Aircraft component with a microstructured surface

Description

[0001] The present invention relates to an aircraft component comprising a microstructured surface with a riblet structure consisting of an embossing lacquer composition which consists of a prepolymer composition.

[0002] Microstructured surfaces are advantageous for a wide variety of applications. Well-known examples include so-called flow-optimized riblet structures on the surfaces of parts moving relative to a fluid or other medium, such as the outer surfaces of aircraft, ships, or wind turbine rotor blades. Such structures are also used, for example, on the inner surfaces of pipelines.

[0003] Nanoimprint lithography (NIL) is suitable for producing such riblet structures. In NIL processes, a polymer composition is structured by mechanical embossing. This is a highly precise embossing method that makes it possible to form even the smallest structures on a suitable substrate. In particular, substrates such as films can be provided with a microstructured surface in this way, which can then be bonded to the desired surface.

[0004] Roll-to-roll UV nanoimprint lithography (R2R-UV-NIL) enables the large-scale, cost-effective, and high-throughput production of micro- and nanostructured surfaces, such as riblet films, for reducing flow friction. In this process, the starting material, which is on a roll, is unwound onto a web of, for example, a substrate film, typically at high speeds, processed, and then rewound. Suitable embossing lacquers can be used as starting materials.

[0005] Such embossing lacquers are known from the prior art. For example, WO 2016 / 090395 A1 discloses a prepolymer composition by which thiol-containing embossing lacquers are obtained. EP 2 590 801 B1 discloses a polymer material comprising at least 60 wt.% oligomer and / or polymer compound, 2 to 40 wt.% reactive diluent, 0.05 to 10 wt.% hydrophobic additives, and 0 to 5 wt.% photoinitiator.

[0006] EP 1 923 406 A1 discloses resin compositions curable by visible light comprising a phosphine oxide compound.

[0007] M. Leitgeb et al., Multilength Scale Patterning of Functional Layers by Roll-to-Roll Ultraviolet-Light-Assisted Nanoimprint Lithography, ACSNANO, Vol. 10, No. 5, April 5, 2016, describes an R2R-UV-NiL embossing process.

[0008] EP 2 848 656 A1 describes the shielding of metal substrates, e.g. aircraft components, by means of curable compositions comprising urethane diacrylate oligomers, reactive diluents and photoinitiator.

[0009] EP 2 502 942 A1 discloses the use of a double-curing polymerizable composition comprising at least one monomer, comprising at least one (meth)acrylate and a combination of at least one photoinitiator and a heat-curing initiator as an embedding material in the material testing of a substrate or in metallography.

[0010] US 2011 / 049089 A1 discloses a UV-curable resin material for pattern transfer, which contains isobornyl acrylate, an acrylate with a fluorine backbone, a polyfunctional acrylate and a polymerization initiator.

[0011] US 2013 / 040073 A1 describes the production of microstructured silicone surfaces.

[0012] Normally, the proportion of reactive diluent in the prepolymer compositions is low, since a higher proportion of diluent usually leads to greater brittleness of the embossing lacquers and therefore the embossing lacquers are not suitable for all areas of application, especially not for outdoor applications.

[0013] The present invention therefore aims to provide an embossing lacquer composition and a microstructured surface that, due to the lower viscosity of the prepolymer composition, enables simple coating while still exhibiting low embrittlement. In particular, (UV-)NIL embossing lacquers are to be made possible for weather-resistant and abrasion-resistant riblet surfaces, for example, riblet films, for reducing the flow friction of commercial aircraft.

[0014] The invention solves this problem by means of an aircraft component comprising a microstructured surface with a riblet structure consisting of a hardened embossing lacquer composition, which consists of a prepolymer composition comprising: a) 1 to 59.5 wt.% of at least one oligomer and / or polymer compound, b) more than 40 wt.% of one or more reactive diluents selected from (meth)acrylate monomers, wherein the reactive diluent is thiol-free, and c) at least one photoinitiator.

[0015] Further preferred embodiments are described in claims 2 to 8.

[0016] First, some terms used in connection with the invention will be explained.

[0017] The term embossing lacquer refers to a composition that can be applied to a substrate surface using a suitable device and cured there to form a microstructured surface, in particular a riblet-structured surface. A prepolymer composition is understood to be a type of intermediate product based on monomers, oligomers, and / or polymers, which only forms the fully polymerized end product through curing (i.e., polymerization).

