Tube furnace for sintering and / or degreasing process

Abstract

The invention relates to the use of a tube furnace in a sintering and / or degreasing process, where the tube furnace comprises a tube (T) comprising an oxide ceramic matrix composite (OCMC). Furthermore, the invention relates to a tube furnace for a sintering and / or degreasing process wherein the tube furnace comprises a tube (T) comprising an oxide ceramic matrix composite material (OCMC). The invention further relates to the use of the tube furnace according to the invention in a method for treating at least one three-dimensional green body (GB).

Description

[0001] The present invention relates to the use of a tube furnace in a sintering and / or debinding process, wherein the tube furnace comprises a tube (T) which contains an oxide ceramic matrix composite (OCMC). Furthermore, the present invention relates to a tube furnace for a sintering and / or debinding process, wherein the tube furnace comprises a tube (T) which contains an oxide ceramic matrix composite (OCMC). Furthermore, the present invention relates to the use of the tube furnace according to the invention in a method for processing at least one three-dimensional green body (GB).

[0002] Tasks which are frequently encountered these days are the production of prototypes, spare parts and models of metal or ceramic bodies, in particular prototypes, spare parts and models having complex geometries. In particular for the production of prototypes, rapid production processes are required. For such so-called "rapid prototyping", different methods are known. One of the most economical methods is the fused filament fabrication process (FFF), also known as "fused deposition modeling" (FDM).

[0003] The fused filament fabrication process (FFF) is an additive manufacturing technique. A three-dimensional object is produced by extruding a thermoplastic material through a nozzle to form layers as the thermoplastic material hardens after extrusion. The nozzle is heated to heat the thermoplastic material to above its melting point and / or glass transition temperature and is then deposited on a substrate by an extrusion head, thereby forming a three-dimensional object in a layer-by-layer manner. The thermoplastic material is typically selected and its temperature is controlled such that it solidifies essentially immediately when extruded or dispensed onto the substrate, and multiple layers are stacked to form the desired three-dimensional object.

[0004] To form each layer, a drive motor is provided to move the substrate and / or the extrusion nozzle (dispensing head) relative to each other in a predetermined pattern along the x-axis, y-axis and z-axis. The FFF process was first described in US 5,121,329.

[0005] Typical materials for the production of three-dimensional objects are thermoplastic materials. It is only possible to produce three-dimensional metal or ceramic objects by fused filament fabrication if the metal or ceramic material has a low melting point such that it can be heated and melted by the nozzle. If the metal or ceramic material has a high melting point, it is necessary to provide the metal or ceramic material to the extrusion nozzle in a binder composition. The binder composition typically contains a thermoplastic material. When a mixture of the metal or ceramic material in the binder is deposited on a substrate, the three-dimensional object formed is a so-called "green body", which contains the metal or ceramic material in the binder. In order to obtain the desired metal or ceramic object, the binder must be removed and finally the object must be sintered. The three-dimensional object formed after removal of the binder is a so-called "brown body"; the three-dimensional object formed after sintering is the so-called "sintered body". The removal of the binder is also referred to as "debinding".

[0006] Typically, the sintering and / or debinding process is carried out in a furnace.

[0007] There are different types of furnaces for different applications. One type of furnace is the so-called tube furnace. It includes a tubular structure with heating elements outside the tube. The tube can be closed at both ends, open at both ends, or closed at one end and open at the other end. With an airtight tube and connectors attached to the tube, a controlled atmosphere can be formed inside the tube. Different types of gases with controlled pressure can be used for heating processes such as three-dimensional printing or sintering and / or debinding processes of metal injection molded parts.

[0008] There are different types of materials that can be used to construct such a tube. It is important that the tube withstands the process conditions without any damage caused, for example, by thermal stress or mechanical stress. For sintering and / or debinding processes of three-dimensional printing or metal injection molded parts, it is additionally desirable that the tube can operate at temperatures > 1250 °C, while the inner diameter of the tube is greater than 90 mm and the heated length of the tube is greater than 300 mm. In the context of the present invention, the term "heated length" means the effective heated length, which means the section in the tube where the temperature deviates from the target temperature setpoint by -10 K to +10 K. Thus, this section is the section where a uniform temperature is achieved.

[0009] However, although a large inner diameter of up to 245 mm can be achieved without introducing critical thermal stress by using materials based on metals or metal alloys as the tube material, higher temperatures or negative pressure (vacuum) at higher temperatures cause deformation of the tube. In addition, the debinding process is usually carried out with acids, especially gaseous acids, which causes corrosion of the tube (e.g., a tube made of titanium or a titanium alloy). On the other hand, by using materials based on ceramic or glass-like materials, temperatures above 1250 °C can be achieved, but an inner diameter greater than 90 mm introduces critical thermal stress and damages the tube.

[0010] So far, none of the known tube materials can meet all the desired criteria, i.e., being non-corrosive in oxidizing and reducing atmospheres, in acidic and alkaline media, being chemically inert, being creep-resistant and fatigue-resistant at temperatures > 1250 °C, while having an inner diameter greater than 90 mm and a heated length greater than 300 mm.

[0011] International patent application WO 2016 / 184776 A1 describes an airtight multi-layer composite tube that has a heat transfer coefficient > 500 W / m 2 / K and includes at least two layers, one layer made of non-porous monolithic oxide ceramic and one layer made of oxide fiber composite ceramic.

[0012] European patent application EP 3 835 639 A1 describes a tube having a heat transfer coefficient > 500 W / m 2An airtight multi-layer composite tube with a heat transfer coefficient of / K, the airtight multi-layer composite tube comprising at least two layers, the composite tube having, in its construction, an integral oxide ceramic without openings in the cross-section of the wall of the composite tube as the inner layer, the inner layer being wrapped by an outer layer of oxide fiber composite ceramic, and wherein an electrical conducting system is embedded in the wall of the composite tube.

[0013] International patent application WO 2019 / 201654 A1 describes a device for sealingly connecting two tubular elements (10, 20), wherein the end face of the first tubular element is sealingly connected to the end face of the second tubular element. Each tubular element has a sleeve (12, 22) extending radially outwards, wherein the second tubular element (20) is made of a ceramic material, the connecting side end thereof being at least partially provided with a circumferential support layer, a sleeve made of a ceramic material surrounding the support layer as the sleeve (22) and being firmly connected thereto, and a connecting element (30) being connected to the outer side of the sleeve. The support layer is made of an oxide ceramic fiber composite material.

[0014] International patent application WO 2020 / 187607 A1 describes an airtight multi-layer composite tube with a heat transfer coefficient of >500 W / m 2 / K, the airtight multi-layer composite tube having, in its structure, an integral oxide ceramic without pores in the cross-section of the wall of the composite tube as the inner layer, the inner layer being wrapped by an outer layer of oxide fiber - composite ceramic, the outer layer having an open porosity of 5% < ε < 50%, and having a plurality of depressions on the inner surface of the composite tube facing the outer wall of the composite tube.

[0015] The disadvantages of the prior art are that the oxide ceramic composite reinforcement only endures up to approximately 1200 °C permanently. In addition, the oxide ceramic composite loses its beneficial mechanical properties, such as strength and quasi-ductility. Therefore, within this temperature range, the oxide ceramic composite gradually becomes brittle and is sensitive to thermal shock.

[0016] Therefore, the object of the present invention is to provide an improved tube furnace that can be used during the sintering and / or debinding process and does not have the above-mentioned disadvantages of the prior art or only has the above-mentioned disadvantages to a significantly reduced extent.

[0017] This object is solved by using a tube furnace during the sintering and / or debinding process, wherein the tube furnace comprises a tube (T) that contains an oxide ceramic matrix composite (OCMC).

[0018] Another object of the present invention is a tube furnace for the sintering and / or debinding process, wherein the tube furnace comprises a tube (T) that contains an oxide ceramic matrix composite (OCMC).

[0019] A further object of the present invention is the use of the tube furnace of the present invention in a method for treating at least one three-dimensional green body (GB), wherein the method comprises at least the following steps

[0020] a) providing the at least one three-dimensional green body (GB), wherein the at least one three-dimensional green body (GB) comprises an inorganic powder (IP) and a binder (B),

[0021] b) providing an acid,

[0022] c) treating the at least one three-dimensional green body (GB) with the acid in the tube furnace to obtain at least one three-dimensional brown body (BB),

[0023] and optionally

[0024] d) sintering the at least one three-dimensional brown body (BB) obtained in step c) in the tube furnace to obtain at least one three-dimensional sintered body (SB).

[0025] It has surprisingly been found that by using a tube furnace (wherein the tube furnace comprises a tube (T) comprising an oxide ceramic matrix composite (OCMC)) during sintering and / or debinding, an inner diameter of the tube in the range from 50 mm to 500 mm, more preferably in the range from 90 mm to 400 mm, particularly preferably in the range from 90 mm to 300 mm and most preferably in the range from 120 mm to 280 mm can be achieved, and the tube (T) can be heated to achieve a uniform temperature over a length of at least 100 mm, preferably at least 300 mm, at a temperature in the range from 1250 °C to 1500 °C without damaging and / or corroding the tube, while the end regions of the tube outside the furnace are cold.

[0026] Furthermore, it has surprisingly been found that the tube furnace of the present invention can also be used in a method for treating at least one three-dimensional green body (GB), wherein the at least one three-dimensional green body (GB) is treated with an acid, in particular a gaseous acid, without corrosion of the tube furnace. The process of treating the three-dimensional green body (GB) with a gaseous acid is also referred to as a catalytic debinding process.

[0027] It is even possible that the debinding process, in particular the catalytic debinding process, and the sintering process take place subsequently or in parallel without any kind of equipment change or intervention. This results in a very fast and robust workflow.

[0028] The present invention is described in more detail below.

[0029] Tube furnace

[0030] A first object of the present invention is the use of a tube furnace in a sintering and / or debinding process, wherein the tube furnace comprises a tube (T).

[0031] Tube (T)

[0032] In a preferred embodiment, the tube (T) comprises an inner tube (IT) and an outer layer (OL), wherein the outer layer (OL) is attached to the inner tube (IT), and wherein the inner tube (IT) comprises a non-porous monolithic oxide ceramic, and the outer layer (OL) comprises an oxide ceramic matrix composite (OCMC).

