WO2009066813A1

WO2009066813A1 - Expandable polystyrene bead including plate-shaped talc coated by resin and production method thereof

Info

Publication number: WO2009066813A1
Application number: PCT/KR2007/005932
Authority: WO
Inventors: Youn Chul Kim; Won Kyong Choe; Jong Hwan Lee; Dong Kwon Jung
Original assignee: Dongbu Hitek Co., Ltd.
Priority date: 2007-11-21
Filing date: 2007-11-23
Publication date: 2009-05-28
Also published as: KR100876211B1; DE112007003709B4; DE112007003709T5

Abstract

Disclosed herein is a method for preparing expandable polystyrene beads, which comprises: mixing platy talc with resin at high speed to coat the platy talc with the resin; and coating the resin-ooated platy talc on polystyrene beads or adding the resin-coated platy talc to a styrene polymerization process for preparing polystyrene beads. The expandable polystyrene beads prepared according to the disclosed method have excellent thermal insulation properties, because either the platy talc penetrated into the polystyrene beads or the platy talc coated on the outside of the polystyrene beads reflects or absorbs incident infrared radiation, thus achieving low thermal conductivity.

Description

EXPANDABLE POLYSTYRENE BEAD INCLUDING PLATE-SHAPED TALC COATED BY RESIN AND PRODUCTION METHOD THEREOF

Technical Field

[1] The present invention relates to expandable polystyrene beads. More particularly, the present invention relates to expandable polystyrene beads containing a platy talc coated with resin and to a preparation method thereof. Background Art

[2] Polystyrene resin, thermoplastic resin, has excellent molding processability, and thus is widely used in various industrial fields, including daily-use articles, electrical/ electronic products, packaging materials and building materials.

[3] Particularly, polystyrene foam produced by expanding polystyrene resin is widely used as thermal insulation materials.

[4] Generally, polystyrene foam is obtained by expanding polystyrene beads impregnated with a blowing agent.

[5] Thermal insulation materials must generally have low thermal conductivity so as to have high thermal insulation properties.

[6] In order to satisfy this requirement of thermal insulation materials, fluorine-based gas was used as a blowing agent in polystyrene beads in the prior art. However, such fluorine-based gas is currently not recommended, because it has problems in that it is the main cause of ozone layer depletion and is released from the polystyrene foam with the passage of time to gradually reducing the thermal insulation properties of the thermal insulation material.

[7] Also, in order to increase the indoor area of buildings, the thickness of thermal insulation materials is preferably thin. However, if the thickness of thermal insulation materials becomes thin, there is a problem in that thermal insulation performance thereof is rapidly reduced.

[8] Thus, it is required to prepare expandable polystyrene beads, which produce polystyrene foam, having high thermal insulation properties even at small thickness, without using fluorine-based gas.

[9] In accordance with such requirements, a method for preparing an expandable polystyrene polymer containing graphite particles is disclosed in, for example, Korean Patent Laid-Open Publication No. 2001-0012557.

[10] However, if this method of increasing thermal insulation properties by adding graphite particles or like is used, the additive added to the expandable polystyrene polymer makes the adhesion between pre-expanded particles difficult, such that low- quality foam can be produced. In addition, such graphite particles have a problem in that they enlarge or destroy the cellular structure of foam to cause fine pores, resulting in an increase in the moisture permeability of the foam. Furthermore, a thermal insulation material containing such graphite particles can act as a black body absorbing sunlight, leading to an increase in the temperature thereof, if it is disposed or placed in an outdoor environment so as to be exposed directly to sunlight. In this case, this increase in temperature can melt the expandable polystyrene polymer contained in the thermal insulation material. In addition, there is a problem in that the additive is not uniformly distributed in the foam, thus making it difficult to ensure uniform thermal insulation properties.

[11] Also, European Patent Publication No. 620246 discloses moldings produced from expanded polystyrene foam containing particulate materials for increasing thermal insulation properties (hereinafter referred to as "athermanous materials"), particularly carbon black, graphite and aluminum particles.

