CA1195488A - Low density, electromagnetic radiation absorption composition - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Low density compositions containing colloidal--size particulates of an ELM absorber such as Fe3O4 and a particulate of an ELM attenuator such as carbonyl iron dispersed in a dielectric matrix such as a styrene/butyl acrylate copolymer provide improved absorption of electromagnetic radiations.

Description

~ ~35~

LOW DENSITY, ELECTROr~GNETIC RADIATION
ABSORPTION COMPOSITION

Back~round of the Invention This invention relates to a composition for suppressing electromagnetic radiation and, particularly, for reducing the reflection of microwave energy.

The use of materials for absorbing electro~
magnetic radiation is wide spread in the coating of (1) military devices which are required to avoid or minimize detection by radar, (2) appliances that employ microwave radiation, and (3) reflectors of ships, airplanes, building and bridges to reduce reflec-tion that often causes navigational errors.

Many materials including natural ones and synthetic ones are known for their ability to surpress electromagnetic radiation in the microwave frequency range. This ability to s~ppress electromagnetic radia-tion enables the absorbing material to dissipate electro-magnetic energy ~ithin the material, thereby reducing the reflection of microwaves.

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2--Of the various absorbing ma-terials, the artificial dielectrics are the most commonly emplo~ed.
Artificlal dielectrics are generally formed by dis-persing a magnetic powder or other natural absorber in a dielectric material, such as plastics including thermoplastics and thermosets, ceramics, waxes and the like. The artificial dielectrics which have been formed by loading the aforementioned dielectric binders with magnetic metals, semi-conductors, ferromagnetic oxides or ferrites have very desirable magnetic and dielectric properties.

The use of solid ferrites, i.e., ferro-magnetic ferrites formed of ferric oxide and other bivalent metal oxides, as sheet materials for reflecting surfaces and objects to suppress or substantially reduce the reflection of electromagnetic energy offers many advantages. I-t has been ~ound that mixed ferrites often provide good absorptive materials over a wide range of microwave frequencies. In addition, ferrites in the form of solid coatings display the higher permeabili-ties which are required for broad band,operation. Such solid ferrite coatings are capable of higher permeabili--ties than those exhibited by the ferrite powders since the magnetic properties of ferrite decline appreciably by grinding it into powder form. Thus, it is found that ferrites that are both non-conductive and ferro-magnetic provide within a single composition the pote~-tially optimum dielectric and magnetic properties.

Unfortunately, it is found that, in the con~entional absorptive coatings that contain ferrites, substantial quantities of the heav~ ferrites is required in order to achieve the desired absorptive capabilit~.

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The resulting dense coati.ngs of such convèntional absorbers are generally undesirable because they are heavy and difficult to fabricate.

In view of the aforementioned deficiencies of the prior art materials or absorbing electromagnetic radiation, it is highly desirable to provide a light-weight absorptive material that can be readily fabri-cated in-to any shape or applied as a coa-ting to any of a variety of substrates which coatings contain rela-tively low concentrations of the heavy magnetic par-ticles needed for a~sorption.

Summary of the Invention The present inven-tion is such a low density ELM absorption composition which exhibits high efficiency in the absorption of electromagnetic radiation, paxticu-larly at microwave frequencies. Such composition ~hereinafter called ELM compositions) comprises (1) a dielectxic material (hereinafter called dielectric matrix) having dispersed therein (2) a colloidal-size particulate of a material capable of absorbing electro-magnetic radiation (hereinafter called ELM absor~er) and (3) a particulate of a metal-contalning material capa.ble of providing increased a-ttenua-tion of elec-tro-magnetic radiation (hereinafter called ELM attenuator).
The concentration of ELM absorber in the ELM composition .s advantageously sufficient to provide a magnetic loss tangent greater than 0.05 at a frequency of 2 gegahertz ~gHz) and a composition thickness of 2 centimeters (cm). The concentration of ELM attenuator is sufficien-t to provide the ELM composition with an attenuation of greater than 0.5 decibels per cent.imeter (dB/cm) under the aforementioned conditions. For the purposes C-29,901 -3-~ ~ ~'3~ ~

of this invention, a low density ELM composition has a density less than 6 gxams per cubic centimeter (g/cm3).
Surprisingly, the low density ELM composition of the present invention exhibits dissipative properties hlgher than would be expected at khe concentrations of ELM absorber being employed.

