Composite bipolar plate and method for the production thereof

Abstract

The invention relates to: a method for joining electrically conductive plastic parts; joined plastic parts; and an apparatus for carrying out said method.

Description

[0001] Composite bipolar plate and methods for its manufacture

[0002] The present invention relates to the technical field of plastics processing, in particular the processing of electrically conductive plastics, as well as fuel cell technology.

[0003] In particular, the present invention relates to a method for joining preferably structured electrically conductive plastic parts and joined electrically conductive plastic parts.

[0004] Furthermore, the invention relates to a device for joining electrically conductive plastic parts.

[0005] The joining of thin-walled plastic parts, such as those used in fuel cells, battery technology and chemical process engineering, represents a crucial step in the production of high-performance and durable components and parts.

[0006] In a fuel cell stack, more than 300 individual cells are electrically connected in series to achieve higher voltage and power output. The bipolar plate component in a fuel cell stack serves as an electrically connecting and materially separating element between two individual cells and functions as a media distribution plate for the homogeneously distributed supply and discharge of the reaction gases and the cooling medium. Three materially separated flow zones are created within the bipolar plate by joining two films or sheets, each with flow structures on its front and back surfaces, using either a force-fit or material-fit connection.

[0007] Currently, only metallic bipolar plates are used to generate high power-to-weight and volume-specific output, as required, for example, in mobile applications (see in particular Yoshida, T.; Kojima, K. (2015): Toyota MIRAI Fuel Cell Vehicle and Progress Toward a Future Hydrogen Society. In: Interface magazine 24 (2), pp. 45-49. DOI: 10.1149 / 2.F03152if and Dana (2016): Dana's metallic bipolar plate technology helps commercialise fuel-cell power trains. In: Sealing Technology 2016 (3), p. 10. DOI: 10.1016 / S1350-4789(16)30062-9). Laser welding is the most common method for joining metallic plates (see in particular Weiss, Tony; Kick, Michael; Grabmann, Sophie; Geiger, Christian; Mayr, Lukas; Wudy, Katrin; Zaeh, Michael F. (2022): A holistic approach for an intelligent laser beam welding architecture using machine learning for the welding of metallic bipolar plates for polymer electrolyte membrane fuel cells. In: Procedia CIRP 111 , pp. 810-815. DOI: 10).1016 / j.procir.2022.08.089 and Dana (2016): Dana's metallic bipolar plate technology helps commercialise fuel-cell power trains. In: Sealing Technology 2016 (3), p. 10. DOI: 10.1016 / S1350-4789(16)30062-9). Laser welding of metallic bipolar plates utilizes the deep penetration welding effect, in which high joining temperatures cause the material at the joint to vaporize, allowing the laser beam to penetrate deep into the component without creating a wide seam (see especially Schuler, Volkmar; Twrdek, Jürgen (2019): Praxiswissen Schweißtechnik. Werkstoffe, Prozesse, Fertigung. 6th, completely revised edition. Wiesbaden, Heidelberg: Springer Vieweg).

[0008] Metallic bipolar plates represent the preferred state of the art for mobile applications, particularly for fuel cells and batteries. The thin weld seams achieved through laser welding enable a flow field design when joining individual components that enhances the fuel cell's performance. The deep penetration welding effect of laser metal welding is utilized to create these thin weld seams.

[0009] However, metallic bipolar plates can also be joined in other ways, especially by resistance welding.

[0010] DE 10 2011 116997 A1 describes a basic structure for an arrangement of bipolar plates, whereby the resistance welding process is used to weld two metallic unipolar plates together to form a bipolar plate in the outdoor area.

[0011] DE 10 2020 112 349 A1 describes a process in which flat metallic bipolar plates are produced by roller embossing and joined together in a subsequent roller welding process.

[0012] German patent DE 102022 204 913 A1 describes a basic structure for an arrangement of bipolar plates, using resistance welding to join metal sheets via tack welds. Since metallic bipolar plates are highly susceptible to corrosion in the acidic environment found in fuel cells, they are provided with protective coatings or manufactured from high-quality corrosion-resistant alloys, resulting in high costs. For this reason, plates with a thickness of 300 pm made of carbon-containing composite materials, consisting of a polymer and electrically conductive particles, have been developed. These offer advantages over metallic plates in terms of both cost and durability (see, in particular, Kurzweil, Peter (2016): Fuel Cell Technology. Fundamentals, Materials, Applications, Gas Generation. With the collaboration of Ottmar Schmid. 3rd ed.).Springer and Hala, Miroslav; Malis, Jakub; Paidar, Martin; Bouzek, Karel (2022): Characterization of Commercial Polymer-Carbon Composite Bipolar Plates Used in PEM Fuel Cells. In: Membranes 12 (11 ). DOI: 10.3390 / membranes12111050). However, with regard to the joining process, the composites cannot be joined using the deep penetration welding effect, as the polymer base exceeds its decomposition temperature before evaporation begins (see especially Schüle, Helmut; Eyerer, Peter (eds.) (2020): Polymer Engineering. Springer-Verlag GmbH. 2nd edition. Berlin, Heidelberg: Springer Vieweg).

[0013] When composites are joined using a laser, the welding process is based on the thermal conductivity of the material. In heat conduction welding, the heat spreads faster across the plane of the plate at the same penetration depth than when using the deep penetration welding effect.

[0014] This results in a wider weld seam, which necessitates wider gas channels in bipolar plates as the joining surface. This negatively impacts the function of the flow field and thus the performance of the fuel cell. Furthermore, laser welding is comparatively slow, as the weld line must be traversed successively with a laser beam. The weld nugget forms on the surface of one joining partner and must grow to the other partner to weld. During this lengthy process, heat can spread across the plane of the plates, resulting in a wide weld seam.

[0015] Composite bipolar plates are fundamentally more cost-effective and durable than metallic bipolar plates, but have the disadvantage that they cannot be joined using the deep penetration welding effect, as previously described. In particular, it is not possible to obtain weld seams with a width of at most 1 mm, preferably well below 1 mm, using laser welding of polymer-containing composite materials. Other methods for manufacturing composite bipolar plates are not yet known.

[0016] Currently, composite bipolar plates are simply stacked and not welded together. However, for mobile or high-performance applications, a connection is necessary, similar to that used with metallic bipolar plates, to reduce contact resistance.

[0017] DE 102009 006 296 A1 describes a basic structure for an arrangement of bipolar plates, in which two unipolar plates are joined at points to form a bipolar plate. It does not explain how composite plates are joined.