[12] Meanwhile, known methods for incorporating such athermanous materials into moldings include a method of coating the surface of pre-expanded polystyrene beads with athermanous materials, a method of introducing athermanous materials into unexpanded polystyrene beads.

[13] However, such methods have problems in that it is difficult to distribute athermanous materials uniformly in expanded foam, the adhesion of pre-expanded granules is significantly reduced to cause low-quality foam, and particles of athermanous materials peel off from the surface of moldings. Disclosure of Invention Technical Problem

[14] The present invention has been made in order to solve the above-described problems occurring in the prior art, and it is an object of the present invention to provide polystyrene beads coated with resin-coated platy talc, which is a novel athermanous material distinguishable from the prior athermanous materials, and a preparation method thereof, in which the polystyrene beads are used to prepare polystyrene resin foam having low thermal conductivity. [15] Another object of the present invention is to provide polystyrene beads, prepared by adding platy talc, which is a novel athermanous material distinguishable from the prior athermanous materials, to a styrene polymerization process, and a preparation method thereof, in which the polystyrene beads are used to prepare polystyrene resin foam having low thermal conductivity.

[16] Still another object of the present invention is to provide polystyrene foam which is prepared from polystyrene beads containing a platy talc coated with resin. Technical Solution

[17] To achieve the above objects, according to the present invention, platy talc is included in polystyrene beads. Accordingly, the inventive polystyrene beads can reflect incident infrared radiation by platy talc, thus showing low thermal conductivity, that is, excellent thermal insulation performance.

[18] In addition, in the present invention, platy talc is coated with resin before use, and thus the platy talc can be distributed uniformly on the outer surface or inside of polystyrene beads.

[19] Accordingly, the present invention allows incident infrared ray to be sufficiently reflected by platy talc distributed uniformly in polystyrene beads, thus ensuring low thermal conductivity, that is, high thermal insulation performance.

[20] According to one aspect of the present invention, there are provided expandable polystyrene beads, the outside of which is coated with a platy talc coated with resin, the polystyrene beads having a diameter of 0.3-3 mm.

[21] According to another aspect of the present invention, there are provided expandable polystyrene beads, in which resin-coated platy talc is distributed, the polystyrene having a diameter of 0.3-3 mm.

[22] According to still another aspect of the present invention, there is provided a method for preparing expandable polystyrene beads, the method comprising the steps of: mixing platy talc with resin at high speed to coat the platy talc with the resin; and coating polystyrene beads, having a diameter of 0.3-3 mm, with the resin-coated platy talc, wherein the content of the platy talc in the expandable polystyrene beads is 0.1-10 parts by weight based on 100 parts by weight of the expandable polystyrene beads.

[23] According to still another aspect of the present invention, there is provided a method for preparing expandable polystyrene beads, the method comprising the steps of: mixing platy talc with resin at high speed to coat the platy talc with the resin; and adding the resin-coated platy talc to a styrene polymerization process to prepare polystyrene beads having a diameter of 0.3-3 mm, wherein the content of the platy talc in the expandable polystyrene beads is 0.1-10 parts by weight based on 100 parts by weight of the expandable polystyrene beads.

[24] According to yet another aspect of the present invention, there is provided a polystyrene foam prepared by expanding the pre-expanded polystyrene beads.

[25] The platy talc that is used in the present invention can reflect incident infrared radiation to improve thermal insulation performance. The content of the platy talc in the polystyrene beads is preferably 0.1-10 parts by weight based on 100 parts by weight of the polystyrene beads.

[26] Such platy talc is not a black body such as graphite powder, but is a reflector. Thus, even if the platy talc is disposed or placed in an outdoor environment, there are no problems in that it absorbs incident sunlight to increase the temperature thereof and feels off from the surface of polystyrene beads.

[27] If the content of the platy talc is higher than 10 parts by weight based on 100 parts by weight the polystyrene beads, the amount of the platy talc is excessively large, and thus the platy talc is difficult to penetrate into the polystyrene bead particles, such that the dispersion stability of the platy talc can be deteriorated. On the other hand, if the content of the platy talc is lower than 0.1 parts by weight based on 100 parts by weight of the polystyrene beads, the dispersion stability of the platy talc is maintained, but the content is insufficient to reflect incident infrared radiation, and thus it cannot achieve desired thermal insulation performance.

