GB2192756A - Energy absorbing coatings and their use in camouflage - Google Patents

Abstract

Bodies are protected from detection by radar, infra-red detectors or the like in a manner which does not hinder their movement by providing on the bodies a covering comprised by a coating applied to the body of a coating composition which undergoes a gradual decrease in impedance throughout its depth from the surface (20) thereof remote from the surface (22) of said body. Such coatings may be applied in one or more layers by conventional coating techniques and, while in an unhardened state caused to undergo migration of components therein to achieve the desired gradual decrease in impedance. <IMAGE>

Description

SPECIFICATION Energy absorbing coatings and their use in camouflage This invention relates to radiant energy absorbing coatings for use in camouflage, particularly camouflage of military targets.

Well established surveillance techniques involve the reflecting from subjects of interest of incident radiation and the detection of the reflected radiation. The pattern of reflected radiation enables the location, size and character of the reflecting target to be characterised.

Radio waves and infra-red radiation are typically employed for this purpose.

A considerable number of proposals have already been made for protecting targets against identification in such manner. The techniques proposed often have much in common with techniques previously adopted for avoiding spurious reflections under laboratory conditions affecting the operation of laboratory equipment. Thus, there have been proposals for providing out of phase reflections the net effect of which is to cancel out the effect of reflections which would be normally received at a receiver device. Reference is made in this connection for example to British Patent Specification No.795,510. Such solutions to the aforementioned problem generally take the form of applying special structures to wall surfaces to be protected.

An alternative technique which again requires the application of a special structure to a surface to be protected is to provide a sandwich construction providing multiple internal reflections, as according to British Patent Specification No.844,140. The success of such a construction is dependent upon choosing of layer thicknesses such as to eliminate reflections as radiation passes from air to a substrate medium. Reflections are eliminated if the thickness is at least one quarter of the wavelength of the incident radiation. Dielectric layers provided for this purpose having a total electrical thickness which is a quarter wavelength at a desired angle of incidence and frequency are generally termed quarter-wave absorbers.This technique may be termed a resonant method in contrast to the non-resonant method which involves covering the reflecting surface with a substance of small reflecting power and small high frequency loss but of a thickness sufficient so that wave penetration through the undersurface and reflection there are sufficiently weakened to be negligible when re-emerging. Unfortunately with this technique, the thickness of such layers must be so great that the method is of very limited practical value.

Of the various methods available, the resonant methods have the advantage of greater efficiency but are generally not suitable for use with targets which are to be moved about.

Moreover, the aforesaid quarter wavelength layers generally lack the ability to absorb a wide range of frequencies, rather being "tuned" to one.

A radiation absorbing material which will operate at a wide frequency range and which prevents the unwanted return of radation from objects in the path of radar beams and the like is provided by British Patent Specification No.2058469. The material proposed here comprises a layer of three dimensional net like structure having an electrically resistive coating applied to one side only so that the resistivity across the thickness of the layer varies substantially exponentially. Notwithstanding the efficiency of this structure for use over a wide frequency range, it is limited in its practical application because of physical constraints. It would appear to be suitable for attachment to the exterior of fixed structures but not suitable for use with moving targets such as tanks.

It is an object of the present invention to provide a radiation absorbing material capable of absorbing over a wide range of wavelengths and which is applied to a potential target as a permanent feature irrespective of whether the target is fixed or capable of powered motion.

According to one aspect of the present invention, there is provided a substrate body to a surface of which is applied a coating composition which undergoes a gradual decrease in impedance throughout its depth. The thickness of this composition is preferably at least one quarter of the wavelength for any type of radiation whose reflection it is desired to provide protection against. The thickness of the composition will thus be directly related to the maximum wavelength of radiation against which protection is to be provided.

