WO2003027195A1

WO2003027195A1 - Coating method by phase equilibrium

Info

Publication number: WO2003027195A1
Application number: PCT/KR2002/001745
Authority: WO
Inventors: Fijeau Kim
Original assignee: Nexsol Technologies, Inc.
Priority date: 2001-09-27
Filing date: 2002-09-18
Publication date: 2003-04-03
Also published as: KR20010100101A

Abstract

The present invention relates to a coating method using the physical and chemical phase equilibrium between soluble resin and insoluble resin.

Description

COATING METHOD BY PHASE EQUILIBRIUM

Technical Field

The present invention relates to a coating method, which utilizes the phase equilibrium between soluble resin and insoluble resin.

Background Art

Coating (painting or surface treatment) means a process of coating a desired material on the surface of an object. Generally, the coating is conducted for the purpose of maintaining the desired lifetime of the object by the protection thereof, and increasing the quality of the object by surface treatment rendering the object beautiful and functional. Also, it is used for various purposes, including protection/beauty treatment, contamination prevention, coloring treatment, electrical insulation, the conferring of water and chemical resistances, the conferring of fireproof, heatproof, soundproof and rustproof properties, and the like.

Nowadays, there are many various kinds, performances and coating methods of paints, coating agents (solutions) and vehicles of forming them. Examples of resins used in the coating agents include thermosetting resin, water- soluble resin, organic solvent soluble resin, and insoluble resins, and the like. Coating methods using such resins will now be described in detail. 1. Brushing:

This is a fundamental method, which has been used for the longest period of time. It has a shortcoming in that it is inefficient and results in the non-uniform surface of a coating after application. Advantages are that the amount of loss of a coating is very low and effective in coating a small area. A brush used in the brushing is a coating tool using natural animal wool, and recently, the use of synthetic fibers was increased in the coating tool because of significant development in artificial fibers.

2. Rolling:

A roller used in this rolling is a coating tool where a cylinder is uniformly covered with fibers absorbing well a coating, and a separate handle is attached to the cylinder. It is a tool, which is capable of coating easily a large area, and simple and inexpensive. The rolling method includes a method where coating is conducted with hands using a roller brush, and a method where coating is mechanically conducted using rubber rollers between which an object to be coated is inserted, etc. The roller brush is a brush coating tool where young sheep' s hairs are wound cylindrically such that an easily rotating tool is obtained. Recently, with progress in studies on artificial fibers, there were developed a roller brush where a cloth implanted with artificial wool or mixed wool of natural and artificial wool had been wound cylindrically. Also, a roller brush, which had been weaved with a mixture of various wools and wound cylindrically, was developed. Thus, the rolling and the brush roller are effective in painting a plane at a large area-.

3. Air-spray coating:

This method is the same as the principle of spray, and performs coating by spraying a coating by means of the force of compressed air. It has characteristics in that it is applicable to most of coatings, inexpensive, simple to handling, and allows a coated film of clear and beautiful appearances to be obtained. However, it has a shortcoming of inferior coating efficiency. Thus, it is broadly propagated and used in all coating fields, and particularly suitable for use in coating of small-sized objects, including parts, automobiles, furniture and the like.

4. Airless coating:

This is a method of performing coating by pressing and spraying a coating agent through small holes by high pressure applied to the coating agent. This is similar to a principle that a person takes a shower by increasing the pressure of service pipes and pushing down the end of a hose line. It has characteristics in that it has excellent operation efficiency and allows even a highly viscose coating agent to be coated, and the amount of loss of the coating agent is low. It is particularly suitable to coat large-sized objects, including ships, bridges, the outer surface of buildings, and a lower body of automobiles.

5. Electrostatic coating: This is a method where the negative electrode is connected to an object to be sprayed, the negative electrode is connected to a device for spraying a coating agent, and high voltages (-60 kV to 120 kV) is applied to form electrostatic field between the positive and negative electrodes, thereby adsorbing the coating agent on the object. It is can be easily understood when regarding magnetic sand as particles of the coating agent and a magnet as the object. This electrostatic coating method exhibits reduced coating agent loss and good operation efficiency, and is suitable for a continuous coating process and thus for coating a standardized product. As a result, this method is suitable to coat objects of complex shapes, and automobiles where much loss of coating agents is caused due to over-spraying.

