JPH0422325B2 - - Google Patents

Description

Claim 1: (1) a polymeric dielectric material in which (2) colloidal-sized particles of an electromagnetic radiation absorber are dispersed;
(3) particles of electromagnetic radiation attenuator and 6 g/cm ³
wherein the polymeric dielectric material containing absorbent particles polymerizes a monomeric material which polymerizes into the polymeric dielectric material in a mixture with the absorbent particles. An electromagnetic radiation absorbing composition characterized in that the absorbent particles are held in discrete spaced apart relationship by a polymeric dielectric material formed. 2. The composition of claim 1 which exhibits a magnetic loss tangent greater than 0.05 and an ELM attenuation greater than 0.5 dB/cm when the composition having a thickness of 2 2 cm is exposed to electromagnetic radiation having a frequency of 2 GHz. 3 It has a density in the range of about 1.5 to about 3 g/ ^cm3 , and 0.2
3. A composition according to claim 2 exhibiting a greater magnetic loss tangent and attenuation greater than 2 dB/cm. 4. The composition according to claim 2, wherein the absorbent is an oxide of a magnetic metal and the damping agent is a magnetic metal or an alloy containing at least one magnetic metal. 5. The composition according to claim 4, wherein the magnetic metal is iron. 6. The composition of claim 5, wherein the absorbent is Fe ₃ O ₄ with a maximum particle size of less than 1 micron and the damping agent is carbonyl iron with an average particle size of greater than 1 micron. 7. The composition of claim 6, wherein _{the Fe3O4} has a maximum particle size ranging from 0.01 to about 0.7 microns and the carbonyl iron has an average particle size ranging from about 2 to about 40 microns. 8 from about 90 to about 15 parts by weight of a dielectric synthetic thermoplastic polymeric material and from about 10 to about 85 parts by weight of an absorbent plus a damping agent, with an absorbent/damping agent weight ratio of from about 90:10 to
8. The composition of claim 7, wherein the ratio is about 60:40. 9. Claim 8, wherein the synthetic thermoplastic polymeric material is a styrene/butyl acrylate copolymer.
Compositions as described in Section. 10. The composition of claim 7, wherein substantially all of the particles of absorbent and damping agent are held in separate and spaced apart relationship by a thermoplastic polymeric material. 11. The composition of claim 7, wherein the composition is a base for a liquid coating formulation. 12 Claim 11 where the coating formulation is a paint
Compositions as described in Section. BACKGROUND OF THE INVENTION The present invention relates to compositions for shielding electromagnetic radiation, particularly for reducing reflection of microwave energy. The use of materials that absorb electromagnetic radiation can be used in (1) weapons that require avoiding or minimizing detection by radar, (2) instruments that use microwave radiation, and (3) often resulting in navigation errors. It is widely used in coating reflectors on ships, aircraft, buildings and bridges to reduce glare. Many substances, both natural and synthetic, are known to have the ability to shield electromagnetic radiation in the microwave frequency range. This ability to shield electromagnetic radiation allows the absorbing material to dissipate electromagnetic energy within the material, thereby reducing microwave reflection. Among the various absorbing materials, man-made dielectric materials are the most commonly used. Artificial dielectric materials are commonly made by dispersing magnetic powders or other natural absorbents into dielectric materials such as plastics, ceramics, waxes, etc., including thermoplastics and thermosets. Artificial dielectric materials prepared by loading magnetic metals, semiconductors, ferromagnetic oxides or ferrites onto the dielectric binders described above have highly desirable magnetic and dielectric properties. The use of ferrite, a ferromagnetic ferrite made from ferric oxide and other divalent metal oxides, as a sheet material on reflective surfaces and objects to shield or substantially reduce the reflection of electromagnetic energy Gives many advantages. It has been found that mixed ferrites often provide good absorption over a wide range of microwave frequencies. Also, ferrite in the form of a solid coating exhibits a high degree of permeability necessary for broadband operation. Such solid ferrite coatings can have a higher degree of permeability than ferrite powders exhibit. This is because the magnetic properties of ferrite are significantly reduced by grinding the ferrite into powder form. It has thus been found that non-conducting, ferromagnetic ferrites provide potentially optimal dielectricity and magnetism in a single composition. Unfortunately, it has been found that in conventional absorbent coatings containing ferrite, substantial amounts of heavy ferrite are required to achieve the desired absorbent capacity. Dense coatings made from such conventional absorbents are generally undesirable because they are heavy and difficult