Polymer powders with core-shell structure for use in additive manufacturing processes

Abstract

The invention relates to polymer powders for use as building materials for the production of three-dimensional objects, which polymer powders have a core-shell structure, wherein the compositions of the core and of the shell are adapted to achieve uniform meltability of the powder particles in an additive manufacturing process. In this way, for example, improved layer bonding is achieved in the context of the production of three-dimensional objects. The present invention further relates to methods for producing such polymer powders, to methods for producing three-dimensional objects from such powders, and to corresponding three-dimensional objects, and to the use of corresponding polymer particles for improving the melting and processing properties when processing the polymer particles to form a three-dimensional object.

Description

[0001] MEISSNER BOLTE

[0002] Widenmayerstraße 47, 80538 Munich

[0003] EOS GmbH Electro Optical Systems September 18, 2024

[0004] M / EOSG-032-PC

[0005] PF / CBE / cbe

[0006] Polymer powders with core-shell structure for use in additive manufacturing processes

[0007] Description

[0008] The invention relates to polymeric powders for use as building materials for the production of three-dimensional objects having a core-shell structure, wherein the compositions of the core and the shell are tailored to achieve uniform meltability of the powder particles in an additive manufacturing process. The present invention further relates to methods for producing such polymeric powders, methods for producing three-dimensional objects from such powders, and corresponding three-dimensional objects, as well as the use of corresponding polymer particles to improve the melting and processing properties when processing the polymer particles into a three-dimensional object.

[0009] State of the art

[0010] Additive manufacturing encompasses various processes in which solid three-dimensional (3D) objects are created layer by layer based on a computer model. Powder-based additive manufacturing processes use powders as build materials, which are applied layer by layer to a substrate during processing. The layers within the objects being built are then solidified. This MEISSNER BOLTE M / EOSG-032-PC

[0011] 2

[0012] Solidification can be achieved, for example, by selective irradiation with electromagnetic radiation or by depositing a liquid ("ink") which is subsequently activated by surface exposure.

[0013] An example of a powder-based additive manufacturing process is laser sintering. In laser sintering, an object is built layer by layer in a thermally controlled process chamber, which is maintained at a temperature just below the melting point of the polymer system used. After a layer of polymer particles has been applied, a laser beam is passed over the surface of the powder layer and switched on and off to selectively sinter or fuse the polymer powder particles into a shape defined by a computer. Taking into account the reduction in thickness resulting from the densification associated with the melting of the particle layers, the object is built from a 3D CAD image that has been decomposed into profile slices.

[0014] In laser sintering, the powder is typically applied in thin layers, approximately 0.01 to 0.30 mm thick, to a build platform housed within a casing. After a layer has been processed, the build platform is lowered, and a new layer of powder is applied over the completed layer. Each newly applied layer is then fused together according to the defined shape to ultimately form the desired three-dimensional object.

[0015] Fig. 1 shows an example of a laser sintering device commonly used with a CO₂ laser, comprising a laser beam and a deflection mirror, which enables the layer-by-layer fabrication of a three-dimensional object. As can be seen in Fig. 1, the device has a container 1. This container is open at the top and bounded at the bottom by a support 4 for carrying an object 3 to be formed. The upper edge 2 of the container (or its side walls) defines a working plane 6. The object is located on the top surface of the support 4 and is formed from a plurality of layers of a powdered build-up material, solidifiable by electromagnetic radiation, extending parallel to the top surface of the support 4. The support is movable in the vertical direction, i.e., parallel to the side wall of the container 1, via a height adjustment device. MEISSNER BOLTE M / EOSG-032-PC

[0016] 3

[0017] This allows the position of the carrier 4 relative to the working plane 6 to be adjusted.

[0018] Above the container 1 or the working plane 6, an application device 10 is provided for applying the powder material 11 to be solidified onto the carrier surface 5 or a previously solidified layer. Furthermore, an irradiation device in the form of a laser 7 is arranged above the working plane 6, which emits a directed beam of light 8. This beam is deflected by a deflection device 9, for example a rotating mirror, as a deflected beam 8' towards the working plane 6. This arrangement is common for a laser sintering system with a CO2 laser. A control unit 40 enables the control of the carrier 4, the application device 10, and the deflection device 9. The elements 1 to 6, 10, and 11 are arranged within the machine frame 100.

[0019] When processing conventional polymer powders using laser sintering, a heating system is also present to heat the polymer powder in the build chamber where the three-dimensional object is produced. Typically, the powder is heated to a temperature close to, but still below, its melting point or, in the case of amorphous polymers, its softening point, thus minimizing the amount of energy required to liquefy the material. Typical heating systems are radiant heaters that emit radiation in the infrared and near-infrared ranges. However, such heating systems must be precisely controlled to prevent unintentional melting of particles or particle regions by the heating system.This problem arises particularly in the processing of polymer powders containing additives to increase the absorption of irradiated energy, since such polymer powders are relatively sensitive to the irradiated energy.

[0020] In the production of the three-dimensional object 3, the powder material 11 is applied layer by layer to the support 4 or a previously solidified layer and solidified with the laser beam 8' at the points in each powder layer corresponding to the object. After each selective solidification of a layer, the support is lowered by the thickness of the next powder layer to be applied. MEISSNER BOLTE M / EOSG-032-PC

[0021] 4

[0022] In selective beam sintering, the powder is deposited and then scanned with a directed energy beam. The energy beam is usually a laser, but electron beams can also be used. In selective inhibition sintering, the powder is first deposited, and then a sintering inhibitor is selectively applied to the powder before the entire layer is irradiated with electromagnetic radiation. In high-speed sintering, a layer of powder is deposited, and then a radiation-absorbing liquid is selectively applied before the layer is irradiated with electromagnetic radiation.

[0023] Furthermore, processing methods are known in which binder is applied to selected areas of the powder layer and sintering only takes place in a final step, or in which selected areas of a layer are irradiated with electromagnetic radiation using a mask.

[0024] In powder-based processes, the required polymer powders are currently mostly produced by precipitation of the polymers from a solution or by milling pellets or coarse powders. However, these processes are associated with the disadvantage of relatively high costs for plant technology and solvents or the milling process and only allow the incorporation of additives into the particles to a limited extent.