6. Dipping:

This is a method where an object to be coated is dipped in a tank containing a coating agent and then pulled up. In this case, the entire surface of the object is coated at a time so that there is little or no loss of the coating agent. This method is suitable to coat objects whose coated appearances are not of importance, such as parts for automobiles, pipes and the like.

7. Electro-deposition coating:

This is a method where an object is immersed in a tank containing a coating agent, and direct current flows through the object, thereby coating the object. It exhibits no loss of the coating agent, and allows the entire surface of the object to be coated. Also, it exhibits reduced flow marks as compared to the dipping method, and allows the thickness of the coating film to be controlled. Although this method is disadvantageous in that it is difficult to achieve uniform coating, it is commercially used for one-sided coating of small-sized object or sheets. Moreover, it is particularly suitable to coat entire metallic products, electrical parts, such as resistors and condensers, etc. 8. Shower coating:

In this method, a coating agent is sucked up by means of a pump and then injected from a nozzle. The entire surface can be coated at one time so that loss of the coating agent is very low. It is suitable to coat an object whose coated appearances are not of importance, such as parts for automobiles, parts for prefabricated houses, and the like. 9. Curtain coating:

This is a method where an object is coated with movement when a coating material streams down like a curtain. It results in loss of the coating material, and has high coating capability. It is used to coat printed plywood, steel plates for a bottom material, and the like.

10. Roll coater:

In this method, an object is passed between rolls so as to be coated. It results in loss of the coating material, and has high coating capability. It is used to coat colored steel plates, printed plywood, a bottom material, and the like.

11. Flow dipping:

In this method, powders become a flow state by blowing of air through a porous plate, and a pre-heated object is introduced into the flowing powders, so that the powders are adhered to the object and melted. This is mainly used with thermoplastic resin-based coatings, such as vinyl chloride or polyethylene, and suitable to coat wires, nets," pipes, and the like.

12. Knife coating:

This method is used to apply a coating on a flat surface at a fixed thickness, or to apply a liquid coating on an object on a roller and then subject the coating to a forced drying process using light or a rapid curing process, or to apply thermosetting resins after being liquefied. It is mainly used for precious coating, such as coating of large-scaled flat surface, or silk-screen printing, etc.

Meanwhile, articles made of starch or pulp molds, which have a sheet shape or other various three-dimensional shapes, for example, trays for food package, packs for egg package, dishes, vessels for noodles, cups, boxes, loose fills for package, network-shaped loose fills for fruits, hinge-lid-containers for hamburgers and sandwiches, have been familiar with us for a long time and can be widely used. There are needs for conferring moisture resistance, i.e., a hydrophobic property or a water repellent property, to the vessels formed of natural biodegradable materials, particularly starches and pulp molds, so that an epoch- making chance is provided such that the products made of starches or pulp molds, which had been currently limited in their use, can be enlarged in their use, and at the same time widely used in everyday life.

Moreover, if commercially used plastics, such as thermally changeable insoluble, which have inexpensive costs and excellent properties but was not used in the field of coatings or paints because of their properties, can be uniformly coated on large-sized structures or buildings, etc., this is believed to be an epoch in enlargement of the coating field. Specifically, methods capable of conferring moisture resistance include a method where water repellent materials, particularly fluorine resin, are added to the inside of an- object to be coated. Also, there is a method where resins or resin powders are added to the inside of the object and then formed into a desired shape, thereby producing a waterproof body. In addition, there is a method where a hydrophobic material is coated on the surface of the object.