to fabricate. In view of the above-mentioned shortcomings of the prior art materials for absorbing electromagnetic radiation, any material that is easy to make into any shape or in a variety of substrates, containing relatively low concentrations of the heavy magnetic particles necessary for absorption, has been proposed. It would be highly desirable to provide a lightweight absorbent material that is easy to coat onto objects. SUMMARY OF THE INVENTION The present invention provides such low density absorbing materials that exhibit high efficiency in absorbing electromagnetic radiation, particularly at microwave frequencies. Such compositions (hereinafter referred to as ELM compositions) consist of (1) a dielectric material (hereinafter referred to as a dielectric matrix) and (2) colloidal-sized particles of a substance capable of absorbing electromagnetic radiation (hereinafter referred to as an ELM absorber). (3)
It consists of particles of metal-containing substances (hereinafter referred to as ELM attenuators) that can increase the attenuation of electromagnetic radiation. The concentration of ELM absorber in the ELM composition is advantageously sufficient to provide a magnetic loss tangent of greater than 0.05 at a frequency of 2 gigahertz (gHz) and a composition thickness of 2 cm.
The concentration of the ELM attenuator is such that the ELM attenuation is greater than 0.5 decibels per centimeter (dB/cm) under the above conditions.
sufficient to provide the composition. For purposes of this invention, low density ELM compositions have a density of less than 6 g/cm ³ . Surprisingly, the low density of the present invention
The ELM composition exhibits higher dissipation than expected at the ELM absorbent concentration used. In another aspect, the present invention provides a stable fluid dispersion of colloidal sized particles of a dielectric matrix containing colloidal or subcolloidal particles of an ELM absorber and an ELM damping agent as described above. Surprisingly, such a dispersion can be applied as a coating and dried to form a continuous film in which the particles of ELM absorbent are held in substantially discrete and spaced apart relationship by a dielectric matrix. It gives Preferably, the particles of ELM damping agent are also substantially held in substantially discrete spaced apart relationship by the dielectric matrix. The ELM compositions of the present invention are suitable for use in paints and coatings for reducing reflection of metal structures such as towers, bridges, ships, etc.; for microwave camouflage and radar camouflage; for appliances in which absorption of microwave radiation is desired, such as They are particularly useful as electromagnetic radiation absorbers in applications such as coatings for microwave ovens and microwave spray equipment; applications involving the transfer of solar energy from space satellites; and the like. The ELM compositions are also well suited for molded articles and for foam and fiber manufacturing. Detailed Description of Embodiments ELM compositions of the present invention have attenuation and magnetic loss tangents as described above. Preferred compositions have (1) a density ranging from about 1.2 to about 5 g/cm ³ , most preferably from about 1.5 to about 3 g/cm ³ , (2) greater than 0.1, most preferably greater than 0.2 under the conditions described above. and (3) an attenuation greater than about 0.1 dB/cm, most preferably greater than about 2 dB/cm. This ELM composition consists of three essential ingredients namely (1)
(2) a dielectric solid matrix that serves as the continuous phase of the composition; (2) granules held in substantially discrete and spaced-apart relationship by the matrix;
Consisting of an ELM absorber and (3) a particulate ELM attenuator. In preferred ELM compositions, the ELM damping agent is also substantially entirely dispersed within the dielectric matrix. The dielectric matrix is preferably any normally solid material that can serve as an insulating matrix (binder) for the ELM absorber. Preferably the dielectric matrix has an electrical resistance greater than 10 ⁶ ohms/cm, more preferably greater than 10 ¹⁰ ohms/cm, and most preferably between 10 ¹⁵ and 10 ²⁰ ohms/cm. Examples of such suitable dielectric materials include:
Examples include glass, ceramics, wax, plastics (including thermoplastic and thermosetting), rubber polymers, and synthetic plastics are preferred. Preferred synthetic plastics are water-insoluble polymers made from hydrophobic monomers that are substantially immiscible with water, i.e., monomers that form a separate phase when 5 g of this monomer is mixed with 100 g of water. This is what I did. Such water-immiscible monomers are polymerized under emulsion polymerization conditions to give water-insoluble polymers that are usually present in the form of stable aqueous colloidal dispersions with the aid of suitable surfactants. ELM absorbers are materials that (1) absorb electromagnetic radiation with frequencies ranging from about 0.3 to about 20 gHz and (2) are in the form of colloidal and subcolloidal sized particles.
