Abstract:
An EMI/RF absorbing film may comprise a polymer resin, and metallic flakes dispersed in the resin. A method for manufacturing an EMI/RF absorbing material. The method may include dispersing metallic flake in the polymer resin, applying the polymer resin and dispersed metallic flake on a releasable medium, curing the polymer resin to form a film, and releasing the cured polymer resin film from the release medium.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 60/790,058, filed on Apr. 6, 2006, the disclosure of which is herein incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates generally to the field of materials used to absorb and shield electromagnetic interference (EMI), radio frequency radiation (RF), or other such phenomena (collectively EMI/RF) and the process for making such materials and in particular to a formed thermoplastic material which incorporates materials designed to absorb and shield electromagnetic interference and radio frequency radiation and the like from the user of an electronic device emitting such interference. 
     EMI and RF waves may be generated both by the internal components of an electronic device and sources external to the device. The EMI and/or RF radiation may cause a decrease in performance of electronic devices and interfere with wireless communication devices. EMI and RF radiation may be especially problematic for consumer electronics devices such as digital cameras, cellular or digital phones, RF transmitters and receivers, personal digital assistants, computers and computer components, and other such devices. The EMI and RF radiation may interfere with wireless transmissions causing an increase in noise that detracts from signal quality. Also, the EMI or RF generated by one electronic component in a device may disrupt the performance of another component in the same device. 
     Many consumer electronic devices, however, are made to be as compact as possible. This limits the ways in which EMI and RF radiation can be dealt with in these devices. 
     Accordingly it would be desirable to provide material or a process for producing a material that is capable of shielding EMI and/or RF that could otherwise interfere with the performance of an electronic device. It would also be desirable to provide a material that may be provided in a variety of configurations such that it may be used in a variety of electronic devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of the process of forming a thin thermoplastic film which incorporates elements to shield and absorb electromagnetic interference, radio frequency radiation, and the like according to one aspect of the innovation; 
         FIG. 2  is a schematic of the recirculation process for the heavy metal flake used to absorb and shield EMI/RF; 
         FIG. 3  is a drawing and brief explanation of the post calendering process for forming the thin film material; 
         FIG. 4  is a drawing and brief explanation of one method of forming a multi-layer product according to the innovation; and 
         FIG. 5  is a drawing and brief explanation of a second method of forming a multi-layer product according to the innovation. 
     
    
    
     SUMMARY 
     Some embodiments relate to an EMI/RF absorbing film, comprising a polymer resin, and metallic flakes dispersed in the resin. The EMI/RF absorbing film may be effective for absorbing at least a portion of electromagnetic waves having a frequency of about 10 mhz to about 40 ghz. The EMI/RF absorbing material may have a surface resistance of at least about 1MΩ. 
     Other embodiments relate to a method for manufacturing an EMI absorbing material. The method includes dispersing metallic flake in the polymer resin, applying the polymer resin and dispersed metallic flake on a releasable medium, curing the polymer resin to form a film, and releasing the cured polymer resin film from the release medium. The EMI/RF absorbing material made by the method may be effective for absorbing at least a portion of electromagnetic waves having a frequency of about 10 mhz to about 40 ghz. 
     Yet other embodiments relate to electronic devices including the EMI/RF absorbing material. 
     DETAILED DESCRIPTION 
     A process for forming a sheet of flexible and moldable EMI/RF absorbing material may generally include coating a thermoplastic resin suspension of an energy absorbing flake onto a release medium and curing in a continuous process (referred to interchangeably as transfer coating or cast coating). The release medium may then be stripped and discarded in subsequent processes to yield the unsupported energy absorbing sheet material. 
     In some embodiments the material may include a flexible, moldable, thermoplastic resin such as a polyurethane resin. The cured resin must be sufficiently strong, but is preferably flexible enough for the material to be used in a variety of electronic devices. One suitable polyurethane resin is SOLUCOTE 6629 available from SOLUOL, INC., of East Providence, R.I. SOLUCOTE 6629 is an aliphatic polyurethane in a solvent blend of toluene, isopropyl alcohol (IPA), and dimethylformamide (DMF). Other commercially available resin systems may be used including polyvinylchloride, polyethylenes, polypropylenes, and other thermoplastic resins. In other embodiments, various natural and/or synthetic polymers, may be used. 
     To provide the material with the desired EMI/RF shielding characteristics, a heavy metal flake may be added to the solvated resin. In some embodiments, a permalloy flake (containing nickel, iron, and optionally molybdenum) is used. Some or all of the permalloy flake may be coated, for example, with a ceramic material. In embodiments where a mixture of coated and uncoated permalloy flake is present, the weight ratio of coated to non coated permalloy flake may be in the range from about 1:10 to 10:1. In some of these embodiments, the ratio may more desirably be from about 2:1 to 8:1, and even more preferably from about 3:1 to 6:1. In other embodiments, other EMI/RF absorbing materials may be used. For example, metal powders or ceramic materials with suitable absorption characteristics may be used. 
     To facilitate mixing of the metal flake in the resin, a dispersing and/or wetting agent may be used. The dispersant may be any known dispersant capable of use in solids suspension mixtures. One suitable dispersing agent is DISPERBYK-111, commercially available from BYK-CHEMIE GmbH of Wesel, Germany. Other suitable dispersants include, but are not limited too; acrylates, alcohols, alkyds, amino compounds, betaines, sulfonates, carboxylic acids, epoxies, fatty acids, glycols, mineral oils, oxazolidines, phosphorus based compounds, silicon based compounds, low molecular weight polymers and waxes. Optionally, other agents that aid in dispersion of the flake may be used. Such agents include leveling aids, defoamers, anti-defoamers, surfactants, and other suitable agents. 
     Additional solvent may be used to obtain a desired viscosity for processing the material. In an exemplary embodiment, a combination of toluene, DMF, and IPA may be used. Alternatively, other solvents that are compatible with the resin may be used. The solvent mixture may be chosen to optimize material flow and curing/drying times. Other suitable solvents may be selected based on the selected polymer. 
     A wide variety of natural and synthetic resins may be used. Each such resin will present unique solvent and dispersant requirements. While the flake loading in the material may be adjusted for specific applications, the use of coated flake without any uncoated flake may yield adequate flake dispersions. However, it may be more cost effective to include a portion of uncoated flake. A slurry of flake, resin, and solvent may incorporate up to 40 wt % or more of metallic flake (i.e. combined coated and uncoated flake). In some embodiments, the material may be made from a mixture comprising the following components: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Weight Percent 
               