[0033] Preferably, the inner tube (IT) comprises a wall (W) having an inner surface (IS) and an outer surface (OS), wherein the outer layer (OL) is preferably attached to the outer surface (OS) of the wall (W) of the inner tube (IT). The outer layer (OL) may cover the outer surface (OS) of the wall (W) of the inner tube (IT) over the entire length of the inner tube (IT) or over a section of the axis of the inner tube (IT).

[0034] Figure 1 The inner tube (IT) is shown, which comprises a wall (W) having an inner surface (IS) and an outer surface (OS). Figure 3 A cross-section of the tube (T) comprising the inner tube (IT) and the outer layer (OL) is shown, wherein the outer layer (OL) is attached to the outer surface (OS) of the wall (W) of the inner tube (IT).

[0035] In the case where the outer layer (OL) partially covers the outer surface (OS) of the wall (W) of the inner tube (IT), i.e., over a section of the axis of the inner tube (IT), there are the following advantages: the cost of the tube (T) is lower, and the heat transfer in the central hot zone of the tube (T) is improved.

[0036] The wall (W) of the inner tube (IT) typically has a wall thickness (WT). The inner tube (IT) may have any desired wall thickness (WT). Preferably, the wall thickness (WT) of the inner tube (IT) is in the range of 0.5 mm to 45 mm, more preferably in the range of 1 mm to 25 mm, and most preferably in the range of 3 mm to 15 mm.

[0037] Figure 2 A cross-section of the inner tube (IT) is shown, which comprises a wall (W) having an inner surface (IS) and an outer surface (OS) and a wall thickness (WT).

[0038] The outer layer (OL) may also have any desired thickness. Preferably, the outer layer (OL) has a thickness in the range of 0.5 mm to 5 mm, more preferably in the range of 0.5 mm to 4 mm.

[0039] The total thickness of the wall thickness (WT) of the inner tube (IT) and the thickness of the outer layer (OL) can also have any desired range. Preferably, the total thickness of the wall thickness (WT) of the inner tube (IT) and the thickness of the outer layer (OL) is in the range of 1 mm to 50 mm, more preferably in the range of 1.5 mm to 29 mm, and most preferably in the range of 3.5 mm to 19 mm.

[0040] Preferably, the tube furnace includes a heating element outside the tube (T). The tube furnace can include any desired heating element. Preferably, the tube furnace includes a metal heating element.

[0041] The tube (T) preferably includes two ends, where the two ends of the tube (T) are closed, both ends are open, or one end is closed and the other end is open.

[0042] The inner diameter of the inner tube (IT) can have any desired range. Preferably, the inner diameter of the inner tube (IT) is in the range of 50 mm to 500 mm, more preferably in the range of 90 mm to 400 mm, particularly preferably in the range of 90 mm to 300 mm, and most preferably in the range of 120 mm to 280 mm.

[0043] In a preferred embodiment, the tube (T) is heated over a length of 100 mm to 1000 mm, preferably 300 mm to 600 mm.

[0044] As defined above, in the context of the present invention, the term "heating length" means the effective heating length, which means the section of the tube (T) in which the temperature in the tube (T) deviates from the target temperature set point by -10 K to +10 K. Thus, this section is the section where a uniform temperature is achieved.

[0045] The tube (T) can be heated to any desired temperature. Preferably, the tube (T) is heated to a temperature in the range of 15 °C to 1500 °C, more preferably heated to a temperature in the range of 100 °C to 1500 °C, and most preferably heated to a temperature in the range of 1250 °C to 1500 °C.

[0046] The tube (T) preferably includes an inner tube (IT) and an outer layer (OL).

[0047] Inner tube (IT)

[0048] The inner tube (IT) contains a non-porous monolithic oxide ceramic. Preferably, the inner tube (IT) is composed of a non-porous monolithic oxide ceramic.

[0049] Non-porous monolithic oxide ceramic

[0050] As the non-porous monolithic oxide ceramic, any non-porous monolithic oxide ceramic known to those skilled in the art can be used.

[0051] In the context of the present invention, the term "non-porous" means that the porosity of the oxide ceramic is preferably < 10%, more preferably < 4%. The porosity is defined as (the ratio of the void volume of the oxide ceramic to the total volume of the oxide ceramic) * 100%.

[0052] In the context of the present invention, the term "monolithic" means that the inner tube (IT) comprising the non-porous oxide ceramic is preferably prepared as one continuous piece from the non-porous oxide ceramic. Methods for the production of monolithic ceramics are known in the art, for example as described in Informationszentrum Technische Keramik [Information Center for Technical Ceramics], IZTK (ed.). (1999). Brevier Technische Keramik [Brevier Technical Ceramics].

[0053] Based on the total weight of the non-porous monolithic oxide ceramic, the non-porous monolithic oxide ceramic preferably comprises at least 90% by weight, more preferably at least 95% by weight and most preferably at least 97% by weight of at least one selected from the group consisting of alumina (Al2O3) and mullite (Al 4+2x Si 2-2x O 10-x ; x≈0.4). As the non-porous monolithic oxide ceramic, Haldenwanger Pythagoras 1800Z TM , Pythagoras 1800 (mullite), Alsint 99.7 TM , Kyocera AL23 or AL24 (alumina) can be used in particular.

[0054] However, the non-porous monolithic oxide ceramic can also be a compound of at least one selected from the group consisting of ZrO2, Y2O3 and MgO. In addition, the non-porous monolithic oxide ceramic can also contain a substantial amount of non-oxide compounds, such as carbides or nitrides, for example SiC, Si3N4, AlN.

[0055] The density of the non-porous monolithic oxide ceramic is preferably greater than the density of the oxide ceramic matrix composite (OCMC). The density of the non-porous monolithic oxide ceramic is preferably in the range of 1000 kg / m 3 to 7000 kg / m 3 and more preferably in the range of 2000 kg / m 3 to 5000 kg / m 3within a range of, for example, 2,800 kg / m for mullite 3 or 3,700 kg / m for alumina (Al2O3) with a purity of > 99.7% 3 .

[0056] Outer layer (OL)

[0057] The outer layer (OL) comprises an oxide ceramic matrix composite (OCMC). Preferably, the outer layer (OL) consists of an oxide ceramic matrix composite (OCMC).

[0058] Oxide ceramic matrix composite (OCMC)

[0059] The oxide ceramic matrix composite (OCMC) preferably comprises

[0060] - a matrix (M), wherein the matrix (M) comprises oxide ceramic particles (P), and

[0061] - fibers (F),

[0062] wherein these fibers (F) are embedded as linear, sheet-like, or three-dimensional textile structures between the oxide ceramic particles (P) of the matrix (M).

[0063] The oxide ceramic particles (P) generally exist in a sintered form, which means they exist as solid blocks.

[0064] In a more preferred embodiment, the oxide ceramic matrix composite (OCMC) consists of

[0065] - a matrix (M), wherein the matrix (M) comprises oxide ceramic particles (P), and

[0066] - fibers (F),

[0067] wherein these fibers (F) are embedded as linear, sheet-like, or three-dimensional textile structures between the oxide ceramic particles (P) of the matrix (M).

[0068] The density of the oxide ceramic matrix composite (OCMC) is preferably in the range of 500 kg / m 3 to 3,000 kg / m 3 .

[0069] Matrix (M)

[0070] The matrix (M) comprises oxide ceramic particles (P). Preferably, the matrix (M) consists of oxide ceramic particles (P).

[0071] The oxide ceramic particles (P) can in principle comprise any desired ceramic oxide. The oxide ceramic particles (P) are preferably particles comprising an oxide of at least one element selected from the group consisting of: Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se and Te, or a mixture of these oxides.

[0072] In the context of the present invention, the term "oxide of at least one element" means that the oxide can exactly comprise one element from the group consisting of: Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se and Te, or the oxide can comprise two or more elements from the group consisting of: Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se and Te. An example of an oxide comprising two or more elements from the group consisting of: Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se and Te is Al2SiO5.

[0073] The oxide ceramic particles (P) are more preferably particles comprising an oxide of at least one element selected from the group consisting of Ti, Zr, Hf, Cr, Fe, Al, Si, Na, K, and most preferably particles comprising an oxide of at least one element selected from the group consisting of Zr, Al, Si.

[0074] In a particularly preferred embodiment, the oxide ceramic particles (P) are particles comprising an oxide of at least one element selected from the group consisting of Zr, Al, and Si.

[0075] In a most preferred embodiment, the oxide ceramic particles (P) comprise a mixture of alumina and silica, preferably, the oxide ceramic particles (P) consist of a mixture of alumina and silica.

[0076] Fibers (F)

[0077] The oxide ceramic matrix composite (OCMC) preferably comprises fibers (F).

[0078] As the fibers (F), in principle, all known fibers can be used. Preferably, the fibers (F) are ceramic fibers (F), more preferably non-oxide and / or oxide ceramic fibers (F), and most preferably oxide ceramic fibers (F).

[0079] The oxide ceramic fibers (F) preferably comprise an oxide of at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se, and Te, or a mixture of these oxides.

[0080] In a more preferred embodiment, the oxide ceramic fibers (F) comprise a compound selected from the group consisting of alumina such as NEXTEL 610 or OxCeFiA99, mullite, a mixture of alumina and mullite such as NEXTEL 720 or OxCeFiM75, zirconia toughened alumina (ZTA), and zirconia toughened mullite (ZTM). More preferably, the oxide ceramic fibers (F) consist of a compound selected from the group consisting of alumina, mullite, a mixture of alumina and mullite, zirconia toughened alumina (ZTA), and zirconia toughened mullite (ZTM).

[0081] The non-oxide fibers (F) preferably comprise at least one compound selected from the group consisting of boron nitride, tungsten carbide, aluminum nitride, barium titanate, lead zirconate titanate, and boron carbide.

[0082] Of course, the fiber (F) may also contain oxides of at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se, and Te, or mixtures of these oxides, and / or at least one compound selected from the group consisting of boron nitride, tungsten carbide, aluminum nitride, barium titanate, lead zirconate titanate, and boron carbide. In this case, the fiber (F) has a non-oxide ceramic portion and an oxide ceramic portion.

