Abstract:
Low radio frequency loss, static dissipative adhesives are disclosed that have at most 2 percent by weight amorphous carbon dispersed in a cyanate ester resin. The adhesive has an electrical conductivity that dissipates static charge with an electrical resistivity of about 3.0×10 5  ohms to about 3.0×10 10  ohms, and low radio frequency loss with a dielectric constant of about 1.3 to about 1.7 and a loss tangent of at most 0.006 measured at 2 gigahertz.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/397,804, filed Mar. 4, 2009. 
    
    
     FIELD 
     The present patent application relates to synthetic resins and, more particularly, to low radio frequency loss, static dissipative adhesives. 
     BACKGROUND 
     Due to random electro-static discharges, all metal parts on a spacecraft are typically grounded to bleed off static charges, thereby avoiding catastrophic damage to the surrounding electronic hardware. For example, a space radar antenna is typically comprised of a plurality of radio frequency (“RF”) radiating elements, commonly referred to as patches, that are supported by lightweight foam tiles. These patches, which are typically made from metal, must be grounded. 
     One known technique for grounding RF radiating patches to lightweight foam tiles involves the use of conductive mechanical fasteners, such as metal pins. Such conductive mechanical fasteners secure the patches to the lightweight foam tiles while at the same time providing a conductive path for dissipating static charges. However, conductive mechanical fasteners add complexity to the design and are not practical for use with non-conductive lightweight foam tiles. 
     An alternative technique for grounding RF radiating patches to lightweight foam tiles involves the use of conductive adhesives. Conductive adhesives are attractive because they simplify the manufacturing process due to their ease of use. Furthermore, like conductive metal fasteners, they secure (i.e., bond) the patches to the lightweight foam tiles while at the same time providing a conductive path for dissipating static charges. However, the large quantities (e.g., 5 to 10 percent by weight) of conductive fillers, such as carbon powder, graphite, ceramic and metal, in conductive adhesives impart the adhesives with high RF loss properties and, therefore, cannot be used in applications, such as space radar antenna, that require low RF loss. 
     Accordingly, there is a need for a static dissipative adhesive that has low RF loss properties. 
     SUMMARY 
     In one aspect, the disclosed low loss static dissipative adhesive may include at most 2 percent by weight amorphous carbon dispersed in a cyanate ester resin. The adhesive has an electrical conductivity that dissipates static charge with an electrical resistance of about 3.0×10 5  ohms to about 3.0×10 10  ohms, and has low radio frequency loss with a dielectric constant of about 1.3 to about 1.7 and a loss tangent of at most 0.006 measured at 2 gigahertz. 
     In one embodiment, the amorphous carbon has an average particle size of at most 25 microns. In another embodiment, the amorphous carbon is carbon black, which may be mechanically processed to an average particle size of less than 30 nm. 
     In another aspect, the quantity of amorphous carbon dispersed in a cyanate ester resin may be at most 1.5 percent by weight of the adhesive or at most 1 percent by weight of the adhesive. 
     In another aspect, the adhesive may be formulated as a liquid, paste, or film. For the liquid and/or paste formulations, the adhesive may include a solvent. In other embodiments, the adhesive may include a pigment. 
     In one embodiment, the cyanate ester resin is a bisphenol-A-based cyanate ester resin. 
     In one embodiment, the amorphous carbon is present in a quantity sufficient to impart the adhesive with an electrical resistance of at most 10 9  Ohms. In another embodiment, the amorphous carbon is present in a quantity sufficient to impart the adhesive with an electrical resistance of at most 10 7  Ohms. 
     Other aspects of the low loss static dissipative adhesives of the present disclosure will become apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphical illustration of return loss versus frequency for an exemplary low loss static dissipative adhesive in accordance with the present disclosure; and 
         FIG. 2  is a graphical illustration of insertion loss versus frequency for the exemplary low loss static dissipative adhesive of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     It has now been discovered that adhesives prepared by dispersing relatively small quantities of amorphous carbon in a cyanate ester resin exhibit desired electrical conductivity and low RF loss properties. 
