Particles of polyamide powder and their use in a powder agglomeration process

Abstract

The present invention relates to seed particles of polyamide powder (PA), which consist of: - a polyamide core with a d50 value ranging from 15 to 60 μm, and - a polyamide shell, characterized in that the shell has a higher intrinsic viscosity in solution and a higher melting point than the core.

Description

Technical Field

[0001] The present invention relates to seeded particles of polyamide powder and their use in a method for the layer-by-layer agglomeration of powders by melting to produce three-dimensional objects that are easy to clean. Background Art

[0002] Techniques for agglomerating polyamide powders are used to produce three-dimensional objects such as prototypes and models, especially in the fields of motor vehicles, nautical, aviation, aerospace, medical (prosthetics, hearing systems, etc.), textiles, clothing, fashion, decoration, electronic housings, telephones, home automation, computing, and lighting. This technique also enables the achievement of fine and complex geometries that cannot be achieved by conventional molding techniques.

[0003] Agglomeration of the powder by melting (hereinafter referred to as "sintering") is caused by radiation (such as a laser beam (laser sintering), infrared radiation, UV radiation, or any electromagnetic radiation source that enables the layer-by-layer melting of the powder to produce a three-dimensional object).

[0004] In the case of laser sintering, a thin layer of polyamide powder is deposited on a horizontal plate that is maintained in a chamber heated to a temperature between the crystallization temperature Tc and the melting temperature Tm of the polyamide powder. The laser enables the melting of the powder particles at various points in the layer, and after the laser passes, the layer slowly crystallizes in correspondence with the geometry of the object, for example, using a computer that stores the shape of the 3D object in memory and reproduces the shape in the form of 2D slices. Subsequently, the horizontal plate is lowered by a value corresponding to the thickness of the powder layer (for example, between 0.05 and 2 mm and typically on the order of 0.1 mm), then a new powder layer is deposited and the laser enables the melting of the powder particles according to the geometry corresponding to this new layer, and this new layer slowly crystallizes according to the geometry corresponding to the object, and so on. The procedure is repeated until the entire object is manufactured. An object surrounded by powder is obtained in the chamber. The non-agglomerated parts thus remain in powder form. After complete cooling, the object is separated from the powder, and the powder can be reused for other operations.

[0005] When the temperature of the powder in the machine is too close to the melting temperature (Tm) of the powder, this results in solidification around the part ("caking" phenomenon), which is manifested by the presence of unwanted powder agglomerates at certain positions on the surface of the object, rather than having a good definition of the final object. At the end of the build (also called "run"), it is therefore necessary to clean the parts to remove this powder that remains attached to the parts before using them again. This cleaning is usually done by sandblasting, which can cause deterioration of some of the fine and / or fragile elements of the built 3D part.

[0006] Currently, apart from cleaning components, no technique is known for solving this problem. Nowadays, the ease and time of cleaning components are two important factors in the total manufacturing cost for all users of 3D machines. In the specific case of polyamide powders, certain grades require particularly long cleaning times. Additionally, certain geometries cannot be cleaned, even though the manufacture of components with complex and intricate geometries is precisely an advantage offered by 3D construction. Summary of the Invention

[0007] The present invention stems from the inventors' unexpected demonstration that seed particles of core / shell polyamide powders (formed by a polyamide shell with a high molar mass and high melting temperature surrounding a polyamide core) facilitate and / or reduce the time for cleaning objects obtained from these powders by additive manufacturing (also known as the term 3D printing). Advantageously, the inventors have also demonstrated that these polyamide powders are easily recyclable.

[0008] Accordingly, a subject of the present invention is a seed particle of a polyamide (PA) powder, which consists of:

[0009] - a polyamide core with a volume median diameter D50 in the range of 15 to 60 μm, and

[0010] - a polyamide shell,

[0011] characterized in that the intrinsic viscosity and melting temperature of the shell in solution are respectively higher than or equal to the intrinsic viscosity and melting temperature of the core in solution.