[0083] The fiber (F) can have any desired diameter. The fiber (F) preferably has a diameter in the range of 5 μm to 15 μm, more preferably in the range of 10 μm to 12 μm.

[0084] These fibers (F) are embedded between the oxide ceramic particles (P) of the matrix (M) as linear, sheet-like, or three-dimensional textile structures. Preferably, they are embedded between the oxide ceramic particles (P) of the matrix (M) as sheet-like or three-dimensional textile structures.

[0085] An example of a linear textile structure is a fiber bundle in which multiple fibers (F) are combined. Such fiber bundles can be wound on bobbins (roving).

[0086] Examples of sheet-like textile structures are woven fabrics, knitted fabrics, braids, or non-woven fabrics, which are processed from fibers (F) (e.g., from linear textile structures such as fiber bundles).

[0087] Examples of three-dimensional textile structures (bulk structures) are textile hoses, which are also processed from fibers (F) (e.g., from linear textile structures such as fiber bundles).

[0088] Suitable rovings are NEXTEL 610 1500 denier, NEXTEL 610 4500 denier, NEXTEL 610 10000 denier, NEXTEL 610 20000 denier, NEXTEL 720 1500 denier, and 10000 denier.

[0089] Suitable sheet-like or three-dimensional textile structures are NEXTEL 610DF11, DF-13-4500, DF19, NEXTEL720EF11, and EF19.

[0090] The fiber (F) may contain

[0091] i) a component identical to the oxide ceramic particles (P), or

[0092] ii) a component different from the oxide ceramic particles (P).

[0093] The tube (T) is preferably prepared by a method comprising at least the following steps i) and ii):

[0094] i) providing an inner tube (IT) comprising a non-porous monolithic oxide ceramic, and

[0095] ii) attaching an outer layer (OL) to the inner tube (IT), preferably by a lamination technique.

[0096] Preferably, step i) comprises at least the following steps:

[0097] i-1) preparing particles of a non-porous monolithic oxide ceramic,

[0098] i-2) preparing a three-dimensional green body from the particles prepared in step i-1), for example by molding, extrusion or isostatic pressing, preferably by extrusion or isostatic pressing,

[0099] i-3) sintering the three-dimensional green body obtained in step i-2) to obtain the inner tube (IT).

[0100] Attaching the outer layer (OL) to the inner tube (IT) according to step ii) is preferably carried out by a lamination technique, which comprises the following steps:

[0101] ii-1) providing fibers (F), wherein these fibers (F) are in the form of a linear, sheet-like or three-dimensional textile structure, preferably in the form of a sheet-like or three-dimensional textile structure,

[0102] ii-2) impregnating these fibers (F) with a slurry, wherein the slurry contains water, oxide ceramic particles (P) and a binder (B1) to obtain impregnated fibers (IF),

[0103] ii-3) laminating the inner tube (IT) with the impregnated fibers (IF) obtained in step ii-2) to obtain a laminated inner tube (LIT),

[0104] ii-4) drying the laminated inner tube (LIT) obtained in step ii-3) at a temperature in the range of 40 °C to 150 °C, preferably at a temperature in the range of 60 °C to 100 °C, to remove at least a portion of the water and obtain a dried laminated inner tube (DLIT), and

[0105] ii-5) Sinter the dried laminated inner tube (DLIT) obtained in step ii-4) at a temperature in the range of 1100 °C to 1300 °C, preferably at a temperature in the range of 1150 °C to 1250 °C, to completely remove the binder (B1) and water and obtain a tube (T), wherein the tube (T) comprises an inner tube (IT) and an outer layer (OL), wherein the outer layer (OL) is attached to the inner tube (IT), and wherein the inner tube (IT) comprises a non-porous monolithic oxide ceramic and the outer layer (OL) comprises an oxide ceramic matrix composite (OCMC).

[0106] Steps ii-1) to ii-5) can be repeated until the desired thickness of the outer layer (OL) is achieved.

[0107] The binder (B1) is preferably at least one compound selected from the group consisting of ZrO2, Al2O3 and SiO2.

[0108] The infiltration in step ii-2) is preferably carried out by dipping or spin coating, preferably by spin coating.

[0109] Another object of the present invention is also a tube furnace for sintering and / or debinding processes, wherein the tube furnace comprises a tube (T) which comprises an oxide ceramic matrix composite (OCMC).

[0110] For the tube furnace, the above embodiments and preferences regarding the use of at least one tube furnace in sintering and / or debinding processes apply analogously.

[0111] The tube furnace is used in sintering and / or debinding processes, preferably in the debinding process of at least one three-dimensional green body (GB) to obtain at least one three-dimensional brown body (BB) and / or in the sintering process of at least one three-dimensional brown body (BB) to obtain at least one three-dimensional sintered body (SB).

[0112] Use of a tube furnace in a method for treating at least one three-dimensional green body (GB)

[0113] Accordingly, another object of the present invention is the use of the tube furnace of the present invention in a method for treating at least one three-dimensional green body (GB), wherein the method comprises at least the following steps

[0114] a) Providing the at least one three-dimensional green body (GB), wherein the at least one three-dimensional green body (GB) comprises an inorganic powder (IP) and a binder (B),

[0115] b) Providing an acid,

[0116] c) Treating the at least one three-dimensional green body (GB) with the acid in the tube furnace to obtain at least one three-dimensional brown body (BB),

[0117] and optionally

[0118] d) sintering the at least one three-dimensional brown body (BB) obtained in step c) in the tube furnace so as to obtain at least one three-dimensional sintered body (SB).

[0119] The method according to the invention for treating at least one three-dimensional green body (GB) comprises at least steps a) to c) and optionally step d).

[0120] Steps a) and b) can be performed simultaneously, but it is also possible to perform step a) before step b) or to perform step b) before step a). Step c) is preferably performed after steps a) and b), and optional step d) is preferably performed after step c), more preferably immediately after step c).

[0121] Step a)

[0122] In step a), at least one three-dimensional green body (GB) is provided.

[0123] The term "at least one three-dimensional green body" according to the present invention specifically means one three-dimensional green body and a mixture of two or more three-dimensional green bodies.

[0124] At least one three-dimensional green body (GB) comprises an inorganic powder (IP) and a binder (B), wherein the binder (B) preferably comprises

[0125] (b1) At least one polyoxymethylene (POM).

[0126] Preferably, at least one three-dimensional green body (GB) comprises 30 to 70% by volume of an inorganic powder (IP) and 30 to 70% by volume of a binder (B), based on the total volume of the at least one three-dimensional green body (GB), wherein the % by volume of the inorganic powder (IP) and the binder (B) as a whole add up to 100%.

[0127] More preferably, at least one three-dimensional green body (GB) comprises 45 to 65% by volume of an inorganic powder (IP) and 35 to 55% by volume of a binder (B), based on the total volume of the at least one three-dimensional green body (GB), wherein the % by volume of the inorganic powder (IP) and the binder (B) as a whole add up to 100%.

[0128] Particularly preferably, at least one three-dimensional green body (GB) comprises 48 to 60% by volume of an inorganic powder (IP) and 40 to 52% by volume of a binder (B), based on the total volume of the at least one three-dimensional green body (GB), wherein the % by volume of the inorganic powder (IP) and the binder (B) as a whole add up to 100%.

[0129] In one embodiment of the present invention, at least one three-dimensional green body (GB) comprises at least one dispersant. Preferably, at least one three-dimensional green body (GB) comprises at least one dispersant in an amount of from 0.1% to 5% by volume, particularly preferably from 0.2% to 4% by volume, and most preferably from 0.5% to 2% by volume, based on the total volume of the at least one three-dimensional green body (GB).

[0130] For a person skilled in the art, it is clear that if at least one three-dimensional green body (GB) comprises at least one dispersant, the % by volume of the inorganic powder (IP), binder (B) and at least one dispersant together generally totals 100%.

[0131] The "at least one dispersant" according to the present invention specifically means one dispersant, as well as a mixture of two or more dispersants.

[0132] Examples of suitable dispersants are oligomeric poly(ethylene oxide) with a low molecular weight of from 200 to 600 g / mol, stearic acid, stearamide, hydroxystearic acid, fatty alcohols, fatty acid esters, sulfonic acid esters, and block copolymers of ethylene oxide and propylene oxide, as well as polyisobutene, which is particularly preferred.

[0133] The components of the at least one three-dimensional green body (GB) are presented in more detail below.

[0134] Inorganic powder (IP)

[0135] At least one three-dimensional green body (GB) comprises an inorganic powder (IP).

[0136] As the inorganic powder (IP), any known inorganic powder (IP) can be used. Preferably, a sinterable inorganic powder (IP) is used. More preferably, the inorganic powder (IP) is a powder of at least one inorganic material selected from the group consisting of metals, metal alloys and ceramic materials, and most preferably, the inorganic powder (IP) is a metal or a metal alloy, and particularly preferably, the inorganic powder (IP) is a metal.

[0137] "Inorganic powder (IP)" specifically means an inorganic powder (IP) and a mixture of two or more inorganic powders (IP). The same applies to the term "inorganic material". "Inorganic material" specifically means an inorganic material and a mixture of two or more inorganic materials. "Metal" specifically means a metal and a mixture of two or more metals. The metal in the present invention can be selected from any metal in the periodic table, which is stable under the conditions of the fused filament fabrication process and can form a three-dimensional object. Preferably, the metal is selected from the group consisting of aluminum, yttrium, titanium, zirconium, vanadium, niobium, chromium, molybdenum, tungsten, manganese, iron, carbonyl iron powder (CIP), cobalt, nickel, copper, silver, zinc, magnesium, tin, and cadmium, and more preferably, the metal is selected from the group consisting of titanium, niobium, chromium, molybdenum, tungsten, manganese, iron, carbonyl iron powder (CIP), nickel, and copper. Particularly preferably, the metal is selected from the group consisting of titanium, iron, and carbonyl iron powder (CIP).

[0138] Carbonyl iron powder (CIP) is a high-purity iron powder prepared by chemical decomposition of purified iron pentacarbonyl.

