EP3914444A1

EP3914444A1 - Riblet film and method for the production thereof

Info

Publication number: EP3914444A1
Application number: EP20703385.3A
Authority: EP
Inventors: Kai-Christoph Pfingsten; Andreas Seul
Original assignee: Lufthansa Technik AG
Current assignee: Lufthansa Technik AG
Priority date: 2019-01-22
Filing date: 2020-01-22
Publication date: 2021-12-01
Also published as: CA3126609A1; WO2020151907A1; AU2020211652A1; CN113646160A; BR112021013849A2; US20220040908A1; AU2020211652B2; DE102019101485A1

Abstract

The invention relates to a riblet film (1) and to a method for the production thereof. The riblet film (1) with a riblet structure (6) on a first side of the film (1) and a fastening surface on the second side of the film (1) is formed from cured embossing lacquer (3), in which a sheet-like textile (4) is embedded and into which the riblet structure (6) is stamped. The production of such a riblet film (1) is performed by the following steps: a) surrounding a sheet-like textile (4) with embossing lacquer (3); b) stamping the riblet structure (6) into the embossing lacquer (3); and c) curing the embossing lacquer (3).

Description

Riblet film and process for its manufacture

The invention relates to a riblet film and a method for its production.

Riblet structures are known from the prior art. Corresponding riblet structures are microstructuring of surfaces in order to reduce the frictional resistance for fluids flowing along the surface - the so-called wall friction. Corresponding structures are fundamentally interesting for those applications in which the wall friction makes up a not insignificant part of the overall flow-related resistance. Riblet structures can be found, for example, on the outer surfaces of aircraft, but also on the rotor blades of wind turbines, on the outer skin of ships or on the outer surfaces of high-speed trains. Such structures are also used, for example, on inner surfaces of pipelines.

Corresponding riblet structures can therefore reduce the wall friction and thus also the overall air resistance of an aircraft in flight, which u. a. leads to lower fuel costs. The same applies to other applications of Riblet structures.

Various possibilities are known in the prior art of how riblet structures can be applied to already existing structures, such as, for example, the painted outer skin of aircraft.

In a first variant, the riblet structure is applied as an additional layer of lacquer to an already existing aircraft lacquer layer, a so-called embossing lacquer being used as the lacquer, in which one is used before or during curing

CONFIRMATION COPY microstructured surface is embossed to form the desired riblets.

A disadvantage of this prior art is that the riblet layer - for example in the event of damage from impinging particles such as grains of sand, hail, etc. - cannot be removed from the aircraft lacquer layer on a regular basis in a non-destructive manner, so that a new riblet structure is fundamentally full constant repainting of the aircraft in this area is required.

Alternatively, it is known to apply the embossing lacquer to an intermediate film and to structure and cure it there before the riblet film thus created is glued onto the outer skin of an aircraft, for example.

Even if this film can be removed from the aircraft paint regularly without damaging it, a riblet structure created in this way is disadvantageous due to the comparatively higher weight provided by the intermediate film. The connection between the intermediate film and the embossing lacquer layer is also often problematic, since the embossing lacquer should on the one hand be firmly connected to the intermediate film, but at the same time it must not adhere to the embossing tool, which also interacts with the embossing lacquer, in order to ensure the best possible microstructuring.

In order to reduce the problem of the connection between the embossing lacquer and the intermediate foil, it is known to provide an additional layer between the embossing lacquer and the intermediate foil, for example an adhesive layer, which on the one hand binds firmly to the intermediate foil and to which the embossing lacquer adheres well. However, such a riblet film also has a comparatively high weight. The object of the invention is to create a riblet film and a method for its production, in which the disadvantages known from the prior art are avoided or at least reduced. This object is achieved by a riblet film according to the main claim, and a manufacturing method according to claim 7. Advantageous further developments are the subject of the dependent claims.

Accordingly, the invention relates to a riblet film with a riblet structure on a first side of the film and a fastening surface on the second side of the film, the riblet film being formed from hardened embossing lacquer in which a sheet-like textile is embedded and in which the riblet structure is embossed. The method according to the invention for producing a riblet film according to the invention comprises the steps: a) surrounding a flat textile with embossing lacquer; b) embossing the riblet structure into the embossing lacquer; and c) curing the embossing lacquer. First, some of the handles used in the invention will be explained.