In another aspec-t, this invention is a stable fluid dispersion of the aforementioned ELM attenuator and colloidal-sized particles of the dielectric matrix containing colloidal or sub-colloidal particles of the ELM absorber. Surprisingly, such a dispersion can be applied as a coating and dried to form a con-tinuous film wherein the particles of the ELM a~sorber are maintained in an essentially discrete spaced apart relationship by the dielectric matrix. Preferably , particles of the ELM attenuator are also substantially maintained in an essentially discrete spaced apart relation by the dielectric matrix.

The ELM composition of this invention is ~0 particularly effective as an electromagnetic radiation absorber in such applications as paints and coatings to be used for reflection reduction for metal structures such as towers, bridges, ships, etc.; microwave camou-flage and radar camouflage; coatings for appliances wherein microwave radiation absorption is desired, such as in microwave ovens and microwave browning devices;
applications related to the transport of solar energy from space satellites; and the like. This ELM composi-tion is also well suited for molding shaped articles and for fabrication into foams and fibers.

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S~f.`3 Detailed Description of Illustra-tive Embodimen-ts The low density ELM composition of the present invention has a density, an attenuation and a magnetic loss tangent as defined hereinbefore. Preferred composi-tions have (1) densities in the range from abou~ 1.2 toabout 5, most preferably fxom about 1.5 to abou-t 3, g/cm3; (2) magnetic loss tangent greater than 0.1, most preferably greater than 0.2 under the conditions speci-fied hereinbefore; and (3) at~enuation greater than 1 dB/cm, most preferably greater than about 2 dB/cm.

The ELM compositions comprises three essential components: (1) a dielectric solid matrix acting as the continuous phase for the composition, (2) a particu-late ELkl absorber that is maintained in an essen-tially discrete, spaced apart relationship by the matrix and

(3) a particulate ELM attenuator. In preferred ELM
composltions, the ELM a-ttentuator is also essentially totally dispersed in the dielectric matrix.

The dielectric matrix is suitably any normally solid material capable of serving as an insulating matrix (binder) for the ELM a~sorber. Preferably, it has an electrical resistivity greater than 106 ohms per centimeter (ohms/cm~, more preferably greater than about 101 ohms/cm, most preferably from about 1015 -to 102 ohms/cm. Examples of such suitable dielectrics include glass, ceramics, waxes, plastics, including thermoplastics and thermosets, rubber polymers and the like, with the synthetic plastics being preferred. Of the synthetic plastics, preferred are polymers that are water-insoluble and are prepared from hydrophobic monomers that are essentially water-immiscible, i.e., C-29,901 -5-the monomer forms a separate phase when 5 grams of the monomer is mixed wi-th 100 grams of water. Such water immiscible monomers will polymerize under emulsion polYmerization conditions to form a water-insoluble pol~mer which will exist in the form of a stable aqueous colloidal dispersion, usually with the aid of suitable surface active agents.

The ELM absorber is a material (1) which absorbs electromagnetic radiation hav.ing frequencies in the range from about 0.3 to about 20 gHz and (2~ which is in the form of a colloidal or sub-colloidal size particulate. Preferred ELM absorbers can be further characterized as paramagnetic or superparamagnetic due to their small size. Examples of such materials are compounds of magnetic metals such as ferromagne-tic oxides or ferrites, e.g., Fe3O~, as well as ferromag-netic ferrites formed of ferric oxide and varlous bivalent metal oxides such as metal oxides of nickel, zinc and manganesei magnetic metals such as iron, cobalt and nickel and their alloys; and other known ~LM
absorbing materials such as carbon black, graphlte and the like. The ELM ab~orber generally contains particles having a maximum dimension less than about 1 micrometer (~m), preferably in the range from about 0.01 to about 0.7 ~mO Of these materials, the magnetic me-tallic compounds are preferred, with Fe304 being most preferred.

