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1、 Expanded polystyrene production process One introduction The 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

2、 of consisting 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

3、 The invention 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 t

4、he functional skin layer 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 function

5、al skin layer according to 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 functiona

6、l additive is selected from the group 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, crosslinki

7、ng agents, fluorescent whitening agents, thermal conductivity-imparting agents, electrical conductivity-imparting agents, permeability modifiers, magnetism-imparting agents, surfactants, stabilizers, excipients, drugs, solvents, hardeners, desiccants, fortifying agents, flavoring agents, antibacteri

8、al agents, and mixtures thereof. A 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: he

9、ating 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 functional add

10、itive 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 layer into in

11、dividual particles, and drying the separated particles. 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 hydrocar

12、bons, or mixtures thereof. The process according to claim 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 rel

13、ease agent includes hydrophilic liquid materials having two or more hydroxyl 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 proce

14、ss for manufacturing a functional expanded polystyrene molded product, comprising: introducing 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

15、particles with functional skin layer to each other through their functional skin layer; 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

16、, the acrylic esters includes ethyl acrylate, butyl acrylate or octyl acrylate, the fumaric acid 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, whe

17、rein the content of the vinyl acetate monomer in the vinyl acetate based polymer is 55 mol % or more. Description TECHNICAL FIELD The present invention relates to expanded polystyrene molded products having various functions, including flame-retardation, antibacterial properties, colorati

18、on properties, etc. More particularly, the present invention relates to (pre-) expanded polystyrene particles having 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 polystyren

19、e particles, functional expanded polystyrene molded products manufactured using the expanded polystyrene particles with 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 c

20、uring the expandable polystyrene resin including expanding agent, such as pentane (C.sub.5H.sub.12) or butane (C.sub.4H.sub.10) 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, therm

21、al insulation properties, sound absorption and buffering properties. Due to these advantages, expanded polystyrene resin is widely used as a packaging material, a construction material, a material for floats, life jackets, decorative articles, insulators, living products, such as food containers and

22、 disposable products, and the like. More detailed explanation regarding a molding process of expanded polystyrene products will 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 hav

23、ing a length of about 2.5 mm is pre-expanded to produce expanded particles (usually in the forms of beads). Thereafter, the expanded particles 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

24、 polystyrene thus produced contains 98 vol % of air and only about 2 vol % of the polymer resin. Further, the expanded polystyrene has a unique expanded structure in which air is enclosed by plastic foams. Based on this structure, since the expanded polystyrene is lightweight and shows superior buff

25、ering properties, thermal insulation properties, and sound absorption, it can be widely utilized in many applications. However, since the expanded structure makes the expanded polystyrene more susceptible to some chemicals than the chemical resistance inherent to the polystyrene resin, the expande

26、d structure is an obstacle in imparting a variety of functions upon the expanded polystyrene particles. That is, the addition of functional components to expanded polystyrenes in order to impart additional functions to expanded polystyrene products presents difficulties that water resistance and h

27、eat resistance of the functional components, and moldability and chemical resistance of the expanded polystyrene must be comprehensively taken into account. Accordingly, functional expanded polystyrene molded products hitherto developed are very limited in the selection of the amount and kind of fun

28、ctional components within the range that does not affect physical and chemical properties during preparation of polystyrene. Particularly, since expanded polystyrenes have relatively low melting point, and the molten polystyrene has low viscosity, molded products made of expanded polystyrene lose

29、their structure and are melted easily by fire, thus being converted to a combustible liquid material. Under these circumstances, regulations including the Fire Services Act, in most developed countries, restrict the use of expanded polystyrenes, which prohibits the development of the expanded polyst

30、yrenes as construction materials. In order to solve these problems, great efforts have been devoted to developing flame-retardant or fire-resistant expanded polystyrenes. For example, U.S. Pat. No. 6,384,094 discloses a process for preparing a fire-resistant expandable polystyrene by addin

31、g 1.about.12 wt % of an expandable graphite or an expanded graphite to a suspension containing a styrene monomer. In addition, the patent publication proves the usefulness of the expandable polystyrene thus prepared. However, the expandable graphite begins to expand at about 900.degree. C., wherea

32、s 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 upon high-temperature heating to perform its thermal insulation, but the graphite loses its structure due to the absence of a material capable of

33、 binding the graphite, resulting in poor fire resistance. On the other hand, Korean Utility Model No. 323680 describes a flame-retardant expanded polystyrene panel manufactured by coating an aqueous sodium silicate solution on the surface of an expanded polystyrene particle, and irradiating

34、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 poor in water resistance and expensive equipment is required for the microwave irradiation. DISCLOSURE OF THE INVENTION Therefore,

35、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 using conventional molding equipment without the need for additional expensive equipment and developed into various functional produ

36、cts superior in flame retardation, fire resistance, antibacterial properties, waterproofness, aromatic properties, coloration properties, etc. The object of the present invention can be achieved by coating the surface of expanded polystyrene particles with a functional coating composition to

37、 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 acetate resin with high affinity to the expanded polystyrene particles and low-temperature melt bonding property so as to

38、 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 polystyrene layer and a functional skin layer wherein the inner expanded polystyrene layer is formed by heating and expanding ex

39、pandable 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 containing 10.about.99 wt % of a vinyl acetate based polymer and 0.1.about.90 wt % of at least one functional additive.

