Method for protecting the surface of an aircraft against contamination with insect residues and/or icing

Abstract

A method of protecting a part of a surface of an aircraft against contamination with insect residues and/or against icing is described. The method includes the following steps: a) coating the part of the surface to be protected with a coating that can, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduce the adhesion to the part of the surface to be protected until detachment occurs on the basis of (i) temperature change and/or (ii) UV radiation and/or (iii) addition of a decomposing component and/or (iv) difference in air humidity, and b) exposing the part of the surface to be protected to (i) a temperature change and/or (ii) UV irradiation (iii) and/or addition of a decomposing component and/or (iv) a difference in air humidity, so that the coating together with any contaminants and/or icing thereon detaches.

Claims

1. An aircraft with a coating on a part of its surface, wherein the coating is designed so that the coating, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduces adhesion to the part of the surface to be protected until detachment occurs on the basis of (i) temperature change, wherein the temperature change is brought about at least partially by changes in environmental conditions due to a change in flying altitude after the aircraft takes off and the coating material is selected from the group consisting of polyurethane dispersions, acrylate dispersions, polyvinyl acetate dispersions and biological binders, (ii) UV radiation using a polymer that contains acid-labile groups in a main chain of a polymer and of photo-acids as initiators as the coating materials, (iii) addition of a decomposing component which is selected from a wetting addition of protein-degrading enzymes, acids and/or bases, and/or (iv) difference in air humidity, wherein the difference in air humidity is brought about at least partially by changes in environmental conditions due to a change in flying altitude after the aircraft takes off, and wherein a respective temporary layer of the coating is part of a system where the decomposition process is initiated owing to decreased air humidity at high altitudes, preferably because of the low humidity at the tropopause, the respective temporary layer becomes so brittle that there is a sufficient loss of adhesion.

2. The aircraft as claimed in claim 1, comprising a device for applying the coating and/or a device for spraying the coating and/or an artificial UV light source for irradiating the coating.

3. The aircraft as claimed in claim 1, wherein: the surface has a laminar flow profile, the coating comprises a temporary protective coating adhesively disposed on the laminar flow profile of the surface, and the temporary protective coating is configured to, during flight, reduce adhesion to the surface and expose the laminar flow profile of the surface to air.

4. The aircraft of claim 3, further comprising at least one of: a first spraying device configured to apply the temporary protective coating; an ultra-violet illumination source configured to irradiate the temporary protective coating during flight; a second spraying device configured to apply a decomposing component to the temporary protective coating during flight; and a heater configured to heat the temporary protective coating during flight.

5. The aircraft of claim 3, wherein the temporary protective coating is configured: to become brittle in response to lower temperature at increasing altitude, and to break up after becoming brittle in response to high air friction during flight.

6. The aircraft of claim 3, wherein the temporary protective coating is configured to protect the surface from at least one of: contaminants and icing.

7. The aircraft of claim 3, wherein the temporary protective coating is configured to remove, during flight, any contaminants present on the surface when the temporary protective coating was applied.

8. An aircraft with a coating on a part of its surface, wherein the coating is designed so that the coating, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduces adhesion to the part of the surface to be protected until detachment occurs on the basis of temperature change, wherein the temperature change is brought about at least partially by changes in environmental conditions due to a change in flying altitude after the aircraft takes off and the coating material is selected from the group consisting of polyurethane dispersions, acrylate dispersions, polyvinyl acetate dispersions and biological binders.

9. The aircraft of claim 8, wherein the coating comprises a temporary layer configured to begin the decomposition process owing to the temperature change at high altitudes such that the respective temporary layer become sufficiently brittle so that there is a loss of adhesion.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the photofragmentation of a primary main chain for a triazene system.

DETAILED DESCRIPTION

(2) The idea behind the present invention is based on the fact that insects only occur up to a certain altitude (see above). Therefore protection against insect contamination is only required in the first few minutes of the flight, to exclude the disadvantages of insect contamination during the main flying time. This also applies correspondingly to icing, which occurs on the ground or during the take-off phase. Advantages of the method according to the invention are that insect contamination and icing can be removed reliably and completely at a variable time point after the take-off phase. Furthermore, the coatings provided for protection do not represent a permanent weight increase for the aircraft. Moreover, the protective layers can be adapted to suitable methods of application, in particular by applying them as hardening liquids.