The ELM attenuator is preferably a ferromag-netic material which is capable of providing microwa~e attenuation as described hereinbefore. The ELM attenua-tor ls in the form of particles having a dimensioIlgreater than 1 ~m, preferably in the range from about 1.5 to about lQ0 ~m, most preferabl~ from about 2 to about 75 ~m. Examples of such attenua-ting materials are iron, cobalt, nickel and other ferromagnetic metals C-29,901 -6-1 L~3S~8~

as well as alloys of such metals. Of these materials, metallic iron is preferred, with carbonyl iron being most preferred. It is understood, however, that in addition to carbonyl iron, metallic iron made by other procedures such as electrolytic iron, reduced ixon and atomized iron are preferred.

In the preparation of -the low density, ELM
absorbing compositions of this invention, it is advant-ageous to disperse the ELM absorber into the dielectric matrix such that dielectric matrix forms a continuous phase that maintains the par-ticles of EI~ absorber in an essentially discrete, spaced apart relationship.
Any of a variety of conventional blending procedures for incorporating a colloidal or sub-colloidal parti-culate into dielectric binders are suitably employedfor this purpose. Preferably, however, the dielectric matrix having the ELM absorber dispersed therein ~herein-after called dielectric/absorber~, is prepared by initially forming an aqueous dispersion of the ELM
a~sorber by contacting colloidal or sub colloidal particles of said absorber with an aqueous solution of a water-soluble surfactant or emulsifier, thereby forming the dispersion which contains from about 5 to about 70 weight percent of the absorber particles.
Examples of preferred aqueous dispersions of ELM absorbers are the so-called ferrofluids such as disclosed in the U.S. Patent No. 3,981,844, preferably those having an average particle diarneter in the range from about 0.05 to about O.l micrometer. Preferably, such fluids are aqueous dispersions of the rnagne-tic metals which are stabili~ed by the presence of surfactants, emulsifiers and/or chemical dispersants as described hereinafter.

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Typically, suitable surface active agents, dispersants or emulsifiers include salts of fatty acids such as potassium oleate, metal alkyl sulfates such as sodium lauryl sulfate, salts of alkyl aryl sulfonic acids such as sodium dodecylbenzene sulfonate, polysoaps such as sodium polyacrylate and alkali metal salts of methyl methacrylate/2-sulfoethyl methacryla-te copolymers and other sulfoalkyl acrylate copolymers, and other anionic surfactants such as the dihexyl ester of sodium sulfosuccinic acid; nonionic surfactants such as the nonionic condensates of ethylene oxide with propylene o~ide, ethylene glycol and/or propylene glycol; and cationic surfactants such as alkylamine guanidine polyoxyethanols, as well as a wide variety of micelle generating substances described by D. C. Blackley in Emulsion Polymerization, Wiley and Sons, Chapter 7 (1975) and other surfactants listed in McCutcheon's Deter~ents and Emulsifiers, 1980 Annual North Americal . _ _ Edition, McCutcheon, Inc., Moxristown, N.J. Also included among the suitable surfactants are the surface active polymers (often called polysoaps), e.g., those described in U.S. Patent No. 3,965,032. Of the suitable surfactants, the anionic varieties such as the potassium salts of functionalized oligomers, e.g., Polywet varie ties sold by Uniroyal Chemical, are preferred. Such surface active agents or emulsifiers are employed in amounts sufficient to provide a stable dispersion of the ELM absorber in water. Preferably, such surface acti~e agents are employed in concentrations in the range from about 0.2 to about 10, most preferably from about 1 to about 6, weight percent based on the aqueous phase. Particularly desirable processes for forming such aqueous colloidal dispersions of the ELr~ absorber C-29,901 -8-~ r~

are described in U.S. Patent Mos. 3,826,667; 3,981,84~;
3,843,540 and Industrial ~ neer1~ Production and Research Devel~ment, Vol. 19, 147-151 (1980).

