Method for producing polyamide powder for 3D printing

Abstract

The invention relates to a method for preparing polyamide powder by anionic polymerization in a solvent, comprising the following steps: a) forming a reaction medium comprising the introduction of at least one lactam monomer into the solvent, the introduction of at least one catalyst into the solvent and the introduction of at least one activator into the solvent at a given temperature; b) polymerizing a lactam monomer into a polyamide in a reaction medium; c) precipitating the polyamide in the form of a powder in the reaction medium; and d) after step a), preferably after step c), heating the reaction medium to a temperature higher than the temperature at which the at least one activator is introduced and in the range of 140-200 DEG C. The invention also relates to a powder obtained by such a method and to the use of such a powder.

Description

Technical Field

[0001] The present invention relates to a process for manufacturing polyamide powder by anionic polymerization in a solvent medium. The invention also relates to the powder obtained by this process, and its use in a process for constructing three-dimensional articles. Background Art

[0002] The construction of three-dimensional (3D) articles can be used for the production of prototypes or various workpieces, for example in the fields of automotive, nautical, aviation, aerospace, medical (especially for manufacturing prostheses, hearing systems, cell tissues, etc.), textiles, clothing, fashion, decoration, electronic enclosures, telephones, home automation, computers, lighting, sports and industrial tools.

[0003] In the techniques for manufacturing 3D articles, the polyamide powder sintering manufacturing process (also known as powder melt agglomeration) is particularly advantageous. This technique makes it possible to achieve fine and complex geometries, which are impossible to achieve by conventional molding techniques. According to this method, a layer of polyamide powder is conventionally selectively and briefly irradiated in a chamber with radiation, usually electromagnetic radiation (such as a laser beam, infrared radiation, UV radiation), with the result that the powder particles are melted by the radiation impact. The molten particles coalesce and solidify to cause the formation of a solid mass. This process can produce a 3D article by repeatedly irradiating a series of newly applied powder layers.

[0004] In the case of selective laser sintering (SLS), the following process is conventionally carried out. A thin layer of polyamide powder is deposited on a horizontal plate held in a chamber, which is heated to a temperature between the crystallization temperature Tc and the melting temperature Tm of the polyamide powder. The laser causes the powder particles to agglomerate at different points in the powder layer according to the geometry of the article, for example using a computer having the shape of the article in its memory and reproducing the shape in the form of slices. Subsequently, the horizontal plate is lowered by a value corresponding to the thickness of the powder layer (for example, between 0.05 mm and 2 mm, and typically on the order of 0.1 mm), then a new powder layer is deposited, and the laser causes the powder particles to agglomerate according to the geometry of this new slice of the object. This process is repeated until the entire article is produced. Then the entire assembly is slowly cooled, and the object solidifies once its temperature drops below the crystallization temperature Tc. Thus, the unagglomerated parts remain in powder form. An object surrounded by powder is obtained inside the chamber. After cooling, the object is separated from the powder, which can be reused for another operation.

[0005] After the action of the laser beam, the temperature of the sample is immediately higher than the crystallization temperature Tc of the powder. However, sometimes adding a new, colder layer of powder causes the temperature of the workpiece to drop rapidly, and when the temperature of the workpiece is lower than the temperature Tc, this causes deformation of the manufactured object ("curling" phenomenon). Similarly, when the temperature of the powder in the machine is too close to the melting temperature (Tm) of the powder, this causes solidification around the workpiece ("caking" phenomenon), which is manifested as the presence of lumps or agglomerates of powder at certain positions on the surface of the object, rather than having a well-defined final object. When the caking phenomenon occurs, before using the workpiece, the workpiece must be cleaned in order to remove the powder that still adheres to the workpiece. This cleaning is usually carried out by sandblasting, which may cause deterioration of some fine and / or fragile elements of the constructed 3D workpiece.

[0006] To avoid these phenomena, it is therefore important to separate the Tc and Tm of the powder as far as possible. The difference Tm - Tc of the powder determines the operating temperature window of the device for agglomerating powder particles by radiation-induced melting. In this text, the expression "operating window" means the range of construction temperatures applicable to the powder when used in 3D printing according to the following definition. The operating window is defined by its upper temperature limit and its lower temperature limit. The upper limit of the operating window corresponds to the temperature of the construction chamber, above which agglomeration or caking occurs. The lower limit of the operating window corresponds to the temperature of the construction chamber, below which distortion or deformation or curling occurs. Having a wide operating window allows for greater flexibility in terms of the constructed object. In addition, this makes it possible to neutralize the high temperature variations that are usually observed in 3D printers, which are typically on the order of ±3 °C.

[0007] If, in order to widen the operating window, a large Tm - Tc difference is desired (since this encompasses the operating window), other parameters also play a role in the definition of the operating window. Therefore, the widening of the operating window is not necessarily caused by an increase in the Tm - Tc difference.

[0008] The widening of the operating window of the powder is determined for a specific 3D printing system; however, this widening of the operating window of the powder will occur on any device, although not necessarily in the same proportion.

[0009] Various methods for preparing or treating polyamide powder have been described in order to obtain a powder suitable for 3D printing.

[0010] Document WO 2013 / 090174 relates to a method for treating a powder of a thermoplastic polymer, in which the polymerized and separated powder is subjected to a heat treatment by heating at a temperature relatively close to its melting temperature for one or more hours, as determined before this operation. This heat treatment process aims to change the melting temperature, recrystallization temperature, and / or melting enthalpy of the polymer.

[0011] Document EP 1571173 describes a process for preparing polyamide 12 powder by anionic polymerization of laurolactam in a solvent of said laurolactam in the presence of a specific proportion of organic or mineral fillers and amides relative to laurolactam. In the examples of this document, the polymerization process comprises the step of heating the polymerization components in the solvent at 120 °C for 2 hours after completion of the introduction of the activator in order to complete the polymerization.

[0012] Document FR 3095205 relates to a process for preparing polyamide powder by anionic polymerization in a solvent in order to obtain a powder in which the particles comprise a polyamide core and a polyamide shell having a melting temperature respectively higher than that of the core both in terms of intrinsic viscosity in solution and melting temperature. In the examples of this document, the polymerization process comprises the step of heating the polymerization medium at 130 °C for 3 hours after completion of the introduction of the activator.

[0013] There is a real need to provide a process for preparing polyamide powder for 3D printing, in particular by sintering, which process makes it possible to obtain an extended working window of the construction temperature for 3D printing devices while remaining easy to implement. Summary of the Invention

[0014] The present invention first relates to a process for manufacturing polyamide powder by anionic polymerization in a solvent, which process comprises the following steps:

[0015] a) forming a reaction medium comprising:

[0016] – introducing at least one lactam monomer into said solvent;

[0017] – introducing at least one catalyst into said solvent; and

[0018] – introducing at least one activator into said solvent at a given temperature;

[0019] b) polymerizing the lactam monomer into polyamide in the reaction medium;

[0020] c) precipitating the polyamide in powder form in the reaction medium; and

[0021] d) after step a) and preferably after step c), heating the reaction medium to a temperature higher than the temperature for introducing at least one activator and being in the range of 140 °C to 200 °C.