(3) On the basis of the present invention and in particular in connection with the present description, a person skilled in the art will be able to carry out a large number of possible embodiments of the invention:

(4) i) Physicothermally Controlled Degradation of a Temporary Layer

(5) The temporary layer (protective layer) can be designed in such a way that, because the air becomes colder with increasing altitude, a temperature difference acts upon the coating after take-off, so that the coating falls below the glass transition temperature. As a result the coating becomes brittle. The brittle coating then no longer has sufficient stability and is broken up by the high air friction occurring during flight.

(6) Alternatively or also additionally, the differential temperature required for embrittlement can also be produced by warming the surface before or during application of the protective layer. In the case of aircraft, this warming is for example conceivable in combination with de-icing, which naturally it is generally useful to carry out before application of a corresponding protective layer.

(7) Examples of coating materials that are suitable for a corresponding temporary layer are polyurethane dispersions, acrylate dispersions or dispersions of thermoplastics, e.g. polyvinyl acetate. In this connection, a UV-curing acrylate system is often preferred. Another option is to use biological binders, e.g. casein.

(8) Of course, additional stresses can be produced in the temporary layer by using a de-icing system during flight, e.g. at an external temperature of 50 C., by heating the coated areas to approx. 80 C.

(9) A particular advantage of thermophysically degradable temporary layers is that the layer can be designed so that in normal flight it completely loses adhesion with the surface to be protected, without the need to take additional measures. Preferably, in this connection, warming, e.g. by the de-icing system, is not required in connection with application or after application.

(10) When selecting the material for the layer, a person skilled in the art can also bear in mind the effects that occur in normal flight, i.e. in particular the shearing action that acts on said layers on account of air friction.

(11) ii) Chemically Controlled Degradation of a Temporary Layer Based an UV Radiation

(12) For chemical degradation of the temporary coating, it is possible for example to use photo-acids as initiators. Photo-acids are substances that form acids under the action of UV radiation. Photo-acids can be divided into ionic and nonionic photo-acids. The ionic photo-acids belong to the group of onium salts, e.g. phenyldiazonium, diphenyliodonium, triphenylsulfonium or triarylphosphonium salts with the cations BF.sub.4.sup., SbF.sub.6.sup., AsF.sub.6.sup. and PF.sub.6.sup.. Nonionic photo-acids are, as a rule, substances that form sulfonic acids under the action of UV radiation.

(13) For systems that use photo-acids as initiator, UV light is used as the trigger. Through the interaction of photo-acids and UV light, protons form, which catalyze acid hydrolysis of the coating. Hydrolysis leads to cleavage of the polymer used in the coating and thus to decomposition (degradation) of the layer.

(14) So that the coating can decompose, a polymer is used that contains acid-labile groups in the main chain of the polymer. An example of this is the implementation of ester groups in the main chain.

(15) In the case of a photo-acid, the acid component is only released on irradiation; therefore the photo-acid can be incorporated in the coating without any problem. A preferred example of a photo-acid is diphenyliodonium hexafluoroantimonate. On irradiation with UV light, HSbF.sub.6 forms, which breaks down into HF (strong Brnsted acid) and SbF.sub.5 (strong Lewis acid). Release of the acid as catalyst takes place irreversibly and is shown in reaction scheme 1.

(16) ##STR00001##

(17) In the case of the ester groups proposed above, cleavage of the main polymer chain takes place via transesterification with an alcohol. The alcohol must be incorporated in the coating. So that the alcohol remains in the layer during application of the layer, preferably a low-volatility alcohol is selected. Primary alcohols are preferred, as these have the highest reaction rate. An example of an appropriate primary alcohol of low volatility is butyl diglycol.

(18) Reaction scheme 2 shows the transesterification in the main chain of the polymer, which leads to chain shortening.

(19) ##STR00002##

(20) A polyurethane system is preferred for this system. Because the hydroxyl groups of the alcohol used would react with the isocyanate groups of the hardener of the polyurethane (PUR) system, it is preferable to polymerize the binder in solution in the presence of butyl acetate and then add the photo-acid (preferably diphenyliodonium hexafluoroantimonate) and the alcohol (preferably butyl diglycol). The coating is then applied as 1K-PUR system (single-component PUR system)see Example 1 for a preferred recipe.

(21) An alternative example of a concept for a coating that can undergo chemically controlled degradation (is designed to be temporary) under the action of UV radiation is the approach of integrating UV-sensitive groups in the main chain of the polymer. These UV-sensitive groups decompose when exposed to UV light. Examples of UV-sensitive groups are aryltriazene-chromophore systems (ArNNN). These groups decompose with formation of nitrogen, lowering the stability of the polymer. This process is shown schematically in FIG. 1.

(22) FIG. 1 shows the photofragmentation of a primary main chain for a triazene system.

