Patent Publication Number: US-2013251977-A1

Title: Conductive polymer foam elastic member

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Korean Patent Application No. 10-2012-0028366 filed on Mar. 20, 2012, and Korean Patent Application No. 10-2012-0031679 filed on Mar. 28, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a conductive polymer foam elastic body, and more particularly, to a conductive polymer foam elastic body pressed with a small force, having excellent restoring force, good elasticity, and good conductivity. 
     Also, the present invention relates to a conductive polymer foam elastic body having conductivity improved while being pressed by an external force. 
     BACKGROUND OF THE INVENTION 
     In order not to send outward electric waves generated by electronic components or modules for high frequency used in electric devices or information communication devices or to protect internal electronic components or modules from external electric waves, electric wave preventing gaskets having excellent restoring force and electrical conductivity, pressed with a small force, are used. 
     Such electroconductive gaskets have low electric resistance to provide high electrical conductivity, for example, lower than 1 ohm, and have excellent elasticity to be well restored against forces repetitively applied. 
     According to typical technology, to provide excellent elasticity and restoring force, a metallic layer is formed using electroless plating on the outside of an elastic foam body formed of polyurethane PU with porous to be conductive. 
     Since the electroconductive elastic foam body has an open cell structure, the metallic layer thinly formed on the outside of the elastic foam body may be easily cut or separated by externally provided repetitive forces in such a way that electrical conductivity of the entire electroconductive elastic foam body becomes lowered and separated metallic plating fragments may be bad influences on peripheral circuits. That is, the metallic layer with no elasticity, exposed outward, is easily cut or separated from the elastic foam body due to externally provided repetitive forces. 
     Also, when a material used at a relatively low temperature such as polyurethane is used as the elastic foam body, it is difficult to endure a temperature condition of reflow soldering performed at a relatively high temperature. 
     According to another typical method, a plurality of holes having a certain size is formed in an insulating elastic body such as a nonfoam silicone rubber sheet having elasticity, and then an electroconductive material such as electroconductive silicone rubber is inserted into the holes to be charged with electricity. 
     However, in this method, it is too expensive to form the plurality of small holes in the nonfoam silicone rubber sheet and it is difficult to uniformly insert the electroconductive silicone rubber into the holes having the relatively small size. 
     Also, restoring force and elasticity are deteriorated and a force needed to press is increased by metal powder such as carbon, nickel, silver, or copper mixed to improve electrical conductivity. 
     In the described above, as an example, an elastic body having electrical conductivity has been described. However, since being manufactured by using the described method, a thermally conductive elastic body well transferring heat may have similar disadvantages. 
     For example, due to ceramic powder such as alumina or boron mixed to improve thermal conductivity or since the silicone rubber sheet is nonfoam, a force needed to press is relatively great and restoring force and elasticity are low. 
     Accordingly, using the typical methods as described above, it is difficult to manufacture conductive elastic bodies having high electrical conductivity or thermal conductivity, pressed with a small force, and having excellent restoring force and elasticity, that is, conductive silicone foam rubber. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a conductive polymer foam elastic body, pressed with a small force, having excellent restoring force, good elasticity, and good conductivity. 
     Another object of the present invention is to provide a conductive polymer foam elastic body capable of being easily manufactured and having excellent productivity. 
     A still another object of the present invention is to provide a conductive polymer foam elastic body whose conductivity increases in a thickness direction while being pressed by an external force. 
     A yet another object of the present invention is to provide a conductive polymer foam elastic body whose conductive material is not damaged although external forces are repetitively applied thereto. 
     A further object of the present invention is to provide a conductive polymer foam elastic body whose heat-resisting temperature is improved. 
     A still further object of the present invention is to provide a conductive polymer foam elastic body capable of being surface-mounted by vacuum-pickup and being reflow-soldered by solder cream. 
     A yet further object of the present invention is to provide a conductive polymer foam elastic body using one of conductive polymer elastic coating layers as a skin layer in such a way that the conductive polymer foam elastic body is well vacuum-picked up, has improved waterproofing properties, is protected from impurities such as dusts, has a beautiful appearance, and a top surface and a bottom surface thereof may be easily distinguished from each other. 
