LU87058A2 - Electromagnetic radiation absorbent coating composition containing metal coated microspheres - Google Patents

Description

Coating composition absorbing electromagnetic radiation and containing metal coated microspheres.

In US Patents 4,624,798 and 4,624,865, new electrically conductive microparticles are described in the form of metal-coated magnetic ceramic microspheres, processes for manufacturing of these microspheres, as well as their usefulness during the formation of composite structures or electrically conductive coatings for different applications. Now, the Applicant has discovered that metal-coated microspheres of the type described in the aforementioned patents could, under certain conditions, be used to form composite structures and robust and very light coatings absorbing electromagnetic radiation.

In the prior art, a number of materials have been described which are capable of absorbing electromagnetic radiation and, in particular, high frequency microwave radiation such as that used for radar. However, these materials have manifested some undesirable properties and their applicability and usefulness have been severely limited to many purposes. Absorbent materials in which iron powder, ferrites and other magnetic materials are used are very heavy, which limits their usefulness in light weight applications, for example, in aeronautics. Plastic foams containing carbon particles, metallic powders and ferrites require a great thickness and are physically too weak for many applications. Resonant absorbers based on a quarter-wave resistive material from a reflector are only effective at a very narrow frequency band. Broadband absorbers based on materials having a relative magnetic permeability equal to the relative electrical permittivity specifically contain ferrites, iron and other high density materials, so that very heavy absorbers result.

Complex systems with several layers of partially electrically conductive materials spaced apart with non-electrically conductive dielectrics with a view to obtaining a laminate having calibrated electrical properties have been produced in the form of tiles having fairly good electrical properties. 'absorption, but it is very difficult to apply these systems to complex structures such as in aeronautics, not to mention that they are very thick and of considerable weight.

The present invention provides a robust and light composition containing metal coated microspheres. This composition has very broad band absorption properties for electromagnetic radiation and it is effective when used in the form of a thin coating or as a structural composite material. It can withstand very high temperatures (900 ° C) and, in this regard, it is generally limited only by the binding agent.

It has the additional advantage of ensuring soundproofing, it is a good thermal insulator and it can be easily applied to a substrate by means of conventional paint spraying equipment.

Summary of the invention.

More particularly, the present invention provides a composition absorbing electromagnetic radiations and comprising microspheres having a diameter of 1 to 350 microns, these microspheres containing at least one material absorbing radiation and chosen from the group comprising carbon, ferrites, lamagnetite, iron, nickel and cobalt, these microspheres comprising a thin coating of metal on their surface, this coating of metal being present in an amount of 0.01 to 22% by weight.

Preferably, the microspheres are ceramic magnetic microspheres derived from the ash of charcoal ovens and they have permanent magnetic properties.

The microspheres can be dispersed in and bound by an organic or inorganic binder and they can be used to form composite materials or coating compositions.

Additional uses, characteristics and advantages of the present invention will be better understood on reading the detailed description below.

Detailed description of the invention.

As will be understood by a person skilled in the art, the expression "microspheres" denotes very small particles of low density in the form of hollow or porous spheres made of a plastic material, glass or a ceramic material.

They are most commonly used as fillers in solid materials such as resins to form composite materials of low density and high strength.

Certain qualities of commercially available ceramic microspheres contain components such as carbon, ferrites, magnetite, iron, nickel and cobalt in the ceramic composition. These components confer magnetic properties to microspheres.

The Applicants have found that, when they contain an extremely thin coating of a conductive metal, the magnetic ceramic microspheres with a granularity lying between approximately 1 and approximately 350 microns and containing the abovementioned components in the ceramic composition, absorb the electromagnetic energy at large. bandaged. This absorption is several orders of magnitude greater than that which should be expected based on the carbon, ferrite, trace metal and metal oxide content found in the ceramic wall of the microspheres. The Applicant has also found that the best results are obtained when the metal coating has a very uniform thickness at such a low point that an appreciable light transmission occurs when the microspheres are examined under the microscope.