Preferred absorbers can further be characterized as paramagnetic or supermagnetic due to their small size. Examples of such particles are compounds of magnetic metals such as ferromagnetic oxides or ferrites such as Fe ₃ O ₄ as well as ferric oxides and various divalent metal oxides (such as metal oxides of nickel, zinc and manganese). ferromagnetic ferrites made of; magnetic metals such as iron, cobalt, nickel and their alloys; and other well known ELM absorbers such as carbon black, graphite, etc.
ELM absorbents generally include particles having a largest dimension of less than about 1 micron (μ), preferably in the range of about 0.01 to about 0.7μ. Among these substances, magnetic metal compounds are preferred, and Fe ₃ O ₄ is most preferred. The ELM attenuator is preferably a ferromagnetic material capable of providing the microwave attenuation described above. The ELM attenuator is in the form of particles greater than 1 micron, preferably in the range of about 1.5 to about 100 microns, and most preferably in the range of about 2 to about 75 microns. Examples of such dampening materials are iron, cobalt, nickel and other ferromagnetic metals and alloys of these metals. Among these materials, metallic iron is preferred, and carbonyl iron is most preferred. However, it will be appreciated that, in addition to carbonyl iron, metallic iron produced by other methods, such as electrolytic iron, reduced iron and atomized iron, is preferred. In preparing the low density ELM absorbent compositions of the present invention, the ELM absorbent is placed in a dielectric matrix such that the dielectric matrix forms a continuous phase that maintains the ELM absorbent in a substantially discrete and spaced apart relationship. To this end, in the present invention, the monomers that are polymerized to form the polymeric dielectric material, ie, the dielectric matrix, are polymerized in a mixture with the ELM absorbent particles. Preferably,
A dielectric matrix with an ELM absorbent dispersed therein (hereinafter referred to as dielectric/absorbent) is prepared by first preparing an aqueous dispersion of the ELM absorbent by contacting colloidal or subcolloidal particles of the absorbent with a water-soluble surfactant or emulsifier. This makes the absorbent particles about 5 to about 70
% by weight. An example of a preferred aqueous dispersion of an ELM absorbent is a so-called ferrofluid as described in U.S. Pat. No. 3,981,844, preferably from about 0.05 to about
It has an average particle size in the range of 0.1 micron.
Preferably, such fluid is an aqueous dispersion of magnetic metal stabilized by the presence of surfactants, emulsifiers and/or chemical dispersants as described below. Typically suitable surfactants, dispersants or emulsifiers include salts of fatty acids such as potassium oleate, metal alkyl sulfates such as sodium lauryl sulfate, salts of arylsulfonic acids such as sodium dodecylbenzenesulfonate; polysoaps such as sodium alkali metal salts of polyacrylates and methyl methacrylate/2-sulfoethyl methacrylate copolymers, and other anionic surfactants such as dihexyl ester of sodium sulfosuccinic acid; nonionic surfactants such as ethylene oxide and propylene oxide, ethylene glycol and and/or nonionic condensates with propylene glycol; and cationic surfactants such as alkylamines, guanidine, and polyoxyethanol; and "Emulsion Polymerization" (by D. C. Blatsky, published by Wiley & Sons) 7th
A wide range of micelle-generating substances as described in ``Detergents and Emulsifiers'' (Chapter 1975) and ``Detergents and Emulsifiers'' (written by Matsukato Chiesson, Morristown, New Jersey, USA).
Published by Incorporated North American
1980 Edition). Also, some suitable surfactants include surface active polymers (often referred to as polysops) such as those described in US Pat. No. 3,965,032. Among these suitable surfactants, the anionic ones are preferred, such as the potassium salts of functionalized oligomers such as the Polywet variety sold by Uniroyal Chemical. Such surfactants or emulsifiers are
Used in an amount sufficient to provide a stable dispersion of ELM absorbent. Preferably, such surfactants are present in an amount of about 0.2 to about 10% by weight, based on the aqueous phase.
