JPS63239420A - Liquid crystal display element using plastic substrate - Google Patents

Abstract

PURPOSE:To make display of high contrast by specifying the helix angle of liquid crystal molecules between substrates, the angle between the orientation direction of the liquid crystal molecules on the substrate surfaces and the transmission or absorption axes of polarizers proximate to the substrates and the product of the refractive index anisotropy of the liquid crystal and the thickness of the liquid crystal to prescribed ranges. CONSTITUTION:The liquid crystal layer having positive dielectric anisotropy is formed between the substrate 11 formed with a transparent electrode 13 and an oriented film 15 and the substrate 21 having a transparent electrode 23 and an oriented film 25. Polarizing plates 41, 43 are disposed on both sides of a liquid crystal cell 10. The liquid crystal molecules are horizontally oriented and the helix angle alpha is set at 180-250 deg.. The angle between the orientation direction of the liquid crystal molecules on the two substrates and the transmission or absorption axes of the polarizers proximate to the substrates is specified to 30-60 deg.. The angle created by the stretching axes of a pair of the substrates and the transmissivity of absorption axes of the polarizers proximate to the substrates is specified within + or -3 deg.. The product of the refractive index anisotropy DELTAn of the liquid crystal at an ordinary temp. and the thickness (d) of the liquid crystal is so selected as to satisfy the equation. The display quality which is good in all of brightness, contrast and time division characteristic is obtd. by this constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid crystal display element having a structure in which liquid crystal molecules are largely twisted between substrates.

TN (twisted nematic) has traditionally been the mainstream
A type of liquid crystal display element is composed of a liquid crystal cell having a liquid crystal layer having a 90 degree spiral structure between two electrode substrates, and a pair of polarizers sandwiching the liquid crystal cell.

However, the number of time divisions has increased with the recent increase in the size of dot matrix type liquid crystal display elements, and the time division characteristics of conventional TN type liquid crystal displays are no longer able to cope with the increase in size. Split driving reduces contrast and viewing angle.

On the other hand, S B E (Super Twisted Bi) has liquid crystal molecules twisted 270 degrees between the substrates.
Refrin-gence effect) type liquid crystal display elements have been proposed, and are reported to be able to provide wide viewing angles and high contrast.
fer et al., SIDDgest, 120 (
I985)], L/But in this case, since a large pretilt angle of around 30 degrees is required, there is a problem in that orientation treatment must be performed by oblique evaporation, which is poor in productivity. Orientation treatment by rubbing produces a scattering structure.

It is not possible to obtain the desired characteristics.

On the other hand, liquid crystal display elements using plastic films are attracting attention from the viewpoints of thinness, weight reduction, strength, workability, and cost reduction.

Among these plastic films, -axially stretched polymer films are particularly preferred from the viewpoint of strength and lack of coloration.

The present invention provides a plastic substrate liquid crystal display that has improved time-division drive characteristics, can be aligned by rubbing, has high brightness when not lit, has a uniform background color, and has high contrast. The purpose is to provide an element.

The plastic substrate liquid crystal display element of the present invention has a structure in which liquid crystal molecules having positive dielectric anisotropy are aligned approximately horizontally with respect to the substrates when no voltage is applied between a pair of stretched plastic film substrates having electrodes. A liquid crystal cell is sandwiched between the two substrates to form a liquid crystal layer, and the liquid crystal molecules are twisted in the thickness direction of the liquid crystal layer between the two substrates. A liquid crystal display device comprising a polarizer is characterized in that it satisfies the following conditions (1) to (4).

■The twist angle α of the liquid crystal molecules is in the range of 180 to 250 degrees.

(2) A uniaxially stretched plastic film is used as a pair of substrates, and the angle between the stretching axis of each substrate and the transmission axis or absorption axis of a polarizer adjacent to the substrate is within ±3 degrees.

■The angle between the alignment direction of liquid crystal molecules on each substrate of the liquid crystal cell and the transmission axis or absorption axis of the polarizer near the substrate is 3
Must be within the range of 0 to 60 degrees.

■The product d・Δn of the refractive index anisotropy Δn of the liquid crystal at room temperature and the liquid crystal layer thickness d is expressed by the following formula (I) % Formula % () Hereinafter, the present invention will be explained in more detail with reference to the attached drawings. .

FIG. 1 is a sectional view showing an example of the structure of a liquid crystal display element of the present invention. An upper substrate 11 on which a transparent electrode 13 and an alignment film 15 are formed, and a lower substrate 21 on which a transparent electrode 23 and an alignment film 25 are formed are arranged to face each other at a distance, and a liquid crystal 31 is sealed between them to form a liquid crystal layer. , and a liquid crystal cell IO is constructed. Transparent electrode 13 of upper substrate 11 and lower substrate 21
The transparent electrode 23 is formed, for example, in a dot matrix shape. 33 indicates a gap material, and 35 indicates a sealant. By sandwiching this liquid crystal cell 10,
An upper polarizing plate 41 and a lower polarizing plate 43 are arranged to constitute a liquid crystal display element.