[0025] WO 2013 / 138204 proposes a process for producing particles for use in polymer sintering processes, in which fibers are first produced and then, in a second step, cut into microparticulate cylinders. According to WO 2013 / 138204, this process should also allow the production of particles with multiple polymer components, shaped as cylindrical particles, which are created by cutting fibers with a core-shell structure. This shape should make it possible to selectively introduce additives such as flame retardants, conductive fillers, or heat stabilizers only into the shell structure of the particles.

[0026] One problem in the production of three-dimensional objects, where powders are softened or liquefied by means of electromagnetic radiation, is the uneven absorption of the irradiated energy from the outside to the inside, which can lead to a particle on the outside being MEISSNER BOLTE M / EOSG-032-PC

[0027] 5. The particle is strongly liquefied, but the core is not yet completely melted. This behavior is particularly exacerbated by the short exposure times and high energy input required in laser sintering or laser melting; increasing the power is usually not possible here because the outer areas of the particles would be overheated, potentially leading to polymer degradation. The inconsistent melting behavior, due to the incompletely melted particle cores, negatively impacts the bonding between different layers. Therefore, there is a need for a powder material that can be liquefied as uniformly as possible in additive manufacturing processes, despite the short exposure times and high energy fluxes, in order to achieve improved component properties, such as increased detail resolution and / or the mechanical properties of the objects.

[0028] There is also a need for methods to incorporate additives into three-dimensional objects produced using additive manufacturing that are unstable during prolonged storage or reactive to the polymer material. Furthermore, there is a need for build materials for additive manufacturing that exhibit previously incompatible properties. Finally, there is a need for build materials for additive manufacturing that enable the production of three-dimensional objects based on cross-linked polymers.

[0029] Amorphous polymer powders soften above their glass transition temperature, so these polymers must be processed below this temperature to prevent the powder particles from clumping together. To achieve sufficiently good particle coalescence and a high melt viscosity during processing, a high energy input from a laser is required. To avoid overheating the particles, which can damage the polymer, a lower laser power with a longer exposure time is typically used. This is obviously associated with a significant time investment for manufacturing the corresponding objects. Against this background, there is a need for methods that allow for faster processing of amorphous polymer powders.

[0030] The present invention addresses the problems described.

[0031] Description of the invention MEISSNER BOLTE M / EOSG-032-PC

[0032] 6

[0033] The investigations underlying this application surprisingly revealed that the concept of WO 2013 / 138204 can be used to produce highly uniform melting in the additive manufacturing of objects, particularly with regard to energy input. For this purpose, a polymer composition is used as the core material of the core-shell particles described in WO 2013 / 138204. This polymer composition softens more than the polymer composition of the shell when exposed to energy of the same power. This compensates for the lower energy input in the core of the particles, which results from attenuation due to partial absorption when passing through the shell material. Furthermore, it even achieves a situation where the core material is more fluid than the shell material at the melting point, thus promoting mixing of the materials.

[0034] According to a first aspect, the present invention therefore relates to a polymeric powder for use as a building material for the additive manufacturing of a three-dimensional object by selectively solidifying a building material at the cross-sectional points of the three-dimensional object in the corresponding layers, wherein the polymeric powder has particles with a core-shell structure in which the composition in the core differs from the composition in the shell by a changed melting or softening behavior, such that the core softens more than the shell when energy is irradiated.

[0035] In the context of the invention described herein, the terms "core-shell structure", "core-shell structure", and "shell-core structure" are used synonymously and refer to materials whose surface is composed of a material that is distinguishable from the material in the interior. This distinguishability can arise from a different composition, a different crystallinity of the polymers, and / or from different viscosities of the polymers at temperatures above the melting and softening temperatures of the material at the surface and in the interior of the material.

[0036] The "core" of the core-shell structure can be in the form of one or more cores, whereby in the case of multiple cores, different core strands can be present, which are embedded in a strand of the shell material and attached to the surface. MEISSNER BOLTE M / EOSG-032-PC

[0037] 7 before the strand is enveloped by it. In particles produced from such strands, a smaller part of the surface can also be formed by the core material, e.g., when strands with a core-shell structure are converted into particles by cutting, so that core material is also present at the cut surfaces of the particles. In the context of the invention described here, however, the larger part of the particle surface, i.e., at least 55%, in particular at least 60%, and more preferably at least 70% of the particle surface, is covered by shell material.

[0038] “More softened” means that when separate core and shell compositions are irradiated with the same energy intensity, the core composition has a lower viscosity (determined under identical measurement conditions for the liquefied core and shell compositions) than the shell composition after a uniformly specified time “t”.

[0039] The terms "shell material", "shell composition", and "shell composition" are used synonymously in the context of this application. Likewise, the terms "core material", "core composition", and "core composition" are used synonymously.

[0040] In the context of this application, "solidification" refers to at least partial melting or remelting of the build-up material followed by solidification or resolidification. The term "selective" in the context of solidification refers to localized irradiation of areas within a continuous layer that are to be solidified, while areas of the layer that are not to be solidified are not irradiated. "Irradiation" here refers to irradiation that is directly responsible for melting the build-up material (i.e., non-selective irradiation for heating purposes, where the build-up material is not heated above its melting point, does not cause "selective" melting and solidification). In contrast to such selective solidification, for example, a technique in which a complete layer is not deposited, but rather the build-up material is partially melted, e.g.,In liquid form, applied only to areas where solidification is desired (e.g., extrusion processes or jet injection). MEISSNER BOLTE M / EOSG-032-PC.

[0041] 8

[0042] Selective solidification therefore comprises localized or layer-by-layer melting by the application of electromagnetic radiation and subsequent solidification of the build-up material by solidification of the melt at the locations corresponding to the cross-section of the three-dimensional object in the respective layer. In contrast, sintering a previously generated body does not constitute selective solidification as defined in the application. Layer-by-layer melting can be achieved, for example, by applying a continuous layer of an additive such as an ink, whereby the layer is irradiated with energy after application, so that areas to which the additive has been applied melt and solidify in the selected cross-sections.

[0043] As mentioned, the compositions of the core and shell in core-shell particles differ in that the core composition or core material softens more upon irradiation than the shell material. A simple and, in the context of the invention described here, preferred measure for adjusting such different melting or softening behavior is to incorporate an absorber material into the core material that, due to its absorption properties, converts more radiation into heat energy than the polymer forming the core or shell material. This allows the core material to be heated more than the shell material upon exposure to the same amount of energy.