[0012] The present invention also relates to a method for preparing polyamide powder particles as defined above by anionic polymerization in a solution in a solvent, which comprises the polymerization of a shell from 2-pyrrolidone (lactam 4), caprolactam (lactam 6), 2-azacyclononanone (lactam 8), laurolactam (lactam 12) or a mixture thereof around a seed in the presence of a catalyst, an activator and at least one amide selected from N,N'-alkylenediamides, said seed being selected from PA4, PA6, PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.13, PA6.10, PA6.6 and PA10.10, which will form the core of the particle.

[0013] The present invention also relates to a method for preparing powder particles according to the present invention by dissolving the polyamide of the shell in an alcohol-based solvent and then precipitating the polyamide of the shell around the core of the particle.

[0014] The present invention also relates to the use of the polyamide powder as defined above in composite materials, substrate coatings, transfer papers or for the manufacture of cosmetic compositions.

[0015] The present invention also relates to the use of a polyamide powder as defined above for the production of an object by agglomeration of said powder by melting induced by radiation selected from a laser beam, infrared radiation or UV radiation.

[0016] The present invention also relates to the use of a polyamide powder as defined above in an additive manufacturing method for reducing the phenomenon of powder agglomeration on the surface of an object.

[0017] The present invention also relates to a method for producing an object by agglomeration of a polyamide powder as defined above, wherein:

[0018] a. A thin layer of powder (layer 1) is deposited on a horizontal plate, which horizontal plate is maintained in a chamber heated to a temperature between the crystallization temperature (Tc) and the melting temperature (Tm) of said powder.

[0019] b. Depending on the geometry corresponding to the object to be produced, a laser or electromagnetic energy input causes the particles to agglomerate by melting at various points in the powder layer (layer 1).

[0020] c. The horizontal plate is then lowered by a value corresponding to the thickness of one powder layer, and a new powder layer (layer 2) is deposited.

[0021] d. Depending on the geometry corresponding to this new slice of the object to be produced, a laser or electromagnetic energy input causes the particles to agglomerate by melting of the powder layer (layer 2).

[0022] e. The previous steps are repeated until the object is constructed.

[0023] f. An object surrounded by powder is obtained in the chamber.

[0024] g. After complete cooling, the object is separated from the powder, and the powder can be reused for other operations. Detailed Description

[0025] Definitions

[0026] Within the meaning of the present invention, the terms "3D printing" or "additive manufacturing" are understood to mean any method for the large-scale production of parts by successive addition or agglomeration of powder. Within the meaning of the present invention, the terms "3D printing" or "additive manufacturing" are also understood to mean selective sintering techniques using an absorber, in particular techniques known under the names "High Speed Sintering" (HSS) and "Multi Jet Fusion" (MJF).

[0027] In the present specification, the term "sintering" encompasses all these methods, regardless of the type of radiation. Even if reference is generally made hereinafter to the selective laser sintering method, what is written for this method applies of course to other manufacturing methods.

[0028] In the present specification of the present invention, including in the following examples, the D50 of the powder (also known as "volume median diameter") corresponds to the particle size value that exactly divides the examined population into two parts. The D50 can be measured according to standard ISO 9276 - Parts 1 to 6: "Representation of results of particle size analysis" or according to standard ISO 13319. Preferably, according to standard ISO 13319:2007. In the present specification, a Multisizer 3 Coulter Counter particle size analyzer from Beckman Coulter is used to obtain the particle size distribution of the powder and the D50 is derived therefrom.

[0029] The intrinsic viscosity in solution (especially of polyamides, powders or parts manufactured by sintering) is measured at a concentration of 0.5% by weight in m-cresol relative to the total weight of the solution, at a temperature of 20 °C, using an Ubbelohde viscometer, according to standard ISO 307:2007.

[0030] The thermal properties of the polyamide are analyzed by DSC according to standard ISO 11357-3 "Plastics – Differential Scanning Calorimetry (DSC) Part 3: Determination of temperature and enthalpy of melting and crystallization". The temperatures that are more particularly relevant to the present invention herein are the first heating melting temperature and the melting enthalpy (Tm1, ΔHf1), and the crystallization temperature (Tc). Preferably, a high melting enthalpy enables better geometric definition of the parts manufactured by the additive manufacturing method.

