INORGANIC COATING COMPOSITION COMPRISING MAGNETITE COATED WITH CARBON AQUEOUS SOLUTION FOR SHIELDING ELECTROMAGNETIC WAVES Technical Field The present invention relates to inorganic coating compositions for shielding electromagnetic waves, which can be applied as an exterior coating on buildings and similar objects. More specifically, the present invention relates to an inorganic coating composition for shielding electromagnetic waves, composed mainly of an inorganic binder, a resin, and a conductive material, characterized in that magnetite coated with a carbon aqueous solution is used as the conductive material, whereby the shielding efficiency against electromagnetic waves is high, thus effectively shielding the electromagnetic waves exposed frequently.

Background Art In general, while electromagnetic waves pass through a medium, transmission, absorption and reflection take place. Such phenomena are affected by the kinds and shapes of the medium, in which the medium is divided into an absorbent and a reflector, depending on electromagnetic properties thereof.

The electromagnetic wave absorbent has absorbable regions more than reflectable regions. Further, the electromagnetic wave absorption is known to be largely influenced by conductive loss, dielectric loss or magnetic loss.

Commonly, the conductive loss is induced from conductive materials, such as metals, and the dielectric loss takes place from dielectric materials, including barium titanate. Also, the magnetic loss occurs from magnetic materials, such as ferrite. It is known that the electromagnetic wave absorption takes place by use of the above material selected property for desired purposes.

Japanese Patent Laid-open Publication No. Hei. 3-217466 discloses a noninflammable inorganic conductive coating composition exhibiting excellent

wetting resistance, comprising conductive powders and blended water-soluble silicate represented by M20-nSiO2 (M: Li, Na and K, n: mol ratio). As for the blended water-soluble silicate, Li salt and Na salt are essentially contained, and the water-soluble silicate has a mol composition of 0.40-0. 75 Li20 0. 25-0.60 (Na20- K2O) 3-3. 5SiO2 (provided that Na2O > K20, Li2O+Na2O+K20=1). In addition, the water-soluble silicate includes the aqueous solution amounting to 25-35 wt%, and a mixing ratio by weight of the conductive powders to the aqueous solution is 0.4-1. 0. The conductive powders are exemplified by silver powders, nickel, copper powders, carbon powders, titanium nitride powders, surface-treated powders thereof, inorganic powders of mica or sericite coated with positive lead metals. Preferably, copper powders and nickel powders are used.

Japanese Patent Laid-open Publication No. Sho. 62-153155 discloses an electromagnetic absorbing material, comprising 25 wt% of Portland blast furnace cement, 55 wt% of water-granulated slag containing graphite having 24 to 50% by volume to the water-granulated slag, and 20 wt% of an aqueous organic polymer emulsion of a liquid phase. However, the above patent is disadvantageous in terms of not having a very high shielding efficiency against electromagnetic waves, due to the use of only graphite as the conductive material.

Korean Patent No. 281697 discloses a conductive composite, comprising particulate carbon having a size of 0.05 to 7.5 mm, and fibrous carbon having a length of 2-40 mm, as a conductive material mixed at a weight ratio of 100: 0.5- 100: 140. Although the above composite is used as the conductive material and exhibits 1 to 10 Q-cm at a level allowable for electric applications, there are not particularly stated electromagnetic frequencies capable of being shielded.

Korean Patent Application No. 10-2001-00773372 discloses an inorganic coating composition for shielding electromagnetic waves of buildings, comprising a conductive material, a binder, a filler, a mixing agent, a resin and an additive, in which the conductive material having carbon powders with a particle size of 0.1 to 1.0 mm and magnetite powders with a particle size of 10 um to 0.5 mm and a ratio of thickness to diameter of 10 to 50, mixed at 1: 0.5-20, is used in the amount of 5 to 50 parts by weight, based on 100 parts by weight of the binder. However, the

used carbon powders have a specific gravity of 0.2 or less that is very quite different from that of other materials, such as the cement binder and the mixing agent, thus decreasing mixing uniformity in a preparation process. Further, since a coating material resulting from the above inorganic coating composition is a powder phase, carbon powders may be floated upon mixing with water on the spots.

Further, as for the magnetite powders and the carbon powders, the amount of the carbon powders depending on the use of the magnetite is limited, attributable to the difference of the specific gravity.

Disclosure of the Invention Therefore, it is an object of the present invention to solve the problems encountered in the related art and to provide an inorganic coating composition for shielding electromagnetic waves, suitable for use in exterior materials of buildings, which is advantageous in that conventional problems related to floating upon mixing of the coating composition with water and difference of specific gravity can be solved, and workability is improved upon mixing of the coating composition with water, by using magnetite coated with a carbon aqueous solution as a conductive material.

Best Mode for Carrying Out the Invention In the present invention, an inorganic coating composition for use in shielding electromagnetic waves includes a conductive material, a binder, a filler, a mixing agent, a resin and an additive, characterized in that magnetite powders coated with a carbon aqueous solution as the conductive material is used in an amount of 4 to 80 parts by weight, based on 100 parts by weight of the binder.

The carbon aqueous solution used to provide electroconductivity to the inorganic coating composition for shielding electromagnetic waves is composed of carbon black, an acrylic resin, and water. As such, in the carbon aqueous solution, carbon black amounts to 5 to 25 parts by weight, and the acrylic resin

amounts to at least 5 parts by weight. Preferably, carbon black is used in the amount of 10 to 15 parts by weight, and the acrylic resin is used in the amount of 8 to 12 parts by weight. If carbon black is used in the amount smaller than 5 parts by weight, shielding efficiency drastically decreases, and shielding function may be lost. Meanwhile, if carbon black is used in the amount exceeding 25 parts by weight, the carbon aqueous solution has high viscosity, and thus it is difficult to coat the magnetite powders, thereby negating economic benefits.

