DE102020108236B4 - Method for joining two parts, adhesive film and use of an adhesive film - Google Patents

Abstract

Method for joining two joining parts (11, 12), in which the joining parts (11, 12) are adhesively connected to one another by means of an adhesive material (13) provided between the joining parts (11, 12), the method comprising the following steps:
- Providing a first joining part (11) and at least one second joining part (12);
- Providing an adhesive material (13) which contains at least one open-pore ventilation textile (14a, 14b) and is provided integrally with the at least one ventilation textile (14a, 14b) in the form of an adhesive film (20);
- introducing the adhesive material (13) together with the open-pore ventilation textile (14a, 14b) between the joining parts (11, 12) in a predetermined joining zone, so that the ventilation textile (14a, 14b) touches the respective joining part (11, 12) with one surface side, wherein the joining zone is tempered at least in a partial area before the surface pressure is applied and subsequently a pressure is applied locally to the joining zone to fix the position;
- creating a vacuum structure (10) such that at least the joining zone with the open-pore ventilation textile (14a, 14b) is sealed in a vacuum-tight manner under the vacuum structure (10);
- evacuating the joining zone under the vacuum structure (10) including the open-pore ventilation textile (14a, 14b) by means of a vacuum pump; and
- Carrying out a curing process for curing the adhesive material (13) introduced between the joining parts (11, 12), wherein, in order to carry out the curing process, the adhesive material (13) is tempered by means of a tempering device only after the at least one open-pore ventilation textile (14a, 14b) has been completely evacuated.

Description

The present invention relates to a method for joining two parts to be joined, in which the parts to be joined are adhesively bonded to one another by means of an adhesive material provided between the parts to be joined. The invention also relates to a film adhesive for this purpose.

Due to their weight-specific strength and stiffness, fiber composites have become indispensable in the manufacture of any component. Fiber composites consist primarily of two essential components: a fiber material and a matrix material that embeds the fiber material. In addition, such fiber composites can contain other components that may be application-specific. When manufacturing fiber composite components from a fiber composite, the fiber material is usually formed into the desired component shape, and then the matrix material that embeds the fiber material is cured. In the vast majority of cases, curing occurs through the application of heat and, if necessary, pressure. Curing forces the load-bearing fibers of the fiber material into their predetermined direction and, together with the cured matrix material, forms an integral load-bearing unit.

Adhesive bonding represents a joining process with particular potential for lightweight construction, not only in the manufacture of complex fiber composite components. With the help of an adhesive bond, two components are firmly and generally irreversibly connected at their respective joining surfaces by means of cohesion and/or adhesion. Further mechanical fastening elements can only be dispensed with if the failure of the adhesive bond is irrelevant to the operation of the underlying technical device. Validating the strength and durability of adhesive bonds represents a core problem of adhesive bonding techniques, as incorrect or inadequate surface treatments, over-aged or improperly processed adhesives, and contamination can significantly reduce bond strength. Particularly in the aviation sector, when manufacturing aircraft, particularly from fiber composite materials, where structural components are also manufactured from fiber composite components using adhesive bonds, it is essential that the adhesive bonds have strength and durability, since in the event of damage to such an adhesive bond, safe flight operations can no longer be guaranteed.

From the DE 10 2013 107 849 A1 A method is known for producing a fiber composite structure from at least two separately manufactured fiber composite components that are to be joined together by means of adhesive bonding as a joining process. Two different adhesive systems are used here, which are introduced into the adhesive bond in such a way that separate adhesive zones are created from the first adhesive system and the second adhesive system. In the solidified state of the second adhesive system, it has a higher fracture toughness than the first adhesive system, whereby the adhesive seam within the adhesive bond has different properties. In the area with low fracture toughness and high strength (first adhesive system), cracks that develop can only propagate into the areas of the second adhesive zone, where the adhesive seam has a high fracture toughness (tough-elastic) but reduced strength and stiffness. Cracks that develop in the adhesive seam can thus be effectively limited to a local area.