In particularly, the first method of adding the water repellent material to the raw material has been widely used in the field of paper and pulp molds. The water repellent materials, which are currently widely used, can include fluorine-based resins and silicon-based products, for example, Lodyne commercially available from Ciba Specialty Chemical, Fluorad commercially available from 3M, and silicon-based oil commercially available from Dow-Corning. However, these materials are unsuitable for the present invention aimed at low cost, since they are expensive in cost and added at a large amount. Also, this method has a simple process as compared to the case of coating the general resins, but it has no competitive price and thus is unsuitable to use. The second method, which confers a hydrophobic (water repellent) property by adding the -resins or resin powders to the starch products or pulp molds, causes a change in physical requirement of the formed products. Thus, it results in inconvenience in a desired initial use, and has a reduced competitive price and a problem of poor biodegradability.

The third method, which is a surface coating method, utilizes a simple application process and thus is widely used. However, in order to coat the highly hygroscopic, porous surface of curved three-dimensional object, the object must be coated .with a coating solution containing solvent soluble vehicles, functional additives and solvents by the spray or dipping process, and then subjected to a drying process. However, the highly hygroscopic porous product is subjected to the spray or dipping process, the following uneconomical factors will be caused. First, since the coating agents are required for application at large amounts, it is never economical. Second, a drying device to remove the solvent of the coating from the starch products (and pulp molds) requires construction of a thermally insulating furnace or tunnel having a length of several to several tens of meters, a plurality of heaters and air blowers . Installation of the drying device requires many cost and large spaces. And, the drying device requires many maintenance costs, since a significant amount of heat needs to be generated in order to dry the solvent of the coating film. Also, the drying process generates the volatile organic compounds (VOC) , the emission into atmosphere of which is rigorously restricted in various countries. Third, serious changes in the shape and dimension of the formed product occur during the drying process of the products. Such shape and dimension changes can reach up to ±5%, so that the specification of the furnished materials can differ from that of the original materials. In addition, the surface of the formed materials becomes rough during their drying process, so that their appearance becomes coarse, thereby reducing a product property and printability.

An example of a method capable of solving such problems without using the drying installation includes a method where thermoplastic resin, for example polyethylene or polypropylene, is thermally melted and coated on a flat surface by the knife coating process. However, since this method can be restrictively applied to only the flat surface, it is difficult for this method to coat the coating material on the surface of a curved three- dimensional vessel at a desired thin thickness of less than several hundreds of microns. Methods other than liquid or powder coating are not physically easy to form a thin and uniform coating film. However, even in the case of the liquid or powder coating, since the starch or pulp products as objects to be coated are not conductive articles, special environment must be made such that the powders can be adhered to the surface of the objects. Furthermore, in this case, a separate curing or drying furnace is additionally required.

Disclosure of Invention

As used herein, the term "turbid solution" means a coating solution having a dispersion equilibrium state, which is a mixture where more than two different materials insoluble in each other are mixed so as to be chemically and physically dispersed for easy coating. This is similar to an emulsion, but may be a solvent on one hand and a solid on the other hand.

The thermally changeable resin generally designates thermoplastic and thermosetting resins. The soluble resin means a resin, which is easily soluble in water and an organic solvent.

The insoluble resin means a resin, .which is not easily soluble in water and an organic solvent.

The vehicle indicates the portion of resin in the turbid solution.

The object means a material to be coated.

A method capable of providing water resistance to a molded article made of starch or pulp mold, i.e., the simplest method capable of uniformly coating on a three- dimensional surface, is a coating method using a solution state. Namely, as described below, the present invention provides a coating method capable of conferring a hydrophobic property to a three-dimensional surface using a turbid solution, which is economical in that it meets the two requirements required in the prior art, i.e., drying of the solvent during a coating process and high productivity, without using the separate drying installation and the drying process.

The present inventors have invented a molding (starch and pulp mold) or coating method (structure) , which comprises the steps of: powdering soluble resin and insoluble resin, particularly thermoplastic and thermosetting resins; mixing the resin with an aqueous solution or organic solvent to form a turbid solution; applying the turbid solution on the surface of an object; subjecting the solvent to a natural drying process or a forced drying process using heat from the object or the outside, thereby curing- the soluble resin to form a coating film having dimensional stability; and melting the thermally changeable resin so as to be cured and adhered.

