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1、Expanded polystyrene production process One introductionThe expanded polystyrene particle with functional skin layer according to claim 1, wherein the vinyl acetate based polymer includes a vinylacetate homopolymer, or a copolymer of vinyl acetate and at least one monomer selected from a group of co
2、nsisting of vinyl esters, acrylic esters, fumaric acid esters, carboxylic acids, vinyl alcohols, butadienes, caprolactones, and a mixture or a blend thereof, and wherein the vinyl acetate based polymer has a degree of polymerization (DP) of 10 to 100,000.TWO the process and technologhy The invention
3、 claimed is: An expanded polystyrene particle with a functional skin layer comprising: an inner expanded polystyrene layer; and a functional skin layer, wherein the inner expanded polystyrene layer is formed by heating and expanding an expandable polystyrene bead or pellet, and the functional skin l
4、ayer is formed by coating the surface of the inner expanded polystyrene layer with a functional coating composition having 10 to 99 wt % of a vinyl acetate based polymer and 0.1 to 90 wt % of at least one functional additive. The expanded polystyrene particle with functional skin layer according to
5、claim 1, wherein the functional skin layer has a thickness corresponding to 0.003 to 10% of the overall diameter of the particle, and has 1 to 95 wt % of the total weight of the particle. The expanded polystyrene particle according to claim 1, wherein the functional additive is selected from the gro
6、up consisting of expanding agents, nucleating agents, lubricants, antioxidants, heat stabilizers, ultraviolet stabilizers, biostabilizers, fillers, reinforcing agents, plasticizers, colorants, impact-resistant agents, flame retardants, antistatic agents, crosslinking agents, fluorescent whitening ag
7、ents, thermal conductivity-imparting agents, electrical conductivity-imparting agents, permeability modifiers, magnetism-imparting agents, surfactants, stabilizers, excipients, drugs, solvents, hardeners, desiccants, fortifying agents, flavoring agents, antibacterial agents, and mixtures thereof. A
8、functional expanded polystyrene molded product manufactured by molding the expanded polystyrene particle with functional skin layer according to claim 1. A process for producing expanded polystyrene particles with functional skin layer, comprising: heating and expanding expandable polystyrene beads
9、or pellets to produce expanded polystyrene particles; applying a functional coating composition to the surface of the expanded polystyrene particles, the functional coating composition being prepared by mixing or dissolving at least one functional additive with a vinyl acetate based polymer solution
10、 to form a functional skin layer; and adding a release agent to the expanded polystyrene particles whose surface is coated with the functional coating composition, to separate the expanded polystyrene particles having the functional skin layer into individual particles, and drying the separated part
11、icles. The process according to claim 6, wherein the solvent used to prepare the vinyl acetate based polymer solution is water, or an organic solvent selected from alcohols, esters, ketones, carboxylic acids, aromatics, and halogenated hydrocarbons, or mixtures thereof. The process according to clai
12、m 6, wherein the vinyl acetate based polymer solution includes 3 to 80 wt % of a vinyl acetate based polymer based on the total weight of the vinyl acetate based polymer solution. The process according to claim 6, wherein the release agent includes hydrophilic liquid materials having two or more hyd
13、roxyl groups (-OH) in their molecular structure, silicone oils, or mixtures thereof. The process according to claim 9, wherein the hydrophilic liquid materials includes water, ethylene glycol or glycerin. A process for manufacturing a functional expanded polystyrene molded product, comprising: intro
14、ducing the expanded polystyrene particles with functional skin layer produced by the process according to claims 6 into a steam molder; applying high-pressure steam to the molder to bond the expanded polystyrene particles with functional skin layer to each other through their functional skin layer;
15、and cooling the bonded expanded polystyrene particles. The expanded polystyrene particle with functional skin layer according to claim 2, wherein the vinyl esters includes vinyl caproate or vinyl stearate, the acrylic esters includes ethyl acrylate, butyl acrylate or octyl acrylate, the fumaric acid
16、 esters includes dibutyl maleate, the carboxylic acids includes maleic acid, acrylic acids, or itaconic acid. The expanded polystyrene particle with functional skin layer according to claim 2, wherein the content of the vinyl acetate monomer in the vinyl acetate based polymer is 55 mol % or more. De