From the DE 10 2017 113 432 A1 A film adhesive for adhesively joining at least two joining partners is known, wherein the adhesive is applied in film form to a textile, flat carrier material. The textile, flat carrier material has at least a first section that borders on at least a second section of the carrier material, wherein the carrier material of the first section is materially, chemically and/or geometrically modified compared to the carrier material of the second section in such a way that, after the adhesive has cured, the adhesive bond in the region of the first section has a mechanical property with regard to elongation at break or fracture toughness that is different from the mechanical property of the adhesive bond in the region of the second section.

Furthermore, for example, in the WO 02/081189 A1 A method for joining two parts is described, in which an adhesive material is introduced that has a textile on at least one side. The aim is to avoid air pockets by evacuating this textile. The reinforcing material can have a ventilation structure to allow gas to escape from the laminate structure. EP 2 324 979 A1 describes a method for producing a preform using adhesive strips between different layers of fiber material, whereby this primarily involves prefabricated elements. For example, the DE 10 2004 002 618 A1 a process for laminating decorative cut-outs or molded parts made of leather or similar materials, in particular vacuum-assisted laminating onto rigid, elastic or Surfaces designed to be elastic using suitable intermediate supports.

Currently, thermoset matrix systems are predominantly used to manufacture fiber composite components and adhesive bonds in aviation, and these typically require curing at elevated temperatures. During this process step, the component is shielded from the atmosphere by means of a vacuum and often additionally subjected to high pressure (e.g., using an autoclave). This high overpressure leads to a strong compaction of the fiber layers or an adhesive layer or adhesive materials applied between two parts to be joined, and compresses trapped air bubbles.

However, in some applications, the use of additional pressure is not possible or undesirable. In these applications, the component is only subjected to a vacuum during the curing of the matrix material, so that the pressure acting on the component only corresponds to the ambient pressure (1 bar). In these applications, porosity often occurs in the components or the adhesive joint.

The cause of the porosity is air inclusions in the matrix system or the adhesive, which are introduced during the impregnation or lamination process. On the one hand, the trapped air bubbles cannot be sufficiently compressed during the curing process due to the low pressure of one bar. On the other hand, the application of a vacuum to the resin, which is liquid at elevated temperature, causes resin flow and, with it, an expansion of existing air inclusions, so that the pores become even larger. When pre-formulated adhesive materials are used, such as film adhesives, air inclusions often arise as a result of the lamination process itself. In this process, the impregnated layers are applied layer by layer, with large air bubbles being removed by rolling or pressing. Often, however, not all air inclusions can be removed and are further trapped by the pressing.

After subsequent curing, voids form in the adhesive joint or seam within the cured adhesive, compromising the stability and strength of the bond. When structurally critical components, particularly in the aerospace industry, are to be bonded together, a reduction in the structural mechanical properties must be avoided. However, at present, especially when using pre-assembled adhesive materials such as film adhesives, it is not possible to reliably guarantee that air inclusions can be prevented during the application of the film adhesive.

It is therefore an object of the present invention to provide an improved method for joining two joining parts, in particular two joining parts made of a fiber composite material, in which the formation of structure-weakening cavities within the adhesive bond due to air inclusions can be reliably prevented.

The object is achieved according to the invention with the method according to claim 1. Advantageous embodiments of the invention can be found in the corresponding subclaims.

According to claim 1, a method for joining two joining parts is proposed, in which the joining parts are to be adhesively connected (joined) to one another by means of an adhesive material provided between the joining parts. The two joining parts can, for example, be fiber composite components formed from a fiber composite material, wherein the fiber composite components can be in an already cured state. It is of course also conceivable that one of the two joining parts or both joining parts are not yet cured, i.e., that the matrix material embedding the fiber material is not yet fully polymerized.

First, the two joining parts are prepared accordingly. The same applies to the adhesive material intended for use. This can, for example, be a matrix material that is also used to manufacture fiber composite components. In particular, this can be the matrix material from which the joining parts were formed, provided they are made of a fiber composite material. Accordingly, the adhesive material can be a matrix material of a fiber composite material. The adhesive materials used can, for example, be thermoplastic or thermosetting adhesives.