[0018] The embossing lacquer is preferably cured by UV radiation. However, all other curing mechanisms known to those skilled in the art can also be used. If necessary, thermal curing can also be added, particularly in so-called dual-cure systems.

[0019] The desired microstructured surface is created in the already applied embossing lacquer before or during its curing. The embossing lacquers are suitable for use in roll-to-roll, roll-to-sheet, or sheet-to-sheet processes. They are particularly preferred for use in roll-to-roll UV-NIL processes and exhibit very good molding properties.

[0020] According to the invention, it is preferred that the prepolymer composition comprises 41 to 53 wt.%, preferably 47 wt.%, of at least one oligomer and / or polymer compound.

[0021] According to the invention, it is further preferred that the prepolymer composition comprises 41 to 53 wt.%, preferably 47 wt.%, of one or more reactive diluents selected from (meth)acrylate monomers.

[0022] According to the invention, it is preferably provided that the mixing ratio of oligomer and / or polymer compound to reactive diluent is 1:1 in the prepolymer composition.

[0023] Within the scope of the present invention, the term (meth)acrylate monomer is understood to mean either a methacrylate monomer or an acrylate monomer. Preferably, the (meth)acrylate monomers are the only reactive diluent in the prepolymer composition. This means that, in particular, no other types of reactive diluents, such as thiol-containing monomers or other monomers serving merely as diluents, are used in the prepolymer composition. Thiol-containing monomers are understood to be monomeric compounds that have at least one thiol group (-SH) as a functional group. The storage stability of thiol-containing UV lacquers is very limited. Thiol-containing lacquers often cure within hours to days, even in the dark. Therefore, it is generally necessary to mix the lacquers only shortly before processing, which is usually undesirable. Furthermore, such lacquers have a strong odor.

[0024] In a preferred embodiment of the invention, one or more (meth)acrylate monomers are selected from the group consisting of hexanediol diacrylate (HDDA), isobornyl acrylate (IBOA), norbonyl acrylate, decanediol diacrylate (DDDA), (2-ethoxyethoxy)ethyl acrylate (EOEOA), phenoxyethyl acrylate (PEA), cyclohexyl acrylate, 2-ethylhexyl acrylate, tetrahydrofuran acrylate and ethoxylated trimethylolpropane triacrylate (TMP(EO)xTA).

[0025] Furthermore, according to the invention, it is preferred that the at least one oligomer and / or polymer compound is selected from the group consisting of polyurethane acrylates, polyacrylates, epoxy acrylates, silicone acrylates, polyether acrylates, and mixtures thereof. In a particularly preferred embodiment, the at least one oligomer and / or polymer compound is a urethane acrylate oligomer. Commercially available oligomer and / or polymer compounds according to the invention are, for example, Ebecryl 8402, Ebecryl 4858, Ebecryl 4680 (Allnex), as well as Miramer PU2200 and Miramer PU2100 (Miwon).

[0026] In an advantageous embodiment, the oligomer and / or polymer compound also has reactive (meth)acrylate groups.

[0027] In another advantageous embodiment, the oligomer and / or polymer compound is acrylic-functionalized.

[0028] Preferably, at least one photoinitiator is selected from the group consisting of (alkyl)-benzoyl-phenyl phosphine oxides, 1-hydroxyalkylphenyl ketones, 2,2-dimethoxy-1,2-diphenylethan-1-one, thioxanthones, ketosulfones, and mixtures thereof. The photoinitiators are preferably used in a weight fraction of approximately 3% by weight in the prepolymer composition. Commercially available photoinitiators according to the invention include, for example, Omnirad TPO-L (IGM), Genocure TPO (Rahn), Genocure LTM (Rahn), and Omnirad 819 (IGM).

[0029] According to the invention, it is preferred that further components of the prepolymer composition are selected independently of one another from: d) UV absorbers, e) radical scavengers and f) non-stick additives.

[0030] In a preferred embodiment, the components photoinitiator, UV absorber, radical scavenger and / or anti-stick additive are included in the overall composition of the prepolymer composition with a weight fraction of 0.1 to 18 wt.%, preferably 0.5 to 7 wt.%, more preferably about 6 wt.%.

