The present invention relates to a filament suitable to be used in a 3D printing device, wherein the filament comprises a ceramic powder, a thermoplastic binder and processing additive(s). The invention also relates to a process for producing a shaped body comprising the step of printing a shaped green body using the filament according to the invention. Also provided is the use of a filament according to the invention in a 3D printing device. The invention also relates to the use of a binder of the invention for the production of a filament for 3D printing devices.

BACKGROUND OF THE INVENTION In the late 1980s powder injection molding processes including metal injection molding

(MIM) and ceramic injection molding (CIM) were established. In these processes finely- powdered metal or ceramic material is mixed with a measured amount of binder material to form a 'feedstock' capable of being handled by plastic processing equipment through a process known as injection mold forming. The molding process allows complex parts to be shaped in a single operation and in high volume. The final products of such processes are commonly component items used in various industries and applications.

In these processes the molding step involves the use of injection molding machines and results in the formation of a so-called green body. This green body undergoes a further step in which the binder is typically at least partially removed before the body is heated to temperatures where the metal or ceramic components are sintered.

A feedstock is required also for more modem processes for forming prototypes such as 3D printers. In some aspects however the feedstock for 3D printing devices has been found to require different properties as will be explained below.

Creating a feedstock for 3D printing devices is not an easy feat as there are multiple parameters that should be adjusted. The final feedstock product must in particular meet the flexibility, stiffness, stickiness and viscosity required for successful 3D printing. In the field of 3D printing the fused deposition modelling (FDM) process, also called fused filament fabrication (FFF), is increasingly being used for manufacturing consumer goods, warranting an improvement in the quality of the 3D printed object output.

Fused deposition modelling (FDM) is an additive manufacturing technology commonly used for modelling, prototyping, and production applications. FDM is a rapid prototyping technique and it is one of mechanical manufacturing technologies, in which the process of extrusion of feedstock materials is involved. Generally, FDM works by laying down material in layers.

It is known that volumetric flow errors compromise the quality of the printed product. The thermoplastic filament itself has a significant effect on the variability in an FDM extruder’s flow. In other words, depending on its material the feedstock filament contributes to volumetric flow errors.

Furthermore, without wanting to be bound by theory it is believed that in terms of mechanical design, the size and tolerance of the filament diameter is found to play a very significant role in determining flow characteristics of the extruder.

Ideally, the diameter of the filament used can be minimized and a filament can be manufactured with tighter diameter tolerances to reduce volumetric flow errors.

Also, in the development of new feedstock composite materials need to be selected with reasonably good mechanical and thermal properties as well as their capabilities of mixing and surface bonding with binders.

WO 2016/004985 discloses sinterable feedstock for use in 3D printing devices.

Furthermore, classical ceramics are formulations which, in addition to aluminosilicates, also contain other sinterable oxides such as zirconium or aluminium (di)oxide. In addition to classic porcelain, technical ceramics such as boron nitride, silicon nitride or silicon carbide are also important representatives of this substance class. The difficulty in processing these materials using 3D printing is that the melting points of the components are so high. This means that 3D printing is usually only possible using a complex laser melting process (SLM). The lasers required for this must have high output powers and are correspondingly complex and expensive. Another disadvantage of the SLM process is the high temperature differences which can lead to distortion of the molded body. Likewise, components with thin wall thicknesses cannot be manufactured using this process. Even different alloys with liquid phases such as hard metals cannot be processed.

The stickiness is at least one property by which a feedstock suitable for 3D printing devices differs from a feedstock that is commonly used in powder injection molding (PIM) or powder extrusion molding (PEM) processes. A feedstock suitable for 3D printing requires good bonding ability of the individual mass strands between each other in order to produce a 3D structure with high resolution and good reproducibility. This property of the feedstock is however not beneficial with powder injection molding (PIM) or powder extrusion molding (PEM), in particular if these applications involve a smoothing calendar, a slit die or similar means. In fact, in PIM or PEM applications such stickiness is rather undesirable, and is generally avoided by preparing a specific feedstock that does not have this property, for example by including anti-adhesion additives.

