COATING MATERIAL

Abstract

A coating material (10) for coating an article is described. The coating material (10) comprises a surface (100) having an optical interference coating (110) thereon. The coating material (10) improves protection of the article from incident electromagnetic radiation having a predetermined wavelength. The coating material (10) may retroreflect at least some of the incident electromagnetic radiation, for example towards a source (e.g. a laser) thereof. An article having a coating provided by such a coating material and methods of providing such coating materials are also described.

Claims

1. A coating material for coating an article, the coating material comprising a surface having an optical interference coating thereon.

2. The coating material according to claim 1, wherein the coating material is arranged to reflect incident electromagnetic radiation.

3. The coating material according to claim 1, wherein the coating material comprises a film comprising the surface having the optical interference coating thereon.

4. The coating material according to claim 3, wherein the film comprises a metamaterial.

5. The coating material according to claim 1, wherein the surface comprises protrusions.

6. The coating material according to claim 1, comprising a flowable formulation including the surface having the optical interference coating thereon.

7. The coating material according to claim 6, wherein the coating material comprises particles and wherein the surface having the optical interference coating thereon is provided by at least a part of surfaces of the particles.

8. The coating material according to claim 7, wherein the at least a part of the surfaces of the particles is a first part of the surfaces of the particles, and wherein an antireflective coating is provided on at least a second part of the surfaces of the particles.

9. The coating material according to claim 7, wherein the particles are beads.

10. The coating material according to claim 7, wherein the particles include fibres.

11. The coating material according to claim 10, wherein the particles include chopped fibres.

12. The coating material according to claim 10, wherein the particles include ground fibres.

13. The coating material according to claim 7, wherein the particles are flakes, platelets or chopped film.

14. An article having a coating provided by a coating material according claim 1.

15. A method of providing a coating material, the method comprising: applying a layer of a polymeric composition comprising a polymer on a particle; floating the particle on a liquid; and irradiating the particle with a laser beam, thereby providing an optical interference coating on the particle, wherein the liquid reflects the laser beam.

16. A method of providing a coating material, the method comprising: providing a film having an optical interference coating on a surface thereof; and indenting the film, thereby providing protrusions and/or depressions therein.

17. The coating material according to claim 5, wherein the protrusions comprise: columns and/or depressions; and/or cube corners.

18. The coating material according to claim 8, wherein at least some of the particles are oriented such that the antireflective coating is outermost or generally outermost.

19. The method according to claim 15, wherein the liquid comprises mercury.

20. The method according to claim 16, wherein providing the protrusions and/or depressions provides a photonic metamaterial.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0087] For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

[0088] FIG. 1 schematically depicts a coating material according to an exemplary embodiment;

[0089] FIG. 2 schematically depicts a coating material according to an exemplary embodiment;

[0090] FIG. 3 schematically depicts a coating material according to an exemplary embodiment;

[0091] FIG. 4 schematically depicts a method of providing a coating material according to an exemplary embodiment;

[0092] FIG. 5 schematically depicts a method of providing a coating material according to an exemplary embodiment;

[0093] FIG. 6 schematically depicts a filter assembly for a coating material according to an exemplary embodiment;

[0094] FIG. 7 schematically depicts a method of providing a filter assembly for a coating material according to an exemplary embodiment;

[0095] FIG. 8 schematically depicts transmission characteristics of a filter assembly for a coating material according to an exemplary embodiment; and

[0096] FIG. 9 schematically depicts transmission characteristics of a filter assembly for a coating material according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

[0097] FIG. 1 schematically depicts a coating material 10 according to an exemplary embodiment.

[0098] In more detail, the coating material 10 is for coating an article. The coating material 10 comprises a surface 100 having an optical interference coating 110 thereon. The coating material 10 comprises a film 120 comprising the surface 100 having the optical interference 110 coating thereon. The coating material comprises a flowable formulation including the surface 100 having the optical interference coating 110 thereon. In use, the flowable formulation cures to a solid. The coating material comprises particles 130 and wherein the surface 100 having the optical interference coating 110 thereon is provided by at least a first part of surfaces of the particles 130.

[0099] In this example, the particle 130 is a bead, particularly a spherical bead. In this example, the particle 130 is completely coated with the optical interference coating 110, particularly a holographic interference coating 110, provided as the film 120 on the surface 100 thereof. The particle 130 is formed from optical glass and has a radius of 100 μm.

