Patent Publication Number: US-2013251355-A1

Title: Coating composition, heater of washing machine having the same, and coating method for the heater

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 102012-0029920, filed on Mar. 23, 2012 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present disclosure relate to a coating composition to prevent contamination of a heater caused by water or steam, a heater of a washing machine having the coating composition, and a method of coating the heater. 
     2. Description of the Related Art 
     In electrical appliances, a heater is used to heat water in a state of being in contact with water or steam. Hereinafter, a washing machine will be described as an example thereof. 
     A washing machine is a machine that uses electric power to wash clothes. A washing machine includes a tub mounted in a housing to contain wash water, a drum rotatably mounted in the tub to be spaced apart from the tub by a predetermined distance, and a heater mounted at a lower portion of a space formed between the tub and the drum to heat wash water contained in the tub. 
     Although a heater improves washing performance of a washing machine by controlling temperature of wash water, minerals such as calcium carbonate (CaCO 3 ) and magnesium hydroxide (Mg(OH) 2 ) dissolved in the wash water are precipitated and accumulate on the surface of the heater after extended use. As scales accumulate on the surface of the heater, performance of the heater deteriorates. Accordingly, the power consumption of the washing machine for heating wash water is increased, thereby lengthening the washing time. In addition, excess accumulation of scales at a predetermined region may cause a short circuit in the heating wire disposed in the heater. Thus, the heater may cease to function properly, resulting in degradation in the performance of the washing machine. 
     In order to prevent this problem, a scale prevention device may be provided thereto, or the surface of the heater is treated with TEFLON or a ceramic composition. 
     However, when the scale prevention device is used, the scale prevention device is disposed between the tub and a drum. Thus, the size of the scale prevention device is limited, and disturbance generated in wash water by a small size scale prevention device is not sufficient to remove scales that have been formed on the heater. 
     In addition, TEFLON needs to be heat-treated at high temperature for a long period of time, and the ceramic compositions are degraded after extended use at high temperature. 
     SUMMARY 
     Therefore, it is an aspect of the present disclosure to provide a coating composition capable of preventing scales from accumulating on the surface of a heater, and a heater of a washing machine having the coating composition. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     In accordance with one aspect, a heater of a washing machine contacting water or steam includes a heating wire disposed at the center, a magnesium oxide (MgO) layer disposed outside the heating wire to surround the heating wire to transmit heat from the heating wire to the outside, and a stainless alloy layer disposed outside the magnesium oxide layer to surround the magnesium oxide layer. The surface of the stainless alloy layer is coated with a coating composition including a silicon resin comprising organopolysiloxane and silicon including polysilsesquioxane. 
     The silicon including polysilsesquioxane may be composed of fine particles. 
     The organopolysiloxane may be represented by Formula 1 below: 
     
       
         
         
             
             
         
       
     
     In Formula 1, R1 to R7 are selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, an aryl group, and an alkoxy group, and S1 may be represented by Formula 2 below: 
     
       
         
         
             
             
         
       
     
     In Formula 2, R8 and R9 are selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, an aryl group, and an alkoxy group or have a repeating unit of Formula 2. Furthermore, in Formula 1, Z1 to Z3 are selected from the group consisting of a hydroxyl group, a vinyl group, and an alkoxy group, n is an integer of 1 to 50,000, m is an integer of 1 to 10,000, and k is an integer of 0 to 10,000. 
     The polysilsesquioxane may be represented by Formula 3 below: 
     
       
         
         
             
             
         
       
     