               
                   
                   
               
             
             
               
                   
                 Resin (including solvent) 
                 20-60%   
               
               
                   
                 Dispersing Agent 
                 0.5-2%  
               
               
                   
                 Uncoated Flake 
                 0-40% 
               
               
                   
                 Coated Flake 
                 0-40% 
               
               
                   
                 Total Flake 
                 4-40% 
               
               
                   
                 DMF 
                 0-30% 
               
               
                   
                 IPA 
                 0-30% 
               
               
                   
                 Toluene 
                 0-30% 
               
               
                   
                   
               
             
          
         
       
     
     In some embodiments, the solvated material is coated onto a transfer medium. The transfer medium is preferably a releasable medium such as release paper. Other mediums may be used provided they have adequate adhesion and release properties. One release medium that is suitable for use in the process is flat paper such as T2000s and other grained paper each coated with polypropylene, silicone or acrylics may work. Another example would be S/K VLC release paper available from S.D. WARREN of Westbrooke, Me. 
     The solvated material may be dispensed from a tank to a coating machine where the solvated material is coated onto the release medium. 
     As shown in  FIG. 1 , system  10  includes a release medium roll  12 , and one or more coating subsystems  14 ,  16 ,  18 , and  20 . Each coating subsystem in turn includes a dispersing tank ( 22 ,  24 ,  26 , and  28 ), a coater ( 30 ,  32 ,  34 , and  36 ), and a curing system shown as ovens  38 ,  40 ,  42 , and  44 . A coated release medium is wound onto an uptake roll  46 . 
     According to this embodiment, a release paper  48  is fed to coater  30  which is shown as a knife and roll type coater. Solvated material is fed to the coater  30  to place a layer of material on the release paper  48 . The paper and solvated material are fed to oven  38  which is shown as a two stage oven. Any of the ovens described herein may be single or multistage ovens. The solvated material is dried leaving a layer of metal flake suspended in a polymer matrix coated onto the release paper. 
     The coated paper may then be passed through subsequent coaters  32 ,  34 , and  36  and ovens  40 ,  42 , and  44  to gradually increase the thickness of the coating on the release paper. The preferred multistage process reduces individual drying times in the oven which may help avoid flake settling within the solvated material. This provides a more uniform distribution of flake throughout the total thickness of the coating. After all the desired layers of material have been applied and dried, the coated release paper is taken up on take-up roll  46  and may be transferred to additional processing steps or stored. Alternatively, the coated release paper may be fed directly to another process (e.g., calendering) without being rolled. 
     In downstream processing, the release paper may be removed to yield an unsupported EMI/RF absorbing film. The film may be calendered to increase the density of the film and improve its EMI/RF absorption or processing characteristics. The film may also be laminated to provide an EMI/RF barrier of a desired thickness. 
       FIG. 2  illustrates a recirculation system that may be used to prevent settling of the suspended flake in the dispersing tanks of system  10  shown in  FIG. 1 . Subsystem  50  includes dispersing tank  52 , outlet  54 , pump  56 , valve  58 , recirculating line  60  and line  62 . Pump  56  draws flake laden resin from dispersing tank  52  and directs the flow to valve  58 . Valve  58  is shown as a three way valve which may be controlled to direct a portion of the flow back to dispersing tank  52 . The recirculating flow keeps the contents of the dispersing tank  52  moving within the tank and aids in preventing settling of the flake. A portion of the flow through valve  58  may also be directed to line  62  which delivers the flake laden resin to a coating head. 
     According to some embodiments, the ratio of material taken off to the coater to material recirculated to the dispersing tank may be from about 0%:100% to 100%:0%. The ratio selected for a given application will depend on a variety of factors including the volume of the dispersing tanks, flow rates required for the coater and other operating parameters. 
       FIG. 3  shows a post calendering process. The film produced by system  10  may not be suitable as a finished product. Accordingly, the film may be calendered to increase density and/or decrease thickness. Film  64  may be passed through rollers  66  to adjust the thickness and/or density of the film to provide a product  68  having adequate properties. Rollers  66  may be heated and/or cooled as needed. While shown as a two roller calender, three or more rolls may be used. Preferably, the multilayer film is calendered to a density of about 1.5 g/cm 3  to 3.5 g/cm 3 . 
       FIG. 4  shows another post calendering process. Multiple films  70   a ,  70   b , and  70   c  may be passed through a calender  72  to provide a product  74  having a desired thickness and/or density. Again calender  72  may be heated and/or cooled, and may include two or more rollers. Heated calenders may improve lamination of layers  70   a - c  with each other. Each of layers  70   a - c  may be the same or different (e.g., the layers may vary in solids content, composition, thickness, etc.). Each of layers  70   a - c  may independently be a monolayer material or a multilayer material (such as those produced by system  10  shown in  FIG. 1 ). While three layers are shown, two layers, or four or more layers may be laminated. 
       FIG. 5  shows yet another post calendering process. Layers  76   a - e  of EMI/RF absorbing material may be laminated in a flat bed laminator  78  to provide product  82 . The layers may be alternated with adhesive web, or the layers may be joined as a result of heat and pressure applied to the layers which include a thermoplastic resin as it passes between belt rollers  80   a  and  80   b.    
     Example 1 
     A four layer film was produced according to the process illustrated in  FIG. 1 . The first layer deposited on the release medium was a low solids content mixture having the following composition: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Weight Percent 
               