[0100] In this way, at least some incident electromagnetic radiation λ.sub.incident reflects off a front surface of a bead 130 (i.e. as reflected electromagnetic radiation λ.sub.reflected, external) while at least some incident electromagnetic radiation λ.sub.incident is transmitted into the bead 130 (i.e. enters the bead 130 as admitted electromagnetic radiation λ.sub.admitted). At least some of the admitted electromagnetic radiation λ.sub.admitted transmitted into the bead 130 is subsequently transmitted out of the bead 130 (i.e. exits the bead as transmitted electromagnetic radiation λ.sub.transmitted) (not shown). However, at least some of the electromagnetic radiation λ.sub.admitted transmitted into the bead 130 is trapped in the bead 130, due to reflection by the optical interference coating, for example the holographic interference coating 110. The trapped electromagnetic radiation λ.sub.reflected,internal may ablate at least a part of the optical interference coating 110, for example the holographic interference coating, and/or the bead 130, thereby absorbing energy of the trapped electromagnetic radiation and protecting the underlying article. At least some of the trapped electromagnetic radiation λ.sub.reflected,internal may exit the bead 130 via the front surface i.e. as reflected electromagnetic radiation λ.sub.reflected. Furthermore, since the at least a part of the optical interference coating 110 may be ablated, access by the electromagnetic radiation to the remainder of the bead 130 is enabled, providing greater retro reflection and trapping less energy than the intact coating. In other words, the bead 130 is sacrificial, being at least partly damaged by the incident electromagnetic radiation, while protecting the article. By providing multiple layers of beads 130, for example, resistance to the incident electromagnetic radiation may be improved.

[0101] In use, the beads 130 are applied to an article such that a surface of the article is covered in many layers of the beads 130, thereby providing a protective layer. This allows for the destruction and ablation of the protective layer by a laser from a weapon without breach of protection. Protection may be required only for a limited time due to a nature of the weapon only being able to target the article for a specific time. The holographic interference coating 110 may be tuned for a predetermined wavelength or a range of predetermined wavelengths. Since lasers from weapons are generally monochromatic, protection may be thus provided against predetermined weapons. In addition, the coating material may retroreflect at least some of the incident electromagnetic radiation, towards a source (e.g. a laser) thereof, thereby potentially compromising, such as damaging, the source or its associated sensors, which may comprise a weapon and/or a sensor.

[0102] FIG. 2 schematically depicts a coating material 20 according to an exemplary embodiment. The coating material 20 is similar to the coating material 10 and like features are denoted by like reference signs. However, in contrast to the coating material 10, a holographic interference coating 210 is provided on at least a first part of surfaces of particles 230 and an antireflective coating 215 is provided on at least a second part of the surfaces of the particles 230. Particularly, half of the surface of the particles 230 is covered by the holographic interference coating 210 and the other half of the surface of the particles 230 is covered by the antireflective coating 215. Coating one side with half of the surface of the particles 230 with the holographic interference coating 210 allows for greater reflection whilst ensuring that energy is allowed to exit the bead whilst the antireflective coating 215 on the front side allows for easier admittance of the electromagnetic radiation. Preferably, some or all of the particles 230 are oriented, for example electrostatically, whereby the antireflective coating 215 is outermost or generally outermost. In this way, retroreflection of at least some of the incident electromagnetic radiation, towards a source (e.g. a laser) thereof, is improved.

[0103] In this way, at least some incident electromagnetic radiation λ.sub.incident is transmitted into the bead 230, admitted therein by the antireflective coating 215 (i.e. enters the bead 230 as admitted electromagnetic radiation λ.sub.admitted). At least some of the admitted electromagnetic radiation λ.sub.admitted transmitted into the bead 230 is subsequently transmitted out of the bead 230 (i.e. exits the bead as transmitted electromagnetic radiation λ.sub.transmitted) (not shown). However, at least some of the electromagnetic radiation λ.sub.admitted transmitted into the bead 230 is trapped in the bead 230, due to the optical interference coating, for example the holographic interference coating 210. The trapped electromagnetic radiation λ.sub.refleded,internal may ablate at least a part of the optical interference coating 210, for example the holographic interference coating, and/or the bead 230, thereby absorbing energy of the trapped electromagnetic radiation and protecting the underlying article. However, reducing and/or minimizing an amount of the trapped electromagnetic radiation may be preferred such that retroflection (i.e. as reflected electromagnetic radiation λ.sub.reflected) is enhanced and/or damage to the coating is reduced and/or life of the coating is extended. In other words, the bead 230 is sacrificial, being at least partly damaged by the incident electromagnetic radiation, while protecting the article. By providing multiple layers of beads 230, for example, resistance to the incident electromagnetic radiation may be improved.