The adhesive material is provided in such a way that the adhesive material has at least one open-pore venting textile. The open-pore venting textile is an integral component of the adhesive material. The adhesive material and the open-pore venting textile are provided as a single, integral unit. The open-pore venting textile is at least partially impregnated with or in contact with the adhesive material.

The provided adhesive material, together with the open-pore ventilation textile, is then introduced into a specified joining zone between the joining parts. This can be achieved, for example, by applying the provided adhesive material evenly to a joining surface of one of the two joining parts and then bringing the applied adhesive material into contact with the joining surface of the other joining part. After this introduction of the adhesive material into the respective joining zone, the adhesive material used is present between the two joining parts in the designated joining zone and is intended to bond the two joining parts together by curing the adhesive in the designated joining zone.

According to the invention, the at least one open-pore ventilation textile, which is an integral component of the adhesive material, touches or contacts the respective joining part at its joining surface with a first surface side. The opposite second surface side of the open-pore ventilation textile is wetted or contacted with the adhesive material.

The joining zone, and thus in particular the at least one open-pored venting textile, is then sealed vacuum-tight under a vacuum setup. At one point in the vacuum setup, there is a connection for a vacuum pump in order to be able to evacuate the part enclosed under the vacuum setup, in particular the joining zone. Between the open-pored venting textile under the vacuum setup and the connection for the vacuum pump there is a vacuum bridge which prevents the vacuum cover (e.g. a vacuum film) from being pressed down by the evacuation of the vacuum setup in such a way that the open-pored venting textile can no longer be evacuated and thus further air inclusions are present within the open-pored venting textile. Consequently, the vacuum setup is created in such a way that a vacuum bridge is created under the vacuum setup between a connection of a vacuum pump and the at least one open-pored venting textile, which enables continuous evacuation of the venting textile by means of the vacuum pump. Such a vacuum bridge can, for example, be a thin, non-deformable hose or tube. However, such a vacuum bridge can also be formed by one or more layers of a venting fabric. Flow aids or peel ply fabrics can also be used, as channels always remain between individual fabric filaments (with a round cross-section), through which a vacuum bridge can be ensured.

Subsequently, the joining zone, and in particular the at least one open-pore venting textile, is evacuated using the connected vacuum pump. This creates a vacuum between the first surface of the respective joining part and the adhesive material, where the open-pore venting textile is located, so that no air pockets can form during the subsequent curing of the adhesive material.

The vacuum setup can be implemented in such a way that it either seals off a mold, thereby completely enclosing both parts to be joined under the vacuum. It is also conceivable, however, that the vacuum setup is sealed off from one of the two parts to be joined, so that at least the other part to be joined is completely enclosed under the vacuum. This is usually advantageous when the first part to be joined is significantly larger than the second part to be joined.

The adhesive material applied between the joining parts is cured together with the open-pore ventilation textile in a curing process.

By providing an open-pore ventilation textile between the joining surface of one of the joining parts and the adhesive material intended for the adhesive bond, it is possible to ensure that trapped air pockets are evacuated during the evacuation of the joining zone between the adhesive material and the joining surface of the joining part, thereby preventing the inclusion of air bubbles. Thus, the method according to the invention for joining two joining parts enables the production of non-porous bonds during curing, in particular without additional pressure application.

During the curing process, the open-pore venting textile is pressed into the adhesive before it fully cures, allowing full contact between the adhesive and the respective joining surface. Since the venting textile is completely evacuated, the cavities formed by the open-pore venting textile can be completely filled by the adhesive without trapping air.

The inventors have recognized that with the help of an open-pore venting textile, the inclusion of air bubbles in the boundary layer between the joining surface of one of the joining parts and the adhesive material can be reliably prevented without significantly impairing the adhesive ability of the adhesive material due to the presence of the open-pore venting textile. Rather, it has been shown that, despite the presence of the open-pore venting textile, a process-reliable bond between the adhesive and the joining surface can still be achieved, allowing the open-pore venting textile to remain in the interface between the adhesive and the joining surface. Furthermore, it has been shown that the presence of the open-pore venting textile allows the venting textile to perform additional functional tasks, such as acting as a spacer or providing mechanical properties in the adhesive bond, such as crack stoppers.