Most preferably it is used at a concentration of about 1 to about 6% by weight. A particularly preferred method for making such aqueous colloidal dispersions of ELM absorbers is described in U.S. Patent No. 3,826,667.
No. 3881844, No. 3843540 and Industrial Engineering Production and Research Development, No. 19
Volume 147-151 (1980). The aqueous dispersion of ELM absorbent is then mixed with water-immiscible monomers as described herein, and both the dispersion and monomer are mixed in a high shear mixing device (e.g., a walking tube) using conventional methods, such as a high shear mixing device (e.g., walking
the desired emulsion by passing it through a blender, homogenizer or ultrasonic mixer). Alternatively, and preferably during the polymerization
Add monomer continuously to the aqueous dispersion of ELM absorbent. Advantageously, the monomers are in the form of an aqueous emulsion of the monomers whose emulsification is maintained by water-soluble monomers and/or water-soluble emulsifiers as described above. Alternatively, aqueous emulsions of ELM absorbers and water-immiscible monomers are
It can be prepared by adding colloidal or subcolloidal particles of ELM absorbent to an existing aqueous emulsion of monomer. In such cases, it is often desirable to add additional emulsifiers or surfactants to the emulsion prior to or simultaneously with the addition of the ELM absorbent particles. underwater
In the emulsion of the ELM absorbent and the water-immiscible monomer, the aqueous phase is present in sufficient proportion to be the continuous phase of the emulsion. The ELM absorbent phase is present in a sufficient proportion to provide the desired dissipative properties of the dielectric/absorbent particles. The water-immiscible monomer is present in a sufficient proportion to envelop or encapsulate the ELM absorbent when polymerized. Emulsifiers and/or surfactants are present to provide an aqueous colloidal emulsion that is sufficiently stable to be subjected to emulsion polymerization conditions. Preferably the emulsion contains from about 0.1 to about 25% by weight ELM absorbent, from about 1 to about 30% by weight ELM absorbent.
% monomer and a residual amount of aqueous phase (including emulsifiers (surfactants), catalysts, etc.). Examples of suitable water-immiscible monomers that may be used to prepare the above dielectric/absorbent agents include monovinylidene aromatics such as styrene, vinyltoluene, t-butylstyrene, chlorostyrene, vinylbenzyl chloride and vinylpyridine;
Alkyl esters of α,β-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; Saturated nitriles such as acrylonitrile; dienes such as butadiene and isoprene; and the like. Among these monomers, monovinylidene aromatics such as styrene, and alkyl acrylates such as butyl acrylate are preferred. In addition to the water-immiscible monomers mentioned above, relatively small proportions of water-soluble monomers may be used, for example less than 10% by weight, preferably less than 5% by weight, based on the total monomer component. Examples of such water-soluble monomers include ethylenically unsaturated carboxylic acids or their salts such as acrylic acid or sodium acrylate, methacrylic acid, itaconic acid and maleic acid; ethylenically unsaturated carboxamides such as acrylamide; vinylpyrrolidone; hydroxyalkyl Acrylates and methacrylates such as hydroxyethyl acrylate,
Hydroxypropyl acrylate, and hydroxyethyl methacrylate; aminoalkyl esters of unsaturated acids such as 2-aminoethyl methacrylate; epoxy-functional monomers such as glycidyl methacrylate; unsaturated sulfoalkyl esters such as 2-sulfoethyl methacrylate; ethylenic Examples include unsaturated quaternary ammonium compounds such as vinylbenzyl-trimethylammonium chloride. In practicing this preferred embodiment, it is important that such water-soluble monomers are not used in amounts sufficient to render the resulting polymer soluble in water. A monomer formulation that is particularly useful in the practice of this invention is about 20 to about 90% by weight styrene, about 10 to about 80% by weight alkyl acrylate (e.g., butyl acrylate), and about
001 to about 2% by weight of an unsaturated carboxylic acid (eg, acrylic acid). Note that these weight percentages are based on the weight of all monomers. The emulsion polymerization conditions used in the practice of this preferred embodiment of the invention are those of conventional free radical type polymerizations carried out in the presence of radical initiators such as peroxygen compounds, azo catalysts, and ultraviolet light. Preferably,
Such polymerization can occur in the presence of water-soluble peroxygen compounds.