     In Formula 3, R10 and R11 are selected from the group consisting of a hydrogen atom, a hydroxyl group, a vinyl group, an alkoxy group, an alkyl group unsubstituted or substituted with a reactive group, and an allyl group unsubstituted or substituted with a reactive group, and j is an integer of 1 to 100,000. 
     The content of the silicon including polysilsesquioxane may be in the range of 0.1 to 50% by weight. 
     A diameter of the fine particles may be about 10 microns or less. 
     The coating composition may further include a transition metal or an acidic catalyst to cure the silicon resin including organopolysiloxane and the silicon including polysilsesquioxane. 
     The stainless alloy layer may have protrusions and grooves on the surface thereof to improve adhesive force of a coating layer formed on the surface of the stainless alloy layer. 
     In accordance with one aspect, a coating composition formed on the surface of a heater contacting water or steam includes a silicon resin comprising organopolysiloxane, and silicon including polysilsesquioxane. 
     The organopolysiloxane may be represented by Formula 4 below: 
     
       
         
         
             
             
         
       
     
     In Formula 4, R1 to R7 are selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, and an alkoxy group, and S1 is represented by Formula 5 below: 
     
       
         
         
             
             
         
       
     
     In Formula 5, R8 and R9 are selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, and an alkoxy group or have a repeating unit of Formula 5. Furthermore, in Formula 4, Z1 to Z3 are selected from the group consisting of a hydroxyl group, a vinyl group, and an alkoxy group, n is an integer of 1 to 50,000, m is an integer of 1 to 10,000, and k is an integer of 0 to 10,000. 
     The polysilsesquioxane may be represented by Formula 6 below: 
     
       
         
         
             
             
         
       
     
     In Formula 6, R10 and R11 are selected from the group consisting of a hydrogen atom, a hydroxyl group, a vinyl group, an alkoxy group, an alkyl group unsubstituted or substituted with a reactive group, and an allyl group unsubstituted or substituted with a reactive group, and j is an integer of 1 to 100,000. 
     In accordance with one aspect, a method of coating a heater includes preparing a coating composition by mixing a silicon resin comprising organopolysiloxane and polysilsesquioxane, surface-treating the heater, forming a coating layer by coating the coating composition on the surface of the surface-treated heater, and curing the coating layer by heat-treating the coated heater. 
     The organopolysiloxane may be represented by Formula 7 below: 
     
       
         
         
             
             
         
       
     
     In Formula 7, R1 to R7 are selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, and an alkoxy group, and S1 is represented by Formula 8 below: 
     
       
         
         
             
             
         
       
     
     In Formula 8, R8 and R9 are selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, and an alkoxy group or have a repeating unit of Formula 8. Furthermore, in Formula 7, Z1 to Z3 are selected from the group consisting of a hydroxyl group, a vinyl group, and an alkoxy group, n is an integer of 1 to 50,000, m is an integer of 1 to 10,000, and k is an integer of 0 to 10,000. 
     The surface-treating of the heater may include forming protrusions and grooves on the surface of the heater by sandblasting or chemical etching. 
     The forming of the coating layer by coating the coating composition on the surface of the heater may include spray coating, dip coating, spin coating, or flow coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is perspective view illustrating a heater according to an embodiment; 
         FIG. 2  is a cross-sectional view of the heater of  FIG. 1  taken along line AA′; 
         FIG. 3  is a flowchart illustrating a method of manufacturing a heater according to an embodiment; 
         FIG. 4  is a cross-sectional view of a washing machine according to an embodiment; 
         FIG. 5  is a cross-sectional view of the heater of  FIG. 1  taken along line AA′ according to one embodiment; 
         FIG. 6  is a photograph illustrating the surface of a heater coated as in Comparative Example 2 in which scales are formed after a test according to Experimental Example 2; and 
         FIG. 7  is a photograph illustrating the surface of a heater coated as in Example 2 in which scales are not formed after a test according to Experimental Example 2. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. Hereinafter, a heater mounted in a washing machine will be described as an example. However, the heater is not limited thereto and may be any heater that heats water in a state of being in contact with water. 
       FIG. 1  is a perspective view illustrating a heater  10  according to an embodiment. 
     As illustrated in  FIG. 1 , the heater  10  is configured to have a predetermined diameter and length. The heater  10  may be configured in a zigzag shape. The heater  10  includes terminals  11  connected to cables in which current flows, a sealing member  12  disposed to be spaced apart from the terminals by a predetermined distance to prevent leakage of air, and a heating element  13  extending from the terminals  11 . The heating element  13  having the predetermined diameter and length is bent a plurality of times. 
       FIG. 2  is a cross-sectional view of the heater of  FIG. 1  taken along line AA′. 
     As illustrated in  FIG. 2 , a heating wire  14  generating heat is disposed at the center of the heating element  13 . A magnesium oxide (MgO) layer  15  and a stainless alloy layer  16  are sequentially disposed outside the heating wire  14 . The stainless alloy layer  16  and the magnesium oxide layer  15  perform a function of transferring heat generated in the heating wire  14  to the outside. The surface of the heating element  13  contacts water or steam. 
     A coating layer  20  is disposed outside the stainless alloy layer  16 . That is, the coating layer  20  is formed on the surface of the heating element  13 . The coating layer  20  may be formed of a coating composition that includes a silicon resin including organopolysiloxane and silicon including polysilsesquioxane. The silicon including polysilsesquioxane may be in the form of fine particles of silicon. In this regard, the fine particles may have a diameter of  10  microns or less. 
     Organopolysiloxane contained in the coating composition according to an embodiment may have a structure represented by Formula 1 below. 
     