17、scriptionTECHNICAL FIELD The present invention relates to expanded polystyrene molded products having various functions, including flame-retardation, antibacterial properties, coloration properties, etc. More particularly, the present invention relates to (pre-) expanded polystyrene particles having
18、 a functional skin layer produced by coating the surface of (pre-) expanded polystyrene particles with a functional coating composition, a process for producing the expanded polystyrene particles, functional expanded polystyrene molded products manufactured using the expanded polystyrene particles w
19、ith a functional skin layer, and a process for manufacturing the polystyrene molded products. BACKGROUND ART Expanded polystyrene is a polymer resin obtained by heating and curing the expandable polystyrene resin including expanding agent, such as pentane (C.sub.5H.sub.12) or butane (C.sub.4H.sub.10
20、) in polystyrene resin or its copolymer resins, to generate foams therein. Expanded polystyrene is white in color, light in weight, and is superior in water resistance, thermal insulation properties, sound absorption and buffering properties. Due to these advantages, expanded polystyrene resin is wi
21、dely used as a packaging material, a construction material, a material for floats, life jackets, decorative articles, insulators, living products, such as food containers and disposable products, and the like. More detailed explanation regarding a molding process of expanded polystyrene products wil
22、l be described below. A polystyrene or styrene copolymer resin containing an expanding agent in the shape of a bead having a diameter 0.2.about.0.3 mm or a pellet having a length of about 2.5 mm is pre-expanded to produce expanded particles (usually in the forms of beads). Thereafter, the expanded p
23、articles are aged and dried. The aged expanded particles are placed in a mold, heated with a high-pressure steam, and thus molded into a desired shape. The expanded polystyrene thus produced contains 98 vol % of air and only about 2 vol % of the polymer resin. Further, the expanded polystyrene has a
24、 unique expanded structure in which air is enclosed by plastic foams. Based on this structure, since the expanded polystyrene is lightweight and shows superior buffering properties, thermal insulation properties, and sound absorption, it can be widely utilized in many applications. However, since th
25、e expanded structure makes the expanded polystyrene more susceptible to some chemicals than the chemical resistance inherent to the polystyrene resin, the expanded structure is an obstacle in imparting a variety of functions upon the expanded polystyrene particles. That is, the addition of functiona
26、l components to expanded polystyrenes in order to impart additional functions to expanded polystyrene products presents difficulties that water resistance and heat resistance of the functional components, and moldability and chemical resistance of the expanded polystyrene must be comprehensively tak
27、en into account. Accordingly, functional expanded polystyrene molded products hitherto developed are very limited in the selection of the amount and kind of functional components within the range that does not affect physical and chemical properties during preparation of polystyrene. Particularly, s
28、ince expanded polystyrenes have relatively low melting point, and the molten polystyrene has low viscosity, molded products made of expanded polystyrene lose their structure and are melted easily by fire, thus being converted to a combustible liquid material. Under these circumstances, regulations i
29、ncluding the Fire Services Act, in most developed countries, restrict the use of expanded polystyrenes, which prohibits the development of the expanded polystyrenes as construction materials. In order to solve these problems, great efforts have been devoted to developing flame-retardant or fire-resi
30、stant expanded polystyrenes. For example, U.S. Pat. No. 6,384,094 discloses a process for preparing a fire-resistant expandable polystyrene by adding 1.about.12 wt % of an expandable graphite or an expanded graphite to a suspension containing a styrene monomer. In addition, the patent publication pr
31、oves the usefulness of the expandable polystyrene thus prepared. However, the expandable graphite begins to expand at about 900.degree. C., whereas the polystyrene leaves 3 wt % of ash behind at 500.degree. C. or above because of its extremely low heat resistance. The expandable graphite is expanded
32、 upon high-temperature heating to perform its thermal insulation, but the graphite loses its structure due to the absence of a material capable of binding the graphite, resulting in poor fire resistance. On the other hand, Korean Utility Model No. 323680 describes a flame-retardant expanded polystyr
33、ene panel manufactured by coating an aqueous sodium silicate solution on the surface of an expanded polystyrene particle, and irradiating microwaves to the coating to heat-bond the sodium silicate to the polystyrene. This technology has disadvantages in that the coating of the sodium silicate is poo