[0031] The term "caking" is understood to refer to the agglomeration phenomenon of the powder, which is manifested by the presence of unwanted powder agglomerates at certain positions on the surface of the object rather than having a good definition of the final object. This phenomenon is observed when the transition temperature in the machine is too close to the melting temperature (Tm) of the powder.

[0032] Polyamide powder particles

[0033] The subject of the present invention is a seed polyamide (PA) powder consisting of a shell made of polyamide and a core made of polyamide, where the core and the shell are either the same type of polyamide but have different weight-average molar masses (Mw) or are different types of polyamide.

[0034] According to one embodiment, the polyamide (PA) is an aliphatic polyamide or an aliphatic copolyamide.

[0035] Preferably, the core and the shell are made of polyamides selected from PA4, PA6, PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.13, PA6.10, PA6.6 and PA10.10.

[0036] Preferably, the shell and the core of the particles according to the invention are as follows:

[0037] - a shell and a core made of PA6, or

[0038] - a shell and a core made of PA12, or

[0039] - a shell and a core made of PA6 / 12.

[0040] Also preferably, the shell and the core are different types of polyamides. Preferably, the shell and the core of the particles according to the invention are as follows:

[0041] - a shell made of PA6 and a core selected from PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.10, PA10.10; or

[0042] - a shell made of PA12 and a core selected from PA6 / 12, PA6.12; or

[0043] - a shell made of PA6 / 12 and a core selected from PA11, PA12, PA6.12, PA10.10, PA6.10.

[0044] Preferably, the core has a volume median diameter (x) between 15 and 60 μm.

[0045] According to an embodiment, the core has the following volume median diameter (x): between 15 and 20 μm, or between 20 and 25 μm, or between 25 and 30 μm, or between 30 and 35 μm, or between 35 and 40 μm, or between 40 and 45 μm, or between 45 and 50 μm, or between 50 and 55 μm, or between 55 and 60 μm.

[0046] Preferably, the shell has a thickness (y) between 1 and 15 μm.

[0047] Preferably, the powder particles according to the invention have a volume median diameter (x + 2*y) between 17 and 90 μm, more preferably between 35 and 55 μm.

[0048] Preferably, the shell has a melting temperature greater than 180 °C, preferably in the range of 183 °C to 185 °C.

[0049] Preferably, the shell has an intrinsic viscosity between 1.20 and 1.50, preferably between 1.35 and 1.45.

[0050] Preferably, the core has a melting temperature greater than 140 °C, preferably between 175 °C and 180 °C.

[0051] Preferably, the core has an intrinsic viscosity between 0.30 and 1.30, preferably between 0.75 and 1.05.

[0052] Preferably, the powder particles according to the invention have a melting temperature greater than 180 °C.

[0053] Preferably, the powder particles according to the invention have an intrinsic viscosity between 1.20 and 1.35.

[0054] Advantageously, the polyamide powder according to the invention facilitates and reduces the time for cleaning objects constructed by an additive manufacturing method. Also advantageously, the polyamide powder according to the invention reduces the risk of breakage when cleaning objects with a very fine geometry.

[0055] Method for preparing polyamide powder particles

[0056] In one embodiment of the invention, the powder particles according to the invention are prepared by dissolving the polyamide of the shell in an alcohol-based solvent and then precipitating the polyamide of the shell around the core of the particle. The dissolution is preferably carried out under pressure and / or with heating. Also preferably, the dissolution is carried out in the presence of the core of the particle suspended in the solvent. Preferably, the precipitation is carried out by lowering the temperature and / or extracting the solvent.

[0057] The subject of the invention is also a method for preparing a powder by anionic polymerization in a solution in a solvent.