       
         
         
             
             
         
       
     
     In Formula 1, R1 to R7 may be selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, an aryl group, and an alkoxy group. Z1 and Z2 may be selected from the group consisting of a hydroxyl group, a vinyl group, and an alkoxy group. In Formula 1, n is an integer of 1 to 50,000, and m is an integer of 1 to 10,000. In addition, S1 of Formula 1 may have a structure represented by Formula 2 below. 
     
       
         
         
             
             
         
       
     
     In Formula 2, R8 and R9 may be selected from the group consisting of a linear chain alkyl group, a branched chain alkyl group, a cyclic alkyl group, an aryl group, and an alkoxy group or may repeatedly have the structure of Formula 2. Z3 may be selected from the group consisting of a hydroxyl group, a vinyl group, and an alkoxy group, and k is an integer of 0 to 10,000. 
     In addition, polysilsesquioxane of the coating composition according to an embodiment may have a structure represented by Formula 3 below. 
     
       
         
         
             
             
         
       
     
     In Formula 3, R10 and R11 may be selected from the group consisting of a hydrogen atom, a hydroxyl group, a vinyl group, an alkoxy group, an alkyl group unsubstituted or substituted with a reactive group, and an allyl group unsubstituted or substituted with a reactive group. In addition, j is an integer of 1 to 100,000. 
     In Formulae 1 to 3, the alkyl group may be a linear chain alkyl group, a branched chain alkyl group, or a ring-shaped chain alkyl group. The number of carbon atoms of the alkyl group is not limited, but may be 1 to 30. Particularly, examples of the alkyl group may include a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, a t-butyl group, an n-pentyl group, an iso-pentyl group, a neo-pentyl group, an n-hexyl group, a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group, but are not limited thereto. 
     In Formulae 1 to 3, the alkoxy group may be a linear chain alkoxy group, a branched chain alkoxy group, or a ring-shaped chain alkoxy group. The number of carbon atoms of the alkoxy group is not limited, but may be 1 to 30. Particularly, examples of the alkoxy group may include a methoxy group, an ethoxy group, an n-propyloxy group, an iso-propyloxy group, an n-butyloxy group, and a cyclopentyloxy group, without being limited thereto. 
     In Formulae 1 and 2, the aryl group may be a single-ring-shaped aryl group or a multi-ring-shaped aryl group. The number of carbon atoms of the aryl group is not limited, but may be 6 to 60. Particularly, examples of the single-ring-shaped aryl group may include a phenyl group, a biphenyl group, a terphenyl group, and a stylbenyl group, but are not limited thereto. Examples of the multi-ring-shaped aryl group may include a naphthyl group, an anthryl group, a phenanthryl group, a pyrenyl group, a perylenyl group, a chrycenyl group, and a fluorenyl group, but are not limited thereto. 
     In Formula 3, the expression “a group unsubstituted or substituted with a reactive group” refers to a group substituted with at least one reactive group selected from the group consisting of a hydroxyl group, a carboxyl group, an isocyanate group, an amine group, an amide group, a carbamate group, a urea group, an urethane group, a vinyl group, and an unsaturated ester group or a group not having any reactive group. 
     The coating composition may further include a solvent to facilitate processing. Examples of the solvent may include hydrocarbons, halogenated hydrocarbons, ethers, ketones, and alcohols. Particularly, 2-propaneol and toluene may be used, but the solvent is not limited thereto. 
     The coating composition may further include a curing catalyst for curing the coating layer. The curing catalyst may be a transition metal or an acidic material. Particularly, examples of the transition metal may include zinc, tin, nickel, and chromium, and examples of the acidic catalyst may include hydrochloric acid, nitric acid, phosphoric acid, acetic acid, potassium hydroxide, amines, hydrogen fluoride (HF), and fluorinated potassium (KF), without being limited thereto. 
     In order to improve adhesive force of the coating composition to the surface of the heater, an adhesion enhancer may be further added to the coating composition. The adhesion enhancer may include a silane compound having an amino group. In particular, N(beta-aminoethyl)-gamma-aminopropyl trimethoxysilane (NH 2 —(CH 2 ) 2 —NH—(CH 2 ) 3 —Si—(OCH 3 ) 3 ) may be used. 
