KR100355280B1 - Liquid crystal display device - Google Patents

Abstract

PURPOSE: A liquid crystal display device is provided to improve the luminance in the white display and the contrast in the black display by polymer liquid crystal, and increase the viewing angle by not using a prism sheet. CONSTITUTION: A liquid crystal display device includes two pairs of substrates(4) respectively having a transparent electrode layer, a first liquid crystal layer formed between a pair of the substrates and containing polymer liquid crystal(5), a second liquid crystal layer formed between the other pair of substrates and containing cholesteric liquid crystal(10), and a light absorbing sheet(7) formed under the second liquid crystal layer. The first and second liquid crystal layers are adhered to each other by a transparent optical adhesive(8).

Description

LCD

The present invention relates to a liquid crystal display device composed of two liquid crystal layers, and more particularly, a polymer liquid crystal composite is formed of a first liquid crystal layer and a second liquid crystal layer of a second liquid crystal layer containing a cholesteric liquid crystal as a second liquid crystal layer. The back scattering formed by the liquid crystal layer of the layer is improved, and the present invention relates to a liquid crystal display device usable as a direct-view display.

Twisted Nematic (TN) or Super Twisted Nematic (STN), which are currently the mainstream of liquid crystal displays, have low light utilization efficiency due to the use of polarizing plates, resulting in poor contrast. The alignment treatment around the device becomes difficult, and the viewing angle is narrow to around 20 °. Therefore, many efforts have been attempted to use a display by using a transmission scattering mode of light without using polarization. That is, recently, a polymer dispersed liquid crystal (PDLC) or a polymer network liquid crystal (PNLC) of light scattering mode in which a liquid crystal is dispersed in a polymer has appeared.

In the PDLC, when the electric field is not applied, the refractions of the liquid crystal and the polymer do not coincide with each other. Thus, the incident light is scattered and the cell appears opaque. When the electric field is applied, the liquid crystal is arranged in the electric field direction so that the cell appears transparent. Since the polarizing plate is not used according to the principle of scattering and transmission of light in this manner, light use efficiency is higher than conventional liquid crystal display elements, and luminance is high, and viewing angle is excellent, and dimming display is possible. However, such a PDLC has a higher driving voltage than a conventional liquid crystal display device, so there is no problem in using a switchable window that can be used only by switching within a range of 100 volts. And fast response speed and high contrast are required.

The first published as such PDLC technology is a NCAP (Neutral Curveliner Aligned Phase) (US Pat. No. 4,435,047) in which a liquid crystal sphere is microencapsulated and dispersed in a mechanical and physical method between transparent polymer resins. This method evenly disperses the liquid crystal in gelatin, gum arabic, or polyvinyl alcohol aqueous solution, uniformly coats 5-20 μm on the transparent conductive support, and then evaporates water to bond another transparent conductive support to the polymer. It is to prepare a liquid crystal complex.

Another more advanced method of preparing a polymer dispersed liquid crystal composite (PDLC) is a phase-separated manufacturing method utilizing the solubility difference between a polymer and a liquid crystal monomer (US Pat. Nos. 4,688,900 and 4,685,771). This is prepared by dissolving the liquid crystal monomer in the monomer or oligomer of the transparent polymer resin and then reducing the solubility of the liquid crystal monomer as the polymerization reaction proceeds by UV or heat to precipitate the liquid crystal monomer in the form of a droplet.

This phase separation method is advantageous for driving characteristics such as low driving voltage and high-speed response, and is currently used in manufacturing methods such as PIPS (Polymerization Induced Phase Separation), SIPS (Solvent Induced Phase Separation), and TIPS (Thermal Induced Phase Separation). Research is actively underway.

