Liquid crystal display element

Abstract: A liquid crystal display element (5) comprises a liquid crystal material (2) having a resistivity ρ LC (Ω·cm) and a polymer matrix material (1) having a resistivity ρ p (Ω·cm). The ratio ρ LC /ρ p is defined to fall within a range of between 1 and 10⁵ (1 ≦ ρ LC /ρ p ≦ 10⁵). Also, the particles diameters of the polymer matrix material (1) or the liquid crystal material (2) are distributed such that at least two peaks each having a half value width of at least 0.5 µm are present within a range of between 0.5 µm and 2.5 µm. Further, a protective layer is formed to cover the outer surface of the liquid crystal material. The particular construction permits manufacturing a polymer dispersion type liquid crystal display element (5), which exhibits a high response speed, which can be operated with a low driving voltage, and which permits a high contrast.

Description: The present invention relates to a polymer dispersion type liquid crystal display element useful as, for example, a TV or flat panel display device. Recently, a polymer dispersion type liquid crystal display element attracts attentions as a new type liquid crystal display element. The new type liquid crystal display element, which was disclosed for the first time in Published Unexamined Japanese Patent Application No. 61-83519, is constructed such that a film prepared by dispersing a liquid crystal material in a polymer matrix material is held between a pair of electrodes. In the polymer dispersion type liquid crystal display element, the liquid crystal material dispersed in the polymer matrix material differs in the refractive index from the polymer matrix material, when an electric field is not applied across the film held between the two electrodes. As a result, scattering of light is brought about so as to make the film opaque. When an electric field is applied across the film by applying voltage between the two electrodes, the liquid crystal molecules dispersed in the polymer matrix material are oriented, with the result that the refractive index of the oriented portion of the liquid crystal is made substantially equal to that of the polymer matrix material. It follows that the film is made transparent. The polymer dispersion type liquid crystal display element, which utilizes the particular operation principle described above, makes it unnecessary to use a polarizer, an orientation film, etc., which are required in the conventional TN type liquid crystal display element used in, for example, a miniature TV receiver. It follows that it is possible to markedly simplify the manufacturing process of the polymer dispersion type liquid crystal display element. In the conventional polymer dispersion type liquid crystal display element, however, the polymer matrix material has a relatively high resistivity, leading to requirement of a high driving voltage. Also, the conventional liquid crystal display element of this type is low in its response speed. Further, the light shuttering function is performed by the scattering of light, leading to a low contrast. The document US-A-4 673 225 describes a liquid crystal display element according to the preamble of claim 1. An object of the present invention is to provide a liquid crystal display element, which can be operated with a low driving voltage, which exhibits a high response speed, and which achieves a good contrast. According to an embodiment of the present invention, there is provided a liquid crystal display element, having two substrates and a film held between the two substrates, the film being made of a material prepared by dispersing a liquid crystal material in a polymer matrix material, wherein at least 80% of the particles of the polymer matrix material or the liquid crystal material have particle diameters falling within a range of between 0.5 µm and 2.5 µm, at least two peaks of the particle size distribution each having a half value width of at least 0.5 µm being present within a range of between 0.5 µm and 2.5 µm. Further, a protective layer may be formed to cover the outer surface of the liquid crystal particle. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic view main portion of a liquid crystal showing display element of the present invention. In a first arrangement, the electrical conductivity of the polymer matrix material used in a polymer dispersion type liquid crystal display element is adjusted so as to permit a ratio in resistivity of the liquid crystal material to the polymer matrix material, i.e., the ratio, ρ LC /ρ p , to fall within a predetermined range, i.e., 1 < ρ LC /ρ p < 10 5 . For allowing the ratio p LC /ρ p to fall within the predetermined range noted above, electrically conductive fine particles may be added to or dispersed in a polymer matrix material. Fine particles of metal oxides such as alumina, metal such as silver, cupper; carbon such