[0104] In use, the beads 230 are applied to an article such that a surface of the article is covered in many layers of the beads 230, thereby providing a protective layer. This allows for the destruction and ablation of the protective layer by a laser from a weapon without breach of protection. Protection may be required only for a limited time due to a nature of the weapon only being able to target the article for a specific time. The holographic interference coating 210 may be tuned for a predetermined wavelength or a range of predetermined wavelengths. Since lasers from weapons are generally monochromatic, protection may be thus provided against predetermined weapons.

[0105] FIG. 3 schematically depicts a coating material 30 according to an exemplary embodiment.

[0106] In this example, a holographic interference coating 330 includes corner reflectors and is applied to a surface of an article.

[0107] Using a corner reflector arrangement, this variation would allow for reflection to occur as the internal surfaces would be made from highly polished metal (such as steel) with the HIC applied to the surface. This would allow for the metallic structure to be cooled using traditional cooling methods such as phase changing, radiators etc. If the laser ablates the HIC the underlying structure provides additional protection. It is envisaged that many small structures will be placed along the asset to be protected to provide full coverage.

[0108] FIG. 4 schematically depicts a method of providing a coating material according to an exemplary embodiment.

[0109] At S401, a layer of a polymeric composition comprising a polymer is applied on a particle.

[0110] At S402, the particle is floated on a liquid, preferably mercury.

[0111] At S403, the particle is irradiated with a laser beam, thereby providing an optical interference coating on the particle.

[0112] The method may include any of the steps described herein. Particularly, the method may include any of the steps described with reference to FIG. 7.

[0113] FIG. 5 schematically depicts a method of providing a coating material according to an exemplary embodiment.

[0114] At S501, a film having an optical interference coating thereon on a surface thereof is provided.

[0115] At S502, the film is indented, thereby providing protrusions and/or depressions therein, for example a metamaterial.

[0116] The method may include any of the steps described herein.

[0117] FIG. 6 schematically depicts the filter assembly 300 for the coating material 10, 20 according to an exemplary embodiment.

[0118] A first notch filter 320 is provided as a layer applied to a first face of a substrate 340 to provide the filter assembly 300 adapted for mitigating laser threats such as dazzle. The substrate 340 is substantially transmissive of visible light (for example it may have a visible light transmission (VLT %) of around 90% of normally incident light) and may be formed for example from a glass or a plastics material such as polycarbonate.

[0119] The first notch filter 320 is an interference filter formed by holographically exposing a photosensitive film with a plurality of lasers having a set of predetermined wavelengths within a selected wavelength band of bandwidth 10 nm or less.

[0120] Conformable photosensitive (e.g. polymeric) films for use in exemplary embodiments of the present invention will be known to a person skilled in the art, and the present invention is not necessarily intended to be limited in this regard. Such photosensitive polymeric films are provided having varying degrees of inherent visible light transmission (VLT), ranging from less than 70% (and possibly, therefore, having a coloured tinge) up to 99% or more (and being substantially colourless and transparent). In respect of the present invention, suffice it to say that a photosensitive flexible/conformable (e.g. polymeric) film is selected having an inherent VLT of, for example, at least 85%. The film typically has a thickness of 1 to 100 micrometers. Thinner, currently known, films may not achieve useful optical densities. Indeed, in respect of currently known photosensitive polymeric films, the degree to which a selected radiation wavelength can be blocked (i.e. the effectiveness of a filter region formed therein) is determined by the thickness and refractive modulation index of the film and, also, by the optical design. Thus, the filter region thickness is ideally matched to the application and the potential power of the source from which protection is required (which may be dictated, at least to some extent, by the minimum distance from the target platform the laser threat may realistically be located and this, in turn, is dictated by application). In general, thicker films and films with higher refractive modulation indices would be selected if it were required to provide protection from higher power radiation sources or to provide greater angular coverage, but this might then have a detrimental effect on the inherent VLT of the film, so a balance is selected to meet the needs of a specific application.

[0121] Thus, once the film has been selected, the required holographic exposure thereof is effected to form the filter regions of a required notch filter region to be provided thereon, as described below with reference to FIG. 7.

[0122] FIG. 7 schematically depicts a method of providing the filter assembly 300 for the coating material 10, 20 according to an exemplary embodiment.