In view of the above, there is a need for new feedstock for 3D printing devices which can be formed into a filament of constant diameter and which also meets the further material requirements such as sufficient hardness, suitable viscosity, good extrusion properties as well as a good adhesion of the printed mass strands to each other.

It was an object of the invention to provide a novel feedstock material suitable for 3D printing devices meeting the above outlined criteria. Especially, a feedstock material shall be provided which can be easily converted in sintered material.

SUMMARY OF THE INVENTION

To solve the aforementioned problems, the present invention provides a filament suitable to be used in a 3D printing device, wherein the filament comprises or consists of (a) a ceramic powder;

(b) a thermoplastic binder comprising a thermoplastic polymer and at least one plasticizer; and

(c) between 0 and 10 wt% of at least one processing additive based on the total weight of the filament. A further aspect of the invention relates to a process for producing a shaped body, the process comprising the following steps:

(i) printing a shaped green body using the filament according to the present invention and a 3D-printing device;

(ii) removing at least partially the plasticizer from the shaped green body; and

(iii) sintering the shaped green body obtained from step (ii) to obtain said shaped body.

Also provided is the use of a filament according to the invention in a 3D printing device.

In another aspect, the invention also relates to a green body producible by mixing a ceramic powder according to the invention and a thermoplastic binder according to the invention.

Also provided is as a further aspect the use of a binder as defined in the invention for the production of a filament for 3D printing devices.

DETAILED DESCRIPTION OF THE INVENTION

Before the present invention is described in detail below, it is to be understood that this invention is not limited to the particular methodology, protocols and reagents described herein as these may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention which will be limited only by the appended claims. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art.

In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. In the following definitions of some chemical terms are provided. These terms will in each instance of its use in the remainder of the specification have the respectively defined meaning and preferred meanings.

The term“alkyl” refers to a saturated straight or branched carbon chain. Preferably, an alkyl as used herein is a C1-C36 alkyl and more preferably is a C1-C10 alkyl, i.e. having 1, 2,

3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,

30, 31, 32, 33, 34, 35 or 36 carbon atoms, e.g. is selected from methyl, ethyl, propyl, iso propyl, butyl, /.v -butyl, /erf-butyl, pentyl or hexyl, heptyl, octyl, 2-ethylhexyl, nonyl decyl, behenyl, isostearyl and stearyl. Alkyl groups are optionally substituted. The term“alcohol” refers to a compound having one or more hydroxyl groups. For example a Cg-C36 alkyl alcohol is a Cg-C36 alkyl substituted with one or more hydroxyl groups. A fatty alcohol as used herein refers to a linear aliphatic primary alcohol.

The present invention provides novel filaments suitable to be used in a 3D printing device such as a fused deposition modelling device. It was unexpectedly found that the filaments exhibit an ideal combination of viscosity and hardness properties as well as sufficient adhesion when printed.

Thus, in a first aspect the invention provides a filament suitable to be used in a 3D printing device, wherein the filament comprises or consists of

(a) a ceramic powder;

(b) a thermoplastic binder comprising a thermoplastic polymer and at least one plasticizer; and

(c) between 0 and 10 wt% of processing additives based on the total weight of the filament.

According to the present invention the components present in the filament add up to 100% by weight, i.e. the sum of all components of the filament is 100% by weight.

In the filament according to the invention the ceramic powder is preferably sinterable. This means that the ceramic powder comprised in the printed shape will form a coherent mass upon heating without undergoing melting. In a preferred embodiment the ceramic powder is selected from the group consisting of porcelain, aluminium oxide, silicon dioxide, silicon carbide, silicon nitride, calcium phosphates, boron nitride, boron carbide, aluminium titanate, zirconium dioxide, amorphous ceramic compounds like glass and glass ceramics, and a mixture thereof, preferably porcelain, aluminium oxide zirconium dioxide and a mixture thereof

In a preferred embodiment at least 90% of the particles of the ceramic powder have a diameter of 20 pm or less, preferably 10 pm or less (measured by laser diffraction).

