KR19980067504A - Polymer dispersed liquid crystal display device - Google Patents

Abstract

The present invention provides an active circuit comprising a thin film transistor and a capacitor, and a first substrate having an ITO transparent electrode; A second substrate having a black matrix and an ITO transparent electrode; And a polymer / liquid crystal composite film interposed between the first and second substrates, wherein the polymer / liquid crystal composite film is a liquid crystal monomer, an ultraviolet curable monomer and an oligomer, an initiator for polymerization, and charge transport. It relates to a polymer dispersed liquid crystal display device, characterized in that it is formed by curing a composition comprising a substance. According to the present invention, an additional charge transport material is added to the composition for forming a polymer / liquid crystal composite film so that even if an uncured initiator remains, the display device exhibits excellent voltage retention by discharging radicals or ions generated by irradiation with ultraviolet rays or light sources without accumulating them. Can be kept constant. As a result, it is possible to improve the deterioration of image quality when used for a long time, and to have an advantage of extending the life.

Description

Polymer dispersed liquid crystal display device

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer dispersed liquid crystal device (PDLCD). More particularly, the present invention relates to a rapid response time, excellent brightness and contrast, and excellent image quality and lifespan. The improved polymer dispersed liquid crystal display device.

In general, liquid crystal (Liquid Crystal) is a material having an intermediate property between the liquid and the solid that combines the liquidity and the optical properties of the crystal, the optical anisotropy can be changed by an electric field or heat. It is known that the use of such a property is representative of flat panel displays in addition to plasma displays and light emitting devices. Liquid crystal display devices have received much attention recently as displays because they consume much less power and are driven at lower voltages compared to other display devices such as CRT (Cathod Ray Tube) and can be made thinner and lighter. The display market, which has been occupied by the method, is rapidly evolving in a short period of time so that you can clearly see that the display market is being reorganized.

TN (Twisted Nematic) type and STN (Super Twisted Nematic) type liquid crystal display devices, which are mainly used, have two polarizers on both sides of the panel. These polarizers reduce the light transmittance, so the luminance is lowered. As the pixel density increases, the alignment process around the thin film transistor element becomes difficult and the viewing angle is narrow. In particular, when the color pillar is provided, there is a problem that the utilization efficiency of light is further lowered, such as transmitted light does not reach 10% of incident light.

Therefore, research on the development of a liquid crystal display device capable of overcoming the problems described above has been widely conducted, and as one of the results, a PDLCD employing a liquid crystal / polymer resin composite film has been developed.

PDLCD uses light scattering and transmission instead of polarizing plate. When no electric field is applied, the liquid crystal and polymer do not have the same regulation rate, so 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. The principle is that the cells appear transparent. Therefore, since the utilization efficiency of light is high compared with the conventional liquid crystal display element which uses a polarizing plate, it is a liquid crystal display device which remarkably improved brightness, viewing angle, and contrast ratio.

The first attempt among PDLCDs is a method of dispersing liquid crystal in a transparent polymer resin (US Pat. No. 4,435,047). According to this method, the liquid crystal is uniformly dispersed in gelatin, gum arabic or polyvinyl alcohol aqueous solution, and then uniformly coated with a thickness of about 5-20 μm on a glass plate or polyester film coated with conductive material ITO, It is a method of manufacturing a liquid crystal display device by evaporating water and bonding another glass plate or polyester film coated with ITO again.

Another method of making PDLCDs is a phase separation process that utilizes the difference in solubility of polymers and liquid crystal monomers (US Pat. No. 4,688,900). This is a method in which a liquid crystal monomer is dissolved in a monomer or oligomer of a transparent polymer resin, and then the liquid crystal monomer is precipitated in the form of a droplet by polymerizing the monomer or oligomer with ultraviolet rays or heat.

The phase separation method is more useful than the method of dispersing the liquid crystal in the transparent polymer resin, which is easy to control the characteristics of the liquid crystal when applied to the display, the assembly of the substrate is simple, the polymerization time is short, and the manufacturing is simple. Because there is.

In addition to the impregnation method and emulsification method to impregnate the liquid crystal monomer in the porous plastic, but there are many practical difficulties.