[0022] Alternatively, the process for manufacturing polyamide powder by anionic polymerization in a solvent comprises the following steps:

[0023] (a) forming a reaction medium comprising:

[0024] – Introduce at least one lactam monomer into the solvent;

[0025] – Introduce at least one catalyst into the solvent; and

[0026] – Introduce at least one activator into the solvent at a given temperature to polymerize the lactam monomer into a polyamide therein, and the polyamide precipitates in the reaction medium in powder form; and

[0027] (b) After step a), heat the reaction medium to a temperature higher than the temperature used for introducing at least one activator and in the range of 140 °C to 200 °C.

[0028] In some embodiments, the heating in step d) is carried out for a time greater than or equal to 2 h, preferably greater than or equal to 3 h, more preferably greater than or equal to 5 h, more preferably 8 to 15 h, and even more preferably 10 to 12 h.

[0029] In some embodiments, step a) further includes introducing at least one filler into the solvent, and the at least one filler is preferably a mineral filler, preferably silica, and / or an organic filler, preferably polyamide powder.

[0030] In some embodiments, step a) further includes introducing at least one amide into the solvent, and the at least one amide is preferably an N,N'-alkylene bisamide, more preferably N,N'-ethylene bisstearamide and / or N,N'-ethylene bisoleamide.

[0031] In some embodiments, the at least one lactam monomer is selected from 2-pyrrolidone, caprolactam, 2-azacyclononanone, lauryl lactam, and mixtures thereof.

[0032] In some embodiments, the at least one catalyst is selected from sodium, potassium, alkali metal hydrides and hydroxides, alkali metal alkoxides, and mixtures thereof, preferably selected from sodium hydride, potassium hydride, sodium, sodium methoxide, sodium ethoxide, and mixtures thereof.

[0033] In some embodiments, the at least one activator is selected from lactam-N-carboxyanilide, (mono)isocyanates, polyisocyanates, carbodiimides, cyanamides, acyl lactams and acyl carbamates, triazines, ureas, N-substituted imides, esters, phosphorus trichloride, and mixtures thereof.

[0034] In some embodiments, the solvent is an alkane fraction having a boiling point temperature range of 120 °C to 170 °C.

[0035] In some embodiments, the heating temperature in step d) is 140 °C to 170 °C, preferably 145 °C to 160 °C, and more preferably 145 °C to 155 °C.

[0036] In some embodiments, the temperature for introducing at least one activator is from 50 °C to 150 °C, preferably from 60 °C to 135 °C.

[0037] In some embodiments, the method further comprises the step of introducing one or more additives selected from pigments, dyes, carbon black, carbon nanotubes, antioxidants, UV stabilizers and plasticizers into the solvent.

[0038] The present invention also relates to a polyamide powder obtained by the manufacturing method as described above.

[0039] The present invention also relates to the use of the powder as described above for constructing three-dimensional articles, preferably layer by layer, more preferably by sintering, even more preferably by sintering mediated by electromagnetic radiation.

[0040] The present invention also relates to the use of the powder as described above for manufacturing composites, substrate coatings, transfer papers, liquid or solid ink compositions, liquid or solid paints, structural adhesives, cosmetic compositions or pharmaceutical compositions.

[0041] The present invention also relates to a method for manufacturing a three-dimensional article, comprising the following steps:

[0042] – manufacturing a powder by the method as described above;

[0043] – depositing the powder, preferably in the form of a layer; and

[0044] – sintering the powder, preferably by electromagnetic radiation.

[0045] The present invention makes it possible to meet the above needs. More specifically, the present invention provides a method for manufacturing a polyamide powder having a wider working temperature window in 3D printing. Thus, when used in 3D printing, this makes it possible to obtain better quality workpieces with better clarity and / or facilitate, limit or even avoid mechanical cleaning that may damage the manufactured workpieces, especially those that are thin. Therefore, the powder prepared according to the present invention allows for the manufacture of a wider range of workpiece geometries by 3D printing.

[0046] This is achieved by performing the step of "curing" the anionic polymerization medium, which comprises heating the reaction medium at a certain temperature after introducing the reactants and other compounds involved in the polymerization. Without wishing to be bound by theory, the inventors believe that the curing step results in physical and chemical modification of the surface of the powder particles, such as improved crystallization at the surface, which allows widening of the working window.

[0047] The heat treatment described in document WO 2013 / 090174 is an additional step which is carried out on the powder which has polymerized and been separated from its synthesis medium after the synthesis of the powder particles. However, the curing step according to the invention is carried out on the reaction medium, in particular at the end of the basic polymerization, to form components of the anionic polymerization stage in the solvent medium and which allows the crystallization of the precipitated powder particles to be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is a photograph showing an illustrative example of a type 1B tensile dumbbell (15 cm in length and 4 mm in thickness) which has undergone the deformation measurement test as described in the "Examples" section below.

[0049] Figure 2 is a photograph showing the measurement of the deformation of the same illustrative dumbbell during the deformation measurement test as described in the "Examples" section below. In this illustrative example, the dumbbell is deformed by 5 mm between these two ends.

[0050] Figure 3 is a photograph showing four examples of workpieces having 10 different-sized holes manufactured by selective laser sintering with polyamide powder as described in the "Examples" section below. These workpieces show various sensitivities to caking and a score which can be assigned to evaluate said sensitivity based on the number of unblocked holes, as described in the "Examples" section below. Workpiece A has a score of 0 / 10, workpiece B has a score of 6 / 10, workpiece C has a score of 8 / 10 and workpiece D has a score of 10 / 10. DETAILED DESCRIPTION

[0051] The present invention will now be described in more detail and non-limitingly in the following description.

[0052] Unless otherwise indicated, all percentages with respect to amounts are by mass.

[0053] In this text, the amounts indicated for a given substance can apply to that substance according to all its definitions (as mentioned in this text), including the more restricted definitions.

[0054] Manufacture of the powder

[0055] The present invention first relates to a method for manufacturing polyamide powder by anionic polymerization in a solvent medium. Unless otherwise stated, the steps of this method, in particular steps a), b) and c) as defined above, can be carried out at least partly simultaneously. In particular, the polymerization generally starts once all the reactants have been introduced into the reaction mixture (and thus once step a) has ended).

[0056] ​​​Anionic polymerization proceeds by ring-opening of lactams. It generally comprises three steps: an initiation step to form a lactam anion, followed by an activation reaction leading to the formation of an acyl lactam, and finally a growth (or polymerization) step.