A “riblet structure” denotes a microstructured surface which is designed to reduce the wall friction in a flow along the surface compared to a corresponding surface without a riblet structure. The riblet structure here comprises arranged on a base surface Surveys, the mutual distance and / or height each if less than 1 mm, often even less than 200 ym if is not even less than 60 ym. The shape and arrangement of the elevations is adapted to the expected flow and, in particular, is designed for a main flow direction, the elevations generally being elongated in the direction of the main flow direction. In the case of a flight, the main flow direction over the outer wall, which varies locally over the entire outer wall of an aircraft, results, for example, from the flow around the aircraft during the flight.

The term “embossing lacquer” denotes a composition that can be applied to a surface by means of a suitable device and cured to form a microstructured surface, in particular a riblet-structured surface. Corresponding embossing lacquers are known in the prior art.

The embossing lacquer is preferably cured by means of UV radiation. However, all other curing mechanisms known to those skilled in the art can also be used. If necessary, thermal curing can also occur, in particular with so-called Dua1-Cure systems.

The desired microstructured surface on the embossing lacquer is generated before or during its curing by a suitable embossing element which has the negative shape of the desired surface structure. The embossing element has an embossing surface to which the embossing lacquer does not adhere or only adheres to a negligible extent.

The invention is based on the finding that known embossing lacquers are in principle suitable for being processed into a film-like element which can then be compared to the riblet films known from the prior art with an intermediate film on a surface, such as in particular the painted outer skin of an aircraft, applied, in particular can be glued on. In order to achieve a sufficient strength of the riblet film, the invention provides for a sheet-like textile to be embedded in the riblet film formed from embossing lacquer. The internal sheet-like textile increases the strength and stiffness of the riblet film, which makes handling easier when applying the film compared to a theoretical riblet film without an internal sheet-like textile. At the same time, thanks to the sheet-like textile on the inside, the riblet film can regularly, if necessary - for example because the riblet structure is damaged - also get back from the surface on which it is attached with adhesive, for example over a large area is, can be solved. Without the appropriate sheet-like textile on the inside, it would be difficult and difficult to remove a riblet film from embossing lacquer, since the riblet film would almost inevitably tear into small parts when attempting to remove it due to the material properties of embossing lacquers.

Through the use of a sheet-like textile according to the invention, which is integrated into the riblet film made of embossing lacquer, a weight advantage can regularly be achieved compared to the known embodiments in which the embossing lacquer is applied to a continuous intermediate film, even if this is of course the case with the material and the design of the flat textile is dependent. The person skilled in the art is fundamentally free to design the textile, but the material must be selected such that it combines well with the matrix consisting of the embossing lacquer.

It is preferred if an adhesive layer is applied to the fastening surface of the riblet film. The riblet film can then be compared to any other adhesive film process and apply in particular on a surface. The adhesive layer preferably allows a firmly bonded material connection for the loads to be expected on the riblet film, which, however, can also be achieved by pulling off the film with a force component perpendicular to the surface to which the riblet film is applied can be released again. The latter removal of the riblet film is actually possible through the embedded sheet-like textile provided according to the invention.

Preferably, the sheet-like textile is a woven or knitted fabric formed from threads, the crossing threads preferably being approximately at right angles to one another and / or the distance between two adjacent, parallel threads running at least five times the thread diameter. Even if other thread arrangements are of course possible, the preferred right-angled arrangement of the crossing threads enables simple and inexpensive production of the textile. Regardless of the final arrangement of the individual threads to each other, a large distance between the individual threads running parallel to the thread diameter offers weight advantages. With a suitable choice of material for the textile, the desired strength of the riblet film can still be easily achieved.

It is preferred if the distance between the second side of the film and the embedded sheet-like textile is at least 5 μm and / or the distance between the base surface of the riblet structure on the first side of the film and the embedded sheet-like textile is at least 15 μm .

The appropriate distances ensure on the one hand that the flat textile is completely embedded in the embossing lacquer. On the other hand, it ensures that the embedded sheet-like textile conforms to the shape of the The surface of the riblet film is not impaired on either side.

It is preferred if the total thickness of the riblet film is between 30 mih and 120 mih, preferably between 50 mih and dqmpi. It has been shown that correspondingly thin riblet films according to the invention can be easily produced, are easy to handle for attachment to surfaces and can also be easily removed from the surfaces.

The riblet film is cut to, preferably square, patches with a size between 0.3 x 0.3 m and 1.0 x 1.0 m. Corresponding patches can be handled well, for example, when equipping the outer surface of aircraft.