However, when the polymer / liquid crystal composite membrane is prepared by the phase separation method, it is most ideal that the added photoinitiator or thermal initiator is completely consumed during the polymerization reaction so that no unreacted initiator remains. However, in the case of almost all polymerization reactions, such polymerization is impossible. In particular, in the preparation of the polymer / liquid crystal composite film, the polymerization is more difficult because complete polymerization is performed. Inevitably, an unreacted polymerization initiator remains. However, the remaining unreacted polymerization initiator is initiated by a small amount of ultraviolet light in the surroundings or by strong light or ultraviolet light generated from a light source when used as a projector to form radicals, and simultaneously polymerize the polymerized part and liquid crystal by aging. Generates. Radicals and ions generated in this way lower the voltage retention of the polymer / liquid crystal composite membrane that has been produced, resulting in a decrease in brightness under the same voltage. Therefore, a problem arises in that the driving voltage must be gradually increased to maintain the same luminance, and eventually, the display device loses its function.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a polymer dispersed liquid crystal display device having excellent contrast ratios, viewing angles, and the like, and having a constant voltage maintenance ratio so that luminance does not decrease under the same voltage.

Technical problem of the present invention includes a first substrate having an active circuit unit and an ITO transparent electrode consisting of a thin film transistor and a capacitor; A second substrate having a black matrix and an ITO transparent electrode; And a polymer / liquid crystal composite film interposed between the first and second substrates, wherein the polymer / liquid crystal composite film is a liquid crystal monomer, an ultraviolet curable monomer and an oligomer, an initiator for polymerization, and charge transport. It can be made by a polymer dispersed liquid crystal display device characterized in that it is formed by curing the composition containing the material.

In the polymer dispersed liquid crystal display device according to the present invention, both the cyan liquid crystal and the fluorine liquid crystal or a mixed liquid crystal thereof may be used as the liquid crystal monomer, but various voltages such as voltage retention, hysteresis and contrast are considered. It is preferable to add a small amount of cyan-based liquid crystals to the fluorine-based liquid crystals or to use only fluorine-based liquid crystals.

In addition, the ultraviolet curable monomers include styrene and its derivatives; 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 Acrylates or methacrylates substituted with the same substituents; 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, Polyfunctional acrylates or methacrylates and derivatives thereof such as pentaerythritol, dipentaerythritol and the like can be used.

Moreover, as an ultraviolet curable oligomer, aromatic urethane acrylate, aliphatic urethane acrylate, epoxy acrylate, polyester acrylate, its derivatives, etc. are mentioned.

As initiators for ultraviolet polymerization, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexylphenyl ketone, 1- (4-isopropylphenyl) -2-hydroxy-2- Methylpropan-1-one, benzyl dimethyl ketal and the like can be used.

Finally, as the charge transport material, an organic low molecular material, an organic polymer material, an organic metal material, etc. may be used. More specifically, an oxadiazole derivative, triphenylmethane, triazo derivative, pyrazoline derivative, carbazole derivative, Arylamine derivatives, stilbenzene derivatives, and the like. Although the mobility of these charge transport materials is about 10 ⁻⁷ cm 2 / Vs, the mobility of the charge transport material is slow, but is sufficient to move the charge and keep it constant without lowering the voltage holding ratio of the display device.

Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to this.

Example

5 g of Aronix M-1700 (manufactured by Toa Gosei Chemical), an ultraviolet curable oligomer, 15 g of ethylhexyl acrylate, 10 g of n-butyl acrylate and 3 g of 2-hydroxyethyl acrylate, a liquid crystal monomer 66.8 g of TL-205 (manufactured by E. Merk), a fluorine-based liquid crystal, 0.2 g of Tarocure-1173 (manufactured by Ciba-Geigy Corporation), which is a polymerization initiator, and a charge transport material

A uniform mixed solution containing 1 g of a triazole derivative (Aldrich) represented by was prepared. The mixture was injected with a homogeneous mixed solution obtained in advance between glass substrates having a cell thickness of 10 占 퐉, and then irradiated with ultraviolet rays having an intensity of 4 mW / cm 2 for 2 minutes in a temperature range of 22 ° C. A display device panel was manufactured. After measuring the driving voltage, response speed and voltage holding ratio of the liquid crystal display panel manufactured as described above, the panel was irradiated with ultraviolet rays having an intensity of 5 mA / cm 2 for 500 hours, and the physical property values were measured again to compare the differences. . The results are shown in Table 1 below.

Example 2

A liquid crystal display panel was prepared in the same manner as in Example 1 except that 50 g of Tl-205 (manufactured by E. Merck) and 16.8 g of BL-001 (manufactured by E. Merck) were used as the liquid crystal monomer. The physical properties were investigated. The driving voltage, response speed, and voltage holding ratio were measured at the initial stage of manufacturing the panel, and after irradiating UV light having an intensity of 5 mW / cm 2 for 500 hours, the physical property values were measured again to compare the differences. The results are shown in Table 1 below.