It is carried out at a temperature ranging from 50° to about 90°C. The emulsion is generally stirred during polymerization to maintain proper feed movement. Catalyst concentration typically ranges from about 0.0005 to about 8% by weight, based on total monomer, preferably about
It ranges from 0.01 to about 5% by weight. Examples of suitable catalysts include inorganic persulfates such as sodium persulfate, potassium persulfate, ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl hydroperoxide, dibenzole persulfate, and dilauroyl peroxide; azo catalysts such as azo bisisobutyronitrile; and other common free radical generating compounds. Various forms of free radical generating radiation are also suitable, such as ultraviolet radiation, electron beam radiation and gamma radiation. Alternatively, redox catalyst compositions can be used. The polymerization temperature at that time is in the range of about 25° to about 80°C. Examples of redox catalysts include combinations of the aforementioned peroxygen compounds, preferably potassium persulfate or t-butyl hydroperoxide, with reducing components such as sodium sulfite and sodium formaldehyde hydrosulfite. Various chain transfer agents such as mercaptans (dodecyl mercaptan), dialkylxanthogen disulfides, diaryl disulfides,
It is also suitable to use other chain transfer agents as described in Chapter 8 of the Blackley Report, cited above, at the concentrations described therein. After emulsion polymerization, the resulting aqueous dispersion of dielectric/ELM absorbent particles can be removed from the polymerization vessel and (1) this dispersion can be used as is or (2) unreacted monomers can be removed from the polymerization vessel. (3) This dielectric/ELM absorbent particle can be prepared by conventional methods such as spray drying or vacuum drying. can be separated from the aqueous phase of the dispersion by means of. If desired, the dielectric/ELM absorber particles preferably have a particle size of about 10 to
about 80% by weight, most preferably about 15 to about 70% by weight
ELM absorbent and about 90 to about 20 weight percent dielectric matrix polymer, most preferably about 85 to about 30 weight percent. In this preferred embodiment, the particles in the form of an aqueous dispersion or of dry colloidal size are then ELM
Mix with a damping agent to form the desired low density ELM absorbent composition. Preferably the ELM damping agent (particles) is dispersed as an aqueous dispersion of dielectric/ELM absorbent;
This allows it to be applied to any substrate as desired.
A coating composition is formed that can be dried into a continuous coating that can absorb ELM radiation. Alternatively, ELM
The damping agent may also be encapsulated in the preferably dielectric material described above prior to mixing with the dielectric/ELM absorber. In this embodiment, the ELM damping agent and dielectric/ELM absorbent can be in the form of an aqueous dispersion and/or in the form of a dry powder when mixed. In dry form, the resulting low density ELM composition can be formed into articles of desired shape by conventional forming techniques such as injection or compression molding, extrusion, and the like. Alternatively, the ELM composition in dry powder form can be dispersed in a non-aqueous liquid and used as a base for, for example, a paint base or other coating formulation, if desired. A preferred low density ELM absorbent composition using colloidal size Fe ₃ O ₄ as the ELM absorber and carbonyl iron as the ELM damping material is approximately
90:10 to about 40:60, most preferably about 80:20 to about
With an ELM absorber:ELM damping agent weight ratio of 55:45. In this preferred ELM composition, (ELM
The weight ratio of absorber + ELM damping agent):dielectric matrix is about 85:15 to about 10:90, most preferably about 70:
30 to about 55:45. Besides the essential ingredients listed above, these compositions optionally contain other ingredients such as stabilizers,
Pigments, fillers, blowing agents, corrosion inhibitors, and other additives commonly used in ELM absorbent compositions may be included. The present invention will be explained in more detail with reference to the following examples, but these examples should not be construed as limiting the scope of the present invention. All parts and percentages are by weight unless otherwise specified. Example 1 A Preparation of an aqueous dispersion of Fe ₃ O ₄ (ELM absorbent) An aqueous dispersion of magnetic iron oxide (Fe ₃ O ₄ ) [ELM absorbent] was mixed with an aqueous solution of a ferrous salt and a ferric iron salt. It was prepared by mixing aqueous solutions of salts in amounts to maintain a Fe ⁺³ /Fe ⁺² molar ratio of approximately 2:1. Then PH
