paint

Abstract

There is disclosed a paint 22 comprising an additive 24 of hydrogel particles. This paint 22 provides improved matching between the reflectance profile of the paint and that of natural vegetation.

Description

[0001] Paint

[0002] Field

[0003] The present invention is concerned with the field of paints, specifically camouflage paints to mimic vegetation.

[0004] Background

[0005] 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.

[0006] 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). Thus a known problem for camouflage paints is that they are not able to mimic vegetation outside of the visible and NIR bands.

[0007] Figure 1 is a graph illustrating reflectance profiles for average vegetation and conventional green 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 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 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.

[0008] Summary

[0009] According to an aspect of the present invention, there is provided a paint comprising an additive of hydrogel particles.

[0010] Although hydrogel is generally known in the art, its use as an additive in paint is considered to be both novel and inventive in its own right.

[0011] Hydrogel, as the name suggests, is a gel-like substance that contains water. Commercially available hydrogels tend to be used in the biomedical industry, e.g. to make contact lenses, corneal prosthesis or wound dressing. However, the Applicant has recognised that the inclusion of hydrogel particles as an additive in paint 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 that are present in the 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.

[0012] Further, the Applicant has had to overcome a number of manufacturing problems associated with how to provide hydrogel in particle form suitable for use as an additive in paint. As will be described in further detail below, the manufacturing method of the present invention is a departure from the widely-held industry practice, whereby a hydrogel is made in bulk form before being shaped or cut down to the size of a final article, e.g. a contact lens.

[0013] It will be appreciated that the advantages of the paint of the present invention, i.e. improved reflectance matching between the paint and vegetation, can be realized with the presence of hydrogel particles 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 hydrogel particles in the total paint formulation (expressed as a percentage of the total weight or volume of the paint formulation). While not essential, this minimum quantity of hydrogel particles has been found to be particularly advantageous in that it yields more clearly identifiable water absorption bands in the reflectance profile.

[0014] The paint can take two forms: a wet form in which it can be easily applied to a substrate or surface; and a dry form whereby the paint has been air dried to the substrate or surface 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.

[0015] The wet or dry paint may comprise up to fifty percent by weight or by volume of hydrogel particles (expressed as a percentage of the total weight or volume of the paint formulation). The (wet or dry) paint may comprise hydrogel particles 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 volume of the paint. The paint (wet or dry) may comprise hydrogel particles in an amount falling within a loading range of 10-45, 15-40, or 20-35, percent by total weight or volume of the paint. While not essential, each one of the above sub-ranges may be 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 hydrogel particles 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. The percentage by weight or volume of the hydrogel particles may be measured and confirmed by one skilled in the art using any known, conventional method or standard for doing so.

[0016] The hydrogel may be coated with a dispersant. This may improve the separation of the hydrogel particles and to prevent their settling or clumping in the wet paint. This in turn may ensure that the hydrogel particles are uniformly spread and provide consistent reflectance properties across the paint, e.g. when dried.

[0017] The paint may further comprise a binder material. The binder material may be a polyurethane based material. One suitable example is a commercially available off-the-shelf (COTS), solvent borne two-pack polyurethane (2KPLI) binder which can be supplied by PPG Industries Inc. or Synthesia Technology, and which comprises 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.

[0018] According to another aspect of the present invention, there is provided an article having a surface region which is coated with the paint of any preceding statement.

[0019] The surface region may be that of fabric, e.g. cotton.

[0020] The paint may be used in combination with a primer to form a multi-layered paint system. For example, the surface region of an article 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. 2KPU.

[0021] 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 hydrogel particle additive).

[0022] The paint system may further comprise an outer protective coating. The coating may comprise polyurethane, e.g. 2KPU, which is applied to the second layer of the dried paint. 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.

[0023] According to another aspect of the present invention, there is provided a method of producing a camouflaged article, comprising coating a surface region of an article with a layer of the paint referred to in any one of the preceding statements.

[0024] 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 article. 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

[0025] 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.

[0026] The Applicant considers the method of manufacturing hydrogel particles to be novel and inventive in its own right. Thus, according to an aspect of the present invention, there is provided a method of making hydrogel particles, comprising cross-linking an emulsion comprising a hydrogel precursor dispersed as droplets in a solvent solution.