[0044] A measure with a comparable effect is the embedding of a larger proportion of absorber material in the core material compared to the shell material. Accordingly, in a preferred embodiment, the composition of the core has a higher proportion of absorber material than the composition of the shell.

[0045] Another measure with a comparable effect is the inclusion of different absorber materials in the core and shell composition, wherein the absorber material included in the core composition can convert more energy into heat than the absorber material included in the shell composition. Accordingly, in a preferred embodiment, the core composition includes an absorber material that can convert more energy into heat than the absorber material included in the shell composition. MEISSNER BOLTE M / EOSG-032-PC

[0046] 9

[0047] Any material suitable as an absorber material exhibits higher absorption (determined photospectrometrically at identical layer thickness) at the incident wavelength than the polymer of the core or shell in which the absorber is embedded. Examples of suitable absorbers include metal particles, organic dyes, pigments (e.g., inorganic pigments or carbon black), salts, minerals, or mixtures of different absorber types. Near-infrared (NIR) absorbing materials (NIR absorbers) are particularly preferred because polymers typically exhibit low absorption in this range. NIR absorbers absorb electromagnetic radiation with wavelengths in the range of 780 nm to 3000 nm. Within the scope of the invention described herein, the term "NIR absorber" refers to a substance or mixture of substances that at least partially absorbs NIR radiation.For the sake of readability, the following explanations refer to a single substance as an NIR absorber. The same applies if a mixture of substances is used as an NIR absorber.

[0048] A preferred NIR absorber in the invention described herein is carbon black, since carbon black exhibits high absorption even at comparatively low concentrations. Particularly suitable types of carbon black for use as NIR absorbers are, for example, those with one or preferably both of the properties described below:

[0049] (i) amorphous industrial carbon black (definition according to EC No. 215-609-0, CAS No. 1333-86-4), preferably with a mean primary particle size, determined by laser diffraction, in the range of 26 to 58 nm;

[0050] (ii) Soot with a carbon content of more than 96% (according to elemental analysis).

[0051] Particularly preferred types of carbon black for use as absorbers in core materials are those described in WO 2020 / 099236 Al.

[0052] For broadband absorbers like carbon black, encapsulating the material in the core of core-shell particles provides an outer material that is (at least largely) free of carbon black, thus suppressing softening and clumping of the particles caused by inhomogeneous heating of the powder bed. This allows the powder bed to be heated to a higher temperature using the heating element, thereby increasing the temperature difference required to melt the particles. MEISSNER BOLTE M / EOSG-032-PC

[0053] The amount of energy required to melt the material can be reduced to 10. This avoids an inhomogeneous thermal stress on the building material, which occurs when a higher amount of energy is required for melting.

[0054] If carbon black is incorporated into the core material of the plastic powder, its proportion can vary widely, up to, for example, 8 wt.%, preferably up to 5 wt.%, more preferably up to 4 wt.%, and even more preferably up to 2 wt.%, wherein the amount is expediently tailored to the proportion of the core material in the polymeric powder according to the invention, so that higher carbon black proportions are achieved particularly with smaller core material proportions. Suitable minimum amounts of carbon black in the core material can be specified as 0.1 wt.% or more, and preferably 0.2 wt.% or more.

[0055] Furthermore, it is preferred if the polymeric powders according to the invention have a total carbon black content of 0.02 wt.% to 1 wt.%, more preferably 0.03 wt.% and / or at most 0.5 wt.% and even more preferably at least 0.04 wt.% and / or at most 0.15 wt.%.

[0056] If the same polymer or a polymer with a comparable melting or softening temperature is used as the base material of the core as for the shell material, it is further preferred if the proportion of the absorber material in the core material is at least 0.01 wt.% and in particular at least 0.03 and / or at most 8.0 wt.% and further preferably at least 0.04 wt.% and / or at most 5.0 wt.% higher than in the material of the shell.

[0057] The term "base material" of the core refers here to the main polymer component in the composition. Preferably, the proportion of the main polymer component, based on the total amount of polymers in the composition, is at least 50 wt.%, more preferably at least 70 wt.%, and even more preferably at least 90 wt.%. In a particularly preferred embodiment, the main polymer component constitutes the entire amount of polymer in the composition.

[0058] The use of an absorber material in the core composition may be unnecessary if the polymer contained in the core composition is inherently MEISSNER BOLTE M / EOSG-032-PC

[0059] 11 exhibits a higher absorption than the polymer contained in the shell composition. Such an embodiment of the polymeric powder according to the invention is hereby included.

[0060] In the case of using absorber materials in the core composition, but also in the alternative embodiments of the core and shell composition described below, it is preferred if the composition from which the core and / or the shell of the polymeric powders according to the invention is formed contains a homopolymer, copolymer, and a polyblend (also referred to as a "polymer blend") as the polymer. A polyblend is understood to be a mixture of two or more different polymers. A polyblend can be a single-phase polyblend (homogeneous polyblend) or a multi-phase polyblend (heterogeneous polyblend). In a multi-phase polyblend, several glass transitions are typically observed by differential scanning calorimetry.

[0061] The polymer can be selected from polyaryletherketones (PAEK), polyarylethersulfones (PAES), polyamides, polyesters, polyethers, polylactides (PLA), polyolefins, polystyrenes, polyphenylene sulfides, polyvinylidene fluorides, polyphenylene oxides, polyimides, polyetherimides, polycarbonates, polyurethanes, polysulfones, polyketones, polyacetals, polysiloxanes and copolymers comprising at least one of the aforementioned polymers or their monomer units, as well as polymer blends of one or more of the aforementioned polymers or copolymers thereof, the selection being, however, not limited to the aforementioned polymers and copolymers and polymer blends thereof.

[0062] Suitable PAEK polymers and copolymers, for example, are selected from the group consisting of polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherketone (PEK), polyetheretherketoneketone (PEEKK), polyetherketoneetherketoneketone (PEKEKK), polyaryletheretheretherketone (PEEEK) and copolymers that include at least one of the aforementioned polymers.

