Spacer particles for a liquid crystal display and liquid crystal display using such particles

Abstract: A liquid crystal display having a structure such that two sheets of substrates 1 each provided with a transparent electrode 2 and an orientation film 3 are disposed opposite to each other through a spacer particle 5, which has a elastic modulus in compression of 370 to 550 kg/mm² at the 10% displacement of particle diameter, and a liquid crystal layer 6 is provided. According to the present invention, a uniform display quality can be realized through a reduction in the cell gap deviation caused when use is made of a thin sheet substrate.

Description: The present invention relates to a spacer particle for a liquid crystal display and a liquid crystal display, particularly to a spacer particle for a liquid crystal display spacer and a liquid crystal display capable of realizing a uniform display quality by reducing cell gap deviation (the variation in the thickness of a liquid crystal layer from place to place) caused particularly when use is made of a thin-sheet substrate having a thickness of 1.0 mm or less. The liquid crystal display of the present invention can be used mainly as a display for OA equipment such as personal computers and word processors. Fig. 1 is a cross-sectional view of an embodiment of the liquid crystal display according to the present invention. Fig. 2 is a diagram showing a cell gap distribution of the liquid crystal display according to the present invention. Fig. 3 is a diagram showing a cell gap distribution of the conventional liquid crystal display. Fig. 4 is a schematic illustration for explaining portions of a cell gap measurement. As shown in Fig. 1, a liquid crystal display has a structure such that two sheets of substrates 1 each provided with a transparent electrode 2 and an orientation film 3 are disposed opposite to each other through a spacer particle 5 and a liquid crystal layer 6. In the prior art, the spacer particle 5 used in the display has been mainly a plastic bead polymerized from mainly divinylbenzene and the like and generally having properties such that the elastic modulus in compression at the 10% displacement of particle diameter is 300 to 350 kg/mm 2 , the mean particle diameter is 3 to 10 µm and the standard deviation of the particle diameter is 10% or less of the mean particle diameter. In the present specification, the elastic modulus in compression at the 10% displacement of particle diameter (elastic modulus in 10% compression) is a value measured by the following method. One sample particle was put on a sample mount and a load was applied toward the center of the particle through the use of a Micro Compression Testing Machine (PCT-200 manufactured by Shimadzu Corporation) to determine the relationship between the load and the compression displacement, thus determining the load at 10% displacement. This load value was substituted in the following equation to calculate the elastic modulus in 10% compression. This procedure was conducted on three sample particles, and the average value was taken as the elastic modulus in 10% compression of the particle. The measurement was conducted at room temperature. E = 3 x F x (1 - K 2 ) 2 x S 1.5 x R wherein E represents the elastic modulus in compression (kg/mm 2 ), F represents the compressive load (kg), K represents the Poisson's ratio of the particle (constant, 0.38), S represents the compression displacement (mm), and R represents the particle radius (mm). In the above-described prior art, no problem occurred when use was made of a glass substrate having a thickness of 1.1 mm or more. However, it has been found that when use is made of a thin substrate (particularly one having a thickness of 1.0 mm or less, for example, a glass substrate having a thickness of 0.3 to 0.7 mm and a plastic substrate having a thickness of 0.1 to 0.7 mm), the difference in the number of spacer particles present among various places is liable to cause the cell gap deviation. Specifically, the cell gap deviation seems to occur through the following mechanism. Because the substrate is liable to bend, the deviation in the number of spacer particles present among various places (for example, 100 ± 30 spacer particles/mm) in the step of injecting a liquid crystal under reduced pressure results in a small deformation in a portion where the number of the spacer particles is large (for example, 130 spacer particles/mm) and a large deformation in a portion where the number of the spacer particles is small (for example, 70 spacer particles/mm). Thus, the cell gap deviation seems to occur due to the variation in the deformation of the spacer particles from place to place. WO 92/06402, which is a document in accordance with Article 54(3) EPC discloses a fine sphere used for a spherical spacer defining a gap dimension between substrates. For example, tetramethylolmethane tetraacrylate alone or in combination with divinylbenzene is polymerized by suspension polymerization. Then, spacers having a mean particle size of about 7 µm are selected by classification. These spacers have a K-value at 10% compression deformation, defined as K=(3/ 2 )FS -3/2 R -1/2 (wherein F and S represent a load value (kgf) and a compression displacement (mm) at 10% compression deformation of said fine sphere, respectively, and R represents a radius (mm) of said fine sphere), of 550 (homopolymer) and 450 kgf/mm 2 (copolymer). EP-A-0 311 441 discloses a liquid crystal display panel comprising at least two base plates spaced by a sealant on the peripheral portion of the plates and liquid crystal in the space between the base plates, wherein said liquid crystal contains as a spacer rigid particles and polymer particles, said polymer particles having 10 to 30% by weight of an epoxy group. An epoxy group containing vinyl monomer may be polymerized to form the polymer particles having an epoxy group. According to the Examples, the polymer particles have an average diameter of 3.5 or 9.0 µm. The present inventors have found that the use of 1, a hardly deformable spacer is effective in solving the problem of the cell gap deviation particularly in the case of a thin substrate, which has led to the completion of the present invention. Thus, the present invention provides a spacer particle for a liquid crystal display as defined in claim 1. Preferred features are described in sub-claims 2 and 3. Furthermore, the present invention provides a liquid crystal display as defined in claim 4. Preferred features thereof are described in the sub-claims. Because the spacer particle of the present invention has an elastic modulus in compression higher than that of the conventional spacer particle, it is less susceptible to deformation when pressed, so that the cell gap deviation can be reduced even when the number of the spacer particles somewhat varies from place to place. However, because the spacer particle is less liable to deformation, there is a tendency that the variation in the particle diameter of the spacer particle brings about the cell gap deviation. For this reason, it is preferred that the standard deviation of the particle diameter be small. For example, when use is made of a thin sheet substrate (for example, a glass substrate having a thickness of 0.3 to 0.7 mm and a plastic substrate having a thickness of 0.1 to 0.7 mm), it is useful to regulate the standard deviation of the particle diameter, a, to 10% or less of the mean particle diameter for the purpose of realizing a uniform cell gap. Polyether sulfone (PES), polyethylene terephthalate (PET), polycarbonate, polyarylate or acrylic resin, are suitable as the material for the above-described plastic substrate. When use is made of a spacer particle having a elastic modulus in 10% compression exceeding 550 kg/mm 2 (for example, a benzoguanamine-based hard plastic bead), because the spacer particle is hardly deformable, it is necessary to regulate the standard deviation of the particle diameter, σ, to substantially zero, which renders such a spacer particle unsuitable for use in the case of a thin substrate. On the other hand, when use is made of a spacer particle having a elastic modulus in 10% compression less than 370 kg/mm 2 , it is as same as the case that the conventional spacer particle is used. As described above, the spacer particle of the present invention has a specified elastic modulus in 10% compression and there is no particular limitation e.g. on the process for the preparation or composition. For example, as a spacer particle, a vinyl plastic bead, particularly a vinyl plastic bead having a high crosslinking density may be used. The vinyl plastic bead having a high crosslinking density is obtainable by dispersion polymerization in the presence of a large amount of a crosslinkable monomer and a large amount of a polymerization initiator. There is no particular limitation on the above-described crosslinkable monomer as far as it has two or more radical-polymerizable unsaturated double bonds. Examples thereof include vinyl compounds such as divinylbenzene, 1,4-divinyloxybutane and divinyl sulfone; allyl compounds such as diallyl phthalate, diallylacrylamide, triallyl (iso)cyanurate and triallyl trimellitate; (poly)oxyalkylene glycol di(meth)acrylates such as (poly)ethylene glycol di(meth)acrylate and (poly)propylene glycol di(meth)acrylate; and pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol penta(meth)acrylate, glycerol tri(meth)acrylate and glycerol di(meth)acrylate. These crosslinkable monomers may be used alone or in the form of a mixture of two or more of them. Divinylbenzene is particularly preferred. All the radical polymerizable monomers other than the crosslinkable monomer may be used as the noncrosslinkable monomer in the production of the above-described vinyl plastic bead. Examples thereof include styrene monomers such as styrene, p-(or m-) methylstyrene, p-(or m-)ethylstyrene, p-(or m-) chlorostyrene, p-(or m-)chloromethylstyrene, styrenesulfonic acid, p-(or m-)tert-butoxystyrene, α-methyl-p-tert-amyloxystyrene and p-tert-amyloxystyrene: (meth)acrylic ester monomers such as ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, hydroxyethyl (meth)acrylate, diethylene glycol mono(meth)acrylate, glycerol mono(meth)acrylate, polyethylene glycol mono(meth)acrylate and butanediol mono(meth)acrylate; unsaturated