GB2295665A

GB2295665A - Infra-red body

Info

Publication number: GB2295665A
Application number: GB9424005A
Authority: GB
Inventors: Donald James Highgate
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1994-11-29
Filing date: 1994-11-29
Publication date: 1996-06-05
Also published as: GB9424005D0

Abstract

A flexible IR camouflage garnish comprising a hydrophilic material such as cross-linked methyl methacrylate N-vinyl2-pyrollidine (MMVP) polymer attached to a support material which provides camouflage in the visual region. In use the garnish is applied to the surface of a body and water is transferred eg from a central spray reservoir to the hydrophilic material of the garnish whereby the water evaporates therefrom cooling the surface of the body and thus reducing the IR emissions from the body.

Description

INFRA-RED BODY The present invention relates to methods of reducing the IR emissions from a body.

As is well known, bodies emit thermal radiation as a result of their own surface temperature. However this radiation is not emitted uniformly across all wavelengths, and the variation in its intensity at a given temperature with the wavelength being detected is described by Planck's curve. For most materials at temperatures between 25 and 1000"C, the wavelengths emitted lie within the IR region between about 1 and 20 pm.

Many modern reconnaissance and weapons systems employ radiation detectors which operate within the IR region (principally between 0.8 to 100 m) in order to aid target acquisition and weapon guidance.

Such detectors, for instance thermopiles or photographic films, may operate by passively receiving thermal radiation emitted by a body which is at a different temperature to its environment.

Alternatively they may passively receive radiation reflected from the surface of the body from other, possibly distant, sources eg. the sky or sun.

It is therefore desirable to provide potential targets with thermal 'IR camouflage' which confuses such detectors by matching the IR image of the body to that of its immediate environment notwithstanding the fact that the bulk of the body is hotter than the ambient temperature. Such camouflage can also be used to modify the IR 'signature' of a body such that its thermal image resembles a less valuable asset.

One type of existing IR camouflage exploits the fact that the thermal radiation emanating from a body is dependent not only on its surface temperature, but also on a number of other factors including the nature of the material forming the surface of the body. For instance thermal radiation may be reflected, transmitted or adsorbed (and re-emitted) by the body's surface depending on the molecular characteristics of the material from which it is composed. These characteristics (reflectance, transmitance, absorbance and emissivity) will have a profound affect on the intensity of the radiation emanating from the material at any given wavelength and temperature.Thus coating a potential target in a material having a low IR emissivity can confuse passive detectors because the target will emit relatively little energy irrespective of the fact that its actual surface temperature is significantly above the local ambient level.

Unfortunately such coatings can only reduce, not eradicate, IR emissions. Thus the camouflage of a body coated in these materials can be undermined when there is an unusually large temperature differential between the body and its surroundings.

Such camouflages will also be greatly undermined by the presence of a surface layer of dirt, which will increase the effective emissivity of the coating.

Additionally, low emissivity materials will also have a high reflectance because these properties are inversely related to each other. Thus the effectiveness of the camouflage provided by such materials is to some degree undermined by reflection of solar radiation (corresponding to a high temperature) and radiation from other parts of the sky (corresponding to a low temperature) which both lead to increased detectability.

Currently, visual camouflage is often deployed in the form of a flexible garnish which can be used with a variety of potential targets. Such garnishes may be continuous sheets or comprise discrete camouflage elements on a 'net' framework.

It would therefore be desirable to have an effective IR camouflage which could be deployed in a similar fashion to, and preferably in conjunction with, existing visual camouflage measures. Ideally such IR camouflage would be cheap to manufacture in bulk, light in weight, easily deployed and require little logistic support to carry out its function. Such characteristics limit the applicability of some sophisticated IR-technology such as variable emissivity materials.

The present invention has now provided methods and apparatus for reducing the IR emissions from a body that address some, and in preferred forms all, of these problems.

According to one aspect of the present invention there is provided a method of camouflaging a body comprising applying a hydrophilic material to the surface of the body and transferring water from a water source to the material whereby the water evaporates therefrom cooling the surface of the body and thus reducing the IR emissions from the body.