Magnetic iron oxide was precipitated at 0-10<0>C by rapid addition of 1N _NH4OH and vigorous stirring until a temperature of 9-10 was reached. Immediately thereafter, the dispersion was introduced with stirring into the aqueous medium containing the precipitated iron oxide, and the mixture was heated at 90° C. for 1 hour. During this period, hydrochloric acid was added until the PH of the mixture reached 7.5. The precipitated iron oxide particles were washed with deionized water and treated with a functionalized oligomeric potassium salt (polywet KX-4, sold by Uniroyal Chemical) at 0.5 g per gram of precipitated iron oxide using a conventional ultrasonic needle. redispersed in deionized water containing The magnetization of the dispersed iron oxide was measured by Colpitts oscillator circuit technique. B. Preparation of Magnetic Latex (Dielectric/ELM Absorbent) In a three-necked flask equipped with a stirrer, two addition funnels, and a condenser, add 28.5% of Fe ₃ O ₄ [200 Gauss; average particle size less than 0.08 micron]. A mixture of 507 g of solid dispersion and 203 g of deionized water was added. The mixture was then heated to 90° C. under a nitrogen atmosphere while stirring. At this temperature of 90°C, the monomer and aqueous surfactant streams were separately introduced into the flask through two addition funnels. Each flow was introduced over 65 minutes at a rate of approximately 6 ml/min. The monomer stream consisted of 64 g styrene, 16 g butyl acrylate, and 3 g t-butyl hydroperoxide. The aqueous surfactant flow was 110 g deionized water, 2.9 g functionalized oligomeric potassium salt (Polywet KX-4)
and 2g of sodium formaldehyde.
It was composed of hydrosulfite.
The resulting reaction mixture was stirred and diluted with another 1/2 ml under nitrogen.
It was kept at 90°C for 2 hours. The resulting 25% solid latex was concentrated by vacuum distillation to obtain a 30.3% solid latex (dielectric/ELM absorbent) in which polymer and magnetic particles were dispersed. The particles of this latex had a narrow particle size distribution and an average particle size of 0.11 microns as measured by hydrodynamic chromatography. This latex remained stable when exposed to a magnetic field of 1800 Gauss, exhibiting properties common to magnetic colloids. That is, such magnetic colloids are magnetic fluids that are demagnetized immediately when a magnetic field is removed, and attract objects when a magnetic field is applied. By the Colpitts oscillator circuit technology described by E.A. Peterson et al. in Journal of Colloidal and Interfacial Science 70 3 (1977),
The magnetization of this latex was estimated to be 135 Gauss. This latex was heated to −80°C under a vacuum of 0.5 mmHg.
The latex particles were recovered by freeze-drying. C Preparation of ELM Composition (Dielectric/ELM Absorber/ELM Attenuator) 50.3 g of the dry powder of the above latex (55.4% dielectric material/44.6% Fe ₃ O ₄ ) with an average particle size of 3-4 microns was added. From GAF Corporation
One ELM composition (Sample No. 1) was prepared by dry blending with 33.5 g of carbonyl iron (ELM damping agent) sold under the tradename Super Fine Special. Blending was carried out in a Brabender mixer, and the resulting blend was then compression molded at 2000 lbs positive pressure and 230°C for 2 minutes into flat plates (0.8 cm thick x 2.6 cm diameter).
I made it. The sample was cooled to room temperature and the pressure on the sample was released. The resulting ELM composition slabs were 2.54 cm in diameter and 0.64 cm and 0.32 cm thick, respectively.
machined into two flat discs of cm. Latex dry powder 56.5g and carbonyl iron
A second ELM according to the method above using 18.8g
A composition (Sample No. 2) was produced. This sample was similarly blended, molded, and processed into disks. For comparison, a third sample of dry latex particles (Sample No. C) was formed into disks by the method described above. All of these samples were tested for ELM absorption and the results are shown in Table I.

【table】

[Table] As is clear from the data shown in Table I,
The compositions of the present invention (Samples No. 1 and No. 2) are Sample No.
It exhibits significantly better attenuation at a given frequency than the composition of C.