     In one aspect, the amorphous carbon useful in the formulations of the present disclosure may be any amorphous carbon having a relatively low average particle size and relatively high surface area. Those skilled in the art will appreciate that typical low average particle sizes for amorphous carbon may range from several microns to several nanometers. For example, the amorphous carbon in the formulations of the present disclosure may have an average particle size of about 1000 nanometers or less, or 500 nanometers or less, or 250 nanometers or less, or 100 nanometers or less, or 50 nanometers or less, or 30 nanometers or less. 
     In one particular aspect, the amorphous carbon useful in the formulations of the present disclosure may be carbon black. An example of a useful and commercially available carbon black is PRINTEX® XE2, which is available from Evonik Industries (formerly Degussa) of Essen, Germany. PRINTEX® XE2 carbon black has an average particle size of about 30 nanometers and a surface area of about 910 square meters per gram. Other useful commercially available carbon black pigments will be readily apparent to those skilled in the art. 
     At this point, those skilled in the art will appreciate that the average particle size of commercially available amorphous carbons may be further reduced by mechanical processing. For example, PRINTEX® XE2 carbon black may be milled with ceramic media to achieve an average particle size of less than 30 nanometers. 
     The cyanate ester resins useful in the formulations of the present disclosure may be any thermosetting resin wherein the hydrogen atom of a hydroxyl group (—OH) is substituted with a cyanide group (—CN) to form a cyanate ester group (—OCN). For example, the cyanate ester resins may be bisphenol-A-based or novolac-based cyanate ester resins. An example of a useful and commercially available cyanate ester resin is EX-1515 resin, which is available from TenCate Advanced Composites USA, Inc. of Morgan Hill, Calif. 
     The disclosed low loss static dissipative adhesives may be prepared by dispersing the amorphous carbon in the cyanate ester resin. A solvent, such as methyl ethyl ketone or toluene, may be used to reduce the viscosity of the resin to assist in dispersing the amorphous carbon therein. Furthermore, solvent may be used to adjust the viscosity of the resulting adhesive depending on whether a liquid adhesive, a paste adhesive or a film adhesive is desired. 
     The amount of amorphous carbon in the adhesive may depend on design considerations, including the required amount of electrical conductivity required and the amount of RF loss that may be tolerated. In one aspect, the disclosed adhesives may include at least about 0.1 percent by weight amorphous carbon and at most about 2 percent by weight amorphous carbon. In another aspect, the disclosed adhesives may include at least about 0.1 percent by weight amorphous carbon and at most about 1.5 percent by weight amorphous carbon. In another aspect, the disclosed adhesives may include at least about 0.1 percent by weight amorphous carbon and at most about 1.0 percent by weight amorphous carbon. In another aspect, the disclosed adhesives may include at least about 0.1 percent by weight amorphous carbon and at most about 0.5 percent by weight amorphous carbon. 
     At this point, those skilled in the art will appreciate that additional components may be included in the adhesive formulation, including additional pigments and resins, without departing from the scope of the present disclosure. Furthermore, a substrate material, such as fiber, may also be added to the adhesive formulation to form a composite material without departing from the scope of the present disclosure. 
     Specific examples of the disclosed low loss static dissipative adhesive are set forth below. 
     EXAMPLES 1-8 
     Eight adhesives were prepared by dispersing various quantities of PRINTEX® XE2 carbon black in EX-1515 cyanate ester resin. Solvent was added to the formulation to reduce the viscosity of the adhesives such that the adhesives may be sprayed with a pneumatic spray gun. The resulting adhesives were then either sprayed or poured onto a D-shaped comb pattern circuit having a 20 mil spacing and the electrical resistance was measured. The results are set forth in Table 1 below: 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Percent 
                   