40、 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 steps of: heating and expanding expandable polystyrene beads or pellets to produce expanded polystyrene particles;

41、 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 to form a functional skin layer; and adding a re

42、lease 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 particles. In accordance with another aspect of the

43、 present invention, there is provided a process for manufacturing the functional expanded polystyrene molded product comprising the steps of introducing the expanded polystyrene particles having a functional skin layer into a molder, and applying high-pressure steam to the molder to bond the expande

44、d 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 polystyrene particles with functional skin layer according to the present invention. A more detailed ex

45、planation 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 having a functional skin layer are spherical particles having a skin-core structure wherein the overall d

46、iameter 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 and about 29.999 mm, and the thickness of the outer skin layer is between about 1 .mu.m and about 100 .m

47、u.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 less, based on the total weight of the expanded polystyrene particles. The expanded polystyrene particle

48、s 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 is formed by coating the surface of expanded polystyrene particles with a functional coating compos

49、ition 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 prepared by dissolving 3.about.80 wt % of a vinyl acetate based polymer in a solvent. The vinyl aceta

50、te 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

51、; carboxylic acids, such as maleic acid, acrylic acids and itaconic acid; vinyl alcohols; butadienes; and caprolactones; or a mixture or a blend thereof. The vinyl acetate based polymer has a degree of polymerization (DP) of 10.about.100,000. It is preferred that the content of the vinyl acetate mon

52、omer in the vinyl acetate based polymer is 55% or more. Examples of suitable solvents to prepare the vinyl acetate based polymer solution include water, and organic solvents such as alcohols, esters, ketones, carboxylic acids, aromatics, and halogenated hydrocarbons. It is preferred to select a so

53、lvent having relatively high volatility and reduced human toxicity, particularly, minimally dissolving or eroding the surface of expanded polystyrene particles. In this respect, alcohols are preferred. The concentration of the resin component is determined within the range defined above, taking into

54、 account the adhesion to the surface of the expanded polystyrene particles and workability upon handling. The functional additive can be added for intended functions without particular limitations so long as it is readily dissolved or dispersed in the vinyl acetate based polymer solution and does

55、not erode or dissolve the surface of the expanded polystyrene particles. Examples of such additives include expanding agents, nucleating agents, lubricants, antioxidants, heat stabilizers, ultraviolet stabilizers, biostabilizers, fillers, reinforcing agents, plasticizers, colorants, impact-resistant

56、 agents, flame retardants, antistatic agents, cross-linking agents, fluorescent whitening agents, thermal conductivity-imparting agents, electrical conductivity-imparting agents, permeability modifiers, magnetism-imparting agents, surfactants, stabilizers, excipients, drugs, solvents, hardeners, des

57、iccants, fortifying agents, flavoring agents, antibacterial agents, etc. These additives may be added alone or in combination as a mixture of two or more agents compatible with each other. In particular, when a flame retardant, such as antimony trioxide, a phosphoric compound, boron, boric acid or

58、 aluminum oxide, is added, a kind of fire wall surrounding the surface of the expanded polystyrene particles is built, which prevents the spread of a fire to sites not directly reached by the flame and maintains the overall shape of the skin layer acting as a skeleton unchanged, leading to a very ef

59、fective flame-retardant expanded polystyrene product. The vinyl acetate based polymer solution containing the functional additive is uniformly applied to the surface of the expanded polystyrene particles. At this step, if the viscosity of the polymer solution is low, it is advantageous to stir the

60、 expanded polystyrene particles while the polymer solution is sprayed on the surface of the particles. Meanwhile, if the polymer solution is highly viscous, the expanded polystyrene particles can be mixed with the polymer solution in a mixer with stirring to apply the polymer solution to the surface

61、 of the particles. The application of the vinyl acetate based polymer solution containing the functional additive to the expanded polystyrene particles may cause the polystyrene particles to be agglomerated due to the viscosity of the polymer solution, which makes it difficult to manufacture a m

62、olded product from the expanded polystyrene particles. Therefore preferably, the polymer solution is uniformly applied to the surface of the expanded polystyrene particles, followed by the steps of separating and drying, to produce the final expanded polystyrene particles having the functional skin

63、layer. As the release agent used for separating, there may be used a liquid material or a solid powder different from the solvent used to prepare the vinyl acetate based polymer solution. When the solid powder is used, the moldability of the expanded polystyrene particles is degraded and dust is c

64、reated during drying. When the liquid material is too lipophilic, it is more compatible with the polystyrene than with the vinyl acetate based polymer, causing the separation of the polystyrene from vinyl acetate based polymer. Accordingly, the use of excessively lipophilic liquid materials should b

65、e avoided. Thus, preferred release agent should be more or less hydrophilic liquid type material such that it can be uniformly applied to the exterior surface of the vinyl acetate based polymer, and should remain until the solvent of the polymer solution is appropriately removed such that it can. Ex

66、amples of suitable release agents include, but are not limited to, hydrophilic liquid materials, such as water, ethylene glycol and glycerin, containing two or more hydroxyl groups (--OH) in their molecular structure, and silicone oils. These release agents can be used alone or as a mixture of two or more thereof. The release agent is used in an amount of 1.about.40 wt % of the vinyl acetate based polymer. The amount of the release agent used is properly selected depending on the surface area of