Method for producing graphitized molded parts using 3D printing

Abstract

The present invention relates to a method for producing graphitized molded parts using 3D printing of commercially available filaments made of plastic. It is desired to provide a simple method for producing graphitized 3D molded parts or other three-dimensional structures made of thermoplastic plastic using 3D printing. A filament made of a meltable and hardenable plastic is melted and deposited layer by layer by a 3D printer until the desired structure is produced. The 3D structure (1) printed from the material of the filament is stabilized by a heat treatment that chemically or crystallographically changes the plastic, such as by pre-stabilizing at temperatures up to 180°C for a relatively long period of time, followed by a stabilization step at temperatures up to 250°C ± 20°C until the printed 3D structure (1) is shape-stable as a stabilized structure (8), and the 3D structure is carbonized or graphitized to produce a graphitized structure (9).

Description

[Technical Field]

[0001] The present invention relates to a method for producing graphitized molded parts using 3D printing with filaments made of meltable plastic. [Background technology]

[0002] Patent document 1 describes a method for manufacturing ceramic objects using 3D printing.

[0003] To do this, a precursor material containing carbon or graphite is mixed with inorganic components, which can be metals such as silicon, titanium, tungsten, or metal oxides, and then heat-treated in a 3D printer to produce a carbide-based ceramic object.

[0004] The precursor material further contains carbon black or graphite powder and a polymer such as polyacrylonitrile.

[0005] The precursor material thus prepared is deposited layer by layer by 3D printing, i.e., fused deposition modeling using a 3D printer, to form the desired object.

[0006] After printing, the stabilization of the object is carried out in air at a temperature of about 160°C to about 250°C, followed by a carbonization process at a temperature of 1000°C to 1500°C.

[0007] Furthermore, patent application WO 02 / 04999 describes a plastic mixture and its use.

[0008] The plastic raw materials include powdered silicon, a water-soluble, cross-linkable thermosetting resin and a carbon precursor with powdered silicon filler, and a water-soluble thermoplastic binder.

[0009] The method for producing honeycomb structures involves mixing materials to produce a plastic material mixture, and subsequent shaping of the green body, preferably by extrusion, drying and curing of the green body.

[0010] The green body is then heated in a nitrogen atmosphere to a temperature sufficient to carbonize the resin and sintered at a temperature sufficient to convert the green body to porous silicon carbide, preferably above 1400°C. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2018 / 196965 [Patent Document 2] WO 01 / 98207 Summary of the Invention [Problem to be solved by the invention]

[0012] The problem underlying the present invention is to provide a method for producing graphitized shaped components in the form of honeycomb structures or other three-dimensional structures made of thermoplastic plastics by means of 3D printing, which is significantly simplified compared to conventional methods, for example for use as supports for catalysts, filters, etc. or other structural components. [Means for solving the problem]

[0013] The problem underlying the present invention is: printing a 3D structure with a 3D printer using a meltable and hardenable filament made of plastic by melting the filament and depositing it in layers until the desired structure is created; Stabilizing 3D structures printed from filament material through a temperature treatment that chemically or crystallographically changes the plastic and Stabilization performing pre-stabilization of the 3D structure at a temperature of 150°C to 180°C for a predefined period of time; Then, a stabilization step is carried out at a temperature of up to 250°C ± 20°C until the 3D structure is sufficiently shape-stable. This is resolved by doing the following.

[0014] Surprisingly, it has been found that it is possible to produce 3D objects, such as honeycomb structures, figurines or the like, by printing them on a 3D printer using plastic filaments without any further admixtures, and then convert them directly into graphitized molded parts by a two-stage stabilization process at high temperatures, while preserving the printed structure.

[0015] In a first development of the invention, the meltable filaments consist of a thermoplastic plastic such as ABS (acrylonitrile butadiene styrene copolymer) or PVA / BVOH (polyvinyl alcohol) or BVOH (butenediol vinyl alcohol copolymer).

[0016] In another configuration of the invention, the printed 3D structure is ramped to the pre-stabilization temperature and the stabilization temperature at a heating ramp rate of 0.2°C / min to 1°C / min, respectively.