[0063] Suitable polyamide polymers or copolymers can be selected from the group consisting of polyamide 6 / 6T, polyamide elastomers such as polyether block amides like PEBAX™-based materials, polyamide 5, polyamide 6, polyamide 66, polyamide 11, polyamide 12, polyamide 510, polyamide 612, polyamide 610, polyamide MEISSNER BOLTE M / EOSG-032-PC

[0064] 12

[0065] 1010, polyamide 1012, polyamide 1212, polyamide PA6T / 66, PA4T / 46, and copolymers comprising at least one of the aforementioned polymers. Suitable polyester polymers or copolymers may be selected from polyalkylene terephthalates (e.g., PET, PBT) or polyarylent terephthalates and their copolymers. Suitable polyolefin polymers or copolymers may be selected from the group consisting of polyethylene, polypropylene, and polybutylene. Suitable polystyrene polymers or copolymers may be selected from the group consisting of atactic, syndiotactic, and isotactic polystyrenes. Suitable polyimide polymers or copolymers may be selected from the group consisting of polyarylamide, polybismaleimide, and, in particular, polyetherimide. Suitable copolymers include, for example, acrylonitrile butadiene styrene (ABS) copolymer, styrene acrylonitrile (SAN) copolymer or acrylonitrile styrene acylate (ASA) copolymers.

[0066] In one embodiment of the invention, the core and / or shell material comprises polymers or copolymers or blends of PAEK, polyamide, or polyetherimide, wherein the PAEK is preferably PEEK, PEKK, PEK, PEEKK, PEKEKK, and / or PEEEK, and the preferred polyamide is polyamide 12 and / or polyamide 11. In a further preferred embodiment, the core and / or shell material comprises a polypropylene polymer or a polypropylene / polyethylene copolymer.

[0067] Within the scope of the invention, it is preferred if the core material and / or the shell material of the core-shell particles are polyamide polymers or copolymers, in particular polyamide 12, polyamide 11 and / or polyamide 1012 and / or a copolymer that includes at least one of the preceding polymers or their monomer units, and / or at least a polymer blend that includes at least one of the aforementioned polymers or copolymers. Alternatively, it is preferred that the core material and / or the shell material of the core-shell particles comprises polyolefins, e.g., in the form of polyethylene, polypropylene or polybutylene, polystyrene, polyester, polycarbonate, acrylonitrile butadiene styrene (ABS) copolymer, styrene-acrylonitrile (SAN) copolymer, an acrylonitrile styrene acylate (ASA) copolymer, polyetherimide, a thermoplastic polyurethane, or a block copolymer of polyamide and polyethers (also referred to as polyether block amide or PEBA). MEISSNER BOLTE M / EOSG-032-PC

[0068] 13

[0069] In an alternative embodiment to the use of absorbers, different melting or softening behavior of the core and shell materials is achieved by the core composition having a lower viscosity at temperature A, at which the core and shell compositions are just in molten form (i.e., a temperature just above the highest melting or softening temperature of the core and shell compositions), than the shell composition (i.e., the core composition is more fluid than the shell composition at this temperature). Accordingly, in a further preferred embodiment, the present invention relates to a polymeric powder with a core-shell structure as described above, wherein the core composition has a lower viscosity at the processing temperature than the shell composition.

[0070] The “temperature A” is usually somewhat higher than the melting or softening temperature of the shell material used, i.e. preferably 5°C higher or 10°C higher.

[0071] The melting temperature or melting point of a polymer, as used in this description, is determined by DSC as the maximum in the melting curve, where the heat flux (in mW / mg) is plotted against the temperature. For this determination, reference can be made, for example, to DIN EN ISO 11357-3: 2018. The softening process is visible as a step in the DSC thermogram, and the softening temperature corresponds to the end set of the melting or softening process according to DIN EN ISO 11357-2 at a heating rate of 20 K / min. The reference point "end set" ensures that the polymer is completely in the fluid phase at this temperature.

[0072] Furthermore, it is preferred that the viscosity difference at temperature A, determined as the viscosity of the core composition / viscosity of the shell composition, is 0.9 or less, and preferably 0.7 or less. The relevant viscosities of the core and shell compositions are to be determined at a temperature 10°C above the melting or softening temperature of the shell composition. The melting or softening temperature of the shell composition is to be determined analogously to the melting or softening temperature of the polymer described above, in accordance with DIN EN ISO 11357. The determination is carried out according to MEISSNER BOLTE M / EOSG-032-PC.

[0073] 14 of the viscosities is preferably to be determined in accordance with ISO 1133 in the context of the invention described herein.

[0074] A lower viscosity can be achieved by using a different polymer, or the same polymer (i.e., a polymer based on the same monomers) with a different chain length or branching, by including viscosity-reducing additives, e.g., plasticizers, in the core material, or by using a different polymer mixture in the core and shell composition.

[0075] Particularly suitable polymers for core-shell particles based on the same polymer in the core and shell composition with different viscosities are, for example, polyamides, such as polyamide 12, which is available as a low-viscosity variant (e.g. commercially as VESTAMID® L1670, Evonik) and as a higher-viscosity variant (e.g. commercially as VESTAMID® L1940, Evonik).

[0076] In a further embodiment, different melting or softening behavior of the core and shell compositions is achieved by including an amorphous polymer in the core composition and a crystalline or semi-crystalline polymer in the shell composition. Unlike amorphous polymers, crystalline or semi-crystalline polymers exhibit a melting peak, or a determinable melting temperature, in a DSC melting test. Since additional energy ("melting energy") is required to break up crystalline structures, less energy is needed to liquefy / soften amorphous materials than crystalline or semi-crystalline materials. Accordingly, an amorphous polymer softens faster than the same polymer in crystalline or semi-crystalline form when irradiated with the same energy. It is preferred that the respective compositions consist only of amorphous or semi-crystalline polymers.Contains crystalline or semi-crystalline polymer as a polymer component.

[0077] In the context of the invention described herein, an amorphous polymer is defined as a polymer with a fusion enthalpy of < 5 J / g and preferably < 1 J / g. A semi-crystalline or crystalline polymer has a fusion enthalpy of more than 5 J / g, in particular at least 10 J / g and preferably at least 50 J / g, wherein the respective fusion enthalpies are to be determined by DSC according to ISO 11357-3. MEISSNER BOLTE M / EOSG-032-PC

[0078] 15

[0079] The polymers in the core and shell composition can be the same or different. Core-shell particles with a shell of crystalline or semi-crystalline polymer and a core of the same polymer in amorphous form can be produced, for example, by controlled action of a crystallizing solvent on completely amorphous particles. Alternatively, core-shell particles with a shell of crystalline or semi-crystalline polymer and an amorphous core made of different polymers can be produced, for example, by depositing a layer of a crystalline or semi-crystalline polymer onto amorphous polymer primary particles.