In a preferred embodiment the porcelain comprises 20 to 70 % by weight kaolin, 8 to 50 % by weight quartz and 20 to 35 % by weight feldspar. In a preferred embodiment the ceramic powder is present in the filament in an amount of 40 to 90 % by volume, preferably 50 to 80 % by volume (based on the total volume of the filament).

In a preferred embodiment the ceramic powder is present in the filament in an amount of 60 to 85 % by weight, preferably 65 to 85 % by weight, preferably 70 to 85 % by weight (based on the total weight of the filament).

In a preferred embodiment the thermoplastic binder is present in the filament in an amount of 60 to 20 % by volume, preferably 30 to 50 % by volume(based on the total volume of the filament).

In a preferred embodiment the thermoplastic binder is present in the filament in an amount of 10 to 30 % by weight, preferably 14 to 25 % by weight, preferably 14 to 24 % by weight (based on the total weight of the filament).

In a preferred embodiment the thermoplastic binder comprises 60 to 80 % by weight, preferably 65 to 75 % by weight, preferably 66 to 74 % by weight of the at least one plasticizer (based on the total weight of the thermoplastic binder). In a preferred embodiment the thermoplastic binder has a melting temperature of from

100°C to 190°C. In a preferred embodiment the at least one thermoplastic polymer is selected from the group consisting of a polyurethane, a polyamide, a polyvinylpyrrolidone, a polyethylene glycol, polyvinyl butyral, a polystyrene, a polyacrylate, a polymethacrylate, a polyolefin and a mixture thereof In a preferred embodiment the polyamide is selected from the group consisting of a copolyamide, polyamide 11, polyamide 12, polyamide 6,6, polyamide 6,12, a polyether block amide and a mixture thereof.

Preferably, said thermoplastic polymer is a polyamide selected from the group consisting of a copolyamide, Polyamide 11, Polyamide 12, polyamide 6,6, polyamide 6,12, a polyether-blockamide and mixtures thereof. The copolyamide which can in one embodiment be soluble in alcohol is preferably produced from a C4-C8 lactam and from a C10-C18 lactam. Most preferably said copolyamide is produced from caprolactam and laurinlactam.

In a preferred embodiment the at least one thermoplastic polymer is present in the filament in an amount of 1 to 10 % by weight, preferably 4 to 8 % by weight (based on the total weight of the filament).

In a preferred embodiment the at least one plasticizer is present in the filament in an amount of 5 to 20 % by weight, preferably 5 to 10 % by weight (based on the total weight of the filament).

In a preferred embodiment the at least one plasticizer is a substituted or non-substituted aromatic or heteroaromatic carboxylic acid ester or a mixture thereof. Preferably, the at least one plasticizer must be compatible with the thermoplastic polymer.

The plasticizer must be preferably well tolerated in the polymer. For the extraction step, the plasticizer should be (readily) soluble and the polymer poorly soluble or insoluble in the extraction solvent. In a preferred embodiment said at least one plasticizer is a hydroxybenzoic acid ester or a mixture of hydroxybenzoic acid esters, especially if a polyamide is used as thermoplastic polymer. In a preferred embodiment the hydroxybenzoic acid esters are esters of hydroxybenzoic acid and a branched or linear alcohol, wherein the alcohol is selected from the group consisting of a branched or linear C8-C22 alcohol and mixtures thereof and preferably selected from 2-propyl heptyl alcohol, isodecyl alcohol, 1-docosanol, 1-octadecanol, 1- dodecanol, 2-ethylhexyl alcohol, fatty alcohol and a mixture thereof. Preferably, the fatty alcohol is an isostearyl, stearyl and/or behenyl alcohol.