In contrast to U.K. Patent Specification No.2058469, there is now provided a coating directly applied to radar or other radiation reflectant surfaces or to components for installation upon objects of a desired low radar or other radiation signature. Advantages of such a coating are ease of application and maintenance, whether on fixed or motively powered structures, robustness, low cost of manufacture, general capacity for customisation and an ability to absorb a wide range of frequencies rather than being tuned to one. It works on a principle which may be established from theoretical considerations that when there is a gradual change in the impedance of a material between free space and a metal reflector, it is possible to achieve total absorption of incident radiation before the metal surface is reached so that reflections do not pose a problem.In theory, the layer can be of totally universal applicability if its thickness is greater than one quarter wavelength in respect of the longest conceivable incident radiation wavelength, preferably being around one half wavelength or more. Radio waves are in the millimetre or centimetre range and relatively thick coatings providing the required gradual decrease in impedance at the depth are a very practical consideration. Infra-red radiation tending to be in the wavelength range 3-5 or 8-14 can also be satisfactorily absorbed even by provision of a relatively thin coating according to the invention. A coating capable of absorbing radio waves will of course also be capable of absorbing waves of shorter wavelength. Any layer gains effectiveness with reduction in wavelength provided the carrier medium remains transparent and structural details therein remain below one tenth wavelength.

A layer applied according to the invention may comprise a low permittivity dielectric carrier medium within which low impedance doping is included, the doping increasing in concentration across the thickness of the layer such that the impedance decreases across its thickness. A low permittivity carrier medium may be polyethylene or polypropylene although polyurethane is a preferred plastic material of low permittivity. The doping preferably comprises particles or fibres which are electrically resistive. Preferably the doping consists of carbon in fine particulate form or metal in fine particulate form or a combination thereof. The gradation in doping may be achieved by exponential variation in numbers of particles of the doping material of like particle size over the thickness of the layer of carrier material.Alternatively there may be equal numbers of particles at different positions through the thickness of the carrier medium but with an exponential gradation in size of the particles through the thickness of the carrier medium.

An alternative means of producing gradation in impedance across a layer is for the carrier medium to be uniformly doped throughout and to have therein bodies of low permittivity material decreasing in concentration across the layer. Such low permittivity material may be provided by solid bodies, for example spheres or even by gas bubbles. As an alternative to providing a decreasing concentration of bodies of low permittivity, it is also possible to employ approxiately the same number of bodies of low permittivity at all depths through the doped carrier medium, with the low permittivity bodies decreasing in forward surface to rearward surface of the layer of carrier medium.

According to a second aspect of the invention, there is provided a method of providing on a surface of a substrate body to be protected, a coating of low radiant energy reflectance which comprises applying to the surface to be protected a curable plastics composition for forming a matrix and containing therein particles of different permittivity to the plastics composition and subjecting the coating to a form of energy such as to cause diffusion of the particles to position them within the coating in such manner as to provide a gradual decrease in impedance through its depth from the surface of the coating to the part thereof adjacent the substrate.

Such diffusion will be initiated in accordance with the type of material to be diffused within the carrier medium provided by the plastics composition. Thus, if it is electrically conductive, then it is possible to apply electrical fields. If it is ferromagnetic, then magnetic fields may be employed. In some circumstances it may even be possible for gravitational fields to be employed. The extent of diffusion will depend upon the physical character of the carrier medium. With thermoplastic materials, melting of the carrier medium may enable the particles to undergo diffusion by heat and be locked into the desired disposition when curing leads to solidification of the plastics carrier medium. Mechanically forced diffusion may take place when the carrier material is subjected to ultrasonic vibration for which purpose the substrate will have to be placed in an ultrasonic bath.Mechanically forced diffusion may even be the result of vibrations caused by loudspeakers of sufficient output, this method being particularly suitable when large scale operation is required.

In a variant of this procedure where gas bubbles are to provide the variation in permittivity across the thickness of the coating, the gas bubbles may be produced in situ by means of a blowing agent in a plastisol which is to undergo curing with the gas bubbles sized and positioned as required.

Application of the compositions from which the coatings are provided can be by brush, roller, spray or pouring, depending upon the consistency of the carrier medium. In general, at least two layers will be applied having initially differing constitution to allow the required diffusion to take place between the layers where particles or bubbles to be diffused are initially present in high concentration and locations where they are initially absent or in low concentration and thereby achieve the exponentially changing permittivity through the thickness of the coating structure. In the simplest case, there will be two layers, one containing diffusible material and the other totally free from this.