34、r in water resistance and expensive equipment is required for the microwave irradiation. DISCLOSURE OF THE INVENTION Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an expanded polystyrene that can be produced usin
35、g conventional molding equipment without the need for additional expensive equipment and developed into various functional products superior in flame retardation, fire resistance, antibacterial properties, waterproofness, aromatic properties, coloration properties, etc. The object of the present inv
36、ention can be achieved by coating the surface of expanded polystyrene particles with a functional coating composition to form a functional skin layer inside the particles, followed by molding. The functional coating composition used herein contains at least one functional additive, and a polyvinyl a
37、cetate resin with high affinity to the expanded polystyrene particles and low-temperature melt bonding property so as to be suitable for steam molding. In accordance with one aspect of the present invention, there are provided expanded polystyrene particles consisting of an inner expanded polystyren
38、e layer and a functional skin layer wherein the inner expanded polystyrene layer is formed by heating and expanding expandable polystyrene beads or pellets, and the functional skin layer is formed by coating the surface of the inner expanded polystyrene layer with a functional coating composition co
39、ntaining 10.about.99 wt % of a vinyl acetate based polymer and 0.1.about.90 wt % of at least one functional additive. In accordance with another aspect of the present invention, there is provided a process for producing the expanded polystyrene particles with a functional skin layer, comprising the
40、steps of: heating and expanding expandable polystyrene beads or pellets to produce expanded polystyrene particles; applying a functional coating composition to the surface of the expanded polystyrene particles, the functional coating composition being prepared by mixing or dissolving at least one fu
41、nctional additive with a vinyl acetate based polymer solution to form a functional skin layer; and adding a release agent to the expanded polystyrene particles whose surface is coated with the functional coating composition, to separate the expanded polystyrene particles having the functional skin l
42、ayer into individual particles, and drying the separated particles. In accordance with another aspect of the present invention, there is provided a process for manufacturing the functional expanded polystyrene molded product comprising the steps of introducing the expanded polystyrene particles havi
43、ng a functional skin layer into a molder, and applying high-pressure steam to the molder to bond the expanded polystyrene particles with each other. In accordance with yet another aspect of the present invention, there is provided a functional expanded polystyrene molded product made of the expanded
44、 polystyrene particles with functional skin layer according to the present invention. A more detailed explanation concerning the structure of the expanded polystyrene particles having a functional skin layer according to the present invention will be given below. The expanded polystyrene particles h
45、aving a functional skin layer are spherical particles having a skin-core structure wherein the overall diameter of the expanded polystyrene particles is between about 1 mm and about 30 mm, the diameter of the expanded polystyrene particles constituting the inner core layer is between about 0.999 mm
46、and about 29.999 mm, and the thickness of the outer skin layer is between about 1 .mu.m and about 100 .mu.m. The expanded polystyrene particles constituting the core layer account for 5.about.99 wt %, the functional skin layer accounts for 1.about.95 wt %, and other residues account for 5 wt % or le
47、ss, based on the total weight of the expanded polystyrene particles. The expanded polystyrene particles constituting the core layer include those previously known in the art. The components and production processes are as explained in the above prior art. On the other hand, the functional skin layer
48、 is formed by coating the surface of expanded polystyrene particles with a functional coating composition containing a vinyl acetate based polymer solution and at least one functional additive, followed by separating (to individual particles) and drying. The vinyl acetate based polymer solution is p
49、repared by dissolving 3.about.80 wt % of a vinyl acetate based polymer in a solvent. The vinyl acetate based polymer can be a vinylacetate homopolymer, or a copolymer of vinyl acetate and at least one monomer selected from: vinyl esters, such as vinyl caproate and vinyl stearate; acrylic esters, such as ethyl acrylate, butyl acrylate and octyl acrylate; fumaric acid esters, such as dibutyl maleate; carboxylic acids, such as maleic aci