[0031] According to the invention, it is preferably provided that the UV absorbers are selected from organic and inorganic UV absorbers. Preferred organic UV absorbers are hydroxyphenyltriazines (HTP), hydroxybenzotriazoles (HBT), and mixtures thereof. The organic UV absorbers are preferably used in a weight fraction of approximately 2 wt% in the prepolymer composition. Commercially available organic UV absorbers according to the invention are, for example, Tinuvin 479 (BASF), Tinuvin 400 (BASF), Tinuvin 328 (BASF), Tinuvin 384-2 (BASF), Tinuvin 982 (BASF), and Tinuvin 1130 (BASF). Preferred inorganic UV absorbers are nanoparticles of TiO₂, ZrO₂, ZnO, CeO₂, and mixtures thereof.

[0032] The addition of UV absorbers significantly increases the UV stability of the embossing lacquers, even with small amounts, preferably around 1 wt%. This allows the embossing lacquers to withstand the extraordinary stress of UV radiation at commercial flight altitudes of over 10,000 meters over the long term. The addition of inorganic UV absorbers, and in particular UV-absorbing inorganic nanoparticles, also offers the advantage of greater UV stability and thus a longer service life. Due to their hardness, they can also further increase scratch resistance and abrasion resistance.

[0033] In a preferred embodiment, the prepolymer composition further comprises at least one radical scavenger. In a preferred variant, the radical scavenger is a HALS (hindered amine light stabilizer). Preferred HALS are bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate, methyl-(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate, bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl)sebacate, 2,4-bis[N-butyl-N[1-cyclohexyloxy-2,2,6,6-tetramethylpiperidin-4-yl)amino]-6-(2-hydroxyethylamine)-1,3,4-triazine, and mixtures thereof. HALS are preferably used in a weight fraction of approximately 1 wt.%. Commercially available HALS according to the invention are, for example, Tinuvin 292 (BASF), CHISORB 292 (Bouble Bond Chemical Ind., Co., Ltd.), Tinuvin 123 (BASF) and Tinuvin 152 (BASF).

[0034] Preferred non-stick additives are selected from the group consisting of alkyl (meth)acrylates, polysiloxane (meth)acrylates, perfluoroalkyl (meth)acrylates, perfluoropolyether (meth)acrylates, alkyl vinyl ethers, polysiloxane vinyl ethers, perfluoroalkyl vinyl ethers and perfluoropolyether vinyl ethers.

[0035] Normally, a higher proportion of reactive diluents leads to greater brittleness of the cured embossing lacquers, so that these are generally not suitable for outdoor applications and especially for the formation of riblet structures.

[0036] Surprisingly, the embossing lacquer composition, with an increased proportion of (meth)acrylate monomer as a reactive diluent and a low proportion of oligomer and / or polymer compounds, meets the exceptionally high requirements for the production of riblet-structured surfaces for commercial aircraft. In this way, UV-NIL embossing lacquers and corresponding microstructured surfaces with excellent weathering stability and abrasion resistance can be obtained, offering high application potential for reducing flow friction in aviation.

[0037] Microstructured surfaces exhibit a structure or topography with raised or recessed areas whose spacing and depth / height are in the submillimeter range. Preferably, the spacing and depth are in the range of 0.5 to 100 µm, more preferably 0.5 to 60 µm. Within the scope of the invention, a microstructured surface is a surface with a riblet structure. A riblet is defined as a surface geometry that reduces frictional resistance on surfaces subjected to turbulent flow in a specific direction.

[0038] The invention will now be described by way of example using some advantageous embodiments.