The aqueous dispersion of the ELr~l absorber is then combined with the water-immiscible monomer as described herein to form the desired emulsion by normal mixing procedures, for example, by passing bo-th the dispersion and monomer through a high shear mixing device such as a Waring blender, homogenizer or ultra sonic mixer. Alternatively and preferably, the monomer is added continuously to the aqueous dispersion of the ELM absorber during the polymerization. Advantageously, the monomer is in the form of an aqueous emulsion of the monomer which emulsion is maintained by a wa-ter-soluble monomer and/or a water-soluble emulsifier such as described hereinbefore. As another alternative, the a~ueous emulsion of the ELM absorber and water-immiscible monomer can be prepared by adding colloidal or sub-colloidal particles of the ELM absorber to an existing agueous emulsion of monomer. In such instances, it is often desirable to add additional emulsifier or sur-factant to the emulsion prior to or simultaneous with the addition of the particles of the ELM absorber. In the emulsion of the ELM absorber and water~immiscible monomer in water, the aqueous phase is present in a proportion sufficient to be the continuous phase of the emulsion. The ELM absorber is presen-t in proportions sufficient to provide the dielectric/absorber particulate with the desired dissipative properties. The water~
immiscible monomer is present in proportion sufficient to enclose or encapsulate the ELM absorber when polym erized. The emulsifier and/or surface ac-tive agent is present to provide an aqueous colloidal emulsion which C-~9, 901 9-.3 is sufficiently stable to be subjected to emulsion polymerization conditions. Preferably, the emulsion contains from about 0.1 to about 25 weiyht percent of ELM absorber, from about 1 to about 30 weight percent of monomer and a remaining amount of the aqueous phase including emulsifier (surfactant), catalyst and the like.

Examples of suitable wa-ter-immiscible monomers that can be employed to prepare the aforementioned dielectric/absorber include monovinylidene aromatic monomers such as stryene, vinyl toluene, t-butyl styrene, chlorostyrene, vinylbenzyl chloride and vinyl pyridene;
alkyl esters of ~ ethylenically unsaturated acids such as ethyl acrylate, methyl methacrylate, butyl acrylate and 2-ethylhexyl acrylate; unsaturated esters of saturated carboxylic acids such as vinyl acetate, unsaturated halides such as vinyl chloride and vinylidene chloride; unsaturated nitriles such as acrylonitrile;
dienes such as butadiene and isoprene; and the like.
Of these monomers, the monovinylidene aromatics such as styrene and the alkyl acrylates such as butyl acrylate are preferred.

In addition to the aforementioned water-immiscible monomer, relatively minor portions, e.g., less than lO, preferably less than 5, weight percent based on tota~ monomer component, of a water-soluble monomer such as an ethylenically unsaturated carboxylic acid or its salt such as acrylic acid or sodium acrylate;
methacrylic acid, itaconic acid and maleic acidi an ethylenically unsaturated carbo~amide such as acrylamide;
vinyl pyrrolidone; hydroxyalkyl acrylates and me-th-acrylates such as hydroxyethyl acryla-te, hydxoxypropyl C-29,901 -10-acrylate and hydroxyethyl rnethacryla-te; aminoal~yl esters of unsaturated acids such as 2~aminoethyl meth-acrylate; epoxy functional monomers such as glycidyl methacrylate; sulfoalkyl esters of unsaturated acids such as 2 sulfoethyl methacrylate; ethylenically unsatur ated guaternary ammonium compounds such as vinylbenzyl trimethyl a~monium chloride may be employed. It is critical in the practice of this preferred embodiment, that such water-soluble monomer~ not be employed in amoun-ts sufficient to render the resulting polymer soluble in water. Particularly effective monomer recipes for the prac-tice of this invention are those containing from about 20 to about 90 weight percent of styrene, from about 10 to about 80 weight percent of alkyl acrylate such as butyl acrylate and from about 0.01 to abou-t 2 weight percent of the unsaturated carboxylic acids such as acrylic acid, with said weight percentages being based on the weight of total monomers.

