METHOD OF PREPARING MONODISPERSE AND HIGH- BRIDGED POLYMER PLASTIC BEAD

Technical Field

[ 1 ] The present invention relates to a method of for the preparation of monodispersed, highly-crosslinked polymeric plastic beads and more particularly, it relates to a method of preparing plastic beads wherein monodispersed beads can be synthesized with a high efficiency by a single process, and a particle size, compressive modulus and recovery rate are freely controlled in the preparation of plastic beads and further, the prepared polymeric plastic beads are especially suitable to be applied to liquid crystal display elements and for the preparation of conductive particles for anisotropic conductive connection. Background Art

[2] Anisotropic conductive connection is frequently used to connect the connection terminals of semi-conductive elements and the connection terminals of substrates for mounting thereon. For this anisotropic conductive connection, there is used a method of inserting fine conductive particles into films dispersed in insulative adhesives or between materials for connecting anisotropic conductive connection materials in the shape of space and then adhere them by applying heat and pressure.

[3] Recently, as a display device, the use of liquid crystal display devices has been rapidly increasing and in general, a liquid crystal display element used in liquid crystal display devices includes spacers as shown in Fig. 1.

[4] This liquid crystal display element comprises a pair of substrate (37, 39), spacers

(38) installed between the pair of the substrates (37, 39) to maintain a constant cell gap between them, a pneumatic liquid crystal (41), a sealing substance (30) filled around the cell gap between the substrates (37, 39), and polarized sheets (42, 43) coated onto the surface of each substrate (37, 39).

[5] A conductive particle used in the anisotropic conductive connection materials comprises, from the inside, a spherical organic bead-coating layer (metal layer) of conductive metal and if necessary, it further comprises an insulative coating layer coated onto the surface of the metal layer. However, as the pattern of connection terminals which are the target of the anisotropic conductive connection becomes gradually finer, there is an increasing concern about the occurrence of shorts during conductive connection. Hence, there have been attempts to raise contact reliability by using conductive particles of uniform size and there is a constant demand on methods for the preparation of spherical organic beads having uniform particle size with a high

efficiency.

[6] Also, in case of the spacers of liquid crystal display elements, organic beads are directly applied thereto or alternatively, organic beads with improved properties by being coated with a metal material are applied.

[7] Accordingly, in case of the conductive particles and spacers, plastic beads having a uniform particle size while having a uniform shape of sphere are in demand. In the prior art, spherical plastic beads were prepared by an emulsion polymerization or suspension polymerization of monomers and stabilizers and then fractionation thereof, but their yield after the fractionation was less than 3 % and the efficiency was thus very low.

Disclosure of Invention Technical Problem

[8] To solve the problems of the prior art as described in the above, it is an object of the present invention to provide a method for the preparation of polymeric plastic beads wherein monodispersed beads can be synthesized with a high efficiency by a single process, and their compressive modulus and recovery rate can be freely controlled in the preparation of beads and the polymeric plastic beads prepared therefrom.

[9] It is another object of the invention to provide a method for the preparation of polymeric plastic beads which have excellent contact reliability, cause no change in the orientation properties of liquid crystal by the damage of orientation-control films, do not degrade the quality of image and do not cause the irregularity of a cell gap when applied to conductive particles for anisotropic conductive connection and liquid crystal display elements, the polymeric plastic beads prepared therefrom, and conductive particles for anisotropic conductive connection and a liquid crystal display element to which the polymeric plastic beads are applied. Technical Solution

[10] To achieve the aforementioned objects, the present invention provides a method of preparing polymeric plastic beads comprising:

[11] a) synthesizing monodispersed beads by mixing 1) an acryl-based monomer, 2) an initiator, and 3) a solvent and polymerizing them using an ultrasonic machine or a device with an agitation function in up-to-down or left-to-right directions; and

[12] b) filtering and drying the monodispersed beads synthesized in step a).

[13] Also, the invention provides polymeric plastic beads prepared by the method above.

[14] Also, the invention provides conductive particles for anisotropic conductive connection comprising the polymeric plastic beads.