               
                   
                   
               
             
             
               
                   
                 Polyurethane Resin (SOLUCOTE 6629) 
                 52.7% 
               
               
                   
                 Dispersing Agent (DISPERBYK 111) 
                 1.57% 
               
               
                   
                 Uncoated Permalloy Flake 
                 5.21% 
               
               
                   
                 Coated Permalloy Flake 
                 20.28%  
               
               
                   
                 DMF 
                 6.57% 
               
               
                   
                 IPA 
                 6.57% 
               
               
                   
                 Toluene 
                 6.57% 
               
               
                   
                   
               
             
          
         
       
     
     The second, third, and fourth layers were a high solids composition of the following composition: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Weight Percent 
               
               
                   
                   
               
             
             
               
                   
                 Polyurethane Resin (SOLUCOTE 6629) 
                 34.82% 
               
               
                   
                 Dispersing Agent (DISPERBYK 111) 
                  0.66% 
               
               
                   
                 Uncoated Permalloy Flake 
                  8.67% 
               
               
                   
                 Coated Permalloy Flake 
                  34.5% 
               
               
                   
                 DMF 
                 13.98% 
               
               
                   
                 IPA 
                  3.68% 
               
               
                   
                 Toluene 
                  3.68% 
               
               
                   
                   
               
             
          
         
       
     
     The viscosity of the high solids composition was adjusted by addition of DMF until a viscosity of about 4,500 to 6,500 cps at ambient temperature was reached. 
     The total amount of material deposited was 560 g/m 2 . Of this, 80 g/m 2  was the low solids first layer. Each of the other three high solids layers were deposited in the amount of about 120 g/m 2 . Advantageously, it has been found that processed-film scrap may be re-solvated and used in one or more of the layers without appreciable negative impact on the properties of the film. The third layer was entirely comprised of recycled processed-film scrap. After each layer was deposited, the coated release paper was passed through a conveyor oven to cure the resin. 
     The film had a thickness of about 13 to 15 mils. The film was then calendered to a thickness of about 9 mils. Post calendering was also performed under heat and pressure. The post calendering temperatures and pressures will depend on the thickness of the film. The post calendering process may aid in aligning the flakes parallel to one another and thus improve the EMI/RF absorption of the film. 
     The multilayered film was tested for EMI/RF absorption and found to be acceptable for shielding electronic components from EMI/RF in the frequency range of about 10 mhz to about 40 ghz. In particular, EMI/RF in the frequency range of about 100 mhz to about 5 ghz was absorbed. The multilayered film had a surface resistance of at least about 1MΩ. 
     Although the foregoing has been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope thereof. For example, although different example embodiments may have been described as including one or more features providing one or more benefits, it is contemplated that the described features may be interchanged with one another or alternatively be combined with one another in the described example embodiments or in other alternative embodiments. The present subject matter described with reference to the example embodiments and set forth in the following claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements. Many other changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of these and other changes will become apparent from the appended claims. The steps of the methods described herein may be varied, and carried out in different sequences.