. The liquid crystal display element 51 is a luminance meter 52
It is arranged in a constant temperature bath 53 at an angle of 10° to 40° relative to the main body. And for the luminance meter 52
A tungsten lamp 54 arranged at an angle of 30 degrees irradiates light onto the liquid crystal display element 51 through a heat-absorbing glass filter 55.
1 is measured by a brightness meter 52. The respective drive waveforms in the case of time division drive 1/3 bias 1/3 duty and 1/2 bias 1/2 duty measured by the method described above are as shown in FIGS. 6 and 7. FIG. 8 shows voltage and brightness characteristics using this waveform. The area is an area that is not lit, and the area is an area where only the selected point is lit.
Desired numbers, characters, etc. can be displayed in this area. The area is an area in which all segments are lit and does not serve as a display function. In other words, this is an area where crosstalk occurs. Vth ₁ is the voltage at the selected point (ON state) at 10% brightness, Vth ₂ is the voltage at the non-selected (OFF state) point at 10% brightness, Vsat ₁ is the voltage at the selected point at 50% brightness,
Vsat ₂ is the non-select voltage at 50% brightness. The operating margin is defined by the following formula. M (%) = Vth ₂ (T = 40, φ = 40°, f = 100) - Vsat ₁
(T=0, φ=10゜, f=550)/Vth ₂ (T=40, φ=4
0゜, f = 100) + Vsat ₁ (T = 0, φ = 10゜, f = 550)
×100……(9) However, T = Temperature (°C) 0 to 40°C φ = Viewing angle (°) 10° to 40° = Frequency (Hz) 100 to 550Hz Therefore, a wide operating margin means a wide area. It is synonymous with. In this way, time-division driving requires driving within a certain voltage width (margin). Further analysis of the operating margin M expressed by equation (9) reveals that M is determined by the three factors (a) to (c) described above. Each factor is quantitatively defined by the following formula. (a) Temperature characteristics of Vth ΔT ΔT=Vth ₂ (T=0℃)−Vth ₂ (T=40℃)/Vth ₂
(T=0℃)+Vth ₂ (T=40℃)×100(%)……(10) However, temperature T is in the range of 0° to 40°C, φ=40°,
Define ΔT as the value at =100Hz. (b) Angular dependence of Vth Δφ Δφ=Vth ₂ (φ=40°)/Vth ₂ (φ=10°)……(11) However, Δφ was defined as the value at T=40°C and =100Hz. . (c) Sharpness of voltage-luminance characteristics γ γ = Vsat ₁ /Vth ₁ ...(12) These three (a) to (c) are the main elements, but in addition to these, the frequency characteristics Δ is generally exist. Δ=Vth ₁ (=550)/Vth ₁ (=100) (13) However, Δ is defined as the value at T=40°C and φ=40°. Furthermore, for convenience in deriving the equation, we define the voltage averaging method margin α: α=Vth ₂ /Vth ₁ ……(14) Here, substitute (10) to (14) into equation (9). If you organize it by
The operating margin M is given by the following equation. M=1-(γ/Δ)Δf/α・A/1+(γ/Δ
)Δf/α・A……(15) However, A=1−ΔT/1+ΔT Generally, γ, Δφ, ΔT, Δ are γ≧1, Δφ≦1,
Takes the value of ΔT≧0, Δ≦1. The operating margin defined here can take various values depending on the liquid crystal material, and a material that can provide a larger margin M is suitable for time-division driving. As is clear from equation (15), in order to expand the operating margin M, the temperature characteristic ΔT is
approaches 0, the angle dependence Δφ, voltage-luminance sharpness γ, and frequency characteristic Δ each decrease to 1.
It is necessary to be close to. In some cases, by introducing a temperature compensation circuit into the device, the influence of the temperature characteristic ΔT can be almost ignored and the operating margin of the device can be expanded. However, providing a temperature compensation circuit inevitably makes the device expensive, so in order to reduce costs, popular products such as calculators are widely used without such extra compensation circuits. It is extremely preferable to use component materials (elements) that allow a sufficient operating margin. Regarding the third requirement, that is, good responsiveness over a wide temperature range, especially at low temperatures, a method can be derived from the following considerations. The response in twisted nematic mode during time division drive operation is given by the following equation. trise∝1/(8/N+1)・η・d ² /K …(16) tfa11∝d ² η/K …(17) η is the viscosity, K is the elastic constant, and d is the thickness of the liquid crystal layer. From the above equation, it can be seen that the response of the liquid crystal is mainly determined by the viscosity of the liquid crystal material. This theoretical formula is said to be in good agreement with actual measurements, and those skilled in the art will readily recognize that responsiveness can be improved by reducing the viscosity of the liquid crystal material. In other words, the key to success or failure in meeting this third requirement lies in finding a liquid crystal material with low viscosity (after satisfying the other first and second requirements, of course). Conventionally, a nematic liquid crystal ( _N2 liquid crystal) whose constituent molecules have negative dielectric constant anisotropy is used as a matrix, and an appropriate amount of liquid crystal is used as a material suitable for the second and third of the above three requirements. Nematic liquid crystals with positive dielectric anisotropy (N _p liquid crystals) and/or liquid crystal analogues with positive dielectric anisotropy (substances whose molecular structure is similar to positive nematic liquid crystals; N _p
Liquid crystal systems have been proposed, in which (N _o + N _p ) liquid crystal systems are added.
It has been revealed that this material has superior time-sharing characteristics compared to materials consisting only of N _p liquid crystal systems. In these liquid crystal systems, the strong N _p liquid crystal added to the N _o liquid crystal system is generally at the upper limit of the liquid crystal temperature range (MR).
T _N- 〓 point (temperature at which nematic liquid crystal transitions to liquid)
In order to compensate for this and to reduce the temperature characteristics of the driving voltage, it is necessary to add a liquid crystal material with a high T _N - point. And this
Liquid crystal materials with high T _N- points, especially T _N- points of about 150℃
The above-mentioned materials have three benzene rings or cyclohexane rings, and are usually called long-molecule materials and are used by being added to a liquid crystal material for time-division driving. However, since such long molecular materials have large molecular weights, they have the drawback of increasing the viscosity of the liquid crystal composition and reducing responsiveness. Therefore, the present invention uses the general formula

