Camouflage paint

Abstract

There is disclosed a paint (22) comprising an additive (24) of hydrated salt, such as a Magnesium Perchlorate based hydrated salt. This paint (22) provides improved matching between the reflectance profile of the paint and that of natural vegetation.

Description

[0001] Camouflage Paint

[0002] Field The present invention is concerned with the field of paints, specifically camouflage paints to mimic vegetation.

[0003] Background Articles can be hidden or concealed by applying camouflage paint to their surfaces. For example, one of the most popular types of camouflage uses blotches of coloured paints including green paint, brown paint and gray paint to mimic vegetation in the visible part of the EM spectrum. Enhancements in sensor technology has moved from sensors capable of simply detecting features in the visible (300nm- 700nm) and near-infrared (NIR) (700nm - 1000nm) bands, to multispectral and hyperspectral sensors that are capable of observing fine (5nm) reflectance features and using contrast within each of these fine bands to differentiate articles from the background. Modern sensors are also sensitive to longer wavelengths e.g. SWIR (1000nm-3000nm).

[0004] Figure 1 is a graph illustrating reflectance profiles for average vegetation and conventional green camouflage paint. As can be seen, inherent reflectance characteristics of vegetation includes a steep increase in reflectance at approximately 0.7 microns (700nm), a phenomenon known in the art as the “red edge”, water absorbance bands (i.e. drops in reflectance values) seen at 0.97 microns (970nm), 1.2 microns (1200nm), 1.45 microns (1450nm) and 1.9 microns (1900nm), as well as the general reduction in reflectance from 1.3 - 2.5 microns (1300nm - 2500nm) ignoring the absorbance bands. The reflectance profile of conventional green camouflage paint only matches the red edge and relatively high reflectance values in the NIR range. The reflectance profile does not include more granular characteristics present in that of vegetation. Traditional green camouflage paint is therefore exposed under multispectral and hyperspectral sensors. Accordingly, there is a need for a paint having a more granular spectral profile that better matches vegetation across an extended spectral range.

[0005] Summary

[0006] According to an aspect of the present invention, there is provided a paint comprising an additive of hydrated salt.

[0007] The Applicant has recognised that the inclusion of hydrated salt(s) as an additive can be used advantageously to provide a camouflage paint that mimics reflectance characteristics that are inherent to vegetation across a wide spectral range. Specifically, the paint of the present invention is able to mimic water absorption bands present in reflectance profiles of vegetation. This is particularly advantageous to combat the advancement of multispectral or hyperspectral sensors, which analyse reflectance profiles to characterise surfaces that lack water absorption bands as manmade or otherwise anomalous within a vegetation containing scene.

[0008] The advantages of the present invention, i.e. improved reflectance matching between the paint and vegetation, can be realized with the presence of the hydrated salt additive in the paint in any quantity or loading, such that the invention is not limited to specific values. However, in preferred embodiments, the paint may comprise at least five, e.g. at least ten, percent by weight or volume of hydrated salt (expressed as a percentage of the total weight or volume of the paint). While not essential, this minimum quantity of hydrated salt additive has been found to be particularly advantageous in that it yields more clearly identifiable water absorption bands in the reflectance profile.

[0009] The paint can take two forms: a wet form in which it can be easily applied to a substrate or surface of an article; and a dry form whereby the paint has been air dried to the substrate or surface of an article to leave a thin coating. The paint in dry form is substantially the same as that in its wet form, except that solvents present in the wet paint will have evaporated from the paint during the drying process to form the dry paint. The wet or dry paint may comprise up to fifty percent by weight or by volume of hydrated salt (expressed as a percentage of the total weight or volume of the paint formulation). The percentage by weight or volume of the hydrated salt may be measured and confirmed by one skilled in the art using any known, conventional method or standard for doing so. The (wet or dry) paint may comprise hydrated salt(s) in an amount falling within a loading range of 5-50 10-50, 15-50, 20-50, 25- 50, 30-50, 35-50, 40-50, or 45-50 percent by total weight or total volume of the paint. The paint (wet or dry) may comprise hydrated salt in an amount falling within a loading range of 5-45, e.g. 30-45, percent by total weight or total volume of the paint. While not essential, each one of the above ranges may be particularly advantageous in that it yields clearly identifiable water absorption bands in the reflectance profile, while providing a high quality and stable paint formulation for most applications. For example, by keeping the percentage by weight or volume of the hydrated salt additive to fifty percent or less, the paint formulation may comprise binder material in sufficient quantities (relative to the additive) to increase the resilience and adhesive properties of the paint. However, the upper limit of 50 percent is not essential.