[0018] US Patent 2007 / 059571 A1 describes a component of an electrochemical cell, comprising a cooling plate and another cooling plate or bipolar plate, both made of polymer and a conductive filler. The plates are designed such that one plate has a profile in a specific area, while the other plate has the corresponding counter-profile. The plates are joined in this area by a welding process, namely resistance welding or vibration welding, creating a sealing seam in the profiled area. This patent focuses on the use of the welding process to create a seal in a designated area and applies exclusively to one cooling plate.

[0019] The state of the art therefore still lacks a simple method for joining thin-walled, structured and electrically conductive plastic parts with which bipolar plates or chemical reactors can be accessed.

[0020] One object of the present invention is therefore to avoid, or at least mitigate, the aforementioned disadvantages associated with the prior art.

[0021] In particular, one object of the present invention is to provide an improved method for joining electrically conductive plastic parts, which is faster than previously known methods of the prior art and leads to significantly improved joined products, in particular bipolar plates.

[0022] Furthermore, it is an object of the present invention to provide an improved bipolar plate for applications in fuel cells and batteries.

[0023] The problem set out above is solved according to a first aspect of the present invention by a method according to claim 1; further, advantageous embodiments of this aspect of the invention are the subject of the relevant dependent claims.

[0024] A further subject matter of the present invention according to a second aspect of the present invention is a joined plastic part according to claim 24.

[0025] Finally, a further subject matter of the present invention, according to a third aspect of the present invention, is a device according to claim 25; further, advantageous embodiments of this aspect of the invention are the subject matter of the relevant dependent claims.

[0026] It goes without saying that special features, characteristics, designs and embodiments as well as advantages or the like, which are subsequently described – for the purpose of avoiding unnecessary repetition – only with regard to one aspect of the invention, naturally apply accordingly to the other aspects of the invention, without the need for any express mention.

[0027] Furthermore, it should be noted that all values ​​or parameters mentioned below, or the like, can in principle be determined using standardized or explicitly specified determination procedures, or using determination methods that are generally familiar to the person skilled in the art in this field.

[0028] Furthermore, it goes without saying that weight- or quantity-related percentages are selected by a professional in such a way that the total results in 100%.

[0029] Having stated the above, the present invention will now be described in more detail. According to a first aspect of the present invention, the subject matter of the present invention is thus a method for joining preferably structured, electrically conductive plastic parts, in particular semi-finished products, preferably for joining structured plastic half-plates to form bipolar plates, wherein the electrically conductive plastic parts are joined by means of electrical resistance welding.

[0030] As the applicant surprisingly discovered, resistance welding of electrically conductive plastics allows for the creation of very narrow and cohesive joints, particularly those that are liquid- and gas-tight. This enables the production of high-performance and corrosion-resistant bipolar plates with optimized flow fields. These plates can then be used, for example, in fuel cells, batteries, or electrolyzers. Furthermore, the resulting joined plastic components are also suitable as heat exchangers or chemical reactors.

[0031] The method according to the invention is not only suitable for producing narrower weld seams or weld points, but is also significantly faster and less complex in design compared to conventional methods for joining plastic parts, such as laser welding, since the predefined weld points or weld lines can be created within a few seconds or even fractions of a second using structured electrodes. In contrast, laser welding requires the laser to trace the weld line, which significantly slows down and increases the cost of the process.

[0032] The present invention makes durable, carbon-based composite bipolar plates based on thermoplastic polymers accessible, particularly for use in fuel cells. The bipolar plate is produced in particular by joining the anode-side and cathode-side components or half-plates of the bipolar plate by resistance welding. In this process, the weld seams grow rapidly at the contact point between the joining partners, enabling the production of particularly thin, material-bonded seams with a width of less than 1 mm.

[0033] The performance of a bipolar plate depends particularly on the joining strategy for electrical contact within the flow field. Welding is a suitable method for joining the two plates, as it significantly reduces the contact resistance between them and thus increases the electrical conductivity of the bipolar plate. The electrical conductivity of the joined component is only slightly lower, in particular less than 5% lower, than the electrical conductivity of the material itself. This means that the inventive method makes it possible to produce electrically conductive plastic parts with fine structures and complex geometries whose electrical conductivities are approximately equal to those of the materials used.

[0034] Furthermore, a high-performance flow field is characterized by numerous thin channels that transport the media homogeneously throughout the cell. The bottoms of the channels preferably serve as joints between the half-plates. For this reason, the weld seams must be designed to be narrow to maximize the channel width. Using the inventive method, thin weld seams can now be produced in composite bipolar plates by means of resistance welding, something that was previously only possible with metallic bipolar plates. These form the necessary condition for generating a high-performance flow field.

[0035] Due to the narrower weld seams, composite bipolar plates obtained using the inventive method can be designed to be significantly more efficient. The use of composite bipolar plates manufactured in this way makes it possible to produce durable fuel cells with high power density.

[0036] When composite bipolar plates are joined using the inventive method, the seams, unlike those formed in other welding processes, are created precisely at the contact surface between the bipolar plates, where they are required. The width of the weld seam is not limited, as in laser welding processes, by the spherical spread of the molten metal through heat conduction on the outer surface. This allows for the production of significantly thinner weld seams than previously possible with these materials.

[0037] Furthermore, the high heat input into surrounding areas, which can unnecessarily impair the structural integrity of the joined plastic parts, especially a bipolar plate, can be reduced. In addition, the surface of the parts being joined is not altered by the joining process, since resistance welding only heats the contact points or areas between the plastic parts being joined, i.e., the joining components. Therefore, melting of the outer surface of the workpieces during the joining process typically does not occur. This allows the established surface properties of the plastic parts, especially the half-plates, to be maintained.

[0038] Furthermore, the process is faster than the laser welding process, with a processing time ranging from fractions of a second to a few seconds.

[0039] Within the scope of the present invention, the resistance welding carried out is generally a direct current resistance welding process and a pressure welding process, i.e. within the scope of the present invention, a direct current is usually applied between 2 electrodes, between which the plastic parts to be joined are located, and pressure is exerted on the plastic parts to be joined in order to obtain the joining connection.

[0040] In the context of the present invention, electrically conductive means a compound or material which has an electrical conductivity of at least 1 • 10⁻⁶ at 20 °C. 11 S • m -1 exhibits.