[0058] A. Aggregated components

[0059] - Monomer

[0060] Preferably, the polymerization is carried out as follows: in the presence of a monomer of laurolactam (lactam 12), caprolactam (lactam 6), 2-pyrrolidone (lactam 4) or 2-azacyclononanone (lactam 8) or a mixture thereof, in a solution in a solvent of the lactam or mixture, in the presence of a seed (or organic filler) and a catalyst, activator and at least one amide selected from N,N'-alkylenediamides, said seed (or organic filler) being particles of a powder of PA4, PA6, PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.13, PA6.10, PA6.6 and PA10.10.

[0061] - Solvent

[0062] The solvent used dissolves the monomers but does not dissolve the polymer particles formed during polymerization. Advantageously, the solvent is a paraffinic hydrocarbon fraction having a boiling range between 120 and 170 °C, preferably between 140 and 170 °C.

[0063] The solvent can be supersaturated with the monomer at the polymerization temperature. Various methods enable the solvent to be supersaturated with the monomer. One of these ways can consist in saturating the solvent with the monomer at a temperature higher than the initiation temperature and then lowering the temperature to the initiation temperature. Another way can consist in substantially saturating the solvent with the monomer at the initiation temperature and then adding, still at this temperature, a primary amide preferably containing 12 to 22 carbon atoms, such as oleamide, N-stearamide, erucamide, isostearamide or N,N'-alkylenediamide, examples of which are given below.

[0064] Polymerization can also be carried out in a solvent not saturated with the monomer. In this case, the reaction medium contains the monomer dissolved in the solvent, the concentration of which is far from supersaturation at the initiation temperature.

[0065] - Catalyst

[0066] Preferably, a catalyst selected from the common catalysts for the anionic polymerization of lactams is used. This is a base strong enough to produce lactamate after reacting with the lactam. Combinations of various catalysts are possible. As non-limiting examples, sodium hydride, potassium hydride, sodium, sodium methoxide and / or sodium ethoxide may be mentioned. The amount of catalyst introduced generally can vary between 0.5 and 3 mol per 100 mol of monomer.

[0067] - Activator

[0068] Preferably, an activator is also added, which has the effect of initiating and / or accelerating the polymerization. The activator is selected from lactam-N-carboxyanilide, (mono)isocyanate, polyisocyanate, carbodiimide, cyanamide, acyl lactam and acyl carbamate, triazine, urea, N-substituted imide, ester and phosphorus trichloride. It can also optionally be a mixture of various activators. The activator can also optionally be formed in situ, for example, an acyl lactam is obtained by the reaction of an alkyl isocyanate with a lactam.

[0069] Preferably, the catalyst / activator molar ratio is between 0.2 and 2, preferably between 0.8 and 1.2.

[0070] - Amide

[0071] Preferably, at least one amide is also added, one of which is always an N,N'-alkylene bisamide. The amount of the introduced N,N'-alkylene bisamide is usually on the order of 0.001 to 4 mol, preferably 0.075 to 2 mol, per 100 mol of monomers. Among the particularly recommended N,N'-alkylene bisamides, mention may be made of the N,N'-alkylene bisamides of fatty acids, and more preferably:

[0072] - the N,N'-ethylene bisstearamide of the formula C 17 H 35 -C(=O)-NH-CH 2 CH 2 -NH-C(=O)-C 17 H 35 , abbreviated as EBS; and

[0073] - the N,N'-ethylene bisoleamide of the formula C 17 H 33 -C(=O)-NH-CH 2 CH 2 -NH-C(=O)-C 17 H 33 , abbreviated as EBO.

[0074] - N,N'-alkylenebispalmitamide, N,N'-alkylenebisgadoleamide, N,N'-alkylenebiscetoleamide, and N,N'-alkylenebis(erucamide).

[0075] Preferably, EBS and / or EBO are used.

[0076] Primary amides preferably containing 12 to 22 carbon atoms may also be added. The amide is preferably selected from: oleamide, N-stearamide, isostearamide, erucamide.

[0077] - Organic fillers

[0078] Regarding organic fillers, this preferably relates to homopolyamide or copolyamide powders, preferably PA4, PA6, PA8, PA11, PA12, PA6 / 12, PA 6.12, PA6.13, PA6.10, PA 6.6, and PA10.10. As an example, mention may be made of the powder from Arkema, the fine powder from Arkema, the powder from Evonik, the powder from Chemopharma, etc.