[0017] Finally, in a further development of the invention, the pre-stabilization and stabilization of the printed 3D structure are carried out after reaching a predetermined temperature and maintaining it constant for several hours to several days, depending on the size of the 3D structure, where a temperature of 180° C. for the pre-stabilization is preferred to accelerate this process.

[0018] It is important for the stabilization that the highest possible temperature in each case, at which no melting or severe deformation occurs, is adapted to the selected filament material, so that the stabilization can be accelerated overall.

[0019] Furthermore, it is advantageous if the printed 3D structure is stabilized by a support device to avoid deformation during pre-stabilization and stabilization, especially in the case of large components.

[0020] As a support device for the printed 3D structure, advantageously a temperature-stable material such as metal or graphite can be used, but it is also possible to use a molded part produced by the method as a support.

[0021] Alternatively, the printed 3D structure can be embedded in a temperature-stable powder, such as graphite dust or finely divided table salt, as a support device.

[0022] In one development of the invention, the stabilized 3D structure is carbonized and finally graphitized at high temperatures, which are carried out at temperatures >1800°C, preferably >2000°C.

[0023] It is understood that graphitization must be carried out under vacuum or under an inert gas such as argon, krypton, xenon or nitrogen to prevent combustion of the carbon.

[0024] During graphitization, a halogen gas such as chlorine can be fed into the furnace atmosphere to simultaneously initiate a cleaning process to remove foreign matter such as metals.

[0025] Another development of the invention consists in coating the three-dimensional structures with pyrolytic carbon after graphitization in order to seal the structures and make them more stable for use as carriers for catalysts, or for use as carriers in the form of skeletons, so that by suitable coating for example a skeleton (for example a honeycomb structure) can be used as a carrier for catalysts.

[0026] Additionally, the graphitized structure can be coated with or converted to silicon carbide, silicon nitride, or tantalum carbide.

[0027] To achieve encapsulation, the graphitized 3D structure can also be impregnated with pitch or resin.

[0028] In a particular embodiment of the invention, in a first step, a support or support structure or frame is first printed and stabilized on a suitable base, which is then used in a second step as a carrier, support or rest for the 3D structure to be printed.

[0029] To enable a printed 3D structure supported by a support or support structure to be easily detached or removed or removed from the support or support structure, after printing of the support or support structure, a release agent is applied to the support or support structure at least in a number of areas that will contact the subsequent 3D structure when the 3D structure is printed.

[0030] Alternatively, the support or support structure can be printed simultaneously with the 3D structure.

[0031] For example, a support structure in the form of a honeycomb structure on the outside and / or inside of the crucible or pot can be printed, resulting in an overall more stable structure. The support structure can then be mechanically removed after graphitization. This can be done by scraping the crucible or pot.

[0032] The invention will now be explained in more detail by way of example with reference to the accompanying drawings, in which: FIG. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is a schematic diagram showing a 3D printer. [Figure 2a] FIG. 10 is a diagram showing a 3D structure as a honeycomb structure after printing. [Figure 2b] FIG. 1 shows the three-dimensional structure after stabilization. [Figure 2c] FIG. 1 is a diagram showing a three-dimensional structure after graphitization. [Figure 3] 1 shows examples of three-dimensional structures produced by the method according to the invention, for example as supports for catalysts or filters, containers and structural elements (cubes). FIG. [Figure 4] FIG. 1 shows typical heating curves for pre-stabilization and stabilization. [Figure 5] FIG. 10 shows another example of a three-dimensional structure in the form of a frog. [Figure 6a] FIG. 1 shows a crucible or pot with a delicate support structure in the form of an inner honeycomb structure after printing and stabilization. [Figure 6b] FIG. 1 shows a crucible or pot in which the support structure has been mechanically removed after graphitization. DETAILED DESCRIPTION OF THE INVENTION

[0034] For producing a molding 1 made of meltable and hardenable plastic, for example in the form of a honeycomb structure (see FIG. 2), or another three-dimensional structure, a conventional 3D printer (FIG. 1) is best suited using a filament made of a heat-meltable plastic. Commercially available filaments can be used as the filament.