carboxylic acid monomers such as (meth)acrylic acid and maleic acid; alkyl vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; vinyl ester monomers such as vinyl acetate and vinyl butyrate; N-alkyl-substituted (meth)acrylamides such as N-methyl(meth)acrylamide and N-ethyl(meth)acrylamide; and nitrile monomers such as (meth)acrylonitrile. These noncrosslinkable monomers may be used alone or in the form of a mixture of two or more of them. Regarding the mixing proportion of the crosslinkable monomer to the noncrosslinkable monomer in the production of the above-described vinyl plastic bead, the proportion of the crosslinkable monomer is 50% by weight or more, particularly preferably 70% by weight or more. When the proportion of the crosslinkable monomer is below this range, the resulting vinyl plastic bead may exhibit no sufficient elastic modulus in compression. The polymerization initiators used are peroxide free radical generators. For example, peroxide radical initiators such as benzoyl peroxide and lauroyl peroxide are preferred from the viewpoint of the purpose of the present invention. The amount of use of the radical polymerization initiator is 3 to 10% by weight, still preferably 4 to 7% by weight based on the polymerizable monomer. No plastic bead having a suitable elastic modulus in compression can be prepared in both the cases where the amount of use of the polymerization initiator is excessively small and excessively large. The aqueous polymerization or the suspension polymerization wherein use is made of the above-described material is conducted according to the conventional procedure at a temperature in the range of from 25 to 100°C, preferably in the range of from 50 to 90°C in the presence of a dispersion stabilizer while stirring the reaction system. Examples of the dispersion stabilizer include surfactants such as sodium lauryl sulfate, sodium laurylbenzenesulfonate and sodium polyoxyethylene lauryl ether sulfate; water-soluble polymers such as gelatin, starch, hydroxyethylcellulose, carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl alkyl ether and polyvinyl alcohol; and sparingly water-soluble inorganic salts such as barium sulfate, calcium sulfate, barium carbonate, calcium carbonate, magnesium carbonate and calcium phosphate. In the liquid crystal display of the present invention, as shown in Fig. 1, the upper and lower light-transmitting substrates 1, each provided with a transparent electrode 2 having a predetermined electrode pattern on its surface and an orientation film 3 covering the transparent electrode 2, are disposed so as to face to each other through a spacer particle 5, which has a elastic modulus in compression of 370 to 550 kg/mm 2 at the 10% displacement of particle diameter, and a liquid crystal layer 6. As shown in this figure, a liquid crystal layer 6 is held by a sealing material 4 such as an epoxy resin. Numeral 7 designates a pair of polarizing films respectively provided on the top and bottom of the device. In the liquid crystal display of the present invention, there is no particular limitation on the above-described light-transmitting substrates 1 as far as they are light-transmittable, at least one of the above-described light-transmitting substrates 1 is preferably a plastic substrate. Examples thereof include polyether sulfone (PES), polyethylene terephthalate (PET), polycarbonate, polyarylate and acrylic resin. In recent years, there is an ever-increasing demand for a liquid crystal display having a reduced thickness and a reduced weight. In the prior art, the cell gap deviation is liable to occur when use is made of a thin sheet substrate, which brings about a problem that the uniformity of the display quality is low. By contrast, the present invention has the effect of realizing a thin liquid crystal display having a uniform display quality. [Examples] The present invention will now be described in more detail with reference to the following Examples. In the Examples, the "part(s)" is part(s) by weight. Synthesis Example 1 A liquid mixture comprising 80 parts of divinylbenzene (DVB-810 having a purity of 81% manufactured by Nippon Steel Chemical Co., Ltd.), 20 parts of ethylene glycol dimethacrylate (NK-ester 1G manufactured by Shin-Nakamura Kagaku Kogyo K.K.) and 5 parts of benzoyl peroxide was finely dispersed in 800 parts of a 3 wt.% aqueous solution of polyvinyl alcohol (GH-17 having a saponification value of 86.5 to 89% by mole manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.) to conduct polymerization in a nitrogen gas stream at 80°C for 15 hr while stirring. The resultant fine particles were washed with deionized water and a solvent, classified, isolated and dried to give fine particles of a crosslinked polymer having a mean particle diameter of 6.0 µm and a standard deviation of a particle diameter of 0.4 µm. The elastic modulus in 10% compression (mean value) of the fine particle of the crosslinked polymer thus prepared was 380 kg/mm 2 . Synthesis Example 2 A fine particle of a crosslinked polymer having a mean particle diameter of 7.5 µm and a standard deviation of a particle diameter of 0.5 µm was prepared in the same