     Meanwhile, the surface of the heater may be processed to have a double protrusion-groove structure as illustrated by  FIG. 2 . For example, the coating layer  20  that constitutes the surface of the heater may be surface-treated to form protrusions and grooves. For example, surface protrusions and grooves  22  may be formed by sandblasting the surface of the heater. In addition, fine protrusions and grooves  21  are formed on the surface of the surface protrusions and grooves  22  by fine particles of polysilsesquioxane. As a result, a double protrusion-groove structure  21  and  22  is formed. The double protrusion-groove structure  21  and  22  improves water repellency and facilitates bubble formation on the surface of the heater when heated by the heater, so that scales may be efficiently detached. The stainless alloy layer  16  may comprise a protrusion and groove structure, as illustrated by  FIG. 5 , to improve the adhesion of the coating layer  20  to the stainless layer  16  as describe below. Wherein the heater may comprise a protrusion and grove structure on both the coating layer  20  and the stainless alloy layer  16 . 
     The coating composition may further include a silicone oil to improve non-stick ability for prevention of scales from sticking to the coating layer. The silicone oil may include reactive silicone oil and/or non-reactive silicone oil. Particularly, examples of the non-reactive silicone oil may include dimethyl silicone oil, phenyl modified silicone oil, and alkyl modified silicone oil, without being limited thereto. Examples of the reactive silicone oil may include amino modified silicone oil, hydroxyl silicone oil, vinyl modified silicone oil, and methyl hydrogen silicone oil, without being limited thereto. 
     The coating composition may include 0.1 to 50% by weight of silicon including polysilsesquioxane. When the content of the silicon including polysilsesquioxane is less than 0.1% by weight, scale inhibition efficiency of the composition may be reduced. On the other hand, when the content of the silicon including polysilsesquioxane is greater than 50% by weight, scale inhibition efficiency of the composition may be reduced in comparison with the amount of the added silicon including polysilsesquioxane, thereby reducing processing efficiency. The weight ratio of the coating composition is applied to both cases with or without a solvent in addition to the silicon resin including organopolysiloxane and silicon including polysilsesquioxane. 
       FIG. 3  is a flowchart illustrating a method of manufacturing a heater according to an embodiment. 
     The method of manufacturing a heater includes preparing a coating composition (S 100 ), forming a coating layer by coating the coating composition on the surface of the heater (S 200 ), and curing the coating layer by heat-treating the coated heater (S 300 ). 
     The method may also include additional operations that will be apparent to those of ordinary skill in the art. 
     The preparation of the coating composition (S 100 ) may include mixing a resin including organopolysiloxane and silicon including polysilsesquioxane. An additive may be added to the coating composition in order to improve functionality of the coating layer. Examples of the additive may include an adhesion enhancer, a curing catalyst, and/or silicone oil, as described above. 
     The adhesion enhancer may prevent the coating layer from being delaminated from the surface of the heater by improving adhesive force of the coating layer to the surface of the heater. As the adhesion enhancer, a silane compound to which an amino group is bonded may be used. The curing catalyst may be a transition metal or an acidic material and may be used to cure the coating layer. In addition, silicone oil may be used in order to improve non-stick ability of the coating composition. 
     In addition, in order to improve processibility of a coating composition in the preparation of the coating composition (S 100 ), a solvent for mixing the resin including organopolysiloxane and the silicon including polysilsesquioxane may be used. Examples of the solvent may include hydrocarbons, halogenated hydrocarbons, ethers, ketones, and alcohols. 
     In the forming of the coating layer by coating the coating composition on the surface of the heater (S 200 ), the coating method is not particularly limited and any coating method may be used in accordance with a surface pattern and size of the heater. Particularly, the coating layer may be formed on the surface of the heater by spray coating, dip coating, spin coating, flow coating, and the like. 