[28] Talc is generally in a powder form, and thus the space between talc particles is large, such that the infrared radiation incident thereon is diffracted or refracted so as to be irregularly reflected in various directions without being reflected in a specific direction. For this reason, even if the powdery talc is contained in polystyrene beads, it has no meaningful effects on the thermal insulation performance of the polystyrene beads.

[29] Also, there are problems in that it is not easy to coat polystyrene beads with this powdery talc and in that the powdery talc does not easily penetrate into polystyrene beads, even though it is added to a styrene polymerization process.

[30] However, the platy talc that is used in the present invention has a platy structure having a particle length-to-thickness ratio of about 100-1000, such that it can reflect effectively the infrared radiation incident thereon in a single direction, thus reducing thermal conductivity and ensuring high thermal insulation performance. This platy talc of the present invention has a lamellar or platy structure, and such lamellar or platy talcs are all referred to as "platy talc" in the present invention.

[31] The platy talc that is used in the present invention has an average particle diameter of 5-50, and preferably 8-15 μm. If the particle diameter of the platy talc is excessively small, the platy talc cannot sufficiently reflect incident infrared radiation, and if the diameter is excessively large, the platy talc can be bent to reduce the effect of reflecting infrared radiation. [32] However, there is a problem in that this platy talc itself is not easily coated on the surface of polystyrene beads or does not easily penetrate into polystyrene beads. [33] For this reason, in the present invention, the platy talc is surface-treated with resin by using, for example, a coating process, such that the resin-coated platy talc can be easily impregnated or densely distributed in manufactured thermal insulation materials, and thus the thermal insulation materials can always show excellent thermal insulation properties regardless of the molded or cut shape thereof. [34] Specifically, when the platy talc is coated with resin as described above, the resin can increase the adhesion or distribution of the platy talc to polystyrene beads, such that the platy talc can adhere well to the surface of polystyrene beads and can penetrate uniformly into polystyrene beads. Accordingly, the content of the platy talc adhered to or contained in polystyrene beads can be increased. [35] Also, when the platy talc is coated with resin, the passage of heat loss can be prevented from occurring due to the contact between the platy talc and the beads. [36] In addition, when the platy talc is coated with resin, it does not cause damage to the surface of polystyrene beads, such that deterioration in foam quality can be prevented. [37] Methods for preparing polystyrene beads containing the resin-coated platy talc of the present invention can be largely classified into a coating method and a polymerization method.

[38] First, the coating method comprises the steps of:

[39] mixing platy talc with resin at high speed to prepare a platy talc coated with resin; and [40] adding 0.1- 10 parts by weight, based on 100 parts by weight of polystyrene beads, of the resin-coated platy talc to a polystyrene bead coating process. [41] The method of coating polystyrene beads with the platy talc comprises mixing polystyrene beads with the platy talc at high speed in a super mixer or a screw mixer. [42] The talc that is used in the present invention preferably has a platy structure. The platy talc has no limitation on the shape thereof and may be a polygonal such as triangular or rectangular, or circular.

[43] The platy talc has an average particle size of 5-50 μm, and preferably 8-15 μm.

[44] The length-to-thickness ratio (aspect ratio) of the platy talc preferably ranges from 100 to 1000. If the aspect ratio is less than 100, the platy structure is small, such that it cannot sufficiently incident infrared radiation, and if it is exceeds 1000, the platy structure is large, but it can be bent to reduce the effect of reflecting incident infrared radiation.

[45] Herein, the resin that is used to coat the platy talc is not specifically limited, as long as it can be adsorbed onto the platy talc to reduce the thermal conductivity of the platy talc. Polypropylene wax, polyethylene wax or a low-molecular- weight polystyrene having a weight- average molecular weight of 500-1000 is preferably used as the resin.