[15] Also, the invention provides a liquid crystal display element comprising the polymeric plastic beads.

Advantageous Effects

[16] The method for the preparation of polymeric plastic beads in accordance with the present invention enables the synthesis of monodispersed beads with a high efficiency through a single process and it can freely control their compressive modulus and recovery rate in the preparation thereof and further it has excellent contact reliability, causes no change in the orientation properties of a liquid crystal by the damage of an orientation-control film, does not degrade the quality of image and does not cause the irregularity of a cell gap when applied to conductive particles for anisotropic conductive connection and liquid crystal display elements. Brief Description of the Drawings

[17] Fig. 1 is a sectional view showing a common liquid crystal display device.

[18] Fig. 2 is a photograph showing plastic beads prepared in accordance with one embodiment of the invention.

[19] Fig. 3 is a photograph showing particles prepared by forming a metal coating layer on the plastic beads prepared in accordance with one embodiment of the invention. Mode for the Invention

[20] The present invention is further described in detail.

[21] The polymeric plastic beads are prepared by a) synthesizing monodispersed beads by mixing 1) an acryl-based monomer, T) an initiator, and 3) a solvent and polymerizing them using an ultrasonic machine or a device with an agitation function in up-to-down or left- to-right directions; and b) filtering and drying the monodispersed beads synthesized in step a).

[22] The following will describe the method for the preparation of the polymeric plastic beads of the invention in detail.

[23] a) Synthesis of Beads

[24] This step is to synthesis monodispersed beads by mixing 1) an acryl-based monomer, 2) an initiator, and 3) a solvent and polymerizing them using an ultrasonic machine or a device with an agitation function in up-to-down or left- to-right directions.

[25] The compressive modulus and recovery rate of the finally produced polymeric plastic beads can be controlled by the type of the acryl-based monomers of 1) used in this step or the mixing ratio thereof if two or more kinds are mixed.

[26] For the acryl-based monomers, there can be used methyl(meth)acrylate, ethyl(meth)acrylate, trimethylomethane tetraacrylate, trimethylomethane triacrylate, trimethylobutane triacrylate, glycidyl(meth)acrylate, and ethyleneglycoldiglycidyl- methacrylate, alone or as a mixture of two or more.

[27] The acryl-based monomers can be further mixed with styrene or divinylbenzene, and the styrene or divinylbenzene forms final polymeric plastic beads by reacting with

the acryl-based monomers.

[28] The acryl-based monomer and styrene or divinylbenzene are mixed preferably in a ratio of 10:90 to 90: 10 by weight, more preferably in a ratio of 30:70 to 70:30 by weight. The mixing ratios within the above ranges are advantageous for the refraction index, intensity and properties of the final polymeric plastic beads produced.

[29] The initiator of 2) used in this step regulates the particle size and particle size distribution of monodispersed beads to be formed.

[30] For the initiator, there can be used azo-based initiators such as

2,2-azobisisobutyronitrile, 2,2-azobis(2-methylbutanenitrile), 3,3-azobis(4)-cyanopentanoic acid, 2,2-azobis(2-methyrbutyronitrile), and 2,2-azobis(2,4-dimethylvaleronitrile) capable of initiating polymerization by thermal degradation in a non-aqueous phase; or peroxide-type initiators such as benzoyl peroxide, lauryl peroxide, octanoyl peroxide, and 3,3,5-trimethylhexanoyl peroxide.

[31] The initiator is contained preferably in an amount of 0.1 to 20 parts by weight of

100 parts by weight of the mixed amount of the acryl-based monomers and styrene- based monomers. If its amount is too small, polymerization does not occur and if it is too much, it fails to form polymeric plastic beads of monodispersed particles.

[32] For the solvent of 3) used in this step, it is preferable to use solvents in which the aforementioned monomers and initiators can be completely mixed.