[0123] Particularly, as shown in FIG. 7, distinct filter regions defining a notch filter region of a predetermined bandwidth (for example 5-10 nm) may be formed by exposing the film to the intersection of two counter propagating laser beams for each of a set of laser wavelengths within the selected wavelength band having a selected spectral bandwidth. Each laser 1000 (of a wavelength within the selected spectral bandwidth) produces a laser beam 120 which is controlled by a shutter 140. The laser beam 120 is directed by a mirror 160 into a beam splitter 180 wherein the beam is divided into equal beam segments 200. Each beam segment 200 passes through a microscope objective 220 and is then reflected by a respective mirror 360 onto a photosensitive polymer film 320 provided on the substrate 340. Other coating materials (not shown) may be provided between the microscope objective 220 and the mirror 360 to, for example, focus or diverge the respective beam segments 200, as required. Furthermore, masking or other limiting techniques may be utilised to limit the extent or thickness to which the film is exposed to the beam segments 200, as will be understood by a person skilled in the art. As a specific (non limiting) example, if it is required to provide a notch filter region of bandwidth 5 nm around 520 nm, then a plurality of lasers 1000 may be used to produce the notch filter region of (purely by way of example) 517.5 nm, 518 nm, 518.5 nm, 519 nm, 519.5 nm, 520 nm, 520.5 nm, 521 nm, 521.5 nm, 522 nm and 522.5 nm. The above-described exposure process may be performed consecutively for each of these laser wavelengths or, in other exemplary embodiments, the exposures may be performed substantially simultaneously. Other apparatus for forming a holographic filter region at each specified wavelength is known and could, alternatively, be used.

[0124] Once the exposure process has been completed, the resultant hologram can be fixed by, for example, a bleaching process.

[0125] FIG. 8 schematically depicts transmission characteristics of the filter assembly 300 for the coating material 10, 20 according to an exemplary embodiment.

[0126] Particularly, FIG. 8 shows the transmission characteristics (which may alternatively be referred to as the transfer function) of visible electromagnetic radiation incident on the first notch filter 320. The transmission intensity relative to incident radiation intensity is shown on the y-axis and the wavelength of the incident radiation is shown on the x-axis.

[0127] As can be seen on the plot, across the range of wavelengths the intensity of the transmitted radiation is close to 100% of that which is incident. In general, a VLT % of 90% would be acceptable if 100% were not feasible. If the coating material is for coating an opaque article, for example, such as a part of a military article as described above, a lower VLT is acceptable, for example a VLT % of at most 50%, at most 40%, at most 30%, at most 20%, at most 10%, at most 5%, at most 1% or 0%.

[0128] There are three distinct notches in the transmission characteristic associated with three wavelength bands. These are in particular a 10 nm band centred on 455 nm, a 10 nm band centred on 532 nm and a 10 nm band centred on 650 nm. In general any three notches from the group consisting of 405 nm, 455 nm, 520 nm, 532 nm, and 650 nm may be selected. Further, notches may be chosen to coincide with any expected laser threat wavelength and/or expected red shift to compensate for blue shift due to the angle of inclination. Still further, the bandwidth may be 5 nm.

[0129] At the centre of each of these bands, the intensity of the transmitted radiation is at a minimum and has an optical density of approximately 3, which is equivalent to 0.1% of the initially incident radiation.

[0130] FIG. 9 schematically depicts transmission characteristics of a filter assembly for an coating material according to an exemplary embodiment.

[0131] Particularly, FIG. 9 shows the measured transmission characteristics of visible electromagnetic radiation incident on the first notch filter 320. The transmission intensity relative to incident radiation intensity is shown on the y-axis and the wavelength of the incident radiation is shown on the x-axis, as described with reference to FIG. 8.

[0132] Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

[0133] In summary, the invention provides a coating material for coating an article, the coating material comprising a surface having an optical interference coating thereon. In this way, in use, the coating material improves protection of the article, coated with the coating material, from incident electromagnetic radiation, particularly incident infra red electromagnetic radiation, having a predetermined wavelength, for example by attenuating the incident electromagnetic radiation having the predetermined wavelength incident on the article c.f. incident on the coating material. In this way, the coating material blocks and/or disrupts incident laser energy, for example. That is, the coating material reduces damage of the article due to the incident electromagnetic radiation, thereby maintaining operation, preserving integrity and/or preventing destruction thereof, thereby increasing resistance to hostile threats. In other words, the coating material wavelength-selectively reduces energy absorbed by the article, thereby extending life of the article. In addition, the coating material may retroreflect at least some of the incident electromagnetic radiation, for example towards a source (e.g. a laser) thereof, thereby potentially compromising, such as damaging, the source, which may comprise a weapon and/or a sensor. An article having a coating provided by such a coating material and methods of providing such coating materials are also provided by the invention.

[0134] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0135] All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

[0136] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0137] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.