     According to an aspect of the present invention, there is provided a conductive polymer foam elastic body comprising: a polymer foam body having an open cell structure in which a plurality of pores connected in a thickness direction is exposed on a top surface and a bottom surface therein; and conductive polymer foam elastic coating layers formed by curing and adhering a conductive liquid polymer mixed with conductive powder to the top surface and bottom surface of the polymer foam body and the inside of the pores to be connected to one another, wherein the conductive polymer foam elastic body has an open cell structure due to the open cell structure of the polymer foam body, and wherein, when pressing the conductive polymer foam elastic body in the thickness direction, conductivity of the conductive polymer flam elastic body increases due to increase of a contact between the conductive polymer elastic coating layers inside the pore caused by a reduction of a floor area ratio of the pore. 
     The conductivity may be at least one of thermal conductivity and electrical conductivity. 
     The polymer foam body may be a conductive polymer foam body plated with a metal on the top surface and the bottom surface thereof and the inside of the pores. 
     The polymer foam body may comprise a penetration hole penetrating the polymer foam body and having a diameter greater than the pore, and the conductive polymer elastic coating layer may be formed on an inner surface of the penetration hole. 
     Preferably, polymer resin of the conductive polymer elastic coating layers is a chemical composite different from polymer resin of the polymer foam body. 
     A heat-resisting temperature of the conductive polymer foam elastic coating layers may be the same as that of the polymer foam body or higher. 
     Preferably, the conductive polymer foam elastic body satisfies a temperature condition of reflow soldering by the conductive polymer foam coating layers. 
     Preferably, the conductive polymer elastic coating layers may be formed of nonfoam resin. 
     The elastic body may further comprise a metal foil, capable of being soldered and adhered to the conductive polymer elastic coating layers by the curing on one of the top surface and the bottom surface of the polymer foam body. 
     The elastic body may further comprise a conductive elastic member adhered to the conductive polymer elastic coating layers by the curing on another of the top surface and the bottom surface of the polymer foam body. 
     The elastic body may further comprise conductive elastic members adhered to the conductive polymer elastic coating layers by the curing on the top surface and the bottom surface of the polymer foam body, respectively. 
     The conductive polymer foam elastic body may be capable of being packed using a reel of tape and vacuum-picked up to be reflow-soldered. 
     According to another aspect of the present invention, there is provided a conductive polymer foam elastic body comprising: a polymer foam body having an open cell structure with a plurality of pores connected in a thickness direction therein; and conductive polymer elastic coating layers formed by curing and adhering a conductive liquid polymer mixed with conductive powder to one surface and another surface of the polymer foam body and the inside of the pores to be connected to one another, wherein the conductive polymer elastic coating layer formed on the one surface forms a skin layer and the plurality of pores are exposed toward only the other surface, wherein the conductive polymer foam elastic body has an open cell structure due to the open cell structure of the polymer foam body, and wherein, when pressing the conductive polymer foam elastic body in the thickness direction, conductivity of the conductive polymer flam elastic body increases due to increase of a contact between the conductive polymer elastic coating layers inside the pore caused by a reduction of a floor area ratio of the pore. 
     Preferably, vacuum-pickup is performed at the skin layer. 
     Preferably, a thickness of the skin layer may be ¼ or less of a thickness of the conductive polymer foam elastic body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and other advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a view illustrating a conductive polymer foam elastic body according to an embodiment of the present invention; 
         FIG. 2  is a view illustrating a conductive polymer foam elastic body according to another embodiment of the present invention; 
         FIG. 3  is a view illustrating a conductive polymer foam elastic body according to still another embodiment of the present invention; and 
         FIG. 4  is a conductive polymer foam elastic body according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a conductive polymer foam elastic body  100  according to an embodiment of the present invention. 
     The conductive polymer foam elastic body  100  includes a polymer foam body  10  having an open cell structure with a plurality of pores  12  connected in a thickness direction and conductive polymer elastic coating layers formed by curing and adhering a conductive liquid polymer mixed with conductive powder to a top surface and a bottom surface of the polymer foam body  10  and an inner surface of the pores  12  to be electrically or thermally connected to one another. 
     The pores  12 , as well known to a person skilled in the art, are formed by using a chemical foaming agent and the like and are formed in the shape of a cave to be connected to one another in the open cell structure, and the conductive polymer elastic coating layer  24  is formed on the inner surface of the pore  12 . 
     Polymer resin forming the polymer foam body  10  may be a chemical composite different from polymer resin forming the conductive polymer elastic coating layers  20 ,  22 , and  24 , and the polymer foam body  10  may be generally but may be conductive if necessary. 
     Conductivity may be one of thermal conductivity and electrical conductivity or may include both the same. In other words, the conductive polymer elastic coating layers  20 ,  22 , and  24  may have one of thermal conductive powder and electrical conductive powder and may have both the same. 