The emulsion polymerization conditions employed in the practice of this preferred embodiment of the invention are generally those of conventional free-radical type polymerizations carried out in the presence of a radical initiator such as a peroxygen compound, an azo catalyst, ultraviolet light and the like. Pre-ferably, such polymerization is carried out in thepresence of a water-soluble peroxygen compound at temperatures in the range from about 50 to about 90C.
The emulsion is generally agi-tated during the po1ymeri-zation period in order to maintain adequate feed -transfer.
The concentration of catalyst is normally in the range from about 0.005 to about 8, preferably from about 0.01 to about S, weight percent based on total momomer.

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Examples of suitable catalysts include inorganic persul~
fate compounds such as sodium persulfate, potassium persulfate, ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl hydroperoxide, dibenzol per oxide and dilauroyl peroxide; azo catalysts such as azobisisobutyronltrile, and other common free~radical generating compounds. Also suitable are various forms of free-radical generating radiatlon means such as ulkraviolet radiation, electron beam radiation and gamma rad~ation. Alternatively, a redox catalyst composition can be employed wherein the polymerization temperature ranges from about 25 to about 80C.
Exemplary redox catalyst compositions include a peroxygen compound as described hereinbefcre, preferably potassium persulfate or t-butyl hydroperoxide and a reducing component such as sodium metabisulfite and sodium formaldehyde hydrosulfite~ It is also suitable to employ various chain transfer agents such as mercap-tans, e.g., dodecyl mercaptan; dialkyl xanthogen disul-fides; diaryl disulfides and others listed in Blackley,su~ra, Chapter 8 in concentrations as described therein.

Following emulsion polymerization, the result-ing aq~eous dispersion of the particles of dielectric/-ELM absorber can be withdrawn from the polymerization vessel and (1) the dispersion is employed as is or ~2) -the unreacted monomer and other volatiles are removed to form a concentrated dispersion and then used as a paint base for the ELM composition or (3) the dielectic/ELM absorber particulate can be separated fxom the a~ueous continuous phase of the dispersion by conventional means such as spray drying or drying under vacuum. If dried, the dielectric/ELr~ absorber particll-late preferably contains from about 10 to about 80, C-29,901 -12-most preferably from about 15 to about 70, weightpercent of the ELM absorber and from about 90 to a~out 20, most preferably from about 85 to about 30, weigh-t percent of dielectric matrix polymer.

In this preferred embodiment, the dielectric/-ELM absorber in the form of an aqueous dispersion ox a dry colloidal-slze particulate is then con~ined with the ELM attenuator to provide the desired low density, ELM absorbing composition. Preferably, the ELM attenuator (particulate) is dispersed as an aqueous dispersion of the dielectric/ELM a~sorber, thereby forming a coating compositions which can be applied to any substrate as desired and dried to a continuous coating capable of absorbing ELM radiation. Alternatively, the ELM attenu-ator may be encapsulated in a suitably dielectricmaterial as defined hereinbefore prior to combination with the dielectric/ELM absorber. In this alternative embodiment, the ELM attenuator and dielectric/ELM
absorber may be in the form of aqueous dispersions and/or in the form of dry powders when combined.

In dry form, the resulting low density, ELM
compositions can be fabricated into an article of desired shape by conventional fabrication techniques such as injection or compresslon molding, extrusion and the like. Alternatively, the ELM composition in the form of a dxy powder is dispersed in a nonaqueous li~uid and employed as desired, e.g., as a paint base or base ior other coating ~ormulations.