[33] For the solvents, there can be used acetonitrile; alcohols such as methanol, ethanol, buthyl alcohol, amyl alcohol, octyl alcohol and benzyl alcohol; or polyhydric alcohols such as ethyleneglycol, propyleneglycol and glycerin. It is preferable to use these solvents by blending one or two more among the solvents having similar solubility with monomers, stabilizers, etc to be used in consideration of the solubility of monomers in the polymerization solvents.

[34] Preferably, the solvents are contained such that the amount of solids, e.g., acryl- based monomers is 0.1 to 5 % by weight.

[35] The aforementioned acryl-based monomers, initiators and solvents are evenly mi xed and then polymerized in an ultrasonic machine or a device with an agitation function in up-to-down or left- to-right directions to form monodispersed beads. This invention enables the formation of monodispersed beads by carrying out polymerization using an ultrasonic machine or a device with an agitation function in up- to-down or left-to-right directions as stated above, without the necessity of carrying out fractionation that was carried out after polymerization step in the prior art.

[36] In a preferred embodiment, after the mixture of the acryl-based monomers, initiators and solvents is poured into a glass bottle with a nitrogen purge system and nitrogen is then purged for 2 to 3 min, it is polymerized while being stirred at 60 to 80 rpm in a device with an agitation function in up-to-down or left-to-right directions (for

example, water bath), or being stirred using an ultrasonic machine. Preferably, the polymerization is carried out for 20 to 30 hours at temperatures of 50 to 90 ⁰ C.

[37] b) Filtering and Drying

[38] This step is to filter and dry the monodispersed beads synthesized in the above step.

[39] The monodispersed beads of which the polymerization is complete in the above step exist precipitated on the bottom of the glass bottle. The precipitated, monodispersed beads are withdrew by filtration through centrifugation or filter papers and then dried for 20 to 30 hours at a vacuum oven of 70 to 80 ⁰ C whereby final polymeric plastic beads can be obtained.

[40] Preferably, the polymeric plastic beads of the invention prepared in the above have an average particle diameter of 1 to 10 um, a compressive modulus of 100 - 700 kgf/ mm and a recovery rate of 10 - 60 %.

[41] If the compressive modulus is less than 100 kgf/mm , it is difficult to control a cell gap when they are used in liquid crystal display element spacers, and if it exceeds 700 kgf/mm , the surface of liquid crystal orientation-control films can be easily damaged during the manufacture of liquid crystal display elements when used in liquid crystal display element spacers, and the spacers of the liquid crystal elements prepared as above hardly change by compression due to the shrinkage of liquid crystals at a reduced temperature and thus bubbles occur by a reduced pressure in liquid crystal cells. Also, if the recovery rate is less than 10 %, the portion to which excessive pressure is applied is not restored to a desired cell gap, and if exceeds 60 %, in case that they are used in liquid crystal display element spacers, the spacers deformed by compression are easily restored to their original configuration by elasticity when a gap between substrates is controlled by applying press during the manufacture of liquid crystal cells and subsequently the pressure is reduced and thus, the cell gap of the obtained liquid crystal cells become unsuitable.

[42] According to the method for the preparation of polymeric plastic beads of the invention as stated above, monodispersed beads can be synthesized with a high efficiency without using a stabilizer and without carrying out fractionation after polymerization, and the control of compressive modulus and recovery rate is possible. Fig. 2 shows polymeric plastic beads prepared by one embodiment of the present invention wherein the polymeric plastic beads of the invention are polymeric plastic beads of spherical shape having uniform size.

[43] Further, the invention provides conductive particles for anisotropic conductive connection comprising the polymeric plastic beads, and the conductive particles comprise preferably the polymeric plastic beads-coating layer of a conductive metal (metal layer) and more preferably, the polymeric plastic beads-coating layer of a conductive metal (metal layer) -insulative coating layer. For the coating layer (metal

layer) and insulative coating layer, any conventional coating layers and insulative coating layers applied to conductive particles for conductive connection can be applied and as a specific example, there can be mentioned a coating layer of Ni/ Au having the thickness of 0.1 to 1.0. Fig. 3 shows conductive particles prepared by forming a metal coating layer on the polymeric plastic beads of the invention and it can be seen that they are spherical particles of uniform size. The use of the conductive particles for anisotropic conductive connection in accordance with the invention can remarkably improve contact reliability in the conductive connection of fine semi-conductive elements.