[0057] The term "polyamide powder" is to be understood to mean a powder comprising at least particles, said particles comprising at least one polyamide, and the term includes homopolyamides and copolyamides resulting from the polymerization of several different comonomers; for the purposes of the present invention, "polyamide powder" may thus contain components other than polyamides (either in the particles containing polyamides or in the particles not containing polyamides).

[0058] Anionic polymerization can be carried out continuously or discontinuously (batch mode). Preferably, it is carried out in batch mode.

[0059] In the process according to the invention, at least one lactam monomer, at least one catalyst and at least one activator, and preferably at least one amide, and preferably at least one filler are brought into contact in a solvent. Herein, the solvent containing at least one of the different compounds involved in the polymerization is referred to as the "reaction medium". The above compounds can be introduced all at the same time, all successively or partly at the same time and partly successively.

[0060] Preferably, simultaneously or successively, the solvent is introduced into any suitable device, such as a reactor, and then the lactam monomer, amide (when present), filler (when present), catalyst and activator are introduced. Advantageously, the solvent, lactam monomer, amide (when present) and filler (when present) are first introduced into the polymerization device, then for example the water present in the reaction medium is removed by azeotropic distillation, and then the catalyst is added to the anhydrous medium. Preferably, the activator is not added all at once to the reaction medium. This makes it possible to prevent solidification or loss of control of the polymerization. Thus, the activator is preferably added in one or more increasing introduction rates or continuously injected over a certain period of time. Preferably, the activator is introduced into the reaction medium after the lactam monomer, amide (when present), filler (when present) and catalyst have been introduced.

[0061] Preferably, the solvent used (at least partly) dissolves the lactam monomer and the amide. However, the solvent does not dissolve the polyamide formed during the polymerization (i.e., the polyamide is insoluble in said solvent under the polymerization conditions). Thus, the polymerization of the polyamide results in its precipitation directly in the form of particles (and thus powder) in the solvent.

[0062] The solvent can be supersaturated with the lactam monomer at the temperature of introduction of the activator. Various means make it possible for the solvent to be supersaturated with the monomer. One of these methods can include the following steps: saturating the solvent with the monomer at a temperature higher than the temperature of introduction of the activator and then lowering the temperature to the temperature of introduction of the activator.

[0063] Alternatively, the polymerization can be carried out in a solvent in which the lactam monomer is not supersaturated. In this case, the reaction medium preferably contains the monomer dissolved in the solvent, and its concentration is far from saturation at the temperature at which the activator is introduced.

[0064] For the lactam monomer, any solvent that is inert to the polymerization reaction can be used. The solvent is preferably an alkane fraction (preferably a mixture of isoparaffins, normal paraffins and naphthenes). Advantageously, its boiling range is between 120 °C and 200 °C, preferably between 140 °C and 170 °C.

[0065] The lactam monomer is preferably selected from lauryllactam (lactam 12), caprolactam (lactam 6), 2-pyrrolidone (lactam 4), 2-azacyclononanone (lactam 8) and mixtures thereof. More preferably, the lactam monomer is lauryllactam and / or caprolactam. Particularly preferably, a lactam mixture mainly containing lauryllactam and a small amount of shorter-chain lactams, especially caprolactam or 2-pyrrolidone (lactam 4) can be used. In this case, it is particularly preferred to use these two lactams in a weight ratio of 90 to 99.999:0.001 to 10, preferably 92 to 99.99:0.01 to 8, especially 94 to 99.9:0.1 to 6. This is because it has been observed that a very small amount of caprolactam can maintain the melting temperature of most of the polymer, but affects its crystallization. Therefore, a decrease in the crystallization temperature and / or a delay in crystallization are observed, both of which reduce the tendency to curl in both cases and thus allow the working window to be widened.

[0066] Very preferably, the amide that can be introduced into the reaction medium comprises, or is one (or more) N,N'-alkylene bisamides. Even more advantageously, the amide is selected from N,N'-alkylene bisamides of fatty acids, and more preferably from: N,N'-ethylene bisstearamide (having the formula C 17 H 35 -C(=O)-NH-CH2-CH2-NH-C(=O)-C 17 H 35 and abbreviated as EBS), N,N'-ethylene bisoleamide (having the formula C 17 H 33 -C(=O)-NH-CH2-CH2-NH-C(=O)-C 17 H 33and abbreviated as EBO), N,N'-alkylenebis(palmitamide) (especially N,N'-ethylenebis(palmitamide)), N,N'-alkylenebis(arachidamide) (especially N,N'-ethylenebis(arachidamide)), N,N'-alkylenebis(cetylamide) (especially N,N'-ethylenebis(cetylamide)), N,N'-alkylenebis(erucamide) (especially N,N'-ethylenebis(erucamide)) and mixtures thereof. More preferably, the amide is selected from EBS, EBO and mixtures thereof. The amide may comprise primary amides preferably containing 12 to 22 carbon atoms, preferably in combination with the N,N'-alkylenebisamides as described above. The primary amide is preferably selected from oleamide, N-stearamide, isostearamide, erucamide and mixtures thereof.

[0067] An amide, such as an N,N'-alkylenebisamide, can be introduced into the reaction medium in an amount of 0.001 - 4 mol, preferably 0.075 - 2 mol per 100 mol of lactam monomer; in particular, the amount of the amide (such as an N,N'-alkylenebisamide) can be 0.001 to 0.05 mol, or 0.05 to 0.1 mol, or 0.1 to 0.5 mol, or 0.5 to 1 mol, or 1 to 1.5 mol, or 1.5 to 2 mol, or 2 to 3 mol, or 3 to 4 mol, based on every 100 mol of lactam monomer. Adding the amide as described above helps to regulate the apparent specific surface area (measurable by the BET method) of the polyamide powder particles. The higher the addition amount of the amide, the higher the apparent specific surface area.

[0068] The filler optionally introduced into the reaction medium is intended to be used as a seed crystal. It can be inorganic or organic, or contain one (or more) inorganic fillers and one (or more) organic fillers. As mineral fillers suitable for the present invention, mention may be made of silica, carbon black and / or talc. Suitable organic fillers that may be mentioned include powders of (thermoplastic or thermosetting) polymers insoluble in the synthesis solvent, more particularly powders of polyamides, especially powders of PA 4, PA 6, PA 8, PA 11, PA 12, PA6 / 12, PA 6.12, PA 6.13, PA 6.10, PA 6.6 and / or PA 10.10. As examples of such polyamide powders, mention may be made of Orgasol® powder from Arkema, Rilsan® fine powder from Arkema, Vestosint® powder from Evonik and MICROPAN® powder from Chemopharma. In some advantageous embodiments, the filler is a mineral filler and more particularly silica. In other advantageous embodiments, the filler is an organic filler and more particularly a polyamide powder, such as PA 12 powder.