The method according to the invention can be used, in particular, to carry out repair processes when the first joining part is already fully cured and a smaller joining part is to be bonded to this already fully cured first joining part. The vacuum structure is then designed such that the second joining part to be bonded, including the adhesive layer, is completely enclosed under the vacuum structure, and the vacuum structure is then sealed off from the first joining part. This allows an adhesive bond to be created in a repair process in which it can be reliably guaranteed that no air inclusions or pore formation occur in the boundary layer between the joining surface and the adhesive material.

The advantage of the invention lies precisely in repair processes in which no additional compaction (> atmospheric pressure) can be achieved by means of a pressure furnace (autoclave).

An open-pore venting textile in the sense of the present invention is understood in particular to mean a venting textile which forms evacuable pores in the boundary layer between the joining surface and the adhesive material, which pores are occupied or filled by the adhesive material in the later course of the process.

According to one embodiment, the adhesive material is provided in such a way that an open-pore ventilation textile is provided both between the adhesive material and the first joining part and between the adhesive material and the second joining part when the adhesive material is introduced into the joining zone. Accordingly, a corresponding evacuation of the interface between the adhesive material and both joining parts is realized by a corresponding open-pore ventilation textile.

According to the invention, the adhesive material is tempered using a tempering device to carry out the curing process. Tempering the adhesive material generally activates it thermally, initiating the chemical crosslinking reaction. In the process, the adhesive material generally becomes softer, so that the cavities formed by the venting textile at the interface between the adhesive material and the respective joining part are completely occupied by the adhesive material. Due to the vacuum created in these cavities of the venting textile, no air is trapped, thus ensuring that the interface of the bond has no porous areas. It can also be provided that the entire joining zone or the entire component formed from the two joining parts is tempered.

According to the invention, the adhesive material is only tempered after the at least one open-pore ventilation textile has been completely evacuated.

According to one embodiment, the vacuum structure is inserted into an autoclave, whereby, to carry out the curing process, an overpressure is generated in the area surrounding the vacuum structure by means of the autoclave in order to apply a surface pressure to compact the adhesive layer. This ensures reliable contact between the adhesive material and the respective surface of the corresponding joining part. It can also be provided that the joining zones are simultaneously tempered in the autoclave.

It is now advantageous if the overpressure in the area surrounding the vacuum build-up is only generated after the complete evacuation of at least one open-pore venting textile in order to reliably avoid air inclusions in the interface.

According to a further advantageous embodiment, the adhesive material is introduced with the at least one venting textile in such a way that, before the surface pressure is applied, the pores of the open-pore venting textile form a cavity between the adhesive material and the respective joining part. By tempering and evacuating the joining zone, the cavity within the pores is evacuated. Due to the melting of the adhesive material, this cavity is then filled by the adhesive material, thus eliminating all air inclusions.

According to the invention, the adhesive material is provided integrally with the at least one ventilation textile in the form of an adhesive film.

According to the invention, the joining zone is at least locally tempered in a certain range, Subsequently, pressure is applied locally to the joining zone to fix the position. This allows the adhesive to be bonded locally to the joining surface for pure positional fixation, thus preventing the adhesive from slipping within the joining zone. Only by evacuating the ventilation textile and curing the adhesive is the entire adhesive layer compacted, eliminating the air pockets within it.

According to one embodiment, it is provided that the entire joining zone and/or the entire surface of one or both joining partners is covered with the open-pore ventilation textile.

The object is also achieved according to the invention with the adhesive film for joining two joining parts according to claim 12, wherein the adhesive film has an adhesive material designed to form an adhesive bond between the joining parts. According to the invention, at least one open-pore venting textile is arranged on the adhesive material of the adhesive film in such a way that the venting textile is not completely saturated with the adhesive material and that the pores of the open-pore venting textile form a cavity for venting when the adhesive film is arranged on the joining parts.