[46] In the present invention, the polystyrene beads can consist only of polystyrene or may be a copolymer of polystyrene with 20 parts by weight, based on 100 parts by weight of the polystyrene beads, of an ethylenically unsaturated comonomer, particularly alkylstyrene, divinylbenzene, acrylonitrile or α-methylstyrene.

[47] The polystyrene beads of the present invention can be prepared by, but not limited to, a styrene suspension polymerization process.

[48] Such polystyrene beads of the present invention have a diameter of 0.3-3 mm, and preferably 0.35-2 mm.

[49] Also, the content of the platy talc coated on polystyrene beads is preferably 0.1- 10 parts by weight based on 100 parts by weight of the polystyrene beads.

[50] Meanwhile, the polystyrene beads may further contain graphite, carbon, aluminum, zinc and the like in an amount of 0.5-2 parts by weight, and preferably 0.5-1 part by weight, in order to increase the thermal insulation and flame-retardant properties thereof.

[51] Also, such polystyrene beads may be additionally coated with a conventional coating agent. Such a coating agent may be, for example, metal stearate, glyceryl ester or fine powder silicate.

[52] Next, the polymerization method comprises the steps of: mixing platy talc with resin at high speed to prepare a resin-coated platy talc; and adding the resin-coated platy talc to a styrene polymerization process for preparing polystyrene beads.

[53] Because the preparation of the resin-coated platy talc is the same as in the above- described coating method, the detailed description thereof will be omitted herein.

[54] By coating the platy talc with resin, the amount of platy talc added in the process of coating the polystyrene beads with the platy talc or in the styrene polymerization process is dramatically increased. Also, if the platy talc that is used in the present invention consists of thinner, wider platy particles, the effect thereof can be maximized. [55] Because the polystyrene beads that are used in the present invention are the same as in the above-described coating method, except that the platy talc is added in the styrene polymerization process, the detailed description thereof will be omitted herein.

[56] Specifically, in the styrene polymerization process for preparing the polystyrene beads, the amount of addition of the resin-coated platy talc is 0.1- 10 parts by weight based on 100 parts by weight of the polystyrene beads.

[57] To increase the thermal insulation and flame-retardant properties of the polystyrene beads, thermal insulation-promoting additives, such as graphite, carbon, aluminum and zinc, may be added during the styrene polymerization process at a polymerization conversion of more than 70% in an amount of 0.5-2 parts by weight, and preferably 0.5-1 part by weight, based on 100 parts by weight of the polystyrene beads. Then, the thermal insulation-promoting additives are located outside the polystyrene beads, and thus incident infrared light can be prevented from being passed therethrough by diffraction or refraction, since incident infrared light is not reflected or scattered into the space between the platy talc particles in the polystyrene beads, leading to an increase in the thermal insulation performance of the polystyrene beads.

[58] Meanwhile, the above-described polymerization process can be carried out by, but not limited to, a conventional process of polymerizing styrene.

[59] In this styrene suspension polymerization process, a suspension stabilizer may additionally be used. As the suspension stabilizer, an inorganic pickering dispersant such as magnesium pyrophosphate or calcium phosphate, or an organic dispersant such as a polyvinylpyrrolidone (PVP), polyvinylalcohol (PVA) or polycellulose-based dispersant, is advantageously used.

[60] When the platy talc is included in the polystyrene beads through such a polymerization method, the excellent stability of the polystyrene beads with time can be expected, because the platy talc is contained in the polystyrene beads, and thus the polystyrene beads are stably maintained even with the passage of time. Also, the uniform thermal insulation performance of the polystyrene beads can be ensured, because the platy talcs are distributed uniformly in the polystyrene beads and are not detached from the polystyrene beads during the processing of the beads.