As used herein, the term "turbid solution" may also include an insoluble resin containing a polymer that is insoluble in a solvent, and means a physical emulsion where the insoluble resin is mixed with the solvent so that dispersion equilibrium between the solvent and the solid resin powder is reached. The turbid solution contains _ a solvent, a vehicle and functional additives. In the case of the general coating process, the soluble resin (main vehicle) is mostly separately used, but in the present invention, the soluble resin may be used in a combination with the insoluble resin (secondary vehicle) . Examples of the soluble resin, which can be cured by drying of the solvent of the turbid solution according to the present invention, include natural biodegradable polymers, including gum rosin, dammar gum, copal gum, alginic acid, agarose, Arabic gum, guar gum, locust gum, nitrocellulose, cellulose acetate butyrate, cellulose acetate propionate, gelatin, glue, casein, chitosan, agar, or mixtures or derivatives thereof; thermoplastic resin, including polystyrene, polycarbonate, polyacetal, polyurethane, polyimide, epoxy, synthetic latex, acrylate, and styrene butadiene; water soluble resin, including polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, polyvinylmethyl ether, polyacryl amide, ethylene oxide polymer, methylolated urea resin, methylolated melanine resin, carboxymethylcellulose (CMC) , natural and synthetic latex, or mixtures or derivatives thereof; fluorine resin and silicon-based resin; and copolymers thereof.

The insoluble resin used in the turbid solution according to the present invention utilizes an insoluble property by which the resin is insoluble in a solvent and can be melted and cured according to only a change in temperature. Example of the insoluble resin includes biodegradable synthetic resin, including polylactic acid, polyhydroxy butylate (PHB), and biodegradable aliphatic polyester resin; thermoplastic resin, including polyethylene and polypropylene; and thermosetting resin, including melamine, urea, phenol resin, unsaturated polyester, polyurethane, and epoxy resin. Thus, the insoluble resin means resins, which generally have an excellent resin property but not easily made into paints or inks so that they are difficult to use as coating agents. However, these resins are present only for the purpose of illustration, but the present invention is not limited to the resins as listed above.

The thermoplastic and thermosetting resins are widely used because of their inexpensive costs and excellent physical properties, and supplied at large amounts. Also, the thermoplastic and thermosetting resins having excellent performance, after being powdered, can be easily used for the coating method according to the present invention. Moreover, they are present in a solution or dispersion to which any curable resins, including thermoplastic resin, thermosetting resin, water-soluble resin and organic solvent soluble resin, may also be added.

Adding resin beads containing a foaming agent or foaming gas to the turbid solution is believed to be a very good attempt capable of increasing performance of the coating solution. Commercially readily available foaming agents and their chemical reaction temperature are as follows :

Azodicarbonamide: 205-215 °C

4, 4-oxybis (benzenesulfohydrazide) : 155-160 C Diphenylsulfon-3, 3-disulfohydrazide: 155 C Trihydrazinotriazine: 275 C p-toluenesulfonylsemicarbazide: 228-235 C 5-phenyltetrazole 240-250 C

Isatoic anhydride: 210-225 C

The resin beads containing the foaming gas are commercially readily available.

. In order to confer necessary functions to the turbid solution during or after a coating process or after curing, fibers may also be added to the turbid solution. Such fibers are used to increase elasticity, ductility, bendability, cohesion, elongation, refractivity, toughness, rupture energy, flexural strength and tensile strength. As long as the fibers can be uniformly dispersed in the solvent, they may be used, without limitation, in the form of pulp, glass fiber, flax fiber, hemp fiber, cotton fiber, wood fiber, Manila hemp, Indian millet, recycled paper, and the like. The fibers have a length of 1-5 mm, and preferably 1-2 mm. Also,, they have an L/D (length by diameter) of 5-100, and preferably 10-25.

In addition, in order to further improve performance of the turbid solution, resin dispersant waxes, a viscosity-adjusting agent, a thixotropic agent, a plasticizer, a crosslinking agent, a foaming agent, and inorganic fillers may be added, if required.