[0069] Preferably, the filler according to the present invention is a filler of finely divided particles, in particular having a volume average diameter of 0.01 to 40 μm, preferably 10 to 30 μm. Such an average diameter range enables powder particles to be obtained having a volume average diameter particularly suitable for methods of constructing 3D articles. The value of the volume average diameter of the particles corresponds to the arithmetic mean of the particle diameters weighted by the volume of the particles. It can be determined according to standard ISO 13319:2007, for example by using a Multisizer 3 Coulter particle size analyzer from Beckman Coulter. The weight ratio (expressed as %) of the filler introduced into the reaction medium to the lactam monomer introduced into the reaction medium can be 0.001% to 65%, preferably 0.005% to 45%, more preferably 0.01% to 30%, even more preferably 0.05% to 20%. In some embodiments, the weight ratio can be 0.001% to 0.1%, or 0.01% to 0.05%, or 0.05% to 0.1%, or 0.1% to 0.3%, or 0.3% to 0.5%, or 0.5% to 1%, or 1% to 2%, or 2% to 5%, or 5% to 10%, or 10% to 20%, or 20% to 30%, or 30% to 45%, or 45% to 65%. The ratio of the amount of the filler to the amount of the lactam monomer and the average diameter of the filler have an impact on the average diameter of the obtained polyamide particles. The lower the ratio of the amount of the filler to the amount of the lactam monomer, the higher the volume average diameter of the powder particles. The larger the volume average diameter of the filler, the higher the volume average diameter of the powder particles.

[0070] The catalyst can be any catalyst that can be used in the anionic polymerization method of lactam. More particularly, the catalyst is a strong enough base to cause the formation of lactam acylates after reacting with lactam. The catalyst can be selected from alkali metals (especially sodium and potassium), alkali metal hydrides and hydroxides, alkali metal alkoxides, and mixtures thereof. As non-limiting examples of suitable catalysts, sodium hydride, potassium hydride, sodium, sodium methoxide, and / or sodium ethoxide can be mentioned. The catalyst according to the present invention can be a mixture of several catalysts, especially the catalysts described above. Advantageously, the amount of the catalyst is 0.1 to 5 mol per 100 mol of lactam monomer, preferably 0.3 to 3 mol, for example 0.1 to 0.3 mol, or 0.3 to 0.5 mol, or 0.5 to 0.7 mol, or 0.7 to 1 mol, or 1 to 1.5 mol, or 1.5 to 2 mol, or 2 to 3 mol, or 3 to 5 mol per 100 mol of lactam monomer.

[0071] The role of the activator is to induce the formation of acyl lactam and control the polymerization. The activator is preferably selected from lactam-N-carboxyaniline, (mono)isocyanate, polyisocyanate, carbodiimide, cyanamide, acyl lactam and acyl carbamate, triazine, urea, N-substituted imide, ester, phosphorus trichloride, carbon dioxide and mixtures thereof. Preferably, the molar ratio of the catalyst to the activator is from 0.2 to 2, preferably from 0.8 to 1.2, for example from 0.2 to 0.5, or from 0.5 to 0.8, or from 0.8 to 0.9, or from 0.9 to 1, or from 1 to 1.1, or from 1.1 to 1.2, or from 1.2 to 1.5, or from 1.5 to 2.

[0072] In some embodiments, other additives can be added to the reaction medium, such as pigments, colorants, carbon black, carbon nanotubes, antioxidants, UV stabilizers and / or plasticizers.

[0073] The polymerization is preferably carried out at atmospheric pressure or even at a slightly higher pressure (partial pressure of the hot solvent). It can be carried out in an inert gas atmosphere, such as under nitrogen. Once the catalyst and the activator are added to the reaction mixture, the polymerization begins.

[0074] Preferably, the temperature applied to the reaction medium during the introduction of the activator (referred to as the "activator introduction temperature") is between 50 °C and 150 °C, and more preferably between 60 °C and 135 °C, more preferably between 75 °C and 125 °C. It can be in particular from 50 °C to 65 °C, or from 65 °C to 75 °C, or from 75 °C to 85 °C, or from 85 °C to 95 °C, or from 95 °C to 105 °C, or from 105 °C to 115 °C, or from 115 °C to 125 °C, or from 125 °C to 135 °C, or from 135 °C to 150 °C.

[0075] The activator introduction temperature can be applied to the reaction medium at any time before or during the addition of the activator. Advantageously, this temperature is applied to the reaction medium after adding the lactam monomer, the amide (when present) and the filler (when present) to the solvent, and before adding the catalyst and injecting the activator. Before applying the activator introduction temperature, the reaction medium can be, for example, at room temperature (i.e., at a temperature of, for example, between 15 °C and 30 °C).

[0076] The activator introduction temperature can be maintained for a certain time after the start of the activator introduction. Preferably, it is maintained until the end of the activator introduction. The activator introduction temperature can be maintained for at least 1 h, preferably 1 - 12 h, preferably 3 - 10 h, for example for 1 - 2 h, or 2 - 3 h, or 3 - 4 h, or 4 - 5 h, or 5 - 6 h, or 6 - 7 h, or 7 - 8 h, or 8 - 9 h, or 9 - 10 h, or 10 - 11 h, or 11 - 12 h. Advantageously, the activator introduction temperature is applied for a period equal to or substantially equal to the activator introduction time.

[0077] The method according to the invention further comprises the step of heating the reaction medium to a temperature of from 140 °C to 200 °C, which step is carried out once the introduction of the lactam monomer, the catalyst and the activator into the reaction medium has been completed and substantial polymerization has occurred. This step is also referred to herein as the "curing step". According to the invention, this curing step is carried out on the reaction medium, that is to say, the polymerized product and components are always in the solvent. Once the activator has been completely added to the reaction medium, the curing step enables the polymerization to continue by means of a chain extension reaction.

[0078] Preferably, the temperature of this curing step (referred to herein as the "curing temperature") is different from the activator introduction temperature and more preferably higher than the activator introduction temperature. Preferably, the curing step as defined herein is carried out after the lactam monomer, the amide (when present), the filler (when present), the catalyst and the activator have been added to the solvent (or are in contact with the solvent).

[0079] The curing step is preferably carried out in the same apparatus (such as a reactor) as the above-described steps. The curing step is preferably carried out after all the catalyst and activator have been added and thus substantial polymerization has occurred. Advantageously, the reaction medium is heated to the curing temperature, preferably gradually, for example over a period of from 15 to 60 min, preferably from 20 to 40 min, in particular about 30 min.