[0027] According to another aspect, there is provided a method of making hydrogel particles, comprising: preparing a hydrogel precursor comprising a polymer and a liquid (e.g. water); mixing the hydrogel precursor with a dispersant and a solvent such that the hydrogel precursor will form droplets or spheres which are suspended in the solution to form an emulsion; and adding a cross-linking agent to the emulsion so as to cross-link the hydrogel precursor while in droplet or spherical form.

[0028] According to a further aspect of the present invention, there is provided a method of making a paint comprising an additive of hydrogel particles, the method comprising: preparing hydrogel particles by cross-linking an emulsion comprising a hydrogel precursor dispersed as droplets in a solvent solution; and adding hydrogel particles to a binder material (e.g. a polyurethane based material) to form the paint.

[0029] The step of preparing hydrogel particles by cross-linking an emulsion comprising a hydrogel precursor dispersed as droplets in a solvent solution may comprise: preparing a hydrogel precursor comprising a polymer and water; mixing the hydrogel precursor with a dispersant and a solvent such that the hydrogel precursor will form droplets or spheres which are suspended in the solution to form an emulsion; and adding a cross-linking agent to the emulsion so as to cross-link the hydrogel precursor while in droplet or spherical form.

[0030] The above methods may further comprise extracting the hydrogel particles from the emulsion.

[0031] The hydrogel precursor may further comprise a desiccant, for example a salt, e.g. Lithium Chloride. The desiccant may be advantageous to stabilize the water in the hydrogel.

[0032] The polymer may be polyacrylamide.

[0033] The dispersant may be a surfactant. The solvent may be Xylene. The method of manufacturing hydrogel particles may have any one or more of the features of the method described herein, in particular with respect to Figure 5.

[0034] Brief Description of the Drawings

[0035] Embodiments of the invention will now be described by way of non-limiting example with reference to the remaining drawings, in which:

[0036] Figure 2 is a schematic drawing illustrating a scene in which the invention may be used;

[0037] 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 article;

[0038] Figure 4 is a block diagram schematically illustrating a method of producing a camouflaged article in accordance with an embodiment of the present invention;

[0039] Figure 5 is a block diagram schematically illustrating a method of making hydrogel particles for use in an embodiment of the present invention; and

[0040] Figure 6 is a schematic diagram illustrating a paint system in accordance with another embodiment of the present invention, when applied to the surface of an article.

[0041] Like reference numerals will be used throughout the detailed description to denote like features of the invention.

[0042] Detailed Description

[0043] 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.

[0044] The scene 10 is of a landscape comprising natural and man-made articles. In the current example, the scene 10 comprises a man-made article 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 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.

[0045] 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.

[0046] 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 4, the primer 20 is applied (at block 40 of Figure 4) directly to the surface 18 of the vehicle 12 and the paint 22 is then applied (at block 42) 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.

[0047] The primer 20 is a novel formulation of polyurethane and an additive of (e.g. Rutile) Titanium Dioxide or Silica (and optionally colorant pigments). The polyurethane may be a two-pack polyurethane (2KPU) primer 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. Applying the primer as an intermediate layer between the exterior surface 18 of the vehicle 12 and the layer of paint 22 improves adhesion and may reduce corrosion. The primer 20 may be a grey colour, and selected to be 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. Furthermore, by adding (e.g. Rutile) Titanium Dioxide or Silica to the primer 20, its reflectance profile will exhibit a general reduction in reflectance from 0.7 micron (700nm) to 2.5 microns (2500nm) ignoring the absorption bands.

[0048] In this embodiment, the paint 22 itself comprises an additive 24 of hydrogel particles, visible colour pigments, and a binder material. The paint (in wet form before application to the article) may further comprise one or more solvents.

[0049] A hydrogel is a hydrophilic polymeric material comprising a porous, permeable solid that can absorb water without dissolving. In a hydrogel material, water molecules are incorporated into a water insoluble three dimensional network of polymers that form the porous, permeable solid. Naturally occurring hydrogels include collagen or gelatine. There are also a number of synthetic hydrogel materials such as polyethylene glycol (PEG) and polyacrylamide based gels. Any of the above types of hydrogels are suitable for use as an additive 24 in the present invention, e.g. when further processed or shaped to form particles suitable for inclusion in the paint.

[0050] By incorporating particles of a hydrogel material as an additive in paint, the present invention provides a paint that has reflectance characteristics which 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.