as graphite; fullerenes such as C 60 , C 78 ; charge-transfer complex, for example, a substance obtained by using hydrazone compounds, styryl compounds or compounds bonded diethylamino group or dimethylamino group thereto as a donor, and by using tetracyanoethane, tetracyanoquinodiethane, or maleic anhydride as an acceptor; organic electrolyte such as tetrabuthylammonium chloride or tetraethylammonium perchlorate can be used as conductive fine particles. It is desirable for the conductive fine particles to have an average particle diameter of 0.01 to 3 µm. If the average diameter is smaller than 0.01 µm, it is difficult to control the particle diameter in the manufacture process. On the other hand, if the average diameter of the conductive fine particles is larger than 3 µm, the display quality is degraded. The amount of the conductive fine particles used is not particularly restricted, as far as the ratio ρ LC /ρ p falls within the range defined above. It is also possible to put conductive fine particles in microcapsules which are immiscible with the liquid crystal material used. In this case, the microcapsules thus prepared are dispersed in the polymer matrix material. It should be noted that conductive fine particles, if mixed with a liquid crystal material, tend to impair the operation characteristics of the liquid crystal material. For avoiding the difficulty, it is desirable to put conductive fine particles in microcapsules immiscible with the liquid crystal material and disperse the microcapsules thus prepared in the polymer matrix material. Alternatively, conductive polymers may be added to or dispersed in a polymer matrix material to permit the ratio ρ LC /ρ p to fall within the range specified in the above. Incidentally, where the polymer itself used exhibits an electrical conductivity, it is possible to use the conductive polymer as the polymer matrix material. In this case, it is unnecessary to apply any treatment to the conductive polymer. The conductive polymers used include, for example, conjugated π aromatic polymer, polypyrrole, poly (N-substituted dipyrrole), poly (3,4-2 substituted pyrrole), polypyridine, polyphenylene, polythiophene, poly (3,4-2 substituted thiophene), polyaniline, polyazulene, polyvinylene, polyvinylcarbazol, poly (N-substituted carbazol), polyselenophene, polyfuran, poly (2,5-furylenevinylene), polybenzothiophene, poly (2,5-thienylenevinylene), polybenzofuran, polyindole, polyisothionatphthene, polypyridazine, polyacetylene, and polysilane. The content of the conductive polymer is not particularly restricted as far as the ratio ρ LC /ρ p falls within the range specified above. It is desirable for the ratio ρ LC /ρ p to fall within a range of between 1 and 10 5 . If the ratio is smaller than 1, the electric field in liquid crystal phase is weak. On the other hand, if the ratio is larger than 10 5 , the conductivity of liquid crystal phase is large in excess. If the resistivity ρ p of the polymer matrix material whose resistivity has been adjusted exceeds 10 10 Ω·cm, a satisfactory effect cannot be expected. On the other hand, if the resistivity ρ p is lower than 10 -4 Ω·cm, the polymer matrix material exhibits an unduly high electrical conductivity, making it impossible to apply an electric field to the liquid crystal phase. In the embodiment of the present invention, the size (or particle size distribution) of the polymer matrix material or the particle size distribution of the liquid crystal material is defined so as to control the threshold value. Where the particle size distribution is defined as specified in the present invention, a change in the light transmittance relative to the applied voltage is rendered relatively moderate and linear so as to improve the steepness. Further, the particle size distribution specified in the present invention makes it possible to provide a polymer dispersion type liquid crystal display element adapted for the active-matrix driving. To be more specific, the particle size distribution of the polymer matrix material or the liquid crystal material is defined in the present invention such that at least two peaks each having a half value width of at least 0.5 µm are present within a range of between 0.5 µm and 2.5 µm in the particle diameter. It is important for a plurality of peaks to be present within a range of between 0.5 µm and 2.5 µm. If the particles of the polymer matrix material are unduly large, the number of light scatterings is diminished, leading to a low light scattering capability. On the other hand, if the particles of the polymer matrix material are unduly small, the particle diameters overlap with the wavelength region of the visible light. In this case, the scattering of light is suppressed, also leading to a low light scattering capability. In addition, the threshold voltage is increased if the particles of the polymer matrix material are