[61] In addition, the melt- adhesion between the polystyrene beads becomes easy, such that the preparation of polystyrene foam can be performed in an easier manner. Specifically, because the talc is distributed in the polystyrene beads, the adhesion of the pre-expanded beads is not problematic

[62] Meanwhile, in the styrene polymerization process for preparing polystyrene beads, a blowing agent may further be added in an amount of 3-10 parts by weight based on 100 parts by weight of the polystyrene beads. The blowing agent may be added before, during or after the polymerization. As the blowing agent, an aliphatic hydrocarbon having 4-6 carbon atoms is preferably used and examples thereof may include iso- butane, n-butane, iso-pentane and n-pentane.

[63] The polystyrene beads of the present invention may further comprise, in addition to the blowing agent, conventional additives, for example, a peroxide initiator, a chain transfer, a blowing aid, a nucleating agent and a plastiάzer.

[64] For the flame-retardant properties of thermal insulation materials, the polystyrene beads of the present invention may further comprise a flame-retardant agent or a flame-retardant synergist. The flame-retardant agent is contained in an amount of 0.6-6 parts by weight based on 100 parts by weight of the polystyrene beads, and the flame retardant synergist is contained in an amount of 0.1-1 part by weight based on 100 parts by weight of the polystyrene beads.

[65] Preferred examples of the flame-retardant agent that is used in the present invention include aliphatic, cycloaliphatic and aromatic bromine compounds, such as hexabro- mocyclododecane, pentabromomonochlorocyclohexane and pentabromopheny- lallylether. As the flame retardant synergist, a labile organic compound containing a C- C- or O-O- functional group, for example, bicumyl or dicumyl peroxide, is preferably used.

[66] After the polymerization process for preparing the polystyrene beads, a step of coating the polystyrene beads with a conventional coating agent, such as metal stearate, glyceryl ester or fine powder silicate, may preferably additionally be carried out.

[67] In addition, a step of pre-expanding the polystyrene beads may additionally be carried out.

[68] The polystyrene beads, obtained by coating them with the platy talc or adding the platy talc during the polymerization process as described above, can be expanded to form polystyrene foam.

[69] The method for expanding the polystyrene beads can be carried out using a conventional expansion method. If the foaming rate is excessively high, the thermal conductivity of the foam is increased, and if the foaming rate is excessively low, polystyrene is used in an excessively large amount, thus causing a problem in economic terms. The polystyrene foam thus formed has a number of expanded pores, in which the platy talc can be located. [70] The platy talc contained in the foam reduces thermal conductivity by reflecting or absorbing incident infrared radiation. Particularly, because the platy talc in the present invention has a platy structure, it has significantly increased infrared radiation reflectivity.

[71] Meanwhile, a method for preparing polystyrene foam using the polystyrene beads of the present invention may comprise pre-expanding the prepared polystyrene beads, and then expanding the pre-expanded polystyrene beads.

Advantageous Effects

[72] As described above, in the polystyrene beads according to the present invention, the platy talc penetrated into the polystyrene beads can reflect or absorb incident infrared radiation to achieve low thermal conductivity, that is, excellent thermal insulation performance.

[73] Also, resin coated on the platy talc does not interfere with the adhesion between polystyrene foam granules, and the platy talc allows the polystyrene foam to show excellent flame retardancy.

[74] The inventive expandable polystyrene beads, prepared through the styrene polymerization process and containing the platy talc, has an advantage in that the stability thereof with time is maintained even with the passage of time.

[75] In addition, the polystyrene beads of the present invention can ensure uniform thermal insulation performance, because the platy talc in the polystyrene beads is distributed uniformly and is not separated from the beads during the processing of the beads. Best Mode for Carrying Out the Invention

[76] Hereinafter, the present invention will be described in detail with reference to examples. It is to be understood, however, that these examples are illustrative only, and the scope of the present invention is no limited thereto.

[77] Example 1

[78] A: Preparation of platv talc coated with resin

[79] Talc ore was placed in a ball mill and ball-milled in the presence of compressed air and steam for 20 hours, and 15 kg of platy talc having an average particle diameter of 8-15 /M and a length-to-thickness ratio (aspect ratio) of 100-1000 was selected from among the ball-milled talc

[80] The obtained platy talc was placed in a super mixer KAWATA SMV 20 together with 5 kg of resin, polyethylene wax and mixed with the resin at a high speed of 600 RPM, thus obtaining platy talc coated with the resin.