[0079] The polyamide powder is preferably finely divided.

[0080] The amount of the organic filler and the diameter of the filler enable the size of the final particles obtained at the end of polymerization to be directed in a desired direction (small particles or large particles).

[0081] - Other fillers or additives

[0082] Any kind of filler (pigments, dyes, carbon black, carbon nanotubes, etc.) or additive (antioxidants, anti-UV agents, plasticizers, etc.) known to those skilled in the art can also be added to the reaction medium, provided that all these compounds are completely dry and inert with respect to the reaction medium.

[0083] B. Aggregation

[0084] The anionic lactam ring-opening polymerization is carried out continuously or preferably batchwise. The solvent is introduced batchwise, and then the monomer, optionally N,N'-alkylenediamide, filler, catalyst, and activator are introduced simultaneously or successively. It is recommended to first introduce the solvent and the monomer, then remove all traces of water, for example, by azeotropic distillation, and then add the catalyst once the medium is anhydrous. The filler can be introduced, for example, after the monomer is introduced. To avoid solidification or a runaway polymerization, it may be advantageous to introduce the activator not all at once but incrementally or at a given introduction rate.

[0085] The polymerization is carried out at atmospheric pressure or at a slightly higher pressure (the partial pressure of the hot solvent) and at a temperature between 20 °C and the boiling temperature of the solvent. The initiation temperature and the polymerization temperature of the lactam are generally between 70 and 150 °C, preferably between 80 and 130 °C. Advantageously, the polymerization temperature of the lactam is below 120 °C and above 70 °C.

[0086] The weight ratio (expressed as %) of [organic filler / monomer] introduced into the reaction medium is between 0.001% and 65%, preferably between 0.005% and 45%, even more preferably between 0.01% and 30%, and advantageously between 0.05% and 20%.

[0087] Preferably, the seed powder obtained at the end of polymerization is insoluble in the solvent of the monomer pre-introduced into the reaction medium.

[0088] Use of polyamide powder particles

[0089] Preferably, the polyamide powder according to the invention is used in a method for manufacturing an object by agglomerating the powder by melting or sintering using radiation. The radiation can be selected from any radiation known to those skilled in the art. As examples of radiation, a laser beam (laser sintering), infrared radiation, UV radiation, or any electromagnetic radiation source that enables the powder to be melted layer by layer to manufacture a three-dimensional object can be mentioned.

[0090] The apparatus used can be any sintering apparatus known to those skilled in the art. By way of example, mention may be made of sintering apparatuses sold by EOS, 3D Systems, Aspect, Trump Precision Machinery, Hewlett Packard, Sinterit, Sintratec, Sharebot, FormLabs, Sonda Sys, Farsoon, Prodways, Ricoh, Wematter3D, VoxelJet, Xaar, etc. By way of example of a sintering apparatus, mention may be made of the EEOSINTP396 and Formiga P100 from EOS GmbH.

[0091] The object manufactured by the agglomeration of the powder according to the invention is preferably a 3D object. Preferably, the object is selected from prototypes, component models ("rapid prototyping"), small series of finished components ("rapid manufacturing") for the automotive, nautical, aeronautical, aerospace, medical (prostheses, hearing systems, etc.), textile, clothing, fashion and decoration fields, casings for electronics, telephones, home automation, computing, lighting, sports and industrial tools.

[0092] Advantageously, the use of the polyamide powder according to the invention in a 3D printing method makes it possible to reduce the phenomenon of powder agglomeration at the surface of the 3D object. The use of the powder according to the invention in additive manufacturing is particularly advantageous because it makes it possible to facilitate and / or reduce the time for cleaning the objects obtained by this technique.

[0093] The use of the powder according to the invention in additive manufacturing is particularly advantageous because it can be recycled several times, either alone or in a mixture. Specifically, the untransformed powder can be recovered by sieving, which thus retains the 3D part and allows the powder to flow through. Preferably, the powder according to the invention can be recycled at least 3 times, preferably at least 5 times and more preferably at least 10 times.