[0041] Within the scope of the present invention, it is preferred if the electrically conductive plastic has an electrical conductivity of at least 1 • 10⁻⁶ at 20 °C. 4 S • m' 1 , especially 1 • 10' 2 S • m' 1 , preferably 1 S • m' 1, exhibits. Particularly good results are obtained when the electrically conductive plastic has an electrical conductivity of at least 1 × 10⁻⁶ at 20 °C. 2 S • m' 1 , in particular 1 • 10 3 S • m' 1 , preferably 1 • 10 4 S • m' 1 , exhibits. In particular, especially with plastics that exhibit the aforementioned conductivities, particularly high-performance bipolar plates can be obtained.

[0042] In the context of the present invention, a plastic, also called a polymer compound, is understood to be, in particular, a polymeric material which, in addition to a polymer or polymer blend, contains further additives, such as fillers, plasticizers, UV stabilizers, etc. Preferably, the plastic parts in the context of the present invention consist of composite materials, i.e., highly filled plastics.

[0043] Typically, bipolar plates, heat exchangers, and chemical reactors are produced using the inventive method. In particular, it is preferably intended that bipolar plates and heat exchangers, and especially bipolar plates, are produced using the inventive method. The bipolar plates can preferably be used in fuel cells, redox flow batteries, or electrolyzers.

[0044] Within the scope of the present invention, it is preferably provided that at least one plastic part to be joined, i.e., at least one joining element, is structured and / or microstructured. Preferably, all plastic parts to be joined are structured and / or microstructured.

[0045] Typically, at least one plastic part to be joined is structured; preferably, all plastic parts to be joined are structured. According to a preferred embodiment, at least one plastic part to be joined is additionally microstructured; preferably, all plastic parts to be joined are additionally microstructured.

[0046] In the context of the present invention, "structured" means that the plastic part—or an electrode—has raised or recessed areas. In the context of the present invention, "microstructured" means that the plastic part has structures that are no higher, deeper, or wider than 100 pm.

[0047] Furthermore, it is preferably provided that the plastic part, in particular the half-plate, has structures which are not wider than 5 mm, in particular not wider than 2 mm, preferably not wider than 1 mm, preferably not wider than 0.6 mm.

[0048] Preferably, the plastic part, in particular the half-plate, has structures with a width of 0.05 to 5 mm, in particular 0.1 to 2 mm, preferably 0.2 to 1 mm, preferably 0.3 to 0.6 mm.

[0049] Likewise, it is possible that the plastic part, in particular the half-plate, has structures that are no higher or deeper than 5 mm, in particular no higher or deeper than 2 mm, preferably no higher or deeper than 1.5 mm, preferably no higher or deeper than 1 mm.

[0050] In this context, it has proven advantageous if the plastic part, in particular the half-plate, has structures with a height or depth of 0.05 to 5 mm, in particular 0.1 to 2 mm, preferably 0.1 to 1.5 mm, preferably 0.1 to 1 mm.

[0051] Furthermore, it is typically provided within the scope of the present invention that the plastic parts to be joined have a thickness, i.e. a material thickness, of at most 5 mm, in particular at most 2 mm, preferably at most 1 mm, preferably at most 0.5 mm.

[0052] Similarly, it can be provided that the plastic parts to be joined have a thickness of at least 0.05 mm, in particular 0.1 mm, preferably at least 0.2 mm, preferably at least 0.25 mm.

[0053] Within the scope of the present invention, it is particularly preferred if the plastic parts to be joined have a thickness in the range of 0.05 to 5 mm, more particularly 0.1 to 2 mm, preferably 0.2 to 1 mm, and more preferably 0.25 to 0.5 mm. It is therefore preferred if the plastic parts are very thin-walled. This ensures that very narrow weld points and weld lines can be obtained with short joining times.

[0054] According to a preferred embodiment of the present invention, the plastic parts to be joined have a thickness in the range of 0.5 to 5 mm, in particular 1 to 3 mm, preferably 1 to 2 mm. These plastic parts with a relatively large material thickness can be used, for example, for the manufacture of heat exchangers.

[0055] According to a particularly preferred embodiment of the present invention, the plastic parts to be joined have a thickness in the range of 0.05 to 1 mm, in particular 0.1 to 0.8 mm, preferably 0.2 to 0.5 mm, and more preferably 0.25 to 0.4 mm. The plastic parts according to this embodiment are ideally suited for the production of bipolar plates. If one or all of the plastic parts to be joined are microstructured, it has proven advantageous for the plastic part, in particular the half-plate, to have microstructures that are no wider than 100 pm, in particular no wider than 90 pm, preferably no wider than 75 pm, and more preferably no wider than 60 pm.

[0056] Preferably, the plastic part, in particular the half-plate, has microstructures with a width of 10 to 100 pm, in particular 20 to 90 pm, preferably 30 to 75 pm, preferably 40 to 60 pm.

[0057] It is also possible that the plastic part, in particular the half-plate, has microstructures which are not higher than 100 pm, in particular not higher than 90 pm, preferably not higher than 75 pm.

[0058] In this context, it has proven advantageous if the plastic part, particularly the half-plate, has microstructures with a height or depth of 10 to 100 pm, particularly 20 to 90 pm, preferably 25 to 75 pm. Microstructures can significantly improve the surface properties, especially the mass transport properties, of the joined plastic parts, as well as their electrical conductivity – as will be explained below. The microstructures can be generated, for example, by laser ablation. A special feature of the method according to the invention is that the microstructures can also be arranged in the immediate vicinity of the welds, i.e., the spot welds and weld seams, and are not destroyed during the joining process.This is due to the low and locally very limited heat input from resistance welding, which results in a pinpoint weld seam without adjacent areas of the plastic parts to be joined being melted or fused.

[0059] The number of plastic parts to be joined can vary. According to the present invention, it is possible to join 2 to 5 plastic parts, particularly 2 to 3, in a single joining process. It is especially preferred that 2 plastic parts are always joined.

[0060] Within the scope of the present invention, it is typically provided that the plastic parts are joined, in particular joined, by at least one weld point and / or at least one weld line. Preferably, the plastic parts are joined by several weld points and / or several weld lines. It is particularly preferred within the scope of the present invention if the plastic parts are joined by several weld lines.

[0061] Preferably, the plastic parts are joined together via 2 to 1,000, in particular 2 to 500, preferably 2 to 300, preferably 10 to 300, particularly preferably 50 to 300, weld lines and / or weld points, preferably weld lines.

[0062] Within the scope of the present invention, it is preferred if the weld line or weld spot has a width of less than 2 mm, in particular less than 1.5 mm, preferably less than 1 mm, preferably less than 0.8 mm, particularly preferably less than 0.7 mm, most preferably less than 0.6 mm.

[0063] Likewise, it can be provided that the weld line or weld point has a width of more than 0.05 mm, in particular 0.07 mm, preferably 0.1 mm, preferably 0.2 mm, particularly preferably 0.4 mm.