Among the phase separation methods, PIPS (Polymerization Induced Phase Separation) is a liquid crystal monomer, a liquid crystal polymer, and an acrylate or methacrylate monomer capable of UV polymerization between two glass plates or polyester films coated with transparent electrodes. After mixing, the solubility of the liquid crystal decreases in phase separation due to polymerization of oligomer and monomer by heat or ultraviolet rays. Solvent Induced Phase Separation (SIPS) dissolves all of the liquid crystal components of oligomers and monomers such as chloroform in a co-solvent to form a homogeneous mixed solution, and uses a bar coater on a glass plate or polyester film coated with conductive material ITO. It is a method of phase-separation by removing the solvent after coating uniformly to a certain thickness. TIPS (Thermal Induced Phase Separation) is a method in which a thermoplastic resin is heated to be melted, mixed with liquid crystals uniformly, and then cooled to phase-separate the resin.

When the refractive index of the transparent polymer resin is n _p , the normal light refractive index of the liquid crystal is n _o , and the ideal light refractive index is n _e , when no electric field is applied, the liquid crystal molecules become disordered and the refractive indices of n _e and n _p become different. Since the incident light is scattered, the film appears opaque. When the electric field is applied, the liquid crystal is arranged in the electric field direction, so that n _o and n _p coincide with each other and the incident light is transmitted.

In addition, research on so-called polymer network liquid crystals (PNLCs), in which a polymer network is formed on a liquid crystal continuum by adding a certain amount of liquid crystal content among UVPS curing among PIPS methods, is being actively conducted. Unlike PDLC, polymers are continuous and liquid crystals form drawlets, PNLC is a three-dimensional network structure in which liquid crystals are continuous and polymers are cross-linked. (Japanese Laid-Open Patent Publication No. 01-198725), the driving voltage is generally lower than that of PDLC.

However, in order to apply such a polymer liquid crystal complex to a high-resolution display, it is also important that the polymer liquid crystal complex is driven within a range of driving ICs capable of driving a polymer dispersed liquid crystal composite film, but it has high response speed, high voltage maintenance, excellent hysteresis and high contrast. Must have

In general, PDLC is more likely to be used as a projector because it has a higher brightness than conventional liquid crystal displays. However, since the PDLC is a transmission / scattering mode, the light is not completely blocked by the leakage of light in the state where no electric field is applied, so the contrast of the black display is insufficient compared to the liquid crystal projectors up to now. Thus, attempts have been made to improve the contrast of the material itself or to improve it using the Schlieren optics system, as shown in FIG. This causes parallel light to enter the PDLC panel 1 as shown in FIG. 1 (B) in the scattering state when the voltage is not applied to the transmission state of FIG. 1A when voltage is applied, thereby transmitting the transmitted light to the lens 3. By condensing on the pinhole 2 by this, it is a method of improving contrast. However, since the PDLC material itself has a scattering mode, even if the contrast of the material itself is increased, there is a limit to using the PDLC as a direct view display. Recently, however, an attempt has been made to apply a direct view using a PDLC panel.

One of these attempts is to synthesize a monomer having a structure similar to that of a liquid crystal monomer having a double bond in the sock end of the polymer monomer, and to prepare a PSTC (Polymer Stabilized Cholesteric Texture) prepared by mixing with a nematic liquid crystal and a cholesteric liquid crystal. Stabilized cholesteric structure). This uses bistable stability of the planar structure and the focalconic structure in three states, such as the planar structure, the focalconic structure, and the homeotropic structure of the liquid crystal layer, and can be used as a direct-view display capable of simple matrix driving. However, according to this method, since the rubbing process must be performed to stabilize the planar structure, the biggest advantage of PDLC disappears, and the driving voltage is too high, about 50V, and color display is difficult.

Another attempt to use the PDLC panel in a direct view is a prism sheet 6 and a light on the back of the PDLC, in which a polymer liquid crystal composite 5 is laminated between upper and lower conductive glass substrates 4, as shown in FIG. The absorption sheet 7 is attached to improve backscattering. Referring to FIG. 2 (A) showing the scattered state (white state), when no voltage is applied, all light passing through the liquid crystal panel is absorbed without the prism sheet 6 as shown on the left side of FIG. The brightness of the white display is low because it is absorbed by the sheet 7 and only the liquid crystal panel contributes to the white display. On the contrary, when the prism sheet 6 is used as shown in the right side of FIG. 2A, light incident on the prism sheet 6 at an angle greater than the critical angle θ _CP is reflected from the rear surface of the prism sheet 6. Because of this, forward scattering by the light behind the panel is added to the original backscattering, thereby further strengthening the backscattering of the panel itself.