Among the commercially available ceramic microspheres having magnetic properties, there are, for example, the qualities "Q-Cell 100, 110 and 120" from "PQ Corporation, ValleyForge, PA"; and "Extendospheres XOL-200, CG, SG, SF-14" as well as others from "PA Industries, Chatta¬nooga, Tennessee and EUA These are light and robust microspheres made up of a ceramic composition comprising mainly aluminum silicates, magnesium silicates, sodium silicates or mixtures of these materials. These microspheres have a hollow ceramic shell; moreover, they are much more robust and much more resistant to abrasion than microspheres Ceramic crucibles (glass) are preferred. The ceramic microspheres which are called in the industry "centospheres" deriving from the ashes of industrial furnaces in which coal powder is burned, since such microspheres contain the desired compounds in sufficient quantity in the wall. ceramic and they are very inexpensive. However, microspheres of ceramic material, glass or plastic can be produced in incorporating the desirable compounds into their wall by means of well-known techniques concerning microspheres. When they are coated with a conductive metal as described in the present specification, these micro-spheres can be advantageously used in accordance with the present invention.

As obtained on an industrial scale, the centospheres produced with coal specifically contain about 1 to about 15% by weight of carbon, ferrites, magnetite, iron, nickel and cobalt. The quantity of these compounds involved in the ceramic composition will vary with the quality of burnt charcoal, so much so that certain qualities of cento-spheres will contain a greater quantity of desirable compounds than others.

For many applications, it may be desirable to refine or enhance the magnetic properties of commercially available microspheres. The more magnetic particles can be separated from the less magnetic particles using a magnet. Likewise, the permanent magnetic domains of commercially available magnetic centospheres can be strengthened by slowly heating these spheres to avoid rupture at a point beyond the Curie Point, then allowing the particles to cool in a strong magnetic field, thereby providing orientation domains in a strong permanent magnetic field.

Microspheres have a thin coating of an electrically conductive metal. Although any conductive metal can be used, preference is given to silver, gold, platinum, palladium and their Alloys are applied. The deposition of metal is applied to the microspheres by adopting the methods described in US Patents 4,624,798 and 4,624,865.

These methods give a very uniform shiny deposit analogous to a mirror uniformly covering the surface of the microspheres. In order to obtain the desirable properties for the absorption of electromagnetic radiation, the metal coating must be very thin and very uniform.

The metal coating is so bad that conventional thickness measurement techniques are inadequate. It is preferable to apply the metal coating on the micro¬ spheres in a simple percentage by weight. The magnetic centers available in commerce can be obtained in various particle sizes and the percentage by weight of the coating metal depends on the particle size of the particles. Specifically, a particulate material with an average diameter of 50 microns will require 2.0 to 3.0% by weight of the coating metal. Microspheres with a smaller average diameter having a larger specific surface area per unit of weight will require a higher percentage by weight of metal coating while a particle material with a larger average diameter will require a lower percentage by weight of coating metal . Microspheres having a diameter in the range of about 1 to about 350 microns will require a coating of metal in an amount of 0.01 to 22% by weight, calculated on the coated microspheres.

The quality and quantity of the coating can be measured by measuring the electrical conductivity of the dried metal coated microspheres. To this end, a small quantity of the dried microspheres is deposited in a rectangular plastic tube having an open internal cross section of 1 cm × 1 cm, providing, in this tube, a tolerance for the fine adjustment of silver-plated brass pistons electrically connected to an ohmmeter.

Weights can be placed on the device to decompress the material between the pistons. If an increase in weight does not appreciably modify the resistance of the test piece, this reading is taken. Since it is difficult to adjust the height of material in the tube, an exact weight is placed in the apparatus, specifically 0.05 g of material. Readings are taken until a slight damage and the addition of a weight (usually about 6.8 kg) gives stable readings. Microspheres coated with metal and having properties for absorbing electromagnetic radiation have specifically an electrical resistance ranging between 0.2 and 200 ohms (measurement carried out by the above method), the preferred interval being between 30 ohms and 40 ohms for absorbing electromagnetic radiation in the wavelengths used by a radar.

A 0.05 g test tube of quality SF-20 microspheres (from "P.A. Industries", Chattanooga, Tennessee) coated with 21 by weight of silver will give a reading of approximately 34 ohms when it is properly prepared.

Mixtures of charges of materials deviating from the optimum characteristic can be brought to lie within this range with good results if the percentage of silver is within the tolerance of 0.02% for each component of the mixture.