It should be noted that the principle of the present invention is applicable to the absorption of any electromagnetic wave, provided that the primary conditions are met, namely 1. the dielectric is transparent to the wavelengths to be absorbed 2. the layer thickness is greater than one quarter wavelength for the particular radiation to be absorbed 3. the structural detail, that is size of particles and separation of particles remains below one tenth wavelength for the particular radiation be absorbed.

Although described herein with reference to "land" structures, in being applicable to the absorption of electromagnetic radiation in general, this invention can also be used for the absorption of sound waves such as to provide anti-sonar coatings upon submarines.

For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made by way of example only to the accompanying drawings wherein: FIGURES 1 to 4 are sections through coatings of different type embodying the invention, and FIGURES 5 to 9 show schematically the production of different types of coating embodying this invention.

Thus, referring to Figure 1, there is shown a coating comprising a carrier medium such as polyurethane, solid plastic material or any low permittivity dielectric the exposed surface of which is shown at 20 and the surface adjacent the substrate of which is shown at 22.

The coating comprises like-sized particles of high permittivity materials such as of carbon whose concentration decreases exponentially from a region of high concentration shown at 12 adjacent the surface 22 to a region 10 of low concentration. Although not shown, a coating of an anti-dip material such as quartz powder may be applied to the surface 20 if the coating is to be subjected to high abrasion. It is naturally a requirement of such additional coating that it should be transparent to the radiation which is to be protected against.

Referring to Figure 2, in which like reference numerals denote like parts in Figure 1, a similar carrier material is applied to a substrate, but here, rather than have particles of the same size throughout the thickness of the carrier medium, the particle size is graded exponentially between particles 11 of larger size adjacent the surface 22 and particles 13 of smaller size adjacent the surface 20.QL The coating of Figure 3 differs from that of Figures 1 and 2 in that the carrier medium here is uniformly doped with a material which is electrically resistive, such as one of those materials proposed herein for use in the embodiments of Figures 1 and 2.The required gradation in impedance is produced by the provision of gas bubbles in the carrier material extending from a region 14 in which the gas bubbles are present in high concentration to a region 15 in which they are present in a low concentration, this latter region being adjacent the surface 22. The bubbles are produced in situ by use of an appropriate blowing agent in the carrier material which produces the gas at the time of heating the carrier material to achieve cross-linking thereof.

Figure 4 shows a similar carrier material to that usable with the embodiment of Figure 3.

A similar profile of total percentage gas bubble cross-section in relation to carrier material depth is present, but here this is achieved, not so much through having a large number of bubbles of like size adjacent the surface 20, with the number of bubbles reducing through the thickness of the carrier material, but rather the same number of bubbles to be found at any layer position through the thickness of the carrier material is the same but with bubbles 16 of largest size appearing adjacent the surface 20 and the finest bubbles 17 lying adjacent against the surface 22.

As an alternative to providing gas bubbles in a carrier material which is uniformly doped as shown in Figures 3 and 4 it is also possible to employ solid materials such as glass beads which possess low permittivity.

The production of radiation absorbing coatings according to the invention can be readily appreciated by reference to Figures 5 to 8.

Figure 5 shows schematically in the left hand drawing a bi-layer coating such as might be applied to a substrate. A lower layer 30 of a material of low permittivity contains carbon particles of similar size substantially uniformly distributed therethrough. An upper layer 31 is formed of the same carrier material but without any carbon particles dispersed therein.

The right hand drawing shows how a single body 32 of carrier material appears to exist as a result of the diffusion of carbon particles across the interface 33 present originally between layers 30 and 31. Figure 6 may be considered to show an intermediate position in the production of the final layer 3 1. On application of heat to the underside of layer 30, convection takes place. This mode of production of a coating embodying the invention will require the layers of 30 and 31 to be in a fluid state at the outset, generally being in the form of a plastisol.