[0094] Preferably, in each build cycle or "run", relative to the total weight of the powder used in the machine each time, the content of the recycled powder is at least 50% by weight, preferably at least 60% by weight, more preferably at least 70% by weight. In other words, relative to the total weight of the powder used in the machine each time, except for the first run using 100% fresh powder, each subsequent run reuses at least 50% by weight, preferably at least 60% by weight, preferably at least 70% by weight of the unsintered powder from the previous run.

[0095] Before use, the object can be easily cleaned using any cleaning technique known to those skilled in the art. For example, the object can be cleaned using a sandblaster.

[0096] The following are non-limiting examples of the present invention. Description of the Drawings

[0097] Figure 1

[0098] Figure 1 Four components constructed by the selective laser sintering (SLS) method are shown, each having 10 holes of different sizes. The cleaning properties of the components were studied using a compressed air blower without sandblasting. A score on a 10-point scale was awarded based on the number of unblocked holes. The score for the first component starting from the top was 0 / 10. The score for the second component starting from the top was 6 / 10. The score for the third component starting from the top was 8 / 10. The score for the fourth component starting from the top was 10 / 10.

[0099] Example

[0100] The present inventors studied the phenomenon of powder agglomeration (caking), which manifested as the presence of powder agglomerates at certain locations on the surface of 3D objects fabricated by additive manufacturing methods.

[0101] 1. Preparation of polyamide powder according to the present invention

[0102] 1.1. Preparation of PA12 powder with PA12-seeds

[0103] 2800 ml of solvent was charged into a reactor maintained under nitrogen, and then 716 g of laurolactam 12, 8.2 g of EBS, and 203 g of 2002ES3Nat 3 (PA12 powder) were successively charged. After starting stirring at 350 rpm, the mixture was gradually heated up to 110 °C, and then 360 ml of the solvent was distilled off under vacuum to azeotropically entrain any trace water that might be present.

[0104] After returning to atmospheric pressure, an anionic catalyst, namely 2.9 g of sodium hydride with 60% purity in oil, was rapidly introduced under nitrogen, and the stirring was increased to 400 rpm at 105 °C for 30 minutes under nitrogen.

[0105] According to the following procedure, the selected activator, namely isocyanatostearate (12.2 g made up to 189.6 g with solvent), was continuously injected into the reaction medium using a small metering pump:

[0106] - 11 g / h of the isocyanate solution for 180 minutes,

[0107] - 45 g / h of the isocyanate solution for 210 minutes.

[0108] Meanwhile, during the injection, the temperature was maintained at 105 °C for 360 minutes, then increased to 130 °C within 30 minutes, and maintained at this temperature for 3 hours after the introduction of the isocyanate was completed.

[0109] At the end of the polymerization, the polyamide powder was dispersed in the synthesis solvent. The reaction medium was cooled to 80 °C to enable the reactor to be emptied: after the solid / liquid separation, the polyamide powder was placed in an oven at 75 °C to dry the solvent.

[0110] 1.2. Properties of the powder

[0111] 1.2.1 Measurement methods

[0112] D50 was measured according to standard ISO 13319.

[0113] The intrinsic viscosity was measured at a temperature of 20 °C at a concentration of 0.5 wt% in m-cresol relative to the total weight of the solution according to standard ISO 307:2007.

[0114] The melting temperature and the enthalpy of fusion were measured by DSC according to standard ISO 11357-3 “Plastics – Differential Scanning Calorimetry (DSC) Part 3: Determination of temperature and enthalpy of melting and crystallization”.

[0115] 1.2.2. Results

[0116] The obtained core / shell polyamide powder particles had a volume median diameter of 43.1 μm, an intrinsic viscosity of 1.27, a melting temperature of 184 °C, and an enthalpy of fusion of 115 J / g.

[0117] The core had a volume median diameter D50 of 30 μm, an intrinsic viscosity of 1.02, and a melting temperature of 177 °C.