[0139] "Metal alloy" specifically means a metal alloy and a mixture of two or more metal alloys. In the context of the present invention, the term "metal alloy" means a solid solution or partial solid solution that exhibits metallic properties and contains a metal and another element. "Metal" specifically means a metal and a mixture of two or more metals as described above. The same applies to "another element". "Another element" specifically means an other element and a mixture of two or more other elements. A solid solution metal alloy exhibits a single solid-phase microstructure, while a partial solid solution metal alloy exhibits two or more solid phases. These two or more solid phases can be uniformly distributed in the metal alloy, but they can also be non-uniformly distributed in the metal alloy. The metal alloy can be prepared by any process known to those skilled in the art. For example, the metal can be melted and other elements can be added to the molten metal. However, in the case where the metal alloy has not been prepared previously, it is also possible that the inorganic powder (IP) contains a metal and other elements. Then, a metal alloy will be formed during the process of preparing the three-dimensional object.

[0140] Regarding metals, the above-described embodiments and preferred options for metals are applicable. Other elements may be selected from the above metals. However, the other elements are different from the metals contained in the metal alloy. The other elements may be selected from any element of the periodic table, which forms a stable metal alloy under the conditions of the fuse manufacturing process, or the other element is stable under the fuse process conditions or forms a stable alloy with the metal. In a preferred embodiment of the present invention, the other elements are selected from the group consisting of the above metals, boron, carbon, silicon, phosphorus, sulfur, selenium, and tellurium. Particularly preferably, at least one other element is selected from the group consisting of the above metals, boron, carbon, silicon, phosphorus, and sulfur. Preferably, the metal alloy according to the present invention includes steel.

[0141] "Ceramic material" exactly means a ceramic material and a mixture of two or more ceramic materials. In the context of the present invention, the term "ceramic material" means a non-metal or a non-metal compound of a first-class metal and a non-metal or a second-class metal.

[0142] "Metal" exactly means a metal and also a mixture of two or more metals. This also applies to "non-metal", "first-class metal", and "second-class metal". "Non-metal" exactly means a non-metal and also a mixture of two or more non-metals. "First-class metal" exactly means a first-class metal and also a mixture of two or more first-class metals. "Second-class metal" exactly means a second-class metal and also a mixture of two or more second-class metals.

[0143] Non-metals are known to those skilled in the art. The non-metals according to the present invention may be selected from any non-metal of the periodic table. Preferably, at least one non-metal is selected from the group consisting of carbon, nitrogen, oxygen, phosphorus, and sulfur.

[0144] Metalloids are also known to those skilled in the art. The first-class metals and the second-class metals may be selected from any metalloid of the periodic table. Preferably, the first-class metal and / or the second-class metal is selected from the group consisting of boron and silicon. It should be clear that the first-class metal and the second-class metal are different from each other. For example, if the first-class metal is boron, the second-class metal is selected from any other metalloid in the periodic table other than boron.

[0145] In one embodiment of the present invention, the ceramic material is selected from the group consisting of oxides, carbides, borides, nitrides, and silicides. In a preferred embodiment, the ceramic material is selected from the group consisting of MgO, CaO, SiO2, Na2O, Al2O3, ZrO2, Y2O3, SiC, Si3N4, TiB, and AlN. Particularly preferably, the ceramic material is selected from the group consisting of Al2O3, ZrO2, and Y2O3.

[0146] In order to prepare the inorganic powder (IP), the inorganic material must be comminuted. Any method known to those skilled in the art can be used to comminute the inorganic material. For example, the inorganic material can be ground. The grinding can be carried out, for example, in a classifier mill, in a hammer mill or in a ball mill.

[0147] Carbonyl iron powder (CIP) is prepared by chemically decomposing purified iron pentacarbonyl.

[0148] The particle size of the inorganic powder (IP) used, measured by laser diffraction, is preferably from 0.1 to 80 μm, particularly preferably from 0.5 to 50 μm, more preferably from 0.1 to 30 μm.

[0149] Binder (B)

[0150] At least one green body (GB) further comprises a binder (B), wherein the binder (B) preferably comprises

[0151] (b1) at least one polyoxymethylene (POM).

[0152] In a preferred embodiment, the binder (B) comprises

[0153] (b1) at least one polyoxymethylene (POM) in an amount of 50% to 96% by weight, based on the total weight of the binder (B),

[0154] (b2) at least one polyolefin (PO) in an amount of 2% to 35% by weight, based on the total weight of the binder (B),

[0155] (b3) at least one further polymer (FP) in an amount of 2% to 40% by weight, based on the total weight of the binder (B), wherein the % by weight of components (b1), (b2) and (b3) together total 100%.

[0156] In a more preferred embodiment, the binder (B) comprises

[0157] as component (b1), at least one polyoxymethylene (POM) in an amount of 60% to 90% by weight,

[0158] as component (b2), at least one polyolefin (PO) in an amount of 3% to 20% by weight, and

[0159] as component (b3), at least one further polymer (FP) in an amount of 5% to 30% by weight,

[0160] each based on the total weight of the binder (B), wherein the % by weight of components (b1), (b2) and (b3) generally total 100%.

[0161] Particularly preferably, the binder (B) comprises

[0162] At least one polyoxymethylene (POM) in an amount of 70% to 85% by weight, based on the total weight of the binder (B), as component (b1).

[0163] At least one polyolefin (PO) in an amount of 4% to 15% by weight, based on the total weight of the binder (B), as component (b2), and

[0164] At least one further polymer (FP) in an amount of 10% to 26% by weight, based on the total weight of the binder (B), as component (b3),

[0165] each based on the total weight of the binder (B), where the percentages by weight of components (b1), (b2) and (b3) total 100%.

[0166] According to the invention, component (b1) is different from component (b2), component (b2) is different from component (b3), and component (b3) is different from component (b1). However, components (b1), (b2) and (b3) may contain the same structural units and may differ, for example, in terms of another structural unit and / or in terms of molecular weight.

[0167] The components (b1), (b2) and (b3) of the binder (B) are described in more detail below.

[0168] Component (b1) / Polyoxymethylene (POM)

[0169] For the purposes of the present invention, the terms "component (b1)" and "at least one polyoxymethylene (POM)" are synonymous and are used interchangeably throughout the present invention.

[0170] Preferably, the binder (B) contains at least one polyoxymethylene (POM) in an amount of 50% to 96% by weight, more preferably 60% to 90% by weight, most preferably 70% to 85% by weight, based on the total weight of the binder (B).

[0171] "At least one polyoxymethylene (POM)" in the present invention precisely means one polyoxymethylene (POM) as well as a mixture of two or more polyoxymethylenes (POM). For the purposes of the present invention, the term "polyoxymethylene (POM)" includes both polyoxymethylene (POM) itself, i.e., polyoxymethylene (POM) homopolymers, and also polyoxymethylene (POM) copolymers and polyoxymethylene (POM) terpolymers. Polyoxymethylene (POM) homopolymers are typically prepared by polymerizing monomers selected from formaldehyde sources (b1a). The term "formaldehyde source (b1a)" relates to substances that can release formaldehyde under the reaction conditions for preparing polyoxymethylene (POM). The formaldehyde source (b1a) is advantageously selected from the group of cyclic or linear formaldehyde acetals, particularly from the group consisting of formaldehyde and 1,3,5-trioxane. 1,3,5-trioxane is particularly preferred.

[0172] Polyoxymethylene (POM) copolymers are known per se and are commercially available. They are generally prepared by polymerizing trioxane as the main monomer. In addition, comonomers are used simultaneously. The main monomers are preferably selected from trioxane and other cyclic or linear formaldehyde acetals or other formaldehyde sources (b1a). The term "main monomer" is intended to indicate that the proportion of these monomers in the total amount of monomers (i.e., the sum of the main monomers and the comonomers) is greater than the proportion of the comonomers in the total amount of monomers. Generally, the polyoxymethylene (POM) according to the invention has at least 50 mol-% of the repeating unit –CH2O- in the main polymer chain. Suitable polyoxymethylene (POM) copolymers are in particular those comprising the repeating unit -CH2O- and 0.01 to 20 mol-%, in particular 0.1 to 10 mol-%, and very particularly preferably 0.5 to 6 mol-% of the repeating unit of the formula (I),

[0173]

[0174] wherein

[0175] R 1 to R 4 are each independently selected from the group consisting of H, C1-C4-alkyl, and halogen-substituted C1-C4-alkyl;

[0176] R 5 is selected from the group consisting of a chemical bond, (-CR 5a R 5b -) group, and (-CR 5a R 5b O-) group,

[0177] wherein

[0178] R 5a and R 5b are each independently selected from the group consisting of H and unsubstituted or at least monosubstituted C1-C4-alkyl,

[0179] wherein these substituents are selected from the group consisting of F, Cl, Br, OH, and C1-C4-alkyl;

[0180] n is 0, 1, 2, or 3.

[0181] If n is 0, then R 5 is a chemical bond between adjacent carbon atoms and an oxygen atom. If R 5 is a (-CR 5a R 5b O-) group, then (-CR 5a R 5bThe oxygen atom (O) of the O-) group is bonded to another carbon atom (C) of formula (I) and not to the oxygen atom (O) of formula (I). In other words, formula (I) does not contain peroxide compounds. This also applies to formula (II).

[0182] In the context of the present invention, a definition such as C1-C4-alkyl, as for example the group R in formula (I) above 1 to R 4 defined, means that this substituent (group) is an alkyl group having 1 to 4 carbon atoms. The alkyl group can be straight-chain or branched and can also be optionally cyclic. An alkyl group having both a cyclic component and also a straight-chain component likewise falls within this definition. Examples of alkyl groups are methyl, ethyl, n-propyl, isopropyl, butyl, isobutyl, sec-butyl and tert-butyl.

[0183] In the context of the present invention, a definition such as halogen-substituted C1-C4-alkyl, as for example the group R in formula (I) above 1 to R 4 defined, means that the C1-C4-alkyl is substituted by at least one halogen. The halogen is F (fluorine), Cl (chlorine), Br (bromine) and I (iodine).