Comparative Example 1

A liquid crystal display panel was manufactured in the same manner as in Example 1, except that the triazole derivative, which was a charge transport material, was not added. Ultraviolet rays having an intensity of cm 2 were irradiated for 500 hours and then measured. The results are shown in Table 1.

Comparative Example 2

A liquid crystal display panel was manufactured in the same manner as in Example 2, except that the triazole derivative, which was a charge transport material, was not added. Ultraviolet rays having an intensity of cm 2 were irradiated for 500 hours and then measured. The results are shown in Table 1.

TABLE 1

Drive voltage (Vrms) Response speed (ms) Voltage retention rate (%) Before ultraviolet irradiation After ultraviolet irradiation Before ultraviolet irradiation After ultraviolet irradiation Before ultraviolet irradiation After ultraviolet irradiation Example 1 5.5 6.0 45 50 95 93 Example 2 5.0 5.2 35 47 86 81 Comparative Example 1 5.5 5.3 47 50 94 55 Comparative Example 2 5.2 5.0 51 53 86 25

As can be seen from the results of the above table, the liquid crystal display device according to the present invention has been found that the driving voltage, response speed and voltage holding ratio of the liquid crystal display device according to the present invention are almost unchanged immediately after the manufacture and after the ultraviolet irradiation, but without the charge transport material added thereto. In the case of the display element, the physical property value after ultraviolet irradiation, especially the voltage retention, was shown to fall remarkably. From this, undesired degradation and shortening of life are expected to occur, such as sudden drop in luminance over time under the same voltage.

According to the present invention, by further including a charge transport material, even if the uncured initiator remains, the display device maintains an excellent voltage holding ratio by discharging the radicals or ions generated by irradiation of ultraviolet rays or light sources without accumulating them. As a result, there is an advantage that the image quality deterioration phenomenon and the lifespan characteristics which appear when used for a long time are improved.

Claims (6)

A first substrate having an active circuit unit consisting of a thin film transistor and a capacitor and an ITO transparent electrode; A second substrate having a black matrix, a color filter, and an ITO transparent electrode; And A polymer dispersed liquid crystal display device comprising a polymer / liquid crystal composite film interposed between the first and second substrates, Wherein the polymer / liquid crystal composite film is formed by curing a composition including a liquid crystal monomer, an ultraviolet curable monomer and an oligomer, an initiator for polymerization, and a charge transport material. The polymer dispersed liquid crystal display device according to claim 1, wherein the liquid crystal monomer is a cyan liquid crystal, a fluorine liquid crystal or a mixed liquid crystal thereof. The method of claim 1, wherein the ultraviolet curable monomer is styrene and its derivatives; Methyl, ethyl, propyl, butyl, pentyl, hexyl, amyl, 2-ethylhexyl, octyl, nonyl, dodecyl, isodecyl, lauryl, hexadecyl, cyclohexyl, benzyl, methoxyethyl, ethoxyethyl, butoxy Ethyl, phenoxyethyl, allyl, methalyl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl, 3-chloro-2-hydroxypropyl, dimethylaminohexyl, diethylaminohexyl and isobornyl Acrylate or methacrylate having at least one substituent selected from the group consisting of; And ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, tripropylene glycol, polypropylene glycol, butylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, butanediol, hexadiol, trimethylol propane And at least one selected from the group consisting of polyfunctional acrylates or methacrylates and derivatives thereof selected from the group consisting of pentaerythritol and dipentaerythritol. The polymer dispersed liquid crystal display device according to claim 1, wherein the ultraviolet curable oligomer is selected from the group consisting of aromatic urethane acrylate, aliphatic urethane acrylate, epoxy acrylate, polyester acrylate and derivatives thereof. The method of claim 1, wherein the ultraviolet polymerization initiator is 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexylphenyl ketone, 1- (4-isopropylphenyl) -2 A polymer dispersed liquid crystal display device, characterized in that it is selected from the group consisting of hydroxy-2-methylpropan-1-one and benzyl dimethyl ketal. The polymer dispersion according to claim 1, wherein the charge transport material is selected from the group consisting of oxadiazole derivatives, triphenylmethane, triazo derivatives, pyrazoline derivatives, carbazole derivatives, arylamine derivatives and stilbenzene derivatives. Type liquid crystal display device.