[0064] Within the scope of the present invention, it is preferably further provided that the weld line or weld spot has a width in the range of 0.05 to 2 mm, in particular 0.07 to 1.5 mm, preferably 0.1 to 1 mm, more preferably 0.1 to 0.8 mm, particularly preferably 0.2 to 0.7 mm, and most preferably 0.4 to 0.6 mm. Typically, the length of a weld spot corresponds to its width, while the length of a weld line is greater than its width.

[0065] Within the scope of the present invention, it may further be provided that gas- and / or liquid-tight, preferably gas- and liquid-tight, weld seams are created in the joined plastic part by the joining process.

[0066] It is also possible that the joining process creates gas- and / or liquid-tight, in particular gas- and liquid-tight, areas in the joined plastic part.

[0067] Within the scope of the present invention, in particular, half-plates can be joined or connected to form a bipolar plate, whereby liquid-tight and / or gas-tight welds and thus liquid-tight and / or gas-tight channels are obtained in the bipolar plate. The weld lines can also be interrupted or the joined plastic parts can be connected via spot welds if a liquid-tight and / or gas-tight connection is not necessary.

[0068] The process makes it particularly possible to obtain flow channels and reactors through which gas or liquid flows can be directed, through the joining process, whereby the flow channels do not have to be linear, but can also take on other shapes, for example be curved.

[0069] Furthermore, the method according to the invention can be used to produce weld seams outside the flow field in the electrochemically passive region of a bipolar plate. The properties of the method also improve upon the state of the art in this area.

[0070] The joining process according to the invention is typically carried out as a spot welding process, a line welding process, or a projection welding process. Particularly good results are obtained within the scope of this application when the joining process is carried out as a line welding process or a projection welding process. In particular, bipolar plates, heat exchangers, or chemical reactors with gas- and liquid-tight channels can be produced using both line welding and projection welding processes.

[0071] By joining the plastic parts using electrical resistance welding, it is possible to weld the entire flow field of a bipolar plate in one joining process.

[0072] As previously explained, the inventive method is carried out as a projection welding process according to a particularly preferred embodiment. Carrying out the inventive method as a projection welding process has the advantage that, particularly when using thermoplastic polymers to manufacture the plastic parts to be joined, very precise and sharp structures can be created in the plastic parts, which are suitable as material constrictions and contact surfaces with the joining partner. In this way, very narrow and precise weld seams can be obtained, while simultaneously allowing the use of relatively wide electrode structures. If very narrow electrode structures, for example with widths of 0.1 mm or less, are used, the process must be carried out with extreme care, as there is a risk that the joining parts will be damaged, in particular cut by the electrode structure.If the joining process is carried out using projection welding, the pressure build-up step is significantly less critical, as wider electrodes can be used.

[0073] The method according to the invention is preferably a static welding process in which the electrodes are not moved relative to the plastic parts to be joined, i.e. preferably the welding points and welding lines are not approached or retracted with one or more electrodes, but are preferably generated simultaneously via structured electrodes.

[0074] Within the scope of the present invention, it is usually provided that the resistance welding is carried out using at least 2 electrodes, preferably using 2 electrodes.

[0075] Within the scope of the present invention, it is usually provided that the joining parts, i.e. the electrically conductive plastic parts, are arranged between 2 electrodes and joined by applying an external voltage and pressure.

[0076] The best results are obtained within the scope of the present invention if at least one electrode is formed as a flat surface. However, it is particularly preferred if at least two electrodes, preferably exactly two electrodes, are formed as flat surfaces. Due to the flat design of the electrodes, the joining process can be carried out within a few seconds or even fractions of a second.

[0077] Furthermore, it is usually provided that at least one electrode is structured. Preferably, at least two electrodes are structured, and in particular, exactly two electrodes are structured. Preferably, the structures or structural elements of the electrodes have a width of less than 2 mm, in particular less than 1 mm, and preferably less than 0.5 mm. Likewise, it is possible for the structures or structural elements of the electrodes to have a width in the range of 0.05 to 2 mm, in particular 0.1 to 1 mm, and preferably 0.1 to 0.5 mm. In spot or line welding processes, the electrode structures are usually narrower and deeper than the structures of the plastic parts to be joined. Within the scope of the present invention, it is preferably provided that the joining process is carried out with at least one structured planar electrode.Preferably, at least two electrodes, and in particular exactly two electrodes, are planar and structured. This makes it possible to create a large number of individual weld spots and / or weld lines in a single joining process.

[0078] Within the scope of the present invention, it is further typically provided that the current density during the joining process is in the range of 100 to 500 A • cm' 2 , especially 150 to 350 A • cm' 2 , preferably 200 to 300 A • cm' 2 preferably 220 to 280 A • cm' 2 , amounts.

[0079] Furthermore, it may be provided that the current density of the joining process exceeds 100 A • cm' 2 , especially more than 150 A • cm' 2 , preferably more than 200 A • cm' 2 preferably more than 220 A • cm' 2 , amounts.

[0080] Similarly, it may be stipulated that the current density of the joining process is at most 500 A • cm' 2, in particular at most 350 A • cm' 2 , preferably no more than 300 A • cm -2 , preferably at most 280 A • cm' 2 , amounts.

[0081] Regarding the contact pressure during the joining process, this can vary considerably. However, it has proven effective if the contact pressure is at most 8 MPa, in particular at most 6 MPa, preferably at most 5 MPa, and preferably at most 4.5 MPa.

[0082] Furthermore, it is preferred that the contact pressure is at least 1 MPa, in particular at least 2 MPa, preferably at least 3 MPa, preferably at least 3.5 MPa, and especially preferably at least 4 MPa.

[0083] Particularly good results are obtained when the contact pressure is 1 to 8 MPa, especially 2 to 6 MPa, preferably 3 to 5 MPa, preferably 3.5 to 5 MPa, and most preferably 4 to 4.5 MPa. The aforementioned pressures ensure that, on the one hand, a sufficiently strong and durable joint is achieved, and on the other hand, the plastic parts are not damaged. Furthermore, it is typically also provided that the joining time, in particular the duration of the application of external stress and pressure, is at most 2 seconds, especially at most 1.5 seconds, preferably at most 1 second, preferably at most 750 ms, most preferably at most 500 ms, and most preferably at most 250 ms.

[0084] Furthermore, it is equally preferred if the joining time, in particular the duration of the application of an external stress and pressure, is at least 10 ms, in particular at least 15 ms, preferably at least 17 ms, preferably at least 10 ms, particularly preferably at least 22 ms, most preferably at least 25 ms.