[0080] In a further embodiment, different melting or softening behavior of the core and shell compositions is achieved by the core composition containing a polymer with a lower melting point or a lower softening temperature than the shell composition, preferably wherein the core composition contains only polymers with a lower melting point or a lower softening temperature than the shell composition.

[0081] To determine the characteristic "lower melting point" or "lower softening point," the melting and softening temperatures, respectively, must be determined under identical conditions. It is preferred that the difference between the melting and softening temperatures of the polymers be at least 5 °C, preferably at least 10 °C, more preferably at least 20 °C, and even more preferably at least 40 °C. A possible upper limit for the temperature difference is 150 °C or 200 °C. Corresponding specifications are preferred for the softening temperatures of the polymers. With very large differences between the melting and softening temperatures, it is possible that at the processing temperature (i.e., the temperature to which the polymer powder can be heated for processing, e.g., by a surface heating system), the core material is already molten or softened, while the shell material is still solid.

[0082] The difference in melting or softening temperature can result from the polymer in the core or shell itself, or from an additive in the composition of the core or shell that regulates the melting or softening temperature of the polymer. Accordingly, such embodiments are also covered by the application, MEISSNER BOLTE M / EOSG-032-PC

[0083] 16 where the composition of the kernel and shell exhibits different melting or softening points due to additives.

[0084] The approaches described above for controlling the melting and softening behavior in the polymeric powders with core-shell particles according to the invention by including absorbers, by different viscosities, melting or softening temperatures, or by a different degree of crystallinity, can be used independently of one another, or several of the approaches can be combined in a core-shell particle product, e.g. by differentiating the core composition by a content of an absorber and by a lower viscosity at the processing temperature.

[0085] Besides the fact that the composition of the core differs from that of the shell due to altered melting or softening behavior, such that the core softens more than the shell upon exposure to energy, the core may contain further functional additives that can act on the overall mixture or specifically on polymers in the shell composition during processing of the powder. For example, the core composition may contain an oxygen- or moisture-sensitive additive, or an additive that decomposes a polymer contained in the shell composition in the presence of moisture and elevated temperature. Additives that decompose or promote the decomposition of polycarbonate contained in the shell composition in the presence of moisture and elevated temperature include, for example, organic phosphites or phosphonites (typical thermostabilizers).By being incorporated into the core composition, such additives are shielded from oxygen and / or moisture by the shell composition and are only released during processing, where they can then exert their effect. With polycarbonate in the shell of core-shell particles according to the invention, crystallization can be achieved by solvent exposure, drying, and annealing to indicate increased crystallinity without any significant risk of degradation.

[0086] Furthermore, this type of encapsulation protects additives that react to environmental influences during storage or heating, especially if their effectiveness would otherwise be impaired or the additive has a damaging effect. An example of a "degrading" additive is MEISSNER BOLTE M / EOSG-032-PC.

[0087] 17. Enzymes can be identified that promote biodegradation under certain pH values ​​and temperatures, whereby the encapsulation ensures that decomposition only occurs after controlled (industrial) composting, and not through environmental influences during storage.

[0088] Other functional additives that can be included in the polymeric powders according to the invention are crosslinking additives, catalytic additives, and additives that act as nucleation agents or nucleation inhibitors in a crystallization process.

[0089] Another additive that may be usefully included in the core composition is a crosslinking agent for a polymer contained in the shell composition. This agent does not react with the other components of the core composition but is released from the core composition as a result of mixing the core and shell compositions during processing of the polymeric powder. Other additives that may be included in the core composition are foaming agents such as sodium bicarbonate, stabilizers, and / or flame retardants.

[0090] An example of a system with an additive and a crosslinkable polymer in the shell composition is a polymeric powder containing a sulfur crosslinker in the core composition and a thermoplastic elastomer, e.g., EPDM (ethylene propylene diene monomer rubber), in the shell composition. Another example of a crosslinking additive is a multifunctional crosslinker, e.g., a polyisocyanate, which can react with functional groups, e.g., OH or NH₂ groups, in a polymer in the shell composition.

[0091] As mentioned above, functional additives can be incorporated into the core composition. These additives can act on the powder during processing, either in the overall mixture or specifically on polymers within the shell composition. In such particles, the functional additives are kept separate from the shell material within the particle core, allowing a specific functional effect to be initiated during processing when the core and shell materials are mixed during liquefaction. MEISSNER BOLTE M / EOSG-032-PC

[0092] 18

[0093] This effect can also be utilized independently of differing softening behavior between the core and shell materials. Accordingly, the present invention relates in a further aspect to a polymeric powder for use as a building material for the additive manufacturing of a three-dimensional object by selectively solidifying a building material at the cross-sectional points of the three-dimensional object in the corresponding layers, wherein the polymeric powder has particles with a core-shell structure in which the composition in the core contains a functional additive that is not included in the composition of the shell. Exemplary functional additives in this case are oxygen- or moisture-sensitive additives, additives that decompose a polymer contained in the shell composition in the presence of moisture and elevated temperature, and additives that cause cross-linking of the polymer in the composition of the shell.The relevant additives are described above.

[0094] The core or shell composition of the particles in the polymeric powder according to the invention may contain further additives for controlling the properties of the particles and / or an object produced therefrom. Examples of such additives include additives selected from the group comprising thermostabilizers, reflective particles, e.g., in the form of TiOz, flame retardants, flow agents (e.g., Aerosil 200), discoloration inhibitors, lubricants, nucleating agents, thickeners, antioxidants (e.g., N,N'-(hexane-1,6-diyl)bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanamide], which is commercially available from BASF as Irganox 1098), antistatic agents, agents for improving biodegradability and / or biocompatibility, preservatives, dyes, fragrances, hydrolysis stabilizers, fillers, fibers, in particular in the form of glass or carbon fibers, or plasticizers.Fillers include, among other things, metal particles, mineral and / or ceramic fillers, or glass beads. Furthermore, the core and / or shell composition may contain indicators, i.e., substances that undergo a structural change under appropriate thermal stress or contact with selected media, thereby, for example, visually indicating this contact. The additives may be incorporated into the core-shell particles, so that they are present in all or only in specific areas of the core-shell structure (i.e., only in the core or the shell), or they may be present separately from the core-shell particles. MEISSNER BOLTE M / EOSG-032-PC.