[81] B: Introduction of platy talc into polystyrene beads by polymerization method

[82] 38 kg of pure water as a dispersant was added in a 0.1 m reactor, 137 g of tricalάum phosphate as a primary stabilizer was suspended therein, and 36 kg of styrene was added thereto. Then, the polymerization of styrene was initiated.

[83] While the content in the reactor was heated to 88 ⁰C, 105 g of benzoyl peroxide as an initiator and 240 g of hexabromocyclododecane as a flame-retardant agent were added thereto, and the polymerization process was continued.

[84] Then, 370 g, based on 1 kg of styrene, of the above-obtained resin-coated platy talc was dissolved for 30 minutes and introduced into the reactor at 1 hour after the start of the polymerization.

[85] Then, 39 g of secondary stabilizer, polyvinylalcohol, already dissolved in 390 g of pure water, was introduced into the reactor at 3 hours after the start of the polymerization.

[86] 2.95 kg of n-pentane as a blowing agent was introduced into the reactor at 6 hours after the start of the polymerization, and the content in the reactor was heated to 123 0C. The content in the reactor was maintained at that temperature for 2 hours and 30 minutes in order to remove unreacted styrene monomer, and then it was discharged, dewatered and dried, thus obtaining desired expandable polystyrene beads containing the resin-coated platy talc therein.

[87] C: Preparation of polystyrene foam

[88] The expandable polystyrene beads thus obtained were placed in a 1000 L pre- expander (Sunghoon Machinery Co., Korea) and applied with 0.32 kg/cm² of steam for 28 seconds to pre-expand the beads to a specific gravity of 15.6 g/1.

[89] After 4 hours, the pre-expanded expandable polystyrene beads were placed in a

Kurtz pad molding machine and applied with 0.6 kg/cm² of primary steam for 5 seconds, and then with 0.75 kg/cm² of secondary steam for 5 seconds, thus obtaining the desired foam product.

[90] Example 2

[91] D: Coating of polystyrene beads with platy talc, by coating method

[92] 7 kg of expandable polystyrene beads (F351, Dongbu Hitek Co. Ltd., Korea) and 70 g of the resin-coated platy talc prepared in Example 1 were placed in a KAWATA 2OL super mixer and rotated at a high speed of 600 rpm for 7 minutes, thus coating the expandable polystyrene beads with the platy talc

[93] E: Preparation of polystyrene bead [94] The coated expandable polystyrene beads were placed in a IOOOL pre-expander

(Sunghoon Machinery Co., Korea) and applied with 0.32 kg/cm² of steam for 33 seconds to expand the beads to a specific gravity of 14.7 g/1. After 4 hours, the pre- expanded expandable polystyrene particles were placed in a Kurtz pad molding machine and applied with 0.6 kg/cm² of primary steam for 5 seconds, and then with 0.75 kg/cm² of secondary steam for 5 seconds, thus obtaining the desired foam product.

[95] Comparative Example 1

[96] Expandable polystyrene beads (Dongbu Hitek Co. Ltd., Korea) were placed in a

IOOOL pre-expander (Sunghoon Machinery Co., Korea) and applied with 0.32 kg/cm² of steam for 28 seconds to pre-expand the beads to a specific gravity of 14.8 g/1.

[97] After 4 hours, the pre-expanded expandable polystyrene beads were placed in a

Kurtz pad molding machine and applied with 0.6 kg/cm² of primary steam for 5 seconds, and then with 0.75 kg/cm² of steam for seconds, thus obtaining the desired foam product.

[98] Comparative Example 2

[99] Platy talc was obtained in the same manner as in Example 1, except that the talc was not coated with resin.

[100] Expandable polystyrene beads were prepared in the same manner as in Example 1, except that 370 g of the obtained platy talc was used in the polymerization reaction.

[101] The prepared expandable polystyrene beads were placed in a IOOOL pre-expander (Sunghoon Machinery Co., Korea) and applied with 0.32 kg/cm² of steam for 29 seconds to pre-expand the beads to a specific gravity of 16.1 g/1.