[0085] However, it is particularly preferred if the joining time, especially the duration of the application of an external stress and pressure, is 10 ms to 2 s, more preferably 15 ms to 1.5 s, preferably 17 ms to 1 s, more preferably 20 ms to 750 ms, more preferably 22 ms to 500 ms, and most preferably 25 ms to 250 ms. With the joining times used in this application, very fast joining and industrial series production can be easily achieved.

[0086] As previously stated, the plastic part, especially the semi-finished product, usually consists of a composite material, i.e., a highly filled plastic.

[0087] The composite material preferably comprises a thermoplastic polymer or polymer blend and optionally additives and electrically conductive particles. Preferably, the composite material consists of a thermoplastic polymer or polymer blend and optionally additives as well as electrically conductive particles.

[0088] Within the scope of the present invention, it is typically provided that the plastic part, in particular the semi-finished product, comprises a thermoplastic polymer.

[0089] The polymer of the electrically conductive plastic is usually selected from the group of polyolefins, in particular polyethylene (PE), polypropylene (PP), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyaryletherketone (PAEK), polyetherketoneketone (PEKK), polyetherimides (PEI), acrylonitrile butadiene styrene (ABS), polyamides (PA), polyphenylsulfones (PPS), polysulfone (PS), polyethersulfone (PES), polyamide imides (PAI), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA) and mixtures thereof.

[0090] Good results are obtained within the scope of the present invention if the polymer of the electrically conductive plastic is selected from the group consisting of polyethylene (PE), polypropylene (PP), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyvinyl chloride (PVC), polyamide (PA) and mixtures thereof, preferably polyethylene (PE), polypropylene (PP), polyetheretherketone (PEEK), polyphenylene sulfide (PPS) and mixtures thereof.

[0091] Within the scope of the present invention, it is typically provided that the electrically conductive particles are selected from the group consisting of carbon-containing materials, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), core-shell structured mesoporous materials, in particular core-shell nanoparticles (CSN), metals, metal oxides, nitrides, carbides, MAX phases, MXenes and mixtures thereof.

[0092] Particularly good results are obtained within the scope of the present invention when the electrically conductive particles are selected from the group consisting of carbon-containing materials, covalent organic frameworks, metals, metal oxides, carbides and mixtures thereof, preferably carbon-containing materials, metals and mixtures thereof.

[0093] The electrically conductive particles are particularly preferably carbon-containing materials.

[0094] If the electrically conductive particles are metal particles, it has proven effective to select the metal from the group of nickel, titanium, platinum, ruthenium and their mixtures.

[0095] If the electrically conductive particles are metal oxides, it has proven advantageous to select the metal oxide from the group consisting of tin(IV) oxide, manganese oxide, silver oxide, antimony oxide, cobalt(II,III) oxide, copper(II) oxide, lanthanum oxide, cerium(IV) oxide, aluminum oxide, titanium oxide, zirconium oxide, niobium oxide, and mixtures thereof. It is particularly preferred if the metal oxide is selected from the group consisting of tin(IV) oxide, manganese oxide, silver oxide, antimony oxide, cobalt(II,III) oxide, copper(II) oxide, lanthanum oxide, and mixtures thereof.

[0096] If the electrically conductive particles are nitrides and / or carbides, the conductive particles are preferably selected from metal nitrides, metal carbides and mixtures thereof.

[0097] If the electrically conductive particles are nitrides and / or carbides, it has proven advantageous if the electrically conductive particles are selected from nitrides and carbides of elements of the group of B, Al, Ti, Ga, In, Ti, Si, Ge, Sn, Pb, P, As, S, Cd, V, Cr, Mn, W, Co, Ni, Zn, Zr and their mixtures, preferably selected from nitrides and carbides of titanium.

[0098] As previously stated, the electrically conductive particles are preferably carbon-containing materials. These carbon-containing materials are, in particular, particles selected from the group consisting of (conductive) carbon black, graphite, carbogels, graphene, carbon nanotubes, onion-like carbons, and mixtures thereof. According to a particularly preferred embodiment of the present invention, the carbon-containing materials are carbon-based particles and are preferably selected from the group consisting of (conductive) carbon black, graphite, carbogels, and mixtures thereof, in particular (conductive) carbon black, graphite, and mixtures thereof.

[0099] Within the scope of the present invention, a carbogel is understood to be a carbon-containing material obtained by gel synthesis, wherein the gel is carbonized and optionally activated. Carbogels within the scope of this invention are, in particular, carbon aerogels, carbon xerogels, and carbon cryogels. Carbogels are porous particles whose specific pore volume of the mesopores, i.e., the pores with a size of 2 to 50 nm, determined by nitrogen adsorption and evaluation according to the Barrett-Joyner-Halenda and / or t-plot method, is at least 0.5 cm³. 3 / g, preferably at least 1.0 cm 3 / g, is the amount.

[0100] Furthermore, it can be provided that the electrically conductive particles contain or consist of MAX phases or MXenes, preferably MXenes. Within the scope of the present invention, MAX phases are, in particular, carbides and nitrides of the general formula M that crystallize in hexagonal layers.n +iAXn is to be understood as n = 1 to 3. M represents an early transition metal from groups 3 to 6 of the periodic table, while A represents an element from groups 13 to 16. A is specifically selected from Cd, Al, Ga, In, Ti, Si, Ge, Sn, Pb, P, As, S and their mixtures, while M is preferably selected from Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and their mixtures. X is either carbon or nitrogen.

[0101] MAX phases often exhibit unusual combinations of chemical, physical, electrical, and mechanical properties, as they display both metallic and ceramic behavior depending on the conditions. This includes, for example, high electrical and thermal conductivity, high resistance to thermal shock, very high hardness, and low coefficients of thermal expansion.

[0102] MAX phases are highly resistant to chemicals and to oxidation at high temperatures.

[0103] MXenes are two-dimensional structures based on metal carbides and / or metal nitrides and can be obtained in particular by etching MAX phases, for example with HF.

[0104] It is particularly preferred if the electrically conductive particles are selected from the group consisting of carbon, graphite, (conductive) carbon black, carbogels, titanium carbide, titanium nitride, metals, metal compounds and their mixtures, in particular graphite, carbon black, nickel, titanium, platinum, ruthenium and their mixtures, preferably graphite, carbon black and their mixtures. Mixtures of graphite and carbon black are especially preferred.

[0105] It has also proven advantageous if the electrically conductive particles have a particle size distribution D90 in the range of 1 to 300 pm, particularly 1 to 200 pm, preferably 1 to 150 pm. Within the scope of this invention, the particle size distributions are determined in particular by means of static laser scattering (SLS).