[Formula] (R ₁ and R ₂ are linear alkyl groups having 1 to 5 carbon atoms),

[Formula] (R ₅ is a straight chain alkyl group having 3 to 7 carbon atoms), and

[Formula] (R ₇ is a linear alkyl group having 3 to 7 carbon atoms) A mixed liquid crystal containing at least a compound represented by the general formula (R ₈ , R ₉ are linear alkyl groups having 1 to 9 carbon atoms) By adding at least one type of compound represented by the following, a liquid crystal composition with improved time division characteristics and responsiveness is provided. The purpose is to The present invention will be explained in detail below using Examples. Example First, general formula The compounds represented by include compounds having a liquid crystal temperature range (MR) shown in Table 1 below.

[Table] Also, as a parent liquid crystal, A mixed liquid crystal consisting of the following composition is used. in this case,
This base liquid crystal has a general formula

ECH (ester/cyclohexane liquid crystal) represented by [formula]
as a compound and are 20% by weight, 10% by weight, and 20% by weight, respectively, and the general formula

PCH (phenylcyclohexane liquid crystal) represented by [formula]
as a compound

[Formula] O call

[Formula] is 20% by weight each, and the general formula

As an additive represented by [formula]

[Formula] and 10% by weight are respectively added and mixed, -
It has a liquid crystal temperature range (MR) of 5-55°C.
Then, to the above mixed liquid crystal, the general formula The compounds shown in Table 1 above are When added in an amount of 1 to 15% by weight, it was possible to significantly improve the viscosity characteristics (characteristics) and operating margin (characteristics') of the host liquid crystal, as shown in Figure 9. In addition, in the same figure, the characteristics , , are comparative examples in which the above-mentioned long molecular material, e.g. (Characteristic), (Characteristic), (Characteristics) shows the conventional viscosity characteristics and operating margin characteristics in which the same weight percent was added. Therefore, as is clear from the figure,
According to the liquid crystal composition having the above structure, the viscosity characteristics and the margin characteristics can be improved, so that time-division driving with good responsiveness is possible. In this case, as an additive compound If the amount added exceeds the upper limit of 15% by weight, becomes difficult to dissolve and a precipitation phenomenon appears, and if it is less than the lower limit of 1% by weight, the above-mentioned effects cannot be obtained. Therefore, if the amount added is about 10% by weight, extremely good effects can be obtained. In addition, in the above examples, the general formula As a compound represented by Although we have explained the case where
The same effect can be obtained with any compound having a linear alkyl group having 1 to 9 carbon atoms as shown in the formula. In addition, these compounds are not limited to one type, but multiple types can be added,
If the total amount is about 15% by weight or less, the same effects as described above can be obtained. In this case, when two or more types of compounds are added, the compatibility can be improved more than when one type of compound is added. Furthermore, when three or more types are added, the compatibility can be improved, but almost the same effect as adding two types can be obtained. As explained above, according to the present invention, the viscosity of the base liquid crystal can be reduced and the responsiveness can be improved, so that a liquid crystal composition with excellent time division characteristics can be obtained. In addition, it has good electro-optical properties and a good margin range, so it is not only the best material for time-division driving, but it is also chemically stable, making it an excellent material for highly reliable liquid crystal compositions. have.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing an example of a liquid crystal display element, Figure 2 is a configuration diagram showing the alignment state of liquid crystal molecules, and Figure 3 is an example of a voltage averaging method (1/3 bias) time-division drive waveform. Fig. 4 is a diagram showing the definition of viewing angle, Fig. 5 is an explanatory diagram of the electro-optical characteristic measuring device, Fig. 6 is a drive waveform of 1/3 bias and 1/3 duty, and Fig. 7 is a diagram showing 1/3 duty. 2 bias, 1/2 duty drive waveform, Figure 8 is a diagram showing the brightness-voltage characteristics when time-division driving, Figure 9 is and FIG. 7 is an electro-optical characteristic diagram for the amount of conventional additives added. 1... Upper plate, 2... Lower plate, 5... Liquid crystal, 6... Electrode,
8, 9...Polarizing plate, 51...Liquid crystal display element.

Claims (1)

[Claims] 1. As a nematic liquid crystal, [Formula] [Formula] and A liquid crystal composition containing at least a compound represented by the general formula (R ₁ , R ₂ are linear alkyl groups having 1 to 9 carbon atoms) 1% by weight of at least one compound represented by
A liquid crystal composition characterized in that it is added in an amount of 15% by weight or less and used in a time division drive system.