The metal-coated microspheres can then be mixed with organic or inorganic binding agents to obtain coating compositions or composite materials. In paint coatings, the weight ratio between the microspheres and the binding agent can vary within wide percentages depending on the yield. desired final physics. At low values of bonding agent, the coating is light, but is not as robust. At very high weight values of binding agent, the coating is very tenacious, but may contain insufficient microspheres to obtain good absorption by the radar. At weight ratios close to 1: 1, the microspheres increase the concentration of specific binding agents for plastic material such as urethanes, epoxy resins or polyesters. This 1: 1 ratio gives coatings with excellent physical properties and excellent absorption properties.

The paints can be sprayed in a conventional device in order to obtain coatings on any practical substrate. As a rule, the absorption properties increase with the thickness of the coating. Specific results at 10 GHz should be -1 db for 0.0254 to 0.0381 mm of coating.

In order to obtain the best results, the coating should be electrically insulated from an electrically conductive substrate by means of a coating of insulating material applied as a primer. The thickness of the bottom layer and its electrical properties become less important as the thickness of the absorbent layer increases. A superior coating can be applied to the absorbent layer to obtain a very stubborn and smooth final surface with any conventional coating which is transparent to radar, for example, an acrylic resin, a urethane or an epoxy resin.

The microspheres can be mixed with self-hardening binding agents such as polyesters and epoxy resins, or with thermosetting binding agents such as phenolic and polyimide resins to obtain very robust and light composite materials. Specifically, the microspheres have a density of 0.7 and the composite materials obtained usually have a density of less than 1. Composite structures such as radar absorption screens generally have a thickness of at least 3.175 mm and they give a high absorption of very wide band radar. The mixtures situated between a fluid state and a state analogous to that of the mastic, which is obtained before the bonding agent is taken, are easily molded into complex configurations. Very robust and tenacious elements are obtained for the absorption of the radar with 10 to 50% by weight of binding agent.

Composite materials containing electromagnetic radiation absorbing microspheres can be mixed with various organic binder agents such as epoxy resins, polyesters, polyimides, resins, plastics and silicones; orwith inorganic binding agents such as lessilicates, clays, borates, aluminates and ceramic materials.

Coating compositions containing the microspheres can be used in a variety of applications. For example, on a canvas or a net, a coating containing the microspheres can be applied in order to give them radar absorption properties. A coating composition containing the microspheres can be applied to the surface of land vehicles such as motor vehicles, on water vehicles such as boats, on air vehicles such as planes, airships and balloons, on space vehicles and on vehicles intended to enter the atmosphere. A piece of furniture or other housing intended to receive an electronic device may be coated with paint or it may have composite walls containing the microspheres in order to form, for the electronic device, a highly effective shielding against electromagnetic frequencies. Waveguides and antennas for electromagnetic energy can be coated with a paint or they can be formed from a composite material containing the microspheres. On a thermo¬ couple, a coating containing the microspheres can be applied to thereby obtain an instrument for detecting electromagnetic radiation fields, since the electromagnetic energy received by the thermocouple will be absorbed, converted into heat and thus detected by lethermocouple. The walls of microwave cooking vessels may contain or be coated with a material containing the microspheres. A liquid containing the microspheres absorbing electromagnetic radiation may be used in various applications, for example, as a heat transfer liquid capable of effecting energy heating. microwave.

The nonlimiting examples below are intended to further illustrate the properties and virtual uses of materials containing the microspheres absorbing electromagnetic radiation according to the invention.

Example 1

"SF-20" quality microspheres from "P.A. Industries", Chattanooga, Tennessee, E.U.A. They were stripped of their scoriae and silvered by the process described in United States patent 4,624,865 to obtain a final product in a greasy powder comprising a deposit of 2% by weight of silver on the surface of the microspheres. This material was dried and mixed in a two-component urethane paint such that after drying, the weight ratio between the urethane plastic and microspheres was 1: 1.

This paint was sprayed onto 13.5 cm x 13.5 cm x 1.4 mm polycarbonate panels to a thickness of 0.76 mm when dry and at about 0.078 g / cm 2.