[0106] However, within the scope of the present invention, it can also be provided that the electrically conductive particles have a particle size distribution D90 in the range of 0.01 to 5 pm, in particular 0.05 to 1 pm, preferably 0.05 to 0.1 pm. According to a preferred embodiment of the present invention, it is provided that the electrically conductive particles have a multimodal, in particular a bimodal, particle size distribution, preferably with the aforementioned particle size distributions. This applies particularly when mixtures of graphite and carbon black are used. The graphite typically has the larger particles, while the carbon black is more finely divided.

[0107] Furthermore, it has proven advantageous within the scope of the present invention if the electrically conductive plastic part contains the polymer in amounts of 1 to 60 wt.%, in particular 1 to 50 wt.%, preferably 2 to 40 wt.%, preferably 10 to 30 wt.%, based on the plastic part.

[0108] Furthermore, it has proven advantageous within the scope of the present invention if the electrically conductive plastic part contains the electrically conductive particles in quantities of 40 to 99 wt.%, in particular 50 to 99 wt.%, preferably 60 to 98 wt.%, preferably 70 to 90 wt.%, based on the plastic part.

[0109] Particularly suitable plastic parts within the scope of the present invention can be manufactured by first producing raw materials or semi-finished products using a powder-to-roll process, as described in WO 2015 / 00744 A1. WO 2015 / 00744 A1 describes in particular a process for producing composite semi-finished products, such as thin layers or films made of composite materials. These thin layers or films can then be formed into structured, flat workpieces or plastic parts by thermoforming or other forming processes, which are joined using the joining method according to the invention.

[0110] Furthermore, at least one plastic part to be joined may be surface-treated, preferably all plastic parts to be joined are surface-treated. Preferably, the plastic part is surface-treated by plasma treatment, corona treatment, or laser treatment, preferably laser treatment. Surface treatment allows the surface properties of the plastic parts to be precisely adjusted, and due to the pinpoint and highly localized energy input during the joining process according to the invention, these surface properties are retained in the joined product. The laser treatment is preferably laser ablation. Laser ablation can be used, on the one hand, to create microstructures on the surface of the plastic parts to be joined, and on the other hand, it can also be used to selectively remove the polymer skin that forms during the forming process, which reduces electrical conductivity.The targeted energy input during the inventive process effectively prevents the polymer skin from reforming, so that joined plastic parts with improved conductivity can be obtained.

[0111] Preferably, the process according to the invention is carried out in such a way that at least 2, preferably planar, structured and electrically conductive plastic semi-finished products are arranged partially overlapping one another, so that at least point or area contact is created between the plastic semi-finished products and the plastic semi-finished products are subsequently joined at least area or point by means of electrical resistance welding.

[0112] The welding process is preferably selected from the group consisting of spot welding, line welding, and projection welding. Line welding or projection welding is preferred.

[0113] Preferably, it is provided that, after the plastic semi-finished products have been arranged partially overlapping one another, they are contacted with electrodes in such a way that the plastic semi-finished products touch each other at least at points or in areas.

[0114] The joining process is carried out by applying an external voltage and pressure, as described above. The pressure is transmitted to the plastic semi-finished products via the electrodes. Preferably, according to the present invention, the plastic parts to be joined are arranged between the electrodes.

[0115] Furthermore, it is preferred that, to carry out the joining process, pressure is first applied to the plastic parts, preferably via the electrodes, and then current is passed through the electrodes and the plastic parts. According to a preferred embodiment of the present invention, the plastic semi-finished products are arranged to overlap completely. In this way, finished components can be obtained directly in one joining process, for example, end seals can also be produced by the weld seams.

[0116] Within the scope of the present invention, it is further possible to fix the plastic parts before the joining process. Fixing them prevents the plastic parts to be joined from slipping or moving relative to each other before or during the joining process. If the plastic parts are fixed before the actual joining process, it is further provided that the plastic parts touch at predetermined points or areas. These areas are, in particular, the areas to be welded subsequently.

[0117] Furthermore, it may be necessary for the components to be joined, particularly the plastic parts, to undergo a preliminary processing step of preparation and / or cleaning. Preparation or cleaning prepares the components for the subsequent joining process.

[0118] It is also possible that the joined plastic parts undergo post-processing after the joining process.

[0119] The method according to the invention can be carried out in particular with the following process steps:

[0120] 1. Preparation of the workpieces: The joining partners are cleaned and brought to the correct dimensions and surface finish.

[0121] 2. Placement of the joining partners between the electrodes: The electrodes are positioned so that they touch the joining partners at the desired points.

[0122] 3. Applying pressure: A specific pressure is applied to the workpieces and electrodes to establish good electrical contact.

[0123] 4. Applying the current: An electric current is passed through the workpieces and electrodes, generating heat. 5. Welding: The heat causes local softening or melting of the workpieces at the points of contact. Once the desired weld depth is reached, the current is switched off.

[0124] 6. Cooling and removal: The welded workpieces are cooled and, if necessary, reworked to achieve the desired strength and quality.

[0125] The figure depictions show, according to

[0126] Fig. 1 : a fuel cell stack which has bipolar plates according to the invention,

[0127] Fig. 2: a device according to the invention, in particular for carrying out the method according to the invention,

[0128] Fig. 3: an enlarged section of a device according to the invention during the joining process,

[0129] Fig. 4A: a schematic representation of a device according to the invention for carrying out a line or spot welding process and

[0130] Fig. 4B: a schematic representation of a device according to the invention for carrying out a projection welding process.

[0131] A further object of the present invention - according to a second aspect of the present invention - is a joined plastic part made of an electrically conductive plastic, in particular joined by means of a aforementioned method, wherein the plastic part has weld points and / or weld lines with a width of at most 2 mm, in particular at most 1 mm.

[0132] For further details on the joined plastic part according to the invention, reference can be made to the above descriptions of the method according to the invention, which apply accordingly to the joined plastic part.

[0133] Yet another object of the present invention - according to a third aspect of the present invention - is a device for joining electrically conductive plastic parts, wherein the device comprises,

[0134] (A) at least 2 electrodes and (B) at least one device for generating pressure.

[0135] In general, the device has two electrodes. The electrodes are preferably metallic. In particular, the electrodes consist of a metal from the group consisting of copper, aluminum, tungsten, molybdenum and their alloys, preferably from the group consisting of copper, aluminum and their alloys.

[0136] The electrodes can be flat or structured. Preferably, at least one electrode is structured.