[0118] The shell had a thickness of 6.5 μm, and the selection target of the synthesis parameters was an intrinsic viscosity of 1.40 and a melting temperature of 184 °C.

[0119] 2. Comparative example

[0120] 2.1. Comparative Example 1: PA 12 powder (PA 2200, a product sold by EOS)

[0121] The PA 2200 powder particles had a volume median diameter of 52.7 μm, an intrinsic viscosity of 1.00, a melting temperature of 186 °C, and an enthalpy of fusion of 125 J / g.

[0122] 2.2. Comparative Example 2: PA 12 powder with silica seeds

[0123] The PA 12 powder particles according to Example 2 of Patent FR2867190 have a volume median diameter of 51 μm (without agglomerates), an intrinsic viscosity of 1.12, a melting temperature of 184 °C, and a melting enthalpy of 118 J / g.

[0124] 3. Evaluation of powder agglomeration

[0125] 3.1. Procedure

[0126] In order to evaluate the phenomenon of powder agglomeration by melting, parts with 10 different-sized holes, particularly parts sensitive to caking, were constructed by the selective laser sintering (SLS) method using the powder particles according to the present invention and the powders according to Comparative Examples 1 and 2.

[0127] The cleaning properties of the parts were studied using a compressed air blower without sandblasting. The more unblocked holes there are in the part, the easier the cleaning. A score on a 10-point scale is awarded based on the number of unblocked holes. Figure 1 Examples of parts with 10 different-sized holes and the scores that can be awarded based on the number of unblocked holes are shown. A score of 10 / 10 is awarded to the powder that is easiest to clean. On the contrary, a score of 0 / 10 is awarded to the powder with strong agglomeration and the most difficult to clean (resulting in a longer cleaning time and possibly deterioration of some fine and / or fragile elements of the constructed 3D part).

[0128] 3.2. Transformation conditions

[0129] The laser conditions used for this test are the conditions recommended for PA 12 powder (PA2200):

[0130] [Table 1]

[0131]

[0132] 3.3. Results

[0133] Under the same operating conditions in a laser sintering (LS) machine:

[0134] - The powder according to the present invention obtained a score of 8 / 10 because 8 out of 10 holes were unblocked;

[0135] - The PA 2200 powder (Comparative Example 1) obtained a score of 7 / 10 because 7 out of 10 holes were unblocked;

[0136] - The PA 12 powder with silica seeds (Comparative Example 2) obtained a score of 0 / 10 because the pores in the part were not unblocked during cleaning.

[0137] Therefore, compared with conventional polyamide powders, the powders according to the invention are less likely to form unwanted powder agglomerates. This is related to the fact that the powders according to the invention form fewer powder agglomerates on the surface of the manufactured object. Therefore, compared with objects made from conventional powders, these objects have a better final definition and are easier to clean.

Claims

1. Seed particles of polyamide (PA) powder, consisting of the following components: - A polyamide core with a volume median diameter D50 in the range of 15 to 60 μm, and - A polyamide shell, where the intrinsic viscosity of the shell in solution is in the range of 1.20 to 1.50, and the shell has a melting temperature (Tm1) greater than 180 °C, characterized in that the intrinsic viscosity and melting temperature of the shell in solution are respectively higher than those of the core in solution, where the intrinsic viscosity in solution is measured at a temperature of 20 °C using an Ubbelohde viscometer at a concentration of 0.5 wt% in m-cresol relative to the total weight of the solution according to standard ISO 307:2007.

2. The particles according to claim 1, wherein the core and shell of the particles are made of polyamides selected from PA4, PA6, PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.13, PA6.10, PA6.6, and PA10.

10.

3. The particles according to claim 1 or 2, wherein the shell and core of the particles have the same properties: - A shell and core made of PA6, or - A shell and core made of PA12, or - A shell and core made of PA6 / 12.