The turbid solution according to the present invention may contain water, an organic solvent or both. A method of coating the turbid solution of the present invention is not limited in a special manner. As long as the solution can be uniformly coated, any coating method including dipping, air-spray coating and brushing may be used. Viscosity of the solution at room temperature, viscosity of the solution when coated on the surface of the object to form a coating film, maintenance of strength of the coating film by increase in viscosity after drying of the solvent, and viscosity of the solution when coming in contact with another object or molded article, are important to stabilization of the coating film during a molding process and performance of the cured coating film. On coating, viscosity of the turbid solution must be suitably maintained by adjusting the amount of the solvent, such that the solvent can easily flow along the surface of the object so as to a uniform coating film.

After the solvent is dried so that the soluble vehicle forms the coating film on the object, the soluble vehicle must be semi-cured while attaining its dimensional stability. On the other hand, the insoluble vehicle contained in the turbid solution must be semi-dissolved on the surface of the object, which has higher temperature than the melting point of the insoluble resin. Thus, the semi-dissolved resin maintains morphological and structural strengths at which the coating film is not changed with respect to its shape even when the coating film is in contact with other molded article in additional processes. In this case, viscosity or strength of the molded article, which is in contact with the coating film in the additional processes, must be lower than those of the coating film, such that it does not affect the coating film. The fibers used in the turbid solution of the present invention serve to stably binds, by means of fiber chains, the vehicles, the thermally changeable .resins and other components in the turbid solution to each other, such that these components have a configuration of non-woven fabrics. The vehicle (soluble resin) dissolved in the solvent, after drying of the solvent, acts to temporarily adhere the fibers to the thermally changeable resin powder, which was not yet thermally dissolved. Also, this vehicle serves to increase physical strength of the molded article such that the article is maintained at a stronger structural shape.

This is also called dimensional stability of the coating film. The dimensional stability means that the coating film has sufficient strength and structural stability at which it can keep up its own weight against gravity, and stand the expansion of destructive evaporation gas, such as water vapor, which can be generated during a drying or forming process, and also it can resist physical deformation requirements in subsequent processes.

The resin, which is used as the soluble vehicle in the turbid solution of the present invention, is easily soluble in the solvent, and even after being cured, it is soluble where it reaches the temperature exceeding its softening point. For this reason, this vehicle resin can be cured with drying of the solvent, but even after being cured, it is softened by heating so as to reduce its viscosity. One example of this resin is cellulose. This is a natural resin, which has excellent physical properties and high solubility in solvents so that it is widely used in inks, paints and lacquers, etc. Examples of this cellulose are as follows:

Cellulose acetate: 230-240 °C Cellulose acetate butyrate, low butyryl: 230-240 °C Cellulose acetate butyrate, high butyryl: 127-142 °C Cellulose acetate propionate: 188-210 °C

This indicates that the vehicle resin is not only chemically easily dissolved in various solvents, but it can be physically melted when being heated to more than its melting point, so that it can selectively melted.

The thermally changeable vehicle is shaped as the soluble vehicle, which had been dissolved in the solvent, is cured with drying of the solvent. Then, if temperature of the object rises to more than melting point of the insoluble vehicle, the thermally changeable vehicle is dissolved together so that it is adhered to its surrounding parts. Thus, when the temperature of the object falls to room temperature, the desired product exhibits a waterproof effect while maintaining a physically and chemically stronger shape.

As described herein, if a suitable vehicle, which was selected according to the process temperature on the basis of two physical and chemical properties, i.e., chemical solubility at room temperature and physical processability, is used, it will be preciously coated on the desired article. In other words, the insoluble vehicle, which remains undissolved in the chemically dissolved soluble vehicle, is heated so that the two vehicles reaches physical phase equilibrium therebetween while being mixed. Then, the mixed vehicles are dried and lowered to room temperature so that they are cured.

Table 1 below shows physical and chemical properties of several resins:

A curing process after coating is conducted as described in the process example 1 below. Process example 1:

1. A turbid solution is coated on an object.

2. The turbid solution is heated due to the temperature of the object.

3. By the heating of the solution, a solvent in the turbid solution is dried and a coating film is formed.

4. By the drying of the solvent, a soluble vehicle in the coating film is cured to reach self-stabilization.

5. As the curing of the soluble vehicle progress, a highly viscous coating film is formed while a thermally changeable vehicle starts to melt.