Advantageous embodiments of the adhesive film can be found in the corresponding subclaims.

The object is also achieved by the use of such an adhesive film in the adhesive joining of two joining parts according to claim 14.

• 1 - Schematischer Aufbau zum Fügen zweier Bauteile vor dem Aushärten;
• 2 - Schematische Darstellung eines Vakuumaufbaus während oder nach dem Aushärten des Klebstoffes.

The invention is explained in more detail by way of example with reference to the accompanying figures. They show:

• 1 - Schematic structure for joining two components before curing;
• 2 - Schematic representation of a vacuum build-up during or after curing of the adhesive.

1 shows a vacuum structure 10 with which a first joining part 11 is to be joined to a second joining part 12 using an adhesive 13 in an adhesive joining process. For this purpose, a film adhesive 20 containing the adhesive 13 and an open-pore venting textile 14a and 14b is first applied to the first joining part 11. The venting textiles 14a and 14b are provided such that the adhesive 13 is provided between the two venting textiles 14a and 14b. Consequently, exactly one surface side of one of the venting textiles 14a and 14b is always impregnated with the adhesive 13, while the other surface side of the respective venting textiles 14a and 14b is provided for contact with the respective joining part 11, 12.

After the film adhesive 20 with the adhesive 13 and the venting textiles 14a and 14b has been applied to the first joining part 11, the first venting textile 14a contacts both the adhesive 13 and the first joining part 11. In the individual pores of the venting textile 14a, cavities 15 are formed, which are due to the fact that the adhesive 13 does not completely soak the respective venting textile 14a, 14b.

Subsequently, the second joining part 12 is placed on the film adhesive 20 applied to the first joining part 11, so that the second venting textile 14b now contacts not only the adhesive 13 but also the second joining part 12. Here, too, corresponding cavities 15 are formed by the pores of the venting textile 14b.

This structure is then covered by a vacuum film 16 and sealed vacuum-tight with respect to the first joining part 11 using a sealing tape 17. Furthermore, the cavity 18 formed beneath the vacuum film 16 communicates with a vacuum connection 19, to which a vacuum pump (not shown) can be connected. This allows the vacuum cavity 18 beneath the vacuum film 16 to be evacuated, ultimately also evacuating the joining zone occupied by the film adhesive 20.

A vacuum bridge 22 is provided beneath the vacuum film 16 between the vacuum connection 19 and the venting textile 14a, 14b to prevent the vacuum film 16 from being pressed against the first joining part 11 in the area of the vacuum bridge 22 during the evacuation of the joining zones, and in particular the venting textile 14a, 14b. This could prevent the venting textile from being further evacuated, and corresponding air pockets could subsequently form in the interface. However, the vacuum bridge can be used to ensure that the venting textile can be completely evacuated.

Now that the 1 Once the vacuum structure 10 shown has been produced in this way, the vacuum cavity 18 is evacuated using a vacuum pump, in order to evacuate in particular the ventilation textile. Furthermore, the entire vacuum structure is tempered, for example, with a tempering device not shown, so that the adhesive 13 of the adhesive layer 21 melts and becomes plastically deformable. Due to the surface pressure and the evacuation of the vacuum cavity 18, as well as the plastic deformability of the adhesive 13, the previously The cavities 15 formed by the ventilation textile 14a, 14b are filled by the adhesive 13, while all air inclusions located in the boundary layer between a joining part 11, 12 and the adhesive layer 21 are completely evacuated due to the vacuum and the surface pressure. The resulting result is shown in 2 shown.

All pores of the ventilation textile 14a, 14b are completely occupied by the adhesive material 13 and thus form a corresponding adhesive connection with the respective joining part 11, 12. In addition, due to the surface pressure, the adhesive layer 21 was compacted in such a way that it was reduced to the size of the ventilation textile, so that the ventilation textiles in this context also serve as spacers to form a defined adhesive layer thickness.