[102] After 4 hours, the pre-expanded expandable polystyrene beads were placed in a Kurtz pad molding machine and applied with 0.6 kg/cm² of primary steam for 5 seconds, and then with 0.75 kg/cm² of secondary steam for 5 seconds, thus obtaining the desired foam product.

[103] Comparative Example 3

[104] Talc ore was placed in a ball mill and ball-milled in the presence of compressed air and steam for about 20 hours, and 15 kg of powdery talc having an average particle diameter of 8-15 μm and a powder shape was selected from among the ball-milled talc.

[105] The obtained powdery talc was placed in super mixer KAWATA SMV 20 together with 5 kg of resin, polyethylene wax and mixed with the resin at a high speed of 600 RPM, thus obtaining powdery talc coated with the resin.

[106] Expandable polystyrene beads were prepared in the same manner as in Example 1, except that 370 g of the obtained powdery talc was used in the polymerization reaction.

[107] The obtained expandable polystyrene beads were placed in a IOOOL pre-expander (Sunghoon Machinery Co., Korea) and applied with 0.32 kg/cm² of steam for 26 seconds to pre-expand the beads to a specific gravity of 15.4 g/1.

[108] After 4 hours, the pre-expanded expandable polystyrene beads were placed in a

Kurtz pad molding machine and applied with 0.6 kg/cm² of primary steam for 5 second, and then with 0.75 kg/cm² of secondary steam for 5 seconds, thus obtaining the desired foam product.

[109] Comparative Example 4

[110] Talc ore was placed in a ball mill and ball-milled in the presence of compressed air and steam for about 20 hours, and powdery talc having an average particle diameter of 8-15 μm and a powder shape was selected from among the ball-milled talc

[111] Expandable polystyrene beads were prepared in the same manner as in Example 1, except that 370 g of the obtained powdery talc was used in the polymerization reaction.

[112] The obtained expandable polystyrene beads were placed in a IOOOL pre-expander (Sunghoon Machinery Co., Korea) and applied with 0.32 kg/cm² of steam for 32 seconds to pre-expand the beads to a specific gravity of 14.1 g/1.

[113] After 4 hours, the pre-expanded expandable polystyrene beads were placed in a Kurtz pad molding machine and applied with 0.6 kg/cm² of primary steam for 5 seconds, and then with 0.75 kg/cm² of secondary steam for 5 seconds, thus obtaining the desired foam product.

[114] Test Example

[115] The polystyrene foam products, prepared in Examples 1 and 2 and Comparative Examples 1 to 4, were dried at 60 ⁰C for 24 hours, and then cut with a hot wire to a size of 200 x 200 x 50 mm, thus preparing samples for the measurement of thermal conductivity.

[116] The prepared samples were left to stand in a dry chamber at 60 ⁰C for 48 hours and taken out from the dry chamber, and the thermal conductivity of the samples was measured with a thermal conductivity analyzer (Anaoon TCA8). The measurement results are shown in Table 1 below.

[117] Table 1 [Table 1] [Table ]

[118] As can be seen in Table 1, the present inventive polystyrene foam of Example 1, prepared by introducing the platy talc in the polymerization process, and the present inventive polystyrene foam of Example 2, prepared by coating the platy talc on the polystyrene beads, showed thermal conductivities significantly lower than that of the conventional polystyrene foam product of Comparative Example 1.

[119] Also, the present inventive polystyrene foam of Example 1, prepared by introducing the resin-coated platy talc in the polymerization process, showed a thermal conductivity significantly lower than that of the polystyrene foam of Comparative Example 2, prepared using the platy talc not coated with resin.

[120] Furthermore, the present inventive polystyrene foam of Example 1, prepared by introducing the platy talc in the polymerization process, showed a thermal conductivity significantly lower than that of the polystyrene foam of Comparative Example 3, prepared using the powdery talc

[121] In addition, the polystyrene foam of Comparative Example 4, which was not coated with resin and was prepared using the powdery talc, which was not platy, had a thermal conductivity lower than that of the conventional polystyrene foam of Comparative Example 1, but showed a thermal conductivity higher than those of the polystyrene foams of Example 1 and Comparative Examples 2 and 3.