[0184] The repeating unit of formula (I) can advantageously be introduced into a polyoxymethylene (POM) copolymer by ring-opening of a cyclic ether as the first comonomer (b1b). Preferably the first comonomer (b1b) of general formula (II),

[0185]

[0186] wherein

[0187] R 1 to R 5 and n have the meanings defined above for general formula (I).

[0188] As the first comonomer (b1b), mention may be made, for example, of ethylene oxide, 1,2-epoxypropane, 1,2-epoxybutane, 1,3-epoxybutane, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepane (= butanediol formal, BUFO) as cyclic ethers and also straight-chain oligomeric or polymeric formaldehyde such as polydioxolane or polydioxepane. 1,3-Dioxolane and 1,3-dioxepane are particularly preferred first comonomers (b1b), very particularly preferred is 1,3-dioxepane as the first comonomer (b1b).

[0189] Polyoxymethylene (POM) polymers obtainable by reacting formaldehyde sources with a first comonomer (b1b) and a second comonomer (b1c) are likewise suitable. The addition of the second comonomer (b1c) enables the preparation of polyoxymethylene (POM) terpolymers in particular.

[0190] The second comonomer (b1c) is preferably selected from the group consisting of compounds of formula (III) and compounds of formula (IV),

[0191]

[0192]

[0193] wherein

[0194] Z is selected from the group consisting of a chemical bond, (-O-) groups, and (-O-R 6 -O-) groups,

[0195] wherein

[0196] R 6 is selected from the group consisting of unsubstituted C1-C8-alkylene and C3-C8-cycloalkylene.

[0197] In the context of the present invention, the definition of C1-C8-alkylene means C1-C8-alkanediyl. C1-C8-alkylene is a hydrocarbon having two free valences and 1 to 8 carbon atoms. The C1-C8-alkylene according to the present invention can be branched or unbranched.

[0198] In the context of the present invention, the definition of C3-C8-cycloalkylene means C3-C8-cycloalkanediyl. C3-C8-cycloalkylene is a cycloalkane having two free valences and 3 to 8 carbon atoms. Hydrocarbons having two free valences, being cyclic and also having a straight-chain component, and having 3 to 8 carbon atoms likewise fall within this definition.

[0199] Preferred examples of the second comonomer (b1c) are ethylene glycol diglycidyl ether, diglycidyl ether, and diethers prepared from glycidyl compounds and formaldehyde, dioxane, or trioxane in a molar ratio of 2:1, and also diethers prepared from 2 mol of glycidyl compounds and 1 mol of an aliphatic diol having 2 to 8 carbon atoms, such as the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol, 1,2-propanediol, and 1,4-cyclohexanediol.

[0200] In a preferred embodiment, component (b1) is a polyoxymethylene (POM) copolymer which is prepared by polymerizing at least 50 mol-% of a formaldehyde source, 0.01 to 20 mol-% of at least one first comonomer (b1b) and 0 to 20 mol-% of at least one second comonomer (b1c).

[0201] In a particularly preferred embodiment, component (b1) is a polyoxymethylene (POM) copolymer which is prepared by polymerizing 80 to 99.98 mol-%, preferably 88 to 99 mol-% of a formaldehyde source, 0.1 to 10 mol-%, preferably 0.5 to 6 mol-% of at least one first comonomer (b1b) and 0.1 to 10 mol-%, preferably 0.5 to 6 mol-% of at least one second comonomer (b1c).

[0202] In a further preferred embodiment, component (b1) is a polyoxymethylene (POM) copolymer which is prepared by polymerizing at least 50 mol-% of a formaldehyde source, 0.01 to 20 mol-% of at least one first comonomer (b1b) of general formula (II) and 0 to 20 mol-% of at least one second comonomer (b1c) selected from the group consisting of a compound of formula (III) and a compound of formula (IV).

[0203] Accordingly, another subject of the present invention is a process in which component (b1) is a polyoxymethylene (POM) copolymer which is prepared by polymerizing:

[0204] - at least 50 mol-% of a formaldehyde source (b1a),

[0205] - 0.01 to 20 mol-% of at least one first comonomer (b1b) of general formula (II)

[0206]

[0207] wherein

[0208] R 1 to R 4 are each independently selected from the group consisting of H, C1-C4-alkyl and halogen-substituted C1-C4-alkyl;

[0209] R 5 is selected from the group consisting of a chemical bond, (-CR 5a R 5b -) group and (-CR 5a R 5b O-) group,

[0210] wherein

[0211] R 5a and R5b each independently selected from the group consisting of H and unsubstituted or at least monosubstituted C1-C4-alkyl, where these substituents are selected from the group consisting of F, Cl, Br, OH and C1-C4-alkyl;

[0212] n is 0, 1, 2 or 3;

[0213] and

[0214] - 0 to 20 mol-% of at least one second comonomer (b1c), which is selected from the group consisting of compounds of formula (III) and compounds of formula (IV)

[0215]

[0216] where

[0217] Z is selected from the group consisting of a chemical bond, (-O-) group and (-O-R 6 -O-) groups,

[0218] where

[0219] R 6 is selected from the group consisting of unsubstituted C1-C8-alkylene and C3-C8-cycloalkylene.

[0220] In a preferred embodiment of the present invention, at least some of the OH-end groups of the polyoxymethylene (POM) are capped. Methods for capping OH-end groups are known to those skilled in the art. For example, the OH-end groups can be capped by etherification or esterification.

[0221] Preferred polyoxymethylene (POM) copolymers have a melting point of at least 150 °C and a weight average molecular weight M in the range of 5,000 g / mol to 300,000 g / mol, preferably 6,000 g / mol to 150,000 g / mol, particularly preferably in the range of 7,000 g / mol to 100,000 g / mol W .

[0222] Particularly preferred are polyoxymethylene (POM) copolymers with a polydispersity (M w / M n ) of 2 to 15, preferably 2.5 to 12, particularly preferably 3 to 9.

[0223] The measurement of the weight average molecular weight (M w ) and the number average molecular weight (M n ) is usually carried out by gel permeation chromatography (GPC). GPC is also known as size exclusion chromatography (SEC).

[0224] Methods for preparing polyoxymethylene (POM) are known to those skilled in the art.

[0225] Component (b2) / Polyolefin (PO)

[0226] In addition, the binder (B) may contain component (b2).

[0227] Preferably, the binder (B) contains 2% to 35% by weight, more preferably 3% to 20% by weight, and most preferably 4% to 15% by weight of component (b2).

[0228] Preferably, component (b2) is at least one polyolefin (PO). "At least one polyolefin (PO)" in the context of the present invention precisely means one polyolefin (PO) as well as a mixture of two or more polyolefins (PO).

[0229] Polyolefins (PO) themselves are known and commercially available. They are generally prepared by polymerizing C2-C8-olefin monomers, preferably by polymerizing C2-C4-olefin monomers.

[0230] In the context of the present invention, C2-C8-olefins mean unsubstituted or at least monosubstituted hydrocarbons having 2 to 8 carbon atoms and at least one carbon-carbon double bond (C-C double bond). "At least one carbon-carbon double bond" precisely means one carbon-carbon double bond as well as two or more carbon-carbon double bonds.

[0231] In other words, C2-C8-olefins mean that the hydrocarbons having 2 to 8 carbon atoms are unsaturated. The hydrocarbons can be branched or unbranched. Examples of C2-C8-olefins having one C-C double bond are ethylene, propylene, 1-butene, 2-butene, 2-methyl-propene (=isobutene), 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 1-hexene, 2-hexene, 3-hexene, and 4-methyl-1-pentene. Examples of C2-C8-olefins having two or more C-C double bonds are allene, 1,3-butadiene, 1,4-pentadiene, 1,3-pentadiene, 2-methyl-1,3-butadiene (=isoprene).

[0232] If the C2-C8-olefin has one C-C double bond, the polyolefin (PO) prepared from these monomers is linear. If there are more than one double bond in the C2-C8-olefin, the polyolefin (PO) prepared from these monomers can be crosslinked. Linear polyolefins (PO) are preferred.

[0233] Polyolefin (PO) copolymers can also be used, which are prepared by using different C2-C8-olefin monomers during the preparation of the polyolefin (PO).

[0234] Preferably, the polyolefin (PO) is selected from the group consisting of polymethylpentene, poly-1-butene, polyisobutene, polyethylene, and polypropylene. Polyethylene and polypropylene, as well as their copolymers, which are known to those skilled in the art and are commercially available, are particularly preferred.

[0235] The polyolefin (PO) can be prepared by any polymerization method known to those skilled in the art, preferably by free radical polymerization, such as by emulsion, bead, solution, or bulk polymerization. Depending on the monomer and the type of polymerization, possible initiators are free radical initiators such as peroxides and azo compounds, and the amount of the initiator is generally in the range of 0.001% to 0.5% by weight based on the monomer.

[0236] Component (b3) / Additional polymer (FP)

[0237] The binder (B) may contain an additional polymer (FP) as component (b3).

[0238] For the purposes of the present invention, the terms "component (b3)" and "additional polymer (FP)" are synonymous and are used interchangeably throughout the present invention.

[0239] Preferably, the binder (B) contains 2% to 40% by weight, more preferably 5% to 30% by weight, and most preferably 10% to 26% by weight based on the total weight of the binder (B) as component (b3).

[0240] Component (b3) according to the present invention is at least one additional polymer (FP). The "at least one additional polymer (FP)" in the present invention specifically means one additional polymer (FP) and also a mixture of two or more additional polymers (FP).

[0241] As described above, the at least one additional polymer (FP) is different from component (b1) polyoxymethylene (POM) and component (b2) polyolefin (PO).

[0242] According to the present invention, the at least one additional polymer (FP) is preferably selected from the group consisting of polyethers, polyurethanes, polyepoxides, polyamides, vinyl aromatic polymers, poly(vinyl esters), poly(vinyl ethers), poly(alkyl (meth)acrylates), and their copolymers.