[0039] Embossing lacquer compositions were prepared, consisting of a prepolymer composition containing equal proportions (1:1 mixing ratio) of oligomer and / or polymer compound to (meth)acrylate monomer as a reactive diluent. In the examples presented, 47 wt% of the oligomer and / or polymer compound and 47 wt% of the (meth)acrylate as the sole reactive diluent were used. Table 1 shows the oligomer and / or polymer compounds and (meth)acrylate monomers used as reactive diluents in the examples. Table 1: Use of different oligomer and / or polymer compounds and reactive diluents. Example number 47 wt% oligomer and / or polymer compound 47% w / w reactive diluent 1 E4858 HDDA 2 E4858 IBOA 3 E4858 Cyclohexyl acrylate 4 E4858 2-Ethylhexyl acrylate 5 E4858 THF acrylate 6 E8402 HDDA 7 E8402 IBOA 8 E8402 Cyclohexyl acrylate 9 E8402 2-Ethylhexyl acrylate 10 E8402 THF acrylate 11 E4680 HDDA 12 E4680 IBOA 13 E4680 Cyclohexyl acrylate 14 E4680 2-Ethylhexyl acrylate 15 E4680 THF acrylate 16 PU2100 HDDA 17 PU2100 IBOA 18 PU2100 Cyclohexyl acrylate 19 PU2100 2-Ethylhexyl acrylate 20 PU2200 THF acrylate 21 PU2200 HDDA 22 PU2200 IBOA 23 PU2200 Cyclohexyl acrylate 24 PU2200 2-Ethylhexyl acrylate 25 PU2200 THF acrylate

[0040] The commercially available compounds Ebecryl 8402 and Ebecryl 4858 (Allnex) are aliphatic urethane diacrylates. Ebecryl 4680 (Allnex) is an aliphatic urethane diacrylate further diluted with 1,6-hexanediol diacrylate (HDDA). The commercially available Miramer PU2100 and PU2200 (Miwon) are urethane diacrylates.

[0041] In the example compositions, the remaining 6 wt% of the prepolymer composition consists of photoinitiator (3 wt% TPO-L), UV absorber (2 wt% Tinuvin 479), and radical scavenger (1 wt% Tinuvin 292). The addition of anti-stick additives is also possible.

[0042] Surprisingly, despite high proportions of reactive diluents, the example formulations exhibit remarkably low brittleness, making the prepolymer compositions suitable as embossing lacquers for riblet structures. The higher dilution also facilitates easier coating of the embossing lacquers onto the substrate film using R2R-UV-NIL embossing processes due to the lower viscosity. The resulting microstructured surfaces are characterized by good weathering and abrasion resistance and are therefore particularly well-suited for outdoor applications.

Claims

1. Aircraft component including a microstructured surface having a riblet structure consisting of a hardened embossing varnish composition, the embossing varnish composition consisting of a prepolymer composition comprising: a) 1% to 59.5% by weight of at least one oligomeric compound and / or polymeric compound, b) more than 40% by weight of one or more reactive diluents selected from (meth)acrylate monomers, the reactive diluent being thiol-free, and c) at least one photoinitiator.

2. Aircraft component according to Claim 1, characterized in that the prepolymer composition comprises 41% to 53% by weight, preferably 47% by weight, of the at least one oligomeric compound and / or polymeric compound.

3. Aircraft component according to Claim 1 or 2, characterized in that the prepolymer composition comprises 41% to 53% by weight, preferably 47% by weight, of the one or more reactive diluents selected from (meth)acrylate monomers.

4. Aircraft component according to any of Claims 1 to 3, characterized in that the mixing ratio of oligomeric compound and / or polymeric compound to reactive diluents in per cent by weight is 1:1 in the prepolymer composition.

5. Aircraft component according to any of Claims 1 to 4, characterized in that the (meth)acrylate monomers are selected from the group consisting of hexanediol diacrylate, isobornyl acrylate, norbornyl acrylate, decanediol diacrylate, (2-ethoxyethoxy)ethyl acrylate, phenoxyethyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, tetrahydrofuran acrylate and trimethylolpropane triacrylate.

6. Aircraft component according to any of Claims 1 to 5, characterized in that the at least one oligomeric compound and / or polymeric compound is selected from the group consisting of polyurethanes, polyacrylates, epoxy acrylates, silicone acrylates, polyether acrylates and mixtures thereof.

7. Aircraft component according to any of Claims 1 to 6, characterized in that further constituents of the prepolymer composition are selected independently from: d) UV absorbers, e) free-radical scavengers and f) non-stick additives.

8. Aircraft component according to Claim 7, characterized in that the photoinitiator, UV absorber, radical scavenger and / or non-stick additive constituents are present in the overall composition of the prepolymer composition with a total proportion by weight of 0.1% to 18% by weight, preferably 0.5% to 7% by weight, more preferably 6% by weight.