[0080] The curing temperature is from 140 °C to 200 °C, preferably from 145 °C to 160 °C, more preferably from 145 °C to 155 °C. In particular, the curing temperature can be from 140 °C to 145 °C, or from 145 °C to 150 °C, or from 150 °C to 155 °C, or from 155 °C to 160 °C, or from 160 °C to 165 °C, or from 165 °C to 170 °C, or from 170 °C to 175 °C, or from 175 °C to 180 °C, or from 180 °C to 185 °C, or from 185 °C to 190 °C, or from 190 °C to 195 °C, or from 195 °C to 200 °C.

[0081] Advantageously, the curing temperature is at least 20 °C higher than the activator introduction temperature, preferably at least 40 °C. In particular, the activator introduction temperature can be at least 20 °C, or at least 30 °C, or at least 40 °C, or at least 50 °C, or at least 60 °C, or at least 70 °C, or at least 80 °C, or at least 90 °C.

[0082] The duration of the curing step (also referred to herein as "curing time") is preferably greater than or equal to 2 h, more preferably greater than or equal to 3 h, even more preferably greater than or equal to 5 h. Even more advantageously, it is 8 to 15 h, more preferably 10 to 12 h. For example, the curing time can be 2 to 3 h, or 3 to 4 h, or 4 to 5 h, or 5 to 6 h, or 6 to 7 h, or 7 to 8 h, or 8 to 9 h, or 9 to 10 h, or 9 to 10 h, or 10 to 11 h, or 11 to 12 h, or 12 to 13 h, or 13 to 14 h, or 14 to 15 h.

[0083] After the curing step, the reaction medium can be cooled, preferably to a temperature from room temperature to 110 °C, more preferably to a temperature from 60 °C to 90 °C.

[0084] The polyamide obtained by polymerization precipitates in the form of a powder in the solvent. Thus, the polyamide powder is dispersed in the solvent (i.e., it is not dissolved in the solvent). The polyamide powder can be separated from the solvent by any solid / liquid separation means known to those skilled in the art, such as by sedimentation, centrifugation, etc.

[0085] The polyamide powder can be subjected to a drying step, in particular a vacuum drying step, to remove solvent residues, for example in an oven.

[0086] The powder obtained by polymerization is preferably a polyamide powder selected from PA 4, PA 6, PA 12, PA 12 / 6, PA 12 / 4, PA 4 / 6, PA 4 / 6 / 12 and mixtures thereof.

[0087] The present invention also relates to a powder obtainable or obtained by the method as described above.

[0088] The polyamide powder preferably has a volume average diameter of 10 to 100 μm, preferably 20 to 80 μm, more preferably 25 to 60 μm, even more preferably 30 to 50 μm. In some embodiments, the volume average diameter of the powder is 10 to 20 μm, or 20 to 30 μm, or 30 to 35 μm, or 35 to 40 μm, or 40 to 45 μm, or 45 to 50 μm, or 50 to 60 μm, or 60 to 70 μm, or 70 to 80 μm, or 80 to 90 μm, or 90 to 100 μm. The volume average diameter of the powder can be determined as described above.

[0089] Preferably, the temperature difference between Tm and Tc of the powder is at least 20 °C, more preferably at least 25 °C, more preferably at least 28 °C, more preferably at least 30 °C, even more preferably at least 33 °C. The crystallization and melting temperatures can be measured according to Standard ISO 11357-3:2018 Plastics - Differential scanning calorimetry (DSC) - Part 3.

[0090] The polyamide powder may have an intrinsic viscosity of 0.8 to 1.7 (g / 100g) -1 , preferably 1.0 to 1.5 (g / 100g). -1 Preferably, in an Ubbelohde viscometer, the intrinsic viscosity is measured according to Standard ISO 307:2019, but using m-cresol as the solvent and a temperature of 20 °C. The intrinsic viscosity has the dimension of the reciprocal of concentration and is equal to the natural logarithm of the relative viscosity, all divided by the concentration of the polymer dissolved in the solvent.

[0091] Use of the powder

[0092] The present invention also relates to the use of the polyamide powder as described above for the manufacture (or construction) of three-dimensional articles.

[0093] The polyamide powder according to the present invention can be used in 3D printing processes. In the context of the present invention, the terms "3D printing" or "additive manufacturing" should be understood to mean any process for the batch manufacturing of workpieces layer by layer by the addition or agglomeration of powder. In the context of the present invention, the terms "3D printing" or "additive manufacturing" are also understood to denote selective sintering techniques using an absorber, in particular techniques known under the names "High Speed Sintering" (HSS) and "Multi Jet Fusion" (MJF).

[0094] Preferably, the polyamide powder according to the present invention is used in a process for manufacturing three-dimensional articles by causing the powder to agglomerate by means of a radiation melting or selective sintering process. The term "sintering" herein includes all processes of 3D printing in which the powder is agglomerated by melting, regardless of the type of radiation.

[0095] More specifically, the method for manufacturing a 3D object according to the present invention comprises:

[0096] a) depositing the powder according to the present invention, preferably in the form of a layer; and

[0097] b) sintering the powder, preferably by means of an electromagnetic radiation beam.

[0098] Preferably, steps a) and b) are repeated to form a three-dimensional article.

[0099] Preferably, the layer deposited in step a) is heated to a temperature called the construction temperature. The term "construction temperature" (also called "temperature of the powder bed") denotes the temperature of the powder bed that heats the constituent layers of the three-dimensional article in the construction during the layer-by-layer sintering of the powder. This temperature is lower than the melting temperature of the polyamide and higher than the crystallization temperature of the polyamide, and very preferably, it is included within the working window of the polyamide powder.

[0100] The method for manufacturing a 3D object according to the present invention may include a step of manufacturing the powder used in step a) according to the method described above.

[0101] The radiation may be selected from any radiation well 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 may be mentioned.

[0102] The device used may be any sintering device well known to those skilled in the art. By way of example, sintering devices sold by EOS, 3D Systems, Aspect, Trump Precision Machinery, Hewlett Packard, Sinterit, Sintratec, Sharebot, FormLabs, Sonda Sys, Farsoon, Prodways, Ricoh, Wematter3D, VoxelJet, Xaar, etc. may be mentioned. As examples of sintering devices, EOSINT P396 and Formiga P100 from EOS GmbH may be mentioned.

[0103] According to an advantageous sintering process, a thin layer of powder is deposited on a horizontal plate held in a chamber heated to the construction temperature. Advantageously, electromagnetic radiation is then provided at various points of the powder layer with the energy required to sinter the powder particles corresponding to the geometry of the object (e.g., using a computer that stores the shape of the object and reproduces the shape in the form of slices). Next, the horizontal plate is lowered by a value corresponding to the thickness of one powder layer, and a new layer is deposited. The electromagnetic radiation provides the energy required to sinter the powder particles according to the geometry corresponding to this new slice of the object, and so on. The process is repeated until the object has been manufactured.