Referring to FIG. 2 (B) showing the transmission state (black state), when there is no prism sheet 6 as shown on the left side of FIG. 2 (B) when the voltage is applied, all the transmitted light is absorbed by the light absorption sheet ( Absorbed in 7), the liquid crystal panel looks black. On the contrary, when the prism sheet 6 is used as shown in the right side of FIG. 2B, the light incident on the prism sheet 6 at an angle smaller than the critical angle θ _CP passes through the prism sheet 6 and is light. Although the liquid crystal panel appears black when absorbed by the suction sheet, light incident at an angle greater than the critical angle is reflected from the back of the prism sheet 6, so that the characteristics of the viewing angle are reduced when viewed in an inclined direction, and adhesion of the prism sheet 6 There are problems such as difficulty.

SUMMARY OF THE INVENTION An object of the present invention is to provide a polymer liquid crystal composite having excellent contrast, a wide viewing angle, and a low driving voltage, which can be used as a direct-view display with improved backscattering.

The object of the present invention described above is to provide light through the PDLC membrane by providing a cholesteric liquid crystal layer as the second liquid crystal layer under the polymer liquid crystal complex (PDLC) as the first liquid crystal layer to improve backscattering of the existing polymer liquid crystal composite. As the reflection is further increased backscattering, there is provided a liquid crystal display device using a conventional polymer liquid crystal composite as a direct view display.

That is, according to the present invention, a second liquid crystal layer including a cholesteric liquid crystal is attached between a first liquid crystal layer containing a polymer liquid crystal mixture and two substrates having a transparent electrode layer between two substrates having a transparent electrode layer. Wherein the second liquid crystal layer has a rubbing layer above and below the cholesteric liquid crystal layer, and the first liquid crystal layer and the second liquid crystal layer are bonded with a transparent optical adhesive, and the second liquid crystal layer The light absorbing sheet is formed below the layer.

Hereinafter, the present invention will be described in detail with reference to FIG.

The polymer liquid crystal composite of the present invention is composed of two liquid crystal layers, and the first liquid crystal layer 20 uniforms a liquid crystal monomer, an ultraviolet curable oligomer or monomer and an ultraviolet initiator between two substrates 4 having a transparent electrode layer. It is a general polymer liquid crystal composite layer prepared by injecting and curing the mixed polymer liquid crystal mixture (5). The second liquid crystal layer 30 rubs two substrates 4 having a transparent electrode layer with polyimide or the like to form a rubbing layer 9, and then the cholesteric liquid crystal or cholesteric liquid crystal and nematic liquid crystal. It is a liquid crystal layer manufactured by injecting a mixed liquid crystal. The two liquid crystal panels thus prepared are adhered to each other using a transparent optical adhesive 8. When the electric field is applied to the lower portion of the second liquid crystal layer 30, the light absorbing sheet 7 is adhered so that incident light transmitted through both the first liquid crystal layer 20 and the second liquid crystal layer 30 can be absorbed.

In the present invention, the substrate 4 having the transparent electrode layer may be made of indium tin oxide (ITO) glass or a transparent polyester film coated with ITO, or SnO ₂ glass or a transparent plastic film. In this case, it is important that the transmittance of the transparent conductive substrate is 80% or more, in particular 85% or more, and the area resistance of the conductive layer is 30 kPa / square or less. In the present invention, a spacer used to maintain a constant cell gap may be alumina, rod-shaped glass fibers, glass beads, polymer beads, micropearles, and the like, and the thickness of the cell manufactured at this time may be about 1 μm to 50 μm.