6. By secondary heating, the soluble vehicle and the thermally vehicle reach the phase equilibrium therebetween while being melt-mixed.

7. The thermally vehicle is completely melted.

8. On the surface of the object, the thermally changeable vehicle is mixed with the soluble vehicle and other additives.

9. The temperature of the resulting structure is lowered.

10. A cured coating film formed on the object is obtained. In the process example 1 as described above, gas beads containing foaming gas, expandable polystyrene (EPS) grains filled with gas, or chemical foaming agents, may also be added to the thermally changeable resin.

A curing process where the thermally changeable resin containing such foaming agents or beads is used is conducted as described in the process example 2 below. Process example 2:

1. A turbid solution is coated on an object.

2. The turbid solution is heated due to the temperature of the object.

3. By -the heating of the solution, a solvent in the turbid solution is dried and a coating film is formed. 4. By the drying of the solvent, a soluble vehicle in the coating film is cured to reach self-stabilization.

5. As the curing of the soluble vehicle progress, a highly viscous coating film is formed while the thermally changeable vehicle to foam with a foaming agent.

7. The thermally changeable vehicle is completely melted foamed.

9. The temperature of the resulting structure is lowered.

10. A cured coating film formed on the object is obtained.

Best Mode for Carrying Out the Invention

Coating and forming methods according to the present invention will now be described in further detail with reference to the following examples.

Example 1: Forming of general three-dimensional articles including vessels, trays and cups, by injection of starch

In this example, starch is modified and added with additives . The mixture is kneaded, thereby obtaining pellets having plasticity and flexibility. The pellets are formed and foamed into a desired shape according to a conventional process using conventional injection and extrusion machines. A mold is immersed in a tank containing a turbid solution so that the surface of the mold is closely coated with the turbid solution. The mold closely coated with the turbid solution is taken out of the tank and left to stand such that an over-coated solution falls down into the tank. At this time, the turbid solution has flowable thin viscosity, so that it forms a coating layer on the mold at a desired thickness while the remaining portion of the turbid solution naturally falls down into the tank. The mold is heated, so that hot air flows into a gate in the mold, whereby a solvent in the turbid solution is evaporated and removed from a cavity. The solvent in the turbid solution is further evaporated by heat from the mold. At this time, more than 60% of the solvent is removed. The turbid solution is dried and thus a soluble vehicle therein is cured so that a coating film is formed on the surface of the mold. After the solvent is evaporated, the soluble vehicle, the thermally changeable resin powder and the fibers form a semi-softened (semi-cured) coating film together and maintain a thixotropic phase equilibrium state. Meanwhile, the starch mixture is kneaded in an injector cylinder at 180 °C or above. The starch mixture is injected on the semi-melted coating film and successively subjected to general injection processes. After the starch mixture is injected, the thermally changeable resin powder in the semi-melted coating film is completely melted due to .heat from the starch mixture of 180 °C or above, so that the resin powder with low viscosity, plasticity and flexibility is soaked into, and adhered to the surface of the injected starch mixture. The temperature of the injected starch mixture is lowered due to low temperature -of the mold, and a molded shape of the starch mixture in the mold is stabilized. Then, the molded article made of the starch mixture is released from the mold. This gives a molded article, which was coated with the resin and thus resistant to water.

Example 2: Forming of three-dimensional articles, including general vessels, trays and cups, by starch slurry

Unlike the starch injection, in this example using starch slurry, starch slurry of relatively high water content, which was mixed with a modifier and additives, is poured into a highly heated mold. The starch slurry is dried by heat from the mold, and foamed and formed into a desired shape under high pressure by the water content in the slurry. This method is very similar to the preparation of Crucian-shaped breads or walnut-shaped cakes.