[0104] The thickness of the powder layer deposited on the horizontal plate (before sintering) may be 20 to 200 μm and preferably 50 to 150 μm. After sintering, the thickness of the layer of agglomerated material may be 10 - 150 μm, preferably 30 - 100 μm.

[0105] Preferably, the composition of the present invention is used in a selective laser sintering process. The composition may also be used in sintering processes of the MJF and HSS (high-speed sintering) types.

[0106] Preferably, the three-dimensional objects manufactured by the method according to the present invention are selected from prototypes, workpiece models ("rapid prototyping"), small series of finished workpieces ("rapid manufacturing"), especially for the fields of motor vehicles, nautical, aviation, aerospace, medical (prosthetics, hearing systems, etc.), textiles, clothing, fashion and decoration, and for the fields of housings for electronics, telephony, home automation, computing, lighting, sports and industrial tools.

[0107] It is advantageous to use the powder according to the present invention in additive manufacturing because the polyamide powder can be recycled in several successive builds. The polyamide powder can thus be used alone or mixed with other recycled or non-recycled powders several times. Specifically, the non-agglomerated powder can be collected by sieving, the sieve retaining the 3D object and allowing the powder to flow through. Preferably, the powder according to the present invention can be recycled at least 3 times, preferably at least 5 times, more preferably at least 10 times.

[0108] Preferably, in each build cycle or "run", the content of the recycled powder is at least 50 wt%, preferably at least 60 wt%, more preferably at least 70 wt% relative to the total weight of the powder used in the machine during each run. In other words, apart from the first run using 100% fresh powder, in each subsequent run, at least 50 wt%, preferably at least 60 wt%, preferably at least 70 wt% of the non-sintered powder from the previous run is reused relative to the total weight of the powder used in the machine during each run.

[0109] Before use, the manufactured 3D 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.

[0110] The present invention also relates to the use of the polyamide powder as described above for manufacturing composites, substrate coatings, especially metal substrate coatings (coil coatings), transfer papers, liquid or solid ink compositions, liquid or solid paints, structural adhesives, cosmetic compositions or pharmaceutical compositions.

[0111] Examples

[0112] The following examples illustrate the invention without limiting it.

[0113] In the following examples, the working windows of various polyamide powders were evaluated.

[0114] The crystallization and melting temperatures and the melting enthalpy of the powder were measured according to standard ISO 11357-3:2018 (DSC), the volume average diameter of the powder was determined according to standard ISO 13319:2007, and the intrinsic viscosity of the powder was determined according to standard ISO 307:2019, but at a temperature of 20 °C and using m-cresol as the solvent.

[0115] Program for evaluating the working window

[0116] Determine the working temperature window in a P100 (EOS) laser sintering machine, operating at a constant energy of the laser and a constant temperature of the removal chamber, so as to vary only the temperature of the construction chamber. The laser conditions for this test are as follows:

[0117] – Temperature of the removal chamber: 140 °C

[0118] – Profile

[0119] - Laser power: 16 W

[0120] - Speed: 1500 mm / s

[0121] – Hatch

[0122] - Power: 18 W

[0123] - Speed: 3000 mm / s

[0124] - Beam offset: 0.20 mm

[0125] - Energy: 0.3 mJ / mm 3

[0126] The temperature (T bc ) of the construction chamber corresponds to the temperature to which the upper layer of the powder bed is heated before the laser passes through. 3D printed constructs of different workpieces have been developed, according to which:

[0127] 1) Deposit 40 layers of powder successively and heat each of them to T bc (for these 40 layers, the laser is not used to selectively melt the powder); then

[0128] 2) Starting from the 41st layer, for each layer, as soon as the temperature of the powder layer reaches T bc , start the 3D construction of selectively fusing the powder by laser, thus constructing a defined workpiece; then

[0129] 3) Deposit 40 layers of powder successively and heat each of them to T bc (for these 40 layers, the laser is not used to selectively melt the powder).

[0130] The workpiece defined in step 2) above is a type 1B tensile dumbbell (length 15 cm, thickness 4 mm) according to ISO 527-2 as shown in Figure 1 , which is particularly sensitive to curling, or a workpiece with 10 holes of different sizes (thickness 4 mm) as shown in Figure 2 , which is particularly sensitive to caking.

[0131] For each powder tested, five tensile dumbbells of type 1B and two workpieces with ten holes were manufactured according to the above model construction.

[0132] Then a two-step protocol based on a temperature scan of the build chamber was carried out to determine the limits of the working window of the powder. These two steps are as follows:

[0133] 1) First, the above model construction was carried out at T bc = 168 °C. Then it was repeated several times, increasing the temperature of the build chamber by 2 °C each time until T bc = 176 °C was reached. Then the machine was cooled and the 3D object was retrieved.

[0134] 2) The above model construction was carried out again at T bc = 168 °C. Then it was repeated several times, decreasing the temperature of the build chamber by 2 °C each time until T bc = 160 °C was reached. Then the machine was cooled and the 3D object was retrieved.

[0135] For each build chamber temperature (T bc ) tested, the following were determined:

[0136] – The deformation of the dumbbells (reflecting the sensitivity to curling), and

[0137] – The ease of cleaning the workpiece with ten holes (representing the sensitivity to caking).

[0138] The deformation of the dumbbells was determined by measuring their flatness. The flatter the specimens (dumbbells), the less obvious the deformation, and thus the manufactured 3D object will correspond to the expected geometry. To determine their flatness, each test sample was placed on a flat surface, a 1 kg weight was placed on the left gripping part of the dumbbell, and the distance between the end of the right gripping part and the flat surface was measured. This measurement was repeated on each of the four longitudinal faces of the dumbbell, and the maximum measured distance corresponded to the deformation adopted.

[0139] For each build temperature tested, the deformation of each of the five workpieces built at that build temperature was measured, and the final deformation was the average of the five measurements taken (this allows for the temperature variations at the surface of the powder bed to be considered).

[0140] A distance less than or equal to 2 mm was considered to meet the geometric requirements of the 3D object and thus corresponded to an unobvious curling phenomenon.

[0141] The lower limit of the working window corresponded to the lowest temperature of the build chamber at which a distance less than or equal to 2 mm was obtained as the final deformation.

[0142] In the absence of sandblasting, a compressed air blower was used to evaluate the ease or difficulty of cleaning a workpiece having 10 holes of different sizes. The more holes of the workpiece that were unblocked using the blower, the easier the cleaning. A score out of 10 was given based on the number of unblocked holes. For example, a powder with 10 unblocked holes was given a score of 10 / 10 and would thus be the easiest to clean. Conversely, a score of 0 / 10 was given to a powder with no unblocked holes and that was thus strongly agglomerated and the most difficult to clean (resulting in a longer cleaning time and potentially deterioration of some delicate and / or fragile elements of the constructed 3D workpiece).