To produce a riblet film according to the invention, the sheet-like textile is introduced into the sen before the embossing lacquer hardens. The subsequent complete hardening of the embossing lacquer, which is also necessary in particular for fixing the riblet structure, then creates a composite of sheet-like textile and embossing lacquer.

In order to be able to ensure a desired position of the flat textile in the riblet film, it is preferred if, before the final introduction of the flat textile into the embossing lacquer - that is, the step in which the textile is enclosed by the embossing lacquer - first a base layer made of embossing lacquer is produced, on which the flat textile is placed, before the textile is completely enclosed by the embossing lacquer with the help of further embossing lacquer to be applied thereon. By providing a base layer, a desired distance of the sheet-like textile from the side of the final finite riblet film formed by the base layer can be ensured. The base layer can preferably be at least partially cured before the sheet-like textile is placed on it. However, it must be ensured that the base layer also connects sufficiently well with the subsequently applied and hardened embossing lacquer in such a case.

It is preferred if, in order to produce the riblet film, the embossing lacquer is first applied to a substrate to which the embossing lacquer does not adhere or only adheres little, at least in the cured state, so that the cured rib let film can be detached from the substrate without destruction. The embossing lacquer is often flowable in the uncured state, which is why it cannot regularly be shaped into a self-supporting film in this state. By using a substrate in the manufacture, from which the Riblet

If the foil can be loosened without damage after the embossing lacquer has hardened, the riblet foil according to the invention can easily be produced. The material that can be used for the substrate is basically the same material that is used for the tool for microstructuring the surface to create the riblet structure. Suitable materials for the different embossing lacquers are known to the person skilled in the art from the prior art.

It is also possible for the substrate in question to be in the form of a film, so that the production of the riblet film according to the invention is in principle also possible on a roll-to-roll basis, in which an embossed lacquer layer on the film-like substrate, possibly with an already integrated one sheet-like textile is applied. If the film-like substrate with embossing lacquer is unwound from a first roll, the sheet-like textile can, if not yet embedded, possibly be introduced into the not yet hardened embossing lacquer and then the surface structured and the embossing lacquer hardened in a known manner before this film-like Substrate with the hardened embossing lacquer which then forms the riblet film is wound onto a second roll.

It is possible that the embossing of the riblet structure into the embossing lacquer and the hardening of the embossing lacquer take place at the same time. For UV-hardening embossing lacquers, for example, the NIL-UV method is known, in which a nanoimprint lithography (NIL) and an ultraviolet are known Radiation are combined.

As already explained above, it is preferred, after completion of the riblet film, to apply an adhesive layer to the side facing away from the riblet structure, which results in an adhesive film.

The invention will now be further described with reference to the accompanying drawings. Show it:

Figure la-e: an embodiment of an inventive

Manufacturing process for a riblet film according to the invention;

Figure 2: the use of the manufactured according to Figure 1

Riblet film; and

3a-d: exemplary representations of possible configurations for the sheet-like textile used in the method according to FIG.

A method according to the invention for producing a riblet film 1 according to the invention (see FIG. 1e) is shown in FIG. The figures la-e each show sectional views.

At the beginning of the method, a 5 μm thick base layer 2 made of an embossing lacquer 3 is formed on a substrate 20 (FIG. 1 a). Not at this stage of the process, at least but not yet fully hardened embossing lacquer 3 is highly viscous at least at room temperature and adheres to the substrate 20 in such a way that the substrate 20 can basically be rolled up together with the base layer 2.

In the step shown in FIG. 1b, a flat textile 4 is placed on the base layer 2. The flat textile 4 is a mesh-like fabric made of threads 5 arranged approximately at right angles to one another, the spacing between two adjacent, parallel threads 5 being approximately five times the thread diameter. The plan view of the flat textile 4 used in FIG. 1 is sketched by way of example in FIG. 3a.

Subsequently, additional embossing lacquer 3 is applied, so that the flat textile 4 previously lying on the base layer 2 is completely surrounded by the embossing lacquer 3 and thus embedded in it (FIG. 1). The embossing lacquer 3 has not yet hardened even at this stage of the process.

The curing of the embossing lacquer 3 (characterized by the changed hatching) takes place together with the embossing of the riblet structure 6 on the side remote from the substrate 20 (FIG. 1d). The methods known from the prior art can be used for embossing and curing. In particular, embossing and curing can take place simultaneously. Suitable methods for this are also known from the prior art. By curing the embossing lacquer 3, he riblet film 1 according to the invention is created.