[0243] Preferably, component (b3), i.e., the at least one additional polymer (FP), is selected from the group consisting of poly(C2-C6-alkylene oxide), aliphatic polyurethane, aliphatic uncrosslinked epoxide, aliphatic polyamide, vinyl aromatic polymer, poly(vinyl ester) of aliphatic C1-C8 carboxylic acid, poly(vinyl ether) of C1-C8 alkyl vinyl ether, C 1-8Group consisting of poly((meth)acrylic acid alkyl esters) and their copolymers with -alkyl.

[0244] The following describes in more detail at least one preferred additional polymer (FP).

[0245] The polyether contains repeating units of formula (V):

[0246]

[0247] where

[0248] R 11 to R 14 each independently of one another are selected from the group consisting of H, C1-C4-alkyl, and halogen-substituted C1-C4-alkyl;

[0249] R 15 is selected from the group consisting of a chemical bond, (-CR 15a R 15b -) group, and (-CR 15a R 15b O-) group,

[0250] where

[0251] R 15a and R 15b each independently of one another are selected from the group consisting of H and unsubstituted or at least monosubstituted C1-C4-alkyl,

[0252] where these substituents are selected from the group consisting of F, Cl, Br, OH, and C1-C4-alkyl;

[0253] n is 0, 1, 2, or 3.

[0254] If n is 0, then R 15 is a chemical bond between adjacent carbon atoms and an oxygen atom. If R 15 is a (-CR 15a R 15b O-) group, then the oxygen atom (O) of the (-CR 15a R 15b O-) group binds to another carbon atom (C) of formula (V) and does not bind to the oxygen atom (O) of formula (V). In other words, formula (V) does not contain peroxide compounds. This also applies to formula (VI).

[0255] Typical polyethers and their preparation are known to those skilled in the art.

[0256] Preferred polyethers according to the invention are, for example, poly(alkylene glycols), also known as poly(alkylene oxides).

[0257] Polyalkylene oxides and their preparation are known to those skilled in the art. They are generally synthesized by the interaction of water and divalent or polyvalent alcohols with cyclic ethers (i.e., alkylene oxides) of the general formula (VI). The reaction is catalyzed by an acidic or basic catalyst. The reaction is a so-called ring-opening polymerization of the cyclic ether of the general formula (VI).

[0258]

[0259] wherein

[0260] R 11 to R 15 has the same meaning as defined above for formula (V).

[0261] The preferred poly(alkylene oxide) according to the invention is derived from monomers of the general formula (VI) having 2 to 6 carbon atoms in the ring. In other words, preferably, the poly(alkylene oxide) is a poly(C2-C6-alkylene oxide). Particularly preferably, the poly(alkylene oxide) is derived from monomers selected from the group consisting of 1,3-dioxolane, 1,3-dioxepane, and tetrahydrofuran (IUPAC-name: oxolane). In other words, particularly preferably, the poly(alkylene oxide) is selected from the group consisting of poly-1,3-dioxolane, poly-1,3-dioxepane, and polytetrahydrofuran.

[0262] In one embodiment, the poly(alkylene oxide) may contain OH-end groups. In another embodiment, at least some of the OH-end groups of the poly(alkylene oxide) may be blocked. The methods for blocking OH-end groups are known to those skilled in the art. For example, the OH-end groups can be blocked by etherification or esterification.

[0263] The weight-average molecular weight of the poly(alkylene oxide) is preferably in the range of 1000 to 150000 g / mol, particularly preferably 1500 to 120000 g / mol and more preferably in the range of 2000 to 100000 g / mol.

[0264] Polyurethanes are polymers having urethane units. Polyurethanes and their preparation are known to those skilled in the art.

[0265] In the present invention, aliphatic polyurethanes are preferred. They can be prepared, for example, by the addition polymerization of aliphatic polyisocyanates and aliphatic polyhydroxy compounds. Among the polyisocyanates, diisocyanates of the general formula (VII) are preferred

[0266] OCN——R 7 —NCO (VII),

[0267] wherein

[0268] R 7 is a substituted or unsubstituted C1-C 20 -alkylene or C4-C20 -A cycloalkylene group, wherein the substituent is selected from the group consisting of F, Cl, Br, and C1-C6-alkyl.

[0269] Preferably, R 7 is a substituted or unsubstituted C 2- C 12 -alkylene or C6-C 15 -cycloalkylene.

[0270] In the context of the present invention, the definition of C1-C 20 -alkylene means C1-C 20 -alkanediyl. C1-C 20 -alkylene is a hydrocarbon having two free valences and a carbon atom number of 1 to 20. The C1-C 20 -alkylene according to the present invention can be branched or unbranched.

[0271] In the context of the present invention, the definition of C4-C 20 -cycloalkylene means C4-C 20 -cycloalkanediyl. C4-C 20 -cycloalkylene is a cycloalkane having two free valences and a carbon atom number of 4 to 20. A hydrocarbon having two free valences, being cyclic and also having a straight-chain component and a carbon atom number of 4 to 20 also falls within this definition.

[0272] Preferred diisocyanates are selected from the group consisting of: hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, 1,2-diisocyanatomethylcyclohexane, 1,4-diisocyanatomethylcyclohexane, and isophorone diisocyanate (IUPAC name: 5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethyl-cyclohexane).

[0273] The diisocyanate can also be used in the form of oligomers, such as dimers or trimers. Instead of the polyisocyanate, a conventional blocked polyisocyanate obtained from the isocyanate, such as by the addition reaction of phenol or caprolactam, can also be used.

[0274] Suitable polyhydroxy compounds for preparing aliphatic polyurethanes are, for example, polyesters, polyethers, polyester amides, or polyacetals, or mixtures thereof.

[0275] Suitable chain extenders for preparing polyurethanes are low molecular weight polyols, especially diols and polyamines, especially diamines or water.

[0276] The polyurethane is preferably thermoplastic and thus preferably substantially uncrosslinked, i.e., they can be repeatedly melted without significant signs of decomposition. Their reduced specific viscosity measured in dimethylformamide at 30 °C is generally from 0.5 to 3 dL / g, preferably from 1 to 2 dL / g.

[0277] The polyepoxide contains at least two epoxy groups. The epoxy group is also referred to as a glycidyl or oxiranyl group. "At least two epoxy groups" specifically means two epoxy groups as well as three or more epoxy groups.

[0278] The polyepoxides and their preparation are known to those skilled in the art. For example, polyepoxides are prepared by the reaction of epichlorohydrin (IUPAC-name: chloromethyloxirane) with diols, polyols or dicarboxylic acids. The polyepoxides prepared in this way are polyethers with epoxy end groups.

[0279] Another possibility for preparing polyepoxides is the reaction of glycidyl (meth)acrylate (IUPAC-name: 2-methyloxirane-2-ylmethyl 2-methylprop-2-enoate) with polyolefins or polyacrylates. This results in polyolefins or polyacrylates with epoxy end groups.

[0280] Preferably, aliphatic uncrosslinked polyepoxides are used. Particularly preferred is the copolymer of epichlorohydrin and 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A).

[0281] Component (b3) (at least one further polymer (FP)) can also comprise polyamides. Preferably aliphatic polyamides.

[0282] The intrinsic viscosity of suitable polyamides is generally from 150 to 350 mL / g, preferably from 180 to 275 mL / g. The intrinsic viscosity is determined here according to ISO 307 at 25 °C from a 0.5% by weight solution of polyamide in 96% by weight sulfuric acid.

[0283] Preferred polyamides are semi-crystalline or amorphous polyamides.

[0284] Examples of polyamides suitable as component (b3) are those derived from lactams having 7 to 13 ring members. Other suitable polyamides are those obtained by the reaction of dicarboxylic acids with diamines.

[0285] Examples of polyamides derived from lactams that may be mentioned are polyamides derived from polycaprolactam, polyoctanamide and / or polylaurolactam.

[0286] If polyamides obtainable from dicarboxylic acids and diamines are used, the dicarboxylic acids that can be used are alkanedicarboxylic acids having 6 to 14 carbon atoms, preferably 6 to 10 carbon atoms. Aromatic dicarboxylic acids are also suitable.

[0287] Examples of dicarboxylic acids that may be mentioned here are adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and also terephthalic acid and / or isophthalic acid.

[0288] Examples of suitable diamines are alkanediamines having 4 to 14 carbon atoms, especially alkanediamines having 6 to 8 carbon atoms, and also aromatic diamines such as m-xylenediamine, bis(4-aminophenyl)methane, bis(4-aminocyclohexyl)methane, 2,2-bis(4-aminophenyl)propane, 2,2-bis(4-aminocyclohexyl)propane, and 1,5-diamino-2-methylpentane.

[0289] Other suitable polyamides are those obtainable by copolymerization of two or more of the above-mentioned and below-mentioned monomers, and mixtures of various polyamides in any desired mixing ratios.

[0290] Preferred polyamides are polyhexamethylene adipamide, polyhexamethylene sebacamide, and polycaprolactam, and also nylon-6 / 6,6, especially having a proportion of caprolactam units of 75% to 95% by weight.

[0291] Particularly preferred is a mixture of nylon-6 with other polyamides, especially with nylon-6 / 6,6 (PA 6 / 66), particularly preferably a mixture of 80% to 50% by weight of PA 6 and 20% to 50% by weight of PA 6 / 66, where PA 6 / 66 contains 75% to 95% by weight of caprolactam units based on the total weight of PA 6 / 66 in the mixture.

[0292] The following non-exclusive list includes the above-mentioned polyamides and other suitable polyamides, and also the monomers contained therein.

[0293] AB polymers:

[0294]

[0295] AA / BB polymers:

[0296]

[0297]

[0298] Preferred polyamides are PA 6, PA 66, and PA PACM 6.

[0299] Vinyl aromatic polymers are polyolefins having unsubstituted or at least monosubstituted styrene as monomer units. Suitable substituents are, for example, C1-C6-alkyl, F, Cl, Br and OH. Preferred vinyl aromatic polymers are selected from the group consisting of polystyrene, poly-α-methylstyrene and their copolymers with up to 30% by weight of comonomers selected from the group consisting of acrylates, acrylonitrile and methacrylonitrile.

[0300] Vinyl aromatic polymers are commercially available and are known to those skilled in the art. The preparation of these polymers is also known to those skilled in the art.