The inside of a mold is closely coated with a turbid solution. The mold is left to stand such that an over- coated solution falls down. At this time, the turbid solution has flowable thin viscosity, so that it forms a coating layer on the mold at a desired thickness while the remaining portion of the turbid solution naturally falls down. Most of a solvent in the coated turbid solution is evaporated due to heat transferred from the mold. The mold is heated at more than 175 °C. However, this heating temperature must not exceed the temperature at which organic materials contained in the solution can be decomposed. The dried coating film on the mold, from which most of the solvent was evaporated, forms a strong coating film on the mold at a significantly dried state. Vehicles, thermally changeable resin powders and fibers in the turbid solution, from which the solvent was evaporated, form a semi-softened (semi-cured) coating film together and maintain a thixotropic phase equilibrium state. The starch slurry is discharged on the semi-melted coating film, and self-foamed by hot heat from the mold and water content in the slurry. Then, it is a drying process where the water content is removed from a cavity under high pressure. After the starch slurry is discharged, the mold rises to a temperature of 180 to 250 °C. The thermally changeable resin powder is melted due to heat from the heated mold, so that the resin powder is adhered to the surface of the dried starch. After the molded starch is lowered to low temperature and stabilized, the molded starch is released from the mold. This gives the molded article of starch, which was coated with the resin and thus resistant to water. Example 3: Forming of three-dimensional articles, including general vessels, trays and cups, by pulp molds

A forming mold for forming pulp molds is equipped with a wire-screen of 40-50 meshes. After a pulp liquid is coated on the forming mold, most of the water content in the pulp liquid is sucked through the wire-screen by vacuum adsorption. After adsorbing the water content, the pulp liquid is dried and formed at high temperature and pressure.

A forming mold is immersed in a tank containing a turbid solution so that the inner surface of the forming mold is closely coated. with the turbid solution. The mold closely coated with the turbid solution is taken out of the tank and left to stand such that an over-coated solution easily falls down. At this time, the turbid solution has flowable thin viscosity, so that it forms a coating layer on the forming mold at a desired thickness while the remaining portion of the turbid solution naturally falls down. A solvent in the coated turbid solution is evaporated by heat from the forming mold. The turbid solution from which the solvent was evaporated naturally forms a semi-melted (semi-cured) coating film. A pulp liquid is coated on another forming mold, and water in the coated pulp mold is absorbed with a vacuum adsorber. After absorbing water, the pulp liquid, which is formed into the shape of a compressed mold maintained at more than 200 °C, is compressed while being strongly absorbed with a vacuum pump. When water content in the pulp mold is dried to more than 70%, it is attached to the formed coating film and compressed by heating at more than

200 °C. The resin powder in the coating film is melted due to heat from the heated forming mold, and adhered to the surface of the pulp mold. Thereafter, the molded article in the mold is stabilized and then released from the mold. This gives the pulp mold, which was coated with the resin and thus resistant to water.

Example 4: Coating of outer surface of large-sized structure

As long as insoluble plastics are heated after being coated, they may also be coated on the surface of a large- sized structure. A turbid solution is closely applied on the surface of an object to be coated. The object is left to stand such that the solvent in the turbid solution is sufficiently dried. As the solvent is dried, a coating film of a soluble vehicle is formed. After the solvent is sufficiently dried, the surface of the coating film is thermally treated with hot air, supersonic waves, laser or a combination thereof, which can heat a thermally changeable resin to its melting point or above. This thermal treatment is to increase the temperature of the coating film to the melting point or above of the thermally changeable resin. By the thermal treatment, the soluble vehicle and the thermally changeable vehicle reach phase equilibrium therebetween, after which they are sufficiently mixed, bound to each other and then cured. This can give a strong coating film.

Example 5: Foaming coating of large-sized structure A turbid solution containing a foaming agent or a foamable resin is closely applied on the surface of an object to be coated. The object is left to stand such that the solvent in the turbid solution is sufficiently dried. As the solvent is dried, a coating film of a soluble vehicle is formed. After the solvent is sufficiently dried, the surface of the coating film is thermally treated with hot air, supersonic waves, laser or a combination thereof, which can heat a thermally changeable resin to its melting point or above. This thermal treatment is to increase the temperature of the coating film to the melting point or above of the thermally changeable resin. By the thermal treatment, the soluble vehicle and the thermally changeable vehicle reach phase equilibrium therebetween, after which they are sufficiently mixed, bound to each other and then cured. This can form a foamed coating film on the outside of the structure.