[0051] Further, a paint comprising an additive 24 of hydrogel particles 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, hydrogel particles are stable such that their use as a paint additive may provide reflectance matching while improving the quality, texture and integrity of the paint as compared to these hypothetical arrangements. Hydrogels may also be advantageous over other water-based particles in that they are much cheaper to manufacture.

[0052] Any conventional colour pigments can be used as the visible colour pigments. In this embodiment, organic colour pigments are used to allow NIR and SWIR transmission through the second layer 22 of the paint system 16 to maximise the depth of film the light has to pass though, thereby increasing the effect of the hydrogel particles in that layer 22. The colour pigments may vary from paint to paint, depending on the application and the visible colour to be represented.

[0053] The binder may be any suitable material or agent that binds the hydrogel particle additive 24 and the colour pigment(s). In preferred embodiments, the binder is a polyurethane binder such as a COTS 2KPU binder.

[0054] Figure 5 is a block diagram schematically illustrating a method of making hydrogel particles suitable for use as a paint additive in accordance with the present invention.

[0055] Firstly, it will be appreciated that hydrogels can occur naturally or may be manufactured in a cross-linking process whereby the network of natural or synthetic polymers is formed by covalent or non-covalent bonding of the polymeric material.

[0056] Known hydrogel products, such as contact lenses, are typically made by crosslinking a precursor material to form a hydrogel material in bulk. The bulk material is then either injection moulded into a shaped product, or cut down to the desired size and shape of the product, e.g. in a lathe-cutting process. However, it is prohibitively difficult to make hydrogel particles using these conventional techniques. This is especially (but not exclusively) the case for hydrogel particles having an average diameter or span of a few microns or less, e.g. particles that are 0.1 to 100 pm in size. For example, it is difficult to make injection moulding tools of the small size needed to produce hydrogel particles, let alone at the scale (in terms of number of particles) required for applications where they are to be used as an additive in paint. Further, owing to the soft, pliable, semi-solid nature of hydrogels, it is difficult to cut down bulk hydrogel materials to the desired size using conventional techniques.

[0057] To address this problem, the manufacturing method developed by the Applicant does not cross-link hydrogel materials in bulk sizes (i.e. greater than that of a particle). Instead, the method produces hydrogel particles (where in embodiments the particles can be as little as 100 microns or less in size) as a direct output from the cross-linking process.

[0058] The manufacturing method begins at block 50, where a hydrogel precursor is prepared. The hydrogel precursor comprises a polymer, a liquid and a desiccant. In this embodiment, where the hydrogel particles are to be used to enhance camouflage paints, the liquid is water so as to ensure that the reflectance characteristics of the hydrogel will comprise water absorption bands. However, other liquids are possible for other applications.

[0059] Any polymer can be used but in the present embodiment it is Polyacrylamide, i.e. a synthetic polymer made of acrylamide (or the combination of acrylamide and acrylic acid). Polyacrylamide may be advantageous over other polymers as it is water-soluble and stable across a wide range of conditions, e.g. temperatures.

[0060] As stated above, in this embodiment the precursor comprises a desiccant, such as salt, specifically Lithium Chloride. The weight ratio between the desiccant and polymer-water mix is 1 :3, although other ratios are possible. The desiccant, especially Lithium Chloride, is hydrophilic and therefore absorbs the water and binds to the polymer. This can increase water retention in the final hydrogel material, and lead to more identifiable absorption bands.

[0061] At block 52, a solution is prepared separately to the hydrogel precursor and comprises a dispersant (surfactant) and a solvent mixture. Any dispersant and solvent can be used, but in this embodiment the solvent is Xylene.

[0062] At block 54, the hydrogel precursor is added to the dispersant solution and they are mixed together using a high shear mixer. In that regard, the hydrogel precursor and dispersant solution are immiscible liquids, such that the step of mixing both together will cause them to emulsify. Specifically, the hydrogel precursor will be dispersed throughout the solution and form small (e.g. micron-sized) droplets or spheres which are suspended in the solution to form an emulsion. Further, by using a solution that contains a dispersant (surfactant), the hydrogel precursor is prevented from settling or clumping together within the solution. In that regard, a surfactant is an organic compound with hydrophilic "heads" and hydrophobic "tails." As the hydrogel precursor is water-based, the surfactant heads will be attracted to the hydrogel precursor while the surfactant tails will interact more strongly with the solvent in the solution. This inhibits the hydrogel precursor droplets from merging into fewer, larger droplets.