[0137] Within the scope of the present invention, it is usually provided that at least 2 electrodes, preferably exactly 2 electrodes, are structured.

[0138] Within the scope of the present invention, it is further preferably provided that the electrode or electrodes have structural elements. The structural elements are structures on the surface of the electrodes, in particular protrusions that project from the base plane of the electrode.

[0139] It is usually intended that the structural elements have a width of less than 2 mm, in particular less than 1 mm, preferably less than 0.6 mm.

[0140] Similarly, it can be provided that the structural elements have a width of at least 0.05 mm, in particular 0.1 mm, preferably 0.2 mm.

[0141] The best results are obtained within the scope of the present invention if the structural elements have a width in the range of 0.05 to 2 mm, in particular 0.07 to 1 mm, preferably 0.2 to 0.6 mm.

[0142] Likewise, it is possible that the structural elements have a height of at most 7 m, in particular at most 5 mm, preferably at most 3 mm, preferably at most 2 mm, and most preferably at most 1 mm.

[0143] Furthermore, the structural elements may have a height of at least 0.05 mm, in particular at least 0.1 mm, preferably at least 0.3 mm, more preferably at least 0.4 mm, and most preferably at most 1 mm. In this context, it has proven advantageous for the structural elements to have a height of 0.05 to 7 mm, in particular 0.1 to 5 mm, more preferably 0.1 to 3 mm, more preferably 0.3 to 2 mm, and most preferably 0.4 to 1 mm.

[0144] Preferably, the structural elements of the electrodes correspond to the flow field, in particular the negative of a flow field, of a bipolar plate.

[0145] In the context of the present invention, it is typically provided that the device includes a direct current source. The direct current source is generally connected to the electrodes.

[0146] Furthermore, it is possible that the device exerts pressure on the electrodes to generate pressure, which in turn exert pressure on the plastic parts to be joined.

[0147] Furthermore, the device may also include at least one means for fixing the plastic parts to be joined. This means the plastic parts can be positioned one above the other and brought into contact, preventing them from slipping during the subsequent joining process or when pressure is applied by the electrodes, thus ensuring consistent and high-quality joins.

[0148] For further details concerning the device according to the invention, reference can be made to the above statements concerning the other aspects of the invention, which apply accordingly to the device.

[0149] The subject matter of the present invention is explained below in a non-limiting manner with reference to the figures.

[0150] Figure 1 shows an arrangement of bipolar plates 1 according to the invention, which are arranged serially one above the other in a fuel cell stack 2. Membrane electrode assemblies 3 are located between each of the bipolar plates 1. The membrane electrode assemblies 3 are electrically connected via the bipolar plates 1. The bipolar plates 1 have flow fields for mass transport. The flow fields of the bipolar plates 1, together with the membrane electrode assemblies 3, form gas channels 6 for the transport of gases, in particular hydrogen and oxygen, in a fuel cell.

[0151] Inside the bipolar plates 1 are channels, in particular liquid channels 5, for the transport of cooling media, especially water. The bipolar plates 1 consist of two half-plates 4A and 4B, which are joined by resistance welding.

[0152] At the contact points between the half-plates 4A and 4B, the bipolar plates 1 according to the invention have weld points or weld seams 7, which were produced by resistance welding. The weld points or weld seams 7 permanently connect half-plates 4A and 4B with a lower electrical resistance than merely stacked or glued half-plates. The weld points or weld seams 7 have a width of less than 2 mm, in particular less than 1.5 mm, preferably less than 1 mm, more preferably less than 0.8 mm, more preferably less than 0.7 mm, and most preferably less than 0.6 mm. The weld points or weld seams 7 of the bipolar plates 1 according to the invention are thus significantly narrower than weld points or weld seams in bipolar plates of the prior art.

[0153] Since the bipolar plates 1 are produced by means of resistance welding, they also have the further advantage that the welding points or weld seams 7 are arranged inside the bipolar plates 1 and, with suitable process control, do not extend to the outer surfaces of the bipolar plates 1, i.e., the surface properties of the half-plates 4A and 4B, which were set during the manufacturing process, are not changed.

[0154] Fig. 2 shows a device 8 according to the invention for carrying out a method according to the invention and for producing bipolar plates 1 according to the invention. The device 8 has, in particular, two electrodes 9, which are preferably planar and structured and have structural elements 10. The electrodes are preferably connected to a direct current source.

[0155] The device 8 preferably further comprises means for generating pressure 11, for example, a pressing device. The means for generating pressure 11 preferably exerts pressure via the electrodes 9 on the workpieces to be joined, in particular the half-plates 4A and 4B, during the joining process. To carry out the method according to the invention, the workpieces to be joined, here the half-plates 4A and 4B, are placed between the electrodes 9 in the device 8. The workpieces, in particular the half-plates 4A and 4B, are usually also structured, which, however, is not shown in Fig. 2 for the sake of clarity. Typically, the half-plates 4A and 4B are flat and have structures.

[0156] In carrying out the method according to the invention, the workpieces, in particular the half-plates 4A and 4B, are arranged such that they touch at predetermined joining points, and pressure is then exerted on the half-plates 4A and 4B via the electrodes 9 by means of the pressure generation device 11. As previously described, the electrodes 9 preferably have structures, in particular structural elements 10, which touch the workpieces, in particular the half-plates 4A and 4B, at predetermined points or areas. At these contact points, when a direct current is applied to the interfaces between the half-plates 4A and 4B, the weld points or weld seams 7 are formed.

[0157] In Fig. 2, the structural elements 10 of the electrodes 9 are shown as parallel raised structures, which are used, for example, to create flow channels in bipolar plates. However, other electrode structures are also conceivable, in particular nonlinear electrode structures, to generate specific mass transport pathways or flow fields.

[0158] Figure 3 shows a section of a device 8 according to the invention with a preferred electrode configuration. The electrodes 9 are arranged parallel, in particular one above the other, and have structural elements 10 in the form of protrusions. Two half-plates 4A and 4B are arranged between the electrodes 9. The plates are in contact, thus forming a bipolar plate. Pressure is exerted on selected contact points or areas of the half-plates 4A and 4B via the pressure-generating device (not shown in Figure 3) through the electrodes 9 and, in particular, through the structural elements 10 of the electrodes 9. By applying a direct current to the electrodes 9, the half-plates 4A and 4B are heated at the contact points, so that the contact surfaces between the half-plates 4A and 4B melt and form a weld spot or weld seam 7. Figure 3 shows...4A represents a preferred embodiment of the method according to the invention as a spot or line welding process. The half-plates 4A and 4B, which are arranged between the electrodes 9, are also shown as a flat surface in this illustration, although they are usually structured.