In a preferred embodiment the hydroxybenzoic acid ester is a p-hydroxybenzoic acid ester.

In a more preferred embodiment of the filament, said at least one plasticizer is an ester mixture produced from p-hydroxybenzoic acid and a mixture of alcohols, wherein the alcohol mixture preferably comprises 2-propyl heptyl alcohol, isodecyl alcohol, 1-docosanol, 1- octadecanol, 1-dodecanol, isostearyl alcohol and/or 2-ethylhexyl alcohol.

In a preferred embodiment the at least one plasticizer an ester which is solid at 20°C or an ester that is liquid at 20°C or a mixture thereof Preferably, the ester solid at 20°C is docosanyl-4-hydroxybenzoeic acid and the ester liquid at 20°C is 2-ethylhexyl-4- hydroxybenzoeic acid.

In a preferred embodiment of the filament of the invention said at least one plasticizer is an ester which is solid at 20°C and/or an ester that is liquid at 20°C or comprises only esters that are solid at 20°C. It was unexpectedly found that including such a mixture in the plasticizer allowed the filament of the invention in particular for filaments based on ceramic powders to stay elastic for a longer time, while at the same time providing sufficient bonding capability upon printing.

In a preferred embodiment in the mixture of esters (a) the ester which is solid at 20°C and (b) the ester that is liquid at 20°C are present in a ratio of 10:1 to 3:1 ((a) : (b)). Preferably, the filament of the invention comprises at most 8, 6, 4or at most 2 wt% of processing additives, based on the total weight of the filament.

Preferably, the at least one processing additive is selected from the group consisting of montan waxes, amide waxes, paraffin waxes, fatty acids, esters of fatty acids and any mixture thereof. In a preferred embodiment the at least one processing additive is a release agent.

In other applications, no additive is necessary. This will maximize the amount of ceramic powder to be included in the filament which is desirable because it will reduce the volume change of the printed prototype upon sintering. If the filament comprises no additives it is preferred that the filament may nevertheless comprise impurities which are typically found in ceramic powders or in the thermoplastic binder of the invention. It is most preferred that the amount of such impurities does not exceed 0.8 wt% of the total mass of the filament of the invention. It is preferred that a filament of the invention does not comprise any anti adhesion additives. In a preferred embodiment the filament of the invention comprises, preferably consists of 60 to 80 % by weight, preferably 70 to 75 % by weight porcelain powder, 3 to 10 % by weight, preferably 5 to 10 % by weight polyamide, 15 to 20 % by weight plasticizer and 3 to 5 % by weight release agent.

In a preferred embodiment the filament of the invention comprises, preferably consists of 80 to 85 % by weight, preferably 80 to 83 % by weight metal oxide powder (preferably aluminium oxide and/or zirconium dioxide powder), 3 to 7 % by weight polyamide, 8 to 12 % by weight plasticizer and 2 to 4 % by weight release agent.

In a preferred embodiment the filament has a diameter of 1 mm to 5 mm, preferably 1.1 to 3 mm, preferably 1.75 or 2.85 mm. In a preferred embodiment the filament is elastic, has a diameter of between 1 mm and 5 mm and a length of at least 10 cm.

According to the present invention a process for producing a shaped body comprises the following steps:

(i) printing a shaped green body using the filament according to the present invention or according to a preferred embodiment according to present invention and a 3D-printing device;

(ii) removing at least partially the at least one plasticizer from the shaped green body; and

(iii) sintering the shaped green body obtained from step (ii) to obtain said shaped body.

The invention-based process for the production of ceramic moulded parts using thermoplastic filaments even allows the use of conventional 3D printers, since it is possible to work in a low temperature range of 60-250° C, in particular 90-200°C. The process is also suitable for the production of ceramic moulded parts using thermoplastic filaments. In this process, the sinterable material is embedded in a thermoplastic binder, which can then be processed thermoplastically in the specified temperature range. It also allows particularly thin-walled and filigree structures to be produced. In addition to the sinterable material, the formulation contains a thermoplastic and a suitable plasticizer. Additional processing aids such as release agents, lubricants and surfactants may also be included.