Referring to Figure 7, where like reference numerals denote like parts in Figures 5 and 6, here it can be seen that vibration schematically denoted by the line 34 bearing oppositely directed arrows causes migration of particles to take place from layer 30 to layer 31 over interface 33.

Next Figure 8 shows in the left hand drawing two relatively thin layers of plastisol 35 and 36 respectively. Both layers have a uniform concentration of dielectric particles such as carbon particles but the lower layer 35 also contains blowing agents. On heating the layers 35 and 36, initially the blowing agent causes expansion of the lower layer 35 as foaming takes place and this foaming spreads into the upper layer 36 as migration of blowing agent and of gas bubbles which have been produced takes place. The gas bubbles produced and the carbon particles become locked in place as subsequent heating leads to curving of the plastics material. A single layer coating 37 as show in the right hand drawing of Figure 8 then results. Gas bubbles exist at all depths in the coating but with the concentration increas ing through the depth of the coating from the external surface 38 thereof.

Finally, referring to Figure 9, there is shown in three drawings a so-called "hairbrush dipping" method for producing a coating embodying the invention. Initially, there is present a bi-layer coating of the type shown in Figure 5 with like reference numerals denoting like parts in Figures 5 and 9. While the layers are not completely cured, a "hairbrush" type tool 40 is pushed into the layers so as to extend to a lower region of the lower layer containing the carbon particles. When, as shown in the lowermost drawing, the "hairbrush" type tool will have been withdrawn, it will have drawn with it into the upper layer quantities of the carbon particle-containing layer. The curing which is taking place will set the drawn up lower layer material within the body of upper layer material as an array of peaks 41 of exponentially decreasing cross-section thereby producing the exponentially decreasing concentration of carbon particles in carrier material of the upper part of the coating.

Claims (39)

1. A substrate body which is provided with a covering for reducing the reflection from the body of energy incident thereon, which covering is comprised by a coating applied to the substrate body of a coating composition which undergoes a gradual decrease in impedance throughout its depth from the surface thereof remote from the substrate body.

2. A substrate body as claimed in claim 1, which is capable of travel from location to location.

3. A substrate body as claimed in claim 1 or 2, wherein the coating is capable of substantially completely absorbing radio waves incident thereon.

4. A substrate body as claimed in any preceding claim, wherein the coating comprises a low permittivity dielectric carrier medium within which low impedance doping is present.

5. A substrate body as claimed in claim 4, wherein said carrier-medium is selected from thermoset and thermoplastic materials.

6. A substrate body as claimed in claim 5, wherein the carrier medium is selected from polyethylene, polypropylene and polyurethane plastics.

7. A substrate body as claimed in any one of claims 4 to 6, wherein the doping comprises particles or fibres of electrically resistive material.

8. A substrate body as claimed in any one of claims 4 to 7, wherein the doping consists of carbon or metal in finely particulate form or of a combination of fine carbon particles and fine metal particles.

9. A substrate body as claimed in any one of claims 4 to 8, wherein the doping of the carrier medium increases in concentration from said surface.

10. A substrate body as claimed in claim 9, wherein the extent of doping increases exponentially through the coating.

11. A substrate as claimed in claim 10, wherein particles or fibres of like size dope the coating, the particles increasing in number per unit cross-sectional area of the coating as the depth in the coating increases.

12. A substrate body as claimed in claim 10, wherein the particles or fibres are present in substantially equal numbers per unit crosssectional area of the coating at all depths therein, the particles or fibres increasing in size through the depth of the coating.

13. A substrate body as claimed in any one of claims 4 to 8, wherein the carrier medium is uniformly doped throughout and has therein bodies of low permittivity for achieving an increase in permittivity from said surface through the coating.

14. A substrate body as claimed in claim 13, wherein the coating shows an exponential increase in permittivity through the depth thereof.

15. A substrate body as claimed in claim 13 or 14, wherein the bodies are of like size and decrease in number per unit cross-sectional area of the coating as the depth in the coating increases.

16. A substrate body as claimed in claim 13 or 14, wherein the bodies are present in substantially equal numbers per unit cross-sectional area of the coating at all depths therein, the bodies decreasing in size through the depth of the coating.