[0095] 19

[0096] With regard to particle shape, the polymeric powders according to the invention are not subject to any relevant restrictions, typically achieving a spherical or cylindrical structure. Spherical or nearly spherical (e.g., potato-shaped) particles can be produced, for example, by first producing primary particles from the core composition, for instance, by precipitation of a mixture to obtain particles, and then coating these particles with the shell composition. Cylindrical particles can be produced, for example, by extruding strands in which the core material is formed in the middle and the shell material on the outside, and then cutting the strands into particles. Alternatively, any other particle shape that can arise during the production of powders suitable for additive processing can also be achieved.

[0097] The particle size of the polymeric powders according to the invention should be within a range suitable for additive processing of the particles into a three-dimensional object. Preferably, the particles have a mean particle size d50 in the range of 10 to 100 pm, more preferably at least 20 pm and / or at most 80 pm, and more preferably at least 30 pm and / or at most 75 pm. This particle size is to be determined according to ISO 13320-1 (wet), e.g., using a CILAS 1064 laser diffraction measuring instrument.

[0098] For cylindrical particles, it is further preferred that they have an aspect ratio (= ratio of the largest to the smallest dimensions of the particles in space, averaged over the total quantity of particles) in the range of 1 to 5, preferably at least 1 and / or at most 3, and even more preferably at least 1 and / or at most 2. Such particles can be produced by first manufacturing continuous strands with a thickness in the specified size range, and then cutting the strands to obtain cylindrical particles.

[0099] Cylindrical particles can also be transformed into particles with rounded edges using appropriate processes.

[0100] The bulk density of the polymeric powder according to the invention is expediently in the range of 300 kg / m³. 3 up to 800 kg / m² 3 and preferably at least 400 kg / m² 3 and / or a maximum of 750 kg / m² 3. MEISSNER BOLTE M / EOSG-032-PC

[0101] 20

[0102] In a further aspect, the present invention relates to methods for producing a polymeric powder, as described in detail above. In one embodiment, such a method involves coating primary particles with a composition that differs from that of the primary particles, forming particles with a core-shell structure. Such coating can be achieved, for example, by multi-stage precipitation, a CVD or PVD process, by melting a second polymer layer onto a core, e.g., in a downer reactor, or by thermomechanical deposition of a second polymer layer onto core particles.In another embodiment, the process involves a multi-stage polymerization in which, in a first stage, primary particles are produced that form the subsequent core of the core-shell particles, and in a second stage, secondary particles are produced in which a layer of shell material has been polymerized onto the core material. In a still further embodiment, such a process involves the formation of fibers with an inner and an outer component that encloses the inner component, e.g., via co-extrusion, and the cutting of the fibers to form particles.

[0103] Corresponding particles are shown schematically in Figures 2 and 3. The particles in Figure 2 consist of a core (gray) surrounded by a shell made of a shell material (white). Figure 3 shows an embodiment with several cores (gray) embedded in a shell material.

[0104] In a further aspect, the present invention relates to a three-dimensional object produced by irradiation at points in space corresponding to the cross-section of the three-dimensional object in the respective layer, wherein a polymeric powder as described above was used as the building material.

[0105] The term "three-dimensional object" is used here synonymously with the term "shaped body".

[0106] In a further aspect, the present invention relates to a method for producing a three-dimensional object, in particular by solidifying a powdered building material at the spatial points corresponding to the transverse MEISSNER BOLTE M / EOSG-032-PC

[0107] 21 section of the three-dimensional object in the respective layer, wherein a polymeric powder as described above is used and preferably the building material is selectively solidified by the action of electromagnetic radiation emitted from a radiation source.

[0108] In a further aspect, the present invention relates to a system for producing a three-dimensional object by solidifying a powdered building material at points corresponding to the cross-section of the three-dimensional object in the respective layer. The system comprises at least one radiation source configured to emit electromagnetic radiation, a process chamber configured as an open container with a container wall, a support located within the process chamber, wherein the process chamber and the support are movable relative to each other in a vertical direction, and a horizontally movable storage container and coating unit, wherein the storage container is at least partially filled with a polymeric powder as described above. Such a system preferably includes an additional surface heating system for heating the polymeric powder on the support.The specified system includes the polymeric powder according to the invention and the described technical device (analogous to a kit comprising both components).

[0109] The radiation source in the prescribed method and the corresponding system is preferably a radiation source that emits electromagnetic radiation with a wavelength in the range of 500 to 1500 nm, preferably in one of the wavelength ranges of 1064 ± 15 nm and / or 980 ± 15 nm and / or 940 ± 15 nm and / or 810 ± 15 nm and / or 780 ± 15 nm and / or 640 ± 15 nm, or electromagnetic radiation with a wavelength of approximately 10.6 pm, in the range of 1.9 to 2.0 pm or in the range of 4.8 to 8.3 pm, and preferably approximately 5 pm. For the specified deviation “±15 nm”, a range of ±10 nm is considered preferred and a range of ±7 nm is considered particularly preferred.

[0110] Additionally or alternatively, it is preferred if the radiation source comprises at least one laser, preferably at least one diode laser.

[0111] In a further embodiment, the present invention relates to the use of polymer particles with a core-shell structure in which the composition in the core differs from the composition in the shell MEISSNER BOLTE M / EOSG-032-PC

[0112] 22 and wherein the core composition can be melted using less energy than the shell composition, to improve the melting and processing properties of the particles. The particles to be used for such a purpose are preferably processed into a three-dimensional object by means of a process as described above.

[0113] Within the scope of the present invention, the terms "comprising" or "containing" and their grammatical variations have the following meanings: In one embodiment, additional elements may be included besides those mentioned. In another embodiment, essentially only the elements mentioned are included. In other words, in addition to their conventional meaning, the terms may, in a particular embodiment, be synonymous with the terms "essentially consisting of" or "consisting of".

[0114] Embodiments described above as preferred for one particular aspect shall also be considered preferred and described for all other aspects, provided that no obvious contradictions arise from the combination.

[0115] Figure 1 shows an example of a conventional laser sintering device for the layer-by-layer production of a three-dimensional object.