                 Electrical 
               
               
                   
                   
                 Carbon 
                 Application 
                 Resistance 
               
               
                   
                 Example 
                 (by weight) 
                 Method 
                 (Ohms) 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 1.20 
                 Sprayed 
                 &lt;10 5   
               
               
                   
                 2 
                 0.80 
                 Sprayed 
                 &lt;10 5   
               
               
                   
                 3 
                 0.55 
                 Sprayed 
                 &lt;10 5   
               
               
                   
                 4 
                 0.50 
                 Poured 
                 &lt;10 5   
               
               
                   
                 5 
                 0.40 
                 Sprayed 
                 &lt;10 5   
               
               
                   
                 6 
                 0.40 
                 Poured 
                 &lt;10 5   
               
               
                   
                 7 
                 0.28 
                 Sprayed 
                 2.2 × 10 5   
               
               
                   
                 8 
                 0.13 
                 Sprayed 
                 6.0 × 10 8   
               
               
                   
                   
               
             
          
         
       
     
     Thus, Examples 1-8 confirm that adhesives prepared with relatively low quantities (e.g., less than 1 percent by weight) amorphous carbon in cyanate ester resin provided electrical conductivity sufficient to effectively dissipate static charges. 
     EXAMPLES 9-19 
     Eleven adhesives were prepared by dispersing various quantities of PRINTEX® XE2 carbon black in EX-1515 cyanate ester resin. Solvent was added to the formulation to reduce the viscosity of the adhesives such that the adhesives may be sprayed with a pneumatic spray gun. The resulting adhesives were then either sprayed or poured onto a trace to trace circuit having 15 millimeter spacing and the electrical resistance was measured. The results are set forth in Table 2 below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Percent 
                   
                 Electrical 
               
               
                   
                   
                 Carbon 
                 Application 
                 Resistance 
               
               
                   
                 Example 
                 (by weight) 
                 Method 
                 (Ohms) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 9 
                 1.20 
                 Sprayed 
                 3.0 × 10 5   
               
               
                   
                 10 
                 0.80 
                 Sprayed 
                 4.5 × 10 5   
               
               
                   
                 11 
                 0.55 
                 Sprayed 
                 1.2 × 10 6   
               
               
                   
                 12 
                 0.50 
                 Poured 
                 1.2 × 10 6   
               
               
                   
                 13 
                 0.40 
                 Sprayed 
                 1.1 × 10 7   
               
               
                   
                 14 
                 0.40 
                 Poured 
                 8.0 × 10 6   
               
               
                   
                 15 
                 0.28 
                 Sprayed 
                 1.6 × 10 7   
               
               
                   
                 16 
                 0.28 
                 Sprayed 
                 1.4 × 10 7   
               
               
                   
                 17 
                 0.28 
                 Sprayed 
                 1.2 × 10 7   
               
               
                   
                 18 
                 0.28 
                 Sprayed 
                 1.3 × 10 7   
               
               
                   
                 19 
                 0.13 
                 Sprayed 
                 3.0 × 10 10   
               
               
                   
                   
               
             
          
         
       
     
     Thus, Examples 9-19 also confirm that adhesives prepared with relatively low quantities (e.g., less than 1 percent by weight) amorphous carbon in cyanate ester resin provided electrical conductivity sufficient to effectively dissipate static charges. 
     EXAMPLE 20 
     Using an appropriate mixer, 1 gram of PRINTEX® XE2 carbon black was dispersed in 249 grams of EX-1515 cyanate ester resin to yield an adhesive having 0.40 percent by weight carbon black. The viscosity of the adhesive was adjusted and the resulting formulation was sprayed onto a DI-STRATE™ 95225 syntactic foam tile having a thickness of 0.1375 inches. DI-STRATE™ 95225 syntactic foam tiles are commercially available from Aptek Laboratories, Inc. of Valencia, Calif. 
     The adhesive coated tile had an average dielectric constant of about 1.4 and an average loss tangent of about 0.005 measured at 2 gigahertz when tested at both a vertical and horizontal polarization. The return loss versus frequency of the tile is shown in  FIG. 1  and the insertion loss versus frequency of the tile is shown in  FIG. 2 . 
     EXAMPLE 21 
     Using an appropriate mixer, 1 gram of PRINTEX® XE2 carbon black was dispersed in 356 grams of EX-1515 cyanate ester resin to yield an adhesive having 0.28 percent by weight carbon black. The viscosity of the adhesive was adjusted and the resulting formulation was sprayed onto a DI-STRATE™ 95225 syntactic foam tile having a thickness of 0.1339 inches. 
     At a vertical polarization, the adhesive coated tile had an average dielectric constant of about 1.4 and an average loss tangent of about 0.004 measured at 2 gigahertz. At a horizontal polarization, the adhesive coated tile had an average dielectric constant of about 1.4 and an average loss tangent of about 0.005 measured at 2 gigahertz. 
     EXAMPLE 22 
     The adhesive of Example 21 was sprayed onto a DI-STRATE™ 95225 syntactic foam tile having a thickness of 0.1367 inches. 
     At both a vertical and horizontal polarization, the adhesive coated tile had an average dielectric constant of about 1.4 and an average loss tangent of about 0.005 measured at 2 gigahertz. 
     Accordingly, adhesives prepared by dispersing relatively small quantities of amorphous carbon in a cyanate ester resin pursuant to the present disclosure exhibit desired electrical conductivity and low RF loss properties. 
     Although various aspects of the disclosed low loss static dissipative adhesives have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.