Thus the evaporative cooling of a hydrophilic material applied to the exposed surface of a body is utilised by the apparatus to provide IR camouflage for, or modify the IR signature of, that body. It is not necessary to cool the bulk of the body - only those surfaces which may be detected by an IR sensor.

The body can be mobile (eg. a vehicle), static (eg. a building) or itself be an element in a camouflage system.

The effectiveness of the IR camouflaging properties of the apparatus will be dependent on the temperature and relative humidity of its surroundings, and will be greatest when the body is at a significantly higher temperature than its environment. It should be noted, however, that the transfer of water from the source to the material could be carried out even when the body is colder than its surroundings.

It should be noted that the term 'hydrophilic material' is intended to embrace all substantially insoluble materials which reversibly bind water.

Preferably the hydrophilic material is a hydrogel polymer; these materials can store water and provide surface evaporative cooling over long periods of time and are well known to those who are skilled in the art. Suitable examples are disclosed by Cordrey et a1. in UK Patent 1439132. This patent discloses methods for conveniently producing copolymers (eg. comprising methyl methacrylate and N-vinyl-2-pyrollidine) which are capable of absorbing upto 10 times their dry weight of water.

UK 1439132 further discloses how the properties of such polymers can be readily altered by adjustment of the ratio of constuents used, or by changing the polymerisation conditions eg. by including a cross-linking agent such as allyl methacrylate. Thus qualities such as biocompatibility, water uptake, swelling, transparency, mechanical strength and elasticity can all be finely tuned according to the precise requirements of the application.

The hydraulic properties of such hydrogel polymers can be further altered by use of the methods disclosed by Highgate in UK Patent 1463301 to form interpenetrated network polymers.

Other types of suitable hydrogel polymer (using amino- and amidoentities) are disclosed by Frankland in UK Patent 1467179.

The hydrophilic material may be permanently or temporarily attached to the surface of the body. Where the hydrophilic material or body is thermoplastic, the material may be hot-rolled onto (and into) the surface of the body.

Ideally the hydrophilic material is present as a continuum over the entire area of the body to be camouflaged, hence maximising the cooling effect. Unfortunately, large continuous layers of hydrophilic material may present problems such as curling if, for example, the material swells when absorbing water. Such problem may be avoided by using non-swelling polymers or by using the material in discontinuous form. Discontinuity may be desirable for compatability with the nature of the body itself (eg. where the body is a flexible visual camouflage garnishes based around netting). It should be noted that the camouflaging effect will extend beyond those zones of the body which are being cooled directly owing to passive heat transfer (eg. by conduction) from the surrounding, warmer, zones.

In preferred embodiments of the method, the hydrophilic material is applied to, or present within, a supporting layer to create a camouflage element which is itself applied to the body to be camouflaged.

In another aspect of the same inventive concept there is provided an element for camouflaging a body to which it is applied comprising a hydrophilic material and a support material for supporting the hydrophilic material such that in use the element is applied to the surface of the body and water is transferred from a water source to the hydrophilic material of the element whereby the water evaporates therefrom cooling the surface of the body and thus reducing the IR emissions from the body.

Preferably the element is flexible and fairly robust, thereby allowing it to be simply and easily utilised and transferred between bodies having different shapes (eg. by using elements in sheet form). Suitable support matrices include polyethylene and polypropylene.

The hydrophilic material may be applied, and preferably laminated, directly to the surface of the supporting material.

Alternatively the hydrophilic material may be present as a dispersion within the support layer. In such embodiments there should be a sufficiently high density of hydrophilic material in the support layer to ensure that that water can be transported between the hydrophilic domains in the matrix and to the surface where it can be lost to the atmosphere. A concentration of 50% or greater (by volume) may therefore be required. It should be noted that the density of the hydrophilic material will also affect the properties (eg. flexibility, adhesiveness, permeability etc.) of the support material in which it is dispersed.

In elements where there is not a contact continuum of hydrophilic domains in the matrix, or where the supporting material or body forms a barrier between the hydrophilic material and the atmosphere, then the support layer must be sufficiently permeable to allow the loss of water vapour from the hydrophilic material. Such configurations may be necessary when the hydrophilic material has undesirable IR characteristics, for instance a high emissivity, which should be shielded from external detectors.

The invention further makes available a body utilising IR camouflage elements as described above.