     Spray coating refers to a method of coating the surface of an object by spraying a low viscosity coating solution through a spray nozzle. A coating layer may be uniformly formed even on a non-uniform surface or on the surface having protrusions and grooves. Generally, spray coating is applied to one surface of the object, and thus a small amount of the coating solution is used, and energy for evaporation is reduced. According to an embodiment of the present invention, when the surface of the heater is treated to form protrusions and grooves, spray coating may be used by adjusting viscosity of the coating composition. 
     Dip coating refers to a method of coating an object by dipping the object in a coating solution for a predetermined period of time and evaporating a solvent component. Dip coating is generally used for coating of an object with a non-uniform surface. Thus, the dip coating may be applied to the heater in accordance with the surface of the heater to which the coating composition according to the present embodiment is applied. 
     Spin coating is generally used to form a thin coating layer since the coating solution is sprayed onto a rotating object, dried and heat-treated. According to the spin coating, the coating solution applied to the object rotated by a spin-coater is spread by centrifugal force. In this regard, the coating composition may be in a solution state or in a liquid state by use of a solvent. Particularly, if the coating composition is in a liquid state by use of a solvent, a film may be formed on the surface of the object by spin coating. 
     Flow coating is a coating method performed by pouring a paint onto an object and may be efficiently used only when a small amount of the object is coated. 
     The curing of the coating layer by heat-treatment of the heater on which the coating layer is formed (S 300 ) may include drying the coating layer. The drying may be performed at a temperature of 25° C. to 60° C. for 1 min to 1 hr. The heat-treatment may be performed at a temperature of 80° C. to 200° C. for 10 min to 24 hr. As the heat-treatment time decreases, the curing of the coating layer may not be completely performed. On the other hand, as the heat-treatment time increases, mass productivity may be reduced. Thus, the heat-treatment may be performed with the range as described above. 
     Meanwhile, the method may further include processing the surface of the heater to improve adhesive force of the coating layer formed on the surface of the heater before forming the coating layer by coating the coating composition on the surface of the heater (S 200 ). For example, the surface of the heater may have protrusions and grooves by surface treatment. For example, the surface of the heater may be modified by sandblasting. Beads used in the sandblasting may include grid glass beads, ceramic beads, and metal beads with a small diameter. The size of the beads may vary, and beads having different sizes and types may be used in combination. In addition, chemical etching may also be used to form protrusions and grooves on the surface of the heater in addition to the sandblasting. 
       FIG. 4  is a cross-sectional view of a washing machine  100  according to an embodiment. 
     As illustrated in  FIG. 4 , the washing machine  100  includes a body  101  defining the external appearance of the washing machine  100 , a tub  102  mounted in the body  101  to contain wash water when performing washing, a drum  103  rotatably mounted in the tub  102  to wash laundry when performing washing. Here, a door  106  is mounted at the front of the body  101  to open and close an opening through which laundry is put into the drum  103 . 
     A water supply pipe  104  and a detergent supply unit  105  to supply wash water and detergent into the tub  102  are mounted in the body  101 . The detergent supply unit  105  has a chamber to contain detergent. In order to allow the user to easily put detergent into the chamber, the detergent supply unit  105  is arranged at the front of the body  101 . In addition, a drainage pump  110  and a drainage pipe  109  are mounted at a lower portion of the body  101  in order to drain wash water from the drum  103 . 
     A motor to rotate the drum  103  in alternating directions is mounted outside of the tub  102 . A flange shaft  108  and a rotary shaft  107  are mounted at the rear of the drum  103  to transmit the rotating force of the motor to the drum  103 . 
     The rotary shaft  107  is coupled to the center of the flange shaft  108  and extends to the outside of the tub  102  to be connected to the motor. The flange shaft  108  has a plurality of blades extending from the center thereof, to which the rotary shaft  107  is coupled, in the radial direction. Ends of the blades are fixed to the drum  103  by fixing members such as bolts. 