Preferred low density, ELI~ absorbing composi-tions that employ colloidal-size Fe304 as the ELr~
absor~er and carbonyl iron as the ELr~ attenuator have C-29,901 13-35~

an ELM absorber:ELM a-ttenuator weight ratio from about 90:10 to about ao: 60, most preferably from about 80:20 to about 55:45. In the preferred ELM compositions, the weight ratio of the sum of ELM absorber and ELM attenuator to the dielectric matrix is from about 85:15 to about 10:90, most preferably from about 70:30 to about 55:45.
In addition to the foregoing critical components, these compositions opt:ionally contain other ingredients such as stabilizers, pigments, fillers, blowing agents, corrosion inhibitors and other additives comrnonly employed in ELM absorbing compositions.

The following examples are given to illus-trate the inventlon and should not be construed as limiting its scope. Unless otherwise indicated, all parts and percentages are by weight.

Example 1 A. Preparation of A~ueous Dispersion of Fe3O4 An aqueous dispersion of magnetic iron oxlde (Fe3O4~ (ELM absorber) is prepared by mixing aqueous solutions of ferric and ferrous salts in amounts to maintain the Fe /Fe molar ratio at ~2:1. Magnetic iron oxide is then precipitated at 0-10C by rapid addition of lN NH40H and vigorous agitation until a pH
of 9-10 is reached. Ir~ediately thereafter, the disper-sant is introduced with agitation to the aqueous mediurn containing the precipitated iron oxide and the mi~ture is heated at 90C f~r one hour. During this period, C-29,901 1 ~3~

hydrochloric acid is added un-til the pH of the mixtuxe reaches 7O5. The particles of precipitated iron oxide are washed with deionized wa-ter and redispersed in deionized water containing ~0.5 g of a potassium salt of a functionalized oligomer (Polywet KX-4 sold by Uniroyal Chemical) per gram of precipitated iron oxide, by using an ultrasonic probe. Magnetization of the dispersed iron oxide is measured by a Collpits oscillator circuit technique.

B. Preparation of Magnetic Latex (Die]ectric/-ELM Absorber~
. .

To a 3-neck flask equipped with a stirrer, two addition funnels and a condenser is added a mix-ture of 507 g of the 28.5 percent solids dispersion of Fe3O4 (200 gauss and average particle size of less than 0.08 micrometer) and 203 g of deionized water. The mixture is then h~ated under nitrogen atmosphere -to 90C while stirring the mixture. At this temperature of 90C, a monomer stream and an aqueous surfactant stream are separately introduced via the two addition funnels in-to the flask, each stream being introduced at the rate of ~6 ml/min over a period of 65 minutes. The monomer stream consists of 64 g of styrene, 16 g of butyl acrylate and 3 g of t-butyl hydroperoxide. The aqueous stream consists of 110 g of deionized water, 2.9 g of the potassium salt of a functionalized oligomer ("Polywet KX-4"3 and 2 g of sodium formaldehyde hydrosulfite.
The resulting reactlon mixture is stirred and main-tained under nitrogen at 90C for an additional half hour. The resulting ~5 percent solids latex is concen-trated by distillation under vacuum to a 30.3 percent solids latex (dielectric/ELM absorber) having dispersed C-29,901 -15-:~.;L~3~

particles with 2 polymeric as ~ell as magnetic cnaracter lstic. The particles of this latex have a narrow par-ticle size distribution and an averaye particle diameter of 0.11 micrometer as determined by hydrodynamic chroma-tography. The latex remains s-table in an applled magnetic field o 1800 gauss and exhibits proper-ties common to magnetic colloids. For example, such magnetic colloids are magnetizable li~uids tha-t are instantly demagnitized upon removal of a magnetic field and levita-te an object upon application of a magnetic field. Magnet1zation of the latex by a Collpits oscil-lator circuit technique, described by E. ~. Peterson et al.
in the Journal of Colloidal and Inter~ac_ 1 Science, 70, 3 (1977~, is estimated to he 135 gauss.