[0010] The hydrated salt may be an alkali salt, i.e. the soluble hydroxide compound of an alkali metal or an alkaline earth metal, or a transition metal salt. The hydrated salt may be a Magnesium Perchlorate based hydrated salt. The hydrated salt may be Magnesium Perchlorate Hexahydrate. These types of hydrated salts may be advantageous in that they are stable at temperatures up to one-hundred-and-fifty degrees Celsius. In embodiments, the paint comprises a single type of hydrated salt, e.g. only Magnesium Perchlorate. This may improve the consistency and thus quality of the paint. Furthermore, the additive of hydrated salt may be in its raw form. That is, in such embodiments, the raw hydrated salt is not treated with a chemical or other material before application in the paint. The paint may further comprise a binder material. The binder material may be a polyurethane based material. One suitable example is the commercially available off- the-shelf (COTS) solvent borne two-pack polyurethane (2KPLI) binder, which is supplied by PPG Industries Inc. in two parts (Part A and Part B) to be mixed to form a single binder material. Alternatively, the binder may be 100% solids paint binder. Additionally or alternatively, the binder may be an epoxy-based binder material. According to another aspect of the present invention, there is provided an object having a surface region which is coated with the paint of any preceding statement.

[0011] The object may have a first surface region which is coated with a first paint in accordance with the paint of the present invention described above, and a second surface region which is coated with a second paint (which may or may not be a paint in accordance with the present invention). The first paint and the second paint may form a camouflage pattern on the object. They may have different colours, for example.

[0012] The surface region may be that of fabric, e.g. cotton.

[0013] The paint may be used in combination with a primer to form a multi-layered paint system. For example, the surface region of an object may be coated with a first layer of primer, e.g. which is in direct contact with the surface region, and a second layer of the paint, which is applied to (e.g. in direct contact with) the primer when dried. The primer may be a polyurethane based material, e.g. 2KPLI.

[0014] The primer may comprise an additive of Titanium Dioxide (e.g. in Rutile form) or Silica (and optionally colorants), irrespective of whether polyurethane is used. In this way, the present invention may ensure that the reflectance profile for radiation interacting with the paint and primer layers in combination includes a general reduction in reflectance with increasing wavelength, e.g. from 1.3 - 2.5 microns (1300nm - 2500nm). This is in addition to the water absorption bands (which is a result of the hydrated salt additive).

[0015] The paint system may further comprise an outer protective coating. The coating may comprise polyurethane, e.g. 2KPLI, and is applied to the second layer of the dried paint.

[0016] The Applicant considers the primer to be novel and inventive in its own right. Thus, according to an aspect of the present invention, there is provided a primer for a camouflage paint system, the primer comprising (e.g. polyurethane and) an additive of (e.g. Rutile) Titanium Dioxide or Silica. Optionally, the primer further comprises one or more colorants or pigments. The primer may have any one or more of the features of the primer described herein with respect to the overall paint system.

[0017] According to another aspect of the present invention, there is provided a method of producing a camouflaged object, comprising coating a surface region of an object with a layer of the paint referred to in any one of the preceding statements.

[0018] The method may further comprise applying a first layer of primer, e.g. directly, to the surface region. The step of coating the surface region with the layer of the paint may comprise applying the paint, e.g. directly, to the first layer of dried primer. The layer of paint may be exposed on the exterior of the object. However, in further embodiments, an outer protective coating may be applied to the layer of paint, such that the layer of paint is intermediate between the primer layer and the outer protective coating layer. The outer layer may be the outermost layer of the paint system, i.e. that which is exposed to the ambient environment

[0019] The surface region may be a fabric, e.g. cotton, surface region. In other words, the paint may be applied directly to a fabric, e.g. cotton, surface region. Brief Description of the Drawings

[0020] Embodiments of the invention will now be described by way of non-limiting example with reference to the remaining drawings, in which: Figure 2 is a schematic drawing illustrating a scene in which the invention may be used;

[0021] Figure 3 is a schematic diagram illustrating a paint system in accordance with an embodiment of the present invention, when applied to the surface of an object;

[0022] Figure 4 is a graph illustrating the reflectance profile of the multi-layered paint system including the paint and primer in accordance with an embodiment of the present invention, as well as that of vegetation; and Figure 5 is a block diagram schematically illustrating a method of producing a camouflaged object in accordance with an embodiment of the present invention.