unduly small. It is necessary that at least 80% of the particles to have particle diameters falling within the range of between 0.5 µm and 2.5 µm because of optimizing the width of driving voltage. A similar situation takes place where the amount of the liquid crystal material is small relative to the amount of the polymer matrix material such that the liquid crystal molecules are dispersed in the polymer matrix material. It should be noted that, where only one peak is present within the range of the particle diameter noted above, a difficulty is brought about that the relationship between voltage and transmittance can not be controlled. Likewise, where the half value width of the peak is smaller than 0.5 µm, a difficulty is brought about that the relationship between voltage and transmittance can not be controlled. The particle size distribution of the polymer matrix material can be controlled by changing the condition of the polymerization. For example, where a thermosetting resin such as an epoxy resin is used as tne polymer matrix material, the particle size distribution of the polymer matrix material can be controlled by adjusting the curing temperature and curing time. To be more specific, the particle diameter of the formed polymer matrix material is diminished with increase in the curing temperature and with decrease in the curing time. The particle diameters of the polymer matrix material can be made nonuniform by controlling the curing treatment such that, in the initial stage, a heat treatment is carried out at a low temperature for a relatively long time, followed by elevating the curing temperature for finishing the heat treatment. Where a photo-setting resin such as an acrylic resin is used as the polymer matrix material, the particle diameters of the polymer matrix material can be controlled by adjusting the curing temperature and the UV energy, as in the case of using an epoxy resin as the polymer matrix material. A polymer matrix material having a wide particle size distribution can be obtained by controlling the curing condition such that, in the initial stage, a UV irradiation is carried out under a low temperature, followed by elevating the temperature before completion of the curing treatment while continuing a UV irradiation. In another arrangement, the outer surface of the liquid crystal material is covered with a protective layer. To be more specific, the liquid crystal material is put first into microcapsules and, then, the microcapsules thus prepared are dispersed in the polymer matrix material. A surfactant series monomer can be used as the material of the protective layer. For example, sodium undecylenate can be used as the material of the protective layer. To be more specific, the surfactant series monomers are polymerized so as to form the protective layer. For example, a mixture consisting of a liquid crystal material and a surfactant series monomer is vigorously stirred and dispersed within water so as to form a layer of the surfactant series monomer in a manner to cover the outer surface of the liquid crystal molecule. Under this condition, a polymerization initiating agent is added to the system, with the result that the monomer layer is polymerized so as to form a microcapsule. The microcapsules thus prepared can be filtered, washed with water, dried, and preserved. The microcapsules can be dispersed in an optional polymer matrix material after applying a suitable treatment to the microcapsule thus obtained or intact. The dispersion means is not particularly restricted as mentioned above though it is desirable to use, for example, an ultrasonic wave. Incidentally, it is desirable to disperse the microcapsules under the condition that the polymer of the microcapsule is not dissolved. The polymerization film thus obtained produces a strong anchoring effect relative to the polymer matrix material unlike the conventional liquid crystal phases which lack an affinity and are formed under a mutually repelling condition. As a result, decay speed of the element is improved in non-application-voltaqe state. It is of course possible to employ the various arrangements and the embodiment described above in combination so as to provide a satisfactory liquid crystal display element. The polymers used in the present invention as the polymer matrix material include, for example, polyethylene, polypropylene, polybutylene, polystyrene, polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride, polyacrylic ester, polymethacrylic ester' polyacrylonitrile, polyvinyl ether, polyvinyl ketone, polyether, polycarbonate, polyester, polyamide, polyvinyl alcohol, diene series polymer, polyurethane series polymer, silicone resin, natural rubber, modified celluloses such as methyl cellulose and ethyl cellulose, phenoxy resin, phenol resin, epoxy resin, photo-setting resin, melamine resin, polyimide resin, polysilane, copolymers