In a preferred embodiment the filament is produced by mixing the components to be present, preferably at a temperature of 20 to 60°C. Subsequently, the mixture is plasticized in a kneader or extruder, preferably at a temperature of 100 to 200°C to form a mass. The mass is then preferably ground or granulated, especially if the mass is produced in the kneader. Subsequently, the (ground or granulated) mass is preferably extruded in a single-screw extruder to form the filament, preferably at a temperature of 100 to 160°C.

In a preferred embodiment in step (i) the filament is extruded through a nozzle having a temperature of 120°C to 180°C, preferably 150°C to 170°C.

In a preferred embodiment in step (i) the filament is extruded through a nozzle having a diameter of 0.3 mm to 1.0 mm, preferably 0.4 mm to 0.6 mm.

In a preferred embodiment in step (i) the filament is extruded into a container or a box, wherein the atmosphere has a temperature of 30°C to 70°C, preferably 55°C to 65°C. Preferably, the filament is extruded onto a printing bed having a temperature of 30°C to 70°C, preferably 55°C to 65°C. Preferably, the filament is extruded into a container or a box having an atmosphere temperature of 30°C to 70°C, preferably 55°C to 65°C and onto a printing bed having a temperature of 30°C to 70°C, preferably 55°C to 65°C.

In a preferred embodiment in step (i) the filament is extruded with a printing velocity of 500 to 4,000 mm/min, preferably 1,500 to 2,000 mm/min.

In a preferred embodiment in step (i) the shaped green body is printed in layers, wherein the thickness of the layers is from 0.10 to 1.0 mm, preferably 0.10 to 0.50 mm. In a preferred embodiment in step (ii) the at least one plasticizer is at least partially removed by at least one extraction step, wherein the at least one plasticizer is soluble in the organic solvent(s) used in the at least one extraction step. In a preferred embodiment in step (ii) the at least one plasticizer is at least partially removed, preferably at least partially extracted, by contacting the green body with at least one organic solvent, preferably at a temperature of 20°C or higher. Preferably, the solvent is acetone, ethylacetate, hydrocarbons, and/or methylethylketone. Preferably, the step (ii) is carried out two or more times, preferably by using different solvents. Preferably, a solvent is used, wherein the at least one plasticizer is (readily) soluble and the thermoplastic polymer is poorly soluble or insoluble. In a preferred embodiment in step (ii) the shaped green body is thermally debound at a temperature of 200°C to 500°C, preferably 300°C to 450°C.

In a preferred embodiment in step (ii) the at least one plasticizer is partially removed by contacting the green body with an organic solvent, preferably at a temperature of 20°C or higher, preferably of 20°C to 50°C, and then the shaped green body is thermally debound at a temperature of 200°C to 500°C, preferably 300°C to 450°C.

In a preferred embodiment in step (iii) the shaped green body is sintered at a temperature of 800°C to 2200°C, preferably 1100°C to 1700°C.

According to the present invention a filament according to the present invention or according to a preferred embodiment is used in a 3D printing device. Preferably, the process for producing a shaped body comprises the following steps:

(i) printing a shaped green body using the filament according to the present invention or according to a preferred embodiment according to present invention and a 3D-printing device, wherein the filament is extruded through a nozzle having a diameter of 0.3 mm to 1.0 mm, preferably 0.4 mm to 0.6 mm and a temperature of 120°C to 180°C, preferably 150°C to 170°C, with a printing velocity of 500 to 4,000 mm/min,

preferably 1,500 to 2,000 mm/min, onto a printing bed having a temperature of 30°C to 70°C, preferably 55°C to 65°C, and wherein the shaped green body is printed in layers, wherein the thickness of the layers is from 0.10 to 0.50 mm, preferably 0.10 to 0.20 mm, (ii) removing at least partially the at least one plasticizer from the shaped green body by contacting the green body with at least one organic solvent, preferably at a temperature of 20°C or higher, preferably of 20°C to 50°C, and then thermally debinding the shaped green body at a temperature of 200°C to 500°C, preferably 300°C to 450°C; and

(iii) sintering the shaped green body obtained from step (ii) at a temperature of 800°C to 2200°C, preferably 1100°C to 1700°C to obtain said shaped body.