[0116] Figure 2 shows particles consisting of a core (grey) surrounded by a shell material (white).

[0117] Figure 3 shows particles with multiple nuclei (grey) embedded in a shell material.

[0118] The advantages realized by the present invention can be summarized as follows:

[0119] By appropriately adjusting the different compositions in the core and shell of the core-shell particles of the present invention, the absorption properties, viscosity, and film-forming properties of the powders can be controlled in a desirable manner. Sensitive components can be MEISSNER BOLTE M / EOSG-032-PC

[0120] 23 are encapsulated in the core of the particles and thus stabilized within them. The core-shell structure also enables a combination of properties that are otherwise not possible (e.g., in physical mixtures of the polymers), and allows the production of objects made of elastomers or thermosets that cannot otherwise be processed using additive manufacturing methods. The more uniform melting and softening behavior achieved results in improved process capability (e.g., through a wider process window) and enhanced component properties, such as improved detail resolution and the mechanical properties of the objects.

[0121] The present invention will be illustrated in more detail below by means of some examples, which, however, should not be regarded as limiting the scope of protection of the application in any way.

[0122] Example 1:

[0123] Low-viscosity polyamide 12 (Vestamid 1700, Evonik) with 0.05 wt% incorporated carbon black (Special Black 4) as an absorber as the core and higher-viscosity polyamide 12 (Vestamid 1900, Evonik) with incorporated Irganox 1098 (0.6 wt%) as a thermal stabilizer as the sheath were processed into fibers with a diameter of 40–70 pm by multi-component melt spinning. A high degree of crystallinity was achieved by stretching the fibers. The fibers were cut into cylindrical particles with a length of approximately 40–70 pm using a guillotine cutting system. After protective sieving and the addition of Aerosil 200 as a flow aid to improve flow properties, the material was processed on a modified EOS P 500 LPF laser sintering system with a diode exposure unit at a nominal wavelength of 980 nm.

[0124] The following properties are achieved with this material combination:

[0125] The absorption of electromagnetic radiation by the exposure unit occurs in the core of the particles, while the radiation can (predominantly) pass through the shell without significantly heating the material. This melts the particles from the inside out, ensuring that the material is completely molten. This process enables improved layer adhesion. MEISSNER BOLTE M / EOSG-032-PC

[0126] 24

[0127] - The low-viscosity core flows well after melting and mixes with the higher-viscosity shell polymer. This allows for the realization of good mechanical properties of a high-molecular-weight polyamide with good particle coalescence and thus good overall mechanical properties.

[0128] Due to the lower absorption of electromagnetic secondary radiation (thermal radiation from the molten structures during cooling) by the shell composition, the particles adhere less to each other and less to the fused / sintered structures. This results in better detail resolution of the components and fewer / no agglomerates when recycling the unfused / sintered powder.

[0129] Example 2:

[0130] Polypropylene (Lumicene MR30MC2, Total Energies) with a carbon black content of 0.33% w / w (Special Black 4, Orion Engineered Carbons) was used as the core composition. Three different polypropylene filament variants were produced using high-temperature melt spinning with a BiKo melt spinning system. These variants differ in the type of core composition added. For variant PI (not according to the invention), a homogeneous melt was produced from 1 part core composition and 3 parts of the polypropylene contained in the core composition (without carbon black) and processed into filaments. In variant P2, filaments were produced with a core of the specified composition and a shell made of the polypropylene used in the core without the addition of carbon black (ratio 1:3; see Fig. 2).The filaments according to variant P3 differ from variant P2 in that several core strands made from the core composition are surrounded by a sheath of carbon black-free polypropylene (see Fig. 3, with 12 cores). In this case as well, the ratio of core to shell composition was 1:3.

[0131] The filaments were manufactured and drawn according to prior art methods for producing multi-component fibers with variable internal structures, resulting in an average filament diameter between 40 and 60 pm. MEISSNER BOLTE M / EOSG-032-PC

[0132] 25

[0133] The filaments were then bundled into loose fiber bundles ("wefted") and cut into particles on a Pierret P26 guillotine cutter at a feed rate of 60–70 pm per cut to obtain particles with a low aspect-to-length ratio. To remove long fibers, the resulting powder was sieved with a 150 pm mesh screen, and the sieve residue was used as the product.

[0134] Powder analysis:

[0135] The obtained filament diameters and corresponding core diameters are given in Table 1. The diameters of the fibers and cores were determined by light microscopy using a Leica MZ16 stereomicroscope with a Leica MC170 HD camera and a Leica KL 1500 LED cold light source on cut and sieved fibers. The LAS 4.1 software was used for this purpose. The values ​​are given as the statistical mean of all measured particles.

[0136] The powders were analyzed for particle size determination using laser diffraction according to ISO 13320. Measurements were performed using a CILAS 1064 instrument from Quantachrome Particle Measurement Technology with a wet dispersion cell in water with the addition of a dispersing medium (surfactant). During wet dispersion, the sample was additionally dispersed using ultrasound. The particle size distribution was evaluated according to the Fraunhofer model. Corresponding distribution parameters are also shown in Table 1.

[0137] From the given particle size values, an estimation of the aspect-length ratio is possible. Due to the cylindrical shape of the particles, the D10 value can be approximated as the diameter and the D90 value as the section length. This results in approximate aspect-length ratios of 1:2–2.5.

[0138] Table 1 MEISSNER BOLTE M / EOSG-032-PC

[0139] Processing of the powders:

[0140] The obtained powders were mixed with 0.1% flow agent AEROSIL® R812 (Evonik / Degussa) for processing on a PBF system. Processing was carried out on a modified EOS P 500 LPF system. The powders were applied in two 50 pm layers (double coating) using an application system, heated to the processing temperature, and then selectively melted using a diode exposure unit. The following preferred results were observed when comparing the three powders:

[0141] The maximum processing temperature, i.e., the temperature to which the polymer powder can be preheated by radiant heating without clumping, is approximately 2°C higher for powders with core-shell structures (P2, P3) than for PI. Due to the reduced interaction of the radiant heating with the shell structure, the outer layer softens less at this temperature than a particle containing carbon black as an absorber in a homogeneous distribution. The total absorber content in the particle, necessary for sufficient interaction with the selective radiation source, can therefore be maintained.