[0023] Like reference numerals will be used throughout the detailed description to denote like features of the invention.

[0024] Detailed Description

[0025] Figure 2 is a schematic drawing illustrating a typical scene 10 in which the camouflage paint system of the present invention is to be applied.

[0026] The scene 10 is of a landscape comprising natural and man-made objects. In the current example, the scene 10 comprises a man-made object in the form of a vehicle 12 which forms part of the foreground of the scene 10, and natural vegetation 14 which forms the background. The vegetation 14 may be part of a woodland which typically comprises green vegetation such as foliage, grass, etc. The vehicle 12 is coated wholly or in parts with a dried, camouflage paint system 16, which forms a large pattern that mimics the background vegetation 14 to disrupt the outline of the vehicle 12 to the observer.

[0027] Figure 3 schematically illustrates a cross-sectional view of the paint system 16 in accordance with an embodiment of the present invention. In contrast to conventional paints, the paint system 16 of the present invention is configured to match characteristics of the average reflectance profile of vegetation across not only the visible and NIR bands, but also the SWIR spectral band.

[0028] As can be seen in Figure 3, a region on an exterior surface 18 of the vehicle 12 is coated with a first layer comprising a primer 20 and a second layer comprising the paint 22. With reference to the method of Figure 5, the primer 20 is applied (at block 50 of Figure 5) directly to the surface 18 of the vehicle 12 and the paint 22 is then applied (at block 52) directly to the outwardly facing surface of the primer 20. The method may comprise the step of waiting for the first layer of primer 20 to dry (or be cured) before the second layer of paint 22 is applied thereto. The layer of paint 22 forms the outermost layer and is exposed and visible on the exterior of the vehicle 12. The primer 20 forms an intermediate layer between the exterior surface 18 of the vehicle 12 and the layer of paint 22.

[0029] The primer 20 is a novel formulation of polyurethane and an additive of (e.g. Rutile) Titanium Dioxide or Silica (and optionally colorant pigments). In this example embodiment, the primer 20 comprises two-pack polyurethane (2KPLI) and the Titanium Dioxide or Silica (and any colorant pigments) may have been added to the polyurethane in its wet form before application to the surface 18 and then drying. Its application as an intermediate layer between the exterior surface 18 of the vehicle 12 and the second layer of paint 22 improves adhesion and may reduce corrosion. The primer 20 may be a grey colour, and is fully opaque in the near infrared (0.7-1 microns) and SWIR (1-3 microns) wavelength bands. It is noted here that by blocking the NIR and SWIR wavelength with the primer 20, it is ensured that the overall reflectance profile from the multilayer paint system will not be determined by the reflectance of the underlying surface 18 of the vehicle 12.

[0030] By adding (e.g. Rutile) Titanium Dioxide or Silica (and optionally colorant pigments), the primer and thus paint system exhibits a general reduction in reflectance from 0.7 micron (700nm) to 2.5 microns (2500nm) ignoring the absorption bands.

[0031] Further, the paint 22 comprises an additive 24 of hydrated salt(s). This may be advantageous in that it gives the paint 22 reflectance characteristics that are inherent to vegetation, specifically absorption bands that are commonly observable in the reflectance profiles of vegetation. The paint 22 may have a formulation or composition that further comprises, but is not limited to: visible colour pigments, and a binder material. The paint (in wet form before application to the object) may further comprise one or more solvents.

[0032] Hydrated salts can occur naturally, such as in freshwater, or may be manufactured. A hydrated salt is a crystalline salt molecule that is attached to one or more water molecules. This is in contrast to an anhydrate, i.e. a salt molecule that is not bound to any water molecules. In a hydrated salt, the water molecules are incorporated into the crystalline structure of the salt. By incorporating hydrated salt, e.g. together with the colour pigment(s) and binder, the present invention provides a paint that has reflectance characteristics that resemble those of water, which is inherently present in vegetation. The reflectance spectra of the paint therefore more closely matches the average reflectance profile of vegetation. This minimises contrast between the paint and vegetation, when imaged (or otherwise viewed) using multispectral and or hyperspectral sensors.