This panel gave an absorption exceeding the range of the Applicant's device (-20 db, less than 1/100 of the normally reflected power), measured compared to a similar non-coated panel placed in front of a metallic reflector which was pointed at by a radar device. at 10GHz (X band).

Example 2

A composite material was prepared by mixing 2% by weight of silver-plated ceramic microspheres with 1/3 by weight of premelanated epoxy resin. The obtained mastic-like material in a thickness of 3.2 mm was applied to a polyethylene sheet by means of a roller and allowed to harden for 48 hours. Thus, a very tough, light and robust composite sheet was obtained when removed from the polyethylene.

This sheet also exceeded the dynamic range of the Applicant's device, measured at 10 GHz.

Example 3

6g of ceramic microspheres containing 2.20% silver were placed in a paper cup. This cup was placed in a commercially available microwave oven (about 6 GHz) and irradiated for 15 seconds.

The heat of absorption has blackened and scorched the cup. Slightly longer exposure times would burn the cup.

Example 4

A coating was applied to a very thin cotton fabric by spraying with a mixture of flexible urethane paint and microspheres containing 2.50% by weight of silver so that the ratio of resin solids to pearls was 3: 2. The star obtained gave an absorption of X-band radard greater than -20 db when it was placed at a distance of 1.27 mm from a metallic reflective panel.

Example 5

On a 13.5 cm × 13.5 cm × 1.5 mm polycarbonate panel, a paint prepared from 2.00% by weight silver microspheres and an amine hardened epoxy resin was applied. The cured final coating contained the microspheres and the epoxy resin in a 3: 2 weight ratio to a thickness of 0.76 mm after curing. Placed on a 100% reflective plate 13.5 cm x 13.5 cm, this panel gave a broadband absorption exceeding -15 db from 8 GHz to 16 GHz most of the absorption exceeding -30 db.

Claims (7)

2. Composition according to claim 1, characterized in that the microspheres have permanent magnetic properties. 3. Composition according to claim1, characterized in that these microspheres are ceramic magnetic microspheres derived from ash from charcoal ovens. 4. Composition according to Claim 1, 2 or 3, characterized in that the thin metal coating comprises a metal chosen from the group comprising silver, gold, tin, chromium, platinum, palladium, nickel, copper and cadmium, as well as alloys of these metals, the coating thus formed being in the form of a glossy deposit similar to a mirror uniformly covering the surface of the microspheres. 5. Composition according to claim1, characterized in that the microspheres have an average diameter of about 50 microns, while the metal coating is present in an amount of about 2 to 3% by weight. 6. Composite material absorbing electromagnetic radiation, characterized in that it includes the microspheres according to claim 1 (dispersed in and linked by an organic or inorganic binding agent. 7. Coating absorbing electromagnetic radiation, characterized in that it comprises the microspheres according to claim 1 in admixture with an organic or inorganic binding agent forming a coating on the substrate. ^ ΪΓ) Fabric comprising a coating containing the microspheres according to claim 1, conferring, on this fabric, properties of absorption of electromagnetic radiation. (Q Laminate containing at least one layer comprising the microspheres according to claim 1. Structure absorbing electromagnetic radiation, characterized in that it comprises a substrate and a coating absorbing electromagnetic radiation deposited on this substrate, this coating comprising a binding agent and several magnetic ceramic microspheres dispersed in and bound by this binding agent, these microspheres having a diameter of 1 to 350 microns and containing at least one material absorbing radiation and chosen from the group comprising carbon, ferrites, magnetite, iron, nickel and cobalt, a thin a radiation absorbing metal deposit being deposited on the surface of the microspheres, this metal deposit being present at 0.01 to 22% by weight in a thickness giving a resistivity of 0.2 to 200 ohms. 11. Structure according to claim10, characterized in that the substrate is an electrically conductive substrate while, in addition, it comprises an upper coating of a composition which does not absorb electromagnetic radiation, covering the coating absorbing radiation electromagnetic and define the extreme outer surface of this structure. Land, nautical, air, space vehicle or intended for re-entry into the atmosphere, this vehicle comprising a coating according to claim 7 or 10. Radar antenna comprising a coating according to claim 7 or 10. ^ TT?) Projectile comprising a coating according to claim 7 or 10.