The particles o the latex are recovered by freeze dryiny the latex at -80C under vacullm at 0.5 mm Hg.

C. Preparation of ELM Composition (Dielec-tric/ELM
~bsor~er/EhM A-ttenuator L_ One ELM composition (Sample No. 1) is pre-pared by dry blending 50.3 g of a dry powder of the aforementioned latex (55.4 percent dielectric~44.6 percent Fe3O4) with 33.5 g of carbonyl iron (ELM attenu-ator) having an average particle size cf 3-4 microrneters and sold-by GAF Corporation under the trade name Super - Fine Special. The blending is carried out on a Brabender mixing apparatus and the resultant blend is then compres-sion molded in-to 1at plates (0.8 cm thickness x 2.6 cm diameter) at 2000 pounds of positive pressure and 230C
for 2 minutes. The sample is cooled to room tempera-ture C-29,901 -16 3t"t~

and the pressure on the sample is released. The resul-tant plate of the ELM composition is machined inko t~70 flat disks having a diameter of 2.54 cm and a thickness of 0.64 cm and 0.32 cm, respectively.

A second ELM composition (Sample Mo. 2) is prepared following the foregoing procedure using 56.5 g of the dry powder of the latex and 18.8 g of the carbonyl iron. The sample is similarly blended, molded and fabricated into disks. For purposes of comparison, a third sample (Sample No. C) of dry particles of the latex is molded and fabricated into disks by the fore-going procedure.

A11 of the foregoing samples are tested for ELM absorption and the results are reported in Table I.

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~20-As evidenced by the data in Table I, the compositions o~ the present invention (Sampl.e Nos. 1 and 2) exhibit significantly better attenuation at - a given frequency than does the composition of Sample.No. C.

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Claims (12)

WHAT IS CLAIMED IS:

1. An ELM absorption composition comprising (1) a dielectric material having dispersed therein (2) a colloidal-size particulate of an absorber for electromagnetic radiation and (3) a particulate of an attenuator for electromagnetic radiation, said composi-tion having a density less than 6 grams per cubic centimeter (g/cm3).

2. The composition of Claim 1 wherein the composition exhibits a magnetic loss tangent greater than 0.05 and an ELM attenuation of greater than 0.5 decibels per centimeter (dB/cm) when the composition having a thickness of 2 centimeters is exposed to electromagnetic radiation having a frequency of 2 gegahertz.

3. The composition of Claim 2 which has a density in the range from about 1.5 to about 3 g/cm3 and exhibits a magnetic loss tangent greater than 0.2 and an attenuation greater than 2 dB/cm.

4. The composition of Claim 2 wherein the absorber is an oxide of a magnetic metal and the attenua-tor is a magnetic metal or an alloy containing at least one magnetic metal.

5. The composition of Claim 4 wherein the magnetic metal is iron.

6. The composition of Claim 5 wherein the absorber is Fe3O4 having a maximum particle dimension less than 1 micrometer and the attenuator is carbonyl iron having an average particle size greater than 1 micrometer.

7. The composition of Claim 6 wherein Fe3O4 has a maximum particle dimension in the range from 0.01 to about 0.7 micrometer and the carbonyl iron has an average particle size in the range from about 2 to about 40 micrometers.

8. The composition of Claim 7 comprising from about 90 to about 15 weight parts of a dielectric synthetic thermoplastic and from about 10 to about 85 weight parts of combined absorber and attenuator wherein the weight ratio of absorber to attenuator is from about 90:10 to about 60:40.

9. The composition of Claim 8 wherein the synthetic thermoplastic is a styrene/butyl acrylate copolymer.

10. The composition of Claim 7 wherein substantially all of the particles of the absorber and attenuator are maintained in a discrete spaced apart relationship by the thermoplastic.

11. The composition of Claim 7 wherein the composition is the base of a liquid coating formulation.

12. The composition of Claim 11 wherein the coating formulation is a paint.