     Consequently, when the rotary shaft  107  is rotated by the motor, the flange shaft  108  coupled with the rotary shaft  107  is rotated. Accordingly, as the drum  103  connected to the flange shaft  108  is rotated, laundry in the drum  103  is washed or spin-dried. 
     The washing machine  100  according to the present embodiment further includes a heater  10  to heat wash water supplied into the tub  102 , thereby improving washing efficiency, and to perform an antibacterial function through boiling washing. 
       FIG. 5  is a cross-sectional view of the heater of  FIG. 1  taken along line AA′ according to an embodiment. 
     The heater according to the embodiment illustrated in  FIG. 5  has the same structure including a heating wire  140 , a magnesium oxide layer  150 , a stainless alloy layer  160 , and a coating layer  200  as the heater according to the embodiment illustrated in  FIG. 2 . However, protrusions and grooves  161  are formed on the surface of the stainless alloy layer  160  in the heater illustrated in  FIG. 5 . 
     The protrusions and grooves  161  may be formed on the surface of the stainless alloy layer  160  by sandblasting. Fine protrusions and grooves  201  may be formed on the surface of the coating layer  200  by fine particles of polysilsesquioxane. 
     The protrusions and grooves  161  formed on the surface of the stainless alloy layer  160  may improve adhesive force of the coating layer  200 . In addition, the fine protrusions and grooves  201  formed on the surface of the coating layer  200  may improve water repellency and facilitate bubble formation on the surface of the heater when heated by the heater, so that scales may be efficiently detached. 
     Now, the embodiments will be described in more detail with reference to the following examples. These examples are only provided to illustrate the present invention and should not be construed as limiting the scope and spirit of the present invention. 
     EXAMPLE 1 
     A water repellent fine powder was added to a coating composition including 20 g of a polysiloxane resin, 1 g of polysilsesquioxane silicon fine particles, 21 g of toluene, 16.7 g of 2-propaneol, and 0.24 g of a curing catalyst. The coating composition was stirred at room temperature for 30 min for uniform mixing to prepare a coating solution. The prepared coating solution was coated on the surface of a heating element of a heater by dip coating and cured at 110° C. for 30 min. 
     EXAMPLE 2 
     A coating was performed in the same manner as in Example 1, except that the heater was surface-treated by use of glass beads #80, and then the coating solution was applied thereto. 
     EXAMPLE 3 
     A coating was performed in the same manner as in Example 2, except that 0.2 g of the polysilsesquioxane silicon fine particles were used. 
     EXAMPLE 4 
     Coating was performed in the same manner as in Example 2, except that 4 g of the polysilsesquioxane silicon fine particles were used. 
     EXAMPLE 5 
     Coating was performed in the same manner as in Example 2, except that a coating solution including 20 g of the polysiloxane resin, 0.1 g of the polysilsesquioxane silicon fine particles, 62.5 g of toluene, 16.7 g of 2-propaneol, and 0.24 g of the curing catalyst was used. 
     EXAMPLE 6 
     Coating was performed in the same manner as in Example 2, except that a coating solution including 20 g of the polysiloxane resin, 20 g of the polysilsesquioxane silicon fine particles, and 0.24 g of the curing catalyst was used. 
     COMPARATIVE EXAMPLE 1 
     A general heater that is not coated was used in Comparative Example 1. The surface of a heating element was formed of a rust-resistant iron plate. 
     COMPARATIVE EXAMPLE 2 
     A coating was performed in the same manner as in Example 1, except that the polysilsesquioxane silicon fine particles were not used. 
     EXPERIMENTAL EXAMPLE 1 
     The amounts of scales accumulating on the surface of the heater of Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated. Hard water having a concentration of 1000 ppm was used and prepared in accordance with IEC 60734, 3rd edition. An accelerated life test was performed 20 times by conducting an operation in hard water for 10 min and stopping the operation for 10 min. The results are shown in Table 1 below. Here, ‘∘’ indicates that scales formed on the surface of the heater are easily removed by weak water flow, ‘Δ’ indicates that scales are partially removed by strong water flow, and ‘×’ indicates that scales are not removed. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example  
                 Example 
                 Example 
                 Comparative 
                 Comparative 
               