The amount of the embossing lacquer 3 applied in the method step outlined in FIG. 1c is selected such that the total thickness of the layer of embossing lacquer 3 after embossing and curing is between 30 pm and 120 pm, preferably between 50 pm and 80 mi ^h . The distance a between the base surface of the riblet structure 6 and the flat textile 4 is at least 15 pm, while the distance b between the flat textile 4 and the contact surface to the substrate 20 - which is set by the choice of the thickness of the base layer 2 can (cf. Fi gur la) - 5 pm or more.

The material of the substrate 20 is selected such that the riblet film 1 can be detached from it without destruction at least after the embossing lacquer 3 has completely hardened. The then accessible side of the riblet film 1 serves as a fastening surface and is provided with an adhesive layer 7, with which the rib let film 1 becomes a self-adhesive film.

The riblet film 1 described according to FIGS. La-e can be cut into patches in the sizes 0.3 x 0.3 m and 1.0 x 1.0 m.

In FIG. 2, the riblet film 1 from FIG. 1 is glued to a painted outer wall 10 of an aircraft with the aid of the adhesive layer 7, the arrangement and orientation of the riblet film 1 being selected in accordance with the prior art in order to prevent air friction in the Reduce cruise.

Various possible configurations of the sheet-like textile 4 are outlined in FIGS. 3a-d.

In Figure 3a, the individual threads 5 of the textile 4 are arranged at right angles to each other. In the exemplary embodiments according to FIGS. 3b and 3c there are three or four main directions for the individual threads 5. The flat textile 4 according to FIG. 3b is knitted. In all exemplary embodiments according to FIG. 3, the distance between adjacent, parallel threads 5 is at least five times as large as the diameter of the threads 5.

Claims

Expectations

1. Riblet film (1) with a riblet structure (6) on a first side of the film (1) and a fastening surface on the second side of the film (1),

characterized in that

the riblet film (1) is made of hardened embossing lacquer (3), in which a sheet-like textile (4) is embedded and in which the riblet structure (6) is embossed.

2. Riblet film according to claim 1,

characterized in that

on the mounting surface an adhesive layer (7) is brought up.

3. Riblet film according to one of the preceding claims,

characterized in that

the sheet-like textile (4) a woven or knitted fabric is formed from threads (5), the threads (5) preferably being approximately at right angles to one another and / or the distance between two adjacent, parallel threads (5) at least the five - times the thread diameter.

4. Riblet film according to one of the preceding claims,

characterized in that

the distance (b) between the second side of the film (1) and the embedded sheet-like textile (4) is at least 5 gm and / or the distance (a) between the base surface of the riblet structure (6) on the first side of the film ( 1) and the embedded sheet-like textile (4) is at least 15 gm.

5. Riblet film according to one of the preceding claims,

characterized in that the total thickness of the riblet film (1) is between 30 mΐh and 120 mih, preferably between 50 mp \ and 80 mih.

6. Riblet film according to one of the preceding claims,

characterized in that

the riblet film (1) is cut to size, preferably square, with a size between 0.3 x 0.3 m and 1.0 x 1.0 m.

7. A method for producing a riblet film (1) according to one of the preceding claims, comprising the steps of: a) surrounding a flat textile (4) with embossing lacquer (3); b) embossing the riblet structure (6) into the embossing lacquer (3); and c) curing the embossing lacquer (3).

8. The method according to claim 7,

characterized in that

the following steps are carried out before step (a):

- Production of a base layer (2) from embossing lacquer (3);

- Preferably at least partially curing the base layer (2); and

- Placing the sheet-like textile (4) on the uncured or at least partially cured base layer

(2).

9. The method according to any one of claims 7 or 8,

characterized in that Applying the embossing lacquer (3) to a substrate (20) follows, on which the embossing lacquer (3) does not adhere, or at least only slightly enough, at least in the hardened state, that the hardened riblet film (1) is detached from the substrate (20) in a non-destructive manner can.

10. The method according to any one of claims 7 to 9,

characterized in that

steps (b) and (c) take place simultaneously.

11. The method according to any one of claims 7 to 10,

characterized in that

after completion of the riblet film (1), an adhesive layer (7) is applied to the side facing away from the riblet structure (6).