     In the case of the conductive polymer elastic coating layer  20 ,  22 , and  24 , when the conductive powder is the electrical conductive powder, electrical resistance may be 1 ohm or less and is not limited thereto and may be 108 ohm to easily remove static electricity. Also, when the conductive powder is the thermal conductive powder, thermal conductivity may be 1 W/m·k or more but is not limited thereto. 
     In the present embodiment, when the conductive polymer foam elastic body  100  is pressed in the thickness direction, conductivity thereof increases. The conductive polymer foam elastic body  100  may be used while being pressed to 5% or more in the thickness direction. 
     The polymer foam body  10  is manufactured by using a material used at a relatively low temperature and available to easily manufacture an open cell structure. A heat-resisting temperature of the material forming the polymer foam body  10  is lower than a heat-resisting temperature of a material forming the conductive polymer elastic coating layers  20 ,  22 , and  24 . 
     The polymer foam body  10  may have the open cell structure with pores continuously formed in a vertical direction in which skin layers are not formed on the top and bottom surfaces thereof. However, as follows, a skin layer may be formed for a certain purpose. 
     The polymer foam body  10  may be formed in the shape of a thin sheet having a uniform thickness, and for example, the thickness of the polymer foam body  10  may be 0.2 mm to 3 mm but is not limited thereto. 
     The polymer foam body  10  may have a flat surface to be easily vacuum-picked up. 
     As described above, the polymer foam body  10  having a relative low heat-resisting temperature and capable of being easily manufactured as the open cell structure is provided as an inner core. 
     In the structure as described above, since the polymer foam body  10  having the relative low heat-resisting temperature and capable of being easily manufactured as the open cell structure is used as the inner core, the conductive polymer foam elastic body  100  is pressed with a small force, has excellent restoring force and elasticity, is easily manufactured, and has excellent productivity. 
     In addition to giving conductivity via the pores  12 , a penetration hole vertically penetrating the polymer foam body  10  and having a diameter greater than the pores  12  may be additionally formed with a conductive polymer elastic coating layer formed on an inner surface thereof, thereby increasing conductivity. 
     The conductive polymer elastic coating layers  20 ,  22 , and  24  may be formed by curing one of an electroconductive liquid polymer with electroconductive powder, such as carbon, nickel, copper, and silver, equally dispersed therein and a thermally conductive liquid polymer with thermally conductive ceramic powder such as alumina and boron, equally dispersed therein. As a liquid polymer, one of silicone rubber and fluoride resin may be used. 
     The polymer resin for the conductive polymer elastic coating layers  20 ,  22  and  24  is thermally cured but is not limited thereto. One of the electroconductive and thermally conductive polymers are self-attached to the polymer foam body  10  by being cured. 
     The conductive polymer elastic coating layers  20 ,  22 , and  24  may be nonfoam coating layers. 
     A heat-resisting temperature of the conductive polymer elastic coating layers  20 ,  22 , and  24  may be the same as the heat-resisting temperature of the polymer foam body  10  or higher than the same, which makes a heat-resisting temperature of the entire conductive polymer foam elastic body  100  high. 
     For example, the polymer resin material for the polymer foam body  10  may be PE having low heat-resisting properties and the polymer resin material for the conductive polymer elastic coating layers  20 ,  22 , and  24  may be silicone rubber having high heat-resisting properties and excellent elasticity. 
     In the case of the structure as described above, when being reflow-soldered or being used at a high temperature, the conductive polymer elastic coating layers  20 ,  22 , and  24  may protect the polymer foam body  10  with the relative low heat-resisting temperature. 
     Accordingly, it is impossible to satisfy a reflow-soldering condition by using only the polymer foam body  10 , but the reflow-soldering condition may be satisfied by using the conductive polymer elastic foam layers  20 ,  22 , and  24  when the conductive polymer elastic foam layers  20 ,  22 , and  24  are adhered to the polymer foam body  10  to be formed as a single body. 
     Also, according to the structure as described above, it is possible to increase a heat-resisting temperature by using the polymer foam body  10  being easily manufactured with relatively low costs and having the open cell structure with no skin layer formed on top and bottom thereof, thereby providing proper quality at a proper price. 
     In addition, regardless of the nonfoam conductive polymer elastic coating layers  20 ,  22 , and  24 , the conductive polymer foam elastic body  100  may be allowed to have an open cell structure by using the open cell structure of the polymer foam body  10  capable of being easily manufactured at relatively low prices. 