[0143] For each build temperature tested, each of the two workpieces built at the build temperature was scored, and the final score was the average of the two scores obtained (this allows for taking into account the temperature variations at the surface of the powder bed).

[0144] A score greater than or equal to 8 / 10 was considered to maintain satisfactory cleanability and thus corresponded to a non-significant caking phenomenon.

[0145] The upper limit of the working window corresponded to the highest temperature of the build chamber at which a score greater than or equal to 8 / 10 was obtained.

[0146] Preparation of polyamide powder

[0147] The powder preparation method described below was repeated until a sufficient amount of powder was obtained to determine the working window.

[0148] Example 1 (comparative): Preparation of PA 12 powder

[0149] This comparative example was similar to Example 4 from document EP 1571173.

[0150] 2800 ml of solvent (an alkane fraction with a boiling range between 145 °C and 160 °C) was introduced into a reactor maintained under nitrogen, followed successively by 899 g of dry lauryllactam, 4.95 g of EBS, and 0.36 g of finely divided and dehydrated silica. After starting stirring at 300 rpm, the mixture was gradually heated to 110 °C, and then 290 ml of solvent was distilled off under vacuum to entrain any trace of water that might be present by azeotropic formation.

[0151] After returning to atmospheric pressure, an anionic catalyst, 1.79 g of sodium hydride with 60 wt% purity in oil, was then quickly introduced under nitrogen, and the stirring was increased to 400 rpm at 110 °C for 30 minutes under nitrogen. Next, the temperature was brought back to 100.5 °C, and a stearyl isocyanate activator was continuously injected into the reaction medium using a small metering pump according to the following procedure:

[0152] –3.6 g of stearyl isocyanate was added over 60 minutes, and then

[0153] –5.9 g of stearyl isocyanate was added over 132 minutes.

[0154] Meanwhile, the temperature was maintained at 100.5 °C for the first 60 minutes, then raised to 120 °C over 30 minutes, and held at 120 °C for 2 hours after the addition of stearyl isocyanate was complete.

[0155] The polymerization was then terminated, the reaction medium was cooled to 80 °C, and the powder was separated by sedimentation and dried.

[0156] The resulting PA 12 powder particles had a volume average diameter of 55 μm, an intrinsic viscosity of 1.48 (g / 100g) -1 a melting temperature of 183 °C and a crystallization temperature of 135 °C.

[0157] The working window was evaluated as described above. It was 168 °C, and model construction was only possible at this temperature.

[0158] Example 2 (according to the invention): Preparation of PA 12 powder

[0159] 2800 ml of solvent (an alkane fraction with a boiling range between 145 °C and 160 °C) was introduced into a reactor maintained under nitrogen, followed successively by 919 g of laurolactam (lactam 12), 15.0 g of EBS, and 3.2 g of silica (SIPERNAT 320DS). After starting stirring at 300 rpm, the mixture was gradually heated to 105 °C, and then 360 ml of solvent was distilled off under vacuum to azeotropically entrain any trace amounts of water that might be present.

[0160] After returning to atmospheric pressure, an anionic catalyst, 2.4 g of sodium hydride with 60 wt% purity in oil, was quickly introduced under nitrogen, and the stirring was increased to 550 rpm at 105 °C for 30 minutes under nitrogen.

[0161] According to the following procedure, the selected activator, stearyl isocyanate (27.3 g in 119.7 g of solvent), was continuously injected into the reaction medium using a small metering pump:

[0162] – A solution of 12 g / h of stearyl isocyanate was added over 180 minutes, and then

[0163] – A solution of 50 g / h of stearyl isocyanate was added over 133 minutes.

[0164] Meanwhile, the temperature was maintained at 105 °C for 313 minutes during the injection, and then, after the introduction of the isocyanate was completed, the reaction medium was heated to 150 °C within 30 minutes and maintained at this temperature for 12 hours.

[0165] 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 its solvent.

[0166] The obtained PA 12 powder particles had a volume average diameter of 38 μm, an intrinsic viscosity of 1.32 (g / 100g) -1 , a melting temperature of 184 °C (associated with a melting enthalpy of 112 J / g), and a crystallization temperature of 146 °C.

[0167] The working window was evaluated as described above. It was 166 °C to 172 °C.

[0168] Example 3 (according to the invention): Preparation of PA 12 / 6 powder

[0169] 2800 ml of solvent (an alkane fraction with a boiling range between 145 °C and 160 °C) was introduced into a reactor maintained under nitrogen, followed successively by 919 g of laurolactam (lactam 12), 4.6 g of caprolactam (lactam 6), 14.0 g of EBS, and 4.2 g of Orgasol® 2001UD Nat 1 (PA 12 powder). After starting stirring at 300 rpm, the mixture was gradually heated to 105 °C, and then 360 ml of solvent was distilled off under vacuum to entrain any trace amounts of water that might be present by azeotropy.

[0170] After returning to atmospheric pressure, an anionic catalyst, 3.9 g of sodium hydride with a purity of 60 wt% in oil, was rapidly introduced under nitrogen, and the stirring was increased to 550 rpm at 105 °C for 30 minutes under nitrogen.

[0171] According to the following procedure, the selected activator, stearyl isocyanate (25.3 g in 185.1 g of solvent), was continuously injected into the reaction medium using a small metering pump:

[0172] – A solution of stearyl isocyanate at 12 g / h for 180 minutes, and then

[0173] – A solution of stearyl isocyanate at 50 g / h for 210 minutes.

[0174] Meanwhile, the temperature was maintained at 105 °C for 390 minutes during the injection, then raised to 150 °C within 30 minutes, and maintained at this temperature for 12 hours after the introduction of the isocyanate was completed.

[0175] At the end of the polymerization, the polyamide powder is dispersed in the synthesis solvent. The reaction medium is cooled to 80 °C in order to be able to empty the reactor: after solid / liquid separation, the polyamide powder is placed in an oven at 75 °C in order to dry its solvent.

[0176] The obtained PA 12 / 6 powder particles have a volume average diameter of 39 μm, an intrinsic viscosity of 1.34 (g / 100g) -1 , a melting temperature of 183 °C (associated with a melting enthalpy of 109 J / g), and a crystallization temperature of 145 °C.

[0177] The working window is evaluated as described above. It is from 164 °C to 172 °C.

[0178] Example 4 (comparative): Preparation of PA 12 powder with subsequent heat treatment

[0179] The particles of the PA 12 powder of Orgasol® 2002ES4 Nat 3 have a volume average diameter of 41 μm, an intrinsic viscosity of 1.02 (g / 100g) -1 , a melting temperature of 177 °C (associated with a melting enthalpy of 112 J / g) and a crystallization temperature of 150 °C.

[0180] The working window is evaluated as described above. The powder has no working window.