Ultraviolet-polymerizable oligomers usable in the present invention may use aromatic urethane acrylates, aliphatic urethane acrylates, polyester acrylates, epoxy acrylates and derivatives thereof.

Monomers for ultraviolet polymerization usable in the present invention include styrene and derivatives thereof; Methyl, ethyl, propyl, butyl, pentyl, hexyl, amyl, 2-ethylhexyl, octyl, nonyl, dodecyl, isodecyl, lauryl, hexadecyl, cyclohexyl, benzyl, methoxyethyl, ethoxyethyl, butoxy Ethyl, phenoxyethyl, allyl, metaallyl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl, 3-chloro-2-hydroxypropyl, dimethylaminohexyl, diethylaminohexyl, isobornyl and the like Acrylate or methacrylate having the same single functional group; Ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, tripropylene glycol, polypropylene glycol, butylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, butanediol, hexanediol, trimethylolpropane And polyfunctional acrylates or methacrylates such as pentaerythritol and dipentaerythritol, and derivatives thereof.

The composition of the polymer compound used in the present invention may include an ultraviolet polymerization initiator and other additives in addition to the liquid crystal polymer and monomer described above. Preferred initiators for ultraviolet polymerization at this time include 2-hydroxy-2-methyl-1-pentylpropane-1-silver (Merck, Darocure 1173), 1-hydroxycyclohexylphenyl ketone (Ciba-6eigy, Irgacure184), 1- (4-isopropylphenyl) -2-hydroxy-2-methylpropan-1-one (Merck, Daroeure 1116), benzyldiethyl metal (Ciba-Geigy, Irgacure651) and the like can be used. .

As the liquid crystal monomer used in the first liquid crystal layer in the present invention, both cyan-based liquid crystals and fluorine-based liquid crystals or liquid crystals mixed with them may be used. However, in consideration of physical properties such as voltage maintenance, hysteresis and contrast, It is preferable to add in small amounts.

In the present invention, both steroidal cholesteric liquid crystals and chiral nematic cholesteric liquid crystals may be used as the cholesteric liquid crystal of the second liquid crystal layer, but it is more preferable to use a chiral nematic cholesteric liquid crystal having a low viscosity. Do. The nautical directionality of the cholesteric liquid crystal is irrelevant to the priority or to the left linearity, and the length of the nautical pitch is set to the ultraviolet region. Nematic liquid crystals are mixed with cholesteric liquid crystals to adjust the pitch of the liquid crystals by adjusting the helical twisting power. The nematic liquid crystals have good viscosity and low viscosity as possible. It is good.

In the present invention, when the liquid crystal monomer is mixed with a monomer capable of ultraviolet polymerization such as acrylate or methacrylate and polymerized by UV polymerization under a photoinitiator to produce a polymer liquid crystal composite, the liquid crystal monomer and the acrylate or methacrylate monomer and A ratio of about 1: 1 is good. However, when acrylate or methacrylate is not mixed well with the liquid crystal monomer, the monomer may be insufficient, so that an acrylate or methacrylate monomer may be added until about 1: 5. As the content of the liquid crystal polymer increases, the driving voltage increases, and as the content of the liquid crystal polymer decreases, the effect of reducing the response speed is reduced. In this case, the total addition ratio of the acrylate or methacrylate monomer to the liquid crystal monomer is preferably used in an amount of about 10 to 50% by weight based on the total composition, as in the case of using the common solvent mixed with the liquid crystal. The weight% is best, and the UV polymerization initiator is preferably used 1 to 5% by weight based on the acrylate or methacrylate monomer.