     Also, due to the open cell structure of the polymer foam body  10 , when pressing the conductive polymer foam elastic body  100  in a vertical direction, the conductivity of the conductive polymer foam elastic body  100  is increased. That is, when the thickness of the polymer foam body  10  is reduced by pressing the same, the air is discharged from the pores  12  and a floor area ratio of the pores  12  is reduced in such a way that a conductive distance becomes shorten and a mutual contact between the conductive polymer elastic coating layers  24  increases inside the pores  12 , thereby increasing the conductivity. 
     In this case, the thickness of the conductive polymer elastic coating layer  24  formed on the inner surface of the pore  12  is not particularly limited. However, when filling the entire pore  12 , conductivity increases but a force need to press increases and restoring force and elasticity are deteriorated. Accordingly, the conductive polymer elastic coating layer  24  may be formed in such a way that a horizontal cross-sectional area of the conductive polymer elastic coating layer  24  is 90% or less of a horizontal cross-sectional area of the pore  12  when it is assumed that the polymer foam body  10  is cut horizontally. Also, a total weight of the conductive polymer elastic coating layers  20 ,  22 , and  24  may be greater than that of the polymer foam body  10 . 
     Although not shown in the drawings, as another embodiment, the polymer foam body  10  may be an electroconductive polymer foam body in which the top and bottom surfaces thereof and the inner surfaces of the pores  12  are plated with a metal. 
     The electroconductive polymer foam body, basically, has excellent electrical conductivity, but as described above, may be easily damaged at a metal plating layer due to an outer environment. However, to the electroconductive polymer foam body, the conductive polymer elastic coating layers  20 ,  22 , and  24  are applied, thereby protecting a conductive material from the outside. For example, although external forces are repetitively applied, the metal of the metal plating layer is not broken and fewer fragments thereof are generated. 
     A method of manufacturing the conductive polymer foam elastic body  100  will no be described as follows. 
     Electroconductive liquid silicon rubber with copper powder coated with silver having a size of 0.035 mm or less equally dispersed therein is poured over the polyurethane (PU) foam body  10  formed in the shape of a sheet having a thickness of about 2 mm and having an entirely open cell structure including top and bottom surfaces, and then the electroconductive liquid silicone rubber is allowed to fully pass throughout the pores  12  of the polymer foam body  10  by squeeze-printing. 
     In this case, pressure and time for squeezing the polymer foam body  10  are accurately controlled or an amount of the electroconductive liquid silicone rubber is controlled to allow the conductive polymer foam elastic body  100  to have low heat-resistance or low electrical resistance and to have excellent restoring force and elasticity. 
     As a result thereof, to the top and bottom surfaces of the polymer foam body  10  and the inner surfaces of the pores  12 , the conductive liquid silicone rubber is continuously and uniformly applied. 
     In this state, when curing the conductive liquid silicone rubber by using heat, the conductive liquid silicone rubber applied to the top and bottom surfaces of the polymer foam body  10  and the inner surfaces of the pores  12  is adhered to the polymer foam body  10  and forms the conductive polymer elastic coating layers  20 ,  22 , and  24 , thereby manufacturing the conductive polymer foam elastic body  100 . 
     The weights of the conductive polymer elastic coating layers  20 ,  22 , and  24  may be 60% or more of the weight of the entire the conductive polymer foam elastic body  100 . 
       FIG. 2  is a view illustrating a conductive polymer foam elastic body  200  according to another embodiment of the present invention. 
     In the present embodiment, a nonfoam conductive polymer elastic coating layer  22  formed on a bottom surface of the polymer foam body  10  forms a skin layer  28  and a plurality of the pores  12  is connected only to a top surface of the polymer foam body  10 . 
     A thickness of the skin layer  28  may be ¼ or less of a thickness of the conductive polymer foam elastic body  200 . When being thicker than this, not only a pressing force and a weight thereof increase but a large amount of a material is consumed, which is uneconomical and an elastic restoring force becomes deteriorated. 
     A skin layer may be formed by using a film base for transferring the polymer foam body  10  in a manufacturing process. That is, to mass-produce the conductive polymer foam elastic bodies  200  by using the polymer foam bodies  10  having a small thickness and being easily torn, the conductive polymer foam elastic bodies  200  are manufactured by using a roll to roll method using a film base. 
     In detail, the polymer foam body  10  is added onto a polymer film such as a PET film to be supplied as a roll to smoothly transfer the polymer foam body  10  and is continuously impregnated with conductive liquid silicone rubber containing conductive powder in such a way that the conductive liquid silicone rubber fully passes throughout the pores  12  of the polymer foam body  10 . Then, a part of the conducive liquid silicone rubber percolating the pores  12  of the polymer foam body  10  is continuously squeezed and removed by a roll or a blade and then is continuously thermally processed and cured, thereby continuously manufacturing the conductive polymer foam elastic bodies  200 . 