[0142] - Due to the reduced interaction of the radiation sources with the shell of particles P2 and P3, especially during selective irradiation of the areas to be melted with the laser diodes, coalescence between the particles occurs only in the irradiated areas. This allows for better detail resolution and more uniform surfaces. The core interacts more strongly with the selectively introduced radiation than the shell and thus becomes less fluid. Through heat conduction to the shell, this liquefies, and the melt flows into each other, or the molten particles liquefy. MEISSNER BOLTE M / EOSG-032-PC

[0143] 27 interact with each other. Non-exposed particles do not receive enough heat to liquefy and flow accordingly. This behavior is not achieved with homogeneously introduced absorbers (PI).

Claims

MEISSNER BOLTE M / EOSG-032-PC 28 Claims:

1. Polymeric powder for use as a building material for the additive manufacturing of a three-dimensional object by selective solidification of a building material at the cross-sectional points of the three-dimensional object in the corresponding layers, wherein the polymeric powder has particles with a core-shell structure in which the composition in the core differs from the composition in the shell by a changed melting or softening behavior, such that the core softens more than the shell when energy is applied.

2. Polymeric powder according to claim 1, wherein the composition in the core differs from the composition in the shell by a higher proportion of an absorber material, preferably wherein the absorber material is only contained in the core composition.

3. Polymeric powder according to claim 2, wherein the core composition includes an NIR absorber, preferably in the form of carbon black, as the absorber material.

4. Polymeric powder according to any one of claims 1 to 3, wherein the composition of the core at a temperature A, at which the core and shell compositions are in molten form, has a lower viscosity than the composition of the shell.

5. Polymeric powder according to at least one of the preceding claims, wherein the core composition comprises an amorphous polymer and the shell composition comprises a crystalline, semi-crystalline or crystallizable polymer, preferably wherein the respective compositions contain only amorphous or crystalline or semi-crystalline polymers as polymer components.

6. Polymeric powder according to at least one of the preceding claims, wherein the core composition contains a polymer with a lower melting point or a lower softening temperature than the shell composition, preferably wherein the core composition contains only polymers with a lower melting point or a lower softening temperature than the shell composition. MEISSNER BOLTE M / EOSG-032-PC 29 7. Polymeric powder according to at least one of the preceding claims, wherein the core composition includes an oxygen- or moisture-sensitive additive, an additive that decomposes a polymer contained in the shell composition in the presence of moisture and elevated temperature, or a crosslinking agent for a polymer contained in the shell.

8. Polymeric powder according to at least one of the preceding claims, wherein the particles of the powder have a spherical or cylindrical structure.

9. Polymeric powder according to at least one of the preceding claims, wherein the particles have a mean particle size d50 in the range of 10 to 100 pm, preferably at least 20 pm and / or at most 80 pm and further preferably at least 30 pm and / or at most 70 pm.

10. Polymeric powder according to at least one of the preceding claims, comprising at least one additive selected from the group consisting of thermostabilizers, reflective particles, flame retardants, flow agents, discoloration inhibitors, lubricants, nucleating agents, thickeners, antioxidants, antistatic agents, biodegradability or biocompatibility enhancers, preservatives, dyes, perfumes, hydrolysis stabilizers, fillers, fibers, in particular in the form of glass or carbon fibers, or plasticizers, wherein the additive(s) are present separately from the particles having a core-shell structure and / or are incorporated into them in all or in individual regions of the core-shell structure.

11. Polymeric powder for use as a building material for the additive manufacturing of a three-dimensional object by selective solidification of a building material at the cross-sectional points of the three-dimensional object in the corresponding layers, wherein the polymeric powder has particles with a core-shell structure in which the composition in the core contains an oxygen- or moisture-sensitive additive, an additive that decomposes a polymer contained in the shell composition in the presence of moisture and elevated temperature, or a crosslinker for a polymer contained in the shell composition, wherein the additive is not contained in the composition of the shell. MEISSNER BOLTE M / EOSG-032-PC 30 12. A method for producing a polymeric powder according to any one of claims 1 to 10, wherein the polymeric powder is produced by coating primary particles with a composition different from the composition of the primary particles, forming particles with a core-shell structure, by multi-stage polymerization, or by forming fibers with an inner and an outer component substantially enclosing the inner component, and cutting the fibers to form particles.

13. Three-dimensional object produced by solidifying a powdered building material at spatial points corresponding to the cross-section of the three-dimensional object in the respective layer, by irradiation, wherein a polymeric powder according to any one of claims 1 to 10 is used as the building material.

14. Method for producing a three-dimensional object, in particular by solidifying a powdered building material at the points corresponding to the cross-section of the three-dimensional object in the respective layer, wherein a polymeric powder according to one of claims 1 to 10 is used as the building material and preferably the building material is selectively solidified by the action of electromagnetic radiation emitted from a radiation source.

15. System for producing a three-dimensional object by solidifying a powdered building material at the points corresponding to the cross-section of the three-dimensional object in the respective layer, wherein the system comprises at least one radiation source designed to emit electromagnetic radiation, a process chamber designed as an open container and having a container wall, a support located in the process chamber, wherein the process chamber and the support are movable relative to each other in a vertical direction, and a storage container and coater movable in a horizontal direction, wherein the storage container is at least partially filled with a polymeric powder according to at least one of claims 1 to 10.

16. Method according to claim 13 or system according to claim 14, wherein the radiation source emits electromagnetic radiation of a wavelength in the Be- MEISSNER BOLTE M / EOSG-032-PC 31 ranging from 500 to 1500 nm, preferably in one of the wavelength ranges 1064±15 nm and / or 980±15 nm and / or 940±15 nm and / or 810±15 nm and / or 780±15 nm and / or 640±15 nm or emits electromagnetic radiation of a wavelength of about 10.6 pm, a wavelength of 1.9 to 2.0 pm or in the range of 4.8 to 8.3 pm.

17. Method according to claim 13 or 15 or system according to claim 14 or 15, wherein the radiation source comprises at least one laser, preferably at least one diode laser.

18. Use of polymer particles with a core-shell structure in which the composition in the core differs from the composition in the shell and wherein the core composition can be brought into a molten state using less energy than the composition of the shell, to improve the melting and processing properties of the particles, wherein the particles are preferably processed into a three-dimensional object using a method according to one of claims 13, 15 or 16.

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