[0033] A paint comprising an additive 24 of hydrated salt is advantageous over hypothetical arrangements in which other types of water-containing additives are used, such as paints comprising dry water (i.e. fine water droplets coated with nanoparticles to render each droplet hydrophobic), and zeolite structures (macro inorganic lattices, with regular and repeating pores throughout the structures). Dry water and zeolite structures suffer from weak and easily reversible bonding to water, such that water will be only temporarily held within these additive components and will be released to the paint formulation over time, likely forming bubbles that could lead to cracking. In contrast to this, hydrated salts have relatively strong bonding between the salt molecule and the water molecules. Accordingly, the use of hydrated salts (particularly alkali or transition metal salts) as a paint additive may provide reflectance matching while improving the quality, texture and integrity of the paint as compared to these hypothetical arrangements.

[0034] The hydrated salt additive 24 may be formed by milling a quantity of the hydrated salt in a solvent (one which the additive does not dissolve in) using a (e.g. motorised) pestle and mortar (although a ball mill or high shear mixer could be used instead) and may account for up to fifty percent by weight of the paint.

[0035] Conventional colour pigments may be used as the visible colour pigments, although in this embodiment organic colour pigments are used to allow NIR and SWIR transmission through the second layer to maximise the depth of film the light has to pass though, thereby increasing the effect of the hydrated salt in that layer. The colour pigments may vary from paint to paint, depending on the application and the visible colour to be represented. The binder may be any suitable material or agent that holds together the hydrated salt additive 24 and the colour pigment(s). In preferred embodiments, the binder is a COTS 2KPLI binder. One preferred example of a hydrated salt that is suitable for use as an additive 24 in the present invention is Magnesium Perchlorate hexahydrate, which may be purchased from Sigma-Aldrich, a subsidiary of Merck Group (KGaA). The chemical formula of this hydrated salt is Mg(CIO4)2'6H2O. Magnesium Perchlorate Hexahydrate is particularly suitable for use in a paint because it is relatively resistant to high temperatures, as compared to other types of hydrated salts. That is, compared to some other hydrated salts, Magnesium Perchlorate Hexahydrate has a relatively strong bond between the salt molecule and the water molecules, such that it is comparatively resistant to high temperatures. It has been found that Magnesium Perchlorate Hexahydrate (particularly the bonds between the salt molecule and the water molecules) is stable at temperatures up to one-hundred-and- fifty degrees Celsius. It is therefore particularly suitable for use as an additive 24 in paint in that the water molecules will not easily be released from their bond with the salt and be released into the paint formulation.

[0036] Further, some methods to synthesise hydrated salts produce large particles which are milled down to an appropriate particle size for use as a paint additive. This presents challenges as conventional milling methods, such as dry ball milling, generate heat which can drive off the bound water. Accordingly, where milling methods are to be used to prepare the hydrated salt for use as a paint additive, a high tolerance to temperatures (such as that provided by Magnesium Perchlorate Hexahydrate) is advantageous in that it is more easily milled and reduces material losses or contamination, thereby increasing the yield of suitable additive from the milling process.

[0037] Figure 4 is a graph illustrating the reflectance profile of the multi-layered paint system including the paint and primer, in accordance with an embodiment of the present invention, as compared to that of average vegetation. In the present embodiment, the primer (in wet form) comprises the following formulation:

[0038] 140 grams (70.56 percentage by weight) of a first part (Part A) of a two- part 2KPLI binder (known as “CV124” and supplied by PPG Industries, for example) • 50 grams (23.5 percentage by weight) of a second part (Part B) of a two- part 2KPU binder (CV124)

[0039] • 12.50 grams (5.88 percentage by weight) of Rutile Titanium Dioxide

[0040] • 0.1 grams (0.05 percentage by weight) of Carbon Black colorant pigment

[0041] An alternative wet primer formulation is:

[0042] • 30 grams (56.20 percentage by weight) of a first part (Part A) of a two-part 2KPU binder (CV124) • 7.5 grams (14.05 percentage by weight) of a second part (Part B) of a two- part 2KPU binder (CV124)

[0043] • 15 grams (28.10 percentage by weight) of Silica

[0044] • 0.45 grams (0.84 percentage by weight) of visible black pigment

[0045] • 0.36 grams (0.67 percentage by weight) of visible green or yellow pigment • 0.07 grams (0.13 percentage by weight) of visible red or violet pigment

[0046] The paint comprises the following formulation in wet form:

[0047] • 50 grams of a first part (Part A) of a two-part 2KPLI binder (CV124) • 0.45 grams of black pigment (commercially known as L0086)

[0048] • 0.36 grams of yellow pigment (K0961)

[0049] • 0.07 grams of red pigment (K3911)

[0050] • 50 grams of Magnesium Perchlorate Hexahydrate

[0051] • 15 grams of a second part (Part B) of the two-part 2KPLI binder (CV124)

[0052] Accordingly, in its wet form the paint formulation comprises 43.15% by weight of Magnesium Perchlorate Hexahydrate, 56.09% by weight of 2KPLI binder, and 0.76% by weight of colour pigments. In its dry form, the loading of the Magnesium Perchlorate Hexahydrate will be greater than 43.15% by weight, owing to the fact that solvents etc. in the binder will have evaporated from the paint during the drying process. This paint yields a green coloured paint with clearly identifiable absorption bands at the same or similar wavelengths as those of the absorption bands present in the vegetation reflectance profile. Specifically, as can be seen in Figure 4, the paint system of the present embodiment has a reflectance profile comprising reflectance characteristics that closely resembles that of average vegetation across the spectral range 0.3-2.5 microns (300 nm to 2500 nm). By virtue of the paint formulation, the reflectance profile of the paint system includes a steep increase in reflectance (from below 10 percent to above 50 percent) at approximately 0.7 microns (700nm), as well as drops in reflectance values in discrete wavelength bands centred at 0.97 microns (970nm), 1.2 microns (1200nm), 1.45 microns (1450nm) and 1.9 microns (1900nm). These drops in reflectance values occur at water absorption bands, which are the bands of spectral wavelengths at which the light (i.e. across 0.3-2.5 microns) impinging on the paint is absorbed by the water molecules contained within the hydrated salt. These characteristic drops in reflectance values are not present in conventional camouflage paint, which does not contain any water based pigments.

[0053] In addition to characteristic drops in reflectance at water absorption bands, there is a general reduction in reflectance from 0.7 micron (700nm) to 2.5 microns (2500nm) ignoring the absorption bands. This feature of the reflectance profile is also characteristic of green vegetation, but is largely attributable to the primer 20, because the paint 22 is transparent in the NIR band (0.7 - 1 microns) and above.

[0054] While the invention has been described above with respect to the paint being applied to a first layer of primer, the first layer of primer is not essential to the invention. It will be understood that the second layer of paint (i.e. the paint containing a hydrated salt additive) could be applied directly to a surface of an object to be camouflaged, without an intermediate primer layer. In those arrangements, the reflectance profile of the paint will still comprise drops in reflectance values in discrete wavelength bands centred at approximately 0.97 microns (970nm), 1.2 microns (1200nm), 1.45 microns (1450nm) and 1.9 microns (1900nm), corresponding to water absorption bands in vegetation, by virtue of the presence of a hydrated salt additive in the paint formulation. Such characteristic reflectance features may still be useful to conceal objects from sensors which are programmed to identify objects in a manner that is agnostic to the general reduction in reflectance from 1.3 microns (1300nm) to 2.5 microns (2500nm), e.g. based instead on whether or not the object being detected exhibits water absorption bands in its reflectance profile. Further still, it has been found that fabric materials such as cotton have reflectance profiles that exhibit a general reduction in reflectance with increasing wavelengths (e.g. from 1.3 microns (1300nm) to 2.5 microns (2500nm)). Accordingly, where a layer of the paint (i.e. that which contains a hydrated salt additive) is applied directly to a fabric surface, e.g. cotton, a primer layer is not required.