     As described above, when the polymer foam body  10  is formed on a film to manufacture the conductive polymer foam elastic body  200  by using the roll to roll method, on a part in contact with the film for the polymer foam body  10 , the pores  12  is filled with the conductive liquid silicone rubber transferred downwardly due to the force of gravity, thereby forming the skin layer  28 . 
     That is, the skin layer  28  is a nonfoam conductive elastic coating layer with no pores  12 , in which the density of the contained conductive powder increases, thereby increasing conductivity. 
     In the case of the roll to roll method, since a polymer film is generally pulled, a slip may occur between the polymer film and the polymer foam body  10  added thereto not to be pulled. To prevent this, the polymer film having a self-adhesive adhesive (PSA) on one surface thereof may be used and the polymer foam body  10  may be added to the adhesive. 
     On the other hand, referring to  FIG. 2 , the conductive powder equally dispersed in conductive liquid polymers are transferred downwardly by own weight thereof in such a way that the density of the conductive powder in the skin layer  28  largely increases, thereby increasing the conductivity. 
     According to the configuration as described above, since the pores  12  are blocked in the skin layer  28 , vacuum-pickup is well performed at the skin layer  28 , water proofing properties are improved, impurities such as dusts are prevented, and an appearance thereof is beautiful. 
     Also, due to the increase of the density of the conductive powder in the skin layer  28  by the own weight thereof, horizontal conductivity is largely increased. 
     Also, due to a difference between degrees of reflecting light by the skin layer  28  and an opposite surface, it is possible to easily distinguish the top and bottom surfaces with naked eyes. 
       FIG. 3  is a view illustrating a conductive polymer foam elastic body  300  according to still another embodiment of the present invention. 
     According to the conductive polymer foam elastic body  300 , a metal foil  30  capable of being soldered is adhered to the top surface of the polymer foam body  10  by curing the conductive polymer elastic coating layer  20  and a nonfoam conductive elastic member  40  such as conductive silicone rubber is adhered to the bottom surface thereof by curing the conductive polymer elastic coating layer  22 . 
     In this case, the metal foil  30  and the nonfoam conductive elastic member  40  may be deposited on the bottom surface and the top surface, respectively, and may be deposited on any one thereof. 
     Also, the nonfoam conductive elastic member  40  may be formed of the same material as the conductive polymer elastic coating layers  20  and  22 . 
     Since the pores  12  in the polymer foam body  10  are not exposed outside the top and bottom surfaces of the polymer foam body  10  by the nonfoam conductive elastic member  40  and the metal foil  30 , an appearance thereof is beautiful and it is possible to prevent water and impurities passing through the top and bottom surfaces. 
     In a manufacturing process, conductive liquid silicone rubber is applied to the top and bottom surfaces of the polymer foam body  10  and the metal foil  30  and the nonfoam conductive elastic member  40  are located on the top and bottom surfaces, respectively, and cured as a single body, thereby adhering the metal foil  30  and the nonfoam conductive elastic member  40  to the conductive polymer elastic coating layers  20  and  22 . 
     Conductivity of the nonfoam conductive elastic member  40  may be one of electrical conductivity and thermal conductivity or may be both the same. 
     The conductive polymer foam elastic body  300  with the metal foil  30  formed on one side thereof is reel-packed by a carrier tape and may be reflow-soldered by using a surface mounting process. 
       FIG. 4  is a view illustrating a conductive polymer foam elastic body  400  according to yet another embodiment of the present invention. 
     In the present embodiment, the nonfoam conductive elastic members  40  such as conductive silicone rubber are adhered to the top and bottom surfaces of the polymer foam body  10 , respectively, by curing the conductive polymer elastic coating layers  20  and  22 . 
     According to the structure as described above, since the nonfoam conductive elastic members  40  having excellent conductivity and elasticity are deposited on both the top and bottom surfaces of the polymer foam body  10 , not only the conductivity increases but elasticity is improved comparing with the embodiment of  FIG. 3 . 
     Since the pores  12  in the polymer foam body  10  are not exposed outside the top and bottom surfaces of the polymer foam body  10  by the nonfoam conductive elastic members  40 , an appearance thereof is beautiful and it is possible to prevent water and impurities passing through the top and bottom surfaces. 
     While the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.