[0063] At block 56, a cross-linking agent (and optionally an accelerator) is added to the emulsion while the hydrogel precursor is suspended as droplets within the solution. The cross-linking agent triggers the cross-linking of the hydrogel precursor droplets and causes their substantially immediate gelation to form hydrogel particles. Examples of known cross-linking agents which may be suitable for use with the present invention include alkylenebisacrylamide or N, N’-methylenebisacrylamide (NMBA). Other cross-linking agents will be apparent to the skilled user.

[0064] At block 58, and after the cross-linking process has finished, the hydrogel particles formed at block 56 are allowed to sediment and settle at the bottom of the solution before being extracted. Active separation methods, e.g. those which use sieving or a centrifuge, may also or instead be used to extract the hydrogel particles from the remaining liquid.

[0065] The hydrogel particles are then ready to be used as an additive in paint. Accordingly, the method of Figure 5 may form part of a wider method of making a paint in accordance with the present invention.

[0066] In view of the above, it can be seen that by mixing the hydrogel precursor with a dispersant solution, and cross-linking the mixture when in the emulsified state, the present invention is able to manufacture hydrogel particles in large quantities with relative ease of manufacture. Further this can be done at low cost as there is no need for complex injection moulding or lathe-cutting machinery typically used for this purpose.

[0067] While the method has been described above with respect to preparing the dispersant solution separately at block 52, before mixing them with the hydrogel precursor at block 54, in other embodiments the dispersant and solvent may be added to the hydrogel precursor directly, i.e. without preparing a solution separately. That is, block 52 may comprise adding the dispersant and solvent to the hydrogel precursor, before they are then mixed using a high-shear mixer at block 54.

[0068] A specific embodiment of a multi-layered paint system including the paint and primer will now be described. In the present embodiment, the primer (in wet form) comprises the following formulation:

[0069] • 150 grams (70.56 percentage by weight) of a first part (Part A) of a two- part 2KPU binder (e.g. that supplied by PPG Industries and known as “CV124")

[0070] • 50 grams (23.5 percentage by weight) of a second part (Part B) of a two- part 2KPU binder (CV124)

[0071] • 12.50 grams (5.88 percentage by weight) of Rutile Titanium Dioxide

[0072] • 0.1 grams (0.05 percentage by weight) of Carbon Black colorant pigment

[0073] An alternative wet primer formulation is:

[0074] • 30 grams (56.20 percentage by weight) of a first part (Part A) of a two-part 2KPU binder (CV124)

[0075] • 7.5 grams (14.05 percentage by weight) of a second part (Part B) of a two- part 2KPU binder (CV124)

[0076] • 15 grams (28.10 percentage by weight) of Silica

[0077] • 0.45 grams (0.84 percentage by weight) of visible black pigment

[0078] • 0.36 grams (0.67 percentage by weight) of visible green or yellow pigment

[0079] • 0.07 grams (0.13 percentage by weight) of visible red or violet pigment

[0080] The paint comprises the following formulation in wet form: • 50 grams of a first part (Part A) of a two-part 2KPU binder (CV124)

[0081] • 0.45 grams of black pigment (commercially known as L0086)

[0082] • 0.36 grams of yellow pigment (K0961)

[0083] • 0.07 grams of red pigment (K3911 )

[0084] • 50 grams of hydrogel particles

[0085] • 15 grams of a second part (Part B) of the two-part 2KPU binder (CV124)

[0086] Accordingly, in its wet form the paint formulation comprises 43.15% by weight of hydrogel particles, 56.09% by weight of 2KPU binder, and 0.76% by weight of colour pigments. In its dry form, the loading of hydrogel particles will be greater than 43.15% by weight, owing to the fact that solvents etc. in the 2KPLI 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.

[0087] Specifically, 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 at approximately 0.7 microns (700nm), as well as drops in reflectance values in discrete wavelength bands corresponding to 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 hydrogel particles. These characteristic drops in reflectance values are not present in conventional paint, which does not contain any water based pigments.