17. A substrate body as claimed in any one of claims 13 to 16, wherein said bodies are spherical.

18. A substrate body as claimed in any one of claims 13 to 17, wherein said bodies are solid.

19. A substrate body as claimed in claim 17, wherein said bodies are gas bubbles.

20. A substrate body which is provided with a covering for reducing the reflection from the body of energy incident therein, which covering is constituted by a coating substantially as hereinbefore described with reference to, and as shown in, any one of Figures 1 to 4 of the accompanying drawings

21. A method of protecting a substrate body from detection which comprises reducing the reflection from the body of energy incident thereon by providing on said surface a coating composition which undergoes a gradual decrease in impedance throughout its depth from the surface thereof remote from the substrate body.

22. A method as claimed in claim 21 which is carried out so that said coating composition shows an exponential decrease in impedance throughout its depth.

23. A method as claimed in claim 21 or 22, wherein a plastics composition comprising particles of doping material of non-uniform distribution is applied to said surface.

24. A method as claimed in claim 23, wherein a two or more layer composition is applied to said surface, the layers containing said particles in different concentrations and/or sizes and said particles being caused to migrate within and/or between the layers while the plastics composition is in an unhardened condition to achieve said gradual decrease in impedance through the depth of the coating.

25. A method as claimed in claim 24, wherein said particles are of differing size and are caused to migrate for there to be present substantially equal numbers per unit cross-sectional area at all depths therein, with an increase in size of particles occurring through the depth of the coating.

26. A method as claimed in claim 24, wherein said particles are of like size and are caused to migrate for there to be present increasing numbers thereof per unit cross-sectional area at increasing depths of the coating from said surface thereof.

27. A method as claimed in any one of claims 24 to 26, wherein the plastics composition comprises a thermosetting plastics matrix whose curing on said surface is not completed until said doping material has undergone said migration therethrough.

28. A method as claimed in any one of claims 24 to 26, wherein the plastics composition comprises a thermoplastic plastics matrix and migration of said particles takes place while the matrix is in a molten condition sufficient to permit diffusion of the particles.

29. A method as claimed in claim 26 or 27, wherein said migration takes place by diffusion under gravity, applied mechanical force or, the particles being electrically conductive or ferromagnetic, by application of an electrical field or a magnetic field respectively.

30. A method as claimed in claim 28, wherein said migration takes place by thermal diffusion of the particles.

31. A method as claimed in claim 23, wherein said composition comprises said particles of doping material uniformly distributed therethrough and, in addition, comprises bodies of low permittivity for achieving an increase in pemittivity through the coating from said surface.

32. A method as claimed in claim 31, wherein a two or more layer composition is applied to said surface, the layers containing solid or gaseous bodies in different concentration and/or size and said bodies being caused to migrate prior to hardening of the plastics material of said composition to achieve said gradual increase in permittivity.

33. A method as claimed in claim 32, wherein said bodies are of differing size and are caused to migrate for there to be present equal numbers per unit cross-sectional area at different depths therein, with an increase in size of body occurring through the depth of the coating from said surface.

34. A method as claimed in claim 31, wherein said bodies are of like size and are caused to migrate for there to be present different numbers per unit cross-sectional area at different depths in the coating from said surface.

35. A method as claimed in claim 32, 33 or 34, wherein gaseous bodies are produced in situ in the composition by decomposition of a blowing agent therein.

36. A method as claimed in any one of claims 21 to 35, wherein said coating is applied by brush, roller, spray or pouring.

37. A method as claimed in any one of claims 21 to 36, wherein the total coating thickness is at least one quarter of the wavelength of the energy whose reflection is to be reduced.

38. A method for providing on a surface of a substrate body to be protected from detection, a covering for reducing the reflection from the body of energy incident thereon, which method is substantially as hereinbefore described with reference to Figures 5 to 9 of the accompanying drawings.

39. A sustrate which has been provided with a covering for reducing the reflection from the body of energy incident thereon by the method claimed in any one of claims 21 to 37.