Patent Number: 062654667
Section: description

EXAMPLES Example 1 Electromagnetic Shielding Effectiveness (EMSE) Five pounds of pelletized polyethylene terephthalate (PET) with fifteen weight percent Graphite Fibril.TM. nanotubes were produced by Hyperion Catalysis International. This Hyperion concentrate of 15 wt % carbon fibers in unspecified Eastman extrusion grade PET polyester resin was used as a master batch for let down (dilution) with neat Natural PET resin 0.85 IV Eastman natural PET. Both resins dried 4.5 hours at 290 F. and kept in sealed glass bottles before use. The 1.5% carbon resin was a 9:1 blend of the concentrate and the neat resin by weight. 2:1 blends of concentrate with natural were made to reduce carbon content from 15% to 10% and again from 10% to 6.7%. In doing so, varying concentrations of nanotubes could be extruded for testing. The master batch and a letdown thereof to the plaque size required for EMI shielding testing were extruded along with a neat PET control. A 3/4 inch Brabender single screw extruder with an engineering (higher compression) screw, run at 110 to 115 rpm screw speed was utilized. A die with a 6 inch width by 0.115" thick slit (with no adjustments for thickness control across extrudate width) was used to form the initial plaques. A shrouded rubber coated belt (with high air ventilation for cooling) for take-up, cooling and draw control was used to elongate the extruder plaques. Belt speed was controlled to induce various shearing loads via elongation. The coated belt effectively cooled the hot extrudate, grabbed onto it and restrained its shrinkage during its travel. The base PET was readily and easily extruded, with no evidence of moisture-related bubbling. From literature, oriented PET dimensionally stabilizes below 70 C., and is drawable (orientable) between about 100 and 150 C. Draw of extrudate occurred in the short distance between the die and the contact point of extrudate with belt. This distance was generally an inch or two. Elongation was controlled in this area by the difference in the speed of the belt versus the speed of extrusion. Die and extrudate temperatures were in the range of 440-450 F. for natural PET. Natural PET extrudate a foot from the die (in contact with the belt) was 135-140 F. By varying the shear rate and concentration of the nanotubes, and by utilizing the neat PET as a control, the EM shielding efficacy of the nanotubes as a function of concentration was determined, as well as the significance of shear on the nanotubes. It was determined that shear is important because, as produced in this test, the nanotubes are agglomerates and exist as curved, intertwined entanglements, somewhat like steel wool pads. By imparting shear in the process, the entanglements are pulled apart, thus increasing the effective aspect ratio of the nanotubes. Electromagnetic Shielding Effectiveness (EMSE) tests between 20 kHz and 1.5 GHz on the PET-1.5 wt. % nanotube plaques and the neat PET were conducted. Testing was performed in accordance with conventional specs: MIL-STD-188-125A, ASTM D4935, IEEE-STD-299-1991, MIL-STD-461 C and MIL-STD-462. The data, normalized for thickness, is shown in Table 1. Testing was performed at 22.degree. C., a relative humidity of 39%, and atmospheric pressure of 101.7 kPa. TABLE 1 Shielding Effectiveness of PET with 1.5 weight percent Nanotubes v. Elongation Shielding Effectiveness Test, dB, at Frequency Sample Loading 20 kHz 0.4 MHZ 15 MHZ 0.2 GHz 1.5 GHz and Elongation Thickness SE.sub.pw SE.sub.m SE.sub.pw SE.sub.m SE.sub.pw SE.sub.m SE.sub.pw SE.sub.m SE.sub.pw SE.sub.m Minimum target 100 100 100 100 100 value 1.5 wt % 10 to 1 1 mm 182 116 180 114 182 116 184 -- 184 -- 1.5 wt % 6 to 1 1 mm 114 48 113 52 116 56 119 -- 120 -- 1.5 wt % slight 1 mm 46 28 46 29 46 29 47 -- 47 -- Neat PET 1 mm 31 17 32 18 32 17 33 -- 34 -- SE.sub.pw - plane wave shielding effectiveness; SE.sub.m - magnetic wave shielding effectiveness Each magnitude of the plane wave (SE.sub.pw) and magnetic wave (SE.sub.m) Shielding Effectiveness (SE) in Table 1 is an average from six (6) runs of the test at a given frequency. The experimental error evaluated by the partial derivatives and least squares methods does not exceed 6%. The linear arrangement of the generator and receiver antennas and the