manner as that of the Synthesis Example 1, except that the benzoyl peroxide was used in an amount of 7 parts. The elastic modulus in 10% compression (mean value) of the fine particle of the crosslinked polymer thus prepared was found to be 420 kg/mm 2 . Example 1 A description will now be made on a plastic liquid crystal display wherein the fine particle of the crosslinked polymer prepared in the Synthesis Example 1 or a conventional plastic bead was used as a spacer particle. Figs. 2 and 3 show the results of a measurement of cell gap wherein a PES substrate and a PET substrate each subjected to hard coating treatment and having a cell size of about 15,24 cm (6 inches) and a thickness of 0.4 mm with the cell gap being set at 6.0 µm were used as the light-transmitting substrate 1 shown in the Fig. 1. In Fig. 3, the conventional plastic bead (Micropearl SP manufactured by Sekisui Fine Chemical K.K. having a elastic modulus in 10% compression of 320 kg/mm 2 , a mean particle diameter of 6.0 µm and a standard deviation of particle diameter, a, of 0.4 µm) was used as the spacer particle, and cell gaps of the liquid crystal display were measured at positions sufficiently distant from the sealed part (See Fig. 4). In Fig. 3, the cell gap (µm) is plotted as the ordinate against the measuring distance (0 - 10 mm) as the abscissa. In the data, the cell gap varies to a great extent in the range of from 5.90 to 6.11 µm around 6.00 µm. On the other hand, Fig. 2 shows the results of measurement in the case where use was made of a plastic bead having a high elastic modulus in compression, as a spacer particle, as prepared in the Synthesis Example 1 according to the present invention. In this case, the cell gap varies only to a small extent in the range of from 5.95 to 6.03 µm around 6.00 µm. This demonstrates the effect attained in the case where use is made of the plastic bead having a high elastic modulus in compression according to the present invention as a spacer particle. Similar measurements were conducted on embodiments wherein a glass substrate having a thickness of 0.7 mm and a plastic substrate having a thickness of 0.4 mm were used respectively as the upper and lower light-transmitting substrates. As a result, good results of substantially the same level as that described above could be obtained. Example 2 The fine particle of the crosslinked polymer prepared in the Synthesis Example 1 was used as a spacer particle, and a glass sheet having a thickness of 0.7 mm was used as a substrate to prepare a super-twisted nematic liquid crystal display having a cell size of 25,4 cm (10 inches) in terms of the diagonal size, the number of dots of 640 x 480 and a cell gap of 6.0 µm. The obtained display exhibited a uniform display quality free from the cell gap deviation. Example 3 The fine particle of the crosslinked polymer prepared in the Synthesis Example 2 was used as a spacer particle, and a glass sheet having a thickness of 0.7 mm was used as a substrate to prepare a 240°-super-twisted nematic liquid crystal display having a cell size of 25,4 cm (10 inches) in terms of the diagonal size, the number of dots of 640 x 480 and a cell gap of 7.5 µm. The obtained display exhibited a uniform display quality free from the cell gap deviation.

Claims: Spacer particles for a liquid crystal display comprising a vinyl polymer or vinyl copolymer wherein, said polymer or copolymer is crosslinked and said particles have a mean particle diameter of 3 to 10 µm, characterized in that said particles display an elastic modulus in compression of 370 to 550 kg/mm 2 at 10% displacement of the particle diameter, and the vinyl polymer or vinyl copolymer comprising the particles is obtained by dispersion-polymerizing a monomer mixture comprising 50% by weight or above of a crosslinkable monomer in the presence of 3 to 10 % by weight based on the monomers of a peroxide compound polymerization initiator. The spacer particles according to claim 1 comprising a vinyl copolymer. The spacer particles according to claim 1 or 2 having a standard deviation of the particle diameter of 10% or less of the mean particle diameter. A liquid crystal display having a structure such that two sheets of substrates, each provided with a transparent electrode and an orientation film are disposed opposite to each other through a liquid crystal layer and the spacer particles as defined in claim 1 or 2. The liquid crystal display according to claim 4 wherein at least one of the two sheets of substrates is a plastic substrate. The liquid crystal display according to claims 4 or 5 wherein the thickness of the substrate sheet is 1.0 mm or less. The liquid crystal display according to claims 4 or 5 which comprises a substrate sheet of glass of thickness 0.3 to 0.7 mm. The liquid crystal display according to claims 4 or 5 which comprises a substrate sheet of plastic of thickness 0.1 to 0.7 mm.

Inventors: NISHIMORI TADASHI, RINDO KATSUHIKO, TAKANASHI HIROSHI, YOSHIMATSU AKIRA, NISHIMORI, TADASHI, RINDO, KATSUHIKO, TAKANASHI, HIROSHI, YOSHIMATSU, AKIRA
Applicants: KAO CORP, SHARP KK, KAO CORPORATION, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G02F1
IPC codes: G02F1