[0181] As described in document WO 2013 / 090174, the PA12 powder is heat-treated at 167 °C for 88 h in a stirred reactor under a nitrogen stream. The powder particles after heat treatment have a volume average diameter of 41 μm, an intrinsic viscosity of 1.02 (g / 100g) -1 , a melting temperature of 182 °C (with an obvious shoulder peak at 180 °C on the DSC curve) associated with a melting enthalpy of 118 J / g, and a crystallization temperature of 150 °C.

[0182] The working window is evaluated as described above. It is 168 °C, and model construction is only possible at this temperature.

[0183] Example 5 (according to the invention): Preparation of PA 12 / 6 powder

[0184] 2800 ml of solvent (an alkane fraction with a boiling range between 145 °C and 160 °C) was introduced into a reactor maintained under nitrogen, followed successively by the addition of 919 g of laurolactam (lactam 12), 4.6 g of caprolactam (lactam 6), 14.0 g of EBS, and 3.0 g of Orgasol® 2001UD Nat 1 (PA 12 powder). After starting stirring at 300 rpm, the mixture was gradually heated to 105 °C, and then 360 ml of solvent was distilled off under vacuum to entrain any trace amounts of water that might be present by azeotropy.

[0185] After returning to atmospheric pressure, an anionic catalyst, 3.9 g of sodium hydride with 60 wt% purity in oil, was rapidly introduced under nitrogen, and the stirring was increased to 550 rpm at 105 °C for 30 minutes under nitrogen.

[0186] According to the following procedure, the selected activator, stearyl isocyanate (25.3 g in 185.1 g of solvent), was continuously injected into the reaction medium using a small metering pump:

[0187] – A solution of stearyl isocyanate at 50 g / h for 253 minutes.

[0188] At the same time, the temperature was maintained at 105 °C for 253 minutes during the injection, then raised to 150 °C in 30 minutes, and held at this temperature for 6 hours after the introduction of the isocyanate was completed.

[0189] 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 solid / liquid separation, the polyamide powder was placed in an oven at 75 °C to dry its solvent.

[0190] The obtained PA 12 / 6 powder particles had a volume average diameter of 42 μm, an intrinsic viscosity of 1.16 (g / 100g) -1 , a melting temperature of 182 °C (associated with a melting enthalpy of 122 J / g), and a crystallization temperature of 149 °C.

[0191] The working window was evaluated as described above. It was 168 °C to 172 °C.

[0192] Results

[0193] It has been found that the powder according to the invention has a wide working temperature window for 3D printing, at least 4 °C when the curing step is carried out at 150 °C for 6 hours and at least 6 °C when the curing step is carried out at 150 °C for 12 hours. In addition, it should be noted that the PA 12 powder according to the invention of Example 2 has a very enlarged working temperature window compared to the PA 12 powder of Comparative Example 1, where the curing step is carried out for a period of 2 h at 120 °C.

[0194] Furthermore, when the powder is post-thermally treated (Comparative Example 4), even if the Tm-Tc difference increases, an enlargement of the working window cannot be achieved. Based on the intrinsic viscosity results before and after the heat treatment, no change in molar mass was observed. These results indicate that the curing step must be carried out on the compound under polymerization conditions in order to obtain an enlargement of the working window.

Claims

1. A method for manufacturing polyamide powder by anionic polymerization in a solvent, comprising the following steps: a) Forming a reaction medium, comprising: – Introducing at least one lactam monomer into the solvent; – Introducing at least one catalyst into the solvent; and – Introducing at least one activator into the solvent at a given temperature; b) Polymerizing the lactam monomer into polyamide in the reaction medium; c) Precipitating the polyamide in powder form in the reaction medium; and d) After step a), and preferably after step c), heating the reaction medium to a temperature higher than the temperature for introducing at least one activator and in the range of 140 °C to 200 °C.

2. The method according to claim 1, wherein the heating in step d) is carried out for a time greater than or equal to 2 h, preferably greater than or equal to 3 h, more preferably greater than or equal to 5 h, and more preferably for a time of 8 to 15 h, more preferably 10 to 12 h.

3. The method according to claim 1 or 2, wherein step a) further comprises introducing at least one filler into the solvent, the at least one filler being preferably a mineral filler, preferably silica, and / or an organic filler, preferably polyamide powder.

4. The method according to any one of claims 1 to 3, wherein step a) further comprises introducing at least one amide into the solvent, the at least one amide being preferably N,N'-alkylenediamide, more preferably N,N'-ethylenebisstearamide and / or N,N'-ethylenebisoleamide.

5. The method according to any one of claims 1 to 4, wherein the at least one lactam monomer is selected from 2-pyrrolidone, caprolactam, 2-azacyclononanone, lauryl lactam, and mixtures thereof.

6. The method according to any one of claims 1 to 5, wherein the at least one catalyst is selected from sodium, potassium, alkali metal hydrides and hydroxides, alkali metal alkoxides, and mixtures thereof, preferably selected from sodium hydride, potassium hydride, sodium, sodium methoxide, sodium ethoxide, and mixtures thereof.

7. The method according to any one of claims 1 to 6, wherein the at least one activator is selected from lactam-N-carboxyanilide, (mono)isocyanate, polyisocyanate, carbodiimide, cyanamide, acyl lactam and acyl carbamate, triazine, urea, N-substituted imide, ester, phosphorus trichloride, and mixtures thereof.

8. The method according to any one of claims 1 to 7, wherein the solvent is an alkane fraction having a boiling temperature range of 120 °C to 170 °C.

9. The method according to any one of claims 1 to 8, wherein the heating temperature in step d) is 140 °C to 170 °C, preferably 145 °C to 160 °C, and more preferably 145 °C to 155 °C.

10. The method according to any one of claims 1 to 9, wherein the temperature for introducing the at least one activator is 50 °C to 150 °C, preferably 60 °C to 135 °C.

11. The method according to any one of claims 1 to 10 further comprises the step of introducing one or more additives into the solvent, said additives being selected from pigments, colorants, carbon black, carbon nanotubes, antioxidants, UV stabilizers and plasticizers.

12. A polyamide powder obtained by the manufacturing method according to any one of claims 1 to 11.

13. Use of the powder according to claim 12 for constructing a three-dimensional article, preferably by layer-by-layer construction, more preferably by sintering, even more preferably by sintering mediated by electromagnetic radiation.

14. Use of the powder according to claim 12 for manufacturing a composite, a substrate coating, a transfer paper, a liquid or solid ink composition, a liquid or solid paint, a structural adhesive, a cosmetic composition or a pharmaceutical composition.

15. A method for manufacturing a three-dimensional article, comprising the following steps: – manufacturing a powder by the method according to any one of claims 1 to 11; – depositing the powder, preferably depositing the powder in the form of a layer; and – sintering the powder, preferably sintering the powder by electromagnetic radiation.

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