[0159] Fig. 4B shows an alternative representation of the inventive method and preferred electrodes, in which the half-plates 4A and 4B are joined by projection welding.

[0160] In this case, the half-plates 4A and 4B have larger structures than in spot or line welding processes. According to this embodiment, the electrodes 9 can also preferably be structured; however, the structures of the electrodes 9 are typically larger and wider than in spot or line welding processes. According to this embodiment, one of the half-plates 4A or 4B has a protrusion 12 at the intended joining point, where the plates 4A and 4B touch. The joining point is defined by the protrusion 12. Therefore, the structural elements 10 of the electrodes 9 do not need to be as finely formed as in spot or line welding processes, but are arranged over the area of ​​the planned weld line. Subsequently, direct current is passed through the half-plates 4A and 4B.Since the electrical resistance is greatest at the contact points of the half-plates 4A and 4B, the strongest heating of the material of the half-plates 4A and 4B also takes place here, so that weld points or weld seams are formed at the contact points.

[0161] Reference numeral list: Bipolar plate 7 Weld seam Fuel cell stack 8 Device Membrane electrode unit 9 Electrode half plate 10 Structural element Liquid channel 11 Device for generating gas channel pressure

Claims

Patent claims:

1. Method for joining preferably structured electrically conductive plastic parts, in particular semi-finished products, preferably for joining structured plastic half-plates to bipolar plates, characterized in that the electrically conductive plastic parts are joined by means of electrical resistance welding.

2. The method according to claim 1, characterized in that the method is used to produce bipolar plates, heat exchangers and chemical reactors, in particular bipolar plates and heat exchangers.

3. Method according to claim 1 or 2, characterized in that at least one plastic part is structured, preferably all plastic parts are structured.

4. Method according to one of the preceding claims, characterized in that the plastic parts to be joined have a thickness of at most 5 mm, in particular at most 2 mm, preferably at most 1 mm, preferably at most 0.5 mm.

5. Method according to one of the preceding claims, characterized in that the plastic parts to be joined have a thickness of at least 0.05 mm, in particular at least 0.1 mm, at least 0.2 mm, preferably at least 0.25 mm.

6. Method according to claim 4 or 5, characterized in that the plastic parts to be joined have a thickness in the range of 0.05 to 5 mm, in particular 0.1 to 2 mm, preferably 0.2 to 1 mm, preferably 0.25 to 0.5 mm.

7. Method according to one of the preceding claims, characterized in that the plastic parts are joined via at least one welding point and / or at least one welding line.

8. Method according to claim 7, characterized in that the weld line or weld point has a width of less than 2 mm, in particular less than 1.5 mm, preferably less than 1 mm, preferably less than 0.8 mm, particularly preferably less than 0.7 mm, most particularly preferably less than 0.6 mm.

9. Method according to claim 7 or 8, characterized in that gas- and liquid-tight seams are produced in the joined plastic part by the joining process.

10. Method according to one of claims 7 to 9, characterized in that the welding process is carried out as a spot welding process, line welding process or projection welding process, preferably as a line welding process or projection welding process.

11. Method according to one of the preceding claims, characterized in that the resistance welding is carried out using at least 2 electrodes, preferably 2 electrodes.

12. Method according to claim 11, characterized in that at least one electrode is formed in a planar shape, preferably 2 electrodes are formed in a planar shape.

13. Method according to claim 11 or 12, characterized in that at least one electrode is structured, preferably 2 electrodes are structured.

14. Method according to one of the preceding claims, characterized in that the current density during the joining process is in the range of 100 to 500 A • cm' 2 , especially 150 to 350 A • cm' 2 , preferably 200 to 300 A • cm' 2 preferably 220 to 280 A • cm' 2 , amounts.

15. Method according to one of the preceding claims, characterized in that the contact pressure during the joining process is 1 to 8 MPa, in particular 2 to 6 MPa, preferably 3 to 5 MPa, preferably 3.5 to 5 MPa, particularly preferably 4 to 4.5 MPa.

16. Method according to one of the preceding claims, characterized in that the joining time, in particular the duration of the application of an external The voltage and pressure is 10 ms to 2 s, in particular 15 ms to 1.5 s, preferably 17 ms to 1 s, preferably 20 ms to 750 ms, particularly preferably 22 ms to 500 ms, most particularly preferably 25 to 250 ms.

17. Method according to one of the preceding claims, characterized in that the plastic part, in particular the semi-finished product, consists of a composite material.

18. Method according to claim 17, characterized in that the composite material comprises a thermoplastic polymer and electrically conductive particles.

19. Method according to claim 18, characterized in that the thermoplastic polymer is selected from the group consisting of polyethylene (PE), polypropylene (PP), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyvinyl chloride (PVC), polyamide and mixtures thereof.

20. Method according to claim 18 or 19, characterized in that the electrically conductive particles are selected from the group consisting of carbon, graphite, carbon black, carbogels, titanium carbide, titanium nitride, metals, metal compounds and mixtures thereof, in particular graphite, carbon black, nickel, titanium, platinum, ruthenium and mixtures thereof, preferably graphite, carbon black and mixtures thereof.

21. Method according to one of the preceding claims, characterized in that at least two preferably planar, structured and electrically conductive plastic semi-finished products are arranged at least partially overlapping one another, so that at least point or area contact is created between the plastic semi-finished products and the plastic semi-finished products are subsequently joined at least area or point by means of electrical resistance welding.

22. Method according to claim 21, characterized in that the plastic semi-finished products are arranged overlapping over their entire surface.

23. Method according to one of the preceding claims, characterized in that the plastic semi-finished products are fixed before the joining process.

24. A joined plastic part made of an electrically conductive plastic, in particular joined by a method according to one of claims 1 to 23, characterized in that the plastic part has weld points and / or weld seams with a width of at most 2 mm, in particular of at most 1 mm.

25. Device (8) for joining electrically conductive plastic parts (4A, 4B), characterized in that the device (8) comprises, (a) at least two electrodes (9) and (b) at least one device (11) for generating pressure.

26. Device (8) according to claim 25, characterized in that at least one electrode (9) is structured, 27. Device (8) according to claim 26, characterized in that 2 electrodes (9) are structured.

28. Device (8) according to claim 26 or 27, characterized in that the electrode (9) and / or the electrodes (9) have and / or have structural elements (10).

29. Device (8) according to one of claims 25 to 28, characterized in that the device (8) has a direct current source, in particular wherein the direct current source is connected to the electrodes.

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