[44] Further, the invention provides a liquid crystal display element comprising the polymeric plastic beads, wherein the liquid crystal display element can be prepared through a conventional method for the preparation of liquid crystal display elements which employs polymeric plastic beads as a spacer. In the liquid crystal display elements in accordance with the present invention, the orientation properties of liquid crystals by the damage of orientation-control films do not change, the quality of image is not degraded, and no irregularity of cell gaps is caused because of monodispersion.

[45] For better understanding of the present invention, preferred examples follow. The following examples are intended to illustrate the invention more fully without limiting the scope of the invention.

[46] EXAMPLES

[47] Example 1

[48] 1 % by weight of a monomer in which 20 parts by weight of methyl(meth)acrylate and 80 % by weight of trimethylomethane triacrylate were mixed and 0.05 % by weight of 2,2-azobis(2-methylbutanenitrile) were added to acetonitrile of remaining amount. After the mixture was poured into a glass bottle with a nitrogen purge system, to which nitrogen was purged for 2 to 3 min., it was polymerized at 70 ⁰ C for 24 hours while being stirred at the speed of 70 rpm in a water bath with an agitation function in left- to-right directions.

[49] After the polymerization was complete, the reactant precipitated in the glass bottle was filtered through centrifugation, washed with water and dried for 24 hours in a vacuum oven of the temperature of 70 - 80 ⁰ C thereby to prepare monodispersed beads of 4 um having a compressive modulus of 580 kgf/mm and a recovery rate of 54 %. The efficiency was 95 %.

[50] Example 2

[51] With the exception that an ultrasonic machine was used instead of the water bath with the agitation function in left- to-right directions in Example 1, the same method as Example 1 was carried out to prepare monodispersed beads of 3.5 um having a compressive modulus of 580 kgf/mm and a recovery rate of 54 %. The ultrasonic

machine was a product by Eltek Co., Ltd. and 20 to 30 hz was applied in the polymerization. The efficiency was 96 %.

[52] Example 3

[53] With the exception that a monomer mixture in which 10 parts by weight of methyl(meth)acrylate and 90 parts by weight of divinylbenzene were mixed was used as a monomer mixture in Example 1, the same method as Example 1 was carried out to prepare monodispersed beads of 4 um having a compressive modulus of 380 kgf/mm and a recovery rate of 38 %. The efficiency was 94 %.

[54] Example 4

[55] With the exception that 2,2-azobisisobutyronitrile was used instead of

2,2-azobis(2-methylbutanenitrile) as an initiator in Example 1, the same method as Example 1 was carried out to prepare monodispersed beads of 4.5 um having a compressive modulus of 580 kgf/mm and a recovery rate of 54 %. The efficiency was 94 %.

[56] Example 5

[57] With the exception that methanol was used instead of acetonitrile as a solvent in

Example 1, the same method as Example 1 was carried out to prepare monodispersed beads of 3.2 um having a compressive modulus of 580 kgf/mm and a recovery rate of 54 %. The efficiency was 96 %.

[58] Comparative Example 1

[59] With the same components as used in Example 1, beads were prepared by a fractionation following a suspension polymerization which is a prior method for the preparation of plastic beads, whereby monodispersed beads of 4 um having a compressive modulus of 438 kgf/mm and a recovery rate of 48 % were prepared. This method involves two steps, i.e., polymerization and fractionation, and the efficiency after fractionation was more or less than 1 % and it was very low. Industrial Applicability

[60] The method for the preparation of polymeric plastic beads in accordance with the present invention enables the synthesis of monodispersed beads with a high efficiency through a single process and it can freely their control compressive modulus and recovery rate in the preparation thereof and further it has excellent contact reliability, causes no change in the orientation properties of liquid crystal by the damage of orientation-control films, does not degrade the quality of image and does not cause the irregularity of cell gaps when applied to conductive particles for anisotropic conductive connection and liquid crystal display elements.