The invention also makes available a flexible camouflage garnish comprising one or more IR camouflage elements as described above and further comprising camouflage material effective in the visual region. In most applications it will be important that the characteristics of the hydrophilic material and support layer should not compromise the visual camouflage and vice versa.

The visual camouflage material may itself provide a supporting material for the hydrophilic material. Thus visual/IR camouflage garnishes could be produced from existing visible camouflage netting by coating one or both faces of the visual camouflage in the netting with hydrophilic materials.

In a further embodiment the IR camouflage element may incorporate its own support material and may be applied to the visual camouflage in the form of a plastic matrix eg. a mixture of hydrophilic material in polyethylene.

In a yet further embodiment, hydrophilic material may be incorporated along with visible camouflage material into a (preferably extrudible) plastic support material during manufacture, so as to produce visible/IR camouflage elements in a single manufacturing step.

The invention further embraces a body employing such camouflage garnish.

As described above, the subject matter of the current invention requires a water source, in order to provide evaporative cooling.

Preferably the water source is carried by the body itself. Where this creates logistic problems the water required can be taken from external sources such as ground water supplies, for instance in applications where the apparatus is being used with static installations or where the hydrophilic material is being 'charged' at intervals. The external sources could also be atmospheric precipitates such as rain or dew, these being utilisable by both dynamic and static bodies. It should be emphasised that there is no necessity for the water used by the apparatus to be pure or clean, which therefore increases the number of potential sources available.

The transfer of water from the source to the hydrophilic material could be either passive or active, and could be either constant or involve intermittent 'charging' of the material. Hydrogel polymers, for instance, have a high capacity to adsorb and retain water, and can therefore function as a reservoir when evaporative cooling is not occuring. In order to reduce any lag in the effectiveness of the apparatus, the hydrophilic material or camouflage element can be 'charged' at the time of manufacture (providing that the manufacturing and storage processes are adapted accordingly). For instance a wet material may attract fungal growth over a period of time, and may therefore need to incorporate anti-fungal measures during storage.

Preferably the transfer rate and distribution of water is controllable to take account of the prevailing conditions, for instance temperature, humidity and availability of water supplies.

In preferred forms the apparatus further comprises an environmental sensor, eg. a thermocouple, IR detector, or hygrometer, and the water transfer rate or distribution is controlled in accordance with data received from the environmental sensor.

One mechanism for transferring water to the material is to use a network of hollow elements extending through the apparatus from the source to the hydrophilic material. Such a system is analogous to the transpiration stream in plants. The use of a strongly hydrophilic materials encourages the passive flow of water from the source to the material as water is lost to the atmosphere. The passive transfer of water can be enhanced by utilising capillary action (through the use of micro-capillary fibres) or gravity (by having an elevated water source). Additionally, active transfer methods can be used, for instance peristaltic pumps.

Transfer of water from the source to the surface of the hydrophilic material can also be by means of an aerosol spray. Where such a mechanism is employed with embodiments of the invention wherein the hydrophilic material is is held under or within a supporting material, then the supporting material must allow entry of liquid-water (in addition to the loss of water vapour).

Preferably the water is evenly distributed from the source over the entire hydrophilic material. However, in embodiments employing continuous hydrophilic materials eg. surface layers or random contact continuum matrices, the water can equilibrate within the hydrophilic material following its distribution thereto. This will enhance the uniformity of the water distribution irrespective of the orientation of the apparatus.

The method and apparatus of the present invention will now be described, by way of illustration only, by reference to the following examples. Other embodiments falling within the scope of the invention will occur to those skilled in the art in the light of these.

ExamDle 1 - A flexible IR camouflage garnish Cross-linked methyl methacrylate N-vinyl2-pyrollidine (MMVP) polymer was prepared and ground in a hammer mill with the discriminator grating set to pass 100 um particles. A thin sprinkling of the dry MMVP powder (which was translucent) was attached using a thin layer of a suitable plastics adhesive (Evostick) to both sides of the fabric camouflage elements of conventional visible camouflage garnish. The elements were irrigated from a central spray reservoir The garnish was visualised using an IR detector which indicated depressed IR emissions from the camouflage elements.