[0035] The 3D printer basically consists of a base plate 2 with threaded posts 3 that are spaced apart, oriented vertically and capable of running in the Y direction, i.e. back and forth, between which horizontal threaded rods 4 are suspended so that they can run vertically in the Z direction on the threaded posts 3 by means of threaded sleeves 5. The threaded rods 4 ultimately support the actual 3D printing head 6, which is capable of running in the X direction (laterally), which prints the 3D structure by means of a molten layer of filament 7 that is fed to the heatable printing head 6. For the very positioning in the X, Y and Z directions, correspondingly controllable step motors are usually acted upon.

[0036] This kind of relatively simply constructed 3D printer, together with a corresponding programmable control device, allows the realization of any 3D structure 1. Examples of such 3D structures can be seen in Figures 2, 3 and 5.

[0037] However, if it is desired that the 3D structure 1 produced in this way be graphitized after printing, a particular problem arises in that the heat-meltable plastics used for printing the 3D structure 1 will re-melt or at least lose their shape at the high temperatures required for graphitization, up to above 2000° C. This means that it is not possible to produce the desired final product consisting only of graphite structures.

[0038] Here, the invention starts by first printing a three-dimensional structure 1 with a 3D printer using a filament 7 made of a suitable plastic by melting the filament and depositing it layer by layer until the desired 3D structure, such as a honeycomb structure, skeleton or frame, is created (FIG. 2a). As plastic materials for the filament, ABS (acrylonitrile butadiene styrene copolymer), PVA / BVOH (polyvinyl alcohol) or butenediol vinyl alcohol copolymer) are particularly suitable.

[0039] TPU (thermoplastic polyurethane) is also suitable, as is ABS, although it will deform at higher temperatures.

[0040] The printed 3D structure 1 is then stabilized by a special temperature treatment by changing the chemical or crystallographic structure of the plastic.

[0041] For this purpose, a pre-stabilization is carried out at temperatures up to 150°C-180°C for a relatively long period until the printed 3D structure 1 is shape-stable, followed by a stabilization step at temperatures up to 250°C ± 20°C until the 3D structure is sufficiently shape-stable so that it can be transported or otherwise handled without being damaged. Temperatures up to 180°C are preferred for pre-stabilization because at this temperature stabilization is accelerated without melting occurring.

[0042] Figure 4 shows the corresponding temperature progression for the complete stabilization process, where the heating of the 3D structure to the pre-stabilization and stabilization temperatures is performed with a heating gradient of 0.2-1 °C / min, preferably 0.5 °C / min, respectively.

[0043] The actual pre-stabilization and stabilization of the printed 3D structure 1 are performed in a long-term process by maintaining the respective temperatures constant for several hours to several days, depending on the size of the 3D structure. The result of this thermal process is a stabilized 3D structure 8, as exemplarily shown in FIG. 2.

[0044] Furthermore, it is advantageous if the 3D structure 1 is stabilized by a support during pre-stabilization and stabilization in order to avoid deformations at least in critical places, which may occur especially if the printed 3D structure is large.

[0045] As a possible support for the 3D structure, a frame made of a temperature-stable material such as metal or graphite can be advantageously used. The frame supports the printed 3D structure 1 at several suitable points, for example, on the outside, underside, or inside. The frame manufactured by the present method can also be used as a support.

[0046] In a first step, a support or support structure or frame is first printed, for example by the methods described, on a suitable base, which support then has the possibility to be used as a carrier, support or rest for the 3D structure to be printed in a second step.

[0047] The support or support structure can also be printed at a higher or lower density than the 3D structure 1, but it is desirable that at least the support or support structure has sufficient mechanical strength, as is the case for example with honeycomb or beehive structures.

[0048] Furthermore, to enable the 3D structure to be easily removed from the support or support structure after stabilization, it must be ensured that the material used to print the support is not able to bond or adhere to the material used to print the 3D structure, or that the support or support structure has a sufficiently low density, such as a honeycomb or beehive structure, so that the support or support structure can be easily mechanically removed from the 3D structure without damaging it.

[0049] After printing of the support or support structure, it is also possible to apply a release agent onto the support or support structure at least in the areas that may come into contact with the 3D structure during subsequent printing of the 3D structure.