As the upper and lower substrates 11.21, -axis stretched plastic films are used. A typical alignment film is a polymer film made of polyamide, polyimide, or the like that has been subjected to a rubbing treatment.

As the liquid crystal, one having positive dielectric anisotropy is used, and a typical example is a mixed liquid crystal in which chiral nematic liquid crystal or cholesteric liquid crystal is mixed into p-type nematic liquid crystal.

- The coefficient of thermal expansion of an axially stretched film is different in the stretching direction and in the direction perpendicular to the stretching direction, but it is necessary to control the liquid crystal filling conditions such as the curing conditions of the sealant when bonding the counter substrate, or the conditions for extruding excess liquid crystal during filling. Therefore, even if the substrate gap in an empty cell is non-uniform, the gap can be controlled to be uniform by the action of the surface tension of the liquid crystal after the liquid crystal is filled.

Moreover, the -axis stretched film is optically anisotropic and has an optical axis in the stretching direction and the direction perpendicular to the stretching direction. Therefore,
It is necessary that the angle formed between the stretching axis of the upper substrate 11 and the transmission axis or absorption axis of the upper polarizing plate 41 is ±3 degrees or less.

Similarly, it is necessary that the angle formed between the stretching axis of the lower substrate 21 and the absorption axis or transmission axis 43a of the lower polarizing plate 43 is ±3 degrees or less. If this angle exceeds ±3 degrees, the contrast of the display will be extremely reduced.

The liquid crystal molecules are oriented substantially horizontally on the substrate surfaces, and have a twisted structure of 180 to 250 degrees, preferably 180 to 230 degrees on both substrates. FIG. 2 is a diagram showing the angular relationship seen from the lower polarizing plate 43, and shows a case where a counterclockwise spiral structure is taken when seen from above. The twist angle α is limited by the scattering structure, contrast, and viewing angle dependence that occur near the threshold voltage. When α is increased, a scattering structure is more likely to occur near the threshold voltage, but this scattering structure does not occur by reducing the ratio d/p of the liquid crystal layer thickness d to the liquid crystal pitch p. However, if d/p is too small, disclination will occur, so d
/p needs to be a value within a range where scattering structure and disclination do not occur. Due to this d/p limitation,
The upper limit of α is 250 degrees. On the other hand, when α is less than 180 degrees, the contrast decreases and viewing angle dependence increases.

Further, the angle formed by the alignment direction of liquid crystal molecules in contact with the upper and lower substrates and the transmission axis or absorption axis of the polarizing plate adjacent to the substrates is in the range of 30 to 60 degrees, preferably 35 to 55 degrees. In FIG. 2, the angle βL between the transmission axis 43a of the lower polarizing plate 43 and the alignment direction of liquid crystal molecules in contact with the lower substrate, that is, the rubbing direction 21r of the upper substrate is.

It is in the range of 30 degrees≦βL≦60 degrees. Similarly, the angle βU between the transmission axis 41a of the upper polarizing plate 41 and the orientation direction of liquid crystal molecules in contact with the upper substrate (ie, the rubbing direction 11r of the lower substrate) is in the range of 30 degrees≦βU≦60 degrees. By setting in this way, it is possible to realize a display that is bright and has a large contrast between 1 selection and non-selection when not selected (when not lit).

Here, both 41a and 43a are used as transmission axes of the polarizing plate, but the effect is the same if both are used as absorption axes.

FIG. 3 shows the value of L' in the CIE color system when not selected when α=220°, using d·Δn as a parameter. It can be seen that the background becomes extremely bright when β (=βL=βU) is in the range of 30 to 60''.

In addition, in FIG. 2, when the liquid crystal display element is viewed from above, the twist direction of the liquid crystal from the bottom to the top is defined as positive, and βL,
Although the case where both U are positive is shown, βL and βU can also have different signs. In this case, light will be transmitted at selected points and will be blocked at non-selected points.

Furthermore, the absolute value of the difference between βL and βU is 1βL−βu1
It is desirable that the angle be ≦20°.

Even if this condition is not met, the brightness when the lamp is not lit hardly changes, but the "darkness" when the lamp is lit once is reduced, and the contrast of the image display is reduced.

Figure 4 shows the light transmittance of 1βL+βu at the time of selection in 1/64 duty drive when α==180@.
-90'' is a graph showing l dependence.

Here, the light transmittance was measured as the transmittance of white light when a visibility filter was used.

Furthermore, it is necessary to set the product d·Δn of the thickness d of the liquid crystal layer and the refractive index anisotropy Δn of the liquid crystal within a predetermined range. d・Δ
The most preferable range of the value of n changes depending on the twist angle α. As a result of plotting and studying various combinations of α and d·Δn, it was found that d·Δn needs to satisfy the following formula (I).

−0,0023α+1.20≦d−Δn≦−0,002
3g +1.50'' (I) More preferable conditions include the following formula (n).