When manufactured by PIPS method, the UV light used for phase separation can be used from several mW to several hundred mW at 365 nm, and the irradiation time of the UV light varies depending on the composition, but it is completely hardened in about 5 seconds to 30 minutes. It's time. In addition, if the intensity of the ultraviolet rays irradiated is too strong, the polymerization reaction to the polymer proceeds so rapidly that all of the liquid crystal monomers dissolved in the monomer and oligomer may not be precipitated. Thus, the morphology of the polymer dispersed liquid crystal composite is polymerized. Appear, the hysteresis increases and the driving voltage increases. In addition, if the intensity of ultraviolet light is too weak, even if the amount of light required for polymerization is irradiated, the size of the liquid crystal droplets precipitated is too large, the drive voltage is low, but the response speed is a problem.

In the present invention, the first liquid crystal layer 20 and the second liquid crystal layer 30 are connected in parallel and simultaneously opened and closed. Considering the operation of the liquid crystal display device of the present invention is as follows. First, in the scattering state (white state) where no voltage is applied as shown in FIG. 3A, the PDLC scatters the light and scatters it in the direction in which part of the incident light is incident, thereby making the liquid crystal panel appear white. In the case of FIG. 2A without the second liquid crystal layer 30, the luminance of the white display is low as described above. However, when the second liquid crystal layer 30 having the planar structure of the cholesteric liquid crystal is omitted as in the present invention, incident light passing through the PDLC in the second liquid crystal layer 30 is reflected, which results in the original panel. Backscattering by the panel itself is enhanced because backscattering by the light behind the panel is added to the backscattering. In the field unapplied state, the higher the backscattering, the higher the luminance when using the reflective display and the higher the cell thickness, the higher the contrast, and the lower the driving voltage.

In the transmission state (black state) to which voltage is applied as shown in FIG. 3 (B), PDLC transmits light, and thus incident light reaches the second liquid crystal layer 30. Since the incident light reaching the second liquid crystal layer 30 is transmitted through the second liquid crystal layer 30 as the planar cholesteric liquid crystal layer changes into a homeotropic structure, the PDLC panel is absorbed. It appears black. When two liquid crystal layers are used as in the present invention, since there is no light returning to forward scattering when voltage is applied, as in the case of using a prism sheet, black display characteristics are excellent when voltage is not applied.

Since the polymer dispersed liquid crystal composite of the present invention does not use a polarizing plate unlike a general TN or STN liquid crystal, light utilization is good and the luminance is high. In addition, the polymer dispersed liquid crystal composite of the present invention has higher brightness due to increased backscattering in the white display, and excellent contrast due to the elimination of forward scattering due to reflection in the black display, which can be used as a direct-view display. Since it is not used, the viewing angle is wide and the driving voltage is lower than that of the conventional PSCT.

Hereinafter, the present invention will be described in detail with reference to Examples, but it should be noted that the present invention is not limited by the following Examples.

Example 1

As the liquid crystal monomer, TL204 (Merck Co., Ltd.), which is a liquid crystal monomer having a fluorine group introduced at its terminal, and E-7 (Merck Co., Ltd.) having a cyan group introduced at the terminal, were mixed in a ratio of 80:20 with respect to the liquid crystal component. A liquid crystal component was prepared. As an oligomer for ultraviolet polymerization, PN393 (Merck Co., Ltd.) and bifunctional or trifunctional acrylate were mixed in the ratio of 50:50 with respect to a polymer component. As an initiator for ultraviolet polymerization, Darocure 1173 (Darocure 1173, Merck) was used in an amount of 1 wt% based on the polymer component. This mixture was uniformly mixed in a handy mixer under water bath at 60 ° C. to prepare a single phase. Subsequently, the liquid crystal component was mixed at a weight ratio of 20/80 to the total polymer component / liquid crystal monomer, and then mixed uniformly by a handy mixer. This mixed solution was injected between two ITO-coated glass plates having a transmittance of 86% and an area resistance of 30 mW / square adjusted to a thickness of 7 m by using a spacer. When the injection was completed, the first liquid crystal layer was prepared by irradiating UV light having a wavelength of 365 nm for 1 minute at an intensity of 10 mW / cm 2 to photocuring.