thereof, mixture of these polymers, and polymer alloy. Example 1 A mixture was prepared first by using 100 parts by weight of Epon 812 (registered trademark of an epoxy resin manufactured by Yuka Shell Inc.), which was used as a polymer matrix material, 100 parts by weight of Capcure 3-800 (trade name of a curing agent manufactured by Yuka Shell Inc.), and 200 parts by weight of E-7 (registered trademark of a liquid crystal manufactured by BDH Inc.). On the other hand, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 20 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was subjected to a heat treatment at 60°C for 30 minutes, followed by elevating the temperature to 100°C for an additional heat treatment for 10 minutes so as to obtain a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 1.0 µm were included within a range of between 1.0 µm and 2.0 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtained were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.30, and the rising (0 to 90%) was also satisfactory. Example 2 Two kinds of resins, i.e., Epon 812 and Devcon 5A (registered trademark of an epoxy resin manufactured by Devcon Inc.) were used as the polymer matrix material. A curing agent Capcure 3-800 was added to these resins such that the weight ratio of Epon 812 : Devcon 5A : Capcure 3-800 was 1 : 1 : 2. Further, a liquid crystal E-7 was added to the resultant mixture such that the weight ratio of the resin mixture to the liquid crystal was 1 : 2. On the other hand, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 20 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was subjected to a heat treatment at 50°C for 20 minutes, followed by elevating the temperature to 90°C for an additional heat treatment for 5 minutes so as to obtain a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 1.0 µm were included within a range of between 1.3 µm and 2.0 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtained were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.25, and the rising (0 to 90%) was also satisfactory. Example 3 A raw material mixture was prepared first by mixing 100 parts by weight of a polymer matrix material consisting of polyesterpolyol and polyurethane acrylate prepared from hydroxypropyl acrylate, 400 parts by weight of a liquid crystal E-7, and 40 parts by weight of 2-hydroxy-2-methyl-1-phenylpropane-1-one, which was used as a photopolymerization initiator. Then, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 10 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was irradiated for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 30 W/cm, 80°C, followed by an additional irradiation for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 30 W/cm, 90°C, so as to prepare a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 0.6 µm were included within a range of between 0.8 µm and 1.7 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtain were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.25, and the rising (0 to 90%) was also satisfactory. Example 4 A raw material mixture was prepared first by mixing 100 parts by weight of a polymer matrix material consisting of polyesterpolyol and polyurethane acrylate prepared from hydroxypropyl acrylate, 400 parts by weight of a liquid crystal E-7, and 20 parts by weight of 2-hydroxy-2-methyl-1-phenylpropane-1-one, which was used as a photopolymerization initiator. Then, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 10 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was irradiated for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 20 W/cm, 60°C, followed by an additional irradiation for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 80 W/cm, 80°C, so as to prepare a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 1.5 µm were included within a range of between 1.0 µm and 2.0 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtained were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.20, and the rising (0 to 90%) was also satisfactory. Example 5 A raw material mixture was prepared first by mixing 100 parts by weight of a polymer matrix material consisting of polyesterpolyol and polyurethane acrylate prepared from hydroxypropyl acrylate, 400 parts by weight of E-44 (registered trademark of a liquid crystal manufactured by BDH Inc.), and 20 parts by weight of 2-hydroxy-2-methyl-1-phenylpropane-1-one, which was used as a photopolymerization initiator. Then, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 10 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was irradiated for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 30 W/cm, 60°C, followed by an additional irradiation for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 80 W/cm, 80°C, so as to prepare a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 1.5 µm were included within a range of between 1.5 µm and 2.0 