[122] From the above test results, it can be seen that the use of the platy talc coated with resin showed the lowest thermal conductivity.

[123] Although the preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Industrial Applicability As described above, the inventive polystyrene foam, prepared from the polystyrene beads prepared through the inventive method, has low thermal conductivity and, at the same time, excellent stability with time. Thus, the polystyrene foam can be widely used as thermal insulation materials, such as materials for buildings.

Claims

[I] Expandable polystyrene beads, the outside of which is coated with a platy talc coated with resin, the polystyrene beads having a diameter of 0.3-3 mm.

[2] Expandable polystyrene beads, in which a platy talc coated with resin is distributed, the polystyrene beads having a diameter of 0.3-3 mm.

[3] The expandable polystyrene beads of Claim 1 or 2, wherein the resin is polypropylene wax, polyethylene wax or a low-molecular-weight polystyrene having a weight- average molecular weight of 500-1000.

[4] The expandable polystyrene beads of Claim 1 or 2, wherein the content of the platy talc in the expandable polystyrene beads is 0.1-10 parts by weight based on 100 parts by weight of the expandable polystyrene beads.

[5] The expandable polystyrene beads of Claim 4, wherein the platy talc has an average diameter of 5-50 /M.

[6] The expandable polystyrene beads of Claim 4, wherein the platy talc has a length-to-thickness ratio (aspect ratio) of 100-1000.

[7] A method for preparing polystyrene beads, the method comprising the steps of: mixing platy talc with resin at high speed to coat the platy talc with the resin; and coating polystyrene beads, having a diameter of 0.3-3 mm, with the resin-coated platy talc

[8] A method for preparing polystyrene beads, the method comprising the steps of: mixing platy talc with resin at high speed to coat the platy talc with the resin; and adding the resin-coated platy talc to a styrene polymerization process to prepare polystyrene beads having a diameter of 0.3-3 mm.

[9] The method of Claim 7 or 8, wherein the resin is polypropylene wax, polyethylene wax or a low-molecular- weight polystyrene having a weight- average molecular weight of 500-1000.

[10] The method of Claim 7 or 8, wherein, in addition to the platy talc, 0.5-2 parts by weight of one or more athermanous materials selected from among graphite, carbon, aluminum and zinc are additionally added.

[I I] The method of Claim 7 or 8, wherein the content of the platy talc in the expandable polystyrene beads is 0.1-10 parts by weight based on 100 parts by weight of the expandable polystyrene beads.

[12] The method of Claim 11, wherein the platy talc has an average diameter of 5-50

/M. [13] The method of Claim 11, wherein the platy talc has a length-to-thickness ratio

(aspect ratio) of 100- 1000. [14] The method of Claim 11, additionally comprising pre-expanding the expandable polystyrene beads. [15] The method of Claim 11, wherein the mixing of the platy talc with the resin is carried out in a super mixer or a screw mixer. [16] The method of Claim 8, wherein a blowing agent, is further added in the styrene polymerization step in amount of 3-10 parts by weight based on 100 parts by weight of the polystyrene beads. [17] The method of Claim 8, wherein 0.6-6 parts by weight of a flame-retardant agent and 0.1-1 part by weight of a flame-retardant synergist are further added in the styrene polymerization step, the parts by weight being based on 100 parts by weight of the polystyrene beads. [18] The method of Claim 8, wherein one or more suspension stabilizers selected from among magnesium pyrophosphate, calcium phosphate, polyvinylpyrrolidone (PVP), polyvinylaloohol (PVA) and polycellulose-based dispersants are further added in the styrene polymerization step. [19] The method of Claim 8, additionally comprising a step of coating the prepared polystyrene beads with metal stearate, glyceryl ester or fine powder silicate. [20] Polystyrene foam which is prepared by expanding the pre-expanded polystyrene beads of Claim 14.