               
                 Item 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 Example 1 
                 Example 2 
               
               
                   
               
             
            
               
                 Amount of scale 
                 0.223 
                 0.13 
                 0.345 
                 0.34 
                 0.36 
                 0.41 
                 1.1 
                 0.86 
               
               
                 accumulation (g) 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Scale removal 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 Δ 
                 x 
               
               
                 efficiency 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, it was confirmed that the amount of scales formed on the heater coated with the coating composition including the polysiloxane resin according to Examples 1, 2, 3, 4, 5, and 6 was relatively low. In addition, when the polysilsesquioxane silicon fine particles were mixed with the polysiloxane resin, the amount of scales was further reduced. In addition, when the surface of the heater had protrusions and grooves formed by treatment with beads, the amount of scales was further reduced. In addition, it was confirmed that, under the same conditions, scale reduction efficiency using the polysilsesquioxane silicon fine particles in an amount less than 1 g (Example 3) or in an amount greater than 1 g (Example 4) was lower than scale reduction efficiency using 1 g of the polysilsesquioxane silicon fine particles (Example 2). In addition, it was confirmed that the formation of scales was reduced by adjusting the amounts of the solvent and the polysilsesquioxane silicon fine particles in the coating solution in terms of the weight% of the polysilsesquioxane silicon fine particles. Scale reduction efficiency using the polysilsesquioxane silicon fine particles in an amount less than 1.7% by weight (Example 5) or greater than 1.7% by weight (Example 6) was lower than scale reduction efficiency using 1.7% by weight of the polysilsesquioxane silicon fine particles (Example 2). 
     When the polysiloxane resin was mixed with the polysilsesquioxane silicon fine particles, the amount of scales was reduced by about 80% in comparison with the heater that is not coated. When the protrusions and grooves were formed on the surface of the heater, the amount of scales was reduced by about 89% in comparison with conventional heaters. 
     In addition, it was confirmed that scales that had already been formed were easily removed by weak water flow when the polysiloxane resin was mixed with the polysilsesquioxane silicon fine particles. 
     As described above, when the polysiloxane resin is mixed with the polysilsesquioxane silicon fine particles, the amount of scales may be reduced and adhesive force of scales may be reduced. 
     EXPERIMENTAL EXAMPLE 2 
     The amounts of scales formed on the heater according to Example 2 and Comparative Example 2 were evaluated. In particular, in order to identify problems occurring after long-term use, long-term accelerated life tests were performed. Hard water having a concentration of 1000 ppm was used and prepared in accordance with IEC 60734, 3rd edition. The accelerated life test was performed 240 times by conducting an operation in hard water for 10 min and stopping the operation for 10 min. These conditions for the accelerated life test correspond to those after 4-year use based on a calculation method. The results are shown in Table 2 below. Here, ‘∘’ indicates that scales formed on the surface of the heater are easily removed by weak water flow, ‘Δ’ indicates that scales are partially removed by strong water flow, and ‘ 33  ’ indicates that scales are not removed. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Item 
                 Example 2 
                 Comparative Example 2 
               