4. The particles according to claim 1 or 2, wherein the shell and core of the particles have different properties: - A shell made of PA6 and a core selected from PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.10, PA10.10; or - A shell made of PA12 and a core selected from PA6 / 12, PA6.12; or - A shell made of PA6 / 12 and a core selected from PA11, PA12, PA6.12, PA10.10, and PA6.

10.

5. The particles according to claim 1, wherein the intrinsic viscosity of the shell in solution is in the range of 1.35 to 1.

45.

6. The particles according to claim 1, wherein the shell has a melting temperature (Tm1) in the range of 183 °C to 185 °C.

7. The particles according to claim 1 or 2, characterized in that the shell is obtained by anionic polymerization.

8. The particles according to claim 1 or 2, characterized in that the shell is obtained by dissolving and precipitating polyamide around the core of the particles.

9. The particles according to claim 1 or 2, characterized in that it has a volume median diameter D50 in the range of 17 to 90 μm.

10. The particles according to claim 1 or 2, characterized in that the shell has a thickness between 1 and 15 μm.

11. A method for preparing seed particles of a polyamide (PA) powder as described in any one of claims 1 to 7 and 10 by anionic polymerization in a solution containing a solvent, which comprises the polymerization of a shell from 2-pyrrolidone, caprolactam, laurolactam, 2-azacyclononanone or a mixture thereof in the presence of a catalyst, an activator, an amide selected from N,N'-alkylenebisamides and an organic filler for the core of the particles, wherein the organic filler for the core of the particles is selected from PA4, PA6, PA8, PA11, PA12, PA6 / 12, PA6.12, PA6.13, PA6.10, PA6.6 and PA10.

10.

12. The method according to claim 11, characterized in that the N,N'-alkylenebisamides are selected from EBS and EBO.

13. The method according to claim 11 or 12, characterized in that, in addition to the N,N'-alkylenebisamides, there are other amides selected from oleamide, N-stearamide, isostearamide and erucamide.

14. A method for preparing seed particles of a polyamide (PA) powder as described in any one of claims 1 to 6, 8 to 10 by dissolving the polyamide of the shell in an alcohol-based solvent and then precipitating the polyamide of the shell around the core of the particles.

15. The method according to claim 14, wherein the dissolution is carried out under pressure and / or under heating.

16. The method according to claim 14 or 15, wherein the dissolution is carried out in the presence of the core of the particles suspended in the solvent.

17. The method according to claim 14 or 15, wherein the precipitation is carried out by lowering the temperature and / or extracting the solvent.

18. Use of the polyamide (PA) powder as described in one of claims 1 to 10 in composite materials, substrate coatings, transfer papers or for the manufacture of cosmetic compositions.

19. Use of the polyamide (PA) powder as described in one of claims 1 to 10 for manufacturing an object by agglomeration of the polyamide (PA) powder caused by melting via radiation selected from a laser beam, infrared radiation or UV radiation.

20. Use of the polyamide (PA) powder as described in one of claims 1 to 10 in a 3D printing method for reducing the clustering phenomenon of the agglomerated powder on the surface of a 3D object.

21. A method for manufacturing an object by agglomeration of the polyamide (PA) powder as described in one of claims 1 to 10, wherein: a. A thin layer 1 of the polyamide (PA) powder is deposited on a horizontal plate, which horizontal plate is maintained in a chamber heated to a temperature between the crystallization temperature (Tc) and the melting temperature (Tm) of the polyamide (PA) powder, b. According to the geometry corresponding to the object to be manufactured, laser or electromagnetic energy input causes the particles to agglomerate by melting at various points in the polyamide (PA) powder layer 1, c. Then the horizontal plate is lowered by a value corresponding to the thickness of one polyamide (PA) powder layer, and then a new polyamide (PA) powder layer 2 is deposited, d. Based on the geometry of the new slice corresponding to the object to be manufactured, the laser or electromagnetic energy input causes the particles to agglomerate through the melting of the polyamide (PA) powder layer 2. e. Repeat the previous steps until the object is constructed. f. Obtain an object surrounded by polyamide (PA) powder in the chamber. g. After complete cooling, separate the object from the polyamide (PA) powder, which can be reused for other operations.

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