The present invention also relates to a green body producible by mixing a ceramic powder according to the present invention or according to a preferred embodiment of the present invention and a thermoplastic binder according to the present invention or according to a preferred embodiment of the present invention.

The present invention also relates to a binder according to the present invention or according to a preferred embodiment of the present invention which is used for the production of a filament for 3D printing devices. Various modifications and variations of the invention will be apparent to those skilled in the art without departing from the scope of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.

The following examples are merely illustrative of the present invention and should not be construed to limit the scope of the invention as indicated by the appended claims in any way.

EXAMPLES

Example 1: Filament with porcelain 1

The individual components as listed below were added together and processed in a Coperion laboratory kneader for 1 h at 130°C to form a homogeneous mixture (approx. 200 g per batch). This mass was then ground at room temperature to a 1-3 mm granulate and then extruded in a single-screw extruder at 120°C to a filament with a diameter of 1.75 mm (suitable for conventional 3D printers). The filament was wound onto suitable bobbins and could then be used in the 3D printing process.

An exemplary release agent consists of 20% ethylenediamine bis-stearamide, 30% paraffin wax and 50% stearic acid.

Example 2: Filament with porcelain 2

The individual components as listed below were extruded as described in Example 1 at 120°C to a filament with a diameter of 1.75 mm (suitable for conventional 3D printers) and can then be used in the 3D printing process.

Example 3: Filament with aluminium oxide

The individual components as listed below were extruded as described in Example 1 at 120°C to a filament with a diameter of 1.75 mm (suitable for conventional 3D printers) and can then be used in the 3D printing process.

Example 4: Filament with zirconium dioxide

The individual components as listed below were extruded as described in Example 1 at 120°C to a filament with a diameter of 1.75 mm (suitable for conventional 3D printers) and can then be used in the 3D printing process.

Example 5: Printing the porcelain green bodies

The filaments of example 1 were processed on a 3D printer of the company FELIX (model FELIX pro 2). A gear wheel with a diameter of 40 mm and a height of 7 mm was printed as a structural element. The following settings were used:

Extruder/nozzle temperature 160°C

Printing bed temperature 60°C

Nozzle diameter 0.5 mm

Print speed 1800 mm/min

Layer thickness 0.15 mm The green bodies obtained had the following properties:

diameter = 40 mm

Weight = 6.31 g

Height = 7 mm

Example 6: Removal of the plasticizer component

The specimen was placed for 20 h at 36°C in 150 ml acetone and then dried for 24 h at room temperature.

Specimen (brownling)

Diameter 38 mm

Weight 5.42 g (weight loss 17%)

height 6.5 mm

Example 7: Sintering of the test specimen

The specimen was then slowly sintered in a furnace made by Nabertherm (type B 180). The following temperature program was used:

Heating up to 200 °C lh

Heating from 200°C to 400°C with 25K/h

Heating from 400°C to 1100°C with 350 K/h

Keep the temperature at 1100 °C for 2 h, then let it cool in the oven.

Test specimen (sintered) has the following parameter:

Diameter = 34 mm

Weight = 4.54 g (weight loss 16.2%)

Height = 6 mm

The other recipes (Examples 2 to 4) can be printed and debound under the same conditions. For aluminium oxide and zirconium oxide, however, different sintering temperatures must be selected because of the higher melting points: Aluminium oxide was debound up to 400°C and sintered at 1600°. Zirconium dioxide was debound up to 400°C and sintered at 1500°.