samples under test meet the requirements of MIL-STD-188-125. The following equipment was used during testing: Generators: Model 650A HP (0.5 kHz to 110 MHZ) and Model 8673 HP (50 MHZ to 18 GHz) PA1 Analyzers: Model 85928 HP and 8593L (both 9 kHz to 22 GHz) PA1 Oscilloscope: ID-4540 HK, Nanoammeter 3503 RU with Metrologic Laser ML869S/C M 11 PA1 Antennas: HP 11968C, HP 11966C, HP 11966D; Dipole Antenna Set HP 11966H PA1 Magnetic Field Pickup Coil HP 11966K, Active Loop H-Field HP 11966A PA1 Dual Preamplifier HP8447F PA1 Coniometer 3501-08 F-DM, Micrometer Hommelwerke (100000 nm), Starrett Dial Indicator 25-109 PA1 Digital Thermometer/Hygrometer Model 63-844 MI PA1 Sample: PET-1.5wt. % NTN PA1 Shape of Test Sample: Cylinder PA1 Volume of Test Sample (Vs): 0.00282 cubic inches PA1 Empty Cavity Resonant Frequency (Fe): 9.263 GHz PA1 Cavity Resonant Frequency, With Test Sample (FS): 9.028 GHz PA1 The Q of the empty cavity is 4308 PA1 The Q of the cavity with the specimen: 25 PA1 Calculated relative dielectric constant, (k): 5.429 PA1 Calculate loss tangent, (tan delta): .6288 PA1 Calculated reflection at 1.5 GHz.: 16% This equipment meets the applicable National Institute of Standard and Technology (NIST), American Society for Testing Materials (ASTM), Occupation Safety and Health Administration (OSHA) and State requirements and was calibrated with the standards traceable to the NIST. The calibration was performed per ISO 9001 .sctn.4.11, ISO 9002 .sctn.4.10, ISO 9003 .sctn.4.6, ISO 9004 .sctn.13, MIL-STD-45662, MIL-I-34208, IEEE-STD-498, NAVAIR-17-35/MLT-1 and CSP-1/03-93. This equipment also passed a periodic accuracy test. As can be seen, shearing is preferred in accordance with this invention. Example 2 Dielectric Testing for Low Observability Correlation In addition to the EMSE testing, dielectric testing to ASTM D2520 "Standard Text Test Methods for Complex Permittivity (Dielectric Constant) of Solid Electrical Insulating Materials at Microwave Frequencies and Temperatures to 1650 .degree. C." was performed. This method uses a waveguide cavity to measure the material at microwave frequencies. The cavity measurement is the most accurate dielectric measurement available at microwave frequencies. Although cavities are designed for a discrete frequency, within the normal microwave range material dielectric properties do not change over frequency, and thus this measurement is fairly accurate for the range. This trend can be noted in the EMSE testing, where shielding effectiveness did not appreciable change over frequency sweep of 20 kHz to 1.5 GHz. The cavity volume used was 0.960 cubic inches and the cavity (Q) equals 4308, based on ambient temperatures and typical test equipment setup. Pertinent test data are as follows: Table 1 shows the shielding effectiveness of 1.5 weight percent multi-walled carbon nanotubes mixed in a base host resin of polyethylene terephthalate (PET) at various frequencies and degrees of orientation. The data is normalized for a thickness of 1 mm and shows a broad band average plane wave shielding effectiveness (SE.sub.pw) of 182 dB for high orientation shielding composite of the present invention at a loading level of only 1.5 wt %. The required broad band shielding effectiveness per MIL-STD-188-125A is 100 dB. The dielectric constant of this material is 5.429. From this dielectric constant, about 16% of the power will be reflected from a plane wave hitting the surface of the material. Correlating this data with that in Table 1 reveals that the primary shielding effectiveness mode of this present invention is absorption. The shielding composite of the present invention clearly offers both electromagnetic shielding and low observability. Aspects of this invention include: An electromagnetic (EM) shiclding composite comprising a polymer and an amount effective for EM shielding of nanotubes, wherein said nanotubes are not bonded or glued together. An electromagnetic (EM) shielding composite comprising a polymer and an amount effective for EM shielding of nanotubes, wherein said composite is subjected to shearing to optimize its EM shielding property. An electromagnetic (EM) shielding composite comprising a polymer and an amount effective for EM shielding of nanotubes which are substantially not in contact with each other, other than