[0301] Preferably, vinyl aromatic polymers are prepared by free radical polymerization, for example by emulsion, bead, solution or bulk polymerization. Depending on the monomer and the type of polymerization, possible initiators are free radical initiators such as peroxide compounds and azo compounds, where the amount of initiator is generally in the range of 0.001% to 0.5% by weight based on the monomer.

[0302] Poly(vinyl esters) and their preparation are known to those skilled in the art. Poly(vinyl esters) are preferably prepared by polymerization of vinyl esters. In a preferred embodiment of the invention, the vinyl ester is a vinyl ester of an aliphatic C1-C6 carboxylic acid. Preferred monomers are vinyl acetate and vinyl propionate. These monomers form poly(vinyl acetate) and poly(vinyl propionate) polymers.

[0303] Poly(vinyl ethers) are prepared by polymerization of vinyl ether monomers. Poly(vinyl ethers) and their preparation are known to those skilled in the art. In a preferred embodiment, the vinyl ether is a vinyl ether of an aliphatic C1-C8 alkyl ether. Preferred monomers are methyl vinyl ether and ethyl vinyl ether, which form poly(methyl vinyl ether) and poly(ethyl vinyl ether) during polymerization.

[0304] Preferably, poly(vinyl ethers) are prepared by free radical polymerization, for example by emulsion, bead, solution, suspension or bulk polymerization. Depending on the monomer and the type of polymerization, possible initiators are free radical initiators such as peroxide compounds and azo compounds, where the amount of initiator is generally in the range of 0.001% to 0.5% by weight based on the monomer.

[0305] The poly((meth)acrylic acid alkyl esters) in the present invention include poly(acrylic acid alkyl esters), poly(methacrylic acid alkyl esters) and their copolymers. The poly((meth)acrylic acid alkyl esters) contain units derived from monomers of formula (VIII),

[0306]

[0307] wherein

[0308] R8 selected from the group consisting of H and C1-C8-alkyl, and

[0309] R 9 is a group of formula (IX)

[0310]

[0311] wherein

[0312] R 10 is C1-C 14 -alkyl.

[0313] Preferably, R 8 is selected from the group consisting of H and C1-C4-alkyl, particularly preferably, R 8 is H or methyl. Preferably, R 10 is C1-C8-alkyl, particularly preferably, R 10 is methyl or ethyl.

[0314] If R in formula (VIII) 8 is H and R 9 is a group of formula (IX) and R in formula (IX) 10 is methyl, then the monomer of formula (VIII) is methyl acrylate.

[0315] If R in formula (VIII) 8 is H and R 9 is a group of formula (IX) and R in formula (IX) 10 is ethyl, then the monomer of formula (VIII) is ethyl acrylate.

[0316] If R in formula (VIII) 8 is methyl and R 9 is a group of formula (IX), then the monomer of formula (VI) is methacrylate.

[0317] Poly((meth)acrylic acid alkyl ester) contains, as monomers, preferably 40% to 100% by weight of methacrylate, particularly preferably 70% to 100% by weight of methacrylate, and more preferably 80% to 100% by weight of methacrylate, each based on the total amount of poly((meth)acrylic acid alkyl ester).

[0318] In another preferred embodiment, the poly((meth)acrylic acid alkyl ester) contains 20% to 100% by weight of methyl acrylate, ethyl acrylate, or a mixture thereof as monomers, preferably 40% to 100% by weight of methyl acrylate, ethyl acrylate, or a mixture thereof, and particularly preferably 50% to 100% by weight of methyl acrylate, ethyl acrylate, or a mixture thereof, each based on the total weight of the poly((meth)acrylic acid alkyl ester).

[0319] Such polymers of monomers of formula (VIII) with or without additional monomers can be prepared by conventional, preferably free-radical polymerization (e.g., emulsion, bead, solution, or bulk polymerization) (see Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, volume 1, pages 330 - 342, volume 18, pages 720 - 755, J. Wiley; H. Rauch-Puntigam, Th. Acryl-und Methacrylverbindungen). Depending on the monomer and the type of polymerization, possible initiators are free-radical initiators such as peroxides or peroxy compounds and azo compounds. The amount of initiator is generally in the range of 0.001% to 0.5% by weight based on the monomers.

[0320] Suitable initiators for emulsion polymerization are, for example, persulfates and redox systems, and for bulk polymerization not only peroxides such as benzoyl peroxide or lauroyl peroxide, but also azo compounds such as azobisisobutyronitrile, similar to the case of solution polymerization or bead polymerization. The molecular weight can be adjusted using conventional regulators, especially thiols such as dodecyl mercaptan.

[0321] Preferably, the polymerization is carried out at elevated temperature (e.g., above 50 °C). The weight-average molecular weight (M W ) is generally in the range of 2,000 g / mol to 5,000,000 g / mol, preferably 20,000 g / mol to 3,000,000 g / mol (determined by light scattering; see Houben Weyl, Methoden der Org. Chemie [Methods of Organic Chemistry], 4th edition, volume 14 / 1, Georg Thieme-Verlag Stuttgart 1961).

[0322] Those skilled in the art know that the monomers used to prepare components (b1), (b2) and (b3) can undergo changes in their structures during the polymerization reaction. Therefore, the structural units of the polymers are not the same as the monomers from which they are derived. However, those skilled in the art know which monomers correspond to which structural units of the polymers.

[0323] Under the conditions of compounding or processing by injection molding or fused filament fabrication, there is little transacetalization between the polyoxymethylene (POM) of component (b1) and at least one other polymer (FP) of component (b3), that is, there is little exchange of comonomer units.

[0324] Three-dimensional green body

[0325] At least one three-dimensional green body (GB) can be prepared by any method known to those skilled in the art, for example, by additive manufacturing processes such as fused filament fabrication process or injection molding. Preferably, at least one three-dimensional green body (GB) is prepared by the fused filament fabrication process.

[0326] The fused filament fabrication process for producing at least one three-dimensional green body (GB) is well known in the prior art. The fused filament fabrication process is also called a 3D-printing process. The filaments can include continuous filaments and rods, pellets and / or powders.

[0327] Preferably, the fused filament fabrication process comprises the following steps:

[0328] i) Providing a mixture (M) to a nozzle, wherein the mixture (M) comprises an inorganic powder (IP) and a binder (B), wherein the binder (B) preferably comprises at least one polyoxymethylene (POM),

[0329] ii) Heating the mixture (M) to a certain temperature (T M )

[0330] iii) Depositing the mixture (M) in a build chamber using a layer-based additive technique to form at least one three-dimensional green body (GB).

[0331] The above embodiments and preferred options regarding at least one three-dimensional green body (GB) comprising an inorganic powder (IP) and a binder (B) similarly apply to the mixture (M), wherein the binder (B) preferably comprises at least one polyoxymethylene (POM).

[0332] The mixture (M) can be prepared by any method known to the person skilled in the art. Preferably, the mixture (M) is produced by melting the binder (B) and mixing it with the inorganic powder (IP) and, if appropriate, at least one dispersant. For example, the binder (B) can be melted in a twin-screw extruder at a temperature preferably between 150 °C and 220 °C, in particular between 170 °C and 200 °C. Subsequently, at a temperature within the same range, the inorganic powder (IP) is metered into the melt stream of the binder (B) in the required amount. The inorganic powder (IP) advantageously contains at least one dispersant on its surface. However, the mixture (M) according to the invention can also be produced by melting the binder (B) and optionally at least one dispersant in the presence of the inorganic powder (IP) at a temperature between 150 °C and 220 °C, preferably between 170 °C and 200 °C.

[0333] A particularly preferred device for metering the inorganic powder (IP) comprises a feed screw as an essential element, which is located in a heatable metal cylinder and conveys the inorganic powder (IP) into the melt of the binder (B). Compared to mixing the components at room temperature and then raising the temperature for extrusion, the advantage of the method described above is that the decomposition of the polyoxymethylene (POM) used as the binder due to high shear forces that occurs in this variant is largely avoided.

[0334] Step b)

[0335] In step b), an acid is provided.

[0336] Suitable acids are, for example, inorganic acids which are gaseous at room temperature or can be vaporized at the temperature of step c) or below. Examples are hydrogen halides and nitric acid. The hydrogen halides are hydrogen fluoride, hydrogen chloride, hydrogen bromide and hydrogen iodide. Suitable organic acids are those which have a boiling point below 130 °C at atmospheric pressure, such as formic acid, acetic acid or trifluoroacetic acid and mixtures thereof. Acids with a boiling point above 130 °C, such as methanesulfonic acid, can also be used when added as a mixture with a lower-boiling acid and / or water. Preferred acids are, for example, nitric acid, a 10% by weight solution of oxalic acid in water or a 50% by volume mixture of methanesulfonic acid in water.

[0337] In addition, BF3 and its adducts with inorganic ethers can be used as acids.

[0338] If a carrier gas is used, the carrier gas generally passes through the acid and is pre-loaded with the acid. The carrier gas pre-loaded with the acid in this way then reaches the temperature at which method step c) is carried out. This temperature is advantageously higher than the loading temperature to avoid condensation of the acid.

[0339] Preferably, the temperature at which method step c) is carried out is at least 1 °C higher, particularly preferably at least 5 °C higher and most preferably at least 10 °C higher than the loading temperature.

[0340] Preferably, the acid is mixed into the carrier gas by a metering device and the gas mixture is heated to a temperature at which the acid can no longer condense. Preferably, the temperature is at least 1 °C, more preferably at least 5 °C and most preferably at least 10 °C higher than the sublimation and / or vaporization temperature of the acid and / or the carrier gas.

[0341] The carrier gas is generally any gas that is inert under the reaction conditions of the degreasing step. A preferred carrier gas according to the invention is nitrogen.

[0342] In a particularly preferred embodiment, in step b), anhydrous oxalic acid is provided in an amount of 0.01% to 5.0% by weight, more preferably 0.05% to 2.5% by weight, and most preferably 0.1% to 1.5% by weight based on the total weight of at least one three-dimensional green body (GB).

[0343] The anhydrous oxalic acid preferably has a purity of ≥ 95%, more preferably ≥ 98%.