In the preparation of the second liquid crystal layer, first, a polyimide solution as an alignment agent was applied to two ITO coated glass substrates by a spin coater. Rubbing was performed after complete curing at 250 ° C. Injecting the mixed liquid crystal of cholesteric liquid crystal and nematic liquid crystal into the empty cell sealed by adjusting the space | interval 5 micrometers of the two glass substrates which were processed by the spacer, and end-seal treatment. At this time, CE2 and CB15 (Merck Co., Ltd.) were mixed in a weight ratio of 1: 2 for cholesteric liquid crystals, and E48 (Merck Co., Ltd.) was used for nematic liquid crystals, and the weight ratio of cholesteric liquid crystals and nematic liquid crystals was used. Was prepared in a ratio of 1: 5. After the formation of the second liquid crystal layer, the light absorption sheet was adhered to the lower portion of the second liquid crystal layer. Subsequently, the first liquid crystal layer and the second liquid crystal layer were bonded with an acrylic optical adhesive having the same refractive index as that of the glass used as the substrate so that no incident light was refracted.

Thus, the polymer liquid crystal composite was prepared and its physical properties are shown in Table 1 below.

Example 2

A first liquid crystal layer was produced in the same manner as in Example 1 except that TL203 (Merck) was used alone as the liquid crystal monomer in the first liquid crystal layer. The second liquid crystal layer was prepared using the same liquid crystal monomer as in Example 1 but having a weight ratio of cholesteric liquid crystal and nematic liquid crystal to 1:10. The following procedure was carried out in the same manner as in Example 1 to prepare a polymer liquid crystal composite and the physical properties thereof are shown in Table 1 below.

Comparative Example 1

A polymer liquid crystal composite was prepared using only the first liquid crystal layer, the prism sheet, and the light absorption sheet without forming the second liquid crystal layer. The polymer liquid crystal mixture of the first liquid crystal layer was prepared in the same manner as in Example 1, and the polymer liquid crystal composite was prepared by adhering the prism sheet and the light absorption sheet to the lower portion of the first liquid bottom layer. It was.

Table 1

1 (A) is a configuration diagram of the transmission state of the slylene optical system,

(B) is a scattered state configuration diagram of the slylene optical system,

2 (A) is a scattering state configuration of a conventional PDLC panel using a prism sheet,

(B) is a configuration of the transmission state of a conventional PDLC panel using a prism sheet,

3 (A) is a scattering state configuration of the PDLC panel of the present invention,

(B) is a block diagram of the transmission state of the PDLC panel of the present invention.

* Description of the symbols for the main parts of the drawings *

1: PDLC panel 2: Pinhole

3: lens 4: substrate

5: polymer liquid crystal mixture 6: prism sheet

7: light absorption sheet 8: optical adhesive

9: rubbing layer 10: cholesteric liquid crystal

20: first liquid crystal layer 30: second liquid crystal layer

Claims (6)

A second liquid crystal layer containing a cholesteric liquid crystal is formed between a first liquid crystal layer containing a polymer liquid crystal mixture and two substrates having a transparent electrode layer between two substrates having a transparent electrode layer, and the first liquid crystal layer And the second liquid crystal layer are bonded with a transparent optical adhesive, and a light absorption sheet is formed under the second liquid crystal layer. The method of claim 1, And said second liquid crystal layer has a rubbing layer above and below the cholesteric liquid crystal layer. The method of claim 1, And the first liquid crystal layer is composed of a kind of liquid crystal monomer selected from the group consisting of cyan liquid crystal and fluorine liquid crystal or a mixed liquid crystal thereof. The method of claim 1, The cholesteric liquid crystal of the second liquid crystal layer is a liquid crystal display device comprising a steroid-based cholesteric liquid crystal or chiral nematic cholesteric liquid crystal. The method of claim 1, And the second liquid crystal layer is composed of a mixture of cholesteric liquid crystal and nematic liquid crystal. The method of claim 1, The second liquid crystal layer has a planar structure in the non-field applied state, and a homeotropic structure in the field applied state.