[0055] It will be appreciated here that the specific percentage by weight or by volume of visible colour pigments may differ from paint to paint and can be tailored to provide a specific colour or shade, as may be desired. In that regard, the present invention is not limited to green paint specifically, but is applicable more generally to paint of any colour but that seeks to mimic the reflectance properties of vegetation across a broad spectrum. In some embodiments the visible colour of the paint may be insignificant, in which case only the reflectance characteristics of the paint are important. In further embodiments, the paint may comprise zero visible coloured pigments. In yet further embodiments, the hydrated salt additive is added to a transparent (in the visible part of the EM spectrum) varnish, which is then applied to a layer of the primer. Furthermore, an object or article may be coated with two or more different paints in respective surface regions, where the paints together form a camouflage pattern on its surface. They may have different colours, for example. In such embodiments, one or more or all of the paints may comprise hydrated salt(s) in line with the present invention described above.

[0056] Further, while the embodiments described above relate to a paint having up to fifty percent by weight or volume of hydrated salt additive, higher or lower quantities (by percentage weight or volume) of hydrated salt may be used whilst still providing the desired reflectance profile. Indeed, specific reflectance characteristics of the paint such as the prominence of absorption bands in the reflectance profile, may be tailored by suitable modification of the percentage by weight or volume of the hydrated salt additive. The thickness of the second layer of paint may also be tailored to control the prominence of the absorption bands. For example, increasing the thickness of the second layer increases the number of hydrated salts and thus water molecules through which light passes, and in turn the extent of absorbance aligning with the water absorbance bands of natural vegetation at 0.97 microns, 1.2 microns, 1.45 microns and 1.9 microns.

[0057] Further still, it will be appreciated that the exact percentage by weight or volume of the primer components may differ from the values stated above. For example, the percentage by weight of (e.g. Rutile) Titanium Dioxide or Silica can be tailored to provide a specific reflectance profile, as may be desired. Furthermore, any crystal structure of Titanium Dioxide can be used in the primer, such that embodiments are not limited to Titanium Dioxide in Rutile form.

[0058] It will be appreciated that whilst various aspects and embodiments of the present invention have heretofore been described, the scope of the present invention is not limited to the embodiments set out herein and instead extends to encompass all methods and arrangements, and modifications and alterations thereto, which fall within the scope of the appended claims.

Claims

Claims1. A paint (22) comprising an additive (24) of hydrated salt.

2. The paint (22) of claim 1 , comprising at least five percent by weight or volume of hydrated salt.

3. The paint (22) of claim 1 or 2, comprising up to fifty percent by weight or volume of hydrated salt.

4. The paint (22) of claim 1 , 2 or 3, wherein the hydrated salt is stable at temperatures up to one-hundred-and-fifty degrees Celsius.

5. The paint (22) of claim 4, wherein the hydrated salt is an alkali salt or a transition metal salt.

6. The paint (22) of claim 5, wherein the hydrated salt is Magnesium Perchlorate Hexahydrate.

7. The paint (22) of any preceding claim, further comprising a binder material.

8. The paint (22) of claim 7, wherein the binder material is a polyurethane based material.

9. The paint (22) of claim 7, wherein the binder material is 2KPLI.

10. A paint system (16) comprising the paint (22) of any preceding claim and a primer (20) comprising an additive of Titanium Dioxide or Silica.

11. An object (12) having a surface region which is coated with the paint (22) of any one of claims 1 to 9.

12. The object of claim 11 , wherein the surface region is a fabric, e.g. cotton, surface region.

13. The object (12) of claim 11 or 12, wherein the surface region is coated with a first layer of primer (20), and a second layer of the paint (22).

14. The object (12) of claim 13, wherein the primer is a polyurethane based material.

15. The object (16) of claim 14, wherein the primer is 2KPLI.

16. The object (16) of any one of claims 13 to 15, wherein the primer comprises an additive of Titanium Dioxide.

17. The object (16) of any one of claims 13 to 15, wherein the primer comprises an additive of Silica.

18. A method of producing a camouflaged object, comprising coating a surface region of an object (12) with a layer of the paint (22) of any one of claims 1 to 9.

19. The method of claim 18, further comprising applying a first layer of primer (20) to the surface region; wherein coating the surface region with the layer of the paint (22) comprises applying the paint (22) to the first layer of primer (20).

20. The method of claim 18, wherein the surface region is a fabric, e.g. cotton, surface region.

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