To coat the magnetite powders with the carbon aqueous solution, while the magnetite powders are stirred by use of a gravity-free mixer equipped with a liquid spraying system, the carbon aqueous solution is sprayed. After completion of the spraying, a stirring process is sufficiently performed. Then, the coated magnetite powders are removed from the gravity-free mixer, and a coated surface thereof is completely dried.

The magnetite powders coated with the carbon aqueous solution are preferably used in the amount of 4 to 80 parts by weight, based on 100 parts by weight of the binder. If the amount of the magnetite powders is less than 4 parts by weight, shielding efficiency drastically decreases. On the other hand, if the amount exceeds 80 parts by weight, economic benefits do not occur.

In the present invention, 2 to 10 parts by weight of the carbon aqueous solution is coated to the magnetite powders. The use of the carbon aqueous solution less than 2 parts by weight results in decreased shielding efficiency, whereas the use of the solution exceeding 10 parts by weight results in coagulated magnetite.

The binder for use in the inorganic coating composition to shield electromagnetic waves is selected from the group consisting of general portland cement, moderate heat cement, high early strength portland cement, super-high early strength portland cement, slag cement, fly ash cement, alumina cement, white cement, rapid hardening cement, expansive cement, high strength cement, and mixtures thereof.

In addition, the resin for use in the inorganic coating composition of the present invention is selected from the group consisting of acryls, vinyl acetates,

and SBR latexes, and is used in the amount of 5 to 50 parts by weight, based on 100 parts by weight of the binder. As such, when the resin is used in the amount less than 5 parts by weight, the resulting composition has a decreased adhesion strength. Further, it is impossible to manufacture a thin coating film due to the change of rheological properties. Whereas, if the amount is larger than 50 parts by weight, viscosity becomes higher and thus, bubbles occur. That is, needful workability is not ensured and a coating film cannot be formed, thus decreasing shielding properties.

Also, the filler is added to increase strength and workability, and is selected from the group consisting of calcium carbonate, siliceous earth, silica, alumina, and mixtures thereof. The filler is used in the amount of 30 to 200 parts by weight, based on 100 parts by weight of the binder.

Further, a fluidizing agent is added to help the dispersion of the conductive material mixed in the coating composition, and is exemplified by naphthalene-and melamine-based materials. 0.3 to 5 parts by weight of the fluidizing agent is used, based on 100 parts by weight of the binder.

Moreover, a thickening agent to control the viscosity of the composition and a defoaming agent to increase the uniformity of the coated surface are used in the amounts of 0.1 to 3 parts by weight and 0.1 to 5 parts by weight, respectively, based on 100 parts by weight of the binder.

In such a case, the filler, the thickening agent, the fluidizing agent, and the defoaming agent mentioned above, which are commonly added to the cement composition, are not particularly limited in the present invention.

A better understanding of the present invention may be obtained through the following examples and comparative examples which are set forth to illustrate, but is not to be construed as the limit of the present invention.

Examples 1 and 2 and Comparative Example 1 Portland cement, redispersible powders based on polyvinylacetate resin, calcium carbonate, siliceous earth, a melamine-based fluidizing agent, a thickening agent, a defoaming agent and magnetite powders were used according

to composition ratios shown in Table 1, below, to prepare an inorganic coating composition for shielding electromagnetic waves, which was then measured for shielding efficiency (ASTM D 4935), mixing time, and workability. The results are given in Table 2, below.

Measurement of Shielding Efficiency against Electromagnetic Wave The shielding efficiency with respect to electromagnetic waves was measured according to ASTM D 4935-99.

A coating process was performed to have a thickness of 0.5 mm, and the used frequency was in the range of 30 MHz to 1.5 GHz. Three days after the coating process was performed, measurement was carried out.

Determination of Mixing Time A mixing time (d) to have the same mixing uniformity was determined by use of a 20 L gravity-free mixer (Sejitech, Korea). The sampling time was at intervals of 15 sec.

Workability A travel time was measured using a Ford Cup &num 4 according to ASTM D 1200. The most desirable results were obtained in 40 to 50 sec based on the travel time. As the travel time was prolonged (high viscosity), water was further added in the amount exceeding a proper ratio to adjust the viscosity on the spots.

However, such additional water results in poor quality.

TABLE 1 Ex. No. C. Ex. No. 1 2 1 Carbon 15.0 20- Carbon Aqueous Acrylic 10. 0 5- Solution Resin Composition Water 75 75- Total 100 100- Coated Magnetite 40 50 35 (5 parts by weight coating) (non-coated magnetite) Cement 100 100 100 Composition Resin 20 15 20 Carbon Powders - - 35 Calcium Carbonate 45 45 45 Siliceous Earth 30 10 30 Additive10 510 TABLE 2 Ex. No. C. Ex. No. 1 2 1 Shielding Efficiency against 18-30 dB 20-34 dB 18-30 dB Electromagnetic Wave Mixing Time 15 sec 15 sec 2 min 30 sec Very Good Very Good Normal Workability (travel time: (travel time: (travel time: 45 sec) 42 sec) 65 sec)

Industrial Applicability As described hereinbefore, the present invention provides an inorganic coating composition for shielding electromagnetic waves, which is advantageous in terms of high mixing uniformity in a preparation process, non-floated carbon powders upon mixing with water on the spots, unlimited use of carbon powders based on the used magnetite, and increased workability upon mixing with water.

Although the preferred embodiments of the present invention have been disclosed 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.