Example 2 - A flexible IR camouflage garnish Cross-linked MMVP was ground in a hammer mill with the discriminator grating set to pass 100 um particles. This was mixed with polyethene in a Z blade for 1 hour. The ratio of MMPV:polythene was 2:1 by weight. The resulting material was extruded using a 24'' single screw extruder equipped with a heated slit die and was air cooled and pressed to form a flat 1 mm thick sheet. The material was hot rolled at 90"C on to both sides of the fabric camouflage elements of conventional visible camouflage garnish. The elements were irrigated from a central spray reservoir The garnish was visualised using an IR detector which indicated depressed IR emissions from the camouflage elements.

Example R - A flexible IR camouflage garnlsh Cross-linked MMVP was ground in a hammer mill with the discriminator grating set to pass 100 urn particles. A thin sprinkling of the dry MMVP powder (which was translucent) was attached using a thin layer of a suitable plastics adhesive (Evostick) to one side of a perforated fabric sheet coloured on the other side so as to provide visible camouflage. The MMVP side was irrigated and the sheet was draped (MMVP side innermost) over a vehicle.

The vehicle was visualised using an IR detector which indicated depressed IR emissions.

Claims

Claims

1. A method of camouflaging a body comprising applying a hydrophilic material to the surface of the body and transferring water from a water source to the material whereby the water evaporates therefrom cooling the surface of the body and thus reducing the IR emissions from the body.

2. A method as claimed in claim 1 wherein the hydrophilic material is a hydrogel polymer.

3. A method as claimed in claim 2 wherein the hydrogel polymer is cross-linked.

4. A method as claimed in any one of the preceeding claims wherein the hydrophilic material is present on the surface of the body as a continuum.

5. A method as claimed in any one of claims 1 to 3 wherein the hydrophilic material is discontinuous.

6. A method as claimed in any one of the preceding claims wherein the hydrophilic material is associated with a support material which is applied to the surface of the body.

7. An element for camouflaging a body to which it is applied comprising a hydrophilic material and a support material for supporting the hydrophilic material such that in use the element is applied to the surface of the body and water is transferred from a water source to the hydrophilic material of the element whereby the water evaporates therefrom cooling the surface of the body and thus reducing the IR emissions from the body.

8. An element as claimed in claim 7 wherein the hydrophilic material is a hydrogel polymer.

9. An element as claimed in claim 8 wherein the hydrogel polymer is cross-linked.

10. An element as claimed in any one of claims 7 to 9 wherein the hydrophilic material is present in or on the support material as a continuum.

12. An element as claimed in any one of claims 7 to 9 wherein the hydrophilic material is discontinuous.

13. An element as claimed in any one of claims 7 to 12 wherein the support material is flexible.

14. An element as claimed in any one of claims 7 to 13 wherein the hydrophilic material is present on a surface of the support material.

15. An element as claimed in any one of claims 7 to 13 wherein the hydrophilic material is present within the support material.

16. An element as claimed in claims 15 wherein the ratio of the volume of hydrophilic material to support material is greater than 1 to 1.

17. An element as claimed in any one of claims 7 to 16 wherein the support layer is permeable to water vapour.

18. An element as claimed in any one of claims 7 to 17 wherein the support layer provides camouflage in the visual region.

19. An element as claimed in any one of claims 7 to 17 furthur comprising camouflage material which is active in the visual region.

20. A method or element as claimed in any one of the preceeding claims wherein the water source is carried by or associated with the body.

21. A method or element as claimed in any one of the preceeding claims wherein the water is transferred to the hydrophilic material intermittently.

22. A method or element as claimed in any one of the preceeding claims wherein the water is transferred to the hydrophilic material continuously.

23. A method or element as claimed in any one of the preceeding claims wherein transfer of water to the hydrophilic material is controllable in accordance with data received from an environmental sensor.

24. A method or element as claimed in any one of the preceeding claims wherein the water is transferred to the hydrophilic material by means of hollow elements.

25. A method or element as claimed in any one of the preceeding claims wherein the water is transferred to the hydrophilic material by means of a spray.

26. A body employing an element as claimed in one of claims 7 to 25.

27. A flexible IR camouflage garnish comprising an element as claimed in any one of claims 7 to 25.

28. A body employing a flexible IR camouflage garnish as claimed in claim 27.