−0,0023α+1.26≦d−Δn≦−0,002
3α+1.45 ・Be■) When d・Δn exceeds the upper limit, the color tone becomes reddish, which is visually undesirable.
”, which significantly impairs display quality (see curve a in FIG. 5), and if it is less than the lower limit.

Cells are colored blue and sufficient contrast cannot be obtained (
(See curve b in FIG. 5) 9 Curve C shows the voltage-luminance characteristic when d·Δn is the above-mentioned appropriate value.

In addition, when measuring one or more of each data, a liquid crystal display device was created using the same materials as in the examples described later.

Highlights According to the present invention, in a liquid crystal display element having a structure in which liquid crystal molecules are largely twisted between substrates, the twist angle α of the liquid crystal molecules between the substrates is close to the alignment direction of the liquid crystal molecules on the substrate surface. By setting the angles βL, βU, and d·Δn values formed by the transmission axis or absorption axis of the polarizer within predetermined ranges, display with high contrast between the non-selected state and the selected state can be achieved. In addition, the steepness of the voltage-transmittance characteristics is good, making high time-division driving possible.Furthermore, since the viewing angle dependence is small,
Display with an extremely wide viewing angle is possible.

Moreover, a large predil angle is not required to achieve such display characteristics.

Highly productive alignment treatment such as alignment treatment by rubbing is possible, and combined with the characteristics of the plastic film substrate, it is extremely useful industrially. 7 Examples Striped ITO transparent electrodes formed with a thickness of 100μ
A polyamide film having a thickness of 1,000 mm was coated as an alignment film on a uniaxially stretched polyethylene terephthalate film substrate having a thickness of 1,000 mm, and was subjected to a rubbing treatment.

E as a nematic liquid crystal between these substrates.

A liquid crystal cell was created by enclosing ZLI-2293 manufactured by Merck, the same <E as a chiral nematic liquid crystal, and a liquid crystal mixed with 8811 manufactured by Merck. , 82-12) to form liquid crystal display elements (see 1-28).

At this time, α, βL, βu, AL, A u, and d・Δn are adjusted as shown in Tables 1 and 2 below, and Ll, maximum contrast ratio, and steepness γ of the CIE display system when not selected are was measured.

Here, the angles Ar and Au respectively indicate the following.

AL: angle between the stretching axis of the lower substrate and the transmission axis of the lower polarizing plate Au: angle between the stretching axis of the upper substrate and the transmission axis of the upper polarizing plate; L@ is the angle formed by measuring the spectral transmittance; The maximum contrast ratio calculated from that value is calculated from the voltage-transmittance characteristics for a white light source using a visibility correction filter, and the ratio a / b of the transmittance a when not selected and the transmittance when selected did.

The steepness is the voltage V□ at which the transmittance changes by 10%.

Voltage V, which changes by 50%. The ratio with V,. /V engineering. And so.

As shown in Table 1 (Examples of the invention) and Table 2 (Comparative examples), the liquid crystal display element of the invention is as follows.

brightness L'''), contrast, time-division drive characteristics (γ
) were both superior to the elements of the comparative example, and it was confirmed that they had high display quality.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the structure of a liquid crystal display element of the present invention.6 FIG. 2 is an explanatory diagram showing the angular relationship in the liquid crystal display element of the present invention. FIG. 3 is a graph showing the relationship between angle β and Ll. FIG. 4 is a graph showing the relationship between 1βL+βu−9061 and transmittance. FIG. 5 is a graph showing voltage-luminance characteristics. 11... Upper substrate 21... Lower substrate 41... Upper polarizing plate 43... Lower polarizing plate 11r...
・Rubbing direction 21r of the upper substrate...Rubbing direction 41a of the lower substrate...Transmission axis 43a of the upper polarizing plate...Transmission axis of the lower polarizing plate Fig. 1 Fig. 2 Fig. 38 β (=seed= βU) Figure 5

Claims (1)

[Claims] 1. Liquid crystal molecules having positive dielectric anisotropy are sandwiched between a pair of stretched plastic film substrates having electrodes so as to be oriented substantially horizontally with respect to the substrates when no voltage is applied to form a liquid crystal layer. and a liquid crystal cell configured such that liquid crystal molecules take a twisted structure in the thickness direction of the liquid crystal layer between both substrates, and a pair of polarizers disposed outside the substrates so as to sandwich the liquid crystal cell. In the liquid crystal display element, the twist angle α of the liquid crystal molecules is in the range of 180 to 250 degrees, and the alignment direction of the liquid crystal molecules on each of the pair of substrates is formed by the transmission axis or absorption axis of the polarizer adjacent to the substrates. The angle is in the range of 30 to 60 degrees, and the angle formed by the stretching axis of each of the pair of substrates and the transmission axis or absorption axis of the polarizer adjacent to the substrate is within ±3 degrees, and the liquid crystal at room temperature is The product d・Δn of the refractive index anisotropy Δn and the liquid crystal layer thickness d is expressed by the following formula (I) −0.0023α+1.2≦d・Δn≦0.0023α
+1.5... A plastic substrate liquid crystal display element characterized by satisfying (I).