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtained were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.20, and the rising (0 to 90%) was also satisfactory. Example 6 A raw material mixture was prepared first by mixing two kinds of polymer matrix materials, i.e., 100 parts by weight of n-butyl acrylate and 200 parts by weight of acrylic oligomer, 400 parts by weight of E-44, and 10 parts by weight of 2-hydroxy-2-methyl-1-phenylpropane-1-one, which was used as a photopolymerization initiator. Then, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 10 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was irradiated for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 20 W/cm, 60°C, followed by an additional irradiation for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 80 W/cm, 80°C, so as to prepare a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 0.8 µm were included within a range of between 1.0 µm and 2.0 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtained were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.25, and the rising (0 to 90%) was also satisfactory. Example 7 A raw material mixture was prepared first by mixing two kinds of polymer matrix materials, i.e., 100 parts by weight of n-butyl acrylate and 200 parts by weight of acrylic oligomer, 400 parts by weight of E-44, and 10 parts by weight of 2-hydroxy-2-methyl-lphenylpropane-1-one, which was used as a photopolymerization initiator. Then, two glass substrates each having an ITO film formed on the surface by means of vapor deposition were disposed to face each other with a spacer of 10 µm interposed therebetween, and the resin mixture noted above was filled into the free space between the two glass substrates. The resultant assembly was irradiated for 3 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 30 W/cm, 60°C, followed by an additional irradiation for 5 minutes with an ultraviolet light emitted from a metal halide lamp under the condition of 30 W/cm, 60°C, so as to prepare a polymer dispersion type liquid crystal display cell. The particle size of the polymer matrix material was found to be distributed such that two peaks each having a half value width of 0.7 µm were included within a range of between 1.0 µm and 1.5 µm. The contrast and the response speed of the polymer dispersion type liquid crystal display cell thus obtained were measured, with the result that the contrast was satisfactory, i.e., V 10 /V 90 was 0.2, and the rising (0 to 90%) was also satisfactory. As described above in detail, the present invention provides a polymer dispersion type liquid crystal display element which exhibits a markedly improved response speed, which can be operated with a low driving voltage, and which permits a high contrast. Naturally, the liquid crystal display element of the present invention is adapted for an active matrix driving. In order to produce the prominent effects described above, the particle diameters of the polymer matrix material or the liquid crystal material are intentionally distributed over a wide range so as to control the threshold value. Further, the liquid crystal material is formed into microcapsules in advance and, then, the microcapsules are dispersed in the polymer matrix material. The use of the liquid crystal microcapsules permits widening the scope of selection of the polymer matrix material and the liquid crystal material.

Claims: A liquid crystal display element, comprising a pair of substrates (4) and a film (5) held between the substrates (4), said film (5) being prepared by dispersing a liquid crystal material (2) in a polymer matrix material (1), wherein at least 80% of the particles of the polymer matrix material or the liquid crystal material have particle diameters falling within a range of between 0.5µm and 2.5µm and, characterised in that at least two peaks of the particle size distribution each having a half value width of at least 0.5µm are present within a range of between 0.5µm and 2.5µm. A liquid crystal display element according to claim 1, wherein a protective layer (3) is formed to cover the outer surface of the liquid crystal material (2). A liquid crystal display element according to claim 2, further characterised in that the material of the protective layer is made of a surfactant series monomer. A liquid crystal display element according to claim 1, wherein said polymer matrix material (1) is an acrylic resin or an epoxy resin. A liquid crystal display element according to claim 1, wherein said element is active-matrix driven.

Inventors: SANO KENJI, SHIROTA KOICHIRO, SUNOHARA KAZUYUKI, SANO, KENJI, SHIROTA, KOICHIRO, SUNOHARA, KAZUYUKI, Sano, Kenji, c/o Intellectual Property Div., Shirota, Koichiro, c/o Intellectual Property Div., Sunohara, Kazuyuki, c/o Intellectual Property Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: G02F1
IPC codes: G02F1, G09F9, C09K19