[0050] Furthermore, the support or support structure can be printed simultaneously, for example as a delicate honeycomb structure and a 3D structure, with the delicate honeycomb structure having the advantage of being particularly stable, yet easily mechanically removed after graphitization without damaging the 3D structure.

[0051] In another alternative, the printed 3D structure 1 can be embedded in a temperature-stable powder such as graphite dust or finely divided table salt as a support.

[0052] A disadvantage here is the relatively poor heat conductivity of the powder, but this is again compensated for by the long stabilization times in any case.

[0053] After the stabilization is complete, the stabilized 3D structure 8 (FIG. 2(b)) can be graphitized in a suitable furnace at high temperatures with high shape stability and exclusion of air, which can be done at temperatures >1800° C., preferably >2000° C. In this process, firstly the carbonization of the stabilized 3D structure occurs, i.e. the plastic is converted into a carbon-containing or amorphous carbon structure, which is finally converted into graphite.

[0054] Obviously, graphitization of the stabilized 3D structure 8 must be carried out in a furnace suitable for graphitization under vacuum or under an inert gas such as argon, krypton, xenon or nitrogen, to avoid oxidation damage, resulting in a graphitized 3D structure 9 (FIG. 2(c)).

[0055] During graphitization, a halogen gas such as chlorine can additionally be supplied to the furnace atmosphere in order to simultaneously initiate a cleaning process to remove foreign matter such as metals from the graphitized 3D structure 9.

[0056] An example of a graphitized 3D structure 9 in the form of a honeycomb structure is shown in Figure 2c. Honeycomb structures have a relatively high mechanical stability and contain particularly little material, but any other 3D structure can of course also be produced by the method according to the invention, see for example Figure 3 or Figure 5 which show a frog produced by the method according to the invention.

[0057] The graphitized 3D structure 9 can also be coated with pyrolytic carbon after graphitization, which seals the structure and makes it more stable, or can be used as a support in the form of a framework.

[0058] Alternative coating with silicon carbide (SiC), silicon nitride or tantalum carbide is also possible, or the graphitized 3D structure can be converted to silicon carbide or tantalum carbide by exposing the 3D structure to gases of the above-mentioned materials, e.g., SiO or Ta2O5, in a furnace.

[0059] Furthermore, the graphitized three-dimensional structure 9 can be impregnated with pitch or resin to achieve sealing, for example for filtration purposes.

[0060] Coating and impregnation are also carried out at high temperatures, so the 3D structure must at least be stabilized beforehand.

[0061] In FIG. 3, some examples of 3D structures produced by the method according to the invention can be seen, such as supports for catalysts or filters (top), containers (middle) and structural elements, such as cubes (bottom).

[0062] To use the graphitized 3D structure 9 as a support for a catalyst, it is only necessary to coat the 3D structure 9 with the material required for catalytic function in a suitable coating device. Known coating processes such as CVD (chemical vapor deposition), PVD (physical vapor deposition), PECVD (plasma-enhanced chemical vapor deposition), etc. are suitable for this purpose.

[0063] Alternatively, a sol-gel process can be used to wet chemically coat the graphitized 3D structures, allowing them to be used as catalysts.

[0064] Figure 6a shows one example of a 3D structure 1 in the form of a crucible or pot 10 and an inner support structure 11 in the form of a honeycomb structure within the crucible or pot, which inner support structure 11 is printed at the same time as the crucible or pot (Figure 6a). Obviously, the 3D structure may have other shapes.

[0065] The delicate support structure 11 can be easily removed mechanically after graphitization. This can be done by scraping the crucible or pot 10.

[0066] 6b shows the crucible or pot 10 from which the support structure 11 has been removed after graphitization. Remains 12 of the removed support are visible on the inside bottom of the crucible or pot 10. [Explanation of symbols]

[0067] 1. Printed 3D structure 2 base plates 3 threaded posts 4 threaded rods 5 Threaded Sleeve 6 print heads 7 filaments 8. Stabilized 3D Structure 9 Graphitized 3D structure 10 crucibles or pots 11 Support structure 12 Traces of removed support

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