[0344] Therefore, the anhydrous oxalic acid preferably contains at most 5% by weight, more preferably at most 2% by weight, most preferably at most 1% by weight, and particularly preferably 0% by weight of water based on the total weight of the anhydrous oxalic acid.

[0345] Step c)

[0346] In step c), at least one three-dimensional green body (GB) is treated with an acid in a tube furnace to obtain at least one three-dimensional brown body (BB).

[0347] By performing step c), part of the binder (B) is preferably removed. Preferably, in step c), at least 90% by weight, more preferably at least 95% by weight of the binder (B) contained in at least one three-dimensional green body (GB) provided in step a) is removed. This can be checked, for example, by the degree of weight reduction.

[0348] After the binder (B) is removed in step c), the resulting three-dimensional object is called a "three-dimensional brown body". The three-dimensional brown body (BB) contains the inorganic powder (IP) and the part of the binder (B) that was not removed during degreasing. Those skilled in the art know that a three-dimensional brown body containing a ceramic material as the inorganic powder (IP) is also called a three-dimensional white body. However, for the purposes of the present invention, the terms "three-dimensional brown body" and "three-dimensional white body" are used synonymously and are interchangeable.

[0349] In a preferred embodiment, at least one three-dimensional green body (GB) formed in step c) comprises 90% to 100% by volume of inorganic powder (IP) and 0% to 10% by volume of binder (B), preferably 95% to 100% by volume of inorganic powder (IP) and 0% to 5% by volume of binder (B), based on the total volume of the at least one three-dimensional green body (GB).

[0350] It is known to the person skilled in the art that at the temperature during step c), the inorganic powder (IP) comprised in the at least one three-dimensional green body (GB) can undergo chemical and / or physical reactions. In particular, the particles of the inorganic powder (IP) can fuse together and the inorganic powder can undergo solid-state phase transitions.

[0351] This also applies to the binder (B). During step c), the composition of the binder (B) can change.

[0352] Thus, in an embodiment of the present invention, the inorganic powder (IP) and / or binder (B) comprised in the at least one three-dimensional green body (GB) is different from the inorganic powder (IP) and / or binder (B) comprised in the three-dimensional brown body (BB) obtained in method step c).

[0353] The debinding step before the sintering process is important for extracting parts of the binder matrix. There are different possibilities to carry out this operation, such as thermal, dissolution with a solvent or chemical debinding. One chemical debinding process is the so-called catalytic debinding process, in which the binder polymer is decomposed by using a gaseous acid.

[0354] In a particularly preferred embodiment, in step c), at least one three-dimensional green body (GB) is treated with anhydrous oxalic acid at a temperature (T1) of < 140 °C in the presence of an inert gas.

[0355] In this case, preferably, in step c), at least one three-dimensional green body (GB) is treated with anhydrous oxalic acid at a temperature (T1) of 110 °C to 135 °C, more preferably at a temperature (T1) of 110 °C to 130 °C.

[0356] The inert gas can be any gas that is substantially free of oxygen and water. The inert gas is preferably selected from the group consisting of hydrogen, nitrogen and noble gases, more preferably from nitrogen and argon.

[0357] Step c) is carried out in a tube furnace of the present invention.

[0358] In a preferred embodiment, anhydrous oxalic acid and at least one three-dimensional green body (GB) are placed in the tube furnace of the present invention. Preferably, the tube furnace of the present invention is heated to a temperature (T1) of <140 °C, which is lower than the sublimation temperature of anhydrous oxalic acid, more preferably heated to a temperature (T1) of 110 °C to 135 °C, and most preferably heated to a temperature (T1) of 110 °C to 130 °C.

[0359] Step d)

[0360] Optionally, an additional step d) can be carried out.

[0361] Preferably, step d) follows step c), in which at least one three-dimensional brown body (BB) is sintered to form at least one three-dimensional sintered body (SB). Method step d) is also referred to as sintering. For the purposes of the present invention, the terms "method step d)" and "sintering" are synonymous and can be used interchangeably throughout the present invention.

[0362] After sintering, the three-dimensional object is a three-dimensional sintered body (SB). The three-dimensional sintered body (SB) contains inorganic powder (IP) and is substantially free of binder (B).

[0363] "Substantially free of binder (B)" according to the present invention means that the three-dimensional sintered body (SB) contains less than 5% by volume, preferably less than 2% by volume, particularly preferably less than 0.5% by volume, and most preferably less than 0.01% by volume of binder (B) based on the total volume of the three-dimensional sintered body (SB).

[0364] As is known to those skilled in the art, during the sintering process, the inorganic powder (IP) is sintered together to obtain a sintered inorganic powder. In addition, during the sintering process, the inorganic powder (IP) can undergo chemical and / or physical reactions. Therefore, the inorganic powder (IP) contained in the three-dimensional brown body (BB) is usually different from the sintered inorganic powder contained in the three-dimensional sintered body (SB).

[0365] In one embodiment of the present invention, after method step c) and before method step d), the three-dimensional brown body (BB) obtained in method step c) is heated at a temperature preferably of 250 °C to 700 °C, particularly preferably 250 °C to 600 °C, for preferably 0.1 h to 12 h, particularly preferably 0.3 h to 6 h to completely remove the residual binder (B).

[0366] The temperature, duration, and atmosphere during method step d) depend on the inorganic powder (IP) contained in the mixture (M). The temperature program, duration, and atmosphere of the sintering process generally adapt to the requirements of the inorganic powder (IP) contained in the mixture (M). Suitable conditions for method step d) are known to those skilled in the art.

[0367] Generally, method step d) is carried out in an inert gas atmosphere that is inert with respect to the inorganic powder (IP) and the binder (B). Typical inert gases are, for example, nitrogen and / or argon.

[0368] Depending on the inorganic powder (IP) comprised in the mixture (M), method step d) can also be carried out in air, under vacuum or in a hydrogen atmosphere.

[0369] The temperature (T2) in method step d) is generally, for example, in the range from 750 °C to 1600 °C, preferably from 800 °C to 1500 °C and particularly preferably from 850 °C to 1450 °C.

[0370] Step d) is carried out in the tube furnace of the present invention, wherein the tube furnace is preferably the same tube furnace in which step c) is carried out.

Claims

1. Use of a tube furnace in a sintering and / or debinding process, wherein the tube furnace comprises a tube (T) which contains an oxide ceramic matrix composite (OCMC).

2. The use according to claim 1, wherein, The tube (T) comprises an inner tube (IT) and an outer layer (OL), wherein the outer layer (OL) is attached to the inner tube (IT), and wherein the inner tube (IT) contains a non-porous monolithic oxide ceramic and the outer layer (OL) contains the oxide ceramic matrix composite (OCMC).

3. The use according to claim 1 or 2, wherein The tube (T) comprises two ends, wherein the tube (T) is closed at both ends, open at both ends, or closed at one end and open at the other end.

4. The use according to claim 2 or 3, wherein, The inner diameter of the inner tube (IT) is in the range of 50 mm to 500 mm.

5. The use according to any one of claims 1 to 4, wherein, The tube (T) is heated over a length of 100 mm to 1000 mm, preferably 300 mm to 600 mm.

6. The use according to any one of claims 1 to 5, wherein The tube (T) is heated to a temperature in the range of 1250 °C to 1500 °C.

7. The use according to any one of claims 1 to 6, wherein The oxide ceramic matrix composite (OCMC) comprises - a matrix (M), wherein the matrix (M) contains oxide ceramic particles (P), and - fibers (F), wherein the fibers (F) are embedded as a linear, sheet-like, or three-dimensional textile structure between the oxide ceramic particles (P) of the matrix (M).

8. The use according to claim 7, wherein, These oxide ceramic particles (P) are particles comprising an oxide of at least one element selected from the group consisting of: Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se, and Te, or a mixture of these oxides.

9. The use according to claim 7 or claim 8, wherein, The fibers (F) are ceramic fibers (F), preferably non-oxide and / or oxide ceramic fibers (F), more preferably oxide ceramic fibers (F), wherein the oxide ceramic fibers (F) preferably comprise an oxide of at least one element selected from the group consisting of: Be, Mg, Ca, Sr, Ba, rare earths, Th, U, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Ga, Si, Ge, Sn, Li, Na, K, Rb, Cs, Re, Ru, Os, Ir, Pt, Rh, Pd, Cu, Ag, Au, Cd, In, Tl, Pb, P, As, Sb, Bi, S, Se, and Te, or a mixture of these oxides, and more preferably, the oxide ceramic fibers (F) comprise a compound selected from the group consisting of alumina, mullite, a mixture of alumina and mullite, zirconia toughened alumina (ZTA), and zirconia toughened mullite (ZTM).

10. The use according to any one of claims 7 to 9, wherein, The fibers (F) have a diameter in the range of 5 μm to 15 μm, preferably in the range of 10 μm to 12 μm.

11. The use according to any one of claims 1 to 10, wherein, The tube furnace comprises a heating element outside the tube (T).

12. The use according to claim 11, wherein, Based on the total weight of the non-porous monolithic oxide ceramic, the non-porous monolithic oxide ceramic comprises at least 97% by weight of at least one compound selected from the group consisting of alumina (Al2O3) and mullite.

13. The use according to any one of claims 2 to 12, wherein, The tube (T) is prepared by a method comprising at least the following steps i) and ii): i) providing an inner tube (IT) comprising a non-porous monolithic oxide ceramic, and ii) attaching an outer layer (OL) to the inner tube (IT), preferably by a lamination technique.

14. A tube furnace for a sintering and / or debinding process, wherein the tube furnace comprises a tube (T) which comprises an oxide ceramic matrix composite (OCMC).

15. Use of the tube furnace as claimed in claim 14 in a method for treating at least one three-dimensional green body (GB), wherein, The method comprises at least the following steps: a) providing the at least one three-dimensional green body (GB), wherein the at least one three-dimensional green body (GB) comprises an inorganic powder (IP) and a binder (B), b) providing an acid, c) treating the at least one three-dimensional green body (GB) with the acid in the tube furnace so as to obtain at least one three-dimensional brown body (BB), and optionally d) sintering the at least one three-dimensional brown body (BB) obtained in step c) in the tube furnace so as to obtain at least one three-dimensional sintered body (SB).

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