[0088] 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. 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 hydrogel particle additive) could be applied directly to a surface of an article, without an intermediate primer layer. In those arrangements, the reflectance profile of the paint will still comprise drops in reflectance values corresponding to water absorption bands, by virtue of the presence of hydrogel particles in the paint formulation. Such characteristic reflectance features may still be useful to conceal articles from sensors which are agnostic to the general reduction in reflectance from 1.3 microns (1300nm) to 2.5 microns (2500nm). 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 hydrogel particle additive) is applied directly to a fabric surface, e.g. cotton, a primer layer is not required.

[0089] While the invention described above with respect to Figure 3 has a layer of paint 22 which forms the outermost layer that is exposed on the exterior of the vehicle 12, this is not required. In other embodiments, such as that shown in Figure 6, the layer of paint 22 is not the outermost layer and is not exposed on the exterior of the vehicle 12. Instead, an outer protective coating 60 is applied to the outwardly facing surface of the paint 22, such that it is the outermost layer and is exposed on the exterior of the vehicle 12. The outer protective coating may be polyurethane. The outer protective coating 60 may be provided irrespective of whether a primer is used.

[0090] It will be appreciated here that the specific percentage by total weight or total 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 hydrogel particle additive is added to a transparent (in the visible part of the EM spectrum) varnish, which is then applied to an article or a layer of the primer.

[0091] Further, while the embodiments described above relate to a paint having up to fifty percent by weight or volume of hydrogel particle additive, higher or lower quantities (by percentage weight or volume) of hydrogel particles 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 hydrogel particle additive in the total paint formulation. The thickness of the layer of dried paint may also be tailored to control the prominence of the absorption bands. For example, increasing the thickness of the paint layer increases the number of hydrogel particles and thus water molecules that the light passes through, and in turn the extent of absorbance aligning with the water absorbance bands of natural vegetation.

[0092] 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 Titanium Dioxide or Silica can be tailored to provide a specific reflectance profile, as may be desired.

[0093] 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 hydrogel particles.

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

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

4. The paint (22) of claim 1 , 2 or 3, wherein one or more of the hydrogel particles comprise cross-linked polyacrylamide.

5. The paint (22) of any preceding claim, wherein the hydrogel particles have an average span of less than 100microns.

6. The paint (22) of any preceding claim, further comprising a binder material, e.g. of polyurethane.

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

8. An article (12) having a surface region which is coated with the paint (22) of any one of claims 1 to 6.

9. The article of claim 8, wherein the surface region is a fabric, e.g. cotton, surface region.

10. The article (12) of claim 8 or 9, wherein the surface region is coated with a first layer of primer (20), and a second layer of the paint (22).1 1. The article (12) of claim 10, wherein the primer is a polyurethane based material.

12. The article (12) of claim 10 or 1 1 , wherein the primer comprises an additive of Titanium Dioxide or Silica.

13. The article (16) of any one of claims 10 to 16, wherein the surface region is further coated with an outermost protective coating layer of polyurethane.

14. A method of making a paint (22) comprising an additive (24) of hydrogel particles, the method comprising: preparing hydrogel particles by cross-linking an emulsion which comprises a hydrogel precursor dispersed as droplets in a solvent solution; and adding the hydrogel particles to a binder material (e.g. a polyurethane based material) to form the paint.

15. The method of claim 14 wherein the step of preparing hydrogel particles by cross-linking an emulsion comprising a hydrogel precursor dispersed as droplets in a solvent solution comprises: preparing a hydrogel precursor comprising a polymer and water; mixing the hydrogel precursor with a dispersant and a solvent such that the hydrogel precursor will form droplets or spheres which are suspended in the solution to form an emulsion; and adding a cross-linking agent to the emulsion so as to cross-link the hydrogel precursor while in droplet or spherical form.

16. The method of claim 14 or 15, wherein the hydrogel precursor further comprises a desiccant, e.g. Lithium Chloride.

17. The method of claim 15, or claim 16 when dependent on claim 15, wherein the polymer is polyacrylamide.

18. The method of claim 15, or claims 16 or 17 when dependent on claim 15, wherein the dispersant is a surfactant.

19. A method of producing a camouflaged article, comprising coating a surface region of an article (12) with a layer of the paint (22) of any one of claims 1 to 6.

20. The method of claim 19, 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).

21. The method of claim 19 or 20, wherein the surface region is a fabric, e.g. cotton, surface region.

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