The venting textile 14a, 14b can, for example, be a knitted fabric, a woven fabric, or a nonwoven fabric. The material from which the venting textiles are formed can be designed to have additional functions. For example, the material could react chemically with the adhesive during curing, thus implementing further advantageous properties of the adhesive bond (e.g., crack-stopping properties). For example, the material could also melt completely or partially during the curing reaction or migrate into the adhesive, so that after the curing reaction, the adhesive and the material of the venting textile are mixed. Furthermore, it is also conceivable that the novel adhesive film, in addition to the venting textile on one or more outer sides, additionally has a conventional spacer fabric in the center.

List of reference symbols

10

Vacuum build-up

11

first joining part

12

second joining part

13

Adhesive material

14a, 14b

Ventilation textile

15

Cavity of the pores of the ventilation textile

16

Vacuum film

17

sealing tape

18

Vacuum cavity

19

Vacuum connection

20

Film adhesive

21

Adhesive layer

22

vacuum bridge

Claims (11)

A method for joining two joining parts (11, 12), in which the joining parts (11, 12) are adhesively connected to one another by means of an adhesive material (13) provided between the joining parts (11, 12), the method comprising the following steps: - Providing a first joining part (11) and at least one second joining part (12); - Providing an adhesive material (13) that contains at least one open-pore ventilation textile (14a, 14b) and is provided integrally with the at least one ventilation textile (14a, 14b) in the form of an adhesive film (20); - Introducing the adhesive material (13) together with the open-pore venting textile (14a, 14b) between the joining parts (11, 12) in a predetermined joining zone such that the venting textile (14a, 14b) touches the respective joining part (11, 12) with one surface side, wherein the joining zone is tempered at least in a partial area before the surface pressure is applied and subsequently, pressure is applied locally to the joining zone to fix the position; - Creating a vacuum structure (10) such that at least the joining zone with the open-pore venting textile (14a, 14b) is sealed in a vacuum-tight manner under the vacuum structure (10); - Evacuating the joining zone under the vacuum structure (10), including the open-pore venting textile (14a, 14b), using a vacuum pump; and - carrying out a curing process for curing the adhesive material (13) introduced between the joining parts (11, 12), wherein, in order to carry out the curing process, the adhesive material (13) is tempered by means of a tempering device only after the at least one open-pore ventilation textile (14a, 14b) has been completely evacuated. Procedure according to Claim 1 , characterized in that the adhesive material (13) is provided in such a way that an open-pore ventilation textile (14a, 14b) is provided both between the adhesive material (13) and the first joining part (11) and between the adhesive material (13) and the second joining part (12). Method according to one of the preceding claims, characterized in that the vacuum structure (10) is moved into an autoclave, wherein, in order to carry out the curing process, an overpressure is generated in the surrounding area of the vacuum structure (10) by means of the autoclave in order to apply a surface pressure for compacting the adhesive layer (21). Procedure according to Claim 3 , characterized in that the overpressure in the surrounding area of the vacuum structure is only generated after the complete evacuation of the at least one open-pore ventilation textile (14a, 14b). Method according to one of the preceding claims, characterized in that the adhesive material (13) with the at least one ventilation textile (14a, 14b) is introduced in such a way that before evacuating the joining zone and/or carrying out the curing process, the pores of the open-pore ventilation textile (14a, 14b) each form a cavity (15) between the adhesive material (13) and the respective joining part (11, 12). Method according to one of the preceding claims, characterized in that the entire joining zone and/or the entire surface of one or both joining partners is covered with the open-pore ventilation textile. Method according to one of the preceding claims, characterized in that one or both joining parts (11, 12) are formed from a fiber composite material. Adhesive film (20) for joining two joining parts (11, 12), wherein the adhesive film (20) has an adhesive material (13) which is adapted to be applied according to a method according to one of the preceding claims. Adhesive film (20) after Claim 8 , characterized in that an open-pore ventilation textile (14a, 14b) is arranged on each surface side of the adhesive film (20). Use of an adhesive film (20) according to one of the Claims 8 or 9 when adhesively joining two parts (11, 12). Use according to Claim 10 , characterized in that at least one joining part (11), preferably both joining parts (11, 12) are or will be produced.