               
                   
               
             
            
               
                 Amount of scale accumulation (g) 
                 1.6 
                 7.9 
               
               
                 Scale removal efficiency 
                 ∘ 
                 x 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, it was confirmed that the amount of scales was reduced when the polysiloxane resin was mixed with the polysilsesquioxane silicon fine particles. When the polysilsesquioxane silicon fine particles were added, the amount of scales was reduced by about 80% compared with the case in which the polysilsesquioxane silicon fine particles were not used. In addition, scales that had already been formed were easily removed by weak water flow. 
     EXPERIMENTAL EXAMPLE 3 
     Adhesive force of the coating compositions according to Examples 1 and 2 to the surface of the heater was evaluated. Hard water having a concentration of 1000 ppm was used and prepared in accordance with IEC 60734 3rd edition. An accelerated life test was performed 20 times by conducting an operation in hard water for 10 min and stopping the operation for 10 min. The results are shown in  FIGS. 6 and 7 .  FIG. 6  is a photograph showing the test result of Comparative Example 2, and  FIG. 7  is a photograph showing the test result of Example 2. 
     As illustrated in  FIGS. 6 and 7 , when the surface of the heater was not treated, the coating was delaminated. When the surface of the heater was treated to form protrusions and grooves, and then coated with the coating composition, the coating was not delaminated. As described above, when the surface of the heater was treated, the coating layer was stably adhered to the surface of the heater. 
     As described above, the amount of scales may be reduced, and the scales may be efficiently removed. In addition, when the surface of the heater is processed to form protrusions and grooves, delamination of the coating may be prevented. 
     Furthermore, cracks may occur on the surface of conventional heaters. Cracks accelerate the formation of scales and increase physical adhesive force of the scales. When additional heat treatment is performed to prevent cracks, non-stick ability may be deteriorated, so that the amount of scales decreases, and adhesive force of the scales increases. Since the coating composition according to an embodiment of the present invention includes polysilsesquioxane silicon mixed with the polysiloxane resin, fine protrusions and grooves are formed by the polysilsesquioxane silicon. In addition, by surface treatment of the heater, additional protrusions and grooves are formed on the surface, thereby reducing the formation of scales. 
     Conventionally, the surface of the heater is degraded after extended use. Although a conventional coating material needs to be cured at a temperature of 200 or higher, the heater cannot be heat-treated at a high temperature of 200 due to the structure thereof. Thus, interaction between the coating material and the heater is not completely performed. Accordingly, degradation of the surface of the heater causes incomplete electron coupling, thereby increasing accumulation of scales. Since the coating composition according to embodiments of the present invention is curable at a low temperature, the surface of the heater is not degraded after extended use. As a result, formation of scales may be prevented. 
     As is apparent from the above description, a coating composition according to the disclosed embodiments may prevent the formation and adhesion of scales on the surface of the heater, and thus defects may be reduced in electrical and electronic appliances. 
     Furthermore, the coating composition according to the disclosed embodiments may be cured at a low temperature and degradation does not occur on the surface of the heater after extended use thereof. As a result, formation of scales may be prevented. 
     Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.