Industrial Applicability

As described above, a most important characteristic of the present invention is that the desired resin can be preciously coated on the molded products at a uniform thickness without injecting, extruding and pressing the thermoplastic and thermosetting resins on the irregular three-dimensional surface. For this reason, an effort of carefully applying the resins with a spray or brush is reduced, and various processes for drying is achieved in a simple, easy and rapid manner using heat generated during the processes without additional equipment investment. This permits constant maintenance of the quality of products and thus provides a great advantage in terms of the control of quality and production cost. Also, since the products are subjected to forming, drying and adhering processes within a forming mold, there is no distortion of a shape after forming and cooling. Moreover, a need for installing a drying furnace (including drying oven and tunnel) in a large area at a large cost is eliminated. In addition, process time can be reduced as compared to that of the conventional method, so that high productivity can be maintained even at mass production.

As apparent from the above results, liquid powder application can be achieved even on objects on which powder coating is impossible. Furthermore, if starch or pulp made of natural polymer materials, which has no water resistance, is subjected to a forming process with the water-resistant coating agent of the present invention according to the method of the present invention, the formed starch or pulp mold will have sufficient water resistance and thus will be widely used.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.

Claims

What Is Claimed Is:

1. A coating turbid solution using phase equilibrium, which comprises the steps of: mixing a soluble vehicle with a thermally changeable resin powder in a solvent, the thermally changeable resin powder being insoluble in the solvent; and mixing the mixture with functional additives, thereby forming a turbid solution having viscosity and conditions suitable for a coating process.

2. The turbid solution of Claim 1, which utilizes phase equilibrium.

3. The turbid solution of Claim 1 or 2, in which the solvent is an aqueous soluble solvent.

4. The turbid solution of Claim 1 or 2, in which the solvent is an organic solvent.

5. The turbid solution of Claim 1 or 2, in which the solvent is a mixed solvent of water and an organic solvent.

6. The turbid solution of Claim 1 or 2, in which a thermoplastic resin is mixed.

7. The turbid solution of Claim 1 or 2, in which a thermosetting resin is mixed.

8. The turbid solution of Claim 1 or 2, in which a foamable resin is mixed.

9. The turbid solution of Claim 1 or 2, in which a foaming agent is mixed.

10. A method of coating a liquid powder using phase equilibrium, which comprises the steps of: coating a turbid solution of Claim 1 or 2 on an object; maintaining the turbid solution at suitable viscosity so that an over-coated solution flows down from the object by it own weight to form a coating film at a desired thickness; drying a solvent so that a soluble resin in the turbid solution is cured to form a coating film; softening a thermally changeable resin in the coating film by heat supplied from the outside; mixing the soluble resin with other resins in the coating film so that it is adhered to the surface of the object while maintaining high viscosity, whereby the coating film is maintained at a thixotropic phase equilibrium state; applying the coating film being cured .and softened, according to a desired purpose or use of the coating film; additionally melting the semi-liquefied vehicles by high temperature from the outside, so that they are mixed with each other with low plasticity, viscosity and flexibility while being adhered to the surface of the object; cooling the coating film and the object by low temperature of the object, thereby curing the coating film and the object; and releasing the cured object from a mold.

11. The method of Claim 10, in which the coating film is applied in an injection molding process of starch.

12. The method of Claim 10, in which the coating film is applied in a molding process of starch slurry.

13. The method of Claim 10, in which the coating film is applied in a molding process of pulp.

14. The method of Claim 10, in which the coating film is applied in a process of forming a coating film on a three-dimensional structure.

15. A coating film coated according to the method of Claim 10.

16. The coating film of Claim 15, in which the coating film has a thickness of 1 to 500 microns.

17. The coating film of Claim 15, in which the coating film has a thickness of more than 500 microns.