(23) The structure of the polymer used is also shown in FIG. 1. An example of a corresponding synthesis is the diazotation of a bifunctional primary amine and subsequent reaction with a secondary bifunctional amine to aryltriazene. Because in this case bifunctional amines are used in each case, this results in formation of a thermoplastic. The schematic representation of the synthesis is shown in reaction scheme 3.

(24) ##STR00003##

(25) A corresponding coating is then applied as a single-component system.

(26) Another variant of a UV-sensitive temporary protective layer is based on radical degradation. Here, the coating is destabilized by a radical reaction in the binder. The radical reaction is triggered by photocatalytically active pigments. An example of a photocatalytically active pigment is titanium dioxide.

(27) The principle of radical degradation of a pigmented coating that is triggered by UV radiation has been known for a long time, by the term chalking. As a result of the photochemical reaction, the pigment particles that are present lose their bond to the coating. On wiping such a surface, one gets the impression that it had been covered with chalk. Normally every effort is made to prevent this effect, but in the sense of the present invention it is expressly desirable.

(28) An example of a corresponding coating system is an epoxy resin based on bisphenol-A. These resins have low UV-resistance, which makes them suitable for use in the sense of the invention.

(29) The pigment preferably used, titanium dioxide, is a polymorphic compound and occurs in the anatase, rutile and brookite phases. Only the anatase and rutile phases are of interest for photocatalytic reactions, as amorphous titanium dioxide is not as a rule sufficiently photocatalytically active. Basically, photocatalytic reactions are initiated when an electron is promoted from a filled valence band into the empty conduction band. During this, the electron absorbs the light energy h, which is equal to or greater than the gap between valence band and conduction band. Through promotion of the electron, a gap is left in the valence band. This reaction thus leads to formation of an electron-hole pair. In the case of titanium dioxide, the required energy is 380 nm (3.23 eV) for anatase and 410 nm (3.10 eV) for rutile. A preferred example of a titanium dioxide pigment is Degussa P25. This product has very high reactivity.

(30) A corresponding coating is used as a 2-component system.

(31) Layers that are to be used in the method according to the invention, which decrease their binding force on account of UV radiation, can be arranged so that this decrease takes place without further measures, owing to increased UV radiation at higher altitudes during normal flight. It is, however, also possible for the aircraft to be specially equipped with an irradiating device, by means of which the required UV radiation is emitted at a desired time point, and leads to decomposition of the protective layer.

(32) iii) Layers to be used According to the Invention, which Reduce Adhesion on the Basis of Decomposing Components

(33) As coatings to be used in methods according to the invention, it is also possible to use layers in which the binder contains polymers which are cleaved by enzymes, so that the adhesion between the layer and the substrate decreases. In this connection it is preferable for the polymers to be biopolymers, e.g. sugars or proteins. Said polymers can be used as the binder component of the coating.

(34) The time point at which the decomposition reaction for the protective layer is to be started can be determined by adding a second (decomposing) component. This can be e.g. within the scope of a wetting addition of protein-degrading enzymes to a protein-based protective layer (e.g. based on degraded casein).

(35) A number of alternatives to biopolymer-based and enzyme-decomposable layers to be used according to the invention are known by a person skilled in the art. As an example, we may for example mention polyesters, which can, by adding bases or acids (in each case as decomposing component), be influenced with respect to their properties of adhesion to the substrate so that adhesion is reduced or even lost completely.

(36) The advantage of these two-component systems is that the exact time point at which decomposition (degradation and hence decrease of adhesion) should begin is defined. Furthermore, part of the coating can be applied long before the aircraft takes off, and the second component can be applied just before take-off or even by means of an on-board spraying device during flight.

(37) iv) Layers to be used According to the Invention, the Adhesion of which Decreases as a Result of a Difference in Air Humidity

(38) Basically it is also possible to design layers so that their adhesion to the substrate decreases on account of differences in air humidity. Systems where the decomposition process is initiated owing to decreased air humidity at high altitudes are preferred for the method according to the invention. For example, because of the low humidity at the tropopause, these layers become so brittle that there is a sufficient loss of adhesion.

(39) Examples of corresponding coating systems are casein-based coating substances with low percentage of fillers.

(40) It will be evident to a person skilled in the art that the effects described above, e.g. decreased adhesion owing to temperature difference and UV irradiation can also be combined, in order to achieve the desired result (detachment of the protective layer from the aircraft).

(41) Preferably, the layer systems are designed so that they can reduce the adhesion of the part of the surface to be protected until detachment occurs, within 2 h, more preferably 1 h, even more preferably 30 min and preferably after (optionally complete) application in each case after the aircraft takes off and in normal flying conditions. Moreover, this condition refers in particular to normal flying conditions, in which other effects, e.g. due to drag, also contribute to detachment.