along their longitudinal areas. An electromagnetic (EM) shielding composite, according to the above, wherein said nanotubes, which are in contact with each other, if any, are not bonded or glued to each other. An electromagnetic (EM) shielding composite, according to the above, wherein said polymer is not carbonizable. An electromagnetic (EM) shielding composite, according to the above, wherein said polymer is not carbonizable. An electromagnetic (EM) shielding composite, according to the above, wherein said composite has been subjected to shearing which disentangles and/or aligns said nanotubes. An electromagnetic (EM) shielding composite comprising a polymer and an amount effective for EM shielding of nanotubes, said nanotubes having an effective aspect ratio of at least 100:1. In an electromagnetic (EM) shielded enclosure comprising an inner space and a surface defining said space, the improvement wherein said surface comprises a layer of aligned nanotubes effective for EM shielding. The electromagnetic shielding composite according to the above, wherein said polymer is derived from a natural source, including cellulose, gelatin, chitin, polypeptides, polysaccharides, or other polymeric materials of plant, animal, or microbial origin. The electromagnetic shielding composite according to the above, wherein said nanotubes are substantially disentangled. An electromagnetic attenuating composite which comprises: a loading of nanotubes substantially aligned in a polymer, wherein the alignment of said nanotubes is created in a shearing process. The electromagnetic attenuating composite according to the above, wherein said loading is about 1.5% or less. An electromagnetic attenuating composite which comprises: a loading of nanotubes substantially disentangled and mixed in a polymer, wherein the disentanglement is imparted by a shearing process. The electromagnetic attenuating composite according to the above, wherein said loading is about 1.5% or less. A method for preparing an electromagnetic (EM) shielding composite comprising a polymer and an amount effective for EM shielding of nanotubes, said method comprising formulating said polymer and said nanotubes and shearing said composite. A method for lowering radar observability of an object comprising partially or entirely surrounding said object with a layer of aligned nanotubes effective for EM shielding. A method for electromagnetic shielding an object or space comprising partially or entirely surrounding said object or space with a layer of aligned nanotubes effective for absorbing electromagnetic energy. A method for producing an electromagnetic shielding composite comprising: providing a source containing nanotubes; providing a source containing a polymer; combining said source of nanotubes and said source of polymer; and, extruding said combination of nanotubes and polymer to impart a shearing force to the composite effective to enhance its shielding properties. The method for producing an electromagnetic shielding composite according to the above, wherein the loading level of nanotubes is from 0.001 to 15 wt. % in the resulting composite. The method for producing an electromagnetic shielding composite according to the above, wherein said extruding comprises imparting shear on said nanotubes so as to cause substantial alignment of said nanotubes. The method for producing an electromagnetic shielding composite according to the above, wherein said extending comprises elongating said combination of nanotubes and polymer so as to control the degree of alignment of said nanotubes. The method for producing an electromagnetic shielding composite according to the above, wherein said extruding comprises substantial disentangling of said nanotubes. The method for producing an electromagnetic shielding composite according to the above, wherein said disentangling results in an increase of the EM shielding effectiveness. A method for electromagnetic shielding, comprising: using a composite of nanotubes in a polymer to absorb electromagnetic radiation and thereby shield an object. The method for electromagnetic shielding according to the above, wherein said composite effectively absorbs electromagnetic radiation in a range of 10.sup.3 Hz. to 10.sup.17 Hz. The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.