(42) As already mentioned, it is preferable that for detachment of the protective layer of the part of the surface of the aircraft to be protected based on a temperature change, UV irradiation and/or a difference in air humidity is brought about at least partially (preferably completely, optionally together with the mechanical stresses in normal flying conditions) by changes in environmental conditions, due to the change in flying altitude after the aircraft takes off.

(43) In many cases it is preferable for the method according to the invention to be supported in that the surface to be protected is coated with or is formed by a non-eroding layer of one of the following materials: metal, e.g. special steel, titanium, aluminum or hard chromium plated metal, metallized plastic, e.g. electrometallized plastic, e.g. hard chromium plated plastic, plasma- or flame-sprayed metal layers e.g. of special steel or titanium, ceramic wear-protecting layers, e.g. of titanium nitride or boron nitride, DLC layers or diamond coatings, elastomeric wear-protecting layers based on rubber, polyurethane or silicone, inorganic layers produced by the sol-gel technique, e.g. silicate coatings, inorganic-organic hybrid layers produced by the sol-gel technique, organic paint coatings plasma-polymer coatings.

(44) These coatings can in particular also serve, in combination with the actual surface shape of the surface of the aircraft, to ensure laminar air flow (after removal of the protective layer and any contamination located thereon). In this case it is possible for a person skilled in the art to adapt the temporary protective layer to the respective surface in a suitable way. This means in particular that the temporary protective layer has sufficient adhesion while the aircraft is climbing, until the desired detachment (i.e. in the period when it must fulfill its protective function against insect strike) and on the other hand decomposes rapidly by a suitable decomposition mechanism, so that, together with the contamination, it loses adhesion to the surface to be protected, after it has fulfilled its function.

(45) It is preferable for the protective layer that is to be used according to the invention to be designed so that insect contamination that was present prior to application of the protective layer also adheres to it. This is in particular the case when the aircraft surface was contaminated with insect residues during landing (when as a rule there is no longer any protective layer present, as this is regularly removed during the flight). With a layer of this design, cleaning of the surface to be coated prior to coating can optionally be omitted.

(46) It is of course evident to a person skilled in the art that the protective layer can only perform its protective function in particular with respect to ensuring laminar flow of the region to be protected if this was either cleaned before application of the protective layer or if preferably the protective layer is designed in such a way that the contaminants present when it was applied are also removed during detachment of the protective layer according to the method according to the invention.

(47) The last-mentioned variant can also be supported by a suitable form of the surface to be protected, e.g. by a hydrophobic design.

(48) In many cases a method according to the invention is preferred in which the coating is applied by a device that is integral with the aircraft. Said device can for example comprise nozzles, which are embedded in the leading edge of the wing and can be brought out for the spraying operation. Alternatively, it can be a device that moves along the surface to be coated and applies the coating material by a spraying process. This device can for example be housed in the side wall of the fuselage during the flight.

(49) The advantage of such devices is that they ensure a constant coating quality and are always available for a coating operation.

(50) Alternatively and for some uses also preferably, application can, for the method according to the invention, take place by means of an external device. Said device can for example be a large hangar equipped with spraying devices. Alternatively it is possible to use devices like the de-icing vehicles that are currently typically used in the prior art. These vehicles are equipped with a lifting platform, on which a spraying device is mounted, so that they can be driven along the surfaces to be coated and the coating can be applied by a spraying process.

(51) It may also be preferable for some applications if the UV radiation, which leads to detachment of the protective layer, is provided at least partially by an artificial UV source, which can be a component of the aircraft.

(52) Preferably the method according to the invention is carried out so that the coating is applied less than 30 minutes, preferably less than 20 minutes, preferably less than 10 minutes before the aircraft takes off.

(53) This is expedient in particular for coatings for which the decomposition process is already begun before take-off, e.g. by adding enzyme solutions.

(54) The invention also relates to an aircraft with a coating on a part of its surface, wherein the coating is designed so that it can, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduce the adhesion to the part of the surface to be protected until detachment occurs on the basis of (i) temperature change and/or (ii) UV radiation and/or (iii) addition of a decomposing component and/or (iv) difference in air humidity.

(55) Aircraft are preferred according to the invention that comprise one of the (temporary) protective coatings described as preferable above and optionally a device for applying the coating and/or a device for spraying the coating (in particular with a decomposing component) and/or an artificial UV light source for irradiating the coating.

(56) The aircraft according to the invention are protected against insect contamination that can arise on the ground, during take-off and during climbing. This also applies correspondingly to icing. A person skilled in the art will of course understand that contamination of the aircraft can first take place on the protective layer, but the aircraft itself (i.e. without the protective layer) is protected from the corresponding contaminants, as these can be removed according to the invention from the aircraft together with the protective layer by triggering a suitable mechanism.

(57) The invention also relates to a device for applying a coating on an aircraft, which is designed so that it can, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduce the adhesion to the part of the surface to be protected until detachment occurs on the basis of (i) temperature change and/or (ii) UV radiation and/or (iii) addition of a decomposing component and/or (iv) difference in air humidity, wherein the device is integral with the aircraft and wherein the device comprises nozzles that can be brought out and/or is designed so that it can move along the surface to be coated.

(58) Nozzles that can be brought out means, in this context, that the nozzles can undergo a change of position relative to the surface of the aircraft, which comprises a perpendicular component relative to said surface. The embodiment wherein the device can move along the surface to be coated means in connection with this patent application that at least one part of the coating device can execute a movement that comprises a parallel component relative to the surface to be coated. Of course, an aircraft with a corresponding coating device also forms part of the invention.

(59) The advantage of such a device has already been described above and consists in particular in that it ensures reproducible application of the coating and constant availability.

(60) A further part of the invention is the use of a coating that is designed so that it can, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduce the adhesion to the part of the surface to be protected until detachment occurs on the basis of (i) temperature change and/or (ii) UV radiation and/or (iii) addition of a decomposing component and/or (iv) difference in air humidity for protecting a part of a surface of an aircraft against contamination with insect residues and/or against icing.

(61) The invention further relates to a use of a coating device for coating a part of the surface of an aircraft with a coating, which is designed so that it can, within a period of 3 hours from take-off of the aircraft and in normal flying conditions, reduce the adhesion to the part of the surface to be protected until detachment occurs on the basis of (i) temperature change and/or (ii) UV radiation and/or (iii) addition of a decomposing component and/or (iv) difference in air humidity.

(62) The periods from take-off of the aircraft and in normal flying conditions described above as preferable are naturally also preferable for the uses according to the invention. This also applies correspondingly to the embodiments of the coating to be used according to the invention that are described as preferable.

(63) The invention is explained in more detail below on the basis of examples.

EXAMPLE 1

Example of a Recipe for a Protective Layer for Detachment by UV Radiation

(64) The following example of a recipe is a single-component system, which can be applied by a spraying process.

(65) TABLE-US-00001 TABLE 1 Recipe Item Raw material Weight [g] 01 Slightly branched polyester containing OH groups 29.9 (Desmophen 670) 02 Butyl acetate 44.9 03 Dicyclohexylmethane diisocyanate (Desmodur W) 9.9 04 Butyl diglycol 15.0 05 Diphenyliodonium hexafluoroantimonate 0.3 Total 100

EXAMPLE 2

Example of a Recipe for a Layer for which Adhesion Decreases Under the Action of UV Radiation by Radical Degradation

(66) The coating is a two-component system.

(67) TABLE-US-00002 TABLE 3.1 Item Raw material Weight [g] Recipe of base 01 Polypox E 064 34.5 02 Degussa P25 43.0 03 Methoxypropyl acetate 19.7 04 n-Butanol 2.8 Total base 100 Hardener 01 Polypox H 147 13.8 Total hardener 13.8

EXAMPLE 3

Example of a Recipe for a Layer that Detaches as a Result of Cold Embrittlement

(68) Application of an aqueous dispersion of polyvinyl acetate (Craymul 2325 from Cray Valley Kunstharze GmbH in Zwickau) with a dry film layer thickness of 15-30 m on aluminum sheets, which can be hydrophobized with a plasma-polymer separating layer.

EXAMPLE 4

Example of a Recipe for a Layer that Becomes Brittle on Drying Out and Loses Adhesion

(69) Application of a casein-based coating on aluminum sheets with a dry film thickness of 15-30 m. The casein coating is prepared as follows:

(70) TABLE-US-00003 Borax solution 5 g 12.8 Low-fat curd cheese 24 g 61.6 Water 10 g 25.6 39 g 100 dissolve 5 g borax in 20 g H.sub.2O mix 5 g of this solution with 24 g of low-fat curd cheese leave to swell for 2 h dilute with 10 g H.sub.2O

(71) After layer formation, all the example recipes detach from the respective substrate in (optionally simulated) take-off/flight conditions in less than 180 minutes, in the case of the example recipes 3 and 4 even in less than 60 minutes.